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Chemical Engineering Journal 172 (2011) 914 923 Contents lists available at ScienceDirect Chemical Engineering Journal jo u r n al hom epage: Shear-induced fl oc structure changes for enhanced dewatering of coal preparation plant tailings Philip Oforia, Anh V. Nguyenb, Bruce Firtha, Clint McNallya, Orhan Ozdemirb,1 aCSIRO Energy Technology, Pullenvale, QLD 4069, Australia bSchool of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia a r t i c l e i n f o Article history: Received 24 May 2011 Received in revised form 30 June 2011 Accepted 30 June 2011 Keywords: Coal tailings Shear-induced fl occulation Fractal analysis Dewatering a b s t r a c t The handling and disposal of industrial waste slurries containing colloidal particulate matter, with par- ticular reference to coal and mineral processing plant tailings with a high proportion of clay materials present signifi cant challenge. To develop effective disposal methods requires an improved understanding of fl oc structure that promote better water recovery and higher sediment density during settling and con- solidation of fl occulated tailings. In this paper, the effect of fl oc structure on coal tailings dewatering and the impact of shear-induced modifi cation of fl oc network structure on additional sediment consolidation and densifi cation have been examined. Four real tailings and a high molecular weight anionic polymeric fl occulant were used. Flocculation tests were conducted under reproducible hydrodynamic conditions in batch mode. Flocculated aggregates have complex structures with different sizes, shapes and voidages. The structures of the fl ocs were characterised by fractal analysis using light scattering technique and the dewatering characteristics of the fl ocs were investigated in terms of settling rates and sediment consol- idation. The fractal analysis by laser scattering identifi ed optimum mixing speed and fl occulant dosage required for the production of fl ocs with compact structure of fractal dimensions greater than 2. It was determined that hydrodynamic conditions during fl occulation have a profound impact on fl oc settling rates and to a lesser extent sediment consolidation. Low mixing speeds produced large fl ocs with high settling rates, while very high mixing speed during fl occulation resulted in irreversible fl oc degradation leading to much reduced settling rates with some additional sediment consolidation. Application of shear to pre-sedimented fl ocs resulted in signifi cant additional sediment consolidation for the tailings samples and fl occulants studied. This approach would allow fl occulation to be optimised for fast settling with subsequent application of shear to the settled sediment to promote the removal of inter and intra-fl oc liquor to improve consolidation and sediment density. Crown Copyright 2011 Published by Elsevier B.V. All rights reserved. 1. Introduction The effective management and disposal of process effl uents containing colloidal particulate matter with particular reference to fi nes rejects with a high proportion of clay minerals gen- erated during coal and mineral processing are important but present considerable challenge. Current practice of storing the waste slurry in tailings dams to settle over long periods is not environmentally sustainable. Better tailings management solu- tions that produce drier end product would reduce environmental impact and also reduce water consumption and increase water recycling. The thickened tailings approach that is increasingly being adopted in the hard rock mineral processing industry could be Corresponding author. E-mail address: philip.oforicsiro.au (P. Ofori). 1 Present address: Istanbul University, Faculty of Engineering, Department of Mining Engineering, 34320, Istanbul, Turkey. an effective tailings management pathway for the coal indus- try. The use of chemical additives, particularly high molecular weight polyelectrolytes is a critical component of effective fl oc- culation and dewatering. The structure of the fl ocs produced strongly infl uences the consolidation behaviour of the settled sed- iment and the density of the dewatered product. In order to improve the functioning of dewatering systems, it is important to understand how the fl oc aggregate network structure, infl u- ences sediment consolidation and the dewatering process and how controlled alterations to the structure could enhance the pro- cess. Shear-induced structural changes to the fl oc network to favour additional consolidation could be a viable approach to improving the density of the dewatered tailings sediment. Previous studies found that fl oc structural changes by application of shear, enhanced dewatering due to aggregate densifi cation 13. Mpofu et al. 4 found improvement in consolidation of pre-sedimented kaolin- ite when fl occulated with polyethylene oxide (PEO) but not with polyacrylamide (PAM). McFarlane et al. 5 found modest improve- 1385-8947/$ see front matter. Crown Copyright 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.06.082 P. Ofori et al. / Chemical Engineering Journal 172 (2011) 914 923915 ments in sediment density by mixing pre-sedimented kaolinite and smectite. Flocculated aggregates are characterised by complex structures with different sizes, shapes and voidages. The optimal dewater- ing conditions and relationships between the size of fl occulated aggregates and the structures of fl occulated aggregates/beds are a function of parameters such as type and concentration of coagu- lants and fl occulants, the colloidal interaction forces, and mixing regime. Large, fl occulated aggregates usually settle very fast but have very loose structure and form less compact beds of consoli- dation with signifi cant inter and intra-fl oc liquor. Small aggregates settle at a slower rate but form more compact structure and sedi- ment