4. RESULTS AND DISCUSION
4.1 Preliminary test work
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Figure 4-1: Mass fraction remaining for 75/65 mm rocks
Figure 4-2: Mass fraction remaining for 65/53 mm rocks y = -0.0012x + 0.9222
R² = 0.9749
0.8 0.82 0.84 0.86 0.88 0.9 0.92 0.94 0.96 0.98 1
0 15 30 45 60 75 90
Mass Fractionn Remaining
Time (min)
y = -0.0011x + 0.9137 R² = 0.9843
0.8 0.82 0.84 0.86 0.88 0.9 0.92 0.94 0.96 0.98 1
0 15 30 45 60 75 90
% Mass Fraction Remaining
Time (min)
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Figure 4-3: Mass fraction remaining for 53/37.5 mm rocks
Figure 4-4: Average mass vs Time
At this stage the results indicated that the smallest average particle size tagged was still within this constant wear region. Smaller particles needed to be tracked in order to find this transition zone. The method of tagging rocks by cutting grooves into the surface was useful as it allowed
y = -0.0011x + 0.9198 R² = 0.9932
0.8 0.82 0.84 0.86 0.88 0.9 0.92 0.94 0.96 0.98 1
0 15 30 45 60 75 90
% Mass Fraction Remaining
Time (min)
0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55
0 15 30 45 60 75 90
Average mass (kg)
Time (min)
75/65 mm 65/53 mm 53/37.5 mm
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for multiple runs to be performed without retagging after each run. However, for smaller rocks this method was not practical. The size of the rocks made it difficult to tag and it was evident that the tagging procedure would create planes of weakness thus affecting the characteristics of the rocks. Spray-painting was used as an alternative method of measuring the wear of samples of the smaller rocks. A different colour was used to identity each size range. The smaller size ranges were 37.5/26.5 mm, 26.5/22.4 mm, 22.4/16 mm.
Figure 4-5 is a graph showing the specific wear rate as a function of average size in screen fractions for various types of rocks. The specific wear rate is defined as the rate of mass loss per unit mass. It shows how fast a rock is abraded away relative to its own mass. The graph shows that larger rocks have a relatively constant specific wear rate. However for a rock size around about 45 mm a change in the trend is observed. A rapid increase in the specific wear rate is observed in a relationship that is inversely proportional to size. The trends observed are similar to that published by Loveday and Whiten (2002). The authors tested ore on a pilot plant scale obtaining specific wear rates between 0.0025 min-1 and 0.0033 min-1. The difference between the two graphs demonstrates the initial fast chipping phase. The fact that the two graphs are equidistant from each other throughout the size range shows that a change in trend at this same size is also observed during the fast chipping phase.
Figure 4-5: Specific wear rate for various types of rocks 0
0.002 0.004 0.006 0.008 0.01 0.012
0 10 20 30 40 50 60 70 80
Specific Wear Rate Rs (1/min)
Size (mm)
Local rocks (Fresh) Local rocks (Rounded) Gold waste rock (Fresh) Gold Waste rock (Rounded) Gold reef rock (Rounded)
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The method of marking the larger rocks enabled the size distribution of marked rock to be tracked as it is worn away. The combined size distribution of the marked is shown in Figure 4-6.
The graph shows that for any given feed size distribution the charge will reach a constant size distribution. The irregular shape of the fresh rock sometimes caused a rock to be held up in an
‗incorrect‘ size fraction. This is quickly corrected by the fast chipping phase which as mentioned breaks off sharp edges and planes of weakness. This allows a rock that was previously held up by a sharp edge to enter a smaller ‗correct‘ size fraction. This phenomenon is evident in Figure 4-6.
It can be seen that the feed size distribution changes rapidly during the first 15 minutes. After that the rocks enter the second phase of wear and thus the gradual change in size distribution.
The main mechanism during this phase is abrasion which is defined the detachment fine material from the surface of larger rocks. Although this process is a relatively slow one it may still cause a large mass to leave a size fraction and thus cause a change in size distribution. This occurs when a rock is close to the lower limit in a size range. Figure 4-6 shows how the gradually change size distribution until a constant size distribution is achieved. It is important to note that this constant size distribution is not a steady-state size distribution because the marked rock was not topped up continually.
Figure 4-6: Change in size distribution with time
A visual examination of the screened charge, was used to determine the size at which pebbles no longer exists. Table 4-1shows that between 16-11.2 mm, some fracture occurs, while below 11.2 mm, no rounded pebbles were found. Thus 11.2 mm was the taken as the lower limit for pebbles and particles between 11.2-3.3 mm were considered to be chips or scats. A large proportion of
0 10 20 30 40 50 60 70 80 90 100
0 10 20 30 40 50 60 70 80
Cummalative % Passing
Size (mm) FEED (t=0)
RUN 1 (t=15) RUN 2 (t=30) RUN 3 (t=60) RUN 4 (t=90)
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these chips are progenies of the fast chipping phase. In a closed circuit the inability to deal with these chips results in an accumulation of chips which may be detrimental to the grinding process.
The average mass allowed the shape factor of the rocks to be traced as a function of time. The shape factor is a good indicator of the rounding procedure of fresh rock. The general idea is shown in Figure 4-7. Figure 4-8 is a plot of shape factor versus time, combining all tagged rocks in the selected size fraction. This plot shows how rocks of a fixed ‗size‘ change shape and hence mass. The rapid shift in the shape factor is due to a rapid change in the rocks average mass. This rapid change takes place during the fast chipping phase and is the probable reason for sudden changes in the charge size distribution. The anomaly observed at the 75/65 mm fraction is thought to be due to the fact that it represents the largest feed size fraction where rocks may leave the size fraction but may not enter since new feed is not added.
Figure 4-7: Rounding of fresh rock (MacLeod, 2002)
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Figure 4-8: Combined change in shape factor for various size fractions 0.8
0.9 1 1.1 1.2 1.3 1.4
0 10 20 30 40 50 60 70 80 90 100
Shape factor (M/Ms)
Time (min)
75/65 mm 65/53 mm 53/37.5 mm
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Table 4-1: Pebble size limit
Size Range (mm) Image Observation
37.5-26.5 Pebbles present
26.5-22.4 Pebbles present
22.4-19 Pebbles present
19-16 Pebbles present
16-11.2 Pebbles present with
some fracture
11.2-3.3 No rounded pebbles
present
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