Sulfuric Acid Baking and Leaching of Rare Earth Elements, Thorium and Phosphate from a Monazite Concentrate

Abstract

Monazite , a rare earth and thorium phosphate mineral, is one of the major minerals processed for extraction of rare earths . Industry practice for treating monazite concentrates is to use either a sulfuric acid bake or caustic conversion route. In the sulfuric acid bake, monazite concentrate is mixed with concentrated sulfuric acid and roasted. The rare earth phosphate mineral is converted to rare earth sulfate which dissolves in a subsequent water leach . As the bake temperature increases above 300 °C, thorium becomes less soluble. Although acid baking is practised in industry, the bake reactions are not well understood. A combination of chemical analysis, XRD and SEM -EDS was used to identify reaction processes occurring during sulfuric acid baking of a 93 wt% monazite concentrate between 200 °C and 800 °C. The effects of these reactions on the leachability of the rare earths , thorium and phosphate were also examined.

Introduction

Rare earth elements have received an increasing amount of attention in recent years due to their use in a large number of modern technological applications. Most of the world’s rare earth resources occur in the form of two minerals , namely monazite [(Ce,La,Th)PO₄], the subject of this study, and bastnasite [(Ce,La)CO ₃F] [1]. Commercial processing of monazite is either by sulfuric acid baking or caustic conversion, with sulfuric acid baking generally preferred for lower grade ores/concentrates [2]. Sulfuric acid baking is currently used by the world’s largest rare earth producer at Bayan Obo in China, for processing Baotou mixed bastnasite/monazite concentrate [3], and also by Lynas Corporation for processing monazite concentrate from Mt. Weld, Australia [4].

Sulfuric acid based treatment of monazite for recovery of rare earths and/or thorium has a long history, going back to the early-mid 1900s, and typically involves treatment with concentrated sulfuric acid at elevated temperature followed by leaching in water or dilute acid [5–7]. Early studies used temperatures of 200–250 °C to achieve virtually complete monazite decomposition [6–8]. Takeuchi [9] reacted monazite with sulfuric acid at temperatures ranging from 200 to 300 °C, and found that thorium extraction decreased if a temperature of 300 °C was used in combination with longer reaction times. This was attributed to formation of insoluble thorium pyrophosphate, although no experimental evidence for this species was provided. In the 1980s, a ‘high temperature ’ acid bake process using temperatures of 500–900 °C was developed for processing of mixed bastnasite/monazite concentrate from the Bayan Obo deposit in China. The high temperature process was reported to reduce extraction of thorium and phosphate, leading to the benefits of a simplified purification process [3]. This demonstrates that bake temperature has an important effect on the performance of the acid bake process; however, a fundamental understanding of the reaction processes driving these effects is lacking.

In this work, the effect of bake temperature on the sulfuric acid baking and leaching of a high grade monazite concentrate was examined in the temperature range of 200 to 800 °C. The focus of the study was characterisation of baked solids and leach residues using chemical analysis, XRD, and SEM -EDS . Extractions of rare earths and thorium in the leach were related to the different phases formed.

Materials and Methods

The monazite sample was a high grade concentrate obtained from an Australian placer type deposit. The elemental composition was determined by x-ray fluorescence spectroscopy (XRF) and digestion followed by ICP-MS. The sample was ground to a particle size of P₈₀ = 44 µm prior to acid bake tests. The acid addition was fixed at 1700 kg/t. For the 200 °C and 250 °C bakes, the sample/acid mixture was placed in a muffle furnace pre-heated to 100 °C, heated to the target temperature at 5 °C/min, held for 2 h, and directly removed from the furnace . For bake temperatures above 250 °C, the samples were first baked at 250 °C for 2 h before heating at 5 °C/min to the final target temperature and holding for 2 h. Baked samples were cooled and ground to a fine powder before leaching at a liquid to solid ratio of 40:1 (w/w) in 0.9 M sulfuric acid at 20–25 °C for 2 h. Solid-liquid separation after leaching was by vacuum filtration .

