Reactivity of lime in paper manufacture

Abstract

In modern pulp and paper manufacturing processes, lime is used in the recovery and recycling of the chemical reagents. After treatment of wood chippings with aqueous sodium hydroxide and sodium sulphite solutions to separate the lignin and the cellulose, the spent liquor is evaporated leaving sodium carbonate which is causticised with hydrated lime, viz., Ca(OH) 2 + Na 2CO 3 → CaCO 3 + 2NaOH. The sodium hydroxide solution is recycled and the lime is recovered by thermally decomposing the calcium carbonate at temperatures up to about 1000 °C. The calcined product (mainly quicklime) is rehydrated and recycled in the causticiser. The present research deals with the reactivity of the lime in the causticiser, its recovery and rehydration. Surface areas and porosities are determined from gravimetric B.E.T. nitrogen gas sorption recorded on a vacuum microbalance (μg to mg sensitivity using 0.25 g samples or less); changes are correlated with experimental conditions. Phase composition and microstructural changes are monitored by thermal analysis. X-ray diffractometry and electron-microscopy. It is important to avoid “dead-burning” the lime when it is recovered from the calcium carbonate in the presence of sodium salts (ca. 2 %) which promote sintering, so that optimum calcination temperatures and times must be determined.