- Email: [email protected]
Experimental design in mullite microfilter preparation
Desalination 184 (2005) 57–64
Experimental design in mullite microfilter preparation T. Mohammadi*, A. Pak, Z. Nourian, M. Taherkhani Research Laboratory for Separation Processes, Faculty of Chemical Engineering, Iran University of Science and Technology, Narmak, Tehran, Iran Tel. þ98 21 7896621; Fax þ98 21 7896620; email: [email protected] Received 27 March 2005; accepted 15 April 2005
Abstract Microfiltration (MF) membranes were previously made of kaolin using electrophoresis. In this study, effects of calcination time and temperature on flux and porosity of microfiltration membranes produced by extrusion were evaluated. To complete mullite phase formation, -alumina was added to the kaolin. Sodium carbonate was also used to increase porosity and flux of the membranes. Many experiments should be carried out to understand effects of different parameters. Logical selection of the experiments can save time and money. To obtain the minimum number of experiments with meaningful results, Taguchi’s method was used. The MF membranes were successfully made of 30% alumina, 3% sodium carbonate and 67% kaolin. The dried MF membranes were calcined at a temperature of 1100 C. These membranes were characterized using XRD and SEM analysis. The results showed that mullite is major phase of the membranes and maximum pore size of the membranes is 5.5 mm as measured by SEM. Water flux and porosity of these membranes were evaluated as 21.5 kg/m2.h and 33.92%, respectively. Keywords: Mullite; Microfilter; Ceramic membrane; High porosity membrane; Membrane synthesis
1. Introduction Recently, membrane technology has been developed due to its applications in different industries having thermodynamic or safety limitations. On the other hand, this process: 1) needs less energy 2) produces material with
*Corresponding author.
higher purity 3) is cleaner in comparison with other conventional processes of separation. MF is a membrane process which most closely resembles conventional coarse filtration. Pore sizes of MF membranes range from 0.05 to 10 (m), making the process suitable for retaining suspensions and emulsions. MF membranes may be prepared from a large number of different materials based on
Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005. European Desalination Society. 0011-9164/05/$– See front matter Ó 2005 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2005.04.037
58
T. Mohammadi et al. / Desalination 184 (2005) 57–64
either organic materials (polymers) or inorganic materials (ceramics, metals and glasses). Frequently inorganic membranes are used instead of polymeric membranes because of their outstanding chemical and thermal resistance. In addition, pore size in these membranes can be better controlled and as a consequence the pore size distribution is generally very narrow. Various techniques can be used to prepare ceramic membranes with some important ones being: sintering, sol/gel process and anodic oxidation. The membranes are made of (g or a)-alumina, but other substances like Carbon, Mullite [Al2O3, SiO2] and porous acetyl are also used [1]. Mullite, which is the only stable crystalline phase in the SiO2-Al2O3 system at normal pressure, is formed as the result of thermal decomposition of alumina silicates. Time-Temperature-Transformation curves were used to investigate transformation/reaction kinetics specially the reaction of silica and a-alumina to form mullite [2]. Besides its importance for conventional ceramics, mullite has become a strong candidate material for advanced structural and functional ceramics in recent years. The reasons for this development are some outstanding properties of mullite: low thermal expansion, low thermal conductivity and excellent creep resistance. Other favorable characteristics of mullite are suitable hightemperature strength and high chemical stability [3]. Among the extensively studied processing routes for mullite formation, sol-gel, hydrothermal and solution co precipitation are essentially synthesis routes. Conversely, conventional processing routes starting from high purity conventional raw materials such as quartz, amorphous silica, alumina and kaolinite are also being studied with the aim to produce more economical mullite for a wider range of applications. Mullite and mullite
based composites using chemical processing routes with hexamethylenediamine were prepared [4]. The kaolinite and alumina sintering reactions were also performed at a range of 980–1600 C with various amounts of alumina resulted in economically practical route to produce mullite [5]. In the kaolinite-alumina system, kaolinite is first dehydrated at 500–600 C during heating and metakaolinite is created and it goes through a series of reactions about 980 C, resulting in mullite crystals (primary mullite), silica and an impurity-containing silica-rich liquid. Secondary mullite formation is upon further heating. Experimental data of the primary and secondary mullite formation in a high purity kaolinite-a-alumina system was reported in details. Porous mullite bodies which retain porosity at elevated temperatures and which are used as catalyst supports and contact materials were also prepared [6,7]. In this study, the aim was to produce a kind of mullite MF membrane from kaolin clay through sintering reaction, which has high strength and suitable porosity and flux. In this research, to improve the completion of mullite formation reaction, the crucial substance was g-alumina and to get better porosity, sodium carbonate was used. In the experiments, the effect of four factors (calcination temperature, calcination time, alumina content and sodium carbonate content) on three levels was investigated. Taguchi’s procedure followed in this study is described as follows: (1) Select the major influential factors involved in this study from both theoretical and empirical viewpoints and set their respective levels (low, medium and high) accordingly and appropriately. (2) Construct an appropriate orthogonal table. (3) Perform experiments in triplicate under the various conditions listed in the orthogonal table and calculate the relevant recoveries
