FIELD OF THE INVENTION
The present invention relates to a carbonation reactor system for precipitation of
impurities present in spent alkali. The invention relates to a process for
regeneration of spent alkali and recycling the regenerated alkali in an alkali
leaching of coal process.
BACKGROUND OF THE INVENTION
Carbonation reactor is used for the gas-liquid operations to provide intimate
contact between two fluids in order to permit interphase diffusion of the
constituents. The rate of mass transfer is directly dependent upon the interfacial
surface exposed between the phases, and the nature and degree of dispersion of
one fluid in the other. Carbonation reactor is a sparged mechanically agitated
vessel. In this reactor, the gas phase is dispersed into bubbles in the liquid
through a sparger. The gas-liquid mixture can be mechanically agitated with an
impeller. The operations of this kind are classified as batch, semi batch with
continuous flow of gas and a fixed quantity of liquid, or continuous with flow of
both phases.
Different type of Gas-liquid equipments such as such as bubble columns and
tray towers can be used based upon single stage or multistage operations.
Carbonation reactor or bubble column is used for semi batch operation with
continuous flow of gas and a fixed quantity of liquid. Tray towers

are used for multistage and counter current operations. Basic schematic diagram
of a known carbonation reactor is shown in figure 1. The reactor comprises two
or three inlets (A, B and C) based on the number of feeds and one product outlet
(F). Provisions are arranged on the reactor for insertion of different instruments.
An impeller (D) stirs the reagents to ensure a proper mixing. Carbon dioxide gas
(E) is injected directly into the reactor to disperse the gas into bubbles and the
desired reaction takes place due to interphase diffusion of one phase in other.
After the gas sparging, the resultant slurry is transferred to a solid-liquid
separation system to separate the solids from the liquid. Essentially, carbonation
tank are used to precipitate impurities from spent alkali solution at a temperature
of up to 100°C and pressure of up to 5 kg/cm2. Further, carbonation reactor acts
as a gas-liquid contact equipment.
However, there are a number of disadvantages with currently adopted
carbonation reactors. The carbonation reactors are of open type, and consists of
turbine agitator blades. The gas dispersed in the liquid is not mixed properly due
to vortex formation, in turn channeling at the impeller shaft through which
sparged gas escapes.
Further, the shape of the bottom dish end of the reactors is either flat or ellipsoid
which allows generation of dead zones which ultimately leads to improper mixing
of reactants. The diameter of the sparger pipe is of uniform cross section which
in turn leads to a non-uniform distribution of gas bubbles at the bottom of the
tank.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved carbonation
reactor system for precipitation of impurities from a spent acid solution which
eliminates prior art disadvantages of improper mixing, vortex formation and
unreacted gas escape from the vessel.
Another object of the invention is to propose an improved carbonation reactor
system that enables regeneration of a spent alkali solution in a coal leaching
process and subsequently, recycling of the spent alkali solution.
A further object of the invention is to propose a process for refining spent alkali
in an improved carbonation reactor system to provide sufficient residence time
for gaseous material to react with liquid substance and maximize gas utilization.
Yet another object of the invention is to develop an economical coal leaching
process of reducing the ash content.
SUMMARY OF THE INVENTION
Accordingly, there is provided an improved carbonation reactor system at pilot
scale for precipitation of impurities present in a spent alkali solution and
regenerating the spent alkali for recycling in a coal leaching process. The current
pilot scale system is capable of handling and treating spent alkali in the quantity

of up to 1000 liters. The carbonation reactor system as per the current invention
comprises a metal vessel, at least one inlet for ingress of the spent alkali and
carbon dioxide gas, at least one valve means disposed on each feed line to
regulate the rate of ingress of the spent alkali and the carbon dioxide gas into
the reactor vessel; a heating jacket provided at an external upper location of the
vessel connected to heat or cool the reactants to a reaction temperature, an
agitator disposed inside the vessel for stirring the reactants to ensure a
homogeneous mixing of the reagents; and a pump for discharge of slurry on a
filter after completion of reactions, and recycling back the condensate inside the
vessel through an outlet line. The reactor system further comprises a plurality of
Rota meters, multiple pressure transmitters, flow transmitters, a number of
temperature transmitters, a plurality of solenoid valves, including a number of
manually operated valves to monitor and control the operation of the
carbonation reactor.
Further, there is provided a process for regeneration of spent alkali and recycling
the regenerated alkali in an alkali leaching of coal process. Regeneration of spent
alkali removes silica and alumina-bearing compounds.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1- shows a schematic diagram of a prior art carbonation reactor assembly.
Fig. 2- shows a diagram of an improved carbonation reactor assembly at pilot
scale according to the invention.
Fig. 3- shows a diagram of sparger pipe in improved carbonation reactor
according to the invention.
Fig. 4- shows a process flow sheet at pilot scale for precipitation process
according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
An improved carbonation reactor of the invention (as shown in fig 2) gives
maximum gas utilization and proper mixing for making homogeneous slurry. The
current pilot scale system is capable of handling and treating spent alkali in the
quantity of up to 1000 liters.
The inventive carbonation reactor (2) has twelve nozzles (Nl to N12).The
functions of each nozzle is given below.
Nl nozzle is a main hole. It is used for maintenance of sparger pipe, shaft, baffle
plates, agitator shaft and other flanges presented inside carbonation reactor.
N2 is used for injecting carbon dioxide gas into the carbonation reactor.

