FIELD OF THE INVENTION
The present invention relates to a process for production of low ash clean
coal with improved coking properties from lignite coal.
BACKGROUND OF THE INVENTION
Due to the expansion of Steel industries, demand for coking coal has
increased drastically. Due to the less availability of captive coking coals,
lignite has become more attractive to convert to coking coal and use in coke
making process. Lignite has a lower rank in the coalification process,
meaning this more recent than bituminous and anthracite coals. Lignite has
some special characteristics which stop us to use in iron and steel making
industry: Low net energy value limits commercial potential, high moisture
content increases CO2 emissions, high VM content which increase wall
pressure during coke making process, and Low coking property.
Due to the limited reserves of bituminous coal, some of the properties need
to be improved for upgrading the lignite coals for coke making application.
Lignite coal has high volatile content due to low molecular weight
hydrocarbon chains. Removal of volatile matter improves the concentration of
aliphatic hydrocarbons and aromatic hydrocarbons thus improving coking
potential. Removal of mineral matter from lignite coal is also difficult due to
the presence of humic substance as volatile matter.
Lignite coal is a soft brown coal formed from naturally compressed peat. It is
considered the lowest rank of coal due to its relatively low heat content.
Lignite has a high content of volatile matter which makes it easier to convert
into gas and liquid petroleum products than high-rank coals. Efficient
processes that remove volatile matter and latent moisture locked within the
structure of brown coal will improve the coking property and calorific value to

the same level as black coal. Indian lignite coals are of high mineral content
and due to the presence of volatile matter as humic acid, it is very difficult to
reduce the mineral content present in the lignite coal by conventional physical
separation methods. Due to the limited reserves of bituminous coal, there is a
need to devise a process for upgrading the lignite coals for coke making
application. Removal of the humic substance from lignite coal, the inventors
noticed that the coking properties including the calorific value of the lignite
coal is improved.
OBJECT OF THE INVENTION
It is therefore an object of the invention to propose a process for production
of low ash clean coal with improved coking properties from lignite coal.
SUMMARY OF THE INVENTION
Lignite coals are of low rank, low calorific value coals with high volatile
material, moisture and mineral content. Due to the presence of volatile
matter as humic substance, the coal is spontaneous for combustion, difficult
to separate the low ash coal by physical methods and lower heat values.
Moisture present in the coal also contributes to the above problems related to
the utilization of lignite coal. A process has been established for separation of
lower ash fractions with improved coking properties from lignite coal, which
involves the following steps.
1. Lignite coal of Indian origin is digested in alkali at a temperature of 700C
to extract humic substance.
2. The extracted humic substance is separated from coal and is neutralized
to precipitate humic acid product. 17.30% yield of humic acid product
obtained from this process. The filtrate is the fulvic acid, it can be used for
agricultural application.

3. The residue after removal of extracted humic substance is washed and
low ash coal is separated using density separation at1.4 specific gravity.
Float contains low ash clean coal at 11.58% ash with 61% yield.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWING AND TABLES
Figure-1: Process Flow sheet for low ash clean coal production from lignite
Table 1: Properties of Indian Lignite Coal
Table 2: Proximate analysis of different materials
Table 3: Yield and composition of humic acid and fulvic acid at different alkali
Concentrations
Table 4: Yield and composition of residue at different alkali concentrations
Table 5: Yield and composition of humic acid and fulvic acid at different
treatment temperatures
Table 6: Yield and composition of residue at different operating temperatures
DETAIL DESCRIPTION OF THE INVENTION
Coke is the most important raw material fed into the blast furnace in terms of
its effect on blast furnace operation and hot metal quality. Introduction of
high quality coke to a blast furnace will result in lower coke rate, higher
productivity and lower hot metal cost. Metallurgical coke is a macro-porous
carbon material of high strength produced by carbonization of coals of
specific rank or of coal blends at temperatures up to 1400 K. The quality and
properties of the resulting coke is inherited from the selected coals, as well as
how they are handled and carbonized in coke plant operations.
In terms of coal properties, coke quality is largely influenced by coal rank,
composition (reactive and inert macerals and minerals), and an inherent
ability when heated to soften, become plastic, and re-solidify into a coherent
mass. Bituminous class coals of high volatile, medium volatile, and low
volatile rank possess these properties, but not all produce a coke of desirable

