Determining Suitable Dredge Size: After the placer deposit is properly prospected the miner has a reasonable knowledge of the volume and grade of the placer together with information on its geometry, formation, and the distribution of values by size and location as well as information on bedrock topography, surface topography, water supply, and a host of other information that is pertinent to future decisions.
If the results are generally favorable and suggest the probability of a profitable operation, then the next step is to test that thesis by a cost analysis to determine the optimum type and size dredge. It is best not to assume that a dredge of standard design will suit all deposits. In general, if the ground is deep, if the gravel is not cemented, and if the property is large, the larger the dredge the lower the unit cost of mining. If the ground is shallow, 30 feet or less, a large dredge will have difficulty or cannot operate. With a hull depth of 10 to 12 feet the pond beneath the dredge can cause flotation trouble. For the same reason, if the ground is shallow, the slope of bedrock becomes a very important factor in pond depth and thereby in the size of the dredge. In one shallow relatively high value dredging operation in the Western United States, 52 inches of placer material was being excavated with a 3-cubic-foot dredge that drew 50 inches of water. Coarse gravel that contained little fines, a good waste-disposal system, and careful advance made dredging possible. High-surface gradients might require additional bedrock to be dug or pond dams to be built. If the ground is tightly cemented the buckets should not be too large, but should be smaller and strongly built. Emphasis should be given to a smaller bucket line such as a 9-cubic-foot line designed for hard digging and equipped with extra horsepower. Where boulders are present and rock-type buckets are necessary, bucket size might actually have to be increased over that first anticipated. Larger buckets in this case designed with a longer than normal pitch and running at a somewhat reduced bucket speed will possibly not materially change the production rate or the need to redesign the capacity of the treating plant.
The capital and operating costs of continuous mechanical dredges should be compared with those of hydraulic or other types that have their treating plants separate from the excavator and which have lower capital costs but higher operating costs. The study should appraise the purchase of used dredges available at lower costs, even though they may be somewhat less efficient.
Size and type of hull, whether standard or portable, for a proposed dredging project require considerable study. The judgment of a qualified engineer experienced in dredging will be particularly valuable.
Gold Dredge Capacity
The estimated annual capacity of a dredge in cubic yards of bank is Cfs x S x M x D/27, where C equals bucket capacity in cubic feet; f equals average estimated fill factor; s equals average estimated swell factor; S equals average bucket-line speed in buckets per minute; M equals estimated average minutes of running time per day; and D equals estimated days of operation per year.
The fill factor might drop to as low as 0.50 when dredging a flowing sand and the ladder is elevated. Usual limits are 0.75 to 1.25, the latter being heaped loads when digging in cohesive clay. When estimating dredge capacity in new ground, a unity fill factor might be the best to use until experience is gained. The swell factor, the reciprocal of one plus the decimal equivalent of percent swell, can vary from 0.72 for a clay and gravel mixture to 0.89 for a loose sand. In new ground an average of 0.80 might be used for a 25-percent swell until more experience is gained.
The running time of a gold dredge is the time when gravel is being dug and treated. The yearly average based on a three-shift, round-the-clock operation will vary as the time required for moving ahead, greasing and oiling, repairing and cleanup, recovery from power failures, and the more unusual disruptions brought on by adverse weather and water conditions cutting into operating time. A placer dredge is usually shut down for cleanup, the collecting of concentrates, once each week. Taking advantage of this 5 to 8 hours of downtime, repairs are made to keep the dredge operating as continuously as possible between cleanup days. For many dredges, digging is hard and it requires all the effort that the dredge is capable of producing. Running under these conditions usually requires a general overhauling every 3 or 4 years not possible during regular cleanup days. Actual yearlong digging time has ranged from 75 percent or less when digging conditions are difficult to about 93 percent when digging conditions are very favorable. The aim of most projects is to attain a yearly average of approximately 21.5 digging hours per day, or approximately 90 percent of available time. However, dredges have been called upon to dig ground so difficult that a running time of 18 hours per day was considered satisfactory. As reported earlier, dredging in the far north requires that the dredge be put in top operating condition during the down months to minimize repair time during the 7- to 8-month operating season. Breakdowns that require more than a few hours of repair time are intolerable. Patty reports that meticulous preseason care has resulted in some dredges compiling running times of as much as 99 percent excepting cleanup time. Winter weather conditions permitting, a dredge normally operates 365 days per year with downtime only on Christmas and Independence Days and then dredges close down for only two of the three shifts. Design capacity of a gold-saving circuit depends upon the estimated percentage of fines at theoretical capacity of the excavator with an added safety allowance, the percentage of blacks within the heavy portion, and the size of the gold particles.
The stress to minimize downtime increases with the size of the dredge. Not only are their direct and indirect costs higher, especially capital-cost recovery, but the larger dredges are often working in lower grade gravel.
There have been cases where an operator mistakenly thought he was exceeding the rated capacity of his dredge when digging in very sandy or loamy overburden, but actually a large percentage of the material was sloughing into the pond and not going through the dredge. When in some clayey formations where dumping is not a problem, heaped buckets can greatly boost dredge output, but where a clay seam is sticky, decreased bucket loads that lessen a dumping problem can in effect maximize a drop in output. Any time when digging is tough and power is insufficient, output will drop. Digging should slow down when crossing a high-grade streak or channel to make sure that the treating plant can make maximum recovery.
Bucket speed will definitely vary as the characteristics of the formation vary. High-speed bucket lines with buckets up to 9-cubic-foot capacity will travel from 28 to 40 buckets per minute. For high-speed lines of larger buckets this becomes 28 to 35 buckets or more per minute. Often when bucket speeds are increased smoother running conditions result.
In determining operating efficiency, therefore, the following conditions are to be considered:
- Lost time to service and repair.
- Lost time to maneuver dredge.
- Lost output when buckets not filled to capacity.
- Lost time due to conditions beyond control such as weather, accidents, and power failures.
When formations can be dug without undue difficulties, the productive capacity for various size buckets generally considered standard are as follows:
Dredges with 54-cubic-foot buckets have been used for harbor excavations but have not been adapted to placering. The monthly production estimates shown for this size dredge are believed conservative. Production will vary from that shown above when variations are necessary to meet special digging conditions.
The above monthly production averages plotted against bucket sizes according to line speeds in figure 4 show a non-proportional relationship in the form of relatively smooth and somewhat similar “S” shaped curves. The averages used are just that, averages, and specific relationships can differ. Monthly production per cubic foot of bucket capacity increases almost proportionately with bucket size up to the 9- to 10-cubic-foot class. The maximum production per unit of capacity occurs at about the 10-cubic-foot size on the slow-speed curve and at about the 9-cubic-foot size on the high-speed curve. Beyond these, the rate of change decreases. This decreasing rate is in some considerable part due to the greater depth at which dredges equipped with larger buckets operate. Slower relative line speeds and longer pitch distances that come with heavier and larger buckets running on longer and more ponderous ladders, which must be supported on larger and less maneuverable hulls, in part exemplify the sequence in design problems that overall have the decelerating effect on production as dredge size exceeds the 9- to 10-cubic foot class.