USE OF HIGH GRADIENT MAGNETIC SEPARATION IN DETAILED CLAY MINERAL STUDIES

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Abstract
High gradient magnetic separation is a simple, inexpensive, nondestructive and rapid means of concentrating iron-bearing minerals to nearly monomineralic levels, particularly those present in undetectable proportions in soil clays. The use of variable magnetic flux density further allows subfractionation of the iron-bearing minerals. Besides iron content, the efficiency of high gradient magnetic separation is highly dependent on the particle size. The stability of suspension, suitable flow rates, contact time and the packing of steel wool are significant factors. The experimental setup used in this study was effective for 2–0.2 μm clays but modifications are necessary to adapt the technique to finer (< 0.2 μm) particle sizes. This resulted in the separation of three distinct mineral groups: (a) smectite, kaolinite, quartz and feldspars, which were entirely associated with the > 1.38 Tesla (T) fraction, suggesting that the smectite and kaolinite present in these soils contain little or no iron; (b) vermiculite, mixed-layer minerals and mica, which were present in all the high gradient magnetic separation fractions; and (c) amphiboles and hydroxy interlayered minerals concentrated only in the < 1.38 T fractions. The contents of hydroxy interlayered minerals and amphiboles increased with decreasing levels of magnetic flux density and concentrated in the < 0.20 T fraction, from which they were further separated into monomineralic separates. A very small proportion of the interlayered mineral present in the total clay had a non-iron-bearing (probably Al-Mg interlayered) counterpart. The iron-bearing vermiculite, mixed-layer minerals (weathering products of biotite) and mica showed different iron contents. Key words: Magnetic separation, iron-bearing minerals, clay mineralogy, X-ray diffraction, scanning electron microscopy

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