WO2003042100A1

WO2003042100A1 - Method and reactor for production of silicon

Info

Publication number: WO2003042100A1
Application number: PCT/NO2002/000409
Authority: WO
Inventors: Jan Arthur Aune; Jon Christian Brinch
Original assignee: Elkem Asa
Priority date: 2001-11-16
Filing date: 2002-11-08
Publication date: 2003-05-22
Also published as: NO20015596L; NO20015596D0; NO318956B1

Abstract

The present invention relates to a reactor for carbothermic production of silicon having (a) a vessel (1) defining a high temperature reduction zone said vessel (1) having an outlet (5) for molten silicon at its lower part, (b) means (10, 101, 11, 111) and (12, 121) for supplying energy to the high temperature reduction zone, (c) a gas/solid reaction shaft (16) for a carbon and SiO¿2? raw material mixture, the shaft (16) being attached to the top of the vessel (1), said shaft (16) having openings in its bottom communicating with the high temperature reduction zone to allow gas from the high temperature reduction zone to enter into the shaft (16). The reactor further comprising means (19) for continuous or intermittent controlled feeding of the carbon and SiO2 raw material mixture to the top of the shaft (16), and means (23, 25) for controlled continuous or substantially continuous controlled supply of reaction products from the bottom of the shaft (16) into the high temperature reduction zone, and where the means for supplying energy to the high temperature reaction zone comprises at least one pair of substantially horizontal electrodes (10, 10?1 11, 111¿) and (12, 121) arranged about the circumference of the high temperature reduction vessel (1) at a level above the outlet (5) for molten silicon. The invention further relates to a method for carbothermic production of silicon by reduction of silicon dioxide comprising : continuous or intermittent controlled supplying of a mixture of a carbon and silicon dioxide raw material mixture to the shaft, where the carbon in the shaft reacts to SiC with contact with SiO-gas flowing from the high temperature reduction zone and upwardly in the shaft; continuous or substantial continuous controlled supplying of gas/solid reaction products from the bottom of the shaft and into the high temperature reduction zone; passing the gas/solid reaction products supplied from the shaft through the electric arc between the at least one pair of substantially horizontal electrodes to produce molten silicon which is collected on the bottom of the high temperature reduction vessel and SiO- and CO gas which flows upwardly through the shaft.

Description

Title of Invention

Method and reactor for production of silicon.

Field of Invention The present invention relates to a method and a reactor for the production of silicon by carbothermic reduction of silicon dioxide.

Background Art

Conventionally, silicon is produced in a submerged electric arc furnace by carbothermic reduction of silicon dioxide (SiO₂). The carbonaceous reduction material has typically been charcoal, coal, coke, wood chips and the like. The overall reduction reaction can be represented by the equation

SiO₂ + 2C = Si + 2CO

It is generally excepted that the reaction involves multiple reactions, the most significant being:

SiO₂ + 3C = SiC + 2CO (1 )

SiO₂ + C = SiO + CO (2)

SiO + 2C = SiC + CO (3)

2Si0₂ + SiC = 3SiO + CO (4), and

SiO + SiC = 2Si + CO (5)

Reaction 1 is endothermic and is estimated to consume as much as 50 % of the energy for the overall reduction reaction.

The practice of feeding SiO₂ and a solid carbonaceous reduction material to a low shaft submerged electric arc furnace has many disadvantages. Thus, controlled mass transfer is difficult when handling the complex reaction system of solid, molten and gaseous reactants, intermediates and products. The conventional smelting reactor design, low shaft submerged arc electric furnaces, do not allow the necessary control of the system to optimize the high temperature reduction zone, the gas/solid reactions and heat transfer in the reactor. This results in the loss of material in the form of gaseous silicon monoxide (SiO) together with the CO-gas produced. Only a part of the SiO- gas produced according to reaction (2) and (4) is consumed in reaction (3) and (5). A further part of SiO-gas dissociates in the upper part of the furnace charge to Si0₂ and Si, while the remaining part of SiO escapes from the furnace and reoxidizes to SiO₂ in the atmosphere above the furnace charge and is lost from the process. The reoxidized SiO₂ may account for 10 to 20 % by weight of the SiO₂ charged to the furnace. The reoxidized SiO₂ particles are very fine and follow the furnace off-gas and has to be recovered in bag- house filters.

