International Journal of Mineral Processing 106–109 (2012) 31–36

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International Journal of Mineral Processing

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Kaolin aerated flocs formation assisted by polymer-coated microbubbles

Cristiane Oliveira, Jorge Rubio ⁎Universidade Federal do Rio Grande do Sul, Departamento de Engenharia de Minas, PPGEM, Laboratório de Tecnologia Mineral e Ambiental, Av. Bento Gonçalves, 9500, Porto Alegre, RS,CEP 91501-970, Brazil

⁎ Corresponding author. Tel.: +55 51 33089479; fax:E-mail address: jrubio@ufrgs.br (J. Rubio).URL: http://www.ufrgs.br/ltm (J. Rubio).

0301-7516/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.minpro.2012.01.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 November 2011Received in revised form 24 January 2012Accepted 28 January 2012Available online 4 February 2012

Keywords:FlocculationFlotationAerated flocsAeroflocsDissolved air flotation (DAF)

Aerated kaolin floc formation was studied in a continuous-flow, laboratory-scale system, and the flocs wereseparated from aqueous solution by flotation. Microbubbles were generated by the depressurisation of dis-solved air in water, and flocculation was achieved in a Floc Generator Reactor (FGR). An aerated floc charac-terisation (AFC) technique was utilised for characterising the aggregates formed. Flocculation–flotationstudies were performed using a non-ionic polymer (920SH SNF-Floerger®) added to a suspension flow(4 L/min) at an air/solid rate (A/S) of 0.02 mg/mg. The main results revealed the existence of an extra floccu-lation mechanism resulting from interactions between particles, bubbles and polymer-coated air microbub-bles. The latter acted as “flocculation nuclei”, where particles aggregated by a sort of “bridge” with thepolymer-coated-bubbles. These polymer-coated bubbles can interact with other bubbles, particles and/orflocs resulting in a large amount of bubbles (of different sizes, depending on coalescence) to the floc struc-tures improving the rising rates of the aerated flocs. It is believed that these findings will contribute to theunderstanding of the formation of the light, aerated flocs and the rapid flocculation–flotation process forsolid/liquid separation.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Aerated floc generation has been well described in the literature(Carissimi et al., 2007; Carissimi and Rubio, 2005a; Colic et al.,2001; Da Rosa, 2002; Da Rosa and Rubio, 2005; Miller, 2001;Oliveira, 2010; Oliveira et al., 2010a; Owen et al., 1999; Rodriguesand Rubio, 2007; Rubio, 2003; Rubio et al., 2007; Rulyov, 1999;Rulyov, 2001). Most of the efforts have been focused on developingand improving the solid/liquid separations of suspended particles inhigh-throughput flocculation–flotation units.

Although important studies have been conducted on the forma-tion of aerated flocs, only a few of those studies have included mech-anisms. Thus, knowledge about the interactions among particles, airmicrobubbles and polymers should increase the potential use of floc-culation–flotation processes in the environmental field (effluent con-trol and water reuse). The advantages of employing these processesinclude a higher loading capacity and cost savings in a very rapidsolid/liquid separation.

The Flocs Generator Reactor (FGR) appears to have a great poten-tial as an innovative helical mixing reactor for rapid aggregation. Inthis “flocculator”, the polymer is dispersed by the flow of slurrythrough the helical tubes, permitting both polymer adsorption andflocculation to take place simultaneously, in a very short amount of

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time (seconds). Air microbubbles are injected, and they can be at-tached, entrapped, occluded and coalesced inside of flocs, formingthe so-called aerated flocs (aeroflocs), which readily float (or levitate)(Carissimi et al., 2007; Carissimi and Rubio, 2005a; Carissimi andRubio, 2005b).

Our research group has recently developed a technique to charac-terise aerated flocs (multiphase systems), which was named aeratedfloc characterisation (AFC) (Oliveira et al. (2010a). AFC uses image-processing measure the size, shape factor, up-rising rate, fractal di-mension and density. In addition, adhered, attached and entrappedbubbles in or onto the flocs can also be monitored.

