Waste-free production of phosphates using dilute sulfuric acid

Abstract

Waste-free phosphoric acid and methods for making and using the same are provided. Aspects of the methods include producing concentrated phosphoric acid by electrochemically producing a dilute sulfuric acid (H2SO4) stream from aqueous sodium sulfate, contacting the sulfuric acid with rock phosphorus to produce dilute phosphoric acid (H3PO4) and a gypsum (CaSO4•nH2O) slurry, and concentrating the dilute phosphoric acid to produce the concentrated phosphoric acid. Also provided are systems for practicing the methods, as well as phosphate products produced by the methods.

Description

[0001] Atty. Dkt. No. TVTI-006WO

[0002] WASTE-FREE PRODUCTION OF PHOSPHATES USING DILUTE SULFURIC ACID

[0003] CROSS-REFERENCE TO RELATED APPLICATIONS

[0004] This application claims the benefit of the earlier filing date of U. S. Provisional Patent Application Serial No. 63 / 761,432, filed February 21, 2025, U. S. Provisional Patent Application Serial No. 63 / 913,534, filed November 7, 2025, U. S. Provisional Patent Application Serial No.63 / 934, 864, filed December 9, 2025, and U. S. Provisional Patent Application Serial No. 63 / 961,053, filed January 15, 2026, all of which prior applications are incorporated herein by reference.

[0005] BACKGROUND

[0006] Global anthropogenic carbon dioxide (CO2) emissions are approximately 50 gigatons per year, and affordable solutions to durably sequester CO2 are needed to prevent catastrophic climate change (Mac Dowell et al., 2017; Sullivan et al., 2021). Recent IPCC projections indicate that around 6 billion metric tons (Gt) per year of direct air capture of CO2 with durable storage (DAGS) are required to reduce atmospheric GO2 concentrations to levels that safely limit global warming (Riahi et al., 2022). Formation of carbonate minerals represents a safe, stable, and geologically permanent way to remove and sequester carbon dioxide (Lal, 2008; Mac Dowell et al., 2022), but mineral carbon sequestration requires both a source of carbon dioxide reactive elements (e.g., calcium and magnesium) and a permanent sink for acidity (i.e., an alkaline material such as rock phosphorus or ultramafic mine tailings). Achieving cost-effective carbon dioxide reduction (CDR) that can be scaled to gigatons per year of CO2 sequestration poses a major technological challenge. Direct air capture of CO2 is energy intensive, and many leading direct air capture technologies require several gigajoules of energy - often as heat - to sequester one ton of CO2 (Zeman, 2007; Osman et al., 2021). Production of valuable co-products, such as cement products (Rau et al., 2013; Rau et al., 2018; La Plante et al., 2021), or coupling carbon removal with existing extractive processes (Lu et al., 2022) can help improve the economic viability of CDR.

[0007] Rock phosphorus represents a major geological alkalinity source that has been largely overlooked for mineral carbon sequestration. Conventional phosphate fertilizer production from rock phosphorus (mainly Cas(PO4)3X, where X is OH-, F’, and / or Cl ) requires large amounts of sulfuric acid and emits carbon dioxide to the atmosphere. Wet phosphoric acid (WPA) isAtty. Dkt. No. TVTI-006WO

[0008] conventionally produced by reacting rock phosphorus with sulfuric acid, recycled phosphoric acid, and water (Davenport et al.,1965). Most sulfuric acid for phosphoric acid production is produced by burning - or oxidizing - elemental sulfur derived from fossil fuel refining.

[0009] Production of phosphoric acid (H3PO4) consumes around 60% of the global sulfuric acid supply and generates 200-300 Mt of waste gypsum (i.e., phosphogypsum, or PG) annually (King & Moates, 2013). Rock phosphorus processing neutralizes sulfuric acid to produce a weak acid, phosphoric acid, by the reaction using a component of rock phosphorus, fluorapatite, as an example,

[0010] Ca5(PO4)3p(s, fluorapatite) + 5H2SO4(aq) + 10H2O(l) - 3H3PO4(aq) + 5CaSO4*2H2O(s, gypsum) + HF(aq).

[0011] Phosphogypsum can be produced in several forms, as dihydrate (gypsum, CaSO4*2H2O), hemihydrate (CaSO4*0.5H2O), or anhydrite (CaSO4), depending on the temperature and water concentration of the phosphoric acid reactor. Most phosphoric acid is produced from sedimentary rock phosphate containing carbonate mineral phases that release carbon dioxide during the sulfuric acid digestion step. Igneous phosphate ore concentrates produce less carbon dioxide then sedimentary phosphate rock during digestion, but both rock sources produce large amounts of phosphogypsum waste.

[0012] SUMMARY

[0013] The present disclosure provides a novel approach for phosphoric acid production with reduced phosphogypsum waste. The inventors have realized that conventional production of phosphoric acid and related products, such as monoammonium phosphate, diammonium phosphate, and ammonium sulfate fertilizers results in an overwhelming mass of phosphogypsum waste, as well as an unacceptable level of CO2greenhouse gas emissions. Both waste products pose significant ecological and environmental risks. As such, an improved process for phosphoric acid production with reduced phosphogypsum waste and CO2emissions is needed. The methods and systems disclosed herein satisfy this need.

[0014] Aspects of the disclosure include methods of phosphoric acid production. Disclosed methods include producing concentrated phosphoric acid by producing a dilute sulfuric acid (H2SO4) stream from aqueous sodium sulfate by electrochemical salt splitting, contacting the sulfuric acid with rock phosphorus to produce dilute phosphoric acid (H3PO4) and a gypsumAtty. Dkt. No. TVTI-006WO

[0015] (CaSO4*nH2O) slurry, and concentrating the dilute phosphoric acid to produce the concentrated phosphoric acid.

[0016] Protocols for electrolyzing the aqueous sulfate can vary. In some cases, electrolytic protocols include producing an acidic solution and / or a basic solution. In some cases, the acidic solution comprises sulfuric acid. In some such cases, methods may include reacting the rock phosphorus with the dilute sulfuric acid solution obtained from electrolyzing the aqueous sulfate to produce the phosphoric acid and the gypsum (CaSO4*nH2O) slurry. In additional embodiments, the H2SO4may be used to produce wet phosphoric acid (WPA) from rock phosphorus, initially producing dilute phosphoric acid, followed by optional purification and / or concentration to produce concentrated phosphoric acid. In some embodiments, methods include concentrating the sulfuric acid obtained from electrolyzing the aqueous sulfate prior to reacting it with the rock phosphorus (e.g., at concentrations ranging from 5 wt.% to 98 wt.%). In such embodiments, wherein dilute electrolytic sulfuric acid is concentrated prior to reaction with rock phosphorus, the dilute electrolytic sulfuric acid may be concentrated by reverse osmosis, evaporation, such as multi-effect evaporation or evaporation with indirect mechanical vapor recompression, or both. In one aspect, such concentration of sulfuric acid prior to phosphate rock digestion simplifies or eliminates subsequent processing steps needed to produce concentrated phosphoric acid.

[0017] In certain embodiments, purifying the dilute phosphoric acid comprises feeding the dilute phosphoric acid through a nanofiltration membrane to produce a nanofiltration permeate and a nanofiltration retentate. In such embodiments, the nanofiltration permeate may be enriched in phosphate. Likewise, the nanofiltration retentate may be enriched in impurities. In one aspect the nanofiltration permeate is fed through multiple nanofiltration membranes to yield, in each instance a permeate further enriched in phosphate. In one embodiment, the nanofiltration permeate has a P2Os concentration of from 15 to 21% P2O5, 5% to 25%, or 5% to 18% (w / w).

[0018] In certain embodiments the retentate is further processed to isolate the impurities, such as wherein the retentate contains magnesium. In one aspect of such embodiments, the retentate is contacted with alkali, such as sodium hydroxide, in one or more steps to produce magnesium hydroxide and calcium phosphate. The calcium phosphate may optionally be recycled to phosphate rock digestion.

[0019] In a further embodiment, concentrating the dilute phosphoric acid comprises contacting the nanofiltration permeate with a reverse osmosis membrane to produce an intermediate phosphate concentrate. Such intermediate phosphate concentrates have increased phosphate content, such as wherein the intermediate phosphate concentrate has a P2Os concentration ofAtty. Dkt. No. TVTI-006WO

[0020] from 15% to 40%, such as 15% to 25%, or 18% to 22% (w / w).

[0021] In one embodiment, the nanofiltration permeate can be concentrated to produce a concentrated phosphoric acid without employing a reverse osmosis membrane. In other embodiments, the nanofiltration permeate is contacted with a reverse osmosis membrane to produce an intermediate concentrate that is further concentrated by evaporation to produce the concentrated phosphoric acid. In certain embodiments disclosed herein, the concentrated phosphoric acid has a P2O5 concentration of from 45% to 83% (w / w) or from 45% to 62% (w / w).

[0022] In embodiments, concentrating the dilute phosphoric acid includes using evaporation, for example, evaporation with indirect mechanical vapor recompression. In such embodiments, the dilute phosphoric acid may be purified or concentrated, or both, by nanofiltration, reverse osmosis, or both prior to concentration via evaporation.

[0023] In one aspect, the disclosed methods may comprise a defluorination step. In one embodiment, the method comprises defluorinating the dilute phosphoric acid, the nanofiltration permeate, the intermediate phosphate concentrate, the concentrated phosphoric acid, or a combination thereof. In some cases, concentration includes defluorination, such as when evaporative concentration is used, for example with indirect mechanical vapor recompression. In such examples, the corrosive HF-steam is isolated from the recompression equipment by routing through a heat exchanger or water scrubber.

[0024] In a further embodiment, the disclosed methods comprise extracting phosphoric acid from the nanofiltration permeate, the intermediate phosphate concentrate, the concentrated phosphoric acid, or a combination thereof.

[0025] In another aspect of the process disclosed herein, the gypsum (CaSO4*nH2O) slurry is further processed to reduce or eliminate the phosphogypsum waste that is a byproduct of contacting rock phosphorus with dilute sulfuric acid. In one aspect, the further processing comprises converting the gypsum (CaSO4*nH2O) slurry to a calcium-containing solid product and thereby generating an aqueous sulfate. The aqueous sulfate produced by processing the gypsum (CaSO4*nH2O) slurry may be recycled back into the process as recycled aqueous sulfate. In such embodiments, the recycled aqueous sulfate may be directed to an electrolysis or electrodialysis unit to produce dilute sulfuric acid via electrolysis or electrodialysis. Recycled aqueous sulfate may be recirculated for electrolysis with or without prior purification or concentration. In one embodiment, the recycled aqueous sulfate is purified prior to electrolysis. In one embodiment, the recycled aqueous sulfate is concentrated prior to electrolysis, such as to a concentration of greater than 1 M. In a further embodiment, the recycled aqueous sulfate is purified and concentrated prior to electrolysis.Atty. Dkt. No. TVTI-006WO

[0026] In one embodiment, producing a dilute sulfuric acid (H2SO4) stream from aqueous sodium sulfate by electrochemical salt splitting, further comprises producing an alkaline stream. In exemplary embodiments, the alkaline stream comprises hydroxide, such as a metal hydroxide. The alkaline stream also may be directed into the disclosed process. For example, in one embodiment, the alkaline stream is contacted with a carbon dioxide (CO2)-comprising gaseous stream to produce a solution comprising an aqueous carbonate and a 002-depleted gaseous stream.

[0027] In a further aspect of the present process, the gypsum (CaSO4•nH2O) slurry, such as is produced by contacting the sulfuric acid with rock phosphorus, is contacted with the solution comprising the aqueous carbonate. In one embodiment, contacting the gypsum (CaSO4•nH2O) slurry with the aqueous carbonate results in precipitating a calcium-containing solid product. In one aspect, precipitating the calcium-containing solid product yields a sulfate brine such as sodium sulfate brine, which may be separated from the calcium-containing solid product and recycled as aqueous sodium sulfate for electrolysis or electrodialysis. In one embodiment of such processes, phosphogypsum waste is substantially avoided.

[0028] In one embodiment, the alkaline stream is contacted with the gypsum (CaSO4*nH2O) slurry, prior to being contacted with a CO2-comprising gaseous stream. In one aspect of such an embodiment, gypsum (CaSO4*nH2O) slurry is contacted with the alkaline stream, to produce slaked lime (calcium hydroxide (Ca(OH)2)). In a further aspect, the calcium hydroxide is precipitated to produce a calcium-containing solid product and a separated alkaline solution containing sodium sulfate and sodium hydroxide. The solution may be recycled for electrolysis to produce dilute sulfuric acid, or optionally, prior to electrolysis, may be contacted with the CO2-comprising gaseous stream to produce a solution comprising an aqueous carbonate and a CO2-depleted gaseous stream.

[0029] The calcium-containing solid product produced in the subject methods may vary, and can include, for example, precipitated calcium carbonate (PCC), calcium hydroxide, or both in the alkaline solid product. Thus, in one embodiment, contacting the gypsum (CaSO4*nH2O) slurry with the aqueous carbonate results in calcium carbonate. In one aspect, the process further involves precipitating calcium carbonate to produce the calcium-containing solid product. In certain versions wherein PCC is produced, methods may further include calcining the PCC, calcium hydroxide, or both, to produce lime (CaO). Other embodiments include making a hydraulic cement from calcium hydroxide produced in the subject methods. In some cases, methods include producing a concrete using the hydraulic cement.

[0030] In certain embodiments, the processes and systems disclosed herein receive CO2 input,Atty. Dkt. No. TVTI-006WO

[0031] and in some such versions, methods of the invention include a CO2 sequestering protocol. Such protocols may include sequestering gaseous CO2 from, for example, a point source (e.g., a flue gas) or from the air via direct air capture (DAC). In particular embodiments, the CO2-comprising gaseous stream comprises greater than 5% (v / v) CO2.

[0032] In certain instances, the CO2 sequestering protocol comprises reacting gaseous CO2 with a base to produce an aqueous carbonate. In some cases, the method comprises reacting the aqueous carbonate with the gypsum (CaSO4’nH2O) slurry to produce the calcium-containing solid product. In other instances, the method comprises contacting gypsum (CaSO4*nH2O) slurry with an alkaline stream, such as a stream produced by electrolysis, to produce the calcium-containing solid product. The amount of CO2 sequestered may vary, and in some cases the processes are net carbon negative.

[0033] In certain embodiments, the dilute sulfuric acid stream produced by electrolysis comprises from 10 weight % to 30 weight % sulfuric acid. At least a portion of the dilute sulfuric acid stream may in some embodiments be used directly in phosphate rock digestion.

[0034] The disclosed methods and systems yield additional products that may be used in the disclosed processes, or may be captured for additional uses. By way of example, electrolyzing the aqueous sulfate includes producing gaseous oxygen (O2) and producing gaseous hydrogen (H2). In certain embodiments, hydrogen and / or oxygen may also be recycled to the anode or cathode, respectively. In other embodiments, evolved hydrogen, oxygen, or both, are captured for other uses. In embodiments, electrolyzing the aqueous sulfate includes electrolysis, electrodialysis, or both. In certain embodiments, the electrolyzing comprises the use of a membrane separating an anode chamber comprising an anolyte and a cathode chamber comprising a catholyte. In some such embodiments, the methods involve the use of an electrochemical salt splitting system such as an anion exchange membrane separated two-chamber cell system, a cation exchange membrane separated two-chamber cell system, a three-chamber cell system containing both an anion exchange membrane and a cation exchange membrane, or a bipolar membrane electrodialysis system comprising a stack of cells with an anion exchange membrane, a cation exchange membrane, and a bipolar membrane. In certain cases, the anion exchange membrane is configured so that sulfate anions cross an anion exchange membrane to the chamber where sulfuric acid is generated. Select embodiments of the methods also include maintaining a concentration of base in the catholyte or the center chamber that is low relative to the concentration of acid in the anolyte or the acid chamber, and recirculating fluid through the cathode chamber and the center chamber when present.Atty. Dkt. No. TVTI-006WO

[0035] Also disclosed herein are certain systems configured to accomplish embodiments of the presently disclosed processes. For example, one embodiment of a system comprises an electrolysis unit configured to electrolyze aqueous sulfate to produce a dilute sulfuric acid stream; a reactor configured to react rock phosphorus with the dilute sulfuric acid to produce phosphoric acid; and a nanofiltration unit configured to receive the phosphoric acid. In one embodiment, the nanofiltration unit receives phosphoric acid directly from the reactor without further processing. In one aspect, the nanofiltration unit separates the phosphoric acid received into a permeate enriched in phosphoric acid and a retentate enriched in impurities. In one aspect the permeate is poor in impurities. In one aspect of systems adapted for nanofiltration, the nanofiltration unit is configured to provide a nanofiltration permeate comprising dilute purified phosphoric acid. By way of example, the nanofiltration permeate may have a P2O5 concentration that is higher or lower than the phosphoric acid received. In certain embodiments, the nanofiltration permeate has a phosphoric acid concentration of from 5% to 22% (w / w). In one aspect of such systems, the nanofiltration unit is configured to return a nanofiltration retentate to the reactor. In one aspect of the systems disclosed herein, the nanofiltration unit is configured to provide the nanofiltration permeate to a reverse osmosis unit. In systems comprising a reverse osmosis unit, the reverse osmosis unit may be configured to produce an intermediate phosphate concentrate, such as wherein the intermediate phosphate concentrate has a P2O5 concentration of from about 15% to about 40% (w / w). In yet further embodiments of systems disclosed herein, the systems may further include a phosphoric acid extraction unit, such as a solvent extraction system.

[0036] In certain embodiments the dilute phosphoric acid produced via rock phosphate digestion is optionally subjected to desulfation. Desulfation as exemplified herein comprises contacting the dilute phosphoric acid with a calcium-containing material, such as a solid calcium-containing material. In such embodiment, desulfation may be followed by calcium removal prior to concentration of the phosphoric acid.

[0037] Embodiments of systems for producing phosphoric acid described herein may further include a precipitator configured to generate a calcium-containing solid product. Typically the precipitator is configured to receive gypsum slurry from the phosphate rock reactor. The precipitator may further be configured to recirculate the aqueous sulfate to the electrolysis unit.

[0038] As noted above, systems also may include a reactor configured to react rock phosphorus with sulfuric acid (H2SO4). Such systems may also be configured to return recycled phosphoric acid to the reactor to produce phosphoric acid (H3PO4) and gypsum (CaSO4*nH2O) slurry, thereby eliminating phosphogypsum waste byproduct. Systems may additionally includeAtty. Dkt. No. TVTI-006WO

[0039] an ammonia synthesizer configured to synthesize ammonia (NH3) using the hydrogen produced by the electrolyzer, and an ammonium phosphate generator configured to produce an ammonium salt from the ammonia.

[0040] In embodiments, systems of the invention include an electrolyzer stack of one or more salt splitting electrochemical cells comprising a two-chamber anion exchange membrane separated cell, a two-chamber cation exchange membrane separated cell, a three-chamber cell containing both an anion exchange membrane and a cation exchange membrane, or a bipolar membrane electrodialysis cell comprising anion exchange membranes, cation exchange membranes, and bipolar membranes. In certain versions of the invention, the precipitator is operably connected to a source of sulfate (e.g., calcium sulfate). In some cases, the gypsum (CaSC ’nHsO) slurry produced may be operably connected to the precipitator as the source of calcium sulfate. Embodiments of the subject systems may also include a hydrogen recovery module configured to receive hydrogen from the electrolyzer stack. In some cases, the systems described herein are configured as a continuous flow systems.

[0041] Also described herein are products of the invention. For example, aspects of the invention include phosphoric acid produced according to the subject methods, a hydraulic cement produced according to the subject methods, a concrete produced according to the subject methods, and / or a built structure produced from such a hydraulic cement produced according to the subject methods.

[0042] BRIEF DESCRIPTION OF THE FIGURES

[0043] The invention may be best understood from the following detailed description when read in conjunction with the accompanying drawings. Included in the drawings are the following figures:

[0044] FIG. 1 provides a block diagram of a system for phosphoric acid production using sulfuric acid electrochemically recycled using a 3-compartment electrolyzer and calcium carbonate using carbon dioxide captured from the air.

[0045] FIG. 2 provides a block diagram of a system for phosphoric acid production using sulfuric acid electrochemically recycled using a 3-compartment electrolyzer and calcium carbonate using carbon dioxide captured from a point source.

[0046] FIG. 3 provides a flow diagram of a system for phosphoric acid production using sulfuric acid electrochemically recycled using a 3-compartment electrolyzer and producing slaked lime.Atty. Dkt. No. TVTI-006WO

[0047] FIG. 4 depicts an exemplary three-compartment electrolyzer 400 comprising an AEM and a GEM.

[0048] FIG. 5 illustrates a system for phosphoric acid production using sulfuric acid electrochemically recycled using a 2-compartment electrolyzer and for production of calcium carbonate using carbon dioxide captured from a point source.

[0049] FIG. 6 illustrates a system for phosphoric acid production using sulfuric acid electrochemically recycled using a 2-compartment electrolyzer and producing slaked lime.

[0050] FIG. 7 provides a schematic diagram of an electrochemical cell, comprising an AEM, suitable for use in the systems according to FIG. 5 and FIG. 6.

[0051] FIG. 8 provides a flow chart illustrating phosphoric acid production using electrochemically recycled sulfuric acid and purification using nanofiltration membranes.

[0052] FIG. 9 provides a flow chart 900 illustrating a process for phosphoric acid production using electrochemically recycled sulfuric acid and purification using nanofiltration membranes including a preliminary reconcentration step using reverse osmosis.

[0053] FIG. 10 provides a flow chart illustrating phosphoric acid production using electrochemically recycled sulfuric acid and purification using solvent extraction.

[0054] FIG. 11 provides a flow chart illustrating phosphoric acid production using electrochemically recycled sulfuric acid and purification using solvent extraction including a preliminary reconcentration step using reverse osmosis.

[0055] FIG. 12 provides a flow chart illustrating phosphoric acid production using electrochemically recycled sulfuric acid and purification using solvent extraction with preliminary defluorination and including a preliminary reconcentration step using reverse osmosis.

[0056] FIG. 13 provides a flow chart illustrating a process for producing phosphoric acid using a concentrated recycled sulfuric acid solution.

[0057] FIG. 14 provides a flow chart illustrating a process for phosphoric acid production using a concentrated recycled sulfuric acid and a sulfuric acid solution concentrated from dilute electrolytic sulfuric acid by reverse osmosis and evaporation with mechanical vapor recompression.

[0058] FIG. 15 provides a flow chart illustrating a process for phosphoric acid production using a concentrated recycled sulfuric acid and a sulfuric acid solution concentrated from dilute electrolytic sulfuric acid by evaporation with mechanical vapor recompression.

[0059] FIG. 16 provides a flow chart illustrating a process for separation of radium during bassanite separation.Atty. Dkt. No. TVTI-006WO

[0060] FIG. 17 provides a flow chart illustrating a process for separation of rare earth elements (REE) from gypsum using a sulfonic acid chelating resin

[0061] FIG. 18 provides a flow chart 1800 illustrating a process for separation of REE from gypsum using oxalic acid chelation to precipitate REE oxalates.

[0062] FIG. 19 provides a flow chart illustrating a process using dilute electrochemically recycled sulfuric acid to produce phosphoric acid including multistage washing of gypsum.

[0063] FIG. 20 provides a flow chart illustrating a process for producing magnesium hydroxide and phosphoric acid using high magnesium content phosphate rock and dilute electrochemically generated sulfuric acid.

[0064] FIG. 21 provides a flow chart illustrating a process for phosphoric acid and magnesium production using high magnesium phosphate rock and a recycled sulfuric acid solution.

[0065] FIG. 22 depicts an exemplary three-compartment bipolar membrane electrodialysis cell 2200 comprising an AEM, a GEM, and a BPM.

[0066] FIG. 23 provides a plot of bipolar membrane component voltage as a function of applied current density comparing a conventional low-current bipolar membrane to two high-current bipolar membranes labeled “BPM 1” and “BPM 2”.

