Abstract
A downcomer, as employed in the Jameson Cell, was considered for producing solvent-coated bubbles for possible application in a technique referred to as “air-assisted solvent extraction” (AASX). It was found that the bubble bed in the downcomer started to collapse upon introducing solvent, evident from decreasing air holdup, decreasing aspirated air rate, and formation of air slugs. Neither changing solvent introduction method, solvent composition nor addition of frother or salt alleviated the collapse, which could become complete (air rate became zero) at about 900 ppm solvent. To explain, the bubble bed, which in the downcomer can approach 60% air holdup (Atkinson et al., 1993; Jameson and Manlapig, 1991), was likened to a foam that is known to collapse in the presence of hydrophobic liquid (oil) droplets due to a bridging/spreading mechanism that expels water between contacting bubbles. Tests with hydrophobic solids, graphite and talc, gave increased bubble bed stability showing it is the liquid nature of the solvent that is the key factor causing the collapse. One combination, talc and MIBC, did provide stability against solvent addition. Froth collapse/stability mechanisms are discussed. It is noted that the Jameson Cell is successfully employed to remove solvent droplets in SX plants. The solvent levels in those cases appear to be less than 200 ppm where collapse is only partial and the phenomenon may not be noticed.
Published Version
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