Quarrying
The mobile approach Metso Minerals’ Jorma Kempas explains some of the cost and
production efficiencies of mobile crushing equipment
‘If the mountain won’t come to Mohammed, then Mohammed must come to the mountain’. This rather well worn expression is perhaps the best way to sum up the current trend for mobile systems in the world of crushing and screening. Well, ‘current’ may be an exaggeration, as mobile tracked crushing plants that work at the rock face have been around for 20 years, but the quarrying and mining businesses doesn’t like to rush into things, and only now are mobile crushers challenging stationary crushing plants.
Stationary primary crushers, as their name suggests, are permanently located in one site, often some distance from the quarry or mine work face, and are serviced with rock/ore by a system of attendant off-highway haulers. Although effective, one issue with this approach is the cost of the haulage – which can represent more than half of all costs (drilling, blasting, loading, crushing etc).
With a continual need to increase efficiency and reduce costs, mine and quarry owners have, rightly, seen haulage as an area where cost reductions can be made. These savings have been achieved by moving the fixed primary crusher into the quarry/pit – thereby reducing the haul distance – and replacing older smaller haulers with larger, newer ones. But this is, at best, only a halfway measure – why not eliminate the haul (and the haulers) altogether? Mobile crushing equipment allows this to happen…
On the move
The main advantages of mobile, track-mounted primary crushing plants are their ability to maximise productivity and reduce operating costs – while at the same time increasing safety and reducing environmental impact. While the concept of mobile and semi mobile primary crushers has been around for a long time, many of these were so heavy (up to 1,500 t) and needed so much planning to move them that they were seldom relocated – making them once again effectively permanent facilities. Mobility is no substitute for effective crushing and tracked mobile crushers should meet the same basic criteria as stationary plants. The ability to crush the largest lumps normally received, to the desired cubicity and at the desired rate are all ‘must-have’ rather than ‘nice-to-have’ attributes. The plants should also be easy to use and maintain - and enjoy high availability and a long life cycle. The basic components of a mobile tracked crushing plant are almost the same as for a stationary one (jaw or impactor crusher, power unit, vibrating grizzly feeder, feed hopper etc) but with the added advantage of complete mobility – even up slopes as steep as 1:10 incline. But it doesn’t have to be just the primary crusher that is mobile – Metso’s Lokotrack mobile crushing plants can be built with two, three or even four different crushing and screening stages. While it is true that in terms of spares and maintenance there are more hydraulics, engine and electronic components with
30 Solids & Bulk Handling • March 2011
Lokotrack LT1110S contractor range unit features an impact crusher and a detachable, two-directional screen
mobile crushers, these are generally well supported by the OEM engine manufacturer or the crusher manufacturers themselves.
Quick and easy positioning
But where track-mounted crushing systems really come into their own is their ability to be positioned right at the work face; and then be relocated (when blasting, for example) – under their own power - in as little as 20 minutes. It is intuitively a good solution, in terms of optimised productivity and lowest operating costs, for the crushing equipment to be sited at the rock face. Using haulers can be very inefficient: especially when the largest haulers can expend up to 60% of their energy just propelling the vehicle’s own weight – with only 40% used for moving the blasted rock. When you also consider that by default the hauler is empty for half its operational cycle their inherent inefficiencies becomes apparent.
Conveyors are much more economical than using haulers (at 80% efficiency) and there is no limit on their length (30 km+ is not uncommon in open cast mines). But even here tracked mobile conveyors can play a part, in providing the flexible link between mobile crushing plant and stationary conveyor. They work over shorter distances (in the Lokolink Belt Conveying System they are built up in 42 m sections) than stationary conveyors and contain far fewer parts - as well as having a lower spare parts consumption – than off highway haulers. The main
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