Effect of pre-roasting on leaching of laterite

Hydrometallurgy 99 (2009) 84–88

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Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t

Effect of pre-roasting on leaching of laterite Jinhui Li a,b, Xinhai Li a,⁎, Qiyang Hu a, Zhixing Wang a, Youyuan Zhou a, Junchao Zheng a, Wanrong Liu a, Lingjun Li a a b

School of Metallurgy Science and Engineering, Central South University, Changsha, Hunan Province, 410083, PR China School of Materials and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou, Jiangxi Province, 341000, PR China

a r t i c l e

i n f o

Article history: Received 23 November 2008 Received in revised form 8 June 2009 Accepted 4 July 2009 Available online 5 August 2009 Keywords: Gamierite laterite Pre-roasting Leaching Hydrochloride acid Phase transformation

a b s t r a c t The effect of pre-roasting on leaching of the gamierite laterite ore, obtained from Yunnan province, China, was investigated in this study. The phase transformation of laterite minerals roasted at different temperatures was investigated with X-ray powder diffraction (XRD). The roasting experiment results show that there are two phase transformation processes of dehydroxylation of goethite and lizardite at roasting temperatures of 277 °C and 610 °C, respectively, which accord with the result of DTA–TG analysis. Preroasting of the laterite not only alters its mineralogical composition but also increases its porosity and surface area, thus making it more amenable to leaching. Compared to the leaching result of raw ore and ores roasted at different temperatures, it indicates that increasing roasting temperature up to 300 °C appears to provide the optimum nickel recovery and further heating appears to be detrimental to the nickel recovery. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Nickel ranks 24th in abundance while elements such as iron, silicon, oxygen and magnesium account for over 90% of the earth's composition. Nickel occurs within both laterite and sulfide ores and it is also associated with deep-sea nodules (Moskalyk and Alfantazi, 2002). Laterite ores consist of the majority (80%) of the world's nickel reserves. Although sulfides are still the primary source of nickel and cobalt, increasing underground costs and decreasing the grades of the sulfide ores attract more attention towards the exploitation of laterite ores (Dalvi et al., 2004; Pickles, 2004). However, only 42% of the world's nickel production comes from nickel laterites (Gleeson et al., 2003; Sudol, 2005). One of the major economic factors, which limit the development of the laterite ores, is that the energy costs for processing these ores are 2–3 times higher than that for the sulfide ores (Thomas, 1995). Therefore, in the long term it will be necessary to develop new processing techniques, which are both technically and economically viable. Roasting with addition of reductant such as sulfur, carbon, and other reducing gas is a popular method in dealing with laterite (Valix and Cheung, 2002; O'Connor et al., 2006) since the standard reductive roast/ammoniacal leach process initially defined by Caron (1924) has proven satisfactory in treating most low-grade nickel laterite oxide

⁎ Corresponding author. Present address: School of Metallurgy Science and Engineering, Central South University, Changsha, Hunan Province, 410083, PR China. Tel.: +86 731 8836633. E-mail address: [email protected] (X. Li). 0304-386X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2009.07.006

ores, and pre-roasting has a significant influence on the following reduction process (Valix and Cheung, 2002; Swamy et al., 2003; O'Connor et al., 2006). Pre-roasting can change the mineral structure (Ruan et al., 2002), and Valix and Cheung (2002) even proved that pre-roasting can lead to the dehydroxylation of the serpentine or magnesium hydrosilicate phase and form an amorphous magnesium silicate phase. And dehydroxylation of goethite due to heating causes a topotactic transformation to hematite which occurs via minor modification in the goethite structure (Landers and Gilkes, 2007). Moreover, due to the removal of free and combined moisture and collapse of partial phase structure, roasting can alter its mineralogical composition and increase the surface and porosity of the raw ore, thus making it more amenable to leaching (Olanipekun, 2000). Although the importance of mineral dehydroxylation and phase transformation in metal leachability of reduced laterite has been suggested by the studies of De Graff (1979, 1980), Kukura et al. (1979), Valix and Cheung (2002) and O'Connor et al. (2006) in the reduction of silicate laterite minerals, the mechanism of pre-roasting in different metals leachability and selectivity of gamierite laterite ores with hydrochloride acid as leachant is seldom studied in literatures. In this present study, an attempt has been made to identify the mechanism of pre-roasting on leaching of laterite in hydrochloride acid solution and efforts have been likewise made to extract nickel selectively over iron by a roast–leach process. Although leaching with various acids was studied by McDonald and Whittington (2008a,b), and due to mainly economical reasons sulfuric acid was preferred. In this leaching process, a particular advantage is that hydrochloric acid as a leachant allows comparatively easier recovery of the useful free acid from its waste solution than does sulfuric acid. Moreover, separation of metal

