Lead Out of Wastewater
Q. We recently received a new plating contract for machined parts. In order to get the parts ready for the specified electroless plating, the parts are soak cleaned, electrocleaned and muriatic acid activated. Shortly after we started running this job, the city sampled our wastewater as part of their normal procedure and found that our lead concentration was over our wastewater discharge limit of 0.4 mg/liter. After some exhaustive investigation, we found fairly high lead concentrations in our electrocleaner and muriatic acid process solutions. After further discussions with our customers, our new customer informed us that their steel parts do have some lead in the alloy in order to help with their machining.
We have a batch treatment system for our concentrated wastes, and our rinse waters receive only pH adjustment. After further testing, we have concluded that while the lead concentration in our rinse waters does increase when we run these parts, it is still within our discharge limit. However, when we process a batch of our concentrated wastes, our lead limit is exceeded. Is there any modification to our existing batch wastewater system that could help us achieve compliance? C.Y
A. While the leaded stock that your customer uses improves their process, it sure can give job shops such as yours wastewater headaches. Since in the electrocleaner, the part is the anode and the current flows from anode to cathode, surfacial lead can be pulled off of the part. You may want to consider turning off the electrocleaner when processing these parts and operate two soak cleaners in series if sufficient part cleaning can still be accomplished. The muriatic acid (aka hydrochloric acid) is dissolving some of the lead off of the part during acid activation.
Removal of lead from your concentrated wastes by neutralization and precipitation is not difficult but does require several steps to achieve consistently good results below your limit. One of the challenges is the minimum solubility of lead hydroxide is about 4 mg/liter. While precipitation with other metals, such as iron, will reduce the solubility further, we have found that one must form a lead sulfide or lead carbonate precipitate in order to consistently achieve compliance with your wastewater discharge limit.
While sulfide precipitation of lead will certainly achieve compliance, we would not recommend it due to its “rotten egg” odor unless, for some reason, carbonate precipitation is not successful.
Here is our suggested procedure:
- If at all possible, neutralize the waste acid using
your waste cleaners; the high iron in the waste
acid seems to help minimize the cleaner's ten-
dency to negatively impact metal precipita-
- Adjust wastewater pH to between 7.0 and 8.0
(for every gallon of 10% muriatic acid, about
0.15 gallons of 50% caustic or 0.4 gallons of 25%
caustic is needed).
- With mixing, add sodium carbonate (aka soda
ash) at a rate of approximately one pound so-
dium carbonate per one pound of lead to be
treated (mg/liter lead x 8.34 x gallons x 10-6 =
pounds of lead).
- If needed, add caustic to raise pH to around 9.0.
- Add an anionic polymer for flocculation.
- Allow solids to settle and decant clear wastewater or pump entire batch to filter press.
Before implementing, we recommend that you perform this batch treatment on a lab scale in order to determine your specific sodium carbonate dosage and final pH.
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