EXTERNAL MAGMA PREPARATION BY A SIMPLE DISSOLUTION PROCESS IN PIPE FLOW

THERE have been previous attempts to simplify the magma preparation process, and these have met with varying levels of success. The standard process involves washing in a batch pan by water addition at raised temperatures. The washing process is followed by a period of crystal growth until the crystals reach high grade seed size. Either a dedicated batch pan is used, or the procedure is undertaken to include the duty of several pans. In any case, part of the pan stage capacity is tied up in magma preparation. An alternative process reported by SRI is the external dissolution and cooling crystallisation method. The problem of magma quality originates in the continuous low grade fugals which can shatter some of the crystals and generate a population of small fragments which, if allowed to grow in the high grade pans, will seriously compromise raw sugar quality. The key to successful removal of these crystals is selective dissolution. CSR has carried out washing trials in stirred tanks and has used other simple washing processes, both external to and within continuous high grade pans, with limited levels of success. Dissolution needs to occur in a batch or plug flow regime. The use of back mix flow (inside or outside of pans) superimposes flow dispersion on the crystal population; small crystals may or may not dissolve at all, and large crystals may dissolve to extinction. In batch or in-pipe flow mode, radial mixing should be complete; each crystal should experience the same saturation levels. Finally, the level of undersaturation and the residence time must be appropriate to achieve selective dissolution; the magma dilution syrup or juice must be correctly metered, mixed and remain in contact for sufficient time. This paper describes a successful implementation of these principles at Victoria mill for preparation of the magma feed to a high grade seed pan. Prepared magma showed an increase in mean aperture of almost 90 microns, and a reduction in the coefficient of variation (CV) of approximately 10 %. This was achieved by in-pipe washing of magma with liquor and evaporator supply juice (ESJ), in which the crystal content was reduced from 61.5% to 27.5% on massecuite.