JPS5934131B2 - Method for producing silicofluoride soda from fluorine-containing exhaust gas - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing highly pure, coarse-grained sodium silicofluoride at low cost by effectively utilizing fluorine-containing exhaust gas.

More specifically, we will produce sodium silicofluoride at an extremely low cost from fluorine-containing exhaust gas generated in wet phosphoric acid, dry phosphoric acid, lime superphosphate, calcined phosphorous fertilizer, melted phosphorous fertilizer, cryolite production, aluminum smelting industry, etc. In addition, the present invention provides a method for effectively utilizing the fluorine content in exhaust gas and extremely reducing the amount disposed of as waste.

Conventional industrial methods for producing sodium silifluoride include a method in which the fluorine content is reacted with a sodium compound such as caustic soda or soda carbonate in the process of refining a wet phosphoric acid solution to obtain sodium silifluoride. It is being

This method is economical in that it makes effective use of the fluorine content, which is an impurity in wet phosphoric acid.However, one raw material, sodium chloride, is replaced with caustic soda or carbonate soda, which does not bring any extra anions to the wet phosphoric acid solution. Therefore, a cheaper source of sodium is desired.

On the other hand, from the perspective of a method for treating fluorine-containing exhaust gas, it is necessary to remove the fluorine content of the exhaust gases generated from the above-mentioned phosphoric acid industries, aluminum smelting industries, etc., which contain fluorine content as an unnecessary substance, to below the regulation value when releasing it into the atmosphere. Conventionally, the fluorine content was separated into compounds such as calcium fluoride, calcium silicofluoride, and sodium silicofluoride by treating with milk of lime or an aqueous solution of caustic soda, but these fluorine compounds It is accompanied by various compounds, is of poor quality, and in many cases has no useful value (was treated as industrial waste).

As a result of intensive studies in view of the above circumstances, the inventors of the present invention have found that by absorbing fluorine-containing exhaust gas into water to form a hydrosilicic acid aqueous solution and reacting this with seawater, it is possible to create coarse particles with high purity and ease of handling. They discovered that it is possible to produce sodium silicofluoride at an extremely low cost, reduce the burden of processing fluorine-containing exhaust gas, and make effective use of the fluorine content, leading to the completion of the present invention. be.

That is, the method of the present invention is a method for producing sodium silicofluoride by absorbing fluorine-containing exhaust gas into water to form an aqueous solution of hydrosilicofluoric acid, and then reacting the hydrofluoric acid with sodium chloride. , the concentration of the hydrosilicofluoric acid aqueous solution is 10 to 30.
When using seawater as a sodium chloride source, the sodium chloride in the seawater is 1.5% by weight per mol of hydrosilicofluoric acid.
This is a method for producing sodium silicofluoride from fluorine-containing exhaust gas, characterized in that the amount is used in an amount of 2.5 mol to 2.5 mol.

The reaction in this method is shown by a reaction formula. After converting the fluorine content in the fluorine-containing exhaust gas into an aqueous solution of hydrosilicofluoric acid using equation (1), the aqueous solution of hydrosilicic acid and sodium chloride in seawater are combined. By reacting according to the formula (2), highly pure, coarse-grained sodium silicofluoride is produced.

2HF + 5IF4 →H2SiF6 ・・・・
... (1) H2SiF6+2NaCl-+Na2Si
F6+2HCl... (2) Fluorine contained in fluorine-containing exhaust gas usually exists in the form of hydrogen fluoride and silicon tetrafluoride, so it can be used as is, but if the hydrogen fluoride component in the exhaust gas is excessive. In the case of , in the process of obtaining an aqueous hydrofluoric acid solution,
For example, an active silica component such as diatomaceous earth may be added to balance the molar ratio by the reaction shown in equation (3).

