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Fundamental study on talc–ink adhesion for talc-assisted flotation deinking of wastepaper
Minerals Engineering 20 (2007) 566–573 This article is also available online at: www.elsevier.com/locate/mineng
Fundamental study on talc–ink adhesion for talc-assisted flotation deinking of wastepaper J. Liu a
a,c
, J. Vandenberghe
a,d
, J. Masliyah a, Z. Xu
a,*
, J. Yordan
b
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2G6 b Luzenac America, Centennial, CO, USA c Nalco Company, Naperville, IL 60563, USA d Syncrude Canada Research, Edmonton, Alta., Canada T6N 1H4 Received 12 August 2006; accepted 6 November 2006 Available online 26 December 2006
Abstract Effective removal of ink particles by froth flotation is of up-most importance in paper recycling. In this study, the potential use of talc particles as carriers of difficult-to-float fine ink particles is investigated. In order to facilitate ink particle removal by carrier flotation, the interactions between talc and oily-ink particles in aqueous solutions containing flotation chemicals are studied by zeta potential distribution measurements and with the impinging jet apparatus. The chemical conditions under which talc–ink aggregation occurs are identified and reported. 2006 Elsevier Ltd. All rights reserved. Keywords: Industrial minerals; Agglomeration; Surface modification; Recycling
1. Introduction In the paper recycling industry, effective removal of stickies and ink particles are critical steps to improve the quality of the recycled fibers. Stickies are almost all removed from the recycled fibers by means of screens, reverse cleaners and dissolved air flotation clarifiers. Talc is an effective control agent commonly used for the removal of stickies to supplement the mechanical removal of stickies and ultimately to keep the paper machine free of tacky deposits. On the other hand, ink particles are mostly removed from the re-pulped paper slurry by froth flotation (Fuerstenau, 1980; Borchard, 1993). The removal efficiency of very fine ink particles by flotation is typically low owing to the low collision-attachment efficiency of small ink particles being floated with flotation bubbles. To improve ink removal efficiency by flotation, either agglomerating ink particles with
*
Corresponding author. E-mail address: [email protected] (Z. Xu).
0892-6875/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2006.11.005
oil (Snyder and Berg, 1994) or using coarse talc particles to carry (piggy-back) ink particles (Maiolo and Pelton, 1998) has been recently considered to be promising avenues. Sparks and Puddington (1976) and Markham et al. (1994) also showed an effective removal of ink with the addition of coarse carrier particles such as talc, clay and calcium carbonate. Developing these novel ideas into a more effective and practical technology requires fundamental understanding of their interactions. The direct surface force measurement using atomic force microscope (AFM) in a model system was used to investigate interactions involving toner/oil (Aston and Berg, 1988), toner/anatase (Azevedo et al., 1999), toner/polyethylene (Drelich et al., 2000), and toner/talc (Chi et al., 2001). However, the requirement of regular geometry for the probing particle and surface limited the use of AFM direct force measurement only to a few well-defined systems. This study is concerned with the use of talc particles as carriers of fine ink particles in flotation. The objective of this investigation is to identify the chemical conditions required to promote aggregation (analogous to hetero-coagulation)
J. Liu et al. / Minerals Engineering 20 (2007) 566–573
of talc and ink particles in carrier (piggy-back) flotation of ink particles. The information derived from this study is vital in making the talc-assisted de-inking flotation concept, a successful commercial technology. In order to accomplish this, the critical chemical conditions leading to aggregation or lack thereof between talc and ink particles were identified from zeta potential distribution measurements made with single component of ink and talc particles as well as with their binary mixtures. The measurements were conducted in chemical environments pertinent to de-inking flotation. The aggregation of talc and ink particles was then visually confirmed by deposition tests conducted with an impinging jet apparatus. 2. Principles of the technique Theoretically, the classical DLVO (Deryaguin–Landau– Verwey–Overbeek) theory considering summation of van der Waals and electrostatic double layer interactions was often used to predict colloidal particle interactions. Calculation of electrostatic double layer and van der Waals interactions is relatively straightforward for well-defined systems with known system parameters such as surface potential of interacting colloidal particles, electrolyte composition and concentration of solution, and Hamaker constant or the parameters needed to calculate the resultant Hamaker constant of the systems. However, in many cases, the classical DLVO could not accurately predict the colloidal interactions and an extended DLVO theory has to be used, which may include repulsive hydration force for hydrophilic surfaces, attractive hydrophobic force for hydrophobic surfaces, repulsive steric force and attractive bridge force for polymer bearing surfaces, and specific acid–base (AB) interactions. The theory for describing these additional non-DLVO forces is less well developed. Alternatively, these forces were often inferred from the deviation of the experimentally measured colloidal forces from those predicted by the classical DLVO theory. In this regard, many experimental methods have been developed to study the colloidal interactions, most of them for welldefined systems. Force measurement using AFM and SFA is the most direct, but rather limited to the systems that can be studied by the requirement of regular geometry and/or optical transparency. Visual observation such as settling test can directly observe coagulation behavior, but is limited to the particle density. In this paper, we take advantage of different surface properties for different particles. Tracing the change of surface properties could provide us with an important avenue to study colloidal particle interactions. Zeta potential is one of the most important surface properties in a colloidal system. Previous researchers (Maiolo and Pelton, 1998; Borchard, 1993; Fuerstenau, 1980; Snyder and Berg, 1994), using conventional electrophoretic zeta potential meters, have investigated electrokinetic properties of talc and ink particles extensively. In these early measurements, only the average electrophoretic mobility or zeta potential for a single component system
567
