Show related SlideShares at end. WordPress Shortcode. Next SlideShares. Download Now Download to read offline and view in fullscreen. Download Now Download Download to read offline. Utsav Kant Follow. Engineer at Tata Steel. Finmet process. Crushing of coal and calculation of size reduction efficiency. What to Upload to SlideShare. Related Books Free with a 30 day trial from Scribd.
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The efficient separation of a broad size range of particles on the basis of particle density in a low-density fluid such as water remains a considerable challenge. There is a growing trend to replace existing technologies for beneficiating fine minerals and coal, nominally less than a few millimetres in size, with technologies based on fluidized beds.
Generally, fluidized beds tend to be hydraulically limited and are constrained by feed conditions, and hence the feed needs to be as concentrated as possible to maximize throughput. The device combines a conventional fluidized bed with sets of parallel inclined plates, as shown in Figure 1. Feed slurry enters below the plates while fluidization water is introduced through a distribution plate in the base. The slurry feed moves downwards into the vessel, forming a bed of particles that is fluidized from below.
High-density particles settle into the lower portion of the bed, and light and fine particles are transported upward, with the majority flow towards the lamellae. The high hydraulic load carries the suspension up into the parallel inclined lamella plates. Here slower settling particles, which are unable to settle against the fluidization water, emerge through the plates and report to the overflow.
Faster settling particles drop out of suspension and onto the plates before sliding back to the zone below. At high bed concentrations in the reflux zone, this reject suspension provides an autogenous dense medium, allowing the separation to proceed largely on the basis of density. When the density of the fluidized bed exceeds the set-point value, a valve opens near the base of the unit and discharges some of the denser particles as an underflow stream.
The inclined channels, the geometry of which is defined by the plate length, L, perpendicular channel spacing, z, and angle of inclination with the horizontal, h Figure 2 , provide a significant hydraulic advantage over conventional fluidized beds, consistent with the well-known 'Boycott Effect' Boycott, ; Ponder, ; Nakamura and Kuroda, ; Zhou et al.
As the fundamental research was funded by the Australian coal industry, all the initial testing and subsequent pilot and full-scale work were performed using Australian coals. Mineral testing started in South Africa many years after development of the first commercial units.
All the development details in this paper are therefore based on work with Australian coals. Following the laboratory and theoretical development Galvin and Nguyentranlam, ; Galvin et al. The unit was installed, commissioned, and studied at a Lower Hunter Valley coal preparation plant. The plates were inclined at an angle of 60 degrees to the horizontal and were spaced at mm intervals. In late , further research conducted by Galvin Galvin et al.
The inclined plates were inclined at an angle of 70 degrees to the horizontal and spaced at 6 mm intervals for this installation. The results of the and tests were reported by Orupold et al.
Figure 3 shows a relatively moderate D 50 shift with change in particle size when compared to other echnologies. Figure 4 shows that Ep values are still very low even at fine particle sizes. The latter are additional safety features and do not preclude in-plant installation of oversize protection and de-aeration of the feed.
Many features are shared with the commercial units, including the lamella plates, pressure probes, and fluidizing nozzles. A number of underflow valve configurations are available to cater for various applications.
A number of plants have in excess of six units. Most applications are as primary separation devices, although a number have been installed as scavenger units to recover lost coal from other processes. However, as noted by Galvin et al. Some segregation would occur. However, the applicant has observed that particles covering a broad range of size tend to remain partially mixed in a conventional fluidized bed.
Hence, in the absence of the plate lamellae a sharp separation may be difficult to achieve. A schematic representation of another embodiment of the particle classifier operating in a continuous mode is shown at in FIG. The particle classifier is readily operated in a continuous fashion by providing a fluidization flow at and pumping a particle feed slurry into an external feed well An internal feed well may be preferred, but may be more difficult to incorporate because of the plate lamellae.
The feed well provides a means for disengaging unwanted entities such as air bubbles from the feed slurry. The feed slurry ideally plunges into the middle zone of the classifier. In this embodiment plate lamellae C may be located above the feed entry position and a second stage lamellae B may be located below the feed entry position. For further refinement additional lamellae such as A and D may be used either side of the feed entry position. A lamellae free zone should exist between each set of lamellae A, B, C, and D.
These zones classify via the reflux mechanism. At the top of the fluidizing chamber liquid containing the finer particles spill over into a launder not shown or are withdrawn via an outlet for recovery in a separate vessel not shown.
Alternatively the solids leave in a more concentrated form at outlet from the zone between C and D. The coarser particles suspended near the base are pumped away to another vessel not depicted via outlet In general, an increase in the suspension concentration below the plates results in higher concentrations within the plates, and hence segregation on the basis of density within the plates.
With such segregation, it is possible to use the device to separate particles on the basis of density. Normally, the lower density particles will report to the overflow, and the higher density particles will report to the underflow. One way to achieve a higher concentration and hence promote this mode of operation is to operate using a relatively low underflow concentration In turn, the system naturally produces higher concentrations.
The higher concentrations could also be achieved using lower fluidization velocities. Therefore, the device can also be used to separate particles on the basis of density. It will be appreciated that solids or particles which are close to the separation condition have many opportunities to report to the correct position within the classifier such as 10 or Further, the existence of plate lamellae such as 12 or effectively amplifies the differences in particle velocities.
Although the fluidization chamber has been described in the embodiments above as typically square in cross-section and of constant cross-section throughout its height, it is also possible to vary the shape and cross-section of the chamber in order to provide additional control. For example, in FIG. By controlling the cross-section in this manner, the fluidization rate through each set of lamellae may be individually controlled even through there is a common fluidization rate supplied at at the bottom of chamber.
For example the side walls in region are angled to conform with the angle of inclination of the plates in lamellae , and similarly the side walls in region are angled to correspond with the angle of inclination in lamellae It is preferred that the zones between lamellae remain with substantially vertical side walls.
The classifier and method of particles segregation or classification is suitable for feeds containing particles up to about 5 mm in diameter although larger particles could also be used. Further it is possible to classify particles into different distinct fractions using either batch or continuous conditions. For ultrafine particles, this is especially attractive. Hence the classifier and method of classification provide an excellent alternative to a conventional cyclosizer. Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described.
The classifier including a fluidization chamber may be of practically any configuration where essentially it operates as an elutriator or fluidized bed with the presence of one or more inclined plates typically arranged in one or more sets of lamellae. The method of segregation or classifying particles may also extend to the following applications:. All such variations and modifications are to be considered within the scope of the present invention the nature of which is to be determined from the foregoing description.
What is claimed is: 1. A classifier for segregating particles by size or density, said classifier comprising:. A classifier as claimed in claim 1 wherein said array of inclined plates comprises an array of parallel equally spaced plates extending across said fluidization chamber so that the plates in the array do not form a stepped arrangement.
A classifier according to claim 1 wherein two or more arrays of inclined plates are provided, each array being vertically spaced from the or each other array, and the arrays dividing the fluidization chamber into zones.
A classifier as claimed in claim 3 wherein the length of each plate is an array, the angle of inclination of the plates, and the spacing between plates in that array are selected to enable particles of a predetermined size or density to pass through the array when elutriated at a predetermined rate by the fluidization fluid, while inhibiting particles of greater size or density from passing through the array.
A classifier as claimed in claim 1 wherein a feed fluid incorporating particles to be classified is fed into the fluidization chamber between two of the arrays of inclined plates. A classifier as claimed in claim 1 wherein the particles are fed into the fluidization chamber with the fluidization fluid.
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