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Comments on “The flow behaviour of bentonite — water suspensions”
Technology. 11 (1975) 95-98 @ Elsevier Sequoia S-A_, Lausanne - Printed in The Netherlands
Powder
Letter to the Editor
Comments
on “The
flow behaviour
of bentonite
-
water suspensions”
D.C.H. CHENG Warren
Spring
Laboratory,
Steuenage.
Herts.
SGI
2BX
(Gt.
Britain)
Received April 22.1974
Sharma et al. published two articles on the flow behaviour of bentonite - water suspensions in 1972 [I] and 1973 121. They used an extrusion rheometer and a pipeline set up and covered concentrations from 1% up to 7% by weight. They found that the rheometer results obtained using tubes of different length to diameter ratios do not coincide and concluded that the bentonite suspensions display rheopexy combined with thixotropy; they derived viscosity values that they claim indicate dilatancy and viscoelasticity. The authors divided their rheometer and pipeline data into laminar and turbulent regimes. The former data were fitted with five-param eter Meter flow curves and the parameters used to correlate the turbulent data. The astonishing thing is that they found that the concentration had very little effect on the results, the data for all seven concentrations bunching together within the experimental scatter. In our work published in 1965 [ 33, a reference quoted by Sharma et al., we found that the viscosity increased by 70 to 80-fold between the concentrations of 1.7% to 7.8%. No comparison between the two sets of data was made by them nor any explanation offered for their observation. It is, of course, well known that the rheological property of bentonite suspensions is very sensitive to the ionic composition of the solids as well as the pH and electrolyte composition of the water used. Depending on the balance of these factors, the interparticle attraction may be strong or non-existent_ If the attraction is strong, the suspension would be timedependent, possess yield stress and be highly viscous even when the solids concentration is
low; the dependence on concentration would be dramatic. But if attraction is absent, the increase of viscosity is small even up to medium solids concentrations; and dilatant beha-viour is certainly not prominent until close-packing of the solid particles is approached at about 60 70% by volume. It seems clear that the reason why no variation with concentration was observed by Sharma et al. was that the inter-particle attraction was negligible in their bentonite suspensions. The viscosity of such a suspension is governed by hydrodynamic interaction between solids and liquid alone and can be calculated using the Einstein equation_ Even for the highest concentration of 7% wt. studied, the volume concentration was only 2.7% and the increase in VkCGSity over that of the water would be 6.8%. Such a small increase was readily and completely masked by the scatter of their results, even though the scatter was not unusually larger than commonly encountered in viscometric and pipeline experiments. In view of this, one has to search for an explanation of the differences in rheometric data, obtained in tubes of different L/D ratio, other than in complex rheological behaviour of rheopexy, thixotropy and viscoelasticity. The explanation is likely to be end-effect. Normally, it is common to assume that the entrance length is about 50 diameters, i.e. 16 cm and 21 cm respectively for the 0.317-cm and O-435-cm tubes. Therefore the tubes need to be, say, ten times these lengths if entrance effect is to be minimised. The lengths of 25 cm and 31 cm used by Sharma et al. would appear to be quite inadequately short. End-effect corrections were shown to be
96
significant even for tubes of 300 cm length in my work on a tube viscometer. A graph of thhcend-correction observed is reproduced as Fig. 1 from my report 141. The clay suspension mentioned was in fact the 1.7% wt. bentoniteGn-water mentiuned in our pipeline paper {3]_ The end-correction obtained for water can be used to estimate the magnitude of end-effect in the tubes used by Sharma et al. To do this, the data on Fig. 1 have to be extrapolated by a considerable extent. A simple linear extrapolation of the turbulent end-correction (the Iaminar and turbutent flow data were separated by the discontinuity shown in the curve) would lead to end-effects that are four times the values finally used. The relation (velocity) (head ) mmHg was fitted to thr?water data on Fig. 1. The correction derived frum this equation was halved before applying to the tubes used by Sharma et al. for the reason that, whereas in their work the tube discharged into the atmosphere, in my experiments the discharge was into a receiver used to collect the fluid. This end-correction was added on tu the pressure ca?cuh&ed for turbulent water fiow using, for convenience, the Blasius equation. The xesufts were plotted on Fig. 2 (if the viscosity and densib> of the 7% bentonite suspension were used in the BE&us equation, the five cal-
culated curves would be raised by not more than 5%) and compared with the experimental data for bentonite suspensions obtained by Sharma et al.
