TEMPERATURE SENSITIVE POLYMERS AS EFFICIENT AND SELECTIVE FLOTATION COLLECTORS E Burdukova1, D Bradshaw2 and G Franks3 ABSTRACT Poly (N-isopropylacrylamide) (PNIPAM) is a temperature responsive polymer that undergoes changes from water soluble hydrophilic molecules to water insoluble hydrophobic colloids at temperatures below and above its lower critical solution temperature (LCST) of 32°C. It has been comprehensively demonstrated that PNIPAM is an effective flocculant, causing the formation of particle aggregates by means of hydrophobic attraction. It has also been demonstrated that PNIPAM has potential to act as a collector in a flotation system. As such, it is potentially suitable for the use in ultrafine flotation systems, which require selective formation of hydrophobic particle aggregates. This study investigates both the efficacy and selectivity of PNIPAM as a flotation collector. This is achieved by investigating the effect of charged PNIPAM polymers on the probability of particle/ bubble attachment of quartz and alumina particles respectively, where the probability of attachment is estimated using induction time measurements. The study also examines the effect of charged PNIPAM polymers on the selective floatability of quartz and alumina particles in a microflotaiton system. The results of this study showed that charged poly(N-isopropylacrylamide) polymers selectively increased both the probability particle/bubble attachment as well as the floatability of mineral particles. These results provide a basis for considering PNIPAM as both an effective and selective collector in a flotation system. Coupled with previous studies that demonstrate the efficacy of PNIPAM as a flocculant in mineral suspensions, these results clearly demonstrate the potential use of PNIPAM as a dual function reagent which acts as both selective flotation collector and flocculant in the flotation of ultrafine particles. Keywords: flotation, selectivity, flocculation, temperature responsive polymers

INTRODUCTION One of the major challenges facing minerals processing operations is the flotation of ultrafine mineral particles. It is well known that the floatability of fine particles is poor, due to the low probability of collision with air bubbles (Trahar and Warren, 1976). However, the continuous depletion of higher grade ores requires the comminution of mineral particles to increasingly finer sizes to achieve mineral liberation. For this reason, producing large aggregates of fine particles is gaining an increasing level of importance in the mineral processing industry. In order for particle aggregation methodologies to be successful in a flotation system, they must satisfy two criteria: Aggregation must be selective and the resultant aggregates must be hydrophobic (Laskowski and Lopez-Vladivieso, 2004). Over the years, the predominant focus of study of particle aggregation in flotation has been on the field of oil agglomeration, a technique which is able to achieve selective formation of hydrophobic aggregates. Oil agglomeration is a complex process involving multiple reagents (such as collectors and extender oils) and requires careful control of the sheer conditions (Lapidot and Mellgren, 1968; Song et al, 2001; Laskowski and Lopez-Vladivieso, 2004; da Rosa and Rubio, 2005). Hydrophobic aggregates can also be obtained with a joint use of flotation collectors and polymeric flocculants. Similarly to the oil agglomeration process, multiple reagents are involved and few studies have demonstrated the efficacy of this process (Sadowski and Polowczyk, 2004). 1. Research Fellow, Department of Chemical and Biomolecular Engineering, University of Melbourne, Vic 3010, Australia. Email: [email protected] 2. Professorial Research Fellow, Julius Kruttschnitt Mineral Research Centre, University of Queensland, Isles Road, Indooroopilly Qld 4068, Australia. Email: [email protected] 3. Associate Professor, Department of Chemical and Biomolecular Engineering, University of Melbourne, Vic 3010, Australia. Email: [email protected]

