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Development of porous ceramic bodies from kaolin deposits for industrial applications
Applied Clay Science 65–66 (2012) 31–36
Contents lists available at SciVerse ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Note
Development of porous ceramic bodies from kaolin deposits for industrial applications Johnson Kwame Efavi a,⁎, Lucas Damoah a, Delali Yaw Bensah a, David Dodoo Arhin a, David Tetteh b a b
Department of Materials Science and Engineering, University of Ghana, Legon, Ghana Ceramic Materials Division, Council for Scientific and Industrial Research, Accra, Ghana
a r t i c l e
i n f o
Article history: Received 15 March 2011 Received in revised form 16 April 2012 Accepted 18 April 2012 Available online 20 July 2012 Keywords: Kaolin Porous Ceramic Bodies Ghana Pore formers Styrofoam
a b s t r a c t The possibility of fabricating porous ceramic bodies from kaolin using maize, rice, saw-dust, charcoal, millet and Styrofoam as pore formers have been explored in this work after chemical analysis of two kaolin deposits located at Assin Fosu and Saltpond both in the Central Region of Ghana. Batch formulations of the Kaolin samples were formed into greenbodies and fired to 1400 °C at a controlled rate. Samples with charcoal, millet, sawdust, maize and rice pore formers showed severe surface cracks and low thermal shock after firing. A control batch process using Styrofoam exhibited uniform surface characteristics with pores, high strength, thermal stability and no surface cracks thereby confirming the viability of the process modules. Apparent porosity as high as 38% has been calculated. © 2012 Published by Elsevier B.V.
1. Introduction Porous ceramics are widely used in various forms and compositions in filtration and separation, diffusion, heat isolation, sound absorption, chemical filling, bioceramics, chemical sensors, etc. (Nettleship, 1996). The development of porous ceramic materials is attractive because porous ceramics are more stable in severe environments and they can be engineered to satisfy specific requirements due to their surface characteristics. A variety of porous media including steel, wood, wire screens, woven fiber glass fabrics, bonded aggregate, pressed ceramic strainers and ceramic foams are available. However, the high temperature stability of porous ceramics and the ability to tailor the pore sizes for specific operations has increased demand for porous ceramics in industrial applications. In metal casting operations for instance, the use of optimized casting systems and best melting practices have not been able to match the efficient techniques of filters made from ceramic materials (Ceramic Foam Filters). In metallurgical casting, ceramic based filters are used to separate solid particles from molten metals and the solid particles are captured on a filter (porous ceramic body) allowing the liquid phase to pass through. Non‐metallic inclusions are detrimental to the intrinsic mechanical properties, surface finish of cast products and as a result contribute to increase scrap rate of metals (Riedel et al., 1999). In the area of insulation, porous ceramics provides a means for energy savings or heat loss minimization, better temperature control and has
the capability to withstand the corrosive nature of slags in smelting operations (Rice, 1996). Porous ceramic bodies are usually made from aluminosilicates and several technologies are available for their production (Chen et al., 1998; Sandford and Sibley, 1996). The nature of raw materials however affects the porosity (pore size), surface, chemical inertness and thermal shock resistance of porous ceramic bodies (Taslicukur et al., 2007). Ghana is endowed with large deposits of ceramic raw materials such as pegmatite, mica, clay, feldspar, quartz and bauxite. Clay mineral deposits have been found in all the regions of Ghana and are mainly high grade clays containing kaolinite Al2Si2O5(OH)4. With the growing demand for porous ceramics in industrial application, several technologies have been developed for fabricating these materials while attempting to control their pore characteristics and properties (Montanaro et al., 1998; Scheffler and Colombo, 2005). This experimental investigation therefore aims to develop ceramic bodies from kaolin deposits located at the Assin Fosu and Saltpond regions of Ghana. Kaolin, a primary clay material with little plasticity, was sampled from Assin Fosu and Saltpond and processed to produce ceramic bodies using additives that burn out during firing of green bodies (combustible pore formers) to demonstrate the feasibility of producing porous ceramic medium from these clay deposits. 2. Experimental method 2.1. Raw material beneficiation and powder preparation
⁎ Corresponding author at: Materials Science and Engineering Department, University of Ghana, Legon, Accra, Ghana. Tel.: +233 244111217. E-mail address: [email protected] (J. Kwame Efavi). 0169-1317/$ – see front matter © 2012 Published by Elsevier B.V. doi:10.1016/j.clay.2012.04.010
The kaolin samples were obtained from Saltpond and Assin Fosu in the Central Region of Ghana. The mined kaolin contained impurities
32
J. Kwame Efavi et al. / Applied Clay Science 65–66 (2012) 31–36
which were removed by washing, sieving and drying of the slurry in Plaster of Paris (POP) mold. The washing was done by adding water to a constantly stirred kaolin blunger to achieve slurry with specific gravity 1.23. The slurry was then passed through a 350 μm sieve to eliminate coarse grained impurities. The slurry filtrate was poured into a plaster of paris (P.O.P) mold. The water was separated from slurry by capillary action. The solid was then further dried in an electric oven, (Gallenkamp Co.), UK at 200 °C for 7 hours. The dried samples were fed into a Thomas Hammer Mill (Arthur H. Thomas Co, USA), to break the soft lumps into powder. The essential properties of the raw material such as mean particle density and particle size ranges were experimentally determined. Four wire mesh sieve of 250, 180, 150 and 125 μm arranged in order of reducing mesh sizes was used to determine the particle size distribution. 5 kg kaolin was used for each batch of five analysis and the sieves were agitated mechanically for 60 minutes after which powders on each sieve was measured and characterized. The mean particle density was determined using the Litzenberger approach (Litzenberger, 2004).
