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Settling and filtration characteristics of fine alumina trihydrate slurry
Int. J. Miner. Process. 49 Ž1997. 107–118
Settling and filtration characteristics of fine alumina trihydrate slurry Indra N. Bhattacharya Regional Research Laboratory, Bhubaneswar-751 013, Orissa, India Received 15 March 1995; accepted 11 August 1996
Abstract The paper presents some aspects of settling and filtration of very fine alumina trihydrate particles in Na-aluminate–caustic liquor. The work carried out on laboratory scale shows that flocculation can be effectively used for improving the settling behaviour. The effect of additive, solids concentration, pressure, etc., on filtration characteristics of trihydrate slurry is also investigated. Application of laboratory scale tests for design and scale up of filtration operation are studied. A concept of an average specific cake resistance term developed from pressure and concentration parameters is proposed to be considered for filtration rate calculations. Keywords: filtration; settling; alumina trihydrate; specific cake resistance; cake porosity
1. Introduction Crystallised alumina trihydrate is the principal constituent of bauxite. This is directly produced from Bayer’s process. The grains of this trihydrate precipitates are spheres measuring 50 to 100 microns. Treatment of the sodium aluminate solution with an aged alumina gel or, in some cases, the self-precipitation of Na-aluminate solution, may lead to the production of very fine trihydrate. This fine alumina trihydrate is used in pharmaceutical and chemical industries. Mrs. National Aluminium Company Ltd., Koraput, Orissa, India has recently developed a process ŽSatapathy and Pattanaik, 1991. for making fine alumina trihydrate by a precipitation technique from Na-aluminate liquor. Only recently there is some interest for using this fine grade alumina trihydrate as a commercial product. Trihydrate slurry for this purpose must be dewatered for recovery of trihydrate particles and Na-aluminatersoda. In the present case, the work involves the following steps: Ža. Settling of alumina trihydrate particles, and Žb. Filtration of alumina trihydrate slurry. 0301-7516r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 3 0 1 - 7 5 1 6 Ž 9 6 . 0 0 0 3 2 - 4
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In the settling study, identification of additives that improve the settling behaviour is carried out. The role of the additive during filtration is also studied. The specific cake resistance a , generally not considered as a design parameter by practising engineers due to lack of confidence in the value of a , is entirely based on applied pressure. In the present study, an attempt is made to find a correlation between a and slurry concentration, and to find an average a av value from pressure and concentration factor for scaling up. 2. Theory 2.1. Scaling up equations for filtration operation Scaling up and design of filtration equipment relies on laboratory scale tests. Only when this method fails, pilot scaling of the filtration processes is carried out. The basic filtration law derived by Poiseuille and Darcy is modified by Carman Ž1938. as follows: dt Õ mR s am c 2 q Ž 1. dÕ AD P A DP For constant pressure filtration, the above equation can be expressed as: am c mR trÕ s Õq Ž 2. 2 AD P 2 A DP where t s filtration time, Õ s filtrate volume, A s filtration area, D p s applied pressure, c s mass of dry solids per unit volume of filtrate, m s filtrate viscosity, a s specific cake resistance, and R s filter medium resistance. In actual filtration operation, medium resistance R is negligible, as cake resistance is the dominant parameter. Therefore, Eq. Ž2. can be rewritten as: am c trÕ s Õ Ž 3. 2 A2D P For designing a filter unit, the filtration rate, FR can be expressed as: wt. of dry cake = no. of cyclesrtime W FR s s Ž 4. filter area APt where W s mass deposited at time t, A s filtration area, and t s filtration time, or: cPÕ FR s Ž 5. APt where W s c P Õ, Õ s volume of filtrate obtained at time t, and c s mass of solids per unit filtrate, or: FR P A P t Õs Ž 6. c Now, substituting Õ in Eq. Ž3., we have:
'
FR s 2 cD Pram t
Ž 7.