[0067] FIG. 24 provides an illustration of sulfuric acid reconcentration system configured as a multi-effect evaporator concentrating sulfuric acid from 12 to 50% w / w H2SO4.

[0068] FIG. 25 provides a plot of phosphoric acid digestion pilot data over time for sedimentary feed rock using a dilute sulfuric acid feed representative of electrochemical salt splitting supply.

[0069] FIG. 26 provides a phase diagram of calcium sulfate in the CaSC -HgPC - W system as a function of temperature and phosphoric acid concentration (% w / w P2O5).

[0070] FIG. 27 illustrates the results of replicate phosphate ore digestion experiments using 12% w / w sulfuric acid showing >90% extent of conversion within 300 minutes.

[0071] FIG. 28 provides two graphs illustrating the dependence of magnesium phosphate solubility on digester phosphoric acid pH (left-hand graph) and excess sulfate concentrations for a range of phosphoric acid concentrations (right-hand graph).

[0072] FIG. 29 provides concentrations of nanofiltration retentate as a function of total P2O5 recovery for high concentration (major) elements. Most elements are plotted in units of ppm, except sulfate, which is plotted on a different scale as %w / w SO4.

[0073] FIG. 30 provides a graph of elemental concentrations of nanofiltration retentate as a function of total P2O5 recovery for low concentration (trace) elements.Atty. Dkt. No. TVTI-006WO

[0074] FIG. 31 provides a graph of elemental concentrations of nanofiltration permeate as a function of total P2O5 recovery for high concentration (major) elements. Elements are plotted in units of ppm, except sulfate, which is plotted on a different scale as %w / w SO4.

[0075] FIG. 32 provides a graph of elemental concentrations of nanofiltration permeate as a function of total P2O5 recovery for low concentration (trace) elements.

[0076] FIG. 33 provides a plot of separation performance as rejection (%) as a function of P2O5 recovery for elements with high rejection.

[0077] FIG. 34 provides a plot of separation performance as rejection (%) as a function of P2O5 recovery for elements with low rejection.

[0078] FIG. 35 provides a plot of rejection versus feed concentration of two highly rejected elements compared to output of the nanofiltration model used to size a commercial-scale nanofiltration system.

[0079] FIG. 36 illustrates a phosphoric acid two-effect evaporation system concentrating phosphoric acid from 20 to 54% w / w P2O5 in stages 1 and 2 with separators E1 and E2.

[0080] DETAILED DESCRIPTION

[0081] The present disclosure provides a novel approach for phosphoric acid production with reduced phosphogypsum waste. The inventors have realized that conventional production of phosphoric acid and related products, such as monoammonium phosphate, diammonium phosphate, and ammonium sulfate fertilizers results in an overwhelming mass of phosphogypsum waste, as well as an unacceptable level of CO2 greenhouse gas emissions. Both waste products pose significant ecological and environmental risks. As such, an improved process for phosphoric acid production with reduced phosphogypsum waste and CO2 emissions is needed. The methods and systems disclosed herein satisfy this need.

[0082] In addition, systems for practicing methods of embodiments of the invention are provided.

[0083] In one aspect the present approach provides waste-free phosphoric acid production, where “waste-free” refers to the substantial conversion of phosphogypsum into value-added products. By way of example, such value-added products include precipitated calcium carbonate and slaked lime.

[0084] Before the present invention is described in greater detail, it is to be understood that this invention is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present inventionAtty. Dkt. No. TVTI-006WO

[0085] will be limited only by the appended claims.

[0086] Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges and are also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention.

[0087] Certain ranges are presented herein with numerical values being preceded by the term "about." The term "about" is used herein to provide literal support for the exact number that it precedes, as well as a number that is near to or approximately the number that the term precedes. In determining whether a number is near to or approximately a specifically recited number, the near or approximating unrecited number may be a number which, in the context in which it is presented, provides the substantial equivalent of the specifically recited number.

[0088] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention, representative illustrative methods and materials are now described.

[0089] All publications and patents cited in this specification are herein incorporated by reference as if each individual publication or patent were specifically and individually indicated to be incorporated by reference and are incorporated herein by reference to disclose and describe the methods and / or materials in connection with which the publications are cited. The citation of any publication is for its disclosure prior to the filing date and should not be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates which may need to be independently confirmed.

[0090] It is noted that, as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as “solely,” “only” and the like in connection with the recitation of claim elements, or use of a “negative” limitation.

[0091] As will be apparent to those of skill in the art upon reading this disclosure, each of theAtty. Dkt. No. TVTI-006WO

[0092] individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present invention. Any recited method can be carried out in the order of events recited or in any other order which is logically possible.

[0093] While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 U. S. C. §112, are not to be construed as necessarily limited in any way by the construction of "means" or "steps" limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 U. S. C. §112 are to be accorded full statutory equivalents under 35 U. S. C. §112.

[0094] METHODS OF PHOSPHORIC ACID PRODUCTION

[0095] The production of phosphoric acid via the "wet process" traditionally involves the digestion of phosphate rock with concentrated sulfuric acid (typically 93% to 98% H2SO4). While established, this conventional approach is characterized by high exothermic heat release that is difficult to manage, often leading to "blinding" of the rock particles with calcium sulfate, which prematurely halts the reaction and reduces phosphorus recovery. Furthermore, the process generates approximately five tons of phosphogypsum waste for every ton of phosphoric acid, creating significant environmental liabilities in the form of "gypsum stacks." These stacks pose a radiological risk primarily due to the presence of Radium-226 (226Ra). It is well established in the art that radionuclides such as226Ra disproportionately partition into the fine-grained fraction of the phosphogypsum matrix (Rutherford et al., 1994). Prior attempts to improve digestion efficiency have utilized multistage reactors to manage slurry flow and facilitate calcium sulfate crystal growth (e.g., U. S. Pat. No. 3,522,003). However, these traditional designs are optimized for concentrated acid feeds. When utilizing dilute sulfuric acid— such as the 10% to 30% w / w streams produced by electrochemical salt splitting or up to 50 to 70% w / w streams with intermediate reconcentration — standard reactor systems cannot maintain the reaction kinetics and precise sulfate saturation levels necessary for efficient digestion and subsequent mineral separation. There remains a need for an integrated system that utilizes an atmospheric multicompartment digestion, filtration, and washing system specifically configured to handle dilute acid streams. Such a system would ideally eliminate the need for energy-intensive acid concentration to 98% H2SO4 and high-pressure vessels, while ensuring the completeAtty. Dkt. No. TVTI-006WO

[0096] conversion of phosphate rock into high-purity phosphoric acid and value-added mineral coproducts, thereby achieving a truly waste-free production cycle.

[0097] As discussed above, aspects of the disclosure include methods of producing phosphoric acid with minimal waste. Aspects of the methods disclosed herein include phosphoric acid production by contacting rock phosphorus with sulfuric acid (H2SO4) produced by electrochemical salt splitting to produce phosphoric acid and a gypsum (CaSO4*nH2O) slurry.

[0098] Unless otherwise indicated, all %- and ppm-concentrations are expressed in weight-% (= wt.%) and ppm in weight (= ppm). Since phosphate can be present in different forms in a solution, the phosphate contents of solutions are expressed in % equivalent P2O5, indicated by % P2O5, as is customary and known to those of ordinary skill in the art. The concentration of phosphates in a solution is sometimes expressed in the art in terms of % equivalent H3PO4. For information, 1% P2O5 corresponds to 1.38% H3PO4.

[0099] By phosphoric acid production, it is meant that end products of the subject methods can include, inter alia, phosphoric acid, which may subsequently be employed in any suitable application. The methods described herein may also be employed to enhance carbon dioxide emissions reduction or to remove carbon dioxide from the atmosphere. For example, in some embodiments, use of the subject methods may reduce carbon dioxide emissions from a given source, such as a point source as compared to conventional methods by 20% or more, such as 25% or more, such as 30% or more, such as 35% or more, such as 40% or more, such as 45% or more, such as 50% or more, such as 55% or more, such as 60% or more, such as 65% or more, such as 70% or more, such as 75% or more such as 80% or more, such as 85% or more, such as 90% or more such as 95% or more, and including by 100%.

[0100] The suitable application in such cases may involve subsequent ammonium salt production, such as by contacting the concentrated phosphoric acid with ammonia to produce an ammonia salt. Accordingly, in certain versions, at least one of these suitable applications is ammonium salt or fertilizer production. In some cases, the protocols of the invention provide economic, environmental, and / or strategic co-benefits for the production of ammonium salts. For example, methods of the invention may be employed to reduce the amount of sulfate wastes that are produced as a result of ammonium salt production relative to conventional or currently available methods, or reduce existing stockpiles of sulfate wastes by 20% or more, such as 25% or more, such as 30% or more, such as 35% or more, such as 40% or more, such as 45% or more, such as 50% or more, such as 55% or more, such as 60% or more, such as 65% or more, such as 70% or more, such as 75% or more such as 80% or more, such as 85% or more, such as 90% or more such as 95% or more, and including by 100%.Atty. Dkt. No. TVTI-006WO

[0101] By “ammonium salt” it is meant a compound comprising ammonium cations (NH ) and one or more suitable anions. Any desirable ammonium salt may be produced via the methods described herein. In some embodiments, the ammonium salt is an ammonium phosphate produced from phosphoric acid as described herein. In the present disclosure, the term “ammonium phosphate” may refer to any salt comprising one or more ammonium cations and phosphate anions (PO43). For example, the ammonium phosphate may be comprised of monoammonium phosphate (MAP; (NH4)H2PO4), diammonium phosphate (DAP; (NFU HPCU), triammonium phosphate ((NH4)sPO4), ammonium polyphosphate ([NH4PO3]n(OH)2), and combinations thereof. In other embodiments, the ammonium salt is ammonium sulfate ((NH4)2SO4).

[0102] Methods of producing the phosphoric acid include reacting rock phosphorus with sulfuric acid (H2SO4) and optionally recycled phosphoric acid (H3PO4) to produce phosphoric acid and phosphogypsum (CaSO4*2H2O). “Rock phosphorus” — sometimes referred to as phosphorite, phosphate rock or rock phosphate — refers to a mineral source of phosphorous, generally corresponding to the formula Ca5(PO4)sX, where X is a suitable anion (e.g., F’, OH-, Br, or Cl ). In some cases, the rock phosphorus comprises fluorapatite (Ca5(PO4)sF), hydroxyapatite (Ca5(PO4)sOH or Caio(P04)6(OH)2), and / or chlorapatite (Ca5(PO4)3CI). In some cases, rock phosphorus may be found in, accompanied by, or interbedded with limestones, mudstones, shales, cherts, limestone, dolomites and sandstone, or may be concentrated from igneous deposits. Phosphogypsum is discussed herein in its conventional sense to describe the calcium sulfate of varied hydration states generally formed as a byproduct of phosphate production protocols. In some embodiments, the reaction of rock phosphorus with sulfuric acid proceeds as follows:

[0103] Ca5(PO4)3X(s) + 5H2SO4(aq) + 10H2O(l) -3H3PO4(aq)+ 5(CaSO4*2H2O)(s) + HX(aq)

[0104] The “X” in the above reaction may, in some cases, be F’, OH-, Br, or Cl’. Phosphogypsum may also include one or more of the following: SiO2, Cd, Al, Ba, Pb, Cr, Se, II, Fe, P, Th, Ra, and rare earth elements. While phosphogypsum is generally referred to herein using the dihydrate chemical formula CaSO4*2H2O, it is to be understood that other forms of phosphogypsum such as hemihydrate (bassanite; CaSO4*0.5H2O) and anhydrite (CaSCh) forms may equally be produced and employed by the present methods and systems instead of or in addition to the dihydrate form. With reference to FIG. 26, formation of the hemihydrate (bassanite) form ofAtty. Dkt. No. TVTI-006WO

[0105] calcium sulfate is favored over gypsum at elevated temperatures and phosphoric acid concentration.

[0106] Techniques for reacting rock phosphorus with sulfuric acid to produce phosphoric acid and phosphogypsum may vary. In some embodiments, methods include an acid leaching protocol. In such cases, the acid leaching protocol is a sulfuric acid leaching protocol, although leaching via other acids (e.g., HCI, H3PO4) is envisioned. In an acid leaching protocol, the rock phosphorus (e.g., in the form of an ore, etc.) may be combined with acid under conditions of high temperature to convert the minerals within the rock phosphorus into soluble salts.

[0107] Phosphoric acid produced according to such protocols may be referred to as wet phosphoric acid (WPA). In embodiments, acid leaching occurs within a reactor system configured as a multistage reactor containing two or more compartments compatible with a dilute sulfuric acid feed stream. In embodiments, acid leaching occurs within a reactor system configured as an autoclave at elevated pressure. Pressures that may be used can vary from ambient to high pressure and include, e.g., 0 MPa or more, 0.3 Mpa or more, 0.4 Mpa or more, 0.5 Mpa or more, 0.6 Mpa or more, 7 Mpa or more, 8 Mpa or more, and including 9 Mpa or more.

[0108] Temperatures that may be used range from, e.g., 330 K to 600 K, such as 330 K to 360 K, such as 350 K to 400 K, such as 430 K to 570 K, such as 440 K to 560 K, such as 450 K to 550 K, such as 460 K to 540 K, and including 470 K to 530 K. Acid leaching is described in further detail in, for example, U. S. Patent Nos. 3,087,809; 3,522,003; 3,741,752; 3,773,891; 3,809,549; 3,880,981; 4,044,107; 4,098,870; 4,284,611; 4,410,498; 4,872,909; 6,383,255; 6,406,676;

[0109] 6,471,743; 7,387,767; 9,255,006; 9,732,400; 10,808,296; and US Patent App. Pub. No.

[0110] 2014 / 0120020; the disclosures of which are incorporated by reference herein in their entirety. In embodiments, methods for phosphoric acid production involve feeding rock phosphorus to a multistage reactor with dilute sulfuric acid produced directly by salt splitting electrolysis. Such dilute sulfuric acid employed herein is typically in the concentration range of 5-30 wt.%, such as 10-30 wt.%. In one embodiment, recycled phosphoric acid and process water recovered from the calcium sulfate solid-liquid separation also is added to the reactor. In another embodiment, sulfuric acid containing dilute phosphoric acid returned from calcium sulfate washing is recycled to the multistage reactor system. In certain embodiments, the dilute sulfuric acid is concentrated to produce concentrated sulfuric acid prior to contacting the rock phosphorus. Such concentrated sulfuric acid is typically in the concentration range of 24-98 wt.%, such as 50 wt.%. Produced wet phosphoric acid concentrations described herein typically are from 5 to 25 wt.% P2O5 prior to purification or concentration. However, in some instances from the wet phosphoric acid concentrations may range higher, such as from 5 to 30 wt.% P2O5, such as between 26-30Atty. Dkt. No. TVTI-006WO

[0111] wt.% H3PO4 (19.5-22.8% P2O5) when higher sulfuric acid concentrations are used in the phosphate rock digestion. In certain embodiments, sulfuric acid is introduced to the process in an acid washing step of intermediate gypsum or bassanite slurry and recycled to the reactor fed with rock phosphorus. According to certain embodiments, when the dilute sulfuric acid is concentrated prior to contacting the rock phosphorus, phosphoric acid is produced at a higher concentration, such as greater than 30%, such as from 35% to 45% P2O5. For example, in certain embodiments of the methods disclosed herein, dilute electrolytic sulfuric acid is concentrated by reverse osmosis, evaporation, or both. In further embodiments, a recycled acid stream is provided from rock phosphorus digestion and is optionally concentrated prior to returning it to the rock phosphorus digestion. Thus, in one embodiment, sulfuric acid recovered from rock phosphorus digestion is recycled by concentrating it to provide a recycled concentrated sulfuric acid solution or makeup stream and returning it to digest rock phosphorus with or without blending it with a concentrated sulfuric acid solution obtained from electrolysis.

[0112] In certain embodiments, the present processes and systems allow the use of low-grade or “off-spec” phosphate rock. Such low-grade phosphate rock may have low phosphorus concentration, such as less than 50% bone phosphate of lime (w / w%) and / or a P2O5 content of 35% or less, such as 20% or less. In another aspect, low-grade phosphate rock has high concentrations of magnesium, such as in the form of dolomite (CaMg(CO3)2). Typically phosphate rock having a high concentration of magnesium, such as greater than about 2.5% w / w MgO, is deemed low-grade or off-spec. Processing of high-MgO rock is not possible in conventional phosphate plants, because the concentration of phosphoric acid in conventional digesters (25% to 40% w / w P2O5) causes precipitation of magnesium orthophosphate minerals (e.g. newberyite), so the phosphate ore cannot be completely dissolved. A high concentration of magnesium in phosphate rock has multiple negative impacts in conventional phosphoric acid plants: (1) loss of both magnesium and phosphate to the phosphogypsum, severely reducing P2O5 yield, and (2) contamination of product phosphoric acid with magnesium, causing struvite precipitation during granulation, leading to plant shutdowns. Methods and systems disclosed herein solve problems associated with high-MgO rock by, for example, digesting phosphoric acid using electrochemically recycled sulfuric acid, avoiding deleterious precipitation of magnesium phosphate in the digestion step while providing comparable P2O5 yield to the conventional process. At the same time, magnesium can be isolated, for example, as magnesium hydroxide, providing another commercial product.Atty. Dkt. No. TVTI-006WO

[0113] Methods of the invention also include converting the gypsum (CaSO4*nH2O) slurry to a calcium-containing solid product and thereby generating an aqueous sulfate. In one aspect of such methods, phosphogypsum waste is substantially reduced or even completely eliminated. The generated aqueous sulfate may vary. In some cases, the aqueous sulfate may be sodium sulfate (Na2SO4), potassium sulfate (K2SO4), calcium sulfate (CaSO4), lithium sulfate (Li2SO4), magnesium sulfate (MgS04), ammonium sulfate ((NH4)2SO4), and the like, or combinations thereof. In certain cases, the aqueous sulfate comprises sodium sulfate. In some cases, the generated aqueous sulfate is electrolyzed, as discussed in further detail below.

[0114] In some cases, the calcium-containing solid product is calcium carbonate (CaCO3), i.e., precipitated calcium carbonate (PCC). “PCC” is discussed herein in its conventional sense to refer to calcium carbonate (CaCO3) that is produced via artificial or synthetic means. Put another way, PCC described in the instant disclosure is distinct from natural ground calcium carbonate (GCC). For example, PCC is not limestone that had been produced (e.g., mined) by natural processes. Additionally, PCC for use in embodiments of the invention may not constitute calcium carbonate that is a product of an organism, including but not limited to gastropod shells, eggshells, and shellfish skeletons. In some cases, the PCC employed in the invention is, at the time of its use, precipitated relatively recently with respect to the geologic time scale, such as 100 years ago or less, 90 years ago or less, 80 years ago or less, 70 years ago or less, 60 years ago or less, 50 years ago or fewer, 40 years ago or less, 30 years ago or less, 20 years ago or less, 10 years ago or less, 5 years ago or less, 1 year ago or less, 6 months ago or less, 3 months ago or less, 1 month ago or less, 15 days ago or less, 10 days ago or less, 5 days ago or less, 1 day ago or less, 10 hours ago or less, 5 hours ago or less, 1 hour ago or less, 30 minutes ago or less, 10 minutes ago or less, and including 5 minutes ago or less. The PCC may consist of any convenient form of calcium carbonate. In some instances, the PCC is in a form selected from calcite, aragonite, vaterite, monohydrocalcite, and amorphous calcium carbonate, or combinations thereof. In some cases, PCC of the invention comprises calcite. In additional embodiments, PCC of the invention comprises aragonite. In still additional embodiments, PCC of the invention comprises vaterite. In still additional embodiments, PCC of the invention comprises amorphous calcium carbonate or a combination of crystalline and amorphous calcium carbonate. In some embodiments, the calcium-containing solid product is a calcium hydroxide alkaline solid product. In other words, the calcium-containing solid product comprises or is formed from slaked lime (Ca(OH)2). Precipitates according to some embodiments are selected from calcite, aragonite, vaterite, monohydrocalcite, disordered dolomite, magnesite, lansfordite, nesquehonite, dypingite, hydromagnesite, as well as combinations thereof.Atty. Dkt. No. TVTI-006WO

[0115] Protocols for precipitating the calcium-containing solid may vary. In some embodiments, PCC formation occurs according to the following reaction:

[0116] XCOs(aq) + CaSC>4(s,aq) ->■ XSO4(aq) + CaCOs(s)

[0117] where X is a suitable counterion. This reaction is referred to herein on occasion as a “metathesis” reaction. The source of carbonate may in some instances vary, however methods according to some embodiments of the processes and systems disclosed herein include receiving the carbonate (XCO3) from the reaction of base with carbon dioxide (e.g., as discussed below) during a carbon capture process. Similarly, while the source of calcium sulfate (CaSC ) may vary, the calcium sulfate may in some cases be from a gypsum source or a phosphogypsum source. In the instant case, the source of calcium sulfate is a gypsum (CaSO4*nH2O) slurry produced as a result of phosphoric acid production (e.g., discussed in detail herein).

[0118] In one embodiment, the calcium-containing solid product includes slaked lime (Ca(OH)2), which can be used to produce further materials, such as cement, for example, Portland cement. In conventional processes for manufacturing Portland cement, energy input from fossil fuels is used to heat the limestone raw materials (CaCOs), to calcine and sinter the materials. The calcining and sintering steps are carried out at high temperatures (ca. 900° C and 1500° C, respectively) and thus use considerable energy. Further, in such conventional processes, CaCOsis decomposed into CaO and CO2. Therefore, CO2is released in both fossil fuel combustion and decomposition of CaCOs during ordinary Portland cement production. About 1 ton of CO2is released for every ton of Portland cement produced in such processes. Using slaked lime as a raw material feedstock for cement production reduces the temperature of calcination substantially to around 800° C. Thus, the present methods for producing (Ca(OH)2reduce overall CO2emissions in ordinary Portland cement production by reducing or eliminating the use of fossil fuels for heating raw materials, eliminating CO2from CaCOs is decomposition, and further by providing for the capture of CO2. In one embodiment, production of cement using the processes disclosed herein results in a reduction of CO2emissions of 50% or greater, 60% or greater or even 70% or greater, relative to conventional processes for producing cement.

[0119] In some cases, methods include leaching phosphogypsum or a gypsum slurry, and using the resulting leachate for precipitation. Precipitation may or may not also involve a separate metathesis step beforehand. For example, in some cases, methods include metathesis in one or more metathesis reactors, where said metathesis is sufficient to cause transformationAtty. Dkt. No. TVTI-006WO

[0120] of the solid sulfate waste to a solid carbonate. Residual material (e.g., calcium, magnesium) in solution may subsequently be precipitated out of the sulfate stream from the metathesis in carbonate reactors. In some cases, there is no bifurcation of metathesis and carbonate reactors, and precipitation occurs in the same precipitator or set of precipitators. In some cases, methods include calcium sulfate leaching in place of the metathesis to supply aqueous calcium sulfate for precipitation.

[0121] In some cases, a solvent may be added to the calcium sulfate (i.e., gypsum (CaSO4*nH2O solid or slurry)) to form a solution, slurry or suspension prior to reacting with the carbonate source. Suitable solvents that may be used for forming a gypsum solution, slurry, or suspension include aprotic polar solvents, polar protic solvents, and non-polar solvents. Suitable aprotic polar solvents may include, but are not limited to, propylene carbonate, ethylene carbonate, butyrolactone, acetonitrile, benzonitrile, nitromethane, acetonitrile, nitrobenzene, sulfolane, dimethylformamide, N-methylpyrrolidone, or the like. Suitable polar protic solvents may include, but are not limited to, water, aqueous sulfate solution, nitromethane, and short chain alcohols. Suitable short chain alcohols may include, but are not limited to, one or more of methanol, ethanol, propanol, isopropanol, butanol, or the like. Suitable non-polar solvents may include, but are not limited to, cyclohexane, octane, heptane, hexane, benzene, toluene, methylene chloride, carbon tetrachloride, or diethyl ether. Go-solvents may also be used. In a certain embodiment, the solvent added to gypsum is water or aqueous sulfate solution. To form a slurry or suspension, an amount of water or aqueous solution is added to partially dissolve the phosphogypsum, such that some of the gypsum is fully dissolved and some of the phosphogypsum remains in solid form. In another embodiment, water or aqueous solution is added to phosphogypsum to form a slurry, wherein the percent of solids in the slurry is 5-50%, or 20-40%, or 30-35%. In another embodiment, the gypsum (CaSO4*nH2O) produced from contacting rock phosphorus with dilute sulfuric acid forms a slurry, wherein the percent of solids in the slurry is 5-50%, or 20-40%, such as 30-35%. In certain embodiments, water or aqueous solution is added to the gypsum (CaSO4*nH2O) produced from contacting rock phosphorus with dilute sulfuric acid to form or further dilute a gypsum slurry.