Table 1 Characteristics of the formulation components. Formulation Sources components
Trade name Physical properties
Starch
Cassava Roots (Asenama, Eastern Region)
Cassava Starch
Charcoal
Wood (Afram Plains, Eastern Region)
Charcoal
Millet
Millet Plant (Upper East Region) Wood Shavings (Afram Plains, Eastern Region)
Millet
Saw Dust
Styrofoam Maize
• Low Tensile Strength • High Water Absorption Rate • Low Moisture Content • Light Weight • Less Fire Resistant • High Level Of Heat Tolerance • Light Weight • Less Fire Resistant • Low Moisture Content • Higher Water Transmission Rate • Highly Bouyant • .Light Weight
Saw Dust
Polystrene Styrofoam Maize Plant Corn (Bosomtwi–Ashanti Region)
2.2. Chemical analysis The chemical analysis of the kaolin powders was obtained with a Spectro XLab 2000 X-Ray Fluorescence (XRF) spectrometer (AMETEK USA), at the Ghana Geological Survey. Four grams of each sample sieved through a 106 μm sieve were mixed with 1 g of Licowax powder that served as a binder. The mixture was then milled for 3 minutes in a Retsch milling machine (model MM 301) and pressed in a XRF pellet press. The powder samples were then loaded into the Spectro XLab 2000 sample holders for the XRF analysis. 2.3. XRD analysis and crystallinity index X-ray powder diffraction patterns were obtained using a Philips PW1820 Automatic Powder Diffractometer and CuKα radiation at 40 kV and 30 mA and a step size of 0.05° 2θ at a counting time of 10 s. Measurements were performed on randomly oriented powder preparations. The diffraction patterns were matched against the ICSD's PDF database and analysed using the X'pert Highscore software (panalytical, Netherlands). The quality (order–disorder) of the kaolinite samples were characterized using the Hinckley index (HI) (Hinckley, 1963). The Hinckley Index (HI) values of the kaolinites were calculated as the ratio of sum of the height above the back and 111 peaks against the band of overlapping ground of the 110 peaks occurring between 20° and 23°, 2θ and then compared to the total height of the 110peak above the background. The average crystallite sizes of the kaolinite samples were also calculated by the Scherrer broadening method using the basal 002 reflection with peak position at (26.8°, 2θ) (Scherrer, 1918). 2.4. Porous body formation
weight of component 100 total weight
Apparentporosity ¼
Ws Wd W si
ð2Þ
2.5. Results and discussion The suitability of porous ceramic bodies for potential use in industrial filtration and molten metal filtration is explained by high volume
The mathematical formula in Eq. (1) was used to determine the individual proportions (kaolin, pore formers) in the batch samples. Experimentally the Assin Fosu kaolin was observed to be more plastic and has therefore been used mainly as a plasticizer in the batch formulations. weight % ¼
summarized in Table 2. The mixture of kaolin and pore formers was then filled into a mold and hand pressed for 20 seconds. The mold was completely filled on all sides to the same level for dimensional homogeneity and to ensure that the powders were well pressed and compact. The pressed samples were removed from the mold and dried at room temperature (ca. 25 °C) on boards for four days after which they were further dried at 105 °C to remove moisture. The dried batch-formulated green bodies were fired to 1400 °C and soaked for 2 hours using a locally CSIR manufactured gas furnace. At 573 °C, the heating rate of 4 °C/min was reduced to 2 °C/min to avoid shattering and crack initiation and propagation, that might be caused from volume change due to transformation of low quartz to high quartz (Heaney et al., 1994; Rykart, 1995). A second set of batch formulations using rice, maize and Styrofoam were also prepared and the compositions of the formulations are presented in Table 3. The Styrofoam with low temperature characteristics has been used as a reference pore former. The process sequence described previously was repeated for these batch formulations as well. The apparent porosity (the amount of void or pores within a volume of porous solid) of the Styrofoam based ceramic bodies was calculated using Eq. (2): weight of dry porous sample (Wd), weight of dry porous sample soaked in water to enable pores to be filled with water (Ws), weight of soaked sample immersed in water again Wsi.
