620 showed that liberated and free precious metals in the regrind cyclone underflow tend to be heavy and coarse (Ref: EHM – Au Deport Mineralogy Report). The accumulation of liberated, high-grade (25-50g/t) gold within the regrind circuit may lead to overgrinding or mechanical distortion of gold particles into elongated or smeared forms, potentially affecting both grindability and flotation recovery. Quarterly mineralogical assessments of selected process streams showed that a significant portion of liberated and free precious metals reporting to the regrind cyclone underflow remained relatively coarse, indicating that valuable material was recirculating rather than being efficiently recovered. Samples collected from the regrind cyclone underflow, overflow, and mill discharge streams were subjected to modal mineralogy analysis. The gold content in the regrind cyclone underflow comprised approximately 60% free gold, with electrum accounting for the remaining 40%. The proportion of liberated precious metals in the regrind cyclone overflow (the feed to the cleaner circuit) was lower at 36%, compared with 67% in the underflow. This disparity suggests that liberated and free precious metals recirculating in the regrind cyclone underflow are relatively coarse and heavy and may benefit from early extraction rather than being subjected to further grinding, where overgrinding or particle distortion could reduce recovery. Several test work programs were undertaken to evaluate options for recovering liberated gold particles and reducing the loss of valuable minerals to the cleaner tailings stream. These include: (i) gravity concentration, (ii) a block plant trial using the gold‑specific collector Aerophine 3418A, (iii) pilot‑scale testing with the IsaMill operating in open circuit, and (iv) cyanide leaching of cleaner tailings. Across these programs, the outcomes either showed negligible improvement in gold recovery or did not deliver a favourable economic return, leaving Ernest Henry Operations in search of a more effective and commercially viable solution. 3. EVALUATION OF FLASH FLOTATION TECHNOLOGY 3.1 Flash flotation technology overview Flash flotation is the rapid recovery of fast‑floating valuable mineral particles from the grinding circuit to produce a suitable concentrate grade. It is typically installed in the primary grinding circuit to recover well‑liberated particles from the cyclone underflow before they are returned to the mill for further grinding, thereby preventing overgrinding of valuable minerals. The flash flotation cell also acts as a buffer within the grinding circuit, reducing feed‑grade fluctuations to the main flotation circuit and improving downstream stability. Flash flotation provides a targeted solution to the challenge of liberated gold losses by recovering fast‑floating gold particles at the earliest possible stage, before they enter the regrind environment. By intercepting these particles directly from the cyclone underflow, flash flotation reduces the circulating load of valuable minerals, preserves particle surface characteristics, and prevents the detrimental effects of overgrinding. This early‑stage recovery not only improves overall gold recovery but also enhances the stability and efficiency of downstream flotation circuits by reducing the burden on the regrind mill and cleaner cells. 3.2 Laboratory-scale flash flotation test work A laboratory test program was initiated following a comprehensive analysis of operational data, plant surveys, and mineralogical characterisation, which identified a consistent pattern of gold losses associated with the regrind circuit. It was important that laboratory flash flotation tests closely replicate the conditions of an operating flash flotation environment. To achieve this, the following guidelines were applied: • Maintain adequate agitation to ensure effective suspension of coarse particles. • Use short conditioning times with minimal collector addition and low frother dosage. • Limit flotation time, typically to no more than 180 seconds, to target fast‑floating particles. • Preserve natural pulp chemistry wherever possible by floating the slurry soon after collection, ensuring the integrity and representativeness of the sample.
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