RU2323875C1 - Method for purifying extraction phosphoric acid - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to methods for purifying evaporated extraction phosphoric acid, produced during apatite concentrate decomposition, to eliminate sulphates and suspended matter. Purifying results in sulphateless phosphoric acid, which is used in purified phosphoric acid production based on solvent extraction with tributyl phosphate, as well as in phosphating agents and other technical grade phosphates production. Method for purifying evaporated extraction phosphoric acid, produced from apatite, from sulphates and suspended matter implies treatment of the acid (which is heated and stirred) with apatite concentrate, clarification in the presence of coagulant and separation of clarified sulphateless acid and concentrated portion thereof. For separation, sulphateless acid, which contains 5-11% of suspended matter, is supplied. Separation is carried out continuously in thin layer, on tilted sedimentation plates, linear speed of suspension being 3-11 m/h, until clarified acid in 60-80% yield (on the basis of Р₂О₅ ) is obtained. Sulphates are eliminated until residual SO₄ ^2- content in clarified extraction phosphoric acid is 0.3-0.5%, and acids are separated until residual content of suspended matter is 0.1-0.5%. Acid is treated with apatite concentrate at 70-95°C.

EFFECT: process simplification and operational costs reduction.

3 cl, 4 ex

Description

The invention relates to methods for purifying one stripped off extraction phosphoric acid, formed upon decomposition of apatite concentrate, from sulfates and suspensions to obtain desulfurized phosphoric acid, which is used in the production of purified phosphoric acid by liquid extraction with tributyl phosphate, as well as in the manufacture of phosphating preparations and other technical-grade phosphates.

Known methods for purification of extraction phosphoric acid (EPA) with a content of 22-64% P ₂ O ₅ from sulfates, including processing it with stirring at a temperature of 75-95 ° C with strontium compounds in an amount necessary to achieve the ratio Sr ²⁺ : SO ₄ ^{2 -} in acid 0.7-1.05, clarification of desulfurized acid, separation of the clarified part by decantation (RF patent No. 2170700, С01В 25/234, 2001, and RF patent №2194667, С01В 25/234, 2002). The proposed methods allow for the deep desulfurization of EPA.

The disadvantages of the methods are the long clarification time (up to 100 hours), the production of strontium sulfate in the form of a finely dispersed easily agitated precipitate, which complicates the separation of clarified and condensed acid and practically makes it unacceptable to use continuous processes for purifying EPA from suspensions.

Closest to the proposed invention is another known method for purifying extraction phosphoric acid obtained from apatite from sulfates and suspensions, including treating it with stirring and an elevated temperature with apatite concentrate, clarification in the presence of a coagulant, and separating the clarified desulfated acid and its condensed part. According to this method, EPA containing 34-40% P ₂ O _{5 is} fed for purification, it is evaporated to 52-57% P ₂ O ₅ while maintaining the F: P ₂ O ₅ and SO ₄ ^2- : P ₂ mass ratios in evaporated acid. O ₅ in the range of 0.003-0.008 and 0.02-0.05, respectively. Processing apatite concentrate one stripped off the acid is carried out at a temperature of 70-75 ° C to a content of SO ₄ ^2- : P ₂ O ₅ in it 0,003-0,010. The clarification of desulfated acid is carried out in the presence of a coagulant, and the separation of the condensed part and the clarified desulfated acid by decantation. The claimed ratios of the components in the EPA solution after evaporation are proposed to be carried out by varying the composition of the acid according to P ₂ O ₅ before evaporation and after evaporation (RF patent No. 2131842, С01В 25/234, 1999).

