paper Farrow 1996

ELSEVI:ER Int. J. Miner. Process. 46 (1996) 263-275

A new procedure for assessing the flocculants

J.B. Farrow, J.D. Swift

performance of

CSIRO Division of Minerals, 7 Co&on Street, Water-ford, W.A., PO Box 90, Bentley, W.A. 6102, Australia

Received 28 July 1994; accepted 20 June 1995

Abstract

Laboratory batch cylinder tests are commonly used to assess the effectiveness of flocculants to enhance the settling rate of suspensions. In this paper, the performance obtained from batch cylinder tests is compared to that from a new flocculation system based upon vertically mounted, concentric rotating cylinders (Couette geometry). It has been shown that the results from batch cylinder tests are very dependent upon factors such as the number of inversions and the cylinder diameter. These limitations are overcome by conducting flocculation in a continuous mode under reproducible mixing conditions such as prevail within a mechanically driven Couette device. Results (obtained with this equipment are found to be of much higher reproducibility than from cylinder tests conducted under set conditions.

1. Introduction

Synthetic flocculants (high molecular weight polyelectrolytes) are key reagents in the mineral processing and other industries. These reagents are used to aggregate fine particles into clusters (floes) which settle faster, enhancing solid-liquid separation in gravity thickeners.

There is a wide range of flocculants available from a number of commercial suppliers. These vary in chemistry, ionic charge, molecular weight, cross-linking, branching and physical form (e.g. emulsion, powder, gel or bead). Perhaps because of this diversity, but also because flocculants are supplied as proprietary reagents, the selection of a flocculant for a particular industrial application is not based upon their specific chemical and physical composition. Instead, performance parameters (e.g. settling rate, residual turbidity, sediment density and sediment rheology) are used to characterise the quality or extent of flocculation achieved with a given flocculant.

0301-7516/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0301-7516(95)00084-4

The process of selecting a flocculant for a specific application is done by screening the available flocculants in laboratory tests, with plant trials being conducted only for the most suitable materials. The traditional laboratory procedure for assessing flocculants is the batch cylinder test (Song et al., 1992; Font, 1991; Bhatty et al., 1982; Michaels and Bolger, 1962; Richardson and Zaki, 1954), in which a suspension is flocculated within a measuring cylinder and the settling rate determined from the rate of fall of the mud-line. This procedure is used by all commercial flocculant suppliers in the evaluation of their products, and is also widely used by researchers (Caskey and Primus, 1986; Henderson and Wheatley, 1987; Gill and Herrington, 1989). The procedure, while having the advantage of being extremely simple, has several problems. Some of these are: - being a batch procedure, whereas in industrial practice flocculation is generally

conducted on a continuous basis. - the dependence on the nature of the agitation. - the dependence on the diameter of the cylinder. - the experimental reproducibility associated with the procedure.

In this paper, the flocculation of an industrial thickener feed suspension, at a set flocculant dosage, has been used to demonstrate the inherent problems associated with the batch cylinder test procedure. An alternative procedure is proposed, in which flocculation is conducted in a continuous manner within the gap between a rotating inner cylinder and a stationary concentric outer cylinder (Couette geometry). Such a device has several advantages over the use of a stirred baffled tank (Waters, 1985; Keys and Hogg, 1978) for continuous flocculation. These include the ability to operate with shear fields ranging from laminar flow to wavey vortex to fully turbulent (Schlichting, 1979 p. 526), a lower probability of particle short-circuiting, greater flexibility in the variation of the residence time and the ability to conduct two stage flocculant addition. The disadvantage is the devices mechanical complexity.

While devices based on Couette geometry have been used previously for both coagulation (Stein et al., 1986; Ives and Bhole, 1977) and flocculation studies (Ivanaus- kas et al., 1987; Muhle and Domasch, 1991; Smith and Kitchener, 1978), their design has been unsuitable for rapidly settling, fragile aggregates such as those formed by the flocculation of mineral suspensions. This has been overcome in this work by the use of a vertical flow-through arrangement.

2. Experimental

2.1. Materials

2.1.1. Mineral suspension All flocculation experiments were conducted using a thickener feed slurry obtained

from the operations of Argyle Diamond Mines Pty. Ltd. near Kununurra in the north of Western Australia.

