Feasibility of recycling pulp and paper mill sludge in the paper and board industries

Abstract

Pulp and paper mills typically generate significant quantities of non-hazardous solid waste which require management as a waste material or as a by-product. Most of these solids are removed after primary mechanical treatment, resulting in a sludge that contains large quantities of fibers, papermaking fillers, or both. Although this primary sludge is commonly landfilled, it could be recycled into production on-site, reused in other pulp and paper mills, or used in other products. In an effort to explore these possibilities, the fiber content, fiber quality, and key physical and chemical properties (humidity, ash content, abrasiveness, drainability, and O2 uptake) of 20 different primary sludges sampled in European mills are reviewed in this paper. Although sludge characteristics are highly variable across pulp and paper mill processes, sludges can be considered to fall into two main types: high-ash sludge (>30% dry weight) and low-ash sludge (<30% dry weight). Results of paper tests (caliper, breaking length, tear index, elongation, bursting strength, stiffness, opacity, whiteness, and porosity) and board tests (ring crush test, Concora medium test, corrugated crush test) suggest that at least 12 of the sludges studied could be reused in the paper and board industry.

The results make it possible to differentiate three primary sludge grades: the first needs little cleaning and has appropriate strength properties to be a component of printing and writing papers, tissues, and wrapping papers; the second requires cleaning, bleaching, or both and has appropriate strength properties for applications that do not require high brightness, such as corrugated board, boxboard, and some tissue grades; the third requires cleaning and has limited strength properties, but could be used in some mills that operate using closed water cycles because the final product can tolerate a certain degree of dirt and contamination, as in some packaging and construction-paper grades. Primary sludges share several features in common with recycled paper, and therefore these two materials could be managed together.