RU2297976C2 - Phosphoric acid purification process - Google Patents

Abstract

FIELD: inorganic compounds technologies.

SUBSTANCE: invention relates to the field of mineral fertilizers, food-quality phosphates, reagents, and detergents based on processing of phosphate mineral stock, in particular to extractive phosphoric acid processing processes having as objective its recovery and purification. Extraction of phosphoric acid id performed with tributyl phosphate. Process according to invention comprises double-step washing of phosphoric acid extract wherein extract is divided after first-step washing operation into two streams. One of the streams is transferred to reextraction of phosphoric acid to form low-grade product and the other stream is subjected to further washing, which is accomplished with high-quality phosphoric acid being neutralized with monovalent cation base during washing operation. Wash solution obtained after second-step washing is diluted with water and added in the first-step washing, Washed second-step extract is reextracted and re-extract is then additionally purified to remove fluorine in evaporation and bottom residue stemming operations resulting high-quality product.

EFFECT: elaborated economically efficient complex extractive phosphoric acid processing scheme allowing production of technical-grade and simultaneously high-grade (up to reagent-grade) phosphoric acid.

6 cl, 2 dwg, 3 tbl, 3 ex

Description

The invention relates to methods for producing purified phosphoric acid (FC) used in the production of mineral fertilizers, food phosphates, chemicals and detergents, using extraction phosphoric acid (EPA) obtained as a starting product from sulfuric acid opening of mineral phosphate raw materials.

The extraction process using organic compounds, in particular tributyl phosphate (TBP), as an extractant includes the operations of extraction of phosphoric acid, washing the extract and the operation of re-extraction [R. Dumon, "Le precedes de production d'acide phosphorique par voie humide", Information Chimie Hebdo, 1977, No. 172; P. Mangin, "Nouveau precédé de purification d'acide phosphorique par voie humide", Information Chimie Hebdo, 1976, No. 152; UK patent No. 1436114, C01B 25/46, 1976; UK patent No. 1596730, C01B 25/46, 1981; French patent No. 2320262, C01B 25/46, 1977]. The extract is washed with aqueous solutions in order to purify the extracted phosphoric acid from impurities, primarily sulfuric acid. In this case, purified FC is obtained with a content of at least 0.2% sulfate ion, which limits its use to industrial detergents or feed additives.

Known methods for producing high-purity phosphoric acid by extraction consist in the sequential purification of EPA, first to obtain a medium-quality product (with an acid concentration of about 40%), then, after evaporating all or part of the obtained product, a cycle of its purification is carried out to obtain food-grade or reagent-grade FC .

To increase the purification from the sulfate ion, the washing of the extract can be carried out with aqueous solutions containing monovalent cations, in particular sodium, potassium, or ammonium. Based on this technique, an EPC purification scheme was created, including extraction, washing, re-extraction and regeneration of the extractant. The specificity of this scheme lies in the fact that extraction is carried out from evaporated FC solutions with a concentration of 15-17 mol / L, the extract is washed using a solution of dipotassium phosphate or diammonium phosphate, and the reextraction is carried out in two stages - first part of the PC is re-extracted with water, and the residual its amount is an excess of alkali solution with simultaneous regeneration of the extractant [RF Patent for the invention No. 2191745 С01В 25/46, BI 30, 27,10,2002)]. This method is taken as a prototype. It proposes the use of pre-evaporated EPA (up to a concentration of phosphoric acid of more than 90%) or industrial superphosphoric acid as feedstock.

In the described method, the degree of FC extraction is limited by the permissible raffinate acidity of about 2 mol / L, since mineral precipitation falls below this acid concentration in the raffinate. Taking into account the return of part of the purified FC to the preparation of the washing solution and when using unpaired EPA as a feedstock, the proportion of FC in the raffinate fed to fertilizers is 20-30% in balance depending on the content of sulfate ion in the feed. In addition, when working with EPA, purification from fluoride ion is unsatisfactory.

The objective of the present invention is the creation on the basis of the prototype of an economical complex scheme for the purification of EPA, which allows to obtain in parallel with the technical product of high purity FC up to acid of reagent quality.

