RU1803384C - Method of silicon dioxide purification - Google Patents

Abstract

Usage: to obtain crystal. SUMMARY OF THE INVENTION: silicon dioxide is dissolved in an alkaline solution, the resulting solution is mixed with a solution of calcium chloride, taken in an amount; providing a CaCl2: F 0-5-0.7 molar ratio in the mixture, the precipitate formed is separated from the solution and conc. HCI with an iron content of not more than 0.009 wt.%, The resulting precipitate was separated, washed, dried and calcined. 2 tab.

Description

The invention relates to the field of refined chemicals, in particular silicon dioxide, and can be used In the preparation of silicon dioxide, it is suitable for the production of crystals (mass and Fe 0.008% by mass). Until now, natural quartz has been used in crystal production, but in the USSR and abroad, there is an increasing shortage of this type of raw material.

; In this connection, other sources of silicon dioxide, in particular, hydrofluoric acid (CPA) HaSiFe, a waste product of the production of phosphorus H | JX fertilizers, attract attention.

The simplest way to obtain silicon dioxide from it is to treat an aqueous solution of CPA with ammonia:

H2SiF6 + 6NH3 + 2H20- Si02 + 6NH4F (1)

The silica emitted by reaction (1) typically contains 2.5-3% by weight of fluorine and 0.1-0.2% by weight of iron. Thus, the use in the glass industry of silicon dioxide formed by reaction (1) is not possible without purification.

A known method of purification of silicon dioxide (waste production of aluminum fluoride) from impurities of fluorine and aluminum by first treating with acid at and t 20-100 ° C for several hours to remove aluminum and other metals and part of fluorine, then an alkali solution at pH 8.5 -11.5 and t 20-100 ° C for several hours to remove most of the remaining fluorine. Silicon dioxide insoluble under these conditions is filtered off, washed with water and dried (US Patent No. 4,557,918, CL 423-335, 1986).

This method of purification of silicon dioxide from impurities of metals and fluorine, as the closest in technical essence and the achieved result, was adopted by us as a prototype.

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However, the method for purifying silicon dioxide described in the prototype method has certain disadvantages. It is known that by washing silica, substantially the main contamination from the surface of non-porous SI02 particles is effectively removed. formed in an acidic environment; impurities from the depths of the silica particles, moreover, if SIF bonds can be formed, are not removed in this way. In this case, it is not possible to completely remove fluorine even when S102 powder is calcined at t 1000 ° C. Silica particles formed by reaction (1) in an alkaline medium have a developed internal structure and, according to our data, cannot be substantially purified from fluorine by washing in alkaline solutions (see Example 7). Another disadvantage of the method is the formation of a large number of low concentration fluorine-free aqueous solutions that are not used.

The aim of the invention is to increase the purity of the product by fluorine and to enable iron removal.

To this end, it is proposed to treat the contaminated silica with sodium hydroxide in an aqueous medium until a homogeneous solution forms. In this case, in addition to the dissolution of SiO2, the destruction of SiF о anions occurs, with the transition of reaction products into solution:

M2SiF6 + 4 #OH + 4NaF -t- + 2MF + 2H20 (2)

The absence of a precipitate in an alkaline solution indicates that metal impurities form a colloidal solution of hydroxides or soluble compounds such as sodium ferrite NaFe02.

The following provides a mixture of the products of the interaction of contaminated silicon dioxide with an aqueous solution of sodium hydroxide, treated with a certain amount of calcium chloride to precipitate F ions as calcium fluoride by the reaction:

CaCl2 + 2F + 2Cr (3)

The fluorine-free solution is treated with hydrochloric acid to precipitate pure silica; while the iron contained in the mixture remains in the solution in the form of chloride. The resulting silica is then washed with demineralized water, dried and calcined to remove residual chlorine.

Of great importance is the choice of process parameters at all stages.

The reaction of contaminated silica (3.2% by mass of fluorine and 0.1% by mass of iron) with an aqueous solution of sodium hydroxide is carried out with slight heating (50 ° C)

at a 20% excess hydroxide compared with the stoichiometry of the process to obtain a homogeneous slightly opalescent sodium silicate solution with a concentration of 13-15% by weight.

