FR2527635A1 - Cobalt prodn. from oxidic material - by smelting using reducing gas plasma jet - Google Patents

Abstract

Cobalt is mfd. by plasma jet heating of oxidic cobaltiferous primary material to the m. pt. of cobalt and reducing the cobalt oxides to cobalt in the presence of a reducing gas, followed by desulphurisation of the resulting metal bath and removal of dissolved gases. The plasma jet is produced by passing the reducing gas across an electric discharge zone and heats the primary material to 1450-1580 deg. C. The amt. of reducing gas is 1.15-1.5 times the stoichiometric amt. for redn. of the cobalt oxides. The process gives high yields of high quality cobalt. Use of a reducing gas, instead of solid carbonaceous reductant of the electric arc and carbothermic redn. processes, gives improved working conditions and less environment pollution (by reducing CO evolution), improves reaction capacity, reduces reduction time and avoids subsequent operation such as decarburisation and deoxidation.

Description

 The present invention relates to the field of plasma metallurgy, and in particular a process for the manufacture of cobalt in the metallic state.

 The invention can be applied with maximum efficiency to the pyrometallurgical reduction of metal oxides, with the action of a plasma at low temperature on these oxides.

 The consumption of cobalt in the metallic state increases from year to year. New factories are commissioned for the production of cobalt. Cobalt is finding increasingly widespread applications in branches of industry, such as steel metallurgy, electronics, electrical engineering, nuclear industry, etc.

Three processes are known for the pyrometallurgical manufacture of cobalt
- reduction of a cobaltiferous raw material oxidized by a solid reducing agent at a temperature of up to 1200 C;
reduction of an oxidized cobaltiferous raw material in a stream of reducing gas (hydrogen), followed by the melting of the cobalt powder in an induction furnace
- reduction of an oxidized cobaltiferous raw material in an electric furnace during fusion with a solid reducing agent, resulting in the production of molten cobalt.

The first of these processes does not make it possible to obtain a high quality metal, because all the sulfur and the other impurities, present in small quantities in the oxidized cobaltiferous raw material, pass entirely into the metal and lower its rate - of cobalt
The second of the aforementioned processes (reduction of the cobaltiferous raw material oxidized by hydrogen without melting the material to be treated) is applied to the development of hard cobalt alloys. Among the drawbacks of this process, it should be pointed out that the long duration of the reduction which takes place at relatively low temperatures.

 The most widely used method is the third: that is, the reduction of the raw material oxy cobaltiferous in the electric furnace during the melting with a solid reducing agent. According to this process, the oxidized cobalt material mixed with the solid reducing agent is loaded into the melting furnace. During the reductive fusion, special attention was paid to the fullness of the reduction of the cobalt compounds, the removal of impurities and the desulturation of the metal. As a reducing agent during smelting, petroleum coke is usually used, sometimes a graphite aggregate.

 The melting is carried out in electric resistance ovens with coal heating elements, or in electric arc furnaces ignited between electrodes.

The reductive fusion of the oxidized fiery cobalti raw material is a periodic process comprising six operations
- load preparation and charging
- merger, reduction
- decarburization
- deoxidation
- desulfurization
- metal casting.

 The production time, depending on the type of oven, is 6 to 8 hours. The charge introduced into the furnace is made up of 75 to 88% of cobaltiferous materials - cobalt oxide-oxide and recycling products (waste, magnetic slag fraction, scrap metal) - and 12 to 25% of reducing agent; petroleum coke, small coke, graphite debris and aggregates.

 if as deoxidizer is added during melting, silicon and / or aluminum is used.

 To decarburize the metal, cobalt protoxide-oxide is added to the bath.

 For desulphurization, 1 to 5 lime slag baths are produced, by loading chalk or limestone, calcium fluoride and soda into the oven.

 The quantity of slag produced goes up to 10, c0 of the quantity of metal produced (merchant cobalt) with a cobalt content of up to 20 0.

 When the fusion is complete, the metal is poured into ingot molds, then, in the form of ingots, it is sent to the barrel for the elimination of the slag.

The extraction of cobalt by the process indicated amounts to 98.7-99%, of which
- in merchant metal 95 to 97, -o - in dust ................. U, 7 to 1.5%
- in the slag. 1.0 to 2.5%
Irretrievable losses amount to 1.0 to 1.3%.

The main disadvantages of the process examined, as well as its technological realization, are
- the long duration of the elaboration
- the need to frequently replace the electrodes
- the low resistance of the lining and certain of its constituent parts
- the irregularity of the electrical regime of fusion
- increased fire losses and increased entrainment of dust containing cobalt
- increased noise (for independent arc furnaces)
- the large quantity of recycled products
- the need to carry out several slag baths to extract the sulfur
- large losses of metal in the lining and slag
- the strong release of smoke.

