The invention relates to a method for processing steel slags
and optionally iron carriers such as, e.g., electric furnace
slags, converter slags, dusts derived from steel production,
mill scale or secondary metallurgical residues.
A method of the initially mentioned type is described, for
instance, in WO 99/14381. In that known method, steel slags
were reduced above an iron bath along with iron carriers and,
in particular, along with fine ore in order to thereby form
environmentally safe slags. To this end, SiO2 for adjusting a
beneficial slag basicity or Al2O3 was also added among others,
wherein the addition of iron carriers such as, for instance,
fine ores in the first place served to appropriately use such
starting materials which are difficult to process and which
are difficult to reduce by conventional methods, in order to
enhance the economoy of the method. Due to the relatively poor
thermal conductivity of slags and a heat capacity that is 1.5
to 2 times larger than that of iron, the heat transfer and
degree of postcombustion to be attained are of essential
importance to the economy of such a method. In any event,
relatively high energy expenditures will have to be taken into
account regarding that type of processing of steel slags aimed
to produce environmentally safe slaps.
The invention aims to improve a method of the initially
defined type with a view to enabling, simultaneously with the
processing of slags, the recovery of valuable raw materials
which will guarantee the economy of the method despite the
elevated consumption of energy involved. To solve this object,
the process according to the invention essentially consists in
that the molten steel slags and iron carriers, respectively,
are supplemented with chromium ores or chromium-containing
and/or nickel-containing dusts in order to adjust a slag
basicity of 1.2 to 1.6, whereby the bath temperature is
maintained at above 1600°C, in particular between 1650°C and
1800°C, and a carbon-containing iron bath is provided or
formed, and that the slag and the ferrochromium alloy formed
are tapped separately. It is true that the use of chromium
ores or chromium-containing and/or nickel-containing dusts
basically involves the risk of an excessive chromium oxide
content within the slag, which might again seriously
jeopardize the environmental safety of the thus processed
steel slags. Yet, by working at bath temperatures of above
1600°C, it has surprisingly become feasible to reach final
chromium contents in the slag of far below 500 ppm and, at the
same time, transform chromium from such chromium ores nearly
quantitatively into the iron bath used for reduction. The
activity of the chromium within the iron bath is considerably
reduced by the formation of chromium carbide, to which end an
iron bath having an accordingly high carbon content must be
provided or formed. In this context, the carbon content of the
iron bath, as a rule, must be kept at above 3 wt.-% C in order
to ensure that the desired chromium carbide formation will be
effected so as to avoid reslagging of the reduced chromium
oxide. Final chromium concentrations of a carbon-containing
iron bath provided in such a manner may comprise up to 60 wt.-
% chromium, thus enabling a high-grade metal regulus in the
form of socalled ferrochromium carbure, in which the portion
of carbon may amount up to 9 wt.-%. Due to that high-grade
metal regulus, the economy of such a mode of procedure will be
safeguarded despite the high temperature level required,
whereby the thus formed pre-alloy, which may be used in the
special steel plant, covers the overall energy costs and
production costs of the slags that have thus become
environmentally safer.
What is essential to the process according to the invention is
the high temperature level of above 1600°C, which will be
ensured by a suitable postcombustipn only. In addition, an
accordingly rapid mass transfer must be ensured. Such
conditions advantageously can be observed in that the chromium
ores or chromium-containing and/or nickel-containing dusts are
top-blown onto the bath via a hot blast lance at particle
sizes of below 4 mm, preferably 0.5 to 2 mm. By blowing in the
chromium ores at such particle sizes by the aid of a jet of a
suitable speed, high turbulences will form, thus subjecting
the bath to intensive agitation. The large relative surface
implied by the particle size and the high shearing forces
acting between the individual phases and, in particular,
between the gas phases, the chromium ores, the slag melt and
the metal melt result in accordingly high energy and mass
transfer rates. The temperature level required in a simple
manner may be ensured in that hot blast at temperatures of
between 1200° and 1600°C is used.
In order to ensure an appropriate postcombustion, the hot
blast advantageously is enriched with oxygen to an oxygen
content of 25 to 40 wt.-%, early incipient solidifying of the
slag by the progressing reduction procedure advantageously
being feasible by mixing the steel slags with the chromium ore
in a suitable mixing vessel. In such a mixing vessel, the
respective basicity and the mixed slag temperature may be
adjusted, whereby the slag mixture may be tapped at
temperatures of, for instance, about 1750°C and supplied to
the consecutively arranged reduction unit. At those high slag
input temperatures prevailing within the reduction converter,
oxygen enrichment of the hot air jet may even be obviated at
all.
Advantageously, it is proceeded in a manner that the molten
steel slag and the chromium ores are mixed in a slag ladle and
are electrically heated using, in particular, graphite
electrodes and applied onto the iron bath at temperatures of
above 1700°C, preferably above 1750°C, whereupon the iron bath
is carburized and hot blast is top-blown. The iron bath in
this case is to be present in a low-carbon form refined by hot
blast in order to reduce the initial reduction speed, with
high bath temperatures being safeguarded at the same time.
