Comparison of atmospheric citric acid leaching kinetics of nickel from different Indonesian saprolitic ores

HYDROM-04252; No of Pages 14 Hydrometallurgy xxx (2016) xxx–xxx

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Comparison of atmospheric citric acid leaching kinetics of nickel from different Indonesian saprolitic ores Widi Astuti a,b,⁎, Tsuyoshi Hirajima a, Keiko Sasaki a, Naoko Okibe a a b

Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan Mineral Processing Division, Indonesian Institute of Sciences (LIPI), Indonesia

a r t i c l e

i n f o

Article history: Received 15 April 2015 Received in revised form 17 December 2015 Accepted 27 December 2015 Available online xxxx Keywords: Citric acid Saprolitic ores Atmospheric leaching Nickel Kinetics

a b s t r a c t Saprolitic ores from Sulawesi Island (SS ore) contain serpentine and goethite as major minerals, whereas the main minerals in saprolitic ore from Halmahera Island (SH ore) are talc and goethite. Most of the nickel was incorporated in a magnesium–silica-containing mineral. The effects on nickel extraction of leaching temperature, citric acid concentration, and ore particle size were determined to investigate the leaching performances and leaching kinetics of the two saprolitic ores. Nickel leaching efficiency from SS ore is always higher than that from SH ore under the same leaching conditions. The mineral contents of the ores significantly affected the leaching performances and mineral dissolution behaviors of the samples. The results of nickel leaching efficiency and analysis of the solid residues suggest that all dissolved nickel originated from serpentine, which is more easily leached than goethite and talc. The rate of nickel extraction for SS ore was faster than that for SH ore. Nickel leaching from SS ore and SH ore followed the shrinking-core model and was controlled by diffusion of a reactant or product through the solid product layer. © 2016 Elsevier B.V. All rights reserved.

1. Introduction In the future, hydrometallurgical methods or aqueous treatments will become the primary techniques for the recovery of nickel and other metals from nickel laterite ores, especially low-grade laterite ores, because they enable valuable metals such as nickel, cobalt, iron, magnesium, chromium, and aluminum to be extracted comprehensively (Watling et al., 2011; Liu et al., 2010; Fan and Gerson, 2013). One hydrometallurgical method that has not yet been used industrially, although its study is becoming increasingly important, is atmospheric acid leaching. Some researchers have investigated the atmospheric acid leaching of nickel laterite using inorganic acids such as sulfuric acid, hydrochloric acid, and nitric acid, and a range of organic acids (MacCarthy et al., 2014; Nosrati et al., 2014; Rice and Strong, 1974a, 1974b; Wang et al., 2012; Wang et al., 2014). The use of organic acids in dissolution of the metals in nickel laterite ores is an alternative method that has advantages in terms of environmental issues. Many researchers have confirmed that citric acid is the most effective organic acid in the leaching of nickel laterites (McDonald and Whittington, 2008b). The widespread presence of citric acid in the animal and plant kingdoms is an assurance of its nontoxic nature. Today, citric acid is industrially produced by fermentation, and

⁎ Corresponding author. E-mail address: [email protected] (W. Astuti).

the filamentous fungus Aspergillus niger is exclusively used due to its high citric acid productivity at low pH without the secretion of toxic byproducts. Glucose and sucrose are usually the main carbon sources in citric acid production. However, a variety of agricultural products and wastes are suited to the production of citric acid by A. niger species including pineapple peels, sugarcane molasses, corn starch, corn cob, corn husk, coffee husk, banana peel, oil palm empty fruit bunch, and tapioca starch. (Kirimura et al., 2011). The use of citric acid in the nickel leaching, therefore, gives some advantages particularly the environmental issues such as providing the leaching reagent that is non-toxic and biodegradable as well as addressing the agricultural waste problems. Moreover, citric acid leaching has high effectiveness and selectivity on the nickel leaching (Mubarok et al., 2011). Therefore, citric acid leaching will be a suitable alternative technique for nickel extraction from Indonesian lateritic ores; it has low capital and operating costs because of the biodiversity and abundant carbon sources in Indonesia. Citric acid contains three carboxyl groups with dissociation constants for the three protons expressed as pKa1 = 3.13, pKa2 = 4.76 and pKa3 = 6.40, respectively, at 25 °C. When this acid is fully dissociated, complexation of a nickel cation with a citrate anion can be expressed as (Behera et al., 2011) C6 H8 O7 ↔ðC6 H5 O7 Þ3− þ 3Hþ

ð1Þ

Ni2þ þ ðC6 H5 O7 Þ3− ↔ðNi C6 H5 O7 Þ− :

ð2Þ

http://dx.doi.org/10.1016/j.hydromet.2015.12.015 0304-386X/© 2016 Elsevier B.V. All rights reserved.

