Tailings Handling Techniques
Tailings are transported to their final storage place in a number of different ways. The most common is slurry transportation in a pipeline from the thickeners (normally located at the processing plant) to deposition points located within or around a surface tailings storage facility. The tailings properties, tonnage and site topography dictate the type of transportation system required to manage the tailings both cost effectively and efficiently.
The types of tailings transportation systems used today are as follows:
- Pumped pipeline systems: Pipeline transportation using centrifugal or positive displacement pumping systems.
- Gravitation pressure pipeline system: Where the storage facility is at a lower elevation compared to the processing plant and the tailings can flow without the need of a pumping system. Energy dissipation such as drop boxes or chokes may be required to prevent high flow velocities and subsequent wear in the pipeline system.
- Channel (launder): An open (or capped) concrete launder system where tailings flow gravitationally with a free surface as a low density slurry (conventional tailings). This transport system is not suitable for high density thickened tailings due to rheological properties.
- Wet cake (filtered tailings):
- Conveyor: A conveyor belt is used to transport filtered tailings (or coarse cyclone sands) as a wet cake (normally less than 18% moisture content) to a disposal point, normally called a dry stack facility. Occasionally, if the tailings are coarse sand, this conveyor product can be blended with waste rock in a waste dump to form a co-disposal facility.
- Truck: Particularly for low tonnage tailings operations, the use of trucks are more viable for haulage to the disposal site. Difficult terrain may not permit construction of a conveyor transport system.
Slurry pumping systems
Tailings pumping systems can be complex. The tailings properties and flow rate for a mining operation are never uniform and so a system has to be installed that allows for variation. Today, with the increase in popularity of High Density Thickened Tailings (HDTT) and paste, these pumping systems are more advanced as principally higher pressures are required to transport a material that is less viscous. In conventional tailings systems, the higher volumes of water (i.e. lower % solids concentration of the slurry) reduces the yield stress of the tailings where-by the water acts as a transporter for the tailings solids. This can be observed in tailings facility failures, where a breach of the facility due to high volumes of water (e.g. overtopping) generally have large run out distances as the water is transporting the tailings solids. In tailings facility breaches where the water contained was not greater than normal conditions (e.g. seismic trigger failure), the run out distances are usually much less.
Pumping using centrifugal style pumps are still the most common type of pumping system used in tailings slurry transportation. Quite often, when a tailings trade-off study for different tailings discharge techniques is performed for a proposed storage facility, the limits of centrifugal pumping are almost always more cost effective than paste transportation (particularly for high tonnage operations). However, there are site specific factors that dictate this (e.g. limited land and water availability where paste is the only viable option).
Paste tailings can be pumped by centrifugal pumps, up to a limit, which is normally determined by the yield stress in the first instance which is a function of the %solids concentration of the slurry. Beyond the limits of centrifugal pumps, positive displacement pumps are required for paste tailings. This is typical done using piston diaphragm or hydraulic piston pumps. Peristaltic (hose) pumps may be used for very low tonnage systems.
Wet cake transportation
Filtered tailings (dry stacking) style of tailings storage is becoming more popular mainly due to the growing need to reduce makeup water costs in the mining industry. Over the recent years there has been a significant advancement in the field of dewatering of tailings using pressure and vacuum filtration techniques. Typical disadvantages to filtration are the associated power and maintenance costs (e.g. filter cloth replacement) that can vary depending on the tailings properties. For this reason it is important to consider laboratory or pilot testing to determine the feasibility of filtration for a particular type of tailings.
The transportation of wet cake tailings is typically achieved by conveyor or truck transportation. Conveyor systems require a minimum tonnage per hour to be economical to install and operate, where-by truck transportation becomes more feasible.
Typically there are two types of conveyor systems used for delivery of wet cake tailings. These are similar to the style of conveyors systems used for stacking and reclaiming coal (bucket wheel excavators), but usually on a much smaller scale:
- Plough: A movable plough is used which directs the wet cake off the main conveyor on to a delivery conveyor, or direct discharge to the storage area. This plough typically advances at a slow rate to allow a small conical pile of tailings to be formed in a line parallel to the conveyor. This style of system is used at La Coipa in Chile.
- Radial Stacker: A radial stacker style of conveyor is mostly suited to low tonnage operations, particularly where support using truck and shovel is required. Occasionally a small radial stacker can be incorporated with a plough style of system to allow better management of the final placement of the wet cake. The radial stacker can rotate, normally to around 120 degrees, and can feed on to separate delivery conveyors if required.
For more information regarding dry stacking of tailings, see the relevant page here.