Literature study of present and new methods for reducing non-process elements in the lime circulation of a kraft pulp mill

摘要

Kraft pulping has for long relied on its chemical recovery system in regenerating cooking liquors.One important process part in this chemical recovery system is the lime cycle, where lime playsan important role. Lime is slaked with green liquor during causticization reaction and converted tocalcium carbonate during the process. The overall efficiency of lime in the chemical recoverysystem is strongly dependant on the amount of different impurities or non-process elements thatare present. These non-process elements are chemical elements or compounds originating fromvarious sources, for example from the wood raw-material, used make-up lime or from the internalprocess sources such as piping corrosion and they do not participate in the main process in anybeneficial way, hence are producing inert dead-load to the circulation, which harmfully effects thewhole operation performance of the system. The aim of this literature study is to present some ofthe current solutions for removing non-process elements and to also consider new possibilities forreducing impurities from the lime cycle.Conventional way of reducing these non-process elements is to simply purge some amount ofspent lime mud from the process from selected purge points within the lime cycle and by doing soalso reduce the amount of circulating impurities. However, where being relatively in-expensivemethod, the purging of lime mud and treating it as a waste looses all the potential of the spentlime mud. Tightening environmental regulations for plant waste generation are also driving theneed of finding alternative ways for doing the recycling of these solid plant wastes such as limemud. Based on the conducted brief economical analysis, depending on the selected method, ifproven to be suitable for process environments, the economical impact can be significant. In theend the spent lime mud can generate positive, negative or zero price depending on its end-of-lifeuse.There exist several possibilities for recycling the spent lime mud either back to process viableraw-material or to potential product used in alternative places. Most of these methods, presentedin this thesis are already widely researched in the past, where some are introduced as new ideasthat would potentially require further studies with possible detailed experimental setups. In thisthesis, the methods were presented as singular process steps and due to the theoretical approachtheir practical functionality wasn’t evaluated. Based on the results some of the presented methodsmight only prove to be functional and effective when used as combination of each other.