Track-mounted crushers are not new. The concept was a natural for European and Scandinavian countries where deposits are small. The units are easily transported between jobs on lowboy trailers, or even using their own bogey wheel attachment. But track-mounted crushing systems really come into their own in their ability to be positioned right at the working face. They can be relocated (when blasting, for example) under their own power in as little as 20 minutes. U.S. contractors used these units in demolition and roadwork in heavily populated areas. Removed material could often be recycled and used on site or, at a minimum, reduced in bulk to save trucking costs.

The next significant advance in track-mounted equipment was development of modular conveyors that are linked to the crusher and to a transfer point that could be repositioned along a stationary field conveyor. Thus equipped, the crusher could advance, retreat and move along the working face as needed, all the while being connected with the downstream process system. This mobile crusher and conveyor system essentially replaced haul trucks and the costs of acquiring, operating and maintaining them. Haul trucks spend half of their working time empty and even when loaded may be only 50% efficient. Conveyors are about 80% efficient.

The next significant advance was the expansion of the product line. Those first jaw plants could move from job to job, laying down crusher run that could be used as is or recrushed later to the desired specifications. New plants with horizontal-shaft impactors, cones, gyratories, and screens gave tracked systems the ability for two, three or even four crushing stages and screening.

Mobility is no substitute for effective crushing and screening. These plants are successful because they can handle the feedstock, make properly shaped product, produce tonnage comparable to their stationary counterpart and have high availability and a long life cycle. Although some concessions must be made to achieve transport dimensions, the basic components of a tracked plant are almost the same as for a stationary plant. Typically the feed hoppers on mobile units are smaller and better suited to an excavator than the typical bucket loader.

But that can be an advantage if the excavator bucket is sized to maximum feed size of the crusher. If material fits in the bucket, it will likely go through the plant without jamming. This selective feeding is particularly helpful in recycle applications. The undercarriage and hydraulics add some complexity, but those components are usually familiar to operators and supported by the manufacturer.

The potential cost savings from using a mobile crushing and conveying system used at the rock face are significant. A study conducted by Tampere University in Finland found a 31% cost saving over semi-mobile installations in similar applications. The savings over using haulers is even 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. Labor costs also are reduced as people are no longer required to operate and maintain haulers, and the excavator or wheel loader operator can control the complete crushing operation via radio remote control. Fuel use and vehicle emissions are drastically reduced and there is no longer a need to build and maintain elaborate haul roads.

With their lower capital and operating costs, flexibility of location and elimination of the need for haulers and haul roads, mobile crushing plants offer an attractive alternative to the traditional stationary crushing and hauling. Mobile systems, however, will not totally replace the stationary variety. In quarries where supply is large and located within a consistently small radius, stationary plants make sense.

Despite the ever-increasing number of track-mounted manufacturers and configurations available, many operators still prefer traditional rubber-tire portable plants. According to Mark Utecht, portable plant product manager for Metso Minerals, that isn't surprising. “Quarry operators are a conservative bunch. They are comfortable with things that work and are familiar to them. The rubber-tired portable still has a lot of things going for it.”

Wheeled plants generally have longer feeders and feeder hoppers, which allow them to be fed from the rear or side with a loader. Because the charge amount is higher, the cycle time of the loader can be longer. This enables a larger area to be worked before moving the equipment. On the track-mounted unit the feeders and hoppers tend to be smaller, requiring that they be fed with an excavator. Once the area within reach of the excavator is worked, the equipment must be relocated. Most operators already have the large loaders to manage stockpiles and load trucks.

Track-mounted equipment is reputed for working in tight areas, even negotiate residential streets. However, that ability decreases as the size of the unit increases. Smaller units move easily and require no disassembly. Larger units have to be split, with the over-run carried on a separate trailer. Many rubber-tire portable plants can be hooked up and towed by a standard semi tractor without major disassembly. With modern hydraulic raising and leveling systems, those portable plants can be making product in a short amount of time following relocation.

The initial purchase price of wheeled units is lower than that of comparable tracked units. The nature of the tracked units is that you need more standardization to be cost effective. There is a wider selection of wheeled units and they have a wider range of options. It is practical to customize units to satisfy user preferences using standard configurations and options. Utecht said, “In the case of a really unusual application, we can make a one-off design in a surprisingly short amount of time without a huge cost premium.”

Rubber-tire systems offer more drive options. Although they can be diesel-powered, they often draw their electric power from 440-volt commercial supplies or from a large generator set. Conveyor and feeder drives are usually electric, as would be found in a stationary plant, rather than hydraulic as found on the tracked units. Diesel engine and hydraulic system maintenance require different skills than those required for the typical rock processing equipment. That is why some operators subcontract their loader and hauler maintenance. Those with a general understanding of mechanical equipment can maintain V-belt and reducer drives on rubber-tire plants.

In the end, the best system is that which offers the most in-spec product with the lowest life cycle cost per ton. It's not an either-or proposition, track mounted or tire mounted. The solution could be a hybrid system with a fully mobile, tracked primary crusher with linked conveyors feeding a tire-mounted crushing and screening system. It might even be a stationary system with haul trucks.

Information provided by Metso Minerals.