BACKGROUND OF THE INVENTION The invention relates to a process to convert copper or nickel mattes into substantially pure metal by blowing oxygen-enriched air into a bath of molten metal. This type of process is usually carried out in a side blown converter such as a Peirce-Smith converter wherein oxygen-enriched air is injected through 20 to 50 single pipe tuyeres, having a diameter between 25 to 50 mm located along the axis of the reactor more or less in a horizontal position. Usually, oxygen is added to the pure air to a maximum enrichment level of less than 30% oxygen in air. Above this level of enrichment, the refractory wear at the tuyere line becomes excessive.

In 1988, a survey of world copper smelters indicated that of 41 smelters using 134 converters, 15 of them used oxygen enrichment of the blast either occasionally or on a regular basis. The maximum reported enrichment level was 28% oxygen.

It is well known by the man skilled in the art that when air comprising about 30% oxygen or more, is injected in a copper converter, the refractory wear at the tuyere tip line becomes excessive. This deterioration has been explained in an article entitled

"Accretion Growth at the Tuyeres of a Peirce-Smith Copper Converter" by A.A. Bustos et al., the Centre for Metallurgical Process Engineering, the University of British Columbia, as published in Canadian Metallurgical Quarterly, Vol. 27, No. 1, pp. 7-21, 1988, Canadian

Institute of Mining and Metallurgy, Pergamon Press pic. In this article is disclosed a mathematical model predicting the size and shape of accretions produced during converting. The results from this model which agree well with industrial experience, indicate that 36% oxygen enrichment level is the maximum theoretical limit compatible with the formation of a protective layer of solid material around the periphery of the tuyere refractory, as explained in said article. Thereafter, the tuyere line is unprotected, which is detrimental to refractory life.

However, in order to avoid the destruction of the refractory lining, as explained hereabove, the industry using copper enrichment converters has not processed an enrichment of more than 28% of oxygen through the air injected through the converter.

U.S. Patent 3,990,899 discloses a converting process for the extraction of heavy non-ferrous metals from their ores, such as copper, nickel, cobalt or lead sulfides.

The conversion is carried out by injecting oxygen

of industrial purity (which means oxygen produced cryogenically from air, i.e., having a purity of about 99.9% vol. 0 ₂ —at least) through the bath of molten metal to make the conversion from ore to pure metal with the formation of, e.g., some slag. The refractory lining in the vicinity of the tuyere output is protected by injecting a shielding hydrocarbon or sulfur dioxide or water. To protect the injectors and refractory lining, a shielding fluid such as sulfur dioxide is injected in the molten bath until the sulfur content of the molten bath is about less than 5%, at which time the sulfur dioxide is replaced with hydrocarbon gas. The injection of both SO2 and hydrocarbon gas, which might generate CO at high temperature, because those two gases are reactive with oxygen, is an environmental issue today.

U.S. Patent 3,990,890 relates to refining molten copper matte with an enriched oxygen blow. This process comprises two phases: during a first phase of the refining, the oxygen content of the oxidizing gas at a pressure in the range of 6 to 20 bar is varied as necessary up to pure oxygen to maintain the temperature of the matte in the range of 1280-C to 1420-C and still promote sufficient stirring of the molten bath by the injected oxidizing gas highly enriched with oxygen for a second phase of the refining while still maintaining the

temperature in the aforementioned range by adjusting the oxygen content of the oxidizing gas. By injecting a cooling agent, which is a carbon containing material, while blowing the oxidizing gas at much higher pressure than those used before, the oxygen content of the oxidizing gas can be increased far beyond the limits previously imposed in copper matte conversion processes. This high pressure oxygen containing gas expands in the tuyere, due to it high pressure, which creates a cooling effect of the tuyere, but this cooling effect might sometimes be too strong and reinforce the clogging effect (mushroom creation) that this process is supposed to reduce.

U.S. Patent 4,023,781 discloses a tuyere made of three concentric pipes for refining metal in molten state by injection of oxygen in the central tuyere while protective medium are injected in the outside tuyeres. These tuyeres are individually movable longitudinally into the vessel in order to avoid their premature replacement.