N3 is used for injecting the compressed air to flush the sparger pipe.
N4 nozzle is used for fixing the level indicator switch for measuring the reactor
contents level.
N5 nozzle is used for fixing the thermocouple for measuring the top of the jacket
temperature.
N6 nozzle is used for fixing the thermocouple to measure the reactor contents
temperature.
N7 nozzle is used for releasing the condensate from the carbonation reactor (2)
jacket.
N8 nozzle is used for discharging the slurry
N9 nozzle is used for injecting the steam in carbonation reactor jacket(HJ) for
heating the slurry to desired temperature.
N10 is sampling line and is specially designed for collecting the samples without
having any back mixing or contamination of previous sample slurry.
N11 nozzle is used for fixing the thermocouple for measuring the lower side of
jacket temperature.
N12 nozzle is kept as additional for cooling water circulation to cool the
carbonation reactor contents.
BF is a body flange which can be used for major maintenance work inside the
reactor.

LS is a leg support plates
AN is anchor type agitator blades which gives homogeneous mixing and
coalesces the gas bubbles. The anchor type agitator having bottom plates with a
minimum clearance of 20 mm from the bottom dish end.
BP is a baffle plate which enhances the mixing
PP is sparger pipe arrangement inside carbonation reactor
SP is additional plates arranged at bottom of anchor to stir the solids which are
settling at the bottom. It helps the solids in slurry form.
A Heating jacket (HJ) is provided to heat/cool the reactants to the reaction
temperature. Steam is injected through the jacket (HJ) of the carbonation
reactor (2). In the carbonation reactor (2), one or more reagents/components
are introduced into a tank of the reactor (2) equipped with an agitator (AA). The
agitator (AA) stirs the reagents to ensure proper mixing.
Diameter of the improved carbonation reactor = 1150 mm
L/D ratio =1.48
Anchor type impeller sweep =800 mm
Length of baffle plates = 1200 mm
Diameter of sparger pipe ring = 900 mm

Angle between Agitator shaft to conical portion of anchor type impeller = 55°
Height of conical portion of carbonation reactor = 250 mm
The sparger pipe arrangement is shown in fig 3.The sparger pipe is 3/4th of a
circle. The sparger pipe has two sections, the first section pipe diameter is 50
mm and the second section pipe diameter is 40 mm. The second section pipe
end is closed with dummy flange. The sparger pipe has holes facing the bottom
portion of the carbonation reactor. This feature allows the sparger pipe holes not
to be blocked with the solids that are being settled during carbonation process.
The hole diameter is 6 mm and the center to center distance between the holes
is 28 mm. The distance from the any nearby flange to the holes is around 128
mm. The reduction in the sparger pipe diameter increases the gas pressure
which will in turn maintain the same gas velocity in first and remaining sections
of the sparger pipe respectively.
Operating conditions of the process with new equipment
a. Carbonation Reactor (1)
Temperature (Maximum) - 100°C
Pressure (Maximum)-3 kg/cm2
b. Carbonation Reactor Jacket (D)
Temperature (Maximum) -150°C
Pressure (Maximum)- 4.5 kg/cm2

Carbonation reactor jacket has means for feeding the steam into the reactor
jacket and a solenoid valve is placed in the same line for on/off the steam
flow and maintaining the desired temperature inside the carbonation reactor.
Carbonation reactor bottom dish end has drain from the shell and valves for
discharging the condensate from the jacket. Sampling port is provided for
collecting the samples during the process.
The Carbonation reactor comprises the following instruments:
i) Pressure transmitters
ii) Flow transmitters
iii) Temperature transmitters
iv) Solenoid valves
v) Manual valves
Process description:
The process flow sheet of carbonation process (white process) is illustrated in
Fig.3. The total feed of spent alkali solution (800 lit) is heated up to 65 °C in
carbonation tank and then the carbon dioxide gas is purged till the pH of the
solution reaches 9.0. Since the reaction is exothermic, heat evolves during the
gas purging while simultaneously precipitating silica-alumino compound. The CO2
gas pressure in the header line is 2.5 kg/cm2.The carbonated solution
temperature reaches to 75-95 °C at pH value 9.0 and the resultant slurry is
filtered through vacuum Nutsche filter to separate the silica-alumino compound

