Tertiary Stage Crushing

Tertiary stage crushing involves crushing the product to its required size, generally 7 to 15 mm, using two or more types of crushers.

If using cone crushers for both the secondary and tertiary duties, it is common to install different head arrangements on each cone crusher. In this case, the standard head cone crusher becomes the secondary while the short head cone becomes the tertiary crusher. This will be discussed in more detail later.

As with secondary stage crushing, tertiary crushing circuits employ a combination of both open and closed circuit crushing. The primary crusher is normally configured in an open circuit arrangement; while the secondary and tertiary crushers are commonly configured in closed circuit arrangement.

In this closed circuit, the product from the crushers are screened using a double-deck screen with the top-deck oversize from the screen being returned to the secondary crusher and the intermediate size (bottom-deck oversize) being delivered to the tertiary crushers.

Again, the screen mesh size and not the crusher CSS’s govern the size of the material leaving the circuit.

The CSS on the secondary crusher can be set higher than in a secondary crushing circuit as this product will undergo further reduction in size through the tertiary crushers.

The three stages allow the practical achievement of fine product on competent rock, as each stage is operating at normal or equipment design reduction ratios.

Further size reduction can be achieved with the adoption of 4-stage crushing, however this is usually limited to ores of extreme hardness, where reduction ratios are low at each stage due to the competency of the rock, or if the product size required is unusually fine.