RU2805533C1 - Method for complex processing of gas cleaning solutions in aluminum production - Google Patents

Abstract

FIELD: chemical technology.

SUBSTANCE: invention can be used for production of commercial products from aluminum production waste. In the method for complex processing of an aluminum production gas purification solution containing NaF, Na₂CO₃, NaHCO₃ and Na₂SO₄, an aqueous solution of Al₂(SO₄)₃ is added and the resulting solution is mixed, to which a flocculant is added, sedimentation and thickening of the solution are carried, the condensed solution is filtered under vacuum and separated into a cake of cryolite and aluminum hydroxide concentrate and a clarified Na₂SO₄ solution. The clarified Na₂SO₄ solution is evaporated and dried to obtain Na₂SO₄ crystals, and the cake of cryolite concentrate and aluminum hydroxide is dried at a temperature of 500-700°C. The cake of cryolite concentrate and aluminum hydroxide is fed into a drum drying oven, in which at a temperature of 500-700°C γ-modification of Al₂O₃ is obtained and crystallization moisture is removed until it takes up 1 wt.%.

EFFECT: it is possible to obtain commercial quality sodium sulfate and cryolite by acting on the gas cleaning solution with aluminum sulfate.

2 cl, 2 ex

Description

The present invention relates to the field of aluminum production and can be used for the regeneration of fluorine contained in the exhaust gases of aluminum electrolyzers. The purpose of the invention is to obtain commercial quality sodium sulfate and cryolite by acting on a gas purification solution with aluminum sulfate.

The invention relates to the electrolytic production of aluminum and can be used for the regeneration of fluorine contained in the exhaust gases of aluminum electrolyzers to produce cryolite and sodium sulfate of commercial quality.

Currently, during the electrolysis of aluminum with fluorine-containing flue gases, 100-300 mg/dm ³ of sulfur dioxide is released, which is captured by a soda solution in a wet gas cleaning system, oxidized to the sulfate form and accumulated in gas cleaning solutions.

Due to the accumulation of sodium sulfate in solutions, firstly, there is a loss of sodium fluoride in wet gas cleaning apparatus, which precipitates in the form of a double salt NaF Na ₂ SO ₄ and is removed with sludge, and secondly, there is a loss of soda and fluorine when discharging mother liquors to sludge fields.

There is a known method for producing sodium sulfate from solutions containing dissolved sodium sulfate and carbonate by cooling them to a temperature from 0ºC to 20ºC (US Patent, N 3508884, class 23-Z02, 1970).

The disadvantage of this method is that it produces a mixture of two salts, which leads to losses of sodium carbonate, and the impossibility of using the resulting precipitate.

There is a known method for isolating sodium sulfate from gas purification solutions for electrolytic aluminum production (Patent RU 2243938).

A method for separating sodium sulfate from electrolytic aluminum production solutions involves saturating a gas purification solution with sodium sulfate, cooling the resulting solution with a cooling agent, and separating the resulting precipitate. Cooling of sulfate-containing solutions to t=8-12°C is carried out in cooling devices through the wall, and cooling to t=0--2°C is carried out by direct contact with the cooling agent.

The disadvantage of this method is the use of cooling devices, equipment costs, as well as labor and energy costs.

The closest in technical essence (Patent No. 2254293) is a method for processing a sodosulfate solution obtained after gas purification of waste gases from electrolysis buildings in the production of aluminum, which includes purification of gas from sulfur oxides and fluoride compounds by sprinkling them with a sodosulfate solution in wet scrubbers, separation from the solution after gas purification the main amount of sodium fluoride in the form of cryolite. The sodosulfate solution, purified from cryolite, is additionally purified from sodium fluoride by treating it at 95-105°C for 1.5-2.0 hours with lime milk introduced into the sodosulfate solution based on the stoichiometric binding of fluorine contained in the solution to CaF ₂ . The sodosulfate solution, purified from fluorine, is then subjected to concentrating evaporation until the density of the evaporated solution reaches 1.37±0.02 g/l and sodium sulfate is separated from it into a precipitate in the form of berkeite salt by introducing carbonate soda into the evaporated solution until the concentration of the titrated alkali in the mother liquor is reached solution of 215-230 g/l Na2 O(t) and the density of the solution in the suspension up to 1.35±0.02 g/l when stirring the suspension at a temperature of 95-100°C for 30-40 minutes. The invention allows for more complete

extraction of sodium sulfate from evaporated sodium sulfate solution in the form of berkeite salt purified from sodium fluoride.

The disadvantages of this method include the use of electricity for heating, incomplete extraction of sodium sulfate and the inability to use calcium fluoride.

The purpose of the present invention is to obtain commercial quality sodium sulfate and cryolite.

