HGMS of tin ore tailings

Two samples of Bolivian tin ore tailings have been subjected to High Gradient Magnetic Separation (HGMS) for the purpose of increasing the tin content, and reducing the iron content which interferes with smelting. The Colavi mine tailing was the discarded product of a froth flotation process and contains about 0.7% by weight of valuable tin. The Atoroma tailing was the discard from mechanical ore cleaning and contains from 0.16 to 0.81% tin depending on the particle size, with most of the tin in the < 53μ size range. One of the best results we have on this latter sample, otherwise discarded, is to recover a magnetic product which assays at 3.25% tin which approaches the requirement for smelting. This result was achieved at a field of 3 kOe and a flow of 13.4 cm/s without chemicals. Higher fields improve the recovery at the expense of grade. Loading was investigated up to about 4.5 times the matrix weight and only a small decrease in recovery was found to occur. Particle surface charges often affect a separation process between mineral species, in this case, cassiterite, SnO2, tourmaline sometimes containing iron, and other impurities including iron minerals. We have varied the pH of the ore slurry from 5.6 to 9 by adding sodium silicate. At a flow rate of 9.5 cm/s, the effect is dramatic. With increasing magnetic field values, the tin recovery in the mags increases as expected but much more so at the higher pH. At a higher flow rate, 15 cm/s, the effect nearly disappears indicating that the surface effects are less important. This sample was from the Colavi mine and at increasing values up to 35 kOe, at least half the iron is removed in our preliminary tests.