Table 2 Composition of Batch formulation I. Sample
ð1Þ
Different sets of batch formulated samples composed of Saltpond kaolin, Assin Fosu kaolin and pore formers (charcoal, sawdust, millet) were mixed with water and kneaded to eliminate lumps, trapped air bubbles and to ensure homogeneity. The sources, trade name and some physical properties of the pore formers used in this investigation are listed in Table 1. The first set of batch formulations are
1a 1b 2a 2b 3a 3b 4a 4b
Composition (%) Saltpond Kaolin
Plasticizer (Assin-FosuKaolin)
Pore formers Sawdust
Charcoal
Millet
Water
65.89 62.05 68.38 64.59 72.15 65.51 63.84 57.31
– 5.84 – 17.36 – 9.20 – 10.19
– – – – 2.53 2.30 2.84 2.55
14.73 13.87 – – – – 12.77 11.46
– – 13.87 12.50 – – 7.09 6.37
19.38 18.25 18.38 17.36 25.32 23.00 13.48 12.10
J. Kwame Efavi et al. / Applied Clay Science 65–66 (2012) 31–36 Table 3 Composition of Batch Formulation II. SAMPLE COMPOSITION (%) Saltpond Plasticizer PORE FORMERS Water Kaolin (Assin-Fosu Kaolin) Rice Maize Styrofoam (Control Process) 1 2 3 4 5
50 50 50 50 50
25 30 30 15 15
25 20 15 30 35
25 20 15 30 35
25 20 30 35 –
25 25 25 25 25
Table 4 Particle size distribution of the materials used in the different experiments. Experiment
Sample weight (g)
Size range (μm)
Weight retained (g)
%Weight retained
1
1500
2
1200
3
1000
4
800
+ 250 μm + 180 μm + 150 μm + 125 μm − 125 μm + 250 μm + 180 μm + 150 μm + 125 μm − 125 μm + 250 μm + 180 μm + 150 μm + 125 μm − 125 μm + 250 μm + 180 μm + 150 μm + 125 μm − 125 μm
490 290 25 9 686 470 240 23 8 459 460 140 22 7 371 440 120 20 6 214
32.67 19.33 1.67 0.60 45.73 39.17 20.00 1.92 0.67 38.25 46.00 14.00 2.20 0.70 37.10 55.00 15.00 2.50 0.75 26.75
33
variable degree of disordered lattice, generally attributed to random displacements of unit layers along the b-axis, in the case of disordered kaolinite. Fig. 2 shows the powder XRD patterns of the starting Saltpond and Assin Fosu kaolinite between 20 and 40° 2θ. The narrow and intense peaks confirm that these kaolinites are well-crystallized. HI calculations, 0.8 for Saltpond and 0.2 for Assin Fosu indicates that the Saltpond kaolin is well crystallized than the Assin Fosu kaolin. HI relates to the physical properties of kaolin, particularly particle size (Hinckley, 1963). Experimentally the Assin Fosu kaolin was observed to be more plastic than the Saltpond kaolin and this observation is attributed to the particle sizes of the sample used: Assin Fosu kaolin is relatively finer than the Saltpond kaolin. This is supported by crystallite sizes of 276.6 Å and 368.7 Å calculated for Assin Fosu and Saltpond respectively using Scherrer broadening method. In general, ordered kaolins are more coarse than disorded kaolinites which tend to be less coarse and therefore have a higher surface area with greater plasticity tendency (Whittaker, 1939). Fig. 3 shows compacted images of dried green bodies ready for firing. The green body strength is important because a weak green body is likely to shatter in the course of sintering. After the drying period, it was observed that the samples with high percentage of pore formers (charcoal, millet, rice and maize) either cracked or warped (twist or bent out). This observation is attributed to the nature of the pore formers used. After firing, the millet, charcoal, saw dust, maize and rice pore formed samples had large surface cracks, trapped pore formers, high porosity, uneven cleavages in broken samples, discoloration and distorted dimension (Fig. 4). These characteristics are attributed to the nature of pore formers used. The popping nature of maize and rice when approaching its charring temperature leads to thermal shock which distorts the shape of the samples leading to cracks. This observation indicates that the disintegration of the fired sample results from release of large volume of CO2, H2O vapor and other gaseous volatiles during firing (reaction III). Δ
of porosity and surface area as well as the physical, chemical and thermal properties of the ceramic bodies. These properties in turn are intrinsically related to the nature of raw materials (kaolin) and the pore formers used. The particle size ranges of the samples after particle size analysis is shown in Table 4. There is a wide range of particle sizes in the samples which is necessary for close packing configuration (Kingery et al., 1976; Richerson, 2006). Table 5 lists the chemical composition of the kaolin samples used in this study. Geologically, both Saltpond and Assin Fosu deposits are associated with the Cape Coast–Winneba Series of the Birimian System (Fig. 1) that consists of intruded K-rich granitoids in the form of muscovite biotite granite and diorite, porphyroblastic biotite gneiss, aplites and pegmatites (Worrall, 1975). Mineralogically, the process by which these kaolins are formed from precursor alumino-silicates of the bed rocks as follows: First pyrophillite, Al2O3∙4SiO2∙H2O, is formed from the potassium feldspar present in the granitic rocks by hydrolysis and release of Si. Kaolinite forms by further release of Si from pyrophillite. Kaolin mineral is classified as either ordered or disordered with relative degrees of crystallinity. X-ray studies indicate that in minerals of such nature there is a
C ðsÞ þ O2ðgÞ → CO2 ↑ 2.6. Millet and saw dust The fact that reaction IV releases more CO2 than reaction III explains why the physical damages in the structure of porous ceramic bodies with charcoal pore formers were minimal in comparison with millet and sawdust. Δ
ðC6 H10 O5 Þnðs Þþ 6O2ðgÞ → 6CO2 ↑ þ 5H2 O It was also observed that samples without plasticizer in their composition were completely disintegrated whilst those with a plasticizer had only few cracks. Most of the porous ceramics also showed discoloration because of the release of large volumes of CO2 from the combustible pore formers. Fig. 5 shows samples of a controlled batch process with Styrofoam as pore formers. The controlled samples after firing exhibited uniform pore distribution, thermal stability and compactness without indications of surface cracks or distortions. This is attributed to lower melting temperatures of Styrofoam, the faster melting rate and the
Table 5 Chemical composition of the kaolin samples. Raw Materials
SiO2
TiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
P2O5
L.O.I
Total
Saltpond Kaolin (Washed) Assin Fosu Kaolin (Washed)
46.21 47.70
0.01 0.01
38.91 33.81
0.58 2.68
0.18 0.92
0.03 1.42
0.69 1.98.