In EP-A-0357395, there is proposed a cyclic two-step batch process in which SiO₂ and SiC are reacted to form molten silicon, SiO and CO, and where SiO is being contacted with a bed of carbon to regenerate SiC. In the process disclosed in EP-A 0357395 the smelting furnace comprises a closed vessel defining a reaction zone for containing solid reactants and molten silicon, which vessel has an energy source being fit into the furnace body. A shaft suitable for containing solid particles of carbon and for passing gases from the furnace through the shaft, is attached to the top of the furnace. The shaft has a bottom plate with openings for passing of gases from the furnace through the shaft.

The process of EP-A-0357395 is carried out batchwise. A feed mixture of silicon carbide and silicon dioxide is placed in the furnace, and the shaft is filled with carbon. Energy is then applied to the reaction zone to effect conversion of the feed mixture to molten silicon. Silicon monoxide and carbon monoxide produced during the reaction, pass into the shaft loaded with carbon particles where silicon monoxide reacts with carbon to form silicon carbide and where carbon monoxide passes through the shaft. The silicon produced is tapped from the furnace. The energy supply is then stopped and the silicon carbide produced in the shaft is charged to the furnace together with further silicon dioxide and the shaft is again filled with carbon particles, whereafter energy again is applied to the reaction zone and the above steps are repeated. The process of EP-A-0357395 has the advantage that the SiO gas is used to produce silicon carbide in the shaft, thus increasing the silicon yield. However, the process disclosed in EP-A-0357395 has some main drawbacks which makes it unsuitable as a process for the commercial production of silicon. First, it is a batch process where a batch of silicon carbide and silicon dioxide is reacted to silicon in the furnace. Thus the furnace has to be stopped when the batch of raw materials has been converted to silicon. The mere need to stop the furnace results in huge losses of heat energy, making it impossible to compete with the conventional submerged arc furnace practice set out above. Further, one has to break the plate in the bottom of the shaft in order to charge the silicon carbide produced in the shaft into the furnace. This means that for each batch a new bottom plate has to be inserted in the shaft. This increases both the loss of heat energy and increases the time between the processing of each batch of raw materials in the furnace. Thus, even though the silicon monoxide is being used to convert carbon to silicon carbide in the shaft and thus increase the yield of silicon, the high energy consumption due to the batchwise operations of the furnace, renders the process of EP-A- 0357395 uneconomical compared to the conventional continuous submerged arc furnace operation. Finally the process of EP-A- 0357395 does not solve the problem of controlling the mass and heat transfer in the reaction zone since the reaction zone still is operated in a submerged electrode configuration.

Description of the Invention

According to the present invention there is provided a process and a reactor for carbothermic production of silicon, which makes it possible to continuously produce silicon with closely 100 % Si recovery and with an energy consumption close to the theoretical value. Further, the process and the reactor of the present invention make it possible to fully control the mass and heat transport in the reactor.

According to a first aspect the present invention relates to a process for the carbothermic production of silicon comprising: Producing a downward flow of dispersed particles, said particles comprising a mixture of carbonaceous reduction material and particulate SiO₂ raw material, providing energy to at least one pair of horizontally oriented electrodes; and passing said downward flow of dispersed particles past said at least one pair of electrodes.

According to a preferred embodiment the mixture includes silicon carbide.

According to a second aspect the present invention relates to a smelting reactor for carbothermic production of silicon having

(a) a vessel defining a high temperature reduction zone said vessel having an outlet for molten silicon at its lower part,

(b) means for supplying energy to the high temperature reduction zone,

(c) a gas/solid reaction shaft for a carbonaceous reduction material and SiO₂ raw material mixture, the shaft being attached to the top of the vessel, said shaft having an opening in its bottom communicating with the high temperature reduction zone to allow reaction gases from the high temperature reduction zone to enter into the shaft,

(d) said reactor further comprising means for continuous or intermittent feeding of the carbonaceous reduction material and SiO₂ raw material mixture to the top of the shaft, and means for continuous or substantially continuous controlled supply of solid reaction products from the bottom of the shaft into the high temperature reduction zone, and

(e) where the means for supplying energy to the high temperature reaction zone comprises at least one pair of substantially horizontal electrodes arranged about the circumference of the high temperature reduction vessel at a level above the outlet for molten silicon.

