Alluvial mining grows in popularity.
Throughout the world, long-established gold alluvials are being reworked, and in many countries small time miners with their pans and sluices are having a field day.
Although gold is the most glamorous placer, and the metal most associated with placer mining, it is by no means the only one. All the world's mineral sands come from alluvial deposits as does much of the world's tin. Alluvial mining methods are also used to mine phosphate, potash, salt, and other industrial minerals.
Alluvial mining is essentially a low grade/high capacity business and the dredge is its workhorse. There are three main types: bucket-line, bucket-wheel and cutter-suction. Depending on grade, volume, and size distribution of the deposit, all have their application.
For gold and tin in difficult ground, the bucket-line dredge with integral treatment plant is the preferred choice. These heavy-duty machines have high digging forces, can cope with a wide variety of ground conditions, and are essential where clean-up of bedrock is required. Availability is high, often as much as 90%; and 600 hr/month is considered normal operating time.
The shearing action of the buckets makes for high energy efficiency, and the on-board treatment plant eliminates materials handling costs, as mining and concentrating is done in one place. These factors combined with high availability give bucket-lines the lowest operating cost of any dredging method. Their main disadvantage is their high capital cost.
Cutter-suction dredges have higher throughputs and lower capital costs than bucket-lines. A 10-ft [sup.3] bucket-line dredge will weigh about 1,200 mt and treat about 200,000 m [sup.3]/month while a similar sized cutter-suction machine will treat about 600,000 m [sup.3]/month.
Their disadvantages are inability to dig boulders or clean up bedrock, limited power inputs, low and variable pulp densities of pumped product, and a tendency for hydraulic classification at the cutter head to leave heavy minerals behind.
Power is limited by pressure limitations of hydraulic drives, and energy efficiency is lower than for bucket-lines because, in place of the shearing action of the buckets, the cutter has to overcome compression forces. Pulp density of product slurry is low and can only be maintained while cutting. Any retreat from the face results in the pumping of water only. Pumps and pipelines are thus subject to higher wear and, consequently, operating costs in terms of power and maintenance can be high.
Availabilities of cutter-suction machines are much lower than for bucket-lines, and their use is limited to removal of barren overburden and freeflowing materials where: there is little debris and, cleaning of bedrock is not important.
Bucket-wheels combine many of the good characteristics of bucket-lines and cutter-suction machines and are widely used for mineral mining. They can scrape the bottom for good clean-up and give higher slurry pulp densities than cutter-suction dredges. Their main disadvantage is that they cannot dig boulders. Big rocks are simply thrown aside. Capital costs are similar to cutter-suction machines and much less than bucket-lines, but operating costs are higher and availabilities lower.
It might seem from the above that selection of a dredge type is a simple trade-off between capital and operating costs, but size of project, and grade, are also important. A typical gold placer will be low grade, of the order of 100 mg/m [sup.3], and the proposed treatment rate seldom exceeds 500 m [sup.3]/hr. On the other hand, heavy mineral deposits are higher grade, usually 1-8% heavy mineral, and lower value. Such projects may require throughputs of over 2,000 m [sup.3]/hr to be attractive. In this case, provided there are few boulders present, a bucket-wheel with its high throughput and low capital cost would be preferred over a bucket-line of the same capacity.
Tin is a case where grade can be important. In low grade alluvials, high recovery is essential and a bucket-line to be preferred, but some Brazilian alluvial tin operations are of such high grade, often 3-4 kg/m [sup.3], that the low capital cost and high throughput of a bucket-wheel has proven more attractive. Lower recovery is compensated for by higher production. Paranapanema has 10 Ellicott bucket-wheels producing around 22,000 mt/yr tin.
For high value minerals in difficult ground, the bucketline dredge with its high availability and low operating cost, remains king. The hull is either a welded structure or a number of pontoons bolted together. Pontoon-based designs are becoming more common as they are easily removed for transport. The bucket-line or ladder is anchored at the top drive tumbler and supported by a hoist which controls the digging depth. Digging depths are usually of the order of 20 m. The steel buckets, which are normally from 6 to 24 ft [sup.3] in size, have integral or replaceable wear-lips and are held together by pins. The complete bucket-line runs around two seven-sided tumblers, the top one of which is driven by a thyristor-controlled de motor.