59
T. Mohammadi et al. / Desalination 184 (2005) 57–64
and means. (4) Justify the above influential factors using an F-test. (5) Recognize the optimal experimental conditions (levels) that achieve the optimum separation among those listed in the orthogonal table. (6) Further justify the above optimal levels by comparing the respective sums of ‘iso-level’ separations. (7) Further justify the above optimal levels by comparing all the (values resulting from various levels associated with the orthogonal table. [Z = S/N = 10 log(1/2), where S/N is signal-to-noise ratio and d is standard deviation]. Efforts were made to maximize the (value and hence minimize the variance of separation data [8]. As a result, the number of experiments needed was 9, the array of which was given in Table 1. The application of this method results in the less consumption of time and money. 2. Experimental 2.1. Materials In this research, MF membranes were prepared from kaolin clay (mean grain size 40 m). The kaolin material used in this stage (SL-KAD grade) was obtained from WBB cooperation. The analysis of the kaolin is listed in Table 2. Sodium carbonate anhydrous extra pure (Na2CO3) (Merck which Table 1 Levels of used parameters Temperature ( C) Time (h) Na2CO3 Alumina Run (%) (%) 1 2 3 3 1 2 2 3 1
1 2 3 2 3 1 3 1 2
1 2 3 1 2 3 1 2 3
1 1 1 2 2 2 3 3 3
1 2 3 4 5 6 7 8 9
Table 2 Analysis of the kaolin Component
Percentage (%)
SiO2 TiO2 Al2O3 Fe2O3 K2O Na2O L.O.I Total
51.9 0.1 34.1 1.4 0.8 0.1 11.6 100
milled and sieved with 60 mesh) and aluminum oxide anhydrous (g-alumina) (Merck with mean size <150 mm) were the other components of preparing mixture. 2.2. Membrane preparation The first stage is producing process of powder mixture consisting of alumina 0–15– 30%, sodium carbonate 1–2–5% (if containing more than 5% sodium carbonate, decomposition in drying stage is occurred) and kaolin (the rest). Homogenization process is completed through 4 times sieving by 60 mesh. The next stage is combining homogenized powder mixture with distillated water so that the final samples consist of 67–75% powder mixture and 33–25% the water. The samples are extruded to produce tubular membranes (OD = 15 mm, ID = 8 mm, L = 15 cm) and then dried up in air and then calcined at temperatures of 1100, 1250, 1300 C for 3, 4, 5 h with an electrical furnace at a rate of 8 ( C/min). Alumina is active only in formation of the second mullite, hence the minimum temperature must be suitable for breaking a˜-alumina crystals. Therefore, the chosen temperature as the preliminary temperature was 1100 C. On the other hand, using sodium carbonate caused the membrane to loose its flux at temperatures above 1300 C because it changes to
60
T. Mohammadi et al. / Desalination 184 (2005) 57–64
porcelain, thus 1300 C was chosen as maximum temperature. 2.3. Flux measurement Using an experimental set up as shown in Fig. 1, the distillated water flux was measured. Pressure and temperature are important factors in flux measurements. They were kept constant during all experiments. Experiments were carried out at a temperature of 30 C and a pressure of 2 bar, which was supplied by a pump (pedrollo, sanBonifacio (VR Italy)). Each test was run for 30 min and repeated 2 times. 2.4. Porosity determination This test was done according to water saturation route based on the weight of absorbed water by the membrane. Porosity was obtained using the volume difference caused by floating of the membrane saturated with water in water [8]. 2.5. Membrane characterization Qualitative X-ray diffraction (XRD) was the major tool used to identify crystalline
T
phases appearing in each sample. XRD was done by Philips-PW3710 with radiation of Cuka. Membrane surface morphology was studied by Scanning Electron Microscopy (SEM). SEM was done by. 3. Results and discussion Variance analysis was used to distinguish effects of each factor. This was done to determine whether the changes in the results are caused by the level changes of the parameters or they are only related to the accidental errors of measurements. As shown in Table 3, the most effective parameter in increasing flux is alumina content, which has the biggest role in flux (51%). The next effective parameter is calcination temperature (25%). Two other factors are less effective (12%) as shown in Fig. 2. SN calculations as shown in Table 4 are consistent with the result of variance analysis. XRD analysis of the sample containing 30% Al and 2% sodium carbonate (Fig. 3), which was sintered at a temperature of 1300 C for 3 h in comparison with two other samples (Figs 4 and 5), only shows mullite crystalline phases and aluminum
Feed
V1
V2
P1
P2 V3
Membrane cell
Fig. 1. Microfiltration set up.
