Interrupted electroosmotic dewatering of clay suspensions

Interrupted electroosmotic dewatering of clay suspensions H. R. Rabie, A. S. Mujumdar, and M. E. Weber Department

of Chemical

Engineering,

McGill

University,

Montreal,

Quebec,

Canada

A new method of electroosomotic dewatering with interrupted DC voltage or current was demonstrated. In this method the electrodes are short circuited during the periods of power interruption. Suspensions of Bentonite, kaolin and red clay were dewatered by electroosmotic dewatering using interruption with a short circuit, interruption with an open circuit and continuous DC. The suspension was held in a vertical j-cm diameter column between two platinum-coated, titanium mesh electrodes, an upper anode, and a lower cathode. In the interrupted regimes the off-time ranged from 0.5-20 s with the on-timeJixed at 30 s. The interrupted process with a short circuit was also applied at the end of a DC run when dewatering had stopped. Interruption with a short circuit applied during a 0.5 s off-time removed20-40% more water than DC or interruption with an open circuitfor equal energy consumption. Interruption with a short circuit at the end of a DC run removed additional water, but the total removal was less than if interruption with a short circuit were applied from the beginning.

Keywords:

dewatering; electroosmosis;

interruption; solid-liquid separation; clays; suspensions;

Bentonite; kaolin;

red clay

Introduction Electroosmotic dewatering (EOD) removes water from colloidal suspensions that are difficult to dewater by mechanical means. The advantage of EOD is that it can remove water with an energy expenditure well below the heat of vaporization, thus making it an economical alternative to thermal drying. Many studies have been conducted on EOD under continuous DC.lm7 The few published investigations of EOD with power interruption, all with an open circuit during power interruption, gave conflicting results. Some worker&” found better performance with interruption than with continuous DC power, whereas others6 found poorer performance. For example, Sprute and Kelsh*-lo dewatered coal waste with interrupted DC and continuous DC at current densities from 1-4 mA/cm2 of bed cross section. The on/off cycle was 30 min with power on-time varying from 12-18 min. Interrupted EOD required less energy than DC for the same water removal. Lockhart and Hat-G6 in experiments on coal washery tailings, showed that interrupted open circuit EOD did not remove more water

Address reprint requests to Dr. M. E. Weber at the Department of Chemical Engineering, McGill University, 3480 University Street, Montreal, Quebec, Canada H3A 2A7. Received 20 April 1993; accepted 15 July 1993

38

Sep. Technol.,

1994, vol. 4, January

for the same energy expenditure if the electrodes were designed so that the water could flow easily out of the suspension. They concluded that previously reported improvements with interruption were caused by poor drainage of water during continuous EOD rather than by more rapid electroosmosis with power interruption. We began a study of the interrupted EOD of clay suspensions. During our early experiments we discovered that better results were obtained when the electrodes were short-circuited during the period of power interruption. Our major focus then became the exploration of this new mode of operation. In interrupted dewatering a constant current or voltage is applied for t, s, the on-time, followed by a period of t, seconds, the off-time, when the power supply is turned off. The pattern is then repeated. Figure I shows the variation of applied DC voltage or DC current as a function of time. During the off-time there is either a short circuit or an open circuit between the electrodes. Interrupted EOD with a short circuit is denoted as “IS” and with an open circuit, as “IO.” The interrupted EOD process may be applied from the beginning of a run or at the end of a DC run when dewatering stops (denoted DC/IS).

Experimental set-up and procedure A schematic diagram of the electroosmotic dewatering apparatus is shown in Figure 2. The suspension was 0

1994 Buttetworth-Heinemann

Electroosmotic

dewatering

of clay suspensions:

H. R. Rabie et al.

14

Figure 1 current.