To achieve this result in the proposed method, including the extraction of FC, washing the extract of FC using a solution of a monovalent cation (dipotassium phosphate and / or diammonium phosphate or an equivalent solution of ammonia with a concentration of 1-6 mol / l per cation) and reextraction of FC, washing is carried out in two stage with the separation of the extract after the first washing in two streams. The ratio of the volume of flows depends on specific tasks, but the optimal ratio is 3: 1. The first of the streams is directed to the re-extraction of the FC with the receipt of a product of technical quality, and the second goes to additional washing, which is carried out by high-purity FC, which is neutralized during washing with a solution of the base of the monovalent cation. An acid obtained in the same process can be used as a high-purity FC. The amount of reagent FC returned to the washing is 15-30% of its total amount and is determined by the content of sulfuric acid in the initial FC. The washing solution after the second washing is connected to the first washing with dilution with water. The washed extract after the second stage of washing is transferred to re-extraction followed by additional purification of the re-extract from fluorine in the operations of evaporation and steaming of the bottom residue and obtaining a high-purity product. After this reextraction, the extractant is supplied for regeneration with an alkaline solution, which is then attached to the raffinate or to purified FC obtained during the first reextraction.

Such a construction of the scheme eliminates the complex and expensive process of intermediate concentration (evaporation) of purified FC, eliminates the second extraction cycle from one stripped off intermediate product and significantly reduces the amount of raffinates and waste.

The proposed process modes are aimed at increasing the efficiency of the FC purification process by conducting fractional washing of the phosphoric acid extract with dividing the extract into two streams and by choosing the ratios of both these streams and the streams and compositions of the washing solutions.

The final purification and concentration of the FC is carried out by evaporation of the aqueous reextract of the second stage with additional steaming through the resulting still water solution, both of which are carried out under vacuum. The rarefaction depth should ensure a reduction in the boiling point of the bottoms solution to prevent the tarring of organic contaminants, in particular dissolved TBP, which are removed in the process of evaporation and steaming with water. At the same time, purification from fluorine-containing impurities is achieved, the hard-to-remove part of which, as shown by studies, is not initially hydrofluoric acid. Concentration of FC up to 85-90% is determined by the requirements of GOST for acid of reagent quality. When carrying out the process of steaming water in a batch mode, a 2-3-fold volume of water is required to remove impurities, and with a continuous process in the column, 0.5-1 volume of water is sufficient.

To increase the extraction of FA, along with its deep purification, sulfuric acid can be used as a salting out agent in the extraction cascade, the flow rate of which should be adjusted taking into account the amount of introduced monovalent cationic phosphate.

As a result of the application of the claimed phosphoric acid purification scheme, a technical FC of “purified” quality is obtained, as well as reagent FC of qualification “CHDA” and higher in an optimum ratio of 4: 1 for the process. Raffinate, if it contains a sufficiently large amount of FA, can be used to produce ammonium or potassium phosphate fertilizers. When carrying out the process with the introduction of sulfuric acid as a salting out agent, the FC extraction reaches 99% while maintaining the quality of the production fractions.

Extraction processes can be carried out in extraction columns, in sump mixers, or in centrifugal extractors.

EXAMPLES

Example 1

The experiment was conducted on a cascade of centrifugal extractors and was aimed at testing the prototype when using EPA as the initial solution. The difference from the prototype in this scheme was to increase the number of washing stages from 4 to 7. The compositions of the products are given in table 1. The total number of steps is 17, of which extraction steps are 4, washings are 7, water stripping is 4, alkali stripping is 2. Flows (ml / h): TBP - 1000, ref. EPC - 160, washing solution (2.5 mol / L (NH ₄ ) ₂ HPO ₄ ) - 65, water - 200, alkali (5 mol / L NaOH) - 150.

The aqueous re-extract was evaporated under vacuum (~ 50 mm Hg) to obtain purified FC with a concentration of H ₃ PO _{4 of} 86%. The volumes of solutions and compositions of the products are given in table 1.

Table 1. Product Volume ml / h The concentration of the component,% of the mass. Density, g / cm ³ Color PO ₄ ^3- SO ₄ ^2- F ^- Fe Original EPC 160 68 3,5 0.46 0.48 1,64 brown Raffinate 105 25 4.9 0.55 0.66 1.29 green FC purified 102 86 0.006 0.008 <0,0005 1.7 colorless Alkaline reextract 185 3,7 0.001 0,025 <0,0005 1.17 St. yellow

This example shows that when using EPA as a feedstock according to the scheme proposed in the prototype, obtaining a reagent grade FC is not achieved.

Example 2

The experiment was conducted on a cascade of centrifugal extractors. The process diagram is shown in figure 1. The total number of steps is 22, of which extraction is 4, the first flushing is 4, the first flashing with water is 4, the second flushing is 4, the second flashing with water is 4, the alkali regeneration is 2. Flows (ml / h): TBP - 1000, ref. EFK - 155, prom. solution - 16, water [1] - 130, water [2] - 46, water [3] - 46, 18% NH ₄ OH - 5, 10% NaOH - 70. The extract stream after the first washing is divided in the ratio 3 :one. Reagent acid was obtained by evaporation of an aqueous reextract under vacuum (~ 50 mm Hg) to a concentration of FC of 87%, followed by periodic steaming of the bottom residue with 2.5 volumes of water.

