Mineral Separation Techniques

Mineral separation prepares purified fractions of individual mineral phases from rock samples for chemical analysis, X-ray diffraction, isotope studies, or other investigations. The technique required depends on the amount of material needed, the physical properties of the target mineral, and the acceptable level of contamination. The standard methods are hand extraction, density separation using heavy liquids, and magnetic separation.

Crushing and Sieving

Before any separation can take place, the rock must be disaggregated into individual mineral grains. Jaw crushers reduce rocks to particles a few millimeters across; rotary mills can grind material to much finer sizes; percussion mortars and steel mortars with pestles are used for smaller samples and for manual control. The crusher must be thoroughly cleaned between samples to prevent cross-contamination. For igneous and metamorphic rocks, the target fragment size is between one-fourth and one-tenth the average grain size of the rock, which ensures that most grains are monomineralic. ^[1]

After crushing, the sample is sieved to select a working grain size. The -100/+170 mesh or -140/+200 mesh fractions are commonly used - these represent particles that pass a 100-mesh sieve (0.150 mm openings) and collect on a 170-mesh sieve (0.090 mm), or that pass 140-mesh (0.106 mm) and collect on 200-mesh (0.075 mm). The grains are examined under a binocular microscope to confirm that they are monomineralic; if not, a finer fraction is required. Crushed and sieved material is then cleaned of fine surface dust by repeatedly agitating in water or acetone and decanting until the liquid runs clear. ^[1]

Density Separation (Heavy Liquids)

Heavy liquids exploit density differences to float low-density minerals and sink high-density minerals. The organic heavy liquids - bromoform, tetrabromoethane, and diiodomethane - are toxic and must be used in a fume hood. Tungstate products such as lithium heteropolytungstate (LST) are much less toxic but have higher viscosity. The density of any heavy liquid can be adjusted by diluting with an appropriate solvent - acetone for the organic liquids, water for the tungstate products - so that its density falls between the densities of the two minerals to be separated. LST density can also be varied by temperature adjustment. ^[1]

Separation is performed in a separatory funnel. Low-density minerals float; high-density minerals sink. Periodic stirring promotes complete separation. When separation is finished, the valve at the base of the funnel is opened and the heavy fraction and part of the liquid drain onto filter paper. The light fraction is then collected on a second filter paper. A centrifuge is often used to speed the process, especially with the more viscous tungstate liquids. Grains are washed with the appropriate solvent to remove heavy liquid residue, and the expensive liquids are recovered by evaporation or liquid washing for reuse. ^[1]

Magnetic Separation

Ferrimagnetic and ferromagnetic minerals such as magnetite can be separated with a simple hand magnet wrapped in plastic wrap. The crushed sample is spread on paper and the magnet is moved through the grains to extract the magnetic particles. The particles are retrieved by stripping the plastic wrap from the magnet. ^[1]

The Frantz Isodynamic Magnetic Separator provides separation of paramagnetic minerals, which are not strongly attracted to a hand magnet but do respond to an intense magnetic field. Grains are fed onto an inclined chute positioned between the poles of a strong electromagnet. Gravity pulls all grains down the tilted chute; the magnetic field deflects paramagnetic grains toward the high side, while diamagnetic grains continue to the low side. A splitting ridge divides the two populations into separate collection containers. Sequential passes at increasing field strengths separate different paramagnetic mineral fractions. After all paramagnetic minerals are removed, the residual fraction typically consists of diamagnetic quartz and feldspar. ^[1]

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