RU2200703C1 - Sodium tripolyphosphate production process - Google Patents

Abstract

FIELD: industrial inorganic synthesis. SUBSTANCE: process of production of sodium tripolyphosphate appropriate for use in chemical, textile, household, and food areas comprises desulfatization of acid with calcium and barium compounds, defluorination with soda, removal of calcium sulfate and sodium silicofluoride precipitates, and neutralizing barium sulfate suspension with soda followed by separation of phosphorus-containing impurity precipitate. Process further envisages neutralization of extraction phosphoric acid with soda conducted until Na/P ratio in liquid phase is reached 1.62-1.63; 0.1-0.3% of activated carbon is added at 70-75 C followed by 25-40-min aging and product after drying containing 70-85% of fraction -4 to +1 mm is subjected to calcination conducted in drum-type calcinator provided with inverse screw and supporting rings at temperature of incoming gases 615-635 C and released gases 450-470 C, residence time product in calcinator being 3-5 h. Degree of conversion of orthophosphates into sodium tripolyphosphate achieves 97.5-98.5%. Product includes 0.003-0.01% iron, 0.03-0.1% calcium oxide, and 0.03-0.1% fluorine. Content of l-form of tripolyphosphate ranges between 35 and 65%. ROH test data: 81-89 C after 1 min and 88-94 C after 5 min. EFFECT: increased yield and purity of product. 2 tbl, 3 ex

Description

 The invention relates to techniques for producing sodium tripolyphosphate used in the chemical, textile, household and food industries.

A known method of producing sodium tripolyphosphate (Pozin ME Technology of mineral salts. M., 1970, p.1076-1082), which consists in neutralizing the extraction of phosphoric acid with soda to pH 6-7 with obtaining solutions of sodium phosphates in which the molar ratio Na ₂ O: P ₂ O ₅ is 5: 3, separation of the impurity precipitate, evaporation of the clarified solution, followed by dehydration of salts at 340-400 ^o C. Get a product containing 92-93% of the basic substance.

 The disadvantage of this method is the low content of the basic substance.

A known method of producing sodium tripolyphosphate (SU, copyright certificate, 352835, class C 01 B 25/30, 1970) by neutralizing thermal phosphoric acid with soda to obtain a solution of a mixture of mono - and disodium phosphates, followed by dehydration. A powdery mixture of phosphates is granulated at 150-200 ^° C and calcined at 470-500 ^° C in a fluidized bed apparatus in the presence of additives that catalyze the calcination process - sodium tripolyphosphate or sodium hexametaphosphate.

 The disadvantage of this method is the multi-stage, the use of expensive thermal phosphoric acid, the instability of the product quality due to the introduction of dry catalytic additives, the low content of I form of sodium tripolyphosphate and the mismatch of the product in terms of hydration rate (RON test) to the best world samples.

The main indicator that is used to characterize the quality of sodium tripolyphosphate is the hydration rate or ROH test. Good sodium tripolyphosphate has a hydration rate, expressed through an increase in temperature as a result of the thermal effect of dissolution in sodium sulfate solution, over 1 and 5 minutes, respectively 81-89 and 88-94 ^o C.

The closest to the proposed technical essence and the achieved result is a method of producing sodium tripolyphosphate (RU, patent, 2102314, class C 01 B 25/41, 1996), including desulfurization of the acid with calcium compounds to obtain a suspension containing 0.4 in the liquid phase -0.8% CaO, neutralization of calcium-containing phosphoric acid solution with soda to pH 0.7-2.5, separation of the precipitate of calcium sulfate and sodium silicofluoride, deep desulfurization of the filtrate with barium compounds, which are taken from the calculation of the molar ratio of BaO: Ca 0.2-0.4, continuous neutralization of phosphoric acid with soda suspension of barium sulphate to a mole ratio Na ₂ O: P ₂ O ₅ and 1,05-1,08 1,63-1,65 at 80-85 ^o C. during 2.0-2.5 hours at each stage. The degree of conversion of orthophosphates to sodium tripolyphosphate is 97-99%. The content of I form of sodium tripolyphosphate up to 3-5%.

 The disadvantage of this method is the mismatch of sodium tripolyphosphate to the best world samples according to the ROH test, the high content of such trace elements as calcium, iron and fluorine in the dried product.

The objective of the invention is to achieve favorable hydration rates (ROH) for 1 min 81-89 ^o C and 5 min 88-94 ^o C, the reduction of microimpurity in the dried product.

