Paper No.192
LEACHING BEHAVIOUR AND PARTICLE INTERACTIONS IN URANIUM LATERITE DISPERSIONS: EFFECT OF ORE MINERALOGY AND TEMPERATURE A Nosrati1,*, J Maccarthy2, W Skinner3 and J Addai-Mensah4 ABSTRACT Due to complex mineralogical and chemical interactions of the value and gangue mineral phases present in uranium-containing lateritic ores, deleterious pulp gelation which impacts on value metal recovery sometimes occur during atmospheric leaching. In this study, the leaching behavior and particle interactions of selected oxide (hematite, quartz) and clay minerals (muscovite, chlorite), which constitute predominant host gangue phases of typical lateritic uranium ores are investigated to provide basic, mechanistic understanding of temporal pulp gelation phenomenon. Isothermal, batch leaching with H2SO4 was carried out on concentrated dispersions (30 – 57 wt.%) of single minerals at pH 1 and 25 or 70 °C and their concomitant solution, speciation and rheology (shear yield stress, viscosity) were characterized. It is clearly shown that despite the high oxide content of lateritic ores, the pulp chemistry and rheology is largely affected by the incongruent leaching behavior of more reactive, acidconsuming aluminosilicate clays. Furthermore, time- and temperature-dependent alumino-silicate species polycondensation interfacial reactions are shown to be responsible for the enhanced particle interactions, reflecting strong pulp rheology in agitation and pumping. If not properly mitigated, high pulp shear yield stress and viscosity have a striking impact on processability and hence, value metal recovery. Keywords: leaching, lateritic ores, rheology, pulp gelation, Si polycondensation
INTRODUCTION In the mining, and minerals industry, high acid consuming clay (e.g., muscovite and chlorite) and oxide (e.g., quartz and hematite) minerals constitute a non-valuable, gangue mineral when associated with complex, low grade lateritic ores containing metals of high economic value (e.g., nickel, uranium) (Macnaughton et al, 1999, 2000). During aqueous processing and solid-liquid separation operations of such ore dispersions (e.g., leaching, counter current decantation and dewatering), pulp processability and handleability sometimes become intractable due to the formation of a viscous gel (Willenbacher, 1996; Xu and Van Deventer, 2000; Michot et al, 2001; Nicolai and Cocard, 2001; Ferse et al, 2007; Labanda and Llorens, 2008). Considerable reduction in valuable metal recovery and significant losses to the tailings dam, increased energy and acid consumption during leaching, higher viscosity and pumping limitations, poor pulp dewaterability and supernatant filterability are some of the main issues associated with pulp gelation phenomenon(Whittington and Muir 2000; Neudorf and Huggins 2006; McDonald and Whittington 2008; Kyle 2010; Nosrati et al, 2011a). The problem, which is mainly linked to the gangue minerals-mediated interfacial changes and particle interactions, leads to significantly increased production costs and markedly reduced productivity. 1.Ian Wark Research Institute, University of South Australia, Mawson Lakes, South Australia, 5095. Email:
[email protected] 2.Ian Wark Research Institute, University of South Australia, Mawson Lakes, South Australia, 5095. Email:
[email protected] 3.Ian Wark Research Institute, University of South Australia, Mawson Lakes, South Australia, 5095. Email:
[email protected] 4.Ian Wark Research Institute, University of South Australia, Mawson Lakes, South Australia, 5095. Email:
[email protected]
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Several studies on sulphuric acid leaching of real uranium laterite and zinc silicate ores and single model mineral dispersions at elevated temperature have shown that the presence of alumina-silicate clay and oxide gangue minerals can result in high acid consumption, elevated concentrations of leached Si(IV) and Al(III) species in the supernatant and development of a highly viscous gel structure (Haque and Lalibete, 1987; Bodas, 1996; Macnaughton et al, 2000; Safarzadeh et al, 2007; Tan et al, 2012, Nosrati et al, 2011b). These studies suggest that the high concentration of leached Si (IV) and Al (III) plays a pivotal role in viscous gel formation, through polymerization reactions. For instance, the polymerization of dissolved silicic acid at low pH and high temperature (70°C) is reported during the acid leaching of silicate ores (Bodas, 1996). Taking into account the complex and diverse nature of low-grade lateritic ores, which is due to the presence of various clay and oxide gangue minerals phases in the ore, unraveling the real cause/s of pulp gelation during their atmospheric acid leaching becomes somehow challenging. Hence, a better understanding of the leaching kinetics, pulp chemistry and rheological behavior of single clay (muscovite, chlorite) and oxide (quartz, hematite) mineral dispersions, which constitute predominant host gangue phases of typical lateritic uranium ores, is necessary to (i) better understand the possible gelation mechanism/s, and (ii) device effective strategies to suppress or mitigate pulp gelation. In the present work, the leaching kinetics and rheological behavior of 30-57 wt. % solid clay (chlorite and muscovite) and oxide (quart and hematite) mineral dispersions were investigated as a function of leaching time (4 h) and temperature (25 and 70 °C) at pH 1. The interplay between pulp mineralogy/chemistry, temperature, supernatant speciation, solid loading and shear rheology were probed; and the observed gelation mechanism/s was discussed in terms of pulp temperature and Si (IV)-induced polymerization reactions.
