Note: Descriptions are shown in the official language in which they were submitted.
13~42~6~
The present invention relates -to calcium-magnesium
alloys for use in the removal of bismuth from lead by the
Kroll-Betterton process, or for similar lead refining
processes which require alkaline-earth metals.
B~CKGROUND
In the Kroll-Betterton process, alkaline earth
metals are added to the lead melt in order to react with the
bismuth therein. One or more alkaline earth metals, usually
magnesium and calcium, are added in either a continuous or
batch fashion to the unrefined lead. The preferred
temperature range for making the addition is between 380C to
500C. Below this tempera-ture range, the reaction is
sluygish while above the range excessive oxidation of
reactive alkaline earth metals, particularly calcium, occurs.
Oxidation gives rise to briyht flaring, excessive ~ume
generation and an overall loss of reagent leading to lower
reagent recoveries, excessive processing costs, unpredictable
final bismuth levels and environmental concerns.
Furthermore, the addition of calcium metal to the lead bath
is often accompanied by an increase in the bulk temperature
of the lead either due to an exothermic release of heat
during the reaction and/or the heat generated by the
oxidation of calcium metal. This increase in bath
temperature may result in additional ca]cium oxida-tion as
well as lengthening the overall processing time since the
melt must be cooled to just above its solidification point
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prior to removing the bismuth rich dross~
Another disadvantage of calcium metal is that it is
highly reactive with atmospheric oxygen and humidity. Hence,
calcium metal must be pac~aged, shipped and stored in such a
way as to eliminate contact with air and moisture. Excessive
con-tact with water will result in heat and hydrogen evolution
which can cause fire and explosion. Mild contamination of
the calcium prior to the lead treatment will result in lower
than expected reagent recoveries and unpredictable final
bismuth levels.
After the lead has been treated with the alkaline
metals, the melt is then cooled to a temperature near its
solidification point which causes the alkaline-earth bismuth
compounds to float up as a solid dross which may be skimmed
from the surface of the melt.
Most commercial debismuthizing processes utilize a
heterogeneous mixture of magnesium and calcium metals. In
the present invention, debismuthizing is carried out with an
alloy comprising of essentially magnesium and calcium with
the ratio of magnesium to calcium on a weight basis being
between 1.2 and 5.2 and in the preferred embodiment of the
invention, between 1.9 and 3Ø
The concept of substituting alloys for metallic
magnesium and calcium was initially suggested by Betterton in
1930, U.S. Patent No. 1,~53,540, who tested alloys comprising
of maynesium and lead and calcium, magnesium and lead.
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'1'.l~.~. Davey "q'he Physical Chemistry of Lead Refining", Lead-
Zinc-rin 1980, edited by J~Mo Cigan et al., Metallurgical
Society of AIME, p. 477, men-tions the use of a 5% calcium-
lead alloy while Kirc1l and Othmer "Lead", Encyclopedia of
C}lemical Technology, Vo. 8, The Interscience Encyclopedia
Inc., New York, 1952,refer to a 3~ calcium--lead alloy. In
all oE these cases, lead is -the principal alloying
consti-tuent and is present to lower the melting point of the
reayent thus promoting dissolution oE magnesium, and in
particular calcium, both of which have melting points
substantially hiyher than the lead bath temperature.
In U.S. Patent No. 2,129,~45, Rehns mentions that
lead can be debi.smuthized by Eloating a calcium-magnesium
alloy on the surface of a mechanically stirred lead bath.
The alloy contained 79.4% magnesium and 20.6% calcium by
weight. Rehns specifically points out that when using a
calcium-magnesium alloy of the cited composition, it is
necessary that the lead bath be raised to a higher
temperature, namely 593C.
~eference to a binary magnesium-calcium phase
diagram shows that the addition of calcium to
magnesium will initially lower the melting point of the alloy
compared to metallic magnesium. However, once the alloy
exceeds lG.2% calcium (i.e. a Mg to Ca ratio of 5.i7), its
melting point begins to rise due to an increasing
concelltration in the eUtectic of the highly stable
,
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intermetallic compound, Mg2Ca. This stable compound has a
melting point of 715C which is about 200 - 300C above
commercial debismuthizing temperatures.