U.S. Patent 4,657,586 discloses copper refining by submerged injection of oxygen and fuel in a concentric tuyere wherein between at least 100% and not more than 150% of the oxygen required for complete conversion of the injected fuel is injected (which means between 66% to 100% of fuel compared to the stoichiometric quantity

of oxygen needed for complete combustion of said fuel) . Most of the oxygen is thus for the purpose of combusting the fuel to create heat in the molten bath which comprises oxygen containing impurities. It is also disclosed a process wherein the amount of oxygen might be less than that required for complete combustion with the fuel which means that the process disclosed in this patent does not need oxygen injection for the purpose of refining copper. U.S. Patent 3,844,768 relates to refining of alloy steels, not to copper or nickel refining. In the steel making industry, it is known, as disclosed in said patent, to use argon (in the so-called AOD process) for decarburization of pig iron, including oxygen and argon mixtures or argon in a shrouding tuyere comprising an inner tuyere wherein oxygen is injected through the molten metal. This type of double tuyere with argon- oxygen injection is disclosed in said patent as being inefficient and there is a strong suggestion of using a double or triple feed tuyere with a peripheral flow of carbon make-up liquid.

U.S. Patent 3,281,236 discloses a method for copper refining wherein a material containing copper sulfide, a flux and an oxygen containing gas, is introduced beneath the surface of a mass or pool of molten copper, matte and slag to bring about reactions with yield metallic

copper, slag and gaseous byproducts. The net heat produced by the reactions is said to be sufficient to produce at least a major part of the heat required by the process. Additional heat can be supplied to the reactor by preheating the air or the charged ore, or both, by enriching the air with oxygen, or by continuously burning a suitable amount of gaseous, liquid or solid fuel within the reactor (e.g., by submerged combustion in the melt) . Off-gases can also be recovered, as well as solids in those gases, for recycling within the process of for other purposes.

In essence, the Meissner document is simply describing a one-step process proposed in the 1960's as a replacement for previous two or three stage processes requiring bulky, high temperature equipment and having serious maintenance problems. It is not directed to increasing the oxygen available for reaction with oxydizable materials, while avoiding unacceptable wear of gas injectors or the refractory lining of the converter, which is associated with higher concentrations of oxygen.

U.S. Patent 4,238,228, including its reissue Re 32234 discloses a process to convert non ferrous metal matte by blowing an oxygen containing gas comprising up to 40 % oxygen in a plurality of unshielded tuyeres at a pressure which is from about 50 to 150 psig.

SUMMARY OF THE INVENTION

There are presently incentives for using oxygen- enriched air in non-ferrous converters but there is presently no efficient and economical process available on the market for that purpose. .Among these incentives are heat conservation in the charge to melt cold materials, increase of converter capacity and productivity and increase of the off-gas strength. It is an aim of the present invention to provide a new process for non-ferrous material conversion, including copper and nickel which allows the use of an oxygen comprising gas which comprises more than about 30% volume of oxygen gas without unacceptable wear of the gas injectors and the refractory lining of the converter, surrounding the injectors.

According to the invention, there is provided a process for converting a copper or nickel matte com¬ prising at least 30% by weight of a copper or nickel based material and a substantial weight amount of an iron based material as one of the impurities to be removed from the matte by oxidation in a converter wherein during a first step of said process substantia¬ lly all of the iron based material is oxidized with an oxygen comprising gas to make ferrous oxide which is thereafter transformed into slag by addition of a solid

material to the molten metal and during a second step of said process, the copper based material is oxidized with said oxygen comprising gas to generate substantially pure copper and residues, said residues being essen- tially in gaseous form, the oxygen comprising gas being injected through an internal injector of a concentric shrouded tuyere comprising the internal injector and an external injector concentrically arranged around said internal injector with a gas passage therebetween, a shrouding substantially inert gas being injected through said gas passage with such flow rate and pressure that between about 5% to about 25% of the heat generated by the exothermic reaction of the oxygen with the copper matte substantially throughout the duration of the process is absorbed by the inert gas flowing through the gas passage.

Oxygen and nitrogen containing gases can be provided by any source or on site generator or plant, in liquid or gaseous state, including the delivery by pipe- lines for very large installations.

