A process for obtaining phosphoric acid and calcium sulphate dihydrate from apatite

Abstract

The disclosure relates to a continuous process for obtaining phosphoric acid and calcium sulphate dihydrate from apatite, and sulphuric acid using hydrochloric acid as an intermediate attacking agent, comprising the steps: Leaching (101) apatite (11) with an aqueous solution of hydrochloric acid (12,13) to solubilize phosphorus compounds contained in the apatite, thereby obtaining a solution (15) containing phosphate (H3PO4) and calcium chloride (CaCl2); extracting (103) said solution (15) with organic solvent (16), yielding an organic extract (17) loaded with phosphate ions, and an aqueous raffinate (21) containing calcium chloride and residual impurities; re-extracting the phosphate ions from the organic extract (17) with water (18), thus yielding an aqueous extract (19) of phosphoric acid, and an unloaded organic phase substantially free of phosphoric acid, reacting (107) the raffinate (21) containing calcium chloride with sulphuric acid (26) to cause precipitation of calcium ions in the raffinate as calcium sulphate dihydrate and to recover chloride ions in the raffinate in the form of hydrochloric acid (12), filtering off (108) precipitated calcium sulphate dihydrate (27) from the previous step (107) and washing it with water, dividing (113) the remaining hydrochloric acid (12) aqueous solution into a primary stream (12a) and a secondary stream (12b); and recycling (112) the primary stream (12a) of the aqueous solution of hydrochloric acid (12) to the step (101) of the leaching apatite, and at least partially removing the secondary stream (12b) from the process.

Description

[0001] A process for obtaining phosphoric acid and calcium sulphate dihydrate from apatite

[0002] Technical field

[0003] The present disclosure relates to a process for obtaining phosphoric acid and calcium sulphate dihydrate from apatite and sulphuric acid, using hydrochloric acid as an intermediate attacking agent.

[0004] Background

[0005] The hydrochloric acid route for phosphate rock is a method that overcomes the limitations of traditional methods for obtaining phosphoric acid from phosphate rock. In this process, phosphate rock, apatite, is leached with hydrochloric acid, which results in the formation of water-soluble calcium chloride and phosphoric acid. The advantage of this method is that it does not require the exhaustive grinding that is necessary in traditional methods, which use sulphuric acid, where gypsum may precipitate on the surface of the apatite grains, thereby hampering the acid attack. However, using hydrochloric acid in place of sulphuric acid overcomes these limitations. The calcium chloride produced is soluble in water, allowing for the breakdown of minerals without restrictions on the initial particle size. By addition of sulphuric acid to the calcium chloride, the process also gives a considerable amount of calcium sulphate dihydrate, which is a valuable product for example within the building industry or in agriculture.

[0006] Summary

[0007] It is an object of the present invention to improve the process, in particular in order to improve the quality of the calcium sulphate dihydrate produced. Although, the quality of the calcium sulphate dihydrate obtained through the hydrochloric acid route is already considerably higher than that obtained from sulphuric acid route for apatite, there is an ongoing interest in improving the process and to keep control of the flow volumes of steps included in the process. According to the present invention there is provided a continuous process for obtaining phosphoric acid and calcium sulphate dihydrate from apatite, and sulphuric acid using hydrochloric acid as an intermediate attacking agent, comprising the steps of: leaching apatite with an aqueous solution of hydrochloric acid to solubilize phosphorus compounds, e.g. in the form of calcium phosphate, contained in the apatite, thereby obtaining a solution containing phosphate and calcium chloride; extracting the solution with organic solvent, yielding an organic extract loaded with phosphate ions, and an aqueous raffinate containing calcium chloride and residual impurities; re-extracting the phosphate ions from the organic extract with water, thus yielding an aqueous extract of phosphoric acid, and an unloaded organic phase substantially free of phosphoric acid, reacting the raffinate containing calcium chloride with to cause precipitation of calcium ions in the raffinate as calcium sulphate dihydrate and to recover chloride ions in the raffinate in the form of hydrochloric acid, filtering off precipitated calcium sulphate dihydrate from the previous step and washing it with water, dividing the remaining hydrochloric acid aqueous solution into a primary stream and a secondary stream, and recycling the primary stream of the aqueous solution of hydrochloric acid to the step of the leaching apatite, and at least partially removing the secondary stream from the process.

