Measurement of pH in high-temperature nickel laterite pressure acid leach process solutions

Hydrometallurgy 105 (2010) 155–160

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Measurement of pH in high-temperature nickel laterite pressure acid leach process solutions Z. Jankovic, V.G. Papangelakis ⁎ Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, ON M5S 3E5, Canada

a r t i c l e

i n f o

Article history: Received 23 May 2010 Received in revised form 3 September 2010 Accepted 7 September 2010 Available online 15 September 2010 Keywords: Nickel laterite Pressure acid leach pH measurement High temperature Yttria-stabilised zirconia

a b s t r a c t The performance of a flow-through electrochemical cell with an yttria-stabilized zirconia (YSZ) sensor for pH measurement in high-temperature nickel laterite pressure acid leach (PAL) process solutions was evaluated. Very good agreement was observed between the measured pH values and those theoretically predicted using the OLI Systems software package calibrated independently based on solubility measurements. PAL process solutions were simplified to ternary MgSO4–Al2(SO4)3–H2SO4 solutions by combining all the divalent metal sulphates into one with the properties of MgSO4. The experimental results support the postulation that the high-temperature behaviour of nickel, cobalt and manganese sulphates can be satisfactorily approximated with that of MgSO4. The experimental findings also support the postulation that acid should be added to a PAL process so that the solution pH is around 1 at the leach temperature, regardless of the feed composition. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The measurement of pH at high temperatures is highly desirable for the control and monitoring of industrial processes. A typical example is in the pressure acid leaching (PAL) of laterite ores, which is conducted in autoclaves at a temperature around 250 °C, to extract nickel and cobalt (Krause et al., 1997; Whittington and Muir, 2000). Limonitic laterites are iron-rich oxide ores with iron content higher than 40 wt.%, magnesium content up to 2 wt.%, while the nickel and cobalt contents are about 1.4 wt.% and 0.15 wt.%, respectively. These ores are commonly processed by pressure acid leaching. Saprolitic laterites are magnesium-rich ores with magnesium content in the range of 10–20 wt.%, iron content is between 10 and 25 wt.%, while the nickel and cobalt contents are about 2.4 wt.% and 0.05 wt.%, respectively. Although saprolites are richer in nickel, the high magnesium content results in higher sulphuric acid consumption, which renders the PAL process less economical. However, an optimal blend of limonites and saprolites may form a high-grade feed that at the same time has an acceptable acid consumption (Rubisov et al., 2000; Whittington and Muir, 2000). More specifically, the sulphuric acid consumption for pressure acid leaching of limonitic laterites is relatively low (180–260 kg per tonne dry ore), whereas limonite/saprolite blends require much higher acid additions (400–500 kg per tonne of dry ore). The extra acid requirement is considerably higher than the stoichiometric requirement to dissolve the magnesium and other soluble constituents in saprolitic laterites. Although

⁎ Corresponding author. Tel.: + 1 416 978 1093; fax: + 1 416 978 8605. E-mail address: [email protected] (V.G. Papangelakis). 0304-386X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2010.09.002

magnesium consumes part of the acid, a significant amount of acid must remain in solution as “free” H2SO4. This free acid is around 20–30 g/L in the case of limonite processing, compared to 50–60 g/L in the case of processing of limonite/saprolite blends (Baghalha and Papangelakis, 1998). However, free acid measured by titration at room temperature does not reflect the true concentration of hydrogen ion (or pH) at elevated temperatures, which in fact controls the kinetics of the PAL process (Rubisov et al., 2000). Acid addition requirements in a PAL process were previously calculated using a speciation approach based on the postulation that high-temperature behaviour of divalent metal sulphates can be approximated with that of MgSO4 (Baghalha and Papangelakis, 1998). These calculations suggested that high nickel and cobalt extractions are achieved when sufficient acid is added to keep the solution pH at around 1 at the leach temperature, regardless of feed composition (Papangelakis et al., 2004). These two postulations were experimentally tested in the present work by direct (i.e. at temperature) pH measurements of PAL process solutions with the aid of an yttria-stabilized zirconia (YSZ) sensor. 2. Methods and materials The yttria-stabilized zirconia electrode is considered a primary pH sensor (Macdonald et al., 1988) and may be used for measuring pH without prior calibration under conditions of thermodynamic equilibrium. When using a flow-through YSZ electrode, however, irreversible thermodynamic contributions should be taken into account (Lvov et al., 1999, 2003; Lvov, 2007). As a result, a particular calibration procedure has been developed (Lvov, 2007; Seneviratne et al., 2003). This procedure involves measuring the potentials of at least

