Refining of Precious Metal Bearing Materials from Secondary Sources-Methanesulfonic Acid Leaching of Raw Silver Granules as a Promising Approach towards a Green Way of Silver Refining

The state-of-the-art technology of raw silver refining in a silver nitrate-based electrorefining process (Moebius-electrolysis) is accompanied by several disadvantages, both from a technological and from an ecological point of view. In addition, increasing concentrations of critical impurities from secondary sources, like palladium, in raw silver are a further challenge for the future of silver refining. Thus, there is strong motivation for the development of an adequate, alternative process of raw silver refining to substitute the existing Moebius-electrolysis. Due to its less environmentally toxic character and the high aqueous solubility of its silver salt, methanesulfonic acid (MSA) is a possible base chemical for the design of an efficient refining method based on leaching of raw silver followed by electrowinning, with less ecological and technological complications. In this paper the results of some fundamental investigations on the leaching of raw silver granules, containing approx. 94% silver, with methanesulfonic acid and hydrogen peroxide as an oxidation agent are presented. Agitation leaching experiments were conducted on a laboratory scale and the effects of the solid concentration, the hydrogen peroxide dosage and the temperature as leaching parameters were studied. The obtained results indicate that silver leaching yields of more than 90% are achievable with leaching at elevated temperatures of 65 °C or 80 °C, solid concentrations of 500 g/L and at a stoichiometric H2O2:Ag-ratio of 3:1. Increased solid concentrations greater than 500 g/L and elevated temperatures of 65 °C or 80 °C additionally improved the selectivity of the process regarding the leaching of Pd.

Reflected polarized darkfield imaging of bone surfaces

Reflected polarized light microscopy (Fig. 1A) was used previously to generate high contrast images of birefringent and light scattering samples, including bone surfaces and autoradiographic specimens. We now present a modification (Fig. 1B) of the Gullberg system with improved sensitivity for the characterization of bone specimens and quantitation of silver granules on autoradiographic specimens. Reflected imaging techniques were useful to generate high contrast images superior to transmitted light strategies, and both of the strategies presented can be adapted easily to any fluorescence microscope.Reflected light produced images free of refractile noise from materials through the thickness of the specimen which detracts from transmission darkfield imaging of silver grains and brightfield imaging of bone surfaces. The use of crossed polars also eliminated noise from stray light reflected off of internal microscope elements. The rotatable lambda/4 plate mounted on the objective front element (Fig. 1A) allowed considerable manipulation of image contrast, permitting dual imaging of silver granules, birefringent tissues in autoradiographic specimens and the surface topography of bone specimens by rotating the lambda/4 plate to 45°.

Serum Concentrations and Accumulation of Silver in Skin During Three Months Treatment with an Anti-smoking Chewing Gum containing Silver Acetate

Silver acetate chewing gum was used for 12 weeks as a smoking deterrent in 21 adults. The effect of silver on serum concentrations, its accumulation in the skin and the risk of developing clinically evident argyria were investigated. Serum concentrations of silver clearly rose after chewing gum use had started, and concentrations quickly returned to normal after use had ceased. In most cases the number of silver granules in skin biopsies, observed by autometallography, increased after the gum had been used for 12 weeks. No one developed clinical signs of argyria. Silver acetate containing remedies can be used as an aid to stop smoking, but the consumption must be monitored to avoid accumulation of larger amounts of silver in the body.

LIGNOFIBRILS ON THE EXTERNAL CELL WALL SURFACE OF CULTURED PLANT CELLS

Small strands and bundles of strands extend from the outside surface of suspension-cultured cells of Daucus, Ipomoea, and Phaseolus into the medium. This fibrous cell coat is present in all samples from various growth stages but appears to increase in quantity in the order Ipomoea < Phaseolus < Daucus. The bundles are often many microns in length and display great variation in frequency, size, and form. Identification of the composition of the strands and bundles as lignin is consistent with the following observations: alkaline nitrobenzene oxidation of the strands to compounds which resemble monomers of wood lignin; resistance of the strands to pronase, trypsin, pectinase, and lipase; strong irreversible adsorption of heavy metals; deposition of silver granules by treatment with silver nitrate-hexamine reagent; extraction of the bundles with aqueous dioxane (Björkman procedure); presence in quantity of a structured form of Klason lignin; and existence of material giving a positive test with the Wiesner reagent. Large individual strands (lignofibrils) from Phaseolus show the form of a flat ribbon with very thin branches at irregular intervals. This form does not vary with preparatory techniques, although its electron opacity does. Intercellular spaces display considerable structure and sometimes contain sheets of fibrillar material merging with both the middle lamella between the cells and the surface bundles facing the medium. These sheets are probably another form of association of the lignofibrils. It is suggested that natural fibrous lignin may be a much commoner component of plant tissue than suspected hitherto.