FIELD OF INVENTION
The present invention relates to a process for recovery of spent acid
solution and separation of by-products during chemical leaching of coal.
BACKGROUND OF INVENTION
Leaching is a method by which solute from a solid is removed by using a
solvent. The dissolved solute is removed along with the solvent. Leaching
is employed for the extraction of valuable metals. Leaching of coal with
inorganic solvents is undertaken for removal of inorganic ash bearing
minerals from coal.
The major inorganic components present in coals in particular Indian coals
are silica and alumina. Silica and alumina both are acidic in nature and can
be separated by treating with an alkali. Hence, coal is initially leached with
aqueous solution of NaOH at different concentrations at elevated
temperature under atmospheric and elevated pressure. After treatment of
coal with caustic solution, the treated coal is filtered and washed with
water to remove traces of alkali. After leaching with alkali, the washed coal
is treated with dilute hydrochloric acid to remove desilication products like
sodalite and iron bearing minerals present in coal.
Coal is basically a non-volatile, insoluble, non-crystalline, extremely
complex mixture of organic molecules varying in size and structure and
inorganic mineral matter embedded in the organic clusters. Indian coals
are generally of drift origin and hence the concentration of mineral matter
is relatively higher than that of coals extracted in other parts of the world,
which interalia makes it unsuitable for efficient utilization, such as
carbonization, gasification, combustion, or liquefaction. As a result,

enormous efforts are necessary to make the Indian coal and related solid
carbonaceous materials, as a better source of energy. Leaching or solid
extraction is done to dissolve mineral matter in coal using a solvent. The
organic and inorganic components (acidic and/or basic components present
in mineral matter) in the coal react with the solvents, gets dissolved and
then removed. Chemical leaching of coal is a known technology to produce
clean coal where the ash content of clean coal is as low as 1.0%, and is
based on mineralogical composition of the feed coal. There are potential
use of this clean coal both as a fuel and nonfuel. The leaching process
involves treatment of coals crushed to 0.2 mm size for removal of ash-
forming minerals. The process includes the steps of a first treatment of the
coal in an aqueous alkaline solution at an elevated temperature under
atmospheric pressure to remove acidic components, followed by a second
treatment with acidic solution to remove the basic components of the coal.
This is a process to produce low ash (-5% ash) clean coal from high ash
Indian coals with 80-85% yield.
Various leaching processes of coal are disclosed in Indian Patent
Application Nos. 621/KOL/07, 1391/KOL/08, 1550/KOL/08 and
1546/KOL/08. A chemical leaching process for treating high ash Indian
coals in a batch of 500 kg of raw coal is further disclosed in Indian Patent
Application NO.1518/KOL/08 .
As described hereinabove, the major components present in mineral
matter/ash of Indian coals are silica and alumina. Silica and alumina both
are acidic in nature and can be separated by treating with an alkali. Hence,
the main impurities present in the spent alkali liquor after leaching of coal
are compounds of silicon and aluminum. The treatment of coal with caustic
solution results in dissolution of silica and alumina followed by re-
precipitation of sodalite-type desilication production. These desilication

products present in coal after the alkali treatment have a negative effect in
ash removal. When the alkali-treated coal products are subsequently
treated with aqueous acidic solution, these desilication products and other
iron bearing components dissolve into acid forming chlorides with
hydrochloric acid solution (Reggel et al., 1976). After treatment of the coal
with acid, the spent acid is separated by filtration. Spent acid after the
treatment with coal contains impurities like silica, alumina, iron, calcium,
sulfur and magnesium. Recovery of the spent hydrochloric acid has a great
importance for the process economy as well as to restrict environmental
hazards.
OBJECTS OF INVENTION
It is therefore an object of the invention to propose a process for recovery
of spent acid solution and separation of by-products during chemical
leaching of coal, which eliminates the disadvantages of prior art.
SUMMARY OF INVENTION
In order to recover the used acid, three different methods are employed,
for example, adsorption by using activated charcoal, ion exchange with
strong anionic exchange resin, and pyro hydrolysis.
In adsorption method, the spent acid solution is passed through a bed of
adsorbent where impurities get adsorbed onto the adsorbent and the
treated solution is separated. In the ion exchange process, the spent acid
solution is treated with ion exchange where, ions of silica, alumina and iron
are neutralized and removed. In pyrohydrolysis process, the spent acid
solution is heated above 110°C and the vapors formed are cooled and
condensed in a condenser unit with water. Condensed liquid is collected as

