RU2819968C1 - Method of producing soda-sulphate mixture from recycled soda-sulphate solutions of gas cleaning of aluminium electrolyzers - Google Patents

Abstract

FIELD: non-ferrous metallurgy.

SUBSTANCE: invention can be used to obtain commercial products from wastes formed during wet gas cleaning of exhaust gases from residues of hydrogen fluoride and sulphur dioxide from electrolysis buildings of aluminium production. Method involves defluorination of the solution with extraction of secondary cryolite, its further clarification with a reagent to reduce suspension of fluorides and gas cleaning sludge to obtain a clarified solution. Evaporation of the clarified solution until a solid phase appears and the obtained pulp of the soda-sulphate mixture is filtered. Clarification is carried out with a synthetic flocculant, and filtration of the soda-sulphate mixture pulp is combined with its washing with cold process water with separation of the filtrate and sediment of the soda-sulphate mixture, which is sent for drying, wherein the dried residue of the soda-sulphate mixture is ground to a size of minus 200 mcm to increase the whiteness factor.

EFFECT: obtaining a soda-sulphate mixture with high whiteness coefficient and low yellowness index.

5 cl, 1 dwg, 3 tbl

Description

Field of technology to which the invention relates

The invention relates to non-ferrous metallurgy and can be used to obtain commercial products from waste generated during wet gas purification of waste gases from hydrogen fluoride and sulfur dioxide residues from aluminum production pots. As a commercial product, a soda sulfate mixture with a high whiteness coefficient is obtained for the production of synthetic detergents.

State of the art

Purification of exhaust gases from aluminum production electrolysis vessels from hydrogen fluoride and sulfur dioxide involves preliminary purification of gases from hydrogen fluoride using a dry method by its adsorption on aluminum oxide. The exhaust gases, purified from the main part of hydrogen fluoride, are then subjected to a second stage of purification from sulfur dioxide and hydrogen fluoride residues using a wet method, i.e. by sprinkling exhaust gases in scrubbers with a soda-sulfate solution circulated in a wet gas cleaning system.

The problem of increased salt concentrations in circulating solutions of wet gas cleaning is especially relevant with the ongoing increase in sulfur content in cokes used for the production of anodes. The accumulation of spent gas cleaning solutions occurs at all aluminum smelters that use wet gas cleaning as one of the stages. The use of evaporation units to remove a mixture of sodium sulfates, carbonates and fluorides from circulating solutions will lead to the formation of tens of thousands of tons of material that requires additional costs for their disposal. These production tailings are not in demand in other industries, incl. for the production of detergents, and therefore belongs to waste. The main reason is related to the properties of this material, incl. low whiteness coefficient, increased yellowness, the presence of carbon particles, caking due to high humidity, etc.

An urgent task when obtaining a sodosulfate mixture is the purification of the circulating solution:

• from chromophoric compounds (i.e. compounds that reduce the whiteness coefficient and increase the yellowness of the resulting product);

• from mechanical suspensions of gas purification sludge and regeneration cryolite

to obtain a soda sulfate mixture of commercial quality, i.e. meeting consumer requirements.

There is a known method for isolating sodium sulfate from gas purification solutions for electrolytic aluminum production (patent RU 94 029709, IPC C01D 5/00, published on August 27, 1996). A method for isolating sodium sulfate from gas purification solutions of electrolytic aluminum production, including saturating the solutions with sodium sulfate, carbonate and bicarbonate, cooling the resulting solution at a temperature from +7 to -5°C for at least 2 hours, separating and dehydrating the resulting precipitate.

Disadvantages of this method:

At low temperatures from +7 to -5°C, a precipitate consisting of sodium mirabilite sulfate decahydrate Na ₂ SO ₄ ⋅10H ₂ O, double salt of sulfate and kogarkoite fluoride NaF⋅Na ₂ SO ₄ , sodium berkeite sodosulfate 2Na ₂ SO ₄ precipitates from the solution ⋅Na ₂ CO ₃ , sodium bicarbonate NaHCO ₃ , etc. The resulting product has no consumer due to low consumer properties, incl. the presence of a large amount of fluorine, variable composition, low whiteness coefficient, etc.

As a prototype, a method is claimed for producing a sodosulfate mixture from circulating sodosulfate solutions for gas purification of aluminum electrolyzers, disclosed in RU 2254293 C2, publ. 06/20/2005 A method for processing a sodosulfate solution obtained after gas purification of exhaust gases from electrolysis buildings in the production of aluminum involves purifying the gas from sulfur oxides and fluoride compounds by sprinkling them with a sodosulfate solution in wet scrubbers, separating 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 a solution of 215-230 g/l Na ₂ O _t and the density of the solution in the suspension is 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 the evaporated sodosulfate solution in the form of berkeite salt purified from sodium fluoride.

