Characterization of Multiple Hindered Settling Regimes in Aggregated Mineral Suspensions

Abstract

Aqueous suspensions of magnesium hydroxide are shown to exhibit low zeta potential behaviour and highly complex settling dynamics. Two distinct regimes of hindered settling behaviour are observed either side of a threshold concentration, φ * , of 2.38 % v/v, which is considerably below the gel point, φ g , observed at 5.4 ± 1.6 % v/v.The low concentration regime was characterised by a very large Richardson and Zaki exponent of 146, a factor of 10 larger than that of the high concentration regime. Michaels and Bolger analysis of the low concentration regime implies settling governed by large, low density macro-aggregates of 138-147 µm diameter and low intra-aggregate packing fractions in the order of 0.05, which is in good agreement with in-situ particle char-

Yield stress dependency on the evolution of bubble populations generated in consolidated soft sediments

“…The brucite test material used in this study is H3 Versamag (Martin Marietta Magnesia Specialties LLC, USA), a fine white precipitated powder with less than 1.2% oxide impurities . The advertised median particle size is 1.09 mm, however, a light scattering particle size distribution using a Malvern Mastersizer revealed a median diameter of 4.4 mm, reflecting the low zeta potential of aqueous magnesium hydroxide suspensions and a tendency for rapid aggregation . Soft sediments were prepared by the addition of tap water and agitation for 30 min using an overhead stirrer with an axial flow impeller.…”

“…47 The advertised median particle size is 1.09 mm, 47 however, a light scattering particle size distribution using a Malvern Mastersizer revealed a median diameter of 4.4 mm, reflecting the low zeta potential of aqueous magnesium hydroxide suspensions and a tendency for rapid aggregation. 48 Soft sediments were prepared by the addition of tap water and agitation for 30 min using an overhead stirrer with an axial flow impeller. In situ gas generation was achieved by addition of 35% w/w hydrogen peroxide (Merck Chemicals, Germany) which decomposes within the bed to form oxygen bubbles according to Eq.…”

Impact of preflocculation on scroll decanter centrifuge separation performance

“…The reader is directed to specialized publications for discussions on terminal settling velocity which take in to account the effects of particle shape and neighbouring suspended solids. [27][28][29] In summary, conducting the preflocculation of the centrifuge feed improves the separation efficiency because: 1) of the higher flocs residence time; 2) introduction into the machine of flocs that settle faster; and, as a consequence, 3) the formation of a pool whose lower solids content improves the settling rates of flocs.…”

Concentration profiling using a novel acoustic backscatter system with single transducers pulsed at multiple frequencies

“…as is the attenuation due to the sediment, kt is the transducer constant, which depends on the system and is determined experimentally, and ks is the particle species backscatter coefficient given by (2). where a is the particle radius, s is the particle density and f is the form function (angled brackets indicate an average over the particle size distribution).…”

“…This legacy is due, in part, to the large cost for storage and processing of waste sludges, because of the high risk of radiological or toxicological contamination making characterization difficult or costly. For this reason, much research has been directed towards developing technologies for the remote characterization of nuclear waste for cleanup processes [2] [3]. Acoustic backscatter systems (ABS) represent such a technology, as they can be used non-intrusively to measure both particle size and concentration depending on prior system knowledge [4].…”