New technology ends the tyranny of chromite in UG2 PGM reef

Published on 17th October 2008

South Africa's platinum-group metals (PGMs) mining sector is set to be revolutionised by minerals and metals beneficiation institution Mintek's ConRoast smelting technology, developed with the financial support of JSE- and London Aim-listed Braemore Resources.

The world's biggest and richest source of PGMs is South Africa's Bushveld Complex, and, within this formation, PGMs are concentrated in two widely exploited reefs - the Merensky and the UG2. However, most of the PGMs in the UG2 reef have been locked into a geological dungeon by a metallic tyrant - chrome - which bedevils conventional smelting.

Thus, until now, the low-chrome-content Merensky reef has been the main source of PGMs. Only limited quantities of lower-chrome-content UG2 ore are currently smelted, and only in blends with Merensky reef ore. But the Merensky reef is dominated by the current ‘big four' South African producers - Anglo Platinum, Impala Platinum, Lonmin and Northam - which means that new entrants and juniors are left with UG2. And the Merensky reef is being used up.

Traditional smelters can handle a maximum chromite content of a mere 2% to 2,5%.

"Chromite has a very resilient crystalline structure," explains Mintek pyrometallurgy division specialist consultant Rodney Jones. "It is stable in nature and is very hard to break down. It doesn't easily melt in conventional furnace slag."

The result is that, in conventional furnaces, chromite tends to form a barrier between the slag (molten waste material) and the matte (also molten, but containing the PGMs). The slag floats above the matte, and the molten PGMs fall through the slag into the matte.

But, with a liquid chromite barrier layer between the slag and matte, smaller globules of the molten PGMs are prevented from falling into the matte, and are thus lost, ending up being disposed of with the slag. This clearly has unwelcome production and financial consequences. Nor is this all, or even the worst - the chromite comes to line the furnace and the tap hole. This, over time, reduces furnace capacity and tap hole flow.

"This chromite lining can be removed, but it is a very time consuming and very expensive process," says Jones. "So existing smelter operators try to keep the chromite input to a minimum. They do this by mixing Merensky reef with UG2, to blend the chromite out.

But today, junior PGM miners are producing only UG2 ore, so this blending option is running out of time. Further, the smelters impose financial penalties on UG2 ore and, if the chromite content of the ore is too high, they won't accept it at all." A conventional furnace can only handle chromite by operating at dramatically higher temperatures than normal. And this is dangerous.

It increases the chances of smelter failures - of blowouts and explosions - and increases the damage (and, sometimes, casualties) caused by such failures. This is because, in this situation, the matte is far above its melting - or, if you prefer, congealing and solidifying - temperature. This makes it very difficult to stop, once it breaks out of the furnace.

There are safety measures that can be put in place, but they are expensive and complex, and, because of their complexity, they are more likely to fail. In sharp contrast, handling a furnace breach when the temperature of the liquid is just above melting point is quite straightforward: blast the escaping liquid with cold air, which quickly pushes its temperature below melting point, so it congeals and hardens.

"There is an alternative strategy, which can easily be applied in dc arc furnaces, but which can only be used in conventional furnaces if they are specifically set up for it from the start - that is, put some carbon (anthracite) into the furnace," he reports. "This changes the oxidation state of the chrome - that is, puts it into a form that is readily soluble in the slag, which makes it no problem at all. But South Africa's PGM smelters are not set up for this, although the country's ferrochrome industry is."

It all started with nickel. About 1994, Mintek started a project to examine every possible method of smelting this metal. "We studied every technology we could conceive of," affirms Jones. "What came out, head and shoulders above the alternatives, was a combination of fluidised-bed roasting and dc arc smelting. This is the essence of ConRoast." In the process, it also became clear that ConRoast held the promise of unlocking the treasure trove of PGMs hitherto trapped in high-chromite UG2 ore.

The project started with computer simulations, which were followed by the construction of a small, laboratory-scale pilot plant. This processed some 500 kg of ore, providing the researchers with a small exploratory test, which proved that the chemistry worked.

The next step was a series of bigger plants, constructed during the late 1990s, each of which processed some 30 t of material over a period of about a week. This phase saw the same cycle repeated more than once - a furnace was designed, built, run, dismantled, and its parts examined.

This process allowed the researchers to ascertain, in detail, how the furnace performed, to improve and refine the design and construction, and to gather the necessary amount of engineering data to patent the overall process, all over the world. "While the process is applicable to PGMs in South Africa, it is also applicable to the nickel sulphide industry around the world and it has certain other niche applications as well," he states.

A true pilot plant followed, with a capacity of some 1 000 t/m, and Mintek was able to source low-sulphur, high-chromite feed material, which allowed them to run the pilot on a sustained basis, for three years, from 2004 to 2007. It was during this phase, in 2006, that Braemore Resources entered the picture, opening negotiations with Mintek.

From 2007, the pilot plant continued to be operated, but now under the auspices of Braemore. "This pilot plant has operated for years, and on a variety of feed materials, so it is effectively a demonstration plant," he emphasises. "We were able to do this because we were working with PGMs. The income gained from selling the alloy produced more than offset the costs of the project." Under the aegis of Braemore, from October 2007 to August 2008, the plant smelted 9 500 t of material, producing more than 16 000 oz of PGMs in alloy, with recoveries of 98,6%.

