Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1~5~
The invention is a process for substantlally lowering
the level of elements such as cobalt and iron, and in addition
lead, contained in a molten nickel matte by selective oxidation
in the presence of a molten slag mixture. The transfer to the
slag of elements which form more stable oxides than nickel and
have a higher affinity for the slag purifies the nickel mattè.
The slag is then subjected to a reduction ~reatment to remove
the metal oxides and thereafter returned for further matte
refining.
In the pyrometallurgical processing of nickel-containing
sulfide concentrates, the concentrate is roasted and smelted to
produce a furnace matte. This matte contains about 15% to 45%
iron and is converted by the addition of a silicious flux, which
serves to oxidize and slag the iron, thereby providing a matte
suitable for further processing. Following this conversion the
matte usually contains about 0.8~ iron, about 0.8~ cobalt~ and
about 0.05% lead as well as any copper present from the furnace
matte. However, it is highly desirable to reduce further the
levels of these elements. For example, since cobalt is physi-
cally and chemically akin to nickel, it is difficult to separate
this element from nickel, but for a number of applications~it is
of great importance to keep cobalt at its lowest practicable
level. Furthermore, the presence of iron, even in small ~
quantities, can adversely affect the quality of nlckel electro-
plate and also, it is an undesirable constituent in some high - -
nickel alloys. Similarly, the deleterious effect of lead on
'~ the weldability of nickel alloys is well documented. Thus, for
these and other reasons, it is advantageous to limit the levels
of impurity elements in the mat e from which the final form of
0 nickel is produced.
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The process of the present invention is intended to
reduce the levels of iron, cobalt, lead and elements that form
more stable oxides than nickel at an earlier stage of the
refining process. Early removal is intended to eliminate the
need for separate refining steps later in the operation such
as those required in the electrorefining of nickel.
Known pyrometallurgical processes, concerned with
removal of elements such as coba:Lt and iron at an early stage
of the nickel refining operation, differ substantially from the
method of the present invention although they too make use of
the principle of selective oxidation. One of these methods
is concerned with the affinity of three metals, i.e., Fe, Ni,
Co, for oxygen and uses an oxide slag for refining nickel-
containing matte. The slag, consisting mainly of iron and
cobalt oxides, is formed in situ by preferential o~idation of
some of the metallic components of the matte at temperatures
of about 1100C to 1200C. The slag from the process which
is enriched in the valuable element cobalt and also contains a
substantial amount of nickel, is subsequently discarded. The
major disadvantage of this process is that the cobalt and nickel
values, contained within the discarded slag, cannot be economi-
cally recovered.
Another prior art process makes use of a slag containing
nickel oxides, oxides of metals less noble than nickel, silica
,
and lime. This slag is brought into contact and reacted with
a molten bath containing nickel and impurity elements. The
impurity elements are preferentially oxidized, transferred to
another location in the reaction vessel, reduced and removed.
This process is, of necessity, operated at a temperature in
' 30 excess of the melting temperature of nickel, above about
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1455~C, and such a high temperature is known to cause deterio-
ration of refractories and in addition requires substantial
quantities of fuel and hence is costly. The high operating
temperature of this process represents a major drawback.
It has now been discovered that -the levels of elements
that form oxides more readily than nickel, such as iron and
cobalt, can be substantially reduced in a molten nickel matte
by selectively oxidizing the matte while in contact with a
molten slag mixture. The molten slag mixture is principally
composed of alkali metal oxides, silica and boron oxide but can
also contain alumina and alkaline earth metal oxides. Oxides
of elements more reactive than nickel transfer by gravity
separation to and dissolve in the oxide slag layer. Also,
surprisingly the lead content of the nickel matte is substan-
tially lowered during treatment. After a suitable contact
period, the slag mixture is removed from contact with the matte -
and subjected to a reduction step wherein the contained impurity
metal oxides are substantially removed from the slag by a
reduction operation. The clean slag is then returned for
refining a new batch of nickel matte.
It is an object of the process of this invention to
provide a means for removing undesired elements, such as cobalt
and/or iron from nickel matte.
It is another Gbject of this invention to provide a
~ more efficient method than presently used for removing one or
J' more elements such as iron, cobalt and lead from a nickel ma~te.
