Method for forming calcium silicate

Abstract

The method includes forming a reaction bed containing the feed aggregates in a reaction chamber by heating the feed aggregates. The individual feed aggregates initially contain gypsum source particles and silicon source particles substantially uniformly distributed throughout the individual aggregates. The gypsum and silicon in the feed aggregates react during heating in the reaction chamber, thereby forming treated aggregates containing calcium silicates and increased amounts of amorphous silicon compared to the feed aggregates prior to heating. The method includes generating off-gases containing oxides of sulfur from the reaction bed and removing the treated aggregates from the reaction chamber.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority under 35 U.S.C. § 119 to U.S. Provisional Patent Application No. 63 / 317,447, entitled "Methods, Compositions, and Systems for Forming Silicates of Calcium," filed March 7, 2022, and to U.S. Provisional Patent Application No. 63 / 236,892, entitled "Methods, Compositions, and Systems for Forming Silicates of Calcium," filed August 25, 2021, each of which is incorporated by reference herein. [Background technology]

[0002] Phosphogypsum (PG) is one of the potential gypsum raw materials that can be used in the methods described herein. However, phosphogypsum is a waste product that has processing problems and characteristics that must be addressed if it is to be used. Phosphogypsum is the main waste by-product of the wet acid process that produces phosphoric acid from phosphate rock. Nearly 50 tons of phosphogypsum are generated every minute in Florida, in addition to which over 1 billion tons of phosphogypsum are already laid down in large stacks, each covering hundreds of acres, some over 300 feet high. Worldwide, 170 million tons of phosphogypsum are discarded annually, but the recycling rate is still extremely low, i.e., less than 5%. The total amount of stored PG is estimated to reach 7-8 billion tons by 2025. At the same time, the recent development of the phosphate industry, coupled with a rapidly growing economy, has led to a significant increase in the production of phosphogypsum, necessitating large dumping areas with meticulous management strategies to avoid contamination of soil, water and air.

[0003] Phosphate rock, due to its chemical nature, may contain significant amounts of naturally occurring radioactive material (NORM). Small amounts of phosphate rock, and therefore NORM, are also present in the phosphorus sand and mud that can be used in the methods herein. Before being converted into fertilizer or other products, phosphate rock is converted to phosphoric acid through a wet process that uses sulfuric acid to dissolve the phosphate rock, creating a solid-liquid mixture (slurry) of phosphoric acid and calcium sulfate (phosphogypsum). The desired phosphate component is separated from the mixture by filtration, leaving phosphogypsum as a waste product. This process concentrates the NORM in the waste product and converts the phosphogypsum to a Technologically Enhanced Naturally Occurring Radioactive Material (TENORM). Over five tons of phosphogypsum are generated for every ton of phosphoric acid produced. Thus, it may be beneficial to reduce the accumulation of phosphogypsum or to address the radiological concerns of phosphogypsum, or both. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] U.S. Pat. No. 4,312,842 [Patent Document 2] U.S. Patent No. 7,378,070 [Patent Document 3] U.S. Patent No. 7,910,080 [Patent Document 4] U.S. Pat. No. 8,734,749 [Patent Document 5] U.S. Pat. No. 9,783,419 [Patent Document 6] U.S. Patent Application Serial No. 16 / 358,504 [Patent Document 7] U.S. Patent Application Serial No. 16 / 914,182 [Non-patent literature]

[0005] [Non-Patent Document 1] Dennis Sebastian et al., A 5-Day Method for Determination of Soluble Silicon Concentrations in Nonliquid Fertilizer Materials Using a Sodium Carbonate-Ammonium Nitrate Extractant Followed by Visible Spectroscopy with Heteropoly Blue Analysis: Single-Laboratory Validation, J. of AOAC Intl., Volume 96, 2nd Edition, March 1, 2013, Pages 251–259, https: / / doi.org / 10.5740 / jaoacint.12-243 [Non-Patent Document 2] Health Physics Society ("Radiation from granite countertops," 2012) Summary of the Invention [Means for solving the problem]

[0006] Some embodiments are described below with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a flow diagram of a method and system for producing calcium silicate. [Diagram 2] This is a phase diagram of CaO-SiO2. [Diagram 3] 1 is a bar graph of radon-222 exhalation rates from the compared materials. [Figure 4] 1 is a graph of plant available silicon content versus silica ratio. [Diagram 5] 1 is a graph of concrete setting time test results (penetration distance vs. time). [Figure 6]1 is a graph of concrete compressive strength versus time. [Figure 7] 1 is a graph of surface resistivity, a measure of water permeability, versus time for concrete. [Figure 8] 1 is a bar graph of sulfur decomposition versus time. [Figure 9] FIG. 1 is a flow diagram of a method and system for producing calcium silicate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] The methods, compositions, and systems herein provide for the production of plant available silicon (PAS) fertilizers and / or supplemental cementitious materials (SCMs), as well as the capture of potentially available sulfur, such as gypsum (CaSO 4 Some areas of discovery include the use of rotary kilns, rotary hearth furnaces, grate kilns, and similar equipment to process calcium oxide (CaSO 3 , calcium source) and silica. The methods, compositions, and systems described herein provide a method for producing CaSO 3 , CaSO 4 , CaSO 5 , CaSO 6 , CaSO 7 , CaSO 8 , CaSO 9 , CaSO 10 , CaSO 11 , CaSO 12 , CaSO 13 , CaSO 14 , CaSO 15 , CaSO 16 , CaSO 17 , CaSO 18 , CaSO 19 , CaSO 20 , CaSO 21 , CaSO 22 , CaSO 23 , CaSO 24 , CaSO 25 , CaSO 26 , CaSO 27 , CaSO 28 , CaSO 29 , CaSO 31 , CaSO 32 , CaSO 33 , CaSO 34 , CaSO 35 , CaSO 36 , CaSO 37 , CaSO 38 , CaSO 39 , CaSO 39 , CaSO 36 , CaSO 37 , CaSO 38 , CaSO 39 , CaSO 39 , CaSO 34 , CaSO 35 , CaSO 36 , CaSO 37 , CaSO 38 , CaSO 39 ...9 , CaSO 39 , CaSO 39 , CaSO 39 , CaSO 39 , CaSO 34 , CaSO 35 , CaSO 3 4 No carbon is utilized as a reducing agent, and the use of melting furnaces, such as those in U.S. Patent No. 4,312,842 to Yin, is avoided to produce solids, as described in the prior patents referenced herein. Instead, silica or silica-containing waste materials (e.g., tailing sands, etc.) are used in predetermined amounts to reduce melting and facilitate achieving desirable product properties and optimizing material recovery. In addition, alternative sources of gypsum (e.g., SO 2 The present invention can also utilize phosphogypsum waste (e.g., waste piles and phosphogypsum filter cakes) produced by wet phosphoric acid processes through radon encapsulation to reduce radon emissions. The methods, raw materials, and systems described individually herein can be used in one or more combinations thereof.

[0009] U.S. Patent Nos. 7,378,070, 7,910,080, 8,734,749, and 9,783,419, and U.S. Patent Application Nos. 16 / 358,504 (filed March 19, 2019) and 16 / 914,182 (filed June 26, 2020), are incorporated by reference into this specification as containing background description of the methods and systems supporting the methods and systems described herein.

