(285c) Compaction of Compressible Filter Cakes By Applying Low Pressure and Oscillatory Shear

Cake filtration is a proven method for separating solid particles from a suspension. After cake formation, there is a particle network whose porse are filled with liquid. To reduce the residual moisture of the filter cake, mechanical dehumidification measures are a more cost-effective alternative to thermal drying. After cake formation, gas pressure is applied to the filter cake, allowing the liquid to be largely displaced from the pores of the particle network. This can lead to the formation of shrinkage cracks, especially in fine-particle products, which result in higher gas consumption and thus higher process costs. It is known from previous research activities that shrinkage cracks during liquid removal can be prevented by pre-compaction of the filter cake. [1-2] Furthermore, filter cake compaction is also a way to displace more liquid from the filter cake. Since the required pressing pressure can be very high, depending on the particle system, and the method is thus not feasible on a continuous technical scale due to the high stress on the apparatus, alternative compaction methods are desirable. An innovative method is the application of an oscillatory shear into the filter cake using significantly lower pressure, which has already been successfully tested on a laboratory scale for mineral materials with low and medium compressibility. [3]

In order to further clarify the applicability of the process, the compaction behavior of other relevant products under oscillatory shear must be investigated. As a result, correlations between the various particle properties (particle size, particle size distribution, agglomeration state) and the compaction behavior under oscillatory shear should be established. It is also necessary to investigate the depth effect of the vibration-enhanced compaction to better understand the compaction mechanism.

Three limestone products with different mean particle sizes and the same particle size distribution width were selected to investigate the influence of particle size on compaction behavior under oscillatory shear. Subsequently, experimental studies under variation of process parameters such as the vibration frequency and the number of applied oscillations were carried out on a laboratory plant (see figure 1 in the attached image). The laboratory plant consists of a vacuum filtration plate, in which a filter cake is first formed. A transfer plate is then placed on the filter cake, which applies oscillatory shear to the filter cake. The superimposed pressure of 80 kPa is achieved during oscillatory shear by a vacuum pump.

The evaluation of the compaction effect under oscillatory sear is possible with the consolidation potential, which is defined as the difference between the porosity after cake formation and the minimum achievable porosity as a result of oscillatory shear. Figure 2 in the attached image shows that higher compaction is achieved for the finer limestones 1 and 2 compared to the coarse limestone 3. While in the low to medium frequency range the compaction effect hardly differs between the fine limestones, the finest limestone indicates significantly higher compactability at a high frequency. It can thus be stated that compactibility by oscillatory shear increases with decreasing mean particle size. This can be explained by the higher mobility of finer particles within the filter cake, which leads to a stronger rearrangement by the vibration input. In individual tests, the filter cake was additionally cut into several layers after compaction and the progress of compaction in the layers was analyzed. It was found that after only a few oscillations applied to the filter cake, compaction can be detected in all layers. The upper filter cake layers undergo the highest compaction.

In the course of the presentation, the influence of the mean particle size on the compaction behavior under oscillatory shear as well as the depth effect of vibration-enhanced compaction will be shown.

References:

[1] Wiedemann, T. Das Schrumpfungs- und Rißbildungsverhalten von Filterkuchen, 1996.

[2] Wiedemann, T.; Stahl, W. Experimental investigation of the shrinkage and cracking behaviour of fine particulate filter cakes. Chemical Engineering and Processing: Process Intensification, 1996, 35, 35–42.

[3] Illies, S. Darstellungen zur Entfeuchtung von zu Rissbildung neigenden Filterkuchen, 2017.