4HF+S i02→S t F4+ H2O...
(3) Furthermore, the molar ratio of hydrogen fluoride and silicon tetrafluoride (HF:SiF4) does not need to be strictly adjusted to 2:1, but is between 1.8 and 2.2. : It is sufficient if it is about 1, and there is no problem in the quality of the obtained sodium silicofluoride and in the treatment of exhaust gas.

Note that the aqueous solution of hydrosilicofluoric acid obtained by absorbing exhaust gas in water is water-soluble even if it contains impurities such as phosphoric acid and iron, as shown in Table 1. If these impurities are dissolved in the P solution after solid-liquid separation of the sodium silicofluoride and removed, the quality of the obtained sodium silicofluoride is hardly affected.

In the method of the present invention, an aqueous solution of hydrosilicofluoric acid and seawater are brought into contact and reacted, but crystals of sodium silicofluoride precipitate at the same time as the two solutions come into contact. The concentration of the hydrosilicofluoric acid aqueous solution and the molar ratio of sodium chloride that are brought into contact with the sodium silicofluoride have a large influence on the quality and yield of the obtained sodium silicofluoride, as well as on the operability of filtration, drying, etc.

That is, in the method of the present invention, it is preferable to adjust the concentration of the hydrosilicofluoric acid aqueous solution to a range of 10 to 30% by weight,
Furthermore, a range of 20 to 25 weight i% is optimal.

When the concentration of the hydrosilicofluoric acid aqueous solution exceeds 30% by weight, the particle size of the resulting sodium silicofluoride becomes extremely small, making solid-liquid separation extremely difficult, and the amount of mother liquor adhering to the crystals increases. This leads to negative effects such as a decrease in drying capacity due to drying, an increase in the energy required for drying, and an increase in impurities in the product.

Furthermore, if the concentration of the hydrosilicofluoric acid aqueous solution is less than 10% by weight,
In addition to the particle size of the produced sodium silifluoride becoming smaller than when the amount exceeds 30% by weight, the yield of sodium silifluoride decreases, although the exact cause is unknown, and the recovery rate of fluorine content decreases. It is not desirable because it will fall off.

Next, it is desirable that the amount of seawater used in the reaction between the hydrosilicofluoric acid aqueous solution and seawater is such that the molar ratio of sodium chloride to hydrosilicofluoric acid is in the range of 1.5 to 2.5. Among them, 20 to 2.3 is preferable.

If the molar ratio is less than 1.5, the fluorine recovery rate will drop significantly, and if the molar ratio is more than 2.5, the purity of the obtained sodium silifluoride will decrease, resulting in poor value as a product and the loss of the sodium silifluoride. The recovery rate also decreases.

The decrease in the purity of sodium silicofluoride due to this molar ratio addition is considered to be due to the influence of impurities other than sodium chloride contained in seawater.

The seawater used only needs to be free of suspended matter, and there are no other special requirements.

The reaction between hydrofluorosilicic acid and sodium chloride proceeds relatively quickly, so a reaction time of 5 minutes or less is sufficient.
There is no harm caused by prolonging the reaction time, and if the reaction temperature is between room temperature and 70°C, the resulting sodium silicofluoride will have a purity of 9.
At 8.5% or more, high quality particles with a particle size of about 20 to 60 μm can be obtained.

The sodium silicofluoride thus produced can be made into a product by separating solid and liquid by centrifugation, filtration, etc., and then drying it.

On the other hand, the furnace liquid can be easily treated by conventionally known methods in which a small amount of dissolved fluorine is coagulated, neutralized, or precipitated with a calcium compound such as milk of lime.

Conventional methods for treating fluorine-containing exhaust gas often use useful raw materials such as milk of lime and caustic soda to turn the fluorine content into industrial waste with no use value, but the method of the present invention , it is possible to obtain high-quality sodium silicofluoride by simply reacting it with seawater without using the above-mentioned raw materials, which also serves as a treatment for fluorine-containing exhaust gas, and its industrial usefulness is It is extremely large.

