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A METHOD AND APPARATUS FOR MAKING LOW PHOSPHOROUS IRON
FIELD OF THE INVENTION
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This invention relates to a method and apparatus for making iron with a low phosphorous
content.
BACKGROUND TO THE INVENTION
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Liquid iron is produced from ore and steel plant wastes, but in these processes almost all of
the phosphorous in the feed material report to the metal, i.e. the liquid iron, and the slag
contains a very low fraction of phosphorous.
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The partition ratio of phosphorous between slag and iron is determined by thermodynamics,
and is generally dependant on factors which include temperature, the carbon content of the
iron, the FeO content of the slag and the basicity of the slag.
Generally, low temperature, high Carbon in the iron, high FeO in the slag and high slag
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basicity is beneficial to increase the fraction of the phosphorous from the feed material that
report to the slag.
According to Fruehan and Lee, the log of the phosphorous partition ratio (%P in the slag
divided by %P in the metal) is given by the following equation:
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LogLp = -12.24 + 20000/T + 2.5Log(%FeO) + 6.65LogB + 0.13[%C]
Where B is defined as: (%CaO + 0.8x%MgO)/(%SiO2 +%Al2O3 + 0.8x%P2O5)
Conventional iron making processes, such as the blast furnace and submerged arc furnaces,
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operate at high temperatures and low FeO content of the slag that result in low phosphorous
partition ratios, so that practically all the phosphorous reports to the metal.
The term liquid bath refers to the liquid contents of the hearth which includes a bath of liquid
metal contained by the hearth and a layer of slag floating on top of at least a portion thereof.
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The term “charge” refers to the raw material that is charged into a furnace for processing,
and which is still in the solid state, and the word “charging” refers to the act of so feeding raw
material into the furnace.
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The term “burden” refers to the total content of a furnace, and includes the liquid iron, the
slag, and the charged material.
OBJECT OF THE INVENTION
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It is an object of the invention to provide a method and apparatus for producing iron which at
least partly overcomes the abovementioned problem.
SUMMARY OF THE INVENTION
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In accordance with this invention there is provided a furnace for producing low phosphorous
iron, the furnace comprising a shell lined with refractory material, the furnace including a
hearth, floor, roof, at least one side wall and preferably four side walls, at least one tap hole,
at least one feed port for charging the furnace and at least one channel type induction heater
located in the floor of the furnace and communicating with the furnace through a throat, with
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the hearth being configured to operatively contain a bath of liquid metal which is at least
partly heated by the induction heater, and the bath having a width of at least 0.8m a depth at
least 0.2m, and the length of the furnace being at least 2m, with the feed port configured to
charge raw material into the furnace distal from the throat and above the bath of liquid metal
such that in combination with the angle of repose of the raw material the raw material covers
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at least 50% of the upper surface of the bath, and the induction heater being configured to
have a power rating of less than about 150 kW/m2 at the interface between the raw material
and the upper surface of the bath, to operatively melt slag with a constitution configured to
have a melting point below 1450°C.
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There is still further provided for the induction heater to be configured to preferably have a
power rating of less than about 100 kW/m2 at the interface between the raw material and the
upper surface of the bath.
In accordance with a further aspect of this invention there is provided a method of producing
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low phosphorous iron which includes the steps of
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mixing a particulate ore or waste material with an amount of carbon bearing material selected
to produce after reduction of iron oxides in the ore or waste material a liquid iron product with
a carbon content between about 1.5% and 4.0%, and flux to produce a furnace charge which
operatively generates a slag with a melting point below about 1450°C and preferably with a
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basicity of between 0.7 and 1.3,
feeding the charge into a furnace as defined above to cover at least 50%, of the upper
surface of a liquid bath in the furnace,
heating the charge by means of contact heat transfer between the liquid bath and the charge,
to achieve at least partial solid state reduction of the charge and to melt the at least partially
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reduced charge,
and continuously or intermittently removing liquid metal and slag from the hearth.
