Crushing is the first mechanical stage in the process of comminution in which the main objective is the liberation of the valuable minerals from the gangue.
It is generally a dry operation and is usually performed in two or three stages. Lumps of run-of-time ore can be as large as 1.5 m across and these are reduced in the primary crushing stage to 10~20 cm in heavy~duty machines.In most operations, the primary crushing schedule is the same as the mining schedule. When primary crushing is performed underground, this operation is normally a responsibility of the mining department; when primary crushing is on the surface, it is customary for the mining department to deliver the ore to the crusher and for the mineral processing department to crush and handle the ore from this point through the successive ore-processing unit operations. Primary crushers are commonly designed to operate 75% of the available time, mainly because of interruptions caused by insufficient crusher feed and by mechanical delays in the crusher.Secondary crushing includes all operations for reclaiming the primary crusher product from ore storage to the disposal of the final crusher product, which is usually between 0.5 cm and 2 cm in diameter. The primary crusher product from most metalliferous ores can be crushed and screened satisfactorily, and secondary plant generally consists of one or two size-reduction stages with appropriate crushers and screens. If, however, the ore tends to be slippery and tough, the tertiary crushing stage may be substituted by coarse grinding in rod mills.
On the other hand, more than two size-reduction stages may be used in secondary crushing if the ore is extra-hard, or in special cases where it is important to minimize the production of fines.A basic flowsheet for a crushing plant is shown in Fig.7.1, incorporating two stages of secondary crushing. A washing stage is included, which is often necessary for sticky ores containing clay, which may lead to problems in crushing and screening.Vibrating screens are sometimes placed ahead of the secondary crushers to remove undersize material, or scalp the feed, and thereby increase the capacity of the secondary crushing plant. Undersize material tends to pack the voids between the large particles in the crushing chamber, and can choke the crusher, causing damage, because the packed mass of rock is unable to swell in volume as it is broken.Crushing may be in open or closed circuit depending on product size (Fig.7.2). In open-circuit crushing, undersize material from the screen is combined with the crusher product and is then routed to the next operation. Open-circuit crushing is often used in intermediate crushing stages, or when the secondary crushing plant is producing a rod mill feed.
If the crusher is producing ball-mill feed it is good practice to use closed-circuit crushing in which the undersize from the screen is the finished product. The crusher product is returned to the screen so that any over-size material will be recirculated. One of the main reasons for closing the circuit is the greater flexibility given to the crushing plant as a whole. The crusher can be operated at a wider setting if necessary, thus altering the size distribution of the product and by making a selective cut on the screen, the finished product can be adjusted to give the required specification. There is the added factor that if the material is wet or sticky (and climatic conditions can vary), then it is possible to open the setting of the crusher to prevent the possibility of packing, and by this means the throughput of the machine is increased, which will compensate for the additional circulating load. Closed-circuit operation also allows compensation for wear which takes place on liners, and generally gives greater freedom to meet changes in requirements from the plant.
Surge bins precede the primary crusher to receive dumped loads from skips or lorries and should have enough storage capacity to maintain a steady feed to the crusher. In most mills the crushing plant does not run for 24 h a day, as hoisting and transport of ore is usually carried out on two shifts only, the other shift being used for drilling and blasting. The crushing section must therefore have a greater hourly capacity than the rest of the plant, which is run continuously. Ore is always stored after the crushers to ensure a continuous supply to the grinding section. The obvious question is, why not have similar storage capacity before the crushers and run this section continuously also? Apart from the fact that it is cheaper in terms of power consumption to crush at off-peak hours, large storage bins are expensive, so it is uneconomic to have bins at the crushing and grinding stage. It is not practicable to store large quantities of run-of-mine ore, as it is “long-ranged”, i.e. it consists of a large range of particle sizes and the small ones move down in the pile and fill the voids. This packed mass is difficult to move after it has settled. Run-of-mine ore should therefore be kept moving as much as possible, and surge bins should have sufficient capacity only to even out the flow to the crusher.