High-Temperature Conductivity Measurements for Industrial

mounted in a 2 L titanium pressure vessel. The effects of the platinization of the electrodes and the level of conductance on the cell constant were e...
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High-Temperature Conductivity Measurements for Industrial Applications. 1. A New Cell Morteza Baghalha and Vladimiros G. Papangelakis* Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3E5

A novel conductivity cell, suitable for high-temperature industrial applications, was developed. The cell body was made of Pyrex glass, and the electrodes were made of pure platinum. It was mounted in a 2 L titanium pressure vessel. The effects of the platinization of the electrodes and the level of conductance on the cell constant were extensively studied. It was found that the cell with the platinized electrodes has a very stable cell constant at 19.97 ( 0.06 cm-1, for conductivity levels ranging from 1000 to 100 000 µS/cm. The change in the cell constant when the temperature changes from 25 to 250 °C is less than 0.1%. The cell is operable at the equilibrium vapor pressure from low to high acidities. To assess the overall performance of the new cell, the conductivities of 0.05 m H2SO4 solutions at temperatures from 15 to 250 °C were measured. These measurements were in excellent agreement with literature values with a difference of less than 1%. Introduction Most industrial applications involve the use of concentrated electrolyte systems to achieve acceptable productivity levels. Concentrated electrolyte solutions are nonideal and difficult to characterize chemically as well as thermodynamically. Moreover, aqueous processes operating at high temperatures (up to 250 °C) pose even more difficulties because of the lack of appropriate sensors for measuring the solution chemistry (by solution chemistry we mean here the identification of solution speciation including hydrogen ion concentration). A typical case is the acid pressure leach process for nickel and cobalt recovery from lateritic ore deposits. It has become the process of choice for nickel and cobalt productions, as is evident by the new fullscale and demonstration plants that recently came into operation particularly in Australia and New Caledonia.1-3 This hydrometallurgical process employs an autoclave at 250-270 °C to conduct a selective leaching of Ni and Co with H2SO4 from a predominantly rich Fe, Al, and Mg matrix.1-8 To operate at the best autoclave conditions, which leads to fast and complete nickel extraction and low solubilities of impurity metals, a continuous monitoring of the solution chemistry is required. This continuous monitoring of the solution chemistry may be achieved by using a conductivity cell. The electrical conductivity of an aqueous solution depends on the equivalent conductivities of all species in the solution.9 As a result, the solution conductivity can be used to monitor the species present in the solution. Historically, conductivity has been one of the first means of probing electrolyte solutions, particularly at high temperatures. Conductivity measurements have been applied recently for neutralization reactions10 and in power plants11 to monitor the calcium ion supersaturation and the corrosion potential of the solution, respectively. In the present paper (part 1), the design and development of * To whom correspondence should be addressed. Tel.: 1-416-978-1093. Fax: 1-416-978-8605. E-mail: papange@ chem-eng.utoronto.ca.

a new conductivity cell are discussed. The objective is to construct a simple yet precise conductivity cell for use with lab-scale autoclaves for high-temperature industrial applications. In part 2,9 this cell is used to measure and interpret the conductivities of solutions relevant to the laterite leach solutions. The first high-temperature conductivity cell was built by Noyes and Coolidge12 in 1903. It was useful for temperatures up to 306 °C and pressures corresponding to the saturation pressure of water. This cell was, in fact, an all-metal pressure vessel, which was internally lined with platinum. It had a low cell constant, on the order of 1 cm-1, and, hence, was not suitable to measure the conductivity of solutions more concentrated than 0.1 N. Franck et al.13 introduced a new cell capable of reaching supercritical conditions. This cell has been in continuous use in the Oak Ridge National Laboratory since 1962.14 This cell consisted of a 61.0 cm length platinum-25% iridium lined pressure vessel. It had a very low cell constant, ∼0.3 cm-1, and was used for very dilute solutions for temperatures up to 800 °C and for pressures up to 4000 bar. Because of the low value of the cell constant, the cell could not be used for highconcentration measurements. In 1993, Bianchi et al.15 introduced a flow-through cell. The cell consisted of four zircaloy cylindrical sections of 12 mm o.d. and 6 mm i.d. The electrodes were platinum disks located between the zircaloy rings. The cell had a small volume, with the solution flowing through it continuously, and was employed for solutions up to 0.015 M. Because of the contamination of the solution and the dependence on the residence time, the solution conductivities could not be measured at temperatures higher than 150 °C.15 Another flow-through high-temperature conductivity cell was recently introduced by the Los Alamos National Laboratory.16 The electrodes consisted of two platinum wires embedded in alumina, except for a 2-mm-long section. This cell was used to measure the conductivities of dilute solutions for concentrations up to 0.01 m at temperatures up to 505 °C and pressures up to 490 bar. According to the authors,16 the cell constant was variable, ranging from