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An alternative methodology for the flotation of mineral aggregates is the use of temperature sensitive polymers, such as Poly (N-isopropylacrylamide) (PNIPAM). At temperatures below its lower critical solution temperature (LCST) of 32°C, PNIPAM is hydrophilic and soluble in water. At temperatures above the LCST, the hydrogen bonds between water molecules and polymer chains are broken and instead, intramolecular and intermolecular hydrogen bonds formed. The formation of such bonds causes the molecule to coil up, exposing its hydrophobic core. This renders the polymers hydrophobic and insoluble in water, which causes them to precipitate from solution in the form of hydrophobic colloidal particles (Saunders et al, 1999; Sakohara, Kimura and Nishikawa, 2002; Sun et al, 2004). The studies of PNIPAM in the context of mineral processing applications originate in its potential use as a temperature responsive reagent for flocculation and solid/liquid separation. It has been comprehensively demonstrated that at temperatures above the LCST, PNIPAM can act as an effective flocculant. The presence of highly hydrophobic polymer colloids on the surfaces of mineral particles facilitates the formation of large particle aggregates through hydrophobic attraction. The resultant particle aggregation significantly improves the settling characteristics of ultrafine mineral suspensions (Deng, Xiao and Pelton, 1996; Sakohara, Kimura and Nishikawa, 2002; Sakohara and Nishikawa, 2004; Franks, 2006; Li et al, 2007; Li and Franks, 2008; Franks et al, 2009; Li, O’Shea and Franks, 2009). As well as being an effective flocculant, preliminary investigations also demonstrated that PNIPAM could act as an effective collector in a flotation system (Franks et al, 2009). At temperatures above the LCST, the presence of PNIPAM was shown to increase the hydrophobicity of silica glass surfaces as well as induce flotation of both silica and kaolinite in a very simple flotation system. The floatability of kaolinite in the presence of PNIPAM was shown to be significantly greater that in the presence of dodecyl amine, a common flotation collector (Franks et al, 2009). The above studies have shown that PNIPAM is able to act as both a flocculant and a flotation collector. Its presence in a suspension of ultrafine hydrophilic particles is able to induce the formation of particle aggregates that are both large and hydrophobic. This satisfies both criteria for successful particle aggregation in a flotation system (Laskowski and Lopez-Vladivieso, 2004). This places PNIPAM in a unique position in the field of particle aggregation aimed at flotation. For this reason it is important to comprehensively characterise PNIPAM as a potential dual function reagent which acts as both flotation collector and flocculant. The aim of this work is to study the effect of different types of PNIPAM on the selective floatability of mineral particles. Floatability is generally discussed in terms of three components: bubble/ particle collision, bubble/particle attachment and bubble/particle detachment (Laskowski, 1986; Ralston, 1992). Of the three components, bubble/particle attachment is the one that is most affected by collectors, as it is chiefly dependent on the surface forces between bubbles and particles and is unaffected by hydrodynamic factors such as cell turbulence and particle size. The probability of bubble/particle attachment is largely dependent on the surface forces existing between a bubble and a particle. Therefore, the probability of bubble particle attachment is subject to surface forces, as described by the modified DLVO theory, mainly: van der Waals forces, electrical double layer repulsion and hydrophobic attraction. It follows that the increase in the probability of bubble particle attachment will likely be caused by an increase in mineral hydrophobicity and a decrease in double layer repulsion between particles and bubbles. Particle/bubble attachment can be characterised using induction time measurements (Nguyen, Schulze and Ralston, 1997). Induction time is defined as the time for the thinning of the intervening liquid film between an air bubble and a hydrophobic particle to a critical thickness at which the film will rupture spontaneously (Laskowski, 1974). Induction time is measured by bringing captive bubbles into contact with a bed of particles for a controlled time interval (typically between ten and 1500 ms), to determine whether or not mineral particles attach to bubbles and can be lifted out of the particle bed. The contacts between bubbles and a bed of particles are repeated numerous times, estimating the probability of bubble/particle pickup at each contact time. The contact time corresponding to 50 per cent probability of particle pickup is termed the induction time. This technique allows for a simple and reliable estimate of the strength of particle/bubble attachment (Eigeles and Volova, 1965; Yordan and Yoon, 1986; Yordan and Yoon, 1988; Holuszko et al, 2008; Burdukova and Laskowski, 2009). XXV INTERNATIONAL MINERAL PROCESSING CONGRESS (IMPC) 2010 PROCEEDINGS / BRISBANE, QLD, AUSTRALIA / 6 - 10 SEPTEMBER 2010

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In this paper, charged PNIPAM polymers are evaluated in terms of their efficacy and selectivity as collectors in a model flotation system. The effect that these polymers have on the properties of mineral surfaces is characterised using zeta potential measurements. The effect of the charged polymers on the probability of bubble/particle attachment is studied using induction time measurements. Finally, the selective floatability of mineral particles in the presence of different types of PNIPAM is tested using microflotation.

