CA1056161A - Production of cobalt from cobalt sulfide - Google Patents

Abstract

ABSTRACT OF DISCLOSURE

A pyrometallurgical process for producing cobalt metal from a sulfidic cobalt melt containing up to, 35% sulfur while inhibiting the formation of substantial amounts of cobalt oxide is provided. The method comprises contacting such a molten bath maintained in a state of vigorous agitation and at a temperature at least 100°C above the melting point thereof with an oxygen-containing gas to remove sulfur therefrom as SO2, while controlling the partial pressure of oxygen in said gas and the temperature of said bath as the percent sulfur in said bath decreases to inhibit the oxidation of cobalt to cobalt oxide.

Description

'[`hi9 inventi()n relates to a novel proce.Ys for the pyrometallurgical conversion of a sul~idic cobalt melt to yield metallic cobalt.
State of the Art It is known to produce metallic nickel from nickel-containing matte by the pyrometallurgical conversion of the matte using an oxygen-containing gas as the oxidant.
A process which has been successfully developed for the treatment of nickel mattes is that disclosed in U.S. Patent No. 3, 069, 254 (assigned to the same assignee). This patent is directed to the recovery of nickel from nickel-containing sulfide .
materials, such as nickel sulfide matte and crude nickel sulfide precipitates, such as those obtained in processing sulfidic and .
oxidic nickel ores. These mattes in the molten condition are con-verted autogenously in a rotary furnace using commercial oxygen or oxygen-enriched air blown onto the surface of the molten matte while maintaining it in a turbulent state. The matte is oxidi~ed to substantially sulfur-free nickel by controlling the oxygen supply and bath temperature while subjecting the bath to strong mechanically- ~
induced agitation. The foregoing permits rapid nickel sulfide-nickel ~.
oxide reaction and high efficiency of oxygen utilization. ~;
Ac~cording to the patent, any cobalt and iron contained in the matte is removed by oxidation. For example, cobalt elimination may be accomplished during routine conversion of the nickel matte for iron removal by selectively oxidizing the cobalt in the presence of a flux, such as silica, the cobalt being then skimmed off as a slag.

