Once aboard the dredge, the dredged material is classified, its valuable portion recovered, and its waste portion disposed in a coordinated recovery disposal system designed to handle the capacity of the digging system. Also on board are the dredging-support equipment of the spuds, winches, and controls, and the water-supply systems. The recovery equipment consists of a hopper, revolving screen, distributor systems, and gold-saving equipment.
The first bucket-line dredges used deck-type shaking screens, some single and others double in tandem. These were inefficient and were soon replaced by inclined revolving screens or trommels that have the dual capacity of sizing and attrition scrubbing. They break down cemented or compacted material and scrub the gold-bearing fines from the oversize. The perforated plates on the revolving screen are either manganese steel cast with tapered holes or abrasive-resistant, low-manganese, high-carbon steel with drilled tapered holes. The cast-manganese steel has proven most economical with coarse gravel and boulders which cause it to work harden. With coarse sand and fine gravel where work hardness is not developed, the cast-manganese plates wear very fast and have proved uneconomical as compared to alloy-steel plates. Yuba Manufacturing Co. of Benicia, Calif., supplied most of these plates to the dredging industry.
Holes are tapered from ¼ inch to 3/16 inch in 3/8-inch plate but in the more generally used ¾-inch plate they tapered from ¾ to 5/8 inch, outside to inside. When in sand and fine gravel in which the minerals to be recovered are less than 1/8 inch, the screen with smaller holes is generally used. For subsequent selective concentration based on particle size, a trommel with graduated hole sizes, increasing downslope, is often used. On dredges where the holes in the trommel are all of the larger size, the materials are distributed from one bulk collection point to the usual jigging system.
To trap the occasional nugget that is larger than the standard-size hole, a set of wider slotted holes is added at the lower end of the trommel. The treating system for this limited area is usually either tables with coarse riffles, cocoa matting with expanded metal, or a heavy-media system.
Clayey placer material must be broken up to free the entrapped gold before concentration. This is a serious problem in any placer operation and one that requires more research.
From the trommel the undersized must be uniformly delivered to the concentrating units. Dewatering, accomplished by dewatering cones or hydrocyclones, is almost always necessary. The hydrocyclone has greater capacity, is far more efficient, and requires less space. Various combinations of jig circuits and auxiliary equipment are used to treat the undersize. The old-fashioned standard gold-saving tables with Hungarian riffles and mercury have been replaced to a large extent with jigging systems because of the inefficiency of the former method, especially in the recovery of fine gold and that which resists amalgamation. For a small one-dredge project it may still be economic to use the old system, but the decision should be based on a detailed study. A riffle has certain limitations as a gold-saving device. A riffle system, to accomplish its primary duty of recovery, must be on the proper slope to first accomplish transport. Riffle design must entrap the gold particles in a current of water swift enough to transport all sizes of material in the feed yet slow enough to permit capture of the gold. Of riffles in general it might be said that the design of the riffle is not so important as are the adjustments of feed, water, and slope with respect to the design in question. Often required to treat volumes larger than its designed capacity, recovery on a riffle drops as higher velocity along with excessive turbulence lowers the chances of capture, especially the fine-particle, coated, rusty, or refractory gold. Increasing the exposure time by lengthening the riffled surface has limited application on dredges. One idea of increasing riffle length was tried in 1914 when a long gold sluice was mounted on a separate hull towed behind the dredge. An enlarged treating plant mounted on a separate hull is presently recovering ilmenite in Florida.
