Combining mineral and clay-based wastes to produce porcelain-like ceramics: An exploratory study
Highlights
► A viable end use was found for wastes from the beneficiation of mineral resources. ► Such end use might lead to the production of waste-only porcelain-like ceramics. ► The choice of compositions and processing parameters was guided by phase diagrams. ► Results suggest the possibility of low temperature fast-firing ceramic processing. ► Initial results might easily be furthered towards a suitable commercial application.
Introduction
Ceramic processes have long been regarded as ideal to accomplish inertization of potentially hazardous wastes (Bingham and Hand, 2006) and, given the composition similarities between some wastes and natural raw materials used in ceramics manufacture, this has led to the general acceptance of ceramic industry, particularly the sector dealing with construction materials, as a potentially high consumer of wastes from other industrial sources (Acchar et al., 2006, Acchar et al., 2009, Correia et al., 2009, Dondi et al., 1997, Menezes et al., 2009, Segadães, 2006, Vieira and Monteiro, 2009). Some wastes often also contain materials that are beneficial for the ceramics manufacture processes (Acchar et al., 2005, Carty and Senapati, 1998, Lee et al., 2007, Leite et al., 2009, Rawlings et al., 2006, Segadães et al., 2005) and, therefore, promoting industrial wastes to alternative ceramic raw materials is both technically and economically attractive.
In recent decades human kind has become growingly concerned with the environment's degradation and wastes disposal, in particular. Industrialization brought about intensive extraction and beneficiation of natural resources, which are examples of activities that can cause landscape and ecosystems destruction and, if not controlled, ecological unbalances with unpredictable consequences, but also a new major concern about secondary pollution induced by end-products resulting from primary waste treatment, such as wastewater treatment sludges (Huang et al., 2001, Oliveira et al., 2007, Qi et al., 2010) and municipal solid waste incineration (MSWI) ashes (Ginés et al., 2009, Monteiro et al., 2008). Nowadays, industries of all kinds have optimized their processes in order to reduce the amount of wastes they produce and the corresponding negative environmental impact but are also looking for alternative, less expensive raw materials.
Replacing natural raw materials by wastes can configure an opportunity to ameliorate today's waste management difficulties. The valorization of industrial wastes and their up-grading to alternative raw materials can present several advantages when compared with the use of primary natural resources, namely reduction in the extraction volume of natural raw materials (resource preservation), lower energy consumption during subsequent processing (reduced costs), and lower pollutant emission levels (improvement of population health and safety). Even if the waste incorporation is done in small amounts, high production rates will translate into significant consumption of waste materials. Moreover, for the industry willing to use them, waste materials might constitute a cheap renewable raw material source (Dondi et al., 1997, Lee et al., 2007, Rawlings et al., 2006, Segadães, 2006, Vieira and Monteiro, 2009).
From this perspective, recycling and finding reuse alternatives for industrial wastes gain added relevance, and a number of potential applications have been proposed, namely in the production of clay-based bricks (Acchar et al., 2006, Huang et al., 2001, Oliveira et al., 2007, Qi et al., 2010), porcelainized tiles (Correia et al., 2009, Menezes et al., 2009, Segadães et al., 2005), mortars and concrete (Ginés et al., 2009, Raupp-Pereira et al., 2007), ceramic pigments (Leite et al., 2009), and glass-ceramics (Mello-Castanho et al., 2011, Monteiro et al., 2008), to name a few. The waste materials involved range from ore tailings and ornamental stone cuttings to potable and residual water treatment sludges, metal anodizing and other surface treatment sludges, MSWI and thermal power plant ashes, and also include agro-industrial and forestry wastes. As a consequence, recycling has become common practice, particularly in those countries where raw materials are strategic and waste disposal is both strict and costly. But also in traditionally more permissive countries the ecological awareness has been awakened and stricter laws for environmental control and licensing of waste generation by industrial activities are being implemented.
