Crushers and Rockbreakers

1. Introduction

Hard rock mines use rockbreakers and crushers for two fundamental reasons.

• To facilitate the transport of ore from the mine to the mill.

• To initiate the size reduction process required to concentrate the ore.

Crushers

Only three types of crushers are normally used in the hard rock mining industry.

• Jaw crushers

• Gyratory crushers

• Cone crushers

In the crushing circuit, gyratory and jaw crushers are normally employed as primary crushers, while cone crushers serve as secondary or tertiary crushers. Primary crushers normally operate in open circuit while cone crushers operate in either open or closed circuit. (The coarser portion of the product is separated and re-circulated through the crusher in a closed circuit.) In some primary crusher installations, fine-sized material is scalped from the feed before it enters the crusher and reunites with the crushed product after bypassing the crusher (short circuit).

Product Size

In the past, primary crushers provided a product of 4-6 inches (100-150 mm) to feed secondary (cone) crushers. Small modern mines continue with the traditional primary crusher product size that is then directed to secondary and tertiary cone crushers to reduce the particle size enough to permit a single grinding stage.

Medium and large sized mines are typically designed to provide feed directly to an autogenous mill. For this reason, the desired product from the primary crusher has increased to 8-12 inches (200- 300mm) to provide a grinding medium (cone crushers are eliminated). However, the cone crusher has found a new role at some larger mines crushing the coarse fraction of the output from an autogenous mill for subsequent re-circulation. This procedure permits the reduction ratio (ratio between size of feed and product) required of a subsequent ball mill to be kept within practical limits.

Before the advent of the modern rockbreaker, over-size material feeding the primary crusher was a greater problem than it is today and primary crushers were designed largely on the basis of gape (minimum dimension of feed opening) rather than capacity. As a result, typical practice underground was to crush the daily requirement in 1 or 1½ shifts and provide sufficient storage in passes and bins to feed the crusher continuously throughout the operating shift(s).

Rockbreakers enable smaller crushers to be employed for the same service, but the old principals of underground primary crusher sizing persist, perhaps to reduce the cost of operating labor and the frequency of maintenance and repairs.

Truck Haulage

Underground mines served by truck haulage from ramp or adit do not require an underground crusher, but there is a limit to the amount of traffic that such an entry can bear. Several current trends are allowing the vertical depth to which ore bodies may be practically exploited by truck haulage to increase.

• Larger trucks (up to 70-tonne capacity)

• Road trains (truck tractor and trailers)

• Twin entries (permitting one-way traffic)

• Block signals (and even transponders) to control traffic

Nevertheless, when high production is to be transported over a long distance, a crusher-fed belt conveyor remains a more economical alternative.