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Influence of Mineral Chemistry on Electrokinetic and Rheological Behavior of Aqueous Muscovite Dispersions Ataollah Nosrati,* Jonas Addai-Mensah, and William Skinner Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia ABSTRACT: The dispersion electrokinetic behavior and rheology of two mineralogically similar muscovites with subtle differences in their bulk chemistry (low-Fe versus high-Fe substitution) were investigated as functions of pH and temperature. At 25 °C, the zeta potentials of both samples were quite similar for dilute dispersions with a common isoelectric point (iep) at pH ≈ 2. For concentrated dispersions, the high-Fe-substituted particles displayed lower zeta potential and higher iep values than the low-Fesubstituted particles. The incongruent leaching of Fe(III), Al(III), Si(IV), and K+ ions from muscovite particles was facilitated by both higher Fe substitution and lower pH. Temperature elevation to 70 °C marginally enhanced the cations’ leaching rates but suppressed Si(IV) release. The yield stresses of the two dispersions were similar in the pH ranges of 49 and 19 at 25 and 70 °C, respectively, and notably characteristic of dispersed sols/weak gels. At 25 °C and pH < 3, however, the high-Fe-substituted dispersion displayed an unusually strong gel structure. In contrast, the low-Fe-substituted dispersion remained dispersed. These observations reflect the different pH- and temperature-mediated muscovite-mineral-specific leaching and pulp chemistry behaviors that prevailed under the investigated conditions.
1. INTRODUCTION The behavior of muscovite, which commonly coexists as a gangue mineral with valuable, base-metal (e.g., copper, nickel, lead, gold) ores in hydrometallurgical operations, is of high interest. Aqueous processing (i.e., leaching, countercurrent decantation, and thickening) of mineral ores rich in reactive clays can often cause intractable handling difficulties.14 A better understanding of the clay gangue (e.g., muscovite) mineral pulp chemical, electrokinetic, and rheological behaviors is essential for the formulation of effective strategies underpinning enhanced processability and increased capacity or production. Muscovite [ideally, KAl2(AlSi3O10)(OH)2], sometimes referred to as sericite, belongs to the mica group of clay minerals with a 2:1 structure. The crystal structure comprises an AlOAl octahedral (O) layer sandwiched between two SiOAl tetrahedral (T) layers. Substitutions of lattice Si4+ by Al3+ in the tetrahedral layer and of Fe3+ or Mg2+ and Ca2+ for Al3+ in the octahedral layer, which invariably occur, result in a permanent net negative charge on the basal surfaces. The isomorphous substitution of Al(III) by Fe(II/III) or Mg(II) can also lead to diverse Fe/(Fe + Al + Mg) ratios and, hence, a variety of mineralogically similar but chemically altered analogues of these mica group minerals. The aluminol (AlOH) and silanol (SiOH) groups exposed at the particle edge faces can protonate or deprotonate, depending on pH, and lead to an anisotropic surface charge.5,6 The bulk and surface chemistries of mineralogically similar muscovite ores can show dependencies on the type and amount of substituted or incorporated lattice elements, reflecting markedly different electrokinetic behaviors and particle interactions.713 Powder X-ray diffraction (XRD) analysis is conventionally used to provide reasonably accurate crystallochemical information on the mineralogy and crystallinity of ore samples. The technique, however, is not diagnostic enough for detecting small or subtle chemical changes in the mineral's bulk chemistry and surface structure that can have a significant impact on leaching Published 2011 by the American Chemical Society
behavior and particle interactions in aqueous media. For some mineralogically similar oxide and clay minerals, the influential role in the electrokinetic and rheological behaviors played by pulp chemistries has been highlighted.1419 For instance, Leong and Boger16 showed that the different rheological behaviors of two types of brown coal dispersions can be directly ascribed to their surface charge densities and ionic strengths. Different rheological and electrokinetic behaviors for two mineralogically similar kaolinite clays that displayed different edge face surface chemistries have also been observed.19 Despite recent rheological investigations of concentrated muscovite dispersions,13,20 there is a paucity of knowledge about the specific influence of mineral chemistry on the clay mineral’s electrokinetic and rheological behaviors. In this study, the particle zeta potentials and rheologies of low-Fe- and high-Fe-substituted muscovite clay mineral dispersions (0.00157 wt % solid) were investigated under well-defined aqueous solution conditions: pH 19 at 25 °C and 103 M KNO3 background electrolyte. Dispersion shear rheology, reflecting pulp chemistry-mediated particle interactions, was probed through the shear yield stress as a function of temperature (25 and 70 °C) in tandem with supernatant speciation analyses. The links between mineral-specific chemistry, leaching, and electrokinetic and rheological behaviors as functions of pH and temperature were established.
