Pit Mines Hard Rock

Mining and Haulage in Hard Rock Open Pit Mines

Mining of medium hard and hard rock is characterized that before loading of the rock blasting is required. For using conveyor haulage in IPCC systems also in-pit crushing is required.

The following mining and haulage systems are available for hard rock mining:

• Conventional truck and shovel
• Fixed crushing station
• IPCC with semi-fixed crushing station
• IPCC with semi-mobile crushing station
• IPCC with fully mobile crushing system

Truck and shovel operation have been the predominating way of mining and haulage in hard rock applications. The blast material is loaded by shovel onto trucks which take the material to a primary crushing station at the exit of the mine respectively to a waste dump area.

In conventional truck and shovel mines the primary crushing station is located at the mine entry. As the crusher will not be relocated it is called fixed crushing station.

Semi-mobile crushing stations can be designed for any rock material and for any crusher type; roller crushers allow a compact design, whereas gyratory crushers require larger ramps due to their size. Semi-mobile crushers are built in modules, mainly apron feeder, crusher, discharge conveyor and service tower module, which can be relocated to a new position in the pit usually by transport crawlers within a few days. Semi-fixed crushing stations are not designed in modules and require therefore much more efforts in disassembly and reconstruction than semi-mobile types.

In contrary to lignite mines continuous mining and/or haulage are comparable new technologies in open pit medium hard and hard rock mining. Several fully mobile crushers with capacities in a range of 3,000 to 10,000 t/h have been implemented since 2002 in medium hard material applications. The systems have proven to operate well and reliable nevertheless there are strong efforts in the industry to further improve fully mobile crushers in this IPCC systems.

In the most applications more than one mining methodology could be considered, probably also a combination of the alternatives listed above. The selection of the mining technology is determined respectively influenced by the following main criteria:

• Geology
• Size, shape and depth of deposit
• Hardness and abrasiveness of material
• Annual production
• Initial investment
• Operating costs
• Lifetime of mine

Typical Hard Rock Mining with Truck and Shovel

The typical hard rock mining arrangement comprises drill rigs for drilling the blast holes, shovels or excavators for loading and trucks for haulage .

The truck and shovel system is very flexible to adapt to changing production capacity. The tuck fleet can be kept small at the begin of the mining and can be increased the deeper the pit develops. Also increase of production capacity and be made by adding additional shovel and trucks.

After blasting the rock loading is made by hydraulic excavator, rope shovel or by draglines. The truck usually is loaded by three or four shovel cycles. Numbers of trucks are used for haulage and the quantity depends on the haul distances and lift. Up to 500 m about three trucks are required and up to 1000 m about six trucks are required per shovel. For distances exceeding 1000 m conveyor systems will be more advantageous.

In-Pit Crushing and Conveying with Semi-mobile Crushing Station

IPCC with semi-mobile crushing stations combines truck and shovel operation at the face area and continuous haulage by conveyors from the face to the waste dump or plant. Crushing is required to limit the maximum lump size for conveyor transportation.

Figure 10 shows the equipment used: At the face shovels or excavators load the trucks which haul the material via ramps to the semi-mobile crushing station. For conveyor transportation the material shall have a maximum lump size of approx. 20 to 25% of the belt width, typically 350 mm for a 1800 mm wide belt. The discharge conveyor transfers the material onto a series of conveyors up to the plant or to shiftable waste dump conveyor and spreader.

Semi-mobile crushing stations as shown in Figure 11 are operated similar to fixed crushing plants, but are located in the mine adjacent to the face. When the distances become too large for economic truck haulage or when the mine develops to larger depth the semi-mobile crusher is relocated and positioned closer to the mine face. The conveyors are extended and relocated accordingly. The relocation takes a few days in which the operation is completely stopped. Transport crawlers are used for the relocation of the semi mobile crusher modules.

