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Enhancement of the Flotation Deinking Selectivity by Natural Polymeric Dispersants Elisa Zeno,*,† Patrick Huber,† Xavier Rousset,† Benjamin Fabry,† and Davide Beneventi‡ Centre Technique du Papier, CTP, B.P. 251, 38044 Grenoble CEDEX 9, France, and Laboratoire de Génie des Procédés Papetiers, LGP2-UMR5518, 461 rue de la Papeterie, DU, BP. 65, 38402 St. Martin d’He`res, France
The impact of natural polymeric dispersants, that is, carboxymethyl cellulose and guar gum, on the flotation selectivity was investigated on a flotation deinking pilot plant. Their effect on pulp flocculation, air bubble size, and gas hold-up was determined by implementing a fiber flocculation sensor, an automated bubble measurement device, and a gas hold-up sensor on the pilot cell. Dispersants reduced pulp flocculation and induced an increase in gas hold-up and a slight decrease in the air bubble size. Their addition promoted ink removal and, at the same time, depressed fiber entrainment and mineral fillers flotation, thus increasing exceptionally the flotation selectivity. 1. Introduction Paper recycling has increased substantially over the past several decades, reaching a current recovery rate of over 60% and 65% in the U.S. and EU, respectively.1,2 This contributes to both the sustainability and the competitiveness of the pulp and paper industry, in the present socio-economic context that promotes the sustainable use of resources, waste reduction, and reuse. Actually, recovered fibers have become the major raw material for paper and board production, even over virgin fibers, at least in the EU.2 More than 35% of the recovered fibers are deinked, commonly by froth flotation, to remove ink besides other contaminants. This allows to fulfill the furnish requirements for the production of some paper grades (printing-writing papers, newsprint, tissue...). Within these categories, however, the utilization of deinked pulp (DIP) can vary from one product family to another: between 92% in case materials and only 10% in graphic papers other than newsprint.2 Actually, the massive use of deinked pulp is still limited for high-quality grades, mainly because of the presence of residual ink, not selectively removed during deinking by flotation. This lack of selectivity entails also a loss of resources, fibers, and mineral fillers, with an immediate economic impact. Additionally, this situation is aggravated by the deterioration of collected waste paper from urban communities (papers more contaminated in ink and other components), which makes it increasingly challenging to produce high-quality grades from recycled fibers. Thus, there is an imperative need to improve the ink removal selectivity, to further enhance the efficiency of recycling technology and the pulp quality. Improvements of the latter are likely to translate into increased rates of fiber reuse (and recovery) with a direct effect on the energy and resources savings. Flotation selectivity generally refers to the process ability to remove a target particle versus the other particles present in the initial mixture. For deinking, it has been formally defined as the ratio of ink removal, expressed by the International Standards Organization (ISO) brightness gain or the reduction in relative effective residual ink concentration (ERIC) and the * To whom correspondence should be addressed. Tel.: 00 33 (0)4 76 15 40 58. Fax: 00 33 (0)4 76 15 40 16. E-mail: elisa.zeno@webctp. com. † CTP. ‡ LGP2.
