FI59077B - FOERFARANDE FOER RENING AV VAOTPROCESSFOSFORSYRA - Google Patents

Description

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Patent · och ragiltantyralaan '' AnsOkan utlagd och uti.skriftan publkarad 27.02. oi (32) (33) (31) Requested atuolkaui — B «| trd pHoritat 26.0U.72 02.IO.72 England-England (GB) 19U76 / 72, U5380 / 72 ^ (71) Albright & Wilson Limited, 1 Knightsbridge Green, London SW1X 7QD,

England-England (GB) (72) Thomas Alan Williams, Beckermet, Cumberland, Frank Michael Cussons,

Little Broughton, Cumberland, England-England (GB) (7U) Oy Kolster Ab (5U) Method for the purification of wet process phosphoric acid - Förfarande för rening av vätprocessfosforsyra

The invention relates to a process for the purification of wet process phosphoric acid, for example suitable for foodstuffs, by a multi-stage process. In the wet process, sulfuric acid is reacted with a phosphate mineral to give phosphoric acid. Such an acid is called "wet process phosphoric acid".

Proposals for the purification of wet process phosphoric acid by solvent extraction are at least Uo years old. The solvents originally proposed (e.g., in U.S. Patent 1,968,5UU) were water-miscible (e.g., alcohols and acetone) that require distillation to liberate the purified acid. Alternatively, water-immiscible, short-chain alcohols were proposed, the efficient extraction of which required several steps.

In recent years, attempts have been made to introduce solvent extraction methods for wet process phosphoric acid. However, these are largely limited to processes (such as the process of GB patent 805 517 using butanol and the process of GB patent 953 378 using alkyl phosphates), 2,59077 in which either chloride ions are specifically added to the acid or chloride ions are present in the production of phosphoric acid using hydrochloric acid instead of sulfuric acid. The presence of chloride ions promotes salting out of solution, which contributes to the transition of phosphoric acid to the organic phase.

Some recent proposals concern the use of certain ethers as solvents, which ethers extract phosphoric acid from aqueous solutions when the phosphoric acid content of the solution exceeds the said limit of 35% H 2 PO 2, but do not extract any acid below this limit. The first of these proposals is contained in GB Patent 1,112,033, which mentions other possibilities in addition to the ethers mentioned above, e.g. ketones. The disclosed method requires that the acid be extracted and released at a substantially different temperature. The extraction is carried out at a relatively low temperature and the extract is heated without water or added slightly to release the acid. Additional Patent GB 1 112 033 GB Patent 1 2k0 285 describes the same process but uses a solvent mixture, one example of which is a mixture of ether and ketone. The preferred ketone is cyclohexanone. A third proposal for a solvent group with a limit of 35% H 2 PO 4 is disclosed in U.S. Patent 3,556,739, which describes the use of a variety of aliphatic esters, aliphatic and cycloaliphatic ketones, and glycol ethers. Although the description relates only to a process of the type of GB patent 1,112,033 in which the extract is heated to release an acid, reference is also made to the possibility of releasing the acid isothermally, whereby the solvent extract is allowed to come into contact with water in one step. However, it is not mentioned which of the many proposed solvents must be used in the temperature-raising process, nor is it mentioned the temperatures associated with the solvent.

GB Patent 1,063,2U8 also describes a process for removing nitric acid from an aqueous mixture of phosphoric acid and nitric acid obtained by decomposing a phosphate mineral with nitric acid. In this method, nitric acid is extracted with a ketone, leaving the phosphoric acid impure in the aqueous phase. Such an acid can only be used to make a fertilizer.

The process according to the invention for purifying wet process phosphoric acid, e.g. suitable for foodstuffs, is characterized by a combination of the following steps: (a) wet process phosphoric acid having an acid content of 70-85% is contacted with methyl isobutyl ketone in a solvent: acid weight ratio of 0.5: 1-2: 1 to give An organic extract containing an acid content of 28% by weight and an aqueous raffinate, (b) the extract is separated from the raffinate, (c) the extract is washed, (59077) (d) the washed organic extract is contacted with water in two or more in the countercurrent step, separating the extract into an aqueous purified phosphoric acid phase containing less than 100 ppm of metal impurities and into an organic phase, (e) separating the aqueous acid phase and the organic phase, (f) raising the concentration of the aqueous phase to Qh, 9-92% H 2 PO 4, (g) the concentrated acid is further purified by crystallization from an acid cooled to below 20 ° C, preferably 8-12 ° C a H 2 PO 2 .1 / 2H 2 O crystals, and the crystals are separated from the impure mother liquor, (h) water is added to the obtained mother liquor, and it is recirculated as the washing solution of step (c).

