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Technical note

Study of pore size distribution of slate ceramic pieces producedby slip casting of waste powders

A.A.P. Mansur, A.E.C. Peres *, L. Palhares, H.S. Mansur

Department of Metallurgical and Materials Engineering, Federal University of Minas Gerais, Rua Esprito Santo, 35/206,

30160-030 Belo Horizonte/MG, Brazil

Received 18 July 2005; accepted 3 October 2005Available online 15 November 2005

Abstract

One crucial problem in quarries associated with mining dimension stones is the generation of enormous amounts of waste, whichstrongly impacts on the environment. Slate mining and processing generate large amounts of fine powder during extraction, cuttingand polishing procedures. In the present work, suspensions were produced by mixing slate powder with different solid fractions in water,and then casting into plaster moulds (slip casting process). The green samples (as cast) were characterised according to densityand pore size distribution by mercury porosity and X-ray diffraction techniques. Ceramic pieces from slate powder, produced throughslip casting were fired at 1000 C/4 h. The results show that slate powder waste has a potential use in the manufacturing of ceramic piecesby the slip casting process. 2005 Elsevier Ltd. All rights reserved.

Keywords: Waste processing; Environmental; Mineral processing

1. Introduction

Brazil is very important in relation to the dimensionstones industry. Slate quarrying generates a higher amountof waste compared to the mining of other minerals, beingestimated as 80 wt.% of total production, with 25% asslurry (Mansur and Mansur, 2004). In order to minimiseclay extraction, alternative ways of using waste materialshave been attempted, including their incorporation inclay-based ceramic products (Moreira et al., 2003).

In the present work, the objective was to evaluate thepore size distribution of ceramic pieces manufactured withslate powder wastes by the slip casting process. Severalcharacterisation tests were conducted before and after fir-ing in order to investigate pore structure changes duringthermal treatment and morphology modifications.

2. Experimental

Slate powder samples were supplied by a mining plantlocated in Pompeu, Minas Gerais, Brazil. Chemical analysiswas performed by aqueous titration using waste powder asthe sample. Mineralogical analysis was carried out by X-raydiffraction (XRD, PHILIPSPW/1710, CuKa = 1.5418 A,JCPDS database). Scanning Electron Microscopy (SEM,model: JEOL-JSM/5410) coupled to EDX microprobe(NORAN, USA) in order to evaluate morphological

aspects of the slate samples (Au-coated). Ceramic pieceswere manufactured by the slip casting process. Aqueoussuspensions of slate powder were prepared with differentsolid fraction (wt%: 48%; 68%; 73%; 78%) and dispersantconcentration (0.5%; 1.0%; 1.5; 2.0%; 2.5%). Poly(ammo-nium acrylate) was used as dispersant and poly(vinyl alco-hol) (PVA, Brazil) was used as binder (2.0% v/v).

After casting, the pieces were dried at room temperaturefor 24 h followed by oven heating at 100 C/24 h. Greensamples were then transferred to a furnace and the temper-ature raised from 100 C to 1000 C (rate = 10 C/min).

0892-6875/$ - see front matter 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.mineng.2005.10.001

* Corresponding author. Tel./fax: +55 31 3238 1717.E-mail address: aecperes@demet.ufmg.br (A.E.C. Peres).

This article is also available online at:

www.elsevier.com/locate/mineng

Minerals Engineering 19 (2006) 525527

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Mineralogical analyses of fired samples were performed byXRD. Porosity and pore size distribution for green sam-ples and for samples after thermal treatment were deter-mined by mercury porosimetry (MICROMERITICS,Autopore-III). Prior to porosity tests, the slate sampleswere oven-dried at 105 C/24 h, the samples then being

placed in a vacuum chamber for 5 min for outgassing(0.050 mm/Hg).

3. Results and discussion

3.1. Microstructural characterisation of slate powder

A typical SEM photomicrograph of slate powder parti-cles is shown in Fig. 1a. The presence of particles with

irregular size, shape and lamellar structure associated withcleavage (arrow, Fig. 1a) can be seen. The chemical analy-sis (equivalent oxide content) has indicated the presence ofmajor components such as silica, alumina and iron oxide(wt%: 63.8% 1.4; 16.5% 0.5; 7.5% 0.5, respectively).Other minor constituents were also detected (Na2O =

1.3%; K2O = 1.6%; MgO = 2.8%; CaO = 1.7%; TiO2 =0.9%).Mineralogical composition obtained by XRD (Fig. 1b)

has revealed the presence of albite, chlinoclore, calcite,hematite, muscovite and quartz as major crystalline phases,similar to those usually reported in the literature (Mooreand Reynolds, 1997). The high amount of quartz in theslate sample would be important in producing low shrink-age during the firing process. Calcite is expected to act as a

Fig. 1. (a) SEM photomicrograph (2000) of slate powder, with lamellar phase (arrow); (b) X-ray diffraction pattern of slate powder sample (Aalbite,Cchlinoclore, Cacalcite, Hhematite, Mmuscovite, Qquartz); SEM photomicrographs of slate ceramics with solid fraction = 73% and

dispersant = 2% after firing at (c) 500 C; (d) 1100 C.

