Operational units
Introduction Operational of Iranian Alumina Company
Bauxite and lime unit
PU 01 Bauxite Receiving and Crushing
Bauxite with maximum lump size 1000 x 800 x 800 mm is transported to the plant by Lorries. Bauxite is crushed primary in jaw crusher, after that is transported to vibratory rough sorter where particles bigger than 20 mm are separated and crushed in impact hammer crusher. The crushed Bauxite with grain size up to 20 mm is by means of conveyors of up 01A delivered into the covered Bauxite storage PU 02.
PU 01A Belt Conveyer Transport
This unit secures the transport of crushed Bauxite from PU 01 to PU 02 (Bauxite storage), homogenization purposes and transport of homogenized Bauxite from storage PU 02 to PU 08 Bauxite wet grinding.
PU 02 Bauxite Storage
The maximum Storage capacity is 66000 t what means one month consumption provided that the plant is supplied by Bauxite with stable or very little changing composition. If the composition is changing what is usual situation the Bauxite should be homogenized. Therefore the storage is divided into three stockpiles with storage capacity of 10 days each. The operation of the unit is as follows:
Bauxite after crushing is conveyed to one of the stockpiles. During this time no Bauxite is taken from this stockpile.
Bauxite for homogenization and for feeding of PU 08 Bauxite wet grinding is taken form the remaining two stockpiles.
In such way the stock of homogenized Bauxite for 10 days operation is created.
PU 03 Limestone Storing
The crushed limestone of two sizes – 30-70 and 70-160 is stored in this unit. To that unit the limestone is transported from crushing unit located outside the plant.
The unit PU 03 is APPI’s delivery and responsibility.
PU 04 Limestone Burning
Two parallel ordered two-shaft regenerative kilns represent a unit for limestone burning. The burning cycle in both shafts is alternated in regular time intervals exactly adjusted to the kiln output capacity.
The normal capacity of the kiln is 310 t/day (max 380 t/day) when burning fraction 70-160mm and approx. 240 t/day (max 260 t/day) when burning fraction 30-70 mm. lime is used for lime milk preparation in PU 05 and in wet grinding unit PU 08.
PU 04B Burnt lime Store
This Unit serves for storing of burnt lime. In case of maintenance of one limestone burning kiln the burnt lime is taken from this store and used together with lime from the kiln which is in operation. The unit consists from covered store and transporting facilities. The stock of lime is sufficient for 20 days shut down of one kiln.
PU 05 Lime Milk Preparation
For preparing of lime milk 4 individual lines are designed. the burned lime is stored in storage tanks with capacity of 1.5 operating days of shaft kilns.
The lime is transported from these tanks by means of vibratory feeders in to a lime slaking tanks where slaking process takes place by reaction with process water. The lime milk flows from the slacking tanks through classifiers into maturing tanks from where it goes to consumers – red mud caustification PU 14 and calcium aluminate preparation in PU 13.
PU 08 Bauxite Wet Grinding
There are three limed for bauxite wet grinding, two lines are in operation, the third is stand-by.
In these unit the bauxite is grinding down to max. 5% particle size over 0.09 mm and bauxite slurry is prepared.
Bauxite is transported from bauxite storage PU02 by belt conveyers to the inlet hopper from where it is fed into the ball mill .weak caustic liquor from PU17A and lime from PU05 are also fed into ball mill.
Grinded bauxite slurry is pumped to PU09 predesilication
Digestion unit
PU09 Predesilication
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.
Red Mud unit
PU 12 Aluminate Slurry Make-up
Digested slurry from Pu 11 is pumped into mixed tanks where dilution of slurry takes place and afterdesilication continues. slurry is diluted by wash water coming from PU 14 red mud washing and filtration to the required concentration.
To keep the temperature of aluminate slurry on desired level, the wash water is heated by steam from PU 24 Boiler house. Aluminate slurry is from PU 12 pumped into sedimentation tanks of PU 13.
PU 13 Aluminate Liquor Separation
Aluminate slurry from PU 12 is fed into sedimentation tanks and flocculant solution is simultaneously added. In the sedimentation tanks the red mud sedimentates and is drawn off to Unit PU 14 in such way to keep steady required solids content of red mud slurry.
Overflow from sedimentation tanks-aluminate liquor is collected in collecting tank from where it is pumped to control filtration where the rest red mud particles are caught.
After control filtration in kelly filters the alumiate liquor is routed to unit PU 17A.
Preparation of flocculant solution takes place in the unit PU 13 too. the concentrated flocculant is diluted to required concentration by overflow wash water from PU 14.
PU 14 Red Mud Washing and Filtration
Thickened red mud from PU 13 contains solids of red mud and aluminate liquor which has to be washed out.
The washing takes place in three thickeners and it is done counter currently by process water coming from PU 35.
Washed red mud is filtered on vacuum drum filters but before filtration red mud is causticized by addition of lime milk to decrease the losses of soda. In the caustification step about 42.5% of Na2O which is contained in red mud is diluted to the concentration of solids about 50% wt. by waste water and pumped away to the disposal area.
Precipitation unit
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.
