Metso Underground Crushers.pdf

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Aspects of

Underground Primary Crusher Plant DesignAuthored by Ari Jaakonmäki, M.Sc. Mining, Metso Minerals Inc, Tampere, Finland

Presented by Darcy Flath, Technical Support Engineer, Metso Minerals Industries,

Inc. Waukesha, WI USA





1

SIZING A CRUSHING PLANTReal estate in an underground mine is usually at a prime cost;

therefore, the design of primary crushing plants faces more limita-

tions than on surface where you could say the sky is the limit.

However, the primary crushing and transportation of the ore to

surface are often dimensioned so that they define the capacity of

the mine; whatever limits the functionality of the plant may limit

the output of the mine.

Capacity

Required capacity defines the size and type of crushing equipment

and also the number of parallel crushers or plants required to meet

the mine production rate. Most suppliers and EPCM providers

have simulation and calculation tools for the basic equipment and

capacity selection, however, especially underground, the selection

criteria gets wider, usually in the direction of securing production

capacity with a larger safety margin.

Jaw crushers are the most common solution when the throughput

is less than 1000 tonnes per hour, depending a bit on the scalping

solutions, which will be discussed later. Above 1000 tph, primary

gyratory crushers begin to look interesting; there are no exact

boundaries but above 2000 tph, jaw crushers get rare except in a

parallel plant layout. Generally jaw crushers are in the clear majori-

ty in underground crushing.

Capacity depends also on the feed size and material type intro-

duced to the plant and required product curve. In most cases,

the first stage of crushing really has two functions:

• To get the material small enough for transporting

out of the mine

• 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 300 – 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 rock characteristics. The belt may be the

means of transporting the crushed ore out of the mine but even

when it’s not, there is usually a conveyor somewhere in the materi-

als handling process, so the requirements need to be met.

There are mines where more than one stage of crushing takes

place underground but mostly the lower cost and higher flexibility

of surface construction result in only primary crushing being done

underground.

Most underground mining methods produce a feed that is f iner

than in a typical open pit mine, resulting in higher capacity

through-plant; however, surprises do happen and there needs to

be a way to handle oversize feed material when the requirement

occurs.

ABSTRACT To fix a primary crushing plant that has been excavated and built underground is complicated and

costly; therefore, careful planning pays good dividends. This paper looks at different aspects of plant

design including capacity, installation, layout, maintenance and safety.

This paper focuses on hard rock applications, however, some of the findings will apply also to crushing

softer minerals using other than compression crushers, for instance, impact crushers, sizers, etc...



Figure 1 The Inmet Mining Corporation Pyhäsalmi Mine underground crushing plant. The plant is automated and is operated from

a surface control room 1.4 km above the crusher. Push feeder into the crusher, no scalping.

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Overhead Crane

Ore Pass + Feeder Ore Pass + Feeder

Water Spray

Push Feeder

Feed Chute

Metso C200 Jaw Crusher

LHD Dump



Scalping is recommended for a jaw crusher because:

• Capacity increases when material below crushersetting bypasses the crusher, also the energy

consumption is lower. In the simulated example

shown in Figure 2, the plant capacity drops over

40% if the scalping is removed; the volumetric load

of the crusher itself remains the same.

• The life of wear parts increases. A jaw crusher usually

wears more manganese steel per crushed ton than a

gyratory crusher, because the kinematics of a

single toggle jaw crusher cause an oval motion and

the fines typically create a grinding action at the

bottom part of the jaw dies.

• 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 scalping can be arranged is with a vibrat-

ing grizzly feeder. However, it is usually beneficial for the process

to use a separate feeder and scalper, in this way both components

can be optimized for improved process control and performance.

Points against scalping:

• If a separate scalping grizzly is used, that means

introducing a new piece of equipment into the

process, it is a capital cost and will require

maintenance. The installation also requires chutes and

other bypass arrangements which may become a

space and maintenance access issue depending on

the layout options. A feeder is required anyway for a

jaw crusher but the scalping functions can be

discussed.

• A common reason against installing a scalper is that

typically in an underground mine the feed is not asclean of foreign objects as in a sur face operation.

Underground mine feed often includes debris such as

rock bolts, wire mesh, cable, pieces of pipe, or wood, etc.,

which may get stuck in the scalping grizzly.

• If the crushing plant is planned to be operated

remotely and as 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. This adds up to

production stoppages, lower long term capacity and

process reliability.

• In practice dealing with issues like debris andoccasional slightly oversize feed can be helped by

selecting a crusher size that is clearly above the

theoretical need and thus less sensitive to variations

in the process.