Please cite this article as: Astuti, W., et al., Comparison of atmospheric citric acid leaching kinetics of nickel from different Indonesian saprolitic ores, Hydrometallurgy (2016), http://dx.doi.org/10.1016/j.hydromet.2015.12.015

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Table 1 List of leaching parameters studied. Experiments

Studied parameters

Fix parameters

Leaching temperature (°C) Citric acid concentration (M) Ore particle size (μm)

30, 40, 60 0.1, 0.5, 1 b75, 75–150, 212–355, 355–850

1 M, 20% (w/v), b75 μm 40 °C, 20% (w/v), b75 μm 1 M, 40 °C, 20% (w/v)

There are two possible mechanisms for metal dissolution by citric acid: (1) attack of acid and metal ion displacement by hydronium ions, and (2) forming soluble metal–ligand complexes by metal chelation (McDonald and Whittington, 2008). Indonesia is one of the largest producers of nickel ore in the world, and has approximately 16% of total global nickel resources in the form of lateritic ores. They are widely distributed and spread among the islands of Kalimantan, Sulawesi, Halmahera, Gag, and Papua (Supriyadi, 2008). The properties of lateritic ores differ, depending on the sample origins (Liu et al., 2010; Watling et al., 2011); this is also the case for all Indonesian saprolitic ores. The chemical and mineral compositions of laterites from different origins or even from different depths in one area can vary significantly. Therefore different extraction methods should be used. Although some nickel lateritic ores have similar mineralogical compositions, they show radically different atmospheric acid leaching performances (Watling et al., 2011). Leaching rates and products will depend not only on the leaching conditions but also on the mineralogy of ores (Senanayake et al., 2011). Specific mineralogical research should therefore always be conducted when a new laterite ore sample is found (Wang et al., 2012). Indonesian nickel laterite ores have been the subject of a number of previous mineralogical studies (Fan and Gerson, 2013; Wang et al., 2012). Considerable effort has been devoted to the study of atmospheric and pressure leaching of laterites, but little emphasis has been placed on understanding the detailed nickel mineralogy on the basis of leaching (Liu et al., 2009), especially citric acid leaching at atmospheric pressure. A general lack of knowledge regarding the mineralogy and chemistry of the leached products also hampers understanding of the leaching behavior and kinetics of the other mineral components present in ores (Fan and Gerson, 2013). Saprolitic ores are silicate laterite type ores, formed after a long period of aggressive weathering and located deeper in the laterite profile than limonitic laterites. Saprolitic ores are usually magnesium rich and the majority of the goethite is replaced by serpentine, with the iron and nickel substituting for magnesium. Typically, most nickel is associated within the saprolitic ore by substituting for magnesium in serpentine minerals. Although goethite is the major component of limonitic ores, some saprolitic ores also contain goethite as a minor component and nickel is sometimes also incorporated into goethite. The aim of this study was to obtain a better understanding of the comparative leaching performances, mineral dissolution behavior, and leaching kinetics of different Indonesian saprolitic ores from two different mining areas, under atmospheric leaching conditions using citric acid. This is important in the design of alternative techniques for

Fig. 1. XRD pattern of raw SS ore.

treating such ores. The findings of the study can be adopted for leaching of other lateritic ores from different locations.

2. Material and methods 2.1. Materials Saprolitic ores from two different mining areas in Indonesia (i.e., Sulawesi Island and Halmahera Island) were used. The samples were mineralogically and chemically characterized. X-ray fluorescence (XRF; ZSX Primus II, Rigaku, Tokyo, Japan) and inductively coupled plasma-optical emission spectroscopy (ICP-OES; Perkin Elmer 8500, Waltham, MA, USA) were used to determine the chemical compositions of these samples. The mineral phases of the raw ore samples were identified by X-ray diffraction (XRD; Ultima IV, Rigaku, Tokyo, Japan), using CuKα radiation, in the 2θ range 5° to 80°. The accelerating voltage and applied current were 40 kV and 40 mA, respectively, with a scanning speed of 2°/min and a scanning step of 0.02°. Additional information on the mineral compositions of the ores was obtained using Fouriertransform infrared spectroscopy (FTIR; 670 Plus spectrophotometer, JASCO, Tokyo, Japan). Thermogravimetric-differential thermal analysis (TG-DTA; 2000SA, Bruker, Billerica, MA, USA) of the ore samples was performed from room temperature to 1250 °C at a heating rate of

Table 2 Chemical compositions of raw ore samples (determined using XRF and ICP-OES). Ore samples

SS ore SH ore

Oxide or element % (w/w) SiO2

Fe

Ni

Co

Mg

Mn

Cr

Al

36.30 34.00

21.64 22.77

1.76 1.28

0.06 0.05

8.44 11.18

0.43 0.34

1.07 1.18

2.04 1.31

Fig. 2. XRD pattern of raw SH ore.