Disadvantages of this method:

- is the presence of the operation of treating the solution with lime milk, which significantly increases OPEX, because requires the purchase of lime from a third-party supplier, as well as the organization of several additional technological operations, including: unpacking big bags or bags of lime, preparation of lime milk by slaking lime in a Mika apparatus, classification of lime milk from non-palm on a spiral or rack classifier , heating and defluoridation of the soda-sulfate solution with lime milk in a stirrer-reactor, filtration of the defluorinated soda-sulfate solution from the formed fluorite CaF ₂ ;

- when defluoridating with lime milk, the soda-sulfate solution is diluted with water contained in the lime milk, which increases the specific steam consumption for crystallization residue.

- when defluoridated with lime milk, calcium carbonate precipitates and to obtain a soda sulfate mixture it is necessary to introduce soda ash, which significantly increases the cost of the resulting product.

Disclosure of the invention

The objective and technical result of the invention is to obtain from circulating (spent) gas purification solutions of aluminum electrolyzers (including those that have undergone fluoridation due to the release of secondary cryolite) a sodosulfate mixture with a high whiteness coefficient and a low yellowness index, i.e. a product suitable for the production of synthetic detergents and competitive in price.

The technical result is achieved and the problem is solved due to the fact that in the method of producing a sodosulfate mixture from circulating sodosulfate solutions for gas purification of aluminum electrolyzers, including its defluorination with the release of secondary cryolite, subsequent clarification with a reagent to reduce suspended fluorine salts and gas purification sludge to obtain a clarified solution, its evaporation until the appearance of the solid phase and filtration of the resulting sodosulfate mixture pulp, according to the claimed invention, clarification is carried out with a synthetic flocculant, while filtration of the sodosulfate mixture pulp is combined with its washing with cold technical water with separation of the filtrate and sediment of the sodosulfate mixture, which is sent for drying, while the dried sediment The sodosulfate mixture is crushed to a particle size of minus 200 microns to increase the whiteness coefficient.

The method is complemented by private distinctive features.

The separated filtrate and industrial water can be returned to gas purification.

The separated filtrate and industrial water can be returned to the circulating sodosulfate solution in an amount of no more than 25% of the original circulating sodosulfate solution.

The recycling sodosulfate solution, purified from cryolite, can be treated with a synthetic flocculant based on polyacrylic acid.

Washing the soda sulfate mixture pulp during filtration with cold technical water can be carried out under pressure through nozzles.

Carrying out the invention

The implementation diagram of the proposed method is shown in Fig. 1

As an example, we considered a gas purification circulating solution, which had the following salt composition (Table 1).

Table 1 - Salt composition of the sulfate solution sample after cryolite regeneration

Content, g/dm ³ Na ₂ CO ₃ NaHCO ₃ Na ₂ SO ₄ Na ₂ S NaF NaCl TO ₂ SO ₄ solid,
g-tv/dm ³ 15.0 23.5 101.3 4.9 3.0 1.7 8.9 1.5

To clean (“clarify”) the circulating sodosulfate gas purification solution from a suspension of fluorine salts, gas purification sludge, an emulsion of a synthetic flocculant based on polyacrylic acid is prepared at a special installation 1 at the rate of 0.5÷0.8 grams per 1 m ³ of the circulating sodosulfate gas purification solution, to ensure suspended fluorine salt content <0.5 g/dm ³ . The recycled sodosulfate solution and flocculant are mixed in thickener 2 . Under the influence of the flocculant, suspended solid particles are flocculated, precipitated and accumulated in the thickener cone 2 . The accumulated sediment from the cone is periodically pumped back to the fluoride salts production site (UPFS). The clarified circulating soda-sulfate solution is unloaded through a drain comb located in the upper part of the thickener 2 into the initial solution tank 3 and pumped out to the evaporation unit 4 .

The clarified circulating soda sulfate solution is evaporated until a solid phase appears (total salts ~ 350 g/dm ³ ) and pumped into a tank of evaporated solution 5 . Afterwards, the soda sulfate mixture pulp is pumped out onto a drum vacuum filter 6 . Filtration of the soda-sulfate mixture pulp using a modernized drum vacuum filter 6 is combined with washing with cold process water. This allows you to remove contaminants located on the surface of the filtered layer. Water is supplied under pressure through nozzles. Next, the sodosulfate mixture is sent for drying.

