Minerals Engineering 70 (2015) 20–25

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Minerals Engineering

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Polymer-bridging flocculation performance using turbulent pipe flow

http://dx.doi.org/10.1016/j.mineng.2014.08.0190892-6875/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +55 55 9685 4999.E-mail addresses: ecarissimi@gmail.com (E. Carissimi), jrubio@ufrgs.br (J. Rubio).

Elvis Carissimi a,⇑, Jorge Rubio b

a Department of Environmental and Sanitary Engineering, Federal University of Santa Maria (UFSM), Roraima ave. 1000, CT, Santa Maria, RS 97105-900, Brazilb Laboratory of Mineral and Environmental Technology, Federal University of Rio Grande do Sul (UFRGS), Bento Gonçalves ave. 9500, Bldg 75, Porto Alegre, RS 91501-970, Brazil

a r t i c l e i n f o

Article history:Received 28 April 2014Accepted 29 August 2014Available online 25 September 2014

Keywords:FlocculationHydraulic flocculationCoiled reactorSerpentine reactorCoalFe(OH)3

a b s t r a c t

Hydraulic flocculation has attracted the attention of many researchers and designers due to its potentialapplications, simple design, small foot print and reduced electrical/mechanical energy consumption.Flocculation studies were conducted with two types of equipment, namely the Flocs Generator Reactor(FGR) and the Flocculation–Flotation (FF), comparing both aggregation devices. FGR is a compact systemwhereby the flocculation of particles occurs through a helical reactor and FF has a serpentine design. Bothdevices have plug flow regimes with suitable hydrodynamics to disperse the polymer and also to gener-ate flocs, enabling solid/liquid separation. This work summarizes recent results on hydraulic flocculationas a function of particle type and concentration (colloidal Fe(OH)3 and coal particles as suspensionmodels), polymer type and dosage. The FF and the FGR were evaluated individually and with the FFplaced ahead of the FGR. The effectiveness of the process was measured by indirect performance criteriasuch as solid/liquid separation. Best efficiency for the Fe(OH)3 flocs generation was obtained using theFGR, reaching settling rates of 22 mh�1. Best results for coal dispersions were obtained with the FF aheadto the FGR, reaching settling rates of 30 mh�1. In all cases, flocculation degrees were higher than 98% andshowed that efficiency is largely dependent on flocs characteristics (size, mass density and water con-tents). These data show a high performance of polymer-bridging flocculation efficiency for both hydraulicflocculation devices evaluated, considering two suspension models (Fe(OH3) and coal particles), andvalidate the potential of the in-line flocs formation prior to solid/liquid separation at high rates.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The aggregation of suspended particles is one of the first pro-cesses used as a pre-treatment step before solid–liquid separationby filtration, sedimentation or flotation. Most dispersed colloidalparticles are aggregated by coagulation and/or flocculation to formrelatively large aggregates that settle, rise (with entrapped air), orproduce permeable filter cakes. The effectiveness of the aggrega-tion process depends basically on two main phenomena: physico-chemical conditions and hydrodynamic parameters (Rulyov,1999a, 1999b; Carissimi and Rubio, 2005; Carissimi et al., 2007;Owen et al., 2008; Oliveira et al., 2010a; Rulyov et al., 2011;Oliveira and Rubio, 2012a, 2012b; Concha et al., 2012).

Physicochemical parameters are related to a series of complexissues such as the use of the appropriate chemical reagents (coag-ulant and/or flocculant type and dosage), molecular architecture ofthe polymer – linear or branched (Larsson et al., 1999; Weir andMoody, 2003; Grabsch et al., 2013), order of addition, interfacialforces, temperature, and pH. Also, they depend on particle

physicochemical characteristics (e.g., surface charge, density, pH).The accurate determination of physicochemical properties of parti-cles in suspension plays a key role for the success of destabilizingparticle suspensions. Some authors (Sato et al., 2004; Zhanpengand Yuntao, 2006; Walaszek and Ay, 2006; Gray and Ritchie,2006) have studied the particles morphology as a new concept offlocculation theory, which investigates the diversity of forms ofcolloidal particles and hydrolyzed coagulants or flocculants inwater during aggregation and the effect on the aggregation processefficiency. According to these authors, the aggregation is greatlyaffected by the particle and aggregate characteristics, includingshape, size, particle diameter, size distribution and space structure,as well as some related chemical factors. The rate at which particle/liquid separation occurs is highly dependent on floc formation(flocculation efficiency) and when high rate (throughput) particleseparation is targeted, the generation of ‘‘resistant’’ flocs is alwaysadvantageous (Gray and Ritchie, 2006; Capponi et al., 2006;Walaszek and Ay, 2006; Carissimi et al., 2007; Rubio et al., 2007;Rulyov et al., 2011; Grabsch et al., 2013). However, conventionalflocculation (mechanically mixed) usually requires high energygradients, which leads to the formation of irregular, fluffy andweak flocs. Flocs size, porosity and density can be controlled by

