Sulphuric acid pressure leaching of laterites — speciation and prediction of metal solubilities “at temperature”

Hydrometallurgy 58 Ž2000. 13–26 www.elsevier.nlrlocaterhydromet

Sulphuric acid pressure leaching of laterites — speciation and prediction of metal solubilities Aat temperatureB D.H. Rubisov, V.G. Papangelakis ) Department of Chemical Engineering and Applied Chemistry, UniÕersity of Toronto, 200 College Street, Toronto, ON, Canada, M5S 3E5 Received 27 April 1999; received in revised form 10 March 2000; accepted 29 March 2000

Abstract Sulphuric acid pressure leaching of nickeliferous laterites has attracted considerable attention from the nickel industry during the past 5 years. The process is especially advantageous for limonitic laterites that mostly contain goethite, because iron precipitates releasing acid and thus rendering low acid consumption. It is also applicable to mixtures of limonites and saprolites, although at a higher acid consumption due to magnesium. Effective process design requires the solubility of metals that precipitate during the process to be known. In the present work, determination of metal solubilities is based on a simple speciation program that assumes the presence of only one dominant complex for each metal. The thermodynamic data for the precipitation reactions are extracted from high-temperature experiments with monometallic systems published previously. The validity of the approach is then tested against mixed bimetallic systems, and finally applied to calculate the solubility of aluminium, iron and magnesium in laterite leaching effluents Aat temperatureB. In both cases of limonitic feed and limoniticrsaprolitic blends, the prediction closely follows metal solubilities measured experimentally at temperatures from 230 to 2708C and at terminal-free acidities ranging from 10 to 75 grl. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Sulphuric acid pressure leaching; Speciation; Solubility

1. Introduction Direct sulphuric acid leaching is the process of choice to recover nickel and cobalt from limonitic laterites. Limonitic laterites are rich-in-iron oxide ores with iron content higher than 40 wt.%. Other elements found in limonites include aluminium, chromium, manganese and silicon. Aluminium in

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Corresponding author. Tel.: q1-416-978-1093; fax: q1-416978-8605. E-mail address: [email protected] ŽV.G. Papangelakis..

particular, is responsible for scale formation during leaching w1x, although it exists in small amounts between 2 and 6 wt.%. These ores contain 1.1–1.4 wt.% of nickel and 0.1–0.2 wt.% of cobalt that are extractable hydrometallurgically. Direct sulphuric acid pressure leaching is the preferred route, because under conditions of 250–2808C and 20–30 grl H 2 SO4 , goethite and gibbsite dissolution is immediately followed by iron and aluminium precipitation as hematite and hydronium alunite, respectively. This precipitation makes it possible to attain a high wNi q CoxrwFe q Alx ratio in the resulting leach liquor w2–4x. Another marked advantage of this process is the relatively low consumption of sulphuric acid due

0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 0 0 . 0 0 0 9 3 - 1

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D.H. RubisoÕ, V.G. Papangelakisr Hydrometallurgy 58 (2000) 13–26

to its regeneration during iron and, to a lesser degree, aluminium precipitation. Saprolitic laterites are rich-in-magnesium oxide ores with magnesium content in the range of 10–20 wt.% and iron content between 10 and 25 wt.%. Although they are richer in nickel Žup to 3 wt.%., the high magnesium content results in higher acid consumption, which renders the process less economical. However, an optimal mixture of limonites and saprolites may form a high-grade feed that at the same time yields an acceptable acid consumption. Previous experience demonstrated that a highly agitated continuous autoclave Žor series of autoclaves. is the best choice for successful nickel and cobalt extraction w5x. Effective process design requires the solubility of metals that precipitate during the process to be known. Moreover, knowledge of the solubility of metals Aat temperatureB is essential for understanding the chemistry that governs the leaching process. For example, the leaching kinetics depend heavily on the concentration of hydrogen ion Aat temperatureB. However, in the absence of suitable sensors, this concentration cannot be monitored effectively. Instead, it may be calculated using a theoretical approach. Such an approach has to take into account all transient metal concentrations Aat temperatureB. The latter depend on metal solubilities Aat temperatureB since both dissolution and precipitation reactions occur simultaneously. Existing theoretical approaches to modelling the high-temperature ionic solution chemistry may be split into two major categories: Ža. ion association Žspeciation., and Žb. ion interaction. The former approach assumes that ions form stable complexes. The free energy of the complex formation reaction may then be evaluated at elevated temperatures by approximating the dependence of ionic heat capacities on temperature from regression Že.g., Ref. w6x. or by considering forces involved in the binding of the ions in complexes w7x. The second approach, pioneered by Pitzer w8x, assumes no association, but interactions between simple ions. These interactions are taken into account by a series of coefficients that can be evaluated directly, analysing high-temperature experiments w9x. Both approaches have already been applied in the past to predict the solubilities of metals in lateritic systems w10,11x.

