Barrick invests heavily in new processing
March 10, 2014
What are roasters and autoclaves?
Roasters and autoclaves are advanced milling processes for recovering gold from refractory ores, or ores that require complex treatments to free the gold molecules from either carbonate or sulfide minerals.
Roasters use fine grinding and heat to burn off the carbonate minerals and recover gold. Traditional autoclaves use a combination of grinding, heat and high pressure to free the gold from sulfide minerals in large pressure vessels similar to the hull of a submarine.
A workforce of nearly 800 construction workers is changing operations at Barrick Gold's Goldstrike complex in Eureka and Elko counties — one of the largest gold mines in North America — and those changes will bring about a whole new method of gold processing in Nevada.
Nearly a dozen cranes dot the skyline at Goldstrike as construction workers implement $550 million in modifications to the autoclave circuit that will allow Barrick to process millions of tons of ore through the revamped facility rather than through its roaster, another large gold mill on the property.
The new process uses tiny resin beads to recover the gold and eliminates the use of cyanide leaching.
"There is a lot of new technology out there, and we are spending a lot of time and effort with our people getting them up to speed so they understand the process and the new flow sheet," says Andy Cole, general manager of the Goldstrike complex.
Barrick's chemists, metallurgists and engineers have been working for more than two decades to perfect the process and bring it to commercial scale. They've honed the process in the lab and in a pilot test plant, but the unique chemistry of using a sodium thiosulfate compound instead of cyanide may pose additional and unforeseeable challenges when the autoclaves are scaled back into full production later this year, Cole says.
The new process became necessary due to changing characteristics of ore mined at the Goldstrike complex.
When surface mining operations at the site started in the late 1980s, the ore was very simple to recover, Cole says. But as the Betze-Post open-pit mine deepened, those easily recovered ores were depleted and the original heap leach processing method became obsolete, prompting Barrick to build its autoclaves to process ores from Betze-Post and the Meikle underground mine.
In the late 1990s, the ore characteristics changed once again. The gold contained too much organic carbon to be processed through the autoclaves and lent itself to processing through a roaster. Barrick completed construction of the roaster at the Goldstrike complex in 2000 at a cost of $330 million.
Goldstrike's autoclaves produced a steady stream of gold until recent years, as supplies of ores best processed in the autoclaves dwindled. Meanwhile, the company stockpiled millions of tons of ore scheduled for processing through the roaster — as much as 20 years worth.
Barrick was faced with two options: shut down the autoclaves — and lay off hundreds of employees — or find a way to process ores through both mills.
"We have tremendous resources of tens of millions of tons of ore that we could put through the roaster, but we had a great investment in our autoclaves," Cole says. "The problem was how to get the gold out. We could build another roaster, but we had all this existing infrastructure and really just needed to tweak one part which would allow us to use the bulk of that existing infrastructure.
"It allows us now to use our autoclaves and process ore that we wanted to process for 20 years," Cole adds. "It allows us to treat double-refractory ore, or ore that has naturally occurring carbon that doesn't need a roast in front of it."
Lou Schack, Barrick's director of communications, says that in addition to employment of hundreds of construction workers, keeping the autoclaves in the company's processing circuit retains as many as 250 people currently employed at Goldstrike. Without the huge renovation project, the autoclaves most likely would have been shuttered by year's end.
"We would otherwise be challenged to keep all the folks in the autoclave circuit employed," Schack says. "With the new process we essentially will be able to keep those facilities going well past 2025."
The change to the autoclaves also allows Barrick to bring millions of gold ounces forward in Goldstrike's production schedule. Bringing those ounces into play in the near-term creates new opportunities for the company, Schack says.
"Goldstrike is the processing center for Barrick, and with the autoclave and roaster circuits we expect to be bringing ore from other locations there. There are a lot of economic trade offs between what you can do now and what you can do in the future."
Construction includes a mix of new and brownfield construction. Barrick had placed the autoclaves in extended care and maintenance status so it could begin the greenfield construction last year. It's currently working on the brownfield construction aspects, as well as performing extended maintenance work.
The autoclaves are scheduled to come back online in April still using the cyanide leaching process for a few months before being shut down and converted to the new process with the goal of completing the first pour of doré bars from the expanded refinery in the fourth quarter.
Cole says the main challenges associated with the job are coordinating the many different construction groups and ensuring job-wide safety. The primary contactors on the job are industrial contractor TIC of Steamboat Springs and AMES Construction.
"We have to make sure people are working and getting home safe," Cole says.