0.02 0.03
Nil 0.08
13.37 11.35
100.00 99.98
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J. Kwame Efavi et al. / Applied Clay Science 65–66 (2012) 31–36
Fig. 1. Location of the Saltpond and Assin Fosu kaolin deposits.
absence of implosion. The characteristics of the samples after firing are summarized in Tables 6 and 7 for the different batch formulations I and II respectively. In addition, the apparent porosity of the ceramic bodies increased with increasing amount of Styrofoam in the samples. Apparent porosity as high as 38% was calculated confirming the potential of the kaolin deposits in forming porous bodies for industrial application. 3. Conclusion The feasibility of producing porous ceramic bodies for industrial applications from kaolin deposits has been successfully explored in
this investigation. HI calculation based on XRD analysis revealed that the Assin Fosu kaolinite is poorly crystallized in structure while that of Saltpond is well crystallized and ordered. The uniform compact green bodies formed become distorted and structural defects were observed after firing at 1400 °C when combustible pore formers are used. A controlled process using Styrofoam as pore former resulted in uniform porous bodies, without discoloration of the medium confirming that the choice and size of pore formers is critical in achieving porous ceramic bodies. Formulations containing 60% kaolin can be used for the production of ceramics with porosities as high as 38% if the right pore formers are used.
80000 70000
Intensity (a.u)
60000 50000
Saltpond Kaolin
40000 30000 20000 Assin Fosu
10000 0 10
15
20
25
30
35
40
2θ (degrees) Fig. 2. XRD patterns of the Saltpond and Assin Fosu kaolin samples.
Fig. 3. Ceramic bodies after drying.
J. Kwame Efavi et al. / Applied Clay Science 65–66 (2012) 31–36
35
Fig. 4. Porous ceramic bodies with combustible pore formers after firing.
Fig. 5. Porous ceramic bodies with styrofoam after firing.
Table 6 Summary of results of fired bodies at 1400 °C for Batch formulation I. Sample ID
1a 1b 2a 2b 3a 3b 4a 4b
Pore former
Saltpond kaolin
Charcoal Charcoal Millet Millet Sawdust Sawdust Charcoal, Millet, Sawdust Charcoal, Millet, Sawdust
AssinFosu kaolin plasticizer
Fired samples characteristics upon visual inspection Cracks
Trapped pore former
Glassy phase
Cleavage
Porosity
No Yes No Yes No Yes No Yes
Few surface cracks Large surface cracks Surface Cracks Few surface cracks Few surface cracks Discoloration, No surface cracks Large surface cracks Large surface cracks
Visible Visible Not Visible Not Visible Not Visible Not Visible Visible Visible
Visible Visible Visible Visible Visible Not Visible Visible Visible
Uneven Uneven Uneven Uneven Uneven No cleavage Uneven Even
Low Low Low High High Very Low Very Low High
Table 7 Summary of results of fired bodies at 1400 °C for Batch formulation II. Sample ID
Pore former
Saltpond kaolin
Plasticizer (Assin-Fosu kaolin)
Fired samples characteristics upon visual inspection Cracks
Trapped pore former
Glassy phase
Cleavage
Porosity
1a 1b 2a 2b 3a 3b 4a
Rice Maize Rice Maize Rice Maize Rice Maize Styrofoam
Yes Yes Yes Yes Yes Yes Yes Yes Yes
Yes
Minor
High and Visible
Visible
Uneven
Low
Yes
Minor
High and Visible
Visible
Uneven
Low
Yes
Large
High and Visible
Visible
Uneven
Low
Yes
Large
High and Visible
Visible
Uneven
Low
Yes
None
None
None
None
High
5
Acknowledgement The authors will like to thank Ghana Geological Survey Department and the Council Centre for Scientific and Industrial Research for making their facilities available for this investigation. The authors are also grateful to University of Ghana for financial assistance received in the course of the work. The authors are most grateful to
Prof. Djibril Diop, of Cheikh Anta Diop Universityin Dakar, Senegal for the XRD measurements of the samples.
References Chen, F., Huang, X., Wang, Y., Hu, Z., 1998. Investigation on foam ceramic filter to remove inclusions in revert alloy. Materials Letters 372–376.
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Heaney, P.J., Prewitt, C.T., Gibbs, G.V., 1994. Silica—physical behaviour, geochemistry and materials applications. Reviews in Mineralogy, Vol.29. Mineralogical Society of America. Hinckley, D., 1963. Variability in “crystallinity” values among the kaolin deposits of the coastal plain of Georgia and South Carolina. Clays and Clay Minerals 11, 229. Kingery, W.D., Bowen, H.K., Uhlmann, D.R., 1976. Introduction to Ceramics. John Wiley & Sons Inc, New York, pp. 209–210. Litzenberger, C.Cg., 2004. Rheological study of kaolin clay slurries. Proceedings of the 16th International Conference of Hydraulic Transport of Solids (Hydrotransport), Cranfield UK, pp. 77–91. Montanaro, L., Jorand, Y., Fantozzi, G., Negroa, A., 1998. Ceramic foams by powder processing. Journal of the European Ceramic Society 18, 1339–1350. Nettleship, I., 1996. Applications of porous ceramics. Key Engineering Materials 122, 305–324. Rice, R.W., 1996. Porosity dependence on the physical properties of materials. Key Engineering Materials 115, 1–20. Richerson, D.W., 2006. Modern Ceramic Engineering-Processing, Properties and Use in Design, Third Edition. Taylor and Francis CRC Press, New York.