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0.178 to 8.48 cm-1, depending on the concentration. This variability in the cell constant resulted in uncertainties beyond 10%. To measure the conductivity of high-temperature concentrated solutions with an acceptable accuracy, a cell with an invariable cell constant is required. Furthermore, from the engineering point of view, it is desirable for this cell to accommodate easy installation inside the process vessels. On the basis of these general features, a novel conductivity cell was built. The design and development of this new cell and its integration with other system components are discussed in this paper (part 1). Development of a Novel Conductivity Cell Design Parameters of the Cell. To be suitable for measuring the conductivities of the laterite leach solutions, the new conductivity cell must operate at temperatures up to 250 °C at the corresponding saturation pressure of water (i.e., 4000 kPa at 250 °C). The solution concentrations are on the order of 1 m, and the solution conductivities are on the order of 100 mS/cm. Hence, a conductivity cell with a high cell constant, e.g., greater than 10 cm-1, is required. A high cell constant minimizes the polarization potential at the electrodes, because the current passing through the cell is small. Furthermore, the variability of the cell constant has a serious negative impact on the accuracy of the measurements. As a result, it was desirable to design a conductivity cell so that its cell constant has minimum variability with changes in concentration and temperature. Description of the Cell and the Setup. The new conductivity cell was constructed from Pyrex (type 7740), supplied by Corning Labware & Equipment, Inc. This is a borosilicate glass with excellent chemical resistance in acidic solutions (except for hydrofluoric and hot phosphoric acids). Furthermore, it has good temperature-resistance properties, with the factory-suggested maximum temperature standing at 490 °C. This material is also an insulator for the electrical current, which is a requirement for measuring the electrical conductivity. The general configuration of the new conductivity cell is shown in Figure 1. The material of the electrodes and the connecting wires was platinum with 99.99% purity supplied by Alfa Aesar (a Johnson Matthey company). The electrodes were square flat plates with a side length of 0.5 cm and a thickness of 0.05 cm. The main part of the cell consisted of two Pyrex tubes with 0.7 cm i.d. and 7 cm length each. The bottom ends of these tubes were connected to two curved Pyrex tubes with 0.5 cm i.d. and a total length of 4 cm. According to the given dimensions, the cross-sectional area of the conduit between the electrodes was designed to be smaller than the surface area of each electrode. As found through testing a variety of designs, this was a necessary requirement for obtaining an invariant cell constant. There was a small gap of 0.2 cm separating the lower tubes in the lowest level. When placed in a solution, the cell was filled through this slot. The platinum wires (0.05 cm diameter) were passed through two thin Pyrex tubes with 0.3 cm o.d. to prevent their contact with the solution. To seal these Pyrex tubes and the platinum wires, cobalt blue borosilicate sealing glass (supplied by Corning Labware & Equipment, Inc.) was applied between them. Cobalt glass was

Figure 1. New conductivity cell, tested for temperatures up to 250 °C.