EXPERIMENTAL METHODS Materials Temperature sensitive poly(N-isopropylacrylamide) (PNIPAM), was used in this work. Cationic and anionic random co-polymers of roughly equivalent molecular weights and charge densities were used. The cationic polymer had a molecular weigh of 1.18 MDa and a charge density of 15 per cent. The anionic polymer had a molecular weight of 1.84 MDa and a charge density of 15 per cent. All the polymers used were synthesised in our laboratories as described in detail by O’Shea et al (O’Shea et al, 2007). Quartz powder (Silica 400G) was obtained from UNIMIN Australia Limited for the use in zeta potential measurements. The particles had a size distribution of d50 10 μm, and a BET surface area of 1.7 m2/g. The powder contained 99.6 per cent SiO2 and traces of alumina, ferric oxide, titania and lime. For the use in induction time and flotation measurements, quartz particles (Silica 100 WQ) were obtained from UNIMIN Australia Limited. The particles were dry sieved to obtain a particle size distribution of 90 μm < Dp < 150 μm. The particles contained 99.5 per cent SiO2 and traces of alumina, ferric oxide, titania and lime. Alpha alumina powder was obtained from Sumitomo, Japan for the use in zeta potential measurements. The particles had a size distribution of d50 1 μm, and a BET surface area of 7 m2/g. The powder contained 99.9 per cent alumina and traces of silica, calcium, fluoride and sulfur. For the use in induction time and flotation measurements, alumina particles (CA 100) were obtained from UNIMIN Australia Limited. The particles were crushed in a ring mill and dry sieved to obtain a particle size distribution of 90 μm < Dp < 150 μm. The particles contained 97.5 per cent Al2O3 and traces of silica, soda, lime and ferric oxide.

Methods Zeta potential measurements Zeta potential measurements were performed using the Zeta Acustosizer, manufactured by Colloidal Dynamics, Sydney Australia. The instrument is equipped with a recirculating water bath, which allowed for the measurement being performed at 50°C. 5 wt per cent suspensions of quartz and alumina respectively were prepared using deionised water containing 40 ppm of PNIPAM (corresponding to 400 g/ton of mineral). The suspensions were allowed to stand for 24 hours prior to measurement. The measurements were performed in a pH range between 2 and 11. 1M NaOH and 1M HCl solutions were used for pH adjustment, with the background electrolyte of 10-2 KCl.

Induction time measurements Quartz and alumina particles were treated with both anionic and cationic PNIPAMs, prior to induction time measurement. 10 grams of mineral were placed in a beaker containing 100 ml of deionised water adjusted to pH 8 using 1M NaOH solution for pH adjustment, with the background electrolyte of 10-2 KCl. The beaker was immersed in a water bath controlled to 50oC. Each suspension was dosed with 40 ppm of PNIPAM, which corresponded to 400 g of polymer per ton of mineral. The suspensions were allowed to equilibrate of one hour, while being continuously agitated with a magnetic stirrer. After one hour, the solids were filtered out and gently washed with deionised water heated to 50°C. The PNIPAM coated solids were placed in the oven to dry at 70oC. Induction time measurements were performed with the MCT 100 Induction Time Meter, provided by the Julius Kruttschnitt Mineral Research Centre, University of Queensland. The temperature of the measurement was kept controlled by a small water jacket, fitted to a measurement vessel. Water from an adjacent recirculating water bath was pumped through the water jacket, maintaining a constant temperature of 50°C (above the LCST). XXV INTERNATIONAL MINERAL PROCESSING CONGRESS (IMPC) 2010 PROCEEDINGS / BRISBANE, QLD, AUSTRALIA / 6 - 10 SEPTEMBER 2010

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FIG 1 - Example of the calculation of the error in the induction time measurement.

