RU2372282C1 - Method of producing trisodium phosphate - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to the technology of producing inorganic phosphorous compounds, and more specifically to a method of producing trisodium phosphate (TSP) from wet-process phosphoric acid. The method of producing trisodium phosphate involves neutralisation of phosphoric acid with soda ash, separation of the solution and residue of trace constituents, neutralisation of separated solution with sodium hydroxide at pH=11.5 to 12.0, obtaining a solution of trisodium phosphate, its subsequent cooling, crystallisation of the final product and separation of the crystallisation mother solution, in which neutralisation of phosphoric acid with soda ash is done at the beginning before formation of a solution containing 12 to 14% monosodium phosphate and 27 to 30% disodium phosphate at temperature of 55 to 62°C, and soda ash is then added until obtaining a disodium phosphate solution with concentration 40 to 42%. Introduction of crystallisation mother solution during neutralisation of the separated solution with sodium hydroxide is preferred.

EFFECT: method increases degree of extraction of P₂O₅, improves quality of product and increases technological effectiveness of the process by increasing efficiency of the filtration stage.

2 cl, 3 tbl, 1ex

Description

The invention relates to a technology for the production of inorganic phosphorus compounds, and in particular, to a method for producing trisodium phosphate (THF) from extraction phosphoric acid (EPA), which finds use as a water softener; scale inhibitor; component of detergents for soaking and washing clothes, work clothes; in energy, metallurgy, mechanical engineering, pulp and paper industry (paper bleach), food industry, etc.

As is known, extraction phosphoric acid obtained by acid decomposition of phosphate raw materials contains a rather large amount of impurities, including fluorine, which is previously separated.

A known method of producing trisodium phosphate, in which, along with it, receive as final products sodium fluoride and free of fluorine and silicon phosphates of iron and aluminum. For this, phosphoric acid is neutralized with soda ash at pH = 0.6-0.9. Then, after separation of the cremnefluoride, soda ash is again introduced, the pH being 4.4-4.6. At the next stage of the process, sodium hydroxide is added to the separated solution. As the results of additional studies conducted by us according to the mode of the described method show, in the second stage a precipitate is formed, consisting mainly of iron, aluminum and calcium phosphates, which is almost free of fluorine and silicic acid, and a significant amount of phosphorus is removed in the form of iron and aluminum phosphates etc. (35.6% of the amount administered with EPA). Part of the sodium ions introduced into the process with soda also becomes precipitate - up to 12.3% of the initial amount. Moreover, the precipitate is very poorly filtered, it is necessary to carry out filtration under pressure, and a significant amount of filtering equipment will be required. In the third stage, the disodium phosphate solution is averaged, as usual, with sodium hydroxide to trisodium phosphate. Unfortunately, the example does not indicate the specific technology for producing TNF, its composition and the extraction of P ₂ O ₅ from EPA into it, which is crucial for establishing the quality of a commercial product.

However, in our studies of the method, it was shown that in this case a TNF containing 18.2% P ₂ O ₅ and 0.24% fluorine is obtained, since preliminary removal of fluorine in the form of sodium silicofluoride does not completely remove fluorine from the system. (USSR author's certificate No. 52922, Cl. 16.4; 12i, 31, 1938).

There are also known methods for the production of trisodium phosphate, based on the stepwise or phased neutralization of EPA by carbonate or sodium hydroxide to a pH of 4.0, to obtain monosodium phosphate (with the precipitation and separation of impurities of iron, aluminum and calcium in the form of their phosphates), pH = 9 to obtain disodium phosphate . The main disadvantage is the low extraction of P ₂ O ₅ in the TNF due to phosphorus losses in the intermediate stages with phosphates of iron, aluminum, calcium, etc. and the low quality of the TNF due to the lack of deep purification of the phosphate solution at pH = 4.0-4, four. (Application of Germany No. 2461064, class C01B 25 / 30.1976; Application of Japan No. 54-103799, class 15 G1, C01B 25/30, 1979).

A known method of producing trisodium phosphate, in which phosphoric acid is first neutralized with soda ash to a pH of a disodium phosphate solution of 8.8-9.2, the resulting suspension is separated on a filter press, and a cake containing impurity components (CaHPO ₄ , MgHPO ₄ , RPO ₄ , fluorides) and a purified disodium phosphate solution, to which a mixture of condensed sodium phosphates is introduced in a ratio of Na ₅ P ₃ O ₁₀ : Na ₄ P ₂ O ₇ equal to 0.5-4: 1.

