Mixing requirement for ultra paste tailings

Abstract

A controlling variable for a thickened tailings scheme design is the beach slope that will be formed by the tailings as they deposit. The slope will be mainly related to the discharge flow rate and the slurry rheology. The latter will, in turn, be dependent on the solids concentration of the thickener underflow. A higher solids concentration can be achieved by combining thickening and filtration. The objective is to still maintain a slurry that is pumpable and self-distributing at the tailings storage facility (TSF), rather than having to rely on trucks or on conveyors and stacking systems to manage a ‘cake’, but one that will deposit at a steeper beach slope than can be achieved by thickening alone. The use of ultra paste has the potential to broaden the range of topographic conditions that will suit thickened tailings discharge. The ultra paste thickened option is when a portion of the tailings is separated and dewatered in a more advanced dewatering unit (in this case filter presses). The filtered cake will then be re-mixed with the paste thickener underflow stream. This paper presents a case study for a large copper mine in which underflow from a series of paste thickeners with a nominal tonnage of 3,660 tph at 59% solids concentration would be mixed with 1,363 tph of filter cake at 80% solids concentration. The combined tailings (ultra paste) would be discharged equally into two open channels (flumes) at the combined solids concentration of 63.5%. The ultimate goal of adopting an ‘ultra paste’ scheme is to produce homogenous consistency tailings, hence proper mixing of the paste and filter cake has to occur. In this study based on a series of tests, the mixing requirement has been assessed. The study also covers a conceptual investigation of natural turbulent mixing (NTM) of the filtered tailings with paste thickened tailings and issues associated with the deposition of the combined tailings, the ultra paste, into the existing TSF. In the conclusion, the study indicates that NTM can possibly occur only for the case in which the total tailings are discharged into one channel, which is not practical due to limitations with regards to tailings management. Hence, mechanical mixing has been recommended in this case.