Scalping or not is mainly a jaw crusher issue. A primary gyratory is

not as sensitive to fines, the machines are large, steep and have a

relatively short stroke compared to setting, allowing fines to flow

though easier. The 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.

Figure 2 Simulated capacity difference between scalping

or not scalping in a primary crushing plant.

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To scalp or not to scalp?

Scalping before a jaw crusher is usually recommended. In most cases, gyratory crushers would be

fed with all the excavated ore, because of the different technical construction and behavior

of the two crushers.



Figure 4 Jaw crusher plant with Apron feeder and scalping with a vibrating grizzly.

Modular bolted construction is a definite advantage from

this point of view. Some jaw crushers are built in this way,

some not, and even if practically all gyratories break intobolt connected modules, the modules can be pretty bulky.

A top shell of a 54 inch machine weighs 60 – 70 tonnes so

lifting capacity is definitely an issue in addition to size. The

heaviest part for maintenance in a Metso C200 jaw crusher

is the pitman assembly which weighs approximately 40

tonnes and requires 5 m of vertical space plus clearances in

lifting.

Ramp access gives some flexibility in the installation phase,

at least comparing to having to sling everything through a

shaft.

Ore transportation is often operating at different hours and

hourly capacity than many other functions in the mine,

therefore it is advisable to consider some buffer capacity.

Often the crusher is built below or on top of an ore pass,

this brings up practical and safety issues in installation and

also in maintenance. For this purpose, there needs to be a

quick and safe way to prevent objects or people falling

through the crusher, especially in a jaw crusher where

changing the wear parts happens quite often.

4

Figure 3 Lifting of the pitman into a jaw crusher.

Installing the Machinery and Plant Layout

The primary crushing plant is most commonly built close to the deepest part of the mine, getting the

machinery to the final location can be quite challenging. The most demanding unit is the crusher

itself; feeders, bins and platework are easier to handle for size and weight.

Ore Pass

Apron Feeder

Grizzly

C140 Jaw Crusher

Ore Pass



If there is an ore pass underneath the crusher, it

increases the height between the crushing section and

the feeders, belts, etc., below, safe and quick personnel

access needs to be planned to the top and bottom of

the installation, a lift might be a consideration.

Cranes and other lifting equipment need to be suitable

mainly for maintenance but can also be used for the

installation. Overhead bridge cranes are most common

in underground crushing plants as they can efficiently

use the available space and have a constant lifting

capacity throughout the area. In selecting lifting

equipment and reserving space for lifting, it is good to

note that over the years, the demands on capacity and

reliability have a tendency to increase the weight and

sometimes the size of key components. The nextmachine you eventually want to use, when time comes

to replace the current one, may be slightly bigger but

would ideally go into the same plant layout without

excavation work or major modifications.

It is difficult to pick debris out of the primary feed, but

if there is a belt after the crusher, that would be a good

place to locate a magnetic separator and suff icient

space around it to get rid of at least the magnetic parts

of unwanted materials, rather than send it to the next

stage of comminution.

Figure 5 Installation of a jaw crusher plant at Newcrest Mining Ltd Cadia Valley Operations Ridgeway Deeps gold mine.

LHD loading into feed hopper, apron feeder to crusher, no scalping.

Figure 6 Jaw crusher plant with scalping with a

vibrating grizzly feeder.

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C140 Jaw Crusher

Apron Feeder

50 t Overhead Crane

LHD Load Points 2 Sides



One often neglected question is that if you lift something out of

the crusher, where do you put it? In practice it means floor space

and headroom, one tip to save space is to build a hole in the floor

for the mainshaft, you lay the pitman from the jaw crusher on its

side to work on bearings or place the gyratory mainshaft upright.

When you are not using the hole you need to cover it.

Preventive and predictive maintenance is largely based on good

planning and follow-up, including the discipline to follow visual

inspection and lubrication schedules. There’s a learning curve on

improving plant availability in specific conditions. One key factor

in plant design is that 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.

Figure 7 Access to service points under crusher at Ridgeway Deeps Gold Mine.

Automatic and centralized lubrication are ways to improve the maintenance process with low cost actions. As many of these under-

ground plants will operate unmanned for most of the time, the automation and diagnostic systems should be utilized to their maximum,

this includes well placed cameras to key points.

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Maintenance

Planning the crushing plant for maintenance is an important part of achieving high process reliability

and availability. Lifting requirements for the major components has been discussed above but it

should also be noted that the routine maintenance includes lifting wear and spare parts, well located

jib cranes or monorails may be the answer. In large crushers, even the tools needed for opening or

tightening bolts can get bulky and benefit from having a lifting device handy.