EXPERIMENTAL METHODS Materials High purity minerals: muscovite (North Mineral Factory, China), chlorite (Ward's Natural Science Est. LLC, US), quartz (Aldrich, Australia) and hematite (Unimin, Australia) were used in this study. Table 1 lists their major oxide compositions determined by X-ray fluorescence (XRF). The 80th percentile particle sizes determined by laser diffraction (Malvern Mastersizer X, Malvern UK) were 13.3, 53.8, 48.9 and 45.6 µm for muscovite, chlorite, quartz and hematite, respectively. Table 1.Measured (XRF) major bulk oxide composition of the mineral samples. Major oxide
Hematite
Muscovite
Chlorite
Quartz
SiO2
2.41
47.00
25.9
99.50
TiO2
0.07
-
2.66
0.02
Al2O3
2.61
31.81
19.90
0.11
Fe2O3
92.14
3.60
28.90
0.02
MnO
-
0.02
0.10
-
MgO
0.14
0.76
12.60
0.02
CaO
0.27
0.11
0.66
0.01
K2O
-
10.00
0.29
-
Na2O
-
0.71
0.45
0.01
LOI*
2.30
6.02
7.69
0.35
Dispersions of 30 – 57 wt.%solid which are found in real plant leaching processes, were used for leaching/aging -3 experiments. They were prepared by adding known masses of dry particles to known masses of 10 M KNO3 solution used as background electrolyte. To study the effect of higher concentration of Si (IV) and Al (III) ions on leach slurry’s gelation behavior, solutions of 0.25 – 1.5 M Si (IV)/Al (III) were also used. The Si (IV)/Al (III) ions -3 were dissolved in 10 M KNO3 background solution at pH 1 before addition of particles. After addition of solids, the slurry pH was monitored and maintained at 1 during all aging tests. Si(IV)/Al(III) solutions (without added particles) were also used for aging tests at pH 1, and different temperatures to investigate these ions’ involvement
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in possible polymerization reactions leading to the supernatant and hence, pulp gelation. Analytical grade potassium nitrate, sodium metasilicatepentahydrate, aluminium nitrate (Chem-Supply Pty. Ltd., Australia) and sulfuric acid (98 wt.%) (ScharlauChemie, Australia) were used in solution preparation, Si (IV) and Al (III) concentration adjustment and slurry/solution pH modification. High purity Milli-Q water (specific conductivity < 0.5 -1 -1 µScm , surface tension at 20 °C of 72.8 mNm and pH of 5.6) was used in preparing all solutions and dispersions.