relative fiber (oven-dry basis) rejection loss.3 It can be also more generally defined as the ratio of ink removal to the loss of solids present in the pulp suspension. Thus, for improving flotation selectivity, it is necessary to enhance ink removal while reducing other solid losses, mainly composed by mineral fillers and cellulosic fibers and fines. This is hardly achieved because usually a reduction in solid losses corresponds to a decrease in the ink removal (and conversely).3,4 It is commonly accepted that ink is removed by its transport from the aerated pulp to the froth by mean of “true flotation”, that is, of the adhesion to the rising air bubbles. The transport of the other particles can be due either to flotation or to entrainment, depending on their surface energy. Hydrophilic particles, such as cellulosic materials, do not adhere to the surface of air bubbles, but they can be entrained by remaining confined in eddies generated behind rising bubbles. However, this depends on the particle size and density.5 It has been shown4 that entrainment of cellulosic material in the bubble wake occurs for sizes below 75-100 µm (i.e., for fines). The same entrainment mechanism can take place also for mineral fillers when their size does not exceed 25 µm. Fibers, because of their larger size, do not remain confined in the eddies, and their transport to the froth is mainly due to the entrapment of air bubbles in fiber flocs, as previously hypothesized by several authors.6-10 It can be supposed that a high level of pulp flocculation would favor this mechanism with a direct negative impact on the fiber losses. On the contrary, the presence of more individualized fibers would decrease the fiber transport to the froth by air bubble trapping. This hypothesis has been proposed11 for explaining the decrease in the fiber entrainment observed at increasing concentrations of a surface active agent, likely behaving as a dispersant. However, any experimental evidence of fiber dispersion and of its relationship with fiber entrainment and losses was provided. To our knowledge, so far, the effect of the fiber flocculation state, varied with a specific fiber dispersant, on flotation yield and mechanisms has not been investigated. Thus, this work aims at elucidating the role of a nonsurface active dispersant on the flotation selectivity and at providing a basic mechanistic explanation. For this purpose, two polysaccharide-based dispersants, currently used as wet-end additives to limit the flocculation of the fiber suspension during the sheet formation and to improve paper stiffness (carboxymethyl cellulose and guar gum),12-15 were used to precondition recovered papers pulp
10.1021/ie100979v 2010 American Chemical Society Published on Web 08/19/2010
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Table 1. Main Characteristics of the Recovered Papers’ Pulp Slurry at the Inlet of the Flotation Cell pulp concentration (%)
pH
fiber fraction (%)
ash content (%)
brightness (%)
ERIC (ppm)
ERICHW (ppm)
1.23 ( 0.03
8.8 ( 0.2
55 ( 0.5
24.5 ( 0.5
45.1 ( 0.7
1100 ( 60
98 ( 7
slurries before flotation. The study was performed on a deinking pilot plant, and conventional flotation efficiency assessments (pulp optical quality, total and specific particle yield) were combined with online measurements of the pulp flocculation, gas hold-up, and average bubble size. Additionally, a specific laboratory flotation cell, coupled with a mathematical model expressly developed,16 was used to estimate the fiber entrainment coefficient φ corresponding to each condition run on the pilot. 2. Experimental Section 2.1. Materials. Raw material for the pilot trial was a mixture of 50% old magazines (∼25% super calendered paper for rotogravure and ∼25% lightweight coated paper for offset printing) and 50% old newsprint papers. A conventional repulping chemistry was applied: 0.45% fatty acid soap (SERFAX MT 90, mainly sodium oleate), 0.7% NaOH, 0.7% H2O2, and 1.2% silicate (dosage given with respect to dry paper). Pulp was diluted with tap water and added with CaCl2 to adjust the calcium ion concentration to 150 mg/L. Fiber dispersants selected for the study were a guar gum (GG), extracted from annual plant, uncharged and with an average MW of ∼2 000 000 (estimation based on average values of native guar gum17), and carboxymethylcellulose (CMC), negatively charged (DS ) 0.7) and with an average MW of 250 000. Both were used as received from Sigma-Aldrich. They were dissolved under gentle stirring in tap water, at room temperature for CMC and at 90 °C for GG during 15 min, both at 0.7%. The main characteristics of the pulp slurry at the inlet of the flotation cell are given in Table 1. 2.2. Deinking Pilot Plant and Ink Removal. The flotation trial was performed on the CTP recycling pilot plant (Figure 1), with a one open loop configuration (using fresh water, with
Figure 1. Simplified scheme of CTP pilot plant.
