MECHANISM RESPONSIBLE FOR FLOCCULATION IN POLY (N-ISOPROPYLACRYLAMIDE) TEMPERATURE RESPONSIVE DEWATERING – ATTRACTIVE INTERACTION FORCES INDUCED BY SURFACE HYDROPHOBICITY G Franks1, E Burdukova2, H Li3, N Ishida4 and J-P O’Shea5 ABSTRACT Temperature responsive polymers such as Poly (N-isopropylacrylamide) (PNIPAM) have been shown to have potential to improve water recovery from mineral tailings by using the polymer as a flocculant at temperature above the lower critical solution temperature (LCST 32°C) and then as a consolidation aid at temperature below the LCST. The mechanism for the flocculation (at high temperature) is shown to be hydrophobic and the mechanism for the repulsion (at low temperature) is shown to be steric repulsion. The contact angle between water and air on PNIPAM covered silica glass surfaces increases with increasing temperature and increases with increasing polymer molecular weight. Atomic force microscope (AFM) colloid probe surface force measurements at temperature below the LCST show steric repulsion which increases with polymer molecular weight. At temperature above the LCST, attraction is observed between the PNIPAM covered surfaces which increases as molecular weight of the polymer increases. The polar acid-base interfacial interaction forces have been estimated using contact angle measurements of three different liquids (water, formamide and diiodomethane) on PNIPAM coated silica glass surfaces. The results of these calculations are consistent with hydrophobic attraction at temperature above the LCST and steric repulsion at temperature below the LCST. Since the mechanism for the attraction is due to an increase in particle hydrophobicity it is also possible to use PNIPAM as a flotation collector for fine particles because it can produce hydrophobic floccs. Keywords: temperature-sensitive polymer, atomic force microscopy, contact angle, flocculation, flotation, dewatering, solid-liquid separation, hydrophobic attraction, polar interfacial interactions

INTRODUCTION A new class of flocculants is being developed which are responsive to various stimuli such as pH, (Vaslin-Reimann et al, 1990; Franks et al, 2006; Naka et al, 2007; Cruz-Silva et al, 2007) shear (Addai-Mensah et al, 2008; McGuire et al, 2008) and temperature (Guillet et al, 1985; Deng et al, 1996; Sakohara et al, 2002, Sakohara and Nishikawa, 2004; Franks, 2005; Li et al, 2007; Sakohara et al, 2008; Li et al, 2009). The most frequently investigated temperature responsive polymer which has been applied as a flocculant is PNIPAM. This polymer is soluble in water at room temperature, but becomes dehydrated and precipitates out of water at temperature above its LCST at 32°C (Sun et al, 2004; Ono and Shikata, 2006). The polymer acts as a dispersant at room temperature and as a flocculant as temperature is raised above the LCST. In addition to acting as a flocculant at high temperature, Franks et al have shown that additional consolidation of the sediment (secondary consolidation) will occur when the temperature is reduced to below the LCST after the settling stage of solid-liquid separation is complete. (Franks 2005; Franks et al, 2008; Li and Franks, 2008; Li et al, 2009) Furthermore, PNIPAM has been shown to be able to be used as a flotation collector 1. Australian Mineral Science Research Institute, Chemical and Biomolecular Engineering, The University of Melbourne, Vic 3010, Australia. Email: [email protected] 2. Australian Mineral Science Research Institute, Chemical and Biomolecular Engineering, The University of Melbourne, Vic 3010, Australia. Email: [email protected] 3. Alberta Research Council, Edmonton AB, Canada. Email: [email protected] 4. Photonics Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8565, Japan. Email: [email protected] 5. Australian Mineral Science Research Institute, Chemical and Biomolecular Engineering, The University of Melbourne, Vic 3010, Australia. Email: [email protected]

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at temperature above its LCST (Franks et al, 2008; Li and Franks, 2008; Franks et al, 2009). The mechanism for the change in particle interaction forces from repulsive to attractive as temperature is changed from below to above the LCST is believed to be a change in the particle surface from a hydrophilic to a hydrophobic state (Burdukova et al, 2010a). In this paper, we measure contact angles of water on PNIPAM coated surfaces in order to confirm the change in hydrophobicity of the surfaces as a function of temperature and polymer molecular weight. Additional contact angle measurements with formamide and diiodomethane provide the information necessary to model the polar acid-base interfacial interaction forces (Van Oss et al, 1987; Van Oss et al, 1988; Van Oss et al, 1990; Van Oss, 1994). These forces combined with the DLVO forces (van der Waals and electrical double layer) can be used to model the total interaction forces between PNIPAM coated silica surfaces at temperature above and below the LCST for a range of polymer molecular weights. These predicted forces are then compared to the actual forces between silica surfaces in PNIPAM solutions measured with the atomic force microscope (AFM) colloid probe method.

