The critical size of the cluster (nc) fixes the free energy change in this region, ∆G* (Moore, 1972). A low-temperature kinetic study of the dissolution of pentlandite from the monosulfide solid solution using a refined Avrami method.
Introduction 1
- Overview 1
- Rate Laws 1
- Transition State Theory 5
- Diffusion 8
- Nucleation 11
- Crystal Growth 15
- References 18
Acknowledgments - The authors would like to thank Barbara Etschmann and Andrew Putnis for assistance with data collection and valuable advice. A Kinetic Study of the Dissolution of Pentlandite (Ni,Fe)9S8 from a Monosulfide Solid Solution (Fe,Ni)S.Am. The intermediate and end products of the oxidation depend on the stoichiometry of mss.
9b shows the time function of the reaction rate (dy/dt) for both samples (the S-rich and non-S-rich). According to the "Transition State Theory", the activation energy required to form an activated complex (an intermediate species between reactant and product) depends on the chemical environment around the reactant atoms.
Mineralogy of the Fe-Ni-S and Ni-S systems 21
Introduction 21
Pyrrhotite Group Minerals 21
- Overview 21
- Polymorphs 22
- Binary Phase Diagram 29
- Ternary Phase Diagram 34
It is shown that the α-transition temperature depends on the starting stoichiometry of the troilite (Fe1-xS, x = 0~0.05). When the temperature decreases, the vacancies begin to organize and the superstructure of the NiAs subcell is formed (Nakazawa and Morimoto, 1975). The superstructure of the NiAs subcell, except 4C and 2C, contains nonintegral X-ray reflections attributed to the intergrowth of differently ordered domains (Pierce and Buseck, 1974).
Some difficulties have also occurred in determining the phase solvus at high temperatures, as some high temperature phases cannot be quenched for room temperature measurements. Kullerud (1963) showed that above 1083oC an immiscible liquid field spans the sulfur-rich part of the ternary system with a centrally located homogeneous liquid field (Figure 2.5a) (Kullerud, 1963).
Nickel Monosulfides 36
Many researchers believe there is no crystal structure modification of the α phase during the metal→semiconductor transition. Due to the higher compressibility of β-NiS (millerite) compared to that of the α-phase, the. The coordination of Ni changes from 5 (in β-NiS structure) to 6 (in α-NiS structure) during the β- to α-phase transition (shown in Figure 2.6).
Since the compositional range of the α-phase is wider than that of the β-phase, the equilibrium compositions of the α- and β-phases depend on whether the transition is from the α- to β-phase or from the β- to α-phase. . The present study attempts to reconcile the ambiguities regarding the stoichiometric effect on the transition kinetics of the α- to β- phase transition at ambient pressure.
The desulfurization and the release of SO2 are responsible for the induction phase of the oxidation[4]. A higher metal content retards the oxidation of the metal sulfide, but facilitates the exsolution of pentlandite from the MSS host [15,48,50]. Comparison of the theoretical and experimental weight fraction of hematite after the oxidation of MSS reached equilibrium.
The first successful implementation of the refined Avrami method was achieved in the study of the kinetics of pentlandite dissolution (from mss host). Another demonstration of the applicability of the refined Avrami method is illustrated by the study of the oxidation of mss, (Fe,Ni)1-xS.
Experimental Methods 48
Introduction 48
Sample preparation 48
The experimental methods used in this work involve sample synthesis, annealing-quenching processes, phase characterization, Rietveld refinement of the XRD profiles, collection of kinetic data and cell parameters for each phase, and compositional analysis. Kinetic studies were performed on solid-state reactions in the Fe-Ni-S and the Ni-S systems. The use of synthetic rather than natural samples allows a choice of bulk compositions and reduces the uncertainty associated with the effects of trace impurities.
Depending on the reaction studied, different annealing/heating schemes have been adopted (details are available in Chapters 4-7). The input is a welding torch used to melt and seal glass tubes under vacuum.
X-ray diffraction and Rietveld refinement 50
NOTE: The statement of authorship appears on the hard copy of the thesis held in the University of Adelaide Library. The activation energy (Ea) decreases from 49.6 to 20.7 kJ mol⫺1 during the course of the reaction. Crystal chemistry and magnetic properties of iron in monosulfide solid solution of the system Fe-Ni-S.Am.
NOTE: The statement of co-authorship appears in the printed copy of the dissertation in the University of Adelaide library. Any variation of the stoichiometry of mss during the course of oxidation will be reflected in changes in the cell parameters. The oxidation of sulfur content in MSS is an exothermic process and is important in flash melting.
NOTE: This edition is included on pages 156 - 160 in the hard copy of the thesis held in the University of Adelaide Library.
Composition Analysis and Surface Morphology 52
This may be caused by the effect of sintering of the finely ground mss (Fe7.9S8) powder during the oxidation. The activation energies of the oxidation for Fe6.4Ni1.6S8 and Fe6.15Ni1.54S8 increase steadily during the oxidation. NOTE: This publication is included on pages 127 - 138 of the printed copy of the thesis held at the University of Adelaide Library.
NOTE: This edition is included on pages 139 – 143 in the hard copy of the thesis held in the University of Adelaide Library. NOTE: This edition is included on pages 144 – 148 in the hard copy of the thesis held in the University of Adelaide Library. NOTE: This edition is included on pages 149 - 155 in the hard copy of the thesis held in the University of Adelaide Library.
NOTE: This edition is included on pages 161 – 166 in the hard copy of the thesis held in the University of Adelaide Library.
