Six ways to take primary crushing underground

Underground mining presents a unique set of challenges, so careful planning pays off. Metso's Eero Hamalainen examines the big decisions in taking primary jaw crushers underground.

Sub-surface mining sets unique requirements on equipment as well as on safety and maintenance issues: excavating for and assembling a primary crushing plant underground is complicated and costly, so what are the big decisions when starting an underground primary jaw crushing plant?

1. Sizing the plant

Real estate in an underground mine is not cheap, so the design of primary crushing plants faces more limitations underground than on the surface.

However, the primary crushing and transportation of ore to the surface are often the determining factor that decides on the capacity of the mine.

The required capacity defines the size and type of crushing equipment and also the number of parallel crushers or plants required to meet the mine's production rate. Most suppliers and EPCM providers have simulation and calculation tools for basic equipment and capacity selection.

2. Jaws or a primary gyratory?

Jaw crushers are the most common solution in underground applications when the throughput is less than 1000 tons per hour, depending somewhat on the scalping solutions. Above 1000 tph primary gyratory crushers begin to look interesting, and after the throughput reaches 2000 tph, jaw crushers are very rare, with the exception of a parallel plant layout. Capacity is also determined by the feed size and material type introduced to the plant, as well as the required product curve. In most cases, the first stage of crushing really has two functions: to get the material small enough to be transported out of the mine, as well as to get a suitable product curve for the next stage of comminution.

In either case, the top size of the material ends up typically in the range of 200 – 350 mm, which is good for belt conveyors and fits into most secondary crushers without causing process risks. To achieve this top size, the crusher setting is in the range of 200 mm or below, depending on the rock characteristics.

If a belt is not used in transporting the crushed ore out of the mine, the other method of transporting ore from underground applications is via skip hoist. Depending on the size of the skip, the ore being transported may need to be finer.

Picking debris out of the primary feed is always a challenge. If there is a belt after the crusher, it makes sense to place a magnetic separator there to get rid of at least the magnetic parts of unwanted materials. This way, it will not get transported to the next stage of comminution.

Underground mining methods mostly produce a feed that is finer than in a typical open-pit mine, resulting in higher capacity through the plant. However, surprises do happen, so be prepared to handle oversize feed material when required.

3. To scalp or not to scalp?

Scalping is usually recommended before a jaw crusher, whereas gyratory crushers can handle the excavated ore as such. This is because primary gyratories are not as sensitive to fines: they are large, steep and have a relatively short stroke compared to settings that allow fines to flow through more easily.

The primary gyratory kinematics produce a straight, compressive crushing function without the grinding action found in a single toggle jaw, therefore the liner wear is not as high per ton of crushed material.

Scalping is recommended for a jaw crusher for three main reasons:

Capacity increases when material below the crusher setting bypasses the crusher; also the energy consumption is lower.

The life of wear parts increases; a jaw crusher usually wears more manganese steel per crushed ton than a gyratory crusher.

Feeding fines into the crusher may increase peak stress loads because of packing, i.e. material being compacted as far as it compacts. This is enhanced by moisture, which again is highest in the fines and quite common in underground mines.

The most compact way to scalp is with a vibrating grizzly feeder. However, using a separate feeder and a scalper allows both components to be optimised for improved process control and performance.

Points against scalping:

A separate scalping grizzly adds a new piece of equipment into the process, increasing capital costs and requiring maintenance. The installation also calls for chutes and other bypass arrangements, which may hinder maintenance access.

In an underground mine, the feed tends to include more foreign objects than in a surface operation. Debris, such as rock bolts, wire mesh, cable, pieces of pipe or wood etc., may get stuck in the scalping grizzly.

If the crushing plant will be operated remotely and as highly automated as possible, the debris can cause unpredictable clogging of the grizzly. Getting someone to the plant to clean things up can take a while, and the cleaning itself takes time. Downtime leads to lower long-term capacity and weaker process reliability.

In practice, issues like debris and occasional slightly oversized feed can be resolved by selecting a crusher size that is clearly above the theoretical need and thus less sensitive to variations in the process.

