Swirl flow agitation for scale suppression

International Journal of Mineral Processing 112–113 (2012) 19–29

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International Journal of Mineral Processing journal homepage: www.elsevier.com/locate/ijminpro

Swirl flow agitation for scale suppression J. Wu a,⁎, G. Lane a, I. Livk a, B. Nguyen a, L. Graham a, D. Stegink b, T. Davis b a b

CSIRO Process Science and Engineering, Minerals Down Under Flagship, Graham Rd, Highett, Victoria 3190, Australia Queensland Alumina Ltd, Parsons Point, Gladstone, QLD 4680, Australia

a r t i c l e

i n f o

Article history: Received 21 November 2011 Received in revised form 8 July 2012 Accepted 14 July 2012 Available online 22 July 2012 Keywords: Scale Scale suppression Erosion Swirl flow Agitator

a b s t r a c t Scale formation is a serious problem in the mineral processing industry. To better understand the options available for mitigating this problem, a novel scale–velocity model is proposed in this paper for slurry systems commonly found in mineral processing plants. The new qualitative scale growth model predicts that at very low fluid velocities the scale growth rate is enhanced by an increase in fluid velocity due to the mass transfer-controlled scale growth. At higher fluid velocities, the scale growth rate decreases with increasing fluid velocity due to the increased flow erosion effect, for slurry systems. This suggests the potential of particulate erosion as a scale suppression mechanism. The model predicts the existence of an optimal slurry flow velocity, where scaling rate and equipment erosion rate are both zero. The optimal fluid velocity value is proposed to be used as the main parameter to improve engineering design of mineral processes in terms of scale suppression. A novel agitator design, swirl flow technology (SFT), developed and patented by CSIRO and Queensland Alumina Ltd (QAL) in Australia was introduced as an agitator design that better meets requirements for scale suppression than a widely used conventional draft-tube agitator system. SFT agitation has been installed in gibbsite precipitators at QAL's alumina plant for a decade. As shown by CFD simulations and laboratory measurements, swirled flow agitation generates more uniform velocity distribution and higher velocity values at the wall than conventional agitators for the same power input; this reduces the maximum growth rate of scaling in the tank leading to significantly prolonged precipitator's service life. Based on the full-scale operational experience at QAL, it can be suggested that SFT agitation roughly halves the scale growth rate as compared to that measured in the conventional draft tube agitator systems. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Scale formation in slurry equipment has been a serious on-going problem for the mineral processing industry. Its enormous cost for the mineral industry is manifested through increased capital expenditure, reduced capacity and throughput, and continuous requirement for human intervention. Scale growth is a major cause of lost production through tank downtime required for de-scaling cleaning operations. Scale is probably a more serious problem in the minerals industry than any other process industries. Slurry tanks are used extensively for hydrometallurgical processing such as leaching, digestion and precipitation. Often a large number of slurry tanks are installed for continuous chemical reactions. In low viscosity Newtonian slurry mixing tank operations, agitators are often designed on the basis of achieving off-bottom solids suspension, as other mixing processes often satisfactorily follow once the solids are suspended. It is useful to mention that, in a fully suspended Newtonian slurry tanks the times required to mix liquid and solids or ⁎ Corresponding author. E-mail address: [email protected] (J. Wu). 0301-7516/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2012.07.007

liquids are typically minutes, which is an order of magnitude shorter than the residence time required for reactions such as leaching or crystallization (Wu et al., 2001, 2010, 2011b), which is typically many hours or even days. Conventionally, axial flow impellers pumping downward with vertical baffles are used for solids suspension operations in slurry tanks. It has been generally accepted that axial flow impellers are more energy efficient than radial turbines in fully baffled tanks, refer to Nienow (1992); Ibrahim and Nienow (1996); Wu et al. (2001), and that the energy efficiency for off-bottom solids suspension is sensitive to the impeller off-bottom clearance and impeller diameter, refer to Nienow (1992), Chapman et al. (1983). Zwietering (1958), Nienow (1992) and Wu et al. (2001, 2002, 2006a, 2007, 2010) have described the basis of solids suspension in mixing tanks. Such conventional agitation designs are mostly developed from the chemical industry experience, where reactions are fast and the tanks used are relatively small compared to the tanks used in the minerals industry, where the reactions are often slow. The scarcity of publications available in the open literature on this topic suggests that in general little consideration has been given to the problem of scale formation in the current slurry tank mixing and agitation design.

