Application of Biomass-based Fly Ash in the Pressate of a Thermomechanical Pulping Process

Abstract

In the thermomechanical pulping (TMP) process wood chips are pretreated with steam prior to refining, and the resultant effluent (TMP pressate) is currently treated in the wastewater treatment facility of the pulp mill. Pulp mills also generate a substantial amount of biomass fly ash as a residue from burning wood and wastewater sludge in boilers and most of the fly ash is landfilled. Strict effluent regulations may require the use of tertiary treatment operations at pulp mills and alternative uses for biomass fly ash are required. In this study, two biomass fly ash samples (FA1 and FA2) were received from a pulp mill, fractionated and characterized. Definitive screening design (DSD) experiments were conducted to evaluate the impact of process parameters (e.g. time, temperature) on the chemical oxygen demand (COD) and lignin contents in a TMP pressate. Model equations were developed and used to optimize the COD and lignin removals from a TMP pressate on the fly ash samples. It was determined that the main factor that impacted the lignocelluloses removal from a TMP pressate was dosage, and FA1 was more effective than FA2. A maximum COD removal by FA1 of 91.3 % was observed under the conditions of a weight-based average particle size of 0.43 mm, a dosage of 70.0 mg/g FA1/TMP pressate and a treatment time of 2 h. The maximum lignin removal from the TMP pressate by FA1 was 95.0 % under the conditions of a weight-based average particle size of 0.11 mm, a dosage of 46.5 mg/g FA1/TMP pressate and a pH of 6. In this study, a biomass fly ash was fractionated and ground to generate five fly ash samples with different compositions of similar particle size, and four fly ash fractions of different sizes of similar composition. Isotherm analysis with respect to the COD and lignin removals from a TMP pressate by the biomass fly ash samples were done and it was determined that the Freundlich model fit the adsorption data best. The Freundlich maximum adsorption capacities for the COD and lignin from a TMP pressate by the biomass fly ash were estimated. Furthermore, the potential impact of various physicochemical properties on the experimentally determined Freundlich parameters were evaluated. In this study, a biomass fly ash was fractionated and ground to produce two fly ash samples (S1 and S2) with a similar particle size. Leaching tests of the biomass fly ash samples were conducted under various conditions in water and in a TMP pressate. It was determined that the leaching characteristics of the biomass fly ash in a TMP pressate differed from that in water and overall it was observed that the total metal content leached from the biomass fly ash was greater in water when compared with that in a TMP pressate under the conditions studied. The potential use of a biomass-based fly ash adsorbent on a TMP pressate was then assessed against the wastewater effluent guidelines of the metal mining sector. In this study, the coagulation performance of various commercial coagulants and a biomass fly ash leachate (FLC) on a TMP pressate was also assessed. The effect of coagulant dosage, pH, agitation speed and treatment time on the chord length distribution of the resultant coagulant/TMP pressate solutions was monitored using focused beam reflectance measurement (FBRM) analysis. The COD and lignin removals of the coagulant treatments on a TMP pressate were also determined. A FLC-based treatment of a TMP pressate removed 18.4 % of the COD and 26.9 % of the lignin under the conditions of 5060 mg/kg FLC/TMP pressate, 200 rpm, pH 12.5 and 298 K for 30 min. Coagulation of a TMP pressate using alum removed 41.2 % of the COD and 40.6 % of the lignin under the conditions of 1000 mg/kg Al2(SO4)3/TMP pressate, 200 rpm, pH 6 and 298 K for 30 min.