The roll-out of new narrow-reef boring technology, which has the potential to boost platinum group metal (PGM) grades, lower unit costs, enhance safety, and streamline downstream materials handling, will commence at Bokoni Platinum Mines in Limpopo in January.

African Rainbow Minerals (ARM) executive: growth and strategic development in the executive chairperson’s office Mike Schmidt outlined this during question time following the JSE-listed company’s presentation of financial results on March 10, covered by Engineering News & Mining Weekly.

A novel tunnel boring machine (TBM) will open up Bokoni’s narrow tabular dipping PGM orebody to allow narrow-reef boring to take place, so that upper group two (UG2) reef extraction is maximised and low PGM content waste minimised.

PGM ounces are concentrated in a very narrow 60 cm reef width. By targeting the high-grade UG2 reef and minimising dilution, the mined stoping grade can be uplifted by as much as 60%.

Bokoni PGM mine’s UG2 is being grasped as the opportunity to implement what is perceived to be a paradigm shifter that will take people out of the stopes, open the way for explosion-free mining, lift grade and even shrink tailings requirements.

Conventionally, only a two-metre advance can be achieved a day, which prompted ARM to tailor TBM technology to suit Bokoni’s narrow tabular dipping UG2 reef.

The ready-built TBM is now scheduled to be mobilised by the end of next month, “at the latest”, and to start cutting by the end of July, paving the way for the introduction of narrow-reef boring in January.

Interestingly, the new technology also comes with distinct downstream handling advantages, reduced tailings management, as well as a propensity to advance from mechanisation to automation.

A benefit of reef boring is consistent rock chip size, which eliminates the need for underground rock breakers and crushers and provides the option to pump rock chips directly to concentrator plants.

Performance targets have been based on a typical mechanised stoping crew at a bord-and-pillar operation where 15 000 t a month would be typically achieved and a grade of about 3.3 g/t, which provides the equivalent of 50 kg of PGMs a month, or 1 600 oz.

ARM has partnered with Herrenknecht, a German company, and Master Drilling, the South African company that specialises in reef boring solutions.

Herrenknecht has addressed the cross- sectional challenge by using three overlapping cut heads in one frame, while Master Drilling has focused on attaining a rectangular cut profile instead of a circular one. Both approaches are aimed at increasing production and reducing waste.