[0122] Following precipitation, the calcium-containing solid product may be subjected to one or more further treatment steps such as washing and solid-liquid separation. In some embodiments, methods include setting the calcium-containing solid product. The initial calcium-containing solid product composition can include not only compounds in the solid state, but also compounds in a liquid state, e.g., liquid water. “Setting” the initial calcium-containing solid product composition is used interchangeably with “drying” the solid composition and includesAtty. Dkt. No. TVTI-006WO

[0123] placing the solid composition in an environment such that there is evaporation of liquid from the solid composition. By removing a liquid from the solid composition, the chemical composition and thereby physical properties of the solid composition can be altered, e.g., a reduced volume of liquid can cause solutes dissolved in the liquid to transition to a solid state. For example, the calcium-containing solid product composition can be placed on a solid surface so that it is not in contact with another liquid, e.g., so that liquid from the solid composition can evaporate and the solid composition will not gain liquid from another liquid. In some cases, the composition is placed within a thickener configured to reduce the liquid content of the composition. Thickeners of interest have an inlet for receiving a slurry of the inorganic alkaline solid product composition and an outlet where processed inorganic alkaline solid product is output with a lower water content. Thickeners may operate by maintaining a fluidized bed of settled slurry particles that pass to a filter press for solid-liquid separation, with thickener overflow water returned to the process. Residual liquid may subsequently evaporate from solids exiting the filter press. In some cases, methods include ways of increasing the rate of evaporation, e.g., flowing a gas past the solid composition, applying a reduced gas pressure to the solid composition, increasing the temperature of the solid composition, or a combination thereof. Flowing the gas past the solid composition can be performed, for example, with a fan. A pump, e.g., a vacuum pump, can be employed to reduce the gas pressure, thereby increasing the rate of evaporation. The temperature of the solid composition can be increased, e.g., using an electric heater or a natural gas heater, to a temperature such as ranging from 25 °C to 95 °C, such as from 35 °C to 80 °C. In embodiments, the setting can be done simply by air drying for 1 -30 days or by drying with elevated temperature (for minutes - hours at 30 - 200 °C).

[0124] In some embodiments, methods include subjecting the calcium-containing solid product composition to a separation process. The term “separation process” is used herein in its conventional sense to refer to the conversion of a mixture of chemical substances to a plurality of different products. As discussed above, products of the precipitation process include a calcium-containing solid and an aqueous sulfate solution. In some cases, the separation process includes separating water from the calcium-containing solid. In additional cases, the separation process includes separating sulfate from the calcium-containing solid product. In still additional cases, the separation process includes separating an aqueous sulfate from the calcium-containing solid product. In select cases, the separation process involves the use of a filter press. Filter presses operate by injecting a slurry into one or more chambers. Pressure in the chambers is increased, and liquid is strained through a filter (e.g., using pressurized air or water). The type of filter press may vary. Examples include plate and frame filter presses,Atty. Dkt. No. TVTI-006WO

[0125] automatic filter presses, recessed plate filter presses, and membrane filter presses. Where aqueous sulfate is separated from the calcium-containing solid product, the aqueous sulfate may in some versions be electrolyzed.

[0126] In some cases, the disclosed processes and systems include CO2capture. In some cases, the CO2capture mitigates or reduces the CO2 output associated with producing phosphoric acid or ammonium phosphates. In some cases the CO2 capture is sufficient to render the process carbon-negative. In other words, production of the phosphoric acid results in a reduction of CO2, including a net reduction of atmospheric CO2. In some such cases involving CO2 capture, methods of the invention include a CO2 sequestering protocol. In some such cases, methods include reacting a basic solution with a CO2 containing gas, such as a CO2-comprising gaseous stream, to form an aqueous carbonate. The basic solution may vary. In some embodiments, the basic solution comprises a metal hydroxide such as sodium hydroxide (NaOH). In select versions, the basic solution is obtained from electrolyzing the aqueous sulfate, as described in greater detail below. The CO2 containing gas may be pure CO2 or be combined with one or more other gases and / or particulate components, depending upon the source, e.g., it may be a multi-component gas (i.e., a multi-component gaseous stream). While the amount of CO2 in such gases may vary, in some instances the CO2 containing gases have a pCC>2 of 103or higher, such as 104Pa or higher, such as 105Pa or higher, including 106Pa or higher. The amount of CO2 in the CO2 containing gas, in some instances, may be 20,000 or greater, e.g., 50,000 ppm or greater, such as 100,000 ppm or greater, including 150,000 ppm or greater, e.g., 500,000 ppm or greater, 750,000 ppm or greater, 900,000 ppm or greater, up to including 1,000,000 ppm or greater (In pure CO2 exhaust the concentration is 1,000,000 ppm) In some instances may range from 10,000 to 500,000 ppm, such as 50,000 to 250,000 ppm, including 100,000 to 150,000 ppm. The temperature of the CO2 containing gas may also vary, ranging in some instances from 0 to 1800°C, such as 100 to 1200°C and including 600 to 700°C.

[0127] In some instances, the CO2-comprising gases are not pure CO2, in that they contain one or more additional gases and / or trace elements. Additional gases that may be present in the CO2 containing gas include, but are not limited to water, nitrogen, mononitrogen oxides, e.g., NO, NO2.and NO3, oxygen, HF, SiF4 and other volatile fluoride compounds, sulfur, monosulfur oxides, (e.g., SO, SO2 and SO3), volatile organic compounds, e.g., benzo(a)pyrene C2OH12, benzo(g,h,l)perylene C22H12, dibenzo(a,h)anthracene C22H14, etc. Particulate components that may be present in the CO2 containing gas include, but are not limited to particles of solids or liquids suspended in the gas, e.g., heavy metals such as strontium, barium, mercury, thallium, etc.Atty. Dkt. No. TVTI-006WG

[0128] In certain embodiments, CO2 containing gases are obtained from an industrial plant, e.g., where the CO2 containing gas is a waste feed from an industrial plant. Industrial plants from which the CO2 containing gas may be obtained, e.g., as a CC>2-comprising gaseous stream waste feed from the industrial plant, may vary. Industrial plants of interest include, but are not limited to, power plants and industrial product manufacturing plants, such as but not limited to chemical, fertilizer, biofuel, and mechanical processing plants, refineries, cement plants, steel plants, etc., as well as other industrial plants that produce CO2 as a byproduct of fuel combustion or other processing step (such as calcination by a cement plant or CO2 off gassing by a phosphoric acid plant). Waste feeds of interest include gaseous streams that are produced by an industrial plant, for example as a secondary or incidental product, of a process carried out by the industrial plant.

[0129] Of interest in certain embodiments are CO2-comprising gaseous streams produced by industrial plants that combust fossil fuels, e.g., coal, oil, natural gas, as well as man-made fuel products of naturally occurring organic fuel deposits, such as but not limited to tar sands, heavy oil, oil shale, etc. In certain embodiments, power plants are pulverized coal power plants, supercritical coal power plants, mass burn coal power plants, fluidized bed coal power plants, gas or oil-fired boiler and steam turbine power plants, gas or oil-fired boiler simple cycle gas turbine power plants, and gas or oil-fired boiler combined cycle gas turbine power plants. In certain embodiments, the CO2-comprising gaseous stream is a waste stream produced by a power plant that combusts syngas, i.e., gas that is produced by the gasification of organic matter, e.g., coal, biomass, etc., where in certain embodiments such plants are integrated gasification combined cycle (IGCC) plants. Of interest in certain embodiments are waste streams produced by Heat Recovery Steam Generator (HRSG) plants. Waste streams of interest also include waste streams produced by cement plants. Cement plants whose waste streams may be employed in methods of the invention include both wet process and dry process plants, which plants may employ shaft kilns or rotary kilns, and may include precalciners. Each of these types of industrial plants may burn a single fuel, or may burn two or more fuels sequentially or simultaneously. A waste stream of interest is industrial plant exhaust gas, e.g., a flue gas. By "flue gas" is meant a gas that is obtained from the products of combustion from burning a fossil or biomass fuel that are then directed to the smokestack, also known as the flue of an industrial plant.

[0130] In some instances, the CO2sequestering protocol comprises direct air capture (DAC). DAC encompasses a class of technologies and methods capable of separating carbon dioxide CO2 directly from ambient air. A DAC system of the invention may be any system that capturesAtty. Dkt. No. TVTI-006WO

[0131] CO2directly from air and generates a product that includes CO2 at a higher concentration than that of the air that is input into the DAC system or that generates dissolved aqueous carbonate solution. DAC systems are systems that extract CO2 from the air using media that binds to CO2 but not to other atmospheric chemicals (such as nitrogen and oxygen). As air passes over the CO2 binding medium, CO2 "sticks" to the binding medium. DAC systems of interest include, but are not limited to: hydroxide based systems and CO2 sorbent / temperature swing based systems. In some instances, the DAC system is a hydroxide based system, in which CO2 is separated from air by contacting the air with an aqueous hydroxide liquid. Examples of hydroxide based DAC systems include, but are not limited to, those described in PCT published application Nos. WO / 2009 / 155539; WO / 2010 / 022339; WO / 2013 / 036859; and

[0132] WO / 2013 / 120024; the disclosures of which are herein incorporated by reference. Where hydroxide-based systems are employed, capture of CO2 in an aqueous hydroxide may proceed as follows:

[0133] CO2(g) + H2O(I) H2CO3(aq)

[0134] H2CO3(aq) + XOH(aq) -XCO3(aq) + H2O(I)

[0135] where X is a suitable counterion. In some cases, the method can use gases containing concentrated carbon dioxide by bubbling gas directly through a solution in which CaCO3precipitation is occurring using a disseminator or other suitable system to produce gas bubbles. In some cases, the DAC system can include an air contactor configured as a cooling tower, except the volumetric flux of air relative to that of hydroxide solution is approximately 50 times higher than standard cooling towers.

[0136] Methods of the invention also include subjecting the aqueous sulfate to electrochemical salt splitting. Protocols used for electrochemical salt splitting of aqueous sulfate may vary. “Electrolysis” and “electrolyzing” are referred to in their conventional sense to describe a chemical reaction that is driven by an electric current. In some embodiments, electrolyzing includes electrolysis, electrodialysis, or both. Accordingly, unless otherwise clearly indicated expressly or by context the term “electrolyzing” refers to electrochemical salt splitting by electrolysis and / or by electrodialysis. In embodiments, the electrolysis reaction proceeds as follows:

[0137] X2 / mSO4(aq) + H2O(l,g) (2 / m) X(OH)m(aq) + H2SO4(aq)Atty. Dkt. No. TVTI-006WO

[0138] where Xm+is a suitable counterion with valence m. Counterions may include, but are not limited to, K+, Ca2+, Na+, Li+, NH4+and Mg2+. Remaining water, hydrogen ions (H+), and sulfate ions (SO42-) comprise a sulfuric acid solution. In addition, hydrogen (H2) and oxygen (O2) gases may be evolved. Hydrogen or oxygen may also be recycled to the anode or cathode, respectively, to lower the cell voltage. Electrochemical salt splitting protocols for use in the subject methods may vary. While the current applied to an electrolyzer in embodiments of the invention may vary, in some instances the applied current ranges from 10 mA / cm2to 3,000 mA / cm2, such as 60 to 600 mA / cm2, and including 150 to 300 mA / cm2. In addition, the cell voltage at which the electrolyzing occurs may vary. In some embodiments, the electrochemical salt splitting occurs at a cell voltage ranging from 1 V to 15 V, such as 2 V to 10 V, and including 2 V to 7 V, such as 2 V to 5 V. Electrolytic protocols may have any convenient source of electricity. In some instances, the source of electricity for the process is a low-carbon energy source generated by solar, wind, hydroelectric, geothermal, hydrogen, nuclear, sulfur, or fusion power plants, with or without battery energy storage, or by conventional gas generation with carbon capture and sequestration, that can optionally be purchased from the electrical grid.

[0139] In embodiments, electrolysis of the aqueous sulfate via the subject methods produces an acidic solution and a basic solution. The acidic solution produced in the electrochemical salt splitting protocol can include, but is not limited to, sulfuric acid (H2SO4), hydrochloric acid (HCI) and hydrofluoric acid (HF), and the like, or combinations thereof. In some embodiments, the acidic solution comprises H2SO4. In additional embodiments, the acidic solution comprises HF or hydrofluorosilicic acid (HFSA). In other embodiments, the acidic solution comprises HCI. The basic solution may likewise vary. In some embodiments, the basic solution is an alkaline solution. Basic solutions of interest may include, but are not limited to, sodium hydroxide (NaOH), potassium hydroxide (KOH), and magnesium hydroxide (Mg(OH)2). In select cases, the basic solution comprises NaOH. In additional cases, the basic solution comprises KOH. In still additional cases, the basic solution includes magnesium hydroxide (Mg(OH)2). In yet additional cases, the basic solution comprises ammonia (NHs).

[0140] Electrolytic protocols of an embodiment of the invention include the use of a stack of one or more electrochemical salt splitting cells. The number of electrochemical cells may vary, and can range from, e.g., 1 to 60, such as 10 to 30. Each electrochemical cell may include an anode within an anode chamber containing an anolyte, a cathode within a cathode chamber containing a catholyte, or it may contain a bipolar membrane separating an anolyte and a catholyte, and each cell includes one or more membranes separating the anolyte and catholyte.Atty. Dkt. No. TVTI-006WO

[0141] As used herein the terms, “compartment” and “chamber” are used interchangeably in reference to electrochemical cells. In embodiments, the electrolyzers contain titanium or carbon mesh or foam anodes coated in precious metal oxide catalyst suitable for the oxygen evolution reaction in acidic solution conditions. The electrolyzers may also contain nickel or stainless steel mesh or foam cathodes, suitable for the water reduction in alkaline solution conditions. In some embodiments, the anode is an acid-resistant anode (e.g., consisting of titanium, platinized titanium, carbon, or other conductive support). In embodiments, the anode includes a catalyst for water or hydrogen oxidation (e.g., platinum, iridium oxide, ruthenium oxide, mixed metal oxide, or other catalyst suitable for water oxidation) deposited on the anode. In some cases, the cathode includes porous titanium, stainless steel, nickel or other material suitable for water or oxygen reduction. The anolyte within the anode chamber may vary. In certain cases, the anolyte comprises water. In some cases, the anolyte comprises the aqueous sulfate such as sulfuric acid or ammonium sulfate. In select cases, the anolyte (e.g., comprising the aqueous sulfate) is recirculated. Similarly, the catholyte within the cathode chamber may vary. In some cases, the catholyte comprises water. In select versions, the catholyte comprises the aqueous sulfate and hydroxide. In certain instances, methods include recirculating the catholyte.

[0142] Membranes for use in electrolysis include anion exchange membranes (AEMs), cation exchange membranes (CEMs), and bipolar membranes (BPMs). As is understood in the electrochemical arts, cation exchange membranes, which primarily consist of negatively charged groups (anions), prevent anions from passing through the membrane while allowing positively charged groups (cations) to pass through. In some cases, the cation exchange membrane is configured so that sodium cation (Na+) crosses the cation exchange membrane to the cathode chamber. As is understood in the electrochemical arts, AEMs, which primarily consist of positively charged groups (cations), prevent cations from passing through the membrane while allowing negatively charged groups (anions) to pass through. In some embodiments, the AEM is configured so that sulfate anion (SO42-) crosses the anion exchange membrane. Bipolar membranes may be configured to split water into protons and hydroxide ions and in some embodiments include a water-splitting catalyst at the bipolar junction.

[0143] In embodiments, methods include the use of one of the following: A stack of anion exchange membrane (AEM)-separated, two-chamber water electrolysis and electrodialysis cells containing an anode for production of acid and oxygen and a cathode for production of base and hydrogen (also referred to as the AEM system); a stack of cation exchange membrane (CEM)-separated two-chamber water electrolysis and electrodialysis cells (also referred to as the CEM system) cells containing an anode for production of acid and oxygen and a cathode forAtty. Dkt. No. TVTI-006WO

[0144] production of base and hydrogen; a stack of three-chamber water electrolysis and electrodialysis cells containing a CEM, an AEM, an anode for production of acid and oxygen, and a cathode for production of base and hydrogen (also referred to as the three-chamber cells); or a stack of bipolar membrane electrodialysis (BMED) cells containing an AEM, a CEM and a bipolar membrane, with a single anode and cathode per stack (also referred to as the BMED system). Any of these electrochemical salt splitting units may also contain additional components that serve to protect the ion exchange membranes and / or electrode components from degradation or that improve the current efficiency of the system by facilitating acid / base separation.

[0145] In some embodiments, electrochemical salt splitting units of interest include the AEM system, i.e., a stack of one or more electrochemical cells comprising an anode within an anode chamber containing an anolyte, a cathode within a cathode chamber containing a catholyte, and an anion exchange membrane separating the anode and cathode chambers. Exemplary electrolysis protocols according to such embodiments may be found in International Application No. PCT / US2022 / 039829, filed on August 9, 2022; herein incorporated by reference in its entirety. In certain cases, the anion exchange membrane is configured so that sulfate anion crosses the anion exchange membrane to the anode chamber where sulfuric acid is generated. Methods may include maintaining a low concentration of base (OH⁻ in the catholyte relative to the concentration of acid (H+) in the anolyte, where in some instances the magnitude of the H+: OH_ratio ranges from 5 to 100,000, such as 10 to 100 and including 2 to 200,000, where the relatively lower concentration of base is provided by flowing the catholyte through the cathode chamber, e.g., as a total stack flow rate ranging in some instances from 300 to 10,000 liters per minute (L / min) such as 500 to 1,000 L / min for a 1 metric ton CO2 mineralization per day system, e.g., by recirculating fluid from the reactor through the cathode chamber.

[0146] In certain embodiments, the electrolyzers for use in the methods are configured as three-chamber systems designed to produce acid solutions at concentrations of [H+] between 0.1 to 5.6 M, such as from 0.1 M to 2.0 M and hydroxide solutions at concentrations [OH-] between 0.5 to 10 M, such as from 0.5 M to 2.0 M with or without production of gaseous hydrogen and oxygen, and with the concentrated hydroxide solution suitable for direct air capture of carbon dioxide using an air contactor or slaked lime precipitation. Similarly, by example, electrodialysis cells can produce concentrated acid and base solutions as described in Chen et al. Journal of Membrane Science and Research 2021, 7, 273-279, which is incorporated herein by reference. The system includes a cell or stack of cells consisting of an anode chamber separated from the sulfate feed solution chamber by an AEM as well as aAtty. Dkt. No. TVTI-006WO

[0147] cathode chamber separated from the sulfate feed solution chamber by a CEM. In still other embodiments, the electrochemical unit configured as a BMED system is designed to produce concentrated acid solutions at concentrations [H+] between 0.1 to 3.8 M, such as from 0.1 M to 2.0 M, and concentrated hydroxide solutions at concentrations [OH-] between 0.1 to 3.8 M, such as from 0.1 M to 2.0 M, and with the concentrated hydroxide solution suitable for direct air capture of carbon dioxide using an air contactor or slaked lime precipitation.

[0148] In embodiments, the process avoids the usual pitfalls of electrochemical acid-base production by maintaining a low concentration of OH' in the feed solution or catholyte contacting the AEM, such that the ratio of sulfate (SO42-) to hydroxide (OH ) in the feed solution or catholyte is greater than 10. This configuration ensures that the flux of sulfate ions across the anion exchange membrane (AEM) is greater than the flux of hydroxide ions, minimizing Faradaic losses and increasing energy efficiency. The precipitation of carbonate, hydroxide, and hydroxycarbonate minerals consumes alkalinity, so the concentration of produced sulfuric acid is greater than the concentration of hydroxide in the catholyte or center chamber solution by a factor of 5 or greater. Suitable AEMs minimize voltage by allowing a sufficiently high sulfate flux, while limiting proton leakage, and are durable over the pH range 0-14.

[0149] In embodiments, methods include maintaining a relatively low concentration of base (OH ) in the catholyte (AEM system) or the sulfate feed solution (three-chamber and BMED systems) relative to the concentration of acid (H+) in the anolyte by recirculating fluid from mineralization through the cathode chamber (AEM system) or the cathode and sulfate feed solution chambers (three-chamber and BMED systems) rather than using the same solution feeds into the cathode and anode chambers, such that although protons and hydroxides are produced at the same rate in the electrochemical cell, the system generates an acid concentration in the anolyte that is much higher than (by at least about 5 times to about 200,000 times) the base concentration in the solution contacting the AEM, because the fluids are circulated separately. This configuration (i) minimizes Faradaic losses by migration of OH' across the anion exchange membrane and resulting loss reaction:

[0150] OH' + H+H2O

[0151] in the electrochemical cell and (ii) protects the anion exchange membrane from degradation in the strong base. In embodiments, methods also include sequestering carbon dioxide as mineralized carbonate, e.g., calcium carbonate, and producing sulfuric acid by reacting sulfate solids, e.g., calcium sulfate solids, with electrochemically produced hydroxide solution contacted with carbon dioxide directly from air or from a more concentrated source. In addition, methods may include recirculating water at a constant rate through the anode chamber to allow forAtty. Dkt. No. TVTI-006WO

[0152] accumulation of sulfuric acid, e.g., at cell flow rate ranging in some instances from 15 to 100 L / min, such as 60 to 90 L / min and including 10 to 300 L / min for a 1 metric ton CO2 mineralization per day system.

[0153] In other embodiments, the electrolyzer of interest configured as a CEM system is designed to produce dilute sulfuric acid at concentrations between 0.05-0.5 M and concentrated hydroxide solutions at concentrations between 0.5 to 10 M with or without production of hydrogen and oxygen, with the hydroxide solution suitable for direct air capture of carbon dioxide using an air contactor. The produced sulfuric acid contains substantial quantities, up to 1M, sodium sulfate salt, so the use of this electrolyzer configuration will be limited.

[0154] Electrolytic protocols that may be adapted for use in the subject methods may be found in PCT patent application serial no. PCT / US2022 / 039829 filed on August 9, 2022, the disclosure of which is incorporated by reference in its entirety. In addition protocols may be adapted from U. S. Provisional Patent Application Nos. 63 / 415,168; 63 / 443,217; and 63 / 443,268; the disclosures of which are herein incorporated by reference in their entirety.

[0155] In embodiments, dilute or concentrated sulfuric acid produced by electrolysis is reacted with rock phosphorus to produce the phosphoric acid. Complete dissolution of phosphate from certain types of rock phosphorus can be achieved using dilute sulfuric acid, with concentrations as low as 100 mM or less than 2 wt. % (Mendes et al., 2020). For example, the range of sulfuric acid concentrations entering the phosphoric acid production process may be from 2-98 wt.%, such as 5-98 wt.%, for example 10-28 wt.% or 60 to 80 wt.% or 80 to 98 wt.%. In particular embodiments described herein, the sulfuric acid produced from electrolysis may be in the range of 2-30 wt.%, such as 10-30 wt. %. Use of dilute sulfuric acid from electrolysis as described herein is advantageous as increased sulfuric acid concentrations require energy- and capital-intensive modifications.

