Engineering & Mining Journal

AUG 2018

Engineering and Mining Journal - Whether the market is copper, gold, nickel, iron ore, lead/zinc, PGM, diamonds or other commodities, E&MJ takes the lead in projecting trends, following development and reporting on the most efficient operating pr

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Page 33 of 107

MINERAL PROCESSING 32 E&MJ • AUGUST 2018 get was filed down to less than 50 mesh sized particles, which were conditioned to round-off edges. The second binary gold standard was made up of silica sand and 35 and 50 mesh particles of placer gold from Washington. A total of three placer standards were evaluated with feed grades varying between 23.7 g/mt and 2.95 g/ mt. The tailings from the 23.7 g/mt high grade run (No. 192) was used as the feed grade (2.95 g/mt Au) for the subsequent gold run (No. 193). Rougher concentrate weights varied between 1% and 1.7% of the initial feed weights. Rougher metal recoveries (including the media residuals) varied between 81.8% and 87.8%. When development of this dry proto- type concept was initiated, a gravity sep- arator processing a bed rather than a film was envisaged. This was done to increase the productive capacity/unit area rate. However, as development work evolved, gold results showed that only a flowing film would provide acceptable metal recoveries. The limiting factor was the process residence time, which was deter- mined to be 0.7 seconds. For acceptable metal recoveries, a residence time of 0.7 seconds was too short of a time interval to effectively process a bed of material. Scale-up for Commercial Operations Wet spiral concentrators have tonnage rate unit area values of 17 mt per hour (mt/h)/m 2 . And wet plane tables (Rand Leases 1950s practice) have values of 16 to 24 mt per h/m 2 . The dry gravity proto- type has a tonnage rate unit area of 1.6 mt per h/m 2 , roughly one-10th of that for the wet spirals and plane tables. When the dry-gravity prototype design was set up, the length to width ratios for air-gravity conveyors was followed. This gave the dry gravity prototype a 15:1 length: width ratio. A scale-up rule of thumb for gravity separators is go longer for grade increases and wider for higher tonnage rates. Since the length of the prototype was producing good concen- trate grades; the scale-up decision was taken to increase the width and retain the length. Aerated bin feeders are available to accommodate these width increases. And, increasing the width brings tonnage rate area values up to commercial levels. Should precious metal mineralogy for a gold deposit be favorable for dry-gravity concentration, lower capital and operat- ing costs favor gravity-concentration cir- cuits over other unit operations. Should gold values be present within complex sulphides; low weight dry-gravity con- centrates could be recovered, sold or transported to other locations for addi- tional processing. Commercial grades of precious metal values that extend into the minus 270 mesh fines, where terminal velocities are too low for gravity recovery meth- ods, can be processed and recovered by employing iron ore pelletizing practices. Those fines will be in a Blaine surface area range that lends itself to balling and cold bond pelletizing. Those pellets can be sold or transported to a water source area for conventional treatment.

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