~, Yet another object of this invention is to use advan~
tageously low operating temperatures which provide substantial
J, savings in energy and, in addition, improve refractory life.
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Generally speaking, the presentinvention contemplates
a process for substantially lowering the levels of elements
that form oxides more stable than nickel and capable of sub-
stantially lowering the lead content of an impure nickel matte
and comprises contacting an impure nickel matte with a slag
mixture containing alkali metal oxides, silicon dioxide and
boron oxide at temperatures of about 650C to about 1350C
- while selectively oxidizing said impure nickel matte suf-
ficiently to convert at least one elemental impurity having
greater affinity for oxygen than nickel, such as cobalt or
iron, contained therein to a metallic oxide form and dis-
solving said metallic oxide form in said slag mixture; removing
said slag mixture containing said metallic oxide form from
contact with the purified nickel mattei reducing said slag
mixture to convert said metallic oxide form to a separable
~orm; and removing said separable form from said slag mixture.
Although the process may be used at temperatures as
high as 1350C, it is preferred that the upper temperature be
limited to about 9S0C. Operation at temperatures below 950C
provides considerable savings in fuel and also serves to extend
refractory life.
In practice, it is preferred that the slag mixture be
in continuous contact with the nickel matte and that its flow
direction be counter-current to the flow direction of the nickel
matte since impurities can thereby be removed to a lower level.
- This may be accomplished by continuously removing unclean slag ~;~
and purified nickel matte from opposite ends of a reaction
.
vessel while charging impure matte and clean slag. The counter-
current flow provides maximum contact of the slag mixture and
.
the molten nickel matte, thus improving the rate at which OXlC es
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~0557~
of impurities are removed from the nickel matte. By using a
continuous process, it has been found that only about 3% of
the nickel is transferred to the slag and a cobalt removal
of 98~ can be achieved whereas only about ~0% of the cobalt
is removed in a single-stage batch operation. The continuous
process offers the advantage of increased throughput, lower
heat losses and better efficiency in materials handling.
The slag mixture is composed of a combination of
ingredients selected to provide a melting temperature roughly
equal to or preferably 50C, above that of the melting tempera-
ture of the nickel matte. An upper limit of 1350C is required
to avoid corrosive attack of the refractories used in the
reaction vessel. A useful slag mixture can contain from about
10% to about 50~ alkali metal oxides (e.g., Na2O, K2O), from
about 5% to about 50~ silicon dioxide (SiO2), from about 20
to about 40% boron oxide (B2O3), from 0~ up to about 15~
alumina (A12O3), and from 0~ up to about 20% alkaline earth
metal oxides (e.g., CaO, BaO). A preferred slag mixture contains
from about 20% to about 35~ sodium oxide, from about 20% to
about 35~ boron oxide and from about 10% to about 45~ silicon
dioxide. The preferred slag can be prepared by melting and
adding an appropriate mixture of sodium carbonate (Na2CO3?,
borax (Na2B4O7.10 H2O) and silica (SiO2) to the molten matte.
Another factor which must be considered in selecting
the proper combination of slag ingredients is the composition
of the nickel matte and in particular the elements desired to
be removed from it. The elements that will be selectively
removed from the nickel matte will have reactivity greater
~ than that of nickel but the reactivity must be less than that
t 30 of, for example, silicon, since otherwise, the silicon dioxide
will be reduced and silicon will dissolye into the nickel matte.~ ~
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Surprisingly, the lead content of nickel mattes is
reduced during the selective oxidation process even though
thermodynamic data suggests that lead should not be removed
since lead has a lesser affinity for oxygen than nickel.
The exact explanation for the observed phenomenon is not
completely understood, however, and without being bound to
any particular theory, it is thought that some lead oxide forms
during the selective oxidation of the matte and separates to
the slag mixture. Subsequently, the lead oxide vaporizes and
is thereby removed from the slag. The vaporization of the
lead oxide from the slag, in turn, provides a driving force
for additional lead oxidation in the impure nickel matte.
In the preferred continuous process, the degree of
purification is controlled by the rates at which slag and
nickel matte are processed and oxidized. A slag may be given
several cleaning treatments and reused with the same batch
of nickel matte or contacted several times with the same batch
of nickel matte without a cleaning step.