very suitable for the sealing, because it has a thermal coefficient of expansion between those of Pyrex and platinum. To fix the relative position of the electrodes, the 0.3 cm Pyrex tubes that hold the electrodes were spottily connected to the 0.7 cm Pyrex tubes using a few beads of Pyrex glass. Finally, the main assembly of the cell was permanently installed into another Pyrex tube with 2.2 cm o.d., as shown in Figure 1. To prevent breakage of this glass cell during installation and operation, part of the Pyrex body was blanketed with a thick block of Teflon (a registered trademark of E. I. du Pont de Nemours and Co.), as shown in Figure 1. Teflon is a relatively soft and temperature-resistant material that has a factorysuggested maximum operating temperature of 255 °C. This is the main restriction that fixes the maximum operating temperature of the cell at 250 °C. The bottom surfaces of the electrodes were electrolytically coated with a platinum-black layer. For this purpose, the cell was immersed at room temperature in a platinizing solution supplied by Yellow Springs Instrument Co., Inc. To avoid platinization of the top surfaces of the electrodes, the solution level was adjusted so that only the bottom faces of the electrodes were covered. The leads of the cell were then connected to a direct current with a voltage of 0.6 V. Each electrode was platinized for 15 min. To stabilize the platinized electrodes, the cell was then immersed in tap water at room temperature and left for 10 h. A 2 L Parr titanium autoclave (a pressure vessel) was used to house the new conductivity cell, as shown in Figure 2. The autoclave consisted of two main parts: a cylindrical bomb (10.2 cm i.d. and 26.5 cm height) and a head assembly. The head assembly consisted of a lid and four autoclave internals connected to the lid. The internals were a stirring shaft at the center of the lid with two turbine-type impellers on it (with an operating range of 0-700 rpm), a thermocouple well, a U-shaped cooling tube, and a 1/4 in. diameter straight titanium tube. A removable Pyrex glass liner was used inside the autoclave bomb to prevent corrosion of the wall. The conductivity cell was permanently mounted on the 1/4 in. titanium tube by applying an aluminum clamp on the Teflon block. The cell was positioned so that there

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Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 Table 1. Calibration of the New Conductivity Cell at 25 °C cell constant (cm-1)

Figure 2. The 2 L Parr titanium autoclave that was used to house the new conductivity cell.

was a 3.6 cm distance between the top of the cell and the autoclave lid and a 2 cm distance between the top of the impeller and the bottom of the conductivity cell. The leads of the conductivity cell were connected to copper wires covered with a thin Teflon layer. As shown in Figure 2, the copper wires were then passed through the autoclave lid through a 1/8 in. NPT thread Conax pressure sealing assembly, supplied by Conax Buffalo Technologies. In a typical experiment, the solution fills the conduit between the electrodes through the 0.2 cm slot up to a level below the Teflon block and above the electrode level. Upon heating, the liquid solution expands. For the maximum design temperature of 250 °C, this expansion is about 25% of the original solution volume. The system was designed such that, at 250 °C, the liquid level is 2 cm below the aluminum clamp. Hence, the clamp would not be contacted. Furthermore, the electrical current can only flow through the bottom faces of the electrodes through the conduit at the lower portion of the cell. Therefore, the liquid level above the electrodes has no effect on the cell constant. The temperature of the solution inside the autoclave was measured by a type J thermocouple with an operating range of 0-600 °C, a resolution of 1 °C, and a system accuracy of (2 °C. The temperature was kept practically constant within (0.25 °C (in every 10 min interval) by a microprocessor-based controller in PID mode. The controlling medium for the heating was an electric heating mantel outside the autoclave, and that for the cooling, a cooling tube passed through the autoclave and cooled by tap water. A model 20 accumet (Denver Instrument Co.) conductivity meter was used to measure the conductivity. According to the manufacturer’s information, the signal generated by the meter was an alternating square-wave voltage with an amplitude of 0.6 V. The frequency of this signal was fixed at 225 Hz. The meter output was the specific conductivity (in mS/cm) which was calculated based on eq 1.

σ)

(R1 )(Al )

(1)

where R (in kΩ) is the solution resistance between the electrodes measured by the meter and l/A (in cm-1) is

conductivity standard, nominal (µS/cm)

