Cake washing

While washing performance in process scale filters can be theoretically predicted a priori from a knowledge of cake and wash liquor properties (see Chapters 6 and 7), laboratory testing is usually concerned with the evaluation of two parameters  

How much of the soluble species will be removed by a given volume of wash liquid  

How long will the wash liquid take to effect such a removal  

Both displacement and reslurry washing can be investigated on a small scale, although only the former is described in detail here. The conditions of test work should replicate as far as is practicable the actual conditions under which any larger scale filter will operate. In addition, the temperature, the pH value, and the composition of the wash liquid and slurry or cake being washed must correspond to the conditions likely to be met in practice, otherwise misleading results may be obtained. 

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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. 

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. 

This allows each section, as it rotates, to go through a series of operations such as filtration, dewatering, cake washing, and discharge. Two basic designs exist, depending on the method of soHds discharge. 

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. 

Rotary Vacuum Disk Filters. An alternative to the dmm filter is the disk filter, which uses a number of disks mounted vertically on a horizontal shaft and suspended in a slurry reservoir. This arrangement provides a greater surface area for a given floor space, by as much as a factor of 4, but cake washing is more difficult and cloth washing virtually impossible. 

Generally, the horizontal vessel, vertical filters with leaves arranged laterally can be designed up to filtration areas of 300 m. Cake washing is possible but must be carried out with caution since there is a danger of the cake falling off... 

Vertical Vessel, Horizontal Leaf Filters. These filters, like all horizontal leaf filters, are advantageous where the flow is intermittent or where thorough cake washing is required. Filtration areas are limited to about 45 m. ... 

The Gaudfrin disk filter is designed for only relatively low pressures of 100 kPa on average and it provides for cake washing in two stages, in two separate compartments within the same vessel. 

As observed from Figure 27, the cake removal by fluid shear is also aided by centrifugal force. Other arrangements include stationary filtration media and rotating disks to create the shear effects, and rotating cylindrical elements it has also been shown how such filters can be used for cake washing. 

Lime-Sulfuric. Recovery of citric acid by calcium salt precipitation is shown in Figure 3. Although the chemistry is straightforward, the engineering principles, separation techniques, and unit operations employed result in a complex commercial process. The fermentation broth, which has been separated from the insoluble biomass, is treated with a calcium hydroxide (lime) slurry to precipitate calcium citrate. After sufficient reaction time, the calcium citrate slurry is filtered and the filter cake washed free of soluble impurities. The clean calcium citrate cake is reslurried and acidified with sulfuric acid, converting the calcium citrate to soluble citric acid and insoluble calcium sulfate. Both the calcium citrate and calcium sulfate reactions are generally performed in agitated reaction vessels made of 316 stainless steel and filtered on commercially available filtration equipment. 

There are a number of commercially available surfactants that can be employed as an aid in filter cake moisture reduction. These reagents can be added to the filter feed sluriy or to the filter cake wash water, if washing is used. Since these reagents have a dispersing effect, flocculation may be required subsequently Typical moisture reduc-... 

It is absolutely necessaiy that a dam be used in all cases, except for roll discharge applications which do not involve cake washing or where the maximum cake thickness is on the order of 2 mm or less. If a dam is not used, filter cake will form past the edge of the leaf in the general shape of a mushroom. When this happens, the total filter area is some unknown value, greater than the area of the leaf, that constantly increases with time during cake formation. 

It is difficult to plan a filtration leaf test program until one test has been run. In the case of a bottom-feed test, the first run is normally started with the intention of using a 30-s cake formation time. However, if the filtrate rate is very high, it is usually wise to terminate the run at the end of 15 s. Should the filtrate rate be very low, the initial form period should be extended to at least 1 min. If cake washing is to... 

When cake washing is involved, it is usually convenient to weigh the wet cake and then repulp it in a known quantity of distilled water or in water at the same pH as the filtrate, if precipitation of... 

In cake-washing tests, it is important that the feed sluriy hquid be analyzed for total dissolved sohds and density as well as the reference constituent. 

Cake Washing Wash efficiency data are most conveniently represented by a semilog plot of percent remaining B as a function of wash ratio N as shown in Fig. 18-103. Percent remaining refers to that portion of the solute in the dewatered but unwashed cake which is left in the washed and dewatered cake. Since a cake-washing operation... 

Practical experience has shown that the most convenient and best means of expressing R is in terms of the solute concentrations in the washed cake hquid the feed liquid (or unwashed cake liquid), and the cake wash hquid. Furthermore, the wash ratio N may also be expressed either as a volume or weight ratio. 

The residence time of the cake-washing fluid within the cake is relatively short and is not normally considered useful for any kind of leaching operation. Therefore, it is assumed that all of the solute is in solution. 

If it were possible to obtain a perfect sing displacement wash, the frac tion remaining would be numerically equal to I minus the wash ratio. This ideal condition is represented by the maximum theoretical line as shown in Fig. 18-103. Since it represents the best that can be done, no data point should fall to the left of this cni ve. Most, but not all, cake-washing curves tend to fall along the heavy solid line shown. In the absence of ac tnal data, one may estimate washing results by using this cni ve. 

Wash Time Cake-washing time is the most difficult of the filtration variables to correlate. It is obviously desirable to use one which provides a single cni ve for all of the data. Filtration theory suggests three possible correlations [Eqs. (18-59) to (18-61)]. These are listed below, beginning with the easiest to use ... 

V = volume of cake wash/nnit area/cycle N = wash ratio... 

Therefore, the cake wash -t- final dry rate is controlling and a cycle time of 1.67 mpr must be used. 

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