[0156] In select cases, the concentration of the sulfuric acid obtained from electrolyzing the aqueous sulfate ranges from 5 wt.% to 80 wt.%. And despite the cost and energetic disadvantages, in some embodiments, the sulfuric acid may be concentrated to up to 98 wt.% with a suitable concentration method such as multi-effect evaporation with up to three effects with or without mechanical vapor recompression to reduce energy consumption. Sulfuric acid concentrations exceeding about 70 wt.% require modifications to the heat exchangers and thus a separate evaporator system must be built to achieve concentrations between about 60 and 80 wt.% or 80 and 98 wt. %. When concentrated, the concentration of the sulfuric acid may be, for example, 0.5 M or more, such as 0.6 M or more, such as 0.7 M or more, such as 0.8 M or more, such as 0.9 M or more, such as 1 M or more, such as 1.1 M or more, such as 1.2 M or more,Atty. Dkt. No. TVTI-006WO

[0157] such as 1.3 M or more, such as 1.4 M or more, and including 1.5 M or more. In other embodiments, produced dilute sulfuric acid may be used to dilute conventional concentrated sulfuric acid from 98 wt. % to 93 wt. %. The phosphoric acid concentration produced by sulfuric acid reaction with rock phosphorus depends on the feed sulfuric acid concentration. For example, a feed sulfuric acid concentration of 10.7 wt.% (e.g., 1M H2SO4) generates a phosphoric acid concentration of 9.8 wt.% when calcium sulfate dihydrate is produced. A feed sulfuric acid concentration of 60 wt.% generates a phosphoric acid concentration of 74.4 wt.%.

[0158] In certain cases, methods of the invention may be characterized as applying an electric current to drive the conversion of calcium sulfate to PCC. For example, methods may include subjecting calcium sulfate (i.e., gypsum (CaSO4·nH2O)), a base (i.e., OH ) and carbon dioxide to electrolysis. In select embodiments, electrolysis proceeds, as follows:

[0159] CaSO4-2H2O + CO2(g) + electricity -> H2SO4(aq) + CaCOs(s) + H2(g) +1 / a O2(g)

[0160] As shown above, sulfuric acid (H2SO4), PCC (CaCOs), hydrogen gas (H2) and oxygen gas (O2) are products of the above-described embodiment of the electrolytic protocol. In other words, sulfuric acid (~1 M), base (e.g., aqueous NaOH), green hydrogen, and oxygen are produced by water electrolysis in aqueous sulfate solution.

[0161] In some cases, methods include cyclic steps of electrochemical production of sulfuric acid at the anode and a hydroxide aqueous solution, e.g., calcium hydroxide aqueous solution, at the cathode, wherein the hydroxide solution is reacted with carbon dioxide to produce solid carbonate, e.g., solid calcium carbonate. In embodiments, methods include cyclic steps of electrochemical production of sulfuric acid at the anode and hydroxide aqueous solution, e.g., calcium hydroxide aqueous solution, at the cathode, wherein the hydroxide solution is reacted with carbon dioxide and divalent cation, e.g., calcium ion, to produce a solid carbonate, e.g., PCC, wherein the sulfuric acid anolyte is recovered, concentrated as desired, e.g., to >70% H2SC>4, and in some instances reacted with rock phosphorus to produce phosphoric acid, calcium sulfate, and HF, wherein the product calcium sulfate is returned to the process to produce calcium carbonate and sulfate solution, wherein the sulfate solution is returned to the electrochemical cell along with water to continue the cycle.

[0162] Phosphoric acid and purified phosphoric acid have a variety of applications including in food and beverages and in battery production, such as lithium iron phosphate batteries or sodium phosphate batteries. In select instances, the phosphoric acid produced herein is converted to an ammonium phosphate salt comprising monoammonium phosphate,Atty. Dkt. No. TVTI-006WO

[0163] diammonium phosphate, or both. As is known in the art, mono- and diammonium phosphate are powdered or granular compositions produced by reaction of ammonia with phosphoric acid in varied proportions, followed by crystallization. Several commercial processes exist for the production of monoammonium phosphate that include from one to three neutralization steps wherein wet phosphoric acid is fed with ammonia for ammoniation. These processes include the Chemico process, the Dorr-Oliver, Process, the Fluor process, and the Knowles Associates Process (Hicks, 2018). In particular, the present methods include producing concentrated phosphoric acid and contacting the concentrated phosphoric acid with ammonia. The phosphoric acid fed to these processes is typically concentrated (50-52.5% H3PO4), and the slurry discharged from the final neutralization step is solidified and granulated. For example, monoammonium phosphate may be prepared by the following exothermic reaction:

[0164] H3PO4+ NH3(NH4)H2PO4,

[0165] Similarly, diammonium phosphate may be prepared by the following reaction:

[0166] H3PO4+ 2NH3(NH4)2HPO4,

[0167] Furthermore, triammonium phosphate may be prepared by treating the phosphoric acid with ammonia such that the following reaction occurs:

[0168] H3PO4+ 3NH3(NH4)3PO4

[0169] As is understood in the art, the proportion of ammonia added relative to phosphoric acid will influence the form of ammonium phosphate produced.

[0170] Methods according to some embodiments also include producing a fertilizer from the ammonium salt (e.g., ammonium phosphate, ammonium sulfate). The term “fertilizer” is employed in its conventional sense to refer to a product that supplies an essential nutrient such as nitrogen, phosphorus and / or sulfur to an agricultural system. In some cases, the fertilizer comprises phosphoric acid or a product generated therefrom. In some cases, the ammonium salt employed in the fertilizer is a carbon-negative ammonium salt. In some such cases, the ammonium salt employed in or as the fertilizer was produced using a CO2sequestering protocol, such as those discussed above. The amount of CO2sequestered per unit mass of ammonium salt may vary. In some cases the ratio of the mass of sequestered CO2to the massAtty. Dkt. No. TVTI-006WO

[0171] of ammonium salt produced may be 0.2:1 or more, 0.3:1 or more, 0.4:1 or more, 0.5:1 or more, 0.6 to 1 or more, and including 0.7:1 or more. In certain instances, the CO2 sequestering protocol comprises sequestering a mass of CO2 per ton of ammonium salt produced that ranges from 0.2 tons to 0.8 tons, such as 0.5 tons to 0.7 tons, and including 0.6 tons to 0.7 tons. In select cases, the CO2 sequestering protocol comprises sequestering 0.6 tons of CO2 per ton of ammonium salt produced. In some embodiments, the CO2 sequestering protocol comprises sequestering 0.63 tons of CO2 per ton of ammonium salt produced. In other cases, the CO2 sequestering protocol comprises sequestering 0.33 tons of CO2 per ton of ammonium salt produced.

[0172] Methods of fertilizer production may vary as desired. In some instances, fertilizer production involves converting the ammonium salt to a form that is salable as a fertilizer. In some embodiments, methods include producing a fertilizer blend, where the fertilizer may include two or more different types of fertilizer that are sold together as a single product.

[0173] Methods of fertilizer production may include but are not limited to steam granulation, chemical granulation, compaction, and bulk blending. In select cases, methods include producing a multinutrient fertilizer comprising two or more nutrient compounds. General classes of nutrient compounds include nitrogen-based compounds, phosphorus-based compounds and potassium-based compounds. Where the ammonium salt is an ammonium phosphate, such salts may be considered multinutrient fertilizers because they include nitrogen (i.e., in the form of ammonium ion) and phosphorus (i.e., in the form of phosphate ion). In select cases, fertilizer production includes adding potassium to the ammonium salt to produce an “NPK” fertilizer. Methods of fertilizer production may also include the addition of one or more micronutrients, such as but not limited to boron, zinc, molybdenum, iron, and manganese.

[0174] Products of the subject methods may have various uses. Fertilizers may be used in one or more agricultural applications. For example, the produced fertilizers may be employed to provide nutrients for use in the cultivation of one or more crops, including but not limited to maize / corn, wheat, fruit and tree nuts, soybeans, rice, and the like. In embodiments, the oxygen gas produced at the anode is off-gassed to the atmosphere, is collected to be compressed and sold, or is used as an oxidant in the sulfuric acid extraction process to avoid sulfate-reducing conditions. In addition to being used to produce ammonia, hydrogen gas may be combusted to produce heat and / or energy that may be supplied to another part of the process (e.g., ammonia generation).

[0175] Sulfuric acid produced as described above may find multiple uses. As described above, sulfuric acid is often employed in phosphate fertilizer production. As such, embodiments of theAtty. Dkt. No. TVTI-006WO

[0176] invention include employing the produced sulfuric acid in phosphate fertilizer production. In some embodiments, methods include concentrating the produced sulfuric acid prior to employing it for phosphate fertilizer production. When concentrated, the concentration of the sulfuric acid may be, for example, 0.5 M or more, such as 0.6 M or more, such as 0.7 M or more, such as 0.8 M or more, such as 0.9 M or more, such as 1 M or more, such as 1.1 M or more, such as 1.2 M or more, such as 1.3 M or more, such as 1.4 M or more, and including 1.5 M or more. As noted above, a byproduct of phosphoric acid and phosphorous fertilizer production is phosphogypsum. Such phosphogypsum may then be used to create more PCC, and so on. Also as described above, a byproduct of phosphoric acid and phosphorous fertilizer production is a gypsum (CaSO4·nH2O) slurry. The sulfate produced mining and fertilizer production, can be substantively recycled as described herein to reduce the accumulation of sulfate wastes.

[0177] The calcium containing solid may also have various uses. In some cases where the calcium containing solid includes precipitated calcium carbonate (PCC), said calcium containing solid may be employed in, for example, iron purification, oil drilling fluids, sugar refining, chalk, paint, resin, ceramic glazes, antacids, calcium supplements, and food additives. In certain cases, PCC may be used as a feedstock for lime (CaO) production. In some cases, said lime is employed in cement production. The production of lime from PCC is described in U. S.

[0178] Provisional Patent Application No. 63 / 443,217 and International Application No.

[0179] PCT / US2023 / 034367; the disclosures of which are incorporated by reference herein in their entirety. The PCC (CaCOs) may be used as an additive to conventional blended hydraulic cements by admixture with ordinary Portland cement at a concentration up to 15 wt.% according to ASTM C595 / C595M Performance Specifications.

[0180] SYSTEMS FOR PHOSPHORIC ACID PRODUCTION

[0181] As discussed above, aspects of the invention include systems for phosphoric acid production and methods for their use. Systems of interest include those comprising a reactor configured to react rock phosphorus with sulfuric acid (H2SO4), such as dilute sulfuric acid produced by electrochemical salt splitting, to produce phosphoric acid (H3PO4) and a gypsum (CaSO4*nH2O) slurry, thereby avoiding or minimizing phosphogypsum waste. Any suitable reactor configured to combine a mineral source including rock phosphorus and an acidic solution under conditions sufficient to extract phosphorus from the mineral source may be employed. In select versions, the reactor is or comprises a reactor system. In some instances, pressures that may be used in the subject reactors can vary and include, e.g., 0 MPa or more,Atty. Dkt. No. TVTI-006WO

[0182] 0.3 MPa or more, 0.4 MPa or more, 0.5 MPa or more, 0.6 MPa or more, 7 MPa or more, 8 MPa or more, and including 9 MPa or more. Temperatures that may be used in the subject reactors range from, e.g., 330 K to 600 K, such as 330 K to 360 K, such as 350 K to 400 K„ such as 430 K to 570 K, such as 440 K to 560 K, such as 450 K to 550 K, such as 460 K to 540 K, and including 470 K to 530 K. Devices and protocols that may be adapted for use in the subject reactor are described in, for example, U. S. Patent Nos. 3,087,809; 3,522,003; 3,741,752;

[0183] 3,773,891; 3,809,549; 3,880,981; 4,044,107; 4,098,870; 4,284,611; 4,410,498; 4,872,909; 6,383,255; 6,406,676; 6,471,743; 7,387,767; 9,255,006; 9,732,400; 10,808,296; and U. S.

[0184] Patent App. Pub. No. 2014 / 0120020; the disclosures of which are incorporated by reference herein in their entirety. Systems of the invention may also include a pulverizer or crusher configured to reduce mineral source to finer particles.

[0185] Systems of the invention also include a precipitator (sometimes referred to as a mineralized carbonate production reactor or precipitation reactor) configured to convert the gypsum (CaSO4*nH2O) slurry to a calcium-containing alkaline solid product and thereby generate an aqueous sulfate. Any device(s) (e.g., reactor) suitable for the precipitation of an inorganic alkaline solid may be employed as the subject precipitator. In some cases, the precipitator is in a gypsum slurry -receiving relationship with the reactor, i.e., such that the gypsum slurry is received in the precipitator from the rock phosphorus reactor. In some embodiments, the precipitator is operably connected to the reactor and a source of carbonate. The precipitator may introduce the reagents in any suitable manner (e.g., disintegration and / or spraying, etc.). In some cases, the precipitator is a continuous flow mixer. Precipitators may additionally include an agitator configured to mix the slurry undergoing precipitation. Agitators of interest may include one or more sets of rotors and blades. Where multiple rotors are employed, embodiments of the precipitation reactor include rotors rotating in opposite directions or in the same directions at different speeds. The blades, or the like, can create shear forces, turbulence and under and overpressure pulses, which grind, or disintegrate and spray the material.

[0186] Precipitators that may be adapted for use are described in, e.g., U. S. Patent No. 8,012.445, the disclosure of which is incorporated by reference herein.

[0187] Systems described herein also include an electrolysis unit consisting of or including an electrolyzer configured to electrolyze the aqueous sulfate to produce sulfuric acid and hydrogen (H2). In embodiments, electrolyzers include a stack of cation exchange membrane (CEM)-separated two-compartment water electrolysis and electrodialysis cells containing an anode for production of acid and oxygen and a cathode for production of base and hydrogen. In embodiments, some such systems are designed to produce both relatively concentrated acidAtty. Dkt. No. TVTI-006WO

[0188] and relatively concentrated base simultaneously at a range of current densities between 10 and 1000 mA / cm2and cell voltages between 2 and 6 V. In embodiments, CEM-separated systems of the invention are configured to produce relatively dilute sulfuric acid, and the produced acid contains a substantial quantity of recirculated salt, with salt concentrations > 0.1 M and produced acid concentrations up to 0.5 M. The CEM-separated system may be configured to produce a relatively concentrated base that may or may not contain a substantial quantity of salt. The range of produced base concentrations includes 0.1 to >1.5 M, with salt concentrations ranging from ppm-level up to saturation with respect to a recirculated soluble sulfate salt such as Na2SO4. Being an electrolysis system, the CEM system produces gaseous hydrogen (H2) and oxygen (O2) products in equimolar quantity to the theoretical acid and base produced but with a higher current efficiency (up to 95-100%) due to lower Faradaic losses of the produced gases compared to the produced acid and base. The system can be configured to recycle hydrogen or oxygen to the anode or cathode, respectively, to reduce the cell voltage.

[0189] The CEM system may be configured for use with a component that removes carbon dioxide from the air or from point sources and produces aqueous carbonate solutions such as Na2CO3, for example using an air contactor in the case of air or a gas disseminator in the case of more concentrated carbon dioxide point sources. The high concentration of produced base in the CEM-separated system enables direct air capture of carbon dioxide, allowing the system to accomplish carbon dioxide direct air capture and geologically permanent sequestration of carbon dioxide in carbonate solid phases. Thus, in one embodiment, the CEM produces an alkaline stream for contacting CO2. In one embodiment, the CEM is in fluid communication with a direct air contactor, for example, a DAC system, such as a hydroxide based DAC system, as described herein.

[0190] In systems where dilute sulfuric acid and concentrated base are produced, CEM electrolyzers may be configured to produce sulfuric acid at concentrations between 0.05-0.5M and concentrated hydroxide solutions at concentrations between 0.5 to >2.0 M with or without production of hydrogen and oxygen. The concentrated hydroxide solution is suitable for direct air capture of carbon dioxide using an air contactor. The produced sulfuric acid can contain a substantial concentration of sodium sulfate salt, from 0.25 to 1M, such as 0.25-0.5M, or 0.4-0.6M or >0.5M.

[0191] Alkaline solutions, such as carbonate or carbonate-free hydroxide solutions may be reacted with a stream containing metal sulfates (e.g. aqueous calcium and / or magnesium sulfate) or solids containing metal sulfates to precipitate solid calcium and / or magnesium-containing phases. The alkaline solutions may be derived directly or indirectly from an alkalineAtty. Dkt. No. TVTI-006WO

[0192] stream produced by electrolysis as described herein. For example, the alkaline stream may be fluidly coupled to a precipitator to which a metal sulfate is delivered. In certain examples, the alkaline stream is first contacted with a CO2-comprising gaseous stream to provide a carbonate solution that is then reacted with the metal sulfate(s). By way of example, the solid calcium and / or magnesium-containing phases may include calcite, aragonite, vaterite, monohydrocalcite, disordered dolomite, magnesite, lansfordite, nesquehonite, dypingite, and / or hydromagnesite. Thus, in certain embodiments the reaction yields a calcium-containing solid product. Such reactions to precipitate solid calcium and / or magnesium-containing phases, yields, via metathesis, an aqueous sulfate phase, such as sodium sulfate. In one embodiment, the reaction of carbonate or carbonate-free hydroxide solutions yields calcium carbonate as the calcium-containing solid product and an aqueous phase comprising sodium sulfate. The aqueous sodium sulfate, in certain examples described herein, is recycled by returning it to an electrolysis unit for the production of sulfuric acid.

[0193] Alternatively, the CEM system is configured to use the alkaline stream to produce solid hydroxide phases such as Ca(OH)2instead of solid carbonate phases. In some cases, the CEM system integrates a water treatment step or steps designed to remove residual alkaline earth elements (especially Ca and Mg) as well as other trace impurities by liming with a portion of the produced base stream, by ion exchange, or by a combination of these and / or other brine treatment steps. The treated brine stream is returned to the electrolyzer in a continuous fashion to enable continuous, integrated operation.

[0194] In some cases, electrolyzers include an anion exchange membrane. Exemplary electrolysis protocols according to such embodiments may be found in International Application No. PCT / US2022 / 039829, filed on August 9, 2022; herein incorporated by reference in its entirety. In certain cases, the anion exchange membrane is configured so that sulfate anion crosses the anion exchange membrane to the anode chamber where sulfuric acid is generated. Systems may be configured to maintain a low concentration of base (OH ) in the catholyte relative to the concentration of acid (H+) in the anolyte, where in some instances the magnitude of the H+: OH' ratio ranges from 5 to 100,000, such as 10 to 100 and including 2 to 200,000, where the relatively lower concentration of base is provided by flowing the catholyte through the cathode chamber, e.g., as a total stack flow rate ranging in some instances from 300 to 10,000 liters per minute (L / min) such as 500 to 1,000 L / min for a 1 metric ton CO2mineralization per day system, e.g., by recirculating fluid from the reactor through the cathode chamber.

[0195] In other embodiments, electrolyzers include a stack of three-compartment water electrolysis and electrodialysis cells containing a CEM, an AEM, an anode for production of acidAtty. Dkt. No. TVTI-006WO

[0196] and oxygen, and a cathode for production of base and hydrogen (also referred to as the three-compartment system). In certain embodiments, the three-compartment system is configured to produce concentrated acid solutions at concentrations between 0.05 to >2.0 M, such as 1 M, and concentrated hydroxide solutions at concentrations between 0.5 to > 2.0 M, such as 1 M, with production of gaseous hydrogen and oxygen. In some embodiments, the gaseous hydrogen or oxygen can be recycled to the anode or cathode, respectively, to reduce the cell voltage. Hydroxide solution concentrations >0.5 M are suitable for direct air capture of carbon dioxide using an air contactor.

[0197] Embodiments include a cell or stack of cells consisting of an anode compartment separated from the sulfate feed solution compartment by an AEM as well as a cathode compartment separated from the sulfate feed solution compartment by a CEM. The systems configured to contain three-compartment electrolysis cells may be designed to produce both relatively concentrated acid and relatively concentrated base simultaneously at a range of current efficiencies between 50-100%. In embodiments, the produced acid and base are both substantively salt-free (ppm range concentrations of recirculated salt).

[0198] Bipolar membranes (BPMs) may likewise be employed in the subject systems. In some such cases, electrolyzers are configured to perform bipolar membrane electrodialysis (BMED). In some cases, the electrochemical unit configured as a BMED system is designed to produce concentrated acid solutions at concentrations between 0.05 to >2.0 M and concentrated hydroxide solutions at concentrations between 0.5-2.0 M.

[0199] Any of the above electrochemical units may also contain additional components that serve to protect the ion exchange membranes and / or electrode components from degradation or that improve the current efficiency of the system by facilitating acid / base separation. Each salt splitting approach has a unique set of embodiments. In some cases the system is configured to recirculate a soluble sulfate salt (e.g., Na2SO4, K2SO4, Li2SO4, (NH4)2SO4, or other sulfate salt), at a concentration between 0.5 M and saturation, which serves the role of supplying a sufficient quantity of sulfate anion and a cation (e.g., Na+) to the electrochemical unit to enable efficient production of acid and base.

[0200] In some embodiments, systems are configured such that calcium sulfate is introduced to one or more precipitators or reactors where it is converted to calcium carbonate by metathesis reaction with carbon dioxide from air and alkalinity produced in a two-chamber or three-chamber water electrolyzer or a BMED system. In an embodiment, calcium sulfate is introduced into a train of from one up to three or more metathesis reactors where solid calcium sulfate is supplied via a suitable method such as a screw conveyor or by pipe as a slurry discharged fromAtty. Dkt. No. TVTI-006WO

[0201] phosphoric acid production. In the metathesis reactors the solid calcium sulfate is converted to calcium carbonate and the final solid product is recovered by a suitable method of solid-liquid separation. The residual aqueous calcium sulfate is then taken to a series of precipitators or reactors that produce either solid calcium carbonate or aqueous calcium chloride, aqueous calcium sulfate, or a combination of products. Effluent from the precipitators or reactors is recirculated through the cathode chamber of the water electrolyzer, where sulfate liberated crosses an anion exchange membrane to gradually accumulate sulfuric acid in a recirculating anolyte solution. During operation, sulfuric acid and calcium carbonate are produced by reacting a calcium sulfate source with electrochemically produced hydroxide contacted with carbon dioxide derived from atmospheric air, although more concentrated sources of carbon dioxide can also be used (e.g., as discussed above).

[0202] Systems described herein also can be used in conjunction with or include an ammonia synthesizer configured to synthesize ammonia (NH3) from the hydrogen as described in U. S. Patent Application No. 19 / 100,054, (the ‘054 application) filed January 30, 2025, which is incorporated herein by reference in its entirety. Ammonia synthesizers and components thereof (e.g., catalysts) that may be adapted for use in the present systems are described in U. S.

[0203] Patent Nos. 9,150,423; 9,272,920; 10,131,545; 10,322,940; 10,597,301; 10,974,970; and 12,528,752 the disclosures of which are herein incorporated by reference in their entirety.

[0204] Systems of the invention also include an ammonium salt generator configured to produce an ammonium salt from the ammonia as described in the ‘054 application. In particular, in some cases, systems are configured to produce ammonium phosphate salts. In such cases, systems are configured to provide dilute or concentrated phosphoric acid to the ammonium salt generator. Feeding the ammonium salt generator with dilute phosphoric acid minimizes the overall energy consumption of the process when dilute sulfuric acid is used to produce phosphoric acid.

