The composition of electroplating wastewater is very complicated. Except for cyanide (CN-) wastewater and acid-alkali wastewater, heavy metal wastewater is a category of potentially hazardous wastewater for electroplating. According to the classification of heavy metal elements contained in heavy metal wastewater, it can be generally divided into chromium (Cr) wastewater, nickel (Ni) wastewater, cadmium (Cd) wastewater, copper (Cu) wastewater, zinc (Zn) wastewater, Wastewater containing gold (Au), wastewater containing silver (Ag), etc. In general, water is highly acidic, and some are alkaline. The heavy metal acid-base content varies with the surfactant, brightener, and production process.
The content of heavy metals in electroplated plastic water is generally low. The metal plating depends on the processed items and quantity. However, the chromium content in electroplated water is generally high. As for the treatment methods, there are the following methods, which are mainly based on cost and water requirements.
1. Chemical precipitation:
The chemical precipitation method is a method for converting a heavy metal in a dissolved state into a heavy metal compound that is insoluble in water, and includes a neutralization method and a sulfide precipitation method.
2. Neutralization precipitation method:
The alkali is added to the heavy metal-containing wastewater to neutralize the reaction, so that the heavy metal forms a water-insoluble hydroxide, and the precipitate is separated. The neutralization precipitation method is simple in operation and is a commonly used method for treating wastewater. Practice has proved that the following points need to be noted in operation:
(1) After neutralization and precipitation, if the pH value of the wastewater is high, it can be discharged after neutralization treatment;
(2) Many kinds of heavy metals coexist in the wastewater. When the wastewater contains amphoteric metals such as Zn, Pb, Sn, and Al, the pH value is too high, and there may be a tendency to re-dissolve. Therefore, the pH value must be strictly controlled and the staged precipitation should be implemented. ;
(3) Some anions in the wastewater such as: halogen, cyanide, humus, etc. may form complexes with heavy metals, so they need to be pretreated before neutralization;
(4) Some particles are small and difficult to precipitate, you need to add flocculant to aid precipitation.
3. Sulfide precipitation method:
Method for adding sulfide precipitant to precipitate and remove sulfide from heavy metal ions in wastewater. Compared with the neutralization precipitation method, the sulfide precipitation method has the advantages that the solubility of the heavy metal sulfide is lower than that of its hydroxide, and the pH value of the reaction is between 7-9, and the treated wastewater is generally not neutralized. The disadvantage of the sulfide precipitation method is that the sulfide precipitation particles are small and easy to form colloids; the sulfide precipitation agent itself remains in the water, and encounters acid to generate hydrogen sulfide gas, which causes secondary pollution. In order to prevent the secondary pollution problem, scholars have developed an improved sulfide precipitation method, which is to selectively add sulfide ions and another heavy metal ion to the wastewater to be treated (the equilibrium concentration ratio of sulfide ions of this heavy metal needs to be removed). (The equilibrium concentration of sulfides of heavy metal pollutants is high). Because the added heavy metal sulfide is more soluble than the heavy metal sulfide in the wastewater, the original heavy metal ions in the wastewater are separated before the added heavy metal ions, and the harmful gas hydrogen sulfide generation and sulfide ion residual problems are prevented at the same time. .
4. Chelation precipitation method:
A chelating precipitating agent (such as DTCR) is added to cause chelating precipitation. The method has the advantages of good effluent stability and standard compliance, wide application conditions, no secondary pollution, low sludge moisture content, easy recovery of sludge, simple equipment requirements, and convenient implementation. The disadvantage is the high price.
5. Redox treatment:
Chemical reduction method: Chemical reduction method is one of the earliest applied treatment technologies to treat electroplating wastewater. It has been widely used in China. Its treatment principle is simple, operation is easy to master, and it can withstand the impact of large amounts of water and high concentration wastewater. According to the addition of reducing agents, it can be divided into FeSO4 method, NaHSO3 method, iron filings method, SO2 method and so on.
The chemical reduction method is used to treat Cr-containing wastewater. Lime is generally used for alkalization, but the waste residue is more. When NaOH or Na2CO3 is used, there is less sludge, but the cost of the chemical is high and the treatment cost is large. This is the disadvantage of the chemical reduction method.