A fundamental study of the heating effect of 2.45GHz microwave radiation on minerals

Harrison, Patrick Charles (1997). A fundamental study of the heating effect of 2.45GHz microwave radiation on minerals. University of Birmingham. Ph.D.

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Abstract

There has been little research into the effects of microwave radiation on minerals, with workers relating microwave heating rates to minerals' chemical structure. This approach fails to explain the behaviour of the 25 minerals studied in this work. Three distinct groups of heating behaviour were attained after 180 seconds at 650W 2.45GHz and compared to an analogy of mineral response to light:
Group I minerals attained high temperatures after exposure to microwaves, with temperatures in the range 175-470°C. They all readily absorb microwave radiation and absorb light as they are all opaque. The members of this group are all ore minerals e.g. Pyrites, Galena, Magnetite, Pyrrhotite, Bomite, Chalcopyrite.
Group II minerals attained medium temperatures in the range of 68-110°C. These minerals do not absorb all the radiation, when in thin section these minerals absorb the light and emit some of the light back (known as internal reflections) They are all ore minerals e.g. Ilmenite, Hematite, Cassiterite, Bauxite
Group III minerals obtained low temperatures with a maximum temperature of 50°C. The members in this group are transparent to microwave radiation, the gangue members are transparent to light and refract it, while the ore minerals in this group possess internal reflections. Members of this group are mainly gangue minerals with some ore minerals e.g. Quartz, Feldspar, Calcite Mica, Barytes, Sphalerite, Rutile.
The Groups I, II and III were subjected to various analytical tests to observe changes in chemical properties after microwave exposure. The minerals in Groups I and II produced much larger changes in particle size, surface area, chemistry, grindability, leachability and magnetic susceptibility compared to minerals in Group III after microwave treatment. These changes in physical and chemical structure may have significance in the downstream processing of these minerals.
A set of experiments were undertaken using microwave energy in order to attempt to address four industrial problems to ascertain whether dielectric heating could produce similar or better results than conventional heating:
Microwave heating failed to improve the separation of clay binding from greensand particles. Favourable results were observed when removing sulphur from leached TiO\(_2\) using carbon as a catalyst. Over 90% of sulphur was removed after a pulse of 6-7 minutes at 650W; this compares to muffle furnace heating results of 1-3 hours.
Microwave pretreatment of coal showed an improvement in grindability of coal (with a decrease in grindability of 30-40%), some desulphurisation, a decrease in moisture content and a decrease in ash content without any change in the calorific value of the coal. The results were better than conventional muffle treatment.
The use of microwaves with Thermal Assisted Liberation has shown that for disseminated ore large changes could be brought about in grindability, as long as the ore is a strong absorber of microwaves. This was shown with Ilmenite Ore which showed a decrease in grindability of 84% after microwave treatment at 2.6kW for 30 seconds.
The results have shown the major advantages microwaves have over conventional ovens are: Microwave ovens can induce large initial temperature rises, especially if a good microwave receptor is present. The energy from microwaves is 'channelled' directly into the substance to be heated, whereas conventional ovens take time to heat up via conduction mechanisms. Microwave heating can be selective; a good microwave receptor can be heated preferentially to a poor receptor to allow the bulk temperature of the sample to remain as low as possible and to minimise energy input.
Any process which involves conventional heating, can well be improved by microwave ovens, but the economics of each process must be addressed.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):