Method for producing a sodium sulphate mixture from spent sodium sulphate solutions

Abstract

The invention relates to non-ferrous metallurgy and can be used for producing a saleable product from the waste formed during the wet scrubbing of exhaust gases from aluminium potrooms to remove residual hydrogen fluoride and sulphur dioxide. The invention makes it possible to use spent aluminium electrolysis cell scrubbing solutions (including solutions that have been defluorinated by the extraction of secondary cryolite) to produce a sodium sulphate mixture having a high whiteness index and a low yellowness index, i.e. a competitively priced product suitable for producing synthetic detergents. A spent sodium sulphate solution is first clarified using a synthetic flocculant to reduce suspended fluorides and scrubbing sludge. Filtration of the sodium sulphate mixture slurry is carried out conjointly with washing using cold process water, and the dried sodium sulphate mixture residue is ground to a fineness of minus 200 μm to increase the whiteness index.

Description

[0001] METHOD OF PRODUCING SODA-SULFATE MIXTURE FROM RECYCLED SODA-SULFATE SOLUTIONS

[0002] Field of technology to which the invention relates

[0003] The invention relates to non-ferrous metallurgy and can be used to obtain commercial products from waste generated during wet gas cleaning of exhaust gases from residues of hydrogen fluoride and sulfur dioxide from electrolysis buildings of aluminum production. The commercial products obtained are a soda sulphate mixture with a high whiteness coefficient for the production of synthetic detergents.

[0004] State of the art

[0005] Cleaning of exhaust gases of electrolysis buildings of aluminum production from hydrogen fluoride and sulfur dioxide, provides for preliminary cleaning of gases from hydrogen fluoride by a dry method by its adsorption on aluminum oxide. Exhaust gases, cleaned from the main part of hydrogen fluoride, are then subjected to a second stage of cleaning from sulfur dioxide and hydrogen fluoride residues by a wet method, i.e. by irrigating exhaust gases in scrubbers with a soda sulphate solution circulating in the wet gas cleaning system.

[0006] The problem of increased salt concentrations in wet gas cleaning circulating solutions is especially relevant given the ongoing growth of sulfur content in cokes used for anode production. Accumulation of spent gas cleaning solutions occurs at all aluminum plants that use wet gas cleaning as one of the stages. The use of evaporation units to remove a mixture of sulfates, carbonates, and sodium fluorides from circulating solutions will result in 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, such as the production of detergents, and therefore are considered waste. The main reason is related to the properties of this material, including a low whiteness coefficient, increased yellowness, the presence of carbon particles, caking due to high humidity, etc.

[0007] An urgent task in obtaining a soda-sulphate mixture is the purification of the recycled solution:

[0008] • from chromophore compounds (i.e. compounds that reduce the whiteness coefficient and increase the yellowness of the resulting product);

[0009] • from mechanical suspensions of gas cleaning sludge and regeneration cryolite to obtain a commercial-quality soda sulphate mixture, i.e. meeting consumer requirements.

[0010] A method for separating sodium sulfate from solutions of gas cleaning in the electrolytic production of aluminum is known (patent RU 94 029709, IPC COID 5 / 00, published on August 27, 1996). The method for separating sodium sulfate from solutions of gas cleaning in the electrolytic production of aluminum includes saturating the solutions with sulfate, carbonate and sodium bicarbonate, cooling the resulting solution at a temperature of +7 to -5 °C for at least 2 hours, separating and dehydrating the resulting precipitate.

[0011] Disadvantages of the method:

[0012] At low temperatures from +7 to -5 °C, a precipitate consisting of sodium mirabilite decahydrate sulfate NaiSOrlOHiO, double salt of sulfate and fluoride kogarkoite NaF-NaiSCA, sodium bicarbonate berkeite 2Na2SO4-Na2COs, sodium bicarbonate NaHCO, etc. precipitates from the solution. The resulting product has no consumer due to its low consumer properties, including the presence of a large amount of fluorine, variable composition, low whiteness coefficient, etc.