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This is the most important equation required by practising engineers for scaling up operation. In the above equation, a and cycle time Ž t . are two most important parameters which affect the measurement of FR . The measurement of total cycle time should be taken up very carefully. In general, cycle time is calculated as follows: total cycle time s cake formation time q cake removal time q dewatering time qfilter unit cleaning time, etc. These different times will vary with type of filter unit used. 3. Experimental The slurry of alumina trihydrate and Na-aluminate liquor containing 110–120 gpl of caustic concentration was supplied by Mrs. National Aluminium Company Ltd., India. The particle size distribution of the alumina trihydrate particles is shown in Fig. 1, where 50% particles passes through 1.94 micron size. The particle size measurement is carried out in a Malvern particle size analyser model 3600. the specific surface area of the particles are directly obtained from the analyser. The characteristics of the slurry like, specific gravity, particle density, viscosity of liquor, etc. are shown in Table 1. The settling experiments are carried out in a standard one litre glass measuring cylinder of 6.38 cm dia and 40 cm height. All filtration experiments are carried out in a 500 ml 3 piece Buchner funnel fitted with a Whatman 42 filter paper under vacuum. The
Fig. 1. Particle size distribution of alumina trihydrate.
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Table 1 Various physical parameters of alumina trihydrate particles and Na-aluminate liquor Specific gravity of liquor Viscosity of liquor pH of the liquor Alumina concentration Caustic concentration Density of the particles Specific surface area
1.136 1.4 cP )14 50–60 grl 110 to 120 grl 2.3 grml 4.05 m2 rml
diameter of the Buchner funnel used for the study is 10 cm. The volume of slurry taken for each filtration experiment is 275 ml. Washing of cake is carried out with 75 ml of de-ionised water immediately after the supernatant liquor disappears from the cake surface. Different concentration slurries are prepared from the supplied slurry by adding liquor or by removing liquor. The slurry concentrations used are: 36 kgrm3, 45 kgrm3, 65 kgrm3, 88 kgrm3 and 170 kgrm3. The concentration of flocculant stock solution prepared is 0.2 gpl. The porosity of filter cakes are measured using the following equation:
as
K 0 S02
rs
ž
1ye
e3
/
Ž 8.
where a s specific cake resistance, S0 s specific surface area, K 0 s Kozeney constant s 5, rs s solid density, e s porosity.
4. Results and discussion 4.1. Settling study The settling work carried out with as received slurry Ž65 kgrm3 solids concentration. collected from Mrs. NALCO shows very poor settling characteristics mainly because of very fine nature of the trihydrate particles. To enhance the settling properties three different flocculating agents namely, Magnafloc 351 Žnonionic., Magnafloc 156 Žanionic., Magnafloc 140 Žcationic. are tested. It is observed that anionic flocculant does not respond at all, the reason being the alumina trihydrate particles show negative zeta potential at this pH range ŽPuttock et al., 1985.. Due to the negative potential of trihydrate particles charge neutralisation with an anionic flocculant is not possible. The mechanism of flocculation by organic flocculants is considered to involve mainly two processes Ža. surface charge neutralisation, and Žb. bridging ŽHughes, 1981.. Both mechanisms can act simultaneously or separately. It is however observed that both cationing and non-ionic flocculants have worked satisfactorily. The effectiveness of cationic and non-ionic flocculants in the slurry is due to charge neutralisation in the former case and bridging in the latter. Fig. 2 shows the settling curves of as received as well as flocculated slurries. The settling rate could not be enhanced at lesser dosage of
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Fig. 2. Settling curves of alumina trihydrate particles in Na-aluminate liquor.
flocculant but it showed improved rate at higher dosage. The settling rate has increased from 0.56 cmrh to 2.4 cmrh when 200 ppm additive is used. The ultimate settled concentration of the trihydrate particles is found to be 170 kgrm3. 4.2. Filtration study Fig. 3 shows the filtration curves for three different concentration slurries without ŽA, B, C. and with ŽD, E. the addition of flocculants. The solids concentration of C is the ultimate settled concentration of the slurry obtained from settling experiments. Comparing the curves A, B and C, it is observed that filtration rate has decreased with increase in slurry concentration. But in case of flocculated slurries the filtration rate has decreased Žcurves B, D and E.. The filtration rate has dropped from 13.6 to 8.64 kg my2 hy1 for slurry B and E. This behaviour is quite opposite to the generally observed behaviour where due to flocculation, filtration rate increases by forming larger particles which in turn increases the porosity. In the present case, probably due to very soft nature of particles, as the cake forms the interparticle gap reduces due to flocculation giving rise to a lower porosity. This fact is supported by the increase in specific cake resistance Ž8.7 = 10 11 mrkg for D and 10.8 = 10 11 mrkg for E. for flocculated slurry compared to 6.9 = 10 11 mrkg for unflocculated slurry ŽB.. The medium resistance has found to be decreased with flocculation ŽB s 13.9 = 10 10 my1 , D s 3.12 = 10 10 my1 and E s 6.14
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Fig. 3. Filtration curves of three different concentration slurries without and with the addition of flocculants.