The disadvantages of the method are:

- conducting the clarification process in periodic mode, which reduces the technological and economic efficiency of the process;

- the complexity of the technological scheme and instrumentation of the clarification process due to the presence of a large number of stages: clarification, decantation, agitation of condensed EPA and its transportation for processing;

- low speed of clarification of desulfurized EPA (0.18-0.24 m / s), due to the practical difficulty of controlling the process due to the content of P ₂ O ₅ in weak and evaporated EPA and suboptimal content of SO ₄ ^2- in production acid (up to 0, fifteen%);

- the use of only hemihydrate EPA (34-40% P ₂ O ₅ ) is envisaged as the initial acid, the use of dihydrate EPA is not expected (and this method gives the bulk of EPA in Russia);

- a change in the content of P ₂ O ₅ in weak EPA in the claimed range (up to 38-40% P ₂ O ₅ ) sharply worsens the main technical and economic indicators of production of weak EPA.

We set the task of intensifying the process of desulfurization of stripped-off EPA while ensuring a consistently high yield of production acid, simplifying the technological scheme and reducing operating costs.

The problem is solved in the proposed method for cleaning one stripped off extraction phosphoric acid from apatite by treating it with stirring and an elevated temperature with apatite concentrate, clarifying in the presence of a coagulant and separating the clarified desulfated acid and its condensed part, in which desulfated acid containing 5-11 is fed for separation % suspensions, and the separation process is carried out continuously in a thin layer on inclined precipitation plates with a linear velocity of the suspension between the plates 3-11 m / h to obtain at the outlet of the clarified acid 60-80% of the load according to P ₂ O ₅ . It is advisable to desulfurize to a residual content of SO ₄ ^2- in clarified EPA 0.3-0.5%, and the separation of acids to a residual content of suspensions of 0.1-0.5%. The acid treatment with apatite concentrate is carried out at a temperature of 70-95 ° C.

The main differences of the proposed method are:

- carrying out the separation of the suspension of desulfurized EPA containing 5-11% of the suspensions, into the clarified desulfated acid and the condensed part in a continuous manner in a thin layer on inclined precipitation plates, with a linear speed of suspension between the plates of 3-11 m / h and the output of clarified acid 60- 80% of the load according to P ₂ O ₅ to the residual suspended matter content in desulfurized EPA 0.1-0.5%;

- desulphurization of the stripped off EPA obtained from apatite concentrate by hemihydrate and dihydrate methods is carried out to a residual SO ₄ ^2- content in clarified acid of 0.3-0.5%.

The essence of the proposed method is as follows.

We propose separation of the suspension in a continuous mode in a thin layer on inclined precipitation plates located at a relatively small distance from each other. The clarification of EPA in a thin layer proceeds at a high speed (1-1.5 orders of magnitude higher compared to clarification in a volumetric sump) due to the fact that particles of the solid phase in a thin layer pass a relatively short path (several centimeters) and come in contact with the surface of an inclined plate and then slide down the inclined plate, and the clarified acid passes up between the plates. In this case, the rate of creep of solid particles is an order of magnitude or more higher than the rate of clarification of particles in the bulk sedimentation tank.

In the proposed method of carrying out the process, a large role is played by the content in the initial stripped-off EPA of suspensions, which is maintained in the range of 5-11%. With this amount of suspensions, a high clarification rate of 0.3-0.6 m / h is achieved. The amount of suspensions in the starting acid depends on various technological parameters - the composition of the stripped-off EPA supplied for desulfurization, apatite consumption and / or can be achieved by recycling part of the condensed EPA from the clarification and separation of the suspension. Deviation from the stated interval of the suspension in suspension leads to a decrease in the rate of sedimentation.

The claimed interval of linear velocity of suspension between the plates of 3-11 m / h is due, on the one hand, to the intensification of the clarification process and, on the other, to the quality of the product in terms of suspension content. An increase in the linear speed of movement leads to an increase in the suspension content in production acid over 0.5%, which substantially complicates its further processing (for example, purification with tributyl phosphate). Maintaining a speed of less than 3 m / h is impractical due to a decrease in the intensity (productivity) of the clarification stage. Comparison of the actually achieved clarification rate (0.3-0.6 m / h) and its linear suspension velocity (3-11 m / h) between the plates many times higher than it clearly demonstrates the high efficiency of the proposed method for the desulfurization of EPA.

