Environmental Case Study
Safe and Responsible Management of Cyanide in Minera Penmont
Sodium cyanide is used in an aqueous solution to dissolve and extract gold from ore-bearing rock. However, at high concentration it can be dangerous to the health of human and many living organisms. We are therefore strongly committed to safe and responsible management of cyanide to guarantee safe conditions for our people, contractors, surrounding communities and to the environment.
The “Cyanide Code”, developed by the International Cyanide Management Institute (ICMI) sets the best practices for transporting, storing, using, and disposing of cyanide. “Minera Penmont” is a signatory of the Cyanide Code. The ICMI monitors adherence to the Cyanide Code through independent third-party audits. Certified operations like “Minera Penmont” are required to be recertified by ICMI every three years. Audit and certification are comprehensive processes that require significant time to companies and auditors to prepare and review all supporting information.
Minera Penmont purchases Sodium Cyanide exclusively from ICMI certified manufacturers (Cyanco and Dupont). Sodium cyanide is transported and delivered to our facilities by ICMI certified transporters, following adequate protocols to safely manage cyanide. Sodium Cyanide is supplied to our operations either as a liquid or as solid briquettes. Liquid Sodium Cyanide (~30% aqueous solution) is transported in tanker trucks and off-loaded onsite into our storage tank. Solid briquettes (~98% purity) are transported in ISO-containers designed so that the cyanide can be safely dissolved in a high-pH solution. The pH value of cyanide solutions is maintained high during dissolution to prevent the volatilization of hazardous hydrogen cyanide (HCN) gas. The resulting solution is then pumped to our storage tank. Throughout the unloading of Sodium Cyanide we apply strict Health and Safety measures to prevent spills and exposure of our employees and contractors to cyanide. Likewise, we mobilize our response teams to be ready to contain releases or respond to incidents causing exposure of our employees and contractors to cyanide.
The process used by Minera Penmont to extract gold from ore is known as “heap leach”. In this process, the ore is staked in layers on an impermeable membrane known as “leach pad”. The cyanide solution is dripped onto to the heap by a drip irrigation system. The solution percolates through the heap leaching the gold from the ore. The cyanide solution applied is indeed water containing a very low concentration of sodium cyanide (~.05% solution). The leach pad is made of an impermeable liner cover made of high-density polyethylene (HDPE) to recover the pregnant solution and protecting groundwater from seepage. The pH is controlled throughout the process (pH > 10.5 - strongly alkaline) using lime to ensure that when the cyanide solution is added, hydrogen cyanide (HCN) gas is not generated and cyanide remains in solution, ready to dissolve the gold. The resultant gold bearing solution is collected on the leach pad and channeled to the Merrill-Crowe plant for further processing
The Merrill-Crowe plant separates gold and silver using a process to filter the pregnant solution, removing oxygen and precipitating the desired metals (using zinc). The gold/silver precipitate is smelted to form crude and impure bars which are sent to a refinery. The residual solution is returned to the heap leach process allowing our operations to reduce the use of new water in our processes. Furthermore, our processing plants have contingency ponds to store the solution when the inflow rate exceeds the processing capacity, for instance when rain increases the percolation volumes in the heap.
Our facilities and processes are designed to prevent the exposure of our employees and the environment to cyanide. Emergency preparedness is a key component of our strategy to manage cyanide responsibly. Certified transporters are trained to be first respondents in case of an incident while transporting cyanide. Our emergency response and rescue teams are trained and equipped to respond in case of accidents involving worker exposure to cyanide. Our crews receive part of their training is the prestigious Texas A&M TEEX training center in Houston. To raise collective awareness, our internal communication strategy disseminates information on how to recognize the early symptoms of cyanide poisoning and how to react promptly.
Moreover, we actively engage emergency preparedness stakeholders such as the Civil Protection and Firefighters Department of the municipality of Caborca to respond effectively to cyanide emergencies. Minera Penmont has an active agreement with the San Javier Hospital in Caborca to respond to medical emergencies related to Cyanide. Furthermore, we actively engage other health stakeholders such as the Red Cross and National Health Authorities (Mexican Social Insurance Institute and the Ministry of Health).
Engaging communities on responsible cyanide management is essential. Our community relations staff implements a permanent program of visits to our facilities, where we explain our approach to safely manage cyanide and how we are prepared for emergencies. As part of our environmental stewardship commitment, the ISO 14,001 Environmental Management System of Minera Penmont supports the requirements of the Cyanide Code to protect the environment. Additionally, we implement measures to protect birds from exposure to cyanide at contingency ponds and monitor the health and integrity of the leaching pad and pipelines to prevent spills. Minera Penmont has a mine closure plan that considers a strategy for neutralizing the mineral stocked in heaps. This plan also considers the necessary measures to dismantle the entire infrastructure related to cyanide management.