RU2115622C1 - Method of preparing potassium carbonate - Google Patents

Abstract

FIELD: metallurgy of non-ferrous metals, more particularly technology of inorganic matter. SUBSTANCE: method of preparing potassium carbonate from soda-potash solutions resulting from manufacture of alumina by agglomeration of nephelines comprises evaporating and cooling solutions to separate deposit of admixtures and double salt and crystallizing potassium carbonate sesquihydrate from the resulting solution, cooling solutions having K₂O molar moiety from total of K₂O and N₂O of at least 70% with 5.5-7.0 H₂O moles per 1 mol of dry salts. EFFECT: more efficient preparation method.

Description

The invention relates to a technology for the production of potassium carbonate from soda-mixed solutions of alumina production in the complex processing of nepheline by sintering and can also be used to obtain K ₂ CO ₃ from other types of raw materials.

The technology of polythermal separation of salts and the production of potassium carbonate from soda-seed solutions for the production of alumina from nepheline is known (see, for example, Abramov V.Ya., Alekseev A.I. Badalyants Kh.A. Complex processing of nepheline-apatite raw materials. Moscow, Metallurgy, 1990, p. .171). The process scheme involves the following basic operations: evaporation of the initial soda-paste solutions with the precipitation of soda, evaporation of the mother liquor of soda to obtain a precipitate of NaKCO ₃ double salt, cooling of the mother liquor of the double salt with crystallization of potash K ₂ CO ₃ • 1.5 H ₂ O , which is separated from the solution, washed, dried and, if necessary, obtain an anhydrous product is additionally calcined.

 The principal drawback of the above scheme for the production of potassium carbonate is the possibility of contamination of the product with various impurities, for example, sulfates, which enter the circulating solutions of alumina production as a result of oxidation of sulfur compounds contained in raw materials and fuel.

To avoid this contamination, several options have been proposed for the purification of soda-potash solutions from impurities before the precipitation of potassium carbonate. So, according to ed. N 1791380 A1 dated 10.31.90, it is proposed to evaporate the solution before the isolation of potassium carbonate to a density of 1.68 g / cm ³ , dilute the resulting suspension with water to a total alkalinity of 300-380 g / l (in terms of Na ₂ O) and cool to 20 -30 ^o C in order to isolate impurities in the solid phase. The precipitate of the latter is separated from the solution, from which potassium carbonate is then obtained.

The authors of the application of the Russian Federation N 93032467 IPC: C 01 D 7/40, published on 06.27.96 in the bulletin "Inventions" N 18 for 1996 and taken as a prototype, in order to clean soda-paste solutions from impurities, they offer to dilute the mother liquor after separation of the double salt with condensate and cool at a speed of 3-17 ^o C per hour, then filter the solution with a filtration resistance of the septum of 0.5-3.0 kg / cm ² , evaporate the clarified solution and crystallize potassium carbonate from it with a reduced content of impurities.

 A common disadvantage of the above methods - analogue and prototype is the need to dilute the solutions before cooling them to remove impurities. The introduction of water or condensate into soda-liquid solutions requires subsequent evaporation of this water, i.e. energy-related operations and therefore quite expensive.

 The technical task of the invention is to develop such a method for producing potassium carbonate from soda-liquor solutions, which, when cleaning solutions from impurities, would not require dilution of the solutions and, therefore, evaporation of water introduced into the dilution.

The problem is solved in that soda-mixed solutions before the separation of potassium carbonate are cooled to separate the precipitate of impurities and evaporated to obtain a reversible double salt, while solutions with a molar fraction of K ₂ O of the sum of K ₂ O + Na ₂ O of at least 70 are sent for cooling % and containing 5.5-7.0 moles of H ₂ O per 1.0 mol of total salt content.

 The need for a combination of these distinguishing features is determined by the behavior of soda-potash solutions with impurities of sulfates and chlorides.

One of these features is that in the temperature range of 20-40 ^o C (the most favorable for the separation of impurities), the solubility of sulfates and chlorides begins to increase rapidly with a decrease in the molar fraction of K ₂ O below 70%. In the solubility diagram of the Na system $\genfrac{}{}{0ex}{}{\text{+}}{\text{2}}$ , K $\genfrac{}{}{0ex}{}{\text{+}}{\text{2}}$ / CO $\genfrac{}{}{0ex}{}{\text{=}}{\text{3}}$ SO $\genfrac{}{}{0ex}{}{\text{=}}{\text{4}}$ , Cl $\genfrac{}{}{0ex}{}{\text{-}}{\text{2}}$ - H ₂ O this is expressed in the fact that the field of crystallization, for example, potassium sulfate, has an almost constant width with a molar fraction of K ₂ / K ₂ + Na ₂ from 100 to 70%, and then substantially narrows. This is a prerequisite for the first distinguishing feature.

 An increase in the solubility of impurities and a deterioration in purification also occurs when the water content in the system increases, i.e. when the solution is diluted. Depending on the composition of the initial solution, this occurs when the water content is about 7.0 moles per 1.0 mole of dry salts. If the water content becomes less than 5.5 mol per mol of salts, then, together with impurities, when cooled, potassium carbonate, i.e. the target product, begins to precipitate from the solution. And only within this interval is there a sufficiently deep purification of the solution from impurities without additional precipitation of the main substance with them. The value of this interval is determined, as already mentioned, by the spread in the composition of the initial soda-potash solutions, in particular, by a change in the content of caustic alkali in them.

