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Monitoring of Dissolved Air Flotation by Focused Beam Reflectance Measurement Ville Saarimaa,* Anna Sundberg, and Bjarne Holmbom Process Chemistry Centre, A° bo Akademi UniVersity, Turku, Finland
Angeles Blanco, Elena Fuente, and Carlos Negro Complutense UniVersity of Madrid, Madrid, Spain
Aggregation and removal of detrimental substances from peroxide-bleached, fiber-free thermomechanical pulp water by dissolved air flotation (DAF) were studied for different chemicals. A focused beam reflectance measurement (FBRM) instrument was used to continuously assess the aggregation and removal of dissolved and colloidal substances (DCS) during batchwise DAF of process water. FBRM is particularly suitable for assessing the aggregation of DCS during the coagulation process prior to flotation. Differences between flocculants, as well as between different flocculant doses, can be observed. The FBRM results were in good agreement with turbidity measurements and with determination of wood resin and pectic acids by gas chromatography. General trends in the flotation process were also observed. However, monitoring the aggregates during flotation is more complicated due to the interference of air bubbles. To determine the overall flotation efficiency, a two-probe system with measurement points before and after actual flotation is proposed. It is essential to ensure good agitation at the measurement points, and if the destabilization of DCS produces sticky flocs, as in the case of cationic polyacrylamide, the position and the cleaning of the FBRM probe in the process pipe must be optimized to minimize the deposition of sticky material on the probe. 1. Introduction Alkaline peroxide bleaching causes significant changes in the composition and amount of components released from thermomechanical pulp into the water phase during processing. Lipophilic wood resin droplets lose their steric stabilization, which leads to increased depositability.1,2 Pectins are demethylated to pectic acids that contribute strongly to the anionic charge of the water, thus consuming cationic process chemicals.3 Apart from these components, alkaline conditions also promote dissolution of lignin and hemicellulose fragments. In general, alkaline peroxide bleaching increases the anionic charge of the released substances. In papermaking, phenomena such as excessive consumption of process chemicals, deposits on machinery, spots in the final product, fluctuation in process dynamics, and other process problems can, in many cases, be traced back to the dissolved and colloidal substances (DCS), some of which are known as detrimental substances.4 Closure of water circuits causes accumulation of these substances and, furthermore, increases the vulnerability of the process. Dissolved air flotation (DAF), membrane filtration, evaporation, centrifugation, and retention of detrimental material on fiber mats are methods practiced in many mills.5 DAF is a process where microscopic air bubbles collect and remove hydrophobic particles from a suspension. To increase the removal efficiency of contaminants, the DCS has to be destabilized and aggregated before flotation. A wide range of chemicals has been developed in order to remove the detrimental material from the system. DAF has recently been beneficially applied for removal of extractives in a eucalyptus kraft mill in order to control pitch deposition.6 In our previous study7 it was shown that aggregated, hydrophobic wood resin can also be efficiently removed from peroxide-bleached thermomechanical pulp (TMP) water by * To whom correspondence should be addressed. Address: Åbo Akademi University, Process Chemistry Centre, Porthansgatan 3, FI-20500 Turku/Åbo, Finland. E-mail:
[email protected].
flocculation and flotation. A set of commercial flocculants was used and compared in terms of water purification efficiency. Traditional methods to determine the DCS load in papermaking process waters and effluents are turbidity measurement, determination of lipophilic extractives and carbohydrates by chromatography, and determination of cationic demand by polyelectrolyte titration or ζ-potential measurement. Chemical oxygen demand (COD) and total organic carbon (TOC) are also often measured. These methods provide much information on the behavior of individual components in the process stream, but may require time-consuming sampling, sample pretreatment, and analysis and interpretation of results. Furthermore, several analytical methods are needed in order to obtain an overall picture of the process water. Focused beam reflectance measurement (FBRM) has traditionally been used to study crystallization processes. However, nowadays it is also being used to optimize and control flocculation in different industries, on the laboratory8-10 and mill scale.11 FBRM has been applied, for example, to study paper machine retention,12 polyelectrolyte-induced aggregation of microcrystalline cellulose,13 and filler flocculation;14,15 to determine the flocculation mechanism of poly(ethylene oxide);16 and as a tool for web break control17 and prediction of product properties.18 In 2000, Dunham et al.19 used FBRM to investigate the effect of DCS on aggregation of a TMP furnish induced by chemicals. Furthermore, in our previous work,7 a FBRM device was successfully used to real-time monitor the chord length distribution of the particles in the suspension, both to select the best chemicals and to determine the chemical doses needed for aggregation of DCS prior to the actual flotation experiments. However, the FBRM can also be used to monitor particle concentrations, dimensions, and size distributions in-line, even in high-consistency suspensions. Therefore, the aim of this study was to determine the applicability of FBRM as a rapid in-line tool to assess the destabilization, aggregation, and removal of DCS in a flotation process, which could facilitate the monitoring of industrial DAF processes.
