Comparative leaching of minerals by sulphuric acid in a Chinese ferruginous nickel laterite ore

Hydrometallurgy 98 (2009) 281–286

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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

Comparative leaching of minerals by sulphuric acid in a Chinese ferruginous nickel laterite ore Kui Liu a,b, Qiyuan Chen a,⁎, Huiping Hu a a b

School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China School of Chemistry and Chemical Engineering, Guangxi Normal University, Guilin 541004, China

a r t i c l e

i n f o

Article history: Received 25 February 2009 Received in revised form 18 May 2009 Accepted 22 May 2009 Available online 30 May 2009 Keywords: Sulphuric acid Leaching Nickel laterite ore Maghemite Lizardite Goethite Chromite Ringwoodite

a b s t r a c t The Yuanjiang nickel laterite ore containing mainly maghemite, goethite and lizardite was leached by sulphuric acid at atmospheric pressure and the residues were characterized using X-ray diffraction and scanning electron microscopy/X-ray energy dispersive spectroscopy. The relationship was discussed between the extraction of nickel, cobalt, iron, magnesium, aluminum, and the dissolution behaviour of the laterite minerals; as well as the extent of congruency of nickel, cobalt and iron extraction. The results show that the solubility of the laterite minerals in sulphuric acid decreases in the following order: lizardite N goethite N maghemite N magnetite ≈ hematite N chromite ≈ ringwoodite. Lizardite dissolved rapidly in 0.6 mol/L sulphuric acid at 60 °C whilst goethite dissolved completely in 2.5 mol/L sulphuric acid at 80 °C. The dissolution of the primary mineral maghemite was slow, but increased with increasing acid concentration and leaching temperature. Magnetite dissolved more slowly than maghemite; and hematite was only dissolved in N 6.2 mol/L sulphuric acid at 105 °C. Chromite and ringwoodite were not dissolved. The leaching behaviour of the laterite minerals may be explained by the bond strength differences of Me–O and the substitution of metal cations in the mineral structure. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Acid leaching has become the primary technology for processing nickel laterite ores because of its advantage that elements such as nickel, cobalt, iron and magnesium can be recovered comprehensively. A proper understanding of mineral dissolution behaviour in acidic solution is helpful for leaching valuable metals from laterite ores. Previous studies have provided some information of the dissolution behaviour of laterite minerals at atmospheric pressure. For example, it was found from the studies of Agatzini-Leonardou and Zafiratos (2004) and Stamboliadis et al. (2004) that sulphuric acid selectively dissolved the magnesium silicates from serpentinic nickeliferous ores while the leaching of hematite was limited. Rice and Strong (1974a,b) and Canterford (1986) also reported that serpentine minerals in nickeliferous laterites were leached relatively easily while other minerals such as magnetite, maghemite and chromite, remained untouched in hydrochloric or sulphuric acid, even after long periods of extraction. Canterford (1978) and Griffin et al. (2002) reported that nickel was more readily leached from clay-like (e.g. smectite, saprolite) ores than limonitic (e.g. goethite) ores. It has been thought from extensive studies (Kumar et al., 1990, 1993; Singh et al., 2002; Pozas et al., 2004; McDonald and Whittington, 2008)

⁎ Corresponding author. Tel.: +86 731 8877364; fax: +86 731 8879616. E-mail address: [email protected] (Q.Y. Chen). 0304-386X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2009.05.015