Departing from carbon contents of, for instance, 0.8 to 1.5
wt.-% C within the iron bath, the carbon content required for
the quantitative separation of chromium will be adjusted only
subsequently by carburizing the bath.
In order to attain a particularly high environmental safety of
the treated steel slag, the basicity advantageously is
adjusted to 1.4 to 1.55.
An accordingly reliable mixing of the bath is feasible in a
particularly simple manner in that the hot blast with the
chromium ores or chromium-containing and/or nickel-containing
dusts is directed onto the bath at jet speeds of more than 500
m/s and, in particular, 700 m/s.
In the context of the method according to the invention,
special steel slags are preferably used as steel slags, yet
also LD slags, OBM slags and electric furnace slags as well as
secondary metallurgical residues may be envisaged as charging
materials. Chromium ores may at least partially be replaced
with dusts highly loaded with chromium and nickel, the
introduction of phosphorus having to be minimized in any
event.
In the main, the method according to the invention, in which
carbon contents of up to 9 wt.-% may be obtained within the
bath, offers the advantage that any reslagging of chromium
within the slag can be largely excluded, residual chromium
oxide contents of less than 3 60 ppm having been observed
within the slag at temperatures of 1650°C.
In the following, the invention will be explained in more
detail by way of an exemplary embodiment. Steel slag having
the following composition:
until a target basicity of approximately 1.5 had been reached.
Chromium ores, as a rule, are acidic such that the respective
basicity can be adjusted by the addition of chromium ores.
The chromium ore was top-blown onto the melt by means of a hot
gas lance with the ore conveying rate having been determined
as the speed-determining step. The reduction itself proceeded
within seconds and was merely dependent on the dissolution
kinetics and hence the particle size of the chromium ores.
Chromium ore having a particle size of 0.5 to 2 mm was used, 1
part by weight of steel slag having been bombarded with 0.442
part by weight of chromium ore in order to attain the target
basicity of approximately 1.5 (CaO/SiO2) . The mixed slag
obtained was composed of 69.35 wt.-% steel slag and 30.65 wt.-
% chromium ore and had the following composition:
This mixed slag was continuously charged onto a carbon-
containing iron bath and reduced, whereupon a slag having the
following composition:
was obtained. From 1 ton of mixed slag was formed 0.688 ton of
such a target slag. At the same time, a high-grade metal
regulus having the following composition:
was formed. Such a ferrochromium carbure could subsequently be
used directly as a pre-alloy in a special steelworks.
In order to maintain the necessary slag treatment temperature
of above 1600°C, it has proved to be advantageous to add coal
dust and coal to the hot blast jet. In this manner, the energy
balance can be economically improved by very cheap thermal
coal, but also by the addition of heavy oil or natural gas.
Claims:
1. A method for processing steel slags and optionally iron
carriers such as, e.g., electric furnace slags, converter
slags, dusts derived from steel production, mill scale or
secondary metallurgical residues, characterized in that the
molten steel slags and iron carriers, respectively, are
supplemented with chromium ores or chromium-containing and/or
nickel-containing dusts in order to adjust a slag basicity of
1.2 to 1.6, whereby the bath temperature is maintained at
above 1600°C, in particular between 1650°C and 1800°C, and a
carbon-containing iron bath is provided or formed, and that
the slag and the ferrochromium alloy formed are tapped
separately.
2. A method according to claim 1, wherein the
chromium ores or chromium-containing and/or nickel-containing
dusts are top-blown onto the bath via a hot blast lance at
particle sizes of below 4 mm, preferably 0.5 to 2 mm.
3. A method according to claim 2, wherein hot
blast at temperatures of between 1200° and 1600°C is used.
4. A method according to claim 1, 2 or 3, wherein
the hot blast is enriched with oxygen to an oxygen
content of 25 to 40 wt.-%.
5. A method according to any one of claims 1 to 4,
wherein the basicity is adjusted to 1.4 to 1.55.
6. A method according to any one of claims 1 to 5,
characterized in that the hot blast with the chromium ores or
chromium-containing and/or nickel-containing dusts is directed
onto the bath at jet speeds of more than 500 m/s and, in
particular, 700 m/s.
7. A method according to any one of claims 1 to 6,
wherein the carbon content of the iron bath is
kept at above 3 wt.-% C.
8. A method according to any one of claims 1 to 7,
wherein the molten steel slag and the chromium
ores are mixed in a slag ladle and are electrically heated
using, in particular, graphite electrodes and applied onto the
iron bath at temperatures of above 17 0 0°C, preferably above
1750°C, whereupon the iron bath is carburized and hot blast is
top-blown.

The invention describes a method for processing steel slags
and optionally iron carriers such as, e.g., electric furnace
slags, converter slags, dusts derived from steel production,
mill scale or secondary metallurgical residues, in which the
molten steel slags or iron carriers are supplemented with
chromium ores or chromium-containing and/or nickel-containing
dusts in order to adjust a slag basicity of 1.2 to 1.6, whereby
the bath temperature is maintained at above 1600°C, in
particular between 1650°C and 1800°C, and a carbon-containing
iron bath is provided or formed. In addition to environmentally
safe slags, a high-grade ferrochromium alloy may be recovered,
thus enhancing the economy of the method.