FIELD OF INVENTION
The present invention relates to a process for removal of silica from the spent
alkali liquor adapting Friedel's salt. More particularly, the invention relates to a
process of treating spent alkali for removal of silica generated during leaching of
coal by adapting NaOH.
BACKGROUND OF INVENTION
A substantial variety of coals being associated with mineral matter, they are
primarily debarred from effective utilization, for example, in the process of
carbonization, gasification, combustion, or liquefaction. Coking industries, and in
particular blast furnaces, are more sensitive to high-ash contents in coals. As a
result, enormous efforts are being extended to make coal and related solid
carbonaceous materials equivalent or better sources of energy. Leaching or solid
extraction to dissolve mineral matter in coal using a solvent is known. The acidic
and/or basic components present in mineral matter react with the solvents, gets
dissolved and then removed. Chemical leaching of coal is a technology to
produce clean coal where the ash content of clean coal is as low as~1.0% based
on mineralogical composition of the feed coal. There are potential use of this
clean coal both as a fuel and nonfuel. A process to decrease the ash content of
high ash content coals up to~5% in clean coal is also known. The process
involves treatment of coals crushed to -30/-72 BS mesh size or fine clean coal
obtained from flotation circuit coal for removal of ash-forming minerals which

includes the steps of treating the coal in an aqueous alkaline solution at an
elevated temperature under atmospheric and elevated pressure followed by
reaction/extraction with an aqueous acidic solution. This is a known process to
produce low ash (~5% ash) clean coal from high ash content coals with 80-85%
yield. Reference is hereby incorporated to Indian Patent Application numbers
621/KOL/07, 1391/KOL/08, 1550/KOL/08 and 1546/KOL/08 which discloses a
plurality of process to decrease ash content from high ash coals. Besides, for
making the chemical leaching process economically feasible, particularly for
treating high ash coals has been disclosed in Indian Patent Application no.
1518/KOL/08 which is hereby incorporated by way of reference.
The major components present in mineral matter/ash of Indian coals are silica
and alumina. Hence, the main impurities present in the spent alkali liquor after
leaching of coal are compounds of silicon and aluminium. Silicon commonly
occurs as kaolinite (AI2O3.2SiO2.2H2O) and quartz in coal. Silica in the form of
quartz only dissolves in caustic liquor at temperature in excess of nearly 180°C,
while silica combined as clay or other silicates, such as kaolinite, is readily
attacked by caustic soda in the digestion units (Oku and Yamada, 1971;
Whittington et al., 1998). The presence of silica in spent liquor causes at least
two major problems in the reactors, i.e.
(i) Dissolution and re-precipitation of silica as sodalite-type desilication
products (DSP), thereby consuming caustic soda;
(ii) Re-precipitation of complex sodalite-type desilication products on plant
surfaces, thereby causing scale build-up, which is quite severe.

There are at least two known methods of removing silicates from solutions. The
first method consists of precipitation of sodium aluminosilicate by increasing the
temperature of the solution, which produces a decrease in silicate concentration
in the solution, the silicate concentration still being too high. Therefore, it seems
reasonable to convert silica into slightly soluble compounds, for example, calcium
aluminosilicates. This can be achieved by introducing chemicals, such as calcium
oxide or calcium hydroxide into the process. Prior art further discloses that
4CaO.Al2O3.CO2.11H2O prepared using calcium hydroxide has higher desilication
capacity (Sizyakov et al., 1984). Development of tricalcium hydroaluminate
(C3AH6) by synthesizing using calcium oxide is also known which is enabled to
remove silicate ions completely from aluminate solutions with high caustic
module (ak, molar ration of Na2O to Al2O3) (Yaun and Zhang 1999).
Friedel's salt (FS) (3CaO.Al2O3.CaCl2.10H2O) was first mentioned by Friedel
(1897), who studied the reactivity of lime with aluminium chloride. This
compound is formed in cements which are rich with tricalcium aluminate
(Suryavanshi et al., 1996; Birnin-Yauri and Glasser, 1998), as in the case of
Portland cement. In this study, Friedel's salt was prepared by reacting calcium
chloride (a waste by-product in the alkali industry) with sodium aluminate.
Friedel's salt displays a hydrotalcite like layered positively charged structure
containing charge compensating anions and water in the interlayer spaces
(Birnin-Yauri and Galsser, 1998). It belongs to the layered double hydroxide
family and would therefore be capable of accumulating anions by ion exchange
(Miyata, 1983).