Calcination unit
PU 15 Alumina Storage and Expedition
Alumina product is transported by means of airlift and air slides from calcination unit PU 21 into two silos where alumina is stored before further expedition.
Alumina for expedition is taken from these silos and by means of pneutransport is loaded into railway wagons.
The silos are designed for one month production of alumina.
PU 19 Aluminium Hydrate Storage
Covered aluminium hydrate storage serves for storing of aluminium hydrate in the case when calcination unit PU 21 is out of operation but the other units operate. The storage is designed for storing capacity of 14 days production. Supply and discharge of hydrate is solved by belt conveyers of PU 20.
PU 20 Outdoor Belt Conveyer of Hydrate
This unit secures the transport of hydrate from PU 17 hydrate filtration to PU 21 calcination, PU 19 hydrate storage and from PU 19 to PU 21 calcination and for loading into wagons or trucks.
PU 21 Calcination
In this unit the aluminium hydrate is calcined into aluminium oxide product of the alumina plant. Aluminium hydrate is transported to PU21 by belt conveyers either from PU17 or from PU 19 or from PU17 and PU 19 simultaneously. For calcination the Gas suspension calciner is used. The most important part of the unit is a rector what is a refractory lined cylindrical vessel with conical bottom. Preheated and partly calcined alumina enters the rector above the inlet of fuel and of preheated combustion air. The velocity of the air at the inlet is sufficient to ensure proper suspension of particles over the entire cross section of the reactor at full and partical capacity.
The calcined alumina is entrained from the reactor by the gaseous mixture of water vapour and combustion products after a few seconds retention time at approx. 1 150 – 1 250 °C. Calcinated Alumina is separated from the hot gases, stepwise cooled down by incoming air and cooling water (in last step) and by pneumatic transport delivered into alumina storage.
Evaporating & Desalting unit
PU 06 NaOH Storage
The liquid caustic soda with concentration of 50% is transported to the plant by railway tankers and from them is unloaded into two storage tanks with capacities of 4000m3 .caustic soda from these tanks is delivered to the unit PU18 for preparation of leaching solution and to unit PU unit PU 16 for preparation of cleaning solution.
PU 18 Evaporating and Desalting
Evaporating and desalting unit protects liquor from dilution and from increasing of sodium salt content above admissible limit.
The unit consists of two evaporating line and one common desalting line. Weak caustic solution coming from PU 17A is thickened in five evaporating stages to concentration at which sodium salt falls out from the solution. No formation and fall out of salts is expected under normal operation conditions provided that specified bauxite is processed, therefore desalting system will be put in operation only rarely.
Sodium salts can be separated from the caustic liquor by filtration on vacuum drum filter. Solid salts are dried and transported for use out of the plant.
Strong caustic solution is discharged to prede-silication unit PU 09.
PU 18A H2SO4 Racking
This unit secures the racking of 96% sulphuric acid from railway tanks to PU 18. The sulphuric acid is used for chemical cleaning in PU 13, 14, 16, 18.
Utility unit
PU22 Compressor Station
Compressor station supplies the whole Alumina plant with compressed air on two levels. Low pressure air (0.18 MPa) is used for drum filters, medium pressure air (0.85 MPa) is used for pneumatic transport, for service purposes and for production of instrument air. The air drying station is also part of this units and instrument air is dried down to the dew point – 20 °C there.
Low pressure air is produced by turbo blower, medium pressure air by screw compressors.
Low pressure air (0.18 MPa)
Production 1150 – 1400 Nm3/h
Medium pressure air (0.85 MPa)
Production 1700 – 2050 Nm3/h
Instrument air production 1000 Nm3/h
PU 23 Water Treatment Unit
This unit is producing treated water for use in steam generation units (PU 24, PU 11), in cooling water systems and in central heating system. Pumps for raw water delivery to process water system and service water pumping station are also part of this unit. Raw water origins from deep wells from where it flows into covered charge reservoir with volume of 8000 m3.
Treatment of water for addition to boiler feed water and addition to cooling water system is based on reverse osmosis. Additional water to boiler feed cooling system undergoes pH correction.
Production of treated water
to BFW 48 m3/h
Production of treated water
to cooling water system 54 m3/h
Production of treated water
for central heating 1.2 m3/h
Pumping of raw water
to process waters 45 m3/h
Service water system 45 m3/h
PU 24 Boiler House
Boiler house consists of three boilers which supply steam for process and other use. The unit PU 24 shall deliver to its battery limit 79 t/hr (max) of steam.
Two boilers are always in operation, the third is stand-by.
The parameters of generated steam at Boiler house battery limit are as follows:
Pressure : 1.32 MPa
Temperature : 220 °C
The boilers are designed for combustion of fuel oil. Thermal treatment of boiler feed water is a part of the Boiler house. The returned condensate and additional treated water are introduced into deaerator and from it are pumped as boiler feed water to individual boilers and also to tube digestion unit PU 11 where the steam is generated in convection section of tube digestion heater.
The unit PU 24 is APPI’s delivery.
PU 25 A Cooling Water System
There are two systems of cooling water:
System I system of clean pressure cooling water for use in water coolers, for cooling water is pumped to consumers and from them is returning back to cooling towers.