Hereinafter, the present invention will be specifically explained with reference to Examples and Comparative Examples.

In addition, in Examples and Comparative Examples, % and linen are based on weight, respectively.

Example 1 100 kg of a hydrosilicofluoric acid aqueous solution having the composition shown in Table 1 obtained by absorbing exhaust gas during cryolite production into water and clear seawater (chlorinated After adding 8,701 ml of sodium (concentration: 21.1 g/l) and reacting at a temperature of 50°C, it was filtered with a rotary vacuum filter and dried to obtain sodium fluorosilicide with a purity of 99.6% and a particle size of 30 to 60μ. 20.4 kg of was obtained.

Example 2 Add 1 part of water to 100 kg of the same hydrosilicofluoric acid aqueous solution as in Example 1.
The same operation as in Example 1 was performed except that 00 kg was added and diluted.

The obtained sodium silicofluoride has a purity of 99.5% and a particle size of 30 to 6.
The yield was 18.1 kg.

Comparative Example 1 The same operation as in Example 1 was carried out except that the effective volume of the reaction tank was changed to 18001 and the seawater volume was changed to 14004.15.
Although 1 kg of sodium silicofluoride was obtained, the purity deteriorated to 98.1% and the JIS standard value (98.5% or more) could not be maintained, the particle size decreased to 20 to 40μ, and the yield also decreased. did.

Comparative Example 2 A solution obtained by absorbing fluorine-containing exhaust gas generated in the wet phosphoric acid concentration process in water was analyzed and found that the fluorine content (as F) was 16.7% and the silicon content (as SiO□) was 5.1%. there were.

Using 100kg of this liquid, clear seawater 1000G sodium chloride content 21. :l/l) and reacted at a temperature of 40℃, filtered, and dried to obtain a mixture with a purity of 98.1% and a particle size of 20-40μ.
9.8 kg of sodium silicofluoride was obtained.

Example 3 Diatomaceous earth was added as activated silica to the same fluorine-containing exhaust gas absorption liquid as in Comparative Example 2, and analysis was performed again to find F16.6%.
, S t 028.5%.

Using this liquid, reaction, filtration, and drying were performed under the same conditions as in Comparative Example 2 to obtain 21.2 kg of sodium silicofluoride with a purity of 99.5% and a particle size of 30 to 60 μm.

Comparative Example 3 Add 2 ml of water to 100 kg of the same hydrosilicic acid aqueous solution as in Example 1.
The same operation as in Example 1 was performed except that 50 kg was added and diluted.

The obtained sodium silicofluoride had a purity of 98.0% and a particle size of 10 to 2.
The yield was 14.9 kg.

Comparative Example 4 The same aqueous hydrosilicofluoric acid solution as in Example 1 was concentrated to obtain a liquid having the composition shown in Table 2. 50 kg of this liquid and 870 kg of clear seawater (sodium chloride concentration 21.1 g/l) were added. As in Example 1, the reaction, filtration, and drying were carried out, and the purity of the sodium silicofluoride obtained was 97.8%, the particle size was 5 to 10 μm, and the yield was 20.1 kg.

Example 4 The same operation as in Comparative Example 4 was performed except that 50 kg of the liquid shown in Table 2 was diluted by adding 35 ky of water.

The obtained sodium silicofluoride has a purity of 99.4% and a particle size of 30 to 6.
0, and the yield was 20.4 kg.

Claims (1)

[Claims] 1. In a method for producing sodium silicofluoride by absorbing fluorine-containing exhaust gas into water to form an aqueous solution of hydrosilicofluoric acid and then reacting the hydrofluoric acid with sodium chloride, the method comprises: The concentration of sodium chloride is 10 to 30% by weight, and seawater is used as a sodium chloride source in an amount such that sodium chloride in the seawater is 1.5 to 2.5 mol per 1 mol of hydrosilicofluoric acid. A method for producing sodium silicofluoride from fluorine-containing exhaust gas, characterized in that: