Perfecting Hard-Rock High Pressure Grinding Performance
With high pressure grinding rolls firmly established on the mineral processing map, advocates of this technology are looking to optimize its benefits
By Kyran Casteel, European Editor
As well as the growing number of installations the support infrastructure for HPGR application and operation has been developing, with more testing facilities, independent R&D programs and manufacturer-supported service operations (for example, Polysius directly and Köppern via IMS Engineering, both in South Africa). And, at the Procemin 2009 Seminar in Santiago this December, six presentations will examine progress to date and directions for further advance.
The acceptance of HPGR technology by major and junior hard-rock mining companies results from several advantageous features, including: better energy efficiency, steady high rate throughputs, low dust and noise production, compact design requiring relatively little space and low installation costs, lower operating costs, more rapid progression from start-up to full capacity (reportedly 14% less than for an equivalent SAG mill1), and simple push-button control. In addition, some of the downside of early machines has been minimized by ongoing development, in particular wear material rates and costs. Operators add that circuit operation is easy and the fine product has advantages in subsequent ball milling.
From a supplier’s point of view, the fact that industrial-scale data has been shown to confirm pilot plant results is a considerable help. This ability to predict results, plus favorable comment from respected users, has made risk assessment easier in customers’ evaluation of comminution process options. The commissioning and operation of four HPGRs at the Cerro Verde mine in Peru during 2006 was significant in this context. Successful operation and a program of product sampling demonstrated the effectiveness of the HPGR in this major copper-gold process plant and helped suppliers to overcome the “unproven technology” barrier.
The uptake of HPGR technology by metallic mineral processors has involved a number of suppliers but primarily KHD Humboldt Wedag and Polysius for the HPGR machines and ABB for the drive systems. Normally, it is the HPGR supplier’s prerogative to choose the drive supplier; the end user normally buys the HPGR with the drive systems included.
In their late 2008 paper HPGR— Revolution in Platinum? Anglo Platinum’s Chris Rule and colleagues stated that the Polysius Polycom HPGR has come of age, offering appropriate roll wear-abatement technology and proving itself as an alternative comminution device for hard-rock applications. The Polycom can be specified as to roll diameter, roll width, roll speed, hydraulic pressure and bearing size. Each model is designated by roll diameter/ roll width and frame size so, for example, a PC 17/12–4 has a 1.7-m-diameter roll that is 1.2-m wide in a size 4 frame.
As of the date the paper was prepared there were: 22 Polysius installations at kimberlite operations ranging from a PC 14/08–2 up to the PC 28/05 B, with the majority being PC 17/08–6 (8) and PC 17/12–4 units; 22 iron-ore installations with units ranging from the PC 11/08–1 to the PC 22/15–9 and PC 24/17–8; and 14 in hard rock applications comprising one each of the PC 09/06–0, PC 17/09–5 and PC 22/16–8, plus 11 x PC 24/17–8 machines. The commissioning of the Cerro Verde installation in Peru during 2006 was followed by two HPGRs at PT Freeport’s Grasberg mine in Indonesia and one at the De Beers Victor diamond mine in Canada during 2007; one at the Anglo Platinum Mogalakwena operation, one at Northam Platinum and one for the Assmang Khumani iron ore mine in South Africa during 2008; and four at Newmont’s Boddington gold mine in Australia earlier this year.
The drive system power chosen for an HPGR installation is partly related to the roll size but also to material characteristics, etc. On the Polycoms mentioned above the installed power ranges from 2 x 400 kW to 2 x 1,350 kW in the kimberlite installations, from 2 x 200 to 2 x 2,300 kW in the iron-ore machines and from 2 x 220 to 2 x 2,800 kW on the hard-rock Polycoms.
ABB’s involvement with HPGR technology dates back to 2002 when its use in the mining industry (diamond, iron ore and hard rock) intensified significantly. The company has also supplied drives for HPGRs to the cement industry and to other manufacturers such as KHD as well as Polysius. ABB believes that the wide acceptance of its HPGR drive system in the market is due to its high reliability and fast dynamic response. These features are crucial for consistent production, with minimal losses from production stoppage or poor product quality.
