Non-ferrous metal wastewater can be divided into acidic wastewater, alkaline wastewater, heavy metal wastewater, cyanide-containing wastewater, and oil-containing wastewater. Acidic wastewater contains a large amount of sulfuric acid, lead, zinc, copper, arsenic, and iron heavy metal ions. If this wastewater enters the environment without treatment, it will cause a decrease in the pH value of the water body, affecting the survival environment of aquatic plants and animals, and disrupting the ecological balance. Heavy metal ions will accumulate in the bodies of plants and animals, harming human health through the food chain. Some substances in wastewater can be utilized; as long as suitable processes are explored, they are resources. This article takes the acidic wastewater of a certain smelting plant as an example, combines the treatment methods of non-ferrous metallurgical wastewater, and utilizes waste acid resources to produce zinc sulfate products. The application of this process is analyzed and discussed.

The sulfuric acid content in the acidic wastewater of a certain smelting plant is about 12～18% (the actual concentration of the sampled waste acid may fluctuate, and the actual determination shall prevail), the chloride ion content is about 3000 mg/L, and the fluoride ion content is about 1000 mg/L. In addition, there are iron, lead, zinc, copper, cadmium, and arsenic heavy metal ions. Wastewater discharged during a certain period was used as a process test sample, and the analysis results are shown in Table 1.

Table 1 Composition of Acidic Wastewater from a Certain Smelting Plant

2.1 Process Flow

The wastewater from various workshops in the plant is mixed evenly in the acidic wastewater mixing tank and reacts with zinc oxide waste residue to produce zinc sulfate. After the heavy metal and defluorination processes, zinc sulfate is precipitated from the solution according to the different solubilities of various salts in the solution, purified, and dried to obtain a high-purity zinc sulfate product. The process flow is shown in Figure 1.

Figure 1 Metallurgical Wastewater Resource Utilization Process Flow

2.2 Zinc Sulfate Production

Zinc sulfate is produced by reacting zinc oxide waste residue with sulfuric acid in metallurgical acidic wastewater. This is the main reaction of the metallurgical acidic wastewater resource utilization technology, making full use of the main components in wastewater and waste residue to produce zinc sulfate. In this step, the pH value must be strictly controlled. After the reaction, the pH value of the solution is around 1, preparing for the removal of heavy metals from the solution in the next step.

2.3 Heavy Metal Removal

Iron, arsenic, cadmium, copper, and lead are present in metallurgical acidic wastewater, and a small amount of copper oxide is also present in zinc oxide waste residue. Copper oxide reacts with dilute sulfuric acid to produce copper sulfate, which will increase the amount of copper ions in the solution.

Under acidic conditions, sulfide ions in sodium sulfide solution can react with most heavy metal ions to form precipitates. Adding an appropriate amount of sodium sulfide to the solution can remove iron, arsenic, cadmium, lead, and copper. After removing heavy metals, the metal sulfide precipitate must be discharged to prevent it from flowing into subsequent processes. Otherwise, as the pH increases, some sulfides will redissolve, and heavy metal ions will return to the solution.

2.4 Defluorination

After removing heavy metals, zinc oxide powder is added to adjust the pH of the solution to around 6. According to the amount of heavy metal ions in the solution, an appropriate amount of lime milk is added, and the solution is stirred thoroughly. Note that the pH of the solution should not exceed 7. Then, aluminum sulfate solution is added, and after a thorough reaction, the precipitate is discharged.

2.5 Zinc Sulfate Product Production

After removing heavy metals and fluoride ions, there will be a small amount of chloride ions and sodium ions in the solution. Through an evaporation crystallization device, MDE evaporation concentration and crystallization can be used when the factory steam is low, and MVR evaporation crystallization can be used when the factory steam is high and the electricity price is low. The solution is evaporated to a certain concentration to precipitate zinc sulfate. The precipitated zinc sulfate crystals are further purified, and after drying, a high-purity zinc sulfate product can be obtained. The condensed water from the evaporation can be reused.

3.1 Heavy Metal Removal Effect

The effluent from the sulfidation reaction tank was taken for analysis of metal ion content, and the results are shown in Table 2.

Table 2 Metal Ion Analysis of Water Quality After Sulfidation Reaction

After defluorination, as the pH of the solution increases, the amount of iron ions also decreases. The iron content in the solution was determined to be <0.001 mg/L. Comparing Table 2 and Table 1, it can be seen that this process has a good effect on removing heavy metals.

3.2 Defluorination Effect

After defluorination, water quality analysis was performed, and the fluoride ion content was measured to be 1.2 mg/L, indicating a good defluorination effect.

3.3 Zinc Sulfate Purity Analysis

Salt analysis of the zinc sulfate product was performed, and the results are shown in Table 3.

Table 3 Salt Analysis of Zinc Sulfate Product

1. Using non-ferrous metallurgical acidic wastewater and zinc oxide waste residue as raw materials, the cost is low, and this process has high economic value.

2. This process combines traditional heavy metal and defluorination processes, with high removal rates of heavy metals and fluoride, simple operation, mature technology, and readily available raw materials.

3. In the zinc sulfate evaporation crystallization process, the condensed water quality of secondary steam is good and can be reused in the production workshop of the plant or as recycled water.

4. The purity of the produced zinc sulfate product can reach 99.90%, meeting the requirements of Class 1 products in the national standard ("HGT 2326-2005 Industrial Zinc Sulfate"). The zinc sulfate product has excellent quality and high added value.

In recent years, with the rapid development of industrialization in China, the non-ferrous metallurgy industry has also developed rapidly. Not only is there a wide variety of products, but the output also ranks among the top in the world. At the same time, a large amount of wastewater discharged during industrial production has led to environmental pollution. The treatment of wastewater is related to the long-term development of enterprises, and the environmental problems caused by wastewater are related to the prosperity, stability, and even survival of human society. National environmental protection policies are becoming increasingly stringent, and the issues of how the non-ferrous metallurgy industry can develop in the long term and how it can achieve green development are imminent.

This article introduces a resource utilization technology for acidic wastewater in non-ferrous metallurgy. It involves using the factory's own zinc oxide waste residue to react and generate zinc sulfate. Then, through heavy metal and fluoride removal processes, high-purity zinc sulfate is prepared using evaporation crystallization technology. From the actual effect of acidic wastewater treatment at a certain factory, this technology can meet the requirements of acidic wastewater treatment in non-ferrous metallurgy. At the same time, it utilizes waste acid to produce high-purity zinc sulfate products, achieving good economic benefits. This technology has opened up a new path for the treatment of acidic wastewater in non-ferrous metallurgy and has important reference significance for the treatment of acidic wastewater in the same industry.