Sketch of the time variation of applied DC voltage or

held in a vertical acrylic cylinder (l), 5 cm ID and 20 cm long with flanges on the lower end. A flanged funnelshaped base (A) was bolted to the lower flange of the cylinder. The base (A) held a drilled acrylic cylindrical plate (2) to support the lower electrode (3). This support plate had 90 holes of 3 mm diameter drilled on a square pattern with 4.8 mm between centers. The plate was 63 mm in diameter and 7 mm thick. An electrical wire was soldered to the edge of the lower electrode and passed through a groove in the top surface of the flange of the base (A). A filter sheet (5) was held in place above the lower electrode by an O-ring clamped between the two lower flanges. The filter sheet, which was made of Nylon with 5.0~pm pores, was supplied by Micron Separation Inc (Westborough, MA). A circular acrylic plate, 0.5 cm thick, with 29 holes of 2-mm diameter drilled through it to match the diamond pattern of opening in the electrode, was fixed to the end of a cylindrical tube. The upper electrode (4) was attached to this plate by three small plastic screws. A wire was soldered to the upper electrode, passed through the tube and was connected to the circuit by a slip ring (10) mounted on the tube. These connections permitted the rotation of the upper electrode, a feature not used in this work. To maintain electrical contact between the bed and the upper electrode, the bed of wet solids was pressed against the upper electrode by two springs (14). The springs were chosen so that there was no significant compression of the bed. Each electrode was a circular piece of titanium mesh coated with platinum to a thickness of 2.5 ,um to pre-

I

1. Aaylicredncyllnda

6. DC pcrwa IU~@Y

11.munl

2Bcdwpport

7. voltmeta

12.ekpporl

3. Luwaduarode

8. Atllmeter

15. Metelk Rod

4. Uppaac4%rode

0. llmer Relay

14.sprlng

s.Rlmpapcr

Figure 2

10.8llp

Electroosmotic

Ring

dewatering

apparatus.

vent electrode corrosion. The wires forming the mesh were flattened in cross section. The openings in the mesh were diamond shaped, and the open area was about 50% of the cross-sectional area. These electrodes were supplied by Engelhard Corporation (Edison, NJ). Because the Bentonite, kaolin, and red clay had negative charges, the upper electrode was made the anode so that water migrated downward toward the cathode. A constant voltage or a constant current was applied to the electrodes by a regulated DC power supply (Model 6201B, Hewlett Packard, Pointe-Claire, QC) (6). Interrupted voltage or current was provided by a timer relay (Model CKK-00060-461, NCC Electrosonic, Ottawa, ON) (9) with a suitable circuit for differ-

Notation initial CaCl, concentration, M dielectric constant of liquid in pores D electric field strength, V E initial field strength, V/cm Eo ECX oxidation potential, V initial bed height, cm Ho current in jth on-time, mA ij number of the cycles volume of water removed by ;! electroosmosis, cm3 initial solid content, wt% SO time, s t CO

t1 t2

u, vo vi

on-time, s off-time, s electroosmotic velocity, m/s applied voltage, V voltage in jth on-time, V

1

Greek letters zeta potential, 5

<@CO

& rl

V length-averaged zeta potential, V permittivity of free space, C*/J . m energy consumption, J liquid viscosity, J/m - s

Sep. Technol.,

1994, vol. 4, January

39

Electroosmotic

dewatering

of clay suspensions:

H. R. Rabie et al.

ent types of interrupted electroosmotic dewatering. Both voltage and current were measured during the on-time by a voltameter (7) and an ammeter (8). During the off-time of IS experiments, the electrodes were short-circuited through a 0.2 fI shunt resistor (11). This resistance was less than 2% of the resistance of the bed, yet it was large enough to permit measurement of the current during the off-time. The variation of the current during both the on- and off-times of the IS experiments was also recorded on a storage oscilloscope (Model 5223, Tektronix Inc., Pointe-Claire, QC) hooked up across the 0.2 fl shunt. The water removed by electroosmosis was collected in a graduated cylinder (not shown in Figure 2) placed below the funnel (A). The volume of water was measured as a function of time. Additional details are available.i2 Purified grade Bentonite powder in the sodium form (Fisher Scientific Co., Pittsburgh, PA), laboratory grade kaolin powder (A & C American Chemicals Ltd., Montreal, QC) and air-floated red clay (Cedar Height Clay Co., Oak Hill, OH) were used for the experiments. These materials contain largely silica and alumina with smaller amounts of the oxides of potassium, titanium, iron, magnesium, calcium, and sodium. The particle size distributions of the materials were measured with a Malvern 2600 Particle Sizer (Malvern Instruments, Malvern, UK). The medium and mean particle sizes were 6.8 E.cmand 8.2 pm for Bentonite, 17 pm and 23.0 pm for red clay, and 46 pm and 81 pm for kaolin, respectively. The zeta potential of the Bentonite particles was negative for pH > 2. As the pH increased, the zeta potential became more negative.’ The electrolyte added in the preparation of the original suspensions of Bentonite was CaCl,. The zeta potential increased from -34 mV in a salt-free suspension to - 14 mV in 10m2 mol/L CaCl, with a pH of 8.5. The zeta potentials of kaolin and red clay suspensions, which were prepared only in distilled water, were -42 mV and -54 mV, respectively. The initial suspensions were prepared from weighed amounts of oven-dried clay and an aqueous solution of reagent grade CaCl, in distilled water for Bentonite and distilled water alone for kaolin and red clay. The solid contents of the initial suspensions were high enough so that no water drained by gravity. The solid was added to the solution gradually while mixing using a magnetic stirrer. The suspension was then poured slowly into the cylinder to the desired height. The upper electrode was fixed so that it contacted the top of the bed. As soon as this electrode was in place, EOD began. Occasionally the pH of the water removed from the Bentonite suspension was measured. In all cases the pH of this water was about 12. In a few runs the cake was removed at the end of dewatering, and the pH of the cake near the electrodes was measured by pressing the sensing element of the pH probe 2 or 3 mm into each end of the cylindrical cake. The resulting pH readings were about 2 for the top of the cake (near the anode) and about 10 for the bottom of the cake (near 40

Sep. Technol.,

1994, vol. 4, January

the cathode). The significance of these values will be discussed later.

Results The independent

variables were

1. Initial solid content of the suspension (S,) 2. Concentration of CaCl, (C,) in the distilled water used to prepare the Bentonite suspensions 3. Initial height of bed (H,) 4. Magnitude of the constant applied DC voltage (VO) or the constant applied DC current (i) 5. On-time and off-time (t, and t2) Values of the first four variables are given in the parentheses in the caption of the figures in the order: S,, C,, H,, V,, or i. The initial bed height ranged from 0.5 to 2 cm. Constant voltages from 2.75 to 5.5 V and a single constant current of 30 mA (1.5 mAlcm2 of bed cross section) were used. The suspensions were prepared with CaCl, in distilled water at concentrations from zero to 10d2 mol/L. The initial solid contents were 9.1 wt% and 15 wt% for Bentonite, 25 wt% for kaolin, and 45 wt% for red clay. Two interrupted EOD processes were studied: a short circuit during the off-time (denoted IS) and an open circuit during the off-time (denoted IO). These processes are abbreviated IS (tllt2) and IO (t,lt2) where I, is the on-time and t2 is the off-time, both measured in seconds. The on-time was fixed at 30 s, whereas the off-time was varied from 0.5 to 20 s. Comparison

of DC, IO, IS

Figure 3 shows the volume of water removed as a function of the cumulative on-time for continuous EOD (denoted DC) and the two modes of interruption, IO and IS. The initial solid concentration was 9.1 wt% Bentonite with no added electrolyte. The initial bed height was 1.0 cm and a constant voltage of 2.75 V was applied. For both interrupted processes the power was on for 30 s and off for 0.5 s, that is, IO (30/0.5)

‘1

CumulatlvaOn-limo (min.) Figure 3 ~S’lO~;

. .

Volume of water removed vs. on-time for Bentonite 0 M; 1.0 cm; 2.75 V) under DC, IO (30/0.5), and IS

Electroosmotic

Cumulatlw On-Time (min.) Figure 4 Volume of water removed vs. on-time for Bentonite (9.1 wt%; 0 M; 2.0 cm; 5.5 V) under DC, IO (30/0.5), and IS (30/0.5).

and IS (30/0.5), thus the power was on for 98.4% of the running time. The volume of water removed was essentially the same for DC and IO, but more water was removed by IS. The DC, IO and IS processes removed 18%, 19%, and 26% of the initial water, respectively. Figure 4 shows similar results with the same material for an initial bed height of 2.0 cm and an applied voltage of 5.5 V. Again, IS removed more water than DC and IO. The final water removals were 47%, 48%, and 53% for DC, IO, and IS, respectively. If there were no water movement during the offtime, the amount of water removed should be a function only of the cumulative on-time. Figures 3 and 4 show that DC and IO water removals are the same for equal total on-times. The small differences between the DC and IO are within the scatter of replicate runs with either method. This agreement shows that there was no impediment to water drainage from the bed,6 and thus the increased water removal with IS was, in fact, due to the short circuit imposed during the off-time.