Buckets discharge into a chute feeding a cylindrical trommel running down the center of the dredge. Spillage is collected by a "save-all" sump below the discharge point and pumped to the jigs. The trommel can be either a straight cylindrical screen, or if there is a large amount of clay in the feed, a combination of washing drum and screen. Small diameter trommels (less than 2 m) can be chain driven. Larger ones are electrically or hydraulically driven from two or four rollers. Hydraulic drives have the advantage of being naturally self-balancing, because they run in parallel off a common circuit, but need slightly more power than electric drives. Trommel oversize is discharged behind the dredge by a stacker.
Trommel undersize goes to two or three stages of jigging, the jigs being mounted on either side of the trommel and fed by pump or gravity. Pumping through hydrocyclones eliminates slimes and excess water to give a steady pulp density and low viscosity to the jigs, but this advantage must be set against the extra operating costs involved. For gold dredges operating in ground with a small percentage of slimes, gravity distribution is often preferred.
Jig tailings can be discharged through launders directly into the dredge pond, but land restitution is an environmental imperative in most places, and it is common for about half the jig tailings to be pumped over the coarse boulders left by the stacker. The fine tailings are then contoured and covered with top soil.
The traditional method of dredge positioning is to use an anchoring spud at the stern and two forward side lines, the side lines cause the bucket-line to cut an arc around the anchoring spud. In many places, spuds have given way to five rope mooring using a head line with two forward and two aft sidelines.
Five line mooring has the advantage of giving a wide cutting face in one pass, frequently 200-300 m, and higher availabilities. Digging time is about 95% of available operating time compared with about 80% for a spud moored dredge. Five lines also allow the dredge to be moved forwards and backwards while digging which makes it easier to maintain a stable face.
In cutter-suction and bucket-wheel dredges the cutter or bucket-wheel is mounted on the end of a ladder supported by a hoist. The cutter or wheel is driven by high-torque/low-speed radial piston motor and the dredged product pumped through a floating pipeline to a shore-based or pontoon-mounted treatment plant. The pump or pumps can be mounted either on the ladder behind the cutter or on the dredge pontoon. For deep dredging twin pumps are used, one behind the cutter and the other on the pontoon.
As with bucket-line dredges, mooring is by spuds or mooring ropes. With spuds, it is common to have a spud carriageway at the stern to provide steady forward motion. Repositioning is done with a second fixed spud. Some designs have anchor booms to facilitate placing of the mooring ropes.
Bucket-wheels are superior to cutter-suction dredges in that they can scrape the bottom for good clean-up and the buckets provide positive feed of material to the pump suction. Spillage is thereby minimized and pulp densities of 50% solids or more can usually be maintained. Pulp density can be controlled by varying wheel rotation speed and/or dredge swing speed to obtain the required solids/water ratio. Bucket spacing can be set to eliminate the danger of oversize damaging the pump and pipeline.
Other advantages are that they can cut equally well in both directions. If the dredge is to be operated on spuds vertically upward motion is usually recommended. If on wires, vertically downwards.
Bucket-wheels are widely used in the mining of heavy minerals, barren overburden, industrial minerals, and to a limited extent in tin and gold mining.
Pay ground often lies below a layer of barren overburden and in some cases it is economic to strip the barren material with a cutter-suction or bucket-wheel dredge before going in with a bucket-line. An interesting operation started this year on the Grey River in New Zealand, where the R A Hanson Co. has built a dredge incorporating both a bucket-line and bucket-wheel. Both excavating mechanisms operate simultaneously, the bucket-wheel removing the overburden and the bucket-line the underlying pay ground. Pay ground passes through the trommel and discharged behind the dredge. Overburden from the bucket-wheel is discharged directly on top of this coarse material and covered by pumped jig tailings before replacing topsoil.
Grey River Gold reports no difficulty in running the two excavating systems simultaneously (this had been expected to be the most difficult part of the operation) but has had problems with the bucket-wheel. The percentage of plus 12-in boulders in the overburden has proved larger than expected, and these have been blocking the pipeline. The company is devising a way of keeping the coarse rocks out of the bucket-wheel and shifting them down to the bucket-line. However, gold recovery, at 95% of indicated values, is good and throughput is expected to reach its planned level once the dredge pond reaches full size.
Deep Water Dredging
Most bucket-line and cutter type dredges are limited to depths of 20-30 m. For greater depths, airlifts, grabs, and sea-bed vehicles are available.