Much of the credit of improved dredge recovery over the last 55 years belongs to the evolution of jigs. Jig improvement in the way of better recovery and larger capacity has been one of the more important reasons why dredging has been able to move into lower grade gravels. One of the earliest, large-scale jig tests on a gold dredge was by J. W. Neill aboard the 3½ cubic-foot Yosemite Dredging and Mining Co. No. 2 dredge in California in 1914. The success of these tests was not so much in the way of better recovery but in the adaptation of heavy and cumbersome milling equipment to a design that could conform to the limited space aboard a dredge. Following these tests, the Natomas Consolidated Co., later Natomas Co., in 1915 made subsequent tests and used modified Neill jigs with Hardinge mills to grind the concentrate on its No. 7 dredge. In both cases the jigs were installed after the riffles to save some of the gold normally lost in the tailings. Hartz-type jigs, a plunger-type, were installed on the Pacific Tin Consolidated Corp. No. 2 dredge to recover cassiterite soon after it was sent to Malaya in 1923. This rebuilt dredge was formerly the Dawson No. 5 of Yukon Gold Co. the predecessor of Pacific Tin. In 1932 the Bulolo Gold Dredging Co. made a successful Bendilari jig (a diaphragm jig) installation aboard a dredge in New Guinea where jigs handled the total output of the dredge. Placer Development Ltd. engineers designed a new jig patented it in 1936, and made an arrangement with Pan-American Engineering Corp. for manufacturing rights. Several dredges in New Guinea and Colombia were later equipped with this newer jig, thereafter called the Pan-American Placer Jig. Manufacturing rights were later sold to Dorr-Oliver with Placer Development retaining the right to use the jig. In 1936 and 1937 Fisher and Baumhoff in Idaho and Yuba Consolidated Goldfields in California made their initial jig installations aboard their dredges. The jig designed by Yuba Manufacturing Division. Yuba Consolidated Industries, Inc., in 1948 is currently being built in Great Britain and is being used mostly on tin dredges. This jig was found to be one of the more practical dredge jigs until the advent of the circular Cleaveland-IHC jig.
Jigs (1) can handle relatively large volume feeds in relatively compact space; (2) can better capture fine-particle gold by reason of quieter action on top of the jig bed; (3) can better capture coated, rusty, and refractory gold, which resists amalgamation when mercury is used in riffles; (4) can better handle large amounts of black sands, which in a riffle can pack the spaces to hinder gold settling, because forced-water pulsations through the jig bed keep the bed open and “live;” (5) can better handle limited surges in the feed which in a riffle can overload and suppress its required action; (6) can be adjusted to more readily meet changing conditions; and (7) require less but a controlled supply of water. The advantages of better recovery are based on the condition that the volume and pressure of the water be properly controlled. Jigging has its limitations too, and trapping all of the finest particle gold is still not economically possible.
Currently most concentrating units are composed of primary or rougher-jig circuits followed by secondary and tertiary circuits. The primary jigs are arranged in parallel banks or flowlines of one to four cells in series. The number of cells in a flowline is a point of much discussion, but for most gold operations two cells are adequate. This is especially so if sufficient flowlines are used to handle the screened materials on an approximate 12½ cubic yards per hour per flowline basis, The flowsheet of the 18-cubic-foot Yuba Consolidated Dredge No. 21 is shown in figure 5.
The concentrates from the secondary system are often treated in a ball-mill amalgamator. If some of the gold is not amalgamable, the tails are further jigged in a small scavenger cell. Usually after the last jigging, the scavenger concentrate is put over a small section of tables with standard Hungarian riffles or cocoa matting overlain with expanded metal.
Recently a circular jig has been patented and placed in tin, gold, and diamond-placer operations. Previously, circular jigs used in onshore treating plants were not practical on floating dredges as they required steady and firm foundations. The new Cleaveland-IHC jig was designed by Norman Cleaveland when he was president of Pacific Tin Consolidated Corporation of New York, a company now engaged in dredging tin in Malaysia. Interestingly, this company with others started the first bucket-line dredging operation for diamonds and byproduct gold in Brazil. Possibly this type of jig will someday replace the standard square-shaped pulsating jigs because of its greater efficiency and capacity, lessening the number required and conserving deck and height space. The jig is fed at the center with the overflow at the periphery. As the feed moves toward the perimeter, its radial velocity drops, increasing the recovery time, a basic aim in any gravity-concentrator design. Radial arms help to move the larger gravels toward the perimeter.
The success of any dredging project rests ultimately with the nature of the deposit to be mined. Seldom are conditions completely favorable. A successful dredge operation begins with an adequate exploration program. The cost to properly evaluate a placer is small with respect to the cost of a dredge, especially if dredging later proves an unfortunate choice. Choosing dredge equipment should be left to those competent for there is no standard dredge. Getting maximum recovery, first from the ground and then in the treating plant, requires continuous effort. The more complicated the recovery problems, the more capable must be the dredgemen. A low-grade placer containing a high-specific-gravity, high-value metal such as gold means an extremely low metal-to-waste ratio, volume-wise. Add to this the usually small sometimes extremely small gold-particle size, its tendency to collect where digging conditions are often most difficult, and its recovery on basically simple gravity equipment that must handle large volumes continuously with little downtime, and an appreciation of gold dredging develops.