 The indicated drawbacks of the process, as well as its low yield, make the problem of the transformation of oxidized cobaltiferous raw materials by reduction with the use of a plasma at low temperature topical.

 Attempts to solve this problem have led to the appearance of the process of carbothermic reduction of the oxidized cobaltiferous raw material, with arc plasma heating in a crucible plasma plant (ingot mold).

 This process consists of the following.

 A granulated charge, with a grain size of 1 to 2 cm, consisting of a cobalt protoxide-oxide, with small coke and a binder, is introduced into a crucible (20 SO of the total weight of the charge) which is placed in a pregnant. The enclosure is placed under a vacuum at 0.67 x 10 2 Pa (0.5 mm Hg) and washed with technical argon, then the plasma generator is activated.

The plasma gas flow rate (argon) and the power are adjusted in the plasma generator. The plasma arc melts the charge poured into the crucible, and, at equal time intervals, the rest of the charge is poured onto the metal bath. The reduction process has a violent character and is accompanied by a release of carbon in the form of black which is deposited on the walls of the enclosure in which the crucible is placed. During the fusion Co, Ni, Fe and Cu pass entirely into the metal. The process is critical as to the carbon / oxygen ratio and the metal can be carbide.

 The practical value of this process is lowered by the necessity of the preliminary preparation of a granulated material and by the use of plasma at low temperature as a heating source only.

 In addition, in the case of the reduction of the cobaltiferous raw material oxidized by a solid reducing agent, it is necessary to introduce into the plasma furnace scorifying, decarburizing and deoxidizing products, ie the organization of the production process in the plasma oven, in the case of the reduction of the cobaltiferous raw material oxidized by a solid reducing agent, differs little from the processes carried out in electric furnaces with indirect heating (resistance electric oven and independent arc electric oven). In this way, this process has all the drawbacks listed above of the reductive fusion in an electric furnace, except as regards the yield.

 It has been proposed to create a process for the manufacture of cobalt in the metallic state from an oxidized cobaltiferous raw material which would have a high yield, would be economically more advantageous and ecologically clean, while increasing the reaction capacity of the reducing gas.

 The solution consists in a process for manufacturing cobalt in the metallic state from an oxidized cobaltiferous raw material, by heating this oxidized raw material using a plasma jet to the melting temperature of the cobalt and reduction of cobalt oxides to metal in the presence of a reducing gas, desulfurization of the uetal bath obtained and elimination of the gases which are dissolved therein, characterized in that according to the invention, the plasma jet is generated in passing the reducing gas through an area an electrical discharge is initiated, the raw material oxide is heated to a temperature of between approximately 1,450 (;; and approximately 1,580 OC, and the quantity of reducing gas necessary for the reduction of cobalt oxides is between about 1.15 and about 1.5 times the stoichiometric amount of reducing gas.

 Such an implementation of the process significantly lowers the level of impurities in the metallic cobalt, thanks to the elimination of the use of a solid reducing agent. In addition, this results in a lowering of the carbon monoxide content in the fumes, which improves the ecology of the environment and the working conditions. The use of the reducing gas as the plasma gas increases its reaction capacity, which shortens the time for reduction of the cobalt oxides to metal.

If the amount of reducing gas used in the process is less than 1.15 times the stoichiometric amount, a metal cobalt with increased oxide content * is obtained, and if the amount of reducing gas used in the process is greater than 1.5 times the stoichiometric amount, an obtains a metallic cobalt with increased rate of rer resulting from the reduction of the oxidized cobaltiferous raw material
The use of an amount of reactive gas in the range indicated, ranging from about 1.15 to about 1.5 times the stoichiometric amount of reducing gas, ensures obtaining a metallic cobalt at minimum rate d impurities, for example iron. The choice of the extreme values of the process temperature, approximately 1450 OC and approximately 1580 DC, is determined by the melting temperature of the cobalt (lower limit) and by the conditions of service of the lining. ceramic crucible and the vault of the oven (upper limit), in order to ensure prolonged service of this filling.

 In addition, in the process described, the hydrogen in the reducing gas participates in the desulfurization of the metal cobalt, by reacting with the sulfur in the bath and forming with it volatile compounds, which results in the elimination of the bath sulfur and lowering the sulfur level in the metallic cobalt obtained. This latter circumstance has the effect of reducing the number of treatment operations, by eliminating, for example, the production of slag baths to remove the sulfur. This results in a reduction in the duration of implementation of the process and a reduction in the losses of metal in the slag.

 The use of a reducing gas lowers the level of carbon in the metal cobalt, which improves the quality of the metal and reduces the duration of the process, thanks to the elimination of treatment operations, such as the decarburization of cobalt. metallic and its subsequent deoxidation.

 it is advantageous to replace the reducing gas, passing through the zone where the electrical discharge is initiated, with a neutral gas for refining the metal cobalt bath, which makes it possible to rid the cobalt bath of hydrogen reducing gas dissolved therein.