Airlift dredges manufactured by Fuchs Systems can reach depths of 150 m. The vertical airlift pipe is raised and lowered by a barge-mounted winch. High-volume low-pressure air introduced at the foot of the pipe reduces the specific gravity of the column interior causing water, sand, and other free flowing materials, to be forced into the airlift pipe by the hydrostatic pressure difference. The rapidly ascending air carries the dredged material to surface. High pressure water jets and/or mechanical diggers can be added to the suction foot to loosen consolidated materials.
Grabs can reach almost any depth but have the disadvantage of being cyclical and until recently were ineffective in compacted ground. IHC Holland has overcome these problems with its IHC-Rohr mineral dredger which uses an active grab, i.e. one which is positively closed by hydraulic rams, to overcome the problem of compacted soils. Automatic positioning and measuring equipment helps achieve consistent digging and capacity can be increased by using two or three grabs. The IHC-Rohr dredger is a recent addition to the IHC range of alluvial mining equipment and has yet to be proven, but should become a reliable option for deep water dredging.
Perhaps the most interesting development in deep water dredging is Alluvial Mining's Tramrod, a remote-controlled sea-bed vehicle capable of operating at depths of 100 m or more. A prototype was used in 1988, to level the sea-bed around an oil platform in the North Sea. This year the same machine, with modifications, is reportedly being used successfully for sea-bed mineral mining in the USA.
Tramrod is a remotely operated tracked dredging vehicle capable of scouring and cleaning up bedrock in conditions where conventional cutter dredges cannot operate. It can negotiate rough terrain and slopes greater than 45 [degrees]. The vehicle is controlled from surface and its progress monitored by two or three underwater TV cameras. The suction head consists of a Plenum jet pump which sucks material from the sea bed and pumps it to a centrifugal pump on surface for onward transfer to the treatment plant. The system is said to handle solids up to 250 mm in diameter at throughputs from 40 to 200 [m.sup.3]/hr, depending upon materials.
The plant used to treat the mineral bearing ground is as important as the method used to mine it. Bucket-line dredges have onboard treatment plants but smaller suction type machines feed separate skid or pontoon-mounted plants. In small alluvial deposits that cannot accommodate a dredger, treatment plants are often fed by a backhoe, also sometimes barge mounted.
For gold, tin and diamonds, treatment plants are similar to those installed on bucket-line dredges, consisting of a trommel to wash the product and usually three stages of jigging. Heavy minerals processing is usually done by Reichert cones and spirals after screening to remove tramp oversize and organic debris.
The jig is the heart of the treatment plant. Three main types are used; Yuba, Pan-American and radial. Yuba jigs are rectangular in shape, with up to four cells, and are pulsated horizontally by rubber diaphragms at either end. Pan-American jigs are also rectangular but are pulsated vertically by a rubber diaphragm on the bottom. However, it is the radial jig that has done most to improve recoveries in alluvial mining, and these are usually the preferred design for primary and secondary concentration.
The radial jig was invented by IHC Holland and variants are made by other manufacturers. Advantages include high capacity per unit area and low power consumption. However, the characteristic which gives the radial jig its high recovery is the trapezoidal shape of the bed. Because the pulp radiates from a narrow feed zone to a wide discharge lip, pulp velocity reduces as it crosses the bed. This allows the jig to treat a wide feed size range and gives more time for valuable minerals to permeate the bed. With a rectangular jig bed pulp velocity actually increases as it moves towards the discharge because of hutch water dilution.
IHC radial jigs are made up of individual modules, each of which can treat about 20 [m.sup.3]/hr. A full 7.75 m circular jig consists of 12 modules and treats about 240 [m.sup.3]/hr. The unique, hydraulically driven, saw-tooth pulsation has a rapid upstroke for good bed fluidization and a slow downstroke for good suction of values. It also uses less hutch water than a simple harmonic pulsation.
Radial jigs from Alluvial Dredges Ltd. are 7-m-dia and a full circle consists of eight cells. The 8-cell design is claimed to give a more even bed pulsation than the IHC. The motion is a simple mechanically operated harmonic as opposed to the IHC saw tooth.
The largest radial jigs are 36 ft diameter 18-cell units built by R A Hanson Co. for their Grey River Gold dredge.
