Sulphuric acid pressure leaching of a limonitic laterite: chemistry and kinetics

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Abstract

Sulphuric acid pressure leaching of limonitic laterites is the process of choice to recover nickel and cobalt from equatorial lateritic ores, replacing the energy intensive pyrometallurgical methods. This process achieves a high nickel and cobalt extraction (more than 95%) with a high selectivity due to simultaneous iron and aluminium dissolution and precipitation. Experiments were carried out using batch pressure leaching techniques. A titanium autoclave equipped with acid injection and sample withdrawal units was employed. Conditions close to the industrial practice were tested: pulp density 30%, acid to ore ratio 0.2 and temperature ranging from 230 to 270°C. Raw limonite and the evolution of the nature of solid products during leaching were characterised using transmission electron microscopy. It was observed that limonite consists of aggregates of needle-like particles of goethite compacted together. Nickel was found to be predominately associated with this phase. During leaching, goethite dissolves continuously liberating nickel whilst iron re-precipitates as dense hematite particles in solution by ex situ precipitation. Several kinetic models for porous solids were also tested. The grain model was finally proposed to best describe nickel dissolution kinetics. The rate-controlling step was suggested to be pore diffusion of sulphuric acid.

Introduction

Laterites are oxide ores widely distributed in the equatorial regions. They were formed during laterization, a weathering process of ultramafic rocks that is favoured by warm climate and abundant rainfall. Lateritic deposits usually consist of three layers, namely the limonitic, the saprolitic and the garnieritic layer. Limonite, which comprises the top lateritic layer, is a homogeneous ore consisting mainly of goethite with which nickel is associated 1, 2, 3.

Sulphuric acid pressure leaching is the preferred process to recover nickel and cobalt from limonitic laterites. This is reflected by the current activity of many companies in Canada and in Australia. Although several projects for Ni and Co recovery from laterites by acid pressure leaching are now under consideration, particularly in Australia, the only plant currently employing this process is located at Moa Bay, Cuba which is operated by Moa Nickel 4, 5. Advantages of the acid pressure leaching process include:

  • 1.

    Low operational cost — sulphuric acid is a cheap raw material — and acid is regenerated in situ.

  • 2.

    No drying and reduction steps are needed, since raw laterite (`as mined') is used.

  • 3.

    High selectivity is obtained due to hydrolytic iron re-precipitation as hematite.

  • 4.

    No sulphur dioxide emissions are produced.

  • 5.

    Recoveries of more than 95% for nickel and more than 90% for cobalt can be achieved.

The process is a `one-pass' with respect to sulphate. As a consequence there is a large volume of sulphate tailings (gypsum) that is generated. Limonitic laterites are ideal for this process due to their low magnesia content and consequently low acid consumption [6]. The chemistry of the process was recently reviewed from an industrial perspective [5]. Sulphuric acid leaching of limonitic laterites is performed at high temperatures (240–270°C) in acid resistant autoclaves. Titanium has been found to be the best material of construction. At these temperatures, equilibrium vapour pressure reaches 33–55 atm. Iron and aluminium (in the trivalent state), follow a dissolution–precipitation path, forming solid products 2, 7.

Iron in the form of goethite and aluminium in the form of boehmite (gibbsite, the major phase of Al in limonite, transforms during slurry heating to boehmite at around 135–155°C 8, 9) dissolve to ferric and aluminium sulphates respectively, according to reactions 1 and 2:FeOOH(s)+3H+→Fe3++2H2OAlOOH(s)+3H+→Al3++2H2ONiO(s)+2H+→Ni2++H2OCoO(s)+2H+→Co2++H2O

Nickel and cobalt in the assumed form of `oxides', dissolve according to reactions 3 and 4 respectively and remain in the aqueous phase as sulphates 7, 10. Ferric cations hydrolyse rapidly after the dissolution of goethite, forming directly hematite according to reaction 5 or basic ferric sulphate (reaction 6), which can transform to hematite (reaction 7). Basic ferric sulphate formation depends upon leaching conditions and it is favoured by very acidic environments (high sulphate contents). High temperatures though, favour the formation of hematite 11, 12. These reactions cause the regeneration of the acid consumed by goethite dissolution in the first place:2Fe3++3H2O→Fe2O3(s)+6H+Fe3++SO2−4+H2O→FeOHSO4(s)+H+2FeOHSO4(s)+H2O→Fe2O3(s)+2SO2−4+4H+3Al3++2SO2−4+7H2O→(H3O)Al3(SO4)2(OH)6(s)+5H+Al3++SO2−4+H2O→AlOHSO4(s)+H+

Aluminium cations also hydrolyse, leading to the formation of solid products. Alunite and/or basic sulphate are formed, according to reactions 8 and 9 respectively. High temperatures (above 280°C) favour the formation of basic sulphate, but this can also form at lower temperatures if the acidity is high 10, 11, 12. Again, most of the acid consumed by boehmite dissolution is regenerated. Finally, an acid-to-ore ratio of around 0.2 was found adequate for Ni and Co leaching of a limonitic laterite by previous investigations 7, 10.

Most of the work done in the past involved bulk measurements of solution and/or solids composition after batch experimentation where all samples were obtained at the end of reaction period after autoclave cool down 7, 10. In the present work, more accurate chemical/mineralogical analysis and experimental procedures were followed that enabled quantitative analysis of the chemistry and the reaction kinetics [13]. In brief, the objectives of this work were:

  • 1.

    Study the evolution of solution and solids chemistry during leaching.

  • 2.

    Derive a simple conceptual model for nickel dissolution kinetics to be used in process modelling studies later.

Section snippets

The ore

The laterite used in this study was an Indonesian limonite and was provided by INCO It was a reddish-brown, clay-like solid, containing 42 to 44% water. A particle size analysis was performed and is shown in Fig. 1. About 50% of the total number of particles had a size of less than 1 μm (median size of 0.97 μm), most of them falling in the range of 0.5 to 1 μm. The mean size of the particles was 1.70 μm. Further analysis of bulk and absolute density, pore volume, pore size distribution, as well

Limonite characterisation

The distribution of each metal in the several mineral phases existing in limonite is described below.

Conclusions

Sulphuric acid pressure leaching was employed for a limonitic laterite supplied by INCO A series of experiments was carried out using a batch reactor technique. A 2-liter titanium autoclave equipped with acid injection and sample withdrawal systems was utilised. Leaching chemistry and the evolution of the nature of the solids during leaching were studied. Several kinetic models for porous solids were tested. The following points summarise the findings.

1. Goethite particles are highly porous

Nomenclature

Afluid reactant
Aggrain external surface area (cm2)
bstoichiometric coefficient
CAconcentration of fluid reactant (sulphuric acid in mol l−1)
CA,fbulk concentration of fluid reactant (mol l−1)
CA,aveaverage sulphuric acid concentration (mol l−1)
cMconcentration of metal M in limonite (wt.%, on a dried basis)
CM,iconcentration of M in sample i (mg l−1)
Deeffective diffusivity of fluid A in porous solid (cm2 s−1)
Fggrain shape factor
Gdimensionless structural parameter of the uniform model (G>0)
kreaction

Acknowledgements

The J. Roy Gordon Research Laboratory of INCO and the Natural Sciences and Engineering Research Council of Canada (NSERC) are gratefully acknowledged for their financial support. The authors also wish to thank Mr. D. Holmyard for his assistance with the TEM technique.

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