FIELD OF THE INVENTION
The present invention relates to an improved agitated pressure Nutsche filter
(ANF) system for separation of solids after leaching of minerals using viscous
solvent. The invention further relates to a process for separation of solids after
leaching of minerals in an agitated pressure Nutsche filter (ANF) System.
BACKGROUND OF THE INVENTION
Filtration is a process of separation of solids from liquid and vice versa. Basically
the known filtration system are of two major types segregated based on the
pressure and vacuum applications i.e. pressure filters and vacuum filters.
Pressure filtration is preferred than the vacuum filtration due to several reasons
for example, a low moisture content of the product cake, capability to remove
residual contaminants to the minimum levels, formation of clear filtrate solution,
provision for heating of the cake. Filter bodies and internals of a filtration system
can be constructed from a wide variety of materials and wide level of automation
can be provided based on the requirement. Vacuum filters are however, useful
for application when functions for example, continuous filtration, intensive
soluble recovery in counter-current washing, relatively clean filtrates, and
convenient access to the cake for sampling, easy control of operating parameters
such as cake thickness or wash ratios, are preferred.
The Nutsche Filter is an industrial version of the well known laboratory scale
Buchner Funnel except that it is configured to act as either a vacuum filter or a
pressure filter. The majority of the Nutsche filter operates, usually under
pressure. Prior art Nutsche Filters are constructed to perform a multitude of
tasks including reaction, filtration, cake washing and thermal drying on a single

unit. As such these are very sophisticated machines with tight process control on
parameters such as pressure, temperature and pH. Nutsche Filters are well
suited for handling flammable, toxic, corrosive and odour-noxious materials since
they are autoclaved and designed for use in hazardous and ex-proof
environments when an extremely safe operation is required.
The Nutsche filter in its full configuration consists of the following major
components:
• A vessel;
• A filter floor and cloth, woven mesh screen or sintered metal plate
medium; and
• Re-slurry arms for cake washing and smoothing.
A schematic diagram of a known agitated pressure nustche filter in a reactor is
shown in figure 1. Slurry is fed to the filter through feel valve (A). After feeding,
the reactor is pressurized using compressed air fed through an inlet valve (C).
Pressure in the reactor is always measured using Pressure gauge (D). During the
filtration stage, the slurry suspension (G) is filtered and the cake (H) is formed
on the filter cloth where as the filtrate (I) is collected from the bottom. S shaped
agitator blade (F) is used in the reactor for mixing of the cake during the
washing stage. After washing, the air is discharged through air vent (B) of the
reactor. After washing, cake is collected through an outlet (J).
The basic purpose of a nustche filter system is separation/filtration of solids from
the solvent after leaching of minerals, washing the filter cake for residual solvent
removal and recovery, drying of filtered cake to reduce the moisture content and
final cake discharge. Pressure is applied in the system using compressed air

during the process for faster and effective filtration and a vacuum is applied for
reducing the moisture content of the final filter cake.
Coal leaching and extraction in a chemical reactor involves handling of a huge
quantity of liquid. Particle size is a major parameter that significantly changes the
performance of leaching and extraction methods and hence the particle size
should be less. Coal leaching process involves leaching the mineral with solvents
(like alkali and acid) followed by a solid-liquid separation in a filter system of the
liquids from coal/minerals. Coal extraction also involves the extraction and solid-
liquid separation.
A prior art process flow sheet is shown in figure 2. As shown in figures 1 and 2 :-
Step 1: The reactor has one inlet from which two streams of the reactants like
coal slurry (A) and alkali solution (B) are fed to the Reactor-l(l). Reactants are
treated at high temperature and pressure using steam (C) injected through a
jacket of the reactor and compressed air (D). 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 flash drum for
capturing solid particles, and the vapors are allowed to pass to a heat exchanger
for condensation. Hence, the Compressed air (D) is injected directly into the
flash drum to pressurize the reactants as well as to maintain the reflux of the
condensed vapour. Coolant water (E) is used for condensation of the alkaline
vapours. After the treatment, the reactants are cooled to reduce the pressure
and then discharged through an outlet line (F) using a pump or pressure.