A dredge treatment plant should be considered a roughing unit and jig concentrates should preferably be cleaned-up on shore. Gold dredges sometimes clean-up on board, using mercury jackpots under the jig spigots and/or amalgam plates, but this is rarely satisfactory. Amalgamation is not as simple as it appears. Mercury often needs conditioning to be effective and tarnished gold is frequently resistant and lost to tailings. Furthermore, final clean-up on board increases risk of theft and inevitably incurs some mercury loss into the dredge pond, an environmental hazard. The Yuba dredge in California actually recovers mercury when reworking old tailings, an indication of how much can be lost in this way.
Alluvial Dredges Ltd. (ADL) is the inheritor of the original Lobnitz design of bucket-line dredge. Today, ADL is consultanting in placer mining and designs and supplies a wide range of dredges and treatment plants for the recovery of alluvial gold, diamonds, tin and other heavy minerals. The range of equipment includes bucket-line, bucket-wheel, and cutter-suction dredges, radial and Yuba type jigs, and small mobile skid or trailer-mounted jig plants.
A recent contract is for a 475-1 bucket-line gold dredge for Minero-Peru's San Antonio de Poto project near Lake Titicaca. Planned throughput is 3.5 million [m.sup.3]/yr and maximum digging depth 20 m. The 525-hp main drive will be a single-sided shaft-mounted dc thyristor unit to give speed control from minus 10 to plus 33 buckets/min and 160% torque at creep speed to facilitate opening and closing of the bucket band.
The treatment plant will consist of a 3-m-dia by 17.5-m trommel screen with 12 mm openings and three stages of jigging. Screen undersize will be pumped to desliming cyclones ahead of jigging. Primary jigs will be three 8-cell 7-m-dia ADL radials, the secondary jig a 4-cell 7 m dia ADL radial, and the tertiary a single 3-cell 42 in. by 42 in. Yuba. Primary jigs will operate at fixed speed but have variable stroke while secondary and tertiary jigs will have variable frequency capability.
The high slimes content of the ground requires special water reticulation. There will be two floating pipelines; one to take slimes to a settling area, and one to bring in clean hutch water for the jigs. Only trommel wash water will be drawn from the dredge pond. Slimes will come from the hydrocyclone overflows and an extra underwater pump on the bucket ladder.
Alluvial Mining Ltd. specializes in offshore dredging and underwater site investigation. The company produces diver operated suction-dredges and the remote controlled Tramrod sea-bed vehicle described.
Boskalis Westminster is a renowned dredging company which undertakes feasibility studies to determine the viability of dredging in a mining context, and contract dredging. Mining projects have included the removal of 13 million [m.sup.3] of overburden at a bauxite mine in Surinam and the stripping of 20-30 m overburden at Mattabi (nickel) in Canada. The most recent mining project was the dredging of 5.3 million [yd.sup.3] of overburden at the Centralia coal mine (Washington state).
Ellicott Machine Corp. constructs cutter-suction and wheel-dredgers. These are fitted with a spud carriage at the stern to allow a number of accurate advances in precise increments, reducing spudding time and permitting repositioning at any point in the swing cycle.
Ellicott's Dual Wheel Excavator (DWE) consists of two sections of eight steel buckets to form an integral unit of two bucket-wheels. The unit is driven by slow speed high torque radial piston hydraulic motors through a gearbox. Conversion to cutting vertically up to vertically down is done simply by exchanging the half sections to opposite sides. DWE's are available in the horsepower range 50-1,500 hp and can service pumping systems from 6 in. through 36 in.
Advantages of the DWE are said to be higher production rates than standard rotary cutters of equal power, equal efficiency on starboard and port swings, and positive feed. Materials excavated are passed through the buckets into the hopper to the pump intake thus maximizing percent solids. Interior scraper plates make the buckets virtually impossible to plug.
A more recent development is the Hoe Dragon that combines the hard digging high volume capacity of the DWE with a submerged dredge pump. The Hoe Dragon can be mounted on any track type backhoe with the capacity of lifting the dead weight of the excavator pump module at the close-in position of the stick. The backhoe can also be mounted on a barge. The mobility of the backhoe makes it possible to undertake projects too small or erratic for normal dredging. Hoe Dragons are available in four basic sizes ranging from the 50 hp HMDE 50 through the 500 hp HMDE 120.
Fuchs Systems produces the Airlift dredges for deep water extraction already described, movable drag scrapers capable of production rates of 65-340 [yd.sup.3]/hr down to depths of 75 ft, and dewatering bucket-wheels with capacities up to 1,100 st/hr.