It
used
is to be rxalked that the end*ormction is only approximate, but the magnitudes
invohFedserve to showthat the-rheometric data of 2hamia et ;al. ‘could reasonS.Ay be expk6ned by en&efR?ct *‘a 1argextent. The way in whi&h ex&$menW iesuits of Sharma ef af; di+erge &om the cakulafzd vaIues for water is nritew&.hy. The 254xn 0.435-em tube data were high compared with the cakxilated curve, whilst the 31-cm 0,317 em tuhe data were almost identical with the because the experhnencaleuJat&%fThisarose tal pressma drop that Sharma et a~?.obtained for the 25-em 0,435-cm tube was actuany larger than that for the 31-fzm 0.435-cm t&e at the same flow rate.) In contrast, the 3%cm 0.317-cm tube data were low compared with the calculated curve- The explanation for these divergences could be found in some other properties of bentonite suspensions, namely their .sensitivity to particle size and method of preparation. Different bat&es of bentonite, even from the same geogfaphid source, can have greatly different pr+erWs. For example, we found a two-fold difference in viscosity between two bat&es of powder used in our work 1;5]. In the relatively small quantities used in the rheometer, one would even expect differences between different aiiquots from any batch by virtue of particle size differences arising fram -on. In addition, the intensity of a@t&ion and ageing of the suspension result in varying degree of water molecule penetration into the bentonite crysuds and so influence ::;r viscosity, It is not clear from the artides by Sharma et al_ whether different batches of hentonite powder or different Iota of suspetiom were wed and in what order the measurem ems were csmied uut. They mentioned repeat runs at each concenfxation, but were these made using dSf&rent Iots of suspension? rf9 in fact,iixwee
'cmswemmedforthe dsfferentbgRic~ 'vediIutiontuobhin fsm?etubea,witbs,exowercon~~ons,this~edurewould weIIaccomtforthediveqpncesnoted~OEherwise,onewor.tIdkiweto lookforgross diflereneesinBmgeometayofthetibesmed.
for 0.8 in. It is seen that the flow eharacteristies of the bentonite suspensions were not greatly different from that for watir. Sharma et nl,offered no explanation for the higher resuhs obtained with the vertical pipe compared with the horizontal pipe. It seems likely that this was due to the different distances between the test section from the disturbance up&earn. The vertical test section started 1 ft. downstream of the flow meter whereas the horizon&d test section was 2 ft. downstream of a bend which was 6 ft. of stm@t pipe away from the fiaw meter. The vertical pipe date must therefore contain a large element of the d’sturbance to flow caused by the Bow meter. Sharma et al. apparently divided their x-esuits, rbeometric and pipetine, into huninar and turbulent regimes, and fitted appropriate equations to them, but they gave no indication as to what criferioa was used to make the distinction. In view of the close agreement hetween their laminar data with turbulent water flow {Reynolds number involved was in the range 6 X 1Oa to 95 X 103], it seems clear that all their data are in fact turbulent. ltn conclusion, it would appear that the results obtained by Sharma ef rrl. were not significantiy different from that of turbulent fiow of water_ Et therefore follows that the data treatment they made and the condusions that they drew regarding the rheoiogicai behaviour of bentonite suspensions are wholly unwarranted.
98
REFERENCES 1 P-V. Sharma, R. Nagarajan and G.S. Davies, Powder Technol., 6 (1972) 103. 2 P-V. Sharma, R. Nagarajan and G.S. Davies, Powder Technol., 8 (1973) 193. 3 D.C.H. Cheng, D.J. Ray and F.H.H. Valentin. Trans. Inst. Chem. Engrs., 43 (1965) T176.
4 D.C.H. Cheng, A vertical capillary tube viscometer, Res. Rept No. RRICEl46, Warren Spring Lab., Stevenage, 1964. 5 D.C.H. Cbeng and D.J. Ray, The flow of a thixotropic suspension througb pipes and pipe-fittings, Res. Rept No. RR/CE/48, Warren Spring Lab., Stevenage, 1964.
