Wastewater from chrome plating and brightening operations contains soluble chrome.
In the case of chrome plating, They use a highly soluble form of chrome known as Hexavalent Chrome or CR+6. In order to remove this form of chrome from water it must first be reduced to trivalent chrome or CR+3. After chrome reduction, the chrome is precipitated, coagulated, flocculated and removed from the waste steam by clarification.
The typical method to reduce and remove CR6+ from wastewater is as follows:
Stage 1 Chrome Reduction:
pH is lowered to pH 2.5 while adding a reducing agent such as sodium metabisulfite or ferric sulfate converting CR+6 to CR+3.
Stage 2 Precipitation:
pH is adjusted upward to a pH of 8-9.5 to the optimum chrome hydroxide precipitation point. Often, a coagulant such as ferric sulfate is added to enhance metal co-precipitation and the formation of “pin floc”.
Stage 3 Flash mix:
The wastewater with it’s precipitated pin floc is introduced to the flash mix zone where a polymer flocculent is added. This stage maximizes flocculent dispersion throughout the coagulated wastewater.
Stage 4 Flocculation:
The wastewater is now introduced to the slow mix zone to agglomerate the pin floc into larger rapid settling particles.
Clarifier, Inclined Plate:
The flocculated wastewater is introduced into the clarifier where the settling particles accumulate in the sludge chamber. The clarified water then exits the clarifier and flows downstream to sewer or further treatment if necessary.
Clarifier Sludge Handling:
The accumulated sludge is periodically removed from the clarifier and sent to a sludge holding tank where it further thickens for disposal or dewatering.
Sludge dewatering is typically handled by a Filter Press. After processing a batch of "sludge" the filter press is emptied of “chrome cake” which is a semi solid of approximately 20-35 % solids. Chrome cake is high in chrome and sulfite and should be disposed of according to environmental regulations.
Plating shops are found in typically two categories, captive and independent shops. Some industries operate their own captive, in house plating operation while others outsource to an independent plating operation.
Typical industries include: