In conventional chrome plating with hexavalent (Cr6+) electrolytes for decorative or hard chrome plating applications, considerable quantities of electrolytes are lost either through drag-over in rinsing, or through spray mist that is generated by the low current efficiency of the process. Only a small amount of this loss can be recovered by way of dragout rinses. The subsequent concentration of these dragouts with vacuum evaporators or open-surface evaporators can reduce this loss; however, the recovery rate is governed by the rate of evaporation in the chrome bath. The rate of evaporation is even less in chrome baths that have improved throwing power characteristics due to their lower operating temperature. Deep tanks with less bath surface, or covered tanks, also have relatively low evaporation losses, permitting only low rates of recovery. In addition, the losses incurred by the air extraction cannot be disregarded, since, depending upon the operating conditions, they can add up to 5 - 20 mg/m3 of electrolyte.
The chromic acid effluent must be detoxified and neutralized. This treatment starts with the reduction of hexavalent to trivalent chrome through the addition of sodium bisulfite or SO2 at a pH of 2 - 2.5. This reaction, which is monitored by way of ORP measurement, requires excess dosage of the reducing agent, which is both costly and increases COD values. After this reduction step, the acidic effluent with the now trivalent chrome must be neutralized. In order to achieve satisfactory precipitation and separation of the Cr (OH)3 sludge, flocculants have to be added. The resulting sludge is dewatered with a filter press into a sludge cake having 30-40% of solids, which must be disposed of as hazardous waste. If all of the other losses are also taken into account, it becomes clear that recovery will pay off within a short period of time. In addition to the costs of the lost electrolyte (chromic acid and catalyst), one must add the expense for acid, reducing agent and alkali, and for flocculants, sludge treatment, transport and hazardous waste disposal. Next to these direct costs, there are numerous indirect expenses (maintenance, energy, electrodes, filter clothes, flocculants, selective ion exchange). This treatment of the chrome effluent also generates increased salinity equivalent to three times the amount of CrO3.