The same phase diagram, also shows that the 79~4~
magnesium, 20.6% calcium alloy sug~ested by Rehns begins to
melt at 516.5C and is fully molten by about 575OC. sy
specifying a lead bath temperature of 593C, Rehns ensures
that this alloy will be fully molten and hence its
dissolution and the resulting reagent recovery will not be
impeded by the presence of any unmelted, highly stable Mg2Ca
intermetallic compound.
Kroll-Betterton type debismuthizing processes
usually operate in the 380C to 500DC range. Rehns specified
temperature, 593C, is thus substantially higher than
reported commercial debismuthizing practices.
In the present invention, magnesium-calcium alloys
with magnesium to calcium ratios on a weight basis between
1.2 and 5.2, and preferably between 1.9 and 3.0, are added to
lead in the commercial temperature range, that is between
380C to 500C. As indicated by the relevant phase diagram,
all of these alloys have melting points in excess of 516.5C
and, in the range of the preferred embodiment, the alloys do
not fully melt until temperatures exceed between 610C to
685C which is substantially above the temperature of the
lead bath. Contrary to the teachings of the Rehns patent,
which ensures that the alloy is completely melted by
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specifying a higher process temperature of 593C, in the
present invention the alloys do not completely melt and hence
the reaction must proceed by dissolving (not melting) a solid
into liquid lead. According to the eutectic composition of
these alloys, this solid phase is essentially the stable,
high melting point Mg2Ca intermetallic compound. Hence, the
present invention differs from that of Rehns since the
mechanism of introducing the reagent into the lea~ is
considerably different, that is melting the reagent alloy in
the Rehns patent and solid-liquid dissolution in the current
invention. In the former case, the rate of reaction depends
only on how fast the alloy melts which in turn depends on the
rate of heat transfer from the bath to the reagent. Once
melted, any ~g2Ca compound present in the alloy is completely
dissociated and hence available ~or debismuthizing.
In the present invention, the rate at which the
solid Mg2Ca phase in the alloys eutectic dissolves into the
liquid lead depends on thermodynamic and kinetic
considerations which are related to the chemical stability of
Mg2Ca relative to magnesium-calcium-bismuth compounds which
form during debismuthizing. The rate of dissolution and
hence the degree of dissociation of Mg2Ca in the alloy has
significant commercial significance as it will determine
processing time and reagent recoveriesO
French Patent Application No. 81 19673 assigned to
Extramet tPublication No. 25614 786, April 22, 1983)
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dlscloses a process for debismuthizing lead by using a
mixture of two types of alloy granules. The first type of
granule comprises a calcium-magnesium alloy near the calcium-
rich eutectic point (approximately 82 weight % calcium~ and
the second alloy comprises a magnesium-calcium alloy near the
magnesium-rich eutectic point (approximately 16.2 weight %
calcium). These two types of granules are mixed together in
the appropriate amounts to give the ratio of the metals for
the best result and are injected into the lead melt to react
lo with the bismuth therein. The composition of the individual
alloys are chosen to ba near the eutectic points so that they
have relatively lower melting points compared to pure
maynesium and calcium metals. It is claimed that this speeds
up the rate oP the reaction at a given processing
temperature. The mixture is injected into the lead bath with
an inert gas. The temperature of the lead bath is maintained
high enough to melt and not simply dissolve the lead
granules.
This heterogeneous mixture of magnesium-rich
calcium-rich alloy granules is still susceptible to poor
reagent recovery because the calcium-rich alloy granules will
behave in much the same way as pure calcium metal. Because
of the composikion of calcium-rich eutectic alloy granules,
the eutectic may contain up to almost 2/3 of finely divided
calcium metal with the remainder being the Mg2Ca
intermetallic compound. The high proportion of calcium metal
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in the eutectic causes the calcium-rich alloy granules to
reac-t with atmospheric oxygen and humidity in much the same
way as calcium metal~ Tests with ingots cast at the calcium-
rich eutectic composition have shown that this alloy reacts
with atmospheric oxygen and humidity and, hence, is not
stable in air.
