Composite mineral particles: analysis by automated scanning electron microscopy

Abstract

A computer control system has been constructed for the collection of composition and geometric data from samples consisting of whole particles or polished cross-sections of multicomponent materials in a Scanning Electron Microscope (SEM). This information was used to quantitatively characterize the degree of liberation as a function of particle size of the products in the various stages of several mineral processing plants. In operation, the SEM electron beam is stepped across the specimen stage in a raster pattern until discriminated signals from any one of three electron and five x-ray channels indicate a change in material composition. An eight-bit composition code and the beam coordinates are then read into computer memory. Each image is recorded as a compressed digital map of line segment end-points. A border-following algorithm determines particle parameters by linking overlapping line segments to establish areas, perimeters, moments and bounds for each particle or sub-area of a composite particle. Whole particle examination in the SEM has provided new information on composite (non-1iberated) particle characteristics. There is photographic evidence that liberation by grain boundary breakage is significant in autogeneous milling. Particle shape and surface nature can be qualitatively correlated with circuit location of flotation concentrates. 'Quantitative examination of mineral particles from industrial flotation plants by the automated SEM technique has made possible the differentiation between entrained gangue (quartz) and floated quartz in composite particles. Most composite particles recovered from the two porphyry copper ores studied, are quartz particles, each with a small surface area of chalcopyrite (of the order of 3% projected sulphide surface area). Sets of plant data have also been collected to provide metallurgical information on the steady-state flotation behaviour of composite particles in a regrind-retreatment circuit.