EXPERIMENTAL Materials A temperature sensitive, non-ionic polymer – PNIPAM, was used in this work. Five molecular weights of this polymer were used. The lowest molecular weight (0.23 MDa) PNIPAM was purchased from Sigma-Aldrich, Australia. The PNIPAMs with medium molecular weights (0.71 and 2.0 MDa) were purchased from Polymer Source Inc, Canada. The PNIPAMs with the highest molecular weights (3.6 MDa and 4.5 MDa) was synthesised in our laboratory as described in detail by O’Shea et al (2009). The polymers were dissolved in deionised water with 0.01 M NaNO3 at room temperature and at pH of 6.0 as a 0.1 wt per cent solution. A silica powder (Silica 400G, >98 per cent pure) was obtained from Unimin Australia Limited and used for sedimentation and flotation studies described in detail elsewhere (Franks et al, 2008; Li and Franks, 2008; Li et al, 2009; Franks et al, 2009). It has a density of 2.62 g/cm3. The particle size distribution is about 80 per cent minus 20 micron, 53 per cent minus 10 micron, and 30 per cent minus 5 micron.

Contact angle measurements The sessile drop technique was used for contact angle measurements (Contact angle system OCA, Dataphysics). For each molecular weight of PNIPAM, three clean silica glass slides (26 × 76 mm) were coated at 0.1 grams of polymer per m2 of glass surface. They were dried in an oven at 80°C for about 30 min. In measuring contact angle, a glass slide coated with polymer was placed on the top surface of an aluminium stage which has its temperature controlled by a small water bath to control the temperature at either 22°C or 50°C. Five minutes was allowed to make sure that the slide had reached 50°C. A liquid drop with 3 μl volume was introduced onto the slide through a micro-syringe controlled by a computer. The measurement was taken ten seconds after the liquid drop was put on the glass slide and the contact angle value was read. The measurement was repeated three times under each condition and the average value of the three measurements is reported. The contact angle of deionised water, formamide and diiodomethane on the PNIPAM coated surfaces were measured at 22°C and 50°C.

AFM force measurements Interaction forces between silica surfaces in PNIPAM solutions were measured using an AFM (SPI3700, Seiko Instruments, Tokyo, Japan). For the force measurements, non-porous, nearly spherical, amorphous silica particles of 8 - 20 μm diameter (Tatsumori, Tokyo, Japan) and silicon wafers (Nilaco, Tokyo, Japan) were used as test surfaces. Prior to use, the particles were boiled in a warm ethanol solution for 10 min and in a 5 vol per cent hydrogen peroxide solution for another 10 min to remove organic contamination. Then they were rinsed twice with warm pure water and dried. Silicon wafers were immersed in a 7:3 mixture of concentrated sulfuric acid and hydrogen peroxide (piranha solution) at 70 - 80°C for 30 min, to remove contamination and ensure an oxide layer on the surface and rinsed with copious amount of pure water. The cleaned wafers and particles were stored in a desiccator until used. For all experiments, triangular silicon nitride cantilevers (Olympus, Tokyo, Japan) were used. Spring constants of cantilevers were determined by measuring their resonance frequency (Cleveland et al, 1993). The silica particle was glued on the cantilever tip with epoxy resin. XXV INTERNATIONAL MINERAL PROCESSING CONGRESS (IMPC) 2010 PROCEEDINGS / BRISBANE, QLD, AUSTRALIA / 6 - 10 SEPTEMBER 2010

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The sizes of the particles were measured with an optical microscope after the force experiments. In order to control the solution temperature during force measurements, a customised fluid cell that consisted of a Teflon cuvette and a copper pipe around the cuvette was used. A mixture of water and ethylene glycol was circulated in the copper pipe from a thermostat to maintain the solution’s temperature in the cell with deviations within 0.1ºC. Solutions of 20 ppm PNIPAM in 10-2 M NaNO3 were prepared at pH 6.0 and introduced into the AFM cell at 22°C. AFM force measurements were made at 22°C. Then the solution in the cell was heated to 50°C and a second set of force curves was measured.

