Reducing costs and logistics of haulage

If the mountain won?t come to Mohammed, then Mohammed must come to the mountain.?

This rather well worn expression best describes 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 do not 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 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 costs can be reduced. 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.

The main advantages of mobile, track-mounted primary crushing plants are their ability to maximise productivity and reduce operating costs – while 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 1500 tonnes) 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 mobile crushers, these are generally well supported by the OEM engine manufacturer or the crusher manufacturers themselves.

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) – in as little as 20 minutes under their own power. 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 per cent of their energy just propelling the vehicle?s own weight – with only 40 per cent 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 become apparent.

Conveyors are much more economical than using haulers (at 80 per cent efficiency) and there is no limit on their length (over 30km 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 42m sections) than stationary conveyors and contain far fewer parts – as well as having a lower spare parts consumption – than off highway haulers. The main benefit, however, is their ability to be quickly relocated – made possible by connecting the Lokolink conveyor to the Lokotrack crushing plant, which in turn moves the whole assembly. This rapid relocation reduces down-time when blasting and changing location within the pit.

The potential cost savings from using a mobile crushing and conveying system based at the rock face are significant. A study conducted by Tampere University in Finland found a 31 per cent cost saving over semi-mobile installations in similar applications. The savings over using haulers is greater, as the excavators or wheel loaders used for feeding the haulers can be downsized (to ones more suited to the crusher rather than the hauler), labour costs are reduced too as there are no longer hauler operators to employ. In terms of manpower, the excavator or wheel loader operator can control the complete crushing operation. Fuel usage is also drastically reduced and there is also no longer a need to build elaborate and well maintained haul roads – a significant additional (and ongoing) expense.

With a similar lifespan to that of stationary systems (the first LT160 unit Metso supplied generated 65,000 hours over 10 years, with still a ?second life? ahead of it away from front line operation) mobile plants have few disadvantages. There are even significant environmental advantages of using a mobile system: eliminating haulers reduces noise, vibrations and emissions – and fuel. Safety too is enhanced, as fast moving vehicles play a significant role in site accident statistics.

Mobile systems will not totally replace the stationary variety. In quarries where supply is large (greater than 500,000 tonnes per annum) and constant in a small radius, the stationary plant is justified. But where supply is not large then mobile plants have the advantage of being able to circulate between locations and build up stockpiles. In mining, the advantages of having a primary crusher feeding a conveyor system are becoming more recognised as an effective solution. Once this is fully established, the industry will move to a mobile second stage and finally third stage of crushing. The UK is already at this third stage, whereas the US has been catching up fast from a late start.

Worldwide, Metso Minerals has sold well over 2500 tracked crushing plants since it introduced the concept in the mid 1980s. The adoption of a Lokotrack/Lokolink/Field Conveyor system at a quarry in the UK for client Tarmac has delivered savings of over 50 per cent compared to the previous blast and haul method.

With their lower capital and operating costs, flexibility of siting and elimination of the need for haulers and haul roads, mobile crushing plants offer an attractive alternative to the traditional stationary crusher and hauler system. The reduction in manpower, fuel usage and pollution is mirrored by an increase in health and safety.

Offering higher productivity and low cost per tonne production, truly mobile tracked crushing plants are destined to become a more common sight in quarries and mines worldwide.

Jorma Kempas is a writer for Metso Minerals, Tampere, Finland.

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