A speciation model developed previously for a pure Al–H 2 SO4 system was used to evaluate the equilibrium constants for the hydronium alunite precipitation, w10,12x. However, our attempt to re-apply this model to predict of the solubility of Al in experiments with a limonitic feed w13x was not successful. This was attributed to the uncertainties of the heat capacity extrapolation techniques, the lack of reference state thermodynamic data Ž298 K. for all species, and the probable formation of totally new aqueous complexes at temperatures between 2308C and 2708C. On the other hand, applications of the ion interaction approach to extract the ion interaction parameters from monometallic systems w9x and then use them for polymetallic lateritic systems w11x showed that only a hybrid of the two approaches Žspeciation and ion interaction. is capable of predicting metal solubilities at elevated temperatures. The complexity of this latter approach suggested us to investigate a simplified variant in order to facilitate its use in process engineering applications. It was decided to employ an ion association Žspeciation. approach that accounts for only one dominant complex for each metal and extract equilibrium constants for the precipitation reactions from high-temperature monometallic experiments to avoid performing any temperature extrapolation. Hence, the present paper, that opens a series of publications on laterite leaching process, describes a solution speciation approach that allows for the prediction of solubilities of Al, Fe and Mg in the temperature range from 2308C to 2708C. It also reviews the process chemistry, demonstrates the extraction of thermodynamic data from high-temperature experiments with monometallic systems, and verifies the speciation approach against real polymetallic Žlateritic. systems. 2. Review of the process chemistry Limonites contain Fe chiefly as goethite Ž aFeOOH.. During high-temperature sulphuric acid leaching, trivalent, ferric iron dissolves according to the following reaction:

° Fe

FeOOHq 3Hq

3q

q 2H 2 O

Ž I.

Saprolitic laterites may also contain as much as half of the iron as goethite. Contrary to limonites, a

D.H. RubisoÕ, V.G. Papangelakisr Hydrometallurgy 58 (2000) 13–26

marked fraction of iron in saprolites is divalent iron that substitutes Mg in serpentine ŽMg 3 Si 2 O5 ŽOH.4 .. This FeŽII. dissolves, following the dissolution of Mg, and then oxidizes to FeŽIII.. Once dissolved, trivalent iron precipitates releasing acid. Two precipitating phases may be formed, basic ferric sulphate and hematite w3,4x. Basic ferric sulphate, FeOHSO4 , is known to be unstable, and according to the Stranski–Ostwald step rule it should be formed first if homogenous nucleation initially prevails: Fe

3q

q SO42yq H 2 O

° FeOHSO Ž s. q H

q

4

Ž II .

Unstable FeOHSO4Žs. should then rapidly convert to hematite: 2FeOHSO4 Ž s . q H 2 O

° Fe O Ž s. q 2SO 2

2y q 4 q 4H

3

Ž III .

The overall reaction is: 2Fe 3qq 3H 2 O

° Fe O Ž s. q 6H

q

2

3

Ž IV .

Previous work w3x indicated that high acidity favours the precipitation of basic ferric sulphate and impedes its conversion to hematite. The question whether hematite may precipitate directly according to Reaction ŽIV. or only through the formation of basic ferric sulphate is still unclear. Theoretically, an increase of the amount of hematite in the system has to favour the secondary nucleation of hematite w14x and consequently the precipitation path according to Reaction ŽIV.. It should also be noted that the formation of FeOHSO4Žs. was never confirmed in experiments at the University of Toronto even in the first 2.5 min of leaching w13,15x. On the other hand, Reaction ŽIII. may be extremely fast. Most of the Ni in limonitic laterites is found in solid solution with goethite. Ni dissolution from goethite is described by the following equation:

° Ni

q

NiO q 2H

2q

q H 2O

Ž V.

Ni in saprolitic laterites is mostly associated with Mg substituting the latter in serpentine. Similarly to divalent Fe, this Ni dissolves following the dissolution of Mg. Mg concentration in limonites is relatively low, normally less than 1 wt.%. Acid balance based on previous experiments w13x suggested that the dissolved fraction of Mg never exceeded 50%. The mean Mg extraction from limonitic samples was

15

25%. The concentration of Mg in saprolites is in the range of 10–20 wt.%. Serpentine dissolves very quickly in sulphuric acid and under normal process conditions the dissolved Mg does not precipitate: 2Mg 3 Si 2 O5 Ž OH . 4 q 12Hq

° 6Mg

2q

q 4SiO 2 Ž s . q 10H 2 O

Ž VI .

