Integrated method for producing ammonium sulfate by reclaiming phosphogypsum waste

Abstract

The present invention relates to an integrated electrochemical process for the production of ammonium sulfate, operating at low temperature and pressure, using only renewable energy sources and reclaiming phosphogypsum waste and CO2 emissions of the phosphate industry.

Description

[0001] INTEGRATED PROCESS FOR THE PRODUCTION OF AMMONIUM SULFATE BY THE VALORIZATION OF PHOSPHOGYPSE ​​WASTE

[0002] FIELD OF INVENTION

[0003] The present invention relates to processes, systems and techniques implemented in particular in the electrochemical production of ammonium sulfate by the use of an electrodialysis cell, and contributing more particularly to the valorization of phosphogypsum waste as well as carbon dioxide emissions from the phosphate industry.

[0004] CONTEXT OF THE INVENTION

[0005] Ammonium sulfate is a fertilizer with a high mass content of nitrogen (21% N) that is widely used in agriculture. Conventional methods for producing this fertilizer are based on the Merseburg process, which often requires the use of ammonia from the combustion of fossil fuels.

[0006] In recent years, the agricultural industry has faced increasing pressure to adopt more sustainable practices to meet growing food demand while minimizing environmental impacts. One innovative solution that has garnered significant attention in this regard is the production of e-fertilizers. This technology relies on producing green ammonia from renewable hydrogen and nitrogen captured from the air to synthesize fertilizers.

[0007] However, this method still relies on the use of the Haber-Bosch reactor for ammonia synthesis. This process uses iron-based catalysts that have limited flexibility and are not suitable for being powered by a variable energy source such as renewable energies.

[0008] Furthermore, existing industrial processes for producing ammonium sulfate often involve multiple production steps and the use of various pieces of equipment. This type of sequential production leads to an inefficient and more capital-intensive method. Moreover, these processes suffer from the high production cost of green hydrogen, unless gray hydrogen derived from fossil fuels is used.

[0009] Therefore, it is of industrial interest to develop integrated production processes for ammonium sulfate-based fertilizers that are less energy-intensive, less expensive, simpler, continuous and better suited to being powered by renewable energy sources.

[0010] SUMMARY OF THE INVENTION

[0011] According to the aspects illustrated below, the present invention describes an integrated process for the production of ammonium sulfate-based fertilizers, using only a carbon dioxide source, a phosphogypsum source, and a water and electricity source.

[0012] The invention relates firstly to a process for the production of ammonium sulfate ((NH4)2SU4) by electrodialysis using a source of ammonium cations (NH4 + ) on the one hand, and sulfate anions (SO4 = ) on the other hand.

[0013] The invention also relates to a method for capturing and fixing carbon dioxide (CO2) from an industrial gaseous effluent rich in carbon dioxide.

[0014] (CO2).

[0015] The particularity of this invention lies in the fact that it uses, for each of its implementations, a common reaction sequence integrating the capture of the carbon dioxide contained in said industrial gaseous effluvium by means of an alkali hydroxide by its conversion into alkali carbonate, followed by the contacting of said alkali carbonate with a source of alkaline earth sulfate and conversion of the latter into alkali sulfate, water-soluble, and into alkaline earth carbonate insoluble or essentially insoluble in water.

[0016] The invention is more precisely identified by the following claims. BRIEF DESCRIPTION OF FIGURES

[0017] FIG. 1 shows the block diagram of the integrated system.

[0018] FIG. 2 shows the variation of the concentration of ammonia / ammonium and carbon dioxide / carbonates as a function of pH.

[0019] FIG. 3 shows the results of the comparison of energy requirements between integrated and conventional pathways.

[0020] FIG. 4 shows a comparison of investment and operating cost results between integrated and conventional routes.

