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Abstract: Canada's 2021 Critical Minerals Strategy underscores the prominence of tungsten as a rare and critical element. Today, tungsten production relies on scheelite, categorized as a secondary resource due to its complex ore composition and lower-grade nature. This phenomenon stems from the depletion and utilization of high-grade wolframite reserves. In response to this context, the significance of synthetic scheelite has ascended, primarily due to its inherent advantages such as its low impurity content and enhanced accessibility, particularly in ongoing laboratory-based leaching investigations. The most recent advancements in laboratory-scale research have yielded a feasible and environment-friendly approach for leaching synthetic scheelite utilizing a mixed acid solution of H2SO4 and H2O2. This process is carried out under normal pressure and moderate temperatures. Despite this notable progress, there persists a dearth of comprehensive inquiries into this novel leaching method. Furthermore, a comparative analysis between processes involving natural and synthetic ores remains underexplored. This existing gap in the literature underscores the need for a collaborative endeavor to achieve a comprehensive understanding of the novel leaching technique and a rigorous evaluation of the possibilities for optimizing operational parameters. Ultimately, the objective is to offer valuable insights and enhancements concerning the utilization of reagents, thereby fostering a more efficient, economically viable, and environmentally sustainable leaching process. To achieve this, a promising 2-step method for scheelite leaching was explored, aiming to minimize the thermal decomposition of H2O2 and subsequently reduce its concentration. The results showed that the 2-step method is infeasible as it requires a longer reaction time compared to the synergistic leaching approach, resulting in reduced productivity and energy efficiency. Subsequently, a detailed investigation into the thermal decomposition process ensued, seeking to extract tungsten without compromising leaching efficiency. This was achieved by optimizing the operational conditions based on established literature for natural ore. The findings unveiled the potential for multiple recycling of lixivium, supplemented with depleted chemicals, thereby promoting an eco-friendly approach while reducing operational expenses. Notably, for an L/S (mL/g) ratio of 10, enhanced H2WO4 crystallization efficiency was achieved by optimizing the thermal decomposition duration, extended up to 6 hours. Moreover, experiments conducted without H2SO4 supplementation revealed the inherent optimization potential within the leaching system. Finally, optimization was undertaken in terms of reagent concentration, involving a reduction of H2SO4 concentration from 3 mol/L to 1 mol/L, an increase in temperature from 45°C to 60°C, and an additional 30-minute leaching duration. This collective effort culminated in proposing an environmentally conscious and cost-effective process for synthetic scheelite leaching in the solution of H2SO4 and H2O2.

Abstract: Tungsten is a critical metal. Critical metals are strategic metals with associated economic, social, energy, geostrategic and environmental issues. As a result, the need to monitor and to secure local supplies of critical elements, including tungsten, has become a strategic priority for many countries. The Canadian Cordillera hosts one of the largest tungsten-producing provinces in North America. Tungsten mineralization in the Cordillera occurs mostly as skarns, which are calc-silicate rocks resulting from the hydrothermal alteration of carbonate rocks by fluids exsolved from an intrusive body. The most significant among the tungsten deposits known in the Canadian Cordillera as well as in the North American Cordillera broadly, include the Cantung and Mactung skarn deposits. Because of their major tungsten endowments, these deposits provide an ideal framework to study the parameters that govern tungsten mineralization. In this dissertation, (1) we evaluate the paragenesis and mineral compositions in tungsten deposits from the Canadian Cordillera to determine the parameters that control the distribution of high- grade tungsten mineralization locally. (2) We constrain the magmatic sources of ore fluids in these deposits and the effects of fluid-rock interaction on the mineralization process and geochemical signatures in the Canadian Cordillera. And (3) we further evaluate the geochemical and paleogeographic controls responsible for the irregular distribution of tungsten in the North American Cordillera. (1) The composition of minerals in tungsten skarn deposits from the Canadian Cordillera are indicative of reduced fluids, which is characteristic of high-grade tungsten deposits. We document that the main tungsten-bearing mineral in these deposits, scheelite (CaWO4), precipitated throughout the entire evolution of the hydrothermal alteration but peaked at different stages for different deposits. Also, the preferential distribution of scheelite at the deposit scale was controlled by parameters such as the permeability and porosity of the host rock. (2) Strontium isotopes and chemical variations in the host rocks reflect fluid-rock interaction processes. Although the geochemical variability of the host rock precludes quantification of chemical exchange between ore fluids and host rock, the distribution of the ore within preferential areas in the host rock suggests a lithologic control on the tungsten mineralization. In addition, strontium isotope data indicates that crust-derived fluids, likely from the granitoids adjacent to the deposits, are the main contributors to the ore fluids. (3) Geochemical fingerprinting using neodymium isotopes reveals that the ore fluids that precipitated tungsten in the North American Cordillera are specifically associated with source materials of Mesoarchean to Paleoproterozoic average age. We propose that tungsten enrichment in the North American Cordillera is the product of intense weathering of the supercontinents Columbia and Rodinia, followed by chemical transport and accumulation of tungsten-rich materials along the Ancient North American craton margin. Subsequent melting of these enriched sediments caused by orogenic heating produced reduced melts that scavenged tungsten and formed the major tungsten deposits found today in North America.

Tungsten, an emerging contaminant, has no EPA standard for its permissible levels in drinking water. At sites in California, Nevada, and Arizona there may be a correlation between elevated levels of tungsten in drinking water and clusters of childhood acute lymphocytic leukemia (ALL). Developing a better understanding of how tungsten is released from rocks into surface and groundwater is therefore of growing environmental interest. Knowledge of tungstate ore mineral weathering processes, particularly the rates of dissolution of scheelite (CaWO4) in groundwater, could improve models of how tungsten is released and transported in natural waters.Our research focused on the experimental determination of the rates and products of scheelite dissolution in 0.01 M NaCl (a proxy for groundwater), as a function of temperature, pH, and mineral surface area. Batch reactor experiments were conducted within constant temperature circulation baths over a pH range of 3-10.5. Cleaned scheelite powder with grain diameters of 106-150µm were brought in contact with 0.01 M, NaCl in a Teflon vessel. Aliquots of solution were taken periodically for product analysis by ICP-OES. Changes in mineral surface characteristics were monitored using SEM and EDS methods.The integral method was used to interpret data from experiments and to develop a rate law. The specific rate for the dissolution of scheelite in groundwater is expressed as: (dM)/(sch overall)/dt = kr(Ksp - IAP)where kr is the reverse rate constant in units of mol/L-sec, Ksp is the solubility product in mol/L , and IAP is the ion activity product for scheelite in mol/L. At pH 6.2 and 20oC, the specific rate law for the dissolution of scheelite in a 0.01M NaCl groundwater proxy is:(dM)/(sch overall)/dt = (9.09x10-8) (2.63x10-10-IAP)The dissolution of scheelite shows variable behavior depending on the pH and temperature. At low pH, the release of Ca and W is disrupted by the formation and adsorption of tungstic acid onto grain surfaces. In groundwater with progressively higher alkalinity, W releases at a higher rate. The reaction also follows typical Arrhenius behavior. These results suggest that the warmer and more basic the groundwater that scheelite is in contact with, the greater the propensity for an elevated level of tungsten within it.