However, AMAX researchers observed Mg precipitation as mono-hydrate sulphate salt: Mg 2qq SO42yq H 2 O

° MgSO P H OŽ s. 4

2

Ž VII.

in continuous autoclaves, treating limoniticrsaprolitic ore blends, if wMgx was above 12 grl w16x. They suggested the use of this precipitation for inhibiting the formation of aluniterhematite scale. Aluminium exists in laterites in several phases. It is chiefly found in the form of gibbsite, AlŽOH. 3 . Al also replaces iron in goethite structures Žsolid solutions Al x Fe1- xOOH.. Some Al is associated with Cr in chromium spinels ŽŽFe,Mg.ŽAl,Cr. 2 O4 . and appears in other phases as well. The major Al phase, gibbsite, transforms to boehmite ŽAlOOH. by dehydration during ore pre-heating w17x. Boehmite reacts with sulphuric acid according to a dissolution reaction similar to that of goethite ŽReaction ŽI..:

° Al

AlOOHq 3Hq

3q

q 2H 2 O

Ž VIII.

Under process conditions, the concentration of dissolved aluminium usually exceeds its solubility and aluminium precipitates as hydronium alunite: Al 3qq 2SO42yq 7H 2 O

° Ž H O. Al Ž SO . Ž OH. Ž s. q 5H

q

3

3

4 2

6

Ž IX .

3. Experimental data and theoretical approach Laterite leaching experiments were conducted with a limonitic feed w13,18x and limoniticrsaprolitic blends w19x. The duration of the experiments was at least 1 h to ensure that the metal concentrations had stabilized and thus were close to the solubility values. Ore samples for these experiments Žy100 mesh, 45–55% moisture. were supplied by Inco Technical Services ŽITSL.. The elemental composition of the feeds is given in Table 1. Leaching tests with laterites were performed in a 2-liter titanium autoclave, manufactured by the Parr Instrument. Temperature was controlled within "28C

D.H. RubisoÕ, V.G. Papangelakisr Hydrometallurgy 58 (2000) 13–26

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

Ni

Limonitic 1.22 Limoniticrsaprolitic I 1.87 Limoniticrsaprolitic II 1.67

Co

Fe

Al

Mg

Mn

Cr

Si

S

0.14 0.13 0.10

47.7 39.4 43.8

1.9 2.16 2.72

1.03 3.85 2.69

0.97 0.75 0.59

1.56 1.79 1.48

3.87 7.35 4.78

0.26 0.30 0.08

by a temperature control system, manipulating both an electrical heating mantle and a water-cooling stream. Agitation was provided by a titanium twin magnetically driven impeller. The autoclave was equipped with an acid injection device. It was also equipped with a dip tube to withdraw samples that were then cooled by a co-current heat exchanger. It is important to stress that liquid (not slurry) samples were taken during all the tests preventing a possible re-dissolution of the precipitates during cooling. This approach ensured that the solubilities measured reflect conditions Aat temperatureB. This procedure also allowed us to identify Mg precipitation inside the autoclave in the case of limoniticrsaprolitic blends. A 30-mm pore graphite filter, manufactured by Union Carbide, was utilised in order to prevent solids from passing through the sampling tube. Solution aliquots were periodically withdrawn and analysed. 6 to 8 ml were taken and discarded to clean the tube, and then 4 ml were withdrawn for the analysis. A total of six samples of 12 ml each, made less than 7% of the initial liquid volume Ž1 l of water plus acid., hence the change of the initial solidsrliquid ratio did not change significantly. Furthermore, production of up to 50 ml of water due to goethite– hematite conversion made this effect even smaller. After dilution, metals were analysed using flame atomic absorption spectroscopy ŽFAAS.. A fully automated instrument ŽVARIAN SpectrAA. 250 Plus. was employed for this purpose. FAAS analysis was followed by complexiometric titration of the stoichiometric free acidity. The metal cations were first chelated in order to prevent them from reacting with NaOH used for titration. Calcium cyclohexane-1,2diaminetetraacetate ŽCa-CDTA. was used as the chelating agent w18x. The free acid measured corresponds to the total sulphate minus that bound to metals stoichiometrically assuming simple sulphates:

w H 2 SO4 x free s w SO4 x total y z ir2 P Me iz iq

Ž 1.

