RU2472865C1 - Method of processing fluorine-containing wastes from electrolytic production of aluminium - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method involves loading material into a metallurgical furnace, heating and holding for 0.5-1.0 hour. Before heating, fluorides of alkali-earth and/or alkali metals are added to the wastes in amount of 1-5%. Heating is carried out at temperature of 1100-1300°C and the wastes are held at this temperature without or with limited access to air while feeding gaseous reaction products into a dry gas cleaning system. Phase separation of the electrolyte and carbon in form of a carbon residue is then carried out.

EFFECT: recycling wastes, extracting valuable components from wastes and returning said components into the technological process.

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Description

 The proposed technical solution relates to the field of non-ferrous metallurgy and can be used in the processing of fluorine and carbon-containing waste from aluminum production - coal foam and other carbon materials impregnated with electrolyte.

The invention relates to the priority area of development of science and technology "Processing of raw materials, except ore, such as scrap, in order to obtain non-ferrous metals or their compounds" [Alphabetical subject index to the international patent classification for priority areas of development of science and technology / Yu.G. Smirnov , E.V. Skidanova, S.A. Krasnov. - M: INITS "PATENT", 2008. - p. 58].

In the electrolytic method for the production of aluminum, solid fluorine-containing wastes are formed, which include: coal foam, carbonaceous waste of the overhaul of electrolyzers (spent lining), tailings of flotation of coal foam, dust and sludge from gas treatment, sludge from sludge fields. Valuable elements are lost with these wastes: fluorine, carbon, aluminum, sodium. Non-recyclable wastes or sludges from their partial processing belong to the third hazard class; expensive sludge storages are used for their storage, which pose an environmental hazard and occupy large areas. Reclamation of sludge storages also requires significant funds.

In connection with the transition to new technologies for the production of aluminum, the main fluorine and carbon-containing wastes remain coal foam and spent lining of electrolyzers.

Currently, these types of waste are mainly disposed of using the flotation method, which is quite expensive and does not allow to achieve high fluorine recovery. Secondary waste after flotation - tailings are stored in significant quantities in sludge fields, which leads to the loss of valuable components (F, Al ₂ O ₃ , soda products), requires a constant fee for storage of waste and free storage space, and also has a negative impact on the environment environment in the area of aluminum smelters.

The technological scheme of coal foam flotation is complex, requires the use of expensive equipment, qualified personnel and is cost-effective only when processing large volumes of coal foam.

At the same time, in connection with the use of new technologies for the production of aluminum (calcined anodes), the volume of coal foam formation decreased from 30-40 kg per ton of aluminum to 1-3 kg. Under these conditions, the use of flotation has become impractical and no flotation department is being built at new plants. Coal foam is stored in dumps. Another way is needed to efficiently process small volumes of coal foam.

For processing waste, various methods are used.

A known method of processing carbonaceous sludge removed from the system for the electrolytic production of aluminum containing carbon particles, various fluoride compounds, cryolite, alumina, with a moisture content of up to 40%, including its mixing with alumina-soda-limestone-containing mixture of alumina production and joint heat treatment, in which carbonaceous sludge is introduced into the alumina-soda-limestone-containing mixture of alumina production in the form of pulp or in a partially dehydrated state in an amount of 2-10% by dry weight, and limestone in the mixture is dosed to form CaF ₂ , CaF ₂ · 3CaO · 2SiO ₂ , 2CaO · SiO ₂ .

In this case, the heat treatment is carried out at a temperature above 1000 ° C in a sintering furnace to obtain a cake, which is subjected to hydrochemical treatment according to known alumina production methods to extract aluminum compounds, alkali metals, and the solid residue after processing is used in the production of cement, silicate brick, road construction [RF patent No. 2167210, C22B 7/00, C01F 7/38, published January 20, 2001].

The signs of the analogue according to the patent of the Russian Federation No. 21727210, coinciding with the essential features of the proposed method are:

- fluorocarbon containing aluminum production wastes is recycled;

- waste is subjected to heat treatment at temperatures above 1000 ° C.

The main disadvantage of this method is that fluorine does not return to the aluminum production process, but is lost in the form of an insoluble precipitate, in addition, the disadvantages are:

- the need to use special equipment and significant energy costs;

- up to 2-5% of fluorine compounds are present in the solid precipitate, which limits the possibility of its use;

- the carbon part of the waste is irretrievably lost;

- for processing waste into the charge, it is necessary to introduce a large amount of additional materials - alumina, soda, limestone.