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Thus, it is quite apparent that cobalt and iron oxidi;~.e preferentially to nickel in the matte.
` It has been reported in the literature that iron sulfide cannot be converted directly to iron metal at practicaL temperatures.
Despite the generally similar chemical behavior of iron and cobalt, ; we have now discovered that it is possible to convert sulfidic cobalt melts containin~ up to 35% sulfur to cobalt metal containing less than 1% sulfur provided particular care is taken to control the oxygçn partial pressure and the bath temperature during desulfuri~ation.
The process is applicable for the recovery of cobalt from cobaltiferous iron sulfides, from cobalt sulfides produced by the sulfide precipitation of cobalt from leach solutions and from cobaltiferous oxide ores or scrap.
Object of the Invention It is thus the object of the invention to provide a pyro~
metallurgical process for converting sulfidic cobalt melts to cobalt ` metal.
Other objects will more clearly appear from the following description taken in conjunction with the accompanying drawing, wherein the attached figure illustrates the relationship between temperature, the sulfur content of the matte and the oxygen in the gas that permit one to avoid forming substantial amounts of cobalt oxide while carrying out the invention. ,~ -Summary of the Invention ; `
The invention is directed to a pyrometallurgical process for converting a sulfidic cobalt melt containing at least about 1% sulfur or z -' "' 5t~
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more usually at least abollt 5'~, ~nd llp to ahout 35~ slllfur to rnetallic cobalt while avoiclin~ sllbstantial rormation Or cohalt oxide. The ;~ process co~mprises estahlishing a molten bath of said sulfidic material at a temperature of at least 100C above the melting point , thereof, vigorously agitating saicl bath, and contacting said bath of vigorously agitated sulfidic material with an oxygen-containing gas to remove sulfur as gaseous sulfur dioxide. Unfortunately, the aforementioned tendency of cobalt to oxidize leads to a competition between cobalt and sulfur to react with the oxygen in the inlet gas.
The formation of cobalt oxide not only lowers the yield of cobalt but, in the case where the oxygen-containing gas is being blown onto the melt surface, an oxide scum can actually slow up or stop the reaction.
Such a scum also severely attacks the refractories of the reaction `
vessel, : :~: .
Whlle cobalt oxide may form as an intermediate step in the `
conversion reactions, generally no difficulty is encountered so long as the following reaction proceeds to the right with reasonable velocity~
S (melt) + 2CoO (solid) ~ 2Co (liquid) +52 (gas).
For any given concentration of sulfur dissolved in the melt, the reaction can be driven further to the right by increasing the melt temperature and/or by lowering the partial pressure of the gaseous ~ SO2 product. One way to accomplish this latter effect is to dilute the ,.' ~ .
oxygen-containing gas used to contact the melt with a gas such as .: .:
nitrogen or argon, At high levels of sulfur in the matte (^~15% S), bath temperature is the more effective means of control. Gases relatively rich in oxygen are employed to generate heat by the ~ -. .
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oxid~tit)n o~ 1r~"- th~rci~y r.li~in~ the hath lemp~rc~ re ~ul~stantia]]y ~vl~ilt~ pro~ llly Ino~ la~ c,r ~ c)~ o(l~(t. ~low~v(:l~
a~ the s~llfur content of ~h(? I~ath (l~r~?ase~ still ~reat~r temper~tllr~;
and/or diiutions of the ~a~ are r~quired to prevent the a~ lmulation of cobalt oxide. The limitations of pre~ently available refractories rul~:
;; out the continuing increase of temperature as a practicable means for promoting the reaction so dilution becomes the more effective m~ans of control. However such dilution tends to lower the bath temp~rature by reducing the amount of oxygen introduc~d per unit time (i. e. reducing the rate of heat generation) and by virtue of the sensible heat removed from the system by the diluting gase~ t lower s~llfur content~ ~ ;
(~lo~/lJ s) heat mttst therefore be supplied from some source other ehan the oxidation of sulfur. This Lan he done ~31ectrically when the proce9s i9 conducted in a ves~;e1 provided with induction heating. It can also be provided chami(:ally by addin~ a strong deoxidi7.ing agent such aff ~ .
silicon to the hath. Or the heat can be provided by the combustion of hydrocarbon fuel~ with a controlled ~xce0~ of air or oxygen. In this last case, the gaseous producta of ~:ombustion advantageously act as diluent~ .
~0 Another very effattiva m~anff for promotin~ tho tonversio without accunlu1atin~ cxee~Hive amount~ of t obalt oxide i~ lo lowor lhe partial pre~ur~ of the product SO~ by e onduetin~ the pro~e~s under vacuum. Thi~ procedure would work at any sulfur eont~nt bue it is not practicable to pump lar~e volume0 of SC);~ through pros~lltly avail- ~
ab1e vacuum YyDtemD. This procadure is therefor~ used only at low ~ ~-~ulfur tevel~ (~ay c5% S adva.ntaKeou~ly ~IV/u S). Thi~ approach ''s'' ' ~'. ~ ' __` . J

onveniently lends itself to heatin~ and stirring the molten hath by electromagnetic means.
It is clear that the successful practice of this invention requires careful control of bath temperature and the oxygen content of the inlet gases. The required sulfur-oxygen-temperature correlation will be apparent from the accompanying drawing which depicts a cobalt-sulfur binary phase diagram with curves of constant oxygen partial pressure superimposed thereon correlated to both the sulfur content of the cobalt melt and the temperature of conversion.

Thus, referring to the Curve B, it will be noted that ~ ;
, when the inlet gas contains 5~0 oxygen, a conversion temperature at ,:, .. . ~ :
least above 1350C, for example, 14nooc and higher is required to ~ ~
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desulfurize a melt containing 20% sulfur. When the melt sulfur is lowered to 14% sulfur with this gas, the bath tempeFature should be ;~
above 1500C to avoid substantial oxide formation.
Similarly, when employing a gas with iO% oxygen `~
(Curve C) to desulfurize a 14% sulfur melt, the bath temperature `
should be above 1540C. Likewise, with a gas containing 21% oxygen ~Curve D) the bath temperature should be above 1560C to desulfurize ~ a bath containing 16% sulfur. Temperatures for desulfurizing cobalt melts of different sulfur levels with inlet gases of other oxygen contents can be interpolated from the drawing. ~ .
As will be appreciated, both the oxygen content of the gaseous stream and the temperature can be varied during the con-version treatment according to the sulfur remaining in the melt. ;
In carrying out the process, the sulfidic cobalt material, however derived, is melted to provide a molten bath. The melting ' -:., . ' .' ' "
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can be carrie(l o~t in any convcnti()nal f~lrnace, sl1cll ,-9 an inc1~1ction ~lrnace, reverberatory f~lrnace, top blown r otary c onverter, or an e1ectric furnace. The melting temperature will clepend on the initial sulfur content of the sulficlic cobalt rnaterial. A cobalt sulfide with a s~llfur content of about 26% has the lowest melting point of about 880C, Sulfidic materials with either higher or lower sulfur contents have higher melting points. On the lower sulfide side, the melting point increases to 1495C, the melting point of pure cobalt. It should be noted that the presence of other impurities can lower the melting point slightly. Preferably, the temperature of the molten matte before treatment should be at least about 1200C and, for example, ~`-at least about 1300C.
The conversion of molten sulfidic cobalt material to metal should be carried out in a vessel in which good stirring of ,~ the sulfide is provided. It can be a top blown rotary converter, an . induction unit with adequate stirring or a bottom blown vessel like the bessemer converter. The vigorous stirring may be produced by any of the well known methods; for example,~ hy mechanical stirring, by electromaKnetic mcans or by using porous plugs or ; 20 tuyeres through which inert or oxidizing gases are blown into the melt below the surface thereof.
The oxidation is carried out advantageously by employing , ~ .
free oxygen-containing gases. For example, when the sulfur content of the matte exceeds 15% by weight, the oxygen content of the gaseous stream is preferably about 10% or more by volume. Oxidants employed include air, oxygen-enriched air, oxygen and combustion gases -6- ;;
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producecl by coml~ustion of fuel with more than stoichiometric amounts of air or oxygen or combinations of air an~l oxygen. The oxidant as mentioned before can be injected through tuyeres or porous plugs or can be lancod on top of the bath with single or multlple lances or by a single lance capable of delivering multiple jets of oxidant over the entire bath surface. When tuyeres or porous plugs are used these may : .
be shielded with hydrocarbons. :~

