RU2495825C1 - Method of purifying sodium chloride - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention can be used in chemical industry. The method of purifying sodium chloride involves purifying aqueous sodium chloride solution, which is saturated at 25°C, from mechanical impurities, evaporating the solution, crystallisation, subsequent separation of sodium chloride crystals by centrifuging and drying the end product. The saturated sodium chloride solution is treated while stirring with hydrochloric acid solution to concentration of 1.0-1.5 wt % of the total weight of sodium chloride, followed by evaporation while boiling 2.0-2.5 times. The crystals are cooled and separated by centrifuging. The separated crystalline sodium chloride is washed with highly pure water and dried at 100-110°C.

EFFECT: invention enables to obtain highly pure sodium chloride with impurity content of 10^-5-10^-6 wt %.

2 cl, 4 ex

Description

The invention relates to methods for the purification of sodium chloride and can be used to obtain a high purity product used in modern fields of science and technology.

In recent decades, the main areas of application of sodium chloride remain medicine and food production, construction and the chemical industry. However, the requirements for the quality of this product in different fields of its application are different. One of the main requirements in any of these areas is the purity of the product, although the degree of purity and limitation of the content of specific impurities in different applications are different. The purity of sodium chloride as a final product directly depends on the feedstock and on the method of its production, or on the selected method of purification of the product.

One of the known and widely described previously methods for the purification of sodium chloride is the method of evaporation of various saline solutions. For example, to obtain table salt for the food industry, the evaporation method is used (RU 2075440, C01D 3/06, 1997). The salt obtained by this method contains impurities of calcium - 0.02 wt.%, Magnesium - 0.01 wt.%, Potassium - 0.02 wt.% And sulfate ions - 0.16 wt.%, Which meets the requirements presented to Extra table salt for the food industry, but does not meet the product requirements for modern branches of science and technology (single crystals, optics, etc.).

Another method that is widely used for the purification of sodium chloride is the ion-exchange purification method, carried out according to known inventions using chelating resins, for example ion exchangers Duocellite ES-467 and Amberlite IRC-718 (US 4119508, C01D 3/16, 1985; US 5578218, C02F 1/42). Using this method, it is possible to obtain sodium chloride with the following content of limited impurities (wt.%): Aluminum - (1.1-7.4) · 10 ^-3 , silicon - (7.7-8.7) · 10 ^-4 .

- 0,0035; K⁺ - 0,0065; Fe₂O₃ - 0,0004; H₂O - 0,1 и получаются кристаллы хлорида натрия размером 1,2-0,2 мм. Однако, как видно из описания данного способа, он применим для получения поваренной пищевой соли, однако не соответствует более строгим по чистоте требованиям для современных отраслей науки и техники (монокристаллы, оптика и др.). Поскольку по своей технической сущности данный способ наиболее близок предлагаемому способу, он выбран в качестве прототипа.Another method used to isolate and purify sodium chloride (sodium chloride) is the crystallization method. As you know, crude solutions of salt obtained from various raw materials contain various impurities. For example, a solution of table salt obtained from gallite raw materials or rock salt contains impurities of calcium, magnesium, potassium, sulfate anions, iron oxide, which, according to the Russian patent (RU 2056355, C01D 3/04, 1996), are separated from the main product by a multi-stage by a method including crystallization. The method of producing sodium chloride (sodium chloride) according to this known invention involves dissolving the feedstock in circulating liquor, separating insoluble impurities from the obtained hot saturated solution, cooling the clarified sodium chloride solution in the presence of sodium chloride crystals in a multi-stage vacuum crystallization unit, thickening and filtering in centrifuges the resulting suspension and the selection of the target product. In this way, sodium chloride is obtained with the content of the main product (NaCl) at the level of 94.8-99.95 wt.%. The optimal result in the known method is achieved using the purest raw materials — rock salt containing 94.8 wt.% Sodium chloride. In this case, the content of impurities is (wt.%): Ca ²⁺ - 0.0055; Mg ²⁺ - 0.002; $${\text{SO}}_{\text{four}}^{\text{2}-}$$

- 0.0035; K ⁺ - 0.0065; Fe ₂ O ₃ - 0.0004; H ₂ O - 0.1 and get crystals of sodium chloride with a size of 1.2-0.2 mm However, as can be seen from the description of this method, it is applicable for the production of table salt, however, it does not meet the more stringent purity requirements for modern branches of science and technology (single crystals, optics, etc.). Since in its technical essence this method is closest to the proposed method, it is selected as a prototype.