We have now found that if methyl isobutyl ketone is used as the solvent, it is possible to efficiently purify the wet process telelic acid in the commercial concentration range. This applies to both extraction and release of acid at or near ambient temperature. This requires that the acid be cooled prior to the extraction step or that the temperature of the extract be raised prior to the release step, whereby the viscosity of the acid solvent system is suitable for carrying out the process.

The ketone used is methyl isobutyl ketone, which is not mentioned in U.S. Pat. No. 3,556,739. · This is unexpected because, in analytics, this ketone is known to extract metallic impurities from the phosphoric acid into the aqueous phase. However, we have found that methyl isobutyl ketone is capable of extracting almost all of phosphoric acid, as defined below, from commercial wet process phosphoric acid having an acid content of about 70-85%.

High molecular weight ketones require a higher phosphoric acid content in the feed to regenerate an equivalent amount of phosphoric acid. Thus, an additional cumbersome acid concentration step should necessarily be performed before using such ketones. An additional advantage of methyl isobutyl ketone is its higher flash point compared to lower molecular weight ketones. It forms an azeotrope with a sufficiently low boiling point to allow easy recovery of the solvent by distillation. In addition, the solubility of methyl isobutyl ketone in aqueous phosphoric acid is low (about 1-2%), so there is little methyl isobutyl ketone in the release step which can be removed relatively easily compared to other ketones due to the composition and boiling point of the azeotrope formed. In addition, it allows the preparation of an aqueous raffinate with a phosphoric acid content of 55-65%, which can be used in the next step after removal of the small amounts of methyl isobutyl ketone present.

Since the 11,59077 H 2 PO 2 R 2 contained in a standard commercial phosphoric acid is almost completely extracted into methyl isobutyl ketone, usually at a comfortable and desirable working temperature (i.e., 25-50 ° C, with a suitable viscosity), it is usually necessary to add a little water to obtain a workable raffinate. This is done either by adding water with or to the feed, or preferably by feeding an aqueous solution of phosphoric acid obtained from the washing step to reduce impurities in the extract, at the beginning of the process in which the ketone is added to the wet process acid.

It is also recommended, in contrast to U.S. Patent 3,556,739, that either the extraction of phosphoric acid from the crude process phosphoric acid into an organic solvent, methyl isobutyl ketone, and the release of the purified acid into water, or both, be performed in several steps. For this reason, we have found that with a certain degree of impurity, the phosphoric acid content of the raffinate is higher in the single-stage extraction than in the two-stage extraction. Thus, in the two-stage extraction, less phosphoric acid enters the raffinate. The two-stage extraction is most conveniently performed by feeding an acid-two-stage countercurrent mixer and settling nozzle to the second mixing unit and the ketone to the second unit. In the washing step, the recycled washing liquid is fed to the same unit as the acid feed. Multi-stage release results in an aqueous solution containing more phosphoric acid.

In such processes, it is common for the reaction of sulfuric acid with a phosphate mineral not to form CaSO 2 .111, but CaSO 2. 1/2 ^ 0 in either one or two steps, i.e. this results in a suspension obtained by crystallization by conventional methods. When methyl isobutyl ketone is used, the acid content of the acid is suitably 70-85% and preferably about 7-79%, calculated as phosphoric acid. In this context, the acid content of an acid is defined as the total percentage of phosphoric acid and sulfuric acid present. This simple addition is possible because both the phosphoric acid and sulfuric acid have a molecular weight of 98. Thus, an acid with% phosphoric acid and 5% sulfuric acid has an acid content of 79%. multi-stage. By doing so, for example, 55-58% of phosphoric acid is obtained from the methyl isobutyl ketone extract.