Liquors were analysed by inductively-coupled plasma optical emission spectroscopy (ICP-OES) and mass spectrometry (ICP-MS) using a Perkin Elmer Optima 5300DV instrument and a Perkin Elmer Elan 9000 instrument, respectively. A Rigaku PrimusII spectrometer was used for analysis of leach residues by XRF. Elemental mass balances were calculated to ensure reliability of the reported elemental extractions.

The baked samples and leach residues were characterised by a combination of XRF, x-ray diffraction (XRD) and scanning electron microscopy with energy dispersive spectrometry (SEM -EDS ). For SEM -EDS , samples were mounted in epoxy resin and analysed using a Quanta 650F electron microscope with dual Bruker XFlash 5030 energy dispersive detectors. XRD analyses were carried out using a Bruker D8 Advance diffractometer with a CuK radiation source.

Results and Discussion

Chemical Analysis and Characterisation of Sample

Table 1 shows the elemental composition of the starting material. Monazite was confirmed as the major phase in the sample by both XRD and SEM -EDS . Point measurements by EDS showed that calcium and silicon were substituted within the monazite mineral structure. Based on the elemental composition, the sample consisted of 93% (w/w) monazite , 5% (w/w) zircon (ZrSiO₄) and 2% (w/w) minor minerals .

Table 1

Elemental composition of monazite sample by XRF and digestion and ICP-MS^a

Elements	P	Si	Th	Zr	LRE^b	HRE^c + Y	TRE
% (w/w)	11.5	1.39	5.97	2.46	46.9	3.21	50.1

^aEu, Ho and Yb were analysed by digest and ICP-MS, all other elements were analysed by XRF

^bLRE: La, Ce, Pr, Nd;

^cHRE: Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu

Effect of Bake Temperature on Leaching

The effect of bake temperature on the dissolution of the rare earth elements , thorium , phosphate and sulfur during leaching is shown in Fig. 1. The sulfur dissolution was calculated with respect to the baked solids and does not include the sulfur volatilised in the bake, while the dissolutions of other elements were calculated with respect to the feed to the bake. The dissolution of rare earth elements was 90% at 200 °C, 96% at 250 °C and 99% at 300 °C, showing that virtually complete reaction of the monazite could be achieved.

../images/468727_1_En_197_Chapter/468727_1_En_197_Fig1_HTML.gif — Fig. 1
Effect of bake temperature on dissolution of major elements after acid baking and leaching of monazite

Between 250 °C and 300 °C the dissolution of thorium and phosphate decreased sharply, and by 500 °C, the rejection of phosphate and thorium to the leach residue was almost complete. For the rare earth elements , increasing the bake temperature above 300 °C resulted in a steady decrease in dissolution to a final value of 52–55% following baking at 800 °C. The dissolution of heavy rare earth elements was slightly more affected by bake temperature , particularly between 400 °C and 650 °C. The dissolution of sulfur from the baked solids was greater than ~ 90% for all bake temperatures.

XRD Analysis

The XRD patterns for baked samples from tests at the various bake temperatures are given in Fig. 2. Hydrated rare earth sulfates, e.g. Ce₂(SO₄)₃.xH₂O (x = 8, 7.5 and 5), were identified in the XRD patterns of solids baked at 200, 250, and 300 °C, showing that rare earth sulfates were formed from the sulfation reaction of monazite . Some hydration of the rare earth sulfates probably occurred during handling of the samples after baking, due to the hygroscopic nature of the low temperature baked samples. This would explain why the rare earth sulfates formed at 300 °C appeared to have incorporated more waters of hydration (x = 8) than the rare earth sulfates formed at 250 °C (x = 5). At 500 °C, anhydrous rare earth sulfates appeared and persisted to temperatures up to 800 °C.

../images/468727_1_En_197_Chapter/468727_1_En_197_Fig2_HTML.gif — Fig. 2
XRD patterns for monazite concentrate after acid baking between 200 °C and 800 °C

Monazite and rare earth trimetaphosphate (PrP₃O₉, a cyclic polyphosphate species) were identified in the sample after baking at 800°C (Fig. 2). These phases were also observed in the leach residue. The re-appearance of monazite after baking at 800 °C could not be due to the presence of unreacted monazite , as virtually complete reaction of monazite was demonstrated to occur in the first stage of baking at 250 °C, and must therefore be due to re-formation of monazite during baking. The formation of the above two species would appear to account for the sharp decrease in rare earth dissolution between 650 °C and 800 °C noted in Fig. 1. For a commercial process, this re-forming of monazite would preclude baking at very high temperatures.

