Dredging is an underwater excavation performed to recover gold from placer deposits. Initially, the excavation need to make a channel, the continuous dredging will let you know the potentiality of the deposit. Obviously, dredging is not exclusive for mining purposes; this operation can be employed to clean lakes and different aquatic environments that must stay safety for navigation purposes.
Essentially, a dredge is a machine that removes material from aquatic environments. In this way, the process involves a digging operation of underwater placer deposits by using two types of equipments, cutter head and cutting bucket line. During the operation, digs, scrapes removes material and send it onto the platform that is located in the dredge. Normally, the ore is treated on the dredge by washing, screening and concentration operations. The non-valuable material is returned to the water. The final place is selected considering environmental aspects.
People have been dredging placer deposits for a long time. At the beginning of the last century, dredges were effective to extract gold. Keeping the dredge in the right place, the extraction was very encouraging and the process has become a mixture of technology and operator skills.Sometimes, people ask about the reason of dredging operation. The reason is based on the concept that as river moves with turbulence and variable velocity, heavy materials such as free gold, will settle at the bottom. It is important to mention that streams have periods of high turbulence during some periods and the formation of gravel bars is common. This material is light and contains silt and clay particles do not settle out until the stream lost its velocity. The material deposited at the bottom is sediment and contains gold particles. Then, the idea is to mine this material.
Dredging Operation
The dredging equipment can be categorized in three groups based on the digging or raising system. The first group is formed by cutterheads and suction pumps; the second one comprises bucket wheel elevators, and the last one by mechanical shovels. Cutterhead pipeline dredges work best in large areas with deep shoals, where the cutterhead is buried in the bottom. Water pumped with the dredged material must be contained in the disposal site until the solids settle out. It is then discharged, usually back into the waterway. This method of dredging is not suitable in areas where sediments are contaminated with chemicals that would dissolve in the dredging water and be spread in the environment during discharge. The second type employ buckets to removes the material and its work is very competitive with the first group. The last type employs a clamshell to extract the material. Although, this dredge is not very used in gold operations, it is important to mention that this dredge lowers the opened bucket from the end of a crane into the sediment. After penetrating the material, the bucket jaws are closed and the material is picked up until the surface and is usually dumped into a barge or the place to be processed the material.
Cutterheads and suction pumps are employed to remove material by using a pumping system. Once the suction device is at the bottom, water is added to fluidize the sand so that it can be pumped to the surface for further treatment. This dredge is simple, but there is a disadvantage, heavy material can be lost during the fluidization and also sometimes, heavy material is difficult to be removed due to its geological formation. Perhaps, one way of reducing this problem is to employ special cutterheads whose duty is to facilitate the movement of material out of the bottom. The cutterheads design has been improved with the years. One design considers cutters with curved blades and the driving mechanism is assembled on a ladder with the suction pipe. The ladder is lowered to the alluvial zone and the digging, fluidization, and pumping actions are performed. Normally, the concentration plant is placed out of the dredge.
Bucket wheel elevators consist of a series of steel buckets supported by a dredging ladder. The dredging ladder transports buckets with material whose treatment is performed by gravimetric devices. Once processed the material, the non-valuable material is discharged. This type of dredges needs a special study due to the investment cost is high. The study must include enough reserves; otherwise the project will produce losses. The selection of a dredging equipment is influenced by the type of material to be dredged, the amount of clays and inorganic material, the rock type, the material formed by glaciations, size deposit, plant treatment location, throughput required to achieve expected incomes, and the type of environment.
Historical Gold Dredging Operations | First Dredges used to Recover Gold | Gold Gravel Processing | Geology of Traditional Dredging Areas in California | Dredge used in California | Dredge Buckets | Dredge Stacker | Screens Employed in Gold Dredging | Gravity Equipment and Gold Dredging Operation | Gold Dredging Costs | Aspects to Study before Dredging Potential Gold Zones | Exploration of Potential Gold Dredging Deposits