Refining molten metal

A method is provided for refining molten metal containing an impurity which is oxidizable in preference to one or more selected constituent elements of the metal and which forms a gaseous oxide, the method comprising injecting oxygen and a diluent gas into the molten metal contained in a vessel so as to reduce the proportion of the impurity in the metal by evolution as the gaseous oxide, weighing the vessel during refining to establish a point at which the weight of the vessel contents changes significantly due to preferential oxidation of the one or more selected constituent elements, initiating at the point a reduction in the ratio of oxygen: diluent gas to as to reduce the rate of oxidation of the one or more selected constituent elements relative to the impurity and continuing injection until the proportion of the impurity is reduced to the desired level.

This invention relates to the refining of molten metal and is particularly 
concerned with the refining of alloy melts. 
Alloy melts such as stainless steel melts and other highly alloyed 
materials such as nickel superalloys are currently produced by a duplex 
process involving melting the basic cold charge in an electric arc furnace 
and in the case of stainless steel adding chromium as high carbon 
ferrochrome and decarburising the liquid steel in a converter vessel. The 
decarburisation is performed by injecting a stream of oxygen and diluent 
gas either singly or combined into the metal held within the vessel. 
The oxygen and the diluent gas may be injected in varying modes. For 
instance the oxygen and the diluent gas may be injected as a mixture 
either through the bottom or the side of the vessel. This is the basis of 
the A.O.D process developed by Union Carbide Corporation, the diluent gas 
in this case being either argon or nitrogen. In the C.L.U. process 
developed by Creusot Loire/Uddeholm the gas mixture is oxygen and steam. 
In other processes the oxygen is injected through the top of the vessel 
and the diluent gas in the form of an inert gas or mixture of inert gases 
is injected through the bottom or side wall of the vessel as described in 
U.K. Patent Application No. 25449/74. These processes generally operate 
over a series of stages in which the volume gas ratio of oxygen: diluent 
gas is successively reduced as the carbon content in the melt is lowered 
in order to reduce oxidation of chromium, iron and other metals while 
maintaining removal of carbon from the melt as a gaseous oxide. 
For instance in one procedure the refining of the charge is begun with the 
ratio set at 3:1 or greater. This ratio is maintained for a period 
determined by the initial hot metal composition. When the carbon content 
has reached approximately 0.3% C the ratio is reduced to 2:1 and the ratio 
is reduced again to 1:2 when the carbon content has reached 0.1%. The melt 
is then finally refined to a desired low carbon level. It has been found 
that this procedure enables chromium oxidation to be kept at a minimum 
while allowing carbon removal to be kept at a maximum. 
In each of these processes in order to achieve maximum economies in terms 
of minimum expensive diluent gas usage, minimum chromium and other alloy 
oxidation, minimum usage of ferrosilicon reductant to recover chromium 
from the slag, minimum process time and minimum refractory consumption, it 
is advantageous to be able to monitor the composition changes occurring in 
the melt throughout the blow in order to select optimum input gas ratios. 
If the relative rates of oxidation of carbon and chromium can be assessed 
throughout refining, the oxygen: diluent gas ratios can be changed without 
process interruption and over a wide range in order to control the 
oxidation of carbon and chromium. 
One method by which carbon removal rate is currently monitored involves the 
use of waste gas composition and flow rate measurements in the waste gas 
duct. The instantaneous carbon content of the melt is calculated by 
difference knowing the quantity of carbon in a sample taken from the 
initial liquid metal charge and the integrated quantity of carbon removed 
in the waste gas. However, this method has not been used widely on 
production plants because the basic design of most of the waste gas 
extraction systems of such plants incorporates a full combustion -- full 
dilution collecting hood in order to cool the waste gases to a temperature 
at which they can be readily used in a simple non water cooled, non -- 
refractory lined system. This is done by diluting the waste gas in the 
duct with air and the result is that frequently the level of carbon 
dioxide in the diluted gas is so low that determination of decarburisation 
rate is inaccurate. 
