Paste Thickener Design to Achieve Downstream Requirements
Paste, or thickened tailings, have become an increasingly important method to address many of the environmental problems facing the mining industry. The term “paste” as used in this discussion applies to the full range of non-settling tailings that exhibit a yield stress such as “thickened tailings” and “mine paste backfill.” Tailings pond life, improved water recovery, tailings dam safety, and tailings reclamation/re-vegetation are some of the major drivers to investigate the use of paste. Compared to conventional thickening that produces settling slurry, paste tailings technology requires an approach that integrates the thickening, pumping, and disposal designs on a rheological basis. Each segment of the process has critical parameters that must be integrated for a successful installation to handle the yield stress. The benefits of an integrated design approach have been documented (Boger et al. 2006, Johnson et al., 2004). The selection of the paste thickener type depends not only on the feed stream parameters and underflow paste characteristics, but on-site pumping and deposition demands.
Paste Thickener Design Variables
Paste thickeners are uniquely different than conventional and high-rate thickeners. High-rate thickeners, that produce a settling slurry underflow, have been in use for decades. Thickener selection is based on feed characteristics and standard bench-scale tests combined with experience. Consideration of the downstream use of the underflow is minimal because the assumption of a settling slurry without significant viscosity (Newtonian) always applies. On the other hand, without consideration of the rheology of downstream processes, paste thickeners will likely be inappropriately designed.
The paste characteristics produced from each paste thickener depend on the thickener design and operating conditions. Thickener design features include: drive torque, sidewall height, floor slope, feedwell design, and underflow discharge arrangements. Target underflow paste characteristics can be the deciding factor for selection of many of these features. Selection of the target underflow paste characteristics is determined by starting with the application requirements, establishing pump and pipe line options, and then choosing the thickener design. This design process is carried out in reverse direction to the process flow sheet. (Johnson et al. 2004).
Design of a system for handling paste downstream of the thickener varies greatly depending on the application. For example, the design of a surface disposal for thickened tailings should consider parameters such as:
- Required disposal foot print
- Existing and greenfield dam construction to contain thickened tailings if disposal is down-valley
- Pumping requirements of higher viscosity thickened tailings from process plant to impoundment site
- Potential capacity of impoundment site
- Distribution of paste over time to optimize capacity of disposal site
- Site environmental requirements (climate, surface water, soil, etc.)
- Customer’s preferences (i.e. aversion to positive displacement pumps, thickener automation requirements)
Through the use of a paste team, composed of the thickener, transportation, and geotechnical designers, the end user can identify the unique design parameters, obstacles, and needs for the site. Paste system evaluation must consider the requirements for each parameter and provide a process design compatible with the requirements of each phase. Conflicts between the application objective and design parameters will arise. A common conflict is between the final angle of repose of the paste to maximize capacity and the transport requirements to the site. Optimization of the site life or capacity is greatly affected by the angle of repose for the deposited paste. The angle of repose is a function of the end-of-the-pipe rheology of the material. Higher angles of repose are produced initially by higher yield stresses and solids concentrations in the paste thickener underflow. Response of the paste rheology (changes in viscosity and yield stress to shear) as a result of pumping and pipe flow must be factored into design. The attractiveness of area reduction by higher angles must be balanced against higher pumping costs or the ability to produce the material. Failure to achieve the target slope significantly increases the area needed (see Fourie et al., 2006 for more detail).
One of the connecting relationships between paste system stages (thickening, transportation, application) is the rheological nature of the paste. For example, the selection of the means to transport the material from the thickener to the disposal site is based on this rheology of the material, distance, and volume. The table below shows a basis for the selection between centrifugal and positive displacement pumps depending on the rheological nature of the material. If high angle of repose (high yield stress and viscosity) for maximum site capacity is selected then the rheology dictates a positive displacement pumps may be required.
Each deposition site solution and transportation solution has a price tag (Cooke, 2005, Paterson, 2005). The overall paste system must balance process requirements with costs to the end user.
The following examples illustrate the effect of the system consideration on selection of the paste thickener.
Iron Ore Tailings
The integration process for the paste system at a South African iron ore plant identified the following high priority design parameters (du Toit, Crozier, 2012):
- A greenfield plant; tailings disposal site must be constructed
- Multiple disposal sites to be considered including new surface disposal and abandoned open pits during the life of the mine
- Water conservation and recovery a major driver
- Distance from plant to disposal site of several kilometers
- Centrifugal pumps preferred
- Paste thickener location 2-3 km from the downhill disposal site
These general issues were identified and investigated by the paste team composed of the thickener designer, pipeline design, and geotechnical consultants. Starting with the deposition design, pumping and pipe and thickener design were evaluated in an iterative fashion to design a system based on the same rheology.
The thickening study identified significant variations in the particle size distribution (PSD) fast settling flocculated solids and high solid specific gravity. The process selected for this site included two 90 m rim driven high rate thickener to be located at the plant. These thickeners recover water and produce an underflow further thickened to a paste in deep cone type paste thickener. A significant operational issue is to provide sufficient storage in the paste thickener during times when underflow to the disposal site is disrupted for changes to the distribution system.
