Bauxite slurry coming from PU08 is gathered in intermediate storage tanks from where it is pumped through heat exchangers where the slurry is heated up to approx. 104 °C into predesilication tanks. In the heat exchangers the slurry is heated by flash vapours coming from PU11.
predesilication is secured at temperature about 104 °C and retention time approx. 8 hours. into the last predesilication tank strong caustic liquor from PU 18 is added.
Predesilicated bauxite slurry is pumped to PU10 high pressure pumping station.
Flash condensate from heat exchanges which could be polluted by caustic is pumped in system of process water.
PU10 High Pressure Pumping Station
Predesilicated bauxite slurry coming from PU09 is collected in two storage tanks from where it is pumped by booster pumps to the suction of high pressure pumps.
There are four pieces of high pressure pumps, there of them are discharging the slurry in three digestion lines and the fourth pump discharge is about 15 Mpa.
PU 11 Tube Digestion
Bauxite slurry coming from PU10 is preheated in tube side of tube heat exchanger by means of vapours coming from flashing tanks. Preheated slurry enters tube digestion heater where it is heated up to digestion temperature which is approximately 310 °C.
Slurry with mentioned temperature flows into tube reactor where digestion goes on. Retention time in tube reactor is approx. 4 minutes.
Heat contained in digested slurry is utilized in the flashing system for preheating of bauxite slurry feed. The flashing system operates in that way that the digested slurry gradually decreases in that way that the digested slurry gradually decreases its pressure (in eleven stage) and in that time steam is generated (flashed). Flashed steam is routed to shell side of heat exchanger where it condensates. The condensate after condensation trap loses pressure and new steam with lower pressure is produced.
Mixture of water and steam goes to separation drum where steam from next slurry stage is added to corresponding heat exchanger. Condensate from a/m drum goes to condensate separation drum with lower pressure.
Steam from last two stages are routed to PU09 for heating of bauxite slurry to predesilication temperature.
Flash condensate which could be polluted by caustic is pumped to the system of process water to PU35.
Digested slurry from last stage goes to the tanks where after desilication takes place. To prevent steaming in those tanks digested slurry before entering the tanks is mixed with wash water coming from PU14.
From this tanks the digested slurry is pumped to PU12.