The Influence of Surface Energy on the Washing Quality of Filter Cakes

doi:10.4236/aces.2011.14036

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Advances in Chemical Engi neering and Science , 2011, 1, 252-255

doi:10.4236/aces.2011.14036 Published Online October 2011 (http://www.SciRP.org/journal/aces)

The Influence of Surface Energy on the Washing

Quality of Filter Cakes

Markus Wilkens, Urs A. Peuker

Technische Universität Bergakademie Freiberg Institute of Mechanical Process Engineering and

Mineral Processing, Freiberg, Germany

E-mail: [email protected]

Received May 1, 2011; revised May 30, 2011; accepted June 9, 2011

Abstract

The washing of filter cakes, especially the displacement washing, represents an important aspect in science

researches and industrial applications. A lot of ongoing researches are focussed on impurities, which are

dissolved in the mother liquid (e.g. sodium chloride) and washed out with the identical pure liquid without

impurities. The project flushing focuses on systems with two chemically different liquids. The main aim is to

exchange an organic solvent by water. This article focuses on the adsorption effects during a washing proc-

ess with solid systems of different wetting behaviours.

Keywords: Filter Cake Washing, Surface Energy Effects, Residual Content

1. Introduction

Washing processes and in this case also flushing proc-

esses always involve the elution of impurities. The aim is

the preferably complete replacement of the mother liquid

by the lowest possible amount of washing liquid.

A possibility to characterise a washing process is the

washing curve (Figure 1), here a concentration ratio c/c0

is plotted as a function of a washing ratio W. The con-

centration ratio is related to the concentration c0 of the

impurity at the beginning of the washing process (for a

flushing process the concentration of the organic solvent

in the mother liquid (normally 100%)). The concentra-

tion c represents the actual concentration of the impurity.

This concentration c can be related to the filtrate or the

filter cake itself. The washing ratio W represents the ratio

of the added washing liquid volume related to the vol-

ume of the pores in a fully saturated filter cake. In case

of a flushing process a mass washing ratio is used, be-

cause of the difficult calculation of the washing liquid

volume for two-liquid-systems. Due to this, the mass of

the added washing liquid MW is related to the mass of

mother liquid in the filter cake pores MV at the beginning

of the flushing process.

W

Because of the fact, that the concentration of an impu-

rity within the filter cake is not detectable, the concentra-

tion ratio in the filter becomes an inaccessible factor. For

the industrial application a possibility has to be found, to

calculate the concentration ratio in the filter cake out of

an accessible factor (e.g. the concentration ratio in the

filtrate). Due to this Heuser [1] defined the residual con-

tent R as an integral factor for the concentration ratio in

the filter cake.





Rc*W

The concentration factor c* represents the concentra-

Figure 1. Ideal and exemplary real washing curve (concen-

tration ratio in the filter cake depending on the washing

ratio).

M. WILKENS ET AL.

253

tion ratio c/c0 in the filtrate and a factor for the adsorp-

tion of the mother liquid in the filter cake.

In Figure 1 an ideal washing curve and an exemplary

real washing curve are shown. For an ideal washing

process or flushing process a complete exchange of the

mother liquid by the washing liquid happens at a wash-

ing ratio of 1. This means it is possible to clean the filter

cake with the same amount of washing liquid as the

amount of mother liquid in the filter cake.

The real washing curve is divided in three characteris-

tic areas [2,3]; the displacement regime (I), the intersti-

tial regime (II) and the diffusion regime (III). During the

displacement regime the washing liquid displaces the

mother liquid in a kind of a plug flow. Due to this only

mother liquid is detected in the washing filtrate. The end

of displacement regime is indicated by the detection of a

concentration ratio lower than one in the washing filtrate.

At this point a mixture of mother and washing liquid

leaves the filter cake and the interstitial regime starts.

During the last regime of the washing process (III) the

removal of the mother liquid is dominated by diffusion

mechanisms (diffusion regime). The remaining impuri-

ties in the filter cake diffuse out of stagnant areas into the

primary stream. This removal often occurs slowly be-

cause of sterical barriers. Quite long time scales are

needed to achieve a significant impurity removal [4].

2. Materials

For the research of the adsorption properties on a wash-

ing process an aluminosilicate (Dorkafill, x50 = 1.65 µm)

in three different surface modifications is used. The sur-

faces vary from hydrophilic (DF600) over hydrophilic-

hydrophobic (DF601) to hydrophobic (DF602). Because

of the identical shape and particle size distribution the

filtration and flushing are only influenced by structure

effects, which result from the different particle-fluid in-

teractions [5].

The two liquids water and methanol are used as

mother and washing liquid. Characteristic of this system

is that the progression of the dynamic viscosity depends

on the mixing ratio of water and methanol, with a maxi-

mum at a methanol percentage of 40 vol.-percent.

3. Methods

3.1. Filtration

The filter cake formation is performed with a lab scale

suction filter (nutsch filter) according to VDI 2762. The

filtration area is 19.64 cm² (diameter 50 mm). The proc-

ess parameters are a solid volume concentration of 0.2

and a filtration pressure of 3 bar. For the displacement

washing a fully saturated filter cake is needed, because

of this the filtration is stopped when the suspension

reaches the filter cake. With the known porosity of each

system it is possible to calculate the amount of liquid

staying in the filter cake, which is necessary for the cal-

culation of the washing ratio during the flushing step.

3.2. Flushing

For the flushing process the filter cake (inside a cake

formation unit) is removed from the suction filter into a

combined compression-washing unit.

Before and during the washing the filter cake is com-

pressed by a pneumatic piston with a constant pressure of

3.5 bar to prevent fissuring and crack formation. The

washing liquid flow is provided by a pressure difference

of 2 bar.

