RU2316473C1 - Method for separating anhydrous sodium sulfate from return solutions of gas scrubbing of aluminum cells - Google Patents

Abstract

FIELD: processing of soda-sulfate-fluoride solutions formed at scrubbing gases of electrolysis housings.

SUBSTANCE: method comprises steps of exhaust gas scrubbing of electrolysis process by means of salt solution; separating from solution after gas scrubbing secondary cryolite; lime causticization of cryolite mother liquor; metering lime for causticization in quantity 100 - 110% of active CaO of stoichiometric quantity necessary for reaction with soda and sodium fluoride; separating mixed deposit of calcite CaCO₂ and fluorite CaF₂ from solution; evaporating prepared sulfate-caustic solution till content of caustic alkali Na₂O in range 150 - 200 g/l at depositing anhydrous sodium sulfate.

EFFECT: elimination of discharging fluoride solution to slime storage, possibility for producing sodium sulfate as commercial product.

1 dwg, 1 ex

Description

The method relates to the field of aluminum production, and in particular to the processing technology of soda-sulfate-fluoride solutions formed during the purification of exhaust gases from electrolysis cells.

When aluminum electrolyzers operate on unburned anodes, the process exhaust gases contain sulfur dioxide and hydrogen fluoride, for the purification of which the gases are irrigated with a soda solution. As a result of this, a circulating solution is formed containing 15-30 g / l of soda (carbonate and bicarbonate), 60-80 g / l of sodium sulfate and 15-30 g / l of sodium fluoride. This solution is directed to the allocation of secondary cryolite from it; After separation of the cryolite with a sodium fluoride content of 4-8 g / l, the mother liquor is returned to the gas treatment (after adjusting its composition with fresh soda), while to prevent the accumulation of sodium sulfate in the gas cleaning solution to an undesirable level, part of the cryolite mother liquor is discharged to the sludge dump. This leads to the loss (per 1 ton of aluminum produced): 5-6 kg of soda, 2.0-2.5 kg of sodium fluoride and 15-20 kg of sodium sulfate. In addition, the discharge of soda-sulfate-fluoride solution into the sludge dump is the cause of environmental pollution. Thus, the problem of cleaning the waste solutions of electrolysis gas purification from these salts is relevant both economically and ecologically.

There is a method of separating sodium sulfate from working solutions of electrolysis gas purification (USSR author's certificate No. 648518 of 05.10.73), the essence of which is to cool the solutions to 0 - minus 5 ° C. In this case, the solubility of sodium sulfate decreases, and it precipitates in the form of decahydrate crystalline hydrate Na ₂ SO ₄ · 10H ₂ O (Glauber's salt or mirabilite).

The disadvantage of this method is the need for deep cooling of the solution. In addition, mirabilite is not a commercial product and requires dehydration and drying. In addition, it is contaminated with sodium fluoride. For these reasons, the method is applied only sporadically in the winter.

There is also a method according to USSR author's certificate No. 739392 of January 15, 1979, according to which the exhaust gases of the electrolysis process are treated with a soda solution, cryolite is isolated from the resulting circulating solution, the mother liquor after separation of the cryolite is divided into parts, one of which is returned to gas treatment, and another causticized with lime. Mirabilite is isolated from the solution after caustification by cooling the solution to minus 5 - minus 12 ° C, the mirabilite precipitate is removed from the process, and the solution after caustification is used to treat solid waste from electrolysis production.

In essence, this method is a complicated version of the previous analogue method, therefore, its inherent disadvantages are also present. In particular, cooling the solution to minus 12 ° C is associated with significant energy costs and requires special equipment. The same applies to the dehydration process of mirabilite containing about 60% water. Therefore, both of the above methods have not found practical application.

