EA041223B1 - METHOD FOR OBTAINING A REDUCER FOR THE PRODUCTION OF COMMERCIAL SILICON - Google Patents

Description

The invention relates to the field of metallurgy, in particular, the technology for the production of reducing agents for the production of technical high-purity silicon.

A known method for the production of silicon, including dosing, mixing, loading and melting in the electric furnace charge of quartzite and charcoal (Ragulina R.I. Emlin B.N. Electrothermal silicon and silumin. M: Metallurgy, 1972, p. 240). However, this method of production is associated with the consumption of an extremely scarce and expensive type of raw material - charcoal. In addition, this method is accompanied by exceptionally large losses of the reducing agent. The latter is explained by the fact that charcoal is very fragile and when introduced into the charge and, especially, when it is overloaded, it is intensively crushed and abraded.

A known method for the production of silicon, including dosing, mixing, loading and melting in the electric furnace charge of quartzite and Colombian coals, which have a very low content of iron and ash (http://www.up-pro.ru/library/production_management/systems /kremnij-kruchkov.html). The disadvantages of this method is the high cost of Colombian coals due to transportation costs.

A known method for producing a briquetted mixture for the production of silicon (patent RU 2528666, C01B 33/025, publ. 05/27/2014), which includes mixing silica-containing raw materials and carbon-containing substances of plant origin (sawdust, wood waste, etc.), their briquetting to obtain briquettes, which are subjected to thermal pyrolysis without oxygen until the pyrogas is completely released.

The disadvantage is low strength, low fixed carbon and high costs for obtaining briquettes.

A known method for producing carbonizate (rexyl) (patent KZ 23615, C10B 57/00, C10B 55/00, publ. 12/15/2010) from non-caking hard coals, which are heat treated at high temperature in an inert atmosphere without air access. The disadvantage of this method is the need to select the feedstock for the production of carbonizate, namely, low-ash coals with a low content of harmful impurities, which after heat treatment are completely converted into carbonizate.

A known method of obtaining a mixture for the smelting of pure silicon (patent RU 2424341, C22C 33/04, C01B 33/025, publ. 07/20/2011), which consists in mixing amorphous silicon dioxide with a carbon-containing reducing agent in the form of soot in a ratio of 1:1, 5 and silicic acid gel, followed by the manufacture of granules or briquettes of various shapes, which are dried and fired at temperatures of 900-1200°C. The disadvantage of this method of obtaining a mixture is the high cost of carbon black, increased yield of volatile silicon monoxide SiO.

A known method of briquetting charcoal (patent RU 2376342, C10L 5/12, publ. 20.12.2009), predominantly lignite, provides for the stage of preparation of the starting components: grinding the charcoal to a size of 0-7 mm, mixing the charcoal with slaked lime with a water-lime ratio of 3 :1 or 5:1, pressing, impregnation of briquettes with liquid glass and drying.

The disadvantage is the high content of ash and undesirable impurities, in particular, iron, which limits the use of reducing briquettes in the production of technical silicon of the highest grades.

A known method includes heat treatment of a layer of coal in a vertical shaft-type apparatus during ignition from the side opposite to the air supply. Coal fraction 20-70 mm is used as raw material, air supply is carried out through a layer of coal with a specific consumption of 70-99.5 m ³ /(m ² xh) depending on the brand of coal. The method allows to obtain a solid product with higher strength, low ash content, as well as a larger average piece size with a simultaneous increased product yield (patent RU 2275407, C10B49/02, C10B53/00, publ. 27.04.2006).

The disadvantage of the known technical solution is the presence of a mineral part in large pieces of heat-treated coal, polluting the final product, which limits their use in the production of silicon.

