Microwave-assisted grinding
The use of rapid microwave heating to stress-fracture ore samples is discussed. Iron ores containing hematite, magnetite, and goethite were subjected to microwave energy in batch operations at 3 kW and heated to average maximum temperatures between 840 degrees C and 940 degrees C. Scanning electron microscope (SEM) photomicrographs verified fracturing along grain boundaries and throughout the gangue matrix. Standard Bond grindability tests showed that microwave heating reduced the work index of iron ores by 10 to 24%. Using a continuous-feed belt in a microwave applicator, samples heated more uniformly and quickly than in batch operations. In a microwave chamber designed to simulate a continuous-throughput operation at 3 kW, the grindability of a taconite ore was improved by 13% at a bulk temperature of 197 degrees C. Because stress cracking occurred at a lower temperature, less energy was consumed, thereby improving cost effectiveness. To further improve the economics, preliminary tests were conducted to increase heating rates by using higher microwave powers. The beneficial effects in the grinding circuit and in the extractive operation are discussed.<<ETX>>
Microwave propagation and absorption and its thermo-mechanical consequences in heterogeneous rocks
A numerical analysis in a two-component model rock is presented including the propagation and absorption of a microwave beam as well as the microwave-induced temperature and stress distributions in a consistent way. The analyses are two-dimensional and consider absorbing inclusions (discs) in a non-absorbing matrix representing the model of a heterogeneous rock. The microwave analysis (finite difference time domain — FDTD) is performed with values of the dielectric permittivity typical for hard rocks. Reflections at the discs/matrix interfaces and absorption in the discs lead to diffuse scattering with up to 20% changes of the intensity in the main beam compared to a homogeneous model rock. The subsequent thermo-mechanical finite element (FE) analysis indicates that the stresses become large enough to initiate damage. The results are supported by preliminary experiments on hard rock performed at 2.45 GHz.
The Relative Transparency of Minerals to Microwave Radiation
AbstractMicrowave radiation has potential applications in the metallurgical industry although little has been published on the reaction of microwaves with minerals. In this paper, 40 minerals have been exposed individually to microwave radiation in air to ascertain their general behaviour. Most silicates, carbonates and sulfates, some oxides and some sulfides are transparent to microwave energy but other oxides are readily heated. Both natural and synthetic jarosite-type compounds are unaffected by microwave irradiation. Most sulfides, arsenides, sulfosalts and sulfarsenides are readily heated and consequently, fuse or dissociate prior to eventual oxidation. Microwave effects are compositionally dependent, and elemental substitutions affect the mineral behaviour. Resume Les radiations d'onde ultra-courte ont un potential d'application dans l'industrie metallurgique bien que peu ont ete publie sur la reaction d'onde ultra-courte sur les mineraux. Dans ce rapport, 40 mineraux ont ete exposes individuellemen...