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two standard solutions and evaluating the value of a calibration coefficient, α, for the YSZ electrode, which takes into account deviation from the ideal behaviour. Once calibrated, the YSZ electrode can be used to measure a test solution potential. The calculation of the calibration coefficient, α, is based on the following equation (Lvov, 2007; Seneviratne et al., 2003)

pH1 −pH2 = −α

ðE1 −E2 Þ 2:303RT F

  2 3 ð1Þ ð2Þ ð1Þ Δ/D −Δ/D aH O 1 2 − log4 ð2Þ 5 + 2:303RT 2 a F

Table 2 Pressure acid leaching conditions. Exp. no.

Ore type

Acid/ore (kg/kg)

Temp. (°C)

Solids (wt.%)

Agitation (rpm)

Time (min)

1 2 3

Limonitic Limonitic/saprolitic Limonitic/saprolitic

0.20 0.50 0.28

260 260 260

19 19 19

450 450 450

90 90 90

ð1Þ

H2 O

where pH1 and pH2 are the pH values of standard solutions 1 and 2; E1 and E2 are the measured potentials of the YSZ electrode versus the (1) (2) reference electrode; Δ/D and Δ/D are the diffusion potentials; and (1) (2) aH2O and aH2O are the activities of water for standard solutions 1 and 2, respectively. The potentials of the standard solutions are experimentally measured, the pH values and water activities are calculated from established and verified thermodynamic data, while the diffusion potentials have to be independently calculated. In this work, pH values of the standard solutions and water activities were calculated using the mixed solvent-electrolyte (MSE) chemical model of the OLI Systems software (Wang et al., 2002) with a calibrated databank based on solubility measurements of Al, Mg and Ni in acidic sulphate solutions (Liu and Papangelakis, 2005). Diffusion potentials were evaluated using the Henderson equation (Hamann et al., 1998):   jzi ju0i B A RT ∑i zi mi −mi ∑ jz ju0 mB  B  ln i i 0i Ai Δ/D = /B −/A = − 0 A F ∑i jzi jui mi −mi ∑i jzi jui mi

ð2Þ

where A and B are the two solutions across the isothermal junction, zi is the charge of ion i, u0i is the limiting ionic mobility of ion i, and mi is the molal concentration of ion i. Concentrations and mobilities of the component ions were taken from the OLI Systems software. The measured potential of a test solution is converted into pH also using Eq. (1). In this case, if solution 1 is the test solution, then solution 2 is one of the standard solutions used for calibration. Pressure acid leaching experiments were conducted with a limonitic ore (originating from Indonesia) and a limonite/saprolite blend (originating from New Caledonia) to produce solutions, the pH of which was subsequently measured at temperature. The elemental composition of the feeds is given in Table 1. Pressure acid leaching conditions are given in Table 2. These conditions have been tested previously (Rubisov et al., 2000) and found to produce very similar kinetic and extraction behaviour between the limonite and the blend. The third experiment was under conditions that produce slow kinetics and low terminal Ni extraction. The acid–ore ratios in experiments with the blend were higher than that with the limonite ore to compensate for the higher magnesium content in the blend. In order to ensure nearequilibrium conditions of the PAL solutions, longer time was allowed during their pressure acid leaching (90 min), compared to the normal time required for N96% nickel extraction, which is 30–45 min. Pressure acid leaching experiments were carried out in a 2 L titanium autoclave (Parr Instruments). Temperature control was achieved by manipulating an electric heating mantle and a water-cooling stream. Agitation was provided by a titanium, magnetically driven, twin impeller. The autoclave was equipped with a dip tube that was used to withdraw filtered leach solutions at the end of the autoclave leach tests. A 10 μm pore graphite filter (Union Carbide) was used to prevent

