AU705242B2 - Oxygen smelting of copper and/or nickel sulphide ore concentrates - Google Patents

Description

WO 97/28288 PCT/GB97/00275 OXYGEN SMELTING OF COPPER AND/OR NICKEL SULPHIDE ORE CONCENTRATES This invention relates to smelting, and is more particularly, but not exclusively, concerned with the oxygen smelting of copper sulphide ore concentrates, nickel sulphide ore concentrates or bulk copper and nickel sulphide ore concentrates (hereinafter referred to simply as "copper and/or nickel sulphide ore concentrates") of high intrinsic energy value.

Depending on the processing conditions employed and especially the chemical composition and impurity levels, the product of smelting an ore concentrate in both copper and nickel smelting may be a low-iron high grade matte (sometimes referred to as "white metal" in copper smelting if the iron content is exceedingly low) or alternatively, a crude metal containing various levels of sulphur and oxygen, which in the case of copper is referred to as "blister copper or "semi-blister". For nickel, the normal product is a furnace matte containing relatively large amounts of iron sulphide. If the iron is reduced to a low level it is sometimes referred to as "Bessemer matte". However, even for nickel, if high temperatures and vigorous agitation are employed, it is possible to produce a metallic nickel product directly from a sulphide concentrate or similar material.

The invention relates equally well to solid sulphide process intermediates, such as granulated and ground low-iron matte of similar physical properties and chemical composition to a low-iron copper ore concentrate or analogous low-iron nickel sulphide material.

W093/24666, whose subject matter is incorporated herein by reference, discloses an oxygen smelting procedure where the copper and/or nickel WO 97/28288 PCT/GB97/00275 2 sulphide ore concentrate of high intrinsic energy value is oxygen smelted by forcibly circulating a molten sulphide carrier composition through a closed loop extraction circuit from which at least one product selected from copper, nickel and sulphides thereof can be continuously extracted at an elevated temperature, introducing the ore concentrate into the molten carrier composition at an ore receiving station so that the ore is dissolved in or melted by the composition, contacting the molten carrier composition containing said ore with an oxidising gas containing at least volume percent oxygen at an oxidation station so as to oxidise at least part of the ore and/or the molten carrier composition, and utilising heat generated during the oxidation step as a result of oxidation of the ore concentrate. In the procedure described in WO 93/24666, the oxidising gas is preferably technically pure oxygen and the heat generated during the oxidation step is utilised either by smelting the copper/nickel sulphide ore concentrate of high intrinsic energy value with another mineral concentrate of low or negative intrinsic energy value or by reducing iron oxide in slag produced in the method to a liquid iron product, and recovering such liquid iron product.

The use of technically pure oxygen, i.e. oxygen of commercial purity or higher) or oxygen enriched air as opposed to air has the advantage that it is not necessary to treat large quantities of nitrogen to remove entrained pollutants such as oxides of nitrogen, oxides of sulphur and volatiles such as arsenic, antimony and bismuth before discharge.

The large amounts of oxides of sulphur which result from the process using technically pure oxygen are relatively undiluted with other gases and can therefore be relatively easily converted. WO 93/24666 describes a procedure where sulphur dioxide produced during the WO 97/28288 PCT/GB97/00275 3 oxidation of the ore concentrate and any excess unreacted oxygen are passed to a sulphuric acid production unit, the separated oxygen being recycled back to lances used in the oxidation step to be mixed with fresh technically pure oxygen.

The present invention is not only applicable to a modification of the procedure described in WO 93/24666, but is also applicable to the enriched oxygen smelting of high intrinsic energy copper and/or nickel sulphide ore concentrates using existing bath and flash smelting operations, and indeed the sulphidic process intermediates already referred to, such as granulated and ground low-iron matte.

According to the present invention, there is provided a process of smelting a metal (preferably copper and/or nickel) sulphide ore concentrate, or similar material, of high intrinsic energy (hereinafter referred to as "metal sulphide material"), comprising the steps of exothermically reacting the metal sulphide material with oxygen in an oxidation station to produce the metal, or a low-iron matte, and sulphur dioxide; and recovering said metal or low-iron matte; wherein sulphur dioxide removed from the oxidation station is oxidised with oxygen externally of said oxidation station to form sulphur trioxide, the sulphur trioxide and further oxygen is passed to the oxidation station, and in said reacting step in the oxidation station, at least some of the sulphur trioxide is endothermically dissociated to form sulphur dioxide and oxygen so as to absorb at least some of the exothermic heat resulting from reaction of the metal sulphide material with the oxygen.

