Large scale semi-mobile gyratory crushing plants – evolution and outlook

The concept of moving the primary crusher into the mine pit, either fully or semi-mobile, is not new. Back in 1956 the first mobile crusher was developed by Krupp and put into operation in a German limestone quarry. The rationale for taking that path is today as valid as it was 55years ago: limitations of the truck fleet. While in 1956 the issue was the trucks’ inability to cope with frozen ramps, today it’s an abundance of challenges for modern mine management such as labour availability and cost, fuel cost, tyre management and carbon emissions.

 

This article shall not discuss the viability of in-pit crushing per se but provide an overview of the evolution of semi-mobile crushing plants over the last decades, which lead to the current state-of-the-art. Currently, this design sets new standards coming into service on sites all over the world including Australia, China, Europe and South America.

 

 

Figure 1 shows a graph of ThyssenKrupp supplied mobile and semi-mobile crushing plants since inventing the principle. Besides the number of units installed the graph also provides an overview of throughput rates. These plants have been designed to reach up to 14,000 tph.

 

 This article will focus on large scale semi-mobile crushing plants equipped with gyratory crushers, since it often seems to be the design of choice for modern mine sites. These are challenged by the need for increasing capacities, due to declining ore grades, and share-holder demand to fully utilise economies of scale.

 

 

Figure 2 shows the design principle of early large scale crushing plants most popular in the1980s. Trucks are tipping into the feed hopper, an apron feeder is feeding the gyratory crusher which is discharging onto a belt underneath a small discharge bin. A large service tower with an operator’s cabin is incorporated into the plant to overlook the hopper, feeder and crusher inlet. The crusher auxiliaries and the plant’s electric equipment are accommodated in the service tower. All equipment rests on a steel structure with large pontoons which distribute the loads to the ground.

 

In contrast, Figure 3 shows a direct fed crushing plant which represented the standard of the 1990s. Trucks tip into a feed hopper with the gyratory crusher directly underneath. The crusher discharges into a discharge hopper onto an apron feeder, which feeds a short conveyor before the material is passed to the ramp conveyor. Again, the large service tower is part of the plant structure. Dumping heights of 26 to 30 m are typical for this type of installation. The largest of these systems in operation are the massive 45m high crushing plants at Freeport-McMoRan’s Grasberg Mine, Indonesia equipped with KB 63-114gyratory crushers achieving capacities in excess of 10,000 tph.

 

 

In 2005, the newest generation of direct fed gyratory crushing plants (Figure 4) went into operation at Shougang’s Shuichang Mine 220 km East of Beijing, China. Major design improvements included the installation of truck ramps which allow simplification of the crusher pocket design and preparation.

 

Another milestone easing the pocket creation was achieved by redundancy of the apron feeder underneath the crusher discharge hopper saving overall installation height. The crusher discharges into the discharge hopper directly onto a low speed heavy duty belt feeder. The state-of-the-art plant does not require a large service tower any more, since modern automation and visualisation as well as sophisticated camera systems allow the operator to control the plant either from a cabin alongside the discharge belt feeder or from the mine’s central control room. In either case operators are not subject to vibration or potential dust and noise emissions.

 

Overall installation heights have been reduced to an absolute minimum while still maintaining optimal equipment protection utilising a fully sized discharge hopper: depending on the truck fleet employed typical installation heights don’t exceed 21m.

 

These design features are the basis of ThyssenKrupp’s recent order intake records with 14 additional installations coming into service until 2012. Most prominent among these are the primary crushing plant for Scandinavia’s largest quarry operation at Norsk Stein’s Jelsa plant commissioned in 2009 and the four in-pit crushing stations at Citic Pacific Mining’s Sino Iron Project at Cape Preston in north west WA, which will form Australia’s largest in-pit crushing system (see Figure 9).

 

Some of the advantages of a semi-mobile crushing plant have already been touched upon. However, some points deserve further accentuation.