[0205] Systems may additionally include one or more brine treatment modules configured to treat the aqueous sulfate prior to its recirculation to the electrochemical salt splitting system. Purification of the brine exiting the precipitation reactor may be included to prevent scaling, fouling, or degradation of electrolyzer components. In addition or alternatively, the brine treatment module may be employed to selectively extract valuable elements or to remove other impurities from the aqueous solution, such that a purified sulfate brine such as sodium sulfate is produced. Modules that may be employed include, for example, filters (e.g., polishing filters) and ion exchange columns.Atty. Dkt. No. TVTI-006WO

[0206] In some cases, systems comprise a CO2 sequestration unit. The CO2 sequestration unit may be operably connected to any source of CO2, including air or a point source (e.g., flue gas). In some cases, the CO2 sequestration unit is an air contactor. Any suitable air contactor may be employed. In some instances the air contactor is a DAC system, such as a hydroxide based DAC system. DAC systems include, but are not limited to, those described in PCT published application Nos. WO / 2009 / 155539; WO / 2010 / 022339; WO / 2013 / 036859; and

[0207] WO / 2013 / 120024. In select cases, the air contactor operates by bubbling gas directly through the precipitation reactor solution using a disseminator or other suitable system to produce gas bubbles. The concentrated hydroxide solution described above is suitable for direct air capture of carbon dioxide using the air contactor.

[0208] Systems described herein optionally include a nanofiltration membrane and / or a nanofiltration unit comprising at least one nanofiltration membrane. In one embodiment, a nanofiltration membrane is contained in a nanofiltration unit configured to receive a phosphate-containing acidic solution. In one embodiment the nanofiltration unit is in fluid communication with a phosphate rock reactor such that the nanofiltration unit receives a phosphate-containing acidic solution directly or indirectly from the phosphate rock reactor. In one aspect, the phosphate-containing acidic solution is produced by phosphate rock digestion with dilute sulfuric acid produced by electrochemical salt splitting as described herein. In certain embodiments, the phosphate-containing acidic solution is subjected to one or more purification steps, such as desulfation, calcium removal, or both, prior to contacting the nanofiltration membrane.

[0209] In one embodiment, of the systems disclosed herein a nanofiltration unit is configured to receive phosphate-containing acidic solution directly from the phosphate rock reactor without intermediate processing.

[0210] Typically, the phosphate-containing acidic solution comprises at least 2% P2O5, such as from 2 to 25% P2O5, or at least 5% P2O5, such as from 5 to 25% P2O5, from 5 to 18%, or from 15 to 21% P2O5, prior to contacting the nanofiltration membrane. In certain cases, the phosphate-containing acidic solution comprises less than 100 ppm of particles larger than 1 pm, such as less than 50 ppm, less than 10 ppm, or even less than 1 ppm of particles larger than 1 pm. Similarly, prior to contacting the nanofiltration membrane, the phosphate-containing acidic solution may comprise, or be purified to comprise less than 3 wt.% total organic carbon (TOC), such as not more than 1 wt% TOC. Likewise, prior to contacting the nanofiltration membrane, the phosphate-containing acidic solution may comprise, or be purified to comprise less than 4 wt.% SO4, or not more than 1000 ppm SOAtty. Dkt. No. TVTI-006WO

[0211] Exemplary nanofiltration processes that can be incorporated into the present methods for producing phosphoric acid are described, for example, in U. S. Patent No. 5,945,000, which describes a nanofiltration process for purifying phosphoric acid solutions using multiple nanofiltration membrane units arranged in series, as described in WO2013133684 and WO 2021 / 254944, both of which are incorporated herein by reference.

[0212] Certain systems may also optionally include a reverse osmosis unit for further purifying and concentrating dilute phosphoric acid. In one embodiment, reverse osmosis follows nanofiltration and produces dilute purified phosphoric acid of 15% or higher, such as 20% or higher, for example from 20% to 40% w / w P2O5.

[0213] The systems described herein also may optionally include further an evaporation unit for concentrating the phosphoric acid to produce a concentrated phosphoric acid. Such an evaporation unit can provide vacuum, and / or evaporative concentration by mechanical vapor recompression. In one embodiment the concentrated phosphoric acid produced herein has a P2O5 concentration of greater than 40%, such as greater than 45%, or from 40% to 62% or from 45% to 83% w / w P2O5.

[0214] In certain examples, solvent-based extraction is used to further purify the phosphoric acid. A suitable extraction solvent, such as tributyl phosphate, is mixed with the aqueous phosphoric acid in a separation unit, such as a mixer-settler or a countercurrent extractor. The solvent extracts the phosphoric acid from the aqueous phase. As is known to those of skill in the art, the duration of mixing and temperature can be optimized for maximum extraction efficiency. The mixture is allowed to settle, resulting in a separate aqueous phase (primarily comprising impurities and water) from the organic phase comprising phosphoric acid. The organic phase is then stripped to recover purified phosphoric acid.

[0215] In certain embodiments the dilute phosphoric acid produced via rock phosphate digestion is optionally subjected to desulfation. Desulfation methods are known to those of ordinary skill in the art and are further exemplified herein. In one embodiment, desulfation comprises contacting the dilute phosphoric acid with a calcium-containing material, such as a solid calcium-containing material. In certain such embodiments, desulfation comprises contacting the dilute phosphoric acid with a solid calcium-containing material comprising calcium carbonate, rock phosphorus, slaked lime, or a combination thereof. Desulfation may be followed by calcium removal prior to concentration of the phosphoric acid. In certain examples of the systems and processes disclosed herein, dilute phosphoric acid is subjected to desulfation and calcium removal prior to concentration and solvent extraction to produce purified phosphoric acid product.Atty. Dkt. No. TVTI-006WO

[0216] FIG. 1 provides a flow diagram of a system 100 for production of using sulfuric acid electrochemically recycled using a 3-compartment electrochemical cell and calcium carbonate using carbon dioxide captured from the air. Electrochemical cell 110 receives water and sodium sulfate brine and outputs sulfuric acid stream 111, sodium sulfate in stream 112 and sodium hydroxide in stream 113. Electrochemical cell 110 is fluidly coupled to reactor 120 via stream 111. The system is configured such that alkaline stream 113 from electrochemical cell 110 is provided to direct air capture unit 130, which is configured to produce an aqueous carbonate (Na2COs) from the alkaline stream and carbon dioxide (CO2), which in this implementation is obtained from air. Reactor 120 is configured to produce dilute phosphoric acid (H3PO4) and a gypsum (CaSO4*nH2O) slurry from the H2SO4 and recycled phosphoric acid and in some instances is configured to carry out multiple digestion steps, such as from 2 to 8 steps. System 100 includes a recycle stream configured to recirculate phosphoric acid to reactor 120 for further concentration. Reactor 120 is configured to provide calcium sulfate-containing gypsum slurry to precipitator 160 after separation from aqueous phosphoric acid in separator 150. Direct air capture unit 130 is configured to deliver aqueous carbonate to precipitator 160, which is configured to convert the gypsum slurry to a calcium-containing solid product (in this case, calcium carbonate) using the aqueous carbonate and thereby generate an aqueous sulfate via metathesis. System 100 further includes brine treatment module 170 configured to remove impurities from the aqueous sulfate prior to its concentration in brine concentration module 180 and return to electrochemical cell 110 for electrochemical salt splitting, thus recycling the aqueous sulfate.

[0217] FIG. 2 provides a flow diagram of a system 200 for phosphoric acid production using sulfuric acid electrochemically recycled using a 3-compartment electrochemical cell and calcium carbonate using carbon dioxide captured from a point source. As illustrated by FIG. 2 electrochemical cell 210 is configured to receive water and recycled sodium sulfate brine and produce sulfuric acid stream 211, and an alkaline stream comprising sodium hydroxide in stream 213. Electrochemical cell 210 is fluidly coupled to reactor 220 via stream 211. The system is configured such that alkaline stream 213 from electrochemical cell 210 is provided to direct air capture unit 230, which is configured to produce an aqueous carbonate (Na2COs) from the alkaline stream and carbon dioxide (CO2), which in this implementation is obtained from a point source. Reactor 220 is configured to produce dilute phosphoric acid (H3PO4) and gypsum (CaSO4*nH2O) slurry from the H2SO4. and recycled phosphoric acid and in some instances is configured to carry out multiple digestion steps, such as from 2 to 8 steps. System 200 includes a recycle stream configured to recirculate phosphoric acid to reactor 220 for furtherAtty. Dkt. No. TVTI-006WO

[0218] concentration. Reactor 220 is configured to provide calcium sulfate-containing gypsum slurry to reactor 260 after separation from aqueous phosphoric acid in separator 250. Point source capture unit 230 is configured to deliver aqueous carbonate to precipitator 260, which is configured to convert the gypsum slurry to a calcium-containing solid product (in this case, calcium carbonate) using the aqueous carbonate and thereby generate an aqueous sulfate via metathesis. System 200 further includes brine purification module 270 configured to remove impurities from the aqueous sulfate prior to return to electrochemical cell 210 for electrochemical salt splitting, thus recycling the aqueous sulfate.

[0219] FIG. 3 provides a flow diagram of a system 300 for phosphoric acid production using sulfuric acid electrochemically recycled using a 3-compartment electrochemical cell and producing slaked lime. As illustrated by FIG. 3, 3-compartment electrochemical cell 310 is configured to receive water and recycled sodium sulfate brine and produce, via electrochemical salt splitting, sulfuric acid stream 311, and an alkaline stream comprising sodium hydroxide in stream 313. Electrochemical cell 310 is fluidly coupled to reactor 320 via stream 311.

[0220] Electrochemical cell 310 is further configured to produce oxygen gas and hydrogen gas.

[0221] Reactor 320 is configured to produce dilute phosphoric acid (H3PO4) and gypsum (CaSO4*nH2O) slurry from the H2SO4. and recycled phosphoric acid and in some instances is configured to carry out multiple digestion steps, such as from 2 to 8 steps. System 300 includes a recycle stream configured to recirculate phosphoric acid to reactor 320 for further concentration. Reactor 320 is configured to provide calcium sulfate-containing gypsum slurry to precipitator 350 after separation from aqueous phosphoric acid in separator 340. Precipitator 350 is configured to receive alkaline stream 313 from electrochemical cell 310 and is further configured to convert the gypsum slurry to a calcium-containing solid product (in this case, slaked lime). Precipitator 350 is further configured to deliver the alkaline liquid phase to a CO2capture module 360 (in this implementation a point source capture module) operably linked to a CO2 source. System 300 further includes brine purification module 370 configured to remove impurities from the aqueous sulfate prior to return to electrochemical cell 310 for electrochemical salt splitting, thus recycling the aqueous sulfate. In one embodiment, the slaked lime product from precipitator 350 is used in carbon-free cement production, such as Portland cement production.

[0222] FIG. 4 depicts an exemplary three-compartment electrochemical cell 400 comprising an AEM and a GEM. The electrochemical cell depicted in this FIG. 4 is suitable for use in the present methods and systems, including, without limitation, the systems illustrated in FIGs. 1-3.

[0223] Electrochemical cell 400 includes exchange elements 401 and 405 having inlets and outlets forAtty. Dkt. No. TVTI-006WO

[0224] conveying liquid (e.g., anolyte and catholyte, as appropriate) to and from the anode chamber comprising anode 404 and cathode chamber comprising cathode 402, respectively. Water (H2O) is provided to the anode chamber, while water, sulfuric acid (H2SO4) and oxygen (O2) are conveyed from the anode chamber. In addition, water and sodium sulfate (Na2SO4) are provided to the cathode chamber, while water, sodium sulfate, sodium hydroxide (NaOH) and hydrogen (H2) are conveyed from the cathode chamber. Cell 400 also includes inlet 406 and outlet 407 configured to pass aqueous sulfate through the cell. During electrochemical salt splitting, sodium ion (Na+) passes through CEM 403a to form sodium hydroxide in the cathode chamber, while sulfate ion (SO42-) passes through AEM 403b to form sulfuric acid in the anode chamber. In some embodiments, catholyte and / or anolyte may be recirculated. In some cases, electrochemical cells include an anion exchange membrane. Exemplary electrochemical salt splitting protocols according to such embodiments may be found in U. S. Application No.

[0225] 18 / 403,666, filed on January 3, 2024; herein incorporated by reference in its entirety.

[0226] FIG. 5 illustrates a system 500 for phosphoric acid production using sulfuric acid electrochemically recycled using a 2-compartment electrochemical cell and for production of calcium carbonate using carbon dioxide captured from a point source. Electrochemical cell 510 is configured to receive water and recycled sodium sulfate brine and produce sulfuric acid stream 511, and an alkaline stream comprising sodium hydroxide in stream 512 via electrochemical salt splitting. Electrochemical cell 510 is fluidly coupled to reactor 520 via stream 511. Electrochemical cell 510 is further configured to produce oxygen gas and hydrogen gas. The system is configured such that alkaline stream 512 from electrochemical cell 510 is provided to carbon sequestration unit 530, which in this implementation is a point source capture unit, configured to produce an aqueous carbonate (Na2CO3) from the alkaline stream and carbon dioxide (CO2) from a point source. Reactor 520 is configured to produce dilute phosphoric acid (H3PO4) and gypsum (CaSO4*nH2O) slurry from the H2SO4. and recycled phosphoric acid and in some instances is configured to carry out multiple digestion steps, such as from 2 to 8 steps. System 500 includes a recycle stream configured to recirculate phosphoric acid to reactor 520 for further concentration. Reactor 520 is configured to provide calcium sulfate-containing gypsum slurry to reactor 550 after separation from aqueous components, such as phosphoric acid in separator 540. Point source capture unit 530 is configured to deliver aqueous carbonate to precipitator 550, which is configured to convert the gypsum slurry to a calcium-containing solid product (in this case, calcium carbonate) using the aqueous carbonate and thereby generate an aqueous sulfate via metathesis. System 500 further includes brine purification module 550 configured to remove impurities from the aqueous sulfate prior toAtty. Dkt. No. TVTI-006WO

[0227] recycling the aqueous sulfate to electrochemical cell 510 for electrochemical salt splitting.

[0228] FIG. 6 illustrates a system 600 for phosphoric acid production using sulfuric acid electrochemically recycled using a 2-compartment electrochemical cell and producing slaked lime. Electrochemical cell 610 is configured to receive water and recycled sodium sulfate brine and produce sulfuric acid stream 611 via electrochemical salt splitting, and an alkaline stream comprising sodium hydroxide in stream 612. Electrochemical cell 610 is fluidly coupled to reactor 620 via stream 611. Reactor 620 is configured to provide calcium sulfate-containing gypsum slurry to precipitator 630 after separation from aqueous phosphoric acid in separator 640. Precipitator 630 is configured to receive alkaline stream 612 from electrochemical cell 610 and is further configured to convert the gypsum (CaSO4•nH2O) slurry to a calcium-containing solid product (in this case, slaked lime). Precipitator 630 is further configured to deliver the alkaline liquid phase to a CO2 sequestration module 650 (in this implementation a point source capture module) operably linked to a CO2 source. System 600 further includes brine purification module 660 configured to remove impurities from the aqueous sulfate prior to recycling to electrochemical cell 610 for electrochemical salt splitting. In one embodiment, the slaked lime product from precipitator 630 is used in carbon-free cement production, such as Portland cement production.

[0229] FIG. 7 provides a schematic diagram of an electrochemical cell 700, suitable for use in the systems according to FIG. 5 and FIG. 6, comprising an AEM. Cell 700 includes exchange elements 701 and 705 having inlets and outlets for conveying liquid (e.g., anolyte and catholyte, as appropriate) to and from the cathode chamber comprising cathode 702 and anode chamber comprising anode 704, respectively. Water (H2O) and sodium sulfate (Na2SO4) are provided to the cathode chamber, while water, sodium hydroxide (NaOH) and hydrogen (H2) are conveyed from the cathode chamber. In addition, water is provided to the anode chamber, while water, sulfuric acid (H2SO4) and oxygen (O2) are conveyed from the anode chamber. AEM 703 is configured such that sulfate ion (SO42-) crosses into the anode chamber and forms an acidic solution comprising sulfuric acid. In some embodiments, catholyte and / or anolyte may be recirculated.

[0230] FIG. 8 provides a flow chart 800 illustrating phosphoric acid production using electrochemically recycled sulfuric acid and purification using nanofiltration membranes.

[0231] Phosphate rock digestion yields dilute phosphoric acid, which is subjected to preliminary purification by desulfation 820 and calcium removal 840, before contacting one or more nanofiltration membranes 850. Retentate from nanofiltration may be recycled into the process,Atty. Dkt. No. TVTI-006WG

[0232] while permeate is passed to evaporative concentration by mechanical vapor recompression 860.

[0233] FIG. 9 provides a flow chart 900 illustrating a process for phosphoric acid production using electrochemically recycled sulfuric acid and purification using nanofiltration membranes, including a preliminary reconcentration step using reverse osmosis. The process illustrated in flow chart 900 differs from that in Fig. 8 in that it provides for further concentration of phosphoric acid via reverse osmosis 960 of the permeate received after nanofiltration.

[0234] FIG. 10 provides a flow chart 1000 illustrating phosphoric acid production using electrochemically recycled sulfuric acid and purification using solvent extraction. The process illustrated in flow chart 1000 provides for further purification of phosphoric acid via desulfation 1020, comprising contacting the dilute sulfuric acid with a calcium-containing solid material, followed by calcium removal 1040 and solvent extraction 1070. Solvent extraction 1070 to purify the phosphoric acid product may be combined with the other processes and systems outlined herein, including those of Figs. 1 -9.

[0235] FIG. 11 provides a flow chart 1100 illustrating phosphoric acid production using electrochemically recycled sulfuric acid and purification using solvent extraction including a preliminary reconcentration step using reverse osmosis. The process outlined in chart 1100 differs from that of FIG. 10 in that it provides for intermediate reconcentration 1145 of the phosphoric acid product prior to evaporative concentration by mechanical vapor recompression 1150 and solvent extraction 1170.

[0236] FIG. 12 provides flowchart 1200 illustrating phosphoric acid production using electrochemically recycled sulfuric acid and purification using solvent extraction with preliminary defluorination and including a preliminary reconcentration step using reverse osmosis. The process of chart 1200 differs from that of FIG. 11 in that it provides for a defluorination step1242 prior to reverse osmosis 1245.

[0237] FIG. 13 provides a flow chart 1300 illustrating a process for phosphoric acid production using a recycled sulfuric acid solution. In this process, indirect mechanical vapor recompression is used to concentrate recycled sulfuric acid solution allowing for a concentrated phosphoric acid product to be formed. The concentrated recycled sulfuric acid is contacted with phosphate rock in digester 1310, yielding an intermediate bassanite phase (hemihydrate, CaSO40.5H2O). Bassanite is separated from the phosphate rock digestion slurry 1311 in separator 1330, giving sulfuric acid makeup stream 1331, dilute phosphoric acid stream 1332, and bassanite slurry 1333. Bassanite slurry 1333 is fed to gypsum reactor 1350 and recrystallized to gypsum (dihydrate, CaSO42H2O). Gypsum is subjected to counter-current washing, and then theAtty. Dkt. No. TVTI-006WG

[0238] purified gypsum is filtered from the slurry at 1360. Intermediate sulfuric acid solution from reverse osmosis is provided to wash gypsum at 1360. Dilute phosphoric acid 1332 is subjected to clarification 1370 and concentrated via indirect mechanical vapor recompression 1380 to provide concentrated phosphoric acid 1381, which is further purified by solvent extraction 1390.

[0239] FIG. 14 provides a flow chart 1400 illustrating a process for phosphoric acid production using a recycled sulfuric acid solution. In this process, evaporation I mechanical vapor recompression is used with reverse osmosis to increase H2SO4 concentration. Dilute sulfuric acid is provided to reverse osmosis module 1410. Intermediate sulfuric acid solution is concentrated via evaporation with mechanical vapor recompression in module 1420, yielding concentrated sulfuric acid solution 1421 to which is optionally added concentrated sulfuric acid makeup stream 1441. The concentrated recycled sulfuric acid is contacted with phosphate rock in digester 1430, yielding an intermediate bassanite phase (hemihydrate, CaSO4•0.5H2O). Bassanite is separated from the phosphate rock digestion slurry 1431 in separator 1440, giving sulfuric acid makeup stream 1441, dilute phosphoric acid stream 1442, and bassanite slurry 1443. Bassanite slurry 1443 is fed to gypsum reactor 1450 and recrystallized to gypsum (dihydrate, CaSC>42H2O). Gypsum is subjected to counter-current washing, and then the purified gypsum is filtered from the slurry at 1460. Dilute phosphoric acid 1442 is subjected to clarification 1470 and purification by nanofiltration 1480 to provide nanofiltration permeate containing dilute purified phosphoric acid 1481, which is concentrated via indirect mechanical vapor recompression 1490.

[0240] FIG. 15 provides a flow chart 1500 illustrating a process for phosphoric acid production using a recycled sulfuric acid solution. The process uses the same steps diagrammed in FIG.

[0241] 14, omitting the reverse osmosis step. Instead, evaporation I mechanical vapor recompression is used increase H2SO4 concentration of a dilute sulfuric acid stream produced by electrochemical salt splitting.

[0242] FIG. 16 provides a flow chart 1600 illustrating a process for separation of radium during bassanite separation. The radium separation step can be integrated into the processes for phosphoric acid production illustrated in FIGs. 13 - 15. Cyclonic separation allows separation of radium fine particles from the bassanite slurry.

[0243] FIG. 17 provides a flow chart 1700 illustrating a process for separation of rare earth elements (REE) from gypsum using a sulfonic acid chelating resin. The process of 1700 can be integrated into the processes for phosphoric acid production illustrated in FIGs. 8 - 15.Atty. Dkt. No. TVTI-006WO

[0244] FIG. 18 provides a flow chart 1800 illustrating a process for separation of REE from gypsum using oxalic acid chelation to precipitate REE oxalates. The process of 1800 can be integrated into the processes for phosphoric acid production illustrated in FIGs. 8 - 15.

[0245] FIG. 19 provides a flow chart 1900 illustrating a process using dilute electrochemically recycled sulfuric acid to produce phosphoric acid including multistage washing of gypsum. Electrochemically produced dilute sulfuric acid stream 1901 is contacted with rock phosphorus in multistage reactor 1910 followed by separation of dilute phosphoric acid solution via centrifugation at 1920. Solids from 1920 are repulped in the presence of acid in repulp wash reactor 1930 followed by filtering with dilute electrochemically recycled sulfuric acid at belt filter 1940 to give first stage filtrate acid repulp recycle stream 1941 for return to reactor 1910. Wash of remaining filter cake at 1940 with minimal water gives displacement wash recycle stream 1942 containing residual sulfuric acid for return to repulp wash reactor.

[0246] FIG. 20 provides a flow chart 2000 illustrating a process for producing magnesium hydroxide using dilute electrochemically generated sulfuric acid. This process also produces phosphoric acid and may be combined with processes and systems for phosphoric acid production described herein. Electrochemically produced dilute sulfuric acid stream 2041 is contacted with high magnesium content rock phosphorus in reactor 2010 followed by separation of dilute phosphoric acid solution 2011 and gypsum slurry 2012 which may be returned to core process 2040 for metathesis to give precipitated calcium carbonate 2042 and sulfate brine for recycling to electrochemical salt splitting. Dilute phosphoric acid 2011 is subjected to nanofiltration to give retentate 2021 and permeate 2022 enriched in phosphoric acid. Permeate is further processed via reverse osmosis 2030 to provide upgraded phosphoric acid 2031. Nanofiltration retentate 2021 is treated with alkali, optionally including excess NaOH 2041, in neutralization step 2050 to precipitate iron and alumina, 2060 to precipitate MgHPO4, and 2170 to convert gypsum and NaH2PC>4 to calcium phosphate 2072 for return to digestion 2010 with the production of sodium sulfate 2074 for return to core process 2040. Optional neutralization 2065 on MgHPO4 solid intermediate converts MgHPO4 to Mg(OH)2.

