Fractional crystallization method and process for extracting high purity sodium sulfate and sodium chloride from high salt wastewater for resource utilization

High-salt wastewater comes from a wide range of sources, mainly concentrated in coal, electric power, oil refining, chemical, metallurgical, paper-making, pesticide and other industries. With the development of modern industrial technology, the volume of high-salt wastewater is growing rapidly, which brings great challenges to the current wastewater treatment and recycling technology. High-salt wastewater contains a variety of substances, including organic matter, inorganic salts, oil, organic heavy metals and radioactive substances, etc., due to the high salt content leads to the difficulty of survival of bacterial strains, so it is difficult to biochemical treatment, and therefore is internationally recognized as one of the difficult to deal with wastewater, and the environment compared with the ordinary wastewater has a greater degree of contamination.

A method for recycling and treating high salt wastewater containing sodium sulfate and sodium chloride, by which the recycling of sodium sulfate and sodium chloride in high salt wastewater can be realized at low cost and high efficiency, the environmental protection pressure of enterprises such as coal chemical industry can be alleviated, the cost of treating high salt wastewater can be reduced, and the comprehensive utilization of wastewater resources can be realized.

The technical solutions used to solve the above technical problems are:

The use of activated carbon for high salt wastewater decolorization pretreatment, has been sent to electrodialysis and mechanical vapor recompression (mvr) device for concentration, using the method of cooling crystallization to recover sodium sulfate in the concentrate, denitrification mother liquor sent to the evaporation crystallizer, the use of two-stage evaporation and crystallization method to recover the denitrification mother liquor concentrate of sodium chloride, the two-stage evaporation of the part of the mother liquor after the salt return to the system and the feeding wastewater mixing recycling After two-stage evaporation and salt extraction, part of the mother liquor returns to the system and mixes with the feed wastewater for recycling, and the rest goes to the evaporation crystallizer for miscellaneous salt.

The method includes the following steps:

(1) Activated carbon is used for decolorization pretreatment to remove insoluble impurities, ca2+, mg2+, and soluble substances of silicate;

(2) After pretreatment, it is successively sent to electrodialysis and mechanical vapor recompression device for concentration, and according to the initial composition of wastewater sodium sulfate and sodium chloride, the content of sodium sulfate and sodium chloride reaches close to saturation through concentration;

(3) Concentrated wastewater is sent to the crystallizer, cooled and crystallized to obtain manganese, manganese is directly extracted, washed and dried to obtain anhydrous sodium sulfate products;

(4) The concentrate of the denitrification mother liquor is subjected to two-stage evaporation and crystallization, and the sodium chloride product is directly extracted, washed and dried;

(5) After the secondary evaporation of salt extraction, part of the mother liquor returns to the system to mix with the feed wastewater for recycling, and the rest of the mother liquor goes to the miscellaneous salt evaporation crystallizer.

The dosage of activated carbon in the pre-treatment of activated carbon decolorization in step (1) of the described method is 7-9 g/l wastewater;

The cod removal rate in the activated carbon decolorization pretreatment in step (1) of the described method is about 75-85%;

The total salt content of sodium chloride and sodium sulfate of the feed wastewater in step (1) of the method described is in the range of 3-10%, and the ratio of sodium chloride to sodium sulfate mass fraction is in the range of 1-3.2;

The feed wastewater in step (2) of the described method is successively concentrated by electrodialysis to a total salt content of about 18-20%, and then concentrated by a mechanical vapor recompression (mvr) device, and the mass fraction of sodium chloride after the two concentrations ranges from 6.5-20.58%, and the mass fraction of sodium sulfate ranges from 6.0-9.0%; and the mass fraction of sodium sulfate ranges from 6.0-9.0%. -After the two concentrations, the mass fraction of sodium chloride ranges from 6.5 to 20.58% and that of sodium sulfate from 6.0 to 9.0%;

The temperature of cooling crystallization for extracting mannite in step (3) of the said method is controlled in the range of -5-0°C, with -5°C being optimal;

The mass fraction of sodium sulfate in the denitrification mother liquor after cooling and crystallizing the extracted manganese in step (3) of the said method is 0.8-1.0%;

The conditions for extracting sodium chloride by first- and second-stage evaporation and crystallization in step (4) of the said method are a heating temperature of 65-70°C and an absolute pressure of 0.05-0.06 mpa;

The ratio of the product mass flow rate of sodium chloride in the two-stage crystallizer in step (4) of the method described to the mass flow rate of the feed liquid to the crystallizer ranges from 0.9 to 1.1;

In step (4) of the method, the amount of evaporated water in the first-stage evaporation and crystallization to extract sodium chloride accounts for 37%-83% of the crystallizer feed, preferably 37.8%-82.3% (according to the composition of the high-salt wastewater feed);

In step (4) of the method, the amount of water evaporated during the extraction of sodium chloride by secondary evaporation and crystallization accounts for 4.0-48% of the crystallizer feed, preferably 4.0-47.7% (determined according to the composition of the high-salt wastewater feed);

The reuse rate of the desalinated mother liquor after secondary evaporation and crystallization in step (5) of the said method ranges from 38-42%.

The described method has the following beneficial effects: an efficient, low-cost and highly stable method of split crystallization of high-salt wastewater is provided, which can effectively recover sodium sulfate and sodium chloride from high-salt wastewater of coal chemical industry and other industries separately, and the quality of the sodium sulfate obtained reaches the standard of first-class product of the second class as stipulated in the national standard (gb/t6009-2014), the sodium sulfate The purity of sodium sulfate is ≥98.5%, and the sodium chloride product reaches the first grade standard of refined industrial salt as stipulated in the national standard of industrial salt (gb/t5462-2015), the purity of sodium chloride is ≥98.5%, and the recovery rate of sodium chloride and sodium sulfate reaches more than 90%, which realizes the comprehensive utilization of wastewater resources. This method has simple process conditions, high process stability, no wastewater discharge except water evaporation during the whole treatment process, realizing zero-discharge treatment of high-salt wastewater, reducing the cost of wastewater treatment, easing the pressure on enterprises, meeting the needs of environmental protection, and having good economic and social benefits.