solids from passing through the dip tube during the solution withdrawal process. The solutions were cooled in situ by a co-current heat exchanger during sample withdrawal. This sample withdrawal technique ensured preservation of solution composition during transfer from the autoclave to the electrochemical cell, particularly during cooling and reheating back to the same temperature of the original leach test. Because of the inverse solubility with temperature of hematite, alunite and all metal sulphates, the collection of samples filtered in situ and at temperature was critical. Metals in these solutions were analyzed using ICP-AES (Optima 3000 DV, Perkin Elmer). ICP-AES analysis was followed by complexiometric titration of the free acidity. Free acid titrations were performed with an auto-titrator (Tirtoline 96, Schott) which had a sensitivity of 0.01 mL of 0.1 mol L− 1 NaOH. The metal cations were first chelated to prevent them from reacting with NaOH used for titration. Calcium cyclohexane1,2-diamine-tetraacetate (Ca-CDTA) was used as a chelating agent (Georgiou, 1995). The free acid measured by this method is equal to the total sulphate minus that bound to metals stoichiometrically assuming simple sulphates. PAL solutions were subsequently transferred to a flow-through electrochemical cell. The pH was then measured at 250 °C. This temperature was intentionally 10 °C below the temperature at which PAL solutions were collected as a safeguard against metal precipitation on the cell walls, which are at a higher temperature due to heat transfer, during the pH measurement. Three different PAL solutions were produced, P#1 to P#3, corresponding to the tests shown in Table 2. Leach solutions P#1 and P#2 were used “as is” in pH measurements. The compositions of leach solutions P#1 to P#3 are given in g/L in Table 3. These concentrations were converted into molalities using density values at 25 °C obtained from OLI, namely 1.025, 1.070 and 1.033 kg/L for solutions P#1, P#2 and P#3, respectively. The concentrations of corresponding metal sulphates are listed in mol kg− 1 in Table 4. In addition, two synthetic solutions, SP#1 and SP#2, were made to test whether the high-temperature behaviour of nickel, cobalt and manganese sulphates can be approximated with that of MgSO4. Magnesium sulphate concentrations in synthetic solutions SP#1 and SP#2 were equal to the combined total divalent metal sulphate concentrations of P#1 and P#2, respectively, as shown in Table 5. Furthermore, in order to test the effect of H2SO4 on pH at a fixed metals concentration, PAL solution P#3 was first diluted and then acid-adjusted. Five solutions, D#1 to D#5 with different acidities were thus made by adding sulphuric acid to diluted P#3. The dilution factor was 3.8 and was chosen in such a way that dilution would not result in metal precipitation at 250 °C at the lowest acidity chosen, that is, 0.15 mol kg− 1. The compositions of solutions D#1 to D#5 are shown

Table 3 Compositions of leach solutions P#1, P#2 and P#3. Solution

Table 1 Elemental composition (wt.%) of laterite feeds. Type of feed

Ni

Co

Mg

Mn

Al

Fe

Limonitic Blend (limonitic/saprolitic)

1.20 1.67

0.16 0.10

0.60 2.69

1.08 0.59

2.05 2.72

48.0 43.8

P#1 P#2 P#3 a

Concentration (g L− 1) Ni

Co

Mg

Mn

Al

Fe

H2SO4

Mg*,a

2.625 3.825 1.437

0.231 0.165 0.072

0.735 5.450 2.650

0.553 1.063 0.437

0.076 0.310 0.032

0.002 0.044 0.002

26.5 58.8 26.5

2.166 7.607 3.476

Equivalent-Mg = sum of concentrations of Ni, Co, Mg and Mn (molar basis).

Z. Jankovic, V.G. Papangelakis / Hydrometallurgy 105 (2010) 155–160 Table 4 Compositions of leach solutions P#1, P#2 and P#3. Solution

P#1 P#2 P#3 a

Table 7 Compositions of binary NiSO4–H2SO4 synthetic solutions Ni#1 to Ni#6.

