chemEcaleb
Chemical
- Oct 1, 2015
- 30
Note: It's a sodium chlorate plant, not exactly a chlor-alkali plant, but the two have a lot of similarities.
The reason I'm posting this is to share a recent success I had this summer with a filtration problem that has haunted the plant since the day it was built 25 years ago. We use candle fliters with polypropylene/felt filter socks as a polishing step after the clarifier. Chlor-alkali plants generally like to have raw salt to have a calcium to magnesium ratio or 3-4 because this improves settling in the clarifier and give a better filter cake. Our plant has only one feasible salt supply which has a calcium to magnesium ratio of .25 which results in a "stick" or "slimy" filter cake. A sticky/slimy filter cake is hard to remove during backwashing of filters.
The end result is that every week, maintenance has to open up these filters and wash off the socks which consumes an entire 6 man hours. I looked in to the following solutions:
* Pall Zylon Filters - supposedly a magical goretex candle filter that will allow you to operate WITHOUT A CLARIFIER. We installed a pilot scale unit in our plant, and the filter socks were irreversibly damaged within a matter of days due to particle penetration and impregnation. Those filters aren't set up to operate with a stick/slimy cake of small particles.
* Dynasand Filters - These are self cleaning, low maintenance, but running high pH brine through them results in silica and other components of the sand dissolving. I experimented with all kinds of sand (including garnet sand which resulted in aluminum impurity). They basically all dissolved impurities in to the brine, and chlor-alkali plants use sand filters, but at our plant we don't do low pH anion exchange like most chlor-alkali, we only have an amino-phosphonic acid resin to remove Ca+Mg. In order to use sand filters we'd need to add another step in purification to remove silica... not feasible.
* Changing the ratio of Ca to Mg with the addition of CaCl2. This is guaranteed to work, but we'd need to install a 30,000 gal tank and a dissolving system. If we were to buy CaCl2 solution that would dramatically increase the cost of the project. This was too expensive.
THE SOLUTION!!!!! The solution was to use an LMI Metering pump to add cellulose fiber filter aid to the feed going to the filters. We already had filteraid on site for the precoat after backwashing. So instead of spending 6 man hours per week cleaning filteraid, operators now just have to make a few more batches of filteraid per week ~30 minutes per week total.
Hope this can help someone dealing with a sticky slimy filter cake.
The reason I'm posting this is to share a recent success I had this summer with a filtration problem that has haunted the plant since the day it was built 25 years ago. We use candle fliters with polypropylene/felt filter socks as a polishing step after the clarifier. Chlor-alkali plants generally like to have raw salt to have a calcium to magnesium ratio or 3-4 because this improves settling in the clarifier and give a better filter cake. Our plant has only one feasible salt supply which has a calcium to magnesium ratio of .25 which results in a "stick" or "slimy" filter cake. A sticky/slimy filter cake is hard to remove during backwashing of filters.
The end result is that every week, maintenance has to open up these filters and wash off the socks which consumes an entire 6 man hours. I looked in to the following solutions:
* Pall Zylon Filters - supposedly a magical goretex candle filter that will allow you to operate WITHOUT A CLARIFIER. We installed a pilot scale unit in our plant, and the filter socks were irreversibly damaged within a matter of days due to particle penetration and impregnation. Those filters aren't set up to operate with a stick/slimy cake of small particles.
* Dynasand Filters - These are self cleaning, low maintenance, but running high pH brine through them results in silica and other components of the sand dissolving. I experimented with all kinds of sand (including garnet sand which resulted in aluminum impurity). They basically all dissolved impurities in to the brine, and chlor-alkali plants use sand filters, but at our plant we don't do low pH anion exchange like most chlor-alkali, we only have an amino-phosphonic acid resin to remove Ca+Mg. In order to use sand filters we'd need to add another step in purification to remove silica... not feasible.
* Changing the ratio of Ca to Mg with the addition of CaCl2. This is guaranteed to work, but we'd need to install a 30,000 gal tank and a dissolving system. If we were to buy CaCl2 solution that would dramatically increase the cost of the project. This was too expensive.
THE SOLUTION!!!!! The solution was to use an LMI Metering pump to add cellulose fiber filter aid to the feed going to the filters. We already had filteraid on site for the precoat after backwashing. So instead of spending 6 man hours per week cleaning filteraid, operators now just have to make a few more batches of filteraid per week ~30 minutes per week total.
Hope this can help someone dealing with a sticky slimy filter cake.
