Starting a smelting process

Abstract

A method of starting a molten bath-based process for smelting a metalliferous material is disclosed. The method includes using the heat flux of water-cooled elements in lower parts of a smelting vessel to provide an indication of molten bath temperature during at least an early part of the start-up method and adjusting injection rates of oxygen-containing gas and/or carbonaceous material into the smelting vessel to control the molten bath temperature during start-up without exceeding critical heat flux levels and tripping the start-up method.

Claims

1. A method of starting a molten bath-based process for smelting a metalliferous material in a smelting vessel and producing molten metal, the smelting vessel including (a) an initially empty smelting chamber having a hearth and a side wall extending upwardly from the hearth, with the side wall including water-cooled elements at least in a lower section of the side wall, (b) a forehearth, and (c) a forehearth connection that interconnects the smelting chamber and the forehearth, with the method including the steps of: (a) supplying a charge of molten metal into the smelting chamber via the forehearth; (b) supplying a solid carbonaceous material and an oxygen-containing gas into the smelting chamber after completing the molten metal charge and igniting the carbonaceous material and heating the smelting chamber and molten metal and forming molten slag and thereafter increasing the amount of molten slag, with the molten metal and the molten slag forming a molten bath in the smelting chamber; (c) supplying a metalliferous material into the molten bath and smelting metalliferous material to molten metal; and wherein, during step (b), the method including controlling the temperature in the molten bath by: (i) monitoring heat flux of water-cooled elements in contact with the molten bath to obtain an indication of the temperature in the molten bath, and (ii) adjusting the supply rates of the solid carbonaceous material and/or the oxygen-containing gas having regard to water-cooled element heat flux to adjust the heat input into the smelting chamber and thereby control the temperature of the molten bath.

2. The method defined in claim 1 includes preheating the smelting chamber, the forehearth, and the forehearth connection.

3. The method defined in claim 1 includes preheating a hearth of the vessel, the forehearth, and the forehearth connection such that an average surface temperature of the hearth, the forehearth, and the forehearth connection is above 1000° C.

4. The method defined in claim 1 includes preheating a hearth of the vessel, the forehearth, and the forehearth connection such that an average surface temperature of the hearth, the forehearth, and the forehearth connection is above 1200° C.

5. The method defined in claim 1 wherein step (a) includes supplying sufficient molten metal so that the level of the molten metal is at least about 100 mm above the top of the forehearth connection.

6. The method defined in claim 1 includes injecting a gas or liquid fuel and an oxygen-containing gas into the gas space above the metal for a period of time after completing the molten metal charge into the smelting chamber to generate heat in the smelting chamber.

7. The method defined in claim 1 wherein step (b) includes supplying flux material into the smelting chamber to promote molten slag formation.

8. The method defined in claim 1 includes injecting slag or slag-forming agents to promote molten slag formation in the molten bath.

9. The method defined in claim 1 includes commencing step (c) of supplying the metalliferous material into the molten bath at any time during the course of step (b).

10. The method defined in claim 1 wherein the molten bath-based smelting process includes the steps of: (a) supplying carbonaceous material and solid or molten metalliferous material into the molten bath and generating reaction gas and smelting metalliferous material and producing molten metal in the bath, (b) supplying oxygen-containing gas into the smelting chamber for above-bath combustion of combustible gas released from the bath and generating heat for in-bath smelting reactions; and (c) producing significant upward movement of molten material from the bath by gas upwelling in order to create heat-carrying droplets and splashes of molten material which are heated when projected into the combustion region in the top space of the smelting chamber and thereafter fall back into the bath, whereby the droplets and splashes carry heat downwards into the bath where it is used for smelting of the metalliferous material.