quality and some even may be detrimental to coke ovens. To compensate for
the lack of individual coals with all the necessary properties, blends of
anywhere from 2 to 20 different coals are used in today's coke making
operations. These coal blends must be managed to optimize coke quality and
reduce the cost of raw materials. Individual coals and coal blends need to
have the proper proportions of reactive and inert components and be
sufficiently thermoplastic to bind all of the components together. Only true
bituminous coals upon coking produced dense and hard coke suitable for
blast furnaces and foundries. Wood, peat, lignite coals, sub-bituminous coals,
semi-anthracites and anthracites upon coking, especially at normal pressure,
give either sandy coke or sintered coke with little if any mechanical strength.
Coals can be classified in various ways. The most widely used classification
schemes are based on the degree to which coals have undergone
coalification. The degree of alteration (metamorphism) during the temporal
history of development determine their position or rank in the coalification
series which commence at peat and extend through lignite to bituminous coal
and finally anthracite. The relative amount of moisture, volatile matter, and
fixed carbon content varies from one to the other end of the coalification
series. With further increase in temperature during coal formation peat is
converted to lignite. Lignite is considered as immature coal. Lignites are
brown coloured, soft, low calorific value coal. It is compact in texture. With
enhancement of rank, moisture and volatile matter decrease while carbon
content increases. Lignite has some special characteristics which stop us to
use in iron and steel making industry i.e.low net energy value limits
commercial potential, high moisture content increases CO2 emissions, high
VM content which increase wall pressure during coke making process, low
coking property.Due to the limited reserves of bituminous coal, some of the
properties need to be improved for upgrading the lignite coals for coke
making application. A process is being developed for improving coking

properties of brown coal in the usual operation of an ordinary coking oven or
retort.
Very important components of lignite is volatile matter, occur naturally in
lignite as humic substance and can account for an important fraction (10 -
80% depending on the maturity level) of the lignite organic matter. Mineral
matter in lignite is largely organically bound and inseparable by standard
washing techniques. The typical composition of Indian lignite coal is
mentioned in the Table 1.
The proposed process contains three step methodology for separation of low
ash clean coal with relatively lower volatile content has been described.
Lignite Coal sample is pulverized, washed and sieved to small sizes from 0.5
mm or less to liberate the dust particles. Then the lignite coal sample
obtained in step 1 is oven dried for 5-6 hours at 1050C. The dried lignite coal
sample is added with aqueous alkali namely NaOH/KOH at different
concentrations. The slurry prepared at 1:10 pulp density and heated at 70-
800C for 2 hours. After treatment, the reactants were cooled and the
supernatant liquid is filtered in this step and the coal residue is washed with
distilled water until the water appears to be clean. The washed sample is
centrifuged to extract maximum quantity of residual liquid in the cake.
The solid coal residue is dried then separated into fractions at a specific
gravity of 1.4. Float obtained at the 1.4 specific gravity is considered as clean
coal, which is having an ash content of 11.58%.
The filtrate obtained after the extraction process is neutralized and then
acidified using H2SO4 till its pH reaches 1. Precipitation of humic acid solid
particles was visualized in the solution. Since humic acid does not dissolve in
strong acids, it tries to precipitate out of the solution. It is allowed to stand
overnight. The precipitated humic acid is centrifuged to some 6000

