RU2383493C1 - Method of carbo-thermal reduction of silicon - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method of silicon reduction out of its oxides consists in preliminary preparation of briquettes containing silica and carbon containing reducer, in supplying charge into ore-thermal furnace, where charge contains briquettes and not briquetted silica, and in successive two-stage charge heating. Contents of silicon dioxide and carbon in briquettes are maintained in stoichiometric ratio in terms of reaction of production of silicon carbide, while contents of not briquetted silica in charge to silicon in a briquette are maintained at ratio 1:2. As a carbon containing reducer there are used reducers with contents of hydrogen not less, than 5% of total mass of the reducer.

EFFECT: increased degree of silicon extraction at reduced consumption of raw materials and power.

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Description

The present invention relates to the field of non-ferrous metallurgy, in particular to a technology for the production of melting silicon in ore-thermal furnaces.

Currently, technical silicon is obtained by carbothermic reduction from silica. Ore-thermal smelting, as a rule, is carried out continuously on the same charge with a constant top level and content blast furnace at an unchanged electric mode, characterized by a constant voltage and current strength, with periodic releases of the melt from the furnace that are repeated at regular intervals.

Various patent methods for conducting electroreduction smelting are known from patent information.

So, the known "Method for the production of silicon" by and.with. No. 420241 (C01B 33/02, priority 06.07.71 g), according to which silicon carbide is introduced into the charge in order to increase the efficiency of the process (to increase silicon recovery, reduce energy consumption, reduce the electrical conductivity of the charge) with a silicon carbide to carbon ratio within 0.6 ÷ 2.9. The disadvantage of this method is a significant loss of silicon in the form of volatile compounds, while the degree of extraction is 76%.

Also known is the "Method of producing silicon by electroreduction melting" by A. with. No. 595942 (C01B 33/02, priority 11.26.76), in which, in order to increase the extraction of silicon, melting is carried out with the introduction of silicon carbide briquetted with silicon dioxide and carbon into the main charge at a ratio of (carbide silicon: silicon dioxide: carbon) = (4-24) :( 3-30): 1. The essence of this method lies in the fact that with the introduction of silicon carbide in the form of a briquette containing silicon dioxide and carbon, silicon monoxide formed during the process by reaction

2SiO ₂ + SiC = 3SiO _gas + CO,

maximally captured in the same briquette by reaction

SiO _gas + 2C = SiC + CO.

The process according to the reactions proceeds mainly locally in the briquette with the claimed ratio of components. The recovery reached 82%.

Known methods of silicon reduction, according to which the process of reducing silicon from silicon dioxide was divided into two stages: obtaining silicon carbide from silica, and then silicon from its carbide. The well-known "Method for producing silicon" by as No. 1579014 (C01B 33/02, priority July 27, 1996). At the first stage, quartzite and a reducing agent are loaded into the furnace and the silica is carbidized. After cooling, silicon carbide is unloaded, crushed and re-fed into a furnace with silica preheated to 1900 ° C. The recovery reached 90%. The method is quite complicated and expensive, so I have not found industrial application.

Also known is the "Method of producing silicon in a low shaft furnace" according to the USSR patent No. 1494861 (patent holder - International Mineral and Chemical Corporation (US), СВВ 33/02, priority November 26, 83). The method involves obtaining silicon by loading briquettes from quartz and carbon, as well as granular (un briquetted) raw quartz, into the furnace. The method is carried out with two-stage heating of the mixture. In the first stage, in the upper part of the furnace, the silicon dioxide of the briquettes is subjected to interaction with an excess amount of carbon relative to the reaction of silicon carbide formation, the first stage being carried out at 1500 ÷ 1600 ° C with an excess of carbon in the briquettes equal to 50 ÷ 90 wt.%, And in the second stage formed in the first stage, silicon carbide is subjected to interaction with unbriquetted quartz at 1800 ÷ 2000 ° C.

By technical nature, the presence of similar essential features of the USSR patent No. 1494861 is selected as the closest analogue (prototype).

