Going Mobile

Mobile crushing equipment can not only replace stationary systems – it can also eliminate the use of dumptrucks and reduce staffing. Metso Minerals’ Jorma Kempas explains some of the cost and production efficiencies

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 crushing and screening systems. The word ‘current’ may be something of an exaggeration, as mobile tracked crushing plants that work at the rock face have been around for 20 years, but the quarrying and mining industries are not known for rushing into things, and it is only now that mobile crushers are really beginning to challenge stationary crushing plants in some parts of the world.

Stationary primary crushers, as their name suggests, are permanently located in one position, often some distance from the quarry working face, and are commonly fed with rock by a team of attendant off-highway dumptrucks. Although effective, a key issue with this approach is the cost of the haulage, which can represent more than half of all operating costs (drilling, blasting, loading, crushing etc).

With a continual need to increase efficiency and reduce operating costs, quarry owners have looked upon haulage as an area where cost reductions can be made. These savings have been achieved by moving the fixed primary crusher into the quarry, thereby reducing the haul distance, and by replacing older, smaller dumptrucks 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 advantage of mobile track-mounted primary crushing plants is their ability to maximize productivity and reduce operating costs, while at the same time increasing safety and reducing environmental impact. Although 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 tonnes) and needed so much planning to move them that they were seldom relocated, making them effectively permanent facilities.

Mobility, however, 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 ‘must-have’ rather than ‘nice-to-have’ attributes. The plants should also be easy to use and maintain, and offer 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 impact 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. Moreover, it does not 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. And while it is true that, in terms of spares and maintenance, there are more hydraulics, engines and electronic components with mobile crushers, these are generally well supported by the OEM engine manufacturer or the crusher manufacturers themselves.

Quick and easy positioning

Where track-mounted crushing systems really come into their own is in their ability to be positioned close to the working face and then relocated (eg, when blasting) under their own power in as little as 20min. It is intuitively a good solution in terms of optimized productivity and lowest operating costs for the crushing equipment to be located at the rock face. Using dumptrucks can be very inefficient; the largest hauler can expend up to 60% of its energy propelling its own weight, with only 40% used for moving the blasted rock. Moreover, by default a dumptruck is empty for half its operational cycle, making the inherent inefficiencies even more apparent.

Conveyors are much more economical than using dumptrucks (at 80% efficiency) and there is no limit on their length (30km plus is not uncommon in opencast mines). But even here tracked mobile conveyors can play a part, providing the flexible link between the mobile crushing plant and the stationary field conveyor. They work over shorter distances (in the Lokolink belt-conveying system they are built in 42m sections) than stationary conveyors and contain far fewer parts, as well as having a lower spare parts consumption, than off-highway dumptrucks. 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 downtime when blasting and changing locations within the pit.

Cost savings

The potential cost savings arising from the use of a mobile crushing and conveying system based at the rock face can be significant. A study conducted by Tampere University in Finland found a 31% cost saving over semi-mobile installations in similar applications. The savings over the use of dumptrucks is greater still, as the excavators or wheel loaders used for feeding the haulers can be downsized to a capacity more suited to the crusher than the hauler. Labour costs are also reduced 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 significantly reduced and there is no longer a need to build elaborate and well-maintained haul roads, which can be another significant additional and ongoing expense. As an example, the adoption of a Lokotrack/Lokolink/field conveyor system at a Tarmac quarry in the UK has delivered savings of over 50% compared with the previous blast and haul method.

With similar lifespans to those of stationary systems – the first Metso LT160 unit achieved 65,000h over 10 years and still had a ‘second life’ away from front-line operations – mobile plants have few disadvantages. There are even significant environmental advantages to be gained by using a mobile system; the elimination of dumptrucks reduces noise, vibrations, emissions and fuel consumption. Moreover, safety is also enhanced, as fast-moving vehicles tend to play a significant role in site accident statistics.

However, mobile systems will not totally replace stationary plants. In quarries where output is large and constant (ie, greater than 500,000 tonnes a year) the stationary plant may well be justified. But where supply is not large mobile plants have the advantage of being able to circulate between locations, building up stockpiles as they go. In quarrying, the advantages of having a mobile primary crusher feeding a conveyor system are becoming increasingly recognized 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.

Conclusion

Worldwide, Metso Minerals have sold well over 2,500 tracked crushing plants since they introduced the concept in the mid-1980s. With their lower capital and operating costs, locational flexibility and elimination of the need for haulers and haul roads, mobile crushing plants offer an attractive alternative to the traditional stationary crusher and dumptruck 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 an even more common sight in quarries and mines around the world in the years to come.