There is further provided for the method to include additionally heating the charge by
combusting reduction gases with preheated air or oxygen or any combination thereof,
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alternatively introducing and combusting combustion fuel into a combustion chamber formed
by the space between the charge and the roof of the furnace, to reduce the charge, and
subsequently providing sufficient heat through the induction heater to melt the at least
partially reduced iron particles into liquid iron.
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There is still further provided for the method to include analysing the slag composition and to
adjust the amount of flux that is mixed into the particulate and carbonaceous material mixture
to ensure a slag composition with a melting point below about 1450°C, and preferably also
with a basicity of between 0.7 and 1.3.
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These and other features of the invention are described in more detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
A preferred embodiment of the invention is described by way of example only and with
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reference to the accompanying drawings in which:
Figure 1
is a side perspective view of a liquid metal bath in a furnace according to a
first embodiment of the invention, showing the area that the induction heater
and a part of the hearth encloses;
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Figure 2
shows a rear perspective view of a furnace of Figure 1; and
Table 1
shows results from trials conducted in the furnace of Figures 1 and 2.
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DETAILED DESCRIPTION OF THE INVENTION
A furnace (1) according to a preferred embodiment of the invention is shown in Figures 1 and
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2. The furnace (1) comprises a shell (2) lined with refractory material (3) to form a
combustion chamber (4) and a hearth (5) which contains the liquid metal (6) and slag (7).
The furnace (1) also includes a roof (22) and walls (8, 9 – the end walls are not shown for the
sake of clarity). It also includes a plurality of feed ports (10) for feeding, from hoppers (not
shown), particulate raw material into the furnace (1) to create a charge (11). The charge (11)
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so fed into the furnace (1) partly rests on a sloping side wall (8) of the furnace (1) and is
partly supported by the bath of liquid metal in the form of a bath of molten iron (6) in the
hearth (5).
The furnace (1) includes a channel type induction heater (12) located below the hearth (5).
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This heater (12) communicates with the liquid bath (6) in the hearth (5) through a throat (not
shown).
The feed ports (10) are located to one side (14) of the furnace (1), and the induction heater
(12) is located at the opposing side (15) of the furnace (1).
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The furnace (1) also includes a plurality of burners or hot air or oxygen ports (16) in one side
wall (9) thereof configured to burn gasses escaping from the charge (11), or to inject fuel,
and one or more of oxygen, hot air, or cold air, through it into the furnace (1) for combustion.
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As mentioned, the furnace (1) is fed with a charge (11). The charge (11) is created by mixing
a particulate ore or waste material with an amount of carbon bearing material selected to
achieve after reduction of iron oxides in the ore or waste material a liquid iron product with
carbon content between about 1.5% and 4.0%. Flux is added to the mixture in an amount
selected to produce a charge which operatively generates a slag with a melting point below
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about 1450°C and basicity between 0.7 and 1.3.
The charge (11) is fed into the furnace (11) by means of a plurality of feed ports (10) which
are located above a sloping part of the side wall (8) of the furnace (1) which extends upwards
and away at an angle away from the hearth (5). The feed ports (10) are located above the
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angled or sloping side wall (8), to operatively charge onto tit. The charge (11) flows down the
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sloping side wall (8) to cover a substantial portion, preferably more than 50%, of the upper
surface (17) of a liquid metal bath (6) in the furnace (1).
As mentioned, a portion of the charge is supported by the sloping side wall (8) and a portion
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is supported by the liquid metal bath (6). With the charge (11) being fed into one side (14) of
the furnace (1) onto the sloping side wall (8), the charge (11) flows down the sloping side (8)
wall onto the liquid metal bath (6). Once an initial charge has been made into the furnace (1)
to cover the sloping side wall (8) and liquid metal bath (6) in this manner, further charging is
then done on top of the then existing charge heaps (18) resting on the sloping side wall (8).
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The induction heater (12) is connected to the furnace (1) through the throat (not shown), with
this located on the opposite side (15) of the furnace (1) with respect to the feed ports (10).
The charge (11) is molten from below by the heat from the induction heater (12).
