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Liberation and Free Surface Area in the Float Feed

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  LIBERATION AND FREE SURFACE AREA IN THE FLOAT FEED By  Al Cropp  July 10, 2013   Flotation, Grade Recovery, Liberation, Process Mineralogy   The importance of the liberation of target minerals in the feed to a flotation circuit is well recognised and understood by process metallurgists. This blog seeks to introduce some of the concepts around: - how liberation is defined    - what the important parameters to understand are       - how liberation is defined by process mineralogists Liberation measurements estimate the volumetric grade distribution of a mineral as a measure of the quality in a processing stream (Spencer and Sutherland, 2000). Put simply, it is based on the area % of the mineral grain in the particle: which brings us to the first key question  –   what is the difference between a “grain” and a “particle”?  The second critical question is to ask whether area % alone is enough to help predict how a particle will behave in a flotation cell  –  what about free surface area? A grain may be defined as 90% liberated, but have no free surface area… so will this recover more quickly or more slowly to the flotation concentrate than say a grain that is 60% liberated but has a high free surface area? What is the difference between a “Grain” and a “Particle”? Typically, a “grain” is classed as a single mineral, whilst a “particle” is made up of one or more mineral grains. The figure below provides an example of a single “particle” that contains four mineral “grains”.    Example of a “particle” con taining five mineral “grains” (4 black and 1 white)   Liberation In process mineralogy, the degree of liberation of a mineral is generally calculated from examining 2D sections of a statistically representative set of particles containing the mineral of interest. The degree of liberation is typically based on the area% of the mineral grain(s) of interest in a particle (figure below). Particles are usually classed in to particle grades of various incremental steps: these steps might be 10% increments (100% liberated, 90-100% liberated, 80-90% liberated etc), or as broader 30% steps such as <30% (locked), 30-60% (middlings) and >60% (liberated).   Particles classified by ore mineral liberation at 100%, 75%, 50% and 25% by area Free Surface Area This calculation of ‘liberation’ is distinctly different to an estimate of the free surface area (or PSSA  –  phase specific surface area) of a mineral in a particle. The free surface area is again typically based calculations from examining 2D sections of a statistically representative set of particles, however as the name suggests it is an estimate of the % of the grain(s) of interest that has a free surface, and therefore have a surface accessible by bubbles in a flotation cell, or acid on a heap leach pad for example (figure below).   Particles classified by % free surface area Understand how the data is derived in order to understand how to interpret it  The figure below combines these concepts, showing example particles containing ore mineral grains classified by both liberation % and free surface area %. These examples serve to highlight the influence of particle and mineral texture for grade and recovery calculations, and how critical it is to understand both degree of liberation and the free surface area. Both these textural classifications are critical for mineral processing, with the ideal particle of course having 100% liberation (and therefore by inference, 100% free surface area).  
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