JOHANNESBURG (miningweekly.com) – Successful removal of coarse waste is beneficial in that it reduces power requirements in the milling circuit, lowers water requirements, and increases platinum group metal (PGM) feed grades.

With preconcentration, a head grade of, for example, 2 g/t can be as much as doubled to 4 g/t, resulting in PGMs being valuably increased in the circuit.

Displayed was a PGM grade of 1.7 g/t being uplifted to 4.8 g/t through dense media separation (DMS), at a flotation loss of around 0.4 g/t.

Mining benefits include being able to lower the cutoff grade, which increases the reserves and provides the opportunity to extend the life of the mine, Mintek physical separation head Gertrude Marape pointed out during Mintek’s PGM Day covered by Mining Weekly. (Also watch attached Creamer Media video.)

Mintek flexibly provides clients with options. “Some clients already know the techniques, the technology that they want to use. Some ask us to advise, but essentially, we work with everyone,” Marape stated in providing comprehensive information on the benefits of upfront waste rejection and chrome removal in PGM circuits.

Marape was one of a dozen Mintek managers, engineers and scientists who presented in the auditorium of this 92-year-old State-owned research organisation, which is situated at 200 Malibongwe Drive in Randburg.

While typical PGM reefs are Merensky, upper group two (UG2) and Platreef, Marape also paid attention to the PGMs present in chrome reefs such as lower group (LG) reef, middle group (MG) reef and, of course, UG2 reef.

Mintek has found that it is easier to introduce a preconcentration stage on a greenfield project than it is at a brownfield project. As much as some brownfield projects were metallurgically suitable, practical implementation has generally been found to be inappropriate.

Clear information was provided on the recovery of PGMs from the tailings of chrome-rich MG and the LG reefs as well as UG2, pursuits to which Mintek has given considerable attention.

Mintek’s studies show that upfront preconcentration is viable but mineralogy dependent.

Sometimes it works, sometimes it doesn't work, but if it works, there's value in it, because then it reduces capital expenditure and operational expenditure of plants, lowers cutoff grade and also provides flexibility in the cases of PGM extraction being amenable to mechanised or conventional mining methods, Marape noted in the PGMs Day covered by Mining Weekly.

Moreover, Mintek’s experience regarding interstage chrome removal is that it provides better yields and higher chrome recovery but that this will be accompanied by some loss of PGMs to chromite concentrate.

In addition, PGMs and chrome are recoverable from MG, LG and UG2 reefs, which uplifts revenue potential.

PGMs come mainly from the Bushveld Igneous Complex's three limbs - the western, eastern, and northern limbs.

While typical reefs are Merensky, UG2 and Platreef, there are also PGMs in chrome reefs such as LG reef and MG reef.

While LG and MG reefs are initially mined mainly for their chrome, PGMs are there for the taking in LG and MG tailings.

Displayed were infographics showing typical upfront coarse waste rejection, done after crushing and before milling.

Most of the reefs have a lot of pyroxenite gangue in them and the rejection of this coarse waste in them can be through DMS or through sorting – dry on-site processing – depending on which is more beneficial and with greater throughput also being taken into consideration.

Mintek used to provide on-site sorters but now has partnerships with various sorting companies and can link clients to these suppliers.

The purpose of coarse waste rejection is to reject the waste, and with it come benefits.

For example, in the event of the DMS option being selected, Mintek will test to see how amenable the ore is to preconcentration by DMS, but for clients opting for sorting because of it involving dry processing, Mintek has partnerships with various sorting companies and can link clients to those sorting service providers.

Mintek will then do various tests; tests basically at the laboratory and at pilot scale to see whether the ore is amenable to preconcentration by DMS, which basically involves rejection of coarse waste to extents that are always liberation-dependent.

Mintek’s experience is that sometimes it works with UG2 and Merensky reefs but whether it works or not boils down to liberation.

As a consequence, Mintek often recommends to clients that different functions be tested to determine which provide better preconcentration, because what is wanted is the upfront rejection of as much coarse waste as possible, but without major PGM loss.

If coarse waste can be rejected, the benefit is the minimising of discard of waste ahead of the entry of the value-containing material into the concentrator.