It is another aim of the invention to provide a process for refining a molten metal (including alloys) wherein said molten metal comprises at least one oxidizable impurity, wherein an oxygen-containing gas is injected in a central (or internal) tuyere and a nitrogen-containing gas is injected in a peripheral (or

external) tuyere concentrically disposed around said central tuyere, at least a portion of the oxygen- containing gas and a portion of the nitrogen-containing gas coming from the same generator, such as a membrane generator or a PSA or a VSA generator or even a cryogenic unit (single or double column, producing one or two products, but also by using the vent gases or these cryogenic units, when their composition is appropriate, if necessary after recompression) . The essence of the invention is to inject through a concentric tuyere oxygen-enriched air comprising preferably between about 30% and 60% volume of oxygen and at the same time injecting an inert gas as a shielding gas in order to absorb some of the additional heat generated by the additional oxygen, but only in the vicinity of the tuyere to avoid overheating of the tuyere and premature destruction of the refractory lining surrounding the tuyere. The concentric tuyere system is disclosed e.g. in French Patent 1,458,718 (Savard & Lee) incorporated herein as a reference.

According to a preferred embodiment of the inven¬ tion, the inert gas, e.g. nitrogen gas, is injected at a pressure which is higher than the pressure of the oxygen-enriched air. By using an inert gas at a higher pressure than the oxygen-enriched air, and preferably an inert gas having

a pressure which is 0.5 bar (or more) higher than that of oxidizing gas, the reaction of between oxygen and oxidizable species in the non-ferrous molten material (which is exothermic) takes place at some distance of the tip of the tuyere which allows to avoid overheating of the tuyere and of the refractory lining surrounding the tuyere.

According to a preferred embodiment of the invention, inert gas flow rate and pressure are such that about 5% to about 15% and more preferably about 10% of the heat generated by the exothermic reaction of 0 ₂ is absorbed by the substantially inert gas.

The inert gas is preferably nitrogen which is inexpensive and has an adequate heat capacity. However, the inert gas can also be sometimes CO ₂ and will be more generally selected from the group consisting of N ₂ , C0 ₂ , argon, neon, krypton, xenon, or any mixture thereof. This inert gas is not however necessarily pure inert gas, but generally comprises an inert gas mixed with another gas. While in the case of nitrogen, air which comprises about 79% volume of nitrogen is generally not adequate (because of its too high content of residual oxygen) , a gas mixture comprising about 90% of inert gas such as nitrogen or the like is usually adequate. This inert gas (mixture) can be obtained from cryogenically distilled air (which means a gas

industrially pure, i.e. containing more than 99.99% vol. of N ) and injected as such (preferably in gaseous form and preferably at least at ambient temperature) or mixed with a certain quantity of pressurized air to achieve the above limit. This nitrogen gas comprising more than about 90% of nitrogen ("impure" nitrogen ) also can be obtained from e.g. a nitrogen membrane generator or a nitrogen Pressure Swing Adsorption (PSA) system or a nitrogen Vacuum Swing Adsorption (VSA) system at adequate pressure.

If liquid nitrogen is used, it should be, of course, usually vaporized before injection through the tuyere and even preheated at a temperature which does not increase the accretion problem (the clogging of the tuyere) but keep the cooling effect of the nitrogen gas. This vaporization and/or preheating are preferably carried out by heat exchange with the molten metal, somewhere in the plant where the heat generated by the molten metal can be easily used to heat the gas. While the difference of pressures between the inert gas and the oxygen containing gas is usually a least about 0.5 bar, it is preferred that the pressure of the inert gas is comprised between about 3 bar and about 6.5 bar absolute. The pressure of the inert gas, e.g. nitrogen, can be sometimes substantially higher than that of the oxygen gas, not only to reduce wear of the

tuyere and the refractory linings as explained above, but also to provide an inert gas in the so-called "jetting" regime, which substantially reduces the clogging of the tuyere, which allows to avoid any punching operation in the tuyere (which problem is disclosed in the article of Brimacombe et al. entitled "Toward a Basic Understanding of Injection in the Copper Convert", Metallurgical Society of AIME, Feb. 1985).

