Cake discharge

The cake discharges at the rear of the beach, between the beach proper and the rear hub. In its simplest form, the cake discharge will he a series of radial holes around the beach end. These holes will usually be lined with some form of erosion protection, quite often in the form of a sintered tungsten carbide 

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As can be seen, for constant cake thickness doubling the feed concentration doubles the yield. So-called high duty vacuum dmm filters use a unique cake discharge method to allow very thin cakes to be discharged and can therefore be operated at very high speeds up to 25 revolutions per minute. 

Optimization of Cycle Times. In batch filters, one of the important decisions is how much time is allocated to the different operations such as filtration, displacement dewatering, cake washing, and cake discharge, which may involve opening of the pressure vessel. Ah. of this has to happen within a cycle time /. which itself is not fixed, though some of the times involved may be defined, such as the cake discharge time. 

The pressure version of the enclosed agitated filter is known as the Rosenmund filter it uses a screw conveyor to convey the cake to a central cake discharge hole. 

A variation on this type of filter is the double tipping pan filter, which is a semicontinuous type consisting of two rectangular pans fitted with a filter cloth and pivoted about a horizontal axis. Slurry is first fed onto one pan, which is turned over for cake discharge at the end of the cycle. The second pan is used for filtration while the first is being discharged. 

No internal piping and no conventional filter valve are needed with single-cell dmm filters where the entire dmm also operates under vacuum. The cake discharge is effected by air blowback from an internal stationary shoe mounted inside the dmm at the point of discharge. There are very close tolerances between the inside surface of the dmm and the shoe in order to minimize the leakage. The inside of the dmm acts as a receiver for the separation of air and filtrate conventional multicompartment dmm filters require a separate external receiver. This type of filter permits operation of the filter with thin cakes so that high dmm speeds, up to 26 rpm, can be used and high capacities can be achieved. Sizes up to 14 m are available. 

Total submergence is used in the vacuum disk filter thickener (Eig. 13) in which the cake discharge, by backwashing with filtrate, occurs as each sector passes through the lowest point of the slurry tank. 

In conventional disk filters, cake discharge is usually performed by a scraper blade, for cakes thicker than 10 mm, or sometimes by a tapered roU air blowback is often used to assist the discharge. High pressure sprays also have been used for cake discharge. 

The Sweedand filter, a significant departure from the standard end-opening design, has the cylindrical shell spHt in a horizontal plane into two parts, where the bottom half can be swung open for cake discharge. The upper half is rigidly supported and both the feed and the filtrate piping are fixed to it. 

Horizontal Vessel, Horizontal Leaf Filters. These filters consist of a horizontal cylindrical vessel with an opening at one end (Fig. 19). A stack of rectangular horizontal trays is mounted inside the vessel the trays can usuaUy be withdrawn for cake discharge, either individuaUy or in the whole assembly. The latter case requires a suitable carriage. One alternative design aUows the tray assembly to be rotated through 90° so that the cake can faU off into the bottom part, designed in the shape of a hopper and fitted with a screw conveyor. 

Another advantage of the membrane plate is its flexibiUty to cake thickness, ie, thinner cakes can be easily handled without loss of dryness. Cake release characteristics are also improved by deflation of the membrane prior to cake discharge. Alternating arrangements, in which the membrane plates and the normal recessed plates alternate, have been used to reduce cost. 

The OMD leaf filter (Stella Meta Filters) is a vertical leaf filter with a mbber diaphragm suspended between the leaves. The cake that forms on the leaves eventually reaches the diaphragm at which point pump pressure is used to inflate the diaphragm and compress the cake. The cake discharge is by vibration. 

Filtration and compression take place with the press closed and the belt stationary the press is then opened to allow movement of the belt for cake discharge over a discharge roUer of a small diameter. This allows washing of the belt on both sides (Fig. 15). Cycle times are short, typically between 10 and 30 minutes, and the operation is fully automated. Si2es up to 32 m are available and the maximum cake thickness is 35 mm. 

A continuous pressure filter may be defined as a filter that operates at pressure drops greater than 100 kPa and does not require intermption of its operation to discharge the cake the cake discharge itself, however, does not have to be continuous. There is Htfle or no downtime involved, and the dry soHds rates can sometimes be as high as 1750 kg/m h with continuous pressure filters. 

The Gaudfrin disk filter, designed for the sugar industry and available in Prance since 1959, is also similar in design to a vacuum disk filter but it is enclosed in a pressure vessel with a removable Hd. The disks are 2.6 m in diameter, composed of 16 sectors. The cake discharge is by air blowback, assisted by scrapers if necessary, into a chute where it may be either reslurried and pumped out of the vessel or, for pasty materials, pumped away with a monopump without reslurrying. 

The patented system (15) has stationary disks mounted inside a pressure vessel (horizontal vessel, vertical disks) which is mounted on rollers and can rotate slowly about its axis. A screw conveyor is mounted in the stationary center of rotation it conveys the cake, which is blown off the leaves when they pass above the screw, to one end of the vessel where it falls into a vertical chute. The cake discharge system involves two linear sHde valves that sHde the cake through compartments which gradually depressurize it and move it out of the vessel without any significant loss of pressure. The system rehes entirely on the cake falling freely from one compartment to another as the valves move across. This may be an unrealistic assumption, particularly with sticky cakes when combined with lots of sliding contact surfaces which are prone to abrasion and jamming, the practicality of the system is questionable. 

Cake discharge occurs at atmospheric pressure by the action of a toU or a scraper, assisted by blowback. The cloth may be washed by a spray before the cycle starts again. Filtering areas range up to 8 m and dmm diameters up to 2 meters. The necessity for large seals limits the operating pressure to less than 300 kPa, typically. Cake thickness can be from 2 to 150 mm, depending on machine size, and the speed of dmm rotation up to 2 rpm, usually from 0.3 to 1 rpm. Apphcations occur in the manufacture of pharmaceuticals, dyestuffs, edible oils, and various chemicals and minerals. 

More recendy, the molten caustic leaching (MCL) process developed by TRW, Inc. has received attention (28,31,32). This process is illustrated in Eigure 6. A coal is fed to a rotary kiln to convert both the mineral matter and the sulfur into water- or acid-soluble compounds. The coal cake discharged from the kiln is washed first with water and then with dilute sulfuric acid solution countercurrendy. The efduent is treated with lime to precipitate out calcium sulfate, iron hydroxide, and sodium—iron hydroxy sulfate. The MCL process can typically produce ultraclean coal having 0.4 to 0.7% sulfur, 0.1 to 0.65% ash, and 25.5 to 14.8 MJ/kg (6100—3500 kcal/kg) from a high sulfur, ie, 4 wt % sulfur and ca 11 wt % ash, coal. The moisture content of the product coal varies from 10 to 50%. 

Thixotropic filter cakes from rotary vacuum filters that cannot be preformed by any of the above methods can often be scored by knives on the filter, the scored cake discharging in pieces suitable for through-circulation diying. 

At the end of the run, measure and record the filtrate volume (and weight, if appropriate), cake thickness, final cake temperature (if appropriate), wet cake weight, and note the cake discharge characteristics (roU, sticks to media, etc.). 

Sc e-up on rate Scale-up on cake discharge Scale-up on actual area... 

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