[0021] DETAILED DESCRIPTION OF THE INVENTION

[0022] The process that is the subject of the invention includes, among other things, the implementation of an industrial installation shown schematically in Fig. 1 and whose operation can be described as follows:

[0023] - An absorber column (100) used for capturing carbon dioxide from a smoke emitting carbon dioxide by introducing an alkaline capture solution of alkali hydroxide (407) and allowing the carbon dioxide to react with the alkali hydroxide to form a carbon dioxide-enriched solution consisting of alkali carbonate (107);

[0024] - An air separation unit (200) to separate nitrogen (202) from carbon dioxide-depleted smoke (103);

[0025] - A carbonation reactor (300) for the recovery of sulfate from phosphogypsum waste (301) in order to produce alkali sulfate (303) while precipitating carbon dioxide in the form of calcium carbonates (302);

[0026] - An electrodialysis stack (400) for the production of ammonium sulfate (409) comprising at least one electrodialysis cell, each cell comprising two bipolar membranes, one anion exchange membrane, two cation exchange membranes, and a pair of electrodes comprising a positive electrode (anode) and a negative electrode (cathode). - The electrodialysis cell comprises six compartments, in which an aqueous solution of metal sulfate (303) flows through a first compartment (401) of the electrodialysis unit to separate the sulfate ions from the metal ions, wherein the first compartment is delimited on the anodic side by an anion exchange membrane and on the cathodic side by a cation exchange membrane, and a fourth compartment (404) is delimited on the anodic side by a cation exchange membrane and on the cathodic side by a bipolar membrane.the bipolar membrane comprising respectively an anion-permeable face and a cation-permeable face oriented to face an anode and a cathode of the electrodialysis cell; and the application of an electrical potential across the terminals of the electrodialysis cell causes a direct current to flow through the bipolar membrane, causing the dissociation of the water in the bipolar membrane into hydrogen protons and hydroxide ions; wherein the hydroxide ions from the fourth compartment (404) react with the metal ions recovered in the first compartment to form an aqueous solution of metal hydroxide (407) which is recycled to the absorber (100); wherein (i) the protons from the fifth (405) compartment are reduced at the cathode to form ammonia (408),(ii) wherein the protons from the third (403) compartment react with the recycled ammonia (408) from the third (403) compartment to form ammonium ions; the ammonium ions then migrate through the cation exchange membrane to the second compartment (402) and react with the sulfate ions to give a solution of ammonium sulfate (409).

[0027] - Ammonium sulfate is finally isolated from the solution by any usual dehydration or desiccation method (e.g. spray drying).

[0028] Description of how the process works

[0029] First, carbon dioxide-rich combustion fumes (101) are directed to an absorber column (100). The carbon dioxide-rich combustion fumes may include atmospheric air or a carbon dioxide flue gas stack from the phosphoric acid production plant or any other carbon dioxide-emitting industry. The carbon dioxide concentration in the rich combustion fumes may be between 1 and 100% of the total gas volume concentrations. The absorber uses an alkali metal hydroxide solvent as its input. The alkali hydroxide solution may include at least sodium hydroxide, lithium hydroxide, potassium hydroxide, or cesium hydroxide.

[0030] Carbon dioxide reacts with the hydroxide solution in the absorber to form an aqueous solution rich in carbon dioxide (107) and releases a smoke depleted in carbon dioxide (103). The alkali carbonate solution (107) may comprise either: sodium carbonate, lithium carbonate, potassium carbonate, or cesium carbonate.

[0031] The alkali carbonate solution (107) is then directed to a carbonation reactor (300). The carbonation reactor takes as input phosphogypsum waste (301) from the phosphate industry. Here, the phosphogypsum is assumed to be cleaned and primarily composed of calcium sulfate. The carbonation reactor (300) extracts the sulfate from the phosphogypsum to form alkali sulfate (303). The alkali sulfate solution (303) can comprise either sodium sulfate, lithium sulfate, potassium sulfate, or cesium sulfate.

[0032] The alkali sulfate is then directed to an electrodialysis stack (400). The electrodialysis stack (400) may include at least one electrodialysis cell.

[0033] Each cell has two cation exchange membranes and one anion exchange membrane, sandwiched between two bipolar membranes. The bipolar membrane has an inner catalyst face and two outer faces that allow the exchange of cations and anions. Applying a voltage across the electrodialysis cell causes a direct current to flow through the bipolar membrane, which dissociates water into hydrogen protons and hydroxide ions. The voltage applied to the bipolar membrane depends on the concentration of hydrogen protons and hydroxide ions (or the pH gradient: V = 0.059 * (PHCEL - PHAEL)).