; Details of the Invention Tests were conducted to determine the relationship between the sulfur content of the sulficlic cobalt material, the oxygen content of the gaseous stream and the melt temperature necessary to - ~ avoid the excessive formation of cobalt oxide on the s~urface of the bath.

- ~ Melts containing the specified amount of sulfur were held , , ~ .
,~ ~ in an g kg induction furnace and, by using a three lance arrangement ` were surface blown with gaseous streams containing a controlled amount of oxygen. ~ For each oxygen content in the stream, the temperature of ! -the melt was adjusted until only a light spot of cobalt oxide persisted on the surace of the melt just below the lance. Thus, for oxygen contents of 10%, the bath temperature had to be increased as the sulfur ~ . .
~ ~ 20 decreased as illustrated by Curve C to avoid excessive accumulation of ~ ~
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~ oxide, For gases richer in oxygen than 10%, the temperature required : : ~
to achieve low sulfur contents exceeded the capabilities of the crucible , :
so further dilution was necessary.
Typical oxygen partial pressures and temperatures , required to convert sulfidic cobalt melts of varying sulfur contents to produce negligible oxide formation are given in the following table:

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Melt '1/,, Oxy~en irl Melt W t ~,~" Oxi di z i ng 'r emp S _ Cl~s C

16 21 15 h O
14 10. ~ 1540 12 10.5 1580 10. 5 1610 _ 8 5. 25 1590 6 5. 25 1620 4 _ 5.25 1660 -0. 6 2. ~ 1700 ~ -' :, " ' .... . .
If either the oxygen partial pressure is increased or the temperature is decreased from the values given in the table at the given sulfur levels, excessive oxide formation occurs. This is confirmed by referring to the accompanying drawing. Curves A to F
~ were obtained by mathematically modelling the experimental data and - extrapola.,ing them to lower/higher sulfur contents and to lower/higher oxygen contents in the oxidizing gas. The resulting cobalt product which may contain about 1% or less of sulfur after the oxidation treatment can be further purified by varlous methods. One such method is vacuum refining of the molten bath. The molten bath from the oxidation treatment is subjected to a vacuum treatment for removal of impurities; e. g. antimony, bismuth, lead, zinc, ~-~ cadmium, selenium, tellurium and for final sulfur elimination.
It is desirable for thermodynamic and kinetic reasons to subject the ;
molten bath to a pressure less than 0. 3 atmosphere and advantageously to a pressure less than about 1 mm of mercury. The oxygen in the bath required for this treatment is about twice the amount of sulfur present.
Since the solubility of oxygen in liquid cobalt is rather limited (0.52 wt % Oat 1700C), it is generally not possible to dissolve , ~OS~

