Historically, mining processes with poor recovery rates have resulted in tailings with significant amounts of recoverable material. The extraction of this material can be achieved using dredges.
There are several factors that justify the extraction and re-processing of tailings:
– Tailings require fewer resources when compared to conventional mining. For example, a dredge can – in a single step – extract and transport the material to a processing plant located even kilometers away. This reduces costs related to capital equipment, labor, fuel and maintenance;
– Advances in re-processing technology;
– Processing of tailing allows for additional production without increasing a mine’s footprint and without requiring additional land permits.
Las Lagunas, Pueblo Viejo Tailings Dam
The project is located in the Dominican Republic, 105km north of Santo Domingo. The project owner is Panterra Gold/Envirogold of Australia. The gold and silver tailings were generated from conventional mining operations between 1992 and 1999. The original mining operation’s recoveries were poor (< 30%) and considerable amounts of ore were placed in the dam. The dam has an estimated 5,137 million metric tonnes of ore, grading 3.8 g/t gold and 38.6 g/t silver.
In 2012, Envirogold purchased one standard Ellicott 370 dredge in order to extract the tailings from the dam and pump them to their processing plant.
The dredge was used to extract material at depths of up to 8m and pump them as slurry a distance between 500m and 800m and a terminal elevation of 11m. The slurry is pumped using a 10-inch HDPE pipe. The dredge generates and pumps slurry with a 28% density. The tailings have a material Specific Gravity of 2.69 and a wet bulk density of 1.71t/m3.
In order to increase production capacity and to provide more flexibility to the operation, Envirogold purchased a second 370 dredge from Ellicott in 2013.
The dredges currently operate to provide 24 hour per day plant feed, alternating dredges to achieve the production, with a crew of one operator per dredge and two assistants per shift.
In order to better manage material and water from the dredging operation, the dam has been split into smaller sub-sections known as holding ponds. Once a section of the dam has been dredged, the void created is used to redeposit processed tailings from the plant, thus maintaining the dam’s footprint. The dredge operation utilizes the existing water at the dam. The dredged water is re-circulated back to the dredging pond so no additional water is required.
In the first three months of 2013, the first dredge extracted and pumped approximately 80,000 m3 of tailings to the processing plant.
The Ellicott Series 370 standard dredge is a diesel powered cutter suction dredge with a 12″x10″ pump and 440 HP of total installed power. It is a very powerful, reliable and portable dredge ideal for this type of application. The first unit was supplied with a conventional cutter, while the second unit included a bucket-wheel cutter.
Through a special operations contract, Ellicott Field Service technicians operated the two dredges for a period of 3 months in 2014. This service allowed the client to maximize the production of the dredges and train its operators on best dredging practices.
Market and technological trends make the re-processing of tailings more and more attractive. Due to its capacity to reliably extract large amounts of material and transport them without the need to double handle, a dredge is the ideal workhorse for these projects.