In-Pit Crushing and Conveying with Fully Mobile Crushing System

IPCC with fully mobile crushing system replaces completely truck haulage. The fully mobile crusher follows continuously the face and the shovel or excavator. After drilling and blasting the rock is loaded by shovels or excavators into the hopper of the fully mobile crusher. From the crusher the material is transferred either by a loading (conveyor) bridge or by a belt wagon onto the shiftable face conveyor. Up to three mobile crushers can operate in multi benches and load onto one face conveyor. High availability is ensured when the systems operates in several passes before the face conveyor needs relocation by so called track shifting. A series of conveyors take the rock either to the plant or to the spreader at the dump area .

Fully Mobile Crusher

The annual capacity of a mine operation with IPCC is mainly determined by the efficiency of the shovel resp. excavator and the fully mobile crusher operating at the face . High efficiency is enabled when the shovel can operate preferable within small swing angles and the fully mobile crusher continuously follows the face advance.

The main components of a fully mobile crushing systems are the hopper with apron feeder, the roller crusher (sizer or hybrid crusher) and the crawler track. Most fabricators consider a discharge boom for transferring onto a belt wagon.

A two crawler track arrangement enables only a small support area. The heavy impact from the unloading of the shovel causes machine vibrations and vertical exitement of the discharge boom. Lifetime of equipment might suffer due to permantent vibrations . Some fabricators provide a temporary support at the hopper. The lifting of this support at nearly empty hopper reduces the mobility and lead to additional down time for relocation.

Ciros developed a new type of fully mobile crusher, the so called fully mobile crusher type PF 300, which avoids the disadvantages of pervious designs. The patent pending PF 300 has two crawlers mounted underneath the hopper and a pair of slewable crawlers underneath the crawler. The crawler tracks are standard type, e.g. from , which are commonly used in other mining equipment. The PF 300 has no inbuilt discharge conveyor; the material is either directly loaded from the crusher onto a self propelled conveyor bridge or could be transferred onto a belt wagon. This new concept enables a very stabile and full mobile operation .

Mine Layout

Truck and shovel allows a very flexible mine operation and is less sensitive on long term mine planning.

In contrary in-pit crushing and conveying depend on careful mine planning. The selection of the equipment is made for a targeted annual capacity. The optimum utilisation of the IPCC system depends on timing of maintenance and mining process.

The following example shows a mine layout for a fully mobile crusher with a conveyor bridge for overburden removal. One process cycle comprises six blocks in three benches. A series of illustrations of procedures are made to demonstrate the operational characteristics and the according equipment arrangement from the first box cut in the first pass ramp to the relocation to lower or upper sub-benches. After the six blocks have been removed the shiftable face conveyor is horizontally relocated by track shifting and the operation continues in the same way .

Conveyor Haulage

Beside crushing the conveying is a key technology in IPCC. In contrary to truck haulage only few people are required to operate and maintain conveyor haulage; the operating cost of conveyors is much lower than for trucks.

A belt conveyor consists mainly of the drive and tail station and conveyor modules. For shiftable conveyors at the face and the waste dump the conveyor modules are mounted on steel sleepers and are equipped with rails for track shifting whereas stationary modules are mounted on concrete sleepers .

Both the face conveyor which is loaded by the mobile crusher and the waste dump conveyor at which the waste spreader operates have to be relocated from time to time to follow the progress of the face development or the waste dump. The proven technology whicb is know from lignite mine for decades is called “track shifting”. A pipe layer is equipped with a track shifting head which is connected to the rail of the shiftable conveyor modules, lift the modules and move it laterally of about one meter while travelling along the conveyor. The pipe layer is travelling along the conveyor, shifting in steps until the modules have reached their new position. Depending on the dump and mine layout the shifting might be required parallel or radial. In radial waste dumps the tail station remains on the spot whereas the conveyor is relocated in radial lines.

Waste Dump

As per the strip ratio in ore and coal mines a multiple of waste has to be removed to get access to the ore or coal. For cost reasons the large volume of overburden shall be dumped not too far from the mine but the waste dump arrangement has also to be planned for the life time of the mine. The stability of the waste material, calculated by geologists, determines the maximum dump height for each level. Usually several levels are used for optimal utilisation of available ground area .