A known method of processing solid waste sludge field of aluminum production, which consists in sintering the sludge, grinding and leaching the cake with water and filtering the pulp, in which the sludge is sintered without first washing from sulfates and carbonates. In this case, the sludge is sintered with constant access of air at 750-850 ° C for 20-40 minutes, the cake is crushed and leached with water at a ratio of T: W from 1: 3 to 1: 5, a solid precipitate containing cryolite and alumina, after drying at 100-150 ° C for 30-60 minutes they are used as raw materials for aluminum production, and a solution containing sodium hydroaluminate is used as an alkaline coagulant [EA No. 003660, C22B 7/00, published on 08.28.2003].

Signs of an analogue that coincide with the essential features of the proposed method are:

- waste is heat treated at high temperature;

- cryolite and alumina are returned to aluminum production.

The main disadvantage of this method is the burning of carbon during processing, which leads to a complete loss of the carbon part of the waste.

In addition, the disadvantages are:

- the complexity of the technological scheme, consisting of several operations: sintering, crushing, leaching and drying;

- low quality solid sediment containing a lot of unburned carbon.

A known method of processing fluorine and carbon-containing waste, including grinding waste, and heating in an electric furnace to 1300 ° C for 20 minutes. At this temperature, free electrolyte is smelted and collected in a crucible. Then increase the temperature in the electric furnace to 2200-2500 ° C and maintain it for 0.5-1.0 hours.

During this period, the fluoride salts contained in the carbon material evaporate, trap and condense [USSR AS No. 269495, C22d, published on 01.01.1970].

According to the purpose - the processing of fluorocarbon-containing waste from the electrolytic production of aluminum, technical essence, the presence of similar features, this solution is selected as a prototype.

The signs of the prototype, coinciding with the essential features of the proposed method are:

- heating the waste to a high temperature;

- smelting of electrolyte and its collection in a crucible.

The main disadvantage of this method is the disposal of fluoride salts only, the carbon part is lost. Another disadvantage:

- the complexity of the technological scheme, requiring the use of heat-resistant equipment up to 2500 ° C.

The objective of the proposed technical solution is to increase the technical and economic indicators of the process of electrolytic production of aluminum.

The technical results are the extraction of valuable components from waste and their return to the process, waste disposal to obtain a carbon residue suitable for use in various industries as an energy carrier, reducing agent, sorbent and other purposes.

Technical results are achieved by the fact that in the method for processing fluorine-containing wastes of aluminum electrolytic production, including loading material into a metallurgical furnace, heating, holding for 0.5-1.0 hours, according to the invention, an alkaline earth fluoride salt is added to the waste before heating and / or alkali metals in an amount of 1-5%, heating is carried out to a temperature of 1100-1300 ° C, holding at this temperature without access to air or with limited access with the supply of gaseous reaction products to the system "dry "Gas purification and subsequent phase separation of the electrolyte and carbon as a carbon residue.

In this case, calcium fluoride is preferably used as an additive in the preparation of the charge, heating is carried out to a temperature of not more than 1300 ° C, heating is carried out in an induction furnace without access of air, and gaseous reaction products are diverted to the system of "dry" gas purification of the main aluminum production.

- waste processing is carried out thermally by heating the starting products to a temperature of more than 1100 ° C;

- separation of the phases of the electrolyte and carbon after heat treatment;

- exposure of waste in a metallurgical unit for 0.5-1.0 hours.

The proposed solution is also characterized by features that are different from the features of the closest analogue:

- the mixture for heat treatment further includes the addition of fluoride salts of alkaline earth and / or alkali metals in an amount of 1-5%;

- heating is carried out to a temperature of 1100-1300 ° C;

- heating is carried out without air;

- waste heating is preferably carried out in an induction furnace.

In addition, the gaseous reaction products are sent to the “dry” gas purification system at the aluminum smelters, which avoids the additional costs and almost completely immediately eliminates the fluorine, aluminum and sodium compounds contained in the gases.

The presence in the proposed technical solution of signs other than those characterizing the closest analogue allows us to conclude that the proposed solution meets the condition of patentability of the invention of "novelty."

The technical essence of the solution is as follows.

In the proposed solution, fluorocarbon-containing wastes are initially mixed with fluoride salts of alkaline earth and / or alkali metals, preferably with calcium fluoride. Moreover, the quality of mixing is not of great importance, since the additives and electrolyte have similar melting points.