Ca2+ concentration adjusted at 150 mg/L for the pulp dilution). The raw material was repulped in a helico pulper (Kadant Lamort, concentration of 13%, temperature of 45 °C, 15 min). The pulp was then coarse-screened, fine-screened (CH3, Kadant Lamort) using 0.2 mm slots, and floated on a 100 L capacity cell, equipped with two tangential Venturi type injectors (Kadant Lamort Verticell type, 1.2% concentration, T ) 45 °C, 150% air ratio, i.e., the ratio between gas flow and pulp flow, feed flow )1.6 m3/h). Dispersant solutions were injected with a metering pump (labeled as dispersants addition in Figure 1) to the pulp flow before dilution to the flotation cell inlet, giving a contact time of 30 s before entering the flotation cell. To quantify the ink removal efficiency, pulp samples were collected after 45 min flotation (under steady-state conditions), and pulp pads of ∼400 g/m2 were prepared by filtrating pulp/froth slurries on a Bu¨chner funnel (Whatman grade 2 paper filter) after preconditioning with aluminum sulfate to adjust pH to 7 and cationic polyacrylamide.18 Pulp pads were also prepared after abundant washing (hyperwashing) on a 200 mesh wire screen to remove free ink particles, mineral fillers, and cellulose fine elements and to evaluate the amount of ink remaining attached to fibers (ERICHW). The residual ink content (ERIC, ISO 22754) in pulp pads was determined by measuring pad reflectance at 950 nm19 (Technidyne, ERIC 950), and, because the amount of ink remaining attached to fibers was not affected by the presence of dispersants (ERICHW ) 98 ( 7 ppm), the CMC and GG effect on ink removal was estimated by using the simplified equation IR )
ERICIN - ERICOUT ERICIN
(1)
where ERICIN and ERICOUT are the residual ink concentration before and after flotation, respectively.
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Figure 2. Comparison of turbulence estimation in the pilot flotation cell and in the flocculation sensor channel (all calculations assume that pulp is air-free).
To evaluate the effect of CMC and GG on ink floatability, the surface tension, and the contact angle of water, CMC and GG solutions (0.24 g/L) on a fully printed paper sheet (offset printed magazine) were measured using an image analysis goniometer (Data Physics, OCA20). 2.3. Flocculation Measurement. A fiber flocculation sensor was installed at the inlet of the flotation cell, just before the injector, to assess the flocculation state of the pulp, at the fiber level, in conditions close to those of the flotation cell. The stock was derived to an overflow tank, from which the static head controlled the pulp flow rate (1.18 m3/h). Next, the pulp suspension was observed in transmitted light through a transparent flat channel (thickness ) 3.5 mm). Floc size distribution is obtained by the image analysis technique:20 the histogram of total floc surface in each size class of equivalent disk diameter is constructed, with flocs being categorized in 20 fixed size bins linearly spaced between 0.7 and 9.1 mm. The flocculation index (FI) is calculated as the raw moment of second order of the floc size distribution. It has dimension of mm2. It can be viewed as the surface of an average floc. The evolution of FI follows that of an rms value, in the absence of light diffusing mineral filler, so that the defined flocculation index is correlated with the visual impression. This technique allows one to assess the extent of fiber flocculation almost independently of mineral filler content, which is essential when working with recycled pulp slurry. Each flocculation measurement is a compilation of about 1000 images (40 × 40 mm2 each). The precision on the fiber flocculation index is about (2%. However, the sensor had to be installed at the flotation cell inlet, and this could result in some difference between the absolute value of the fiber flocculation index measured and that of the fiber suspension in the cell, mainly because of the potentially different levels of turbulence. So, it is important to compare the turbulence level in the flotation cell and in the flocculation sensor channel, as fiber flocculation directly depends on it. Typically, a minimum energy is required to start fluidizing the pulp,21 and then the fiber flocculation almost linearly decreases with shear rate.22 Above a certain energy input to the suspension, the fiber flocculation remains stable, depending on pulp concentration.22,23 Therefore, the turbulence level was estimated within the flotation cell, and then flow conditions in the flocculation sensor channel were adjusted to approximately match similar energy input to the pulp (Figure 2). Calculation of Reynolds number in the flotation cell gives a highly turbulent value at the outlet of the injectors (Re ) 57 743), and then a laminar regime value if considering an upward motion of the pulp within the cell (Re ) 690). The
