Table of Contents
At present dredge mining in this country is at a low level. The last gold dredge in California stopped operating in October 1968. Gold dredging in the adjacent States had already ceased because of increasing operating costs, decreasing placer values, and the fixed price of gold. Two gold dredges and one platinum dredge were still operating in Alaska.
Because of the increasing national requirements for gold the Bureau of Mines in 1966 initiated a Heavy Metals study to investigate and appraise potential gold-bearing deposits that might contribute to domestic gold production. Emphasis was given to large deposits which would permit the economies of large-scale production methods.
This report is focused on placer mining by dredging. It brings together the old and proven dredging concepts with newer and more current techniques integrating these ideas with actual cost experience standardized to 1967. It provides data concerning current application and identification of the natural and unchangeable environmental conditions to be considered in selecting a system. Like any mineral deposit, a placer deposit should be evaluated on the basis of the most practical and efficient system suited to it. The risk involved can be reduced by a thorough exploration program balanced with a sound engineering study. There are no current publications that describe modern design and operation of dredges. Charles Janin’s “Gold Dredging in the United States” is an excellent general reference although written 50 years ago. Peele’ s Mining Engineers’ Handbook is another general reference of considerable value. Two of the more current papers that cover foreign gold dredging are McFarland’s paper on dredging in the Yukon, Canada, and O’Neill’s paper on dredging in Colombia and Bolivia, South America. The bibliography in this report was prepared primarily as a source of references for placer exploration and dredging.
History of Equipment Development
The first floating bucket-line dredges were developed in Europe during the 16th century to excavate harbors. Over time their design and efficiency improved. Their adaptation to placer mining began in New Zealand in 1882 when gold-saving equipment was added behind the excavators. The first successful dredging operation in the United States started at Bannock, Mont., in 1895. The dredge design incorporated the bucket-line equipment already found successful in the Eastern United States. To it was added a washing and treating system similar to that used in New Zealand. The first gold dredge to operate in California was a New Zealand type at Oroville in 1898. Not versatile enough to operate under the varied and often rugged dredging conditions in California, it was soon discarded and a variation called the California type was developed after the Montana design.
The California—type dredge, because of the need to dig in gravel containing large boulders and in cemented, clayey, or tightly compacted formations, was strongly built and was equipped with spuds, heavy vertical movable piles, to hold the dredge firmly in place. It had short tail sluices and relatively long stackers to dispose of oversize gravel, usually plus 5/8 inch. The New Zealand dredges, which were lighter in construction, were designed primarily to excavate softer formations. They used headlines instead of spuds because of the easier digging and because there were no high banks ahead to hinder their use. Often long sluices were used to dispose of the waste. In California stackers combined with sluices were found more practical. The California dredge produced more yardage and operated more efficiently considering all types of formations. Long stackers for gravel disposal made it possible to dredge relatively deep placers and to work against high banks not possible with a headline system.
After a very successful and profitable period of mining from 1895 to 1942, cost gold-placer operations in the United States were closed down by Government Order L-208 issued during World War II. The dredging industry continued to produce war-needed scheelite in Montana, platinum in Alaska, and columbium in Idaho. Some gold dredging operations were resumed after the war, but most remained closed because of the widening gap between mining costs and the fixed price of gold. The last gold dredge of the more than 60 operating in California in the 1930’s was shut down on October 1, 1968. Its earlier version is shown in figure 1. In September 1969, two gold dredges and me platinum dredge were operating in Alaska. One gold dredge was on Bear Creek, Hogatzu River, and the other on the Ungalik River, and the platinum dredge was in the Goodnews Bay district.
Dredging systems are classified as hydraulic or mechanical depending on the method of digging; both are capable of large production. A floating dredge consists of a supporting hull with a mining-control system, excavating and lifting mechanism, beneficiation circuits, and waste-disposal systems all designed to work as a unit to dig, classify, recover values, and dispose of waste. Design and operation of a dredge will not be discussed except as they Influence the choice of system.
