Cake filtration or cake-forming filtration is a mechanical process for the separation of solid-containing liquids. It pertains to surface filtration like cross-flow filtration. In contrast to depth filters, the actual separation effect does not take place through the filter medium but through a filter cake, which forms on the filter cloth. As one of the oldest filter units, the filter press is the most important and efficient process pressure filter. Especially when it comes to the highest level of filtration efficiency and maximum degree of dewatering of various filter media. Depending on the application, filtration with filter presses has different functions as a mechanical separation process:

• • Purification of raw materials and products as well as separation of by-products

• • Recycling of solvents and unreacted reactants

• • Removal of impurities from waste water

• • Extraction of recyclable materials and raw materials

WORKING PRINCIPLE OF A FILTER PRESS

During discontinuous cake filtration with filter presses, the suspension to be filtered is pumped and concentrated via the turbidity inlet into the filter plate chambers. The driving force for the liquid transport is provided by a feed pump. The filter plates are lined with special filter cloths according to the respective application. These cloths retain the solids in layers during the flow. A layer of the solids to be separated accumulates over time. This layer is known as the filter cake, which remains in the plate chamber as a solid mass and makes a significant contribution to the filtration of the liquid. The filtrate, which penetrates the filter cloths, is discharged along a channel system via the filtrate drain introduced into the filter plates. As the layer to be flowed through grows, the flow through resistance increases and with it the pressure loss, with a concomitant decrease in throughput and flow rate. The permeability of the cake decreases with compressible products. This leads to inefficient and uneconomically long filtration times. With periodic cake filtration, the filtration process is interrupted once a predetermined pressure has been reached. If necessary, the filter cake is squeeze-treated (membrane technology) and then removed by opening the filter press. After optional cleaning of the filter cloth and closing of the unit, filtration starts again from the beginning. The filtration cycle is essentially divided into four phases:

The decisive parameters for assessing a filtration process with filter presses are the filtration pressure and the throughput. In an ideal filtration process, the filtrate can drain off freely. Only the building up filter cake is what offers the filtrate resistance to the filtrate flow. This means that the filtration pressure only increases significantly when the filter cake is compressed in the chambers of the filter press. Since the resistance of a filter cake depends on numerous factors, such as particle size distribution, particle shape and interaction between the particles, a filter cake resistance and the associated filtration process cannot simply be predicted by theoretical approaches. Preliminary tests are therefore necessary to determine the filter cake resistance. In our laboratory, we create the basics for the best possible filtration on your filter press using laboratory filter presses.