RU2085324C1 - Silicon pouring ingot mold - Google Patents

Abstract

FIELD: metallurgy, in particular, thermoelectric production of crystalline silicon. SUBSTANCE: ingot mold has metal housing with peripheral stepped lining extending on longitudinal sides of ingot mold from bottom to top of longitudinal walls. Lining is made from material having low silicon melt wettability and providing high chemical resistance of this part of lining. Additional stepped metal walls are mounted within metal housing to define closed spaces between metal walls and metal housing. These walls are adapted to carry peripheral lining masonry. Closed spaces are filled with powdered heat-insulating material, such as diatomite, pearlite etc. Stepped peripheral lining is fabricated from fireclay brick impregnated with compound containing, % by weight: coal dust 5-15; ground silicon 10-30; binder, such as chromo-aluminaphosphate binder. Service life of ingot mold is increased by 1.2 times. Specific consumption of coal product is reduced by 50%. EFFECT: increased efficiency, prolonged service life and improved quality of silicon. 9 cl, 1 dwg, 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 silicon into molds.

 A well-known mold of cast iron (see L. S. Kokhan and others. Mechanical equipment of non-ferrous metallurgy plants, M. "Metallurgy", 1988, part 2, p. 256).

 The disadvantage of this mold is its low life due to the low chemical resistance of cast iron with respect to the molten silicon, as well as the low quality of the cast silicon from clogging it with an admixture of iron. These disadvantages are less evident when applying protective coatings, but the life of the mold still remains low. In addition, the small capacity of such molds adversely affects the conduct of silicon production technology, because this requires a more private release of silicon from the ore-thermal furnace.

 The closest in technical essence and the achieved result to the invention is a mold, including a metal casing and lining of its bottom and side walls (bevels) of coal plates (Technological Instruction BrOO OJSC 25204.00001 "Electrothermal production of crystalline silicon", 1993, p.30- 31, clause 5.3.8.2).

 The use of such molds allows to pour silicon in a single volume from the furnace in significant volumes, which helps to maintain the technological mode of smelting at optimal parameters.

 A disadvantage of the known mold is the insufficiently long lining time due to the fact that the silicon fill, applied before pouring the melt (used to take heat from the melt to be poured as a result of melting this fill) covers only the horizontal part of the hearth, but it does not hold on flat bevels resulting in premature wear of the lining in this part of the mold. The destructive effect is aggravated by the high temperature difference of the center and periphery of the lining due to poor thermal insulation along the sides of the mold, which leads to cracking in the lining.

 The aim of the invention is to increase the service life of the lining of the mold by reducing its temperature after pouring the melt, reducing the temperature difference of the center and periphery of the lining and increasing the resistance of the latter from the influence of the melt while reducing the wettability of the lining by this melt.

 The drawing shows a mold for casting silicon, section.

 The mold consists of a metal casing 1, which can be of a collapsible type, a fireclay base 2, a hearth lining 3 of coal blocks, a step-like lining 4, made of fireclay brick impregnated with composition 5, which provides low wettability and high chemical resistance to molten silicon, metal step-like partitions 6, backfill 7 from heat-insulating powdery material (diatomite, perlite, etc.). Before pouring silicon on the surface of the hearth 3 and the stepped lining 4, a layer of bedding 3 made of silicon fines is applied.

In the process of casting silicon after it is discharged from an ore-thermal furnace of the RKO-25KrI type with a temperature of 1600-1700 ^o C, the melt comes into contact with a bed of silicon fines, then with the surface of the lining and crystallizes as it cools. Since the design of the mold provides the creation of a layer of silicon fills over the entire surface of the lining that is in contact with the melt, heat transfer from the melt increases, as a result of which the thermal impact on the lining is reduced, which leads to a decrease in the wear of the lining from chipping of the upper layers of the lining.

 Peripheral lining along the longitudinal sides of the mold from the bottom to the top of the longitudinal walls of the metal casing in the form of steps and in the given sizes allows the addition of silicon fines not only to the central part of the hearth, but also to the inclined peripheral lining, which expands the contact zone of the molten melt with the bed As a result, the temperature of molten silicon at the melt-lining boundary decreases, which leads to a decrease in wear of the mold lining.

 The presence of spaces formed by additional metal partitions and filled with powdery refractory heat-insulating material, such as diatomite, perlite, etc. helps to reduce the temperature difference of the center and periphery of the lining, which leads to a decrease in crack formation in the lining. These partitions at the same time contribute to the simplification and facilitation of the installation of a stepped peripheral lining.

 Impregnation of fireclay brick, of which the stepped lining is made, with a composition containing coal dust, crushed silicon (waste products of aluminum and silicon, respectively) and a binder, for example, AXPS in predetermined proportions, increase the chemical resistance and decrease the wettability of the peripheral lining by melt, which reduces wear surface of the latter. The dust-like and ground state of carbon and silicon, respectively, makes it possible to effectively carry out the process of impregnating fireclay bricks, used for laying a peripherally stepped lining.

