Aqueous Geochemistry and Mineralogy of Tungsten with Emphasis on Mine Wastes

摘要

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 pre-dict 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, geo-thermal waters, and mine tailings. Although progress has been made, the mechanisms of scheelite and wolfram-ite 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 mineral-ogical 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 under-standing 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.