John Langhans, Nathan Stubina and Mitch Cheng (from left to right) at the Barrick Technology Centre in Vancouver, working on the thiosulphate process | Courtesy of Barrick Gold
Once a retrofitted thiosulphate leach circuit goes into full-scale production in 2014, Barrick Gold’s Goldstrike mine will be the largest scale use of the
alternative lixiviant the world has ever seen. The result of decades of R&D, the cyanide-free circuit that is being built will rely on a careful
chemical balance and on lessons learned during years of work scaling up the technology.
The processing plant at Goldstrike in Nevada includes two separate circuits: one that has a roaster for carbonaceous double refractory ores, and one with
autoclaves used to treat the rest of the refractory sulphide ore body. This layout made sense until recently, but Goldstrike has eaten through all of the
single refractory ore. With conventional methods unable to treat double refractory ore in the autoclave circuit, the mine was faced with a choice: “Come up
with some technology, or shut down the autoclaves,” remembers Barrick’s John Langhans. He is project manager of the team working to retrofit the pressure
oxidation circuit to use thiosulphate instead of cyanide to process double refractory ore. The thiosulphate circuit will have its own tailings pond, which
promises environmental benefits, as the leaching agent is non-toxic and is commonly used as a fertilizer.
Through luck or strategy or both, Barrick inherited considerable thiosulphate expertise when it acquired Placer Dome, and the Placer Dome technology
centre. But success in this project at Goldstrike has depended on a lot of collaboration: between the corporate office in Toronto, regional offices in
Nevada and Utah, what is now Barrick’s technology centre in Vancouver, and also with researchers around the world.
Barrick’s work with SGS Lakefield and the University of British Columbia’s materials engineering department has been crucial, as have partnerships with the
Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Perth, Australia, and Ausenco for engineering support.
For many who have spent years in R&D, as Langhans did, the project has been deeply satisfying. He spent four years studying thiosulphate with the U.S.
Bureau of Mines in the 1990s, before joining Barrick in 1999. At the time, he had no idea his expertise would help see the technology into commercial
production. “It’s a once-in-a-career opportunity,” he says.
Ion exchange resins crucial
Newmont had some experience in the 1990s with a demonstration heap-leach thiosulphate circuit, but complications resulted in its eventual shutdown in 1999.
The technology never proved economical. Barrick, however, is in a unique position, having already paid for Goldstrike’s autoclaves. Operating much the same
as they did before, the autoclaves make the work easier for thiosulphate, which is added after the crushed ore has been treated. According to Peter Kondos,
Barrick’s director of strategic technology solutions, the innovations in the new technology are “focused on the use of resin, the resin-loading strategy
and its elution.”
Thiosulphate is relatively good at picking up gold in the leach tanks – creating a gold-thiosulphate complex – but the challenge has been getting that gold
out of solution. Thiosulphate cannot be used in a traditional carbon-in-leach or carbon-in-pulp circuit like cyanide can. Instead, strong base ion
exchange (IX) resins are used, to which the gold-thiosulphate adsorbs. However, the use of ion exchange resins has some issues, says Paul Breuer, principal
research scientist at CSIRO: “The IX resins pick up the gold-thiosulphate out of solution, but once you get that on the resin, you need to get it off. The
elution system is not simple.”
CSIRO has been at the forefront of this research and, according to Breuer, “It was only with [CSIRO’s] discovery of the benefit of sulphite as an additive
to the IX elution process that it became easier.”
“CSIRO was very critical,” says Yeonuk Choi, Barrick’s manager of technology development, who gives credit to decades of research done in Australia. “Paul
and CSIRO have been working on the elution process for many years.”
Since Barrick brought in the IX technology from CSIRO in 2010, Choi has worked to support Langhans as he used it to build and operate a five-tonne-per-day
demonstration plant on site at Goldstrike. But even with CSIRO’s help, consistent performance was difficult to accomplish. Nearly a year of demonstration
plant operation revealed how much chemical control is necessary to keep the thiosulphate circuit working properly. “It went up and down a number of times
from operating properly to being highly problematic, particularly with changes in ore feed,” Breuer recalls.
Langhans and the Barrick team worked on a special step in the leach circuit to deal with unwanted compounds in the leach solution. “There’s an intermediate
kind of conditioning that can take place if we need to have it, in order to adjust the chemistry,” Langhans says. “On the resin, it’s a chemical
equilibrium. During the leach cycle, if you have too much of one chemical in the system, it can either push gold off, or preferentially load itself instead
of gold onto the resin.”
In the thiosulphate circuit, polythionates, in particular, accumulate as the lixiviant breaks down over time.