NON-PROCESS ELEMENTS IN A BLEACHED KRAFT PULP MILL WITH INCREASED SYSTEM CLOSURE

摘要

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 modern 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 with increasing levels of chloride and potassium. Recovery boiler dust is discharged when there is a need to purge chloride and potassium from the system. Aluminium, silica and calcium are components in scales formed on heat transfer surfaces in the digester and/or the evaporation of black liquor. Calcium and barium compounds can also be expected to precipitate and form deposits in a bleach plant with low or no effluent unless calcium and barium are removed from the pulp prior to the bleaching, e.g. in an acidification stage. Transition elements catalyse the decomposition of bleaching chemicals such as hydrogen peroxide. The dead load of inerts tends to increase in a closed cycle system. Nutritional elements, i.e. N and P, and harmful metals, e.g. Cd and Pb, have an effect on the environment if bleach plant effluents are discharged to the receiving waters.