Hi Friends,
After a long gap, I have appeared before you to interact on a
small but very critical process issue of Bayer Alumina Plant.
In the past, we have always discussed about the negative
impact of high silica in bauxite specifically with regard to operational
problems, process control issues, higher consumption of input materials and deterioration
in quality of product hydrate or alumina. In previous articles, this issue was
elaborated at length with measureable / quantifiable approach so as to focus on
its importance in Bayer Alumina Refineries.
In present post, we will briefly discuss about the advantages
and disadvantages of low silica in processing bauxite for production of
calcined alumina. The question mark (?)
on the subject itself forces us to think and re-think about the positive and
negative aspects of low silica in bauxite.
It’s a well-known fact that all Alumina producers wish to
have low silica bauxite for obvious reasons discussed earlier. Now, before
coming to the subject directly, following questions arise-
(i) What
type of silica is harmful for Bayer’s process?
(ii) What
should be the cut-off silica content in bauxite for acceptable quality of
alumina?
Though we have discussed these issues in several articles, still it is
prudent to freshen up with salient features as outlined below-
We are well aware that predominantly there are two types of silica in bauxite designated with
regard to Bayer process of alumina (a) Non-reactive silica and (b) Reactive
silica. ROM bauxite mined adopting either open cast or underground mining
methodology contains a number of metallic oxides including silica. This mined
silica as percentage of total dry bauxite is termed as total silica. The part
of total silica which reacts with caustic soda and forms sodalite complex
through sodium silicate route is termed as Reactive silica. Majority of
Reactive silica is found in form of Kaolinite. And thus, the non-reacting
component of total silica with caustic soda is popularly known as Non-reactive
silica. Quartz is non-reactive silica. The silica being abrasive in nature
causes erosion in components of various process equipment like heat exchangers,
pipelines, valves, pump casing and impellers etc. Thus higher silica content
affects the capital investment of Alumina refinery because robust material of
construction for equipment component in contact with bauxite slurry. Hence
higher silica in bauxite causes higher initial investment for the plant and
higher repair maintenance cost for routine replacement of critical components
of equipment.
Generally, it has been seen from bauxite analysis that
Reactive silica in bauxite ranges from 50% to 80% out of Total silica in
bauxite. Plant operation is affected because of high silica in bauxite as it
has high settling velocity and abnormally high abrasive nature.
Now coming to the topic with raised question mark, it’s true
that low reactive silica in bauxite is considered as a serious process problem
as it directly affects the quality of product alumina because of higher silica
content in alumina. The SiO2 content in calcined alumina produced in
all modern alumina a refinery is controlled at around 0.015%. The alumina
having higher silica content is considered as inferior quality. For achieving
0.015% SiO2 in alumina, the SiO2 in aluminate liquor
feeding to crystallizers / precipitators should be around 0.45 gpl. The ineffective desilication because of delay
in achieving super-saturation of silica across desilicators, gpl SiO2
in liquor increases to much higher level which ultimately deteriorates the
product alumina quality.
The above elaboration concludes that moderate level of
Reactive silica in bauxite at around 1.5% in bauxite is considered to be the
best with regard to plant efficiencies and product quality as well.
Trust, the subject has
been covered here to your desired level of satisfaction in understanding the
criticality of the subject issue. We would welcome your valued comments / remarks to further
substantiate the subject.
Regards.
Kunwar Rajendra
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