Concentration (mol kg− 1)

Solution

NiSO4

CoSO4

MgSO4

MnSO4

Al2 (SO4)3

Fe2 (SO4)3

H2SO4

MgSO4*,a

0.0438 0.0612 0.0238

0.0038 0.0026 0.0012

0.0295 0.2107 0.1058

0.0098 0.0181 0.0077

0.0014 0.0054 0.0006

0.0001 0.0004 0.0001

0.270 0.593 0.269

0.0869 0.2926 0.1385

Equivalent-MgSO4 = sum of concentrations of Ni, Co, Mg and Mn sulphates.

Ni#1 Ni#2 Ni#3 Ni#4 Ni#5 Ni#6 a

Table 5 Compositions of synthetic solutions SP#1 and SP#2. Solution

SP#1 SP#2 a

157

Concentration (mol kg− 1) NiSO4

CoSO4

MgSO4

MnSO4

Al2 (SO4)3

Fe2 (SO4)3

H2SO4

MgSO4*,a

0.0 0.0

0.0 0.0

0.0869 0.2926

0.0 0.0

0.0014 0.0054

0.0001 0.0004

0.270 0.593

0.0869 0.2926

Equivalent-MgSO4 = sum of concentrations of Ni, Co, Mg and Mn sulphates.

Concentration (mol kg− 1) NiSO4

CoSO4

MgSO4

MnSO4

Al2 (SO4)3

Fe2 (SO4)3

H2SO4

MgSO4*,a

0.01 0.04 0.07 0.05 0.10 0.15

0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0

0.10 0.10 0.10 0.30 0.30 0.30

0.01 0.04 0.07 0.05 0.10 0.15

Equivalent-MgSO4 = sum of concentrations of Ni, Co, Mg and Mn sulphates.

constant pressure of 8 MPa. The potential difference between the YSZ electrode and the reference electrode was measured using an electrometer (617, Keithley) with an input impedance of 1014 Ω. Further experimental details are given elsewhere (Seneviratne et al., 2003).

3. Results and discussion 3.1. Pressure acid leach solutions

in Table 6. Subsequently, five synthetic solutions, SD#1 to SD#5, were made to further test the hypothesis that the behaviour of nickel, cobalt and manganese sulphates can be approximated with that of MgSO4. Magnesium sulphate concentrations in synthetic solutions SD#1 to SD#5 were equal to the combined total divalent metal sulphate concentrations in diluted, acid-adjusted leach solutions D#1 to D#5, respectively. Finally, six synthetic binary NiSO4–H2SO4 solutions, Ni#1 to Ni#6, were prepared as shown in Table 7. Although the pH of these solutions was previously measured (Jankovic et al., 2009), the pH of these solutions was recalculated as if they were binary MgSO4–H2SO4 solutions to check whether the behaviour of NiSO4 is similar to that of MgSO4. The flow-through electrochemical cell used for pH measurements consisted of a cylinder made of titanium Grade 5 (Lvov and Palmer, 2004; Seneviratne et al., 2003; Jankovic et al., 2009) into which a flowthrough yttria-stabilized zirconia pH electrode (Lvov, 2007; Lvov et al., 2003) and a flow-through external pressure-balanced Ag/AgCl reference electrode (Lvov, 2007; Lvov et al., 1998) were sealed. The flow-through YSZ pH electrode was made of a ZrO2 (8 mol% Y2O3) tube from CoorsTeck, Hg/HgO paste, and a Pt wire according to procedures published elsewhere (Lvov et al., 2003). Three HPLC pumps were employed to pump the solutions at a fixed flow rate of 0.5 mL min− 1. The reference solution (0.1 mol kg− 1 LiCl) was constantly pumped through the flow-through Ag/AgCl reference electrode. During calibration, standard solutions were pumped through the YSZ electrode. After calibration, pumping of the standard solutions was stopped and test solutions were pumped through the inlet of the cell. Measurements were performed at a