Effect of off-time on IS Figure 5 shows the volume of water removed from a Bentonite suspension as a function of total on-time for DC and for IS with three off-times. The on-time was 30 s with off-times of 0.5, 3, and 20 s. All data are for an initial bed height of 1.0 cm, 9.1 wt% initial solid content with no electrolyte and a voltage of 2.75 V as in Figure 3. The largest volume of water was removed for the shortest off-time. The amount of water removed decreased as the off-time increased from 0.5-20 s. Interruption with an off-time of 20 s removed less water than DC. Because equipment limitations prevented the use of a short circuit with off-times less than 0.5 s, IS (30/0.5) was used for subsequent runs.

dewatering

Figure 6 compares the effect of the initial bed height for IS (30/0.5) and DC. In all cases the original suspensions contained 9.1 wt% Bentonite and no electrolyte. A

H. R. Rabie et al.

different voltage was applied for each height so that the initial overall field strength, defined as the applied voltage divided by the initial bed height, was 2.75 V/cm. The figure shows the volume of water removed as a function of total on-time for various initial bed heights. For each height, there was a DC and an IS run. For the 2.0 cm initial bed height, dewatering stopped after about 46% and 53% water removal for DC and IS, resepctively. For the 1.5 cm and 1.0 cm initial bed heights, water removals were about 43% and 52% and 18% and 26%, respectively. The percent increases in the final water removal by IS relative to DC were 44%, 20%, and 14% for the 1.O, 1.5, and 2.0 cm initial bed heights, respectively. When the initial field strength was fixed, IS was more effective for thinner beds.

Effect of initial electrolyte and solid concentrations Figures 7and 8 show the effects of the initial electrolyte and solid concentrations on EOD of Bentonite using DC and IS (30/0.5). All data are for an initial bed height

OK 0

’

20

’

’

s

’

’

’

40 a 60 100 CumulatlvaOn-nmo (min.)

’

’

120

Figure 5 Volume of water removed vs. on-time for Bentonite (9.1 wt%; 0 M; 1.0 cm; 2.75 V) under DC and IS (30/0.5, 3, 20). 24,

_-

Effect of bed height

of clay suspensions:

I

0204oa

60 100 120 CumulathmOn-Tim0 (min.)

140

Figure 6 Volume of water removed vs. on-time for Bentonite (9.1 wt%; 0 M) with an initial overall field strength of 2.75 V/cm. Open symbols refer to DC; filled symbols refer to IS (30/0.5).

Sep. Technol.,

1994, vol. 4, January

41

Electroosmotic dewatering of clay suspensions:

H. R. Rabie et al.

6

e 4

2

0 0

50

30

90

120

130

130

210

CumulatlvsOn-Tlmr (min.) Figure 7

Volume

(9.1 wt%;

1.0 cm; 2.75 V). Open symbols refer to DC; filled sym-

of water removed

vs. on-time for Bentonite

(open symbols). For each condition of voltage or current, two runs are shown. One is a run with interruption applied from the beginning (triangles), whereas the other is a DC run (circles) in which interruption was applied only after dewatering stopped (squares). In all cases the interruption was IS (3010.5). As previous figures show, IS, when applied from the start, removed more water than DC at the same cumulative on-time. Interruption with a short circuit was also effective when applied at the end of a DC run using either constant voltage or constant current. For 30 mA, dewatering stopped under DC at about 160 min. Application of IS (30/0.5) for 40 additional min of ontime removed more water so that the final amount of water removed was close to, but less than, that removed when IS was applied from the beginning. The results for an applied voltage of 2.75 V were similar.

bols refer to IS (30/0.5).

