RU2378192C1 - Method of producing food grade sodium tripolyphosphate - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to chemical engineering of food grade products which can be used in meat and diary and fish industries, cheese-making and other similar industries, and specifically to methods of producing food grade sodium tripolyphosphate. The method of producing food grade sodium tripolyphosphate involves formation of anhydrous sodium tripolyphosphate based on phosphoric acid and soda, its hydration in a water-salt solution in the presence of a nucleating agent, separation of the suspension which contains ammonium and/or alkali metal sulphate salts, and hexahydrate of sodium tripolyphosphate, as well as drying the latter. The hexahydrate is dried in two steps, using a hexhydrate suspension containing 10-25 wt % water at the first step, and keeping temperature in the 80-100°C range at the first step, and in the 150-200°C range at the second step.

EFFECT: invention allows for achieving up to 56,3-57,0% diphosphorus pentoxide in the end product, increasing content of polyforms to 97,9-98,2%, and reduce pH of 1% aqueous salt solution to 8,9-9,2.

3 tbl, 6 ex

Description

The present invention relates to the field of chemical technology of food qualification products that can be used in the meat and dairy and fish industries, cheese making and other similar fields, in particular to methods for producing sodium tripolyphosphate of food qualification, the raw materials for the production of which are phosphoric acid and soda (caustic and / or calcined).

Numerous methods are known for producing technical anhydrous sodium tripolyphosphate based on phosphoric acid and soda (M.E. Pozin. Technology of mineral salts. Part 2, Chemistry, 1974, pp. 1078-1082). The anhydrous salt formed as the target product is metastable and, when stored in air or when added to water, crumbles, sticks together and sets (US Patent No. 4790984 “Method for the Preparation of Sodium Tripolyphosphate Powder”, MKI C01B 25/41, published 10.08.87, publ. 12.13.88), which degrades consumer qualities, for example, of detergent compositions, which include such sodium tripolyphosphate. It is preferable to use sodium tripolyphosphate hexahydrate (Na ₅ P ₃ O ₁₀ · 6H ₂ O), which is a water-stable form.

A known method of producing sodium tripolyphosphate hexahydrate by forming an anhydrous salt, dissolving it in water, separating impurities, followed by vacuum crystallization to isolate crystals from the mother liquor and purified target product and drying (K.S. Zotova, I.M. Hoffman. Fertilizer industry and sulfuric acid, NIITEKHIM, No. 4, 37, 1966). The disadvantages of the process are its complexity associated with the need to operate vacuum crystallization equipment, as well as the low drying intensity, which, in order to avoid the conversion of tripolyforms to ortho or pyroforms (degradation), is proposed to be carried out using moistened flue gases (W Groves, US Patent No. 3063801, 1962) or water vapor (F. Rodis, Y. Krause, K. Beltz, German Patent No. 1007748, 1961). It is known that sodium tripolyphosphate hexahydrate can be precipitated from a concentrated aqueous solution of anhydrous salt by saturating the solution with sodium chloride (Van Weser. Phosphorus and its compounds. - M .: Publishing House of Foreign Literature, 1962, p. 495) or by treating it with acetone or ethanol (ibid.). In the first case, the product is contaminated with sodium chloride, in the second process it is associated with a significant complication of the technology due to the need for regeneration of the organic phase.

A known method for producing purified sodium tripolyphosphate hexahydrate, including the formation of anhydrous tripolyphosphate based on phosphoric acid and soda, its hydration in a water-salt solution in the presence of a seed, separation of the suspension of the mother liquor and the target product, and drying of the latter in air at room temperature and a relative humidity of 40- 60% (Van Weser). With the comparative simplicity of the technology, it is characterized by low intensity both at the dissolution stage and at the drying stage. Equilibrium values are achieved over a long time of tens of hours, despite the use of sodium tripolyphosphate hexahydrate in the process of seeding.

