Sulfuration treatment of electroplating wastewater for selective recovery of copper, zinc and nickel resource

In the electroplating process, various metal salts are used and the residues on the surface of
electroplated materials are rinsed out as wastewater, followed by hydration in a wastewater
disposal step. Although a variety of metals are concentrated in the sludge, the mixed
heterogeneous state of these metals in sludge makes it difficult to recycle and reuse them. From
the viewpoint of environmental protection as well as resource saving, effective methods for
recycling and reuse of these mixed metal sludge are required to be developed urgently.
In the present work, selective recovery of the metals from mixed-metal wastewater by sulfuration
treatment is proposed. Sulfuration treatment is characterized by low solubility of metal sulfides;
metal sulfides are, in general, lower in solubility than that of metal hydroxides. For the experiments,
three metals of copper, zinc and nickel which are commonly used in the electroplating process
were chosen. Separation of these metals from the mixed solution in various pH ranges was
conducted by employing three sulfurating agents: sodium sulfide (Na2S), sodium disulfide (Na2S2)
and sodium tetrasulfide (Na2S4). Aqueous solutions of CuSO4, ZnSO4 and NiSO4 and those of
real plating wastewater, of which the initial concentrations were adjusted to 100-300 mg dm-3 were
employed.
By sulfuration treatment of the simulated solution using each sulfurating agent without adjusting pH,
CuS was first precipitated, but ZnS and NiS were also precipitated at the same time. In addition, pH
value increased with the amount of sulfurating agents in the solution. The results demonstrated that
the formation behavior of metal precipitates depended on pH value of the solution. For the
sulfuration with three kinds of sulfurating agents, copper was first separated from the solution as
CuS in pH=1.4-1.5, then ZnS was precipitated in pH=2.4-2.5, followed by the precipitation of nickel
sulfide, NiS in the residual solution at pH=5.5-6.0. It was also found that Na2S is most effective for
selective precipitation of metal sulfides, among the three sulfurating agents of Na2S, Na2S2 and
Na2S4. The selectivity of CuS in the filtrated cake obtained by the sulfuration treatment in
pH=1.4-1.5 was about 94 % and was sufficiently high, in terms of allowable metal content level for
recycling. The sequent selectivity of ZnS and NiS in the cake after the sulfuration treatment in
pH=2.4-2.5 were 75-77 % and about 20 %, respectively. The selectivity of NiS and ZnS in the cake
after the sulfuration treatment in pH=5.5-6.0 was 64-66 % and 33-35 %, respectively.