Effectiveness

of IS for other clays

Figure 10 compares dewatering with DC and IS (30/ 0.5) for kaolin clay. The volume of water removed is ,::t

6 4

2

0

0

30

30

90

120

130

130

210

CumulativeOn-lime (min.) Figure 8 Volume of water removed vs. on-time for Bentonite (0.01 M; 1 .Ocm; 2.75V). Open symbols referto DC; filled symbols refer to IS (30/0.5).

of 1.0 cm and a voltage of 2.75 V. In Figure 7 the volume of water removed is plotted as a function of on-time for two initial salt concentrations, 0 and lo-* M CaCl, with an initial solid content of 9.1 wt%. For both concentrations IS removed more water than DC. With no electrolyte, 18% and 26% of initial water was removed by DC and IS, respectively. For an initial CaCl, concentration of lo-* M, DC removed 33% of the initial amount of water while IS removed 44%. Figure 8 shows similar data for two initial solid contents: 9.1 and 15.0 wt% Bentonite with an initial electrolyte concentration of lo-* M. More water was removed by IS than DC for both initial solid contents. Application

03060

90 120 130 180 Cumulatbm On-Tim0 (min.)

210

240

Figure 9 Volume of water removed vs. on-time for Bentonite under IS (30/0.5) and DC followed by IS (30/0.5). Open symbols refer to (9.1 wt%; 0.01 M; 1.5 cm; 30 mA); filled symbols refer to (9.1 wt%; 0.01 M; 1.0 cm; 2.75 V).

12 87

,/-

w 8 10 -

of IS after DC

Figure 9 shows the volume of water removed as a function of total on-time for an initial Bentonite concentration of 9.1 wt%, an initial CaCl, concentration of IO-* M and an initial height of 1.5 cm. The figure shows four runs: two with an applied voltage of 2.75 V (filled symbols) and two with an applied current of 30 mA 42

_-

Sep. Technol.,

1994, vol. 4, January

0

20

40 80 CumulativeOn-Time (min.)

100

Figure 10 Volume of water removed vs. on-time for kaolin (25 wt%; 0 M; 1.0 cm; 5.0 V) under DC and IS (30/0.5).

Electroosmotic

shown as a function of cumulative on-time. All data are for an initial suspension of 25 wt% kaolin with a height of 1.O cm, no electrolyte, and an applied voltage of 5.0 V. More water was removed by IS than DC. The final percent removal for IS was 73%, yielding a bed with an average solid content of 56%. For DC the comparable figures were 62% and 46%. Similar experiments were performed for red clay with 45 wt% initial solid content, no electrolyte, l.O-cm initial bed height, and an applied voltage of 5.5 V. With IS (30/0.5), 65% of the initial water was removed compared with 59% with DC. The final average solid contents were 70 and 66 wt% for IS (30/0.5) with DC, respectively. The results for kaolin and red clay are similar to those presented for Bentonite: IS was more effective than DC. IS and energy consumption

dewatering

of clay suspensions:

100

200

MO

400

500

H. R. Rabie et al.

600

700

800

Energy Conwmsd (J) Figure 12 IS (30/0.5) 2.75 V).

Difference in the volume of water removed between and DC vs. energy consumed for Bentonite (1.0 cm;

The cumulative energy supplied by the DC power supply, E, was computed from the current, voltage and on-time by

(1) where at = number of on/off cycles ij = current in jth on-time Vj = voltage in jth on-time and the cumulative on-time is nr,. Figure 11 shows the cumulative water removed as a function of the cumulative energy consumed for Bentonite with DC, IO (30/0.5), IS (30/0.5), and IS (30/ 20). These data come from runs shown in Figures 3 and 5. The DC and IO (30/0.5) runs removed essentially the same amount of water for equal energy consumption in agreement with the findings of Lockhart and Hart6 For a fixed energy consumption, IS (30/0.5) removed the most water, whereas IS (30/20) removed

Energy Conrumed (J) Figure 11 Volume of water removed vs. energy consumption for Bentonite (9.1 wt%; 0 M; 1.0 cm; 2.75 V) under DC, IO @O/0.5) and IS (30/0.5, 20).

