Assessment of Lithium Pegmatite Ore Bodies to Determine Their Amenability to Processing for the Extraction of Lithium

Abstract

Various processes have been developed using a combination of elevated temperature and chemical treatment processing to recover Li from silicate minerals . To facilitate further process development, a comprehensive understanding of the deportment of Li and associated minerals in ore bodies is essential to allow the industry to predict the response of ore reserves to metallurgical treatment options. This paper describes results from the integrated use of the John de Laeter Centre’s state of the art analytical and mass spectrometry techniques to characterise a selection of Li bearing ore bodies and determine their amenability to potential processing options for the extraction of Li. The mineralogy , mineral associations, and liberation characteristics of ore-bearing and gangue minerals were characterised using a combination of the TIMA and XRPD studies. The Li content and distribution within minerals were defined using LA-ICPMS and field emission scanning electron microscopy techniques (EBSD, ToF-SIMS) and atomic probe microscopy.

Introduction

Granitic pegmatites are well known important source of rare metals including lithium , tin, tantalum , niobium , beryllium, cesium, rubidium, scandium , thorium , uranium , and rare earths [1]. Although granitic pegmatites are common and widespread, only approximately 0.1% are rare element pegmatites, with lithium -rich pegmatites only making up a fraction of rare metal pegmatites [2]. Australia’s economically defined lithium resource is estimated to date at 11.6 M tonnes occuring only within hard rock pegmatite deposits in Western Australia. The lithium minerals which are considered of commercial value are spodumene (6.0–7.5% Li₂O), petalite (3.5–4.5% Li₂O), and the Li-bearing micas (polylithionite, trilithionite, lepidolite , zinnwaldite , 2.0–7.7% Li₂O). While spodumene concentrates are produced from pegmatite deposits at Greenbushes, Mt Cattlin, and in the future at Pilgangoora, the processing of lithium -rich micaceous deposits has yet to be developed on a commercial scale in Australia. Lithium micas most commonly occur as mixtures in LCT Pegmatites and it is not possible to identify with certainty individual lithium mica species in the field.

Various processes have been developed and reported in the literature to recover Li from silicate minerals and some approaches are dependent on the mineralogy [3–7]. Processing of hard rock material normally requires physical beneficiation (grinding, sizing, flotation , gravity, reflux classifier and/or heavy media /magnetic separation ) to liberate and concentrate the Li-bearing minerals from gangue minerals . Lithium is extracted from concentrate usually by a combination of elevated temperature and chemical treatment processing (e.g. sulphation, carbonation, or chlorination roasting) to recover lithium from silicate minerals .

To facilitate further process development, a comprehensive understanding of the deportment of lithium and associated minerals in potential ore bodies and linking the mineralogy to the fundamental processes of minerals liberation and extraction is essential to allow the industry to predict the response of ore reserves to metallurgical treatment options.

This paper describes results from the integrated use of the John de Laeter Centre’s state of the art analytical and mass spectrometry techniques to characterise a selection of samples from Li bearing ore bodies and determine their amenability to potential processing options for the extraction of Li.

Methodology

Definition and Calibration of Mineral Constituents for Analysis

To quantify the different lithium bearing minerals in ores, the chemistry and structure characteristics of a suite of individual lithium mineral phases were examined prior to examining the ore material. Mineral specimens of different Li minerals were collected from sites around Western Australia and characterised using wet chemical and laser ablation ICPMS techniques to generate a mineral database. Minerals commonly found in LCT pegmatites and used as mineral reference specimens in this study, together with their average chemical composition, are listed in increasing Li content in Table 1.

Table 1

Common lithium -bearing and associated minerals found in pegmatites with their average chemical compositions

