Effect of pulp fibres on gas holdup in a flotation column

Abstract

Column flotation is an emerging technology in the deinking of recycled paper. Deinking efficiency depends primarily on the effect of four physical variables: gas and accepts flowrates, bubble size distribution, and pulp consistency. Experiments to establish operating ranges for these variables by determining their effect on gas holdup were conducted in a laboratory column installed at Bowater's Gatineau (Quebec) mill which processes a pulp (70% ONP/30% OMG) with about 1% consistency. The results showed the general trends expected from a review of previous studies: gas holdup increases with gas and accepts flowrates and with smaller bubble size, and decreases for higher pulp consistencies. However, the review also demonstrated that the published information is of limited application as the bubble size was much larger than that encountered in industrial flotation practice, primarily because of the use of pure water. Industrial pulps contain in most cases surface active agents which have a substantial effect on forming and preserving a small size bubble population.La flottation en colonne est une technologie en emergence pour le desencrage du papier recycle. L'eficacite de desencrage depend principalement de I'effet de quatre variables physiques : les debits de gaz et d'alimentation, la distribution de taille des bulles et la consistance de la plte. Afin de determiner les g a m e s de fonctionnement et leur effet sur la retention de gaz, des experiences ont ete menees dans une colonne de laboratoire installee a l'usine de Bowater a Gaineau (Quebec) qui traite une plte a papier (70% ONP/30% OMG) d'une consistance de 1%. Les resultats illustrent les tendances generales previsibles d'apres un examen d'etudes anterieures : la retention de gaz augmente avec les debits de gaz et d'alimentation et avec la diminution de la taille des bulles, et elle diminue avec I'augmentation de la consistance de la plte. Toutefois, I'etude bibliographie revele que les donnees publiees sont d'une application limitke du fait que la taille des bulles est plus grande que celle qu'on rencontre dans la pratique industrielle en matiere de flottation, surtout a cause du recours a de I'eau pure. Dans la plupart des cas, les pltes industrielles contiennent des surfactifs qui ont un effet significatif pour former et conserver une population de bulles de petite taille.

Effect of fiber type on gas holdup in a cocurrent air–water–fiber bubble column

“…Changes in the fluid properties can affect bubble size and bubble behavior. This can occur with certain types of cellulose [58] or synthetic [20] fibers. The surface-active agents usually cause a decrease in liquid surface tension, and produce a smaller, more stable bubble (i.e., one less prone to coalescence).…”

Gas holdup in a gas-liquid-fiber semi-batch bubble column

“…This trend has been observed by Went et al (1993), Lindsay et al (1995), and Janse et al (1999) in GLF bubble columns. This decrease in gas holdup is characterized by a larger bubble size due to the increasing fiber mass fraction, which leads to faster bubble rise velocities and smaller bubble residence times.…”

“…They also observed that the fiber distributed uniformly in the axial direction when the fiber mass fraction was low; however, fiber tended to build up at the bottom of the column when the fiber mass fraction was high. A decrease in gas holdup caused by fiber addition was also observed by Janse et al (1999), and they pointed out the effect of fiber addition was more pronounced at higher superficial gas velocities.…”

“…Some researchers recorded gas holdup decreases with increasing fiber mass fraction (Went et al, 1993;Reese et al, 1996;Janse et al, 1999), but some others observed that gas holdup in a GLF suspension can be significantly higher than that recorded in a GL system operating under similar conditions (Lindsay et al, 1995;Schulz and Heindel, 2000). Hence, it is necessary to further study the effect of fiber addition (mass fraction) on gas holdup and determine factors that influence the results.…”

“…Fibers also tend to trap bubbles and hinder bubble rise, resulting in enhanced bubble coalescence (Walmsley, 1992;Went et al, 1993;Lindsay et al, 1995;Reese et al, 1996;Janse et al, 1999).…”