Step-2 : After alkali and acid leaching, the leached clean coal is separated from
the alkaline/acidic solution and washed to remove the residual alkali/acid.
Aqueous alkali solutions are usually very viscous and the viscosity increases with
increase in alkali concentration. Alkali solution forms a layer or coating on the
coal particles due to which, the filtration rate becomes very slow. During acid
leaching, fines generation is more due to the particle breakage/disintegration
after removing the ash bearing components. Filtration is again a major challenge
due to the reduction in the particle size and washing of the residual acid.
Reduction in particle size also contributes to the increase in moisture content of
the product coal cake. The agitated pressure nutsche filter assembly is used for
the above purpose. In the present case, the filtration operation has been
described in details for separation of coal after leaching with alkali (NaOH).
However, this assembly can also be used for other minerals as well. Nearly 6000
litres of coal slurry containing up to 14% solids in 5-45% NaOH solution can be
filtered with a 1-10 micron screen at a rate of 6 m3/h. Treated coal slurry (F) is
fed to the filter through feed inlet. Compressed air is injected for pressure
filtration. The percentage of moisture is usually less than 15% in the final
product coal cake. The filtrate (G) solution is transferred to a storage tank. After
filtration, washing of the coal cake can be done with water so that, the
concentration of the alkali in the cake is less than 0,1%. Facility has been
provided for recirculation of the wash water.
Step-3 ; After completion of washing, based on the process requirement, either
the coal/mineral cake can be collected as product (H) or the cake can also be
reslurrified with water/alkali (I) for second round of treatment in Mixing tank (3).

Step-4 : After reslurrification of the cake, the slurry (J) is sent to Reactor-2 (4)
for further treatment. Reagents (K) is added to the reactor for treatment. After
treatment, the treated slurry (L) is drained for further processing.
However, the technical disadvantages of prior art nutsche filter in a chemical
reactor are that provisions for re-slurrification of the coal as well as for the slurry
transportation for further operations including the second stage reaction with
solvent/reactants, are not available.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose an improved
agitated pressure Nutsche filter (ANF) system for separation of solids after
leaching of minerals using viscous solvent, which enables re-slurrification of
solids including transportation of slurry for subsequent processing.
Another object of the present invention is to propose an improved agitated
pressure Nutsche filter (ANF) system for separation of solids after leaching of
minerals using viscous solvent, which allows a second stage reaction of the slurry
with solvents.
A further object of the present invention is to propose a process for separation of
solids after leaching of minerals in an agitated pressure Nutsche filter (ANF)
System.

SUMMARY OF THE INVENTION
Accordingly, there is provided an improved agitated pressure Nutsche filter (ANF)
system for separation of solids after leaching of minerals using viscous solvent,
comprising :a drive system consisting of an agitator motor, a gearbox, tools for
supporting lantern, stuffing box/shaft and blades; a hydraulic power pack with
following control means :- Hydraulic motor start/stop push buttons means for
opening/closing of discharge valve, and direction control valve lever, leaver for
agitator up/down DC valve, an anti-clockwise rotation push button, adjustment
means for lantern stool down pressure, a controller for lantern stool down
speed, a plug type discharge valve along with sealing ring for wet cake product
discharge; a polypropylene (PP) filter cloth fitted in the filter; gland ropes; one
each feed inlet, air inlet, and air vent; and a cake discharge outlet, a slurry
(reslurrified slurry) discharge outlet, and a filter outlet. Agitated Pressure
Nutsche filter (ANF) of the invention is configured to filter large amounts of slurry
under pressure or vacuum and to dry wet cake under vacuum only. It ensures
clean and fast filtration. Thorough washing of cake is done in a controlled
quantity of washing cycle and effluent discharge. The results of filtration are
better than centrifuges. The discharge of cake is automatic. The closed operation
ensures odourless and contamination free working conditions. This unique ANF
saves costly solvents, power, labour, and floor space as well as time. It is well
suited for filtration of even highly viscous liquids if accompanied with detachable
bottom bed type arrangement. It provides economical and pollution free working
environment.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Shows a prior art nutsche filter system.
Figure 2 - Shows a prior art process flow-sheet for coal leaching.
Figure 3 - Shows an improved Nutsche filter system according to the invention.
Figure 4 - Shows an improved process flow-sheet for coal leaching according to
the invention.
DETAIL DESCRIPTION OF THE INVENTION
An agitated pressure Nutsche filter assembly generally consists of the following
components :
- a drive system consisting of an agitator motor, a gearbox, tools for
supporting lantern, stuffing box/shaft and blades;
- a hydraulic power pack with following control means :-

