Process engineer Ashwin Gupta does a final inspection of the two-inch fluid bed components, before performing a gold ore roasting experiment | Courtesy of Kingston Process Metallurgy

Collaboration between Kingston Process Metallurgy (KPM) and Barrick Gold has resulted in the development of a two-inch continuous fluid bed roaster that should allow inexpensive testing of small ore samples, like those from drill core from prospective projects.

“Our two-inch roaster can process a small amount of material and get representative results to quantitatively evaluate a new ore behaviour or the valuation of an ore body,” says Alain Roy, co-founder of KPM.

In a paper KPM presented at the Conference of Metallurgists 2014, the company describes two testing campaigns that show their new equipment can both obtain results representative of production-scale roasters and help optimize the process parameters for unique ore compositions. Each test run, meanwhile, consumes as little as a few hundred grams of sample material, making testing faster and cheaper.

“We wanted to find a process that would give us more scalable results, use less sample, have a faster turnaround time, and a lower cost,” says Peter Lind, manager of the Barrick Technology Centre in Vancouver, describing why his company was interested in developing effective smaller-scale testing technology. “We want to test 100 samples of the ore body, instead of one big composite where all our uncertainty is focused on that one sample.”

Refractory ores

One of the major challenges to economically recovering gold from many ore bodies is the presence of certain impurities, like sulphides, organic carbon or arsenic, which can make the ore resistant to normal processing techniques like cyanidation and carbon adsorption. These so-called refractory ores require pre-treatment to allow effective recovery of the gold.

During roasting, a common pre-treatment method, finely milled ore is exposed to oxygen at high temperatures. Chemical reactions take place between the gas and the solid refractory ore, converting the impurities within the ore into gaseous compounds, such as sulphur dioxide, carbon dioxide and arsenic oxides, which are released from the rock and expelled from the roaster, leaving behind ore that can be easily processed by standard methods.

“The idea during the roast is to find a condition where we can remove the arsenic and remove the carbon without causing any other problems,” explains Roy. “If you just heat it up and aren’t careful with what you do, you will form some arsenic minerals that will be stable in the ore, and then you cannot extract your gold.”

The right treatment can increase gold recovery to upwards of 90 per cent. But the wrong treatment – at too high a temperature or with too much oxygen – can cause unwanted solid compounds to form in the ore, limiting gold recovery to as little as 10 per cent or less.

Finding the right treatment has not traditionally been cheap. Production-scale roasters, which may measure upwards of 10 feet in diameter and stand several storeys tall, should not be used for testing and development. Pilot roasters, typically six to eight inches in diameter, consume hundreds of kilograms of ore to perform a single test. Running optimization tests quickly becomes prohibitively expensive or even impossible, if the source material is drill core samples from a prospective site.

Until now, the alternative was to use bench-scale batch rotary roasters, although they are known for giving highly variable results when treating arsenic-rich ore, most likely due to imperfect gas-solid mixing in the roaster and lack of accurate control of the test parameters. “People were doing small-scale batch roasting in rotary reactors,” says KPM project leader Trevor LeBel, “but nobody would really trust it for arsenic containing material because you could do it three times and get three different results.”

An alternative is to use a small-scale, two-inch fluidized bed reactor, in which gas is blown upwards through a bed of fine particulates, causing the solid particles to behave much like a fluid. Fluid bed reactors, more than batch rotary roasters, generate good gas-solid mixing and allow for good control of the key process parameters. This was previously attempted in other laboratories, but without meeting expectations. KPM was challenged by Pascal Coursol, then with Barrick, to build and demonstrate a two-inch fluid bed reactor that would reliably produce results representative of full-scale operation.

It can’t be done!…

One of the main obstacles to building a small fluidized bed roaster is maintaining sufficient control of pressures and temperatures to sustain fluidization and ensure even heating. “People have done this successfully before in a roaster that was much larger than ours, like four inches or six inches in diameter,” adds Roy. “That’s better than a plant, but it still needs tens of kilos or hundreds of kilos of the ore.” When working from drill core samples from new projects, that is still too much material. If the test uses concentrate produced from flotation of drill core ore samples, the cost increases even more.

“It was a question of saying, ‘It has to be possible,’” says Roy. “There’s no physical reason why it would not be possible, and we’ll make it work. But we had to prove to people that it works. We designed carefully, using a large number of instruments and controls. But you have to have ways to measure your key parameters without putting a bunch of probes directly in the bed,” which would disturb the bed and prevent fluidization.

So building the bench-scale fluidized bed roaster came down to solving one fairly prosaic problem: How do you include all the necessary instrumentation in a two-inch-wide chamber?

…Or can it?

The answer was not anything revolutionary: persistence and creativity. Careful consideration of the possible configurations – and several rebuilds, trying different configurations – eventually led KPM to success.

In the tests described in its paper, the company aimed first to replicate the results that Barrick obtained in its plant-scale roasters, ensuring its base case was representative of full-scale roasting.

Having reliably replicated Barrick’s results, KPM’s team then explored the conditions that would permit the best extraction of arsenic from the ore and subsequently allow the best gold recovery during normal processing. Two-phase roasting – one lower-oxygen phase to drive off the arsenic, then a higher-oxygen phase at higher temperature to burn off the remaining carbon – was found to maximize gold recovery as was expected.

By succeeding at replicating Barrick’s results, Roy says, KPM has proved the mining giant can now send a small sample to test and accurately predict what the recovery rate would be in its plant. “Otherwise, they have to run it in their plant, and if they get bad results, it costs millions of dollars,” says Roy. “They can send us one kilogram of a sample, we run it, and they have something that will represent what they would get in their plant.”

A fruitful collaboration

Throughout the development process, the two companies maintained a close and complementary working relationship. Each company brought its expertise to bear – Barrick supplied its knowledge of mineralogy and hydrometallurgical processing, and the KPM team provided its proficiency at pyrometallurgical processing and the development of custom laboratory equipment. Both companies expect to benefit from the collaboration.

“For us, there’s a pretty keen interest to work this out,” says Lind. “Roasting is becoming of more interest for a lot of projects, because there are benefits in terms of capital costs and operating costs.”

“So far with this unit, most of the work we’ve done has been on gold roasting, but there are lots of other opportunities for the fluid bed to test out reactions,” adds KPM’s LeBel. “We definitely expect to gain some new business with this roaster.”