Mineral	Ideal formula	Source	Li	Na	Al	Si	K	Mn	Fe	Rb	F	Be	B	Mg	Ca	P	Ti	Zn	Nb	Cs
Mineral	Ideal formula	Source	%									ppm
Siderophyllite	KLiFe₂²⁺Al(AlSi₃O₁₀)(F,OH)₂	Unknown			8.3	15.5	7.2	0.4	18.6	0.7	0.2
Albite	NaAlSi₃O₈	Cocanarup (Ravensthorpe)	0.0	7.9	9.9	29.8	0.1					4	14	1.4	1024	786	1.2			2
Microcline	KAlSi₃O₈	Spargoville (Widgiemooltha)	0.0	0.6	9.5	29.7	13.1			0.5		3	11		113	656	1.2	0.8		197
Muscovite	KAl₂(AlSi₃O₁₀)(OH,F)₂	Mt Cattlin (Ravensthorpe)	0.15	0.4	16.9	21.1	8.0	0.2	1.9	1.5	0.7	16	337	495	1046	145	611	353	58	513
Biotite	K(Mg,Fe²⁺)₃(AlSi₃O₁₀)(OH,F)₂	Yinnietharra (Gascoyne)	0.18	0.2	11.6	19.4	8.5	1.4	36.8	0.9		5	2	1034	1349	39	5082	998	659	82
Beryl	Be₃Al₂Si₆O₁₈	Spargoville (Widgiemooltha)	0.19	0.7	9.5	29.7			0.4			4.6%		736			111	269		2010
Lithian Muscovite	KLiAl₂(AlSi₃O₁₀)(OH,F)₂	Cocanarup (Ravensthorpe)	0.57	1.0	14.7	21.0	7.5	0.4	0.2	1.5	1.3	8	127	546	639	61	17	237	78	523
“Zinnwaldite ”	Siderophyllite-polylithionite series	Cocanarup (Ravensthorpe)	0.90	0.3	14.3	19.0	7.6	1.1	0.7	1.2	4.6	19	189	94	0	77	198	1166	87	440
Trilithionite	K(Li_1.5Al_1.5)(AlSi₃O₁₀)(F,OH)₂	Londonderry (Coolgardie)	0.91	0.1	15.7	21.0	8.0	1.7	0.3	2.8	2.7	11	22	46	87	94	90	1148	39	2247
Elbaite	NaLi_1.5Al_1.5Al₆Si₆O₁₈(BO₃)₃(OH)₄	Mt Cattlin (Ravensthorpe)	1.06	1.5	23.7	19.1	0.01	0.1				18	3.5%	2	2985	173	3	3	3
“Lepidolite ”	Trilithionite-polylithionite series	Cocanarup (Ravensthorpe)	2.59	0.2	17.5	28.5	10.7	0.3	0.2	2.7	7.3	31	325	65		149	21	118	138	2608
Polylithionite	K(Li₂Al)(AlSi₃O₁₀)(F,OH)₂	Grosmont (Coolgardie)	3.35	0.3	14.4	28.3	10.6	0.5	0.1	1.4	9.6	75	11	90			847	44	174	1819
Petalite	LiAlSi₄O₁₀	Londonderry (Coolgardie)	2.09		8.7	36.7
Spodumene	LiAlSi₂O₆	Ravensthorpe	3.36	0.09	14.2	29.9	0.09	0.1	0.2			5	18	61	131		35	19		7
			3.52	0.06	14.0	29.8		0.04	1.5			1	10	6			21	3

Lepidolite represents solid solution series intergrowths between the Al-bearing micas of polylithionite and trilithionite, whereas zinnwaldite are the Fe bearing micas ranging from trilithionite—polylithionite series to siderophyllite [8]. Polylithionite represents the high-grade source of Li and trilithionite the low grade in tri-octahedral micas. The mica minerals also have high concentrations of Rb and F, with moderate concentrates of Cs. Li muscovite represent di-octahedral mica solid solution series between trilithionite and the pure Li poor muscovite end member.

Spodumene , with the highest Li and lowest impurity content, is a well-known source of Li. While not explicitly lithium minerals , other minerals found in LCT pegmatites such as biotite, topaz, beryl, and tourmalines also contain concentrations of Li. A solid solution series exists between the Fe containing schorl and Al containing rubellite tourmalines which can vary in Li content. Tourmaline and beryl are also high in B and Be respectively. This database was then used to define and search for the minerals in pegmatite samples. A study on the Li bearing micas have been described elsewhere [9].

Ore Samples

The location of sample taken from three pegmatite deposits in Western Australia are listed in Table 1. One sample represents a LCT-Complex spodumene -type pegmatite and the other two are classified as a LCT Complex Lepidolite pegmatite, having abundant lepidolite and high boron content in tourmaline [10].

Sample Preparation of Ores

The samples were first crushed to pass a 3.5 cm screen size. They were then subjected to electrodynamic fragmentation (using SelFrag™) and screened to pass a 4 mm stainless-steel sieve. The electrodynamic fragmentation technique preserves the original crystal morphology and shape, crystal structure , and physical and textural features, allowing for a study of the mineral in its natural form, which is often sacrificed during grinding and milling. Representative subsamples were split for detailed mineralogical evaluation and chemical analyses. Further subsamples were sieved through a series of standard screens to yield eight size distributions for further evaluation of mineral liberation characteristics. Polished section mounts of the sized fractions were prepared and studied separately to determine the relationship of gangue minerals to the lithium bearing silicates and localisation of lithium , as well as quantification of compositions of different ore -forming minerals .

