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Introduction and Overview

1.1 General Aspects of Solid-Liquid Separation in General and Cake Filtration in Detail

Cake filtration represents one of the several different mechanical methods to separate particles from liquids. Cake filtration offers, in comparison to other mechanical particle-liquid separation techniques, the advantageous possibility of direct solids posttreatment. This enables particle washing by cake permeation and the comparatively lowest mechanically achievable solid moisture contents. Particularly, if low residual moisture content of the solids is an important issue, cake filtration is the preferred technique. Unfortunately, not in every case, cake filtration can be realized from the technical and/or physical point of view. Figure 1.1 shows the principal steps of a cake filtration cycle.

Figure 1.1 Process steps of a cake filtration.

After feeding the filter apparatus with slurry, a filter cake is formed under the influence of a pressure difference above and beneath the filter medium. If necessary, the cake can be washed in the next step to get rid of soluble substances in the liquid, which are still present in the wet cake. Finally, the filter cake is deliquored to displace further liquid from its porous structure. At the end of the process, the cake is discharged from the filter apparatus, and if necessary, the entire apparatus or the filter medium has to be cleaned.

The residual moisture content of the separated solids is considerably influencing the efficiency of a subsequent thermal drying and thus the energy consumption of the whole separation process. As a rule, the thermal methods are usually quite energy-intensive compared with the mechanical liquid separation. In the literature, guiding numbers of more than factor 100 can be found between the energy demands of mechanical and thermal deliquoring [1,2]. In comparison to mechanical methods, thermal methods require not only heating of the wet system to the boiling point of the liquid but also a phase transition from the liquid to the gaseous aggregate state. The appropriate vaporization enthalpy must be raised. For this reason and also because of the often-undesirable load of temperature-sensitive products, it is in most cases advantageous to separate as much liquid as possible at low temperatures by mechanical means. For physical reasons, a final rest of liquid remains in any case after the mechanical liquid separation in the particle structure. However, this portion of liquid can only be removed from the solid material by thermal means. If a completely dry powder is required as the final product, one of the tasks for the optimization of the whole separation process consists in determining the most favorable point of transfer from the mechanical to the thermal separation step. This interconnection point is very variable and defined by the requirements of the selected thermal drying process. For spray drying, a pumpable and sprayable slurry is still required, whereas the solids should be deliquored to the mechanical limit for a fluidized bed drying because the cake behaves brittle and powdery in that case. At the interface of these two basic processes for solid-liquid separation, combined mechanical-thermal processes have also been developed and established such as centrifuge dryers, nutsche dryers, and in recent times continuously operating steam pressure filters. The advantages of these systems consist in synergies, which result in energy conservation and compact and simplified process design.

As mentioned before, the application of cake filtration is of course not possible in every case but limited by some boundary conditions. For example, in the case of submicron particles and very low solid concentration in the slurry, cake filtration makes no sense because of very high cake pressure loss of the cake and large quantities of liquid, which must penetrate such a tight cake structure. If the boundary conditions for cake filtration are not fulfilled, alternative separation techniques such as density separation, depth, cross-flow, or blocking filtration must be used. To enhance the filtration performance, electric or magnetic particle properties may be utilized additionally by the realization of an electric or magnetic field in the process room. In the literature, several comprehensive descriptions of more or less the entire technology are published [3-8]. Figure 1.2 gives an overview of the different available physical principles of mechanical particle-liquid separation.

Figure 1.2 Physical principles of solid-liquid separation.

Density separation is based on a difference of density between solid particles and liquid. If the solid density is greater, the particles are settling into the direction of gravity or a centrifugal field and are deposited at a solid wall as sediment, whereby the liquid is displaced to the opposite direction. Static continuously operating thickeners or clarifiers and many types of batch and continuous solid bowl centrifuges are based on this separation principle. If the particles have less density than liquid, they will float on the liquid surface. Also, particles of greater density than liquid float by froth flotation. For this purpose, gas bubbles are generated in the slurry, and if the particles are hydrophobic, they can adhere to the bubbles and float on the surface. This technique is often used to separate hydrophobic from hydrophilic particles of different materials in sorting processes of mineral beneficiation or waste paper recycling. If necessary, the flotation conditions regarding the wetting behavior of particles can be influenced by various surfactants. These are water-soluble molecules with polar (hydrophilic) and nonpolar (hydrophobic/lipophilic) parts. For example, the polar part of a surfactant adheres at the surface of a hydrophilic particle and thus the particle appears hydrophobic from outside. This process runs spontaneously because the systems decrease its free energy. If all particles are hydrophobic as in the case of organic matter, the complete solids can be floated and thus separated very effectively.

Filtration in contrast to density separation is based on the presence of a porous filter medium. Particles and liquid are moving under the influence of a gas pressure difference, a mechanical, hydraulic, hydrostatic, or centrifugal pressure toward the filter medium. The liquid penetrates the filter medium whereby the particles are retained inside the structure or on the surface of the filter medium.

In depth filtration processes highly diluted slurries of very small particles usually in the µm- or sub-µm range are separated. The particles are deposited inside of a three-dimensional network of pores. The pores of the filter medium must be much greater than the particles to be separated in order to minimize the flow resistance for the liquid and to allow the particles to enter the structure and to accumulate inside. The slurry concentration must be very low to prevent the filter from becoming spontaneously blocked by pore bridging at the filter media surface. The filter media can consist of a disperse particle layer from various types of materials such as sand, gravel, activated carbon, diatomaceous earth, and others or premanufactured filter elements made of fibrous materials such as cellulose, carbon, polymers, metal, and others.

Surface filtration can be subdivided principally into blocking, cross-flow, and cake filtration. Blocking or sieve filtration means that single and low concentrated particles are approaching the filter medium and plugging single pores. In the cases of sieve filtration, the pores of the filter medium must be smaller than all particles, which should be separated completely. Not in all cases, a total separation of particles is aimed, but only the retention of oversized particles to protect subsequent separation apparatuses such as hydrocyclones or disc stack separators. These apparatuses are discharging the separated solids highly concentrated through nozzles, which are in danger to become blocked by oversized particles. The particle spectrum, which can be handled by different types of blocking filters, is very broad from the cm to the µm range.

In cross-flow filters, a low concentrated slurry of small particles less than about 10?µm is pumped across a microporous membrane and is consecutively concentrated, whereas the filtrate (permeate) is discharged through the membrane. The formation of a highly impermeable particle layer (filter cake) is prevented as completely as possible by the cross-flow, which should wash away the deposited particles permanently. It depends on the force balance around a particle, whether it is sheared off or adheres at the membrane surface. The particle spectrum to be separated by cross-flow filters ranges from some µm down to small molecules such as Na+ or Cl- ions. As an example for reverse osmosis, "poreless" membranes are used to separate salt ions from seawater in order to produce drinking water. In such cases, not a convective liquid transport through real pores but a diffusive transport of water molecules through the molecular structure of the membrane takes place. If the cross-flow only by pumping the slurry across the membrane is not sufficient to limit the particle deposition dynamic cross-flow filters can be applied. Here, the shear forces between membrane and liquid are generated by a rotor/stator system (membrane/membrane or membrane/stirrer)...