(718f) Effluent Reduction in Pulp and Paper Manufacturing: White Water Recycle in Bleaching Section

The pulp and paper industry is one of the largest consumers of water in the US industrial sector. Water is an integral part of paper manufacturing, starting from wood preparation to fiber conditioning and classification. Over the past thirty years, mills across the United States have made great strides to reduce their freshwater usage. In recent years, there has been an increase in the industry for the adoption of waste minimization strategies covering various aspects and avenues in mill operations. Of the various process areas in a typical integrated bleached Kraft mill, the paper machine and pulp production areas account for around 75% of the total mill freshwater consumption, making it a low hanging fruit for water conservation.

In this work, we used Windows General Energy and Material Balance Systems (WinGEMS) to develop a process simulation model of an integrated softwood line of a Kraft pulp mill. The model includes the bleaching section, the stock preparation section, and the paper machine section. The simulation model estimates the material flows, including Non-Process Elements (NPEs) and water, and the energy flows. We investigated the impact of replacing freshwater use in the final bleaching stage shower washer with white water from the paper machine on the overall freshwater consumption and on the NPEs flowrates.

Steady-state equations for the Elemental Chlorine Free (ECF) bleaching sequence D₀EopD₁EpD₂ are included in the simulation. Industry and literature data were used to verify the ECF bleaching simulation model, and the difference was in the range of 1.8 to 5.5% for final stage brightness and the post first extraction pulp kappa number, respectively. The simulation model prepared can predict changes in the kappa number, brightness and the chemical oxygen demand (COD) values with changes in the operating parameters. Simulation of the stock preparation section includes the filler (CaCO₃) addition, cleaners and screening compound blocks. The paper machine section is simulated with the approach system, headbox and sheet forming section as compound blocks. The next step includes the use of Visual-MINTEQ (V-MINTEQ) software to determine the distribution of Ca²⁺ ions in the paper machine white water. This comprehensive model for the bleaching, stock preparation and paper machine section along with Ca²⁺ ion distribution allows the assessment of the extent and impact of white water recycling from the paper machine section into the bleaching section without compromising the final paper brightness.