Mineralogical and textural parameters affecting the production of refractory chromite sands: the Khajeh-Jamali enrichment plant, Southern Iran

Chromite foundry sands form the mold in which molten metal is contained until it solidifies in the desired shape. Chromite is used because of its resistance when exposed to heat and is chosen over typical silica sand when a higher casting temperature is required. Chromite sands must be as pure as possible as small amounts of low-T melting gangue minerals can damage the resins that bind sand. Moreover, a very narrow sand grain size range is required. In refractory industry Acid Demand (AD) test, a titration method that keeps into account the amount of acid consumed at three different pH levels, is used as a proxy of the sand-resin reaction potential, while for grain-size the Fineness Index (FI) test is used. At Cheshmeh Bid chromite Mine, Southern Iran, refractory sands are produced for the internal foundry market. In the plant a high-Cr2O3 chromite lumpy is crushed and grinded and the resulting sand is split in two feeds: coarse (0.2-2mm) and fine (<0.2mm). Each feed is enriched using a shaking table that produces a concentrate and a tailing. For the present work feeds, concentrates and tailings from the plant were sampled and the concentrate yield of each table was estimated through flow rate measure. AD and FI were determined together with bulk composition (XRF), mineralogy (XRD) and mineral chemistry (SEM). Liberation degree was assessed by means of modal analysis under reflected light microscopy on epoxy resin incorporated sand thin sections and Separation Efficiency (SE) was calculated according to Wills and Napier-Munn (2006). While only the coarser concentrate falls in the suitable range for FI, neither do match the AD quality parameters, with a worse performance for the coarser one. SE is much higher for the fine sand (93.3%) than for the coarse one (33.2%). Liberation degree analysis shows that this is due to a much higher middlings content in the latter. When compared with South African reference foundry sand, the results of bulk composition show that difference in SiO2 content cannot explain the wide gap in AD values. XRD and SEM data suggest that a different gangue mineralogy can be the main factor affecting the different AD performance. Serpentine is the main gangue mineral of Iranian chromite sands with only rare relict olivine grains, while South African gangue is mainly pyroxene with minor amphibole and rare apatite. Further AD tests on pure pyroxenite and serpentinite show that the second one is more reactive than the first one. In conclusion neither sands meet quality parameters for European market but coarse one quality could be improved by a better tuning of shaking table and a further grinding to a lower grain size. However serpentine gangue of Iranian sands negatively affects AD values and hence their concentrate has to reach lower SiO2 contents than South African one to meet market parameters. REFERENCES Wills, B.A., Tim Napier-Munn, 2006. Mineral processing technology: An introduction to the practical aspects of ore treatment and mineral. Maryland Heights, MO: Elsevier Science & Technology Books.