A description of the sedimentation process during dynamic thickener operation

Transcription

1 Paste 2014 R.J. Jewell,.. Fourie and. Van Zyl (eds) 2014 ustralian entre for Geomechanics, Perth, ISN xx-x-x description of the sedimentation process during dynamic thickener operation.j. Vietti Paterson & ooke, South frica F. unn Paterson & ooke, South frica bstract n understanding of the sedimentation process has focused largely on settling under static batch conditions however all installed thickeners are operated under dynamic conditions in which slurry is fed to and withdrawn from continuously. description and comparison of the sedimentation process under static batch and two dynamic modes is presented. The two dynamic modes are defined as dynamic batch mode which typifies the thickener under start-up conditions in which only feed reports to the thickener and no underflow is withdrawn and dynamic continuous mode which typifies the thickener during continuous operation conditions in which both the feed and underflow slurry streams are operational. The paper indicates that when operating a thickener under dynamic continuous mode by maintaining the mud bed interface level at a set height, a steady state is achieved in which the underflow density is lower than that achieved when the thickener is operated in dynamic batch mode at equal mud bed height. These findings imply that in order to predict underflow densities which more accurately reflect the real operating conditions of thickeners, consideration should be made for test work to be conducted under dynamic continuous sedimentation conditions rather than the current batch methods. 1 Introduction Eleven years after the development of the continuous thickener by orr in 1905, oe and levenger described and classified the sedimentation process under static batch settling conditions into four zones from the top down i.e. a clear water zone; an initial slurry concentration zone; a transition zone and a compression zone (oe and levenger, 1916). Through a series of simple static batch settling tests at different slurry solids concentrations and by applying a mass balance analysis, a rudimentary thickener diameter sizing for any particular tonnage throughput could be determined. further 36 years were to elapse before the first mathematical description of the sedimentation process was provided by Kynch who proposed a kinematic theory of sedimentation based on the propagation of sedimentation waves in a suspension. Sedimentation was regarded as a process of propagating concentration changes upwards from the bottom of a settling vessel as a result of the downward movement of solids (Kynch, 1952; oncha and urger, 2002). He also mathematically defined a batch flux density function and based on the new theory, Talmage and Fitch devised a test method by measuring the liquid/solid interface settling velocity from one static batch settling test to provide all the data for determining the diameter of a thickener for a given tonnage duty (Talmage and Fitch, 1955). This method which determines the thickener area required to produce a sediment of a given solids concentration at a given solids throughput (i.e. a unit area function in m 2 /t/d) is still widely used to size thickeners. espite the wide use of the Talmage and Fitch method, there are certain fundamental shortcomings on which the technique is based. Firstly, the sedimentation theory described by Kynch only applies to the static batch settling of equally sized small ridged solid spheres and secondly it does not account for flocculated suspensions forming compressible sediments. It was only during the 1970s that sediment Paste 2014, Vancouver, anada 1

2 description of the sedimentation process during dynamic thickener operation.j. Vietti and F. unn consolidation theory was applied to the settling of compressible slurries (Shirato et al., 1970) which led directly to the development of modern mathematical models to describe the settling; accumulation and compression behaviour of suspensions (oncha and urger, 2003; Farrow and Swift, 1991). However the process of sedimentation in a thickener under continuous operation remains relatively poorly understood and consequently thickener operation remains problematic. This paper provides a description and a comparison of the sedimentation process in thickeners operated under both static as well as dynamic conditions and provides a basis for allowing thickener operational performance to be predicted under controlled steady state conditions. 2 Sedimentation under static batch conditions Once a flocculated slurry at a given solids concentration is mixed in a cylinder and allowed to remain quiescent the static batch sedimentation process begins. Initially as the solids settle due to gravity, a clear water zone () is separated from the remaining settling slurry column zone (). Throughout zone, the flocculated aggregates settle at the same rate, and hence the solids concentration within the zone is constant with the initial slurry solids concentration. The liquid/solid interface between zones and is generally distinct and therefore the interface height as a function of time is the primary parameter for measuring sedimentation rate in all static batch settling tests. Simultaneously with the above process, a second separation takes place at the base of the cylinder where the solids at the bottom of zone encounter a discontinuity. The downward basal collapse of zone coincides with an upward accumulation of a sediment transition zone () in which extensive dewatering takes place so that the solids concentration in this zone is intermediate between the initial slurry solids concentration () and the final compacted sediment of the compression zone () located below the transition zone. The interface between zones and is for all practical purposes indistinct; however the interface between zones and is distinct and is termed the mud bed interface. Sedimentation continues with the progressive downward collapse of zone and the progressive upward accumulation of zone until the two zones merge at a level known as the ritical Sedimentation Point (SP) (Figure 1). The rate of sedimentation is described by plotting the liquid/solid interface height over time. In theory, the initial slurry sedimentation velocity is a linear relationship with time until the SP is reached and is sometimes referred to as the free settling phase. Once the SP has been reached the sedimentation rate becomes non-linear as the slurry enters a transitional phase in which zone collapses into zone. Thereafter, the sedimentation rate is dependent on the rate at which liquid can escape from zone which is defined by the permeability and compressibility of the mud bed. ompaction of zone will continue until the forces due to the weight of the overlying solids are in equilibrium with the compressive yield stress forces of the mud bed. lternatively if the slurry is non-compressible no further compaction takes place (kers, 1980). It follows that once the SP has been reached any further volume change in zones and with time will define the solids concentration within the sediment bed so that a compaction profile for the slurry can be determined as a function of time. 2 Paste 2014, Vancouver, anada

