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Flow mapping of full scale solvent extraction settlers using pulsed Doppler UVP technique
Chemical Engineering Science 104 (2013) 925–933
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Flow mapping of full scale solvent extraction settlers using pulsed Doppler UVP technique K. Mohanarangam a,n, W. Yang a, K.R. Barnard b, N.J. Kelly b, D.J. Robinson b a b
CSIRO Process Science and Engineering/CSIRO Minerals Down Under National Research Flagship, P.O. Box 312, Clayton South, VIC 3169, Australia CSIRO Process Science and Engineering/CSIRO Minerals Down Under National Research Flagship, P.O. Box 7229, Karawara, WA 6152, Australia
H I G H L I G H T S
A technique for the measurement of fluid flow patterns in opaque solutions presented. This believed unique capability is achieved using pulsed Doppler techniques. Application in commercial solvent extraction (SX) operations demonstrated. Variation in SX settler flow patterns observed with settler ‘furniture’ modification.
art ic l e i nf o
a b s t r a c t
Article history: Received 5 July 2013 Received in revised form 7 October 2013 Accepted 14 October 2013 Available online 22 October 2013
Details behind the experimental technique of ultrasonic velocity profiling (UVP) to obtain in situ velocity measurements of both aqueous and organic phases in commercial solvent extraction SX settlers are presented. The commercial applicability and benefit of the process is demonstrated via the on-site analysis of two settlers at a commercial copper solvent extraction operation. The two settlers had the same dimensions and were operated under essentially equivalent conditions but differed in terms of being assessed before and after routine maintenance, with the internal ‘furniture’ configuration of the latter also being modified. UVP-determined organic and aqueous flow pattern results from the two settlers are reported and compared in relation to the furniture configuration/maintenance status of each. The results highlight the potential for UVP analysis to ascertain flow patterns in commercial solvent extraction settlers to benefit industrial operations directly and to enable the development of improved mathematical (computational fluid dynamics) modelling of flow patterns in settlers. Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved.
Keywords: Solvent extraction Ultrasonic velocity profiling (UVP) Flow pattern Full-scale measurement
1. Introduction 1.1. Solvent extraction Liquid–liquid separation or solvent extraction (SX) is an important step in hydrometallurgical processing of base metals which facilitates selective separation of one or more metal ions of interest from an aqueous solution. It relies on the immiscible nature of organic (typically kerosene-based) and aqueous phases. The organic phase contains an extractant which, under suitable operating conditions, is capable of removing metal ions from the aqueous phase into the organic phase and, under strip conditions, transferring them from the organic phase back into a different aqueous phase. There are various extractants used commercially in SX operations for different metal recovery processes. For instance,
n
Corresponding author. Tel.: þ 61 3 9545 8677. E-mail address: [email protected] (K. Mohanarangam).
the SX extractants of choice for copper operations are phenolic hydroxyoxime-based. These are capable of extracting copper via the reaction shown in the following: Cu2 þ 2LHðorgÞ ⇌CuL2ðorgÞ þ 2H þ
ð1Þ
where LH represents the neutral hydroxyoxime extractant and the subscript ‘org’ indicates the species is present in the organic phase. It can be seen from Eq. (1) that this equilibrium reaction will be affected by the operating pH, and this is indeed used to control whether extraction or stripping takes place. An acceptable rate of metal ion transfer between the immiscible phases in SX is achieved by their vigorous mixing which results in an unstable emulsion containing droplets of one phase in the other, being either aqueous droplets in the organic phase (‘organic continuous’), or organic droplets in the aqueous phase (‘aqueous continuous’). The smaller the droplet size, the greater the rate of metal ion transfer (Miller, 2006). However, subsequent to mixing, the phases need to be separated as completely as possible to minimise contamination and/or wastage during the subsequent
0009-2509/$ - see front matter Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ces.2013.10.015
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purification steps. As per Stokes law, smaller droplets separate more slowly than larger droplets and as such require a longer residence time for separation if increased entrainment is not to result. The conflicting demands of the mixing and subsequent separation stages are an integral and often repeated step in SX operation. The type of equipment and how it is used in these processes is therefore extremely important as it impacts the mixing and separation phenomena and thus the operation of the plant. 1.2. SX contactors and phase separation There is no single SX contactor that is best for all situations. The metal transfer kinetics along with dispersion and coalescence behaviour can influence the choice of contacting equipment. Continuous contacting equipment used in the industry for solvent extraction can be broadly classified into two groups; stage-wise or differential, based on their mode of operation (Ritcey and Ashbrook, 1979). Mixer–settlers and inline mixers are examples of stage-wise contactors wherein the aqueous and the organic phases are mixed and then separated in a large settling area before the next stage. Columns and centrifuges are classified as differential contactors requiring a smaller area (‘footprint’) than mixer– settlers but more height. Mixer–settlers are currently used in a large number of plants as they are relatively easy to operate, reliable, flexible and fairly simple to design. The liquid–liquid mixture is created within the mixer box with the aid of a suitable impeller. In many operating plants, the mixer box is fitted with a pump mixer which, in addition to creating the droplets, also works to draw the organic and aqueous solutions into the mixer box. As requirements for the mixers differ, appropriate impeller design is required to maximise mass transfer as well as decrease entrainment (Kehn and Kontur, 2011). Advances in this area also include DOPs (Dispersion Overflow Pump) pumping unit and SPIROXs mixing unit from Outotec that controls the formation and the size of the generated droplets (Hakkarainen et al., 2011). In many operating plants, an auxiliary or a secondary mixer which receives the overflow from the primary mixer box also works to provide additional mixing to prevent phase separation or conversely facilitate some coalescence before the mixture overflows into the settler. The unstable and therefore temporary emulsion emanating from the mixer is then with time able to naturally separate out into discrete organic and aqueous layers, with the droplets of the discontinuous phase also coalescing during this time. The two liquid phases are recovered at the far end of the settler via overflow (organic) and underflow (aqueous) weirs. Ideally the conditions within a settler will enable complete separation of the organic and aqueous phases, although rarely if ever does this occur. Whereas some process parameters affecting settler behaviour such as throughput rates can be readily varied, others such as the dimensions of a constructed settler cannot. In attempting to maximise phase separation, one very important aspect to consider is optimising the residence time available to all liquid molecules in the settler. This is best achieved by having a flow pattern close to plug flow in the settler. This is because, for a given average settler residence time, the benefit of increased residence and thus disentrainment time for some molecules in a non-plug flow system is outweighed by the detrimental effect of necessarily decreased residence time for other molecules (‘shortcircuiting’). Flow patterns and disentrainment behaviour in settlers can be affected by a variety of factors. For example, improvements in settler performance can be achieved by minimising the velocity differential between the organic and aqueous phases, improving the inlet feed distribution from the mixer box to the
settler by eliminating reverse flows and macro eddies, improved coalescence with the aid of settler internals or ‘furniture’ like picket fences and coalescence packs currently used in the SX industry as well as preferential design and placement of furniture in areas of deep emulsion bands (Miller, 2006; Poulter et al., 2011). Measurement of fluid flow patterns within large scale process vessels like SX settler units is therefore an important step in understanding the effect of operating conditions and equipment design (e.g. picket fences) on flow behaviour in a settler and provides the opportunity to test the effectiveness of design changes and to optimise operating performance. 1.3. Computational modelling of liquid flow Patterns in SX Settlers Computational modelling of various phenomena including liquid flow patterns in SX settlers has become more prevalent with the advent of more powerful computers over the past decade or so. This computational fluid dynamics (CFD) modelling provides a relatively effective way of examining computationally derived flow patterns for a given settler design, and assessing the impact of design changes on the resulting flow patterns. CFD typically arrives at a solution by solving partial differential equations for mass, momentum and turbulence and iterating it to the lowest possible errors. Miller (2006) provided a good insight into the CFD generated flow patterns within a settler. Kankaanpaa (2005) through his CFD studies showed that in the absence of picket fences, the inlet jet from the mixer to the settler protruded for long distances into the settler while a shallow inlet depth results in reversed flows of the aqueous phase from half way down the settler back to the inlet. Through its CSXT (Customised Solvent Extraction Technology) programme, Hatch has been using CFD as a design tool to develop and implement alternative inlet launder design called the smooth flow solvent extraction feed launder, CSXT feed distribution array as well as the CSXT bull nose phase splitter with the aim of improving the flow pattern and having effective separation between the phases (Poulter et al., 2011). However, it should never be forgotten that CFD is simply portraying a mathematical model of what is predicted to occur in reality. Ultimately, experimental validation of CFD-determined outcomes greatly increases the value of a particular CFD model as a tool. As such, reliable data from an operating site is required. 1.4. Liquid flow measurement using ultrasonic velocity profiling The UVP technique employs an ultrasonic Doppler method to obtain a one-dimensional velocity profile along its beam path from the reflected echoes of tracer particles present within the measurement medium. Traditionally, this technique was developed to measure the profile of blood flow in a blood vessel (Satomura, 1959). It employs the ‘pulsed echo’ method, detecting the Doppler shift in the echo as a function of time after pulse emission and reflection of the ultrasound that comes from the interface of blood cells. Takeda (1986) first applied this technique to measure onedimensional liquid velocities in Poiseuille and Taylor vortex flows. Since then numerous studies have been conducted to measure velocities of liquids like mercury (Takeda and Kikura, 2002), body lotion (Brunn et al., 2004), suspensions (Ouriev and Windhab, 2003; Xu and Aidun, 2005) as well as void fraction of bubbles in water (Murai et al., 2009). UVP uses two fundamental principles to measure the velocity as well as the location of the fluid motion. A ‘time-of-flight’ technique is used to ascertain the spatial measurement location while the Doppler shift frequency is used to attain instantaneous flow velocity. For any measurement scenario using UVP, factors like selection of ultrasonic basic frequency, transducer settings and presence of tracer particles need to be carefully chosen. Position
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information is obtained from the time lapse of echo reception after the pulse emission, τ; which relates to the time-of-flight of the ultrasonic pulse. The position x is determined using the velocity of sound c as follows: x¼
cτ 2
ð2Þ
The denominator arises as a result of path length of the pulse travel is both for emission and return. The Doppler shift frequency fD at the instant gives the velocity (V) information at that location from the following relationship: V¼
cf D 2f o
ð3Þ
where fo is the basic frequency of the ultrasonic transducer. In summary, instantaneous velocity is obtained by measuring the instantaneous frequency as a function of time after the pulse emission. Fig. 1 shows the schematic of the measurement principle in channel geometry. In this case, the transducer is located outside the top channel wall with some coupling agent (usually ultrasonic gel) to provide acoustic coupling (Fig. 1a). The ultrasonic echoes as received by the transducer when switched to the receiving mode are shown in Fig. 1b. The echoes generated within the flow are due to the presence of tracer particles with additional strong echoes reflected from near and far walls of the channel geometry. A series of echo signals at fixed time instants from the tracer particles are used detect the Doppler shift frequency which provides the velocity at which it is travelling. Since the Doppler shift frequency is directly related to the velocity value, the UVP system does not require a calibration procedure. Fig. 1c shows the typical velocity profile after relevant post-processing steps to obtain velocity as well as spatial information. The spatial information is obtained from time lapse between the emitted and received pulse. More information on this technique can be found elsewhere (Takeda, 2012).