Riedel, U.T., Morgan, J.E., McMahon, C.A., Guild, F.J., Bleck, W., 1999. The detrimental effect of non-metallic inclusions during metal forming processes. Proceedings of the Institution of Mechanical Engineers 213, 427–430. Rykart, R., 1995. Quarz-Monographie- Die Eigenheiten von Bergkristall, Rauchquarz, Amethyst, Chalcedon, Achat, Opal und anderenVarietäten. Sandford, P., Sibley, S.R., 1996. Optimization of Al casting productivity using foam filter technology and application. AFS Transaction 1063–1068. Scheffler, M., Colombo, P., 2005. Cellular Ceramics: Structure, Manufacturing, Properties and Applications. Wiley, U.S.A. Scherrer, P., 1918. Nachr.Ges.Wiss. Göttingen. Mathematisch-Physikalische Klasse 2, 98. Taslicukur, Z., Balaban, C., Kuskonnaz, W., 2007. Production of ceramic foam filters from molten filtration using expanded polystyrene. Journal of the European Ceramic Society 27, 637–640. Whittaker, H., 1939. Effect of particle size on plasticity of kaolinite. Journal of the American Ceramic Society 22 (1–12), 16–23. Worrall, W.E., 1975. Clays and Ceramic Raw Materials. Applied Science Publishers Ltd, London, pp. 27–31.
Contents lists available at SciVerse ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Note
Development of porous ceramic bodies from kaolin deposits for industrial applications Johnson Kwame Efavi a,⁎, Lucas Damoah a, Delali Yaw Bensah a, David Dodoo Arhin a, David Tetteh b a b
Department of Materials Science and Engineering, University of Ghana, Legon, Ghana Ceramic Materials Division, Council for Scientific and Industrial Research, Accra, Ghana
a r t i c l e
i n f o
Article history: Received 15 March 2011 Received in revised form 16 April 2012 Accepted 18 April 2012 Available online 20 July 2012 Keywords: Kaolin Porous Ceramic Bodies Ghana Pore formers Styrofoam
a b s t r a c t The possibility of fabricating porous ceramic bodies from kaolin using maize, rice, saw-dust, charcoal, millet and Styrofoam as pore formers have been explored in this work after chemical analysis of two kaolin deposits located at Assin Fosu and Saltpond both in the Central Region of Ghana. Batch formulations of the Kaolin samples were formed into greenbodies and fired to 1400 °C at a controlled rate. Samples with charcoal, millet, sawdust, maize and rice pore formers showed severe surface cracks and low thermal shock after firing. A control batch process using Styrofoam exhibited uniform surface characteristics with pores, high strength, thermal stability and no surface cracks thereby confirming the viability of the process modules. Apparent porosity as high as 38% has been calculated. © 2012 Published by Elsevier B.V.
1. Introduction Porous ceramics are widely used in various forms and compositions in filtration and separation, diffusion, heat isolation, sound absorption, chemical filling, bioceramics, chemical sensors, etc. (Nettleship, 1996). The development of porous ceramic materials is attractive because porous ceramics are more stable in severe environments and they can be engineered to satisfy specific requirements due to their surface characteristics. A variety of porous media including steel, wood, wire screens, woven fiber glass fabrics, bonded aggregate, pressed ceramic strainers and ceramic foams are available. However, the high temperature stability of porous ceramics and the ability to tailor the pore sizes for specific operations has increased demand for porous ceramics in industrial applications. In metal casting operations for instance, the use of optimized casting systems and best melting practices have not been able to match the efficient techniques of filters made from ceramic materials (Ceramic Foam Filters). In metallurgical casting, ceramic based filters are used to separate solid particles from molten metals and the solid particles are captured on a filter (porous ceramic body) allowing the liquid phase to pass through. Non‐metallic inclusions are detrimental to the intrinsic mechanical properties, surface finish of cast products and as a result contribute to increase scrap rate of metals (Riedel et al., 1999). In the area of insulation, porous ceramics provides a means for energy savings or heat loss minimization, better temperature control and has
the capability to withstand the corrosive nature of slags in smelting operations (Rice, 1996). Porous ceramic bodies are usually made from aluminosilicates and several technologies are available for their production (Chen et al., 1998; Sandford and Sibley, 1996). The nature of raw materials however affects the porosity (pore size), surface, chemical inertness and thermal shock resistance of porous ceramic bodies (Taslicukur et al., 2007). Ghana is endowed with large deposits of ceramic raw materials such as pegmatite, mica, clay, feldspar, quartz and bauxite. Clay mineral deposits have been found in all the regions of Ghana and are mainly high grade clays containing kaolinite Al2Si2O5(OH)4. With the growing demand for porous ceramics in industrial application, several technologies have been developed for fabricating these materials while attempting to control their pore characteristics and properties (Montanaro et al., 1998; Scheffler and Colombo, 2005). This experimental investigation therefore aims to develop ceramic bodies from kaolin deposits located at the Assin Fosu and Saltpond regions of Ghana. Kaolin, a primary clay material with little plasticity, was sampled from Assin Fosu and Saltpond and processed to produce ceramic bodies using additives that burn out during firing of green bodies (combustible pore formers) to demonstrate the feasibility of producing porous ceramic medium from these clay deposits. 2. Experimental method 2.1. Raw material beneficiation and powder preparation
⁎ Corresponding author at: Materials Science and Engineering Department, University of Ghana, Legon, Accra, Ghana. Tel.: +233 244111217. E-mail address: [email protected] (J. Kwame Efavi). 0169-1317/$ – see front matter © 2012 Published by Elsevier B.V. doi:10.1016/j.clay.2012.04.010
The kaolin samples were obtained from Saltpond and Assin Fosu in the Central Region of Ghana. The mined kaolin contained impurities
32
J. Kwame Efavi et al. / Applied Clay Science 65–66 (2012) 31–36
which were removed by washing, sieving and drying of the slurry in Plaster of Paris (POP) mold. The washing was done by adding water to a constantly stirred kaolin blunger to achieve slurry with specific gravity 1.23. The slurry was then passed through a 350 μm sieve to eliminate coarse grained impurities. The slurry filtrate was poured into a plaster of paris (P.O.P) mold. The water was separated from slurry by capillary action. The solid was then further dried in an electric oven, (Gallenkamp Co.), UK at 200 °C for 7 hours. The dried samples were fed into a Thomas Hammer Mill (Arthur H. Thomas Co, USA), to break the soft lumps into powder. The essential properties of the raw material such as mean particle density and particle size ranges were experimentally determined. Four wire mesh sieve of 250, 180, 150 and 125 μm arranged in order of reducing mesh sizes was used to determine the particle size distribution. 5 kg kaolin was used for each batch of five analysis and the sieves were agitated mechanically for 60 minutes after which powders on each sieve was measured and characterized. The mean particle density was determined using the Litzenberger approach (Litzenberger, 2004).