10.1021/ie060250+ CCC: $33.50 © 2006 American Chemical Society Published on Web 09/09/2006
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Figure 1. Dissolved air flotation represented as particle counts/s monitored by FBRM. The curves were adjusted to same time intervals in order to facilitate the interpretation.
Figure 2. Flow of colloidal material at the FBRM probe before and after addition of dissolved air (“phase 1” adapted from Lumpe et al.17). Table 1. Specifications of the Aggregation Chemicals name
chemical character
av molar mass, g/mol
cationic charge, mequiv/g
PAC poly-DADMAC C-PAM1 C-PAM2 bentonite
polyaluminum chloride polyidallyldimethylammonium chloride cationic polyacrylamide cationic polyacrylamide alkali-activated montmorillonite clay
medium 7 million 9 million
medium basicity 2.6-2.9 ∼1 ∼2
2. Experimental Section Materials. Thermomechanical pulp was obtained from a mill in southern Finland using two-stage refining of Norway spruce (Picea abies). The pulp was freeze-dried before use to prevent degradation due to transport in the summer. The specifications of the different chemicals used for aggregation are given in Table 1. CaCl2 of analytical grade was provided by Panreac Quimica SA, and Kemira Oyj provided the other commercial chemicals. Preparation of Peroxide-Bleached TMP Water. Portions of TMP (100 g, oven dried) were mixed with bleaching chemicals in polyethylene bags at 10% consistency and the initial pH was adjusted to about 11 according to Sundberg et al.2 The peroxide dose was 3% on an oven-dried pulp basis. The bags were kept in a water bath at 60 °C for 1.5 h. After bleaching, the samples were acidified to pH 5 with SO2-water.
The water was then pressed from the pulp and centrifuged for 30 min at 1500 rpm. The supernatant, free of fibers, containing DCS and microfines, was diluted to 1:10 with distilled water to represent typical TMP process water from a bleaching plant. Flotation Experiments. A DAF unit model Flotatest FTH3 manufactured by Orchidis Laboratoire was used. It consists of three cylinder-conical flotation vessels of 1300 mL, agitators, a speed-controller, a saturation unit, and a valve for adjusting the air-saturated water stream. The inlet of the air-saturated water and the outlet of the sample are located at the bottom of the vessels. A flotation vessel was filled with 1000 mL of peroxidebleached TMP water at 60 °C. The aggregation chemical was added during rapid agitation (180 rpm) to increase the flocculant interaction with the DCS. After 0.5 min, the agitation was set to 60 rpm for 6.5 min to facilitate the growth of formed flocs.
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Figure 3. Monitoring of the influence of CaCl2 dose on the dissolved air flotation process by FBRM represented as particle counts/s.
Figure 4. Monitoring of the influence of poly-DADMAC dose on the dissolved air flotation process by FBRM represented as particle counts/s.
Figure 5. Residual wood resin, turbidity, and GalA after DAF with CaCl2. Blank 0 corresponds to the initial suspension and Blank corresponds to the initial suspension treated by flotation without chemicals.
In the case of the dual system cationic polyacrylamide (C-PAM)/ bentonite, the chemicals were added with a time gap of 1 min, rapid agitation being maintained for efficient mixing. Following that, the agitation was switched off and the air-saturated water was introduced into the vessel. Samples were left to stand for
10 min to let the aggregates rise to the surface, followed by sampling of the purified water. Determination of Chemical Doses. The results from a previous study were utilized.7 The chemicals were tested at four different doses close to the optimal flocculation. The doses were 5, 10, 15, and 20 mmol/L of CaCl2; 40, 60, 70, and 80 mg/L of PAC; 20, 28, 35, and 40 mg/L of poly-DADMAC; 30, 40, 45, 50 mg/L of C-PAM1; and 30 mg/L of C-PAM2, and 20 mg/L of bentonite was used with C-PAMs. Measurements and Analyses. In-line monitoring of detrimental substances was carried out by a S400 FBRM probe (Mettler-Toledo, Seattle, WA). The methodology is based on a highly focused laser beam scanning across particles in a suspension at a fixed speed. The time duration of the backscattered light from these particles is measured to calculate the chord length of the particles, and each measured chord length is considered a count. Each second thousands of chords are measured. As a result, a measure of the size distribution of the particles in the suspension is obtained. Statistical parameters from the chord size distribution, for example, the mean chord size and the number of total counts, are monitored. Particles
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Figure 6. Particle size distribution after aggregation with CaCl2. Note the low counts/s.
