Hi Friends,
It’s proven fact that improved quality of products /
services have the edge over normal quality. Quality always plays vital role in
value addition and there by attributes in sustained business attracting
confidence of customer. This paper describes key operational and process
control measures to produce consistent quality of hydrate as well as calcined
alumina in any Alumina plant. Major reasons affecting the quality of product are
highlighted along with key control measures for improvement.
There are two main factors which badly affects the quality
of product as listed below-
1.
High soda content in
product hydrate and alumina
2.
High suspended
solids in Thickener overflow pregnant liquor.
Step-I: In different Alumina refineries across the globe, we have
experienced high soda content in product hydrate as one of the basic reasons
for deterioration of product quality. The identification of this issue is
confirmed through simple analytical technique
by analyzing the Na2O content in product hydrate
ready for dispatch to respective customers. If total Na2O content in
Hydrate is found more than 0.30% then it confirms that Hydrate generated in decomposers
has not been washed properly on product filter wherein the maximum component of
leachable soda is removed. After observing this high soda content, vigorous washing
of hydrate using hot water at temperature around 95-98oC helps in
achieving total Na2O content below 0.30%. For further improvement on
washing efficiency of hydrate, steam-hood at the discharge side of product
filter has been found to be more advantageous.
Step-II: High Fe2O3 content
in product hydrate above 0.010% is the indication of high suspended solids
in filtered aluminate liquor feeding to Decomposers (Crystallizers /
Precipitators). The suspended solids must be controlled below 10 mgpl in
pregnant liquor in feed to decomposers. Higher solids in pregnant liquor
happens because of inadequate settling of suspended red mud particles in Liquor
thickener for the major reasons listed below-
(a) High generation of fines during grinding of Bauxite:
Control fineness of
Bauxite at optimum level to ensure higher settling rate of red mud particles in
thickener. Generally, minus 60 mesh fraction in ground bauxite slurry is
maintained more than 85%. Check and control it accordingly.
(b)
Higher viscosity of
thickener overflow liquor because of its lower temperature:
(c) Temperature of Thickener overflow liquor must be controlled
at around 100-102oC if the concentration of liquor in thickener
overflow is at more than 145 gpl Na2O. High concentration profile is
the need of modern Alumina refinery in order to have improved productivity so
as to minimize thermal and electrical energy consumption.
(d)
Selection of right
settling agent:
Settling agent used
for faster settling of residue particles play vital role in achieving desired
quality of product. There are different types of natural and synthetic high
density polymers are available for different applications. It has been observed
that mixed natural starch with particular type of synthetic flocculent are cost
effective for Alumina refinery.
(e)
Selection of proper
filter cloth for polishing filtration equipment:
Selection of proper
filter cloth is equally essential for controlling suspended solids in filtered
aluminate liquor fed to crystallizers. Polypropylene filter cloth of more
than 30 EPI and 32 PPI have been found to be the most suitable cloth for
security filtration.
(f) Controlling P2O5 concentration
in aluminate liquor:
At times, high P2O5 content
in Bauxite affects the settling rate of bauxite residue (Red mud) in decanters.
To eliminate this issue, small quantity of Burnt lime (CaO) may be added with bauxite
being fed to Grinding mills. This results in reducing the Phosphorus content in
aluminate liquor forming Calcium Phosphate.
(g) Control Filter feed pressure below 2.0 kg/cm2.g:
Lower filtration
pressure results in better filtrate quality. Pressure around 2.0 kg/cm2.g
at inlet of security filtration system has been found to be optimum for
security filtration.
(h)
Control solids in
Thickener overflow Liquor well within 100 mgpl:
Suspended solids in
feed liquor to polishing filter should be controlled well within 100 mgpl so as
to avoid extra load of residue on polishing filtration unit thereby improving
the filtrate quality to decomposers,
(i)
Optimize dosing of
TCA:
TCA (Tri calcium
aluminate) addition to filter feed liquor helps in getting improved filtrate
quality. It has been established in various plants across the globe that mass
flow of TCA equal to 10 times of suspended solids in decanter overflow liquor
is optimum. This is considered as main controlling parameters for reducing the
impurities in filtered aluminate liquor and subsequently the quality of product
hydrate and calcined alumina. TCA dosing has been found to be the most
effective step in improving the filtrate clarity.
Step-III: Replacement of Filtration Equipment:
Even after checking and monitoring all the above mentioned
parameters stated under Step-II, if there is no appreciable improvement in
filtrate clarity, then it concludes that something wrong with filtration
equipment itself. In such situation, replacement of existing Polishing filters
with suitably sized and designed security filters is the ultimate solution to
this problem. Since this step requires additional capital expenditure for the
plant, thus decision needs to be taken only after thorough investigations of
results obtained under the guidance of process expert.
We would welcome your comments / remarks for further improvement.
Regards.
Rajendra Kunwar
rajendra@ceti.co.in
www.ceti.co.in
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