Continuous tapping of melts such as molten metals, liquid slags, fused salts and molten mattes is already practiced widely in the metallurgical industry, but interruptable tapping encompassing the whole spectrum from batch to continuous operation and in particular the ability to be able to restart immediately after a planned or temporary shutdown is not a feature of present technology. The present invention provides the means for achieving all of these very worthwhile objectives.

Besides melts which are physically quiescent such as most slags and molten metals such as aluminium, copper, lead and zinc, there is another category of metallurgical melts which contain dissolved elements capable of producing gaseous reaction products under certain process operating conditions. For example, carbothermic reduction of metallic oxides such as iron, cobalt and nickel produces melts which contain dissolved carbon, and it is a feature of such molten materials that during refining carbon is removed by oxidative methods, resulting in both oxygen and carbon being dissolved in the melt with the potential of reacting therein to form gaseous carbon monoxide. If such reaction occurs beneath the surface of the melt, subsurface bubbles are formed which grow by diffusive processes and ultimately burst forth through the liquid surface accompanied by the generation of micro-spray. This is a situation well understood in steelmaking, which is used to good effect in the highly intensive processes referred to generically as vacuum degassing. However, micro-spray generation is also the precursor to copious fume generation caused by the oxidation and further explosive disintegration of very small melt droplets. In transporting a melt either within the process as in a melt circulation loop or in tapping a product continuously from the reactor, sub-surface generation of gas bubbles can be highly disruptive and ideally should be avoided if at all possible. The present invention addresses this aspect in a straightforward and reliable fashion.

Vacuum lift and gas lift pumping of molten steel is now a mature technology as evidenced by the widespread use of what is known as RH or circulating degassing.

Accordingly, there is no difficulty in adapting such technology to the massive forced circulation of melt within a melt circulation loop. Current technology covers the range from about 20 tonnes to 200 tonnes per minute or even higher. Such high circulation rates mean that there is a vast amount of sensible heat in the circulating melt and therefore reasonable heat loses can normally be tolerated without risk of the melt freezing. This is not, however, the case for the actual flow or throughput of product stream, where for example, a 100,000 tonnes per year of metal production equates to a mere 0.19 tonnes per minute and even a moderately sized ironmaking process, producing say 500,000 tonnes per year, is equivalent to a metal discharge of less than one tonne per minute.

Accordingly, new technology is required to ensure that the product stream can be continuously transported between reaction or refining steps or eventually continuously tapped as product in a reliable and trouble-free fashion. This present invention focuses on this immediate requirement.

Particular examples of the invention as applied to direct coal-based steelmaking will now be described with reference to Fig. 1, Fig. 2 and Fig. 3 of the accompanying drawings in which- Fig. 1 is a schematic sectional elevation of a molten metal siphon connecting two furnace hearths at approximately the same level, which constitute adjacent arms of two different melt circulation loops, in which the product flow is a very small in proportion to the forced melt circulation imposed by other means individually on both loops.

Fig. 2 is a schematic plan view of the arrangement shown in Fig. 1 illustrating the molten metal siphon straddling the corners of the two hearths containing molten metal in two side-by-side rectangular channels (not shown in full).

Figure 3, is a schematic arrangement of the gas-cooled area of one leg which is submerged just beneath the surface of the melt to effect the establishment of a sealant rim of frozen material, which precludes adventitious ingress of the ambient furnace gas atmosphere into the siphon legs.