WO 97/28288 PCT/GB97/00275 4 The oxygen is preferably supplied to the process in the form of an oxidising feed gas containing at least 30% oxygen, and is more preferably supplied as technically pure oxygen (95% purity or higher).

Most preferably, at least 99 percent pure oxygen is supplied to the process. In such a case precautions will be taken to avoid ingress of air.

Advantageously, this makes the sulphide smelting facility and its ancillary sulphuric acid plant (or other facility for fixing sulphur) a virtually zero gas emission operation. The only gas eventually discharged or bled from the system will be equivalent to the nitrogen in combined gases S0 2 /0 2 /N2, having a nitrogen content equivalent to the nitrogen arising from the very small amount of nitrogen impurity in the high quality tonnage oxygen provided, which can be scrubbed chemically clean.

Modern smelting processes are almost invariably characterised by high levels of oxygen efficiency. Thus, there is normally very little residual 02 from conventional flash smelters, cyclone type flash smelters and submerged injection bath smelting processes and it is reported that in some flash smelters, for example, additional oxygen is added to the furnace gas offtake to prevent downstream problems with sulphur deposition. When such processes are fed with a high intrinsic energy feed material such as a chalcopyritic ore concentrate to produce a high grade matte, for example, a vast amount of heat is generated by oxidation of iron and sulphur so the benefits of virtually zero gas emission already referred to cannot be attained readily. The problem is exacerbated if one wishes to produce metallic copper directly from chalcopyrite given product fire-refining capabilities for dealing with impurities such as arsenic and antimony or in the relatively rare case WO 97/28288 PCT/GB97/00275 where the ore concentrate is exceptionally clean. High levels of arsenic, antimony and bismuth tend to concentrate in the blister copper in single step smelting processes because their elimination is generally lower than by conventional smelting routes. Generally, arsenic levels need to be around 0.1% or lower before a copper concentrate is classed as clean.

It has long been recognised in pyrometallurgy that gas recirculation is a viable means for dealing with excessive heat generation. For copper and nickel smelting, the presence of sulphur trioxide in smelter gas streams upstream of the sulphuric acid catalytic converter is viewed traditionally with disfavour. However, the present invention draws attention to the very large benefits to be secured in promoting sulphur trioxide formation at relatively higher temperature than used in normal acid plant converters. Energy is released very conveniently in a fluid bed heat recovery steam generator (HRSG) so that high pressure steam can be made available to the new generation of high performance steam turboalternators characteristic of advanced power generation.

Effecting

SO

2 to SO 3 oxidation at higher temperature and the very significant benefit to electricity generation that this provides over current acid plant technology, becomes feasible in a closed gas recirculation system, which is advantageous in itself, irrespective of environmental considerations.

Preferably, the exothermic reaction step is effected in the presence of an excess amount of oxygen so that unreacted oxygen leaves the oxidation station with the sulphur dioxide.

WO 97/28288 PCT/GB97/00275 6 Preferably, some of the sulphur dioxide and oxygen removed from the oxidation station are reacted with an alkaline earth metal compound eg calcium oxide derived from dolomite or limestone, in a sulphation reactor to form an alkaline earth metal sulphate, the remaining sulphur dioxide being subjected to said oxidation to sulphur trioxide.

The process of the present invention is most preferably applied to a smelting process involving forcibly circulating a molten sulphide carrier composition through a closed loop extraction circuit from which said metal can be continuously extracted at an elevated temperature, said closed loop extraction circuit including said oxidation station and a receiving station; and introducing the metal sulphide material into the molten carrier composition at said receiving station so that the metal sulphide material is dissolved in or melted by the composition and is passed to said oxidation station. Such process may be of the type disclosed in WO 93/24666 but where, instead of utilising the exothermic heat by co-smelting a high intrinsic energy ore concentrate with an ore of low intrinsic energy or converting iron in an ore concentrate to metallic iron, the exothermic heat is absorbed at least partly by the above-mentioned endothermic dissociation of sulphur trioxide.

In the accompanying drawings:- Fig 1 is a flow diagram showing an embodiment of the present invention, wherein a chalcopyritic ore concentrate is smelted using technically pure oxygen in a closed loop gas recirculation mode, in association with a closed loop melt circulation extraction circuit, and Fig 2 is a flow diagram of an embodiment of the present invention wherein a clean copper concentrate is smelted with technically pure oxygen in a flash smelter, cyclone smelter, bath smelter or other WO 97/28288 PCT/GB97/00275 7 established smelting process referred to hereafter as a primary smelter, using technically pure oxygen in a closed loop gas circulation mode.

