Assessing the mixing quality in pulp suspensions is very important to pulp and paper processing because inhomogeneities lead to a low quality product. In the past, mixing has been assessed using a number of techniques including:​ The uniformity of inert tracers added to the suspension;​ the distribution of residual bleaching chemical following mixing and reaction;​ the temperature uniformity around the periphery of process piping;​ and by benchmarking the bleaching stage using various pulp quality parameters. Many of these techniques require removing and testing samples, which is a tedious procedure and it gives no indication of the variability of the mixing process over time. Furthermore, the major disadvantage with most of the techniques is that the response or sampling time is too large. Therefore, the objective of this work is to develop an on-line sensor to assess mixing quality.

An on-line technique is described using an in-situ fibre optic probe. The technique measures the concentration of an inert ultraviolet fluorescent tracer added to the pulp suspension at the ppm level. The method has been tested in the laboratory and has been shown to give mixing quality indices that agree with other methods. The applicability of this technique in a mill is discussed;​ however, there are limitations in using this technique in an aggressive bleaching environments, especially in the presence of C102. This technique can easily be applicable in assessing mixing in stock chests, in the headbox of the paper machine, and in the extraction stage of the bleach plant.