Note: Descriptions are shown in the official language in which they were submitted.
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~ lis invention relates to fin~-grain desulEurizing mixtures Eor
the treatment of lron and steel melts based on alkaline earth carbonates,
and to the production and use of such mixtures.
Because of the decreasing quality of available ores and reducing
agents such as coke and heavy fuel oil, the desulfurizing of pig iron
and steel is becoming increasingly more important in the continued produc-
tion of high quality iron based products of consistent quality.
The carbonates of the alkaline earth metals, either those which
occur naturally or which are produced industria:lly in chemical reactions,
are particularly suitable and inexpensive. However~ large quantities of
iron are expelled from the treatment ladles when these compounds are
employed because of the large quantities of gas generated when fine-grained
carbonates are introduced into iron based melts at the temperatures of
1200 - 1700 C. This problem of uncontrolled gas generation renders the
inexpensive alkaline earth carbonates, (otherwise environmentally unobjec-
; tionable and available in sufficient quantities in the natural state)
commercially unsatisfactory for desulfurizing such melts.
This is the more unfortunate, because as is generally known, thefreshly-formed alkaline earth oxides illustrated in Equations (1) and (2)
are particularly reactive because of their small crystal size.
CaCO3~ CaO -~ CO2 ; L~ Hl = ~ 42,8 kcal/Mol (1)
~IgCO3~ MgO + C02 , ~ H2 = ~ 24,3 kcaljMol (2)
A further disadvantage in using alkaline earth carbonates as
desulfurizing agents for iron based melts results from the endothermic
evolution of the carbon dioxide (see Equation (1) and (2), which causes
noticeable cooling of the melt. Moreover, the resulting reaction products,
e.g. calcium oxide, magnesium oxide or calcium sulphide cause hardening
of the slag present on the surface on the melt, by increasing its melting
temperature and thereby make slag removal from the treatment ladles more
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In all o~ the technical processes used today to deacidi~y alkaline
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earth carbonates, e.g. by lime calcining or burning, the time that tlle result-
ing alkaline earth oxides remain at the required deacidification temperature
is,relatively speaking, several orders of magnitude greater than it is when
it is air injected into a melt. Thus, industrial:Ly, calcined lime, dolomite
or magnesite, because of the recrystallization of the primary resulting
oxide crystallite into larger crystals, are considerably morç inert than the
same alkaline earth oxides freshly formed by deacidification during
pneumatic injection of the corresponding alkaline earth carbonates into a
melt, which then, when rising up through the melt can react within one to
five seconds with contained sulfur.
If, instead of alkaline earth carbonates, alkaline earth oxides
produced industrially from such carbonates are used, it is possible to
avoid the disadvantage of uncontrolled gas generation. However, the
desulfurizing effect of alkaline earth oxides used in any process familiar
up to now is unsatisfactory, because even when a soft-burnt alkaline earth
`; oxide is used, the oxide crystals are too large and hence too unreactive.
Only part of the alkaline earth oxide reacts with the sulfur contained in
the melt.
It is an object of the invention to develop new desulfurizing
; mixtures for iron based melts proceeding from products which, technically
speaking, are easily accessible and available in practically unlimited
quantities,which with specific additives will ensure good desulfurizing
values at high reaction speeds, with scarcely increased production costs.
Here described is a process involving the use of fine-grain
desulfurizing mixtures based on alkaline earth carbonates, containing a
reducing metal which causes superheating oE the high]y active alkaline
earth oxide formed in situ in an iron based melt and which also suppress -
gas generation problems.
; It has been found that in the presence of reducing metals and at
approximately 1100 - 1300 C, alkaline earth carbonates react together
exothermically. Surprisingly a large quantity of carbon is formed and
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hardly any gas is generatecl.
When alkaline earth carbonates are injected pneumatically into
iron based melts the life of the ensuing alkaline earth oxides and carbon
dioxide formed by thermal disassociation is only a few seconds, even when
the injecting lance is submerged to a depth of only two to four metres.
The carbon dioxide is captured in exothermic reaction by the addition of
a reducing metal as here described. The resulting gas bubbles collapse
almost immediately and the alkaline earth oxide which is greatly superheated
in the exothermic reaction between the carbon dioxide and the metal reacts
vigorously with the sulfur contained in the melt, since no recrystallization
or increase in grain size takes place in the short time taken by the
oxide to rise to the surface of the melt. The formation and the collapse
of the gas bubbles promotes agitation and mixing of the melt.