= 10 10 my1 .. The moisture content of the filter cakes obtained at different applied pressures are shown in Table 2 and found to be decreasing with applied pressure. The medium resistance R should normally be constant but it varies due to penetration of some solids into the medium and also due to applied pressure. The variation of R with pressure is studied using only Na-aluminate liquor which is shown in Fig. 4. The increase in R with applied pressure is due to the squeezing of filter medium only. The variation of R in slurry filtration with pressure is shown in Fig. 5 and Table 3. Variation of R with pressure ŽFig. 5. is shown for two different concentration Ž36 kgrm3 and 65 kgrm3 . slurries and both found to follow the straight line correlation with equal slope
Table 2 Variation of moisture content with pressure Pressure, kPa
Moisture, %
33 47 67 80 93 100
63 62 57 55 54 49
I.N. Bhattacharyar Int. J. Miner. Process. 49 (1997) 107–118
Fig. 4. Variation of medium resistance R with pressure for Na-aluminate liquor.
Fig. 5. Variation of medium resistance R with pressure for two different concentration slurries.
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Table 3 Variation of medium resistance with slurry concentration at 80 kPa Slurry concentration, kgrm3
Medium resistance, my1
36 45 65 88 170
8.5=10 10 14=10 10 12.2=10 10 20.2=10 10 26.8=10 10
but having different intercepts. The increase in R with concentratin ŽTable 3. may be due to the penetration of more particles in the filter medium. 4.3. Specific cake resistance as design parameter — an inÕestigation The most important factor which is considered here for investigation is the specific cake resistance. Bartolomeu et al. Ž1989. successfully used bench scale tests for two extremely different sludges using Eq. Ž7. and found comparable results to those presented in literature for industrial operations. The specific cake resistance considered in the above case is the apparent resistance using Eq. Ž3.. However, most of the design engineers use average specific resistance as scaling up parameter, which is much lower than apparent resistance. It is therefore observed that a decrease or an increase in a by 4 times means double or halve the filtration rate calculation. Therefore, the value of a has to be measured carefully. Now, as liquid flows through a compressible bed of particles viscous drag on the particles produces compressive pressure which causes a to increase and e to decrease towards the filter medium ŽWakeman, 1981.. Figs. 6 and 7 show the variation of specific cake resistance a and porosity e with applied pressure for two different concentration slurries in log–log plot respectively. The straight line plot obtained for both the concentrations are found to be parallel with different a 0 and e 0 . Therefore, it indicates that for a particular slurry the compressibility index is constant inspite of whatever slurry concentration is chosen. Similar observation is obtained for porosity also. The correlation equation between a and pressure and e and pressure followed most often quoted form ŽShirato et al., 1964; Shirato et al., 1968; Rushton and Wakeman, 1977. as:
a s a0 Ž D P .
s
Ž 9.
and
e s e0 Ž D P .
yl
Ž 10 .
where s s compressibility index and l s porosity index, and a o and eo are cake resistance and porosity at unit applied pressure respectively. As most cakes are compressible and their specific cake resistance changes with pressure drop across the cake. In such cases average specific cake resistance aD P would replace a in Eq. Ž7., which is calculated as aD P s Ž1 y s . a 0 Ž D P . s.
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Fig. 6. Plot of log a versus log D P for two different concentration slurries.
Fig. 7. Plot of ylog e versus log D P for two different concentration slurries.
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From Figs. 6 and 7, s and l are found to be 0.64 and 0.355 respectively, which indicates that s q l s 0.64 q 0.355 , 1 or s s 1 y l. Therefore, once s is known l can be found out and vice versa. It is also felt that a not only changes with pressure but also changes with cake depth. As the thickness of cake depends on concentration of the particles in a slurry, there is possibility of some correlation involving a and slurry concentrations. Slurry concentration effects on the values of a and e are also reported by Rushton et al. Ž1980. and Wakeman Ž1972.. Fig. 8 shows the plot of a vs. slurry concentrations. It is observed from the figure that with increase in concentration a decreased. The log–log plot of Fig. 8 is shown in Fig. 9 which follow a straight line correlation with a negative slope as shown below.
a s a 0 cym 1
Ž 11 .
where m1 is found to be 0.29. A similar approach is made for finding out relationship between e and concentration and found to be as:
e s e0 c m2
Ž 12 .
where m 2 is 0.68. It is observed in this case also that m1 q m 2 s 0.68 q 0.29 , 1, or m1 s 1 y m 2 . Rushton et al. Ž1978. have shown the effect of velocity and concentration on specific cake resistance on several inorganic materials. They found that a in a constant pressure
Fig. 8. Variation of a with slurry concentration Ž c ..