Another method for determining the carbon removal rate is the sampling of 
waste gases from within the vessel itself and prior to dilution by 
entrained air. This method however requires the introduction into the 
vessel of a water-cooled gas sampling probe of sophisticated design. This 
probe requires careful handling and maintainance to work reliably in the 
arduous steelmaking environment and this is not possible in most cases. 
Further disadvantages of the use of methods based on waste gas analysis 
for controlling and inhibiting chromium oxidation are that the rate of 
chromium oxidation cannot be measured directly nor can the relative rates 
of oxidation of carbon, chromium and/or other metallic elements be 
measured directly. It is essential to correlate carbon removal rate with 
metal temperature, reactive gas input rate and chromium oxidation rate 
before a satisfactory gas ratio point can be selected. 
A further method for determining the rate of decarburisation is simple 
chemical analysis of samples removed from the melt at varying intervals 
during refining. This is more accurate than the above methods as the rates 
both of carbon and chromium oxidation can be measured directly but has the 
disadvantage that there is a delay between the point at which the sample 
is taken and the point at which the analysis is available so that control 
action in response to the analysis is also necessarily delayed. 
It is therefore an object of the present invention to provide a method 
overcoming the disadvantages inherent in the above methods. 
According to one aspect of the present invention a method is provided for 
refining molten metal containing an impurity which is oxidisable in 
preference to one or more selected constituent elements of the metal and 
which forms a gaseous oxide, the method comprising injecting oxygen and a 
diluent gas into the molten metal contained in a vessel so as to reduce 
the proportion of the impurity in the metal by evolution as the gaseous 
oxide, weighing the vessel during refining to establish a point at which 
the weight of the vessel contents changes significantly due to 
preferential oxidation of the one or more selected constituent elements, 
initiating at the point a reduction in the ratio of oxygen: diluent gas so 
as to reduce the rate of oxidation of the one or more selected constituent 
elements relative to the impurity and continuing injection until the 
proportion of the impurity is reduced to the desired level. 
The significant change in the weight of the vessel contents may take the 
form of a significant inflexion in the shape of a curve recording the 
weight of the vessel contents. The significant change in the weight of the 
vessel contents may be more clearly established from a determination of 
the rate at which the weight of the vessel contents is changing. 
Preferably the vessel is weighed continuously during refining and suitably 
during refining the reduction in the ration of oxygen: diluent gas is 
continued as one or more further points representative of significant 
changes in the weight of the vessel contents are established. Conveniently 
the reduction is continued until a final ratio is achieved. The object in 
this case is to maximise the rate of oxidation of the impurity element and 
minimise the oxidation of the one or more constituent elements. 
The molten metal may be an alloy of, for example, high chromium iron 
destined to become stainless steel in which the impurity is carbon and the 
one selected constituent element is chromium. In this case the reduction 
in the ratio of oxygen: diluent gas may be performed in two stages 
corresponding to two points at which sharp increases in the weight of the 
vessel contents are found to occur. Preferably the initial ratio is at 
least 3:1 and is reduced in the first stage to an intermediate ratio of 
2:1. In the second stage the ratio may be reduced from 2:1 to a final 
ratio of 1:2. The initial ratio may be held constant during the initial 
refining period until the occurrence of the first point at which a 
significant change occurs in the weight of the vessel contents.

Referring to FIG. 1 a converter 1 is mounted on conventional right angled 
pedestals 2 which are pivotally connected at the outer extremity of their 
lower arms 3 to a pivotal mounting member 4. The upper arms 5 of the 
pedestals 2 are provided with bores in which are disposed the converter 
trunnions 6 which rest on bearings 7 (shown in broken line). The trunnions 
6 are connected to a tiltable circular frame 8 upon which the converter 1 
rests. The pedestals 2 themselves are supported on force transducers or 
load cells 9 which are located at the inner end of the lower arms 3. The 
arrangement of load cells 9 and the pivotal mounting enables the converter 
1 to float freely so that the weight of the converter 1 and its contents 
can be measured via the load cells 9 and associated electrical bridge 
equipment (not shown). 
