Printech Archive, April, 1999: Silver Recovery

Silver Recovery

gjonesprinting@aol.com
Wed, 17 Mar 1999 08:30:58 EST

To All:

I forgot to post the original question to the answer I gave yesterday on
silver recovery. Therefore, I am reposting the question along with the answer.

Gary Jones
GATF

On behalf of one of our newspaper members, what equipment needs to be
installed to meet a silver discharge limit of 0.86 ppm? What do they have
to do to get into compliance with the regulation? At the present time, they
have a silver recycling unit and a tailing unit.

Secondly, is there a list of silver recovery equipment manufacturers
and vendors?

Thanks

It is not clear from your email address as to where the 0.86 ppm silver
discharge limit is being applied at the processor or at the sewer connect.
This is critical as it does make a difference. I do not think it is possible
to treat the silver bearing wastes to achieve a 0.86 ppm limit at the
processor, but meeting it at the sewer connect can usually be achieved. Most
limits for discharges are enforced at the sewer connect (e.g., where the
printers sewer line meets the POTW's sewer line).

In terms of silver recovery, the best approach for tight limits would be to
focus on both the fix and the wash water. The type of silver recovery needs to
be matched to both the concentration and flow of silver bearing wastes. Silver
recovery is influenced by pH and residence time. For an operation that does
not have low flow the use of in-line recovery units is the best choice. The
two principal silver bearing streams for black and white operations are the
fix and washwater. Operations with low flows are best handled via a batch
approach. It is important to note that batch processing of silver bearing
waste streams could be subject to permits as hazardous waste treatment since
this approach does not meet the "closed looped" exemption requirements. Batch
treatment should be performed by using an electrodepositon unit followed by
one or two chemical recovery cartridges (AKA polishing units or steel wool
buckets). It is also critical to note that in low flow or batch situations,
the chemical recovery cartridges are subject to channeling and will not be
efficient. For facilities with moderate flow, the first step for fix is to use
an electrodepositon unit followed by one or two chemical recovery cartridges
(AKA polishing units). If additional silver recovery is desired, then the
solution can be passed through an ion exchange unit. Since washwater contains
about 3-5 ppm, using an ion exchange unit will reduce the silver from this
wastestream as well.

Since residence time and pH are important factors, it may be prudent to
collect the fix and washwater, especially the fix, in holding tanks prior to
silver recovery. The silver bearing solutions can then be metered via small
pumps at a slower rate into the recovery system to allow for maximum residence
time. The metering rate will depend upon the manufacturer's recommendation.
Since silver recovery is reported to be more efficient at a pH of 8, the pH of
the fix can be also be raised to allow for more recovery. This can be
accomplished in the holding tank. In the systems I have seen, pH adjustment is
usually not needed and the metering will accomplish the goal of reducing
silver concentrations.

The other option to consider is to install either a fix and/or washwater
recycling unit. These units are equipped with silver recovery and will reduce
both the amount of silver and solution being discharged.

Another option would be to collect the solutions and send them off-site for
treatment. This is the most expensive option and the fix would have to be sent
under a hazardous waste manifest. The weight of fix would be counted towards
the generator status of the facility and would most likely cause them to be
classified as either a small or large quantity generator.

The other option would be to have the local POTW adopt the Code of Management
Practice. This is a performance based standard that eliminates the need for a
numeric standard. It is based on the total monthly flow of silver bearing
waste streams and the recovery is either 90, 95 or 99%. If you want more
information about this program, let me know and I will send a book on it.

I do not have a list of silver recovery vendors, but have made a request for
one. I will forward if I get one.

Gary Jones
GATF

 

PNEAC

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