Gibbsite scaling on polymer surfaces

Gibbsite scale build up was studied over a range of polymer surfaces in real Bayer process conditions. For all samples there was an initial slow rate of scale growth and there was evidence that this growth proceeded along scratches on the polymer surfaces. The high density polyethylene, acrylonitrile butadiene styrene and polypropylene homopolymer took up to 3 days to scale while the polyvinylchloride, Nylon 6 and ultra high molecular weight polyethylene required over 7 days to fully scale. Once scale had formed, the mass gain proceeded at a linear rate with time, indicating scale on scale growth. Polishing the surface of the polymers had a dramatic effect on the scale formation. Indeed little or no scaling was observed for the polished samples following 7 days exposure, with the exception of the acrylonitrile butadiene styrene sample. The scaling observed by the acrylonitrile butadiene styrene sample may be related to the instability of this polymer in the conditions observed in the Bayer process. This was the only sample that showed considerable degradation under these conditions. The strength of attachment of the scale to the polymer samples was significantly lower than that observed for steel samples.