a regenerated solution. Condensed solution is tested for presence of
impurities.
In order to recover the spent acid, three different methods are employed.
Adsorption by using activated charcoal, ion exchange with strong anionic
exchange resin, and pyro hydrolysis. Hydrolysis is a method by which, a
certain molecule is split into two parts by the addition of a molecule of
water. Dissociation of water particle into ions takes place to increase the
pH and strength of a mixture containing aqueous solution of alkali or acid.
Water particle are separated from a mixture by chemical reaction in the
form of ions. Hydrolysis is basically a method of separation by chemical
reaction. Hydrolysis process is mainly used in the conversion of sodium
acetate to pure acetic acid, saponification of esters, conversion of amide
into carboxylic acid etc. Hydrolysis has a wide variety of applications in
metabolism of plants and animals, where hydrolysis of polysaccharides
results into mono or disaccharides.
Pyrohydrolysis of hydrochloric spent liquor from carbon steel pickling liquor
is a hydrometallurgical reaction where iron chloride converts to iron oxide
by releasing chloride ions. Chloride ions react with the hydrogen cations
and form hydrochloric acid. Pyrohydrolysis is inverse of picking process.
Pyrohydrolysis has better effect on the recovery of the acid.
Spent acid after the treatment with the coal contains impurities like silica,
alumina, iron, calcium, sulphur and magnesium. After demineralisation of
the coal with NaOH, an intermediate compound (Sodalite) is formed which
is a mixture of sodium oxide, silicon dioxide, aluminum oxide. Hydrochloric
acid has a tendency to dissolve sodalite compounds into silicon chloride,
aluminum chloride etc. Acid also helps in leaching of fluorapatite, pyrite
minerals which can be transformed into ferrous/ferric chloride, and

hydrogen sulphide compounds. By pyrohydrolysis method, 99% removal of
silicon chloride, aluminum chloride, ferric chloride and calcium chloride etc.
are observed. Ion exchange method and adsorption with activated charcoal
removes alumina and silica respectively but the percentage of removal is
limited upto 40% and 50% respectively. In spent acid, silicon tetrachloride
is formed which is an unstable compound. This reacts with atmospheric
oxygen present in air and is finally precipitated as silicon dioxide.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWING
Figure 1 : represents a flow sheet depicting regeneration of spent acid and
separation of byproduct.
DETAIL DESCRIPTION OF THE INVENTION
For chemical leaching test, two types of Indian coals are used. Medium
Coking Indian Coal is used for leaching test. The ash constituents present
in the coal are listed in Table 1. Silica and alumina are the main
constituents of ash in Indian coals. Silica and alumina are acidic in nature.
Hence, during alkali leaching of coal, most of these components present in
coal reacts with alkali, and either get removed or form sodalite like
complex compounds. Alkali leaching of coal is conducted with different
alkaline solutions like aqueous sodium hydroxide, potassium hydroxide and
calcium hydroxide solutions etc. It has been found that sodium hydroxide is
having higher effect on the step of removal of ash bearing minerals present
in Indian coals. In the process, the Silica reacts with sodium hydroxide and
forms sodium silicate (Equation-1). Alumina forms sodium aluminates

according to the reaction (Equation-2). It has been observed that, at high
concentration of sodium hydroxide, sodium silicate and sodium aluminates
reacts together and forms sodium aluminum silicate (sodalite). After alkali
leaching, the reagent is separated from the reacted coal using a known
filtration technique. But, the sodalite being jelly material, thus can only be
partly separated along with the reagent by filtration. In order to remove
the alkaline minerals like iron oxides and sodalite, the acid leaching is
conducted at room temperature. Dilute HO at different concentrations is
used for acid leaching process. In the acid leaching process, iron bearing
minerals react with hydrochloric acid and form ferric chloride. Sodalite
dissociates into silica and alumina ions. Silica reacts with chlorine and
forms silicon tetrachloride, which is not a stable compound. Aluminate ion
forms aluminum trichloride with HCI during the leaching. After acid
leaching of the alkali treated coal, the spent acid is separated from the coal
by filtration. The spent acid is analyzed for impurities by using a known
Inductively Coupled plasma spectrometer (ICP-MS). Concentration of
different elements present in the spent acid are listed in Table 2.

