Process:

The client's process involved caustic digestion of used x-ray film followed by reduction of the resultant emulsion with Sodium Borohydride. After the reduction process was finished, the solids were settled out with a polymer and the supernatant sent to sewer and solid phase sent to centrifugation for rapid solids drying.

Problem:

Production was slowed down due to Zinc, Silver, and Chrome being frequently over the City's limits set for discharge. When the supernatant was found to be out of spec, the supernatant was pumped to another tank where Sodium Diethyldithiocarbamate was added to the tank and the residual metals precipitated and settled out with more polymer. The extra step of retreating the supernatant was severely hampering production and running up chemical costs.

Resolution:

The treatment with carbamate was added to the reduction tank eliminating the extra step. Out of spec analyses were reduced 90% due to the efficiency of the high solids increasing the carbamate metal complex precipitation.

• Improved filtration of the supernatant to sewer due to elimination of frequent polymer overdosing.
• Reduction in use of Sodium Borohydride due to increased efficiency of the carbamate in heavy metal removal.
• Sharp decrease in heavy metals going back into solution at the centrifuge reslurry operation.

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