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Surface chemistry of flotation deinking: Agglomeration kinetics and agglomerate structure
YKI – Ytkemiska institutet.
1996 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 11, no 2, 74-85 p.Article in journal (Refereed)
Abstract [en]

Agglomeration kinetics of dried and aged offset ink particles in the flotation deinking of recycled printed papers has been studied using a photometric dispersion analyser. The model ink particles had a volume mean diameter of 1 µm and calcium oleate was used as collector. The agglomeration speed increased with increasing agitation rate, increasing pH and increasing concentrations of sodium oleate, calcium chloride, sodium chloride, sodium metasilicate or pectic acid. The age of the print, the temperature, the amount of hydrogen peroxide had no or very little influence on agglomeration. The agglomeration decreased with increasing concentrations of non-ionic surfactant, starch or carboxymethylcellulose. Theoretical calculations in a deinking system shows that virtually all sodium oleate forms calcium oleate which precipitates at a pH above 6.5. The pH decreases slightly with increasing calcium concentration. Surface tension measurements in the calcium chloride - sodium oleate system indicate that the precipitation of calcium oleate is a rapid process. The equilibrium between free oleate anions and calcium oleate seems to be reached almost completely after 20 minutes of diffusion-controlled mixing. An analysis of the ink agglomerate structure by determining the boundary fractal dimensions showed that they were built up of smaller agglomerates. Flotation deinking is the most commonly used method for ink removal from waste paper in European paper recycling plants. The waste paper is dispersed in the pulper by mechanical agitation and chemical treatment, after which the ink particles released are separated through flotation. The fundamental chemical stages of ink removal during deinking may be identified as ink detachment, ink particle re-deposition, ink particle agglomeration, attachment of ink agglomerates to air bubbles and flotation. Efficient ink detachment and low re-deposition are of course desired. These processes are believed to be promoted by a high pH which swells the fibre wall and increases the negative surface charge of both fibre and ink. They are also promoted by detergent chemicals such as soaps and synthetic surfactants, which lower the inkwater interfacial tension and improve the colloidal stability of the ink particles. Ink flotation is favoured by large ink particles or ink agglomerates and by a high ink-water interfacial tension i.e. by hydrophobic particles. It is quite obvious that the ink particle properties required to improve ink detachment and decrease re-deposition are undesirable during flotation. In parti cular, this appli es to the ink particle si ze and surface hydrophobicity. Very important sub-processes are thus the surface modification and agglomeration of small ink particles. The understanding of ink particle agglomeration and flotation is limited and various mechanisms have been proposed in the literature (Schweitzer 1965; Bechstein, Unger 1972; Ortner et al. 1975; Ortner 1981; Fischer 1982; Larsson et al 1982, 1984a; Hornfeck 1982; Putz et al. 1991). The aim of our work is to improve the fundamental understanding of these sub-processes through studies in well-defined model systems. In this paper, we concentrate upon ink agglomeration. A number of studies of model systems have been reported (Larsson et al. 1982, 1984a-b; Milanova, Dorris 1993; Dorris, Nguyen 1995, Epple et al. 1994). Larsson et. al. worked with an aqueous dispersion of letterpress ink. Their work suggests that a calcium soap collector acts through the precipitation of small calcium soap particles and co-agglomeration of ink particles and calcium soap particles. Calcium soap particles adsorb onto the ink particles rendering the agglomerate hydrophobic and floatable. Milanova, Dorris and Nguyen have worked with aqueous dispersions of offset lithographic inks and flexographic inks. They found that calcium soaps of fatty acids were effective collectors and flotation agents for flexographic ink. This indicated that problems associated with the deinking of flexographic inks depend on other parameters in the system. Redeposition onto fibres is one possible explanation of the poor deinking ability of flexographic inks (Dorris, Nguyen 1995). Epple, Schmidt and Berg studied the flotation of dispersed xerographic toners. The toner used was not printed before dispersion. Then results showed not only that the type of surfactant was important but also that there was an optimal dosage for each surfactant. Adsorption of anionic, non-ionic and cationic surfactants onto the toner particles all occurred tail-on. To yield practicai information, the basic knowledge derived from model systems should be verified with laboratory and pilot plant trials. Hence, the model system should approach as closely as possible the industrial process. For offset inks, this implies that ink oil should be separated from the pigment and binder, and that oxidation reactions within the binder should have taken place i.e. the ink used for the preparation of an ink suspension should have been applied to a paper, dried and aged. In this paper, we describe a method of preparing such an ink suspension. The ink suspension collected and used in our model experiment has a volume mean particle diameter of 1µm, with all particles less than 8µm, which makes it particularly suitable for agglomeration studies since these small particles have a very low flotation separation efficiency unless they are agglomerated (Larsson et al. 1984b; Marchildon et al. 1989; Saint Amand, Perrin 1991; NauJock et al. 1992). This paper concentrates upon calcium oleate as collector and consists of three parts. The main part deals with agglomeration kinetics. Effect of agitation, pH, process chemicals and substances entering the system with coated papers, e.g. carboxymethylcellulose and starch, is studied. The paper also comments on the equilibrium formation of various chemical species, calcium soap formation and the structure of ink agglomerates as revealed by fractal analysis.

Place, publisher, year, edition, pages
1996. Vol. 11, no 2, 74-85 p.
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:ri:diva-27346OAI: oai:DiVA.org:ri-27346DiVA: diva2:1054350
Note
A1012Available from: 2016-12-08 Created: 2016-12-08Bibliographically approved

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