Salt additives can simply be loaded into a metallurgical furnace at the same time as waste. Further, the waste is heated in a metallurgical furnace, preferably in the crucible of an induction furnace, to a temperature of 1100-1300 ° C. At this temperature for 0.5-1.0 hours without oxygen, the pieces of waste and the addition of fluoride salts melt. The electrolyte contained in the waste and fluoride salts are deposited on the bottom of the furnace as a separate phase, and the carbon component in the form of a lighter phase floats to the surface and can be easily separated from the electrolyte. Due to the absence of moisture in the process, the sublimation of fluoride compounds is negligible. The lower the temperature, the less the transition of fluoride compounds into the gas phase. Reducing the temperature and the quality of the separation of electrolyte and carbon is achieved by introducing additives of fluoride salts, which provide an increase in fluidity of the electrolyte and a decrease in the wettability of carbon, which makes it possible to obtain a carbon material with a low fluorine content, as well as easy separation of the electrolyte and carbon.

The composition of the exhaust gases is close to the composition of gases from aluminum electrolysis cells, while, according to the material balance, the volume of these gases is 0.5% of the volume of gases of the main production. Thus, when located next to the main production, the construction of additional gas treatment is not required.

Conducting the process at a temperature below 1100 ° C does not allow qualitative separation of the electrolyte and carbon, in addition, a lot of fluorine remains in carbon.

An increase in temperature above 1300 ° C does not improve technological indicators, while more fluorine passes into the gas phase and energy consumption increases.

It is advisable to conduct the process without access of air in order to avoid burning of valuable carbon and increasing the volume of exhaust gases. When the volume of additives is less than 1% of the weight of the waste, the effect of their use is negligible, with the introduction of more than 5% of additives, the quality of the electrolyte decreases, which limits the possibility of its further use.

The most applicable waste processing unit is an induction furnace, which has the following advantages:

- low energy consumption for heat treatment;

- simplicity of design, one-stage technological scheme;

- the possibility of processing both large and small volumes of fluorocarbon-containing waste;

- the possibility of processing both fine and bulk material.

The result of the proposed method of thermal processing of waste is the complete utilization of all valuable components and the absence of secondary waste, that is, non-waste technology. The main product - an electrolyte containing fluorine, aluminum and sodium, is returned to the process of producing aluminum.

Carbon material with a carbon content of 93-98% without environmentally harmful impurities can be used as fuel, reducing agent, sorbent, raw materials for the production of electrode products and other purposes.

A comparative analysis of the proposed technical solutions with other known solutions in this field revealed the following.

A known method for producing agglomerated materials, including mixing, forming and heat treating a mixture containing agglomerated material, a carbon component, flux and finely dispersed fluorocarbon-containing aluminum waste in the form of flotation tailings of coal foam in an amount of 0.3-2.5 wt.% [RF Patent No. 2291208, C22B 1/245, published January 10, 2007]. In a known solution, this type of waste is used as a reducing agent (carbon part) and flux (sodium fluoroaluminates).

By known technology, only a limited amount of flotation tailings can be processed; the resulting product, a flux in the form of sodium fluoroaluminate, has low consumer properties.

A widely known method of extracting fluorine from solid waste by hydrolysis. The method consists in extracting fluorine in the form of hydrofluoric acid from solid fluorocarbon-containing waste, comprising heating the waste to a temperature of 1000-1400 ° C. and passing a mixture of steam with dilution gas through them. The method has many modifications, for example, in one of the methods pyrohydrolysis is carried out in the presence of calcium carbonate in an amount of 16-27.5 wt.% At 1250-1300 ° C. In the process of pyrohydrolysis, carbon burns out. The target product of pyrohydrolysis is hydrofluoric acid [AS No. 1836462, C22B 7/00, 08/23/1993].

The known method is complicated in hardware design and allows only fluoride compounds to be disposed of, as a result of the method, secondary waste is generated with which fluorine and aluminum are lost.

In the proposed solution, firstly, most types of fluorocarbon-containing waste are used, and secondly, the products obtained as a result of processing are completely utilized, that is, the technology is non-waste.

It was not revealed in the process of searching and comparative analysis of technical solutions that are characterized by a combination of features similar to the proposed solution, allowing to obtain similar results when using, which allows us to conclude that the proposed technical solution meets the patentability condition of the invention “inventive step”. The proposed technology is as follows.

Example 1

1 kg. Coal foam containing, wt.%, 39.9 F, 14.6 Na, 14.9 Al, 1.2 Ca, 24.5 C were mixed with 40 g of calcium fluoride containing, wt.%, 95 CaF ₂ and heated in a crucible laboratory induction furnace to a temperature of 1230 ° C. After exposure at this temperature for 45 minutes, the emerging foam was removed and the electrolyte was poured.

