How to Select a Fusion Flux Most fusions involve the use of lithium tetraborate (Li2B4O7, M.P. 920°C), lithium metaborate (LiBO2, M.P. 845°C), or a mixture of the two. As a rule lithium tetraborate is better suited for the dissolution of basic oxides, and is preferred for cement and most ores. Lithium metaborate or a “met/tet” mixture is more suitable for acidic oxides, and hence for silicate rocks, silica-alumina refractories, and similar samples. Individually or together, these lithium borates will dissolve almost anything with bonded oxygen: oxides, carbonates, silicates, sulfates, etc. Metals, sulfides, nitrates, carbides, phosphides, etc. cannot be fused directly in lithium borate fluxes, and will often attack platinum-gold crucibles, or alloy with them. However, many of these materials can be first calcined with standard techniques and then successfully fused. Katanax has developed a unitary procedure for fusing sulfide-rich material by mixing it with borate flux and an oxidant (lithium or sodium nitrate), heating the mix just enough to oxidize the sample, and then raising the temperature to melt the flux and complete the fusion.
Other fluxes include sodium tetraborate (Na2B4O7), sodium metaphosphate (NaPO3), and potassium pyrosulfate (K2S2O7). These have lower melting points then the lithium borate fluxes, and more specialized applications. Alternate fluxes and fluxing techniques are discussed in Bock (1979) and in Sulcek and Povondra (1989), both cited in the bibliography at the back of this handbook. Melting point may be a factor in the selection of a flux, as the higher the temperature of a fusion, the greater degree of volatilization. However, the utility of lithium tetraborate and lithium tetraborate/metaborate mixtures is so great that most analytical fusions are carried out with these fluxes at temperatures of between 1000° and 1100°C.
SPEX SamplePrep offers pure and ultra-pure lithium tetraborate, lithium metaborate, sodium tetraborate, and several lithium tetraborate/metaborate blends. All are granular, free-flowing, pre-fused fluxes which are supplied with a certificate of analysis of trace metal impurities
and loss on ignition. Pre-fused fluxes are uniform in composition at the level of individual particles, unlike blended powders which can separate during transportation and storage. Granular fluxes are easier to weigh and handle than fine-grained “fluffy” fluxes, which are also
prone to absorb moisture because of their large surface area. Finegrained fluxes, unless dried and kept in sealed bottles, generally haveloss-on-ignition problems; they also can erupt from the crucible when absorbed water is heated and driven off. The coarse-grained fusion fluxes available from SPEX SamplePrep simply do not have the problems associated with water absorption.
All fusion fluxes are sold exclusively by SPEX SamplePrep. |
