Resource utilization refers to the secondary processing of waste salt using integrated processes, and the by-product salt obtained is further processed and used as industrial raw materials and auxiliaries for recycling.

In China, waste salt treatment is more inclined towards resource utilization. Relevant technologies mainly include dry thermal treatment, washing and separation, metathesis, and advanced oxidation. Among them, thermal treatment has the highest degree of industrialization. The advantages and disadvantages of each method are shown in the table below.

In summary, due to different production processes in different industries, the properties of waste salt produced vary in terms of purity, chroma, organic matter content, etc., and require differentiated treatment.

1. Pretreatment: Pretreat the waste salt, such as crushing large pieces.

2. Incineration or pyrolysis: Incinerate or pyrolyze the pretreated waste salt to decompose organic matter into harmless substances and recover the heat energy.

3. Dissolution: Dissolve the incinerated or pyrolyzed waste salt in water to form a brine solution.

4. Purification: Purify the brine solution to remove impurities and harmful substances, such as suspended solids and heavy metals.

5. Concentration: Concentrate the purified brine solution to increase the salt concentration.

6. Crystallization: Crystallize the concentrated brine solution to form salt crystals.

7. Drying: Dry the salt crystals to remove moisture and obtain a pure salt product.

The specific treatment process should be determined according to the nature and treatment requirements of the waste salt. The treatment process of waste salt comprehensive utilization projects should comply with relevant laws, regulations, and standards to ensure environmental safety and personnel health.

The main goal of waste salt resource utilization is to obtain pure industrial salt. After reducing the total organic carbon content (TOC), it can enter market circulation. Taking a chemical company in Jiangsu Province signing a waste salt acceptance index with a nearby chlor-alkali plant as an example, the main performance indicators of resource-based products are shown in the table below.

Resource-based products can be reused in the two-alkali, road, dyeing, construction, metallurgy, leather, and other industries. The table below lists the utilization methods of resource-based products. For sodium chloride-type waste salt, due to its large output and simple composition, it can be reused in the chlor-alkali industry and de-icing agents after appropriate treatment. For sodium sulfate-type waste salt, it can be processed and used as soda ash or sodium sulfide.

China's two-alkali industry has the largest salt consumption, with an annual NaCl consumption of over 40 million tons, which is the main outlet for future bulk waste salt resource products. Organic matter and ammonia nitrogen are the main control indicators for resource-based products, but currently, there is a lack of relevant national, local, and industry circulation standards for resource-based products.

Currently, most projects under construction use GB/T 5462—2015 "Industrial Salt" as the product standard, but this standard does not specify the characteristic factors TOC, ammonia nitrogen, and heavy metals discussed above. In addition, since April 2019, standards such as "Sodium Sulfate from Coal Chemical Industry," "Sodium Chloride from Coal Chemical Industry," and "Industrial Salt Standard-Sodium Chloride from Glyphosate By-products" are being developed.

On the other hand, from the perspective of resource management, at present, some projects under construction are designed and constructed according to HJ/T 176—2005 "Technical Specifications for Construction of Hazardous Waste Concentrated Incineration Treatment Projects." The residence time in the high-temperature gas phase zone of the secondary combustion chamber, the heat loss rate of the slag, the oxygen content of the exhaust gas, and the residence time in the rapid cooling zone are all selected according to this standard. Relevant standards and specifications for waste salt organic matter thermal treatment are also urgently needed.

1. Optimize production processes in various industries to reduce waste salt generation at the source

Since the quality of waste salt is generally poor and the organic pollutants it contains pose significant environmental risks, the first consideration should be to "detoxify" chemical waste mother liquor, etc., by further optimizing production processes to improve product recovery rates and increase pretreatment measures to significantly reduce the content of toxic substances in the waste mother liquor, thereby reducing pollutants in waste salt at the source.

2. Improve relevant laws and standards and promote the application of waste salt resource utilization

China still lacks relevant technical specifications for the systematic utilization and disposal of by-product industrial salt, as well as benchmark standards for refined industrial salt products after treatment. At present, it is recommended to accelerate the development of technical specifications for the thermal treatment of by-product industrial salt, specifying equipment requirements, technical routes, pollution control, and control requirements for harmful substances in the salt after treatment. At the same time, conduct environmental risk assessments based on the resource utilization pathways of refined salt after treatment. It is suggested that the product standards for industrial salt after treatment can be jointly formulated with the two-alkali industry, and the limit values of harmful substances in the treated products can be clarified according to the source of by-product industrial salt and the impurities contained.

3. Innovate regulatory measures and mechanisms to promote the professional disposal of industrial salt

To better address the issue of by-product industrial salt, it is recommended that, at the park level, enterprises be encouraged to establish by-product industrial salt resource utilization and disposal centers. These centers would be responsible for the unified harmless treatment and resource utilization of by-product industrial salt, achieving professionalization and scale in its utilization and disposal. Especially in regions with a high concentration of pesticide companies, such as Jiangsu, Shandong, and Zhejiang, centralized deployment should be based on the number and distribution of chemical enterprises, as well as the production and disposal of by-product industrial salt. This will allow for unified planning and centralized processing of by-product industrial salt in the park and surrounding areas.

4. Develop a catalog of advanced technologies and products, and rationally develop new technologies and equipment.

Provide policy support for the resource utilization of by-product industrial salt. Formulate and publish a catalog of advanced technologies and products for the utilization and disposal of by-product industrial salt. Promote the application of pilot projects for the recycling of by-product industrial salt, and accelerate the technological advancement and maturity of resource utilization technologies for by-product industrial salt. Encourage the research and development and demonstration application of technologies that can effectively and cost-effectively remove organic pollutants from waste mother liquor, and develop efficient and low-energy-consuming equipment for the treatment of organic matter in by-product industrial salt.

It is believed that in the near future, with the continuous investment in industrial waste salt resource utilization projects, existing technical problems will be continuously improved and perfected, and the future industrial waste salt disposal industry will gradually form mature and stable resource utilization technologies.

(Source: Environmental Protection Knowledge Resource Library)