Examples of suitable alkaline earth carbonates are calclum carbo-
nate, magnesium carbonate, dolomite and semi-calcined dolomite and diamide
of lime, which can occur naturally or be produced synthetically. Diamide
of lime is produced as a residue, consisting in the main of calcium
carbonate and graphitic carbon during the production of cyanimide or
`~ dicyanadiamide from crude calcium cyanamide. On contact with the hot iron
based melt which is at a minimum of approximately 1200 C the carbonates
decompose into calcium oxide and carbon dioxide. At the moment of formation,
- this calcium oxide is a highly effective desulfurizing agent. The evoIu- ;
tion of large quantities of carbon dioxide consumes a great amount of heat
; ~ (compare Equations (1) and (2)),which reduces the temperature of the melt.
However, this reduction in temperature is more than compensated
by the addition of a metallic reducing agent. The exothermic reaction of
the reducing agent - which under the conditions in question must be
greater than free carbon or its combinations with hydrogen - with the
carbon dioxide evolved from the carbonate,~ has a positive effect on the
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~ thermal balance loaded negatively by -the alkaline earth carbonate (Equations
(3) and (~1)).
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CO -~ Si- -~ SiO -~ C ~H = -123,7 kcal/Mol (3)
1,5 C02 ~ 2 ~ A1203 ~ 1,5 C ~ H = -258,9 kcal/Mol (4)
The purpose of the reducing agent is thus to bind the carbon
dioxide, generated by the thermal decomposition of the alkaline earth
carbonate, in an exothermic chemical reaction in such a manner that only
intermediate gas generation takes place and that such gas generation
agitates the melt but does not result in any expulslon of metal from the
ladle. The overall thermal balance is also made positive. rl~he pneumat-
ically injected desulfurizing agent is thereby superheated in relation
to the melt, with a certain amount of desirable gas generation to ensure
sufficient mixing of the melt.
Reducing agents which fulfill these conditions are, for example,
silicon, aluminum, alloys of silicon and aluminum, mangagese silicide,
ferrosilicon with a silicon content of 15 - 98%, as well as mixtures of
; the foregoing substances, and powdered metal containing wastes from
comminution processes, aluminum grindings, etc.
The proportion of such reducing agents in the desulfurizing
mixture can amount to 90%-wt. A content of 5 to 85%-wt is preferred~
A further purpose of the reducing agent, if it is oxidized by
carbon monoxide and/or carbon dioxide is to form additional alkaline earth
; oxide that then has a similar desulfurizing effect.
The following reaction Equations are given as examples:
CO ~ Ca~ CaO ~ C ~ H = -125 kcal/Mol (5)
C2 ~ 2 Mg---?2 MgO ~ C A H = -193 kcal/Mol (6)
As examples, the following are reducing agents which meet both
conditions: calcium silicide, barium calcium silicide, magnesium ferro-
silicon, calcium, magnesium, strontium, barium, alIoys of calcium,
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~ magnesium, strontium and barium, magnesium calcium silicide, ferrocalcium
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sillcide, aluminum calclum silicide. Mixtures of these and other substances,
particularly with lron, can be used. Up to 90%-wt, preferably 10 to 25%-wt
should be in the desulfurizing mixture, for effective results.
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After ox:idation by the carbon dioxide or carbon monoxide, preEerably the
reducing agent should contribute to a reduction of the melting point of
the slag from the desulfurizing reaction without increasing any erosion
of the lining oE the processing vessel by added fluxes such as, for example,
fluorspar or colemani~e. Thus, these compounds should be those that are
present in the slag before desulfurization, e.g. in the case of pig iron,
compounds such as silicon dioxide, aluminum oxide or silicates containing
these or other oxides. Reducing agents which satisfy this requirement
are, for example, calcium silicide, magnesium ferrosilicon, silicon,
; 10 aluminum, alloys of silicon and aluminum, manganese si]icide, manganese
calcium silicide, magnesium ferrosilicon, aluminum calcium silicide, as
well as mixtures of the foregoing substances and others. The composition
of the reducing agent should be selected according to the type of slag on
the melt prior to the desulfurizing treatment, in such a way that the slag
does not harden during the treatment.
The quantity of reducing agent of the foregoing type in the
desulfurizing mixture can amount to 90%-wt. It is preferable that a
proportion of approximately 10 - 80%-wt be used.
All the reducing agents cited herein can be industrial grade
products and contain the usual impurities associated with their manufacture.
