FIELD OF THE INVENTION
The present invention relates to an improved pressure reactor assembly for
chemical leaching of minerals at high temperature. The invention further relates
to a process for leaching of coal using alkali to separate mineral matter at
elevated pressure and temperature.
BACKGROUND OF THE INVENTION
Chemical reactors constitute metal vessels to conduct chemical reactions, with
the technical concept of maximizing net present value for any given reaction,
including the highest reaction efficiency towards producing the highest yield of
product at optimum cost. Normal operating expenses of the process include
energy input, energy removal, raw material costs, and labor. Energy changes can
come in the form of heating or cooling, pumping to increase pressure, frictional
pressure loss (such as pressure drop across a 90° elbow or an orifice plate), and
agitation. Reactors are basically divided into three different types such as batch
reactor, plug flow reactors and continuous stirred tank reactors based on the
residence time and materials flow. Batch reactors are used for high residence
time reactions and temperature and pressure sensitive reactions. For pressure
reactions basically autoclaves are being used. Basic schematic diagram of a

batch pressure reactor is shown in Fig. 1. A reactor comprises one or two inlets
(A and B) based on the number of feeds and one product outlet (F). Provisions
are arranged on the reactor for insertion of different instruments. A heating
jacket is usually provided to heat/cool the reactants to reaction temperature.
Steam or thermic fluid (D) is injected through the jacket of the reactor. In a
reactor, one or more reagents/components are introduced into a tank having an
impeller (C ) while the reactor product is taken out. The impeller (C ) stirs the
reagents to ensure a proper mixing. Compressed air (E) is injected directly into
the reactor to pressurize the reactants to desired pressure where the reaction
kinetics enhances the process. After the treatment, the reactants are cooled to
reduce the pressure and then discharged through the outlet line (F) using a
pump or pressure. For cooling the reactants, cooling water is used, which passes
through the same jacket where steam injection takes place.
Basic purpose of the chemical reactors constitute the following:-
a. Leaching of minerals at elevated pressures of up to 10 kg/cm2,
b. Using the reactor as a digester
Leaching is a process of removal of a soluble fraction, in the form of a solution,
from an insoluble, permeable solid with which it is associated. Thus, the basic

purpose of a chemical reactor is leaching of coal using alkali to separate mineral
matter or ash at an elevated pressure of about 10 kg/cm2 and temperature of up
to 180°C. Nearly 0.5 to 1.0 tonne of coal can be treated per batch using a known
pressure reactor of given size under controlled condition. This equipment can
however, also be used for other minerals such as iron ore.
A process flow sheet for a known chemical leaching process for coal is shown in
Fig 2.
Step-1: Initially fine coal powder (A) having required size fraction is fed to a coal
slurry preparation tank (1). Coal powder is mixed with water (B) using an
agitator. After preparing a homogenous coal slurry, the coal slurry (C) is
transferred to the pressure reactor. Alkali lye (D) is added to the coal slurry and
mixed with the agitator to form a homogeneous mixer of the alkalized coal
slurry. Alkalized coal slurry is heated to a temperature less than the boiling point
of the slurry by using steam (E) injected through the reactor jacket at
atmospheric pressure and above the boiling point with pressure more than the
vapour pressure of the slurry (example: at 120°C, the vapour pressure of the
solution is 3kg/cm2 and the external pressure should be more than 3kg/cm2 to
suppress the boiling).

Step-2: In order to maintain the pressure above the vapour pressure of the
solution, compressed air (F) is injected directly into the reactor. The excess
vapour generated from the reactor then enters into a heat exchanger (2) for
getting cooled and condensed. Cooling water (G) is injected from tube side of
the heat exchanger. After the reaction is over, the reactants are cooled by
passing cooling water through the reactor jacket. After the cooling, the treated
coal slurry (H) is discharged to the filter for solid liquid separation. In the filter,
the treated coal slurry is filtered to collect the coal cake (J), and the filtrate (I).
Disadvantages of prior art pressure reactors
a. If the vapour pressure of the alkalized coal slurry is more than the
external pressure, vapour generation starts and the vapour flows into the
heat exchanger. As the coal particles are fine, particles entrain with the
vapour, which causes choking and scaling of the heat exchanger tubes.
b. In coal leaching process, only the inorganic solid fraction is separated with
the help of inorganic solvent. Mixing of the alkalized coal slurry gives
better leaching performance because of the improvement in the reaction
kinetics. Coal particles have a tendency to settle down in absence of
mixing. In auto claves, there is however, no possibility of mixing as it is a
closed system.