In the present work, the thermodynamic properties of ions and complexes formed at autoclave temperatures were extracted from high-temperature experiments with monometallic systems. To achieve this, a speciation program was developed. The program was based on the following simplifying assumptions: Ža. each metal forms only one dominant complex, Žb. equilibrium constants for precipitation reactions are given by an extended Debye–Huckel ¨ equation ŽVasil’ev’s approach, w20x.: log b s

Ž yD HT orT q D ST o . 2.3 R

q

(

a Ic

(

1 q B Ic

q bIc

Ž 2. and Žc. only the first step of the sulphuric acid dissociation is complete. In Eq. Ž2., D HT o and D ST o are the change of standard enthalpy and entropy for the precipitation reaction at temperature T, R is the universal gas constant, Ic is the ionic strength Žmolrl., B and b are parameters, a s AD z 2 , A is the Debye–Huckel constant Žat the temperature of ¨ interest, molarity units., and D z 2 is the algebraic sum of the squares of charges of ionic species in the precipitation reaction. It should be noted that A and D z 2 can be calculated for ion complexation reactions. For precipitation reactions though, their values are expected to be a more complicated issue. Since most precipitation processes involve surface complexation, D z 2 depends on the surface charge density w21x. Therefore, the parameter a was treated as adjustable. The value of parameter B s 1.6 was adopted following previous work w12,22x. A general speciation model was developed for a mix of metal sulphates typical for laterite leaching. Before applying it to lateritic solutions, it was used to verify the existence of assumed dominant complexes in monometallic solutions Žcontaining only Al, Fe and Mg. and to evaluate equilibrium constants of precipitation reactions Aat temperatureB. A de-

D.H. RubisoÕ, V.G. Papangelakisr Hydrometallurgy 58 (2000) 13–26

tailed description of the speciation program is given in the Appendix A. 3.1. Aluminium Thermodynamic data for aluminium were extracted from a series of experiments with a synthetic Al-sulphuric acid system conducted previously at the University of Toronto w9,10,12x in the range of 0.25 to 0.75 molrl initial acidity and temperatures from 230 to 2708C. As suggested before w9x, a neutral aluminium sulphate complex, Al 2 ŽSO4 . 3 Žaq., predominates at 2508C. In the present work, this assumption was extended for the whole range of autoclave temperatures tested Ž230–2708C.. The precipitation Reaction ŽVIII. in terms of the ions and complexes present in the solution can be written as follows: 3Al 2 Ž SO4 . 3 Ž aq . q 14H 2 O

° 2Ž H O. Al Ž SO . Ž OH. Ž s. q 5H q 5HSO q

3

3

4 2

6

y 4

Ž X.

Fitting Eq. Ž2. into the experimental data yielded the following values of the parameters: log b s log

w Hq x T5r2 HSO4y Al 2 Ž SO4 . 3 Ž aq .

5r2 T 3r2 T

( 1 q 1.6(I 14.21 Ic

y 1.667Ic

solid precipitate Žhematite, basic sulphate. were attempted to satisfy Eq. Ž2.. The best fit was obtained with the neutral ferric sulphate ŽFe 2 ŽSO4 . 3 . as the aqueous species and basic ferric sulphate ŽFeOHSO4 . as the solid phase. This may indirectly support the precipitation pathway proceeding through the formation of FeOHSO4, i.e., Reactions ŽII. and ŽIII.. The precipitation reaction is then written as: Fe 2 Ž SO4 . 3 Ž aq . q 2H 2 O

° 2FeOHSO Ž s. q HSO q H y 4

4

q

Ž XI .

The enthalpy and entropy changes for Reaction ŽXI. obtained by fitting Eq. Ž2. to the experimental data yielded D H T o s 52.39 kJrmol, D ST o s 205.0 JrŽmol K.. Eq. Ž2. for Reaction ŽXI. becomes:

w Hq x T HSO4y T log b s log Fe 2 Ž SO4 . 3 Ž aq . T s y2740rTq 10.72 y

( 1 q 1.6(I 9.905 Ic

q 0.0294 Ic

Ž 4.

c

3.3. Magnesium

s y4464rTq 3.774 q

17

Ž 3.

c

That is, D HT o s 85.37 kJrmol and D ST o s 72.17 JrŽmol K. for the precipitation Reaction ŽX.. Concentrations in square brackets hereafter are in molrl, the subscript T means that the concentrations are Aat temperatureB. 3.2. Iron Data for the FeŽIII.-sulphuric acid system have been published in the past w23x, although for temperatures below 2008C. These measurements were confirmed by experiments at the University of Toronto w22x. Various combinations of the aqueous species ŽFeSO4q, FeSO4 HSO4Žaq., Fe 2 ŽSO4 . 3 Žaq.. and the

Data on the solubility of magnesium sulphate for a wide range of temperatures and concentrations were collected three decades ago at the Oak Ridge Laboratory, USA w24x. An attempt to use known Mg complexes existing at room temperature to fit the experimental data was not successful. Mg species identified by Baghalha and Papangelakis w9x were not capable of providing any better result. Moreover, Baghalha and Papangelakis w9x suggested the existence of an extra Mg complex in this system. Therefore, a very general ionic complex in the form k - n.q MgŽOH. k ŽHSO4 .Ž2was assumed to dominate in n the system. It should be noted that this complex converts to a sulphate–bisulphate complex, or basic sulphate–bisulphate complex by removing k molecules of water if k - n and k ) n, respectively. Thus, it covers practically any Mg-bearing species that may be formed in the system. Parameters k and n were treated as adjustable when fitting Eq. Ž2. to