The selective oxidation of the nickel matte may be
accomplished in a number of ways. One means consists of adding
a controlled amount of nickel oxide to the oxide slag mixture.
At the interface of the slag and the nickel matte, the nickel -
oxide reacts with the metal sulfides having greater affinity
for oxygen than nickel, giving up oxygen to these elements
which then transfer and dissolve in the slag layer.
Another means for oxidizing the nickel matte is to
introduce air or oxygen through a lance or tuyeres under the ~
surface o~ the nickel matte. This leads, in part, to direct ~ -
formation of oxides of elements having greater affinity~for
oxygen than nickel and also, the formation of nickel oxide
within the nickel matte which in turn also reacts with the
contained metal su1fides having gxeater afinity or oxygen.
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The slag layer is preferably removed from the vessel
where it has been in contact with the impure nickel matte,
and transferred to a separate vessel for temporary storage to
compile a suitable quantity for the reduction step; also, a
continuous reduction process may be effected in the second
vessel. The invention also contemplates the situation wherein
the purified matte is removed in its entirety from the reaction
vessel and the slag is reduced within the same vessel used for
the initial oxidation step. The slag can be cooled,crushed
and stored until such time as a sufficient quantity is available
for reduction. When a sufficient quantity is obtained, carbon
and/or sulfur can be blended with the slag prior to heating to
the reduction temperature.
Although a continuous operation is preferred, the present
invention can be carried out in conjunction with a conventional
converting operation. The conventional converting operation is
stopped when the matte contains about 5~ iron rather than the
usual 0.8~. In the conventional Bessemer converter or top-`
blown rotary converter, the iron silicate slag can be removed
~0- and replaced by the alkali metal oxide, silica, boron oxide
mixture. The selective oxidation of the matte is then carriéd
out by introducing oxygen or air through submerged tuyeres or
by top-lancing or by the addition of metal oxides such as
nickel oxide. During such an operation, it has been found that
about 80% of the cobalt can be transferred from the matte to ~-
the slag while only about 3~ of the nickel is transferred.
During a conventional refining treatment utilizing iron silicate
slag, about 30% of the nickel would be transferred to the slag
-- in order to obtain 80% cobalt removal from the matte~ Also,
in the conventional operation, it is dif~icult to produce
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mattes which contain less than 0.5~ iron whereas with the
process of the present invention, iron levels below 0.1% can
be achieved. The refining can also be carried out in batches
by multiple additions of clean slag with subseq~lent oxidation
and slagging.
A suitable reduction process may consist of lancing
the oxide slag with hydrogen sulfide. Sulfides of the metallic
elements, such as cobalt and iron, form and sink to the bottom
of the slag mixture due to their greater density. The purified --
slag mixture is subsequently separated from the metal sulfides
and returned to the first reaction vessel for further contact
wlth the nickel matte.
Another means for purifying the slag mixture is carbo-
thermic reduction. A fuel such as pulverized coal or ~aseous
hydrocarbons may be introduced to the oxide slag in the second
reaction vessel and used to reduce the metallic oxides contained -
' therein to metallic form. The elemental metals separate to the
bottom of the second reaction vessel where they are removed for
additional refining operations and the purified oxide slag
returned to the first reaction vessel.
A reactive element such as aluminum may be used to reduce
; the metallic oxides to elemental form. However, this type of - :
addition agent must be chosen with care since the oxides that
are formed may contaminate the oxide slag, possibly resulting in
increased slag melting temperature. Another element useul for
~ this purpose is silicon. Since silicon dioxide is one of the
-i compounds used to formulate the oxide slag, it would not serve ~:
as a contaminant, however, it would be necessary to adjust
~' the composition of the slag to allow for dilution by silica.
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1~557~
Another means for purifying ~he oxide slag contemplated
by this invention is electrolytic reduction. With this method,
the metallic contaminants are removed by plating on a suitable
electrode.
In order to give those skilled in the art a better
understanding of the invention, the following illustrative
examples are given which demonstrate the process of purifying
a nickel matte by selective oxidation and reaction with a
molten slag mixture.
