Bauxite ‘red mud’ in the ceramic industry. Part 1: thermal behaviour

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Abstract

Samples of red mud, by-products of alumina production from bauxite, are studied in the 120–1400°C interval. An extensive characterization was performed by thermal and X-ray diffraction analyses. The identification of gaseous species released upon heating was carried out by coupling the thermal analizer with a gas-chromatographic/mass spectrometer. Density evolution was also determined as a function of the heat treatment. Results indicate primary H2O release from aluminium hydroxides, followed by carbonate decomposition with CO2 evolution below 900°C. Alkaline oxides, mainly CaO and Na2O, lead to the formation of Ca3Al2O6 and NaAlSiO4 between 900 and 1100°C. At the highest temperatures, reduction of Fe3+ to Fe2+, involving O2 release, promotes the formation of Fe2TiO4, with the disappearance of the rutile-TiO2 phase. The various solid state reactions, ascertained at different stages of the heating process, and possible mass balances are discussed with reference to the state diagrams of principal red mud components.

Introduction

The activity of primary industries often yields substantial amounts of by-products. The disposal in the original industrial site is favoured by economic reasons  energy savings, transport, management and productivity  though traditional storage in nearby dumps can be impractical owing to the considerable masses involved and environmental restrictions. The local exploitation of these by-products is therefore a growing technological aspect of basic industries and one tenable option is their re-use as starting materials for other productions.

An emblematic case is the ‘red mud’ discharged by industry producing alumina from bauxite: alkaline digestion of 2.5 t of bauxite affords alumina and ≈1.5 t of red mud,1, 2, 3, 4, 5 so that an average Al2O3 productivity of 5×105 t year−1 involves a mass of by-products of ≈7.5×105 t year−1 discharged as slurry retaining variable water contents. This amount is composed of Fe and Ti oxides, behaving as chemically inert matter, with variable percentages of nominal SiO2, Al2O3 and Na2O. The material is available as a watery mixture which settles slowly and may easily be conveyed from station to station by continuous fluid-carrying machinery.

Several re-use plans have been advanced. Some fundamental studies concerning the extraction of single oxides  Fe2O3 or TiO2  are economically unsustainable.2, 3 As an example of possible applications requiring simple dewatering, we quote the use as acidic amender or bottom sealant (after stabilization with lime) in the construction of disposal sites.2, 6, 7 The recycling of the mud, after curing or high temperature annealing  up to 1200°C  for large-rate daily mass consumption industries such as bricks and tile kilns has been put forward in a number of papers.2, 3, 8, 9, 10, 11, 12

Most of the above reports appear fragmentary and, to some extent surprising, characterization work is limited to the elemental analysis of the raw material and the identification of the crystalline phases in dried samples. However, the definition of thermal behaviour in a wide working range of temperatures appears mandatory for a feasible exploitation of the mud in high temperature applications. Indeed, the reactivity of red mud components on heating may promote ceramization and shrinkage and, apart from other qualities, may affect the mechanical features of clay-based items fabricated with bauxite-waste addition.

Accordingly, we focus here on the thermal behaviour of the mud, the solid-state transformations and solid–liquid phase transitions within the interval 120–1400°C. The use of thermal analysis coupled with a gas–mass spectrometry for detecting possible gas release, and of X-ray diffraction methods seemed well suitable for the problem at hand. The present study is a part of a long-term project on the exploitation of red mud as a clay additive for the ceramic industry or as a compound for self-binding mortars in the fabrication of stoneware. The results collected in this work regarding the evolution of various crystalline phases, occurrence of liquid phases and variations in colour will be used in the second part of the paper13 more specifically addressed to technical aspects.

Section snippets

Experimental procedure

The red mud studied in this work was supplied by Eurallumina (Porto Vesme, Cagliari, Italy) as a mixture containing about 60% of solids, obtained immediately after alumina recovery from the digestion process. This material was heated for 2 h at 120°C, affording a reddish-orange powder.

Chemical analysis was performed by X-ray fluorescence (X’ Unique II, Philips, The Netherlands).

Thermal analysis was performed on a Netzsch (Germany) STA 409 simultaneous analyser. Thermogravimetric (TG) and

Results

The chemical composition of red mud dried at 120°C is reported in Table 1.

The thermogravimetric (TG) plot [Fig. 1(a)] shows a continuous weight loss distributed in the 100–1350°C interval. The high modulated trend of the TG derivative (DTG) curve [Fig. 1(b)] reveals the different contributions in detail: a broad band is centered at 140°C, followed by two intense peaks at 285 and 320°C and a plateau up to about 500°C (weight loss =8.6%); a second, less pronounced weight loss, characterized by a

Discussion

The large amount of information resulting from the complementary techniques employed allowed a comprehensive view of the red mud behavior under heating.

As an introductory comment one notices a high reactivity essentially due to the extreme dispersion of red mud particles still observable in the powder dried at 120°C. Grain dimensions do not exceed 0.1 μm as shown in Fig. 7. This appears to be an ordinary feature of red mud obtained by alkaline digestion of bauxite, as observed in a number of

Conclusions

The thermal evolution of bauxite-derived red mud was studied in this work by different complementary techniques. Dried red mud is substantially inert up to 900°C, the loss of H2O from aluminium hydroxides and of CO2 from silico-alumino-carbonates being the only detectable effects. At 900°C red mud is dark red in colour. In the 900–1100°C interval, samples are involved in the formation of Nepheline and Na2Si2O5 which are responsible for the softening of samples at ≈1200°C. Fundamental components

Acknowledgements

This work was supported by EMSA (Cagliari, Italy) under Contract EMSA-INCM ‘Riutilizzo di fanghi rossi’. The authors would like to thank Miss Alexia Conci for the contribution in the experimental work. Dr. Ullu (EMSA, Cagliari, Italy) and Dr. Teodosi (Eurallumina, Porto Vesme, Cagliari, Italy) are also acknowledged for their collaboration.

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