Experimental Leaching/aging behavior Dispersion leaching and solution aging behavior were isothermally investigated in a well sealed, acid-resistant, 2 3 dm , baffled, cylindrical borosilicate glass vessel stirred at 500 – 800 (±2) rpm by a central, 4-blade overhead impeller powered by a variable speed motor. The vessel, connected to an auto titration acid-base device (Metrohm, Switzerland) to monitor and control pH, was immersed in thermostatically controlled water bath which maintained a constant temperature of 25 or 70 (±1) ºC. Upon pH and temperature stabilization and Si(IV)/Al(III) concentration adjustment (approximately within 10 min) during leaching/aging tests, appropriate amounts of slurry or solution were periodically removed using calibrated syringes for rheological and supernatant speciation analyses. For slurry samples, the solid-supernatant separation was achieved by centrifugation at 12,000 rpm for 7 min. The resulting supernatants were analyzed by inductively coupled plasma (ICP) spectroscopy.
Rheological measurements For rheological studies in this work, shear stress versus shear rate curves and low shear yield stress values (< 20 Pa) of all dispersions were determined using a couette concentric rheometer (Haake RV1). This instrument consists of a moving bob of diameter 34 mm and length 51 mm which rotates in a fixed cup of diameter 36.88 mm (Z34 DIN 53019 Serie 1). To minimize slip effect and the uncertainties in data analysis and subsequent interpretation, the shearing surface of the bob was roughened. Furthermore, a lower weighting factor was -1 3 assigned to the data in the low shear rate (< 50 s ). For each measurement, 40 cm of dispersion was loaded in the cup and the bob lowered until the correct measurement gap was reached. All slurry samples were pre-1 -1 -1 sheared at 400 s for 30 s prior to measurement. The measurements started from 400 s , decreasing to 0 s and -1 then increased back to 400 s in 60 s, whilst the resulting shear rate and shear stress data were automatically recorded and subsequently analyzed by a computer interfaced with the instrument.The resulting flow curves which revealed the non-Newtonian behavior were fitted with a simple two-parameter Bingham plastic model (Eq. 1) to estimate low shear yield stresses by extrapolation of their linear part. (1) -
where (Pa) is the shear stress, (Pa) is the Bingham shear yield stress, (Pa.s) is the plastic viscosity and (s 1 ) is the shear rate. The higher shear yield stresses (> 20 Pa) were measured by the vane technique (Haake VT550). The advantage of this technique is the minimized slip between the sample and the instrument fixture, giving a more accurate yield value particularly for structured dispersions (Nguyen and Boger, 1983, Addai-1 Mensah and Ralston, 2004). In these measurements, the vane was rotated at constant slow rate ( = 0.021 s ) and the yield stress estimated from the maximum torque (T m) using the following equation: (2) where D and H are the vane diameter and height (m), respectively, and T m (N.m) and (Pa) are maximum torque and shear yield stress, respectively. The flow properties of aging supernatants were investigated (to probe the occurrence of polymerization reactions) by measuring their absolute viscosity as a function of time. Kinematic viscosities were determined using a BS/U-tube glass capillary viscometer (Canon instrument, USA) with a 2 2 calibration constant of 0.034 mm /s (mPa). Solutions with different concentrations (0.1 to 1.5 M) of dissolved Si (IV) and Al (III) were prepared at pH 1 and 25 or 70 °C using sodium metasilicatepentahydrate and
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aluminiumnitrate. The solutions were aged for 4 h and flow times were measured to within 0.01 s with a stopwatch and solution viscosity calculated at different aging times.
RESULTS AND DISCUSSION Rheological behavior: effect of mineralogy, temperature, time and solid loading The effect of pulp mineralogy, temperature and time on rheological behavior of slurries leaching at pH 1 is shown in Figs. 2 and 3. The lower solid loading (30 wt.%) used for muscovite dispersion, compared with 57 wt.% solid used for the other samples, was mainly due to its significantly finer particle size. At 25 °C, all dispersions exhibited Bingham plastic fluid behavior over a 4 h leaching period (Fig.2).
