RU2033965C1 - Method of phosphoric acid purification from arsenic - Google Patents

Abstract

FIELD: chemical technology. SUBSTANCE: method involves treatment of acid with reductant. Depleted iron-containing catalyst CA-1 used in ammonia synthesis is used as reductant, and evolving gases were absorbed with acid aqueous solutions with pH 1-3. EFFECT: increased purification degree, diminished energy consumption. 1 tbl

Description

 The invention relates to a method for purifying phosphoric acid from impurities, in particular arsenic, and can be used in the production of feed phosphates and nutrient solutions for the microbiological industry.

Known and most widely used is a method of extracting arsenic and heavy metals from phosphoric acid by alkali metal sulfides (Na ₂ S) or P ₂ S ₅ [1]
This method in all its varieties includes the following main stages: precipitation of arsenic with sodium sulfide solution; further precipitation of insoluble arsenic sulfide and desorption of excess hydrogen sulfide from the filtered acid.

H₂S

2H⁺+S^2-;
2As⁵⁺+5S^2-__→ As₂S

+2S

+8H₂O.This method is based on hydrogen sulfide formation reactions; its dissociation to sulfide ion S ^2- ; reduction of arsenic As ⁵⁺ to As ³⁺ and the formation of insoluble arsenic sulfide:
2H ₃ PO ₄ + Na ₂ S __ → 2NaH ₂ PO ₄ + H ₂ S

H ₂ s

2H ⁺ + S ^2- ;
2As ⁵⁺ + 5S ^2- __ → As ₂ S

+ 2S

+ 8H ₂ O.

However, the use of this method is associated with the costs of the purchase and delivery of Na ₂ S, environmental degradation due to the occurrence of hydrogen sulfide in gas emissions, and difficulties arising in the disposal of the resulting precipitation.

Closest to the proposed is a method of purification of phosphoric acid from arsenic and other undesirable impurities [2]
The essence of the known method is that the acid to be purified is sequentially treated with a reducing agent, alkaline earth compounds in the presence of absorbents and ether. Compounds containing C ₄ -C ₁₂ in the alkyl residue (octyl-isooctyl; 2-ethyl-hexyl ether), which are responsible for the release of arsenic ions from the acid, are used as ether.

 The presence of a reducing agent contributes to a more effective removal of arsenic. In the role of a reducing agent are: iron powder, zinc, red phosphorus, sodium hypophosphite, etc.

The cleaning process is carried out according to the following scheme: 0.1-3.0% of the reducing agent by weight of the acid is added to the acid and at t = 60 ^o C it is stirred for one hour; Further, an absorbent (carbon or silicate-containing agents) is added to the solution in an amount of 0.1-2.0% and alkaline earth compounds in an amount of 0.5-5.0% by weight of the acid. The resulting mixture is stirred for another hour at t = 70 ^o C; the mixture is filtered; 1% perlite and 0.05-0.5% ether are added to the filtrate, after which the new mixture is filtered through a layer of perlite.

The change in acid composition during this treatment is presented below:
Acid Composition Components Initial After Stage III Final P ₂ O ₅ , 51.5 50.3 50.2 SO ₄ , 4.3 0.15 0.15 As, ppm. 5 5 0.1 C organic, ppm 1600 130 140
An analysis of the data given in the table shows the presence of P ₂ O ₅ losses during the process and the absence of arsenic ion emission before ether was introduced.

 The disadvantage of the above method is its technological complexity and high cost. The unresolved problem is the disposal of precipitation.

 The aim of the invention is to simplify the technology of purification of phosphoric acid from arsenic, reducing the cost of its implementation in obtaining a residual content of arsenic, which allows the use of acid in the production of feed phosphates and nutrient solutions (liquid ammonium phosphate JAF) for the microbiological industry.

 The goal is achieved by the fact that the purified thermal phosphoric acid is passed through a layer of a reducing agent, which is used as a spent catalyst for the synthesis of ammonia CA-1.

 The process takes place in a vertical cylindrical apparatus, which is made of a material resistant to phosphoric acid. The device has fittings for entering, leaving and maintaining the level of acid; hatches for loading the catalyst (and unloading if necessary). The upper part of the apparatus is equipped with a pipeline for the removal of reaction gases. The amount of catalyst loaded is related to the size of the apparatus and the desired frequency of the new load. When the unit starts up, the acid is not supplied in full to prevent the release of a large amount of gas.

 During operation of the apparatus, an acid layer above the catalyst should be absent. In this case, the catalyst above the acid level will act as a droplet eliminator.

 The influx of fresh acid supports the reaction until the catalyst is completely depleted.

 The catalyst is used without additional crushing.

The process is carried out at 20-75 ^C. The rate of expiration acid unit is selected according to the required extent (completeness) purification.

The reaction gases formed during the reactions contain a mixture of hydrogen and arsine (AsH ₃ ).