and sodium carbonate solution. The silica-alumino precipitate is washed with hot
water in 2:1 ratio (water to solids).The filtrate obtained after filtration is
causticized for recovering the hydroxide from sodium carbonate solution. The
silica-alumino compound is obtained as by-product.
Experimental Results:
Operating Conditions:
i) Spent alkali solution as feed= 800 lit
ii) Alkali Concentration = 10%
iii) Reaction temperature = 90°C
iv) Reaction time = 1.0 h


WE CLAIM :
1. An improved carbonation reactor system for precipitation of impurities
present in a spent alkali and regenerate the spent alkali for recycling in a
coal leaching process, the reactor system comprising:
a shaped metal vessel having at least one inlet for ingress of the spent
alkali and carbon dioxide gas, at least one valve means disposed on each
feed line to regulate the rate of ingress of the spent alkali and the carbon
dioxide gas into the metal vessel;
a heating jacket (HJ) provided to cover the metal vessel externally, the
heating jacket (HJ) being operably connected to heat or cool the reactants
to a reaction temperature;
an anchor type agitator (AA) for stirring the reactants to ensure a
homogeneous mixing of the reagents, the anchor type agitator (AA)
having agitator blades (AN);
a bottom sparger with a sparger pipe comprising a first section of pipe
(FL1 to FL2 ) with a diameter of 50 mm and a second section of pipe (FL2
to FE) with a diameter of 40 mm, the sparger pipe being 3/4th of a circle
in shape; and
a pump for discharging slurry on a filter after the completion of reactions
and recycling back the condensate inside the vessel through an outlet line.

2. The system as claimed in claim 1, wherein the heating jacket (HJ) has
means for feeding steam into the heating jacket (HJ) and a solenoid valve
is placed in the heating jacket (HJ) to control the steam flow and maintain
the desired temperature inside the carbonation reactor.
3. The system as claimed in claim 1, wherein the anchor type agitator having
bottom plates with a minimum clearance of 20 mm from the bottom dish
end.
4. The system as claimed in claim 1, wherein hole diameter of the sparger is
6mm and the hole is made at the bottom of the sparger.
5. A process for regenerating a spent alkali and recycling the regenerated
alkali in an alkali based coal leaching process, the process comprising the
steps of :
- heating the spent alkali solution in a carbonation reactor up to a
temperature of about 65°C;

- purging carbon dioxide gas into the heated spent alkali solution till pH-
value of the solution reaches around 9.00, the purging of carbon dioxide
gas evolving heat including precipitating silica-alumino compounds;
- filtering the resultant slurry through a vacuum Nutsche filter to separate
sodium carbonate solution and the silica-alumino compounds;
- washing the silica-alumino precipitate from filtering with hot water in a
ratio of 2:1; and
- causticizing the filtrate from said filtering to recover hydroxide from the
sodium carbonate solution.

6. The process as claimed in claim 5, wherein the pressure of carbon dioxide
gas is maintained about 2.5 kg/cm2.
7. The process as claimed in claim 5, wherein the carbonated solution
reaches to a temperature between 75-95°C at ph value of 9.00.
8. The process as claimed in claim 5, wherein the maximum temperature
and pressure of the reactor is maintained at 100°C and 3 kg/cm2
respectively.

9. The process as claimed in claim 5, wherein the maximum temperature
and pressure of the heating jacket of the reactor is maintained
respectively at 150°C and 4.5 kg/cm2

ABSTRACT

The invention relates to an improved carbonation reactor system for precipitation of impurities present in spent alkali and regenerate the same for recycling for coal leaching process, comprising a shaped metal vessel having at
least one inlet for ingress of the spent alkali and carbon dioxide gas, at least one each valve means disposed on each feed line to regulate the rate of ingress of the spent alkali and the carbon dioxide gas into the reactor vessel; a heating jacket provided at an external upper location of the vessel being operably connected to heat or cool the reactants to a reaction temperature, the heating of the reactants allow injecting steam or a thermic fluid into the vessel; an impeller disposed inside the vessel for stirring the reactants to ensure a homogeneous mixing of the reagents; and a pump for discharge of slurry on a filter after
completion of reactions and recycling back the condensate inside the vessel through an outlet line.