Process for removing ash from coal

A process for removing ash from coal comprising the steps of pulverizing the coal to fine particles, admixing water with the finely divided coal to prepare an ash-containing slurry of finely divided coal, mixing with the slurry an oil and seeds in the form of oleophilic solid grains and serving as granulating nuclei to granulate the finely divided coal, separating the resulting granules from the mixture and washing the granules with water to remove the ash, and disintegrating the washed granules to obtain a deashed coal and recover the seeds for reuse.

BACKGROUND OF THE INVENTION 
This invention relates to a process for treating ash-containing coal to 
remove the ash therefrom. 
Naturally occurring coals have relatively high ash contents. Usually coal 
contains about 7 to about 25% of ash which consists chiefly of silica 
(SiO.sub.2), alumina (Al.sub.2 O.sub.3), etc. When observed by an X-ray 
microanalyzer, the ash in coal is in the form of particles about 5 .mu.m 
to tens of .mu.m in size. It is already known to remove such ash from coal 
by pulverizing the coal, mixing the pulverized coal with water to obtain a 
slurry, adding to the slurry an oil serving as a binder to form the coal 
and the oil into granules, and separating the granules from the 
ash-containing aqueous medium. The granules separated from the ash are 
pellets of oil-containing deashed coal useful as a fuel. However, the 
conventional process has the problem of being unable to fully remove the 
ash from coal, because for the full removal of the ash, the coal must be 
pulverized to very fine particles as small as ash particles contained 
therein, i.e. about 5 .mu.m to tens of .mu.m. Nevertheless, if coal is 
divided exceedingly finely, the particulate coal has an increased surface 
area, requires the use of an increased amount of the binder oil and takes 
a greatly prolonged period of time for granulation. Consequently it is 
impossible to pulverize coal to very fine particles and therefore to deash 
the coal to a full extent. 
SUMMARY OF THE INVENTION 
The present invention provides a process for removing ash from coal free of 
the foregoing problems. According to this invention, ash-containing coal 
is pulverized to very fine particles, which are granulated with use of 
seeds in the form of oleophilic solid grains. The coal particles can 
therefore be granulated with use of a greatly reduced amount of oil within 
a short period of time. Since only a small amount of ash is incorporated 
into the granules during the granulation step, the present process affords 
deashed coal having a very low ash content. The deashed coal can be 
obtained in the form of an oil-containing deashed coal or coal-oil 
mixture. Accordingly when the deashed coal, oil-containing coal or 
coal-oil mixture prepared by the process of the invention is used as a 
fuel, the coal burns easily with stability, gives off an increased amount 
of heat per unit quantity and produces only a greatly reduced amount of 
ash. 
The invention will be described below with reference to the accompanying 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to FIG. 1 showing a first embodiment of the invention, 
ash-containing coal material is deashed by the following four steps to 
afford oil-containing deashed coal. 
(1) Preparation of finely divided coal slurry 
Naturally occurring ash-containing coal is pulverized, for example, by a 
wet-type ball mill to fine particles of hundreds of .mu.m to tens of .mu.m 
in size. While the coal material usually contains 7 to 25% by weight of 
ash, the ash is also finely divided by pulverization. The finely divided 
ash-containing coal is then mixed with water to obtain a coal slurry. When 
desired, the coal may be pulverized to superfine particles of several 
.mu.m. Preferably the coal material is pulverized in water, and a required 
quantity of water is added to the resulting slurry for the adjustment of 
the concentration. 
(2) Granulation 
An oil and seeds in the form of oleophilic solid grains and serving as 
granulating nuclei are admixed with the finely divided coal slurry for 
granulation. Examples of useful oils are kerosene, gas oil, fuel oil, 
residuum oil, vegetable oils, etc. The oil is used, for example, in an 
amount of about 10 to about 30% by weight based on the coal. To render the 
oil easily dispersible in the slurry, a small amount of surfactant may be 
used. The seeds serving as granulating nuclei must be oleophilic and must 
be in the form of solid grains so as to be recoverable with ease later. 