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Fig. 1. Relationship between the precipitation or chemical reaction driven scale growth rate and fluid velocity. The actual velocity values are system dependent and can vary for different minerals processing slurries.

While there are extensive literature studies on scale formation, most of the studies appear to be related to fouling in heat exchangers and membranes used in the reverse osmosis (RO) processes in desalination plants (Yu et al., 2005; Neofotistou and Demadis, 2004; Coletti and Macchietto, 2011). Relatively little work has been done on scale formation in slurry pipes and slurry tanks used in the minerals processing industry, as noted by Hoang et al. (2011). Fundamentals of scale formation, as taking place in gibbsite precipitators, were studied in the work at CSIRO by Loan et al. (2008) using a laboratory flow-through scale cell. By observing initial stages of scale formation by X-ray photoelectron spectroscopy (XPS) it was demonstrated that in initial stages scale is formed as a result of precipitation reaction and not via solids settling. Scale studies by different authors (Hoang et al., 2011; Yu et al., 2005; Xing Xiaokia et al., 2005; Nawrath et al., 2006) suggest that the scale growth in processing equipment is affected by a number of factors, including supersaturation in solution, phase transformation phenomena and contact times. Clearly, these physico-chemical factors can be affected by fluid dynamics through its effect on heat and mass transfer. Supersaturation, for example, is strongly dependent on temperature and concentration of solute, and the contact time is directly related to velocity gradients. It is therefore not surprising that in scale literature the fluid velocity is commonly suggested to critically affect

the scale growth. However, there is a considerable confusion on whether increasing velocity enhances or suppresses scale growth (Hoang et al., 2011). This paper is concerned with scale suppression in agitated slurry tanks containing chemical solutions and solid particles from the perspective of the fluid dynamics. A fundamental scale–velocity mechanism and the full-scale industrial application experience are presented to justify and demonstrate the case of employing slurry erosion for scale suppression. 2. Fundamentals of scaling According to the classic work of Epstein (1983) on fouling in heat exchangers, the scaling process can be characterized by the interactions between the scaling fundamental mechanisms and the sub-processes involved in the scale deposition. The formation of scale can involve a number of different mechanisms, including: • • • • •

Precipitation, Chemical reaction, Particulates, Corrosion, or Bio-scaling.

Erosion Fig. 2. Scale growth rate vs. fluid velocity, based on tests using pipes in the precipitation area at QAL by Nawrath et al. (2006), with their measurements denoted by triangle symbols. Dotted vertical line is the wall velocity in the precipitators with SFT agitation at QAL. Courtesy from Nawrath.

Scale formation Fig. 3. Coexistence of scale formation and erosion on an axial flow impeller, after operating in a Ni-laterite HPAL autoclave for an extended period of time.

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At different stages of the process, the formation of scale is driven by a range of different sub-processes: • • • • •

Initiation of scaling, Transport of molecules or particulates, Attachment to surfaces, Removal of scale, and Aging of scale.

