The deterioration in mechanical properties, mass loss, and the appearance of various forms of localized corrosion are documented for the exposure of a representative group of unstressed wrought aluminum alloys at three test sites over a ten-year period. Loss of ductility is the chief form of mechanical property damage observed. Among the forms of localized corrosion observed, intergranular corrosion, which led to partial sheet delamination or exfoliation, is the most damaging to the ductility of materials. Intergranular corrosion without a strong element of orientation in the wrought direction is less damaging, though more damaging than pitting without associated intergranular attack. A study of the effect of a sacrificial cladding layer indicated that loss of ductility can be completely eliminated when 7072 alloy is clad on 3004-H36 and 7178-T6 alloys, and 1230 alloy is clad on 2024-T3 alloy.
An increasing amount of nickel is currently being extracted from the nickeliferous laterite ores, rather than from the sulfide deposits. Consequently, new processes are being developed to extract the nickel from these ores and selective reduction is one possibility. In these processes, it has been shown that the addition of sulfur can improve the grade and/or the recovery. In the current work, the mechanism of the carbothermic reduction of a sulfurcontaining nickeliferous limonitic laterite ore was investigated by utilising TGA/DTA and both thermodynamic and kinetic analyses. It was found that, in addition to dehydroxylation and reduction, the reaction sequence also consisted of the following major stages: (1) conversion of pyrite to pyrrohtite and (2) subsequent desulfurisation. Analyses of the reacted samples showed that both the particle size and the amount of ferronickel increased with both particle growth temperature and retention time at temperature. The thermodynamic predictions were in general agreement with the experimental findings and were consistent with the postulated formation of an iron-sulfur-oxygen solution.
The recovery of nickel from the oxidic nickeliferous laterite ores is receiving increasing attention due to the difficulty of recovering this metal from the sulphide ore deposits. One possible solution is to selectively reduce the nickel oxide in the ore, which could then be upgraded by, for example, magnetic concentration. In this article, a thermodynamic study was performed on the reduction of a limonitic laterite ore by methane. Methane was selected as the reducing agent as it has a lower environmental impact than carbon due to the reduced carbon dioxide emissions. The effects of temperatures and methane additions on the nickel recovery and nickel grade were investigated. High recoveries of over 95 % were predicted, but the grades were limited to about 2.5 % due to the formation of magnetite. The thermodynamic simulations for reduction by methane were in agreement with the experimental results in the literature for other reducing agents, reflecting the fact that the nickel oxide in the limonitic ore is relatively unstable. Thus, high recoveries could be achieved irrespective of the reducing agent involved.
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