René Derie

Shape effects on crystallite size distributions in synthetic hematites from X-ray line-profile analysis

Abstract Synthetic acicular hematite shows a nonuniform broadening of the X-ray lines in a wide temperature range. The factors affecting the peak broadening were studied by recording the powder line profiles of the (012), (104), (110), (024), and (116) reflections. The orientation relationships of these planes vs the main particle axes were previously checked by electron microdiffraction. On the other hand, various hematites having different particle sizes and shapes (spheres, slabs, needles) were investigated as well. The mean crystallite sizes were determined from a classical Fourier transform analysis whereas the size distributions were computed by a smoothing procedure described by A. Le Bail and D. Louer ( J. Appl. Crystallogr. 11, 50–55, 1978). The results show that the nonuniform broadening is ascribed to the shape anisotropy of the particles rather than to strains and faults broadening since (i) this last phenomenon is negligible in spherical-shaped particles, (ii) nonuniform broadening remains in acicular particles after further heating at 600°C, and (iii) a good estimate of the ratio of the particle width and thickness has been found.

The region of stability of green rust II in the electrochemical potential-pH equilibrium diagram of iron in sulphate medium

Abstract The oxidation of Fe(OH) 2 , by slow air bubbling, can lead to the formation of oxyhydroxides. In sulphate medium the following succession was observed: Fe(OH) 2 → green rust II → FeOOH. Green rust II is unstable in sulphate-free solutions; its composition is 2Fe(OH) 3 .4Fe(OH) 2 .FeSO 4 . × H 2 O. From the values of electrode potential and pH corresponding to the equilibrium between the solution and the solids, GR II and α-FeOOH, the value of free enthalpy of GR II has been evaluated to — 1,020,400 ± 4,600 calories/mole. The probable area of stability of the GR II in the tension—pH equilibria diagram for iron has been established.

Analysis of mineral suspensions by d.c. plasma emission spectrometry

Abstract Dilute aqueous suspensions of sub-sieve fractions of various minerals are analyzed for their major constituents by d.c. plasma emission spectrometry. In favourable cases (e.g., determination of zinc in sphalerite or smithsonite powders below 5 μm), the suspensions may be analyzed quantitatively simply by comparison with standard solutions, and with a relative error less than 10%. In other cases (e.g., determination of beryllium in beryl powders), the required element is so poorly atomized that the analysis is impossible, even when very fine-grained suspensions are used. As expected, the atomization efficiency for a particular element decreases with increasing particle size, and with increasing free energy of formation of its oxide at high temperatures. The region extending along the vertical branch of the plasma is a convenient excitation zone for suspensions containing elements that are difficult to atomize but the high background in this region limits the analytical possibilities.

Construction of an isothermal balance with two symmetrical pans for operation under pressure

Abstract A thermobalance with two symmetrical pans was constructed in order to avoid thermogravimetry errors under pressure. The balance functions at 1000°C under 50 atm of hydrogen. The influence of parameters such as pressure, temperature and gas flow rate on the baseline deviation was studied. An example is given of the application of this balance to the hydrogenation of lignite char.

Textural and Structural Parameters Influencing the Ageing of Cu/ZnO L.T.S. Catalysts.

Publisher Summary This chapter discusses the textural and structural parameters influencing the ageing of copper/zinc oxide (Cu/ZnO) low temperature shift (L.T.S) catalysts. Two different classes of catalysts are considered in the chapter that are prepared in the laboratory by calcination (1) at 250°C of mixed precipitates and coprecipitates of copper and zinc hydroxides and (2) at 600°C of mixed organic salt precipitates or amorphous organic complexes. Cu/ZnO L.T.S catalysts, resulting from the calcination of organic material, possess a higher specific activity than those made from hydroxide precipitates; they are also more resistant towards ageing. The high specific activity, however, is counterbalanced by a lower specific area, so that the activity per unit volume of these catalysts remains inferior to that of the best commercial ones. In the absence of external poisons, the ageing process itself has been attributed to two distinct phenomena: (1) the coalescence of the catalyst crystallites, as well the crystallites of the active element, copper, than those of the support, zinc oxide, and (2) the internal poisoning of the copper surface by zinc atoms resulting from the partial reduction of the support.