RFACE=CHEMICAL OF FILLER FLOTATION

Transcription

1 RFACE=CHEMICAL OF FILLER FLOTATION Results are dependent on filler type and particle size BY M. LIPHARD, B. SCHRECK AND K. HORNFECK I N EUROPE, flotation is routinely used to de-ink waste paper that is to be recycled. Flotation may also be used to reduce filler content depending on the type and quality of the recycled paper. At present, for instance, in tissue production the filler content is usually reduced by means of an additional washing process, but because flotation has proved to be superior to washing in terms of water consumption and fibre loss, new reagents have recently been specifically develope flotation El]. governed by the surface properties of the suspended solids involved, and in turn these properties are influenced by the surface active substances present in the pulp. This work is mainly concerned with the flotation characteristics of fillers and gives basic information on how to control the filler content of recycled paper. OASIC ASPECTS Many paper characteristics, e.g. degree of brightness, opacity and smoothness, improve with decreasing filler particle size. Therefore, the pigments contained in the paper or in the pulp are generally very fine; this complicates their removal by flotation. The problems seen in the flotation of very fine particles have been known for a long time in mineral processing. The physicochemical fundamentals, however, have only partially been examined. A summary of the current state is given by Sivamohan [2]. Two of the most important features for particle flotation are their size and the hydrophobicity of the particle surface. They influence the major sub-processes which determine the rate of flotation and the recovery of fillers in different ways: The collision between air bubbles and particles; The drainage and rupture of the remaining thin water film between bubble and particle; The stability of the bubble/particle aggregate. The relationship between the flotation rate and particle size is complex. The particle size particularly influences the first sub-process, i.e. the collision between the particles and the air bubbles. Parallels can be drawn to the aggregation processes of particle dispersions. Here the collision between particles is also a prerequisite for the aggregation. The aggregation process is determined by several different inter- actions between the particles caused by internal and external forces [3]. In the very small particle size range (colloidal systems) the influence of the Brownian motion predominates and the corresponding aggregation rate is independent of the particle size [4]. External influences, such as stirring or sedimentation by gravity, also affect the aggregation. This influence is very small in the colloidal range, but the aggregation rate increases strongly'with increasing particle diameter [4]. Theoretical calculations allow the assumption that even a very low velocity gradient, caused by external forces, has a much greater influence on the aggregation rate for particle sizes above 1 pm than the Brownian motion [3,5,6]. Van der Waals' attraction and repulsion between particles due to electrical double layers also increase with increasing particle diameter [3]. For particle sizes larger than 1 pm the inter-actions depending on the particle size become dominant, and the aggregation rate increases strongly. The situation should be similar for the first sub-process of the flotation, that is for particles larger than 1 pm the flotation rate and therefore the yield increase strongly with an increasing particle size. This explains why very fine particle flotation can be improved considerably by a prior agglomeration process. The second sub-process, the drainage of the film, mainly depends on the hydrophobicity of the surface [2,7]. Hydrophilic surfaces show good wettability due to the formation of hydrogen bonds or dipole/dipole interactions which allow stable films T218 &I PULP & PAPER CANADA 94:8 (1993) 27

2 fibre loss I ash [%] I tl-1 -- :- no sodium silicate 2 X sodium silicate I I I I I

