Treatment of biomass ashes for sustainable use in geopolymer composites and zeolites

Abstract

The goal of this research work is to find novel treatment and recycling methods for solid secondary materials from the pulp and paper and mining industries, i.e., biomass fly ash and bottom ash, coal fly ash, green liquor dregs, mine tailings, and electric arc furnace steel slag. For this purpose, these materials were characterized and pre-treated for the preparation of geopolymer composites and zeolites. The first geopolymer composite recipe (GP1) employed biomass bottom ash and mine tailings as aggregates and metakaolin as a binder. In the second recipe (GP2), biomass fly ash was used as a binder with commercial coal fly ash and mine tailings as aggregates. The 28-day compressive strength and flexural strength of GP1 was 25.1 MPa and 8.1 MPa respectively, which is sufficient for use as a construction material. GP2 exhibited compressive strength and flexural strength of 14.1 MPa and 3.5 MPa respectively and may find application in paving flags and concrete curbs. Two biomass fly ashes (FA1 and FA2) from different sources were used in another study which investigated the dissolution kinetics of aluminosilicates in alkali solutions. The effects of NaOH concentration, dissolution time, and temperature were investigated. In addition to its faster kinetics, the maximum dissolved Al concentration was higher than that of Si, independent of the applied conditions. Precipitation of Al and Si, which limits their simultaneous dissolution at high temperatures, was prevented at a constant temperature of 30 °C. The best-fitting models for the dissolution of Al were the Kabai and 1-dimensional diffusion (1DD) models, while the Jander and Ginstling-Brounstein models best explained Si dissolution. Another recycling method for the chosen biomass fly ash (FA2) was through synthesis of zeolite adsorbents and their use in the removal of Cu2+, Cd2+, Zn2+, and Pb2+. The optimum NaOH/ash ratio and crystallization temperature for NaP (Zeolite P) synthesis was 2/1 and 120 °C, respectively. The optimum NaOH/ash ratio, crystallization temperature, and time for LTA (Zeolite A) synthesis were 1/1, 90 °C and 12 h, respectively. The maximum adsorption capacities of NaP and LTA zeolite respectively were 43 mg/g and 140 mg/g for copper, 117 mg/g and 223 mg/g for cadmium, and 534 mg/g and 851 mg/g for lead. The adsorption kinetics of all four metal ions were explained with the pseudo-second-order model for both zeolite types; the Sips and Redlich-Peterson models described the adsorption isotherm for all investigated metal ions.