before platinization

after platinization

1 000 10 000 100 000

19.94 ( 0.01 20.97 24.36

19.91 ( 0.01 19.94 20.03

the cell constant (obtained experimentally through calibration and stored in the meter memory). Calibration of the Cell. In a typical calibration, the cell was rinsed three times with a fresh conductivity standard solution. The cell was then immersed in the fresh conductivity standard at 25 °C and left for 1 min. The cell constant was calculated by the conductivity meter after entering the value of the conductivity standard to the meter. The cell was calibrated at 25 °C by using three different conductivity standards, supplied by Fisher Scientific with nominal conductivities of 1000, 10 000, and 100 000 µS/cm. The cell constant was obtained before and after the electrodes were platinized. The results are shown in Table 1. As seen in Table 1, for the platinized electrodes, the cell constant changes only about 0.6% when the conductivity varies by a factor of 100. For the unplatinized electrodes, the accuracy of the cell constant is variable. At low conductivities (1000 µS/cm), the obtained value of the cell constant is acceptable; i.e., it is comparable to that for platinized electrodes. However, at higher conductivities, it shows a 22% overestimation. This is due to the fact that, at high conductivities, the total resistance of the cell is low. Hence, the electrical current passing through the electrodes is high. This, in turn, produces a high polarization potential at the electrodes’ interface. To reduce this polarization potential, the effective surface areas of the electrodes must be increased; i.e., the electrodes must be platinized. As a result, the cell constant at 100 000 µS/cm before platinization was almost restored to its true value after platinization of the cell (Table 1). Because the conductivities of the desired solutions for this study were expected to be on the order of 100 000 µS/cm, the electrodes in the platinized mode were used. Furthermore, to have the most representative cell constant, the 100 000 µS/cm conductivity standard at 25 °C was ultimately used to calibrate the cell. Experimental Procedure A total of 1100 mL of solution was placed in the Pyrex glass liner inside the autoclave bomb. The autoclave internals (connected to the autoclave head), including the permanently installed conductivity cell, were vertically inserted into the autoclave bomb. The autoclave head was then closed, and the bomb was heated by an electrical heating mantel to 25 °C (the initial solution temperature was less than 20 °C). The specific conductivity of the solution at 25 °C was then recorded. The autoclave bomb was heated and reached the temperature of 250 °C in about 50 min. The specific conductivity of the solution at 250 °C was then recorded. Using tap water passed through the cooling coil through the autoclave, the solution was then cooled to 225 °C, and the specific conductivity was recorded. This procedure was repeated for other temperatures at 25 °C intervals down to the temperature of 25 °C and, finally, to the temperature of 15 °C. At each desired temperature, the conductivity reading stabilized in less than 1 min and

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was recorded at least 10 min later. At some temperatures, the conductivity measurement was repeated up to four times through heating and cooling the solution within (25 °C. These repeated measurements were used to estimate the reproducibility range. The pressure in the autoclave was the equilibrium vapor pressure of the solution, which was close to that of pure water for all of the solutions. Agitation, in general, had no effect on the measured conductivity values. However, to prevent any fluctuations in the conductivity measurements, it was necessary to maintain two different levels of agitations, depending on the temperature. At temperatures from 250 to 175 °C, the agitation required to be fixed at 600 rpm, probably to prevent build up of small water vapor bubbles on the surfaces of the electrodes. At temperatures lower than 175 °C, it was necessary to operate at lower agitations, around 200 rpm. At these temperatures, because the liquid level is low, the air gas bubbles would probably be sucked into the liquid from the gas phase at high agitations. These air gas bubbles would then block part of the conduit for the electrical current. The platinum-black layers on the electrodes could be deactivated or dissolved to some extent during the course of the experiments. In such cases, according to the data in Table 1, the specific conductivity could be underestimated up to 22%. To monitor this possibility, the measured solution conductivities at 25 °C, both at the start and at the end of the high-temperature experiment, were compared. Any difference between these two measurements (beyond the reproducibility range) would indicate that the platinum-black layer was deactivated and/or the solution was contaminated. The deactivation of the platinum-black layer would be further verified through measurement of the conductivity of the 100 000 µS/cm conductivity standard. In the case of deactivation, the electrodes were replatinized, the cell was recalibrated, and the experiment was repeated. Also, the possibility of solution contamination (caused by dissolution of metal surfaces in contact with the solution) was checked by comparing the conductivities of a sample of the original solution (set aside before the experiment) and a sample of the final solution (from after the experiment). For this purpose, a standard conductivity cell with a nominal cell constant of 10 cm-1, supplied by Fisher Scientific, was employed to measure the conductivities. In the case of a conductivity reading outside the reproducibility range, the experiment was repeated. Furthermore, the proper functionality of the cell was rechecked after three consecutive experiments. For this purpose, the conductivity of the 100 000 µS/cm conductivity standard at 25 °C was measured. As long as the conductivity reading of the standard solution remained within the reproducibility range, the cell was neither replatinized nor recalibrated. Specific Conductivities of Pure Aqueous H2SO4 Solutions To assess the overall performance of the new cell, the conductivities of H2SO4 solutions were measured and compared with the literature values.17-19 First, the conductivities of aqueous H2SO4 solutions at 25 °C and at concentrations from 0.01 to 0.45 m were measured. These measurements along with the relevant literature values at 25 °C17,18 are shown in Figure 3. Second, the conductivities of 0.05 m H2SO4 solutions at temperatures up to 250 °C were also measured. These data are