SEM-EDS Analysis

The leach results in Fig. 1 show that the dissolution of thorium and phosphate from monazite after baking at 300 °C was limited to 65 and 21%, respectively, while 99% of the rare earth elements dissolved. Results from SEM -EDS analysis of the sample after acid baking at 300 °C showed that the rare earth elements were present as sulfates, while thorium was present in an amorphous phosphate type phase. In the leach residue, thorium was also present as a phosphate type species, but with a significantly different composition and appearance to the thorium phosphate phase in the baked sample (Fig. 3). It is reasonable to conclude that the thorium phosphate in the leach residue was a separate phase formed during the leach as a precipitate, rather than a reaction product formed during the bake. When examined under high magnification this phase was found to be composed of agglomerates of very fine particles, which is also more consistent with its formation as a precipitate rather than a product of baking. The average composition by EDS was indicative of thorium pyrophosphate (ThP₂O₇) with some substitution of silica . The presence of thorium pyrophosphate in the leach residue suggested that by 300 °C a significant portion of the orthophosphoric acid had been dehydrated to pyrophosphoric acid (H₄P₂O₇) in the bake via Eq. (1), which then reacted with thorium during leaching via Eq. (2).

../images/468727_1_En_197_Chapter/468727_1_En_197_Fig3_HTML.gif — Fig. 3
BSE image and EDS spectrum of thorium phosphate phase in residue after leaching 300 °C bake solids

$2 {\text{H}}_{ 3} {\text{PO}}_{4} \to {\text{H}}_{4} {\text{P}}_{2} {\text{O}}_{7} + {\text{H}}_{2} {\text{O(g)}}\; ( {\text{in}}\;{\text{bake,}}\;\Delta {\text{G}}_{\text{reaction}}^{^\circ } = \; - 5 3\;{\text{kJ}}/{\text{mol}}\;{\text{at}}\; 3 0 0\,\;^\circ {\text{C)}}$

(1)

${\text{H}}_{4} {\text{P}}_{2} {\text{O}}_{7} ( {\text{aq)}} + {\text{Th(SO}}_{4} )_{2} ( {\text{aq)}} \to {\text{ThP}}_{2} {\text{O}}_{7} ( {\text{s)}} + 2 {\text{H2}}_{2} {\text{SO}}_{4} ( {\text{aq)}}\; ( {\text{in}}\;{\text{leach,}}\;\Delta {\text{G}}_{\text{reaction}}^{^\circ } = - 6 7\;{\text{kJ}}/{\text{mol}}\;{\text{at}}\; 2 5\;^\circ {\text{C}}\; )$

(2)

Examination of the solids after baking at 400–500 °C showed that some rare earth elements were incorporated into an amorphous phase containing thorium and phosphorus, which explained the decrease in rare earth extraction after baking at higher temperatures (Fig. 1). The average atomic ratio of cations to phosphorus in this phase by EDS was indicative of a polyphosphate type species.

Analysis of the 800 °C baked sample by SEM -EDS showed that, in addition to the thorium polyphosphate type species formed at 400–500 °C, a new rare earth phosphate phase was present with a composition closely matching that of the original monazite . This supported the conclusion that the monazite identified by XRD was a product of reactions during the bake rather than unreacted remnants of the starting material.