(a) Hydraulic motor start/stop push buttons,
(b) means for opening/closing of discharge valve, and direction control valve
lever,
(c) leaver for agitator up/down DC valve,
(d) an anti-clockwise rotation push button,
(e) adjustment means for lantern stool down pressure,
(f) a controller for lantern stool down speed,

- a plug type discharge valve along with sealing ring for wet cake product
discharge;
- a polypropylene (PP) filter cloth fitted in the filter;
- gland ropes;
- one each feed inlet, air inlet, and air vent; and

- a cake discharge outlet, a slurry (reslurrified slurry) discharge outlet, and a
filter outlet.
As shown in figure 3, the agitated pressure Nustche filter of the invention is
improved by providing a discharge nozzle (K) at the casing of the filter shell
above the filter bed level for transportation of slurry to the second stage
operation. The nozzle is connected to a slurry pump to enhance the flow rate of
the slurry. The nozzle and pipeline are made of metal capable to avoid the
corrosion of the pipelines. Besides, the filter has an arrangement of detaching
the bottom bed from the main unit which facilitates faster cleaning/changing of
filter cloths. Because of this improvement, the filter can be simultaneously used
for different types of minerals, solvents and particle sizes without affecting the
filtration rate.
Operation of the Nutsche filter of the invention
The operation of the device is quite simple and can be monitored by one person
who is familiar with the filtration process. All controls are on a panel board. The
filtration process can be viewed through a sight glass.
A clockwise motion of the agitator cut and discharge the cake product, and an
anticlockwise motion press and the smoothen the product. The maximum wet
cake height should not exceed the top of the discharge opening. The discharge
valve can be operated hydraulically or manually. Prior to filtration, rubber ring
and its seating groove of the discharge valve is cleaned to ensure leak proof
operation. After filtration is over, the agitator is kept in the top most position.

Charging and filtration
The slurry charging valve (A) of the chemical reactor is opened and the slurry is
transferred to the vessel by gravity, or pumping or air (inert gas) pressure (C).
Filtrate starts passing through the filter cloth which can be viewed through the
sight glass. To enhance filtration rate and to keep the filter media clean due to
initial sedimentation, the agitator (F) is brought down closest to the filter media
and is kept rotating in clockwise "stir" direction. For products having very high
setting rate of solid particles, the stirrer is to be taken upward gradually to keep
it clean for immersion into thick slurry/cake and cake height can be built up in
successive steps.
Once filtration has slowed down or cake formation is completed, air (gas)
pressure (or vacuum at the drain vacuum receiver when designed to filter under
vacuum), is applied. Thus filtration rate disallows the pressure to hold on the
cake resulting slowing down of filtration rate and also wastage of compressed air
(gas). In order to eliminate such phenomenon, squeezing operation is performed
which is done as follows :
The agitator is started in anti clockwise direction and brought down on the semi
solid cake to penetrate it by 2-3 mm. The rotating rounded edges of the agitator
blades smoothen and fill the cracks in the cake. Thus air (gas) pressure holds on
the cake and dewatering continues for little longer period.
During this operation the agitator is brought down, as required, and the cake
level falls down in the vessel because of dewatering. The filtration is allowed to
proceed to the desired level.

The maximum cake (H) height should not exceed the top of the discharge
opening. At this juncture if the cake built on filter plate is less, some more slurry
may be charged. For this, the agitator is taken up, air(gas) pressure (vacuum) is
released, slurry is recharged and the filtration is carried till the cake is formed
and filtration has reached to the desired level.
Filtration is said to reach the desired level, when the filtrate coming out of the
outlet(I) slows down considerably or in some cases the cake particles loosen
from the cake surface and starts rotating along with the rotating blades.
Washing of the coal cake
The cake is made free of unwanted impurities by washing it with a suitable
liquid. In this machine, washing is very effective and quantity of wash liquid can
be controlled. The washing can be done.
(i) By displacing the impurities and
(ii) By dissolving the impurities.
i) Displacing the Impurities
Wash liquid is allowed to enter into the vessel, and accumulate on the wet cake.
Now, the air pressure is applied to the vessel so that the wash liquid passes
through the cake and gets discharged through a liquor discharge valve. While
passing through the wet cake, the impurities are displaced.