According to the method of the prototype at a temperature of 1500 ÷ 1600 ° C, the reaction SiO ₂ + 3C = SiC + 2CO takes place in the briquette, which takes place in the upper part of the furnace with an excess carbon content. As a result of the formation of silicon carbide, the initial briquettes acquire a sponge-like structure, due to which the briquettes actively absorb ascending from the lower part of the furnace, where the mixture is heated to 1800 ÷ 1900 ° С, gaseous silicon monoxide, so that it can react with excess carbon. At this temperature, in the lower part of the furnace, as the authors indicate, there are reactions of carbon with silicon monoxide and dioxide, as well as the reaction of silicon carbide with silicon dioxide.

Consider the modern concept of the mechanism of the carbothermal process for producing silicon. Three main reaction zones can be distinguished in the crucible of an ore-thermal furnace: low (up to 1500 ° С), medium (up to 1900 ° С) and high (more than 1900 ° С) temperatures. In the first and second zones of the furnace, the formation of silicon carbide occurs, and in the second zone, the carbon of the reducing agent must be completely spent on the formation of silicon carbide. This is due to the fact that unreacted carbon in the sintered layer of the mixture (medium temperature zone) when it enters the gas cavity (high temperature zone) causes an increased consumption of thermal energy and reduces the concentration of silicon monoxide in the gas phase. As a result, silicon reduction conditions are worsened and furnace performance is reduced.

In the third zone, under the influence of an electric arc, silicon monoxide is formed and silicon is restored by the total reaction:

SiO ₂ + 2SiC = 3Si + 2CO.

Particular attention should be paid to the role of hydrogen in solid-state carbon reduction processes. When the starting material to be recovered is in a solid-state state of aggregation, chemical processes are carried out with the participation of hydrogen acting as a transporting agent. The role of solid carbonaceous reducing agents in these processes is reduced to the interaction with water and to the removal of the latter from the reaction zone in the form of carbon monoxide and hydrogen. From the aforesaid, it follows that both hydrogen and solid carbon reducing agents take part in the so-called solid-phase reduction of oxides, and therefore it is impossible to “solid-phase” reducing oxides with high efficiency by either a solid carbon reducing agent or only gaseous - hydrogen.

From the position of the modern concept in the method according to the USSR patent No. 1494861 there are a number of significant disadvantages:

- due to a significant excess of carbon in the briquettes, unreacted carbon in the sintered layer inevitably enters the high-temperature zone of the furnace, which, as indicated above, reduces the productivity of the furnace;

- due to the content in briquettes of a significant amount of excess reducing agent, their strength is reduced;

- with an excessive content of reducing agent increases the ablation of raw materials due to the increased formation of gaseous products;

- no attention was paid to the distribution of silica between the briquettes and the non-briquetted part of the mixture, which leads to poor control and management of the chemistry of the process and does not provide the conditions for the predominant occurrence of target reactions: in low and medium temperature zones - silicon carbidization, in the high temperature zone - silicon reduction. This is confirmed by the statement of the authors of the prototype method that in the lower part of the furnace there are reactions of carbon with silicon monoxide and dioxide, as well as the reaction of silicon carbide with silicon dioxide;

- not paid attention to the quality of the reducing agent in terms of hydrogen content, which negatively affects the efficiency of the initial stage of recovery.

The task of the invention is to provide a method for carbothermal reduction of silicon with a high degree of its extraction with minimal consumption of raw materials and electricity.

The technical result of the implementation of the proposed method is the following:

- the possibility of dosing charge materials in an optimal ratio, corresponding to the necessary and sufficient number of charge components, ensuring a phased flow of target chemical processes, which can significantly increase the extraction of silicon;

- significantly reduces the consumption of carbon-containing reducing agent in terms of solid carbon;

- the proposed method eliminates the use of expensive charcoal;

- reduced specific energy consumption.