was reported, even though binary systems were studied. In the case of a binary mixture system, the measured average zeta potentials of the two components often either over or under estimate the electrical behavior of the system, thereby leading to misguided information regarding the interpretation of interactions between the two components. With the capability of measuring electrophoretic mobility or zeta potential distributions, it is possible to identify the deposition conditions of one component on the other in a binary suspension system. Based on this idea, zeta potential distribution measurement was recently developed into a practical technique for studying particle interactions (Liu et al., 2002a,b, 2004a,b, 2005; Xu et al., 2003). This simple surface properties-based technique does not suffer from the limitations of particle density and geometry. In this study, we demonstrate the application of this technique to investigation of talc–ink particle interactions. The details on interpreting zeta potential distribution data in the context of particle interactions were reported elsewhere (Liu et al., 2002b). Briefly, let’s take a mixture of two components, each with its distinct zeta potential distribution peak as an example. If only a single modal zeta potential distribution was obtained with the mixture, we know the particles are hetero-coagulated to form composite aggregates. The location of the distribution peak depends on the nature and relative abundance of the particles. On the other hand, if we obtain a bimodal zeta potential distribution of the mixture, with the two peak positions located close to the values of individual components, the two components are not coagulated with each other. In this case, the piggy-back of one component on the other is not expected. Although the theory of zeta potential is well developed for individual colloidal particles (Hunter, 1981), the quantitative description of zeta potential distribution in a coagulated colloidal system is rather complex if not impossible to comprehend from the first principle and is beyond the scope of this communication. Nevertheless, the measured zeta potential distribution pattern of a complex colloidal system does provide an important indication with regard to particle interactions. This approach is applied in this paper to studying interactions of talc particles with oily ink particles, pertinent to the talc-assisted ink removal by froth flotation. 3. Experimental procedures 3.1. Materials The talc samples used in this study were commercially available products from Luzenac America, USA. Talc A is a naturally occurring, finely ground talc product whereas Talc B is the talc treated with a proprietary cationic chemicals. Both of these two types of talc are currently used or tested as an effective control agent for the removal of stickies in pulp industry. The red color oily ink used in the study was supplied by Rieger Printing Inks (Edmonton, Alberta, Canada). Reagent grade HCl and NaOH (Fisher
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Scientific) were used as pH modifiers. Ultrahigh purity KCl (>99.999%, Aldrich) was used as supporting electrolytes, while reagent grade CaCl2 (99.9965% Fisher) was used as the source of calcium ions. Sodium oleic acid (NaOl, 98%) was purchased from Aldrich Chemicals. Reagent grade toluene (Fisher) was used as the dilution solvent. The de-ionized water with a resistivity of 18.2 MX cm, prepared with an Elix 5 followed by a Millipore-UV plus ultra water purification system was used throughout this study. 3.2. Talc slurry preparation The dry talc particles were first wetted with 1 mM potassium chloride solution to form a paste. The obtained paste was slurried in a potassium chloride-containing aqueous solution to approximately 10 wt% solids under high shear mixing for several minutes to ensure proper talc particle dispersion. This concentrated suspension was further diluted as needed for the zeta potential distribution measurement and the impinging jet experiments. 3.3. Ink suspension preparation The oily ink was dispersed in a potassium chloride solution to contain about 2 wt% ink using an ultrasonic bath. The prepared suspension was diluted as required for the zeta potential distribution measurements. 3.4. Preparation of oily ink surface For preparation of the talc particle collecting surface used in the impinging jet experiments, the ink sample was dissolved in toluene at ca. 10 mg ink per ml toluene dilution ratio. About 0.5 ml ink-in-toluene solution was placed drop-wise onto a 5 · 5 cm transparent, clean glass slide spinning at 5000 rpm for 1 min. To remove toluene from the ink, the ink-coated surface was dried for about one hour in a particle-free horizontal laminar hood under ambient conditions. The ink-coated glass slide had a smooth, reddish appearance, yet sufficiently transparent to be seen through under an optical microscope. The uniform coating of ink was confirmed by a water contact angle value of 73 measured with a drop shape analyzer (Kruss, USA). The surface was used as the collecting surface for talc particles in the impinging jet experiment. 3.5. Zeta potential distribution measurements The zeta potential distribution of the talc slurries and ink suspension was measured with a Zetaphoremeter (SEPHY/CAD). The operating system in this instrument allows tracking 40–100 particles simultaneously and provides a zeta potential distribution for these particles (Liu et al., 2002a,b, 2004a,b, 2005; Xu et al., 2003). If these particles are homogeneous, the distribution peak is narrow. Otherwise, the peak would be broad. A 40 ml suspension
containing 0.1% solids was conditioned for 5 min and then used to fill the electrophoresis cell. The suspensions of ink and talc were measured first using a single component suspension, then a mixture of ink and talc at an ink to talc mass ratio of 1:1. The average of four independent measurements was reported. 3.6. Impinging jet measurements The impinging jet measurement was carried out to visually observe the deposition of talc particles on the ink surface with an impinging jet apparatus (Yang, 2000; Sanders et al., 1995). The apparatus consists of a flow system with a precision flow rate control device, an impinging jet cell with a capillary tube, a collector (glass slide coated with probing sample) and a view system (optical microscope with a CCD camera and video recorder). The suspension flows from the capillary tube in the impinging jet cell at a constant flow rate, and impinges on the collecting surface. In this manner, the particles in suspension may deposit on the collecting surface, should there exist an attractive and adhesion force between the collecting surface and impinging particles. The particle deposition can be observed through the recorded images. About 1.5 L talc suspension at 0.01 wt% solids was used for each test with a pressure driven flow at a Reynolds number of 180. Impinging process over the first 3 min was recorded for the analysis. The deposition over the stagnation region (near the center of impinging jet) was used for deposition analysis. 4. Results and discussion 4.1. Talc A 4.1.1. Zeta potential distributions The interaction between the as-received talc and ink particles in KCl solutions was studied by zeta potential distribution measurement. In this study, the pH of suspension was fixed at 4 unless otherwise stated. The results obtained are presented in Fig. 1. As noted from Fig. 1a (top), both the talc A and ink particles are negatively charged with the zeta potential distributions peaking at 35 mV and 10 mV, respectively. Also included in Fig. 1a (bottom) is the zeta potential distribution of their binary mixture, which exhibits two distinguishable peaks at 30 mV and 10 mV, respectively. The observed characteristics of the zeta potential distributions for the binary mixture is indicative of non-coagulating nature of talc and ink, as schematically illustrated in the insert of the figure. It is evident that under the chemical environment investigated, there is no attraction leading to adhesion between talc and ink particles. This observation was expected as both the talc and the ink particles carry sufficiently high negative surface charges to make the talc and ink particles repel each other. Shown in Fig. 1b are the zeta potential distributions of talc and ink particles, individually and their binary mixture
J. Liu et al. / Minerals Engineering 20 (2007) 566–573
20
Individual
Individual Talc A
Talc A
Ink
10 Frequency (%)
569
Ink
0 20
Mixture
Mixture
10
0 -60
-40 -20 0 Zeta potential (mV)
-60
-40 -20 0 Zeta potential (mV)
Fig. 1. Measurements of zeta potential distribution for talc A and oily ink particles in 1 mM KCl solution (a) and 1 mM KCl and 0.1 mM sodium oleate solution (b) at pH 4.