3 Section name Interface Liquid/solid ritical Sedimentation Point Incompressible Mud bed ompressible Time Figure 1 onceptual model of the static batch sedimentation process 3 Sedimentation under continuous feed dynamic conditions Two thickener operational modes can be defined to describe the sedimentation process under continuous feed dynamic conditions: 3.1 ynamic batch mode This operating mode typifies a thickener during start-up and assumes the following: The thickener is initially filled with water. The feed is flocculated. The feed solids mass flow per unit area per unit time (solids flux rate) is constant. The flow of solids to zone is endless. The underflow pump is off. In this mode the sedimentation process begins when a flocculated slurry at a given solids concentration and flow rate exits the feed well. liquid/solid interface separates zone which reports to the overflow from zone which settles due to gravity to the base of the tank. In reality, this interface is not as well defined as that which develops under static batch sedimentation conditions however for the purposes of illustration it shall be defined as a sharp interface. Once the leading edge of zone encounters the discontinuity at the base of the tank it begins to collapse resulting in the formation of a mud bed interface which defines the top of an upwardly accumulating transition zone from which solids begin to compact into zone. From observations it appears that the depth which zone attains is determined by the mineral properties and solids flux rate of the slurry being treated. In some cases a relatively deep zone may develop while in other cases it may not exist at all and zone appears to transition directly onto zone. If the thickener is fed indefinitely under dynamic batch mode conditions, the mud bed interface will rise within the tank and eventually overwhelm the liquid/solid interface and spill to the overflow (a condition known as sliming ). The time frame in which this event will occur is a function of the mud bed interface Paste 2014, Vancouver, anada 3

4 description of the sedimentation process during dynamic thickener operation.j. Vietti and F. unn rise rate and the available side wall height of the containing tank. If feed to the thickener is discontinued under this mode, the sedimentation process reverts back to that described under static batch mode (Figure 2). No YNMI TH MOE STTI TH MOE Interface Height Liquid/solid Mud bed ritical Sedimentation Point Time Figure 2 onceptual model of the dynamic batch sedimentation process For a given thickener solids flux or throughput the rate of rise of the mud bed interface is constant with time and therefore it is a useful parameter to measure for controlling the level of the mud bed within the thickener (Figure 3). 4 Paste 2014, Vancouver, anada

5 Mud ed Interface Rise Rate (m/h) Mud ed Interface Height (mm) Section name t/m2.h 0.6 t/m2.h 0.8 t/m2.h Time (h) Figure 3 Relationship between mud bed interface height and time at increasing solids throughput (copper tailings in a 350 mm diameter paste thickener) Importantly, it has been shown that under dynamic batch mode, the mud bed interface rise rate is also linearly proportional to the thickener feed solids flux rate for a given slurry type. Furthermore, this relationship appears to be a function of the mineral and particle properties of the solids. Specifically, at a given solids flux rate, the mud bed interface rises more rapidly with clay containing slurries than non-clay containing slurries (Figure 4) Platinum Mill Platinum Tails Gold Tailings Gypsum Precipitate Kaoline iamond Tailings oal Fine Tailings ndalusite Tailings Solids Flux Rate (t/m2.h) Figure 4 Relationship between mud bed interface rise rate and solids throughput for different slurry types Referring back to Figure 2, under both static batch and dynamic batch conditions, zone undergoes compaction from the moment of its formation due to the accumulation of solids mass above it. onsequently, the longer the sediment is allowed to accumulate within the tank, the longer zone is able to compact under the combined forces of gravity and compression until they are in equilibrium with the Paste 2014, Vancouver, anada 5

