The Fusion of Difficult Materials Including Chromite, Cassiterite and Reduced Sulphur

Abstract

The practice of adding alkali metal nitrates such as sodium nitrate to borate fluxes and pre-oxidizing samples at low temperature has been taken a logical step further by the synthesis of sodium borate fluxes via the use of intimate mixtures of sodium nitrate and boric acid. The process of low-temperature pre-firing/decomposition followed by melting at normal high temperature gives enormous oxidative power yet does not give rise to crucible attack and provides a convenient route to produce fluxes from cheap and highly pure starting materials. An improvement in the efficacy of attack for a number of sample types has been observed, allowing for much smaller dilution ratios than have hitherto been achieved for chromite and cassiterite samples, for instance. Further, the inherent ease of formulation allows for the final stoichiometry to be achieved in stages whereby a liner can be made in a platinum crucible consisting of all of the boric oxide and a small amount of the sodium oxide. The remaining sodium oxide is added in the form of sodium peroxide mixed with the sample, which is sintered at a low temperature followed by safe decomposition and fusion at normal temperature. The use of pure alumina crucibles is shown to be possible for samples of such a nature that attack on platinum ware is to be feared and has allowed the use of sodium peroxide for the fusion of ferro-chrome and ferro-niobium. The complete retention of pyritic sulphur in the above methods and with lithium carbonate–tetraborate fluxes has been chemically proven. © 1997 by John Wiley & Sons, Ltd.