inlet

effluent + polymerflow direction

airmicrobubbles

length

outerdiameter

exit

Fig. 1. Flocs Generator Reactor.

E. Carissimi, J. Rubio / Minerals Engineering 70 (2015) 20–25 21

the flocculant type and flocculation conditions, like flocculationtime and energy dissipation (Adachi and Tanaka, 1997; Gorczycaand Ganczarczyk, 1999; Rulyov, 1999a, 1999b; Rulyov, 2001;Hopkins and Ducoste, 2003; Swetland et al., 2013).

Hydrodynamics depends on both the design (geometry mainly)of the aggregation unit and on mixing intensity (impeller speedand type, for mechanical units and feed flow rate for hydraulicunits). Particle collisions are caused by their relative motion,concentration, type and concentration of destabilizing agents,superficial interactions and contact time (Rattanakawin andHogg, 2001). Rulyov et al. (2005) studied the impact of thehydrodynamic treatment of varying intensity on the process offlocculation and the subsequent sedimentation separation offine-disperse (<1 micron) suspensions. Results showed that theapplication of the vigorous hydrodynamic treatment, characterizedby the medium gradient velocity (G of 2000–4000 s�1), followed bya later gentle hydrodynamic treatment of stepwise decreasingintensity (G of 1000–30 s�1) the flocculation and subsequentsedimentation increased the separation rate of the flocculatedsuspensions; decreased the residual concentration of solids inwater supernatant; decreased the consumption of flocculants;and enhanced the densification of the solid phase residue. A num-ber of works (Rulyov, 1999a, 1999b, 2001; Rulyov et al., 2005;Carissimi and Rubio, 2005; Carissimi et al., 2007; Concha et al.,2012) showed that at high shear rates (large velocity gradients)the mixing time in flocculation can be reduced from minutes toseconds by the appropriate hydrodynamic treatment of the sus-pension. This was named ‘‘ultra-flocculation’’ (Rulyov et al., 2005,2011) and the great advantage is that it ensures a good mix ofsmall and large particles and enables the formation of large anddense flocs before settling, thus providing a fast sedimentationand high degree supernatant clarification. Further studies of hydro-dynamic modeling, physical and chemical processes involved inthe flocculation were widely reviewed by Bridgeman et al. (2009).

Lately, the use of in-line mixing devices for hydraulic floccula-tion has shown a great potential due to the smaller foot print arearequired and also simple design, high throughput, a plug flowregime (fewer short circuits or dead zones) with reduced electricaland mechanical energy consumption (Carissimi, 2003; Carissimiand Rubio, 2005; Rosa and Rubio, 2005; Carissimi et al., 2007;Grabsch et al., 2013).

The aim of this study was to evaluate the flocculation perfor-mance as a function of particles type and concentration (colloidalFe(OH)3 and coal particles as suspension models), polymer typeand dosage for two hydraulic flocculation devices. All experimen-tation was carried out using two plug-flow hydraulic flocculators,namely, the Flocs Generator Reactor – FGR – and the Floccula-tion–Flotation– FF – device, individually and with the FF aheadthe FGR. The effectiveness of the aggregation process wasmeasured by indirect performance criteria such as settling rate ofthe flocs formed.

2. Experimental

2.1. Materials

The reagents employed were: FeCl3 anhydrous (EMD™ – FW162.21) for generation of the colloidal precipitates, NaOH (AR™)from Mallinckrodt Chemicals, for pH adjustments, and coalparticles (25–45 lm). All the synthetic solutions were preparedwith tap water (potable water quality for human consumption)immediately prior to each set of experiments. Diverse polyacryla-mides of different charges were evaluated: CD 650 and CP904 - cationics, NF 201 and NF 301 - non-ionics, AF 314 and AF367 – anionic, all of them supplied by Hyperfloc™, with concentra-tions ranging from 1 to 8 mg L�1.