OBJECTS OF THE INVENTION
It is therefore the object of the invention to propose an improved pressure
reactor assembly for leaching of minerals at elevated pressure and
temperature, which eliminates the disadvantages of choking of the condenser
with entrapped mineral particles.
Another object of the invention is to propose an improved pressure reactor
assembly for leaching of minerals at elevated pressure and temperature,
which uses an alkali solution for improved leaching of the minerals, in
particular coal.
A further object of the invention is to propose a coal leaching process in an
improved pressure reactor assembly to provide higher removal of soluable
fraction from an insoluble permeable solid.
SUMMARY OF THE INVENTION
Accordingly, there is provided in one aspect of the invention, An improved
pressure reactor assembly for high-temperature chemical leaching of
minerals, comprising a shaped metal vessel having at least one inlet for
ingress of a coal slurry and an alkali solution, at least one each valve means

disposed on each feed line to regulate the ingress of the coal slurry and the
alkali solution 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 the reaction temperature which further allows 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; a
compressed air inlet to allow injection of compressed air to pressurize the
reactants so as to maintain the reflux of the condensed vapor; a coolant
water inlet to supply cooled water for condensation of the generated alkali
vapor; a pump to the slurry discharge on a filter after the leaching cooling,
and de-pressurizing, and recycling back the condensate inside the vessel
through an outlet line; and characterized in that a flash drum having at least
one baffle plate is interposed between the reactor vessel and the heat
exchanger and connected to control choking of the heat exchanger by the
fine mineral particles associated with the alkali vapor produced through the
reaction between the reagents at a temperature above the boiling point of
the homogeneous mixture.

In a second aspect of the invention, there is provided the assembly as claimed in
claim 1, wherein a plurality of temperature sensors, pressure transmitters, flow
transmitters, temperature transmitters, solenoid valves, pneumatic control
valves, and manually operated valves are provided to monitor and control the
operation of the pressure reactor assembly.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1- shows a schematic diagram of a prior art pressure reactor assembly.
Fig. 2- shows a prior-art process flow sheet for coal leaching process
Fig. 3- shows a schematic diagram of an improved pressure reactor assembly
according to the invention.
Fig. 4- shows a process flow sheet for coal leaching according to the present
invention.
Fig. 5- shows a process and instrumentation diagram for the pressure reactor
assembly of the invention.
Detail description of the invention.
An improved pressure reactor assembly of the invention (as shown in fig-3)
enables treatment of slurry having fine powder at high pressure. Fig. 4 shows a

schematic diagram of an improved pressure reactor assembly with a flash drum
according to the invention.
The inventive pressure reactor (1) has one inlet from which two streams of the
reactants for example, coal slurry (A) and alkali solution (B) are fed to the
reactor (1). This reduces the number of openings to the reactor (1). Two number
of valves (V1,V2) are used to switch on/off the two streams (A,B). A Heating
jacket (HJ) is provided to heat/cool the reactants to the reaction temperature.
Steam or thermic fluid (D) is injected through the jacket (HJ) of the reactor (1).
In the reactor (1), one or more reagents/components are introduced into a tank
of the reactor (1) equipped with an impeller (AA). The impeller (AA) stirs the
reagents to ensure proper mixing.
It is known that if the reaction temperature is above the boiling point
temperature of the solution, the vapour pressure generates and the vapours are
allowed to pass through a heat exchanger (3) for condensation. In order to
control the choking of the heat exchanger (3) with the entrapped coal particles
associated with alkali vapours, a flash drum (2) with umbrella-shaped baffle
plates (BP) is incorporated in between the reactor (1) and the heat exchanger
(3). The flash drum (2) is provided with the baffle plates (BP) with a mesh to