For the composition analyses of the washing liquid

(concentration ratio in the filtrate) samples in an interval

of ΔW = 0.05 are taken. A Karl-Fischer-Titrator and a

gas chromatograph (GC) are used for the analysis.

4. Results

For the technical control of a flushing process the con-

centration of the impurity (organic solvent) in the filtrate

is in most cases the only possibility to get online infor-

mations of achieved washing qualities after a certain

time or washing ratio.

Figure 2 shows the concentration ratio c/c0 in the fil-

trate depending on the mass washing ratio for the hy-

drophilic DF600.

Due to the hydrophilic surface of DF600 the surface

energy between water and DF600 is higher than between

methanol and DF600. The definition of the surface en-

ergy as the ratio of the needed work to create a new sur-

face and the new surface itself shows, that there is a

Figure 2. Flushing process of DF600 (hydrophilic) with

water and methanol.

254 M. WILKENS ET AL.

preferential exchange of a liquid with a poor wetting

behaviour by a liquid with a good wetting behaviour.

This fact correlates with the lower reachable concen-

tration ratios of the washing process with methanol as

mother liquid and water as washing liquid (grey plot).

Between the two washing directions (1 mother liquid

water, 2 mother liquid methanol) a shift of the washing

regimes can be observed. For methanol as mother liquid

the displacement regime ends at a washing ratio of 0.81,

for water at a washing ratio of 0.96. Because of the dif-

ferent structures of the filter cakes (porosities, pore size

distribution) this shift can not be directly ascribed to the

different surface energies. If no preferred adsorption of

the two liquids exists, there should be no shift between

the washing regimes and the washing quality, repre-

sented by the concentration ratios.

The system DF601 with a hydrophilic-hydrophobic

surface shows such behaviour.

Both flushing steps show an identical washing curve,

with the end of the displacement regime in a range of W

= 0.87.

This fact leads to the assumption that the better the

wettability of a system the better is the washing result,

independent on the structure behaviours.

Until now only the concentration ratios in the washing

filtrate are presented. The more interesting concentration

in the filter cake can be calculated by the residual content.

For the experimental work a possibility is found to ex-

tract the liquid of a flushed filter cake by a freezing unit,

which is based on the principle of a vacuum cooling trap.

Figure 4 shows the residual content of the hydrophilic

system DF600 with methanol as mother liquid and water

as washing liquid.

A comparison between the residual content according

to Heuser (black) and the residual content experimentally

determined shows a difference of nearly one dimension

for high washing ratios. This would lead to a wrong

technical dimensioning of flushing processes. The resid-

ual content calculated in consideration of the influence of

the volume flow fluctuation [6,7] during a flushing proc-

ess leads to a relative good fit between calculation and

experiment. This new kind of residual content is based

on the residual content expressed by Heuser.



 



max

,max















Wt Filtrate mm

new WFiltrate m

VWt

RR VW W





Thought has been given to an amplification of the re-

sidual content R by a normalised volume flow. In future

works more attention will be paid to the volume flow

fluctuation and the effects which cause the fluctuation.

Figure 3. Flushing process of DF601 (hydrophilic- hydro-

phobic) with water and methanol.

Figure 4. Residual content/concentration ratio in the filter

cake for DF600 (hydrophilic); mother liquid methanol, wa-

shing liquid water.

5. Conclusions

The flushing of filter cakes is an important technique for

industrial applications. This works shows the importance

of wetting behaviours between the solid system and the

mother liquid and washing liquid involved in a flushing

process. It has been shown that the washing quality and

the washing regimes depend on the surface energy of the

system.

Another interesting and not to be undervalued fact is

the difference between the detectable concentrations

within a filter cake and the calculated concentration (re-

sidual content) out of the concentration ratio in the fil-

trate. An amplification of the residual content by a nor-

malised volume flow eliminates the shift between ex-

periment and calculation to a minimum.

6. References

[1] J. Heuser, “Filterkuchenwaschprozesse unter Besonder-

er Berücksichtigung Physikalisch-Chemischer Einflüsse,”

Shaker-zgl. Diss. U. Karlsruhe, Aachen, 2003.

[2] W. Bender, “Filtrieren, Auswaschen und Entfeuchten

feindisperser Feststoffe,” Chemie Ingenieur Technik, Vol.

M. WILKENS ET AL.

255

48, No. 4, 1976, pp. 270-281. doi:10.1002/cite.330480404

[3] F. Hardekopf, “Experimentelle und Theoretische Unter-

suchungen zum Waschen von Filterkuchen auf Filter-

pressen,” Chemie Ingenieur Technik, Vol. 64, No. 11,

1992, pp. 1041-1044. doi:10.1002/cite.330641123

[4] F. Russlim, “Flow and Mass Transfer Phenomena in Fil-

ter Cake Washing Driven by Mass Force,” Dissertation

University, Karlsruhe, 2009.

[5] S. Neubauer and U. Peuker, “Flüssigkeitsmischungen als

Suspensionsflüssigkeit-Wirkung auf die Filtrationspara-

nmeter,”Chemie Ingenieur Technik, Vol. 79, No. 2, 2007,

pp. 1753-1758. doi:10.1002/cite.200700128

[6] M. Wilkens and U. Peuker, “Flushing-Washing Behav-

iour of Filter Cakes,” 13th Nordic Filtration Symposium,

Iappeenranta, 10-11 June 2010.

[7] M. Wilkens and U. Peuker, “Filter Cake Washing with

Non-Aqueous Liquids—The Influence of the Surface

Properties,” Filtech Conference 2011, Wiesbaden, 22-24

March 2011.