Research and development activities in gold and silver mining have been motivated by decreasing grade of deposits, more complex mineralogy, increasing labour and energy costs, efforts to reduce the footprint of the process and protection of environment. This paper discusses three emerging technologies developed in the last twenty years: processing of sulphide bearing gold ores containing aurostibite, leaching of ultrafine grind sulphide flotation concentrates, and extraction of gold and silver from high grade deposits with a new technology called CELP (CANMET Enhanced Leaching Process). The aurostibite ore contains 8.00 g/t Au as well as native gold and electrum. Sulphide minerals are pyrrhotite (2.8%), several antimony minerals (0.8% berthierite and gudmundite, 0.18% native antimony and stibnite) and preg-robbing properties. The 82% gold extraction was obtained using 250 ppm NaCN, 500 g/t lead nitrate, 30 g/t kerosene, DO 10 ppm and pH 10.5 in 96 hours. The removal of coarse gold (0.57% weight, 27.2%) was performed with a Knelson concentrator and a Mosley table. Cyanidation of the gravity residue increased the overall extraction to 90.4% (leach residue at 0.77 g/t Au). The long retention time was, in part, related to the presence of coarse gold. The Kalgoorlie flotation concentrate contains pyrite and the Eleonore concentrate contains pyrrhotite. The optimum leaching conditions to extract gold from the Kalgoorlie flotation concentrate were: 8-hour pretreatment, DO 1 ppm, 40 kg/t lime, 3 kg/t lead nitrate followed by 72 hours cyanidation, 12 g/L activated carbon, 2000 ppm NaCN, DO 3-5 ppm produced a gold extraction of 95.8% (leach residue at 3.94 g/t) and 8 kg/t NaCN. The findings were confirmed by a plant trial. For the Eleonore concentrate, 97.0% gold extraction was obtained a 16-hour pre-treatment with 6.0 kg/t lead nitrate and 24-hour cyanidation with 800 ppm NaCN. The associated cyanide consumption was 1.2 kg/t NaCN. The limited addition of oxygen in the pretreatment was an indication of its efficiency. The plant trial, performed to validate the parameters, used 24 h pre-treatment, addition of 5 kg/t lead nitrate and 1130 ppm NaCN. Gold extraction and cyanide consumption were 95% and 7.4 kg/t. The Kupol mill was the first commercial application of the CELP. The plant processed a high grade free milling gold/silver ore containing 28.2 g/t Au, 324.8 g/t Ag and approximately 0.5% pyrite during 2008 operation. Gold predominantly occurs as electrum in the ore and silver is predominantly present in the forms of acanthite, stephanite, pyrargyrite and proustite. Mill start-up was in mid-May 2008 and the mill reached the design throughput of 3,000 tpd in October. The ore was ground to approximately 70% -53 microns. A Knelson Concentrator recovered 9.8% gold and 0.5% silver. The gravity tails reported to a cyanidation circuit that has 14 hours pre-treatment and 116 hours leaching. The Kupol mill overall gold and silver recoveries for 2008 were 95.4% and 85.6% respectively, which were 2.8% and 7.5% higher than targets. Gold and silver extractions in the leach circuit were 94.2% and 84.5% respectively. Leach profiles indicated that gold extraction reached a plateau in the 4th tank while minor silver dissolution occurred in the 5th tank. With the CCD circuit included, total gold and silver extractions increased to 95.4% and 86.6%, respectively. Optimisation of the leaching strategy made possible to reduce sodium cyanide concentration as low as 410 ppm without compromising precious metals extractions. The average cyanide consumption for 2008 was 1.3 kg/t.
CITATION:Deschenes, G, 2019. Emerging technologies in cyanidation of gold and silver: a glance at some of the latest contributions, in Proceedings World Gold 2019, pp 156171 (The Australasian Institute of Mining and Metallurgy: Melbourne).