The Pueblo Viejo mine is a sulfide refractory deposit located in the centre of the Dominican Republic in the province of Sanchez Ramirez. The ore body contains gold, silver, copper and zinc. The property had been mined by the previous owner until the oxide portion of the deposit was depleted. It was shut down after the low gold and silver recoveries from the sulfide portion of the deposit made operation uneconomical.
The Pueblo Viejo processing plant was commissioned in 2012 and is based on a complex flowsheet. Pueblo Viejo is one of the only mines in the world currently producing over 1 million ounces of gold annually. The ore at Pueblo Viejo is double refractory, where gold is mainly associated with pyrite as submicroscopic particles and in solid solution. The deposit also contains significant amounts of silver, copper and zinc. After the acquisition of the project, Barrick continued further testwork to add value to the project by incorporating silver and copper recovery to the flowsheet.
AuTec played a major role in the development of the Pueblo Viejo process flowsheet. The work started in 2002 when several options were evaluated, including bio-leaching in heap and stirred tank reactors, flotation, and pressure oxidation. Pressure oxidation was chosen as the preferred option due to high overall recoveries.
Following the bench scale studies, a continuous pressure oxidation pilot campaign was conducted to test different lithologies. It was during the pilot plant campaign that other problems were encountered downstream. The high concentration of acid in the autoclave favoured the generation of basic ferric sulfates. This drastically increased the lime consumption during the neutralization stage to more than 150 kg/t. Further tests to rectify this problem led to a patented “Hot Cure” process where the autoclave discharge slurry was held at high temperature to dissolve the basic ferric sulfate. The hot cure process was designed to reduce the lime addition required for neutralization ahead of cyanidation. The pressure oxidation and hot cure processes encouraged the formation of silver jarosite which resulted in silver not being recoverable from the cyanide-in-leach circuit (CIL).
AuTec also performed work aimed at overcoming the silver jarosite problem. The lime boil process was tested in bench scale and found to enable the recovery of silver. This was followed by several pilot plants testing different ore types to determine operating conditions and the variability of recoveries. After successful pilot plant runs the process flowsheet was finalized to include the lime boil process after hot cure and pressure oxidation. This added additional value to the Pueblo Viejo project as silver could be economically recovered.
In addition to the silver, the ore at Pueblo Viejo has considerable amounts of zinc and copper. During pressure oxidation, both of these metals dissolve and are present in the acidic liquor. Pilot plant tests continued in 2006 to 2008 to test different options for recovering copper and zinc. Bench-scale and continuous pilot plant studies of sulfide precipitation were conducted for the recovery of copper and zinc. After several campaigns, copper sulfide precipitation using biologically produced sulfide gas was selected and added to the flowsheet.
In addition to the recovery of metals, several pilot plants involving different unit operations were also performed at AuTec. These included cyanide detox, iron removal and neutralization of acidic solutions and acid-rock-drainage treatment. AuTec personnel also travelled frequently to site during the commissioning stage and used their extensive bench scale and piloting experience to assist the operations at Pueblo Viejo, and are currently still assisting on site as required.
The experience gained from the Pueblo Viejo project from bench scale tests to continuous piloting and on-site commissioning is extremely rare in the mining industry. The technicians and metallurgists involved with this project have gained valuable knowledge of what it takes to bring a project from bench scale testing to commercial stage.