CN112429797A - Method for separating and purifying sodium nitrate, sodium chloride and sodium sulfate in industrial mixed salt wastewater - Google Patents

Abstract

The invention discloses a method for separating and purifying sodium nitrate, sodium chloride and sodium sulfate in industrial mixed salt wastewater, which comprises the following steps: a. a preheating stage; b. anhydrous sodium sulfate evaporation crystallization stage; c. a mixed salt returning stage; d. anhydrous sodium chloride evaporation crystallization stage; e. crystallizing sodium nitrate; d, immediately cooling the evaporation mother liquor obtained in the step d, and crystallizing to 20 ℃ or below to separate out sodium nitrate crystals; and d, simultaneously mixing the cooling mother liquor with the freezing mother liquor obtained in the step c, and then entering the step d, thereby reducing the accumulation of sodium sulfate and discharging waste liquid. The method can control the primary evaporation rate to be 72 percent, the freezing temperature to be-5 ℃, the secondary evaporation rate to be 77 percent, the purity to be 97.7 percent of sodium sulfate, 96.3 percent of sodium chloride and 98.9 percent of sodium nitrate, and the product reaches the industrial standard, thereby solving the problem of hazardous waste of miscellaneous salt and eliminating the problem of secondary pollution.

Description

Method for separating and purifying sodium nitrate, sodium chloride and sodium sulfate in industrial mixed salt wastewater

Technical Field

The invention relates to a chemical purification process, in particular to a method for separating and purifying sodium nitrate, sodium chloride and sodium sulfate in industrial mixed salt wastewater.

Background

In recent years, with the rapid development of coal chemical industry, the problems of water resources and water environment are increasingly prominent. The traditional high-concentration brine treatment process unit generates crystallized miscellaneous salts which cannot be recycled, and the crystallized miscellaneous salts mainly comprise sodium chloride, sodium sulfate and a small amount of sodium nitrate. Environmental entry conditions (trial) for modern coal chemical engineering construction projects issued by the ministry of environmental protection qualitatively identify them as hazardous wastes. However, the cost of hazardous waste treatment is high (about 3000 yuan/t), common enterprises are difficult to bear, and the evaporation rate of sodium nitrate, sodium chloride and sodium sulfate in the industrial mixed salt wastewater for separation and purification is low at present, so that the purity of the sodium sulfate, sodium chloride and sodium nitrate obtained by separation and purification is not high, and the problems of miscellaneous salt hazardous waste and secondary environmental pollution elimination cannot be fundamentally solved.

Disclosure of Invention

The method aims to solve the technical problems that the purity of sodium sulfate, sodium chloride and sodium nitrate obtained by separation and purification is not high and the environmental pollution caused by miscellaneous salts cannot be fundamentally solved due to the low evaporation rate of the sodium nitrate, the sodium chloride and the sodium sulfate in the industrial mixed salt wastewater in the prior separation and purification.

According to an aspect of the present invention, in order to achieve the above technical effects, the present application provides the following technical solutions: a method for separating and purifying sodium nitrate, sodium chloride and sodium sulfate in industrial mixed salt wastewater is designed, and comprises the following steps:

a. a preheating stage: preheating the salt-mixed wastewater to 100 ℃ through noncondensable gas, condensed water and mother liquor to ensure that the evaporation temperature is reached;

b. anhydrous sodium sulfate evaporation crystallization stage: introducing the preheated mixed salt wastewater into an anhydrous sodium sulfate evaporation crystallizer in a sodium sulfate crystallization area for evaporation, controlling the evaporation rate to the margin of the mixed salt area, and separating out anhydrous sodium sulfate;

c. and (3) salt mixing and returning stage: immediately cooling and freezing the high-temperature mother liquor obtained in the step b to-5 ℃, separating out mixed salt of sodium sulfate and sodium chloride dihydrate, and returning the mixed salt into the mixed salt wastewater for mixing so as to stabilize the dynamic concentration of the mixed salt wastewater;

d. anhydrous sodium chloride evaporation crystallization stage: continuously heating the frozen mother liquor to 100 ℃, introducing the frozen mother liquor into an anhydrous sodium chloride evaporation crystallizer in a sodium chloride crystallization area for evaporation, controlling the evaporation rate to the margin of a mixed salt area, and separating out anhydrous sodium chloride crystals;

e. crystallization stage of sodium nitrate: d, immediately cooling the evaporation mother liquor obtained in the step d, and crystallizing to 20 ℃ or below to separate out sodium nitrate crystals; and d, simultaneously mixing the cooling mother liquor with the freezing mother liquor obtained in the step c, and then entering the step d, thereby reducing the accumulation of sodium sulfate and discharging waste liquid.

According to the solubility curves of sodium sulfate, sodium chloride and sodium nitrate and a phase diagram of a Na +// Cl-, SO 42-H2O ternary water salt system, a salt separation crystallization process scheme is formulated, and the influence of parameters such as primary evaporation rate, freezing temperature and secondary evaporation rate on the separation effect is examined. The experimental results show that: the primary evaporation rate was controlled to 68%, the cooling crystallization temperature was controlled to <20 ℃, the secondary evaporation rate was controlled to 70%, the purity was 96.7% sodium sulfate, 94.3% sodium chloride and > 98% sodium nitrate. The product reaches the industrial standard, solves the problem of hazardous waste of miscellaneous salt, eliminates secondary pollution and truly realizes the resource utilization of salt. Aiming at the separation of sodium sulfate and sodium chloride, the separated sodium chloride and sodium sulfate are prepared into industrial salt or other uses, and the zero emission and resource utilization of high-salinity wastewater are realized.

Drawings

FIG. 1 is a process flow diagram of the method for separating and purifying sodium nitrate, sodium chloride and sodium sulfate in industrial mixed salt wastewater.

Detailed Description

The present invention will be described in further detail with reference to examples. The application discloses a method for separating and purifying sodium nitrate, sodium chloride and sodium sulfate in industrial mixed salt wastewater, which comprises the following steps: a. a preheating stage: preheating the salt-mixed wastewater to 100 ℃ through noncondensable gas, condensed water and mother liquor to ensure that the evaporation temperature is reached; b. anhydrous sodium sulfate evaporation crystallization stage: introducing the preheated mixed salt wastewater into an anhydrous sodium sulfate evaporation crystallizer in a sodium sulfate crystallization area for evaporation, controlling the evaporation rate to the margin of the mixed salt area, and separating out anhydrous sodium sulfate; c. and (3) salt mixing and returning stage: immediately cooling and freezing the high-temperature mother liquor obtained in the step b to-5 ℃, separating out mixed salt of sodium sulfate and sodium chloride dihydrate, and returning the mixed salt into the mixed salt wastewater for mixing so as to stabilize the dynamic concentration of the mixed salt wastewater; d. anhydrous sodium chloride evaporation crystallization stage: continuously heating the frozen mother liquor to 100 ℃, introducing the frozen mother liquor into an anhydrous sodium chloride evaporation crystallizer in a sodium chloride crystallization area for evaporation, controlling the evaporation rate to the margin of a mixed salt area, and separating out anhydrous sodium chloride crystals; e. crystallization stage of sodium nitrate: d, immediately cooling the evaporation mother liquor obtained in the step d, and crystallizing to 20 ℃ or below to separate out sodium nitrate crystals; and d, simultaneously mixing the cooling mother liquor with the freezing mother liquor obtained in the step c, and then entering the step d, thereby reducing the accumulation of sodium sulfate and discharging waste liquid.

The main experimental data for the separation of sodium sulfate, sodium chloride and sodium nitrate are given in the following table:

according to the solubility curves of sodium sulfate, sodium chloride and sodium nitrate and a phase diagram of a Na +// Cl-, SO 42-H2O ternary water salt system, a salt separation crystallization process scheme is formulated, and the influence of parameters such as primary evaporation rate, freezing temperature and secondary evaporation rate on the separation effect is examined. The experimental results show that: the primary evaporation rate was controlled to 68%, the cooling crystallization temperature was controlled to <20 ℃, the secondary evaporation rate was controlled to 70%, the purity was 96.7% sodium sulfate, 94.3% sodium chloride and > 98% sodium nitrate. The product reaches the industrial standard, solves the problem of hazardous waste of miscellaneous salt, eliminates secondary pollution and truly realizes the resource utilization of salt. Aiming at the separation of sodium sulfate and sodium chloride, the separated sodium chloride and sodium sulfate are prepared into industrial salt or other uses, and the zero emission and resource utilization of high-salinity wastewater are realized.

What has been described above are merely some embodiments of the present invention. It will be apparent to those skilled in the art that various changes and modifications can be made without departing from the inventive concept thereof, and these changes and modifications can be made without departing from the spirit and scope of the invention.

Claims (1)

1. A method for separating and purifying sodium nitrate, sodium chloride and sodium sulfate in industrial mixed salt wastewater comprises the following steps:

the method is characterized by comprising the following steps:

a. a preheating stage: preheating the salt-mixed wastewater to 100 ℃ through noncondensable gas, condensed water and mother liquor to ensure that the evaporation temperature is reached;

b. anhydrous sodium sulfate evaporation crystallization stage: introducing the preheated mixed salt wastewater into an anhydrous sodium sulfate evaporation crystallizer in a sodium sulfate crystallization area for evaporation, controlling the evaporation rate to the margin of the mixed salt area, and separating out anhydrous sodium sulfate;

c. and (3) salt mixing and returning stage: immediately cooling and freezing the high-temperature mother liquor obtained in the step b to-5 ℃, separating out mixed salt of sodium sulfate and sodium chloride dihydrate, and returning the mixed salt into the mixed salt wastewater for mixing so as to stabilize the dynamic concentration of the mixed salt wastewater;

d. anhydrous sodium chloride evaporation crystallization stage: continuously heating the frozen mother liquor to 100 ℃, introducing the frozen mother liquor into an anhydrous sodium chloride evaporation crystallizer in a sodium chloride crystallization area for evaporation, controlling the evaporation rate to the margin of a mixed salt area, and separating out anhydrous sodium chloride crystals;

e. crystallization stage of sodium nitrate: d, immediately cooling the evaporation mother liquor obtained in the step d, and crystallizing to 20 ℃ or below to separate out sodium nitrate crystals; and d, simultaneously mixing the cooling mother liquor with the freezing mother liquor obtained in the step c, and then entering the step d, thereby reducing the accumulation of sodium sulfate and discharging waste liquid.

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