D.H. RubisoÕ, V.G. Papangelakisr Hydrometallurgy 58 (2000) 13–26

18

the experimental data. The values of k s 0.4 and n s 1.6 Ž"0.7%. were obtained. This result suggests that the complex Mg 5 ŽSO4 . 2 ŽHSO4 . 6 Žaq. possibly dominates in the system. It should be noted that if two water molecules are added, then this complex becomes Mg 5 ŽHSO4 . 8 ŽOH. 2 Žaq.. However, it is unknown presently which one really exists although both complexes are equivalent thermodynamically. In any case and similarly to the Al–sulphate and Fe–sulphate systems, a neutral complex provides the best fit of the speciation model to experimental data. The precipitation reaction is then written as:

°5MgSO

Mg 5 Ž SO4 . 2 Ž HSO4 . 6 Ž aq . q 5H 2 O P H 2 O Ž s.

4

q 3HSO4yq 3Hq

Ž XII.

Thermodynamic parameters for Reaction ŽXII. were extracted similarly to Al and Fe–SO4 systems and Eq. Ž2. becomes:

w Hq x T3 HSO4y T3 log b s log Mg 5 Ž SO4 . 2 Ž HSO4 . 6 Ž aq .

T

s y1312rTy 9.369 q

( 1 q 1.6(I 34.62 Ic

y 1.484 Ic

Ž 5.

c

i.e., D HT o s 25.12 kJrmol, D ST o s –179.4 JrŽmol K..

The extracted thermodynamic data and the developed speciation program were applied then to predict Žand compare with measured. solubilities in real polymetallic systems, i.e., laterite leaching experiments.

4. Solubilities of Al, Fe and Mg Cat temperatureD during laterite leaching 4.1. Verification of the approach In order to test our speciation approach, the speciation program was first verified against a ternary Mg–Al–H 2 SO4 system at 2508C. The concentrations of Mg were predicted with a maximum relative error of 9.5% and the concentrations of Al were predicted with a maximum relative error of 31% for the data set reported by Baghalha and Papangelakis w9x. Further, the model predicts the concentrations of Al and Mg with an accuracy of "6.35% and "14.3%, respectively, with a confidence level of 70%. It should be noted that the highest relative error of 31% in the prediction of Al concentration occurred at the lowest wAlx value, and the absolute error for this experiment was only 0.003 molrl. Hereafter concentrations without index T imply that they are concentrations Aat temperatureB re-calculated to am-

Fig. 1. Calculated hydrogen ion concentration as a function of free acidity in ternary Al–Mg–SO4 solutions saturated with aluminium sulphate.

D.H. RubisoÕ, V.G. Papangelakisr Hydrometallurgy 58 (2000) 13–26

19

Fig. 2. Calculated solubility of Al and Fe for the limonitic feed Žlines. as compared to batch experiments w18x Žpoints.. 22% solids, 2308C.

bient conditions by accounting for the change of the solution density. The values of the acidity Aat temperatureB obtained from the speciation model were also calculated and plotted in Fig. 1. The calculated values were compared with those evaluated using an ion interaction approach used by Baghalha and Papangelakis w9x. Units of molality were used instead to allow for a direct comparison. As seen in Fig. 1, a

similar trend in the dependence of the wHqx T on the concentration of dissolved MgSO4 as in w9x was observed. The values of wHqx T calculated in the present work are, however, significantly higher. This difference is due to the selection of the dominant Mg-containing complexes in the two models. Baghalha and Papangelakis w9x assumed that Mg exists in the solution as a simple ion and a neutral sulphate complex ŽMg 2q and MgSO4Žaq.., whereas

Fig. 3. Calculated solubility of Al and Fe for the limonitic feed Žlines. as compared to batch experiments w18x Žpoints.. 22% solids, 2508C.

