FIELD OF THE INVENTION
The present invention generally relates to a process for reducing ash content from high ash coal as a result of silica alumina precipitation of the ash content of the coal. More particularly, the invention relates to a method for extraction of pure silica from the mixture of coal and iron ore in a process of chemical reaching of coal.
BACKGROUND OF THE INVENTION
Leaching of coal with organic solvent is known. The major disadvantages of the know process is the process feasibility. All chemical treatment processes entail generally a huge cost and considered to be unviable except when the end product has a high market value. The process cost normally increases due to the following factors:
1. Inefficient regeneration process.
2. Costly regeneration process (like evaporation etc.)
3. Stringent environmental norms (most of the chemicals are hazardous)
4. Unrecovery of costly chemicals.
It is known that concept of chemical beneficiation of minerals by chemical leaching originates from the limitations of physical beneficiation process. It is further known that a chemical beneficiation process is possible through chemical leaching or extraction of gangue matter present in ore with aqueous solution of different mineral acids and alkali. However, the operating cost of this process is high because of cost of chemical and energy requirement of the process. According to prior art, impurities like ash in coal and gangue matter in the iron ore is removed

before commencing a steel making process in blast furnace. Chemical beneficiation may be an alternate technology to improve the ore purity. According to the known chemical beneficiation process, mineral matters mainly silica and alumina are dissolved in caustic solution. The leachant so produced contains mostly unreacted caustic with dissolved sodium silicate and aluminates. The major problem of this known process is the multiple limitations in regeneration of caustic, for example limitation in regeneration efficiency of the process. Another problem of recovery is the use of other chemical like acid which is also very difficult to recover before final discharge. Incase of higher ash content coal the mineral matter a from 20 to 30%. The coal quality and the beneficiation process directly depends on the amount of coal ash. For Steel industry, such coals which is known as inferior quality can not be directly used due to its higher ash content. Prior art teaches several processes for cleaning coal by both physical and chemical operations. Coal containing substantial amount of ash ranging from 20% to 40% is not susceptible to physical cleaning due to certain limitation for example, ultra clean coal can not be produced, and therefore, the physically cleaned high ash coal is to be blended for improving coke quality. So Chemical beneficiation of coal is required for obtaining ultra clean coal. Further, the major part of coal ash is silica and alumina, the approximate amount of these two minerals in total ash content is around 60% and 20% respectively. Thus, silica having the major contribution (60%) in ash constituents, and accordingly extraction of substantial amount of silica from coal is expected to allow production of ultra clean coal from high ash coal.
WO2008119212 describes a method of extracting silica and alumina from fly ash which comprises the steps: leaching fly ash with a NaOH solution with a concentration more than 40% by mass, wherein silicon is leached in the form of sodium silicate from the fly ash, thereby obtaining the sodium silica solution and the alkaline residue with a Al/Si ration no less than 2; concentrating the resulting sodium silicate solution by evaporation, to obtain sodium silicate solution with a

variety of concentrations, or preparing silica by a carbon - decomposing process; preparing AI203 by the conventional process from the alkaline residue; and preparing filler or cement from the final residue after leaching the aluminium. Instead of directly leaching alumina from the fly ash as of prior art, a process route, wherein silicon is leached at first, and then aluminium is leached, may be adopted in the cited reference which can improve the Al/Si ratio in the alkaline residue, simplify process route, and improve the extraction efficiency of the fly ash. Further, said method can be applied for coal gangue with high aluminium content, kaolinite and middle and low grade bauxite, which have been calcined at a temperature of 900° to 1100°C.
Wo2004073600 teaches a process for the manufacture of precipitated silica from rice husk ash, having a surface area ranging from 50-400m<2>/g and tap density of 80-600kg/m<3> having multiple applications in the field of rubbers and plastics, paints, toothpastes, catalyst, carrier agent, insulation, stabilizing and desiccants. The process for silica precipitation is the chemical used and making it a closed loop operation. The extraction process through digestion, precipitation and regeneration are done based on the application specifies, so as to get the required particle size and densities.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method for extraction of pure silica as a byproduct from the mixture of coal and iron ore in a process of chemical reaching of coal.
Another object of the invention is to propose a method for extraction of pure silica as a byproduct from the mixture of coal and iron ore in a process of chemical reaching of coal, which exhibits more than 99% regeneration efficiency.

