Mineralogical investigation of the Nepheline syenite Franspoort, South Africa for beneficiation

Abstract

Mamelodi Quarries, which currently mine the Franspoort nepheline syenite, produces aggregate and crusher sand for the local building industry. The mine is located northeast of Pretoria, South Africa on the road to Cullinan. The aim of this study is to investigate the use of the Franspoort nepheline syenite as an alumina and alkali resource for the glass and ceramics industry at Mamelodi Quarries, and to evaluate the production of a concentrate of zircon and rare earth elements as economic by-products. International standards require a nepheline product with a ferric oxide content of less than 0.35 weight percent. The Franspoort nepheline syenite contains 3.37 weight percent of ferric oxide. The iron-containing minerals present are aegirine, aegirine-augite, magnetite, ilmenite and pyrite. The removal of iron-bearing minerals was attempted by high-intensity wet magnetic separation, low-intensity dry magnetic separation, spiral gravity separation, and heavy liquid separation to produce a saleable nepheline product. This product is the cleaned final concentrate, of the different separation tests, which contains the lowest iron concentration for application in the glass and ceramic industry. The mineral assemblage was determined with a petrographic study as well as X-ray diffraction and electron microprobe analyses. Material from the different separation tests was analysed with X-ray fluorescence to obtain the chemical composition and to evaluate the final iron content of the nepheline product. The dry magnetic separation method produced the best results. The nepheline product has a ferric oxide content of 0.68 weight percent compared to the starting concentration of 3.37 weight percent. The ferric oxide concentration is, however, above the accepted levels for the glass and ceramics industry. The ferric oxide content is attributed to small iron-rich mineral inclusions, which are locked in feldspar and nepheline. The final nepheline product is not suitable for the use in the glass and ceramic industry. Zircon was concentrated the best by the use of heavy liquid separation. Zircon is in most cases locked in the minerals albite, microcline, and nepheline. The rare earth elements are mostly associated with zircon and fluorite and therefore it will not be viable to produce it as a by-product. Copyright