PanTerra Gold reported in early July that the Albion oxidation circuit at its Las Lagunas project for reprocessing high-grade refractory tailings from the historic Pueblo Viejo mine in the Dominican Republic was approaching design oxidation levels. The oxidized slurry is treated in standard carbon-in-leach (CIL) tanks; the gold and silver are stripped from the carbon; and doré bars are produced to complete the process.
The Pueblo Viejo tailings were generated between 1992 and 1999 through the processing of refractory ores by Rosario Dominicana and are impounded in a purpose-built valley-catchment dam. The refractory nature and metallurgical complexity of the ore resulted in poor recoveries (<30%) of gold and silver when treated by the conventional carbon-in-leach/cyanidation process plant then in place for oxide ore, resulting in significant tonnages of tailings containing +3.5 g/mt gold being stored in the Las Lagunas dam.
PanTerra Gold’s subsidiary EnviroGold (Las Lagunas) Ltd. won an international tender and signed a contract with the Dominican state in 2004 giving it the right to reprocess the tailings under a profit-sharing agreement with the government. The project involves the reclamation of the tailings and concentration of gold-bearing sulphides through flotation, followed by sulphide oxidation using the Albion process and extraction of gold and silver utilizing standard carbon-in-leach cyanidation.
The Las Lagunas project has a JORC indicated resource of 5.137 million mt, grading 3.8 g/mt gold and 38.6 g/mt silver. The Albion/CIL plant will process 800,000 mt/y of tailings for 6.5 years. PanTerra anticipates annual production of 69,000 oz of gold and 630,000 oz of silver at operating costs of $331/gold equivalent oz.
PanTerra has thus far spent approximately $72 million developing the Las Lagunas project, including acquisition costs, resource definition, metallurgical testwork, pilot-plant studies, feasibility studies, engineering, site works, procurement of mechanical and electrical equipment, plant construction, project management, and holding costs. Total development costs are expected to come in at about $84 million.
The Albion process being applied at Las Lagunas is a sulphide oxidation process developed in Australia by Mount Isa Mines, now Xstrata Plc, to oxidize refractory ores such that the recovery of precious and base metals can be achieved using conventional extraction technology, such as cyanide leaching in the case of gold. The process is a combination of two technologies, ultrafine grinding and oxidative leaching at atmospheric pressure, which results in the sulphide particles being oxidized, facilitating metal recovery using conventional oxide extraction processes such as cyanide leaching.
Ultrafine grinding is essential to the process, as it increases the surface area of the sulphide particles, enhancing the rate of downstream chemical reactions as well as distorting the minerals’ crystal lattice, lowering the required activation energy for the chemical reaction to follow. Ultrafine grinding is achieved through the use of an IsaMill.
A joint venture of Barrick (60%) and Goldcorp (40%) is nearing completion of a project to re-develop the Pueblo Viejo mine, with ramp-up to full production expected in 2013. That redevelopment project and PanTerra’s tailings reprocessing project are independent of each other.