[0010] Phosphogypsum, due to its presence in phosphate rocks, contains significant amounts of uranium and its decay products such as radium-226 and radon-222. Uranium in phosphate rocks found in the United States ranges in concentrations from 0.26 to 3.7 becquerels per gram (Bq / g) (7 to 100 picocuries per gram (pCi / g)). (Source: United States Environmental Protection Agency (USEPA)).

[0011] During the wet processing process, radionuclides present in the phosphate rock are selectively separated and concentrated. Approximately 80 percent of the radium-226 is concentrated in phosphogypsum. Radium concentrations in phosphogypsum range from 0.4 to 1.3 Bq / g (11 to 35 pCi / g). (Source: USEPA)

[0012] The decay product of radium, radon-222, can be found emitted from phosphogypsum. The average radon flux is 0.06 to 0.44 Bq / m per second. 2 (1.7~12pCi / m 2 ) and is in the range of 12.6 Bq / m per second. 2 (340pCi / m 2 ) per second, up to 0.25 Bq / m 2 (6.8pCi / m 2 ) (Source: USEPA)

[0013] Efficient purification techniques for removing radium from phosphate rock and phosphogypsum are lacking. Radium present in phosphate rock remains present in phosphogypsum.

[0014] Due to concerns about elevated concentrations of radionuclides in phosphogypsum, the USEPA mandates that phosphogypsum be managed in engineered stacks that are designed to limit public exposure from the release of radon and other radionuclides. Owners and operators must monitor and report radon emissions from unused stacks and take steps to maintain radon emissions within the regulatory limit of 20 picocuries per square meter per second.

[0015] Fortunately, the methods and systems herein convert gypsum into useful products. Additionally, if the gypsum is phosphogypsum or otherwise exhibits harmful radon exhalation rates, the conversion method can reduce the radon exhalation rate. Gypsum reacts with gypsum through the following reaction: CaSO 4 +SiO 2 +Heat → SO 3 (g) + CaSiO 3 (s) (Reaction 1) to produce sulfur trioxide and calcium silicate. In this example, the reaction gives monocalcium silicate. Other potential reaction products include dicalcium silicate (Ca 2 SiO 4 ) and tricalcium silicate (Ca 3 SiO 5 or Ca 3 S 2 O 7 Any one or more of these four silicates of calcium may result from the conversion of gypsum, and all are encompassed herein by the term "calcium silicate" unless otherwise noted.

[0016] The conversion of gypsum takes place via the following reaction: Ca 10 (PO 4 ) 6 F 2 +9SiO 2 +15C+heat → 3P 2 +9CaSiO 3 +15CO+CaF 2 (Response 2) These reactions can be compared and contrasted with the chemical reduction of apatite to produce phosphorus and calcium silicate in reaction 1. In both reactions, the reactants include calcium and silica heated to similar temperatures, and the product includes calcium silicate. However, in reaction 1, the sulfur in calcium sulfate is liberated as an oxide, while in reaction 2, the phosphorus in calcium fluorophosphate is chemically reduced to elemental phosphorus. Also, reaction 2 requires carbon as a reducing agent (carbon is oxidized), while reaction 1 does not require carbon or another reducing agent.

[0017] Silicon fertilizer is one useful product that can be produced from the method and system of the present specification.Silicon fertilizer helps plants to resist disease and insect attack, helps plants to resist unfavorable weather conditions, enhances the physical and chemical properties of soil, and maintains plant nutrients.Silicon-based fertilizers lower soil pH, increase mineral absorption from soil, and help plants overcome deficiencies that are mainly caused by lack of suitable nutrients in soil.In addition, these products enhance the structural properties of plants by improving elasticity and rigidity, protecting plants from weather and infection.

[0018] Most of the silicon on Earth is SiO 2In most cases, silicon is in the form of crystalline silica (e.g., quartz), but in this state silicon is inactive and not readily soluble for uptake by plants. Biogenic silicon, also known as bioactive silicon, is usually in the form of amorphous silicates, such as calcium silicate. Laboratory testing of biogenic silicon is reported as "plant-available silicon" (PAS). A suitable test method used to obtain PAS data herein can be found in Dennis Sebastian et al., A 5-Day Method for Determination of Soluble Silicon Concentrations in Nonliquid Fertilizer Materials Using a Sodium Carbonate-Ammonium Nitrate Extractant Followed by Visible Spectroscopy with Heteropoly Blue Analysis: Single-Laboratory Validation, J. of AOAC Intl., Vol. 96, 2nd Edition, March 1, 2013, pp. 251-259, https: / / doi.org / 10.5740 / jaoacint.12-243. A correlation exists between the amount of PAS measured in soil and the amount of biogenic silicon present in plants.

[0019] Silicon fertilizers are currently produced from steel slag, which is a by-product from the melting of pig iron and / or scrap iron in the production of molten steel. Steel slag contains silicon along with various heavy metals that may be released into the environment if used as silicon fertilizer. At sufficient concentrations, some heavy metals may be toxic environmental pollutants, such as chromium, arsenic, cadmium, mercury, lead, manganese, cobalt, nickel, copper, zinc, selenium, silver, tin, antimony, thallium, etc. Preliminary analysis of the chemical composition of the silicon product produced by the method and system herein indicates that it exceeds the specification (2.3%) of PAS currently produced and sold as silicon fertilizer derived from steel slag and contains fewer heavy metals (see Table 1).

[0020] [Table 1]

[0021] Supplemental cementitious materials (SCMs) are another useful product that may result from the methods and systems herein. Coal fly ash and blast furnace slag are widely used SCMs. It is estimated that over 50% of concrete in the United States contains fly ash as an SCM. These materials often replace about 20-30% of the cement, but up to 70% is added to mass concrete used in dams, roller compacted concrete pavements, and parking lots. Replacing cement with these SCMs in concrete can increase the strength of the concrete, improve resistance to harmful chemical reactions, improve workability for good concrete pours, and delay curing times to allow more time for concrete pouring and surfacing. Many Departments of Transportation (DOTs) in all U.S. states have officially mandated the use of SCMs such as fly ash in concrete for a wide range of applications, and it is used worldwide. However, due to the recent closure of many coal-fired power plants, coal fly ash is currently in short supply. The supplemental cementitious materials produced by the methods and systems described herein are as good as or better than cement or fly ash.

[0022] The discoveries described herein identify several solutions that can be implemented in the systems and methods also described herein. Solutions can be combined for implementation, resulting in still other systems and methods. The inventors expressly contemplate that the various options described herein for the individual systems and methods are not intended to be so limited, except where inconsistent with other systems and methods. The features and advantages of the individual systems herein can also be used in combination with the methods and other systems described herein, even if not otherwise specifically indicated. Similarly, the features and advantages of the individual methods herein can also be used in combination with the systems and other methods described herein, even if not otherwise specifically indicated.

