Conditioning circuit analysis for slimes management in quarries

Driven by increasingly stringent environmental regulations on water usage, many European quarries are in the process of adding thickening circuits to their wash plants for managing clayey tailings. One of the critical components of this circuit is slurry conditioning. With typical water consumption over 1 m(3) per ton of washed aggregates, quarries produce dilute slime streams that are conditioned with high flocculant dosages to maintain water clarification rates as high as possible. At present, conditioning systems used in the quarrying industry are designed from elementary rules-of-thumb derived from experience. Practitioners acknowledge that conditioning system design criteria should be investigated further in order to design more efficient full-scale conditioning systems. This work focuses on weir-based conditioning tanks used in European quarries. The paper presents an applied analysis of such conditioning systems based on a comparison between a full-scale conditioning circuit and a controlled laboratory conditioning set-up. Within the range of plant operating conditions, this approach shows that full-scale conditioning is essentially governed by average shear rate and conditioning time. However, fine floc size distribution measurements reveal that quarry slimes conditioning is a dynamic process. This explains the high sensitivity of the process to variations in hydrodynamics, and the challenges of industrial conditioning system design.