3 8.5) was stirred with the respective collec'tor for 10 minutes and then floated for five minutes. The floated fillers were subsequently filtered; the filtrate was dried at 150 to 180 C and weighed to determine percent recovery. Filler/wood pulp mixtures: Fresh spruce wood pulp was mixed with the corresponding quantity of filler or slurry (weight ratio of the solid materials 80:20), and the respective chemicals were added. It was then disintegrated at a stock consistency of 4% for 10 minutes. After dilution to 1% and adjustment of the ph-value to 8.5 the stock was floated for 10 minutes at 45 C. To specify the ash content or the ash reduction, the dried paper and/or the filler/wood pulp mixture was ashed and tempered at 6OOOC for 90 minutes. Particle size measurement: The particle size distributions were determined by means of laser diffraction (particle size analyzer, Sympatek, model Helos KA). The corresponding filler suspensions were made under the same conditions as the corresponding flotation tests (ph, water hardness, collector concentrations, stirring conditions). The solid material concentration was between 40 and 100 mg/l. Fillers: The following commercially available fillers were used: Kaolin Spex IF, calcium carbonates of the type Kreide OM 50 and Hydrocarb OG 90 (powder and slurry). (All calcium carbonates by Pluess and Staufer, Oftringen, Switzerland). Waste paper: Mixture 1 consisted of 50% rotogravure magazines and 50% letterpress newsprint, 18% ash content, almost exclusively kaolin. Mixture 2 was 50% highly coated rotogravure magazines, 25% letterpress newsprint and 25% offset newsprint, 21% ash content, predominantly (>80%) calcium carbonate. Both paper mixtures were aged artificially for 144 hours at 6OOC. Reagents: A commercially available fatty acid (Olinor 4020, mainly C18,) was used for de-inking. The filler collectors are laboratory products, an anionic surfactant (RS 3030) and/or a cationic polymer (RS 3110). OATER HARDNESS PH When de-inking with the standard formulation, the ph-value in the pulper is 9.5 to 10 depending on the stock consistency, and approximately 8.5 in the flotation cell. For filler flotation the ph-value should be slightly alkaline; otherwise the calcium carbonate may start to dissolve. Usually the separation of printing ink is supported by an increased ph value since fibres and printing ink are both negatively charged. Also the commercially available fillers used here are negatively charged above ph 7 El]. Therefore, the complex surface charge models of kaolinite [9,10] and calcite are not dealt with here. The negative surface charge is also important in the adsorption of the collectors, particularly in the case of anionic surfactants. In model tests sulphonate- and phosphate group containing types have proven to be particularly suitable [ 11. Water hardness plays a special role: Calcium ions adsorb on the mineral surfaces and reduce the negative surface charge. The negatively charged surfactant can thus adsorb more easily on the surface if there is no loss of reagents due to the precipitation of insoluble calcium surfactant salts [ll], or if, as is the case for de-inking, the calcium soap flakes support the flotation of the printing ink [12 to 151. The anionic surfactants mentioned above form slightly soluble calcium salts and are therefore especially suitable as a collector for calcium carbonate and kaolin. OODlUM SILICATE Sodium silicate is used in typical formulations for flotation de-inking. It supports the separation of printing ink from fibres [ and stabilizes the peroxide bleach so that a yellowing of the fibres under alkaline conditions can be avoided. Tests on a kaolincontaining paper mixture show that the brightness increases, as expected, during the flotation with an anionic surfactant (RS 3030) when sodium silicate is added, Fig. 1. A second positive effect of sodium silicate is a decrease in fibre loss, i.e. the improved selectivity during the sep aration of printing ink and fibre, Fig. 1. However, the flotation of fillers decreases, and the ash content in the de-inked paper increases (Fig. 1). Sodium silicate is known in mineral processing as a so-called depressant for silicious minerals and minerals of the salt type [ 19,201. The mode of action is still debated [20,21]. There is probably a competitive adsorption between the hydrophilic sodium silicate and the anionic surfactant on the mineral surface so that it is made less hydrophobic. Parallel to the direct flotation of waste paper, tests were carried out with aqueous filler dispersions. These model systems have shown that the lower ash removal can be attributed to the depressant effect of the sodium silicate: The flotation recovery of kaolin ilk ffot.tion [X] 0 "30 As3110 Assoso RS3110 RS3110 plre filler dispemion fillerhood pulp mixture Kreide OM 50 Hydmcad~ SO 00 Slurry Kaolin T PULP & PAPER CANADA 94:8 (1993) 29

