High pressure grinding rolls technology is now widely accepted for multiple hard rock mining.
Whether extracting copper in Argentina or coal in China, the small, low-tonnage mining operation has virtually disappeared. The mining industry has shifted to high-tonnage, low-grade operations, which require larger equipment to accommodate the operational shift.
Manufacturers of mining equipment, including Weir Minerals, have also needed to keep pace with this operational shift.
One example of the adoption of new technology is high pressure grinding rolls (HPGR), which was first introduced to the mining industry in the processing of kimberlite to liberate diamonds. HPGR technology is now widely accepted for multiple hard rock mining applications in iron ore, copper ore, platinum ore, and gold ore.
HPGR Basics
The HPGR delivers a finer mill feed and greater efficiency than can be achieved with conventional cone crushers.
High pressure grinding works with an advanced grinding roll. Contrary to conventional crushing rolls, the particles are broken by compression in a packed particle bed, and not by direct nipping of the particles between the two rolls.
This particle bed is created between two choke-fed, counter-rotating rolls. Between these rolls, a particle bed is pressed to a density of up to roughly 85 per cent of the actual material density.
With advancements in wear surfaces and operating practices, the HPGR is capable of continuous production over a long period time without major replacement of wear surfaces. In hard competent materials of typical high abrasion, the grinding rolls have demonstrated a total service life exceeding 8000 operating hours.
Applications
The HPGR is ideal for hard competent ores with high resistance to traditional methods of comminution, which rely on impact or abrasion for liberation. The HPGR takes on a combined role of preparing mill feed that displaces any option for considering a combination of tertiary and quaternary cone crushers in meeting a comparable product. In soft or high clay content ores, a conventional SAG mill with or without crushing of pebbles by cone crushers is more suitable than a HPGR.
Operational needs
When incorporating a HPGR system, mine operators need to consider cost and need for suitable feed preparation. The high tonnages required for low-grade ore bodies mean that large and powerful cone crushers must be operated in closed circuit with vibrating screens to produce an optimum feed. A fine feed of less than 40mm is preferred but a 50mm feed is acceptable. It is important the incoming feed is 100 per cent finer than the working or operating gap (2.5-3 per cent of the roll diameter) of the HPGR.
In addition, the secondary cone crushers should be operated in closed circuit with vibrating screens to ensure that both the feed top size is properly constrained and that any uncrushable tramp material that passes to the grinding rolls will not be of sufficient size to damage the HPGR.
With a HPGR system, it is imperative that the cone crushers and vibrating screens are of adequate capacity and durability. For this reason, plant designers have preferred single stages of both crushing and screening and the vibrating screen operates as both a coarse scalping screen and a sizing screen.
While cone crushers have continued to increase with the largest in operation currently operating at 1,865 kilowatts, vibrating screens remained smaller than 4.3 meter wide. Manufacturers have struggled to successfully develop a 5-meter wide screen capable of operating at required specifications of a HPGR feed preparation circuit.
Screens and HPGR technology
Screens play an important role in a comminution process that employs HPGR technology. A screen built too light for both the scalping and sizing duty will fail prematurely. Conversely, a screen built too heavy may not be able to sustain the necessary G force to prevent blinding of the deck or properly stratify the material for separation.
Besides the screens that are used in the feed preparation process, vibrating screens are required for the final stage of separation following the HPGR stage with the oversize returning to the HPGR feed stream. Large, single deck multi-slope banana screens or single deck screens up to five meters in width are typically best suited for these applications.
The next generation
The challenge facing screening technology manufacturers is to design systems that vibrate larger with heavier screens without sacrificing ease of operation and maintenance.
The interdependence of the individual components of a mill circuit requires that all processes are optimized and aligned for maximum capacity, efficiency and utilization. By carefully designing and integrating equipment, capital investments can be minimized without compromising productivity or increasing maintenance costs to ensure the continued efficiency of mine operations.
Manufacturers of mill circuit mining equipment have recognised the value of HPGR technology and are expanding their portfolios to accommodate the shift in mining production needs. For example, Weir Minerals is investing in:
* An expanded KHD HPGR platform,
* A new heavy-duty, high-capacity range of Enduron Screens,
* New, longer lasting elastomers, and
* New cone crusher technology.
With advancements in wear surfaces and operating practices, the HPGR is capable of continuous production over a long period time without major replacement of wear surfaces and is ideal for multiple hard rock mining applications in iron ore, copper ore, platinum ore, and gold ore.
