At present, the chromium plating solution used in production is mainly hexavalent chromium solution. Hexavalent chromium is a substance that is highly harmful to human health. The pollution in chromium plating production mainly comes from hexavalent chromium in rinse water and exhaust gas. The following methods can be used to reduce this pollution.
(1) Change the traditional chromium plating process
1) Reduce the concentration.
For a standard solution with CrO3 at 250 g/L, the content can be reduced to 180–200 g/L without affecting the properties of the chromium layer. Reducing the concentration reduces the chromic acid carried out by parts, eases the burden of rinse water treatment, and also lowers the chromium mist content in exhaust gas. Using a lower chromic acid concentration can also improve the current efficiency of chromium plating.
For general decorative chromium plating processes with a concentration of 300–350 g/L, when a rare earth catalyst is used, the CrO3 concentration can be reduced to 120–200 g/L, thereby reducing the chromium content in discharged water and gas.
2) Lower the bath temperature.
This reduces the evaporation of chromic acid, greatly lowering the chromium mist content in exhaust air and thus reducing air pollution. For example, decorative chromium plating using rare earth additives can lower the process temperature from 40–50 °C to 20–35 °C.
3) Select appropriate catalysts.
Conventional chromium plating has low current efficiency. A common way to increase the deposition rate is to raise the current density, but this intensifies hydrogen evolution and increases the chromium mist in exhaust air.
If composite chromium plating or rare earth additives are used, a chromium layer of the same thickness can be obtained in the same time because the current efficiency is improved, allowing a lower current to be used. Therefore, hydrogen evolution is greatly reduced, thus reducing chromium mist emissions.
(2) Reduce the amount of solution carried out by parts.
Reducing the amount of solution carried out by parts is an effective way to lower rinse water concentration and ease water pollution.
The simplest and most reliable method is to allow parts to drip after being lifted from the solution. Holding for 8–10 seconds can reduce the solution adhering to the parts, and the dripped solution is concentrated. This lightens the burden of subsequent rinsing. Adopting a reasonable racking method allows most or all of the solution carried by parts to drip away.
(3) Adopt recovery and countercurrent rinsing.
Generally, after chromium plating, a five-stage recovery or countercurrent rinsing system is set up.
After five stages of rinsing, the hexavalent chromium content in the fifth-stage water can reach 50–100 mg/L. The closer the rinse tank is to the plating tank, the higher the chromic acid concentration; the farther away, the lower the concentration. During operation, the plating tank needs frequent water replenishment; at this time, the first-stage rinse water with the highest concentration can be added. This both replenishes the bath and recovers a large amount of chromic acid. The other four stages are replenished forward in sequence, with only the last stage needing pure water. When parts are of simple shape, no discharge may be required; even if discharge is needed, the wastewater is low in chromic acid, stable in concentration, and easy to treat.
(4) Recover chromium mist.
Exhaust air contains a large amount of chromic acid. A chromium mist recovery device can be installed after the exhaust hood of the plating tank. By slowing the airflow, chromium mist deposits on the recovery mesh, which is periodically rinsed with water and directly returned to the plating tank.
If there is concern about contaminating the plating bath or accumulating impurities, this chromic acid can also be directly used in zinc plating passivation. The mist recovery device should preferably be installed as close as possible to the chromium plating exhaust hood; otherwise, chromium mist will settle in the duct between the recovery device and the edge hood, making recovery difficult and possibly causing new pollution.
For decorative chromium plating, a mist suppressant can be used to reduce mist overflow. However, hard chromium plating, due to the thick deposit, is prone to pitting at the solution interface, which affects quality, so mist suppressants should not be used.
(5) Adopt trivalent chromium plating.
Trivalent chromium plating has made considerable progress, and its appearance is very close to that of hexavalent chromium plating. It has been applied on a large scale in decorative chromium plating, with mature processes available on the market. This is a practical way to solve the pollution problem.
(6) Use substitute coatings.
For example, tin-cobalt-zinc alloy, but its lack of wear resistance limits its promotion.