It has now been several decades since the high-pressure grinding roll (HPGR) made its debut in mineral processing from its cement-based origins. With a number of hard-rock mining applications in large concentrators now several years into production, the time has come to break away from the concept of simply using HPGR in minerals applications, and instead to start focusing on further developmental paths that lie ahead as the technology is optimised specifically for the hard-rock mining industry.
Size of units
HPGRs in today’s minerals applications commonly feature studded rolls with a diameter of 2.4m. One unit this size can typically be used for a 30,000t/d plant. Larger plants would then typically require two, or more, lines.
HPGRs are now available based on current technology with roll diameters of up to 3m, a similarly proportioned roll width, and an installed drive capacity up to 11,000kW. These larger units will be able to handle throughputs in the range of 120,000-140,000t/d. Such a unit can therefore reduce the number of lines installed by half when compared with the smaller ‘typical’ unit.
However, cost savings associated with high-capacity equipment have to be carefully compared with the extra effort required in handling the increased weights of the associated roll assemblies during replacement, which can reach 200t.
The weight of these large rolls also has an impact on the design of the building, associated crane design/capacity and the logistics of servicing these machines, physical handling of the rolls, transportation to/from service centres, etc.
Design of units
Since the initial design of HPGRs for minerals applications, the units have grown steadily larger. While the details of the design have been improved, the basic design principles remain essentially unchanged. Recent innovations are making their way into the market with the goal to:
- Improve the efficiency of HPGRs;
- Emphasise safety; and
- Optimise maintenance.
One supplier has introduced an arch frame design that eliminates skewing of the rolls and introduces shoulders at the roll edges. This idea was adopted to increase the efficiency of the operation by a better distribution of the pressure within the operating gap of the rolls.
FLSmidth’s new Express Frame design addresses the need for a more maintenance-friendly design of the HPGR. In addition to a fast turnaround time for roll replacement, it specifically addresses the issue of a safer work environment when conducting routine maintenance. Design features such as hinged hydraulic rams, an integrated roll-extraction system and oil-lubricated bearings are a result of this safe and efficient mindset.
Wear-protection concepts that had proven to be successful in the cement industry ultimately failed in most minerals applications, with few exceptions.
Without question, the wear-protection system of HPGRs has been the most critical element in attaining successful application in the field of hard-rock mining. The introduction of hard cemented carbide pins, or ‘studs’, embedded in the wear surfaces changed the landscape completely, although many factors influence the wear life and cost.
After a steep learning curve in recent years, a certain saturation point has been reached. Today, optimisation of the wear components will focus not only on the mechanical aspect (for example, stud geometry, stud quality, cemented carbide selection), but more on matching the ore properties (such as the mineralogy and its variation, feed size and size distribution) to the cemented carbide and stud geometry selected for the roll surfaces.
Integrating the mineralogy into the picture not only considers the properties of the actual sample provided to the test facility, but allows for variation of the orebody to a much greater degree.
The operating cost savings of HPGRs are frequently made up not only by a reduction in the power consumption, but by a reduction in wear costs. This is an often-overlooked fact and stresses the importance of the wear-protection system and its optimisation. This requires ongoing assistance through the expertise of the OEM.
By conducting mineralogical ore-characterisation studies, we can establish early, reliable and cost-efficient wear assessments without having to use conventional wear index testing. For this purpose, quantitative mineralogical data will be used to determine the chemical composition and grain size of the studs and related wear design.
When applied to concentrators, HPGRs are in most cases operating in a closed circuit via wet or dry screens. Applications using HPGRs in quaternary mode in open circuit are also working successfully. The prime motivation in these applications is to reduce power consumption and wear cost.
Screen apertures in the case of closed-circuit operations typically range from 5mm to 6mm in the case of wet screens, and typically 10mm for dry screens. Reported power savings are in the range of 15-25% for hard, competent ores when compared with traditional semi-autogenous grinding (SAG)/ball mill, or stage crushing/ball mill circuits.
Future applications may feature even finer screen sizes to enhance the energy efficiency of such a circuit. The moisture content of the screen oversize and the size of the screens themselves will be critical to the success of such installations.
HPGR as metallurgical tool
The process of high-pressure grinding and the properties of its product are very different from those of conventional crushers or grinding mills, and the industry has learned to use this to its advantage in new applications.
Today, the iron-ore industry uses HPGRs to prepare pellet feed for finishing grinding after wet-ball milling and filtering. The wet-ball mill product is more readily filtered without the need to grind it to the final product size. Since HPGRs can process fine ores at moistures of up to 10%, they are used to perform the final stage of grinding.
Water is becoming an increasingly scarce resource, and its consumption in mineral-processing plants plays a key role in the development of new mines. The HPGR technology offers several options to reduce the water consumption within a plant.
Dry finish grinding plants
In addition to eliminating water requirements for the comminution process, higher reductions in plant operating costs can be achieved in cases where the HPGRs are utilised to finish the grind in conjunction with air separators.
Air separators only work in dry mode with a maximum moisture content of around 1.5%. Since drying of the ore is not cost-effective when the downstream process is wet, such a circuit would only be economically feasible in cases of ores with either a low moisture content, or when off gases would be available. Aside from significant reductions in energy and water consumption, such circuits provide other benefits to the downstream process:
- The finished, ground material can be immediately stored. Thus the downstream plant can be fed at a very consistent rate from this created buffer capacity; and
- Air separators provide a product with a narrower size distribution.
In the case of iron ore where part of the downstream process is dry, a significant amount of water can be saved. A portion of the material stream can be subjected to dry magnetic separation before the final grind size is reached. Thus, part of the tailings is removed without the need to conduct finish grinding, which saves a significant amount of energy and water in the downstream (wet) beneficiation process.
Heap leaching of ores
A significant reduction in water consumption and capital expenditure can be achieved in cases where a heap-leach process can be applied. Here, the unique properties found in HPGR product streams offer new and improved options featuring higher recoveries and faster leach kinetics than conventional crushing plants.
Few heap-leaching plants today are operating with HPGRs; however, one successful installation located in Mexico has seen significant improvements in gold recovery since the addition of an HPGR to its stage crushing/leaching circuit.
The typical application of HPGR in heap leaching will be replacing the conventional tertiary crusher. With the larger, super HPGR systems of the future, additional metallurgical options exist, shifting more of the crushing load to the HPGR.
When processing leachable ores through an HPGR, micro-cracks are created. These cracks in the ore particles coupled with the higher amount of fines (compared with conventional crushers) leads to a higher recovery of the valuable constituents of the ore.
Although typical HPGR product includes large amounts of fines when compared with conventional crushers, excessive fines generation can be minimised with the correct usage of HPGR and adequate pressure applications. However, agglomeration is still needed in most cases before placing it on a heap.
Today the mining industry is challenged in many ways:
- Declining ore grades;
- More complex and highly variable ores;
- Harder and deeper ores;
- Rising power and wear costs; and
- Availability of water.
As mentioned above, the HPGR process and product properties are very different from conventional comminution equipment. It provides a new option in hard-rock mining's toolbox, which is badly needed to meet the specific challenges of today's mineral-processing industry.
Johann Knecht is consultant – HPGR and Joe Dziedzina is global product manager – HPGR, both at FLSmidth. See: www.flsmidth.com