[0013] The claimed prototype is a method for processing sodium bicarbonate mixture RU 2188794 (IPC C01D5 / 00, published on 10.09.2002). The method makes it possible to obtain a high-quality product - sodium sulfate, which complies with GOST and is suitable for the production of detergents. The method for processing sodium bicarbonate mixture includes dissolving it, treating with a precipitating reagent, separating the precipitate, evaporating the sodium sulfate solution, and separating the sodium sulfate crystals. In this case, the solution of sodium bicarbonate mixture is treated with sulfuric acid with a decrease in pH to 7.0-^5.8 with preliminary heating to a temperature of 60-95 °C. Additionally, the solution is treated with intensive stirring for at least 15 minutes with holding after the end of the process at 85^-108 °C for at least 20 minutes. The invention makes it possible to obtain a high-quality product - sodium sulfate, which complies with GOST and is suitable for the production of detergents.

[0014] Disadvantages of the method:

[0015] - Purification of the soda sulphate mixture from impurities through dissolution, precipitation of impurities with a precipitating reagent, filtration and repeated evaporation significantly complicates the equipment and process flow diagram, increases the Nut and Capex, i.e. makes the resulting product uncompetitive.

[0016] - Treatment of the soda sulphate mixture with sulphuric acid significantly increases the cost of the resulting product, which makes it uncompetitive in relation to natural sodium sulphate.

[0017] 1 GOST 22567.13-82 Synthetic detergents. Method for determining color. In GOST 6318-77 Sodium sulfate technical in section 3.13 - Determination of whiteness - according to GOST 22567.13-82 For the production of detergents, it is preferable to use not sodium sulfate thenardite Na2SO4^ but the soda sulfate mixture berkeit 2Na2SO4-Na2COs, since Na2CO3 (calcined soda) is a component in detergents.

[0018] Disclosure of the essence of the invention

[0019] The objective and technical result of the invention is to obtain from the recycled (spent) solutions of gas cleaning of aluminum electrolyzers (including those that have undergone defluorination due to the release of secondary cryolite) a soda sulphate 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 its price.

[0020] The technical result is achieved and the problem is solved due to the fact that in the method for obtaining a soda sulphate mixture from recycled soda sulphate solutions, including treating the soda sulphate mixture solution with a precipitating reagent, filtering and evaporating the recycled solution, separating the sediment, according to the claimed invention, the recycled soda sulphate solution is preliminarily clarified with a synthetic flocculant to reduce the suspension of fluoride salts and gas cleaning sludge, while the filtration of the soda sulphate mixture pulp is combined with washing with cold process water, and the dried sediment of the soda sulphate mixture is ground to a size of minus 200 μm to increase the whiteness coefficient.

[0021] The method is supplemented by specific distinctive features.

[0022] The resulting filtrate and industrial water can be returned to gas purification.

[0023] The obtained filtrate and industrial water can be returned to the initial soda-sulfate solution in an amount of no more than 25% of the initial soda-sulfate solution. The soda-sulfate solution can be treated with a synthetic flocculant based on polyacrylic acid.

[0024] Washing of the soda-sulphate mixture pulp during filtration with cold technical water can be carried out under pressure through nozzles.

[0025] Implementation of the invention

[0026] The scheme for implementing the claimed method is shown in Fig. 1

[0027] As an example, a gas cleaning circulating solution was considered, which had the following salt composition (Table 1).

[0028] Table 1 - Salt composition of the sample - sulfate solution, after regeneration of cryolite

[0029] To clean (“clarify”) the recycled soda-sulphate solution of gas cleaning from suspended fluoride salts and gas cleaning sludge, an emulsion of synthetic flocculant based on polyacrylic acid is prepared at a special installation 1 at a rate of 0.5-10.8 grams per 1 m3. 3 recycled gas cleaning solution to ensure the content of suspended fluoride salts <0.5 g / dm3 3 . The recycled sodium sulfate solution and flocculant are mixed in thickener 2. Suspended solids flocculate under the action of the flocculant, settle and accumulate in the cone of thickener 2. The accumulated sediment is periodically pumped back from the cone to the fluoride salt production section (FPS). The clarified solution is discharged through a drain comb located in the upper part of thickener 2 into the feed solution tank 3 and pumped to the evaporation unit 4.