The latter at 70-100 ° C is neutralized with a solution of caustic soda (46% NaOH) to obtain a solution of trisodium phosphate (~ 35.1% Na ₃ PO ₄ ). After cooling and centrifugation, THF (18.5% P ₂ O ₅ ) is obtained, hygroscopicity which at 20 ° C is 77-82%, the content of SO ₃ in trisodium phosphate is 0.08-0.25%.

However, with such a process, the filterability of the insoluble precipitate is 1.2 t / m ² · day, which leads to the need to install a large amount of filtering equipment and increase the loss of P ₂ O ₅ with insoluble sediment to 42.4% due to its high humidity (64, 8%), i.e. reducing the yield of TNF to 57.6% (these results were obtained by testing the method according to ed. certificate. No. 2145572 in laboratory conditions). Moreover, the fluorine content in TNF is 0.3%, and the yield of insoluble residue is 0.08%, which is almost 3 times higher than required. (RF patent No. 2145572, CL. С01В 25 / 30,2000).

As a prototype, we have chosen a known method for the production of trisodium phosphate, including the neutralization of phosphoric acid with soda ash, separation of the solution and the precipitate of impurity components, the neutralization of the separated solution with sodium hydroxide at pH = 11.5-12.0 to obtain a solution of trisodium phosphate, followed by cooling, crystallization of the finished product and separation of the mother liquor crystallization.

The known method involves carrying out a preliminary stage of purification of acid from fluorine by introducing soda ash in it in an amount providing pH = 0.6-0.9 (similar to the stage according to the method described in A.S. No. 52922, where the initial content of F 1.2-1.5%).

Due to the fact that we are sending phosphoric acid purified from a large amount of fluorine for processing, this stage is not carried out and the methods are compared starting from the next stage, as it were, “basic neutralization of EPA with soda ash”.

The main stage of neutralization with soda ash is carried out until no more than one hydrogen ion is replaced by sodium, i.e. before the formation of monosodium phosphate, the pH of the formation of which is 4.4. The process is conducted at a temperature of 75-90 ° C. In this case, phosphates of iron, aluminum and other compounds precipitate.

Then sodium hydroxide is added to the solution (as one of the options). To obtain trisodium phosphate, the pH is maintained between 11.5 and 12.0. Since the description of the method does not contain examples of its implementation, the method was reproduced and the following data were obtained: the extraction of P ₂ O ₅ in TNF is 65.7%, since part of P ₂ O ₅ (2.6%) is precipitated with sodium cremofluoride moisture and a significant amount of P ₂ O ₅ (31.7%) with phosphates of iron, aluminum, calcium and other compounds. In this case, get TNF, which contains 92% of the basic substance, 18.0% P ₂ O ₅ ; 0.35% fluorine and 0.8% insoluble residue.

The content of P ₂ O _{5 is} lower than that required by GOST (at least 18.5%), and the insoluble residue is much higher than the required (0.03%), since not all impurities of trivalent metals and calcium precipitate during the formation of monosodium phosphate (pH = 4 , 4), i.e. when one atom is replaced with a hydrogen ion, the remaining ions precipitate from the so-called purified phosphoric acid solution in TNF, including including a significant amount of water-insoluble residue; the same applies to the fluorine content: upon preliminary removal of fluorine in the form of sodium silicofluoride and at the stage of neutralization to monosodium phosphate, fluorine does not completely withdraw from the system, and it passes into the precipitate, being a ballast for P ₂ O ₅ . (AC USSR No. 62810, 12i, 31.1941).

Our goal is to increase the degree of extraction of P ₂ O ₅ while significantly improving the quality of the product and increasing the processability by increasing the productivity of the filtration stage.

The problem is solved in the proposed method for producing TNF, including neutralization of phosphoric acid with soda ash, separation of the solution and precipitate of impurity components, neutralization of the separated solution with sodium hydroxide at pH = 11.5-12.0 to obtain a solution of trisodium phosphate, followed by cooling, crystallization of the finished product and separation crystallization mother, in which the neutralization of EPA with soda ash is carried out first until a solution containing 12-14% monosodium phosphate and 27-30% disodium phosphate is formed at a temperature Ur 55-62 ° C and then calcined soda are added to obtain a disodium phosphate solution with a concentration of 40-42%. It is advisable to enter the crystallization mother liquor at the stage of neutralization of the separated solution with sodium hydroxide.

