QUESTIONS ON ELECTROWINNING

Some questions about our metal recovery systems reoccur with such regularity that we have written this short question and answer brief to cover the most commonly asked questions.

Q. Why should I electrowinn?

A. Electrowinning high metal content solutions will reduce the loading on your present treatment system. It will also recover metals in a compact and easily handled form that can easily be recycled.

Q. What should I electrowinn?

A. Our rule of thumb is that solutions over 500 ppm usually are effectively electrowinned. Lower metal content solutions can be used but the efficiency is reduced as the metal content is decreased. In a printed circuit board manufacturing facility, cleaners, microetches, dragouts, electroless dumps and many other solutions are effectively electrowinned. Copper, Lead, and Cadmium are easy to electrowinn. Nickel is difficult due to internal plating stresses. Chrome is not economic to electrowin. Tin is possible from plating type baths.

Q. How does Electrowinning work?

A. Electrowinning is another term for plating. The solution to be electrowinned is recirculated (or agitated) past an anode (+) and cathode (-). A low voltage direct current is applied through the solution and metal ions are reduced at the cathode and water or another ion is oxidized at the anode. If a metal that can be plated is in solution, is forms a coating on the cathode. Usually oxygen is formed at the anode from the oxidation of water molecules. In cyanide solutions, the cyanide is oxidized at the anode to cyanate then CO2 and NO2.
The formula for electrowinning Copper sulfate is:
CuSO4 + H2O Cu0 + H2SO4 + O2
The half reactions are:
At the Anode -
2H2O 2 O2 + 2H= + 2e-

At the Cathode -
CuSO4 + 2e- Cu0 + SO4=
Q. What determines electrowinning efficiency?

A. Efficiency is the result of dividing the theoretical current to plate out a give weight of metal by the actual current used. If it takes 80 amp hours to plate out a give amount of metal and it should take 60, the efficiency is 60/80 or 75%. The efficiency is determined by the ability of the metal ion in solution to be reduced at the cathode. The lower the concentration of metal, the less likely it will be at the cathode to be reduced and hence lower efficiency. This result is due to the DIFFUSION LAYER EFFECT which is described as follows:

At the surface of a cathode is a thin film of water which is still. The thickness of this layer is determined by the agitation and temperature of the solution. Because this layer is still, the rate at which the metal ions transfer through by diffusion determines plating rate. The higher the metal concentration, temperature, and/or agitation, the higher the diffusion rate and the plating rate. Low concentrations, temperature, and agitation will reduce the plating rate.

Q. I have some difficult to treat solutions that I would like to electrowin, how can I determine if I can electrowin them?

A. In theory, most solutions can be electrowinned to some extent. The closer you get to no metal in solution, the more current it takes to get the next increment of metal out. It is the law of diminishing returns. Some systems overcome this effect somewhat by placing very large cathode areas in the solution. Another way to overcome this effect is through very high agitation rates. The more solution you pass over the cathode, the more likely it is that a metal ion will be available to be reduced.

Q. How much floor space does an electrowinning system occupy?

A. Integrated systems are compact. A 30 pound per day Copper recovery system will occupy an area 6' x 3'. No storage tank is needed except for your holding tank for future loads. Systems are designed to hold a single or double dump load (200-400 gallons) and recirculate this until it is plated out sufficiently.

Q. How much does it cost to run an electrowinning system?

A. The cost to operate is a total of three variables. These are, power, chemistry (pH adjust) and labor.
Power Costs-
The power requirement is for one 3/4 hp pump and 600 amps of rectification for 30 pounds per day of metal recovered. This is about 5.0 kW total power usage at 80% rectifier efficiency.
Chemistry Costs-
The only chemistry cost is that for adjusting to a optimal pH for plating. This can be overcome by mixing various dumps (microetch and electroless) to optimize the pH.

Labor costs-
Labor costs for the system is usually very low due to the automated nature of the systems. If the proper segregation is used, little or no maintenance is required between loads except for monitoring the operation. Once the operational profiles are learned, the amount of monitoring is reduced. The newer barrel style systems are lower in maintenance due to the lower number of anodes, simpler loading/unloading systems, and the natural tendency to minimize dendritic Copper formation.

Q. How low can I plate a solution?

A. For a Copper or Nickel system, plating to about 50 ppm in a barrel or parallel plate system is quite low. You can get lower but it will take a lot longer to get to 1 or 2 ppm from 50 ppm that it did to get from 5000 ppm to 50. In a production operation, the real question is how long can you wait to plate out a bath and how much energy do you want to expend to get that last few ppm?

