TITLE:
Process for purification of impurities from spent hydrochloric acid by oxidation
FIELD OF INVENTION:
Most of the mineral processing industries are based upon leaching technology due
to the process efficiency and product quality. Hydrochloric acid is one the mostly
used leaching agent/ solvent because of advantages of using hydrochloric acid
over other acids for leaching of different minerals are the ease of filtration of
slurries, the ease of metals removal and the insolubility of other oxides, which is
present in many minerals. Spent Hydrochloric Acids of various origins may
contain organic and inorganic impurities, which must be removed prior to being
re-utilized.
Chloride system in hydrometallurgy has been used for the treatment and
recovery of precious metals for a number of years. Chloride waste contains many
impurities like silica, alumina, iron, phosphorous and magnesium. Removal of
impurities from the leaching waste is very important for cost effectiveness of the
leaching process and other environmental regulations. Usually diluted
hydrochloric acid was used for leaching processes (concentration ranging from
5% to 20.4% w/w) due to which water consumption is also very. high.

Neutralization process is used in most of the industries, but the water recovery is
not possible by this method. For pickle liquor generated in cold roll milling of steel,
pyro hydrolysis method is used for recovery of iron and hydrochloric acid, which is
high energy intensive method. But most of the leaching processes also contain
impurities like silica, alumina, calcium and magnesium, whose oxidation
temperature is between 150-350 deg.C. Development of oxidation techniques are
of high importance for removal of other impurities than iron.
Coal leaching process involves two stage removal of minerals in which
second stage involves leaching of alkaline minerals and alkali leaching
byproducts. Alkali leaching byproduct is a mixture of silica, alumina and sodium
oxides and the alkaline minerals are iron, calcium, magnesium and phosphorous
bearing compounds. During the acid leaching process, the above said
compounds are dissolved into hydrochloric and by leaching coal. The spent
liquid analysis is shown in Table 1.


Spent hydrochloric acid contains silica, alumina and iron as major impurities and
calcium, phosphorous, manganese as minor impurities. Silicon impurity exists in the
form of silicon tetrachloride and aluminum as aluminum trichloride. Removal of
these compounds is the prime concern for the re-utilization of the hydrochloric
acid.
OBJECTS OF THE INVENTION:
An object of the present invention is to propose a process purification of impurities
from spent hydrochloric acid by oxidation.
Another object of the present invention is to propose a process which achieves
87% of the recovery of the spent hydrochloric acid after the removal of the
impurities.
Further, object of the present invention is to propose a simple process for the
maximum recovery of the spent hydrochloric acid.
Still another object of the present invention is to propose a process for the
recovery of the spent acid and re-utilizing the same.

BRIEF DESCRIPTION OF THE INVENTION:
This invention relates to a process for the recovery of spent hydrochloric acid by
oxidation of the impurities comprising:
preparing a mixture of sulfuric acid and hydrogen peroxide;
adding the said mixture to the spent hydrochloric acid;
cooling the spent hydrochloric acid to room temperature to precipitate the solids;
adding flocculating agent to the spent hydrochloric acid to separate the
precipitated said from the liquid;
subjecting the HCI to the step of decantation.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWING
Figure 1: Flow sheet of the process for purification of impurities from spent
hydrochloric acid by oxidation
DETAILED DESCRIPTION OF THE INVENTION
Indian coals are generally low-sulfur bituminous varieties with unusually high ash
content. Low ash coal is required for blast furnace operation and hence washing
of coals using different methods are in practice. Due to yield constraint, Physical
beneficiation process may not be suitable for improving highly intermixed coals.

In a coking coal washery, the run of mine coal is ground, using primary and
secondary crushers. Fine coal powder (-0.5 mm) is washed, using flotation
technique whereas the fraction -13+0.5 mm size material is washed in dense
media (DM) cyclone. In a typical DM cyclone, middlings are generated as a
byproduct having 40-45% ash. Since physical techniques for coai cleaning are not
very effective in separating the mineral matter from the organic part, chemical
beneficiation methods are being explored. Demineralization by chemical leaching
has been developed to remove a maximum of 70% minerals from Indian coals.
Coal leaching involves two stages of treatment, alkali leaching with dilute caustic
soda for removal of acidic minerals like silica and alumina followed by acid
leaching with hydrochloric acid for removal of alkaline minerals like iron, calcium
and magnesium. After alkali leaching, reagent has been separated from the
reacted coal using filtration. But Sodalite is a gel like material, which is formed as
a byproduct of alkali leaching is partly separated along with the reagent by
filtration. Inorder to remove the alkaline minerals like iron oxides and sodalite,
acid leaching has been conducted at room temperature. Dilute HC1(5% w/w
concentration) at different concentrations used for acid leaching process. In acid
leaching alkaline minerals reacts with hydrochloric acid and forms ferric chloride,
calcium chloride, phosphorous chloride and magnesium chloride. Sodalite
dissociates in to silica and alumina ions and forms silicon tetrachloride and
aluminum tri-chloride. After ieaching operation, spent acid has been separated
from the coal by filtration.

