Ore Handling

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

Keywords

Ore transportation; conveyor; feeders; ore storage; self-heating

2.1 Introduction

Ore handling is a key function in mining and mineral processing, which may account for 30–60% of the total delivered price of raw materials. It covers the processes of transportation, storage, feeding, and washing of the ore en route to, or during, the 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 widely. Ore that has been well fragmented can be transported by trucks, belts, or even by sluicing, but large lumps of hard ore may need secondary blasting. Developments in nonelectric millisecond delay detonators and plastic explosives have resulted in more controllable primary breakage and easier fragmentation of occasional overly-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.

Ores are by and large heterogeneous in nature. The largest lumps blasted from an open pit operation may be over 1.5 m in size. The fragmented ore from a blast is loaded directly into trucks, holding up to 400 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 wide particle size range 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 on the grizzly feeding the primary crusher.

The operating cycle of an underground mine is complex. Drilling and blasting are often performed in one or two shifts; the blasted ore is hoisted to the surface during the next couple of shifts. The ore is transported through the passes via chutes and tramways and is loaded into skips, holding as much as 30 t of blasted ore, to be hoisted to the surface. Large boulders are often broken up underground by primary rock 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. The storage and feeding is usually easier, and indeed essential, due to the intermittent arrival of skips at the surface.

2.2 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 it is ground into a fine pulp and causes choking or blocking of screens, flotation cell ports, etc. Wood pulp may also consume flotation reagents by absorption, which reduces mineral floatability. Clays and slimes adhering to the ore are also harmful as they hinder screening, filtration, and thickening, and again may consume flotation reagents.

All these tramp materials must be removed as far as possible at an early stage in treatment. Removal by hand (hand sorting) from conveyor belts has declined with the development of mechanized methods of dealing with large tonnages, but it is still used when plentiful cheap labor is available.

Skips, ore bins, and mill equipment can be protected from large pieces of “tramp” iron and steel, such as rockbolts and wire meshes, by electromagnets suspended over conveyor belts (guard magnets) (Figure 2.1). The magnets are generally installed downstream of the primary crusher to protect skips and ore bins. These powerful electromagnets can pick up large pieces of iron and steel travelling over the belt. They may operate continuously (as shown) or be stationary and, at intervals, are swung away from the belt and unloaded when the magnetic field is removed. Guard magnets, however, cannot be used to remove tramp iron from magnetic ores, such as those containing magnetite, nor will they remove nonferrous metals or nonmagnetic steels from the belt. 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 triggers an alarm, the belt automatically stops and the object can be removed. With nonmagnetic ores it is advantageous to precede the metal detector with a guard magnet, which will remove the ferromagnetic tramp metals and thus minimize belt stoppages.

Large pieces of wood that have been flattened 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. (On cold nights the collected wood might find a use.)

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 (Chapter 14) 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 undersize product is usually directed to the grinding mills and thus the screen apertures are usually of similar size to the particles in the feed to the grinding mills.

Ore washing is sometimes assisted by adding scrubbers in the circuit. Scrubbers are designed to clean crushed ore, sand, and gravel, but they can also upgrade an ore by removing soft rock by attrition. Scrubbers are self-aligning, steel trunnions supported on flanged railroad type bearings, and driven by a saddle drive chain.

In the circuit shown in Figure 2.2, material passing over the screen, that is, 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, that is, 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 Figure 2.2, the thickener overflows are used to feed the high-pressure washing sprays on the screens. Water conservation in this manner is practiced 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.

2.3 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...