The selected conditions are limited by the following factors:
a decrease in the thickness of the stepped lining less than 1.1 of the thickness of the hearth lining 3 leads to a decrease in the mechanical strength of the lining, and an increase of more than 1.2 reduces the peripheral thermal insulation, which leads to an increase in the temperature difference between the center and periphery of the lining;
a decrease in the width in the stepped lining of less than 0.2 of the mold width leads to a decrease in the thermal insulation of the periphery of the lining, and an increase of more than 0.3 leads to an excessive decrease in the mold capacity;
coal dust reduction of less than 5 wt. or from crushed silicon less than 10 wt. leads to an increase in wettability and due to this - welding of the ingot to the surface of the peripheral lining, and the increase, respectively, more than 15 wt. or 30 wt. reduces the degree of impregnation of fireclay bricks (by reducing the fluidity of the impregnating composition), and hence the chemical resistance of the peripheral lining.

 Before pouring silicon, thermocouples are installed in the mold to determine the temperature difference in the center and periphery of the lining. After removing the ingot and cooling the molds, the condition of the lining is fixed (chipping of the upper layers and cracking). In order to determine the service life of the lining, the number of silicon fillings in the molds is fixed.

 The implementation of the proposed mold is as follows.

Example 1
On the bottom of a metal casing with a width of 1350 mm and a length of 2650 mm - a basement of one row of fireclay bricks is laid, on which coal blocks 200 mm thick and 810 mm long are laid in the center. Inside the metal casing along the longitudinal sides from the bottom of the bottom to the top of the longitudinal walls of the metal casing, step-shaped metal partitions are made, forming spaces between them and the casing, which are filled with powdery heat-insulating material, for example diatomite, through the openings on the horizontal stages of the steps. On metal partitions having steps, a step-like lining of chamotte brick is laid out, pre-impregnated with a composition that gives the lining low wettability and high chemical resistance to molten silicon.

 The width of the stepped lining is 270 mm or 0.2 of the mold width.

 The thickness of the stepped lining is 220 mm or 1.1 the thickness of the hearth.

 The composition for impregnating fireclay bricks contains, by weight.

Coal dust 5
Crushed silicon 10
AHFS 85
In examples 2 and 3, the mold is mounted and tested analogously to example 1 with the following parameters.

Example 2
1. The width of the stepped lining is 350 mm or 0.26 of the width of the mold.

 2. The thickness of the stepped lining is 230 mm or 1.15 of the thickness of the hearth.

 3. The composition for impregnation contains, by weight.

Coal dust 10
Crushed Silicon 20
AHFS 70
Example 3
1. The width of the stepped lining is 405 mm or 0.3 of the width of the mold.

2. The thickness of the stepped lining is 240 mm or 1.2 the thickness of the bottom
3. The composition for impregnation contains, by weight.

Coal dust 20
Crushed silicon 30
AHFS 50
In examples 4-11, the mold is mounted and tested similarly to examples 1-3 outside the declared intervals.

 The mold has been completed and tested by a well-known solution.

 The test results are shown in the table.

 The table shows that the use of the mold for casting silicon (according to examples 1-3) ensures the integrity of the lining of the mold for a longer period by lowering the temperature of the lining after pouring molten silicon into the mold, reducing the temperature difference between the center and periphery of the lining, and reducing wettability increase the chemical resistance of the latter with respect to molten silicon, which helps to reduce the degree of wear of the lining and to reduce cracking in it. The service life of the lining is increased by 1.2 times. The consumption of refractory materials per ton of cast silicon is reduced by 20%. The specific consumption of expensive and scarce coal products is reduced by 50%

Claims (8)

 1. The mold for casting silicon, comprising a metal casing and a lining of refractories, characterized in that the peripheral lining along the longitudinal sides of the mold from the bottom to the top of the longitudinal walls of the metal casing is made stepwise.  2. The mold according to claim 1, characterized in that it is equipped with step-like partitions located inside the metal casing for the formation of closed cavities between them and the casing and bearing the masonry of the peripheral lining.  3. The mold according to claim 1, characterized in that the closed cavity is filled with a powdery refractory heat-insulating material.  4. The mold according to claim 3, characterized in that the closed cavity is filled with diatomaceous earth.  5. The mold according to claim 3, characterized in that the closed cavity is filled with perlite.  6. The mold according to claim 3, characterized in that the closed cavity is filled with a mixture of diatomite and perlite.  7. The mold according to claim 1, characterized in that the stepped lining is made with a thickness of 1.1 to 1.2 the thickness of the hearth lining and a width of 0.2 to 0.3 the width of the mold.  8. The mold according to claim 1, characterized in that the stepped lining is made of fireclay bricks, impregnated with a composition containing coal dust, crushed silicon and a binder in the following ratio, wt. Coal dust 5 15
Crushed silicon 10 30
Binder Else
9. The mold of claim 8, characterized in that the binder is used alumina-chromophosphate binder.

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