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To overcome limitations in current technology and to open the door to breakthroughs in the extraction and recycling of metals, melt circulation within closed loops is proposed. The general features of generic melt circulation technology, particularly for massive reductions in energy consumption, are highlighted. Reference is made to the recently published paper on lower-energy primary aluminum. More detailed attention is then focused on coproduction of steel and titanium metal directly from ilmenite concentrates. The energy consumption is projected to be less than one third of the best available technology (Kroll process). Next, new copper smelting concepts based on melt circulation are introduced because current advanced processes are judged to be, without exception, energy inefficient.

INTRODUCTION

The metal-producing industries are responsible for significant greenhouse gas emissions globally. Also, in cost terms, there is increasing awareness that efficient use of energy in general must be a top priority. Since retiring from the University of Birmingham (United Kingdom) in 1998, the author has made a concerted effort to address these issues by conference presentations and published technical papers. The focus has been on the metal industries, not only in extraction of metals from primary resources, but also in recycling secondary materials, such as steel scrap, as illustrated in Figure 1.¹

BACKGROUND

An overview of generic melt circulation technology was presented at a Canadian Institute of Mining, Metallurgy and Petroleum conference in 1996² and prior to this a short review article was published on the same subject in 1994.³ More recently in 2003, a general paper on melt circulation and conductive heating was published.⁴ However, the origins of the technology can be traced back much further to the published account of discussion following a presentation on coal-based ironmaking.

Coal-based ironmaking using the smelting reduction approach was pioneered in Sweden. The “father” of smelting reduction is widely acknowledged to be Sven Eketorp, a professor with the Royal Institute of Technology, Stockholm. Eketorp presented a paper on the “direct use of coal for production of molten iron” to a conference in London in 1981.⁵ He cited four different processes attempted in Sweden. They all failed. Eketorp’s published response to a question⁶ that the author asked in 1981 was: “In reply to Professor N A Warner, there is very little hope of finding a process whereby we can deliver energy to the bath directly, i.e. by burning CO to CO₂ . . .”. He went on to say: “If energy is produced by an oxidizing reaction such as combustion of CO, the problem is to separate heat transfer and mass transfer. The lining could not withstand heat pulsation and the FeO rich slag. . .”.

Obviously the present author does not accept either of the above statements as the final word on smelting reduction. However, in attempting to assess the likelihood of new ironmaking processes replacing the iron blast furnace it is necessary to keep Eketorp’s comments in mind and to realize that to date there has been a whole catalogue of unsuccessful attempts. Without exception the reactors employed have all been incapable of complete so-called “post combustion” of CO to CO₂ within the ironmaking reactor itself without overheating and damaging the refractory lining and thus making the process inoperable. The accumulation of slag within the reactor is the root cause of the problem. Slag accumulation creates a thermal barrier, which inhibits efficient heat transfer back to the site of the endothermic iron producing reaction in the liquid metal bath.

Melt circulation at a rate many times the production rate of metals allows operation of two side-by-side furnace hearths at slightly different levels so that melt overflows continuously from one to the other. If the higher hearth is where, for example, oxygen top blowing takes place, this provides the mechanism for floating slag away as soon as it is formed without allowing an appreciable layer thickness to ever build up.

Melt circulation also provides the means for maintaining one side of the reactor under neutral or reducing conditions while oxygen is blowing on the other. Furthermore, melt circulation allows the transference of heat from exothermic reactions (such as iron slagging and copper conversion) to the endothermic site (such as zinc gas formation and charge assimilation) by using sensible heat transported by the circulating melt. In addition, fuel can be combusted and energy can be transmitted directly at high intensity to what is effectively a slag-free surface. Also forcibly circulating a melt within a reactor overcomes limitations inherent in conventional pyrometallurgical furnaces such as back-mixing and non-countercurrent contacting.

