GB2197343A - Operation of zinc-smelting blast furnaces - Google Patents

Abstract

The efficiency of a zinc-smelting blast furnace wherein oxidic zinc or zinc/lead feed and carbonaceous reductant/fuel are fed in lump form to the top of the furnace shaft is improved by also feeding particulate oxidic material and fuel into tuyeres or lances which direct the material on or into the slag pool at the furnace bottom. This is preferably augmented by the use of oxygen-enriched blast through the tuyeres.

Description

SPECIFICATION Operation of zinc-smelting blast furnaces The blast-furnace smelting of zinc is now a well-established (and well-documented) industrial process (See Morgan & Woods; Metallurgical Review November 1971, 16 pages 161-174).

The blast-furnace shaft was originally conceived essentially as a gas/solid reactor with gangue and molten metallic phases draining into the hearth after gas/solid reactions had taken place in the shaft followed by gangue matrix melting. The furnace charge is prepared, either by sintering or by briquetting, in the form of a high-porosity and high permeability mixed zinc/lead oxidic material to aid diffusion of gaseous species to and from sites of reaction.

As intensity of operation increased by increasing blast rate through the blast furnace tuyeres it became established practice to recover zinc dissolved in the slag by interacting the blast of the tuyeres with the slag by allowing the slag pool level to rise near to or even above the bottom of the tuyeres. This procedure has the effect of chemically reducing the zinc oxide content of the slag by the action of gas containing a high ratio of carbon monoxide to carbon dioxide and physically sweeping the zinc vapour out of the slag. It also provides heat for the reaction by combustion of coke in the raceways of the tuyeres which is transferred to the melt by radiation from the incandescent coke surfaces and by forced convection from the gas stream.An additional feature of this gas/liquid interaction is the creation of a spray of slag droplets which circulate around the lower levels of solid charge in the shaft, hence enhancing surface area for dissolved zinc species reduction.

Further improvement of the technique for reducing zinc levels in slag was achieved by increasing the angle of attack of the blast so that slag/gas interaction was increased. The methods are covered in UK patents 1,458,869 and 2,004,355. These consist of means for changing the angle of the tuyeres to steeper inclinations either by having a steeper straight tuyere or by modifying tuyeres by installing bent tips to direct the blast in a more downward direction onto the slag.

A practical limit to the technique described above is the requirement that heat is supplied mainly by the convection and radiation mechanisms of the raceway. This limits the extent to which tuyeres can be submerged because increasing submergence reduces heat input and chills the slag pool.

A further disadvantage of great tuyere submergence with present practice is that the requirements of raceway generation and slag interaction are to an extent incompatible, from an aerodynamic viewpoint. It is known that as these phenomena occur there is a tendency for raceways to be deflected and altered from their normally roughly spherical shape such that hearth and raceway behaviour is less predictable. Deleterious effects such a blast channelling through the shaft and increased heat load on the furnace lower water-cooled casing are known to occur.

Despite the increased interaction of slag and gas, according to known practice, the phenomenon is known to be predominantly a surface effect with limited circulation and stirring at depth in the slag pool. Commercial equipment is available to fume zinc from slags such as lead blast furnace slags, this operation being conducted by submerged tuyeres with solid fuel injection, the latter to provide combustion heat and a solid reductant. The product is a zine oxide fume produced by the action of oxidising gases on the metallic zinc fume within or above the slag bath surface.

In more recent zinc blast-furnace practice, slag-fall is increasing from the tendency to use lower grade, dirtier charge materials that are considerably cheaper than the high grade concentrates used hitherto. This approach has penalties from the increased use of high cost metallurgical coke needed to melt the gangue and the decreased zinc recovery due to an increased zinc level in slag. Therefore an improved method is necessary to decrease the zinc proportion in slag and reduce dependence on high grade metallurgical coke.