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1.5. On-site assessment of SX settler behaviour An on-site qualitative assessment of any settler design modification (including general maintenance) can be obtained by comparing organic entrainment losses before and after completion of a particular modification. Whilst such modifications may be predicated on CFD and/or physical model tests, the entrainment analysis approach only provides an overall outcome, and cannot provide detailed information in regard to specific effects such as the flow patterns within the settler. Although laser-based techniques such as Particle Image Velocimetry (PIV) and Laser Doppler Velocimetry (LDV) have been used to assess fluid flows including in laboratory-based SX systems (Bujalski et al., 2006; Drumm et al., 2011), they are restricted to analysis in transparent (for the laser) liquid systems, and are not particularly suited for analysis in most or all commercial SX systems. With the advent of the ultrasonic velocity profiling (UVP) technique, which relies on echoes from particles within the flow medium, real time velocity measurements can now be made inside operational solvent extraction settlers. Measurement using the UVP technique (Mohanarangam et al., 2010; Morgan et al., 2011; Yang et al., 2011) not only provides an insight into flow patterns within a given settler design, but also provides the necessary data to validate and refine CFD models leading to more accurate predictions of flow behaviour. The combination of both UVP and CFD analysis by CSIRO at another commercial operation (Minara Resource's Murrin operation in Australia) has recently been disclosed (Lane et al., 2012).
1.6. Aim The present work details the experimental technique of UVP and its application to obtain velocity measurements of both aqueous and organic phases in commercial SX settlers. This is demonstrated by the assessment of two solvent extraction settlers at a commercial copper SX operation. The assessed settlers were both in the first extraction stage within separate SX trains (trains A and D) namely E1A and E1D which were run ostensibly under the same operating conditions. Measurements of the 2D flow structures in a horizontal slice in each of the organic and aqueous phases were obtained in each settler in order to compare flow structures at the selected levels between settler E1A with old furniture and settler E1D with new furniture. Additionally, the E1A settler was assessed prior to routine maintenance, meaning there was scope for operationally undesired aspects such as damaged/ blocked pickets and sediment build up. The E1D settler on the other hand had just been cleaned and fitted with new furniture.
2. Experimental technique 2.1. UVP analysis equipment
Fig. 1. Schematic of the measurement principle. (a) Measurement system. (b) Ultrasonic echo signal. (c) Measured velocity distribution (Takeda, 2012).
2.1.1. UVP system, ultrasonic transducer and cabling A Met-Flow UVP system (Model: UVP-DUO MX) was used to determine the velocity field inside the settlers. The UVP system used during this measurement campaign had an integrated multiplexer which can serially scan quickly through up to 20 transducers and still retain time resolution. A maximum number of up to six transducers were utilised at any one time during the velocity field measurements inside the settler. The UVP software was installed on a laptop which was used to control the UVP hardware via the Ethernet. All measurement files were also saved in the same laptop which was processed later to provide two-dimensional maps.
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Six ultrasonic transducers with a low frequency of 0.5 MHz (outer diameter 23 mm and 40 mm long) were chosen for the current trial for their good propagation abilities and lower signal power losses, since the attenuation of acoustic energy is proportional to the square of frequency. Under the circumstance of relatively low fluid velocity inside the settler, the maximum measuring distance of the 0.5 MHz transducer was approximately three metres along its axis. Additionally, two transducers with frequency of 1 MHz (outer diameter 13 mm and 40 mm long) were also chosen for vertical UVP measurements in order to measure the normal or rise velocities in both the organic and aqueous phases at various points in the settler. As the transducers were relatively small compared to the scale of the industrial settler, they could be placed inside the liquid flow without generating a large disturbance in the measured flow field. Measurement errors incurred using any ultrasonic Doppler technique like UVP system to measure flow velocities are relevant to this current work. Errors specific to this work include the ability of the UVP system to measure low velocity flows. Based on this, the on-axis velocity resolution that could be achieved for the current work was 1.4 mm/s using 0.5 MHz transducer at its full penetration depth. The other source of errors includes the transducers beam size induced due its half-angle and the presence of enough particles (to act as tracers) as well as their motion at very low velocities. The latter was monitored using the validity rate (%) provided by the UVP software. In addition, a custom modification was carried out to extend the standard transducer cable length of 4–50 m due to access and safety requirements regarding potential ignition (sparking) sources requiring the UVP-DUO monitor and laptop computer to be located a large distance away from the settler. Laboratory tests showed that only limited noise was introduced with the addition of a 50 m RG58 extension cable. By optimising the parameter settings on the UVP system, it was estimated that the errors caused by the 50 m extension cable could be kept to less than 5%. 2.1.2. Transducer holder A custom-designed transducer holder consisting of a 400 mm square stainless steel (316L) base plate with a 20 mm diameter stainless steel threaded pipe attached to its bottom was used. Two Perspex disks, one for organic flow and other for aqueous flow measurement, were mounted on the rod which carried the UVP transducers. The disks could be moved along the pipe and fixed at any location with the aid of a shaft (attached to the disk) and bolt arrangement. Each disk was designed to hold four transducers aligned in three perpendicular directions. At any measurement location inside the settler one 0.5 MHz transducer was used to measure the longitudinal velocities or two 0.5 MHz transducers measure transverse velocities (one after the other). The alignment at a specific location inside the settler was carried out manually. 2.2. Settler configuration and operating conditions: Multiple trains of mixer settler units as shown in Fig. 2 are used at the commercial operation. The identically sized E1 settlers of trains A and D (i.e. E1A and E1D) were nominated for fluid flow characterisation using the UVP system. The copper-rich aqueous flow from the PLS (Pregnant Leach Solution) pond enters the E1 mixer along with organic coming from the E2 settler which is arranged in series with E1. Settling tests were conducted each day prior to beginning the flow mapping. For both E1A and E1D, a representative sample was collected from the mixer box in a 1000 ml cylinder which enabled visual confirmation of aqueous continuous operation. The resulting O/A ratio value combined with the known aqueous flow rate obtained from an on-site flow metre
Fig. 2. Operational flow sheet for trains A and D at the commercial SX operation (E1, E2 and EP – extraction stage settlers, W – wash stage settler, S – strip stage settler, Elec. – electrolyte, raff – raffinate, PLS – pregnant leach solution).