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neomineralisation agent, after its decomposition to CaO byreacting with quartz and other minerals, producing cal-cium-silicates phases. In addition, XRD results indicateda considerable amount of fluxing agents (Na2O + K2O +Fe2O3 = 10.4%) in the crystalline form of feldspar, hema-tite and muscovite. As a consequence, a eutectic tempera-

ture ranging from 985 C to 900 C would be expected.Therefore, the possibility of forming liquid during thesintering process in the ceramic production is an importantproperty of the slate powder as a potential substitute forclays.

3.2. Characterisation of slate samples after firing

XRD results on slate samples after firing have revealedthe presence of albite, gehlenite, muscovite, mullite, quartzand spinel phases. Albite was also observed in the rawmaterial. The formation of gehlenite was expected, due tothe presence of calcite as raw material. Gehlenite is formed

when a calcium oxide nucleus reacts with the Si and Alfrom the decomposed clay (Jordan et al., 2001). Theremaining mica peak detected was probably due to theresidual muscovite flakes. It is believed that neomineralisa-tion of mullite was caused by high concentrations of quartzand alumina. Based on XRD results on slate and fired cera-mic pieces, it was possible to identify several importantchanges during firing that may have contributed to creationof porosity. Evaporation of free water, both physically andchemically adsorbed (250 C and 320 C, respectively),dehydroxylation of clays with elimination of structuralwater (muscovite and chlinoclore, 570580 C) and calcite

decomposition with CO2 release (800950 C) are eventswhich increase porosity (Mansur and Souza, 2004).Another important aspect is related to the gaseous prod-ucts resulting from burnout of polymers used as dispersantand binders. Poly(vinyl alcohol) initially loses its hydroxylside groups (150250 C), with the elimination of watervapour, followed by the degradation of the hydrocarbonchain at temperatures from 150 C to 500 C, causing vol-ume expansion (Reed, 1988).

SEM micrographs of slate samples fired at temperaturesof 500 C and 1100 C are shown in Fig. 1c and d, respec-tively. At higher temperature (Fig. 1d), the occurrence ofsintering with the presence of a liquid phase, reduction ofgrain boundaries and remaining porosity are all clearlyseen.

Mercury porosimetry intrusion curves of slate samplesas cast indicated that the higher solid content (48%,63% and 73%) the lower was the porosity, while keepingthe concentration of dispersant and binder constant. It is

believed that this is due to the smaller amount of watereliminated during the casting and drying processes. Thepore distribution curve for the slate sample (as cast)was multi-modal, varying in the range of macroporous(length > 0.05 lm) and mesoporous (0.05 lm < length< 0.002 lm), with average pore size ofl = 0.30 lm. After

sintering, porosity was reduced but the pore size remainedmostly in the macroporous range, with a higher averagesize (0.70 lm).

The effect of PVA as dispersant on ceramic pore distri-bution was investigated and the results have indicated thatthe average pore size was reduced by increasing the concen-tration of dispersant from 0.5% to 1.5%. It is believed thatthe dispersant acted as a deflocculant agent, improving thepacking density of slate particles in green samples. Forthis reason, ceramic pieces were produced with low poros-ity after sintering with high dispersant concentration.

4. Conclusions

The slip casting process using slate powder wastes pro-duced green ceramic pieces with high porosity. After fir-ing, the novel slate based ceramics had lower porosity withhigher average pore size, a consequence of the sinteringdensification process. It was also observed that the porosityof slate ceramics can be optimised by an adequate choice ofsolid fraction and dispersant content.

Acknowledgement

Gratitude is expressed to CNPq/FAPEMIG/CAPES for

supporting this work.

References

Jordan, M.F., Sanfeliu, T., Fuente, C. de la, 2001. Firing transformationsof tertiary clays used in the manufacturing of ceramic tile bodies. Appl.Clay Sci. 20, 8795.

Mansur, H.S., Souza, L.P., 2004. Production and characterization ofceramic pieces obtained by slip casting using slate powder wastes.J. Mater. Process. Technol. 145, 1420.

Mansur, A.A.P., Mansur, H.S., 2004. Characterization and production ofnovel ceramic roof tiles obtained using slate powder waste. In: Pecchio,M. et al. (Ed.), Applied MineralogyDevelopments in Science andTechnology, ICAM, Sao Paulo, pp. 10131016.

Moore, M.M., Reynolds, R.C., 1997. X-ray Diffraction and the Identi-fication and Analysis of Clay Minerals, second ed. Oxford UniversityPress, Oxford.

Moreira, J.M.S., Freire, M.N., Holanda, J.N.F., 2003. Utilizacao deresduo de serragem de granito proveniente do estado do EspritoSanto em ceramica vermelha. Ceramica 49 (312), 262267.

Reed, J.S., 1988. Introduction to the Principles of Ceramic Processing,second ed. John Wiley & Sons Inc., New York.

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