Mining of valuable minerals or metals usually involves the removal of vast quantities of earth from the ground in order to obtain a relatively small amount of the desired material. The material left over after the refining process is referred to as mine tailings.
Mine tailings typically consist of process effluent and finely-ground rock generated at the mine processing plant. Tailings have no financial gain to a mine or mineral operator and as such, tailings are usually stored in the most cost-effective way possible to meet site specific factors as well as adhere to relevant environmental regulations.
Tailings can be quite abrasive and often corrosive. The selected pipelines and equipment must therefore be able to adequately withstand the effects of the tailings. Valves used in tailings transportation systems are often selected such that they feature an enclosed operating mechanism to ensure the valve will be operational when operated.
The physical properties of the tailings, tonnage and local site topography often dictate the type of transportation system required to manage the tailings both cost effectively and efficiently. Tailings are sometimes used as backfill in underground mines, stored in open pits, dried and stacked, or pumped through pipelines into surface tailings ponds on site to allow the solid particles to separate from the water and settle on the bottom. The often-remote location of tailings ponds means that reliability, safety and low maintenance are vital since access is not always straightforward or convenient.
Increasingly strict environmental regulations as well as challenging economic conditions demand tailings piping systems that transport tailings at higher concentrations, travel over longer distances and are designed to have a longer lifespan.
The tailings slurry consistency is dependent upon the mass percentage of solids, particle size distribution (PSD), type of tailings, specific gravity (SG) as well as the extent of thickening at the processing plant. Tailings are often pumped at a solid content of 25 % for low specific gravity coal tailings to over 50 % for hard rock metalliferous tailings.
Prior to the transfer of tailings to a pumped storage facility, tailings often undergo a thickening process. This thickening step facilitates the recycling and reuse of the process water which is transferred back to the mineral processing plant. This reduces water losses and reduces the overall water demand at the processing plant.
The ability of a pumped piping system to transport the tailings slurry is a function of the slurry rheology. In order to determine if tailings can be pumped with existing pumping equipment, the viscosity and yield stress need to be determined. The higher the slurry yield stress, the more difficult it is to pump the slurry (for a given pump type). The yield stress is strongly dependent on the solid content of the slurry.
The design of pumped tailings pipelines can be quite complex owing to the variability of tailings properties and flow rate for the mining operation. A system must therefore be designed to allow for this variation in operating conditions. The design of these systems, usually over large distances with different terrains and elevations can become quite complicated and extremely difficult to design accurately by hand or using a spreadsheet.
The use of centrifugal pumps is still quite a common method of pumping tailings in piping systems. Many of our customers use FluidFlow to automatically size slurry pumps which can be costly plant items. As such, it is important to ensure the pump sizing and selection process has been completed accurately and that the pump(s) are operating at a safe, reliable and efficient duty point.
The use of an appropriate software tool such as FluidFlow allows engineers to design, model and optimise tailings transportation pipelines and consider variability of tailings concentrations and properties. Any risk of saltation or pipe blockage is identified helping the designer develop a safe, reliable and effective system. Solids derating of centrifugal pumps can also be considered and evaluated.
FluidFlow has been used successfully in the mining and metals industry for many years to design and model slurry transportation systems as well as mine dewatering systems, mine venting systems to name a few. Figure 1
Some of our users include BHP Billiton, Rio Tinto, Rusal Aughinish, Aurecon, WorleyParsons, Emirates Global Aluminium (EGA), Ambatovy and JACOBS. The Ambatovy site in Madagascar features a pumped tailings pipeline which transfers the tailings over a distance of 15km to a 750 ha storage facility.
- Paterson and Cooke.