Fig. 3. Plan view of the settlers with major internal features before (left) and after (right) installation of settler furniture (not to scale).
enabled calculation of the organic flow rate. The operating conditions of both settlers were kept as similar as possible to assess the impact on flow patterns of the old furniture before maintenance against the cleaned settler containing newly installed furniture. Fig. 3 is a schematic illustrating one half of each settler with their (symmetric) internal configuration. Although the specific dimensions of the settler remain confidential, it can be stated that length and width both exceeded 20 m. For both settlers, the organic and aqueous mixture exiting from the mixer is introduced centrally into the settler; with a chevron type picket fence located a short distance downstream of the inlet. The E1A settler, which contained one picket fence located downstream of the chevron, was assessed prior to routine maintenance. As such, there was scope for operationally undesired aspects to be present in this settler, such as damaged/blocked pickets and sediment build up. In contrast, the E1D settler had been cleaned beforehand, and was fitted with newly designed furniture comprised of a picket fence located further downstream (in comparison to E1A), followed by a coalescence pack. Organic and aqueous flow velocities were measured inside settlers E1A and E1D with the aid of the UVP system. The velocity measurement heights for the organic and aqueous layers were set at 50 mm below the organic top surface and 250 mm above the
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Free-surface Measures V velocity
ORGANIC Organic disk
Emulsion band Measures U velocity
Measures U velocity
Aqueous disk
AQUEOUS 0.5 MHz transducers
Fig. 4. Schematic of the setup with UVP transducers.
bottom of the settler, respectively. These measurement heights were carefully chosen to make sure that velocities measured were of their respective phases as well as to avoid any ingress of the other phase. The UVP measurement positions were chosen to obtain 2D velocity components in both X and Y directions at grid points at an appropriate distance apart to give a good representation of both organic as well as aqueous flow structures. For both settlers, in excess of 200 UVP measurement locations were assessed. The measurements were carried out sequentially one transducer at a time using the multiplexing option provided by the hardware. The settlers were nominally divided into various sections depending on their internal furniture location. For settler E1A, the area between the chevron and the picket fence was labelled as Section 1, with Section 2 being between the picket fence and the weirs. For settler E1D, Section 1 spanned between the chevron and the picket fence, Section 2 between the fence and the coalescence packs, and Section 3 from the packs to the weir system. Fig. 4 shows the schematic of the setup used in this work with UVP transducers mounted on them.
3. Results and discussion 3.1. Organic velocity flow field of settler E1A Fig. 5 shows the 2D velocity vector and streamline plot of the organic layer along a plane at a height of 50 mm below the top organic surface of the E1A settler. In this and several subsequent Figures, the small black circles along the centreline represent circular concrete pillars that support the roof over the settler. With the aid of a reference vector of 100 mm/s, the 2D vectors in Sections 1 and 2 indicate the direction and magnitude of the flow velocity in this plane at each grid point. The flow streamlines
Fig. 5. 2D velocity vectors, streamlines and longitudinal velocity contours of organic layer inside settler E1A. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)
shown as blue lines are instantaneously tangent to the velocity vectors in the flow field, and show the direction of fluid movement at any point in time. It can be seen that organic flow is highly nonuniform in Section 1 between the chevron and the picket fence. The majority of the flow experiences a strong sideways movement with some backflow very close to the chevron exit. The organic flow exiting the chevron has a stronger transverse motion (U velocity) along the X direction compared to longitudinal movement (V velocity) along Y direction. At this level for organic flow the magnitude of transverse motion is non-symmetric, being far greater on the right side of the section compared to the left. The maximum transverse velocity on the right side of 137 mm/s is almost 11 times the longitudinal velocity at the same grid point. The velocity magnitude on the left of Section 1 is not as high as the right which gives rise to a rather small recirculation pattern close to chevron near the side wall. Fig. 5 also provides a colour contour map of the longitudinal (Y direction) component of organic velocity V, inside settler E1A. Under the convention used here, flow towards the outlet weir has a negative velocity, whereas reverse flow back towards the chevron is registered as a positive velocity. In Section 1, the organic flow field is partly coloured by velocities ranging from 0 to 35 mm/s, with the rest of the section coloured pale blue, indicating a backward flow directed towards the chevron. Overall the flow moves downstream towards the picket fence with a small magnitude. The observed non-symmetric flow behaviour in this plane prior to the picket fence is possibly attributable to obstructions on the left hand side chevron which forces most liquid to favour passage through the right side of the chevron. The resistance of the picket fence which is located only a short distance downstream of the chevron apex inhibits longitudinal motion of this liquid. Instead, the restricted flow through the left side of the chevron could create an area of lower pressure which would encourage the observed transverse flow of liquid from the right hand side back towards the middle and slightly across into the left side of Section 1. However, other phenomena could also account for this flow behaviour, such as
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the effect of flow motion below the measurement section as well as possible non-uniform distribution of feed entering the settler inlet from the mixer. Section 2 of the settler extends from the exit of the picket fence up to the weirs with no furniture in-between. Within the first half of Section 2 downstream of the picket fence, the bulk of the organic flow moves in the reverse direction, i.e. a backflow. A slight transverse motion tending to favour flow towards the central line of the settler is also apparent. The reversed flow behaviour is suspected to be caused by a strong upward flow of organic from deeper in the settler which was observed visually in the settler during the measurement campaign. This is discussed further in Section 3.3 below. The majority of organic flow for the second half of Section 2 flows uniformly and in a more expected manner towards the weirs except for a prominent recirculation region found a couple of metres upstream from the weir on the right hand side of the settler. The presence of several circular pillars located down the middle of the settler seem to have minimal effect on the overall flow pattern. An interesting feature is highlighted by the streamlines at the proximity of the third pillar downstream of the picket fence is a point where liquid flows radiate outwards in all directions. There was also a backflow region (blue) near the right side wall which was also visible from the aqueous flow contours discussed further below (Fig. 6). The organic flow gained momentum as it reached the exit of the settler leading to the overflow weir, as expected.
3.2. Aqueous velocity flow field of settler E1A The aqueous flow velocity measurements for settler E1A were carried out at a height of 250 mm from the bottom of settler. Fig. 6 illustrates the 2D velocity vectors, streamlines and longitudinal velocity contours of the aqueous layer in Sections 1 and 2. In Section 1, the flow exits from the chevron and flows in a transverse manner to both sides due to the resistance offered by the picket fence directly in front of it. The maximum transverse
Fig. 6. 2D velocity vectors, streamlines and longitudinal velocity contours of aqueous layer inside settler E1A. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)
velocity experienced on the left side of the settler is around 212 mm/s which is almost 10 times greater than the longitudinal velocity experienced at the same grid location. A small flow recirculation is also observed in the right side of Section 1. From the contour plot showing the aqueous flow in Y direction, the majority of Section 1 is depicted in green signifying a forward flow velocity of up to 35 mm/s directed towards the picket fence. The blue regions within this layer indicate a reversed flow back towards the chevron. The aqueous flow for Section 2 from the exit of the picket fence to the weirs depicts a relatively uniform albeit rapid average flow of 200 mm/s downstream along the width of the settler, with a centrally localised jet exiting at 340 mm/s. There is some crossover of aqueous flow from the right side of the settler to the left which associates with the recirculation flow pattern at the far left end of the settler near the weir. This recirculation is believed to be attributable to restricted longitudinal flow arising from the presence of an outlet pipe which passes downwards through the settler in this region. Another large recirculation zone is present near right side wall of the settler a short distance upstream of the weir. This recirculation zone occurs directly below the same feature observed in the organic layer (Fig. 5) and is considered to be due to the presence of a mechanical structure close to the proximity of where the measurement was taken. The longitudinal flow velocity decreases as it reaches the outlet, indicating the underflow weir does not reach this height, i.e. the flow exits below the measurement plane. Overall, the 2D results for the organic and aqueous flow patterns in the E1A settler as measured at these two heights indicates non-uniform and thus non-ideal flow behaviour. 3.3. Investigation of reversed flows in the organic layer for E1A Given the reversed flows measured in the organic plane (as well as noticed visually on the surface), additional 2D flow velocity measurements were obtained at various heights inside a targeted section of the settler. These detailed measurements were conducted in the Y–Z plane very close to the centreline of the settler (across its width). The first measurement grid point along its length was at 27% (location expressed as a percentage normalised against the total length of the settler) from the inlet and the final at 55%. The measurement grid for measuring flow velocities across the settler's depth is shown in Fig. 7a. Five equidistant measurement points (Y1–Y5) were chosen along the longitudinal section of the settler. The normal or ‘rise’ velocities W (along the Z direction) were measured using the downward pointing 1 MHz transducer from the organic disk (positioned 50 mm below the top surface, as described in Section 2.1). Normal velocities were also measured using another 1 MHz transducer pointing up from the aqueous disk (positioned 250 mm above the bottom of the settler). The longitudinal velocities were measured at five different heights by changing the location of the transducer. A total of 30 UVP holder positions were used to obtain 2D velocity components in both Y and Z directions. Fig. 7b shows the 2D velocity vector and streamline plot across the depth of the settler, with the contour plot shown in Fig. 7c. The flow in the specific region analysed experiences very high normal (rise) velocities averaging 56 mm/s at 150 mm from the organic surface. A clear distinction in normal velocity (Z direction) can be seen with top half of all the vertical velocities measured moving towards the free-surface. However, below this the flow is dominated by the longitudinal velocity, with a sharp drop in normal velocities. Fig. 7c shows the plot of longitudinal velocity with the aid of a colour contour map. The top half of the section up to two measurement grid points coloured blue experience a backward longitudinal flow directed towards the settler inlet consistent with
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Fig. 7. (a) Measurement grid across the depth of the E1A settler, (b) two-dimensional vectors and streamlines, (c) vertical/rise velocity and (d) longitudinal velocity.
that indicated in Fig. 5. Deeper in the settler, a more uniform longitudinal flow (green colour) with a velocity in excess of 75 mm/s up to a maximum of 225 mm/s is observed. This analysis aligned with the on-site observations and the results shown in Fig. 5 highlights the potential to use UVP to measure flow velocity in planes other than the horizontal.
