Microstructure analysis of mineral ore agglomerates for enhanced processability Ataollah Nosrati 1 *, William Skinner 1 , Jonas Addai-Mensah 1 , David J. Robinson 2 and John Farrow 2 1 Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia 2 CSIRO Minerals Down Under National Research Flagship Australian Minerals Research Centre, PO Box 7229, Karawara W.A. 6152, Australia Corresponding author: [email protected]ABSTRACT Agglomeration is a process that is widely used in several industries (e.g., pharmaceuticals, fertilizers, minerals, food) to produce large single or multiphase granules with desirable attributes from fine particles. In the minerals industry, multi- mineral phase agglomerates are used in heap leaching operations to recover valuable metals such as copper, nickel and gold from finely ground, low grade ores. The key structural features of agglomerates that lead to enhanced leaching performance within full-scale heaps have been identified as their size and its distribution, internal porosity, bed permeability and mechanical strength. Knowledge of the agglomeration mechanisms and kinetics together with the characterization of granules’ internal microstructures (e.g., porosity, spatial distribution of different mineral phases, pores and liquid phase) are essential for greater understanding and correlation with the concomitant mechanical properties and robustness. In this paper, we demonstrate how a non-destructive micro-tomography analysis is used to study nickel laterite ore agglomeration behaviour, specifically its internal, multi- mineral component granule structure and pore volume spatial distribution. The 3-D analysis was in good agreement with laboratory measurements and valuable for both the agglomeration process optimization and improving final granule quality. INTRODUCTION Heap leaching is a well known process used in the mining industry to recover valuable metals such as copper, nickel, gold and uranium from complex, low grade ores (Chamberlin, 1981, Chamberlin, 1986). Producing robust agglomerates or pellets with desirable porosity and permeability is one of the major challenges to achieve efficient heap leaching operations. For variable, low grade nickel laterite ores, the formation of granules with various bulk properties, depending upon variables such as binder composition, moisture/binder content, agglomeration time and post-agglomeration drying conditions (temperature and time) is common place (Watling et al., 2010; Readett and Fox, 2010). A better understanding of the effect of these variables on the microstructure of agglomerates is crucial to control and optimize the granules bulk properties (i.e., density, porosity, stability and strength).
Transcript
Microstructure analysis of mineral ore agglomerates for enhanced processability
Ataollah Nosrati1*, William Skinner1, Jonas Addai-Mensah1, David J. Robinson2 and John Farrow2
1Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095,
Australia
2 CSIRO Minerals Down Under National Research Flagship Australian Minerals Research Centre, PO Box 7229, Karawara W.A. 6152, Australia
ABSTRACT Agglomeration is a process that is widely used in several industries (e.g., pharmaceuticals, fertilizers, minerals, food) to produce large single or multiphase granules with desirable attributes from fine particles. In the minerals industry, multi-mineral phase agglomerates are used in heap leaching operations to recover valuable metals such as copper, nickel and gold from finely ground, low grade ores. The key structural features of agglomerates that lead to enhanced leaching performance within full-scale heaps have been identified as their size and its distribution, internal porosity, bed permeability and mechanical strength. Knowledge of the agglomeration mechanisms and kinetics together with the characterization of granules’ internal microstructures (e.g., porosity, spatial distribution of different mineral phases, pores and liquid phase) are essential for greater understanding and correlation with the concomitant mechanical properties and robustness. In this paper, we demonstrate how a non-destructive micro-tomography analysis is used to study nickel laterite ore agglomeration behaviour, specifically its internal, multi-mineral component granule structure and pore volume spatial distribution. The 3-D analysis was in good agreement with laboratory measurements and valuable for both the agglomeration process optimization and improving final granule quality.
INTRODUCTION
Heap leaching is a well known process used in the mining industry to recover valuable metals such as copper, nickel, gold and uranium from complex, low grade ores (Chamberlin, 1981, Chamberlin, 1986). Producing robust agglomerates or pellets with desirable porosity and permeability is one of the major challenges to achieve efficient heap leaching operations. For variable, low grade nickel laterite ores, the formation of granules with various bulk properties, depending upon variables such as binder composition, moisture/binder content, agglomeration time and post-agglomeration drying conditions (temperature and time) is common place (Watling et al., 2010; Readett and Fox, 2010). A better understanding of the effect of these variables on the microstructure of agglomerates is crucial to control and optimize the granules bulk properties (i.e., density, porosity, stability and strength).
