Mondi Ltd. operates a deinking plant at its Merebank mill. The plant recycles 85 000 tons per annum of flat news and magazine to produce a furnish for its newsprint papermachines. A review of the relevant pulping and flotation chemistry literature revealed that the role played by sodium silicate appears to be multi-faceted and in some dispute. Sodium silicate has an undisputed role in pH buffering, hydrogen peroxide stabilisation and the prevention of fibre yellowing. However, its role in deinking is said to be that of an ink collector or alternatively an ink dispersant. 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 in the laboratory under conditions similar to those encountered in a deinking pulper. The resultant dispersions were studied using turbidity and particle size analysis. The morphology of the ink particles was determined using a scanning electron microscope. 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 were studied in an attempt to elucidate the role of sodium silicate. A model system consisting of the sodium salt of collector soap, calcium ions and sodium silicate was studied under the conditions that prevail in a typical newsprint deinking pulper. It was found that the soap and the sodium silicate compete for the calcium ions, and sodium silicate showed a measurable chelating e:ffect on calcium ions. Thus, increasing levels of sodium silicate lead to an increase in the concentration of sodium soap in solution. It was hypothesised that this effect would lead to better dispersion of ink particles and improved deinking performance. This chelating effect was evaluated in laboratory deinking studies. Samples of newsprint were pulped in a 251 Lamort laboratory pulper under a variety of conditions, viz. with fresh water, with an excess of soap, with an excess of calcium, with and without sodium silicate. The pulps were floated in a 201 flotation cell. The brightness and colour of the unfloated and floated pulps were measured. The level of the final brightness after flotation was taken as a measure of deinking efficiency. The highest final brightness was achieved when there was an excess of sodium soap and a low Ca hardness in the pulper. Softening the water used in pulping without adding excess sodium soap did not significantly improve pulp brightness. The lowest final brightness occurred in the presence of an excess of calcium in the pulper. Calcium in the pulper in the presence of sodium silicate did not result in a significantly lower final brightness. The results support the hypothesis that sodium silicate sequesters the soluble calcium in a pulping system, thereby increasing the sodium soap concentration and the resultant deinking performance. Apart from sodium silicate's chief role as a peroxide stabiliser, the sequestering action on calcium appears to be its main mechanism of action in a deinking system. An appreciation of this role will facilitate the optimisation of deinking systems with respect to calcium hardness and silicate concentration. To this end it was recommended that the Merebank deinking plant should evaluate the use of water with a low calcium ion concentration and the addition of some soap into the pulper to improve their deinking plant performance. / Thesis (M.Sc.)-University of Natal, Durban, 2003.

In the absence of any significant legislation, paper recycling in South Africa has grown to a respectable recovery rate of 43% in 2008, driven mainly by the major paper manufacturers. Recently introduced legislation will further boost the recovery rate of recycled paper. Domestic household waste represents the major remaining source of recycled paper. This source will introduce greater variability into the paper streams entering the recycling mills, which will result in greater process variability and operating difficulties. This process variability manifests itself as lower average brightness or increased bleaching costs. Deinking plants will require new techniques to adapt to the increasingly uncertain composition of incoming recycled paper streams. As a developing country, South Africa is still showing growth in the publication paper and hygiene paper markets, for which recycled fibre is an important source of raw material. General deinking conditions pertaining to the South African tissue and newsprint deinking industry were obtained through field surveys of the local industry and assessment of the current and future requirements for deinking of differing quality materials. A large number of operating parameters ranging from waste mixes, process variables and process chemical additions, typically affect the recycled paper deinking process. In this study, typical newsprint and fine paper deinking processes were investigated using the techniques of experimental design to determine the relative effects of process chemical additions, pH, pulping and flotation times, pulping and flotation consistencies and pulping and flotation temperatures on the final deinked pulp properties. Samples of recycled newsprint, magazines and fine papers were pulped and deinked by flotation in the laboratory. Handsheets were formed and the brightness, residual ink concentration and the yield were measured. It was determined that the type of recycled paper had the greatest influence on final brightness, followed by bleaching conditions, flotation cell residence time and flotation consistency. The residual ink concentration and yield were largely determined by residence time and consistency in the flotation cell. The laboratory data generated was used to train artificial neural networks which described the laboratory data as a multi-dimensional mathematical model. It was found that regressions of approximately 0.95, 0.84 and 0.72 were obtained for brightness, residual ink concentration and yield respectively. Actual process data from three different deinking plants manufacturing seven different grades of recycled pulp was gathered. The data was aligned to the laboratory conditions to take into account the different process layouts and efficiencies and to compensate for the differences between laboratory and plant performance. This data was used to validate the neural networks and select the models which best described the overall deinking performances across all of the plants. It was found that the brightness and residual ink concentration could be predicted in a commercial operation with correlations in excess of 0.9. Lower correlations of ca. 0.5 were obtained for yield. It is intended to use the data and models to develop a predictive model to facilitate the management and optimization of a commercial flotation deinking processes with respect to waste input and process conditions. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2011.