Figure 7. Residual wood resin, turbidity, and GalA after DAF with PAC.
with dimensions over 0.5 µm can be detected. Therefore, the dissolved substances and small colloidal particles will only be detected after aggregation when aggregates larger than 0.5 µm are formed. After filling the flotation vessel, the FBRM probe was submerged in the sample. Monitoring was carried out during the flocculation and flotation processes until sample withdrawal. Turbidity was assessed with a Hanna LP 2000-11 turbidimeter. Lipophilic extractives were determined by extraction with methyl tert-butyl ether (MTBE), followed by silylation and analysis by gas chromatography.20 Carbohydrates were determined using acid methanolysis to obtain monomeric methyl glycosides of neutral sugar units and methyl ester glycosides of uronic acids, followed by gas chromatography to determine monomer concentrations.21 3. Results and Discussion Flocculation and Flotation Monitoring by a FBRM Probe. At the beginning of the DAF experiments, there was a 1-min period of agitation before the chemical addition, to check the FBRM baseline. After that, the DAF process can be divided in three phases, as illustrated in Figure 1. The chemical addition occurred at 1 min and the addition of dissolved air at 8 min. The flow of colloidal material past the FBRM probe window during the different phases is depicted in Figure 2. During the first phase, the DCS is aggregated by the cationic chemicals.
By efficient agitation, the aggregated material is forced into horizontal movement in the suspension. A continuous stream of aggregates sweeps by the probe, enabling efficient detection of the particles that are carried with the flow close to the probe window. The flow also prevents the sticky material from forming deposits on the window.17 This first phase of detection provides valuable information on aggregation mechanisms and kinetics, since efficient aggregation is a prerequisite for successful flotation and subsequent removal of suspended material. Considering “phase 1” in the FBRM data (Figure 1), clearly different behavior can be seen between the chemicals. The inorganic coagulants, CaCl2 and PAC, form a small number of visible aggregates, while poly-DADMAC and C-PAM/bentonite give significantly higher amount of flocs. This can be explained by the different aggregation mechanisms induced by the chemicals. Inorganic coagulants (calcium and PAC) decrease the thickness of the electrostatic double layer surrounding the wood resin droplets or neutralize their charge, allowing the actuation of the attractive forces between particles and causing their aggregation into small coagula. Most of these flocs are smaller than 0.5 µm and they are not detected. Furthermore, the dissociation of these flocculants, which takes place at high pH, could reduce their efficiency, limiting the particle aggregation. However, organic polymers (poly-DADMAC and the dual system C-PAM1/bentonite) form detectable flocs by patching or bridging mechanisms. Polyacrylamides7 and poly-DADMAC are effective over a wide pH under papermaking conditions, since their cationic charge is due to a quaternary amine group, which is not hydrolyzed.22 Highly charged poly-DADMAC forms patch-like cationic sites on particles, which interact with the surfaces free of polymer of other particles forming “soft” flocs. Although the mean size of these flocs is not as large as the bridging flocs, many of them are over 0.5 µm and can be detected. C-PAMs have a lower charge density and a longer polymer chain than poly-DADMAC. Bridging is the dominating mechanism and the formed flocs are large and “hard”. High shear forces can break the large flocs, and they are not formed again because the C-PAM chain remains adsorbed on the colloidal particles with flat conformation that avoids bridging.15 On the other hand, when bentonite is added, it interacts with the cationic moieties of adsorbed C-PAM, forming bridges between these particles and leading to formation of large flocs. These flocs are large enough to be detected by the FBRM device.23,24
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Figure 8. Particle size distribution after aggregation with PAC.
Figure 9. Residual wood resin, turbidity, and GalA after DAF with polyDADMAC.