0

200

s ’ ’ 500 500 1,000 1.200 1,400 EllBm ConBunWd(.!j

400

Figure 13 Difference in the volume of water removed between IS (30/0.5) and DC vs. energy consumed for Bentonite (9.1 wt%; 0.01 M; 1.5 cm; 30 mA).

the least. The data for the intermediate condition, IS (30/3), were very close to those for DC, except near the end of the run when IS (30/3) removed more water. Figure 12 shows the difference between the volume of water removed by IS (30/0.5), denoted Q(N), and the volume of water removed by DC, denoted Q(DC), as a function of the cumulative energy supplied. Data are shown for two electrolyte concentrations and two initial Bentonite concentrations. In all cases IS (30/05) finally removed about 20-40% more water than DC. With an initial solid content of 9.1 wt% the additional water removed by IS was larger for the lower electrolyte concentrations except at high energy consumptions. At an electrolyte concentration of lo-* M the additional water removed by IS was larger for the higher initial solid content except at high energy consumptions. The final additional water removal was essentially the same for both solid concentrations. Data from Figure 9 are plotted in Figure 13 as the extra volume of water removed versus the energy consumed. The two runs with an applied current of 30 mA Sep. Technol.,

1994, vol. 4, January

43

Electroosmotic dewatering of c/a y suspensions: H. R. Rabie et al. are shown: one with IS (30/0.5) from the start and the other, denoted DC/IS, with IS (30/0.5) applied after dewatering stopped under DC alone. In the latter run the additional water removed was zero until dewatering stopped with DC at an energy consumption of 940 J. With the expenditure of approximately 250 J during the IS portion of the run, an additional 2 cm3 of water were removed. The average energy expenditure to remove this water, 125 kJ/kg, was less than 6% of the heat of vaporization. Although this is a low energy expenditure, it is even more energy efficient to apply IS (30/0.5) from the beginning. Figure 9 shows that IS (30/0.5) removed approximately 15.5 cm3 of water, and the energy expended was roughly 600 J (see Figure 13). This gives an average energy of removal of about 40 kJ/kg, a value less than 2% of the heat of vaporization of water.

Discussion While the power was on, the reaction at the anode was 20H-+kO,

+ H,O + 2e-

(1)

while the reaction at the cathode was 2H,O + 2e-+

Hz + 20H-

(2)

The low pH measured near the anode and the high pH measured near the cathode are consequences of these electrode reactions.‘,” Changes in pH produced by electrode reactions affect the rate of water removal by changing the zeta potential. At the end of EOD we measured pH values of about 2 near the upper electrode (anode) and 10 near the lower electrode (cathode). The zeta potential of Bentonite was negative at the initial condition of the suspension, where the pH was approximately 9.” The zero point of charge for Bentonite is in the range of pH = 2-3, depending on salt concentration.’ Near the anode, the magnitude of the zeta potential decreases, and the potential may even become positive as a result of the decrease in pH produced by reaction (1). Near the cathode the pH remains high while the power is on, and the zeta potential changes little. The result is that the zeta potential is not uniform across the bed. Anderson and Idol’* determined the effect of an axially variable zeta potential on the electroosmotic velocity in a cylindrical capillary. Their analysis, which is an extension of the standard Helmholtz/Smoluchowski development, yields the following expression for the electroosmotic velocity:

u, =

-<{>DE&

(3)

rl

where < 4 > is the length-averaged value of the zeta potential. In EOD the magnitude of < 5 > decreases with time as the pH at the top of the bed decreases; hence, the water flux decreases with time. Because kaolin and red clay have zeta potential-pH relation44

Sep. Technol.,

1994, vol. 4, January

ships that are similar to Bentonite, their EOD behavior is similar. The decline in flux during EOD is caused by pH changes induced by electrochemical reactions at the electrodes. This conclusion is supported by the electroosmosis experiments of Shapiro and Probstein.r9 In these experiments an aqueous solution was drawn into a saturated clay column through the anode to replace the solution that was expelled from the cathode. When a NaCl solution was fed at the anode, the electroosmotic flux decreased with time; however, when a caustic solution (pH = 12) was used, the flux remained constant.

