FIELD OF THE INVENTION
The present invention relates to a process of precipitation of silica from extracted
caustic spent of low grade coal by using carbon dioxide.
BACKGROUND OF THE INVENTION
The prior art process involves precipitated silica from spent liquid of
aluminosilicate extract from coal by caustic leaching. The main advantage of the
prior art process is that it is also chemically beneficiated coal for reducing the
ash content. So the twofold advantage of the process is 1) Extraction of
precipitated silica from coal 2) Chemical beneficiation of coal.
Indian Coal always comes with higher ash content. The amount of mineral
matter in coal is varied from 20 to 30%. The coal quality and process directly
depends on the amount of coal ash. For Steel industry these coals can not be
directly used due to its higher ash content. Overall it can be concluded that coal
ash makes the coal inferior. A number of attempts have been initiated for

cleaning the coal by both physical and chemical operations. Indian coal contains
substantial amount of ash ranging from 20% to 40%. Physical cleaning of coal
has some limitation and it cannot produce ultra clean coal which needs to be
blended for improving coke quality. Chemical beneficiation of coal is required for
obtaining ultra clean coal. The major part of coal ash is silicon and aluminum.
The approximate amount of these two in total ash content is around 60% and
20% as an oxide.
In coal-ash, silica has major contribution (60%), and accordingly, it is a long-felt
need to extract substantial amount of silica from coal, which allows cleaning of
the coal through leaching-out the major ash constituent, i.e silica.
Application of Silica
Precipitated silica has a huge market demand as it has a multiple application in
the field of rubber, plastic, paints, catalysts and many more.
1. Paints and Plastics, Polymer Compounds, Rubber, Sealants and
Adhesives, Crystalline silica, as quartz and cristobalite, in its finest flour
form is used as reinforcing filler. Silica flour provides resistance against

abrasive actions and chemical attack. Self-cleaning exterior wall
coatings and heavy-duty offshore or marine paints are typical
examples. The intrinsic properties of silica flour promote its use in
plastics for encapsulating electronic.
2. Agriculture: Silica is used in farming, market gardening, horticulture
and forestry in applications such as soil conditioner or carrier for
fertilizer and animal feed additives.
3. Chemicals: Cristobalite sand and high purity quartz are used to
produce a range of silicon chemicals including sodium silicate, silica
gel, silicones, silicon tetrachloride, silanes and pure silicon. Pure silicon
is used for silicon chips, the heart of the computer world. Silicon
products are used in the production of detergents, pharmaceuticals
and cosmetics.
4. Metallurgical Industry: Quartz is the raw material for the production of
silicon metal and ferrosilicon. Silicon metal is used in the production of
alloys based on aluminum, copper and nickel. Ferrosilicon is a major
alloying ingredient for iron and steel. Metal ores are purified in the
furnace by silica sand that is transformed into metal slag.

The major drawbacks of prior art processes are:
(A) Synthesis of industrial silica
1. The conventional process of manufacturing of silica from sand
by fusion method is substantially costly being a high energy
process implemented at around 1500°C.
2. Industrial silica is obtained from sodium silicate by sulfuric acid,
but it generates a large amount of sodium sulphate as waste
which requires treatment to maintain environment standard.
3. The convention process of silica generation is from silica leach
spent but does not separate from aluminosilicate spent.
(B) Regeneration of Alkali!
1. The method of removal of ash from bituminous and subs -
bituminous coals with regeneration of alkali with caustic soda,
generates a large amount of sludge.
2. Caustic regeneration from aluminosilicate spent, produced
during coal extraction with sodium hydroxide by carbon dioxide
gas, is not enabled to separate alumina.

OBJECTS OF THE INVENTION
It is therefore, an object of the invention to propose a process for extraction of
pure silica from low grade coal and other low grade silicious mineral ores.
Another object of the invention is to propose a process for extraction of pure
silica from low grade coal and other low grade silicious mineral ores, which
allows regeneration of chemicals in a close loop operation.
A further object of the invention is to propose a process for extraction of pure
silica from low grade coal and other low grade silicious mineral ores, which
produces clean coal as the by-product.
SUMMARY OF THE INVENTION
According to the invention, a substantial amount of silica can be leached out by
alkali solution. Caustic solution is used here as alkali to take out the silica, and

alumina content of the coal. As the mineral matter mainly silica, alumina are
separated from the coal by this process, the coal gets automatically cleaned and
thus produces a costly by-product. In leaching operation the caustic solution
leaches out the silica and alumina of coal at a desired pressure, temperature
time, feed size and NaOH concentration. The dissolved mineral matter can be
precipitated by exposing the solution at CO2 environment, and injecting CO2 at a
desired flow rate. First precipitation step provides a mixture of precipitated silica
and alumina which can also be considered as by-product of the process. After
separating almost all alumina and equivalent amount of silica, the leached liquor
still contains dissolved aluminates which are precipitated in a second step by the
injection of CO2 once more. The second precipitate contains almost pure silica
which is the main product of the process. After separating out the silica the
filtrate contains mainly Na2CO3 which produces NaOH by lime water treatment.
Accordingly, about 95% of the solvent can be recycled back. Lime stone or
CaCO3 is also another by-product of the process as CO2 gas is converted to
CaCO3.

