Chromate Decontamination and Neutralization Precipitation with Carbonic Acid

Cause

In a metal processing company that specializes in investment casting, castings with tolerances of <0.1 mm are produced. The molten chromium-containing steel is poured into a special ceramic mold, which is then removed by chipping. The castings are pickled in a sodium hydroxide melt to remove the oxide layer and ceramic residues. The pickling process is followed by cascade rinsing with drinking water. Due to the minimization of rinsing water and thus also waste water, this produces a highly alkaline waste water containing chromate (chromium(VI) oxide - toxic) of 8 m³ per day (in 8 h). The pH value in the inflow is 12.5-13.5, the chromium (VI) concentrations are 16.0-20.0 mg/l and the chromium (total) concentrations are also 16.0-20.0 mg/l. The limit values to be achieved are pH 6.5-9.5, 0.1 mg/l for chromium(VI) and 0.5 mg/l for chromium(total). In order to comply with these limits, WEIL Wasseraufbereitung was commissioned to develop a suitable wastewater treatment concept for the company.

Chromate decontamination and neutralization

Based on the results of the previous laboratory tests, WEIL developed a concept for the treatment of 8 m³ of wastewater per day. This concept includes the conversion and reuse of the existing sedimentation tanks. The wastewater is pumped from the collection tank into the treatment tank in batches. The fully automatic treatment program for detoxifying and neutralizing the wastewater then runs automatically. First, iron II sulphate solution is added to reduce chromate. Once the reaction is complete, the detoxified wastewater is circulated via a pipe coil (pipe neutralization system). At the beginning of the pipe coil, CO2 gas is optimally mixed into the wastewater via a special injector and fully utilized for neutralization.

Once the target pH value has been reached, neutralization is stopped and a polyelectrolyte solution is dosed into the treatment tank as a flocculant. The entire batch is then filtered through a chamber filter press, where the iron and chromium hydroxide sludge is separated.