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|Incorporation of clay-based ceramic formulations containing different solid wastes|
|M.J. Ribeiro1, J.M. Ferreira2, J.A. Labrincha2
1ESTG, Polythecnique Institute of Viana do Castelo, 4900 Viana do Castelo, Portugal
The ceramic industry, particularly the structural sector, might constitute an interesting alternative to cement-making plants for incorporation or reuse of different waste materials. In fact, actual production levels involve the consumption of huge amounts of mineral resources (e.g., 500 ton./day of clays in a medium-sized brickmaking unit), and the heterogeneous character of natural raw materials, the use of several formulations and, not less important, the firing process might ensure the desired inertness of final waste-containing products with respect to the ceramic matrix.
This paper reviews some industrially-oriented studies conducted in our lab on the incorporation of wastes in structural ceramics. A careful characterisation of each residue and a detailed identification of possible pre-treatment needs for their correct use is always necessary, as usually happens for each raw material. Moreover, correct prediction of their effects on processing conditions and/or on changes of typical final characteristics are also evaluated.
Generally, samples are prepared according to the scheme shown in Figure 1, as close as possible to real experimental conditions. The origin of the wastes might be divided in two categories: (i) arising from the own ceramic sector (fired glazed crock, sludges, etc); (ii) generated by different sectors (e.g., sludges from plating or surface treatments (Al-anodising), sludge from potable water filtration, foundry sand, glass containers, etc). Two representative cases are now discussed in more detail.
The Al-rich sludge comes from an aluminium anodising industry (Extrusal, S.A., Aveiro). Table 1 gives the average chemical composition of the dried sludge, confirming its aluminous character. After calcining above 1100°C the aluminium content might reach levels higher than 30%, corresponding to alumina levels higher than 90 wt.%. Calcium, sodium, iron and cromium are the main impurities, being the last one responsible for the light pink couloring effect.
Table 1 – Chemical composition (XRF) of the dried and calcined (at 1600°C) sludge, with respect to the relevant elements.
The leaching behaviour of the dried sludge confirms its non-toxic character. XRD shows the amorphous nature of the dried sludge, with aluminium hydroxide and sulphide being the main constituents. The appearance of crystalline phases (mainly alumina) is only detected above 1100°C, after the occurrence of sulphate decomposition at about 800°C, as confirmed by DTA/TGA. Any reusing alternative of the as-received sludge as a raw material for traditional or new products is seriously affected by excessive moisture levels, that will cause high shrinkage upon drying and firing and require high energy consumption for its decomposition (strong endothermic reactions). This problem is strongly minimised by assuring a previous drying step, preferably in the producer’s. Vacuum and pressure drying methods should be explored by the anodising industries, whenever thermal processes are not currently used.
Table 2 summarises the effects of sludge addition on tile and brick ceramic formulations. Generally, it can be observed that small additions (up to 5 wt.%) of pre-treated sludge do not induce significant deleterious effects on physical and mechanical characteristics of ceramic samples. Some remaining problems that deserve a complemetary study might be related with higher shrinkage values shown by residue-containing samples upon firing, certainly related with the aforementioned late decomposition reactions of the sludge. Those reactions tend to open the microstructure, especially if the resulting main phase is not reactive (as happens with alumina formation). Those problems are more evident in as-received sludge-containing samples. The incorporation of Al-rich anodising sludge in common ceramic products seems to be functionally viable, and interesting for environmental and economical viewpoints. The addition of small amounts (2-3 wt.%) of dried and milled sludge only causes slight changes in the normal processing conditions and in the final properties of the material.
The formation of mullite-based refractory ceramics incorporating Al-rich anodising sludge up to levels of 70-wt% might be an alternative and more valuable way to reuse this residue.
Table 2 – Effect of sludge addition (wt.%) on typical ceramic formulations used for the production of bricks and tiles. As-received and wet or previously dried and milled sludge was used.
The current waste is a non-toxic sludge collected in the wastewater treatment plant of a sanitaryware industry. It is generated in several routine processing operations: (i) washing of the mills; (ii) collected rejects from the glaze applying cabins or in-line application tests, etc. Its average chemical composition is given in Table 3.
Table 3 – Chemical composition (XRF) of the dried sludge, with respect to the relevant elements.
As happens with other several sludges, grain size is very small and permits its direct incorporation in ceramic formulations. Table 4 shows results of several stoneware formulations.
Table 4 – Effect of sludge addition (wt.%) on typical stoneware formulations. As-received and wet or previously dried sludge was used.
In general, the addition of sludge up to levels of 5 wt-% does not induce significant changes in the relevant functional characteristics and is enough to consume the total amount generated in each industry. However, one can notice a tendency for a general degradation of properties (lower mechanical resistance) with sludge additions, due to its refractory nature. Despite careful sorting and mixing requirements, undesirable colouring effects might be also promoted.
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