The above mechanisms and sub-processes of scaling are affected in a complex way by a range of different process properties, such as solution concentration, temperature, rheology and hydrodynamics, interfacial and surface properties, phase stability, and equipment geometry. For scaling in precipitators, studied in this work, the first three fundamental scaling mechanisms are the most relevant. In a precipitation based scaling process, which is central to this work, three main precipitation mechanisms play a crucial role in controlling the rate of scaling, namely, nucleation, crystal growth, and agglomeration. For the purpose of further discussion, it is worth noting that the main driving force for the three precipitation mechanisms is supersaturation in solution (Mullin, 2001), which typically is a strong function of solute concentration, temperature, and chemical composition. Increasing supersaturation in solution will result in increased rates of main precipitation mechanisms and increased rate of scaling. In real systems, however, the net effect of scaling is strongly affected by other physico-chemical properties of the system, such as slurry physical properties and hydrodynamics. In order to explain the complex interactions between different scaling mechanisms and hydrodynamics, a novel mechanistic model is proposed in this work. The new qualitative model that elucidates the relationship between the rate of scale growth and system hydrodynamics, i.e. fluid velocity, is described in the next section. 3. Scale–velocity model In an attempt to clarify the relationship between the fluid flow and scale formation, as reported in literature and observed in practice, we propose a new qualitative scale-velocity model that is schematically llustrated in Fig. 1. According to this model, a total of 4 separate velocity regimes can be identified: 3.1. Regime A — mass transfer controlled As shown in Fig. 1, at zero fluid velocity the scale growth rate is very low as a result of a molecular diffusion-rate controlled process. As the fluid velocity rises, the scale growth rate starts rapidly increasing due to an increased effect of convective mass transport. At the same time, with increasing fluid velocity the thickness of the stagnant layer around solid surfaces reduces, which accelerates the diffusion based mass transfer. Therefore, in regime A the scale growth rate increases with increasing fluid velocity. This behavior is consistent, for example, with the results reported by Hoang et al. (2011) showing an increase in gypsum scale growth rate with increasing fluid velocity in a range from 0 to 0.07 m/s. The fluid flow in this regime is practically stagnant, and therefore of little relevance to industrial slurry flow systems such as pipes and stirred tanks.

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In other cases, when the chemical reaction is relatively fast, the mass transfer controlled regime can extend across the regime B. Under those conditions, the rate of scale growth shows a continuous increase with increasing fluid velocity, instead of a constant scale growth rate. This behavior was reported, for example, by Xing Xiaokia et al. (2005) who by studying calcium carbonate scale deposition in a single phase water flow observed an increase in the scale growth rate when increasing fluid velocity in the test range from 0.3 to 0.9 m/s. 3.3. Regime C — suppression by erosion At higher fluid velocities, removal of deposit can occur simultaneously with material deposition as a parallel process to slow down the rate of scale growth, as shown in regime C of Fig. 1. In this regime, an increase in fluid velocity results in more erosion, which slows down the scale growth. This behavior can be exploited as a technique for effective scale suppression. Nawrath et al. (2006) conducted extensive plant-scale tests on scale growth of a supersaturated aluminate solution in a precipitation circuit at Queensland Alumina (QAL) refinery in Australia. Measurements of scale growth conducted in pipes of various diameters enabled Nawrath et al. to conclude that the scale growth decreases with increasing slurry velocity in the range from 0.5 to 1.7 m/s. Their data on the rate of scale growth as a function of velocity are presented in Fig. 2, courtesy from Nawrath. It can be seen that the data are consistent with the prediction of the scale–velocity model in the regime C. They predicted a zero scale growth at a velocity of around 2.9 m/s. In comparison, the swirl flow agitation technology, operating in the same precipitation circuit at QAL, generates a near wall velocity1 of approximately 0.8–0.9 m/s, as indicated in Fig. 2. It should be pointed out that regime C does not exist if only a single phase liquid is present; there is no slurry erosion without solid particles of significant sizes (e.g. ~ 0.10–0.20 mm). 3.4. Regime D — erosion damage In this regime, the effect of erosion exceeds scale growth and the material surface suffers net loss. Shown in Fig. 3, is the erosion damage near the tip of an impeller after operating in a slurry vessel over a period of time (reproduced from Wu et al., 2011a). The erosion increases with the blade radius, due to increased tip velocity. Close to the impeller hub region, the scale growth is clearly visible; this corresponds to reduced scale suppression effect by erosion at lower velocities, following the behavior in regimes A or B. A somewhat complex, opposing effect by the fluid flow, ignored in Fig. 1 is related to the hydrodynamic lift, studied by Adomeit and Renz (2000), Yiantsios and Karabelas (2003). They showed that deposition of non-reacting micro silica particles (~1.5 μm) is inhibited by increased drag and lift forces at higher velocities. This effect is likely to offset the increase scale growth due to the enhanced mass transfer at high velocities. To understand the relationship between the slurry flow velocity and the erosion damage effect, it is useful to refer to the erosion model of the following form widely accepted in the literature, due originally to Finnie (1960): n

