RU2077969C1 - Silicon ingot manufacture method - Google Patents

Abstract

FIELD: metallurgy, in particular, electrothermal production of crystalline silicon. SUBSTANCE: method involves periodically applying oxygen-containing boron compound to bottom having temperature exceeding melting temperature of this compound, for instance boric anhydride, boric acid or borax, by 50-250 C, with amount of oxygen-containing boron compound being within the range of 0.5-5 kg per sq m of bottom area; delivering silicon fines onto bottom. EFFECT: increased efficiency by improved continuity of lining and prolonged service life due to preventing ingot from welding to bottom surface and oxidation of bottom surface at high temperatures. 6 cl, 1 tbl

Description

 The invention relates to the field of metallurgy, in particular to the electrothermal production of crystalline silicon and can be used in the casting of liquid silicon in the mold.

 A known method of casting silicon, including pouring silicon into cast iron molds, cooling the melt and removing a silicon ingot from the mold (L. Kokhan and others. Mechanical equipment of non-ferrous metallurgy plants. M. "Metallurgy", 1988, part 2. p.256 )

 The disadvantage of this method is the low life of the mold due to the dissolution of cast iron in molten silicon. At the same time, dissolved iron clogs the resulting silicon (it is a harmful impurity). Protective coatings to some extent reduce the manifestation of these shortcomings, however, the life of the mold remains insufficiently high.

The closest in technical essence and the achieved result to the proposed invention is a method of casting silicon into steel welded molds lined with refractory materials inside, including placing a layer of silicon fines on the mold bottom, pouring silicon into the mold, crystallizing the ingot and removing it from the mold (Technological instruction JSC "BrAZ", 25204.00001 "Production of technical silicon", 1993, clause 5.3.8.2 ^o C5.3.8.4. From 38-39). An addition of silicon fines to the hearth is used to remove heat from the melt due to the fusion of silicon fines and to prevent the ingot from welding to the hearth.

 The disadvantage of this method is the insufficiently high service life of the lining of the mold, because when the ingot is removed from the mold, the lining surface is destroyed due to the partial welding of the ingot to the bottom. In addition, for the same reason, labor costs for removing the ingot from the mold increase.

 The aim of the invention is to increase the service life of the lining of the mold and reduce labor costs for removing the ingot from the mold.

This goal is achieved by the fact that by the method of casting silicon into the mold, including placing on the bottom of the mold before pouring silicon a layer of silicon fines, pouring silicon into the mold lined with refractory materials inside, crystallizing the ingot and removing it onto the mold, periodically before placing an oxygen-containing boron compound is applied on it from a silicon fines at a bottom surface temperature exceeding the melting temperature of this compound by 50-250 ^o C, 0.5-5 kg / m ² area di hearth molds.

The application on the surface of the hearth periodically before placing a layer of silicon fines on it of an oxygen-containing boron compound at a hearth surface temperature exceeding the melting temperature of this compound by 50-250 ^o C, 0.5-5 kg / m ^{2 of} the hearth area provides an increase in lining life molds and reducing labor costs for removing the ingot from the mold.

This is achieved by the fact that the oxygen-containing boron compound deposited on the surface of the hearth of the mold is expanded, because the surface temperature of the hearth exceeds the melting point of this compound by 50-250 ^o C, and having a sufficiently high fluidity, fills the pores of the refractory material and the joints in the hearth. When pouring another portion of silicon into the mold (with a temperature of 1600-1700 ^o C), a liquid layer is formed, which prevents the ingot from welding to the bottom. This condition persists until the ingot is removed from the mold, because the temperature of the surface of the hearth exceeds the melting temperature of the oxygen-containing boron compounds by at least 50-250 ^o C. Therefore, the removal of the ingot from the mold is carried out without violating the hearth. In addition, the liquid layer protects the hot surface of the hearth from oxidation, one hundred reduces its wear. The lining life of the mold is increased. Prevention of welding the ingot to the bottom facilitates and accelerates the removal of the ingot from the mold, which helps to reduce labor costs for this operation.

 The selected conditions are limited by the following factors.

A decrease in the amount of oxygen-containing boron compound applied by less than 0.5 kg / m ^{2 of} the bottom area does not achieve the goal, because welding of the ingot to the hearth is not prevented, and an increase of more than 5 kg / m ^{2 of} the hearth area is impractical due to the overexpenditure of the oxygen-containing boron compound without the additional effect of increasing the life of the mold lining.

