Filter Press

Introduction:

A filter press is a tool used in separation processes, specifically in solid/liquid separation using the principle of pressure drive, provided by a slurry pump. The filter press is used in fixed-volume and batch operations, which means that the operation must be stopped to discharge the filter cake before the next batch can be started.

The major components of a filter press are:

  • The skeleton
  • The filter pack.

The skeleton holds the filter pack together while pressure is being developed inside the filtration chamber. The chamber however can only hold a specific volume of solids.

 

Concept behind filter press technology

Generally, the slurry needed to be dewatered is injected into the center of the press and each chamber filled. The filling time should be as quick as possible in order to avoid cake formation in the first chamber before the last chamber is filled. While the chambers are being filled up, the pressure inside the system will increase due to the formation of thick sludge. Then, the liquid is filtered out through the filter cloths by adding streams of compressed air or water. The use of pressurized water requires more time to pass into the chamber compared to pressurized air, however this method is much more cost efficient.

 

Types of filter press

There are three main basic types of filter presses:

  • Plate and Frame Filter Press.
  • Recessed Plate and Frame Filter Press
  • Automatic Filter Press.

 

Plate and frame filter press

A plate and frame filter press is refer as a “membrane filter plate”. This type of filter press consists of many plates and frames assembled alternately with the supports of a pair of rails. The presence of a centrifuge pump ensures the remaining suspended solids which  do not settle in the system, and its main function is to deliver the suspension into each of the separating chambers in the plate and frame filter. For each of the individual separating chambers, there is one hollow filter frame separated from two filter plates by filter cloths. The introduced slurry flows through a port in each individual frame, and the filter cakes are accumulated in each hollow frame. As the filter cake becomes thicker, the filter resistance increases as well. So when the separating chamber is full, the filtration process is stopped as the optimum pressure difference is reached. The filtrate that passes through filter cloth, which is collected through collection pipes and stored in the filter tank. Filter cake (suspended solid) accumulation occurs at the hollow plate frame, then being separated at the filter plates by pulling the plate and frame filter press apart. The cakes then fall off from those plates and are discharged to the final collection point.

 

Automatic Filter Press

An automatic filter press has the same concept as the manual filter and frame filter, except that the whole process is fully automated. It consists of larger plate and frame filter presses with mechanical “plate shifters”. The function of the plate shifter is to move the plates and allow rapid discharge of the filter cakes accumulated in between the plates. It also contains a diaphragm compressor in the filter plates which aids in optimizing the operating condition by further drying the filter cakes. An animation showing this principal of operation can be seen here.

 

Recessed Plate Filter Press

A recessed plate filter press is made up of polypropylene squares at about 2 to 4 feet across with a concave depression and a hole in the center of each. Two plates join together to form a chamber to pressurize the slurry and squeeze the filtrate out through the filter cloth lining in the chamber. It is capable of holding 12 to 80 plates adjacent to each other, depending on the required capacity. When the filter press is closed, a series of chambers is formed. The differences with the plate and frame filter are that the plates are joined together in such a way that the cake forms in the recess on each plate, meaning that the cake thickness is restricted to 32mm unless extra frames are used as spacers. However, there are disadvantages to this method, such as longer cloth changing time, inability to accommodate filter papers, and the possibility of forming uneven cake.

 

Applications:
  • Ultrafine coal dewatering
  • filtrate recovery in coal preparation plants.
  • Municipal waste sludge dewatering,
  • Mix concrete water recovery,
  • Metal concentrate recovery,
  • Large-scale fly ash pond dewatering.

 

Main Process Characteristics

Table 1 Classification of filter press.

Specifications Parameters Units
Scale 1-100 m3/h
Operation Batch, Continuous
Objectives Clarification, Solids Recovery
Slurry settling characteristics
Rate <0.1 to >5 cm/s
Clarification Poor
Proportion of sludge < 2% vol to > 20% vol
Slurry filtering characteristics Slow at the rate of cm/h, up to 10 h