Figure 2. Shear stress versus shear rate curves (up branches) for high solid loading (A) muscovite, (B) chlorite, (C) quartz and (D) hematite dispersions in 10-3M KNO3 solution at pH 1 and 25 °C and different aging times. This behavior was accompanied by distinctly, pulp mineralogy-dependent, extrapolated yield stress values. The data also show that the initially greater extrapolated yield stresses of clay dispersions, compared with oxides, slightly decreased upon leaching time, whilst that of oxide slurries was generally time-independent. Figure 3 shows that the rheological strength of all dispersions was initially decreased at 70 °C. Upon leaching, muscovite, quartz and hematite dispersions displayed almost time-independent rheology in the course of 4 h (Figs. 3A, 3C and 3D). In contrast, chlorite dispersion started to thicken after 2 h and became a strong gel (impossible to be agitated with overhead stirrer) within the next 30 min (refer Figure 3B). Figure 4 clearly shows the dramatic and fast increase of pulp shear yield stress for 57 wt.% solid chlorite dispersion during gel formation period (as measured directly by vane method). The data also show the solid loading-dependency of pulp rheology and gel
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structural strength, where40 wt.%solid chlorite dispersion started to thicken after 4 h and led to slightly weaker gel compared with that of 57 wt.% solid dispersions.
Figure 3. Shear stress versus shear rate curves (up branches) for high solid loading (A) muscovite, (B) chlorite, (C) quartz and (D) hematite dispersions in 10-3 M KNO3 solution at pH 1 and 70 °C and different aging times.
Figure 4. Shear yield stress of 57 wt.% and 40 wt.% solid chlorite dispersions in 10-3 M KNO3 solution as a function of leaching time at pH 1 and 70 °C
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Leaching behavior To understand the influence of mineralogy, temperature and time on observed rheological behavior of clay and oxide dispersions leaching at pH 1, their corresponding pulp chemistries (i.e., leached ions’ concentration in slurry) are shown in Figs. 5 and 6.At 25 °C, incongruent leaching of major constituent elements (Mg, Fe, Al and Si + + from chlorite, Al Si, Fe and K from muscovite, Fe, Al, Mg and Si from hematite and Si, Mg and Na from quartz) albeit at different extents were observed for all dispersions (refer Figure 5). Chlorite appeared to be the most reactive mineral as it released significantly higher concentrations of elements into the supernatant upon 4 h leaching. The concentration of leached species from clay and oxide particles, reflecting their reactivity in acidic media, and their acid consumption capacity decreased in the order of chlorite, muscovite, hematite and quartz.
Figure 5. Concentration of leached ions as a function of leaching time for(A) muscovite, (B) chlorite, (C) quartz and (D) hematite dispersions in 10-3 M KNO3 solution at pH 1 and 25 °C. Figure 6 shows that the concentrations of species that leached out of chlorite and hematite as well as their acid consumption capacity dramatically increased at 70 °C (compared with 25 °C) whilst the leaching rate increase for muscovite and quartz was subtle. This is indicative of chlorite and hematite particles being more reactive at elevated temperature to leach in acidic media. The comparison of leached ions’ concentration data in Figs. 5 and 6 indicate a significant pulp mineralogy and temperature-dependent leaching behavior at pH 1. For clay samples, slower release of Si into solution compared with other elements (e.g., Mg, Al) may be attributed to the initial proton attack of the lattice aluminium-oxygen bonds which leads to fast release of the latter followed by the slower release of the Si(Chou and Wollast, 1984; Oelkers, 2001; Lowson et al, 2005; 2007; Tan et al, 2009). Significantly faster release of Fe from hematite at 70°Cis also attributed to its high reactivity at high temperature which is in good agreement with reported studies (Whittington and Muir, 2000).The highly refractory nature of quartz is also evidenced with its poor leaching performance.
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Figure 6. Concentration of leached ions as a function of aging time for (A) muscovite, (B) chlorite, (C) quartz and (D) hematite dispersions in 10-3 M KNO3 solution at pH 1 and 70 °C.
