APSC 150 Lectures-Part II

APSC 150 Lectures-Part II - FROTH FLOTATION A...

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Unformatted text preview: FROTH FLOTATION A physicochemical process that depends upon the selective attachment to air bubbles of some mineral species (valuables) and the simultaneous wetting by water of others (tailings). Mineral particles that easily attach themselves to air bubbles are called hydrophobic (water-repelling) particles. Mineral particles that do not attach themselves to air bubbles are called hydrophilic (water-attracting) particles. Flotation of hydrophobic particles should not be confused with flotation of light particles in gravity separation methods. Gravity based methods rely on differences in bulk properties of solids, mainly density. Froth flotation relies on differences in surface properties of minerals with density being of secondary importance. For a mineral particle to float with air bubbles, the surface of the mineral must show certain properties that make attachment of the particle to an air bubble very easy. The main surface properties of interest in froth flotation are: 1)The wettability of the surface by water, 2)The surface charging characteristics of the particles (positive, negative, or zero) Blue particles - valuables Red particles tailings dispersed in water forming a slurry with a known solids content. The slurry is placed in a tank, referred to as a flotation cell. Continuous sparging system supplying compressed air. As aeration continues, air bubbles migrate upward through the slurry, collide with ALL particles, but only the valuable (hydrophobic) particles can attach themselves to the bubbles, and are so brought up to the surface. At the end, the valuable particles form a froth (mix of solid particles, air bubbles and some water), while unwanted minerals remain in the cell as tailings slurry. The froth is then mechanically skimmed into a launder to obtain a flotation concentrate. Quartz particles attached to air bubbles. Flotation of galena (PbS) particles. Bubble sizes are around 2-3 mm. Flotation of quartz (SiO 2 ) particles. Chalcopyrite Myra Falls (BC) Molybdenite Highland Valley Copper (BC). Apatite Kapuskasing (ON) Coal Fording River (BC) Sphalerite Olkusz (Poland) Sylvite Vanscoy (SK) Sylvite Vanscoy (SK) Sylvite Vanscoy (SK) Problem: How big an air bubble does it take to float a spherical galena particle with a diameter of 50 micro-meters (m) ? Density of air at T = 250C and p =101.3 kPa is 0.0011839 g/cm3. Density of galena (PbS) is 7.6 g/cm3. In essence, the problem is identical to the gold nugget problem. We can write: Using volume fractions of air ( x ) and galena ( 1-x ), we can derive: PbS Air PbS PbS air Air PbS Air V V d V d V d + + =- PbS Air PbS PbS Air d d d d x-- =- Our galena-bubble aggregate will float only if its density is lower than the density of water (1.0 g/cm3). So, putting 1.0 g/cm3 for d Air-PbS , we find that x = 0.869. In other words, 86.9% of the total volume of our aggregate should be air and 13.1% PbS....
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This note was uploaded on 11/14/2010 for the course APSC 150 taught by Professor Muhan during the Spring '10 term at Wooster.

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APSC 150 Lectures-Part II - FROTH FLOTATION A...

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