20
Individual
Individual Talc A
Talc A
Ink
Ink
Frequency (%)
10
0 20
Mixture
Mixture
10
0
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-40 -20 0 Zeta potential (mV)
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Fig. 2. Measurements of zeta potential distribution of talc A and ink particles in 1 mM KCl solution containing 0.5 mM calcium chloride (a) and 1 mM KCl solution containing 0.5 mM calcium chloride and 0.1 mM sodium oleate (b) at pH 4.
in the presence of 0.1 mM sodium oleate. Sodium oleate is a commonly used fatty acid family surfactant in paper de-inking practice. Note that the addition of sodium oleate shifted the zeta potential distributions of talc and ink particles (Fig. 1b top) slightly to the more negative direction. As a result, when the talc and ink particles were mixed together in the presence of sodium oleate, a bimodal zeta potential distribution was again observed as is shown in Fig. 1b (bottom). The results indicate that the addition of sodium oleate alone could not induce the hetero-coagulation of talc particles with ink particles. The lack of attraction was expected between talc and ink particles due to the
electrostatic repulsion between the negatively charged talc and ink particles. Calcium chloride is a common additive used in paper deinking flotation. Therefore, it would be important to study the effect of calcium addition on talc–ink particle interactions. As shown in Fig. 2a (top), the addition of calcium ions to the talc–ink suspension results in a noticeable shift in zeta potential distributions of both talc A and ink particles, rendering the particles less negatively charged. In fact, adding 1 mM calcium ions caused zeta potential reversal of ink particles to +5 mV. In this case, only a single zeta potential distribution peak was observed for the mixture
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as shown in Fig. 2a (bottom). Clearly, calcium addition induced hetero-coagulation of talc A with ink particles, as anticipated for a binary mixture system of particles carrying opposite electrical charges. A more common practice in de-inking flotation is the coaddition of sodium oleate and calcium chloride as process aids. This co-addition is anticipated to increase the hydrophobicity of ink particles by precipitation of calcium carboxylates on toner particles (Pletka et al., 2000), resulting in an improved ink removal efficiency by flotation. It is therefore important to study the effect of this practice on the talc–ink interactions. The results of the zeta potential distributions obtained with such a system are included in Fig. 2b. As expected, the co-addition of calcium and sodium oleate induced the hetero-coagulation of talc and ink particles, as shown by a single modal zeta potential distribution in Fig. 2b for the mixture of the two. Surprisingly, talc–ink hetero-coagulation occurred despite a marginal negative surface charge carried by ink particles. It is important to point out that in both cases shown in Fig. 2 the talc coated the ink particles with a close-to-full coverage as schematically illustrated in the insert of Fig. 2a. It appears that ink particles are more hydrophobic than talc as such that coating of talc on ink particles would minimize the exposure of hydrophobic ink surfaces to aqueous environment and hence system energy. The zeta potential distribution data presented above demonstrates the feasibility of enhancing ink flotation with talc as a carrier. One way of adhering talc to ink particles is through the addition of chemical additives such as calcium or calcium/sodium oleate mixtures. 4.1.2. Impinging jet measurements The zeta potential distribution results presented in the foregoing section suggested that a negligible interaction exists between the talc and ink particles in KCl aqueous solutions due to strong electrostatic repulsive forces. In order to confirm this observation, impinging jet measurements were conducted in a similar environment. The result
of such a measurement is presented in Fig. 3a. Note that the deposition of talc A particles on the ink-coated collecting surface is marginal, confirming the observations made in the zeta potential distribution measurements. Similarly, the zeta potential distribution measurements presented in the foregoing section suggested that it would be feasible to enhance the deposition of negatively charged talc on the ink particles by the co-addition of sodium oleate and calcium cations. In order to confirm the inferred hetero-coagulation of talc and ink particles from the results of zeta potential distribution measurements, impinging jet experiments were conducted using talc suspensions containing 0.5 mM calcium and 0.1 mM sodium oleate. The recorded image in Fig. 3b shows a significant increase in talc deposition as compared with the case without calcium and sodium oleate addition. In the presence of calcium and sodium oleate, the talc deposited on the ink-coated collecting surface as large aggregates. It is evident that under this condition, calcium induced the adsorption of oleic acid (probably as calcium oleate precipitate) on talc, rendering the talc surface more hydrophobic. The addition of calcium and sodium oleate may also increase surface hydrophobicity of ink particles by calcium aleate precipitation on ink surfaces (Pletka et al., 2000), leading to a strong hydrophobic interaction between the two surfaces. Therefore, the hydrophobic force between talc and ink surface appears to be responsible for the improved talc–ink adhesion. It should be noted that the analysis of impinging jet test results is rather qualitative in this study. Detailed quantification can be done with imaging analysis procedures (Yang, 2000; Sanders et al., 1995). The discussion of interaction in this paper is also limited to a qualitative nature. The calculation can be performed using the DLVO theory to quantify the contributions of hydrophobic interactions in de-inking system using carrier flotation. With the quantification of deposition, a phenomenological relationship can be derived for practical purposes, which is beyond the scope of this work.
Fig. 3. The visual observations of talc A particles on the ink-coated collector surface with the impinging jet cell in 1 mM KCl solution (a) and 1 mM KCl solution containing 0.5 mM calcium chloride and 0.1 mM sodium oleate (b) at pH 4.
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between positively charged talc and negatively charged ink particles. Similar observations are made at pH 6 and 9 as shown in Fig. 4b and c, respectively. One common trend is that increasing pH increases gap between the two distribution peaks measured with individual component suspensions. As a result, a narrower distribution peak with increasing suspension pH is observed in the mixture. Clearly, increasing electrostatic attraction with increasing suspension pH contributed to a stronger hetero-coagulation of ink with talc particles. The effect of sodium oleate addition on ink-particle interaction studied by zeta potential distribution is shown in Fig. 5. Although the zeta potential of talc B is significantly reduced with sodium oleate addition for all the pHs investigated, the feature of the measured zeta potential distribution is essentially the same as those without sodium oleate addition shown in Fig. 4 and can be interpreted similarly.
