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Bucketwheel Suction Dredges however would tally less than one hundred throughout the world. The Neumann Group have produced 11 units and Ellicott, IHC Holland and O. & K. Germany have produced various models.
Some of the reasons there are so few Bucketwheel Suction Dredges are:-
- It is a relatively recent development.
- The problems associated with encouraging the mining and gravel recovery industries to accept the bucketwheel approach.
We, at Neumanns in Australia, being a reasonably small Company, were encouraged to develop the Bucketwheel system for a number of reasons:-
An Associate Company within the Neumann Group Companies is an active dredging contractor and to be competitive and keep ahead of the competition they have to use the best equipment and best approaches to the problems of contract dredging. The Associate Dredging Contracting Company operates five Bucketwheel Suction Dredges and we believe would be the largest single contractor to do so. They also operate a number of Cutterhead Dredges, usually as supports to the Bucketwheel Suction Dredge in the contract.
Semi sandstone material which has proved the end of many a Cutterhead Dredge – the Bucketwheel properly designed will cut through indurated sand or hardpan with barely a hiccup.
Again properly designed bucketwheels will effectively collect a wide range of these materials within the bucketwheel and direct them the short distance to the suction pipe and thereby via the ladder pump into the delivery pipeline.
Mount Morgan Mines Application
When we think of dredging for the mining industry we must not only think about the ability to dredge difficult conditions but also, and this is the most important, we must consider the ability to feed a mining treatment plant with a consistant feed rate. The bucketwheel by virtue of its design and application, when properly operated, acts as an efficient collector as we 11 as a cutter, to take the material within its buckets and direct it into the suction pipe. Pulp densities of up to 55% of solids by weight have been obtained in the Mount Morgan Mines gold tailings recovery plant.
The bucketwheel design for this particular project required special design consideration. Our client had conducted physical tests to determine the necessary break out force required to break the Rock Salt from the parent mass. We proceeded to design a bucketwheel drive and gearbox system to accommodate almost twice the specified force because it was most important for the Dredge to be capable of dredging beyond the specified figures. We all know it is the unknown and unaccountable obstacles that can ruin a project.
This Dredge is a development of the past few years technology in the design and use of Bucketwheel Suction Dredges and illustrates further the potential for Bucketwheel Suction Dredges in the world of mining..
Mining Dredging Potential
The techniques developed in the other uses of Neumann Bucketwheel Suction Dredges can be readily applied to the mining industry.
The ability to dredge sand, gravel and clay combinations render them most suitable for alluvial dredging in river beds where the valuable minerals are likely to appear.
The ability to handle submerged vegetation such as tree trunks, and the like, open a large area of opportunity in other mining leases.
The Bucketwheel Suction Dredge lends itself admirably to bench type dredging to obtain maximum pulp densities and high recovery rates and with its moveable spud feed control system makes it ideal for complete recovery of all the valuable materials in the dredge path.
The Bucketwheel Suction Dredge would be the most efficient pond bottom cleaner in the business. By simply reversing the moveable spud from the feed mode to the withdrawal mode by the flick of a control lever, the Dredge is in position within seconds to carry out a bottom cleaning sweep which, in some cases, is the most important cut of the cycle as any heavy minerals that may have escaped the ravenous suction and cutting action of the bucketwheel on its way through the forward and sweeping operation cycle are picked up during the clean up cut.
Buckecwheels have been used for many years in the dry reclaimer field with quite respectable success and dredge manufacturers utilised many of the ideas developed by the reclaimer manufacturers and adapted bucketwheel digging wheels to the suction dredging business and, of course, introduced a number of ideas of their own.
In the development of the bucketwheel design as now in use on the Neumann Dredge, the initiai design was based on the conventional bucketwheel reclaimer, using spaced, closed bottom buckets. While this design showed some advantages over the ‘rotary’ basket or rose type cutter, analysis its performance showed some disadvantages when the actual dredging operation was considered.
Unlike the usual suction cutter, the bucketwheel, with its rotary motion combined with slewing, acts in a manner best compared with a modern milling machine fitted up with a side and facing cutting.
The prototype bucketwheel used eight closed bottom buckets with a 2.400m (8ft.) outside diameter. The pulsations due to the engagement/disengagement of the buckets was most marked, resulting in a repetative flexure in both the ladder and the spud.