The arrangement is similar to that shown in U.K. Pat. No. 1,373,652 and as 
in that arrangement the load cells 9 can be located within the bores 
between the bearings 7 and the pedestals 2 rather than underneath the 
pedestals 2. 
The converter 1 is of the type where oxygen is injected through the top of 
the converter 1 via a lance (not shown) and the diluent gas (in this case 
argon) is injected into the bottom of the converter 1 via a pipe 10. The 
pipe 10 is supplied with the argon via the trunnion 5 which is hollow and 
which is connected to a union 11 into which argon issues from a supply 
pipe 12. 
Referring to FIGS. 2 and 3, the weight records with respect to time of two 
melts of similar composition are shown. The initial charge weight in each 
case was 1150 Kg and this was taken as the mean weight shown as zero in 
FIGS. 2 and 3. 
Referring to FIG. 2 the initial composition of the molten iron charge was 
1.4% C, 0.045% P, 0.47% Si, 0.01% S, 0.80% Mn, 8.3% Ni, 17.4% Cr, the 
balance being iron, and incidental impurities. The charge was poured into 
a converter at a temperature of 1530.degree. C. Oxygen was injected into 
the charge at 180 m.sup.3 /h through the top of the converter and air and 
nitrogen (the diluent gas) were injected through the base of the 
converter. The air flow through the pipe core was 35m.sup.3 /hour and the 
nitrogen flow rate through the pipe annulus was 8.5m.sup.3 /hour, that is, 
the initial ratio of oxygen: diluent gas was greater than 3:1.8 Kg of lime 
was added to the charge at the start of the blow. 
During refining the weight of the vessel and its contents was continuously 
monitored and the weight changes occurring to the contents were plotted 
against time as shown in FIG. 2. It will be seen from FIG. 2 that apart 
from the initial increase in weight at the start of refining of about 12 
Kg as oxygen was absorbed initially by the various elements of the charge 
e.g. iron, silicon, etc. the weight of the contents remained virtually 
constant until point A. This showed that the weight gain due to the 
oxidation of iron, silicon etc. which formed a slag was being balanced by 
the weight loss due to the evolution of carbon from the charge as CO and 
CO.sub.2. At point A however which occurred some 9.4 minutes after the 
start of refining there was a sudden and dramatic increase in the weight 
of the vessel contents. This showed that virtually all the silicon had 
been oxidised and that now oxidation of chromium (forming an oxide in the 
slag) was proceeding at a rate exceeding that of carbon. An analysis of 
the melt at this stage produced the following results: 0.37% C, 0.042% P, 
0.10% Si, 8.3% Ni, 0.44% Mn, 16.7% Cr, the balance being iron and 
incidental impurities. The temperature of the melt was 1730.degree. C. 
At point A the top lance was removed and subsequent stages were bottom 
blown. The gas mixture flow to the pipe core was 60m.sup.3 /hour oxygen 
and 21.5m.sup.3 /hour argon, with a further 8.5m.sup.3 /hour argon being 
blown through the pipe annulus to replace nitrogen. That is the ratio of 
oxygen: diluent gas was reduced to 2:1. This enabled the carbon and 
chromium oxidation rates to be balanced so that the weight of the vessel 
contents remained roughly constant until point B was reached when once 
again there was a sudden and dramatic increase in the weight of the vessel 
contents. This point occurred some 6.1 minutes after point A and showed 
that the rate of oxidation of chromium was now exceeding that of carbon. 
The analysis of the charge at this point showed the following: 0.12% C, 
0.041% P, 0.04% Si, 0.36% Mn, 8.6% Ni, 16.2% Cr the balance being iron and 
incidental impurities. 
At point B the oxygen rate was immediately reduced to 30 m.sup.3 /hour and 
the argon rate was increased to 60 m.sup.3 /hour that is the ratio of 
oxygen: diluent gas was reduced to 1:2, injection was continued for a 
further 6.1 minutes until the final carbon level was 0.03%, 20 Kg of Fe Si 
were added and the melt was stirred with argon at 28.5 m.sup.3 /hour for 5 
minutes to recover chromium from the slag. The final composition was 0.03% 
C, 0.011% S, 0.04% P, 0.16% Si, 0.49% Mn, 8.5% Ni, 17.2% Cr and the 
balance iron at a temperature of 1650.degree. C. The total loss in 
chromium to the slag was therefore only 0.2%. This is significantly less 
than is at present obtainable with conventional techniques. 