[0247] FIG. 21 provides a flow chart 2100 illustrating a process for phosphoric acid and magnesium production using high magnesium phosphate rock and a recycled sulfuric acid solution. Dilute sulfuric acid stream 2110 (from core electrochemical salt splitting process) is provided to optional concentration module 2120. Module 2120 may concentrate sulfuric acid via reverse osmosis module to give an intermediate sulfuric acid solution and / or via evaporation with mechanical vapor recompression in module. Sulfuric acid stream 2121 may be dilute sulfuric acid from electrochemical salt splitting, intermediate sulfuric acid solution such asAtty. Dkt. No. TVTI-006WG

[0248] concentrated by reverse osmosis, or concentrated sulfuric acid solution, such as provided by evaporation with mechanical vapor recompression. To stream 2121 is optionally added concentrated sulfuric acid makeup stream 2141. The combined recycled sulfuric acid 2122 is contacted with phosphate rock in digester 2130, yielding an intermediate bassanite phase (hemihydrate, CaSO4•0.5H2O). Bassanite is separated from the phosphate rock digestion slurry 2131 in separator 2140, giving sulfuric acid makeup stream 2141, dilute phosphoric acid stream 2142, and bassanite slurry 2143. Bassanite slurry 2143 is fed to gypsum reactor 2150 and recrystallized to gypsum (dihydrate, CaSO4•2H2O). Gypsum is subjected to counter-current washing, and then the purified gypsum is filtered from the slurry at 2160. Dilute phosphoric acid 2142 is subjected to clarification 2170 and purification by nanofiltration 2175 to provide nanofiltration retentate 2176 and nanofiltration permeate containing dilute purified phosphoric acid 2177, which is concentrated via indirect mechanical vapor recompression 2180.

[0249] Nanofiltration retentate 2176 is treated with alkali in neutralization step 2190 to precipitate iron and alumina, 2192 to precipitate MgHPO4, and 2194 to convert gypsum and NaH2PC>4 to calcium phosphate 2196. Optional neutralization 2193 on MgHPO4 solid intermediate converts MgHPO4 to Mg(OH)2. Calcium phosphate is recycled to digestion 2130, and sodium sulfate 2195 is recycled to the core electrochemical salt splitting process described herein.

[0250] FIG. 22 depicts at left an exemplary three-compartment bipolar membrane electrodialysis cell 2200 comprising an AEM, a GEM, and a BPM. The electrochemical cell depicted in this FIG. 22 is suitable for use in the present methods and systems, including, without limitation, the systems illustrated in FIGs. 1-3. Electrochemical cell 2200 includes bipolar membrane 2201, exchange elements 2202, 2203 and 2204 having inlets and outlets for conveying liquid (e.g., anolyte, sodium sulfate, and catholyte, as appropriate) to and from the anolyte compartment comprising anion exchange membrane 2205 and catholyte compartment comprising cation exchange membrane 2206, respectively. In some embodiments, catholyte, anolyte and / or salt solution may be recirculated.

[0251] FIG. 23 provides a plot of bipolar membrane component voltage as a function of applied current density comparing a conventional low-current bipolar membrane to two high-current bipolar membranes labeled “BPM 1” and “BPM 2”. The high current membranes reduce the overvoltage at the bipolar junction, reducing energy loss in the cell configured as depicted in FIG. 22.

[0252] FIG. 24 illustrates sulfuric acid reconcentration system configured as a multi-effect evaporator concentrating sulfuric acid from 12 to 50% w / w H2SO4.Atty. Dkt. No. TVTI-006WO

[0253] FIG. 25 provides a plot of phosphoric acid digestion pilot data over time for sedimentary feed rock using a dilute sulfuric acid feed representative of electrochemical salt splitting supply. Samples were taken from the first and second reactors (R-100 and R-101 respectively). The feed ratios of sulfuric acid, water, and rock were varied over time to study reaction kinetics.

[0254] FIG. 26 provides a phase diagram of calcium sulfate in the CaSO4-H3PO4-H2O system as a function of temperature and phosphoric acid concentration (% w / w P2O5) based on the phase stability data from Slack (1968, Phosphoric Acid, Vol. 1, Part I. Marcel Dekker, New York), indicating a phase transition from gypsum (dihydrate) to bassanite (hemihydrate) at lower temperatures for higher P2O5. Digestion reactor conditions for systems reported here are indicated as open circles.

[0255] FIG. 27 provides results of replicate phosphate ore digestion experiments using 12% w / w sulfuric acid showing >90% extent of conversion within 300 minutes.

[0256] FIG. 28 illustrates dependence of magnesium phosphate solubility on digester phosphoric acid pH (left) and excess sulfate concentrations for a range of phosphoric acid concentrations, including the conventional case typical of most wet phosphoric acid plants using the dihydrate process. Lower pH and higher excess sulfate is required in the conventional case to maintain under-saturation with respect to magnesium phosphate during digestion.

[0257] FIG. 29 provides concentrations of nanofiltration retentate as a function of total P2O5 recovery for high concentration (major) elements. Most elements are plotted in units of ppm, except sulfate, which is plotted on a different scale as %w / w SO4.

[0258] FIG. 30 provides a graph of elemental concentrations of nanofiltration retentate as a function of total P2O5 recovery for low concentration (trace) elements.

[0259] FIG. 31 provides a graph of elemental concentrations of nanofiltration permeate as a function of total P2O5 recovery for high concentration (major) elements. Elements are plotted in units of ppm, except sulfate, which is plotted on a different scale as %w / w SO4.

[0260] FIG. 32 provides a graph of elemental concentrations of nanofiltration permeate as a function of total P2O5 recovery for low concentration (trace) elements.

[0261] FIG. 33 provides a plot of separation performance as rejection (%) as a function of P2O5 recovery for elements with high rejection.

[0262] FIG. 34 provides a plot of separation performance as rejection (%) as a function of P2O5 recovery for elements with low rejection.

[0263] FIG. 35 provides a plot of rejection versus feed concentration of two highly rejected elements compared to output of the nanofiltration model used to size a commercial-scale nanofiltration system.Atty. Dkt. No. TVTI-006WO

[0264] FIG. 36 Illustration of phosphoric acid two-effect evaporation system 3600 concentrating phosphoric acid from 20 to 54% w / w P2O5 in stages 1 and 2 with separators E1 and E2.Atty. Dkt. No. TVTI-006WO

[0265] WORKING EXAMPLES EXAMPLE 1 - PRODUCTION OF SULFURIC ACID AND CALCIUM CARBONATE

[0266] A pilot run was conducted using gypsum fed to an electrochemical salt splitting system to produce dilute sulfuric acid and precipitated calcium carbonate, using carbon dioxide captured from the air and from a point source. Gypsum with a particle size d80 = 70 urn was fed to a slurry tank at a rate of 0.15 tons per day and mixed with recycled process water to a solid suspension density of 30% w / w. The solid slurry was fed to the first in a series of three metathesis reactors and combined with 2.5 M sodium carbonate and 0.5 M sodium hydroxide solution produced by reacting electrochemically produced caustic solution with carbon dioxide supplied from a point source tank as well as from a cross-flow cooling tower enabling capture from the air. The metathesis slurry mixture produced a product solid slurry containing solid calcium carbonate. The slurry was pumped to a thickener, and the thickener underflow was pumped to a filter press. Solids recovered from the filter press had an average d90 of two lots equal to 83.3 urn. The solid product contained 0.06% residual gypsum determined by semi-quantitative X-Ray diffraction. Thickener overflow solution containing 1.1 M sodium sulfate solution at pH 12 was neutralized to pH 10 using a stream of acid from the electrochemical salt splitting system and continuously pumped to a cation exchange column, where residual cations, including 40 ppm calcium, were removed from solution. The resulting purified brine was continuously pumped to a vessel, from which it was recirculated through an electrochemical salt splitting system. In this pilot run, the system was configured as a stack of three 3-compartment salt splitting electrolyzer cells, with active areas of 0.9 m2per cell operated at a current density of 240 mA / cm2. Sodium sulfate was continuously recirculated through the center compartment of the reactor, where sodium ions permeated through a cation exchange membrane to a catholyte compartment, where water splitting on the cathode produced a sodium hydroxide (caustic) solution. Oxygen was recirculated through the cathode to suppress hydrogen formation and reduce the cathodic overpotential. Sulfate ions from the center compartment were continuously permeated through the anion exchange membrane of each cell to an anolyte compartment, where water splitting on the anode produced a dilute sulfuric acid solution of 14.8% w / w H2SO4 at 50°C. The sulfuric acid solution was continuously pumped into a recirculating vessel, where purified water was added to maintain a constant sulfuric acid concentration. A stream of sulfuric acid was pumped to a storage vessel for use. The sodium hydroxide solution was diluted to 3M NaOH and continuously pumped from an intermediate storage vessel to the direct air capture system configured as a cross-flow cooling tower. TheAtty. Dkt. No. TVTI-006WO

[0267] sodium carbonate and sodium hydroxide mixture pumped out of the cross-flow cooling tower was contacted with 100% carbon dioxide at ambient pressure in a continuously stirred tank reactor to simulate point source capture to produce the final sodium carbonate and sodium hydroxide mixture pumped to the first metathesis reactor and reacted with a gypsum slurry, continuing the circular process. The precipitated calcium carbonate was dried and blended at a 20% w / w mixing rate into cement to produce Type 1 L Portland Limestone Cement.

[0268] In commercial embodiments, the point source capture step is optionally configured as a packed tower enabling efficient capture of carbon dioxide at lower concentration gas streams, such as 5-30% v / v CO2 streams. These gas streams may be contaminated with various gases, such as volatile fluorinated compounds, such as hydrofluoric or fluorosilicic acid, sulfur oxide compounds, or nitrous oxide compounds. Contacting the carbon dioxide gas streams with the caustic solution scrubs the contaminant compounds into the produced sodium carbonate solution.

[0269] Alternatively, the metathesis reactor is configured as a slaked lime precipitation reactor, where the 3M concentration sodium hydroxide solution produced in the electrochemical salt splitting system is pumped directly to the first reactor to react with the gypsum slurry. The product of the metathesis system is precipitated slaked lime, which is separated from the residual sodium sulfate and calcium hydroxide solution in a thickener. The thickener overflow solution is contacted with a carbon dioxide gas stream, such as 5-100% v / v CO2 to precipitate residual aqueous calcium hydroxide as calcium carbonate. The resultant slurry is then separated using a combination of a thickener and filter press, and the sodium sulfate solution is continuously pumped back to the electrochemical salt splitting system to continue the circular process. Precipitated calcium hydroxide is used as a raw material feedstock to cement production or as an industrial chemical for various applications in the mining, water treatment, and chemicals industries.

[0270] A bench-top scale run was conducted to produce sulfuric acid and caustic with processcompatible concentrations. A bipolar membrane electrodialysis system compatible with current densities up to 600 mA / cm2(FIG. 22) was configured with a bipolar membrane, an anion exchange membrane, and a cation exchange membrane and operated at fixed temperature (40°C) and ambient pressure, with an active area of 6.54 cm2. Sulfate ions permeated through the anion exchange membrane, where they combined with protons formed by water dissociation at the bipolar junction to produce a 12.7% w / w sulfuric acid solution. Sodium ions permeated through the cation exchange membrane, where they combined with hydroxide ions formed by water dissociation at the bipolar junction to produce a 7.4% w / w sodium hydroxide solution. TheAtty. Dkt. No. TVTI-006WO

[0271] experiments were conducted using a commercial (low-current) bipolar membrane and two novel high-current bipolar membranes over a range of current densities, from 0 to 600 mA / cm2. The measured polarization curves quantifying the contribution of the bipolar membranes to the cell voltage is given in FIG. 23. The high-current membranes showed much lower Ohmic losses and water dissociation overpotentials than the commercial low-current membrane at the same current density. Examples of high-current bipolar membranes are described in U. S. Patent Application Pub. Nos. 2019 / 0134570 and 2020 / 0370188, and U. S. Patent No. 11,268,200. The current efficiencies of acid and base production were measured as the amount of acid / base actually produced relative to the amount that could theoretically be produced given the operating current density. These current efficiencies were compared directly to current efficiencies measured using a 3-compartment salt splitting electrolysis system constructed using the same inactive components, the same anion and cation exchange membranes, and with the same intra-membrane distances. The current efficiencies of the bipolar membrane electrodialysis systems were comparable to one another, and to the 3-compartment electrolysis system, for both acid and base production.

[0272] EXAMPLE 2 - SULFURIC ACID RECONCENTRATION

[0273] In this example, a process for the reconcentration of dilute sulfuric acid is performed using a triple-stage, forced-circulation vacuum evaporation train as illustrated in FIG. 24.

[0274] Sulfuric acid concentration is described in, for example, U. S. Patent No. 4,547,353. A dilute sulfuric acid feed stream, comprising approximately 12% w / w H2SO4 is introduced into the system at a continuous flow rate of 244 t / hr, equivalent to roughly 256,000 T / yr of 100% H2SO4, which is sufficient to produce approximately 100,000 T / yr P2O5 phosphoric acid. This stream is first processed through an acid preheater, which applies a thermal duty of 17 MW to prepare the feed for the initial high-pressure evaporation stage. The preheated acid is circulated through a primary graphite heat exchanger energized by an external steam supply providing a thermal duty of 37 MW. The heated acid enters a first separation vessel (E1) maintained at a pressure of approximately 35 psia and an operating temperature of 128°C. Approximately 60 t / hr of water vapor is evolved and recovered via the overhead of E1. This stage yields an intermediate acid stream at a flow rate of 184 t / hr with a concentration of 16% w / w H2SO4. The intermediate acid (16 wt% w / w H2SO4) is directed to a second evaporation loop. The 60 t / hr of vapor evolved from E1 is utilized as the primary heating medium in a secondary heat exchanger, delivering 34 MW of thermal duty. The second separation vessel (E2) is operated at a reduced pressure of 10 psia and a temperature of 93°C. This stage evolves an additional 64 t / hr of vapor, producing aAtty. Dkt. No. TVTI-006WO

[0275] second intermediate acid stream at a flow rate of 120 t / hr and a concentration of 24 % w / w H2SO4. Process condensate from the heat exchanger is recovered at 126°C. The second intermediate acid stream is directed to a final evaporation loop. The 64 t / hr of vapor from the second stage is utilized as the heating medium in a tertiary heat exchanger, delivering 40 MW of thermal duty. The final separation vessel is operated under deep vacuum at 2.5 psia and a temperature of 79°C. An additional 62 t / hr of vapor is evolved and subsequently condensed in a final condenser with a cooling duty of -42 MW, yielding process condensate at 57°C. Following the triple-stage evaporation process, a final concentrated sulfuric acid product is recovered. The process yields approximately 58 t / hr of product with a final concentration of 50 wt% w / w H2SO4. The recovered process water condensate (pH ~4) is combined and returned to the electrochemical salt splitting system for acid production.

[0276] In a further example, a process for the reconcentration of 256,000 T / yr sulfuric acid is performed using a modified evaporation train using mechanical vapor recompression (MVR) to recover latent heat by steam recompression and achieve a product concentration of 50% w / w H2SO4. Heat recovery by mechanical vapor recompression is described in, for example, U. S. Patent No. 4,303,468. The process is conducted in a dual-stage evaporation system where both the primary and secondary stages utilize MVR to achieve a final product of 50% w / w H2SO4 from a 12% w / w H2SO4 feed. In the first stage, which has lower boiling point elevation (BPE), a dilute sulfuric acid feed (244 t / hr, 12 wt% w / w H2SO4) is introduced into the first separation vessel. The vessel is maintained at a pressure of 10 psia and a temperature of 93°C.

[0277] Approximately 122 t / hr of water vapor is evolved, and a single-stage centrifugal compressor is utilized. The compressor boosts the vapor pressure from 10 psia to approximately 15 psia, raising the saturation temperature sufficiently to drive the heat transfer in the primary graphite exchanger. This stage yields an intermediate acid stream of 122 t / hr at 24% w / w H2SO4. The intermediate acid (24 wt%) is transferred to the second separation vessel for final reconcentration. The vessel is maintained at a deep vacuum of 3 psia and an operating temperature of 83°C to keep the acid below corrosive thresholds for the internal components. Approximately 64 t / hr of vapor is evolved to reach the final product concentration. The BPE of 50% w / w acid necessitates a three-stage compression approach with a compression ratio of 1.5 in each stage achieving a final pressure of 10 psia. Desuperheating is performed after each stage by injecting process condensate to prevent excess temperature in the following compression stage. The final desuperheating of the compressed steam reduces the steam temperature to 93°C prior to introduction into the exchanger where it condenses at 90°C. The system produces 58 t / hr of 50% w / w H2SO4. Materials of construction are selected to avoidAtty. Dkt. No. TVTI-006WO

[0278] corrosion. Tubes are constructed of impervious graphite, high-nickel alloys, or zirconium while separator vessels are lined with fluoropolymers, rubber, or acid resistant brick. Stainless steel is used in the compressor construction, although alternative materials such as titanium impellers could be selected to resist the impact of any acid carryover in the vacuum vapor stream.

[0279] EXAMPLE 3 - PHOSPHORIC ACID PRODUCTION USING DILUTE SULFURIC ACID

[0280] Case 1. 20% P2O5 Laboratory Pilot. A continuous 96-hour pilot experiment was conducted to produce 20% w / w phosphoric acid from rock phosphorus containing 31.1% P2O5, 48.3% CaO, 2.9% F, 1.2% SiO2 and 0.058 MER using dilute (50% w / w) sulfuric acid. In the pilot, a constant supply of rock concentrate milled to a d50 = 145 pm was continuously dosed to the first in a series of two stirred digestion reactors at a rate of 1,150 g / hr. The dilute sulfuric acid solution was continuously pumped into the first stirred 1.9L reactor (attack reactor) to form an acidulation slurry with a retention time of 40 minutes to obtain a steady state concentration of phosphoric acid of 20% w / w P2O5, an optimal sulfate concentration of 0.5% SO4, and a constant temperature of 80°C. The slurry was conducted via a gravity cascade to a second 9.3L stirred reactor vessel with a retention time of 200 minutes maintained at an optimal sulfate concentration of 1.5-2.5% SO4. The product phosphoric acid and gypsum slurry were continuously conducted via gravity cascade from the second reactor into an intermediate storage vessel. The slurry was manually vacuum filtered every 25 minutes using filter cloth, and the filtration rate was measured to ensure good pilot performance. The filter cake was washed with water and the wash water was recycled to the attack reactor, yielding an overall sulfuric acid content of 26.7% w / w fed to the digestion system. The product phosphoric acid was collected for purification. The average phosphate yield was 94.7% and the average final residual phosphate in gypsum was 1.2%. A plot of the product acid composition over time is shown in FIG. 25. The key variable optimized during the pilot run to maximize filtration efficiency was excess sulfate concentration via phosphate rock to sulfuric acid feed ratio. A typical excess sulfate of ~2% is maintained in phosphate digestion reactors to maintain optimal gypsum particle size and morphology (Becker, 1983).

[0281] In a commercial embodiment scaled for 100,000 T / yr P2O5, a multistage reactor is fed with sulfuric acid produced by electrochemical salt splitting and with a stream of acid concentrated to 50% w / w H2SO4. A fraction of the sulfuric acid is introduced to the reactor system in the gypsum wash stage and recycled back with filter wash water to the first and second stages of the reactor to achieve an overall P2O5 concentration of 20% w / w at 80°C (or 90°C in the case of a bassanite digestion system; FIG. 26). Milled phosphate rock is fed to theAtty. Dkt. No. TVTI-006WO

[0282] reactor at a rate required to maintain an excess sulfate concentration of 0.5% in the first reactor (the attack reactor) and a final sulfate concentration of 1.5-2.0% in the subsequent reactors. By maintaining an optimal sulfate concentration in the mother liquor (20% w / w P2O5), corresponding to a mass ratio of SO4 / P2O5 of 0.075-0.1, this system promotes the growth of equant gypsum crystals (>30 pm) suitable for fast filtration and for improved P2O5 yield in nanofiltration for phosphoric acid purification.

[0283] Case 2. 40% P2O5 Example. In another configuration at commercial scale, 500,000 T / yr P2O5 phosphoric acid is produced at a concentration of -40% P2O5 using a series of reactors with intermediate evaporative reconcentration of the recirculating solution. In the system, phosphate rock is introduced to a stirred reactor at a rate of 4,660 T / day and combined with a recycled phosphoric acid stream and a stream of dilute (50% w / w) sulfuric acid recycled from a downstream washing of the solid bassanite product. The slurry is reacted with a retention time of 180 minutes to obtain a steady state concentration of phosphoric acid of 40% w / w P2O5, a sulfate concentration of 2% SO4, and a constant temperature of 90°C. The elevated P2O5 reduces water activity driving formation of calcium sulfate as the phase hemi-hydrate (bassanite; FIG. 26). The concentrated slurry is pumped back to the first reactor to enhance hydraulic mixing and narrow the retention time distribution to allow adequate time for crystallization of particles. The product phosphoric acid and bassanite slurry is continuously pumped from the second reactor into a solid-liquid separation system configured as a centrifugal separation system. The centrate phosphoric acid is collected for purification. The concentrated bassanite slurry is pumped to a repulp recrystallization reactor, where it is slurried with a liquor produced from subsequent filtration where a mixture of 50% w / w sulfuric acid and process water are introduced in a counter current fashion at different stages. Bassanite recrystallizes to gypsum in the reactor maintained at 70°C and ambient pressure. The recrystallized gypsum and sulfuric acid slurry is pumped to a thickener, where a fraction of the thickener overflow sulfuric acid is recycled to the recrystallization reactor, and a fraction of the overflow sulfuric acid is pumped to the first phosphoric acid stirred reactor. The thickener underflow is pumped to a three-stage belt filter to reduce sulfuric acid losses and recover residual P2O5 in the integrated process. For example, a gypsum filter cake with 30% moisture as 50% sulfuric acid equates to an overall sulfuric acid loss of 27% relative to total production in the electrochemical salt splitting system, so water washing is required. Fresh 50% sulfuric acid is introduced at the second stage and returned as wash acid to the first stage. The filtrate from the first stage is continuously pumped to an intermediate reconcentration system to concentrateAtty. Dkt. No. TVTI-006WO

[0284] the recycled stream from 18% to 21% and 21% to 25% aqueous P2O5 and SO4 respectively, and this concentrated stream is then fed back to the bassanite digester. The third stage of filtration consists of a single pore volume displacement wash to recover sulfuric acid from the wet gypsum filter cake. The washed solid filter cake is slurried with process water, neutralized with caustic solution, and continuously pumped to a system to convert the gypsum to calcium carbonate and sodium sulfate as in Example 1.

[0285] Case 3. 6% P2O5 Example. In a third example, dilute (6% w / w P2O5) phosphoric acid was produced by digesting phosphate rock using dilute 12% w / w sulfuric acid. In a proof of concept experiment, sedimentary phosphate rock (29.3 % P2O5) was slurried with 664 g of 12% w / w H2SO4 in a vessel maintained at 80°C. The slurry was sampled approximately every 60 minutes, and the extent of digestion was determined to be greater than 90% based on a mass balance of the produced acid concentration shown in FIG. 27.

[0286] Case 4. Digestion of Off-Specification Phosphate Rock

[0287] The feasibility of digesting off-spec phosphate rock containing greater than 2.5% w / w MgO using a dilute sulfuric acid digestion was evaluated using a series of chemical speciation calculations. Precipitation of solid magnesium phosphate in the digestion reactor must be avoided, because it consumes P2O5 and forms a problematic sludge that incorporates in the phosphogypsum. A set of calculations was performed using PHREEQc geochemical speciation software (Parkhurst & Appelo, 2013) to investigate reactor conditions required to maintain a condition of undersaturation (supersaturation index, SI, with respect to Newberyite (Mg(HPO4).3H2O) < 0) in the digestion reactor. Digestion reactor phosphoric acid compositions representative of producing acid at 5%, 22% and 30% w / w P2O5 were simulated, which are representative of digesting phosphate rock using two different dilute sulfuric acid concentrations and using the conventional wet phosphoric acid feed acid concentration (-98% w / w H2SO4) and water balance, respectively. The simulated impurity content of the phosphoric acid was scaled to the dilution of the product acid. A series of calculations were performed to investigate the dependence of magnesium phosphate solubility on the pH and excess sulfate in the digestion reactor system. At higher P2O5 concentrations in the digestion reactor, higher excess sulfate and lower pH must be maintained to preserve a reactor solution that is undersaturated with respect to solid magnesium phosphate (e.g., SI Newberyite < 0). Results of the calculations are shown in FIG. 28. The optimal excess sulfate concentration (1-2%) can only be achieved for the dilute digestion cases (e.g. 5% and 22% P2O5), while optimal excess sulfate cannot beAtty. Dkt. No. TVTI-006WO

[0288] maintained in the conditions typical of conventional wet phosphoric acid production (e.g. 30% w / w P2O5 or higher). The minimum mass ratio of SO4 / P2O5 required to maintain undersaturation is 0.13 in the 30% P2O5 case and is only 0.07 in the 22% P2O5 case, indicating that less excess sulfate and downstream desulfation is required in the dilute acid digestion case, making it more optimal for higher MgO feed materials. Accordingly, the present process for employing dilute sulfuric acid is superior to conventional processes for off-specification, high magnesium content phosphate rock.