6 Underflow Solids (%m) description of the sedimentation process during dynamic thickener operation.j. Vietti and F. unn compressive yield stress forces of the mud bed. t this time the maximum underflow solids concentration of the slurry at the base of the tank is attained. Thus the compaction time available for a mud bed element at the base of the tank to attain maximum underflow density in dynamic batch mode is a function of the mud bed interface rise rate (which in turn is a function of the solids flux rate and solids properties of the slurry) and the available thickener tank side wall height. It follows that for a thickener with a given side wall height, the underflow density achieved by feeding at a lower flux rate will be higher than that achieved by feeding at a higher flux rate when operated under dynamic batch mode. Given sufficient side wall height however, it is evident that the underflow density achieved by feeding at high flux rate will match that achieved by the lower solids throughput since the compaction profile of the slurry is the same (Figure 5) t/m2.h 0.6 t/m2.h 0.8 t/m2.h Mud ed ompaction Time (h) Figure 5 ompaction profile of a slurry element at the base of the tank when operating a thickener in dynamic batch mode at increasing solids throughput (copper tailings in a 350 mm diameter paste thickener; mud bed height of 1.6 m) 3.2 ynamic continuous mode ynamic continuous mode follows directly on from dynamic batch mode and describes a thickener during continuous operation. It assumes the following: The feed solids mass flow per unit area per unit time (solids flux rate) is constant. The flow of solids to zone is endless. The mud bed interface height is controlled at a constant level The underflow pump is on. Figure 6 describes the sedimentation process under dynamic continuous operating mode. In this mode the sedimentation process begins as described for the dynamic batch mode until the mud bed interface rises to a desired level within the tank. Once at this level, the underflow pump is activated so that the thickener is able to deliver a continuous discharge of underflow while being continuously fed. It is clear that it is only possible to maintain the mud bed interface level at a constant level within the tank by displacement of a volume of slurry to the underflow equal to the volume of slurry accumulating onto the mud bed interface. 6 Paste 2014, Vancouver, anada

7 Underflow Solids (%m) Section name onstant Mud ed Interface Height YNMI TH MOE YNMI ONTINUOUS MOE Liquid/solid Interface Height Mud bed Figure 6 onceptual model of the dynamic continuous sedimentation process Time It would appear logical to assume that once the mud bed interface had achieved a specific height (or alternatively that the slurry column had been exposed to a specific compaction time) within the thickener tank under dynamic batch mode, that the underflow solids concentration should remain constant once the thickener was set to a dynamic continuous operating mode. This, however, is not the case as Figure 7 demonstrates t/m2.h 0.6 t/m2.h 0.8 t/m2.h ynamic ontinuous Mode Mud ed ompaction Time (h) Figure 7 Effect on underflow solids concentration when operating a thickener initially in dynamic batch mode and then in dynamic continuous mode at increasing solids throughput (copper tailings in a 350 mm diameter paste thickener; mud bed height of 1.6 m) Paste 2014, Vancouver, anada 7

8 Slurry olumn Height (mm) description of the sedimentation process during dynamic thickener operation.j. Vietti and F. unn hanging operating modes from dynamic batch to dynamic continuous mode involves maintaining the mud bed interface at a fixed height while continuously discharging underflow at a constant rate to allow the mud be interface to remain static. In this mode the solids concentration in the underflow (when sampled from the base of the thickener over time and measured gravimetrically), was shown to go through a period of transition from high density before equilibrating to a lower density. Once constant underflow solids concentration is achieved the thickener can be said to be operating in a steady state. It is also evident that when operated at low flux rates the differential in underflow solids concentration between the two operating modes is small, and that the differential increases with increasing flux rate i.e. the period of transition to a steady state is shorter at lower flux rates. Since the sedimentation process is dynamic, there is a continual progression of compaction and density downwards through the slurry column irrespective of the mode of operation of the thickener. In other words the void ratio of an element of slurry of given volume at the top of the slurry column is higher than the void ratio of an element of slurry of equal volume at the bottom of the slurry column. Under dynamic batch mode the density of the slurry sampled at the discharge of the thickener at incremental mud bed height intervals represents the density of an element of slurry at the base of the thickener which has been exposed to the maximum compaction time and in which the void ratio is low. When operational mode is switched to dynamic continuous mode this element will be the first to be discharged. The density of the slurry exiting the thickener thereafter will represent elements of slurry within the density profile of the slurry column which have been exposed to less compaction time and hence will have a lower density and a higher void ratio (Figure 8). The density profiles illustrated in Figure 8 are similar in shape to thickened mud bed density profiles collected from previously reported data although the explanation for these changes differ from the current paper (Farrow et al., 2000) t/(m2.h) 0.6 t/(m2.h) 0.8 t/(m2.h) Underflow Solids (%m) Figure 8 ensity profiles of the slurry column within the thickener showing the density transition between dynamic batch mode and dynamic continuous modes of operation at increasing solids throughput (dotted lines for illustrative purposes) Once the period of transition is completed, the underflow discharge density at steady state will reflect the true residence time of an element of slurry of given volume as it passes through the slurry column from top to bottom. This residence time is therefore expressed as the ratio of the volume of the slurry column within the tank to the volume of slurry discharged at any given flux rate (Figure 9). 8 Paste 2014, Vancouver, anada