According to another embodiment of the invention, the oxygen comprising gas comprises more than about 30% volume of oxygen gas, and preferably between about 40% volume to about 60% volume of 0 ₂ .

According to another preferred embodiment of the invention, the pressure of the oxygen containing gas is comprised between about 3 and 6 bar.

While a higher pressure of oxygen containing gas might sometimes help for reducing clogging of the tuyere (injection under jetting conditions) , this is usually not necessary because the injection of nitrogen at higher pressure is usually sufficient to reduce or avoid the punching of the tuyere.

The oxygen gas is either liquid oxygen, vaporized and even heated in a heat exchanger (and preferably in heat exchange with the molten metal when the gas is heated or preheated) or gaseous oxygen (which can be preheated too) . Gaseous oxygen can be provided, e.g.

from an oxygen PSA or VSA generator. Also, under certain conditions, both the product and the residue gas of membrane generators or PSA or VSA generators can be used simultaneously. For example, when an 0 ₂ PSA (or VSA) generator is used to enrich the air compressed and injected in the inner tuyere (the mixing of the gases being made before air compression if the 0 ₂ pressure is too low or after compression if it is high enough) , the residue gas of the generator (which is usually vented) has usually a concentration in nitrogen which is high enough to be injected as such (sometimes after compression) as the inert gas in the outer tuyere or mixed with another source of nitrogen to be injected in said outer tuyere. Similarly, a nitrogen membrane generator or a nitrogen PSA or VSA can be used as the nitrogen source (mixed or not with air) for the outer tuyere while the residue gas of this generator which is enriched in 0 is used either as such (usually after recompression) or mixed with another source of oxygen to feed the inner tuyere.

It is also possible to use the oxygen containing gas and nitrogen containing gas produced by a cryogenic air separation unit, whether it is a simple column system such as an high purity nitrogen unit (HPN) , with or without liquid assist feeding or a double column system which produces both nitrogen and oxygen as

products or even to use the respective by-products of a cryogenic unit, after recompression if necessary through compression means, which are usually vented.

Among the advantages of the process is the possible recovery of off-gases from the converter to produce sulfuric acid when the matte (particularly copper matte) converted is rich in sulfur.

When air comprising up to 28%-30% vol. 0 ₂ is used for example, in a Peirce-Smith converter with a copper matte (which comprises sulfur) , the off-gases usually comprise about 3% vol. of S0 . While this is a source of pollution which means that these off-gases need to be treated before being vented, there is not enough S0 ₂ in these off-gases to transform them into sulfuric acid at a low production cost. Sulfuric acid products usually use gases containing at least 8% of S0 ₂ . When using oxygen-enriched air and depending on the sulfur content in the molten bath, it is possible to obtain off-gases containing more than 8% vol. of S0 ₂ , and preferably between 8% and 12% vol.

Also, the invention makes it possible to use more scrap material, because there is more heat available due to a higher oxygen concentration. It is also possible, on the contrary, to reduce the number of tuyeres in the converter thus reducing the cost. (The material of the tuyere must be, of course, adapted to the slag

composition to avoid corrosion, premature destruction or wear) . When using the same number of tuyeres (or sometimes even less) as compared with air injection, the temperature of the molten metal, particularly at the end of the heat is higher (e.g. 1200°C compared to 1100°C) . Accordingly, more impurities such as metallic impurities comprising As, Sb, Bi or the like can be eliminated in the vent gases, which means that the metal obtained according to the process of the invention is more pure.

BRIEF DESCRIPTION OF THE DRAWING :

Figure 1 is a schematical representation of a particular installation using a membrane generator suitable for carrying out the process according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS :

Figure 1 shows a tuyere (1) comprising an internal injector (3) and an external injector (12) concentrically disposed around the internal injector, defining between them a gas passage (2) .

This gas passage (2) is supplied with a nitrogen- containing atmosphere, which is generated by a nitrogen membrane generator which exit (5) is in fluid connection with a membrane unit (4) which is fed by compressed air (11) . The membrane unit (4) is well known in the art and

can be any type of membrane, such as, e.g., a polymeric hollow fiber membrane made from polya ide, polyimide, polysulfone, etc. , including all known materials from the art. On the permeate side of this membrane unit (4) , an oxygen-enriched mixture (6) is recovered which is used to supply the internal injector (3) .