Thus, no special requirements for purity are imposed. Iron, in particular,
may be present as an impurity.
The metallic reducing agents described herein thus completely
fulfill other purposes than the reduction agents in desulfurizing agents
described in the prior art. Because of the liberation of hydrogen,
carbon dioxide, carbon monoxide or another gas which has a reducing effect,
these prior art agents merely create an atmosphere which protects the
actual desulfurizing agent, e.g., calcium carbide, against oxidation.
As described herein, however, the added metallic reducing agents bind the
carbon dioxide liberated durlng the decomposition of the alkaline earth
carbonates in an exothermic reaction, supply additional active alkali~e
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earth oxide in situ, and reduce the melting point of the slag. The
active superheated calcium oxide reacts in the familiar way wlth the
sulfur dissolved in the melt, for example, in the case of iron according
to the following Equation:
CaO ~ S (dissolved) -~ C (disso:Lved) ~CaS -~CO (7)
The calcium sulfide that forms accord:Lng to Equation (7) i8 absorbed
by the slag floating on the melt.
The melting points of the oxide or oxide mixtures resulting from
the desulfurizing mixtures can be varied by using various metals in the
alkaline earth carbonate and in the metallic reducing agent. This also
makes it possible to vary the melting point and consistency of the slag
floating on the melt. In addition, the desulfurizing mixtures can also
contain a specific proportion of fluxes such as inorganic fluorides or
borates.
In order to further increase the exothermy of the desulfurizing
mixture added to thè melt and thereby increase the superheating of the
desulfurizing alkaline earth oxide, chosen quantities of known thermite
mixtures, containing fine-grained iron oxide and aluminum powder, can also
be included.
When mixtures which consist for the most part of calcium carbonate
and ferrosilicon are used, i~ can be expedient to add a few percentage
; parts by weight of a calcium magnesium or calcium silicon alloy.
Such desulfurizing agents, as here described, can be manufactured
by simply mixlng the individual substances together in the appropriate
proportions. However, it is preferred, after strong drying of the
alkaline earth carbonate, that the components be ground together as the
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~ resultant lumps with one or more lump size reducing agents and reduced to
-` a grain size of 3 mm, preferably less than 0.3 mm. The grain size of the
alkaline earth~carbonate can be coarser than that of the reducing agent,
which should~be of the finest possible grain size. This fine-gralned
mixture of alkaline earth carbonate snd reduc mg metal can be delivered
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pneumatically and is injected into iron based melts in the blast furnace
crucible in open ladles, torpedo ladIes, or mixers. It is particularly
advantageous for the reaction between the released carbon dioxide and the
reducing metal, for the desulfurizing mixtures to be injected through an
injection lance placed as deeply as possible in the melt. The ferrostatic
pressure or the applied over-pressure oE the gas atmosphere is advantageous in
accelerating the reaction between the carbon dioxide and the metal.
Should it be expedient to do so, instead of producing a mixture of
alkaline earth carbonate and reducing metal, the individual components can be
10 stored separately, measured separately, delivered pneumatically and then combined
into a mixture shortly before entering, or when actually in, the lance.
More particularly in accordance with one aspect of the invention there
is provided a fine grain cesulfurizing mixture for iron based melts, comprising
an alkaline earth metal carbonate and from 10 to 90% by weight of a reducing
metal for superheating of highly active alkaline earth metal oxide formed in situ
from said carbonate in the melt and for suppressing attendant gas generation.
The alkaline earth metal carbonate may be selected from calcium carbonate,
magnesite, dolomite and diamide of lime with the reducing metal being selected
from calciumJsilicon~aluminum, magnesium and mixtures or alloys of these.
In accordance with a second aspect of the invention there is provided
the process of desulfurizing an iron based melt comprising the step of injectinga mixture of an alkaline earth metal carbonate and from 10 to 90% by weight of
a reducing metal through a lance pneumatically into said melt, highly active
alkaline earth metal oxide being formed in situ from said carbonate, said metal
effecting superheating and suppressing attendant gas generation. The steps
may be included of separately measuring chosen quantities of the alkaline earth
metal carbonate ancl the metal reducing agent, individually delivering these
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quantities pneumatically to the lance, and combining the qu~ntities to form the
mixture at the lance.
The following examples illustrate embodiments of the invention in
greater detail without in any way being restrictive to the specific compositions
of the mixtures, their manufacture or use.
EXAMPLE N0. 1
Desulfurization of pig iron, using a mixture consisting of magnesium
powder and calcium carbonate.
