It has been found in industrial practice that the efficiency of high tension separation of mineral sand grains is influenced by thermal pretreatments. This phenomenon was systematically investigated by the treatment of mineral sands in CO/C 0,2 and H2/H20 gas mixtures at temperatures between 300-900°C . Laboratory scale tests were carried out using. It has been found in industrial practice that the efficiency of high tension separation of mineral sand grains is influenced by thermal pretreatments. This phenomenon was systematically investigated by the treatment of mineral sands in CO/C 0,2 and H2/H20 gas mixtures at temperatures between 300-900°C . Laboratory scale tests were carried out using a high tension roll separator. Measurements of the electrical resistivity were carried out and microstructural changes in the minerals were examined using both optical and electron microscopy techniques.
The recovery of the Rutile mineral as a conductor after high tension separation was found to be strongly influenced by the pretreatment conditions of the mineral. High temperatures. long residence times and low oxygen potentials favoured the recovery of the Rutile as a conductor.
The recovery of the Rutile mineral was found to be strongly related to the measured mineral resistivity. Decreases in the resistivity of several orders of magnitude were obtained with heat treatment under reducing conditions. The microstructural investigations have shown that there is a strong relationship between the resistivity of the mineral and it's crystal structure.
Two distinct regions of mineral resistivity behaviour were observed :
i) At high oxygen potentials the mineral behaved as a nonstoichiometric oxide with the deviations from stoichiometry being accommodated by point defects distributed throughout the oxide. The variations of the conductivity of the Rutile as a function of the oxygen potential were described by the relationship, σ∝pO2 -1/1 1.0. This behaviour is consistant with the charge carriers being extrinsic acceptor type defects.
ii) At lower oxygen potentials the Rutile mineral conductivity increases (resistivity decreases) rapidly with the decreasing oxygen potential. This increase in the conductivity has been shown to be associated with the formation of (101) microtwin faults in the mineral lattice as a result of the reduction process.
These microtwins form lathe type precipitates which extend through the mineral grain. The growth of these phases creates an electrically parallel circuit consisting of the microtwins in a nonstoichiometric Rutile matrix; since the conductivity of the twin boundary is very high this arrangement produces a large decrease in the resistivity of the Rutile mineral for the formation of a relatively small volume fraction of the microtwins(<0.5%).
The present study shows that significant increases in the efficiency of the high-tension separation of the mineral sands can be achieved by thermal pretreatment under controlled conditions. The important process variables are shown to be the oxygen potential of the gas mixture, treatment temperature and the treatment time.