Using statistical experimental design techniques to determine the most effective variables for the control of the flotation deinking of mixed recycled paper grades

Abstract

Waste paper recycling in South Africa has grown to a respectable recovery rate of 43% in 2008. Pending 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 waste streams entering the recycling mills, which will result in greater process variability, lower quality and operating difficulties. In this study, a typical deinking process was 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 newsprint, magazines, and two grades of mixed office waste were pulped and deinked by flotation and washing in the laboratory. Handsheets were formed and measured for brightness, residual ink concentration and yield. A Plackett-Burman experimental design was applied and the net effects of the various process parameters were determined. The net effects for all grades and process parameters were ranked in order of magnitude. It was determined that the grade of recycled paper had by far the greatest influence on final brightness, followed by the effect of the deinking process and then by the effect of the individual process variables. Within the process variables, the order of influence on brightness was flotation conditions, alkalinity and addition of bleach. The residual ink concentration was largely determined by flotation conditions and alkalinity. The yield was determined only by flotation conditions. Pulping time and pulping temperature were involved in many interactions between variables. The results are expected to be useful in developing a neural network control strategy to deal effectively with the challenges associated with a variable waste paper raw material stream.