According to a preferred embodiment the smelting reactor is sealed and intended to be operated at a pressure above atmospheric pressure. According to another preferred embodiment the means for supplying energy to the high temperature reaction zone comprises three or more pairs of substantially horizontal electrodes arranged about the circumference of the high temperature reduction vessel. Most preferably the electrodes are arranged at an equal circumferential distance.

When more than one pair of electrodes are used, the pairs of electrodes are preferably arranged at the same vertical level in the high temperature reduction zone, but the pairs of electrodes may also be arranged at different vertical levels.

When more than one pair of electrodes are used, a regulating unit for electric current to the pairs of electrodes are arranged, which regulating unit more or less constantly shifts the electric current between the pairs of electrodes in such a way that only one pair of electrodes is supplied with electric current at the same time. The regulating unit may shift the electric current from one pair of electrode to another pair of electrode based on the frequency of the electric current supplied. Thus if the electric current supplied has a frequency of 50 Hz, the current is shifted between the pairs of electrodes 50 times a second. In this way the area in the high temperature reduction zone is heated to a more or less equal temperature over the cross-section of the high temperature reduction zone.

Alternatively, when more than one pair of electrodes are used each pair of electrode may be equipped with a separate supply of electric energy, to ensure that the electric arc strikes between the two electrodes in a pair and not to an electrode in another pair of electrodes.

The electrodes are preferably graphite electrodes, but prebaked carbon electrodes or inert electrodes can also be used.

According to another preferred embodiment at least one of the electrodes are hollow and are equipped with means for supplying solid materials through the electrodes and into the high temperature reduction zone. The means for supply of solid materials through at least one electrode is preferably an inert gas injection means, and the solid materials fed by the gas injection means are one or more of silicon carbide, SiO₂ and carbonaceous reduction material. The solid materials are supplied through the one more hollow electrode in order to adjust the overall ratio of carbon to SiO₂ in the high temperature reduction zone. Alternatively a separate opening is arranged for supplying the solid materials directly to the high temperature reduction zone.

The means for continuous or intermittent feeding of the carbon and Si0₂ raw material mix to the top of the shaft is preferably an air-tight double bell supply means equipped with means for flushing inert gas in order to prevent air from entering into the shaft, even though other conventional feeding means where the raw materials can feed without air flowing into the shaft can be used.

One of the important aspects of the present invention is the flow of gas upward from the high temperature reduction zone into the particulate solid materials in the shaft; the flow of particulate solid material downwards into the high temperature reduction zone; and the flow of any liquid formed in the shaft downward into the high temperature reduction zone.

The means for controlled feeding of the particulate solid material downward and allowing the gas to flow upward is designed to allow free flow or unrestricted flow of the gas or gases upward from the high temperature reduction zone into the particulate solid material in the shaft. This feeding means is also designed to allow for a controlled rate of flow downward of the particulate solid material from the shaft to the high temperature reduction zone and the free flow or unrestricted flow of liquid downward from the shaft into the high temperature reduction zone. The amount or volume of liquid that flows downward from the shaft to the reduction zone is small compared to the amount or volume of particles that flow downward.

The feeding means needs to be able to withstand high temperature because it is situated above the high temperature reduction zone.

One embodiment of the feeding means comprises a circular rotatable or oscillating horizontal disc having a diameter corresponding to the inner diameter of the shaft and resting upon an inwardly extending flange at the lower end of the shaft, said disc having a central opening in the form of a polygon and where the central opening of the disc is partly covered by a cone- shaped baffle being suspended in a vertical member extending above the top of the shaft. The cone-shaped baffle prevents the free flow of particulate solid material from the shaft to the high temperature reduction zone.

The cone-shaped baffle has the peak of the cone pointed upward into the shaft so as to deflect particulate solid material towards the downward sloping sides of the flange. The rotating or oscillating disc provides an inward force to the particulate solid material so that it flows downward toward the opening in the flange into the high temperature reduction zone. This arrangement also allows both the free flow of gases upward and the free flow of liquids downward.

The means for rotating or oscillating the disc preferably comprises one or more rack and pinion arrangements having drives entering the wall of the shaft through gas tight cooled seals. A syncronized drive means is arranged outside the shaft. The drive means may be electrical, hydraulic or pneumatic.