Thermogravimetric and Differential Scanning Calorimetry (TG-DSC) Analysis

The thermogravimetric analysis of a monazite /sulfuric acid mixture with an acid addition of 1400 kg/t is presented in Fig. 4. The DSC curve shows four main endothermic events, each with an associated mass loss. The initial weight gain of 5.7% below 100 °C was attributed to uptake of water by sulfuric acid . The first endothermic event, at 123 °C, was attributed to evaporation of water and the reaction of monazite with sulfuric acid , generally accepted to occur via the reaction in Eq. (3) [10]:

../images/468727_1_En_197_Chapter/468727_1_En_197_Fig4_HTML.gif — Fig. 4
TG-DSC analysis of monazite with sulfuric acid in air (heating rate = 1 °C/min)

$2 {\text{LaPO}}_{4} + 3 {\text{H}}_{ 2} {\text{SO}}_{4} \to {\text{La}}_{2} ( {\text{SO}}_{4} )_{3} + 2 {\text{H}}_{3} {\text{PO}}_{4} \quad (\Delta {\text{G}}_{\text{reaction}}^{^\circ } = - 1 0 7\;{\text{kJ}}/{\text{mol}}\;{\text{at}}\; 2 0 0\;^\circ {\text{C)}}$

(3)

Although there is no directly associated mass loss, the dehydration of orthophosphoric acid is reportedly initiated, albeit slowly, from 100 °C [11].

A second endothermic event and mass loss occurred between 170 and 260 °C. The majority of the associated mass loss was attributed to decomposition of excess sulfuric acid via Eq. (4), for which the predicted theoretical mass loss was 19.3%. The third endothermic event, appearing to consist of two overlapping endotherms at 300–370 °C, was attributed to formation of the thorium polyphosphate phase identified by SEM -EDS in the 400 °C bake.

${\text{H}}_{ 2} {\text{SO}}_{4} ( {\text{l)}} \to {\text{SO}}_{3} ( {\text{g)}} + {\text{H}}_{2} {\text{O(g)}}$

(4)

The final thermal event was highly distended, occurring between 400 and 850 °C. On the basis of identification of monazite at 800 °C by XRD and SEM -EDS , this event may be largely due to the re-forming of monazite . Possible general reactions for this process are given by Eqs. (5) and (6). The polyphosphate species in these reactions (Th(PO₃)₄ and La(PO₃)₃) do not represent the actual polyphosphates formed in this study, which were amorphous and could not be fully characterised using the techniques available. These species are used here as a general representation of thorium and rare earth polyphosphate species, to illustrate the type of reaction proposed for re-forming of monazite . Further work is required to confirm the specific reactions.

$3 {\text{Th(PO}}_{3} )_{4} + 4 {\text{La}}_{2} ( {\text{SO}}_{4} )\to 8 {\text{LaPO}}_{4} + {\text{Th}}_{3} ( {\text{PO}}_{4} )+ 1 2 {\text{SO}}_{3} ( {\text{g)}}\; (\Delta {\text{G}}^\circ_{\text{reaction}} = - 9 0 0\;{\text{kJ}}/{\text{mol}}\;{\text{at}}\; 8 0 0\;^\circ {\text{C)}}$

(5)

${\text{La(PO}}_{3} )_{3} + {\text{La}}_{2} ( {\text{SO}}_{4} )_{3} \to 3 {\text{LaPO}}_{4} + 3 {\text{SO}}_{3} ( {\text{g)}}\; (\Delta {\text{G}}^\circ_{\text{reaction}} = - 1 4 4\;{\text{kJ}}/{\text{mol}}\;{\text{at}}\; 8 0 0\;^\circ {\text{C)}}$

(6)

Conclusions

The results of the present study have confirmed that bake temperature has a profound effect on the reaction processes occurring during sulfuric acid baking of monazite . The reaction of monazite with sulfuric acid to form soluble sulfates was practically complete at 250 °C, and was accompanied by >90% dissolution of the rare earths , phosphate, and thorium during leaching . Increasing the final bake temperature to 300 °C resulted in the formation of an insoluble, amorphous thorium phosphate type precipitate during leaching , while the dissolution of rare earth elements increased to 99%. Analysis of the leach residue by EDS was indicative of a thorium pyrophosphate. Further increasing the bake temperature to 400 °C resulted in formation of a rare earth containing thorium polyphosphate. Baking at 800 °C led to the re-formation of monazite , with a corresponding sharp decrease in the dissolution of rare earth elements . The re-forming of monazite would appear to preclude baking at very high temperatures in a commercial process.

Acknowledgements

This work was funded by an Australian Government Research Training Program (RTP) scholarship to John Demol for PhD research through Murdoch University and by ANSTO Minerals , and conducted at ANSTO.

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