Effects of fragmentation on production at Peak mine Midlands, Zimbabwe

Abstract

Over the years, the cost of production at Peak Mine Midlands, Zimbabwe, has been high due to oversized rock fragments. This led to increased mining costs far above what was planned by the mine. Initial investigations showed that valuable production hours were lost in trying to deal with the oversized rock fragments at the draw points. This study was conducted at Peak Mine Midlands, Zimbabwe, on 1210 level. The methods used to assess the effects of fragmentation on production at the mine included visual assessment, analysis of the drill and blasting costs of the production data, 2D image analysis, sieve analysis, time and motion studies, analysis of the explosive’s consumption, mining blasting trials and post-mine blast experiments. The results show that the current ring hole designs used in the sublevel open stopes at Peak Mine do not give optimal particle size distribution after the primary blasts as the sieve analyses show that 50% of the sizes of materials at the draw points are larger than 1,000 mm. While 4 secondary blasts were budgeted per shift to reduce the sizes of the boulders to sizes that will pass through the grizzly’s sieves, 7 secondary blasts were done per shift and this led to about 75% increase in the cost of explosives consumed by the mine. The results also show that 35.96 minutes of production time were lost on each secondary blast conducted. Four trial blasts conducted on different ring hole patterns show that ring #1 which has a burden of 1.2 m instead of 1.8 m in the other ring holes led to a significant reduction in drilling and blasting costs from USD $33.79 in trial #4 (the control case) to $28.80 in trial #1. The sieve analyses also show that the optimal fragment sizes from primary blasts are in the range of X = +200 mm and X = +300 mm. So, the designed ring hole patterns in the sublevel open stopes should produce particles with sizes in the range of  150 mm and  +350 mm from the primary blasts.