In Fig 1, a primary closed loop extraction circuit for a molten carrier material (matte) of copper sulphide is established through first and second refractory hearths 10 and 12 by means of an R-H degassing unit 14 having an inlet snorkel 16 extending into the molten matte in a slag separation zone 17 of the first hearth 10 and an outlet snorkel 18 which supplies molten matte into the second hearth 12. The slag separation zone 17 is connected with the remainder of the first hearth 10 by an overflow weir 19 for matte and slag. Molten matte circulates back to the first hearth 10 from the second hearth 12 via a connecting passage which is effectively an overflow attached to hearth 12 but free to move with respect to the first hearth 10. This allows differential expansion between the two hearths to be accommodated without damage to the refractory material of the hearths. The passage 20 is fitted with means such as a labyrinth seal 21 for preventing the gases in the respective hearths 10 and 12 from intermixing. As will be understood from the above, the first hearth 10 is lower than the second hearth 12.

Pelletised chalcopyritic ore concentrate is passed via multiport feeds 24 into either side of the first hearth 10 of the primary smelting circuit. The ore concentrate moves down sloping side walls of the first hearth 10 for a substantial distance as a relatively thin layer before entering the layer of molten matte which is moving at high rate through the first hearth Thus, the first hearth 10 defines an ore receiving station. During its passage down the sloping walls, the pellets of ore concentrate are exposed to radiant heat within the first hearth 10 which, like the other hearth 12, is rendered substantially gas tight by a refractory roof. Instead WO 97/28288 PCT/GB97/00275 8 of using sloping side walls, the ore concentrate may be transported using a heat-resistant belt conveyor within the high temperature region of the hearth, which takes the concentrate along the entire length of the hearth before discharge into the circulating molten matte. The ore concentrate is thereby heated to a temperature of about 1000 K, at which temperature labile sulphur is volatilised as sulphur vapour and arsenic and antimony, present as impurities in the ore concentrate, are also volatilised in the form of their sulphides. Such vapours are removed from first hearth 10 via line 26 leading to a condenser (not shown). The ore concentrate, from which the arsenic etc have been volatilised, dissolves in and is melted by the circulating layer of molten matte and passes over weir 19 together with slag and into the slag separation zone 17. The matte is then transferred to the second hearth 12 through the R- H degassing unit 14. Since the inlet snorkel 16 extends below the layer of slag which has formed on top of the molten matte layer M, relatively clean matte is transferred to the second hearth 12 by the R-H unit 14.

Slag is removed and passed either to a conventional slag granulation system employing water, or via path 28 for dry granulation.

The molten matte transferred to the second hearth 12 of the primary smelting circuit and which contains the dissolved ore concentrate is then subjected to oxidation at an oxidation station 29 using technically pure oxygen in admixture with sulphur dioxide and sulphur trioxide supplied via line 30 to top blow nozzles 32. Top blowing is controlled so as to convert the copper sulphide to copper metal. The oxygen utilisation efficiency is arranged so that gases which exit from the oxidation station 29 via a line 36 contain excess oxygen, sulphur dioxide and a reduced amount of undissociated sulphur trioxide.

WO 97/28288 PCT/GB97/00275 9 The copper produced is bled off via line 34 as blister copper whilst the matte layer passes back to the first hearth 10 via passage 20. An in situ slag cleaning operation (eg using iron pyrites) is effected at this location in hearths 10 and 12.

The gases exiting from the oxidation station 29 via line 36 pass into a fluidised bed 38 containing preheated (1200 K) silica sand flux which is fluidised by such gases. Slag may be passed from path 28 into the bed 38 to be dry granulated therein and discharged via line 40. The gases are de-fumed by agglomeration of fume on the bed of sand and leave the bed 38 to enter a sulphation reactor 42. If high purity value-added products are required from the sulphation reactor 42, or to ensure environmentally benign disposal of reacted solids at the mine-site, for example, it may be desirable to capture halides and volatile As 2 03 by contacting the gases with calcined limestone/dolomite upstream of the main sulphation reactor 42 where temperature and gas composition is such that neither calcium sulphite nor calcium sulphate are stable. This would necessitate combustion of the excess oxygen leaving the smelter with elemental sulphur condensed out from the inert side of a melt circulation reactor. The solid product of this pre-treatment of virtually pure sulphur dioxide would comprise calcium arsenate and calcium halides which can be readily separated, with the calcium arsenate being an ideal material for incorporation into a vitreous slag matrix. This is conveniently done by assimilating calcium arsenate into the fayalite slag produced in the smelter. Provided the slag is water-quenched, it can be expected to pass strict environmental regulations such as leachability tests.

