Measurement of free lime content in lime mud

Overliming and the consequent presence of unreacted lime (or free lime) in the lime mud are commonly believed to be the cause of many problems in the operation of causticizing plants and lime kilns in kraft pulp mills. The free lime content in lime mud is typically determined in mill laboratories using a so-called ammonium chloride (NH4Cl) method and in commercial laboratories using a thermal decomposition (TD) method. Over the years, we analyzed many lime mud samples from mills and found that the free lime content was consistently low, < 3 wt% calcium hydroxide (Ca(OH)2), even in cases where overliming was suspected to have caused problems. A systematic study was therefore conducted to investigate the validity of free-lime measurement methods, the reason for the consistently low free lime content in lime mud, and if free lime values can be used to indicate overliming. The results show that the NH4Cl method is not suitable for determining free lime. The TD method is good, but the possible interference of magnesium hydroxide (Mg(OH)2) must be taken into account. Since most pulp mills perform their free lime analysis on mud samples collected from pre-coat filters which have been washed, the resulting free lime value is low, and thus, cannot be used to assess the extent of overliming in the causticizing plant.