Metallurgical testing using a high-pH caustic pre-leaching process on Sihayo Gold’s starter project in North Sumatra has the explorer pointing towards a significantly better cashflow than forecast. The process should enable the company to recover far more gold from the differing ore types found at its Indonesian site.
Oxide and transition ore makes up about 37 per cent of the feed for the proposed plant whilst fresh ore, with a highly variable recovery rate from less than 10 per cent to 90 per cent, comprises about 25 per cent of the ore reserve.
The recent pre-leaching test work suggests an increase in gold recovery of between 8 per cent and 61 per cent for transition mineralisation and between 21 per cent and 74 per cent for fresh mineralisation at the starter project.
Sihayo says the introduction of caustic pre-leaching may mean overall recoveries at the starter project could rise from 71.2 per cent to more than 80 per cent.
Fresh and transition ore types contain pyrite crystals rimmed with gold-rich arsenian pyrite that with direct conventional cyanide leaching can have as low as 20 per cent recoveries. The effect of pre-leaching at a high pH is thought to break down the arsenian pyrite rims, releasing the gold for subsequent cyanidation.
Sihayo’s 2022 feasibility study update estimates a pre-tax life of mine cashflow of US$156 million or an annual average of US$24 million from an initial mine life of 6.5 years.
Back-of-the-envelope calculations from the company suggest the increased gold recoveries could increase the life of mine pre-tax cash flows more than 75 per cent.
The most recent testing suggested 40 kg/t was required to achieve the optimum gold recovery, with only 10 kg/t of the caustic consumed, allowing the remainder to be recycled and a reduction of cyanide needed in subsequent processing. The company estimated the additional processing cost of high pH leaching is estimated to be about US$4.50 per tonne.
Sihayo Executive Chairman, Colin Moorhead said: “The metallurgical test work program for high pH pre-leaching, which commenced in 2021, has been a major success for the company. This is a truly transformational change for the project and we look forward to providing an update on the ore reserves and project’s economics in the near future.”
Moorhead says the program has demonstrated the potential to introduce a high-pH pre-leaching step in the processing flowsheet to increase recoveries above those outlined in Sihayo’s 2022 feasibility study update for its starter project.
The increased recovery rates translate to better project economics. Sihayo Gold has started further geometallurgical studies that will drive a revision of the project’s mineral resources and ore reserves estimates. The company expects its studies to be completed by the end of next month.
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By Matt Birney
