Magnesia graphite metals

Typically, Be-containing alloys and intermetallic phases have been prepared in beryllia or alumina crucibles Mg-containing products have been synthesized in graphite, magnesia or alumina crucibles. Alloys and compounds containing Ca, Sr and Ba have been synthesized in alumina , boron nitride, zircon, molybdenum, iron , or steel crucibles. Both zircon and molybdenum are satisfactory only for alloys with low group-IIA metal content and are replaced by boron nitride and iron, respectively, for group-IIA metal-rich systems . Crucibles are sealed in silica, quartz, iron or steel vessels, usually under either vacuum or purified inert cover gas in a few cases, the samples were melted under a halide flux . 

Metal-support interactions can be defined as being weak, medium or strong.28 Non-reducible metal oxides such as silica, alumina, and magnesia as well as carbon or graphite are considered to exert only a weak influence on the metal and, thus, exhibit only a weak metal-support interaction (WMSI). Zeolites (Chapters 10 and 13) exert a medium metal-support interaction (MMS1)28 while metals supported on reducible oxides when reduced at high temperatures exhibit a strong metal-support interaction (SMSI).27-32... 

Fig. 1.18. Cell for three-layer refining 1-cathode bus bar, 2-cathode hanger, 3-graphite cathode, 4-refininged metal, 5-electrolyte, 6-anode alloy, 7-forewell, 8-refractory lining (magnesia), 9-refractory lining (chamotte), 10-steel shell, 11-adode connection, 12-anode buss bar, 13-C bottom (ref]...

The cell which holds the molten salt and metal can be made of a variety of materials, such as iron, graphite, fire brick, alumina, magnesia, molybdenum or tantalum. The cathode is usually molybdenum, tantalum or tungsten and sometimes iron, cobalt or nickel. The choice of the cell and cathode materials will depend on the product desired. Graphite is almost always the choice for the anode because it is relatively inert to the halide gases (F2 or CI2) or oxygen liberated at this electrode. 

As an illustration. Table 10.7 shows the numerous reduetion reactions which take place in carbon magnesia refractories at high temperatures. These reactions bring into play carbon (originating from the bond and the graphite), periclase grains, lime siUcates which constitute the bonds between the magnesia crystals and the metal additions (which act as carbon antioxidants). 

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