11. A method of starting a molten bath-based smelting process for a metalliferous material in a smelting vessel that defines a smelting chamber and producing molten metal, with the method including supplying a charge of molten metal into the smelting chamber, supplying feed materials including solid carbonaceous material and oxygen-containing gas into the smelting chamber and generating heat and forming molten slag and thereafter increasing the amount of molten slag in the smelting chamber, with the molten metal and the molten slag forming a molten bath in the smelting chamber, monitoring heat flux of a side wall of the vessel in contact with the molten bath to obtain an indication of the temperature in the molten bath as the amount of slag increases towards a suitable slag inventory, and adjusting the supply rates of solid carbonaceous material and/or oxygen-containing gas into the smelting chamber to adjust heat input into the smelting chamber and thereby control the temperature of the molten bath.

12. The method defined in claim 11 includes injecting a gas or liquid fuel and an oxygen-containing gas into the gas space above the metal for a period of time after completing the molten metal charge into the smelting chamber to generate heat in the smelting chamber.

13. The method defined in claim 11 includes injecting slag or slag-forming agents to promote molten slag formation in the molten bath.

14. The method defined in claim 11 wherein the molten bath-based smelting process includes the steps of: (a) supplying carbonaceous material and solid or molten metalliferous material into the molten bath and generating reaction gas and smelting metalliferous material and producing molten metal in the bath, (b) supplying oxygen-containing gas into the smelting chamber for above-bath combustion of combustible gas released from the bath and generating heat for in-bath smelting reactions; and (c) producing significant upward movement of molten material from the bath by gas upwelling in order to create heat-carrying droplets and splashes of molten material which are heated when projected into the combustion region in the top space of the smelting chamber and thereafter fall back into the bath, whereby the droplets and splashes carry heat downwards into the bath where it is used for smelting of the metalliferous material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An embodiment of the method of starting a molten bath-based smelting process in a smelting vessel in accordance with the present invention is described with reference to the accompanying drawings, of which:

(2) FIG. 1 is a cross-sectional view of the smelting vessel of a smelting apparatus for producing molten metal in accordance with the HIsmelt process which illustrates the molten metal level in the vessel after supplying molten metal to the vessel during the course of one embodiment of a method of starting up a smelting process in the vessel in accordance with the invention;

(3) FIG. 2 is a cross-sectional view of the smelting vessel shown in FIG. 1 which illustrates the molten metal and slag levels in the smelting vessel at the end of a successful method of starting up the smelting process in the vessel in accordance with the invention; and

(4) FIG. 3 is a diagrammatic view of one embodiment of an HIsarna apparatus for smelting a metalliferous material and producing molten metal in accordance with the HIsarna process.

DESCRIPTION OF EMBODIMENT(S)

(5) FIGS. 1 and 2 show in very diagrammatic and simplified form a smelting vessel for smelting metalliferous material to molten metal in accordance with the HIsmelt process.

(6) As is indicated above, the HIsmelt process is an example of a molten bath-based smelting process for producing molten metal from a metalliferous feed material in a smelting vessel that has a strong bath/slag fountain generated by gas evolution in the molten bath, with the gas evolution being at least partly the result of devolatilisation of carbonaceous material into the molten bath. As is also indicated above, the HIsmelt process is described in a considerable number of patents and patent applications in the name of the applicant. By way of example, the HIsmelt process is described in International application PCT/AU96/00197 (WO1996/032627) in the name of the applicant. The disclosure in the patent specification lodged with the International application is incorporated herein by cross-reference. The metalliferous material may be any suitable material. Iron-containing material such as iron ore is one type of metalliferous material of particular interest to the applicant.

(7) FIGS. 1 and 2 show the vessel at different steps in a method of starting up the HIsmelt process in the vessel.

(8) With reference to FIGS. 1 and 2, the vessel defines a smelting chamber and has a refractory-lined hearth 1, water-cooled solids injection lances 2, a water-cooled top lance 3 for oxygen-containing gas, and a water-cooled side wall 4. Water-cooled side wall 4 typically comprises an outer steel shell (not shown) and a plurality of water-cooled elements (not shown) in the form of panels having metal water-cooled tube sections on the inside and frozen slag on the side of the panels facing into the vessel and either frozen slag or castable refractory material (or a combination) between the water-cooled tubes and the outer shell. The above-mentioned International application provides further details of typical water-cooled panels. The vessel also comprises a forehearth 5 that defines a forehearth chamber 8 and a forehearth connection 6 that includes a passageway that interconnects the smelting chamber and the forehearth chamber.

