Aqueous Geochemistry and Mineralogy of Tungsten with Emphasis on Mine Wastes

Abstract

Tungsten is a valuable industrial metal with limited effective substitutes for its many industrial uses. Tungsten is classified as a critical mineral by many countries, including Canada and the United States. As such, tungsten mining is expected to increase in the future, generating waste with potential as point sources of metal leaching to the environment. Concerns about the toxicity of tungsten have recently emerged, but the element is not regulated in mine waste. The geochemical behavior of tungsten is understudied, hampering the ability to predict tungsten leaching from past, present, and future mining operations. The low-temperature aqueous mobility of tungsten is pH dependent, with adsorption as a primary factor limiting its dissolved concentrations in the environment. Tungsten, which forms strictly anionic species, adsorbs to mineral surfaces at low pH, and is most mobile in alkaline waters. The speciation of tungsten has been shown to affect its adsorption propensity, with WO42- more strongly adsorbed than polymeric and thiolated species. Aqueous tungsten concentrations generally decrease with decreasing pH in surface waters, groundwaters, geothermal waters, and mine tailings. Although progress has been made, the mechanisms of scheelite and wolframite dissolution are still poorly understood. Mine waste generated from processing tungsten-rich ores contains elevated tungsten concentrations; the weathering behavior of scheelite and wolframite need more research to mitigate the potential for tungsten leaching during the long-term storage of mine waste. Identifying mineralogical changes to scheelite and wolframite that may occur during ore processing and weathering of mine waste would be beneficial for understanding the leaching behavior of these minerals. Based on current knowledge, it is likely that tungsten is mobilized as both dissolved anions and adsorbed to colloidal particles. Improved understanding of the environmental geochemistry of tungsten will help ensure its sustainable supply in the future and permit more effective remediation at abandoned and existing mines.

Tungsten tailings issues and reprocessing solutions

“…The study of Kazamel et al points out that the geochemical behavior of tungsten is not well studied, hindering the prediction of W leaching from ongoing and former mining operations [41] . Limited studies have examined the behavior of W in mine wastes [67] . Kazamel et al concluded that the mobility of tungsten at low temperatures is primarily controlled by pH and its adsorption behavior [41] .…”

“…Their research explains the limited solubility of CaWO 4 in neutral or acidic mine waters, and the mobility of the small amount of W released is governed by the presence of Fe-oxyhydroxide minerals, the stability of the depositional environment, and the transport of colloidal material in surface water. Few studies have addressed the mobility of W in mine waste [67] .…”

Metabolic evolution of pyranopterin-dependent biochemistry

“…In hydrothermal systems, W can even exceed Mo concentrations (23) . These differences in environmental abundances are likely due to several factors, including the higher reduction potential of molybdenum relative to tungsten compounds or increased likelihood of thiomolybdate formation and precipitation (24–26). The VI to IV reduction potential is typically higher for Mo than for W regardless of the ligand system (27), indicating that in ecosystems with low environmental reduction potential ( E h ), Mo(VI) may be reduced to Mo(IV), while W(VI) remains oxidized.…”

“…The VI to IV reduction potential is typically higher for Mo than for W regardless of the ligand system (27), indicating that in ecosystems with low environmental reduction potential (E h ), Mo(VI) may be reduced to Mo(IV), while W(VI) remains oxidized. Equilibrium models of tungstate and molybdate/molybdenum sulfides suggests the selective depletion of molybdate at concentrations of sulfide between ~10 uM to 1 mM, while tungsten disulfides will form only in environments with >1 mM sulfide in marine environments (24)(25)(26), suggesting that tungstate may have been more available in both the ferruginous Archean and plausible euxinic Phanerozoic marine environments than in modern day oceans. Additionally, the lower reduction potentials of W complexes compared to Mo suggest that W-pterin may be the preferred cofactor for biochemical reactions involving lower potentials, which have been hypothesized to be more common in the earliest phases of metabolic evolution (14,16).…”