For the purification according to the invention, phosphoric acid is obtained by filtration or other isolation of calcium sulphate from a suspension obtained from the reaction of sulfuric acid and a phosphate mineral. As such, it is rich in impurities, some of which, e.g. fluorides, sulphate and dissolved organic matter, can be removed from the acid before solvent extraction, if desired. However, other impurities such as iron, chromium, magnesium and other metal cations cannot be removed by such pretreatment.

5 59077

It is most convenient to cool the acid from the reaction step as much as possible to post-precipitate the impurities present. Usually the acid is cooled to 25-30 ° C.

The optimum solvent extraction temperature depends on the acid concentration. However, we have found that temperatures below 50 ° C, i.e. 25-50 ° C is normally suitable for all acid concentrations. A particularly suitable temperature for methyl isobutyl ketone when using an acid with an acid content of 70-85 ° C is 35 ° C.

The acid used in the present process is either obtained in the required concentration or is concentrated. level, e.g. by vacuum distillation before purification. An acid with an acid content of 7 to 79% is the most commonly available commercial acid today. Thus, an acid with an acid content of about 79% is usually used, e.g. 75% phosphoric acid and 1 *% sulfuric acid or 70% phosphoric acid and 1% sulfuric acid. The extraction ketone is added to an aqueous solution of wet process phosphoric acid in a weight ratio of 0.5: 1 to 2.0: 1, preferably in a weight ratio of 1.0: 1 to 1.5: 1, with an optimum ratio of 1.2: 1 to 1, U: 1 in a single-stage extraction.

A higher ratio of 2.0: 1 can be used, but this requires larger containers. In two-stage extraction, a slightly larger ketone feed can be used, i.e. 1.3: 1 to 1.6: 1. We have found that in a minimum size plant, the purest aqueous phosphoric acid solution can be obtained per phosphoric acid feed unit if the ketone phase has a weight of 28-Uo /? phosphoric acid.

The ketone can be added to the acid in conventional equipment, e.g., settling mixers and countercurrent columns. Normally, for the reasons mentioned above, an extraction device with two or three, preferably two actual or theoretical countercurrent stages is used, although single-stage operation is possible.

Alternatively, other equipment can be used, e.g. extraction equipment of the shale, rotating disk or pulsating column type containing the desired number of theoretical steps.

The extract containing very pure phosphoric acid is washed repeatedly to allow it to come into contact with a small amount of washing liquid, preferably a very pure aqueous phosphoric acid solution for extracting the cationic impurities into the aqueous phosphoric acid phase. Alternatively, water can be used which, on first contact, re-extracts some phosphoric acid, after which it is allowed to come into contact with an additional impure extract, whereupon it acts as a phosphoric acid washing liquid. The impurity content of the phosphoric acid used for washing must be relatively low in order to be able to extract impurities from the ketone extract.

6 59077

The phosphoric acid content must be less than 56% in order to prevent the transfer of phosphoric acid from the washing liquid to the extract. Most preferably, the mother liquor obtained from the release step of the process is used for washing, to which water is added and recirculated as a washing solution.

The amount of phosphoric acid used as a washing liquid in the organic extract, formed by the partial extraction of phosphoric acid into water, is normally at least 10% of the amount of phosphoric acid in the organic phase. The recommended amount is 10-50%. This means that the weight ratio of washing liquid to extract is normal. 0.5 to 0.5: 1 when hexanone, a methyl isobutyl ketone, has a phosphoric acid content of about 56%

After washing, the phosphoric acid is released from the organic phase by allowing it to come into contact with water. Most commonly, multi-stage release is used, normally two real or theoretical stages. The amount of water used must be such that the phosphoric acid is almost completely transferred from the organic phase to the aqueous phase. It is desirable that at most 3%, preferably at most 1% of the phosphoric acid remains in the organic phase. Using two-stage countercurrent extraction for the release, the concentration of the aqueous phosphoric acid solution is 55-59%, depending on the temperature, e.g. 56% at a water temperature of 20 ° C. If desired, water at other temperatures can be used, e.g. condensate from the heat extraction device of the vacuum concentrator. The typical temperature range for the release step is 20 ° -Uo ° 0. In the two-stage release, the extract is fed to the mixing unit of the two-phase countercurrent mixer-settling unit and water to the second unit. Although this concentrated acid can be used for many purposes, it is normally desirable to concentrate it for transport. The amount of water used in the two-stage release may be 0.10: 1 to 0.3: 1 by weight of the extract. It is, of course, clear that if more dilute acid is desired, more water must be used.