20

D.H. RubisoÕ, V.G. Papangelakisr Hydrometallurgy 58 (2000) 13–26

a formation of a sulphate-bisulphate complex is assumed here. The existence of this complex has yet to be confirmed by direct in situ measurements. 4.2. Prediction of metal solubility in laterite leach solutions After the verification, the speciation program was applied to laterite leaching experiments. Solubilities of Al and Fe were calculated for the three feed compositions given in Table 1. Equilibrium concentrations were calculated Aat temperatureB and then were re-calculated to room temperature to allow for a direct comparison with experiments. Figs. 2–4 give these comparisons plotted versus the acidrore ratio for the limonitic laterite feed at 2308C, 2508C and 2708C, respectively. Two lower axes in the same plots also show calculated free acidity and wHqx T that should correspond to the acidrore ratio shown at the upper axis. It is important to note that the solubility prediction for Al is entirely based on the data extracted from monometallic systems. Contrary to Al, predictions of iron concentration based on the data extracted from monometallic systems were more than an order of magnitude below the experimental results. Most probably, this discrepancy is due to the fact that monometallic experiments were conducted at the temperature below

2008C, whereas laterite leaching tests were at the temperature range of 230–2708C. It is not clear whether at higher temperatures Reaction ŽXI. becomes invalid, or the equilibrium constant of Reaction ŽXI. changes so dramatically with temperature. We have assumed the latter at this point. This assumption remains to be confirmed by further experiments. Mean ratios of measured over predicted iron concentrations for the limonitic feed were 25.9 at 2308C, 31.3 at 2508C and 24.9 at 2708C. Since the above values do not show a temperature correlation, a mean value of 28.3 was used as a correction factor to switch from the thermodynamic data obtained from experiments at 150–2008C to those at autoclave temperatures, 230–2708C. This correction changes Eq. Ž4. as follows: log b s log

w Hq x T HSO4y T Fe 2 Ž SO4 . 3 Ž aq . T

s y2740rTq 9.268 y

( 1 q 1.6(I 9.905 Ic

q 0.0294 Ic

Ž 6.

c

The predicted solubilities plotted in Figs. 2–4 were calculated using Eq. Ž6.. The fact that a constant

Fig. 4. Calculated solubility of Al and Fe for the limonitic feed Žlines. as compared to batch experiments w18x Žpoints.. 22% solids, 2708C.

D.H. RubisoÕ, V.G. Papangelakisr Hydrometallurgy 58 (2000) 13–26

21

Fig. 5. Calculated solubility of Al, Fe and Mg for the limoniticrsaprolitic blend I Žlines. as compared to batch experiments w19x Žpoints.. 27% solids, 2508C.

factor Ž28.3. may be used to correct the concentration implies that the change in entropy, but not enthalpy, for Reaction ŽXI. is particularly dependent on the temperature range. Figs. 5 and 6 show calculated and measured solubilities of Al and Fe for the limoniticrsaprolitic blend I at 2508C and 2708C. The solubilities for the blend II are shown in Fig. 7. Experiments with both

blends were conducted at 27% of solids w19x. The same correction factor as for the limonitic feed Ž28.3. was also used to predict iron solubility in the case of blends. Precipitation of Mg was observed experimentally for the limoniticrsaprolitic blend I only. Therefore, the solubility of Mg was also calculated and plotted in Figs. 5 and 6. The comparison of this prediction

Fig. 6. Calculated solubility of Al, Fe and Mg for the limoniticrsaprolitic blend I Žlines. as compared to batch experiments w19x Žpoints.. 27% solids, 2708C.

22

D.H. RubisoÕ, V.G. Papangelakisr Hydrometallurgy 58 (2000) 13–26

Fig. 7. Calculated solubility of Al and Fe for the limoniticrsaprolitic blend II Žlines. as compared to batch experiments w19x Žpoints.. 27% solids, 2708C.

with measured Mg concentrations w19x in these plots confirms that the thermodynamic data for Mg precipitation obtained from monometallic systems are also applicable to complex solutions resulted from the treatment of laterites. A comparison of Figs. 5 and 6 suggests that predicted and measured Al and Fe solubilities are practically temperature-independent at acidrore ratios of 0.32 or lower. Calculations for the blend II at 230–2708C showed that a similar temperature-independence of the solubilities is observed at acidrore ratios of 0.25 or lower. This should be attributed to Mg precipitation at low acidities. At higher temperatures, Mg solubility is lower. Hence, when Mg precipitates, its concentration decreases at constant free acidity releasing bisulphate ions and thus also increasing wHqx T . Higher wHSO4yx T and wHqx T allow more Al and Fe to stay in solution thus compensating the direct effect of temperature on the solubilities of Al and Fe. An interesting conclusion may be derived when comparing metal solubilities obtained in experiments with the limonitic feed and with the blends at the same temperature. For example, comparing Fig. 3 with Fig. 5 one may observe that Al solubility of 2 grl at 2508C is achieved at an acidrore ratio of 0.25 in the case of the limonite and at an acidrore ratio of 0.4 in the case of the blend I. The correspondent values of the free acidity also differ significantly