IHC Holland has been building bucket-line dredges since the end of the 19th century and, in 1979, built the world's largest bucket-line dredge in combination with a Singapore shipyard for offshore tin mining in Indonesia. This has 30 [ft.sup.3] buckets for dredging to 46 m below waterline and is equipped with swell compensation for offshore working. It is now dredging gold off Nome, Alaska.
Other products include bucket-wheel, plain suction and cutter-suction dredges, and a range of modular skid or pontoon mounted treatment plants. The latest addition to the program is the IHC-Rohr active-grab-type dredge for deep digging.
The IHC dredging wheel has a large number of buckets close together from which the bottom and rear have been omitted. This makes the area within them an integral part of the pump suction mouth with the result that cut material moves immediately within suctionable range, giving very low spillage. IHC has recently received an order for the world's largest bucket-wheel dredge for ACM's Eneabba West mineral sands project in Western Australia.
Treatment plants consist of combinations of modular scrubber/screen and jig units based on the IHC radial jig. A standard jig plant consists of primary, secondary, and, on larger units, tertiary jigs, complete with piping, tanks, pumps and electrical controls. The smallest unit is a 20 [m.sup.3]/hr 2-stage plant. One skid can accomodate up to six primary jig modules and for more capacity extra jigs can be mounted on a separate auxiliary skid linked to the main skid. In this way, transportable standard jig plants of up to 12 primary modules can be obtained with a capacity of up to 240 [m.sup.3]/hr. Scrubber/screen plants and a standard IHC jig unit can be installed together on a floating pontoon.
Neumann Equipment produces and operates its own bucket-wheel dredges. Particular attention is paid to ease of operation, accessibility to engines and equipment, operator comfort and control flexibility. Compactness is a major feature and modular design makes for portability.
Spud carriage-ways allow a controlled advance of 2.5 to 3.5-m into the face before resetting with the aid of fixed spud, and anchor booms allow repositioning of slewing anchors with minimal time loss. These features give Neumann dredges a high degree of maneuverability.
The Series 200, 250, 350, and 500 dredges can be diesel or electric powered, have maximum digging depths from 7 to 23-m, and are capable of throughputs of 175-2000 mt/hr at up to 40% solids. A range of bucket-wheels is available for different applications.
Orenstein & Koppel produces a range of dredging equipment including bucket-line, bucket-wheel and cutter-suction dredges, pontoon-mounted hydraulic shovels, pumps and winches. Small units are dismountable.
Osborne Chappel Payne's PP Series bucket line dredges are designed to meet the need for small bucket-line dredges purpose built for alluvial mining and designed for ease of transport. They can be used for bulk sampling and pilot operations, can dig to 15 m below waterline and come with a variety of mooring options and a choice of treatment plant for gold, tin or diamonds. Every dredge is supplied in kit form for assembly on site. Nominal capacities for the three standard designs, PP100, PP175 and PP250 range from 1.0-2.5 million [m.sup.3]/yr. For a given unit ownership and operating cost the PP100 said to have a higher throughput and be able to dig to greater depths than a bucket-wheel and attendant treatment plant under similar conditions.
PHOTO : Boskall's cutter-suction dredge is removing coal mine overburden, totalling 5.3 million
PHOTO : [yd.sup.3], near Seattle, Wash.
PHOTO : The Neumann Series bucketwheel-suction dredge is doing land reclamation, creating a
PHOTO : prestige residential waterfront development, near Robina, Queensland.
PHOTO : A Tramrod sea-bed dredging vehicle is used for remotely-controlled dredging at 100 m plus
PHOTO : depths.
PHOTO : Elliott's Hoe Dragon combines hard-digging capability with a submerged pump.
PHOTO : The IHC-Rohr dredge is a new machine using an active grab for deep water applications.
PHOTO : Two or three grabs can be used to add capacity.
PHOTO : IHC's 30 [m.sup.3]/hr treatment plant consists of a trommel screen and two-stage jig unit,
PHOTO : all mounted on a skid.
PHOTO : Elliott's Dual Wheel Excavator has two sections of eight buckets each in sizes from 50 to
PHOTO : 1,500 hp and pumping systems from 6 to 36-in dia.
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|Author:||Suttill, Keith R.|
|Publication:||E&MJ - Engineering & Mining Journal|
|Date:||Nov 1, 1989|
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