[0023] Method A includes forming a reaction bed containing feed aggregates in a reaction chamber by heating the feed aggregates. Each feed aggregate initially contains gypsum source particles and silicon source particles substantially uniformly distributed throughout the individual aggregates. The gypsum and silicon in the feed aggregates react during heating in the reaction chamber, thereby forming treated aggregates containing calcium silicates and increased amounts of amorphous silicon compared to the feed aggregates before heating. Method A includes generating off-gases containing oxides of sulfur from the reaction bed and removing the treated aggregates from the reaction chamber.

[0024] The term "reacting bed" as used herein refers to the portion of the bed of feed aggregates where chemical reactions are occurring, and the term "substantially uniformly distributed" as used herein refers to a distribution of particles that is not necessarily completely uniform throughout the individual aggregates, but is sufficiently uniform in distribution that the rate of reaction of the gypsum and silicon is not limited due to insufficient particle distribution.

[0025] Additional features may be implemented in method A. As an example, the gypsum source may be phosphogypsum. The feed aggregate may exhibit a radon-222 emissivity due to phosphogypsum, and method A may further include reducing the radon-222 emissivity in the treated aggregate compared to the feed aggregate prior to heating. As an example, the radon-222 emissivity of the treated aggregate after cooling to ambient temperature is 0.013 Bq / g or less. Thus, the treated aggregate may be safer for disposal than phosphogypsum. Analysis of radon emissivity of other standard materials is described below and shown in FIG. 3.

[0026] The feed aggregates may initially lack sufficient components to effect carbothermic reduction during heating of the feed aggregates, in contrast to processes involving carbothermic reduction of phosphate ore. It follows that the feed aggregates may initially lack substantial amounts of phosphorus or carbon or both. As the term "lacking substantial amounts" is used herein, it means that the amount of phosphorus or carbon or both present, if any, is not sufficient to effect carbothermic reduction of phosphorus. Substantial amounts of phosphorus or carbon or both would be sufficient to effect carbothermic reduction. Amounts that are not substantial may be insufficient due to physical gaps between reactant particles.

[0027] The individual aggregates can initially provide a silica ratio, defined as the formula ratio (also molar ratio) of silicon dioxide to calcium oxide + silicon dioxide, ranging from 0.2 to 0.8. Thus, "silica ratio" = (wt.% SiO 2 / 60) / [(wt%CaO / 56)+(wt%SiO 2 / 60)]. The weight percent (wt%) of calcium sources other than calcium oxide is converted to CaO before calculating the silica ratio. The wt% of silicon sources other than silicon dioxide is also converted to SiO 2 The silica ratio may range from 0.5 to 0.8, such as from 0.64 to 0.80. The feed aggregate may include 30-50 wt% gypsum and 40-60 wt% sand. Excess SiO 2The presence of may enable the production of a product that is suitable as both a PAS fertilizer and an SCM.

[0028] Method A may further comprise maintaining a temperature in the reaction chamber along at least a portion of the reaction bed at between 1200° C. and 1400° C. The feed aggregate may be maintained at said temperature for a period of 20 minutes or more, including between 20 and 30 minutes or between 30 and 60 minutes. For example, the temperature may be maintained at between 1250° C. and 1300° C. for a period of 30 minutes or more, including between 30 and 60 minutes.

[0029] Method A can further include selecting a combination of silica ratio and temperature such that aggregate melting in the reaction chamber is reduced compared to a higher temperature or a lower silica ratio or both, the selected combination can avoid aggregate melting in the reaction chamber.

[0030] The feed aggregate may initially lack a catalytic additive that would increase the reactivity of gypsum with silicon. As the term "catalytic additive" is used herein, it is an additive that is not naturally present in the gypsum or silicon source and that exhibits catalytic properties for the reaction of gypsum with silicon. The feed aggregate may additionally lack a substantial amount of a material that functions as a naturally occurring catalyst. As the term "lacking a substantial amount" is used herein, it means that the amount of naturally occurring catalyst, if any, is not sufficient to increase the reactivity of gypsum with silicon. A substantial amount of naturally occurring catalyst will be sufficient to increase the reactivity of gypsum with silicon.

[0031] The use of a catalyst can alter the beneficial properties of the methods and systems herein and the products therefrom. Depending on the amount and quality (i.e., chemical composition) of the catalyst, the amount and amorphous form of silicon present in the treated aggregates can be lowered or altered, rendering the aggregates unsuitable as a SCM or PAS, as well as reducing the level of radon encapsulation. Analysis of treated aggregates without any added catalyst indicates that the treated material safely encapsulates radon and is useful as a SCM or PAS.

[0032] Other gypsum pyrolysis methods have been proposed as far back as 1916, which used a carbon source (e.g., coke, coal, lignite, fuel oil, etc.) to reduce calcium sulfate in rotary kilns (Muller-Kuhne and OSW-Krupp), circulating fluidized beds (ISU and Lurgi), or circular grate sinters (proposed by DMC / FIPR) for sulfur destruction. 2 , Al 2 O 3 and / or Fe 2 O 3 Additives, including , were mixed with the feedstock to reduce operating temperatures or added to the discharged clinker solids prior to downstream processing. The properties of the SCM and PAS in the clinker solids and the radon encapsulation capabilities of these processes are unknown because the processes were developed before SCM and PAS products were first commercially available and before radium and radon levels in phosphogypsum were first subject to regulation. However, it is expected that reducing the operating temperature may have a similar effect as described for the use of catalysts, potentially changing the amount and amorphous form of silicon present in the treated aggregates or reducing the level of radon encapsulation.

[0033] The silicon source may be silica, which may be composed of various feed materials, including some feed materials described herein as examples.In order to reduce the physical limitations on the reaction rate of gypsum and silicon, at least 80% of the particles of the gypsum source and the particles of the silicon source may have a particle size of less than 74 micrometers (μm) (200 mesh).

[0034] Optionally, the heating of the feed aggregates may be carried out in an oxidizing atmosphere, which is often less difficult to maintain than a reducing atmosphere. The reaction chamber may be constituted by a rotary kiln, a rotary hearth furnace, a tunnel kiln, a straight grate known for iron ore pelletizing, or other heating device.

[0035] In the off-gas generated from the reactor, the oxides of sulfur include sulfur trioxide (SO 3 ) or sulfur dioxide (SO 2 ) or both. Method A may further include recovering sulfur from the off-gas. For example, the sulfur may be recovered as sulfuric acid or ammonium sulfate.

[0036] In the treated aggregate, the calcium silicates may include dicalcium silicate or tricalcium silicate or both. The treated aggregate may exhibit pozzolanic properties suitable for supplemental cementitious materials, at least when ground to a particle size distribution in which at least 80% of the particles have a particle size less than 74 μm (200 mesh). The treated aggregate may exhibit a plant available silicon content of 2.3 wt% or greater.

[0037] Further described features of Method A may also be implemented in other systems and methods herein.