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Fig. 9. Plot of log a versus log c.
experiment go through a maximum with increasing concentrations and beyond this concentration the resistance decreases exponentially. As it is observed that a depends both on pressure and concentration parameters, the average value of a should be determined by taking into consideration both the parameters. Therefore, it is proposed that the specific cake resistance value to be taken for FR calculation should be the average value of aD P and a c obtained from pressure and concentration parameter, as shown below: a av s Ž aD P q a c . r2 Ž 13 . s ym 1 where aD P s Ž1 y s . a 0 Ž D P . and a c s Ž1 q m1 . a 0 Ž c . , where a c is the average specific cake resistance from concentration parameter. Table 4 shows the values of a obtained from trÕ vs. Õ plot, aD P , a c and a av at 80 kPa pressure. Table 4 Values of different a ’s at 80 kPa pressure Slurry conc., kgrm3
a , mrkg
aD P , mrkg
a c , mrkg
a av , mrkg
36 45 65 88 170
7.22=10 11 6.33=10 11 6.0=10 11 5.5=10 11 4.52=10 11
2.67=10 11 2.28=10 11 2.16=10 11 1.98=10 11 1.63=10 11
9.2=10 11 8.6=10 11 7.76=10 11 7.1=10 11 5.8=10 11
5.9=10 11 5.4=10 11 4.96=10 11 4.54=10 11 3.71=10 11
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The above equation has been found to be valid for the system alumina trihydrate particles–sodium aluminate liquor, and may not be valid for other systems, but the same concept may be utilised for finding out average a values in other systems also.
5. Conclusions Settling characteristics of alumina trihydrate particles in Na-aluminate liquor is very poor but can be improved by adding cationic or nonionic flocculants. The filtration rate has dropped due to the addition of flocculants. The increase in medium resistance during filtration may be due to the penetration of particles in the filter medium and squeezing of medium with applied pressure. Moisture content of the filter cake has decreased with increased pressure. It is observed that the sum of exponents in specific cake resistance and porosity equations equals to one. A realistic approach for measurement of filtration rate Ž FR . is considered by proposing the a av term.
Acknowledgements The author is thankful to Mrs. National Aluminium Company Limited, Damanjodi, Orissa, India, for supplying the material. The author also wishes to thank Prof. H.S. Ray, Director, Regional Research Laboratory, Bhubaneswar, for providing the facilities and Dr. R.P. Das, Head, Hydro and Electrometallurgy Division, for his encouragement during the work.
References Bartolomeu, F., Chambino, T. and Carrondo, M.J.T., 1989. Filtrat. Sep., ŽJulyrAugust.: 288. Carman, P.C., 1938. Trans. Inst. Chem. Eng., ŽOctober.. Hughes, M.A., 1981. Coagulation and flocculation. In: L. Svarovsky ŽEditor., Solid–Liquid Separation. Butterworth, p. 64. Puttock, S.J., Fane, A.G., Fell, C.J.D., Robins, R.G. and Wainwright, M.S., 1985. A.I.Ch.E.J., 31Ž7.: 1213. Rushton, A. and Wakeman, R.J., 1977. J. Powder Bulk Solids Technol., 1: 58. Rushton, A., Hosseini, M. and Hassan, I., 1978. In: Proc. Symp. on Solid–Liquid Separation Practices, Leeds. Yorkshire Branch of I.Chem.E., p. 27. Rushton, A., Hosseini, M. and Hassan, I., 1980. J. Sep. Process. Technol., 1: 35. Satapathy, B.K. and Pattanaik, S.C., 1991. Light Met.: 145. Shirato, M., Sambuichi, M. and Murase, T., 1964. Mem. Fac. Eng. Nagoya Univ., 16: 68. Shirato, M., Aragaki, T., Mori, R. and Sawamoto, K., 1968. J. Chem. Eng. Jpn., 1: 86. Wakeman, R.J., 1972. Proc. 2nd World Filtration Congress, London, p. 57. Wakeman, R.J., 1981. Trans. I.Chem.E., 59: 147.
