SYSTEM CLOSURE POSSIBILITIES FOR MODERN ALKALINE PULPING PROCESSES

The closure of the loop of pulp mills means that not only the pulping chemicals and the alkali used in the bleach plant have to be recovered , but in addition non-process elements have to be removed from the cyc le. For closing the loop, it is important to perform pulp bleaching wh ile maintaining a homogeneous pH and temperature profile in the differ ent stages. These prerequisites can be achieved more easily in TCF tha n in ECF bleaching. The application of catalysts in peroxide containin g bleaching stages renders possible a bleaching sequence which accompl ishes these prerequisites. The fully bleached pulps have brightnesses which correspond to the market re-quirements, and their strength prope rties are equivalent to those of ECF-bleached pulps. For ASAM as well as for Kraft pulping the replacement of black liquor burning in a redu cing atmosphere by a pyrolysis technique is favorable. By splitting th e process chemicals into sodium and sulphur compounds it is possible t o apply consequently the principles of extended delignification, and a ll bleaching stages can be supplied with alkali In the ASAM process, t he application of pyrolysis technique facilitates the chemical recover y process.