Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HOE 79/H 051
The present invention relates to a process for the
manufacture of desulfurizing agents containing 1 - 6 %
by weight of chemically bonded water, based on calcium
oxide-containing calcium carbide, for crude iron and
steel melts.
The desulfurization of crude iron and steel melts
by means of calcium carbide (referred to hereinafter as
carbide) containing calcium oxide (briefly referred to
hereinafter as lime) is generally known (DE-PS 1 160 457,
DE-PS 20 37 758).
Heretofore, it has been customary to prepare these
desulfurizing agents by processes, wherein the required
amount of finely divided lime was introduced into molten
carbide in order to produce a homogeneous mixture in the
melt, which was then allowed to cool and was comminuted.
Preferably, the finely divided lime was introduced
into the stream of carbide tapped of~ from the furnace.
Although the amount of lime that can be introduced into
the carbide melt is limited and there are risks associated
with working with liquid carbide, it has long been hel~
in the art that this method could not be avoided since
it was thoughtthat only a mixture of CaC2 and CaO pro-
duced in the melt was ideally suited for desulfurizationof metal melts.
Desulfurizing agents based on calcium carbide that
contain substances which split off water at the temperature
'~
of the metal melt have already been described in DE-AS
22 52 795. These agents, which are mixtures of customary
commercial carbide with, for example, Ca(OH)2 as the
substance that splits off water, have the disadvantage
that they are mixtures produced by mechanical mixing
processes in which carbide particles exist separately from
the Ca(OH)2 particles, so that the use of theæ products
leads to higher consumption, irregular and vigorous gas
reactions and a large variation as regards the desulfuriz-
ing efficiency, rendering difficult any controlled use ofthese agents.
The present invention now unexpectedly provides a
process for making a highly effective desulfurizing agent
for crude iron and steel melts which process permits the
disadvantages of prior art methods to be avoided and which
comprises: thermally producing from lime and coke a molten
starting mi2ture of carbide and calcium oxide with a CaO
content varying within the range 20 to 80 %; allowing
the mixture to cool and solidify into a block; rough-
crushing the solidified block while it still has anaverage temperature of more than 400 C, preferably of
between 400 C and the solidification temperature of the
melt, to particles with a size of less than 150 mm and
calciu~ oxide; and admixing the comminuted mixture, which
still has a temperature of at least 400 C, with a
quantity of calcium oxide, necessary to establish in the
mixture a total content of CaO corresponding to the CaO
content desired in the end product. Preferably, calcium
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oxide is added in a quantity necessary to establish
in the mixture a total content of CaO of more than 45 O~G
up to 90 % by weight. Next, the mixture is gro~nd with
intensive mixing and in the presence of air or nitrogen
having a moisture content of 5 to 20 g/m3 (at 1.013 bar
and 273.15 K) at temperatures below 100 C, preferably
at 10 - 50 C, to particles with a size of less than
10 mm, preferably of less than 100 /um.
A preferred feature of the present process provides
for the molten starting mixture of calcium carbide and
calcium oxide to contain 20 to 45 % by weight of CaO,
the mixture having been obtained ~rom lime and coke in
known manner by thermal treatment. It is also possible,
however, initially to produce a molten starting mixture
of calcium carbide and calcium oxide with a CaO content
of more than 45 % up to 80 % by weight, by introducing
finely divided calcium oxide into an existing calcium
carbide melt containing up to 45 % by weight of CaO
until establishment of a maximum CaO content of up to
80 % by weight, then allowing the whole to solidify into
a block, and rough-crushing the latter at temperatures of
above 400 C.
The present process permits inhibiting mixing the lime with
a carbide melt and the di~ficulties associated therewith.
In addition to this, the present process has, inter alia,
the following beneficial effects: It is not necessary for
the composition of the burden to be set in each particular
case ~or producing the carbide melt, nor is it necessary
6~
for the lime to be first ground to a certain particle
size. Use can rather be made of a carbide block with
a CaC2 : CaO weight ratio which may vary within wide
limits, In other words, the carbide block may contain
CaC2 and CaO in practically any ratio, and the lime can
even be used in the form of coarse particles with a size
within the range 8 to 60 mm, for example.
A further beneficial effect of the agent produced
in accordance with this invention resides in the fact that
on the surface each individual particle of Ca(OH)2 lies
close to CaC2, with the result that the desulfurizing
reaction is started very early and regularly. As a re-
sult of this, smaller amounts of desulfurizing agents
are required to be used for producing comparable de-
sulfurization results, which incidentally are controllable.
The following Examples illustrate the invention:
EXAMPLE 1
The starting material was a melt as customarily usedfor the manufacture of commercial carbide, the melt con-
taining 80 % by weight of CaC2 and 20 % by weight of CaO.A carbide block of corresponding composition was produced
in known manner in a crucible by allowing this melt to
cool.
After the block had cooled to an average temperature
of approximately 600 C, it was rough-crushed to particles
with a size of less than 150 mm and the carbide, still
with a temperature of 500 C, was covered with a layer
of sufficient lime with a particle size of 8 to 60 mm
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that the resulting mixture had a total CaO content of
50 ~ by weight.
Next, the mixture was ground in a rotary mill, while
1500 m3/h of air with a moisture content of 10 g/m3 (at
15 C) was passed therethrough. The throughput was 50Q kg/h
at 50 C, and the mixture was ground to particles with a
size of up to 100 /um.
1500 kg of this product was used for customary desul-
furization of 300 000 kg of a crude iron melt th~ contained
0.03 % by weight of sulfur. The iron so treated contained
0.005 % by weight.
EXAMPLE 2
Calcium carbide was thermally produced in known
manner from lime and coke, the lime/coke mi,~ture in the
total burden being set at a weight ratio of approximately
110 : 40, corresponding to a carbide with a CaO content of
approximately 45 % by weight.
CaO of a particle size of 3 - 8 mm was metered into
the carbide tapped off, the CaO being used in a quantity
necessary to establish an average content of approximately
80 % by wei~ht of CaO in the discharge crucible (approxi-
mately 1.2 - 1.3 t of CaO per metric ton material tapped
off).
After cooling the crucible to an average temperature
of no less than 600 C, which was the case after 4 hours,
the block was rough-crushed to particles with a size of
less than 150 mm and a layer of sufficient lime of
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particles with a size of 8 to 60 mm was added to the
hot mixture that the average CaO content was 90 % by
weight. Next, the mixture was ground in a rotary mill,
while 1500 m3/h of air with a moisture content of
10 g/m3 (at 15 C) w~s passed therethrough. The through-
put was 500 kg/h at 50 C and the material was ground
to particles with a size of less than 100 /um.
The mixture permitted the same desulfurization
results to be produced, based on the calcium carbide
content, as described in Example 1.