Table 1 Characteristics of the formulation components. Formulation Sources components
Trade name Physical properties
Starch
Cassava Roots (Asenama, Eastern Region)
Cassava Starch
Charcoal
Wood (Afram Plains, Eastern Region)
Charcoal
Millet
Millet Plant (Upper East Region) Wood Shavings (Afram Plains, Eastern Region)
Millet
Saw Dust
Styrofoam Maize
• Low Tensile Strength • High Water Absorption Rate • Low Moisture Content • Light Weight • Less Fire Resistant • High Level Of Heat Tolerance • Light Weight • Less Fire Resistant • Low Moisture Content • Higher Water Transmission Rate • Highly Bouyant • .Light Weight
Saw Dust
Polystrene Styrofoam Maize Plant Corn (Bosomtwi–Ashanti Region)
2.2. Chemical analysis The chemical analysis of the kaolin powders was obtained with a Spectro XLab 2000 X-Ray Fluorescence (XRF) spectrometer (AMETEK USA), at the Ghana Geological Survey. Four grams of each sample sieved through a 106 μm sieve were mixed with 1 g of Licowax powder that served as a binder. The mixture was then milled for 3 minutes in a Retsch milling machine (model MM 301) and pressed in a XRF pellet press. The powder samples were then loaded into the Spectro XLab 2000 sample holders for the XRF analysis. 2.3. XRD analysis and crystallinity index X-ray powder diffraction patterns were obtained using a Philips PW1820 Automatic Powder Diffractometer and CuKα radiation at 40 kV and 30 mA and a step size of 0.05° 2θ at a counting time of 10 s. Measurements were performed on randomly oriented powder preparations. The diffraction patterns were matched against the ICSD's PDF database and analysed using the X'pert Highscore software (panalytical, Netherlands). The quality (order–disorder) of the kaolinite samples were characterized using the Hinckley index (HI) (Hinckley, 1963). The Hinckley Index (HI) values of the kaolinites were calculated as the ratio of sum of the height above the back and 111 peaks against the band of overlapping ground of the 110 peaks occurring between 20° and 23°, 2θ and then compared to the total height of the 110peak above the background. The average crystallite sizes of the kaolinite samples were also calculated by the Scherrer broadening method using the basal 002 reflection with peak position at (26.8°, 2θ) (Scherrer, 1918). 2.4. Porous body formation
weight of component 100 total weight
Apparentporosity ¼
Ws Wd W si
ð2Þ
2.5. Results and discussion The suitability of porous ceramic bodies for potential use in industrial filtration and molten metal filtration is explained by high volume
The mathematical formula in Eq. (1) was used to determine the individual proportions (kaolin, pore formers) in the batch samples. Experimentally the Assin Fosu kaolin was observed to be more plastic and has therefore been used mainly as a plasticizer in the batch formulations. weight % ¼
summarized in Table 2. The mixture of kaolin and pore formers was then filled into a mold and hand pressed for 20 seconds. The mold was completely filled on all sides to the same level for dimensional homogeneity and to ensure that the powders were well pressed and compact. The pressed samples were removed from the mold and dried at room temperature (ca. 25 °C) on boards for four days after which they were further dried at 105 °C to remove moisture. The dried batch-formulated green bodies were fired to 1400 °C and soaked for 2 hours using a locally CSIR manufactured gas furnace. At 573 °C, the heating rate of 4 °C/min was reduced to 2 °C/min to avoid shattering and crack initiation and propagation, that might be caused from volume change due to transformation of low quartz to high quartz (Heaney et al., 1994; Rykart, 1995). A second set of batch formulations using rice, maize and Styrofoam were also prepared and the compositions of the formulations are presented in Table 3. The Styrofoam with low temperature characteristics has been used as a reference pore former. The process sequence described previously was repeated for these batch formulations as well. The apparent porosity (the amount of void or pores within a volume of porous solid) of the Styrofoam based ceramic bodies was calculated using Eq. (2): weight of dry porous sample (Wd), weight of dry porous sample soaked in water to enable pores to be filled with water (Ws), weight of soaked sample immersed in water again Wsi.