A further object of the invention is to propose a method for extraction of pure silica as a byproduct from the mixture of coal and iron ore in a process of chemical reaching of coal, which is cheaper and echo-friendly.
SUMMARY OF THE INVENTION
Ash removal of coal according to the invention is done by caustic leaching process. The ash material mainly consists of silica and alumina (55% silica and 35% alumina). In a caustic leaching process, the influencing factor is the regeneration of alkali. Generally leachant contains sodium aluminosilicate which can be precipitated by varying the pH of the system. A general method of reducing pH is acid dosing which however, poses additional problem of acid recovery. Acid recovery is itself a challenging task which substantially enhances the cost. The inventive method is based on carbonation technique which is established in benchscale level. The caustic used for leaching is recycled to a substantial amount. Further advantage of the method is that it produces clean coal. The inventive method is comprised of three basic steps :-
1. Digestion
2. Precipitation
3. Regeneration
One of the major advantages of the inventive method is that C02 instead of any costly chemical is used which is available free from boiler flue gas. As the inventive method requires a boiler to supplement energy, the required CO2 is captured from the boiler flue gas. The invention proposes an echo friendly regeneration method having an efficiency of >99%. The invention proposes a method for regeneration of alkali during ash removal from coal by caustic leaching.

The main advantages of the method are 99% regeneration efficiency, and use of a cheaper raw material. The material of regeneration is CO2 which is recovered from the boiler flue gas.
According to the invention, a substantial amount of silica and alumina can be leached out by alkali solution. Caustic solution is used as alkali to take out the silica and alumina of the coal. As the mineral matter mainly constitute silica and alumina, these are separated from the coal during the coal leaching process, the costly separated mineral materials (silica and aluminium) being the byproducts of the cleaning process. In the leaching, caustic solution leaches out the silica and alumina out 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 maintaining CO2 injection at a desired flow rate. First precipitation formed is a mixture of precipitated silica and alumina which may also be considered as a by product of the process. After separating almost all alumina and equivalent amount of silica, the leached liquor still contains dissolved aluminates which are again precipitated by injection of CO2 once more. The second precipitate contains almost pure silica which is the main by product of the process. After separating out the silica the filtrate contains mainly Na2CO3 which NaOH by lime water treatment. So this way, almost 95% of the solvent can be recycled back. Lime stone or CaCO3 is another by product of the process as CO2 becomes CaCO3.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Shows a process flow diagram of the inventive method.
Figure 2 - Shows regeneration yields of the method under different trials of the inventive method.

DETAILED DESCRIPTION OF THE INVENTION
According to the inventive method, the precipitation technique is unique because the amount of chemical used is substantially recycled. Further, the method of the invention simultaneously produces clean coal. Clean coal obtained here constitutes a vital by - product. Another byproduct of the method is the silica, alumina mixture which can be used for refractory application. A third by product of the method is lime stone or CaCCO3. CO2 used here for precipitations is converted to CaCO3 in course of implementation of the inventive method.
As described hereinbefore, the inventive method comprises three basic steps :
1. Digestion
2. Precipitation
3. Regeneration
Step 1: Digestion
In a Digester pulverized coal is treated with caustic solution. The concentration of alkali is varied depending on solubility of the caustic solution (NaOH) at desired operating temperature. The amount of NaOH used is directly related to the feed coal ash. 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 30% ash among which 55 to 65 is silica and 20 to 25% is alumina. With an approximation, silica and alumna content of the coal can be considered as

respectively 20% and 8%. The reaction of alkali with those mineral matters is limited and around 60% conversion can be expected. The major reactions of the ash digestion can be expressed as:
1 NaOH + Ash ► Na2SiO3 + H2O + Insoluble
2 NaOH + Ash ► NaAI02 + H2O + 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 here as a by product of the digester.
Step 2: Precipitation
The filtrate coming out from the digester is first exposed to CO2 for several minutes. The temperature of the process is 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 calculated amount of water. The precipitate contains both alumina and silica. The precipitation Reactions are:
3. Na2SiO3 + CO2 Energy Na2CO3 + (SiO2 + Others Insoluble)
4. NaAIO2 + CO2 Energy Na2CO3 + (AI2O3 + Others Insoluble)
►
Yield : more than 99% Conversion of Al203.