beds. The challenge has been to create fl occulated aggregates having both desired characteristics, i.e., big size and compact struc- ture, without changing and modifying the devices. Two approaches may be taken to optimise the fl oc size and structure. The fi rst is by application of chemical dewatering agents to modify the chemical interaction forces followed by hydrodynamic agitation during fl oc- culation to produce the desired compact fl oc structure for increased sediment consolidation. This approach usually requires a trade-off between large, bulky aggregate structure with fast settling rates but poor compaction and more compact fl ocs with lower settling rates but good consolidation of sediment beds. The second approach is to use gentle mixing during fl occulation to form large, fast-settling but loose fl ocs during fl occulation and apply shear to the settled fl ocs to enhance dewatering by expelling more inter and intra- fl oc liquor. Both approaches were experimentally explored in this study. While research on polyelectrolyte fl occulation is extensive, e.g., Hogg 6, Dentel et al. 7 and Scott et al. 8, relatively little research has focussed on the fl oc structures formed during fl occulation and the changes to the aggregate structures induced by mixing. The rea- son for this is thought to be due to the lack of suitable methods of properly describing the complex structures of particle aggregates until recently 9. Fractal analysis techniques have been used to characterise fl oc structures in colloidal systems using light scatter- ing 10,11 which have provided indications of the structure of the aggregates. The aim of this work was to examine the structure of fl occulated tailings and the application of shear-induced structural changes in fl ocs for improved dewatering results. This will provide insights into the relationships between the size of fl occulated aggregates and the structures of fl occulated aggregates/beds as a function of type and concentration of fl occulants and the infl uence of agitation. The knowledge gained would assist in the use of chemical additives to effectively control the sedimentation and consolidation process to achieve desired dewatering outcomes. The ultimate aim is to use this knowledge to dramatically reduce the volume of waste slurry currently produced in coal processing for a more environmentally sustainable industry. 2. Experimental 2.1. Materials and characterisation Coal preparation plant tailings containing colloidal particu- late matter in water suspension from four operating mines in Australia were used in this study with two of the mines located in Hunter Valley, New South Wales and two in Bowen basin, Queensland, Australia. The samples were taken from the thick- ener feed stream before fl occulants had been added to minimise the complication of residual fl occulants, and were designated as samples A, B, C, and D. The solids concentrations of the tail- ings were 27.7% for tailings sample A, 19.3% for tailings sample B, 23.3% for tailings sample C and 18.8% for tailings sample D. The tailings samples were characterised for size distribution, chemical assay, mineralogical composition, particle surface (zeta) potentials, and chemical composition of the associated process water. The particle size distributions of the tailings samples were deter- mined by an optical method based on light scattering analysis using a laser particle size analyser (Mastersizer E2000, Malvern Instru- ments Ltd., Worcestershire, UK) and by sieve analysis. The particle size fractions obtained by sieving were analysed for ash content by ashing at 815C. The tailings were also characterised by X-ray fl uo- rescence (XRF) spectrometry using a Philips PW 2400 spectrometer (the ashed samples were each fused with lithium metaborate and cast into a disc for XRF). The mineralogy of each tailings sample was analysed by X-ray powder diffraction using a Phillips Xpert diffractometer with copper K? radiation, and the minerals present identifi ed by reference to the ICDD Powder Diffraction File. Quan- titative analyses of the crystalline mineral phases in each sample were made using SIROQUANTTMcommercial interpretation soft- ware written by CSIRO 12 based on the Rietveld XRD analysis technique. The particle surface potential measurements were carried out using a Malvern Zetasizer Nano-ZS instrument (Malvern Instru- ments Ltd., Worcestershire, UK). Suspensions of 0.5 wt% were prepared from the main tailings suspensions. Further size reduc- tion down to 5 microns to meet the Nano-ZS requirement was conducted by wet grinding using a mortar and pestle. For zeta potential measurements as a function of pH, a 20 mL sample of representative suspension was taken from the main suspension, and mixed for about 5 min in order to reach the equilibrium point after adding desired amount of acid (HCl) or base (NaOH). The experiments were carried out at room temperature. The concen- trations of major cations in the process water were determined by Atomic Absorption Spectrometry (AAS) and anions concentrations were determined by Inductively Coupled Plasma Mass Spectrome- try (ICP-MS). A high molecular weight partially hydrolysed anionic polyelec- trolyte was used. The selected fl occulant was designated as F1. 