that metal cations exist in nickeliferous laterites in two modes, (a) weakly adsorbed to the mineral surface and (b) as a substitute in the mineral structure. The extent of substitution has great impact upon the dissolution behaviour of laterite minerals. Lim-Nunez and Gilkes (1987) reported that chromium-containing goethites dissolved at about one tenth the rate of un-substituted and nickel- and aluminium-containing goethites; whereas cobalt- and manganese-containing goethites dissolved at about twice this rate. Landers et al. (2009) reported that the presence of aluminum and chromium in goethite appeared to reduce dissolution rate possibly through the higher M3+–OH (M= Al or Cr), bond strength relative to Fe3+, Ni2+–OH. The Yuanjiang laterite ore is a primary resource of nickel oxide in China located in Yunnan province. The estimated nickel reserves of the Yuanjiang laterite ore is 0.43 million tons and the average nickel and cobalt contents are 0.83% and 0.08%, respectively. The main mineral components of the ore are maghemite, lizardite and goethite together with hematite, magnetite and chromite; which all contain nickel, cobalt, iron, magnesium and aluminum (Wang and Jiang, 2005). Hence, the recovery of valuable metals such as nickel and cobalt from this ore is largely dependent on the complete dissolution of the nickeliferous minerals. Until recently, however, there has been no systematic research on the dissolution behaviour of the nickeliferous minerals in Yuanjiang laterite ore during acid leaching. The aim of this study was to investigate the dissolution behaviour of Yuanjiang laterite minerals during sulphuric acid leaching at atmospheric pressure. Particular interest was devoted to the dissolution

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Table 1 Chemical analysis of the three ore samples (wt.%). Ore sample

Ni

Co

Mg

Fe

Al

Cr

Mn

Si

1# 2# 3#

0.96 1.23 1.22

0.1 0.09 0.088

1.12 6.39 5.48

48.5 33.5 32.98

3.94 2.89 4.28

0.71 0.68 0.88

0.79 0.7 0.63

3.85 12.05 9.16

behaviour of maghemite, lizardite and goethite. A leaching procedure was designed to investigate the dissolution behaviour of the minerals under different leaching conditions. X-ray diffraction, scanning electron microscopy and X-ray energy dispersive spectroscopy were employed to characterize the residues. Furthermore, the extraction of nickel, cobalt, magnesium, aluminum, iron; and the substitution of nickel, cobalt in the laterite minerals were investigated to support the interpretation of their dissolution behaviour. 2. Experimental 2.1. Materials Fig. 2. XRD patterns of the three ore samples. M: maghemite, G: goethite, L: lizardite.

Three Yuanjiang laterite samples were selected as representative ore samples in this study. They were yellow, filemot or rufous soil, with traces of grey rock slivers. Ore samples were dried overnight in an oven at 100 °C, then ground in a disc crusher for a few minutes to mill the rock slivers, followed by screening to − 165 μm. The chemical analysis and particle size distribution of the screened ore samples are given in Table 1 and Fig. 1. The mineral phases of Yuanjiang laterite were analysed by a D/max 2550 X-ray diffractometer (XRD) with Cu Kα radiation from 10° to 85° and a LEICA DM4500P optical microscope. The major phases in the ore are maghemite, lizardite and goethite, as shown in Fig. 2. Besides these minerals, significant amounts of magnetite, small amounts of chromite and traces of hematite in sample 3# were observed under the optical microscope. Additionally, traces of lizardite, magnetite, hematite and chromite were observed in sample 1# as well as traces of magnetite, hematite and chromite in sample 2#. Hematite and chromite are not identified by XRD because their contents in the ore were too low to be detected. The mineral composition of sample 1# and sample 2# was determined by semi-quantitative analysis of XRD as shown in Table 2. However,

because the magnetite peaks almost coincide with those of maghemite, the mineral composition of sample 3# was calculated according to the results of chemical analysis of iron. Both these two methods are of low accuracy. The nickel, iron, magnesium and aluminum contents in the different minerals were determined by using an EDX–GENESIS60S X-ray energy dispersive spectroscope as shown in Table 3. Aluminum exists predominantly in chromite (Table 3), whereas aluminum content is much higher than chromium content particularly in sample 3# (Table 1), indicating that other aluminum-containing mineral undetected by XRD and optical microscopy may be present in the ore samples.