Because of the reactive nature of the calcium-rich
granules, the heterogeneous granule mixture of magnesium-rich
granules and calcium-rich granules must be packaged under
dry, inert gas in a similar fashion to calcium metal.
Contamination of the calcium-rich granules with oxygen or
moisture prior to treatment will result in lower reagent
recoveries and unpredictable final bismuth levels. The
calcium-rich granules are also susceptible to oxidation
during treatment with the lead in much the same way as
calcium metal, especially if they float to the surface before
- they have completely reacted due to large differences in
density between lead and calcium. The injection of the
granules into the lead bath with an inert gas carrier adds
additional turbulence to the melt, increasing the amount of
oxidation and emissions from the lead bath.
In the present invention, the difficulties
associated with the use of calcium metal or granular mixtures
containing calcium-rich alloy granules are avoided by using a
single magnesium-calcium alloy o~ the desired composition.
In this invention, the alloy is primarily made up of
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magnesium and calcium hut may contain one or more minor
amounts of other alloying elements.
The present invention will now be described in more
detail in conjunction with the accompanying drawings, in
which:
Figure 1 is the known binary magnesium-calcium phase
diagram;
Figure 2 is a graph showing the e~fect of the Mg/Ca
ratio on the quantity of alloy required to reduce the bismuth
concentration to prescribed amounts;
Figure 3 is a graph showing the effect of the same
ratio on incremental cost;
F'igure 4 is a graph showing the effect of the same
ratio on the melting temperature of the alloy; and
Figure 5 is a graph showing the effect of the same
ratio on the percentage of MgzCa intermetallic compound
contained in the alloy.
In the present invention, an alloy for use in lead
refining is provided which is rich in maynesium and has
magnesium to calcium ratios on a weight basis between 1.2 and
5.2; the lower ratio corresponding to the intermetallic
compound Mg2Ca. In a preferred embodiment of the invention,
the alloy has a magnesium to calcium ratio between about 1.9
to 3Ø
Fiyure 1 illustrates the binary magnesium-calcium
phase diagram and shows that the addition of calcium to
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magnesium will initially lower the melting point of the alloy
compared to metallic magnesium. However, once the alloy
exceeds 16.2% calcium (i.e. a Mg to Ca ratio of 5.17), its
melting point begins to rise due to an increasing
concentration in the eutectic of the highly stable
intermetallic compound, Mg2Ca. This stable compound has a
meting point of 715~C which is between about 200 - 300~C
above commercial debismuthizing temperatures.
In Kroll-Betterton processes, magnesium and calcium
are first dissolved in liquid lead at temperatures usually
between ~15~C to 500C. Subsequent cooling of the lead
precipitates a solid compound, CaMg2Bi2, which is separated
out in the dross. The lead is eventually cooled to just
above its liquidus temperature; however, some calcium,
magnesium and bismuth will still be retained in solution in
the lead.
T.R.A. Davey in "The Physical Chemistry of Lead
Refining" published in 1980 by The Metallurgical Society of
the ~IME indicates that at a specific final bismuth
concentration, the amount of calcium and magnesium retained
in solution in the lead at the liquidus temperature is given
by equation (1):
log (%Ca) -~ 2 log(%Mg) + 2 log (%Bi) = -7.37 ...(1)
The inventors have calculated the theoretical alloy
requirements to chemically remove bismuth, based on the
stoichiometry of the bismuth containing intermetallic,
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CaMg2Bi2, and the solubility relationship given in equation
(1) -
Figure 2 illustrates the effects of alloy
composition on the quantity of alloy needed to remove bismuth
to 0.005% and 0.020% which represents the range of final
bismuths in most commercial treatments.
As indicated in Figure 2, for both final bismuth
levels, the amount of alloy required increases exponentially
as the calcium content of the alloy decreases below 35% (a Mg
to Ca weight ratio of about 1.9). Conversely, a higher
calcium content (i.e. 40~ Ca) does not significantly reduce
the quantity of alloy needed to remove bismuth. I[ence, based
on this analysis, an alloy with a Mg to Ca weight ratio of
about 1.9 is chemically optimum for removing bismuth from
lead.
