Holistic Nitrogen Management Throughout the Project Life Cycle
Creators: name: Mutsaerts, Rachael
name: Kratochvil, David
Publisher: name: The University of British Columbia
Year: 2023

description: Nitrogen species management and treatment is an increasingly important aspect of mine development and environmental compliance. Ammonium, nitrite, and nitrate are introduced into mine water from ammonium nitrate fuel oil (ANFO) used for blasting. At precious metals mines that use cyanide for metal leaching, cyanide destruction and/or natural degradation are additional sources of ammonium, while the leaching process may produce thiocyanate. Additionally, the increasing complexity of ore bodies and metallurgical flowsheets has resulted in thiocyanate becoming a more common constituent of concern. Concentrations of these nitrogen species in mine impacted waters depend on multiple site-specific factors, and in many instances can provide useful insights into blasting practices and help improve water management. For instance, the ratio of NH₄₋N to NO₃₋N can aid in estimating the efficiency of ANFO use, or point to poor materials handling practices as a potential source of ammonium nitrate loss. Since most aqueous nitrogen species have either specific discharge limits or are regulated indirectly through effluent toxicity, water management may have a significant influence on the life cycle cost of treatment. Initial water management and treatment strategies are often based on water quality predictions relying on precedence from other projects. However, site-specificity makes these predictions subject to change, which creates the need for adaptive water management during mine development, operations, and closure. A case study of an underground gold mine in Northern Ontario is presented where the nitrogen balance and management are reviewed, including an examination of the relationship between nitrogen management and water treatment capacity. The mine utilizes a non-biological removal process involving an ion exchange process to remove and concentrate ammonium from their mine impacted water into a brine stream. The brine is then treated via air stripping to achieve net ammonia removal. The ion exchange media in use is naturally occurring zeolite, which exchanges sodium ions (Na⁺ ) for ammonium (NH₄⁺ ) in the water. Operational challenges and successes of the ion exchange system are discussed and plans for upgrades based on an in-depth analysis of the plant operations data and bench scale tests are reviewed. A separate case study involving treatability tests for thiocyanate (SCN- ) removal from mine tailings water using ion exchange combined with electro-oxidation will also be presented. Ion exchange removes thiocyanate from water and concentrates it into a small volume of highly electrically conductive brine solution. Electro-oxidation transforms thiocyanates into cyanide suitable for re-use, and the sulphate that is also generated can be precipitated as solid gypsum suitable for on-site disposal with tailings. The results suggest that ion exchange paired with electro-oxidation is a viable technology for removing thiocyanate from mine water, making it a possible alternative to biological processes. Overall, this paper explores realworld applications to build the case for a holistic approach to managing nitrogen species throughout the full life of the mine, which includes investments in monitoring and technological innovation.