61
T. Mohammadi et al. / Desalination 184 (2005) 57–64 Table 3 Variance analysis P
F
SS0
V
SS
10
Average flux
Levels
Parameters
50.95183
1421.7192
2820.758
1411.372
2822.744
2
336.314
665.745
333.8667
667.7334
2
12.17497
340.48
674.22
338.0037
676.0075
2
24.5284
685.342
1358.723
680.354
0 15 30 1 2 3 3 4 5 1100 1250 1300
Alumina (%)
12.33036
1.89 2.21 28.61 4.47 9.16 19.08 9.13 19.13 4.44 22.88 2.52 7.3
0.014 100
-
25.8108 5527.192
0.9927220 -
1360.708
8.9345 9 5527.192 17
The effect of parameters
12%
Al
12%
Temperature 51%
25%
2
Na2CO3 (%) Time (h) Temperature ( C)
Errors Sum
oxide. This means that silica phases of the mixture were eliminated and on the other hand, the amount of alumina was excess and some of it was not used. Morphology of a samples was shown in Fig. 6. Pores of the membrane can be measured using SEM.
Time Na2O3
1.1. Effect of alumina content As expected, by adding alumina to the mixture, flux and porosity increase. This is due to the fact that the size of alumina particles in comparison to the size of kaolin particles is bigger. Also, alumina prevents formation of a mild liquid phase flowing through the pores and blocking them after getting cold. Alumina reacts with the liquid phase and changes to mullite. This phenomenon can be seen from the intensity of mullite peek in Fig. 5. Enhancement of membrane flux and porosity, as shown in Table 5, confirms the results.
Fig. 2. Effect of parameters.
Table 4 SN analysis SN (Average)
Level
Parameter
21.67 1.95 26.35 7.97 0.69 5.96 9.55 4.42 2.43 21.84 11.9 7.21
0 15 30 1 2 5 3 4 5 1100 1250 1300
Alumina (%)
Na2CO3 (%) Time (h) Temperature ( C)
1.2. Effect of calcination temperature As seen in Table 6, flux and porosity decrease with increasing up to 1250 C. It is
62
T. Mohammadi et al. / Desalination 184 (2005) 57–64
Fig. 3. XRD of a sample containing 30%Al, 2% Na2O3.
due to increase of glass phases. Porosity above 1250 C does not change with increasing temperature. However, flux increases above 1250 C and it is due to the formation of thinner membranes.
phases increases. The double effect of sodium carbonate results in (1) production of CO2 bubbles and increasing porosity (2) rapid formation of glass phases and producing unexpected results as shown in Table 8.
1.3. Effect of calcination time
4. Conclusion
As seen in Table 7, mullite formation does not change after 3 h [8,9]. Porosity and flux change significantly and little differences can be due to experiments errors.
Addition of alumina causes formation of mullite phase, higher strength and higher flux. Impurities of kaolin and sodium carbonate which are added to the feed provide a situation at which increasing temperature leads to formation of glassy phases which block the pores. The extent of heating has no great effect on the membrane flux after 3 h. Sodium carbonate has a double effect. Increasing temperature increases CO2 formation and this increases the membrane porosity. On the other hand, this causes formation of glassy phases which lessen membrane
1.4. Effect of sodium carbonate Sodium carbonate was used to increase porosity. However, formation method and long retaining period in furnace influence the function of this substance. Sodium oxide drops melting point and possibility of diffusion of alkali phases in aluminum and mullite
T. Mohammadi et al. / Desalination 184 (2005) 57–64
Fig. 4. XRD of a sample containing kaolin.
