Filter cake washing

Extra-Fine Precipitated Hydroxide. Very fine (< 1 /im-diameter) particle size hydroxide is produced by precipitation under carefully controlled conditions using specially prepared hydroxide seed. Precipitation is usually carried out at low (30 —40°C) temperatures causing massive nucleation of fine, uniform hydroxide particles (Fig. 5). Tray or tumiel Ape dry ers are used to dry the thorouglily washed filter cake to a granular product wliich is easily pulverized to obtain the fine hydroxide. Alternatively, the washed product is spray dried. Precipitation from an organic-free aluniinate Hquor, such as that obtained from the soda—sinter process, fields a very wliite product. Tlie fine precipitated hydroxide is used by the paper and plastic industries as fillers. 

A valuable variant of the conventional centrifuge is the horizontal spindle centrifuge, especially the Heinkel type. This centrifuge has a unique offloading feature in that the filter cloth loaded with washed filter cake is pulled out to allow a reversed centrifugal removal of the solid as illustrated in Figure 10. 

Similarly, sodium chloride is formed in the absorber by the reaction of chloride, present in the flue gas as HC1 vapor, with the alkaline sodium solutions. The level of sodium chloride in the system builds up to a steady state concentration, such that the rate at which sodium chloride leaves the system with the washed filter cake is equivalent to the rate at which it is absorbed by the process liquor in the absorber. 

Concentrated (15 M) aqueous ammonia is diluted with an equal volume of water, and about 5% more than the theoretical quantity is placed in a polyethylene beaker. This solution is stirred vigorously, either manually or with a plastic-coated magnetic stirrer, and the clear fluoride solution from procedure B is added drop by drop. The resulting finely divided, white precipitate is removed by filtering, and the filter cake is washed by slurrying with water and refiltering. The washed filter cake is about 80% water and represents 65 to 75% of the initial titanium-(IV) oxide content of the mineral. 

It is easy to find out which of these mechanisms is dominating by performing washing trials. One should check high-performance liquid chromatography purity of the wet filter cake, washed filter cake, mother liquor, and extremely washed filter cake with about 30% and then 70% of the API washed off. Data from these experiments will provide a clear picture of the mechanism of incorporation, which can suggest corrective measures. 

The diuranate precipitate is separated from the mixed NaG and Na SO4 salt solution by a system of thickeners and filters. Filter cake from the first filter is washed with water, reslurtied with water, filtered a second time, and washed again to reduce its content of NaG and Na2S04. When it is necessary to reduce the amount of sodium diuranate, a third stage of filtration is used, and the filter cake is reslurried with ammonium sulfate instead of water to replace most of the sodium with ammonium ion. Washed filter cake is dried by heating to 160 to 180°C. 

WASHING FILTER CAKES. To wash soluble material that may be retained by the filter cake after a filtration, a solvent miscible with the filtrate may be used as a wash. Water is the most common wash liquid. The rate of flow of the wash liquid and the volume of liquid needed to reduce the solute content of the cake to a desired degree are important in the design and operation of a filter. Although the following general principles apply to the problem, these questions cannot be completely answered without experiment. ... 

Just like washing generally can be done by washing filter cakes in situ (whilst they are still on the filter medium), by reslurrying of filter cakes or sediments, or by successive dilutions of slurries, the same is done in counter-current washing. Applications and examples exist in industry for all of the above three types. 

To filter a solution one attaches a vacuum and pours the solution into the Buchner funnel. All the solution will go whoosh into the flask leaving what is called a filter cake in the funnel. The liquid that has collected in the flask is now called the filtrate. Usually, the filter cake is then washed with a little bit of clean what-... 

The soiution is aliowed to cool and the crystals of the P2P-bisulfite addition compound are then separated by vacuum filtration, washed with a little clean dH20 then washed with a couple hundred mLs of ether, DCM or benzene. The filter cake of MD-P2P-bisulfate is processed by scraping the crystals into a flask and then 300mL of either 20% sodium carbonate solution or 10% HCi soiution are added (HCI works best). The soiution is stirred for another 30 minutes during which time the MD-P2P-bisulfite complex will be busted up and the P2P will return to its happy oil form. The P2P is then taken up with ether, dried and removed of the solvent to give pure MD-P2P. Whaddya think of that ... 

C. Big secret 2...Quack ...One MUST filter this 600+ ml of ether...but a duck can t do this all at once...so one must fitter in vacuum filter in 200 ml portions...changing the duck paper every time and wash the filter cake with ether...Dr. Quack thinks a vacuum filter (apirator) at this stage is a must..Quack ... 

D. Now the ether will be a deep reddish yellow. Distill off the ether...quack...and take the temp up to 170 C to drive off any other volatiles. Should recover 90%+ of the original weight of oil. Now add 500 ml of saturated bisulfite and stir for 1.5 hours...Quack Vacuum Filter, the duck fat crystals Wash with water and ether, yield dull fine ppt in the filter cake...stable bisulfite addition product...can be stored forever...QuackU Yield -50 to 80% depending on a ducks technique ... 