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chlorides through solvent extraction (SX) is much easier than from sulfate medium (Olanipekun, 2000; Gibson and Rice, 1997), and the highly corrosive nature of the hydrochloric acid was overcome by using aceramic-lined continuous digester (McDonald and Whittington, 2008b). In this study, some determination methods are carried out to study the mechanism, such as TG (thermo gravimetric), XRD (X-ray diffraction), AAS (Atomic Absorption Spectrometry) and SEM (scanning electron microscope). Results of this research may assist in the development of more efficient procedures for processing oxide-type lateritic nickel ores. 2. Experimental 2.1. Materials preparation and characterization The ores tested in this study originated from Yunnan province in China, where the laterite mainly consisted of three layers in order of increasing depth: a hematite cap, a limonitic laterite deposit and finally garnieritic ore. After drying in vacuum overnight at 105 °C, the ores were ground to 100% passing 100 mesh. The typical chemical analysis of these ores is shown in Table 1. The main minerals, which are evident from the X-ray diffraction pattern shown in Fig. 1, are lizardite (Mg3Si2(OH)4O5, referred to the JCPDF file No. 86-0404), goethite (FeO(OH), referred to the JCPDF file No. 81-0462), hematite (Fe2O3, referred to the JCPDF file No. 89-0597) and quartz (SiO2, referred to the JCPDF file No. 83-0539). With considering chemical composition of raw ore shown in Table 1, the ores tested are mainly garnieritic laterite ore. 2.2. Roasting and leaching Roasting was conducted in a fixed tube furnace. The temperature of the ores during roasting was monitored using a sheathed thermocouple inserted into the bed. The air was metered into the reactor using rotameters before the temperature began to increase. After roasting was completed, the Ar gas was metered into the reactor continuously until the ores cooled down. The cooled roasted ore was then transferred into a leaching pod containing 4 mol/l hydrochloride acid solution which had been heated to the desired temperature and the timing was initiated. The stirring speed (300 rpm) which gave sufficient mixing was kept constant in all the experiments. The leaching experiments were performed in a 250-ml round-bottom flask with 3 holes. The reaction temperature was maintained constant with a thermostatically controlled water bath, equipped with a digitally controlled thermometer (within ± 0.5 °C). For minimizing aqueous loss when the system was heated, a reflux condenser was mounted on top of the cell. And the effect of leaching time on nickel extraction was also investigated, the result was calculated in formula (1).

V− XM;i =

iP −1 i=1

! vi CM;i +

iP −1 i=1

vi CM;i

mðcM = 100Þ

:

ð1Þ

X is the metal dissolution, M represents metal, where V is the initial volume (ml) of the solution, νi is the volume (ml) of sample i withdrawn each time, CM,i is the concentration of M (Ni, Co and etc.) in sample i (mg l− 1), m is the initial mass of laterite in g (on dried basis) added into the reactor and cM is the concentration of M in laterite (wt.% dried solids). Table 1 Chemical composition of raw ore. Constituent

NiO

CoO

MnO

Fe2O3

CuO

MgO

Al2O3

SiO2

Content (wt.%)

0.60

0.042

0.174

15.01

0.007

26.4

0.346

57.43

Fig. 1. Powder XRD pattern of raw laterite.