The measurements in pressure acid leach solutions were performed with an yttria-stabilized zirconia electrode referred to as YSZ-3. Two sets of measurements were done, namely runs 1 and 2. The calibration coefficient, α, was evaluated using Eq. (1) based on diffusion potentials calculated using the Henderson equation (Eq. (2)). The values of the calibration coefficient were 0.878 and 0.852 for runs 1 and 2, respectively. The comparison between experimentally derived pH values and theoretical pH values (OLI Systems software) for leach solutions P#1 and P#2 is given in Fig. 1. Measured potentials were converted into pH using Eq. (1). The lines represent theoretical pH values (obtained using OLI) as a function of H2SO4 concentration at various MgSO4 concentrations up to saturation. Solid lines refer to solutions containing no aluminium, whereas dotted lines represent solutions under aluminium saturation (with hydronium alunite). By comparing the pH values given in Fig. 1, it can be seen that the differences between the measured and theoretical pH are 0.05 and 0.17 pH units for leach solutions P#1 and P#2, respectively.

Table 6 Compositions of diluted, acid-adjusted leach solutions D#1 to D#5. Solution

D#1 D#2 D#3 D#4 D#5 a

Concentration (mol kg− 1) NiSO4

CoSO4

MgSO4

MnSO4

Al2 (SO4)3

Fe2 (SO4)3

H2SO4

MgSO4*,a

0.0063 0.0063 0.0063 0.0063 0.0062

0.0003 0.0003 0.0003 0.0003 0.0003

0.0272 0.0271 0.0270 0.0269 0.0268

0.0020 0.0020 0.0020 0.0020 0.0020

0.0001 0.0001 0.0001 0.0001 0.0001

0.0000 0.0000 0.0000 0.0000 0.0000

0.151 0.201 0.252 0.302 0.405

0.0359 0.0357 0.0356 0.0355 0.0353

Equivalent-MgSO4 = sum of concentrations of Ni, Co, Mg and Mn sulphates.

Fig. 1. Variation of pH as a function of H2SO4 concentration at different MgSO4 concentrations for pressure acid leach solutions P#1 and P#2 as well as synthetic solutions SP#1 and SP#2. The lines represent theoretical pH values (calculated with OLI). Solid lines refer to solutions containing no Al, whereas dashed lines represent solutions under Al saturation (with hydronium alunite). Symbols refer to experimental measurements: (△) P#1; (■) P#2; (▲) SP#1; and (□) SP#2.

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Two conclusions can be drawn. One is that simplification of PAL solutions to equivalent-MgSO4-Al2(SO4)3-H2SO4 solutions as was proposed by Baghalha and Papangelakis (1998) is acceptable. The second is that the pH values are around 1 for both feeds. This finding supports with measurements a previous conclusion based on speciation calculations (Papangelakis et al., 2004). That is, acid is added to a PAL process so that the final solution pH at temperature is around 1 regardless of the feed composition and the solubility of the constituent minerals. The experimental pH values for synthetic solutions SP#1 and SP#2 are also illustrated in Fig. 1. By comparing the data, it is evident that the pH values measured for PAL solutions are close to those measured for synthetic solutions containing equivalent-MgSO4 only. 3.2. Diluted, acid-adjusted pressure acid leach solutions The measurements in diluted, acid-adjusted leach solutions D#1 to D#5 are shown in Fig. 2. The maximum difference between the average measured pH and the theoretical pH is 0.11 pH units. The experimental pH measurements for synthetic solutions SD#1 to SD#5 are also given in Fig. 2. By comparing the data given in Fig. 2, it can be seen that the pH values measured for diluted, acid-adjusted leach solutions D#1 to D#5 and synthetic solutions SD#1 to SD#5 are similar. This finding again supports the postulation that the hightemperature behaviour of nickel, cobalt and manganese sulphates can be satisfactorily approximated with that of MgSO4*. 3.3. Synthetic NiSO4–H2SO4 solutions

Fig. 3. Experimental and theoretical pH values for binary NiSO4–H2SO4 synthetic solutions Ni#1 to Ni#6 along with recalculated experimental and theoretical pH values for binary MgSO*4–H2SO4 synthetic solutions Mg*#1 to Mg*#6, where the NiSO4–H2SO4 solutions were treated as though they were binary MgSO4–H2SO4 solutions. The lines represent theoretical pH values (calculated with OLI). Symbols refer to experimental measurements: (○) Ni#1; (●) Ni#2; (□) Ni#3; ( ) Ni#4; (△) Ni#5; (■) Ni#6; (x) Mg*#1; (✳) Mg*#2; (▲) Mg*#3; ( _ ) Mg*#4; (+) Mg*#5; and (×) Mg*#6.

