FIELD OF THE INVENTION
The present invention relates to a process for separation of Silicon/Aluminum
based Compounds from spent Hydrochloric Acid in a coal leaching process.
BACKGROUND OF THE INVENTION
Coal is a heterogeneous material containing mineral matter embedded in a
homogenous organic cluster. The mineral matter is distributed in the coal seam
both in vertical and horizontal directions. High mineral matter in coal makes it
unsuitable for efficient industries, thermal power plants and other industries,
which require coals with low ash content. Due to yield constraint, chemical
beneficiation is more efficient compared to the physical beneficiation. The
mineralogical analysis of coals from Indian origin shows presence of mineral
matter mainly divided into two groups. The list of ash constituents and
composition is listed in Table-1. Group I minerals, including silica, alumina, clay,
etc. reacts with alkaline reagents such as Caustic soda, soda ash etc. however,
group II minerals, mainly iron, phosphorous, Calcium, Magnesium containing
species, are removed by acid leaching.
Table-1: List of ash constituents in the coal and composition of ash
constituents



According to the prior art (PCT/IN09/000328; Dt. 05.06.09), chemical leaching is
carried-out in two stages - alkali leaching followed by acid leaching. It can be
observed from Table-2 that, removal of group-I and group-II minerals is minimum
in alkali leaching due to the formation of Sodalite. The reactions in alkali leaching
stage are depicted in Equation 1, 2 and 3. Silica reacts with alkali forms sodium
silicate and alumina forms sodium aluminate. But at higher temperature and
concentration conditions, sodium silicate and sodium aluminate forms an insoluble
sodium alumino silicate, due to which the ash reduction is minimum. The
reduction is much higher in acid leaching stage. In acid leaching stage, sodium
alumino silicate dissociates in to the acid (Equation 4) and subsequently, reduces
ash in the coal. Based on the analysis, it is clear that, the major impurities are
recovered by acid and hence acid requirement is quite high. The major
constituents in acid are both group-I and group-II minerals. The concentration of
minerals present in the acid is high and the HCI concentration is only 5%, hence
regeneration by evaporation of spent acid is costly as 95% of the spent acid
contains water. Removal of impurities from acid is very important for coal leaching
process in terms of cost and effluent disposal issues are concerned.



In coal leaching process, silica and alumina based compounds are acidic in
nature and reacts with alkaline materials such as Sodium hydroxide (NaOH) to
form un-dissolved solid like Sodalite. Sodalite is a mixture of Sodium, Silicon and
Aluminum oxides, which are completely soluble in hydrochloric acid.
Spent hydrochloric acid is generated in coal leaching process as effluent. Spent
hydrochloric acid contains silica, alumina, iron, calcium as major impurities and
also contains unreacted acid and water. Concentration of Hydrochloric acid is
only 5% (wt/wt) and the rest is water. Recycling of the spent hydrochloric acid is
important as the process economics and environmental issues are concerned. As
the spent acid contains silica and alumina based compounds, by product
generation by value addition of the Silica/Alumina product improves the
economics of the leaching process.

Acid leaching of coal not only involves leaching of group-II minerals but also
dissociation of sodalite salt and hence, a major composition is due to silica,
alumina and iron. Silica and alumina are considered as major impurities because
of the negative impact of these on coal property. Hence, the removal of these
compounds is of great importance. The sodalite dissolution is expected to takes
place during acid leaching process.
Silicon chloride is an unstable compound which is converted to silicon
oxide/hydroxide by reacting with oxygen present in the water. However, the rate
of oxidation is a very slow process and depends on percentage silica and
concentration of acid.
OBJECT OF THE INVENTION
It is therefore an object of the invention is to propose a process for separation of
Silicon/Aluminum based Compounds from spent Hydrochloric Acid in a coal
leaching process.
Another object of the invention is to develop a process for removing impurities
from spent hydrochloric acid in a coal leaching process.
Another object of the invention is to develop a process that enables recycling of
the spent hydrochloric acid in a coal leaching process.
Still another object of the invention is to develop an economic coal leaching
process of reducing ash content in the coal.

SUMMARY OF THE INVENTION
According to the invention, the spent hydrochloric acid solution is heated to
different temperature conditions to obtain an improved oxidation behavior. The
improved oxidation behavior is attributed to poly-condensation of silica in the
solution at temperatures above 80 °C. After precipitation of the solution, the
silica is separated from the acid and the liquid is mixed with Sulphuric acid for
alumina separation. Two layers are formed and it has been observed that, top
layer is yellow in color and the bottom layer is white in color. The layers are
separated and analyzed. It has been found that, more than 99% of silica
removal and 80% alumina removal is possible from the spent acid.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 - Flow sheet for regeneration of spent acid and byproduct separation
according to the invention
DETAIL DESCRIPTION OF THE INVENTION
Coal is a heterogeneous rock consisting of organic macerals, inorganic minerals
and extensive pore structure. Indian coals are of drift origin and the ash content
in Indian coals is high compared to other parts of the world. Due to yield
constraint in coal washing, chemical beneficiation is more efficient compared to
the physical beneficiation. Indian coal contains silica, alumina as main substitutes
of ash. Silica and alumina are acidic in nature. Hence, alkali leaching of coal has
a better effect on removal of ash constituents. Alkali leaching test has been
conducted with different alkaline solutions like aq. Sodium hydroxide, Potassium
hydroxide and Calcium hydroxide solutions and found that sodium hydroxide is
having better effect on the removal of ash bearing minerals. Silica reacts with

sodium hydroxide and forms sodium silicate. Alumina forms sodium aluminates
according to the reaction. 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, reagent has
been separated from the reacted coal using filtration. But Sodalite is a gelly
material, which can be partly separated along with the reagent by filtration.
Precipitated sodalite is separated with coal. After separation, the coal is treated
with dilute acid to recover precipitated salts and also to leach group-II minerals.
Acid leaching is carried out with dilute Hydrochloric acid at 5% (w/w)
concentration. Silica reacts with chlorine forms unstable silicon tetrachloride and
aluminate ion forms aluminum trichloride during the HCI leaching. Group-II
minerals such as iron, phosphorous also dissolve in hydrochloric acid. After
leaching operation, spent acid is separated from the coal by filtration. Spent acid
has been analyzed for impurities by using Inductively Coupled plasma
spectrometer (ICP-MS). Elemental composition of spent hydrochloric acid is
mentioned in Table-3.