Please cite this article as: Astuti, W., et al., Comparison of atmospheric citric acid leaching kinetics of nickel from different Indonesian saprolitic ores, Hydrometallurgy (2016), http://dx.doi.org/10.1016/j.hydromet.2015.12.015

W. Astuti et al. / Hydrometallurgy xxx (2016) xxx–xxx Table 3 Relative abundance of mineral in each ore. Mineral name

Serpentine Goethite Talc Chromite Quartz

Chemical formulae

Mg3Si2O5(OH)4 FeOOH Mg3Si4O10(OH)2 FeCr2O4 SiO2

3

Table 4 Frequencies and assignments of the bands in the IR spectra. Relative abundance in each ore SS ore

SH ore

Major Major – Minor Minor

Minor Major Major Minor Minor

Peak position (cm−1)

Assignment

Reference

3200 950–1030 780, 884 630–650 610–620

ν(OH) Si–O stretching δdeform–OH Si–O bending OH vibration

Prasada et al. (2006), Cambier (1986) Yariv and Heller-Kallai (1973) Prasada et al. (2006) Golightly and Arancibia (1979) Golightly and Arancibia (1979)

Table 5 Chemical analyses of different sieve fractions of raw ore samples.

10 °C/min to clarify the mineral contents of each sample. Scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS; VE-9800 SEM, Keyence, Osaka, Japan, and EDAX Genesis, Ametek, NJ, USA) was used to determine the nickel contents of the different minerals by metal mapping.

Ore samples

Size (μm)

SS ore

b75 75–150 212–355 355–850 b75 75–150 212–355 355–850

SH ore

2.2. Leaching experiments Chemical leaching experiments were performed using 300 mL flasks in a shaker at 200 rpm and atmospheric pressure. At the shaker rate of 200 rpm, the slurry of ores and leaching reagent has a good suspension. It was ascertained that the slurry has been mixed well. Analytical-grade citric acid (Wako Pure Chemicals Industries, Ltd., Tokyo, Japan) was used to prepare the solutions. The effects of leaching temperature, citric acid concentration, and ore particle size (Table 1) were evaluated to investigate the nickel-leaching behavior of each ore. Pulp density was kept constant at 20% (w/v). The extent of leaching was monitored by sampling the slurry periodically; the pH was measured using a pH meter (pH meter/ORP/EC, MM60R Type, DKK-TOA, Tokyo, Japan) and

Oxide or element % (w/w) SiO2

Fe

Ni

Co

Mg

Mn

Cr

Al

41.98 46.65 46.50 47.00 37.12 38.23 39.43 40.12

17.96 11.73 11.02 8.53 23.56 21.98 19.54 18.76

1.71 1.59 1.53 1.47 1.31 1.27 1.23 1.20

0.03 0.02 0.02 0.02 0.02 0.02 0.01 0.01

9.12 11.78 12.54 13.47 10.34 11.67 13.76 14.56

0.28 0.23 0.20 0.15 0.26 0.21 0.19 0.14

0.81 0.77 0.71 0.64 1.15 1.10 0.96 0.92

2.04 2.07 2.10 2.18 1.29 1.31 1.34 1.37

the dissolved metals were analyzed by ICP-OES using standard procedures. The XRD and FTIR analyses were performed on the leaching residues to determine the mineral phases after leaching and to study the mineral dissolution behaviors. Mineralogical analyses of the raw laterite and leach residues, as well as the dissolution of pure goethite using 1 M citric acid were performed in the current study to investigate the leachability of goethite in citric acid solution. Kinetic analyses were performed based on the experimental data for the dissolution of nickel from each saprolitic ore sample.

Fig. 3. FTIR spectra of raw ore samples.

Please cite this article as: Astuti, W., et al., Comparison of atmospheric citric acid leaching kinetics of nickel from different Indonesian saprolitic ores, Hydrometallurgy (2016), http://dx.doi.org/10.1016/j.hydromet.2015.12.015

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Fig. 4. TG-DTA patterns: (a) SS ore and (b) SH ore.