As a prototype of the proposed method, the patent of the Russian Federation No. 2254293 of 05/05/2003, "Method for the processing of soda-sulfate solution obtained after gas purification of electrolysis cells in the production of aluminum," was adopted. The method includes cleaning electrolysis gases in wet scrubbers with a soda-sulfate solution and isolating the main amount of sodium fluoride in the form of cryolite from the solution after gas purification. The method is characterized in that the soda-sulfate solution, purified from cryolite, is additionally purified from sodium fluoride at a temperature of 95-105 ° C for 1.5-2.0 hours with milk of lime introduced into the solution based on stoichiometric fluorine binding, after which purified from fluorine soda-sulfate solution is subjected to concentrating evaporation to a density of 1.37 ± 0.02 g / cm ³ and a precipitate of berkeitic salt is isolated from it by introducing carbonate soda into an evaporated solution until the titrated alkali concentration in the mother liquor reaches 215-230 g / l and density uspenzii 1.35 ± 0.02 g / cm ^3, and stirring the suspension at a temperature of 95-100 ° C for 30-40 min.

The disadvantages of the method:

- additional consumption of soda for the allocation of berkeitic salt. This salt (berkeite 2NaSO ₄ · Na ₂ CO ₃ ) is removed from the process, and the soda included in its composition is lost for production;

- the need for almost complete evaporation of the solution after additional cleaning it from fluorine. So, according to the balance cited by the author, the initial 30 m ³ / h of the mother liquor after purification from sodium fluoride is evaporated to 6 m ³ / h, i.e. five times in volume. Such a deep evaporation will require an appropriate flow of heat and electricity;

- insufficient purification of the mother liquor from sodium fluoride;

- when lime is fed into the soda-fluoride solution, mainly soda is causticized, since its concentration is much higher, so the berkeite will be contaminated with sodium fluoride, and the calcium fluoride obtained by this scheme will contain more than 50% CaCO ₃ calcite.

The objective of the invention is the elimination of the discharge of fluoride solution into the sludge depot, purification of the circulating solution from sodium sulfate, which will allow to obtain sodium sulfate in the form of a commercial product and return soda, lost with the dump solution, in the process of gas purification.

The technical result is achieved by the fact that in the method for separating anhydrous sodium sulfate from working solutions of gas purification of aluminum electrolysis cells, including purification of waste gases from the electrolysis process with a soda solution, separation of secondary cryolite from the solution after gas cleaning, and lime caustification of the mother liquor of cryolite, lime is dosed for caustification in an amount of 100- 110% of active CaO from stoichiometrically necessary for the reaction with soda and sodium fluoride, I separate the precipitate of calcite CaCO ₂ and fluorite CaF ₂ t from the solution, and the resulting sulfate-caustic solution is evaporated to a caustic alkali content of Na ₂ O _ky = 150-200 g / l with the release of anhydrous sodium sulfate.

A dosage of lime in an amount of 100-110% of the stoichiometrically necessary amount of Na ₂ CO ₃ and NaF solution required for causticization provides almost complete cleaning of the solution from soda and fluoride. As a result, the soda-sulfate-fluoride solution turns into a sulfate-caustic solution, because by reactions

Na ₂ CO ₃ + Ca (OH) ₂ = CaCO ₃ = 2NaOH,

2NaF + Ca (OH) ₂ = CaF ₂ + 2NaOH

caustic alkali and a mixed precipitate of calcite and fluorite are formed; while sodium sulfate is not causticized and remains in solution unchanged. After separation from the solution and washing of the mixed precipitate, the latter can be removed from the process.

Since fluorine in the composition of the precipitate is bound in an insoluble form, this precipitate is environmentally neutral. It is possible to use a mixed precipitate of CaCO ₃ + CaF as a melt in the steel industry, in cement, alumina production and in other industries.

Anhydrous sodium sulfate is precipitated from the sulfate-caustic solution by evaporation at a temperature above 30 ° C. Due to the fact that the content of soda and sodium fluoride in the solution after causticization decreases to 1.0-1.5 g / l and 0.2-0.3 g / l, respectively, the Na ₂ SO _{4 released} is quite clean and corresponds to GOST 6318 .