The declared output range of the clarified desulfurized EPA is due to the qualitative composition of the initial stripped-off EPA supplied to desulfation (EPA production method, composition based on SO ₄ ^2- and suspensions, content of P ₂ O ₅ , etc.), and the physicomechanical properties of condensed EPA ( viscosity, solids content, fluidity, ability to inlay, etc.). As a rule, a yield of more than 80% is associated with the production of condensed acid with a high content of suspensions, which makes it difficult to remove it from the sump and further transportation to processing. A yield of about 60% is typical for the desulfurization of evaporated EPA containing significant amounts of SO ₄ ^2- and suspended in evaporated acid. A yield of less than 60% is not economically feasible, because increases the equipment volume of the clarification unit and makes it difficult to process large volumes of condensed acid.

The accepted range of residual SO ₄ ^2- content provides the formation of relatively large crystals of calcium sulfate hemihydrate, which allows to achieve a high rate of sedimentation of the suspension. The upper boundary of the interval is due, on the one hand, to the damping dynamics of improving the quality of crystals of calcium sulfate hemihydrate, and on the other hand, to the requirements for consumer properties of desulfurized acid. When the content of SO ₄ ^{2 is} less than 0.3%, the crystal size of calcium sulfate hemihydrate is significantly reduced. In addition, with an excess of apatite, finely dispersed monocalcium phosphate crystals appear in the suspension, which sharply reduces the rate of clarification of the suspension.

At the stage of desulfurization, the initial stripped-off EPA obtained from the apatite concentrate by the dihydrate or hemihydrate method is treated at a temperature of 70-95 ° C. The expansion of the temperature range of the process compared with the prototype allows to obtain larger crystals of hemihydrate calcium sulfate. An increase in temperature of more than 95 ° C is impractical because equipment corrosion is significantly increased. Lowering the process temperature below 70 ° C leads to the crystallization of small crystals of calcium sulfate.

The proposed method for purification of evaporated EPA from apatite concentrate from sulfates and suspensions in comparison with the known method allows for the purification of EPA in a continuous way, simplifies the purification process by reducing the stages of the process and optimizes the output system of clarified and condensed EPA, stabilizes the yield and quality of desulfurized clarified acid, reduce operating costs.

The method is illustrated by the following examples.

Example 1

1000 kg of extraction phosphoric acid obtained by the dihydrate method containing 4.0% suspensions, 52.5% P ₂ O ₅ and 4.2% sulfates, calculated as SO ₄ ^2- in the liquid phase, is treated at a temperature of 95 ° C with stirring apatite concentrate in the amount of 65 kg. To maintain the desired temperature, 92 kg of hot steam is introduced into the reaction volume. In this case, the sulfate content in terms of SO ₄ ^2- in the clarified part of desulfurized acid decreases to 0.40%, and the suspension content in the suspension increases to 11%. A coagulant (e.g. polyacrylamide) is added to the suspension and clarified. The rate of deposition of solid particles in a solution of extraction phosphoric acid is 0.4 m / h Clarification with the separation of clarified and condensed acids is carried out in a continuous manner on inclined precipitation plates with a linear speed of suspension between the plates of 5 m / h and the output of clarified desulfurized EPA 60% of the load of P ₂ O ₅ (596 kg of clarified acid). In this case, the residual suspended matter in clarified acid is maintained at 0.5%. Suspension content in condensed EPA (503 kg of condensed EPA) is 21.3%.

Example 2

21000 kg / h of extraction phosphoric acid obtained by the dihydrate method containing 3.0% suspensions, 52.3% P ₂ O ₅ and 3.0% sulfates in terms of SO ₄ ^2- in the liquid phase, is treated at a temperature of 70 ° C and stirring with apatite concentrate in an amount of 896 kg / h. To maintain the desired temperature, 714 kg / h of hot steam is introduced into the reaction volume. In this case, the sulfate content in terms of SO ₄ ^2- in the clarified part of desulfurized acid decreases to 0.30%, and the suspension content in the suspension increases to 7.9%. A coagulant (e.g. polyacrylamide) is added to the suspension and clarified. In this case, the rate of deposition of solid particles in a solution of extraction phosphoric acid is 0.3 m / h. Clarification with the separation of clarified and condensed acids is carried out in a continuous manner on inclined precipitation plates with a linear speed of suspension between the plates of 3 m / h and the output of clarified desulfurized EPA 65% of the load according to P ₂ O ₅ (13558 kg of clarified acid). In this case, the residual suspended matter in clarified acid is maintained at 0.1%. Suspension content in condensed EPA (9052 kg) is 19.5%.