A proposed melt circulation approach for continuous coal-based ironmaking is shown in Figure 2.⁷ The remaining challenge facing the ferrous industry is the introduction of an energy-efficient integrated process for continuous steelmaking using coal and iron ore fines as feed material and ending up with ultra-low carbon steel, if so desired. In 2003, the author published three papers on this subject using generic melt circulation technology.^8–10 The key requirement is to balance gas phase mass transfer, interfacial chemical kinetics, and liquid phase mass transfer in order to prevent homogeneous nucleation of CO beneath the melt surface. H. Bessemer himself, in his autobiography paints the picture of what happens if this is disregarded. Regrettably, failure to ameliorate CO bubble eruptions has contributed to a lack of success in all previous attempts with continuous steelmaking.

To emphasize just how disastrous sub-surface nucleation and growth of CO bubbles can be, Figure 3 shows the explosive disintegration of molten steel under stream vacuum degassing conditions, following CO/CO₂ (10/1) gas injection at the same rate for the time intervals indicated. Clearly, such high reaction intensity has to be moderated if successful continuous steelmaking is ever to be achieved, even if this means larger surface area reactors have to be employed. On the other hand, natural-gas-based steelmaking using iron-ore fines eliminates the need for any decarburization whatsoever.¹¹ Refined molten iron is produced directly with carbon transfer to the melt prevented by a thin layer of molten iron oxide. Conventional oxygen steelmaking as such is eliminated and sub-surface nucleation and growth of CO is thus longer an issue.

MATTE CIRCULATION TRIALS

High-temperature semi-pilot-scale trials were conducted at the University of Birmingham employing a closed loop molten matte circulation system at temperatures around 1,200°C–1,250°C. Use was made of a gas-lift system closely resembling RH steel vacuum degassing technology. Ruhrstahl Heraeus (RH) steel degassing is a mature technology used worldwide for batch vacuum degassing of liquid steel. Melt merely overflowed from a high-level hearth into a lower-level hearth to be pumped back with the RH-type device. Figure 4 is a schematic plan view of the plant, which consisted of two refractory-lined rectangular hearths side by side but at different levels, with a vacuum lift pump (RH system) connecting the hearths at one end to transfer matte from the lower to the upper hearth. A passage at the other end was to allow matte to flow back to the lower hearth under gravity, thus completing the closed loop. The hearths were contained within a stainless steel furnace shell with a detachable lid with a sand seal around its perimeter to make it effectively gas tight. The furnace was vented to a caustic soda scrubber for removal of sulfur dioxide, water vapor, and other gaseous products. The furnace shell was force cooled with ducted air to ensure that the melt freeze line was within the inner magchrome brick, which was backed by magnesite and super-duty firebrick. The scale of the operation can be gauged from the plant photographs shown in Figures 5 and 6.

The twin hearths contained a known amount of copper-saturated copper sulfide matte, or “white metal.” The temperature of the matte was raised to about 1,250°C by means of direct-resistive or so-called conductive heating. Electric currents as high as 7,000 A were passed through the copper matte between graphite electrodes at either end of the hearths. Power to each hearth was supplied by two 180 kVA welding transformers with parallel primary windings, and with their secondary windings in series, in order to supply sufficient voltage, and thus power, to melt the matte. The hearths were designed so the matte depth for the lower hearth, at about 280 mm, was greater than the upper hearth at 150 mm.

The RH-type vacuum system connected the hearths via two inclined snorkel legs, which were lowered into the molten matte immediately prior to circulation. Inert gas (nitrogen) was injected into the up leg of the vessel (in the deep, lower hearth), which as it expanded forced the liquid upward under a two-phase, bubble flow regime, into the main RH chamber. Under vacuum, the entrained gas was evolved and the melt returned to the shallow, higher hearth through the other leg by gravity.

The vacuum system was provided by a mechanical rotary pump/booster combination. Various water and nitrogen cooling circuits served to prevent overheating of critical components. Incorporated into the plant was a vacuum- activated system for emptying the molten matte from the hearths into a holding vessel, or dump tank, via a siphon pipe system. Initially it was proposed that this dumping system would be employed after every trial and the hearths refilled with matte. This did not prove necessary, as it was shown possible to re-heat solidified matte in-situ without adverse effects.