4 declines with increasing sodium silicate concentration, Fig. 2. The respective yield depends on the co1lector:sodium silicate ratio. The situation is similar for the flotation of CaC03 dispersions. Here, an increased concentration of anionic surfactant was more effective in compensating for the depressant action of the sodium silicate than in the flotation of kaolin. With the fillers used here the flotation recovery with anionic surfactants is higher for calcium carbonate than for kaolin, especially in the usual application concentration range (20 mg/l) Fig. 2. However, the particle size of the fillers also plays an important role, which will be explained later. The effect of anionic surfactants depends on the type of filler. This dependance also holds for ash reduction from paper mixtures with different filler types via the standard fbrmulation. In the presence of sodium silicate the ash content of the paper mixture containing calcium carbonate can be reduced to a greater extent (from 21 to 5.4%) than in the kaolin-containing mixture (from 18 to 10% ash). OD PULP SUBSTANCES Surface active substances in fibrous materials, e.g. saponified resin and fatty acids [22], can also influence flotation results. This was examined in filler/wood pulp mixtures. Figure 3 shows the ash reduction from corresponding systems with a 20% filler portion during flotation versus increasing concentrations of the anionic surfactant Rs 3030 in the presence of sodium silicate (standard formulation). vet6 que le r6sultat de la flpqti.. Even without surfactant addition an ash reduction of approximately 30% was achieved, which can probably be attributed to the anionic surface active substances in the wood pulp. Using RS 3030 as a filler collector, the anionic surfactant:sodium silicate ratio is thus more favorable for flotation as in the case of pure filler suspensions. Corresponding to the results mentioned previously for an increase in the filler removal larger amounts of anionic surfactant are required for kaolin than for calcium carbonate, Fig. 3. Figure 4 shows a comparison of the ash removal without surfactant versus addition of Rs 3030 along with the fatty acid which is usually used for de-inking. It becomes clear that the fatty acid does not contribute to the flotation of the fillers but that the surfactants already contained in the wood pulp are responsible for this. This also explains why de-inking with fatty acids results in noticeable ash removal in spite of their low effect as filler collectors [23]. The mode of action of this printing ink collector is different from that of the filler collectors. It is assumed that an agglomeration takes place between the hydrophobic printing ink and the hydrophobic calcium soap flakes, thus improving the flotation of very fine printing ink particles [ The difference in the behavior of the three calcium carbonate types has to be attributed to their different particle size. They are: A calcium carbonate which is available as a powder (Kreide OM 50) ; A finer powder (Hydrocarb 90 OG), spray dried from a slurry; Abstract: Investigations were carried out on model systems such as filler suspensions or artificial mixtures of wood pulp and fillers. Particle size distributions and flotation recovery rates were determined as a function of the concentration of the major dissolved substances in the pulp. These measurements were correlated with results obtained from filler containing waste paper. It was found that the flotation results are significantly dependent on the type of filler, its particle size and the presence of dispersants like sodium silicate in the pulp. Reference: LIPHARD, M., SCHRECK, B., HORNFECK, K. Surface chemical aspects of surface flotation in waste paper recycling. hlf~ Paper Can 94(8):T (August 1993). Paper presented at the 1st Research Forum on Recycling of the Technical Section, CPPA, at Toronto, Ontario, on October 29 to 31, Not to be reproduced without permission. Manuscript received September 1,1991. Revised manuscript approved for publication by the Review Panel August 27,1992. Keywords: FILLERS, FLOTATION, PARTICLE SIZE DISTRIBUTION, SURFACTANTS, FIBRE RECOVERY, WASTE PAPERS, ANIONIC COMPOUNDS, CATIONIC COMPOUNDS, SODIUM SILICATE, TEST METHODS. The slurry which contains an anionic dispersing agent. For the measurement of the particle sizes, the filler suspensions were made in the same concentrations as the collector, same water hardness and same stirring conditions as in the flotation process. This procedure is used to measure the particles present under flotation conditions, 'including agglomerates. The mean particle size decreases in the following order: Kreide OM 50 (3,2 pm), Hydrocarb 90 OG (2 pm) and Slurry Hydrocarb 90 OG (1,3 pm). As was expected, the filler recovery during flotation also decreases in the same order, Fig. 4. To compensate for the negative effect of the small particle sizes, specific cationic polymers were developed as filler collectors. Due to their positive charge they adsorb on the negatively charged solid surfaces. Besides their hydrophobizing effect they cause agglomeration, i.e. an increase in particle size. The mean particle size of all three calcium carbonate types increases to about 10 pm in the presence of the cationic polymer (RS 3110). For the flotation of pure filler dispersions without wood pulp, the effect of cationic polymer RS 3110 is clearly seen as compared to flotation with RS The filler yield is high for all calcium carbonate types, independent of their original particle size, Fig. 5. A clear increase in filler flotation can also be seen for kaolin. In the presence of wood pulp this high yield is maintained for kaolin, while the ash removal is lower for calcium carbonate, Fig. 5. The presence of the anionic surfactants in wood pulp can possibly limit the adsorption of the polymer. Figure 6 shows some of the results when sodium silicate is used in filler/wood pulp mixtures. Only slight differences in ash reduction were obtained with anionic surfactant and cationic polymer in the case of calcium carbonate, while clear differences result in the case of kaolin. As in the other systems, the use of sodium silicate in the filler/wood pulp mixtures results in decreased fibre losses. As already mentioned, the ash content obtained with RS 3030 was considerably lower in waste paper mixtures with a high calcium carbonate content than mixtures containing only kaolin. Using the standard formulation, the polymer RS 3110 has a comparably good effect both for kaolin- and calcium carbonate-containing waste papers; the ash content is reduced from 18 to 2% or from 21 to 3%, respectively. OOlCLUSlONS Specific anionic surfactants and cationic polymers can be used as filler & T221