The closest in technical essence and the achieved positive effect (prototype) is a method for producing purified (food) sodium tripolyphosphate, including the formation of anhydrous sodium tripolyphosphate on the basis of phosphoric acid and soda, its hydration in water-salt solution in the presence of seed, separation of the suspension of the mother liquor, containing sulfate salts of alkali metals and / or ammonia and purified (food) sodium tripolyphosphate hexahydrate, as well as drying the latter at 55-110 ° C (RF Patent No. 2148011, CL 25B 25/41, Dec. 17.12.98, publ. 27.04.2000, Bull. No. 12). The disadvantage of this method is the low quality of the product, expressed in its low content of P ₂ O ₅ (43-44% by mass in a natural product) and high hydrogen index of a 1% aqueous solution equal to 9.5-10.0 pH units.

The objective of the proposed technical solution is to improve the quality of the target product by increasing the content of P ₂ O _{5 in it} and decreasing the pH value of its 1% aqueous solution.

The problem is achieved in that in a method for producing sodium tripolyphosphate of food grade, including synthesis based on phosphoric acid and soda of anhydrous sodium tripolyphosphate, its hydration in a water-salt solution in the presence of a seed, separation of a suspension of a mother liquor containing sulfate salts of alkali metals and / or ammonium and tripolyphosphate hexahydrate, as well as drying the latter, drying the hexahydrate is carried out in two stages, using the suspension of hexahydrate containing 10-25 wt.% water in the first stage, and odderzhivaya temperature in the first drying step in the range 80-100 ° C, a second drying stage is carried out at 150-200 ° C.

The inventive method allows to improve the quality of the target product by increasing the content of P ₂ O _{5 in it} to 56.3-57.0% and reducing the pH value to 8.9-9.2 pH units.

The basis for the creation of technical elements of the novelty and usefulness of the claimed method, as well as the choice of the ranges of variation of the physicochemical parameters presented in the characterizing part of the claims, were the studies performed by the authors at the State Technological Institute (St. Petersburg) on the synthesis of purified six-water sodium tripolyphosphate crystalline hydrate (Na ₅ P ₃ O ₁₀ · 6H ₂ O) and the establishment of the necessary drying parameters (temperature and humidity of the suspension in the first stage of removal of absorbent water), both sintering the preservation without dehydration of tripolyforms (P ₃ O ₁₀ ^5- ) and obtaining ultimately anhydrous purified highly concentrated (according to P ₂ O ₅ ) food grade salts.

It was found that the decisive factor determining the receipt of a product of the claimed quality in terms of P ₂ O ₅ and the value of the hydrogen index is the maintenance of the claimed parameters in the drying process by the moisture content of the suspension and the temperature in the second stage of drying. In this case, the second stage of dehydration should be carried out under conditions of the practically absence of ammonium salts in the dried material (in the second stage).

The test results are shown in tables 1-3. The data shown in table 1 indicate that the maximum moisture content of the suspension at which purified sodium tripolyphosphate hexahydrate is obtained, which enters the first stage of drying, should not exceed 25% in terms of water. At higher humidity in the process, there is a loss of tripolyform. A decrease in the moisture content of a suspension of hexahydrate of less than 10% requires the use of relatively low-productivity and complex centrifuges, which is not advisable for the process of separation of such suspensions.

Example 1 (prototype). 35 t / h of extraction phosphoric acid (27% P ₂ O ₅ , 0.3%

SO ₃ ) is treated with 11.2 t / h of soda ash (97.5% Na ₂ CO ₃ , 2.5% K ₂ SO ₄ ). As a result of the subsequent separation of impurities by filtration, drying and calcination of solid material, 14.4 t / h of anhydrous sodium tripolyphosphate (TPPN) of technical qualification are obtained and containing 55.8% P ₂ O ₅ , 1.9% K ₂ SO ₄ and 1.2 % Na ₂ SO ₄ . The level of the first form of Na ₅ P ₃ O ₁₀ after calcination is 28-32%. 12.4 t / h TPFN is used as raw material for synthetic detergents (CMC), 2 t / h of material is treated with 7 t / h of seed - six-water (hexahydrate) sodium tripolyphosphate (44% Р ₂ О ₅ ) and for 90 minutes carry out hydration against the background of potassium and sodium sulfates (0.49% K ₂ SO ₄ and 0.31% Na ₂ SO ₄ ) in the liquid phase, respectively. 9.5 t / h of the suspension are separated on a vacuum filter, obtaining 2.12 t / h of sediment (humidity 20%) of sodium tripolyphosphate and 7.31 t / h of the mother liquor containing 13% Na ₅ P ₃ O ₁₀ , which is disposed of in the production cycle of a technical product (form 0.95 t / h Na ₅ P ₃ O ₁₀ ). When drying the crystalline hydrate precipitate, 1.69 t / h of a salt of the composition Na ₅ P ₃ O ₁₀ · 6H ₂ O and containing 44% P ₂ O ₅ and 98% tripolyform are obtained. The hydrogen index of a 1% aqueous salt solution corresponds to 9.8 pH units.

