The behaviour of gold in Hemlo Mines Ltd. grinding circuit

CIM Bulletin, Vol. 84, No. 955, 1991
S. Banisi, A.R. Laplante, Department of Minng and Metallurgical Engineering, McGill University, Montreal, Quebec, and J. Marois, Hemlo Gold Mines Ltd., Marathon, Ontario
This paper compares the behaviour of gold and ore in the grinding circuit of Hemlo Mines Ltd. Particular attention is given to grinding kinetics, classification behaviour, and liberation. For grinding kinetics, laboratory grinding of gold flakes and silica, 840 \im to 1200 \an in size (14/20 mesh), was also performed to provide a standard for comparison and to study in a controlled environment the mechanisms of breakage, folding, smearing and impacting.
Grinding kinetics were characterized at laboratory scale with breakage and selection functions. The breakage function showed that 75% of gold disappearing from the 840 \an to 1200 \an class reported to the 600 \an to 840 pm; this figure is much higher than for silica (46%) or most other minerals, and implies that comparatively fewer fine fragments are produced from the breakage of gold flakes, on account of gold's malleability. The selection function of silica was four times that of gold. At plant scale, the ratio of the selection function of ore and gold increased from 6 for at 50 \an to 100 nm to 20 at 500 urn to 1000 \an.
Observation of original flakes and their fragments with the scanning electron microscope showed that the main mechanism of "grinding" from the 840 pm to 1200 /j.m to the 600 \an to 840 p.m class was folding rather than actual breakage. This was confirmed by a measure of individual flake weight. The postulated breakage cycle is that relatively globular flakes become increasingly serrated through grinding; they can then regain their original shape through folding and reflattening into cylindrical and spherical shapes, as the majority does; however, some fragments will at this point undergo actual breakage. Photograhic evidence is presented to support the proposed mechanisms.
Smearing of gold unto silica was observed when silica was ground after a gold, and embedding of silica into gold flakes when gold was ground after a silica. Both mechanisms occur whenever gold and harder minerals are jointly ground, and result in "un-liberation" of gold. It is postulated that smearing of gold onto other minerals plays an important role in explaining how gravity recovery can increase over-all gold metallurgy, especially when used ahead of flotation.
Classification of gold in the primary hydrocyclones takes place at a much finer cut-size than ore, 20 jj.m vs 57 \an. This and the slower grinding kinetics result in high gold circulating loads in the primary loop, 6700%, and a gold content in the primary cyclone underflow ((PCUF) is twenty-four times that of the ore. The gold size distribution for primary cyclone overflow is much finer than that of the ore, although they are essentially similar in the PCUF.
Liberation, or more accurately "gravity recoverability", was assessed with a 7.5 cm (3 in.) Knelson concentrator. It varied between the thirties for the primary mill discharge and two cyclone overflows, to highs of 89% and 92 % for the PCUF and secondary mill discharge.
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