Electroplating wastewater treatment process and ion exchange treatment method (1)?

The composition of electroplating wastewater is very complex. Except for cyanide (CN-) wastewater and acid-base wastewater, heavy metal wastewater is a potentially hazardous wastewater category in the electroplating industry. Classified according to the heavy metal elements contained in heavy metal wastewater, it can generally be divided into chromium (Cr) wastewater, nickel (Ni) wastewater, cadmium (Cd) wastewater, copper (Cu) wastewater, zinc (Zn) wastewater, Gold (Au) wastewater, silver (Ag) wastewater, etc.

In general, the water is acidic and has a small amount of alkaline. The content of heavy metals varies with surfactants, brighteners, and production processes. Usually the precious metal plating manufacturers do metal recycling, and the water is also used for water reuse. The general heavy metal content of the plastic plating is relatively low. It is a kind of water. The metal plating depends on the processed items and quantity, but usually the chromium content in the electroplating water Are relatively high
As for the treatment methods, there are the following, mainly according to the cost and water requirements.

Chemical precipitation

The chemical precipitation method is a method for converting heavy metals in a dissolved state into heavy metal compounds that are insoluble in water, including neutralization precipitation and sulfide precipitation methods.

Neutralization precipitation method

Add alkali to the waste water containing heavy metals to carry out the neutralization reaction, so that the heavy metals are separated in the form of precipitation of hydroxides that are insoluble in water. The neutralization sedimentation method is simple to operate and is a commonly used method for wastewater treatment. Practice has proved that the following points should be paid attention to in the operation[1]: (1) After neutralization and precipitation, if the pH value in the wastewater is high, it needs to be neutralized before it can be discharged; (2) There are often multiple heavy metals in the wastewater. When the wastewater contains amphoteric metals such as Zn, Pb, Sn, Al, the pH value is too high, and there may be a tendency to redissolve. Therefore, it is necessary to strictly control the pH value and implement segmented precipitation; (3) There are some anions in the wastewater such as halogen, Cyanide, humus, etc. may form complexes with heavy metals, so pretreatment is required before neutralization; (4) Some particles are small and difficult to precipitate, and flocculants need to be added to assist precipitation.

Sulfide precipitation method

A method of adding sulfide precipitant to precipitate and remove heavy metal ions from wastewater. Compared with the neutralization precipitation method, the advantage of the sulfide precipitation method is that the solubility of 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 generally does not need to be neutralized. The disadvantages of the sulfide precipitation method are [2]: the sulfide precipitate has small particles and is easy to form colloids; the sulfide precipitation agent itself remains in the water and generates hydrogen sulfide gas when it encounters acid, causing secondary pollution. In order to prevent the problem of secondary pollution, British scholars have developed an improved sulfide precipitation method, that is, sulfide ions and another heavy metal ion are selectively added to the waste water to be treated (the sulfide ion balance concentration of the heavy metal is more than that required The removed heavy metal pollutants have a high equilibrium concentration of sulfides). Because the added heavy metal sulfide is more soluble than the heavy metal sulfide in the waste water, the original heavy metal ions in the waste water are separated out before the added heavy metal ions, and at the same time, the harmful gas hydrogen sulfide generation and the residual sulfide ion problems are prevented .
Add chelating precipitation agent (such as DTCR) to make chelating precipitation. The method has the characteristics of stable effluent water up to standard, wide application conditions, no secondary pollution, low sludge moisture content, easy recovery of sludge, simple equipment requirements, and convenient implementation. The disadvantage is that the price is high.

Chemical reduction method

The Cr in the electroplating wastewater mainly exists in the form of Cr6+ ions. Therefore, after adding a reducing agent to the wastewater to reduce Cr6+ to slightly toxic Cr3+, adding lime or NaOH to produce Cr(OH)3 precipitation and separation and removal. The treatment of electroplating wastewater by chemical reduction method is one of the earliest treatment technologies, and it has a wide range of applications in my country. Its treatment principle is simple, the operation is easy to master, and it can withstand the impact of large amounts of water and high-concentration wastewater. According to the difference of adding reducing agent, it can be divided into FeSO4 method, NaHSO3 method, iron filing method, SO2 method and so on.
The chemical reduction method is used to treat Cr-containing wastewater. Lime is generally used for alkalization, but there are more waste residues; NaOH or Na2CO3 is used to reduce sludge, but the chemical cost is high and the treatment cost is high. This is the shortcoming of the chemical reduction method.

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