Referring now to Fig. 1, the shallow pools of molten iron-carbon melt, hereinafter referred to as liquid steel 1 and 2, shown only partially in the diagram, are interconnected by a siphon upleg 3 and a siphon downleg 4 by a reservoir body 5 having a chamber 5a therein into which liquid steel is sucked by maintenance of a reduced pressure connection to a vacuum system (not shown) and in which the level of liquid steel is maintained just below an electrical heater 6 fabricated from graphite or similar material, which is controlled so that the liquid steel is kept molten under all conditions, whether that be in actual production or in a stand-by mode. For reactive melts such as liquid steel, as already explained, it is vitally important that the reaction between dissolved carbon and oxygen is not permitted to take place in an explosive or active mode but rather whatever degassing that may occur is constrained principally to the exposed surface rather than via sub-surface bubble formation. Consequently, the elevation of liquid steel into the reservoir body is designed such that the absolute minimum suction is applied, so that vacuum degassing is not facilitated in this key area of the melt circulation loop or indeed as the product stream is removed for continuous casting or downstream processing to ultra-low carbon steel, for example. It will be appreciated tha ! ; some degassing will always occur so that a connection to a vacuum system is necessary to prevent accumulated gases from eventually destroying the siphonic action. Accordingly, the profile of the siphon assembly in elevation is as low as possible, preferably but not necessarily an inverted Y configuration as shown in Fig. 1. The siphon upleg 3 and siphon downleg 4 dip into the liquid steel pools 1 and 2 only so far as is required for practical operation but, in any event, deep enough to ensure that the ambient gas atmospheres 7 and 8 are not permitted to be drawn into the liquid steel contained in the legs within relatively narrow concentric conduits (or bores) 9,10 encased in dense refractory material 11, probably monolithic but not necessarily so. Admission of gas into either leg will result in a decrease in effective density and the liquid seal and siphonic action will cease, be impaired or at worst flow reversal will take place with the siphon acting as a gas lift pumping device. For purposes of illustration the liquid steel is contained in hearths of frozen shells of steel in Fig. 1, which means that the melt temperature has to be close to the liquidus temperature and therefore any temperature drop has to be minimised or reduced to an absolute minimum. This is particularly the case because of the relatively very low liquid steel throughputs involved. Accordingly, positive steps have to be taken not only to add heat to the liquid reservoir body 5 via the electrical heater 6 as already referred to, but also the liquid steel in both legs has also to be heated to counter inevitable heat losses by passage of a low-voltage heavy current through the liquid steel to effect direct resistance electrical heating or so-called conductive heating. It will be understood that pool 2 is slightly lower than pool to enable the siphoning action to be achieved. The outer metal walls 20 of the upleg and downleg are of double-walled construction with a narrow cavity being defined the walls for reasons which will be explained below with refernce to Fig 3.

In certain cases is may be advantageous to incorporate impermeable refractory tubes into the monolithic dense refractory lining of the upleg 3 and downleg 4 in order to lengthen the useful service life of the conduits through which the melt passes at a velocity of somewhere in the region of 0. 5m/s. For lower melting point liquid metals,"vitreosil"or fused quartz is particularly useful for this purpose because of its excellent thermal shock resistance. For liquid steel, however, thermal shock resistance will probably preclude the use of fused alumina tubes, although some experiments with mullite impervious tubes were inconclusive in this regard and should not ruled out as a possibility. If impermeable or indeed porous refractory such as alumina or silicon carbide tubes are used, these can be inserted directly into the upleg and downleg melt flow conduits via two ports welded at the appropriate angle in the side of the reservoir body to permit periodic direct introduction or replacement without the necessity of dismantling the whole siphon assembly.

Referring now to Fig 2, the means for introducing a current, a crucial component of the present invention, is via at least one electrical conductor 12, which is connected to a mains frequency electrical supply such that a voltage difference can be maintained between the melt in the reservoir body 5 and the liquid pools 1 and 2 so that controllable heavy current can flow in parallel to the required extent individually through both legs, whether the liquid steel is in motion or is merely held-up in the standby mode, if for operational reasons melt flow should inadvertently cease or otherwise be stopped as part of a planned shutdown. In certain circumstances phase differences can be used to establish the required potential difference and in others a"floating voltage"will need to be introduced via the current input conductor 12. The design of the current input conductor 12 is crucial to the successful operation of the molten metal siphon. There are two conflicting considerations, a solution to which is afforded by the present invention.

Firstly, current has to be introduced at relatively low current density with minimal electrical contact resistance. Second, the high interfacial area implied to effect low contact resistance means that a relatively large area is available for conducting heat away from the reservoir body, placing undue demands on the electrical heater 6. Accordingly, the hot end of the input conductor 12, itself has to be at the liquidus temperature with a frozen crust of solid steel being established in contact with the liquid steel and according to operational requirements, either intentional or otherwise, the thickness of the frozen material will either grow or shrink to reach steady state. This is achieved in the present invention by the provision of a computed length of copper or other material of low electrical resistivity for the input connection, either of circular or rectangular cross section, such that controlled cooling of the exposed cooler end does not result in excessive heat losses, but at the same time provides a positive means for ensuring the integrity of the input conductor itself at the hot end and the establishment of what is effectively a welded interface between the current connector and the metal comprising the melt under consideration in more general terms.

A preferred embodiment of the present invention is to have two diametrically opposed current input conductors 12 and 13 arranged at 90 degrees to the centre line of the adjacent upleg 3 and downleg 4 of the siphon. Also it may be advantageous for these current input conductors 12 and 13 to be not horizontal but rather to be at an angle to the reservoir body 5, so that the cooler ends are located beyond the thermally insulated roof or wall enclosures associated with the liquid pools 1 and 2. Such an arrangement facilitates connection to the electrical mains power supply as well as simplifying the provision of the requisite amount of cooling dictated by the thermal flux necessary to establish a stable layer of solidified melt at the hotter ends.

