October 21, 2011
The return of roasting?
Extraction of harder-to-treat gold ores are enticing operators to take another look at this once-popular technology.
During the past decade, North American mining companies have shied away from using roasting, a heating process used to treat refractory gold ores in domestic operations. This once-popular process lost much of its lustre, following implementation of increasingly tougher emission controls standards during the past decades.
However, according to Arthur Barnes, principle extractive metallurgy consultant at Xstrata Process Support, there have been growing signs that this once-disparaged technology could be making a comeback. “New technologies, such as second stage after-burners and oxidizers, are now available to address environmental concerns,” says Barnes. “As a result, much of the current reluctance to use roasting in North America is due to institutional inertia.”
Roasting, which can be liked to “cracking the shell,” to allow refractory gold to be exposed to subsequent cyanidation, has a long history in mining operations. At the turn of the century the heating process was conducted out in the open, which in turn led to substantial emissions. Here in Canada, concerns about unstable cyanide effluent at the Giant Mine in Yellowknife, caused considerable environmental concerns, and contributed in large part to an abandonment of roasting here in North America.
As a result, right now, most North American mine operators rely on pressure oxidation processes to treat ores that contain pyrite, arsenopyrite and carbonaceous material for domestic operations (that’s less true in Europe and Asia where roasting remains a viable alternative). However pressure oxidation requires major investments in capital equipment and operating costs to get running, so many operators are considering alternative procedures.
“Increasingly, gold producers are forced to treat ever more refractory ores,” explains Barnes. “The new bodies that companies are looking to bring onstream during coming years, will require considerably more effort to make them profitable.” The main contributors to refractoriness are sulphides and carbons, and to remove these from the ores, mining companies have a new incentive to consider roasting, which in many cases can be cheaper and easier to implement than alternative techniques.
However according to Barnes, although traditional opposition to roasting came from its bad reputation among environmentalists, much of these concerns have been assuaged. These days, much of the resistance to adopting roasting technologies stems from a reluctance to change existing habits. “A huge percentage of mining sector personnel, with the exception of a few veterans, have used pressure oxidation processes during their entire careers,” he explains. “And when you get in a habit of doing something one way, it is very hard to convince people to consider new alternatives - even if there may be a better option available.”
Xstrata Process Support, a Sudbury-based unit of Xstrata, provides “technical expertise for orebody characterization, operational support, and growth initiatives,” for project developments in the nickel, copper, zinc PMGs, industrial minerals and gold sectors, to its parent company and other mining sector players. According to Barnes, the unit which was once part of Falconbridge eventually emerged as a standalone technology center. Soon after, it began to get attention from mine developers, who in tandem with roasting equipment suppliers such as Outotech and Technip, wanted help to test if they could make gold recoverable from complex ores, using the roasting process.
Much of the incentive is economic. During times of extremely high gold prices, the focus for miners is on getting product into the market as fast as it can be extracted and processed. But when gold price levels return to their historical averages (as they are likely to at some point), then cost containment becomes more of a factor.
Xstrata Process Support operates four fluid-bed reactors that it uses to test whether roasting can be used to treat highly refractory ore samples. Clients consist primarily of companies that want to conduct preliminary mineralogical modelling of both the physical and chemical aspects of the roasting process from samples drawn from ore bodies outside of North America, where roasting use more widespread says Barnes. The facility operates under conditions of strict confidentiality, so Barnes refuses to disclose specifics, other than to say that a large variety of samples have been tested, and that the process has been “encouraging.”
The flexible range of reactor units, which can operate in a variety of modes (such as “bubbling bed,” circulating bed, two stage, sub-stoichiometric and combinations of these) provides Xstrata Process Support the ability to accommodate a range of sample sizes ranging from a few kilograms to a few tons.
“The facilities are busy,” says Barnes. “However because of the difficulty that many clients have in sending us large batches of testing material, we will be mothballing the larger unit, and working mostly with the two smaller ones” (there is also little demand for the center’s batch processing unit).
Assembling the ten kilograms of concentrate to use as test material is not easy. That’s because the main samples available from mine sites prior to the commencement of commercial drilling are generally drill cores. To process, say, ten kilograms of concentrate for testing purposes you need close to a ton of core material says Barnes. Once the testing is completed, Barnes generally recommends that the mine developer start talking with a commercial roasting facility developer.
One such company is Outotech, which develops and provides a variety of technology solutions to process natural resources. These include a gold roasting process that is based on circulating fluidized bed technology (which allows the ore to pass through the heating process multiple times) that is used as part of the process to remove sulphur and carbon from gold ores and concentrates. Marcus Runkel, a senior process engineer at the company, agrees that demand for roasting installations could be poised for a comeback.
“During the 1950s and 1960s Outotech installed about five plants each year. This doubled to ten plants a year during the 1970s and 1980s before falling off to about two plants a year in the 1990s and then to just one plant a year now,” says Runkel. “Much of the existing demand is in China and India.” (Proponents deny that roasting installations are popular there only because of these countries’ weak environmental laws, but rather attribute roasting’s popularity there to the fact that those are places where new mines are being built.)
However mining sector operators are increasingly trying to extract more highly complex ores, due to the fact that the more attractive deposits have already been developed. As a result, Runkel believes that roasting is a valuable alternative for conducting hydro-metallurgical processes and expects that many developers will be giving the technology a new look.
For his part Barnes admits that there are considerable challenges in getting roasting technology accepted again. For example, feeds are far finer these days than they were decades ago, as mineral processing operations seek to maximize gold recovery from supply batches. When processing finer feeds, operators need to limit the maximum velocity of the fluidizing gases used in order to maintain a stable bed, with a distinct “fluid” boundary.
In addition, maintaining a proper energy balance when conducting roasting operations is particularly important, and not easy to do. That is particularly true when processing highly refractory gold ores, especially ones containing arsenic. As a result Barnes notes that “all roasting predictions and calculations need to be confirmed or modified (using) carefully controlled and monitored testing.”
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