**1.3. Economical evaluation**

In the second half of 20 century, many researchers studied on the economical aspect of conventional heat treatment. As early as 1962, it was known that the effect of thermal treatment on ore strength varies with ore mineralogy, and that fluorites and barites, in particular, are susceptible to this effect, but studies showed that the process of thermal treatment was uneconomical when compared to the use of conventional grinding alone (Prasher, 1987)9, due to the enormous energy requirements associated with heating the bulk ore to the required temperatures, where Wills et al., 198710, report that other workers have calculated that the cost of heat treatment and subsequent grinding could be as high as 6 times that of conventional grinding alone (Scheding et al., 1981)11.

### **1.4. Water quenching after heat treatment**

Some researchers studied on the effect of water quenching after heat treatment to reduce the economic costs of heat treatment process.

Kanellopoulos and Ball, 197512, studied the effect of heat treatment on crushing and grinding of quartzite samples. Their investigations showed that heat treatment above 400°C improves the comminution of the ore, but that the best results are obtained after heating the quartzite to temperatures above the α-β phase transition temperature of quartz (i.e. 573°C), at which a sudden volumetric expansion (i.e. a volume increase of 0.86%) of quartz crystals occurs. Comparative testing of material which was slow cooled from 680°C to ambient, and material which was shock cooled through water quenching, showed no difference in the product size distribution of the material after milling. Comparisons of results obtained from the same heat treatments after comminution by slow crushing, however, indicate that quenching the ore results in a change in the product particle size distribution, with significantly less material passing at larger sizes with the difference in passing size decreasing with particle size, thus resulting in a finer product without a significant increase in the production of very fine material. This was the first indication that the manner of the post-processing of the material may be as important as the thermal treatment itself.

Pocock et al., 199813, investigated the use of various quenching solutions to ascertain whether any improvement could be seen from using acid, alkali or salt solutions instead of water. It was found that all of these showed improvements in grinding energy reduction over the use of water, and of these, it was found that the use of acid or alkali solutions provided the best results. At the same time, it was seen from UFLC tests that as comminution of the treated particles continued (i.e. as the particles become smaller), the observed effects of the thermal pretreatment are reduced. What this indicates is that as the easily exploited newly formed fractures are used up, the strength of the ore begins to once again approach that of the untreated ore.

## **1.5. Minerals liberation and heat treatment**

Wills et al., 198710, investigated the thermally assisted liberation of cassiterite in an ore mined at South Crofty. Previous work on this ore (Sherring, 1981)14 had shown a 55% reduction in grinding resistance when the ore was heated to 650°C and then rapidly cooled, however, this was greatly offset by the energy required to heat the material. It was suggested by Manser, 198315, that an increase in tin recovery of 1% would offset this cost in the case of the South Crofty ore, due to the value of the recovered minerals. Employing similar conditions in their work, and heat treated polished sections of the ore which could be photographed before and after the treatment to look for any induced fractures which might indicate that this increase in liberation may be possible. Their results showed that while some intergranular fracturing was observed as a result of their heat treatment, in most of the cases extensive transgranular fracturing occurred, and later separation tests showed no enhanced liberation or recovery of this material with heat treatment.
