Calcium–aluminothermal production of niobium and mineral composition of the slag

The process of niobium pentoxide calcium–aluminothermal reduction in pilot plant and full scale conditions was studied at Silmet JSC, Estonia. In the paper the construction and operating parameters of the pilot plant shaft melting furnace and the composition of raw materials mixture as well as of the product (niobium ingot) are described. The optimum ratio of Ca/Al to obtain the best quality niobium ingot was revealed in pilot plant melting tests, and the results of analyses of the mineral and phase composition of the slag are presented. Addition of metallic Ca to the raw materials mixture with 10% excess enabled to reduce the Al content in ingot to 4.4% and increase the Nb content > 90%. In the optimum conditions of the reduction process a slag with the composition of 3CaO·5Al2O3 is formed. Also different niobium containing oxides and metallic phases may be present in the slag. The results obtained enable to optimize the niobium production process and to elaborate an effective protective layer for the graphite facing of the furnace as well as to make some suggestions for possible utilization of the waste slag in the building materials industry.