The inwardly extending flange at the lower end of the shaft is preferably sloping downwards at an angle towards the center of the shaft to allow any liquid reaction products formed in the shaft to drip off into the high temperature reduction zone.

Other types of mechanical arrangements for controlled solid particle movement can be employed, provided they can withstand the high temperatures and provide the necessary flow characteristics. For example, a rotatable, corrugated roller can be employed with a funnel-shaped flange such that the roller blocks the free flow of particulate solid material from the shaft to the high temperature reduction zone, but allows for controlled feed of particles downward, free flow of liquid downward, and free flow of gas upward. The one or more corrugated rollers are positioned horizontally to block the opening of the funnel while one or more rollers are rotated to control the feed of the solid particulate into the high temperature reduction zone. Specifically, the rollers are cooled in a conventional manner to withstand the high temperatures. As will be appreciated by those of skill in the art, any feeding device which can withstand the heat and provide the proper flow characteristics between the shaft and the high temperature reduction zone may be employed in the present invention.

According to a further embodiment the shaft has means for extracting undiluted CO gas from the top of the shaft for further processing.

According to a third aspect, the present invention relates to a method for carbothermic production of silicon by reduction of silicon dioxide in the smelting reactor of the present invention, the method comprising;

- continuously or intermittently supplying of a mixture of a carbonaceous reduction material and silicon dioxide raw material mixture to the shaft, where the carbonaceous reduction material in the shaft reacts to SiC with contact with SiO-gas flowing from the high temperature reduction zone and upwardly in the shaft,

- continuously or substantially continuously controlled supplying of solid reaction products from the bottom of the shaft and into the high temperature reduction zone,

passing the solid reaction products supplied from the shaft through the electric arc between the at least one pair of substantially horizontal electrodes to produce molten silicon in the high temperature reduction vessel which molten silicon is collected at the bottom of the high temperature reduction vessel and SiO- and CO gas which flows upwardly through the shaft,

continuously or intermittently tapping molten silicon from the high temperature reduction vessel.

The raw material mixture of carbonaceous reduction material and silicon dioxide is preferably supplied to the shaft in a molar ratio of carbon to SiO₂ of about 1.8:1 to about 2.2:1 , and more preferably at a molar ratio of about 2:1. The raw material mixture is preferably supplied to the shaft in the form of agglomerates of carbonaceous reduction material and silicon dioxide. The agglomerates are preferably pellets or briquettes and are produced in conventional way using conventional binders.

According to another preferred embodiment solid materials selected from silicon carbide, SiO₂ and carbonaceous reduction materials are supplied to the high temperature reduction zone through one or more hollow electrodes in order to adjust the overall ratio of carbon to SiO₂ in the high temperature reduction zone. Alternatively, silicon carbide, SiO₂ and carbonaceous reduction materials for adjusting the overall ratio of carbon to SiO₂ may be supplied to the high temperature reduction zone through a separate opening.

In order to further control the process a lance for sampling and chemical analysis from the high temperature reduction zone is preferably inserted into one or more hollow electrodes or through a separate opening.

The method and the reactor for carbothermic production of silicon according to the present invention makes it possible to fully control the mass and heat transport in the reactor. The raw material mixture of carbonaceous reduction material and a SiO₂ source are supplied to the shaft. In the shaft the carbonaceous reduction material will be preheated and will react with SiO-gas entering the shaft from the high temperature reduction zone to form SiC according to the reaction SiO + 2C = SiC + CO. Most of the carbon in the carbonaceous reduction material will thus be converted to SiC in the shaft. However, some of the SiO-gas may condense in the shaft to form Si0₂ and molten Si according to the reaction 2SiO = SiO₂ + Si. Thus a very small amount of liquid Si may form in the shaft.