Settling and filtration characteristics of fine alumina trihydrate slurry Indra N. Bhattacharya Regional Research Laboratory, Bhubaneswar-751 013, Orissa, India Received 15 March 1995; accepted 11 August 1996
Abstract The paper presents some aspects of settling and filtration of very fine alumina trihydrate particles in Na-aluminate–caustic liquor. The work carried out on laboratory scale shows that flocculation can be effectively used for improving the settling behaviour. The effect of additive, solids concentration, pressure, etc., on filtration characteristics of trihydrate slurry is also investigated. Application of laboratory scale tests for design and scale up of filtration operation are studied. A concept of an average specific cake resistance term developed from pressure and concentration parameters is proposed to be considered for filtration rate calculations. Keywords: filtration; settling; alumina trihydrate; specific cake resistance; cake porosity
1. Introduction Crystallised alumina trihydrate is the principal constituent of bauxite. This is directly produced from Bayer’s process. The grains of this trihydrate precipitates are spheres measuring 50 to 100 microns. Treatment of the sodium aluminate solution with an aged alumina gel or, in some cases, the self-precipitation of Na-aluminate solution, may lead to the production of very fine trihydrate. This fine alumina trihydrate is used in pharmaceutical and chemical industries. Mrs. National Aluminium Company Ltd., Koraput, Orissa, India has recently developed a process ŽSatapathy and Pattanaik, 1991. for making fine alumina trihydrate by a precipitation technique from Na-aluminate liquor. Only recently there is some interest for using this fine grade alumina trihydrate as a commercial product. Trihydrate slurry for this purpose must be dewatered for recovery of trihydrate particles and Na-aluminatersoda. In the present case, the work involves the following steps: Ža. Settling of alumina trihydrate particles, and Žb. Filtration of alumina trihydrate slurry. 0301-7516r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 3 0 1 - 7 5 1 6 Ž 9 6 . 0 0 0 3 2 - 4
I.N. Bhattacharyar Int. J. Miner. Process. 49 (1997) 107–118
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In the settling study, identification of additives that improve the settling behaviour is carried out. The role of the additive during filtration is also studied. The specific cake resistance a , generally not considered as a design parameter by practising engineers due to lack of confidence in the value of a , is entirely based on applied pressure. In the present study, an attempt is made to find a correlation between a and slurry concentration, and to find an average a av value from pressure and concentration factor for scaling up. 2. Theory 2.1. Scaling up equations for filtration operation Scaling up and design of filtration equipment relies on laboratory scale tests. Only when this method fails, pilot scaling of the filtration processes is carried out. The basic filtration law derived by Poiseuille and Darcy is modified by Carman Ž1938. as follows: dt Õ mR s am c 2 q Ž 1. dÕ AD P A DP For constant pressure filtration, the above equation can be expressed as: am c mR trÕ s Õq Ž 2. 2 AD P 2 A DP where t s filtration time, Õ s filtrate volume, A s filtration area, D p s applied pressure, c s mass of dry solids per unit volume of filtrate, m s filtrate viscosity, a s specific cake resistance, and R s filter medium resistance. In actual filtration operation, medium resistance R is negligible, as cake resistance is the dominant parameter. Therefore, Eq. Ž2. can be rewritten as: am c trÕ s Õ Ž 3. 2 A2D P For designing a filter unit, the filtration rate, FR can be expressed as: wt. of dry cake = no. of cyclesrtime W FR s s Ž 4. filter area APt where W s mass deposited at time t, A s filtration area, and t s filtration time, or: cPÕ FR s Ž 5. APt where W s c P Õ, Õ s volume of filtrate obtained at time t, and c s mass of solids per unit filtrate, or: FR P A P t Õs Ž 6. c Now, substituting Õ in Eq. Ž3., we have:
'
FR s 2 cD Pram t
Ž 7.