effective Re within the cell must be between these extreme values, but it is difficult to estimate it precisely (moreover, it is likely that turbulence decreases from the bottom high shear to the top zone). The particular cell geometry actually causes a strong rotation pattern to the pulp, at least in the bottom zone. By comparison, the flow regime in the flocculation sensor channel has an intermediate Reynolds number (Re ) 7823), which is normally representative of the onset of turbulent flow, but may still be considered as laminar flow here, because fiber suspension at high concentration (12 g/L) considerably dampens the flow field.24,25 The estimation of energy dissipation in both cases was also attempted. For the flotation cell, it is assumed that the kinetic energy of the jet at the outlet of the injectors is fully dissipated in the flotation cell volume. This gives ε ) 0.31 W/kg applied on pulp within the whole cell. This value is likely underestimated as we have neglected the mixing effect of injected air bubbles. On the other hand, the energy dissipation in the flocculation sensor channel is estimated at 0.44 W/kg, if considering that the flow is still laminar (ε ) νG2, where ν (m2/s) is the kinematic viscosity of the pulp, and G (1/s) is the shear rate). Thus, in terms of overall energy dissipation, the flow in the flocculation sensor channel roughly represents the flow in the flotation cell. 2.4. Air-Content Measurement. The air content of pulp (expressed as gas hold-up) was measured directly in the flotation cell with a specific sensor (multiple channel system ACS 8-P, from PMC, Danbury CT, USA).26 It consists of a rigid pole, with two special pressure gauges, immersed in the pulp. The pressure difference is used to calculate the air content of the pulp in real time, as the apparent pulp density varies with air content. Knowing the distance between the two gauges (50 cm), and the angle of the probe, the system compares the actual pressure difference with the theoretical one (in the absence of air), and then gives the air content of the pulp. The air probes are calibrated with degassed pulp directly in the cells (after the circulation pump of the flotation cell has been shut down for more than 5 min). The air probes are installed vertically in the flotation cell. In the following, all air content measurements have been performed at approximately 12 cm below the froth level (as depth of top pressure probe). The air content signal was recorded at a 1 Hz frequency, then smoothed with a moving average over 1 min, and then resampled at 1 min frequency. The reported air content is the average over the last 5 min of each trial. 2.5. Bubble Size Measurement. The air bubble size in the collecting zone was evaluated using an intrusive bubble sampling technique based on bubble directing in a viewing chamber filled with a non coalescing liquid.27,28 As shown in Figure 3, rising air bubbles in the collecting zone were separated from the fiber slurry by a 3 cm diameter and 30 cm length vertical pipe filled with the pulp slurry filtrate and directed to a 5 × 5 cm viewing window with 6 mm gap between glass plates. The cross-sectional area of the sampling pipe plunged in the pulp slurry was partially sealed to limit bubble concentration in the viewing chamber, prevent bubble overlap, and allow recording of individual bubble pictures in light transmission mode using a CCD camera. A commercial image analysis package (Sherlock 7) was used for automated image processing. Under the tested conditions, the air bubble diameter never exceeded the gap size in the viewing window, most of the air bubbles were smaller than 2.5 mm, and the projected image of air bubbles slightly deviated from the disk shape. Thereafter, the bubble perimeter determined by image analysis was used to calculate the corresponding equivalent disk diameter, which
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Figure 5. Pulp flocculation index as a function of the CMC or GG dosage.