WO 97/28288 PCT/GB97/00275 In the sulphation reactor 42, dolomite is calcined to form calcium oxide which then reacts with a proportion of the sulphur dioxide and oxygen to form anhydrite which is removed as a product. The sulphation reactor 42 and calciner are preferably of the type disclosed in British Patent Application No. 9602037.5. The gases (oxygen, unreacted sulphur dioxide and some sulphur trioxide) are passed to a fluid bed HRSG 44, in which the sulphur dioxide is reacted with some of the oxygen to form sulphur trioxide.

Steam generating coils immersed in a fluidised bed of solids is well proven technology and is a very effective way of capturing the exothermic heat arising from the oxidation of SO 2 to SO 3 The bed of solids does not need to possess the highly catalytic activity of a conventional sulphuric acid plant converter, but catalytically active solids are preferred. For example, if a fluid bed of a chrome-ferric oxide mineral such as chromite fines at, for example 950 K is operated under near equilibrium conditions to produce a gas mixture containing 1 mol

SO

3 0.54 mol 02 and 0.50 mol SO 2 such a gas mixture can be used to cool down a flash smelter operating with chalcopyrite to produce liquid copper without overheating. To achieve the same effect by recirculating only S02 with feed 02, both at room temperature, would require approximately twice the molar gas flow rate at the entrance to the flash smelter reaction shaft. Additionally, dust carry over, a recognised problem in both bath smelting and flash smelting, is reduced when a lower gas volume is used. A lower gas volume also proportionately reduces downstream gas cleaning requirements, reflected in decreases both in capital and operating costs. In relation to bath smelting, a reduction in volume both within the smelting bath and exiting from the reactor, helps reduce excessive splashing, foaming and what is known as WO 97/28288 PCT/GB97/00275 11 "slopping". For waste minimisation reasons, it may be preferable to modify operating conditions so that a fluid bed of iron ore fines rather than chromite is used. The spent iron oxide material could then periodically be assimilated into the smelting slag without producing a solid waste of possible environmental concern.

The resultant gas mixture leaving the HRSG 44 is passed to a hot gas eductor 46. In the hot gas eductor 46, the gases are mixed with technically pure oxygen and the resultant mixture passed via line 30 to the nozzles 32.

A relatively static or slow-moving layer of molten copper is maintained in the hearths 10 and 12 below the rapidly circulating molten matte so as to protect the hearths against severe erosion which would otherwise occur if the circulating matte were directly in contact with the hearth.

The chemical reactions taking place in the smelting circuit per mole of chalcopyrite are:- CuFeS 2 1/2 CuS FeS 1/4 S 2 FeS 3/2 02 FeO+SO 2 1/2 Cu 2 S+1/2 0 2 Cu+1/2SO 2 CuFeS 2 +2 02 Cu+3/2

SO

2 +FeO+1/4 S2 Reaction takes place under neutral or reducing conditions so that one quarter of the sulphur contained in chalcopyrite is recovered in the elemental form, whilst three quarters is released as SO2. The overall heat balance for the circuit is given in Table 1. The outlet gas temperature of WO 97/28288 PCT/GB97/00275 12 1200 K is based on heat and mass transfer correlations for top blowing in a non-splash mode.

Table 1 Heat Balance Matte Circuit Enthalpy datum 298 K Basis: one tonne of copper Chemical Reaction Enthalpy Input at 298 K CuFeS,+2 0,+1/2 SiO2=Cu+3/2 SO+ 1/4 S2+1/2 Fe 2 SiO 4

AH

298 -121.2 kcal,

AH

298 Enthalpy In above 298 K

SO

3

/SO

2

/O

2 Gases Recirculated at 710 K Chalcopyrite Preheated to 500 K Silica sand Flux Preheat to 1200 K TOTAL ENTHALPY IN Enthalpy Out above 298 K Liquid Copper at 1600 K SO3/S2/02 Gases at 1200 K S2 gas at 1000 K (Sensible heat only) Fayalite Slag at 1600 K