Table 2 Composition of Batch formulation I. Sample
ð1Þ
Different sets of batch formulated samples composed of Saltpond kaolin, Assin Fosu kaolin and pore formers (charcoal, sawdust, millet) were mixed with water and kneaded to eliminate lumps, trapped air bubbles and to ensure homogeneity. The sources, trade name and some physical properties of the pore formers used in this investigation are listed in Table 1. The first set of batch formulations are
1a 1b 2a 2b 3a 3b 4a 4b
Composition (%) Saltpond Kaolin
Plasticizer (Assin-FosuKaolin)
Pore formers Sawdust
Charcoal
Millet
Water
65.89 62.05 68.38 64.59 72.15 65.51 63.84 57.31
– 5.84 – 17.36 – 9.20 – 10.19
– – – – 2.53 2.30 2.84 2.55
14.73 13.87 – – – – 12.77 11.46
– – 13.87 12.50 – – 7.09 6.37
19.38 18.25 18.38 17.36 25.32 23.00 13.48 12.10
J. Kwame Efavi et al. / Applied Clay Science 65–66 (2012) 31–36 Table 3 Composition of Batch Formulation II. SAMPLE COMPOSITION (%) Saltpond Plasticizer PORE FORMERS Water Kaolin (Assin-Fosu Kaolin) Rice Maize Styrofoam (Control Process) 1 2 3 4 5
50 50 50 50 50
25 30 30 15 15
25 20 15 30 35
25 20 15 30 35
25 20 30 35 –
25 25 25 25 25
Table 4 Particle size distribution of the materials used in the different experiments. Experiment
Sample weight (g)
Size range (μm)
Weight retained (g)
%Weight retained
1
1500
2
1200
3
1000
4
800
+ 250 μm + 180 μm + 150 μm + 125 μm − 125 μm + 250 μm + 180 μm + 150 μm + 125 μm − 125 μm + 250 μm + 180 μm + 150 μm + 125 μm − 125 μm + 250 μm + 180 μm + 150 μm + 125 μm − 125 μm
490 290 25 9 686 470 240 23 8 459 460 140 22 7 371 440 120 20 6 214
32.67 19.33 1.67 0.60 45.73 39.17 20.00 1.92 0.67 38.25 46.00 14.00 2.20 0.70 37.10 55.00 15.00 2.50 0.75 26.75
33
variable degree of disordered lattice, generally attributed to random displacements of unit layers along the b-axis, in the case of disordered kaolinite. Fig. 2 shows the powder XRD patterns of the starting Saltpond and Assin Fosu kaolinite between 20 and 40° 2θ. The narrow and intense peaks confirm that these kaolinites are well-crystallized. HI calculations, 0.8 for Saltpond and 0.2 for Assin Fosu indicates that the Saltpond kaolin is well crystallized than the Assin Fosu kaolin. HI relates to the physical properties of kaolin, particularly particle size (Hinckley, 1963). Experimentally the Assin Fosu kaolin was observed to be more plastic than the Saltpond kaolin and this observation is attributed to the particle sizes of the sample used: Assin Fosu kaolin is relatively finer than the Saltpond kaolin. This is supported by crystallite sizes of 276.6 Å and 368.7 Å calculated for Assin Fosu and Saltpond respectively using Scherrer broadening method. In general, ordered kaolins are more coarse than disorded kaolinites which tend to be less coarse and therefore have a higher surface area with greater plasticity tendency (Whittaker, 1939). Fig. 3 shows compacted images of dried green bodies ready for firing. The green body strength is important because a weak green body is likely to shatter in the course of sintering. After the drying period, it was observed that the samples with high percentage of pore formers (charcoal, millet, rice and maize) either cracked or warped (twist or bent out). This observation is attributed to the nature of the pore formers used. After firing, the millet, charcoal, saw dust, maize and rice pore formed samples had large surface cracks, trapped pore formers, high porosity, uneven cleavages in broken samples, discoloration and distorted dimension (Fig. 4). These characteristics are attributed to the nature of pore formers used. The popping nature of maize and rice when approaching its charring temperature leads to thermal shock which distorts the shape of the samples leading to cracks. This observation indicates that the disintegration of the fired sample results from release of large volume of CO2, H2O vapor and other gaseous volatiles during firing (reaction III). Δ
of porosity and surface area as well as the physical, chemical and thermal properties of the ceramic bodies. These properties in turn are intrinsically related to the nature of raw materials (kaolin) and the pore formers used. The particle size ranges of the samples after particle size analysis is shown in Table 4. There is a wide range of particle sizes in the samples which is necessary for close packing configuration (Kingery et al., 1976; Richerson, 2006). Table 5 lists the chemical composition of the kaolin samples used in this study. Geologically, both Saltpond and Assin Fosu deposits are associated with the Cape Coast–Winneba Series of the Birimian System (Fig. 1) that consists of intruded K-rich granitoids in the form of muscovite biotite granite and diorite, porphyroblastic biotite gneiss, aplites and pegmatites (Worrall, 1975). Mineralogically, the process by which these kaolins are formed from precursor alumino-silicates of the bed rocks as follows: First pyrophillite, Al2O3∙4SiO2∙H2O, is formed from the potassium feldspar present in the granitic rocks by hydrolysis and release of Si. Kaolinite forms by further release of Si from pyrophillite. Kaolin mineral is classified as either ordered or disordered with relative degrees of crystallinity. X-ray studies indicate that in minerals of such nature there is a
C ðsÞ þ O2ðgÞ → CO2 ↑ 2.6. Millet and saw dust The fact that reaction IV releases more CO2 than reaction III explains why the physical damages in the structure of porous ceramic bodies with charcoal pore formers were minimal in comparison with millet and sawdust. Δ
ðC6 H10 O5 Þnðs Þþ 6O2ðgÞ → 6CO2 ↑ þ 5H2 O It was also observed that samples without plasticizer in their composition were completely disintegrated whilst those with a plasticizer had only few cracks. Most of the porous ceramics also showed discoloration because of the release of large volumes of CO2 from the combustible pore formers. Fig. 5 shows samples of a controlled batch process with Styrofoam as pore formers. The controlled samples after firing exhibited uniform pore distribution, thermal stability and compactness without indications of surface cracks or distortions. This is attributed to lower melting temperatures of Styrofoam, the faster melting rate and the
Table 5 Chemical composition of the kaolin samples. Raw Materials
SiO2
TiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
P2O5
L.O.I
Total
Saltpond Kaolin (Washed) Assin Fosu Kaolin (Washed)
46.21 47.70
0.01 0.01
38.91 33.81
0.58 2.68
0.18 0.92
0.03 1.42
0.69 1.98.
