Effect of a synthetic zeolite on free calcium ion build-up in white water

Transcription

1 p Chabot 3/12/08 1:47 PM Page 39 T49 Effect of a synthetic zeolite on free calcium ion build-up in white water By B. Chabot, J.B. Thibodeau and C. Daneault Abstract: Calcium carbonate fillers can be used in manufacturing wood-containing paper to improve optical and printing properties. However, dissolution of the filler is still limiting its implementation in these paper grades. Although technologies to reduce dissolution of calcium carbonate filler are available, free calcium ion build-up in white water must be overcome to further implement this technology. We have studied the effect of a synthetic zeolite to control the free calcium ion concentration in white water. Handsheets were prepared using recirculated white water containing calcium carbonate filler under various papermaking conditions to simulate commercial papermachine operations. Results show that the zeolite is very effective at limiting the free calcium ion build-up during sheet making. Retention of the zeolite in the sheet also improved brightness and opacity. AOLIN CLAY REMAINS the dominant K filler for groundwood publication papers in North America. However, better optical properties will be needed to meet paper quality standards requested for specialty grades in the 21 st century. Calcium carbonate fillers are actually used in many commercial applications, especially in Europe. Improved paper brightness and opacity are achieved due to the higher light scattering power of calcium carbonate fillers compared to clay. In spite of the benefits obtained with calcium carbonate, the high alkalinity of the filler and the necessity to operate the paper machine at neutral ph conditions are still limiting their use in North America. Conversion from traditional acid to neutral papermaking conditions is challenging and depends on many factors, such as paper grade, pulp furnish, paper mill configuration, and also on the method used to control the ph [1,2]. The paper machine wetend ph has a very significant effect on calcium carbonate solubility. Under acidic conditions, calcium carbonate is totally decomposed into free calcium ion and carbon dioxide. It is known that high calcium ion levels in white waters can have very detrimental effects on papermaking operations and paper quality [3,4]. Consequently, free calcium ion build-up must be addressed to overcome these problems. Various approaches to control dissolution of calcium carbonate filler have been studied in the last 15 years. Passaretti [5] has devel- oped an acid-tolerant precipitated calcium carbonate filler (AT-PCC) that can be used in groundwood-containing grades. This process requires the continuous addition of phosphoric acid in the white water to maintain the ph between A chelant, such as sodium hexamethaphosphate, is generally required to prevent calcium phosphate formation (hydroxyapatite), which can be responsible for scale formation on paper machine parts [6]. The chelant stabilizes the soluble species formed by the reaction between the phosphoric acid and the calcium carbonate. Compton and Brown investigated successful strategies to inhibit calcite (a form of calcium carbonate) by precipitation of magnesium ions or dicarboxylic acids [7,8]. Pang et al [9] and Jaakkola [10] have also studied various methodologies to inhibit calcium carbonate dissolution. Most of these studies have shown that treatment methods containing phosphoric acid/phosphate are the most effective. Carbon dioxide has also been used to inhibit calcium carbonate dissolution during papermaking. It is used to adjust the ph and has been found to prevent calcium carbonate dissolution by reversing equilibria towards solid calcium carbonate. Pakarinen et al [11] have demonstrated that properly dissolved carbon dioxide is effective at retarding dissolution of calcium carbonate in a pulp suspension. Two theories have been proposed to explain the stabilization mechanism of calcium carbonate by phosphoric acid. The first B. CHABOT, Université du Québec à Trois-Rivières Trois-Rivières, QC bruno.chabot@uqtr.ca J.B. THIBODEAU, Domtar Inc. Windsor, QC C. DANEAULT, Canada Research Chair in Value-added Paper Université du Québec à Trois-Rivières Trois-Rivières, QC PULP & PAPER CANADA 109:3 (2008) 39

