The utilisation of a coiled plug flow reactor for the flocculation of kaolin slurry

Transcription

1 The utilisation of a coiled plug flow reactor for the flocculation of kaolin slurry Tutun Nugraha Andy Fourie School of Civil and Resource Engineering University of Western Australia Yee Kwong Leong Lavanya Avadiar School of Mechanical and Chemical Engineering University of Western Australia

2 Introduction In many mine tailings processing operations, kaolinite is a common clay mineral that can comprise a substantial proportion of the tailings materials. In water, the kaolinite clay forms a stable colloidal system that is difficult to separate from water synthetic anionic flocculants are capable of binding clay particles together into large flocs, thus, facilitating water/clay separation through sedimentation SEM image of Kaolin

3 Introduction Flocculants, which are high molecular weight polymers, with significantly long chain lengths, are added into the slurry Aggregates of fine kaolin particles will then form. The thickening and dewatering of the tailing slurries will then take place SEM image of flocculated Kaolin

4 Scope of Study a coiled reactor was designed to provide an in-line flocculation system for settling and consolidation study The behaviour would approach a typical Plug Flow Reactor (PFR), with some differences due to the slightly different nature of the flow within the helical reactor The reactor could provide flocculated slurry of kaolin with good control of the amount of shear during the flocculation process Upon reaching the outlet, the slurry is directly sent to the settling column to avoid alteration in the properties of the slurry during transfer.

5 Parameters Slurry settling characteristics were investigated using settling rates as the main parameter Evolution of % solid as settling/consolidation occurs Evolution of pore water pressure with time Particle size distribution Variables studied effects of the concentration of flocculants (26, 50 and 75 g/ton) Effects of initial kaolin concentration (3% and 8% w/w) the number of split injection of flocculants (2 and 4 injection ports) the effects of raking on consolidation Materials: Kaolinite was obtained from Prestige NY (Australia) Flocculant used was Magnafloc 336, a linear anionic polyacrylamide of medium ionic content, with very high molecular weight (BASF) All experiments were using deionized water

6 Experimental Setup Slurry mixing tank and slurry settling column

7 Gamma Radiation base Density Gauge To measure slurry density across height Acrylic settling Column, 140cm height, 20.2 cm OD, 17.3 cm ID The Cs-137 source holder & detector are placed on moving platform

8 Experimental Setup Built using Transparent Plastic tube 2.54cm ID (1 ) reinforced with steel wire, 10 m long Coiled into 20 cm Dia. forming helical shape Tightly coiled, with each layer of coil touching each other: pitch = tube diameter Pressure probes installed at inlet & outlet to measure pressure drop across the reactor 2 or 4 flocculant injection ports, thus the 10m reactor is built into either 2 x 5m (50% split) or 4 x 2.5m (25% split)

9 Characterisation of the Coiled Reactor: Operating Parameters Reynolds number The main parameters used is the Reynolds number which will give measurement of the level of turbulence within the tube (to induce good mixing) Re was kept high at 17,128, i.e. in turbulence region for good mixing between kaolin slurry and flocculant Note: transition to turbulence in coiled tube is higher due to the effect of curvature from Re. 2,000-4,000 (straight) to Re 7,000-12,000 (curved) In Coiled tube, mixing is not only measured by Re but also vortex that occurred as measured by Dean Number

10 Dean Number and Dean Vortex To measure the effect of curvature Dean Number is used. Dean Number provide some measurement of the appearance of Dean vortex Dean Vortices: Twin vortices that occur within flow, and improve mass transfer/mixing (De = 6098) D= Coil Diameter, d = tube dia., p = pitch and Blood Flow in the Rabbit aortic arch and descending Thoracic Aorta, P.E. Vincent et.al., Journal of the Royal Society Interface

11 Torsion effect: Germano number (Gn) To quantify the effect of torsion: dimensionless Germano number (Gn) is used Gn would promote turbulence

12 Magnitude of Shear (G) Also, Shear (G, s -1 ) was evaluated. The values gives measurement of the applied shear: energy for mixing and flocculation Measured based on pressure drop across reactor, affected by % solid within flow & the number of flocculant split injection ports

13 Results: Settling rates & consolidation As the flocculated slurry was sent to the settling column, settling would begin to occur The lowering of the slurry interface was manually recorded to facilitate the calculation of the initial settling rates % Solid varied with time & heights Effect of Rake on consolidation

14 Settling rates as Function of % Solid, Flocculant Conc. & Split injection At lower % initial solid (3% w/w) settling rate increased with flocculant doses, but not sensitive to flocculant split injection At higher % initial solid (8% w/w) settling rate also increased with flocculant doses, but also very sensitive to flocculant split injection Split doses of flocculant appeared to improve mass transfer and mixing/ contact between growing aggregates & fresh flocculant molecules

15 Optical Microscope: Flocs physical appearance 26g/ton split4 75g/ton split4 Aggregates with fast settling rate showed different physical appearance: Larger flocs facilitate faster settling rates Also, differences in final % solid after consolidation

16 Flocs physical appearance: with/without flocculant Settling rate 0.6 m/hour Versus 25.1 m/hour

17 Particle size distribution (8% initial kaolin) There is a clear shift to larger particle size upon addition of flocculant Differences could be observed between 2 & 4 injection point at 8% solid more efficient shift toward higher size aggregates with 4 ports Consistent with settling rate data: higher settling rate with 4 injection port of flocculant

18 Flocs Growth with subsequent injection of flocculant Aggregates were initially very small, but continuously grow during the flow & with subsequent flocculant injection The observed growth would be a balance between growth & breakage For aggregates with sufficiently large size, movement restriction due to its size limit possible contact between aggregate/fresh flocculant Growth required the meeting of two floc aggregates & Fresh flocculant Subsequent injection ports will supply fresh flocculant to allow further growth of flocs, turbulence at injection port will further assist

19 Effects of raking (%solid & pore pressure changes) Flocs contact Flocs breakage & resuspension Alternate higher/lower pressure expel water from aggregate

20 Conclusion Flocculation of kaolin clay slurry was carried out using in-line coiled flocculation reactor, operated within turbulent region The flocculant was injected into the flow of kaolin slurry along the reactor, using 2 or 4 flocculant injection points at flocculant doses: 25, 50 and 75g/ton) Despite the short residence time (14.8s), the reaction could occur to a sufficient extent to produce slurries with settling rates between m/hour (initial kaolin 3% w/w), and between 0.6 to 25.1 m/hour (initial kaolin 8% w/w) The impact of changing the number of flocculant injection points from 2 to 4 ports was found to be more significant at a higher initial kaolin concentration

21 Acknowledgment The authors would like to acknowledge the Australian Research Council s Discovery Projects that has supported this research. We also would like to acknowledge BASF Performance Chemicals for valuable discussions