Particle Characterization in Mineral Processing
Particle Properties in Mineral Processing
At the heart of mineral processing unit operations are particles of various sizes and mineralogical compositions. These particles are the only physically separate entities in a particulate material, with minerals locked within them. The behaviour of particles in a processing stage is governed by their relevant properties. Therefore, the evaluation of particle properties is a vital aspect of mineral processing.
Key Particle Properties for Mineral Processing
Some of the particle properties of importance include:
- Physical Properties: size, shape, specific surface area (external surface area per unit particle volume or weight), surface morphology (or surface roughness), porosity, density, and crystal structure.
- Chemical Properties: metal content, mineral chemistry (the chemical compositions of individual mineral species), and deleterious impurity content.
- Mineralogical Properties: identity of each mineral species, the relative proportions of different minerals (modal analysis), and mineralogical texture (the size and shape of the mineral phases within individual particles and the extent of their association and intergrowth).
In particulate materials processing, particles are brought into contact with liquid or gaseous environments. Therefore, in addition to the aforementioned intrinsic properties, their interaction with these environments is also crucial.
Impact of Mineralogical Composition and Texture
Mineral deposits with a high content of valuable elements may be worthless if they cannot be treated by known mineral separation methods due to their unfavourable mineralogical compositions or textures. Conversely, deposits with a low content of valuable elements, but favourable mineralogy, may be exploited profitably.
Quantifying Particle Characteristics
This section deals with the quantification of particle size and shape, and of the mineralogical composition and texture, because these are the most important properties which lead to differences in the physical properties of the particles.
Particle Size and Size Distribution
Particle size is one of the most commonly used particle properties in mineral processing because of its relative ease of measurement. It is frequently used as a control measure for size reduction processes.
The definition of particle size is simple for geometrically simple shapes. For example, the size of a sphere is clearly defined by its diameter, and a cube by the length of its edge. In both examples, the volume of the object can be calculated if its size is known. However, particles of a broken mineral are seldom simple geometric objects and exist in all possible irregular shapes that cannot be accurately defined. Although a single irregular particle has an infinite number of linear dimensions, three mutually perpendicular dimensions (Fig. 2-1) may be determined as follows:
Defining Irregular Particle Dimensions
- Thickness (T):
- The minimum distance between two parallel planes (one being the plane of maximum stability) which are tangential to opposite surfaces of the particle.
- Breadth (B):
- The minimum distance between two parallel planes which are perpendicular to the planes defining the thickness and tangential to opposite sides of the particle.
- Length (L):
- The distance between two parallel planes which are perpendicular to the planes defining thickness and breadth and are tangential to opposite sides of the particle.
Nominal and Equivalent Diameters
Multi-parameter size measures, however, are not too practical and are seldom used. It is much more convenient to use a single quantity to describe the particle size in terms of a characteristic dimension, which is referred to as the nominal diameter or the equivalent diameter.
Figure 2-1. Three linear measures of an irregularly shaped particle.
A number of equivalent diameters can be defined either in terms of some real property of the irregular particle, such as its volume or surface area (geometric similarity), or in terms of the behaviour of the particle in a specific environment or process.