The desulfurizing mixture was manufactured in a tube mill by
simultaneously grinding limestone previously, reduced to 0 - 5 mm, with magnesium
powder with a grain size of less than 1 mm, using nitrogen as a protective
atmosphere.
'L'wo hundred and three tons of pig iron were treated in a 230-ton
torpedo ladle, using a mixture of 40%-wt magnesium powder and 60æ-wt ground
limestone; the mixture was pneumatically injected at 1.85 m depth through a dip
lance, using argon as the carrier gas.
The iron melt was at a temperature of 1310 C. The delivery rate
throttled down until no significant flaming caused by the burning magnesium could
be observed on the surface. This occurred at 17 kg/min.
The pig iron melt had an initial sulfur content of SA = 0.042%.
After treatment lasting ll mlDuteS, 198 kg of desulfurizing agent had been
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pneumatically injected. This corresponds to 0.98 kg/t pig iron. AEter
treatment the sulfur content was S~ = 0.005%.
This amounts to a total conversion factor of 46% of the calcium
oxide (formed from the calcium carbonate) to calcium sulfide, the magnesium
oxide (freshly formed during the reaction) to magnesium sulfide and the
excess magnesium to magnesium sulfide.
EX~PLE N0. 2
Desulfurization of a steel melt us:ing a mixture of calcium carbonate,
calcium silicide and aluminum.
The steel at a temperature of 1620 C contained
0,07%-wt carbon
0.13%-wt silicon
0.35%-wt manganese
with a sulfur content 0.024 - 0.033%-wt, to be reduced to an average
0.005%-wt by desulforization treatment.
In order to reduce the oxidizing effect on the steel bath a
desulfurizing mixture with a deficiency of calcium carbonate was selected.
The small amount of aluminum supplement content was to assure the desired
aluminum content of the steel and bring aluminum oxide into the slag by
partial reaction with the calcium carbonate.
~ A desulfurizing mixture consisting of
; 37%-wt calcium carbonate
60%-wt calcium silicide
30%-wt aluminum
in combination with a low-oxide slag cover containing fluorspar was
injected pneumatically into the steel melt in a 70-ton ladle. Six to ten
litres of argon per kilogram of desulfurizing mixture was used as the
carrier gas. The following table shows the results of the individual
treatments.
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Quantity of
Treatmen~ Initial Final Change Mixture
No. S~ SE ~S kg/t c~
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1 0.031 0.005 0.026 2.~ 0.55
2 0.027 0.003 0.02~ 2.4 0.60
3 0.030 0.002 0.028 2.7 0.58
4 0.019 0.003 0.016 2.1 0.78
0.013 0.002 0.011 2.0 1.09
6 0.016 0.002 0.014 2.1 0.9
7 0.033 0.008 0.025 2.2 0.84
0.032 0.0l1 0.0ZI 1.8 0.86
~ is a coefficient representing the specific consumption oE desulfurizing
mixture in kg per ton oE iron and per 0.01% reduction of S content.
Ihe consumption of an average oE 2.2 kg desulfurizing mixture per
ton of steel indicates 50 - 60% better utilization of the desulfur:izing
mixture in comparison with those mixtures usually employed.
EXAMPLE NO 3
Desulfurization of pig iron using a mixture consisting of powdered
calcium silicide and calcium carbonate. A mixture of 28.6% industrial
grade calcium silicide and 71.4% calcium carbonate was made. The
industrial grade calcium silicide contained 30.1% calcium and 60.3% silicon.
The calcium carbonate was a precipitated product produced synthetically.
Ihe mixture was made in a 3-chamber tube mill. The grain size of the
mixture of calcium carbonate and industrial grade calcium silicide leaving
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the mill was 98% less than 0.1 mm.
One hundred and ninety-six tons of pig iron in a torpedo ladle
was treated with this mixture. Air was used as the injection gas.
The ch rge was 12 Nl air/kg mixture. 325 kg of the mixture was
injected pneumatically iD 9.5 min.
Before treatment, the sulfur content was 0.047%, and after
treatment it was 0.013%. Thus, 0.034% sulfur was removed; this corresponds
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to 72% desulfurization. The yield of the reaction of the desulfurizing
agent to the calcium sulfide, related to the total weight of calcium, was
` 30 59.8%. The consumption of desulfurizing agen-t amounted to 0.5 kg/t pig
iron and 0.01~ of sulfur removed.
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