Hi Friends,
In previous papers, we have covered variety of technological, design, engineering and operational issues of Alumina refinery. It’s true that there is no end to advancements and thus tremendous scope for enriching our knowledge base. In continuation to our approach and efforts, we will present here in brief about Tube digestion technology as an alternative to Double digestion process for extraction of alumina.
In previous papers, we have covered variety of technological, design, engineering and operational issues of Alumina refinery. It’s true that there is no end to advancements and thus tremendous scope for enriching our knowledge base. In continuation to our approach and efforts, we will present here in brief about Tube digestion technology as an alternative to Double digestion process for extraction of alumina.
The digestion of predominantly Boehmitic Bauxite is
carried out at an elevated temperature ranging from 220 deg C to 280 deg C,
thus total thermal energy requirement for processing Boehmitic Bauxite is very
high. As such Double digestion technology coupled with Pressure decantation
system is a proven and established technology and many Alumina refineries are
operative with this technology. The estimated overall energy consumption in
efficient Alumina refinery with high temperature digestion technology is about
15 GJ per tonne of calcined alumina.
Another efficient digestion technology which was
developed sometime in 1960s is popularly known as Tube Digestion
Technology. In this process, wet ground Bauxite in caustic liquor followed
by desilication is passed through long jacketed tubes / pipes at high
velocity. The particle size of the Bauxite is controlled at around 100% minus
1000 microns. The tube jackets contain high pressure saturated steam for
raising the temperature of reaction mixture indirectly to ensure dissolution of
alumina. The higher velocity of slurry in the pipelines (tubes) enhances the
heat transfer coefficient and thereby heat transfer rate resulting in lower
digestion time requirement. The retention time during digestion is generally
not dictated by high dissolution rate of alumina rather dependent on chemical
kinetics of quartz with caustic liquor. It has been practically observed in
laboratory as well as plants that higher retention time across tubes during
digestion at higher temperature above 250 deg C causes Quartz attack due to
dissolution of Quartz with caustic soda which increases the formation of DSP
and Cancrinite and thus additional consumption of caustic soda for alumina
production. As per my personal experience with Boehmitic Bauxite, 15 minutes of
digestion time in tube digesters will be adequate. The outcome of thermal
energy calculations reveal that energy requirement with Tube digestion
technology is comparatively less than that for other high temperature digestion
technology.
Advantages of Tube digestion
technology:
There is a growing trend in adoption of Tube
digestion technology in a few Alumina refineries in planning and execution
stages in the World. An Alumina refinery of 1.8 Mtpa production capacity has
been newly constructed and commissioned a few years back in Saudi Arabia adopting
tube digestion technology. The plant has adopted this technology with Basic
engineering developed jointly by two leading engineering consultancy companies.
This will be the biggest Alumina refinery in the World adopting Tube digestion
technology.
Tube digestion technology has following major
advantages over other high temperature digestion technologies available in the
World-
·
Lower thermal energy consumption
for digestion,
·
Easy maintenance or
replacement of equipment,
·
Lower retention time for
digestion,
·
Flexibility in processing wide
range of Bauxite,
·
Lower evaporation requirement,
·
Lower capital investment and
·
Marginally lower operating
cost.
Total thermal energy consumption
for alumina production adopting Tube Digestion Technology works out to about 12
GJ/t alumina which accounts for 9.0 GJ/t alumina for Bayer process (Hydrate)
area and 3.0 GJ/t for Calcination area of Alumina refinery.
Disadvantages of Tube
digestion technology:
The scaling rate in inner surface of
tubes will be high because of Boehmite reversion in tube digestion stage
because of high alumina loading in liquor in presence of solid Boehmite. As
such, the hardness of scale will be very high due to presence of DSP and
Cancrinite. Also, caustic soda consumption will be higher due to dissolution of
quartz and formation of Cancrinite as well. Thus following are the
disadvantages of Tube digestion technology-
·
High scaling rate in
tube digesters and
·
High caustic soda
consumption.
At times, certain chemicals, like MgO,
are added to bauxite slurry before entry to tube digesters for making porous
scales for easy dislodging.
Since high
temperature digestion beyond 250oC causes conversion of quartz to
reactive silica, thus it is always recommended to maintain the digestion
temperature at around 240-245oC and should avoid crossing 250oC
to combat high caustic soda consumption for alumina production.
The statements presented here are
purely based on personal experience and technical studies of the author on the
subject. Hence, it is advised to carry out detailed evaluation of process by
thorough scrutiny of process control data and related efficiency
figures before implementation in any plant for commercial production.
We solicit your valued comments /
remarks for further improvement from our end.
Regards.
Rajendra Kunwar
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