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), wherein the reaction of the raffinate (21) containing calcium chloride with sulphuric acid (26) involves intermittent agitation of the raffinate, 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, especially 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.

[0006] 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.

[0007] Summary

[0008] 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, preferably 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 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, wherein the reaction of the raffinate containing calcium chloride with sulphuric acid involves intermittent agitation of the raffinate, 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.

[0009] The intermittent agitation of the raffinate during the reaction with sulphuric acid allows for control of the size and morphology of the precipitated calcium sulphate dihydrate crystals, and can thus give larger crystals, having higher purity. This will be described in more detail below.

[0010] The intermittent agitation of the raffinate preferably involves an agitation sequence at least including a first period of agitation, followed by a second period of no agitation, said agitation sequence being repeated until substantially all calcium has been precipitated as calcium sulphate dihydrate, to alternatingly allow re-distribution of the substances in the thereby improving crystal growth. The first period of agitation preferably continues for 5-30 minutes, and the second period of no agitation preferably continues for 5-60 minutes. A suitable temperature of the aqueous raffinate containing calcium chloride during precipitation of calcium sulphate dihydrate may be 30-80 °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 vessels may preferably have a diameter to height ratio of 1:2 to 1:10 and 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. Brief descriptions of the drawings

[0011] 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.

[0012] Figure 1 shows a process scheme according to the present invention.

[0013] Figure 2 illustrates an example of an intermittent agitation sequence.

[0014] Figure 3 shows a process scheme for a process according to the present invention, including additional steps.

[0015] Detailed description

[0016] 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.

[0017] 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 phosphorus 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.

[0018] 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 remaining chloride ions after precipitation of calcium sulphate dihydrate are recovered in the form of hydrochloric acid. The reaction of the raffinate containing calcium chloride with sulphuric acid involves intermittent agitation of the raffinate. 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. The intermittent agitation of the raffinate containing calcium chloride during the reaction with sulphuric acid allows for control of the size and morphology of the precipitated calcium sulphate dihydrate crystals, and can thus give larger crystals, having higher purity. This will be described in more detail below. The sulphuric acid having a concentration of 1-18 M is added in an amount giving stoichiometric excess of calcium to sulphuric acid in the reactor. In some situations, it may be beneficial to use sulphuric acid of 1-12.5 M, especially 5-12.5 M, and particularly 10-12.5. In other situations, concentrated sulphuric acid (15-18 M) may be preferred.

[0019] The intermittent agitation of the raffinate containing calcium chloride during the reaction with sulphuric acid preferably involves an agitation sequence at least including a first period of agitation, followed by a second period of no agitation, said agitation sequence being repeated until substantially all calcium (preferably 90 wt% or more of the calcium in the calcium chloride solution) has been precipitated as calcium sulphate dihydrate, to alternatingly allow redistribution of the substances in the thereby improving crystal growth. The first period of agitation preferably continues for 5-30 minutes helps in ensuring a uniform reaction and promotes the mixing of reactants. The second period of no agitation preferably continues for 5- 60 minutes to allow the crystals to grow to a desired extent. 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. During acid addition, agitation power input may be maintained at 0.5-1.0 W / kg (e.g., 80-120 rpm for typical industrial impellers) to ensure homogenization. Immediately after dosing, agitation is reduced to minimal levels for 10-30 minutes, permitting orderly dihydrate crystal growth. Pulsed agitation cycles, for example with agitation ON for 3-15 minutes suitably at 0.3-0.7 W / kg, followed by OFF for 20-45 minutes, to limit secondary nucleation, reduce crystal breakage, and enable phase-pure gypsum formation especially at temperature >50 °C. It may further be advantageous to apply elevated impeller speed for the first agitation period (e.g. first 4-6 minutes) to homogenize reactants, then reduce to 20-40 rpm during crystal growth, to minimize shear-induced secondary nucleation and favor dihydrate crystal precipitation.

[0020] The growing of the calcium sulphate dihydrate crystals is believed to be a result of Ostwald ripening, which is a phenomenon in which larger particles grow at the expense of smaller particles in a dispersed system.

[0021] 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, preferably a supersaturation ratio of 1.05-1.20 in the bulk liquid. The nucleation process involves the assembly of these ions into a stable crystal structure. The absence of stirring during the second period can lead to localized variations in supersaturation, affecting the nucleation rate. 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. During the repeated periods without stirring, 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.

[0023] The temperature of the aqueous raffinate containing calcium chloride during precipitation of calcium sulphate dihydrate is preferably maintained within a range that ensures reliable formation of the dihydrate phase of calcium sulphate. 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.

[0024] 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 have a diameter (D) to height (H) ratio of 1:2 to 1:10, preferably 1:2-1:1, 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.

[0025] 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 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. When precipitation is carried out in two or more reactor vessels in series, a first reactor vessel (nucleation zone) may provide 0.5-1.0-hour residence, followed by one or more growth reactor vessels providing e.g. 3-10 hours combined residence time to secure the desired phase purity for the calcium sulphate dihydrate. Sulphuric acid feed 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-favouring microenvironments. Sulphuric acid injection point may suitably be 10-50 centimetres below an impeller blade.

[0026] 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.

[0027] 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, preferably apatite, and sulphuric acid using hydrochloric acid as an intermediate attacking agent, comprises the steps of:

[0028] - 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;

[0029] - 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;

[0030] - 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;

[0031] - 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, wherein the reaction of the raffinate 21 containing calcium chloride with sulphuric acid 26 involves intermittent agitation of the raffinate,

[0032] - Filtering off 108 precipitated calcium sulphate dihydrate 27 from the previous step 107 and washing it 109 with water 28;

[0033] - Recycling 112 the recovered hydrochloric acid 12 to the step 101 of the leaching apatite.

[0034] The intermittent agitation of the raffinate 21 suitably involves an agitation sequence 120 at least including a first period 121 of agitation, followed by a second period 122 of no agitation, as illustrated in Fig. 2, said agitation sequence being repeated until substantially all calcium has been precipitated as calcium sulphate dihydrate. Said first period 121 of agitation may continue for 5-30 minutes and said second period of no agitation 122 may continue for 5-60 minutes. The washing 109 of precipitated calcium sulphate dihydrate is preferably performed using countercurrent 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.

[0035] As illustrated in Fig. 3, 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. 3, 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.

[0036] 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.

[0037] 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) 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), wherein the reaction of the raffinate (21) containing calcium chloride with sulphuric acid (26) involves intermittent agitation of the raffinate,- 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 intermittent agitation of the raffinate (21) involves an agitation sequence (120) at least including a first period (121) of agitation, followed by a second period (122) of no agitation, said agitation sequence being repeated until substantially all calcium has been precipitated as calcium sulphate dihydrate.

3. The process of claim 2, wherein said first period (121) of agitation continues for 5-30 minutes and said second period of no agitation (122) continues for 5-60 minutes.

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 vessels have a diameter to height ratio of 1:2 to 1:10 and the raffinate containing calcium chloride and the sulphuric acid are fed to an upper part of the 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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