Washing the Cake

The solid particles separated by filtration often must be washed to remove filtrate from the pores. There are two processes involved in washing. Much of the filtrate occupying the voids between particles may be removed by displacement as clean solvent is passed through the cake. Removal of filtrate held in less accessible regions of the cake and from pores in the particles takes place by diffusion into 

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The older methods have been replaced by methods which require less, if any, excess sulfuric acid. For example, sulfonation of naphthalene can be carried out in tetrachloroethane solution with the stoichiometric amount of sulfur trioxide at no greater than 30°C, followed by separation of the precipitated l-naphthalenesulfonic acid the filtrate can be reused as the solvent for the next batch (14). The purification of 1-naphthalenesulfonic acid by extraction or washing the cake with 2,6-dimethyl-4-heptanone (diisobutyl ketone) or a C-1—4 alcohol has been described (15,16). The selective insoluble salt formation of 1-naphthalenesulfonic acid in the sulfonation mixture with 2,3-dimethyl aniline has been patented (17). 

If the cake is to be washed, the slurry heel can be blown from the filter and wash liquor can be introduced to refill the shell. If the cake tends to crack during air blowing, it may be necessaiy to displace the shiny heel with wash gradually so as never to allow the cake to diy. Upon the completion of filtration and washing, the cake is discharged by one of several methods, depending on the sheU and leaf configuration. 

M Preparation of isopropyiidene peniciiiamine hydrochioride To the filtrate obtained In step (b) is added at 20°C to 25°C a total of 85 g of hydrogen sulfide. The precipitated HgS is filtered off and the filtrate is concentrated under reduced pressure to a volume of 200 to 500 ml. Following e polish filtration, the product-rich concentrate is mixed with 1.5 liters of isobutyl acetate. The mixture is refluxed at about 40 C under reduced pressure in equipment fitted with a water separation device. When no further water separates, the batch is cooled to 30t and filtered. The reactor is washed with 1 liter of acetone, which Is used also to wash the cake. The cake is further washed with 200 ml of acetone. The acetone washes are added to the isobutyl acetate filtrate and the mixture is refluxed for 20 to 30 minutes. After a holding period of one hour at 5°C, the crystals of isopropyiidene penicillamine hydrochloride are filtered and washed with 200 m of acetone. On drying for twelve hours at 25°C this product, containing 1 mol of water, weighs about 178 g (73%). 

Weigh 50 g of the ground hulled rice, citrus flesh, or tomato sample (20 g citrus peel or rice straw) into a 500-mL Erlenmeyer flask and extract with 250 mL of acetone (or methanol for rice straw) by shaking for 1 h. Filter by suction and collect the extract in a 500-mL round-bottom flask. Wash the cake with 100 mL of acetone (or methanol for rice straw) and filter off. Combine the filtrates and concentrate to around 2 mL at 40 °C with a rotary evaporator. 

The washed carbon cake on which the capreomycin is adsorbed is washed with 200 L of 0.05 N aqueous hydrochloric acid. The acid wash is discarded. The washed carbon cake is eluted during a one-hour period with 400 L of an aqueous acetone containing 1.65 L of 11.7 N hydrochloric acid and 80 L of acetone. The filter cake is further eluted by washing the cake with 200 L of an aqueous acetone containing 825 ml of 11.7 N hydrochloric acid and 40 L of acetone during a 15-minute period. The combined eluates, having a total volume 575 L, are concentrated in vacuo to 52.5 L. The concentrate is added... 

Washing the cake with solvent removes impurities, and does not disrupt surface or edges of cake. 

For the cake, if the mother liquor in the slurry contains soluble substances, it may be necessary to wash the cake. Deliquoring is a process after cake washing. Decreasing the liquid content of cakes has become a major objective of many SLS processes. 

Despite washing the cake, a small portion of the sodium salts remains occluded within the calcium-sulfur salts or trapped in interstices of agglomerates which cannot be practically washed from the waste cake. The amount of sodium lost will depend primarily upon the total sodium concentration in the process liquor (which is a function of the amount of oxidation and the chloride content of the coal) and the extent of cake washing. The sodium losses in the filter cake are made up by the addition of sodium carbonate to the system. Typically, the sodium carbonate makeup should amount to less than 5 mole % of the S02 removed. 

Assuming that the actual rate of washing is 85 percent of the theoretical rate, how long will it take to wash the cake in the press of Prob. 30.4 with a volume of wash water equal to that of the filtrate ... 

This elementary estimate of horizontal belt dimensions highli s the inqtortance of having available information on the process of cake washing, in terms of time or delivered volume of wash. In the problem, it was decided to wash the cake co-currently wifli two void volumes. 

Iron residue slurry is pumped to a thickener where the underflow is pumped through a filter feed tank to residue filtration. After the filtration and the cake washing, the cake is collected into a container or dumper. Iron residue is transported to the lined disposal pond. Overflow of the thickener is pumped to the solvent extraction circuit. 

Note that = 0 kg and, as no gas deUquoring takes place prior to washing, the cake saturation 5=1 and = W. From equation (6.61), the mass of solute in the cake ... 

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