ELECTROLYTIC RECOVERY TECHNOLOGY FOR METAL CYANIDE RECYCLING

Revision Date: 8/01
Process Code: Navy/Marines: SR-15-99, ID-04-99; Air Force: MT05, FA09; Army: ELE
Usage: Navy: Medium; Marines: Low; Army: Medium; Air Force: Medium
Alternative for: Wastewater disposal, Conventional end-of-pipe treatment
Applicable EPCRA Targeted Constituents: Silver (CAS: 7440-22-4), Cadmium (CAS: 7440-43-9), Cyanide (CAS: 57-12-5)
Overview: Electrolytic recovery technology uses an electrical current to plate out the metals and oxidize the cyanides in the rinse waters from electroplating. The metal is recovered from the electrolytic recovery unit (ERU) as a foil that can be returned to the cyanide plating bath as an anode source. The purity of the recovered metal should meet the specifications for anode purity as long as the water from the rinse tank is used to rinse parts that are only plated in the cyanide tank. The ERU is plumbed to a stagnant rinse tank (the first rinse tank that the plated parts would see) in a closed loop fashion. The cyanides are partially oxidized to cyanates in the ERU. Electrolytic recovery technology can remove more than 90 percent of the metal in the rinse stream and oxidize up to 50 percent of the cyanides.

Wastewater generated from the rinsing of metal cyanide plated parts contains metals (primarily cadmium, copper, and silver) and cyanide containing compounds (cyanides). Many metals and cyanides are toxic hazardous materials. The waste stream requires pretreatment to reduce these toxic materials prior to discharge. This treatment requires the use of hazardous chemicals including acids, alkalis, and chlorine-containing chemicals.

The primary ERU components consist of an electrolytic chamber, power supply (with rectifier) and pumps necessary for the intake, circulation and output of electroplating rinsewater. The electrolytic chamber contains the anodes and cathodes, which are connected to the rectifier in parallel.

Recent innovations in design of electrowinning equipment extend the operating range of the process to include wastewaters that are more dilute (<500 mg/L) in terms of metal and cyanide concentrations. Advanced ERU design features may be selected to optimize electrolytic recovery for specific rinsewater applications.

The selection of commercial ERUs should include the following minimum design features: 
  1. A minimum ERU circulation rate of 5 gpm using a sealed magnetic drive pump.
  2. Corrosion-resistant anodes.
  3. An open electrolytic cell for easy access for removal of individual metal loaded cathodes and for replacement by new cathodes without interrupting the recovery process.
  4. Materials of ERU construction in contact with the plating solution that are highly corrosive resistant, such as polypropylene, ceramic, and viton.
  5. A DC power supply (rectifier) that provides for the proper combination of voltage and amperage for the size of the electrolytic cell.

     
Compliance Benefit: Electrolytic recovery technology uses an electrical current to plate out the metals (which can be recovered) and oxidize the cyanides in rinse waters from plating operations. Electrolytic recovery technology can remove more than 90 percent of the metal in the rinse stream and oxidize up to 50 percent of the cyanides thus reducing the use of hazardous chemicals including acids, alkalis and chlorine-containing chemicals to treat rinse waters. Additionally, since the metal is recovered the volume of metal-containing hazardous sludge at the wastewater treatment plant is reduced.

The reduction of hazardous waste helps facilities meet the requirements of waste reduction under RCRA, 40 CFR 262, Appendix, and may also help facilities reduce their generator status and lessen the amount of regulations (i.e., recordkeeping, reporting, inspections, transportation, accumulation time, emergency prevention and preparedness, emergency response) they are required to comply with under RCRA, 40 CFR 262. 

In addition, this technology generates a less contaminated rinse water and thereby reduces the amount of treatment chemicals used be a treatment facility. Because less chemicals are used the possibility that a treatment facility would meet any of the reporting thresholds of SARA Title III (40 CFR 300, 355, 370, and 372) and EO 12856 is decreased.

The compliance benefits listed here are only meant to be used as a general guideline and are not meant to be strictly interpreted. Actual compliance benefits will vary depending on the factors involved, e.g. the amount of workload involved.
Materials
Compatibility:
No materials compatibility issues identified.


 
Safety and Health: Silver is known to be a mild skin irritant. It is also an equivocal tumorgenic agent. Cyanide compounds are very poisonous in nature, and prolonged exposure to high concentrations can be detrimental to human beings as it can be absorbed through skin and through inhalation. Proper
personal protective equipment should be used.

Consult your local industrial health specialist, your local health and safety personnel, and the appropriate MSDS prior to implementing this technology.
Benefits:
  • Reduction in the use of treatment chemicals for cyanides and heavy metals in the wastewater treatment plant. 
  • The recovery and reuse of heavy metals, which reduces the costs for anodes or chemicals. 
  • Reduction in the volume of metal-containing hazardous sludge at the wastewater treatment plant. 
  • This technology is applicable to other plating baths, such as nickel, zinc, and lead.
Disadvantages:
  • The initial cost of purchasing the equipment may be prohibitive. 
  • The metal recovered from the still rinse can only be recycled if it is of sufficient purity. 
  • The ERU is not used for treatment of general discharges of wastewater, as the ERU cannot differentiate among various types of metals for selective recovery. Thus, the electrolytic process is most applicable to a single metal rinsewater to permit recycling or selling to a reclaimer. 
Economic Analysis: Variables affecting capital cost, operation & maintenance costs, and the hazardous waste disposal costs with regard to the addition of an EN bath life extension system include:
  • Capacity and size of the plating bath. 
  • Quantity of work to be processed. 
  • Ability to stabilize bath characteristics and remove impurities. 

The cost to implement an ERU assumes that the rinse system for the metal cyanide plating line is a dedicated rinse (only used to rinse metal cyanide plated parts) and that the rinse system consists of a two-tank rinse system (a stagnant rinse followed by a final rinse connected to a wastewater treatment plant). The following economic analysis based on operations occurring at the Norfolk Naval Shipyard:

Assumptions:

  • Capital costs are based on an 800-gallon silver cyanide plating bath. 
  • Electrolytic Recovery Unit - $15,000 
  • Labor and Installation Costs - $3,000 
  • Facility generates 300,000 gallons of silver cyanide rinse waters a year. 
  • Labor = $45/hour. 
  • Labor requirements for operating and maintaining ERU system are approximately 30 hours per year. 
  • Electricity cost = $0.07/kWh. 
  • ERU uses 192 watts of energy. 
  • Pump uses 112 watts of energy. 
  • Unit on 365 days per year 24 hours per day. 
  • Waste treatment costs including chemical, labor, and energy costs is $0.05 per gallon. 


Cost Comparison for Electrolytic Unit vs. Treatment in IWTP

  ERU IWTP
Capital and Installation Costs: $18,000 $0
Operational Costs:    
Operation and Maintenance Costs: $1,350 $0
Waste Treatment and Disposal Costs: $0 $15,000
Utility Costs (electricity): $186 $0
Lab Analysis Costs: $2,000 $0
Total Costs (not including capital and installation costs): $3,536 $15,000
Total Income: $0 $0
Annual Benefit: - $3,536 -15,000


Economic Analysis Summary

Annual Savings for ERU: $11,464
Capital Cost for Diversion Equipment/Process: $18,000
Payback Period for Investment in Equipment/Process: 19 months

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Approving Authority: Approval is controlled locally and should be implemented only after engineering approval has been granted. Major claimant approval is not required.
NSN/MSDS:
Product NSN Unit Size Cost MSDS*
None Identified         


*There are multiple MSDSs for most NSNs.
The MSDS (if shown above) is only meant to serve as an example.
Points of Contact: None Identified.


 
Vendors: ECO-Tec Limited
1145 Squires Beach Road
Pickering, Ontario, L1W3T9
Canada
Phone: (905) 427-0077

This is not meant to be a complete list, as there may be other suppliers of this type of equipment.
Sources: None listed.

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