Get Better Recovery From Heap-leach Systems
From comminution to pad design, miners are reviewing options for precious metals
By Steve Fiscor, Editor
Pad designs vary, but there are predominantly two types of heaps: static and dynamic. With dynamic heaps (or on/off pads), the leached ore is removed and fresh ore is placed on the pad after each leach cycle. From a design perspective, the primary difference between dynamic and static heaps is the thickness of the liner. The pads for the dynamic heaps tend to use slightly thicker geomembranes and much thicker over-liner layers, which often consist of two layers. The bottom layer is similar to the over-liner layer for a static heap, and the upper layer consists of a coarse stone (primary crushed ore).
Among the world’s gold producers, Peru has some of the most rugged terrain and those operations tend to use valley leach pads, where the ore is stacked against the mountain. These mines will also use a compacted clay liner or a geosynthetic clay liner, or in some cases, both. Valley leach pads also use geocomposites under the geomembrane to protect the liner from an aggressive subgrade.
The technique for placing ore on the pad can also vary, but many mines prefer slewing stacker systems that build the heap to a consistent level. Large heaps are built in lifts with some mines using as many as three 50- to 60-m lifts. The ideal particle size within the heap is also an area of debate. While most mines rely on traditional secondary or tertiary crushers, one U.S. gold miner is now using high pressure grinding roll (HPGR) technology and getting good results.
Heap-leach Pad Design
Without proper engineering and design, many variables can affect heap-leach systems and possibly jeopardize the economic viability of the project. With tight construction schedules and pressure to begin loading ore on the pad, some engineers and project managers are tempted to skip steps, which could be a costly move. Sharing the lessons they have learned from the design, construction and operation of numerous heap-leach pads worldwide, SRK offered several recommendations for a successful system.
The design should account for site-specific conditions such as topography, climate, geotechnical stability, environmental considerations and closure options. A rigorous sampling program should be developed for characterizing metallurgy, which also assesses ore variability. When selecting the appropriate type of heap-leach pad, engineers should also consider containment and ore loading systems.
The design should also consider other operations nearby. As an example, flyrock from a poorly designed blast could easily ruin an exposed liner. Engineers should also anticipate potential lifecycle changes for the operation, especially closure conditions. The solution management system, i.e., irrigation, collection and water balance, is equally important and it should take extremes into consideration, such as dry conditions or high levels of rainfall.
Golden Queen’s HPGR
Golden Queen Mining commissioned its gold/silver heap-leach operation, the Soledad Mountain mine, during the first quarter of 2016. This is the first commercial heap-leach operation in North America to use HPGR technology to size and prepare ore for leach pads. Located five miles south of Mojave in Kern County, California, the mine uses conventional open-pit mining methods and cyanide heap leach to recover gold and silver from crushed, agglomerated ore. Gold and silver production is projected to average approximately 74,000 oz and 781,000 oz, respectively, per year over the course of 11 years.
Speaking at the Vancouver Resource Investment Conference during February, Thomas Clay, chairman and CEO for Golden Queen Mining, said they had just completed two-thirds of the life of mine heapleach pad capacity for Soledad Mountain. “We use conveyors and radial stackers to heap material onto the pad,” Clay said. “We will start to add space for another pad over the next few years. We can extend mine life by adding pad capacity.
The ore is crushed in three stages: a primary jaw crusher, a secondary cone crusher and a tertiary HPGR, Clay explained. The HPGR consists of two counter-rotating rolls, one fixed roll and a floating roll. Grinding forces are applied by four hydraulic rams. Ore is choke-fed to the gap between the rolls.
The potential benefits of using HPGR technology includes faster and higher recoveries due to the formation of micro- cracks in the ore particles. The process should also demonstrate higher agglomeration due to a more favorable overall particle-size distribution. This would also impact the flow rate of solution through the heap. This flowsheet also offers lower capital costs than a four-stage, conventional crushing-screening plant. Dust control is better managed with fewer transfer points.
“We found that our ore gave significantly better recoveries as well as lower operating costs due to the efficient use of power,” Clay said. “Our recoveries have been tracking very well and we believe that we will be able to achieve our feasibility study targets for recovery rates of 75% to 80%.”
Thyssenkrupp designed and built Golden Queen’s HPGR, which has the ability to operate with a specific press force of 609 psi (4.2 N/mm2) as the key operating parameter. A higher specific press force gives a finer overall particle-size distribution and leads to a greater density of microcracks and this will directly affect tails and recoveries. The conclusion is that the specific press force is the determining operating parameter in an application of the HPGR as the comminution equipment.
Applying the Technology to Heap Leach
At the 2018 Society for Mining, Metallurgy and Exploration (SME), Gerhard Sauermann, business development manager for thyssenrupp Industrial Solutions, presented two case studies for HPGRs being used in heap-leach operations: Soledad Mountain and Gold Fields’s Tarkwa mine in Ghana.
The Tarkwa installation established the benefits of HPGR in heap-leach operations, Sauermann explained. He has overseen the POLYCOM HPGR set up at several sites including Soledad Mountain and Tarkwa. “The use of HPGRs has grown steadily and now they are being incorporated into heap-leach processing,” Sauermann said. “Iron ore miners adopted the technology in the early 1990s. The systems have been in use at hard rock operations since thyssenkrupp developed the technology to overcome the wear rates.”
Queen City uses its HPGR in an open configuration. “The development of the Soledad Mountain coincides with the development of the HPGR,” Sauermann said. “Their machine was commissioned in early 2016 and has been operating since. It processes 800 tons per hour (t/h). There was a lot of test work with this unit.”
Comparing the HPGR results with that of a vertical shaft impactor (VSI), Sauermann said the HPGR had high recovery rates and higher total recovery even though the HPGR produces a coarser product than the VSI.
“With HPGRs, people are always talking about the energy savings or the recovery,” Sauermann said. “With these units, the availability is also very high. After 25 months of operation, the primary jaw crusher and the secondary crusher both required maintenance. The POLYCOM is still using the same set of tires. We expect to change those tires sometime soon.” The plant is working on its third lift and then the total stack height will be 190 m. HPGR performance is exceeding expectations, 70%-75% of output passing 1/4-in. The availability of the heapleach pad has been extremely high.
Tarkwa installed its HPGR as a technology demonstration for the final six months of the mining operation, Sauermann said. “Four years later, it is still running as they treat old heaps.” For years, there has been a lot of excitement about the possible use of HPGRs for heap-leach operations. The benefits over conventional crushing technology for heap-leach applications are becoming clear, Sauermann explained, which includes the microcracking effect. The next HPGR advancement for heap leach might be in copper.