Water balance is an essential design requirement for modern mineral processing facilities.
Water efficiency has become dominant in project design and regulatory approvals, with the response of flowsheet designers and plant operators to discharge higher density slurry to residue storages. Thickener technology and flocculant performance have advanced as water balance and conservation demands on projects have increased. This paper discusses the changes to testwork and design methods that are required for thickeners and flocculants to successfully produce and deliver high density residues.
To meet the needs of operators, the thickener designers and flocculant makers have adapted through conventional, high rate, high compression and more recently paste configurations. As underflow densities trend higher, so rise rates have increased by an order of magnitude from below 1 m/h to over 10 m/h, and test procedures have become more involved. Exponential increases of rake torque to cope with high density, non-Newtonian slurry have been accommodated by improvements in drive and structural components. The high shear requirements that follow high flocculant doses influence discharge geometries and pump selections.
The interaction of high density slurry characteristics with flocculant selection, equipment design and pumping requirements are discussed to provide background for plant designers and operators.
Due to increasing environmental standards, and a more general desire for sustainable resource development, water consumption or savings and recycling of process water are increasingly prominent in design and operation of modern mineral processing facilities. Estimates of water balance are essential design requirements for these facilities which will determine consumption, discharge and recycle arrangements for project development and ultimately during plant operation. In most mineral process flowsheets the overall water consumption or recycle are managed by thickener design, installation and performance. Construction and operation of tailings or residue storage and impurity removal requirements will also be driven by final slurry characteristics and hence the dewatering ability of thickeners. In particular the pulp discharge density from the metallurgical plant will largely determine water flow to tailings and hence overall consumption.
Tailings thickeners can control the density of the solids to be sent to a tailings storage facility (TSF). During beach deposition the density difference between the discharge slurry and initial settlement, at which water decant or leakage finishes, will largely determine the rate or proportion of water that may be available for decant after beach and decant pool evaporation. Return water may be available for recycle to the plant if its salinity, or other soluble and suspended load, is compatible with the metallurgical processes.
Concept or scoping studies will usually have few details available on slurry thickening such as settling curves. Samples of suitably representative feed or discharge slurry usually become available in later project development stages. However, the process design engineers can target higher underflow densities than would traditionally be assumed thanks to evolution of thickener theory and practice over the past twenty years in two particular areas:
- flocculation performance testing
- thickener scale-up and design.
Mineral processing operators have investigated, trialed and applied new or different flocculants in an attempt to increase thickener underflow density, thickener overflow clarity or decrease the flocculant dose or process consumption. Plant metallurgists or operators have also searched for cost effective ways to upgrade or modify their existing thickeners for the same reasons.
Water usage efficiency has become dominant in project design and regulatory approvals, and in response flowsheet designers and plant operators have discharged higher density slurry to residue storages.
The thickening process
This section summarises the wealth of available literature since the first mechanical thickeners were patented in early 20th century and commercialised with various mechanical arrangements (Dorr and Bosqui, 1950). The process of slurry thickening involves concentration by gravity settling of solid or flocculated suspended particles. The primary function of a continuous thickener is to perform this particulate concentration so that steady-state material balance is achieved in a controllable and predictable manner. The concentrated solids being withdrawn from the thickener in the underflow is at a rate that matches the more dilute feed supply over selected time frames.
The thickening process is characterised by pulp density profiles that transition between three vertical regions or zones, namely:
- Free settling, where separated particles or floccules allow unhindered settling as represented by Stokes Law for the nominated size distribution. Convention effects may be ignored in laboratory testing, but must be factored into commercial designs.
- Hindered settling, where the particles are self-constrained within the bed by their interaction during accumulation, clustering or agglomeration at rates that reflect solids concentration, floccule formation rate and particle density.
- The compression zone, where the particle settling rate is constrained by mechanical support of the thickener walls and hydraulic pressure or compressive head of overlying slurry and liquor, balanced by underflow withdrawal and upward percolation of water released from the bed.
The cross-sectional area of a thickener, balanced by the slurry feed rate, determines the rise rate which is an important parameter in determining the clarity of the overflow. An inventory or depth of compressed pulp is maintained to reach the desired underflow pulp concentration.
A thickener has several basic components:
- a tank to contain and compress the slurry including bottom cone or discharge drum
- feed piping and a feedwell often incorporating feed slurry dilution, to create optimum flocculation conditions
- a rake and drive mechanism to both enhance the dewatering of the settled bed and move the solids towards the underflow cone
- an underflow withdrawal system occasionally with additional shear features prior to discharge pumps
- an overflow launder and water or liquor discharge system
- a bridge to support the above mechanical systems and arrangements.