2.2. Equipment

The Flocs Generator Reactor (FGR) as well as the Flocculation–Flotation (FF) are two mixing devices, with Brazilian trademarks,designed in the Laboratory of Mineral and Environmental Technol-ogy – Brazil (Carissimi and Rubio, 2005; Rosa and Rubio, 2005;Rubio et al., 2007) and due to their potential and successfulapplications were chosen for the hydraulic flocculation studiesreported in this work.

The FGR was designed and constructed for the flocs generationprior to the solid–liquid separation stage. The semi pilot scale FGRwas constructed with a Tygon tube (B-3603) with an innerdiameter of 1.25 cm and a length of 12 m, wrapped around a10 cm diameter polyvinyl chloride (PVC) column (Fig. 1).

The FF mixing device (serpentine design) with 10 elements(detailed in Rosa and Rubio, 2005) was used in order to improvepolymer dispersion and evaluate the flocculation efficiency forboth particle systems (Fe(OH)3 and coal) (Fig. 2). In the FF process,a pneumatic in-line flocculation occurs whereby the polymerdiffusion and bridging adsorption are rapidly ensured by theshearing forces and head loss in the flocculator (minimum 0.5 to1.0 kgf cm�2).

The system rig used was composed of a tank (capacity of 200 L)for the Fe(OH)3/coal (suspensions models) preparation and storage,two peristaltic pumps, a Masterflex feed pump, model 7549–60and a Masterflex polymer dosing pump, model 7562–00, providedby Cole Palmer Instrument Company. The FF and the FGR wereevaluated individually and with the FF placed before the FGR.The hydrodynamic conditions (flow rate, velocity gradient – G,and residence time, t) are summarized in Table 1. Flow metersand gauge pressures were used to control the flow and measurethe pressure, respectively. The velocity gradient was calculatedfrom the head loss in the equipment (FGR and/or FF) with a5 L min�1 feed rate applying Eq. (1). Measurements were madeusing manometers between the FGR inlet and outlet flows (thehead loss at the entrance was negligible).

G ¼

ffiffiffiffiffiffiffifficHllt

sð1Þ

where G = velocity gradient (s�1); c = specific weight of water(998.2 kgf m�3 at 68 �F); Hl = head loss (m); l = absolute viscosityof water (1.029 � 10�4 kgf s m�2 at 68 �F); t = time (s).

2.3. Methods

Flocculation: Flocculation experiments were carried out atroom temperature (60 �F), using Fe(OH)3 colloidal precipitatesformed from the FeCl3 dissolution and subsequent precipitationwith NaOH at pH 7.5 ± 0.5 (monitored with an Accumet™ pHmeter, model AR25). Initial size range of the Fe(OH)3 colloids was

Flocculant

Compressed Air

Flowmeter

Pressure gageFlocculator MS-10

InPressure gage

Sample withdraw

Out

Fig. 2. Flocculation–Flotation (FF) system.

Table 1Hydrodynamic conditions in the hydraulic flocculation devices (Flowrate = 5 L min�1).

Device G (s�1) t (s)

FF 1400 5FGR 1600 17FF + FGR 2300 23

Table 2Flocs characterization by X-ray CT. Conditions: energy setting = 50 kV, exposuredtime = 12 s; scan time = 2 h 30 min; reconstruction time = 3 h 30 min, reconstructionresolution = 1 voxel (10 � 10 � 10) lm, spatial resolution = 10 lm.

Density(g cm�3)

Watercontent (%)

Particlescontent (%)

Feret diameter(lm)

Fe(OH)3 1.109485 80.8 19.2 50–300Coal 2.898938 63.0 37.0 30

22 E. Carissimi, J. Rubio / Minerals Engineering 70 (2015) 20–25

reported in the literature between 0.4 and 0.04 lm (Deng, 1997).For the second prepared medium, coal particles (size diameterbetween 25 and 45 lm) were used as suspension model. Eachpolymer solution was prepared with deionized water using aCaframo mechanical stirrer, model RZR50, and added in-line tothe Fe(OH)3 or coal dispersion and pumped to the hydraulicflocculator for the flocs generation. The flow exiting from theFGR fed the solid–liquid separation vessels (columns) wherebythe settling rates were evaluated. These rates were calculated bymonitoring the times needed for the flocs to travel a fixed distance(settling velocity). Time measurements were made by directobservation (randomly) of many individual flocs. The performanceevaluation of the flocculation efficiency, with or without the FFmixer (serpentine) prior to the FGR under the distinct experimen-tal studies was made by monitoring the settling rate of the flocsformed (criterion used for optimal conditions).

