Gold Dredging Methods
A gold dredge combines the four basic principles of mining: digging, classifying materials, gold saving, and disposing of waste materials or tailings. A generic description explains it as a combined excavating and concentrating plant like an animated houseboat. An endless chain of mammoth steel buckets – a hundred or more, each weighing more than a ton – digs the gravel and delivers it to the upper end of a revolving screen through which the gold laden gravel passes to tables or riffles. The oversize gravel is discharged onto an inclined belt-conveyor called the stacker, which carries it to the tailings pile.
Dredges fall into four basic types: (1) flume; (2) screen and flume; (3) combination; and (4) stacker. Each works by means of an endless chain of buckets, linked one behind the other, rotating around a digging ladder that is capable of being raised and lowered as necessary. The modern equivalent in principle although not in use, and on a much smaller scale, is the trenching machine known as a Ditch Witch. In both cases a series of buckets (or shovels) move in an endless chain around a solid arm, constantly bringing more and more material up the chain as it moves forward into new ground.
Flume-Sluice Dredge. On a flume dredge, the buckets dump directly onto the head of a long sluice running down the axis of the boat. Essentially providing a mechanical means to bring large quantities of gravels from in front of the dredge and run it through a sluice. It dumps far enough astern to avoid interfering with its operation and to keep the dredge afloat. Flume dredges are only useful in shallow ground with material that is both small in size and easily washed. Their biggest benefit came in working narrow, rich wet pay streaks. In most cases, flume dredges were small, with buckets ranging from one and a half to three cubic foot. According to Charles Herbert, this type of dredge was capable of processing no more than 500 cubic yards of material daily. Their small size often meant low cost, which allowed operators to get into the dredging business with less capital than that required for larger operations.
Screen and Flume Dredge. With a screen and flume dredge, the buckets deliver the gravel into the head of a revolving screen that separates the larger gravel from the smaller material and sends it off the stern by means of a chute. In some cases, rather than using a revolving, tumbler-type of a screen, dredges used a flat, table-like screen (called a "shaker" or "shaker deck"). The purpose here was to break up the larger pieces of dirt and classify materials allowing the finer particles to pass through the sluice for gravity concentration. Advantages of this type of dredge include: the ability to work ground with larger gravels and boulders; and the revolving screen to help wash the gravels, separating the sticky mud, etc. from it before sending it down the sluices. The chief disadvantages of the screen flume dredge are that it is limited to only digging shallow ground.
Combination Dredge. The third type of dredge, the combination dredge, combines the revolving screen with a mechanical stacker (basically a conveyor belt) to remove the coarse material well astern of the boat. The smaller gold bearing gravels pass through the screen, fall onto a wide riffled sluice that discharges onto additional sluices connecting to two long flumes on either side of the hull. Although this dredge design is both light and inexpensive, it was recommended for use only in shallow ground and because of the nature of the sluices. It had the same inherent problems of both the sluice dredge, and the screen and sluice dredge. It could only operate during mild weather to prevent the flumes from icing over.
Table Stacker Dredge. The table stacker type of dredge is the most familiar to people living around or visiting the gold fields. By far the majority of the large dredges built for Alaskan operations represent this type. In this case, the bucket line delivers the material to a hopper at the head of a revolving screen. As it passes through the screen, the smaller materials fall through a series of holes – smaller holes are located toward the head and larger toward the tail of the screen. The material too large to fall through moves off the stern by means of a mechanical stacker/conveyor as in the combination dredge. The material that continues through the dredge falls onto one or more banks of riffled sluices called tables. From there, it passes down each side into additional sluices. Then the fine material (gravel and sand) washes off the stern by means of short tailings chutes. In the case of this type of dredge, the flumes do not freeze up as readily as the others do. The stacker removes the bulk of the waste material. The flumes are shorter and housed almost entirely within the dredge housing itself. Like much of the mining industry in general, technological advances were few until very late in the twentieth century. Most advances in dredging came principally in the use of alloy metals, incorporating electrical equipment into their operation, and advances in the design of the bucket line. The basics remained constant throughout the dredge era. Material was scooped out of the ground, processed through the screen and sluices, then removed and dropped behind the dredge with the finer gravels processed through a system of sluices to recover the gold.