Table-1: Analysis of ash constituents present in the coal
Methodology followed according to the invention for acid recovery
during chemical leaching process:
For regenerating the spent acid after the acid leaching process, different
process steps are adopted. A first step constitutes adsorption using
activated charcoal. Activated charcoal has an improved effect in removal of
silicate ions and sulphur bearing compounds in coal. As the concentration
of silicate ion is more in acid, the step of adsorption have been carried out
at different parametric conditions using activated charcoal. Thus, activated
charcoal is filled into a glass column having a specific bore diameter and its
bottom end is closed with a poly propylene filter cloth having a
corresponding aperture size, and supported by a rubber gasket. Spent acid
is fed from the top of the column at different flow rates in order to
maintain different residence times. After adsorption, the acid is collected
from the bottom of the column. After this step, the acid is analyzed for
impurities present in it using inductively coupled spectrophotometer. The
impurities present in the spent acid after coal leaching, and the acid after
adsorption with the charcoal in a ratio of approximately 1:10 with a
residence time of about 6 hrs are compared and listed in Table 3. From
Table 3, it is observed that maximum of 40% silica, 46% Mg, 55% Ca and
70% P removal is achieved in this process keeping the concentration of
other elements same.
Table-3: Comparison of concentration of elements present in the spent acid
and regenerated acid after adsorption with char coal.
A second step adapted for regeneration of the spent acid after leaching
constitutes an ion exchange step using highly anionic exchange resin.
Dowex-II (30 mesh) is used for this technique in order to remove silica and
alumina bearing compounds in the spent acid. A sample of ion exchange
resin for example, 500 gm by weight is filled in a glass column having
identical dimensions as were used for adsorption method. Spent acid is fed
from the top of the column at different flow rates. The regenerated acid
passing through the ion exchange process is collected from the bottom of
the column and analyzed for impurities present in an inductively coupled
spectrophotometer. Impurities present in the spent acid after coal leaching,
and the regenerated acid after ion exchange treatment for about 6 hrs are
compared and are listed in Table 4. From Table 4 is observed that
maximum of only 37% alumina and 35% of iron bearing compounds is
removed during the process.
Table-4 : Comparison of concentration of elements initially present in
spent acid and regenerated acid after ion exchange method.

