It is of particular importance in older mines where tonnage has been reduced, spare mills are available and savings in operating costs are essential to ensure the financial success of the mine. This phenomenon is believed to be due to the removal of the larger stones, resulting in a decrease in impact forces on the pebbles, and thus a lower rate of surface wear.
INTRODUCTION
However, this means that the rate of consumption of the smaller pebbles increases, as well as the amount of particles of critical size, as the rocks undergo an initial rounding phase, which produces pebbles but also chips. However, by changing the natural size distribution of the pebbles and setting a new optimized size distribution, it was anticipated that the aforementioned problems could possibly be overcome.
LITERATURE REVIEW
Introduction
Analysis of comminution methods
- Single-Stage Ball mill grinding
- Rod mill - Ball mill grinding
- SAG- Ball mill grinding
- SABC Grinding
- ABC Grinding
- Traditional two stage autogenous grinding
- Boliden grinding circuit
- High pressure roll grinding
- Outogenous grinding method
- Rod mill - Pebble mill grinding
- South African single-stage ROM Mills
Less expensive than a rod mill – ball mill grinding circuit due to the savings in balls in the secondary mill. To ensure that the mill operates within the maximum power range, a 40% or more volumetric fill is maintained and the mill is operated at relatively high speed (70-90% critical) which is therefore the characteristics of the South African ROM mill determined. (Powell et al., 2001).
Autogenous grinding
- History of autogenous and semi-autogenous grinding
- The change to Pebble Milling
- Ore variation
- Critical particle size
- Size of grinding media
- Theory of pebble wear
According to Wipf (1996) "the critical size of material is the dominant factor when mined ore is hard, competent and susceptible to impact fracture to an acceptable size". 1996) defines the critical size fraction as the size of pebble that is unable to break up smaller pebbles and cannot be broken up by larger particles. According to Loveday and Whiten (2002), the wear rate of stone (or pebble) in an AG/SAG mill determines the capacity and efficiency of the mill.
Charge Motion
The distance between the CoC and the center of the mill is known as the radial position of the CoC. The angular velocity of the load and the mill shell may not be the same.
Mechanisms
The test results are shown in Figure 2-22, which shows the linear decrease in weight due to wear. According to the authors, ―results showed that pure abrasion produced relatively low rates of weight loss, so abrasion and crushing were combined to obtain a total crushing-abrasion rate.‖ Figure 2-23 shows a graphical summary of the process.
The Gold Mine
METHODOLGY
Experimental Equipment
- The 1.2 m diameter mill
- Power measurement
- Torque Calibration
The no load force (NLP) which is defined as the force used to rotate the mill cap. This was done by hanging different masses from the shell of the mill as shown in Figure 3-4.
Experimental procedure
- Preliminary Tests
- Gold ore preliminary tests
- Gold ore tests
- Tests at 40% volumetric filling
The mill was then operated for 30 minutes at 83.5% of the critical speed to pass the initial rock rounding stage. This process was repeated until the fill mass and charge size distribution reached a steady state.
Volumetric filling and Speed Testing
- Experimental Procedure
Comparison to steel balls
- Experimental procedure
The run was repeated to determine the correct peak of the fresh stones and so that an average could be obtained.
RESULTS AND DISCUSION
Preliminary test work
The method of marking the larger rocks allowed the size distribution of marked rocks to be monitored as they wore away. The graph shows that for any given feed size distribution the load will reach a constant size distribution. It can be seen that the feed size distribution changes rapidly during the first 15 minutes.
After that, the rocks pass into the second phase of wear and thus gradual changes in size. Although this process is relatively slow, it can still cause a large mass to leave the size fraction and thus cause a change in the size distribution. This rapid change takes place during the rapid crushing phase and is the likely reason for the sudden changes in the charge size distribution.
Preliminary tests on gold ore
The mass of fresh rock fill as a function of time is depicted in Figure 4-12. It can be seen that during the fast chipping phase (which lasts about 30 minutes) a peak in the feed rate is observed, after which the replenishment rate remains relatively constant. This change in fresh rock loading until a steady state is reached is related to the change in the size distribution of the charge until a steady state size distribution is reached as shown in Figure 4-11.
