Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
1999-05-14
2001-12-25
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C075S502000
Reexamination Certificate
active
06332910
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Filed of the Invention
The invention relates to a process for working up steel slags and iron carriers such as, e.g., electric furnace slags, converter slags, fine ores, dusts from steel production, mill scales for obtaining pig iron and environmentally safe slags.
2. Prior Art
From PCT/AT 96/00019 a process for producing pig iron or steel and cement clinker from slags has become known, in which iron-oxide-containing liquid slags such as, for instance, steel works slag were mixed with iron oxide carriers and lime, whereupon a ferrite slag was formed. After this, that ferrite slag was reduced in a reduction reactor under the formation of an iron bath and a sinter phase while burning carbon, whereupon the sinter phase was discharged as a clinker.
Slags have a relatively poor thermal conductivity and a thermal capacity approximately 1.5 to 2 times higher than that of iron. What is essential to the economy of a process of this kind is the heat transfer to be attained or what is called degree of afterburning. The degree of afterburning is defined as follows:
Degree
of
afterburning
=
CO
2
+
H
2
O
CO
+
CO
2
+
H
2
+
H
2
O
gaseous
phase
By the known modes of procedure, only an insufficient degree of afterburning has been ensured so far. Nor do the known modes of procedure offer a
Heat
transfer
=
1
-
H
g
-
H
b
H
pc
H
g
…
Enthalpy
of
gas
(
at
gas
temperature
)
H
b
…
Enthalpy
of
gas
(
at
melting
temperature
)
H
pc
…
Combustion
enthalpy
(
at
melting
temperature
)
that is sufficient for an economic mode of procedure.
Thermal efficiencies of far above 70% are attained neither in conventional blast furnace technologies nor in other processes such as, for instance, fluidized bed processes. Thus, it has already been known to blow prereduced and at least partially preheated charges into a fluidized bed along with coal, wherein coal is gasified in a fluidized bed under reduction of the charge and sponge iron is. melted and drawn off. To make things worse, such meltdown gasification reactions, as a rule, are optimized with a view to the pig iron output sought so that no environmentally safe slag is formed.
The present invention aims at providing a process of the initially defined kind, by which the thermal yield and hence the efficiency are substantially enhanced as compared to known processes. To solve this object, the process according to the invention essentially consists in that the volume ratio of molten slag to iron bath is chosen to be larger than 0.5 to 1 and, preferably, 0.8:1 to 1.5:1 and that the slags are supplemented with SiO
2
carriers such as, e.g., foundry sands, metallurgical sands and/or fine ores so as to adjust a slag basicity (CaO/SiO
2
) of between 1.0 and 1.8 and, preferably, 1.2 and 1.8 at an Al
2
o
3
content of between 10 and 25% by weight, based on the slag, wherein hot blast is top-blown and coal, optionally along with an inert gas and, in particular, nitrogen and, furthermore, optionally oxygen or hot air is blown through the iron bath. By using an extremely high specific amount of slag, which is substantially elevated as compared to known processes, the slag is able to assume the function of a heat transfer medium from the gas space to the slag iron melt mixture. The thermal capacity of the slag, which is 1.5 to 2 time higher than that of iron, allows for a high heat transfer, thereby avoiding the blowing through of coals and hence too small an exchange surface between the reductant carrier iron bath and the oxide carrier slag due to an accordingly high iron bath portion. By adding SiO
2
carriers to the slags for adjusting a defined slag basicity of from 1 to 1.8 and, in a particularly preferred manner, between 1.3 and 1.6 as provided by the invention, it is feasible to directly produce environmentally safe slags, wherein the direct usability of such environmentally safe slags may be enhanced even further by adjusting the Al
2
O
3
content to values of between 10 and 25% by weight, based on the slag.
By top-blowing hot blast or hot air, the heat transfer is additionally increased, the top-blowing of hot blast in combination with the blowing in of coal and N2 and, separately, oxygen into the iron bath enabling the intensive thorough mixing of the slag with the iron bath and hence an elevated heat transmission in the slag and iron bath suspension formed by thorough mixing.
In order to definitely prevent the risk of blow-throughs, it is advantageously proceeded in a manner that the melt bath height corresponds to at least 20 times and, preferably, 30 to 60 times the diameter of the submerged tuyeres. This is, at the same time, beneficial to appropriate fluidization as well as the formation of a slag and iron bath suspension, thus enhancing heat transmission in the liquid phase.
Heat transfer may be further improved by the impact pulse of the hot blast jet, such an impact pulse further improving the intensive thorough mixing of the slag bath with the iron bath. Advantageously, it is proceeded in a manner that the speed of the hot blast is chosen between 0.4 and 0.8 Mach.
An environmentally safe slag product capable of being utilized in a particular suitable manner may be produced within the scope of the process according to the invention in that the slags are granulated forming a glass content of more than 90% and, preferably, more than 93%.
In addition to the selection of the optimum hot blast speed, heat transfer may still be enhanced by increasing the specific bath surface. Such an increase in the specific bath surface may, for instance, be effected by droplets flying into the gas space of the converter using high-pressure submerged tuyeres, thereby being able to obtain an increase in the bath surface by a factor 20 as compared to a calm slag melt. And his is exactly what is reached by dimensioning the height of the melt bath relative to the diameter of the submerged tuyeres.
Unlike usual blast furnace processes and conventional steel or pig iron production processes, which aim for specific slag amounts as small as possible, the process according to the invention in a particularly advantageous manner renders feasible the conversion of a number of hitherto hardly usable iron carriers to pig iron in an energetically particularly favourable and hence economic manner. To a high degree, this applies to fine ores, which have been difficult to work up in an economic manner so far. Also dusts from steel production, which have relatively high heavy metal contents in addition to high iron contents, may be readily utilized within the scope of the process according to the invention. Naturally, this applies also to mill scales, the conventional utilization of which partially involves greater problems due to adhering oils, which do not constitute any difficulties within the scope of the process according to the invention. (Addition through central tube of hot blast lance).
The process according to the invention in the first place is of particularly great interest to electric steel works, the latter using no pig iron technology and, in particular, no blast furnace technology. Electric steel works, as a rule, must buy pig iron at relatively high prices if higher-quality products are to be produced. This holds, in particular, if the contents of copper and tin dragged into an electric furnace through scrap steel are to be reduced by dilution. Besides the favourable utilization and disposal of metallurgical residual substances such as electric furnace slag and dusts, scales as well as optionally foundry sand, also aluminium (oxide) containing grinding dusts as well as dried red muds occurring in the recovery of bauxite by the Bayer process and other sources of residual
"Holderbank" Financiere Glarus AG
Andrews Melvyn
Winthrop LLP Pillsbury
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