9 Underflow Solids (%) Section name t/(m2.h) 0.6 t/(m2.h) t/(m2.h) Mud ed Residence Time at Steady State (h) Figure 9 Mud bed residence time when operating a thickener under steady state conditions in dynamic continuous mode (copper tailings in a 350 mm diameter paste thickener; mud bed height of 1.6 m) 4 onclusion The observations made of the sedimentation process conducted under dynamic conditions are in agreement with the Kynch theory of sedimentation in which the sedimentation process can be likened to a slinky spring collapsing onto itself from a height. Under dynamic batch mode, the liquid/solid interface represents the upper most extended end of the spring below which the coils are equally spaced (Zone ), while the base of the thickener represents a solid surface (or discontinuity) onto which the spring collapses to its most compact state (Zone ). The mud bed interface is represented by the section of the spring which is intermediate between the fully expanded coils and the fully compacted coils. When operated under dynamic continuous mode, the discontinuity at the base of the thickener is effectively removed and the compacted coils are stretched from the bottom to a state which is less compact than what is achieved under batch mode; the more the spring is stretched the less compact the spring will be. It is typical during start-up projects that the thickener sizing test procedures and subsequent predictions of underflow density at full scale are in all cases conducted under batch sedimentation conditions (either static or dynamic). Invariably the maximum underflow density prediction is derived from a residence time curve which describes the relationship between slurry density and time. The density specified under this sedimentation condition relates to an element of slurry at the base of a slurry column in a settling vessel which has undergone compaction for a maximum length of time. This density value then generally forms the design basis for all further downstream processes. The current paper describes a sedimentation process conducted under dynamic continuous conditions under which all thickeners should normally operate. In this mode, a steady mud bed interface level is maintained by displacing a volume of slurry to the underflow equal to the volume of slurry accumulating onto the mud bed interface from the feed. The volume which accumulates is a function of the static mud bed rise rate (which itself is directly related to the thickener solids throughput) and the tank diameter. When operated under this condition, the underflow density was found to decrease from that achieved under batch sedimentation conditions. Under steady state dynamic continuous sedimentation conditions, the density of the slurry exiting the thickener at the point of discharge represents an element of slurry Paste 2014, Vancouver, anada 9

10 description of the sedimentation process during dynamic thickener operation.j. Vietti and F. unn within the density profile of the slurry column which has been exposed to a specific compaction time i.e. it represents the true residence time of the mud bed within the thickener. onsideration should be made for test work to be conducted under dynamic continuous sedimentation conditions rather than the current batch methods. cknowledgement The authors would like to acknowledge and thank ntofagasta Minerals for some of the data which was used in this paper. References kers, R.J. (1980) The compression of flocculated sediments, Filtration and Separation, Vol. 17, pp oe, H.S. and levenger, G.H. (1916) Methods for determining the capacity of slime settling tanks, Trans. IME, Vol. 55, pp oncha, F. and urger, R. (2003) Thickening in the 20th century: historical perspective, Minerals and metallurgical Processing Vol. 20, pp orr, J.V.N. (1915) The use of hydrometallurgical apparatus in chemical engineering. J. Ind. Eng. hem., Vol. 7, pp Farrow, J.. and Swift, J.. (1991), Improving thickening technology, Proc. Extractive Metallurgy onf., Perth, ustralia, pp Kynch, G.J. (1952) theory of sedimentation, Trans. Faraday Soc., Vol. 48, pp Farrow, J.., Johnston, R.R.M., Simic, K. and Swift, J.. (2000) onsolidation and aggregate densification during gravity thickening, hemical Engineering Journal, Vol. 80, pp Shirato, M., Kato, H., Kobayashi, K. and Sakazaki, H. (1970) nalysis of settling of thick slurries due to consolidation, J. hem. Eng. Japan, Vol. 3, pp Talmage, W.P. and Fitch, E.. (1955) etermining thickener unit areas, Ind. Eng. hem., Vol. 47, pp s 10 Paste 2014, Vancouver, anada