Q. What do I do with the depleted solution?

A. Spent solutions are usually bled into the existing waste water treatment system or batch treated. Usually these solutions are high in acid content and may be used for pH adjust for other solutions.

Q. What types of electrowinning cells are there?

A. There are many types of cells that have been tried over the years. There are presently 3 types in wide use that seem to be reasonably efficient and cost effective.

Parallel plate systems use an array of parallel anode and cathode pairs to provide a high surface area for electrowinning. These systems are inexpensive to design but have high labor costs associated with servicing the many contacts required. They are quite efficient down to about 50 ppm with high pumping rates.

Very High Surface Area Parallel plate systems use a mesh cathode to increase the surface area of the cathode. The contact maintenance problem is similar to the standard parallel plate system. The costs incurred with the mesh cathodes is very high. The are efficient to very low metal levels. Some of the major problems are plugging and very high costs associated with high metal recovery rates. These are best used in a polishing applications where metal content is low and you have a clean stream.

Barrel plating systems use a polypropylene barrel with a "dangler" contact inside and a single anode outside. Mesh or drilled panels are used for solution transfer. The barrel has "BB" sized metal balls inside and a through wall cathode "dangler" to get the current inside the barrel. A comformal anode is mounted below the barrel. The barrel is rotated in the solution and this increase the mass transfer rate. High efficiency rates can be achieved with augmented solution transfer and the system can plate to very low levels. Only one anode and cathode contact are required to be maintained so the maintenance level is low. The barrel is emptied by lifting it up and dumping the contents into a trough that empties into the barrel.

Q. How long does electrowinning take?

A. The length of time to plate out a solution varies on the ion content, metal content, temperature, current density, and agitation. Ideally, Copper plates at about 4.2 ounces using 100 amps for 1 hour (theoretical). A 400 amp rectifier with a properly designed cell would plate up to 1 pound per hour.

Q. Do I have to pH adjust my solutions?

A. pH adjusting helps the efficiency of the process. Usually a pH between 2.5 and 4.5 is optimum for Copper plating. Nickel can be plated at the higher range. Gold is plated at very high pH. For cyanide destruct systems, the cyanide is stepped through several pH ranges for total oxidation to carbon dioxide and water.

Q. How automatic is automatic?

A. Full automatic systems are available including sensors to determine the pH, metal levels in the tank, automatic dumps and fills, Full data monitoring, storage and analysis is available as are network interfaces.

Q. Do I have to segregate baths?

A. Some baths may be combined to increase the efficiency of the system. With each system comes a manual with suggested processing parameters for your particular waste.

Q. What types of problems will I have?

A. The main problem we see is low maintenance of the contacts. The contacts should be cleaned and recoated regularly (weekly). Metals should be unloaded before the weight destroys the cathode mounting supports and breaks the operators back (parallel plate systems). Danglers should be cleaned weekly (barrel systems). Prefilters should be changed weekly (all systems). The most common problems are pump plugging due to sloughing of dendritic copper. Most users push the current on inefficient baths which causes poor metal adhesion and lots of sloughing. The copper falls to the bottom and eventually plugs the pump.

Proper contact maintenance and operating the rectifier in the proper range will minimize 90% of the problems associated with electrowinning.

Q. How long will my anodes last?
A. Anode replacement in 1-2 years is usual. Running a system at overpotential (generating lots of gas) will destroy the anodes sooner. Plating at higher metal levels increases anode life. A barrelwinner uses fewer anodes than a parallel plate system.

Q. How big of a system will I need?

A. Systems are sized on recovery requirements. When you provide flow data for each type of batch dump with a general analysis of the level of contamination, it is relatively easy to calculate the size of the system required. Usually a 30-50% fudge factor is included due to the low efficiency when plating to a low level. A small system is used in conjunction with a metal recover ion exchange system because the MRIX can strip metals to very low levels efficiently and plating down to low levels is not necessary.

Q. Can I get a small system to close loop a dragout?

A. Systems are available in many sizes. Small barrel, high surface area and parallel plate systems are available for plating out dragouts at the source. The area required for mounting will vary with the metal recovery rate required. We build a very compact over-the-side parallel plate unit in a 2" thick package for low recovery rates. Higher recovery rates use a small barrel system with a separate tank and pump.