J.B. Farrow, J.D. Swif/lnt. J. Miner. Process. 46 (1996) 263-275 265

This material consisted of a mixture of quartz, feldspar, hematite, goethite, montmo- rillonite and other minor components in aqueous suspension at ambient temperature. The solids were nominally below 100 pm in size with a d,,, d,, and d,, (by laser light diffractilan, Malvem Mastersizer) of 1.5, 10.8 and 69 pm, respectively. The average density was N 2500 kgme3.

All tests were conducted with suspensions containing 3.09 wt% solids, which after the addition of the flocculant solution produced a suspension of 3.00 wt%. The 3.09 wt% suspensions were produced by dilution of a concentrated sample (45 wt% solids) obtained from the plant. The diluent used was a synthetic supematant containing MgSO, (0.096 gL_), MgCl, (0.076 gL_), CaCl, (0.061 gL_) and NaHCO, (0.168 gL_) formulated to match the chemical analysis of the plants process water. Under these conditions the suspension was weakly coagulated.

2.1.2. Flocculant The flocculant used was Magnafloc 333 (Allied Colloids, Bradford UK), a high

molecular weight, dry powder polyacrylamide. A concentrated flocculant solution (1 wt% active) was prepared by first wetting the

surface of the flocculant (2.00 g) with a small quantity of ethanol, and then adding water (198 g). The sample was then gently mixed on an orbital shaker for 2 h to assist with flocculant dissolution. This concentrated flocculant solution was stored in the dark at 4C.

Flocculant feed solutions (0.01 wt%) were prepared daily by dilution of the concentrated flocculant solution with deionised water. The sample was gently mixed on an orbital shaker for 15 minutes prior to use.

A flocculant dosage of 100 g t- (grams of active flocculant per tonne of dry solids) was used in all tests.

2.2. Equipment

2.2.1. Cylinders The ,volumes of the cylinders (standard stoppered glass B-grade measuring cylinders)

used were 50, 100, 250, 500 and 1000 mL. These had internal diameters of 12, 26, 37, 50 and 61 mm, respectively.

2.2.2. Concentric cylinders mixing device The Iconcentric cylinders mixing device (termed a shear vessel) is shown schemat-

ically in Fig. 1. It consisted of an external fixed cylinder (I.D. 210 mm) and an internal rotating cylinder (O.D. 200 mm, length 120 mm) driven by a variable speed motor (N lo-800 + 1 rpm). The volume of the vertical annular gap between the cylinders was 0.350 L. The conical section at the base of the cylinders had a pitch of 45, maintaining the gap width of 5 mm. The volume of this section was 0.165 L.

In all shear vessel tests, the inner cylinder rotation speed was fixed at 200 rpm. Under these conditions, turbulent flow conditions were approached (Schlichting, 1979, p. 526). These conditions were selected since flocculation of most mineral suspensions occurs in practice under reasonably turbulent conditions.

266 J.B. Farrmv, J.D. S~?fi /ht. J. Miner. Process. 46 (19961 263-275

Fig. 1. Schematic layout of the equipment used for the continuous flocculation of suspensions under controlled mixing conditions.

2.3. Flocculation procedures

2.3.1. Cylinder tests The 3.09 wt% thickener feed slurry was added to each cylinder, the amount being

97% of the cylinders volume. The required quantity (3% of the cylinders volume) of the 0.01 wt% flocculant solution was then added from a plastic syringe. The time required to add the flocculant was about 3 seconds.

The slurry and flocculant were then mixed by hand inversion. This involved turning the stoppered cylinder end-over-end for a specified number of times, at a rate of 20 inversions per minute.

2.3.2. Shear vessel tests Feed suspension (3.09 wt%) was delivered from an agitated baffled feed tank (8 L)

via a peristaltic pump to the top of the shear vessel at a constant rate of 125 mL min- . Flocculant solution (3.86 mLmin_) was pumped via another computer controlled peristaltic pump from a holding tank (1 L) into the shear vessel through a hole in the outer wall, 40 mm below the slurry surface. The flocculated slurry was drawn down through the shear vessel by an underflow peristaltic pump. The underflow pump rate (129 _t 1 mLmin_ > was controlled by a computer in response to the output from a conductivity liquid level sensor (Fig. 1).