E ¼ kV f ðα Þ

ð1Þ

3.2. Regime B — chemical reaction controlled At higher fluid velocities, e.g. larger than 0.1 m/s, the mass transfer rate may not anymore represent the slowest step in the overall process, and the chemical reaction rate may start to control the overall rate of scaling. Under these conditions, increasing fluid velocity does not affect the overall rate of scale growth, resulting in a constant scale growth rate in this regime, as shown in Fig. 1. So far, no direct measurement evidence in the literature could be found to validate for this anticipated behavior.

where E is the mass eroded divided by the total mass of particles impinging on a material surface; k is a constant that depends on material properties (e.g. hardness); V is the particle impingement velocity; n is the empirical coefficient (n = 1.8–2.3 for ductile material, n = 2.0–4.0 for brittle materials); and α is the particle impingement angle and f(α ) is a dimensionless wear function, with values from 0 to 1, dependent on material. 1

Defined at the edge of the flow boundary layer.

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Fig. 4. Swirl flow technology, showing the intense inner vortex and high wall velocities.

To comprehend the concept of “scale suppression by erosion”, as an illustration in using Eq. (1): a 50% increase in the slurry flow velocity (V) over the scaling material surface, the erosion will increase by ~3 times, assuming the scale is of brittle material behavior (e.g. n = 3.0). A challenge is to design a flow pattern such that the slurry velocities operate in a regime between C and D (refer to Fig. 1). It is useful to compare the erosion effect with the hydrodynamic lift on the scale formation. It is elementary to show that the hydrodynamic lift or drag forces on depositing nucleation particles is represented by: f n ∝μVdn :

ð2Þ

Based on this analysis, regimes C and D are more important in terms of scale suppression via the fluid dynamics design. Thus hydrodynamic lift and drag forces acting on the nucleation particles are considered less effective to inhibit scale growth. This is the basis of this paper. Finally, it should be stressed that the model outlined in Fig. 1 is very much conceptual and should be calibrated against process conditions. It is probably obvious that the actual velocity values in the model can vary for different minerals processing slurries. E.g., for systems with harder scales, higher slurry velocities are required to suppress the scale growth, consequently higher velocities are expected at the ideal point in-between regimes C and D.

And the erosive force by particles: 2 2

f e ∝ΔρV d :

ð3Þ

Where μ is the viscosity, dn is the size of the deposition particles, typical of nucleation dimension, with a size in the order of ~ 1 μm, d is the slurry particle size of around 100 μm, V is the velocity of particles, Δρ is the density difference between the solids and the fluid and the subscripts n and e denote nucleation particles, and erodent solids (e.g. mineral ore particles) respectively. Different flow regimes were used here: Eq. (2) being laminar flow due to small particle size, and Eq. (3) being turbulent, with the solids in the slurry suspension. Assuming V ~ 1 m/s, the fluid is of water viscosity, it can be estimated that: fe 4 ∝10 : fn

ð4Þ

Therefore, the hydrodynamic forces associated with lift or drag acting on the nucleation particles at the deposition stage are negligible, in comparison with the erosive forces from the impact of the particles in the slurry suspension.

Fig. 5. Industrial application of SFT in an alumina precipitator vessel at QAL, tank: 11 m in diameter and 28.3 m in height, SFT-FS36 rotor diameter: 3.13 m, impeller speed: 28 rpm, power: 54 kW.

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Fig. 6. Test tank facility at CSIRO: (a) schematic of the rig, and (b) mixing tank.

Fig. 7. Test configurations, (a) A310 with baffles, (b) draft tube agitator, and (c) swirl flow.

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4. Swirl flow agitator design