Step 3: Regeneration
The last step of the process is implemented in a regeneration unit, where almost
all the used alkali is regenerated and recycled back. The filtrate coming out from
the precipitation unit mainly contains Na2CO3 which produces NaOH after
reacting with lime water solution. The reactions involved in regeneration step
are:

Step 4: Washing
The efficiency of regeneration is entirely depends upon the washing efficient. A counter current washing method is employed to recover maximum possible amount of NaOH. In figure 3 the washing cycle is shown along the entire process flow diagram of the operation. Counter current washing reduces the consumption of water drastically. Currently the amount of water consumption is 8 times of the amount coal being introduced for the processing. After caustisizing a huge amount of unreacted CaO is present with CaCO3 which is a by produced during final regeneration reaction. This solid contains >30% free lime along with the CaCO3. To make an efficient use of this sludge fresh co2 is purged again to convert the unreacted lime into CaCO3. This way precipitated CaCO3 is formed. The purity of the precipitated CaCO3 is more than 98%. Thus a valuable by product is also formed during regeneration.

We claim:
1. A method for extraction of pure silica as a byproduct from the mixture of
coal and iron ore in a process of chemical leaching of coal, the method
comprising the steps of :
- treating in a digester pulverized coal with caustic (NaOH) solution at a process temperature between 90°C to 99°C, the amount of caustic solution in relation to ash content of the coal maintained at a ratio between 1:2 and 1:4, the concentration of the solution being varied depending on solubility of the NaOH;
- recovering clean coal from the digester;
- precipitating the filtrate recovered from the digester in a precipitation unit by injecting CO2 at a pressure around 6 kg under process temperature of about 90°C, and adding water to agglomerate the produce;
- recovering mineral materials consisting of silica and aluminum oxides;
- regenerating in a regeneration unit the used chemical by reacting the filtrate from the precipitating unit with lime water solution; and
- preparing CaCo3 from the solid rejects in the regeneration unit by mixing in a reactor and injecting CO2 gas for about 45 minutes.

2. The method as claimed in claim 1, wherein the regeneration of alkali is about 99% of the used chemical.
3. The time of co2 purging can be reduced to 5 minutes by maintain mixture temperature around 70 °C which claims significant reduction in co2 consumption by maintaining the temperature of the solution.
4. The Counter current washing method reduced the water consumption by more than 80% in compare to normal washing procedure.

5. The method as claimed in claim1, wherein the purity of precipitated silica is
more than 98%.
6. The method as claimed in claim 1, wherein the purity of the precipitated
CaCO3 is more than 98%.

ABSTRACT

A method for extraction of pure silica as a byproduct from the mixture of coal and iron ore in a process of chemical reaching of coal, the method comprising the steps of treating in a digester pulverized coal with caustic (NaOH) solution at a process temperature between 90°C to 99°C, the amount of caustic solution is relation to ash content of the coal maintained at a ratio between 1:2 and 1:4, the concentration of the solution being varied depending on solubility of the NaOH; recovering clean coal from the digester; precipitating the filtrate recovered from the digester in a precipitation unit by injecting Co2 at a pressure around 6 kg under process temperature of about 90°C, and adding water to agglomerate the produce; recovering mineral materials consisting of silica and aluminum oxides; regenerating in a regeneration unit the used chemical by reacting the filtrate from the precipitating unit with lime water solution; and preparing CaCo3 from the solid rejects in the regeneration unit by mixing in a reactor and injecting Co2 gas for about 45 minutes. The amount of co2 cosumption is reduced by 80% by providing the temperature to the solution. The key feature in regeneration is the recycling of wash water. Process chemistry is known and already patented. The entire process operation is set in such a way that a maximum amount of recycled water can be obtained. The result of regeneration is given in figure 2.