Deep and efficient removal of lead ions by zinc-graphite combination purifying agent from ZnCl2-NH4Cl-H2O system

Abstract

Efficient recovery of metals from secondary resources is essential to address resource shortages and environmental crises. Recovery of secondary zinc resources is usually treated by hydrometallurgical processes, with ammonia treatment being a preferred option due to its advantages of selective leaching and recycling. However, ammonia leach solutions often contain high levels of lead, which hinder zinc electrowinning and require urgent purification. In this paper, a novel proposal was made to use a zinc-graphite composite purifier to remove lead ions from ammonia leach solution in depth and with high efficiency. Experimental results demonstrate significant zinc savings and lead removal efficiencies, the use of zinc-graphite to purify the solutions containing 0.075 and 0.2 g/L of lead can save 33.3 % and 56.7 % of zinc, respectively, and achieve a lead removal ratio of more than 98.7 %. The enhancement mechanism of graphite on lead ion removal efficiency may exist for two reasons. Firstly, when graphite is present, the purification residue forms a flocculent, loose and porous precipitate, which reduces the wrapping of zinc particles by the purification residue. In addition to this, the added graphite is electrically conductive and has the effect of transferring electrons, accelerating the replacement efficiency of zinc for lead. Overall, this work proposes a deep and highly efficient scheme for the removal of lead ions from ammoniacal leach solution by adding graphite, and the enhancement mechanism of graphite is also analyzed. The scheme presents an innovative and widely applicable solution for lead ion removal, with potential applications in other solution purifications.