Fig. 5. XRD of a sample containing 1% Na2O3.
63
64
T. Mohammadi et al. / Desalination 184 (2005) 57–64 Table 6 Results of tests (effect of calcination temperature) Temperature ( C)
flux (kg/m2.h)
Porosity (%)
1100 1250 1300
22.80 2.52 7.30
39.26 23.53 23.55
Table 7 Results of tests (effect of calcination time) Fig. 6. SEM of a sample containing pure kaolin.
Calcination time (h)
flux (kg/m2.h)
Porosity (%)
3 4 5
9.53 9.31 9.14
34.29 32.22 31.36
Table 5 Results of tests (effect of alumina content) Al (%)
flux (kg/m2.h)
Porosity (%)
0 15 13
1.83 2.18 28.50
22.52 26.03 38.08
porosity and membrane flux. To more investigate the effect of sodium carbonate, it needs more detailed tests. References [1] T. Mohammadi and A. Pak, Effect of calcination temperature of kaolin as a support for zeolite membrane. Separation and Purification Technology, 30 (2003) 241–249. [2] K.C. Liu, Time-Temperature-Transformation curves for kaolinite-a- Alumina. J. Am. Ceram. Soc., 77 (1994) 1545–1552. [3] H. Schneider, K. Okada, J. Pask and S. Rahman, Mullite and Mullite Ceramics, John Wiley & Sons, New York, Toronto, London, 1994. [4] C. Maria and H. Jimenez, Preparation of mullite ceramics form coprecipitated aluminum hydroxide and kaolinite using
Table 8 Results of tests (effect of sodium carbonate content) Na2O3 (%)
flux (kg/m2.h)
Porosity (%)
1 2 5
4.47 19.08 9.16
40.21 24.84 32.82
[5]
[6] [7] [8]
[9]
hexamethylenediamine. J. Am. Ceram. Soc., 83 (2000) 2677–80. C.Y. Chen, Preparation of Mullite by the Reaction Sintering of Kaolinite and Alumina. Journal of the European Ceramic Society, 20 (2000) 2519–2525. B.K. Speronello, Porous Mullite, US Patent, No. 4, 601, 997. B.K. Speronello, Porous Mullite, US Patent, No. 4, 628, 042. W. Asmyn, DM. Bass Jr and R.L Whiting, Petroleum Reservoir Engineering Physical Properties, MC Graw-Hill Book Company, New York, 1960. D.C. Montgomery, Design and Analysis of Experiments, 3rd edn. John Wiley & Sons, New York, Toronto, London, 1991.
Experimental design in mullite microfilter preparation T. Mohammadi*, A. Pak, Z. Nourian, M. Taherkhani Research Laboratory for Separation Processes, Faculty of Chemical Engineering, Iran University of Science and Technology, Narmak, Tehran, Iran Tel. þ98 21 7896621; Fax þ98 21 7896620; email: [email protected] Received 27 March 2005; accepted 15 April 2005
Abstract Microfiltration (MF) membranes were previously made of kaolin using electrophoresis. In this study, effects of calcination time and temperature on flux and porosity of microfiltration membranes produced by extrusion were evaluated. To complete mullite phase formation, -alumina was added to the kaolin. Sodium carbonate was also used to increase porosity and flux of the membranes. Many experiments should be carried out to understand effects of different parameters. Logical selection of the experiments can save time and money. To obtain the minimum number of experiments with meaningful results, Taguchi’s method was used. The MF membranes were successfully made of 30% alumina, 3% sodium carbonate and 67% kaolin. The dried MF membranes were calcined at a temperature of 1100 C. These membranes were characterized using XRD and SEM analysis. The results showed that mullite is major phase of the membranes and maximum pore size of the membranes is 5.5 mm as measured by SEM. Water flux and porosity of these membranes were evaluated as 21.5 kg/m2.h and 33.92%, respectively. Keywords: Mullite; Microfilter; Ceramic membrane; High porosity membrane; Membrane synthesis
1. Introduction Recently, membrane technology has been developed due to its applications in different industries having thermodynamic or safety limitations. On the other hand, this process: 1) needs less energy 2) produces material with
*Corresponding author.