(9) Slag-zone coolers 7 are positioned at the top of the hearth refractory material. The slag zone coolers may be of any suitable construction. One example of a suitable slag zone cooler is described in International application PCT/AU2007/000688 (WO2007/134382) in the name of the applicant. The disclosure in the patent specification lodged with the International application is incorporated herein by cross-reference.

(10) The slag-zone coolers 7 and the water-cooled panels of the side wall 4 that are immediately above the slag zone coolers 7 are considered to be water-cooled panels in the “lower parts” of the vessel.

(11) In this embodiment, the maximum allowable heat flux for the water-cooled panels is 500 kW/m.sup.2. As indicated above, the maximum allowable heat flux for the panels in any given situation depends on a range of factors such as different smelter constructions and different metalliferous and other feed materials and can readily be determined.

(12) One embodiment of the method of starting up a HIsmelt smelting process in the vessel in accordance with the present invention includes a first step of preheating refractory in the vessel, including the forehearth chamber 8 and the forehearth connection 6. The preheating temperature and time are a function of a number of factors including but not limited to the type and the amount of refractory material in the vessel.

(13) When the preheating step is completed, a charge of an externally prepared hot metal (such as molten iron) is then poured into the smelting chamber via the forehearth 5 in such a quantity that the metal level is at least about 100 mm above the top of the forehearth connection 6. This step results in a metal inventory 9 in the smelting chamber as shown in FIG. 1.

(14) Injection of carbonaceous material in the form of coal in the case of this embodiment and fluxes via lances 2 is then initiated. At the same time, injection of an oxygen-containing gas in the form of a hot air blast via lance 3 is initiated. The injection of these feed materials results in the formation of a molten slag 10 on the hot metal charge. The hot metal and the slag form a molten bath in the vessel. The amount of molten slag increases as the injection of coal, fluxes and hot air continues. Metal splashing begins with the injection of coal, fluxes and hot air and, at this point, the panels in the lower parts of the vessel show high heat fluxes wherever metal splash occurs—this need not be uniform around the circumference, and the effect may be concentrated in regions that are more or less on the opposite side of the injection lances. Non-uniformity may also arise from splash patterns being asymmetric and hot combustion flames from lance 3 being directed preferentially to regions of low splash intensity.

(15) As is indicated above, high heat fluxes in the lower parts of the vessel are a concern because of the risk of tripping the sequence of the start-up method in the vessel.

(16) As is indicated above, the applicant has found that (1) heat flux of the panels in the lower parts of the vessel provides an indication of molten bath temperature particularly when there is a small amount of molten slag in the vessel and, (2) using this information it is possible via manipulation of the injection rates of coal and/or hot air to control the molten bath temperature and avoid exceeding a critical flux level and tripping the start-up method, leading to a shut-down. In this embodiment, the critical heat flux level is 500 kW/m.sup.2. The heat flux in the water-cooled panels in the lower parts of the vessel may be determined by monitoring the inlet and outlet water temperatures and flow rates for the water-cooled panels and making heat flux calculations based on this data. All of the water-cooled panels may be monitored. Alternatively, a selection of the water-cooled panels may be monitored. These selected water-cooled panels may be in sections of the vessel that are known to be highly susceptible to splashing that causes high heat fluxes in those sections. Alternatively, the selected water-cooled panels may be representative of the overall heat flux in the lower parts of the vessel and the data may be used as a basis for heat flux calculations for all of the water-cooled panels in the lower parts of the vessel. The heat flux monitoring may be continuous or periodic.

