RU2621334C1 - Wet scrubbing method of potroom exit gases of aluminium production - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: wet scrubbing method of potroom exit gases of aluminium production comprises a gas scrubbing of hydrogen fluoride and sulfur compounds by its irrigation by soda-sulfate solution, separation of anhydrous sodium sulfate in evaporator, in this case, gas irrigation by soda-sulfate solution is carried out to obtain a saturated soda-sulfate solution, a portion of which is fed in the evaporator and concentrated until a limiting concentration of sodium sulfate, and the remaining solution is sent to sedimentation, followed by returning to the irrigation stage, and the solution evaporated with sodium sulphate is directed to dewatering and drying followed by obtaining the finished sodium sulfate, and after evaporation, the mother liquor is re-directed to the evaporator.

EFFECT: improved quality of sodium sulfate.

4 cl, 1 dwg

Description

The invention relates to non-ferrous metallurgy and can be used for purification of waste gases from electrolysis aluminum production cells from the remains of hydrogen fluoride and sulfur compounds to produce sodium sulfate as a commercial product.

A known method of wet cleaning of exhaust gases from electrolysis aluminum production buildings (patent RU 2254293, C01D 5/00, C01F 7/54, publ. 11/27/2014), including the cleaning of gas from sulfur oxides and fluoride compounds by irrigation with sodosulfate solution in wet scrubbers, the selection of the main amount of sodium fluoride in the form of cryolite from the solution after gas purification. The saturated solution, purified from cryolite, is further purified from sodium fluoride by treating it at 95-105 ° C for 1.5-2.0 hours with milk of lime introduced into the saturated solution based on the stoichiometric binding of fluorine contained in the solution in CaF ₂ . The saturated solution purified from fluorine is then subjected to a concentrated evaporation until the density of the evaporated solution reaches 1.37 ± 0.02 g / l and sodium sulfate in the form of a berkeitic salt is precipitated from it by introducing carbonate soda into the evaporated solution until the titrated alkali concentration in the mother liquor is reached a solution of 215-230 g / l Na ₂ O and the density of the solution in suspension up to 1.35 ± 0.02 g / l with stirring the suspension at a temperature of 95-100 ° C for 30-40 minutes.

The disadvantages of this method include:

- a decrease in the efficiency of the evaporation process due to the introduction of milk of lime and, as a result, overgrowth of the stiffness salts of the heat exchanger tubes of the evaporator — a decrease in its service life;

- a significant increase in heat energy production costs for the allocation of fluorine in water of calcium fluoride.

A known method for wet cleaning of exhaust gases from electrolysis aluminum production buildings (Patent RU 2320539, C01D 5/00, C01D 5/16, published March 27, 2008), including cleaning gas from sulfur oxides and fluoride compounds by spraying them with soda-sulphate solution in wet scrubbers, the selection of the main amount of sodium fluoride in the form of cryolite from the solution after gas purification. In a method for processing a sodosulfate solution obtained after cleaning exhaust gases from electrolysis vessels in aluminum production, which includes cleaning gas from fluoride compounds and sulfur dioxide by irrigation in wet scrubbers with a sodosulfate solution, isolating sodium fluoride in the form of calcium fluoride from a portion of the circulating soda sulfate solution from calcium mother liquor co-sulfate solution after its purification from sodium fluoride, precipitated in the sediment purified from impurities coarse crystalline anhydrous sodium sulfate tions by uparki mother liquor by adjusting the concentration of titratable alkali carbonate solution evaporated to 75-100 g / l, based on Na ₂ CO ₃ and evaporated solution density 1,30-1,31 g / l at a temperature of 70-105 ° C.

The limits of temperature, density and concentration of carbonate soda in one stripped off solution are selected depending on the value of the residual concentration of sodium fluoride in soda-sulfate solution, which after its purification from sodium fluoride with lime milk can fluctuate in the range of 0.5-1.0 g / l in terms of NaF.

When the residual concentration of sodium fluoride in the sodosulfate solution purified with milk of lime is not higher than 0.5 g / l NaF, the density and concentration of carbonate soda in the stripped off solution can be allowed up to 1.31 g / l and 100 g / l Na ₂ CO _3, respectively the temperature of the solution is 70-80 ° C, which provides a deeper separation of sodium sulfate from one stripped off soda-sulfate solution, while avoiding contamination with carbonate soda and sodium fluoride due to its possible precipitation from the evaporated solution into the precipitate in the form of a double salt of fluoride sodium with sodium sulfate (NaF, Na ₂ SO ₄ ).

When increasing the residual concentration of sodium fluoride in sodosulfate solution purified with milk of lime to one or more grams per liter, to eliminate the possibility of contamination of sodium sulfate precipitated with sodium fluoride, it is necessary to reduce the density and concentration of carbonate soda in one stripped off sodosulfate solution to 1.30 g / l and 75 g / l Na ₂ CO ₃ and increase the temperature of the solution to 90-100 ° C.

The disadvantages of this method include:

- mixing one stripped off mother liquor with gas purification solutions leads to an increase in the concentration of sodium sulfate;

- a significant increase in heat energy production costs for the allocation of fluorine in the form of calcium fluoride;

- a decrease in the efficiency of the evaporation process due to the introduction of milk of lime and, as a result, overgrowth of the stiffness salts of the heat exchanger tubes of the evaporator — a decrease in its service life.

There is a method of separating sodium sulfate from gas purification solutions of aluminum electrolytic production (patent application RU 94029709, C01D 5/00, publ. 08.27.1996), which consists in the crystallization of sodium sulfate by cooling a saturated solution in cascaded crystallizers at a temperature of 7 - (- 5 ) ° C, and the temperature of each previous mold is maintained at such a level that it differs from the temperature of the subsequent mold by 4-6 ° C.

The method is difficult to operate and is associated with high heat and energy costs during the deep cooling of the solution with the precipitation of ten-water glauber salt (Na ₂ SO ₄ ⋅ 10 N ₂ O) with its subsequent drying. For this reason, the method is not used in aluminum plants.

As the closest analogue (prototype), the method according to patent RU 2363525, C01D 5/00, publ. 08/10/2009, according to which wet purification of waste gases from aluminum electrolysis vessels includes purification of gas from hydrogen fluoride and sulfur dioxide by irrigation in wet scrubbers with a sodosulphate solution, purification of the sodosulphate solution from fluorine with lime milk to produce calcium fluoride, and isolation from the mother liquor purified from fluorine sodosulfate solution of anhydrous sodium sulfate by evaporation, while cleaning from fluorine and the allocation of calcium fluoride is subjected to 4-10% sodosulfate solution e wet gas purification, and the remaining solution is recycled to the gas purification, wherein the soda-sulfate solution is purified by fluorine, is subjected to evaporation with separation of anhydrous sodium sulfate, The evaporated mother liquor is mixed with a freshly prepared solution sodoschelochnym and returned to the wet gas cleaning together with the basic soda-sulfate solution.

The disadvantages of the method are:

1) the specified input concentration of Na ₂ SO ₄ 250 g / l is unattainable when cleaning exhaust gases from aluminum electrolysis vessels. Since when the concentration of Na ₂ SO ₄ reaches 200 g / l (n), the gas treatment equipment ceases to function;

2) a large number of redistribution (purification from sodium fluoride) associated with an increase in solution circulation at the enterprise;

3) the use of expensive raw materials - caustic soda to convert bicarbonate soda to carbonate.

The objective of the invention is to increase the efficiency of gas purification equipment and the exclusion of the construction of new and conservation of used sludge fields to obtain dry sodium sulfate for disposal or sale - is aimed at eliminating the above disadvantages.

In this case, the technical result is the implementation of the task, namely improving the quality of sodium sulfate obtained for the further sale in a closed cycle: gas purification - sludge field.

Common with the prototype are the signs:

- gas purification from hydrogen fluoride and sulfur dioxide is carried out by irrigation with sodosulfate solution;

- the allocation of anhydrous sodium sulfate is carried out by evaporation in the evaporator.

Features:

- elimination of the stage of purification from sodium fluoride, reducing the number of redistribution;

- use for irrigation of soda solution after settling it in the sludge field;

- submission for evaporation of solutions with a lower concentration of sodium sulfate (70-170 g / l).

The technical result is achieved due to the fact that in the method for wet cleaning of exhaust gases from aluminum electrolysis vessels, including the purification of gas from hydrogen fluoride and sulfur compounds by irrigation with a sodosulfate solution, the allocation of anhydrous sodium sulfate in an evaporator, according to the claimed invention, gas is irrigated with a sodosulfate solution with obtaining a saturated soda-sulfate solution, part of which is fed to the evaporator and evaporated to reach the maximum concentration of sulfate and sodium, and the remaining solution is sent to sedimentation, after which it is returned to the irrigation stage, while one stripped off sodium sulfate solution is sent to dehydration and drying, followed by obtaining the finished sodium sulfate, and the mother liquor after evaporation is re-sent to the evaporator.

It contributes to the achievement of the technical result by the fact that 3-6% of a saturated soda-sulfate solution is supplied for evaporation, the sodium sulfate content in a saturated soda-sulfate solution when feeding it to the evaporator is 70-170 g / l, and evaporation is carried out to a sodium sulfate concentration of 190-450 g / l

The efficiency of gas purification from sulfur compounds and the efficiency of gas purification is increased by removing the equilibrium or excess amount of sodium sulfate coming from the raw material, and the ability to remove from the general balance the gas purification-sludge field of sodium sulfate in dry form and in the liquid part in the form of juice juice discharge and condensate evaporator.

Part of 3 to 6% of the volume of a saturated soda sulfate solution circulating in the gas treatment system with a concentration of Na ₂ SO _{4 of} 70-170 g / l, NaHCO _{3 of} 15-40 g / l, NaF 5-7 g / l and Na ₂ CO ₃ 5- 25 g / l is evaporated in an evaporation apparatus until the maximum concentration of Na ₂ SO _{4 is} reached 190-450 g / l, and then the evaporated solution is supplied for dehydration and drying, and the mother liquor is returned to the evaporation apparatus.

The concentration limit of Na ₂ SO ₄ 70-170 g / l, in contrast to the prototype, is the most optimal, since higher concentrations lead to the cessation of the functioning of the "wet" stage of the gas treatment plant.

The supply to the evaporation stage of part of the volume within 3-6% is due to the amount of SO ₂ entering the gas treatment as part of the waste gases of the electrolysis production, taking into account the incomplete absorption of sulfur dioxide by soda solution, in terms of the amount of formed Na ₂ SO ₄ tons per hour, which must be removed from the volume of solution circulation.

Achieving the concentration of Na ₂ SO ₄ upon evaporation below 190 g / l is impractical due to the fact that with further dehydration of the resulting product, additional condensation equipment will be required, and at a concentration above 450 g / l, the evaporation apparatus will decrease in efficiency that a high concentration of the solution will lead to a shift in the boiling zone of the liquid from the separator into the heat exchange tubes, which will lead to the rapid deposition and inlay of sodium sulfate on the heat exchange surface.

The technological scheme for the implementation of the proposed method is presented in the drawing.

The operation of aluminum electrolyzers is accompanied by the formation of exhaust gases containing CO ₂ , SO ₂ , SO ₃ , HF, as well as various organic compounds. According to the existing technology, the gases are sent to gas purification, where these impurities are absorbed and neutralized with the help of a soda sulfate solution. In this case, sulfur dioxide SO ₂ is oxidized to SO ₃ . The latter after interaction with soda forms sodium sulfate. The accumulation of sulfate in working gas cleaning solutions leads to overgrowth of equipment with salt crusts and inlays.

The gas purification of the electrolytic production of aluminum in the apparatus for wet gas purification is provided by irrigation of the gas passing through the apparatus with a soda-sulfate solution.

Sodosulfate solution used to trap hydrogen fluoride HF, sulfur compounds (SO ₂ , SO ₃ ) from the exhaust gases of electrolysis production, has the following composition, g / dm ³ :

Soda ash (Na ₂ CO ₃ ) 15-35 Sodium Bicarbonate (NaHCO ₃ ) 3-12 Sodium Sulfate (Na ₂ SO ₄ ) up to 120 Sodium Fluoride (NaF) 4-10

The process of absorption of HF, SO ₂ , CO ₂ proceeds in the foam layer as a result of the following chemical reactions:

Na ₂ CO ₃ + HF = NaF + NaHCO ₃

NaHCO ₃ + HF = NaF + H ₂ O + CO ₂

Na ₂ CO ₃ + CO ₂ + H ₂ O = 2NaHCO ₃

Na ₂ CO ₃ + SO ₂ + ½O ₂ = Na ₂ SO ₄ + CO ₂

The saturated sodosulfate solution obtained in gas treatment plants contains sodium fluoride NaF, soda ash Na ₂ CO ₃ , sodium bicarbonate NaHCO ₃ , sodium sulfate Na ₂ SO ₄ , as well as particles of trapped dust. The clarification (settling) of the gas treatment solution is carried out to separate and dispose of gas treatment sludge (solid particles). The pulp from the apparatus of the "wet" stage of gas purification is pumped out to settle on the sludge field. The solution clarified after settling from the sludge field is sent for gas purification to prepare the soda-sulfate solution.

The method of wet cleaning of exhaust gases of electrolysis cells allows to obtain high-quality sodium sulfate by increasing the concentration of sodium sulfate in saturated soda sulfate solutions of gas purification and effective capture of sulfur compounds, due to the reusable circulation of solutions with a reduced concentration of sodium sulfate (in the system "wet gas-slurry field").

Claims (4)

1. The method of wet cleaning of exhaust gases from electrolysis cells of aluminum production, including cleaning gas from hydrogen fluoride and sulfur compounds by irrigation with a soda-sulfate solution, the allocation of anhydrous sodium sulfate in an evaporation apparatus, characterized in that the gas is irrigated with a soda-sulfate solution to obtain a saturated soda-sulfate solution, part of which is fed to the evaporator and evaporated until the maximum concentration of sodium sulfate is reached, and the remaining solution is sent to sedimentation, after go back to the irrigation stage, while one stripped off sodium sulfate solution is sent to dehydration and drying, followed by obtaining the finished sodium sulfate, and the mother liquor after evaporation is re-sent to the evaporator. 2. The method according to p. 1, characterized in that for evaporation serves 3-6% saturated sodosulfate solution. 3. The method according to p. 1, characterized in that the content of sodium sulfate in a saturated soda sulfate solution when it is fed to the evaporator is 70-170 g / L. 4. The method according to p. 1, characterized in that the saturated sodosulfate solution is evaporated to a concentration of sodium sulfate 190-450 g / L.

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