5. Processing

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The purpose of the aggregate processing plant is to prepare the rock or mineral in a form suitable for its use as aggregate, commonly defined in terms of particle size and size distribution, particle shape and mechanical properties, e.g. compressive strength. As a result, the process plant usually contains only the unit processes of crushing and grinding (comminution) and sizing together with materials handling and transportation equipment such as conveyors and feeders. The use of water and wet processing techniques facilitates the sizing of fine particles (classification) and the dispersal and subsequent rejection of finely sized mineral particles, e.g. clays. Consequently, the process plant may frequently also contain pumping and slurry handling equipment and unit processes of solid-liquid separation for final dewatering of the aggregate products and even waste products. Mineral separation processes are occasionally employed to reject material of undesirable physical or chemical properties. In this respect the process of sizing is commonly used adventitiously or deliberately to separate a particular mineral fraction of the aggregate as will be discussed below. Comminution is an energy intensive and relatively expensive process whose use must be minimized and the agglomeration of fine particles to create larger sizes is rarely if ever economically justified. Therefore, as discussed in Chapter 3, in the case of sand and gravel deposits it is important to determine the relative proportions of the ‘sand’ and ‘gravel’ sizes.