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) having a concentration of 1-12.5 M 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, recycling (112) the recovered hydrochloric acid (12) to the step (101) of the leaching apatite.

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 quality of the calcium sulphate dihydrate produced. 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 sulphuric acid having a concentration of 1-12.5 M, preferably 5-12.5 M, most preferably 10-12.5 M, 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, recycling the recovered hydrochloric acid to the step of the leaching apatite. The sulphuric acid is suitably added until an amount giving stoichiometric excess of calcium to sulphuric acid has been added to the reactor. The sulphuric acid may be added at multiple feed points. A suitable temperature of the aqueous raffinate containing calcium chloride during precipitation of calcium sulphate dihydrate may be 30-60 °C, to give a desired growth rate for the calcium sulphate dihydrate crystals. The precipitation of calcium sulphate dihydrate suitably takes place in one or more reactor vessels having a volume of 200-1500 m3, to facilitate crystallization. The raffinate containing calcium chloride and the sulphuric acid is fed to an upper part of the vessel, so as to promote the formation of larger crystals. 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.

[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 a process scheme according to the present invention.

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

[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. 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. This regeneration prevents the formation of liquid effluents (bleed). Likewise, the soluble phosphorus which may remain in this solution is recovered as it can be recycled as phosphoric acid.

[0014] 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, i.e. comprising less than 5% wt, preferably less than 1 %wt, of phosphoric acid, to be suitable for recycling. 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 remaining chloride ions after precipitation of calcium sulphate dihydrate are recovered in the form of hydrochloric acid. If desired, 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 hydrochloric acid is recycled to the step of leaching apatite.

[0015] It is common in the art to perform the precipitation with concentrated sulphuric acid (18M), since that has been regarded as most efficient. However, it may be a viable alternative to perform the precipitation of calcium sulphate dihydrate using sulphuric acid having a concentration of 1-12.5 M, since this can balance the requirements of obtaining high quality calcium sulphate dihydrate. Using sulfuric acid within this range helps ensure that the precipitation occurs gradually, leading to better crystal formation and purity of the gypsum. The sulphuric acid may suitably have a concentration of 5-12.5 M to somewhat speed up the process. Most preferably a concentration of 10-12.5 M for the sulphuric acid may give a high quality calcium sulphate dihydrate and reduce rapid exothermic reactions. The sulfuric acid solution may contain gypsum washing water that is returned back to the process, and can be mixed into the sulfuric acid solution just at the entry point of sulphuric acid to precipitation reactor.

[0016] A critical parameter for the continuous operation of the integrated loop is the concentration of the sulphuric acid used in the precipitation step. It has been determined that a sulphuric acid concentration in the range of 1 M to 12.5 M provides a technical optimum that overcomes the limitations of using highly concentrated industrial acid (approx. 18 M) or dilute acid (e.g., <1 M).

[0017] The use of sulphuric acid with a concentration below 1 M is generally disadvantageous for the integrated process. Introducing significant volumes of water at this stage dilutes the recovered hydrochloric acid in the raffinate. Since this recovered acid is recycled directly to the leaching step (where a concentration of typically 10-30 wt% is required), excessive dilution would necessitate energy-intensive evaporation of the recycled stream to restore its leaching potency. Therefore, a concentration of at least 1 M, and more preferably at least 5 M, is maintained to preserve the water balance of the closed loop without excessive energy penalties. Conversely, the use of standard concentrated sulphuric acid (e.g., 98% or approx. 18 M) may create regions of extreme local supersaturation at the injection point. This rapid supersaturation can lead to 'flash precipitation' of extremely fine calcium sulphate particles or, in severe cases, the formation of a viscous, gel-like gypsum phase that is difficult to filter and wash. By limiting the concentration to a maximum of 12.5 M, the process ensures that the acid disperses sufficiently fast into the calcium chloride solution. This controlled dispersion moderates the local supersaturation levels, promoting the growth of larger, well-defined dihydrate crystals (e.g., needles or platelets) rather than amorphous fines or gels.

[0018] As mentioned, the sulphuric acid concentration may preferably be maintained between 10 M and 12.5 M. This specific range represents a synergistic operating window that is not immediately apparent from the prior art. First, it minimizes the water input to the lowest practical level, thereby maximizing the concentration of the recovered hydrochloric acid. Second, unlike concentrations above 12.5 M, this range allows for manageable viscosity and mixing dynamics that prevent the formation of gel-like phases, provided that adequate agitation is employed. Further, the exothermic heat of dilution generated by 10-12.5 M acid is sufficient to maintain the reactor temperature within the optimal crystallization range of 30-60°C, without inducing the violent boiling or uncontrolled thermal spikes associated with 18 M acid.