Fixed-speed drives can be used to power HPGRs but, says ABB, because variable- speed drive systems are able to adjust the speed of the rotating rolls they are ideal for optimizing the HPGR to actual conditions. Additionally, variable-speed drive systems are capable of compensating the reduction of the circumferential speed caused by roll wear, by increasing the roll revs. This enables the HPGR to maintain constant throughput throughout the life time of the rolls. Depending on the power demand of the HPGR either a low- or medium- voltage variable-speed drive system can be used, each comprising the following specially designed components: voltage source inverter, transformer and squirrel cage induction motor.
In an HPGR application it is important that the circumferential speed of the two rolls is very close in order to minimize wear. It is also important, ABB points out, to equalize the torque of both rolls in order to avoid permanent overloading of one of the rolls and the drive train. Therefore, the drive system has built-in fiber optically connected master-follower control modes for load sharing, which is the best way to meet these requirements.
The compression and extension of the Cardan shaft, caused by the horizontal back and forth motion of the HPGR’s floating roll, generate large axial and radial forces that are transmitted to the motor. ABB supplies specially designed motors to withstand these forces. The company’s supply capability also provides HPGR users with options for the design of the power supply and control center: integrating the power, control and ancillaries into existing E-rooms, or a self-contained, pre-commissioned containerized Ehouse. This fully air conditioned unit includes the power supply and controls for the mill auxiliaries such as motor control center, PLC and visualization system.
Utilizing the features of the ABB drive system, operators can optimize the grinding process to a significant extent. Setting the appropriate speed according to the material properties fed from the hopper optimizes operating conditions. Slightly decreasing the speed without stopping the HPGR prevents high vibration that can be caused by incorrect material feed. Smooth starting at high torque eliminates damage to rotating rollers and reduces mechanical stress on the gearbox. Setting torque limits in the drive system protects the drive train including the gearbox. Drive parameters can easily be made available to the overriding control system and parameter trends can be used for further process optimization.
Load sharing is particularly challenging in an HPGR application, says ABB. This is because the drive train experiences frequent load transients during normal operation. ABB converters equipped with Direct Torque Control (DTC) technology are best suited for this challenge because they have the fastest torque/speed response time available in the market. This, in turn, provides practical benefits such as improved process control that ensures consistent material quality; fast reaction to load or voltage variations so that the converter does not trip; less stress on the mechanics because backlash problems and torque spikes are minimized, contributing to the overall availability of the HPGR.
ABB says that variable speed drives offer high overall efficiency and are network friendly. The current and voltage harmonic content of an HPGR drive system can be kept within the limits of IEEE 519 by selecting the appropriate rectifier configuration. The power factor is greater than 0.95 through the entire speed range so no separate investment in power factor correction required.
ABB points out two typical examples of recent HPGR installations with variablespeed drive systems—the Cerro Verde copper mine in Peru and Adanac’s Ruby Creek molybdenum operation in Canada. The Cerro Verde Polysius HPGRs have 2 x 2,500-kW VSD Twin drives offering an operational speed range of 360–1,200 rpm and a power factor of 0.96. At Ruby Creek two Humboldt Wedag HPGRs each have a 2 x 1,150-kW VSD Twin drive system with an operational speed range 800–1,790 rpm. In all, ABB has supplied drive systems for HPGR installations at 15 operations since 2002.
For both companies, Western Australia has become a particular hot spot. As well as the start-up of Newmont’s Boddington goldcopper mine in the south of the state earlier in 2009, the Karara Mining joint venture iron-ore project in the central region started mobilization and site clearance in the first week of November this year. Polysius has already delivered four ball mills, and the two HPGRs should arrive in March of 2010. The JV partners, Perth-based Gindalbie Metals and China’s Anshan Iron & Steel (Ansteel), plan to start production of direct shipping hematite in early 2011 and of magnetite concentrate by mid-2011. Moly Mines also has three HPGR units ready for the 10-million-mt/y Spinifex Ridge molycopper project located 50 km northeast of Marble Bar in the Pilbara region of Western Australia. In mid-November the Australian government approved an investment by the Chinese private sector group Sichuan Hanlong, through an Australian subsidiary, which might enable construction to start by mid-2010.