rpm for 10-15 min and the precipitate is filtered of and washed with distilled
water. The gel like humic acid is dried in oven at 600C and stored in
desiccators. The commercial name of the remaining liquid is the fulvic acid.
Total process flow sheet has been depicted in Figure 1. Proximate analysis of
all the solid materials is given in the Table 2.
An optimization study has been conducted for alkali concentration,
temperature for extraction and lignite coal particle size. The analysis of the
product coal is mentioned in the Table 3-6. It was found that, aqueous NaOH
with 0.5% (w/w) is suitable for better extraction of humic substance from the
coal. It was also found that, temperature has insignificant effect and 40OC
temperature is optimum for higher yield of fulvic acid.
The separation of low ash clean coal with good yield was obtained due to the
removal of humic substance from the lignite coal, which causes improved
liberation of ash bearing particles in the coal. The interface between the coal
maceral and the mineral is also loosened by removal of humic substance and
hence, the wash-ability characteristics of the treated lignite are improved.
During the humic acid separation, most of the carboxylic functional groups
are removed from the lignite coal. Since the carboxylic groups in the coal
hinder the swelling property of the coal, the swelling property of the clean
coal is improved after separation of humic substance. The effect of ash also
plays a major role in plastic property of the coal and hence the clean coal
with low ash is suitable for coke making application. The coking property is
further expected to improve by addition of organic binders to the clean coal.
Methodology
A three step methodology has been developed for the extraction of humic
acid, and fulvic acid followed by the separation of humic acid from fulvic acid

and finally separation of low ash clean coal from the residue. The clean coal
obtained by this process has been tested for coking properties and it was
found that, the crucible swelling number (CSN) of the clean coal is improved
by 1 point compared to the feed lignite coal. Description of the three step
process is as follows :
(I) Lignite coal of Indian origin is grounded to fine size having an upper size
limit of -0.5 mm to improve the effectiveness of extraction. The finely
grounded coal is mixed with alkali and digested in alkali at a temperature
of 70-800C for 2 hours to extract humic substance.
(II) The extracted humic substance is separated from coal by centrifugation
and the clear liquid obtained is neutralized and acidified using dilute acid
to precipitate humic acid product. Any normal Sulphuric acid is used for
acidifying the liquid. After acidification, humic acid is precipitated out by
leaving the fulvic acid in the liquid. The humic acid is separated by
centrifugation and is washed with water to remove the residual acid.
17.30% yield of humic acid product obtained from this process. The
filtrate is the fulvic acid, it can be used for agricultural application.
(III) The residue after removal of extracted humic substance is washed and
low ash coal is separated using density separation at1.4 specific gravity.
Float contains low ash clean coal at 11.58% ash with 61% yield. The float
is collected and is dried for testing to check the coking property. Standard
method used to analyze the crucible swelling number (CSN) of the lignite
feed coal, residue and the clean coal. It was found that button was
formed for clean coal with a CSN value of 1. For other coals, no button
was formed.

ormed.

WE CLAIM :
1. A process for production of low-ash clean coal with improved coking
properties from lignite coal, comprising :-
- subjecting lignite coal lumps to pulverizing, washing and sieving to
small particles of size from 0.5 mm or less to liberate dust particles;
- oven drying the coal particles for 5-6 hours at 1050C;
Characterized by comprising the steps of :-
- adding aqueous alkali to the dried lignite coal to form a slurry with
about 1-10 plup density;
- heating the slurry at a temperature of about 600C to extract humic
substances;
- cooling the reactants and separating the extracted humic substances
from coal by filtration;
- repeatedly washing the coal residue with distilled water; and
- separating the low ash coal by density separation at 1.4 specific
gravity.

2. The process as claimed in claim 1 wherein the humic acid product
obtained at 17.30% yield from lignite coal.
3. The process as claimed in claim 1 wherein fulvic acid generated by the
filtration of acidified liquid containing humic substance, which can be
used for agricultural application.

4. The process as claimed in claim 1 wherein the low ash clean coal at
11.58% ash is separated from residual coal with 61% yield.
5. The process as claimed in claim 1 wherein the produced low ash clean
coal is having improved coking properties with 1 point increment in
crucible swelling number.