[0038] Method B includes the steps of forming a reaction bed containing the feed aggregates in a reaction chamber by heating the feed aggregates; and maintaining a temperature in the reaction chamber from 1200°C to 1400°C along at least a portion of the reaction bed for a period of 20 minutes or more. Each feed aggregate initially contains gypsum source particles and silicon source particles substantially uniformly distributed throughout the individual aggregates. Each feed aggregate also initially provides a silica ratio, defined as the formula ratio of silicon dioxide to calcium oxide plus silicon dioxide, ranging from 0.2 to 0.8. Additionally, each feed aggregate initially lacks sufficient components to perform carbothermic reduction during heating of the feed aggregates. Additionally, each feed aggregate initially lacks a catalytic additive that would increase the reactivity of gypsum with silicon. The gypsum and silicon in the feed aggregate react during heating in the reaction chamber, thereby forming a treated aggregate containing calcium silicate and an increased amount of amorphous silicon compared to the feed aggregate before heating. Method B includes generating an off-gas containing oxides of sulfur from the reaction bed and removing the treated agglomerates from the reaction chamber.

[0039] Additional features may be implemented in system / method B. As an example, the gypsum source may be phosphogypsum and the silicon source may be silica. The temperature may be from 1250° C. to 1300° C. and the silica ratio may range from 0.5 to 0.8. Method B may further include selecting a combination of silica ratio and temperature that avoids aggregate melting in the reaction chamber.

[0040] The treated aggregate may exhibit pozzolanic properties suitable for use as a supplemental cementitious material when ground to a particle size distribution in which at least 80% of the particles have a particle size less than 74 μm (200 mesh). The treated aggregate may exhibit a plant available silicon content of 2.3 wt% or greater.

[0041] The further described features of Method B may also be implemented in other systems and methods herein.

[0042] Method C includes forming a reaction bed containing the feed aggregate in a reaction chamber by heating the feed aggregate; and maintaining a temperature in the reaction chamber from 1200° C. to 1400° C. along at least a portion of the reaction bed for a time period of 20 minutes or more. Each feed aggregate initially contains particles of phosphogypsum and particles of silica substantially uniformly distributed throughout the individual aggregate. Each feed aggregate also initially provides a silica ratio, defined as the formula ratio of silicon dioxide to calcium oxide plus silicon dioxide, ranging from 0.2 to 0.8. The feed aggregate exhibits a radon-222 emission rate attributable to phosphogypsum. The phosphogypsum and silica in the feed aggregate react during heating in the reaction chamber, thereby forming a treated aggregate containing calcium silicates and an increased amount of amorphous silicon compared to the feed aggregate prior to heating. The reaction reduces the radon-222 emission rate in the treated aggregate compared to the feed aggregate prior to heating. Method C includes generating an off-gas containing oxides of sulfur from the reaction bed and removing the treated agglomerates from the reaction chamber.

[0043] Additional features can be implemented in Method C. As an example, the temperature can be from 1250° C. to 1300° C. and the silica ratio can range from 0.5 to 0.8. Method C can further include selecting a combination of silica ratio and temperature that avoids agglomerate melting in the reaction chamber.

[0044] The treated aggregate may exhibit pozzolanic properties suitable for use as a supplemental cementitious material when ground to a particle size distribution in which at least 80% of the particles have a particle size less than 74 μm (200 mesh). The treated aggregate may exhibit a plant available silicon content of 2.3 wt% or greater.

[0045] The further described features of method C may also be implemented in other systems and methods herein.

[0046] According to one method and system 30 shown in FIG. 9, a gypsum source 31 and a silicon source 32 are combined in an aggregate 35 to obtain a feed aggregate. The feed aggregate undergoes a heating step 37 in a reaction chamber to produce an off-gas. The off-gas 40 from the heating step 37 can be collected and further processed into a saleable or reusable form such as sulfuric acid or ammonium sulfate. The dashed line connecting the off-gas 40 to the heating step 37 indicates that the collection of the off-gas is optional. The heat-treated aggregate is removed from the reaction chamber by a removal step 38 to a PAS product 41, such as a fertilizer or SCM product 42, or both. The dashed lines connecting the PAS product 41 and the SCM product 42 to the removal step 38 indicate that obtaining the two products is optional.

[0047] More specifically, some methods and systems described herein include the use of rotary hearth furnaces, rotary kilns, and / or grate kilns, although other implementations are contemplated. The methods and systems can reduce radon emissions from phosphogypsum and allow for safer landfill disposal and / or recovery of usable sulfur for further processing into silicon fertilizer products, auxiliary cementitious materials production, and other useful products.

[0048] According to another method and system 20 shown in FIG. 1, raw materials such as a gypsum source 1 and a silicon source 2 are mixed 3 and ground 4. The raw materials are mixed 3 to a desired silica ratio prior to grinding 4. As used herein, the terms "silica ratio" or "SR" are defined as the formula ratio (also molar ratio) of silicon (on a silicon dioxide basis) to calcium (on a calcium oxide basis) plus silicon. Mathematically, SR=(wt% SiO 2 / 60) / [(wt%CaO / 56)+(wt%SiO 2 / 60)]. As a result, gypsum (CaSO 4 The wt% of calcium sources, such as calcium carbonate, calcium phosphate, calcium phosphate phosphate phosphate phosphate phosphate phosphate phosphate, calcium ... 2 is converted into a standard.

[0049] Agglomeration 5 uses either a balling drum, balling pan, or briquette to produce 3 / 8 to 5 / 8 inch (in.) diameter pellets or 1 to 2 in. briquettes. The agglomerates are dried 6 in a grate dryer, fluidized bed dryer, grate kiln, or the like. The dried agglomerates are heated 7 in a rotary kiln, rotary hearth furnace, or other heating device such as a tunnel kiln, or a straight grate known for iron ore pelletization. When using a straight grate method, the drying, heating, and cooling steps are combined. The heat treated agglomerates are cooled 8, and the cooled agglomerates are processed in a solids handling step 9 for packaging and sale as a PAS product 11 or SCM product 12, or both. Gases from the heating step 7 are collected and can be further processed in a gas handling step 10 into a saleable or reusable form in a gas handling step 13, such as sulfuric acid, ammonium sulfate, or other sulfate based products. Although not shown in the drawings, the final product may undergo further processing during the cooling or granulation steps to add other plant nutrients to the PAS product (e.g., phosphorus, potassium, nitrogen, iron, etc.). Waste heat 7 from the kiln, furnace, etc. may be captured and used for drying the aggregates 6.

[0050] Gypsum source 1 includes phosphogypsum, gypsum filter cake from the wet acid process that produces phosphoric acid, and known SO 2 The waste gypsum by-product from a lime scrubber system, regular gypsum, or a combination thereof. Silicon source 2 includes sand, tailings sand, or a combination thereof, and may include other silicon-containing materials. The powdered PAS11 or SCM12 product contains 80% minus 200 mesh material (at least 80% of the particles have a size less than 74 micrometers (μm), i.e., they will pass through a 200 mesh screen).

[0051] The process may use rotary kilns, rotary hearth furnaces, grate kilns, etc., to produce calcium silicate as a means of providing energy and temperature for the reaction of calcium and silicon. Melting in these vessels is controlled in a manner that allows for continuous removal of treated material so that no treated material remains in the vessel and causes a process shutdown. Exhaust heat from the kiln or furnace may be used to dry moisture-containing raw materials and / or aggregate feeds.