Powder
Letter to the Editor
Comments
on “The
flow behaviour
of bentonite
-
water suspensions”
D.C.H. CHENG Warren
Spring
Laboratory,
Steuenage.
Herts.
SGI
2BX
(Gt.
Britain)
Received April 22.1974
Sharma et al. published two articles on the flow behaviour of bentonite - water suspensions in 1972 [I] and 1973 121. They used an extrusion rheometer and a pipeline set up and covered concentrations from 1% up to 7% by weight. They found that the rheometer results obtained using tubes of different length to diameter ratios do not coincide and concluded that the bentonite suspensions display rheopexy combined with thixotropy; they derived viscosity values that they claim indicate dilatancy and viscoelasticity. The authors divided their rheometer and pipeline data into laminar and turbulent regimes. The former data were fitted with five-param eter Meter flow curves and the parameters used to correlate the turbulent data. The astonishing thing is that they found that the concentration had very little effect on the results, the data for all seven concentrations bunching together within the experimental scatter. In our work published in 1965 [ 33, a reference quoted by Sharma et al., we found that the viscosity increased by 70 to 80-fold between the concentrations of 1.7% to 7.8%. No comparison between the two sets of data was made by them nor any explanation offered for their observation. It is, of course, well known that the rheological property of bentonite suspensions is very sensitive to the ionic composition of the solids as well as the pH and electrolyte composition of the water used. Depending on the balance of these factors, the interparticle attraction may be strong or non-existent_ If the attraction is strong, the suspension would be timedependent, possess yield stress and be highly viscous even when the solids concentration is
low; the dependence on concentration would be dramatic. But if attraction is absent, the increase of viscosity is small even up to medium solids concentrations; and dilatant beha-viour is certainly not prominent until close-packing of the solid particles is approached at about 60 70% by volume. It seems clear that the reason why no variation with concentration was observed by Sharma et al. was that the inter-particle attraction was negligible in their bentonite suspensions. The viscosity of such a suspension is governed by hydrodynamic interaction between solids and liquid alone and can be calculated using the Einstein equation_ Even for the highest concentration of 7% wt. studied, the volume concentration was only 2.7% and the increase in VkCGSity over that of the water would be 6.8%. Such a small increase was readily and completely masked by the scatter of their results, even though the scatter was not unusually larger than commonly encountered in viscometric and pipeline experiments. In view of this, one has to search for an explanation of the differences in rheometric data, obtained in tubes of different L/D ratio, other than in complex rheological behaviour of rheopexy, thixotropy and viscoelasticity. The explanation is likely to be end-effect. Normally, it is common to assume that the entrance length is about 50 diameters, i.e. 16 cm and 21 cm respectively for the 0.317-cm and O-435-cm tubes. Therefore the tubes need to be, say, ten times these lengths if entrance effect is to be minimised. The lengths of 25 cm and 31 cm used by Sharma et al. would appear to be quite inadequately short. End-effect corrections were shown to be
96
significant even for tubes of 300 cm length in my work on a tube viscometer. A graph of thhcend-correction observed is reproduced as Fig. 1 from my report 141. The clay suspension mentioned was in fact the 1.7% wt. bentoniteGn-water mentiuned in our pipeline paper {3]_ The end-correction obtained for water can be used to estimate the magnitude of end-effect in the tubes used by Sharma et al. To do this, the data on Fig. 1 have to be extrapolated by a considerable extent. A simple linear extrapolation of the turbulent end-correction (the Iaminar and turbutent flow data were separated by the discontinuity shown in the curve) would lead to end-effects that are four times the values finally used. The relation (velocity) (head ) mmHg was fitted to thr?water data on Fig. 1. The correction derived frum this equation was halved before applying to the tubes used by Sharma et al. for the reason that, whereas in their work the tube discharged into the atmosphere, in my experiments the discharge was into a receiver used to collect the fluid. This end-correction was added on tu the pressure ca?cuh&ed for turbulent water fiow using, for convenience, the Blasius equation. The xesufts were plotted on Fig. 2 (if the viscosity and densib> of the 7% bentonite suspension were used in the BE&us equation, the five cal-
culated curves would be raised by not more than 5%) and compared with the experimental data for bentonite suspensions obtained by Sharma et al.