SO

3 dissociation

AH

29 8 Radiation Convection Losses TOTAL ENTHALPY OUT MJ per tonne Cu 7973 1618 337 471 10,399 MJ per tonne Cu 795 3936 99 2633 1389 154Z 10,399 WO 97/28288 PCT/GB97/00275 13 To effectively capture the excess energy released by smelting of the ore concentrate, use is made of the endothermic dissociation of sulphur trioxide in the recirculated gas. This can be effected by restricting the cooling of gases before recirculation only down to about 900 K within the HRSG 44 or similar device, in which the exothermic recombination of 02 and SO 2 is facilitated and the energy recovered initially from within the smelting furnace and supplemented further in the sulphation reactor 42 appears eventually as steam for electricity generation.

Referring now to Fig 2, there is illustrated a primary smelter 48 in which excessive heat generated over and above the basic requirements of smelting a sulphidic material to either a high grade matte or metal, is removed by circulating a gas principally comprising

SO

3 /S0 2 /0 2 and residual N 2 via a line 50 to a reaction shaft, cyclone reactor, top blown lance(s) or submerged tuyeres (not shown) of the primary smelter 48.

Because of the high oxygen efficiency of these processes, almost all of the oxygen added directly and in addition that resulting from the dissociation of SO3 is reacted within the smelter 48 so that the gases leaving the smelter 48 via a line 52 contain little if any excess 02. The gases leaving via line 52 are therefore composed principally of SO,, a relatively smaller amount of nitrogen and perhaps a small amount of water vapour from residual moisture in the materials fed to the smelter 48. These gases are also associated with smelter fume and some particulates resulting from solidified slag droplets and entrained dust stemming from the solids charged. In this embodiment, the return of fume and particulates to the smelter 48 is an essential requirement so gases are passed to a hot gas cleanup station 54 to effect the desired solid/gas separation. Hot gas cleaning is a rapidly expanding field, particularly in advanced power generation, where candle ceramic filters WO 97/28288 PCT/GB97/00275 14 and moving granular bed filters, for example, are under intense development for removing particulates at high temperature from fuel gas prior to its combustion in gas turbines. More conventionally, the hot gas cleanup station 54 would partially cool the gases in a waste heat boiler followed by a hot gas precipitator. The principal gas flow is passed without further cooling via a line 56 to eductors 58 activated with high pressure oxygen from an air separation plant (not shown). After passing through the eductors 58, the combined gases now containing practically all the oxygen requirements of the smelting operation are passed via a line 60 to a Heat Recovery Steam Generator (HRSG) 62, preferably of the fluid bed type, probably containing catalytically active solids to effect conversion of SO 2 to SO3 in the presence of excess 02 at a relatively high temperature compared with sulphuric acid manufacture.

Depending on the sulphur fixation option being pursued, some SO 2 and 02 may also be added to HRSG 62 via a line 64, or alternatively directly back to the smelter 48 via a line 66. To maintain the sulphur balance, an amount of SO2 equivalent to the sulphur in the feed sulphidic material to the smelter 48 is removed from the principal gas recirculation system via a line 68. For example, line 68 may be connected with a conventional sulphuric acid production plant 70 wherein the gases in line 68 are cooled and extensively cleaned. It is of concern in current smelting technology, if extra SO3 enters the acid plant 70 upstream of the catalytic converters. This extra SO 3 requires extra impure dilute sulphuric acid "blowdown" during the gas cleaning operation, clearly worsening a waste disposal problem. The current proposals do not exacerbate this situation because the high temperatures back in the primary smelter 48 ensure almost complete dissociation of SO3 back to SO, and 02. Also with the gas recirculation system, it is no longer WO 97/28288 PCT/GB97/00275 necessary to strive for ultra-high conversion efficiency in the acid plant because the SO 2 not converted is merely recycled along with any acid mist not eliminated. To control build-up of non-condensables in the gas recirculation loop, it is necessary to bleed off, through a vent line 72, gases containing an amount of nitrogen equivalent to that added in the tonnage oxygen or that due to ingress of air.