The bucket-line dredge is the traditional gold-placer tool. It was developed for gold placers. Capable of continuously mining and treating low-value placer material at a unit cost lower than that for any other type of mining, the bucket-line dredge is a completely mechanized, large-volume, self- contained, coordinated mine-mill unit that digs, treats, and backfills its waste in a matter of minutes. A flexible system, the bucket-line dredge has qualified to mine other types of onshore and offshore placer and bulk-product deposits and to strip the waste that overlies them.
Gold placers are seldom easy to dig. The digging mechanism on a spud-equipped, bucket-line dredge is a rugged but relatively simple piece of machinery that evolved from many successes and failures. It has the ability to run almost continuously under difficult digging conditions, sometimes requiring all the effort it is capable of producing. It is the best and sometimes the only system that can dig in extremely tough clays, cemented gravels, oversized boulders and buried timber debris, and most important in hard, often blocky, and irregular bedrock, on or in which the gold is often concentrated. Digging equally well in both swing directions, the bucket ladder has distinct advantages over cutterhead hydraulic systems that inherently dig less effectively in one direction. The treating plant aboard a bucket-line dredge is a compact gravity mill matched to the capacity of its digging system and the nature of the placer being worked. Not confronted with dewatering problems of the magnitude encountered on hydraulic dredges, it is built to work efficiently in limited space. Progress in dredge-developed gravity equipment has played an important part in keeping gold dredges working while operating costs rose, placer values dropped, and the price of their product stayed the same.
Hydraulic dredging, also a continuous system that can develop a good mining pattern, is particularly adaptable to the more easily dug bulk deposits and to the more easily dug and evenly distributed placer deposits of lower specific gravity minerals that do not require stringent cleanup of bedrock to make dredging successful. Improved digging and depth capabilities developed since World War II have come about primarily from better designs to handle bigger and more difficult earth-moving jobs. The principal advantage of the hydraulic system, its built-in facility to transport the mined material to an off-dredge recovery or collection site, usually becomes a disadvantage when the recovery site is aboard the dredge. Dewatering a large volume of variable- size placer material from a relatively high-velocity stream in limited space and yet not lose its small-fraction gold is difficult.
Some of the future of dredging will lie in overburden stripping and marine mining. Dredging unconsolidated overburden from mineral deposits can often be done more cheaply and with considerable less effort if the material to be stripped is water saturated or lies wholly under water. The outstanding efficiency records set by large dragline equipment in most dry formations can hardly be touched by dredges. However, because of these records there has been a tendency to use dragline equipment in stripping and mining underwater formations that often could have been done more efficiently with dredges. Too, there have been other cases where dry formations could have been ponded, stripped, and mined more efficiently with dredges. In saturated deposits, especially in agricultural or low-lying coastal areas, some problems can develop when the water table is lowered to prepare for dry stripping. A lowered fresh-water table might complicate nearby agricultural use of the land, might increase the cost of potable water, and in some instances can initiate its replacement with salt or brackish water.
Mounting interest in offshore mining has accelerated new thinking in dredge developments, primarily in hydraulic designs. This could be the system with the most promise to someday reach many of the unconsolidated mineral deposits on the continental shelf. The First World Dredging Conference held in New York in May 1967 brought to light the world-wide interest in new dredging ideas, applications, and developments keyed primarily to marine use. Similarly, the Offshore Technological Conference held in Houston in May 1969 is an example of the national effort to pool present engineering knowledge on the development and recovery of marine resources. The growing awareness of the importance of one nation’s position in the sea-dredging industry and the emphasis needed to keep abreast with world-wide dredging research is outlined in a current paper.
Although the use of bucket-line dredges has slowed, as gold dredging has in North America since World War II, these machines will always have an application in mining. New bucket-line developments in depth, capacity, and recovery, which are bound to take place if the system is to continue, will come in such forms as enlarged bucket capacities, faster line speeds, better support of the bucket chain, continued improvements in the use of more sophisticated electric-control gear, conversion to strong light-metal alloys, and improvements in scrubbing and treating efficiency especially in the treatment of clayey materials. New approaches that combine the rugged mechanical capabilities of a bucket-wheel digging system and the efficient transport capabilities of either the hydraulic or modern bucket-chain system should develop more versatile machines.