Before reaching the internal injector (3) , the mixture (6) is recompressed (the permeate side of a membrane being obtained at a relatively low pressure) using a compression means (9) (for example, a compressor or even a venturi) . Usually, the permeate gas flowing from the permeate side of a membrane is vented because it is not useful (low pressure, low oxygen concentration) . According to the invention, this permeate is used as at least a portion of the oxygen- containing gas injected in the internal tuyere. Figure 1 illustrates one possible embodiment wherein the permeate, after recompression is mixed with oxygen coming from a liquid tank (7) to reach the necessary oxygen concentration to feel the internal tuyere. As shown on Figure 1, it may be useful (but not necessary) to add compressed air coming from the air compressor (8). When a venturi is used as a compression means, the 0 ₂ -Air additional mixture may be advantageously used to

simultaneously adjust the residual oxygen concentration of the permeate and generate the venturi effect in the compression means (9) .

Figure 1 is only one of the possible embodiments of the invention and not necessarily the best one in all situations wherein gas consumptions may vary according to the refining installations. Other embodiments have not been represented (e.g. with PSA, VSA, cryogenic units, pipe line connection) because it is usually a simple connection between the generator, the plant or even a liquid storage and the tuyere, sometimes with compression means and/or mixing means to adapt the pressure and composition of the gas mixture) .

EXAMPLE 1;

A matte composition comprising about 45% Cu, 16% S, 8% Fe, a high content of Pb and slight amounts of Ni, Zn and As was used for carrying out this example. Oxygen- enriched air (comprising 50% 0 ₂ ) has been used as the oxidizing gas, injected in the inner tuyere at a pressure of about 4 bar while nitrogen at a pressure of about 5 bar was injected in the outer tuyere.

The pressure of nitrogen (and of oxygen, also) was enough to generate a jetting injection regime. There was no clogging of the tuyere throughout the entire conversion process (approximately one hour) . The

temperature of the molten metal rapidly reached about 1200°C which is about 100°C higher than with air blowing.

Several blows (more than 5) were accomplished and the tuyeres (injectors) cooled with nitrogen remained intact (no erosion and the refractory linings were not damaged) . No punching has been necessary.

By comparison and in the same conditions, but injecting air (about 25% 0 ₂ ) in single injectors, each injector has to be replaced at the end of every converting cycle and punching (usually manually) is necessary from time to time.

The blowing rate according to the invention is also greatly reduced (about 50%) which means energy savings and less splashing risks (in case of air injection, 75% of the gas injected is not useful—N ₂ in air—while in the case of the invention only about 50% of the gas injected in the inner tuyere is nitrogen, while the peripheral injection of nitrogen is much less than the difference) . EXAMPLE 2 :

The results reported in Example 1 can also be obtained in a different way, using an installation such as the one described in figure 1 i.e. the combined use of the permeate of a membrane generator to supply the inner tuyere and of the exit side of the membrane to

supply the outer tuyere.

The following data illustrate the characteristics of composition and flow rate of each gaseous mixture. In each case, a flow rate range is given to take into account two typical sizes of converter : exit of the separator : 17 - 61 Nm ³ /mn of nitrogen containing 7% residual oxygen, permeate side of the separator : 17 - 60Nm ³ /mn of nitrogen containing 38 to 40% residual oxygen, - the liquid tank delivers 25 to 90 Nm ³ /mn of pure oxygen,

- the compressor delivers 11 to 39 Nm ³ /mn of air. The outer tuyere is therefore supplied with the exit mixture of the separator, and the inner tuyere with a global mixture of 50%N ₂ -50%0 ₂ at a 53-189Nm ³ /mn flow rate.

This solution allows reaching an interesting technico-economical compromise, thanks to the use of a membrane to supply the nitrogen-rich annular mixture, and saving liquid oxygen using the permeate of this separator as a part of the global 0 ₂ enriched inner mixture.

Of course those date are given for only one tuyere (injector) , the low-size converter illustrated here comprises 6 injectors, whereas the high-size converter comprises 13 injectors.