Alternatively, it may be desirable to treat the strong SO gases withdrawn through line 68 using other options than acid plant 70 for fixing sulphur, such as liquefaction or elementary sulphur production, both processes benefitting from the availability of high strength sulphur dioxide. It will be appreciated that all these various options, including sulphuric acid manufacture, can comply with present environmental regulations using current technology, so that for retrofitting an existing plant, the teachings of the present invention are seen to be a means for increasing throughput by oxygen enrichment well above the levels presently available because of excessive heat generation. To give an immediate insight into retrofitting an existing smelter in the context of the present invention, it is worth noting that theoretical calculations indicate that autothermal conditions are achieved for pure chalcopyrite, for example, if gases containing 2.4 mol SO and 1.3 mol 02, and associated gases at 950 K are recirculated to the smelter per mol of chalcopyrite being smelted.

An example illustrating the general principle of energy transfer via SO 3 formation and dissociation will now be given. Consider the formation of one mole of copper from chalcopyrite. Reaction stoichiometry demands the addition of 2.75 mol of 02 for every mol of copper produced. Of this total, 2 mol are consumed in reacting with chalcopyrite and 0.75 mol are needed for reaction of 1.5 mol SO 2 with calcium oxide. Say that WO 97/28288 PCT/GB97/00275 16 2.75 mol of new technically pure oxygen is delivered from an air separation plant (not shown) at a pressure of 20 p.s.i.g. to the throat of the eductor 46. Using data in trade literature for air-operated eductors to produce 24 inch water gauge differential pressure, this 2.75 mol of 02 is more than adequate to recirculate 0.05 mol 02, 1.60 mol SO 2 and 1.40 mol SO 3 at 900 K back to the top blow nozzles 32 in the oxidation station 29. The O2/SO0/SO3 gas mixture is essentially at equilibrium at 900 K as it leaves the HRSG 44 and the effect of the cold oxygen addition is to lower the temperature of the recycled gas stream to about 710 K. Because of its short residence time at the lower temperature level and the progressively less favourable SO3 conversion kinetics at lower temperatures, the gas composition entering the oxidation station 29 principally reflects dilution with oxygen rather than a significant increase in the mols of SO 3 The gas discharging from the top blow nozzles 32 thus contains 2.80 mol 02, 1.60 mol SO 2 and 1.4 mol SO3. By appropriate design of the nozzles 32, oxygen utilisation efficiency in the top blow region can be arranged so that the gases exiting from the smelter at say 1200 K contain 1.25 mol 02 4.00 mol SO 2 and 0.5 mol SO3, again essentially in an equilibrium state. The endothermic dissociation of 0.9 mol SO 3 within the oxidation station 29 performs the useful task of removing a significant proportion of the excess heat generated when chalcopyrite is contacted with oxygen. In the sulphation reactor 42, 1.5 mol SO2 and 0.75 mol 02 are removed and, if they exit at say 1400 K, very little SO 3 remains and the gases directed to the HRSG 44 contain 3 mol SO, and 0.75 mol 02. Within the HRSG 44, some 1.40 mol SO3 are formed as the gas mixture cools from 1400 K to 900 K releasing exothermic heat far in excess of that associated with the sensible heat change in the absence of chemical reaction. We are now back to the starting point of this illustrative example and have a gas WO 97/28288 PCT/GB97/00275 17 mixture at 900 K, essentially in an equilibrium state containing 0.05 mol 02, 1.60 mol SO2 and 1.40 mol SO 3 For one tonne of copper produced the steam credit associated with the recycle gas stream containing 0 2

/SO,/SO

3 is shown in Table II.

From Table II the sensible heat recovered from gases is (3322-1618) 1704 MJ per tonne Cu, whilst heat recovery associated with SO 3 formation is some 1.27 times greater at 2160 MJ. These figures illustrate the relative importance of energy transfer via SO3 formation in the primary copper smelting system under consideration and should be assessed in conjunction with the figures given in Table 1 showing the not insignificant cooling effect brought about by SO 3 dissociation.

IabJle Overall Heat Balance on the Heat Recovery Steam Generator A Enthalpy datum 298 K Basis: one tonne of copper Chemical Reaction Enthalpy Input at 298 K

SO

2 +1/2 02= SO3

AH

29 8 =-23.45 kcal.