It is already known to introduce reductant/fuel in both solid, liquid and gaseous form into the blast tuyeres of a zinc-smelting blast furnace, to supplement the fuel fed to the top of the furnace shaft and to give a more reducing environment in the area of the raceway. This enables low cost reductant/fuel to replace, approximately quantitatively, high cost metallurgical coke in the shaft burden but has the disadvantage that there is a penalty from the heat sink of reductant/fuel injected at ambient temperature. Also as injectant replaces metallurgical coke in the shaft the shaft mixture becomes under-fuelled with respect to carbon to zinc ratio if the reductant/fuel injection mechanism fails for any reason, and the greater proportion of metalliferous burden in the shaft becomes more susceptible to inter-fusion either on the shaft walls or as burden hang-up in the shaft.

It is also already known to introduce additional oxygen with the heated blast admitted through the tuyeres to the zinc blast furnace (See U.K. patent specifications 1,047,757 and 1,470,722).

This method increases production rate by combusting more burden carbon per unit of blast input thus liberating more heat and producing a possibly higher reduction potential for the smelting of metalliferous charge. However, beyond a certain point there are recorded difficulties which include furnace casing overheating from higher temperature raceway combustion, higher furnace back-pressure possibly from a sharper burden melting zone and increased production of speiss as a third phase in the hearth between the lead bullion and slag phases. The speiss phase is a combination of iron in a predominantly metallic state (typically between 60 and 70%) with arsenic and other components which indicates localized over-reduction to metallic iron and formation of the inter-layer that is not readily oxidised into the slag.This material is viscous at the operating temperture of the hearth and causes difficulties in removal phases from the furnace and separation of phases outside the furnace.

Thus the use of reductant/fuel and oxygen injection gives benefits which are worthwhile yet limited in scope.

This invention consists in a process for reducing zinc oxide material in a blast furnace, wherein an oxidic zinc or zinc/lead feed material and a carbonaceous reductant/fuel are fed, in lump form, to the top of the furance shaft and a blast of pre-heated oxygen-containing gas is blown in through tuyeres at the furnace bottom; characterized in that at least part of the solid oxidic feed and part of the reductant/fuel is supplied to the furnace bottom either through the main blast tuyeres or through supplementary tuyeres or lances and that such tuyeres or lances are oriented to direct oxidic material and fuel to impinge on or into the slag-pool at the furnace bottom.

Preferably the oxygen-containing gas is oxygen-enriched with respect to atmospheric air.

The reductant/fuel may be solid, liquid or gaseous.

The combination of reductant/fuel and oxidic charge injection into the zinc blast furnace is a means of breaking the dependence of injected fuel on burden fuel and enables the introduction of fine metalliferous-bearing oxidic material to the zinc blast furnace without the disadvantages of solid carryover with the furnace off-gas to the condenser for zinc vapour, and decrease in burden permeability.

Prior to this invention it has been necessary to agglomerate metalliferous-bearing oxidic feed material to the zinc blast furnace by the use of sintering for metalliferous concentrate and briquetting for secondary oxidic materials or zinc calcines. It is now possible to consider the use of a fluid bed roaster to produce calcines for direct injection to the zinc blast furnace as a supplement to the conventional agglomerated feed to the shaft. This possibility is particularly useful when the sintering or agglomerating plants associated with the blast furnace are restricted in output and cannot match the requirements of the blast furnace.

The injection of oxidic metalliferous-bearing material into the zinc blast furnace can take place through the existing raceway-forming tuyeres or through separate tuyeres or lances. These two cases will now be separately considered: Case 1-Tuyere Raceway Injection It is well known that according to the present art the zinc blast furnace operates with tuyere raceways, which are effectively bubbles blown in a bed of coke, typically between 350mm and 550mm diameter. These are typically one third the diameter of raceways blown in iron blast furnaces and the residence time from tuyere mouth to raceway wall in straight line of flight is around 0.003 seconds.

The injection of reduction/fuel into the raceways of the blast furnace will combust with the preheated air to create a flame consisting of hot reducing gases which pass into the active zones around the raceways and then into the furnace burden. The temperature of the raceways may not be so high as for standard air-blown raceways burning coke preheated on its descent through the shaft, due to the previously mentioned sensible heat sink of the reductant/fuel, injected at ambient temperature.Adding oxygen to the combustion air will increase flame temperature to the point where operational problems occur as previously mentioned but this excess exotherm can be utilized by the additional injection of calcine or other metalliferousbearing oxidic material such that in-flight melting and smelting will occur to an extent depending on: -the flight time of the particles -the size of particles -preheating of injectants prior to emergence in the raceway -the temperature and reduction potential of the raceway.

