PU 16 Hydrate Precipitation and Classification
The aluminate (pregnant) liquor from PU 17A is fed into first precipitation stage which is formed by two precipitation tanks equipped with agitators. Fine seed hydrate is also added into the tanks. Retention time of the first precipitation stage is about 8 hours and temperature range approx. 74 °C.
The hydrate suspension from first precipitation stage is cooled down to approx 60 °C and fed to the second precipitation stage which takes place in 17 agitated tanks arranged in series (there are total 21 precipitation tanks in both stage – 19 are in operation ,2 are stand-by). Coarse seed hydrate is added into the first tank of second precipitation stage is about 70 hours. Second precipitation stage consists of two parallel lines.
Part of hydrate slurry from the last precipitation tank is routed to PU 17 for coarse seed filtration and another part is pumped into hydrocyclones where product hydrate is separated.
Separated product hydrate from bottom of fine hydrate slurry separated in hydocyclones goes to thickener where fine hydrate is PU 17 for filtration. All weak caustic liquor streams separated from solid parts of hydrates are introduced into these thickeners too. Weak caustic liquor from thickeners is pumped to Unit PU 17A for heat exchange with hot aluminate liquor.
PU 17 Hydrate Filtration
Product hydrate slurry coming from PU 16 goes to two stage washing and filtration. In the first stage is product hydrate washed on the vacuum drum filters by the wash water from the second stage. In the second stage is the product hydrate washed on the vacuum drum filters by clean condensate. Filter cake product hydrate from the second stage filters is transported by belt conveyers to the calcination unit PU 21. Weak caustic liquor from the first stage filtration goes to PU 13.
Coarse hydrate slurry from PU 16 is washed and filtered on vacuum disc filters and coarse hydrate after diluting goes as seed into second precipitation stage.
Fine hydrate slurry from thickener of PU 16 is filtrated on vacuum drum filter. Fine hydrate cake is diluted by aluminate liquor from PU 13 and is fed as fine seed into first precipitation stage.
Fine hydrate slurry from thickener of PU 16 is filtrated on vacuum drum filter. Fine hydrate cake is diluted by aluminate liquor from PU 13 and is fed ad fine seed into first precipitation stage.
All filtrates from PU 17 filters weak caustic solutions are pumped to PU 16 into fine hydrate thickener. Weak caustic from PU 16 undergoes control filtration on filter presses of PU 17 and is transported to PU 17A.
PU 17A Aluminate Liquor Cooling
Aluminate liquor coming from PU 13 is cooled by weak caustic liquor to the temperature approx. 74 ˚C in this unit. Plate heat exchangers are used for this purpose.