2 p Chabot 3/12/08 1:47 PM Page 40 T50 theory suggests that the reduction in calcium carbonate solubility is achieved through precipitation of an additive to form a barrier at the surface of the calcium carbonate [9,10]. From X-ray photoelectron spectroscopy and transmission electron microscopy, Pang et al [9] have shown that the inhibition mechanism involves the formation of a thin calcium phosphate transition phase on the calcium carbonate surface, which then acts as a barrier and blocks the dissolution sites. The second theory proposes a different stabilization mechanism which is based on the formation of buffering species generated during the reaction of calcium carbonate and the weak acid and is also dependent on chelant addition levels [6]. The chelant has been found to stabilize the buffered system through both ion sequestration and scaling inhibition. Although these technologies are effective at controlling calcium carbonate dissolution under neutral papermaking conditions, free calcium ion build-up in white water must still be overcome to further implement this technology for groundwood-containing papermaking. This is particularly important for integrated or closed paper mills. We have previously investigated the properties of a synthetic zeolite (type 4A) to remove free calcium ions from water containing calcium carbonate filler [12]. Results showed that this zeolite was very effective at removing free calcium ions. However, the effect of free calcium ion build-up in the water phase was not studied. The objective of this paper is thus to determine the effect of a synthetic zeolite at controlling free calcium ion build-up during white water recirculation. Several ph control methodologies will be investigated during sheet making to determine their effect on calcium carbonate dissolution and the corresponding response of the zeolite on free calcium ion removal. Experimental Materials The zeolite used was a sodium aluminosilicate hydrated type Na-A zeolite (Valfor 100). It was supplied by PQ Corporation. The nominal pore size diameter was 4 Angstroms (Type 4A). The calcium carbonate (GCC) slurry was supplied by Omya (Omyapaque). The solid concentration was 65%. Unbleached thermomechanical pulp (TMP) was used to prepare handsheets. The pulp was sampled at an Eastern Canadian paper mill. Handsheet preparation with white water recirculation Figure 1 presents the experimental set-up for handsheet preparation with white water recirculation. For each series of experiments, 20 handsheets (approximately 1.2 g each) were prepared using white water from the previous handsheet. For each handsheet, a volume of 500 ml of white water (or tap water for the first handsheet) was used to dilute 120 ml of a TMP pulp slurry (at 1% consistency). The diluted slurry was then transfered to the mixing vessel. The stirrer was adjusted at 500 rpm. The ph of the diluted slurry was controlled at 7 (5 in several experiments) using FIG. 1. Experimental set-up for handsheet preparation with white water recirculation. FIG. 2. Effect of acid type and white water recirculation on GCC dissolution at a constant ph of 5 for low calcium carbonate addition. a ph controller. Different acids were used to adjust the ph of the slurry. We have compared the effect of hydrochloric, sulphuric, and phosphoric acids on calcium carbonate dissolution. The proper amount of GCC (10% or 50% filler based on dry fibers) was added to the slurry and mixed for 1 minute. Then, the zeolite was added when required and the slurry was mixed for another 5 minutes. No retention aids were used during handsheets making. After mixing, the slurry was poured into the modified handsheet former. This former was designed to allow sheetmaking at up to 0.5% consistency. The modified handsheet former was based on the machine developed by Nazhad and Kerekes [13]. The modification consisted of replacing the standard 300 mm deep cylinder with a 75 mm deep cylinder. This shallower cylinder was designed to allow sheet forming at higher slurry consistency. However, it was necessary to fill the space underneath the screen of the sheet former before pulp slurry transfer. This was required to pre :3 (2008) PULP & PAPER CANADA

