Heini Elomaa

Purpose: The soaring demand for cobalt for lithium-ion batteries has increased interest in the utilization of non-conventional cobalt sources. Such raw materials include complex ores containing minerals such as cobaltite and skutterudite, which, while rare, occur around the world, including in Finland, Canada, and the USA. The goal of this study was to evaluate the cradle-to-gate impacts of cobalt sulfate recovery from unutilized cobalt- and gold-bearing ores with the use of process simulation. Methods: A literature analysis was conducted to establish the state-of-the-art processing methods for complex cobalt ores containing significant amounts of gold. The drafted process was simulated using HSC Sim software to obtain a mass and energy balance, which was compiled into a life cycle inventory (LCI). The environmental impact categories (global warming, acidification, eutrophication, ozone depletion, photochemical smog creation, water use) were calculated in GaBi software. Uncertainty regarding the possible future raw material composition was studied, and the simulation was used to investigate process performance and to evaluate the effect of variation in the process parameters on the environmental impact indicators. Results and discussion: The results indicated that the main cobalt mineral type (cobaltite, linnaeite) had only minor effects on the evaluated impact categories. With cobaltite-dominated ores (High As case), the global warming potential (GWP) was estimated to be 20.9 kg CO₂-eq, of which 12.7 kg CO₂-eq was attributed to the hydrometallurgical process. With linnaeite-dominated ores, the equivalent values were 20.4 kg CO₂-eq and 11.0 kg CO₂-eq. The production of a high grade concentrate was observed to greatly decrease the impacts of the hydrometallurgical process, but the cobalt losses in the beneficiation stage and the mineral processing impacts would likely increase. The simulation showed that there is still potential to improve the cobalt recovery (to approximately 96%), which would also affect the indicator values. Conclusions: The impacts were estimated prior to intensive metallurgical testing to determine the possible high impact areas in the process. Based on this, it is suggested that, during hydrometallurgical processing, improved treatment of cobalt-containing wash waters and the optimization of oxygen utilization efficiency in pressure leaching are the most significant ways to decrease the environmental impacts. Optimal solutions for the concentrate could be found when experimental data on the minerals processing steps becomes available.

Purpose: Currently, almost all cyanide-free gold leaching processes are still in the development stage. Proactively investigating their environmental impacts prior to commercialization is of utmost importance. In this study, a detailed refractory gold concentrate process simulation with mass and energy balance was built for state-of-the-art technology with (i) pressure oxidation followed by cyanidation and, compared to alternative cyanide-free technology, with (ii) pressure oxidation followed by halogen leaching. Subsequently, the simulated mass balance was used as life cycle inventory data in order to evaluate the environmental impacts of the predominant cyanidation process and a cyanide-free alternative. Methods: The environmental indicators for each scenario are based on the mass balance produced with HSC Sim steady-state simulation. The simulated mass balances were evaluated to identify the challenges in used technologies. The HSC Sim software is compatible with the GaBi LCA software, where LCI data from HSC-Sim is directly exported to. The simulation produces a consistent life cycle inventory (LCI). In GaBi LCA software, the environmental indicators of global warming potential (GWP), acidification potential (AP), terrestrial eutrophication potential (EP), and water depletion (Water) are estimated. Results and discussion: The life cycle assessment revealed that the GWP for cyanidation was 10.1 t CO₂-e/kg Au, whereas the halogen process indicated a slightly higher GWP of 12.6 t CO₂-e/kg Au. The difference is partially explained by the fact that the footprint is calculated against produced units of Au; total recovery by the halogen leaching route for gold was only 87.3%, whereas the cyanidation route could extract as much as 98.5% of gold. The addition of a second gold recovery unit to extract gold also from the washing water in the halogen process increased gold recovery up to 98.5%, decreasing the GWP of the halogen process to 11.5 t CO₂-e/kg Au. However, both evaluated halogen processing scenarios indicated a slightly higher global warming potential when compared to the dominating cyanidation technology. Conclusions: The estimated environmental impacts predict that the development-stage cyanide-free process still has some challenges compared to cyanidation; as in the investigated scenarios, the environmental impacts were generally higher for halogen leaching. Further process improvements, for example in the form of decreased moisture in the feed for halide leaching, and the adaptation of in situ gold recovery practices in chloride leaching may give the cyanide-free processing options a competitive edge.