2.2. Methods A 10% w/v mixture of a screened ore sample and sulphuric acid solution (ranging in concentration from 0.6–7.5 mol/L) was placed in conical flask fitted with a water condenser. It was then heated to the desired temperature (ranging from 50–105 °C) and magnetically stirred at 750 rpm for 2 h. The filtered residue was washed with distilled water, dried for 4 h at 100 °C and re-ground in a mortar prior to XRD analysis. The filtrate was analysed for nickel, cobalt and magnesium by a WFX320 atomic absorption spectrophotometer. Total iron was analysed by standard potassium dichromate titration after prior reduction of Fe(III) to Fe(II) by excess SnCl2 and final HgCl2 addition. Aluminum was analysed by EDTA titration and residual acid by titration with sodium hydroxide using methyl orange indicator (pH 3.1–4.4) and hydroxylamine hydrochloride (NH2OH.HCl) to reduce Fe(III) to Fe(II) and mask the otherwise interference of the dissolved iron(III). The amount of acid produced from the reduction reaction (2NH3OH+ + 4Fe3+ = 4Fe2+ + N2O + 6H+ + H2O) was subtracted. Some residues were taken for crystallinity analysis using the software of Jade 5.0 and some were analyzed by a JSM6360LV scanning

Table 2 Mineral composition of the three ore samples (%).

Fig. 1. Particle size analysis of the three ore samples.

Mineral

Sample 1#

Sample 2#

Sample 3#

Maghemite (γ-Fe2O3) Goethite (FeO(OH)) Lizardite (Mg3Si2O5(OH)4) Magnetite (Fe3O4) Hematite (α-Fe2O3) Chromite (FeCr2O4)

70 24 Trace Trace Trace Trace

50 14 28 Trace Trace Trace

33 12 26 9 Trace 3

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283

Table 3 Elemental content of major minerals in Yuanjiang laterite (%). Element

Magnetite

Maghemite

Goethite

Lizardite

Chromite

Ni Fe Mg Al

0.97 68.56 0.43 0.13

0.95 66.55 0.46 0.16

1.25 48.66 2.89 1.58

0.74 2.30 20.5 2.39

0.19 18.95 6.29 10.03

electron microscope combined with X-ray energy dispersive spectroscope (SEM/EDS).

3. Results 3.1. XRD analysis of the residues A series of leaching experiments on the three ore samples were carried out at 80 °C, 90 °C and 105 °C in acid concentrations ranging from 0.6 mol/L to 7.5 mol/L and the XRD patterns of the residues were analysed as summarized in Table 4. Fig. 3 shows the typical change in XRD patterns of sample 3# with increasing acid concentration at 105 °C, which illustrates that maghemite and hematite are difficult to completely dissolve in sulphuric acid. Ringwoodite (Fe2(SiO4)) and magnetite peaks appeared when the sulphuric acid concentration was increased to 5.0 mol/L at 105 °C, indicating that these minerals dissolve more slowly than maghemite. Since the magnetite peaks almost coincide with those of maghemite, it is possible that both minerals are present in b5.0 mol/L sulphuric acid. The peak intensity of maghemite became weaker with sulphuric acid concentration increasing from 2.5 mol/L to 7.5 mol/L (Fig. 3) particularly at 105 °C compared to 80 °C, indicating that maghemite was only gradually dissolved with increasing sulphuric acid concentration and leaching temperature. The peak intensity of hematite at 33°(2θ) was not observed to become weaker with increasing sulphuric acid concentration (Fig. 3), indicating that hematite is hardly dissolved at 80–105 °C. Hematite in the ore samples is undetectable by XRD because of its extremely low content (Table 2), the small hematite peaks were only observed after goethite and lizardite had been partially dissolved. However, these peaks disappeared in N6.2 mol/L acid at 105 °C (Table 4 and Fig. 3(f) and (g)), indicating that hematite could be dissolved under these conditions. Minor goethite peaks were observed when leaching with 0.6 mol/L and 1.2 mol/L sulphuric acid, but disappeared when leaching with 2.5 mol/L sulphuric acid at 80 °C and 90 °C (Table 4), indicating that goethite can be completely dissolved under these conditions. The absence of the goethite peak in Fig. 3 implies that goethite was dissolved in preference to maghemite and hematite. There was no lizardite in the residues of samples 2# and 3# at 80– 105 °C even when the sulphuric acid concentration was decreased to 0.6 mol/L (Table 4), indicating that lizardite was readily dissolved. In order to determine the minimum temperature required to completely dissolve lizardite, several leaching experiments below 80 °C were carried out on sample 2#. The results presented in Table 5 reveal that