RESULTS AND DICUSSION Contact angle measurements and force modelling Figure 1 shows photos of water droplets on silica glass surfaces coated with 0.1 g/m2 PNIPAM at both 22 and 50°C for various molecular weight of PNIPAM. The contact angles measured are presented in Table 1. At 22°C the surfaces are reasonably hydrophilic (Θ <45° even for the highest molecular weight investigated) while at 50°C the surfaces become hydrophobic (Θ >45° even for the lowest molecular weight investigated). In addition, the contact angle increases with increasing polymer molecular weight. According to Young-Dupré equation, expressed by Bangham and Razouk (1937), the contact angle and the interfacial energy (surface tension) are related to the total energy of cohesion between the TOT GSL ): solid and liquid (ΔG

22oC

50oC 0.23 0. 23 MD MDa a

0.7 0.71 1 MDa MDa

2.0 MD 2.0 MDa a

3.6 MD 3.6 MDa a

FIG 1 - Photographs of water droplets on the surface of silica glass surfaces coated with 0.1 g/m2 PNIPAM of various molecular weights as indicated at 22°C (top row) and 50°C (bottom row). TABLE 1 Values of contact angle, components of interfacial interaction energy (γS) of the surface tension of PNIPAM coated surfaces and the polar AB interfacial interaction energy (ΔG SLAB) of the surfaces in water. MW MDa

Temp. °C

W θ°

FM θ°

DM θ°

(γSLW)1/2

(γS+)1/2

(γS-)1/2

ΔG SLAB mJ/m2

0.23

22

17.4

31.1

48.7

5.914

0.820

7.798

46.492

0.71

22

26.2

34.4

49.3

5.885

0.789

7.444

40.797

2.00

22

27.5

32.6

51.0

5.804

1.000

7.234

35.384

3.60

22

41.5

44.9

48.0

5.946

0.357

6.751

31.930

0.23

50

49.3

15.7

47.4

5.974

2.093

4.291

-8.973

0.71

50

56.9

24.9

48.5

5.923

2.048

3.619

-17.180

2.00

50

71.8

35.6

50.7

5.819

2.116

1.963

-36.240

3.60

50

85.9

44.9

47.2

5.983

1.862

0.328

-60.210

Note: W = deionised water, FM = formamide and DM = diiodomethane.

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TOT LW AB cosθ )γ L = − ΔG GSL = − GSSL − ΔGSSL S

(

(1)

where: θ = contact angle γL = surface energy (surface tension) of the liquid TOT The total energy of cohesion between the solid and liquid ( ΔG GSL ) is composed of two components. LW GSL is the apolar component of the interfacial interaction energy (Lifshitz-van der The expressionΔG AB GSL is the polar component of the interfacial interaction energy (Lewis acid-base, Waals, LW) and ΔG AB). Van Oss et al (1987; 1988) suggested that the polar component of the interfacial interaction energy AB ΔG GSL is largely responsible for hydrophobic attraction and structural (steric) repulsion. Van Oss et al (1987; 1988) used the symbol γiAB to represent the polar component of the surface tension of compound i. In addition, γi- represents the contribution due to electron donors or proton acceptors, and γi+ represents the contribution due to electron acceptors or proton donors. They derived the following relationship: ccos osθ )γ L

(

( γ SLW γ LLW + γ S+ γ L− + γ S− γ L+ )

(2)

Using Equations 1 and 2, they determined that the AB interfacial interaction energy between similar solid surfaces in a liquid can be obtained using the following equation: AB ΔG GSL = −4( γ L+ γ L− + γ S+

− S

− γ S+

− L

− γ S− γ L+ )

(3)