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Several quantitative, analytical techniques and methods including wax immersion, pyknometry, mercury porosimetry, uniaxial compression or tension tests, submerging in solution (i.e., soak test) and percolation flooded column test are commonly used to characterize the agglomerates bulk and mechanical properties (Pietsch, 1991; Fayed and Oten, 1997; Gotoh et al., 1997; Bika et al., 2001; Bouffard, 2005; Forsmo and Vuori, 2005; Forsmo et al., 2006; Forsmo et al., 2008; Lewandowski and Kawatra, 2008; Maeda et al., 2009; Turchiuli and Eduardo, 2009). Whilst a plethora of reported agglomeration studies focus on the effect of process variables on the products macroscopic properties, agglomerate micro-structural characterization studies are limited (Itatani et al. 2004; Ennis et al., 1991; Golchert et al., 2004). Fundamentally, the microstructure characteristics of the agglomerate reflect its porosity, permeability, compressive strength and robustness (Chawla et al., 2009). Advanced, high resolution X-ray micro, computer tomography (micro-CT) is a non-destructive technique commonly used for 3D and 2D visualization and quantitative analysis of powder metallurgy steels, multiphase mineral particles and agglomerates in mineral processing operations (Miller et al., 1990; Vagnon el al., 2006; Gimenez, et al., 2006; Grader et al., 2009; Miller and Lin, 2010). Unlike electron microscope, the samples can be scanned for X-ray microtomographs at normal atmospheric pressure and under controlled temperatures. This technique enables the development of a full model of the material (e.g., agglomerate) from millimeter down to nanometer scale and as a result, the material micro-structural properties (e.g., pores distribution and interconnectivity) and defects (e.g., cracks and voids) can be visualized at these length scales. Moreover, the structural parameters such as porosity and pores surface/volume are calculated either in direct 3D based on a surface-rendered volume model, or in 2D from individual binarised crossection images (Chawla et al., 2009; Pringle et al., 2009; Grader et al., 2009). Its use in mineral processing for the analysis of packed particle beds and 3D characterization of particle damage during breakage and constructing grade/recovery curves also have been reported (Miller and Lin, 2009). In such applications, accurate and consistent selection of region/volume of interest inside scanned objects is essential in obtaining accurate and meaningful data. In this paper, X-ray micro-CT analysis is used to probe real low grade nickel laterite mineral ores agglomeration behaviour (e.g., granule size growth mechanism) and products’ microstructure (e.g., macro-porosity, cracks and pores interconnectivity). It was also demonstrated that the permeability of agglomerates can be probed by utilizing a contrasting agent/medium such as CsCl (Shapiro, 1956; Tay et al., 2010; Ott et al. 2010) which creates enhanced contrast between the accessible and non-accessible spots/regions inside the agglomerates when irrigated by leach solution.
MATERIALS AND METHODS
Materials used and single agglomerate preparation The ~10 mm agglomerates used in this study (Figure 1) were produced from run-of-mine, polydispersed, low grade siliceous goethitic nickel laterite ore (~1.1 wt.% Ni) from Western Australia. The feed ore particles had a size range of 1 – 2000 µm (dry sieve analysis) where 85% of the mass was smaller than 600 µm. Quantitative X-ray powder
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diffraction and QEMSCAN analysis showed complex mineral associations where the quartz, goethite, nontronite and serpentine comprise the dominant and hematite, asbolane and kaolinite comprise the minor mineral phases (Table 1). The finer particles are associated with the friable or softer clay and Mg-silicates whilst the coarse fraction comprised hard minerals such as quartz. The binder used for drum agglomeration was 30% w/w H2SO4 solution at 20 wt.% of total feed mass. The details of agglomeration procedure are reported elsewhere (Nosrati et al., 2011). For micro-structure analysis, some agglomerates were scanned moist whilst others were oven dried for 24 h in 50 °C before scanning. Table 1: Mineralogical composition of nickel laterite feed ore used for agglomeration.