Pulp gelation mechanism and kinetics The results shown in Figures. 2-5 suggest that the dramatically higher concentrations of leached species, specifically Si, from chlorite particles at 70 ºC are responsible for gelation observed only for chlorite dispersion at 70 ºC. Low yield stresses exhibited by muscovite dispersion and the absence of gelation at both temperatures is mainly due to the lower concentration of leached Si species (Nosrati et al, 2011b). The link between pulp’s solution chemistry (speciation) and gelation is supported by the fact that only the supernatant sample obtained from chlorite leach slurry (at 70 ºC) turned into gel upon aging in a plastic container at ambient temperature (refer Figure 7). The observed supernatant gelation is attributed to slow, time-dependent, Si-mediated polycondensation reactions at ambient temperature. Hence, the same mechanism albeit with faster (accelerate) rate at 70 ºC may happen and lead to pulp gelation during leaching. To further investigate the effect of temperature, leach solution’s Si(IV) concentration and solid loading on pulp gelation, 30 wt.% solid muscovite dispersions prepared in 1 M Si(IV) solutions and were agitated (aged)for 4 h at pH 1 and 25 and 70 °C.At 25 °C, no shear yield stress enhancement or gelation was observed for muscovite dispersion in the course of 4 h aging and in the presence of 1 M dissolved Si.In contrast, significant gelation occurred for the same dispersion within 2 h when the temperature was increased to 70 °C (refer Figure 8A). Furthermore, increasing the suspension solid loading from 30 to 45 wt.%, in the presence of 1 M Si(IV), reduced the gelationinduction time to 24 h), this observation reveals very slow polymerization rate of monomeric Si (IV) at ambient temperature. At 70 ºC, however, the viscosity enhancement started after 2 h aging and led to a viscous gel formation within 4 h. This phenomenon was completely absent at the same conditions when lower Si (IV) concentrations (e.g., 0.25 or 0.5 M) were tested. The results indicate the central role of Si (IV) concentration in gelation and also accelerated polymerization rate of monomeric Si (IV) at elevated temperature. Furthermore, presence of Al (III) in solution showed synergistic effect on polymerization reactions and reduced the gelation induction time. The data in Figure 9 clearly show that by the addition of 0.5 M Al (III) to 1 M Si (IV) solution aging at pH 1 and 70 ºC, the viscosity enhancement started after 1 h and led to a complete gel formation within 3 h. The data also indicate that Si (IV) polymerization at pH 1 strongly depends on aging time, temperature and Si (IV)/Al (III) concentration.
CONCLUSIONS The influence of pulp mineralogy/chemistry, temperature (25 and 70 ºC) and solid loading on atmospheric acid leaching kinetics and rheological behavior of single clay (muscovite and chlorite) and oxide mineral (quartz and hematite)dispersions was studied. The results revealed that:
Leaching of all clays and oxides that proceeded through incongruent release of their constituent elements (e.g., Si, Al, Fe, Mg and K), was strongly temperature- and mineral composition-dependent. At 25 ºC, the clay minerals displayed faster leaching rates compared with the oxides. Elevated temperature led to enhanced leaching kinetics, the extent of which decreased in the order chlorite>hematite>muscovite>quartz.
Despite significantly different leaching behavior and kinetics observed for oxide samples, their dispersions exhibited similar, time independent rheological behavior with low yield stresses at 25 and 70ºC.
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At 25 ºC, both muscovite and chlorite dispersions displayed similar leaching trends (albeit with slightly faster release rates of ions for the latter), and time-dependent rheological behaviors as their yield stress attenuated upon 4 h leaching.Elevated temperature slightly enhanced the leaching kinetics of muscovite whilst decreased its viscosity. In contrast, the ions’ leaching rate from chlorite dramatically increased at 70 ºC leading to pulp gelation within 2.5 h.
High concentration of released Si species in leach slurry and their polycondensation reactions, which are accelerated at elevated temperature, seems to play a central role in silica-based pulp gelation. Variables such as slurry’s solid loading and concentration of other species (e.g., Al) also facilitate enhanced particle interactions.
During atmospheric acid leaching of complex, low grade lateritic ores, combination of (i) high content of reactive, acid-consuming clay and oxide phases, (ii) high slurry solid loading, and (iii) elevated temperature are conducive to enhanced pulp rheology.For gelling lateritic ores, decreasing (i) process temperature, and(ii) slurry solid loading, may help to prevent pulpgelation.
ACKNOWLEDGEMENTS The financial supports of Ian Wark Research Institute, University of South Australia, University of Mines and Technology, Ghana and Ghana Government are gratefully acknowledged.
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