Table 4 Mineral phases of the residues under the different leaching conditions. Sulphuric acid concentration (mol/L)

Sample 1# 80 °C

90 °C

105 °C

Sample 2# 80 °C

90 °C

105 °C

Sample 3# 105 °C

0.6 1.2 2.5 3.7 5.0 6.2 7.5

M,H,G M,H,G M,H M,H M,H M,H M,H

M,H,G M,H,G M,H M,H M,H,S M,H,S M,H,S

M,H M,H M,H M,H M,H M,H,R M,R

M,H M,H M,H M,H M,H M,H M,H,S

M,H M,H M,H M,H M,H M,H M,H

M,H M,H M,H M,H M,H M,S M,R,S

M,H M,H M,H M,H M,R Mt,R Mt,R

M: maghemite, H: hematite, G: goethite, S: silicon oxide, L: lizardite, Mt: magnetite, R: ringwoodite.

Fig. 3. XRD patterns of the residue from sample 3# after leaching with sulphuric acid at 105 °C. Acid concentration is (a) 0.6 mol/L, (b) 1.2 mol/L, (c) 2.5 mol/L, (d) 3.7 mol/L, (e) 5.0 mol/L, (f) 6.2 mol/L, (g) 7.5 mol/L. M: maghemite, H: hematite, R: ringwoodite, Mt: magnetite.

lizardite was completely dissolved even at 60 °C in 0.6 mol/L sulphuric acid but goethite remained present, showing that lizardite dissolved faster than goethite. Silicon oxide (SiO2) characteristic peaks appear near 22°, 33°and 37° (2θ) and were observed occasionally (Table 4). The XRD peaks from 15–30° in Fig. 3 are irregular, blurred and overlapped, exhibiting poor crystallization of the material particularly in Fig. 3(e), (f) and (g) from sample 3# in which the lizardite content is 26%. In order to determine the amount of the substance with poor crystallization, the crystallinity analysis of the residues leached with 7.5 mol/L sulphuric acid at 105 °C was performed. The results show that the crystallinity of the residues was 12.5% for sample 1#, 7.0% for sample 2# and 9.2% for sample 3#, respectively, meaning that the majority of the residues was amorphous material. The lower crystallinity values corresponded to the ore samples with the higher silicon content (Table 1), indicating that there are large amounts of amorphous silica existing in the residues. As there is no silica in the ore samples (Table 2), the silica in the residues may result from the decomposition of silicate minerals such as lizardite. 3.2. SEM/EDS analysis of the residues From the above XRD analysis, the dissolution behaviour of maghemite, goethite, lizardite, hematite and magnetite in sulphuric acid was determined. Further investigation of the residues by SEM/EDS analysis was conducted to study the dissolution behaviour of other minerals such as chromite and ringwoodite. The results in Fig. 4 and Table 6 show that the silicon and oxygen contents were all very high while iron content was very low in the residues of the three ore samples, indicating that the main component of the residues was silica, which is consistent with the conclusions drawn by XRD and the crystallinity analysis of the residues. It also confirms that the majority of maghemite and goethite dissolved when the ore samples Table 5 Mineral phases of the residues of sample 2# (0.6 mol/L sulphuric acid). Temperature (°C)

50

60

70

80

90

105

Mineral phase

M,H,L,G

M,H,G

M,H,G

M,H

M,H

M,H

M: maghemite, H: hematite, G: goethite, L: lizardite.