From a commercial standpoint, however, calcium is
between 1.5 to 2.0 times more costly than magnesium. Hence,
the most cost effective commercial alloy will depend both on
the chemical requirements to remove bismuth and the
proportion o~ costly calcium relative to less expensive
magnesium in the alloy.
Figure 3 illustrates the effects of alloy
composition on the percentage change in the lead refiners'
cos-t relative to an alloy containing 60~ calcium. These data
are based on the amount of alloy required to chemicall~
remove bismuth and the cost of the magnesium and calcium
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components in the alloy. It can be seen that, depending on
the final bismuth level, the lead refiners' costs are lowest
for alloys containing between 25% to 35~ calcium (a Mg to Ca
weight ratio between 3.0 to 1.9).
Hence, based on both chemical and cost
considerations, alloys containing between 35~ to 2~ calcium
(i.e. Mg to Ca weight ratios between 1.9 to 3.0) are optimum.
In addition to minimizing the alloy requirements
needed to chemically remove bismuth, the dissolving rate of
the alloy at conventional debismuthizing temperatures has
significant commercial implications since it will determine
the amount of alloy that can be recovered during the allotted
processing time.
As indicated in Figure 4 (which was derived from the
phrase diagram, Figure 1), all of the alloys in the present
invention have final melting points in excess of the eutectic
temperature, 516.5C, and do not fully melt until
temperatures exceed between 610C to 685~C which is
substantially above the temperature o~ the lead bath.
As a result, in the present invention the alloys do
not completely melt and hence the reaction proceeds by
dissolving (not melting) a solid into liquid lead. ~ccording
to the eutectic composition o~ these alloys, this solid phase
is essentially the stable, high melting point Mg2Ca
intermetallic compound.
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In the present invention, th~ time required for the
alloys to reac-t depends on the dissolving rate of the stable,
high melting point of Mg2Ca which in turn depends on
thermodynamic and kinetic considerations rela-ted to the
stability of Mg2Ca relative to the CaMg2Bi2 dross.
Table I summarizes the results of laboratory tests
to determine the effects of composition, temperature and
agitation on the dissolving rate of these alloys:
Table I: Results of Laboratory tests to Examine the Effects
of Alloy Composition, Temperature and Agitation on
the Dissolving Rate of Mg-Ca Alloys in Liqui~ Lead
loy Temperature Agitation Dissolving Rate
3_~9_ ~ Ca Mq/Ca _ C _ _ gm~cm2/hr
5.6 425 No 3.5
2.3 425 No 1.0
2.3 500 No 4.0
2.3 425 Yes 3.5
These tests indicate that at 425C, an alloy
containing 15% calcium (i.e. a Mg to Ca weight ratio of about
5.6) dissolves about 3.5 times faster than an alloy
containing 30% calcium (i.e. a Mg to Ca ratio of 2.3).
As indicated in Figure 4, the 15% calcium alloy is
fuIly molten at 530C which is 120C below the melting point
for the 30% calcium alloy.
As shown in Figure 5, this lower melting point and
hence faster dissolving time can be attributed to the fact
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13
that the 15% calcium alloy contains only 33% of the high
melting point ~g2Ca intermetallic in its eutectic compared to
66% ~g2Ca for the 30% calcium alloy.
The alloy's dissolving rate is also depe~dent on the
temperature of the lead bath. The results shown in Table I
indicate that the dissolving rate of a 30% calcium alloy (a
Mg to Ca weight ratio of 2.3) increases by about 4 times when
the lead temperature is increased from 415~C to 500C which
covers the range of processing temperatures for most
commercial debismuthizing operations. Agitating the lead
will also increase the alloy's dissolving rate.
To summarize, magnesium rich-calcium alloys wi-th Mg
to Ca weight ratios between 1.9 to 3.0 are superior to other
a]loy compositions since they combine the optimum chemical
reactivity and dissolving characteristics.
