Resource utilization of pickling wastewater (waste acid)

Acid pickling wastewater (waste acid) refers to a kind of liquid waste produced in the process of steel rolling, metal surface treatment, electronic component manufacturing, etc., which is listed in the national list of hazardous wastes.
According to the different chemical composition, acid pickling wastewater (waste acid) can be divided into three types: hydrochloric acid pickling wastewater (acid), sulfuric acid pickling wastewater (acid) and mixed acid pickling wastewater (acid).

Among them, hydrochloric acid pickling wastewater (acid) contains hydrochloric acid, ferrous chloride, etc.; sulfuric acid pickling wastewater (acid) contains sulfuric acid and ferrous sulfate, etc.; mixed acid pickling wastewater (acid) contains hydrochloric acid, sulfuric acid and corresponding iron salt.

Status of acid pickling wastewater (waste acid) treatment process

1.1 bipolar membrane method

Bipolar membrane method is a kind of membrane separation method, which is suitable for the treatment of acid pickling wastewater (waste acid) with high acid concentration.
Membrane separation is a highly efficient process for the separation of metal ions and acids with high selectivity of membrane to ions, which can fully recover acids and metal salts.
Common membrane technologies include bipolar membrane, microfiltration, ultrafiltration, electrodialysis, diffusion dialysis, ceramic membrane, reverse osmosis and nanofiltration.
Bipolar membrane process is simple, low investment cost, high recovery rate and high purity of recovered acid.
However, compared with other membranes, the operation cost of this method is higher, and because of the membrane material problems, there will be the phenomenon of proton penetrating the anion exchange membrane, thus affecting the concentration of recovered acid and current efficiency.

1.2 crystallization

Crystallization mainly includes concentrated crystallization and freeze crystallization. Salt crystals are analyzed by evaporation of acid group, and by freezing crystallization, the solubility of inorganic salt in waste acid decreases with the decrease of temperature. The solubility of metal salt is reduced by low temperature, and salt crystals are separated to achieve the purpose of acid salt separation.
The advantage of crystallization method is that the treatment process does not need to consume new acid, and the recovered acid can be directly used in the acid consumption section.
The disadvantage is that there are many equipment, large investment and high energy consumption. This method has obvious advantages in both environmental benefits and technical feasibility.

1.3 neutralization method

Neutralization is the most commonly used method to treat pickling wastewater (waste acid). The commonly used neutralizing agents are limestone, soda, sodium hydroxide, fly ash, etc.
The process of lime neutralization + three effect evaporation is adopted. Limestone is used as neutralizing agent to improve the pH value of waste acid, forming the coprecipitation of calcium sulfate and metal hydroxide. The filtrate is evaporated and discharged to the standard.

1.4 spray sintering process

The regeneration of volatile acids such as hydrochloric acid and nitric acid can be realized by roasting.
The roasting method is to evaporate the acid washing wastewater (waste acid) under high temperature, recover the acid through the absorption tower, the metal ions in the waste acid liquor will undergo hydrolysis oxidation reaction under high temperature to generate metal oxide, and enter the powder bin through the transmission pipeline at the furnace bottom, so as to achieve the purpose of separating heavy metals and recovering acid.
The main equipment of roasting process includes roasting reaction furnace, cyclone separator, preconcentrator, liquid drop separator, washing tower, etc. its advantages are high acid regeneration efficiency, high acid concentration of regeneration, and its disadvantages are large facility investment, high operation cost, difficult maintenance, technical difficulty, high energy consumption and secondary pollution.
Taking the treatment of waste acid containing iron as an example, although the regeneration and recovery of acid are realized, the utilization value of acid pickling iron oxide produced is not high.
Spray roasting is widely used in the regeneration process of waste acid in large iron and steel works, but it is not suitable for the treatment of acid pickling wastewater (waste acid).

1.5 ion exchange method

A large number of pickling wastewater (waste acid) will be produced in iron and steel processing enterprises.
The concentration of heavy metal ions in this kind of wastewater is high. As long as the concentration of heavy metal ions can be reduced, the regenerated acid can be returned to the pickling section for recycling.
At present, ion exchange resin is the most commonly used technology for heavy metal wastewater treatment.
The ion exchange resin is mainly composed of monomers, crosslinkers and exchange groups.
Its structure mainly includes polymer framework, ion exchange group and pore.
At present, ion exchange resin is the most commonly used technology for heavy metal wastewater treatment. The ion exchange resin is mainly composed of monomers, crosslinkers and exchange groups.
Its structure mainly includes polymer framework, ion exchange group and pore. Ion exchange resin can be divided into macroporous resin and gel resin according to pass.
The mechanism of macroporous resin is the intermolecular Fan Dehua force, which can adsorb macromolecular organic matter. The gel type resin is a polymer framework, which expands and produces many pores and can absorb inorganic ions.
Ion exchange resin is a renewable material and has good adsorption effect on heavy metal ions. Therefore, gel resin is widely used in heavy metal wastewater treatment.

1.6 chemical conversion

Acid pickling wastewater (waste acid) comes from a wide range of sources. The acid pickling wastewater (waste acid) produced by steel rolling, metal surface treatment and heat treatment industry is mainly hydrochloric acid wastewater and sulfuric acid wastewater.
The impurities in the waste acid are mainly Fe2 +, Fe3 +, and a small amount of zinc, copper and other heavy metal ions.
Direct disposal of such waste acid is not only expensive and difficult, but also can not effectively use the metal resources.
According to the characteristics of the waste inorganic acid, it can not only reduce the difficulty of waste acid treatment, but also bring some economic benefits.
Flocculant is a kind of high polymer inorganic flocculant which appeared in 1980s, mainly including aluminum flocculant and iron flocculant.
The most commonly used aluminum flocculants in water treatment are polyaluminium chloride and polyaluminium sulfate.
Polyaluminium chloride is widely used in water treatment because of its fast hydrolysis speed, high density of flocs, wide range of pH value, large specific surface area, strong adsorption capacity and low treatment cost.
The raw materials for the preparation of polyaluminium chloride mainly include aluminum ash, coal gangue, bauxite, kaolin and calcium aluminate powder.
The preparation methods include thermal decomposition, electrolysis, membrane reactor and acid dissolution.
Acid dissolution is the most commonly used method for the production of polyaluminium chloride at present. Using bauxite as raw material, the synthesis of polyaluminium chloride by acid dissolution method is mainly divided into three steps: leaching bauxite, adjusting the basicity, removing insoluble impurities and heavy metals.

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