Figure 3. Measured conductivities of H2SO4 at 25 °C as compared to literature data.17,18

Figure 4. Measured conductivities of 0.05 m H2SO4 at temperatures up to 250 °C as compared to literature data19 up to 156 °C.

plotted in Figure 4. The literature values19 at temperatures up to 156 °C (along the vapor-liquid saturation curve) are also shown in Figure 4. As seen from Figures 3 and 4, there is an excellent agreement between the measured conductivities of H2SO4 and the literature values.17-19 The average difference between the values measured in this work and the literature values17-19 at all temperatures was within (1%. To obtain the reproducibility of the measurements, as explained in the experimental procedure, some of the measurements were repeated up to four times. On the average, the reproducibility was found to be within (0.5%. Discussion The overall performance of the new cell was found to be consistent with the literature data up to 156 °C, as shown in the previous section. Here, some details of the new cell and the setup are further examined. Correction Factor Due to Water Evaporation. In the present study, concentrations were considered in

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Table 2. Correction Factor To Estimate the True Solution Molalities temp (°C)

correction factor

temp (°C)

correction factor

25 50 75 100 125

1.000 1.000 1.000 1.000 1.001

150 175 200 225 250

1.001 1.002 1.004 1.005 1.007 ( 0.0005

terms of molalities. Theoretically, they must remain constant, i.e., independent of temperature. In practice, however, a small portion of liquid water evaporates into the vapor phase at high temperatures. As a result, the initial molalities change. To compensate for it, a correction factor for each temperature was calculated and is given in Table 2. As seen from this table, the largest correction factor is associated with the highest operating temperature, i.e., 250 °C. Hence, a H2SO4 solution at 250 °C, for instance, with 1 m nominal concentration has a true molality of 1 × 1.007 ) 1.007. The correction factors given in Table 2 were calculated based on the amount of H2O evaporated to the vapor phase inside the autoclave. At 250 °C, for example, the vapor volume was estimated by accounting for the initial solution volume (1100 mL at 25 °C) and a 25% volume increase due to thermal expansion (from 25 to 250 °C).20 Furthermore, the volumes of the autoclave bomb and its internals were measured to be 2150 and 330 mL, respectively. As a result, the vapor volume was estimated to be 2150 - 330 - 1100 × 1.25 ) 445 mL. On the other hand, the specific volume of water vapor at 250 °C was approximated using the specific volume and the vapor pressure of pure saturated water vapor (0.05 m3/kg and 40 bar, respectively) and the partial pressure of water vapor at 1 m H2SO4 (33.5 bar at 250 °C).20 Hence, the specific volume of water vapor was estimated as (40/33.5) × 0.05 ) 0.060 m3/kg (≡L/g) (the calculation was based on the assumption of equal gas compressibility factors for both conditions, i.e., P1V1 ) P2V2). Therefore, the weight of water vapor was estimated to be 0.445 (L)/0.060 (L/g) ) 7.4 g. For the given 1 kg of total water, then, the water remaining in the solution was 1000.0 - 7.4 ) 992.6 g. Hence, for a solute with a molality m, a correction factor of 1/(992.6/1000) ) 1.007, as given in Table 2, was calculated. These calculations were repeated for lower temperatures with values as given in Table 2. No correction factor for temperatures less than 100 °C was required. The Cell Constant. When the temperature of the solution changes from 15 to 250 °C, the dimensions of the cell change because of the thermal expansion. Hence, because of these geometrical changes, the cell constant also changes. According to the manufacturer’s information (Corning Glassware Catalog), Pyrex has a thermal coefficient of expansion of 32.5 × 10-7 cm/cm/ °C. For a temperature change of 225 °C, the expansion of the cell in any given dimension is about 32.5 × 10-7 × 225 ) 0.0007 cm/cm ≡ 0.07%. Therefore, it is concluded that the expansion of the cell has virtually no effect on the cell constant value. As reported in Table 1, when the conductivity changes by a factor of 100, the cell constant changes only by 0.6% (in the case of the platinized electrodes). For the performed experiment, as shown in Figure 4, the conductivities changed by a factor of less than 2 when the temperature increased from 15 to 250 °C. As a result, the change in the cell constant due to the change in the conductivity level is negligible.