Preheating of injectants will decrease the in-raceway flight time to achieve reaction temperature. Means for carrying out preheating may be the use of pre-combustion, or plasma, heating chambers on the tuyeres.

The products of the in-flight processing will pass in combinations of gaseous, vapour, liquid and solid form into the active zone surrounding the raceway or into the slag pool interacting with the raceway. Both routes provide a capture mechanism for the liquid and solid components by wetting or adhesion to melting surfaces.

In the case of the active zone, this consists of jostling lumps of carbonaceous material (usually high grade metallurgical coke) being consumed by oxygen and carbon dioxide gases from the raceway and the ash-component being melted into the slag pool. The active zone is also irrigated with molten metallic lead and gangue formed higher in the shaft, en route to the slag and bullion pools, plus molten slag splashed into the lower layers of the charge column by the interaction of raceways and slag pool. Hence solid and liquid particles of carbonaceous, metalliferous and gange material entering the active zone will be captured and the process of combustion, reduction and melting will continue under the influence of high temperature and reduction potential.

In the case of passage of raceway products into the slag pool, the efficiency of capture of solid and liquid particles will depend on the fluid dynamics and physical properties of the particles. Thus it is probable that liquid particles will be readily captured and that melting particles such as gangue or ash-containing materials will also be assimilated into the slag bath.

Solid particles with high interfacial tension compared with that of the slag pool will be more difficult to capture but as their residence time increases, melting and fusion will occur making capture highly probable. Any material not captured by the slag pool will enter the active zones and lower furnace charge levels where further capture opportunities will occur as described above.

The equipment for injecting material into the raceways of tuyeres is not restricted to the tuyeres that form the raceways. For example it may be beneficial to separately or in-combination inject certain components such as reductant/fuel and oxygen down lances within the racewayforming tuyere and to inject certain components such as oxidic metalliferous-bearing material separately or in-combination through lances disposed to direct the injectants into preferred regions of the raceway and thereby take advantage of flow patterns, temperature distributions or other characteristics.

Case 2-Slag Pool Injection The zinc blast furnace operates, according to present art, with a reject slag containing typically between 5 and 10% zinc over the long term depending on operating conditions and certain control parameters. It has already been explained how furnace operation and construction has evolved to increase the proportion of zinc reduced in the slag pool compared with that reduced in the shaft.

The injection of oxidic metalliferous material into the slag pool-either as a consequence of incomplete reduction when admitted to the raceways as explained above, or as a direct injection into the slag pool-will increase the zinc reduction load on the slag pool beyond that manageable by the present zinc elimination mechanism and zinc content of reject slag will increase beyond economic levels, therefore additional measures are necessary.

It is well known from the prior art that zinc can be fumed from slags, for example lead blast furnace slags by submerged injection of carbonaceous solids, liquid and gaseous hydrocarbons and air. This process is carried out at many metallurgical sites on molten slag that are initially relatively high in zinc and are processed until the zinc content of the slag is reduced to a level whereby it is no longer economic to continue, say 1 to 4% by weight zinc in discard slag. The zinc is reduced from the oxidic state by, for example, solid carbon dispersed in the bath and is liberated from a multitude of small gas bubbles with a reducing atmosphere as these break the surface of the bath, carried by a circulating slag stream in the centre portion of the bath. The majority of the tuyere gas stream rises as an oxdising, large bubble swarm close to the slag bath wall.

The prior-art operation described above differs from the operation in the slag pool of the zinc blast furnace in that: -the fuming operation is carried out in batch mode -the fuming operation produces an oxide fume -a proportion of the iron content of the fuming operation slag is consecutively oxidised to ferric form by the tuyere bubble swarm then reduced to ferrous form by dispersed reductant in the slag.

-the fuming operation has no reducing bed of coke and gas above the slag bath.