Hydraulic dredging systems, whether the lifting force is suction, suction with hydrojet assistance, or entirely hydrojet, have been used much less in placer mining than mechanical systems. However, in digging operations where mineral recovery is not the objective, the hydraulic or suction dredge has greater capacity per dollar of invested capital than any mechanical system because the hydraulic system both excavates and transports. Thus the hydraulic dredge is superior when the dredged material must be moved some distance on the point of processing. Because it is much more economical to treat the placer gravels aboard the dredge the hydraulic systems with their inherent dewatering problems are at a disadvantage.
Hydraulic digging is best suited to relatively small-size loose material, it has the advantages over mechanical systems in such ground when the material must be transported from the dredge whether by pipeline or barge. In easy digging, excavation by hydraulic systems has reached depths of about 225 feet, but excavation for mineral recovery to date has been much less, only about one-quarter of that depth. In the same way as equipment developments benefited open-pit mining, hydraulic mining dredges have benefited greatly from the developments brought about by the need to solve civil-engineering problems, the mounting interest around the world in offshore mining has spurred research and development of hydraulic dredging equipment considerably.
Even with efficiently designed units and powerful pumps, the size of the gold that can be captured by hydraulic dredging is limited. The ability of a hydraulic system to pick up material in large part depends upon intake and transport velocities that must be increased relative to specific gravity and size of the particles. If the gold occurs as nuggets, especially large nuggets, the velocity required for capturing the gold can cause excessive abrasion in the entire system. In addition, higher velocities require more horsepower. On the other hand, when the flake size of the gold is very fine, higher velocities make gold recovery very difficult during dewatering.
The digging power of hydraulic systems has been greatly increased with underwater cutting heads. One disadvantage of a cutterhead is that it must be designed with either right- or left-hand cutting rotation, which results in less efficient digging when the dredge is swung in one direction, especially in tough formations. As digging becomes more difficult and the cutterhead is swung across the face in the direction so that its blades are cutting from the old face to the new, the cutterhead tries to climb onto and ride the scarp. This produces considerable impact stress through the power-delivery system and reduces the capacity of the cutter. Davis and McKay in a creditable up-to-date paper on potential dredge applications discuss this difficulty. Because hydraulic dredges, even with cutterheads, dig less effectively than mechanical dredges, gold particles trapped in bedrock crevices are more difficult to recover. Disadvantages include—
- High wear when endeavoring to capture large-nugget gold.
- Dewatering problems when trying to capture small-fraction gold.
- Less digging capacity when swinging in one direction.
- Less hard digging ability especially where needed on bedrock.
- Relatively low power efficiency because of the large volume of the water that must also be transported.
- The problem of moving and mining around sunken logs, bedrock pinnacles, and oversize material. The latter tends to collect along the face, interfering with good bedrock cleanup, often so vital in gold placers.
- Loss of some material left as a ridge on the bottom each time the dredge is stepped forward to start a new cut.
- Detrimental effect in the treating plant caused by mud-ladened water pumped off the bottom of the pond.
- Fluctuations in the feed to the treating plant.
The principal uses of hydraulic dredges have been for nonmining jobs such as in digging, deepening, reshaping, and maintaining harbors, rivers, reservoirs, and canals; in building dams and levees; and in landfill and reclamation projects. Hydraulic systems in mining have been used to produce sand and gravel, to mine marine shell deposits for cement and aggregate, to reclaim mill tailings for additional mineral recovery, to recover coal from streams, and to mine deposits containing diamonds and minerals of tin, tungsten, columbium-tantalum, titanium, monazite, and rare earths. An interesting example of everyday dredge mining in this country is the offshore shell industry, concentrated mainly along the gulf coast although some is being done off California and the mid-east coast. Dredging the buried reef deposits of dead oystershells is a simple and inexpensive method of mining, and barging the shells to coastal market areas is an inexpensive and flexible method of transportation. From Texas to Florida, an average of 25 small hydraulic cutterhead dredges have been recovering an average of over 25 million cubic yards of shell annually over the last 5 years. Using suction pumps up to 18 inches in size, a few of which are equipped to go as deep as 50 feet, these dredges are the prime suppliers of high-grade calcium carbonate for a number of cement and lime plants along the coast, and of roadstone and aggregate to coastal areas that lack stone.