Mineral phase Mass %
Quartz 36.06
Kaolinite 0.21
Mg-bearing silicates 8.71
Nontronite (smectite group) 18.77
Goethite 27.43
Hematite 2.81
Asbolane 0.40
Total Nickel 1.1
LIO* 5.64
* Loss on ignition
X-ray micro-CT scans The X-ray tomography analysis was conducted on ~10 mm wet and dry agglomerates (Figure 1) using Skyscan 1072 high-resolution X-ray microtomograph (Skyscan, Belgium) equipped with 1024×1024 CCD detector (coupling to x-ray scintillator). Agglomerate were scanned at 100KV/98 µA power setting (source sample distance: 203 mm) in the 0 – 180 ° interval rotation using a 0.675 ° scan rotation step (20 s exposure time for each projection ) and images with 18.6 µm pixel size were produced. Cross-sectional images were reconstructed using Skyscan standard software.
Figure 1: Digital images of wet (A) and oven-dried (B) agglomerates used for micro-CT studies.
A B 10 mm
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RESULTS AND DISCUSSION
Visualization of agglomerate internal structure Figure 2 shows an example of micro-CT positive and negative slice images (created by image processing software) of a typical internal structure of a dry 10 mm nickel laterite granule. The darker spots/areas in the image appear where the sample mostly absorbs the X-ray whilst the brighter spots/areas appear where the sample mostly transmits the X-ray. Another feature which results in good contrast in the image is the fact that X-ray absorption is generally higher for elements with higher atomic number and also increases upon increasing density and thickness of the sample. These features lead to producing images from which valuable information can be gleaned. For instance, in positive slice image (Figure 2A), the distribution of mineral phases with higher average atomic number (i.e. Fe-rich mineral) is distinguishable from those with distinctly lower atomic number (i.e., Mg-silicates). On the other hand, the pore spaces and cracks (channels) are also clearly visible in negative slice image (Figure 2B). Furthermore, both the core-shell and conglomerated nature of the agglomerate structure is discernible in these images. It appears that the majority of pores and cracks develop in sub-agglomerates’ contact region. It is worth mentioning that 3D volume rendering of pore/channel networks and high-Z phase distribution inside agglomerate is possible via processing of 2D images by instrument’s CT-analyzer and CTVol programs. The resulting information can be closely-coupled and correlated with agglomerate mechanical and percolation studies data.
1 mm
Fe-rich
Pore spaces
Figure 2: Positive and negative “slice” images through CT scan of dry agglomerate.
Agglomeration behaviour The size enlargement phenomenon during agglomeration occurs via wetting, nucleation, growth, pseudo-layering and coalescence. The most dominant of these mechanisms define the granule structure characteristics as a function of time. The 2D projections of
A B
Core
Sub-agglomerates
Shell
Mg-silicate
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the four wet agglomerates produced at different times are presented in Figure 3. The results indicate that the agglomerates are irregularly-shaped at early stage of agglomeration process (2 and 8 min) compared with near spherical shape at longer times (> 14 min). After 8 min, an apparent “core-shell” structure developed possibly due to coalescence of two separate granules. The core appears more densely packed and drier than the shell which seems to be derived from finer material and is yet to consolidate to the same extent. After 14 min, uniform mass distribution and relatively smooth spherical shape and tumbled appearance emerges. Upon prolonged compaction and deformation which led to internal separation of material, development of macro-cracks and large pores (Figure 3D) occur.
Figure 3: The micro-CT images of the internal structure of wet nickel laterite granules after 2 (A), 8 (B), 14 (C) and 24 min (D) agglomeration time.
Spatial distribution of different mineral phases Images of spatial (3D) distribution of different mineral phases (with different density) within agglomerate are displayed in Figure 4. The images reveal that that: (i) large particles of medium to high density competent minerals (e.g., quartz and hematite) are randomly distributed, (ii) low density, fine particles (e.g., clays) dominate the granule’s internal part (core) most likely due to their more reactive nature and readiness to join the growing agglomerate,, (iii) particles with medium to high density dominate the
A B
C D
1 mm
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agglomerate’s outer layer (shell) possibly due to lesser reactivity and tendency to join the growing agglomerate. The results also suggest that the bulk structural properties of agglomerates are controlled mainly by bonding properties of finer low density particles.
Figure 4: Spatial distribution of low and medium to high density particles inside agglomerate formed at 14 min revealed by micro-CT “slice” image.