Example 3

36000 kg / h of wet-process phosphoric acid obtained by the hemihydrate process slurries containing 2,5%, 52,3% P ₂ O ₅ and 2.2% of sulfate in terms of SO ₄ ^2- in the liquid phase, is treated at a temperature of 80 ° C and stirring with apatite concentrate in an amount of 1262 kg / h. To maintain the desired temperature, 1008 kg / h of hot steam is introduced into the reaction volume. In this case, the sulfate content in terms of SO ₄ ^2- in the clarified part of desulfurized acid decreases to 0.30%, and the suspension content in the suspension increases to 6.3%. A coagulant (e.g. polyacrylamide) is added to the suspension and clarified. The rate of deposition of solid particles in a solution of extraction phosphoric acid is 0.5 m / h. Clarification with the separation of clarified and condensed acids is carried out in a continuous manner on inclined precipitation plates with a linear speed of suspension between the plates of 8 m / h and the output of clarified desulfurized EPA 70% of the load according to P ₂ O ₅ (25132 kg of clarified acid). In this case, the residual suspended matter in clarified acid is maintained at 0.1%. Suspension content in condensed EPA (13138 kg) is 18.1%.

Example 4

36000 kg / h of extraction phosphoric acid obtained by the hemihydrate method, containing 2.0% suspensions, 52.3% P ₂ O ₅ and 1.4% sulfates in terms of SO ₄ ^2- in the liquid phase, is treated at a temperature of 70 ° C and stirring with apatite concentrate in an amount of 519 kg / h. To maintain the desired temperature, 432 kg / h of hot steam are introduced into the reaction volume. In this case, the sulfate content in terms of SO ₄ ^2- in the clarified part of desulfurized acid decreases to 0.50%, and the suspension content in the suspension increases to 3.75%. In order to achieve a solids content of 5% (providing a high clarification rate), part of the condensed EPA (4590 kg) obtained in the clarification step is added to the suspension. A coagulant (for example, polyacrylamide) is also added to the suspension and clarified. The rate of deposition of solid particles in a solution of extraction phosphoric acid is 0.6 m / h Clarification with the separation of clarified and condensed acids is carried out in a continuous manner on inclined precipitation plates with a linear speed of suspension between the plates of 11 m / h and the output of clarified desulfurized EPA 80% of the load according to P ₂ O ₅ (28592 kg of clarified acid). In this case, the residual suspended matter in clarified acid is maintained at 0.5%. Suspension content in condensed EPA (12950 kg of condensed EPA, including 4590 kg of recycled EPA) is 14.9%.

Claims (3)

1. The method of purification of one stripped off extraction phosphoric acid (EPA) obtained from apatite from sulfates and suspensions, including treating it with stirring and an elevated temperature with apatite concentrate, clarification in the presence of a coagulant and separation of clarified desulfurized acid and its condensed part, characterized in that desulfurized acid containing 5-11% of suspensions is fed to the separation and the separation process is carried out continuously in a thin layer on inclined precipitation plates at a linear speed of enzii plates between 3-11 m / hr to obtain at the outlet of the clarified acid 60-80% by uploading P ₂ O _5. 2. The method according to claim 1, characterized in that the desulfurization is carried out to a residual SO ₄ ^2- content in clarified EPA of 0.3-0.5%, and the separation of acids is carried out to a residual suspension of 0.1-0.5%. 3. The method according to any one of claims 1 and 2, characterized in that the acid treatment with apatite concentrate is carried out at a temperature of 70-95 ° C.