Pumping and pipe line design from the paste thickener were based on rheological testing. Centrifugal pumps in series deliver the paste to multiple port distribution line along a constructed dike. The target underflow paste was set at a low-yield stress paste to accommodate the transport by centrifugal pumps and the distribution of the paste in the disposal site.
Two 18m-diameter by 12m-sidewall height Deep Bed™ Paste Thickeners were used for this project. The floor slope was 45 degree, the feedwell provided with self dilution ports and the rake drive selection provided excess torque. This thickener design was accommodates the extreme feed variability of the tailings produced by the beneficiation plant.
Bauxite Ore Case Study
The integration process for the paste system at a Brazilian bauxite ore plant identified the following high priority design parameters:
- Greenfield project; retaining dam to be constructed
- Approximately 3 km distance from plant to preferred disposal site
- Preference of centrifugal pumps for tailing transport
- Location of the thickener at the plant rather than at the disposal site
- Disposal site in a shallow narrow valley
- High tonnage
- Very fine particle size distribution
A multidiscipline team was formed with the customer’s geotechnical department, a pumping and pipeline specialist and the thickener manufacturer. Pilot scale tests where conducted to determine thickener characteristics and produce material for deposition slope tests. The target underflow was selected to be a low-yield stress thickened tailings. This allowed the transport of the underflow the 3 km to the disposal site with centrifugal pumps. There is sufficient area for a low angle of repose at the disposal site that is appropriate for the slope of valley. The thickener options for high tonnage application are either multiple deep cone thickeners or a single high density thickener. This plant target low-yield stress underflow would not require the higher yield stress and underflow solids produced by a deep cone type paste thickener. Therefore two 45m-diameter WesTech HiDensity™ Thickeners are used.
Reclamation of Coal from Existing Tailings Pond
A plant to reprocess coal refuse (tailings) to recovery the fine coal was plagued by deposition problems of the reprocessed tails. The process design includes dredging an old coal pond, processing the slurry to recovery the fine coal, and replacing the new tailings back into the same pond. The dredging of the pond is being conducted near the dam where the finer particles had settled. The new process tails were to be placed toward the opposite end of the pond where the coarser particles deposited during the original placement.
The startup of this reprocessing plant used high rate thickeners to dewater the tailings stream and deposited a slurry in the pond. This plan of dredging from one side of the pond and returning the tailings to the other side of the pond resulted in a large fines recirculating load as the tailings did not settle but remain suspended and flowed to the dredging location. These recirculating fines overloaded the plant, greatly reducing recovery and grade of the fine coal product. It was determined through testing that the reprocessed tails could be thickened to paste and deposited subaqueous in the pond. The paste does not re-suspend when entering the water in the pond but flows subaqueous on top of the old coarse tails. An 18m diameter Deep Bed Paste thickener was installed. The image below shows the pond being processed (dredging right of center near dam) and the delta formed by subaqueous deposited paste tailing (center bottom). The process plant is in the center top and the paste thickener located toward the other end of the pond (left of center).
The thickener manufacture or consultant has several thickener types from which to select. Each thickener type also has several design features that must be finalized for each application. Many of these features are dependent on the duty the thickener has to perform. The target underflow characteristics play a very important role in selecting the thickener type and many of the features.
The downstream system requirements are used to select the target underflow from the thickener. Installed paste thickener tailings handling systems have repeatedly demonstrated the correct approach to design is to use a multi-disciplinary team of the manufacturer, transportation specialist, and geotechnical consultant. The thickener designer must consider the downstream rheological requirements and constraints to choose the correct type and size.
Boger, D., Scales, P. and Sofra, F. (2006) Rheological concepts. Paste and Thickened Tailings – A Guide. R.J. Jewell and A.B. Fourie (eds), Australian Centre for Geomechanics, p. 27.
Cooke, R. (2005) High concentration tailings transportation system optimisation. International Seminar on Paste and Thickened Tailings, Santiago, Chile, April, 2005.
Fourie, A., Bentel, G., Williams, P. and McPhail, G. (2006) Above ground disposal. Paste and Thickened Tailings – A Guide. R.J. Jewell and A.B. Fourie (eds), Australian Centre for Geomechanics, p. 152.
Johnson, J.L., and Slottee, S.J. (2004) Paste Technology: Success is in the Approach, Tailings and Mine Waste 2004, Vail, October 2004, 7 p.
Paterson, A.J.C. (2005) Determining the optimum location of a high rate thickener for a thickener tailings system. International Seminar on Paste and Thickened Tailings, Santiago, Chile, April, 2005.
du Toit, T., Crozier, M., Khumani Iron Ore paste disposal and water recovery system, The Journal of The South African Mining and Metallurgy Industry, Volume 112, March, 2012