In recent years hydraulic systems have been used to strip unconsolidated overburden–(1) often more cheaply, (2) often with considerably less effort, and (3) often as the only logical method when the material to be stripped is water saturated or lies wholly under water-from ore, coal, and aggregate deposits in North and South America and Europe in preparation for conventional open-pit mining systems. Dredge stripping is not new. It was first tried in 1914 when a method cheaper than steam-shovel stripping had to be found to uncover a marginal iron-ore deposit on the Mesabi Range. Stripping overburden from offshore tin,deposits has been done using both hydraulic and bucket-line systems.
Some examples of how hydraulic dredges have performed mine stripping seem in order. In Surinam (formerly Dutch Guiana) a 24-inch cutterhead dredge was used to strip the overlying silt, sand, and clay from a 2,200- by 4,000-foot bauxite ore body at an average two-shift rate of about 300,000 cubic yards per month and pumped a maximum of about 2 miles to the spoil area. On the Steep Rock iron range in Ontario, Canada, a large river was diverted, a large lake drained, and up to 400 feet of overburden removed from three ore bodies laying beneath Steep Rock Lake. The A and G, or Hogarth and Roberts, ore bodies were together stripped of about 106 million cubic yards of silt up to 300 feet thick with two 900-ton cutterhead dredges. At the C, or Caland, ore body 162 million cubic yards of silt, sand, gravel, and clay overburden were stripped by two 36-inch hydraulic dredges and pumped a maximum of about 6 miles against static heads totaling about 780 feet. Again a river was diverted and a lake drained at the Black Lake project in Quebec, Canada, where 35 million cubic yards of silt, sand, gravel, and glacial till were stripped from atop an asbestos deposit by a 30-inch hydraulic dredge and pumped over 3 miles against a static head of 200 feet. About 15 million cubic yards of muskeg, silt, and clay will be stripped from an area 3,700 by 2,000 feet to expose a nickel ore deposit in northern Manitoba. In Scotland the National Coal Board stripped up to 40 feet of peat and sand cover to expose four coal seams across some 270 acres with a hydraulic dredge that averaged 90,000 cubic yards per week. Two general papers that cover hydraulic dredging developments are by Kaufmann and Giroux. The proceedings of the first World Dredging Conference, held in New York in 1967, cover a variety of technical subjects primarily on hydraulic dredging.
Mechanical Continuous System
Digging systems on continuous mechanical dredges can be a bucket-ladder, rotary-cutter, or bucket-wheel excavator, each with advantages peculiar to specific situations. The bucket-ladder or bucket-line dredge has been the traditional placer-mining tool, and is still the most flexible method where dredging conditions vary. Placer dredges, rated according to bucket size, have ranged from 1½ to 20 cubic feet, although larger equipment has been used in harbor work. Lines of 34- and 54-cubic-foot buckets on the dredge Corozal were used in digging the Panama Canal in 1913-15. Excavation equipment consists of a chain of tandem digging buckets that travel continuously around a truss or plate-girder ladder, scooping a load as they are forced against the mining face while pivoting around the lower tumbler, and then dumping as they pivot around the upper tumbler. The ladder is raised or lowered as required by a large hoisting winch through a system of cables and sheaves. Before the development of the deep digging dredges, the maximum angle of the ladder when in its lowest digging position was usually 45° below the horizontal. During the last few years in Malaysia, 18-cubic-foot dredges digging from 130 to 158 feet below water level have often been operating at angles of 55° and sometimes more. At its upper position the ladder inclines about 15° below the horizontal. Figure 2 is a side elevation of the 18-cubic- foot Yuba Manufacturing Division, Yuba Industries, Inc., No. 110 dredge that was designed to dig 85 feet below water level.