3 p Chabot 3/12/08 1:47 PM Page 41 peer-reviewed T51 FIG. 3. Effect of acid type and white water recirculation on GCC dissolution at a constant ph of 5 for high calcium carbonate addition. FIG. 4. Effect of acid type and white water recirculation on GCC dissolution at a constant ph of 7. vent premature draining through the screen during filling the handsheet former. Therefore, a volume of 500 ml of the same white water used to prepare the pulp slurry was used to completely fill this space. After draining, the white water was collected in a receiving container. From this, 50 ml was sampled to determine free calcium ion concentration using EDTA titration with hydroxynaphtol blue indicator. The sample was first filtered on a 0.8 µm microporous membrane filter and the filtrate was used for titration. Calcium ion concentrations were reported as CaCO 3. The residual white water was used to prepare the next handsheet following the same procedure. For each series of experiments, the final handsheet (20 th ) was saved for brightness, opacity, and ash content determinations. Results and discussion We have studied the effect of two strong acids (sulphuric and hydrochloric) and a weak acid (phosphoric) on the dissolution behaviour of calcium carbonate filler during papermaking with white water recirculation. Figure 2 presents the effect of those acids for experiments carried out at a constant ph of 5 with a low calcium carbonate addition (10%) for each handsheet. This condition was used to simulate papermaking conditions generally found in North America for groundwood grades at a low filler content. Results show that stronger acids had a very detrimental effect on GCC dissolution, as depicted by the increasing free calcium ion content with white water recirculation. We can also see that the free calcium ion content after the 20th handsheet reaches a maximum value ranging from mg/l, indicating that a dynamic equilibrium is achieved by the system. This high level of free calcium ion is unacceptable for papermaking operations due to possible interactions of free calcium ions and dissolved materials from wood that are known to promote pitch deposition and runnability problems. On the other hand, controlling the ph with phosphoric acid had a totally different effect on filler dissolution. In this case, the free calcium ion content remained almost constant at about 100 mg/l. This low calcium level was due to the protecting effect of phosphoric acid for calcium carbonate filler. As discussed previously, phosphoric acid addition produces a buffered system involving complex equilibria between solid calcium carbonate and phosphoric acid, through formation of buffering species such as calcium di-hydrogen phosphate and calcium hydrogen phosphate [11]. As expected, this is in complete agreement with results obtained in commercial paper mill applications with PCC filler [2]. Similar experiments were carried out with higher calcium carbonate addition (50%), as shown in Fig. 3. The same behaviour was observed, except that free calcium ion levels were higher for strong acids. The free calcium ion level achieved with phosphoric acid was also similar to that obtained at lower calcium carbonate addition. Again, this can be attributed to the high inhibiting power of phosphoric acid compared to stronger acids. It is well known that calcium carbonate dissolution is strongly dependent on ph [14]. As shown previously, an acidic ph is very prone to dissolve calcium carbonate filler. It is also known that calcium carbonate added to a pulp slurry tends to stabilize the ph at about 8. However, this high equilibrium ph value has a very detrimental effect on groundwood pulp brightness (alkaline darkening effect) [1]. To avoid this problem, the pulp slurry ph must be controlled to Therefore, we have carried out several experiments with the ph maintained at 7 using the same acids. Experiments were done with a high calcium carbonate level (50%). In this series of experiments, we have also studied the effect of the zeolite on free calcium ion removal for the same papermaking Figure 4 shows the effect of a strong and a weak acid on GCC dissolution during handsheet making with white water recycling. In this case, no zeolite was added to the mixture. First of all, we can see that free calcium ion levels are lower at neutral ph than at acidic ph, note comparison with Fig. 3. The effect of phosphoric acid was also sim- PULP & PAPER CANADA 109:3 (2008) 41

4 p Chabot 3/12/08 1:48 PM Page 42 T52 FIG. 5. Effect of the zeolite (0.22 g/l) on free calcium ion content. FIG. 6. Effect of the zeolite (0.5 g/l) on free calcium ion content. ilar to trials at ph 5, as shown in Fig. 3. In both cases, the free calcium ion level remained low at 100 mg/l. This is also a good indication of the phosphoric acid capacity to protect calcium carbonate dissolution under different papermaking Figure 5 presents the effect of a synthetic zeolite on free calcium ion content in the pulp slurry under the same papermaking As for previous results, we can see that the calcium ion content was slightly increasing with recirculation for both strong acids used to maintain the ph at 7, but to a lesser extent than trials without zeolite addition, as shown in Fig. 4. For the phosphoric