Introduction

Ore handling, which may account for 30–60% of the total delivered price of raw materials, covers the processes of transportation, storage, feeding, and washing of the ore en route to, or during, its various stages of treatment in the mill.

Since the physical state of ores in situ may range from friable, or even sandy material, to monolithic deposits with the hardness of granite, the methods of mining and provisions for the handling of freshly excavated material will vary extremely widely. Ore that has been well broken can be transported by trucks, belts, or even by sluicing, but large lumps of hard ore may need individual blasting. Modern developments in microsecond delay fuses and plastic explosive have resulted in more controllable primary breakage and easier demolition of occasional very large lumps. At the same time, crushers have become larger and lumps up to 2 m in size can now be fed into some primary units.

Open-pit ore tends to be very heterogeneous, the largest lumps often being over 1.5 m in diameter. The broken ore from the pit, after blasting, is loaded directly into trucks, holding up to 200 t of ore in some cases, and is transported directly to the primary crushers. Storage of such ore is not always practicable, due to its “long-ranged” particle size which causes segregation during storage, the fines working their way down through the voids between the larger particles; extremely coarse ore is sometimes difficult to start moving once it has been stopped. Sophisticated storage and feed mechanisms are therefore often dispensed with, the trucks depositing their loads directly into the mouth of the primary crusher.

The operating cycle on an underground mine is complex. Drilling and blasting are often performed on one shift, the ore broken in this time being hoisted to the surface during the other two shifts of the working day. The ore is transported through the passes via chutes and tramways and is loaded into skips, holding as much as 30 t of ore, to be hoisted to the surface. Large rocks are often crushed underground by primary breakers in order to facilitate loading and handling at this stage. The ore, on arrival at the surface, having undergone some initial crushing, is easier to handle than that from an open pit mine and storage and feeding is usually easier, and indeed essential, due to the intermittent arrival of skips at the surface.

The removal of harmful materials

Ore entering the mill from the mine (run-of-mine ore) normally contains a small proportion of material which is potentially harmful to the mill equipment and processes. For instance, large pieces of iron and steel broken off from mine machinery can jam in the crushers. Wood is a major problem in many mills as this is ground into a fine pulp and causes choking or blocking of screens, etc. It can also choke flotation cell ports, consume flotation reagents by absorption and decompose to give depressants, which render valuable minerals unfloatable.

Clays and slimes adhering to the ore are also harmful as they hinder screening, filtration, and thickening, and again consume valuable flotation reagents.

All these must be removed as far as possible at an early stage in treatment.

Hand sorting from conveyor belts has declined in importance with the development of mechanised methods of dealing with large tonnages, but it is still used when plentiful cheap labour is available.

Crushers can be protected from large pieces of “tramp” iron and steel by electromagnets suspended over conveyor belts (Figure 2.­1). These powerful electromagnets can pick up large pieces of iron and steel travelling over the belt and, at intervals, can be swung away from the belt and unloaded. Guard magnets, however, cannot be used to remove tramp iron from magnetic ores, such as those containing magnetite, nor will they remove non-ferrous metals or non-magnetic steels from the ore. Metal detectors, which measure the electrical conductivity of the material being conveyed, can be fitted over or around conveyor belts. The electrical conductivity of ores is much lower than that of metals and fluctuations in electrical conductivity in the conveyed material can be detected by measuring the change that tramp metal causes in a given electromagnetic field.

When a metal object causes an alarm, the belt automatically stops and the object can be removed. It is advantageous with non-magnetic ores to precede the metal detector with a heavy guard magnet which will remove the ferromagnetic tramp metals and thus minimise belt stoppages.

Large pieces of wood which have been “flattened out” by passage through a primary crusher can be removed by passing the ore feed over a vibrating scalping screen. Here the apertures of the screen are slightly larger than the maximum size of particle in the crusher discharge, allowing the ore to fall through the apertures and the flattened wood particles to ride over the screen and be collected separately.

Wood can be further removed from the pulp discharge from the grinding mills by passing the pulp through a fine screen. Again, while the ore particles pass through the apertures, the wood collects on top of the screen and can be periodically removed.

Washing of run-of-mine ore can be carried out to facilitate sorting by removing obscuring dirt from the surfaces of the ore particles. However, washing to remove very fine material, or slimes, of little or no value, is more important.

Washing is normally performed after primary crushing as the ore is then of a suitable size to be passed over washing screens. It should always precede secondary crushing as slimes severely interfere with this stage.

The ore is passed through high-pressure jets of water on mechanically vibrated screens. The screen apertures are usually of similar size to the particles in the feed to the grinding mills, the reason for which will become apparent.

In the circuit shown in Figure 2.­2 material passing over the screen, i.e. washed ore, is transported to the secondary crushers. Material passing through the screens is classified into coarse and fine fractions by a mechanical classifier or hydrocyclone (Chapter 9) or both. It may be beneficial to classify initially in a mechanical classifier as this is more able to smooth out fluctuations in flow than is the hydrocyclone and it is better suited to handling coarse material.

The coarse product from the classifier, designated “washing plant sands”, is either routed direct to the grinding mills or is dewatered over vibrating screens before being sent to mill storage. A considerable load, therefore, is taken off the dry crushing section.

The fine product from classification, i.e. the “slimes”, may be partially dewatered in shallow large diameter settling tanks known as thickeners (Chapter 15) and the thickened pulp is either pumped to tailings disposal or, if containing values, pumped direct to the concentration process, thus removing load from the grinding section. In the circuit shown, the thickener overflows are used to feed the high-pressure washing sprays. Water conservation in this manner is practised in most mills.

Wood pulp may again be a problem in the above circuit, as it will tend to float in the thickener, and will choke the water spray nozzles unless it is removed by retention on a fine screen.

Ore transportation

In a mineral processing plant, operating at the rate of 400,000 t d−1, this is equivalent to about 28 t of solid per minute, requiring up to 75 m3 min−1 of water. It is therefore important to operate with the minimum upward or horizontal movement and with the maximum practicable pulp density in all of those stages subsequent to the addition of water to the system. The basic philosophy requires maximum use of gravity and continuous movement over the shortest possible distances between processing units.

Dry ore can be moved through chutes, provided they are of sufficient slope to allow easy sliding, and sharp turns are avoided. Clean solids slide easily on a 15–25° steel-faced slope, but for most ores, a 45–55° working slope is used. The ore may be difficult to control if the slope is too steep.

The belt conveyor is the most widely used method of handling loose bulk materials. Belts now in use are with capacities up to 20,000 t h−1 and single flight lengths exceeding 15,000 m (“­Bulk Materials Handling”­, 2005), with feasible speeds of up to 10 m s−1.

The standard rubber conveyor belt has a foundation of sufficient strength to withstand the driving tension and loading strains. This foundation, which may be of cotton, nylon, or steel cord, is bound together with a rubber matrix and completely covered with a layer of vulcanised rubber.

The carrying capacity of the belt is increased by passing it over troughing idlers. These are support rollers set normal to the travel of the belt and inclined upward from the centre so as to raise the edges and give it a trough-like profile. There may be three or five in a set and they will be rubber-coated under a loading point, so as to reduce the wear and damage from impact. Spacing along the belt is at the maximum interval which avoids excessive sag. The return belt is supported by horizontal straight idlers which overlap the belt by a few inches at each...