All experimental conditions were tested in triplicate, with atleast 30 times measurements of separation in each experimentalset. An arithmetic average of all the values obtained were calcu-lated and statistically analyzed.

2.4. Physical and chemical analyses

An Aracor™ high-resolution 3D-X-ray microtomography (XMT)system installed at the University of Utah (USA) was employed forthe Fe(OH)3 and coal flocs characterization in order to determinethe flocs size, mass density and water content. Differentiation offlocs characteristics was carried out by the measurements of thelinear attenuation coefficient (l), which provides the necessaryaccuracy and quantitatively describes the system. One greatadvantage is that this method avoids possible incorrectness of datafrom free settling analysis due to the irregular shape andflow-through flocs phenomena. The porosity is then usuallycalculated as a ratio of the area occupied by pores to the total crosssection of the flocs (Walaszek and Ay, 2006).

Further information about the X-ray micro-CT (ComputedTomography) and correlated parameters are detailed given by Linand Miller (1996) and Miller et al. (2003). Image analyses wereprocessed using the ImageJ 1.36b (software in the public domain).

Flocculation efficiency was monitored by means of iron removalfrom water analyzing the residual iron concentration in thesupernatants after flocculation with an Induced Coupled Plasma(ICP-Emission Spectrometer), Perkin Elmer, model P400 or by theresidual turbidity of the coal particles monitored using a Hachmodel 18,900 turbidimeter.

3. Results and discussion

Table 2 summarizes the Fe(OH)3 and coal flocs density, water/particles content and Feret diameter characterization by X-raycomputed tomography (CT). Results show that the Fe(OH)3 flocspresent high water contents – 81%, higher porosity when com-pared to the coal flocs (63%) and a bigger Feret diameter (reachingsize values 10 times superior than the coal flocs). This agrees withthe high-resolution 3D X-ray microtomography analysis, whichshows a characteristic ‘‘spongey’’ structure for the Fe(OH)3 flocsand a more ‘‘compacted’’ structure for the coal flocs.

Fig. 3 shows the coal (a) and Fe(OH)3 (b) cross-sectional X-rayimage analysis in one slice from a total of 530 slices of the 3D dataset for the original CT image. Coal flocs can be easily distinguished,and the Fe(OH)3 flocs tend to form clusters, increasing their size.The dark region is the aqueous media and the ‘‘whiter’’ intensecolor in the flocs section means a higher density (or less porosity)in the cross-section. The average density value obtained by theX-ray CT is close to the value obtained by free settling data(Carissimi and Rubio, 2005).

3.1. Polymer efficiency

High molecular weight polyacrylamides were evaluated for thecoal and Fe(OH)3 flocs generation with a dosage of 1 mg L�1 andthe results are summarized in Table 3.

The cationic high molecular flocculant (CP904) was selectedthrough jar test studies as the best (optimal) polymer for boththe Fe(OH)3 and the coal particles flocculation with a lower resid-ual turbidity, and it was selected for all flocculation studies. TheFe(OH)3/coal flocs generated with this polymer were bigger whencompared to the other polyacrylamides evaluated. Franks et al.(2005) reported a similar result for flocculation of coal with a highweight cationic polymer.

3.2. Fe(OH)3 solid–liquid separation

Fig. 4 shows the settling rate of the Fe(OH)3 flocs as functions ofthe concentrations of both cationic high molecular weight poly-acrylamide [CP 904] and Fe(OH)3 without the FF ahead the FGR.Results show that higher settling rates are achieved with increasedFe(OH)3 concentrations and polymer dosages. The hydraulic dragseems to predominate in this case, with the formation of a ‘‘mesh’’,where the small flocs are entrapped and dragged by the biggerflocs. The same effect is shown when the serpentine is prior theFGR (Fig. 5), however with lower settling rates. It may be due theFe(OH)3 flocs break-up inside the serpentine during the in-lineflocculation. The flocculation studies in the FGR individually seem

Fig. 3. Cross-sectional images of the coal (a) and Fe(OH)3 (b) colloidal particles from the X-ray analysis in the ImageJ software.

