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operationbasedonthecapitalinvestmentcostsofthebucketladderdredgein1980.Koesmadietal. 975! suggested that prior to planning an offshore dredging operation at least 10 years' life of the reserve must be proven. Bucketladderdredgesandcuttersuctiondredgesarethe maindredgetypesusedinminingplacerdeposits. Of the other alternatives, clamshells, dragheads and jet lifts have a limited scope for mining deposits of loose, uncohesive alluvium Macdonald, 1983; Cross, 1979; Herbich, 1978; and others!. The bucket ladder dredges are preferred when mining tight and hard formations having clay lenses and bedrock outcrops, whereas the cutter suction dredge is used mostly for civil applications and overburden removal IHC Holland, 1983!, Some of the rationale for this preference is based on the following: ~ The bucket exerts mainly anupward shear force on the bank, while the blades of the cutter work against a resistance in compression, thus increasing the power consumption. ~ The cutterhead digs efficiently only in its undercutting swing, whereas the bucket ladder dredge is digging in both swing directions, ~ Power and maintenance costs incresases with hydraulic dredges when digging in compact, coarse, more abrasive and high density mineral deposits, The hydraulic dredges tend to produce turbidity and a hydraulic classification of the material around the suction inlet, e.g. heavy gold particles settle out faster andfinesmaybefloatedawaybytheturbidity,whilethesandsandgravelarepulledintothesuction. ~ It is more difficult to maintain a constant slurry density feed to the processing plant from cutter suction dredgesthanfrombucket ladderdredges,Hydraulicsystemsoftenproducequantitiesofwaterinexcess of that needed for the processing operations. ~ Plugging of the cutter or pump with buried fibrous roots and timber creates disruptions to the operation. A bucket ladder dredge system presents some problems also, such as: ~ Losses of valuable minerals from the bucket during the discharge into the drop chute at the top tumbler. Save-aHdevices must be installed to catch the spilled material. ~ The initial capital costs and maintenance costs of a bucket ladder dredge are higher than a hydraulic dredge. ~ The maximum dredging depth is limited, compared to hydraulic dredges, ~ A bucket ladder dredge usually requires more personnel than other types of dredges. Mechanical components and electronic control equipment, i.e. dredge profile monitors, drive systems, mooring techniques, processing plant systems and equipment, and tailings discharge systems have all been developed or evolved towards higher dredge performance, deeper dredging and greater economic recover- ies. A new concept inbucket chain designincreased the bucket capacity to a maximum of 0.88m'of water depth Donkers, 1980!. For easy handling, transportation and lower maintenance costs, modular dredging units have been recently marketed by IHC Holland as pump unit, cutter unit and main and side pontoon modules. To combine advantages from the bucket ladder and hydraulic dredges, a new type of bucket was designed {Figure 1! for the conventional bucket wheel dredge IHC Holland, 1983!, The characteristics of four com- mercial types of IHC bucket wheel dredge are presented in Table 4, and the schematic features in Figure 1. Model number indicates totals machinery output in hpPDF Image | Dredging Applications of High Density Polyethylene Pipe
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