At the end of the first phase, the agitation is stopped and the agitator is removed from the flotation vessel. At the second phase dissolved air is added, followed by removal of the major part of suspended material. As shown in Figure 2, the flow pattern differs from the preceding phase: instead of a horizontal, well-defined stream caused by the agitation forces, the microscopic air bubbles create a vertical, upward-directed stream. In practice, the air bubbles gather the suspended material and start pushing it toward the surface. At some point, a dense layer consisting of aggregated material and air bubbles passes the probe window, causing a peak in the number of particle counts. Because of the more uncontrolled flow pattern during phase 2, only general trends of the flotation process can be obtained. The peak height, however, seems to be higher for the blank and the coagulants than for C-PAM and poly-DADMAC. In the case of the blank sample, the registered counts are due to air bubbles, being smaller than 100 µm. In the presence of flocculants, the agglomerated DCS collected by the air bubbles favors the interaction between these bubbles and reduces the total number of counts. When organic polymers that form large flocs of DCS are used, the interactions between DCS and bubbles are stronger because of the larger floc size. Therefore, large aggregates of flocs and bubbles are formed. This explains why the lowest effect of bubbles on the total counts took place when the dual system C-PAM/bentonite was used. The third phase basically describes the sedimentation of residual, heavy aggregates that were not removed by flotation. Sometimes the formation of deposits on the probe window
during flotation was observed. This was due to the weak agitation forces present in the suspension after flotation in the laboratory vessel. This could be avoided by optimizing the position of the FBRM in the process unit, by installing a cleaning sequence, or by installing a secondary FBRM probe after the flotation vessel in a process part where agitation is provided. The previously discussed phases of the studied flocculationflotation process are also demonstrated in Figures 3 and 4, where the different addition levels of calcium and poly-DADMAC can be seen. The peak height (phase 2) in Figure 3 seems to decrease at higher calcium doses, which could be due to the aggregation of bubbles. The ionic strength increases with the calcium concentration, and therefore, the bubbles’ affinity for surfaces and other bubbles increases too. Therefore, the coalescence of bubbles increases (decreasing the peak height) and the attachment of bubbles on the FBRM probe window also increases. Therefore, it is difficult to draw any firm conclusions about the water purification efficiency, since the final counts greatly exceed the initial amount or the calcium-induced amount of registered colloidal particles due to the fouling of the probe window. Figure 4 shows that the flocculation of DCS to form flocs over 0.5 µm increases with the poly-DADMAC dose, reaching a maximum value at 35 mg/L of flocculant and decreasing at higher dose. The lowest poly-DADMAC dose gave the poorest flocculation of DCS in phase 1 and the highest peak after addition of air-saturated water. It also seems to give the poorest flotation efficiency because the amount of residual particles is much higher than for the other doses. When the experimental doses were higher than 20 mg/L, the flotation removed almost all the flocs produced during phase 1, as shown by the almost zero final value of the total number of counts. In this case, the fouling of the probe window was not observed. In conclusion, an overall picture of the DAF process can be obtained by investigating the FBRM data on phases 2 and 3. To acquire a more accurate relationship between the FBRM analysis and the DAF process, the correlation between FBRM data after flocculation but before addition of dissolved air (phase 1) and the true purification efficiency of DAF, measured by traditional techniques, was studied. In the following, these data will be discussed separately for each coagulant and flocculant, comparing the achieved results with conventional methods to study DCS in process waters. DAF with CaCl2. As is well-known, calcium contributes strongly to aggregation of DCS in mechanical pulp suspen-
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Figure 10. Particle size distribution after flocculation with poly-DADMAC prior to flotation.
Figure 11. Residual wood resin, turbidity, and GalA after DAF with different doses of C-PAMs and 20 mg/L bentonite.
sions.1,2 Calcium chloride alone is barely utilized for coagulation purposes in the paper industry, mostly because the aggregate size is far too small. Nevertheless, at present, the effect of calcium ions on mechanical pulping process water is an interesting topic because white waters from paper machines that use calcium carbonate or calcium sulfate as fillers are often reused in mechanical pulping.25 Figure 5 shows that the removal of colloidal wood resin as determined by gas chromatography, turbidity measurements, as well as removal of dissolved pectic acids all correlate well. Thus, these three methods could all be used to assess the flotation efficiency. For CaCl2, the maximum removal of detrimental material was already achieved at low calcium concentrations. Flotation without chemical addition (blank) resulted in about 30% removal, which is in accordance with earlier findings.7 Selective removal of galacturonic acids, building blocks of pectic acids, was obtained, as in the previous study. Colloidal wood resin was removed about to the same extent as pectic acids. In 1995, Sundberg et al.26 suggested that interactions between calcium and pectic acids are essential in order to bring about efficient flotation. The calcium-induced aggregation can be studied more closely in Figure 6. Compared to the blank sample, any calcium dose causes a remarkable particle size growth and a moderate increase in the total number of particles due to the destabilization of DCS smaller than 0.5 µm, which became visible. Significant differences can be observed between doses.
Both counts and particle size increase with an increase in calcium concentration up to 10 mmol/L, which can be considered as some kind of optimum, but then the number of counts decreases again when the calcium dose increases to 15 mmol/ L. According to the coagulation mechanism, this is due to the restabilization or dispersion of the DCS aggregates due to the excess of calcium. Despite the aggregation trend shown in Figure 6, the flotation efficiency induced by calcium is only slightly reduced at 20 mmol/L (Figure 5) at the experimental conditions. Therefore, as long as the DCS is efficiently aggregated, it will be efficiently removed by flotation. DAF with PAC. Aluminum compounds are traditional papermaking chemicals used, for instance, in coagulation of pitch and resin, in wet-strengthening, and in retention and drainage systems in acid papermaking. PAC concentrations of 40 and 60 mg/L resulted in slightly better water purification than at the higher doses, which showed fluctuation in flotation efficiency (Figure 7). In general, galacturonic acids were removed more efficiently than wood resin and, at the best doses, even better than with calcium. The poorer removal at higher doses is also reflected in Figure 8, where a build-up of a small material (