EX~PLE 1
A nickel matte containing 0.54% copper, 69.2% nickel,
3.5% cobalt, 2.2% iron and 21.9~ sulfur, shown in Table I, was
treated with an oxide slag at 860C. This composition is
representative of a matte derived from a lateritic ore in which
the converting operation has been interrupted at the 2.2% iron
level, rather than the more conventional 0.8~ level, and as a
consequence also contains higher cobalt. The refining operation
was accomplished in three stages in which oxygen was lanced into
the matte for 45 minutes at a rate of 100 milliliters per minute. ~ -
200 grams of matte were treated in this manner while in contact -
with 400 grams of slag which contained 32~ Na2O, 35.5% B2O3 and
32.5% SiO2 and was prepared from a mixture of sodium carbonate,
sodium borate and silica. The melting point of this slag was
~; about 650C and it is non-corrosive to fire-clay and chrome
magnesite refractories up to about 1350~C. As shown in Table I,
the copper content of the matte remained the same during the
three stages of treatment since copper has less affinity for ;~-
oxygen than nic}cel. The cobalt content of the matte was reduced
from 3.5~ to 1.5% (57% removal) during the first stage of the
operation. Simllarly, the iron content of the matte was reduced
from 2.2% to 0.049~ (97.8~ removal).
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When 150 grams of the first stage matte was contacted
with 300 grams of new slag, the cobal~ content was further
reduced to 0.33~ (about 35% additional removal) and the iron
to 0.01%. In the third stage, 100 grams of nickel matte from
the second stage was placed in contact with 200 grams of the
new oxide slag. The cobalt content was reduced further to
0.014%. The overall efficiency for cobalt removal for the
three stage process was 99.6% and for iron, 99.5%.
TABLE I
REFINING NICKEL MATTE WITH CLEAN OXIDE SLAG
: gm : % Cu : % Ni : % Co : % Fe : % S
Stage I . : : : : :
Matte In : 200: 0.54 : 69.2 : 3.5 : 2.2 : 21.9
Slag In : 400: - : 4.0 : 0.4 : 0.1
Matte Out: : 0.55 : 73.5 : 1.55 : 0.049: 23.0
Slag Out : : 0.008: 3.88: 1.15 : 1.02 : - :-.
Stage II :
` Matte In : 150: 0.55 : 73.5 : 1.55 : 0.049: 23.0
Slag In : 300: - :` 2 : 0.02 :
Matte Out: : 0.54 : 74.0 : 0.33 : 0.01 : 23.4
Slag Out : : 0.01 : 2.79: 0.65 : 0.071:
Stage III: : : : : : .
Matte In : 100: 0.54 : 74.0 : 0.33 : 0.01 : 23.4
Slag In : 200:
Matte Out: : 0.58 : 73.4 : 0.014: 0.01 : 24.0
' Slag Out : : 0.01 : 2.3 : 0.13 : 0.05 :
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EXAMPLE 2
A slag to matte ratio of 1:1 was used in this example
at a reaction temperature of 840C. The o~ide sla~ composition
was the same as that described in Example I. The same slag was
kept in contact with the nickel matte for two hours without
intermediate purification treatment. Oxygen was blown through
the matte at a rate of 100 ml/min. Samples were withdrawn from
the nickel matte and the oxide s]ag layer at four intervals of
30 minutes. Chemical analysis of the nickel matte and oxide
slag are shown in Table II. It was found that, as in Example 1,
the copper content of the matte remained the same during the
-: entire two hour period of contact. Cobalt content of the matte
was reduced substantially from 3.08% to 0.65% (78.9% removal).
The iron content of the matte was lowered from 0.19% to 0.014%
t92-6~ removal). The lead content of the matte was reduced from
0.059% to 0.038% (35.6% removal). This example illustrates the
need to allow sufficient time ~or oxidation and removal of the
impurity elements to the oxide slag layer.