2. EXPERIMENTAL METHODS 2.1. Materials. Two high-purity (>99%) polydisperse muscovite minerals were used in this study as received. They were characterized and are referred to as low-Fe-substituted muscovite (North Mineral Factory, Hebei, Shijiazhuang, China) and Received: July 20, 2010 Accepted: September 6, 2011 Revised: September 1, 2011 Published: September 06, 2011 11087
dx.doi.org/10.1021/ie101548f | Ind. Eng. Chem. Res. 2011, 50, 11087–11096
Industrial & Engineering Chemistry Research
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Figure 1. (A) Particle size distributions and SEM images of as-received (B) low-Fe-substituted and (C) high-Fe-substituted muscovite particles.
high-Fe-substituted muscovite (Geological Specimen Supplies, Brisbane, Queensland, Australia). The particles’ specific surface areas measured by the BrunauerEmmettTeller (BET) method21 were 17.65 and 12.7 m2/g for the low-Fe and highFe samples, respectively. The 10th-, 50th-, and 90th-percentile particles size (D10, D50, and D90, respectively), determined by laser diffraction (Malvern Mastersizer X, Malvern Instruments, Malvern, U.K.), were 1, 4, and 20 μm, respectively, for the low-Fe sample and 3, 30, and 140 μm, respectively, for the high-Fe sample. The high BET specific surface areas reflect the high micro-/mesoporosities of the muscovite mineral particles used, which is characteristic of layered clay minerals.2,3 Moreover, the higher specific surface areas observed for the low-Fe-substituted muscovite particles are attributed to their smaller size and higher porosity. Scanning electron microscopy (SEM) imaging revealed their polydispersity, granular sizes, and quite similar platy morphologies (Figure 1). Table 1 lists the major oxide compositions as determined by X-ray fluorescence (XRF) for both samples. XRD analysis confirmed their mineralogical similarities, and electron microprobe (CAMECA SX51) analysis revealed their different crystallochemical compositions as follows: low-Fe muscovite K 0:90 Na0:08 ½ðAl1:73 Fe0:26 Mg0:01 ÞðAl0:91 Si3:09 O10 ÞðOHÞ1:97 ; F0:03
ð1Þ
high-Fe muscovite K 0:89 Na0:11 ½ðAl1:26 Fe0:68 Mg0:06 ÞðAl1:04 Si2:93 O10 ÞðOHÞ1:8 ; F0:2
ð2Þ
Dispersions with different solid contents were used for zeta potential measurements and rheology investigations as deemed
Table 1. Measured (XRF) Major Bulk Oxide Compositions of the Low-Fe- And High-Fe-Substituted Muscovite Samples
*
major oxide
low-Fe muscovite (% oxide)
high-Fe muscovite (% oxide)
SiO2
47.00
44.70
Al2O3
31.80
32.20
K2O
10.00
10.10
Fe2O3 MnO
3.60 0.02
5.33 0.01
MgO
0.76
0.66
CaO
0.10
0.07
TiO2
0.62
Na2O
0.70
0.58
LIO*
6.02
5.73
Loss on ignition.
