Abstract
Black liquor is a by-product of wood digestion process in pulp and paper mills and it is composed by organic and inorganic products. After this digestion process, black liquor has a solid content of 15 wt/% and to be used as fuel in the recovery boiler it is necessary to be concentrated above 75 wt/% of solids. This concentration process is performed in an evaporation plant using multiple effect evaporators. Some black liquor physical properties, such as density, solids content, viscosity and sodium salts content, are strongly dependent on the kind of wood processed (hardwood or softwood) and on operating conditions during the digestion process. Knowledge and comprehension of the relationship between these physical properties of black liquor are essential for studies aiming at a greater energetic performance of the black liquor evaporation unit. When black liquor reaches higher solids content (above 50 wt/%), scaling formation is observed on the heat transfer surfaces of evaporators and concentrators, due to the precipitation of sodium salts, reducing the overall efficiency of this equipment. The aim of this work is to investigate and comprehend the mechanism of formation and precipitation of sodium salts in eucalyptus black liquor evaporators, through the study of the relationship between some physical properties (density, viscosity, total solids content) and the solubility of sodium salts, evaluating the influence of these variables. In this study industrial samples of black liquor taken from different process streams of the evaporation plant were used. A greater comprehension of the mechanism which is responsible for the reduction of the solubility of these salts is very important for the development of alternative methods to avoid or reduce scaling formation. Experimental data were used to develop a model based on chemical equilibrium of salts dissociation reactions, considering also temperature and pH of the system, which is capable of estimating sodium sulphate solubility limit and comprehend the process that causes the formation of the first scaling and the behaviour of the salts in the system. This model may be used to establish some operating parameters, trying to avoid or control scaling formation in industrial evaporators.