TABLE II
REFINING NICREL MATTE BY PROLONGED CONTACT WITH A SLAG MIXTURE
: Weight Percent
: Cu : Ni : Co . Fe : S : Pb
Initial Matte : 0.58 : - : 3.08 : 0.19 : 22.4 : 0.059
Matte 30 min. : 0.58 : - : 1.96 : 0.026 : 22.0 : 0.050
Slag 30 min. : 0.029 : 1.10 : 1.06 : 0.31 : : 0.012 -
Matte 60 min. : 0.58 : - : 1.26 : 0.019 : 22.9 : 0.046
, Slag 60 min. : 0.015 : 2.05 : 1.55 : 0.26 : : 0.016
Matte 90 min. : 0.59 : - : 0.88 : 0.017 : 23.2 : 0.043
i~ Slag 90 min. : 0.019 : 3.35 : 1.90 : 0.26 : : 0.019
30Matte 120 min.: 0.60 ~ - : 0.65 : 0.014 : 23.9 : 0.038
Slag 120 min.: 0.038 : 6.00 : 2.00 : 0.33 : : 0.024
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EXAMPLE 3
A synthetically prepared nickel matte containing 0.50~
copper, 71.8% nickel, 1.51% cobalt, 0.32~ iron and 26.7% sulfur,
shown in Table III, was treated wlth a slag prepared from a
mixture of sodium carbonate, sodium borate and silica. This
composition was equivalent to 34% Na2O, 23% B2O3 and 43% SiO2.
The 6000 gram matte was treated continuously for five hours at
temperatures from 800C to 950C with the 3000 gram slag mixture.
Oxygen was lanced into the matte at a flow rate of 2.5 liters
per minute. The slag mixture and matte were kept molten with a
natural gas burner.
During treatment, the copper content of the matte
remained essentially unchanged and there was no pick-up of this
element by the slag mixture. The cobalt content of the matte
was reduced by 76% and the iron content by 97% while the nickel
content of the slag mixture was increased by only 3.84% during
the five hour oxidation treatment.
TABLE III
REFINING NICKEL MATTE BY PROLONGED CONTACT WITH A SLAG MIXTURE
Time:Temp.,: Matte Composition, ~ : Slag Composition, %
hr.: C : Cu : Ni : Co : Fe : S : Cu : Ni : Co : Fe
:
0 : 800 :0.50:71.8:1.51:0.32:26.7: ~ : 0 : 0 : 0
2 : 800 : :72.2:0.35:0.09:27.7:<0.005 : 2.98: 1.33: 0.54
3 : 840 : :72.6:0.45:0.0~:27.5:~0.005 : 3.07: 1.53: 0.58
4 : 920 : :73.0:0.44:0.09:26.6:C0.005 : 3.64: 1.95: 0.~7
5 : 950 :0.54:73.4:0.36:0.01:27.9:~0.005 : 3.84: 2.13: 0.65
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EXAMPLE 4
An 80 gram slag containing 33% Na2O, 23% B2O3 and 44%
SiO2, was used to treat a 400 granl nickel-copper matte. This
matte contained 27.7% copperr 47.~% nickel, 0.719~ cobalt, 0.94%
iron, 22.4% sulfur and 0.052% lead as sho~n in Table IV. Air
was introduced to the matte through a lance at a rate of 0.1
liters per minute. The matte was treated for one hour at a
temperature of 950C. During this treatment, 59% of the cobalt,
87% of the iron and 54% of the lead were removed from the matte
while the slag mixture only picked up 3.0% nickel.
TABLE IV
REFINING OF NICKEL-COPPER MATTE
Matte Composition, ~ : Slag Composition, %
Cu : Ni : Co : Fe : S : Pb : Cu : Ni : Co : Fe : Pb
27.7: 47.8: 0.71: 0.94: 22.4: 0.052: 0 : 0 : 0 : 0 : 0
27.7: 46.9: 0.29: 0.12: 23.0: 0.024: 0 : 3.0: 1.8: 4.2: 0.048
EXAMPLE 5
-
This example shows that nickel oxide dissolved in the
oxide slag can be used as an oxidant instead of air or oxygen
lanced into the matte. As shown in Table V, when an oxide slag
containing 3.38% nickel was contacted with molten nickel matte, --
the nickel level of the slag was lowered. As a consequence,
the cobalt level of the slag increased from 1.15% to 1.8% and
the iron content of the slag lncreased from 1.02% to 2.06%.
This treatment was performed at a temperature of 850C with a
slag to matte ratio of 2:1.