3.4. Organic velocity flow field of settler E1D Fig. 8 shows the 2D velocity vectors, streamlines and longitudinal velocity contours of the organic phase at the height of 50 mm below the organic surface for settler E1D. In Section 1, the motion of the organic flow close to the exit of the chevron is relatively symmetric. A couple of recirculation regions are evident as illustrated by streamlines, and the 2D contour plot indicates the central region adjacent to the chevron (shaded blue) to have some reverse flow. However, the flow is generally in the desired direction. As the flow reaches the picket fence downstream it continues to move forward but with some transverse motion directed towards the centreline of the settler due to the aforementioned reverse flow. The maximum longitudinal velocity
experienced very close to the entry of the picket fence is 100 mm/s. The majority of organic flow in Section 2 between the picket fence and the coalescing packs experiences a uniform longitudinal flow heading downstream. Examination of the velocity vectors at the entry and exit of the picket fence reveals that whilst the longitudinal velocity remains similar (as shown by the similar colours on the contour plots), the transverse flow is materially reduced by passage through the picket fence, an expected outcome. The longitudinal organic flow remains relatively uniform throughout Section 2, a feature that persists in Section 3 of the settler as it moves beyond the coalescing packs towards the overflow weir. The flow gains momentum as it approaches the overflow weir, particularly on the right side of the settler. The organic flow exits the settler with an average velocity of 100 mm/s and a maximum velocity of 170 mm/s. 3.5. Aqueous velocity flow field of settler E1D Aqueous layer velocity measurements were made at a plane 250 mm from the bottom of the E1D setter. Fig. 9 represents 2D flow features of the aqueous flow in Sections 1–3. In Section 1,
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Fig. 8. 2D velocity vectors, streamlines and longitudinal velocity contours of organic layer inside settler E1D. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)
momentum with an averaged longitudinal velocity of 62 mm/s just before entering the picket fence. As seen in the organic layer, progression of the aqueous phase through the picket fence materially reduced transverse flow, resulting in the desired outcome of a relatively uniform longitudinal flow in Section 2. The longitudinal flow velocity in this section decreased from about 121 mm/s upon passage through the picket fence to about 54 mm/s entering the coalescing packs. This reduction in entry and exit velocities is due to the increased resistance caused at the entry of the coalescing packs. There is an increase of longitudinal velocity in Section 3 at the exit of the packs after which the flow moves downstream until it reaches the weir system where it is slowed down to 38 mm/s. This reduced longitudinal flow velocity is consistent with the behaviour seen in E1A and is explained similarly. Marked aqueous flow velocity differences seen at the entry and the exit of the packs is attributed to the design of the packs whose entry and exit do not match exactly with the aqueous layer measurement height of 250 mm from the settler floor. While there are some flow reversals in Section 1, the flow in Sections 2 and 3 is almost uniformly directed downstream as shown by the coloured contour plot. Overall, the 2D results for the organic and aqueous flow patterns as measured at these two heights in settler E1D indicate more ideal flow pattern behaviour compared to the E1A settler. One of the reasons for this improvement could be attributed to picket fence designs deployed in the settlers. It can be concluded from the previous work within our group (Lane et al., 2012) and elsewhere that picket fence design plays a pivotal role in facilitating a good flow distribution across the settler. Whilst the specific details of the chevron and picket fences cannot be disclosed, it can be stated that the pickets in E1A provided a greater degree of flow blockage relative to those in E1D. The lesser blockage of E1D resulted in a drop in velocities immediately exiting these fences which helped to eliminate strong jets. The observed reduction in short circuiting within the settler relative to E1A could be attributable to either fence design, maintenance protocol or both. 3.6. Operational outcome The results presented in the previous section clearly demonstrate the improved and streamlined flow patterns for E1D settler both for organic as well as aqueous phases in the horizontal planes of measurement compared to E1A settler. The sum of the modifications undertaken for the E1D settler resulted in flow patterns closer to plug behaviour at the levels assessed relative to the E1A settler. Plug flow behaviour is expected to yield an optimum separation of each phase, and therefore would be expected to result in decreased entrainment. In practice, the actual benefit of all the modifications was found by the company in question to result in a 60% decrease in organic entrainment, resulting in a substantial financial benefit.
Fig. 9. 2D velocity vectors, streamlines and longitudinal velocity contours of aqueous layer inside settler E1D. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)
a very high velocity of aqueous outflow is observed at the centre of the settler very close to chevron's apex. The flow reaches a maximum velocity of 370 mm/s with an averaged velocity of 185 mm/s at the chevron exit. The flow exiting the chevron at the centre splits into two streams with a ‘jetting effect’ with both strong longitudinal and transverse motion. This causes the flow to be preferentially directed towards the sides of the settler, an intended outcome from the chevron design. A region of recirculation is apparent on the left side of the settler as depicted by streamlines. Further downstream of Section 1, the flow loses
4. Conclusion A detailed description of the pulsed Doppler UVP technique and demonstration of its application to measuring in situ flow patterns in commercial solvent extraction settlers has been presented via two-dimensional flow mapping of settlers E1A and E1D at a commercial copper SX operation. These settlers operated under near identical inlet flow conditions, but differed in their internal furniture configuration. Measurements were carried out along a plane on both the aqueous and organic phases. Clear differences in the flow patterns of the two settlers were detected by the UVP instrument. The E1D settler, which showed superior organic and aqueous flow behaviour, had the new and improved
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furniture with a chevron, picket fence and a coalescence pack installed inside it in comparison to E1A which had only a chevron and picket fence. Given the relatively constant nature of the other operating parameters, it is concluded that the design and physical location of the furniture within E1D settler coupled with them being freshly installed and therefore not affected by any undesired aspects such as damaged/blocked pickets and sediment build up lead to the improved performance in flow patterns. The increased understanding of settler flow pattern behaviour afforded to the operating company by UVP analysis helped them to improve the economics of their process. Besides the benefit afforded by UVP analysis for understanding existing flow patterns in commercial SX settlers, the results can also be used to enable the development of improved CFD modelling of flow patterns in settlers.
Acknowledgements The authors would like to acknowledge the strong support they received from site management, technical staff and support personnel of the commercial copper SX operation both prior to and during the site work. The authors would also like to thank MERIWA and the commercial sponsors of the Solvent Extraction Technology projects (SXT and SXT2) for their support and also acknowledge the support of the CSIRO Minerals Down Under National Research Flagship and Parker CRC for Integrated Hydrometallurgy Solutions. References Brunn, P.O., Wunderlich, T., Muller, M., 2004. Ultrasonic rheological studies of a body lotion. Flow Meas. Instrum. 15, 139. Bujalski, J.M., Yang, W., Nikolov, J., Solnordal, C.B., Schwarz, M.P., 2006. Measurement and CFD simulation of single-phase flow in solvent extraction pulsed column. Chem. Eng. Sci. 61, 2930. Drumm, C., Hlawitschka, M.W., Bart, H.J., 2011. CFD simulations and particle image velocimetry measurements in an industrial scale rotating disc contactor. AIChE J. 57, 10.