[0052] The raw materials may be ground to at least 80% minus 200 mesh before being mixed and agglomerated, then layered on the hearth bed or fed to the kiln. The feed agglomerates for the rotary hearth furnace may be heated at a rate of 200-300°C per minute, for example 250°C per minute. The process temperature of the rotary hearth furnace may be maintained in the range of 1200°C-1350°C for 20-30 minutes. The temperature, heating rate, and holding time in the kiln are in the same range of 1200°C-1350°C, but slower (100-200°C per minute) and longer (30-60 minutes).

[0053] The interior of the furnace or kiln may be heated by indirect heaters or by direct combustion of fuel gas, natural gas, or fuel oil using port air and / or oxygen-enriched combustion air injected through the roof and walls of the furnace. Radiation is the primary mode of heat transfer from the gases and furnace walls to the coagulated bed. Secondary heat transfer mechanisms are convection from the gases and conduction from the bed and walls.

[0054] The processed material exiting the kiln or furnace may be cooled and the cooled product may be used as is or may be subsequently ground and granulated into the final product.

[0055] Off-gas can be collected to produce sulfuric acid and / or ammonium sulfate using known technology. Dust collection or scrubbing systems to remove particulates for reuse, and / or CO from the combustion of natural gas. 2Carbon dioxide capture systems for the

[0056] Figure 2 shows the CaO-SiO 2 1 is a phase diagram of the above-mentioned eutectic point for various silica ratios and process temperatures.

[0057] The most desirable silica ratio for the method and system herein starts at 0.64 and ends at approximately 0.8 for temperatures above 1250° C. In some cases, even a small amount of melting and stickiness may be undesirable in a rotary kiln. Rotary hearth furnaces with a bed that is stable and running under a heat source are more resistant to melting, in which case a small amount of melting is acceptable.

[0058] As shown herein, a silica ratio of approximately 0.5 can be used which produces amorphous silicon at temperatures just above 1250° C. without significant melting.

[0059] Radiation testing. It can be demonstrated that calcium silicate products containing radium-226, through their use as supplemental cementitious materials (SCMs), lightweight aggregates, soil stabilizers, or plant available silicon (PAS) fertilizers, do not represent a permanent source of radon that would increase external gamma dose rates.

[0060] From a public health perspective, exposure to radon gas is the most recognized potential health effect of most NORM-containing products. It is possible to determine how much radon is emitting from the surface of the product. This is often measured by the "radon flux" technique, which captures and measures the radon released from a material of known surface area into a container of a specific volume that seals the material for a specific time. To know what percentage of the radon contained in the calcium silicate produced from phosphogypsum is represented by the amount released, one needs the activity concentration in the calcium silicate of its "parent" radon generator, radium-226, which gives the maximum amount of its "daughter" radon-222 that can be present. In this case, the activity concentration of the radon daughter never exceeds that of its radium parent, but they become equal in a closed system after 7 half-lives of the daughter (or 3.82 days x 7 ≒ 27 days).

[0061] Radiation tests were performed to characterize the activity concentration of radionuclides in the produced calcium silicate and the proportion of radon gas it releases. The results of these tests are used to determine the potential dose to the public in the vicinity of the unprocessed produced calcium silicate. Radiation dose to the public occurs because of direct radiation from sources external to the body or internal dose from ingestion or inhalation. The known main contributors are dose from inhalation of radon gas and its solid progeny radionuclides, and direct exposure to gamma radiation from sources close to the body.

[0062] The Saskatchewan Research Council (SRC) performed radiological analyses of the raw material inputs and calcium silicate outputs. SRC analyzed the activity concentration of radium-226 (226Ra) and radon-222 (222Rn) emissions in becquerels per gram (Bq / g). Of particular interest to this project, SRC measured the radon emission fraction, which is often called the radon exhalation fraction or radon exhalation coefficient (E). The exhalation fraction is the rate at which radon is produced within the radium (Ra-226)-carrying particles of the material and escapes into the pores of the material. It is therefore a measure of the amount of radon that can be released from the material into the air and potentially become a source of radon exposure. The results of the radiological analyses are shown in Table 2.

[0063] Phosphogypsum processors can demonstrate that radium-226-containing products, through their use as supplemental cementitious materials (SCM), lightweight aggregates, soil stabilizers, or plant available silicon (PAS) fertilizers, do not become permanent sources of radon that would increase external gamma dose rates. These radiation tests show that very little radon is released from the calcium silicate and that the 222Rn activity concentration is close to secular equilibrium; that is, 226Ra and all its descendants down the decay chain will have approximately equal activity concentrations.

[0064] For comparison, the rate of radon escape from granite has been measured to be between 0.03 and 0.28, with a nominal value of 0.1 (or 10%) from radiation safety experts, the Health Physics Society (Radiation from granite countertops, 2012). If we consider granite countertops at the higher end of this range, say 0.3 Bq / g 226Ra, then only 10% or 0.03 Bq 222Rn per gram of granite would be available to the surrounding air. Under a conservative scenario of a small kitchen with a large amount of granite, this would be considered safe for domestic use. The radon released per gram of granite countertop is roughly 10 times more than the radon released per gram of calcium silicate produced.

[0065] As an additional point of reference, SRC also performed radiation testing on other materials. The SRC radiation test results for process inputs, process outputs, and other materials are included for comparison purposes and are summarized below in Table 2 and Figure 3. In addition, the concrete control sample had a radon-222 exhalation rate of 0.014 Bq / g.

[0066] [Table 2]

[0067] The Federal Regulations, 40 CFR, section 61.204 governs the distribution and use of phosphogypsum for outdoor agricultural purposes. The regulation states that the average radium-226 (solid) concentration at the location in the stack from which the phosphogypsum is removed shall not exceed 10 pCi / g. Table 2 shows results for phosphogypsum, phosphorus clay, phosphate rock, and calcium silicate that exceed this standard. Radium exists as a solid in these materials, and the radium limit for phosphogypsum is intended to ensure a safe level of radon emission (as a gas) from the radium solid contained in the phosphogypsum.

[0068] The release rate of radon radiation (gas) from calcium silicate produced by using phosphogypsum and phosphate sand as raw materials is believed to be bound at the molecular level by the "house of glass" that is created when calcium silicate is formed during the pyrolysis of these materials. Two physical parameters (porosity and specific surface area) appear to control the release of radon from calcium silicate products. The appearance of a low permeability barrier appears to be a potential immobilization matrix for radon from phosphate rock and phosphogypsum.

[0069] As shown in Figure 3, the radon exhalation rate of calcium silicate is lower than that of all raw material inputs, as well as that of phosphate rock, phosphorus-bearing clay, granite, and natural soil.

[0070] Based on the sale and use of granite in kitchen countertops, and the radon emissions from the produced calcium silicate compared to granite and natural soil, the calcium silicate produced by the methods herein is believed to be safer for landfill disposal (compared to phosphogypsum). It is also believed to be safe for use as a plant-available silicon fertilizer, supplemental cement-based material, lightweight aggregate, and soil stabilizer. For each of these uses, the exposure pathways can be limited, and the radon emissions and doses produced can be less than those of the materials shown in FIG. 3.