By contrast the blast furnace slag pool has constant addition of zinciferous material from shaft burden and potential injectants and iron is present essentially in the ferrous, or wustite, state without ferric content. Therefore conditions in the blast furnace are conducive to the constant rate production of zinc vapur from the slag pool rather than zinc oxide.

The conditions favouring elimination of zinc vapour from the slag pool are: -high temperature -high reduction potential -good slag/gas/solid contact -a fluid slag -good slag mixing within the pool -a dispersed carrier gas to sweep product zinc vapour.

The means by which this invention permits enhanced zinc production from the slag pool to permit the injection of zinciferous material into the slag pool are the optional injection of: fine solid carbonaceous reductant/fuel liquid hydrocarbon reductant/fuel gaseous hydrocarbon reductant/fuel gaseous carbon monoxide-containing reductant/fuel -air, oxygen enriched or pure oxygen gas streams -fine fluxing agent.

It is not intended that desired combinations of injectants are necessarily admitted through the same injection device. For example, reference has been made to the separate injection of materials within lances in the raceway-forming tuyere and this method is one non-exclusive means for introducing injectants to the slag pool. A further embodiment of this method is the staggered injection of materials into the slag pool. For example, carbonaceous material might be injected through one tuyere and zinciferous material through an adjacent or opposing tuyere.

It is not necessary to restrict injection to the slag pool only via the raceway-forming tuyeres.

In a further embodiment, the invention requires the installation of separate slag pool tuyeres or lances installed beneath, above or alongside the line of existing raceway-forming tuyeres such that the former discharge material into the body of the slag pool either by being partially or totally submerged in the slag pool, or by being above the slag pool but injecting material at high velocity into the slag pool. The slag pool depth can be controlled to suit the characteristics and requirements of the injection.

Use of air, oxygen-enriched air or commercially pure oxygen gas streams as injectant into the slag pool as a co-injectant with zinciferous material and reductant/fuel will be necessary when the heat requirement of the slag pool reactions is no longer met by that supplied by radiation from the incandescent raceway region above the slag pool and by interaction of the raceway blast with the slag pool.

The objective of oxygen-containing gas injection is to partially combust fuel so that the products of combustion are reducing towards the zinciferous content of the slag pool. It is also preferable that contact is minimised between oxidising gases and the slag pool as combustion proceeds, to avoid re-oxidation of zinc vapour and slag components. Some re-oxidation of zinc vapour escaping from the slag pool would be acceptable because this would be re-reduced by the coke-filled region above the bath, but as a general principle double reduction of zinc in this manner should be avoided as it is inefficient in the use of reductant.

Measures that can be taken to avoid undue re-oxidation of zinc involve injection of reductant/fuel as an outer envelope within which oxygen-containing gas is co-injected so that the latter is screened from the slag pool until combustion is essentially complete. This form of shrouded combustion is well-known and practised in the art of metallurgy, being accomplished by means of various types of burner, tuyere or lance usually consisting in arrangements of concentric tubes with various means of cooling the device to prevent failure in the high temperature environment.

Shrouding with gaseous reductant/fuel is more readily achieved than with liquid or solid reductant/fuel but technology is advancing towards the latter such that its use can be considered in the present invention.

Injection of gaseous CO or CO/CO2 containing mixtures may be economic when these gases are available as a product of another process or, more likely, as a recirculation stream from the blast furnace off-gas (See UK patent specification 1,470,722). The advantage of this injectant over hydrocarbon gases such as methane and propane is the absence of hydrogen content which forms a chemical equilibrium between H2, H2O, CO and CO2 in the furnace shaft gases and influences condensation of zinc vapour. Carbon monoxide is also a primary reductant of zinciferous material in the slag pool and its use does not involve the absorption of heat for vapourization or cracking of hydrocarbons.

An advantage of gas injection within the slag pool is the agitation and mixing produced and creation of interfacial areas for reduction to an extent depending on the tendency for gas bubbles to break-up and disperse in the slag. The ultimate contact area is provided if the slag "foams", which is known in certain slag fuming operations.

The aspect of mixing within the slag pool is important to ensure an even composition with respect to zinc. It is known that analyses of zinc within the slag pool of a zinc blast furnace can be variable and that over-reduction of slag to produce metallic iron can occur, causing separation of a third layer of high iron and arsenic material (speiss) which accumulates between the bullion and slag pools as previously explained.