3. Results and discussion 3.1. Ore characteristics The chemical contents of the ores, determined using ICP-OES and XRF analyses (Table 2), showed that the raw ore samples were both typical low-grade saprolitic laterite with the nickel contents 1.76% (w/w) and 1.28% (w/w) for the saprolite samples from Sulawesi (SS ore) and Halmahera (SH ore), respectively. The XRD patterns of the saprolitic ores shown in Figs. 1 and 2 and minerals listed in Table 3 suggest that even though both samples have similar elemental assays the mineralogical composition differs. The SS ore has serpentine and goethite as major minerals, whereas the main minerals in SH ore are talc and goethite. The compounds on the laterite particle surfaces were investigated using FTIR spectroscopy; the spectra are shown in Fig. 3. The peak assignments are given in Table 4. These observations confirm that SS ore has a higher silicate mineral (serpentine) content that was indicated from peak at 984 cm−1 (Yariv and Heller-Kallai, 1973). On the other hand, SH ore contains more goethite that was observed from two IR bands at 780 and 884 cm−1 (Prasada et al., 2006) (Table 5). The TG-DTA technique was used to confirm the FTIR and XRD data. Results from DTA show the temperatures at which thermal reactions such as phase transformation and thermal decomposition take place in a material when it is heated continuously to high temperature, and also the intensity and general characteristics (endothermic or exothermic) of such reactions (Speyer, 1994). Results from TGA show the weight gain or loss of a material (e.g., a mineral) as a result of absorption or gas release as a function of temperature. The TG-DTA patterns of the samples (Fig. 4) show that SS ore and SH ore have different thermal decomposition and phase transformation behaviors. The TG-DTA trace from SS ore (see Fig. 4(a)) has four endothermic peaks, at 70, 260, 567, and 1165 °C, and an exothermic peak at 805 °C. The endothermic peaks at 70 and 260 °C are associated with moisture evaporation and goethite decomposition, respectively. The endothermic peak at 567 °C arises from dehydroxylation of serpentine, whereas the exothermic peak at 805 °C is associated with recrystallization of forsterite (Mg2SiO4); these observations confirm the presence of serpentine minerals (Brindley and De Souza, 1975; Fan and Gerson, 2011; Tartaj et al., 2000). The endothermic peak at 1165 °C is assigned to the

dehydroxylation of talc sheets, accompanied by the formation of enstatite (Mg2Si2O6) and silica. The TG-DTA pattern of SH ore (see Fig. 4(b)) shows that its thermal decomposition behavior differs to some extent from that of SS ore. For SH ore, there are only three endothermic peaks, at 50, 250, and 1150 °C. An exothermic peak is also observed, at 838 °C, but it is much smaller than the exothermic peak for SS ore. This observation indicates that the serpentine content of SH ore is lower than that of SS ore. The TG-DTA results support those obtained using XRD and FTIR. The XRD patterns (Figs. 1 and 2) show no independent peaks for nickel compounds. Information on the nickel-bearing minerals present is important in studying nickel-leaching behavior. The SEM/EDS analysis is a simple method for element mapping and the data can be used to predict the minerals associated with nickel and other metals. Figs. 5 and 6 show the metal mapping for each sample, obtained using SEM/ EDS analysis. The figures show that most of the nickel in both samples is closely associated with magnesium- and silica-containing minerals such as serpentine and talc. 3.2. Comparison of nickel-leaching behaviors of different saprolitic ores 3.2.1. Effect of leaching temperature The results (Fig. 7) show that for both samples, nickel leaching efficiency in the first 24 h increased with increasing leaching temperature The nickel leaching efficiency increased after leaching for 15 days when the leaching temperature was increased from 30 to 40 °C. However, increasing the temperature from 40 to 60 °C did not significantly increase the leaching efficiency. A lower leaching temperature is preferable, technically and economically, therefore a leaching temperature of 40 °C was used for subsequent leaching experiments. The current results also show that the rate of nickel leaching from SS ore was faster than that from SH ore, and the maximum nickel leaching rate achieved from SS ore was higher than that from SH ore at all leaching temperatures, under the same leaching conditions. 3.2.2. Effect of citric acid concentration The results shown in Fig. 8 indicate that the maximum nickel extraction was achieved when the citric acid concentration was 1 M. Nickel extraction increases with increasing citric acid concentration because

Please cite this article as: Astuti, W., et al., Comparison of atmospheric citric acid leaching kinetics of nickel from different Indonesian saprolitic ores, Hydrometallurgy (2016), http://dx.doi.org/10.1016/j.hydromet.2015.12.015

W. Astuti et al. / Hydrometallurgy xxx (2016) xxx–xxx

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Fig. 5. SEM/EDS metal mapping of SS ore.

of the increase in the hydrogen ion (H+) activity, which leads to dissolution of nickel-containing materials. In addition, the nickel leaching efficiency is plateaued at 0.1 M acid due to insufficient acid to leach nickel. Moreover, it was found that 1 M is the optimum citric acid concentration because the use of 2 M did not cause a significant increase of nickel leaching. For SS ore, around 79% of nickel can be leached after 15 days of leaching period when 1 M of citric acid was used. However, only around 81% of nickel can be leached using 2 M of citric acid. In terms of SH ore, the nickel leaching efficiency achieved after 15 days of leaching time using 1 M of citric acid was around 68%; whereas, 73% of nickel was dissolved by 2 M of citric acid. It was also found that solutions of citric acid concentration higher than 1 M were more viscous, so that the shaking of slurry was less effective. Therefore, it can be concluded that 1 M is the optimum citric acid concentration that can be used for leaching process. The nickel leaching efficiency from SS and SH ores showed similar trends, but the maximum leaching efficiency achieved from SS ore was higher than that from SH ore at all citric acid concentrations.