Preferably, the seeds have a specific gravity approximate to that of coal, 
i.e. to 1.4. Examples of useful seeds are granules or grains of synthetic 
resin, such as rigid polyvinyl chloride, coarse coal grains, etc. which 
are about 1 to 10 mm in size. Seeds larger than 10 mm in grain size are 
not desirable since ash particles will easily adhere to such seeds along 
with coal particles. Preferably the seed to coal ratio is usually 1:1. 
This ratio is of course slightly variable provided that the combined 
amount of the seeds and the finely divided coal is up to 30% by weight 
based on the combined amount of the seeds and the slurry. 
The oil and seeds are admixed with the coal slurry by one of the following 
three methods. 
(1) The seeds are admixed with the coal slurry, and the oil is then admixed 
with the resulting mixture. 
(2) The oil is added to the seeds to apply the oil to the surfaces of the 
seeds, and the seeds are then admixed with the coal slurry. 
(3) The oil is admixed with the coal slurry, and the seeds are thereafter 
admixed with the resulting mixture. 
The mixture thus prepared is agitated for granulation with use of a 
granulating apparatus having stirring blades of metal net or other known 
granulating machine. In this step, the oil adheres to the surfaces of the 
seeds in the form of oleophilic solid grains, and fine coal particles 
adhere in increasing amounts to the oil coating the seeds serving as 
nuclei, whereby the fine coal particles are clustered and formed into 
granules very rapidly in an accelerated fashion. Accordingly the 
particulate coal is granulated within an extremely short period of time. 
Furthermore the use of the seeds greatly reduces the amount of oil to be 
used. On the other hand, the ash pulverized approximately to the same size 
as the coal particles remains in the water. 
When the coal material contains a large amount, e.g. about 20% by weight, 
of ash, the mixture may be subjected to a primary deashing treatment prior 
to the granulation. More specifically, when the seeds and oil are admixed 
with the finely divided coal slurry, the resulting mixture separates into 
a liquid phase containing the particulate coal, seeds and oil, and a 
slurry in the form of ash-containing water. The ash-containing slurry is 
separated off from the coal-containing liquid phase, and the liquid phase 
is washed with fresh water, whereby a considerable amount of ash is 
removed. 
(3) Washing 
The granules composed of the finely divided coal, oil and seeds are then 
separated from the resulting mixture and washed to remove the ash. This 
step is performed, for example, by passing the mixture over a filter while 
spraying washing water from above. When desired, the separated granules 
are centrifuged to remove water to a greater extent. The granules composed 
of the finely divided coal, oil and seeds and washed are dried to 
completely remove water. The ash-containing slurry fraction is led into a 
water treating apparatus equipped with a thickener, filter, etc. is 
thereby separated into the ash and water. 
(4) Separation of seeds 
The dried granules are disintegrated, for example, by vibration and thereby 
separated into the seeds and oil-containing deashed coal in the form of 
pellets to obtain the deashed coal and to recover the seeds for reuse in 
the second step of granulation. The seeds, bearing some amount of oil, may 
be returned to the granulation step directly or with further application 
of oil. 
The oil-containing deashed coal thus prepared has been fully deashed and is 
effectively usable as a fuel. 
With reference to FIG. 2 showing a second embodiment of the invention, an 
ash-containing coal material is deashed by the following six steps to 
afford a deashed coal. 
(1) Preparation of finely divided coal slurry 
A slurry of finely divided coal is prepared in the same manner as in the 
first embodiment. 
(2) Granulation 
A low-boiling oil and seeds in the form of oleophilic solid grains and 
serving as granulating nuclei are admixed with the finely divided coal 
slurry for granulation. Useful low-boiling oils are those small in latent 
heat of vaporization and low in viscosity, such as kerosene, gas oil, 
gasoline, etc. The low-boiling oil is used, for example, in an amount of 
about 10 to about 30% by weight based on the finely divided coal. A small 
amount of surfactant may be used conjointly to render the oil dispersible 
in the slurry easily. The same seeds as used for the first embodiment are 
used as granulating nuclei. The coal slurry, low-boiling oil and seeds are 
mixed together in the same manner as in the first embodiment for 
granulation. 
(3) Washing 
The granules formed are washed in the same manner as in the first 
embodiment. 
(4) Heating for evaporation 
The granules washed and composed of the finely divided coal, low-boiling 
oil and seeds are heated to evaporate the oil and water. Steam or electric 
heat is used as the heat source. 