Therefore, by measuring contact angles with three different liquids (two of them must be polar) with known γLLW, γL+, γL- and γL values, applying Equation 2 three times, the γSLW, γS+, γS- values for any solid can be determined. Then it is possible to use Equation 3 with known γL+, γL- (such as water) to determine AB AB ΔG GSL of the solid surface in liquid: L (such as water). Once the value of ΔG GSL is known it can be used to evaluate the influence of polar hydrophilic/hydrophobic interactions induced by PNIPAM of various AB GSL <0, the interaction is attractive and believed to be responsible for the molecular weights. When ΔG AB GSL >0, the interaction is repulsive and is believed to represent hydrophobic interaction and when ΔG steric repulsion and short ranged hydrophilic structural interactions. The approach has been used for the study of hydrophobic aggregation of alumina in a surfactant solution (Hu and Dai, 2003). Table 1 presents the measured contact angles for deionised water, formamide and diiodomethane on a glass slide coated with 0.1g/m2 of PNIPAM (with four different molecular weights) at 22 and 50°C. The values of γLLW, γL+, γL- and γL for water, formamide and diiodomethane are known from Van Oss et al (1990; 1994) and have been used to calculate γSLW, γS+, γS-. These values are then used AB GSL for the PNIPAM coated surfaces in with the known values of γL+, γL- for water to determine ΔG water at both 22°C and 50°C. It is clear that the values of the polar AB interfacial interaction energies for the four PNIPAM coated surfaces at 22°C are positive, indicating a hydrophilic surface and steric repulsion. Compared to that, the polar AB interfacial interaction energies for the four PNIPAM coated surfaces at 50°C are negative, indicating that the surfaces have become hydrophobic. The total interaction forces between two silica surfaces in PNIPAM solutions can be estimated by considering the van der Waals interaction, the electrical double layer interaction and the polar acid-base interfacial interaction energy. It has been found that the classic DLVO theory can not be used alone to describe the interactions between hydrophobic surfaces (Pashley et al, 1985; Meyer et al, 2006; Hu and Dai, 2003). Instead, the extended DLVO force including the polar interfacial interaction force should be an improvement. In this work, all the forces are presented as normalised by the radius of the particle in order to make comparison between AFM colloid probe measurements and theoretical calculations. The van der Waals interaction force as a function of particle surface to surface separation distance (D) will be estimated with: FvdW R

=−

A 12 D 2

(4)

where the Hamaker constant (A) of silica is taken as 0.5 × 10-20 J. The electrical double layer force can be estimated using:

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FEDDL R

(5)

= 2πεε oκΨ 2o e −κ D

where: surface potential (Ψo) can be estimated from the measured zeta potentials (Burdukova et al, 2010a) ε = relative permittivity of water εo = permittivity of free space which is 8.854 × 10-12 C2J-1m-1 κ = inverse Debye length, which is 3.29 × 108 m-1 for 10-2 M salt concentration The interaction force as a function of distance between two particles due to the polar acid-base interfacial interactions can be determined using (Hu and Dai, 2003): FAB R

⎛ D0 D ⎞ ⎜ ⎟ AB ⎝ λ ⎠ SL

(6)

= 2πΔG e

where: D0 = minimum equilibrium contact distance between particles taken as = 0.2 nm λ = decay length typically in the range, about 1 - 10 nm for hydrophobic systems (Israelachvili and Pashley, 1983; Pashley et al, 1985; Meagher and Craig, 1994; Rabinovich and Yoon, 1994) It is reasonable to assume λ is proportional to the molecular weight of the polymer, so in our work, we assume λ is 3.60, 2.00, 0.71 and 0.23 nm for 3.60, 2.00, 0.71 and 0.23 MDa PNIPAM polymers respectively. Although the choice of these values is somewhat arbitrary, the reason they were selected is because they provide a good fit to the experimental data. The total interaction forces calculated by summing Equations 4, 5 and 6 are shown in Figure 2 for 22°C and in Figure 3 for 50°C for each of the polymer molecular weights investigated. The results in Figure 2 indicate that at temperature below the LCST, PNIPAM produces repulsion between silica surfaces and acts as a dispersant due to the positive polar interfacial interaction. Increasing PNIPAM molecular weight increases the range of the polar (steric) repulsion at room temperature. The results shown in Figure 3 indicate that at temperature above the LCST, PNIPAM of sufficiently high molecular weight (>about 1 MDa) produces attraction between silica surfaces due to the polar attraction. This attraction is a result of the increased hydrophobicity of the surface and results in particle aggregation. If the polymer molecular weight is less than about 1 MDa no strong long range attraction appears to be present because the electrical double layer repulsion is sufficiently strong to overwhelm the polar attraction. 1.50