A known method for producing semi-coke, including the stages of heating, drying and carbonization of solid carbon-containing raw materials using air blast, to which combustible gas is added, having an exit temperature from the apparatus, in a concentration not exceeding the lower limit of ignition of the gas. Preferably, the amount of added gas is 8-10% of the volume of combustible gas produced. The carbonization temperature is 920-950°C. EFFECT: improving the quality of semi-coke by reducing the yield of volatiles in the product, increasing the yield of semi-coke by about 5-6%, reducing the processing time of raw materials (patent RU 2169166, C10B49/04, publ. 20.06.2001).

The disadvantage of analogue is that in the mineral part there are iron compounds, which degrade the quality of the obtained technical silicon.

The closest in technical essence is a method for processing coal crushed to 10 mm, which is carried out by simultaneous thermal-oxidative treatment of coal at a temperature of 700-950 ° C due to partial oxidation of coal with air and separation of the mineral part of coal, more

- 1 041223 heavier than coal. Blast air for fluidization of the layer is supplied in the amount of 2500-4200 m ³ /(m ² xh). Finely dispersed coal, steam-gas products of thermal decomposition of coal, as well as coke gasification products are partially burned in a fluidized bed, ensuring its operating temperature. Heavier rock particles settle in the lower part of the fluidized bed and are removed from the apparatus through the failed grate. Combustion products are fed into the waste heat boiler to generate thermal energy. EFFECT: invention makes it possible to obtain high-calorie low-ash heat-treated solid fuel from high-ash coals without preliminary enrichment of the initial coal (patent RU 2401295, C10 B49/10, publ. 10.10.2010).

The disadvantage of the prototype is the inefficient separation of coal from rock particles and high iron content, which limits the use of briquettes-reducing agents in the production of technical silicon of the highest grades.

The objective of the proposed technical solution is to create a method for obtaining a reducing agent for the production of silicon, which makes it possible to replace scarce and expensive reducing agents used in the production of high-quality silicon.

The technical result when using the invention is to reduce the iron content in the reducing agent for the production of technical silicon of high quality.

The specified technical result is achieved by the fact that the method of processing low-ash brown coal with an iron content in the mineral part of coal of not more than 5 wt.% to obtain a reducing agent for the production of technical silicon includes its heat treatment in a fluidized bed at a temperature of 700850 ° C by high-speed blasting with a mixture of air and water steam to ensure the transition of iron-containing compounds in the mineral part of brown coal into a magnetic form, cooling the obtained reducing agent and its subsequent magnetic separation for 100-120 hours immediately after the cooling stage, while the magnetic field induction is at least 1.1 T.

The lower the content of the mineral part in the original coals and the concentration of iron in it, the higher the probability of obtaining a product with a low iron content. Therefore, the use of low-ash, with a low iron content (not more than 5 wt.%), inexpensive brown coal contributes to the achievement of the specified result.

In the original carbonaceous raw materials iron is contained mainly in the form of pyrite, marcasite, siderite and hematite. In light fractions, iron is represented by hydroxides in the composition of clay minerals. These mineral substances in the process of obtaining semi-coke from brown coals pass into the composition of the final material and undergo phase transformations, depending on the conditions for the occurrence of redox processes at high temperatures.

So, the pyrite (marcasite) FeS ₂ present in the original coal - iron disulfide, when heated, decomposes into iron sulfide and sulfur. Reactions that occur when iron sulfide is heated in oxygen:

4FeS + 7О ₂ = 2Fe ₂ O ₃ + 4SO ₂ (1)

3Fe ₂ O ₃ + H ₂ \u003d 2 Fe ₃ O ₄ + H ₂ O (2)

Fe ₂ O ₃ + 4H ₂ \u003d 3Fe + 4H ₂ O (3)

Siderite (spar iron ore) contains iron mainly in the form of carbonate (FeCO ₃ ). Under the influence of high temperatures, the escaping carbon dioxide makes the material porous, while it is easily recoverable.

Hematite and magnetite (Fe ₂ O ₃ , Fe ₃ O ₄ ). When hematite and magnetite are reduced when heated with carbon, iron oxide is obtained in a finely crystalline state, and easily turns into Fe. The FeO to Fe transition zone practically merges with the Fe3O4-FeO zone. It has been found that the rates of the Fe2O3-Fe3O4 and Fe3O4-FeO reactions at temperatures of 700-750°C are close.