With ceramics manufacture in mind, the present work was aimed not only at finding a viable end use, other than landfilling, for abundant non-hazardous mineral and clay-based wastes, but also, and most importantly, at meeting the challenge of producing waste-only ceramics with higher added value.
Thus, wastes were selected based on location and availability, in order to minimize transportation costs, and in the roles raw materials must play in ceramics manufacture (i.e. plastics, inerts and fluxes). To ensure the necessary plasticity, the sludge from the purification/clarification of potable water and one clay mining tail, regarded also as waste, were selected (plastic components); between the two, there should be enough sand to fulfil the need for inert components; to guarantee low temperature firing (fluxing components), two sludges from industrial stone cutting processes (gneiss and varvite) were selected. Although such waste materials have already been object of some investigation as individual additives in ceramic formulations, to the best of our knowledge there is no published work dedicated to their combined use as ceramic raw materials.
Potable water preparation processes for human consumption generates high volume of a waste sludge that consists basically of clay minerals, calcium hydroxide, aluminium sulphate, calcite and sand, with a moisture level generally above 80 wt.%. Several published studies evaluated the effects of its addition to ceramic mixtures and found that, although the final mechanical properties were changed, in general the product remained within the set limits for building construction applications (Huang et al., 2001, Oliveira et al., 2007, Qi et al., 2010, Raupp-Pereira et al., 2007, Teixeira et al., 2006).
Similarly, surface layers of clay deposits that are usually abandoned and discarded as waste due to the reddish coloration they impart to fired ceramic tiles, were shown to be adequate in the production of good quality building products (Menezes et al., 2009, Modesto et al., 2003).
Gneisses are metamorphic rocks, which can develop from a wide variety of igneous and sedimentary materials. Generally, gneisses show a coarse granular texture in alternating dark and light bands of minerals. Varvite is a special kind of layered sedimentary rock whose formation was controlled by the yearly succession of seasonal deposits. Works reported in the literature (Cunha, 2007, Moreira et al., 2008) showed that, upon firing at the adequate temperature, ceramic building materials containing comparatively high contents of gneiss or varvite wastes could be produced with suitable properties (linear shrinkage, water absorption and mechanical strength).
Based on the promising results of those studies on the individual wastes, the present work explores the possibility of combining them in such a way that novel ceramic products can be obtained, seeking a waste-only porcelain-like formulation and simultaneously one way to reduce both the manufacturing costs and the negative environmental impact caused by industrial activity.
Section snippets
Experimental
The sludge from potable water preparation processes (PWS) was provided by potable water treatment plant in Blumenau (SAMAE, producing an average of 2400 l per month of a slurry containing ~ 63 wt.% water). The residual clay was supplied by Eliane (Cocal do Sul, circa 3000 tons accumulated residual clay). The two sludges from industrial stone cutting processes (gneiss and varvite) were supplied by Pedras Morro Grande (Araquari, which generates an average of 700 m3 of damp waste per month) and Ramos
Results and discussion
The chemical compositions of the dried waste materials obtained by X‐ray fluorescence are gathered in Table 1. It can be observed that varvite presents the highest SiO2 content (74.32 wt.%) whereas the highest Al2O3 contents can be found in PWS (due to the addition of aluminium sulphate, used for agglomeration of solid particles on the settling tanks) and clay (22.10 and 19.55 wt.%, respectively). On the other hand, the highest alkaline contents (Na2O and K2O) are found in gneiss (fluxing
Conclusions
This exploratory study suggests that the manufacture of ceramic tiles using only wastes is possible and porcelain-like ceramics can be produced.
The phase diagram was successfully used to guide mixture formulation and to help choosing the process parameters. In all cases, it provided a basis for making predictions of material behavior under several conditions of service or processing. Technical characteristics such as the relatively homogeneous particle size distribution of the wastes promote
Acknowledgments
The authors acknowledge the financial support from the Brazilian funding agencies CAPES and CNPq.
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