3.60 MDa

0.71 MDa 2.0 MDa 2.00

Force/Radius (mN/m)

1.00

no polymer

0.50

0.23 MD 0.00 0

5

Separation Distance (nm) -0.50

FIG 2 - Inter-particle interaction forces normalised by radius calculated as the sum of the van der Waals, electrical double layer and polar interfacial interactions for silica surfaces coated with 0.1 g/m2 PNIPAM of various molecular weights at 22°C.

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1.00

Force/ Radius (mN/m)

0.23 MDa no polymer

0.50

Separation Distance (nm)

0.00 0

5

0.71 MDa

2.00 MDa

-0.50

3.60 MDa

-1.00

FIG 3 - Inter-particle interaction forces normalised by radius calculated as the sum of the van der Waals, electrical double layer and polar interfacial interactions for silica surfaces coated with 0.1 g/m2 PNIPAM of various molecular weights at 50°C.

Comparison to measured force curves Figure 4 shows the results of the measured interaction forces between a silica plate and colloid probe upon approach in 20 ppm PNIPAM solutions in 0.01 M NaNO3 in deionised water at pH 6 at 22°C. (Burdukova et al, 2010a) At room temperature the surfaces repel each other at all separation distances and the repulsion increases upon closer approach. When no polymer was added, the repulsion is due to electrical double layer repulsion. Upon addition of low molecular weight polymer, 0.23 MDa, a slight reduction in electrical double layer repulsion results which is nearly compensated by a slight increase in steric repulsion and there is little difference in the forces when such low molecular weight 1.50

Force/Radius (mN/m)

1.00

0.71 MDa

4.50 MDa

0.50

2.00 MDa 0.23 MDa 0.00 0

5

no polymer

10

15

20

25

30

35

40

Separation Distance (nm)

-0.50

FIG 4 - Inter-particle interaction forces upon approach normalised by colloid probe radius measured using an Atomic Force microscope for silica surfaces in 20 ppm solutions of PNIPAM of various molecular weights at 22°C at pH 6 in 0.01 M NaNO3. Adapted from Burdukova et al, 2010a. XXV INTERNATIONAL MINERAL PROCESSING CONGRESS (IMPC) 2010 PROCEEDINGS / BRISBANE, QLD, AUSTRALIA / 6 - 10 SEPTEMBER 2010

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polymer is used. Slightly higher molecular weight (0.71 MDa) has only slightly larger effect in terms of creating a slightly more significant steric repulsion. When the high molecular weight polymers (2.0 and 4.5 MDa) are used, significant steric repulsion develops resulting in long range repulsion between the silica surfaces which increases with polymer molecular weight. These experimental results are in qualitative agreement with the results of modelling surface forces using the extended DLVO theory incorporating the polar acid-base interfacial interactions shown in Figure 2. There are significant differences in the theoretical results and the experimental results, particularly at smaller separation distances where the model over predicts the magnitude of the repulsion. It is unclear why the model has this effect, although difficulties in experimentally determining the contact position (determining 0 separation distance) are likely to contribute to poor agreement between the model and experiment at small separation distances. Nonetheless the model is useful in predicting the trends observed experimentally. Figure 5 shows the results of the measured interaction forces between a silica plate and colloid probe upon approach in 20 ppm PNIPAM solutions in 0.01 M NaNO3 in deionised water at pH 6 at 50°C (Burdukova et al, 2010a). At 50°C (T >LCST) two types of behaviour are observed. When no polymer or low molecular weight polymer (0.23 and 0.71 MDa) is added, the surfaces interact with repulsion. The repulsion is similar to that observed at room temperature where no polymer or very low molecular weight polymer is used. The repulsion is likely to be primarily due to the electrical double layer repulsion which overwhelms the weak hydrophobic attraction. A different type of interaction occurs when the polymer is greater than about 1 MDa. For these polymers (2.0 and 4.5 MDa) there is a long range attraction which causes surfaces to snap together from a distance of about 30 to 35 nm into an attractive minimum. Upon further approach, the polymer layer compresses resulting in a weak steric repulsion upon close approach. Again the predictions of the extended DLVO model qualitatively agree with the experimental results in many ways. The model also predicts that the lower molecular weight polymers do not have sufficient hydrophobic attraction to overwhelm the electrical double layer repulsion while the polymers with more than about 1 MDa molecular weight cause the surfaces to become sufficiently hydrophobic that the hydrophobic (polar acid-base interfacial interactions) dominate. The model over predicts the magnitude of the attraction at smaller separation distances. This is likely due to the fact that the model does not account for the compression regime of the steric polymer interaction. Despite this, overall the comparison between modelling and experimental results indicate that the increased attractive interaction at T >LCST and with increasing molecular weight is due to the increased hydrophobicity of the surfaces.