It
used
is to be rxalked that the end*ormction is only approximate, but the magnitudes
invohFedserve to showthat the-rheometric data of 2hamia et ;al. ‘could reasonS.Ay be expk6ned by en&efR?ct *‘a 1argextent. The way in whi&h ex&$menW iesuits of Sharma ef af; di+erge &om the cakulafzd vaIues for water is nritew&.hy. The 254xn 0.435-em tube data were high compared with the cakxilated curve, whilst the 31-cm 0,317 em tuhe data were almost identical with the because the experhnencaleuJat&%fThisarose tal pressma drop that Sharma et a~?.obtained for the 25-em 0,435-cm tube was actuany larger than that for the 31-fzm 0.435-cm t&e at the same flow rate.) In contrast, the 3%cm 0.317-cm tube data were low compared with the calculated curve- The explanation for these divergences could be found in some other properties of bentonite suspensions, namely their .sensitivity to particle size and method of preparation. Different bat&es of bentonite, even from the same geogfaphid source, can have greatly different pr+erWs. For example, we found a two-fold difference in viscosity between two bat&es of powder used in our work 1;5]. In the relatively small quantities used in the rheometer, one would even expect differences between different aiiquots from any batch by virtue of particle size differences arising fram -on. In addition, the intensity of a@t&ion and ageing of the suspension result in varying degree of water molecule penetration into the bentonite crysuds and so influence ::;r viscosity, It is not clear from the artides by Sharma et al_ whether different batches of hentonite powder or different Iota of suspetiom were wed and in what order the measurem ems were csmied uut. They mentioned repeat runs at each concenfxation, but were these made using dSf&rent Iots of suspension? rf9 in fact,iixwee
'cmswemmedforthe dsfferentbgRic~ 'vediIutiontuobhin fsm?etubea,witbs,exowercon~~ons,this~edurewould weIIaccomtforthediveqpncesnoted~OEherwise,onewor.tIdkiweto lookforgross diflereneesinBmgeometayofthetibesmed.
for 0.8 in. It is seen that the flow eharacteristies of the bentonite suspensions were not greatly different from that for watir. Sharma et nl,offered no explanation for the higher resuhs obtained with the vertical pipe compared with the horizontal pipe. It seems likely that this was due to the different distances between the test section from the disturbance up&earn. The vertical test section started 1 ft. downstream of the flow meter whereas the horizon&d test section was 2 ft. downstream of a bend which was 6 ft. of stm@t pipe away from the fiaw meter. The vertical pipe date must therefore contain a large element of the d’sturbance to flow caused by the Bow meter. Sharma et al. apparently divided their x-esuits, rbeometric and pipetine, into huninar and turbulent regimes, and fitted appropriate equations to them, but they gave no indication as to what criferioa was used to make the distinction. In view of the close agreement hetween their laminar data with turbulent water flow {Reynolds number involved was in the range 6 X 1Oa to 95 X 103], it seems clear that all their data are in fact turbulent. ltn conclusion, it would appear that the results obtained by Sharma ef rrl. were not significantiy different from that of turbulent fiow of water_ Et therefore follows that the data treatment they made and the condusions that they drew regarding the rheoiogicai behaviour of bentonite suspensions are wholly unwarranted.
98
REFERENCES 1 P-V. Sharma, R. Nagarajan and G.S. Davies, Powder Technol., 6 (1972) 103. 2 P-V. Sharma, R. Nagarajan and G.S. Davies, Powder Technol., 8 (1973) 193. 3 D.C.H. Cheng, D.J. Ray and F.H.H. Valentin. Trans. Inst. Chem. Engrs., 43 (1965) T176.
4 D.C.H. Cheng, A vertical capillary tube viscometer, Res. Rept No. RRICEl46, Warren Spring Lab., Stevenage, 1964. 5 D.C.H. Cbeng and D.J. Ray, The flow of a thixotropic suspension througb pipes and pipe-fittings, Res. Rept No. RR/CE/48, Warren Spring Lab., Stevenage, 1964.