5. Experimental and CFD-modeling set-up A mixing research rig, shown in Fig. 6(a) and (b), of 1000 mm in diameter located at CSIRO in Melbourne (Australia) was used in the research and optimization of the SFT agitation design. To minimize optical distortion, the cylindrical tank was placed inside a water-filled, glass tank. Test impellers were mounted on the central shaft, which was equipped with an Ono Sokki torque transducer and speed detector. The speed and torque were logged using a personal computer for estimating parameters including the shaft power, power number, and other basic agitator operational data. The velocity of the liquid was measured using a laser Doppler velocimetry system (Wu et al., 2006a, 2006b). Fig. 7(a), (b) and (c) show laboratory test configurations used in the physical tests. These model configurations were based on separate full-scale precipitation installations.2 The liquid and solids flows in the tank bottom were observed through the transparent tank walls and the tank floor. The sedimentation bed height (HB) was recorded as the impeller speed was reduced from a full solids suspension condition. The curves of the sedimentation bed vs. speed (power) were used to compare the off‐bottom solids suspension performance. Refer to Wu et al. (2010, 2011b) for a more detailed description of the experimental methods. The test solids were glass particles of SG = 2.5 g/cm 3 at a concentration of 600 g/L and the test liquid was water. Particle size d75 = 0.120 mm. CFD simulations were conducted on a full-scale draft tube precipitation tank and two full-scale swirl flow precipitation tanks (a conebottom and a flat bottom) at QAL. The fluid flow was simulated by solving the Reynolds-averaged equations for mass and momentum conservation using the ANSYS CFX software (for further details see Lane, 2006). From the wall shear stress, the near-wall velocity profile has been calculated, assuming the log‐law equation for the dimensionless velocity in the boundary layer (refer to Tennekes and Lumley, 1972). The velocity at the top of the boundary layer was assumed to correspond to a y+ value of 300. Here, y+ represents the distance from the

Swirl Flow rotor draft tube agitator

0.20

HB/H

3xA310

0.10

0.00 0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

P/Ms (W/kg) Fig. 8. Normalized off-bottom solids sedimentation bed height vs. power input, for the three different agitator configurations tested in the laboratory.

surface, normalized by the friction velocity, and the fluid viscosity and density (Tennekes and Lumley, 1972). 6. Experimental and CFD modeling results 6.1. Suspension bed height measurements in laboratory-scale precipitators Fig. 8 shows the normalized sedimentation bed height (HB/H, where HB is defined as the settled bed height) measured as a function of the power per tank slurry mass, for the three test configurations including 3 × A310 impellers with baffles (× 4 vertical baffles), a draft tube agitator system, and swirl flow installation, as shown in Fig. 7. In general, it is necessary for agitated tanks to operate with solids fully suspended from the bottom, i.e. HB =0. This can be achieved if the power is increased beyond a critical value. For example it requires approximately P/Ms =0.06–0.07 W/kg, where P is the power and Ms is the slurry mass, for the draft tube system or the swirl flow design to suspend solids fully from the bottom in the laboratory tests, while the case with baffled 3× A310 impellers configuration requires a higher unit power input, i.e. P/Ms ~0.09–0.10 W/kg for complete off-bottom suspension. It should be noted that an agitator can be considered more energy efficient, if its sedimentation bed height vs. power curve is located on the left-hand side of the curves presented in Fig. 8. It may be concluded that the swirl flow design is better or similar to the draft tube agitator in their energy efficiency for off-bottom solids suspension. 2.5 Draft tube agitator C110, D/T=0.28 SFT, D/T=0.28

2

1.5

z/T

In 1997, CSIRO and Queensland Alumina Limited (QAL) developed the swirl flow technology (SFT) based on an invention by Martin Welsh (2002) to address the scaling problem within precipitation tanks. In contrast to conventional agitation approaches, SFT employs a pump impeller located at the top of the tank to draw the slurry up to the center of the tank, and discharge the slurry from the impeller with a swirling motion, as shown in Fig. 4. The swirling flow pattern can suspend solids from the tank bottom in a similar way to tornados lifting up sands or gravels in the nature environment. At the time of writing this paper, QAL has converted 20 gibbsite precipitation tanks (28 m high × 11 m diameter, ~3000 m3) to SFT agitation since the first installation in 1997. An inside view of a precipitator fitted with the SFT rotor at the top is presented in Fig. 5. As reported by Stegink et al. (2012), the QAL's experience is that SFT agitation approximately doubles the service life between de-scaling operations compared to the draft tube agitators. In addition, it provides superior re-suspension capabilities after power failures (the order of a few hours for SFT vs. at least several days for draft tube agitators), lower capital cost due to simplified support structures, and health and safety benefits due to the convenience of performing agitator maintenance without entering the tank.

1

0.5

0

0

1

2

3

4

5

2

The difference in the tank aspect ratio (Fig. 7(b) and (c)) is expected to cause some variation on the results, although minimum due to data normalization (e.g. power per slurry mass, non-dimensional velocity). In any case, the performance data presented here on SFT is more conservative due to the higher tank aspect ratio used based on a study at CSIRO.