Force/ Radius (mN/m)

1.00

0.50

0.23 MDa

no polymer Sepa p ration Distance (nm)

0.00 0

5

0.71 MDa

10

15

20

25

30

35

40

2.00 MDa 4.50 MDa

-0.50

-1.00

FIG 5 - Inter-particle interaction forces upon approach normalised by colloid probe radius measured using an Atomic Force microscope for silica surfaces in 20 ppm solutions of PNIPAM of various molecular weights at 50°C at pH 6 in 0.01 M NaNO3. Adapted from Burdokuva et al, 2010a. XXV INTERNATIONAL MINERAL PROCESSING CONGRESS (IMPC) 2010 PROCEEDINGS / BRISBANE, QLD, AUSTRALIA / 6 - 10 SEPTEMBER 2010

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Although the polar interfacial interaction approach has not been fully validated and a complete understanding of hydrophobic attraction and short ranged structural repulsion is still elusive, the approach, at least qualitatively, produces the correct trends for our system. As mentioned earlier, it was necessary to assume the decay length of the polar interaction was proportional to the polymer molecular weight. Although this fact seems reasonable, the reason for the molecular weight dependence of the hydrophobic attraction is still not completely understood. Other AFM results (Burdukova et al, 2010a) show behaviour upon retraction that is similar to those observed for polymer bridging. It is likely that upon separation, the surface interactions are related to phenomena such as polymer chain entanglement that are more complicated than can be handled by the polar interfacial interaction approach. Finally and most importantly, the correlation between contact angle and inter-particle forces (both calculated and measured) verify that the changes in force from repulsive to attractive are associated with changes in the surface from hydrophilic to hydrophobic.

Implications for flocculation and flotation

Initial settling rate, m/h

The flocculation, sedimentation rates, supernatant clarity and sediment bed densities have been shown to be a strong function of both PNIPAM molecular weight and temperature (Li et al, 2009). Figure 6 shows an example of how the polymer molecular weight influences the initial settling rate of 10 micron diameter (average size) 5 wt per cent silica suspensions at 50°C when 20 ppm PNIPAM has been added. The results are consistent with the AFM force measurements and polar interfacial interaction modelling which indicate that strong attraction resulting in large aggregate formation and rapid sedimentation occurs at 50°C for polymers of molecular weight greater than about 1 MDa. The sedimentation studies also confirm that at 22°C the suspensions are stable and do not settle rapidly in the presence of PNIPAM. This is consistent with the repulsive forces observed by AFM and predicted by the extended DLVO modelling at temperatures less than the LCST of PNIPAM. PNIPAM has also been found to be an effective flotation collector for fine minerals (Franks et al, 2008; Li and Franks, 2008; Franks et al, 2009). Recent induction time measurements (Burdukova et al, 2010b) indicate that the induction time for bubble-particle attachment at 50°C decreases as polymer molecular weight increases. These results suggest that higher molecular weight PNIPAM is likely to be a better collector than low molecular weight polymer. Results shown in Figure 7 of flotation recovery of a 10 micron diameter (average size) silica (5wt per cent solids suspensions) at 50°C made in a simple laboratory column described elsewhere (Franks et al, 2008; Li and Franks, 2008; Franks et al, 2009) confirm that increasing PNIPAM molecular weight improve the recovery of fine silica particles in a flotation column. In these results there is once again a clear difference in flotation recovery between cases where polymer molecular weight was less than 1 MDa and low recovery (<5 per cent) was achieved and cases where greater than 1 MDa polymer was used and recovery was between about 30 and 50 per cent. These results are consistent with the AFM force measurements and polar interfacial interaction modelling which indicate that the hydrophobicity of the surfaces increases with increasing polymer molecular weight at temperatures above the LCST. When flotation was attempted at room temperature recovery was very low, with only one or two per cent recovery by entrainment observed regardless of polymer molecular weight.

