3. METHODOLGY
3.2 Experimental procedure
3.2.2 Gold ore preliminary tests
41
42
Figure 3-9: Screen on rubbish bin during sreening
The sample of gold ore was made up of 700 kg of waste rock, 100 kg of reef rock, and 57 × 20 kg of rod mill product. The rod mill product which is actually the feed to the pebble mill was simulated by further crushing of the ore. This gave a product with a top size of 3.3 mm. The fines were conveniently packed in 20 kg bags. Once again complete analysis of each bag was not possible so sub samples of a bag were taken using the cone and quartering method and the sample was screened to determine the per cent passing 75 µm. Various random samples of bags were also tested to check if the size distribution varied from bag to bag. The screen sizes used for the charge and the fines size distribution are shown in Table 3-1.
Table 3-1: Screen sizes used for sizing
Fines screen sizes
(μm) Charge screen sizes (mm)
3300 65
1180 53
425 44
300 35
212 28
150 16
106 11.2
75 3.3
43
The mill was then loaded with a distribution of waste rocks between 65 mm and 35 mm in size.
The density of the gold ore was 2644 kg/m3 and 132.27 kg of the ore ensured that mill was volumetrically filled to 30%. The feed to the mill consisted of 20 kg of rod mill product and 25 l of water. The mill was then run for 30 minutes at 83.5 % of the critical speed in order to pass the initial rounding phase of the rocks. After the first run the contents of the mill were screened using a 3.3 mm screen, in an effort to separate the fines. A screen size of 3.3 mm was chosen as an arbitrary split between fines and pebbles. A size distribution was performed on the +3.3 mm particles which were eventually returned to the mill. The -3.3 mm particles were discarded. The total mill charge was weighed and fresh make up added with a distribution of pebbles between 65 mm and 35 mm. A fresh sample of rod mill product was used for the next run which had duration of 30 minutes. This process was repeated until the mass of top up and the size distribution of the charge reached a steady state.
Now that the charge had reached a steady state, the next step was to determine the time of grind by producing a milling curve. As before fresh make up rock was added to rounded pebbles and a fresh sample of rod mill product used. A milling curve is basically a plot of milling time against per cent passing 75 µm. The mill was then run for 15 minutes at 83.5 % of the critical speed after which the pulp was passed through a 3.3 mm screen as before. However, now a particle size distribution (PSD) was done on the -3.3 mm particles in an effort to determine the per cent passing 75 µm. (For ease of understanding, the experimental procedure for the screening of fines will be discussed later). Now that the per cent passing 75 µm of fresh rod mill product (i.e.
milling time of zero) and the per cent passing 75 µm after 15 minutes of milling was determined, an estimation of the required per cent passing 75 µm (i.e. 75%) was obtained by assuming a linear fit. The time required was estimated to be approximately 35 minutes which was the duration of the next run. The process was repeated and the per cent passing 75 µm was found to be 76.2%. The mill was now operating at the mines operating conditions producing approximately the correct per cent passing 75 µm.
The next step was to determine the specific wear rate of the rocks which was an important factor used in the model to predict steady state size distribution. The groove cutting method used in preliminary testing was not practical because the rocks were now a lot smaller. Instead, marking was achieved by spray painting rocks with different colours where each colour represented a different size fraction. The first step was to determine the specific wear rate of rounded waste rocks. Rounded waste rock was separated into various size fraction and random samples of rocks were taken from each size fraction. The rocks from each size fraction were then counted and
44
weighed in order to determine the average mass of a rock within that respective size fraction.
The shape factor was also determined for the waste rocks. The rocks were then painted using an aerosol paint and left in the sun to dry for an hour as shown in Figure 3-10. The marks rocks were then milled for 35 minutes using the exact same procedure as mentioned before. After milling, the fines were discarded and each rock was washed and examined for remnant traces of paint in order to identify the size of the rock as shown in Figure 3-12. After all painted rocks were recovered, the rocks were recounted and reweighed and the average mass determined.
Knowing the time of grind and the average mass loss of the rock within a given size fraction, a specific wear rate graph could be plotted for rounded waste rocks. This procedure was repeated for fresh waste rock in order to determine the specific wear rate during the initial rounding phase.
The painted fresh waste rock is shown in Figure 3-11. In order to investigate whether the reef rocks and waste rocks wear at the same rate the entire process was repeated for rounded reef rocks.
Figure 3-10: Marked rounded rock ready for milling
45
Figure 3-11: Marked fresh rock ready for milling
Figure 3-12: Recovered rock showing remanant paint in grooves
46