2.2. Fractals of fl occulated aggregates The size and structure of fl occulated aggregated were statis- tically characterised by fractal (self-similarity) analysis using the light scattering technique to measure and analyse intensity of laser light scattered from an aggregated sample as a function of incident angle. The fractal measurement was carried out using a Malvern Mastersizer S instrument (Malvern Instruments Ltd., Worcester- shire, UK). The experimental set-up is schematically shown in Fig. 1. The experimental procedure was as follows. First, the required amount of fl occulant was added to a 100 mL of tailing suspension and stirred at 400 rpm for 2 min (it is important to note that the tailing suspensions before addition of the fl occulant were mixed using an impeller at 2000 rpm). Then, the suspension was allowed to settle for 30 min. Finally, a small amount of fl ocs at the bottom of the beaker was taken with a wide-diameter pipette (in order to avoid breaking the fl ocs) and added into the 1 L of deionised water. The fl ocs suspensions were then stirred gently using an overhead impeller at 400 rpm or 800 rpm (in order to keep the fl ocs in the suspension during the measurements) and fed to the measurement cell of the Malvern instrument. In the measurement, the size of fl occulated aggregates was determined based on the scattering of a laser light (of a wave length ? = 527 nm) which was directed onto the aggregates under differ- ent incident angles, ? (from 0.015to 40.5). The scattered light intensity, I, was measured as a function of angle ?. An example of the dependence of the measured intensity on the incident angle 916P. Ofori et al. / Chemical Engineering Journal 172 (2011) 914 923 Fig. 1. A schematic of the experimental set-up used to determine the structure and fractal dimension D of fl occulated aggregates by means of fractal analysis. is shown in Fig. 2, where the incident angle is converted to the scattered wave vector Q as follows: Q = |Q| = 4?n sin(?/2) ? (1) where n = 1.33 is the refractive index of the liquid medium. The intensity of scattered radiation as a function of the scattering angle provides information about aggregate structures as a function of their length scale. For example, in the limit for the wide angle regime, the scattered light intensity is logarithmically proportional to the scattered wave vector and decays exponentially with the (mass) fractal dimension, D, of the aggregates as I(q) qDand D can be obtained from the slope of the logarithmic plots of I versus Q. In Fig. 2, the slope at wide angle (i.e., at q from 1 104to 1 102) gives the fractal dimension D = 1.93. 2.3. Tailings fl occulation and dewatering tests Sub-samples of real plant tailings obtained from four coal prepa- ration plants with the process water were used in the fl occulation and sedimentation tests. The tailings were fi rst fl occulated with the selected fl occulant at varying fl occulant concentrations ranging from 0 to 600 g/t of solids. The tailings samples were prepared by adding the required amount of their respective high ionic strength process water to the slurry and mixing for 30 s followed by the addi- tion of fl occulant from a 0.5% stock solution and mixing for a further minute and half. Tailings-fl occulant mixtures were prepared in Fig. 2. An example of the loglog plot for the measured scattered laser light inten- sity, I, versus scattered wave vector, Q, obtained using tailings sample B, 600 g/t fl occulant F1 and 400 rpm stirring speed. glass beakers and then transferred into 500 mL graduated cylinders. Tailings suspensions were fl occulated under the selected repro- ducible agitation conditions to examine the infl uence of agitation and fl occulant dosage on aggregate settling rates and consolidation behaviour of the sediments as determined by sediment solids con- centration after decanting the supernatant. Agitation intensities of 150 rpm and 300 rpm were used as well as a very high agitation intensity of 10 000 rpm to determine if signifi cant improvement in sediment consolidation could be achieved by shear-induced struc- tural changes to the fl ocs. Settling tests were conducted at room temperature. Prior to starting measurements the samples were mixed by inverting the measuring cylinder containing the fl occulated slurry twice to ensure the fl ocs/particles were well dispersed. In the sedimentation experiment the decent of settling front at the interface between the supernatant and the particle-rich sediment was tracked by capturing digital images of the experimental system at regular intervals, typically 30 s. An image analysis approach was used to extract the position of the settling front as a function of time. The information was used to generate sedimentation profi les for each experiment, from which the initial settling rates were extracted from the slope of the initial linear portion of the plot. Typically the profi les were characterised by sharp initial settling rates followed by an intermediate region of reduced settling rate and then a period of consolidation of the sediment. 3. Results and discussion 3.1. Properties of plant tailings samples The pertinent size parameters and particle size distribution curves of the four samples as determined by light scattering and by sieving are presented in Fig. 3 and Table 1, respectively. It is seen that tailings sample A was much coarser than the others with about 64% of the material coarser than 0.125 mm. Tailings sample D was the fi nest with 73% of the material less than 0.038 mm. Table 1 also shows that most of the material in tailings sample D that was coarser than 0.038 mm is coal of very low ash especially for the size fractions coarser than 0.063 mm. The XRF results (for chemical analysis) as expressed by per- centages of the major element oxides in the dry ash sample are presented in Table 2. The major oxides are SiO2and Al2O3presum- ably originating from quartz and the silicates and aluminosilicate clay minerals. The XRD results (for mineralogical analysis) are summarised in Table 3 that lists the estimated weight percentage of the crystalline phases recognised in each sample. The mineralogical results indi- cate that kaolinite was the major clay mineral in all four tailings P. Ofori et al. / Chemical Engineering Journal 172 (2011) 914 923917 Table 1 Sieved size and ash distributions of tailings samples. Size fraction (mm) Sample A Sample B Sample C Sample D Mass% %Ash Mass% %Ash Mass% %Ash Mass% %Ash +1.00 11.7 35.5 1.00 + 0.500 17.7 36.4 0.500 + 0.250 18.8 32.5 9.0