Heap leaching has taken pole position on the list of possible answers to the riddle that is the development of Olympic Dam. Photo: Supplied
Five years ago, BHP Billiton had a clear view about the viability of using heap leaching to extract copper and other precious minerals from the extraordinary ores of Olympic Dam.
“At this early stage, recoveries of copper and uranium from heap leaching appear too low,” BHP said in a 2009 environmental impact study.
The document was designed to argue the merits of the Olympic Dam expansion plan that BHP was spruiking at the time – a plan that would see more than $US30 billion ($32.2 billion) spent digging the world’s biggest open pit in the South Australian outback, and the construction of expensive new smelters and mills.
But despite its reservations about heap leaching, Australia’s biggest miner was not entirely dismissive of the ancient method, which separates minerals from wastes by soaking piles of the raw ore with acids and chemicals.
“This option continues to be investigated,” the company said in the same 2009 document.
Fast forward to July 2014, and heap leaching has taken pole position on the list of possible answers to the riddle that is the development of Olympic Dam.
BHP revealed this week laboratory work in the Adelaide suburb of Wingfield, and help from local universities, had progressed far enough to warrant a trial of heap leaching at the existing Olympic Dam underground mine.
No surprise in study
While the study of heap leaching was no surprise, the fact it had progressed far enough to warrant a three-year trial at the actual mine site did surprise some geologists with knowledge of Olympic Dam.
The particles of copper, gold, silver and uranium that make Olympic Dam valuable are spread diffusely throughout the ore body, meaning they appear in lower grades than many other Australian mines.
Heap leaching is a cheap and low-tech way to separate valuable minerals from worthless waste rock, but it is far from the most effective method, and almost never extracts 100 per cent of the valuable minerals.
Despite efforts to drip the acids and chemicals thoroughly through the ore over hundreds of days, the fluids can sometimes fail to get to every part of the ore, and getting consistency across the pile is difficult.
There are also certain types of minerals, such as the copper mineral chalcopyrite, which do not respond well to the process.
The University of South Australia’s Professor Bill Skinner said Olympic Dam contains some of the copper minerals that are difficult to heap leach.
“At Olympic Dam one of the predominant copper minerals is chalcopyrite, and then there is bornite and there is chalcocite,” he said.
“Most of the chalcocite and the bornite will be dissolved in the heap leach, and so will a portion of the chalcopyrite before it becomes passive.
“When they become passivated they don’t leach any further because there are chemical reactions occurring which put layers on the surface, which block the surface from the acid.”
Needing to recover the vast majority of the valuable minerals, particularly the copper and uranium, to make the Olympic Dam expansion economically viable, BHP previously felt heap leaching was not a complete solution.
Perhaps highlighting that view, the existing mine at Olympic Dam does not use heap leaching.
The current set-up sees the ores crushed and milled down to a small size, then floated through a bubbling fluid, during which most of the copper, gold and silver rises to the top.
The remainder, called flotation tailings, contains the uranium, and is taken away into a separate stream from the precious metals stream.
The two streams are separately given bouts of leaching with acids inside tanks, with the precious metals stream then continuously refined by a concentrator, a smelter and eventually electrolysis for final separation.
The uranium stream travels through its own process of decanting and electrolysis to eventually emerge as yellowcake.
The expansion plan that BHP spruiked between 2009 and 2012 basically suggested an expensive enlargement of the aforementioned system, where three extra mills, and one extra smelter would be built to handle the increased volumes of ore coming from one of the world’s biggest open pits.
Famously, that plan proved too expensive once commodity prices started to slide.
The new plan, which is barely in its infancy, is to combine the two processes which were previously rivals of sorts.
The trial will see heap leaching of the ore conducted prior to the existing system of floatation, smelting, electrolysis and so on. The testing at Wingfield appears to have improved the recovery rate and the consistency of heap leaching, and it is hoped up to 80 per cent of the uranium and about half the copper can be extracted at this stage.
Most of the minerals missed by the heap leach should be captured by the existing processing system on site, which may not need to be expanded because the heap leach will cut down the volume of ore required to go through the milling, floating and smelting stages.
“If you do the heap leach first, the residue left over to put through the floatation stage is easier to float because potentially interfering material has been removed,” Mr Skinner said. “As far as I’m aware, the recoveries of residual copper through floatation of the residue are excellent.”
If it works, BHP may be able to mine and process the untapped part of the Olympic Dam deposit without needing to build the expensive extra mills and smelters.
Trial untested commercially
BHP stressed this week that the trial was untested on a commercial scale, nor is it the only new technique being put on trial by its Olympic Dam team.
‘‘A number of new mining and processing technologies are being evaluated to determine the best development path,’’ the miner said in a statement.
In a best-case scenario, the famous mine appears unlikely to be developed until well after 2020.
Mineral economist Richard Schodde has been closely associated with Olympic Dam over several decades of work for both BHP and the company that discovered the deposit, Western Mining Corporation.
Mr Schodde said even if the heap leaching trial is a success, BHP will need many stars to align before it can approve development of the entire Olympic Dam deposit.
“Basically the company has to win the trifecta where commodity prices are rising, construction costs are under control and they have a breakthrough in processing technology,” said Mr Schodde. “Time will tell if they have luck on their side.”