Approximately 2 g of solids were placed in the measurement vessel (enough to cover the vessel bottom) and covered with deionised water adjusted to 50°C. The bubble was brought into contact with the particle bed at set contact times ranging between 10 and 1200 ms. For each contact time, the probability of particle pickup was estimated by counting the number of times particles were picked up by a bubble out of 40 to 80 contacts. The induction time was estimated as the contact time at which the pickup probability was 50 per cent. The error in the induction time value was estimated by calculating the 95 per cent confidence interval of the linear regression of contact times as a function of pickup probability as shown in Figure 1 (Weisberg, 2005).

Microflotaiton The floatability of quartz and alumina particles was tested in a microflotaiton column developed and manufactured at the Centre of Minerals Research, University of Cape Town (Bradshaw and O’Connor, 1996). Air was supplied to the base of the 200 ml column using a syringe, at a rate of 15 ml/min. 16 ppm of Methyl isobutyl carbinol (MIBC) was used as a frother in order to ensure the formation of small bubbles. The mineral pulp inside the column was kept in suspension by recirculating it with the aid of a peristaltic pump, set at 100 rpm. The flotation column was fitted with a re-circulation water jacket, connected to a water bath in order to keep the vessel temperature at 50°C. Flotation tests were performed using 2 grams of mineral (quartz and alumina respectively) in the electrolyte solution containing 10-2 M KCl, adjusted to pH 8. The minerals were first conditioned for 5 minutes at room temperature in 10 ppm solution of PNIPAM (1000 g/ton). The suspension was then transferred to the flotation column where it was conditioned for a further five minutes to bring it up to 50°C. The flotation concentrate was collected, filtered, dried and weighed after five minutes of flotation.

RESULTS Effect of charged polymers on mineral zeta potential The selectivity of PNIPAM was tested using two types of polymer, cationic and anionic, with similar charge densities and molecular weighs. The selectivity of these two polymers was tested using two minerals: quartz and calcined alumina. These minerals were chosen for the differences in their surface charge distributions. The zeta potentials of both minerals as a function of pH are presented in Figure 2. Please note that the error bars represent the 95 per cent confidence interval of the mean value. The figure clearly shows that the zeta potential of quartz is highly negative in the entire tested pH range. On the other hand, alumina is positively charged in an acidic pH range, with the iso-electric point at circa pH 8.5. At higher pH, the charge of alumina becomes negative. At pH 8 (a common pH condition for a variety of flotation systems), quartz carries a strong negative charge exceeding -40 mV, while alpha alumina is approaching its point of zero charge. It is therefore likely that at pH 8, the charges of the two minerals are sufficiently different from one another to XXV INTERNATIONAL MINERAL PROCESSING CONGRESS (IMPC) 2010 PROCEEDINGS / BRISBANE, QLD, AUSTRALIA / 6 - 10 SEPTEMBER 2010

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FIG 2 - Zeta potential of quartz and alumina particles at 50°C in 10-2 KCl solution.

enable selective adsorption of charged polymers onto their surfaces. Furthermore, at pH 8 quartz and alumina do not carry strong opposing charges. This means that they would not heterocoagulate if placed together in an aqueous suspension. For this reason, all further experiments were carried out at pH 8. Figure 3 demonstrates the effect that the addition of 400 g/ton of different types of PNIPAM has on the zeta potential of quartz and alumina particles, at pH 8 and 50°C (above the LCST). The addition of cationic PNIPAM has an effect of significantly reducing the negative zeta potential of quartz surfaces (by ≈ 15 mV), indicative of strong adsorption of positively charged polymer onto the negatively charged quartz surfaces. On the other hand, the addition of cationic PNIPAM has relatively little effect on alumina surfaces. The presence of polymer causes a very small increase in the positive zeta potential of alumina particles; however this increase is not significant on a 95 per cent confidence interval. This indicates that positively charged PNIPAM adsorbs poorly onto weakly positively charged alumina. These results strongly suggest that at pH 8, cationic PNIPAM is selective towards negatively charged quartz particles. Figure 3 also shows the effect that anionic PNIPAM has on both quartz and alumina surfaces. The addition of 400g/ton of anionic PNIPAM to quartz results in a slight increase in the negative zeta potential of quartz surfaces. However, the increase is quite small (less than 5 mV). This indicates that while anionic PNIPAM does adsorb onto negatively charged quartz surfaces, the degree of adsorption is small. On the other hand, the presence of anionic PNIPAM causes a large shift in the zeta potential of alumina surfaces, causing it to change from positive to negative. These results strongly suggest that at pH 8, anionic PNIPAM is selective towards positively charged alumina particles.