Bulk Analyses

Chemical analyses on both mineral standards and ore samples were undertaken using a combination of standard mixed acid digestion—peroxide fusion techniques and ICP-MS or ICP-OES spectrometry to determine major elements (Ca, Fe, K, Al, Mg, Mn, P, S, Si, Ti, and Ba) and lithium contents using the services of Bureau Veritas Minerals Pty Ltd. X-ray powder diffraction studies were performed on pegmatite samples using Cu Kα radiation and a Bruker D8 Advance diffractometer with a graphite diffracted beam monochromator. The minerals in samples were identified using the search/match software Diffrac Eva with ICDD powder diffraction data base. The phase abundance was quantified by Rietveld refinement using Topas™ software (version 5.0, Bruker Advanced X-ray Solutions).

Analytical Approach

A combination of the TESCAN Integrated Mineral Analyser (TIMA), Tescan Lyra—FIB-SEM with Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were used to characterise the mineralogy . The TIMA is a high-resolution Field Emission Scanning Electron Microscope (FESEM) equipped with four fully integrated silicon drift Energy Dispersive Spectroscopy (EDS) detectors designed for automated mineralogy . The TIMA measures mineral abundance, size-by-size liberation , and mineral associations, and carries out searches for mineral phases automatically on multiple samples.

Lithium is not detected using standard energy dispersive X-ray spectrometers mounted on electron microscopes due to its low energy characteristic X-rays. Therefore, the detection of lithium in minerals was assessed by ToF-SIMS mounted on the Tescan Lyra—FIB FESEM. The Tescan Lyra—FIB FESEM has a high spatial resolution down to nanometres (Lateral resolution <50 nm, Depth resolution <20 nm, Detection limit <10 ppm) [11]. Surface profile mapping was performed using high resolution Ga + focused ion beam ablation by rastering over selected areas on the polished mount. The ablated surface is dispersed and analysed by ToF-SIMS to characterise the lithium content and distribution of other elements of interest (e.g. Fe, Mn, F).

Based on the mineralogy identified with the TIMA and ToF-SIMS analysis, selected grains were evaluated by LA-ICP-MS to quantify the concentrations of lithium and the distribution of associated elements. LA-ICP-MS analysis was carried out using a Resonetics Resolution 193 nm excimer laser ablation system coupled with an Agilent 7700 s quadrapole ICP-MS (Conditions: 75 μm diameter spot, frequency 7 Hz, fluence of 3.2 j/cm², 35 s ablation, 20 s baseline). Ablation was performed in an ultra-high purity He atmosphere, and the resulting aerosol was mixed with an Ar carrier gas before introduction to the ICP-MS (Table 2).

Table 2

Pegmatite deposits examined in this study from Western Australia

Deposit	Field	Location
Lepidolite Hill	LCT Complex Lepidolite pegmatite	15 km south of Coolgardie
Mount Day	LCT Complex Lepidolite pegmatite	440 km east of Perth
Pilgangoora	LCT-Complex spodumene -type	120 km from Port Hedland

Results and Discussion

Bulk Chemical Composition

The chemical composition of the main elements found in the three samples is shown in Table 3. The Li grade for Pilgangoora and Lepidolite Hill sample are 1.5 wt%, whereas the Mt Day sample is 1.4 wt%. Pilgangoora sample contains higher Ca, Be, Fe, Mg, Cs, and Zn contents, but lower Ta, Nb and Na contents relative to the micaceous samples. The F contents is low for the Pilgangoora sample (0.11 wt%), but higher for the micaceous samples (4 wt%). The Rb content is also high (>2 wt%) in Lepidolite Hill and Mt Day samples. The concentration of Nb and rare earths is low but the concentration of Ta observed in the Mount Day samples is high.

Table 3

Bulk chemical analysis of samples

Element	Majors				Element	Minors
	Units	Pilgangoora	Lepidolite hill	Mt Day		Units	Pilgangoora	Lepidolite Hill	Mt Day
Li	%	1.5	1.52	1.4	Rb	ppm	2700	22,000	26,800
Si	%	29.7	25.3	24.7	Mn	ppm	2020	4760	2260
Fe	%	0.12	0.06	0.05	Cs	ppm	233	2300	4560
Al	%	9.5	13.4	13.5	Be	ppm	585	89	16
Ca	%	1.4			Sn	ppm	220	365	190
Mg	%	0.2	0.01	0.05	P	ppm	150	150	150
Na	%	0.8	1.16	0.19	Ta	ppm	108	203	566
K	%	1.8	6.2	6.94	Zn	ppm	130	40	130
C	%	0.4			Nb	ppm	25.5	50	38.5
F	%	0.11	4.31	4	B	ppm		20	60

Mineral Abundance

The mineralogical composition of the samples acquired from TIMA and XRPD analysis is shown in Table 4. The mineralogical result for each sample from the TIMA mineral analysis and XRPD measurements are generally in good agreement. XRPD cannot distinguish accurately the difference between the different Li bearing micas. Therefore, the mica value shown for XRD presented in the table represents a combination of the different micas, whereas the TIMA mica data are characterised based on chemical signatures in the mineral classification scheme. The mineral mass of a phase by TIMA is calculated based on area % in polish sectioned mounts using an assumed density for each identified mineral.