[0008] Brief descriptions of the drawings

[0009] The present invention will be more fully appreciated by reference to the following illustrative and non-limiting detailed description of example embodiments, when taken in conjunction with the accompanying drawings.

[0010] Figure 1 shows an overview of the process for obtaining phosphoric acid and calcium sulphate dihydrate from apatite.

[0011] Figure 2 shows a process scheme for a process according to the present invention.

[0012] Detailed description

[0013] The present disclosure will become apparent from the detailed description given below. The detailed description and specific examples disclose preferred embodiments of the disclosure by way of illustration only. Those skilled in the art understand from guidance in the detailed description that changes and modifications may be made within the scope of the disclosure.

[0014] The process for production of phosphoric acid from phosphate rock, apatite, using hydrochloric acid or mixtures of hydrochloric acid as intermediate attacking agent was developed in the 1980:s by Habashi et.al., see e.g. "The Hydrochloric Acid Route for Phosphate Rock"; Habashi et.al; J. Chem. Tech. Biotechnol. 1987, 38, 115-126. Phosphate rock is a sedimentary phosphorus-bearing rock that contains at least 15% phosphorus on the basis of weight, often more, such as up to 40%. The phosphorus content in these rocks is mainly derived from the presence of apatite minerals. Apatite is a group of phosphate minerals, present in phosphate rock, usually as hydroxyapatite, fluorapatite, or chlorapatite, with high concentrations of OH-, F-, and Cl- ions, respectively, in the crystal. The leaching or attack of the phosphate rock with a strong mineral acid converts the phosphate mineral, apatite, into a soluble form. The solubilization of phosphorus, e.g. in the form of calcium phosphate, with hydrochloric acid is carried out by means of reactions solubilizing the phosphorus in the form of phosphoric acid or monocalcium phosphate. The concentration and the quantity of hydrochloric acid used basically depends on the desired concentration of the liquor obtained after leaching and on the content of phosphorous in the apatite used as raw material. Another advantage of the process is the regeneration of the attacking hydrochloric acid by means of the addition of the sulphuric acid to convert the calcium chloride formed in the reaction into calcium sulphate dihydrate, once the H3PO4 has been removed from the aqueous medium in an extraction step using solvents (preferably an organic solvent such as tributyl phosphate). Thus, the raw materials used in the procedure are the apatite and sulphuric acid. Likewise, the soluble phosphorus which may remain in this solution is recovered as it can be recycled as phosphoric acid.

[0015] According to the present invention there is provided a continuous process for obtaining phosphoric acid and calcium sulphate dihydrate from apatite, and sulphuric acid using hydrochloric acid as an intermediate attacking agent. Apatite is leached with an aqueous solution of hydrochloric acid, suitably having a concentration of 10-30 weight % to solubilize phosphorus compounds, such as calcium phosphate contained in the apatite. The solution obtained after leaching contains phosphate and calcium chloride, which is extracted with an organic solvent to retrieve the phosphoric acid. The extraction yields an organic extract, loaded with phosphate ions, and an aqueous raffinate, containing calcium chloride and residual impurities. The phosphate ions are re-extracted with water from the organic extract, thus yielding an aqueous extract of phosphoric acid, and an unloaded organic phase substantially free of phosphoric acid. After the extraction step, the raffinate containing calcium chloride is reacted with sulphuric acid to cause precipitation of calcium ions in the raffinate as calcium sulphate dihydrate (gypsum). The sulphuric acid, typically having a concentration of 1-18 M preferably 15-18 M, is added in an amount giving stoichiometric excess of calcium to sulphuric acid in the reactor, suitably while at least intermittently stirring the solution. In some situations, it may be beneficial to use sulphuric acid of 1-12.5 M, e.g. 5-12.5 M or 10-12.5. In other situations, more concentrated sulphuric acid (15-18 M) may be preferred. The remaining chloride ions after precipitation of calcium sulphate dihydrate are recovered in the form of an aqueous solution of hydrochloric acid. The reaction of the raffinate containing calcium chloride with sulphuric acid may involve intermittent agitation of the raffinate to control of the size and morphology of the precipitated calcium sulphate dihydrate crystals, and can thus give larger crystals, having higher purity. The precipitated calcium sulphate dihydrate is then filtered off and washed with water. The recovered aqueous solution of hydrochloric acid is recycled to the step of leaching apatite.

