*Article* **Recovery of Cobalt from the Residues of an Industrial Zinc Refinery**

**Laurence Boisvert 1, Keven Turgeon 1, Jean-François Boulanger 2, Claude Bazin 1,\* and Georges Houlachi <sup>3</sup>**


Received: 4 November 2020; Accepted: 19 November 2020; Published: 22 November 2020

**Abstract:** The electrolytic production of metallic zinc from processing zinc sulfide concentrates generates a residue containing cadmium, copper, and cobalt that need to be removed from the electrolytic zinc solution because they are harmful to the zinc electro-winning process. This residue is commonly sent to other parties that partly recover the contained elements. These elements can generate revenues if recovered at the zinc plant site. A series of laboratory tests were conducted to evaluate a method to process a zinc plant residue with the objective of recovering cobalt into a salable product. The proposed process comprises washing, selective leaching, purifying and precipitation of cobalt following its oxidation. The process allows the production of a cobalt rich hydroxide precipitate assaying 45 ± 4% Co, 0.8 ± 0.2% Zn, 4.4 ± 0.7% Cu, and 0.120 ± 0.004% Cd at a 61 ± 14% Co recovery. Replicating the whole process with different feed samples allowed the identification of the critical steps in the production of the cobalt product; one of these critical steps being the control of the oxidation conditions for the selective precipitation step.

**Keywords:** zinc residue; cobalt hydroxide; cementation; leaching; oxidative precipitation

#### **1. Introduction**

The conventional roast-leach zinc extraction process yields a solid residue consisting of a mixture of zinc, copper, cadmium, and cobalt. Some zinc smelting plants process that residue [1–3] to recover the contained valuable metals. The high value of cobalt makes it an excellent candidate for a first step in the development of a process to recover the metals contained in that residue [1]. Indeed, cobalt is currently considered as a critical material [4] as it is used in the making of Li-ion batteries [1,5], increasingly strategic for the shift toward green energy or more precisely toward a 100% electric vehicle market [4]. The «critical» status of cobalt is related to uncertainties about the supply of the metal. In fact, 60% of the world's cobalt is mined in the Congo and 80% of its production is processed in China [4]. In the case of a cobalt supply disruption due to a natural disaster, a change of government or a boycott [6], a zinc residue that contains more than 2% Co [1] becomes an interesting alternative to primary cobalt.

The extraction of cobalt from zinc plant residues is not discussed in many papers except in a recent review [1]. Few papers [2,3,7,8] were found to describe processes to recover cobalt from zinc residues. Wang and Zhou described a process [2] to treat a zinc residue containing active carbon and organic compounds used to capture the cobalt and the manganese from the zinc solution prior to the Zn electro-winning step. The process developed for that residue consists of a washing stage followed by two roasting steps at different temperatures, leaching, and precipitation of iron and manganese followed

by anion exchange and solvent extraction of cobalt using extractant P507. Fattahi et al. described a process [3] to extract cobalt from the zinc residues of Iranian Zn smelters that add permanganate to the zinc solution in order to oxidize Co(II) to Co(III) which is precipitated by increasing the pH. The precipitated zinc residue contains about 2% Co and more than 10% Mn. The authors [3] propose a reductive leaching followed by a precipitation of cobalt sulfide (CoS) using Na2S to selectively recover the cobalt from the manganese. Although the conventional approach for the purification of zinc solution is the cementation of Cu, Cd, and Co onto zinc dust [1,9], it was not possible to find in the Western literature a process dedicated to the treatment of this type of residue, although a qualitative description of a possible process can be found in [1]. Li et al. [10] studied the rate of leaching of a zinc residue but did not indicate if the residue is obtained by cementation on zinc dust and the authors did not attempt to process the leach solution to obtain a salable cobalt product.

This paper describes a processing scheme to produce a cobalt rich compound from a zinc plant residue produced by the cementation process. The objective of the test work presented in this paper is not to optimize an existing flowsheet but to propose a process to obtain a salable cobalt compound from a zinc residue produced by cementation on zinc dust. As requested by the industrial partner, the proposed process should use only open tank reactors for leaching and precipitation without resorting to solvent extraction nor ion exchange to obtain the cobalt product. This last constraint complicates the process to be developed as selective extractants are available for cobalt [1,7]. However, if the economics of the process are not favorable, the use of SX will be investigated but it is unlikely that the addition of a SX plant could move the process economics toward more profitable conditions.