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Hakkarainen, J., Villarroel, A., Sotomayor, A., Rodriguez, C., Dellarossa, R., 2011. Technological advances implemented by Outotec in recent copper solvent extraction projects. In: Fernando Valenzuela, L., Moyer, B.A. (Eds.), Proceedings of the 19th International Solvent Extraction Conference ISEC 2011, October 2011. Gecamin Ltd., Santiago, Chile, p. 19 (Chapter 1). Kankaanpaa, T., 2005. Studying solvent extraction settler process by using CFD. In: Schlesinger, M.E. (Ed.), EPD Congress, TMS. Kehn, R.O., Kontur, J., 2011. Proper agitator design for pumper and auxiliary tanks in copper solvent extraction plants. In: Fernando Valenzuela, L., Moyer, B.A. (Eds.), Proceedings of the 19th International Solvent Extraction Conference ISEC 2011, October 2011. Gecamin Ltd., Santiago, Chile, p. 117 (Chapter 4). Lane, G., Mohanarangam, K., Yang, W., 2012. Assessment of the flow pattern in a solvent extraction settler. In: 9th International Conference on CFD in the Mineral and Process Industries, 10–12 December. CSIRO, Melbourne, Australia. Met-Flow, 2002. Met-Flow UVP-DUO Monitor User's Guide. Miller, G., 2006. Design of mixer-settlers to maximise performance, ALTA Copper 2006. ALTA Metallurgical Services, Melbourne, Australia. Mohanarangam, K., Yang, W., Simic, K., 2010. Application of UVP in the Minerals Industry. In: Proceedings of the 17th Australasian Fluid Mechanics Conference, Auckland New Zealand, Paper 164. Morgan, J., Cashwell, W., Doole, S., Steeples, J., 2011. Entrainment reduction at Freeport-McMoRan Copper & Gold Morenci operations. Solvent Extr. Ion Exch. 29, 854. Murai, Y., Ohta, S., Shigetomi, A., Tasaka, Y, Takeda, Y., 2009. Development of an ultrasonic void fraction profiler. Meas. Sci. Technol. 20 (11), 114003. Ouriev, B., Windhab, E., 2003. Transient flow of highly concentrated suspensions investigated using the ultrasound velocity profiler-pressure difference method. Meas. Sci. Technol. 14, 1963. Poulter, S., Robles, E., Panaou, C., Gottliebsen, K., 2011. Hatch customised solvent extraction technology. In: Fernando Valenzuela, L., Moyer, B.A. (Eds.), Proceedings of the 19th International Solvent Extraction Conference ISEC 2011, October 2011. Gecamin Ltd., Santiago, Chile, p. 13 (Chapter 1). Ritcey, G.M., Ashbrook, A.W., 1979. Solvent Extraction: Principles and Applications to Process Metallurgy, 1st edition Elsevier Scientific, Amsterdam. (Part II). Satomura, S., 1959. Study of the flow patterns in peripheral arteries by ultrasonics. J. Acoust. Soc. Jpn. 15, 151–158. Takeda, Y., 1986. Velocity profile measurement by ultrasound Doppler shift method, Int. J. Heat Fluid Flow 8, 313. Takeda, Y., Kikura, H., 2002. Flow mapping of the mercury flow. Exp. Fluids 32, 161. Takeda, Y., 2012. Ultrasonic Doppler velocity profiler for fluid flow. In: Fluid Mechanics and Its Applications Series, vol. 101, Springer. ISBN: 978-4-43154025-0 (Print) 978-4-431-54026-7 (Online). Xu, H.J., Aidun, C.K., 2005. Characteristics of fiber suspension flow in a rectangular channel. Int. J. Multiphase Flow 31, 318. Yang, W., Simic, K., Rajasingam, R., Chovar, M., Schwarz, M.P., 2011. Characterisation of flows in copper settlers. In: Fernando Valenzuela, L., Moyer, B.A. (Eds.), Proceedings of the 19th International Solvent Extraction Conference ISEC 2011, October 2011. Gecamin Ltd., Santiago, Chile, p. 123 (Chapter 4).
Contents lists available at ScienceDirect
Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces
Flow mapping of full scale solvent extraction settlers using pulsed Doppler UVP technique K. Mohanarangam a,n, W. Yang a, K.R. Barnard b, N.J. Kelly b, D.J. Robinson b a b
CSIRO Process Science and Engineering/CSIRO Minerals Down Under National Research Flagship, P.O. Box 312, Clayton South, VIC 3169, Australia CSIRO Process Science and Engineering/CSIRO Minerals Down Under National Research Flagship, P.O. Box 7229, Karawara, WA 6152, Australia
H I G H L I G H T S
A technique for the measurement of fluid flow patterns in opaque solutions presented. This believed unique capability is achieved using pulsed Doppler techniques. Application in commercial solvent extraction (SX) operations demonstrated. Variation in SX settler flow patterns observed with settler ‘furniture’ modification.
art ic l e i nf o
a b s t r a c t
Article history: Received 5 July 2013 Received in revised form 7 October 2013 Accepted 14 October 2013 Available online 22 October 2013
Details behind the experimental technique of ultrasonic velocity profiling (UVP) to obtain in situ velocity measurements of both aqueous and organic phases in commercial solvent extraction SX settlers are presented. The commercial applicability and benefit of the process is demonstrated via the on-site analysis of two settlers at a commercial copper solvent extraction operation. The two settlers had the same dimensions and were operated under essentially equivalent conditions but differed in terms of being assessed before and after routine maintenance, with the internal ‘furniture’ configuration of the latter also being modified. UVP-determined organic and aqueous flow pattern results from the two settlers are reported and compared in relation to the furniture configuration/maintenance status of each. The results highlight the potential for UVP analysis to ascertain flow patterns in commercial solvent extraction settlers to benefit industrial operations directly and to enable the development of improved mathematical (computational fluid dynamics) modelling of flow patterns in settlers. Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved.
Keywords: Solvent extraction Ultrasonic velocity profiling (UVP) Flow pattern Full-scale measurement
1. Introduction 1.1. Solvent extraction Liquid–liquid separation or solvent extraction (SX) is an important step in hydrometallurgical processing of base metals which facilitates selective separation of one or more metal ions of interest from an aqueous solution. It relies on the immiscible nature of organic (typically kerosene-based) and aqueous phases. The organic phase contains an extractant which, under suitable operating conditions, is capable of removing metal ions from the aqueous phase into the organic phase and, under strip conditions, transferring them from the organic phase back into a different aqueous phase. There are various extractants used commercially in SX operations for different metal recovery processes. For instance,
n
Corresponding author. Tel.: þ 61 3 9545 8677. E-mail address: [email protected] (K. Mohanarangam).
the SX extractants of choice for copper operations are phenolic hydroxyoxime-based. These are capable of extracting copper via the reaction shown in the following: Cu2 þ 2LHðorgÞ ⇌CuL2ðorgÞ þ 2H þ
ð1Þ
where LH represents the neutral hydroxyoxime extractant and the subscript ‘org’ indicates the species is present in the organic phase. It can be seen from Eq. (1) that this equilibrium reaction will be affected by the operating pH, and this is indeed used to control whether extraction or stripping takes place. An acceptable rate of metal ion transfer between the immiscible phases in SX is achieved by their vigorous mixing which results in an unstable emulsion containing droplets of one phase in the other, being either aqueous droplets in the organic phase (‘organic continuous’), or organic droplets in the aqueous phase (‘aqueous continuous’). The smaller the droplet size, the greater the rate of metal ion transfer (Miller, 2006). However, subsequent to mixing, the phases need to be separated as completely as possible to minimise contamination and/or wastage during the subsequent
0009-2509/$ - see front matter Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ces.2013.10.015
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purification steps. As per Stokes law, smaller droplets separate more slowly than larger droplets and as such require a longer residence time for separation if increased entrainment is not to result. The conflicting demands of the mixing and subsequent separation stages are an integral and often repeated step in SX operation. The type of equipment and how it is used in these processes is therefore extremely important as it impacts the mixing and separation phenomena and thus the operation of the plant. 1.2. SX contactors and phase separation There is no single SX contactor that is best for all situations. The metal transfer kinetics along with dispersion and coalescence behaviour can influence the choice of contacting equipment. Continuous contacting equipment used in the industry for solvent extraction can be broadly classified into two groups; stage-wise or differential, based on their mode of operation (Ritcey and Ashbrook, 1979). Mixer–settlers and inline mixers are examples of stage-wise contactors wherein the aqueous and the organic phases are mixed and then separated in a large settling area before the next stage. Columns and centrifuges are classified as differential contactors requiring a smaller area (‘footprint’) than mixer– settlers but more height. Mixer–settlers are currently used in a large number of plants as they are relatively easy to operate, reliable, flexible and fairly simple to design. The liquid–liquid mixture is created within the mixer box with the aid of a suitable impeller. In many operating plants, the mixer box is fitted with a pump mixer which, in addition to creating the droplets, also works to draw the organic and aqueous solutions into the mixer box. As requirements for the mixers differ, appropriate impeller design is required to maximise mass transfer as well as decrease entrainment (Kehn and Kontur, 2011). Advances in this area also include DOPs (Dispersion Overflow Pump) pumping unit and SPIROXs mixing unit from Outotec that controls the formation and the size of the generated droplets (Hakkarainen et al., 2011). In many operating plants, an auxiliary or a secondary mixer which receives the overflow from the primary mixer box also works to provide additional mixing to prevent phase separation or conversely facilitate some coalescence before the mixture overflows into the settler. The unstable and therefore temporary emulsion emanating from the mixer is then with time able to naturally separate out into discrete organic and aqueous layers, with the droplets of the discontinuous phase also coalescing during this time. The two liquid phases are recovered at the far end of the settler via overflow (organic) and underflow (aqueous) weirs. Ideally the conditions within a settler will enable complete separation of the organic and aqueous phases, although rarely if ever does this occur. Whereas some process parameters affecting settler behaviour such as throughput rates can be readily varied, others such as the dimensions of a constructed settler cannot. In attempting to maximise phase separation, one very important aspect to consider is optimising the residence time available to all liquid molecules in the settler. This is best achieved by having a flow pattern close to plug flow in the settler. This is because, for a given average settler residence time, the benefit of increased residence and thus disentrainment time for some molecules in a non-plug flow system is outweighed by the detrimental effect of necessarily decreased residence time for other molecules (‘shortcircuiting’). Flow patterns and disentrainment behaviour in settlers can be affected by a variety of factors. For example, improvements in settler performance can be achieved by minimising the velocity differential between the organic and aqueous phases, improving the inlet feed distribution from the mixer box to the