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This is the most important equation required by practising engineers for scaling up operation. In the above equation, a and cycle time Ž t . are two most important parameters which affect the measurement of FR . The measurement of total cycle time should be taken up very carefully. In general, cycle time is calculated as follows: total cycle time s cake formation time q cake removal time q dewatering time qfilter unit cleaning time, etc. These different times will vary with type of filter unit used. 3. Experimental The slurry of alumina trihydrate and Na-aluminate liquor containing 110–120 gpl of caustic concentration was supplied by Mrs. National Aluminium Company Ltd., India. The particle size distribution of the alumina trihydrate particles is shown in Fig. 1, where 50% particles passes through 1.94 micron size. The particle size measurement is carried out in a Malvern particle size analyser model 3600. the specific surface area of the particles are directly obtained from the analyser. The characteristics of the slurry like, specific gravity, particle density, viscosity of liquor, etc. are shown in Table 1. The settling experiments are carried out in a standard one litre glass measuring cylinder of 6.38 cm dia and 40 cm height. All filtration experiments are carried out in a 500 ml 3 piece Buchner funnel fitted with a Whatman 42 filter paper under vacuum. The
Fig. 1. Particle size distribution of alumina trihydrate.
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Table 1 Various physical parameters of alumina trihydrate particles and Na-aluminate liquor Specific gravity of liquor Viscosity of liquor pH of the liquor Alumina concentration Caustic concentration Density of the particles Specific surface area
1.136 1.4 cP )14 50–60 grl 110 to 120 grl 2.3 grml 4.05 m2 rml
diameter of the Buchner funnel used for the study is 10 cm. The volume of slurry taken for each filtration experiment is 275 ml. Washing of cake is carried out with 75 ml of de-ionised water immediately after the supernatant liquor disappears from the cake surface. Different concentration slurries are prepared from the supplied slurry by adding liquor or by removing liquor. The slurry concentrations used are: 36 kgrm3, 45 kgrm3, 65 kgrm3, 88 kgrm3 and 170 kgrm3. The concentration of flocculant stock solution prepared is 0.2 gpl. The porosity of filter cakes are measured using the following equation:
as
K 0 S02
rs
ž
1ye
e3
/
Ž 8.
where a s specific cake resistance, S0 s specific surface area, K 0 s Kozeney constant s 5, rs s solid density, e s porosity.
4. Results and discussion 4.1. Settling study The settling work carried out with as received slurry Ž65 kgrm3 solids concentration. collected from Mrs. NALCO shows very poor settling characteristics mainly because of very fine nature of the trihydrate particles. To enhance the settling properties three different flocculating agents namely, Magnafloc 351 Žnonionic., Magnafloc 156 Žanionic., Magnafloc 140 Žcationic. are tested. It is observed that anionic flocculant does not respond at all, the reason being the alumina trihydrate particles show negative zeta potential at this pH range ŽPuttock et al., 1985.. Due to the negative potential of trihydrate particles charge neutralisation with an anionic flocculant is not possible. The mechanism of flocculation by organic flocculants is considered to involve mainly two processes Ža. surface charge neutralisation, and Žb. bridging ŽHughes, 1981.. Both mechanisms can act simultaneously or separately. It is however observed that both cationing and non-ionic flocculants have worked satisfactorily. The effectiveness of cationic and non-ionic flocculants in the slurry is due to charge neutralisation in the former case and bridging in the latter. Fig. 2 shows the settling curves of as received as well as flocculated slurries. The settling rate could not be enhanced at lesser dosage of
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Fig. 2. Settling curves of alumina trihydrate particles in Na-aluminate liquor.
flocculant but it showed improved rate at higher dosage. The settling rate has increased from 0.56 cmrh to 2.4 cmrh when 200 ppm additive is used. The ultimate settled concentration of the trihydrate particles is found to be 170 kgrm3. 4.2. Filtration study Fig. 3 shows the filtration curves for three different concentration slurries without ŽA, B, C. and with ŽD, E. the addition of flocculants. The solids concentration of C is the ultimate settled concentration of the slurry obtained from settling experiments. Comparing the curves A, B and C, it is observed that filtration rate has decreased with increase in slurry concentration. But in case of flocculated slurries the filtration rate has decreased Žcurves B, D and E.. The filtration rate has dropped from 13.6 to 8.64 kg my2 hy1 for slurry B and E. This behaviour is quite opposite to the generally observed behaviour where due to flocculation, filtration rate increases by forming larger particles which in turn increases the porosity. In the present case, probably due to very soft nature of particles, as the cake forms the interparticle gap reduces due to flocculation giving rise to a lower porosity. This fact is supported by the increase in specific cake resistance Ž8.7 = 10 11 mrkg for D and 10.8 = 10 11 mrkg for E. for flocculated slurry compared to 6.9 = 10 11 mrkg for unflocculated slurry ŽB.. The medium resistance has found to be decreased with flocculation ŽB s 13.9 = 10 10 my1 , D s 3.12 = 10 10 my1 and E s 6.14
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Fig. 3. Filtration curves of three different concentration slurries without and with the addition of flocculants.
