A control room operator monitors the Albion Process/CIL plant built by PanTerra Gold to recover gold from high-grade
tailings at the Pueblo Viejo mine in the Dominican Republic. (Photo courtesy PanTerra Gold)

The Pueblo Viejo tailings were generat-ed between 1992 and 1999 through the processing of refractory ores by Rosario Dominicana and are impounded in a pur-pose-built valley-catchment dam. The refractory nature and metallurgical com-plexity of the ore resulted in poor recover-ies (<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 oper-ating costs of $331/gold equivalent oz.

PanTerra has thus far spent approxi-mately $72 million developing the Las Lagunas project, including acquisition costs, resource definition, metallurgical test-work, 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 leach-ing in the case of gold. The process is a com-bination of two technologies, ultrafine grind-ing and oxidative leaching at atmospheric pressure, which results in the sulphide parti-cles being oxidized, facilitating metal recov-ery 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, lower-ing the required activation energy for the chemical reaction to follow. Ultrafine grind-ing 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 reprocess-ing project are independent of each other.