Mineral to clay, waste to waste - industrial by-products replace virgin material

Materials World magazine
5 Feb 2013

In the ceramics industry, using waste by-products as additives is
nothing new, but a recent study now shows it is possible to produce
ceramics using only these by-products.

The exploratory study, carried out by researchers at universities in Brazil and Portugal, has focused on producing porcelain-like ceramics such as floor tiles for the construction industry, although they claim the general methodology could be adapted to any part of the ceramics sector using other types of waste.  

The team chose four non-hazardous industrial wastes – a clay mining tail, the sludge from potable water treatment, and two sludges from gneiss and varvite stone-cutting processes – to give the necessary blend of plastic, inert and fluxing components. Each one was dried, ground and sieved, and the resulting homogenous powders were then characterised in terms of chemical and mineral compositions, thermal behaviour up to 1,000°C and particle size distribution.  

Using the phase diagram of the SiO2-Al2O3-K2O system, the team formulated four mixtures – denoted F1 to F4 – located within the wastes-defined polygon, then pressed and fired them for 40 minutes at 900–1,150°C.  

The results showed that a firing temperature of 1,150°C gave the highest flexural strengths, above 3MPa, with the F2 and F3 formulations performing the best. F2 consisted of 20% clay, 65% potable water sludge, 10% gneiss and 5% varvite, while for F3 the respective proportions were 20, 10, 5 and 65%.  

This strength is some way below the 18MPa of commercial floor tiles, but the researchers say that might be explained by the presence of significant closed porosity at this firing temperature. The materials may therefore need strengthening or reinforcement, but corresponding author of the research paper Professor Ana Segadães, of the University of Aveiro, Portugal, says, ‘There is always a significant difference between results obtained from laboratory test pieces and those for full-size commercial products. There might, therefore, be no need for any strengthening, it may just be a case of adjusting the firing temperature and time.’  

The environmental benefits are obvious though, as are the cost benefits. As Segadães explains, ‘In the case of the particular wastes investigated in this work, anyone who generates them will no longer need to allocate land space to deposit them or pay landfill taxes to get rid of them. For those ceramics industries willing to use them, they will be much cheaper (for some time at least) than the usual raw materials, particularly feldspars. Thus the cost benefits are obvious. Energy-wise though, I would say that the costs would be similar, as processing generally remains unaltered.’  

Although the methodology is a general one and could include other types of waste, Segadães says each case would need to be investigated to find the best composition. ‘You could use rock wastes – ornamental stones such as granite, basalt and agate are generally very benign, but others can be (and have been) explored, each of which will have its own difficulties and/or limitations.  

‘For instance, you could use power plant ash, municipal waste incinerator ash, construction and demolition waste, ceramic wastes from chemical plants such as spent catalyst supports, sludges from metal plating industries and wastes from agricultural industrie

The team is now carrying out further work to observe changes that might be introduced by modified green processing – the processing involved before firing – but no results from that are available as yet.