Goldcorp’s Marlin gold-silver mine in Guatemala might be the only operation in the world reclaiming its mined-out open pit by mixing cement into tailings from underground mining and using them as backfill. The space-saving process has extended the mine’s life by at least three years at a lower cost than anticipated. The tailings filter plant, designed by Samuel Engineering and equipped with Metso filters, came online in 2010. Marlin buys Type 1 construction cement from Guatemala’s sole supplier and adds it to the filtered tailings in a pugmill, a machine that mixes its contents and forms marble-sized pellets out of the tailings. The cement-tailings mixture is conveyed to a covered stockpile and cured for 24 hours to allow the cement to hydrate; it is then trucked to the pit and spread layer by layer.
Christian Roldan, mine manager, expects the US$72-million plant to pay for itself in two years because the mine has an extended life. The extra space it has created could eventually add a total of 15 years to the life of the mine if additional reserves are discovered.
Three years ago, Marlin faced a problem: Its tailings dam was running out of space. The dam had already been expanded once and building a new one would take too long to permit. “We considered adding chemicals to the tailings dam to achieve a faster settlement of the tailings and higher density, but the results of the test were not as good as expected,” says Roldan.
Marlin personnel decided to adapt the method used at Goldcorp’s El Sauzal mine in Mexico, which filters its tailings and stacks them dry as backfill. But the backfill needs to be strong enough to drive a truck over without sinking; if there is too much moisture in the tailings, they cannot form a solid surface. Marlin’s clay-rich soil holds moisture too effectively to rely solely on filters, so Goldcorp experimented with adding cement as well.
The filtering of the tailings was piloted by Metso Minerals in its own plant. Marlin’s engineers, working with independent consultant Rob Dorey, used a combination of lab and field tests to develop a mixture with a moisture content of 19 per cent and an undrained shear strength of 100 kilopascals. They began by dropping small tailings samples into a lab-scale mixer with different percentages of cement, letting them cure for 12 hours and doing lab tests for moisture content. They checked the shear strength using a laboratory shear vane.
They then used 10- by 15-metre outdoor pads and 1.5-metre-high lifts to fine-tune the deposition process, test-drive dozers and trucks across the deposited tailings, and measure shear strength again. The field results were confirmed back at the lab. “We also did some compaction tests, but we figured that with the amount [of cement] we were using we didn’t even need the compactors,” says Manuel Aparicio, Marlin’s chief of civil works.
Small amounts of cement
Within the first six months of full-scale operation, personnel at Marlin had a pleasant surprise: they had begun by using a conservative mixture of three per cent cement by weight but found that the moisture content and strength targets required far less. In Guatemala’s dry season, the tailings could be deposited without cement, while in the May to November rainy season, adding one per cent cement did the trick. According to Dorey, adding cement has about a one-to-one effect on moisture content: each additional percentage point of cement lowers the moisture content by one to two per cent.
In 2013, the total average cost of the cemented tailings, when one per cent cement is used, was US$6.80 per tonne. The cost of electricity to run the plant accounts for US$1.40 of that total, and another US$1.90 goes to buying the cement. At a three per cent composition, the cost of the cement itself would have been US$5.60 per tonne, bringing the total to around US$10.50 per tonne.
Other surprises were less pleasant. “The moisture content was quite difficult to control,” says Aparicio. “Especially during the rainy season, it was very hard because the tailings get wet during the haul to the pit.” The mine checks for moisture content at the plant and at the storage building before the tailings are hauled and more or less cement is added accordingly.
Roldan explains that Marlin also had to add cement more generously when it mixed the lower tailings layers. “We needed more cement at the bottom,” he says, “to form a strong base so that the bottom of the pit didn’t have any subsidence from underground.”
Staff at Marlin did the usual troubleshooting inside the plant. The first pugmill chute was too small to process at the desired volumes, so it was replaced with a larger one. Sprays of water were installed to deal with the dust the cement generated when it was added. The cloth material in the filters has also been altered to work more efficiently. But on the whole, scaling up to full operation posed few problems.
An unusual solution
Marlin produces 5,000 tonnes of tailings every day. Half of these tailings are cemented and used as backfill, but half go straight to the tailings dam. Roldan says the company’s goal is to fill the dam with tailings and do a dry closure on top – an economical approach to waste storage and reclamation.
Dorey says the mine was fairly unique in its situation: the combination of high-clay ore, the climate, and the underground mine feeding the open-pit backfill was rare enough that he could not think of a similar solution elsewhere.
But those considering cement addition may be pleased to know that it is not a particularly problematic process. The amounts of cement used at Marlin are small enough that any chemical reactions during the cement-setting have negligible effects on the tailings, according to Dorey. The important thing to watch out for is the quality of the cement. “Cement is not cement, particularly if you’re in Latin America,” he notes. “You have to be cautious that the supplier has a relatively clean product.”
One last environmental advantage emerged during Marlin’s testwork: The alkalinity of the cement helps to neutralize acid generation, and tests showed the added material also inhibits the release of metals, cyanide, nitrates and ammonia.