In this context, Oliveira (2010) has discussed the interactions be-tween polymers and bubbles–particles, forming activated carbonand/or kaolin aerated flocs, as shown in Fig. 1. These flocs havelarge bubbles inside their structures resulting from the coalescenceof the air microbubbles, appearing either entrapped or attached inthe flocs structures. This fact has also been discussed by other authors(Carissimi and Rubio, 2005a, 2005b; Oliveira, 2010; Oliveira et al.,2010a; Rodrigues and Rubio, 2007) proving that this mechanism ishighly consistent following the generation of microbubbles (meanbubble size of 65 μm), and not large bubbles after depressurizationof dissolved air in water (Rodrigues and Rubio, 2003).

Consequently, the air bubbles appear to grow enhancing the rela-tive air volume inside the flocs, decreasing their density and leadingto high up-rising rates. This fact has been proved by Oliveira (2010)showing the presence of large bubbles, sizing up to 230 μm (Fig. 1).More, it has been suggested a concomitant formation of polymer-

A

A

Clusters of bubbles Bridges of bubbles Trens of bubbles

erated carbon floc

erated kaolin floc

Fig. 1. Activated carbon and kaolin aerated flocs – Details to large bubbles and mechanisms of bridges of bubbles, trains of bubbles and clusters of bubbles (Oliveira, 2010; Oliveira etal., 2010a).

32 C. Oliveira, J. Rubio / International Journal of Mineral Processing 106–109 (2012) 31–36

coated bubbles which are able to interact with particles, bubbles and/or other particle-coated bubbles.

As a result, this finding led to further investigations involving in-teractions between polymers and bubbles in a continuous systemfor generating aerated flocs assisted by polymer-coated bubbles. Ac-cordingly, the goal of this work was to evaluate these phenomena inkaolin dispersions, in a flow system constituted by an FGR, dissolvedair flotation (DAF) and the aerated flocs characterization (AFC) rig.

2. Experimental

2.1. Materials and reagents

A high-purity kaolin (99% of kaolinite) sample was procured fromCadam S/A Brazil. The mean kaolin particle diameter was determinedto be 0.9 μm; the specific surface area measured by the BET method

was 17 m2/g and the specific mass was 2.3 g/cm3. Reagent-grade alu-minum sulphate (Al2(SO4)314H2O) from Cynth® was used to pro-mote the coagulation. Flocs were formed using a high–molecularweight non-ionic polyacrylamide (920SH) from SNF-Floerger®. Thispolymer was characterized by Oliveira et al. (2010b), who describedit as a high molecular weight of 6.02×106 g/mol having a negativecharge in the pH range of 2–10. The zeta potential value of about−10 mV was measured at pH between 5 and 7, used in this work.According to the literature, the non-ionic polymers are far frombeing pure substances having fractions of anionic groups (in therange of 1%–3%) due to hydrolysis of the amides groups. This phe-nomenon appears to be related to specific conditions of the polymer-ization reactions (Luttinger, 1981) and to the presence of monomersand polymers chains of varying molecular weights. For example,Vorchheimer (1981) obtained hydrolyses grades below of 1% aftercontrolling the monomers concentration, pH and temperature.

[1]

[2]

[3]

[4]

[5] [6]

[7][8]

[9]

[10]

[11]

Fig. 2. Flocculation–flotation rig equipped with an aerated floc characterisation (AFC) system. [1] and [2] polymeric solution tanks; [3] kaolin suspension tank; [4] stereomicroscopewith a digital camera; [5] light sources; [6] screen; [7] tube of the AFC technique; [8] digital camera; [9] flotation cell; [10] Flocs Generator Reactor (FGR®); [11] saturator vessel.

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2.2. Methods

A continuous ring system composed of a Flocs Generator Reactor(FGR) and dissolved air flotation (DAF) techniques was designed toconduct flocculation–flotation studies (Fig. 2). To obtain a full charac-terisation of the flocs and the multiphase systems (water/air bubbles/flocs), aerated floc characterisation (AFC) equipment (developed anddescribed by Oliveira et al. (2010a)) was coupled with this apparatus.

Well-dispersed suspensions of kaolin particles were prepared in5 L of water in an ultrasonic bath for 30 min. The resultant suspensionwas transferred to a tank containing water, where the particle con-centration was set up at 1 g/L. In the coagulation step, an optimal con-centration of aluminium sulphate (2.5×10−5 mol Al3+/g) was added

Table 1Mode of polymer addition — condition III.