3.2.3. Effect of ore particle size The results summarized in Fig. 9 show that the maximum nickel leaching efficiencies for both samples were achieved after leaching for 15 days, with a particle size of 212–355 μm. The results also show that nickel leaching efficiency from SS ore was higher than that from SH ore under all leaching conditions. After leaching for 15 days, around 96% nickel was extracted from SS ore when 1 M citric acid was used, but only 73% nickel was leached from SH ore under the same leaching conditions. 3.3. Dissolution behavior of different metal ions Nickel is assumed to be incorporated in serpentine, goethite, and talc; therefore, nickel dissolution must always coincide with the dissolution of the other metals contained in these minerals, particularly magnesium, iron, and aluminum. The saprolitic ore samples used in this study contained both serpentine and goethite. Goethite particles are smaller than serpentine particles (Cerpa et al., 1999). The data in

Please cite this article as: Astuti, W., et al., Comparison of atmospheric citric acid leaching kinetics of nickel from different Indonesian saprolitic ores, Hydrometallurgy (2016), http://dx.doi.org/10.1016/j.hydromet.2015.12.015

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Fig. 6. SEM/EDS metal mapping of SH ore.

Fig. 7. Effect of leaching temperature for (a) SS ore and (b) SH ore ([citric acid]: 1 M, ore particle size: b75 μm, pulp density: 20% (w/v), shaker speed: 200 rpm).

Please cite this article as: Astuti, W., et al., Comparison of atmospheric citric acid leaching kinetics of nickel from different Indonesian saprolitic ores, Hydrometallurgy (2016), http://dx.doi.org/10.1016/j.hydromet.2015.12.015

W. Astuti et al. / Hydrometallurgy xxx (2016) xxx–xxx

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Fig. 8. Effect of acid concentration for (a) SS ore and (b) SH ore (temperature: 40 °C, ore particle size: b75 μm, pulp density: 20% (w/v), shaker speed: 200 rpm).

Table 2 show that the smaller ore particles had higher iron contents, and it can be predicted that this iron originated from goethite. However, goethite is very difficult to dissolve in atmospheric leaching even with acid solutions of high concentration of citric acid. Fig. 10 shows that iron leaching from pure goethite by 1 M citric acid solution was very low compared with those from SS and SH ores under the same leaching conditions. These results are consistent with those for the dissolution of natural goethite using HCl and HNO3 under atmospheric conditions reported by Abdus-Salam and Adekola (2006). The presence of H+ should contribute in breaking the hydroxyl groups of goethite leading to easy dissolution (Valix et al., 2001). However, goethite is stable at low temperature and dehydrates to hematite only at 130 °C (Ponsjak and Merwin, 1922). It can therefore be concluded that the higher goethite

content decreases the effectiveness of the leaching process if nickel is predominantly associated with goethite. This may be one of the reasons why the highest nickel recovery was achieved with a particle size of 212–355 μm for each sample. It can also be assumed that the serpentine content of the particles of size of 212–355 μm is higher than that of the smaller particles. The slowest nickel leaching was observed for particles of size 350–855 μm, i.e., the largest ore particles, because these particles have the smallest surface areas. Fig. 11 shows the dissolution of nickel and other metals in SS and SH ores under the optimum leaching conditions. It can be seen that the nickel leaching efficiencies from SS and SH ores are both much higher than those of magnesium and iron. Accordingly, it can be predicted that the nickel originates not only from magnesium-bearing minerals

Fig. 9. Effect of particle size for (a) SS ore and (b) SH ore (temperature: 40 °C, [citric acid]: 1 M, pulp density: 20% (w/v), shaker speed: 200 rpm).

Please cite this article as: Astuti, W., et al., Comparison of atmospheric citric acid leaching kinetics of nickel from different Indonesian saprolitic ores, Hydrometallurgy (2016), http://dx.doi.org/10.1016/j.hydromet.2015.12.015

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Fig. 10. Comparison of iron extraction from pure goethite, and SS and SH ores under same leaching conditions ([citric acid]: 1 M, temperature: 40 °C, pulp density: 20% (w/v), ore particle size: b75 μm).

