Metals Recovery from Wastewater Treatment Sludge
Q. I work at a metal finishing facility that performs nickel, copper, gold, etc. plating, mostly on zinc die-cast parts. Also, we strip and re-plate small items, such as shoe lace eyelets, that are made of brass or steel. Because of our processes, our wastewater treatment sludge contains nickel, zinc, copper and iron.
In our wastewater pretreatment system, cyanides are broken down, so we do not expect them to be present in the sludge. The sludge is green in color so we think it contains significant amount of nickel. Could we plate out the nickel? I.L.
A. The short answer to your question is “technically, yes,” however, the cost to do so is prohibitively high. In essence, you would have to install a process that would prepare your sludge like it is a mined metal ore.
Instead of “mining” the nickel or other metals out of your sludge, or as we like to say, filter press cake, we recommend that you investigate keeping the metals out of your rinsewater in the first place and returning them to your process tanks. The cost of nickel anodes, as well as other metals, has been on the rise, so the reuse, recovery, and/or recyclable opportunities need to be evaluated.
Before beginning to evaluate recovery opportunities, analyze at least three batches of your wastewater sludge for the metals you desire to recover, such as nickel, copper and gold, so as to determine the value of what is lost. This data will help as you evaluate economic feasibility.
There are two options for onsite recovery systems. The first is what we call simple recovery systems, which involve using one to multiple counter-flow rinse recovery tanks that are made up with DI water. This rinsewater flows from the tank farthest from the plating tank back to the rinse tank right after the plating tank, hence, counter flow to the direction of work. The tank right after the plating tank contains the highest concentration of metal and is used to make up the plating tank losses due to evaporation. In addition to counter-flow recovery rinse tanks, final flowing rinse tank(s) maybe needed. For this system to work, you will need to know your daily dragout rate and evaporation loss. Evaporation loss from the plating tank typically needs to exceed the dragout rate in order for this concept to be worthwhile.
A good reference for these types of systems is Water and Waste Control For The Plating Shop by J.B. Kushner and A.S. Kushner. Chapters five and six are particularly helpful.
The second option is what we call advanced recovery systems. Typically these systems combine simple recovery methods with other technologies. These other technologies either increase the evaporation rate of the process or plating tank, increase the concentration of the metal in the recovered rinsewater solution, or both.
Other recovery technologies include, but are not limited to atmospheric evaporation, vacuum evaporation, ion exchange, electrowinning, electrodialysis, and reverse osmosis. These technologies can be used by themselves or in combination. For example, ion exchange can be used to concentrate the target metal in the rinsewater and then plate it out by electrowinning.
Water and Waste Control For The Plating Shop provides description of some of the technologies and how they can be used. Another good source is my contribution to PF’s 2011 Directory & Technology Guide – “Recycling/Recovery Methods for Platers.”
Lastly, now more than ever there are a number of service providers available that can take spent plating solutions or filter press cake and recovery the metals. They are set up to run high volumes to economically justify the metal extraction technologies used. Depending on the metal and concentration, you may be charged for their service, but at a faction of the cost you would have to pay to do it yourself.
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