[0016] After the precipitation of calcium sulphate, the remaining solution primarily consists of hydrochloric acid. Once the precipitated calcium sulphate dihydrate has been removed by filtration, the remaining solution, which is an aqueous solution of hydrochloric acid, is divided in a primary stream that is recycled back to the leaching of apatite, and a secondary stream that is at least partially removed from the process, to keep control of the volumes in the process. Thereby, there will be no need for removing water from the process at any other position in the process and all valuable elements and compounds in the various process streams can thus be recovered. The removal of the secondary stream further serves as a selective purging mechanism for multivalent metallic cations and other soluble impurities that are not effectively removed during the solvent extraction or primary gypsum precipitation stages. While the primary stream is optimized for reagent recycling to the leaching step, the secondary stream prevents the progressive, cumulative buildup of non-target ions, such as aluminium, magnesium, and manganese, within the process loop. This purge ensures that the ionic strength and viscosity of the circulating hydrochloric acid solution remain within specified steady-state parameters, thereby preventing the degradation of leaching kinetics and avoiding coprecipitation of these impurities into the final calcium sulphate dihydrate product. Accordingly, an improved process is maintained wherein efficient leaching is ensured, and at the same time the quality of the resulting calcium sulphate dihydrate is improved.

[0017] The volumetric division between the primary stream and the secondary stream is a critical operational parameter that is dynamically adjusted based on the specific composition of the raw apatite and the desired purity of the resulting calcium sulphate dihydrate. The secondary stream is suitably 8-25 % of the total combined volume of the primary stream and the secondary stream to enable prevention of impurity accumulation and control overall water balance in the process.

[0018] When a primary operational objective is the prevention of impurity accumulation, specifically multivalent cations such as aluminium, magnesium, and manganese, the secondary stream is preferably maintained at approximately 8-12% volume. This specific ratio is suitable to purge non-target soluble cations, particularly aluminium, magnesium, and manganese, at a rate that prevents their concentration from reaching levels that would otherwise inhibit the crystallization kinetics of the calcium sulphate dihydrate or increase the viscosity of the solution to a degree that impairs solvent extraction performance. Furthermore, especially in the absence of an evaporation stage for the aqueous phosphoric acid, the secondary stream can serve as a primary water-balance regulator for the entire process loop. The volumetric flow of the secondary stream is specifically calibrated to offset the cumulative water ingress from the hydrochloric acid, the sulphuric acid, and the gypsum wash water. By adjusting the secondary stream removal to between 15% - 25% of the total regenerated acid volume, the process can maintain a constant system volume and a stable steady-state concentration of hydrochloric acid without requiring thermal evaporation, which significantly reduces the energy intensity of the operation, and at the same time accumulation of impurities can be prevented as discussed above.

[0019] Before the secondary stream is discarded, it is preferably treated to recover valuable chemical elements. Sulphuric acid may be added to the secondary stream to recover HCI therein. This is done by means of addition 114 of sulfuric acid, preferably in an approximately stoichiometric ratio to the hydrochloric acid which is present in the secondary stream. Sulphuric acid acts as a dehydrating agent to concentrate the aqueous solution of hydrochloric acid. Preferably, sulphuric acid of high concentration such as 15-18 M is preferred to avoid unnecessary addition of water in this step. The sulphuric acid is added to the aqueous solution of hydrochloric acid, slowly and with constant stirring, to ensure even mixing and to control exothermic reaction. In the reaction, the sulphuric acid binds with water to form sulphuric acid monohydrate, and HCI gas is released:

[0020] HCI (aq) + H2SO4 - HCI (g) + H2SO4-H2O

[0021] The released HCI gas can be collected and re-dissolved in water to form a more concentrated hydrochloric acid solution. This step can suitably be performed in a gas absorption equipment where the HCI gas is bubbled through water to achieve the desired concentration. The more concentrated hydrochloric acid solution may the suitably be recycled back to the process and be used to top up the primary stream of hydrochloric acid or alternatively be sold off.

[0022] Sulfuric acid monohydrate is a highly concentrated form of sulfuric acid with a small amount of water typically having a concentration of 15-16 M and may thus advantageously be recycled back to process and be used in the step of precipitation of calcium sulphate, or alternatively be sold off.