Over a six-hour period some 300 tonnes to 500 tonnes of matte were circulated past a given point inside a closed loop comprised of two interconnected side-by-side open channels, each 0.35 m wide × 4.0 m long. This constitutes proof of concept not only of melt circulation but also electrical conductive heating for melting the crushed solid matte charged initially into the channels and the ability of this mode of heating in keeping the melt in a molten state over the six-hour period of the trial. Accounts of this trial and associated developments have been published elsewhere.^4,12

NEW HORIZONS

In the non-ferrous field, the original melt circulation research was targeted at direct smelting of complex sulfides to produce zinc, lead, and copper simultaneously in the one reactor.¹³ Gradually the emphasis changed to direct smelting of bulk zinc-lead concentrates and finally to zinc concentrates in general.¹² The technology has received some notoriety for proposed zinc metal production, due principally to the efforts of P.M.J. Gray,^14–16 who aptly referred to the zinc adaptation as the “Warner Process,” but the process remains commercially unproven. The general features are illustrated in Figure 7.

The major waste product or solid emission from smelting is slag. Given appropriate technology, it can be argued that the natural products to be derived from copper minerals are metallic copper and metallic iron. Reduction of iron in slag is a highly endothermic chemical reaction, so clearly the possibility exists of using the excess heat generated in direct oxygen smelting to sustain the iron production reaction. A melt circulation reactor employing twin loop circuits is proposed, one for copper matte and the other for fayalite slag, which in principle could eventually lead to low energy simultaneous production of copper and iron with virtually zero solid-waste generation.¹⁷ The general features are shown in Figure 8.

The examples quoted so far have been well documented and will not be considered further in the present paper. Hopefully, there are now improved prospects for ultimate commercialization of energy-saving technologies. Together with perceived threats of global warming and climate change, the stage is set for a major multi-national company to get involved in the generic technology. The philosophy now is to intentionally seek out what can only be referred to as true breakthroughs with radically innovative technology to tackle some of the more obvious shortcomings of the presently accepted status quo.

There is most certainly a need for an improved aluminum process. Titanium metal will not fulfill the ambitious expectations projected for it over many years, unless there is a real breakthrough. The proposed technology for continuous smelting of ilmenite directly to titanium metal potentially offers energy consumption less than one third of the current best available technology (Kroll process). This is before claiming a credit for the co-produced liquid steel.

Primary zinc metal production is currently tied too closely to electrowinning. Thus cheap electrical power is essential. This is unlikely to be available at the mine site without massive capital investment, requiring amortization over periods probably greater than the life of the mine itself.

Finally, and probably unexpectedly, there is great scope for improving the energy efficiency of copper smelting. The current cutting-edge copper smelting processes have made tremendous gains over the traditional processing route employing reverberatory furnaces and Peirce-Smith converters. They are, however, without exception, extremely energy inefficient.

Generic melt circulation offers the prospects of major breakthroughs in aluminum, titanium, zinc, and copper production. The conceptual design aspects of two of these advances will now be considered. The melt circulation process for aluminum has been detailed already this year.¹⁸ Also, exciting prospects for massive reduction in the energy requirements for primary zinc have been identified based on generic melt circulation technology. It would be inappropriate to summarize these in the present paper, as a manuscript on the theoretical foundations has been submitted to Metallurgical and Materials Transactions.

Continuous smelting of ilmenite is proposed, eliminating chlorine-based technology except perhaps as a means for dealing with associated radioactivity.¹⁹ Concentrates are fed into the first of three melt circulation loops, in which optionally both the oxidic melt and the liquid steel layers may be circulated. The molten iron formed joins the circulating bottom layer of liquid steel, which extracts impurities including chromium, silicon, manganese, vanadium, niobium, aluminum, and phosphorus. Liquid steel is withdrawn for refining prior to continuous casting. The oxidic melt has its carbide content increased in the second of the melt circulation loops. Continuous vacuum refining is then conducted in the third loop operating at 10^–4 MPa using a vacuum steel degassing steam jet ejector systems, yielding a theoretical (TiO₂ + ZrO₂) equivalent in the titanium oxycarbide melt of “four nines” (99.99%) purity at 84% recovery.

