Leaching Behaviour and Speciation of Pb, Zn and Cu in Stabilized Gold Mine Tailings
[1]
Restituta Paul Mapinduzi, Department of Transportation and Geotechnical Engineering, University of Dar es Salaam, Dar es Salaam, Tanzania.
[2]
Pancras Mugishagwe Syldion Bujulu, Department of Transportation and Geotechnical Engineering, University of Dar es Salaam, Dar es Salaam, Tanzania.
[3]
William John Senkondo Mwegoha, Department of Environmental Engineering and Management, University of Dodoma, Dodoma, Tanzania.
Leaching and speciation of heavy metals in stabilized gold mine tailings were investigated by using leaching tests and leaching model (Visual MINTEQ 3.1®) respectively. Results indicate that the Stabilized/Solidified (S/S) tailings has high acid neutralizing capacity, which is dependent on binder content used. The dissolution of alkaline metals and oxyanions (such as SO42-, HCO32-) raises both the pH and electric conductivity of the leachant and, in-turn, affects leaching of the heavy metals. Higher leaching rates were observed with low pH leachants. Speciation modeling reveals that under acidic conditions, the solution is dominated by (Cu2+, CuCl and CuSO4(aq)), (Zn2+, ZnCl+, ZnCl42-(aq), ZnCl3-, ZnCl2(aq)) and Zn(SO4)2-) and (Pb(OH)2(aq) and PbOH+) for Cu, Zn and Pb respectively. Under alkaline condition, metal hydroxides are mostly leached into the solution. Adsorption simulation results reveal that only <1% of each metal is immobilized on either HFO or Gibbsite, with a pH dependent sorption process. The maximum adsorption occurs between pH 6 and 9. Based on the observed cumulative amount of metals sorbed, the affinity trend appears to be Pb2+ > Cu2+ > Zn2+ for HFO and Zn2+ > Pb2+ > Cu2+ for Gibbsite. It was further observed that the major leaching mechanism for Cu2+, Zn2+ and Pb2+ in stabilized, non-dissolving cylindrical monolithic tailing samples is surface washout, which is dominant during the whole leaching period. It was also observed that the leaching of the soluble fraction of metals is through surface washout. It can be concluded that that the leaching of the three metals in both granular and monolithic material is pH dependent.
Heavy Metals, Leaching, Stabilization, Speciation, Tailings
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