The solid reaction products in the shaft, silicon carbide and SiO₂, are continuously or substantially continuously supplied from the bottom of the shaft to the high temperature reduction vessel at a rate corresponding to the heat energy supplied to the high temperature reaction zone through the electrodes in order to obtain a steady state operation without accumulating unreacted material in the bottom of the high temperature reduction vessel. The amount of solid reaction products supplied to the high temperature reduction zone from the shaft, is closely regulated preferably by regulating the speed of rotation of the disc arranged in the bottom part of the shaft. In this way the amount of solid materials entering the high temperature reduction zone can be adjusted according to the energy supplied to the high temperature reduction zone in order to avoid accumulation of unreacted material in the high temperature reduction zone. Further, since the reaction product in the form of particles or agglomerates of SiC and Si0₂ fall by gravity from the shaft and down through the high temperature reduction zone, each particle or agglomerate will, when it enters the high temperature reduction zone in the area of the electrodes, react according to the reaction 2SiO₂ + SiC = 3SiO + CO. Gases from this reduction process, SiO-gas and CO, will expand in all directions and create an overpressure in the high temperature reduction zone. The major part of the SiO gas will react with SiC according to the reaction SiO + SiC = 2Si + CO. The produced Si accumulates mainly as liquid Si in the bottom of the vessel and is tapped continuously or intermittently, while the remaining part of the SiO gas and the CO will flow freely upwardly and enter into the shaft where the SiO gas will react with carbon in the carbonaceous reduction material to form SiC. Further, the high heat content of the gases entering the shaft will preheat the raw material mixture in the shaft.

The SiO-gas will be consumed in the shaft, whereby a near 100 % yield of Si is obtained. The relatively pure CO-gas is extracted at the top of the shaft and can either be recovered as liquid CO, be used as a process gas for chemical purposes or can be burned to produce heat.

Short description of the drawings

Figure 1 shows a vertical cut through a smelting reactor according to the present invention,

Figure 2 shows a horizontal view taken along line A - A of figure 1 , and where,

Figure 3 shows a horizontal view taken along line B - B of figure 1. Figure 4 shows a part of the high temperature vessel of Figure 1 having pairs of electrodes at two vertical levels.

Detailed description of the invention

On figure 1 to 3 there are shown an embodiment of a smelting reactor according to the present invention.

The smelting reactor comprises a vessel 1 defining a high temperature reduction zone. The vessel 1 consists of a lower part 2 comprising an outer steel shell 3 having a refractory lining 4 at its sidewalls and bottom. An outlet 5 for produced silicon is arranged in the sidewall of the lower part 2 of the vessel 1. An upper part 6 of the vessel 1 comprises cooled panels 7. The panels 7 are preferably cooled by circulating an oil through internal channels (not shown) in the panels 7, but the panels 7 may be cooled in any conventional way, such as by evaporation cooling or the like. The panels 7 are preferably made from copper, but other metals or alloys can be used as well.

At its upper end, the upper part 6 has an outwardly extending horizontal flange 8 defining a substantial circular opening 9 in the center of the top of the upper part 6.

In the lower end of the upper part 6 there are arranged three pairs of electrodes 10, 10¹, 11 , 11¹ and 12, 12¹. Even though three pairs of electrodes are shown in the embodiment in the figures, the smelting reactor according to the present invention has at least one pair of electrodes, but may also have two pairs of electrodes or more than three pairs of electrodes, such as four, five or six pairs of electrodes.

In the embodiment shown in figure 3 the electrodes are arranged at an equal circumferential distance, but it is within the scope of the invention to arrange the electrodes at different circumferential distances.

The electrodes 10, 10¹, 11 , 11¹ and 12, 12¹ are arranged about the circumference of the vessel 1 and are substantially horizontal. The electrodes

10, 10¹, 11 , 11¹ and 12, 12¹ are preferably made from graphite, but prebaked carbon electrodes or inert cooled electrodes may also be used. The electrodes 10, 10¹, 11 , 11¹ and 12, 12¹ are inserted into the vessel 1 through openings in the upper part 6 of the vessel 1. Electrode seals 13 are arranged both to support the electrodes and to provide a gas tight seal between the electrodes and the openings in the upper part 6 of the vessel 1. The electrodes are via conductors 14 via a regulating unit 15 connected to an electric power source (not shown).

The supply of electric current to each pair of electrodes is regulated by means of the regulating unit 15. The regulating unit 15 operates in such a way that electric current is only supplied to one pair of electrodes at the same time. The supply of electric current is shifted between the pairs of electrodes more or less continuously by means of the regulating unit 15. This can be done based on the frequency of the electric current in such a way that the regulating unit 15 shifts the supply of electric current from one pair of electrodes to another pair of electrodes based on the frequency. Thus if the electric current supplied to the electrodes has a frequency of 50 Hz, the regulating unit 15 shifts the current between the pairs of electrodes 50 times each second. When electric current is supplied to a pair of electrodes, an electric arc strikes between the two electrodes. By more or less continuously shifting the supply of electric energy between the pairs of electrodes, a very high temperature is obtained in the area between the electrodes over the complete cross-section of the high temperature reduction zone.