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Fig. 4. SEM of the residue from sample 3# showing spots A, B, C examined by microprobe (after dissolution at 105 °C in 7.5 mol/L sulphuric acid).

were leached with 7.5 mol/L sulphuric acid at 105 °C. The iron content in the residues of sample 3# was the highest (7.8%) (Table 6) which was due to the presence of small amounts of undissolved magnetite, hematite or maghemite. Whilst the iron content in the residues of sample 2# was the lowest (1.1%) due to the lower overall maghemite and magnetite content in the sample. There are several regions in the residues where iron, chromium and either manganese or magnesium and aluminum enrich simultaneously, such as regions A and B of Fig. 4 (Table 6). This indicates that chromite was completely undissolved and had manganese, magnesium and aluminum incorporated into its crystal lattice. Region C in Fig. 4 is rich in iron, silicon and oxygen (Table 6), indicating that ironrich ringwoodite was also undissolved. 3.3. Extraction of nickel, cobalt, magnesium, aluminum and iron 3.3.1. Relationship between metals extraction and dissolution of minerals Generally, when the mineral is completely dissolved in the acidic solution, nickel, cobalt, iron, magnesium and aluminum, contained in the mineral, are released into the leach liquor. Therefore, the extraction of these metal ions from the three ore samples was investigated at 80 °C and 105 °C in the various acid concentrations. The results in Figs. 5–8 show the increase of iron extraction with increasing sulphuric acid concentration, implying that maghemite, goethite and magnetite were gradually dissolved. The iron extraction reached 95%, consistent with the low iron content of the residues, but could not reach 100% due to undissolved chromite, ringwoodite and small amounts of undissolved maghemite, magnetite and hematite. Besides iron, N90% nickel and cobalt were extracted from the three ore samples in 5.0 mol/L sulphuric acid at 105 °C (Figs. 5–8), indicating that the majority of nickeliferous minerals dissolved.

Fig. 5. Metal ions extracted from sample 1# at 105 °C with sulphuric acid of different concentrations.

Magnesium and aluminium extractions from samples 1# and 2# were greater than 80% in 0.6 mol/L sulphuric acid, up to 90% in 1.2 mol/L sulphuric acid at 105 °C (Figs. 5 and 7). Since magnesium exists predominantly in lizardite, whilst aluminium exists mainly in lizardite, goethite and chromite (Table 3), this extraction reflects the easy dissolution of lizardite and goethite. However, for sample 3#, 35% aluminium was not extracted, probably due to incorporation in undissolved chromite, magnetite, or other aluminium-containing non-crystalline mineral. In 5.0 mol/L sulphuric acid at 105 °C, the overall acid consumption for leaching sample 1#, 2# and 3# was 1.47 kg H2SO4/kg dry ore, 1.64 kg H2SO4/kg dry ore and 1.51 kg H2SO4/kg dry ore, respectively. The sulphuric acid consumption depends mainly on magnesium content in the ore sample, which is highest for sample 2# (Table 1); as well as the iron content, which is highest for sample 1#.

Table 6 EDS analysis of the residues (105 °C, 7.5 mol/L sulphuric acid). Element

Residues of sample 1#

Residues of sample 2#

Residues of sample 3#

Region A of Fig. 6

Region B of Fig. 6

Region C of Fig. 6

O Mg Al Si S Cr Mn Fe Co Ni

39.34 00.24 00.86 48.40 02.89 01.25 00.32 05.66 00.47 00.56

43.67 00.06 00.61 51.69 01.25 00.70 00.52 01.14 00.19 00.17

43.10 00.47 00.92 42.22 01.49 02.44 00.45 07.79 00.64 00.49

17.95 00.89 00.83 02.90 00.33 40.65 05.76 29.20 00.71 00.78

29.77 08.14 14.87 03.17 00.49 26.13 01.26 14.93 00.66 00.59

29.15 00.83 00.45 13.22 00.80 00.46 00.51 52.35 01.08 01.16

Fig. 6. Metal ions extracted from sample 1# at 80 °C with sulphuric acid of different concentrations.

