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Solid-Liquid Separation '02
The first Minerals Engineering Solid-Liquid Separation conference, held in Falmouth, UK, from the 18th to 20th June 2002, was attended by roughly 50 delegates from 15 countries. The conference was jointly sponsored by Larox Corporation, who have developed a range of high-performance pressure filters, and by Axsia-Mozley, who offer both advanced hydrocyclone circuits for de-sliming and equipment for fine particle separation. The technical programme featured presentations on a) techniques for fine particle separation, thickening, de-watering, and moisture measurement, b) additives to enhance the settling rate of solids, and c) modelling of settling behaviour and flow phenomena in separation techniques.
Solid-liquid separation techniques can be subdivided into the following classes: centrifugal techniques with or without moving internals, gravity thickeners, and filtration devices using pressure or vacuum as driving force. Of the centrifugal techniques with moving internals, the screen bowl and the Hydroplex counterflow classifier were featured. While the screen-bowl centrifuge is the most widely used technique in the US for fine-coal dewatering, optimization of the design and the operating parameters is ongoing to cut the loss of fine coal particles (-45 m) without increasing the moisture content of the product coal (M.K. Mohanty). The Hydroplex counterflow classifier is designed for accurate classification of particles between 2 and 100 m. Successful applications of this technique were described (G. Adam).
The hydrocyclone is a prime example of a centrifugal technique without moving internals. A hydrocyclone without a vortex finder ("stub cyclone") was deemed the most effective technique for treating Sas-Chesmeh copper anode slimes to reduce the content of barite, which was mainly present in the +10m fraction (M. Abdollahy). Similarly, a case study at a mine in the Andes using data from an integrated circuit consisting of Axsia-Mozley hydrocyclones with optimized spigot sizes can effectively reduce the fraction of fines (-11 m) reporting to the flotation circuit (J. Turner). The diameter of the vortex finder and the spigot, the feed density and size distribution, and the flow rates all influence the hydrocyclone performance (B. Firth). Steady-state Computational Fluid Dynamics simulation can reveal many features of particle separation in hydrocyclones, provided rules are set up for decoupled Lagrangian particle tracking and a suitable turbulence closure model is selected. An RSM closure model should outperform the standard k- closure model (C.A. Petty).
In the field of gravity thickening, the development of the Eimco High-Density and Deep Cone thickeners appears to reduce the required surface area and increase the underflow solids concentration. In addition, preparation of the feed through the E-Duc unit is recommended (L. MacNamara). Recognizing the significance of feed preparation, the mixing in feedwells was studied with three-dimensional computational fluid dynamics (T. Nguyen). Establishing the surface area of a thickener using Kynch's theory is suspect in the case of flocculated suspensions. New software based on a model developed by Bürger and Karlsen is now available. Future extension of the software focusses on including the effect of the rake on the underflow solids concentration (J.F. Concha). A rake moves sediment to the underflow and assists in sediment dewatering. A tentative explanation obtained by flow visualization and Computational Fluid Dynamics simulation is that particles are pulled down into the low-pressure wake behind the rake (J.B. Farrow). An interesting option to enhance the settling rate could be the application of ultrasound. For de-watering clay prior to ceramic processing, where addition of flocculants is chemically undesirable, trials with ultrasonic treatment showed positive results (G. Onal).
A common method to enhance the settling rate of sediments is to add a flocculant. Natural polymers, such as nirmali nut (India), burn-corn starch (Peru), tuna cactus (Chile), guar gum, or gelatin, were traditionally used. The advent of synthetic flocculants revolutionized flocculation by offering relatively high molecular weight flocculants and increased chemical functionality. Further development has lead to flocculants with advantageous narrow molecular weight distributions (M. Pearse). The selection of a suitable type and dose of flocculant is influenced by the mixing energy. An optimum settling rate is observed for a mixing energy where undermixing of flocculant and floc attrition are balanced. Measurements suggest that the latest flocculants from Ciba disperse more readily and make stronger flocs (S. Weir).
While measurement of the settling rate ignores mixing, variation of the shear rate, and is sensitive to operator skill, a new technique to assess flocculant performance was developed. It consists of a "shear" vessel with an inner rotating cylinder creating Couette flow conditions. The floc dimensions are continuously monitored through measurement of the chord length with Focussed Beam Reflectance measurement (P.D. Fawell). An alternative technique to assess flocculant performance uses a baffle reactor with a Rushton impeller with external measurement of the floc density. A comparison of settling behaviour with and without a flocculant showed that flocculant addition improved the underflow solids concentration in a permeability-controlled regime although this was not necessarily true for high underflow solids concentrations in a compressibility controlled regime (R.G. de Kretser). While many synthetic flocculants are toxic, a harmless bioflocculant could prove attractive. A strain of Bacillus Firmus was isolated which shows high flocculating activity in kaolin suspensions (M. Abdollahy).
Filtration is generally the final stage of de-watering. A new generation of pressure filters was developed which appear to combine modern design with increased reliability and ease of maintenance (I. Townsend). During pressure filtration, the initial stages of cake formation are important. A micropressure transducer capable of measuring the liquid pressure profile in the cake was developed and initial measurements show that stepped pressure tests, equivalent to variable pressure filtration, are promising (E.S. Tarleton). Related experiments involving de-watering coal fines by vacuum filtration showed that intermittent release of the vacuum significantly increased the subsequent filtrate flow (Q.P. Campbell). A decline of filtrate flow may occur during prolonged filtration due to fouling of the filter. While a regular backflush, preferably with water, can alleviate this effect, using filters with slotted instead of circular pores is also beneficial (R.G. Holdich). A large fines fraction, typically in excess of 10 % , can significantly reduce the permeability of the filter cake. An interesting option to maintain sufficient permeability is to utilize a filter aid with a distinctly different particle size (A. Yelshin). As an industrial case study, the beneficiation of ore and mine brine at Cleveland Potash was described. A horizontal belt filter and an E-cat thickener improved the efficiency of brine saturation prior to separation and reduced the loss of brine from the tailings (R. McConnell). In some cases, precipitation of particles through control of pH and Eh combined with underflow recycle seeding could prove effective (P. Evans).
In general, prediction of the performance of de-watering techniques can benefit from integration of measurements with stepped pressure filtration and equilibrium batch settling and measuring across a wide range of solids concentrations (R.G. de Kretser). For rapid selection of suitable equipment, an expert system is available free of charge at www.filtration-and-separation.com (R.G. Holdich). For on-line measurement of the moisture content in granular materials on a conveyor belt, a novel method was developed which measures the drag force on a probe which is subsequently interpreted (P.A. Cancilla).
The social programme featured a gala dinner in the Falmouth Beach Hotel, a coastal walk led by Barry Wills, and a boat trip kindly provided by Axsia-Mozley. The boat excursion offered picturesque views of the Fal estuary, live jazz music, and the characteristic Cornish pasties and saffron cakes. A stop in St. Mawes allowed for exploration of this pleasant village, which houses a number of historic pubs. All these activities contributed to the animated atmosphere which is certainly a feature of the Minerals Engineering Conferences.
Papers from the conference will be published in Volume 16 Number 2, a special 'Solids-Liquid Separation' issue of Minerals Engineering.
Prof. H.J. Glass, Camborne School of Mines, Redruth, UK. Email: H.J.Glass@csm.ex.ac.uk
A further report on the conference can be found in the October 2002 issue of World Mining Equipment (pps. 25-31).
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