= 10 10 my1 .. The moisture content of the filter cakes obtained at different applied pressures are shown in Table 2 and found to be decreasing with applied pressure. The medium resistance R should normally be constant but it varies due to penetration of some solids into the medium and also due to applied pressure. The variation of R with pressure is studied using only Na-aluminate liquor which is shown in Fig. 4. The increase in R with applied pressure is due to the squeezing of filter medium only. The variation of R in slurry filtration with pressure is shown in Fig. 5 and Table 3. Variation of R with pressure ŽFig. 5. is shown for two different concentration Ž36 kgrm3 and 65 kgrm3 . slurries and both found to follow the straight line correlation with equal slope
Table 2 Variation of moisture content with pressure Pressure, kPa
Moisture, %
33 47 67 80 93 100
63 62 57 55 54 49
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Fig. 4. Variation of medium resistance R with pressure for Na-aluminate liquor.
Fig. 5. Variation of medium resistance R with pressure for two different concentration slurries.
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Table 3 Variation of medium resistance with slurry concentration at 80 kPa Slurry concentration, kgrm3
Medium resistance, my1
36 45 65 88 170
8.5=10 10 14=10 10 12.2=10 10 20.2=10 10 26.8=10 10
but having different intercepts. The increase in R with concentratin ŽTable 3. may be due to the penetration of more particles in the filter medium. 4.3. Specific cake resistance as design parameter — an inÕestigation The most important factor which is considered here for investigation is the specific cake resistance. Bartolomeu et al. Ž1989. successfully used bench scale tests for two extremely different sludges using Eq. Ž7. and found comparable results to those presented in literature for industrial operations. The specific cake resistance considered in the above case is the apparent resistance using Eq. Ž3.. However, most of the design engineers use average specific resistance as scaling up parameter, which is much lower than apparent resistance. It is therefore observed that a decrease or an increase in a by 4 times means double or halve the filtration rate calculation. Therefore, the value of a has to be measured carefully. Now, as liquid flows through a compressible bed of particles viscous drag on the particles produces compressive pressure which causes a to increase and e to decrease towards the filter medium ŽWakeman, 1981.. Figs. 6 and 7 show the variation of specific cake resistance a and porosity e with applied pressure for two different concentration slurries in log–log plot respectively. The straight line plot obtained for both the concentrations are found to be parallel with different a 0 and e 0 . Therefore, it indicates that for a particular slurry the compressibility index is constant inspite of whatever slurry concentration is chosen. Similar observation is obtained for porosity also. The correlation equation between a and pressure and e and pressure followed most often quoted form ŽShirato et al., 1964; Shirato et al., 1968; Rushton and Wakeman, 1977. as:
a s a0 Ž D P .
s
Ž 9.
and
e s e0 Ž D P .
yl
Ž 10 .
where s s compressibility index and l s porosity index, and a o and eo are cake resistance and porosity at unit applied pressure respectively. As most cakes are compressible and their specific cake resistance changes with pressure drop across the cake. In such cases average specific cake resistance aD P would replace a in Eq. Ž7., which is calculated as aD P s Ž1 y s . a 0 Ž D P . s.
I.N. Bhattacharyar Int. J. Miner. Process. 49 (1997) 107–118
Fig. 6. Plot of log a versus log D P for two different concentration slurries.
Fig. 7. Plot of ylog e versus log D P for two different concentration slurries.
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116
From Figs. 6 and 7, s and l are found to be 0.64 and 0.355 respectively, which indicates that s q l s 0.64 q 0.355 , 1 or s s 1 y l. Therefore, once s is known l can be found out and vice versa. It is also felt that a not only changes with pressure but also changes with cake depth. As the thickness of cake depends on concentration of the particles in a slurry, there is possibility of some correlation involving a and slurry concentrations. Slurry concentration effects on the values of a and e are also reported by Rushton et al. Ž1980. and Wakeman Ž1972.. Fig. 8 shows the plot of a vs. slurry concentrations. It is observed from the figure that with increase in concentration a decreased. The log–log plot of Fig. 8 is shown in Fig. 9 which follow a straight line correlation with a negative slope as shown below.
a s a 0 cym 1
Ž 11 .
where m1 is found to be 0.29. A similar approach is made for finding out relationship between e and concentration and found to be as:
e s e0 c m2
Ž 12 .
where m 2 is 0.68. It is observed in this case also that m1 q m 2 s 0.68 q 0.29 , 1, or m1 s 1 y m 2 . Rushton et al. Ž1978. have shown the effect of velocity and concentration on specific cake resistance on several inorganic materials. They found that a in a constant pressure
Fig. 8. Variation of a with slurry concentration Ž c ..