4.2. Talc B 4.2.1. Zeta potential distributions The zeta potential distributions of ink and talc B suspensions, individually or in their mixture at pH 4, 6 and 9 are shown in Fig. 4. At pH 4 (Fig. 4a), the zeta potential distribution of ink particles exhibited a peak at a small negative value of 8 mV while talc B, at a large positive value of 65 mV. With the mixture of the two, a single broad distribution from 30 to 65 mV was observed. This distribution characteristic suggests a hetero-coagulation of ink particles with talc B. From the disappearance of distribution peak of ink particles and with the single distribution peak near the value measured with single talc particles, it can be inferred that talc particles are partially coating the ink particles as schematically shown in the insert of Fig. 4a (bottom). The observed interaction is within the expectation by the electrostatic attraction
20 Individual Ink
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-80
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Fig. 4. The zeta potential distribution measurement of talc B and ink particles in 1 mM KCl solution at pH 4 (a); pH 6 (b) and pH 9 (c).
Ink
Frequency (%)
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Fig. 5. The zeta potential distribution measurement of talc B and ink particles in 1 mM KCl solution containing 0.1 mM sodium oleate at pH 4 (a), pH 6 (b) and pH 9 (c).
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Fig. 6. The visual observation of talc B on the ink-coated collector surface with the impinging jet cell in 1 mM KCl solution (a), and 1 mM KCl solution containing 0.1 mM sodium oleate (b) at pH 9.
As in the case of talc A, ink particles are covered by talc B, even though talc B was modified by cationic polyelectrolytes to render them positively charged. It should be noted that with talc B, the composite particles are positively charged. Rendering positive charge of ink-talc composite particles may facilitate bubble-particle attachment as bubbles are in general negatively charged. Such a feature is desirable for ink removal by flotation.
4.2.2. Impinging jet measurements Note that talc B is positively charged over a broad pH range. On the other hand, ink particles are known to be negatively charged at pHs greater than 4. Therefore, a strong attraction between the two is anticipated to exist at pH between 4 and 9 as confirmed in the above zeta potential distribution measurement. Extensive talc deposition on ink-covered collecting surfaces (white spots) was observed in impinging jet test as shown in Fig. 6a. An increased number of talc particles were observed to deposit on the collecting surface with increasing suspension pH, reaching a maximum at pH 9. This observation is consistent with the anticipated increase in electrostatic attraction between ink and talc particles with increasing suspension pH. This observation confirms the interpretation of the
results from zeta potential distribution using single components and their binary mixtures described above. At a given pH, sodium oleate addition was found to enhance talc B deposition on the ink-coated collecting surface as shown in Fig. 6b. The observed improvement in deposition of talc particles on ink-coated collecting surfaces by sodium oleate addition could not be accounted for by simply considering electrostatic attraction, as the reduction of positive charge on talc surface by sodium oleate addition would result in a reduced electrostatic attraction between talc B particles and ink surfaces. Clearly hydrophobic attraction between talc B particles and ink surfaces plays an important role in promoting talc deposition on ink surfaces. The talc particles appeared as aggregates in this case, in particular at higher pH, with the highest deposition appearing at pH 6. The aggregation among talc particles could be attributed to the adsorption of oleate surfactant on positively charged talc, rendering overall talc surface more hydrophobic by masking its positively charged hydrophilic edges. 5. Conclusions This study showed that zeta potential distribution measurement is a simple, yet effective method to study particle
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interactions in a colloidal system of complex nature, such as the talc-assisted de-inking flotation. The zeta potential distribution measurement showed that chemical conditions such as calcium and sodium oleate addition or surface modification on talc could promote the adhesion of talc to ink particles. Impinging jet tests confirmed the deposition of talc particles on ink surfaces, which was inferred from the zeta potential distribution measurements conducted with individual components and their binary mixtures. Acknowledgment The financial support for this work from Natural Sciences and Engineering Research Council of Canada (NSERC) and Luzenac America, Centennial, Colorado, USA, is greatly appreciated. References Aston, D.E., Berg, J.C., 1988. Fluid interfacial separations for secondary fiber recovery as probed with atomic force microscopy. Journal of Pulp and Paper Science 24, 121–125. Azevedo, M.A.D., Drelich, J., Miller, J.D., 1999. The effect of pH on pulping and flotation of mixed office wastepaper. 25(9), 317–320. Borchard, J.K., 1993. Paper de-inking technology. Chemistry and Industry 19, 273–276. Chi, R., Xu, Z., Difeo, T., Finch, J.A., Yordan, J.L., 2001. Measurement of interaction forces between talc and toner particles. Journal of Pulp and Paper Science 27, 152–157. Drelich, J., Nalaskowski, J., Gosiewska, A., Beach, E., Miller, J.D., 2000. Long-range attractive forces and energy barriers in de-inking flotation: AFM studies of interactions between polyethylene and toner. Journal of Adhesion Science and Technology 14, 1843–1929. Fuerstenau, D.W., 1980. Fine particle flotation. In: Somasundaran, P. (Ed.), Fine Particle Processing. SME, New York, NY, USA, pp. 669–705.