sufficient o~;ygen in the bath to react with all the dissolved s~llfllr, It is possible to saturate the system with the stoichiometric amount of required~ oxygen but this solid oxide phase proves troublesome in operation. It frequently sticks to the crucible walls and tends to react with them. It is far more desirable to add oxygen to the system as the process proceeds exercising reasonable care not to form excessive amounts of solids. This can be done with oxygen-bearing materials such as cobalt oxide or compounds heat decomposable - ~ thereto. Alternatively, gaseous oxygen, e. g. in the form of air, can ~ 10 be introduced into or on to the molten bath to overcome any oxygen . ~ :
- ~ deficiency. During vacuum refining, more favorable kinetics can ~
- be obtained by~vigorous agitation, either by electromagnetic means ~;
`; or by purging with inert gases, and by controlling the oxygen content ~ ~ .
of the melt between 1. 5 and 4. 0 times the sulfur content. -It has been found that the kinetics of sulfur removal is ; ~ dependent on the ratio of volume/area of the melt and decreases with ~ ~
, . ' .: , increasing values of this ratio. Therefore the choice of the vessel size should be such as to optimize refining kinetics and vessel costs.
Another alternative is to desulfurize the final melt using v such desulfurizing slag additions as: CaO (l~me slag); CaO +AI;
CaFz + CaO +Alz03; CaO + Mg; CaC2, Mg, Ca etc. The metals Al - ~ ~ and Mg act as efficient reductants which aid the desulfurizing reaction.
`' In addit~on Mg acts as a desulfurizer and can thus be used alone for this purpose. The amount of desulfurizing metal added shauld be at least stoichiometrically sufficient to combine with the sulfur, taking into account the loss of some metal by oxidation.

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As ill~l~strative of the varic)lls errlhodiments of the invention, tllc fol]owin~ exa~llplcs .are given: ~ ' E XA M l~L I~ I ' A 3. 6 kg charge of cohalt sulfide containing 22. 4% S ~ ,' was melted and vigorously stirred by induction heating. The surface of the meLt was blown with a gaseous stream containing 21% oxygen by volume at a starting temperature of about 1300C. The oxidation was carried out for 1. 6 hours, during which period the temperature of the melt was gradually raised to about 1500C. During this period.
the sulfur was lowered to 15. 5% and the formation of cobalt oxide was '~
substantialIy avoided. The oxygen content of the gaseous stream was decreased to 10. 5% and the melt was oxidi;~ed for another 0. 75 hours ,`
during which period the sulfur content of the melt was reduced to 9. 5'~
and the melt temperature was increased to 1620C. The oxygent content of the oxldizlng stream was further decreased to 5. 25% and the melt , was blown for another 0. 9 hours, during which period the sulfur content of the melt was~ reduced to 4. 2% and the melt temperature was increased to about 1650C. A cobalt melt containing 4. 2% S was blown with a , gaseous stream containing 2. h% oxygen for 1. 5 hours while the melt temperature was increased to about 1700C. This produced a cobalt " ;, metal containing 0. 61% S and the formation of the cobalt oxide was substantially inhibited during the entire blow.
'.' :' EXAMPLE lA -' , In a comparison test similar to that described in Example I. ,~
an iron sulfide matte containing 14% S was blown with a gaseous stream ' containing oxygen at levels as low as 2% while maintaining the melt at a : ~ ' ' `' ' ' :`, . ~ . - : . -'-: ~ , , '': , ~ ' ? , : , '':. '' . ' . ' ' '' ' '',' ' ;..'. ., "'' '. ' ;:'", '' ' ' , ,. . ' '" ', :' ' ''' ' ' ; ' : : " "' te~lperature of about Ih()0C' ~o l64n~C, C,ontinuous oxide formation took plac e whic}l causecl corrosion of the crucible resultin~ in a rnel~ ;
break-out within 30 minutes from the start oL the oxidation.
.

The purpose of this example is to show that following the , removal of sulfur from sulfidic cobalt melts to a level below 1% S, ' the resulting product can be further purified by vacuum refining as follows.
In this instance, an 18 kg melt of cobalt containing 0. 09%
- 10 S was refined at 1650C by introducing 0. Z3% oxygen into the melt and subjected tc a vacuum treatment at a nominal chamber pressure of about 0. 6 mm of mercury. After 90 minutes of treatment, the sulfur content o~ the melt was lowered to less than 0. 0001% S and the oxygen decreased to 0. 06%.

.:.~ : , This example illustrates the use of slagging techniques for lowering the final sulfur content of cobalt. In this case, a 5 kg -melt containing 0. 12% S was desulfurized by adding 2% by weight of :: :
CaO and 0. 6% by weight of aluminum at a melt temperature of 1600C
,~ 20 to yield a cobalt metal product containing 0.01~/o S.

Another 5 kg of cobalt melt containing 0. 12% S was desulfurized at 1600C by adding 4% by weight of a slag containing 61. 8% CaO, 22. 5% SiO2, 7. 5% A1203, 3. 5% Fe304, 2. 5% MgO and 2%
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K2O, the desulfurizing being further aided by the addition of 0. IZ% by weight Al. The resulting cobalt metal product analyzed 0. 02% S.

