*2.1. Instrumentation and Reagents*

The solid and liquid samples are assayed using a MP-AES 4200 (microwave-plasma atomic emission spectrometer) from Agilent Technology (Santa Clara, CA, USA). For solid samples, 0.5 g sub samples are taken and digested in aqua regia for analysis using the MP-AES. The pH and Eh of the solution are, respectively, measured using a Fisher Scientific accumet XL600 pH-meter (Waltham, MA, USA), an Orion pH probe, and an Orion Oxidation Reduction Potential (ORP) probe from Thermo scientific (Waltham, MA, USA). Table 1 presents the reagents used for the experimentation.


**Table 1.** Reagents used for the experimentation.

#### *2.2. Provenance of the Zinc Residue*

The zinc residue used for the test work is provided by the CEZinc refinery [11] in Valleyfield, QC, Canada. Figure 1 shows the zinc extraction process and identifies the origin of the Co-bearing zinc residue. The plant processes zinc sulfide concentrates assaying more than 50% Zn, 0–5% Pb, less than 2% Cu, 0.5% Cd and from 50–200 g/t Co. The main impurities are iron (>8%) and sulfur (>30%). The extraction process of zinc begins by a roasting of the zinc sulfide concentrate to oxidize sulfur into SO2 that is subsequently converted into sulfuric acid (H2SO4). The roasting also transforms the zinc sulfide into zinc oxide which is soluble in weakly acidic solutions. The roasted product is leached in sequence with weak and strong sulfuric acid to solubilize the zinc oxide. The metallic impurities (Fe, Cu, Cd, Co) follow the zinc into the solution. The solution then undergoes a neutralization, during which the solubilized iron is precipitated as jarosite [8]. The purification of the iron-free solution from

the remaining metallic impurities is done using cementation on zinc dust [9] where copper, cadmium, and cobalt displace the zinc of the zinc powder through the reaction:

$$\rm{M}\_{aq}^{2+} + \rm{Zn}\_{s} \rightarrow \rm{Zn}\_{aq}^{2+} + \rm{M}\_{s} \tag{1}$$

where *M* stands for copper, cadmium, or cobalt. The reduced copper, cadmium, and cobalt are plated onto the surface of the zinc powder. The cemented powder is separated from the solution by a leaf press filter. The recovered solid is the «Zinc Plant Residue» (ZPR) considered in the following study. This residue is currently transferred to another plant for further processing. Zinc is finally electro-won from the purified solution (See Figure 1). The spent electrolyte is recycled back to the leaching step.

**Figure 1.** Simplified flowsheet of the zinc extraction process.

#### *2.3. Characterization of the Zinc Plant Residue (ZPR)*

A 20 kg sample of ZPR was collected by the CEZinc personnel at the discharge of the press filter. The sample was shipped wet to the laboratory for the test work. The received sample was split wet into ten parts after spreading the material onto a plastic sheet. One 2 kg sample was put in an oven for drying overnight and the remaining material was kept wet for the subsequent test work. A portion of the dried sample is shown in Figure 2a. The material is strongly agglomerated with practically unbreakable lumps. Sulfuric acid and sulfates are likely responsible for the observed particles binding. It rapidly appeared that this material could not be characterized (chemical composition, size distribution, etc.) as is and it was decided to wash the residue in water to remove any excess of sulfuric acid before drying. Washing was carried out by mixing 500 g of wet residue in 2700 mL of water (15% solids in mass) in a beaker for 90 min at room temperature (25 ◦C). The washed residue was separated from the solution by vacuum filtration and dried overnight in an oven. The dried washed residue is shown in Figure 2b and is found to be more amenable to characterization than the raw residue.

**Figure 2.** ZPR as is and after washing with water. (**a**) Raw residue after drying; (**b**) Residue after washing and drying.

#### 2.3.1. Specific Gravity

The measured specific gravity (Gas pycnometer, HumiPyc model 2 from Instruquest, Coconut Creek, FL, USA) of the washed ZPR is 3.95 g/cm3. Since the densities of zinc and of the cemented metals are all above 5 g/cm<sup>3</sup> this result indicates that the residue is not made exclusively of pure metals.