Drying of the filtered and washed sodosulfate mixture (SSM) is carried out in an electric shelf (hearth) dryer 7 to a humidity W ≤ 0.5%, to ensure the flowability of the powder. After the hearth dryer 7, the soda-sulfate mixture consists of loose pieces up to 3 cm in size, which are fed for grinding. Grinding of the dried soda-sulfate mixture to increase the whiteness coefficient to 70-80% is carried out to ≥ 80% of the fraction minus 200 microns.

At the first stage, a hammer crusher 8 is installed to obtain a fraction of 100% minus 1 mm; the second stage is a roller crusher 9 for grinding to ≥ 80% of the fraction minus 200 microns. Next, the SSS enters the finished product bunker 10 . Packaging of finished SSS with a whiteness coefficient ≥ 80% in big bags.

Table 2 shows the results of evaporation of a soda sulfate solution with different suspended matter contents. The amount of suspension in the sodosulfate mixture was regulated by dosing the flocculant.

After evaporation, the suspension was filtered, and the resulting precipitate of the soda-sulfate mixture was dried to constant weight without removing it from the filter. Then the whiteness index was determined in the dry sediment of the sodosulfate mixture.

Table 2 - Effect of the amount of suspended fluorine salts contained
in the original clarified soda-sulfate solution for whiteness indicators of the SSS

Regeneration cryolite sediment content WI(E313-98)(D65) - indicator
whiteness YI(E313-98)(D65) - indicator
yellowness Flocculant content in solution
g/dm ³ in the sediment
SSS, % g/m ³ 0 0 83.77 5.26 0.7 83.12 5.40 83.90 5.21 83.27 5.36 3.0 4.7 76.15 4.12 0.3 73.77 4.62 73.41 4.70 75.19 4.11 5.0 7.8 41.28 8.68 0 41.15 8.76 42.76 7.52 42.46 7.56

Adding the resulting filtrate and saturated industrial water to the circulating sodosulfate solution from the previous cycle in the subsequent cycle increases the productivity of the evaporation battery by ~ 10% and reduces the specific steam consumption per 1 ton of evaporated water, but may affect the whiteness of the resulting product, so the filtrate was dosed into the circulating sodosulfate solution from the previous evaporation cycle in an amount of no more than 25% of the amount of the original circulating sodosulfate solution.

As a result of 4 cycles of evaporation with the return of the resulting filtrate and saturated industrial water from the previous cycle to the subsequent cycle, similar samples of the sodosulfate mixture were obtained with similar whiteness values (with a coefficient of ≥ 80%). Grinding of the dried soda-sulfate mixture to increase the whiteness coefficient to 70-80% is carried out to ≥ 80% of the fraction minus 200 microns.

Table 3 - Effect of grinding on whiteness indicators of CCC

Name Data on the composition of the population
draft whiteness index, % Precipitate of sodosulfate mixture I 0.5-1.6 mm 61.0 60.21 62.94 62.55 Ground to - 200 microns 82.85 82.62 82.35 82.32 Precipitate of sodosulfate mixture II 0.5-1.6 mm 63.71 64.45 64.78 65.30 Ground to - 200 microns 82.28 82.76 81.81 82.55 Precipitate of sodosulfate mixture III 0.5-1.6 mm 63.13 64.44 56.67 58.54 Ground to - 200 microns 80.40 81.93 80.17 81.79 Precipitate of sodosulfate mixture IV 0.5-1.6 mm 61.63 64.28 65.78 67.78 Ground to - 200 microns 83.58 84.26 83.71 85.7

Claims (5)

1. A method for producing a sodosulfate mixture from a recycled sodosulfate solution for gas purification of aluminum electrolyzers, including its defluorination with the release of secondary cryolite, subsequent clarification with a reagent to reduce suspended fluorine salts and gas purification sludge to obtain a clarified solution, its evaporation until a solid phase appears and filtration of the resulting sodosulfate mixture pulp, characterized in that clarification is carried out with a synthetic flocculant, while filtration of the sodosulfate mixture pulp is combined with its washing with cold technical water with separation of the filtrate and sediment of the sodosulfate mixture, which is sent for drying, while the dried sediment of the sodosulfate mixture is crushed to a fineness of minus 200 to increase the whiteness coefficient µm. 2. The method according to claim 1, characterized in that the separated filtrate and industrial water are returned to gas purification. 3. The method according to claim 1, characterized in that the separated filtrate and industrial water are returned to the circulating sodosulfate solution in an amount of no more than 25% of the circulating sodosulfate solution. 4. The method according to claim 1, characterized in that a flocculant based on polyacrylic acid is used as a synthetic flocculant. 5. The method according to claim 1, characterized in that the pulp is washed with a sodosulfate mixture during filtration with cold industrial water under pressure through nozzles.