(17) During this period of injection of coal, fluxes and hot air, if the heat flux calculations indicate that the heat flux has increased to or is increasing towards unacceptably high amounts, the feed material injection conditions are adjusted as required to reduce the heat generated in the lower parts of the vessel. Typically, this involves reducing the injection flow rates of coal and/or hot air.

(18) This period of coal and flux injection with hot air is maintained for about 30-60 minutes and typically, during this period, heat fluxes generally increase.

(19) Once heat fluxes in the lower parts of the vessel are generally above 200 kW/m.sup.2, injection of metalliferous material, such as iron ore, is started. Heat flux monitoring continues during this period. Coal and hot blast rates continue to be modulated to keep the maximum heat flux below 500 kW/m.sup.2, whilst slowly increasing ore injection rates.

(20) Initially, this phase of the start-up method is sensitive and heat fluxes can “spike” if, for example, coal and/or metalliferous material feed rates experience any type of flow disturbance. Such disturbances are possible, since metalliferous material feed (in particular) is at a small percentage of its nominal design rate and solids feeding devices often experience difficulty in maintaining smooth flow under such conditions.

(21) Over the next 1-3 hours the slag inventory increases and, as a consequence, the process slowly becomes less sensitive to high heat flux spikes. As the nature of the splash changes from predominantly metal to a mixture of metal and slag, and from there ultimately to predominantly slag, the panels in the lower parts of the vessel become insulated with frozen slag on the exposed surfaces of the panels and heat fluxes drop. At this stage, heat flux monitoring is less important. Once a (calculated) slag level of about 0.8-1.5 m (depending on vessel size) has been established in the vessel, lower panel heat fluxes are likely to have fallen below 200 kW/m.sup.2 and the process is considered to have safely passed through the start-up method. This condition is illustrated in FIG. 2 which shows the vessel with slag layer 10 in place.

(22) As described above, the method of starting a molten bath-based direct smelting process in accordance with the invention is applicable to the HIsmelt and HIsarna processes, as well as other molten bath-based direct smelting processes.

(23) With reference to FIG. 3, the HIsarna process smelts metalliferous feed material and produces process outputs of molten metal, molten slag, and an off-gas. The following description of the HIsarna process is in the context of smelting metalliferous material in the form of iron ore. The present invention is not limited to this type of metalliferous material.

(24) The HIsarna apparatus shown in FIG. 3 includes a smelt cyclone 2 and a molten bath-based smelting vessel 4 of the type described with reference to FIGS. 1 and 2 having a smelting chamber 19 located directly beneath the smelt cyclone 2, with direct communication between the chambers of the smelt cyclone 2 and the smelting vessel 4.

(25) With reference to FIG. 3, during steady-state operation of a smelting campaign, a blend of magnetite-based ore (or other iron ore) with a top size of 6 mm and flux such as limestone 1 is fed, via an ore dryer, and with a pneumatic conveying gas 1a, into the smelt cyclone 2. Limestone represents roughly 8-10 wt % of the combined stream of ore and limestone. Oxygen 8 is injected into the smelt cyclone 2 via tuyeres to preheat and partly melt and partly reduce the ore. The oxygen 8 also combusts combustible gas that has flowed upwardly into the smelt cyclone 2 from the smelting vessel 4. The partly melted and partly reduced ore flows downwardly from the smelt cyclone 2 into a molten bath 25 of metal and slag in the smelting chamber 19 in the smelting vessel 4. The partly melted and partly reduced ore is smelted to form molten iron in the molten bath 25. Coal 3 is fed, via a separate dryer, to the smelting chamber 19 of the smelting vessel 4. The coal 3 and a conveying gas 2a are injected via lances 35 into the molten bath 25 of metal and slag in the smelting chamber 19. The coal provides a source of a reductant and a source of energy. FIG. 3 shows the molten bath 25 as comprising two layers, of which layer 25a is a molten metal layer and layer 25b is a molten slag layer. The Figure illustrates the layers as being of uniform depth. This is for illustration purposes only and is not an accurate representation of what would be a highly agitated and well-mixed bath in operation of the HIsarna process. The mixing of the molten bath 25 is due to devolatilisation of coal in the bath, which generates gas, such as CO and H.sub.2, and results in upward movement of gas and entrained material from the molten bath into a top space of the smelting chamber 19 that is above the molten bath 25. Oxygen 7 is injected into the smelting chamber 19 via lances 37 to post-combust some of these gases, typically CO and H.sub.2, generated in and released from the molten bath 25 in the top space of the smelting chamber 19 and provide the necessary heat for the smelting process in the bath.

