The role of sodium

The role of sodium silicate in the flotation deinking of newsprint at Mondi Merebank Jimmy Pauck and Professor Jeremy Marsh The deinking process employed at Merebank is outlined and illustrated. A brief literature review of the relevant pulping and flotation chemistry is presented, with particular reference to the role played by sodium silicate. Sodium silicate's role appears to be multi-faceted and in some dispute. The mill's own experience has shown that the sodium silicate plays a vital role in the deinking process. Sodium silicate's ability to disperse ink, both alone and in the presence of calcium ions and fatty acid soaps, was investigated using a model ink system. A representative newsprint ink base was dispersed under a variety of laboratory conditions and the resulting dispersions were studied. Sodium silicate proved to be a poor disperser of ink particles, but nevertheless appeared to greatly influence the dispersing properties of the soap in the presence of calcium ions. The nature of the interactions between sodium silicate, calcium ions and the collector soap is being studied in an attempt to elucidate the role of sodium silicate. Although the work is not complete, early indications are that the sodium silicate competes with the soaps for calcium ions by virtue of the common ion effect. This results in a higher concentration of soap in the pulping process than would be the case if sodium silicate were not present. It is postulated that this would lead to improved dispersion of the ink and hence improved removal in the flotation process. The recycled fibre plant at Mondi Paper's Merebank mill was built in 1990. The plant consists of equipment supplied mainly by LaMort and Voith. The plant recycles 85 000 tons per annum of waste paper and produces about 70 000 tons per annum of deinked newsprint pulp.