674
L. Zheng et al.
changes in deposit morphology [8]. Cobalt and germanium impurities often remain
during the zinc leaching process. They affect the quality of cathode zinc obviously
and cause low current efficiency with high power consumption [9]. At present, the
zinc-antimony salt purification method is widely used for removal of cobalt and germanium in the second-stage purification of zinc sulfate solution. Various parameters,
such as cementation time, temperature, solution pH, concentration of activators, and
concentration of zinc ions in the electrolyte, may have influence in this process [10].
Metallic zinc is a good reactant for purification of zinc sulfate solutions. In humid
air, zinc powders stored in the natural state are easily oxidized to form dense oxide
films on their surfaces. These oxide films will change the kinetic reaction characteristics during the two-stage solution purification process, thereby increasing difficulties in removing cobalt and germanium by zinc itself. Also, the zinc oxidation
often causes increase of zinc consumption, difficult hydraulic filtration after purification, high filtration fabric consumption, and rapid accumulation of residue in the
purification container which needs proper treatment in time.
The aim of this study was to determine the effect of oxidation of zinc powder on
the purification of high-cobalt and high-germanium zinc sulfate solutions by adding
zinc powder with different amounts of zinc oxide powder to the solutions for test.
Experimental
Materials
The main materials for the study, zinc powders and zinc sulfate solutions, were
collected from a plant in China. Table 1 shows the main compositions of the two
types of zinc powder samples. Their zinc contents were more than 95%, and the lead
contents were higher than 1.5%. Figure 1 shows the phase compositions of the zinc
powders. It was obvious that they were mainly consisted of metallic lead and zinc.
The zinc sulfate solutions had the following compositions: 112.7 g/L Zn
2+ , 3.1 mg/L
Co
2+ , 0.1 mg/L Ge
4+ , 0.49 g/L Cd
2+ , and 0.17 g/L Cu
2+ . The other raw materials
used for purification are summarized in Table 2.
Method
The zinc powders and zinc oxide were firstly pre-dried and then grounded using a
ball mill. The Co
2+ and Ge
4+ concentrations were adjusted to 25 mg/L and 1 mg/L,
respectively, using CoSO 4 · 7H 2 O and GeO in the zinc sulfate solution. A 4 g/L zinc
A with particle size less than 0.05 mm or zinc B with particle size between 0.074 and
0.1 mm, as well as 0.3 g/L CuSO 4 and 7.5 mg/L Sb 2 O 3 were used for purification
with the stirring speed of 300 r/min and pH 5.0. The addition of ZnO (0.5 wt%, 1
L. Zheng et al.
changes in deposit morphology [8]. Cobalt and germanium impurities often remain
during the zinc leaching process. They affect the quality of cathode zinc obviously
and cause low current efficiency with high power consumption [9]. At present, the
zinc-antimony salt purification method is widely used for removal of cobalt and germanium in the second-stage purification of zinc sulfate solution. Various parameters,
such as cementation time, temperature, solution pH, concentration of activators, and
concentration of zinc ions in the electrolyte, may have influence in this process [10].
Metallic zinc is a good reactant for purification of zinc sulfate solutions. In humid
air, zinc powders stored in the natural state are easily oxidized to form dense oxide
films on their surfaces. These oxide films will change the kinetic reaction characteristics during the two-stage solution purification process, thereby increasing difficulties in removing cobalt and germanium by zinc itself. Also, the zinc oxidation
often causes increase of zinc consumption, difficult hydraulic filtration after purification, high filtration fabric consumption, and rapid accumulation of residue in the
purification container which needs proper treatment in time.
The aim of this study was to determine the effect of oxidation of zinc powder on
the purification of high-cobalt and high-germanium zinc sulfate solutions by adding
zinc powder with different amounts of zinc oxide powder to the solutions for test.
Experimental
Materials
The main materials for the study, zinc powders and zinc sulfate solutions, were
collected from a plant in China. Table 1 shows the main compositions of the two
types of zinc powder samples. Their zinc contents were more than 95%, and the lead
contents were higher than 1.5%. Figure 1 shows the phase compositions of the zinc
powders. It was obvious that they were mainly consisted of metallic lead and zinc.
The zinc sulfate solutions had the following compositions: 112.7 g/L Zn
2+ , 3.1 mg/L
Co
2+ , 0.1 mg/L Ge
4+ , 0.49 g/L Cd
2+ , and 0.17 g/L Cu
2+ . The other raw materials
used for purification are summarized in Table 2.
Method
The zinc powders and zinc oxide were firstly pre-dried and then grounded using a
ball mill. The Co
2+ and Ge
4+ concentrations were adjusted to 25 mg/L and 1 mg/L,
respectively, using CoSO 4 · 7H 2 O and GeO in the zinc sulfate solution. A 4 g/L zinc
A with particle size less than 0.05 mm or zinc B with particle size between 0.074 and
0.1 mm, as well as 0.3 g/L CuSO 4 and 7.5 mg/L Sb 2 O 3 were used for purification
with the stirring speed of 300 r/min and pH 5.0. The addition of ZnO (0.5 wt%, 1