TABLE V
OXIDATION OF NICKEL MATTE BY NICKEL OXIDE
_ ADDITION TO OXIDE SLAG
-~ 30 : - Weight, %
Cu : Ni Co : Fe : S
- Matte In 0.54 :69.2 3.5 : 2.22 : 21.9
Slag In :0.008: 3.38: 1.15: 1.02 :
; Matte In :0.55 :74.1 : 2.23: 0.07 : 22.4
~ Slag Out :0.009: 2.13: 1.80: 2.06
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E~LE 6
This example illustrates a method used to remove metallic
oxide from a non-metallic oxide slag. A 330 ~ram char~e of an
oxide slag containing 24.6% Na2O, 10.0% SiO2, 35.4% B2O3, 10%
A1203 and the balance metallic oxides was heated to 900C and
hydrogen sulfide was introduced therein at a flow rate of 100
cubic centimeters per minute. As shown in Table VI, slag
samples taken every hour during treatment exhibited a gradual
reduction of nickel, cobalt and iron contents so that after
six hours of treatment, the nickel content was lowered from
3.6% to 0.11% (96.9% removal efficiency), the cobalt content
was lowered from 0.99% to 0.038% (96.2% removal efficiency) and
the iron content was lowered from 0.20% to 0.055% (72.5% removal
efficiency). The resultant oxide slag was considered suf-
ficiently clean at this point and suitable for additional
processing of impure nickel matte. A 25 gram matte recovered
from the cleaned oxide slag contained 31.3% sulfur, 45.0% nickel,
14% cobalt and 0.041% iron from which the nickel and cobalt
values could be readily recovered by conventional techniques.
TABLE VI
REDUCTION OF OXIDE SLAG LAYER TO REMOVE METAL OXIDE~
Time, E~r. : % Cu : % Ni : ~ Co : % Fe
0 : 0.016 : 3.60 : 0.99 : 0.20
1 : 0.0075 : 0.59 : 0.58 : 0.21
' 2 : 0.012 : 0.056 : 0.084 : 0.13
3 : 0.012 : 0.24 : 0.067 : 0.25
4 : 0.016 : 0.71 : 0.20 : 0.14
S : 0~035 : 0.25 : 0.070 : 0.081
6 : 0.004 : 0.11 : 0.038 : 0.055
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EXAMPLE 7
Slag mixtures that had been used for purification of
nickel ~atte were treated by three carbothermic reduction
techniques as shown in Table VII. In treatment A, a 2.9~, by
weight, coal addition was added to a 200 gram charge at 1500C.
After holding for one hour, chemical analysis showed that the
slag had been sufficiently cleaned to be suitable for recycling
to the refining operation. In treatment B, an addition of 3%
coal plus 6% sulfur was made to 200 grams of the used slag.
This charge was held at 1200C for one hour in a magnesia
crucible and this treatment was also found to be effective for
reprocessing the used slag. Treatment C consisted of 3~ coal
plus 12.3% pyrrhotite (FeS) addition to a 200 gram used slag
sample. The slag was also effectively reprocessed by this
treatment at 1200C for one hour. The mattes that resulted
from the three treatments were well suited for subsequent
refining operations in which the nickel and cobalt values would -
be recovered.
TABLE VII
CLEANING OF A SLAG MIXTURE
Treat-:Identity: Cu : Ni : Co : Fe : S :Si0 :Na 0:B 0
ment : 2 2 2 3
:Used : : : : : : : :
: Slag : 0.08: 5.41: 1.94: 0.58: :42.8:21.3:24.5
A :Sulfide : 1.0 :66.0 : 27 : 5.4 :
- :Cleaned :
: Slag : 0.01: 0.28: 0.15: 0.18: :45.9:25.7:29.6
B :Sulfide : 0.8 :50.4 :13.5 : 2.3 :32.9 :
`~ :Cleaned : : : : : : : : `
: Slag :0.004: 0.40: 0.38: 0.50: 0.59:39.8:25.1:28.9
C :Sulfide :1.4 :37.1 : 9.2 :21.1 :31.0 :
, :Cleaned : : : ~ : : : : :
Slag :0.045: 0.75: 0.69: 9.41: 1.15:38.2:21.6:24.8
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Although the present invention has been described in
conjunction with preferred embodiments, it is to be understood
that modifications and variations may be resorted to without
departing from the spirit and scope of the invention, as those
skilled in the art will readily understand. Such modifications
and variations are considered to be within the purview and scope
of the invention and appended claims.
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