Types of thickeners
Ultra-high rate thickeners
These are typically tall tanks without a mechanism but utilising vendor-specific internal distribution structures. They may operate at very high rise rates (20m/h) usually accomplished with higher flocculant dosage. The tall tank provides both a deeper compression zone to compensate for the lack of dewatering effect of the mechanism and larger clarification zone required for the high rise rate. They offer an advantage of smaller footprint and simplicity of operation and maintenance.
Conventional and high rate thickeners
Care needs to be taken with the nomenclature of conventional or high rate thickeners. The critical difference is the use of feed dilution and flocculation in high rate thickeners over conventional thickeners; however, the high rate thickener has become so ubiquitous in the mining industry it has become the ‘conventional’ choice.
Large diameter to height ratios are a feature of conventional thickeners, which can have diameters beyond 100 m. High rate thickeners can handle more solids tonnage per unit area due to a faster settling rate than conventionally designed thickeners and in all but the most extreme cases are limited to around 50 m diameter.
To achieve a similar bed residence time and therefore similar underflow density to a conventional thickener the smaller diameter high rate thickener may have a deeper bed. Designs vary with the arrangement of bridge or column mounted drives with or without a lift mechanism, and may use a variety of rake configurations. Feedwells are usually variations of a cylinder, often with internal proprietary appurtenances, to contain and channel the feed into the tank.
Extensive research has been undertaken over the past 20 years to understand the effects and mechanism of flocculant interaction under various shear conditions. More detailed explanations of these interactions between flocculation and thickener design are described by various authors, eg Perry and Green (1997), Heath et al (2006), Fawell et al (2009), and Scales et al (2015).
Almost all mineral slurries have an optimum feed solids concentration that will maximise the settling flux. At concentrations higher than optimum a hindered settling function causes a lower settling flux. At a concentration lower than the optimum excess flocculant may be consumed in an effort to ‘bridge’ between widely disparate particles. Most vendors will offer proprietary automatic feed dilution systems for times when the feed solids exceeds the optimum and in the rare case of feed solids being too low an underflow recirculation loop may be employed. The optimum solids may vary from 3-15 wt per cent solids and be dependent on particle SG, particle size distribution and mineralogy. The optimum solids for each case is typically assessed as part of the testwork program. According to Perry and Green (1997) most applications have a threshold flocculant dosage at which noticeable increase in settling capacity occurs, although more is not always better with lower underflow density can be experienced at higher rates due to over-flocculation.
Diluted flocculant solution (perhaps tenfold prior to injection) can be added to a thickener either in the feed line or feedwell, or in multiple points. Proprietary feedwell designs are offered by vendors of high rate thickeners that are claimed to help optimise flocculation. High rate thickener designs allow flocculated particles to exit the feedwell under low shear conditions into a free settling zone directly above the hindered settling zone. This arrangement allows flexibility for operators to optimise flocculant dose and position across the feedwell to control of the bed level/inventory within the thickener, and so optimise the relationship between the settling flux and density.
These operational controls will maximise floccule formation rate and size to boost thickener performance. Plant and laboratory tests or trials can be conducted to investigate these effects.
High density/high compression thickeners
High density or high compression thickeners (HDT) are designed to produce underflow rheological yield stress of up to 100 Pa. HDT’s accomplish this effect by using deeper mud bed heights up to circa 2 meters and above to enhance compression and a rake mechanism with pickets to create dewatering channels in the consolidating bed. The mud level hence tank wall height is greater than high rate thickeners. The feed and flocculation conditions are consistent with high rate thickeners.
The thickener mechanism generally will have more robust rake design with increased torque capacity compared to a high rate thickener with the same diameter (Bojcic, 2000). The rake torque factor can be three to four times higher and will vary with underflow density according to an empirical relationship that depends on rake shape and drive configuration. The required rake torque for design is assessed from yield stress rheological measurements at the target underflow solids.
Underflow slurries will approach concentrations five to 10 per cent below a vacuum filter cake formed from the same material. Ultimate underflow density at discharge can be limited by rake strength, drive capacity or underflow delivery capabilities. Pumping requirements to deal with more viscous underflow slurry are necessary for delivery over significant distances, due to high yield stresses.
High density/high compression thickeners are often designed to produce slurry rheology compatible with centrifugal pumping capabilities.