To obtain a high-purity product, the quality of which meets the requirements for high-purity chemical products, a purification method is proposed in which the initial sodium chloride solution saturated at 25 ° C, purified from mechanical impurities, is treated with stirring with a solution of hydrochloric acid to a concentration of 1.0 -1.5% of the total weight of the sodium chloride solution, after which the treated solution is evaporated 2.0-2.5 times, cooled and the precipitated sodium chloride crystals are separated by centrifugation, then washed of high purity water and dried at a temperature of 100-110 ° C.

Centrifugation is carried out at a speed of 500-2000 rpm.

The proposed method has stages similar to the prototype of the process: preliminary purification from mechanical impurities, crystallization from a saturated solution of sodium chloride, the selection of the target product by centrifugation and drying. Purification from mechanical impurities can be carried out by any known methods of separating mechanical impurities, for example, by filtration. But crystallization in the prototype is carried out on a multistage vacuum crystallization unit, and in the proposed method, in a reactor made of glass, equipped with a stirrer and electric heating, which greatly simplifies the process.

A significant difference is the introduction of an additional stage, namely, the stage of preliminary processing of sodium chloride solution with hydrochloric acid to a concentration of 1.0-1.5% of the total weight of the sodium chloride solution. This treatment allows one to reduce the adsorption of impurities (Fe, Ni, Co, Cu, Pb, Ca, K, As, etc.) on sodium chloride crystals. The quality of the resulting process is affected by the selected amount of hydrochloric acid, namely 1.0-1.5 wt.%. As additional experimental studies show, an overestimation of the amount of hydrochloric acid does not increase the purity of sodium chloride, and underestimation does not lead to a deterioration in the quality of the finished product.

Pre-treatment of the initial solution of reactive quality, containing impurity ions at the level of 0.001 wt.%, With hydrochloric acid in combination with other processing stages (filtration, crystallization) ensures its deep cleaning to obtain high-purity sodium chloride.

After treatment with a solution of hydrochloric acid, the sodium chloride solution is evaporated 2.0-2.5 times, and then cooled to 20-25 ° C with constant stirring. Evaporation of the solution by less than 2.0 times leads to a significant decrease in the yield of the product, and evaporation of the solution by more than 2.5 times leads to a decrease in the volume of the mother liquor and, consequently, to a deterioration in the quality of the obtained product.

After the evaporation and centrifugation steps, the precipitated sodium chloride precipitate is washed with highly pure water. The category of high purity products usually refers to chemical compounds containing limited impurities at the level of 10 ^-5 -10 ^-6 mass% or less. In the inventive method, high-purity water is used as a washing liquid, for example, a domestic product is high-purity water of the grade of special grade 27-5 or distilled water produced in accordance with GOST 6709. This stage of the process is carried out to avoid contamination of the finished product.

Known methods can be used to separate crystals of sodium chloride from the mother liquor. Optimal, as shown by experimental studies, in this case is centrifugation, which allows the most complete separation of the mother liquor from the crystals. The centrifugation step in the method under consideration can be carried out using a wide range of centrifuge rotation speeds (from 500 to 2000 rpm). The magnitude of the rotation speed used in the process depends on the choice of one type or another of centrifuges, which can be the centrifuges of the OPN-3M, SM-6MT brand.

The drying stage of the sodium chloride crystals is carried out at a temperature of 100-110 ° C, which ensures complete removal of moisture. Known methods are acceptable for carrying out the drying process, for example drying in a shelf oven, in a fluidized bed dryer.

