Filtration


Introduction:

Filtration is the process of separating solids from liquids by means of a porous medium which retains the solid but allows the liquid to pass through it. The conditions for better Filtration are many and the choice of the suitable equipment depends upon more number of factors. What ever the type of equipment is used, a filter cake gradually develops and the resistance to flow progressively increases through out the operation. The factors which affects rate of filtration and the choice of the particular equipment are discussed below.

Rate of Filtration:

The rate of filtration depends upon the following factors.
  1. The filter area.
  2. The difference in pressure between the two sides of the filter.
  3. The average cross section of the pores within the filter cake.
  4. The number of pores per unit area of the septum.
  5. The thickness of the filter cake.
The effect of each of the above factors is obvious, i.e. the rate of filtration increases directly with (1), (2) & (4), inversely as (5), and directly as some power of (3). Factors (1) & (2) controlled by the device in use, (5) by the way in which the device is operated, and (3) & (4) by the character of the pulp.
Filtration in mineral processing applications normally follows Thickening. The Thickened pulp may be fed to storage agitators from where it is drawn off at uniform rate filters. Flocculants are added to agitators in order to aid Filtration. Slimes have an adverse effect on filtration, as they causes to blind the filter medium. Flocculation reduces this and increases the voidage between particles, making Filtrate flow easier. Flocculants with lower molecular weight are preferable in filtration rather than high molecular weight product. This is due to flocks formed by high molecular weight products are relatively large, and entrain the water with in the structure; it causes more content in the Filter cake.


Filter Medium:

While selecting the Filter medium, the most important consideration is assuring efficient operation of Filter. The medium should be selected primarily for its ability to retain solids without blinding. It should be mechanically strong, corrosion resistant, and little resistant to flow of filtrate. Filter media are manufactured from cotton, wool, silk, glass fiber, porous carbon, metals, rayon and other synthetics and miscellaneous materials such as porous rubber. Cotton fabrics are the most common type of medium, because of their low initial cost and availability is more.

Types of Filters:

In Mineral processing, cake filters are the types which are most frequently used. The main requirement in cake filter is recovery of large amount of solids from concentrated slurries. Cake filters can be classified into Pressure filters and Vacuum filters.

Pressure filters:

Pressure filters have certain advantage over Vacuum Filters due to virtual in compressibility of solids. By using high pressure, which results in better washing, higher flow rates and drying. The vast majority of pressure filters operate as batch units.
The most frequently used type of pressure filters are Filter presses. Filter presses are again divided into two forms; those are (a) Plate & Frame press (b) Recessed or Chamber press.

Plate and Frame Filter press:

It consists of plates and frames arranged alternatively. The hollow frame is separated from the plate by filter cloth. The filter press is closed by means of a hydraulic piston devise and compression of the filter cloth between plates and frames help to prevent leakages. A tight chamber is therefore formed between each pair of plates. The slurry is introduced to the empty frames of the press through a continuous channel formed by the holes in the corners of the plates and frames. The filtrate passes through the cloth and runs down the grooved surfaces of the plates and these removed through a continuous channel.

Plate and Frame Press
Fig: Plate and Frame filter press


The cake remains in the frame and, when frame is full the filter cake can be washed, after which the pressure is released and the plates and frames separated one by one. The filter cake in the frames can then be discharged, the filter press closed again and cycle repeated.

Chamber Filter Press:

It is similar to the plate and frame type except for the fact the filter elements consists solely of the recessed filter plates. The individual filer chambers are therefore formed between successive plates. All the chambers are connected by means of a comparatively large hole in the center of each plate. The filter cloth with a central hole covers the plate and slurry is led through the inlet channel. The clear filtrate passing through the cloth is removed by means of smaller holes in the plate, the cake gradually depositing in the chambers.
Filtration - Mineral Process Engineering



Fig. Chamber or recessed plate filter press


Vacuum Filters:

In vacuum filters the filter media will be incorporated on the Drainage system, beneath which the pressure is reduced by connection to a vacuum system. Vacuum filters may be Batch or Continuous.

CONTINUOUS VACUUM FILTERS: These are again classified into drums, discs, and horizontal filters.

Horizontal belt Filter:

It consists of rubber drainage deck supporting a separate belt made from suitable filter cloth. Initially in the horizontal travel, slurry flows by gravity on to the belt. Filtration immediately commences due to gravity and vacuum through the suction boxes which are in contact with the under side of the drainage deck during the course of its travel. The cake which forms is dewatered and dried by drawing air through it, and then the cake discharged as the belt reverses over a small diameter roller.



Filtration - Mineral Process Engineering

Fig. HORIZONTAL BELT FILTER
The applications for horizontal belt filters are increasing. These are particularly suited for hydro metallurgical circuits where the metal values are recovered from waste solids by filtration of leached slurry and countercurrent washing. These are also suited for concentrated slurries of fast settling products, where efficient washing is required. When compared with disk. drum, roller, press filters these filters offer a particularly cost- effective and reliable solution for filtration problems.



Question and Answers

(1).What is Filtration?
Ans: Filtration is the process of separating solids from liquids by means of a porous medium which retains the solid but allows the liquid to pass.

(2).Mention at least three factors which affects rate of filtration?
Ans: (a).The filter area.
(b).The thickness of the filter cake.
(c).The difference in the pressure between two sides of the filter.

(3).What is the classification of filters?
Ans: Pressure filters & Vacuum filters.

(4).What are the different types of vacuum filters?
Ans: Drum filters, Disc filters, Horizontal filters.

(5).What are the different types of pressure filters?
Ans: Filter and frame press & chamber press.


References:

  • Principles of Mineral Dressing by A.M.Gaudin
  • Mineral Processing Technology by B.A.Wills.

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