Recovery of spent hydrochloric acid is a process of oxidation of dissolved
minerals. Sulfuric acid (98% w/w concentration) acts as a week oxidizing agent
and the recovery of 20% can be obtained using the sulfuric acid alone. Hydrogen
Peroxide(35% w/w concentration) is a strong oxidizing agent, but the oxidation
rate is very less. A mixture of sulfuric acid and hydrogen peroxide commonly
known as piranha solution is a super strong oxidizing agent which can be used
for oxidation of leaching waste impurities. Up to 87% of the recovery obtained
using piranha solution.
Alkali leaching reactions:
SiO2 + 2NaOH → Na2Si03 + H20
Al2O3 + 2NaOH → 2NaAl02+H20
Na2SiO3+NaAlO2 → Nax(Al2O3)y(SiO2)z
Acid leaching reactions:
Nax(Al2O3)y(SiO2)z+HCl→NaCl + AlCl3+SiCl4+H20
Fe2O3 + 6HCl →2FeCl3+3H2O

Acid regeneration:
SiCl4 +3H2 O →H2-SiO3 +4HCl
2AlC13+ 3H2S04 → Al2 (S04)3 + 6HCl
A mixture of sulfuric acid and hydrogen peroxide is prepared carefully at different
proportions and is mixed with spent liquid. The spent liquid is cooled to room
temperature to precipitate the solids. The precipitated solids form gel like material
which is dispersed in the liquid ad it was observed that, the separation of the solid
and liquid is very poor. Poly acrylamide flocculants (Commercial name FL 340)
made by SNF Polymers Ltd., India is used for separation of the precipitated
solids. A clear separation has been observed and the liquid has been separated by
decantation.
Performance of the oxidation method at different proportion of sulfuric acid was
tested and reported in the Tablc-2 and 3. It was observed that, 400ml sulfuric
acid and 25 ml hydrogen peroxide per liter of spent_acid is the optimum addition
ratio. From Table 2, it was also observed that, the Caustic recovery is very less
using this method.

A maximum of 87% regeneration achieved at optimum addition of piranha
solution. It was also observed that, a temperature of 80-90 deg. C will be
generated by addition of piranha solution due to which the silica precipitation
is improved and due to the addition of sulfuric acid, the aluminum is
precipitated in aluminum sulphate form. Composition of the precipitate is
mentioned in Table 4. Different dosages of flocculants has been tried and is
tabulated in Table 5. It was observed that, 9g/l is the optimum flocculant
addition for maximum separation of precipitated solids.

METHODOLOGY TO BE FOLLOWED FOR ACID RECOVERY BY OXIDATION
PROCESS
Dilute HCI (5%, w/w) was used for acid leaching process of coal. In acid leaching
alkaline minerals reacts with hydrochloric acid and forms ferric chloride, calcium
chloride, phosphorous chloride and magnesium chloride. Sodalite, which is
...produced as a by-product in alkali leaching process dissociates in to silica and
alumina ions and forms silicon tetrachloride and aluminum tri-chloride.

Recovery of spent hydrochloric acid is a process of oxidation of dissolved
minerals. A mixture of sulfuric acid and hydrogen peroxide commonly known as
piranha solution is a strong oxidizing agent which can be used for oxidation of
leaching waste impurities. Up to 87% of the recovery obtained using piranha
solution.
A mixture of sulfuric acid and hydrogen peroxide is prepared carefully at
different proportions and is mixed to spent liquid. The spent liquid is cooled to
room temperature to precipitate the solids. The precipitated solids form gel like
. material which is dispersed in the liquid ad it was observed that, the separation of
the solid and liquid is very poor. Poly acrylamide flocculants (Commercial name
FL 340) made by SNF Polymers Ltd., India is used for separation of the
precipitated solids. A clear separation has been observed and the liquid has been
separated by decantation at 9 g/1 dosage of flocculant.
Performance of the oxidation method at different proportion of sulfuric acid has
been tested and found that, 400ml sulfuric acid and 25 ml hydrogen peroxide per
liter of spent acid is the optimum addition ratio. A maximum of 87% regeneration
achieved at optimum addition of piranha solution.

Major waste in the mineral processing industries is spent chemicals. Recycling of
the spent chemicals is a major cost component in most of the mineral processing
industries. Chemical leaching process used hydrochloric acid to remove alkaline
minerals such as iron, phosphorous, calcium and magnesium along with alkali
leaching by product. The impurities present are silicon, aluminum, iron,
phosphorous, calcium and magnesium containing chlorides. Recovery of the
spent hydrochloric acid (5% w/w concentration) is most important to reduce
process water consumption and operating cost of the process. Oxidation of the
impurities can precipitate the compounds, and will be removed from the solution.
Sulfuric acid (98% w/w concentration) and hydrogen peroxide (35% w/w
concentration) alone acts as oxidizing agents, but the effectiveness of
regeneration is less. Mixture of sulfuric acid and hydrogen peroxide commonly
known as piranha solution, which is strong oxidizing agent used for oxidation of
the impurities and upto 87% of the recovery of hydrochloric acid was achieved.
Precipitation obtained by the oxidation process is a gel like material dispersed in
the solution, is very difficult to separate and hence poly acrylamide flocculants
ere used to get dear separation of the precipitated oxides. Sodium containing
salts could not be removed from the solution.

WE CLAIM:
1. A process for the recovery of spent hydrochloric acid by oxidation of the
impurities comprising:
preparing a mixture of sulfuric acid and hydrogen peroxide;
adding the said mixture to the spent hydrochloric acid;
cooling the spent hydrochloric acid to room temperature to precipitate the
solids;
adding flocculating agent to the spent hydrochloric acid to separate the
precipitated said from the liquid;
subjecting the HCI to the step of decantation.
2. The process as claimed in claim 1, wherein the ratio of spent acid: sulfuric
acid: hydrogen peroxide is 1:0.4:0.025.
3. The process as claimed in claim 1, wherein the said oxidation precipitate
is in the form of a gel and the flocculating agent poly acrylamide is used at
9 g/l dosage rate for the clear separation of the oxide precipitation.

4. The process as claimed in claim 1, wherein the temperature of the spent
hydrochloric acid rises to 80-90°C upon addition of the sulfuric acid and
hydrogen peroxide.