Mercur mine helping to pioneer treatment by pressure oxidation

The Mercur mine, near this small western community, is as rich in history as in gold. Although local residents are likely aware of the mine’s colorful past, few realize that Mercur is still making history by being among a handful of pioneers in the use of pressure oxidation to treat refractory gold ores. Owned and operated by a unit of American Barrick Resources (TSE), Mercur is one of less than a half dozen mines to use pressure oxidation to treat ores from which a sizable percentage of gold cannot be recovered by conventional cyanidation. But within the next several years a growing number of new plants — including several along Nevada’s Carlin Trend — will be utilizing this method to treat refractory sulphide ores because of its favorable environmental considerations over more traditional methods such as roasting.

Mercur, at an elevation of about 7,000 ft., is processing 4,200 tons per day of oxide ore and about 800 tons per day of refractory ore through its mill circuit.

During 1989, 1.63 million tons of ore grading an average of 0.074 oz. gold per ton were milled to produce 94,275 oz. gold. The open pit mine also includes a heap leach operation using drippers that turned out 23,261 oz. from a single dump to boost total 1989 gold production to 117,536 oz.

Despite the mix of processing methods to treat a variety of ores mined from a relatively low-grade and complex orebody, the average cash cost at Mercur was a modest US$253 per oz. in 1989.

Because oxide, non-oxide and heap leach reserves are all currently mined from one pit, the Marion Hill, The Northern Miner learned during a recent tour that extensive and constant testing is required to confirm the grades and types of ore mined from various zones in the pit.

Frank Wicks, general manager at Mercur, told The Northern Miner that ore is separated by grade, amenability and grindability with control maintained by assaying and from mill amenability tests on the blast hole cuttings.

“Grade control and good mining practices are the mainstay of this mine,” he emphasized, adding that a sophisticated, computerized program is used for mine planning. “We have some hard ores in the Marion Hill pit, so we also try to separate ores so that we can blend hard and soft material to get the best throughput.”

Typically, mining takes place on 20-ft. benches with pit slopes averaging 40 degrees , while blasting uses an average pattern of 18×18 ft. Over the life of the mine, the strip ratio is expected to average about 4-to-1 waste to ore.

Recoveries of the disseminated gold vary, depending on the process used. The refractory reserve has the highest recovery rate at 81.4%, followed by the oxide milled ore at 76.7% (together they average 77.8%) and finally the heap leach material which averages a 54.7% recovery rate.

As one of the pioneers in pressure oxidation, Wicks said Mercur had its share of bumps on the learning curve during the first few months after the pretreatment step was engineered and integrated into the Mercur plant in 1987. But without it, Barrick was looking at a recovery rate of about 35% for refractory reserves, which explains why this material was largely stockpiled in the past.

“Since then, including all of 1988, the autoclave has more than met our expectations,” Wicks said. “We’ve gone beyond the capacity of the unit, we’ve exceeded the metallurgical projections and we’ve also met the operating cost parameters that we set out for ourselves.”

Although pressure oxidation has been used since the mid-1980s in the U.S., Mercur was the first to use the alkaline pressure oxidation process.

“We use it because unlike some other mines that use pressure oxidation our ore has very low sulphur,” explained Wicks. “We form only a small amount of sulphuric acid that is neutralized very quickly by the lime rock at Mercur. It remains then over a pH of 7 on the alkaline side.”

Pressure oxidation, which has been aptly likened to a giant pressure cooker, is introduced after SAG and ball mill grinding and thickening at Mercur. On average milled ore is ground to 75% minus 200 mesh.

After several stages of heating, the ore in slurry form spends about 70 minutes in the horizontal, cylindrical 12×44-ft. autoclave, where it is mixed with oxygen and steam until the sulphides in the ore are oxidized and the fine gold is ready for cyanidation. (The autoclave operates at 437 degrees F under a pressure of 475 lb. per square inch absolute.) After pressure let down and cooling, the slurry re-enters the conventional milling process at the carbon-in-leach stage.

The heap leach mine at Mercur was established by Barrick in 1985, just after the company acquired the mine-and-mill complex at a cost of US$40 million by acquiring the shares of Getty Gold Mines from Texaco. Getty, which spent US$100 million at Mercur, had previously acquired the project from Gold Standard Inc. which still holds a 15% net profits interest.

Although exploration to find new reserves is ongoing, including an effort to outline sulphide reserves at depth and a regional program, current reserves in all categories (including possible) are believed sufficient to sustain the mine life for at least 10 years.

At the end of 1989, proven and probable oxide mill reserves stood at 9.02 million tons grading 0.071 oz. gold per ton, while oxide heap leach reserves were 6.49 million tons grading 0.033 oz. gold per ton. Proven and probable refractory reserves are 4.78 million tons grading 0.057 oz.

The mine has a workforce of 230, largely drawn from nearby communities. A seasonal on-site visitors’ centre, which includes artifacts from Mercur’s underground mining past as well as details of its current operations, is a popular attraction for tourists.