Žapproximately 30 and 57 grl, respectively.. However, the calculated hydrogen ion concentrations Aat temperatureB are very close, i.e., approximately 0.18 and 0.20 molrl. This means that the wHqx T is a very powerful parameter that make it possible to compare experiments with different solid loads and feed compositions. 4.3. Calculation of free acidity The speciation program given in Appendix A and used in this work is based on the assumption that the free acidity measured by chelation and titration of liquid samples is the total aqueous sulphate less aqueous sulphate that is bound stoichiometrically to the dissolved metals, i.e., it is given by Eq. Ž1.. This statement was confirmed in our laboratory by analysing synthetic metal sulphate solutions. In order to verify that this is also applicable to real laterite leach solutions, values of free acidity calculated from Eq. Ž1. and shown in the solubility diagrams were compared with those obtained experimentally by titration. The comparison is given in Fig. 8. Calculated curves in Fig. 8 closely follow experimental points. It is interesting to note that the values of free acidity do not show temperature dependence. The explanation to this fact is similar to that given in the previous section to explain temperature-independent

D.H. RubisoÕ, V.G. Papangelakisr Hydrometallurgy 58 (2000) 13–26

23

Fig. 8. Calculated free acidity assuming simple sulphates ŽEq. Ž1.. Žlines. as compared to measured by titration w18,19x Žpoints..

solubility of Al and Fe at low acidities. At higher temperatures, metal solubilities are lower. On the other hand, at lower solubility, more precipitate forms and thus more sulphate is rejected from the solution with the solids. That is, both terms in the righthand-side of Eq. Ž1. are equally decreased when temperature increases. It is seen in Fig. 8 that the only condition when the calculated free acidity under-predicts the measured values is in the case of the blend I and acidrore ratio of 0.2. Most probably, this means that at a very low acidity Žcalculated wHqx T - 0.01 molrl., the formation of neutral complexes is not possible and the speciation becomes different from that assumed in the speciation model. Finally, it should be noted that the solubility diagrams generated here show the metal concentrations inside the autoclave Aat temperatureB. Hence, these values will increase if the slurry is cooled down before and after liquidrsolid separation due to re-dissolution of the precipitates.

5. Conclusions The solubilities of Al, Fe and Mg in simple monometallic systems were used to extract equilibrium constants for their precipitation reactions at temperatures relevant to the acid pressure leaching of

laterites. This information was then applied to laterite leach experiments using a speciation analysis Aat temperatureB. The speciation approach employed was mostly simplified assuming that only one complex for each metal dominates in the solution. Extraction of thermodynamic data directly from hightemperature experiments allowed us to avoid errors associated with their extrapolation from the reference state Ž298 K.. This approach demonstrated a good predictive ability for real process solutions resulting from sulphuric acid pressure leaching of limonitic laterites and limoniticrsaprolitic ore blends. Al, Fe and Mg solubility diagrams were then generated for three lateritic feeds for the temperature range from 2308C to 2708C and acidrore ratios of up to 0.5 that corresponded to free acidities of up to 75 grl. Similar diagrams can also be constructed for laterites with other chemical compositions. These diagrams can be readily used to predict metal solubilities in leach liquors in batch reactors. Moreover, combined with leaching kinetics, they may also be used to predict metal concentrations in continuous autoclaves. At temperatures of 2308C to 2708C Al and Fe tend to form neutral sulphate complexes. Also, the formation of a new Mg-bearing neutral sulphate-bisulphate complex ŽMg 5 ŽSO4 . 2 ŽHSO4 . 6 Žaq.. was proposed. The existence of this complex, however, has yet to be confirmed by direct in situ measurements.

D.H. RubisoÕ, V.G. Papangelakisr Hydrometallurgy 58 (2000) 13–26

24

Acknowledgements This work was financially supported by ITSL and the Natural Science and Engineering Research Council of Canada. The authors wish to thank the personnel of ITSL for the continuous support, vivid interest and fruitful discussions over the course of the project.

complexes was as follows: Hq, HSO4y, Al 2 ŽSO4 . 3Ž aq ., Fe 2 ŽSO 4 . 3 Žaq ., Mg 5 ŽSO 4 . 2 ŽHSO 4 . 6 Žaq ., N i 5 Ž SO 4 . 2 Ž H SO 4 . 6 Ž aq . , Co 5 Ž SO 4 . 2 Ž H SO 4 . 6 Žaq., Mn 5 ŽSO4 . 2 ŽHSO4 . 6 Žaq. and HCrO4y. The concentrations of dissolved metals and free acidity coupled with the charge balance provides nine equations necessary to calculate the concentrations of the nine ions and complexes listed above.Eq. Ž1. in the presence of bichromate can be written as:

w H 2 SO4 x free s w SO4 x total q w Cr x total y z ir2 P Me iz iq Ž A1.

