The behaviour of wash water injected into a froth

Wash water is introduced into flotation froths to reduce the amount of entrained material reporting to the product. However, the behaviour of wash water in a froth is not well understood at present, and detailed information is sparse. The design of wash water delivery systems is currently based largely on trial and error. In the present study, experiments were conducted in a two-dimensional froth column to observe the movement of wash water in a froth, using various types of injector. Thus wash water was injected through a vertical pipe placed above the froth; through the same pipe immersed in the froth; and through a tee that resulted in two opposed horizontal jets. The movements of the wash water were observed, and quantitative liquid-fraction maps obtained, for each system. With the horizontal jets, vortex pairs were observed to form in the froth as a result of the wash water injection. In terms of their ability to spread wash water in the horizontal direction, the horizontal jets performed better than the vertical jet within the froth, which gave better results than the same jet falling on the free surface of the froth. The results show that whatever system is used, considerable back-mixing occurs between the froth and the added wash water, in the froth layer. Thus it is never possible to remove all the gangue particles from the flotation product. The best that can be done is to minimise the recovery of gangue by finding the optimum combination of the volume of froth beneath the injection point, the volume of froth above the injection point, and the gas and water flow rates. These observations are independent of the type of wash water distributor used. (c) 2006 Elsevier B.V. All rights reserved.