[0071] Further testing and analysis are planned to determine the mechanism, thermal decomposition, and reformulation of uranium and radium. Further characterization of radon release from the calcium silicate product will be conducted to support USEPA approval of the use of phosphogypsum in the methods herein.

[0072] Other laboratory tests. Sulfur decomposition / generation versus temperature. Laboratory experiments were conducted using phosphogypsum, waste phosphorus sand, and waste phosphorus clay over a range of silica ratios and temperatures to determine sulfur decomposition yields versus temperature. Waste phosphorus clay was used as both the binder and silica source. Carbon was not used as a reductant. Temperatures ranged from 900°C to 1300°C with silica ratios ranging from 0.29 to 0.78. Each sample was held at the target temperature for 30 minutes. Samples were evaluated for any melting or adhesion characteristics. Samples were crushed and then analyzed using a sulfur analyzer and inductively coupled plasma (ICP) available from LECO Corp., St. Joseph, Michigan. A ceramic castable crucible was used since it was believed that the carbon crucible would affect the reactivity. Pure gypsum samples were prepared and tested to obtain a baseline for when decomposition begins to occur. Previously published data indicates 1250°C.

[0073] The data demonstrates that the rate of sulfur evolution is affected by the amount of silica present. The higher the % of silica, the more sulfur is evolved at any given temperature. Reaction 1 above illustrates this. Excess SiO 2 drives the reaction, CaSiO 3 (monocalcium silicate) followed by SO 3 A silica ratio above 0.5 means there is too much SiO in the mixture. 2 has.

[0074] SiO 2 The presence of SiO not only initiates the decomposition of sulfur at a lower temperature, but also 2 Without CaSO 4 This may allow the use of different furnace types (kiln, hearth, straight grate) without operational problems. Excess SiO 2 The presence of also allows for the production of a product that is suitable as both a PAS fertilizer and a cement substitute.

[0075] [Table 3]

[0076] Based on this data, the desired feed aggregate contains 30-50% gypsum, 40-60% sand, and 10% clay with operating temperatures between 1250-1300°C. The sulfur removal results for these ranges are shown in the double-bordered cells. This is an equivalent SR range of 0.64-0.8. Phosphogypsum mixtures in the range of 30-50% gypsum by weight (30 / 60 / 10 to 50 / 40 / 10) appear to produce solid products that are suitable as both a cement replacement material and plant available silicon. Cells with dashed borders have mixtures that melt or have lower sulfur removal rates.

[0077] Laboratory analysis of silicon fertilizers. Laboratory testing has shown that the presence of more soluble forms of silicon, such as calcium silicate, rather than insoluble crystalline forms such as quartz, is evidenced by the amount of plant available silicon present in the final product. The higher the level of plant available silicon in the silicon fertilizer, the more desirable the material. The chemical and physical properties of the product of the system of FIG. 1 can be controlled by controlling the quality, adjusting the amount of input streams, and varying the processing temperature, as described below.

[0078] The production system of FIG. 1 maintains specific silica ratios of various raw materials and uses heat to produce dicalcium silicate (Ca) by controlling the melting of the feed materials at various operating temperatures. 2 SiO 4 ) and / or tricalcium silicate (Ca 3 SiO 5 or Ca 3 S 2 O 7 ) and small amounts of monocalcium silicate (CaSiO 3The method is based on the use of heat to favor the production of amorphous silicon in the form of calcium carbonate and quartz (i.e., silicon dioxide). Usable silicon can be produced from various combinations of calcium and silicon sources contained in tailings sands, gypsum, and phosphogypsum when used as a feedstock to hearth furnaces, kilns, or other heating devices. For silicon fertilizers, the silica ratio may be maintained between 0.64 and 0.80.

[0079] Bench-scale and pilot plant tests have shown that the level of available silicon produced can be controlled by the characteristics of the feedstock, the chemistry (e.g., silica ratio), and the process temperature. As an example, the feed components (a combination of phosphogypsum and phosphorus-bearing tailing sand) contain a silica ratio of 0.2 to 0.8 and may be heated to at least 1200°C to produce a plant-available silicon fertilizer product.

[0080] PAS to silica ratio. While there are many variables that affect the outcome, in general, production of calcium silicate as amorphous silicon (measured as PAS) varied with the silica ratio, as shown in laboratory and pilot-scale testing of the raw materials in the feed. In the examples below, feed aggregates were prepared according to the method shown in Figure 1 and described above to produce dry pellets 3 / 8 to 5 / 8 in. in diameter or 3 / 4 in. briquettes. The silica ratios listed were calculated for "green balls" based on the mass of the ingredients and the compositional assays of the ingredients.

[0081] PAS results were obtained using an experimental furnace to heat various combinations of phosphogypsum and phosphorus-bearing tailing sand (silicon source), which produced plant available silicon at both higher and lower silica ratios when held at a temperature of 1200°C for 30 minutes, as shown in Table 4.

[0082] [Table 4]

[0083] This is a positive outcome since the higher silica ratio mixes still produced acceptable PAS products and were suitable as cement substitutes. As discussed above, silica ratios in the range of 0.64 to 0.8 were not expected to melt at the desired temperatures, while the mix with a silica ratio of 0.42 also did not melt.

[0084] FIG. 4 shows the amount of PAS versus the molar ratio (SiO 2 / (SiO 2 +CaO)) (SiO 2 / (SiO 2 To illustrate the effect of the molar ratio of gypsum to calcium, calcite (CaCO 3 ) was used. It is clear that as it reaches 0.5 and continues to move to the right and begins to produce less calcium silicate, the amount of PAS drops. Below 0.5 and making 100% calcium silicate, the PAS peaks and levels off or may even start to drop.

[0085] The level of PAS in the raw material. As a baseline, the following silicon-containing materials were analyzed for PAS. The results indicate that these materials contain small amounts of PAS, as noted in Table 5. None of these materials are useful alone as silicon fertilizers.

[0086] [Table 5]

[0087] Testing of cement substitutes. As shown in Table 6 and Figures 5-7, all data to date (using a silica ratio of 0.7 and waste rock phosphate as a gypsum substitute) qualify the calcium silicate treated aggregate product (referred to as J-ROX™ in Figures 5-7) as a high quality SCM with improved set time, compressive strength and durability properties that are equal to or better than those of cement or fly ash (partial replacement of cement) in concrete mixes. The cementitious properties of calcium silicate produced from processing either waste rock phosphate or phosphogypsum are presumed to be identical. However, such testing will be repeated using phosphogypsum instead of waste rock phosphate.

[0088] [Table 6]

[0089] Pilot scale testing. Pilot scale testing was performed using a 5 meter rotary hearth furnace (RHF) heated with natural gas. As will be understood from the following description, the pilot scale testing confirmed the validity of the laboratory tests as far as the mixture (silica ratio), temperature, time, and sulfur removal listed in Table 3 above are concerned. Although the furnace is not described in great detail herein, the function of the components can be understood, in combination with the knowledge of one skilled in the art, from a similar RHF as described in U.S. Patent Application No. 16 / 914,182 (filed June 26, 2020), listed above as incorporated by reference.