capture the entrained coal particles so that, there is no particle passage to the
heat exchanger (3).
As the pressure inside the heat exchanger (3) is less than that of the reactor (1),
it stops the flow of the condensed vapour to the reactor (1). And hence,
Compressed air (E) is injected directly into the flash drum (1) to pressurize the
reactants to required pressure as well as to maintain the reflux (CSR) of the
condensed vapour. Coolant water (G) is used for condensation of the alkaline
vapours. After the treatment, the reactants are cooled and the pressure inside
the reactor (1) is gradually reduced and then discharged through the outlet line
(H) using a pump (P) or pressure. For cooling the reactants, cooling water is
used, which passes through the same jacket (D) where the steam injection takes
place.
Operating conditions of the process with new reactor:
a. Reactor (1)
Temperature (Maximum)- 180°C
Pressure (Maximum)-15 kg/cm2
b. Reactor Jacket (P)
Temperature (Maximum) -180°C
Pressure (Maximum)- 20 kg/cm2

c. Condenser (3) and Flash Drum (2)
Temperature -180°C
Pressure -15 kg/cm2
As shown in fig-4, the pressure reactor assembly consists of a reactor (1), a
condenser (3) and a flash drum (4). Since it is particularly meant for leaching
of coal using alkali (NaOH) solution, the material of construction (MOC) is SS-
316. However, MOC may vary with the mineral being treated, solvent type
being used and operating conditions being employed for the process. In
general, the reactor description has been given for chemical leaching of coal
using alkali for removing mineral matter/ash. The reactor top dish end
generally has radar type level transmitter, five point thermo well for
measuring the temperature along the length of reactor cylindrical portion, on-
off gate valve in feed line to allow or stop the flow. According to the
invention, although not shown , a thermo-siphon arrangement for cooling the
double mechanical seal of the agitator assembly (AA), pressure transmitter
(PTS) and temperature transmitter (TTS) assembly for monitoring the
temperature and pressure at various points of the equipments including an
orifice plate arrangement in the compressed air line are provided for

measuring the air flow rate and volume of air used in the system. Control
valve as known, has been fixed in the compressed air line for regulating the
flow depending on the requirement of the reactor. Vent valve for releasing
the air at condenser and safety relief valve at flash drum are also provided as
known in the art to release the pressure in case of excess air pressure inside
the reactor.
Reactor jacket has means for feeding the steam into the reactor jacket and a
control valve is placed in the same line for regulating the steam flow and
maintaining the desired temperature inside the reactor. Separate pipe lines
are used in circulating the coolant to the jacket side for cooling the material
present in reactor during the cooling cycle.
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 pressure reactor comprises the following instruments:
i) Radar level sensor
ii) Pressure transmitters
iii) Flow transmitters
iv) Variable frequency drive
v) Temperature transmitters

vi) Solenoid valves
vii) Pneumatic control valves
viii) Manual valves
Process description:
The coal/mineral slurry of desired dilution is supplied to the reactor (1) as a feed.
The pressure reactor (1) has at least three feed inlet lines into the reactor (1).
Coal slurry feed line is provided with at least three solenoid valves followed by
one each flow transmitter and control valve. Make-up water is added to the
reactor (1) as per the process requirement by triggering the solenoid valve. Alkali
and water flow rates are regulated by the control valve.
The reactor (1) is provided with a jacket (HJ) for heating and cooling purposes.
There are at least five temperature transmitters mounted inside the reactor (1)
to monitor and record the reactor temperatures at different points. The reactor
(1) is also provided with a pressure transmitter to monitor the reactor pressure.
Variable frequency drive is used to change the stirrer (AA) speed as per the
process requirement. Compressed air is used for pressurizing the reactor (1).
The compressed air feed line consists of a pressure transmitter, a flow
transmitter followed by a control valve and a temperature transmitter.

Compressed air flow rate is regulated by a control valve. Compressed air is
introduced at top of the flash drum (2) which is connected to the reactor (1).
The flash drum (2) controls the fine particle carrying over during process
operation. The vapour devoid of any fine particles which comes out of the flash
drum (2) then enters into the condenser (3) . In the condenser (3), the latent
heat of the vapour is removed by circulating cooling water in tube bundles. The
cooling water line is also provided with flow transmitter and temperature
transmitter.
The condensate outlet temperature is measured by a mounted temperature
transmitter. The condensate is refluxed back to the reactor (1). After nearly 4-5
hours of treatment (which depends on the mineral to be treated), the mineral
slurry is discharged to a filter (4) for separating the solids from the solvent. If
the level in the reactor reaches (during discharge time) low-low alarm limit, then
the pump (P) is switched off. Two valves (solenoid and manual valve) are
provided for sampling. Samples may be collected at regular intervals for analysis.
a. If the vapour pressure of the alkalized coal slurry is more than the
external pressure, vapour generation starts and the vapour flow into the
heat exchanger (3). As coal particles are fine particles entrain with the
vapour, which cause choking and scaling of tubes of the condenser.

