Ian Hamilton, Robert Adrien, Daniel Baer, Geoff Covey,
Dennis Creasy and Steven Harper
A laboratory method has been developed which can reproduce the most important factors in the depositformation in the sulphite stage of a magnefite chemical recovery system. By adjusting conditions it is possible toproduce: scale-free operation, formation of free granules (‘sugar’), or severe scaling. The apparatus can also be used to study or confirm the effects of adding dispersants, changing operating temperature, and changing gas composition.
It was found that the presence of magnesium sulphite hexa-hydrate is a sign of conditions likely to result in fouling. The causes of fouling are complex and are dependent on temperature, organics in the distillate, operating pH, sulphur dioxide concentration in the gas in the mono-sulphite section, and (possibly) inorganic impurities in the magnesia. As such, it is unlikely that a full understanding ofthe process can be obtained at a satisfactory cost. HOWEVER, results to date strongly indicate that some comparatively simple and inexpensive adjustments to plantoperation could greatly reduce the severity of the fouling problem. Figure 2. Build up of fouling on a vessel wall.
Keywords scaling, magnesium sulphite, crystallisation
The Tantanoola mill of Kimberly Clark Australia uses a Wagner Biro magnefite chemical recovery unit. Fouling isalways a potential problem in the high pH stages of magnefite recovery. Most other recovery systems counter this by trying to minimise the formation magnesium mono-sulphite. In contrast, the Wagner Biro system encourages the formation of the mono-sulphite to give multiple nucleation sites so that solids are encouraged to grow in the bulk of the liquid rather than on the walls.
The recovery process includes a number of venturi-style scrubbers, through which the flue gas and slurry are contacted in a counter-current fashion so as to effectively remove the SO2 from the flue gas (see Fig. 1). The bulk of the problems encountered have occurred in the final two scrubbers, units S1 and S2 and associated vessels and pipe-work. The unit S2 is the last contact stage before the flue gas is vented to the atmosphere.
Within the units S1 and S2, the addition of slurry is controlled to promote the formation of magnesium sulphite. The sulphite product produced in these two units then proceeds to further stages where the sulphite is transformed into the required bisulphite. However, at times the solid magnesium sulphite does not stay in suspension as a slurry, but crystallises on the interior walls of the units. At other times a coarse granular material (‘sugar’) is produced. “Scaling” is the result of crystals adhering to surfaces. Build-up of this solid formation can reach levels where the recovery process must be shut down to facilitate cleaning oft he units. Such cleaning often requires physical removal of the deposits which can introduce additional risks to personnel involved in that operation (Fig 2 and 3) and results in disruption to production.
The cause of this solid build-up on surfaces is not clear and may be due to a large number of interconnected variables. The main areas from which the problem may stem from are the slurry introduced into the system, the recycling of condensate from the multiple effect evaporators, and the concentration of SO2 in the flue gas. Within these two broad areas there are many variables that could each contribute to the problem. Therefore, the following objectives were set:
(1) determine the exact nature of the solid material;
(2) examine the effects of components in the plant condensate which could facilitate or inhibit nucleation and crystal growth;
(3) investigate the role of some inorganic species in the magnesium hydroxide slurry, which may affect nucleation and crystal growth;
(4) investigate the effect of the partial pressure of SO2 in solid formation.