Compared with any hydraulic system the bucket-line dredge is more efficient in capturing values that lie on bedrock or in scooping up the material which sloughs or falls from the underwater face very similar to how metal detectors for the underwater need to be completely protected from water. It is more efficient when digging in hard formations, because its heavy ladder can be made to rest on the buckets providing them with more ripping force. Bucket size and speed can be varied with formation changes in the deposit according to the volume of material that can be processed through the gold-saving plant. Most bucket line dredges used in placering have compact gravity-system processing plants mounted on the same hull as the excavating equipment. The waste stacking unit, also mounted on the same hull, combines with other dredge functions to make the dredge a complete and efficient mining unit. The advantages of an integral waste distributing system trailing behind the excavator become readily apparent because up to 10,000 cubic yards of oversize waste must be disposed of each day on a large dredge. To assure a high percentage of running time, dredge components must be designed for long life and relatively easy and quick replacement of parts. Dredging experience has shown that most parts need to be larger and heavier than theoretical engineering designs indicate, and the simpler their design, the less their replacement and installation costs.
Summing up, the advantages of the bucket-line dredge as compared to the hydraulic dredge are as follows:
- It lifts only payload material, whereas a hydraulic system expends considerable energy lifting water.
- It loses fewer fines, which contain most of the fine or small fraction gold.
- It can dig more compact materials.
- It can clean bedrock more efficiently.
- It allows more positive control of the mining pattern.
- It has a simpler waste disposal system as compared to a hydraulic system with an onshore treatment plant.
- It requires less horsepower.
The disadvantages with respect to hydraulic systems are as follows:
- It requires more initial investment capital per unit of capacity.
- It requires a secondary pumping system if the excavated material must be transferred to a treating plant distant from the dredge.
The main and interrelated reasons why bucket capacity has not increased during the last half century are as follows:
- Larger placer deposits, which could justify the added expense of larger equipment, have not been found.
- Treatment-plant capacity and recovery need to be bettered to deal with the more complex treatment problems, which have developed as placer values have decreased.
- New equipment development has been retarded by the lower cost of available secondhand equipment.
To date a bucket-wheel excavator has not been used as part of a mining dredge, but conceivably if integrally designed into the total unit it could have distinct advantages. Bucket-wheel control would be similar to that of a bucket line, its ladder maneuvered vertically by a winch-cable-sheave system. Its outstanding advantage on land, to discharge directly onto its ladder conveyor, cannot be fully utilized to dig underwater unless the diameter of the bucket wheel is sufficiently large with respect to the depth of the gravel and possibly unless the bucket transfer and conveying systems are modified. The bucket wheel would seem to have its greatest promise on a hydraulic dredge to replace the cutterhead. With hydraulic lift and transport it should compare favorably with the bucket-line system. Capable of working in either direction, it could overcome the weakness of the cutterhead, which can operate efficiently in only one direction, and in tough formations it should increase output.
Mechanical Repetitive System
Repetitive mechanical excavators such as shovels, draglines, and clamshells have a place in placer mining primarily in small operations where the conditions are less favorable for continuous machines. The principal disadvantage of repetitive digging is lack of mining control. A dual clamshell mounted on a ship-type hull was used to mine offshore tin deposits in Thailand, but after 9 years it was replaced by a much more efficient bucket-line dredge. Even in its best year, 1962, it proved more expensive and less efficient than its replacement. In that year it recovered only 65 percent of the yardage available on the sea bottom while the bucket line made nearly complete recovery. Working underwater, particularly where conditions are not uniform, the operator of a repetitive excavator is unable to see how well his excavator is performing or how to reposition his excavator for maximum recovery.
Onshore placer deposits have been exploited using dragline excavators, shovels, and clamshells. Treating plants, separate from the excavating equipment, can be either floated on a pond or placed onshore, but in almost every instance the treating plant is portable. The onshore dragline excavator feeding a floating treatment plant was very popular in California during the depression after the price of gold was increased to $35 per ounce. It was reported that over 150 of these dragline dredges were operating in gold-bearing gravels in the United States during the late 1930’s. Their principal drawback was a general inefficiency.
Advantages of a dragline excavator operation are as follows:
- Less setup and moving time, factors particularly suitable to mining small and scattered placers.