Iron oxide reduction process:

3Fe ₂ O ₃ + CO \u003d 2 Fe ₃ O ₄ + CO ₂ (4)

Fe ₃ O ₄ + CO \u003d 3FeO + CO ₂ (5)

FeO + CO \u003d Fe + CO ₂ (6)

FeO + C \u003d Fe + CO (7)

The process of iron reduction from oxides proceeds stepwise by sequential transition from higher oxides to lower ones according to the scheme Fe ₂ O ₃ - Fe ₃ O ₄ - FeO - Fe (above 570 ° C) or Fe ₂ O ₃ - Fe ₃ O ₄ - Fe (below 570°C). During the low-temperature reduction of hematite (<800°C), magnetite, as an intermediate oxide stage, has a composition that differs markedly from the stoichiometric one. An analysis of the reduction products showed that there is a whole set of magnetites with different stoichiometry. The appearance of nonstoichiometric magnetite at low temperatures and its absence at high temperatures of hematite reduction confirms the existence of a limiting effect of solid-phase processes in reduction products.

The recovery rate depends on the nature of the oxide, the nature and properties of the reducing agent, which, for example, is brown coal and its derivatives (charcoal). So, when making good contact

- 2 041223 oxides with char carbon, an avalanche-like increase in the recovery rate is observed. Thus, when using the formed semi-coke in the process of brown coal carbonization, the efficiency of the process, as in any solid-state reaction, will depend on the contact between the semi-coke and the reduced oxide, which is intergrown with carbon. The reactivity of brown coal semi-coke and the content of volatile components in it also affect the recovery process. Any brown coal can be used as a carbon-containing raw material for the production of a reducing agent. However, the most promising is the brown coal of the Kansk-Achinsk basin due to significant reserves (forecast resources - 309.9 billion tons or 23.4% of the coal reserves of Russia), low ash content (3.4-22.3%) and low iron content.

It is known that materials containing iron impurities in the form of Fe ₃ O ₄ magnetite, which has strong magnetic properties, are best subjected to magnetic enrichment. In the proposed technical solution, the reduction reactions of iron oxides to magnetite (and / or any other magnetic form of processed carbon-containing raw materials) proceed when carbon-containing raw materials, for example, brown coal, are heated in a fluidized bed at temperatures of 700-850 ° C, forming a reducing agent (semi-coke), which has in total the following properties (composition, wt.%):

ash ( ^Ad ) 5.0 - 8.0%; content of Fe ₂ 0z in ash 0.8-1.2%; volatiles ( ^Vdaf ) 13.0-15.5%; fixed carbon (C) 75.0 - 80.0%; reactivity not less than 8.1 cm ³ /(ghs); electrical resistivity not less than 1.37χ 103 Ohmcm:

specific area 120-500 m ² /g.

The method is carried out as follows. Carbon-containing raw materials (lignite) with a particle size of up to 10 mm are fed by a screw onto a gas distribution grate. Coal is subjected to heat treatment (semi-coking) in a fluidized bed at a temperature of 700-850°C. Fluidization of coal is carried out by a mixture of air and water vapor, which are fed under the gas distribution grate. The supply of water vapor under the grate prevents its slagging in case of violations of the temperature regime of the process.

The particles of the initial coal entering the lower part of the fluidized bed are subjected to high-speed heating, during which water vapor, tar and decomposition gases are released from them. Moreover, each particle of coal is surrounded by a cloud of protective atmosphere, which prevents the access of atmospheric oxygen to its surface. The combustion of released gases and vapors is carried out at the periphery of the gas cloud. Thus, in the lower part of the fluidized bed, the surface of the coal particles is not subjected to ashing by air oxygen, which ensures the required level of the reducing carbon content (good yield).