2.3. Analysis For all the experiments, analysis of Ni, Mn and Co employing standard procedures was carried out by atomic absorption techniques type of Ruili-wfx120. Volumetric analysis (K2Cr2O7 titration) was employed for Fe. The analysis of the final solid residue provided a confirmation of the overall metal recovery after leaching. The solid residue was washed with distilled water and dried in an oven overnight at 105 °C. The dried residue was digested in aqua regia solution (1:3 HNO3/HCl) for 2 h (O'Connor et al., 2006). The digest solutions were also analysed for dissolved metals using atomic absorption spectroscopy and volumetric analysis. A computerized X-ray diffraction equipment (XRD, Rint-2000, Rigaku) using CuKa radiation was used to identify components of raw ore and roasted ores. The specific surface area of the ores was determined through BET nitrogen–helium absorption method. 3. Results and discussion 3.1. Mineralogical analysis The DTA–TG curve of the raw ore is shown in Fig. 2. The effect of roasting on mineral transformation of laterite ores under atmospheric pressure at different temperatures is shown in Fig. 3. DTA–TG analysis shows two endothermic peaks at about 277 °C and 607 °C due to the release of adsorbed water, dehydroxylation of goethite and lizardite, respectively (Tartaj et al., 2000). The weight loss corresponding to these two processes are 4.341% and 7.618%, respectively. In reference to Fig. 3, there are some reactions and structure changes that occurred. At 277 °C, goethite transforms to hematite as shown in Eq. (2) (Valix and Cheung, 2002), which can be observed from the difference between Fig. 3(a) and (b). This decomposition temperature is low compared to the decomposition temperature of 385 °C (Schwertmann et al., 1982) and 337 °C (Swamy et al., 2003) often associated with highly crystalline goethite. The lower decomposition temperature of laterite in this study is indicative of poorly crystalline goethite structure. At roasting temperature of 607 °C, lizardite (Mg3Si2(OH)4O5) discomposes and forms an amorphous magnesium silicate phase. This is evident by the formation of broad peaks and eventual loss of X-ray diffraction signals for the lizardite minerals at 2θ angle of 12.1°, 18.64°, 24.38° and 60.06°. It appears that the lizardite dehydroxylation is

86

J. Li et al. / Hydrometallurgy 99 (2009) 84–88

Fig. 2. TG–DTA curves of laterite.

completed at temperature about 607.5 °C which can be discovered from Figs. 2 and 3(d) and (e), at higher temperature, above 610 °C, the amorphous magnesium silicate phase appears to have recrystallised as forsterite (Mg2SiO4) and enstatite (MgSiO3) shown in Fig. 3(e) and (f). Similarly, the weight loss is 7.618% due to the loss of water which can be explained in Eq. (3). On heating, DTA–TG displays an additional exothermic peak at 807 °C associated with forsterite (Mg2SiO4) recrystallisation (Wei et al., 2008). 2FeO·OH→Fe2 O3 þ H2 O↑

ð2Þ

Mg3 Si2 ðOHÞ4 O5 →Mg2 SiO4 þ MgSiO3 þ 2H2 O↑

ð3Þ

3.2. Leaching results The cooled roasted ores were then transferred into a leaching pod containing 4 mol/l hydrochloride acid solution, at fixed leaching conditions: Solid/liquid ratio = 1:6, temperature = 50 °C, leaching time = 1 h. In contrast, the raw ore was also tested in the same

Fig. 3. Powder XRD pattern of minerals induced by heat at atmospheric pressure (a: raw ore, b: roasted at 300 °C, c: roasted at 400 °C, d: roasted at 500 °C, e: roasted at 610 °C, f: roasted at 700 °C, g: roasted at 800 °C).