Table 4

Modal mineral abundance from TIMA and XRD

Primary phases/mass of phase (%)	Pilgangoora		Lepidolite		Mt Day 2
Primary phases/mass of phase (%)	TIMA	XRD	TIMA	XRD	TIMA	XRD
Albite	9.94	7	9.97	19	0.44	4
K-Feldspar	6.27	5	0.01		3.99	10
Quartz	27.3	27	7.28	5	8.62	9
Spodumene	37.4	44	0.07	2	0.1
Lepidolite	4.79	8	58.4	74	64.2	75
Trilithionite	3.36		17.0		14.0
Polylithionite	0.21		3.36		4.4
Muscovite	2.70		1.02		2.09
Petalite	0.04		0.06		0.12
Elbaite/Cookeite	0.02		0.17		1.83
Topaz			0.19		0.11	1
Beryl	0.83		0.17		0.04
Anorthite	0.46				0.01
Calcite	2.52	<2
Columbite-Tantalite	0.04	3	0.01
Hematite/Magnetite	0.02		0.02
Cassiterite	0.01		0.01
Others	2.47				0.05
Total	98		100		100

The Pilgangoora sample is made up of predominately spodumene , quartz and feldspars. Minor contents of micaceous minerals and calcite are also evident. The beryl content reflects the high Be observed in Table 3. The Lepidolite Hill and Mt Day samples are predominately Li bearing micas also containing feldspars, quartz and muscovite as the main gangue minerals . Traces of spodumene are present. Mt Day also has a high content of Elbaite and K-feldspar. Lepidolite Hill has mainly albite and only traces of K-feldspar. The mineralogy reflects closely with the chemical composition observed in Table 3.

The accuracy and consistency of the derived data calculated by TIMA was compared with the whole rock bulk chemistry and observed to be in good agreement (not shown here).

Morphology and Texture of Particles

Mineral composition maps showing the cross section of Pilgangoora, Lepidolite Hill, and Mount Day grains is illustrated in Fig. 1. The morphology and texture of minerals in different samples varies considerably and the features observed are related to the minerals crystallising at different stages from aqueous fluids or magma, under changing conditions during the formation of the pegmatites, or post alteration processes. At the particle size of P₁₀₀ of 4 mm used to prepare these samples, many of the minerals are present as discrete liberated particles or attached to coarse gangue minerals such as quartz or feldspar.

../images/468727_1_En_190_Chapter/468727_1_En_190_Fig1_HTML.gif — Fig. 1
TIMA mineral composition maps of the +1400 µm size fraction particles showing a spodumene in Pilgangoora sample, b lepidolite intergrowths with quartz and feldspar in Lepidolite Hill sample and c lepidolite and in Mount Day sample

The Pilgangoora sample consists of spodumene associated with quartz , spodumene -quartz -feldspar intergrowths and spodumene with calcite, micas, quartz , and feldspar inclusions (Fig. 1a). The Lepidolite Hill sample is made up of fine complex intergrowths of Li bearing micas with different compositions (Fig. 1b). The Mount Day sample contains lepidolite interlocked with feldspar and quartz (Fig. 1c). The other Li bearing minerals trilithionite and polylithionite appear in low abundance as discrete phases. Micas in all samples show complex textural intergrowths and composition variations within mica particles. The lepidolite grains are mica composites made up of intergrowths of micas with varying Li composition ranging from polythionite to Li muscovites (see colour variation particles in Fig. 1b). Electron backscatter diffraction (EBSD) analysis of a lepidolite particle from Lepidolite Hill was undertaken to investigate the texture. The data showed that the mica grains ranged from coarse (250–500 µm) to fine (1–10 µm), had cleavage fracture lines within individual grains and the orientation of the grains were random, indicating random growth patterns of micas occurred during formation.