To do this, in the method of producing sodium tripolyphosphate, including desulfurization of the acid to a content of 0.6-1% CaO, its defluorination to a concentration of 0.1-0.3% fluorine in the solution, the separation of calcium sulfate and sodium silicofluoride, deep desulfation of a phosphoric acid solution to the content of 0.1-0.15% SO ₃ in the liquid phase, without separation of barium sulfate, purification of macro- and microimpurities as a precipitate when neutralizing acid with soda, separation of pulp by filtration with separation of sludge from impurities, and addition of 1-3% sodium tripolyphosphate into the resulting slit sodium phosphate, their concentration, drying, calcination of the dried product, neutralization of phosphoric acid with soda lead to an atomic ratio of Na: P = 1.62-1.63 and 0.1-0.3% activated carbon is added to the resulting pulp before filtration and incubated at 70-75 ^o C for 25-40 minutes, and the calcination of the dried product containing 70-85% fractions from -4 to +1 mm is carried out in a drum-type calciner with a reverse screw and retaining rings while maintaining the temperature of the incoming gases 615 -635 ^o C, and the exhaust gases - 450-470 ^o C and the residence time of the product in calcium inator 3-5 hours

The amount of impurities and their nature contained in the dried product have an important effect on the performance of the ROH test. The neutralization of extraction phosphoric acid to an atomic ratio of Na: P (metal titer) of 1.62-1.63 and the addition of 0.1-0.3% activated carbon to the pulp at a temperature of 70-75 ^o C and holding for 25-40 minutes allows for optimal pH to further precipitate calcium, fluorine and iron. The presence in the dried product of a small excess of sodium dihydrogen phosphate (after calcination - sodium metaphosphate), as well as a decrease in the content of trace elements, allows to obtain sodium tripolyphosphate that meets the requirements of the ROH test, even with a relatively small content of I form (30-50%).

 The lower limit of the metal titer (1.62) is determined by the yield of the main product sodium tripolyphosphate, which should not be lower than 97%. Above the upper limit of the metal titer (1.63), it is not possible to stably obtain a product that meets the ROH test.

 The amount of activated carbon (0.1-0.3%) is limited, on the one hand, by the adsorption capacity, and on the other hand, by economic feasibility (increasing the content of activated carbon does not lead to a significant effect - the concentration of impurities varies little).

 The selected temperature regime and the exposure time of the suspension with activated carbon make it possible to have an optimal result on the adsorption of impurities with the specifics of this process: the temperature dependence of the solubility and chemical composition of the components of the sludge.

Calcination is carried out in a drum with a reverse screw and retaining rings. After drying, the product should contain 70-85% fractions from -4 to 1 mm. The gas temperature at the inlet to the calciner is 615-635 ^o С, and at the outlet - 450-470 ^o С. The residence time of sodium tripolyphosphate in the calciner is 3-5 hours.

 The use of a coarse-grained product for calcination allows the product to be uniformly heated to obtain an approximately constant composition over the cross section of the granules. A finely dispersed product, falling into one or another zone of the calciner, is more prone to fluctuations in content, in particular I form. The content of 70-85% of the large fraction allows you to stably obtain a product with desired properties. An increase in granule size of more than 4 mm will require an increase in the heating time or the residence time of the product in the calciner. In addition, an unfavorable (uncontrolled) redistribution of the I form occurs in the volume of the granule. When the size is less than 1 mm, granules can be melted due to the large specific heat load on the surface at the entrance to the calciner drum.

The temperature regime of the inlet flue gases (615-635 ^o C) allows you to intensively carry out the calcination of the product, balancing near the melting point of sodium tripolyphosphate - 625 ^o C. The temperature of the exhaust gases (450-470 ^o C) is associated with both an enantiotropic temperature (425 ^o C) the transition of the II form to the I form, and with the size of the granules of the product and its residence time in the calciner. Under all other conditions, a calcination time of 3-5 hours is sufficient to obtain sodium tripolyphosphate of a given quality.

