Refractories crucible mixes

Preparation of Amorphous Boron from Boric Acid Anhydride. (Each student performs only one of the experiments for preparing amorphous boron.) Put 2 g of boric acid anhydride into an iron crucible or onto an iron plate and roast it if the boric acid anhydride contains moisture, then when it is heated with magnesium an explosion may occur ). Triturate the boric acid anhydride as rapidly as possible (why ) in 4 mortar, weigh 1 g and mix it with 2 g of a magnesium powder. Place the mixture into a refractory test tube and fasten it in an inclined position in a clamp of a stand in a fume cupboard . First carefully heat the test tube with the mixture with the flame of a burner, and then strongly heat the bottom part of the tube. What do you observe ... 

Modifications of the Goldschmidt thermite process may also be employed for the preparation of the iron-vanadium alloy. The crushed vanadates or vanadium pentoxide are mixed with the necessary amount of iron scalings or turnings and fluxes, and introduced into a gas-fired open-hearth furnace or into an iron crucible provided with a refractory lining and previously heated to redness. The reactions taking place are ... 

B.t.u. per cu. ft. heating value properly mixed with air very high temperatures are attainable, limited chiefly by the melting point of the refractory. With a furnace of the type shown it has been possible to fuse platinum foils (m. p. 1755° C.) in the crucible. The temperature of the refractory granules themselves is somewhat higher than this even. 

Weigh the sample in a nickel crucible and mix with six fold weight of the fusion mixture (5 parts of K2CO3 + 4 parts Na2C03). Fuse the mixture for 10 min at red heat. Cool, add 5 mL of deionised water and allow to stand for 60 min. Slowly heat to dissolve the contents completely and transfer to a volumetric flask. Rinse and make upto volume with ultrapure water. The solution can be used for measuring Si, Al and other refractory elements (Burr, 1976). 

The lining of induction furnaces is made from ramming mixes (water content below 1 %) as well as plastic mixes (water content below 6%) lining by concretes is rarely made. It is necessary to have the pattern of the crucible. Also, it is necessary to have a specialized heating element for preheating. Yet the sintering of the refractory layer takes place only in service, so it is recommended to make exposure of the furnace with metal for 3 h without electricity and then at a temperature of 750-800 °C for a day. 

The usual shapes in alumina are tubes and crucibles. Fused alumina is mixed with clay to produce alundum cement. This cement is tempered with water and can be hand-molded. It is useful in laboratories. On firing, it shrinks a little and gives rise to a porous refractory. It also spalls easily and is inexpensive. Depending on the grade, it is useful in the range of 1500°C-1700°C. 

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