On the formation of CoFe_2O_4 when drilling hematite with WC-Co drill bit inserts

摘要

It is commonly observed that WC-Co drill bit inserts wear more rapidly when drilling hematite ore than when drilling other ores of comparable hardness, e.g. ores that contain high percentages of SiO_2. This investigation was carried out to assess if chemical wear is part of the wear process of these inserts, i.e. if chemical reactions occur between hematite (alpha-Fe_2O_3) and WC-Co drill bits, during drilling. The drilling conditions were simulated by high energy-milling of WC-Co and hematite powders mixtures. The effect of milling time on the products formed was monitored. During a total milling time of 30 h, samples were collected at 5 h intervals. The powder samples were analysed by X-ray diffraction (XRD) and Mossbauer spectroscopy (MS) to detect the possible formation of new compounds. The XRD results showed that a new phase, which was identified as cobalt ferrite (CoFe_2O_4), was formed after 25 h of milling a Co/hematite powder mixture. This phase was confirmed by MS which showed a growing magnetic sextet that finally collapses with increasing milling time to a super-paramagnetic doublet [Tsjuji T. Inagaki H. Relation between Coercivity and Mossbauer Spectrum of Cobalt-Adsorbed gamma-Fe_2O_3. ICAME Conf Proc, 1996; 50: 255-8: Kundig W. Bommel H, Constabaris G, Lindquist RH. Some properties of supported small alpha-Fe_2O_3 particles determined with the Mossbauer effect. J Phys Rev 1966; 142:327.]. Results from both analytical techniques showed an increase in abundance of the new phase with increasing milling time. The formation of this phase may contribute to the wear of the WC-Co drill bits when drilling hematite ore.