Hi Friends,
In previous post, we have discussed about the Advancements in polishing filtration system wherein we have described the design and operational features of Auto dumping vertical leaf pressure filters over manual security filters. There are two basic objectives of polishing filtration unit in Alumina refinery-
- Maximum filtration rate at lowest filtration pressure and
- Improved filtrate clarity with minimum suspended solids.
In present post, we will discuss about various operational problems of Auto dumping pressure filters and remedial actions to prevent such operational difficulties for smooth and trouble-free operation of polishing filtration system. We are well aware that Auto dumping pressure filters are called under different brand names by different designers and suppliers of this equipment. Typical schematic view of Auto dumping vertical leaf pressure filter is given below just to have a physical feel of equipment and sequence of normal operation. I am thankful to my friend who took pain in developing the schematic view of Auto dumping pressure leaf filter.
The operation of the equipment is generally smooth during normal condition with steady process conditions in the upstream and sound functioning of the automation and controls. But, even during minor fluctuations in process conditions, the equipment behaves abnormally creating acute operational and process control problems. In this post, the remedial measures have been highlighted to prevent two major operational problems as elaborated in following paragraphs-
1.Fast drop in filtration rate : The basic objective polishing filtration unit is to ensure high filtration rate with desired level of filtrate clarity containing maximum impurities within 20 mgpl. Presence of higher suspended solids in filter feed liquor adversely impacts the filtration rate, filtrate clarity and filtration cycle time. Fine grinding of bauxite, inconsistent flow of flocculent to settlers and higher viscosity of feed liquor are the main reasons for higher suspended solids in settlers. This problem can be easily prevented by ensuring proper grinding of bauxite, adequate flow of flocculent, proper cooking of flocculent, controlling the temperature profile of settler overflow liquor by monitoring the dilution intake in digester blow-off slurry.
2. Poor filtrate clarity: At times wide variations in filtrate clarity is observed with fluctuations in suspended solids in filtrate which is not desirable. Such fluctuation mainly occurs due to inadequacy of filter aid in filter feed liquor. It has been observed that TCA addition is done by plant operating personnel in Settler which is not the right way as TCA acts as settling aid in settlers in stead of filter aid in pressure filters. Thus continuous addition of TCA to the suction line of filter feed pumps resolves this problem. As per rule of thumb, the mass flow of TCA in filter feed liquor should be 10 times the mass flow of suspended solids in liquor. In addition, quality of burnt lime and correct method of TCA solution preparation at around 80 degree Centigrade for one hour cooking time to be ensured. Condition of filter leaves also has the vital role as hard leaves cause high filtration pressure and thereby ingress of suspended solid through pores. Thus periodic caustic wash of filter leaves is also essential.
Hope, the topic has been covered to the extent possible. Please put your views / suggestions / remarks / comments, if any.
Regards.
Kunwar Rajendra
The operation of the equipment is generally smooth during normal condition with steady process conditions in the upstream and sound functioning of the automation and controls. But, even during minor fluctuations in process conditions, the equipment behaves abnormally creating acute operational and process control problems. In this post, the remedial measures have been highlighted to prevent two major operational problems as elaborated in following paragraphs-
1.Fast drop in filtration rate : The basic objective polishing filtration unit is to ensure high filtration rate with desired level of filtrate clarity containing maximum impurities within 20 mgpl. Presence of higher suspended solids in filter feed liquor adversely impacts the filtration rate, filtrate clarity and filtration cycle time. Fine grinding of bauxite, inconsistent flow of flocculent to settlers and higher viscosity of feed liquor are the main reasons for higher suspended solids in settlers. This problem can be easily prevented by ensuring proper grinding of bauxite, adequate flow of flocculent, proper cooking of flocculent, controlling the temperature profile of settler overflow liquor by monitoring the dilution intake in digester blow-off slurry.
2. Poor filtrate clarity: At times wide variations in filtrate clarity is observed with fluctuations in suspended solids in filtrate which is not desirable. Such fluctuation mainly occurs due to inadequacy of filter aid in filter feed liquor. It has been observed that TCA addition is done by plant operating personnel in Settler which is not the right way as TCA acts as settling aid in settlers in stead of filter aid in pressure filters. Thus continuous addition of TCA to the suction line of filter feed pumps resolves this problem. As per rule of thumb, the mass flow of TCA in filter feed liquor should be 10 times the mass flow of suspended solids in liquor. In addition, quality of burnt lime and correct method of TCA solution preparation at around 80 degree Centigrade for one hour cooking time to be ensured. Condition of filter leaves also has the vital role as hard leaves cause high filtration pressure and thereby ingress of suspended solid through pores. Thus periodic caustic wash of filter leaves is also essential.
Hope, the topic has been covered to the extent possible. Please put your views / suggestions / remarks / comments, if any.
Regards.
Kunwar Rajendra
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