[0034] As shown in Figure 2, a pH of approximately 6–7 at the cation exchange layer (CEL) is sufficient to convert all the ammonia to ammonium ions (Figure 2a). Similarly, an alkaline solution with a pH of 8.4–9 at the anion exchange layer (AEL) is sufficient to capture all the carbon dioxide as carbonate at the absorption column (Figure 2b). Therefore, a voltage of 0.059–0.17 V must be applied across the bipolar membrane.

[0035] The electrodialysis cell further comprises six compartments: an anode (405); a cathode (406); an alkaline compartment (404) defined between a bipolar membrane and a cation exchange membrane; an acidic compartment (403) defined between a bipolar membrane and a cation exchange membrane; and two additional compartments (401 and 402) each defined between an anion exchange membrane and its adjacent cation exchange membrane.

[0036] One embodiment of the integrated method for the synthesis of ammonium sulfate-based fertilizers includes: supplying the electrodialysis cell with an energy source to generate the necessary electrons in the anode, cathode, and bipolar membranes; passing water through the bipolar membranes to separate it into hydroxide anions (OH-) and hydrogen cations (H+).

[0037] ; the introduction of a nitrogen flow (202) into the electrodialysis anode compartment (405) to reduce it to ammonia (408) using hydrogen cations and electrons supplied by an energy source; thus recycling the ammonia obtained (408) into the acid compartment (403) which neutralizes the acids (H+ cations) leading to the formation of ammonium cations; the ammonium cations pass selectively through the cation exchange membrane into the adjacent compartment (402);The introduction of an aqueous solution of metal sulfate (303) into a first compartment (401), in which metal cations selectively pass through the cation exchange membrane into the adjacent alkaline compartment (404) to regenerate the alkaline metal hydroxide solution for the absorption column (100), and sulfate anions selectively pass through the anion exchange membrane into the adjacent compartment (402) and react with ammonium cations to form an ammonium sulfate solution. At the end, three separate streams exit the electrodialysis process: a solution of alkaline metal hydroxide (407); a pure oxygen gas stream (410); and an ammonium sulfate solution (409). Advantages of the process;

[0038] The integrated ammonium sulfate production process described above has several advantages listed below compared to conventional production routes, but is not limited to them.

[0039] (1) This integrated route is less energy-intensive thanks to the use of an energy-efficient electrodialysis process for the synthesis of ammonium sulfate-based fertilizers. As shown in Figure 3, the integrated route requires 20% less energy compared to conventional processes (Haber-Bosch and electrochemical routes). The electrodialysis process requires approximately 3.18 kWh / kg DA, which comes mainly from ohmic losses in the ion-exchange membranes (1.74 kWh / kg DA). The integrated route avoids the use of energy-intensive PEM electrolysis and the CO2 desorption step, resulting in significant energy savings.Further energy reductions in the integrated route are possible through: the use of more efficient cathode electrocatalysts that kinetically promote the nitrogen reduction reaction for ammonia production; improvements in the stability and efficiency of ion-exchange membranes, including lower resistivity, higher permeability, and greater stability; and the use of more efficient and inert electrolytes that have higher electrolytic conductivities while eliminating the formation of byproducts. (2) This integrated route is less expensive (Figure 4): the invention presented here involves the use of less processing equipment compared to prior art processes. Consequently, the capital expenditure (CAPEX) of the process is lower than that of conventional processes.Furthermore, since the invention does not require the use of PEM electrolyzers and CO2 desorption steps, its operating costs (OPEX) are significantly lower. Overall, the integrated route disclosed herein enables cost savings of over 26% (Figure 4). (3) More flexible: Electrochemical processes are generally better suited to operation powered by renewable energy sources. This flexibility eliminates the need to integrate energy storage with renewable energy, thus reducing the overall system cost. On the other hand, chemical synthesis reactors (such as Haber Bosch) require special attention when powered by intermittent sources, as this can lead to permanent damage to their catalysts.

[0040] (4) In simpler terms: the embodiment described herein can be used in both intermittent and continuous production modes, under ambient temperature and atmospheric pressure conditions. Furthermore, the present invention requires no additional external raw materials other than carbon dioxide, water, electricity, and phosphogypsum. Other necessary reagents (such as ammonia, nitrogen, and alkaline solutions) are produced in-situ within the integrated process, thereby increasing the energy efficiency of the process and reducing transport losses compared to conventional processes.