Effect of charged polymers on selective particle/bubble attachment To test whether or not charged PNIPAM is able to induce selective particle/bubble attachment of quartz and alumina particles, induction time measurements were performed on both minerals using 400 g/ton of cationic and anionic polymers. Figure 4 represents the probability of pickup of quartz

FIG 3 - Zeta potential of quartz and alumina particles at 50°C in 10-2 KCl solution at pH 8, in the presence and absence of 400 g/ton of different types of PNIPAM.

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FIG 4 - Pickup probability of quartz and alumina partices as a function of bubble/particle contact time at 50°C in 10-2 KCl solution at pH 8, in the presence and absence of 400 g/ton of different types of PNIPAM.

and alumina particles by an air bubble as a function of increasing bubble/particle contact times. The figure demonstrates that in the absence of polymer, both quartz and alumina exibit low particle pickup probabilities. In the case of quartz, the addition both anionic and cationic PNIPAM resutls in an increase in the probabilty of particle pickup, but the responces of the individual polymers are vastly different from one anohter. The addition of cationic polymer has an effect of only sligtly inreasing the particle pickup probability, making it roughly equvalent to that of alumina in the absence of polymer. However, the addition of anionic polymer increases the pickup probability by an order of magnitude. The reverse appears to be true of alumina particles. The addition of positively charged polymer, which yielded the best performing condition in the case of quartz, appeared to make little difference to alumina attachment probability. On the other hand, the addition of anionic PNIPAM resulted in the largest improvement in the bubble/particle attachment probability of alumina, having failed in the case of quartz. The obtained pickup probability vs. contact time curves were analysed in a manner described in Figure 1, to obtain the estimates of induction time (contact time corresponding to 50 per cent pickup probability). The results are summarised in Figure 5 (note the logarithmic scale on the induction time axis). The figure clearly shows that in the absence of polymer, both alumina and quartz exhibit very high induction times, indictative of very low probabilities of particle/bubble attachment. However in the case of alumina, the induction time is substantially lower than that of quartz. At pH 8, the zeta potential of alumina particles is very close to the point of zero charge, while that of quartz is strongly negative (see Figures 2 and 3). This means that at this pH, the double layer repulsion between bubbles and particles is significantly lower in the case of alumina, resulting in reduced induction times. With the addition of 400 g/ton of anionic PNIPAM, the induction times of both quartz and alumina undergo a decrease. In the case of quartz, this decrease is small; with the resulting induction time in excess of 600 ms. This indicates a small increase in the hydrophobicity of quartz particles, promoting

FIG 5 - Induction times of quartz and alumina partices 50°C in 10-2 KCl solution at pH 8, in the presence and absence of 400 g/ton of different types of PNIPAM.

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an increase in the probability of particle/bubble attachment. In the case of alumina, the decrease in induction times is much more pronounced, indicating a much stronger increase in hydrophobicity. This is consistent with an earlier study which showed that presence of PNPAM on mineral surfaces results in enhanced surface hydrophobicity (Burdukova, E et al, 2009). The increase in hydrophobicity results in a large increase in the probability of particle/bubble attachment. These results demonstrate that anionic PNIPAM selectively promotes significant increases in the probability of attachment of alumina particles to air bubbles. Similarly, the addition of 400 g/ton of cationic PNIPAM results in a decrease in the induction times of both quartz and alumina. However in this instance, the decrease in induction time is significantly greater in the case of quartz, indicating a greater increase in hydrophobicity. These results illustrate that cationic PNIPAM selectively promotes significant increases in the probability of attachment of quartz particles to air bubbles. The above results demonstrate that both anionic and cationic PNIPAM can be utilised to selectively promote the particle/bubble attachment of different minerals, under the right set of pH conditions. Probability of bubble/particle attachment has a direct effect on the overall probability of flotation (Laskowski, 1986; Ralston, 1992). These results therefore indicate that the presence of charged PNIPAM in a flotation system is potentially capable of inducing selective flotation of previously hydrophobic and non-floatable minerals.