Fig. 9. Laboratory precipitators: non-dimensional velocity efficiency parameter profile along the tank height, measured near the wall (outside the flow boundary layer), with z the vertical distance from the tank bottom, and T the tank diameter. The dotted line is anticipated but not measured.

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Conventional baffled axial flow impellers agitation although considered by the literature being energy efficient to suspend solids for short tanks (e.g. tank height to diameter ratio from 1 to 1.5) is less employed in the alumina precipitation tanks in the Western countries. This appears to be related to the fact that the alumina precipitation tanks are of tall aspect ratios (e.g. tank height to diameter ratio from 2.5 to 3.0). This practice appears to be justified, as the draft tube agitator system is more efficient for solids suspension as evident from Fig. 8.

In the case of the draft tube, approximately ~ 70% of the tank experienced significantly lower velocities, in comparison with those measured in the SFT configuration. It can be concluded that SFT provides a more uniform velocity distribution with higher absolute values adjacent to the tank surface when compared with the conventional agitator design configurations. This is advantageous, as the maximum scale growth on the tank wall, which occurs at the low wall velocities, usually determines the de-scaling operation, and thus the tank downtime.

6.2. Wall velocity measurements in laboratory-scale precipitators

6.3. CFD-modeling predictions for full-scale precipitators

For suppression of scale growth, the velocity near the tank wall surface, immediately adjacent to the boundary layer, is a critical factor. This requirement has been established based on long-term operational experience at QAL and is also clearly illustrated by the scalevelocity model shown in Fig. 1. To compare the velocity on a power input basis, defined as power per mass of solids, a velocity circulation efficiency parameter as described in Wu et al. (2006a) should be used:

CFD modeling studies have been carried out on full-scale precipitation tanks operating with the conventional draft-tube agitator, and the swirl flow agitator of cone-bottom and flat-bottom tanks. Fig. 10(a) and (b) shows the predicted flow field and the turbulent shear stress on the tank wall, for the full-scale draft‐tube agitator configuration. For the swirl flow configuration used at QAL, the flow field in a conebottom tank is illustrated in Fig. 11(a) and the wall shear stress distribution is shown in Fig. 11(b). The flow field in a flat-bottom (with 45° filet) tank is shown in Fig. 12(a); the wall shear stress is calculated and shown in Fig. 12(b). Fig. 11(c) and Fig. 12(c) show the swirl velocity vectors at two different levels of 4 m and 25 m from the tank bottoms for swirl flow agitation configurations. The strong swirling flow velocities are responsible for the erosion scaling suppression effect. The swirl velocity component in the draft tube precipitation configuration is practically zero and is therefore not shown here. For each full-scale tank, the dimensionless velocity efficiency parameter η has been calculated from the near-wall velocity and other data including power, slurry density and tank dimensions, and vertical profiles (assuming circumferential averaging) are plotted in Fig. 13, similar to that for the laboratory scale data (Fig. 9). It can be seen that the curves are broadly similar to those for the laboratory tanks of corresponding design, although the laboratory measurements did not appear to capture an increased η at the upper part of the tank using swirl flow design. The increased velocity at the lower

η¼

V ðP=ρAÞ1=3

where η is the non-dimensional velocity efficiency parameter, P is the agitator power input (W), ρ is the slurry density (kg m−3), and A is the tank wetted surface area excluding the bottom (m2), and V is the velocity, determined outside the flow boundary layer (m s−1). It is necessary that η as a non-dimensional velocity be used to compare the velocities on a power per tank surface area basis. The larger the η, the higher the velocity produced for a given power input, for the same tank internal surface and slurry properties. Fig. 9 shows the distributions of the non-dimensional velocity efficiency parameter, η, along the normalized height, where z is the height (m), and T is the tank diameter (m). The velocities were measured using a laser Doppler velocimetry for the draft agitator configuration and the swirl flow agitator configuration in the laboratory.