[0023] The growth of crystals may be influenced by factors such as temperature and concentration, which can affect the size, shape, and quality of the gypsum crystals. Lower temperatures generally result in slower crystal growth, while higher temperatures can lead to faster growth. A suitable temperature of the aqueous raffinate containing calcium chloride during precipitation of calcium sulphate dihydrate is 30-60 °C. The precipitation of calcium sulphate dihydrate suitably takes place in one or more reactor vessels having a volume of 200- 1500 m3, preferably 800-1200 m3. Large vessels with greater surface area can accommodate more crystal growth without encountering physical constraints. The vessels may preferably have a diameter (D) to height (H) ratio of 1:2 to 1:1, in order to balance settling time with mixing efficiency. This is advantageous because it provides a sufficient settling height for the crystals to grow as they descend, while maintaining a cross-sectional area that ensures uniform upward flow velocity of the liquid phase, thereby preventing short-circuiting of the feed to the outlet. Alternatively, the D / H ratio may preferably be 1.0-1.8, more preferably 1.2-1.5 to minimize dead zones without requiring excessive mixing power.

[0024] The raffinate containing calcium chloride and the sulphuric acid is suitably fed to an upper part of the vessel, so as to promote the formation of larger crystals by allowing for, increased residence time, longer diffusion paths and more gradual changes in concentration gradients along the height of the vessel. High vessels can also minimize stagnant regions where reactants may not mix effectively heat transfer and provide improved temperature control due to increased surface area.

[0025] The washing of precipitated calcium sulphate dihydrate may advantageously be performed using counter-current flow, to obtain efficient washing and avoid unnecessary consumption of water.

[0026] Figure 1 shows an overview of the process for obtaining phosphoric acid and calcium sulphate dihydrate from apatite. The continuous process for obtaining phosphoric acid and calcium sulphate dihydrate from apatite, and sulphuric acid using hydrochloric acid as an intermediate attacking agent, comprises the steps of:

[0027] - Leaching 101 apatite 11 with an aqueous solution of hydrochloric acid 12,13 to solubilize phosphorus , e.g. in the form of calcium phosphate, contained in the apatite, thereby obtaining a solution 15 containing phosphate H3PO4 and calcium chloride CaCh;

[0028] - Extracting 103 the solution 15 with organic solvent 16, yielding an organic extract 17 loaded with phosphate ions and an aqueous raffinate 21 containing calcium chloride and residual impurities; - Re-extracting 110 the phosphate ions from the organic extract 17 with water 18, thus yielding an aqueous extract 19 of phosphoric acid, and an unloaded organic phase substantially free of phosphoric acid;

[0029] - Reacting 107 the raffinate 21 containing calcium chloride with sulphuric acid 26 to cause precipitation of calcium ions in the raffinate as calcium sulphate dihydrate and to recover chloride ions in the raffinate in the form of hydrochloric acid 12,

[0030] - Filtering off 108 precipitated calcium sulphate dihydrate 27 from the previous step 107 and washing it 109 with water 28;

[0031] - Dividing 113 the remaining hydrochloric acid 12 aqueous solution into a primary stream 12a and a secondary stream 12b;

[0032] - Recycling 112 the primary stream 12a of the aqueous solution of hydrochloric acid 12 to the step 101 of the leaching apatite, and at least partially removing the secondary stream 12b from the process.

[0033] Sulphuric acid 35 can be added 114 to at least a part of the secondary stream 12b of aqueous solution of hydrochloric acid to form sulphuric acid monohydrate 31 and gaseous hydrochloric acid 32. The gaseous hydrochloric acid 32 thus obtained can be absorbed 115 in water 33 to form an aqueous hydrochloric acid solution 34 of higher concentration than in the primary stream 12b. This aqueous hydrochloric acid solution 34 of higher concentration can be forwarded to storage, and then be sold off, and thus be removed from the process, or alternatively, advantageously be recycled in the process by adding it to the step of leaching 101 of apatite 11.

[0034] The sulphuric acid monohydrate 31 can be forwarded to storage and then be sold off and thus be removed from the process, or alternatively, advantageously at least partially, be recycled back to the process at the step 107 of reacting the raffinate 21 containing calcium chloride with sulphuric acid.

[0035] The washing 109 of precipitated calcium sulphate dihydrate is preferably performed using counter-current flow. The leaching step 101 may involve of precipitation of fluoride. The fluorine precipitate is suitably separated together with non-dissolved residues 14 of the apatite, e.g. by filtration.