such as serpentine and talc, but also from other minerals in the ore, e.g., goethite. However as the leachability of goethite in 1 M citric acid solution is very low (Fig. 10), iron and aluminum dissolved in the pregnant leaching solution may not originate from goethite. Some studies have shown that iron and aluminum can be associated with silicate minerals such as serpentine as well as talc (Apostolidis, 1978; Suarez et al., 2008; Yoo et al., 2009). The nickel in the silicate ores, which are present in the brucite layer of the clay minerals, is weakly bound and therefore easily dissolved (Valix et al., 2001). Although serpentine and talc are silicate minerals, they have different structures and physicochemical properties. In the serpentine structure, one octahedral layer is bound to one tetrahedral layer (Sufriadin et al., 2011). The octahedral sheet is mainly formed by Mg–O bonds and the tetrahedral sheet consists of Si–O bonds. Other cations such as Ni2+, Co2+, Al3+, and Fe3+ may replace Mg2+ in the octahedral sheets. Accordingly, to leach valuable metals (Ni, Co, Al, Fe, and Mg) from these structures, it is necessary to break the octahedral sheet without

disruption of the tetrahedral sheet. Acid attack on octahedral sheets may take place through both edge and gallery access mechanisms. In contrast, talc is a 2:1 layered silicate in which one octahedral sheet is sandwiched between two tetrahedral layers. Similar to serpentine, the octahedral sheet in talc consists of Mg–O bonds and the tetrahedral layer mainly contains Si–O bonds. The dissolution rate of talc is slower than that of serpentine because the leaching layer forms only at the grain edges (Sufriadin et al., 2011). In addition, the basal faces of the layers in talc, which account for 90% of the total surface, do not contain hydroxyl groups or active ions, and the edge faces contain only a few – SiOH and –MgOH groups (Chabrol et al., 2010). Consequently, talc is hydrophobic and chemically inert (Chabrol et al., 2010). This is the reason for the low leachability of talc in the current experiments. In conclusion, the differences between the serpentine and talc contents of SS and SH ores, shown by the TG-DTA, XRD, and FTIR spectroscopic results for the raw ore samples, may be the reason for the differences between the leaching performances of the samples. It can also be concluded that all the nickel dissolved in the leach liquor should originate from serpentine, for both SS and SH ores. These results are consistent with the XRD patterns and FTIR spectra of the solid residues, shown in Figs. 12–14. Figs. 12 and 13 show the XRD patterns of the solid residues from SS and SH, respectively, after leaching under the optimum conditions. A comparison of Figs. 12 and 1 shows that the serpentine mineral peaks in the solid residue from SS ore were significantly smaller than those for the original sample, whereas the goethite and talc peaks remained unchanged. Similar results were obtained for SH ore, as can be seen by comparing Figs. 13 and 2. The serpentine mineral peaks for the solid residue from this sample decline significantly, but those for goethite and talc remain unchanged. A comparison of the FTIR spectra in Figs. 14 and 3 shows that the amount of silicate phases in the solid residues decreased for both samples, i.e., the amount of serpentine decreased. In addition to strengthening these conclusions, the differences between the changes in the solution pH during the leaching process for each sample (Fig. 15) show that the decrease in the amount of H+ in the leaching solution of SH ore is slower than that in the case of SS ore. This means that there are smaller amounts of easily dissolved minerals such as serpentine in SH ore than in SS ore. These results reinforce the finding that nickel can be extracted only from serpentine when citric acid solution is used in atmospheric leaching of saprolite.

Fig. 11. Metal dissolutions under optimum leaching conditions: (a) SS ore and (b) SH ore (temperature: 40 °C, [citric acid]: 1 M, ore particle size: 212–355 μm, pulp density: 20% (w/v), shaker speed: 200 rpm).

Please cite this article as: Astuti, W., et al., Comparison of atmospheric citric acid leaching kinetics of nickel from different Indonesian saprolitic ores, Hydrometallurgy (2016), http://dx.doi.org/10.1016/j.hydromet.2015.12.015

W. Astuti et al. / Hydrometallurgy xxx (2016) xxx–xxx

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Fig. 12. XRD patterns of solid residue of SS ore under optimum conditions (temperature: 40 °C, [citric acid]: 1 M, ore particle size: 212–355 μm, pulp density: 20% (w/v), shaker speed: 200 rpm).

3.4. Kinetic analysis of nickel leaching

serpentine by citric acid can be described as:

The relative dependence of nickel leaching on nickel containing mineral dissolution can be further examined on the basis of heterogeneous kinetic models that can be represented by the general reaction aA(aq) + bB(s) → products (Senanayake et al., 2015). Kinetic analyses were performed using the experimental data for nickel dissolution from saprolitic samples. As discussed previously in Section 3.2, in citric acid leaching experiments, all the nickel dissolved in the pregnant leaching solution originated from serpentine. Nickel extraction from

ðMg; Ni; Fe; AlÞ3 Si2 O5 ðOHÞ4ðsÞ þ 6Hþ ðaqÞ →3Mg2þ ðaqÞ þ 3Ni 3þ

þ 3Fe

ðaqÞ

2þ

ðaqÞ

þ 3Al

3þ

ðaqÞ

þ 2SiO2ðsÞ þ 5H2 OðlÞ :

ð3Þ The kinetics of nickel dissolution can be represented by two solid– liquid kinetic models, proposed by Levenspiel (1999), known as the shrinking-core model (SCM) and the shrinking-particle model

Fig. 13. XRD patterns of solid residue of SH ore under optimum conditions (temperature: 40 °C, [citric acid]: 1 M, ore particle size: 212–355 μm, pulp density: 20% (w/v), shaker speed: 200 rpm).