[0030] The clarified solution is evaporated until a solid phase appears (total salts ~ 350 g / dm3 3) and pumped into the evaporated solution tank 5. Afterwards, the soda sulphate mixture pulp is pumped onto the drum vacuum filter 6. Filtration of the soda sulphate mixture pulp on the modernized drum vacuum filter 6 is combined with washing with cold process water. This allows removing contaminants located on the surface of the filtered layer. Water is supplied under pressure through nozzles. Then the soda sulphate mixture is fed for drying.

[0031] Drying of the filtered and washed sodium sulfate mixture (SSM) is carried out in an electric shelf (floor) dryer 7 to a moisture content of W < 0.5%, to ensure powder flowability. After the floor dryer 7, the sodium sulfate mixture consists of loose pieces up to 3 cm in size, which are fed for grinding. Grinding of the dried sodium sulfate mixture to increase the whiteness coefficient to 70-80% is carried out to > 80% of the fraction minus 200 μm.

[0032] 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 µm. Then the dry mix goes to the finished product bin 10. The finished dry mix with a whiteness coefficient of > 80% is packed in big bags.

[0033] Table 2 shows the results of evaporation of soda-sulphate solution with different suspended matter content. The amount of suspended matter in the soda-sulphate mixture was regulated by dosing the flocculant.

[0034] After evaporation, the suspension was filtered, the resulting sediment, without removing from the filter, was dried to a constant weight. Then, the whiteness index was determined in the dry sediment. Table 2 - Effect of the amount of suspended fluorine salts contained in the initial soda-sulphate solution on the whiteness index of dry chemical mixtures

[0035] Adding the obtained filtrate and saturated industrial water to the initial soda-sulphate solution (from the previous cycle to 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 can affect the whiteness of the resulting product, therefore, the filtrate from the previous evaporation cycle was dosed into the initial soda-sulphate solution in an amount of no more than 25% of the amount of the initial soda-sulphate solution.

[0036] As a result of 4 evaporation cycles with the return of the obtained filtrate and saturated industrial water from the previous cycle to the subsequent cycle, identical samples of the sodium sulfate mixture were obtained, having similar whiteness indices (with a coefficient of > 80%). Grinding of the dried sodium sulfate mixture to increase the whiteness coefficient to 70-80% is carried out to > 80% of the minus 200 μm fraction. Table 3 - Effect of grinding on the whiteness indices of dry mixes

Claims

Invention formula 1. A method for obtaining a soda-sulfate mixture from recycled soda-sulfate solutions, comprising treating the soda-sulfate mixture solution with a precipitating reagent, filtering and evaporating the recycled solution, and separating the precipitate, characterized in that the recycled soda-sulfate solution is preliminarily clarified with a synthetic flocculant to reduce the suspension of fluoride salts and gas cleaning sludge, while the filtration of the soda-sulfate mixture pulp is combined with washing with cold process water, and the dried precipitate of the soda-sulfate mixture is ground to a size of minus 200 µm to increase the whiteness coefficient.

2. The method according to paragraph 1, characterized in that the obtained filtrate and industrial water are returned to gas purification.

3. The method according to item 1, characterized in that the obtained filtrate and industrial water are returned to the initial soda-sulfate solution in an amount of no more than 25% of the initial soda-sulfate solution.

4. The method according to paragraph 1, characterized in that the treatment of the soda sulphate solution is carried out with a synthetic flocculant based on polyacrylic acid.

5. The method according to item 1, characterized in that the washing of the soda-sulfate mixture pulp during filtration with cold industrial water is carried out under pressure through nozzles.

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