KEYNOTE: HPGR in Gold Leach Applications – an Overview
Authors: Holger Plath, Holger Plath, Christoph Beyer, David Kruth, Johann Knecht, Wolfgang Baum
49th Annual Conference of Metallurgists, Vancouver, Canada, August 27-30, 2017.
Although HPGR technology has been recognized since the late 1980s as a potential powerful tool for heap leaching, operational applications did not proceed until the most recent years. Today, at least four gold operations have implemented HPGR systems from three manufacturers (thyssenkrupp, KHD and FLS) into their crushing circuits. They include Tarkwa (now closed), Lindero, Cierro Prieto and Soledad Mountain. Select published testing and/or operating data from these operations will be discussed. All of those applications confirm the early assessments, i.e. using HPGR within the crushing stages (most typically as a tertiary crusher) shows significant benefits for improving the overall hydrometallurgical parameters. The following presentation will discuss the current experience and provide an outlook for HPGR in heap leaching. HPGR may constitute the key to processing remaining reserves which, with conventional crushing technology, show only marginal metal extraction and borderline economics.
The presentation will also address concerns in the industry regarding the generation of excessive amounts of fines. These concerns have been addressed via a combination of efforts including better ore characterization, optimal HPGR operation, agglomeration, heap construction, leach practices and avoidance of heap compaction. Although HPGR’s major benefit to hydrometallurgy is its micro-fracturing of the rock matrix, it does generate some finer product sizes which are inherently beneficial for gold heap leaching. This fines generation effect does not constitute a problem for other leach applications, specifically heap leaching of copper ores. Extensive testing between 1996 and 2016 has documented that HPGR technology, when utilized under best-practice operating conditions including the adequate pressure settings, will not introduce excessive amounts of finer material.
Prior to the introduction of the HPGR, heap leaching of finely crushed materials has been routinely practiced using Barmac vertical shaft impactors and even dry grind ball mills, with the resulting material agglomerated and conveyor stacked. The fine material produced by an HPGR will have similar process characteristics, with the added advantage that micro-fracturing results in a smaller apparent crush size. Other positive aspects of HPGR use in leaching increasingly confirm the value of this technology not only as a high(er) throughput, power- and steel-wear saving comminution system but as a metallurgical tool. These include but are not limited to the following: • Higher availability than any conventional crusher. • Studded tyre wear protection achieved lifetime of the rolls of more than 10,000 operating hours. • Possible rapid adjustments of pressure, gap and roll speed to match the geo-metallurgical variance of the ore. • Indications that HPGR use will result in both lower retained and saturated moisture in the heap (depending on the ore/rock type). • Even with variable rock types and alteration, HPGRs can generate discharge products which (a) exhibit good agglomeration and (b) show favorable PSDs for geo-technical heap stability. • Faster leach kinetics. • Substantial increases of gold extractions with up to 20% higher than achieved via conventional crushing (e.g. St. Ives). • Recent publications suggest that HPGR may even offer a good potential for heap leaching Witwatersrand gold ores.
Robust HPGR testing will continue to be of critical importance. The leach test facilities of Kappes, Cassiday & Associates in Reno include a thyssenkrupp pilot HPGR (“PILOTWAL”). This unit can handle samples as small as 100 kg for trade-off and feasibility studies, but it can process up to 20 tonnes per hour for larger scale testing and semi-commercial purposes.
Re-thinking the standard approach of leaching gold ores with new HPGR flowsheet and process concepts may actually achieve metallurgical results thus far believed to be out of reach. The concept of de-sliming prior to leaching, which was advocated by Herkenhoff and Dean in 1987, has become of considerable significance with the use of HPGR. Finally, larger HPGR units could become a cutting-edge optimization for bio-leaching of low-grade refractory ores using a combination of finer blasting and a primary crusher followed by in-pit HPGR. This paper will review the encouraging operational data and illustrate innovative use of HPGR for various heap- and stockpile leach ores.
CEEC acknowledges and thanks the Canadian Institute of Mining, Metallurgy and Petroleum (CIM), the Metallury and Materials Society (MetSoc) and organising committee for organising the COM 2017 Conference of Metallurgists hosting World Gold and Nickel-Cobalt.
Abstracts can be found at the CIM website (http://web.cim.org/com2017/conference/SessionPapers.cfm#).
Full papers published in the Conference Proceedings will be available on onemine.org, and CEEC directs readers to http://www.onemine.org/ to access and purchase published papers.