The technical result is achieved by the fact that in the method of carbothermal reduction of silicon from its oxides, which includes the preliminary preparation of briquettes containing silica and a carbon-containing reducing agent, feeding a mixture containing briquettes and non-briquetted silica into the ore-thermal furnace, sequential two-stage heating of the mixture, and, in the first stage, silicon dioxide into briquettes are subjected to interaction with carbon to form silicon carbide, and in the second stage, non-briquetted silica is subjected to interaction with the formed silicon carbide to obtain silicon, in the briquettes, the content of silicon dioxide and carbon is maintained in a stoichiometric ratio in terms of the reaction for producing silicon carbide, and the content of unbricked silica in the charge to silica in the briquette is maintained in a ratio of 1: 2, while as a carbon-containing reducing agents use reducing agents with a hydrogen content of at least 5% of the total mass of the reducing agent.

The technical essence of the proposed solution is as follows.

1. In contrast to the prototype in the proposed solution, the initial components of the briquettes (silicon dioxide and carbon) are blended in a stoichiometric ratio in terms of the reaction SiO ₂ + 3C = SiC + 2CO, which takes place at a temperature of 1300 ÷ 1600 ° C. Currently, the mechanism of silicon carbide formation is generally recognized, according to which the latter is obtained as a result of the interaction of a gaseous siliconizing agent (SiO ₂ , SiO, Si) with a solid carbon material. Initially, cryptocrystalline silicon carbide occurs around carbon grains and along cracks in them. Then, as a result of diffusion of the siliconizing agent, the process of degeneration of the carbon material goes deeper and only relics remain from the latter. As a result, the carbonaceous material is completely replaced by silicon carbide.

Thus, in each individual briquette, the process of formation of silicon carbide takes place. Moreover, the stoichiometric ratio of silicon dioxide and carbon in the briquette provides, as noted above, the complete substitution of the carbon reducing agent for silicon carbide and the briquette acquires a sponge-like structure, which subsequently absorbs gaseous silicon monoxide and dioxide with their reduction to silicon at a temperature of 1800 ÷ 1900 ° С .

The use of the formed silicon carbide for the reduction of the non-briquetted part of silica can significantly reduce the consumption of carbon reducing agent (in terms of solid carbon) in the production of silicon.

2. According to the proposed method, the content of non-briquetted silica in the charge is one third of the total mass of silica, which corresponds to the stoichiometry of the reaction of reduction of silicon by silicon carbide from its dioxide. It is known that in the process of industrial production of silicon there are significant losses of both a reducing agent and silicon in the gas phase. According to the proposed method, the process of carbidization of silicon dioxide in the briquette takes place at a low temperature, about 1300 ° C, which corresponds to the initial phase of the formation of silicon monoxide. Thus, losses of silicon into the gas phase in the form of monoxide can be practically neglected; therefore, when determining the content of unbriquetted silica in the charge, it was assumed that silicon losses during the low-temperature carbidization process are insignificant.

Experimental tests have proved that a guaranteed stable process of silicon reduction is provided when unbriquetted silica is fed into the furnace in an amount equal to one third of the total amount of silica. With a decrease or increase in the content of non-briquetted silica in the charge, relative to 1/3 of the total amount, lower extraction of silicon was observed.

3. At present, it has been proved that the efficiency of the process of silicon carbide formation is determined by the presence of a sufficient amount of gaseous hydrogen-containing substances in the reaction zone, since hydrogen is a transporting agent between solids. In the presence of a transporting gas phase (hydrogen), the interaction of solids occurs at significantly lower temperatures and can be significantly accelerated.

In the initial (low-temperature) period of the solid-phase carbon reduction process, hydrogen in the charge is usually present directly in the carbon material and in the form of moisture in the starting components. But the process that began in connection with the loss of charge materials of hydrogen and hydrogen-containing volatile substances during heating dies over time. Theoretically, to ensure the further efficient course of the process (acceptable speeds and sufficient completeness at low temperatures), it is necessary to continuously introduce additional amounts of hydrogen into the reaction zone, which, as a rule, is not technologically advanced under industrial conditions.