settler by eliminating reverse flows and macro eddies, improved coalescence with the aid of settler internals or ‘furniture’ like picket fences and coalescence packs currently used in the SX industry as well as preferential design and placement of furniture in areas of deep emulsion bands (Miller, 2006; Poulter et al., 2011). Measurement of fluid flow patterns within large scale process vessels like SX settler units is therefore an important step in understanding the effect of operating conditions and equipment design (e.g. picket fences) on flow behaviour in a settler and provides the opportunity to test the effectiveness of design changes and to optimise operating performance. 1.3. Computational modelling of liquid flow Patterns in SX Settlers Computational modelling of various phenomena including liquid flow patterns in SX settlers has become more prevalent with the advent of more powerful computers over the past decade or so. This computational fluid dynamics (CFD) modelling provides a relatively effective way of examining computationally derived flow patterns for a given settler design, and assessing the impact of design changes on the resulting flow patterns. CFD typically arrives at a solution by solving partial differential equations for mass, momentum and turbulence and iterating it to the lowest possible errors. Miller (2006) provided a good insight into the CFD generated flow patterns within a settler. Kankaanpaa (2005) through his CFD studies showed that in the absence of picket fences, the inlet jet from the mixer to the settler protruded for long distances into the settler while a shallow inlet depth results in reversed flows of the aqueous phase from half way down the settler back to the inlet. Through its CSXT (Customised Solvent Extraction Technology) programme, Hatch has been using CFD as a design tool to develop and implement alternative inlet launder design called the smooth flow solvent extraction feed launder, CSXT feed distribution array as well as the CSXT bull nose phase splitter with the aim of improving the flow pattern and having effective separation between the phases (Poulter et al., 2011). However, it should never be forgotten that CFD is simply portraying a mathematical model of what is predicted to occur in reality. Ultimately, experimental validation of CFD-determined outcomes greatly increases the value of a particular CFD model as a tool. As such, reliable data from an operating site is required. 1.4. Liquid flow measurement using ultrasonic velocity profiling The UVP technique employs an ultrasonic Doppler method to obtain a one-dimensional velocity profile along its beam path from the reflected echoes of tracer particles present within the measurement medium. Traditionally, this technique was developed to measure the profile of blood flow in a blood vessel (Satomura, 1959). It employs the ‘pulsed echo’ method, detecting the Doppler shift in the echo as a function of time after pulse emission and reflection of the ultrasound that comes from the interface of blood cells. Takeda (1986) first applied this technique to measure onedimensional liquid velocities in Poiseuille and Taylor vortex flows. Since then numerous studies have been conducted to measure velocities of liquids like mercury (Takeda and Kikura, 2002), body lotion (Brunn et al., 2004), suspensions (Ouriev and Windhab, 2003; Xu and Aidun, 2005) as well as void fraction of bubbles in water (Murai et al., 2009). UVP uses two fundamental principles to measure the velocity as well as the location of the fluid motion. A ‘time-of-flight’ technique is used to ascertain the spatial measurement location while the Doppler shift frequency is used to attain instantaneous flow velocity. For any measurement scenario using UVP, factors like selection of ultrasonic basic frequency, transducer settings and presence of tracer particles need to be carefully chosen. Position
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information is obtained from the time lapse of echo reception after the pulse emission, τ; which relates to the time-of-flight of the ultrasonic pulse. The position x is determined using the velocity of sound c as follows: x¼
cτ 2
ð2Þ
The denominator arises as a result of path length of the pulse travel is both for emission and return. The Doppler shift frequency fD at the instant gives the velocity (V) information at that location from the following relationship: V¼
cf D 2f o
ð3Þ
where fo is the basic frequency of the ultrasonic transducer. In summary, instantaneous velocity is obtained by measuring the instantaneous frequency as a function of time after the pulse emission. Fig. 1 shows the schematic of the measurement principle in channel geometry. In this case, the transducer is located outside the top channel wall with some coupling agent (usually ultrasonic gel) to provide acoustic coupling (Fig. 1a). The ultrasonic echoes as received by the transducer when switched to the receiving mode are shown in Fig. 1b. The echoes generated within the flow are due to the presence of tracer particles with additional strong echoes reflected from near and far walls of the channel geometry. A series of echo signals at fixed time instants from the tracer particles are used detect the Doppler shift frequency which provides the velocity at which it is travelling. Since the Doppler shift frequency is directly related to the velocity value, the UVP system does not require a calibration procedure. Fig. 1c shows the typical velocity profile after relevant post-processing steps to obtain velocity as well as spatial information. The spatial information is obtained from time lapse between the emitted and received pulse. More information on this technique can be found elsewhere (Takeda, 2012).
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1.5. On-site assessment of SX settler behaviour An on-site qualitative assessment of any settler design modification (including general maintenance) can be obtained by comparing organic entrainment losses before and after completion of a particular modification. Whilst such modifications may be predicated on CFD and/or physical model tests, the entrainment analysis approach only provides an overall outcome, and cannot provide detailed information in regard to specific effects such as the flow patterns within the settler. Although laser-based techniques such as Particle Image Velocimetry (PIV) and Laser Doppler Velocimetry (LDV) have been used to assess fluid flows including in laboratory-based SX systems (Bujalski et al., 2006; Drumm et al., 2011), they are restricted to analysis in transparent (for the laser) liquid systems, and are not particularly suited for analysis in most or all commercial SX systems. With the advent of the ultrasonic velocity profiling (UVP) technique, which relies on echoes from particles within the flow medium, real time velocity measurements can now be made inside operational solvent extraction settlers. Measurement using the UVP technique (Mohanarangam et al., 2010; Morgan et al., 2011; Yang et al., 2011) not only provides an insight into flow patterns within a given settler design, but also provides the necessary data to validate and refine CFD models leading to more accurate predictions of flow behaviour. The combination of both UVP and CFD analysis by CSIRO at another commercial operation (Minara Resource's Murrin operation in Australia) has recently been disclosed (Lane et al., 2012).
1.6. Aim The present work details the experimental technique of UVP and its application to obtain velocity measurements of both aqueous and organic phases in commercial SX settlers. This is demonstrated by the assessment of two solvent extraction settlers at a commercial copper SX operation. The assessed settlers were both in the first extraction stage within separate SX trains (trains A and D) namely E1A and E1D which were run ostensibly under the same operating conditions. Measurements of the 2D flow structures in a horizontal slice in each of the organic and aqueous phases were obtained in each settler in order to compare flow structures at the selected levels between settler E1A with old furniture and settler E1D with new furniture. Additionally, the E1A settler was assessed prior to routine maintenance, meaning there was scope for operationally undesired aspects such as damaged/ blocked pickets and sediment build up. The E1D settler on the other hand had just been cleaned and fitted with new furniture.
2. Experimental technique 2.1. UVP analysis equipment
Fig. 1. Schematic of the measurement principle. (a) Measurement system. (b) Ultrasonic echo signal. (c) Measured velocity distribution (Takeda, 2012).
2.1.1. UVP system, ultrasonic transducer and cabling A Met-Flow UVP system (Model: UVP-DUO MX) was used to determine the velocity field inside the settlers. The UVP system used during this measurement campaign had an integrated multiplexer which can serially scan quickly through up to 20 transducers and still retain time resolution. A maximum number of up to six transducers were utilised at any one time during the velocity field measurements inside the settler. The UVP software was installed on a laptop which was used to control the UVP hardware via the Ethernet. All measurement files were also saved in the same laptop which was processed later to provide two-dimensional maps.
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Six ultrasonic transducers with a low frequency of 0.5 MHz (outer diameter 23 mm and 40 mm long) were chosen for the current trial for their good propagation abilities and lower signal power losses, since the attenuation of acoustic energy is proportional to the square of frequency. Under the circumstance of relatively low fluid velocity inside the settler, the maximum measuring distance of the 0.5 MHz transducer was approximately three metres along its axis. Additionally, two transducers with frequency of 1 MHz (outer diameter 13 mm and 40 mm long) were also chosen for vertical UVP measurements in order to measure the normal or rise velocities in both the organic and aqueous phases at various points in the settler. As the transducers were relatively small compared to the scale of the industrial settler, they could be placed inside the liquid flow without generating a large disturbance in the measured flow field. Measurement errors incurred using any ultrasonic Doppler technique like UVP system to measure flow velocities are relevant to this current work. Errors specific to this work include the ability of the UVP system to measure low velocity flows. Based on this, the on-axis velocity resolution that could be achieved for the current work was 1.4 mm/s using 0.5 MHz transducer at its full penetration depth. The other source of errors includes the transducers beam size induced due its half-angle and the presence of enough particles (to act as tracers) as well as their motion at very low velocities. The latter was monitored using the validity rate (%) provided by the UVP software. In addition, a custom modification was carried out to extend the standard transducer cable length of 4–50 m due to access and safety requirements regarding potential ignition (sparking) sources requiring the UVP-DUO monitor and laptop computer to be located a large distance away from the settler. Laboratory tests showed that only limited noise was introduced with the addition of a 50 m RG58 extension cable. By optimising the parameter settings on the UVP system, it was estimated that the errors caused by the 50 m extension cable could be kept to less than 5%. 2.1.2. Transducer holder A custom-designed transducer holder consisting of a 400 mm square stainless steel (316L) base plate with a 20 mm diameter stainless steel threaded pipe attached to its bottom was used. Two Perspex disks, one for organic flow and other for aqueous flow measurement, were mounted on the rod which carried the UVP transducers. The disks could be moved along the pipe and fixed at any location with the aid of a shaft (attached to the disk) and bolt arrangement. Each disk was designed to hold four transducers aligned in three perpendicular directions. At any measurement location inside the settler one 0.5 MHz transducer was used to measure the longitudinal velocities or two 0.5 MHz transducers measure transverse velocities (one after the other). The alignment at a specific location inside the settler was carried out manually. 2.2. Settler configuration and operating conditions: Multiple trains of mixer settler units as shown in Fig. 2 are used at the commercial operation. The identically sized E1 settlers of trains A and D (i.e. E1A and E1D) were nominated for fluid flow characterisation using the UVP system. The copper-rich aqueous flow from the PLS (Pregnant Leach Solution) pond enters the E1 mixer along with organic coming from the E2 settler which is arranged in series with E1. Settling tests were conducted each day prior to beginning the flow mapping. For both E1A and E1D, a representative sample was collected from the mixer box in a 1000 ml cylinder which enabled visual confirmation of aqueous continuous operation. The resulting O/A ratio value combined with the known aqueous flow rate obtained from an on-site flow metre
Fig. 2. Operational flow sheet for trains A and D at the commercial SX operation (E1, E2 and EP – extraction stage settlers, W – wash stage settler, S – strip stage settler, Elec. – electrolyte, raff – raffinate, PLS – pregnant leach solution).