The milling curve obtained for this ore under these milling conditions is shown in Figure 4-13. The rounded pebbles were mainly greystone, but also had some reef stones that were deliberately fed with the fresh top up. The rapid chipping phase of the fresh rock for both gray rock and local ore is also shown in Figure 4-5.
Model for predicting steady state pebble size distribution
As previously discussed, the shape factor for a group of rocks in a given size fraction is defined as the ratio of the average rock mass to that of a sphere having a radius equal to the average of the upper and lower size fractions . Total mass of rocks initially sampled from size range i. kg) = Number of rocks in the sample. So the initial mean mass (t=0) initially from size interval i as defined by equation 4.1 allows the initial number of pebbles in the sample initially from size interval i.
Using the total rock mass at time t originally from size interval i, the specific wear rate (Rs) was used to determine the total mass at next time interval. This then allowed the total mass of rocks at time t+1 originally from size interval i to be converted to an average mass at time t+1 originally from size interval i. After all size fractions were modeled for wear, the total mass of rocks in each size interval i was determined by summing the total mass of rocks in each size interval that had an average diameter between the upper and lower limits of size interval i has.
Tests on Gold
- Product size distribution analysis
The pebble wear rate, or better defined as pebble consumption, is plotted in Figure 4-16 as a function of velocity for both pebble feed sizes. Since the wear from the impact grinding process is greater than the wear from the wear/wear process, the overall wear rate increases. However, it is believed that the low energy loss of the smaller rocks combined with the removal of the larger rocks results in less violent impacts in the crushing zone, and thus a reduction in the rate of wear is observed.
The lower wear rate in the absence of larger rocks means that the specific wear rate (Rs) is also a function of the load size distribution. The correction factor (ks) is obtained by taking the ratio of the total wear rate observed in the new load size distribution to the total wear rate observed in the reference load size distribution. The new specific wear rate chart is created by multiplying each individual wear rate in the reference load size distribution by the correction factor (ks).
Tests at higher volumetric filling
The rate of pebble wear at a higher volumetric fill is shown as a function of mill speed in Figure 4-26. This served to disprove the notion that low volumetric fill was responsible for the exaggerated wear rate of the larger pebbles, thus confirming the original hypothesis that a reduction in the wear rate of the smaller pebbles is due to rock removal. larger than the load. . To investigate how the change in volumetric filling affected the wear of the pebble, the wear rate per unit of energy consumed was plotted as a function of speed shown in Figure 4-27.
To introduce a dimensionless way of representing pebble wear, the wear rate of pebbles expressed as a percentage of the total material entering the mill was plotted as a function of velocity in Figure 4-28. By expressing the wear rate of pebbles in this way, the effects of volumetric filling on the wear rate could now be investigated. When the wear rate was as predicted and the percentage passing 75 µm was matched, the mass of material finer than 75 µm produced in the higher volumetric fill followed the same trend as the lower volumetric fill.
The effects of mill speed and volumetric filling on power draw
Comparison to steel balls
The constant energy input per tonne of incoming fine material method was used to determine the time of grinding to ensure that the percentage passing 75 µm was within a good range. The constant energy input per ton of incoming fine material method was used to determine the time of grinding to ensure that the percentage passing 75 µm is the same as the pebble runs. The energy input per ton of fine material entering the smaller mill is plotted in Figure 4-35 for pebbles and steel balls as a grinding medium.
This additional energy input can be seen in Figure 4-36, which shows the -75 µm pass percentage as a function of mill speed for pebbles and steel balls in a smaller mill. The rate of rock wear was an important factor in downsizing from the larger 1.2 m diameter mill to the smaller 0.3 m diameter mill. The graph shows a slight drop in percentage wear relative to the equivalent condition in the larger mill.
Implications
CONCLUSIONS
It was gratifying to note that the gravel consumption in the Nkomati mine is of the same order (about 30% of the total production). Pebble consumption was reduced when the stable grind fill and feed was changed from a simulated feed of 65/35 mm to a feed of 44/28 mm. This is believed to be due to the reduction of impact forces in the mill.
Tests repeated at a higher volumetric fill showed similar trends in terms of pebble wear, percentage passing 75 µm and power consumption. Tests at higher speeds and volumetric fills were used to illustrate how maximum power could be achieved. The comparison with steel balls showed that pebbles grind with a greater efficiency than steel balls, but a large decrease in throughput is observed.
RECOMMENDATIONS