higher purity 3) is cleaner in comparison with other conventional processes of separation. MF is a membrane process which most closely resembles conventional coarse filtration. Pore sizes of MF membranes range from 0.05 to 10 (m), making the process suitable for retaining suspensions and emulsions. MF membranes may be prepared from a large number of different materials based on
Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005. European Desalination Society. 0011-9164/05/$– See front matter Ó 2005 Elsevier B.V. All rights reserved doi:10.1016/j.desal.2005.04.037
58
T. Mohammadi et al. / Desalination 184 (2005) 57–64
either organic materials (polymers) or inorganic materials (ceramics, metals and glasses). Frequently inorganic membranes are used instead of polymeric membranes because of their outstanding chemical and thermal resistance. In addition, pore size in these membranes can be better controlled and as a consequence the pore size distribution is generally very narrow. Various techniques can be used to prepare ceramic membranes with some important ones being: sintering, sol/gel process and anodic oxidation. The membranes are made of (g or a)-alumina, but other substances like Carbon, Mullite [Al2O3, SiO2] and porous acetyl are also used [1]. Mullite, which is the only stable crystalline phase in the SiO2-Al2O3 system at normal pressure, is formed as the result of thermal decomposition of alumina silicates. Time-Temperature-Transformation curves were used to investigate transformation/reaction kinetics specially the reaction of silica and a-alumina to form mullite [2]. Besides its importance for conventional ceramics, mullite has become a strong candidate material for advanced structural and functional ceramics in recent years. The reasons for this development are some outstanding properties of mullite: low thermal expansion, low thermal conductivity and excellent creep resistance. Other favorable characteristics of mullite are suitable hightemperature strength and high chemical stability [3]. Among the extensively studied processing routes for mullite formation, sol-gel, hydrothermal and solution co precipitation are essentially synthesis routes. Conversely, conventional processing routes starting from high purity conventional raw materials such as quartz, amorphous silica, alumina and kaolinite are also being studied with the aim to produce more economical mullite for a wider range of applications. Mullite and mullite
based composites using chemical processing routes with hexamethylenediamine were prepared [4]. The kaolinite and alumina sintering reactions were also performed at a range of 980–1600 C with various amounts of alumina resulted in economically practical route to produce mullite [5]. In the kaolinite-alumina system, kaolinite is first dehydrated at 500–600 C during heating and metakaolinite is created and it goes through a series of reactions about 980 C, resulting in mullite crystals (primary mullite), silica and an impurity-containing silica-rich liquid. Secondary mullite formation is upon further heating. Experimental data of the primary and secondary mullite formation in a high purity kaolinite-a-alumina system was reported in details. Porous mullite bodies which retain porosity at elevated temperatures and which are used as catalyst supports and contact materials were also prepared [6,7]. In this study, the aim was to produce a kind of mullite MF membrane from kaolin clay through sintering reaction, which has high strength and suitable porosity and flux. In this research, to improve the completion of mullite formation reaction, the crucial substance was g-alumina and to get better porosity, sodium carbonate was used. In the experiments, the effect of four factors (calcination temperature, calcination time, alumina content and sodium carbonate content) on three levels was investigated. Taguchi’s procedure followed in this study is described as follows: (1) Select the major influential factors involved in this study from both theoretical and empirical viewpoints and set their respective levels (low, medium and high) accordingly and appropriately. (2) Construct an appropriate orthogonal table. (3) Perform experiments in triplicate under the various conditions listed in the orthogonal table and calculate the relevant recoveries
59
T. Mohammadi et al. / Desalination 184 (2005) 57–64