The volumes of solutions and compositions of the products are given in table.2.

Table 2. Product Volume ml / h The concentration of the component,% of the mass. Density, g / cm ³ Color H ₃ RO ₄ SO ₄ ^2- F ^- Fe Original EPC 155 68 3,5 0.46 0.48 1,64 brown Raffinate 143 16.5 4.8 0.17 0.67 1.27 green FC technical 215 39 0.011 0.024 <0.001 1.25 colorless FC reagent 22 87 0.001 0.00015 0,0005 1.71 colorless FC negotiable eleven 39 0,0004 0.014 <0,0002 1.25 colorless

This example shows the possibility of obtaining a reagent acid from the original low-quality EPA and obtaining a raffinate suitable for the preparation of fertilizers.

Example 3

The experiment was also conducted on a cascade of centrifugal extractors. A distinctive feature of this experiment was the introduction of sulfuric acid as a salting out agent to increase the extraction of phosphoric acid in the extract. The process diagram is shown in figure 2. The total number of steps is 34, of which extraction is 12 (H ₂ SO ₄ is fed to step 4), the first flush is 6, the first flashing with water is 4, the second flushing is 6, the second flashing with water is 4, and alkali regeneration is 2. Flows (ml / h): TBP - 1000, ref. EPC - 160, concentrated H ₂ SO ₄ - 15, prom. solution - 18, water [1] - 130, water [2] - 46, water [3] -46, 18% NH ₄ OH - 5, 10% Na ₂ CO ₃ - 40. The extract stream after the first washing is separated in a ratio of 3: 1. Reagent acid was obtained by evaporation of an aqueous stripping under vacuum (~ 50 mm Hg) to a concentration of FC 86%, followed by continuous countercurrent steaming of the bottom residue in a laboratory plate column with 2.5 volumes of water.

The volume of solutions and the composition of the products are given in table.3.

Table 3. Product Volume ml / h The concentration of the component,% of the mass. Density g / cm ³ Color H ₃ RO ₄ SO ₄ ^2- F ^- Fe Original EPC 160 73 1.93 0.25 0.39 1,63 brown Raffinate 115 1.4 19.3 but 0.75 1.17 green FC technical 235 38 0.19 0,022 <0.0001 1.24 colorless FC reagent 24 86 0.002 0,0002 0,0005 1.70 colorless FC negotiable eleven 38 0,0005 0.014 <0,0002 1.25 colorless

This example shows the possibility of obtaining a reagent acid with a high degree of extraction of phosphoric acid from the original EPA.

Claims (6)

1. The method of purification of phosphoric acid (FC) obtained by sulfuric acid opening of mineral raw materials by extraction with tributyl phosphate, comprising washing the extract of FC with an aqueous solution containing a monovalent cation, and sequential reextraction of FC with water and an alkaline solution, characterized in that the washing of the FC extract is carried out in two stages with the separation of the extract after the first washing into two streams, the first of which enters the stripping of purified FC, and the second to the additional washing, which is carried out partly by sweat reagent FC, which is neutralized during washing with a solution of the base of the monovalent cation, moreover, the washing solution after the second washing is transferred to the first washing with dilution with water, and the washed extract of the second stage is transferred to the re-extraction of the reagent FC with its subsequent purification from fluorine by evaporation and subsequent evaporation of bottoms of the residue, and if necessary, deep extraction of FC in the process of extraction for salting out injected concentrated sulfuric acid, the flow rate of which must take into account the different voltage acid phosphate with the release of a monovalent cation in free form FC and its subsequent extraction. 2. The method according to claim 1, characterized in that the FC extract after the first washing, aimed at obtaining purified and reagent FC, is divided in a ratio of 3: 1. 3. The method according to claim 1, characterized in that the amount of reagent FC returned to the washing is 15-30% of its total amount and is determined by the content of sulfuric acid in the original FC. 4. The method according to any one of claims 1 to 3, characterized in that the amount of sulfuric acid introduced for salting out should provide a content of free sulfuric acid in the raffinate of at least 1 mol / L. 5. The method according to any one of claims 1 to 3, characterized in that the alkaline reextraction is carried out with a solution of ammonia or sodium hydroxide, or soda using a reextract to obtain pure phosphate salts or by attaching it to a raffinate or purified FC, depending on nomenclature of products. 6. The method according to claim 1, characterized in that to obtain a reagent FC with a concentration of 85-90%, the stripping is evaporated under vacuum, followed by steaming the still solution with 2-3 volumes of water in a batch mode or 0.5-1 volumes of water in a column in continuous mode.

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