At least one of the electrodes 10, 10¹, 11 , 11¹ and 12, 12¹ may be hollow and may have means for supplying additional solid materials selected from silicon carbide, SiO₂ and carbonaceous reduction material through the at least one hollow electrode for adjustment of the carbon to SiO₂ ratio in the high temperature reduction zone.

Finally, conventional means (not shown) are arranged for moving the electrodes inwardly into the vessel 1 in order to compensate for electrode consumption. On the top of the flange 8 there is arranged a shaft 16. The shaft 16 has a substantial circular cross-section and comprises an outer steel shell 17 having a refractory lining 18 on its sidewalls.

At its top the shaft 16 has at least one opening 19 for air-tight supply of a mixture of quarts and carbon. At its upper end the shaft 16 further has an outlet opening 20 for reaction gas.

At its bottom the shaft 16 has a inwardly extending flange 21 defining a central opening 22 having a diameter equal to or smaller than the opening 9 in the top of the upper part 6 of the vessel 1. Preferably the flange 21 is sloping downwards at an angle towards the center of the shaft 16 to allow any liquid reaction products formed in the shaft 16 to drip off into the high temperature reduction zone.

Resting on the downward sloping flange 21 there is arranged a horizontal circular disc 23 having a central opening in the form of a polygon with sides 24 shown in figure 3. Means are arranged to continuously or intermittently rotate or oscillate the disc 23 about its vertical axis. Centrally in the shaft 16 there is arranged cone 25 with its peak pointing upwards. The cone 25 is suspended from the top of the shaft 16 by means of a member 26. The lower horizontal face of the cone 25 has a diameter which are somewhat greater than the central opening 22 defined by the flange 21. The member 26 preferably has internal channels circulating a cooling liquid for cooling the lower end of the cone 25.

The means for rotating or oscillating the disc 23 preferably comprises one or more rack and pinion arrangement 27 having drives entering the wall of the shaft 16 through gas tight cooled seals. A syncronized drive means is arranged outside the shaft 16. The drive means may be electric, hydraulic or pneumatic.

Figure 4 shows part of figure 1 having the electrodes arranged at two different vertical level. Parts of figure 4 corresponding to parts on figure 1 have the same reference numerals. On the embodiment shown in figure 4 one pair of electrodes 50, 50¹ is arranged at a lower vertical level than the pair of electrodes 10, 10¹. Electric current is supplied to the pair of electrodes 50, 50¹ in the same way as described above in connection with figure 1 - 3. With the electrode configuration shown in figure 4, the vertical extension of the high temperature zone in the area of the electrodes is increased, whereby the retention time of the cloud of solid particles supplied from the shaft 16 is increased. This will ensure that the solid particles entering the high temperature reduction zone will stay in the high temperature until they have been completely reacted.

The smelting reactor described above in connection with figures 1 to 4 is operated as follows:

A mixture of carbonaceous reduction material and quarts are filled into the shaft 16 through the opening 19 up to a level indicated by reference numeral 40 on figure 1. Preferably the mixture of carbonaceous reduction material and quarts are in the form of briquettes or pellets. The mixture of carbon and quarts has preferably a molar ratio of carbon to quarts of about 2:1.

The raw material supplied to the shaft 16 will fall into the space between the outer end of the lower part of the cone 25 and the inside of the central opening in the disc 23 and rest on the top of the flange 21.

Now, electric current is supplied to the pairs of electrodes 10, 10¹ , 11 , 11¹ and 12, 12¹ as explained above. In order to obtain an electric arc between each pair of electrodes when the reduction zone is cold, a plasma gas may supplied through the electrodes if hollow electrodes are used, or alternatively, the electrodes in each pair are moved towards eachother such that the electrode tip of each electrode in a pair are very close to each other. When a stable electric arc is obtained between each pair of electrodes, the disc 23 is being rotated or oscillated. Due to the polygonformed central opening in the disc 23, the solid materials resting on the flange 21 will be forced horizontally towards the center of the shaft 16 and will thus fall by gravity through the openings 22 and 9 and will pass through the area between the electrodes 10, 10¹, 11 , 11¹ and 12, 12¹ as a cloud of separate particles. The amount of material supplied to the high temperature reduction zone is closely controlled in relation to the heat energy supplied by the electrodes by controlling the rotation or the oscillation of the disc 23.