It is known that in closed equilibrium chemical systems cyclic hydrogen regeneration occurs in the reaction zone, which ensures the completeness of the process without introducing additional volumes of hydrogen. The method proposed by the authors includes briquetting components, and each briquette can be considered as a closed reaction zone with the content of reacting components in an amount necessary and sufficient to ensure the completeness of the carbidization process. Moreover, due to more efficient conservation of the hydrogen-containing gas phase within the briquette, the carbidization process in it occurs at a higher rate and at lower temperatures.

In the course of the experiments, it was determined that to ensure the beginning of the solid-phase carbon reduction process, it is necessary to use reducing agents with a hydrogen content of at least 5% of the total reducing agent mass as a carbon-containing reducing agent. With a decrease in this indicator, the process of silicon reduction is carried out with less completeness with an increase in the expenditure indices of the process both in electric power and in electrode materials.

The closest analogue and the proposed technical solution are characterized by the following common features:

- both methods are carbothermic reduction of silicon from its oxides;

- in both methods, part of the silica is briquetted together with a carbon reducing agent, and part is loaded into the furnace in an un briquetted form;

- briquettes and non-briquetted silica are loaded into the furnace at the same time;

- the reduction process is carried out in such a way that the mixture successively passes through two stages of heating, each of which ensures the occurrence of the target reaction: at the first stage, the process of carbidization of briquettes, at the second, the process of producing silicon.

Presented by the authors of the claims claimed the following distinctive features from the prototype signs:

- in the briquettes, the content of silicon dioxide and carbon is maintained in a stoichiometric ratio in terms of the reaction for producing silicon carbide. In the prototype, the briquettes contain an excess of carbon in the amount of 50-90 wt.% Relative to the carbide formation reaction;

- the content of non-briquetted silica in the mixture to silica in the briquette is maintained in a ratio of 1: 2. In the prototype method, the distribution of silica between the briquetted and un briquetted parts of the mixture is not regulated. As a result, not only targeted reactions occur at both stages of the process;

- as a carbon-containing reducing agent use reducing agents with a hydrogen content of at least 5% of the total mass of the reducing agent. In the prototype, the quality of the reducing agent in terms of the content of the transporting agent is hydrogen, there are no requirements.

The presence in the proposed technical solution of the above signs that are different from the signs of the closest analogue allows us to conclude that it meets the condition of patentability “novelty”.

In order to determine the “prior art”, a search was conducted on patent and scientific and technical literature. In the process of analysis carried out on the selected array, partially given in the description section - “analogues”, it was revealed that fundamentally separate signs of the object of protection are known:

- carbothermal reduction of silicon dioxide from its oxides with a detailed description of the process mechanism in terms of the significance of the transporting hydrogen-containing gas phase is described in the scientific works of V.V. Digonsky, S.V. Digonsky, A.V. Digonsky, etc. VO Nauka, Novosibirsk, 1993), Unknown Hydrogen (St. Petersburg, Nauka, 2006); Chernyakhovsky L.V. "The role of hydrogen in carbothermal reduction of metals" (J. "Non-ferrous metals", 2001, No. 4); Popova S.I. “Silicon metallurgy in three-phase furnaces” (Irkutsk, 2004);

- briquetting of carbothermal reduction components of silicon is known, and the composition of briquettes is very diverse, namely:

- briquettes contain carbon reducing agent and silicon dioxide: USSR patent No. 1091849 "Method for producing silicon-carbon-containing raw materials"; A.S. USSR No. 1666443 "Method for the preparation of a mixture for smelting silicon";

- briquettes contain carbon reducing agent, silicon dioxide and silicon carbide: and.with. USSR No. 595942 "A method for producing silicon by electroreduction melting" (11.26.76);

- briquettes contain a carbon reducing agent: EPO patent No. 1333229 "Method for the production of silicon"; RF patent No. 2032616 "Method for preparing a carbonaceous reducing agent for smelting silicon";

- briquettes contain carbon reducing agent, silicon dioxide, silicon and liquid glass: patent RU No. 2042721 "Method for the preparation of the mixture for smelting silicon"; RF patent №2103386 "Method for producing agglomerated charge";

- briquettes contain carbon reducing agent, silicon dioxide, silicon and sodium alkali: patent RU No. 2228376 "Blended charge for smelting silicon", RF patent No. 2049057 "Blending the charge for smelting silicon";

- briquettes contain carbon reducing agent, silicon dioxide and a concentrate of sulphite-alcohol stillage as a binder: and.with. USSR No. 1656881 "Method for the preparation of charge materials for smelting silicon."