Threshold voltage and open circuit potential There was no dewatering and very low current through the bed when the applied voltage was below about 2 V. To investigate this phenomenon we measured the potential between the electrodes during EOD when the power was switched off to an open circuit. Voltages were measured during the off-time of IO runs as well as in the middle of DC runs. The measured open circuit potential ranged from 1.8 to 2.3 V with the lower electrode at the higher potential and with no clear trend with electrolyte concentration, initial solid content or initial bed height. Over 2-3 min, the open circuit potential remained constant. It then decreased slowly over the next 20-30 min to values between 0.7 and 1 V and remained constant for hours thereafter. Additional experiments were performed on aqueous solutions (without solids) retained in the column by clamping a plastic sheet below the lower electrode. Solutions of CaCl, identical to those used in preparing the suspensions were tested at heights matching those of the initial suspensions. The open circuit potentials were also between 1.8 and 2.3 V, thus suggesting that this potential originated from the electrochemistry of the system rather than from the dewatering. If a potential below the open circuit potential is applied, little current should flow as we and Yoshida2 observed.

Electrochemistry

of platinum electrodes

The performance of platinum electrodes in reactions involving the liberation of gases is affected by the tendency of the electrodes to develop surface oxide films when anodized. The presence of oxide films on platinum anodes was confirmed by Anson and Lingane.” Following a rapid initial oxidation, a further increase in surface oxidation occurs as the anodic potential increases. A considerable lowering of potential is required for the reduction of the oxide film.r4 Pourbaix” proposed the anode potential-pH relationship shown in Figure 14. The main species are Pt(OH),, Pt02, and PtO,. The electrode reactions and the oxidation potentials are

Pt + 2H20+

Pt(OH),

+ 2H+ + 2e-

E,, = 0.980 - 0.0591 pH

(4)

Electroosmotic

dewatering of clay suspensions:

H. R. Rabie et al.

voltage of 2.75 V. Figure 15 shows the current during a cycle 20 min after the start of EOD of Bentonite suspensions for IS (30/0.5), IS (30/3), and IS (30/20). Figure I6 shows current-time traces for columns of liquids that IS (30/20) for 0, 10e3, and lo-’ M CaCl,. When the power was on, the current was positive, that is, it flowed through the column from the upper electrode to the lower electrode. During the short circuit the current flowed in the opposite direction. The similarity between the current-time traces when Bentonite was present (Figure 15) and when it was not (Figure 16) confirms that the effect of IS is due to electrochemical reactions rather that to dewatering. In EOD of Bentonite the maximum current at the start of the on-time was larger for longer off-times, and the minimum current at the beginning of the off-time was smaller for the longer off-times. The rapid change

0

0

2

4

8

8

10

12

14

10

PH

Figure 14

Potential-pH

diagram for the platinum-water

Pt(OH), + PtO, + 2H+ + 2e-

system.

(5)

E,, = 1.045 - 0.0591 pH PtO, + H,O-,PtO,

+ 2H+ + 2e-

I --a’--+--

(6)

K?X= 2.000 - 0.0591 pH When platinum is used as an anode, it generally becomes covered with platinum oxide (PtO, or Pro, or their mixture) giving a higher potential than pure platinum at the same pH. When used as a cathode, platinum generally adsorbs a considerable quantity of hydrogen. Platinum is stable under the conditions of potential and pH corresponding to the equilibrium state of the following reaction H 2~2H+

+ 2e-

f power on -

-short

circui tA

I

I

I

1

10

20

20

40

1 I

60

Time (8) Figure 15 Variation of current with time after 20 min of dewatering for Bentonite (9.1 wt%; 0 M; 1.0 cm; 2.75 V) under IS. 0, IS (30120); A, IS (3013); n IS (30/0.5).

(7)

E,, = -0.0591 pH This reaction occurs nearly reversibly on the surface of platinum.‘5 As noted earlier, the pH was about 2 near the anode and about 10 near the cathode. Point a in Figure 14 represents the condition at an anode covered with Pt 0, at pH = 2, whereas point b represents the condition at a cathode that is pure platinum at pH = 10. The difference in potential between these two points is 2.4 V, which is in good agreement with the largest measured open circuit potentials. The agreement could be improved if the anode coating were assumed to be a mixture of PtO, and PtO,.

W-

(20) -

-

(-)o

I poweron . I-I . 10

I+rhottoircu~ . I I so

40

Ea

Tim0 (m)

Short circuit current Figures 25 and 16 show the variation of current with time during one cycle of several IS runs with an applied

20

Figure 16 Variation of current with time in aqueous solutions (1.0 cm; 2.75 V) under IS (30/20). 0, distilled water, A, IO-3 M CaCI,; n , 10m2 M CaCI,.