### 2.3.2. Size Distribution

Figure 3 shows the particle size distribution obtained by sieving the washed ZPR on a Tyler screen series from 1.2 mm down to 0.038 mm. The ZPR is coarse with a D80 of about 900 μm. The material coarseness will pose a problem for the sampling for assaying of the residue. Indeed, since assaying of the sample implies collecting a 0.5–1.0 g sample for the digestion prior to analysis using the emission spectrometer (Section 2.1), one can expect a significant variability in the assays of the residue due to the fundamental error of sampling [12]. For ore type material, this error is reduced by pulverizing the sample prior to sampling. However, the metallic and ductile nature of the ZPR makes impossible the pulverization of the sample, and thus one should expect a variability in the assays that data reconciliation [13–16] as applied here should be able to attenuate.

**Figure 3.** Size distribution of the residue.

#### 2.3.3. Chemical Composition

Table 2 gives the chemical composition of the ZPR sample. The measured assays of the sample are compared (see Table 2) to the typical composition of the ZPR provided by the CEZinc plant. The general proximity of the sample composition with the typical composition of the residue provided by CEZinc confirms that the received sample is representative of the residue usually released by the plant. The difference in the composition is attributed to the washing as discussed later. The Co content is consistent with typical ZPR from other zinc smelters [1–3].


**Table 2.** Composition of the ZPR.

\*: Average ± standard deviation of three samples.

#### 2.3.4. X-ray Diffraction (XRD)

Figure 4 shows the XRD pattern (Instrument: Aeris, Malvern Panalytical (Malvern, UK) obtained for the residue. The XRD shows the presence of metallic zinc, copper, and possibly metallic lead. The peaks for Cd and Co are not visible due to the low contents of these elements. The presence of Pb is confirmed by the assays (see Table 2) and is reported under the form of PbO2 for the residue of an Iranian zinc smelter [3] and under the form of PbSO4 [10] for a Chinese zinc residue. The XRD results of Figure 4 also show the presence of copper, zinc, and calcium sulfates likely responsible for the observed low specific gravity of the residue (Section 2.3.1). These sulfates, except gypsum, could be completely removed by a longer or slightly acidic or hot water wash of the residue as reported in [2].

**Figure 4.** XRD powder pattern of the CEZinc ZPR.

#### **3. Results**

#### *3.1. Washing of the ZPR*

The first step of the proposed process is the washing of the ZPR. Initially done to eliminate the sulfuric acid from the ZPR, the washing step provides an economic way to pre-concentrate the cobalt by allowing a partial elimination of the soluble zinc and cadmium sulfates contained in the ZPR. The results of a washing test are shown in Figure 5 and Table 3. Washing is carried out at room temperature using water at 15% solids in mass. Figure 5 shows the measured concentrations of Zn, Cd, Ca, Co, and Cu during washing. The solution pH falls from 7.2 to 6.3 during the washing step due to the release of the residual sulfuric acid. Table 3 gives the proportions of metals removed from the ZPR calculated using:

$$D\_m = 100 \frac{V \text{x}\_m}{V \text{x}\_m + W\_{\text{ZPR}} \text{y}\_{\text{ZFR}}} \tag{2}$$

where *Dm* is the dissolved proportion of metal *m*, *V* is the volume (L) of the wash solution, *xm* the metal content (g/L) in the solution. *WZPR* and *yZPR* are, respectively, the weight (g) and the fraction of metal *m* in the dried washed ZPR. Results presented in Table 3 show that, respectively, 20% and 43% of the zinc and cadmium contained in the raw ZPR are removed by a 30 min wash. About 4% of the cobalt contained in the ZPR is lost during the operation. Higher impurity removal can be achieved by washing the ZPR for 90 min but at increased cobalt losses as shown in Figure 5. The zinc is likely under an insoluble metallic form as metallic zinc dust is used for the cementation, while cadmium is likely present as a sulfate. The 4% Co dissolution is an indication that some cobalt is under the form of sulfate, with the remaining being metallic. The washing step can be viewed as a selective leaching operation for which the leaching conditions are adjusted to target specific metals [1,2]. The use of diluted acid in replacement of water could yield a better elimination of zinc and cadmium at the expenses of more important cobalt losses into the wash solution [10].

**Figure 5.** Dissolved proportion of metals in time (25 ◦C, 15% solids in mass).

**Table 3.** Dissolved metals after 30 min of washing with water (25 ◦C, 15% solids in mass).


\*: average ± standard deviation of three tests.
