Can We Use Vacuum Filters for Polishing Filtration ?

Hi Friends,

One of our friends of Alumina fraternity has raised a very interesting question before us. Can we use vacuum filters for polishing filtration in Alumina refinery? The simple answer is 'No chance'. Now let us analyse the pros and cons with technical reasons to clearly understand this issue.

In Alumina refinery, the polishing filtration of settler overflow liquor is done for achieving following three basic objectives-

• Improved filtrate clarity : Suspended solids in filtrate has direct impact on product quality particularly with respect to impurities like SiO₂ and Fe₂O_{3 .}
• High filtration rate : Plant production rate is directly proportional to liquor filtration rate, hence it is aimed to have the specific filtration rate between 1.50 to 2.00 m³/m².hr.
• Negligible  temperature drop : Since the temperature drop of aluminate liquor may cause loss of alumina with residue cake due to auto precipitation during filtration. Hence all possible measures are taken to minimize the temperature drop of aluminate liquor across filtration unit within 2 to 3 degree centigrades. 

Pressure filters meet all the above process requirements. On the other hand, vacuum filters can not meet any of these three process requirements because the driving force is the vacuum. Applied vacuum for liquor filtration will generate cooling affect on liquor and there by substantial increase in viscosity of saturated aluminate liquor, thus marginal auto precipitation of hydrate in residue cake. The increase in viscosity of liquor will cause increase in filtration pressure ultimately affecting the filtration rate and filtrate clarity as well.

Hence, vacuum filters can not be used for polishing filtration of aluminate liquor in Alumina refinery and therefore, Pressure filters are the only option for this unit operation.

Hope, I have clarified the issue to the extent possible. however, if you have some other reasons to explain, please feel free in sharing your views.

Regards.

Kunwar Rajendra