Real and Potential Metallurgical Benefits of HPGR in Hard Rock Ore Processing

Significant improvements in design and wear abatement technology combined with escalating energy and steel media costs are making High Pressure Grinding Rolls (HPGR) increasingly attractive for applicable “hard rock” ore applications relative to SAG-Ball milling. In addition, HPGR offers significant metallurgical benefits, which is the topic addressed in this paper. As early as 1988 it was shown that HPGR comminution resulted in improved gold leaching even at considerably coarser particle size distributions than the conventional minus-74 micron grind. Dramatic heap leach recovery and leach rate improvements were demonstrated in column leaching of South African gold ores also in 1988. Randol (1993) reported that Anglo American Corporation had demonstrated flotation recovery benefits of HPGR. Smit (2005) showed that grade-recovery curves can be significantly optimized by adjusting the HPGR pressure. Unfortunately, the metallurgical benefits of HPGR were not aggressively pursued in the 1990’s when it was found that wear rates and operational availability presented challenges in the treatment of “hard” ores using HPGR designs available at that time. Following improvements in HPGR wear abatement technology and design, industry was reminded of the metallurgical benefits of HPGR by Baum (1997) on the basis of photo-micrograph and petrologic observations on the product of benchscale HPGR. Operational availability and wear-rate abatement improvements have been conclusively proven in three semi-commercial HPGR demonstrations on two gold mining and one platinum mining operations. HPGR was successfully demonstrated on extremely tough ores that are less amenable to SAG milling. Metallurgical benefits of HPGR stem from the inter-particle breaking characteristics of this type of comminution, and include: improved liberation of valuable mineral grains for better flotation and gravity recovery as well as improved and faster heap leach gold and copper recoveries. Test results suggest that mineral recovery benefits can be enhanced if HPGR-liberated sulfide particles are recovered early, i.e. by flash flotation before the HPGR product is ground in a subsequent ball mill. HPGR deserves also to be tested as a means of accelerating bio-heap oxidation of sulfide ores as well as for pretreatment benefits in the reprocessing of tailings by flotation or gravity and for fine-grinding applications, possibly in conjunction with Isamills. HPGR benefits and applicability are, as one might expect, ore-specific and every ore needs to be carefully tested.