- briquetting a mixture based on a silica-containing material and a carbon reducing agent in a stoichiometric ratio: RU patent No. 2042721 "Method for preparing a mixture for silicon smelting";

- the content in the reducing agent of volatile hydrocarbon-containing substances was limited in the patent of the Russian Federation No. 2228376 "Blended charge for smelting silicon";

- the high-temperature interaction of silicon carbide with silica in a stoichiometric ratio is known from RF patent No. 2026814 "Method for producing high-purity silicon";

- use as a reducing agent of hydrolytic lignin with a high content of hydrogen and hydrogen-containing volatile substances: USSR No. 1512110 "A method of producing a carbonaceous reducing agent for the production of silicon."

During the search and comparative analysis, no technical solutions were identified that are characterized by identical or equivalent features to the proposed solution, namely:

- the content of non-briquetted silica in the charge to silica in the briquette is maintained in a ratio of 1: 2;

- as a carbon-containing reducing agent use reducing agents with a hydrogen content of at least 5% of the total mass of the reducing agent.

Given the foregoing and taking into account the fact that the use of a combination of known and unknown features characterizing the proposed technical solution in the amount presented in the claims allows us to reach a new level of carbothermal reduction of silicon with a high degree of its extraction with minimal consumption of raw materials and electricity, it is possible to conclude that the proposed solution meets the patentability condition "inventive step".

Compliance with the patentability condition "industrial applicability" is proved by the fact that this method is carried out on traditional ore-thermal furnaces using well-known raw materials, while the technological parameters of the recovery process practically correspond to the traditional ones.

To select the optimal parameters of the recovery process, the authors conducted a wide range of pilot tests on a pilot furnace with a capacity of 300 kW with one electrode. In the process of testing, silica with a silicon content of about 99% was used. Various materials were used as the carbon-containing reducing agent, which allowed us to conclude that, in principle, with the qualified selection and calculation of the charge, any carbon material can be used.

The mixture in all the examples below contains 350 kg of silica, of which 235 kg is in briquettes. In briquettes, also 150 kg of reducing agent in terms of solid (non-volatile) carbon. For comparison: in the examples of the prototype, a charge is given in which the mass ratio of the solid carbon of the reducing agent to silicon dioxide is 0.48 kg / kg, and in the examples according to the claimed method it is 0.4285 kg / kg.

In the examples below, the difference between the charge is only in the nature of the reducing agents and, accordingly, in their quantity. Moreover, the batch of the reducing agent was carried out taking into account the content of total hydrogen in the reducing agent.

Example 1

The composition of the mixture: silica (as described above); petroleum coke with a solid carbon content of 90%, in an amount of 17 kg; coal with a solid carbon content of 60%, in the amount of 187.5 kg; wood chips with a solid carbon content of 10%, in an amount of 225 kg.

The components of the reducing agent are thoroughly mixed and briquetted with 235 kg of silica. 115 kg of non-briquetted silica are added to the briquettes and sent to an electric furnace.

The yield of silicon was 93.5%; specific energy consumption - 10400 kWh / t silicon; reducing agent consumption - 1.31 kg / kg of silicon.

Example 2

The composition of the mixture: silica (as described above); petroleum coke with a solid carbon content of 90%, in an amount of 33 kg; brown coal with a solid carbon content of 40%, in the amount of 187.5 kg; sawdust with a solid carbon content of 10%, in an amount of 450 kg.

The components of the reducing agent are thoroughly mixed and briquetted with 235 kg of silica. 115 kg of non-briquetted silica are added to the briquettes and sent to an electric furnace.