Effect of charged polymers on selective mineral floatability Having established the effect that charged PNIPAM polymers have on the probability of particle/ bubble attachment, their effect on the overall particle floatability can now be tested. To achieve this, both quartz and alumina particles were floated in a microflotaiton column both in the presence and absence of anionic and cationic PNIPAM. Figure 6 shows the effect of 1000 g/ton of different types of PNIPAM on the flotation recovery of both quartz and alumina at 50°C (above the LCST), with the error bars representing the 95 per cent confidence interval of the mean value. The results clearly demonstrate that in the absence of PNIPAM both quartz and alumina are characterised by a very low floatabilities. The floatability of quartz appears practically negligible; however a small fraction of alumina particles (circa 4 per cent) is recovered. This is in agreement with the induction time measurements, which show that alumina particles have a higher probability of bubble/particle attachment than quartz (see Figures 4 and 5). The addition of anionic PNIPAM has an effect of slightly increasing the recovery of quartz, however the recovery is still very low (circa 12 per cent). On the other hand, the addition of anionic polymer has an effect of dramatically increasing the floatability of alumina particles, bringing its recovery up to approximately 80 per cent. These results are consistent with induction time measurement and clearly demonstrate that negatively charged PNIPAM polymer selectively induces a high degree of floatability in previously hydrophilic alumina particles, by increasing the probability of bubble/ particle attachment.

FIG 6 - Flotation recovery of quartz and alumina particles after 5 minute of flotation at 50°C in 10-2 M KCl solution at pH 8, in the presence and absence of 1000 g/ton of different types of PNIPAM.

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Conversely, the addition of cationic PNIAPM has only a small effect on the floatability of alumina, with the resulting recoveries increasing up to 20 per cent. However, the effect of cationic PNIPAM has a dramatic effect on the floatability of quartz particles, increasing their recoveries up to circa 67 per cent. These results are once again in agreement with induction time measurement and clearly demonstrate that positively charged PNIPAM polymer selectively induces a high degree of floatability in previously hydrophilic quartz particles, by increasing the probability of bubble/ particle attachment.

CONCLUSIONS The results of this study demonstrate that charged poly(N-isopropylacrylamide) polymers selectively adsorb onto charged mineral surfaces. At temperatures above the lower critical solution temperature (LCST), the presence of these polymers on the surfaces of mineral particles significantly increases the probability of attachment of these particles to bubbles. The increase in particle/bubble attachment probability leads to an overall selective increase in particle floatability. These results provide a fundamental basis for considering PNIPAM as both an effective and selective collector in a flotation system. Coupled with previous studies that demonstrate the efficacy of PNIPAM as a flocculant in mineral suspensions (Deng, Xiao and Pelton, 1996; Sakohara, Kimura and Nishikawa, 2002; Sakohara and Nishikawa, 2004; Franks, 2006; Li et al, 2007; Li and Franks, 2008; Franks et al, 2009; Li, O’Shea and Franks, 2009), these results clearly demonstrate the potential use of PNIPAM as a dual function reagent which acts as both selective flotation collector and flocculant in the flotation of ultrafine particles.

FUTURE WORK The effect of PNIPAM will be tested in a system containing ultrafine mineral particles (<10 μm), where its dual function as a collector/flocculant can be tested. The collecting efficiency of PNIPAM will be compared to that of an industrial collector, such as dodecylamine hydrochloride.

ACKNOWLEDGEMENTS The authors would like to acknowledge the following for their helpful contribution to this work: • The Australian Research Council, AMIRA International, BHP/Billiton, Rio Tinto, Orica, Anglo Platinum, Xstrata, Freeport McMoran and Areva Inc, through the Australian Minerals Science Research Institute (AMSRI) (LP0667828), for their financial support. • Mr Boris Albijanic of Julius Kruttschnitt Mineral Research Centre, for assisting with the induction time measurements. • Mr J P O’Shea, for synthesising the polymers used in this study. • A/Prof Greg Qiao, for his help with polymer synthesis and characterisation. • Mr Hengbao ‘Alex’ Zhang, for assistance in the laboratory.

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