Improved dewatering characteristics of uranium tailings through controlled neutralization for paste disposal

Tailings from a uranium mine have been tested to improve the dewatering properties and final weight percent (wt%) solids of the neutralized tailings. The objectives were to maximize water recovery due to the scarcity of water in the project area and to produce a suitable cemented paste fill to minimize ore dilution underground in order to maximize the head grade to the mill. The applicability of the controlled neutralization process, in which gypsum would precipitate as crystalline particles thereby improving the dewatering properties of the tailings through the use of a deep bed paste thickeners was tested. The acid leach slurry samples used were obtained from the end of the leaching circuit, before uranium recovery and lime neutralization. The tests conducted indicated that the controlled neutralization process greatly reduced the volume of neutralization precipitates which typically results in a higher final density and wt% solids. Additionally it improved the dewatering (filtration) properties of the material. Controlled neutralization also reduced the amount of sulphate in the solution close to that of its saturation limit, which would reduce the amount of scaling that could occur during full scale applications and would increase the recyclability of the water back to the process. Deep bed paste thickening also improved the final wt% solids of the underflow. The test work demonstrated that it was possible, at the bench scale level, to improve final density and recover additional water. The increase in the filterability of the material provided a good indication that further increase in wt% solids and water recovery could be obtained in full scale applications. Unconfined Compressive Strength (UCS) tests improved for samples with reduced gypsum concentrations. While the strength achieved remained low, there was a substantial increase. Although further studies are required, neutralization of uranium tailings under controlled conditions to form gypsum as crystalline particles and/or reduce the gypsum content could possibly provide a viable option in the preparation of suitable paste material. (author)