Hi Friends,
Bauxite residue is the waste generated in Alumina refinery.
The residue generation rate is dependent of Bauxite quality and alumina
extraction efficiency. In Bayer process, Bauxite with around 40% Al2O3 and less
than 4% total silica (Module > 10) is considered as metallurgical
grade bauxite for production of calcined alumina. The alumina digestion process
results in dissolution of bauxite in caustic soda and causes generation of
about 1.15 to 1.35 tonnes of dry residue as waste per tonne of alumina.
It is a known fact that storage and disposal of Bauxite
residue is a big problem for all Alumina producers due to environmental
problems in the vicinity because of dust generation and percolation of caustic
to natural water streams. Thus stress and emphasis of concerned Governments is
always given to store it in well protected area so as to combat probable
environmental problems and health hazard for the community around because of
the passage of alkaline water to natural streams and acute dusting in the area
during summer. Though attempts are being made by researchers and scientists to
find commercial utilization of residue but very limited utilization has been
made possible so far because of economic disadvantages. The laboratory scale
development of various processes were done till 1980s for its utilization in
manufacture of red mud bricks, cement, red mud PVC plastic, pigment and many
other products but only a few has been commercialized till date.
In present post, we will discuss the key contributing
parameters to improve the settling characteristics of Bauxite residue
in Alumina refinery. As such, there are unlimited variables which affect the
settling rate of residue but we will like to highlight the major parameters to
improve the settling rate of residue particles in settlers and washers of
Alumina refinery thereby improving the production rate due to steady operation,
improved product quality due to less carry over of suspended solids in filtrate
during polishing filtration and lower soda loss with residue due to increased
residue washing efficiency in residue washers. The major parameters are briefly
described in subsequent paragraphs-
1.
Mineralogical
composition of Bauxite:
The mineralogical composition of Bauxite clearly reflects
that oxide of iron is present in the form of hematite, goethite and siderite whereas
Oxide of titanium in the form of anatase and rutile in addition to oxides of
other minor metals. The presence of goethite badly affects the settling rate of
residue. Thus the ratio of hematite to goethite is preferably maintained more
than 3 by adopting suitable blending techniques before processing in the Bayer
circuit.
2.
Granulometry
of Bauxite in ground slurry:
The specific gravity of Bauxite ranges from 2.50 to 2.75
depending on the chemical and mineralogical composition of Bauxite. As such,
the bauxite particle of minus 60 mesh has been found to be okay for
satisfactory extraction as well as recovery of alumina but to ensure the same
the granulometry in ground slurry is controlled over 85% of minus 60 mesh
fraction. The slurry contains about 1 to 2% of plus 10 mesh fractions. With
this particle size distribution, the specific surface area of dry bauxite is
observed between 12 to 15 m2 per gram. This figure is also used as a guideline
for required granulometry in ground slurry. Over grinding of bauxite is avoided
as it badly affects the settling rate of residue because of further
disintegration of particles during digestion and slurry flashing circuit of
Alumina refinery.
3.
Selection
of suitable flocculents:
It has been seen that mixed natural and synthetic flocculent
gives better result than any individual flocculent for desired level of
overflow liquor clarity as well as compactness of underflow residue. However,
the ratio of mixture varies widely depending on mineralogical composition as
well as settling characteristics of Bauxite. Thus, the ratio of mixing is
finalized by carrying out settling rate test in Laboratory with digested
blow-off slurry under the plant operating conditions. The dosing rate of
natural flocculent generally varies from 0.25% to 1% of dry bauxite residue
entering the settlers and washers.
4.
Temperature
profile of Settling and Washing circuit:
Though, there may not be wide fluctuations in temperature
profile of thickeners and washers but at times even the minor deviations in
temperature profile either due to ignorance of operating personnel or malfunctioning
of instrumentation and control system may cause serious problem in the settling
rate as well as overflow liquor clarity due to increase in viscosity of
aluminate liquor at lower temperature. Thus close watch on steady temperature
profile is considered as one of the major factors.
5.
Monitoring
and control on P2O5 content in process liquor:
This is one of the very important parameters which requires
constant vigil for steady control within limits by taking required preventive
measures in the upstream of the process. For the purpose, suitable chemical is
added continuously to the process circuit.
It has been experienced that close watch on these critical
parameters results in trouble-free as well as steady and efficient operation of
Alumina refinery. We would appreciate your valued comments.
Best regards.
Rajendra Kunwar
Principal Consultant-Engineering
CETI ENTERPRISES, GURGAON, INDIA.
www.ceti.co.in
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