606
Discussion
For the alum process:- one floculation device with pH regulation; - one scraped sedimentation
tank, calculated on a load of 0,4 - 0,5 m
3 /m
2 /hr; - one scraped thickening tank calculated on a 40
kg SM/m
2 /day basis; - two centrifuges, ,0 43 cm continute decanters each allowing to treat 300 kg
SM/hr, - one rotary kiln treating 2300 kg of wet solids (25% dryness) per hour at 750°C with dust
washing of fume, - one alum recovery unit with reagents tanks, dosering pump, stirrer, etc.
For the lime process: - one lime dosing device; - one scraped sedimentation tank, calculated on a
load of 1,2 - 1,5 m
3 /m
2 /h; - one 6000 m
3 high-pressure blower for injection of dry flue gases rich of
CO2, Counterpressure : 4,5 m of water; - one second scraped sedimentation tank calculated on a load
of 0,8 - 1 m
3 /m
2 /h; - two rotary vacuum filters with accessories, calculated on a 25 kg dry
matters/m
2 /h basis; - one rotary kiln treating 5500 kg DM/hr at 1000°C.
it is to be noted that contrary to Mr. Gould's belief, we did not think that the alum sludge organic
solids has a calorific power higher than the sludge coming from lime treatment, but only that alum
sludge are richer in organic solids (70%) than lime sludge (25%). This is why it is possible to approach
autocombustibility with a dryness lower than that obtainable with lime sludge.
As for the working expenses, 92% efficiency for alum recovery is an estimated one, for we
obtained in the laboratory easy 100% recovery. We hope to reach 97% recovery.
It will be pointed out that we calculated the sedimentation tank to be more efficient with lime
than alum. But from our experience and that of French pulp mill owners, this can only be applied if
treatment with a lime and Ca carbonate mixture is used.
We have had to use 5500 ppm lime and 8000 ppm Ca carbonate doses in sodation effluent - 3000
and 4000 ppm in chlorination - sodation mixed effluent, and obtained only respectively 86 and 83%
color reduction in these two effluents. So we are far from the 95% efficiency mentioned by Mr Sanks.
We think that the economical comparison, favourable to alum process, applies when a high color
removal efficiency is wanted.
We were recently informed that in the USSR in Brask near by the Baikal Lake, of an alum process
similar to ours. They required a very high efficiency process to preserve their lake and have retained
the alum process.
Discussion
For the alum process:- one floculation device with pH regulation; - one scraped sedimentation
tank, calculated on a load of 0,4 - 0,5 m
3 /m
2 /hr; - one scraped thickening tank calculated on a 40
kg SM/m
2 /day basis; - two centrifuges, ,0 43 cm continute decanters each allowing to treat 300 kg
SM/hr, - one rotary kiln treating 2300 kg of wet solids (25% dryness) per hour at 750°C with dust
washing of fume, - one alum recovery unit with reagents tanks, dosering pump, stirrer, etc.
For the lime process: - one lime dosing device; - one scraped sedimentation tank, calculated on a
load of 1,2 - 1,5 m
3 /m
2 /h; - one 6000 m
3 high-pressure blower for injection of dry flue gases rich of
CO2, Counterpressure : 4,5 m of water; - one second scraped sedimentation tank calculated on a load
of 0,8 - 1 m
3 /m
2 /h; - two rotary vacuum filters with accessories, calculated on a 25 kg dry
matters/m
2 /h basis; - one rotary kiln treating 5500 kg DM/hr at 1000°C.
it is to be noted that contrary to Mr. Gould's belief, we did not think that the alum sludge organic
solids has a calorific power higher than the sludge coming from lime treatment, but only that alum
sludge are richer in organic solids (70%) than lime sludge (25%). This is why it is possible to approach
autocombustibility with a dryness lower than that obtainable with lime sludge.
As for the working expenses, 92% efficiency for alum recovery is an estimated one, for we
obtained in the laboratory easy 100% recovery. We hope to reach 97% recovery.
It will be pointed out that we calculated the sedimentation tank to be more efficient with lime
than alum. But from our experience and that of French pulp mill owners, this can only be applied if
treatment with a lime and Ca carbonate mixture is used.
We have had to use 5500 ppm lime and 8000 ppm Ca carbonate doses in sodation effluent - 3000
and 4000 ppm in chlorination - sodation mixed effluent, and obtained only respectively 86 and 83%
color reduction in these two effluents. So we are far from the 95% efficiency mentioned by Mr Sanks.
We think that the economical comparison, favourable to alum process, applies when a high color
removal efficiency is wanted.
We were recently informed that in the USSR in Brask near by the Baikal Lake, of an alum process
similar to ours. They required a very high efficiency process to preserve their lake and have retained
the alum process.