As the slurry exited the base of the shear vessel it passed through a cylindrical tube (the settling column, 14 mm internal diameter, 220 mm in length) before entering the underflow pump. By simultaneously closing the valves A and B (Fig. 1) fitted at the top and bottom of the settling column and also simultaneously opening the valve C, the flow could be diverted and flocculated suspension isolated within the settling column. A

J.B. Furrow, J.D. Swifr/Int. J. Miner. Process. 46 (1996) 263-275 261

period of 1.5 minutes was allowed for the establishment of steady state conditions prior to the first measurement. Once measurements of the flocculation state were completed (outlined below), valve C was closed and valves A and B opened. A period of 5 minutes was then allowed to regain steady state conditions before making another measurement.

In some cases, the flocculated suspension passing from the shear vessels underflow pump was used in cylinder tests. The time taken to fill a cylinder ranged from 23 seconds for 50 mL cylinders to 465 seconds for 1000 mL cylinders. Once filled, each cylinder was mixed by hand using two end-over-end inversions at the rate of 20 inversions per minute prior to measurement.

2.4. Me(zsurement qf the flocculation state

In all tests, the flocculation state of the suspension achieved under the applied mixing conditions was assessed by measurement of the settling rate and the residual turbidity of the supematant. These are detailed below for each type of test.

2.4.1. Cylinder tests After mixing by end-over-end inversion, measurements were made of the height of

the mud-line (the demarcation line between the settling solids and the residual super- natant) as a function of time. The settling rate (in m h- , the usual units for settling rate in the mineral processing industry) was determined from the period where there was a linear ra.te of fall of the mud-line. Typical data is shown in Fig. 2. The initial non-linear section :relates to the subsidence of turbulence due to mixing.

After 2 minutes settling, a calibrated nephelometer (Analite, Australia) was inserted to a distance of 50 mm below the meniscus to record the residual turbidity (in units of NTU) of the supematant at this point. The measuring head of this unit had a diameter of 20 mm which restricted its use to cylinders with a larger internal diameter.

0 10 20 30 40 50 60

Time (s)

Fig. 2. Typical data from the measurement of the position of the mud-line versus time. The settling rate is calculated from the slope of the linear section of the curve, as indicated by the solid line.

268 J.B. Farrow~, J.D. .S~~ft/lnr. J. Miner. Process. 46 (lYY61 263-275

2.4.2. Shear vessel tests The settling rate of the flocculated suspension within the settling column was

determined from the linear section of the mud-line height versus time curve. The data was of the same general form as that shown in Fig. 2. The measured settling rate was expressed in units of m hh . The turbidity (in NTU) of the supematant was measured after 2 minutes using a modified Analite nephelometer fitted flush to the settling columns side wall 50 mm below valve A (see Fig. 1).

2.5. Temperature

All cylinder and shear vessel tests, although not directly thermostatted, were conducted in a room controlled at 22 + 2C.

2.6. Measurement errors

The errors associated with the settling rate and turbidity measurements have been calculated at the 95% confidence level.

3. Results and discussion

3.1. Flocculation tests using cylinders

3.1.1. effect of number of inversions The results from flocculation tests conducted in 26 mm and 50 mm cylinders are

shown in Fig. 3 in terms of the settling rate as a function of the number of end-over-end inversions of the cylinder. It can be seen that the settling rate achieved is strongly

Settling rate (m h-1)

4o I 8 l/ W 26mm I 0 50mm

A------ 0 2 4 6 a 10 12

Number of inversions

Fig. 3. Hindered settling rate as a function of the number of end-over-end inversions using either 26 mm or 50 mm cylinders.

J.B. Farrow, J.D. Swijt / ht. J. Miner. Process. 46 (19961 263-275 269

1200

1000 -

Turbidity (NTU) 800-

800 -

400 -

200 - 26mm I H

0 50mm

0 I 1 I * I * I * 0 2 4 8 8 IO 12

Number of inversions

Fig. 4. Residual turbidity as a function of the number of end-over-end inversions using either 26 mm or 50 mm cylinders. (The 50 mm values for 1 to 5 inversions have been horizontally off-set slightly for clarity.)

dependent upon the number of inversions for both cylinder sizes, decreasing as the number of inversions increases.