Paste thickeners are further extension to the yield stress/solids concentration continuum as shown in Figure 2. They are designed to have compression zones deeper than either high rate or high compression thickeners, generally 3 m or higher bed depth. The greater retention of compressed slurry requires design allowance to deliver selected underflow density for a given flux rate.
This increasing density phenomenon arises from three factors which work together to increase pulp density:
- Longer bed settling time. Settling flocs move relative to one another and release interstitial fluid when they are in compression. Increasing bed residence time achieves higher density underflow as more fluid is removed.
- The net weight of the bed creates a compressive force on the settling flocs. This force increases with depth, so that the floccules at the bottom of the bed experience the highest compressive force. This represents the driving force to expel interstitial fluid and hence increase bed density.
- The influence of a mechanism passing through the consolidating bed is postulated to create dewatering channels for the easier release of the interstitial fluid.
The deep cone style paste thickener was developed in the 1960s and 1970s in the British coal industry (Abbott, 1973). Paste thickeners were introduced by Alcan into the alumina industry. FLSmidth (previously Dorr Oliver Eimco) licensed the technology and now market them as Deep Cone® thickeners, while other vendors offer competitive, proprietary paste thickener designs,
Applications such as laterite thickeners, paste backfill and Bayer washing circuits are examples of recent paste thickening applications. Over 200 installations of paste thickener applications have been documented. Most applications are for tailings disposal requirements, and have been described in other publications, (eg Bedell, 2006), but water savings projects or density improvements within the processing plant have also been the objective of increased slurry thickening.
Because paste thickeners produce high apparent underflow viscosity, the consequential slurry delivery at concentrations which avoid segregation of fines and coarse particles is important. The formation of a free-liquid pond or decant pool on the TSF surface can be minimised or avoided. This practice of paste layering, also known as thin layer deposition, is useful for enhanced storage or underground paste-fill operations with milled tailings.
The maximum design size of paste thickeners, currently up to 40 m diameter, can be limited by the height of tank required and the cost associated with this. The largest, installed, elevated tank configuration is 24 m. Once this overall height is exceeded economic considerations strongly favour an on-ground tank option that is only practical with sufficient topographic availability. More emphasis can be given to specific testwork during design for individual applications (Roebl, 2007) to address these issues.
Thickener evolution, type of thickener and resulting underflow solids production is summarised schematically in Figure 1. The importance of the relationship between underflow density and yield stress in thickener applications is illustrated in Figure 2.
Abbott, J, 1973. Coal preparation plant effluent disposal by means of deep cone thickeners, 6th International coal preparations congress, Paris, October 1973.
Bedell, D, 2006. Chapter 7, Paste and Thickened Tailings – A Guide. Jewell, R.. and Fourie, A, Editors. Australian Centre for Geomechanics. 2006.
Bojcic, P, 2000. Development of a model for predicting thickener rake. PhD Thesis, Julius Kruttschnitt Mineral Research Centre, The University of Queensland
Dorr, J, and Bosqui, F, 1950. Cyanidation and Concentration of Gold and Sliver Ores, 2nd Ed., MGraw-Hill Book Company
Fawell, PD, Farrow, JB, Heath, AR, Nguyen, TV, Owen, AT, Paterson, D, Rudman, M, Scales, PJ, Simic, K, Stephens, DW, Swift, JD, &sher, SP 2009, ‘20 years of AMIRA P266 “Improving Thickener Technology” ‐ how has it changed the understanding of thickener performance?’, in RJ Jewell, AB Fourie, S Barrera & J Wiertz, Proceedings of the 12th International Seminar on Paste and Thickened Tailings, Australian Centre for Geomechanics, Perth, pp. 59‐68.
FL Smidth, formerly Dorr Oliver Eimco 2007. Promotional literature and private communication to URS
Heath, AR, Bahri, PA, Fawell, PD & Farrow, JB 2006a, ‘Polymer flocculation of calcite: experimental results from turbulent pipe flow’AIChE Journal, vol. 52, pp. 1284‐1293.
Perry, R and Green, D, 1997. Perry’s Chemical Engineers Handbook, McGraw-Hill International Edition 1997.
Roebl, A, 2007. The Design and Commissioning of a Scale Thickener Test Laboratory for Measurement and Control Testing, Paste 2007 Conference, AGC Perth.
Scales, P, Crust, A, and Usher,S, 2016, Thickener Modelling – from laboratory experiments to full-scale prediction of what comes out the bottom and how fast, Proceedings of the 18th International Seminar on Paste and Thickened Tailings, Australian Centre for Geomechanics, Perth.