In the current study, platinum - present as a negligible component (below 1 ppb, the detection limit of the HR-ICP-MS at the dilutions used) in real industrial hydrometallurgical process solutions was recovered by an electrodeposition − redox replacement (EDRR) method on pyrolysed carbon (PyC) electrode, a method not earlier applied to metal recovery. The recovery parameters of the EDRR process were initially investigated using a synthetic nickel electrolyte solution ([Ni] = 60 g/L, [Ag] = 10 ppm, [Pt] = 20 ppm, [H₂SO₄] = 10 g/L) and the results demonstrated an extraordinary increase of 3·10⁵ in the [Pt]/[Ni] on the electrode surface cf. synthetic solution. EDRR recovery of platinum on PyC was also tested with two real industrial process solutions that contained a complex multi-metal solution matrix: Ni as the major component (>140 g/L) and very low contents of Pt, Pd and Ag (i.e. [Pt]/[Ni] enrichment ratio of 10¹¹ compared to the industrial hydrometallurgical solution. Furthermore, other precious metallic elements like Pd and Ag could also be enriched on the PyC electrode surface using the same methodology. This paper demonstrates a remarkable advance-ment in the recovery of trace amounts of platinum from real industrial solutions that are not currently considered as a source of Pt metal.

In the current study, electrodeposition-redox replacement was applied to a hydrometallurgical solution with the main elements of Ca (13.8 g L⁻¹), Al (4.7 g L⁻¹), Cu (2.5 g L⁻¹), Zn (1.2 g L⁻¹), Fe (1.2 g L⁻¹), S (1 g L⁻¹), Mg (0.8 g L⁻¹), P (0.5 g L⁻¹) and Ag (3.5 ppm). The solution originated from the leaching experiment of incinerator plant bottom ash, which was dissolved into 2 M HCl media at T = 30 °C. The resulting deposit on the electrode surface was analysed with SEM-EDS and the observed Ag/(Cu + Zn) ratio (0.3) indicated remarkable enrichment of silver on the surface, when compared to the ratio of these elements (Ag/(Cu + Zn)) in the solution (6.8 × 10⁻⁵). The enrichment of Ag vs. (Cu + Zn) could be demonstrated to increase ca. 4500 fold compared to the ratio of the elements in solution.

As a refractory gold mineral, pyrite needs to be oxidised prior to gold leaching. In this study, the effect of [Cl⁻] concentration (40.6–149.8 g/L), [Cu²⁺] concentration (0.8–31.6 g/L), pH (1.5–2.5) and temperature (25–90 –C) on the pyrite leaching rate was investigated. In addition, the open circuit potential (OCP) values of pyrite in cupric chloride solution were investigated. A linear regression model was constructed to predict pyrite dissolution rate i.e. corrosion current density. It was shown that the temperature had a significant positive effect on pyrite dissolution, while increased cupric ion concentration was also shown to provide some dissolution enhancement. According to the regression analysis, pH had no effect on the corrosion current density at OCP. Dissolution rates of pyrite varied between 0.05 and 2.9 µm/h. The activation energy values varied from 20 to 90 kJ/mol, indicated that the pyrite dissolution reaction rate was controlled by the chemical reaction or mixed mechanism rather than diffusion alone. The simultaneous increase in corrosion potential and corrosion current density indicated that the anodic pyrite dissolution reaction was rate determining at OCP.

Silver is a metal widely applied in renewable energy applications and therefore subject to resource scarcity. The paper presents a new approach for recovering silver from zinc-containing solutions mimicking hydrometallurgical base metal process solution. By nature, silver present in ores or concentrates is more noble than zinc and not effectively leached into the sulfate media during zinc hydrometallurgical processing. This paper presents a novel approach for concentrating and recovering silver present in minor amounts in zinc sulfate media. The electrodeposition–redox replacement (EDRR) method was investigated in synthetic zinc sulfate solutions ([Zn] = 60 g/L, [Ag] = 1 ppb–250 ppm, [H₂SO₄] = 10 g/L) containing silver as low as 1 ppb. The deposited metal coating was analyzed by electrochemical techniques and SEM-EDS. As a result, an enrichment of silver as nano- and microparticles on electrodes was evident. With the application of multiple EDRR steps (n = 160), the method was shown to result in a high purity Ag layer (Ag/Zn ratio ≈ 1500 in the product) from solution with minor Ag content (Ag/Zn ratio ≈ 0.0017 in solution). Moreover, at the concentration levels studied, the EDRR method was shown to outperform conventional electrowinning (EW).