For the micaceous pegmatites, trace amounts of spodumene , petalite, zircon, iron oxides, and pyrochlore were measured. Fine grains of spodumene were associated with albite and mixed grain composites in the mica samples. Elbaite grains were associated with lepidolite , quartz , and albite grains. In the Mt Day sample, discrete grains of elbaite were also observed in the coarse fractions. Minor amounts of discreet grains of beryl, which contains the majority of Be in the samples, were exposed with some minor associations in albite and lepidolite grains. Quartz and feldspar grains were coarse with a high proportion moderately liberated.

Li Bearing Mineral Compositions

The chemical composition of Li bearing minerals were analysed by LA-ICP-MS.

Spodumene

The average chemical composition from the LA-ICPMS analysis of spodumene grains in the Pilgangoora sample is shown in Table 5. The spodumene in the Pilgangoora samples contained around 3 wt% Li (Li₂O 5.75 wt%), which is less than the maximum reported in the literature (Li₂O 8 wt%) [12]. The spodumene contain notable concentrations of Na, Mn and Fe. Some high concentrations of Na in secondary spodumene after petalite have been reported in the early literature [13]. Impurities of iron and manganese , which substitute for Al in the crystal structure yield yellow to green coloured and pink to lilac spodumene respectively. Some of the impurities detected (Rb, Cs, K) are associated with fine inclusions of other minerals within the spodumene grains which have been ablated along with spodumene .

Table 5

Average chemical composition of Pilgangoora sample from LA-ICPMS analysis

Major elements
Mineral	Li wt%	Al wt%	Si wt%	Na wt%	Mn wt%	Fe wt%
Spodumene	3.13	14.1	30.2	0.078	0.088	0.058
Minor elements
Mineral	K ppm	Mg ppm	Ca ppm	Ti ppm	Rb ppm	Cs ppm
Spodumene	63	16	26	2	5	1

Mica Compositions and Elemental Trends

Mica grains classified by TIMA were analysed by LA-ICPMS. The correlation of lithium with Al:Si ratio and major impurities in micas is shown in Fig. 2.

../images/468727_1_En_190_Chapter/468727_1_En_190_Fig2_HTML.gif — Fig. 2
Correlations between Li, Al/Si ratio, Na, Mn, Cs, and Rb from analyses of discrete mineral grains in the Lepidolite Hill sample

Figure 2 represent the compositional range set up in the mineral classification scheme for defining the different micas using the TIMA. The correlation between lithium and Al:Si ratio is apparent. Strong correlations between Li, Rb and Cs exist and have an inverse correlation with Na. An example showing the average elemental composition of mica grains from the lepidolite Hill sample are illustrated in Table 6. The F content obtained from EDS analysis and the Li values obtained from LA-ICPMS analysis showed strong correlations and confirmed Li’s association with F in micas.

Table 6

Average composition of micas in Lepidolite Hill sample from LA-ICPMS analysis

Major Elements
Mineral	Li %	Na %	Al %	Si %	K %	Mn %	Rb %	Cs %	F %
Muscovite	0.5	0.5	18.0	20.7	8.3	0.2	1.6	0.2	2.6
Trilithionite	1.3	0.4	15.4	21.1	7.7	0.6	2.4	0.2	4.3
Lepidolite	1.6	0.4	14.8	21.4	7.6	0.4	2.6	0.3	5.0
Polylithionite	2.3	0.2	13.0	23.0	7.9	1.1	2.7	0.6	8.0
Minor elements
Mineral	Fe ppm	B e ppm	Sn ppm	Mg	P ppm	Ti ppm	Zn ppm	Nb ppm
Muscovite	271	20	340	19	64.4	10.2	42.3	23.8
Trilithionite	117	19	382	40	69.6	18.9	73.8	39.5
Lepidolite	219	19	345	42	84.3	14.4	24.9	49.3
Polylithionite	343	15	212	55	66.1	17.9	117.8	41.0

The difference in Li bearing micas compositions have been discussed elsewhere [9]. Lepidolite represents solid solution series intergrowths between the Al-bearing micas of polylithionite (K(Li₂Al)(AlSi₃O₁₀)(F,OH)₂) and trilithionite K(Li_1.5Al_1.5)(AlSi₃O₁₀)(F,OH)₂ [7]. The muscovite is distinguished from the Li-bearing micas as containing little or no detectable Li, low F content, high Na content and a high Al/Si ratio in composition [9].

Assessment of Mineral Constituents Associated with Spodumene and Mica Grains

The compositional variation in particles at a higher magnification was examined in detail by ToF-SIMS analysis. The elemental spectrum maps for Pilgangoora and Lepidolite Hill samples are shown in Fig. 3.