Example 1. 2500 kg of phosphoric acid containing 27% P ₂ O ₅ , 1.8% F, 2% SO ₃ , 0.2% Fe, 0.24% Al, mixed with 78 kg of apatite concentrate (39% P ₂ O ₅ , 52% CaO), incubated for 1.5 hours at a temperature of 80 ^o C, add 83.7 kg of anhydrous soda (200% of stoichiometry), incubated for 1 h at a temperature of 70 ^o C. Suspension containing in the liquid phase 0, 8% CaO, 0.4% SO ₃ and 0.2% F, are filtered, the precipitate is washed with 600 kg of hot water, 253 kg of wet sediment (W = 40%, including 66 kg of sodium silicofluoride) with a content of 2, 5% P ₂ O _{5 total} sent to the dump. Wash solutions containing phosphoric acid are used either in the washing phase of phosphogypsum in the phosphoric acid extraction unit, or the first filtrate obtained is diluted. The preliminary stage of defluorination and desulfurization of phosphoric acid can be organized in another way, namely: first, phosphoric acid is defluorinated with soda to separate sodium silicofluoride, which after washing with water can be used as a by-product, and then gypsum is deposited, followed by washing on the filter from phosphoric acid with water. When the stage is divided, the water balance is maintained. In 2974 kg of defluorinated and desulfurized phosphoric acid containing 23.4% P ₂ O ₅ and 0.35% SO ₃ , 18.3 kg of barium carbonate are added. The resulting suspension of barium sulfate (2988.3 kg) is treated with a circulating neutralized pulp from the second stage to a metal titer of 1.1. Anhydrous soda in the amount of 800 kg is introduced into the second stage to maintain the metal titer (Na: P) in the liquid phase of the pulp equal to 1.62. Before filtering, 3.46 kg of activated carbon (0.1%) is added to the production pulp of the second stage (3456.3 kg) and kept at a temperature of 70 ^{° C.} for 25 minutes. The separation of the pulp on the filter and washing the sludge with hot water (873 kg) gives 770 kg of cake (W = 60%, including 119 kg of P ₂ O ₅ ). B 3562.5 kg of liquors (I filtrate) containing 16% P ₂ O ₅ , 0.003% Fe, 0.03% CaO, 0.03% F, 73 kg of sodium tripolyphosphate are introduced both from the calcination stage and in the form of absorption solutions from the stage of water washing of the exhaust gases from the drying drums, then evaporate to 20% P ₂ O ₅ using the exhaust gases of the calciner, add 11 kg of ammonium nitrate and sent for spraying into drum granulator dryers. After drying the sodium phosphate solutions, the product is sieved with the separation of a 70% fraction from -4 to +1 mm. To this product, 30% of the fine fraction (-1 mm) is added and sent to the calcination stage. 1123 kg of the dry product is introduced into the calciner, into which the direct gases are supplied with flue gases with a temperature of 615 ^o C. Product losses at the drying stage are 48 kg. During calcination, 98 kg of water is removed and 25 kg of sodium tripolyphosphate is lost with the exhaust gases. At the outlet of the calciner, the exhaust gas temperature is maintained at 450 ^° C. A return screw is installed in the head of the calciner to recirculate the product, and two retaining rings are used along the length of the drum. The residence time of the product at the dehydration stage is 3 hours. 1000 kg of sodium tripolyphosphate are cooled and sent to the mill. The finished product after grinding contains not more than 5% of the fraction +0.5 mm and not less than 45% of the fraction -0.25 mm. The content of the basic substance is 97.5%, and the amount of I form in sodium tripolyphosphate is 33%. The amount of basic impurities: 0.01% Fe, 0.1% CaO, 0.1% F. RON test ( ^o С): after 1 min - 81, after 5 min - 88.

Example 2. The main defluorination and desulfurization of extraction phosphoric acid is carried out as in example 1. In 2974 kg defluorinated and desulfurized phosphoric acid containing 23.4% P ₂ O ₅ , 0.35% SO ₃ , add 18.3 kg of barium carbonate. 2988.3 kg of a suspension of barium sulfate is treated with a circulating neutralized pulp from the second stage to a metal titer of 1.1. Anhydrous soda in an amount of 805 kg is introduced into the second stage to maintain the metal titer (Na: P) in the liquid phase of the pulp equal to 1.63. Prior to filtration, 10.4 kg of activated carbon (0.3%) was added to the production pulp of the second stage (3459.1 kg) and kept at a temperature of 75 ^° С for 40 min. As a result of separation of the pulp on the filter and washing the slurry with hot water (873 kg), 777 kg of cake are obtained (W = 60, including 119 kg of P ₂ O ₅ ). 3565.5 kg of liquors (I filtrate) containing 16% P ₂ O ₅ , 0.001% Fe, 0.02% CaO, 0.01% F, mixed with 73 kg of sodium tripolyphosphate from the stages of drying and calcination of the product, evaporated to 20 % P ₂ O ₅ in evaporators, add 11 kg of ammonium nitrate and sent to BHS (drum granulator dryer). After drying the sodium phosphate solution, the product is sieved with the separation of 85% of the fraction from -4 to + 1 mm. To this product is added 20% fine fraction (-1 mm) and sent to the calcination stage. 1126 kg of the dry product is introduced into the calciner, into which the flue gases with a temperature of 635 ^o C are fed directly. At the outlet of the calciner, the gas temperature is maintained at 470 ^o C. The product remains at the dehydration stage for 5 hours. 1000 kg of sodium tripolyphosphate are cooled and sent to the mill. The grinding fineness of the product is the same as in Example 1. Ready sodium tripolyphosphate (TPPF) contains: 98% of the main substance, 0.003% Fe, 0.07% CaO, 0.03% F, 65% I form TPPN. RON test ( ^o С): after 1 min - 89, after 5 min - 94.