 The gas mixture is diverted to absorption by aqueous solutions with a pH of 1-3. As absorption solutions, for example, acidic solutions formed after washing phosphogypsum in the production of extraction phosphoric acid can be used. The washing solution, according to the technological scheme for the production of EPA, enters the reverse cycle and returns to the extractor.

 The arsenic content in the wash solution (table) allows to obtain extraction phosphoric acid without violating the requirements of TU 6-08-342-76 (as amended).

 Comparative analysis with the prototype allows us to conclude that the inventive method for purification of phosphoric acid from arsenic differs from the known simplicity of technology and equipment; the ability to use ammonia production waste as a raw material; lack of solid waste requiring additional treatment and disposal.

 The table shows the results of laboratory experiments on the purification of thermal phosphoric acid from arsenic and the preparation of model solutions of liquid ammonium phosphate brand 8:24 and 10:34 from purified and unrefined acid.

PRI me R 1 (see table, sample N 3). TFA cleaned with a concentration of 64.6% P ₂ O ₅ content of As = 0,0093% for 15 minutes at 30 ^{° C} was passed through a glass reactor filled with spent catalyst SA-1. Evolved gases are discharged to the absorption of aqueous solution at pH 2.7 (solution temperature is in all cases 25 ° ^C) and discharged to the atmosphere through a glass tube overlain bromnortutnoy paper. By the presence of paper staining, the completeness of the absorption of arsenic by an aqueous solution and its content in exhaust gases were judged.

The analysis of purified acid showed that at a concentration of 64.6% P ₂ O ₅ the arsenic content in it was 0.0068%. The degree of acid purification from arsenic was 27%. The content of arsenic in the aqueous solution was 0.0024%. There was no paper staining, which indicates the content arsenic in exhaust gases not more than 0,0003% Loss of arsenic during the process 1%
On purified acid, a sample of LCA grade 10:34 containing 0.0038% As was prepared. In the sample JAF 10:34, obtained from untreated TFA, the arsenic content was 0.0054% (sample N 1).

 PRI me R 2 (sample N 4). The described and all subsequent experiments were carried out according to the scheme presented in example 1.

The initial TFA contained 64.6% P ₂ O ₅ and 0.0093% As.

 The acid treatment time with the reducing agent was 30 minutes.

The temperature of the acid is 30 ^o C.

 The purified acid contained 0.0042% As.

The degree of purification 55%
In an absorption solution with a pH of 2.9, the arsenic content was 0.0051%
In order to check the stability of aqueous solutions containing arsenic, depending on pH values, a precipitating agent containing sulfide ions was added to the absorption solution. The formation of insoluble arsenic compounds was not observed.

 The color of bromnortutnoy paper has not changed.

 Losses of arsenic during the experiment were not detected.

In the sample of JAF grade 10:34 made from purified acid, the arsenic content was 0.0025%
PRI me R 3 (sample N 5). Starting TFA: 62.2% P ₂ O ₅ and 0.0067% As.

 Processing time 15 min.

The temperature of the acid is 40 ^o C.

Purified acid: 62.2% P ₂ O ₅ and 0.0047% As.

The degree of purification: 30%
The absorption solution had a pH of 1.7.

The arsenic content in the solution was 0.0018%. There was no staining of bromnorthortic paper, which indicates an arsenic content in the exhaust gas of less than 0.0003%. Arsenic loss during the process 3%
PRI me R 4 (sample N 6). Starting TFA: 62.2% P ₂ O ₅ and 0.0067% As.

 Processing time 15 min.

The temperature of the acid is 60 ^o C.

Purified acid: 62.2% P ₂ O ₅ and 0.0045% As.

The degree of purification 33%
The absorption solution had a pH of 1.9.

The arsenic content in the solution was 0.0022%. Bromo-mercury paper is not colored, which corresponds to the arsenic content in the exhaust gas 0.0003%
Losses of arsenic were absent.

PRI me R 5 (sample N 7). Starting TFA: 62.2% P ₂ O ₅ and 0.0067% As.

 Processing time 15 min.

The temperature of the acid is 80 ^o C.

Purified acid: 62.2% P ₂ O ₅ and 0.0042% As.

The degree of purification of 37%
The absorption solution had a pH of 2.3.

The arsenic content in the solution of 0.0020%
Bromo-mercury paper is painted pale yellow, which corresponds to the content of arsenic in the exhaust gases of 0.0010%
Loss of arsenic during the experiment amounted to 7%
PRI me R 6 (sample N 8). Starting TFA: 62.2 P ₂ O ₅ and 0.0067% As.

 Processing time 15 min.

The temperature of the acid is 15 ^o C.

 The purified acid contained 0.0056% As, an absorption solution with a pH of 1.0 0.0010% As.

The degree of purification 16%
Bromo-mercury paper has not changed its color.

Loss of arsenic 1%
PRI me R 7 (sample N 9). Starting TFA: 54.3% P ₂ O ₅ and 0.0085% As.