Fig. 3. Plan view of the settlers with major internal features before (left) and after (right) installation of settler furniture (not to scale).
enabled calculation of the organic flow rate. The operating conditions of both settlers were kept as similar as possible to assess the impact on flow patterns of the old furniture before maintenance against the cleaned settler containing newly installed furniture. Fig. 3 is a schematic illustrating one half of each settler with their (symmetric) internal configuration. Although the specific dimensions of the settler remain confidential, it can be stated that length and width both exceeded 20 m. For both settlers, the organic and aqueous mixture exiting from the mixer is introduced centrally into the settler; with a chevron type picket fence located a short distance downstream of the inlet. The E1A settler, which contained one picket fence located downstream of the chevron, was assessed prior to routine maintenance. As such, there was scope for operationally undesired aspects to be present in this settler, such as damaged/blocked pickets and sediment build up. In contrast, the E1D settler had been cleaned beforehand, and was fitted with newly designed furniture comprised of a picket fence located further downstream (in comparison to E1A), followed by a coalescence pack. Organic and aqueous flow velocities were measured inside settlers E1A and E1D with the aid of the UVP system. The velocity measurement heights for the organic and aqueous layers were set at 50 mm below the organic top surface and 250 mm above the
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Free-surface Measures V velocity
ORGANIC Organic disk
Emulsion band Measures U velocity
Measures U velocity
Aqueous disk
AQUEOUS 0.5 MHz transducers
Fig. 4. Schematic of the setup with UVP transducers.
bottom of the settler, respectively. These measurement heights were carefully chosen to make sure that velocities measured were of their respective phases as well as to avoid any ingress of the other phase. The UVP measurement positions were chosen to obtain 2D velocity components in both X and Y directions at grid points at an appropriate distance apart to give a good representation of both organic as well as aqueous flow structures. For both settlers, in excess of 200 UVP measurement locations were assessed. The measurements were carried out sequentially one transducer at a time using the multiplexing option provided by the hardware. The settlers were nominally divided into various sections depending on their internal furniture location. For settler E1A, the area between the chevron and the picket fence was labelled as Section 1, with Section 2 being between the picket fence and the weirs. For settler E1D, Section 1 spanned between the chevron and the picket fence, Section 2 between the fence and the coalescence packs, and Section 3 from the packs to the weir system. Fig. 4 shows the schematic of the setup used in this work with UVP transducers mounted on them.
3. Results and discussion 3.1. Organic velocity flow field of settler E1A Fig. 5 shows the 2D velocity vector and streamline plot of the organic layer along a plane at a height of 50 mm below the top organic surface of the E1A settler. In this and several subsequent Figures, the small black circles along the centreline represent circular concrete pillars that support the roof over the settler. With the aid of a reference vector of 100 mm/s, the 2D vectors in Sections 1 and 2 indicate the direction and magnitude of the flow velocity in this plane at each grid point. The flow streamlines
Fig. 5. 2D velocity vectors, streamlines and longitudinal velocity contours of organic layer inside settler E1A. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)
shown as blue lines are instantaneously tangent to the velocity vectors in the flow field, and show the direction of fluid movement at any point in time. It can be seen that organic flow is highly nonuniform in Section 1 between the chevron and the picket fence. The majority of the flow experiences a strong sideways movement with some backflow very close to the chevron exit. The organic flow exiting the chevron has a stronger transverse motion (U velocity) along the X direction compared to longitudinal movement (V velocity) along Y direction. At this level for organic flow the magnitude of transverse motion is non-symmetric, being far greater on the right side of the section compared to the left. The maximum transverse velocity on the right side of 137 mm/s is almost 11 times the longitudinal velocity at the same grid point. The velocity magnitude on the left of Section 1 is not as high as the right which gives rise to a rather small recirculation pattern close to chevron near the side wall. Fig. 5 also provides a colour contour map of the longitudinal (Y direction) component of organic velocity V, inside settler E1A. Under the convention used here, flow towards the outlet weir has a negative velocity, whereas reverse flow back towards the chevron is registered as a positive velocity. In Section 1, the organic flow field is partly coloured by velocities ranging from 0 to 35 mm/s, with the rest of the section coloured pale blue, indicating a backward flow directed towards the chevron. Overall the flow moves downstream towards the picket fence with a small magnitude. The observed non-symmetric flow behaviour in this plane prior to the picket fence is possibly attributable to obstructions on the left hand side chevron which forces most liquid to favour passage through the right side of the chevron. The resistance of the picket fence which is located only a short distance downstream of the chevron apex inhibits longitudinal motion of this liquid. Instead, the restricted flow through the left side of the chevron could create an area of lower pressure which would encourage the observed transverse flow of liquid from the right hand side back towards the middle and slightly across into the left side of Section 1. However, other phenomena could also account for this flow behaviour, such as
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the effect of flow motion below the measurement section as well as possible non-uniform distribution of feed entering the settler inlet from the mixer. Section 2 of the settler extends from the exit of the picket fence up to the weirs with no furniture in-between. Within the first half of Section 2 downstream of the picket fence, the bulk of the organic flow moves in the reverse direction, i.e. a backflow. A slight transverse motion tending to favour flow towards the central line of the settler is also apparent. The reversed flow behaviour is suspected to be caused by a strong upward flow of organic from deeper in the settler which was observed visually in the settler during the measurement campaign. This is discussed further in Section 3.3 below. The majority of organic flow for the second half of Section 2 flows uniformly and in a more expected manner towards the weirs except for a prominent recirculation region found a couple of metres upstream from the weir on the right hand side of the settler. The presence of several circular pillars located down the middle of the settler seem to have minimal effect on the overall flow pattern. An interesting feature is highlighted by the streamlines at the proximity of the third pillar downstream of the picket fence is a point where liquid flows radiate outwards in all directions. There was also a backflow region (blue) near the right side wall which was also visible from the aqueous flow contours discussed further below (Fig. 6). The organic flow gained momentum as it reached the exit of the settler leading to the overflow weir, as expected.
3.2. Aqueous velocity flow field of settler E1A The aqueous flow velocity measurements for settler E1A were carried out at a height of 250 mm from the bottom of settler. Fig. 6 illustrates the 2D velocity vectors, streamlines and longitudinal velocity contours of the aqueous layer in Sections 1 and 2. In Section 1, the flow exits from the chevron and flows in a transverse manner to both sides due to the resistance offered by the picket fence directly in front of it. The maximum transverse
Fig. 6. 2D velocity vectors, streamlines and longitudinal velocity contours of aqueous layer inside settler E1A. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)
velocity experienced on the left side of the settler is around 212 mm/s which is almost 10 times greater than the longitudinal velocity experienced at the same grid location. A small flow recirculation is also observed in the right side of Section 1. From the contour plot showing the aqueous flow in Y direction, the majority of Section 1 is depicted in green signifying a forward flow velocity of up to 35 mm/s directed towards the picket fence. The blue regions within this layer indicate a reversed flow back towards the chevron. The aqueous flow for Section 2 from the exit of the picket fence to the weirs depicts a relatively uniform albeit rapid average flow of 200 mm/s downstream along the width of the settler, with a centrally localised jet exiting at 340 mm/s. There is some crossover of aqueous flow from the right side of the settler to the left which associates with the recirculation flow pattern at the far left end of the settler near the weir. This recirculation is believed to be attributable to restricted longitudinal flow arising from the presence of an outlet pipe which passes downwards through the settler in this region. Another large recirculation zone is present near right side wall of the settler a short distance upstream of the weir. This recirculation zone occurs directly below the same feature observed in the organic layer (Fig. 5) and is considered to be due to the presence of a mechanical structure close to the proximity of where the measurement was taken. The longitudinal flow velocity decreases as it reaches the outlet, indicating the underflow weir does not reach this height, i.e. the flow exits below the measurement plane. Overall, the 2D results for the organic and aqueous flow patterns in the E1A settler as measured at these two heights indicates non-uniform and thus non-ideal flow behaviour. 3.3. Investigation of reversed flows in the organic layer for E1A Given the reversed flows measured in the organic plane (as well as noticed visually on the surface), additional 2D flow velocity measurements were obtained at various heights inside a targeted section of the settler. These detailed measurements were conducted in the Y–Z plane very close to the centreline of the settler (across its width). The first measurement grid point along its length was at 27% (location expressed as a percentage normalised against the total length of the settler) from the inlet and the final at 55%. The measurement grid for measuring flow velocities across the settler's depth is shown in Fig. 7a. Five equidistant measurement points (Y1–Y5) were chosen along the longitudinal section of the settler. The normal or ‘rise’ velocities W (along the Z direction) were measured using the downward pointing 1 MHz transducer from the organic disk (positioned 50 mm below the top surface, as described in Section 2.1). Normal velocities were also measured using another 1 MHz transducer pointing up from the aqueous disk (positioned 250 mm above the bottom of the settler). The longitudinal velocities were measured at five different heights by changing the location of the transducer. A total of 30 UVP holder positions were used to obtain 2D velocity components in both Y and Z directions. Fig. 7b shows the 2D velocity vector and streamline plot across the depth of the settler, with the contour plot shown in Fig. 7c. The flow in the specific region analysed experiences very high normal (rise) velocities averaging 56 mm/s at 150 mm from the organic surface. A clear distinction in normal velocity (Z direction) can be seen with top half of all the vertical velocities measured moving towards the free-surface. However, below this the flow is dominated by the longitudinal velocity, with a sharp drop in normal velocities. Fig. 7c shows the plot of longitudinal velocity with the aid of a colour contour map. The top half of the section up to two measurement grid points coloured blue experience a backward longitudinal flow directed towards the settler inlet consistent with
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Fig. 7. (a) Measurement grid across the depth of the E1A settler, (b) two-dimensional vectors and streamlines, (c) vertical/rise velocity and (d) longitudinal velocity.