F. Cardarelli²⁰ describes a method for electrowinning titanium metal or alloy from titanium oxide containing compounds in the liquid state, which is claimed to have significant advantages over other emerging technologies. It involves direct electrowinning of titanium from molten titanium mixed oxide compounds. The preferred electrolyte is molten calcium fluoride. During the electrochemical reduction, droplets of liquid titanium metal are produced at the oxide/electrolyte interface and sink by gravity, settling to the bottom of the electrochemical reactor, forming after coalescence a pool of liquid titanium metal or alloy. The liquid metal is continuously siphoned or tapped under an inert atmosphere and cast into dense titanium metal ingots.

The initial production of metallic droplets of impurities such as metallic iron and other transition metals more noble than titanium (e.g., manganese, chromium, vanadium, etc.) detracts seriously from the process in that cross-contamination would inevitably occur between the initial impure metal and the product titanium. Also, the process is not truly continuous, a shortfall self-evident in the batch charging of molten titanium slag or other molten materials into the electrochemical reactor.

A truly continuous process is not yet available which is capable of accepting ilmenite mineral concentrates or titaniferous magnetite at one end of the spectrum right through to synthetic rutile or solid upgraded titanium slag at the other. Their transformation in-line to high-purity liquid titanium II oxide as the preferred continuous feed for titanium metal production is the essence of the proposed melt circulation route to titanium metal. At the same time, it is desirable to co-produce liquid steel in a state ready for continuous refi ning in advance of continuous casting.¹⁹

The principle enunciated for smelting right up to the engineering limit of structural graphite, say 2,200°C in terms of stability and mechanical strength, is that ultra-high-temperature operations must be conducted relatively close to thermodynamic equilibrium between the phases in contact with each other. Normally in pyrometallurgy these phases are molten slag, molten metal, the solid in contact therewith, and the associated gas phase. Of these, the interaction of the slag and solid phase is crucial but the liquid metal/solid hearth contact must also be considered.

It is accepted that relatively large areas are required if close to equilibrium conditions are to apply, and this implies the use of what have been termed previously by the author as “swimming pool reactors.” It is, of course, quite obvious that swimming pool reactors can only be entertained if the processing technology is truly fully continuous without cyclic variation and with provision for the withdrawal upward of certain plant items when processing is interrupted before the melt freezes over.

For large swimming-pool-sized reactors, monolithic linings composed of various grades of titanium oxycarbide are needed. The ductile-to-brittle fracture transition for such materials is very favorable and they would appear to be able to operate over a temperature range from above 2,000°C to, say, 800°C. This facilitates maintenance of the unmelted shell rather than attempting to use so-called skull formation with water-cooled hearths. Transition metal carbides have the ability to deform plastically above a given temperature, referred to as the ductile-to-brittle transition temperature. Below that temperature titanium carbide fails in a brittle manner, while above it, it shows ductile behavior and undergoes plastic deformation. For TiC this is in the region of 800°C and because of the cubic structure of titanium oxycarbide over the whole range of solid-state stability, high temperature linings of titanium oxycarbide can reasonably be expected to behave in a similar fashion.

This is extremely helpful in terms of accommodation of thermal stresses resulting from thermal expansion without fracturing or forming cracks in the lining, which could lead to problems with melt containment and is also very desirable in maintaining the electrical and thermal conductivity integrity of the titanium oxycarbide solid lining. Water-cooling with traditional skull formation, on the other hand, does not secure these beneficial attributes as the region of brittle fracture replaces plastic deformation once the temperature drops much below the critical transition temperature.

For ilmenite processing, the first of the sub-processes is the formation and recovery of a molten iron alloy for subsequent continuous processing to liquid steel ready for continuous casting. Thus a gas phase and two liquid phases are involved. All of these must be essentially at equilibrium with each other throughout the associated melt circulation loop. The reactor hearth, walls, and contact areas of equipment such as lances, snorkels, and overflow and underflow weirs immersed in the melt must all be prefabricated from material of the solidus composition.

Liquid phases must be close to the liquidus temperature and if two liquid phases are involved, it follows that composition and temperature gradients within the bulk phases must also be eliminated and preferably each independently circulated under turbulent flow conditions to promote good mixing. This leads to what is believed to be a totally new approach to pyrometallurgy: forced circulation of both slag and metal phases at relatively high rates.