[0090] The feed to the furnace contained a mixture of ground (at least 80% minus 200 mesh) ordinary gypsum (45 wt%), phosphorus-containing sand (52 wt%), and bentonite (3 wt%), used to make small briquettes with a silica ratio of 0.72. The gypsum in this test was shown to be essentially free of phosphorus at 0.0 wt%. The phosphorus-containing sand often contains 6.6 wt% P. 2 O 5 In this test, the feed briquette contains P 2 O5 The briquettes contained 3.5% bentonite. Bentonite was useful in making the briquettes for this test, but may be reduced in amount or eliminated as briquetting skills improve, such as during commercial operation. Phosphorous clay may be substituted for bentonite. The briquettes weighed approximately 3 grams each and measured approximately 0.75 inches long by 0.5 inches wide by 3 / 8 inches thick.

[0091] Approximately 700 pounds of briquettes were fed into the furnace for 150 minutes. The furnace was operated with an estimated bed temperature of 1275°C and a hot zone hold time of 30 minutes to obtain the desired sulfur decomposition. The "hot zone" is the location where exposure to sufficient temperature for reaction occurs. This temperature was based on readings from a series of thermocouples used to measure the temperature of the air in the reaction chamber at six (6) locations within the hot zone of the furnace. The average temperature for all six thermocouples ranged between 1281°C and 1307°C. There was no significant melting of the briquettes during the run, and as a result, melting did not interfere with operation. Total natural gas usage during the run was 3,300 standard cubic feet. The briquettes processed were P 2 O 5 % by weight, the increase in percentage due to mass loss indicating that there is no substantial carbothermal reduction of phosphorus.

[0092] Based on the difference in sulfur levels measured in the furnace feed and furnace exhaust, sulfur decomposition in the furnace feed was up to 93% and averaged 86% during operation, as shown in Figure 8. This allowed the release of sulfur gases in the furnace exhaust gases for subsequent recovery as sulfuric acid and other sulfur-containing compounds, including ammonium sulfate.

[0093] Although minimum and maximum values ​​are recited for the above ranges and other ranges specified herein, it is understood that the narrower included ranges may also be desirable and may be distinguishable from the prior art, and the processing principles discussed herein may provide further basis for the smaller included ranges.

[0094] In accordance with the statute, the embodiments have been described in generally specific language with respect to structural and organizational features, but it is to be understood that the embodiments are not limited to the specific features shown and described. The embodiments are therefore claimed in any of those forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents. [Explanation of symbols]

[0095] 1 Gypsum source 2. Silicon Source 3. Mixed 4. Shattered 5 Coagulation 6. Dried, drying process 7 Heated, heating process, waste heat 8. Cooled 9. Solids Handling Process 10 Gas handling process 11 PAS products 12 SCM products 13 Gas handling processes into saleable or reusable forms 20 Alternative methods and systems 30 A method and system 31 Gypsum source 32 Silicon Source 35 Aggregates 37 Heating process 38 Removal process 40 Offgas 41 PAS products 42 SCM products

Claims

1. (a) forming a reaction bed containing feed aggregates in a reaction chamber by heating the feed aggregates, wherein the individual feed aggregates initially contain gypsum source particles and silicon source particles substantially uniformly distributed throughout the individual aggregates; wherein the individual aggregates initially provide a silica ratio greater than 0.5, up to a maximum of 0.8, representing excess silica, the silica ratio being defined as the formula ratio of silicon dioxide to calcium oxide plus silicon dioxide, and wherein the weight percent of calcium sources other than calcium oxide is converted to a CaO basis, and the weight percent of silicon sources other than silicon dioxide is converted to a SiO ２ wherein the feed aggregate is initially devoid of a catalytic additive that will increase the reactivity of gypsum with silicon; (b) reacting the gypsum and silicon in the feed aggregates during heating in the reaction chamber, thereby forming treated aggregates containing calcium silicates and an increased amount of amorphous silicon compared to the feed aggregates before heating; (c) generating an off-gas from the reaction bed containing oxides of sulfur; (d) removing the treated aggregate from the reaction chamber; A method comprising:

2. 10. The method of claim 1, wherein the gypsum source is phosphogypsum.

3. 3. The method of claim 2, wherein the feed aggregate exhibits a radon-222 emissivity attributable to the phosphogypsum, and further comprising reducing the radon-222 emissivity in the treated aggregate compared to the feed aggregate before heating.

4. 4. The method of claim 3, wherein the radon-222 emanations of the treated aggregate after cooling to ambient temperature are less than or equal to 0.013 becquerels per gram (Bq / g).

5. 10. The method of claim 1, wherein the feed aggregates are initially devoid of components sufficient to effect carbothermic reduction during heating of the feed aggregates.

6. 10. The method of claim 1, wherein the silica ratio ranges from 0.64 to 0.

8.

7. 10. The method of claim 1, further comprising maintaining a temperature in the reaction chamber along at least a portion of the reaction bed at 1200°C to 1400°C.

8. 8. The method of claim 7, wherein the feed aggregate is maintained at the temperature for a time period of from 20 minutes to 60 minutes.

9. 10. The method of claim 1, further comprising maintaining a temperature in the reaction chamber along at least a portion of the reaction bed at 1250°C to 1300°C for a time period of 30 minutes or more.

10. The gypsum source is CaSO ４ and the silicon source is SiO ２ Contains CaSO ４ and SiO ２ 10. The method of claim 1, wherein is the only essential reactant.

11. The method of claim 1 further comprising recovering sulfur from the off-gas.

12. The method of claim 1 , wherein the heating of the feed aggregates occurs in an oxidizing atmosphere.

13. The method of claim 1 wherein the silicon source is silica.

14. 2. The method according to claim 1, wherein the reaction chamber is constituted by a rotary kiln, a rotary hearth furnace, a tunnel kiln, or a straight grate known for iron ore pelletization.

15. the feed aggregates initially lack sufficient components to effect carbothermic reduction during heating of the feed aggregates; the individual aggregates initially provide a silica ratio, defined as the formula ratio of silicon dioxide to calcium oxide plus silicon dioxide, in the range of greater than 0.5 up to 0.8; the method further comprising maintaining a temperature in the reaction chamber along at least a portion of the reaction bed between 1200°C and 1400°C; the feed agglomerate is maintained at the temperature for 20 minutes to 60 minutes; and The gypsum source is CaSO ４ and the silicon source is SiO ２ Contains CaSO ４ and SiO ２ is the only essential reactant, The method of claim 1.

16. 16. The method of claim 15, wherein the gypsum source is phosphogypsum and the silicon source is silica.

17. the gypsum source is phosphogypsum; the feed aggregate exhibits a radon-222 exhalation rate attributable to the phosphogypsum, the method further comprising reducing the radon-222 exhalation rate in the treated aggregate compared to the feed aggregate before heating; the individual aggregates initially provide a silica ratio, defined as the formula ratio of silicon dioxide to calcium oxide plus silicon dioxide, in the range of greater than 0.5 up to 0.8; the method further comprising maintaining a temperature in the reaction chamber along at least a portion of the reaction bed between 1200°C and 1400°C; the feed agglomerate is maintained at the temperature for a time period of 20 minutes to 60 minutes; and The silicon source is silica. The method of claim 1.