The essence of the method is as follows. The process of neutralizing phosphoric acid with soda ash is carried out as if "in stages." First, neutralization of the extraction phosphoric acid with soda ash is carried out until a solution containing 12.0-14% monosodium phosphate and 27.0-30.0% disodium phosphate is formed at a temperature of 55-62 ° C, and then soda ash is added until a disodium phosphate solution with a concentration of 40-42% and they separate the solution and the precipitate of impurity components, neutralize the separated solution with sodium hydroxide at pH = 11.5-12.0 to obtain a solution of trisodium phosphate, its subsequent cooling, crystallization.

The neutralization of extraction phosphoric acid with soda ash is carried out first until a solution containing 12.0-14% monosodium phosphate and 27.0-30.0% disodium phosphate is formed, because under these conditions phosphoric acid is neutralized with soda ash with complete quantitative separation of impurities from phosphoric acid, and a temperature of 55-62 ° C provides the creation of conditions for the formation of well-filtering precipitates, which are a mixture of phosphates of iron, aluminum, calcium phosphate, calcium fluoride and silicon dioxide.

So, with the exclusion of this stage of neutralization and the process until the formation of the disodium phosphate solution, the filtering properties of the impurity precipitate deteriorate to 1.8 t / m ² · day; the extraction of P ₂ O ₅ from phosphoric acid in THF is reduced to 94.4%, and the loss of sodium carbonate is 4.8%.

When the process is conducted in a mode in which only monosodium phosphate is formed (pH = 4.2-4.5), the quantitative precipitation of the impurity precipitate, in particular fluorine and calcium ions, proceeds to the stage of the formation of disodium phosphate only, which reduces the filterability of the impurity precipitate pulp up to 4.3 t / m ² · day. This reduces the yield of P ₂ O ₅ (97.7%) and an increase in the loss of sodium carbonate to 1.9%. In addition, the concentration of monosodium phosphate and disodium phosphate must be strictly determined. At a concentration of sodium phosphate solution in the liquid phase of a mixture of mono- and disodium phosphate in the ratio of 16% MNF and 32% DNP, sodium phosphates are salted out into the impurity precipitate, while the yield of P ₂ O ₅ in TNF is 97.6%, and the loss of sodium carbonate - 1.7%.

At a concentration of a sodium phosphate solution in the liquid phase of a mixture of mono- and disodium phosphate in the ratio of 10% MNF and 25% DNP, the system is diluted by introducing excess water into it, which in subsequent operations leads to the formation of excess water in the form of phosphoric acid solutions, which must be removed from the process; in the case when there is no other production facility for the disposal of these salts nearby, it is necessary to carry out the processes of evaporation of these solutions and return them to the production cycle of TNF, which requires certain energy and production costs.

A very important parameter is to maintain the process temperature at the stage of formation of a mixture of mono-and disodium phosphate, equal to 55-62 ° C.

An increase in temperature to 70-90 ° C under the same conditions leads to a significant deterioration in the filterability of sludge pulp (2.1 t / m ² · day), apparently due to the transfer of trace amounts of silicon dioxide into a colloidal form, at which filter cloth is coated. the press, which requires frequent replacement and is associated with process shutdowns and a decrease in its productivity.

When the temperature drops to 45-50 ° C (it is necessary to cool the system to maintain it), the kinetics of the neutralization process slows down, while the duration of the carbon dioxide removal process in the first stage increases from 30 to 45 minutes, which reduces the productivity of the process. In addition, due to an increase in the viscosity of the suspension, the filterability of the impurity sludge pulp decreases to 5.6 t / m ² · day. The concentration of the disodium phosphate solution in the second stage of neutralization with soda ash is also regulated, because when the concentration of DNF in the solution is less than 40%, for example 38%, with the further introduction of sodium hydroxide into it, a 25.1% solution of TNF is formed, i.e. more diluted than in the optimal mode (1.8%), from which less TNF crystals are released upon cooling. Therefore, with the crystallization mother, a larger amount of sodium phosphate goes to the stage of neutralizing the DNF solution with hydroxide, which increases the load on the neutralization unit and requires a larger amount of equipment.

When the concentration of DNF in the solution is more than 42%, for example 44%, due to the supersaturation of the solution with DNF, DNF crystals begin to precipitate, which negatively affects the subsequent stage of neutralization of the DNF solution with hydroxide: precipitated DNF crystals may not pass into the TNF crystals, impairing the quality of the finished product product.