Conducting pyrometallurgical operations in relatively low-intensity reactors rather than using high-intensity reactors currently in vogue presents opportunities just not available in compact reactors. Radiative post combustion is an important case in point. Admittedly, reactors of Olympic swimming pool dimensions are going to be needed for very large-scale operations. If these large reactors are lined with unmelted solid shells of the material being processed, the cost implications can be assessed in terms of the interest lost on the cash flow not realized because of the hold-up of product within the process. This has to be balanced against the costs involved in conventional refractory lining of the reactors and the fact that unmelted shells of product material are indestructible. They can be replenished in situ during continued operations by controlled melting or freezing, employing electro-conductive heating in conjunction with steam rising, or other heat removal means at high temperature.

Figure 9 is an overview of the plant for continuous smelting of ilmenite concentrates employing three melt circulation loops in series to feed an electrochemical deoxygenating reactor to produce titanium metal.

Figure 10 shows a sectional elevation of a melt circulation loop containing a single oxidic melt liquid phase showing the “cavity-wall” type of construction. This construction is comprised of an inner hot face lining of solidus composition titanium oxycarbide, a free space containing support skids to allow unimpeded thermal expansion and contraction of the lining, boiler tubes for steam raising, superheating or closedloop steam reheating service as appropriate on safety grounds, so positioned that they receive direct thermal radiation from the cooler face of the oxycarbide lining, and an outer backing of conventional refractory and insulating materials, all encased externally in a gas-tight steel shell.

Special measures need to be taken in the design of such furnaces to accommodate differential expansion internally while keeping the outside surfaces of the furnaces moderately cool. Freedom of the hearth to expand or contract without excessive friction is crucial to the success of the proposed continuous smelting technology. Conventional skid mounting on heat-resistant alloy shells encasing the cooler faces of the oxycarbide linings for mechanical integrity and structural stability, or perhaps more sophisticated “bogey” rail tracking may be necessary for this purpose so that the rather long hearths involved can expand or contract freely. In this connection it must be borne in mind that unscheduled shutdowns have to be accommodated and the prospect of the hearths cooling to room temperature must be addressed at the design stage. Also, sufficient clearances must be provided inside the furnace interiors to permit free expansion and contraction to take place differentially with respect to the outer steel encasement or associated pressure/vacuum vessels.

The refractory roof and its associated structural steel work is supported on steel pontoons floating in launders or troughs on each side of the hearth containing the melt and extending the full length of the furnace. By pumping liquid in and out of these launders, the pontoons can be made to float and thus during heating up from room temperature to, say, 2,000°C at the hot face of the lining, the structure is free to expand both longitudinally and laterally across the width of the hearth. When operating temperature is reached, liquid can be partially removed from the troughs so that the pontoon-supported structures no longer float but rather can weigh down onto refractory fiberboard in a controlled fashion to form a gastight seal. If the plant is to be shut down from operating mode, the pontoons can be floated again by pumping liquid back into the troughs so that the roof structure and its associated refractory flat arch can return eventually to the cold position.

Highly intensive reactors, such as Noranda, Tienente, Mitsubishi, Isasmelt, and Ausmelt do not capture effectively the inherent energy in mineral sulfide concentrates, which is clearly vital for energy-efficient processing. Vast amounts of energy are wasted. Current technology, therefore, normally needs a degree of oxygen enrichment to ensure autogenous operations even when smelting high-energy-content copper or nickel concentrates. Ideally, when such materials are smelted in isolation, excess energy should be available for recovery by steam-based electric power generation to satisfy in-plant requirements and possible export to national grids, if such are available.

According to U.S. patent 5,607,495,21 heat generated during melt circulation smelting can be efficiently utilized by smelting copper/nickel sulfide ore concentrates of high intrinsic energy value with another mineral concentrate of low or negative intrinsic energy value (e.g., high-grade zinc concentrate, high-grade lead concentrate, or even a bulk flotation concentrate containing both lead and zinc, preferably low in gangue oxides for highest thermal efficiency). With such a process, metallic copper, metallic zinc, and metallic lead can all be obtained as products in the primary smelting circuit employing forced circulation of copper/nickel sulfide through various extraction zones.