It must be emphasized that both the molar fraction of K ₂ O and the water content in the system separately are necessary but not sufficient conditions for the most rational solution cleaning process. And only the combined action of both features provides the maximum cleaning depth (which determines, in turn, the quality of potassium carbonate) at minimal cost.

Example. Potassium carbonate is produced from soda-mixed solutions of the Pikalevo Alumina Association. As the source used soda mother liquor composition, g / l: Na ₂ O = 93.6; K ₂ O = 472.8; Na ₂ O _ku = 14.9; Al ₂ O ₃ = 7.1; K ₂ SO ₄ = 9.9; KCl = 5.7; pier the proportion of K ₂ O / K ₂ O + Na ₂ = 77.2%; density γ = 1.54 g / cm ³ ; the water content of 6.04 mol per 1 mol of dry salts.

 1. Obtaining potassium carbonate by the prototype method.

The initial solution is evaporated to γ = 1.67-1.68 g / cm ³ , the precipitate of the double salt is separated off, the resulting mother liquor of the double salt is diluted 1.5 times with water (water consumption 0.5 l per 1.0 l of the mother liquor), cooled at a speed of 10-15 ^o C per hour to a temperature of 30-35 ^o C and filtered at a pressure drop of 600-650 mm RT. column (0.8-0.9 kg / cm ² ). The resulting filtrate was evaporated to obtain a solution of the composition, g / l: Na ₂ O = 10.2; K ₂ O = 340.5; Na ₂ O _ku = 51.1; Al ₂ O ₃ = 31.8; K ₂ SO ₄ = 1.61; KCl = 20.8; MO K ₂ O / K ₂ O + Na ₂ O = 95.8%, density γ = 1.6 g / cm ³ .

The resulting solution is cooled to 55-60 ^o C, while a precipitate of sesquioxide potassium carbonate precipitates from it, which, after filtration and washing, has the following impurity composition, wt.%: Na ₂ CO ₃ = 0.2; K ₂ SO ₃ = 0.2; K ₂ SO ₄ = 0.05; KCl = 0.02; Al ₂ O ₃ = 0.03.

 2. Obtaining potassium carbonate by the proposed method.

2.1. The initial solution containing 6.04 mol of H ₂ O per mole of salts and having MO K ₂ O / K ₂ O + Na ₂ O = 77.2% is cooled (without dilution) to 30-35 ^o C and filtered off from the precipitate of impurities (consisting mainly of potassium sulfate). The filtrate was evaporated to separate a double salt precipitate. The resulting solution in composition and concentration is similar to the solution from example 1, directed to the crystallization of production of potassium carbonate. Therefore, it is natural that the composition of the final product obtained according to this embodiment (K ₂ CO ₃ • 1.5 H ₂ O) is almost the same as the prototype product.

2.2. The experiment is repeated according to option 2.1, but soda mother liquor with M.O. K ₂ O / K ₂ O + Na ₂ O = 67.5%, i.e. less than 70%. It turns out a solution that goes to the crystallization of the final product, similar to that of example 2.1, but containing 4.9 g / l K ₂ SO ₄ (instead of 1.61 g / l of example 2.1). Obtained from this solution K ₂ CO ₃ • 1,5 H ₂ O contains 0.2% K ₂ SO ₄ (instead of 0.05 in example 2.1), which significantly reduces the grade of the product and affects its consumer qualities.

2.3. The experiment is repeated according to option 2.1, but the initial is taken as an unpaired soda mother liquor containing 7.5 mol of water per mole of salts (above the limit of 7.0) with a MO K ₂ O / K ₂ O + Na ₂ O of 77.2 % It turns out a solution for crystallization of potassium carbonate with a content of K ₂ SO ₄ = 5.5 g / l, which leads to the same contamination of the product as in option 2.2.

2.4 The experiment is repeated according to option 2.1, but the paired soda mother liquor containing 5.2 moles of water per mole of salts (below the limit of 5.5) is taken as the source. A good final product is obtained from this solution (as in option 2.1), but the precipitate of impurities contains only 6.2% K ₂ O ₄ and 89.2% K ₂ CO ₃ . In other words, the precipitate of impurities consists mainly of the target product, which reduces its yield.

 Thus, the fulfillment of the conditions according to the invention allows to obtain a product of the same quality as the method of the prototype, but to avoid dilution of the solution and the need for evaporation.

Claims (1)

A method for producing potassium carbonate from soda-potassium alumina production solutions by sintering nepheline raw materials, which includes cooling solutions and separating impurities from them, evaporating purified solutions with separation of the reverse double salt and crystallization from the resulting solutions of potassium hydrogen carbonate, characterized in that the solutions are directed to cooling, having a molar fraction of K ₂ O of the sum of K ₂ O + Na ₂ O of at least 70% with a content of 5.5 to 7.0 moles of H ₂ O per 1.0 mol of dry salts.