The injection of materials into the slag pool as described above, especially gases, will increase the mixing of the slag pool. However it is a purpose of this invention to enhance the degree of mixing and circulation by arranging that raceway-forming tuyeres and supplementary tuyeres and lances are angled in a horizontal plane so that a component of gas momentum is imparted to the slag pool, causing circulation patterns along the longitudinal axis of the furnace. The angle of horizontal displacement to achieve this purpose will not be beyond practical possibility and angles within the range 5% to 25% should be sufficient.

It is recognised that injection of materials into the bottom of the zinc blast furnace according to the invention may affect the heat balance in the furnace shaft. It is important that the heat carried in the ascending gas stream exceeds the heat demand of the descending solids, otherwise the pattern of counter-current heat exchange and chemical reduction is inconsistent. It is also important that as zinc tenor of ascending gases increases, there is a high CO/CO2 ratio to prevent reversion of zinc vapour to zinc oxide. The method proposed according to this invention to control the mass and heat transfer effects within the furnace shaft is to alter the input of oxidic metalliferous material in relation to carbonaceous material at the top of the shaft so that shaft mechanisms are in balance.

Preferably the zinc:carbon weight ratio of materials fed to the shaft is between 0.1 and 1.2.

As increasing proportions of the total zinc blast furnace oxidic metalliferous input is made to the bottom of the furnace the majority of reduction reactions will occur in the raceway region or slag bath. However, the presence of a column of shaft material, especially carbonaceous material, will always be required to provide the filtration and heat sink duties plus the formation of a consumable "roof" for the high temperature smelting zone.

Claims (13)

1. A process for reducing zinc oxide material in a blast furnace, wherein an oxidic zinc or zinc/lead feed material and a carbonaceous reductant/fuel are fed, in lump form, to the top of the furnace shaft and a blast of pre-heated oxygen-containing gas is blown in through tuyeres at the furnace bottom; characterized in that at least part of the solid oxidic feed and part of the reductant/fuel is supplied to-the furnace bottom either through the main blast tuyeres or through supplementary tuyeres or lances and that such tuyeres or lances are oriented to direct oxidic material and fuel to impinge on or into the slag-pool at the furnace bottom.

2. A process as claimed in claim 1 wherein the reductant/fuel supplied to the furnace bottom is in solid, liquid or gaseous form.

3. A process as claimed in claims 1 or 2 wherein the blast is oxygen-enriched with respect to atmospheric air.

4. A process as claimed in claims 1, 2 or 3 wherein oxidic feed and reductant/fuel are fed to raceway-forming tuyeres so that reduction occurs at least partially in the raceway volume.

5. A process as claimed in claims 1, 2 or 3, wherein oxidic feed and reductant are fed, by lances, into the slag-pool at the furnace bottom so that the injectants are blown into the slag pool.

6. A process as claimed in claims 1, 2 or 3 wherein the oxidic material and reductant are separately injected into the slag-pool.

7. A process as claimed in claims 1 to 6 wherein fluxes are injected via tuyeres or lances into the furnace bottom.

8. A process as claimed in claim 1 wherein the ratio of oxidic to reductant material fed into the blast furnace shaft is controlled to produce furnace off-gases having a CO/CO2 ratio and temperature suitable for zinc condensation.

9. A process as claimed in claim 8 wherein the zinc:carbon weight ratio of the solids fed to the blast furnace shaft is between the values of 0. 1 and 1.2.

10. A process as claimed in claims 1 to 6 wherein the slag-pool is. agitated by the injection of gases into the slag-pool so as to produce a slag-pool with a substantially homogeneous composition, reduction capabiiity and ability to eliminate zinc vapour.

11. A process as claimed in claims 1 to 6 wherein the tuyeres or lances are oriented so at to promote circulation of slag around the furnace hearth.

12. A process as claimed in claim 11 wherein the tuyeres or lances are arranged an an acute horizontal angle to the furnace casing.

13. A process for reducing zinc oxide in a blast furnace substantially as hereinbefore described.