Synthesis and characterization of hydrotalcite from South African Coal fly ash

Abstract

The synthesis of adsorbents such as hydrotalcites (HT) from mineral waste (e.g. blast furnace slag) is receiving increasing attention. This study reports a novel procedure to prepare HT from a South African coal-combustion fly ash, an ubiquitous waste generated by coal-fired power stations. The second point of novelty is the optimization process, which highlights the boundary conditions for crystallization of this mineral phase. The parameters investigated for the optimization of HT synthesis from fly ash included HCl concentration, aging time and temperature, pH during the aging step, and crystallization time and temperature. The most suitable synthesis conditions were: HCl concentration of 3 M, aging time of 30 min, aging temperature of 65 °C, pH of 11.5 during aging, crystallization time of 12 h, and crystallization temperature of 70 °C. HT had a large external surface area and very low microporosity. Morphologically, synthesized HT was mostly made up of sub-micron plate-like particles. The structural characteristics of HT synthesized from fly ash were similar to those of HT produced from analytical grade chemicals, except for the presence of calcite. A novel use of the waste South African Class F fly ash was a suitable feedstock for the synthesis of high quality HT under the optimized process conditions that minimized the formation of secondary undesired mineral phases such as calcite or hydrogarnet