Following the analogy of the machine shop milling cutter the number of buckets was doubled to sixteen, with the buckets themselves being fabricated, instead of cast, from mild steel place, cut to shape and bent to form an open bottomed bucket. Adaption of this form of construction allowed considerable scope for variation of widths and bucketwheel side flank lengths to determine the optimum, by experiments in the field.
Adaption of this method of bucket manufacturing allowed the use of standard teeth and mounting, commonly used on earth moving equipment, backhoes, drotts, etc. without the necessity of hard facing bucket edges, or using cast on teeth as often seen on the conventional suction cutter.
Experiments with various rotational speeds on the cutter, compared with the total fraction dredged, were carried out in heavy mineral dredging some years ago on the east coast of Australia. The results did not affect the total recovery, however the finer ore fraction of -250 mesh decreased with increasing cutter RPM and slewing speed. While this did not depress our heavy mineral recovery to any extent, the placer miners will appreciate this point, since the carbon-in-pulp recovery of gold has allowed very fine gold particles indeed to be recovered-demonstrated most ably by Mount Morgan Limited as in particular with their tailings the result of fine grinding – the gold contained being too fine for recovery prior to the development of the carbon-in-pulp recovery.
It should be noted that as material becomes more free flowing, i.e. working on high faces with the material rilling down, the suction cutter improves in performance and can approach but not exceed an equivalent bucketwheel. Fine particle loss can still occur.
Additionally due to the inward slope of the bucket end face, an inward velocity is imparted to the particle by the rotation of the bucketwheel. Little or no inward velocity can be impressed by the suction cutter due to blade aperture considerations. A further factor being the pump suction influence dispersed over the swept surface area of the suction cutter.
Major Dredge Design Considerations
There are two basic types of bucketwheels used in the suction dredging configuration.
- The closed or curved bottom type whose shape closely resembles the dry reclaimer type.
- The open bottom bucket type which would be a later development on the closed bottom variety.
The closed bottom type was, we believe, the first type to be used in practical suction dredging and was a big improvement over the common conventional cutterhead digging wheel.
The open bottom type is the one we currently favour in our Dredge applications as it has, in our opinion, many advantages over other designs.
When designing a bucketwheel to suit a particular application consideration is given to:
- Shape of buckets, their cutting angle into the face and the collection facility required.
- The spacing of the buckets to provide a sizing operation to particles being dredged and thus protecting the important dredge pump and pipelines from blockage and damage.
- The diameter of the bucketwheel is to be sufficient to obtain the required solids capacity and is an important primary consideration of the bucketwheeI design.
- The material to be dredged will influence the decision on the installation of replaceable cutting tines, or teeth, onto the bucket components in applications where a cutting action is required, or wear plates to extend the bucket life when the easier sand or soft materials are to be dredged.
Where consistant material conditions apply for long periods the individual design concept would be one that behaves best in that particular environment.
In the case of a free flow sand situation the bucketwheel would have open bottomed buckets and would be of plain configuration as it is known that digging teeth do not improve the bucketwheel’s operational efficiency in free flowing sand and only add a cost item to the assembly with no worthwhile return. Buckets are usually fitted with replaceable wear plates or tips to extend the bucketwheel life factor and limit downtime during changeovers.
Where the material is particularly hard, such as rock salt, indurated sand (semi-sandstone or hard pan) or coral, the bucketwheel must be sufficiently rugged in mechanical construction to withstand the reactive loads and abrasion factors. Replaceable teeth components form an important part of this particular design, and material from which the bucket items are manufactured should be wear resistant steel press worked to the appropriate shape. Cast steel bucket components also have a place provided that sufficient quantities are involved to make them economic.
A number of factors are involved in the selection of a Dredge for a particular duty. In the Dredge Contracting business it is necessary, of course, to obtain a Dredge which has a wide range of versatility to cope with the various work duties, whereas the mining application is more often within relatively close parameters.
An important basic decision in Plane design is the size and planned capacity of the Dredge unit.
The Dredge unit should not be the limiting factor in plant production. Flowsheet engineers and plant designers are normally conservative in their approach to plant capacity, and it is often experienced, when once the operation of the Plant is stabilised, its capacity is increased by virtue of improvement to the flowsheet, improved process equipment operation and higher plane operator efficiency. It is desirable to have excess capacity available in the Dredge unit.