Vacuum melting

Purification using carbon is accompHshed by addition to the melt or to the soHd charge before vacuum melting. Pressure rises as a result of the reaction of the carbon with dissolved oxygen. Completion of the deoxidation process is marked by a rapid pressure drop indicating when the evolution of CO is complete. 

The vacuum melting process can upgrade chromium at a modest cost the other purification processes are very expensive. Thus iodide chromium is about 100 times as expensive as the electrolytic chromium and, therefore, is used only for laboratory purposes or special biomedical appHcations. 

Potassium cyanide [15]-50-8 M 65.1, m 634 , d 1.52. A saturated solution in H20-ethanol (1 3) at 60° was filtered and cooled to room temperature. Absolute EtOH was added, with stirring, until crystallisation ceased. The solution was again allowed to cool to room temperature (during 2-3h) then the crystals were filtered off, washed with absolute EtOH, and dried, first at 70-80° for 2-3h, then at 105° for 2h [Brown, Adisesh and Taylor J Phys Chem 66 2426 7962]. Also purified by vacuum melting and zone refining. HIGHLY POISONOUS. 

Maraging steels have been produced both by air and vacuum melting. Small amounts of impurities can decrease toughness significantly, sulphur in particular is detrimental and should be kept as low as possible. Silicon and manganese also have a detrimental effect on toughness and should be maintained below a combined level of 0-20%. Such elements as C, P, Bi, O2, Nj and Hj are kept at the lowest levels practicable. 

Two other factors are noteworthy the deleterious effects on chemical and mechanical properties of small amounts of impurities residual from extraction of the metal, and its toxicity. The first of these factors is obviated by vacuum melting the raw metal (for purification) as an essential prerequisite to further processing. The toxicity of beryllium is essentially a pulmonary problem and great care must be taken in handling the finely divided metal or its compounds. In practice, this type of activity is usually carried out under well-ventilated conditions. Certain tolerance levels for atmospheric beryllium are now internationally accepted and merit careful study before work on beryllium is embarked upon. 

Reduction of the fluoride Generally, Ca with RF3 is heated up to 1450°C in a Ta crucible. After cooling, the slag and the reduced metal are easily separated. The metal obtained contains Ca, some fluoride and Ta, which can be removed by vacuum melting (for La, Ce, Pr, Nd) or by vacuum melting plus distillation (Gd, Tb, Dy, Ho, Er, Lu, Sc, Y). Especially pure Ca must be used, such as triple-distilled Ca further re-distilled under low He pressure and handled in He-filled glove boxes. 

Vacuum melting This is used to reduce the volatile impurities (such as H, CaF2, RF3) if present after the metallothermic reduction. 

Potential hazards associated with electron beam vacuum melting of metals include the possibility of ignition of metal aerosols or condensate films in the event of sudden loss of vacuum at elevated temperatures. 

Vacuum Melting without Distillation of the Product Metal... 

Terephthaloyl dichloride is distilled in vacuum. Melting point 82-83 °C. 

It finally is purified by either vacuum melting or chelation with an organophosphate reagent followed by liquiddiquid extraction. 

Others maintain that so much air is entrained in the casting process that the value of vacuum melting is negligible. A possible explanation for this difference of opinion is the difference in techniques that can be applied in various types of operation... 

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