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:: ' As wiL1 be apparent from the foregoing cxanlples, the sulfur can he substantially completely removed from a sulfidic cobalt melt~to form cobalt metal while inhibiting substantially the formal:ion of cobalt oxide.
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 depart-ing from the spirit and scope of the invention as those skilled in the ~ ~
; art will readily understand. Such modifications and variations are ~ ~ -10 considered to be within the purview and scope of the invention and :
~ the appended claims. `
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Claims (13)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A pyrometallurgical process for desulfurizing a sulfidic cobalt melt while inhibiting the formation of cobalt oxide which comprises, establishing a sulfidic cobalt melt containing above about 1% to about 35% sulfur by weight, vigorously agitating said melt, contacting said vigorously agitated melt with an oxygen-containing gas to oxidize and remove the sulfur therein as SO2 while converting said melt to cobalt metal, the temperature of conversion and the oxygen content of the inlet gas being correlated with the sulfur content of the melt, the temperature of conversion being substantially above the temperature at which cobalt oxide forms as determined by the bath sulfur-oxygen-temperature curves of the accompanying drawing, and continuing said conversion treatment wherein the conversion temperature is increased as the sulfur content of said melt decreases as determined by said bath sulfur-oxygen-temperature curves of the accompanying drawing and interpolations thereof until the melt has been substantially converted to cobalt metal.

2. The process of Claim 1, wherein following reduction of the sulfur content in the molten bath to about 1%, the molten bath is further purified by vacuum treatment at a pressure less than about 0.3 atmosphere, during which treatment the oxygen content in the bath is maintained at about 1.5 to 4 times the sulfur content.

3. A process as described in Claim 2 wherein the vacuum treatment is at a pressure of less than about 1mm of mercury.

4. A process as described in Claim 2 wherein the vacuum treatment is carried out for removal of impurities other than sulfur.

5. The process of Claim 1, wherein following the reduction of the sulfur content in the molten bath to below 1%, the molten bath is further purified by removing said sulfur using a desulfurizing slag.

6. The method of Claim 5, wherein said desulfurizing slag is substantially a lime slag.

7. The method of Claim 6, wherein said lime slag includes a small but effective amount of a desulfurizing metal selected from the group consisting of aluminum and magnesium, the amount of said desulfurizing metal being at least stoichiometrically sufficient to combine with said sulfur.

8. A pyrometallurgical process for desulfurizing a sulfur-containing cobalt melt while inhibiting the formation of cobalt oxide which comprises, establishing a molten bath of sulfidic cobalt containing about 1% to about 35% sulfur by weight, vigorously agitating said bath, contacting said vigorously agitated bath with an oxygen-containing gas to oxidize and remove the sulfur therein as SO2 while converting said bath to cobalt metal, the temperature of conversion and the oxygen content of the inlet gas being correlated with the sulfur content of the bath, the temperature of conversion being substantially above the temperature at which cobalt oxide forms as determined by the bath sulfur-oxygen-temperature curves of the accompanying drawing, controlling the oxygen-temperature relationship of said process by causing the temperature of said bath to increase as said sulfur content decreases, the percent oxygen in said inlet gas being substantially proportionately decreased with the increase in said temperature as determined by the inhibition of cobalt oxide formation in accordance with the accompanying drawing.

and continuing said conversion whereby the temperature is caused to increase to over about 1600°C and not substantially exceeding 1700°C
as the sulfur content of said bath decreases to below 3% and the oxygen in the inlet gas is decreased to below about 4% until the sulfur in said bath is finally reduced to below about 1%.

9. The process of Claim 8, wherein following reduction of the sulfur content in the molten bath to below 1%, the molten bath is further purified by vacuum treatment at a pressure less than about 0.3 atmosphere, during which treatment of the oxygen content in the bath is maintained at about 1.5 to 4 times the sulfur content.

10. The process of Claim 9, wherein the oxygen content of said bath is controlled by addition of an oxygen bearing solid selected from the group cobalt oxide or compounds decomposable to cobalt oxide.

11. The process of Claim 8, wherein following the reduction of the sulfur content in the molten bath to below 1%, the molten bath is further purified by removing said sulfur using a desulfurizing slag.

12. The method of Claim 11, wherein said desulfurizing slag is substantially a lime slag.

13. The method of Claim 12, wherein said lime slag includes a small but effective amount of a desulfurizing metal selected from the group consisting of aluminum and magnesium, the amount of said desulfurizing metal being at least stoichiometrically sufficient to combine with said lime.