The proposed method is illustrated by the examples below.

- 0,001.Example 1. The initial 25% solution of sodium chloride (500 ml), purified by filtration from mechanical impurities, is poured into a reactor made of glass, equipped with a stirrer and electric heating. 6.0 ml of a concentrated (70%) solution of hydrochloric acid, which is 1.0% of the total mass of the solution, is added to the reactor, stirred, after which the solution is evaporated by boiling (106 ° C) 2.5 times, cooled and the precipitated crystals are separated by centrifugation at a centrifugation speed of 500 rpm, the pressed crystals are washed with water 27-5, squeezed and dried in an oven at a temperature of 100 ° C. The resulting product contains impurities (wt.%): Fe - 0.000005; Ca -0.000001; Mg - 0.000001; Pb - 0.00001; Co - 0.00000035; Cu - 0.000001; As - 0.00001; K is 0.001; Ni - 0.000001; $${\text{SO}}_{\text{four}}^{\text{2}-}$$

- 0.001.

- 0,001.Example 2. Carried out analogously to example 1, only in the original 25% solution of sodium chloride (500 ml) contribute 8.4 ml of a concentrated (70%) solution of hydrochloric acid, which is 1.5% of the total weight of the solution, mix , then the solution is evaporated at the boil (106 ° C) 2.0 times. Then it is cooled and the precipitated crystals are separated by centrifugation at a centrifugation speed of 2000 rpm, the pressed crystals are washed with water, especially 27-5, squeezed and dried at a temperature of 110 ° C. The resulting product contains impurities (wt.%): Fe - 0.000005; Ca - 0.000001; Mg - 0.000001; Pb - 0.00001; Co - 0.00000001; Cu - 0.000001; As - 0.00001; K 0.00093; Ni - 0.000001; $${\text{SO}}_{\text{four}}^{\text{2}-}$$

- 0.001.

-0,003.Example 3. Carried out analogously to example 2, only the prepared solution is evaporated at boiling (106 ° C) 2.8 times, the precipitated crystals are separated by centrifugation at a centrifugation speed of 2000 rpm, the pressed crystals are washed with water 27-5, squeezed and dried in an oven at a temperature of 110 ° C. The resulting product contains impurities (wt.%): Fe - 0.00001; Ca - 0.00001; Mg - 0.000005; Pb - 0.00005; Co - 0.000001; Cu - 0.00001; As - 0.00005; K is 0.001; Ni-0.000005; $${\text{SO}}_{\text{four}}^{\text{2}-}$$

-0.003.

-0,005.Example 4. Carried out analogously to example 3, only the wrung out crystals after crystallization are washed with distilled water, re-wrung out and dried at a temperature of 110 ° C. The resulting product contains impurities (wt.%): Fe - 0.00005; Ca - 0.00005; Mg - 0.00001; Pb -0.00005; Co - 0.000005; Cu - 0.00005; As - 0.00005; K is 0.003; Ni - 0.000005; $${\text{SO}}_{\text{four}}^{\text{2}-}$$

-0.005.

Claims (2)

1. The method of purification of sodium chloride, including cleaning saturated at 25 ° C aqueous solution of sodium chloride from solids, evaporation of the solution, crystallization, subsequent separation of the crystals of sodium chloride by centrifugation and drying of the target product, characterized in that the pre-saturated solution of sodium chloride is treated with stirring hydrochloric acid solution to a concentration of 1.0-1.5 wt.% of the total weight of the sodium chloride solution, after which it is evaporated 2.0-2.5 times at boiling, the precipitated crystals are cooled, the prices are separated by trifugation and the separated crystalline sodium chloride is washed with high purity water and dried at 100-110 ° C. 2. The method according to claim 1, characterized in that the precipitated crystalline sodium chloride is separated by centrifugation, carried out with a stirring speed equal to 500-2000 rpm.