RU2098499C1 - Method for processing gas cleaning slime in production of primary aluminum - Google Patents

Abstract

FIELD: processing the waste of gas cleaning in aluminum production. SUBSTANCE: method includes subjection of slime to magnetic separation with magnetic field intensity of at least 1000 Oe, and then to high-temperature oxidizing roasting with introduction of modifying additive and subsequent desulfurizing of return product with water. Modifying additive is used in the form of lithium compound in the amount of 7.5-10.0 mas.% in terms of Li₂O, or sodium compound in the amount of 10.0-15.0 mas.% in terms of Na₂O. The invention makes it possible not only to reduce fluorine losses and content of impurities in the form of iron and sulfur in return product in processing wastes of primary aluminum production, but also to increase technical-and-economic indices of roasting process because this process is conducted at lower temperatures (500-600 C). EFFECT: provision of producing secondary raw materials from slimes of gas treatment system in aluminum production with use of self-baking anodes and reduced losses of fluorine in production of return product. 4 cl, 2 tbl

Description

 The invention relates to ferrous metallurgy and can be used in the processing of waste gas purification of aluminum production.

During the electrolysis of aluminum, a large amount of gas is released from the electrolysis cells, which carries solid particles with it. The main components of the gas phase are carbon oxides and volatile fluorides. Particulate matter consists of alumina, cryolite, carbon, organic substances and contains in the form of impurities compounds of iron, calcium, magnesium and sodium sulfate. Modern environmental requirements can be ensured using sophisticated and expensive equipment for the treatment of exhaust gas from electrolysis cells, including dry cleaning in electrostatic precipitators and washing with liquids. In this case, a significant amount of fine solid sludge is formed, which contains valuable components (sodium, aluminum, fluorine) contaminated with impurities [1]
A known method of processing waste from aluminum production, including dust collection, gas neutralization, treatment of coal foam with a mother gas cleaning solution, obtaining cryolite and returning the clarified solution to a gas cleaning system [2] The method has been developed in domestic practice by mixing solutions with gas cleaning sludges, coal foam flotation tailings from subsequent treatment with a solution of hydroxide concentration of 1.0-1.6 g / l, separation of the solution from the precipitate while maintaining the ratio of the volumes of gas cleaning solutions calcium hydroxide in the range 1: (1-2) [3] The disadvantages of the present methods is the high loss of fluorine and complexity of the processes.

The closest to the proposed method for the maximum coincidence of essential features should be recognized the process [4] which involves the oxidation of a slurry of sludge containing aluminum oxide and cryolite in a fluidized bed reactor with simultaneous agglomeration of a product suitable for returning to the aluminum production process. Oxidation of sludge in air is carried out at 770-800 ^o C, preferably at 785-795 ^o C, followed by cooling. The method provides a high yield of the return product, but is not applicable when using self-baking anodes due to the high content of sulphates, iron and carbon-containing substances in the sludge. In addition, the process of high-temperature treatment of sludge is accompanied by significant losses of fluorine, which in this case are not controlled in any way. The method adopted for the prototype.

 The technical result of the proposed technical solution is to obtain secondary raw materials from the sludge of a gas treatment system for aluminum production using self-baking anodes and to reduce fluorine losses upon receipt of a return product.

This is achieved by the fact that before the oxidation of the carbon-containing part of the sludge is subjected to magnetic separation with the subsequent introduction of a modifying additive. After oxidative calcination, desulfurization of the obtained product is carried out by washing soluble sulfates with water. The distinguishing features of the method also include the fact that magnetic separation is carried out at a field strength of at least 1000 E. Lithium compounds (LiOH, Li ₂ CO ₃ ) with a content of 7.5-10.0 wt. in terms of Li ₂ O. In addition, as a modifying additive, sodium compounds (NaOH, Na ₂ CO ₃ ) are used with a content of 10.0-15.0 wt. in terms of Na ₂ O.

The essence of the proposed method is that the solid phase of the sludge contains iron compounds with high magnetic susceptibility due to the recovery medium due to the presence of free carbon in the sludge. Particles enriched in iron are formed as a result of corrosion of structural materials. Magnetic sludge separation is most effective for cleaning iron compounds. It was found that after oxidative treatment of sludge, the efficiency of cleaning by magnetic separation is sharply reduced. The introduction of a modifying additive is necessary in order to increase the oxidation rate of the carbon-containing component of the sludge and to prevent the loss of fluorine in the form of volatile compounds during firing. In addition, the additive provides a decrease in fluorine loss during desulfurization by binding fluorine to sparingly soluble compounds. Thus, the introduction of lithium compounds reduces the solubility of fluorine and aluminum compounds due to the formation of LiF, Li ₃ AlF ₆ , Li ₂ O • Al ₂ O ₃ . This option can be used as one of the ways of introducing lithium additives into the electrolyte during aluminum electrolysis. Further desulfurization of the return product by washing the well-soluble Na ₂ SO ₄ in the upward flow of water does not lead to a significant loss of valuable components. With a linear water flow rate of 0.1-0.8 m / s and a flow rate of 0.5-1.5 m ³ / t of product, the residual sulfate content does not exceed 2.0 wt.

The introduction of sodium compounds reduces fluoride losses during firing and subsequent desulfurization due to the formation of compounds such as NaF, Na ₃ AlF ₆ . Although fluorine losses during desulfurization are higher when sludge is modified with sodium compounds than lithium, sodium can be used as a universal modifying additive.

Example 1. A mixture of sludge from electrostatic precipitators and wet cleaning systems containing 1.9 wt. iron, dried at 100-120 ^o C, after which it is subjected to magnetic separation to remove magnetic compounds of iron. Magnetic separation is carried out at a magnetic field strength of 700-2000 E. In the table. Figure 1 shows the yield of the magnetic part and the content of iron compounds in it, as well as the residual iron content in the sludge, depending on the magnetic field strength.

 From the data table. 1 it follows that obtaining a magnetic fraction with a high iron content is preferable when the field strength is less than 1000 Oe. Obtaining the minimum iron content in the return sludge is possible when the field strength is more than 1000 Oe

Example 2. After magnetic separation into sludge with a fluorine content of about 16.0 wt. lithium or sodium salts are introduced by mixing and subjected to oxidative calcination at a temperature of 550 ^o C. In the table. Figure 2 shows the fluorine loss during firing of conventional and modified gas purification sludges at various contents of the modifying additive.

Example 3. A mixture of sludge from electrostatic precipitators and wet cleaning systems with a fluorine content of about 16 wt. iron 1.9 wt. and sulfates 10 wt. dried at 100-120 ^o C and subjected to magnetic separation at a magnetic field strength of 1000 E. The output of the magnetic fraction was 2.3 wt. In the sludge with a residual iron content of 0.4 wt. by mixing was introduced 10 wt. lithium salts, after which the sludge was subjected to oxidative firing at a temperature of 550 ^o C. The relative loss of fluorine during oxidative firing was 0.8 wt.

The calcined product was washed with an upward flow of water at a casting speed of 0.6 m / s and a flow rate of 1.5 m ³ / t of product. Under these conditions, the sulfate content decreased from 10.0 to 2.0 wt.

 Example 4. The processing of sludge was carried out as in example 3, but as an additive was introduced 15 wt. sodium salts. The relative losses of fluorine during oxidative firing amounted to 3.4 wt. The desulfurization of the calcined product was carried out as in example 3. Under these conditions, the sulfate content also decreased from 10.0 to 2.0 wt.

As follows from the above examples, the optimal content of the additive to effectively reduce fluoride losses during firing is 7.5-10.0 wt. for lithium and 10.0-15.0 wt. for sodium. Desulfurization of the burnt sludge is carried out by an upward flow of water at a linear flow rate of 0.6 m / s and a flow rate of 1.5 m ³ / t of product. With the optimal content of the modifying additive under the indicated conditions of washing with water, the sulfate content decreases from 10 to 2.0 wt.

Thus, the proposed technical solution allows not only to reduce fluorine losses and the content of impurity compounds of iron and sulfur in the return product during the processing of primary aluminum waste, but also to increase the technical and economic parameters of the roasting process, since it is carried out at lower temperatures (500- 600 ^o C) compared with the prototype.

Sources of information
1. Sittig M. Extraction of metals and inorganic compounds from waste / Handbook. M. Metallurgy, 1985.125 p.

 2. Pat. 2231305, USA, 1937.

 3. A.S. 1129270, USSR, 1984.

 4. Pat. 4053375, USA, 1977. prototype.

Claims (4)

 1. A method of processing sludge from gas purification of primary aluminum production, including high-temperature oxidative firing to obtain secondary raw materials for aluminum production, characterized in that the magnetic material is preliminarily separated and the firing is carried out in the presence of a modifying additive followed by desulfurization of the return product with water.  2. The method according to claim 1, characterized in that the magnetic separation is carried out at a field strength of at least 1000 E. 3. The method according to claim 1, characterized in that as a modifying additive using lithium compounds in an amount of 7.5 to 10.0 wt. in terms of Li ₂ O. 4. The method according to claim 1, characterized in that as the modifying additives use sodium compounds in an amount of 10 to 15 wt. in terms of Na ₂ O.

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