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Chrome recovery ReChrom
Overview
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.
The solution
The Gütling chromic acid recovery process conserves valuable resources, and it assists waste minimization efforts in preventing wastewater generation, sludge formation and environmental pollution; the process yields a purified electrolyte that runs under uniform operating conditions.
Process description
The system represents an ideal combination for the recovery of chromic acid from rinse water and exhaust air streams, and the exhaust air discharge meets regulatory emission standards. The evaporation and purification technology of this
water-conserving rinse technique provides electrolyte cooling at no additional cost.
The Gütling chromic acid recovery system is fully integrated into the chrome plating plant;
it consists of the following components:
- Water-saving immersion and spray rinsing combinations
- Air extraction of the chrome bath area, including air purification and chromic acid recovery
- Multi-stage atmospheric evaporator, integrated into the exhaust air stream
- Electrolyte recirculation with evaporative cooling and temperature control of the chrome bath
- Continuous removal of the metallic impurities
- Recovery of the chromic acid from rinses and exhaust air
Advantages
The Gütling system thus offers the following benefits:
- A substantial reduction in effluent loading
- Labor savings at the effluent treatment plant
- Savings in chemical reagents
- Compliance with the regulatory provisions for chrome emissions
- Resource recovery
- No sludge generation signifies extra savings
- Short pay-back period
- Reduction in environmental pollution
- Resource conservation and waste minimization
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