Treatment of acid-containing wastewater

Acidic wastewater is the wastewater with pH value less than 6. According to the different types and concentrations of acids, acidic wastewater can be divided into inorganic acid wastewater and organic acid wastewater. Strong acid wastewater and weak acid wastewater; Monoacid wastewater and polyacid wastewater; Low concentration acidic wastewater and high concentration acidic wastewater. Usually acidic wastewater, in addition to containing some acid, often also contains heavy metal ions and their salts and other harmful substances. Acidic wastewater comes from a wide range of sources, including mine drainage, hydrometallurgy, steel rolling, surface acid treatment of steel and non-ferrous metals, chemical industry, acid production, dyes, electrolysis, electroplating, artificial fibers and other industrial sectors. The common acidic wastewater is sulfuric acid wastewater, followed by hydrochloric acid and nitric acid wastewater. Every year, China is about to discharge nearly one million cubic meters of industrial waste acid, if these waste water is discharged directly without treatment, it will corrodes pipelines, damage crops, harm fish, damage ships, and destroy environmental health. Industrial acid wastewater must be treated to meet the national discharge standards before discharge, acid wastewater can be recycled and reused. When treating the waste acid, the methods can be selected include salt treatment, concentration method, chemical neutralization method, extraction method, ion exchange resin method, membrane separation method, etc.

1. salt out recycling

The so-called salting out is to use a large amount of saturated salt water to precipitate almost all organic impurities in the waste acid. However, this method will produce hydrochloric acid and affect the recovery and utilization of sulfuric acid in the waste acid, so the method of salting out the organic impurities in the waste acid with sodium bisulphate saturated solution was studied.
The waste acid contains sulfuric acid and various organic impurities, which are mainly a small amount of 6-chloro-3-nitrotoluene-4 sulfonic acid and various isomers other than 6-chloro-3-nitrotoluene-4-sulfonic acid produced by toluene in the process of sulfonation, chlorination and nitrification. The salting out method is to use a large amount of saturated salt water to precipitate almost all organic impurities in the waste acid. The salt-out recycling method can not only remove various organic impurities in the waste acid, but also recover sulfuric acid to put into the cycle production, saving cost and energy.

2. Roasting method

Roasting method is applied to the volatile acid such as hydrochloric acid, which is separated from the solution by roasting to achieve recovery effect.

3. Chemical neutralization method

The basic acid-base reaction of H+(aq)+OH-(aq)=H2O is also an important basis for the treatment of acid-containing wastewater. The common methods for treating acid-containing wastewater include neutralization and recycling, mutual neutralization of acid-base wastewater, drug neutralization, filtration neutralization, etc. In the early days of some iron and steel enterprises in China, most of them used the method of acid-base neutralization to treat the waste liquid of hydrochloric acid and sulfuric acid pickling, so that the pH value reached the discharge standard. Sodium carbonate (soda ash), sodium hydroxide, limestone or lime as raw materials for acid-base neutralization, the general use is cheap, easy to make lime.

4. Extraction method

Liquid-liquid extraction, also known as solvent extraction, is a unit operation that uses the difference in solubility of the components in the raw material liquid in the appropriate solvent to achieve separation. In the treatment of acid-containing wastewater, it is necessary to make the acid-containing wastewater and the organic solvent fully contact, so that the impurities in the waste acid are transferred to the solvent. The extractant requirements are:(1) for the waste acid is inert, does not chemically react with the waste acid, and does not dissolve in the waste acid; (2) The impurities in the waste acid have a high partition coefficient in the extractant and sulfuric acid; (3) The price is cheap and easy to get; (4) Easy to separate from impurities, small loss when stripping. Common extractants include benzene (toluene, nitrobenzene, chlorobenzene), phenols (creosote crude diphenol), halogenated hydrocarbons (trichloroethane, dichloroethane), isopropyl ether and N-503.

5. ion exchange resin method

The basic principle of treating organic acid waste liquid by ion exchange resin is that some ion exchange resins can absorb organic acids from waste acid solution and exclude inorganic acids and metal salts to achieve the separation of different acids and salts.

6. membrane separation method

For acidic waste liquid, membrane treatment methods such as dialysis and electrodialysis can also be used. Membrane recovery of waste acid mainly adopts the principle of dialysis, which is driven by concentration difference. The whole device is composed of diffusion dialysis membrane, liquid dispensing plate, reinforcing plate, liquid flow plate frame, etc., and achieves separation effect by separating substances in waste liquid.

7. cooling crystallization method

Cooling crystallization method is a method to reduce the temperature of the solution and precipitate the solute. It is used in the waste acid treatment process that the impurities in the waste acid are cooled out to recover the acid solution that meets the requirements and can be reused. For example, the waste sulfuric acid discharged from the acyl-washing process of a rolling mill contains a large amount of ferrous sulfate, which is treated by the process of concentration-crystallization and filtration. After the removal of ferrous sulfate by filtration, the acid can be returned to the steel pickling process for continued use.
Cooling crystallization has many industrial applications, which are illustrated here by the pickling process in metal processing. In the process of steel and mechanical processing, sulfuric acid solution is commonly used to remove the rust on the metal surface. Therefore, the recycling of waste acid can greatly reduce costs and protect the environment. Cooling crystallization is used in industry to achieve this process.

8. Oxidation method

This method has been used for a long time, and the principle is to decompose the organic impurities in the waste sulfuric acid by oxidizing agents under appropriate conditions, so that it can be converted into carbon dioxide, water, nitrogen oxides, etc., and separated from the sulfuric acid, so that the waste sulfuric acid can be purified and recovered. Commonly used oxidants are hydrogen peroxide, nitric acid, perchloric acid, hypochlorous acid, nitrate, ozone and so on. Each oxidizer has its advantages and limitations.