5 how distinct advantages and disadvantages depending on the filler type used and the type of dissolved substances in the flotation pulp. Anionic surfactants act as collectors by hydrophobizing the surface of the filler particles by adsorption. They can be added in flotation as a collector and they are already present in wood pulp. Since the adsorption onto the calcium carbonate surface occurs at lower concentrations than onto the kaolin surface, anionic surfactants are generally better suited for waste paper systems containing calcium carbonate than for those with kaolin. However, the particle size also plays an important role. Specific cationic polymers are preferable for very fine fillers, since they lead to the agglomeration of the filler particles and thus to a larger particle size which can be better floated. When comparing the different /filler types the cationic polymer shods certain advantages in the removal of kaolin. The effect of collectors can be impaired by sodium silicate due to their competitive adsorption. Sodium silicate is a strong depressant especially for the kaolin flotation in combination with anionic surfactant-type collectors. The mutual interference of the collectors and sodium silicate is also dependent on the concentration ratio of the individual reagents. Thus, a desired higher adsorption can be achieved by a concentration increase. In the case of the cationic polymertype collector the combination with sodium silicates is reasonable, since lower fibre losses can be achieved together with good filler removal. This is important since the new filler collectors lead to increased fibre loss compared with classic flotation de-inking formulations. OEFERE WCES 1. HORNPECK, K., LIPHARD, M., SCHRECK, B. W~flrPapicrfosri.k.ti.n 118 (1990) SIVAMOHAN. R. Intm J. Miner. Process. 28 (1990) P WARREN, LJ. Gem Awl. 4-4 (1977) IVES, ICJ., in: K.J.Ives (Editor), The Scientific Bash of Flocculation, Nato Advances Studies Institutes Series, Series E Applied Science No. 27, Sijthoff & Noordhoff, Alphen an den Rijn (1978). p.37 ff. 5. WARREN, LJ. 7kam. Imt. Min. MeW. 84 (1975) c LASKOWSKI, J., in: E. Matijevic (Editor), Surface and Colloid Science, Plenum Press, New York, Vol. 12 (1982). p. 315 ff. 7. FISHER LR, MITCHELL, E.E., HEWIT, D., W O N, J.. WOLFE, J. Colloidr # Surfaces 52 ( 991) KUaZENDORFER, C.P. hg. COIloid #Polymer sd 69 (1984) van OLPHEN, H. Introduction to Clay Colloid Chemistry, Wilqr Interacience, New York SCHOFIELD, RK., SAMSON, H.R Disc. Faraday Soe 18 (1954) BURG, B., LIPHARD, M., SCHRECK, B., SPECKMA", H.D. ColW Polymer Sci. 83 (1990) LARSSON, A., STENIUS, P., ODBERG, L Wochenblalt JiirPaprerfainikahon 1!4 (1986) LARSSON, A., STENIUS, P., ODBERG, L. Sumsk Pappersttdntng18 (1984) R 158, 14. LARSSON, A,, STENIUS, P., ODBERG, L. Sumsk Pappersttdntngl8 (1984) R FISCHER, S. Wochenblatl fur Papzerfabnkalzon 110 (1982) BECHSTEIN, G. Das Osterrezchrsche Papaer 12 (1975) BECHSTEIN, G., UNGER, G. Zellsloflund Papaer, 10 (1972) 297. Mobil puts it on the line H.L. Wochmbatt fur RAO, KH., ANTTI, B.M., FORSSBERG, E. I;;; J. Miner: Process. 26 (1989) RAO, KH., ANTTI, B.M., FORSSBERG, E. Inl. J. Miner ColloidsSurf: 34 (1988/89) MARINAKIS, K.I., SHERGOLD, H.L. Inl. J. Miner: Process. 14 (1985) AUHORN, W. Wochablatl fir Papierlahikation 112 (1984) LIPHARD, M., HORNFECK, K SCHRECK, B. Proceedings of the 1991 TAPPI Pulping Conference, Orlando, TAPPI Press, Atlanta 1991, p Seven reasonswhy Mobil is the gas supplier of choice Mobil has the gas. 1. Eight trillion cubic feet located throughout North America providing vast choices of supply and reliability. And we're discovering and developing new fields, adding new reserves. 2;. ble;!i;.as the Professionals experienced in all phases of the natural gas business committed to providing you the best service possible. 3m Mobil has the flexibility. Our diverse supply portfolio has access to a vast pipeline network, which gives us a choice of routes to deliver your supplies anywhere in North America. 4.:KzEl accounting. GAMMA generates accurate, comprehensive invoices packed with gas movement information, including delivery points, volumes by delivery point, meter numbers, contract and transportation reference numbers. Mobil has the 7. dedication. Our Mission Statement sums it up best: "We are committed to being the industry leader in providing our customers with reliable gas supplies and qualityservice... These commitments will be achieved through ethical and innovative application of talent, teamwork and technology." For more information, contact: Mobil Natural Gas Inc. tailor our agreement to suit Greensooint Dr.. your special needs -from Houston, TX day interruptible to Mobil (713) or long-term warranted. (800) MobilNG ( ) Mobil has GAMMA. Fax: (713) Our innovative and Mobil Natural Gas proprietary GAS Marketing Canada Ltd. MAnagement software The Supplier of Choice 330 Fifth Avenue, S.W. system enables us to Mobil Tower, Calgary Place track, coordinate and Calgary Alberta, service your nomination T2P OL4 Canada requests. (403) or (800) MobilNG ( ). Fax: (403) ~~~2','~~ T PULP & PAPER CANADA 94:8 (1993) 31

6