18. 18. The method of any one of claims 1, 15, or 17, wherein the temperature is from 1250°C to 1300°C and the silica ratio is in the range of from 0.64 to 0.

8.

19. 20. The method of any one of claims 1, 15, or 17, wherein the treated aggregate exhibits pozzolanic properties suitable for use as a supplemental cementitious material, at least when ground to a particle size distribution in which at least 80% of the particles have a particle size less than 74 μm (200 mesh).

20. 20. The method of any one of claims 1, 15, or 17, wherein the treated aggregate exhibits a plant available silicon content of 2.3 wt% or greater.

21. The method of claim 1, further comprising the steps of: heating the feed aggregate; maintaining a temperature in the reaction chamber along at least a portion of the reaction bed between 1200°C and 1400°C; and maintaining the feed aggregate at the temperature for a time period of between 20 minutes and 60 minutes; forming a reaction bed containing the feed aggregates within the reaction chamber, the individual feed aggregates initially containing gypsum source particles and silicon source particles substantially uniformly distributed throughout the individual aggregates; reacting the gypsum and silicon in the feed aggregate during heating in the reaction chamber, thereby forming a treated aggregate containing calcium silicates and an increased amount of amorphous silicon compared to the feed aggregate before heating; generating an off-gas from the reaction bed containing oxides of sulfur; removing the treated aggregate from the reaction chamber after the 20 to 60 minute period and without further heating at the temperature; A method comprising:

22. A process for forming a reaction bed containing feed aggregates in a reaction chamber by heating said feed aggregates, wherein each feed aggregate initially contains particles of a gypsum source containing CaSO and particles of a silicon source containing SiO distributed substantially uniformly throughout each aggregate, and wherein said feed aggregates are initially devoid of a catalytic additive that will increase the reactivity of the gypsum with silicon; reacting CaSO4 and SiO2 in the feed agglomerates as the only essential reactants during heating in the reaction chamber, thereby forming treated agglomerates containing calcium silicates and an increased amount of amorphous silicon compared to the feed agglomerates before heating; generating an off-gas from the reaction bed containing oxides of sulfur; removing the treated aggregate from the reaction chamber; A method comprising:

23. The method of claim 21 or 22, wherein the gypsum source is phosphogypsum.

24. The method of claim 23, wherein the feed aggregate exhibits a radon-222 emissivity attributable to the phosphogypsum, and further comprising a step of reducing the radon-222 emissivity in the treated aggregate compared to the feed aggregate before heating.

25. The method of claim 24, wherein the radon-222 emission rate of the treated aggregate after cooling to ambient temperature is 0.013 becquerels per gram (Bq / g) or less.

26. The method of claim 21 or 22, wherein the feed aggregates initially lack sufficient components to effect carbothermic reduction during heating of the feed aggregates.

27. The method of claim 21 or 22, wherein the individual aggregates initially provide a silica ratio of from 0.2 to 0.8, said silica ratio being defined as the formula ratio of silicon dioxide to calcium oxide plus silicon dioxide, and wherein, prior to calculating said silica ratio, the weight percentage of calcium sources other than calcium oxide is converted to a CaO basis and the weight percentage of silicon sources other than silicon dioxide is converted to a SiO2 basis.

28. The method described in claim 27, wherein the silica ratio is greater than 0.5 and ranges up to 0.8, representing excess silica.

29. The method of claim 27 when dependent from claim 22, further comprising the step of maintaining a temperature in the reaction chamber between 1200°C and 1400°C along at least a portion of the reaction bed.

30. The method of claim 29, wherein the feed aggregate is maintained at the temperature for a period of 20 minutes or more.

31. A method as described in claim 21 or claim 22, further comprising the step of maintaining a temperature in the reaction chamber between 1250°C and 1300°C along at least a portion of the reaction bed and maintaining the feed aggregates at said temperature for a period of at least 30 minutes, wherein the temperature is between 1250°C and 1300°C and the period of time is between 30 minutes and 60 minutes.

32. The method of claim 21, wherein the feed aggregate is initially devoid of a catalytic additive that would increase the reactivity of gypsum with silicon.

33. The method of claim 21 or 22, further comprising recovering sulfur from the off-gas.

34. The method of claim 21 or 22, wherein the heating of the feed aggregates is carried out in an oxidizing atmosphere.

35. The method of claim 21, wherein the silicon source is silica.

36. A method as described in claim 21 or 22, wherein the reaction chamber is constituted by a rotary kiln, a rotary hearth furnace, a tunnel kiln, or a straight grate known for iron ore pelletization.

37. The feed aggregates are initially devoid of components sufficient to effect carbothermic reduction during heating of the feed aggregates; wherein each aggregate initially provides a silica ratio, defined as the formula ratio of silicon dioxide to calcium oxide plus silicon dioxide, ranging from 0.2 to 0.8, and prior to calculating said silica ratio, the weight percentage of calcium sources other than calcium oxide is converted to a CaO basis, and the weight percentage of silicon sources other than silicon dioxide is converted to a SiO2 basis; 23. The method of claim 22, further comprising maintaining a temperature in the reaction chamber along at least a portion of the reaction bed between 1200°C and 1400°C; 23. The method of claim 22, wherein the feed aggregate is maintained at the temperature for 20 minutes or more; and The feed aggregate is initially devoid of catalytic additives that would increase the reactivity of gypsum with silicon; 23. The method of claim 21 or 22.

38. The method of claim 37, wherein the gypsum source is phosphogypsum and the silicon source is silica.

39. The gypsum source is phosphogypsum; the feed aggregate exhibits a radon-222 exhalation rate attributable to the phosphogypsum, the method further comprising reducing the radon-222 exhalation rate in the treated aggregate compared to the feed aggregate before heating; wherein each aggregate initially provides a silica ratio, defined as the formula ratio of silicon dioxide to calcium oxide plus silicon dioxide, ranging from 0.2 to 0.8, and prior to calculating said silica ratio, the weight percentage of calcium sources other than calcium oxide is converted to a CaO basis, and the weight percentage of silicon sources other than silicon dioxide is converted to a SiO2 basis; 23. The method of claim 22, further comprising maintaining a temperature in the reaction chamber along at least a portion of the reaction bed between 1200°C and 1400°C; 23. The method of claim 22, wherein the feed aggregate is maintained at the temperature for a period of 20 minutes or more; and 22. The method of claim 21 , wherein the silicon source is silica.

23. The method of claim 21 or 22.

40. A method described in any one of claims 21, 37, or 39, wherein the temperature is from 1250°C to 1300°C and the silica ratio is greater than 0.5 and in the range of up to 0.8, representing excess silica.

41. The method of any one of claims 21, 37, or 39, wherein the treated aggregate exhibits pozzolanic properties suitable for use as a supplemental cementitious material when ground to a particle size distribution in which at least 80% of the particles have a particle size less than 74 μm (200 mesh).

42. The method of any one of claims 21, 37, or 39, wherein the treated aggregates exhibit a plant available silicon content of 2.3 wt% or more.

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