OBJECTS OF INVENTION
It is therefore an object of the invention to propose a process for treating spent
alkali for removal of silica generated during leaching of coal by adapting NaOH,
which eliminates the disadvantages of prior art.
Another object of the invention is to propose a process for treating spent alkali
for removal of silica generated during leaching of coal by adapting NaOH, which
allows the spent alkali to be reused for leaching process on removal of the silica.
SUMMARY OF THE INVENTION
It is known that compounds containing silica are formed during chemical
leaching of coal. Controlling silica level in spent liquor is very important in order
to prevent scaling in reactor and evaporator. The conventional method of
removing silicates or sodium aluminosilicates is causticization with quick lime or
slaked lime. In the present innovation, Friedel's salt (FS:
3CaO.Al2O3.CaCl2.10H2O) has been used to remove the silica content in the
spent alkali liquor by up to 85%. Friedel's salt is a mineral anion exchanger
belonging to the layered double hydroxides (LDHs) which is prepared by adding
calcium chloride to sodium aluminate over the temperature range of 50-90 °C
FS prepared at 50 °C has a relatively high dedication capacity. Accordingly, to
the invention, the experimental parameters affecting the desilication, such as

synthesis temperature of Friedel's salt, dedication time, desilication temperature
and FS dosage have been systematically considered.
Accordingly, there is provided a process for treating spent alkali for removal of
silica generated during leaching of coal by adapting NaOH which comprises the
steps of (i) preparing a Friedel's salt in slurry form by mixing 300 ml of 0.25 M
sodium aluminate solution (ak=3.2) in equal volume of 0.5 M with calcium
chloride solution; (ii) stirring the prepared slurry substantially at 300 rpm for
about 1.0 hr; (iii) filtering the resultant white precipitate and washing with water
for removing excess ions present; (iv) drying said precipitate at around 50 °C for
about 10 hr; (v) desilicating the spent alkali containing silica by adding Friedel's
salt and stirring the slurry at around 300 rpm for 0.5 - 4.5 hr at about 90 °C;
and (vi) filtering the slurry and washing with distilled water, wherein the residue
being dried at around 80 °C for about 10 hr.
In an advantageous embodiment of the invention, said spent alkali is generated
during leaching of coal using the alkali.
Another aspect of the invention provides that said Friedel's salt is prepared by
pre-heating 0.5 M CaCl2 (300 ml) to a temperature of 50 °C and adding an
equal volume of 0.25 M NaA102) (ak=3.2) at a rate of 5 ml/min, while stirring at
around 300 rpm for about 1.0 hr.
A further aspect of the invention proposes the dry sample of Friedel's salt (30-50

g/1) is added to 500 ml of silicate containing spent liquor and the mixture is then
heated to a temperature of about 90 °C.
In a still advantageous embodiment of the invention the silica concentration in
the spent alkali is nearly 1100 ppm which reduces to nearly 250 ppm in the
treated spent alkali.
In another advantageous embodiment, said process temperature is maintained
at around 90 °C during removal of silica.
Yet another advantageous aspect of the invention that provides said treated
alkali can be reused in the alkali coal leaching.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Graphically depicts the effect of FS (Friedel's Salt) Dosage on silica
removal
Figure 2 - Graphically illustrates the effect of FS Dosage on % Na2O.
DETAIL DESCRIPTION OF THE INVENTION
According to the Invention, the apparatus used for preparation of Friedel's salt is
a glass mixed tank reactor which is equipped with a non-flat-blade turbine mixer.