(3) According to the invention, (Fig. 3), the Flash drum (2) is provided to
control the fine particle carry over during process operation. The reactor
vapour line is connected to the flash drum (2) followed by the condenser (3).
The vapors usually carry fine particles along with them because of high
pressure inside the reactor (1). The flash drum (2) has an umbrella shaped
baffle (BF). The fine particles get separated from the vapour after contact
with the baffles (BF) inside the flash drum (2) and get settled at bottom of
the drum (2). The compressed air directs the settled particles back to the
reactor (1).
b. In coal leaching process, only the inorganic solid fraction is separated with
the help of inorganic solvent. As opposed to the known coal leaching
procees, mixing of an alkalized coal slurry gives better leaching
performance because of the improvement in the reaction kinetics. Coal
particles have a tendency to settle down in absence of mixing. According
to the invention, this settling down phenomenon is avoided because the
inventive reactor operates as a closed system. The agitator has been
provided to avoid the settling of the particles and to improve the reaction
kinetics of the leaching process. A double mechanical seal for the agitator
assembly is provided to handle the pressure inside the reactor (Fig 3).

Thermosiphon arrangement provided allows cooling of the double mechanical
seal of the agitator assembly.
Experimental Results:
Operating Conditions:
i) Feed coal = 400 kg of West Bokaro Clean coal
ii) Feed coal size = -0.5 mm
iii) Alkali Concentration = 10%
iv) Coal to Alkali Reagent ratio= 1:10
v) Reaction temperature = 120°C
vi) Operating pressure = 5.0 bar
vii) Reaction time = 2.5 h


We Claim:
1. An improved pressure reactor assembly for high-temperature chemical
leaching of minerals, comprising:-
- a shaped metal vessel having at least one inlet for ingress of a coal slurry
and an alkali solution, at least one each valve means disposed on each
feed line to regulate the ingress of the coal slurry and the alkali solution
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 the reaction
temperature which further allows 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;
- a compressed air inlet to allow injection of compressed air to pressurize
the reactants so as to maintain the reflux of the condensed vapor;
- a coolant water inlet to supply cooled water for condensation of the
generated alkali vapor;
- a pump to the slurry discharge on a filter after the leaching cooling, and
de-pressurizing, and recycling back the condensate inside the vessel
through an outlet line; and characterized in that a flash drum having at

least one baffle plate is interposed between the reactor vessel and the heat
exchanger and connected to control choking of the condenser by the fine
mineral particles associated with the alkali vapor produced through the
reaction between the reagents at a temperature above the boiling point of
the homogeneous mixture.
2. The assembly as claimed in claim 1, wherein a plurality of temperature
sensors, pressure transmitters, flow transmitters, temperature
transmitters, solenoid valves, pneumatic control valves, and manually
operated valves are provided to monitor and control the operation of the
pressure reactor assembly.
3. The assembly as claimed in claim 1, comprising a variable drive to
operate the impeller for proper mixing of the reagents inside the reactor
vessel.
4. The assembly as claimed in claim 1, wherein the baffle plates of the flash
drum is provided with at least one mesh to capture the entrained coal
particles.

5. A process for high temperature chemical leaching of minerals in an
improved pressure reactor assembly as claimed in one of claims 1 to 4,
the process comprising the steps of:-
- providing mineral slurry including mineral and alkali solution at desired
dilution through feed lines into the reactor's vessel as a feed, each feed
line having a solenoid valve including a flow transmitter and a control
valve;
- adding make-up water into the reactor by triggering the solenoid valve;
- stirring the solution by the impeller drive inside the reactor based on the
data captured from the temperature/pressure/flow transmitters
representing temperature and pressure of the solution, the reactor
assembly being provided with a jacket for heating and cooling of the
reagents;
- injecting compressed air at a regulated flow at top of the flash drum
connecting the reactor
- controlling the entrained mineral particles in the generated vapour by the
flash drum, and allowing the vapor to enter into the heat-exchanger;
- circulating cooling water through the tubes of the heat exchanger to
remove the latent heat from the vapor;

- after leaching, operation is discharging the mineral slurry to a filter for
separating the solid from the solvent by operating the pump; and
- refluxing the condensate back to the reactor.