Electrical Circuit. The copper wires, connecting the cell to the conductivity meter, have a total resistance of about 1 Ω, as measured by a standard multimeter. Hence, for the present cell with a cell constant of about 20 cm-1 and for a solution with a conductivity of 100 000 µS/cm, the total cell resistance is 200 Ω ()20/(100 000 × 10-6)). Therefore, the wire resistance is only 0.5% ()[1/200] × 100) of the total resistance in the circuit. Furthermore, during the calibration process, all of the resistances in the circuit were considered as “solution resistance”. As a result, the effect of the wire resistance is included in the cell constant and, hence, no correction factor is needed for that effect. To minimize the polarization potentials at the interfaces of the electrodes and the solution, the conductivity measurements must be theoretically made at infinite frequency of the input electrical signal.21 In practice, this is done through measurement of the conductivities at various finite frequencies and then extrapolation to the infinite frequency. However, as observed by Quist et al.,21 when the electrodes are platinized, the effect of frequency is so small that it can be neglected. The conductivity meter employed in the present study operated at a fixed frequency of 225 Hz. Because the electrodes were platinized, very little frequency effect is expected. This is partially justifiable through the data reported in Table 1. As seen from this table, low conductivities (with little frequency effect due to the low polarization potentials at the interfaces) and high conductivities (with a higher frequency effect) result in cell constants that only differ by 0.6%. Furthermore, because the same frequency was used during both calibration and measurements, the frequency effect was, in fact, included in the cell constant. According to the manufacturer (Denver Instrument Co.), the electrical circuit inside the conductivity meter can measure the resistance of an electronic resistor with an accuracy of (0.5%. Hence, the lower boundary of the accuracy range for the measured conductivities is (0.5%. Conclusions A novel Pyrex conductivity cell suitable for hightemperature measurements was developed. The effects of the platinization of the electrodes and the conductivity level on the cell constant were extensively studied. For the case of the bare platinum electrodes (before platinization), the cell worked properly at ∼1000 µS/ cm conductivity levels with a cell constant ∼20 cm-1. However, as the conductivity level increased to 100 000 µS/cm, the cell constant was increased by about 22%. In the case of platinized electrodes, the cell constant was very stable at 19.97 ( 0.06 cm-1, for conductivity levels ranging from 1000 to 100 000 µS/cm. It is envisaged that this cell can be suitable for conductivity measurements in concentrated industrial systems at temperatures up to 250 °C. It can be operated in acidic media at ionic strengths as high as 1 m. To assess the overall performance of the new cell, the conductivities of 0.05 m H2SO4 solutions at temperatures from 15 to 250 °C were measured. These measurements were in excellent agreement with reported literature values having an average difference of less than 1%. Acknowledgment The Center for Chemical Process Metallurgy of the University of Toronto and the Natural Sciences and

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Engineering Research Council of Canada (NSERC) are acknowledged for providing financial support. Literature Cited (1) Krause, E.; Singhal, A.; Blakey, B. C.; Papangelakis, V. G.; Georgiou, D. Sulfuric acid leaching of nickeliferous laterites. In Proceedings of the Nickel-Cobalt 97 International Symposium; Cooper, W. C., Mihaylov, I., Eds.; Canadian Institute of Mining, Metallurgy and Petroleum: Montreal, Canada, 1997; Vol. I, p 441. (2) Motteram, G.; Ryan, M.; Weizenbach, R. Application of the Pressure Acid Leach Process to Western Australian Nickel/Cobalt Laterites. In Proceedings of the Nickel-Cobalt 97 International Symposium; Cooper, W. C., Mihaylov, I., Eds.; Canadian Institute of Mining, Metallurgy and Petroleum: Montreal, Canada, 1997; Vol. I, p 391. (3) Faris, M. D.; Collins, M. J.; Becker, G. S.; Matheson, P. J.; Lennard, G. A. The Calliope Project: Pressure Acid Leaching of Nickel Laterte Ores from New Caledonia. In Proceedings of the Nickel-Cobalt 97 International Symposium; Cooper, W. C., Mihaylov, I., Eds.; Canadian Institute of Mining, Metallurgy and Petroleum: Montreal, Canada, 1997; Vol. I, p 409. (4) Carlson, E. T.; Simons, C. S. Pressure Leaching of Nickeliferous Laterites with Sulfuric Acid. In Extractive Metallurgy of Copper, Nickel, and Cobalt; Queneau, P., Ed.; Interscience Publishers: New York, 1960; p 363. (5) Chalkley, M. E.; Toirac, I. L. The Acid Pressure Leach Process for Nickel and Cobalt Laterite. Part I: Review of Operations at Moa. In Proceedings of the Nickel-Cobalt 97 International Symposium; Cooper, W. C., Mihaylov, I., Eds.; Canadian Institute of Mining, Metallurgy and Petroleum: Montreal, Canada, Vol. I, 1997; p 341. (6) Chou, E. C.; Queneau, P. B.; Rickard, R. S. Sulfuric Acid Pressure Leaching of Nickeliferous Limonites. Metall. Trans. B 1977, 8B, 547. (7) Georgiou, D.; Papangelakis, V. G. Sulfuric-Acid Pressure Leaching of a Limonitic Laterite: Chemistry and Kinetics. Hydrometallurgy 1998, 49, 23. (8) Queneau, P. B.; Doane, R. E.; Cooperrider, M. W.; Berggren, M. H.; Rey, P. Control of Autoclave Scaling during Acid Pressure Leaching of Nickeliferous Laterite Ore. Metall. Trans. B 1984, 15B, 433. (9) Baghalha, M.; Papangelakis, V. G. High-Temperature Conductivity Measurements for Industrial Applications. 2. H2SO4Al2(SO4)3 Solutions. Ind. Eng. Chem. Res. 2000, 39, 3646. (10) Klepetsanis, P. G.; Dalas, E.; Koutsoukos, P. G. Role of Temperature in the Spontaneous Precipitation of Calcium Sulfate Dihydrate. Langmuir 1999, 15, 1534.

