Refractory oxides, fusion

Choice of a dissolution method also depends on the range of substances to be analyzed, but in some specific instances the chemical form of the element is of even more importance. For example, particles formed at high temperature usually contain refractory oxides and silicates which may require fusion with either an acidic or basic flux, or hydrofluoric acid treatment to remove silica, or a combination of these techniques. 

Some instrumental methods (INAA, XRFA) analyse directly the aerosol filter or portions of filters. Recently such approach is intensively tested for other methods especially ETAAS. For the majority of instrumental methods however treatment of the filter is required leading to a liquid extract. Wet ashing with different acidic mixtures is very popular though dry ashing in a furnace, followed by acid dissolution is also used. The choice of the method depends not only on the measurement technique and the range of substances to be analyzed but also on the chemical form of the element (Kneip and Kleinman, 1982). For example high temperature secondary aerosols may contain refractory oxides or silicates and the treatment requires fusion or hydrofluoric acid dissolution. 

Decomposition of Zircon. Zircon sand is inert and refractory. Therefore the first extractive step is to convert the zirconium and hafnium portions into active forms amenable to the subsequent processing scheme. For the production of hafnium, this is done in the United States by carbochlorination as shown in Figure 1. In the Ukraine, fluorosiUcate fusion is used. Caustic fusion is the usual starting procedure for the production of aqueous zirconium chemicals, which usually does not involve hafnium separation. Other methods of decomposing zircon such as plasma dissociation or lime fusions are used for production of some grades of zirconium oxide. 

Method by Scorifaation,—Scarification, or in other words, tho conversion of the silica and other impurities into a scoria, is simply another method of fusion, In. which, however, metallic lead, instead of its oxides, is employed and the oxidation of the various substances to be removed is produced by the aid of atmospheric air, whilst the litharge necessary for the fusion of the earthy and siHcious mattors is formed by the oxidation of a portion of the metallic lead. For this operation, instead of a crucible, a shallow cup-like vessel, termed a tcorifter, is employed. This vessel, the form of which is represented in Fig. 23S, should Pig, sec, be made of refractory clay, aud... 

Oxide Aluminum oxide, alumina AI1O3, white solid, insoluble, melting point 2020 C. formed by heating aluminum hydroxide to decomposition when bauxite is fused in the electric furnace and then cooled, there results a very hard glass ( alundum ), used as an abrasive (hardness 9 Mohs scale) and heat refractory material. Aluminum oxide is the only oxide that reacts both 111 H20 medium and at fusion temperature, 10 form salts with both acids and alkalis. 

Refractory species (such as zirconium oxide) are incompletely atomized at the temperatures of a flame or a muffle furnace. If the radionuclide of interest may be in a refractory form, the material must be mixed with fusion reagents and melted at a high temperature. After cooling, the solidified melt is dissolved in a HNOj solution. Fusion mixtures that have been tested for many types of soil are listed in Table 4.2. Sample masses are limited by practical consideration of the final sample size, taking into consideration the large amounts of fusion reagents added to the sample. 

Some materials are particularly resistant to acid digestion, e.g., certain rocks, mineral oxides, phosphates, and some iron alloys. For these samples, high-temperature fusion with an acidic or basic flux such as lithium metaborate (LiBOs) in the molten state can be used to render such materials soluble in water or dilute acid. Fusion decompositions are the most rigorous methods available and all silicate materials, including refractory substances like zircon and cassiterite, can be dissolved completely when fused with an appropriate flux. However, there are several disadvantages to this method including the introduction of additional salts into the final solution... 

Yttrium Oxide. Y2O3 m.p. approx. 2450°C sp. gr. 4.84 thermal expansion (25-1500°C) 9 X 10-6. An oxide extracted from MONAZiTE (q.v.) though not itself, strictly, a rare earth. An infra-red transmitter in the wavelength range 1 to 8 pm. ZAC Block. A zirconia alumina fusion-cast refractory, resistant to corrosion in glass tanks. 

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