A decrease in the temperature of the surface of the hearth over the melting temperature of the oxygen-containing boron compound by less than 50 ^o C reduces the efficiency of filling the pores of the refractory material and joints in the hearth, which leads to an increase in the degree of wear of the hearth, and an increase of more than 250 ^o C is impractical due to overspending of the oxygen-containing boron compounds without additional effect on increasing the life of the mold lining.

 As a result of a search in the patent and scientific and technical literature, no technical solutions were found with features distinguishing the proposed object of the invention from the prototype, namely: allowing to create a liquid layer that acts as a buffer between the ingot and the bottom of the mold, thereby preventing the ingot from welding to the surface of the bottom .

 The implementation of the method is carried out during the casting of silicon after its release from the ore-thermal furnace type RKO-25 KrI.

 Example 1

After removing the ingot of the previous silicon pouring into the mold, on the surface of the hearth at its temperature exceeding the melting temperature of the oxygen-containing boron compound (boric anhydride) by 50 ^° C, boric anhydride is applied in an amount of 0.5 kg / m ^{2 of} the hearth area. Boric anhydride melts the pores of the refractory material and the joints of the hearth. As the hearth cools, the boric anhydride hardens with the formation of a thin layer onto which a layer of silicon fines is placed according to the usual technology, after which a new portion of silicon is poured into the mold with a temperature of 1600-1700 ^o C. Boric anhydride is melted (has a high boiling point), As a result, the resulting liquid layer interferes with the welding of the ingot to the bottom. This state is preserved when the ingot is removed from the mold, it helps to maintain the integrity of the hearth. In addition, the liquid layer simultaneously protects the hot surface of the hearth from oxidation by atmospheric oxygen. The service life of the lining is increased by 14% (the number of silicon fillings in the mold before replacing the lining was 40). Labor costs for removing the ingot from the mold were reduced by 2.5 times.

 In examples 2-4, the application of an oxygen-containing boron compound to the bottom surface is carried out analogously to example 1 with the following parameters.

 Example 2

The temperature of the surface of the hearth above the melting temperature of the applied oxygen-containing compound of boron (boric acid) is 100 ^o C.

The consumption of an oxygen-containing compound of boron (boric acid in terms of boric anhydride) is 2.0 kg / m ^{2 of} the bottom area.

 Example 3

The excess surface temperature of the hearth over the melting temperature of the applied oxygen-containing boron compounds (borax in terms of boric anhydride) is 150 ^o C.

The consumption of oxygen-containing boron compounds (borax in terms of boric anhydride) is 3.5 kg / m ^{2 of} the bottom area.

 Example 4

The temperature of the surface of the hearth above the melting temperature of the applied oxygen-containing boron compound (boric anhydride) is 250 ^o C.

The consumption of oxygen-containing compounds of boron (boric anhydride) 5 kg / m ² bottom area.

 In examples 5-10, the application on the surface of the hearth of an oxygen-containing boron compound is carried out outside the declared intervals.

 Carry out the filling of silicon into the mold according to the well-known solution.

 The test results are shown in the table.

 The table shows that the use of the method of casting silicon into the mold (according to examples 1 to 4) ensures the integrity of the lining of the mold for a longer period by preventing the ingot from welding to the hearth and protecting the surface of the hearth from oxidation at high temperatures. The service life of the lining is increased by 1.25 times, which leads to a reduction in the consumption of expensive and scarce refractory materials, in particular coal stoves. The labor required to remove the ingot from the mold is reduced by 2.9 times.

Claims (6)

 1. The method of producing silicon ingots, including placing on the bottom of the mold before pouring silicon a layer of silicon fines, pouring silicon into the mold lined inside with refractory materials, crystallizing the ingot and removing it from the mold, characterized in that it is periodically placed on the surface of the hearth before being placed on it a layer of silicon fines is applied oxygen-containing boron compound at a surface temperature of the hearth above the melting temperature of this compound. 2. The method according to claim 1, characterized in that the temperature of the surface of the hearth exceeds the melting temperature of the oxygen-containing boron compounds by 50 - 250 ^o C. 3. The method according to claim 1, characterized in that the oxygen-containing boron compound is applied to the surface of the hearth in an amount of 0.5 to 5 kg in terms of boric anhydride per 1 m ^{2 the} area of the mold bottom.  4. The method according to claim 3, characterized in that boric anhydride is used as an oxygen-containing boron compound.  5. The method according to claim 3, characterized in that boric acid is used as an oxygen-containing boron compound.  6. The method according to claim 3, characterized in that a borax is used as an oxygen-containing boron compound.

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