Availability of accurate models for prediction of the viscosity of black liquor (BL) from the chemical pulping of pine will facilitate its online monitoring and control and subsequently the optimisation of combustion in a recovery boiler (RB). While in some parts of the world, BL viscosity models have been developed, New Zealand (NZ) BL viscosity data is limited and no predictive model is available.

In this paper, the results of the rheological characterization of NZ BL at low solid concentrations (SCs) are presented. A bulk sample (SC>50%) was obtained from a New Zealand pulp and paper mill. From this batch, dilute samples (~15-44%SCs) were obtained. Their viscosities were determined using a Haake RV20 Viscometer at(25-85)oC and shear rates up to ~2000s-1. The samples showed Newtonian behaviour (shear-independent).

Linear correlations proposed by Zaman & Fricke,and Ghosh were used to fit the NZ BL viscosity data as a function of SC and temperature. Accuracies of the models were examined for both the log-transformed and the untransformed viscosity data using coefficient of correlation (R) and maximum absolute relative error (MARE) (between the actual and predicted viscosities) respectively as indices. While these models fit NZ BL viscosity data well when they were log-transformed, they performed poorly when not transformed. Consequently, a new binomial model (linear) was proposed to fit the data; an accurate third-degree polynomial (R=.9997; MARE=5.7%) was found.

It is concluded that at low SCs, NZ BL is a Newtonian fluid whose viscosity can be accurately predicted using a third-degree binomial model.