ii) Dissolving the impurities
The agitator in its top most position is stopped, air (gas) pressure (vacuum) is
released, and was liquid spray pipe is introduced. A pre-decided quantity of wash
liquid is allowed to enter into the filter. The agitator is rotated in cut direction
(clockwise) and slowly lowered to cut through the cake. The rotating agitator
reslurrifies the cake with wash liquid, resulting in highly efficient washing of the
mass by through mixing, dissolving and/or extracting impurities. The agitator
upward movement and filtration continues. Pressure or vacuum is applied again
as explained earlier for demoisturising wet cake to an optimum level.
After washing is completed, the cake is formed in the filtration operation.
Washing may be done twice or thrice as desired. The air (gas) pressure
(vacuum) at the end of each cycle is released.
Blending:
The unit is capable of blending/homogenizing the wet cake. After completion of
the washing cycle and prior to discharge of the wet cake or drying (as the case
may be), the blending operation can be carried out. For homogenizing the wet
cake, the agitator is rotated in clockwise direction (cut direction) and slowly
lowered to its lowest position. The agitator rotating in clockwise direction peels
and cuts the wet cake in to small pieces. The agitator is kept rotating in the
same direction for about 10-15 minutes at the lowest position. This results in a
homogenous blend of the wet cake. If desired, additional air can be passed
through the cake to reduce the moisture content. As soon as the homogeneous
cake is formed, the agitator is brought up to the top most level and then
stopped.

Experimental implementation of the inventive process with coal and
alkali
The modified flow sheet of the invention is shown in figure 4, which interalia
illustrates the filtering process.
Step 1: The modified reactor has one inlet from which two streams of the
reactants like coal slurry (A) and alkali solution (B) are fed to the Reactor-1 (1).
Reactants are treated at high temperature and pressure conditions using steam
(C) injected through the jacket of the reactor including compressed air (D). 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 the
flash drum for capturing the solid particles, and the vapour transmitted to a heat
exchanger for condensation. And hence, the Compressed air (D) injected directly
into the flash drum to pressurize the reactants to a required pressure as well as
to maintain the reflux of the condensed vapour. Coolant water (E) is used for
condensation of the alkaline vapours. After the treatment, the reactants are
cooled to reduce the pressure and then discharged through outlet line (F) using
a pump or pressure.
Step-2 : After alkali and acid leaching, the leached clean coal is separated from
the alkaline/acidic solution and washed to remove the residual alkali/acid. The
agitated pressure nutsche filter assembly is used for the above purpose. In the
present case, the filtration operation has been described in details for separation
of coal after leaching with alkali (NaOH). Treated coal slurry (F) fed to the filter
through the feed inlet. Compressed air is injected for pressure filtration. The
filtrate (G) solution is transferred to a storage tank. After filtration, washing of
the coal cake can be done with water so that concentration of t he alkali in the
cake is less than 0.1%. Means for recirculation of the wash water is arranged.

Step-3 : After completion of washing, based on the process requirement, either
the coal/mineral cake can be collected as product (H) or the cake can also be
reslurrified with water/alkali (I) for a second round of treatment. After
reslurification of the cake, the slurry (J) is sent to another reactor for further
treatment.
Step-4 : After reslurrification of the cake, the slurry (J) is sent to Reactor-2(3) for
further treatment. Reagents (K) added to the reactor for treatment. After
treatment, the treated slurry (L) is drained out for further processing.
Advantages of the invention
The agitated pressure Nutsche filter (ANF) of the invention allows
separation/filtration of solids from solvent after leaching of minerals, washing of
the filter cake for residual solvent recovery, cake drying to reduce moisture
content, cake discharge reslurrification of the cake for further processing and
transportation of the slurry. Re-slurrification facility also incorporated in the same
filter, so that extra mixing tank is eliminated. Besides, the filter has an
arrangement of detaching the bottom bed from the main unit which facilitates
faster cleaning/changing of filter cloths. Because of this facility, the filter can be
simultaneously used for different types of minerals, solvents and particle sizes
without affecting the filtration rate.

Results and discussion
Case study for filtration of coal slurry after alkali leaching is described herein by
way of example. The system is made of SS-316 for filtration of alkalized coal
slurry. The filter is connected with a receiver.
The slurry from the leaching reactors (where coal and alkali solvent are treated)
is fed to the alkali pressure Nutsche filter. The alkali Nustche filter bottom is
made of polypropylene (PP) filter cloth of 1-10 micron porosity and capable of
withstanding feed temperatures of up to 85 °C. A compressed air line is provided
for pressurizing the system. Compressed air having 3 kg/cm2 pressure is used for
the filtration process. After filtration, as vacuum is applied to further reduce the
moisture content of the coal cake. After completion of filtration, wash water is
added to the filtered cake. An agitator assembly is used for mixing and can be
moved up and down by hydraulic pressure for better mixing of the slurry. After
complete mixing, the slurry is filtered using the compressed air and then is dried
using the vacuum. The wash water is stored in wash water tank for reuse. After
complete washing, the cake is mixed with water or alkali and is drained out to
the desired storage tanks. The cake is reslurrified with water or desired solvent
and sent to the designated plate.