Example 3. Sulfate defluorination and desulfurization of extraction phosphoric acid with apatite is also carried out as in Example 1. 18.3 kg of barium carbonate are added to 2974 kg of sulphate and fluorine acid purified and sent to the soda neutralization stage, where 803 kg of Na ₂ CO are introduced. ₃ to obtain the ratio Na: P = 1.625. Before filtration, 6.9 kg of activated carbon (0.25%) was added to the production pulp of the second stage (3462) and kept at a temperature of 72 ^° C for 30 minutes. As a result of separation of the pulp on the filter and washing the sludge with hot water (871 kg), 777 kg of cake are obtained (W = 60%, including 119 kg of P ₂ O ₅ ). B 3556 kg of liquors (filtrate) containing 16% P ₂ O ₅ , 0.003% Fe, 0.02% CaO, 0.02% F, 73 kg of sodium tripolyphosphate are introduced from the stages of drying and calcination of the product, evaporated to 20% P ₂ O ₅ , add 11 kg of ammonium nitrate and sent to GHS. After drying the sodium phosphate solutions, the product is sieved with a separation of 77% of the fraction from -4 to +1 mm. 23% of the fine fraction (-1 mm) is added to this product and sent to the calcination step. 1126 kg of the dry product is introduced into the calciner, into which the flue gases with a temperature of 625 ^o C are fed directly. At the outlet of the calciner, the gas temperature is maintained at 460 ^o C. The product remains at the dehydration stage for 4 hours. 1000 kg of sodium tripolyphosphate are cooled and fed to the mill. The grinding fineness of the product is the same as in example 1. Ready sodium tripolyphosphate contains: 98.5% of the basic substance, 0.01% Fe, 0.07% CaO, 0.07% F, 40% I form of TPPN. RON test ( ^o С): after 1 min - 83, after 5 min - 89.

According to the proposed method, the degree of transition of orthophosphates to sodium tripolyphosphate is 97.5-98.5%. The amount of basic impurities in the product is in the range: iron - 0.003-0.01%, calcium oxide - 0.03 - 0.1%, fluorine - 0.03-0.1. RON test indicators ( ^o С): after 1 min - 81-89, after 5 min - 88-94, I form of sodium tripolyphosphate contains from 35 to 65%.

 A comparison of the indicators of the ROH test and the content of impurities in the silks between the prototype and the proposed method are shown in table 1a. Table 1b shows a comparison of the content of the I form and indicators of the ROH test after the calcination stage between the prototype and the claimed method.

Claims (1)

A method for producing sodium tripolyphosphate from extraction phosphoric acid and soda ash, comprising desulfurizing the acid to a content of 0.6-1.0% CaO, defluorizing it to a concentration of 0.1-0.3% fluorine in a solution, separating calcium sulfate and sodium silicofluoride, deep obessulfachivanie phosphoric acid solution to a content in the liquid phase 0,1-0,15% SO ₃ without separation of barium sulphate, purification by macro- and trace as a precipitate upon neutralization acid soda, filtering the slurry separation with separation of sludge and impurities ADDED 1-3% sodium tripolyphosphate into the obtained sodium phosphate liquor, their concentration, drying, calcination of the dried product, characterized in that the neutralization of phosphoric acid with soda is carried out to an atomic ratio of Na: P = 1.62-1.63 and into the resulting pulp before 0.1-0.3% activated carbon is added by filtration and kept at 70-75 ^o C for 25-40 minutes, and calcination of the dried product containing 70-85% fractions from -4 to +1 mm is carried out in a drum calciner type with reverse screw and retaining rings while maintaining the temperature of the incoming gases 61 5-635 ^o C, and the exhaust gases - 450-470 ^o C and the residence time of the product in the calciner 3-5 hours

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