The resulting electrolyte in an amount of 752 g. Contains, wt.% 49.2 F, 17.9 Na,

18.0 Al, 4.5 Ca, 0.12 C.

The carbon product in an amount of 183 g contains, wt.% 95.8 C and 0.25 F.

The fluorine yield in the electrolyte was 90.9%, in the gas phase 8.99%, in the carbon product 0.11%. Carbon burn amounted to 28.5%.

Example 2

1 kg of spent coal lining composition, wt.% 15.8 F, 10.0 Na, 10.2 Al,

0.94 Ca, 46.61 C were mixed with 40 g of calcium fluoride containing, wt.% 95 CaF ₂ .

Further processing was carried out analogously to example 1.

The resulting electrolyte in an amount of 450 g contains, wt.% 36.6 F, 21.8 Na, 22.2 Al, 6.5 Ca, 0.32 C.

The carbon product in the amount of 382 g contains, wt.% 92 C and 0.5 F. The fluorine yield in the electrolyte was 93.5%, in the gas phase 5.42%, in the carbon product 1.08%. Carbon burn amounted to 22.7%.

The technological parameters of waste treatment according to the proposed technology and the results are shown in table 1. According to this technology, pilot tests were also conducted at the Kandalaksha aluminum plant.

The proposed technology for processing fluorocarbon-containing waste allows you to dispose of all the valuable components of the waste without the formation of secondary waste.

In this case, the electrolyte containing fluorine, aluminum, sodium and calcium, is fully returned to the electrolysis process.

Partially graphitized carbon material with a low fluorine content is a valuable raw material for the electrode industry, and can also be used as a reducing agent for the preparation of sorbents and fuel.

Gas pastures containing fluorine, aluminum and sodium are fed to a “dry” gas treatment, where they are sorbed by alumina and returned directly to aluminum production.

Table 1. The results of the processing of fluorocarbon-containing wastes of aluminum electrolytic production (waste composition, wt.% 39.9 F, 14.6 Na, 14.9 Al, 1.2 Ca, 24.5 C) Experience number Processing parameters The fluorine yield,% Content% Carbon burnout,% T, ° C Holding time, hour Additive CaF ₂ ,% In electrolyte In the gas phase F in carbon Ca in electrolyte one 2 3 four 5 6 7 8 9 one 1050 1,0 4.0 89.8 8.1 4.7 4.3 21,2 2 1100 1,0 4.0 90.8 8.7 10.98 4,5 27.1 3 1250 1,0 4.0 91.0 8.9 0.25 4,5 28.5 four 1300 1,0 4.0 90.5 9.45 0.12 4.45 30.1 5 1400 1,0 4.0 88.2 11.77 0.1 4.4 32,7 6 1250 0.2 4.0 84.5 3.8 5.2 4,58 18.1 7 1250 0.5 4.0 90.9 9.0 0.25 4.47 28.6 8 1250 1,0 4.0 90.8 9.1 0.2 4,5 29.0 9 1250 0.5 88.22 11.1 1,56 1.9 30,2 10 1250 0.7 1,0 88.78 10.8 0.96 2.24 29.1

End of table 1 eleven 1250 0.7 2.0 89.14 10.5 0.84 2,8 28.8 12 1250 0.7 4.0 91.0 8.9 0.25 4,5 28.5 13 1250 0.7 5,0 91.09 8.8 0.19 4.83 28,2 fourteen 1250 0.7 7.0 91.1 8.8 0.15 6.12 28.0

Claims (5)

1. A method of processing fluorine-containing wastes of aluminum electrolytic production, including loading material into a metallurgical furnace, heating, holding for 0.5-1.0 hours, characterized in that an alkali-earth and / or alkali metal fluoride salt is added to the waste before heating to the amount of 1-5%, heating is carried out to a temperature of 1100-1300 ° C, holding at this temperature without access to air or with restriction of access with the supply of gaseous reaction products to the dry gas treatment system and the subsequent phase separation of the electric lithium and carbon as a carbon residue. 2. The method according to claim 1, characterized in that calcium fluoride CaF _{2 is} used as an additive. 3. The method according to claim 1, characterized in that cryolite Na ₃ AlF _{6 is} used as an additive. 4. The method according to claim 1, characterized in that the heating is carried out to a temperature exceeding the melting point of the material. 5. The method according to claim 1, characterized in that the waste is heated in an induction furnace.