The yield of silicon was 94.2%; specific energy consumption - 10350 kWh / t silicon; reducing agent consumption - 2 kg / kg of silicon.

Example 3

The composition of the mixture: silica (as described above); shale coke with a solid carbon content of 70%, in an amount of 17 kg; coal with a solid carbon content of 60%, in the amount of 187.5 kg; lignin with a solid carbon content of 20%, in an amount of 127.5 kg.

The components of the reducing agent are thoroughly mixed and briquetted with 235 kg of silica. 115 kg of non-briquetted silica are added to the briquettes and sent to an electric furnace.

The silicon yield was 92.1%; specific energy consumption - 10500 kW · h / t silicon; reducing agent consumption - 1.025 kg / kg of silicon.

Example 4

The composition of the mixture: silica (as described above); petroleum coke with a solid carbon content of 90%, in an amount of 25 kg; gas coal with a solid carbon content of 65%, in an amount of 161.5 kg; rice husk with a solid carbon content of 20%, in an amount of 112.5 kg.

The components of the reducing agent are thoroughly mixed and briquetted with 235 kg of silica. 115 kg of non-briquetted silica are added to the briquettes and sent to an electric furnace.

The yield of silicon was 91.8%; specific energy consumption - 10540 kW · h / t silicon; reducing agent consumption - 0.93 kg / kg of silicon.

Example 5

The composition of the mixture: silica (as described above); petroleum coke with a solid carbon content of 90%, in an amount of 33 kg; lamp black with a solid carbon content of 70%, in an amount of 171 kg.

The components of the reducing agent are thoroughly mixed and briquetted with 235 kg of silica. 115 kg of non-briquetted silica are added to the briquettes and sent to an electric furnace.

The silicon yield was 95.2%; specific energy consumption - 10300 kWh / t of silicon; reducing agent consumption - 0.61 kg / kg of silicon.

The above examples of the implementation of the proposed method showed that the consumption of reducing agent (in terms of solid carbon) is reduced, and the specific energy consumption is also reduced (see table 1).

Table 1 Technological indicator According to the prototype In the claimed decision The consumption of carbon-containing reducing agent in terms of solid carbon, kg C / kg SiO ₂ 0.48 0.4285 Specific energy consumption, kW · h / t silicon 10485 ÷ 10700 10300 ÷ 10540

The above examples of the implementation of the claimed method show the fundamental possibility of using any carbon-containing reducing agent while providing a qualified calculation of the composition of the charge.

In the above examples, the reduction part of the charge was analyzed for the total hydrogen content. These indicators are summarized in table 2.

table 2 The serial number of the pilot test example The total hydrogen content in the total mass of the reducing agent,% The yield of silicon,% Example 4 3,7 91.8 Example 3 3.9 92.1 Example 1 5.1 93.5 Example 2 6.09 94.2 Example 5 8.5 95.2

The above results show a direct dependence of the results of the silicon reduction process on the hydrogen content in the reducing agent.

The hydrogen content in the reducing agent was adjusted by selecting the components of the reducing agent.

This method can be recommended for widespread implementation on an industrial scale as a highly efficient method that does not require capital costs for the modernization of existing silicon production workshops.

Claims (1)

A method for carbothermal reduction of silicon from its oxides, including the preliminary preparation of briquettes containing silica and a carbon-containing reducing agent, feeding a mixture containing briquettes and non-briquetted silica to the ore-thermal furnace, sequential two-stage heating of the mixture, and in the first stage, silicon dioxide in the briquettes is reacted with carbon to form silicon carbide, and in the second stage, non-briquetted silica is subjected to interaction with the formed silicon carbide to obtain silicon, characterized in that in the briquettes, the content of silicon dioxide and carbon is maintained in a stoichiometric ratio in terms of the reaction for producing silicon carbide, and the content of un briquetted silica in the charge to silica in the briquette is maintained in a ratio of 1: 2, while as a carbon-containing reducing agent use reducing agents with a hydrogen content of at least 5% of the total mass of the reducing agent.