The role of different alkali sources in deinking and bleaching processes

This thesis is divided into two parts. The first and major part deals with deinking and the second part deals with peroxide bleaching. DEINKING The use of deinked fibre in the paper industry has increased dramatically in recent times. Enormous progress has been made in recent years in improving the deinking process. However, deinking chemistry remains one area which is not well understood and not completely optimized. There is widely held belief that the inclusion of approximately 10% ash in the feed is desirable for effective removal of ink from newsprint. However, some recently reported studies have found that the inclusion of magazines does not enhance deinking of newsprint. Many factors can have an influence on the efficiency of flotation deinking of newsprint and magazines. In order to have a better understanding of this complex process, the influence of some important process variables (flotation time and pH, different magazine type and quality, influence of sodium silicate and water hardness) during flotation deinking of newsprint and magazines have been examined in this thesis. No evidence could be found to support the idea that ash components from magazines facilitate the removal of ink from newsprint. There has been significant recent interest in attempting to improve the strength and optical properties of recycled fibre. However, no study has been reported which investigates comprehensively the effect of dein king on the properties of recycled paper after the deinking stages of pulping and flotation. This work also addresses the effects of different stages of deinking on the physical and optical properties of recycled paper. Furnishes of mixed magazine (coated) and newsprint were used to investigate changes in properties of recycled paper during deinking. In most cases, optical and physical properties showed reverse trends after pulping and flotation stages. These differences can be explained by the change in proportions of mechanical and chemical fibre, filler, fines and ink during flotation. After the pulping stage, trends in strength properties were influenced by detrimental effects of filler from magazines. However, after the flotation stage, due to loss of most of the filler and fines, an enhancement in all strength properties occurred. Brightness and light absorption coefficient appeared to be influenced more by ink content, when large amounts of filler are present. The light scattering coefficient was dominated by the fibre type. Extended periods of flotation (greater than ten minutes) had no significant effect on properties of the recycled paper investigated. Studies have also been carried out to examine the effects of the various alkali sources on flotation deinking of newsprint and coated magazines. At the same time, the effects of each alkali used in the deinking process with regard to the strength, optical and surface properties of recycled paper have been investigated. This study also addresses the effect of sodium silicate on the deinking efficiency of each alkali. Significant differences in the strength and optical properties of recycled paper occurred with the use of different alkalis. These observed differences can be explained by the hydrolysing effect of the alkalis. BLEACHING Recently there has been interest in the use of other sources of alkali in peroxide bleaching, particularly magnesium oxide and calcium oxide. These bases may offer the advantage of decreased pollution problems associated with salinity, which arise when discharging conventional effluents into a fresh water system. Magnesia as an alternative alkali source may be particularly attractive in the Australian context as there are very large deposits of high quality mineral available at Kunwarara in Northern Queensland. In this study, sodium hydroxide and magnesium oxide have been compared as alkali sources in single-stage and two-stage bleaching of Eucalyptus regnans cold caustic soda (CCS) and Pinus radiata thermomechanical pulp (TMP). The effects of peroxide stabilizers have also been studied in the presence of individual alkali. Several studies have been reported on recycling spent liquor in the bleaching process for mechanical pulps using sodium hydroxide as an alkali source. However, no work has been reported in the literature using magnesium oxide as an alkali source employed in multi-stage bleaching of mechanical or chemical pulps. In this work, the possibilities of internal recycling of residual liquor during sequential two-stage peroxide bleaching processes using either sodium hydroxide or magnesium oxide as an alkali have been investigated. This study includes the investigation of various recirculation strategies for bleaching of Pinus radiata TMP and Eucalyptus regnans CCS pulp, without altering the pulp concentration in the first and second stage of the bleaching processes. In general, there was a small to no improvement in brightness response in going from one-stage to a two-stage process, although some exceptions show interesting results. Under certain conditions, recycling residual peroxide produces detrimental effects on the bleaching response of Pinus radiata TMP.