Table 3Evaluation of the best polymer for the coal and Fe(OH)3 flocculation.

Polymer Charge Residual turbidity – coalsystem, NTU

Residual turbidity –Fe(OH)3 system, NTU

CD 650 Cationic 12 4CP 904 Cationic 4 2NF 201 Non-

ionic34 16

NF 301 Non-ionic

46 20

AF 314 Anionic 52 23AF 367 Anionic 55 30

0

5

10

15

20

25

30

35

1 2 4 6 8

[CP 904], mgL-1

Settl

ing

rate

, mh-1

19 mg/L Fe(OH)3

58 mg/L Fe(OH)3

86 mg/L Fe(OH)3

Fig. 4. Settling velocity of the Fe(OH)3 flocs as a function of the cationic highmolecular weight polyacrylamide [CP 904] and Fe(OH)3 concentration at pH7.5 ± 0.5 and no FF ahead the FGR.

0

5

10

15

20

25

30

35

1 2 4 6 8

[CP 904], mgL-1

Settl

ing

rate

, mh-1

19 mg/L Fe(OH)3

58 mg/L Fe(OH)3

86 mg/L Fe(OH)3

Fig. 5. Settling velocity of the Fe(OH)3 flocs as a function of the cationic highmolecular weight polyacrylamide [CP 904] and Fe(OH)3 concentration at pH7.5 ± 0.5 and with the FF ahead the FGR.

0

5

10

15

20

25

30

19 58 86

[Fe(OH)3], mgL-1

Settl

ing

rate

, mh-1

FFFGRFF+FGR

Fig. 6. Settling velocity of the Fe(OH)3 flocs as a function of the ‘‘flocculatorconfiguration’’ and the Fe(OH)3 concentration, [CP904] = 1 mg L�1.

E. Carissimi, J. Rubio / Minerals Engineering 70 (2015) 20–25 23

to form more ‘‘compact’’ flocs that are consolidated by the rollingmovement in the helical tube reactor. However with the FF alone(Fig. 6) the flocs break-up effect seems to be more evident, result-ing in lower settling rates.

The settling rate data shown in Fig. 5 for the hydraulic systemwith the FF prior the FGR are lower than the settling rate withthe FGR alone, especially for the higher Fe(OH)3 concentration,where the difference between the FF presence or absence aheadthe FGR is more relevant. It may be due the Fe(OH)3 flocs break-up inside the serpentine during the in-line flocculation. In all casesthe settling rates for the highest polymer dosages (6–8 mg L�1)were very similar, showing that the higher flocculant dosages used

in this work did not promote bigger Fe(OH)3 flocs. As shown inTable 1 the Fe(OH)3 flocs present a high porosity (81% of watercontent) which can increase the probability of break-up duringthe impacts onto the serpentine walls, due its big size and ‘‘spon-gey’’ structure. As a general rule, high molecular weight polymers,

0

5

10

15

20

25

30

5 10 20

[CP 904], mgL-1

Settl

ing

rate

, mh-1

0.5 g/L Coal

1.5 g/L Coal

3.0 g/L Coal

Fig. 7. Settling velocity of coal flocs as a function of the cationic high molecularweight polyacrylamide [CP 904] and particle concentrations, at pH (8.3 ± 0.2) andno serpentine.

0

5

10

15

20

25

30

0.5 1.5 3[Coal], gL-1

Settl

ing

rate

, mh-1

FF

FGR

FF+FGR

Fig. 9. Settling velocity of the coal flocs as a function of the flocculation system andthe coal concentration, [CP904] = 5 mg L�1.

24 E. Carissimi, J. Rubio / Minerals Engineering 70 (2015) 20–25

as used in this work, form large size but less compact flocs, whichare more susceptible to rupture. According to Oliveira et al.(2010b) large flocs formation are highly influenced by the struc-tural differences of the polymeric chains (linear, branched or acombination of both) besides the steric interactions promoted bythe polymer charge density.

The Fe(OH)3 flocculation efficiency evaluating different designflocculators is shown in Fig. 6. The FF and FGR individually andthe FF ahead the FGR were evaluated for three Fe(OH)3 concentra-tions and a fixed polymer dosage of 1 mg L�1. When the FF isplaced before the FGR, it causes break-up of the colloidal flocs,yielding lower settling rates (in the order of 10 mh�1 for the high-est Fe(OH)3 concentration). This compares poorly with the FGRalone where settling rates in the order of 17 mh�1 were achievedfor the highest Fe(OH)3 concentration. Again, the FF alone alsoshows the effects of flocs break-up in the low settling rates.