 

 

Figure 5 illustrates the requirements of a semi-mobile crushing plant in terms of foundation design. The plant’s pontoons are designed to take up all static and dynamic loads occurring in the structure and just require the base to allow for certain ground pressures. In most cases, a bed of compacted gravel is all that is required to ensure a suitable foundation.

 

When utilising the state-of-the-art design, the only concrete required for a primary crushing station are two slabs to ensure levelled surface under the truck ramps and blocks for their dead man straps. Depending on the project location this design feature can become of major importance besides the obvious benefit of cost savings for foundation work. Since the gravel bed underneath the crushing plant is acting like a buffer and cannot get damaged, the semi-mobile design is especially suitable for mine sites affected by frequent seismic activity.

 

Generally, the primary quality of the crushing stations discussed in this article is their ability to be moved by transport crawlers or self propelled modular transporters (SPMT).Usually, after a period between two and five years in one location, a semi-mobile crushing plant follows the shovel deeper into the pit to minimise truck transport distances. Figure 6 shows an example of previous relocations executed either by crawlers or SPMTs. Again, the Freeport installations set the benchmark, with more than 1,200 tons to be carried in one lift.

 

With today’s state-of-the-art plant design the large service tower as an integral part of the plant structure has become obsolete. The result is not only a more simplified crushing station but also an improvement in occupational health and safety. Local electric infrastructure such as transformers and motor control centres as well as local control panels and visualisation is accommodated in electrical cabinets, which are located next to the discharge belt feeder on the bottom level of the crushing station.

 

Thus, the plant main service and operating areas are completely separated and independent from the truck dumping level. By separating the service areas access to the plant has been eased and vibration, dust and noise levels affecting the mine’s personnel have been reduced significantly.

 

 

 

Today’s mine sites and mining projects demand the fastest possible ramp up times and optimised equipment installation. That’s why ThyssenKrupp took these challenges into account at the very beginning when designing its new standard semi-mobile crushing plant range. It has been the objective to deliver the largest practicable modules to site depending on local requirements and limitations. Figure 7 shows how the modularization of plants is incorporated into their 3D-modelling.

 

Figure 8 shows an extreme case of pre-assembled modules where a fully pre-assembled steel structure has been shipped to site in one piece.

 

Historically, their ability to be relocated to follow the mining progress reducing truck transport distances and thus costs was the main reason mining companies decided to install semi-mobile crushing plants. Nowadays, the reasons are much more diverse based on the major design developments which have been pointed out before.

 

These design developments bear potential cost savings even if the primary crusher is positioned at a strategic point between open pit and concentration plant and thus does not require to be moved to another location during the life of the mine.

 

It has been observed that more and more projects consider the utilisation of a semi-mobile crushing plant as an economically superior option compared to building a conventional fixed concrete structure. Steel structures can be manufactured worldwide and, given today’s logistics network, transportation can be organised to the most remote location at low cost.

 

Building large concrete structures, however, requires substantial infrastructure on its own. Since most mine sites operate in remote areas this is not only applicable for equipment but especially for labour. By choosing a semi-mobile crushing plant made of steel the work can be performed where the workforce is employed most efficiently. The amount of infrastructure and personnel required to be brought to site at high cost can be reduced considerably.

Rising costs for labour, fuel and consumables and the trend to more and more stringent governmental regulations and taxation schemes related to greenhouse gas emissions are the driving force for modern mining companies to rethink their traditional truck-shovel mining methods.

 

In combination with the need to increase extraction rates to utilise economies of scale and to react to declining ore grades, large scale semi-mobile crushing plants could be the face of future mining. Currently, ThyssenKrupp is cooperating with university researchers on analysing the energy balance and carbon foot-print of today’s primary crushing alternatives with results to be published by the end of 2011.

 

 In this article it has been shown that based on experiences over the past decades significant design improvements have been realised which justify detailed investigation of the feasibility of large-scale semi-mobile crushing plants for nearly every major mining project.