Examples 2-6 show the implementation of the process according to the described method.

Example 2. 35 t / h of extraction phosphoric acid (27% P ₂ O ₅ , 0.3% SO ₃ ) are treated with 11.2 t / h of soda (97.5% Na ₂ CO ₃ , 2.5% K ₂ SO ₄ ). As a result of the subsequent separation of impurities by filtration, drying and calcination, 14.4 t / h of technical anhydrous TPPN are obtained containing 55.8% P ₂ O ₅ , 1.9% K ₂ SO ₄ and 1.2% Na ₂ SO ₄ . The level of the first form of tripolyphosphate after calcination is 30%. 12.4 t / h TPFN is used as a raw material for obtaining CMC, a 2 t / h is treated with 7 t / h of water, 0.5 t / h of seed are added for six-water sodium tripolyphosphate (44% P ₂ O ₅ ) and for 1 , 5 hours carry out hydration against the background of potassium and sodium sulfates (0.49 and 0.31%) in the liquid phase, respectively. 9.5 t / h of the suspension are separated on a vacuum filter, obtaining 2.12 t / h of wet sediment (humidity 20%) and 7.31 t / h of a mother liquor containing 13% sodium tripolyphosphate. Next, the precipitate is dried in two stages. The first stage is carried out at 95 ° C for 2 hours, the second for 0.5 hours at 170 ° C. In the first stage, in addition to water, ammonium nitrates completely pass into the gas phase. Ammonium nitrate is absent in the material to be dried. 0.42 t / h of water is also removed to the gas phase in the first drying stage, and 0.39 t / h, respectively, in the second stage. The result is 1.3 t / h anhydrous food grade sodium tripolyphosphate containing 56.3% P ₂ O ₅ and 3% potassium and sodium sulfates. The pH of a 1% aqueous solution of salt is 9.0 pH units. The content of tripolyforms in the target product is 98%. 6.45 t / h of the mother liquor are disposed of upon receipt of anhydrous sodium tripolyphosphate of technical qualification, receiving an additional 0.84 t / h of technical product. The total productivity of a technical product with a coefficient of equipment utilization of 90% is 104,000 tons / year, for salt - 10,200 tons / year.

Example 3. The process is carried out analogously to example 2 with the only difference that the second stage of drying is carried out at 150 ° C for 45 minutes. The result is 1.3 t / h of anhydrous food grade sodium tripolyphosphate. The hydrogen index of a 1% aqueous solution is 8.9 pH units, the content of P ₂ O ₅ in tripolyphosphate is 56.8 wt.%.

Example 4. The process is carried out similarly to examples 1 and 2 with the only difference that the second stage of drying is carried out at 200 ° C for 20 minutes. The result is 1.3 t / h of anhydrous food grade sodium tripolyphosphate. The hydrogen index of a 1% aqueous solution is 9.2 pH units, the content of P ₂ O ₅ in tripolyphosphate is 57.0 wt.%.

Example 5. The process is carried out analogously to example 2 with the only difference that when vacuum-filtered suspension receive 2.27 t / h of sediment with a moisture content of 25%. The precipitate is dried initially at 100 ° C for 2 hours. In the gas phase, 0.58 t / h of water is removed. In the second stage, drying is carried out for 0.5 h at 170 ° C, removing 0.39 t / h of water and get 1.3 t / h of anhydrous food grade sodium tripolyphosphate containing 56.3% P ₂ O ₅ . The yield of tripolyform in the target product reaches 97.9%. The pH of a 1% aqueous solution of edible salt is 9.0 pH units.

