Hexavalent Chrome Waste Treatment
& Chrome Reduction Systems

Electroplating metals is a common way to provide corrosion resistant properties to products. After the electroplating is completed, the plated parts are rinsed with water. This rinse water will become contaminated with time and must be replaced. The rinse water will have a high concentration of toxic material including chromates, copper, or zinc that must be treated before dumping to the sewer or reusing.

Case study

A plating facility in California has an acidic waste stream (pH 3.5) high in hexavalent chrome, cadmium, copper, nickel, lead and zinc. The waste stream also contains various complexing agents. Chromate waste is typically treated in two stages. The facility which has a batch treatment system, utilizes this conventional treatment process for hexavalent chrome removal. In the first stage, 50% sulfuric acid is added to lower the pH further to pH 1 -2. Sodium meta-bisulfite is then added to the waste stream to reduce hexavalent chrome to trivalent chrome. A bi-product of the treatment is a bad odor from the sodium metabisulfite that permeates the plant. Sometimes sulfur dioxide or sodium bisulfate is used instead of sodium meta-bisulfite with the same results. The batch system is allowed to mix and react for 10 – 15 hours. In the second stage, the chromium hydroxide precipitation occurs when 50% caustic soda is then added to raise the pH to 8.0 - 9.5 with the addition of calcium hydroxide. Further reaction time of 1 - 2 hours is needed. Two other commodity chemicals, in a home grown formula, are further used to enhance precipitation, coagulation and flocculation. This facility uses a total of six different chemicals for their chrome reduction treatment. The sludge produced is slimy and need filter aids like diatomaceous earth to keep the filter press from getting blinded. The company still cannot meet their local discharge limits for total chrome and the plant is looking for a more efficient technology.

The MetFloc™ Technology

MetFloc™ technology is a highly efficient and cost effective advanced treatment process for total chrome reduction that eliminates the multiple treatment steps associated with conventional processes. The residence time is very short, eliminating the need for large holding tanks. The intermediate step of reducing hexavalent chrome to trivalent chrome is eliminated. MetFloc works with the initial waste, in this case, with a pH of 3.5 and converts chromium to a stable hydroxide. All metals of concern in this case are eliminated in one treatment step to below the local discharge limits. Secondary pH adjustment is also eliminated since MetFloc raises the pH to above pH 6. The MetFloc™ treatment process also produces 20 – 50% less sludge. The flocs are dense, large and highly dewaterable. Pin flocs are eliminated. No filter aid is required for the sludge in the filter press. The sludge produced is highly stable and passes the Toxicity Characteristics Leaching Procedure (TCLP).

Some discharge authorities are now testing for total chrome and not just hexavalent or trivalent chrome (Cr +6 or Cr +3.)  The MetFloc process is uniquely capable of reducing total chrome below discharge limits. The reduction of chemicals, ease of operation, cost advantages, faster treatment times and better sludge make this revolutionary process worth evaluating.