A method for complex processing of an aluminum production gas purification solution containing NaF, Na ₂ CO ₃ , NaHCO ₃ and Na ₂ SO ₄ , characterized in that an aqueous solution of Al ₂ (SO ₄ ) ₃ is added and the resulting solution is mixed, to which a flocculant is added, produced settling and thickening of the solution, the condensed solution is filtered under vacuum and divided into a cake of cryolite and aluminum hydroxide concentrate and a clarified solution of Na ₂ SO ₄ , while the clarified solution of Na ₂ SO ₄ is evaporated and dried to obtain Na ₂ SO ₄ crystals, and the cake of cryolite concentrate and aluminum hydroxides are dried at a temperature of 500-700°C.

The cake of cryolite and aluminum hydroxide concentrate is fed into a drum drying oven, in which, at a temperature of 500-700°C, the γ-modification of Al ₂ O ₃ is obtained and crystallization moisture is removed up to 1 wt.%.

This goal is achieved by the fact that in the method of isolating sodium sulfate from gas purification solutions of electrolytic aluminum production, consisting of sulfates, bicarbonates, carbonates and sodium fluorides, aluminum sulfate is used as a precipitant.

Aluminum sulfate is added to the sodosulfate solution obtained after separation of the main amount of sodium fluoride in the form of cryolite from the solution after gas purification, according to the stoichiometry of the reactions below (according to the analysis of the gas purification solution) (reactions 1-3).

Aluminum sulfate binds into insoluble compounds, sodium fluoride, sodium carbonate and sodium bicarbonate.

Al(OH) ₃ , when interacting with sodium fluoride, forms cryolite and sodium hydroxide, which, interacting with carbon dioxide, again forms sodium carbonate, which can be used.

During the experiments, foaming occurs; after settling, a milk suspension is obtained, for the sedimentation of which a flocculant was used. The suspension was left for a day. The sediment turned out to be loose. The solution was separated from the precipitate by decantation. The solution was evaporated. The dry residue after evaporation consisted of 100% Na ₂ SO ₄ (various modifications). The sediment after decantation consists of cryolite and aluminum hydroxide, which at temperatures above

600ºС decomposes into alumina and water. Cryolite and alumina are coming back into production.

Example 1.

The aluminum production gas purification solution had the following concentrations of dissolved components: 7.5 kg/m ³ NaF, 120.0 kg/m ³ Na ₂ SO ₄ , 10.0 kg/m ³ Na ₂ CO ₃ , 25.0 kg/m ³ NaHCO ₃ .

A solution of aluminum sulfate was taken with a concentration of 37.89 kg/ ^m3 .

1 dm ³ of aluminum production gas purification solution was mixed with 1 dm ³ of aluminum sulfate solution at a temperature of 25-35 ºС. A solution of Magnofloc flocculant was added to the resulting solution mixture at the rate of 50 g per cubic meter of the solution mixture. Settling and thickening were carried out. The condensed solution was filtered under vacuum and separated into a cake of cryolite and aluminum hydroxide concentrate and a clarified Na ₂ SO ₄ solution. The cake of cryolite and aluminum hydroxide was dried in an oven at a temperature of 500-700ºC. The clarified Na ₂ SO ₄ solution was evaporated until Na ₂ SO ₄ crystals were obtained.

The result was: 167.22 g of Na ₂ SO ₄ and 17.63 g of Na ₃ AlF ₆ + Al ₂ O ₃ . A certain discrepancy of 15.54 g was obtained, due to the release of carbon dioxide during the interaction of the components.

Example 2.

At the same concentrations of components in solutions and mixing volumes as in the previous example, a flocculant was used. "Alklar." As a result, 168.53 g of Na ₂ SO ₄ and 17.3 g of Na ₃ AlF ₆ + Al ₂ O ₃ were obtained.

A preliminary technical and economic assessment of obtaining a product that complies with GOST 6318-77 shows that the profit for each ton of product (sodium sulfate) will be:

- from the sale of sodium sulfate 2,526 thousand rubles.

- from the sale of cryolite 1.5 thousand rubles.

- from the sale of alumina 0.52 thousand rubles.

Total: 2.526 + 1.5 + 0.52 = 4.546 thousand rubles.

Claims (2)

1. A method for complex processing of an aluminum production gas purification solution containing NaF, Na ₂ CO ₃ , NaHCO ₃ and Na ₂ SO ₄ , characterized in that an aqueous solution of Al ₂ (SO ₄ ) ₃ is added and the resulting solution is mixed, to which a flocculant is added , the solution is settled and thickened, the condensed solution is filtered under vacuum and separated into a cake of cryolite and aluminum hydroxide concentrate and a clarified Na ₂ SO ₄ solution, while the clarified Na ₂ SO ₄ solution is evaporated and dried until Na ₂ SO ₄ crystals are obtained, and the cake cryolite concentrate and aluminum hydroxide are dried at a temperature of 500-700°C. 2. The method according to claim 1, characterized in that the cake of cryolite concentrate and aluminum hydroxide is fed into a drum drying oven, in which, at a temperature of 500-700°C, the γ-modification of Al ₂ O ₃ is obtained and crystallization moisture is removed up to 1 wt.% .