Silicon chloride is an unstable compound and it will convert to silicon
oxide/hydroxide by reacting with oxygen present in the water (Equation-5). The
rate of oxidation is very slow process and depends on % Silica and concentration

of acid. In order to improve the oxidation behavior, the solution was heated to
different temperature conditions and it was observed that, the oxidation is very
fast. This is attributed to the poly-condensation of silica in the solution at
temperatures 80 deg. C or above. After precipitation the silica is separated from
the acid. The separation of the silica with increase in temperature and heating
time is listed in Table-4. It has been observed that, 80 deg. C -90 deg. C is the
optimal separation temperature. The spent acid regeneration is in the range of
30-50% depending upon the coal used and percentage of different elements
present in the coal. At least 40 % of the spent acid is regenerated during the
heating step. Further, in an embodiment of the invention, the spent acid
regeneration varies in the range of 20%-40%. The percentage regeneration
values for a given set of experiments are mentioned in table Table-5.



After separating silica, acid filtrate is then mixed with Sulphuric acid for alumina
separation (Equation-6). Two layers were formed and it has been observed that,
top layer is in yellow color and the bottom layer is white in color. The layers were
separated and found that the bottom layer is alumina rich compound (Table-6).
From Table-6, in top layer considered to the regenerated acid contains 38 ppm
of silica and 1567 ppm of alumina. More than 99% of the silica removal and 80%
alumina removal from spent acid is achieved using the heating step followed by
Sulphuric acid addition. For a set of experiments, percentage regeneration
calculation based on free chloride content at different sulphuric acid dosage rates
is depicted in Table 7. The spent acid is recovered in the range of 30% to 70 %.
At least 60% of the spent acid is recovered during the sulphuric acid addition
step. The sulphuric acid dosage is used in the range of 100-500 ml/L and
preferably in the range of 350-450 ml/L. Table 7, provides examples of the spent
acid regeneration using different dosage of the sulphuric acid solution.

The precipitates at different stages were analyzed and listed in Table 9. It has
been observed that, the precipitate obtained in heating stage (silica precipitate)
is having 95.3% silica, 2.9% alumina and rest are iron and other compounds.
Whereas, the precipitate during sulphuric acid addition (alumina precipitate) is
having 62.5% alumina, 12.5% iron oxide and rest are silica and other
compounds.


From the results, it has been found that, at least 90% of the silica removal is
removed from the spent acid solution using the process as per the current
invention. Further, at least 80% alumina removed from spent acid. At least 60%
of the spent acid can be regenerated for further use and recycle and hence, it
add significant advantages to the process. Silica and alumina are considered as
major impurity because of the negative impact on coal property due to which,
the removal of these compounds has great importance for improving the overall
economics and efficiency of the process.

WE CLAIM:
1. A process for separating Silicon and Aluminum based compounds from a
spent acid solution in a coal leaching process, the process comprising:
heating the spent acid solution to a temperature range of 80 °C or higher;
filtering a precipitate from the heated spent acid solution;
adding sulphuric acid solution to the heated spent acid solution; and
filtering a precipitate and collecting the spent acid solution for further re-
use.
2. The process as claimed in claim 1, wherein the spent acid is hydrochloric
acid.
3. The process as claimed in claim 1 and claim 2, wherein the spent acid
solution is heated preferably in the range of 80 °C to 90 °c.
4. The process as claimed in claim 1, wherein the spent acid solution includes
compounds of Titanium (Ti), Aluminum (Al), Manganese (Mn), Silicon (Si),
Calcium (Ca), Iron (Fe), Chromium (Cr) and Phosphorous (P).
5. The process as claimed in claim 1, wherein at least 40% of the spent acid is
regenerated during the heating step.
6. The process as claimed in claim 1, wherein at least 60% of the spent acid is
regenerated after the sulphuric acid addition step.
7. The process as claimed in claim 1, wherein the precipitate obtained after
heating the spent acid solution comprises Silica.

8. The process as claimed in claim 1, wherein the precipitate obtained after
adding the sulphuric acid solution to spent acid solution comprises Alumina.
9. The process as claimed in claim 1, wherein at least 90% of silica is removed
from the spent acid solution.
10. The process as claimed in claim 1, wherein at least 80% of alumina is
removed from the spent acid solution.
11. The process as claimed in claim 1, wherein the sulphuric acid is added to the
spent acid solution in the concentration range of 100 to 500 ml/liter of the
spent acid solution.
12.The process as claimed in claim 1, wherein the sulphuric acid is added to the
spent acid preferably in the concentration range of 350-450 ml/L of the spent
acid solution.

ABSTRACT

The present invention relates to a two-stage process for Separation in a coal
leaching process Silicon/Aluminum based Compounds from Spent Hydrochloric
Acid, in which the solution is heated in a first stage to a temperature of around
80 °C to convert the unstable silicon tetrachloride to silicon oxide, and treating
the acid with Sulphuric acid to remove alumina based compounds. The Silicon
chloride is converted to silicon oxide/hydroxide by reacting with oxygen present
in the water, and wherein the regeneration comprises mixing of acid with
Sulphuric acid for alumina separation.