Grants and Contributions:

Grant or Award spanning more than one fiscal year. (2017-2018 to 2022-2023)

The focus of this discovery grant proposal is advancing the fundamental science for the development of hydrometallurgical leaching and separation technologies. Four separate discovery projects have been identified for investigation. These include (1) the control of gold mobility in high temperature, acid autoclave treatment of refractory carbonaceous gold ores containing soluble chloride, (2) selenium removal from waste water and solid selenium stabilization, (3) development of anodes and control of anode chemistry for electrowinning of lead from methane sulfonic acid solutions and (4) the use of mixed, non-cyanide lixiviants for leaching of gold and silver.

Refractory sulfide gold ores and concentrates are commonly oxidized in autoclaves at 210-230°C to destroy pyrite and arsenopyrite prior to cyanidation. One of the limitations of the technology occurs where the gold ore also contains organic matter and the autoclave solutions contain small amounts of chloride (typically 10-50 ppm). A chemical system has tentatively been identified that is expected to precipitate the chloride and prevent this mechanism of gold loss.

Selenium is often present in mine-affected wastewater as selenite (IV) and selenate (VI). The precipitation of selenite is accomplished by co-precipitation with ferric hydroxide. The removal of selenate from wastewater is much more difficult, even more so due to chemical similarity to high levels of sulfate that are often present. The target level for selenium removal from wastewater is 1 ppb. An inorganic layered double hydroxide adsorbent has been identified that has the ability to adsorb selenate into the solid matrix structure. This is very promising but there is a need to study the solubility of the layered double hydroxide, as this will impact the long-term storage of the selenium-loaded adsorbent. A solvent extraction system has also been identified with some selectivity for selenate over sulfate from wastewater.

RWe have developed three processes for lead recovery or refining using methane sulfonic acid solution chemistry. The process is dependent on lead electrowinning at the cathode and oxygen evolution and acid regeneration at the anode. The anode electrochemistry of the lead-MSA system must be studied to identify anode systems and electrolyte chemistries that avoid lead dioxide formation and regenerate acid for leaching.

The use of non-cyanide lixiviants for gold is under worldwide investigation to provide alternatives to the use of cyanide for gold and silver leaching. The obstacles that prevent wider adoption of alternatives to cyanide leaching are numerous including high reagent consumption, slow kinetics of leaching of gold, difficult recovery of gold from the leach solution, etc. In many cases it is believed that some of these difficulties may be overcome by introduction of mixed lixiviant systems.