Referring to FIG. 3 the initial composition of the melt was 1.6% C, 0.01% 
S, 0.045% P, 0.28% Si, 0.81% Mn, 8.3% Ni, 17.8% Cr, the balance being iron 
and incidental impurities. The charge was poured into the converter at a 
temperature of 1520.degree. C. The initial injection rates and ratios for 
oxygen air and nitrogen were the same as for the process described in 
relation to FIG. 2. This stage of the process was terminated at point A', 
10.8 minutes after the start of refining, where the temperature of the 
melt was 1720.degree. C. and the analysis was 0.27% C, 0.01% S, 0.06% Si, 
0.33% Mn, 8.7% Ni, 17.1% Cr, the balance being iron and incidental 
impurities. The analysis thus indicated that the carbon content was 
approximately 0.30% and this was why the stage was terminated at this 
point. The oxygen: diluent gas (argon) ratio was then set at 2:1 and the 
injection rates were 60 m.sup.3 /hour and 30 m.sup.3 /hour respectively as 
in the process described in relation to FIG. 2. This stage of the process 
was terminated in line with conventional practice at point B' after a 
further 6.2 minutes when the analysis indicated that the carbon content of 
the melt was 0.10%, the temperature of the melt then being 1690.degree. C. 
The analysis was in fact 0.10% C, 0.04% Si, 0.01% S, 0.18% Mn, 8.5% Ni, 
15.7% Cr, the balance being iron and incidental impurities. The oxygen: 
argon ratio was then reduced to 1:2, the injection rates being 
respectively 30 m.sup.3 /hour and 60 m.sup.3 /h. The refining process was 
then finally terminated after a further 8 minutes and 20 Kg Fe/Si were 
added to the melt which was stirred with argon for 5 minutes at a rate of 
28.5m.sup.3 /hour. The final analysis of the melt was 0.04% C, 0.01% S, 
0.25% Si, 0.32% Mn, 8.9% Ni, 16.9% Cr the balance being iron and 
incidental impurities and the temperature of the melt was 1660.degree. C. 
The total loss of chromium to the slag was thus 0.9% which is much higher 
than than obtainable with the process described in relation to FIG. 2. It 
will be seen from FIG. 3 that the reductions in the ratios of oxygen: 
diluent gas were both made after the sudden increase in the weight of the 
vessel contents when oxidation of chromium is occurring at an increasing 
rate and this explains why the loss of chromium in the conventional 
process is much higher than that in the process of the present invention. 
The great advantage of the present invention is that the correct points 
for reduction in the ratios can be instantly identified from FIG. 2. 
More importantly however the points at which significant changes in weight 
occur indicate clearly the stages at which oxidation of carbon is slowing 
down and oxidation of chromium is increasing so that action can be taken 
instantly to rectify this situation and ensure preferential oxidation of 
carbon. The fact that the differential rates of chromium and carbon 
oxidation are measured directly enables the effects of temperature 
variations in the chemical equilibrium to be compensated for. 
It will be appreciated that the invention has application to the refining 
of metals and alloys other than stainless steel such as non-ferrous metals 
like copper in which the impurity is sulphur, copper being the selected 
constituent element. 
It will be further appreciated that the ratio of oxygen: diluent gas can be 
continuously varied during the whole of the refining process in accordance 
with the weight changes occurring in the vessel contents and this 
procedure would if optimised still further reduce oxidation of chromium. 
In this case of course it would be necessary to determine an optimum gas 
ratio for minimum chromium oxidation at selected periods during refining 
and relate this to the weight change graph for a melt of stainless steel. 
While FIGS. 2 and 3 are plots of weight change against time, it will be 
appreciated that the weight changes at points A, A', B and B' are more 
clearly shown up if rate of weight change is plotted against time and 
where possible this is a recommended practice.