Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 102-105
102
Physicochemical Aspects of the Filtration of Aqueous Suspensions of Fibers and Cement. 4. Influence of the Í* Potential of the Fibers on Filtration Efficiency Jacques Schultz,
*
Eugene Paplrer, and Michel Nardin
Ind. Eng. Chem. Prod. Res. Dev. 1983.22:102-105. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/26/19. For personal use only.
Centre de Recherches sur la Physico-Chimie des Surfaces Solides, CNRS, 68200 Mulhouse, France, and Laboratoire de Recherches sur la Physico-Chimie des Interfaces de I'Ecole Nationale Superieure de Chimie de Mulhouse, 68093 Mulhouse Cedex, France
In the fourth part of this general study of the filtration of aqueous suspensions of fibers and cement, the influence on filtration efficiency of the surface charge of fibers of different nature was studied. Appropriate surface treatments were performed on the fibers in order to modify, over a large range, their £ potential in the highly alkaline medium of the cement suspension. It could thus be shown that the filtration efficiency is optimum when the £ potential of the fibers is close to zero. This phenomenon is related to the better ability of the noncharged fibers to form a felt during filtration. The effect on filtration efficiency of all the determining parameters is also summarized.
Introduction In the previous parts (Schultz et al., 1983a,b,c) of this general study on the filtration of aqueous suspensions of fibers and cement, the influence on the filtration efficiency of various parameters has been studied. In particular, the effects of the composition of the suspension (fiber content and solids concentration Cs), the nature of the fibers
adsorbed on the fiber surface is (1.05 ± 0.15) X ecules/nm2.
Surface Modification of Fibers with
y have been quantitatively established. Since the fiber-cement suspension to be filtered is an alkaline medium of high ion concentration, it is justifiable to study in addition the surface charge of the fibers. The £ potential of the fibers will, therefore, be determined by the streaming potential method and tentatively related to
Results and Discussion Potentials of Fibers in
£
potential of the
fibers is measured by the streaming potential method. The apparatus used and the experimental conditions have been described elsewhere (Nardin et al., 1982). This apparatus is a slightly modified version of the one used for permeability measurements (Schultz et al., 1983c). The £ potential is given by the Helmholtz-Smoluchowski
equation
where De is the dielectric constant of the solution, is its viscosity, and is its conductivity. Ea, the streaming potential, is the difference in potential observed between the extremities of the fibrous plug when a hydrostatic pressure
difference ( ) is applied. If p is the slope of the linear relationship of the £ potential, in mV, is expressed by £
=
Ea
=
f{AP),
1.4411
when p is in mV X (cm H20)_1 and In eq 2, Da = 80 and = 10'2 P.
(2)
in ( '1 cm'1) X
106.
Surface Treatments of the Fibers. Surface Modification of Glass Fibers by Ca2+ Adsorption. The glass
fibers, a and c, have been treated by Ca(OH)2 as described by Nardin et al. (1982). The average value of Ca(OH)2 0196-4321/83/1222-0102$01.50/0
(Papirer
an
Alkaline Medium.
Ce-
ment suspensions contain essentially high quantities of Can ions at very high values of pH (pH 12.3). Therefore, the £ potential of the fibers has to be measured in a medium ressembling a cement suspension as closely as possible. Previous studies (Nardin et al., 1982) have shown that the £ potential is highly dependent on the Ca11 concentration of the solution. Thus, in this study a constant concentration of Ca11 in the solution was adopted, i.e., [Ca11] = 10'3 M. Concerning the pH of the solution, £ potential measurements cannot be achieved in solutions of pH 12.3. Therefore, a tentative evaluation of the £ potential of fibers at this pH has been made according to the following method. The streaming solution of pH 12.3 ± 0.2 is obtained by filtration of a suspension of cement at 200 g L'1. This solution flows through the fibrous plug for a time tc, called the contact time. The streaming is then stopped and the plug washed with solutions of Ca(N03)2 (10'3 M) having either pH 11 or pH 9.8. The pH of these solutions was adjusted with NaOH. These latter solutions were used for the determination of the £ potential of the fibers. The time of treatment of the fibers by the cement solution can be increased by repeating this operation. Under these conditions, it is supposed that the measured £ potential reflects the surface properties of the fibers acquired during contact with the cement solution. Figure 1 shows the £ potential of glass fibers a, measured with the two pH 9.8 and 11, as a function of the contact time tc. It can be seen that at pH 11, the £ potential increases with tc. This phenomenon is attributed to the adsorption of Ca11 entities on the fibers during contact with
the filtration efficiency.
