Experimental results obtained in laboratory leaching studies of primary copper sulfide ore in sulfuric acid systems pressurized with oxygen are interpreted by a computerized geometric model involving the movement of a reaction zone through the ore fragments. Physical properties of the ore, including size, shape, and mineral content, are considered. The leaching mechanism involves mixed kinetics and includes a surface reaction within a moving reaction zone plus pore diffusion of dissolved oxygen through the reacted portion of the ore fragment to the reaction zone. The results are applicable to conditions that would exist innuclear solution mining or similar processes in which the ore is converted into rubble and then inundated by a leach solution adequately supplied with oxidants. Experimental results at 90°C are correlated with the model, which includes temperature-dependent parameters.

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In-place leaching of primary sulfide ores: Laboratory leaching data and kinetics model

The first European mission dedicated to the exploration of Jupiter and its icy moons (JUpiter ICy moons Explorer—JUICE) will be launched in 2022 and will reach its final destination in 2030. The main goals of this mission are to understand the internal structure of the icy crusts of three Galilean satellites (Europa, Ganymede, and Callisto) and, ultimately, to detect Europa's subsurface ocean, which is believed to be the closest to the surface among those hypothesized to exist on these moons. JUICE will be equipped with the 9 MHz subsurface-penetrating radar RIME (Radar for Icy Moon Exploration), which is designed to image the ice down to a depth of 9 km. Moreover, a parallel mission to Europa, which will host onboard REASON (Radar for Europa Assessment and Sounding: Ocean to Near-surface) equipped with 9MHz and 60MHz antennas, has been recently approved by NASA. The success of these experiments strongly relies on the accurate prediction of the radar performance and on the optimal processing and interpretation of radar echoes that, in turn, depend on the dielectric properties of the materials composing the icy satellite crusts. In the present review we report a complete range of potential ice types that may occur on these icy satellites to understand how they may affect the results of the proposed missions. First, we discuss the experimental results on pure and doped water ice in the framework of the Jaccard theory, highlighting the critical aspects in terms of a lack of standard laboratory procedures and inconsistency in data interpretation. We then describe the dielectric behavior of extraterrestrial ice analogs like hydrates and icy mixtures, carbon dioxide ice and ammonia ice. Building on this review, we have selected the most suitable data to compute dielectric attenuation, velocity, vertical resolution, and reflection coefficients for such icy moon environments, with the final goal being to estimate the potential capabilities of the radar missions as a function of the frequency and temperature ranges of interest for the subsurface sounders. We present the different subsurface scenarios and associated radar signal attenuation models that have been proposed so far to simulate the structure of the crust of Europa and discuss the physical and geological nature of various dielectric targets potentially detectable with RIME. Finally, we briefly highlight several unresolved issues that should be addressed, in near future, to improve our capability to produce realistic electromagnetic models of icy moon crusts. The present review is of interest for the geophysical exploration of all solar system bodies, including the Earth, where ice can be present at the surface or at relatively shallow depths.

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Dielectric properties of Jovian satellite ice analogs for subsurface radar exploration: A review

The kinetics of reaction between Cr23C6 particles and Cr2O3 particles which yields metallic chromium was studied. This reaction is one of the basic reactions in the Simplex process and is also related to the carbon reduction of metal oxide. The rate of reaction between these particles in a pellet of the mixture was measured by thermogravimetry in vacuum at 1050, 1075, and 1100°C. The molar ratio of Cr23C6 to Cr2O3 in the pellet was maintained at 1:8. At the earlier stages of the reaction, a rate equation for interfacial reaction control was applied and the rate constant α was observed to be inversely proportional to the reciprocal effective radius of Cr23C6 particles. The indirect gaseous reactions are predominant: 1/6 Cr23C6 + CO2 = 23/6 Cr + 2CO 1/3 Cr2O3+ CO = 2/3 Cr + CO2 The reduction of Cr2O3 with CO gas is presumed to proceed so rapidly and the equilibrium is attained instantaneously. The reaction of CO2 with Cr23C6 particles occurs in the sequence co2
 (g) +c< ⇋ c(o) + co(g) c(o) → CO(g) +c< The latter reaction was thought to be the rate determining step, and the activation energy of this reaction was estimated to be approximately 60 kcal per mole.

Kinetic studies of the reaction between Cr23 C6 particles and Cr2O3 particles

The reduction kinetics of Brazilian hematite by CO is investigated in a Micro Fluidized Bed Reaction Analyzer (MFBRA) using an analyzing method based on Johnson-Mehl-Avrami (JMA) model at temperatures of 973 K (700 °C), 1023 K (750 °C), 1073 K (800 °C), and 1123 K (850 °C). The solid products at different reduction stages are evaluated by SEM/EDS and XRD technologies. Results indicate that the reduction process is better to be discussed in terms of a parallel reaction model that consists of the reactions of hematite to wustite and wustite to iron, rather than a stepwise route. Meanwhile, the controlling mechanism of the reduction process is found to vary with temperature and the degree of conversion. The overall process is controlled by the gas–solid reaction occurring at the iron/wustite interface in the initial stages, and then is limited by the nucleation of wustite, and finally shifts to diffusion control. Moreover, the reactions of hematite to wustite and wustite to iron take place simultaneously but with different time dependences, and the apparent activation energies of hematite to wustite and wustite to iron are determined as 83.61 and 80.40 KJ/mol, respectively.

Multistep Reduction Kinetics of Fine Iron Ore with Carbon Monoxide in a Micro Fluidized Bed Reaction Analyzer

Experimental results obtained in room temperature laboratory leaching studies of lowgrade chrysocolla are interpreted by a kinetic equation of the form 1−(1−R)1/3=(1/αKr
 c) ln (αkKC
 0t+1) derived on the basis of a special form of the shrinking-core model and assuming chemical-reaction control. The experimental program encompasses a large range of particle sizes and reaction times and significantly extends previous experimental data on similar ore systems. The kinetic model, shown to be consistent with the experimental data, assumes the ore-particle structure to be characterized by pores and channelized regions which extend from the outer surface through a reacted zone into the unreacted core. Scanning electron microscope observations of unreacted particle surfaces as well as reacted particle surface structures lend support to this concept of leaching. The reaction rate is observed to vary inversely with initial mean particle radius, and diffusion in any form appears to be negligible by comparison with surface reaction phenomena.

P.K. Strangway

Papers

Studies on kinetics of iron oxide reduction