- More utility in narrow, shallow, or bouldery deposits.
- Ready adaptability to irregular deposits where the variable length of cast can be advantageous.
- More adaptability to irregular and steep topography.
- Generally advantageous to the smaller miner because of lower capital costs and the possibility of buying used equipment.
Disadvantages are as follows:
- Inefficiency in gold recovery introduced by surges in feed.
- Higher unit operating costs than for continuous systems.
- More difficult to fully recover the gravel because of less control in bucket positioning, especially as depth increases the maximum depth of efficient digging with a dragline is approximately 60 feet.
Power shovels, because of their relatively low capital cost and proven design as an excavating tool, were often mounted on floating dredges to mechanize placer operations between 1890 and 1910. Primarily because of their relatively low and intermittent capacity to supply the treating plant, their high energy consumption per unit of capacity, and their lack of control to effectively clean up bottom gravel, shovels have not proven successful. In the 1930’s G. E. Becker and H. H. Hopkins of San Francisco, Calif., invented and patented a modified shovel excavator which, mounted on a floating hull, had its bucket at the end of a pivoting flume instead of a conventional dipper stick. Moving vertically, the bucket emptied its load at its elevated position into the tilted flume from where it was sluiced to the trommel. One of the first units was worked in Alaska and reported very successful. The Yuba Manufacturing Co. then obtained the manufacturing rights and an operation was started on Butte Creek, Tehama County, Calif. It proved unsuccessful because intermittent feed to the hopper and screen adversely affected the treating system, which in turn resulted in poor recovery and high operating costs. The power shovel, however, does have distinct advantages in tight and bouldery formations where its greater digging capabilities can be put to maximum use.
In operation, the deeper digging dredges have not produced as much in proportion to their bucket capacity as have the smaller size dredges working in shallower depths mainly because the operator does not have the control he does at the shallower depth, his line speeds are usually slower, it takes longer to raise the ladder, and downtime to service and repair is longer. Other things being equal, the deeper digging dredge has produced less per cubic foot of its capacity because of the increased lag time before corrections can be made for partially filled buckets. As more modern electrical controls take over the responsibility of keeping the buckets full, the effect of lag time will lessen.
The approximate percentage increase in production that comes with the change in line speeds is shown by the upper curve in figure 3. This curve indicates that with present dredging technology faster line speeds produce the greater gain for the middle-size dredges. This curve together with the lower two points up the difficulties in lineally scaling production with bucket capacity and at the same time designing for greater depth.
One of the initial adaptations to automation was the use of electrical sideline winch controls early in the 1930’s. As more automatic and precise controls can be built into dredge systems, production, especially for the larger dredges, will increase.
Experience has proven that the dredge with 9- to 10-cubic-foot buckets is generally the most economical size, but digging depth should not exceed 80 feet. In Malaysia a 9-cubic-foot bucket dredge is digging to 125 feet, but its efficiency is probably less than if larger buckets were used. Care muse be taken to be sure that demand for increased production does not result in decreased recovery. In one case where the operation was under good management, the decision was made to greatly increase the yardage with the accepted loss of approximately 5 percent in gold recovery. The end result was an annual increase in the total ounces of gold produced at lower unit operating costs. Opposite to this is the case of a large dredge that was digging in a clayey formation. The dredge was averaging about 125,000 cubic yards weekly, but without the expected recovery. When the yardage was cut to 90,000 cubic yards, the losses were reduced. The manager explained that he was mining for gold and not trying to make high-yardage records. Maximum yardage that can be efficiently treated with minimum loss is the prime objective, not maximum digging. In still another case where management’s policy was high yardage, the dredging crew under pressure to produce did not go to bedrock, especially the crew on the third or graveyard shift. Later, when the same area was redug in crossing to an unmined section, it was found that approximately 3 feet above bedrock had not been dug. Gold recovery on this second pass equaled that on the first. In some foreign dredging fields, inefficient equipment first used and lack of attention to mining patterns have warranted redredging a second and sometimes a third time.