acid trial, the free calcium ion content decreased below 100 mg/l compared to tests without zeolite addition, Fig. 4. This slight reduction can be attributed to the chelating capacity of the zeolite and is dependent on the amount added to the system. Studies in the detergents field have shown that the zeolite 4A has a high exchange capacity for calcium ions. The rate of calcium ion exchange is also very high [15]. Therefore, the concentration of free calcium ions should be strongly reduced. Although the free calcium ion level was below 100 mg/l with phosphoric acid, compared to trials without zeolite, see Fig. 4, it seems that the performance was reduced since the free calcium ions remaining in the white water were not completely suppressed. Detergents containing zeolite 4A generally include additives (co-builders) such as polycarboxylates and citrate to boost the detergency performance. Obviously, no co-builders were added in our experiments and this can partly explain the lower efficiency obtained. It is also possible that the presence of competing ions or other constituents of the pulp slurry may have affected the removal efficiency of the zeolite. Further experimentation will be needed to determine this. Experiments were also carried out with a higher zeolite dosage (0.5 g/l), as shown in Fig. 6. Under these conditions, the free calcium ion content remained almost constant for both strong acids, while it was further decreased with phosphoric acid, compared to trials at a lower zeolite dosage, due to the presence of more zeolite in the system (higher exchange capacity). From these experiments, it is difficult to quantify the performance of the zeolite 4A based on the theoretical calcium uptake value reported by the supplier (270 mg CaCO 3 /g zeolite) [16]. According to our results, it seems that the calcium uptake is very dependent on factors such as the ph, the acid used to control the ph, and the amount of zeolite added. However, it seems that for strong acids, the more zeolite added, the faster the equilibrium concentration of free calcium ion is achieved during white water recirculation. Figures 7 and 8 present the effect of experimental conditions used to make handsheets with white water recirculation on the brightness and opacity of the last handsheet (20 th ). Results show that brightness is strongly dependent on experimental conditions, Fig. 7. The brightness gain can be attributed to several factors. Neutral papermaking conditions improved brightness. Even though it is known that increasing the ph of thermomechanical pulps has a negative effect on brightness (pulp darkening), it is clear that the brightness loss was compensated by the higher amount of filler remaining in the handsheet. This is particularly the case since calcium carbonate filler is dissolved to a lesser extent at neutral conditions compared to acidic Of course, optical properties of handsheets are also dependent on ash content. Since no retention aids were used during handsheet making, the ash content of the last handsheet for trials at low initial filler level (10%) varied from 3 to 5% for trials at ph 5 and ph 7, respectively. For trials at higher initial filler level (50%), the ash content varied from 15 to 22% at ph 5 and ph 7, respectively. Inhibition of calcium carbonate dissolution by using phosphoric acid also had a significant effect on brightness, since more GCC was retained as solid particles in the handsheet compared to trials using stronger acids (hydrochloric and sulphuric acids). The effect of the zeolite also seems important since this material is very bright, with a brightness value of 98% [16]. Although the ash content of the last handsheets were very similar to :3 (2008) PULP & PAPER CANADA

5 p Chabot 3/12/08 1:48 PM Page 43 peer-reviewed T53 FIG. 7. Effect of operating conditions on brightness improvement for the last handsheet. experiments without zeolite additions, it is clear that addition of the zeolite improved both brightness and opacity. We have demonstrated this phenomenon in a previous work [17]. Figure 8 presents the same results for the opacity of the last handsheet. Although the opacity increased depending on papermaking conditions, there was no significant difference related to the acid used to control the ph. As previously discussed for brightness, the retention of both solid particles also contributed to opacity improvements. Conclusions This study showed that zeolite 4A can be used to control the free calcium ion content in the white water during handsheet making with white water recirculation under various papermaking The optical properties (brightness and opacity) are improved due to retention of both solids fillers (calcium carbonate and zeolite). These results indicate that this technology could be used for the production of wood-containing papers. Acknowledgements The authors gratefully acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Research Chair in Value-added Paper for their financial support. Thanks are also offered to Omya Inc. (GCC), PQ Corporation (Zeolite 4A) and Kruger Trois-Rivières (pulps) for providing samples. Literature 1. EVANS, D.B., DRUMMOND, D.K., KOPPELMAN, M.H. PCC Fillers for Groundwood Papers. Tappi Papermakers Conf. Proc., pp (1991). 