The
mol-
et al., 1976; Touray et al., 1980). Glass fiber c and chrysotile fibers have been treated with a solution of aluminum trichloride having the following composition: 2.67 g of A1C13, 80 mL of toluene, 20 mL of dinitrobenzene, and 0.37 mL of HC1 (37%). One gram of the fibers is treated for 2 h with 5 mL of this reagent (10'3 M A1C13) at room temperature for glass fibers and at 80 °C for chrysotile. It has been verified that these treatments do not significantly alter the length of the fibers.
(glass fibers, a, b, c, d, e, carbon fibers, rayon spun fibers, asbestos ...), their morphological and mechanical properties (diameter d, length l, and modulus E), and the grid opening
Experimental Section £ Potential Determination.
A1C13
103
©
1983 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No.
3. The potential AlClg-treated chrysotile.
Figure
Figure
1.
The
f potential of glass fibers a
vs.
vs.
1,
1983
103
pH: (A) untreated chrysotile; (b)
time of contact (tc),
at pH 9.8 and 11.
Figure 2. Calculated potential of glass fibers a vs. time of contact (tc), at pH 12.3. cement solutions. At pH 9.8, the potential is independent of tc, the adsorbed species being desorbed at this lower pH. The value of the f potential at pH 9.8 constitutes an invariant reference value. In order to determine, on a semiquantitative basis, the value of the f potential at pH 12.3 corresponding to the cement suspension, linear extrapolations were carried out using the values obtained at pH 9.8 and 11 for different contact times. Figure 2 shows the variation of the ' potential of glass fibers, at pH 12.3, as a function of tc. The potential varies from -6 to a constant value of +6 mV, the change of sign occurring for tc 20 min. It appears that the adsorption kinetics and thus the surface modification of the fibers are rather slow. However, it should be stressed that the absolute values of the f potential of the fibers in the cement suspension are slightly lower since the Ca11 concentration is significantly higher. It can be therefore concluded that in the aqueous suspension of cement, under the conditions of filtration where the contact time is about 10 min, the f potential of glass fibers is not significantly altered by contact with the cement suspension and is of the order of -5 to 0 mV, i.e., very close to their isoelectric point (IEP). This conclusion can be extended to other fibers, i.e., glass fibers a, b, c, asbestos, carbon, and rayon-spun fibers. The potential at pH 12.3 is of the same order of magnitude as that of
Figure 4. The f potential vs. pH: (A) untreated glass fiber c; (B) AlClg-treated glass fiber c. Table I. f Potential at pH 12.3 of Untreated and Surface-Treated Glass and Chrysotile Fibers nature of fibers
fibers untreated fibers treated with Ca(OH). fibers treated with A1C13 chrysotile untreated chrysotile treated with A1C13
glass glass glass
? at pH 12.3, mV -5 + 18 -22.5
0
-18
~
glass fiber
a.
In order to study the influence of the f potential of the fiber on filtration efficiency, the surface properties of the fibers have been modified leading to a large range of positive and negative values of potentials in alkaline media.
f Potentials of Surface Modified Fibers.
Two
treatments were performed: (1) adsorption of Ca(OH)2 in the case of glass fiber c and (2) surface modification
through A1C13 in the case of glass fibers c and chrysotile. The adsorption of Ca(OH)2 leads to a drastic increase in f potential at pH 12.3 (from -6 to +18 mV). The AICI3 treatment also changes the f potential markedly, but in the opposite direction. This change is seen in Figure 3 which shows the variation of the f potential of chrysotile with pH. The { potentials of nontreated and A1C13 treated chrysotile both decrease with increasing pH, but become equal to zero (IEP) respectively at pH 12.3 and 9.1. It should also be noticed that the f potential of the treated asbestos at the reference value of pH 12.3 is equal to about -18 mV. By analogy with the behavior of alumina (Robinson et al., 1964), this change may be attributed to the partial replacement of the outer Mg(OH)2 layer of the chrysotile by an aluminum hydroxide layer. A similar trend is observed in Figure 4 giving the variation of ' potential of untreated and AlCl3-treated glass fibers c with pH. The IEP’s are respectively equal to about 13 and 7.8. These different treatments thus allow the preparation of fibers having very different potentials at pH 12.3, as recalled in Table I.