In the process of high-speed heating, intensive grinding of coal particles occurs as a result of their thermal destruction and destruction during collisions. This ensures good contact of iron oxides with carbon, which leads to high rates of their reduction and the transfer of iron compounds in carbon-containing raw materials into magnetic forms.

The grinding of particles is accompanied by a decrease in their apparent density due to the release of moisture and resin vapor from the pores of the particles. Therefore, the crushed and lightened particles of the resulting semi-coke move to the upper part of the fluidized bed, where semi-coking continues, during which decomposition gases and tar vapors are released from them, but not as rapidly as in the lower part of the fluidized bed.

If in the lower part of the fluidized bed the coal particles are in a vapor-air medium containing a significant amount of oxygen, then in the upper part the gaseous medium consists of combustion products with an admixture of semi-coking gas. The oxygen content in this medium is low and, therefore, the surface of the particles of the formed semi-coke does not become ashed.

In the upper part of the fluidized bed, under the action of water vapor, the char is activated, during which the char surface increases and it becomes more active in chemical interaction reactions.

Together with coal, rock and coal intergrowths enter the lower part of the fluidized bed, which, due to their increased density, are removed from the fluidized bed to the gas distribution grate and periodically unloaded from it.

The low-calorie gas leaving the fluidized bed enters the separation space of the apparatus. At the same time, large semi-coke particles carried away from the bed fall back into the fluidized bed, and the gas leaves the apparatus and is cleaned of semi-coke dust in a cyclone. Semi-coke dust is added to the semi-coke unloaded from the apparatus, and the resulting low-calorie gas is burned in the waste heat boiler furnace.

A further magnetic separation process in the separator reduces the iron content to an acceptable level, which ensures a high quality of the reducing agent.

Example 1. To obtain a reducing agent, brown coal of the Berezovsky deposit was used,

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Claims (1)

fed into the fluidized bed boiler. The material was processed at various temperature intervals: 550-700, 700-850 and above 850°C. At temperatures of 550-700°C, a reducing agent (charcoke) was obtained with an insufficient degree of pyrolysis, a high content of volatiles, and a low magnetic susceptibility. At temperatures above 850°C, the yield of the product dropped. Therefore, the temperature range from 700-850°C should be considered optimal. Experiments have shown that during long-term storage of pyrolyzed brown coal semi-coke, the efficiency of the subsequent separation of the iron-containing magnetic fraction from brown coal semi-coke decreased. Example 2. Immediately after pyrolysis, the maximum decrease in the iron content in the carbon material (up to 0.11 wt.%) was noted after it passed through the magnetic separator. After 5 days, the efficiency of iron separation decreased to 0.15 wt.%. When separating a pyrolyzed product with a shelf life of less than 100 hours, the iron separation efficiency was less than 0.11 wt.%. The efficiency of iron extraction depended on the magnitude of the magnetic field induction. There is an optimal level of magnetic induction, with a decrease in which the efficiency of separation of magnetic fractions decreases. An excessively high level of magnetic field induction is not economically feasible, as the cost of equipment increases. At a low value of magnetic induction, the iron content in the magnetized reducing agent increases. Example 3. The resulting reducing agent was subjected to separation in a magnetic field. With a magnetic induction value above 1.1 T, the iron content in the magnetized char was 0.11 wt.%. At a lower value of magnetic induction (0.8 T), the iron content was 0.2 wt.%. CLAIM A method for processing low-ash brown coal with an iron content in the mineral part of coal of not more than 5 wt.% to obtain a reducing agent for the production of technical silicon, including its heat treatment in a fluidized bed at a temperature of 700-850 ° C by high-speed blasting with a mixture of air and water vapor to ensure the transition iron-containing compounds in the mineral part of brown coal into a magnetic form, cooling the resulting reducing agent and its subsequent magnetic separation for 100-120 hours immediately after the cooling stage, with a magnetic field induction value of at least 1.1 T. Eurasian Patent Organization, EAPO Russia, 109012, Moscow, Maly Cherkassky per., 2