The waste dump system consists of the shiftable dump conveyor and the spreader with tripper car. The lengths of the receiving and discharge boom length of the spreader shall be optimised to the planned dump height. Longer boom lengths will enlarge the time interval to the next track shifting. The spreader can operate in two directions, the down cast but also the high casting .

IPCC competing with Truck and Shovel

Conventional truck and shovel operation will remain in specific deposit configurations in hard rock mining where high flexibility is essential, but there is increasing competition from IPCC. The main disadvantages of truck operations are the high operating costs, the large demand on manpower for operation and maintenance of the equipment and the mine roads.

IPCC either with semi-mobile or with fully mobile crushing system becomes increasingly important in the mining industry. The advantages in operating costs and reduced man power are so significant that break even of investment is within a few years.

Figure 23 shows the main advantages and disadvantages of truck and shovel and IPCC.

Truck and Shovel

Advantages

• lower initial capital costs
• hard rock and soft rock
• flexible in capacity
• selective mining possible

Disadvantages

• low or medium capacity
• discontinuous material flow
• large fleet required
• high manpower requirement
• expensive truck road preparation
• high operating costs
• large dead load of trucks
• inefficient empty travelling of trucks
• high amount for maintenance
• shortage of tires availability
• fuel cost usually high
• exhaust of diesel engines
• low life time

In Pit Crushing and Conveying IPCC

Advantages

• high capacity and output
• continuous operation
• hard rock and soft rock
• less manpower
• low operating costs
• low amount for maintenance
• long life time
• fully electrical powered
• electricity usually cheap
• less mine road preparation

Disadvantages

• higher initial capital costs
• in-pit crushing required for conveying (hard rock) even if not needec (overburden)
• less flexible in mining layout
• less flexible in capacity

The comparable young IPCC methodology is competing with conventional truck and shovel operation. IPCC with conveyor systems use mainly electrical energy, which can be produced more environmental friendly than utilisation of diesel driven equipment. The ratio of payload to total load is extremely inefficient for truck haulage.

In the last four years Ciros has made more than 15 scoping and prefeasibility studies for customers in coal, copper, iron ore, nickel, gold and diamond mines. The studies evaluated for the capital and operating costs of an IPCC system versus truck and shovel.

Except two studies the IPCC has proven as superior technical and economic solution. The characteristic results of the studies were:

• Best cash cost reduction: range US$ 0.18/t to US$0.82/t
• Best net present value NPV improvement: range US$ 75m to US$ 800m
• Typical reduction in manning: averages 6.5 persons per truck saved
• Typical reduction in ancillary equipment: 25 – 30%
• Reduction in infrastructure costs

Overall capital for long life mines is generally neutral (when truck replacements are taken into account).

The following aspects were found favourable for IPCC versus truck and shovel:

• new (green field) mine operations or expansion, rather than steady state operation.
• minimum 4 years but better 10 years life time is favourable for IPCC
• High capacity mines with at least 10m tons per year (prefer 25m tpa)
• Minimum 100m long mine
• Restrictions of carbon emissions
• Restrictions in water availability and in dust generation
• Automation & safety
• Higher energy efficiency of conveyors versus trucks
• Electricity price ($/kWh) less than 25% of diesel price (S/l)
• Man power restrictions
• More than 25 minutes truck cycle time

There are also world wide trends and economic constraints which promote the increased implementation of in pit crushing and conveying systems for the mining industry:

• limitation of mineral resources will require deeper deposits, higher strip ratios and/or require exploration of lower grade deposits
• restrictions in available fuel and other energy sources
• global environmental restrictions with respect to emissions
• future penalties on C02 footprint of mining operation
• all aspects leading into economic constraint of increasing productivity

Categories: Mining Equipment Tags: Conventional truck , conveyor systems , face , hard rock mining , relocation , waste