The electroplating wastewater in the electroplating centralized control park is one of the difficult types to treat in various industries in China. The fundamental reason is that the characteristic pollutants and related pollutant indicators in the electroplating wastewater in the park are relatively complex, with high pollutant concentrations and harmful substance content. In view of this, this article focuses on exploring analytical chemical precipitation methods and analyzing and exploring them based on actual project operations.

Source of electroplating wastewater

The main sources of electroplating wastewater in the electroplating control area include the following methods:

(1) Pre treated oily wastewater comes from various oil substances coated on the surfaces of metal plating parts, automotive parts plating parts, plumbing plating parts, iron parts, etc. Pre treated oily wastewater accounts for about 30% of electroplating wastewater;

(2) Plating cleaning wastewater, including copper containing wastewater, nickel containing wastewater, chromium containing wastewater, zinc containing wastewater, cyanide containing wastewater, pyrophosphate wastewater, etc;

(3) Waste plating solution, also known as electroplating tank solution, is mainly composed of some liquids with high impurities that precipitate at the bottom of the plating tank, as well as inevitable infiltration between the filtering machinery and water pump, which can also cause the production of waste plating solution. Waste plating solution will constitute electroplating wastewater.

Application Principle

1. Principle of Hydroxide Precipitation

Chemical precipitation method mainly refers to the addition of appropriate amounts of caustic soda, calcium oxide and other substances to electroplating wastewater, and the chemical reaction between heavy metal ions in electroplating wastewater and hydroxide ions of alkaline substances under alkaline conditions to form insoluble hydroxides, thereby achieving the purpose of treating electroplating wastewater through this method.

In the actual production process, reasonable addition of calcium oxide, caustic soda, and liquid alkali can effectively improve the pH level of electroplating wastewater, and remove chromium ions, copper ions, and nickel ions from complex electroplating wastewater in the electroplating control area. The operational test results show that the precipitation using chemical precipitation method can remove 98.77% of chromium ions, 99.83% of nickel ions, and 99.88% of copper ions from electroplating wastewater.

2. Principle of sulfide precipitation

Sulfide chemical precipitation method mainly refers to adding an appropriate amount of sulfides, such as sodium sulfide or sodium sulfide, to electroplating wastewater in the presence of certain weak acids or weak bases, to promote the corresponding sulfide chemical reaction of heavy metal ions and produce precipitation substances.

There are two key details to pay attention to during the chemical reaction process: firstly, the particle diameter of sulfide precipitation is usually small. When filtering the precipitation, it is necessary to add sufficient flocculants to the mixed solution of electroplating wastewater to effectively enhance the precipitation and filtration effects;

The second is to strictly control the amount of sulfides when adding them, in order to avoid heavy metal ions re dissolving in electroplating wastewater due to excessive sulfide addition.

3. Principle of Ferrite Precipitation

The ferrite chemical precipitation method mainly refers to adding an appropriate amount of iron salt to electroplating wastewater, while adjusting the pH value appropriately and controlling the temperature of the electroplating wastewater within a reasonable range.

Under the conditions of ferrite chemical precipitation reaction, heavy metal ions will react chemically with iron salt substances to form oxidized precipitates. After the chemical reaction between the electroplating wastewater mixed solution and the iron salt substance is completed,

The precipitation system can be used to precipitate the obtained mixture, thereby achieving the separation of precipitated sludge and mixed solution, and removing heavy metal ions from various electroplating wastewater mixed solutions, meeting the requirements of relevant specifications and standards.

4. Principle of Barium Salt Precipitation

The barium salt chemical precipitation method is mainly suitable for removing chromium ions from electroplating wastewater.

During precipitation, appropriate amounts of barium salts such as barium sulfide and barium carbonate are usually directly added to chromium containing wastewater. The chromium ions in the chromium containing electroplating wastewater undergo a series of chemical reactions with the barium salt substances, ultimately producing precipitated sludge. Separating the precipitated sludge from the chromium containing electroplating wastewater solution can obtain a relatively pure electroplating wastewater solution.

Residual barium ions can be removed by adding an appropriate amount of calcium sulfate solution to the tail water for reaction.

Operation case analysis

1. Project Overview

The chromium containing wastewater in the regulating tank of the electroplating wastewater treatment plant in the electroplating centralized control area was subjected to corresponding removal and treatment analysis. The concentration of chromium containing wastewater was determined to be 368.4mg/L hexavalent chromium, 647.8mg/L total chromium, and pH value was 2.6.

2. Process flow

The process flow of chemical precipitation method for electroplating wastewater in the high concentration electroplating control area is shown in Figure 1. The chromium containing wastewater is rapidly reduced to trivalent chromium under acidic conditions, and then added to a multi-stage A/O biochemical tank for chemical precipitation to achieve standard discharge and meet all the requirements of the "Electroplating Pollutant Discharge Standards" (21900-2008).

3. Operational Practice Analysis

Taking the chromium containing wastewater mentioned above as the analysis object, the specific process requirements for treating hexavalent chromium in electroplating wastewater are as follows:

(1) Place 30% hydrochloric acid solution, 32% sodium hydroxide solution, and 20% sodium bisulfite solution in corresponding storage tanks, add reducing agent sodium bisulfite for reduction reaction, with a wastewater flow rate of 35m ³/h, and then stir uniformly for 10 minutes to fully reduce hexavalent chromium ions;

(2) Add 32% sodium hydroxide reagent, 15% polyaluminum chloride, and 0.25% polyacrylamide solution to the first stage precipitator for three-stage stirring and thorough reaction for 20 minutes, and precipitate through a tilted plate precipitation device;

(3) Polyaluminum chloride and polyacrylamide solutions were sequentially added to the secondary precipitant for fine control of the precipitation reaction. After three-stage stirring, the precipitation was carried out using an inclined plate precipitation device.

Conclusion

Through the analysis of the actual operation of the electroplating centralized control area sewage treatment plant, the chemical precipitation method has a high treatment effect on most toxic and harmful substances in electroplating wastewater, including heavy metal indicators such as copper, nickel, zinc, chromium, and total phosphorus.

With the in-depth research and analysis of chemical precipitation method, the fine treatment of complex electroplating wastewater in the electroplating centralized control area is carried out, and the treatment effect of various pollutant indicators in electroplating wastewater treatment is more likely to meet the relevant standard requirements.