25 20 15 10 5 0 None

0.23

0.71

2.0

3.6

PNIPAM molecular weight, MDa FIG 6 - Initial settling rates of 10 micron silica suspensions (5 wt per cent solids) flocculated at 50°C with 20 ppm PNIPAM of various molecular weights.

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Silica recovery, %

60 50 40 30 20 10 0 None

0.23

0.71

2.0

3.6

PNIPAM molecular weight, MDa FIG 7 - Flotation recovery of 10 micron silica after 5 minutes flotation at 50°C with 10 ppm PNIPAM of various molecular weights.

CONCLUSIONS Contact angle measurements, atomic force microscopy colloid probe force measurements and polar acid-base interfacial interaction modelling all confirm that temperature responsive polymers such as PNIAPM induce attractive interactions between silica particles due to the hydrophobic nature of the surface when the solution temperature is raised above the LCST. The magnitude of both the attraction and the hydrophobicity (as judged by contact angle), increase with increasing polymer molecular weight. Below the LCST, PNIPAM acts as a dispersant by creating steric repulsion between the particles. The change in interaction force from repulsive to attractive induced by adsorbed PNIPAM as temperature is increased can be used to flocculate suspensions at temperature above the LCST and consolidate sediments at temperature below the LCST. The change in surface character from hydrophilic to hydrophobic as temperature is increased can be used to induce aggregation of fine particles to produce hydrophobic flocs which are amenable to recovery by flotation.

ACKNOWLEDGEMENTS The authors would like to acknowledge financial support from the Australian Research Council, AMIRA International, BHP/Billiton, Rio Tinto, Orica Explosives, Anglo Platinum, Xstrata Technology, Freeport McMoran and AREVA NC through the Australian Minerals Science Research Institute and from the Core-to-Core Program promoted by Japan Society for the Promotion of Science. Also thanks to Greg Qiao, for his help with polymer synthesis and characterisation and Hengbao ‘Alex’ Zhang, for assistance in the laboratory.

REFERENCES Addai-Mensah, J, Bal, H and Yeap, K Y, 2008. Polyelectrolyte enhanced flocculation, particle interactions and dewaterability of fine gibbsite dispersions, Asia-Pacific Journal of Chemical Engineering, 3:4 - 12. Bangham, D H and Razouk, R I, 1937. Adsorption and the wettability of solid surfaces, Transactions of the Faraday Society, 33:1459 - 1463. Burdukova, E, Li, H, Ishida, N. O’Shea, J P and Franks, G V, 2010a. Temperature Controlled Surface Hydrophobicity and Interaction Forces Induced by Poly (N-Isopropylacrylamide), Journal of Colloid and Interface Science, 342:586 - 592. Burdukova, E, Li, H, Bradshaw, D J and Franks, G V, 2010b. The Efficacy of Poly (N-Isopropylacrylamide) (PNIPAM) as a Flotation Collector: Effect of Temperature and Molecular Weight, in Minerals Engineering. Cleveland, J P, Manne, S, Bocek, D and Hansma, P K, 1993. A non-destructive method for determining the spring constant of cantilevers for scanning force microscopy, Review of Scientific Instruments, 64:403 - 405. Cruz-Silva, R, Escamilla, A, Nicho, M E, Padron, G, Ledezma-Perez, A, Arias-Martin, E, Moggio, I and Romero-Garcia, J, 2007. Enzymatic synthesis of pH-responsive polyanailine colloids by using chitosan as steric stabilizer, European Polymer Journal, 43(8):3471 - 3479.

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