leaching conditions. In order to investigate the effect of leaching time on nickel extraction, a series of leaching experiments were performed with the leaching time varied from 5 to 60 min under the same experimental conditions too. The leaching results were shown in Table 2. It can be seen that the nickel dissolution can attain the maximum value with the ore roasted at 300 °C. It indicates that Ni is uniformly incorporated in the goethite structure which comprises the dominant Ni host mineralogy (Senanayake, 2007; Whittington and Muir, 2000; Brand et al., 1998). And nickel bound within goethite grains usually requires complete dissolution of the grain to achieve high nickel extraction (Senanayake and Das, 2004). Therefore, goethite transforms into hematite roasted at about 277 °C leading that the structure of original mineral is changed or destroyed, which is shown in Figs. 2 and 3, this results in the release of nickel or more nickel is exposed to the reaction interface which is more susceptible to attack. Due to the phase transformation, the specific surface area of ore roasted at 300 °C increases to 21.04 m2/g compared to 16.03 m2/g of raw ore shown in Table 3. And it also can be found through Fig. 4(a) and (b) that there are more fine pores and scraps on the surface of ore roasted at 300 °C than that of raw ore surface. Chander and Sharma (1981), studies of the characteristics of the limonite minerals with temperature, suggested that increasing the temperature could reduce the porosity of the ore, so the specific surface area of ores roasted at 400 °C and 500 °C decreases to 19.21 and 19.13 m2/g slightly. Due to the inclusion of nickel in the recrystallised iron oxide (O'Connor et al., 2006), the nickel recovery decreases slightly. When roasting temperature is 610 °C, the main mineral lizardite (Mg3Si2(OH)4O5) begins to discompose, leading to mineral structure destroyed, the specific surface area attains the maximum 26.45 m2/g. And more fine pores and scraps can be found on the surface from Fig. 4(c) compared to that of Fig. 4(a) and (b). But the unexpected result is that nickel recovery decreases to 77%. On the one hand, it is postulated by O'Connor et al. (2006) that the lower nickel recovery is due to the inclusion of nickel in the recrystallised iron oxide and the leaching conditions are not sufficient. On the other hand, it may be due to nickel loss partly in the leach solution (Wei et al., 2008). With leaching of the ore roasted at 610 °C, plenty of silicic acid particles characterized by a very large surface area are dispersed in the leaching solution and Wei et al. (2008) ever confirmed the formation of amorphous silica from XRD pattern and FTIR spectrum of the leach residues. On the surface are highly reactive groups, silicon, oxygen and hydrogen (Si–O–H). This large surface area and surface energy means that colloidal silicic acid is an excellent adsorbent which takes up ions or molecules of gas or liquid (Reig et al., 2002). Whittington et al. (2003) and Whittington and Johnson (2005) examined nickel losses in the leach residue by pressure acid leaching of arid-region laterite ore and found that cations like Ni2+ were adsorbed on amorphous silica, which had a strongly negative surface charge, even in the presence of Mg2+ cations. It can be seen through Fig. 5 that the nickel extraction of leaching ore roasted at 610 °C can reach about 83% after leaching for 20 min and then somewhat decrease which suggests that the nickel loss is due to the nickel absorption by amorphous silica in the solution. Literature on the acid extraction of saprolitic ores under AL conditions implies that serpentine minerals dissolve by partial decomposition of the silicate structure (known as incongruent dissolution), in which metal cations are leached without appreciable release of silica (Kosuge et al., 1995; Lin and Clemency, 1981). This is why there is no nickel loss by leaching ores roasted at different temperatures (300 °C, 400 °C and 500 °C). As roasting temperature increases to 700 °C and even 800 °C, the specific surface area decreases to 18.61 m2/g and 13.23 m2/g, respectively, it may be due to some fine particles reunite together (Chander and Sharma, 1981), which is shown in Table 3. At the same time, the nickel recovery decreases to 31.25% and 31.1%. It suggests that further heating appears to be detrimental to the nickel recovery.

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Table 2 Effect of roasting temperature on leaching results after 60 min. Sample Raw ore 300 °C 400 °C 500 °C 610 °C 700 °C 800 °C

% Ni

% Co

% Mn

% Fe

Solution

Residue

Solution

Residue

Solution

Residue

Solution

Residue

33 93.1 92.7 90 77 31.25 31.1

67.1 7.8 7.5 10 24.08 68.8 70.2

60.4 61.6 61.54 61.6 60.92 59.23 59.3

40.3 40.1 39.8 39.2 40 40.1 40

99.1 98.9 98.24 98.4 98.3 98.5 95

0.82 0.97 2.1 2.43 1.76 1.46 4.45

78 65 32.5 30 26 29 20

22 34.9 67.35 71.3 75.7 71.6 80.56

Table 3 Specific surface area of raw ore and ores roasted at different temperatures. Raw ore 300 °C 400 °C 500 °C 610 °C 700 °C 800 °C Specific surface area (m2/g) 16.03