Figure 3. Schematic picture of the bubble size measurement installed in the pilot flotation cell.
was considered as representing the diameter of a spherical bubble. Approximately 7000 bubbles were counted for each tested condition. 2.6. Entrainment Coefficient Determination. The fiber entrainment coefficient was estimated by flotating the pulp sampled at the inlet of the pilot flotation cell on a specific laboratory continuous flotation column and by fitting the experimental data with a mathematical model, both described elsewhere.16 In the column (Figure 4), the pulp slurry has a counter current contact with rising gas bubbles, injected by tangential Venturi aerators. The froth is removed by using an adjustable reverse funnel connected to a vacuum pump, with the possibility to vary the froth retention time. For each sample collected on the pilot, the flotation cell was fed with a constant pulp flow of 2 L/min and an air stream of 4 L/min. Flotation was run at a froth removal thickness of 2 cm, and the accept was collected each 90 s up to the steady state (approximately 15 min). Next, the froth removal thickness was increased to 3 cm and the accept collected only once at the steady state, after 15 min. This was repeated for froth thicknesses of 4 and 5 cm. For each removal thickness, a froth sample was collected during 5 min after 10 min flotation to collect a sufficient froth volume.
For each sample (accept and reject), the total solid, ash, fiber, and fine consistencies were determined. Fiber and ash fractions in pulp/froth samples were determined by weighing long fibers remaining on a 200 mesh wire screen after abundant washing, and residual inorganic material after dry pulp/froth ignition and overnight storage in an oven at 425 °C, respectively. Actually, the fiber concentration and the water upward flow in the froth are directly correlated with the fiber hydraulic transfer. The fiber rate of removal due to entrainment (re) can be described by the equation16 re )
φ · Qf0 c V
(2)
where φ ) c0f/c is the entrainment coefficient, cf0 is the fiber concentration at the pulp/froth interface, c is the fiber concentration in the pulp slurry, Qf0 is the water upward flow in the froth in the absence of drainage, and V is the pulp volume in the flotation cell. Equation 2 was used to interpret experimental data and calculate the fiber entrainment coefficient. 3. Results and Discussion 3.1. Pulp Flocculation. The addition of the two selected dispersants, guar gum (GG) and carboxymethyl cellulose (CMC), induced a decrease in the pulp flocculation level, as measured by the flocculation sensor (Figure 5). The dispersing effect, well-known in the literature for CMC29,30 and GG31 with virgin fibers, was confirmed with the secondary fibrous suspension (containing also fillers, ink, dissolved, and colloidal substances), in the operating conditions of the pilot plant. Recovered fibers were dispersed to approximately the same
Figure 4. Scheme of the laboratory continuous flotation column used to evaluate the fiber entrainment coefficient.
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Figure 7. Correlation between gas hold-up and flocculation index. Figure 6. Gas hold-up and average bubble diameter as a function of the CMC or GG dosage.
extent as virgin fibers (according to preliminary tests, not reported here); the flocculation index decreased by -19% to -24%, for 1% and 2% GG dosage, respectively, and by -15% to -25% for 1% and 2% CMC dosage. Despite the recognized dispersing effect of such polymers, the mechanisms involved have not been clearly identified, even if several authors agree on the ability of these polymers to reduce interfiber friction through forming water gel layer, thus reducing the extent of fiber flocculation.13,32 This capacity could be ascribed both to the presence of the polymer in solution and to its adsorption onto the cellulose fibers. Actually, CMC can be irreversibly adsorbed on cellulose at certain conditions,33 when the electrostatic repulsion between CMC and cellulose is screened. GG sorption is also believed to be irreversible; very low amounts of GG can be desorbed, even if it can depend on temperature and high alkalinity.34,35 The sorption onto fiber surfaces is thought to take place through hydrogen bonding between hydroxyl groups on the cellulose main chain and polar groups onto the cellulose derivative or onto hemicellulose.36 Additionally, the similarity in conformation of backbone allows a very intimate contact between the two polymers. Whatever the mechanism involved, their efficiency as dispersing agent of cellulose fibers was confirmed, once more. In addition, the fiber deflocculation effect was similar for both CMC and GG, and considering that guar gum is not charged, it seems to confirm that the dispersion mechanism is not charge related, so that variations of conductivity at flotation should not affect the dispersing effect (this was confirmed with virgin pulp in experiments not reported). 