What is the filter press working principle ?

Filter press is simply a kind of machine, which performs Filtration & separation during its Pressing operation.

Type of Filter Press  : Plate and Frame Filter Press , Chamber Filter Press , Membrane Filter Press , etc. 

A filter press consists of a series of chambers containing square or rectangular filter plates supported in a frame. Once the filter chambers are loaded with slurry, the plates are forced together with hydraulic rams that generate pressures typically in the region of 100 pounds per square inch (70,000kg per m2). For comparison, a car tyre would be
inflated to around 30 pounds per square inch.

Each plate is covered by a material or membrane that acts as the initial filter when the press is in operation. As the solid filter cake builds up, the cake adds to the removal of fine particles. The solution coming through the filter, called the filtrate, will be very pure. If it is not wanted the filtrate can be drained away for safe disposal.

At the end of the compression, the solid filter cake can be removed. The whole process is often computer controlled to make it automatic or semi-automatic.