../images/468727_1_En_190_Chapter/468727_1_En_190_Fig3_HTML.gif — Fig. 3
Mineral composition and elemental distribution maps from ToF-SIM analysis for a Pilgangoora sample showing element distributions associated with lepidolite (brown) and spodumene (olive green). Other minerals present are calcite (light pink), quartz (blue), feldspar (reddish pink) and muscovite (darker pink); and b Lepidolite Hill showing lithium associated with F, Cs, Mn and Rb rich zones. Li-poor zones are associated with high Na and Al regions (key to mineral composition maps: Pink—muscovite, Brown—lepidolite , Blue—quartz ) (Size of analysis area 50 x 50 µm)

The composition and elemental distribution of micas and calcite minerals associated with fractures and veinlets in spodumene grains is shown in Fig. 3a. The lepidolite veinlets and aggregates contain K, Rb, Cs, F, and moderate amounts of Mn, disseminated with Li, which are typical composition for Li bearing micas. Mg is also enriched and associated with Rb and Cs. The K, Mg, Cs, F, and Rb composition in lepidolite grains varies within the veinlets. The surrounding spodumene grains consist of Li, Al, and Si and are barren of the other elements. ToF-SIMS analysis of calcite veinlets detected F and Mg contents (not shown here). The lithium content measured by LA-ICP-MS analysis in of some trilithionite and lepidolite grains within spodumene was high at 1.3 and 1.86 wt% respectively.

In the lepidolite Hill sample, strong correlation of Li associated with F, Cs, and Rb rich zones are evident (Fig. 3b). Li-poor zones correlate with high Na and Al zones that are associated with the mineral muscovite. These compositional trends are in agreement with those observed in the mineral specimens and LA-ICP-MS data. Similar trends between chemical compositions were seen in the Mt Day sample.

Lithium Deportment

The Li deportment was calculated by evaluating the laser ablation data and relating the measured Li concentrations in discrete Li-containing minerals to the TIMA modal abundance of the minerals in the sample. Figure 4 shows that the majority of Li is associated with spodumene particles, and notable amounts of lepidolite , trilithionite and polylithionite in the Pilgangoora sample. The lithium in the Lepidolite Hill and Mt Day samples is predominately in the Li-bearing micas. Small amounts of elbaite and beryl make up the remaining Li content.

../images/468727_1_En_190_Chapter/468727_1_En_190_Fig4_HTML.gif — Fig. 4
The distribution of lithium in pegmatite minerals in all three samples

Mineral Liberation

To concentrate the Li bearing minerals it is desirable to remove the quartz , feldspar, and muscovite minerals to reduce their influence in the subsequent treatment process for extracting lithium .

Figure 5 illustrates the different degrees of locked and liberated mineral distributions by mass for spodumene and lepidolite in the three samples. The level of locking is expressed as a function of the surface area of a particular mineral which may be exposed to a leach solution. Mineral liberation expresses how much (or degree) of a mineral of interest is liberated from other minerals . The liberation of Li bearing minerals of interest were computed based on the surface area of mineral particles observed in polished mounts. The amount of liberation is expressed as the length fraction on the outer perimeter of the particle covered by the Li bearing mineral with respect to the whole outer perimeter of the particle as a percent. In this study the liberation classes were classified based on the surface area of the main Li bearing minerals as liberated (≥90%), mostly liberated (<90%, ≥70%), middling (<70%, ≥30%) or locked (<30%).

../images/468727_1_En_190_Chapter/468727_1_En_190_Fig5_HTML.gif — Fig. 5
Mineral locking **a, c, e** and liberation characteristics **b, d, f** for spodumene and lepidolite in the three samples for each size fraction. Mass of Li bearing mineral is related to the percentage in the whole sample and liberation is based on surface area of the Li bearing mineral particles (Liberated ≥90%; Mostly liberated <90%, ≥70%; Middling <70% , ≥30%; Locked <30%)

Spodumene in Pilgangoora Sample

At a P₁₀₀ of 4 mm, around 70% of spodumene grains have free, liberated surfaces not in contact with other minerals (Fig. 5a) and would make them accessible to flotation reagents for concentrating. The graph in Fig. 5a illustrates that most of the spodumene particles reside in the coarse fractions. Around 30% of spodumene grains are in contact with Li bearing micas, muscovite, calcite, feldspars and quartz (Fig. 5a).