4. Don't forget to plan for future capacity

Installing the machinery can prove quite challenging, as the primary crushing plant is usually built close to the deepest part of the mine. Ramp access gives some flexibility in the installation phase, at least compared to having to sling everything through a shaft.

The most demanding unit is the crusher itself; feeders, bins and plate work are easier to handle when it comes to size and weight. From this perspective, modular bolted construction is a definite advantage. Some jaw crushers are built in this way, and practically all gyratories break into bolt-connected modules. Although the modules themselves can be pretty bulky: a top shell of a 54-inch machine weighs approximately 85 tons, so lifting capacity is definitely an issue, in addition to size. The heaviest part of a Metso C200 jaw crusher for maintenance is the pitman assembly, which weighs approximately 40 tons and requires 5 meters of vertical space plus clearances for lifting.

Although designed for maintenance, cranes and other lifting equipment can also be used for the installation. Overhead bridge cranes are the most common in underground crushing plants as they can utilize the available space and have a constant lifting capacity throughout the area.

In selecting lifting equipment and reserving space for lifting, keep in mind that future demands on capacity and reliability have a tendency to increase the weight and sometimes the size of key components. When eventually replacing the current machine, you are likely to opt for a slightly bigger one, and it should fit into the same plant layout without excavation work or major modifications.

Often, the crusher is built below or on top of an ore pass, which brings up practical and safety issues in installation and also in maintenance. Safe and quick personnel access needs to be planned on the top and bottom of the installation, and a lift might be a consideration.

5. A maintenance-friendly plant design maximises availability

In order to achieve high process reliability and availability, it's important to consider future maintenance needs already when planning the crushing plant. Routine maintenance includes lifting wear and spare parts, in which case well-located jib cranes or monorails may be the answer. For large crushers, even the tools needed for opening or tightening bolts can get bulky and benefit from having a lifting device handy.

One question often overlooked is: if you lift something out of the crusher, where do you put it? Sufficient floor space and headroom are essential. One tip for saving space is to build a hole in the floor for the main shaft. Then, you lay the pitman from the jaw crusher on its side to work on bearings or place the gyratory main shaft upright. When the hole is not in use, it must be covered.

Preventive and predictive maintenance is largely based on good planning and follow-up, including the discipline to follow visual inspection and lubrication schedules. If a service point is not readily accessible, there is a temptation to neglect it. With today's 3D CAD tools, it is quite simple to review service access to various points before construction.

Automation and centralised lubrication are cost-effective ways to improve the maintenance process. Underground plants generally operate unmanned, so make sure to utilise the automation and diagnostic systems, like placing cameras at key points.

6. Put safety first

Safety starts with the plant design. When the majority of service procedures can be carried out from the ground level or from proper service and access platforms, climbing onto wet or dusty surfaces can be avoided. Preventing objects or people from falling through the crusher, especially in a jaw crusher, where wear parts are changed quite often, is also crucial.

Surprisingly, medium-sized objects are the most dangerous ones to lift. Large components seem to get a higher level of attention and planning, and small parts don't cause as much reported damage. There is often limited time for routine maintenance procedures and these procedures do not always receive the necessary attention. Typical issues include not using the proper lifting devices or procedures, which often results in hand or foot injuries and sometimes even more serious consequences. Instructions, training, discipline and availability of information play a key role in injury prevention, as does the availability and condition of the correct tools, such as lifting equipment.

Dust suppression on the feed side of a primary plant is always a challenge. In practice, a proper water mist is the only universal solution. In more confined spaces, such as transfer points under or after the crusher, it is easier to install suction and filtering to keep the air quality under control.

Dust gathering on horizontal surfaces over time must be washed away. This requires floor inclinations, drainage and potentially sumps and pumps. If heavier accumulation of material is expected, e.g. under conveyors, leave space for a skid-steered loader.

Fire suppression and fire safety procedures should be given careful consideration, not because fires in crushing plants are common, but because any fire underground can have severe consequences. 

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