The purpose of brown stock pulp washing is to remove soluble impurities from a pulp suspension, recover cooking chemicals and incinerate organics for energy recovery. Brown stock washing is a notable sub-process in chemical pulping because it has effects on the subsequent treatments of the pulp, and it is also the first step in the chemical recovery cycle. Without effective washing, the economic viability of chemical pulp production is affected. The performance of washing can be controlled in two main ways: using wash loss, which describes the amount of washable compounds in the pulp suspension which could have been removed in washing; or the dilution factor, which represents the net amount of water that is added during washing. The amount of sodium in the pulp suspension after washing has typically been used as an indicator of wash loss, usually expressed as kg Na_2SO_4/oven dry ton of washed pulp. Other common measurement methods are conductivity, COD, TOC, dissolved dry solids and lignin concentration. In this study, we investigated Scandinavian pulp mill softwood and hardwood fiber lines using various measurement methods in the pulp phases. The purpose was gain a better understanding of wash loss. The concentration of dissolved dry solids in the streams was measured using a refractometer and laboratory scale. In addition, we measured pH, conductivity, lignin content, TOC and COD to look for any correlations. The results indicated that using different methods, the level differences remain quite constant, but using individual measurement it is difficult to control the washing process correctly. The measurement methods must also be continuous and reliable in order to be used to monitor and control washing.
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