„The cake always tells a story“
When the filtration industry is talking about “cake” it is hardly grandma’s delicious sweets, but rather filtered solids which are left inside a filtration chamber after the filtration process. This cake can be product or waste and its consistency and structure is of essence for the result of the filtration.
Analysing the filter cake for thickness, structure, moisture and evenness will tell us its story and show potential improvements to the process we can apply.
But first things first.
How is filter cake produced?
In a filter press slurry is pumped under pressure into filtration chambers. These chambers are made up from the installed filter plates. Inside these chambers the filter cloth is responsible for the initial solids retention, resulting in the solids accumulating in front of the filter cloth. Once an initial layer of cake is formed on the filter cloth the efficiency of the cloth takes a back seat to the characteristics of the cake, as the cake takes over the job of filtration. That is why it is also characterised as “cake filtration”. From here on, the thickness of the cake is steadily increasing until the filtration chamber inside the filter plates is filled and the filter press needs to be emptied.
Before discharging the cake it can be washed inside the filter press to remove unwanted residue. This step is called “cake washing”. Furthermore, compressed air can be introduced into the cake to drive out any remaining free water from the cake and increase the dry matter content of the cake. This step is called “cake dry blowing”.
How filter cake is created inside a filter press
How do I achieve a low remaining moisture content in my filter cake?
To answer this question, one must understand principal differences between chamber plate filtration and membrane plate filtration regarding their method of dewatering.
A chamber filter press is filled until the filter cake takes up the complete filtration chamber. The filtration pressure is increased until it reaches a set point, thus creating a force that acts upon the filter cake and dewaters it. Therefor, a chamber filter plate has to be filled completely to achieve a dry cake.
However, the filtrate leaving the filter press is becoming consistently smaller the thicker the cake already is whereas at the same time fewer new cake is being built up. This reduces the efficiency of filtration.
Contrary, Filter presses with membrane filter plates follow a different principal. The chambers are filled as long as an efficient filtration is possible. After the filtration cycle a squeezing medium is introduced behind the membranes causing the membranes to move. This movement mechanically dewaters the cake. This allows us to stop the filtration cycle at any point in time and still achieve a dry cake. An overall increase in productivity and lower remaining moisture content inside the cake can be achieved.
Furthermore, through clever process control, better cake washing results can be achieved e.g. through pre-squeezing the filter cake.
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