104
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No.
1,
1983
5. Filtration efficiency (e) vs. fiber content ( ) of the suspension for glass fibers c: (A) untreated; (B) treated with Ca(OH)2; (C) treated with A1C13.
Figure
7. Reduced filtration efficiency vs. £ potential at pH 12.3 of untreated and treated glass and chrysotile fibers.
Figure
Hence surface charge effects concern only the interactions between fibers. In fact, repulsive forces will be developed between charged fibers (i.e., treated fibers), whereas no electrical interactions will occur between the noncharged, untreated fibers. Although better dispersion in the suspension is observed for charged fibers, it seems clear that this surface charge will decrease the ability of the fibers to form a nonpermeable felt and, as a consequence, reduce filtration efficiency. zero.
Conclusion
6. Filtration efficiency (e) vs. fiber content ( ) of the suspension for chrysotile: (A) untreated; (B) treated with A1C13.
Figure
Relation between £ Potential and Filtration Efficiency. Given fibers with a large spectrum of values of £ potential, the effect of surface charge of the fibers on filtration efficiency may be investigated. Figure 5 relates the filtration efficiency e to the fiber content for glass fibers c untreated, treated with Ca(0H)2, and treated with A1C13, respectively. For both treatments leading to either largely positive or negative values of £ potential, the efficiency of filtration is lower than the one observed with untreated fibers which have £ values close to zero. It must be added that this effect becomes negligible for glass fibers of smaller diameters, the morphology of the fiber being the overriding factor of filtration (Schultz et al., 1983c). The same phenomenon is observed in the case of chrysotile as shown in Figure 6. Again, the fiber of highly negative £ value has lower filtration efficiency than the
untreated fiber. The effect of the surface charge of the fibers on their filtration efficiency is summarized in Figure 7. This figure connects the reduced efficiency, i.e., the ratio of the efficiencies for treated and untreated fibers at a given fiber content ( = 1%) to their £ potential, at pH 12.3. It is clear that the filtration efficiency, for all fibers, is optimum when the £ potential is equal to zero, i.e., at the IEP of the fibers. This observation is in agreement with work done by Lindstróm and Soremark (1975) on cellulose. A possible explanation of this result is the following. As shown by Daimon and Roy (1978, 1979) the £ potential of cement particles, in the suspension, is close to
The role of the surface charge of glass and chrysotile fibers in filtration has been studied using £ potential measurements. In order to change the £ of the fibers over a large range, surface modifications, i.e., Ca(OH)2 adsorption and A1C13 treatment, were made. It has been shown that the filtration efficiency is lowered when the £ potential of the fibers is significantly different from zero. This phenomenon has been explained by repulsive forces between the fibers decreasing the ability of the fibers to form an efficient felt during filtration. However, it should be noted that the surface charge is only a second-order effect. As a general conclusion of the four parts of this study, a model has been developed relating filtration efficiency e (or filtration rate) to all the factors governing the filtration of aqueous suspensions of fibers and cement, in a static mode. The main features of this model can be summarized in the equation e
k0
2 *
)
+
k0ypíW/sji/a
*
"I
J This equation takes into account the influence of: (a) the composition of the suspension defined by the fiber content ( ) and the concentration (Cs) of solids (cement and fibers); (b) the geometry of the grid defined by its opening ( ); (c) the properties of the fibers such as length ((), diameter (d), specific gravity (p), and Young’s modulus (.E); (d) k0 is a constant equal to 0.77 ± 0.07 SI units and, according to this fourth part of the study, depends on the surface properties of the fibers expressed by their £ potential in the suspension of cement. The principal interest of such a model stems from its usefulness in predicting quantitatively the two fundamental characteristics of a filtration process (filtration efficiency and filtration rate) from readily measurable
Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 105-110
parameters. An extension of the foregoing work to encompass dynamic filtration could perhaps be of use in industrial applications.