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Hunter, R.J., 1981. Zeta Potential in Colloid Science: Principle and Applications. Academic Press, London. Liu, J., Zhou, Z., Xu, Z., 2002a. Electrokinetic study of hexane droplets in surfactant solutions and process water of bitumen extraction systems. Industrial and Engineering Chemistry Research 41, 52–57. Liu, J., Zhou, Z., Xu, Z., Masliyah, J., 2002b. Interactions between clays and bitumen in aqueous media from the measurement of zeta potential distributions. Journal of Colloids and Interface Science 252, 409– 418. Liu, J., Xu, Z., Masliyah, J., 2004a. Role of clay fines in the bitumen extraction from oil sands. AIChE Journal 50 (8), 1917–1927. Liu, J., Xu, Z., Masliyah, J., 2004b. Interaction between bitumen and fines in oil sands extraction system: implication to bitumen recovery. Canadian Journal of Chemical Engineering 82, 655–666. Liu, J., Xu, Z., Masliyah, J., 2005. Interaction forces in bitumen extraction from oil sands. Journal of Colloids and Interface Science 287, 507–520. Maiolo, J., Pelton, R., 1998. Aerosol-enhanced flotation – a possible approach to improve flotation deinking. Journal of Pulp and Paper Science 24, 324–328. Markham, L.D., Ala, M., Srivatsa, N.R., 1994. Methods for Removing ink from printed paper using agglomeration agent, followed by addition of talc. US Patent 5,340,439. Pletka, J., Gosiewska, A., Chee, K.Y., McGuire, J.P., Drelich, J., Groleau, L., 2000. Interfacial effects of a polyalkylene oxide/fatty acids surfactant blend in flotation deinking of mixed office papers. 9(2), 40–48. Sanders, R.S., Chow, R.S., Masliyah, J.H., 1995. Deposition of bitumen and asphaltene-stabilized emulsions in an impinging jet cell. Journal of Colloids and Interface Science 174, 230–345. Snyder, B.A., Berg, J.C., 1994. Liquid bridge agglomeration: a fundamental approach to toner deinking. Tappi Journal 77, 79–84. Sparks, B.D., Puddington, I.E., 1976. Deinking of waste news by adsorption of contaminants on a hydrophobic, particulate solid. Tappi Journal 59, 117–119. Xu, Z., Liu, J., Choung, J., Zhou, Z., 2003. Electrokinetic study of clay interactions with coal in flotation. International Journal of Mineral Processing 68, 183–196. Yang C., 2000. Attachment of fine gas bubbles onto a solid surface and electrokinetics of gas bubbles, Ph.D. Thesis, University of Alberta.
Fundamental study on talc–ink adhesion for talc-assisted flotation deinking of wastepaper J. Liu a
a,c
, J. Vandenberghe
a,d
, J. Masliyah a, Z. Xu
a,*
, J. Yordan
b
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2G6 b Luzenac America, Centennial, CO, USA c Nalco Company, Naperville, IL 60563, USA d Syncrude Canada Research, Edmonton, Alta., Canada T6N 1H4 Received 12 August 2006; accepted 6 November 2006 Available online 26 December 2006
Abstract Effective removal of ink particles by froth flotation is of up-most importance in paper recycling. In this study, the potential use of talc particles as carriers of difficult-to-float fine ink particles is investigated. In order to facilitate ink particle removal by carrier flotation, the interactions between talc and oily-ink particles in aqueous solutions containing flotation chemicals are studied by zeta potential distribution measurements and with the impinging jet apparatus. The chemical conditions under which talc–ink aggregation occurs are identified and reported. 2006 Elsevier Ltd. All rights reserved. Keywords: Industrial minerals; Agglomeration; Surface modification; Recycling
1. Introduction In the paper recycling industry, effective removal of stickies and ink particles are critical steps to improve the quality of the recycled fibers. Stickies are almost all removed from the recycled fibers by means of screens, reverse cleaners and dissolved air flotation clarifiers. Talc is an effective control agent commonly used for the removal of stickies to supplement the mechanical removal of stickies and ultimately to keep the paper machine free of tacky deposits. On the other hand, ink particles are mostly removed from the re-pulped paper slurry by froth flotation (Fuerstenau, 1980; Borchard, 1993). The removal efficiency of very fine ink particles by flotation is typically low owing to the low collision-attachment efficiency of small ink particles being floated with flotation bubbles. To improve ink removal efficiency by flotation, either agglomerating ink particles with
*
Corresponding author. E-mail address: [email protected] (Z. Xu).
0892-6875/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2006.11.005
oil (Snyder and Berg, 1994) or using coarse talc particles to carry (piggy-back) ink particles (Maiolo and Pelton, 1998) has been recently considered to be promising avenues. Sparks and Puddington (1976) and Markham et al. (1994) also showed an effective removal of ink with the addition of coarse carrier particles such as talc, clay and calcium carbonate. Developing these novel ideas into a more effective and practical technology requires fundamental understanding of their interactions. The direct surface force measurement using atomic force microscope (AFM) in a model system was used to investigate interactions involving toner/oil (Aston and Berg, 1988), toner/anatase (Azevedo et al., 1999), toner/polyethylene (Drelich et al., 2000), and toner/talc (Chi et al., 2001). However, the requirement of regular geometry for the probing particle and surface limited the use of AFM direct force measurement only to a few well-defined systems. This study is concerned with the use of talc particles as carriers of fine ink particles in flotation. The objective of this investigation is to identify the chemical conditions required to promote aggregation (analogous to hetero-coagulation)
J. Liu et al. / Minerals Engineering 20 (2007) 566–573
of talc and ink particles in carrier (piggy-back) flotation of ink particles. The information derived from this study is vital in making the talc-assisted de-inking flotation concept, a successful commercial technology. In order to accomplish this, the critical chemical conditions leading to aggregation or lack thereof between talc and ink particles were identified from zeta potential distribution measurements made with single component of ink and talc particles as well as with their binary mixtures. The measurements were conducted in chemical environments pertinent to de-inking flotation. The aggregation of talc and ink particles was then visually confirmed by deposition tests conducted with an impinging jet apparatus. 2. Principles of the technique Theoretically, the classical DLVO (Deryaguin–Landau– Verwey–Overbeek) theory considering summation of van der Waals and electrostatic double layer interactions was often used to predict colloidal particle interactions. Calculation of electrostatic double layer and van der Waals interactions is relatively straightforward for well-defined systems with known system parameters such as surface potential of interacting colloidal particles, electrolyte composition and concentration of solution, and Hamaker constant or the parameters needed to calculate the resultant Hamaker constant of the systems. However, in many cases, the classical DLVO could not accurately predict the colloidal interactions and an extended DLVO theory has to be used, which may include repulsive hydration force for hydrophilic surfaces, attractive hydrophobic force for hydrophobic surfaces, repulsive steric force and attractive bridge force for polymer bearing surfaces, and specific acid–base (AB) interactions. The theory for describing these additional non-DLVO forces is less well developed. Alternatively, these forces were often inferred from the deviation of the experimentally measured colloidal forces from those predicted by the classical DLVO theory. In this regard, many experimental methods have been developed to study the colloidal interactions, most of them for welldefined systems. Force measurement using AFM and SFA is the most direct, but rather limited to the systems that can be studied by the requirement of regular geometry and/or optical transparency. Visual observation such as settling test can directly observe coagulation behavior, but is limited to the particle density. In this paper, we take advantage of different surface properties for different particles. Tracing the change of surface properties could provide us with an important avenue to study colloidal particle interactions. Zeta potential is one of the most important surface properties in a colloidal system. Previous researchers (Maiolo and Pelton, 1998; Borchard, 1993; Fuerstenau, 1980; Snyder and Berg, 1994), using conventional electrophoretic zeta potential meters, have investigated electrokinetic properties of talc and ink particles extensively. In these early measurements, only the average electrophoretic mobility or zeta potential for a single component system