appropriate. The adequacy of the dispersion solid loading for rheological measurements was verified by particle zeta potential analysis and determination of the solid concentration by isokinetic withdrawal of slurry samples from different zones of the suspension agitated at 600 rpm. The high magnitude of the zeta potential (>|20 mV|) and the agitation rate ensured not only complete homogeneity of the suspensions, but also good dispersion of the particles with no concentration gradients or aggregation effects. The slurries were prepared by adding a known mass of dry muscovite particles to a known mass of 103 M KNO3 solution used as background electrolyte. Analytical reagent grade potassium nitrate (KNO3), potassium hydroxide solution (1.0 M KOH) (both Chem-Supply Pty. Ltd., Gillman, SA, Australia), and nitric acid solution (1.0 M HNO3) (Scharlau Chemie, Gillman, SA, Australia) were used in solution preparation and pH modification. 11088
dx.doi.org/10.1021/ie101548f |Ind. Eng. Chem. Res. 2011, 50, 11087–11096
Industrial & Engineering Chemistry Research Dispersions were homogenized at their natural pH (∼9) by mixing for 10 min using an overhead stirrer at 600 rpm before alteration of the pH. High-purity Milli-Q water (specific conductivity, 102 M). Fresh 8, 30, and 57 wt % solid dispersions in 1 103 M KNO3 electrolyte were used with the initial pH decreased from high (∼9) to low (∼2) using 1 M HNO3 solution, with the zeta potential measured at different pH values. Depending on the flow behavior of the suspension, agitation rates between 400 and 800 rpm were used. Dilute muscovite dispersions, 103 wt % solid in 1 103 M KNO3 solution, were used for zeta potential measurements to minimize metal-ion leaching/specific adsorption effects during the pH sweep.7,20 A Nano-ZS Zetasizer apparatus in electrophoretic light scattering mode (Malvern Instruments, Malvern, U.K.) was employed for the measurements. To prepare the 103 wt % muscovite dispersions, first 200 cm3 of dilute 0.1 wt % suspension was made in 1 103 M KNO3 electrolyte at pH 5.5. This suspension was allowed to stand quiescently over 2 h to enable free settling of the coarse particles (>5 μm). Thereafter, a portion of the supernatant containing suspended colloidal particles was carefully siphoned off and made up to 100 cm3 of 0.001 wt % solid using 1 103 M KNO3. For each new pH value (adjusted by adding 0.1 M HNO3 or KOH), the suspension was allowed to equilibrate for 5 min prior to injection of the supernatant into a disposable capillary cell for zeta potential determination. 2.3. Investigation of the Effects of Dispersion Temperature and pH. The effects of temperature and pH on the shear rheology and leaching behavior of fresh and aging dispersions were isothermally investigated in a 2-dm3 well-sealed, acidresistant, baffled, cylindrical borosilicate glass vessel stirred at 500800 ((2) rpm by a central, 50-mm, 45°-pitch, four-blade overhead impeller powered by a variable-speed motor. The vessel, connected to an autotitration acidbase device (Metrohm, Herisau, Switzerland) to monitor and control pH, was immersed in a thermostatically controlled water bath maintained at a constant temperature of 25 or 70 ((1) °C. Upon stabilization of the dispersion’s pH and temperature (within approximately 5 min) during various tests, appropriate amounts of slurry were periodically removed using calibrated syringes for rheological and supernatant speciation analyses at each pH. Solid supernatant separation was achieved by centrifugation of slurry samples at 12000 rpm for 7 min. The resulting supernatants were filtered through a 0.2-μm membrane to remove possible suspended particles and were then analyzed by inductively coupled plasma (ICP) spectroscopy. To ensure good reproducibility of the results, each experimental run and each rheological measurement was replicated at least three times, and the pure errors were determined and reported at the 95% confidence interval.
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2.4. Rheological Measurements. The rheological characterization of concentrated suspensions with devices such as Couette concentric viscometers (and rheometers) can face the problem of wall slip, especially at low applied shear rates.23 The slip effect typically manifests itself in different observed viscosities for different-sized geometries or underestimated apparent viscosities or lower Newtonian plateaus at low shear rates. To minimize slip effects and uncertainties in data analysis and subsequent interpretation, the walls of the concentric vessel were roughened, and a low weighting factor was assigned to the data in the low-shearrate (