(5) Separation of seeds 
The dried granules free from the low-boiling oil are disintegrated, for 
example, by vibration and thereby separated into the seeds and deashed 
coal in the form of pellets to obtain the deashed coal and to recover the 
seeds for reuse in the second step of granulation. The seeds may be 
returned to the granulation step with or without the low-boiling oil 
applied thereto. 
(6) Condensation and separation 
The evaporated gaseous mixture of the low-boiling oil and water is 
subjected to condensation and thereafter separated into liquid low-boiling 
oil and water. The liquid low-boiling oil is returned to the granulation 
step. 
The deashed coal thus prepared has been fully deashed and is effectively 
usable as a fuel. 
With reference to FIG. 3 showing a third embodiment of the invention, an 
ash-containing coal material is deashed by the following five steps to 
afford a coal-oil mixture. 
(1) Preparation of finely divided coal slurry 
(2) Granulation 
(3) Washing 
These three steps are conducted in the same manner as in the first 
embodiment. 
(4) Mixing of heated oil 
An oil heated to 70.degree. to 150.degree. C. is admixed with the granules 
washed and composed of the finely divided coal, oil and seeds, and the 
mixture is subjected to suction to cause water to evaporate off. Examples 
of oils useful for this step are fuel oil, residuum oil, etc. having a 
relatively high boiling point. When kerosene, gas oil or like low-boiling 
oil is used as the oil for the granulation step, the oil will be partly 
evaporated off by the suction along with water. When desired, the 
evaporated gaseous mixture of such low-boiling oil and water may be cooled 
for condensation to separate the low-boiling oil in a liquid state from 
the water for recovery. The recovered oil is then returned to the 
granulation step for reuse. The heated oil is used in such an amount that 
the coal to oil ratio of the coal-oil mixture eventually obtained will be, 
for example, 1:1. Since up to 30% by weight of oil is added to the slurry 
in the granulation step, at least 70% by weight of the heated oil is used 
based on the finely divided coal so that the combined amount of the oils 
will be approximately equal to the amount of the finely divided coal. When 
the coal to oil ratio of the coal-oil mixture is to be varied, the amount 
of the heated oil is suitably varied accordingly. 
(5) Separation of seeds 
The granules containing the heated oil are disintegrated and are thereby 
separated into the seeds and a mixture of deashed coal and oil to obtain 
the mixture and to recover the seeds for reuse in the second step of 
granulation. The seeds, bearing a small amount of oil, is returned to the 
granulation step with or without oil further added thereto. 
The coal-oil mixture thus obtained contains fully deashed coal and is 
therefore advantageously usable as a fuel. 
The present invention will be described with reference to the following 
examples, to which the invention is not limited. 
EXAMPLE 1 
An oil-containing deashed coal was prepared by the process shown in FIG. 1. 
Blair Athol coal (occurring in Australia) having an ash content of 7.02% 
by weight was pulverized to fine particles not larger than 200 mesh, i.e. 
up to 74 .mu.m, in size. The finely divided coal was mixed with water to 
obtain a coal slurry. Seeds were then admixed with the coal slurry. The 
seeds were made of rigid polyvinyl chloride, were in the form of short 
cylinders about 3 mm in diameter and about 1.5 mm in height, and had a 
specific gravity of 1.4 approximate to that of the coal. The mixture 
contained 5% by weight of the coal and 5% by weight of the seeding 
material. Subsequently kerosene was admixed with the mixture in an amount 
of 20% by weight based on the finely divided coal. The resulting mixture 
was fed to a granulating machine for granulation. The granules were 
separated from the aqueous phase and washed with water to remove the ash. 
The granules were then dried and disintegrated by vibration to obtain an 
oil-containing deashed coal in the form of pellets. The seeds separated 
from the coal were recovered and returned to the granulation step for 
reuse. To determine the ash content of the oil-containing deashed coal 
thus prepared, the oil was extracted from the coal with a solvent. The 
resulting coal was found to contain only 1.74% by weight of ash. For 
comparison, the same coal material as used above was deashed by the 
conventional process without using any seed. The oil-containing deashed 
coal obtained was found to contain 3.32% by weight of ash based on the 
finely divided coal. The results are listed in the table given later. 