The product obtained may contain a small amount of ketone used. If desired, this can be liberated e.g. by distillation. Thus, the process of the invention normally produces two phosphoric acid streams at this stage. Another solvent-derived solution containing about 50-70% of the phosphoric acid in the starting material and having a total metal content of less than 100 ppm based on phosphoric acid and a more impure stream containing phosphate which cannot be extracted into the organic phase and which can be used to prepare a fertilizer. Alternatively, a purified but impure stream containing, e.g., 95% of the initial phosphoric acid content can be prepared using a high concentration of acid feed and little washing.

Purified phosphoric acid containing less than 100 ppm of metal impurities according to the invention is further purified to suit e.g. foodstuffs τ 59077 by recrystallization of the acid. .9-88%. Alternatively, the phosphorus pentoxide obtained by combustion of the elemental phosphorus can be dissolved in the purified acid by solvent extraction or the concentrated phosphoric acid obtained by the thermal process can be mixed with the purified acid to achieve the desired concentration. In any case, the acid of the desired concentration obtained is cooled, e.g., to a temperature below 20 ° C to form H 2 PO 4, 1 H 2 H 2 O crystals. The preparation of phosphoric acid crystals has recently been described in Japanese Patent Publication No. 1 to 692, published 1969. , in which crystallization has sought to improve the previously desulfurized and fluorinated wet process phosphoric acid. In fact, this proposal cannot be used to prepare pure phosphoric acid crystals because the starting material is too impure. In addition, crystals are difficult to isolate from a highly viscous mother liquor. However, it is known that by cooling an aqueous acid solution below the concentration of 92% phosphoric acid, it crystallizes in the form of H 2 PO 4. 1/2 ^ 0 (see Thorpe’s Dictionary of Chemistry, Fourth Edition, Volume IX, p. 503) this applies to an acid produced by a thermal process. Since the acid thus prepared does not need to be purified, the question is purely academic.

The crystals are separated from the impure mother liquor. After dilution to a suitable concentration, the mother liquor of the crystallization can be advantageously used as a washing liquid for the solvent extract. Since the acid for the preparation of the original wet process phosphoric acid from the phosphate mineral is sulfuric acid, the solvent-extracted acid contains a substantial proportion of sulfuric acid, i. 0.5-1.0% sulfuric acid based on 85 ~ phosphoric acid. We have found that this impurity does not interfere with crystallization. However, if an unnecessarily large amount of liquid, which means a high concentration, adheres to the crystals from the mother liquor, e.g. by centrifugation during isolation, it can be easily removed by washing.

The method of the invention is illustrated by the following examples. The composition of the acid feed used in the examples was as follows: Density 1.66 H 3 POu 77, k% H 2 SO 4 1.6%

Fe 0.23%

Mg 0.25% 8 59077

Example 1 A. Extraction step Option 1

The description is made with reference to the flow chart of Figure 1. In the process, both the initial extraction of phosphoric acid into methyl isobutyl ketone and the final release into water take place in a two-stage settling mixer (Figures 1 and 2). The solvent extract is isolated from the aqueous solution of the pure product and washed as it is fed through the settling mixer set 3 to extract cationic impurities from the solvent extract. The washing liquid is the pure acid obtained from the release settling mixer 2.

The acid is fed to the first settling mixer at a temperature of about 25 ° C.

The temperature rises because when phosphoric acid is extracted into the ketone, heat is released. ~~

The flow chart shows the exact conditions for each step of the method.

Option 2

The first settling mixer can also be single-stage. This is illustrated in Figure 2. The temperature of the feed acid was again 25 ° C. The exact conditions of the compositions of the different phases and flows are shown in Figure 2.