A low-temperature kinetic study of the exsolution of pentlandite from the monosulfide
Experimental 57
- Synthesis 57
- X-ray Diffraction 58
- Chemical Analysis 58
- The Kinetic Model 59
- Kinetic Analysis Using the Refined Avrami Method 61
- The Effects of S Fugacity on the Kinetics of Exsolution 65
Note that the cell parameters for pentlandite did not change significantly during the course of the reaction, indicating that its composition remains the same. At 473 K the reaction is less rapid and the pentlandite fraction increases steadily to 33 wt% over 24 h. This value is comparable to the value of Ea (20.7 kJ.mol⫺1) in our present study as the reaction approaches completion.
As the reaction proceeds, the concentration of metal vacancies in mss/pyrronite increases and the diffusion path for metal ions becomes easier. The observation that the reaction rate (dy/dt) slows toward the end of the reaction does not contradict the conclusion that Ea decreases.
Conclusion 65
For the Ni-free sample Fe7.9S8, oxidation of Fe (within the first hour of reaction) leaves a much more porous surface than that of the sample Fe6.15Ni1.54S8. Crystal structure comparison between iron-nickel mss (Fe, Ni)S and nickel sulfide Ni17S18. NOTE: This edition is included on pages 80 – 101 in the hard copy of the thesis held in the University of Adelaide Library.
NOTE: This publication is included on pages 102 – 116 in the printed copy of the dissertation in the University of Adelaide Library. The compositions of the two MSS hosts converge towards the end of the reaction, due to the depletion of metal content in the host. Abundantly released sulfur, which reacts with oxygen at the surface of the particles, increases the surface temperature and accelerates the oxidation of iron to hematite.
It is likely to relate the change in activation energy to variations in the composition of non-stoichiometric compounds during the reactions.
Phase evolution and kinetics of the oxidation of monosulfide solid solution under
Synthesis 68
The charges were removed from the tubes and ground to a fine powder under acetone, ensuring the homogeneity of the mss. The charges were then resealed in silica tubes and heated at 1373 K for 2 h, cooled to 1173 K, annealed for 7 days, and then quenched in a large volume of cold water.
Oxidation 68
X-ray diffraction 69
SEM examination 69
Vyazovkin explained the dependence of the activation energy on the extent of the reaction caused by changing the physical and mechanical properties of the reaction medium [30]. Vyazovkin discussed isoconversion methods that use the notion of the dependence of the activation energy on the extent of the reaction to predict the kinetic behavior of the reaction outside the range of experimental temperatures [31]. Model-free methods can be used to avoid the problems associated with choosing an overly simplistic kinetic model.
Oxidation mechanism 70
Oxidation reactions 70
For Fe-Ni mss compositions, hematite and Ni17S18 were observed as the final oxidation products for experiments performed un-. Magnetite, if present, is below the detection limits of Rietveld profile fitting methods (about 1 wt% of the product). Apart from these main oxidation products, up to 5.7 wt% of an intermediate phase, pentlandite, was observed for sample Fe6.4Ni1.6S8.
Summary of cell parameters and unit cell volume for each phase: wt% pent., wt% mss, wt% hem. Stack of X-ray powder diffraction patterns showing the progress of phase evolution during the oxidation of Fe7.9S8.
Phase evolution 73
In the nickel-rich sample, on the other hand, there is relatively much oxygen on the surface of the particle due to the lower concentration of iron in the mss. This study also showed that increasing the sulfur content of mss accelerates the oxidation rate of mss. Wang Haipeng, Pring Allan, Ngothai Yung and O’Neill Brian (2006) Kinetics of the α → β transition in synthetic nickel monosulfide.
The difference in the bulk composition of the two mss hosts determines their specific activation energy profiles in the nucleation phase, as a higher metal content facilitates nucleation. Consequently, the Ea in the crystal growth phase differs for the two host mss. In addition to the common oxidation products of hematite and Ni17S18, mikasite (Fe2(SO4)3) was observed during the oxidation of Fe7,9S8, while pentlandite was formed in the case of Fe7,9S8.
This study also showed that increasing sulfur content in the MSS accelerates the oxidation rate of MSS.
Structure modification 74
Oxidation kinetics 76
Chamberlain and Dunn suggested that abundantly developed sulfur reacting with oxygen on the surface of the particle increases. Pring, The Effects of S Fugacity on the Exsolution of Pentlandite (Fe, Ni)9S8 from the Monosulfide Solid Solution (Fe, Ni)S, in: Proceedings of the 31st Annual Australian Chemical Engineering Conference, Adelaide, Australia, 2003. In Mineral Reactions of this type, the true functional form of the reaction model is almost never known, and the Arrhenius parameters determined by the classical Avrami method are biased to compensate for errors in the model.
The study of the variation in reaction rate with oxidation time illustrates the optimal oxidation time zone for each. The kinetic results produced by the refined Avrami method agree with those produced by the classical Avrami method when the assumption of constant Ea does not introduce significant deviation in the linearity of lnln(1/(1- y))~lnt curve (y, the degree of reaction; t, the reaction time). A kinetic study of the formation of NL layers on the leaching rate of the nickel-containing sulfides could be of significant economic interest.
Model-free kinetics of pentlandite exsolation from monosulfide solid solution 7th World Congress of Chemical Engineering, 2005, Glasgow, U.K.
Conclusion 78
The kinetics of the α → β transition in synthetic nickel monosulfide 80
- The mechanism and kinetics of α-NiS oxidation in the temperature range 670-700 o C
- Summary and Conclusions 117
- Introduction 117
- Exsolution of (Fe,Ni) 9 S 8 from (Fe,Ni) 1-x S 118
- Oxidation of (Fe,Ni) 1-x S 119
- α → β transition in (Fe,Ni) 1-x S 120
- Oxidation of NiS 121
- Kinetic Implications 121
- Suggestion for Further Work 122
- References 126