Acknowledgment We acknowledge with thanks the support of this project by both Saint-Gobain Recherche and Everitube and the cooperation and advice of Drs. J. J. Massol, F. Naudin, and A. Sabouraud. Registry No. A1C13, 7446-70-0; Ca(OH)2, 1305-62-0.
105
Lindstróm, T.; Sóremark, C. Das Papier 1975, 29(12), 519. Nardin, M.; Papirer, E.; Schultz, J. J. Colloid Interface Sci. 1982, 88(1), 204. Papirer, E.; Dovergne, G.; Siffert, B.; Leroy, P. Clays Clay Miner. 1976, 24, 101. Robinson, M.; Pask, J. A.; Fuerstenau, D. W. J. Am. Ceram. Soc. 1964, 47, 516. Schultz, J.; Papirer, E.; Nardin, M. Ind. Eng. Chem. Prod. Res. Dev. 1983a. Part 1 in this issue. Schultz, J.; Papirer, E.; Nardin, M. Ind. Eng. Chem. Prod. Res. Dev. 1983b. Part 2 in this Issue. Schultz, J.; Papirer, E.; Nardin, M. Ind. Eng. Chem. Prod. Res. Dev. 1983c. Part 3 in this Issue. Touray, J. C.; Thomassin, J. H.; Baillif, P.; Scherrer, S.; Champomier, F.; Naudin, F. J. Non-Cryst. Solids 1980 38, 643.
Literature Cited
Received for review February 19, 1982 Accepted August 23, 1982
Dalmon, M.; Roy, D. M. Cem. Conor. Res. 1976, 8(6), 753. Daimon, M.; Roy, D. M. Cem. Conor. Res. 1979, 9(1), 103.
Bench-Scale Studies To Recover Alumina from Clay by Hydrochloric Acid Process
a
J. A. Elsele,* D. J. Bauer, and D. E. Shanks Reno Research Center, Bureau of Mines, U.S. Department of the Interior, Reno, Nevada 89512
As part of its goal of producing cell-grade alumina from clay, the Bureau of Mines, U.S. Department of the Interior, conducted bench-scale cyclic tests of the Bureau's proposed clay-HCI leachlng-HCI sparging process and Investigated in detail the crystallization of aluminum chloride hexahydrate. The composition of recycled leaching liquor was determined for two HCI sparging crystallization conditions: 36% HCI and 26% HCI. Crystallization research showed that cell-grade alumina could not be produced without a recrystallization step.
Introduction Alumina feed for electrolytic cells in the production of aluminum metal is obtained from bauxite by the Bayer process. The United States has very little bauxite and is dependent on imports. As part of its goal of decreasing dependence on foreign resources, the Bureau of Mines
implemented a program for producing cell-grade alumina from domestic aluminous material. The United States has more than 3 billion tons of high-grade kaolinitic clay that cannot be treated by the Bayer process because of high Si02 content (Bureau of Mines, 1967). Extensive research has been conducted to devise an acid process to recover alumina (Peters and Johnson, 1974). The most promising process is HCI leaching of the clay and crystallization of A1C13-6H20 (ACH) by sparging with HCI gas (Bengtson et al., 1978). The Bureau of Mines performed bench-scale and miniplant-scale research to provide essential information for design of a 25-ton-per-day HCI process pilot plant (Bengtson et al., 1980; Bisele, 1980; Maysilles et al., 1981; Poppleton and Sawyer, 1977; Shanks et al., 1981). In this process, calcined clay is leached with 26% HCI and the clarified liquor is treated by solvent extraction to remove iron. The purified pregnant liquor is evaporated to increase the A1C13 concentration, and ACH is crystallized by injecting HCI gas. The crystals are calcined to alumina, and HCI is recovered from the offgases. An important parameter is the purity of the product alumina. Data on the purity of alumina produced by a continuous process can only be obtained by knowledge of the concentration of impurity elements in the feed stream going to crystallization. Since none of the previous work
Table I.
Composition of Calcined Clay
compd or element ai203
tío2
Fe203 P205
MgO
K20
CaO Na2G
%
43.0 3.27 1.26 0.087 0.087 0.063 0.034 0.032
compd or element Cr203
ZnO NiO MnO CuO
so4
F Si02
%
0.014 0.012 0.0079 0.0037
