Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a novel com-
pacted unit of metallurgical grade powdered silicon or
ferrosilicon and to a method for its preparation.
BACKGROUND OF THE INVENTION
Silicon and ferrosilicon are produced in the
form of ingots and are subjected to a crushing operation
so that they are available to users in smaller size.
During the crushing operation a very fine powder of
silicon or ferrosilicon is formed and recovered from the
filtering system. Through recent environmental regu-
lations the amounts of silicon and ferrosilicon dusts has
risen continuously thus creating a disposal problem for
the manufacturers of silicon and ferrosilicon.
Attempts to find a use for such silicon and
ferrosilicon powders have had little success because it
must be appreciated that any agglomeration process must
insure that the purity of the silicon or ferrosilicon is
substantially retained. Accordingly most processes
developed in the prior art have produced a product which
is not equivalent in purity to the original product so
that such agglomerated product can not be used instead of
the pure product itself.
USP. 4,128,417, Dec. 5, 1978 describes a process
for the preparation of SiC and/or FeSi-containing fine
materials to form metallurgically adjustable silicon
bearers by binding the SiC and/or SiFe fines with a
hydraulic binding agent such as cement or lime. This
method produces a product containing 8-10% calcium oxide
or 5,6 to 7% of calcium which amount renders it unsuitable
to be used as pure silicon or ferrosilicon.
It is an object of the present invention to
provide a metallurgical grade compacted unit of fine
powdered silicon or ferrosilicon where the amount of
impurities originally present have not been increased to
a point where the product still retains the chemical
qualities of the original silicon or ferrosilicon.
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SUMMARY OF THE INVENTION
In accordance with the present invention there
is now provided a compacted and shaped unit of metallurgi-
cal grade granular material selected from silicon and
ferrosilicon comprising a compressed homogeneous mixture
of a fine powder of a silicon material selected from the
group consisting of silicon and ferrosilicon having a
grain finer than 160 ~m, sodium or calcium bentonite in an
amount of from 2 to about 5% by weight of the silicon
material and sodium or calcium lignosulfonate in an amount
of from 2 to about 5% by weight of the silicon material.
The novel compacted and shaped unit of silicon
or ferrosilicon is characterized by being adapted for
packaging and transportation. The shaped unit is such
that it can be readily broken up for metallurgical or
chemical uses. The shaped unit is also characterized by
the fact that the silicon or ferrosilicon retains all its
original chemical properties and the amount of impurities
of the silicon or ferrosilicon remain within the acccepta-
20~ ble limits of the original silicon or ferrosiliconmaterial.
DETAILED DESCRIPTION OF THE INVENTION
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The compacted unit of silicon or ferrosilicon is
prepared by first mixing together the silicon or ferro-
silicon in fine powdered form (finer than 160 ~m) with 2
to about 5% by weight of sodium or calcium bentonite, the
mixing being carried out at high speed such as from 1500
to 2000 r.p.m. until a homogeneous mixture is obtained.
There is then sprayed over this mixture while continuing
to mix at high speed an aqueous solution of a sodium or
calcium lignosulfonate, the amount of this latter compound
representing from 2 to about 5% by weight of the silicon
material. There is then obtained a plastic homogeneous
paste which is fed to a briquetting machine where bri-
quettes of the silicon or ferrosilicon are formed and
subsequently dried.
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The novel silicon or ferrosilicon briquettes
obtained in accordance with the present invention are
easily packaged and can be transported to other sites for
use in industrial processes requiring substantially pure
silicon or ferrosilicon.
The silicon or ferrosilicon powder used in
accordance with the present invention is obtained as a
fine powder from the crushing of pure silicon or ferro-
silicon and is usually collected from special filters.
The silicon or ferrosilicon powder is composed of grains
finer than 160 ~m and have the grain size distribution
being shown in Table I:
TABLE I
DISTRIBUTION OF GRAIN SIZE OF
SILICON AND FERR~SILICON POWDER
Grain size in ~m 315 160 ~0
Cumulative % passing 100 98 86
for silicon
for ferrosilicon 100 ~7
The two mixing steps used in the process of the
present invention should be carried out at high speed of
from 1500 to 2000 r.p.m. preferably about 1700 to 1800
r.p.m. As an example of a suitable mixer there may be
mentioned the Eyrich mixer R-7 which is adapted to
operate at a high speed of 1760 r.p.m.
As far as the addition of the aqueous solution
of sodium or calcium lignosulfonate is concerned this
solution should be sprayed on the mixture of silicon or
ferrosilicon and sodium or calcium bentonite to insure
that a plastic homogeneous paste is obtained. The
incorporation of the lignosulfonate solution to the
mixture as such will not yield the desired plastic paste.
The present invention will be more fully under-
stood by referring to the following ~xamples which are
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given to illustrate the invention only.
EXAMPLE 1
There is introduced in an Eyrich mixer R-7
50 kg of silicon dust having a grain size of from 80 to
315 ~m and 2 kg of calcium bentonite. The mixer is
operated at a speed of 1760 r.p.m. for about 2 minutes
whereby there is obtained a homogeneous mixture. 2
Liters of a solution of sodium lignosulfonate sold under
the trade mark LIGNOSOL by LIGNOSOL CHEMICALS LTD is
mixed with 14.5 liters of water. The 16.5 liters of the
lignosulfonate solution obtained is sprayed of the
mixture of silicon and bentonite while continuing to mix
at a speed of 1760 r.p.m. After two minutes of mixing
there is obtained a homogeneous plastic paste which is
then fed to a charcoal briquetting machine. These
briquettes can readily be packaged and shipped. It has
also been found that the addition of calcium bentonite
has not substantially altered the chemical purity of the
original silicon powder.
EXAMPLE 2
By proceeding in the same manner as in Example
; 1 but substituting ferrosilicon dust for the silicon
~ dust, ferrosilicon briquettes similar to the silicon
- briquettes are obtained.
EXAMPLE 3
ANALYSIS
The silicon briquettes of Example 1 and ferro-
silicon briquettes of Example 2 were analyzed to de-
termine the amounts of Fe, Al, Ca and Ti present in
order to compare these impurities with the limits gener-
ally requested by the specifications of the customer's
dusts used in the preparation of the briquettes.
Results are reported in Table II:
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TABLE II
Acceptable
Maximum impurities in %
in certain industrial uses
. Fe . Al Ca ¦ Ti
Maximum percent permissible
by the specifications of 1.0 1.5 0.5 0.5
the customer s for silicon
powder
. Silicon briquettes Ex. 1
: Run #1 0.64 0.97 0.40 0.20
Run #2 0.63 1.02 0.35 0.18
Maximum percent permissible
by the specifications of _ 2.5 0.5 0.3
the customer's for ferro-
silicon powder
Ferrosilicon briquettes 0.32 0.06
Run #1 _ 2.05
Run #2 _ 1.97 0.36 0.07
It will be seen from the above analysis that the
amounts of Fe, Al, Ca and Ti impurities in the silicon
ferrosilicon briquettes of the present invention are below
the maximum impurities permissible in pure silicon and
ferrosilicon for their respective industrial uses.
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