[0041] (5) The invention further utilizes well-known and commercially available components: ion exchange membranes, catalyst, absorber column, carbonation reactor and air separation unit.

Claims

Demands 1. Integrated process for the production of ammonium sulfate ((NH4)2SO4) by electrodialysis using a source of ammonium cations (NH4) on the one hand, and sulfate anions (SO4) on the other, characterized in that - the source of ammonium cations is purified nitrogen gas extracted either from an industrial gaseous effluent rich in carbon dioxide (CO2), or from the atmosphere, or from any other stream containing N2; - the sulfate cation source is an alkali metal sulfate obtained from a reaction sequence comprising the following steps: i) capture of the carbon dioxide contained in said industrial gaseous effluvium by means of an alkali hydroxide or mixture of alkali hydroxides, by its conversion into alkali carbonate or mixture of alkali carbonates, and then ii) bringing the said alkali carbonate(s) into contact with a source of alkali-earth sulfate or mixture of alkali-earth sulfates and conversion of the latter into alkali sulfate or mixture of alkali sulfates on the one hand, and into alkali-earth carbonate or mixture of alkali-earth carbonates on the other.

2. A method according to claims 1 and 2, wherein the alkali hydroxide is selected from lithium, potassium, sodium and cesium hydroxides, used alone or in combination.

3. A process according to at least one of claims 1 and 2, wherein the alkaline earth sulfate is a calcium sulfate contained in or obtained from gypsum, phosphogypsum, phosphogypsum industrial waste or any equivalent ore containing calcium sulfate, used alone or in combination.

4. A process according to at least one of claims 1 to 3, wherein the reaction sequence according to i) and ii) is carried out in aqueous medium, under conditions ensuring the solubility of the reactants as well as that of the resulting alkali sulfates.

5. A process according to at least one of claims 1 to 4, wherein the alkaline earth sulfate is calcium sulfate (CaSCM) and the alkaline earth carbonate obtained according to ii) is calcium carbonate (CaCOs), the latter being separated from the reaction mixture by precipitation.

6. A method according to at least one of claims 1 to 5, wherein gaseous nitrogen is introduced into the cationic chamber of an electrodialysis stack to be converted therein into ammonium cations (NH4).

7. A method according to at least one of claims 1 to 6, wherein the alkaline sulfate solution is introduced into the anionic chamber of the electrodialysis stack to be converted therein into sulfate anions (SO4).

8. Ammonium sulfate as obtained by means of the process according to claims 1 to 7.

9. Use of ammonium sulfate according to claim 8 as a fertilizing agent, alone or in combination with other fertilizers or active agents or carriers or diluents.

10. A process for capturing and fixing carbon dioxide (CO2) from an industrial gaseous effluent rich in carbon dioxide (CO2), characterized in that it comprises the following reaction steps: - capturing the carbon dioxide contained in said industrial gaseous emanation or from the atmosphere by means of an alkali hydroxide or a mixture of alkali hydroxides, by its conversion into a carbonate or a mixture of alkali carbonates, and then - contacting the said alkali carbonate(s) with a source of sulfate or mixture of alkaline-earth sulfates and converting the latter into sulfate or mixture of alkali sulfates on the one hand, and into carbonate or mixture of alkaline-earth carbonates on the other.

11. A method according to claim 10, wherein the alkali hydroxide is selected from lithium, potassium, sodium and cesium hydroxides, used alone or in combination.

12. A process according to at least one of claims 10 and 11, wherein the alkaline earth sulfate is a calcium sulfate contained in or obtained from gypsum, phosphogypsum, phosphogypsum industrial waste or any equivalent ore containing calcium sulfate, used alone or in combination.

13. A process according to at least one of claims 10 to 12, wherein the reaction sequence according to i) and ii) is carried out in aqueous medium, under conditions ensuring the solubility of the reactants as well as that of the resulting alkali sulfates.

14. A process according to at least one of claims 10 to 13, wherein the alkaline earth sulfate is calcium sulfate (CaSO4) and the alkaline earth carbonate obtained according to ii) is calcium carbonate (CaCOs), the latter being separated from the reaction mixture by precipitation.

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