What Chris is referring to is the fact that the metal hydroxides may fail the EPA testing requirement of TCLP (Toxic characteristic leaching procedure) this process simulates the effect of acid rainwater on solids likely to end up in landfills. There are 8 RCRA metals they look for with this test.
Metals dropped from a mixed metal waste treatment by what is called the classic destruct process (precipitation of metal hydroxides by raising the pH) are likely to contain at least 1 of the metals contained in this waste solid. There are chelating agents which can be added in the process to keep some of these metals from leaching out of the solid form and thereby pass the TCLP testing. But on any commercial scale the material should be tested by a certified lab at least once annually to confirm (or deny) compliance. Then the waste can be shipped as non hazardous unless, of course, the metals cannot be successfully chelated with your process.
The 8 RCRA metals and their allowable concentrations are;
Arsenic (As), 50 ppm
Barium (Ba), 100 ppm
Cadmium (Cd), 1 ppm
Chromium (Cr), 5 ppm
Lead (Pb), 5 ppm
Mercury (Hg), 0.2 ppm
Selenium (Se), 1.0 ppm
Silver (Ag). 5 ppm
The one exception to this is the exemption afforded to refiners for shipping wastes for precious metal recovery. Most refiners dry, (usually by incineration) crush, and sift, their hydroxides and ship the material to a refiner in Europe who will pay for all PM's and copper. It never fails to result in a payable return despite the fees for processing. A refiner that treats his or her waste stream, tests the effluent, and ships their processed hydroxides to a refiner, does not generate any wastes and is only responsible to prove the efficiency of their system if the proper authorities request.
