BUCKET-WHEEL EXCAVATOR
Bucket-wheel
excavators (BWEs)
are heavy equipment used in surface
mining and mechanical engineering/civil
engineering. The primary function of BWEs is to act as a continuous
digging machine in large-scale open pit
mining operations. What sets BWEs apart from other large-scale
mining equipment, such as bucket chain excavators, is their use of a large
wheel consisting of a continuous pattern of buckets used to scoop material as
the wheel turns. They are among the largest vehicles ever constructed, and the
biggest bucket-wheel excavator ever built, Bagger 293,
is the largest terrestrial (land) vehicle in human history according to the
Guinness Book of World Records.
History
Bucket-wheel excavator in the open-pit
mining Garzweiler
(Video, 1:40 Min., ca. 9 MB)
Bucket-wheel
excavators have been used in mining for the past century, with some of the
first being manufactured in the 1920s.[They are used in conjunction with many other pieces of mining machinery (conveyer
belts, spreaders, crushing
stations, heap-leach systems, etc.) to move and mine
massive amounts of overburden (waste). While the overall concepts that go into a
BWE have not changed much, their size has grown drastically. BWEs built since
the 1990s, such as the Bagger 293, have reached sizes as large as 96 metres
(314.9 feet) tall, 225 metres (738.2 feet) long, and as heavy as
14,200 tons (31.3 million lb). The bucket-wheel itself can be over 70 feet in
diameter with as many as 20 buckets, each of which can hold over 15 cubic
metres of material. BWEs have also advanced with respect to the extreme
conditions in which they are now capable of operating. Many BWEs have been
designed to operate in climates with temperatures as low as -45°C (-49°F).
Developers are now moving their focus toward automation and the use of
electrical power.
Structure
A bucket wheel
excavator (BWE) consists of a superstructure
to which several more components are fixed.
The bucket
wheel from which the machines get their name is a large, round wheel with a
configuration of scoops which is fixed to a boom and is capable of rotating.
Material picked up by the cutting wheel is transferred back along the boom. In
early cell-type bucket wheels, the material was transferred through a chute
leading from each bucket, while newer cell-less and semi-cell designs use a
stationary chute through which all of the buckets discharge.
A discharge
boom receives material through the superstructure from the cutting boom and
carries it away from the machine, frequently to an external conveyor system.
A counterweight
boom balances the cutting boom and is cantilevered either on the lower part of
the superstructure (in the case of compact BWEs) or the upper part (in the case
of mid-size C-frame BWEs). In the larger BWEs, all three booms are supported by
cables running across towers at the top of the superstructure.
Beneath the
superstructure lay the movement systems. On older models these would be rails
for the machine to travel along, but newer BWEs are frequently equipped with crawlers,
which grant them increased flexibility of motion.
To allow it to
complete its duties, the superstructure of a BWE is capable of rotating about a
vertical axis (slewing). The cutting boom can be tilted up and down (hoisting).
The speeds of these operations are on the orders of 30 m/min and 5 m/min,
respectively. Slewing is driven by large gears, while hoisting generally makes
use of a cable system.
Size
The scale of
BWEs varies drastically and is dependent on the intended application. Compact
BWEs designed by ThyssenKrupp may have boom lengths as small as
6m, weigh 50 tons, and move 100 fm3/hr of earth. Their larger models reach boom
lengths of 80m, weigh 13,000 tons, and move 12,500 fm3/hr.
The largest BWE ever constructed is TAKRAF's Bagger 293,
which weighs 14,200 tons and is capable of moving 240,000 cubic metres of
overburden every day.
Excavations of 380,000 cubic metres per day have been recorded.The BWEs used in the United States tend to be smaller than those constructed in
Germany.
Bucket chain
excavators
Bucket chain
excavators (BCEs) are similar in structure and function to BWEs. However,
instead of the buckets being placed in a ring, they are strung out in a manner
reminiscent of a trencher. They remove material from below their
plane of movement, which is useful if the pit floor is unstable or underwater.
TAKRAF's BCEs travel on rails rather than crawlers.
Operation
BWEs are used
for continuous overburden removal in surface mining applications. They use
their cutting wheels to strip away a section of earth (the working block)
dictated by the size of the excavator. Through hoisting, the working block can
include area both above and below the level of the machine (the bench level).
By slewing, the excavator can reach through a horizontal range.
The overburden
is then delivered to the discharge boom, which transfers the cut earth to
another machine for transfer to a spreader. This may be a fixed belt conveyor
system or a mobile conveyor with crawlers similar to those found on the BWE.
Mobile conveyors permanently attached to the excavator takes the burden of
directing the material off of the operator.The overburden
can also be transferred directly to cross-pit spreader which reaches across the pit and
scatters overburden at the dumping ground.
Automation
Automation of
the BWEs requires integrating many sensors and electrical components such as GPS, data acquisition
systems, and online monitoring capabilities. The goal of these systems is to
take away some of the work from the operators in order to achieve higher mining
speeds. Project managers and operators are now able to track crucial data
regarding the BWEs and other machinery in the mining operations via the
Internet. Sensors can detect how much material is being scooped onto the
conveyor belt, and the automation system can then vary the speed on the
conveyor belts in order to feed a continuous amount of material.
Applications
Bucket wheel
excavators and bucket chain excavators take jobs that were previously accomplished
by rope shovels
and draglines.
They have been replaced in most applications by hydraulic excavators, but still
remain in use for very large-scale operations, where they can be used for the
transfer of loose materials or the excavation of soft to semi-hard overburden.\
Lignite mining
The primary
application of BWEs is in lignite (brown coal) mining, where they are used for soft rock
overburden removal in the absence of blasting. They are useful in this capacity
for their ability to continuously deliver large volumes of materials to
processors, which is especially important given the continuous demand for
lignite.
Because of the
great demand for lignite, lignite mining has also been one of the areas of
greatest development for BWEs. The additions of automated systems and greater
maneuverability, as well as components designed for the specific application,
have increased the reliability and efficiency with which BWEs deliver
materials.
Materials
handling
Bucket wheel
technology is used extensively in bulk materials handling. Bucket wheel reclaimers
are used to pick up material that has been positioned by a stacker for
transport to a processing plant. Stacker/reclaimers, which combine tasks to
reduce the number of required machines, also use bucket wheels to carry out
their tasks.
In shipyards,
bucket wheels are used for the continuous loading and unloading of ships, where
they pick up material from the yard for transfer to the delivery system. Bucket
chains can be used to unload material from a ship's hold. TAKRAF's continuous
ship unloader is capable of removing to 95% of the material from a ship's hold,
owing to a flexibly-configured digging attachment.
Heap leaching
An extension of
their other uses, BWEs are used in heap leaching
processes. Heap leaching entails of constructing stacks of crushed ore, through
which a solvent is passed to extract valuable materials. The construction and
removal of the heaps are an obvious application of stacking and reclaiming
technology.
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