3.2. Gas Hold-Up and Average Bubble Size. The gas hold-up in the pulp slurry increased when adding the dispersants, coherently with the evolution of the average bubble diameter, db, which decreases from 0.6 to 0.5 and 0.4 mm after the addition of CMC and GG, respectively (Figure 6). However, when considering the terminal velocity, UT, of spherical rising bubbles in a Newtonian fluid,37 UT =
4gdb 3CD
(3)
where CD is the drag coefficient of air bubbles, the measured variation of bubble diameter slightly affects the bubble velocity calculated in water (CD ≈ 0.95), which decreases from 9 to 8 and 7 cm/s. This is not consistent with the large increase of the gas hold-up from 2.6% to 4.2% and 5% measured after the addition of CMC and GG, respectively
(Figure 6). The correlation between the bubble diameter and the gas hold-up, εg, yielded from the drift flux model38 UT(1 - εg)2 )
JP Jg + εg 1 - εg
(4)
where Jg and Jp are the gas and pulp superficial velocity, and eq 3 were used to estimate the relative variation of the bubble drag coefficient showing up to a 130% increase in CD when adding CMC and GG. Thereafter, the direct correlation between the pulp flocculation index and the gas hold-up shown in Figure 7 was associated with an increase in the bubble drag force due to the onset of different bubble/ fiber interaction mechanisms. This is consistent with previous literature results, that showed how an increase in the pulp concentration, which corresponds to an increase in the pulp flocculation,39 can lead to a gas hold-up decrease,26 especially above a certain fiber concentration (>0.5%).40 However, in those studies, no direct flocculation measurements were reported, and, in addition, it should be considered that an increase in the pulp concentration causes also the variation of other parameters, such as the concentration in the liquid phase of colloidal and dissolved substances. Some of the latter are surface active and can influence the air bubble size, with a direct influence on the gas hold-up, and more generally on the phenomena occurring. In the present study, the pulp flocculation level was varied only by adding a specific fiber dispersant, at a fixed pulp concentration, and any other change in the liquid phase composition took place. The relationship pointed out between fiber flocculation and gas hold-up is also consistent with some hypothesis reported in the literature to explain the influence of the fibers on the pulp aeration and comprehensively reviewed by Tang and Heindel.40 To summarize, it is proposed that fibers can reduce the bubble contact chances, acting as a sort of wall and leading to the suppression of the bubble coalescence.41 Moreover, the fiber networks could hinder bubble motion and trap the air bubbles, thus increasing the bubble residence time.42 Both of these mechanisms would lead to an increase in the gas hold-up. On the other hand, as the network can slow down smaller bubbles, this would favor the bubble coalescence43 and thus the gas holdup decrease, the latter being also caused by the gas channelling, favored by the fibrous network.46 Each mechanism described takes place in specific conditions of gas superficial velocity, bubble size, and fiber mass fraction. This means that the final impact of the fiber dispersion state on the pulp aeration can hardly been predicted, as the effects of the fiber network on the gas hold-up can be even opposite, as discussed above. In the present case, the gas hold-up increase with the addition of the dispersants seems to be dominated by the reduction of
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GG and CMC. Actually, the rate of ink removal by flotation is described by the typical first-order equation rf ) kc
(5)
where c is the ink concentration in the aerated pulp and k is the flotation rate constant, which, using the conventional linear correlation with the surface area flux44,45 and eq 3, can be directly correlated to the gas hold-up, the bubble diameter, and the drag coefficient: k∝ Figure 8. Floated pulp optical properties ERIC (ppm) and brightness (%).
Figure 9. Correlation between ink removal and gas hold-up.
gas channelling around dense fiber flocs and the increase in the bubble drag force due to bubble interaction with the homogeneous fiber network generated by flocs disruption, with the slight decrease of bubble diameter (
