Non-Process Elements in a Bleached Kraft Pulp Mill with Increased System Closure, 1996 Minimum Effluent Mills Symposium Proceedings

Per Ulmgren

STFI

A large number of non-process elements (NPE’s) is taken into the kraft pulp mill mainly with the wood chips. However, what is taken in must also be taken out, but in a controlled manner. “Kidneys”, i.e. means for purging NPE’s, are essential in a highly closed system. Beside the existing kidneys in a modem kraft pulp mill, e.g. equipment for removing green liquor dregs, lime mud, precipitator dust and bleach plant effluents, there is a need for new kidneys in order to avoid problems arising from the accumulation of NPE’s in a closed cycle kraft pulp mill. In a system with a high degree of closure, the levels of organic and inorganic substances and the need for cooling tend to increase. The elements of concern are primarily those that do not form insoluble inorganic compounds in alkaline solutions, i.e. Al, Si, Cl and K. But transition elements, i.e. Mn, Cu and Fe, and the alkaline earth metals Ca and Ba also have to be handled in an adequate manner. In a new kidney, acidic (from acid treatments, ozone stages and chlorine dioxide stages) or neutral (from chelating treatments) bleach plant effluents are cleaned from many NPE’s by pH adjustment to 10 to 12 by the addition of lime mud, lime and/or some other alkali source. Many NPE’s are then precipitated as insoluble inorganic ‘compounds which can be separated by filtration and purged from the system together with the green liquor dregs. Bleaching effluents rich in NPE’s can in this way be cleaned of thees NPE’s soluble in weak alkaline solutions, i.e. Al, Si, Ca, Ba, Mn, etc, in the new kidney before further usage e.g. in the chemical recovery area or in the bleach plant. There are many types of process problem associated with NPE’s. The main problems associated with higher levels of chloride and potassium in the liquor cycle of the chemical recovery area are corrosion and plugging of the flue gas passages in the recovery boiler. This is due to the decrease in the melting and “sticky” temperatures of the deposits on the tubes...