Canadian miner Nevsun Resources has ensured improved zinc recoveries at its zinc/copper/gold Bisha mine, in Eritrea, through its decision in August 2017 to allocate additional capital to the mine and delay the delivery of the prefeasibility study (PFS) for its Serbia-based Timok copper/gold to the first quarter of 2018.
“We remain focused on ensuring our Bisha mine delivers maximum value and takes advantage of the current strong zinc price environment by both continuing to improve current metallurgical performance and increasing material movement capability in the mine,” says Nevsun CEO Peter Kukielski.
In August, Mining Weekly reported that Nevsun Resources’ Timok PFS – previously scheduled for delivery in September 2017 – would be preceded by an updated preliminary economic assessment (PEA) in October last year to help with risk mitigation for the new project. The delay of the PFS enabled Nevsun to provide an additional $24-million for Bisha to assist in solving the two most pressing operational challenges: improving the recovery of copper and zinc and respective concentrate grades; and increasing the total material (ore and waste) movement to meet feed and stripping requirements for the updated life-of-mine plan.
Zinc recoveries at Bisha, subsequently, averaged 74% for the quarter ended September 30, 2017, and 43.4-million payable pounds of zinc were sold in zinc concentrate at C1 cash costs of $0.84 a payable pound sold. Nevsun also sold four-million payable pounds of copper from Bisha in copper concentrate at C1 cash costs of $1.70 a payable pound sold. The company declared a third quarterly dividend for 2017 of $0.01 a share and ended the period with cash and cash equivalents of $151-million.
“Zinc recoveries further benefited from the primary ore metallurgy improvement programme on site. “Additionally, several boundary material stockpile trial campaigns resulted in strong zinc recovery at the expense of copper recovery as portions of this material are not amenable to copper flotation. The mobile fleet expansion at Bisha, announced in the second quarter of 2017 and funded entirely through operating cash flow, is expected to be completed by the end of the first half of 2018.”
A summary of the Timok PEA results was further released on October 26, 2017, with plans on track to deliver the Timok PFS in the first quarter of 2018.
Preparing for 2018
Bisha’s strip ratio increased in the third quarter of last year as mining benches, walls and faces in the Bisha main pit were re-established to better prepare for a planned increase in mining rate and a more efficient mining schedule from 2018. This lower mining rate (compared with previous quarters) and altered mining pattern schedule was expected to continue and be finished by the end of the fourth quarter of last year.
The lower mining rate in the third quarter of last year and resulting in-pit primary ore mill feed shortfalls led to trials and campaigns of a portion of the previously stockpiled boundary material with high zinc quantities but with minor activated copper quantities. This resulted in better-than- expected zinc recoveries and zinc production but disappointing, although not unexpected, lower copper production.
Nevsun says it is encouraged by these recent results with particular focus now applied to investigating about 30% of the boundary ore stockpiles for future zinc-only processing. The company is still uncertain if the metallurgical performance from these recent boundary material trials can be consistently achieved for the remaining material as these stockpiles are not homogenous with known variations in grades and mineralogy, and may have been subject to detrimental oxidation during surface stockpiling.
A skilled team of employees and external consultants at the Bisha mine continued to make meaningful progress with the primary ore metallurgy rectification programme, as of October 2017. The programme focuses on minor plant modifications, reagent trials, water treatment options and oxidation, and grind size analysis. Nevsun still expects to achieve 70% recovery of copper-to-copper concentrate and 77% recovery of zinc-to-zinc concentrate over the primary reserve life.
“The company remains committed, with critical expert resources, to upgrading the quality of its copper and zinc concentrates to enhance marketability and realise better commercial terms,” says Nevsun.