Agglomerate micro-structural changes: effect of drying Subtle morphological and structural changes are expected to occur during different stages of agglomeration and post-agglomeration treatment (e.g., drying, aging). These changes can significantly impact on individual agglomerates attributes, processability and overall heap leaching performance. Monitoring the agglomerate internal structure as a function agglomeration time and agglomerate dryness provides valuable information to optimize/improve their processability during heap leaching and increase value metal recovery. Figure 5 clearly shows that macro-pores and cracks develop inside agglomerate upon drying which leads to higher porosity. The dry agglomerates are observed to disintegrate dramatically faster than wet agglomerates upon submersion in acidic solution such as that used in heap leaching (Nosrati et al., 2011).
Low density particles
Medium to high density particles
A B
C D
1 mm
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Figure 5: 2D micro-CT images of the internal cross sectional area of wet (A) and oven-dried (B) agglomerates formed after 14 min.
Agglomerate permeability and porosity To investigate the permeability of dry agglomerate, it was partially wetted by dripping CsCl solution 10 min before micro-CT analysis. Whilst dilute CsCl solution (e.g., 0.3 – 1 M) did not provide noticeable contrast between accessible (i.e., wetted by solution) and non-accessible (dry) areas within agglomerate, clear contrast was achieved when higher concentration of CsCl (~7 M) was used (Figure 6). The results also revealed the high permeability of dry agglomerates upon contact with acidic leach solution which is indicative of high porosity. Finally, to better understand and visualize the porosity of dry agglomerate, a random volume of interest inside agglomerates was selected and analyzed on 3D basis. Figure 7 clearly shows the distribution of micro pores (blue) and solid phase (red) within agglomerate where good interconnectivity exists between the pores. It is worth mentioning that the data can also be used to accurately measure the volume percentages of micro-pores (porosity). This information has significant implications in evaluation and optimization of agglomerate bulk and micro-structural properties.
Figure 6: micro-CT slice image of the internal cross sectional area of partially wetted agglomerate with ~7 M CsCl solution.
Areas reached by leach solution
Areas not reached by leach solution
A B 1 mm
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Figure 7: The micro-CT 3D image showing the distribution of solid (red) and pore (blue) phases in selected volume of interest inside agglomerate.
CONCLUSION X-ray microtomography was used as a useful technique to investigate nickel laterite agglomeration behaviour and granule microstructural properties. The results revealed that:
• The granule growth during agglomeration process mainly proceeds via coalescence of separate large and smaller agglomerates leading to a core-shell structure.
• Low density, fine clay particles bond to growing agglomerate faster than those with medium to high density and dominate the granule’s internal part (core) whilst the latter slowly layer around the growing agglomerate and dominate its outer layer (shell).
• Majority of pores and cracks within agglomerate appear in the contact region of coalesced sub-agglomerates.
• Prolonged agglomeration time leads to separation of material within already formed agglomerates and hence, development of macro-cracks and large pores.
• Larger cracks and pores also develop in agglomerates upon drying. • 7 M CsCl solution is an appropriate contrasting medium to investigate the
agglomerate permeability. • High porosity and good pore interconnectivity observed for dry agglomerates lead
to substantially high porosity.
ACKNOWLEDGEMENTS Financial support provided under CSIRO Minerals Down Under Cluster project funding and technical assistance in gathering micro-CT scans by Ms Ruth Williams are gratefully acknowledged.
1.0 mm
0.8 mm
0.5 mm
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BRIEF BIOGRAPHY OF PRESENTER
Ataollah Nosrati obtained his BSc(Hons) and MSc degrees in Chemical Engineering from Petroleum University of Technology, Iran (1996) and Tarbiat Modares University, Iran (1999). He worked for several years in different industrial (anti-corrosion coating, automotive paint shops) and R&D (petrochemical) environments before commencing his PhD at University of South Australia in 2007. His PhD thesis that he completed in 2011, was titled “Interfacial chemistry, particle interactions and processability of aqueous muscovite clay mineral dispersions”. Ataollah joined the The Wark as a Research
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Associate in September 2010 and currently is part of CSRIO minerals down under flagship collaboration research project titled: Preconcentration and agglomeration to enhance heap leaching of nickel laterite.