2. AIN, R.L., LALEG, M. Mill Experiences with AT Precipitated Calcium Carbonate (PCC) in Papers Containing Mechanical Pulp. Pulp Paper Can. 98(12): (1997). 3. PHIPPS, J., LORUSSO, M. Dissolution Behaviour of Calcium Carbonate in Mildly Acidic Conditions. 12 th Fund. Res. Symposium, The Pulp and Paper Fundamental Research Society, Oxford, pp (2001). 4. ALLEN, L.H. The Importance of ph in Controlling Metal-Soap Deposition. Tappi J. 71(1):61-64 (1988). 5. PASSARETTI, J.D. Acid-Stabilized Calcium Carbonate, Process for its Production and Method for its Use in the Manufacture of Acidic Paper. U. S. Patent, 5,043,017, August 27, (1991). 6. EVANS, B., SLOZER, M. Neutral Groundwood papers: Practical and Chemical Aspects. 5 th Int. Paper Coating Symp.Proc., Montreal, pp (2003). 7. COMPTON, R.G., BROWN, C.A. The Inhibition of Calcite Dissolution/Precipitation: Mg 2+ Cations. J. Colloid Interface Sc., 165: (1994). 8. COMPTON, R.G., BROWN, C.A. The Inhibition of Calcite Dissolution/Precipitation: 1,2-Dicarboxylic acids. J. Colloid Interface Sc., 170: (1995). FIG. 8. Effect of operating conditions on opacity improvement for the last handsheet. 9. PANG, P., KHOULTCHAEV, K., ENGLEZOS, P. Inhibition of the Dissolution of Papermaking Grade Precipitated Calcium Carbonate Filler. Tappi J., 81(4): (1998). 10. JAAKKOLA, P. Comparison of Methods for Stabilization of PCC Against Dissolution at Acid Papermaking Conditions. Nordic Pulp Paper Research J. 16(2): (2001). 11. PAKARINEN, H., LEINO, H. Benefits of Using Carbon Dioxide in the Production of DIP Containing Newsprint. PTS-CTP Deinking Symposium 2000, Paper 19, 18 p., (2000). 12. THIBODEAU, J.-B., CHABOT, B., DANEAULT, C. Calcium Ion Removal by a Synthetic Zeolite in the Manufacture of Mechanical Grade Papers. Pulp Paper Can. 106(3):42-45 (2005). 13. NAZHAD, M.M., KEREKES, R.J. Producing Handsheets at Headbox Consistency. 85 th Annual Meeting Conf. Proc., pp. A203-A208, Montreal (1999). 14. LAUFMANN, M., HUMMEL, W. Neutral Manufacture of Groundwood-Containing Papers in the Nineties, Tappi Papermakers Conf. Proc., Seattle, (1991). 15. KURZENDORFER, C.P., LIPHARD, M., RYBINS- KI, W., AND SCHWUGER, M.J. Sodium-Aluminium- Silicates in the Washing Process. Part. IX: Mode of Action of Zeolite A Additive Systems. Colloid & Polymer Sci., 265: (1987) CHABOT, B., LORANGER, E. The Effect of Zeolite on Value-Added Paper Manufacturing. Pulp Paper Can 105(3):29-32 (2004). Résumé: Le carbonate de calcium peut être utilisé dans la fabrication des papiers à base de pâte mécanique pour améliorer les propriétés optique et d imprimabilité. Toutefois, la dissolution du pigment limite encore son utilisation dans ces grades. Bien que plusieurs technologies pour réduire la dissolution du pigment soient disponibles, il est nécessaire de limiter l accumulation d ions calcium dans l eau blanche pour favoriser l implantation de cette technologie. Nous avons étudié l effet d une zéolite synthétique pour contrôler l accumulation des ions calcium dans l eau blanche. Des formettes ont été fabriquées sous différentes conditions d opération en recirculant l eau blanche contenant du carbonate de calcium. Les résultats montrent que la zéolite permet de limiter efficacement l accumulation des ions calcium dans les eaux blanches durant la fabrication du papier.la rétention de la zéolite dans la feuille a également permis d augmenter la blancheur et l opacité. Reference: CHABOT, B., THIBODEAU, J.B., DANEAULT, C. Effect of a synthetic zeolite on free calcium ion build-up in white water. Pulp & Paper Canada 109(3):T49-53 (March 2008). Paper presented at the 92nd Annual Meeting in Montreal, QC, February 6-10, Not to be reproduced without permission of PAPTAC. Manuscript received on November 14, Revised manuscript approved for publication by the Review Panel on January 14, Keywords: ACCUMULATION, CALCIUM, IONS, WHITE WATER, ZEOLITES, FILLERS, CALCIUM CARBONATE. PULP & PAPER CANADA 109:3 (2008) 43