Changing from Lime To Magnesium Hydroxide
Q. We use a heavy metal precipitation process consisting of sulfuric acid to lower our tumbling wastewater to a pH of 4.0 to break chelating agents, chrome reduction with sulfuric acid and sodium metabisulfite, lime slurry to raise pH to 9.5, addition of flocculant, then clarification. We are considering switching to magnesium hydroxide since jar tests performed by a chemical supplier indicate a 30% reduction in filter press cake at a pH of 8.5. Our flow consists of 2/3 tumbling and 1/3 nickel-chrome plating with a combined flow of 60 gal/min. Is this realistic? J.S.
A. A change from lime to magnesium hydroxide is certainly worth investigating. Its advantages include lower soluble metals, better floc formation, lower solids generation and, generally, higher solids content in the filter press cake. However, magnesium hydroxide has a very slow reaction time, up to 60 minutes, requiring periodic jar tests to determine feed rate. Most metal finishing wastewater treatment systems do not have this amount of time for pH adjustment at design flows, so the magnesium hydroxide slurry needs to be fed as far upstream of pH adjustment as possible, and this may not be feasible.
If you have a sump that collects your acidic tumbling and chrome reduction wastewaters for pumping to pH adjustment, this could be a good location for the magnesium hydroxide and use lime slurry or sodium hydroxide to “top off” your pH control at your pH adjustment tank.
Also, be sure your magnesium hydroxide feed system has a water flush in the feed line that is isolated from the feed tank and will flush the magnesium hydroxide away from the feed tank. Why? Magnesium hydroxide is a slurry that contains chemicals that help maintain the solids in suspension. If these chemicals are diluted too much by flush water back into the feed tank, the solids will be difficult to keep in suspension without constant mixing. Periodic mixing of this slurry throughout the day is also highly recommended.
Based upon your information, your filter press cake is a RCRA-listed hazardous waste (EPA hazardous waste F006) due to the nickel-chrome plating wastewaters, and is expensive to dispose properly. Have you seriously investigated a separate wastewater pretreatment system for the tumbling wastewater, which would very likely not produce a listed RCRA-hazardous waste? Mixing of the tumbling and nickel-chrome wastewaters causes your filter press cake to be hazardous waste.
While tumbling wastewaters are usually very high in solids due to the breakdown of the media, they are typically low in heavy metals (except iron) unless large amounts of stainless steel, brass, or bronze parts are processed. Even here, the metals are in a particulate form and from our experience resist leaching of the acidic extraction fluid when subjected to the Toxicity Characteristic Leaching Procedure. All the tumbling wastewater treatment filter press cakes we have analyzed have been RCRA non-hazardous waste and capable of disposal into a permitted sanitary landfill at significant costs savings as well as reduction in regulatory compliance costs. We recommend that you sample and analyze your tumbling wastewater and compare it against your wastewater discharge permit limitations to determine what level of pretreatment you really need. You may find that this wastewater only needs screening for media recovery, free oil removal and pH adjustment.
Separate treatment also has these advantages: elimination of acid pretreatment of chelating agents likely coming from tumbling, significant reduction in the amount of RCRA-hazardous waste generated, reduced alkali demand for pH control due to elimination of acid pretreatment, and increased recycling of the nickel-chrome wastewater filter press cake because of the removal of the “junk” tumbling solids.
The wastewater from the two treatment trains would then be combined before the sampling point and discharge.
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This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2012 in Las Vegas, Nev., on June 12, 2012.