Tests Polymer concentration (mgpolymer/gkaolin)

Directly to the flow from a polymersolution

Added to the saturationvessel

(a) 0.63 1.25(b) 1.25 1.25(c) 1.25 0.63

to the kaolin suspension. This optimal value has been previously de-termined by Oliveira (2010), who found a slight negative zeta poten-tial of −7,4 mV, of the coagulated kaolin particles and a low residualturbidity (153 NTU) of the supernatant. In addition, the fine particlesof kaolin did not be flocculated, neither with polymers nor with inor-ganic salts (coagulation). A combination of these reagents (sensitiza-tion mechanism) to form the aggregates and, therefore, aerated flocs,was essential.

After 10 min of mixing (to promote the adsorption and coagu-lation), the coagulated suspension was pumped to the FGR at aflow rate of 4 L/min (Qs). The DAF system operated at 4 atm pres-sure to generate air microbubbles to give a 0.02 mg/mg air/solidrate. The polymer was added to the system into two differentmanners:

Condition I: a polymeric solution (1.25 mg/g) directly in the flowline (only water in the saturator vessel);Condition II: placing the polymer solution at different concentra-tions (0.5, 1.25, 1.5, 2.0, 5.0, 10 and 30 mg/g) inside the saturatorvessel, before depressurization;Condition III: conditions I and II together, using different polymerconcentration (Table 1).

After mixing the kaolin particles, the polymer and the air bubblesinside the FGR, the aggregates (floc-bubbles) moved to the flotationcell, for solid/liquid separation (aggregates/water) to take place.

(b)

(c)

(j)

(g)

(f) (e)

(h)

(d)

(a)

(i)

Fig. 3. Photomicrographs of polymer-coated bubbles interacting with flocs and/or other bubbles.

34 C. Oliveira, J. Rubio / International Journal of Mineral Processing 106–109 (2012) 31–36

Fig. 4. Photomicrographs of air bubbles coated with polymer–kaolin particles. (a) [920SH]: 0.5 mg/g; (b) [920SH]: 30 mg/g.

35C. Oliveira, J. Rubio / International Journal of Mineral Processing 106–109 (2012) 31–36

After system stabilization, the tube of the aerated floc characterization(AFC) technique was opened for sampling the rising aggregates, whichwere characterized by image capture and processing, as in Oliveira et al.(2010a). Subnatant liquid samples were collected, and the residual tur-bidities were measured in a turbidimeter (Hach®2100 N) and expressedin nephelometric turbidity units (NTU). All of the experiments and mea-sures were done in duplicate.

3. Results and discussion

The results obtained, following experimental condition I, generatedaerated kaolin flocs having uprising rates ranging between 70 and150 m/h,which account for about 83% of the flocs population. These ag-gregates are composed of large bubbles adhered, entrained, entrappedand by clusters of bubbles, trains of bubbles and/or polymer-coatedbubbles, confirming the forms of aerated flocs formation, proposed inthe series (Oliveira et al., 2010a).

Therefore, the data show the importance of good mixing and tur-bulence inside the FGR, where polymers appear to interact simulta-neously with particles and air bubbles.

Fig. 5. Photomicrographs of the non-spherical polymer–partic

Additionally, the photomicrograph of the multiphase systemallowed evidence for a new contribution to flocculation mechanismfrom interaction between particles and polymer-coated bubbles(Fig. 3) which become the bubbles in flocculation nuclei, where par-ticles flocculated generating new structures. The polymer-coatedbubbles act as flocculation nuclei for particles, generating new struc-tures. These polymer-coated bubbles can interact with other bubbles,particles and/or flocs resulting in a large amount of bubbles (of differ-ent sizes, depending on coalescence) to the floc structures improvingthe rising rates of the aerated flocs.

This mechanism of coated-polymer bubbles was confirmed byOliveira and Rubio (2011) that showed changes of the zeta potentialof the bubbles in the presence of different polymers. In fact, the bub-bles zeta potential values are changed in the presence of polymers. Inthis case, the authors showed that the non-ionic polymer (920SH)provided a lightly increase in the negative charge of the bubbles, indi-cating the existence of an interaction with these chains, which have aslight negative charge density, as showed by Oliveira (2010).