Because of the so-called skin effect in alternating current conductors, rectangular copper busbars wide in relation to thickness are superior to circular current input feeders. This is especially the case in the present invention, because bulk copper away from the effective thickness, actually carrying the bulk of the electrical current, is still equally as effective as the outer layers for thermal conduction. Accordingly, heat losses from the internal region of the reservoir body 5 are exacerbated, if very large circular current input leads are used to counteract adverse fR losses, bearing in mind that the requisite formation of a given thickness of solidified melt at the hot end, is a function the thermal flux but independent of the electrical current density, provided that the contact resistance is negligibly small.

Referring now to Fig. 3, which shows in detail part of Fig. 1, forced cooling of the annular steel rim 14 is effected by gas impingement at high velocity through a multiplicity of small bore metal tubes located in the cavity between the double outer walls 20, two of which 15 and 16 are shown in the diagram or alternatively via a slot placed adjacent in close proximity to the gas side of the rim 14. It is important to ensure that the other side of the rim 14, which is in contact with the melt, has unimpaired access to the melt at the instant when the siphon legs 3 and 4 are first submerged. This is achieved by the provision of a cut-out 22 in the block 17. If a frozen layer is not immediately formed, either because the melt freezes away from the exposed rim or, alternatively, the exposed solid metal rim is contaminated such that a coherent bonded deposit is not formed instantaneously, then gas ingress into the liquid metal conduits of the upleg 3 and the downleg 4 becomes a strong possibility and siphonic action will either not be initiated or will be terminated if already commenced. It is the integrity of the solidified metallic non- porous seal established on the annular metal rim, which is the key to success. Provided this is attained, gas ingress through the refractory block 17 from the ambient furnace atmospheres 7 and 8 is not possible, provided the metal rim itself is fully submerged in the melt 18. The refractory block (endcap) 17, which must be provided in order to protect other steel surfaces, remote from the gas-cooled rim, from excessively high temperatures in the case of liquid steel operations, for example, is preheated to as close as possible to the temperature of the melt by holding the siphon just above the melt surface for as long as necessary before immersion. Also it is advantageous to profile the refractory block in the immediate vicinity of the exposed rim so that melt enters the region through an entrance area which is greater than the area of the exposed steel on which the melt is to be solidified. Thus, the cut-out 22 is of frusto-conical section tapering inwardly towards the rim 14. Finally, steps must be taken to ensure that the refractory block 17 is submerged only to the extent necessary to provide complete submergence of the solidified metal rim formed on the steel rim 14. This is particularly the case at steelmaking temperatures, where the already rather hostile environment must be constrained as far as practically feasible.

For liquid steel operations, upleg 3 and downleg 4 are fabricated with double walls from concentric steel pipes with only a relatively small clearance between them to provide an annulus for coolant gas leaving the gas impingement area to return to a header or discharge manifold located remote from the high temperature region, probably but not necessarily via a double wall construction for the reservoir body 5, also needed to facilitate heat extraction so that mild steel or low alloy steel can be used throughout rather than more expensive heat resistant alloys.

It will be appreciated that emerging technologies for iron and steel making are increasingly moving towards reduction in greenhouse gas emissions or perhaps even toward zero gas emissions, where this is practical to implement. Accordingly, more iron and steelplants in the future will have access to on-site air separation to provide oxygen for iron and steelmaking. This means that nitrogen will be available for the gas coolant referred to in this invention. However, other gases can be used more generally provided their reactivity with the siphon's materials of construction permits this. At lower temperatures for many liquid metal applications this means that simple compressed air would be suitable.

Semi-pilot scale operations have been carried out to provide the background information for the present invention. The high temperature system employed was molten copper matte, a commercially available material comprised principally of cuprous sulphide and which is a process intermediate in conventional copper smelting. The trials were conducted employing a closed loop melt circulation system at temperatures around 1200°C and over a six hour period, some 500 tonnes of melt were circulated past a given point. Use was made of a gas-lift system closely resembling RH steel vacuum degassing technology, a mature technology used worldwide in the steel industry. A hot metal. siphon was not employed in these trials, but rather melt merely overflowed from a high level hearth into a lower level hearth to be pumped back with the RH type device. An account of these trials, performed in the context of development of a new zinc smelting process, has been given in"Direct Zinc Smelting with Virtually Zero Gas Emission", Proceedings of the Second International Symposium on Metallurgical Processes of the Year 2000 and Beyond, San Diego, California, September 20-24,1994, Ed. H. Y. Sohn, Vol. 2,333-349, The Minerals, Metals and Materials Society, Warrendale, Penn. 1994 (ISBN 0-87339-241-8). From the experience gained in these high temperature experiments, the requirements for a commercially-sized hot metal siphon for steelmaking have been assessed and are given in Example 1.