21.04

19.21

19.13

26.45

18.61

13.23

Kukura et al. (1979) and Hayashi (1971) inferred that nickel became unstable after dehydroxylation and it was at this stage that the fate of nickel was decided. The experiment results show that heating may lead to the incorporation of nickel into the magnesium silicate phase (Mg,Ni)3SiO2 followed by recrystallisation into the forsterite phase shown in Fig. 3(f) and (g), which renders it inert and hinders further release of nickel trapped in this phase (Valix and Cheung, 2002; O'Connor et al., 2006). The leaching of cobalt and manganese is not influenced markedly by roasting temperature shown in Table 2. This implies that the structure which comprises the dominant Co and Mn host mineralogy in the ore is not changed or destroyed significantly by roasting. As for Fe, its extraction shows a decreasing trend with the increase of

roasting temperature, it indicates highly crystalline hematite transformed from goethite shown at 2θ angle of 33° with heat increasing in Fig. 3. Especially when roasting temperature is over 610 °C while lizardite begins to dehydroxylate, it will probably lead to the incorporation of iron into the magnesium silicate phase (Mg,Fe)3SiO2 followed by recrystallisation into the forsterite phase which hinders further release of iron trapped in this phase, which is similar as nickel dissolution behavior. Because pre-roasting opens the main gangue mineral structure and goethite, thus allows a rapid interaction between the leachant and the nickel species during leaching. As is shown in Fig. 5, the nickel leaching rate of ore roasted at 610 °C is the maximum. The order of leaching rate can be well in accord with the order of specific surface area. 4. Conclusions The results suggest that roasting temperature can have a significant effect on the nickel metal recovery because it mainly exists in goethite phase. Increasing the temperature up to 300 °C for gamierite laterites which originated from China appears to provide the optimum nickel recovery, and further heating is detrimental to the

Fig. 4. SEM image of laterite (a: raw ore, b: roasted at 300 °C, c: roasted at 610 °C, d: roasted at 800 °C).

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Fig. 5. Effect of leaching time on Ni extraction (a: raw ore, b: roasted at 300 °C, c: roasted at 400 °C, d: roasted at 500 °C, e: roasted at 610 °C, f: roasted at 700 °C, g: roasted at 800 °C).

nickel recovery. At the same time, the leaching of iron is suppressed with increase of temperature. On this basis it may be economically and/or technologically appropriate to leach at lower temperature for a limited time with lower acid concentration after pre-roasting than to leach directly at higher temperature with higher acid concentration for a longer time. Acknowledgements The project was sponsored by the National Basic Research Program of China (973 Program, 2007CB613607) and the National Natural Science Foundation of China (50864004). References Brand, N.W., Butt, C.R.M., Elias, M., 1998. Nickel laterites: classification and features. AGSO J. Aust. Geol. Geophys. 17, 81–88. Caron, M.H., 1924. US Patent 1 487 145. Chander, S., Sharma, V.N., 1981. Reduction roasting/ammonia leaching of nickeliferous laterites. Hydrometallurgy 7, 315–327. Dalvi, A.D., Bacon, W.G., Osborne, R.C., 2004. Past and Future of Nickel Laterite Projects [C]// International Laterite Nickel Symposium. Charlotte, North Carolina, USA, pp. 23–27. De Graff, J.E., 1979. The treatment of lateritic nickel ores—a further study of the Caron process and other possible improvements: Part 1. Effect of reduction conditions. Hydrometallurgy 5, 47–65. De Graff, J.E., 1980. The treatment of lateritic ores—a further study of the Caron process and other possible improvements: Part II. Leaching studies. Hydrometallurgy 5, 255–271.

