Face cleaning

5. In-stope transportation

5.3 Rock handling

5.3.4 Face cleaning

Face cleaning rates (Figure 5-22) are based on empirical studies done in the late 1980s by COMRO (Pickering, 1987; Morris et al., 1988; Pickering et al., 1988).

The work indicates that as panels become longer, the face cleaning rate drops accordingly. In addition, the cleaning rate is dependent on the face conditions and cleaning method, e.g. utilising water-jet-assisted cleaning, and on the application of good mining practices, such as the correct fragmentation, the use of blast barricades, attention to face shapes, etc. Typical cleaning rates of 200 tons- metres per hour (t-m/h) can be expected for faces cleaned without water jets and rates of 400 t-m/h for water-jet-assisted cleaning, while rates as high as 600 t-m/h

practices.

Figure 5-22: Various face cleaning rates (Morris et al., 1988)

Table 5-4 illustrates the impact that face length has on the cleaning rates and thus on the total cleaning time, emphasising that the total cleaning time increases in line with the face length.

Table 5-4: Face cleaning parameters

Face length 25m 40 m

Face cleaning pull length 28,1 m 43,1 m

Average face cleaning length 14 m 21,5 m

Face tonnage 103 tons per blast 165 tons per blast Cleaning rate 42,8 tons per hour 28 tons per hour

Cleaning time 144 minutes 353 minutes

5.3.4.1 Face conditions Face shape

Face shape plays a vital role in the cleaning performance of the face scraper.

This is indicated by the potential cleaning rate of 600 t-m/h, which is based on several factors, of which face shape is, perhaps, the most important. Longer panels become more prone to poor face shapes due to out-of-sequence firing,

misfires and geological disturbances. Bends in the face cause the scraper scoop to hang up, thus requiring rerigging or water jets to move the broken rock to the scraper.

Rigging

The poor positioning of equipment, such as scraper ropes and scraper scoops, can make cleaning difficult when it is left in a position where it can be buried or damaged by blasted rock. Efficient cleaning is further complicated when rig holes are not drilled or are blasted.. It must be noted that rigging is seldom done correctly, which contributes to the poor cleaning rates commonly achieved.

Preparation plays a critical role in the cleaning process. For example, when the equipment is left in the vicinity of the face, it is easy for the stoping crew to commence cleaning operations very quickly. However, when the face has not been prepared, the available cleaning period is shortened as the cleaning crew is required to spend more of the shift on setting up. Also, sharing equipment between faces is a practice that should be discouraged as this results in unnecessary manhandling and transportation of equipment.

Scraper ropes should preferably be rigged into the face and secured to the top of the permanent support prior to the blast, thus obviating the need for the cleaning crew to "snake" down the blasted panel to rig the scraper rope. To further improve efficiencies, all equipment, such as snatch blocks, pins, hammers, hoses and temporary support, should be placed in convenient positions near the stope face.

Placement of support

Incorrectly positioned support (Figure 5-23) can influence cleaning rates by obstructing the scraper path, causing delays and thereby shortening the time available to clean. The positioning of the final row of supports should take account of the face advance and the width of the scoop. Generally, support should be placed a minimum distance of 1,2 m from the face (before the blast) to allow for the free movement of the scoop, which is normally 1,0 m to 1,5 m in width.

Figure 5-23: Face scoop blocked by furthermost prop

Blast barricades

Blast barricades are an effective method of confining the ore on the face, thus reducing the number of re-riggings and the area to be cleaned by the water jet.

Local conditions influence the type of barricade selected, as well as its position relative to the face. There is a wide range of preferences for the type and position of blasting barricades. What is important is that a barricade should be used to confine the broken rock and thereby increase the rate of face cleaning.

5.3.4.2

Water jetting

Water jetting plays an important role in face cleaning as it allows a higher cleaning rate by moving the rock into the path of the scraper, thus reducing the number of rerigs involved in the cleaning process. There are several advantages to using water jets for stope cleaning, as listed below:

• A clean face, as no rock is left on the face, removing the need for sweeping, and resulting in better gold recovery and an improved mine call factor (MCF).

• Higher face cleaning rates, resulting in more blasts and an increase in the face advance rate.

• Reduced water consumption when managed properly.

When used correctly, water jets result in lower water consumption compared with low-pressure water systems. The water flow from a 25-mm-diameter hose is approximately 6 litres per second compared with 2 litres per second for a high-

pressure, low-volume water jet and 3,4 litres per second for a low-pressure, high- volume water jet. In addition, most water jetting systems have automatic shut-off valves that close off the water when not in use.