Rock and Mineral Texture: Controls on Processing

The texture of an ore will define: the grain size distribution(s) and P₈₀ target grind size; the grindability of the ore; the degree of liberation of the target mineral(s); the phase specific free surface area of the target mineral(s); the amount of fines; and the number of coarse composite particles.  These factors will play a major influence on the process flowsheet developed for an ore, from mining strategy through to blending, processing, target grade and recovery, and tailings management.  Understanding these will aid the processing engineer when trying to unlock the maximum value from the rocks, with the minimum of effort, cost and environmental impact.

Texture, in the context of geometallurgy, simply refers to the relationship between the minerals of which a rock is composed (Wikipedia definition).  It includes the size, shape, distribution and association of the minerals in the rock.  All textures, including crystallinity, grain boundary relations, grain orientations, fractures, veinlets etc have a bearing on processing ores, but the sizes of the mineral grains, and the bonding between the grains are the main characteristics that influence ore breakage and mineral liberation (Petruk, 2000).  Understanding the geology and history of an ore will help unravel the complex nature of the textures that may be encountered during processing.

Example of textural changes due to oxidation and deformation (Butcher 2010).

Butcher (2010) presents two broad classes of texture by describing the size distribution of minerals in a rock as either equigranular (grains of similar size) or inequigranular (grains with different size distributions).   This description can be used to describe the size distribution of a particular mineral, which may have grains that are all of a similar size, or have grains with several size distributions.  Subsequent alteration of the rock will then overprint any existing textures. These broad classifications can be expanded by showing how later ground water and oxidation can result in rims and altered surfaces on ore grains, and how hydrothermal alteration and metamorphism can further change the texture.

To further complicate the challenge presented by nature, it may be that the element of interest is distributed across multiple minerals, each potentially with different size distributions.  By way of example, Evans (2010) highlights how copper deportment may be across multiple primary and secondary Cu-minerals.

Example of how copper may deportment across multiple minerals and therefore present an inequigranular texture for the Cu-minerals (Evans 2010)

References:

Butcher, A.R., 2010. A practical guide to some aspects of mineralogy that affect flotation, in: Flotation Plant Optimisation, Spectrum Series. pp. 83–93.

Evans, C.L., 2010. Development of a methodology to estimate flotation separability from ore microtexture (PhD Thesis). The University of Queensland, Australia.

Petruk, W., 2000. Applied Mineralogy in the Mining Industry. Elsevier Science B.V.