A reexamination of the performance of demagnetizing coils on finely ground natural magnetite

Since the last period of intensive study of the demagnetization process for magnetite during the 1950s, our understanding of the physical basis of the magnetism of small particles has increasing exponentially. In light of this improved understanding, the demagnetization process, particularly as applied to finely ground particles of natural magnetite, was reexamined. Magnetometer measurements showed that the coercivity and remanent magnetization increase markedly as the particle size decreases below 10 μm. Although industry-standard 60-Hz demagnetizing coils reduced the remanent magnetization of these particles to a significant extent, the particles remained strongly magnetized, forming magnetic agglomerates of 100 particles or more under quiescent conditions. Although in previous work with high-coercivity synthetic magnetite, it had been found that increasing the strength and frequency of the applied field gave improved demagnetization, applying the same conditions to finely ground natural magnetite gave no significant improvement. The sizable remanent magnetization of the fine natural magnetite particles after all known demagnetization processes is a fundamental property related to the limited number of magnetic domains in each particle. Whether these particles behave as individual particles or as larger agglomerates in any particular unit operation depends upon the amount of shear stress to which the particles are subjected.