Zinc is the third most non-ferrous metal following aluminum and copper. About 13.8 million tons of zinc was produced in 2018. Out of the total zinc production, 90% of zinc is produced by hydrometallurgical route using primary or secondary resources. The hydrometallurgical processing option involves leaching and a few purification steps producing high pure zinc solution suitable for electrowinning. During this operation, various kinds of waste residues are generated at different stages. These waste residues contain toxic but valuable heavy metals. Due to the potential release of toxic heavy metals into the surface and ground water, these waste residues are classified into a hazardous category. Therefore, such wastes are securely stored inside the industrial premises, encroaching valuable land mass. However, these wastes may serve as the secondary resources of valuable metals if technologically and economically feasible extraction techniques are developed.
The hydrometallurgical extraction of metals from low grade ores, secondary resources and wastes is a highly attractive process. This attractiveness is due to economic, environmental and technical reasons. Reports are available on hydrometallurgical recovery of zinc and other valuable metals from industrial wastes [1
]. In a research conducted by Kul and Topkaya [3
], the extraction of germanium and other valuable metals was studied, whereas in the study of Wang and Zhou [4
], a hydrometallurgical process was reported for the recovery of cobalt oxide from zinc plant residue. A process was developed by Moradkhani et al. [5
] for the recovery of cobalt and manganese from zinc plant residue. The process involved reductive leaching of zinc plant residue followed by solution purification by cadmium, and cobalt-manganese separation by precipitation of cobalt with beta-napthol. The sulfuric acid leaching of cadmium from zinc plant has been examined by Garabaghi et al. [6
]. In the study of Palencar et al. [7
], the purification of steel making dust by cementation was described and the highest removal efficiency (>99%) of cadmium and lead was achieved from chloride solution.
A zinc ore concentrate obtained from various mines in and out of Iran, mixed in different ratios, is processed by the leach-electrolysis process to produce a high purity zinc metal. The process flowsheet followed by the industry is given in Figure 1
. The zinc concentrate processing generates a variety of solid wastes. These solid wastes contain a number of toxic but valuable metals. These metals must be recovered prior to waste disposal for environmental and economic benefits.
As shown in the process flowsheet (Figure 1
), zinc from the concentrate is recovered by leaching with dil. sulfuric acid at pH 1. After leaching, the slurry is neutralized to pH 5 by adding lime. The slurry is then filtered to generate pregnant leach solution and a solid waste (Cake leach). The cake leach is washed with water. The washing at pH 2.5 is recirculated to initial leaching stage. The wastes cake leach and cake wash may mainly contain gypsum and minor quantities of minerals of zinc concentrate.
The pregnant leach solution (PLS), apart from zinc, contains other impurity elements like Co, Ni, Cd, etc. Cobalt from the PLS is removed by oxidative precipitation with KMnO4 and lime at pH 3.7. At this stage, cobalt is precipitated as cobalt cake. This waste is expected to contain Co(OH)3, gypsum as major phases.
Nickel and cadmium impurities in the PLS are removed by cementation using zinc powder at 65–70 °C. The waste (Ni-Cd cake) generated at this stage is separated through filtration. The Ni-Cd cake residue contains zinc, nickel and cadmium. The purified leach solution is then electrowon to produce zinc metal at the cathode, which is then re-melted to produce zinc ingot. During the melting process, a slag is generated.
The solid wastes generated during the zinc concentrates processing include: Cake leach, Cake wash, Cobalt cake, Ni-Cd cake and Slag. Chemical analyses of the residues are given in Table 1
. Out of the above waste residues, with respect to valuable metal contents, cobalt cake, Ni-Cd cake and slag are worth investigating for the development of process to recover metal values. In this paper, development of a process for recovery of metal values from Ni-Cd cake is presented.
A.C. was in charge of supervising this project, he contributed to experimental design and he was involved in the redaction of this paper; S.K.S. designed and performed experiments, and he wrote the present paper; M.K.R. contributed in the redaction of the paper and she brought her knowledge of this tailing; M.B. used his network to provide the tailing and he contributed to the redaction of this paper. All authors have read and agreed to the published version of the manuscript.
This work was supported by the national program “Investissements d’avenir” with the reference ANR-10-LABX-21-RESSOURCES21.
One of the authors S.K. Sahu is thankful to the Director, CSIR-National Metallurgical Laboratory, Jamshedpur, India for granting sabbatical leave to carry out this work. Furthermore, authors want to acknowledge Miss Zoreh Kazemi for her support.
Conflicts of Interest
The authors declare no conflict of interest
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General flowsheet of ziterite process by hydrometallurgical route.
SEM micrograph with overlaid EDX (Energy Dispersive X-ray Spectroscopy) mapping of Ni-Cd cake waste residue.
SEM image with over laid EDS mapping of leach residue obtained after of Ni-Cd cake waste residue with mixture of hydrochloric acid and sulfuric acid.
Effect of equilibrium pH on the extraction of zinc and cadmium from the leach solution with 1 mol/L HDEHP in presence of various concentration of TEHA (0–1M TEHA) in kerosene.
SEM image with over laid EDS mapping of cement material obtained after cementation of cadmium from leach solution.
Effect of phase volume ratio (O/A) on the extraction of zinc from purified leach solution.
Mc-Cabe Thiele diagram for the solvent extraction of zinc from purified leach solution.
(a) Simulation model for counter current extraction of zinc from purified leach solution using mixture of 1 mol/L HDEHP and 0.5 mol/L tris-2-ethylhexyl amine (TEHA) as extractant. (b) Counter current extraction showing concentration of metal ions in the aqueous and loaded organic phases at different stages.
Effect of equilibrium pH on the extraction of nickel with mixture of HDEHP and TEHA.
Complete process flowsheet for the recovery of metal values from Ni-Cd cake from an Iranian zinc plant.
Chemical analyses of zinc process residues in % (weight).
Chemical composition of Ni-Cd waste residue, leach solution and leach residue.
|Element||Ni-Cd Cake Waste Residue (%)||Leach Solution (mg/L)||Leach Residue (%)|
Chemical composition of leach solution, purified leach solution and cement material.
|Element||Leach Solution (mg/L)||Purified Leach Solution (mg/L)||Cement Material (%)|
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