Gibson, R.W., Rice, N.M., 1997. A hydrochloric acid process for nickeliferous laterites. In: Cooper, W.C., Mihaylov, I. (Eds.), Hydrometallurgy and Refining of Nickel and Cobalt, vol. 1. Canadian Institute of Mining and Metallurgy, Montreal, QC, pp. 247–261. Gleeson, S.A., Butt, C.R.M., Elias, M., 2003. Nickel laterites: a review. SEG Newsl. 54, 9–16. Hayashi, M., 1971. Effect of phase transition of reductive roasting of nickel bearing serpentine. MA Thesis, University of Utah. Kosuge, K., Shimada, K., Tsunashima, A., 1995. Micropore formation by acid treatment of antigorite. Chem. Mater. 7 (12), 2241–2246. Kukura, M.E., Stevens, L.G., Auck, Y.T., 1979. Development of UOP process for oxide silicate ores of nickel and cobalt. In: Evans, D.J.I., Shoemaker, R.S., Veltman, H. (Eds.), International Laterite Symposium. SME-AIME, New York, USA, pp. 527–552. Landers, M., Gilkes, R.J., 2007. Dehydroxylation and dissolution of nickeliferous goethite in New Caledonian lateritic Ni ore. Appl. Clay Sci. 35, 162–172. Lin, F.C., Clemency, C.V., 1981. The dissolution kinetics of brucite, antigorite, talc, and phlogopite at room temperature and pressure. Am. Mineral. 66 (7–8), 801–806. McDonald, R.G., Whittington, B.I., 2008a. Atmospheric acid leaching of nickel laterites review. Part I. Sulphuric acid technologies. Hydrometallurgy 91 (1–4), 35–55. McDonald, R.G., Whittington, B.I., 2008b. Atmospheric acid leaching of nickel laterites review. Part II. Chloride and bio-technologies. Hydrometallurgy 91 (1–4), 56–69. Moskalyk, R.R., Alfantazi, A.M., 2002. Nickel laterite processing and electrowinning practice. Miner. Eng. 15, 593–605. O'Connor, F., Cheung, W.H., Valix, M., 2006. Reduction roasting of limonite ores: effect of dehydroxylation. Int. J. Miner. Process. 80, 88–99. Olanipekun, E.O., 2000. Kinetics of leaching laterite. Int. J. Miner. Process. 60, 219–221. Pickles, C.A., 2004. Microwave heating behaviour of nickeliferous limonitic laterite ores. Miner. Eng. 17, 775–784. Reig, F.B., Adelantado, J.V.G., Moya Moreno, M.C.M., 2002. FTIR quantitative analysis of calcium carbonate (calcite) and silica (quartz) mixtures using the constant ratio method. Application to geological samples. Talanta 58 (4), 811–821. Ruan, H.D., Frost, R.L., Kloprogge, J.T., Duong, L., 2002. Infrared spectroscopy of goethite dehydroxylation. II. Effect of aluminium substitution on the behaviour of hydroxyl units. Spectrochim. Acta, Part A 58, 479–491. Schwertmann, V., Schulze, D.G., Murad, E., 1982. Identification of ferrihydrite in solids by dissolution kinetics, differential X-ray diffraction and Mossbauer spectroscopy. Soil Sci. Soc. Am. J. 46 (4), 869–897. Senanayake, G., 2007. Review of theory and practice of measuring proton activity and pH in concentrated chloride solutions and application to oxide leaching. Miner. Eng. 20 (7), 634–645. Senanayake, G., Das, G.K., 2004. A comparative study of leaching kinetics of limonitic laterite and synthetic iron oxides in sulfuric acid containing sulfur dioxide. Hydrometallurgy 72 (1–2), 59–72. Sudol, S., 2005. The thunder from down under. Everything you wanted to know about nickel laterites but were afraid to ask. Can. Min. J. 126 (5), 8–12. Swamy, Y.V., Kar, B.B., Mohanty, J.K., 2003. Physico-chemical characterization and sulphatization roasting of low grade nickeliferous laterites. Hydrometallurgy 69, 89–98. Tartaj, P., Cerpa, A., Garcia-Gonzalez, M.T., Serna, C.J., 2000. Surface instability of serpentine in aqueous suspensions. J. Colloid Interface Sci. 231 (1), 176–181. Thomas, F.T., 1995. Comparative costs of nickel sulphides and laterites. Res. Policy 21 (3), 179–187. Valix, M., Cheung, W.H., 2002. Study of phase transformation of laterite ores at high temperature. Miner. Eng. 15, 607–612. Wei, L., Qiming, F., Leming, O., Guofan, Z., Yiping, L., 2008. Fast dissolution of nickel from a lizardite-rich saprolitic laterite by sulphuric acid at atmospheric pressure. Hydrometallurgy 96 (1–2), 171–175. Whittington, B.I., Johnson, J.A., 2005. Pressure acid leaching of arid-region nickel laterite ore. Part III: Effect of process water on nickel losses in the residue. Hydrometallurgy 78 (3–4), 256–263. Whittington, B.I., Muir, D., 2000. Pressure acid leaching of nickel laterites: a review. Miner. Process. Extr. Metall. Rev. 21, 527–600. Whittington, B.I., McDonald, R.G., Johnson, J.A., Muir, D.M., 2003. Pressure acid leaching of arid-region nickel laterite ore — Part I: effect of water quality. Hydrometallurgy 70 (1–3), 31–46.