[0019] Thus, the selection of the 1-12.5 M range, and specifically the 10-12.5 M sub-range, serves the dual technical purpose of maintaining the process water balance while ensuring the morphological quality and filterability of the calcium sulphate dihydrate product.

[0020] The sulphuric acid having a concentration of 1-12.5 M is suitably added to multiple feed points until an amount giving stoichiometric excess of calcium to sulphuric acid has been added to the reactor. By adding sulfuric acid at multiple feed points to the calcium chloride solution to ensure a controlled reaction, keeping control of heat generation and localized supersaturation. The sulphuric acid can be added in a slow, steady stream while continuously stirring the solution can help maintain uniform mixing and prevent hot spots. The reaction of the raffinate containing calcium chloride with sulphuric acid may suitably involve agitation of the raffinate to improve crystallisation.

[0021] Larger crystals often settle more rapidly and can be easier to filter, thus allowing efficient filtration, reducing processing time and costs. Larger crystals may also have a more regular and well-defined structure, which may result in a purer product with fewer impurities or defects. Nucleation is an initial stage of crystal formation, where the crystals can nucleate from a supersaturated solution, where there is an excess of calcium and sulphate ions. The nucleation process involves the assembly of these ions into a stable crystal structure. Higher supersaturation levels in certain regions may promote the formation of numerous small nuclei, potentially limiting the maximum size of individual crystals. Once nucleation occurs, the crystals continue to grow by the addition of more ions from the surrounding solution. By adding sulphuric acid in lower concentration in multiple feed points, the crystal growth may proceed more slowly, allowing crystals to grow larger before reaching equilibrium.

[0022] The growth of crystals may also 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 may be 30-80 °C, preferably 30-60 °C. This temperature range may improve stable formation of the dihydrate form (gypsum) rather than hemihydrate (which might form at higher temps) or slow reaction kinetics (at lower temps). The temperature during precipitation may be 30-50 °C to minimize formation of metastable calcium sulphate phases. At an upper end of the temperature range, such as 70-80 °C, it may be beneficial to apply residence times of 6-10 hours to ensure complete conversion to the dihydrate end phase.

[0023] 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. The sulphuric acid is suitably fed to the uppermost 50% of the vessel first reactor vessel and may suitably be split into 2-4 inlets around an upper circumference of the reactor vessel to avoid concentrated reaction zones and ensure uniform bulk supersaturation. Further, the sulphuric acid may be introduced below an impeller blade to achieve instantaneous dilution and mixing, suppressing anhydrite-favoring microenvironments. Sulphuric acid injection point may suitably be 10-50 centimetres below an impeller blade.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.

[0025] Figure 1 shows a process scheme illustrating the process of the present invention. 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:

[0026] - 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;

[0027] - 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;

[0028] - 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] - Recycling 112 the recovered hydrochloric acid 12 to the step 101 of the leaching apatite.

[0032] The reaction of the raffinate 21 containing calcium chloride with sulphuric acid 26 preferably involves agitation of the raffinate to improve crystallisation.

[0033] 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.

[0034] 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 (Ars) 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.

[0035] 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.

[0036] The aqueous raffinate 21 containing calcium chloride may be evaporated 106 as needed to keep the water balance in the system.

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) having a concentration of 1-12.5 M 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;- Recycling (112) the recovered hydrochloric acid (12) to the step (101) of the leaching apatite.

2. The process of claim 1, wherein the sulphuric acid (26) has a concentration of 5-12.5 M.

3. The process of claim lor 2, wherein the sulphuric acid (26) is added gradually to the aqueous raffinate (21) containing calcium chloride until an amount giving stoichiometric excess of calcium to sulphuric acid has been added to the reactor.

4. The process of any one of claims 1-3, wherein the temperature of the aqueous raffinate (21) containing calcium chloride during precipitation (107) of calcium sulphate dihydrate is 30-80 °C.

5. The process of any one of claims 1-4, wherein the precipitation of calcium sulphate dihydrate takes place in one or more vessels having a volume of 200-1500 m3.

6. The process of claim 5, wherein said raffinate containing calcium chloride and sulphuric acid are fed to an upper part of the one or more vessels, preferably to the upper 50% of the reaction vessel.

7. The process of claim 1, wherein the washing of precipitated calcium sulphate dihydrate is performed using counter-current flow.

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