The invention finds application in precipitation of pure silica from other silicious
minerals matter extractable from caustic leaching such as low grade iron and
manganese ore. The inventive process for silica precipitating from the spent
liquid mixture of sodium aluminosilicate, and enabled to regenerate the caustic
used in close circulating operation with capturing of CO2.
The chemical used here is getting recycled with an appreciable amount. The
impact of the process is more profound as it produces clean coal or by product.
The product and the by-product of the invention both are although costly, it
raises a high industrial importance. The process has five different stages.
1. Digestion
2. Captured CO2 from boiler flue gas.
3. Precipitation Stage I (Precipitated Silica alumina mixture)
4. Precipitation Stage II (Precipitated silica)
5. Regeneration

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 - shows a schematic of the process flow sheet according to the
invention.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention, in the process of precipitation the quantity of
chemical used get substantially recycled. The process further produces clean coal
as a first by-product. The clean coal obtained here can be used for coking
purpose. A second by-product of the process is silica, the mixture of and alumina
which can be used for refractory application. A third by-product of the process is
the lime stone or CaCO3 which additionally restrict emission of substantial
amount of CO2 to the environment and accordingly, the process can be
considered as a green project.

As the main product and the by-products both are costly in this process, the
invention is environmentally visible. Figure 1 shows a schematic process flow
diagram. The entire operation is comprised of five steps.
6. Digestion
7. Capturing CO2 from boiler flue gas.
8. Precipitation Stage I (Precipitated Silica alumina mixture)
9. Precipitation Stage II (Precipitated silica)
10. Regeneration
Step 1: Digestion
In a Digester the pulverized coal is treated with caustic solution. The
concentration of alkali is variable depending upon the solubility of the NaOH at
that operating temperature. The amount of NaOH used is directly related to the

ash-content of feed coal. Generally 1:2 to 1:4 coal ash to NaOH ratio is
maintained. The temperature of the process is kept between 90 to 99°C. Feed
coal contains in an average around 30% ash among which 55 to 65% is silica
and 20 to 25% is alumina. The silica and alumina content of the coal is around
20% and 8% respectively. The reaction of alkali with the mineral matters is
limited and around 60% conversion can be achieved. The major reactions of the
ash digestion can be expressed as:
1. NaOH + Ash → Na2SIO3 + Insoluble
2. NaOH + Ash → NaAIO2 + Insoluble
Yield: 60% Conversion of Ash
Soluble components are aluminates, silicates and aluminosilicates and the
insolubles are coal and unreacted mineral matters. The clean coal obtained bore
as the by-product of the digester.

Step 2: CO2Capture
The filtrate from the digester contains unreacted NaOH2 aluminates, silicates and
some complex aluminosilicates. All the complex compound of silica and alumina
has to be precipitated first. Thus, purging of CO2 into the system is arranged for
precipitation of silica and alumina. A constant source of CO2 is provided for
maintaining the continuous process operation. Boiler flue gas generally contain
excess O2, CO2, and N2 which is the source of CO2. For capturing CO2 from the
boiler flue gas, the sorbent technology is used.
Step 3: First Precipitation
The filtrate coming out from the digester is first exposed to CO2 for several
minutes. The temperature of the process is maintained around 90°C and the

Pressure of CO2 injection is around 6 Kg. After CO2 injection, a white gelatinous
precipitation is formed which agglomerates further with the addition of a
calculated amount of water. The precipitate contains both alumina and silica.
Table 1 gives the composition of the precipitate.
Table 1: compositions of precipitates obtained during carbonation at different
stage
The precipitation Reaction are:
3. Na2SiO3 + CO2 → Na2CO3 + (SiO3 + Others Insoluble)
4. NaAIO2 + CO2 → Na2CO3+ (AI2O3 + Others Insoluble)
Yield: more than 99% Conversion of Al2O3.

Step 4: Second Precipitation
The diluted filtrate coming out from the first precipitation unit is further exposed
to CO2 for several minutes. This time carbonation period is longer than before.
All the silicates converted to precipitated silica and excess NaOH converted to
N2CO3. The second precipitate contains almost pure silica with a trace amount of
other impurities. The precipitated silica is filtered off and the filtrate is sent to a
regeneration unit.
5. Na2SiO3 + CO2 → Na2CO3 + (SiO2 + Others Insoluble)
Step 5: Regeneration
The last step of the process is regeneration where almost all alkali is regenerated
and recycled back. The filtrate coming out from second precipitation unit mainly
contains Na2CO3 which produces NaOH back after reacting with lime water
solution.

We claim:
1. A process for extraction of pure silica from low grade coal and other low
grade silicious mineral ores comprising the steps of:
a) extracting of metal inclusion present in the coal by addition of
sodium hydroxide in a ratio varying from 1:2 to 1:4 at temperature range of 80-
95°C;
b) extracting silica and alumina in the form of aluminosilicate by 60%
in the reaction conditions as defined in step (a); and
c) maximizing the extraction at atmospheric pressure and
temperature of 95°C with reaction time of 2 hours.

2. The process as claimed in claim 1, wherein pure silica is obtained by
preferential separation of alumina in step (a).
3. The process as claimed in claim 1, comprising alumina separation from
mixture of sodium aluminates and sodium silicates of spent alkali solution.

4. The process as claimed in any of the preceding claims, wherein the process
is applicable to low grade silicious minerals like low grade iron ore, low grade
manganese ore, and any other mineral matter containing silica and alumina, and
wherein the extraction is implemented by caustic leaching.
5. The process as claimed in any of the preceding claims, wherein caustic
extraction of silica from iron ores - precipitation of silica from sodium silicate is
implemented with carbon dioxide.
6. The process for extraction of pure silica from low grade coal and other low
grade silicious mineral, substantially as herein described and as illustrated in the
accompanying drawings.

A process for extraction of pure silica from low grade coal and other low grade
silicious mineral ores comprising the steps of: a)extract of metal inclusion
present in the coal by addition of sodium hydroxide in a ratio varying from 1:2 to
1:4 at temperature range of 80-95°C; b) extracting silica and alumina in the
form of aluminosilicate by 60% in the reaction conditions as defined in step (a);
(c) maximizing the extraction at atmospheric pressure and temperature of 95°C
with reaction time of 2 hours.
Fig. 1.