Many mills experience smelt flow problems, which could lead to dissolving tank explosions and plugged smelt spouts. Liquor chemistry affects the smelt composition, causing changes in the smelt melting point and its viscosity. Higher melting point smelts tend to have higher viscosity at a given temperature. The sulphide content of the smelt has the most significant effect on the smelt’s melting point. The melting point and viscosity decrease as the sulphidity increases to about 40%, but increase rapidly when the sulphidity increases over 40%. Also, when the boiler is operating at very high reduction efficiencies (i.e., low Na2SO4 content in the smelt) the smelt melting point increases significantly. K and Cl enrichment reduces the smelt melting point and the viscosity.
Other factors, such as boiler operating conditions and design may also have an effect. Smelt spout feed water temperatures below BLRBAC guidelines (60-65 oC) can result in low smelt temperatures and high viscosity, or in the solidification of smelt in the spouts. High furnace drafts, and excessive air infiltration around the smelt spouts, could also cool the smelt, with the same result.
Incomplete combustion of the BL organics, or firing a highly viscous BL, could lead to high carbon content in the smelt, raising the smelt melting temperature and viscosity. This can be verified by measuring the suspended solids in the unclarified green liquor. If the suspended solids are over 1200 ppm, this is an indication of unburned carbon or a dregs related problem [1]. Increased BL viscosity can result in the formation of larger droplets, that fall closer to the walls and increase the carbon or dregs content of the smelt. Other factors influencing smelt flow include bed temperature, excessive metals build-up in the smelt, a change in chemical makeup, upsets or control problems with practices that add Cl to the system, and a change of furnish.
1. Karidio, I, et al, A review of the conditions in chemical recovery boilers that result in poor-flowing smelt, 2004 Int. Chem. Rec. Conf.
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