Fig. 4 shows the residual turbidity as a function of number inversions from the tests corresponding to the data given in Fig. 3. With both the 26 mm and 50 mm cylinders, the residual turbidity decreases as the number of inversions is increased from one to five. The residual turbidity was measured for additional inversions in the 50 mm cylinder only, and was found to increase with increasing number of inversions. This behaviour probably indicates that efficient mixing of the flocculant and the suspension in the cylinder requires about four to five inversions, thereafter with additional inversions the aggregates start to break-up, resulting in higher residual turbidities.

From the data presented in Figs. 3 and 4, it is clear that the optimum number of cylinder inversions depends upon the parameter by which flocculation efficiency is assessed. In this case, the maximum settling rate is achieved with only one inversion whereas the minimum residual turbidity requires four inversions. In all subsequent cylinder test work it was decided to use four inversions during the mixing stage, since this was considered to correspond to maximum flocculation.

Although not demonstrated in this study, the rate of inversion also has a significant effect upon the flocculation efficiency. Consequently, the inversion rate in all tests was kept as constant as possible ( _ 20 inversions per minute) as outlined in the experimental section.

3.1.2. Ejyect of cylinder diameter The effect of cylinder diameter on the flocculation state in terms of settling rate and

residual turbidity is shown in Figs. 5 and 6. It was not possible to determine the residual turbidity in the 12 mm cylinder because of the size of the turbidity sensors measurement head (20 mm).

Fig. 5 shows that the diameter of the cylinder used for the flocculation test has a significant effect upon the flocculation efficiency as assessed by the settling rate. The

210 J.B. Farrow. J.D. Sw(ft/ ht. J. Minrr. Process. 46 f I9961 263-275

I I 0 10 20 30 40 50 60 70

Cylinder diameter (mm)

Fig. 5. Hindered settling rate measured in different diameter cylinders for suspensions flocculated within the cylinder.

settling rate decreases markedly as the diameter of the cylinder increases, falling from N 17 mh- with the 12 mm cylinder to - 11 m h- with the 61 mm diameter cylinder. This dependence of the settling rate on cylinder diameter is of the opposite trend expected for wall effects which would cause slower settling in smaller diameter cylinders. It probably relates to the efficiency of mixing of the flocculant with the suspension. It would be expected that as the cylinder diameter decreases, the mixing efficiency achieved by inversion would increase. This would result in more uniform distribution of the flocculant amongst the suspension and thus promotion of flocculant bridging over single particle adsorption, thereby producing better flocculation.

The residual turbidity data (Fig 6) obtained in four cylinders of different diameters

Turbidity (NW)

700 . , I, . . . . . , I , ,

600 - I 500 - I 400 -

I I 300 -

I 200 -

100 -

0 "*""'.','* 0 10 20 30 40 50 60 70


Fig. 6. Residual turbidity measured in different diameter cylinders for suspensions flocculated within the cylinder.

J.B. Farrow, J.D. Swij?/lnt. J. Miner. Process. 46 (1996) 263-275 271

Table 1 Mean settling rate and mean residual turbidities measured by three operators for equivalent flocculation tests conducted in 26 mm and 50 mm cylinders mixed by inversion

Operator No. 26 mm cylinder 50 mm cylinder

Mean Standard Mean Standard Mean Standard Mean Standard settling error of turbidity error of settling error of turbidity error of rate the mean (NTLJ) the mean rate the mean (NTU) the mean (mh-1 (%I (%b) (mh-) (%c) (%I

1 a 18.2f 1.7 9.3 454k64 14.1 15.3 f 1.9 12.4 477+111 23.2 2 18.1 f 1.6 8.8 410&71 17.3 15.1 f 1.2 7.9 406*50 12.3 3 16.6+ 1.8 10.8 400 f 82 20.1 12.2+ 1.1 9.0 626 f 164 26.2

Average b 17.6+0.9 5.1 420*37 8.8 14.2kO.9 6.3 500+70 14.0

a 10 measurements. b 30 measurements.

was too scattered for any clear interpretation of a trend. However, the measured values did vary considerably, from _ 600 NTU in the 37 mm cylinder to N 400 NTU in the 61 mm cylinder, indicating a strong effect of cylinder diameter on the residual turbidity.

3.1.3. keproducibility of cylinder tests The reproducibility associated with cylinder tests was assessed by three operators

each conducting 10 repeat flocculation tests using first 26 mm then 50 mm cylinders. Each operator carefully followed the same procedure, using a mixing regime of four end-over-end inversions at the uniform rate of 20 inversions per minute. The results are shown in Table 1.

