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

Abstract

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 plays an important role. Lime is slaked with green liquor during causticization reaction and converted to calcium carbonate during the process. The overall efficiency of lime in the chemical recovery system is strongly dependant on the amount of different impurities or non-process elements that are present. These non-process elements are chemical elements or compounds originating from various sources, for example from the wood raw-material, used make-up lime or from the internal process sources such as piping corrosion and they do not participate in the main process in any beneficial way, hence are producing inert dead-load to the circulation, which harmfully effects the whole operation performance of the system. The aim of this literature study is to present some of the current solutions for removing non-process elements and to also consider new possibilities for reducing impurities from the lime cycle.

Conventional way of reducing these non-process elements is to simply purge some amount of spent lime mud from the process from selected purge points within the lime cycle and by doing so also reduce the amount of circulating impurities. However, where being relatively in-expensive method, the purging of lime mud and treating it as a waste looses all the potential of the spent lime mud. Tightening environmental regulations for plant waste generation are also driving the need of finding alternative ways for doing the recycling of these solid plant wastes such as lime mud. Based on the conducted brief economical analysis, depending on the selected method, if proven to be suitable for process environments, the economical impact can be significant. In the end the spent lime mud can generate positive, negative or zero price depending on its end-of-life use.

There exist several possibilities for recycling the spent lime mud either back to process viable raw-material or to potential product used in alternative places. Most of these methods, presented in this thesis are already widely researched in the past, where some are introduced as new ideas that would potentially require further studies with possible detailed experimental setups. In this thesis, the methods were presented as singular process steps and due to the theoretical approach their practical functionality wasn’t evaluated. Based on the results some of the presented methods might only prove to be functional and effective when used as combination of each other.