that indicated in Fig. 5. Deeper in the settler, a more uniform longitudinal flow (green colour) with a velocity in excess of 75 mm/s up to a maximum of 225 mm/s is observed. This analysis aligned with the on-site observations and the results shown in Fig. 5 highlights the potential to use UVP to measure flow velocity in planes other than the horizontal.
3.4. Organic velocity flow field of settler E1D Fig. 8 shows the 2D velocity vectors, streamlines and longitudinal velocity contours of the organic phase at the height of 50 mm below the organic surface for settler E1D. In Section 1, the motion of the organic flow close to the exit of the chevron is relatively symmetric. A couple of recirculation regions are evident as illustrated by streamlines, and the 2D contour plot indicates the central region adjacent to the chevron (shaded blue) to have some reverse flow. However, the flow is generally in the desired direction. As the flow reaches the picket fence downstream it continues to move forward but with some transverse motion directed towards the centreline of the settler due to the aforementioned reverse flow. The maximum longitudinal velocity
experienced very close to the entry of the picket fence is 100 mm/s. The majority of organic flow in Section 2 between the picket fence and the coalescing packs experiences a uniform longitudinal flow heading downstream. Examination of the velocity vectors at the entry and exit of the picket fence reveals that whilst the longitudinal velocity remains similar (as shown by the similar colours on the contour plots), the transverse flow is materially reduced by passage through the picket fence, an expected outcome. The longitudinal organic flow remains relatively uniform throughout Section 2, a feature that persists in Section 3 of the settler as it moves beyond the coalescing packs towards the overflow weir. The flow gains momentum as it approaches the overflow weir, particularly on the right side of the settler. The organic flow exits the settler with an average velocity of 100 mm/s and a maximum velocity of 170 mm/s. 3.5. Aqueous velocity flow field of settler E1D Aqueous layer velocity measurements were made at a plane 250 mm from the bottom of the E1D setter. Fig. 9 represents 2D flow features of the aqueous flow in Sections 1–3. In Section 1,
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Fig. 8. 2D velocity vectors, streamlines and longitudinal velocity contours of organic layer inside settler E1D. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)
momentum with an averaged longitudinal velocity of 62 mm/s just before entering the picket fence. As seen in the organic layer, progression of the aqueous phase through the picket fence materially reduced transverse flow, resulting in the desired outcome of a relatively uniform longitudinal flow in Section 2. The longitudinal flow velocity in this section decreased from about 121 mm/s upon passage through the picket fence to about 54 mm/s entering the coalescing packs. This reduction in entry and exit velocities is due to the increased resistance caused at the entry of the coalescing packs. There is an increase of longitudinal velocity in Section 3 at the exit of the packs after which the flow moves downstream until it reaches the weir system where it is slowed down to 38 mm/s. This reduced longitudinal flow velocity is consistent with the behaviour seen in E1A and is explained similarly. Marked aqueous flow velocity differences seen at the entry and the exit of the packs is attributed to the design of the packs whose entry and exit do not match exactly with the aqueous layer measurement height of 250 mm from the settler floor. While there are some flow reversals in Section 1, the flow in Sections 2 and 3 is almost uniformly directed downstream as shown by the coloured contour plot. Overall, the 2D results for the organic and aqueous flow patterns as measured at these two heights in settler E1D indicate more ideal flow pattern behaviour compared to the E1A settler. One of the reasons for this improvement could be attributed to picket fence designs deployed in the settlers. It can be concluded from the previous work within our group (Lane et al., 2012) and elsewhere that picket fence design plays a pivotal role in facilitating a good flow distribution across the settler. Whilst the specific details of the chevron and picket fences cannot be disclosed, it can be stated that the pickets in E1A provided a greater degree of flow blockage relative to those in E1D. The lesser blockage of E1D resulted in a drop in velocities immediately exiting these fences which helped to eliminate strong jets. The observed reduction in short circuiting within the settler relative to E1A could be attributable to either fence design, maintenance protocol or both. 3.6. Operational outcome The results presented in the previous section clearly demonstrate the improved and streamlined flow patterns for E1D settler both for organic as well as aqueous phases in the horizontal planes of measurement compared to E1A settler. The sum of the modifications undertaken for the E1D settler resulted in flow patterns closer to plug behaviour at the levels assessed relative to the E1A settler. Plug flow behaviour is expected to yield an optimum separation of each phase, and therefore would be expected to result in decreased entrainment. In practice, the actual benefit of all the modifications was found by the company in question to result in a 60% decrease in organic entrainment, resulting in a substantial financial benefit.
Fig. 9. 2D velocity vectors, streamlines and longitudinal velocity contours of aqueous layer inside settler E1D. (For interpretation of the references to colour in this figure caption, the reader is referred to the web version of this article.)
a very high velocity of aqueous outflow is observed at the centre of the settler very close to chevron's apex. The flow reaches a maximum velocity of 370 mm/s with an averaged velocity of 185 mm/s at the chevron exit. The flow exiting the chevron at the centre splits into two streams with a ‘jetting effect’ with both strong longitudinal and transverse motion. This causes the flow to be preferentially directed towards the sides of the settler, an intended outcome from the chevron design. A region of recirculation is apparent on the left side of the settler as depicted by streamlines. Further downstream of Section 1, the flow loses
4. Conclusion A detailed description of the pulsed Doppler UVP technique and demonstration of its application to measuring in situ flow patterns in commercial solvent extraction settlers has been presented via two-dimensional flow mapping of settlers E1A and E1D at a commercial copper SX operation. These settlers operated under near identical inlet flow conditions, but differed in their internal furniture configuration. Measurements were carried out along a plane on both the aqueous and organic phases. Clear differences in the flow patterns of the two settlers were detected by the UVP instrument. The E1D settler, which showed superior organic and aqueous flow behaviour, had the new and improved
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furniture with a chevron, picket fence and a coalescence pack installed inside it in comparison to E1A which had only a chevron and picket fence. Given the relatively constant nature of the other operating parameters, it is concluded that the design and physical location of the furniture within E1D settler coupled with them being freshly installed and therefore not affected by any undesired aspects such as damaged/blocked pickets and sediment build up lead to the improved performance in flow patterns. The increased understanding of settler flow pattern behaviour afforded to the operating company by UVP analysis helped them to improve the economics of their process. Besides the benefit afforded by UVP analysis for understanding existing flow patterns in commercial SX settlers, the results can also be used to enable the development of improved CFD modelling of flow patterns in settlers.
Acknowledgements The authors would like to acknowledge the strong support they received from site management, technical staff and support personnel of the commercial copper SX operation both prior to and during the site work. The authors would also like to thank MERIWA and the commercial sponsors of the Solvent Extraction Technology projects (SXT and SXT2) for their support and also acknowledge the support of the CSIRO Minerals Down Under National Research Flagship and Parker CRC for Integrated Hydrometallurgy Solutions. References Brunn, P.O., Wunderlich, T., Muller, M., 2004. Ultrasonic rheological studies of a body lotion. Flow Meas. Instrum. 15, 139. Bujalski, J.M., Yang, W., Nikolov, J., Solnordal, C.B., Schwarz, M.P., 2006. Measurement and CFD simulation of single-phase flow in solvent extraction pulsed column. Chem. Eng. Sci. 61, 2930. Drumm, C., Hlawitschka, M.W., Bart, H.J., 2011. CFD simulations and particle image velocimetry measurements in an industrial scale rotating disc contactor. AIChE J. 57, 10.
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