While the Pilgangoora sample has spodumene particles with high surface exposure (70%), as evaluated from the locking assessment, liberation analysis of particles indicate a high proportion of the particles representing 47% of the spodumene fall into the middling class and 10% of particles fall into the locked class (Fig. 5b). The liberated and mostly liberated classes makeup 10 and 34% of spodumene particles respectively. The locked spodumene particles have mineral compositions consisting of 4 wt% spodumene associated mainly with quartz (57 wt%) and moderate amounts of feldspars (21 wt%) and micas (13 wt%). The spodumene particles in the middling class are made up of 70 wt% spodumene containing micas (12 wt%), calcite (7 wt%) and feldspars (3 wt%). The mostly liberated and liberated class are predominately spodumene (92 and 98 wt% respectively) with minor amounts of calcite, micas, quartz , and feldspars in the mostly liberated class and trace amounts in the liberated particles class. The majority of micas in the middling class are made up of lithium bearing micas of lepidolite composition and trilithionite. Muscovite, the lithium poor mica , is mainly present in the locked class. To liberate the spodumene in the middling class would require further particle size reduction to reject the mica , quartz and feldspar.

Lepidolite in Lepidolite Hill and Mt Day Samples

At a P₁₀₀ of 4 mm the majority of the lepidolite particles in the Lepidolite Hill have free surfaces not in contact with other minerals (Fig. 5c). Grains of the trilithionite and polylithionite were also liberated in the fine size fractions and free of major inclusions. With the coarser fractions, a proportion of the lithium minerals are still in contact with quartz , albite, and with the other micas.

The majority of lepidolite particles reside in liberated (56 wt%) and mostly liberated (30 wt%) classes with only a minor amount residing in the middlings (11 wt%) and locked (2 wt%) classes (Fig. 5d). All classes have a high abundance of lepidolite with the liberated and mostly liberated classes containing over 94% lepidolite . The locked and middlings classes contain albite (5–12 wt%) and quartz (10–17 wt%) associations and a minor muscovite content (1–2 wt%).

For the Mount Day sample, which have lepidolite grains intergrown with quartz and feldspar, much of the lepidolite grains surfaces are exposed ((Fig. 5e). However, the distribution of lepidolite is spread roughly a third between liberated, mostly liberated, and middlings classes (Fig. 5f). The lepidolite particles in the locked class, which represents only about 3 wt% of the sample, consists of high abundance of Li bearing micas at 62 wt%, associated mainly with quartz (15 wt%) and moderated amounts of feldspars (7 wt%). The lepidolite particles in the middling class have a high lepidolite and trilithionite content of 70 wt% and contain moderate quartz (8 wt%) and feldspars (6 wt%) contents and a minor muscovite (2 wt%) content. The mostly liberated class is rich in lepidolite at 90 wt% and contains minor concentrations of feldspar (1 wt%), quartz (2 wt%), and muscovite (2 wt%), whereas the liberated class consists of 98 wt% lepidolite with minor muscovite (1 wt%) and trace amounts of quartz and feldspar. To liberate the lepidolite in in the middling class and improve the yield in the mostly liberate class would require further particle size reduction to reject the mica , quartz and feldspar.

Grade-Recovery

Overall, high grade mineral recoveries in the high 90% can be expected for all three samples.

At 90% spodumene recovery the lithium grade is upgraded from 1.5 to 3.0 wt% (6.5 wt% Li₂O). To liberate spodumene grains associated with lepidolite and calcite grains in the middlings class would require a finer grind size of +125 μm. Grinding the Pilgangoora sample to +125 μm at 90% spodumene recovery would increase lithium grade to 3.15 wt% (6.8 wt% Li₂O).

A calculated theoretical grade-recovery for minerals lepidolite and combined trilithionite and polylithionite in Lepidolite Hill indicated that best results for major Li-bearing mineral recovery occurs in the sieve fraction −355 to +180 μm with the rejection of the gangue, which make up around 20% of the bulk sample. This would upgrade the Li grade from 1.52% to 1.75% (3.8 wt% Li₂O).

For the Mt Day sample, the best Li-bearing mineral recovery also occurs in the sieve fraction −355 to +180 μm with the rejection of the gangue, which make up around 22% of the bulk sample. This would upgrade the Li grade from 1.4% to 1.85% (4.0 wt% Li₂O).

Process Considerations

The upgrade of spodumene can be achieved by either or a combination of dense media separation and flotation techniques commonly used in the industry [14]. The coarse nature of liberated spodumene particles and differences in specific gravities between spodumene and feldspars, micas, and quartz minerals allow dense media separation approach to be used. The predicted upgrade of spodumene concentrate to around 6.0 wt% Li₂O in this study is consistent with expected grades in the concentrator being built at Pilgangoora [15]. Upgrading of spodumene particles further, which contain veinlets of calcite and lepidolite grains or feldspar and quartz observed in the middling classes, would require further grinding to liberate. The iron content (81–1475 ppm) in the spodumene is low and therefore make these spodumene concentrates suitable for use in ceramic and glass applications.