(11) MacDonald, D. Personal communication, Materials Science and Engineering, The Pennsylvania State University, University Park, PA, Nov 10, 1999. (12) Noyes, A. A.; Coolidge, W. D. The electrical conductivity of aqueous solutions at high temperatures, I. Description of the apparatus. Results with sodium and potassium chloride up to 306°. J. Am. Chem. Soc. 1904, 26, 134. (13) Franck, E. U.; Savolainen, J. E.; Marshall, W. L. Electrical conductance cell assembly for use with aqueous solutions up to 800 °C and 4000 bar. Rev. Sci. Instrum. 1962, 33, 115. (14) Mesmer, R. E.; Palmer, D. A.; Simonson, J. M.; Holmes, H. F.; Ho, P. C.; Wesolowski, D. J.; Gruszkiewicz, M. S. Experimental studies in high-temperature aqueous chemistry at Oak Ridge National Laboratory. Pure Appl. Chem. 1997, 69, 905. (15) Bianchi, H.; Corti, H. R.; Fernandez-Prini, R. A cell for the study of the electrolytic conductivity at high temperature in aqueous solutions. Rev. Sci. Instrum. 1993, 64, 1636. (16) Goemans, M. G. E.; Funk, T. J.; Sedillo, M. A.; Buelow, S. J.; Anderson, G. K. Electrical conductances of aqueous solutions of inorganic nitrates at 25-505 °C and 100-490 bar. J.Supercrit. Fluids 1997, 11, 61. (17) Arifin, J. Conductivities of Mixed Electrolytes Containing Sulfate Ions. B.A. Sc. Thesis, University of Toronto, Toronto, Ontario, Canada, 1992. (18) Hinatsu, J. T.; Tran, V. D.; Foulkes, F. R. Electrical conductivities of aqueous ZnSO4-H2SO4 solutions. J. Appl. Electrochem. 1992, 22, 215. (19) Noyes, A. A. The Electrical Conductivity of Aqueous Solutions, A report; The Carnegie Institution of Washington: Washington, DC, 1907. (20) Liley, P. E.; Reid, R. C.; Buck, E. Physical and Chemical Data. In Perry’s Chemical Engineers’ Handbook, 6th ed.; Perry, R. H., Green, D. W., Maloney, J. O., Eds.; McGraw-Hill: New York, 1984; Section 3. (21) Quist, A. S.; Franck, E. U.; Jolley, H. R.; Marshall, W. L. Electrical conductances of aqueous solutions at high temperature and pressure. I. The conductances of potassium sulfate-water solutions from 25 to 800° and at pressures up to 4000 bar. J. Phys. Chem. 1963, 67, 2453.

Received for review February 9, 2000 Revised manuscript received June 22, 2000 Accepted July 4, 2000 IE000214P