In a flask the chemist mixes 50g piperonal into 200mL glacial acetic acid, then adds 45mL nitroethane and 17g ammonium acetate. The solution is then refluxed 4 hours and takes on the color of yellow to yellow-orange. After 4 hours and cooling, yellowish crystals of p-nitropropene will spontaneously form. If not, the solution can be diluted with 50ml of dHjO and chilled in an ice bath for an hour to form the crystals with some slushy glacial acetic acid and water intermixed. The mass of crystals is broken up and plopped into a Buchner funnel to be vacuum filtered. The filter cake is washed with a little extra acetic acid or water. All of the filtrate is saved. 

The way the chemist knows that she has methylamine and not ammonium chloride is that she compares the look of the two types of crystals. Ammonium chloride crystals that come from this reaction are white, tiny and fuzzy. The methylamine hydrochloride crystals are longer, more crystalline in nature and are a lot more sparkly. The chemist leaves the methylamine crystals in the Buchner funnel of the vacuum filtration apparatus and returns the filtrate to the distillation set up so it can be reduced one last time to afford a second crop. The combined methylamine hydrochloride filter cake is washed with a little chloroform, scraped into a beaker of hot ethanol and chilled. The methylamine hydrochloride that recrystallizes in the cold ethanol is vacuum filtered to afford clean, happy product (yield=50%). 

A filter cake from the wringer is washed to remove absorbed acid, transferred to a slurry tank of water, and quickly submerged, after which the nitrocellulose is pumped to the stabilization operation as a diluted water slurry. Exhaust systems are installed to protect personnel and equipment from acid fumes, and water sprays and cyclone separators are used for acid fume recovery before venting to the air. 

The three types of washing are washing of filter cakes by displacement, washing by reslurrying of cakes or sludges, and washing by successive dilution. 

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 filter cake can then be washed either by displacement or by reslurrying. Reslurrying is easily accompHshed using the stirring action of the rotor blades when the rotor is lowered into the cake. The cake may also be dried in situ by the passage of hot air through it, or may be steam distilled for the recovery of solvent. 

In general, pan filters are selected for freely filtering soHds and thick filter cakes. Cake washing can be introduced easily. Most appHcations are in the mining and metallurgical industries for small-scale batch filtration. 

Despite their theoreticaUy poor washing performance, due to uneven wash distribution and excessive mn-off because the filter surface is not horizontal, many multicompartment dmm filters continue to be used as cake washing filters. Effective washing of the filter cloth can be done only with the belt discharge type, where the cloth leaves the dmm for a brief period and can thus be washed on both sides. 

Some attempts have been made to reslurry the filter cake without having to open the filter press. However, a number of problems appear, eg, bending of the plates due to uneven cake deposition or cavitation, uneven dewatering and washing within the frames, and plugging of the inlet ports. 

The KDF filter was first tested in prototype on a coal mine in northern Germany. It was installed in parallel with existing vacuum filters and it produced filter cakes consistendy lower in moisture content by 5 to 7% than the vacuum filters. Two production models have been installed and operated on a coal mine in Belgium. The filter is controlled by a specially developed computer system this consists of two computers, one monitoring the function of the filter and all of the detection devices installed, and the other controlling the filtration process. The system allows optimization of the performance, automatic start-up or shut-down, and can be integrated into the control system of the whole coal washing plant. 

Reaction times can be as short as 10 minutes in a continuous flow reactor (1). In a typical batch cycle, the slurry is heated to the reaction temperature and held for up to 24 hours, although hold times can be less than an hour for many processes. After reaction is complete, the material is cooled, either by batch cooling or by pumping the product slurry through a double-pipe heat exchanger. Once the temperature is reduced below approximately 100°C, the slurry can be released through a pressure letdown system to ambient pressure. The product is then recovered by filtration (qv). A series of wash steps may be required to remove any salts that are formed as by-products. The clean filter cake is then dried in a tray or tunnel dryer or reslurried with water and spray dried. 

The softened seawater is fed with dry or slaked lime (dolime) to a reactor. After precipitation in the reactor, a flocculating agent is added and the slurry is pumped to a thickener where the precipitate settles. The spent seawater overflows the thickener and is returned to the sea. A portion of the thickener underflow is recirculated to the reactor to seed crystal growth and improve settling and filtering characteristics of the precipitate. The remainder of the thickener underflow is pumped to a countercurrent washing system. In this system the slurry is washed with freshwater to remove the soluble salts. The washed slurry is vacuum-filtered to produce a filter cake that contains about 50% Mg(OH)2. Typical dimensions for equipment used in the seawater process may be found in the Hterature (75). 

Mercuric Sulfate. Mercuric s Af2iX.e.[7783-35-9] HgSO, is a colorless compound soluble ia acidic solutions, but decomposed by water to form the yellow water-iasoluble basic sulfate, HgSO 2HgO. Mercuric sulfate is prepared by reaction of a freshly prepared and washed wet filter cake of yellow mercuric oxide with sulfuric acid ia glass or glass-lined vessels. The product is used as a catalyst and with sodium chloride as an extractant of gold and silver from roasted pyrites. 

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