The third step constitutes pyrohydrolysis, where the spent acid is stripped
in a glass column at different temperatures and the vapours at different
temperatures are absorbed onto the water and separated. Stripping of the
acid is conducted at different temperatures based on the boiling point of
different constituents present in the spent acid. At around 92°C, water
starts to vaporize. For every 5°C of temperature rise, the samples are
collected and are analyzed. Concentration of elements at different
temperatures is listed in Table 5. From Table 5, it is observed that more
than 99% of the impurities are removed by this method. From the data of
Table 5, it is observed that the concentration of silica and iron is increased
slightly because of the entrainment of the solid particles with the acid
vapours. At 130°C, formation of some solid particles is observed. From the
published non-patent literature, it is found that at a temperature of above
130°C, the aluminum chloride gets converted to aluminum oxide which is a
solid. After complete vaporization, the solid material is collected, washed
with water, dried and analyzed. Composition of the solid material found is
given in Table 6. From Table 6, it is seen that alumina and iron are the
main constituents and silica is also present in the solid material. The solid
material fuses above around 600°C and the iron chloride present gets
converted to iron oxide. The fused solid sample is tested qualitatively using
XRD and the presence of iron oxide is confirmed.
Silicon tetrachloride is an unstable compound and is the major silicon
compound initially found in the final solid material. The spent acid is kept
aside without mixing for about 72 hours and it is observed that, the silica
bearing solid compound is formed in the spent acid. This solid is separated
from the spent acid by filtration and the filtrate is used for the regeneration
by pyrohydrolysis method. Filter cake is washed with water, dried, weighed
and analyzed. Results of chemical analysis are given in Table 7. More than
75% of the precipitate is of silica and a study of the qualitative nature of
the silica shows that it constitutes silicon dioxide. Silicate ion in the spent
acid reacts with the oxygen from atmospheric air and forms silicon dioxide.
Table - 5 : Concentration of elements initially present in the spent acid and
regenerated acid at different vaporization temperatures during
pyrohydrolysis.
Initially, a sample of spent acid solution for example 500 ml after leaching
with coal powder is kept for digestion without any mixing for about 72
hours. After around 72 hours, a gel formation is observed in the acid. The
solid material is separated from the spent acid solution by filtration
technique as a byproduct. Then the solid material is washed with water to
remove traces of acid and air dried at 105°C. The spent acid filtrate is
heated to different temperatures and the vapours are cooled in a
condenser and then passed directly through the cooling media (water) for
complete condensation and dissolution. After Pyrohydrolysis process, the
solid precipitate is removed and washed with water to remove traces of
acid and then the solid is air dried. At different temperature and time, the
condensed samples are collected and analyzed for impurities using ICP-MS.
The solid samples like precipitate after digestion including pyrohydrolysis,
are analyzed using X-ray Diffractometer in an X-ray Florescence technique.
A process flow sheet for regeneration of spent acid is shown in Figure 1.
WE CLAIM :
1. A process for treating spent acid for removal of impurities
consisting of silica, alumina and iron bearing compounds
generated during leaching method of coal with Hydrochloric acid,
the process comprises :
(i) implementing an adsorption step adapting activated
charcoal in which silicate ions and sulphur bearing
elements present in the coal is removed;
(ii) carrying-out a step of ion exchange using a highly anionic
exchange resin which enables removal of silica and
aluminium bearing compounds in the spent acid; and
(iii) applying pyrohydrolysis at temperatures above 1100C, for
removal of impurities and reuse of the spent acid in coal
leaching process.
2. The process as claimed in claim 1 wherein said spent
hydrochloric acid solution is generated during the acid leaching
of coal.
3. The process as claimed in claim 1 wherein said silicon from the
spent acid in coal leaching process is partially removed by using
the adsorption step with activated charcoal, and wherein
approximately 50% of the silicate is removed by the step of
adsorption.
4. The process as claimed in claim 1 wherein said alumina and iron
bearing compounds from the spent acid produced in coal
leaching process is removed by the ion exchange step, and
wherein an ion exchange resin is used which enables removal of
aluminates from the spent hydrochloric acid solution.
5. The process as claimed in claim 1 wherein 99% of the spent acid
regeneration is achieved in the step of pyrohydrolysis in which
water is separated out at a temperature above 100°C, acid
vapours are separated out from the spent acid in a temperature
range between 100° - 130°C, and the vapours are absorbed on
to the water to from regenerated hydrochloric acid, the
aluminum chloride converting into alumina precipitate by
recovering the chloride ions.
6. The process as claimed in claim 5 wherein a final precipitate
after separating out of the vapour, contains iron chlorides which
is converted into iron oxides in a step of fusing the mixture
above 600°C, the byproducts consisting of alumina and iron
oxide contain impurities like calcium and silica.
7. The process as claimed any of the proceeding claims, wherein
the sodalite dissociates into silicate and aluminate ions during
the hydrochloric acid treatment, the silicate ion forming silicon
tetrachloride which is an unstable compound, and wherein the
silicon tetrachloride gets converted into silicon dioxide by
reacting with the atmospheric air.
8. A process for treating spent acid for removal of impurities
consisting of silica, alumina and iron bearing compounds
generated during leaching method of coal with Hydrochloric acid,
as substantially described and illustrated herein with reference to
the accompanying drawings.

The invention relates to a process for treating spent acid for removal of
impurities consisting of silica, alumina and iron bearing compounds
generated during leaching method of coal with Hydrochloric acid, the
process comprises : implementing an adsorption step adapting activated
charcoal in which silicate ions and sulphur bearing elements present in the
coal is removed; carrying-out a step of ion exchange using a highly anionic
exchange resin which enables removal of silica and aluminium bearing
compounds in the spent acid; and applying pyrohydrolysis at temperatures
above 110°C, for removal of impurities and reuse of the spent acid in coal
leaching process.