Alloys containing about 35% calcium (i.e. a Mg to Ca
weight ratio of 1.9) are the most chemically effective since
they m:inimize the amount of alloy needed to remove bismuth
from lead. However, the slow dissolving rate of this alloy
limits its use commercially to practices which operate at
high temperatures (about 500C) with aggressive agitation.
Conversely, for debismuthizing practices operating
at lower temperatures and/or with less agitation, alloys
containing as low as 25% calcium (i.e. a Mg to Ca weight
ratio of 3.0) are more commercially attractive since they
offer significantly faster dissolving rates at an acceptable
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chemical reactivity with bismuth (see Figures 2 and 3).
Magnesium rich-calcium alloys with Mg to Ca weight
ratios outside the 1.9 to 3.0 range are inferior for removing
bismuth because they are either too rich in calcium leading
to inordinately long processing times and high pracessing
costs or too rich in magnesium to be sufficiently reactive
with bismuth.
The alloys of the present invention are prepared by
melting the appropriate proportions of calcium and magnesium
lo metals under a protective atmosphere and pouring and
solidifying the alloy in the same or similar protective
atmospheres. The protective atmosphere may comprise
nitrogen, argon or any other gases which are protective or
non-reacti~e when in contact with magnesium and calcium. The
tempera-ture used to melt the metals and prepare the alloy is
preferably but not necessarily in the range of 680 - 7500C.
In a further aspect of the present invention, a
method for achieving the solution of calcium in lead
resulting in high recoveries is provided. This method
comprises the steps of providing a magnesium and calcium
alloy ~hich has a magnesium to calcium ratio between 1.2 and
5.2, and adding this alloy to a lead bath.
Since these magnesium-rich alloys consist of
eutectic structures which contain mostly finely divided
magnesium metal and Mg2Ca intermetallic with the complete
absence or only minor quantities of finely divided calcium
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metal, they are not subject to the aforementioned
difficulties associated with calcium metal or calcium-rich
alloy granules.
In the present invention, these alloys are stable in
air. Since -the alloy does not oxidize or hydroxylize in air,
it does not require special packaging or protective
atmospheres. There is no danger of fire or explosion if
these alloys come in contact with moisture.
When added to liquid lead, these alloys react with minimal or
no oxidation. The reaction is often accompanied by a minor
degree of bubbling; however, there is essentially little or
no flaring or fume generation. Since the alloys are not
prone to contamination from contact with air prior to
treatment, reagent recoveries are higher and more predictable
than with other reagents. Further, since the alloys do not
oxidize readily even if they float to the surface, provided
the bath is being agitated no excessive flaring or fuming
occurs, which would lead to lower recoveries. This
substantially increases the predictability of achieving the
desired final bismuth level which is particularly important
when aiming at low bismuth levels of less than 0.01~.
The alloy is preferably added to the lead bath in
the form of large ingots. Under some circumstances, smaller
inyots, large chunks, granules or powder may also be used.
The alloys can be added either by plunging or supplied to the
surface of an agitated lead bath.
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When the alloy is added to the lead bath, the bulk
temperature of the melt does not increase as is often the
case with calcium metal additions. In this invention, the
alloys can be added at commercial debismuthizing temperatures
that are between about 380C to ~00C and are not restricted
to the higher temperatures needed to fully melt the alloy as
in the case of the prior art discussed. In general, the
dissolution rate of these alloys increases with increasing
temperature and by agitation. Since there is virtually no
flaring and related fume generation with this alloy, even at
temperatures as high as 530C and with agitation, no special
fume collection system is required to contain emissions.
Agitation is sometimes avoided when calcium metal is utilized
as it increases oxidation and flaring.
After the alloy has been added to the lead melt and
the dissolution is complete, the lead melt is allowed to cool
in the customary fashion of the Kroll-Betterton process to
separate out the solid bismuth-rich dross.
The ~ollowing examples are given to demonstrate the
high reagent recoveries that are possible with this alloy.
Refined lead low in bismuth was used in all tests to enable
investigation of the effects of process conditions on alloy
dissolution recoveries without the complications of side
reactions with bismuth.
