Colloids and Surfaces A: Physicochem. Eng. Aspects 449 (2014) 157–169 Contents lists available at ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects journa l h om epage: www.elsevier.com/locate/colsurfa Wall effects during settling in cylinders Benjamin Buratto, Shane P. Usher ∗ , David Parris, Peter J. Scales Particulate Fluids Processing Centre, Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia h i g h l i g h t s • Compared settling of alumina slurries in cylinders and cylinders with verti- cal rods. • Polymer ﬂocculated solids shrank from rod and settled faster: Shear densiﬁcation. • Salt coagulated solids clung to the rod and settled slower: Syneresis. • Aggregation state inﬂuences settling rate changes in presence of vertical rods. g r a p h i c a l a b s t r a c t B – Shear densiﬁcaon A - Syneresis Rod Suspension Cylinder Liquid a r t i c l e i n f o Article history: Received 4 September 2013 Received in revised form 16 February 2014 Accepted 17 February 2014 Available online 28 February 2014 Keywords: Densiﬁcation ﬂocculation settling tests syneresis wall effects a b s t r a c t The phenomena of syneresis and shear densiﬁcation were analysed through the sedimentation of alumina suspensions. Experiments compared cylinder settling tests with a rod inserted in a cylinder against free settling without a rod for the cases of polymer ﬂocculated and salt coagulated suspensions. Additionally, dye intrusion was used to visualise the ﬁnal bed structure to identify the inﬂuence of cylinder walls and the potential presence of aggregate arching structures. It was found that, for various cylinder diameters, polymer ﬂocculated alumina suspensions settled more quickly when a rod was placed in the centre of the settling cylinder. This is attributed to shear induced densiﬁcation of the ﬂocculated aggregates during the settling process, as it was observed that the solids shrank away from the rod, creating a channel which allowed water to escape more easily than when the rod was absent. The reverse result was observed when a rod was placed in an unﬂoccu- lated/coagulated alumina suspension; the solids stuck to the rod, resembling the syneresis mechanism of isotropic shrinkage towards the rod. Industrially these results are signiﬁcant since inserting rods into sedimentation devices such as a thickener can result in a ﬂocculated suspension settling further and at a faster rate, such that rods can provide an alternative to mechanical devices such as rakes. Dye intrusion tests indicated that there was arching in the network bed, resulting in small pockets of water being trapped in the structure. An equation was derived to predict the maximum diameter arch that could be supported, taking into account the shear yield stress of the suspension. The solution to this equation was supported by observed cavities at the cylinder wall of up to 3 mm. These results suggest that cylinder walls may have a negative effect on the extent to which a suspension dewaters. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Solid-liquid separation techniques are used extensively in industry in a variety of applications. There are two main ∗ Corresponding author: Tel.: +61 3 8344 5592; fax: +61 3 8344 4153. E-mail address: spusher@unimelb.edu.au (S.P. Usher). mechanisms through which separation occurs; ﬁltration and sed- imentation. Filtration is based on separation by a physical barrier, as the liquid moves through the ﬁlter the solids are impeded. Sed- imentation is the movement of solids due to a density difference under the inﬂuence of a body force such as gravity or centrifugal acceleration. The focus herein is on sedimentation as it occurs in thickeners and clariﬁers typical of the minerals industry with the addition of ﬂocculants to aggregate ﬁne particles and achieve http://dx.doi.org/10.1016/j.colsurfa.2014.02.045 0927-7757/© 2014 Elsevier B.V. All rights reserved.