At a dosage of lime for caustification in an amount of less than 100% of stoichiometry in the amount of Na ₂ CO ₃ + NaF, the solution of soda and fluoride will be incomplete, and the sulphate precipitate released during evaporation of the sulphate-caustic solution will be contaminated with these compounds. Dosage of lime in excess of 110% of the amount required for the reaction is impractical due to overuse of lime. The optimum flow rate is 102-105%.

The evaporation of a caustic sulphate solution to concentrations of less than 150 g / l in Na ₂ O _ky results in a decrease in the degree of Na ₂ SO ₄ precipitation and a decrease in the efficiency of the purification scheme. Increasing the concentration of Na ₂ O _ky over 200 g / l is also undesirable, because increases the viscosity of the solution, the magnitude of depression and T: W sulfate pulp.

The caustic solution after evaporation and separation of the sulfate precipitate from it can be used in the process of cooking the secondary cryolite or again by gas purification, where at first NaOH is hydrocarbonized to soda, and the soda will normally interact with SO ₃ and HF of the exhaust gases, in any variant of soda from the solution discharged according to the existing scheme to the sludge repository is returned to the process.

Thus, this scheme makes it possible:

- return in the process of gas purification soda dump solution, simultaneously turning it into a caustic alkali;

- convert sodium fluoride to environmentally friendly insoluble CaF ₂ , which in a mixture with calcium CaCO ₃ can be used in ferrous metallurgy, cement and other industries;

- to exclude the discharge of toxic liquid waste electrolysis gas treatment to the sludge storage facility;

- clean the circulating solutions of sodium sulfate and get it in the form of a commercial product.

Example.

Take 1 liter of a solution of the following composition, g / l:

Na ₂ SO ₄ = 65

Na ₂ CO ₃ = 30

NaF = 5.0

This solution in chemical composition corresponds to the mother liquor after cooking secondary cryolite at the Bratsk aluminum plant.

20 g / l of CaO active in the composition of milk of lime (105% of stoichiometry in the amount of Na ₂ CO ₃ and NaF in solution) are dosed per 1 liter of solution, the process is carried out at t = 90 ° C for 2 hours. After that, the causticization sludge containing 19% CaF ₂ (from the sum of CaCO ₃ and CaF ₂ ) is separated, washed and in an amount of 32 g (dry) removed from the process.

Pro-causticized solution of the composition, g / l:

Na ₂ SO ₄ = 64

Na ₂ O _ky = 20

Na ₂ CO ₃ = 1.4

NaF = 0.2

evaporated to Na ₂ O _ky = 160 g / l. It turns out sulfate pulp with a sediment (anhydrous sulfate) content of about 500-600 g / l in an amount of 0.15 l. Sulfate precipitate after separation from the solution has a composition, wt.%:

Na ₂ SO ₄ = 97.1

Na ₂ CO ₃ = 1.6

NaF = 0.2

Na ₂ O _ky = 1.1

Humid = 4.5

The amount of sulfate obtained is 50 g. The degree of purification of the initial solution from sulfate is about 77%.

The caustic solution (mother liquor of sulfate) has the following composition, g / l:

Na ₂ O _ky = 160

Na ₂ SO ₄ = 75

Na ₂ CO ₃ = 11.2

NaF = 01.5

The amount of solution is 0.12 l.

The technological scheme of the processing of sulfate solutions of electrolysis gas purification is shown in the drawing. Flows are given based on 1 m ³ mother liquor cryolite

Claims (1)

The method of separation of anhydrous sodium sulfate from working solutions of gas purification of aluminum electrolysis cells, including purification of waste gases of the electrolysis process with a soda solution, separation of secondary cryolite from the gas purification solution and lime caustification of the mother liquor of cryolite, characterized in that lime is dosed for caustification in an amount of 100-110% active CaO from stoichiometrically necessary for the reaction with soda and sodium fluoride, the mixed precipitate of calcite CaCO ₃ and fluorite CaF _{2 is} separated from the solution, and the resulting sul The caustic sulphate solution is evaporated to a caustic alkali content of Na ₂ O _ky = 150-200 g / l with anhydrous sodium sulfate precipitated.