The removal and upgrade of Li-bearing micas can be achieved by flotation techniques [14] or reflux classifier as used by Galaxy Resources at Mt Cattlin at Ravensthorpe WA, Australia. The benefits of slightly upgrading the mica for treatment may be marginal, where direct leaching of micaceous pegmatite ore may be considered more economical.

However, the degree of how much gangue minerals that have to be rejected will be dependent upon the method used to extract lithium in the downstream processing.

The majority of the methods reported for treating Li-bearing mineral pegmatitic ores for the extraction of lithium involve high temperature (800–1500 °C) roasting in chloride , sulfate, or sulphuric acid media , leaching lithium in water and recovering lithium by precipitation , resin or solvent extraction approaches [5, 16–21]. Factors which need to be considered in treating Li bearing, based on mineralogical observations in the current study, are as follows:

The presence of disseminated feldspar minerals within some Li bearing mineral particles of the Pilgangoora and Mt Day samples will limit the roasting temperature used to decompose the Li bearing minerals . The decomposition of feldspars at elevated temperatures (≥∼1035 °C) causes fine grained particles to form large agglomerates due to the partial melting and reduces lithium yield in the downstream leaching process [22].
The Pilgangoora sample contains around 2.6 wt% calcite which will be consumed in any acidic process (∼25.6 kg/tonne of sulphuric acid).
At P₁₀₀ of 4 mm, the high exposed surfaces of Li minerals observed in the locking measurements, even in the coarse particles, indicate minerals are exposed to both flotation reagents used in the beneficiation process and in the chemical reagent contact for dissolution processes for lithium extraction .
The micas exist as intergrowths of different polytypes as identified by XRPD. Further work is required to understand the reactivity of the different micas under different processing conditions. The roasting processes require an effective mixing of reagents and ore prior to roasting. Particle feed size needs to be optimised to allows contact between reagent and mica grains.
Li content of mica ores is directly related to F content and the higher the Li grade the more F that will be processed. Hence hydrofluoric (HF) emissions require managing [16, 17]. From an environmental and safety concern, F will require stabilising and removal from waste streams. Calcium carbonate or lime have been used as additives to the roast to capture and remove F and so in this case the presence of calcite in the Pilgangoora may help facilitate fixing of some of the fluorine. Alternatively, the presence of fluorine minerals can be conditioned and used to decompose micas. The halogen based SiLeach™ process uses the addition of ground fluoride minerals followed by sulphuric acid to process slurry to generate F⁻ in solution and preferentially react with silicates without any accumulation of HF in the slurry [23].
The high concentration of Rb (0.9–3.6 wt%) and Cs (0.1–0.8 wt%) in micas make them a favourable resource for these elements and ultimately need to be recovered along with Li. There is limited discussion in the literature on the optimum conditions required in order to recover these elements.
The chemical breakdown of micas to liberate Li also results in the dissolution of Fe, Al, Mn, and monovalent ions K and Na which require stabilising and removal from recycle streams. In particular, Mn is an environmentally regulated element requiring removal to less than 1 mg/L before the treated liquid stream can be safely discharged to the environment .

Conclusions

A combination of analytical microscopy and mass spectrometry techniques were used to characterise samples from the Pilgangoora, Lepidolite Hill and Mt Day pegmatite deposits in Western Australia, Australia.

Based on mineralogical observations and TIMA analysis the majority of the main gangue minerals , quartz , feldspar and albite can be rejected at a coarse grind size (−4 mm in this study), to recover 90% of the spodumene with Li upgrade from 1.5 to 3.15 wt% (6.8 wt% Li₂O). The iron content (580 ppm) in the spodumene is low and therefore make these spodumene concentrates suitable for use in ceramic and glass applications.

The benefits of upgrading the mica pegmatite for treatment may be marginal. The micas can be classified and grouped based on their compositions (Al/Si ratio; F, Na content) and used to distinguish different micas with different lithium grades. Micas exist as different polymorphs that are generally related to composition and also geological environment . Lithium extraction processes will have to consider the influence of the liberation and treatment of F, Fe, Al and Mn from both a safety and an environmental point of view. The recovery of Rb and Cs in the process will also have to be considered in process development.

Acknowledgements

My colleagues who assisted in the experiments (Kelly Merigot, William Rickard, Noreen Evans and Bradley McDonald). This study was supported by Lithium Australia NL. The Tescan Integrated Mineral Analysis (TIMA) instrument was funded by a grant from the Australian Research Council (LE140100150) and is operated by the John de Laeter Centre at Curtin University with the support of the Geological Survey of Western Australia, University of Western Australia and Murdoch University. The use of the Tescan Lyra was supported by the Science and Industry Endowment Fund (SIEF).

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