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was reported, even though binary systems were studied. In the case of a binary mixture system, the measured average zeta potentials of the two components often either over or under estimate the electrical behavior of the system, thereby leading to misguided information regarding the interpretation of interactions between the two components. With the capability of measuring electrophoretic mobility or zeta potential distributions, it is possible to identify the deposition conditions of one component on the other in a binary suspension system. Based on this idea, zeta potential distribution measurement was recently developed into a practical technique for studying particle interactions (Liu et al., 2002a,b, 2004a,b, 2005; Xu et al., 2003). This simple surface properties-based technique does not suffer from the limitations of particle density and geometry. In this study, we demonstrate the application of this technique to investigation of talc–ink particle interactions. The details on interpreting zeta potential distribution data in the context of particle interactions were reported elsewhere (Liu et al., 2002b). Briefly, let’s take a mixture of two components, each with its distinct zeta potential distribution peak as an example. If only a single modal zeta potential distribution was obtained with the mixture, we know the particles are hetero-coagulated to form composite aggregates. The location of the distribution peak depends on the nature and relative abundance of the particles. On the other hand, if we obtain a bimodal zeta potential distribution of the mixture, with the two peak positions located close to the values of individual components, the two components are not coagulated with each other. In this case, the piggy-back of one component on the other is not expected. Although the theory of zeta potential is well developed for individual colloidal particles (Hunter, 1981), the quantitative description of zeta potential distribution in a coagulated colloidal system is rather complex if not impossible to comprehend from the first principle and is beyond the scope of this communication. Nevertheless, the measured zeta potential distribution pattern of a complex colloidal system does provide an important indication with regard to particle interactions. This approach is applied in this paper to studying interactions of talc particles with oily ink particles, pertinent to the talc-assisted ink removal by froth flotation. 3. Experimental procedures 3.1. Materials The talc samples used in this study were commercially available products from Luzenac America, USA. Talc A is a naturally occurring, finely ground talc product whereas Talc B is the talc treated with a proprietary cationic chemicals. Both of these two types of talc are currently used or tested as an effective control agent for the removal of stickies in pulp industry. The red color oily ink used in the study was supplied by Rieger Printing Inks (Edmonton, Alberta, Canada). Reagent grade HCl and NaOH (Fisher
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Scientific) were used as pH modifiers. Ultrahigh purity KCl (>99.999%, Aldrich) was used as supporting electrolytes, while reagent grade CaCl2 (99.9965% Fisher) was used as the source of calcium ions. Sodium oleic acid (NaOl, 98%) was purchased from Aldrich Chemicals. Reagent grade toluene (Fisher) was used as the dilution solvent. The de-ionized water with a resistivity of 18.2 MX cm, prepared with an Elix 5 followed by a Millipore-UV plus ultra water purification system was used throughout this study. 3.2. Talc slurry preparation The dry talc particles were first wetted with 1 mM potassium chloride solution to form a paste. The obtained paste was slurried in a potassium chloride-containing aqueous solution to approximately 10 wt% solids under high shear mixing for several minutes to ensure proper talc particle dispersion. This concentrated suspension was further diluted as needed for the zeta potential distribution measurement and the impinging jet experiments. 3.3. Ink suspension preparation The oily ink was dispersed in a potassium chloride solution to contain about 2 wt% ink using an ultrasonic bath. The prepared suspension was diluted as required for the zeta potential distribution measurements. 3.4. Preparation of oily ink surface For preparation of the talc particle collecting surface used in the impinging jet experiments, the ink sample was dissolved in toluene at ca. 10 mg ink per ml toluene dilution ratio. About 0.5 ml ink-in-toluene solution was placed drop-wise onto a 5 · 5 cm transparent, clean glass slide spinning at 5000 rpm for 1 min. To remove toluene from the ink, the ink-coated surface was dried for about one hour in a particle-free horizontal laminar hood under ambient conditions. The ink-coated glass slide had a smooth, reddish appearance, yet sufficiently transparent to be seen through under an optical microscope. The uniform coating of ink was confirmed by a water contact angle value of 73 measured with a drop shape analyzer (Kruss, USA). The surface was used as the collecting surface for talc particles in the impinging jet experiment. 3.5. Zeta potential distribution measurements The zeta potential distribution of the talc slurries and ink suspension was measured with a Zetaphoremeter (SEPHY/CAD). The operating system in this instrument allows tracking 40–100 particles simultaneously and provides a zeta potential distribution for these particles (Liu et al., 2002a,b, 2004a,b, 2005; Xu et al., 2003). If these particles are homogeneous, the distribution peak is narrow. Otherwise, the peak would be broad. A 40 ml suspension
containing 0.1% solids was conditioned for 5 min and then used to fill the electrophoresis cell. The suspensions of ink and talc were measured first using a single component suspension, then a mixture of ink and talc at an ink to talc mass ratio of 1:1. The average of four independent measurements was reported. 3.6. Impinging jet measurements The impinging jet measurement was carried out to visually observe the deposition of talc particles on the ink surface with an impinging jet apparatus (Yang, 2000; Sanders et al., 1995). The apparatus consists of a flow system with a precision flow rate control device, an impinging jet cell with a capillary tube, a collector (glass slide coated with probing sample) and a view system (optical microscope with a CCD camera and video recorder). The suspension flows from the capillary tube in the impinging jet cell at a constant flow rate, and impinges on the collecting surface. In this manner, the particles in suspension may deposit on the collecting surface, should there exist an attractive and adhesion force between the collecting surface and impinging particles. The particle deposition can be observed through the recorded images. About 1.5 L talc suspension at 0.01 wt% solids was used for each test with a pressure driven flow at a Reynolds number of 180. Impinging process over the first 3 min was recorded for the analysis. The deposition over the stagnation region (near the center of impinging jet) was used for deposition analysis. 4. Results and discussion 4.1. Talc A 4.1.1. Zeta potential distributions The interaction between the as-received talc and ink particles in KCl solutions was studied by zeta potential distribution measurement. In this study, the pH of suspension was fixed at 4 unless otherwise stated. The results obtained are presented in Fig. 1. As noted from Fig. 1a (top), both the talc A and ink particles are negatively charged with the zeta potential distributions peaking at 35 mV and 10 mV, respectively. Also included in Fig. 1a (bottom) is the zeta potential distribution of their binary mixture, which exhibits two distinguishable peaks at 30 mV and 10 mV, respectively. The observed characteristics of the zeta potential distributions for the binary mixture is indicative of non-coagulating nature of talc and ink, as schematically illustrated in the insert of the figure. It is evident that under the chemical environment investigated, there is no attraction leading to adhesion between talc and ink particles. This observation was expected as both the talc and the ink particles carry sufficiently high negative surface charges to make the talc and ink particles repel each other. Shown in Fig. 1b are the zeta potential distributions of talc and ink particles, individually and their binary mixture
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20
Individual
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Fig. 1. Measurements of zeta potential distribution for talc A and oily ink particles in 1 mM KCl solution (a) and 1 mM KCl and 0.1 mM sodium oleate solution (b) at pH 4.