and means. (4) Justify the above influential factors using an F-test. (5) Recognize the optimal experimental conditions (levels) that achieve the optimum separation among those listed in the orthogonal table. (6) Further justify the above optimal levels by comparing the respective sums of ‘iso-level’ separations. (7) Further justify the above optimal levels by comparing all the (values resulting from various levels associated with the orthogonal table. [Z = S/N = 10 log(1/2), where S/N is signal-to-noise ratio and d is standard deviation]. Efforts were made to maximize the (value and hence minimize the variance of separation data [8]. As a result, the number of experiments needed was 9, the array of which was given in Table 1. The application of this method results in the less consumption of time and money. 2. Experimental 2.1. Materials In this research, MF membranes were prepared from kaolin clay (mean grain size 40 m). The kaolin material used in this stage (SL-KAD grade) was obtained from WBB cooperation. The analysis of the kaolin is listed in Table 2. Sodium carbonate anhydrous extra pure (Na2CO3) (Merck which Table 1 Levels of used parameters Temperature ( C) Time (h) Na2CO3 Alumina Run (%) (%) 1 2 3 3 1 2 2 3 1
1 2 3 2 3 1 3 1 2
1 2 3 1 2 3 1 2 3
1 1 1 2 2 2 3 3 3
1 2 3 4 5 6 7 8 9
Table 2 Analysis of the kaolin Component
Percentage (%)
SiO2 TiO2 Al2O3 Fe2O3 K2O Na2O L.O.I Total
51.9 0.1 34.1 1.4 0.8 0.1 11.6 100
milled and sieved with 60 mesh) and aluminum oxide anhydrous (g-alumina) (Merck with mean size <150 mm) were the other components of preparing mixture. 2.2. Membrane preparation The first stage is producing process of powder mixture consisting of alumina 0–15– 30%, sodium carbonate 1–2–5% (if containing more than 5% sodium carbonate, decomposition in drying stage is occurred) and kaolin (the rest). Homogenization process is completed through 4 times sieving by 60 mesh. The next stage is combining homogenized powder mixture with distillated water so that the final samples consist of 67–75% powder mixture and 33–25% the water. The samples are extruded to produce tubular membranes (OD = 15 mm, ID = 8 mm, L = 15 cm) and then dried up in air and then calcined at temperatures of 1100, 1250, 1300 C for 3, 4, 5 h with an electrical furnace at a rate of 8 ( C/min). Alumina is active only in formation of the second mullite, hence the minimum temperature must be suitable for breaking a˜-alumina crystals. Therefore, the chosen temperature as the preliminary temperature was 1100 C. On the other hand, using sodium carbonate caused the membrane to loose its flux at temperatures above 1300 C because it changes to
60
T. Mohammadi et al. / Desalination 184 (2005) 57–64
porcelain, thus 1300 C was chosen as maximum temperature. 2.3. Flux measurement Using an experimental set up as shown in Fig. 1, the distillated water flux was measured. Pressure and temperature are important factors in flux measurements. They were kept constant during all experiments. Experiments were carried out at a temperature of 30 C and a pressure of 2 bar, which was supplied by a pump (pedrollo, sanBonifacio (VR Italy)). Each test was run for 30 min and repeated 2 times. 2.4. Porosity determination This test was done according to water saturation route based on the weight of absorbed water by the membrane. Porosity was obtained using the volume difference caused by floating of the membrane saturated with water in water [8]. 2.5. Membrane characterization Qualitative X-ray diffraction (XRD) was the major tool used to identify crystalline
T
phases appearing in each sample. XRD was done by Philips-PW3710 with radiation of Cuka. Membrane surface morphology was studied by Scanning Electron Microscopy (SEM). SEM was done by. 3. Results and discussion Variance analysis was used to distinguish effects of each factor. This was done to determine whether the changes in the results are caused by the level changes of the parameters or they are only related to the accidental errors of measurements. As shown in Table 3, the most effective parameter in increasing flux is alumina content, which has the biggest role in flux (51%). The next effective parameter is calcination temperature (25%). Two other factors are less effective (12%) as shown in Fig. 2. SN calculations as shown in Table 4 are consistent with the result of variance analysis. XRD analysis of the sample containing 30% Al and 2% sodium carbonate (Fig. 3), which was sintered at a temperature of 1300 C for 3 h in comparison with two other samples (Figs 4 and 5), only shows mullite crystalline phases and aluminum
Feed
V1
V2
P1
P2 V3
Membrane cell
Fig. 1. Microfiltration set up.
61
T. Mohammadi et al. / Desalination 184 (2005) 57–64 Table 3 Variance analysis P
F
SS0
V
SS
10
Average flux
Levels
Parameters
50.95183
1421.7192
2820.758
1411.372
2822.744
2
336.314
665.745
333.8667
667.7334
2
12.17497
340.48
674.22
338.0037
676.0075
2
24.5284
685.342
1358.723
680.354
0 15 30 1 2 3 3 4 5 1100 1250 1300
Alumina (%)
12.33036
1.89 2.21 28.61 4.47 9.16 19.08 9.13 19.13 4.44 22.88 2.52 7.3
0.014 100
-
25.8108 5527.192
0.9927220 -
1360.708
8.9345 9 5527.192 17
The effect of parameters
12%
Al
12%
Temperature 51%
25%
2
Na2CO3 (%) Time (h) Temperature ( C)
Errors Sum
oxide. This means that silica phases of the mixture were eliminated and on the other hand, the amount of alumina was excess and some of it was not used. Morphology of a samples was shown in Fig. 6. Pores of the membrane can be measured using SEM.
