Manufacture of Ceramic Products Using Inertized Aluminum Sludges

Abstract

The aim of the present research work was to study the effect of mixing aluminum hydroxide sludges with wastes resulting from the cutting and polishing of dimensional stones, which have high content of alumino-silicates, in order to develop mullite. This study shows that, after sintering different mixtures to temperatures up to 1300ºC, there is an important increase of secondary mullite phase in the resultant material, particularly in the mixes 2:1 (alumina:silica ratio) simultaneously with a significant decrease of silica phases. The presence of important quantities of -alumina was also detected with the increasing of sintering temperature. Therefore the product obtained after sintering was a composite of mullite and alumina. The mullitisation behaviour was studied using X-ray diffraction and microstructural analysis, which have confirmed the increase of mullite attaining a maximum when the sintering temperature was 1270ºC. The composite formed during sintering was responsible for a flexure modulus higher than 100 MPa, with a Weibull Modulus typical of technical ceramics, without degrading other properties, like water absorption. The new developed material was found to be inert after leaching tests carried out according to DIN standard 38414.

Mullitization kinetics from silica- and alumina-rich wastes

“…The characterization of this kind of sludges revealed their strong shrinkage behaviour [19,22] and their particle fineness [19,20]. The high content of alumina in heat-treated aluminium sludges acted as a drivingforce for the attempts of re-use them as a raw material in the ceramic industry [17,20,22,23]. The addition of this kind of sludges to slate wastes resulted in mullite-a-alumina composites and an important increase of mullite content (secondary mullite) was observed, simultaneously with a significant decrease of silica phases [17].…”

“…The aluminium hydroxides were either obtained during bauxite refining processes or synthesized from aluminium salts, but also by-products from aluminium industry, namely aluminium sludges produced by physico-chemical treatment of wastewater generated by electrochemical treatment of aluminium surfaces [17,19]. These aluminium sludges are mainly composed by water (75-85 wt.%) and colloidal aluminium hydroxides.…”

“…Besides alumina polymorphs, aluminium hydroxides have also been used as alumina sources in aluminium silicate-based mullite-forming mixtures [14,15,17]. The aluminium hydroxides were either obtained during bauxite refining processes or synthesized from aluminium salts, but also by-products from aluminium industry, namely aluminium sludges produced by physico-chemical treatment of wastewater generated by electrochemical treatment of aluminium surfaces [17,19].…”

“…In fact, recent studies on the production of mullite rich-ceramics from aluminium silicate minerals focused the recovery of wastes from dimensional stones exploitation facilities, namely slate wastes [16,17]. Slate waste powders have been submitted to thermal treatments under several atmospheres and using different heating velocities [18].…”

Novel mullite-based ceramics manufactured from inorganic wastes

“…Attempts were made to improve both the refractoriness and the mechanical resistance of slate-based materials by adding ␥-alumina (Martins et al, 2002) and aluminium anodising sludges (Martins et al, 2004). Again, poor mechanical properties (<50 MPa in three-point bending) were achieved by adding up to 30% ␥-alumina to slate powders sintered at 1250 • C, in spite of the fact that an increase in mullite formation (33-40 wt% as compared to 19-21 wt% obtained for slate after a soaking time of 6 h) has occurred (Martins et al, 2002).…”