RU2355634C1 - Method of high-purity silica preparation - Google Patents

Abstract

FIELD: chemistry. ^ SUBSTANCE: invention can be applied in chemical and electronic industries. The silica is prepared by the electrolytic reduction of the ammonium silicofluoride water solution. The electrolysis is carried out in pH range 3-7.5. ^ EFFECT: invention allows to simplify the process scheme and to decrease the power consumption of the process and the product cost. ^ 4 ex

Description

The invention relates to chemical technology for the production of highly pure silicon and can be used for the manufacture of semiconductors and photovoltaic cells.

A known method of producing semiconductor silicon [RU 2035397], including the interaction of a fluorine-containing compound with a reducing agent. As a fluorine-containing compound, hydrofluoric acid obtained by treating with deionized water preliminarily purified by freezing or by contacting with activated carbon silicon tetrafluoride formed by the interaction of sand with a solution of hydrofluoric acid in the presence of concentrated sulfuric acid, atomic hydrogen is used as a reducing agent.

The disadvantages of the method: an expensive stage of purification of hydrofluoric acid by freezing; low silicon yield associated with a rapid loss of atomatization.

The known method [RU 2272785], selected as a prototype, including fluorination of silicon dioxide with elemental fluorine, purification of silicon tetrafluoride from impurities, saturation of molten salts with it, electrolytic reduction of silicon with the associated production of reverse fluorine, separation of silicon from molten salts.

The disadvantage of this method is the multi-stage technology, a low degree of saturation with silicon tetrafluoride of a molten salt, the complexity of the hardware design and the high cost of the processes of fluorination of silicon dioxide and purification of silicon from the electrolyte.

The objective of the invention is to simplify the technological scheme, reducing energy costs and cost of the final product.

By the action of ammonium fluoride on silicon dioxide or silicates, ammonium hexafluorosilicate ((NH ₄ ) ₂ SiF ₆ ) is obtained, which is readily soluble in water, a non-aggressive substance with a sublimation temperature of 320 ° C, which allows purification from volatile impurities, including phosphorus compounds and boron (boiling points PF ₃ , PF ₅ , BF ₃ less than -85 ° C). Silicon as a semiconductor has high demands on the content of phosphorus and boron, which can be easily removed by fluorine-ammonium processing of the feedstock, in contrast to the use of other fluorinating agents (F ₂ , HF, HF _aq ), with which silicon tetrafluoride with a boiling point is formed -80 ° C. Thus, in the processing of quartz raw materials by ammonium fluorides, a complete separation from impurities is possible.

The invention consists in the electrolytic reduction of silicon from an aqueous solution of ammonium silicofluoride. The initial silica sand is mixed with a fluorine-containing ammonium salt, the mixture is heated to form ammonium cremne fluoride, which is separated from impurities by sublimation, followed by dissolution of the sublimate.

The fluorine-containing ammonium salt is taken in an amount of 50-125% of the stoichiometric ratio, taking into account the course of reactions:

SiO ₂ + 6NH ₄ F = (NH ₄ ) ₂ SiF ₆ + 4NH ₃ + 2H ₂ O,

SiO ₂ + 3NH ₄ HF ₂ = (NH ₄ ) ₂ SiF ₆ + NH ₃ + 2H ₂ O.

As a result of the electrolysis of ammonium silicofluoride, silicon is obtained with a purity of at least 99.9%. The main impurity is fluorine, which is separated by recrystallization. Electrolysis is carried out at the highest rate at a solution pH of 3-7.5. At pH greater than 7.5, a silica gel precipitate (SiO ₂ · nH ₂ O) is formed in the electrolyte, which reduces the concentration of dissociated ions. At a pH of less than 3, the deposition rate of silicon at the cathode decreases due to partial interaction with fluoride ions.

Example 1

Quartz sand is mixed with ammonium hydrodifluoride in a mass ratio of 1 : 3 , the mixture is heated to 150 ° C, kept at this temperature for 0.5 hours for more complete interaction, then the hexafluorosilicate is distilled off at a temperature of 350 ° C for an hour. Sublimation is dissolved in water, the resulting solution with a volume of 1 l with a concentration of ammonium hexafluorosilicate 100 g / l (or 0.56 mol / l) is placed in the electrolysis bath. A plate of steel is taken as the cathode, and the anode is graphite. The electrolysis is carried out at a cathode current density of 50 mA / cm ² , the voltage at the electrodes is 9 V, the pH of the solution 6. The process takes 3 hours, after which the silicon layer is separated from the electrode, washed with water and dried. The mass of silicon obtained is 4.7 g (30% recovery from solution). Silicon is obtained in an amorphous form with a purity of not less than 99.9%.

Example 2

It differs from Example 1 in that an electrolyte using a solution of ammonium hexafluorosilicate with a concentration of 178 g / l (or 1 mol / l). Electrolysis is carried out at a pH of 6.5 for 1 hour. The mass of silicon obtained is 3.64 g (13% recovery of silicon from solution).

Example 3

It differs from Example 2 in that the electrolysis is carried out at a cathodic current density of less than 20 mA / cm ² , ultimately producing a layer of polycrystalline silicon at the cathode.

Example 4

It differs from Examples 1, 2, 3 in that the anode is used from a noble metal or from another metal coated with a noble metal.

Claims (1)

A method of obtaining highly pure silicon, including the restoration of silicon by electrolysis, characterized in that an aqueous solution of ammonium silicofluoride is used as the electrolyte;