WE CLAIM :
1. An improved agitated pressure Nutsche filter (ANF) system for separation of
solids after leaching of minerals using viscous solvent, comprising :
- a drive system consisting of an agitator motor, a gearbox, tools for
supporting lantern, stuffing box/shaft and blades;
- a hydraulic power pack with following control means :-

(a) Hydraulic motor start/stop push buttons,
(b) means for opening/closing of discharge valve, and direction control valve
lever,
(c) leaver for agitator up/down DC valve,
(d) an anti-clockwise rotation push button,
(e) adjustment means for lantern stool down pressure,
(f) a controller for lantern stool down speed,

- a plug type discharge valve along with sealing ring for wet cake product
discharge;
- a polypropylene (PP) filter cloth fitted in the filter;
- gland ropes;
- one each feed inlet, air inlet, and air vent; and
- a cake discharge outlet, a slurry (reslurrified slurry) discharge outlet, and a
filter outlet,
the improvement is characterized by comprising :
- a discharge nozzle provided at the filter sheet above the filter bed level for
transportation of the slurry to a second-stage operation, the slurry line and
the nozzle is formed of corrosion resistant metal, wherein the nozzle is
operably connected to a pump to enhance the flow-rate; and

- the bottom bed of the system is configured to be detachably attached to
the main body to allow faster cleaning/changing of the filter cloths
including multipurpose use of the system irrespective of types of minerals,
solvents, and particle sizes.
2. A process for separation of solids after leaching of minerals in an agitated
pressure Nutsche filter (ANF) system, comprising the steps of :
- feeding the coal slurry after alkali and acid leaching in a chemical reactor
through an inlet to the filtration system;
- injecting compressed air into the coal slurry for pressure filtration;
- transferring the filtrate solution to a storage tank;
- washing the coal cake after filtration to maintain the concentration of the
alkali in the cake to less than 0.1%;
- collecting the product (cake) after completion of washing step, or
transferring the product for re-slurification with water/alkali for a second
stage treatment;
- transferring the slurry after the second stage of treatment to a second
reactor for further treatment through adding of reagents; and
- draining out the further-treated slurry from the second reactor.

3. The system as claimed in claim 1, wherein the bottom portion of the system
is made of polypropylene filter cloth.
4. The system as claimed in claim 1, wherein an agitator assembly vertically
morable under hydraulic pressure is disposed in the filter system for
homogeneous mixing of the slurry.

5. The system as claimed claim 1, wherein a vacuum is provided to reduce the
moisture content of the coal cake.

ABSTRACT

The invention further relates to an improved agitated pressure Nutsche filter
(ANF) system for separation of solids after leaching of minerals using viscous
solvent, comprising : a drive system consisting of an agitator motor, a
gearbox, tools for supporting lantern, stuffing box/shaft and blades; a
hydraulic power pack with following control means : (a) Hydraulic motor
start/stop push buttons, (b) means for opening/closing of discharge valve,
and direction control valve lever, (c) leaver for agitator up/down DC valve,
(d)an anti-clockwise rotation push button, (e)adjustment means for lantern
stool down pressure, (f) a controller for lantern stool down speed, a plug type
discharge valve along with sealing ring for wet cake product discharge; a
polypropylene (PP) filter cloth fitted in the filter; gland ropes; one each feed
inlet, air inlet, and air vent; and a cake discharge outlet, a slurry (reslurrified
slurry) discharge outlet, and a filter outlet, the improvement is characterized
by comprising : a discharge nozzle provided at the filter sheet above the filter
bed level for transportation of the slurry to a second-stage operation, the
slurry line and the nozzle is formed of corrosion resistant metal, wherein the
nozzle is operably connected to a pump to enhance the flow-rate; and the
bottom bed of the system is configured to be detachably attached to the
main body to allow faster cleaning/changing of the filter cloths including
multipurpose use of the system irrespective of types of minerals, solvents,
and particle sizes.