Please cite this article as: Astuti, W., et al., Comparison of atmospheric citric acid leaching kinetics of nickel from different Indonesian saprolitic ores, Hydrometallurgy (2016), http://dx.doi.org/10.1016/j.hydromet.2015.12.015

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Fig. 14. FTIR spectra of solid residues from SS and SH ores under optimum conditions (temperature: 40 °C, [citric acid]: 1 M, ore particle size: 212–355 μm, pulp density: 20% (w/v), shaker speed: 200 rpm).

(SPM). According to the calculated regression correlation coefficients (Tables 6 and 7), the experimental data best fitted the integral rate equation for diffusion through solid product layers (Figs. 16 and 17), where XNi in those figures represents the nickel fraction reacted. The apparent rate constants (k) were determined from the slopes of the lines in Figs. 16(b) and 17(b) and used to determine the activation energy using the Arrhenius equation:   Ea k ¼ exp − RT

Table 6 Multiple regression coefficients for experimental kinetic data fitted to unreacted core models of constant size spherical particles (SCM) for all leaching parameters studied. Sample

Model

Equation

R2min–R2max

SS ore

Product layer diffusion

0.8567–0.9945

SH ore

Chemical reaction Film diffusion Product layer diffusion

kt = 1 − 3(1 − XNi)2/3 + 2(1 − XNi) kt = 1 − (1 − XNi)1/3 kt = XNi kt = 1 − 3(1 − XNi)2/3 + 2(1 − XNi) kt = 1 − (1 − XNi)1/3 kt = XNi

ð4aÞ

Chemical reaction Film diffusion

0.6561–0.9513 0.6442–0.8528 0.8848–0.9942 0.6850–0.8987 0.6736–0.8621

The k values represent the apparent rate constants for different control steps that can be expressed by these following equations (Levenspiel, 1999). Product layer diffusion: k¼

2M B DC A ρB ar 20

Chemical reaction: k¼

K c MB C A ρB ar 0

Fluid film diffusion: k¼

Fig. 15. Comparison of pH changes during leaching process for SS and SH ores under optimum conditions (temperature: 40 °C, [citric acid]: 1 M, ore particle size: 212–355 μm, pulp density: 20% (w/v), shaker speed: 200 rpm).

MB K g C A ρB ar 0

where XNi the nickel fraction reacted, kc the kinetic constant, MB the molar mass of dissolving B, CA the concentration of dissolved lixiviant A in the bulk of solution, ρB the density of the ore, a the stoichiometric coefficient of the reagent in the leaching reaction, r0 the initial radius of the solid particle, t the reaction time, kg the mass transfer coefficient between fluid and particle, D the diffusion coefficient in the porous product layer.

Please cite this article as: Astuti, W., et al., Comparison of atmospheric citric acid leaching kinetics of nickel from different Indonesian saprolitic ores, Hydrometallurgy (2016), http://dx.doi.org/10.1016/j.hydromet.2015.12.015

W. Astuti et al. / Hydrometallurgy xxx (2016) xxx–xxx Table 7 Multiple regression coefficients for experimental kinetic data fitted to shrinking spherical particles model (SPM) for all leaching parameters studied. Sample SS ore SH ore

Model

Equation

R2min – R2max

Chemical reaction Film diffusion Chemical reaction Film diffusion

kt = 1 − (1 − XNi)1/3 kt = 1 − (1 − XNi)2/3 kt = 1 − (1 − XNi)1/3 kt = 1 − (1 − XNi)2/3

0.6561–0.9513 0.6502–0.8993 0.6850–0.8987 0.6793–0.8809

lnk ¼ ln A−

Ea : RT

ð4bÞ

The three data points in Figs. 16(b) and 17(b) are not linear because there is a different trend between the increase of nickel leaching rate from 30 °C to 40 °C and that from 40 °C to 60 °C. The results show that the activation energies of the leaching process for both samples are low, around 6–49 kJ/mol. This low-activation energy corresponds to a process controlled by diffusion of a reactant or product through the