Access to Service

Points Under Crusher

Push Plates to Cover Surge

Bin Maintenance



Most lifting incidents appear to happen in connection with medi-

um size objects around the plant. The big components seem to

get a higher level of attention and planning and small parts don’t

cause as much reported damage. Routine maintenance proce-

dures may take place under time pressure and are often thought of

as not requiring so much specific attention every time. Typical

issues are not using the proper lifting devices or procedures, result-

ing in hand or foot injuries, sometimes even more serious conse-

quences. Instructions, training, discipline and availability of infor-

mation are key in prevention, as well as availability and condition

of correct lifting devices.

Access to maintenance points has been pointed out earlier; how-

ever, the safety aspect to this is to design as much of the service

procedures to be done from ground level or proper service and

access platforms. Climbing on wet or dusty surfaces should be

avoided whereever possible, this is partly an equipment engineer-

ing issue but has also a strong connection to plant design.

Control of the excavation and ore loading and hauling processes

should remove most of oversize handling or move it to safe loca-

tions. As the world is not perfect, corrective actions are occasional-

ly needed, the most efficient way is to operate a hydraulic boom

and hammer remotely to break up oversize or arching before or in

the crusher. A serious risk of accidents exists when this is done

manually; also if you need to send someone down to clear things

up it cuts production time.

Removing debris is related to the previous topic, except that it can

get even more improvised and require the use of the crane and

other devices in addition to the boom and hammer. An additional

flavor of this problem is that especially steel objects can get stuck

under tension so that when released they jump into unwanted

directions.

Moving parts as flywheels, belts, etc., should be protected with

proper guards. Sometimes the guard designs are clumsy or get

damaged so that there is a temptation not to replace them after

repairs or sometimes they have not been installed at all. Local safe-

ty regulations usually cover this area quite closely and technical

solutions are not overly complicated.

Dust suppression on the feed side of a primary plant is always

demanding, especially if you need to maintain the option to feed

the plant with trucks or LHD’s. In practice, water mist is the only

universal solution. Most underground plants plan to operate

unmanned and when personal presence is required it would most-

ly be in a pressurized and filtered control cabin. When the operator

or service person needs to move in the plant itself personal protec-

tive equipment should be used.

In more confined spaces like transfer points under or after the

crusher, it is easier to install suction and f iltering to keep the air

quality under control.

Eventually dust gathers on horizontal surfaces and needs to be

cleaned away. This means usually washing, in which case floor incli-

nations, drainage and potentially sumps and pumps need to be

thought of. If heavier accumulation of material is expected e.g.

under conveyors, space should be considered for a skid-steered

loader.

Fire suppression and fire safety procedures should be carefully con-

sidered, not so much because fires in crushing plants would be

very common but because any fire underground can have severe

consequences. The fire load normally consists of rubber in convey-

ors or v-belts and lubricants, in some occasions the crushed mate-

rial itself presents a potential hazard, e.g. high sulphur content dust

can be inflammable in certain conditions.

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SAFETY

In our experience, some of the most common types of safety issues

that affect a crushing plant are connected with:

• Lifting wear and spare parts

• Slipping on or off machines

• Clearing blockages in the crushing chamber

• Removing unwanted material from the feeder,grizzly or crusher

• Protection of moving parts

• Dust



However, there are ore deposits that spread out over large horizon-

tal distances and are mined by, e.g. room-and-pillar methods, withhigh production rates, these mines have a different logistical chal-

lenge. Moving blasted ore with LHD equipment gets increasingly

costly and inefficient after 400 – 500 m and, in most cases the dis-

cussion turns into trucks but also belt conveyors are used success-

fully. Compared to trucks, belt conveyors are less flexible but have

clear advantages in operating and maintenance costs and are envi-

ronmentally more correct than diesel driven equipment. But, as

pointed out earlier, to put material on a belt, it needs to be primary

crushed.

The technical solution is to use rubber tired towable crushing

equipment or a crawler mounted self propelled unit, an example isshown below. Due to space limitations underground, these units

are small to medium in size but to optimize the mining sequence

at different areas, you can have several of them. The same technol-

ogy is used in surface mining, the difference is mostly in making

the size more compact and running on electric power instead of

diesel.

Figure 8 Underground application of a portable primary crushing plant with feed hopper unit and jaw crusher unit.

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MOBILE SOLUTIONS

Most underground mines try to keep their infrastructure as compact as possible and use gravity where

they can. In practice, this means locating the primary crushing plant close to the bottom of the mine

with ore passes coming down towards it and a silo under the crushing plant to buffer capacity

fluctuations. If ore is hoisted through a shaft the crusher is close by to shorten belt conveyors.



Figure 9 Portable underground primary crushing plant. For transportation the crusher section and feed hopper part are separated

and towed to new location separately with LHD’s.

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