Based on several existing data indicating some interactions betweenpolymers and air bubbles, the present work considers that the interac-tion arises from van der Waals and hydrophobic forces between air

le-coated air bubbles using the 920SH non-ionic polymer.

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bubbles and hydrophobic moieties present in the backbone of the poly-meric chains. This hydrophobicity of the polyacrylamides has been de-scribed by some authors (Ghannam, 1999; Oliveira, 2010) and thehydrophobic portion in the non-ionic polymer (920SH) has been con-firmed by measurements of contact angle of polymer solution at thesolid Teflon/liquid interface, showing a decrease in the water dynamiccontact angle (122°±2.6°) to 100°±2.1 Oliveira (2010).

Thus, such interactions and phenomena shown in the presentwork and other investigations appear to be very important to explainthe presence of large amount of bubbles in aerated flocs causing anincrease of the rising rates of these light aggregates.

The captured images shown in Fig. 3, illustrate the flocculation in-volving polymer-coated bubbles strongly interacting with particles,free bubbles, other polymer-coated bubbles and aerated flocs insuspension.

Fig. 3 (a), (b), (c) and (d) shows aerated kaolin flocs containing alarge polymer-coated bubble covered with flocs and (e), (f), (g), (h),(i) and (j) illustrate polymer-coated bubbles (covered with either floc-culated particles or not) interacting with flocs and other polymer-coated bubbles, forming bridges between them.

An attempt to improve the formation of the polymer-coated bubbleswas made by injecting the polymer flocculant directly to the air satura-tor vessel just prior to the bubble generation in the nozzle (conditions IIand III). However, contrary to expectations, this resulted in poor floccu-lation efficiency, causing high turbidity residual values (greater than479 NTU). This behavior may be due to the strong interactions betweenpolymeric macromolecules and air bubbles, resulting in a high adsorp-tion and a lack of polymer molecules free to interact with particles togenerate flocs. This consideration is consistent with data reported byOliveira and Rubio (2011)whomeasured the air bubbles zeta potentialsin the presence of polymeric macromolecules. These authors showedthat different polyacrylamides, such as the 920SH non-ionic polymermay interact with air bubbles, changing the charge density of thesebubbles.

The studies including addition of a non-ionic polymer (920SH)both in the flow line and inside the saturator vessel (mode of polymerdosage (a), (b) and (c)) showed removal efficiencies of 25%, 35% and63%, respectively. These results may be due to the repulsion forces be-tween air bubbles and kaolin flocs, both having the same polymeradsorbed. Herein, it is concluded that should the polymer concentra-tion increases the repulsion between the polymer coated bubbles andthe particles of the flocs also increases following the so-called stericeffect (Hunter, 2001).

In addition, the floating product obtained in these conditions (IIand III) had a “plastic” aspect (because of the polymer–particle layerformed on the bubble surface) and a low moisture content.

Despite flocculation was not efficient under these conditions (IIand III), the captured images of the multiphase system clearly showthe interactions between particles and air bubbles, very likely coatedwith polymer molecules. In this case, at the lower polymer concentra-tion (10 mg/g in condition II), this effect was less significant than theeffect at high concentrations (30 mg/g in condition II), as shown inFig. 4.

Likewise, non-spherical polymer–particle-coated air bubbles werefound (Fig. 5), probably because of the formation of a polymeric filmon the bubbles in which the particles are adsorbed and adheredstrongly, creating a polymer–particle layer, which might prevent thespherical form of the air bubbles.

4. Conclusions

A detailed experimental system was set up to characterize and vali-date thefinding that polymer-coated bubbles act as “nuclei” or “bridges”

between particles,flocs or other similar bubbles. These units assist in theformation of aeratedflocs, with structures having a large amount of bub-bles of varying sizes due to coalescence. Results obtained widen theknowledge of the mechanisms and interactions leading to aerated flocgeneration, which is responsible for rapid flocculation–flotation inwater and wastewater treatment.

Acknowledgements

The authors would like to thank the Cadam® and Floerger® Corpo-rations for their technical information and for providing the kaolinand polymer samples. The authors would also like to thank all ofour colleagues at the LTM and all Institutions supporting research inBrazil.

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