EXAMPLE 2 
An oil-containing deashed coal was prepared from Daido coal (occurring in 
China) containing 12.14% by weight of ash in the same manner as in Example 
1 except that seeds of rigid polyvinyl chloride to which kerosene was 
applied were admixed with a slurry of finely divided coal. The 
oil-containing deashed coal obtained was found to have the ash content 
listed below. The table also shows the result achieved by the conventional 
process with use of the same coal material. The ash contents listed are 
based on the coal free from the oil. 
EXAMPLE 3 
An oil-containing deashed coal was prepared from the same Daido coal as 
used in Example 2 in the same manner as in Example 1 with the exception of 
admixing a specified amount of kerosene with a slurry of finely divided 
coal and thereafter mixing seeds with the mixture. The following table 
shows the result achieved and also the result attained by the conventional 
process without using any seed. 
______________________________________ 
Ash content of coal (% by weight) 
Conventional 
Process of 
Example Material process invention 
______________________________________ 
1 7.02 3.32 1.74 
2 12.14 5.58 3.17 
3 12.14 5.58 4.78 
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EXAMPLE 4 
A deashed coal was prepared by the process shown in FIG. 2. Blair Athol 
coal (occurring in Australia) having an ash content of 7.02% by weight was 
pulverized to fine particles not larger than 200 mesh, i.e. up to 74 
.mu.m, in size. The finely divided coal was mixed with water to obtain a 
coal slurry. Seeds were then admixed with the coal slurry. The seeds were 
made of rigid polyvinyl chloride, were in the form of short cylinders 
about 3 mm in diameter and about 1.5 mm in height, and had a specific 
gravity of 1.4 approximate to that of the coal. The mixture contained 5% 
by weight of the coal and 5% by weight of the seeding material. 
Subsequently gasoline was admixed with the mixture in an amount of 20% by 
weight based on the finely divided coal. The resulting mixture was fed to 
a granulating machine for granulation. The granules were separated from 
the aqueous phase and washed with water to remove the ash. The granules 
were then heated to evaporate the gasoline and water. The gasoline-free 
granules were disintegrated by vibration to obtain a deashed coal in the 
form of pellets. The seeds thus separated were recovered and returned to 
the granulation step for reuse. The evaporated gaseous mixture of gasoline 
and water was subjected to condensation and then separated into liquid 
gasoline and water. The gasoline was returned to the granulation step for 
reuse. The deashed coal thus prepared was found to contain only 1.86% by 
weight of ash. For comparison, the same coal material as used above was 
deashed by the conventional process without using any seed to prepare a 
deashed coal, which was found to contain 3.32% by weight of ash. 
EXAMPLE 5 
A coal-oil mixture was prepared by the process shown in FIG. 3. Blair Athol 
coal (occurring in Australia) having an ash content of 7.02% by weight was 
pulverized to fine particles not larger than 200 mesh, i.e. up to 74 
.mu.m, in size. The finely divided coal was mixed with water to obtain a 
coal slurry. Seeds were then mixed with the coal slurry. The seeds were 
made of rigid polyvinyl chloride, were in the form of short cylinders 
about 3 mm in diameter and about 1.5 mm in height, and had a specific 
gravity of 1.4 approximate to that of the coal. The mixture contained 5% 
by weight of the coal and 5% by weight of the seeding material. 
Subsequently fuel oil was admixed with the mixture in an amount of 20% by 
weight based on the finely divided coal. The resulting mixture was fed to 
a granulating machine. The granules formed were separated from the aqueous 
phase and washed with water to remove the ash. Bunker fuel heated to about 
90.degree. C. was mixed with the granules in an amount of about 80% by 
weight based on the finely divided coal. The mixture was subjected to 
suction to evaporate water and a small amount of oil. The oil-containing 
granules were then disintegrated to separate the granules into the seeds 
and a mixture of deashed coal and oil in an approximate ratio of 1:1. The 
seeds were recovered and returned to the granulation step for reuse. To 
determine the ash content of the deashed coal in the mixture, the oil was 
extracted from the mixture with a solvent. The deashed coal was found to 
contain only 1.74% by weight of ash. For comparison, the same coal 
material as used above was deashed by the conventional process without 
using any seed. The oil-containing deashed coal obtained was found to 
contain 3.32% by weight of ash based on the finely divided coal.