eRocks

1.2 Liberation
Liberation involves the release of “values” from within a rock matrix, and is usually achieved by size reduction. The occurrence of “values”, usually in the form of locked particles and the degree to which they need to be liberated, depends on the subsequent processes that are employed for their recovery. For example, gravity separation ideally requires relatively clean individual particles for production of a concentrate but a process such as leaching may only require a small proportion of the value to be exposed in order to be effective.

The physical properties of the mineral assemblage will dictate how well a given constituent can be liberated, and principally depends on the size of the grains present within the matrix. If the grain size of the value is large in comparison to the run of mine rock, then little size reduction is required and liberation can be achieved relatively easily. Conversely, if the grain size of the value is small in comparison, effective liberation will be more difficult to obtain. This concept is represented in Figure 1.1 below, where the black circles represent grains of the desired value. It can be seen that to liberate the grains on the B side would require greater size reduction when compared to the A side.

Figure 1.1 Liberation From a Matrix

In practice it is rarely possible to achieve 100% liberation due to the size of the value compared to the matrix and a point is reached where the cost of further size reduction outweighs the additional income from improved recovery. At fine particle sizes material can be lost to the tailings due to the inefficiencies of separation processes and therefore recovery decreases. The physical entrainment of fine particles can also decrease the grade of final concentrates.

1.2 Grindability Determination
Crushing and grinding are the principal methods of reducing the size of a mine product. Wide ranges of equipment types are available within both categories and choices are dependant on the properties of the rock to be treated. The grinding process is geared towards finer particle sizes and as a result is more energy intensive. It is for this reason that a suitable means of estimating power consumption is utilised so the correct equipment can be chosen. An error at the design stage could lead to a huge additional capital cost as a result of underestimation and increased working cost as a result of overestimation. A number of tests have been developed to estimate the grinding energy.

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