Chemical, physical and biological technologies for removal of sulphate from mine tailings pond water (8 g SO₄²⁻/L) were investigated. Sulphate concentrations of approximately 1,400, 700, 350 and 20 mg/L were obtained using gypsum precipitation, and ettringite precipitation, biological sulphate reduction or reverse osmosis (RO) after gypsum pretreatment, respectively. Gypsum precipitation can be widely utilized as a pretreatment method, as was shown in this study. Clearly the lowest sulphate concentrations were obtained using RO. However, RO cannot be the only water purification technology, because the concentrate needs to be treated. There would be advantages using biological sulphate reduction, when elemental sulphur could be produced as a sellable end product. Reagent and energy costs for 200 m³/h tailings pond water feed based on laboratory studies and process modelling were 1.1, 3.1, 1.2 and 2.7 MEur/year for gypsum precipitation, ettringite precipitation, RO and biological treatment after gypsum precipitation, respectively. The most appropriate technology or combination of technologies should be selected for every industrial site case-by-case.

Microorganisms enter the flotation process mainly from intake water and ore material. The diversity and number of microorganisms can vary significantly from mine to mine. In flotation, the conditions including oxygen levels, temperature, and nutrients from ore, intake water, and reagents are often favorable for the microbial growth. The mining industry aims to close the water loops, which is expected to result in the accumulation of microorganisms in the process waters with potential effects on flotation performance. Bioflotation, bioleaching, and bio-oxidation have been studied for decades as tools for concentrating and dissolving minerals. In contrast, there is limited scientific literature or industrial knowledge about microorganisms that naturally inhabit and prevail in minerals processing applications over a wide pH range. Microorganisms affect minerals when they selectively attach to the surfaces, produce extracellular polymeric substances (EPS) and polysaccharides, oxidize or reduce the minerals, change the pH and Eh of the process solution, and degrade organic flotation chemicals. Microorganisms contain different structural components that affect their surface chemistry, charge, and behavior in flotation, but these properties may also change via adaptation and solution conditions. Almost all studies on flotation have focused on chemical and physical parameters, and the role of naturally occurring microorganisms has remained underexplored. Advances in genomics and proteomics offer possibilities to describe not only which microorganisms are present, but also which physiological functions are being exercised. This article reviews the current knowledge of microorganisms in various mineral processes, identifies potential microbe–mineral interactions in flotation, describes the gaps in current knowledge, and concludes with the potential effects of microorganisms on flotation, especially in closed water loops.

Acid bioleaching of a low-grade and polymetallic sulfide concentrate was studied, in order to determine suitable feed material particle size and pH for efficient leaching of valuable metals. The sulfide concentrate consisted of pyrite (50 wt %), pyrrhotite (31 wt %), quartz (10 wt %) and lower amounts of cobalt, nickel, zinc and copper (each <1 wt %). After adaptation of microorganisms in shake flasks, stirred tank tests were conducted at different pH levels and supplementing feed material at different particle sizes (milled to d80 < 150 µm, <50 µm, <28 µm, <19 µm). The operation at pH 1.8 was seen prone to iron precipitation, while this was not observed at a pH between 1.3 and 1.5. Additional milling to decrease particle size from the initial d80 < 150 µm had a major positive effect on cobalt- and nickel-leaching yields, proposing that at least d80 < 28 µm should be targeted. The best leaching yields for the main economic elements, cobalt and nickel, were 98% and 94%, respectively, reached with d80 < 19 µm at pH 1.3. However, it was considered that at pH 1.5, similar results could be obtained. This research sets the basis for continuing the experiments at a continuous pilot scale.

Copper slags produced in vast quantities in smelting operations could be considered as secondary material sources instead of stockpiling them in landfills. This study investigates the recovery of valuable metals from copper slag and the valorisation of the leach residue as construction material in line with the principles of a circular economy. By taking into account that the environmental characterization of the as-received copper slag did not allow its disposal in landfills without prior treatment, chemical and biological leaching were tested for the recovery of metals. Pre-treatment with acids, namely HNO₃ and H₂SO₄, resulted in the extraction of several target metals and the production of an almost inert waste. Despite the clearly better oxidative conditions prevailing in the bioleaching reactors, chemical leaching resulted in the higher dissolution of Cu (71% vs. 51%), Co (70% vs. 36%), and Zn (65% vs. 44%). The acid consumption was much lower during the bioleaching experiments compared to the chemical leaching. The bioleach residue was suitable for its use as supplementary cementitious material, showing a better performance than the reference sample without causing any detrimental effects to the calcium aluminate cement (CAC) quality. The complete valorisation of copper slags is expected to improve the economics of the process, by avoiding landfill costs and producing saleable products with high added value.

It has only recently been discovered that naturally prevailing microorganisms have a notable role in flotation in addition to chemical process parameters and overall water quality. This study’s aim was to assess the prevailing microbial communities in relation to process chemistry in a zinc and copper mineral flotation plant. Due to the limitations of cultivation-based microbial methods that detect only a fraction of the total microbial diversity, DNA-based methods were utilised. However, it was discovered that the DNA extraction methods need to be improved for these environments with high mineral particle content. Microbial communities and metabolism were studied with quantitative PCR and amplicon sequencing of bacterial, archaeal and fungal marker genes and shotgun sequencing. Bacteria dominated the microbial communities, but in addition, both archaea and fungi were present. The predominant bacterial metabolism included versatile sulfur compound oxidation. Putative Thiovirga sp. dominated in the zinc plant and the water circuit samples, whereas Thiobacillus spp. dominated the copper plant. Halothiobacillus spp. were also an apparent part of the community in all samples. Nitrogen metabolism was more related to assimilatory than dissimilatory nitrate and nitrite oxidation/reduction reactions. Abundance of heavy metal resistance genes emphasized the adaptation and competitive edge of the core microbiome in these extreme conditions compared to microorganisms freshly entering the process.

Municipal solid waste incineration bottom ash (MSWI BA) is the main output of the municipal solid waste incineration process, both in mass and volume. It contains some heavy metals that possess market value, but may also limit the utilization of the material. This study illustrates a robust and simple heap leaching method for recovering zinc and copper from MSWI BA. Moreover, the effect of autotrophic and acidophilic bioleaching microorganisms in the system was studied. Leaching yields for zinc and copper varied between 18–53% and 6–44%, respectively. For intensified copper dissolution, aeration and possibly iron oxidizing bacteria caused clear benefits. The MSWI BA was challenging to treat. The main components, iron and aluminum, dissolved easily and unwantedly, decreasing the quality of pregnant leach solution. Moreover, the physical nature and the extreme heterogeneity of the material caused operative requirements for the heap leaching. Nevertheless, with optimized parameters, heap leaching may offer a proper solution for MSWI BA treatment.