So how does ConRoast work? "The ConRoast process doesn't need sulphur to collect the valuable metals from the ore. The concentrates are roasted to remove sulphur prior to the material going into the smelter. Sulphur emissions from traditional smelting processes are a huge problem," elucidates Jones. "Then the material is smelted in a dc arc (also known as a plasma) furnace. A small amount of iron is generated from the con- centrate. That molten iron collects the PGMs, and base metals, of value. Then, from the furnace, the molten alloy is tapped."

That alloy can be poured into ingots, or produced in granular form, or be subjected to water (or gas) atomisation. This last process involves spraying the molten alloy stream from the furnace with a very high-pressure (up to 200 bar) water jet (or gas jet; but water atomisation requires a simpler and cheaper infrastructure). This results in a very fine alloy powder, with particle sizes between 20 µm and 40 µm. "Whether ingots, granules, or powder, this alloy is a saleable commodity," he highlights, "and Braemore has already sold all the alloy that has been produced to date, both locally and overseas." The alloy, in whichever form, is then refined to separate the PGMs, base metals and iron.

The ConRoast technology has three significant advantages over the existing smelting technologies. The first advantage is, of course, its ability to handle high-chromite- content ores. Mintek has so far smelted materials with a chromite content of as much as 15% - remember, traditional smelters can handle a chromite content of, at most, 2,5%. "Our plant has no maximum chromite content level," he asserts. "As long as it has PGMs in it, we can smelt it."

A second advantage is an environ- mental one - it is a much cleaner technology, producing only one hundredth of the sulphur dioxide emissions generated by traditional smelting methods. In the new technology, the sulphur is captured in the roasting stage of the process, and can be used as a feedstock for the chemicals industry - for example, in the production of sulphuric acid. The environmental advantages of the technology are clearly illustrated by the fact that the demonstration plant is located on Mintek's campus in Randburg, in north-west Johannesburg.

The third major advantage is that ConRoast uses an iron-based alloy to collect the desired metals. This alloy has a higher melting temperature than sulphide-based mattes, which makes it safer. Should a furnace breach occur, the escaping alloy is not far above its melting (or solidifying) point and so can be contained much more easily and safely.
Extensive and complex cooling systems are not required. The result is a more robust and reliable furnace.

The new technology also enjoys a number of other benefits. For example, being a simpler process, using fewer processing steps, it also uses less energy.

Braemore Resources is a junior mining and minerals processing company, currently developing operations in Australia and South Africa. In Australia, the company will reprocess nickel tailings dumps in Western Australia, with production due to start in 2010 - these dumps are estimated to contain more than 480 000 t of nickel.

In South Africa, the company plans to enter the PGM sector through ConRoast.

"Initially, we'll act as a processing company in South Africa," explains Braemore CEO Leon Coetzer. "We will negotiate joint ventures (JVs) with junior and medium PGM-miners to give them access to the ConRoast facility and technology." (Phase two of the company's programme in South Africa will see it take part in selected mining projects, resulting, in time, in its being able to process its own concentrate.)

The company provided the finance which allowed the upgrading of Mintek's pilot plant, which effectively doubled its capacity to about 2 000 t/m of material. In return, Braemore has been granted an exclusive global licence for the use of ConRoast for three years of development and ten years in total, both locally and abroad, for PGM projects. The company will pay licence fees to Mintek, and have access to the agency's facilities and technical services. Braemore also agreed to list itself on the JSE as well as on Aim, and has under- taken to ensure up to 50% participation in its South African operation by historically disadvantaged South Africans.

While operating the existing, now upgraded, plant at Mintek on a commercial basis, Braemore has had a feasibility study executed on a 10-MW plant, to be located near Rustenburg, on the Western Limb of the Bushveld Complex (the Mintek facility is now, after the upgrade, a 3,2-MW plant).

Beyond that, Braemore plans a 35-MW ConRoast plant capable of processing 30 000 t/m of material, with an annual production capacity of one- million ounces of PGMs, 30 000 t of nickel, and 15 000 t of copper, depending on the feedstock. This plant could be created on a modular basis, with 10-MW to 12-MW units, rather than with a single 35-MW unit.

"This technology offers a process that addresses key technology challenges in traditional smelting," avers Coetzer. "Chromite-containing concentrates will increasingly be coming on stream in the coming years, and there will be a need to be able to process higher chromite ores. ConRoast offers a very cost-effective solution. In comparison to traditional smelters, it needs less capital expenditure, and less operational expenditure. It will offer an alternative to junior miners.

Instead of just selling their concentrate to the majors, they will be able to participate with us in beneficiation JVs, and so get greater income. It will no doubt offer solu-tions for mines that couldn't previously be developed because of the chromite content of the UG2 reef. I believe that ConRoast is going to start opening up the market."

‘Start' is the operative word. ConRoast is not the end of Mintek and Braemore's cooperation, nor of their desire for PGM beneficiation. "Phase three of our involvement in South Africa is now in development at Mintek - refining," he reports. "We're not going to stop at smelting. We want to fully beneficiate in South Africa, to produce the metals here."