[0289] EXAMPLE 4 - PHOSPHOGYPSUM REPULP WASHING AND METATHESIS

[0290] This example demonstrates the feasibility of acid washing gypsum produced by digestion using dilute sulfuric acid for P2O5 recovery and REE (rare earth element) recovery. This example further demonstrates that the final alkaline mineral products produced from conversion of the gypsum to calcium carbonate or calcium hydroxide contain low concentrations of phosphorus and fluoride following washing. The experiments were initiated by hydrating a dry phosphogypsum feedstock containing 1.5% P2O5, 0.1% F, and 28% CaO using 12% w / w sulfuric acid (H2SO4). The sulfuric acid was heated to 60°C and combined with the PG for five minutes in a 20L vessel to ensure the slurry was well mixed. The slurry was then transferred to a Buchner funnel and filter flask, where the solid and aqueous materials were separated; the hydration filtrate was massed and retained. The gypsum cake was washed with heated 12% w / w H2SO4 to displace pore fluid from the hydration step, which contains soluble impurities. The use of sulfuric acid leverages the common ion effect to minimize the dissolution of gypsum itself while effectively purging impurities from the cake. The wash filtrate was massed and retained for mass balance. The deportment of phosphorus, fluoride, and rare earth elements were quantified. The washing method recovered 62.2% of the initial P2O5 and 34.5% of the initial F. The wash also liberated >10% of neodymium (Nd) into the sulfuric acid stream, resulting in a sulfuric acid leach liquor containing 323 ppm total REEs, suitable for selective separation by various methods.

[0291] Following the acid wash, the cake was removed from the funnel, massed, and transferred to a clean 20L reactor vessel and combined with ultrapure water under agitation to form a homogeneous slurry. The water addition was carefully controlled to a solid concentration of 24% w / w solids. The slurry was neutralized to pH 7 using 50% w / w reagent grade sodium hydroxide (NaOH). Batchwise metathesis experiments were performed, and depending on the desired solid product, sodium carbonate (Na2CO3) was added to produce calcium carbonate (CaCO3), or NaOH was added to generate slaked lime (Ca(OH)2). The chosen reagent andAtty. Dkt. No. TVTI-006WO

[0292] slurry were slowly combined using peristaltic pumps to transfer solutions in a 50 L reaction vessel simultaneously. The materials were added gradually to allow the slurry to serve as seed material for the remaining reagents. After all additions were complete, the reaction proceeded for 2 hours, consistent with optimal process conditions, producing solids and aqueous sodium sulfate solutions. Solid products were filtered and analyzed, and the recovered aqueous sodium sulfate solution was shown to be compatible with brine purification to recycle back to electrochemical salt splitting. The product solid calcium carbonate contained 0.4% P2O5, <0.1% F, and 51.5% CaO and contained >95% calcite as determined by X-ray diffraction. The sulfate yield of the carbonate metathesis reaction was 96.7%. The product solid calcium hydroxide contained portlandite (94.6% Ca(OH)2) as identified by X-ray diffraction with low impurities, including 0.2% P2O5, <0.1% F and 71.6% CaO.

[0293] EXAMPLE 5 - CENTRIFUGAL FRACTIONATION FOR RADIUM ABATEMENT AND CALCIUM SULFATE PURIFICATION

[0294] In this example, a centrifugal separation system is integrated into the digestion circuit to produce a low-radium gypsum product suitable for feeding to repulp washing and metathesis for construction-grade alkaline mineral products. The use of centrifugal force to separate radium in phosphate systems is described in U. S. Patent No. 4,328,193. This example describes the operation of a hydrocyclone bank scaled for a 100,000 T / yr P2O5 production facility. At a production scale of 100,000 metric tons P2O5 per year (operating at -8,000 hours / year), the system processes approximately 12.5 tons of P2O5 per hour fed as 20% P2O5 phosphoric acid slurry with 1-2% excess sulfate. To achieve the sharp cut-point (dso= 25 pm) required to isolate radium-enriched fines from the high-density slurry, a multi-cyclone manifold is employed. A bank of six 6-inch (150 mm) high-efficiency hydrocyclones is utilized, with five cyclones operating in parallel with one unit on standby. The system operates at a pressure drop of 15-20 psi. To maintain fractionation efficiency at high solids loading, a portion of the 20% P2O5 product acid or a recycled wash stream is injected into the hydrocyclone feed header to maintain an optimal feed percent solids of 30-35%. The centrifugal field effectively segregates the Radium-226, which is primarily associated with the fine-grained fraction (<20 pm) and insoluble impurities (e.g., barite), suspended in the phosphoric acid overflow. To recover the product acid, this overflow is routed to a secondary separator (e.g., a polishing filter or decanter) which isolates the radioactive fines as a concentrated cake. The underflow contains radium-depleted gypsum slurry, which is pumped to a sulfuric acid washing stage prior to being converted to the final calcium carbonate or calcium hydroxide alkaline mineral product.Atty. Dkt. No. TVTI-006WO

[0295] EXAMPLE 6 - PHOSPHORIC ACID PURIFICATION

[0296] A pilot run was conducted using a single membrane element nanofiltration system. The pilot system consisted of a 4” spiral wound membrane element fed by a 100L HDPE recirculating tank at a pressure of 400 psig. Phosphoric acid (22% P2O5) at room temperature was first pumped through an ultrafiltration system to remove fine particles and collected into the recirculating tank. Then, an antiscalant was dosed to the tank at a concentration of 40 mg antiscalant / kg phosphoric acid solution to suppress gypsum formation in the membrane element. The phosphoric acid was fed to the nanofiltration membrane element in a recirculating fashion at 1200 psig, with retentate recycled to the feed tank. The membrane permeate was continuously collected and both permeate and retentate were intermittently sampled for elemental composition. Phosphoric acid permeate and retentate compositions vs. total P2O5 yield in the permeate are given in FIGs. 29-32. The nanofiltration permeate had a final P2O5 concentration of 24%, and the system was effective at removing impurities, with separation efficiencies (rejections) as a function of P2O5 recovery given in FIGs. 33-34. Rejection (%) was calculated as 1 -(permeate concentration / feed concentration) and is plotted for two key impurity elements as a function of feed concentration in FIG. 35. Higher phosphate yields can be achieved because of the higher SO4 / P2O5 ratio and lower pH in the raw acid product of dilute phosphoric acid digestion compared to conventional acid digestion diluted to the target P2O5 concentration (e.g. SO4 / P2O5 of 0.07-0.1 instead of -0.05).

[0297] EXAMPLE 7 - DILUTE PHOSPHORIC ACID INTERMEDIATE CONCENTRATION

[0298] In this example, a dilute phosphoric acid stream is concentrated using a three-stage osmotically-assisted reverse osmosis (OARO) system. The general systems and processes of OARO are described in, for example, U. S. Patent WO2017213992A3. The example system consists of 85 pressure vessels, each containing 6 standard 8” reverse osmosis membrane elements with approximately 37m2area per element. A draw solution consisting of a recycled concentrated phosphoric acid stream is circulated on the permeate side of the membranes, maintaining a net osmotic pressure differential ( IT) of less than 30 bar across the membrane interface. A feed inlet stream of 323 t / hr at 6% w / w P2O5 is pressurized to 65 bar and flows to a system of 35 pressure vessels arranged in parallel. No osmotic assistance is required on the permeate side in the first stage, and the acid is concentrated to -10.5% w / w P2O5. In the second stage, the 10.5% w / w P2O5 concentrate from the first stage is pressurized to 78 bar and flows toAtty. Dkt. No. TVTI-006WO

[0299] a system of 30 pressure vessels. A portion of the first stage concentrate (~ 10.5% w / w P2O5) is circulated on the permeate side of the second stage membranes as a draw solution. The presence of the 10.5% w / w P2O5 draw solution provides approximately 30 bar of osmotic assistance, allowing the system to concentrate the feed to 14.5% w / w P2O5 while staying within mechanical limits. In the final stage, the 14.5% w / w P2O5 concentrate is pressurized to 80 bar and flows to a system of 20 pressure vessels. The final product (18.1% w / w P2O5) is used as the draw solution, overcoming an osmotic pressure of ~95 bar. By providing 60 bar of osmotic assistance, the net driving pressure remains at ~45 bar. The final product is recovered at 108 t / hr at 18.1 % w / w P2O5.

[0300] EXAMPLE 8 - PHOSPHORIC ACID EVAPORATIVE CONCENTRATION

[0301] In this example, a process for the concentration and defluorination of phosphoric acid is performed using a dual-stage, forced-circulation vacuum evaporation system as illustrated in FIG. 38. A dilute phosphoric acid feed stream, comprising approximately 20% w / w P2O5 and 538 ppm F is introduced into the system at a continuous flow rate of 115 t / hr. This feed stream is first passed through an acid preheater, where it is subjected to a thermal duty of 8 MW to elevate the stream temperature to a point suitable for vacuum evaporation. The preheated acid is then introduced into a first evaporation loop. The acid is circulated through a primary graphite heat exchanger energized by a steam supply, providing a thermal duty of 24 MW. The heated acid enters a first separation vessel (E1 ) maintained at a vacuum pressure of 11 psia and an operating temperature of 95°C. Under these conditions, approximately 36 t / hr of water vapor containing 661 ppm F is evolved and removed via the overhead of E1. This yields an intermediate acid stream at a flow rate of 78 t / hr with an increased concentration of 29% w / w P2O5 and a decreased fluorine concentration of 272 ppm F. The intermediate acid stream (29% w / w P2O5) is subsequently directed to a second evaporation loop. In this stage, the 36 t / hr of vapor evolved from E1 is utilized as the heating medium in the secondary heat exchanger, delivering approximately 23 MW of thermal energy to the intermediate acid. This integration allows for the recovery of latent heat, resulting in a process condensate discharge at 92°C. The second separation vessel (E2) is operated at a higher vacuum of 5 psia and a reduced temperature of 85°C. An additional 36 t / hr of vapor is evolved, which is subsequently condensed in a final condenser with a cooling duty of -21 MW, producing a final condensate at 74°C containing 262 ppm F. Following the dual-stage evaporation process, a concentratedAtty. Dkt. No. TVTI-006WO

[0302] phosphoric acid product is recovered from the bottom of E2. The process yields approximately 43 t / hr of product with a final concentration of 54 % w / w P2O5 and 13 ppm F.

[0303] In another example, the first evaporator loop is replaced with an indirect mechanical vapor recompression (MVR) system. 115 t / hr of 20% w / w P2O5 and 538 ppm F is introduced into the primary graphite heat exchanger energized by a make-up steam supply and compressed steam condensate vapors at 15 psia. The steam condensate is then fed to a secondary graphite heat exchanger at 7 psia. The evaporative cooling of the steam condensate is used to condense the process vapors from the primary graphite heat exchanger. That low pressure steam is then compressed and reintroduced into the make-up steam supply for the primary graphite heat exchanger. This yields an acid stream similar to the above example of between 29-31% w / w P2O5. The compressor would require 2.0 MW of power at isentropic conditions to produce 38 metric t / hr of 15 psia steam (24 MW), with an overall coefficient of performance of 12.0.

[0304] Alternatively, the 2-effect evaporator is replaced with a single-effect evaporator with indirect MVR. In this example, 115 t / hr of 20% w / w P2O5 and 538 ppm F would be concentrated to 40-54% w / w P2O5 in a single evaporation step at 11 psia and 98-104°C. 54-72 t / hr steam condensate at 7 psia would be used to condense the process vapors via evaporative cooling. The resulting steam condensate vapors would then be compressed to 17-20 psia in a two-stage compressor to provide heat (34-45 MW) to the primary graphite heat exchanger. The compressor would require 3.2 to 5.2 MW of power, for a coefficient of performance of 8.7 to 10.5, and would eliminate the requirement for makeup steam. The resulting P2O5 product would have 22-45 ppm F, and the process condensate would have 820-1030 ppm F.

[0305] CERTAIN EMBODIMENTS

[0306] The present disclosure contemplates, among other things, the following numbered embodiments:

[0307] 1. A method of producing a concentrated phosphoric acid (H3PO4), the method comprising:

[0308] electrolytically producing a dilute sulfuric acid stream from aqueous sodium sulfate; contacting the dilute sulfuric acid stream with rock phosphorous to produce a dilute phosphoric acid and a gypsum (CaSO4·nH2O) slurry;

[0309] concentrating the dilute phosphoric acid to produce the concentrated phosphoric acid.

[0310] 2. The method of embodiment 1, further comprising purifying the dilute phosphoric acidAtty. Dkt. No. TVTI-006WO

[0311] by feeding the dilute phosphoric acid through a nanofiltration membrane to produce a nanofiltration permeate.

[0312] 3. The method of embodiment 2, wherein the nanofiltration permeate has a P2O5 concentration of from 5% to 18% (w / w).

[0313] 4. The method of embodiment 2, further comprising contacting the nanofiltration permeate with a reverse osmosis membrane to produce an intermediate phosphate concentrate.

[0314] 5. The method of embodiment 4, wherein the intermediate phosphate concentrate has a P2O5 concentration of from 15% to 40% (w / w).

[0315] 6. The method of any one of embodiments 2 - 5, further comprising concentrating the nanofiltration permeate, the intermediate phosphate concentrate, or both, to produce the concentrated phosphoric acid.

[0316] 7. The method of embodiment 6, wherein the concentrated phosphoric acid has a P2O5 concentration of from 45% to 62% (w / w).

[0317] 8. The method of any one of embodiments 2 - 7, further comprising defluorinating the nanofiltration permeate, the intermediate phosphate concentrate, the concentrated phosphoric acid, or a combination thereof.

[0318] 9. The method of any one of embodiments 2 - 8, further comprising extracting phosphoric acid from the nanofiltration permeate, the intermediate phosphate concentrate, the concentrated phosphoric acid, or a combination thereof.

[0319] 10. The method of embodiment 1, wherein concentrating the dilute phosphoric acid comprises solvent extraction of phosphoric acid.

[0320] 11. The method of embodiment 1 or embodiment 10, further comprising desulfating the dilute phosphoric acid.Atty. Dkt. No. TVTI-006WO

[0321] 12. The method of embodiment 11, wherein desulfating comprises contacting the dilute phosphoric acid with a calcium-containing solid material.

[0322] 13. The method of embodiment 12, further comprising removing calcium from the dilute phosphoric acid.

[0323] 14. The method of embodiment 1, wherein the aqueous sodium sulfate comprises recycled aqueous sodium sulfate.

[0324] 15. The method of embodiment 1, wherein electrolyzing the aqueous sodium sulfate further comprises producing an alkaline stream.

[0325] 16. The method of embodiment 15, wherein the alkaline stream is contacted with a carbon dioxide (CO2)-comprising gaseous stream to produce a solution comprising an aqueous carbonate and a CO2-depleted gaseous stream.

[0326] 17. The method of embodiment 16, wherein the CO2-comprising gaseous stream is air.

[0327] 18. The method of embodiment 16, wherein the CO2-comprising gaseous stream comprises CO2 from a point source.

[0328] 19. The method of any one of embodiments 16 - 18, wherein the CO2-comprising gaseous stream comprises greater than 5% (v / v) CO2.

[0329] 20. The method of any one of embodiments 16 - 19, further comprising contacting the solution comprising the aqueous carbonate with the gypsum (CaSO4*nH2O) slurry.

[0330] 21. The method of embodiment 20, further comprising precipitating calcium carbonate (CaCO3).

[0331] 22. The method of embodiment 20, wherein contacting the solution comprising the aqueous carbonate with the gypsum (CaSO4*nH2O) slurry produces recycled aqueous sodium sulfate.Atty. Dkt. No. TVTI-006WO

[0332] 23. The method of embodiment 1, wherein the aqueous sodium sulfate comprises the recycled aqueous sodium sulfate of embodiment 22.

[0333] 24. The method of embodiment 15, further comprising contacting the gypsum (CaSO^nfW) slurry with the alkaline stream to produce a calcium-containing solid product.

[0334] 25. The method of embodiment 24, further comprising separating the calcium-containing solid product from the alkaline stream to produce a separated alkaline solution.

[0335] 26. The method of embodiment 24 or embodiment 25, wherein the calcium-containing solid product comprises calcium hydroxide (Ca(OH)2).

[0336] 27. The method of embodiment 25, wherein the separated alkaline solution comprises sodium sulfate and sodium hydroxide.

[0337] 28. The method embodiment 25, further comprising contacting the separated alkaline solution with a C02-comprising gaseous stream to produce a 002-depleted gaseous stream and a sodium sulfate brine.

[0338] 29. The method of embodiment 28, wherein the CO2-comprising gaseous stream comprises greater than 5% (v / v) CO2.

[0339] 30. The method of embodiment 28, further comprising electrolyzing the sodium sulfate brine.

[0340] 31. The method of embodiment 1, wherein the aqueous sodium sulfate comprises the sodium sulfate brine of embodiment 28.

[0341] 32. The method of embodiment 1, wherein the dilute sulfuric acid stream comprises from 10 weight % to 30 weight % sulfuric acid.

[0342] 33. The method of embodiment 1, wherein electrolyzing the aqueous sodium sulfate produces hydrogen (H2).Atty. Dkt. No. TVTI-006WO

[0343] 34. The method of embodiment 1, wherein electrolyzing the aqueous sodium sulfate produces oxygen (O2).

[0344] 35. The method of any one of the preceding embodiments, further comprising contacting the concentrated phosphoric acid with ammonia to produce an ammonium salt.

[0345] 36. A system comprising:

[0346] an electrolysis unit configured to electrolyze aqueous sulfate to produce a dilute sulfuric acid stream;

[0347] a reactor configured to react rock phosphorus with the dilute sulfuric acid to produce phosphoric acid; and

[0348] a nanofiltration unit configured to receive the phosphoric acid.

[0349] 37. The system according to embodiment 36, wherein the nanofiltration unit is configured to provide a nanofiltration permeate comprising dilute purified phosphoric acid.

[0350] 38. The system according to embodiment 36, wherein the nanofiltration permeate has a P2O5 concentration of from 5% to 18% (w / w).

[0351] 39. The system according to any one of embodiments 36 - 38 wherein the nanofiltration unit is configured to provide a nanofiltration retentate to the reactor.

[0352] 40. The system according to any one of embodiments 36 - 39, wherein the nanofiltration unit is configured to provide the nanofiltration permeate to a reverse osmosis unit.

[0353] 41. The system according to embodiment 40, wherein the reverse osmosis unit is configured to produce an intermediate phosphate concentrate.

[0354] 42. The system according to embodiment 41, wherein the intermediate phosphate concentrate has a P2O5 concentration of from about 15% to about 40% (w / w).

[0355] 43. The system according to any one of embodiments 36 - 42, further comprising a phosphoric acid extraction unit.Atty. Dkt. No. TVTI-006WO

[0356] 44. The system according to embodiment 36, further comprising a precipitator configured to generate a calcium-containing solid product.

[0357] 45. The system according to embodiment 44, wherein the precipitator is configured to recirculate the aqueous sulfate to the electrolysis unit.

[0358] 46. The system of embodiment 36, further comprising a GO2 sequestration unit.

[0359] 47. The system according to embodiment 46, wherein the CO2 sequestration unit is in fluid communication with a source of CO2.

[0360] 48. The system according embodiment 47, wherein the source of CO2 is air.

[0361] 49. The system according to embodiment 48, wherein the source of CO2 is a point source.

[0362] 50. The system according to embodiment 49, wherein the point source is a flue gas.

[0363] 51. The system according to any one of embodiments 36 - 50, wherein the electrolysis unit is configured to provide an alkaline stream.

[0364] 52. The system according to embodiment 51, wherein the electrolysis unit is configured to provide the alkaline stream to the CO2 sequestration unit.

[0365] 53. The system of any one of embodiments 36 - 52, further comprising an evaporation unit for concentrating dilute phosphoric acid to produce concentrated phosphoric acid.

[0366] 54. A system comprising:

[0367] an electrolysis unit configured to electrolyze aqueous sulfate to produce a dilute sulfuric acid stream;

[0368] a reactor configured to react rock phosphorus with the dilute sulfuric acid to produce phosphoric acid; and

[0369] a phosphoric acid extraction unit.Atty. Dkt. No. TVTI-006WO

[0370] 55. The system according to embodiment 54, further comprising a precipitator configured to generate a calcium-containing solid product.

[0371] 56. The system according to embodiment 55, wherein the precipitator is configured to recirculate the aqueous sulfate to the electrolysis unit.

[0372] 57. The system of embodiment 54, further comprising a GO2 sequestration unit.

[0373] 58. The system according to embodiment 57, wherein the CO2 sequestration unit is in fluid communication with a source of CO2.

[0374] 59. The system according embodiment 58, wherein the source of CO2 is air.

[0375] 60. The system according to embodiment 59, wherein the source of CO2 is a point source.

[0376] 61. The system according to embodiment 60, wherein the point source is a flue gas.

[0377] 62. The system according to any one of embodiments 54 - 61, wherein the electrolysis unit is configured to provide an alkaline stream.

[0378] 63. The system according to embodiment 62, wherein the electrolysis unit is configured to provide the alkaline stream to the CO2 sequestration unit.

[0379] 64. The system of any one of embodiments 54 - 63, further comprising an evaporation unit for concentrating dilute phosphoric acid to produce concentrated phosphoric acid.

[0380] In at least some of the previously described embodiments, one or more elements used in an embodiment can interchangeably be used in another embodiment unless such a replacement is not technically feasible. It will be appreciated by those skilled in the art that various other omissions, additions and modifications may be made to the methods and structures described above without departing from the scope of the claimed subject matter. All such modifications and changes are intended to fall within the scope of the subject matter, as defined by the appended claims.

[0381] It will be understood by those within the art that, in general, terms used herein, andAtty. Dkt. No. TVTI-006WO

[0382] especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations.

[0383] However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to embodiments containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and / or “an” should be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “ a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and / or A, B, and C together, etc.). In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “ a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and / or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and / or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.”

[0384] In addition, where features or aspects of the disclosure are described in terms of Markush groups, those skilled in the art will recognize that the disclosure is also therebyAtty. Dkt. No. TVTI-006WO

[0385] described in terms of any individual member or subgroup of members of the Markush group.

[0386] As will be understood by one skilled in the art, for any and all purposes, such as in terms of providing a written description, all ranges disclosed herein also encompass any and all possible sub-ranges and combinations of sub-ranges thereof. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art all language such as “up to,” “at least,” “greater than,” “less than,” and the like include the number recited and refer to ranges which can be subsequently broken down into sub-ranges as discussed above. Finally, as will be understood by one skilled in the art, a range includes each individual member. Thus, for example, a group having 1-3 articles refers to groups having 1, 2, or 3 articles. Similarly, a group having 1-5 articles refers to groups having 1, 2, 3, 4, or 5 articles, and so forth.

[0387] Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it is readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes and modifications may be made thereto without departing from the spirit or scope of the appended claims.

[0388] Accordingly, the preceding merely illustrates the principles of the invention. It will be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and conditional language recited herein are principally intended to aid the reader in understanding the principles of the invention and the concepts contributed by the inventors to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions.

[0389] Moreover, all statements herein reciting principles, aspects, and embodiments of the invention as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents and equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims.