Calcium chloride is added to the solution as a saturated aqueous solution at room temperature while stirring the mixture. The optimal amount of calcium chloride is 0.5-0.7 mol per 1 g

fluorine ion, With a decrease in this ratio of reagents, the amount of fluorine remaining in silicon dioxide increases significantly; an increase in the amount of calcium chloride compared to the optimal

leads to a significant decrease in the fluorine content, stabilizing at the level of 0.5% of the mass. This is apparently due to the simultaneous processes of coprecipitation of calcium silicates)

the calcium fluoride precipitate obtained during the precipitation process always contains significant amounts of silicon; can be used as a mineralizer in the cement industry).

The separation of silicon dioxide from a sodium silicate solution was carried out by adding concentrated hydrochloric acid to an acidic reaction of the medium. The iron content in the resulting silicon dioxide is strongly influenced by the purity (in iron) of the acid used: to obtain in the process of silicon dioxide with an iron content of 0.008% by mass, the iron content in the used hydrochloric acid is not

should exceed 0.009% of the mass. Thus, not only reactive hydrochloric acid according to GOST 3118-77, but also synthetic technical grades A and B (highest category) according to GOST 857-78 can be used, which

expands the raw material base of the process.

The yield of purified silica under optimal conditions is 92-94% by weight of the starting dioxide; losses are mainly due to the precipitation of calcium silicate with calcium fluoride.

Example 1. In a reactor equipped with a stirrer, a thermometer and a reflux condenser, 5 g of silicon dioxide containing 3.2% by weight of fluorine and 0.1% are placed

masses of iron, and a solution of 7 g of sodium hydroxide (h) in 65 ml of distilled water is added. At t 40-50 ° C and stirring, a homogeneous solution forms, which is then cooled to room temperature. To the resulting solution was added 0.61 g of calcium chloride (h) as a saturated aqueous solution (molar ratio of CaCl2: F 0.7), the mixture was stirred for half an hour and then the precipitate was filtered off. Concentrated sulphic acid with a gelphase content of 0.009 wt% to pH 5 was added to the filtrate to precipitate silica gel. The silica precipitate is filtered off, washed with 2 p4 for 30 ml with distilled water, dried and calcined at 400-500 ° С for 1.5-2 hours. 4.63 g (yield 92.6%) of purified silica with content are obtained. fluorine 0.6% of the mass and iron 0.007% of the mass.

Other examples are given below.

Example 6. Under the conditions of Experiment 1, purification of silicon dioxide from fluorine (initial content of 2.47 wt%) without the addition of calcium chloride. The content of Hi / e fluorine in purified silica is 1.55% by mass, the degree of purification

37

.2%.

Example. 2 g of silicon dioxide with an initial fluorine content of 2.47% of the mass is treated with stirring 50 g

0 5

0

5

A 2.5% NajCO solution, (pH 9.5) at a temperature of 50 ° C for 2 hours. Then, silicon dioxide is isolated as described in Example 1. The fluorine content after processing is 1.09% by mass, the degree of purification is 55.9%.

Claims (1)

Thus, the proposed method for purification of silicon dioxide allows removing impurities from the particle volume S102 and obtaining a product satisfying the requirements of crystal production. SUMMARY OF THE INVENTION A method for purifying silicon dioxide, comprising sequential treatment with alkaline and acidic solutions, characterized in that, in order to increase the degree of purification of the product from fluorine and to ensure purification from iron, silicon dioxide is dissolved in an alkaline solution, the resulting solution is mixed with a solution of calcium chloride, taken in an amount providing a CaCteiF molar ratio of 0.5-0.7 in the mixture, the precipitate formed is separated from the solution and concentrated hydrochloric acid with an iron content of not more than 0 is added to the solution. 009 wt.%, The resulting precipitate was separated, washed, dried and calcined. Effect of the amount of calcium chloride on the degree of purification of silicon dioxide from impurities of fluorine (initial fluorine content of 3.2% by mass) Table 1 The effect of the iron content in the initial hydrochloric acid on the quality of the obtained silicon dioxide (iron content in the initial S102 0.1% mass) Note: the iron content in aqueous solutions was determined by EPR using a signal from the complex. In order to determine the iron content of silicon dioxide, the samples were previously dissolved in hydrofluoric acid pure in iron. Table 2