The required heat to this reactor is supplied by a rotamantle and the solution
temperature is monitored by a digital thermometer.
According to the inventive process, the Friedel's salt is prepared by pre-heating
0.5 M CaCl2 (300 ml) to a temperature of 50 °C and adding an equal volume of
0.25 M NaA102) (ak=3.2) at a rate of 5 ml/min using a pipette, while stirring at
around 300 rpm. Once the addition of NaAlO2 is complete, the mixture is further
stirred for about 1.0 hr. This results in the formation of a white colour precipitate
which is then collected, filtered off, and washed with distilled water three times
to remove residual ions before it is finally dried in an oven at around 50 °C for
about 10 hr.
As shown in Figures 1 and 2, desilication effects are also determined in the same
reactor as mentioned above Dry sample of Friedel's salt (30-50 g/1) is added to
500 ml of silicate containing spent liquor (silica ~ 1100 ppm) generated during
the leaching of coal using NaOH. The mixture is then heated to a temperature of
90 °C. Samples of 5 ml each are collected starting from 30 min to 4.5 hr at every
30 minutes time interval. The samples collected are then filtered immediately.
The liquor is analyzed for its silica content. The silica content in treated spent
alkali reduces to ~250 ppm after two hour of treatment when the Friedel's slat
addition is 30 gl/1. The graphical illustration in figures 1 and 2 respectively
provides the effect of Friedel's salt dosages on silica removal, and the effect of
Friedel's salt on Na2O (percentage).

WE CLAIM
1. A process for treating spent alkali for removal of silica generated during
leaching of coal by adapting NaOH which comprises the steps of (i) Preparing
a Friedel's salt in slurry form by mixing 300 ml of 0.25 M sodium aluminate
solution (ak=3.2) in equal volume of 0.5 M with calcium chloride solution; (ii)
stirring the prepared slurry substantially at 300 rpm for about 1.0 hr; (iii)
filtering the resultant white precipitate and washing with water for removing
excess ions present; (iv) drying said precipitate at around 50 °C for about 10
hr; (v) desilicating the spent alkali containing silica by adding Friedel's salt
and stirring the slurry at around 300 rpm for 0.5 - 4.5 hr at about 90 °C; and
(vi) filtering the slurry and washing with distilled water, wherein the residue
being dried at around 80 °C for about 10 hr.
2. The process as claimed in claim 1 wherein said spent alkali is generated
during leaching of coal using alkali.
3. The process as claimed in claim 1 wherein said Friedel's salt is prepared by
pre-heating 0.5 M CaCl2 (300 ml) to a temperature of 50 °C and adding an
equal volume of 0.25 M NaA1O2) (ak=3.2) at a rate of 5 ml/min, while stirring
at around 300 rpm for about 1.0 hr.
4. The process as claimed in claim 1 wherein the dry sample of Friedel's salt
(30-50 g/1) is added to 500 ml of silicate containing spent liquor and the
mixture is then heated to a temperature of about 90 °C.
5. The process as claimed in claim 1 wherein the silica concentration in the
spent alkali is nearly 1100 ppm which reduces to nearly 250 ppm in the
treated spent alkali.
6. The process as claimed in claim 1 wherein said process temperature is
maintained at around 90 °C during removal of silica.
7. The process as claimed in claim 1 wherein said treated is reusable in the
method of alkali coal leaching.

The present invention relates to a process for treating spent alkali for removal of
silica generated during leaching of coal by adapting NaOH which comprises the
steps of (i) Preparing a Friedel's salt in slurry form by mixing 300 ml of 0.25 M
sodium aluminate solution (αk=3.2) in equal volume of 0.5 M with calcium
chloride solution; (ii) stirring the prepared slurry substantially at 300 rpm for
about 1.0 hr; (iii) filtering the resultant white precipitate and washing with water
for removing excess ions present; (iv) drying said precipitate at around 50 °C for
about 10 hr; (v) dedicating the spent alkali containing silica by adding Friedel's
salt and stirring the slurry at around 300 rpm for 0.5 - 4.5 hr at about 90 °C;
and (vi) filtering the slurry and washing with distilled water, wherein the residue
being dried at around 80 °C for about 10 hr.