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Fig. 2. Measurements of zeta potential distribution of talc A and ink particles in 1 mM KCl solution containing 0.5 mM calcium chloride (a) and 1 mM KCl solution containing 0.5 mM calcium chloride and 0.1 mM sodium oleate (b) at pH 4.
in the presence of 0.1 mM sodium oleate. Sodium oleate is a commonly used fatty acid family surfactant in paper de-inking practice. Note that the addition of sodium oleate shifted the zeta potential distributions of talc and ink particles (Fig. 1b top) slightly to the more negative direction. As a result, when the talc and ink particles were mixed together in the presence of sodium oleate, a bimodal zeta potential distribution was again observed as is shown in Fig. 1b (bottom). The results indicate that the addition of sodium oleate alone could not induce the hetero-coagulation of talc particles with ink particles. The lack of attraction was expected between talc and ink particles due to the
electrostatic repulsion between the negatively charged talc and ink particles. Calcium chloride is a common additive used in paper deinking flotation. Therefore, it would be important to study the effect of calcium addition on talc–ink particle interactions. As shown in Fig. 2a (top), the addition of calcium ions to the talc–ink suspension results in a noticeable shift in zeta potential distributions of both talc A and ink particles, rendering the particles less negatively charged. In fact, adding 1 mM calcium ions caused zeta potential reversal of ink particles to +5 mV. In this case, only a single zeta potential distribution peak was observed for the mixture
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as shown in Fig. 2a (bottom). Clearly, calcium addition induced hetero-coagulation of talc A with ink particles, as anticipated for a binary mixture system of particles carrying opposite electrical charges. A more common practice in de-inking flotation is the coaddition of sodium oleate and calcium chloride as process aids. This co-addition is anticipated to increase the hydrophobicity of ink particles by precipitation of calcium carboxylates on toner particles (Pletka et al., 2000), resulting in an improved ink removal efficiency by flotation. It is therefore important to study the effect of this practice on the talc–ink interactions. The results of the zeta potential distributions obtained with such a system are included in Fig. 2b. As expected, the co-addition of calcium and sodium oleate induced the hetero-coagulation of talc and ink particles, as shown by a single modal zeta potential distribution in Fig. 2b for the mixture of the two. Surprisingly, talc–ink hetero-coagulation occurred despite a marginal negative surface charge carried by ink particles. It is important to point out that in both cases shown in Fig. 2 the talc coated the ink particles with a close-to-full coverage as schematically illustrated in the insert of Fig. 2a. It appears that ink particles are more hydrophobic than talc as such that coating of talc on ink particles would minimize the exposure of hydrophobic ink surfaces to aqueous environment and hence system energy. The zeta potential distribution data presented above demonstrates the feasibility of enhancing ink flotation with talc as a carrier. One way of adhering talc to ink particles is through the addition of chemical additives such as calcium or calcium/sodium oleate mixtures. 4.1.2. Impinging jet measurements The zeta potential distribution results presented in the foregoing section suggested that a negligible interaction exists between the talc and ink particles in KCl aqueous solutions due to strong electrostatic repulsive forces. In order to confirm this observation, impinging jet measurements were conducted in a similar environment. The result
of such a measurement is presented in Fig. 3a. Note that the deposition of talc A particles on the ink-coated collecting surface is marginal, confirming the observations made in the zeta potential distribution measurements. Similarly, the zeta potential distribution measurements presented in the foregoing section suggested that it would be feasible to enhance the deposition of negatively charged talc on the ink particles by the co-addition of sodium oleate and calcium cations. In order to confirm the inferred hetero-coagulation of talc and ink particles from the results of zeta potential distribution measurements, impinging jet experiments were conducted using talc suspensions containing 0.5 mM calcium and 0.1 mM sodium oleate. The recorded image in Fig. 3b shows a significant increase in talc deposition as compared with the case without calcium and sodium oleate addition. In the presence of calcium and sodium oleate, the talc deposited on the ink-coated collecting surface as large aggregates. It is evident that under this condition, calcium induced the adsorption of oleic acid (probably as calcium oleate precipitate) on talc, rendering the talc surface more hydrophobic. The addition of calcium and sodium oleate may also increase surface hydrophobicity of ink particles by calcium aleate precipitation on ink surfaces (Pletka et al., 2000), leading to a strong hydrophobic interaction between the two surfaces. Therefore, the hydrophobic force between talc and ink surface appears to be responsible for the improved talc–ink adhesion. It should be noted that the analysis of impinging jet test results is rather qualitative in this study. Detailed quantification can be done with imaging analysis procedures (Yang, 2000; Sanders et al., 1995). The discussion of interaction in this paper is also limited to a qualitative nature. The calculation can be performed using the DLVO theory to quantify the contributions of hydrophobic interactions in de-inking system using carrier flotation. With the quantification of deposition, a phenomenological relationship can be derived for practical purposes, which is beyond the scope of this work.