Attention should also be paid to the fact that it is advisable according to the proposed method to obtain a solution of TNF by neutralizing a solution of disodium phosphate with sodium hydroxide in the presence of a crystallization mother liquor, which creates the conditions for obtaining such a modification of TNF crystals that practically does not cake, which was established experimentally. The mechanism of this phenomenon requires a separate study. However, under these conditions, the introduction of an expensive conditioning additive is not required.

The proposed method has been tested in laboratory conditions. The following are the test results (example 1).

Example 1

In 762 kg of phosphoric acid containing 26.0% P ₂ O ₅ , 234.0 kg of soda ash (99% Na ₂ CO ₃ ) was added at a temperature of 58 ° C for 30 minutes, while a suspension of mono - and disodium phosphate solution, in the liquid phase of which the content of monosodium phosphate (MNF) was 13.0%, and disodium phosphate (DNF) 28.5%; then, 50.0 kg of soda ash was added to it at a temperature of 63 ° C for 30 minutes, while a suspension of disodium phosphate solution was formed, in the liquid phase of which the content of DNP was 41.0%. Next, the resulting system (930 kg) was sent to the separation of the solution and the precipitate of impurity components by vacuum filtration on a suction filter. An impurity precipitate in an amount of 5 kg (3.6 kg dry) was removed from the system, dried and analyzed for the content of P ₂ O ₅ , Na, F, W, and in 925 kg of a DNF solution, a crystallization mother liquor from previous experiments (1727 kg) was introduced and neutralized the resulting mixture of 234 kg of a 45% sodium hydroxide solution at a temperature of 75-80 ° C for 60 minutes. The resulting solution of trisodium phosphate (26.9% THF) in the amount of 2887 kg was cooled to a temperature of 25-27 ° C. After that, the system was kept for 6 hours and sent for centrifugation. TNF crystals (1159 kg) were removed from the system, and the crystallization mother liquor was returned to the stage of DNP neutralization with a solution of sodium hydroxide, etc. After reaching a constant technological mode, the yield of all products was determined and the TNF crystals (1000 kg) dried at 45–50 ° С and the mother liquor were analyzed for the required parameters. Table 1 presents the experimental results.

Table 1 The test results of the proposed method for producing TNF (in terms of dry product) Name Structure, % Extraction in TNF,% P ₂ O ₅ But. pH 1% solution W Na F P ₂ O ₅ Na Trisodium phosphate 1000 kg (after drying at 45-50 ° C) 18.7 0.02 12.0 3.4 18.2 0.001 99.0 99.3 Impurity sediment 3.5 kg (in terms of dry) 56.6 30,0 24.4 8.1 1,0 0.7

Note. The filtration performance of the disodium phosphate suspension was 7.9 t / m ² · day with a sediment layer height on the filter of 20 mm. From table 1 it follows that as a result of the experiment obtained TNF corresponding to GOST 201-76 (see appendix).

Table 2 shows the results of other experiments conducted by the proposed technology.

Thus, the use of the proposed method will increase the degree of extraction of P ₂ O ₅ from EPA in TNF from 65.7% (according to the prototype) to 98.8-99%, improve the quality of TNF to a level corresponding to GOST 201-76 (attached), increase filtration performance up to 7.5-8.4 t / m ² · day.

application Characterization of trisodium phosphate (GOST 201-76) No. p / p Name of indicator Norm one. Appearance Crystals or flakes capable of caking 2. Mass fraction of total P ₂ O ₅ ,%, not less than 18.5 3. pH of 1% aqueous solution 11.5-12.5 four. Mass fraction of residue insoluble in water,%, no more 0,03

Claims (2)

1. A method of producing trisodium phosphate, including the neutralization of phosphoric acid with soda ash, separation of the solution and the precipitate of impurity components, neutralization of the separated solution with sodium hydroxide at pH 11.5-12.0 to obtain a solution of trisodium phosphate, its subsequent cooling, crystallization of the finished product and separation of the crystallization mother liquor characterized in that the neutralization of phosphoric acid with soda ash is carried out first until a solution containing 12-14% monosodium phosphate and 27-30% disodium phosphate is formed at a temperature of 55-62 ° C, and then soda ash is added to obtain a solution of disodium phosphate with a concentration of 40-42%. 2. The method according to claim 1, characterized in that the crystallization mother liquor is introduced into the stage of neutralization of the separated solution with sodium hydroxide.