Pulp and Paper Mill Fly Ash: A Review

Abstract

The continual growth of pulp and paper industry has led to the generation of tremendous volumes of fly ash as byproducts of biomass combustion processes. Commonly, a major part of it is landfilled; however, updated environmental regulations have tended to restrict the landfilling of fly ash due to rising disposal costs and the scarcity of suitable land. The pulp and paper industries are therefore urgently seeking energy-efficient mechanisms and management for the beneficial use of fly ash in an ecological and economical manner. This paper offers a comprehensive review of existing knowledge on the major physicochemical and toxicological properties of pulp and paper mill fly ash to assess its suitability for various bound and unbound applications. The current state of various methods used for the valorization of pulp and paper mill fly ash into more sustainable geomaterials is briefly discussed. This paper also presents promising and innovative applications for pulp and paper mill fly ash, with particular reference to agriculture and forestry, the construction and geotechnical industries, and the immobilization of contaminants. It was identified from a literature review that modified pulp and paper mill fly ash can be environmentally and economically advantageous over commercial coal-based fly ash in various sustainable applications.Sustainability 2019, 11, 4394 2 of 16 use of compost products, etc., are drawbacks of composting [10]. Contaminant-free PPFA has great significance as a soil liming agent and fertilizer in agriculture and forestry [6,11]; however, its direct application requires safety precautions due to its undesirable handling and spreading characteristics and associated health risks [12,13]. Further, high pH and electrical conductivity values of pore water in wood ash might have perturbing effects on the microbial community [14]. In order to withstand extreme conditions, the bacteria may enter a dormant stage, thereby decreasing the microbial population and diversity and ultimately disturbing the ecological balance. PPFA and value-added materials also have multifunctional engineering applications, such as supplementary cementitious material (SCM) in concrete systems, aggregate in pavement construction, binder for soil stabilization, and adsorbent for the immobilization of toxic heavy metals [15][16][17][18][19][20][21][22][23]. Based on a statistical survey conducted on the management and utilization of boiler ashes generated at Canadian pulp and paper mills, it was reported that more than 50% of ash is landfilled, 20-25% is used as a soil amendment (direct application or compost), and less than 20% is used for other beneficial applications such as the construction of embankment fills, the stabilization of pavement layers, and the solidification of wastes. [1,2].This paper reviews the existing knowledge on physicochemical and toxicological properties of PPFA in order to assess its suitability as a sustainable geomaterial for various bound and unbound applications. The bound applications of fly ash in...

Inorganic Waste Generated in Kraft Pulp Mills: The Transition from Landfill to Industrial Applications

“…Moreover, alumina (Al 2 O 3 ), iron oxide (Fe 2 O 3 ), periclase (MgO) and sylvite (KCl) also constitutes the BFA in variable amounts. In fact, BFA from coniferous trees tends to contain a higher amount of Si and Ca when compared to hardwood trees and relatively low K and S content [32]. The high calcite and aluminosilicate contents seem suitable as a cement replacement material and as aggregates for road construction [32].…”

“…In fact, BFA from coniferous trees tends to contain a higher amount of Si and Ca when compared to hardwood trees and relatively low K and S content [32]. The high calcite and aluminosilicate contents seem suitable as a cement replacement material and as aggregates for road construction [32]. Attention should be paid to sulfate and chloride content, which can be harmful in concrete applications.…”

“…However, transportation costs can be higher, as well as the difficulties in mixing with other powdered materials (for example, clay, and cement). BFA is usually generated with very low moisture, while showing high hydrophilic properties and easily forming agglomerates [32]. A key property to define the best application of a specific material is the pH.…”

“…However, this property may also be positive in certain applications, for example, to be used as neutralizing agents, liming applications or as operating supply in applications that normally use alkaline raw materials (e.g., cement). BFA reveals an average pH of 11, but can vary between 8 and 13 [32].…”

“…Nevertheless, sulfate can exceed the limit for inert waste landfills (1000 mg/kg dw), since the value found was 5250 mg/kg. Additionally, Cherian and Siddiqua [32] concluded that fly ash from pulp and paper mills is in general considered non-hazardous, since PTM are mainly held in the amorphous aluminosilicate phases, and thus with low solubility. Ribeiro et al [49] produced a glass from BFA and the material exhibited a satisfactory leaching behavior.…”