Enthalpy In MJ per tonne Cu 0 2 /SO2/SO 3 at 1400 K 3111

AH

298 for SO 3 formation 2160 5271 Enthalpy Out 2/SO,/SO 3 at 900 K 1618 Heat Losses at 10% 527 Enthalpy in Steam 3L26 5271 WO 97/28288 PCT/GB97/00275 18 It is important to recognise the full significance of the energy transfer scheme suggested involving the key role of SO 3 formation and dissociation. Unlike all other copper smelting processes, only relatively gentle top blowing with oxygen onto an essentially slag-free matte surface is proposed so that the oxygen utilisation efficiency is purposely considerably less than 100 percent, ensuring enough excess oxygen leaves the smelter along with sulphur dioxide to facilitate SO 3 formation in the HRSG 44 after first supplying the needs of the SO,/O 2 reaction with limestone or dolomite in the sulphation reactor 42 and also ensuring that sufficient SO3 is contained in the gases recycled to the oxidation station 29 to assist in providing in situ cooling of systems involving high intrinsic energy sulphides being smelted with technically pure oxygen.

A. Yazawa and C. Acuna, "Copper Smelting by Use of Calcium Ferrite Slag and Recycling High Strength SO 2 Exhaust Gas", Copper '87 Volume 4 Pyrometallurgy of Copper, (Santiago, Chile, 1988), 305-317, point out that using pure oxygen, as in the Inco Flash Furnace with typical copper concentrates, restricts the matte grade to around 50 percent copper because of excessive heat evolution. They propose smelting with oxygen and recycling of high strength SO 2 exhaust gas as a means for producing white metal (75% Cu) in a new copper smelting process using calcium ferrite slag. These authors indicate that for exhaust gas recycling, taking as a base 1,000 Kg of dry chalcopyrite concentrate, some 466 m 3 of gases leave the smelter comprising 423 m 3 of SO 2 and 43 m 3 of H 2 0.

There is no excess 02 involved in such proposal. Some 193 m 3 of SO2 and 20 m 3 of H 2 0 are recycled to the smelter, whilst 253 m 3 of SO and 43 m 3 of H 2 0 go forward to gas purification.

SO

3 formation and dissociation does not feature in their discussion.

Claims (9)

1. A process of smelting a metal sulphide ore concentrate, or similar material, of high intrinsic energy (hereinafter referred to as "metal sulphide material), comprising the steps of exothermically reacting the metal sulphide material with oxygen in an oxidation station to produce the metal, or a low-iron matte, and sulphur dioxide; and recovering said metal; wherein sulphur dioxide removed from the oxidation station is oxidised with oxygen externally of said oxidation station to form sulphur trioxide, the sulphur trioxide and further oxygen is passed to the oxidation station, and in said reacting step in the oxidation station, at least some of the sulphur trioxide is endothermically dissociated to form sulphur dioxide and oxygen so as to absorb at least some of the exothermic heat resulting from reaction of the metal sulphide material with the oxygen.

2. A process according to claim 1, wherein the metal is copper and/or nickel.

3. A process according to claim 1 or 2, wherein the oxidation of the sulphur dioxide to sulphur trioxide is effected in a fluid bed heat recovery steam generator, and excess oxygen and sulphur dioxide from the generator is passed to the oxidation station.

4. A process according to any preceding claim, wherein the oxygen is supplied as technically pure oxygen.

WO 97/28288 PCT/GB97/00275 A process according to any preceding claim, wherein at least 99% pure oxygen is supplied to the process and precautions are taken to avoid ingress of air.

6. A process according to any preceding claim, wherein the exothermic reaction step is effected in the presence of an excess amount of oxygen so that unreacted oxygen leaves the oxidation station with the sulphur dioxide.

7. A process according to claim 6, wherein some of the sulphur dioxide and oxygen removed from the oxidation station are reacted with an alkaline earth metal compound in a sulphation reactor to form an alkaline earth metal sulphate, the remaining sulphur dioxide being subjected to said oxidation to sulphur trioxide.

8. A process according to any preceding claim, wherein a molten sulphide carrier composition is forcibly circulated through a closed loop extraction circuit from which said metal can be continuously extracted at an elevated temperature, said closed loop extraction circuit including said oxidation station and a receiving station; and introducing the metal sulphide material into the molten carrier composition at said receiving station so that the metal sulphide material is dissolved in or melted by the composition and is passed to said oxidation station, and wherein the exothermic heat is absorbed at least partly by the above-mentioned endothermic dissociation of sulphur trioxide.

9. A process according to any preceding claim, wherein the metal sulphide material of high intrinsic energy is at least one material selected WO 97/28288 PCT/GB97/00275 from the group of copper sulphide ore concentrate, nickel sulphide ore concentrate, and solid sulphide process intermediates. A process according to any preceding claim conducted in a closed gas recirculation system.