Appendix A. Speciation program This speciation program calculates wHqx T and wHSO4yx T based on the stoichiometric free acidity and metal concentrations measured for liquid samples withdrawn Aat temperatureB. The complexity and mathematical instability of speciation programs at elevated temperatures that involve multiple equilibria, has already been reported w22x. To overcome this problem, the solution chemistry was mostly simplified. Only one complex for each metal was assumed to be present in the system. The complexes containing Al, Fe and Mg were neutral ion associations of aqueous Al 2 ŽSO4 . 3 , Fe 2 ŽSO4 . 3 and Mg 5 ŽSO4 . 2 ŽHSO4 . 6 , respectively. Based on the similar chemical behaviour of Mg, Ni, Mn and Co, Baghalha and Papangelakis w11x proposed to combine the concentrations of these metals in a generalized divalent metal concentration. Following this approach, it was assumed in the present speciation program that the four divalent metals form complexes similar to that formed by Mg, i.e., Me 5 ŽSO4 . 2 ŽHSO4 . 6 , where Me stands for any divalent metal. It was further assumed that only the first step of the sulphuric acid dissociation is complete, and that the concentration of hydroxyl ion is negligible. Finally, the presence of chromium as HCrO4– was assumed w5x. Thus, the system of ions and

Other equations used in the speciation program were set as follows. The total sulphate concentration for the given system of complexes is:

w SO4 x total s HSO4y q 1.6 Ž w Ni x total q w Cox total q w Mg x total q w Mn x total . q 1.5 Ž w Al x total q w Fe x total .

Ž A2.

Combining Eqs. ŽA1. and ŽA2., the bisulphate ion concentration was calculated from: HSO4y s w H 2 SO4 x free y 0.6 Ž w Ni x total q w Co x total q w Mg x total q w Mn x total . y w Cr x total

Ž A3. Then a charge balance was applied to evaluate the hydrogen ion concentration:

w Hq x s HSO4y q w Crx total

Ž A4.

Before the calculated concentration of hydrogen and bisulphate ions can be used to predict the metal solubilities, they should be corrected for the value of the solution density. The density of the solution at autoclave temperatures is considerably lower than at ambient conditions. A widely used equation relating the solution density to the electrolyte molar concen-

Table 2 Coefficients c i , j for the calculation of the solution density Electrolyte

c1,1

c1,2

c1,3

c2,1

c 2,2

c 2,3

H 2 SO4 Al 2 ŽSO4 . 3 NiSO4 MgSO4 MnSO4

70.60 369.9 177.2 143.7 155.1

y0.2367 y0.6816 y0.2278 y0.6531 y0.1258

1.676 = 10y3 5.346 = 10y3 0.2337 = 10y3 5.263 = 10y3 y0.5341 = 10y3

4.903 55.22 0 23.28 0

y0.05698 y0.3907 0 y0.3911 0

3.985 = 10y4 3.300 = 10y3 0 2.783 = 10y3 0

D.H. RubisoÕ, V.G. Papangelakisr Hydrometallurgy 58 (2000) 13–26

tration for a solution of a single electrolyte is as follows w25x:

w3x

r s r w q k 1C y k 2 C 3r2 k i s c i ,1 q c i ,2 TC q c i ,3 TC2

Ž A5.

w4x

Here, r w and C are the water density and the molar concentration of the electrolyte at temperature TC . For a complex solution involving n electrolytes with concentrations C j Ž j s 1, . . . ,n., Eq. ŽA5. can be generalized w25x:

w5x

n

r s rw q

i s 1,2

3r2

n

ž

Ý k 1, j C j y Ý js1

k 22r3 , j Cj

js1

/

Ž A6.

w6x

The following extrapolation formula for the water density was used w10x:

w7x

r w s 15.81747 q 9.87802T y 0.035239T 2

w8x

q 5.38051 P 10y5 T 3 y 3.2612 P 10y8 T 4 Ž A7 . where T is expressed in K, and r w is in kgrm3. The values of the coefficients c i , j ŽTC in Eq. ŽA5. is in 8C. are given in Table 2. It should be noted that those coefficients were evaluated processing experimental results from 0 to 1008C w25x and thus their application to the autoclave temperatures Ž230– 2708C. is suspect. They can be used as estimation though, until more reliable data become available. When the solution density is evaluated from ŽEqs. ŽA5. Eqs. ŽA6. Eqs. ŽA5., the concentrations of hydrogen and bisulphate ions at temperature are calculated as follows: rT w Hq x T s w Hq x 258C r 258C HSO4y T s HSO4y

rT 258C

r 258C

Ž A8.

w9x

w10x

w11x

w12x

w13x

w14x

w15x

w16x

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