◇

Fig. 3. By comparing the data shown in Fig. 3, it is evident that the original and recalculated pH values are similar. Therefore, it can be concluded that MgSO4 behaves similarly to NiSO4 in H2SO4 solutions at 250 °C in the concentration range investigated. 3.4. PAL process implications

The measurements in synthetic NiSO4–H2SO4 solutions were performed with an yttria-stabilized zirconia electrode denoted YSZ-1. The calibration data for electrode YSZ-1 are provided elsewhere (Jankovic et al., 2009). Two sets of measurements were done, referred to as runs 3 and 4. The calibration coefficient, α, was evaluated using Eq. (1) based on diffusion potentials calculated using the Henderson equation (Eq. (2)). The values of the calibration coefficient were 1.073 and 1.030 for runs 3 and 4, respectively (Jankovic et al., 2009). The measured and theoretical pH values for synthetic binary NiSO4–H2SO4 solutions Ni#1 to Ni#6 are illustrated in Fig. 3. The pH of these solutions was recalculated as if they were binary MgSO4–H2SO4 solutions. The recalculated experimental pH values are also shown in

Krause et al. (1997) proposed an empirical Eq. (3) to calculate the required sulphuric acid addition to obtain high nickel extraction as a function of feed composition in a fresh water PAL process at around 30% solids. Papangelakis et al. (2004) concluded that Eq. (3) essentially suggests the appropriate amount of acid to maintain solution pH at 1 at the leach temperature, regardless of feed composition. wt:%H2 SO4 ¼ 4 þ 6ðwt:%MgÞ þ 2:4ðwt:%Al  0:8Þ þ3ðwt:%Ni þ Co þ MnÞ þ 4ðwt:%CO2 Þ

ð3Þ

In the present work, a method is proposed to predict the required free H2SO4 concentration (as measured at room temperature) to achieve high nickel extraction (≥96%) in a fresh water PAL process as a function of the equivalent-Mg (i.e. Mg + Ni + Co + Mn) concentration in solution measured at room temperature. This method is based on an equation that gives the free acid necessary (as measured at room temperature) to maintain pH at 1 at the leach temperature as a function of the equivalent-Mg concentration (also as measured at room temperature). This equation has a general form as follows: 2

½H2 SO4  ¼ A½“Mg” þ B½“Mg” þ C

ð4Þ

where [H2SO4] is the concentration of free H2SO4 in g/L, [“Mg”] is the concentration of equivalent-Mg in g/L, while A, B and C represent constants. The values for the constants A, B and C depending on the leach temperature are reported in Table 8. Eq. (4) is shown in Figs. 4 and 5 for temperatures of 250 °C and 270 °C. Sulphuric acid concentrations in this equation were calculated with OLI by using the trial and error method. That is, equivalent-

Fig. 2. Variation of pH as a function of H2SO4 concentration at different MgSO4 concentrations for diluted, acid-adjusted solutions D#1 to D#5 as well as synthetic solutions SD#1 to SD#5. The lines represent theoretical pH values (calculated with OLI). Solid lines refer to solutions containing no Al, whereas dashed lines represent solutions under Al saturation (with hydronium alunite). Symbols refer to experimental measurements: (○) D#1; (△) D#2; (□) D#3; (+) D#4; ( ) D#5; (●) SD#1; (▲) SD#2; (■) SD#3; (✳) SD#4; and (x) SD#5.

