Feed slurry

In apphcations where the fraction of fine particles in the soHds of the feed slurry is low, a simple and relatively cheap vacuum filter can yield cakes with moisture contents comparable to those discharged by pressure filters. Vacuum filters include the only truly continuous filters built in large sizes that can provide for washing, drying, and other process requirements. 

One common design is typified by a mbber belt mounted in tension. The belt is grooved to provide drainage toward its center. Covered with cloth, the belt has raised edges to contain the feed slurry, and is dragged over stationary vacuum boxes located at the belt center. Wear caused by friction between the belt and the vacuum chamber is reduced by using replacable, secondary wear belts made of a suitable material such as PTFE, terylene, etc. 

As the vessel is only about half filled with slurry, the disks become coated with the cake when immersed, the cake is dewatered when the disks emerge from the slurry, and scraped or blown off, by reverse blow, into the central conveyor which takes the cake to one end of the vessel. The planetary action and the slow movement of the disks through the feed slurry ensure exceptionally good homogeneity of the cake which is critically important for good dewatering characteristics the typical speed of rotation of the planetary system of shafts is from 0.8 to 1 rpm. 

The disk filter is similar to the dmm in operation, but filtration is conducted using a series of large diameter filter disks that carry the filter medium on both sides of the disk. They are connected to the main horizontal shaft and partly immersed in the feed slurry. The central shaft is connected by a set of valves which serve to provide vacuum and air as in dmm filters. As the disk sections submerge during rotation, vacuum is appHed to form a cake on both sides of the disk. The cycle of operation is similar to that in a dmm filter. One unit can have as many as 12 disks of up to 5-m diameter. Disk filters, both compact and cost effective, are used extensively in the iron ore industry to dewater magnetite concentrates. 

Particle Size Distribution. Almost every feed slurry is a mixture of fine and coarse particles. Performance depends on the frequency of distribution of particle size ia the feed. Figure 5 shows that whereas all of the coarse particles having a diameter greater than some are separated, fewer of the very fine particles are, at any given feed rate. The size distribution frequency of particles ia feed and centrate for a fine and coarse feed are quite different. More coarse particles separate out than fine ones. Classification of soHds by size is often done by centrifugal sedimentation. 

Tanks. Three basic tank types have been used in ore concentration concurrent, where feed slurry moves in the same dHection as the dmm rotates (Fig. 

The KEN-FLOTE column (11) is one of several column flotation processes based on a countercurrent principle. The feed slurry containing reagents is iatroduced iato the column just below the froth zone. Air is iujected at the bottom of the column via an air sparger. Wash water is sprayed within the froth zone to reject the entrained impurities from the froth. Test results on this column iadicate that a 6% ash product coal having a combustible-recovery of 75—80% can be obtained. A 70—80% pyrite reduction is also claimed. Figure 2 shows the operation of such a column. 

Another modification is the use of microbubble column flotation (13). In this process, smaller bubbles are generated to enhance the recovery of micrometer-sized particles. A countercurrent flow of feed slurry is also used to further enhance the bubble—particle attachment. The process is capable of produciug ultraclean coals containing less than 0.8% ash. 

Vacuum or Pressure The vast majority of all continuous filters use vacuum to provide the driving force for filtration. However, if the feed slurry contains a highly volatile hquid phase, or if it is hot, saturated, and/or near the atmospheric pressure boiling point, the use of pressure for the driving force may be required. Pressure filtration might also be used where the required cake moisture content is lower than that obtainable with vacuum. 

Feed Slurry Temperature Temperature can be both an aid and a limitation. As temperature of the feed slurry is increased, the viscosity of the hquid phase is decreased, causing an increase in filtration rate and a decrease in cake moisture content. The limit to the benefits of increased temperature occurs when the vapor pressure of the hquid phase starts to materially reduce the allowable vacuum. If the hquid phase is permitted to flash within the filter internals, various undesired resiilts may ensue disruption in cake formation adjacent to the medium, scale deposit on the filter internals, a sharp rise in pressure drop within the filter drainage passages due to increased vapor flow, or decreased vacuum pump capacity. In most cases, the vacuum system should be designed so that the liquid phase does not boil. 

In some special cases, steam filtration can be used to gain the advantages ot temperature without having to heat the feed slurry. 

Construction is similar to that of other drum filters, except that vacuum is applied to the entire rotation. Before feeding slurry a precoat layer of filter aid or other suitable solids, 75 to 125 mm (3 to 5 in) thick, is apphed. The feed slurry is introduced and trapped in the outer surface of the precoat, where it is removed by a progressively advancing doctor knife which trims a thin layer of solids plus precoat (Fig. 18-126). The blade advances 0.05 to 0.2 mm (0.002 to 0.008 in) per revolution of the drum. When the precoat has been cut to a predefined minimum thickness, the filter is taken out of service, was ned, and freshly precoated. This turnaround time may be 1 to 3 h. 

The feed slurry is introduced into the lower portion of the bowl through a small orifice. Immediately downstream of the orifice is a distributor and a baffle assembly which distribute and accelerate the feed to circumferential speed. The centrate discharges from the top end of the bowl by overflowing a ring weir. Solids that have sedimented against the bowl wall are removed manually from the centrifuge when the buildup of solids inside the bowl is sufficient to affect the centrate clarity. 

Types of Jigs A jig is essentially an open tank filled with water and provided with a horizontal screen on the top and a hutch compartment fitted with a spigot (Fig. 19-27 ). A layer of coarse, heavy particles, known as ragging, is placed on the top of the screen onto which the feed slurry is introduced. The feed moves over the ragging and the separation takes place as the bed is pulsated by a different mechanical device. The heavy particles are collected into the hutch compartment and removed through the spigot while the hghter particles are made to overflow from the top or the tank. 

If the feed rate exceeds the maximum of the design, particulate matter are unable to settle out of the normal clarification zone. Hence, there is an increase in the solids concentration, resulting in hindered settling. The result Is a corresponding decrease in the sedimentation rate below that observed for the feed slurry. 

In determining the proper size and number of cyclones required for a given application, two main objectives must be considered. The first is the classification or separation that is required, and the second is the volume of feed slurry to be handled. In the case of hydroclones, before determining whether these objectives can be achieved, it is necessary to establish a base condition as follows Feed liquid - water at 20 C. Feed solids - spherical particles of 2.65 specific gravity Feed concentration - less than 1 % solids by volume Pressure drop - 69 kPa (10 psi) Cyclone geometry - "standard cyclone" as described above. 

Another important objective which must be considered is to provide adequate cyclone capacity for the application. The volume of feed slurry that a given cyclone can handle is related to the pressure drop across the cyclone. The relationship between flow rate and pressure drop for several different sizes of standard cyclones is shown in Figure 56. As shown, the flow rate increases as the pressure drop increases. In order to utilize this graph, the pressure drop used for calculating the separation is used to determine the flow rate for the cyclone diameter which was... 

Once the precoating stage is completed the process slurry is pumped into the filter, the forming cake is retained on the plates and the filtrate flows to further processing. When the solids are fine and slow to filter a body-aid is added to the feed slurry in order to enhance cake permeability. However, it should be kept in mind that the addition of body-aid increases the solids concentration in the feed so it occupies additional volume between the plates and increases the amount of cake for disposal. Likewise, for all those applications when the cake is the product, precoat and filter-aid may not be used since they mix and discharge together with the cake. 

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