3.3. Coal flocs solid–liquid separation

The settling velocity of the coal flocs as a function of the cationichigh molecular weight polyacrylamide [CP 904] dosage and theflocculation efficiency without the FF located before the FGR isshown in Fig. 7. Results show that the higher the coal concentra-tion (0.5, 1.5 or 3 g L�1), the higher the settling rate, however there

0

5

10

15

20

25

30

35

5 10 20

[CP 904], mgL-1

Settl

ing

rate

, mh-1

0.5 g/L Coal1.5 g/L Coal3.0 g/L Coal

Fig. 8. Settling velocity of coal flocs as a function of the cationic high molecularweight polyacrylamide [CP 904] and particles concentration at natural pH(8.3 ± 0.2) and with FF.

is no distinction between the highest polymer dosages (10 and20 mg L�1). The same effect is seen with the FF, but in this casethe settling rates are higher when compared to the FGRalone (Fig. 8). It may be due the coal flocs better aggregation insidethe serpentine during the in-line flocculation. In this case, due thelower porosity of the coal flocs (63% of water content) and smallerflocs size than the Fe(OH)3 flocs, the FF helps to consolidate thecoal flocs and increase the mass density, promoting resistant flocswith higher settling rates than the FGR individually.

Sabah and Cengiz (2004) and Sabah and Erkan (2006) reportedsimilar results for coal aggregation, however with lower settlingrates (in the order of 18 mh�1) than this work. These authors pos-tulate that the electrostatic patching mechanism rather than thebridging mechanism predominates for the cationic polymerdespite its long chain. They suggest that the cationic polymer mol-ecules adsorb in a flat configuration onto particle surfaces due tostrong electrostatic attraction and poor ability of polymer to bridg-ing. The charge patch mechanism is reported to usually producerelatively small flocs and slow settling rates. The coal flocculationefficiency using different design for the hydraulic flocculation isshown in Fig. 9. The FF and FGR, and the FF placed before theFGR were evaluated for three coal concentrations and a fixedpolymer dosage of 5 mg L�1. With the FF placed ahead of the FGRthere is an enhancement in the coal flocs growth, yielding highersettling rates (in the order of 25 mh�1) when compared to the FFor the FGR. The smaller size and small porosity of the coalflocs when compared to the Fe(OH)3 flocs may explain the bestflocculation efficiency with the FF.

4. Conclusions

The hydrodynamic performance considering the flocculationefficiency according to the particles type and concentration (colloi-dal Fe(OH)3 and coal particles as suspension models), polymer typeand dosage was investigated using two hydraulic flocculators,namely the Flocs Generator Reactor (FGR) and Flocculation–Flota-tion (FF). The FF and the FGR were evaluated individually and withthe FF prior to the FGR. The effectiveness of the aggregation processwas measured by indirect performance criteria considering settlingrates. Results showed that the flocculation efficiency is largelydependent on the particle characteristic (porosity/water content,size, etc) and the hydraulic flocculation system design. The Fe(OH)3

showed a higher porosity with a ‘‘spongey’’ structure (81% watercontent) and a larger size than the coal flocs (more compact and63% water content) yielding flocs more susceptible to break-up.The best hydraulic flocculation efficiency for the Fe(OH)3 flocs

E. Carissimi, J. Rubio / Minerals Engineering 70 (2015) 20–25 25

generation were obtained using the FGR, yielding settling rates inthe order of 22 mh�1 for a Fe(OH)3 concentration of 58 mg L�1.Results for the coal flocs showed that FF ahead of the FGR producedbetter aggregation efficiency, reaching settling rates in the order of30 mh�1, for a coal particle concentration of 3 g L�1. The floccula-tion efficiency was over 98% in all cases. These data show howimportant it is to characterize the particles in suspension prior todesigning a hydraulic flocculation system for particle aggregationin order to obtain the best flocculation efficiency.

Author disclosure statement

Elvis Carissimi and Jorge Rubio disclose any commercial associ-ations that might create a conflict of interest in connection withthe submitted manuscript.

Acknowledgements

The authors are grateful to CNPq and CAPES for the financialsupport received.

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