Nanofiltration is very efficient in separating heavymetals from water solutions, even if they contain strong acids.Desalination System, Inc., has many years of experience in thisarea, and this paper deals with three different such cases.The first case deals with concentrating the heavy metals in thepregnant leach solution from a copper dump leaching site with asurplus water problem. The increased copper concentration inthe pregnant leach solution increases the capacity of thefollowing copper extraction stage. The nanofiltration permeateis polished with a biomass adsorption column to remove thesmall amounts of remaining heavy metals, before beingdischarged. This method eliminates sludge handling anddisposal, which are quite costly with the alternative precipitation process.In the second case, a complicated precipitation method was usedto remove heavy metals from the waste stream of a specialtymetals manufacturer. Nanofiltration was installed to increase themetals concentration four times, which greatly simplified theprecipitation process. The chemical consumption and sludgeproduction decreased by more than 80 percent, and the laborcost decreased by 66 percent.In the third case, nanofiltration was used to purify waste sulfuricacid solutions containing heavy metals, to meet thespecifications for technical grade acid. Sulfuric acidconcentrations above 30 weight percent were successfullypurified in a multiple pass membrane system.
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