In late November 2014, the Goldstrike mine poured a small but significant bar of gold. At 107 ounces, the pour amounted to just one-eighth the size of a typical doré gold bar, but it marked the first time the mine had produced gold using its patented thiosulfate processing method. In fact, it marked the first time any company in the western world had successfully produced gold using thiosulfate.
Long viewed as a potential alternative to cyanide, which is typically used to recover gold that is trapped inside ore, thiosulfate is a difficult chemical to master. Barrick spent more than two decades perfecting its thiosulfate processing method and relied on an unmatched level of scientific and technological expertise.
“It’s the culmination of years of hard work and a good example of how our partnership culture is manifesting itself on the ground,” says Goldstrike General Manager Andy Cole. “This was a huge initiative, and it would not have succeeded if it weren’t for the collaboration, trust and accountability that developed between our project team, the construction group and the Goldstrike operations team.”
The patented thiosulfate process uses no cyanide and accelerates production from four million stockpiled ounces.
The initiative, known as the TCM (total carbonaceous matter) project, involved a $620 million investment that included major upgrades to the Goldstrike leaching circuits and new infrastructure at site. Part of that infrastructure is a thiosulfate plant that will ensure that a steady supply of the chemical is readily available.
A new water treatment plant will allow large volumes of thiosulfate to be recycled, which is critical because it will reduce the cost of purchasing new thiosulfate and make the project economically viable. The project also required the construction of dozens of large storage tanks to house solutions and chemicals, and a labyrinth of pipes to transport those materials—water, natural gas, compressed air and calcium thiosulfate—around the site.
Tony Carroll, Project Manager at Goldstrike, discusses the many benefits of the TCM project to the state of Nevada. Carroll oversaw construction of the project, which at peak activity involved more than 1,400 workers from more than 20 companies, the majority from Nevada.
At the peak of construction during the summer of 2014, the TCM project site resembled a very busy ant colony. More than 1,400 workers performed numerous tasks such as laying electrical wiring, welding steel and fitting pipes. At least a dozen cranes transported heavy parts to their designated areas. Trucks roared, machines hummed, and huge banners permeated the site reminding workers that their safety must always come first. “Safety is about doing the right thing even when no one is looking,” one banner read.
All told, the TCM project required 4,000 tons of steel, 20,000 cubic yards of concrete, more than 900,000 feet of cable and 200,000 feet of piping. At peak construction, more than 20 companies toiled on the project, the majority of them from Nevada.
“It’s been a huge boon to the area,” says Tony Carroll, a veteran project manager at Goldstrike, who oversaw the construction work.
One of the first things visitors to the thiosulfate processing area see is 14 new stainless steel tanks, some as high as 55 feet and 50 feet in diameter. This is the new resin-in-leach circuit, a critical component of the thiosulfate process. Gold-bearing ore is pumped here from the autoclaves in the form of a slurry – a thick, liquefied combination of ore, air, water and limestone.
Once inside the tanks, the slurry interacts with thiosulfate and resin, a fine, bead-like substance that literally collects the gold from the slurry. The slurry passes through seven tanks in total, with the process repeating itself in each tank to maximize gold recovery. At full capacity, the leaching process will occur simultaneously in two sets of seven tanks, with 13,400 tons of ore being processed daily.
Previously, ore that went through the autoclaves – large pressure chambers in which gold-bearing ore is heated to 225 degrees Celsius to facilitate the leaching process – was transferred into a cyanide-based carbon-in-leach circuit. However, as Goldstrike matured, new, complex sulfur-carbon ores were being mined that could not be processed profitably using this method.
“Cyanide doesn’t leach gold from this type of ore very effectively because the carbon in the ore literally steals the gold from the cyanide solution, a phenomenon known as preg-robbing,” says Yeonuk Choi, Senior Manager in Barrick’s Strategic Technology Solutions group. “In practical terms, this means gold recovery rates using this leaching process on this ore type are too low to be economic.”
While the new ore type, known as double-refractory ore, can be processed at another facility at Goldstrike known as a roaster, to do so would delay processing until near the end of the mine’s life, years down the road. In the interim, Goldstrike would have had to close its six autoclaves and eliminate the 300 positions required to operate them.
Goldstrike’s operators were well aware of the dilemma long before they began mining double-refractory ore. To address the problem, they worked closely with Barrick’s Strategic Technology Solutions group, which, as its name suggests, is tasked with finding solutions to complex operational problems. The group consists of a team with expertise in key areas of mining such as energy, mineral processing, flotation and environmental science.
“After our initial research, we concluded that the most promising solution was to use thiosulfate,” says Choi, who holds a doctorate in metallurgical engineering. “The beauty of thiosulfate is that, unlike cyanide, it can handle the carbon in double-refractory ore. Gold doesn’t get preg-robbed by the carbon.”
In other words, if Barrick could perfect thiosulfate processing, gold recovery rates of double-refractory ore sent through the autoclaves would be high enough to sustain this key infrastructure along with hundreds of jobs and technological expertise that took years to build. But that was a big if.
“Using cyanide is quite straightforward, but thiosulfate is not,” says Choi. “The thiosulfate can break down into numerous derivative chemicals when it’s inserted into the resin-in-leach circuit and the process won’t work. So it required a lot of lab-testing and manipulations.”
The TCM project started in 2009, once important chemical parameters had been defined in the laboratory tests. “The project was on a fast-track, due to the limited amount of ore remaining that could be economically processed through the autoclaves and the imminent shut-down of the facility,” says John Langhans, who served as Process and then Commissioning Manager for the TCM project.
The project team decided in the early stages to use calcium thiosulfate rather than sodium or ammonium thiosulfate, in no small part because it’s the most environmentally friendly of the three. “Calcium thiosulfate is used as a fertilizer in the agricultural industry,” Choi says.
In 2010, Barrick approved construction of a demonstration plant at Goldstrike to prove the process could work in a real-world setting. The plant operated for 16 months and helped define the process and chemical parameters required to ensure successful design of the full-scale facility. “We ran this plant through the paces, changing parameters on an almost daily basis,” Langhans says.
In 2011, the company approved construction of the main processing area; it took a year for key permits to be approved. Larry Enloe, Director of the TCM project and a 30-year mining industry veteran, says the construction and commissioning of the TCM processing area is the biggest, most challenging and most rewarding project he’s ever undertaken. “We had to go through a steep learning curve,” he says. “A lot of this is brand new, but we also had to incorporate those new processes with the old processes and make them come together, and do it safely.”
The success at Goldstrike is an important milestone that shows that thiosulfate is a viable option for certain complex ores, Enloe adds. “It can be done,” he says. “We’ve proven that.”
The success of the TCM project will allow Barrick to realize cash flow from about four million ounces of stockpiled ore that would otherwise have been processed at the end of Goldstrike’s mine life. Over the first five years of full production, thiosulfate processing is expected to contribute an average of 350–450 thousand ounces of annual production at a cost similar to the overall operation.
The success of the project also means that Barrick is able to keep a large plant operating in Nevada, Carroll says. “It means the state still has revenue coming in, which is important for a state that’s still struggling economically in the aftermath of the recession,” he says. “It means jobs are preserved, expertise is honed here in Nevada, and not just among our employees, but also among the local firms who worked on the project. It’s been a long road, but we’re proud of what we’ve accomplished.”