[0390] The scope of the present invention, therefore, is not intended to be limited to the exemplary embodiments shown and described herein. Rather, the scope and spirit of present invention is embodied by the appended claims. In the claims, 35 U. S. C. §112(f) or 35 U. S. C.Atty. Dkt. No. TVTI-006WO

[0391] §112(6) is expressly defined as being invoked for a limitation in the claim only when the exact phrase "means for" or the exact phrase "step for" is recited at the beginning of such limitation in the claim; if such exact phrase is not used in a limitation in the claim, then 35 U. S. C. § 112 (f) or 35 U. S. C. §112(6) is not invoked.

Claims

Atty. Dkt. No. TVTI-006WOWhat is claimed is:

1. A method of producing a concentrated phosphoric acid (H3PO4), the method comprising: subjecting aqueous sodium sulfate to electrochemical salt splitting to produce a dilute sulfuric acid stream;contacting the dilute sulfuric acid stream with rock phosphorous to produce a dilute phosphoric acid and a gypsum (CaSO4·nH2O) slurry;concentrating the dilute phosphoric acid to produce the concentrated phosphoric acid.

2. The method of claim 1, further comprising purifying the dilute phosphoric acid, the concentrated phosphoric acid, or both.

3. The method of claim 2, wherein purifying the dilute phosphoric acid, comprises feeding the dilute phosphoric acid through a nanofiltration membrane to produce a nanofiltration permeate.

4. The method of any one of claims 1 - 3, wherein the dilute phosphoric acid has a sulfate to phosphorus pentoxide ratio (SO4 / P2O5) of 1:15 or more.

5. The method of claim 4, wherein the sulfate to phosphorus pentoxide ratio of more than 1:15.

6. The method of claim 4 or 5 wherein the sulfate to phosphorus pentoxide ratio ranges from 1:5 to 1:15.

7. The method of any one of claims 4 - 6, wherein the sulfate to phosphorus pentoxide ratio is 1:10 or greater.

8. The method of any one of claims 4 - 7, wherein the sulfate to phosphorus pentoxide ratio is 1:5 or greater.

9. The method of any one of claims 3 - 8, wherein the nanofiltration permeate has a P2O5 concentration ranging from 5% to 22% (w / w).Atty. Dkt. No. TVTI-006WO10. The method of any one of claims 3 - 9, further comprising contacting the nanofiltration permeate with a reverse osmosis membrane to produce an intermediate phosphate concentrate.

11. The method of claim 10, wherein the intermediate phosphate concentrate has a P2O5 concentration ranging from 15% to 40% (w / w).

12. The method of any one of claims 3 - 11, wherein concentrating comprises concentrating the nanofiltration permeate, the intermediate phosphate concentrate, or both, to produce the concentrated phosphoric acid.

13. The method of any one of claims 1 - 12, wherein the concentrated phosphoric acid has a P2O5 concentration ranging from 45% to 83% (w / w).

14. The method of any one of claims 3 - 13, further comprising defluorinating the nanofiltration permeate, the intermediate phosphate concentrate, the concentrated phosphoric acid, or a combination thereof.

15. The method of any one of claims 3 - 14, further comprising extracting phosphoric acid from the nanofiltration permeate, the intermediate phosphate concentrate, the concentrated phosphoric acid, or a combination thereof.

16. The method of any one of claims 1 - 15, wherein concentrating comprises evaporation.

17. The method of any one of claims 1 - 16, wherein concentrating comprises evaporation with indirect mechanical vapor recompression.

18. The method of any one of claims 1 - 17, wherein concentrating comprises reverse osmosis.

19. The method of any one of claims 1 - 18, further comprising purifying the dilute phosphoric acid, the concentrated phosphoric acid, or both.Atty. Dkt. No. TVTI-006WO20. The method of claim 19, wherein purifying comprises solvent extraction of phosphoric acid.

21. The method of claim 19 or 20, further comprising desulfating the dilute phosphoric acid.

22. The method of claim 21, wherein desulfating comprises contacting the dilute phosphoric acid with a calcium-containing solid material.

23. The method of claim 22, further comprising removing calcium from the dilute phosphoric acid.

24. The method of any one of claims 1 - 23, wherein the aqueous sodium sulfate comprises recycled aqueous sodium sulfate.

25. The method of claims 1 -24, wherein electrochemical salt splitting of the aqueous sodium sulfate further comprises producing an alkaline stream.

26. The method of claim 25, wherein the alkaline stream is contacted with a carbon dioxide (CO2)-comprising gaseous stream to produce a solution comprising an aqueous carbonate and a COg-depleted gaseous stream.

27. The method of claim 26, wherein the CO2-comprising gaseous stream is air.

28. The method of claim 26, wherein the C02-comprising gaseous stream comprises CO2 from a point source.

29. The method of any one of claims 26 - 28, wherein the CO2-comprising gaseous stream comprises greater than 5% (v / v) CO2.

30. The method of any one of claims 26 - 29, further comprising contacting the solution comprising the aqueous carbonate with the gypsum (CaSO4*nH2O) slurry.

31. The method of claim 30, further comprising precipitating calcium carbonate (CaCO3).Atty. Dkt. No. TVTI-006WO32. The method of claim 30, wherein contacting the solution comprising the aqueous carbonate with the gypsum (CaSO4*nH2O) slurry produces recycled aqueous sodium sulfate.

33. The method of claim 32, wherein the recycled aqueous sodium sulfate is subjected to electrochemical salt splitting to produce the dilute sulfuric acid stream.

34. The method of any one of claims 25 - 33, further comprising contacting the gypsum (CaSO4*nH2O) slurry with the alkaline stream to produce a calcium-containing solid product.

35. The method of claim 34, further comprising separating the calcium-containing solid product from the alkaline stream to produce a separated alkaline solution.

36. The method of claim 34 or 35, wherein the calcium-containing solid product comprises calcium hydroxide (Ca(OH)2).

37. The method of claim 35, wherein the separated alkaline solution comprises sodium sulfate and sodium hydroxide.

38. The method of claim 35, further comprising contacting the separated alkaline solution with a CO2-comprising gaseous stream to produce a CO2-depleted gaseous stream and a sodium sulfate brine.

39. The method of claim 38, wherein the CO2-comprising gaseous stream comprises greater than 5% (v / v) CO2.

40. The method of claim 38, further comprising subjecting the sodium sulfate brine to electrochemical salt splitting.

41. The method of claim 40, wherein the subjecting the sodium sulfate brine to electrochemical salt splitting produces the dilute sulfuric acid stream.

42. The method of claim 1 or 41, wherein the dilute sulfuric acid stream comprises a sulfuric acid concentration ranging from 10 weight % to 30 weight % sulfuric acid.Atty. Dkt. No. TVTI-006WO43. The method of claim 1, wherein subjecting the aqueous sodium sulfate to electrochemical salt splitting produces hydrogen (H2).

44. The method of claim 1, wherein subjecting the aqueous sodium sulfate to electrochemical salt splitting produces oxygen (O2).

45. The method of any one of the preceding claims, further comprising contacting the concentrated phosphoric acid with ammonia to produce an ammonium salt.

46. The method of claim 1, further comprising separating the gypsum slurry from the phosphoric acid.

47. The method of claim 46, further comprising removing radium from the gypsum slurry to produce a radium-depleted gypsum.

48. The method of claim 47, wherein the radium is removed via cyclonic separation.

49. The method of claim 46, further comprising repulping and washing the gypsum with the dilute sulfuric acid solution yielding a sulfuric acid filtrate.

50. The method of claim 49, further comprising recirculating the sulfuric acid filtrate.

51. The method of claim 50, wherein recirculating the sulfuric acid filtrate comprises combining the sulfuric acid filtrate with the dilute sulfuric acid stream, contacting the sulfuric acid filtrate with rock phosphorus, or both.

52. The method of any one of claims 49 - 51, further comprising recovering a rare earth element from the sulfuric acid filtrate.

53. The method of claim 52, wherein recovering the rare earth element comprises contacting the sulfuric acid filtrate with a chelator.

54. The method of claim 53, wherein the chelator is selected from a sulfonic acid chelating resin, oxalic acid, or a combination thereof.Atty. Dkt. No. TVTI-006WO55. A system comprising:an electrolysis unit configured for electrochemical salt splitting aqueous sulfate to produce a dilute sulfuric acid stream;a reactor configured to react rock phosphorus with the dilute sulfuric acid to produce phosphoric acid; anda nanofiltration unit configured to receive the phosphoric acid.

56. The system according to claim 55, wherein the nanofiltration unit is configured to provide a nanofiltration permeate comprising dilute purified phosphoric acid.

57. The system according to claim 56, wherein the nanofiltration permeate has a P2O5 concentration of from 5% to 22% (w / w).

58. The system according to any one of claims 55 - 57 wherein the nanofiltration unit is configured to provide a nanofiltration retentate to the reactor.

59. The system according to any one of claims 55 - 58, wherein the nanofiltration unit is configured to provide the nanofiltration permeate to a reverse osmosis unit.

60. The system according to claim 59, wherein the reverse osmosis unit is configured to produce an intermediate phosphate concentrate.

61. The system according to claim 60, wherein the intermediate phosphate concentrate has a P2O5 concentration of from about 15% to about 40% (w / w).

62. The system according to any one of claims 55 - 61, further comprising a phosphoric acid extraction unit.

63. The system according to claim 55, further comprising a precipitator configured to generate a calcium-containing solid product.

64. The system according to claim 63, wherein the precipitator is configured to recirculate the aqueous sulfate to the electrolysis unit.Atty. Dkt. No. TVTI-006WO65. The system of claim 55, further comprising a CO2 sequestration unit.

66. The system according to claim 65, wherein the CO2 sequestration unit is in fluid communication with a source of CO2.

67. The system according claim 66, wherein the source of CO2 is air.

68. The system according to claim 67, wherein the source of CO2 comprises a point source.

69. The system according to claim 68, wherein the point source is a flue gas.

70. The system according to any one of claims 55 - 69, wherein the electrolysis unit is configured to provide an alkaline stream.

71. The system according to claim 70, wherein the electrolysis unit is configured to provide the alkaline stream to the CO2 sequestration unit.

72. The system of any one of claims 55 - 71, further comprising an evaporation unit for concentrating dilute phosphoric acid to produce concentrated phosphoric acid.

73. The system of claim 72, wherein the evaporation unit is an indirect mechanical vapor recompression evaporator.

74. A system comprising:an electrochemical unit configured for electrochemical salt splitting of aqueous sulfate to produce a dilute sulfuric acid stream;a reactor configured to react rock phosphorus with the dilute sulfuric acid to produce phosphoric acid; anda phosphoric acid extraction unit.

75. The system according to claim 74, further comprising a precipitator configured to generate a calcium-containing solid product.Atty. Dkt. No. TVTI-006WO76. The system according to claim 75, wherein the precipitator is configured to recirculate the aqueous sulfate to the electrochemical unit.

77. The system of any one of claims 74 - 76, further comprising a CO2 sequestration unit.

78. The system according to claim 77, wherein the CO2 sequestration unit is in fluid communication with a source of CO2.

79. The system according claim 78, wherein the source of CO2 is air.

80. The system according to claim 79, wherein the source of CO2 is a point source.

81. The system according to claim 80, wherein the point source is a flue gas.

82. The system according to any one of claims 74 - 81, wherein the electrochemical unit is configured to provide an alkaline stream.

83. The system according to claim 82, wherein the electrochemical unit is configured to provide the alkaline stream to the CO2 sequestration unit.

84. The system of any one of claims 74 - 83, further comprising an evaporation unit for concentrating dilute phosphoric acid to produce concentrated phosphoric acid.

85. A method of producing a concentrated phosphoric acid (H3PO4), the method comprising: concentrating a recycled sulfuric acid stream to produce a concentrated recycled sulfuric acid solution;contacting the concentrated recycled sulfuric acid solution with rock phosphorous to produce a phosphoric acid solution and the recycled acid stream; andconcentrating the phosphoric acid solution to produce the concentrated phosphoric acid.

86. The method of claim 85, wherein concentrating the recycled sulfuric acid stream comprises reverse osmosis.Atty. Dkt. No. TVTI-006WO87. The method of claim 85, wherein concentrating the recycled sulfuric acid stream comprises evaporation with direct or indirect mechanical vapor recompression.

88. The method of claim 85, further comprising producing a dilute sulfuric acid by electrochemical salt splitting; andcombining the dilute sulfuric acid with the recycled sulfuric acid stream.

89. The method of claim 85, further comprising producing a dilute sulfuric acid by electrochemical salt splitting;concentrating the dilute sulfuric acid to provide a concentrated sulfuric acid solution; and combining the concentrated sulfuric acid solution with the concentrated recycled sulfuric acid solution.

90. The method of any one of claims 85 - 89, wherein the phosphoric acid solution has a P2O5 concentration ranging from 15% to 45% (w / w).

91. The method of any one of claims 85 - 90, wherein the phosphoric acid solution has a P2O5 concentration of greater than 30%.

92. The method of any one of claims 85 - 91 wherein the phosphoric acid solution has a P2O5 concentration ranging from 35% to 45% (w / w).

93. The method of any one of claims 85 - 92, wherein concentrating the phosphoric acid solution comprises feeding the phosphoric acid solution through a nanofiltration membrane to produce a nanofiltration permeate.

94. The method of claim 93, wherein the phosphoric acid solution has a concentration ranging from 18% to 22%.

95. The method of claim 93 or 94, further comprising contacting the nanofiltration permeate with a reverse osmosis membrane to produce an intermediate phosphate concentrate.

96. The method of claim 95, wherein the intermediate phosphate concentrate has a P2O5 concentration ranging from 22% to 40% (w / w).Atty. Dkt. No. TVTI-006WO97. The method of any one of claims 85 - 96, further comprising concentrating the phosphoric acid solution, the nanofiltration permeate, the intermediate phosphate concentrate, or a combination thereof, to produce the concentrated phosphoric acid.

98. The method of claim 97, wherein the concentrated phosphoric acid has a P2O5 concentration ranging from 56% to 83% (w / w).

99. The method of claim 98, wherein concentrating comprises evaporation and defluorination.

100. The method of any one of claims 97 - 99, further comprising purifying the phosphoric acid solution, the nanofiltration permeate, the intermediate phosphate concentrate, or a combination thereof.

101. The method of claim 100, wherein purifying comprises solvent extraction.

102. The method of claim 85, wherein contacting the concentrated sulfuric acid solution with rock phosphorus produces phosphoric acid and a bassanite (CaSC ’O. S W) slurry.

103. The method of claim 102, further comprising separating the bassanite slurry from the phosphoric acid.

104. The method of claim 102, further comprising removing radium from the bassanite slurry to produce a radium-depleted bassanite.

105. The method of any one of claims 102 - 104, further comprising producing gypsum (CaSO4*2H2O) from the bassanite.

106. The method of claim 105, wherein the gypsum is produced in a gypsum reactor.

107. The method of claim 105 or 106, further comprising washing the gypsum with the sulfuric acid solution yielding a sulfuric acid filtrate.Atty. Dkt. No. TVTI-006WO108. The method of claim 107, further comprising recirculating the sulfuric acid filtrate to the gypsum reactor.

109. The method of claim 107 or 108, further comprising recovering a rare earth element from the sulfuric acid filtrate.

110. The method of claim 109, wherein recovering the rare earth element comprises contacting the sulfuric acid filtrate with a chelator.

111. The method of claim 110, wherein the chelator is selected from a sulfonic acid chelating resin, oxalic acid, or a combination thereof.

112. The method of any one of claims 107- 111 further comprising recycling the sulfuric acid filtrate by concentrating the sulfuric acid filtrate to produce a recycled concentrated sulfuric acid solution.

113. The method of claim 112, further comprising contacting the recycled concentrated sulfuric acid solution with rock phosphorus.

114. A system comprising:a concentration unit configured to receive a recycled sulfuric acid stream and to produce a concentrated recycled sulfuric acid solution;a reactor configured to react rock phosphorus with the concentrated recycled sulfuric acid solution to produce a phosphoric acid solution and a bassanite slurry; anda multi-stage separator configured to separate the phosphoric acid solution from the bassanite slurry and the recycled sulfuric acid stream from the bassanite slurry.

115. The system of claim 114, wherein the reactor is configured to produce the phosphoric acid solution at a concentration of from 15% to 45% (w / w).

116. The system of claim 114 or 115, wherein the reactor is configured to produce the phosphoric acid solution at a P2O5 concentration of greater than 30%.Atty. Dkt. No. TVTI-006WO117. The system of any one of claims 114 - 116, wherein the reactor is configured to produce the phosphoric acid solution at a P2O5 concentration of from 35% to 45% (w / w).

118. The system of claim 114 or 115, further comprising a nanofiltration unit configured to receive the phosphoric acid solution and produce a phosphoric acid permeate.

119. The system of any one of claims 114 - 118, further comprising an evaporation unit configured to receive the phosphoric acid solution or the phosphoric acid permeate and to produce concentrated phosphoric acid.

120. The system of claim 118, further comprising an evaporation unit configured to receive the phosphoric acid permeate and to produce concentrated phosphoric acid.

121. The system of claim 119 or 120, wherein the concentrated phosphoric acid has a P2O5 concentration of from 56% to 83% (w / w).

122. The system of any one of claims 114 - 121, comprising a separator configured to separate radium from the bassanite slurry.

123. The system of claim 122, wherein the separator is a cyclonic separator.

124. The system of claim 123, further comprising a secondary separation unit configured to receive a hydrocyclone overflow from the cyclonic separator.

125. The system of claim 124, wherein the secondary separation unit is configured to separate radium-enriched fines from the slurry.

126. The system of claim 124 or 125, wherein secondary separation unit comprises a centrifuge, a pressure filter, or both.

127. The system of any one of claims 114 - 126, further comprising a gypsum reactor configured to receive the bassanite slurry and produce gypsum.Atty. Dkt. No. TVTI-006WO128. The system of claim 127, wherein the gypsum reactor is configured to receive concentrated sulfuric acid solution.

129. The system of claim 127 or 128, further comprising a filtration unit configured to receive gypsum from the gypsum reactor.

130. The system of claim 129, wherein the gypsum reactor is configured to receive a sulfuric acid filtrate from the filtration unit.

131. The system of any one of claims 127 - 130, further comprising a precipitator configured to convert the gypsum to a calcium-containing solid product.

132. The system of claim 131, wherein the precipitator configured to receive an aqueous carbonate.

133. The system of claim 131 or 132, wherein the precipitator is configured to provide an aqueous sulfate to the electrochemical unit.

134. The method of any one of claims 1 - 54 or 85 - 113, wherein the rock phosphorus is less than 50% bone phosphate of lime (w / w%).

135. The method of any one of claims 1 - 54 or 85 - 113, wherein the rock phosphorus has a P2O5 content of 35% or less.

136. The method of claim 134 or 135, wherein the rock phosphorus has a magnesium oxide (MgO) concentration of 2.5% (w / w%) or greater.

137. The method of claim 136, wherein the ratio of sulfate to phosphorus pentoxide in the dilute phosphoric acid and a gypsum (CaSO4*nH2O) slurry is 0.13 or less.

138. The method of claim 136, wherein the ratio of sulfate to phosphorus pentoxide in the dilute phosphoric acid and a gypsum (CaSO4*nH2O) slurry is 0.07 or less.Atty. Dkt. No. TVTI-006WO139. The method of any one of claims 3 - 54, wherein producing the nanofiltration permeate produces a nanofiltration retentate.

140. The method of claim 139, further comprising contacting the nanofiltration retentate with alkali.

141. The method of claim 140, wherein contacting the nanofiltration retentate with alkali comprises multi-step neutralization of the retentate.

142. The method of claim 141, wherein the alkali comprises sodium hydroxide produced by electrochemical salt splitting of aqueous sodium sulfate.

143. The method of any one of claims 140 - 142, wherein contacting the retentate with alkali produces solid magnesium phosphate, calcium phosphate, magnesium hydroxide, or a combination thereof.

144. The method of claim 143, further comprising recycling the calcium phosphate.

145. The method of claim 144, wherein recycling the calcium phosphate comprises contacting the calcium phosphate with dilute sulfuric acid.

146. The method of claim 144 or 145, wherein the recycling the calcium phosphate comprises combining the calcium phosphate with rock phosphorus.

147. A method of producing magnesium hydroxide, the method comprising: subjecting aqueous sulfate electrochemical salt splitting to produce sulfuric acid in a dilute sulfuric acid stream;contacting the sulfuric acid with rock phosphorous to produce a phosphoric acid; feeding the phosphoric acid through a nanofiltration membrane to produce a nanofiltration permeate and a nanofiltration retentate;contacting the retentate with alkali to produce magnesium hydroxide.

148. The method of claim 147, wherein the rock phosphorus is less than 50% bone phosphate of lime (w / w%).Atty. Dkt. No. TVTI-006WO149. The method of claim 147 or 148, wherein the rock phosphorus has a P2O5 content of 35% or less.

150. The method of any one of claims 147 - 149, wherein the rock phosphorus has a magnesium oxide (MgO) concentration of 2.5% (w / w%) or greater.

151. The method of claim 150, wherein ratio of sulfate to phosphorus pentoxide in the phosphoric acid is 0.13 or less.

152. The method of any one of claims 147 - 150, wherein the alkali comprises hydroxide produced by electrochemical salt splitting of the aqueous sulfate.

153. The method of any one of claims 147 - 152, wherein contacting the retentate with alkali comprises multi-step neutralization of the retentate.

154. The method of any one of claims 147 - 153, wherein contacting the sulfuric acid with rock phosphorous produces a gypsum (CaSO4*nH2O) slurry.

155. The method of any one of claims 147- 154, further comprising concentrating the sulfuric acid prior to contacting with rock phosphorous.

156. The method of claim 155, wherein contacting the sulfuric acid with rock phosphorous produces a bassanite (CaSO4*0.5H2O) slurry.

157. The method of claim 156, further comprising producing a gypsum (CaSO4*2H2O) slurry from the bassanite slurry.

158. The method of claim 154 or 157, further comprising contacting the gypsum (CaSC>4*nH2O) slurry with an aqueous carbonate.

159. The method of claim 158, further comprising precipitating calcium carbonate (CaCO3).Atty. Dkt. No. TVTI-006WO160. The method of claim 158 or 159, wherein contacting the gypsum (CaSO4*nH2O) slurry with an aqueous carbonate produces recycled aqueous sodium sulfate.

161. The method of claim 160, further comprising subjecting the recycled aqueous sodium sulfate to electrochemical salt splitting.

162. The method of claim 147 or 161, wherein electrochemical salt splitting of the aqueous sodium sulfate, the recycled aqueous sodium sulfate, or both produces an alkaline stream.

163. The method of claim 162, further comprising contacting the alkaline stream with a carbon dioxide (CO2)-comprising gaseous stream to produce a solution comprising the aqueous carbonate and a CO2-depleted gaseous stream.

164. The system of any one of claims 55 - 73, wherein the nanofiltration unit is configured to provide a nanofiltration permeate.

165. The system of claim 164, further comprising a neutralization unit is configured to receive the nanofiltration permeate.

166. The system of claim 165, wherein the neutralization unit is further configured to receive an alkaline stream from the electrochemical unit.

167. The method of any one of claims 36 - 40, further comprising producing cement from the Ca(OH)2.

168. The method of any one of claims 85 - 113, further comprising producing a calcium-containing product.

169. The method of claim 168, wherein the calcium-containing product is a solid product.

170. The method of claim 168 or 169, wherein the calcium-containing product comprises Ca(OH)2.Atty. Dkt. No. TVTI-006WO171. The method of claim 170, further comprising producing cement from the Ca(OH)2.

172. The method of any one of claims 1 - 54, 85 - 113, or 167 - 171, wherein electrochemical salt splitting comprises electrodialysis, electrolysis, or both.

173. The method of any one of claims 147- 172, wherein electrochemical salt splitting comprises electrodialysis.

174. The system of claim 55 or 74, wherein the electrolysis unit configured for electrochemical salt splitting is a bipolar membrane electrodialysis system.

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