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1994, vol. 4, January

45

Electraosmotic dewatering of clay suspensions:

H. R. Rebie et al.

in current at the start of the on- and off-times was a consequence of concentration polarization phenomenon.i6 The short circuit current results from the discharge of an electrochemical cell. There are several possible reactions. For example, reactions 1 and 2 could run in the reverse direction or the electrodes might participate through the oxidation of Pt at the lower electrode and the reduction of Pr-oxides at the upper electrode.

Mechanisms

for IS

The effect of IS on dewatering is related to the pH changes that are produced by reactions occurring during the short circuit. All combinations of reactions 1, 2, and the PtlPt-oxide reactions produce the same result: the pH is increased at the top electrode (by the liberation of OH- or the consumption of H+), and the pH is decreased at the lower electrode (by the consumption of OH- or the liberation of H+). The increased pH at the upper electrode prevents the zeta potential from approaching zero or becoming positive as rapidly as it would using DC or IO. Changes in pH have a major effect on the zeta potential near the zero point of change. The zeta potential near the anode during IS thus retains its original negative sign and large magnitude through a longer time than it would if the run were DC or IO, hence IS dewatering rates are higher. Because these phenomena should occur any time a short circuit is applied, IS is effective when applied at the end of a DC run. The short circuit reactions also decrease the pH at the lower electrode, although at high pH values the zeta potential is not too sensitive to small changes in pH. If the reactions occur for a sufficient time, however, the pH at the lower electrode can decrease sufficiently to reduce the magnitude of the zeta potential and, thus, slow dewatering. If the electrodes are shortcircuited for a long time, IS dewatering rates could be reduced below DC rates. This suggests that there may be an optimum short-circuit time. We found that dewatering rates are above DC rates for IS (3010.5) and below DC rates for IS (30/20).

for too long. The IS mode also removes additional water if applied at the end of dewatering with DC.

Acknowledgment This work was made possible by the financial support from the Natural Sciences and Engineering Research Council of Canada and by a scholarship to H.R. Rabie from the Ministry of Culture and Higher Education of Iran.

References 1.

2.

3. 4.

5.

6.

7.

8.

Reporr of Investigations 9.

13.

15.

Electroosmotic dewatering can remove significant amounts of water with energy expenditures per unit mass of water well below the heat of vaporization. Operation in the interrupted mode has no beneficial effect if there is an open circuit during the off-time (IO). However, if the electrodes are short-circuited during the off-time (IS), dewatering rates and final water contents are larger than those with DC for equal energy expenditure if the short circuit is not applied

46

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1994, vol. 4, January

16. 17. 18.

of Investigations

1976, -8197, l-68

Yankovskii. A.A., Khrustalev. Yu. Y. and Zavialova. N.A. Experimental investigations of electroosmosis in peat’at impulse electric field (translated from Russian by Kudra, T., in a personal communication). Torphouaya Promyshfenost (Pear Industry)

12.

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Sprute, R.H. and Kelsh, D.J. Dewateting and densification of coal waste by direct current laboratory tests. U.S. Bureau of Mines Report

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Snrute, R.H. and Kelsh, D.J. Limited held tests in electroki&tic densification of mill tailings. U.S. Bureau of Mines Report of Investigations

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Conclusion

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Rabie, H.R. Continuous and interrupted electroosmotic dewatering of clay suspensions. M. Eng. diss., 1992, McGill University, Montreal, Quebec. Anson, F.C. and Lingane, J.J. Chemical evidence for oxide films on platinum electrometric electrodes. J. Am. Chem. Sot. 1957, 79,4901-4904 Laitinen, H.A. Electroanalytical chemistry of surface monolayers. Anal. Chem. 1961,33: 1458-1464 Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions (translated from French by Franklin, J.A.).-Houston, TX: National Association of Corrosion Enaineers. 1974 Hampel, C.A. The Encyclopedia of Electrochemistry.’ New York: Reinhold Publishing, 1964 Ju, S. Electroosmotic dewatering of Bentonite suspensions. M.Eng. diss., 1990. McGill University, Montreal, Quebec Anderson, J.L. and Idol, W.K. Electroosmosis through pores with nonuniformly charged walls. Chem. Eng. Commun. 1985, X3,93-106

19.

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283-291