 Processing time 15 min.

Temperature 20 ^o C.

 The purified acid contained 0.0063% As.

 Absorption solution with a pH of 1.4 0.0023% As.

The degree of purification 26%
Bromorto paper did not change color.

 Losses of arsenic were absent.

 In the sample of FAA grade 8:24, obtained on the basis of purified acid, the arsenic content was 0.0025%. The sample of FAA of grade 8:24 from crude acid contained 0.0036% (sample No. 2).

PRI me R 8 (sample N 10). Starting TFA: 54.3% P ₂ O ₅ and 0.0085% As.

 Processing time 15 min.

The temperature of the acid is 85 ^about C.

 The purified acid contained 0.0049% As, an absorbent solution with a pH of 1.2 0.0013% As.

The degree of purification of acid from arsenic is 42%
Bromo-mercury paper had a tan color, which corresponds to an exhaust gas content of up to 0.0018% As.

Loss of arsenic during the experiment amounted to 27%
A sample of LAF grade 8:24, obtained from purified acid, contained 0.0019% As.

PRI me R 9 (sample N 4). Starting TFA: 50.1% P ₂ O ₅ and 0.0072% As.

 Processing time 30 min.

The temperature of the acid is 55 ^o C.

The arsenic content in purified TPA is 0.0028%
The degree of purification of 61%
Arsenic content in an absorption solution with a pH of 3.6 0.0043%
When precipitator was added to the absorption solution, precipitation was detected.

 Bromorto paper did not change color.

Loss of arsenic amounted to 1%
PRI me R 10 (sample N 12). Starting TFA: 50.1% P ₂ O ₅ and 0.0072% As.

 Processing time 30 min.

The temperature of the acid is 80 ^o C.

Purified TFA: 50.1% P ₂ O ₅ and 0.0016% As.

The degree of purification of 78%
Arsenic content in an absorption solution with a pH of 3.2 0.0045%
When a precipitant is added to the absorption solution, a small amount of precipitate forms.

 Bromo-mercury paper had a pale yellow color, indicating an exhaust gas content of 0.0010% As.

Loss of arsenic was 15%
PRI me R 11 (sample N 13). Starting TFA: 68.7% P ₂ O ₅ and 0.0114% As.

 Processing time 30 min.

Temperature 75 ^o C.

Purified TFA: 68.7% P ₂ O ₅ and 0.0032% As.

The degree of purification 72%
Arsenic content in an absorption solution with a pH of 1.5 0.0079%
Bromorto paper did not change color.

Loss of arsenic 3%
PRI me R 12 (sample N 14). Starting TFA: 68.7% P ₂ O ₅ and 0.0114% As.

 Processing time 60 min.

The temperature of the acid is 75 ^o C.

The arsenic content in purified acid was 0.0009% and in an absorption solution with a pH of 2.5 0.0096%
The degree of purification of TPA from arsenic 92%
Bromo-mercury paper had a lemon color, which corresponds to the content of 0.0008% of arsenic in the exhaust gases.

The loss of arsenic during the experiment was 8%. The chemical composition of the spent catalyst for the synthesis of ammonia CA-1, used as a reducing agent: Fe 85-96% Ca 0.7-2% Al 0.3-3% K 0.2-0.4 %
Based on the results of the experiments, the following conclusions can be drawn.

The spent catalyst for the synthesis of ammonia CA-1 can be used to isolate arsenic from phosphoric acid in the form of arsine AsH ₃ (gas) (examples 1-12).

 With increasing temperature of the acid, the degree of purification increases with the same reaction time with a reducing agent (examples 3, 4, 5).

It is advisable to apply for cleaning acid at a temperature not lower than ²⁰ ° C, since otherwise dramatically increases the time required to achieve the same degree of purification (Examples 6, 7).

It is not desirable to use an acid having a temperature above 75 ^{° C,} as an increasing number of non-absorbed gases that might impair health standards (Examples 5, 8, 10-12).

 As absorption solutions, acidic aqueous solutions with a pH of 1-3 may be recommended (examples 1-12).

 At the indicated pH values, arsenic-containing aqueous solutions remain stable even with the addition of a precipitant, which guarantees against possible precipitation during further processing of the solutions (examples 2, 9, 10).

 Thus, the proposed method allows you to clean phosphoric acid from arsenic impurities to the values required when using acid in the production of feed phosphates and nutrient solutions for the microbiological industry; simplify technology; reduce costs.

Claims (1)

METHOD FOR CLEANING PHOSPHORIC ACID FROM ARSENINE, comprising treating an acid with a reducing agent, characterized in that the treatment is carried out by passing an acid with a temperature of 20 75 ^o C through a reducing agent layer, which uses spent iron-containing ammonia synthesis catalyst CA-1 and the gases emitted are quenched with acid aqueous solutions with a pH of 1 to 3.

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