Due to the very high temperature which exists in the electric arcs between each pair of electrodes, the free falling materials will be reduced to Si, SiO gas and CO. The produced liquid Si together with any unreacted raw materials will be collected in the bottom of the lower part 3 of the vessel 1 while the part of the SiO which has not reacted to Si, will, together with CO gas formed, move freely upwards in the vessel 1 and into the shaft 16 where the SiO gas will react with carbon in the raw materials to form SiC, while the CO gas will move up through the shaft 16 and out through the gas outlet opening 20. The gas leaving the shaft 16 through the gas outlet opening 20 will essentially be CO- gas which either can be recovered as liquid CO, be used a process gas for chemical purposes or can be burned to produce heat. Any unreacted SiO-gas will condense to SiO₂ and Si in the rather cold upper part of the shaft 16.

As the carbon in the raw materials in the shaft 16 is being converted to SiC by reaction with SiO-gas entering into the shaft from high temperature reduction zone, the material charged to the vessel 1 by rotating the disc 23 will basically be SiC and SiO₂. Thus the high heat energy in the SiO gas entering the shaft is utilized to carry out the endothermic reaction SiO + 2C = SiC + CO and to preheat the raw materials. This substantially lowers the energy needed to produce silicon in the high temperature reduction zone in the vessel 1. Further due to the electrode configuration in the smelting reactor of the present invention, the temperature in the whole area between the electrodes will be very high which facilitates a very fast melting and reaction of the raw material particles as they fall by gravity as a cloud of solid particles into the high temperature reduction zone between the electrodes. The electrode configuration of the smelting reactor according to the present invention further makes it possible to supply a large amount of energy to a very small space. This will substantially increase the rate of production per unit reactor volume and thus reduce the overall heat losses from the smelting reactor. Further, since the SiO-gas is used to preheat the raw material charged to the shaft and to react carbon to SiC, no SiO-gas is lost from the smelting furnace. The yield of Si will therefore be increased to a near 100 % yield.

By regulating the supply of materials from the shaft to the high temperature reduction zone in accordance with the energy supplied to the high temperature reduction zone, a balance between the raw material supply and energy input is achieved at all times.

Finally, compared to the conventional submerged arc furnaces, which today are used for the production of metallurgical silicon, the investment costs will be substantially reduced as the smelting reactor of the present invention needs much less space than a submerged arc furnace producing the same amount of silicon.

Claims

CLAIMS:

1. A process for the carbothermic production of silicon comprising:

producing a downward flow of dispersed particles, said particles comprising a mixture of carbonaceous reduction material and particulate SiO₂ raw material;

providing energy to at least one pair of horizontally oriented electrodes; and

passing said downward flow of dispersed particles past said at least one pair of electrodes to produce silicon.

2. The process of claim 1 , c h a r a c t e r i z e d i n that said mixture includes silicon carbide.

3. A reactor for carbothermic production of silicon having

(a) a vessel (1 ) defining a high temperature reduction zone said vessel (1 ) having an outlet (5) for molten silicon at its lower part,

(b) means (10, 10¹, 11 , 11¹ and 12, 12¹) for supplying energy to the high temperature reduction zone,

(c) a gas/solid reaction shaft (16) for a carbonaceous reduction material and SiO₂ raw material mixture, the shaft (16) being attached to the top of the vessel (1 ), said shaft (16) having an opening in its bottom communicating with the high temperature reduction zone to allow gas from the high temperature reduction zone to enter into the shaft (16), c h a r a c t e r i z e d i n that the reactor further comprising means (19) for continuous or intermittent feeding of the carbonaceous reduction material and SiO₂ raw material mixture to the top of the shaft

(16), and means (23, 25) for continuous or substantially continuous controlled supply of solid reaction products from the bottom of the shaft (16) into the high temperature reduction zone, and where the means for supplying energy to the high temperature reaction zone comprises at least one pair of substantially horizontal electrodes arranged about the circumference of the high temperature reduction vessel (1) at a level above the outlet (5) for molten silicon.

