In the tailings thickening continuum, filtered tailings are typically seen to be at the opposite end of the spectrum from conventional slurry tailings
laboratory testing and analytical predictions, for example if particle breakdown is foreseen( since particle breakdown may result in decreased permeability and a possible development of a phreatic surface).
As discussed by Davies and Rice( 2001) the parameters that need to be understood include: n Shear strength and shearing behaviour; n Hydraulic conductivity characteristics; n Moisture-density relationship.
Davies and Rice( 2001) continued that the moisture-density relationship indicates in-situ density expectations for compacted materials, and that there is often a sensitivity of that degree of compaction for a given moisture content, recognising that the optimal degree of saturation is usually between 60 and 80 %.
The present authors would like to add to this list of important soil parameters – the Atterberg limits. With the ever-improving technology of filter plants, clayier tailings are now being produced more and more often using filtering technologies. Designers should consider and indeed anticipate changes in orebodies and assess seemingly subtle changes in Atterberg limits and the associated impacts of those changes on the material shear strength, shearing behaviour( ie contractive versus dilatant or ductile versus brittle behaviour), and permeability.
At one mine site the author has worked at, the tailings were identified as being low to nonplastic based on geological considerations. However, once in production, the tailings were found to have a greater amount of clay, and the design target moisture content fell between the liquid limit and the plastic limit; thus, the tailings at that moisture content act as a plastic, rather than as a granular material. To compound the problem, the filters could not remove as much moisture as designed due to the higher fines and clay content. This combination of higher moisture content and higher plasticity is far from optimum and the deposited filtered
tailings behaved adversely to the design intent, requiring a significant amount of engineering analyses, instrumentation and insitu testing.
This is a convenient place to discuss moisture content. There are two forms of the gravimetric water content equation. Metallurgists usually use moisture content by total weight, and geotechnical engineers use moisture content by weight of solids. The difference in these formulas can lead to considerable confusion unless this difference is understood by the process plant designer and the geotechnical engineer and the difference is considered in the design. The difference in these two moisture contents may be minor, but if working with a material that may exhibit a plastic behaviour, the difference can be very meaningful.
A Davies and Rice( 2001) report on the trafficability of newly placed filtered tailings, provided the following observations. Filtered tailings are generally produced at or slightly above their optimum moisture content and as such trafficability problems can arise especially in wet environments. The tailings surface can quickly become un-trafficable resulting in significant placement problems. Facility designers must take this matter into account to ensure the operation is not unduly hampered during the wet season.
The degree of compaction required to achieve trafficability can be higher than the compaction required to achieve slope stability goals. This can work favourably for geotechnical requirements.
In some designs, there is a requirement to compact the tailings to a higher density in at least the outer reaches of the facility. This requirement increases the need for construction quality control.
For facilities designed to accommodate materials that are produced during seasonally unfavourable periods, such as rainy seasons and / or cold seasons( usually this requires storage in a portion of the facility that is remote from the outer slopes of the facility) these trafficability issues during unfavourable weather may be greatly reduced or eliminated altogether.
Project experiences
Technology for preparing filtered tailings is currently considerably more expensive per tonne of tailings stored than for conventional slurry systems. Filtered tailings facilities often have regulatory attraction, and smaller footprints can be accomplished with this type of facility( Davies and Rice, 2001). A significant benefit of filtered tailings is the probable ease of reclamation of the facility, compared to conventional or thickened tailings facilities, and possibly for paste tailings facilities too. The possibility of placing a closure cover immediately upon cessation of operations can be a very attractive option in a closure plan.
The authors have gained experience with a variety of different applications for filtered tailings during the past few years, including designs, construction and operation. Three of those applications are worthy of a brief discussion here.
One project completed for a confidential client was the design of a filtered tailings facility in Latin America that would be built upon an existing, though inactive, tailings facility. The current inactive facility is a typical spigotted side-hill style‘ sand dam’ where tailings had been spigotted onto a beach, and the outer‘ wall’ was raised by excavating portions of the sandy beach material for use in constructing a small berm adjacent to and upstream of the wall, upon which the slurry deposition line was placed. By continuing this operation slowly and allowing the beach to adequately desiccate, a stable structure can be developed. When design work commenced on this project the facility had been inactive for several years, and the upper surface had become quite firm. This condition gave the owner a false sense of comfort as the underlying silty portions of the impounded tailings upstream of the sandy beach mass still exhibited a phreatic surface and was still in a loose, contractive state.
The owner modified a planned site investigation which had included cone penetration tests( CPTs). The modified site investigation included only standard penetration tests( SPTs) together with sampling and laboratory testing. The use of the CPT( an electronic instrument equipped with strain gauges and a transducer to measure and record resistance to penetration at its tip and sleeve as the probe is pushed through a soil deposit as well as pore-pressure both during penetration and during pauses in the penetration process) is widespread in assessing materials such as those present in a tailings facility.
As the SPT approach was taken rather than using the CPT, little information was gained regarding the in-situ pore pressure regime within the facility. The owner agreed to allow piezocone testing during upcoming design stages. The selection of filtered tailings for this project was made based on the following design considerations.
The limited amount of space available for tailings storage made the higher density achieved by filtered tailings very desirable. To reduce the potential for a rapid loading condition in the underlying tailings slimes, filtered tailings was the
P2 International Mining | JUNE 2017 Supplement