3. Crystallization step

The separated aqueous purified phosphoric acid product obtained from the solvent extraction of Option 1 above was 33.6 l / h steam-distilled on a packed countercurrent column to give 31.8 l / h of 1.1 * 3 acid with 1 * 2.0% phosphorus pentoxide and less. than 50 ppm ketone. 0.9 l / h of both ketone and water were recovered saturated with each other.

This almost solvent-free phosphoric acid is concentrated on an accelerated rotary evaporator at 80 ° C and in vacuo to give 18.2 l / h of product containing 62.0% P 2 O 5 1.0% so 3

1 * 0 ppm Fe less than 10 ppm Mg less than 50 ppm F density 1.70 / 20 ° C

and which, when cooled to 30 ° C, was fed at 8 ° -12 ° C to a continuous crystallizer. The resulting suspension was centrifuged to give 11 *, 2 kg / h of crystalline phosphoric acid hemihydrate (H 2 PO 2 .1 / 2H 2 O) having a composition of 66% P 2 O 5 less than 500 ppm SO 2 less than 5 ppm Fe, Mg, F

9 59077 as well as 16.2 kg / h of an aqueous solution of phosphoric acid with a composition of 58% Pg0 1.9 1 SO3

less than 100 ppm Fe, Mg, F

The purity of the crystalline product can be further improved by washing it with a portion of water in which the crystals have been dissolved. This, of course, reduces the crystal yield.

The crystals are melted by transferring them to a stirred tank maintained at a temperature above 30 ° C to transport the phosphoric acid in liquid form. The acid forming the mother liquor can be sold as such.

Example 2 This example was for the recovery of "improved" total phosphoric acid as crystals According to the extraction step option 1 of Example 1, the solvent-extracted phosphoric acid was 66.8 l / h steam distilled and concentrated to give 36 .mu.l / h of a product having a composition of

61.5% P 2 O 5 1.55% SO 3 25 ppm Fe 10 ppm Mg less than 50 ppm F density 1.70 / 20 ° C

and which, after cooling to 30 ° C, was fed to a crystallizer consisting of a stirred tank and a recycle line connected thereto via a grooved surface heat exchanger. The salt recycle solution at -5 ° C kept the temperature at about 10 ° C.

The resulting suspension was centrifuged to give 29.1 kg / h of phosphoric acid hemihydrate crystals and 32.7 kg / h of aqueous phosphoric acid. The crystals were dissolved in 5.0 kg / h of water to give 5.1 kg of an aqueous solution of phosphoric acid, which was washed in a centrifuge.

The washings were combined with isolated aqueous phosphoric acid and diluted with water at 14.1 L / h to give 36.8 L / h of acid to wash the solvent extract. The acid contained U 1.0 JS PgO. and 2.1% SO3>

The crystals formed at 25.0 kg / h were melted in a stirred tank maintained at 30 -U0 ° C to give a commercially available liquid phosphoric acid having a composition of 66% PgO.

less than 200 ppm SO 3 less than 5 ppm Fe, Mg, F.

Claims (1)

Claim: A process for purifying wet process phosphoric acid, e.g. suitable for foodstuffs, characterized by a combination of the following steps: (a) wet process phosphoric acid having an acid content of 70-85 # is contacted with methyl isobutyl ketone in a solvent: acid weight ratio of 0.5: 1-2: 1 to give an organic extract containing H 2 PO 2 having an acid content of 28-1 + 0 by weight and an aqueous raffinate, (b) the extract is separated from the raffinate, (c) the extract is washed, (d) the washed organic extract is contacted with water in two or more countercurrent steps, the extract separating into an aqueous purified phosphoric acid phase containing less than 100 ppm of metal impurities and an organic phase, (e) separating the aqueous acid phase and the organic phase, (f) raising the concentration of the aqueous phase to 81 +, 9-92% , (g) the concentrated acid is further purified by crystallization from an acid cooled to below 20 ° C, preferably 8-12 ° C. crystals of H 2 PO 3 / PH 2 O, and the crystals are separated from the impure mother liquor, (h) water is added to the obtained mother liquor, and it is recirculated as the washing solution of step (c).