K. Liu et al. / Hydrometallurgy 98 (2009) 281–286

Fig. 7. Metal ions extracted from sample 2# at 105 °C with sulphuric acid of different concentrations.

3.3.2. Congruency of nickel, cobalt, iron extraction There have been some research reports stating that the location of metal cations in the minerals could be assessed by the congruency extent of metal extraction (Wells, 1998; Senanayake and Das, 2004; Perrier et al., 2006). If iron and associated metals dissolve in identical proportions, it can be postulated that the metal is uniformly distributed throughout the iron oxide crystals. If their dissolution is not congruent, either the distribution of the metal is not uniform or the iron and other metal are not present in a single mineral (Landers and Gilkes, 2007). Cobalt and iron extractions were found to exhibit a linear correlation (slope 1.05≈1) in the range of 30–90% at 80 °C (Fig. 9), meaning that cobalt and iron were extracted simultaneously when goethite and maghemite dissolved partially or completely. Therefore, cobalt mostly isomorphously displaces iron in the structure of goethite and maghemite.

Fig. 8. Metal ions extracted from sample 3# at 105 °C with sulphuric acid of different concentrations.

285

Fig. 9. Nickel, cobalt extraction versus iron extraction from data in Fig. 6.

Nickel extraction was higher than cobalt and iron in 0.6–2.5 mol/L sulphuric acid at 80 °C (Figs. 6 and 9), indicating that nickel is more susceptible to acid attack and easier to be extracted than cobalt and iron. Only goethite dissolved under these conditions, therefore, nickel is not bound within the crystal lattice and may be adsorbed on the surface of the goethite crystal (Landers et al., 2009). However, in N2.5 mol/L sulphuric acid which dissolves maghemite, nickel extraction was not significantly higher than iron, cobalt extraction and exhibits linear correlation (slope 0.97 ≈ 1) with iron extraction (Figs. 6 and 9), indicating that rather more nickel was uniformly substituted isomorphously for iron in the structure of maghemite. 4. Discussion The results of this study and XRD analysis confirm the observations of Büyükakinci and Topkaya (2009) and Agatzini-Leonardou and Zafiratos (2004) that goethite and serpentine minerals are leached preferentially to hematite with sulphuric acid depending on the concentration and temperature. Nevertheless, the observation that hematite of Yuanjiang laterite can be dissolved in N6.2 mol/L sulphuric acid at 105 °C has not been reported in the previous studies. Furthermore, the maghemite in Yuanjiang laterite was gradually dissolved with increasing acid concentration whereas Canterford (1986) found that maghemite in saprolite ores was not dissolved in sulphuric acid. However, as laterite deposits from different regions vary in composition and reactivity, it is reasonable that the rate of leaching of laterite minerals differ from one deposit to another. The fast dissolution of lizardite in this investigation is in accord with the recent studies by Luo et al. (2009) and the established behaviour of serpentine minerals (Rice and Strong, 1974a; Lin and Clemency, 1981; Hirasawa and Horita, 1987; Kosuge et al., 1995). Lizardite displays a 1:1 planar structure, which is composed of one tetrahedral sheet containing silicon and one octahedral sheet containing magnesium (Auzende et al., 2004). These two sheets are easily disconnected and when attacked by acid, the Mg–O bond is ruptured and Si–O bond remains in the tetrahedral sheet producing an amorphous silica residue (McDonald and Whittington, 2008). It is not surprising that chromite is not attacked by sulphuric acid at atmospheric pressure, even in concentrated sulphuric acid. This is consistent with the investigation of Rice and Strong (1974a,b), Canterford (1986) and Stamboliadis et al. (2004). Whittington and Johnson (2005) reported that chromite was not dissolved even in high