The error in the mean settling rate determined by each operator was quite high, ranging from 7.9% for operator 2 to 12.4% for operator 1, both with the 50 mm cylinder. The mean settling rates calculated from all 30 measurements were 17.6 m hh (26 mm cylinder) and 14.2 mh- (50 mm cylinder) with corresponding errors of 5.1% and 6.3%, respectively.

The measured residual turbidities showed greater variation, with errors in the mean value for each operator ranging from 12.3% for operator 2 to 26.2% for operator 3 both with the 50 mm cylinder. The error in the average of the 30 residual turbidity measumments was 8.8% for the 26 mm cylinder and 14.0% for the 50 mm cylinder.

3.2. Flocculation tests using the shear vessel

3.2.1. t

212 J.B. Farrow. J.D. Swijt/ Int. J. Miner. Process. 46 (I9961 263-275

20 - T T

1 A Settling rate

,5 _

(m h-1) 10 -

5-

0 ".'.'*"'~" 0 10 20 30 40 50 60 70


Fig. 7. The settling rate measured in different diameter cylinders for suspensions flocculated in the shear vessel.

ranging from 122 NTU (26 mm cylinder) to 117 NTU (both 37 mm and 50 mm cylinders).

The independence of the settling rate on the cylinder diameter for material prepared by flocculation in the shear vessel (see Fig. 7) is in contrast to the strong dependence observed when flocculation was conducted in the cylinder itself (see Fig. 5). It is the general belief in the mineral processing industry that large diameter cylinders (> 50 mm) must be used in flocculation test work to obtain reliable results. The results in Fig. 7 prove that this is not true when flocculation is conducted under reproducible conditions (in this case, within the gap of the shear vessel) prior to the assessment of the flocculation state in a cylinder.

Turbidity (NW)

-0 10 20 30 40 50 60 70


Fig. 8. The residual turbidity measured in different diameter cylinders for suspensions flocculated in the shear vessel.

J.B. Farrow, J.D. Swift/lnt. J, Miner. Process. 46 (1996) 263-275 273

Table 2 Settling rate and residual turbidities measured by three operators for equivalent flocculation tests conducted using the shear vessel

Operator No. Shear vessel (14 mm diameter cylinder)

Mean settling rate (mh-)

Standard error of the mean (SC)

Mean turbidity (NTU)

Standard error of the mean (%I

1 a 36.8 + 0.6 1.6 123+14 11.3 2 38.3 + 1 .o 2.6 114+14 12.2 3 36.3 to.4 1.1 90*4 4.4

Average b 37.1+0.5 1.3 110*9 8.2

a 10 me,asurements. b 30 measurements.

The dependence of settling rate upon cylinder diameter for tests in which flocculation is conducted within the cylinder (see Fig. 5) is most likely due to differences in the mixing conditions during flocculation, and not to wall effects as is commonly stated in the lmineral processing and flocculant sales industries. With the end-over-end inversion method, the mixing conditions achieved within the stoppered cylinder are governed by the movement pattern of the trapped air pocket through the suspension as the cylinder is inverted. The nature of this movement depends upon the diameter of the cylinder, the inversion rate and the inversion angle, therefore producing different mixing conditions in cylinders of different diameters. The influence of wall effects would be expected to reduce the settling rate as the diameter of the cylinder decreased; clearly this was not the case over the range of diameters investigated (see Fig. 7) and for the size of the aggregates produced ( < 1 mm by visual estimation).

3.2.2. Reproducibility of shear vessel tests The reproducibility associated with the flocculation tests conducted with the shear

vessel was assessed in the same way as with the cylinder tests, using three operators each conducting 10 repeat flocculation tests. Measurements of the settling rates and turbidities of the flocculated suspensions were made within the 14 mm settling column shown in Fig. 1 using the procedures described earlier in the experimental section. The results are given in Table 2.

The error in the mean settling rate determined by each operator ranged from 1.1% for operator 3 to 2.6% for operator 2. The error in the mean settling rate determined from the 30 measurements made by the three operators was 1.3%. This compares to errors of 5.1% a.nd 6.3% in the mean settling rate for the 30 batch cylinder tests conducted with 26 mm and 50 mm cylinders (see Table 1). Clearly, better reproducibility is obtained in the settling rate measurements using the shear vessel compared to batch cylinder tests.

