Removal of dissolved heavy metals by chemical coagulation, magnetic seeding and high gradient magnetic filtration

Abstract

A rapid and highly efficient treatment process for waste waters containing heavy metals was investigated by combining the processes of chemical coagulation, magnetic seeding and high gradient magnetic filtration (HGMF). Chemical coagulation was performed with ferric chloride (FeCl 3), sodium sulfide (Na 2S) and FeCl 3, or calcium polysulfide (CaSx, x ≒ 4) and FeCl 3. Magnetite was used as seeding material, to whose surface non-magnetic particles adhere. The removal tests of cadmium was performed first by varying the pretreatment conditions, such as the amount of each additive, order and time interval of addition, pH and other factors. Consequently, the desirable conditions to establish such a high removal efficiency, close to 100%, were obtained by batch experiments for each of the combined processes. Based on the above results, removal of heavy metals, including mercury, in the gas scrubbing waste water from a municipal solid waste incineration plant, was examined by the process consisting of the coagulation with CaSx and FeCl 3, the magnetite seeding, and HGMF. A removal efficiency high enough to assure the effluent standards of Japan was established by monitoring the reaction in this process using a sulfide ion electrode. The CaSx added to the feed water was regenerated in a purity of nearly 100%. Almost all of the magnetite used was also recovered. About 70% of heavy metals on the magnetite was removed simultaneously.