[0036] As illustrated in Fig. 2, aqueous extract 19 of phosphoric acid obtained after the reextraction step 110 may be evaporated 111 to give a more concentrated phosphoric acid 20. As further illustrated in Fig. 2, the process may further include precipitation 102 of arsenic (As) 30 by adding a sulphide 29, such as sodium sulphide (Na2S) or hydrogen sulphide (H2S), and separating the precipitate 30, e.g. by filtration. The process may further include precipitation step 104 of rare earth elements (REE) by adding precipitation agent 22, e.g. a base, such as calcium hydroxide or calcium carbonate, to the raffinate 21, said precipitation step 104 including separation of the REE precipitate 23, e.g. by filtration. A step 105 of removing further impurities, such as aluminium, magnesium or manganese may be included, and may involve adding 24 a suitable additive, and separating the precipitate 25, e.g. by filtration.

[0037] The physical removal of the secondary stream 12b operates in technical synergy with the chemical precipitation of impurities 105. By continuously removing a portion of the aqueous phase, the process ensures that even those impurities that do not reach their solubility limit for precipitation are purged from the system. This dual-action impurity management is vital for maintaining the morphology and crystal size of the calcium sulphate dihydrate 27, as it limits the concentration of lattice-distorting ions that would otherwise impair the quality of the commercial-grade gypsum. Accordingly, calcium sulphate dihydrate of improved quality is obtained from the process.

[0038] Performing the precipitation 102 of arsenic prior to the extraction step 103 can provide the technical advantage of preventing arsenic from co-extracting into the organic phase, thereby ensuring the production of high-purity phosphoric acid suitable for sensitive applications. Furthermore, removing rare earth elements 104 and other impurities such as aluminium, magnesium, or manganese 105 from the raffinate 21 before the precipitation of calcium sulphate dihydrate 107 is critical for the quality of the by-product. This sequence reduces the risk of these metal impurities co-precipitating with the calcium sulphate, so that a calcium sulphate dihydrate product of high whiteness and purity is obtained, which is suitable for use in the building industry, while also preventing the build-up of impurities in the recycled hydrochloric acid loop.

Claims

CLAIMS1. A continuous process for obtaining phosphoric acid and calcium sulphate dihydrate from apatite, and sulphuric acid using hydrochloric acid as an intermediate attacking agent, comprising the steps:- Leaching (101) apatite (11) with an aqueous solution of hydrochloric acid (12,13) to solubilize phosphorus compounds contained in the apatite, thereby obtaining a solution (15) containing phosphate (H3PO4) and calcium chloride (CaCh);- Extracting (103) the solution (15) with organic solvent (16), yielding an organic extract (17) loaded with phosphate ions and an aqueous raffinate (21) containing calcium chloride and residual impurities;- Re-extracting the phosphate ions from the organic extract (17) with water (18), thus yielding an aqueous extract (19) of phosphoric acid, and an unloaded organic phase substantially free of phosphoric acid;- Reacting (107) the raffinate (21) containing calcium chloride with sulphuric acid (26) to cause precipitation of calcium ions in the raffinate as calcium sulphate dihydrate and to recover chloride ions in the raffinate in the form of an aqueous solution of hydrochloric acid (12),- Filtering off (108) precipitated calcium sulphate dihydrate (27) from the previous step (107) and washing it with water;- Dividing (113) the remaining hydrochloric acid (12) aqueous solution into a primary stream (12a) and a secondary stream (12b);- Recycling (112) the primary stream (12a) of the aqueous solution of hydrochloric acid (12) to the step (101) of the leaching apatite, and at least partially removing the secondary stream (12b) from the process.

2. The process of claim 1, wherein the secondary stream (12b) constitutes 8-25 % by volume of the total combined volume of the primary stream (12a) and the secondary stream (12b).

3. The process of claim 1 or 2, wherein sulphuric acid (35) is added (114) to the secondary stream (12b) of aqueous solution of hydrochloric acid to form sulphuric acid monohydrate (31) and gaseous hydrochloric acid (32).

4. The process of claim 3, wherein the gaseous hydrochloric acid (32) is absorbed (115) in water (33) to form an aqueous hydrochloric acid solution (34).

5. The process of claim 4, wherein the aqueous hydrochloric acid solution (34) is recycled by adding it to the step of leaching of apatite (11).

6. The process of any one of claims 3-5, wherein the sulphuric acid monohydrate (32) is at least partially recycled back to the process at the step of reacting (107) the raffinate (21) containing calcium chloride with sulphuric acid.

7. The process of claim 6, wherein sulphuric acid monohydrate (32) is at least partially removed from the process.

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