With the residual turbidity measurements, the difference in the reproducibility was not as large. The shear vessel tests resulted in a mean residual turbidity of 110 f 9 NTU

274 J.B. Farrow, J.D. Scv~ft/Int. J. Miner. Process. 46 (19961 263-275

(error 8.2%). This compares to 420 k 37 NTU (error 8.8%) and 500 $- 70 NTU (error 14.0%) for the batch cylinder tests conducted with 26 mm and 50 mm cylinders.

It is interesting to note that faster settling rates and lower residual turbidities were obtained when flocculation was conducted within the shear vessel and assessed within the in-line settling column (37.1 + 0.5 m hh, 110 & 9 NTU, see Table 2) compared to the case when flocculation was conducted and assessed within 26 mm or 50 mm cylinders (17.6 k 0.9 m hh , 420 k 37 NTU and 14.2 * 0.9 m h- , 500 f 70 NTU, see Table 1). As the data in Figs. 5 and 7 show, the difference in settling rates is not due to the differences in the diameter of the measurement cylinder. Batch cylinder tests could produce the same settling rate as the shear vessel tests if only one end-over-end inversion was used (e.g. 26 mm cylinder, see Fig. l), but the residual turbidity was always much higher indicating inferior flocculation.

The data given in Tables 1 and 2 shows that more appropriate mixing for flocculation is achieved with the continuous shear vessel method than in batch cylinder tests. Not only is the mixing more reproducible, leading to higher reproducibility in the measured settling rate and residual turbidity, but the mixing regime also promotes better flocculation of the suspension.

The shear vessel results presented in this paper have all been obtained at a fixed rotation speed of the inner cylinder, i.e. under a given agitation regime. Further work is being conducted at other shear vessel rotation speeds to investigate the effect that different agitation conditions have on the final flocculation state. This work is also investigating the effect of residence time upon the flocculation state achieved. This will be published shortly.

4. Conclusions

It has been shown that a continuous flocculation device based upon vertically mounted concentric cylinders (Couette geometry) is a reliable method for laboratory flocculation test work. This method overcomes most of the problems associated with batch cylinder tests, which are commonly used in industry for assessing the performance of flocculants. In particular, the method overcomes the strong dependence that batch cylinder tests have upon factors such as the number of cylinder inversions, the rate of inversion and the cylinder diameter.

The continuous flocculation test method using the shear vessel produces results of higher reproducibility compared to those obtained in batch cylinder tests, both for settling rate measurements and residual turbidity measurements. For the particular suspension used in this work, the error in the settling rate measured with the shear vessel was 1.3% compared to 5.1% and 6.3% for batch cylinder tests using 26 mm and 50 mm cylinders, respectively.

The common belief that large diameter cylinders are required in flocculation test work to avoid wall effects is not supported by this study. Providing a suspension is flocculated prior to introduction into a cylinder, then the settling rate shows little or no dependence on the diameter of the cylinder used (in the range 26 to 61 mm). When flocculation is done within a cylinder then the settling rate achieved does depend

J.B. Farrow, J.D. Swift/ ht. J. Miner. Process. 46 (1996) 263-275 275

strongly upon the diameter of the cylinder, but this is because of variations in the mixing conditions during flocculation and not because of wall effects.

Acknowledgements

The authors would like to thank Bill Hutton of Argyle Diamond Mines for his assistance with sample collection and also Ben Fletcher and Warren Jones for their assistance in conducting the experiments.

The support of this work by the following members of the Australian Mineral Industry Research Association is appreciated: Alcoa of Australia Ltd.; Argyle Diamond Mines Pty. Ltd.; Coal & Allied Operations Pty. Ltd.; Comalco Aluminium Ltd.; Cyanamid Australia Pty. Ltd.; Dorr-Oliver Pty. Ltd.; Kidd Creek Mines, Falconbridge Inc.; MIM Holdings Ltd.; Nalco Australia Pty. Ltd.; Pasminco Metals - BHAS; Pasminco Metals - EZ; Pasminco Mining - Broken Hill; RTZ Services Ltd.; Westralian Sands Ltd.

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