0.02 0.03
Nil 0.08
13.37 11.35
100.00 99.98
34
J. Kwame Efavi et al. / Applied Clay Science 65–66 (2012) 31–36
Fig. 1. Location of the Saltpond and Assin Fosu kaolin deposits.
absence of implosion. The characteristics of the samples after firing are summarized in Tables 6 and 7 for the different batch formulations I and II respectively. In addition, the apparent porosity of the ceramic bodies increased with increasing amount of Styrofoam in the samples. Apparent porosity as high as 38% was calculated confirming the potential of the kaolin deposits in forming porous bodies for industrial application. 3. Conclusion The feasibility of producing porous ceramic bodies for industrial applications from kaolin deposits has been successfully explored in
this investigation. HI calculation based on XRD analysis revealed that the Assin Fosu kaolinite is poorly crystallized in structure while that of Saltpond is well crystallized and ordered. The uniform compact green bodies formed become distorted and structural defects were observed after firing at 1400 °C when combustible pore formers are used. A controlled process using Styrofoam as pore former resulted in uniform porous bodies, without discoloration of the medium confirming that the choice and size of pore formers is critical in achieving porous ceramic bodies. Formulations containing 60% kaolin can be used for the production of ceramics with porosities as high as 38% if the right pore formers are used.
80000 70000
Intensity (a.u)
60000 50000
Saltpond Kaolin
40000 30000 20000 Assin Fosu
10000 0 10
15
20
25
30
35
40
2θ (degrees) Fig. 2. XRD patterns of the Saltpond and Assin Fosu kaolin samples.
Fig. 3. Ceramic bodies after drying.
J. Kwame Efavi et al. / Applied Clay Science 65–66 (2012) 31–36
35
Fig. 4. Porous ceramic bodies with combustible pore formers after firing.
Fig. 5. Porous ceramic bodies with styrofoam after firing.
Table 6 Summary of results of fired bodies at 1400 °C for Batch formulation I. Sample ID
1a 1b 2a 2b 3a 3b 4a 4b
Pore former
Saltpond kaolin
Charcoal Charcoal Millet Millet Sawdust Sawdust Charcoal, Millet, Sawdust Charcoal, Millet, Sawdust
AssinFosu kaolin plasticizer
Fired samples characteristics upon visual inspection Cracks
Trapped pore former
Glassy phase
Cleavage
Porosity
No Yes No Yes No Yes No Yes
Few surface cracks Large surface cracks Surface Cracks Few surface cracks Few surface cracks Discoloration, No surface cracks Large surface cracks Large surface cracks
Visible Visible Not Visible Not Visible Not Visible Not Visible Visible Visible
Visible Visible Visible Visible Visible Not Visible Visible Visible
Uneven Uneven Uneven Uneven Uneven No cleavage Uneven Even
Low Low Low High High Very Low Very Low High
Table 7 Summary of results of fired bodies at 1400 °C for Batch formulation II. Sample ID
Pore former
Saltpond kaolin
Plasticizer (Assin-Fosu kaolin)
Fired samples characteristics upon visual inspection Cracks
Trapped pore former
Glassy phase
Cleavage
Porosity
1a 1b 2a 2b 3a 3b 4a
Rice Maize Rice Maize Rice Maize Rice Maize Styrofoam
Yes Yes Yes Yes Yes Yes Yes Yes Yes
Yes
Minor
High and Visible
Visible
Uneven
Low
Yes
Minor
High and Visible
Visible
Uneven
Low
Yes
Large
High and Visible
Visible
Uneven
Low
Yes
Large
High and Visible
Visible
Uneven
Low
Yes
None
None
None
None
High
5
Acknowledgement The authors will like to thank Ghana Geological Survey Department and the Council Centre for Scientific and Industrial Research for making their facilities available for this investigation. The authors are also grateful to University of Ghana for financial assistance received in the course of the work. The authors are most grateful to
Prof. Djibril Diop, of Cheikh Anta Diop Universityin Dakar, Senegal for the XRD measurements of the samples.
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