The zinc and/or lead formation reactions consume thermal energy and so if the ore concentrates are added in the correct proportions, the excess energy released on direct smelting of copper/ nickel concentrate using technically pure oxygen can be balanced against the endothermic requirements of zinc and/or lead production. This has the advantage that the energy required for zinc and lead production is provided in situ within the smelter so that no external fuel is required and all the benefits of virtually zero gas emission smelting are secured. Preferably, copper is extracted as the metal, while nickel is extracted as high-grade nickel sulfide.

In contradistinction to the preceding paragraphs, it is now considered that for single copper sulfide concentrate of high intrinsic energy value, use of oxygen enrichment or technically pure oxygen should be avoided in the interest of energy efficiency if the sole purpose is to produce copper in the most efficient way possible.

Notwithstanding energy considerations, it is recognized from the outset that a move away from high-intensity reactors could significantly increase the amount of saleable material held up within the process. For example, if there is a significant increase in the hold-up of molten copper matte within the circuit, this will have immediate financial implications. Interest will be lost on the equivalent value of copper metal not sold to customers. Clearly, to minimize the hold-up of matte, the first prerequisite, if large surface-area reactors are dictated on other grounds, the flowing melt streams must be very shallow. In the present context, for smelting high-intrinsic-energy sulfide, emphasis is directed toward melt depths in topblown zones not to exceed about 5 cm to 10 cm in swimming pool reactors.

The corollary to the above is that top air blowing must avoid jet penetration into the melt and be conducted in what is known as the non-splash mode. The critical conditions necessary to enforce this requirement have been the study of numerous research projects over the years. For the present purpose based on available data for melt properties and likely scenarios for multiple top jetting of molten cuprous sulfide, the critical depth of the melt cavity beneath each jet before splashing commences is in the region of 1.5 cm to about 1.75 cm.

Recently M. Campforts et al.²² have expressed the view that the formation of a freeze layer has to be guaranteed for high-intensity processes conducted in furnaces with refractory walls. Such an approach is compatible with expert opinion, as expressed by K.M. Donaldson et al.²³ for modern high-productivity pyrometallurgical furnaces, “. . . bottom leaks invariably lead to catastrophic run-outs and because the hearth brickwork is not readily accessible for repairs, proper attention to the design and erection of the hearth arches is the single most important aspect of furnace construction.” In the context of continuous copper smelting, this means that conditions need to be established by appropriate design to ensure a freeze lining of cuprous sulfide is maintained throughout to protect the hearth or other melt containment walls in their entirety. Also, as matte oxidation generates iron-oxide-containing slags, the sidewalls clearly need protection by a freeze lining, which is preferably contiguous with that on the hearth or containment vessel.

For low-intensity continuous copper smelting, there are two principal melt circulation loops. The first essentially replaces the continuous high-intensity matte-producing reactor systems of current technology. To ensure effective removal of minor impurity elements such as arsenic, antimony, and bismuth, this first melt circulation loop has a carrier melt of cuprous sulfide with only a relatively minor level of ferrous sulfide but most importantly a relatively small thermodynamic activity of dissolved copper metal. This is where the incoming moderately preheated (before chalcopyrite decomposition) copper concentrate feed is dispersed into the slag, produced initially on the oxidizing side of the melt circulation loop, and then floated into the neutral or reducing side. To effect dispersion, mechanical agitation or alternatively inert gas sparging is required and then phase disengagement must be achieved by gravity separation, possibly enhanced by electromagnetic means. This is an extremely effective procedure for reducing the copper oxide dissolved in the slag and can be regarded as in-situ slag cleaning.

The melt is then returned to the matte oxidation zone containing the extensive array of air top-blowing jets. It is crucially important that the positioning of the jets in terms of spacing and height above the shallow melt surface be such that in the region of 80–90% oxygen utilization is achieved whilst operating in the non-splash mode.

The second melt circulation loop replaces the multiple-batch Peirce–Smith converters of the traditional converter aisle and it again is low intensity in terms of the air top-blow arrangements. The matte in this loop is copper-saturated and a separate bottom layer of molten copper is formed. This flows by gravity down a gently sloping hearth of frozen cuprous sulfide so that it does not accumulate in the shallow hearth itself but rather is collected as a pool down one end. In the event of a temporary shutdown, electrical conductive heating can keep the extensive shallow layer of molten cuprous sulfide matte, again about 5 cm or so in depth, in the liquid state until melt circulation is resumed.

