Note: Descriptions are shown in the official language in which they were submitted.
1101137
Field of th _ nventjon,
THIS invention relates to coal beneficiation processes in which
coal fines are separated into a fraction which is low in ash
and a fraction which is high in ash.
Background of the Invention.
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Coal fines with a high ash content can be upgraded by means of
a known dense medium separation method. In this method the
coal fines are added to a suspension made up of dense medium
particles, e.g. magnetite particles, in water, the density of
the slurry being adjusted to be about 1,4. The m;xture of
coal and the suspension is then fed to a cyclone separator.
.The light or overflow fraction from the cyclone contains a high
proportion of coal fines lo~ in ash together with some dense
i` medium particles, while the dense or underflow fraction conta;ns
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a high proportion of dense medium particles, high ash coal and
ash constitutents.
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Having obtained these two fractions one then has to recover the
~ ~ dense medium particles so that they may be recirculated for
; use in the process. With coarse coal th;s is readily achieved
~y screening and washing. With ~jne coal particles other
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` measures have to ~e resQrted to.
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Since coal particles are easily rendered hydrophobic, an
obvious other measure is to float the coal particles from the
slurries, leaving heavy medium particles behind - see British
patent No. 827 443. However, with fine high density particles,
such as magnetite fines, these float with the coal.
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Summary of the Invention.
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According to the invent;on a method of benefic;at;ng coal fines
of a particle s;ze less than 0,5 mm comprising the steps of
lo introducing a ~ixture of coal and a suspension of water and
particles of a dense medium to a gravity separation in which
high ash coal is obtained as an underflow slurry and low ash
coal is obtained as an ov~rflow slurry~ and the dense medium
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~ particles from both the underflow and the overflow slurries
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~:` 15 are recovered to be reused to make up the suspension; ;s
characterised by the step of submitting either or both of the
underflow slurry and the overflow slurry to a first froth
flDtation process in which a wetting agent adapted to render
~ ` the surfaces of the coal particles hydrophil1ic is added to
- ;~ 20 the slurry and the slurry is subjected to froth flotation so
that the dense medium particles report in the float fraction
of the first flotation process and are then used to make up
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the suspension.
The wetting agent may be an e~ulsifjer such as a sulphonic acid
type anionic emulsifier. A salt of R SO2 NH2 CH2 COOH, wherein
R is alkyl, has proved to be highly effective.
Virtually only dense medium particles report in the float of
the first froth flotation process. However, some of the
dense medium partic1es remain in the tailings and need to be
recovered. This recovery may be done by a second froth~flotation
process after rendering the coal particle hydrophobic as by
treating the tailings with a petroleum hydrocarbon oil.
- Alternatively the tailings may be treated in a magnetic
separation process to recover dense medium particles. Of
course, in the latter case the dense medium particles must be
magnetic or magnetizable.
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;~ lS Description of the Drawing.
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It shows a flow sheet of a process according to the invention
for treat;ng coal fines.
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Description of a~Preferred Embodiment~
In the illustrated flow sheet coal fines of particle size minus
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0,5 mm is first mixed at lO wlth a suspension of magnetite
in water made up to give the correct separation density, $ay,
1,4. The mixture is then ~ed to a cyclone separator ll.
The overflow and the underfiow from the separator ll are
treated in an identical manner in two separate circuits.
Dealing first with the overflow it is fed to a conditioner
12 where a cationic emu1sifier is added as well as a suitable
frothing agent such as methyl isobutyl carbinol. From the
cond;tioner l2 the overflow slurry passes to a rougher
flotation section 13. The float from this section, which is
` substantially pure magnetite is returned to the suspension
~, ~ preparation section 14. The tailings go to a further conditioner
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In the conditioner l5 a hydrocarbon oil is added to render the
~ coal particles hydrophobic and the conditioned product passes
to a cleaner flotation section 16. Here a clean coal product
is the float and the tailings is substantially pure magnetite
which is also returned to l4.
~ Likewise the underflow from the cyclone ll passes through a
``; 20 conditioner 17, a rougher flotation section 18~ a cond;tioner
~ ~ l9 and a c1eaner flotation section 20; The float from the
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~1~1137
section 18 and the tailing from the section 20 go to the
secti~n 14. The float from the section 20 represents the
high ash coal fraction, which is generally discarded. The
product of the process is the float from the section 16 from
which fine coal is obtained in a conventional manner.
Example:
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In an example of the invention coal fines less than 0,5 mm in
size were treated in a heavy medium suspension made up of
magnetite of which 95 % was less that 44 microns. The
suspension was prepared in the conventional manner to achieve
an apparent density of about 1,4.
The mixture of coal fines and suspension was fed to a standard
hydrocyclone separator to produce an overflow slurry containing
63,5 % magnetite and 36,5 % 1GW ash coal fines and an underflow
slurry containing 84,6 % magnetite and 15,4 % high ash coal
fines all by weight.
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The two slurries were conditioned separate1y by adding to them
0,5 kg/t of an ionic emulsifier, Emu~sogen STH obtained from
Hoechst as well as methy~ isobutyl carbin~l as a frothing
agent.
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~1~1137
The conditioned slurries were separately treated in rougher
froth flotation cells, In the case of the ~Yerflow slurry
89,9 % of the magnetite present reported in the float and the
case of the underf~ow slurry 84,9 % of the magnetite present
reported in the float. These f70ats were returned to the
suspension make up section.
The tailings from the rougher flotation sections were then
separately conditioned with 0,8 kg/ton of dry feed of kerosene
to render the coal fractions hydrophobic. They were then
separately subjected to froth flotation in cleaner flotation
cells. In the case of the,overflow slurry 7,1 % and in the
case of the underflo~ slurry 17,5 % of the magnetite calculated
on the original feed, reported in the tailings.
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, In the case of the underflow 99,4 % of the magnetite was
reco,vered and in the case of the overflow 96,9 % was recovered.
-~ ' The loss of magnetite to the coal fractions is thus negligible.
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`, The key to the high magnetite recovery is to float it from the
t!~lo coal fractions. Thereafter the remainder can be recovered
in the tailings after floating off the coal or by means of a
magnetic separation. The low concentration of magnetite in
~ ~ ~ the product being treated makes magnetic separation highly
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11~1137
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economjcal 4ver the tre~at~ent of the whole Qf the underflo~
or the overflow, as the case may be, in a magnetic separation
process. It is well kno~m that magnetic separation from a
suspension in which the magnetic fractions is highly diluted
is more ef~icient and economical than the recovery from a
suspension in which it is highly concentrated.
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In an experiment the tailings from a rougher flotation cell
was subjected to a double stage magnetic separation. The
result was that between the magnetic separator and the cleaner
flotation 100 / of the magnetite was recovered.
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