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The weight of the charge (11) displaces liquid slag (7) to ensure that a contact surface (19) is
formed between liquid metal (6) and charge (11). Since more weight is acting on surface
(19) farther away from wall (19) towards wall (8) the contact surface (19) is inclined as
shown. The drawing indicates a straight line, but in practice the contact surface (19) is
curved and the shape is constantly changing as the charge (11) is melted to join the liquid
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metal (6) and slag (7) bath, and the charge (11) subsides under its own weight.
Due to the density difference between the slag (7) and the liquid iron (6), the slag (7) will
naturally tend to rise up against the slope of the contact surface (19) to collect in the open
area (21) of the bath above the induction heater (12), being replaced by denser liquid iron
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(6). This process is continuous with slag (7) being formed the whole time due to the flux that
is mixed in with the charge (11).
Since the charge (11), at the interface (19) consist of iron and slag forming components that
are being melted, a composite metal and slag sintered layer is formed at the interface (19).
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The heat transferred from the induction heater (19) to the molten iron (6) is transferred
through the thin sintered layer at the interface (19) to the charge through direct contact
between these. If the melting temperature of the slag forming components in the sintered
layer is high, the metal temperature increases to melt the higher temperature melting slag. It
is therefore important to ensure a low melting slag is formed at the interface (19) by adjusting
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the fluxes mixed in with the feed material to form the charge (11). The temperature of the
metal (6) is therefore controlled by controlling the melting point of the slag (7) and the contact
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surface area (by adjusting the size of the charge in the furnace), or by adjusting the power
input to the inductor(s).
The furnace (1) is continuously charged from above and the iron particles in the charge (11)
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are reduced and melted into the liquid bath. At the same time flux is also included in the
charge which creates a liquid slag layer (7) on the liquid iron bath. This continuously
increases the volume of material making up the liquid bath, which causes the level of the
liquid bath to increase in the hearth (5).
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The furnace (1) is thus continuously or intermittently tapped to remove liquid iron (6) and slag
(7) from the hearth (5). The slag (7) is tapped as often as is necessary to maintain the
thickness of the slag layer (7) in the open area (21) below about 200mm, and preferably
below 150mm.
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The slag (7) is tapped from a tap hole or weir located higher in height than a second tap hole
for the liquid iron (not shown). The slag weir is maintained open so that slag can flow out
continuously or intermittently when there is sufficient slag to maintain flow. The metal weir is
also maintained open at all times (unless it has to be closed for maintenance work, for
example) for metal to flow out when there is sufficient metal accumulated. The difference in
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level between the slag weir and metal weir determines the slag thickness.
As mentioned the primary importance of this furnace (1) and its method of operation is
producing liquid metal, in this instance liquid iron, with a very low amount of phosphorous
therein, alternatively the use of high phosphorous content raw materials to produce regular
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iron.
The partition of phosphorous between iron and slag in iron making furnaces has been
investigated by Fruehan and Lee, rendering the following equation within the range of
conditions considered:
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LogLp = -12.24 + 20000/T + 2.5Log(%FeO) + 6.65LogB + 0.13[%C]
where the basicity B is defined as: (%CaO + 0.8x%MgO)/(%SiO2 +%Al2O3 +
0.8x%P2O5)
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Trial campaigns have been conducted using a test furnace according to the invention. The
results are shown in Table 1.
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According to Fruehan and Lee, the phosphorous partition ratio should have ranged from 0.22
to 3.98 and the respective metal phosphorous levels should have been 0.071% and 0.038%.
The results show much lower phosphorous levels in the metal and partition ratio’s over 5 in
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all cases.
The thermodynamics and mechanism which prevents a large proportion of phosphorous that
is contained as some compound in the ore to be transferred to the metal is not fully
understood and will be the subject of further study.
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The applicant believes it is the unique combination of a power rating below about 150 kW/m2,
the control over the slag composition, the coverage of the metal bath with charge, and the
configuration of the furnace that allow this unexpected result to be achieved.
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It will be appreciated that variations to the embodiment described above is possible without
departing form the scope of the invention.