Washing and Deliquoring

The term post-treatment refers to processes vdiich are used after the principal solid-liquid separation has been achieved. The final step in the latter operation is usually some type of cake filtration which will leave a cake that may be fully saturated with liquid or partially drained and it is extremely unlikely that the solids are in an acceptable condition for any subsequent operation such as thermal drying. Thus the post treatment processes are washing and deliquoring which are concerned req>ectively with removing the soluble solids from the liquid remaining in the cake and then purging the major proportion of the liquid fi om the cake pores. 

Figure 4.18 shows state-of-the-art, laboratory scale experimental apparatus capable of automated data acquisition during sequential filtration, washing and deliquoring phases of a filter cycle the level of hardware and software... 

The use of computer control allows sequential filter cycle data to be acquired in a repeatable and reliable manner with a minimum of operator interference. By defining the desired cycle phases through a software algorithm, a cake formation phase can be directly followed by the chosen combination of washing and deliquoring. The real time measurement of experimental parameters also allows continuous display of results and the use of on-line analysis techniques as an experiment proceeds. 

Table 6.2 Potential operations in a relatively complex batch filter cycle with single washing and deliquoring phases (adapted from Purchas and Wakeman, 1986).

In many processing sequences a filter is preceded by a particle formation unit such as a crystalliser or precipitator, the product from which can have a profound effect on filter cycle performance. While the effects of particle size on washing and deliquoring are explicit in the equations, correlations and design charts relating to those processes, the effects on cake formation need to approximated for the purpose of simulation. Although ideally the variation of specific resistance and porosity with particle size would be assessed experimentally, for the current purpose approximations... 

It is difficult to represent on a single graph the results of sequential filtration, washing and deliquoring phases in a filter cycle. Perhaps the best method is... 

Membrane Filter Press. Membrane filter presses use impermeable, flexible membranes, or diaphragms to squeeze the cake for further cake deliquoring, as shown in Fig. 15. This type of filter provides less dead time in a filtration cycle, better washing, and drier cake compared to traditional plate-and-frame and recessed plate filter presses. A comparison of a recessed press operated at 100 psi and a membrane filter operated at 25psi for sludge dewatering is shown in Table 4. ... 

Figure 1.22 The typical operating cycle of a multi-element Moore s vacuum filter, (a) filtration (b) washing (c) deliquoring prior to discharge. Vacuum needs to be maintained during transit of the leaves and cakes between tanks.

Although variants exist, the basic unit shown in Figure 1.31 comprises sequences of narrow vertical chambers lined on both sides by filter cloths. The chambers, formed between hollow frames and flat filter plates or between adjacent recessed filter plates, allow for filter cake formations as well as washing and gas-blown deliquoring operations. Suspension is fed to the square, rectangular or circular chambers through a variety of plate porting... 

Figure 4.18 Schematic of a state-of-the-art apparatus for investigating the filtration, displacement washing and gas deliquoring phases of the filter cycle. (1) suspension feed vessel (2) wash liquor feed vessel (3) filter cell (4) rotary index table (5) electronic balance (6) pressure regulator. The inset photograph shows fully automated apparatus for obtaining filtration and deliquoring data including facility for transient measurements of cake growth and state.

While valuable information on settling, filtration and cake post-treatments such as washing and gas deliquoring can be obtained from individual tests, in order to subsequently simulate filter performance it is usually necessary to evaluate so-called scale-up coefficients from sequences of tests. These empirical coefficients principally relate to cake formation (compressibility) and compression dehquoring (consolidation), as it is currently impossible to predict either from a knowledge of fundamental solid and liquid properties. Many filter cakes are compressible to some extent, and increases in filtering pressure lead to less porous and more resistant cakes. For these systems data are needed which relate the specific resistance, oc, a measure of cake structure such as solids volume fraction, and where appropriate the modified consolidation coefficient, Q, to variations in the plied pressure difference Ap. It is conventional practice to assume that Q and Q are solely functions of Ap. 

The methodologies presented in this chapter illustrate how characterising parameters for cake formation, compression and gas deliquoring, washing and... 

As described in Section 6.1, the cycle for a batch filter can comprise one or more cake formation phases followed by any sequential combination of consolidation, displacement washing and gas deliquoring phases. While a complex batch cycle may involve the list of operations shown in Table 6.2, a more typical cycle can be represented by... 

The mass of solids in the cakes remains constant throughout washing and is equal to the value at the end of compression deliquoring (M = 4527 kg). The mass of liquid in the cakes, as given by equation (6.63), also remains the same as the density of filtrate and wash are equal (M = 3920 kg). The data sequences shown in Table 6.8 provide information for the washing phase where ... 

Variations in upstream formulation mean that crystallisation of the j8-form, which is more difficult to filter, can occur in place of the a-form. In each batch, 50 kg of solids are present at a concentration of 6% v/v and it is envisaged that cake formation will occur to a maximum depth of 50 mm. In order to meet product specifications this new filter installation requires a sequential cycle comprising filtration, displacement washing and gas deliquoring. Preliminary... 

For a filter cycle following the sequence filtration - washing - gas deliquoring, and with reference to the procedures outlined in Section 6.2, calculations... 

With the horizontal belt filter cycle an endless cloth supported on a perforated belt is driven around two rollers and across a sequence of evacuated suction boxes (see Figure 7.1 and Section 1.4.1.3). The feed suspension is introduced toward one end of the filter and processed at constant vacuum to form cake which is subsequently washed and/or gas deliquored theoretically as many... 

As previously described, the cycle for a continuous filter typically comprises a cake formation phase followed by a combination of sequential displacement washing and gas deliquoring phases, potentially in any order. If a cycle is assumed to comprise the sequence filtration-washing-deliquoring and the subscripts /, d and w, respectively denote values for these phases, then the total time (tj) devoted to a cycle is given by... 

Alternatively, the curvefit correlations of dimensionless instantaneous solute concentration ( ) V5. W (see Appendix B and Wakeman and Tarleton, 2005a) can be numerically integrated to give F. If a gas deliquoring phase precedes washing then the cake saturation (S) will be less than unity at the start of washing and it is necessary to correct each value of W (= Ws=i) calculated by equation (7.30) according to... 

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