◇

Table 8 Values of A, B and C in Eq. (4) depending on the leach temperature. Type of equation

pH (250 °C) = 1 equation at 25 °C pH (270 °C) = 1 equation at 25 °C

Constant A (g− 1 L)

B

C (g L− 1)

-0.0377 -0.0483

4.732 4.952

19.553 21.252

Z. Jankovic, V.G. Papangelakis / Hydrometallurgy 105 (2010) 155–160

159

and Papangelakis, 1998). Hydronium alunite dissolves during cooling according to the following equation (Baghalha and Papangelakis, 1998):

2H3 OAl3 ðSO4 Þ2 ðOHÞ6 ðsÞ þ 5H2 SO4 ¼ 3Al2 ðSO4 Þ3 þ 14H2 O

Fig. 4. Variation of free H2SO4 as a function of equivalent-Mg at various temperatures. Symbols refer to experimental data at: (△) 245 °C (Tuffrey et al., 2009); (□) 250 °C (Krowinkel, 1997; Rubisov et al., 2000); (●) 260 °C (this work); (○) 260 °C (Collins et al., 2004); (■) 270 °C (Krowinkel, 1997; Rubisov et al., 2000); and (▲) 270 °C (Rubisov and Papangelakis, 2000). Free acidities measured are corrected for Al and Fe. Equivalent-Mg includes Ni, Co, Mg and Mn.

MgSO4 concentration was kept constant at different levels while H2SO4 concentration was varied until pH = 1 was attained at each equivalent-MgSO4 level at either 250 °C or 270 °C. Molal concentrations were converted into grams per litre by using densities at 25 °C calculated with OLI. In order to make the proposed method universally applicable, aluminium concentration is excluded from the equation, and the effect of aluminium on the measured free acid is accounted for separately. The correction for aluminium can be calculated based on the assumption that there is very little soluble aluminium at temperature in the last autoclave compartment (but not necessarily in the first). That is, aluminium at temperature is in the form of hydronium alunite, H3OAl3 (SO4)2(OH)6(s). This assumption is supported by solubility experiments, which indicate that the solubility of Al2(SO4)3 in H2SO4 drops to essentially zero in the presence of saturated MgSO4 at 250 °C (Baghalha

Fig. 5. Variation of free H2SO4 as a function of equivalent-Mg at various temperatures. Symbols refer to experimental data at: (△) 245 °C (Tuffrey et al., 2009); (□) 250 °C (Krowinkel, 1997; Rubisov et al., 2000); (●) 260 °C (this work); (○) 260 °C (Collins et al., 2004); (■) 270 °C (Krowinkel, 1997; Rubisov et al., 2000); and (▲) 270 °C (Rubisov and Papangelakis, 2000). Free acidities are corrected for Al. Equivalent-Mg includes Ni, Co, Mg, Mn and Fe.

ð5Þ

Accordingly, for every 3 moles of Al2(SO4)3 released into solution during flash depressurization and cool down of the slurry, 5 moles of H2SO4 will be consumed. In other words, for every 1 g/L of aluminium present in solution as measured at room temperature, 3 g L− 1 of H2SO4 should be added to the amount of free acid measured by titration to account for acid levels at temperature (inside the autoclave). The effect of iron on the measured free acid can also be accounted for. It should be noted that Eq. (3) implies that all of the iron in solution is in the ferric state. Although many limonitic laterite feeds do not generate ferrous iron in solution, it is not unusual for the ferrous iron concentration to be equal to, or even greater than, the ferric iron concentration in laterite leach liquor, particularly when non-limonites are leached. If we then assume that all Fe(III) at temperature is in the form of hematite, Fe2O3(s), then the hematite will dissolve during cooling according to the following equation (Baghalha and Papangelakis, 1998):