(26) After start-up, normal operation of the HIsarna process during a smelting campaign involves (a) coal injection via lances 35 and cold oxygen injection via lances 37 into the smelting chamber 19 of the smelting vessel 4 and (b) ore injection 7 and additional oxygen injection 8 into the smelt cyclone 2.

(27) The operating conditions, including but not limited to, coal and oxygen feed rates into the smelting chamber 19 of the smelting vessel 4 and ore and oxygen feed rates into the smelt cyclone 2 and heat losses from the smelting chamber 19, are selected so that offgas leaving the smelt cyclone 2 via an offgas outlet duct 9 has a post-combustion degree of at least 90%.

(28) Offgas from the smelt cyclone 2 passes via an offgas duct 9 to an offgas incinerator 10, where additional oxygen 11 is injected to burn residual CO/H.sub.2 and provide a degree of free oxygen (typically 1-2%) in the fully combusted flue gas.

(29) Fully combusted offgas then passes through a waste heat recovery section 12 where the gas is cooled and steam is generated. Flue gas then passes through a wet scrubber 13 where cooling and dust removal are achieved. The resulting sludge 14 is available for recycle to the smelter via the ore feed stream 1.

(30) Cool flue gas leaving the scrubber 13 is fed to a flue gas desulphurisation unit 15.

(31) Clean flue gas is then vented via a stack 16. This gas consists mainly of CO.sub.2 and, if appropriate, it can be compressed and geo-sequestered (with appropriate removal of residual non-condensable gas species).

(32) The smelting vessel 4 includes a refractory-lined hearth 33 and side walls 41 defined predominantly by water-cooled elements in the form of water-cooled panels that define the smelting chamber 19. The smelting vessel 4 also includes a forehearth 21 which is connected to the smelting chamber 19 via a forehearth connection 23. As indicated above, the smelting vessel 4 is of the type described with reference to FIGS. 1 and 2. Moreover, the embodiment of the method of starting up a HIsmelt smelting process in a vessel in accordance with the present invention as described with reference to FIGS. 1 and 2 may be used to start-up the smelting process in the vessel 4.

(33) During the course of a smelting campaign of the HIsarna process, molten metal produced in the smelting chamber 19 discharges from the smelting chamber 19 via the forehearth connection 23 and the forehearth 21. Under steady-state normal operating conditions, the forehearth 21 and the forehearth connection 23 contain molten metal. The normal manometer overflow system functions via “excess” metal (from production) spilling over forehearth lip 5 to keep the molten metal level in the smelting chamber 19 substantially constant.

(34) Many modifications may be made to the embodiment of the process of the present invention described above without departing from the spirit and scope of the invention.

(35) By way of example, whilst the smelting vessels shown in the Figures include a forehearth, it is noted that the process start-up method of the invention is not confined to vessels that include forehearths.

(36) In addition, whilst the smelting vessels shown in the Figures include water-cooled elements, including water-cooled panels of the side wall 4 and slag-zone coolers 7 at the top of the hearth, it is noted that the process start-up method of the invention is not confined to vessels that include these elements. The side walls of the smelting vessels may be any suitable construction whereby the heat flux from the side walls of the vessels in contact with the molten baths provides an indication of the temperature of the molten baths.

(37) In addition, whilst the embodiments focus on smelting metalliferous material in the form of iron-containing material, it is noted that the invention extends to smelting other materials.