An Assessment of Reuse of Light Ash from Bayer Process Fluidized Bed Boilers in Geopolymer Synthesis at Ambient Temperature

Received Mar 31, 2018; Accepted May 20, 2018

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1. Introduction

The chain for producing aluminum from bauxite to primary aluminum includes refining using the Bayer process. One of the main waste products of the Bayer process is the fly ash collected from the cyclone overflow of the fluidized bed boilers at operational conditions of 900°C of temperature and 120 kPa of pressure. The pressure and temperature conditions influence the specification and morphology of the ash waste generated [1].

In the Bayer process, the circulating-type fluidized bed boilers operate at specific conditions of 900°C and 120 kPa as a means of guaranteeing maintenance of the bed and generate a specific fly ash characteristic of that process. Fly ashes are classified into two types. These are C type class with a higher amount of calcium oxide and F type class with a lower amount of calcium oxide [2].

When the sum of the levels of silica, alumina, and iron oxide is greater than 70%, the fly ash is classified as class F [3].

Geopolymers are synthesized at a different temperature by the alkaline activation of aluminosilicates derived from natural minerals, calcined clay or industrial by-products 2008. This activation is generally done with metakaolin silicates of sodium or potassium. Geopolymers are inorganic ligands with good resistance to high temperatures and degradation by acid, as well as good mechanical properties. They are thus an attractive alternative to Portland-type cement, and their use makes it possible to recycle large quantities of industrial wastes. The mechanical properties of geopolymeric materials depend upon the alkaline cation (Na⁺), the SiO₂/Al₂O₃ molar relation as used by Davidovits and the conditions under which the reaction occurs [4, 5].

The term “geopolymer” describes the chemical properties of aluminosilicate-based inorganic polymers. Geopolymers present properties of cement and therefore have great potential for use in the civil construction industry [6].

The water trapped in a network of geopolymers generates porosity, which results in a reduction of mechanical properties. Their three-dimensional structure is made up of SiO₂ and MAlO_4t tetrahedrons, where...