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Fig. 9. Plot of log a versus log c.
experiment go through a maximum with increasing concentrations and beyond this concentration the resistance decreases exponentially. As it is observed that a depends both on pressure and concentration parameters, the average value of a should be determined by taking into consideration both the parameters. Therefore, it is proposed that the specific cake resistance value to be taken for FR calculation should be the average value of aD P and a c obtained from pressure and concentration parameter, as shown below: a av s Ž aD P q a c . r2 Ž 13 . s ym 1 where aD P s Ž1 y s . a 0 Ž D P . and a c s Ž1 q m1 . a 0 Ž c . , where a c is the average specific cake resistance from concentration parameter. Table 4 shows the values of a obtained from trÕ vs. Õ plot, aD P , a c and a av at 80 kPa pressure. Table 4 Values of different a ’s at 80 kPa pressure Slurry conc., kgrm3
a , mrkg
aD P , mrkg
a c , mrkg
a av , mrkg
36 45 65 88 170
7.22=10 11 6.33=10 11 6.0=10 11 5.5=10 11 4.52=10 11
2.67=10 11 2.28=10 11 2.16=10 11 1.98=10 11 1.63=10 11
9.2=10 11 8.6=10 11 7.76=10 11 7.1=10 11 5.8=10 11
5.9=10 11 5.4=10 11 4.96=10 11 4.54=10 11 3.71=10 11
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The above equation has been found to be valid for the system alumina trihydrate particles–sodium aluminate liquor, and may not be valid for other systems, but the same concept may be utilised for finding out average a values in other systems also.
5. Conclusions Settling characteristics of alumina trihydrate particles in Na-aluminate liquor is very poor but can be improved by adding cationic or nonionic flocculants. The filtration rate has dropped due to the addition of flocculants. The increase in medium resistance during filtration may be due to the penetration of particles in the filter medium and squeezing of medium with applied pressure. Moisture content of the filter cake has decreased with increased pressure. It is observed that the sum of exponents in specific cake resistance and porosity equations equals to one. A realistic approach for measurement of filtration rate Ž FR . is considered by proposing the a av term.
Acknowledgements The author is thankful to Mrs. National Aluminium Company Limited, Damanjodi, Orissa, India, for supplying the material. The author also wishes to thank Prof. H.S. Ray, Director, Regional Research Laboratory, Bhubaneswar, for providing the facilities and Dr. R.P. Das, Head, Hydro and Electrometallurgy Division, for his encouragement during the work.
References Bartolomeu, F., Chambino, T. and Carrondo, M.J.T., 1989. Filtrat. Sep., ŽJulyrAugust.: 288. Carman, P.C., 1938. Trans. Inst. Chem. Eng., ŽOctober.. Hughes, M.A., 1981. Coagulation and flocculation. In: L. Svarovsky ŽEditor., Solid–Liquid Separation. Butterworth, p. 64. Puttock, S.J., Fane, A.G., Fell, C.J.D., Robins, R.G. and Wainwright, M.S., 1985. A.I.Ch.E.J., 31Ž7.: 1213. Rushton, A. and Wakeman, R.J., 1977. J. Powder Bulk Solids Technol., 1: 58. Rushton, A., Hosseini, M. and Hassan, I., 1978. In: Proc. Symp. on Solid–Liquid Separation Practices, Leeds. Yorkshire Branch of I.Chem.E., p. 27. Rushton, A., Hosseini, M. and Hassan, I., 1980. J. Sep. Process. Technol., 1: 35. Satapathy, B.K. and Pattanaik, S.C., 1991. Light Met.: 145. Shirato, M., Sambuichi, M. and Murase, T., 1964. Mem. Fac. Eng. Nagoya Univ., 16: 68. Shirato, M., Aragaki, T., Mori, R. and Sawamoto, K., 1968. J. Chem. Eng. Jpn., 1: 86. Wakeman, R.J., 1972. Proc. 2nd World Filtration Congress, London, p. 57. Wakeman, R.J., 1981. Trans. I.Chem.E., 59: 147.