In this invention the establishment of stable frozen steel protective layers is readily achieved for both the sealant rim immersed in the molten steel pool and also for the current input busbars located a small distance back from the liquid steel flowing through the reservoir body 5 of the siphon. Off-setting the copper busbars back a short distance into the refractory lining away from the bulk of the melt in the reservoir body 5 means that effectively a short length of solid steel ingot is formed at steady state within the interconnecting refractory passage way, probably of the same cross-sectional rectangular dimensions as the copper busbar itself. At the hotter end of this solid ingot, the face temperature will be close to the liquidus temperature and the liquid metal heat transfer will be determined predominantly by natural convection rather than forced convection, which would pertain if the solidified material was actually projecting out into the bulk flow. However, it is vitally important to ensure that the interface between the short length of steel ingot and the copper busbar is properly bonded so that interfacial resistance is minimised. In some cases it may be necessary to load initially a composite busbar comprised of copper with a bonded steel end cap and then in actual operation ensure that the temperature of the solid interface between copper and steel never exceeds a predetermined temperature, compatible with the metallurgical properties of the interfacial material. In any event, it is likely that the absolute maximum temperature of the copper/steel interface should be no higher than about 1000°C. However, in the first instance it should be established whether or not the particular liquid steel being processed can bond satisfactorily onto the copper busbar under chemically-clean non-oxidative conditions so that a composite busbar is created in-situ immediately on first contact between the melt and the copper busbar. Physical strength at high temperature is not an issue for the copper busbar as it will be fully encased in supportive refactory material.

For the sealant rim of solidified material in contact with the main pool of liquid steel, as already discussed, it is important to observe the suggested profiling of the refractory in the immediate vicinity of the exposed steel surface on which the solidified material is deposited. Under steady state conditions, the predominate liquid metal heat transfer mechanism has to be natural convection, a situation created by offsetting the exposed steel surface into the refractory away from bulk movement of liquid metal and its associated eddies and forced convection.

The further the liquid metal temperature deviates from the liquidus temperature, the more difficult it becomes to maintain a stable frozen layer at an acceptable heat flux extraction rate under operating conditions. Therefore, if process requirements downstream demand a higher temperature, then the teachings of this invention are clear. It is better to operate close to the liquidus temperature for liquid steel siphons and then downstream use electrical conductive heating or other appropriate means to raise the liquid steel temperature to that required for subsequent processing.

The preferred way to bring a new or replacement metal siphon on-line is to use sacrificial heating elements to preheat both arms of the siphon as well as the reservoir body. For the present invention this is a very straight forward procedure. Steel pipes or rods can be inserted into the legs and connected together either within the reservoir body or externally, with the bottom contact finally being made by dipping the rods or pipes into the liquid steel pools once the refractory blocks and snorkel externals have been preheated above the melt surface. Electrical conductive heating can then be undertaken, but to reduce to an acceptable level the associated thermal shock, it will probably be necessary for the first part of preheating to be conducted with the sacrificial pipes or rods connected together with a bridging conducting member, whilst preheating above the melt takes place, but then to accept destruction of this member once the snorkels are lowered into the melt. For liquid steels with low carbon content close to the liquidus temperature, dissolution or melting of sacrificial pipes or rods will be delayed long enough to achieve a final top-up preheat, before sucking up the melt into the reservoir body to commence siphonic action with the full input then of the conductive heating circuit, as well as that provided by the heater within the reservoir body itself.

Example 1 Molten Metal Siphon for 500, 000 tpa Liquid Steel Establishment of Protective Solidified Rim Design Parameters Jet diameter = 3mm Jet throat to steel surface = 6 mm Nitrogen throat velocity = 300 m/s Calculated heat transfer coefficient = 1722 W/m2s Temperature range across frozen rim = 1536°C - 450°C Coolant convective heat flux = 0. 73 MVV/m2 Included angle of splayed legs = 120° Number of gas jets for 2 legs = 220 Total nitrogen mass flow = 0.534 kg/s Estimated thickness of sealant solidified rim = 4. 0 cm Conductive heating (2 legs in parallel) = 80.kW Equivalent non-inductive circuit 20, 000 A at 4 v Current input leads to reservoir body = 2 copper busbars Copper busbar dimensions = 50 mm thick x 300 mm wide x 1200 mm long Temperature range for copper busbars = 1000°C - 200°C Total 19 loss for 2 copper busbars = 1. 40 kW Temperature range for busbar frozen steel ends = 1536°C - 1000°C Estimated thickness of busbar frozen steel ends = 6. 8 cm Total I2R loss for 2 busbar frozen steel ends 1. 16 kW Total thermal conduction to cooling system through copper busbars = 8. 40kW