Fig. 10. Full-scale alumina precipitator with draft tube agitator — (a) projected mean velocity vectors (excluding the component normal to the plane) in a vertical plane through the center of the tank; (b) turbulent shear stress on tank wall. Tank: 14 m in diameter and 36 m in height, shaft speed: 30.8 rpm, power: 55 kW.

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a

b

25m

4m

c

4m above bottom

25m above bottom

Fig. 11. Full-scale gibbsite precipitator with swirl flow impeller and conical bottom — (a) projected mean velocity vectors (excluding the component normal to the plane) in a vertical plane through center of tank; (b) turbulent shear stress on tank wall; (c) horizontal plots of the swirl velocity at 4 and 25 m from the tank bottom. Tank: 11 m in diameter and 28.3 m in height, SFT-FS36 rotor diameter: 3.13 m, impeller speed: 27.5 rpm, power: 54 kW.

part of the cone-bottom swirl flow tank corresponds to reduced radius effect. This is consistent with a laboratory observation that swirl flow agitation in cone bottom tank is more energy efficient than in flat bottom tanks; for example it was found that a cone bottom tank requires significantly less power than a flat bottom tank to suspend solids when operating with the same swirl flow agitation design. 7. Discussion Scaling at a material surface is perhaps one of the most serious maintenance problems facing the minerals processing industry. Given the big dimensions and large number of slurry tanks used, scale cleaning operations are costly and are inherently hazardous in many mineral processing plants. Ironically, a slurry equipment suffering a scaling problem could also experience material erosion damage within the same equipment, albeit at a different surface position. This is illustrated in Fig. 3, where scaling and erosion co-exist. It is therefore thought that these two undesirable processes could be engineered to cancel each other, i.e. to achieve a “zero” scale growth

rate, as suggested in a regime between C and D in Fig. 1. It is telling to see in Fig. 3, that the middle part of the impeller blades (in-between the tip and hub) was free of either scaling or erosion damage. This is very encouraging, as it is suggesting such an ideal operating regime could indeed exist in the real world. In practice, the concept of scale suppression via erosion can only be implemented within engineering constraints, e.g. the agitator power limit. Thus a lower practical velocity than ideal has to be used in the slurry tanks, to minimize the scale growth, rather than eliminating it completely. In the case of the swirl flow technology jointly developed by CSIRO and QAL, the wall velocity was estimated to be approximately ~ 0.80– 1.0 m/s, which is still within regime C in the scale-velocity model. In this instance, the increased erosion by the swirled flow pattern suppresses the scaling growth, by approximately halving the scaling rate (Stegink et al., 2012).3 This is still far from the “zero” growth point, as

3 Actual scaling suppression effect varies from tank to tank, due to variation in temperature, super-saturation levels.

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a

27

b

25m

4m

c

4m above bottom

25m above bottom

Fig. 12. Full-scale alumina precipitator with swirl flow impeller and flat bottom — (a) projected mean velocity vectors (excluding the component normal to the plane) in a vertical plane through center of tank; (b) turbulent shear stress on tank wall. (c) horizontal plot of the velocity at 4 and 25 m from the tank bottom. Tank: 11 m in diameter and 28.9 m height, SFT-BS464 rotor diameter: 3.49 m, shaft speed of 28.4 rpm, power of 70 kW.

further increase in the velocity is not feasible, due to the constraint of the available motor power. Nevertheless, it is useful to point out that, in theory, it is possible to increase the erosion effect also by modifying the slurry properties. For example, blending some coarse particles could lead to increased erosion, though this is yet to be demonstrated in the full-scale operations. The patented swirl flow technology has an advantage of producing a relatively uniform tank surface velocity distribution with a flow pattern similar to a tornado in the natural environment, such that the highly swirling flow results in an increased erosion effect. At the same time, the central upward vertical motion provides solids transportation critical for off-bottom solids suspension. In addition to these performance advantages, the swirl flow technology requires a lower capital cost due to being free of baffles, draft tubes,

4

This is a more recent design version of SFT.

