high pressure grinding rolls for gold copper applications

High-pressure grinding rolls (HPGRs) are well established in the diamond and iron ore industries. Process advantages of HPGRs had been recognised by the minerals industry for many years. However, unresolved issues pertaining to wear have made the industry reluc- tant to adopt this technology.

Starting in 2003, a successful pilot-plant demonstration on an exceptionally hard and abrasive gold ore proved that the wear issues could be resolved by the design of an appropriate wear-protection system, and availabilities in excess of 90% could be achieved. The pilot-plant results built up confidence in the minerals industry and a sec- ond pilot-plant trial was conducted on another extremely hard ore with the aim of deter- mining if anything could break the machine. The machine demonstrated even higher availabilities than in the previous case.

A commercial breakthrough then came when one of the world’s leading copper pro- ducers decided to build a new concentrator in South America based on HPGR technology. Four Polycoms, 24/16 in size, each equipped with two 2,500-kW motors, will be used in tertiary crushing duty in closed circuit with wet screens. Shortly thereafter, a second major copper producer ordered two large Polycom 20/15 units for an existing copper concentrator in Indonesia.

In both cases, it was the energy savings and low operating costs of the HPGRs that attracted the producers. This paper examines the conditions (such as the press force nec- essary) that lead to energy savings, lower operating costs, and the optimum performance of the HPGRs.

Wide variations occur in ores, even within one deposit. These variations, insofar as they affect the performance of an HPGR, need to be quantified with meaningful HPGR indices. Two such indices are the ATWAL Wear Index (ATWI) for wear due to abrasion and the Polycom Grinding Index (PGI) for quantifying the fines production.

A laboratory ball mill test, the Labmill test, is described to overcome the uncertainty about the energy required for ball milling after an HPGR. The test is aimed to deal specif- ically with the special features of an HPGR product (i.e., microcracking and the high amount of fines in the product).

The first serious application of HPGRs in the hard-rock mining industry was the installa- tion of an HPGR in a copper concentrator in the United States in 1994.

The expected performance in terms of throughput, fines production, and energy consumption was met. However, the hardness and abrasiveness of the ore was by far higher than that of ores treated in HPGRs previously. It soon became apparent that the wear protection of HPGRs, in particular, the stud technology, was not advanced enough at that time to allow for a smooth and easy transition into continuous operation. Different stud qualities had to be tested and changed in order to suit the requirements of the ore. The change-outs were facilitated by having the rolls equipped with segments; however, these also contributed to wear problems. Finally, stud qualities were found that provided a reasonable lifetime at low cost (~0.10 to 0.15 US$/t) even under these difficult circum- stances. In the end, the unit was decommissioned after treating more than 7,000,000 t of ore when the initial investment plans for the mine were abandoned. Despite the positive operating results, this installation was widely viewed by the industry as a failure of HPGR technology, and its acceptance was set back for years.

Following the Cyprus Sierrita demonstration, there was little progress made towards improving the technology or improving the wear protection for hard-rock applications. The next milestone in HPGR development came in April 2003, when Newmont Mining Corporation began a 3-month trial of a pilot-sized HPGR. Polysius designed a new roll surface specifically for the grinding of hard and abrasive copper and gold ores (Figure 2). The new roll surface consisted of a replaceable shrink-fitted tyre, armed with a new design of tungsten carbide studs, and a new edge-protection system intended to eliminate repair welding of the roll edges. The rolls were also provided with cheek plates to con- tain the material within the gap.

The roll dimensions were 950 mm diameter 350 mm width. Each roll was driven by a 160-kW motor and was operated at a speed of 21 rpm. The capacity of the unit was about 80 tph. The unit was run in closed circuit with an 8-mm square-mesh screen, and was protected from tramp metal by an overhead magnet and a metal detector on the main conveyor belt.

The machine was operated 24 hours a day, 7 days a week, for a period of 87 days with no mechanical downtimes. The initial operational availability of the unit was 89% due to a programming glitch, which occurred after startup, after which a 92% availabil- ity was achieved.

The Lone Tree trial was a true milestone, as no other pilot unit previously had been oper- ated continuously on a 24/7 basis, and no mechanical or welding repairs had been required. The operators and maintenance staff were encouraged by the operation of the HPGR.

No stud failures occurred during the more than 1,600 operating hours of the demon- stration trial. In a commercial-scale application on a similar hard, abrasive ore, an HPGR would have achieved more than 3,000 hours of service and run considerably longer on a less competent and less abrasive ore.

Relationships between wear and particle-size distribution were obtained that will improve future understanding of wear and wear life, benefiting the industry as a whole. The Polysius ATWAL laboratory abrasion test accurately predicted the wear rate in the trial. Individual tests, conducted over the course of 1 year on several representative samples obtained prior, during, and after the trial, were found to be reproducible within 10% of each other, validating the method used for determining and predicting wear in larger units.