2~ EX~PLE 1:
Approximately 98.8 grams of a magnesium-calcium
alloy with a magnesium to calcium ratio of 2.7 was plunged
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into a 20 kilogram quiescent lead melt at 419C. No flaring,
oxidation or fume generation was observed. Approximately 45%
of the alloy dissolved after 30 minutes with essentially 100%
reagent recovery. Einal magnesium and calcium analyses were
0.16% and 0.06% respectively.
EXAMPLE 2-
Approximately 98.7 grams of a magnesium-calcium
alloy with a magnesium to calcium ratio of 3.0 was plunged
into a 20 kilogram agitated liquid lead melt at 415C. No
flaring or fume was observed. Approximately 98% of the alloy
dissolved after 23 minutes of stirring with essentially 100
reagent recovery. The final magnesium and calcium analyses
were 0.33% and 0.11%, respectively.
_AMPLE 3:
Approximately 98.8 grams of a magnesium-calcium
alloy with a magnesium to calcium ratio of 2.7 was plunged
into a 20 kilogram quiescent lead melt at 432C.
Approximately 90% of the sample had dissolved after 30
minutes with essentially 100% reagent recovery. No flaring
or fume was observed during the treatment. The final
magnesium and calcium analyses were 0.32% and 0.12%
respectively.
EXAMPLE 4:
Approximately 97.7 grams of a magnesium calcium
alloy with a magnesium to calcium ratio of 3.0 was plunged
into a 20 kilogram quiescent liquid lead melt at 500C. The
reaction was characterized by heavy bubbling; however, no
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flaring, oxidation or fume generation was evident. The alloy
was completely dissolved after 12 minutes with essentially
100% recovery at 0.38% magnesium and 0.13% calcium. Black
dross was observed to form on top of the melt after 22
minutes which was accompanied by a 13 - 15% fade in the
dissolved magnesium and calcium after 30 minutes to 0.33%
magnesium and 0.11% calcium.
In summary, this application has disclosed an
invention which improves the dissolution characteristics of
magnesium and calcium in lead at commercial debismuthizing
temperatures thereby improving the efficiency of b:ismuth
removal from lead. This alloy is stable in atmospheric air
and humidity and requires no special protective packaging as
does calcium metal. When added to liquid lead at commercial
processing temperatures, the alloy dissolves with essentially
no oxidation, flaring and Eume generation. This results in
higher and more consistent reagent recoveries and more
predictable final bismuth levels which are particularly
important when aiming for final bismuth levels less than
about 0.01%. The virtual absence of fume precludes the need
for special fume collection systems. The absence of flaring
and oxidation enables the alloy to be added with agitation
and, if desired, at higher processing temperatures than is
customary with calcium metal.
Thus, the present application describes the use of
certain magnesium-calcium alloys in Kroll-Betterton type
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processes for the removal of bismuth impurities from lead.
The inventors have found that the use of certain magnesium
rich-calcium alloys at commercial debismuthizing temperatures
results in a more efficient process since;
(i) in the preferred compositional range, the
amount of alloy required to remove bismuth is
minimized and the alloy's dissolving rat~s are fast
enough for commercial debismuthizing operations.
(ii) with these alloys there is essentially no
burning, flaring or fuming during the lead treatment
which results in higher, more preclictable reagent
recoveries.
(iii) the alloys are resistant to atmospheric oxygen
and humidity and, hence, do not require special
pac~aging or protective atmospher~s.
(iv) the alloys are sufficiently strong and ductile
to enable casting and shipping as ingots of a
consistent weight and size, thereby permitting
precise addltions to the lead bath.
These magnesium~calcium alloys are superior to other
alloy compositions since this preferred range minimizes the
amount of alloy required to remove bismuth and gives alloy
dissolving rates which are acceptable at commercial
debismuthizing temperatures.
The present invention has been described using
preferred ratios of magnesium to calcium. Clearly, minor
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variations in these ratios may be made within the scope of
the lnvention. The alloy may contain other constituents,
such as different alkali earth metal, which do not affect the
essential nature of the metallurgical process herein
disclosed.
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