This study focused on the characterisation and utilisation of coal fly-ash and other waste materials as a raw material to develop cost effective and production friendly procedures for the production of refractory thermal insulating products by casting.
The properties of the newly developed geopolymeric porous insulating refractory material are compared to traditionally manufactured insulating refractory materials as well as to the specifications for porous insulating refractory materials (Table 5.1).
Table 5.1: Comparison of physical properties of traditional and geopolymeric porous insulating refractories
Property Geopolymeric Porous Insulating Refractory (GPIR) Traditionally manufactured porous ceramic material Specification for porous insulating refractories Comments on compliance to specification
Total linear shrinkage
(%) 1.32 4 1 to 7
GPIR the best
Strength (MPa) 13.84 7.89 0.31 GPIR the
best
Density (g.cm-3) 0.58 0.94 0.74 GPIR the
best
Porosity (%) 51 53 45 to 75 GPIR within
the limits
Service temperature (°C) 1220 1450 1150 to 1261 GPIR within
the limits Thermal conductivity
@1100 °C ( W.m-1K-1) 0.462 1.068 2.6 – 2.8
GPIR the best
The comparison shows that the geopolymeric porous insulating refractory (GPIR) values comply with the specifications for porous insulating refractories.
The outcome of this study provided an insulating refractory material to be used in applications up to 1250 °C. The geopolymeric methods drastically increased the strength of the insulating refractory materials to make automatisation of the process possible. The process is rapid and the in situ foaming of the geopolymer results in high closed porosities and thus good thermal conductivities (Figure 5.1). In situ forming of monolithic geopolymeric porous insulating refractory, benefits material storage, handling and mould availability.
Figure 5.1: Thermal conductivity of insulating fire brick and insulating
castables (Carniglia & Barna, 1992). STL indicating the Service Temperature limit (in °F) of the Insulating Fire Brick (IFB) and Geopolymeric Porous Insulating
Refractory (GPIR)
The process is economical, as the major raw materials are inorganic waste materials. The use of these materials further helps to resolve the problem of waste disposal and benefits the environment.
The utilisation of these inorganic waste materials for the manufacturing of light- weight porous insulating refractory materials, should lead to the preservation of natural resources. The environment will benefit from this utilisation as the demand on mining activities for raw materials will be reduced and discarded waste that degrades the environment will be utilised.
A geopolymeric porous insulating refractory was successfully developed using the inorganic waste material, coal fly-ash, as the main ingredient. Phosphogypsum and iron-rich waste is not utilised in the manufacturing of insulating refractory materials as it impacts negatively on the thermal properties of the insulating material. These other waste materials are successfully incorporated in water filters from inorganic waste materials – another project within the research group. This developed insulating refractory material complies with the specifications (Table 5.1) laid down for a porous insulating refractory material. The developed process delivers a very good product without shifting the waste disposal problem by creating more or different waste products detrimental to the environment and the developed manufacturing process is economical.
The specific objectives of the project were all achieved, namely
• Characterising inorganic waste materials with regard to their physical and chemical properties relevant to refractories.
• Investigation of different mixtures to result in a mix formulation complying with the laid down specifications and
• An economical manufacturing route for porous insulating refractory materials made of inorganic waste was found that lend itself to the automatisation of the manufacturing process.
5.2 RECOMMENDATIONS
By employing the following process steps in the geopolymerisation of the inorganic waste materials a very good economical porous insulating refractory material would be available on the market.
• Dissolve 10.53 % sodium hydroxide into 10.53 % of water. This reaction is exothermic.
• Leave the solution to cool. • Add 15.78 % sodium silicate.
• Add 10.53 % of meta-kaolin to the solution. • Stir for 5 minutes.
• Add 55.63 % coal fly-ash while stirring the mixture. • Add 0.005 % aluminum powder as the porogen.
• Cast the geopolymeric material into position or into moulds. • Setting will occur within 3 hours.
• Leave to dry at room temperature for ± 24 hours.
• A heating procedure of 2.5 °C per minute may follow up to service temperature of the product.
• The shrinkage which will occur will be as little as 0.3 % allowing for the formwork to be removed if needed before firing.
The hypothesis of developing a geopolymeric porous insulating refractory using inorganic waste materials as the main ingredient was successfully accomplished.
The successfully developed porous insulating refractory material from inorganic waste materials is a suitable candidate to replace fireclay insulating materials or other insulating materials in high temperature application vessels, used in industry. The newly developed procedure is cost effective and manufacture can be easily automated.
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