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solid product layer. These results are in agreement with those reported by Luce et al. (1972) and Teir et al. (2007), who found that either diffusion of ions in the mineral lattice itself or through a product layer is the rate-controlling mechanism for serpentine dissolution. The undetected silicon in the pregnant leach liquor, based on ICP-OES analysis in the current study, supports the proposal that a product layer of silica builds up on the particles. The current analysis also shows that the kinetic behaviors of nickel leaching from both SS and SH ores are similar, although the nickelleaching performances of the samples are different. These results support the finding that for both samples all the dissolved nickel originated from the same mineral, i.e., serpentine. It can therefore be concluded that the performances in atmospheric leaching of nickel from saprolitic ores by citric acid depend on the serpentine contents, and the serpentine dissolution (Eq. (3)), where the silica in serpentine is the solid product during and after leaching. It was consistent with the microstructure of serpentine that was evaluated before and after leaching using SEM-EDS as shown in Fig. 18. It can be seen that serpentine mineral after leaching process becomes porous with an increase in silica content. Astuti et al. (2015) have reported the kinetic models of nickel

Fig. 16. (a) Plot of 1 − 3(1 − XNi)2/3 + 2(1 − XNi) as a function of leaching time at different temperatures for extraction of nickel from SS ore and (b) Arrhenius plot for nickel leaching (1 M citric acid, 20% (w/v) pulp density, b75 μm particle size, 200 rpm shaker speed).

Please cite this article as: Astuti, W., et al., Comparison of atmospheric citric acid leaching kinetics of nickel from different Indonesian saprolitic ores, Hydrometallurgy (2016), http://dx.doi.org/10.1016/j.hydromet.2015.12.015

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W. Astuti et al. / Hydrometallurgy xxx (2016) xxx–xxx

Fig. 17. (a) Plot of 1 − 3(1 − XNi)2/3 + 2(1 − XNi) vs leaching time at different temperatures for extraction of nickel from SH ore and (b) Arrhenius plot for nickel leaching (1 M citric acid, 20% (w/v) pulp density, b75 μm particle size, 200 rpm shaker speed).

leaching from serpentine by citric acid. Furthermore, based on some of the results of the present study, a scheme for the leaching reaction of some metals i.e. not only nickel but also magnesium, iron, and aluminum from serpentine saprolitic ores using citric acid solution at atmospheric pressure is proposed in Fig. 19.

4. Conclusions The chemical contents of the ores indicated that both samples were typical low-grade saprolitic laterite. Although both samples have similar chemical contents, the mineral composition differs. The SS ore contains serpentine and goethite as the main minerals, whereas the major phases in SH ore are talc and goethite. The maximum leaching efficiency of nickel was achieved from SS ore (96%) and SH ore (73%) under the same leaching conditions, i.e., citric acid concentration 1 M, leaching temperature 40 °C, leaching time 15 d, ore particle size 212–355 μm, shaking speed 200 rpm, and pulp density 20% (w/v). Nickel leaching rate from SS ore is always higher than that from SH ore under the same leaching conditions.

The results of nickel leaching efficiency and analysis of the solid residues using XRD and FTIR spectroscopy showed that all dissolved nickel originated from serpentine, which is more easily leached than goethite and talc. Because of the lower serpentine content of SH ore, nickel leaching rate from this sample was lower than that from SS ore. In terms of the kinetics of nickel extraction, the results for both ore samples were similar, and showed that nickel leaching from SS and SH ores follows the SCM and is controlled by diffusion of a reactant or product through the solid product layer. This finding is significant for the design of nickel-leaching processes for different lateritic ores using citric acid at atmospheric conditions and low temperature. Acknowledgment The authors gratefully acknowledge a JSPS KAKENHI(Grants-in-Aid for Scientific Research(A)) Grant Number 15H02333, the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) Japan for financial and other supports, and Dr. M. Zaki Mubarok and Mr. Fikri Irsyad (Bandung Institute of Technology) for the supply of Indonesian saprolitic ore samples.

Please cite this article as: Astuti, W., et al., Comparison of atmospheric citric acid leaching kinetics of nickel from different Indonesian saprolitic ores, Hydrometallurgy (2016), http://dx.doi.org/10.1016/j.hydromet.2015.12.015

W. Astuti et al. / Hydrometallurgy xxx (2016) xxx–xxx

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Fig. 18. Microstructure and EDS analysis of serpentine in (a) raw ore and (b) solid residue.

Fig. 19. Schematic diagram of nickel-leaching process from serpentine based on shrinking-core model: (a) initial state and (b) state at time t.

Please cite this article as: Astuti, W., et al., Comparison of atmospheric citric acid leaching kinetics of nickel from different Indonesian saprolitic ores, Hydrometallurgy (2016), http://dx.doi.org/10.1016/j.hydromet.2015.12.015

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Please cite this article as: Astuti, W., et al., Comparison of atmospheric citric acid leaching kinetics of nickel from different Indonesian saprolitic ores, Hydrometallurgy (2016), http://dx.doi.org/10.1016/j.hydromet.2015.12.015