Toward closed-cycle kraft : Canadian mill case studies

Three case studies were developed for retrofit of closed-cycle designs into existing Canadian bleached kraft mills. These were based on a technical and engineering analysis of four partner mills. These mills presented a range of process and equipment ages from the 1960s to the late 1980s. The case studies consider both integrated and separate bleach effluent recovery from ECF and TCF bleach plants. Development of these case studies indicated that implementation of closed-cycle operation in existing mills may involve substantial capital costs, with the case study estimates at CAD 75-90 million (in 1995 dollars) per mill or US$ 216-415 per annual airdried ton. For these case studies, the increase in operating costs was generally that required for servicing the cost of capital, as other operating costs remained similar. Closed-cycle operation appears feasible, although there are still a number of technical issues to resolve. These issues include pitch, scales, corrosion, and operating dynamics. Application : the case studies provide a validated approach and some initial chemical and cost data to assess and compare ECF and TCF closed-cycle technologies in existing mills. The relationships between the fiber line, evaporation, recovery, and recausticizing are considered and explored.