4. A reactor according to claim 3, characte rized i n that the reactor is sealed and is intended to be operated at a pressure above atmospheric pressure.

5. A reactor according to claim 3, ch aracterized i n that the means (10, 1θ\ 11, 11¹ and 12, 12¹) for supplying energy to the high temperature reaction zone comprises three or more pairs of substantially horizontal electrodes arranged about the circumference of the high temperature reduction vessel (1).

6. A reactor according to claim 5, ch aracterized i n that the pairs of electrodes are arranged at the same vertical level in the high temperature reduction zone.

7. A reactor according to claim 5, characterized i n that the pairs of electrodes are arranged at different vertical levels in the high temperature reduction zone.

8. A reactor according to claim 3, characterized i n that the electrodes are made from graphite.

9. A reactor according to claim 13, characterized in that the electrodes are prebaked carbon electrodes.

10. A reactor according to claim 3, characte rized i n that the electrodes are inert electrodes.

11. A reactor according to claim 3, characterized in that at least one of the electrodes are hollow and equipped with means for supply of solid materials through the hollow electrodes and into the high temperature reduction zone.

12. A reactor according to claim 3, characterized in that a lance for sampling and chemical analysis of material in the high temperature reduction zone is inserted into one or more hollow electrodes (10, 10¹ , 11, 11¹ and 12, 12¹).

13. A reactor according to claim 3, ch a ra cte rized in that the means (19) for continuous or intermittent feeding of the carbonaceous reduction material and SiO₂ raw material mixture to the top of the shaft (16) is an air-tight double bell supply means (28) equipped with means (29) for inert gas flushing.

14. A reactor according to claim 3, cha racte rized in that the means for continuous or substantial continuous controlled supply of solid reaction products from the bottom of the shaft (16) to the high temperature reduction zone comprises a horizontal circular rotatable or oscillating disc (23) having a diameter corresponding to the inner diameter of the shaft (16) and resting on an inwardly extending flange (21 ) at the lower end of the shaft (16), said disc (23) having a central opening in the form of a polygon and where the central opening of the disc (23) is partly covered by a cone-shaped baffle (25) suspended in a vertical member (26) extending above the top of the shaft (16).

15. A reactor according to claim 14, characterized in that the flange (21) is sloping downwards at an angle towards the center of the shaft (16).

16. A reactor according to claim 14, characterized in that the means for rotating or oscillating the disc (23) comprises at least one rack and pinion arrangement having drives entering the wall of the shaft (16) through gas-tight cooled seals.

17. A reactor according to claim 3, cha racterized in that the shaft (16) has means (20) for extracting undiluted CO gas from the shaft (16) for further processing.

18. A method for carbothermic production of silicon by reduction of silicon dioxide in the reactor of claim 3; the process comprising

continuous or intermittent supplying of a mixture of a carbonaceous reduction material and silicon dioxide raw material to the shaft, where the carbon in the shaft reacts to SiC with contact with SiO-gas flowing from the high temperature reduction zone and upwardly in the shaft,

continuous or substantial continuous controlled supplying of solid reaction products from the bottom of the shaft and into the high temperature reduction zone,

- passing the solid reaction products supplied from the shaft through the electric arc between the at least one pair of substantially horizontal electrodes to produce molten silicon in the high temperature reaction vessel which molten silicon is collected of the bottom of the high temperature reduction vessel and SiO- and CO gas which flows upwardly through the shaft.

19. Method according to claim 18, characterized in that the raw material mixture of carbon and silicon dioxide is supplied to the shaft in a molar ratio of about 1.8:1 to 2.2:1.

20. Method according to claim 18, characterized in that the raw material mixture is supplied to the shaft in the form of agglomerates of carbonaceous reduction material and silicon dioxide.

21. Method according to claim 20, characterized i n that the agglomerates are pellets.

22. Method according to claim 20, cha racterized i n that the agglomerates are briquettes.

23. Method according to claim 18, characterized in that solid materials selected from silicon carbide, SiO₂ and carbonaceous reduction material are charged to the high temperature reduction zone through one or more hollow electrodes in order to adjust the carbon to SiO₂ ratio in the high temperature reduction zone.