Fig. 3. The visual observations of talc A particles on the ink-coated collector surface with the impinging jet cell in 1 mM KCl solution (a) and 1 mM KCl solution containing 0.5 mM calcium chloride and 0.1 mM sodium oleate (b) at pH 4.
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between positively charged talc and negatively charged ink particles. Similar observations are made at pH 6 and 9 as shown in Fig. 4b and c, respectively. One common trend is that increasing pH increases gap between the two distribution peaks measured with individual component suspensions. As a result, a narrower distribution peak with increasing suspension pH is observed in the mixture. Clearly, increasing electrostatic attraction with increasing suspension pH contributed to a stronger hetero-coagulation of ink with talc particles. The effect of sodium oleate addition on ink-particle interaction studied by zeta potential distribution is shown in Fig. 5. Although the zeta potential of talc B is significantly reduced with sodium oleate addition for all the pHs investigated, the feature of the measured zeta potential distribution is essentially the same as those without sodium oleate addition shown in Fig. 4 and can be interpreted similarly.
4.2. Talc B 4.2.1. Zeta potential distributions The zeta potential distributions of ink and talc B suspensions, individually or in their mixture at pH 4, 6 and 9 are shown in Fig. 4. At pH 4 (Fig. 4a), the zeta potential distribution of ink particles exhibited a peak at a small negative value of 8 mV while talc B, at a large positive value of 65 mV. With the mixture of the two, a single broad distribution from 30 to 65 mV was observed. This distribution characteristic suggests a hetero-coagulation of ink particles with talc B. From the disappearance of distribution peak of ink particles and with the single distribution peak near the value measured with single talc particles, it can be inferred that talc particles are partially coating the ink particles as schematically shown in the insert of Fig. 4a (bottom). The observed interaction is within the expectation by the electrostatic attraction
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Fig. 4. The zeta potential distribution measurement of talc B and ink particles in 1 mM KCl solution at pH 4 (a); pH 6 (b) and pH 9 (c).
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Fig. 5. The zeta potential distribution measurement of talc B and ink particles in 1 mM KCl solution containing 0.1 mM sodium oleate at pH 4 (a), pH 6 (b) and pH 9 (c).
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Fig. 6. The visual observation of talc B on the ink-coated collector surface with the impinging jet cell in 1 mM KCl solution (a), and 1 mM KCl solution containing 0.1 mM sodium oleate (b) at pH 9.
As in the case of talc A, ink particles are covered by talc B, even though talc B was modified by cationic polyelectrolytes to render them positively charged. It should be noted that with talc B, the composite particles are positively charged. Rendering positive charge of ink-talc composite particles may facilitate bubble-particle attachment as bubbles are in general negatively charged. Such a feature is desirable for ink removal by flotation.
4.2.2. Impinging jet measurements Note that talc B is positively charged over a broad pH range. On the other hand, ink particles are known to be negatively charged at pHs greater than 4. Therefore, a strong attraction between the two is anticipated to exist at pH between 4 and 9 as confirmed in the above zeta potential distribution measurement. Extensive talc deposition on ink-covered collecting surfaces (white spots) was observed in impinging jet test as shown in Fig. 6a. An increased number of talc particles were observed to deposit on the collecting surface with increasing suspension pH, reaching a maximum at pH 9. This observation is consistent with the anticipated increase in electrostatic attraction between ink and talc particles with increasing suspension pH. This observation confirms the interpretation of the
results from zeta potential distribution using single components and their binary mixtures described above. At a given pH, sodium oleate addition was found to enhance talc B deposition on the ink-coated collecting surface as shown in Fig. 6b. The observed improvement in deposition of talc particles on ink-coated collecting surfaces by sodium oleate addition could not be accounted for by simply considering electrostatic attraction, as the reduction of positive charge on talc surface by sodium oleate addition would result in a reduced electrostatic attraction between talc B particles and ink surfaces. Clearly hydrophobic attraction between talc B particles and ink surfaces plays an important role in promoting talc deposition on ink surfaces. The talc particles appeared as aggregates in this case, in particular at higher pH, with the highest deposition appearing at pH 6. The aggregation among talc particles could be attributed to the adsorption of oleate surfactant on positively charged talc, rendering overall talc surface more hydrophobic by masking its positively charged hydrophilic edges. 5. Conclusions This study showed that zeta potential distribution measurement is a simple, yet effective method to study particle
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interactions in a colloidal system of complex nature, such as the talc-assisted de-inking flotation. The zeta potential distribution measurement showed that chemical conditions such as calcium and sodium oleate addition or surface modification on talc could promote the adhesion of talc to ink particles. Impinging jet tests confirmed the deposition of talc particles on ink surfaces, which was inferred from the zeta potential distribution measurements conducted with individual components and their binary mixtures. Acknowledgment The financial support for this work from Natural Sciences and Engineering Research Council of Canada (NSERC) and Luzenac America, Centennial, Colorado, USA, is greatly appreciated. References Aston, D.E., Berg, J.C., 1988. Fluid interfacial separations for secondary fiber recovery as probed with atomic force microscopy. Journal of Pulp and Paper Science 24, 121–125. Azevedo, M.A.D., Drelich, J., Miller, J.D., 1999. The effect of pH on pulping and flotation of mixed office wastepaper. 25(9), 317–320. Borchard, J.K., 1993. Paper de-inking technology. Chemistry and Industry 19, 273–276. Chi, R., Xu, Z., Difeo, T., Finch, J.A., Yordan, J.L., 2001. Measurement of interaction forces between talc and toner particles. Journal of Pulp and Paper Science 27, 152–157. Drelich, J., Nalaskowski, J., Gosiewska, A., Beach, E., Miller, J.D., 2000. Long-range attractive forces and energy barriers in de-inking flotation: AFM studies of interactions between polyethylene and toner. Journal of Adhesion Science and Technology 14, 1843–1929. Fuerstenau, D.W., 1980. Fine particle flotation. In: Somasundaran, P. (Ed.), Fine Particle Processing. SME, New York, NY, USA, pp. 669–705.
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