Fe2 O3 ðsÞ þ 3H2 SO4 ¼ Fe2 ðSO4 Þ3 þ 3H2 O

ð6Þ

It can be readily calculated that 2.6 g/L of H2SO4 should be added to the amount of free acid measured at room temperature for every 1 g/L of Fe(III) in solution. The behaviour of ferrous iron is different from that of ferric iron; in fact, ferrous iron would be expected to behave in the same manner as Mg, Mn, Ni and Co. That is, the concentration of Fe(II) can be added to the concentration of equivalent-Mg. Also shown in Figs. 4 and 5 are the measured free H2SO4 concentrations as a function of the measured equivalent-Mg concentration for different PAL solutions. For comparison, also illustrated in Figs. 4 and 5, are the curves that give the required free acid to maintain pH at temperature at 0.8 and 1.2 as a function of equivalent-Mg. In addition to the data for PAL solutions P#1 and P#2, the pH of which was measured in this work, the data were also examined for PAL solutions obtained during batch PAL tests at the University of Toronto (Krowinkel, 1997; Rubisov et al., 2000) and Sherritt (Tuffrey et al., 2009), as well as for those obtained during continuous miniplant PAL testwork at Vale Inco (Rubisov and Papangelakis, 2000) and Sherritt (Collins et al., 2004). The PAL solutions reported in the literature were obtained from limonitic ores and various limonite/ saprolite blends originating from Soroako, Indonesia (Krowinkel, 1997; Rubisov et al., 2000); Goro, New Caledonia (Krowinkel, 1997; Rubisov and Papangelakis, 2000; Rubisov et al., 2000); Moa, Cuba (Tuffrey et al., 2009); and Ambatovy, Madagascar (Collins et al., 2004). The operating conditions were: temperatures from 245 °C to 270 °C, acid additions from 200 to 500 kg per tonne of ore, pulp densities from 20 to 34 wt.% solids, and retention times from 30 to 70 min. Nickel extraction was equal to or in excess of 96% in all the cases. In Fig. 4, the free acid is corrected for both alunite and hematite dissolution (all of the iron in solution is assumed to be in the ferric state), and the equivalent-Mg includes Mg, Ni, Mn and Co. In Fig. 5, the free acid is corrected for alunite dissolution only, i.e. the iron in solution is assumed to exist as Fe(II), and the equivalent-Mg includes Mg, Ni, Mn, Co and Fe. As can be seen in Figs. 4 and 5, most of the data fall in the pH range of 0.8 to 1.2 at the leach temperature. Also, the differences between the predicted and measured free acid are within about ±10 g/L for most of the experimental data.

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4. Conclusions A flow-through electrochemical cell with an yttria-stabilized zirconia (YSZ) sensor was used for in situ measurement of pH of nickel laterite pressure acid leach (PAL) solutions at 250 °C. Good agreement was observed between the measured pH values and those theoretically predicted using the OLI Systems software package calibrated independently based on solubility measurements. PAL process solutions were simplified to a ternary electrolyte system by combining all the divalent metal sulphates into one with the chemical and thermodynamic properties of MgSO4. The simplified PAL process solution was assumed to contain H2SO4, Al2(SO4)3 and equivalent-MgSO4. The experimental results support this postulation and also that acid should be added to a PAL process so that the solution pH is around 1 at the leach temperature, regardless of the feed composition. Most of the experimental data reported in the literature falls in the pH range of 0.8 to 1.2 at the leach temperature. There is good agreement between the predicted and measured free acid, within about ±10 g/L, for most of the experimental data. This result can, therefore, be used as a control parameter in determining acid addition levels in PAL autoclaves. Leach solutions high in magnesium and other divalent metals need extra acid to maintain a low pH at temperature which is required to solubilise the metal sulphates and still provide sufficient driving force to ensure an acceptable rate and overall extraction of nickel and cobalt. Acknowledgements The authors gratefully acknowledge financial support by the Natural Sciences and Engineering Research Council of Canada (NSERC). The authors also thank Dr. Michael J. Collins, Sherritt International Corporation, for useful discussions during the course of this work. References Baghalha, M., Papangelakis, V.G., 1998. The ion-association-interaction approach as applied to aqueous H2SO4–Al2(SO4)3–MgSO4 solutions at 250 °C. Metall. Mater. Trans. B 29B, 1021–1030. Collins, M.J., Barta, L.A., Buban, K.R., Kalanchey, R., Owusu, G., Raudsepp, R., Stiksma, J., Masters, I., 2004. Process development studies carried out by Dynatec as part of the Ambatovy Nickel Project evaluation. In: Collins, M.J., Papangelakis, V.G. (Eds.), Pressure Hydrometallurgy 2004. The Canadian Institute of Mining, Metallurgy and Petroleum, Montreal, Canada, pp. 259–276. Georgiou, D., 1995. Kinetics of nickel dissolution during sulphuric acid pressure leaching of a limonitic laterite. Master's Thesis, University of Toronto, Canada. Hamann, C.H., Hamnett, A., Vielstich, W., 1998. Electrochemistry. Wiley-VCH, Weinheim.

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