Example 6. 35 t / h of extraction phosphoric acid (27% P ₂ O ₅ and 0.3% SO ₃ ) are treated with 11.2 t / h of soda (97.5% Na ₂ CO ₃ and 2.5% K ₂ SO ₄ ). As a result of the subsequent separation of impurities, drying and calcination in the presence of NH ₄ NO ₃ supplied to the process in an amount of 75 kg / h, 14.4 t / h of technical anhydrous TPPN containing 55.9% P ₂ O ₅ , 1.9 are obtained % K ₂ SO ₄ , 1.2% Na ₂ SO ₄ and 100 ppm nitrates (in terms of nitrogen). The level of the first form of tripolyphosphate in the product after calcination is 10%. 13 t / h TPFN is used as a raw material for obtaining CMC, and 1.4 t / h are treated with 5 t / h of water, 0.35 t / h of seed is added - hexahydrate sodium tripolyphosphate (44% P ₂ O ₅ ) and during 1.25 hours carry out hydration against the background of potassium sulfate, sodium and ammonium nitrate. A suspension separation step is carried out on a press filter at a pressure of 12 bar. As a result, the moisture content of the sediment can be reduced to 10% H ₂ O, and its mass is 1.90 t / h. The precipitate is dried in two stages. The first stage is carried out at 80 ° C for 2 hours, removing 0.20 t / h of water and 52 kg / h of NH ₄ NO ₃ in the gas phase. The second stage is carried out in the absence of nitrates for 0.5 h at 170 ° C, removing 0.39 t / h of water, and 1.3 t / h of food grade sodium tripolyphosphate containing 56.4% P ₂ O _{5 is} obtained. The yield of tripolyforms in the target product is 98.0%. The pH of a 1% aqueous solution of edible salt is 9.0 pH units.

Thus, the proposed method allows to bring the level of diphosphorus pentoxide (P ₂ About ₅ ) in the target product to 56.3-57.0% instead of 43-44% in the product of the prototype. The content of the polyform increased to 97.9-98.2%, and the pH of a 1% aqueous salt solution decreases to 8.9-9.2 pH units instead of 9.5-10.0 pH units for tripolyphosphate obtained in a known manner .

Table 1
The effect of humidity of the initial suspension supplied to the first stage of drying (temperature 95 ° C) on the yield of tripolyform after salt dehydration The moisture content of the suspension, wt.% H ₂ About The yield of tripolyforms, after removal of hygroscopic moisture,% 10 98.2 fifteen 98.0 twenty 98.0 25 97.9 27 90.0 thirty 62.0

table 2
The effect of drying temperature in the second stage of dehydration on the content of P ₂ O ₅ in the product (drying time 90 minutes) and the pH of a 1% solution of dried salt (NH ₄ NO ₃ in the material is absent) The temperature of the 2nd stage of drying, ° C Wt.% P ₂ O ₅ in dehydrated tripolyphosphate PH value, pH unit 140 54.5 8.8 150 56.8 8.9 175 57.0 9.0 200 57.0 9.0 210 57.1 9.5 230 57.1 9.6

Table 3
The effect of ammonium nitrate additives on the dehydration of sodium tripolyphosphate during drying of hexahydrate salt (temperature 150 ° С) The amount of additive NH ₄ NO ₃ in the dried material, ppm The content of P ₂ O ₅ in anhydrous sodium tripolyphosphate,% wt. Lack of 56.8 one hundred 55.0 500 54.8 1000 54.1

Claims (1)

A method for producing food grade sodium tripolyphosphate, including synthesis based on phosphoric acid and soda of anhydrous sodium tripolyphosphate, its hydration in an aqueous salt solution in the presence of a seed, separation of a suspension of a mother liquor containing sulfate salts of alkali metals and / or ammonium, and sodium tripolyphosphate hexahydrate, as well as drying the latter, characterized in that the drying of the hexahydrate is carried out in two stages, using in the first stage a suspension of hexahydrate containing 10-25 wt.% water, and maintaining those perature in the first drying step in the range 80-100 ° C, a second drying stage is carried out at 150-200 ° C.