High vacuum indirectly-heated rotary kiln for the removal and recovery of mercury from air pollution control scrubber waste

In the mining industry, mercury is often associated with various ores, particularly copper and lead. To meet Environmental Protection Agency (EPA) and State air emission regulations, smelters must remove mercury vapors from hot exhaust gases, usually through the use of scrubbing solutions. Most of the mercury is precipitated out of the waste scrubbing solutions as the sulfide and the sludge dewatered in a filter press. SepraDyne Corporation (Denton, TX, USA) has conducted pilot-scale treatability studies of dewatered acid plant blowdown sludge generated by a copper smelter using its recently patented high temperature and high vacuum indirectly-heated rotary retort technology[1]. This unique rotary kiln is capable of operating at internal temperatures up to 850°C with an internal pressure of 50 torr and eliminates the use of sweep gas to transport volatile substances out of the retort. By removing non-condensables such as oxygen and nitrogen at relatively low temperatures and coupling the process with a temperature ramp-up program and low temperature condensation, virtually all of the retort off-gases produced during processing can be condensed for recovery. The combination of rotation, heat and vacuum produce the ideal environment for the rapid volatilization of virtually all organic compounds, water and low-to-moderate boiling point metals such as arsenic, cadmium and mercury. Atmospheric oxygen can readily be reduced to very low concentrations within the kiln so that combustion does not take place. Initial concentrations of mercury in the waste ranged from 565 to 1260 mg/kg. The waste was processed in a 1.5 cubic foot capacity batch pilot unit at temperatures ranging from 427 to 649°C at a pressure of 76 torr. Total processing time ranged from 1.5 to 4.0 h. The processed material contained total mercury concentrations ranging from 0.065 mg/kg when processed at 649°C to an average of 2.2 mg/kg when processed at 427°C. The vaporized mercury was recovered as elemental mercury in a low temperature condenser[2]. Mercury emissions averaged approximately 6.5 μg/M3, significantly below the Maximum Achievable Control Technology (MACT) standard of 40 μg/M3. Based upon the success of these tests, a contract was let and a commercial scale unit was built. The 2-ton per batch commercial unit was installed at the copper smelter and began operations in late June 1998. As of the end of October, approximately 500 tons of mercury-contaminated acid blowdown sludge had been processed to below the specified treatment criteria of 10 mg/kg total mercury.