HEAP LEACHING & SX
FLSmidth has recently updated its mobile stacking conveyor line to offer solutions tailored to the ground conditions of each site but consisting of standardised and modularised components that are easy to replace and service
Leveraging the leach
Oxide orebodies across the globe are dwindling , leading miners to look for new economic ways to increase utilisation at SX-EW plants . At the same time , these operators are chasing higher yields during solvent extraction with cleaner and greener solutions . Dan Gleeson covers all this and more
As gold and copper prices tracked higher
during the early stages of the COVID-19 pandemic , time-hardened investors and analysts came out in force to warn mining companies against complacency .
The mining community – like the economic cycle that underpin its fortunes – has a habit of repeating mistakes when commodity prices run higher but , for the time being , mining companies have kept the cash in their coffers and are content successfully mining the conservative vein .
Still , these companies want increased exposure to rising commodity prices , meaning brownfield investments with low cost and low risk are the way forward for operations with existing infrastructure . This is most certainly the case for the heap leaching sector , which is often constrained by the amount of viable oxide material within the resource and reserve inventory .
These miners are regularly faced with the dilemma of suffering drops in recoveries from continuing the same leaching process using increased amounts of reagents or building a sulphide processing line that comes with substantial capital expenditure ( capex ) requirements .
In recent years , options have emerged to help solve this dilemma .
Positive discrimination
Molecular imprinted polymers ( MIPs ) have been around for some time , but it is only recently that their abilities have been seen on an industrial scale , detecting IEDs and explosives out on the battlefield . MIPs have now found their way into mining . Sixth Wave Innovations President & CEO , Dr Jon
Gluckman , said MIPs ’ ability to “ discriminate ” between metals of significance / insignificance can offer gold miners a process route that is a cheaper , more effective alternative to the conventional use of activated carbon in the elution of adsorbed gold .
While activated carbon has been the media of choice for the gold sector for over a century , it suffers from several shortcomings , according to Sixth Wave .
“ Its selectivity for gold is not exclusive ,” the
company states . “ It attracts other metals from the pregnant leach solution in addition to gold , including copper , mercury , and other base metals , which reduces its capacity to capture gold . In addition , activated carbon suffers from mechanical breakdown over time , with the result that some gold captured by the product becomes unrecoverable .”
As a result of this and other inefficiencies associated with activated carbon , 5-10 % of gold contained in a cyanide leach solution can routinely remain in the solution after cyanidation .
“ Breakdown of activated carbon also results in carbon fines – small particles that range in size from dust to 0.7 mm – that are not captured by screens and are therefore lost to the overall process ,” Sixth Wave said .
Sixth Wave ’ s IXOS ® purification beads do not come with the same drawbacks .
The technology hinges on a cavity in a polymer matrix that has an affinity for the target molecules . These cavities are created by including the target
compound ( or a similar proxy ) as a template during
the polymerisation process , which is then removed to create the active MIP .
The IXOS purification beads , according to Dr Gluckman , work in many respects like ion exchange resins but leverage the MIP as a mechanism to help reject the non-gold and non-silver metal ions in solution .
“ We still use the same kinds of attractive forces to make the metals interact with the polymer as ion exchange does , but we use that ‘ pocket ’ to discriminate ,” he told IM .
IXOS patented beads have demonstrated recoveries of up to 99 % of gold from cyanide leach solutions in laboratory and mine site testing , representing a material increase relative to activated carbon , which recovers between 85-95 % of the gold , Sixth Wave says .
On top of this , the technology is much more cost effective than the equivalent activated carbon route , Dr Gluckman noted .
“ On an operating expenditure ( opex ) basis , we are around 30 % of the opex of activated carbon ,” he said . “ There are significant reasons why .” The process of stripping gold from carbon usually requires the solution to go into an acid wash to remove scale and a pressure zadra system – a high pressure ( 50 psi ), high temperature (~ 160 ° C ) system – over a 24-48 hour period . Following this , a regeneration kiln operating at 500-900 ° C is needed to bring the activated carbon system back to peak operation .
IXOS requires no regeneration or descaling , comes with a three-hour cycle time , has a reusable eluent , requires less medium for an adsorption , desorption and recovery ( ADR ) system and operates at 60 ° C , according to the company .
It also comes with significant capital expenditure benefits .
“ The entire ADR plant can be about 30 % of the size of an activated carbon plant because you can cycle that many more times before you have to be ready to put the beads back into the adsorption circuit ,” Dr Gluckman said . “ We actually demonstrated that at our pilot plant at Kinross ’ Bald Mountain mine .”
The company added : “ Additionally , the increased selectivity of IXOS results in the capture of fewer other competitive base metals . Testing at multiple mine sites has shown more than 90 % gold purity captured on IXOS , as opposed to 40 %-70% for those generated by activated carbon , resulting in efficiencies in the refining process used to generate higher purities of gold .”
Such discrimination could be particularly useful in heap leach applications where recoveries and operating cost are the two factors usually determining if the operation makes a profit or loss .
It is no surprise that the company ’ s first tests were with a heap leach operation – Kinross ’ Bald Mountain mine .
Sixth Wave had six 40 litre columns in the collection circuit at the Bald Mountain pilot plant to prove IXOS was up to the task .
“ It was a counter-current , gravity flow system –
NOVEMBER / DECEMBER 2020 | International Mining 57