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pressure acid leaching at 250 °C, demonstrating that chromite was strongly resistant to sulphuric acid. Ringwoodite is a high-pressure polymorph derived from olivine (Binns et al., 1969; Colemann, 1977) that has not been found in Yuanjiang laterite ore in previous studies. In the weathering process, olivine is transformed to serpentine minerals while ringwoodite resists erosion and is preserved in the laterite ore in trace amounts. It was only detected by XRD after the bulk of the other minerals were dissolved. The dissolution behaviour of laterite minerals can be related to the metal-oxide bond strength (Landers et al., 2009). Both the Al–O bond strength (512.1 kJ/mol) and Cr–O bond strength (429.3 kJ/mol) are higher than the Fe–O bond strength (390.4 kJ/mol) (Weast, 1989–1990); therefore, the substitution of iron with aluminum and chromium stabilize goethite against proton attack (Schwertmann, 1991). This may be one of the reasons why Al-rich chromite in Yuanjiang laterite is more difficult to be dissolved than other iron minerals such as maghemite, magnetite and hematite. Though nickel and cobalt have slightly lower oxide bond strengths (382.0 kJ/mol for Ni–O and 384.5 kJ/mol for Co–O) to iron, Singh et al. (2002) and Manceau et al. (2000) reported that the atomic distances for Ni-substituted goethite were shorter than those for pure goethite, resulting in a Ni–O bond stronger than the Fe–O bond. The combination of these two factors may negate any overall change in bond strength resulting from the substitution of Fe3+ by Ni2+ (Landers et al., 2009). This was evident from the observation of Lim-Nunez and Gilkes (1987) that the dissolution of Ni-substituted goethites did not increase with increasing nickel substitution. Nickel and cobalt are isomorphously substituted for iron in the structure of maghemite (Fig. 9), accordingly, it is reasonable to assume that maghemite in Yuanjiang laterite is unlikely to show preferential acid attack of any Me–O bond or more rapid dissolution. Some elements such as nickel are not bound within the crystal lattice of goethite (Fig. 9), indicating that there probably exists some disordered, microporous or defective structure easy to be attacked by acid (Landers and Gilkes, 2007), therefore goethite dissolved faster than maghemite in Yuanjiang laterite. Lizardite exhibits fast incongruent dissolution, with magnesium readily extracted leaving silica, because the Si–O bond strength is much higher (799.6 kJ/mol) than that of Mg–O (363.2 kJ/mol), which is even lower than the Fe–O, Ni–O, Co–O bond strengths. 5. Conclusions Yuanjiang laterite minerals exhibit significantly different dissolution behaviour during sulphuric acid leaching at atmospheric pressure. The results from the investigation of nickel, cobalt, magnesium, aluminum, iron extraction and the residues characterization using XRD and SEM/ EDS analysis show that the leaching of Yuanjiang laterite minerals in sulphuric acid decreases in the following order: lizardite N goethite N maghemite N magnetite ≈ hematite N chromite ≈ ringwoodite. Lizardite can be completely dissolved at 60 °C in 0.6 mol/L sulphuric acid whilst goethite dissolves in 2.5 mol/L sulphuric acid at 80 °C and maghemite, hematite and magnetite all dissolve slowly. Hematite only dissolves in N6.2 mol/L sulphuric acid at 105 °C but ringwoodite, first found in trace amounts, and chromite cannot be dissolved. The congruency of nickel and cobalt versus iron extraction indicates that nickel and cobalt substitute for iron in the maghemite and goethite structures and complete mineral dissolution is required to extract these valuable metals. Since maghemite is the primary nickeliferous mineral in Yuanjiang laterite, its slow dissolution in sulphuric acid limits the application of atmospheric acid leaching. Acknowledgements This work is financially supported by National Basic Research Program of China (973 Program) (No. 2007CB613601). The authors wish to thank

Prof. Guizhi Zhou and Jing Fang for the assistance of residues' characterization. The authors also thank the two reviewers for their constructive comments.

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