Elsewhere, two Polysius HPGRs at Vale’s Salobo Metais copper project, located in the Tapirapé-Aquiri National Forest, Pará, Brazil, should be commissioned during 2010; ABB’s scope of supply comprises two MV ACS1000 VSD systems, including motors and transformers. Each drive system has a power of 2 x 1,800 kW. Goldcorp’s Penasquito development in Mexico will commission one HPGR during 2010.
An extension of the Polycom size range could, for example, use a size 9 frame with 2.6 x 1.75-m rolls, a press force of 20,000 kN and 2 x 3,300-kW motors for a throughput of 3,000 mt/h. Methods to allow additional use of HPGR in a comminution circuit include using both a tertiary and a quaternary HPGR ahead of the ball milling stage, using one HPGR with both dry- and wet-screening before ball milling, or HPGR plus dry/wet classification to produce a 125-micron finished product.
The authors commented that the HPGR in quaternary mode is a powerful tool for capacity increase and that efficient classification is the key for further energy savings. However, tertiary plus quaternary HPGR use is less efficient than fine screening at the tertiary stage, while the limits on fine wet screening have yet to be determined. Assessing ore properties thoroughly before final flowsheet selection is essential.
The paper mentioned an estimate— included in a presentation by John Marsden at the SME Hydrometallurgy 2008 conference—that using HPGR to prepare low-grade ore for heap leaching is estimated to achieve an overall energy saving of 13% compared with SAG milling, of which >50% is the energy equivalent reduction in steel wear and the remainder is lower electric power consumption. (HPGR’s potential for low grade gold ore heap leaching operations was also addressed by Brian McNab in a paper presented to the IIR Comminution Conference in Australia during 2006.3)
This year’s seminar, to be held in Santiago, Chile, from December 2–4, includes six HPGR papers in a single session.
Chris Morley, with Ausenco Minerals, will discuss HPGR Flowsheets—The Next Generation, examining the practicalities of simplifying the design of HPGR-based circuits by adopting open-circuit operation for the secondary and/or tertiary crushing stages.
Addressing the question How Have HPGRS Fared in the Current Financial Crisis? Rene Klymowsky and his Polysius colleagues will suggest that financial and commodity demand problems have slowed the use of HPGRs but have not dampened interest in the technology.
KHD Humboldt Wedag, with Cia Minera Huasco and Negroni, will discuss Success and Reliability of HPGR Crushing at Compañia Huasco in Chile. And in a presentation titled HPGR—The Journey Over the Rockies Newmont’s Robert Dunne will review the application of HPGRs with the latest design features, wear materials and wear components at projects such as Newmont’s Boddington.
Meanwhile, in an interesting commentary for the South African Institute of Mining and Metallurgy, R.E. Robinson applied Thomas Jefferson’s view that a revolution must lead to reformation to the current scientific environment, arguing that it implies a lead into new fundamental advances and new freedom in creative thinking. He hoped that HPGR development will stimulate new thinking in mineral dressing and extractive metallurgy, and not the least in gold and platinum processing, and especially in the processing of the UG2 reefs of the Bushveld Igneous Complex.
The advantages offered by HPGR could well be termed a revolution leading to a reformation in mineral processing concepts. Can we afford not to pursue such possibilities and do we have the organizations with the “mission” and the expert manpower to lead the reformation, Robinson asked.
1. R. Klymowsky, Patzelt N. Knecht J. and Burchardt E.: HPGR Technology: Present and Future. Presented at Procemin 2008.
2. C. M. Rule, Minnar D. M., and Sauermann G. M.: HPGR—Revolution in Platinum? Paper presented to SAIMM Platinum Conference–Platinum in Transition, October 6-9, 2008.
3. B. McNab: Exploring HPGR Technology for Heap Leaching of Fresh Rock Gold Ores. Paper presented to IIR Crushing & Grinding Conference 2006, Townsville, March 29–30, 2006.