 It is recommended, during the refining of the metal cobalt bath, to raise its temperature to the casting temperature of up to about 1,650 OC, in order to obtain a quality metal after its casting in molds.

 The other aims and advantages of the invention will be made more comprehensible by the concrete example of embodiment given below and by FIG. single drawing of the accompanying drawing which schematically shows a plasma melting furnace with a ceramic crucible and a hearth electrode, for the implementation of the cobalt manufacturing process according to the invention, in longitudinal section.

 The cobalt manufacturing process is carried out in a plasma melting furnace 1, comprising a ceramic crucible 2, closed by a cover 3 with lining which comprises a pipe 4 for evacuating the smoke. In a hole 5, made in the central part of the cover 3, is placed a plasma generator 6, the working electrode of which is connected to the negative pole of a source of electrical energy (not shown). The positive blade of the electric power source is connected to an electrode 1, which is located in the tang of the crucible 2. In the side wall of the crucible 2 is formed an opening 8 for casting provided with a spout 9.

 The process is carried out as follows. The starting material, an oxidized cobaltiferous raw material, is introduced into the ceramic crucible 2. It is heated there by the heat of the plasma jet II of the plasma generator 6, to a temperature of between approximately 1,450 ° C. and approximately 1,580 C. The plasma jet It is generated by passing a reducing gas , for example hydrogen, raw natural or conversion, mixed with a neutral gas, through the area where an electrical discharge is initiated. The electric discharge is initiated between the working electrode of the plasma generator and the bath 12.

 The cobalt oxides are reduced to metal under the action of the reducing gas with increased reaction capacity, introauit in an amount ranging between about 1.15 and about 1.5 times the stoichiometric amount. At the same time the metal cobalt bath is desulphurized, thanks to the formation of hydrogen sulphide which is evacuated with the smoke by the pipe 4. The necessary level of temperature, from 1450 to 1580 is maintained during the reduction by regulation of the power of the plasma o generator.

 After completion of the processes for reducing the cobalt gold to metal and desulfurization of the metal cobalt bath obtained, the cobalt bath is refined, to remove the hydrogen which is dissolved therein. For this, the reducing gas is replaced, passing through the zone where the electrical discharge is initiated, by a neutral gas, for example argon. During the refining, the temperature of the cobalt bath is raised to approximately 1,680 c (the temperature of casting of the metal) to obtain, after casting molds, a quality metal.

EXAMPLE
An oxidized cobaltiferous raw material having the following composition: Co = 71,%, Ni = 0.25%, Fe = 0.28 = 0, cu = 0.02%, Mn = 0.07%, O s 0.012%, S = -0.2%, with particle size # 3 mm, was reduced in the plasma oven shown in the figure, with a power of the plasma generator of 70 kW. Through the plasma generator, a gas containing hydrogen H 2 or a synthesis gas (CO + H 2) was introduced, in an amount equal to 1.4 times the stoichiometric amount. The temperature of the bath was maintained at the level of 1480 to 1520 C.

The reduction process proceeded calmly, without ejection of raw material;
The end of the reduction process was determined from the increase in the hydrogen level in the fumes, after which the metallic cobalt was freed of H2 and the non-metallic impurities from the packing, by introduction of a neutral gas (argon) through the plasma generator 6.

 During the refining, the metallic cobalt was heated to a temperature of 1,620 to 1,650 C, then it was poured from the oven into molds.

The composition of the metal obtained was: Co = 99.5%,
Ni = 0.3%, Fe w 0.09%, Cu = 0.03 case C = 0.01%, S = 0.003%, Mn = 0.004%.

 The process for the production of cobalt which is the subject of the present invention can be carried out in the plasma furnace with a ceramic crucible described in detail in the orifice of the United States of America bare 4,002,466.

Claims (3)

R X V E N I) 1 C A r I O N S  1 - Process for the manufacture of cobalt in the metallic state from an oxidized cobaltiferous raw material, by heating this oxidized raw material using a plasma jet to the melting point of the cobalt and of reduction of cobalt oxides to metal in the presence of a reducing gas, desulfurization of the metal bath obtained and elimination of the gases dissolved therein, characterized in that the plasma jet is generated by passing the reducing gas through an area or is primed an electric discharge, the oxidized raw material is heated to a temperature between approximately 1,450 OC and approximately 1,580 OC, and the quantity of reducing gas necessary for the reduction of the cobalt oxides is chosen between approximately 1.15 and about 1.5 times the stoichiometric amount of reducing gas.  2 - Method according to claim 1, characterized in that one replaces the reducing gas, passing through the area where the electrical discharge is initiated, by a neutral gas for refining the bath of metallic cobalt.  3 - Process according to claim 1 or 2, characterized in that, during the refining of the cobalt bath, its temperature is raised to the casting temperature up to about 1,650 OC.