and simple mechanical engineering due to the cantilever shaft structure (thus free of the bottom shaft support bearing). The total weight of a SFT installation can be significantly less than that of a conventional installation for the same process duty, possibly allowing for lighter support structures. Access is from above, eliminating the need for tank entry for agitator maintenance. As the SFT impeller is located in the top part of the tank, there is no “bogging” issue as can occur with conventional agitators, particularly after a power failure. Full-scale experience at QAL suggests that SFT impellers restart easily and re-suspend all the solids in much shorter time than a draft tube agitator system (Stegink et al., 2012). There exists a considerable confusion in the literature regarding the effect of the velocity on scale formation. It is suggested in this paper that the confusion is probably caused by the fact that the erosion effect of the solid phase has been largely ignored in the literature, particularly in laboratory studies. In this paper, we have shown that fluid velocity is critical to the effect of hydrodynamic forces associated

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3

velocities at the wall than conventional agitators for the same power input, as shown by CFD simulations and laboratory measurements. Erosion as a scaling suppression mechanism in gibbsite precipitators is demonstrated through a decade‐long industrial experience at QAL of operating SFT agitated precipitators. The full-scale operational experience clearly demonstrates that this technology results in roughly halving the scale growth rate as compared to that measured in conventional draft tube agitation systems. This practical experience clearly supports the validity of the new scale-velocity model.

2.5

z/T

2

1.5

Draft tube agitator

Swirl with cone bottom Swirl with flat bottom

1

0.5

0 0

1

2

3

4

5

6

Fig. 13. CFD modeling results: full-scale precipitators: normalized efficiency parameter based on the velocity near the edge of the boundary layer plotted along the vertical position in the tank.

with lift and drag. In the case of micron-size particles the effect is negligible in comparison to the impact of erosion. The same forces, however, can greatly impact on particles of larger sizes, e.g. around 0.1–0.2 mm. It should be commented that for slurry suspension with very low solid concentrations or very small particle size, the scale suppression via erosion may not be practical, even at very high velocities (e.g. 2–4 m/s). However, with growth of scales in size, the hydrodynamic forces acting on the scales may not be negligible anymore. It is conceivable that the hydrodynamic lifting could be effective for “loose scales”. More research is required to determine this effect. Finally, it should be emphasized that the fluid velocity values defining the different regimes in Fig. 1 are very preliminary, with the values expected to vary widely with the chemistry, scale material hardness, and slurry properties. For instance, it is probably self-evident that an increase in the solids particle size would lead to a reduction of the velocity value at the point between regimes C and D. Therefore the fluid velocity values defining each regime are expected to vary from case to case. It is suggested that these velocities could be determined for a given system to optimize operating parameters so as to minimize scaling. Critical to this will be a need to account the material properties of the scale, as clearly the hardness of the scale will affect its erosion rate. Calibration testing to account for the material properties of both the surface and the solid particles in the erosion model is a major on-going research topic at CSIRO currently. This will hopefully shed more light in the future on the applications of the scale–velocity model proposed in this paper. 8. Conclusions A new qualitative model that relates the scale growth rate with fluid velocity by defining 4 main scale growth regimes is proposed in this paper. At very low fluid velocities (regime A), the scale growth rate is enhanced by increased fluid velocity, as the mass transfer controls the overall process rate. In the case where scale growth is via chemical reaction (regime B), which is slow and process-rate determining, the scale growth rate is predicted to remain constant regardless of the fluid velocity. At higher velocities (regime C and D), the scale growth rate decreases with an increased velocity due to scale growth suppression by erosion. A negative scale growth rate in regime D implies a net material loss due to the erosion damage to the equipment — which should obviously be avoided. A patented swirl flow technology (SFT) has been introduced to tackle the scale problem in mineral processing industry. Swirled flow agitation generates more uniform velocity distribution and higher fluid

Nomenclature A tank surface area (m 2) E mass eroded divided by total mass of particles impinging on a material surface (−) d particle diameter f(α) wear function in the erosion model (−) fe erosive forces on scale (kg) fn hydrodynamic forces on nucleation particles (kg) HB sedimentation bed height at the tank bottom (m) H liquid height in tank (m) k a constant that depends on material properties in the erosion model (−) Ms slurry mass or solid mass (kg) n empirical coefficient in the erosion model, or subscript for nucleation P shaft power (kW) SFT swirl flow technology T tank diameter (m) V velocity (m s −1) z vertical distance from the tank bottom y+ non-dimensional distance from the surface η velocity efficiency parameter ρ density (kg/m 3).

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