Molten copper siphoned out or otherwise withdrawn continuously from the accumulated pool becomes the feed to two relatively intense contactors in series based on non-wetted irrigation of packed beds. Counter-current flow of inert gas with the controlled minor addition of air in the first is followed then by reformed natural gas as the continuous phase in the second packed bed.

The purpose of the first contactor is to remove the residual sulfur in the blister copper by the reaction S + 2O = SO_2(g), whilst the second reduces residual oxygen down to specification limits by the two reactions: O + H_2(g) = H₂O_(g) and O + CO_(g) = CO_2(g), where the underlining represents the elemental species dissolved in liquid copper.

The gas phase is maintained as a continuum and high-intensity heat and mass transfer are achieved by allowing a liquid-metal stream to disintegrate and flow downward as droplets or rivulets by gravity, countercurrent to an upward flow of gas within a bed of solid packing material.

The design of the reactor for melt deoxidation must take the possibility of longitudinal mixing in the gas phase fully into account, because countercurrent conditions are vital in the interests of low natural gas consumption and hence retention of high energy efficiency. In this reactor the two liquid phases involved are molten cuprous sulfide and copper metal under strongly reducing conditions. It is well established that neither of these melts are aggressive toward high-alumina refractory or spinel direct-bonded brick (71% Al₂O₃; 28% MgO) SP. Also C.A. Gonzales et al.²⁴ have reported that there is zero penetration of Cu₂S into SP at 1,300°C and p_O2 = 10^–8 MPa. About Al₂O₃, R. Parra et al.²⁵ state that the contact angle of molten Cu₂S on Al₂O₃ at 1,200°C is 105° and hence this system exhibits non-wetting behavior. For melt deoxidation in continuous copper smelting, it is difficult to imagine a better system than straightforward gas/liquid contacting employing a compact and well-insulated packed bed with true countercurrent driving forces throughout.

ACKNOWLEDGEMENTS

The research behind the developments described in this paper has been funded by the Science and Engineering Research Council, the Department of Trade and Industry, the Commission of the European Communities, the British Technology Group, the Mineral Industry Research Organisation, and the Engineering & Physical Sciences Research Council.

REFERENCES

1. N.A. Warner, “Continuous Oxygen Steelmaking with Copper-, Tin-, and Zinc-Contaminated Scrap,” Metall. and Materials Transactions B, 35 (4) (2004), pp. 663–674.
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20. F. Cardarelli, “Method for Electrowinning of Titanium Metal or Alloy from Titanium Oxide Containing Compound in the Liquid State,” U.S. patent application 2004/0194574 A1.
21. N.A. Warner, “Oxygen Smelting of Copper or Nickel Sulfides,” U.S. patent 5,607,495 (1997).
22. M. Campforts et al., “On the Microstructure of a Freeze Lining of an Industrial Nonferrous Slag,” Metall. and Materials Trans. B, 38B (2007), pp. 841– 851.
23. K.M. Donaldson, F.E. Ham, and J.G. Schofield, “Design of Refractories and Bindings for Modern High-Productivity Pyrometallurgical Furnaces,” Non- Ferrous Pyrometallurgy: Trace Metals, Furnace Practices and Energy Efficiency, ed. R. Bergman et al. (Montreal, Canada: Metallurgical Society of CIM, 1992), pp. 491–505.
24. C.A. Rodríguez González, W.F. Caley, and R.A.L. Drew, “Copper Matte Penetration Resistance of Basic Refractories,” Metall. and Materials Trans. B, 38B (2007), pp. 167–174.
25. R. Parra, R. Voytovych, and N. Eustathopoulos, “Wetting of MgO by Cu2S-FeS Melts,” Metall. and Materials Trans. B, 38B (2007), pp. 347–349.

Noel A. Warner is professor emeritus at the University of Birmingham, Chemical Engineering Department, Edgbaston, Birmingham, U.K. B15 2TT, and can be reached at warnerna@btopenworld.com.