Time Na2O3
1.1. Effect of alumina content As expected, by adding alumina to the mixture, flux and porosity increase. This is due to the fact that the size of alumina particles in comparison to the size of kaolin particles is bigger. Also, alumina prevents formation of a mild liquid phase flowing through the pores and blocking them after getting cold. Alumina reacts with the liquid phase and changes to mullite. This phenomenon can be seen from the intensity of mullite peek in Fig. 5. Enhancement of membrane flux and porosity, as shown in Table 5, confirms the results.
Fig. 2. Effect of parameters.
Table 4 SN analysis SN (Average)
Level
Parameter
21.67 1.95 26.35 7.97 0.69 5.96 9.55 4.42 2.43 21.84 11.9 7.21
0 15 30 1 2 5 3 4 5 1100 1250 1300
Alumina (%)
Na2CO3 (%) Time (h) Temperature ( C)
1.2. Effect of calcination temperature As seen in Table 6, flux and porosity decrease with increasing up to 1250 C. It is
62
T. Mohammadi et al. / Desalination 184 (2005) 57–64
Fig. 3. XRD of a sample containing 30%Al, 2% Na2O3.
due to increase of glass phases. Porosity above 1250 C does not change with increasing temperature. However, flux increases above 1250 C and it is due to the formation of thinner membranes.
phases increases. The double effect of sodium carbonate results in (1) production of CO2 bubbles and increasing porosity (2) rapid formation of glass phases and producing unexpected results as shown in Table 8.
1.3. Effect of calcination time
4. Conclusion
As seen in Table 7, mullite formation does not change after 3 h [8,9]. Porosity and flux change significantly and little differences can be due to experiments errors.
Addition of alumina causes formation of mullite phase, higher strength and higher flux. Impurities of kaolin and sodium carbonate which are added to the feed provide a situation at which increasing temperature leads to formation of glassy phases which block the pores. The extent of heating has no great effect on the membrane flux after 3 h. Sodium carbonate has a double effect. Increasing temperature increases CO2 formation and this increases the membrane porosity. On the other hand, this causes formation of glassy phases which lessen membrane
1.4. Effect of sodium carbonate Sodium carbonate was used to increase porosity. However, formation method and long retaining period in furnace influence the function of this substance. Sodium oxide drops melting point and possibility of diffusion of alkali phases in aluminum and mullite
T. Mohammadi et al. / Desalination 184 (2005) 57–64
Fig. 4. XRD of a sample containing kaolin.
Fig. 5. XRD of a sample containing 1% Na2O3.
63
64
T. Mohammadi et al. / Desalination 184 (2005) 57–64 Table 6 Results of tests (effect of calcination temperature) Temperature ( C)
flux (kg/m2.h)
Porosity (%)
1100 1250 1300
22.80 2.52 7.30
39.26 23.53 23.55
Table 7 Results of tests (effect of calcination time) Fig. 6. SEM of a sample containing pure kaolin.
Calcination time (h)
flux (kg/m2.h)
Porosity (%)
3 4 5
9.53 9.31 9.14
34.29 32.22 31.36
Table 5 Results of tests (effect of alumina content) Al (%)
flux (kg/m2.h)
Porosity (%)
0 15 13
1.83 2.18 28.50
22.52 26.03 38.08
porosity and membrane flux. To more investigate the effect of sodium carbonate, it needs more detailed tests. References [1] T. Mohammadi and A. Pak, Effect of calcination temperature of kaolin as a support for zeolite membrane. Separation and Purification Technology, 30 (2003) 241–249. [2] K.C. Liu, Time-Temperature-Transformation curves for kaolinite-a- Alumina. J. Am. Ceram. Soc., 77 (1994) 1545–1552. [3] H. Schneider, K. Okada, J. Pask and S. Rahman, Mullite and Mullite Ceramics, John Wiley & Sons, New York, Toronto, London, 1994. [4] C. Maria and H. Jimenez, Preparation of mullite ceramics form coprecipitated aluminum hydroxide and kaolinite using
Table 8 Results of tests (effect of sodium carbonate content) Na2O3 (%)
flux (kg/m2.h)
Porosity (%)
1 2 5
4.47 19.08 9.16
40.21 24.84 32.82
[5]
[6] [7] [8]
[9]
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