GB2428428A

GB2428428A - Methods of operating a zinc-producing blast furnace

Info

Publication number: GB2428428A
Application number: GB0514632A
Authority: GB
Inventors: Michael William Gammon; Joseph Philip Evans; Philip John Gabb
Original assignee: Individual
Current assignee: Individual
Priority date: 2005-07-16
Filing date: 2005-07-16
Publication date: 2007-01-31
Also published as: GB0514632D0

Abstract

In a first method of operating a zinc-producing blast furnace 1, solid carbon-containing materials 3 and oxidic zinc-containing materials 2 are charged into the top of the furnace 1 and a blast of preheated air 4, recycled furnace gases 14 and commercial oxygen 10 are introduced into the bottom of the furnace 1. In a second method, carbon-containing materials 3 and oxidic zinc-containing materials 2 are charged into the top of the furnace 1 and a blast of preheated recycled furnace gases 14 and commercial oxygen 10 are introduced into the bottom of the furnace 1. Surplus furnace gases 14 are used to preheat furnace carbonaceous materials 18 and the furnace blast 19. The raw material may additionally contain lead ore, thus also resulting in the production of lead bullion.

Description

Operation of zinc-producing blast furnaces

Description

This invention relates to zinc-producing blast furnaces. Objects of the invention are to increase the throughput of such blast furnaces and to reduce the ratio of solid carbonaceous material (such as metallurgical coke) consumed to zinc produced, compared with the prior art in which zinc is smelted with high-temperature blast air with additional limited quantities of pure oxygen.

In a zinc-producing blast furnace, as customarily operated, the charge materials consist of oxidised zinc and lead materials and solid carbonaceous reductant and fuel. Carbonaceous material, usually in the form of metallurgical coke, is fed hot, typically between 600 C and 800 C, and oxidised zinc and lead materials, usually in the form of zinc/lead sinter, is fed at up to 300 C. Fluxing agents, such as lime or silica, are incorporated with the zinc/lead charge in order to provide a fluid slag of low zinc content for tapping from the furnace hearth. Blast air at high temperature is introduced through tuyeres near the bottom of the furnace. The air is normally preheated in regenerative Cowper Stoves at temperatures typically around 1000 C by combusting low calorific value furnace offgases with air. The blast air mains are refractory-lined to reduce heat losses and protect the outer steel containment. The furnace tuyeres are water-cooled to maintain the steel surfaces within acceptable temperatures.

In a zinc-producing blast furnace, the gases leaving the furnace charge contain the main components of nitrogen, carbon monoxide, carbon dioxide and zinc vapour.

There is also a small proportion of hydrogen determined by volatiles and water inputs in solid and gaseous charge materials. The ratio of carbon monoxide to carbon dioxide leaving the furnace charge is an indicator of the efficiency of the use of carbon in the process, the lower the ratio the higher the efficiency. Heat must be supplied to the furnace to raise the charge materials to reaction temperature and provide the energy for endothermic chemical reactions and melting of metallic and slag phases.

A certain amount of heat is lost to the walls of the furnace depending on the volume of the furnace and its surface area. Large furnaces are more efficient due to the lower proportion of the total heat input that is lost through the walls. Near the bottom of the furnace, carbon is combusted to carbon dioxide (equation 1) and part of the carbon dioxide is further reacted with coke to form carbon monoxide (equation 2).

C+O2=C02 (1) C02+C=2C0 (2) Higher up in the furnace, zinc in oxidic form is reduced to metallic zinc (equation 3).

ZnO + CO = Zn + CO2 (3) Carbon dioxide formed by reactions I and 3, can further react with carbon higher in the shaft to regenerate carbon monoxide (as equation 2). A considerable proportion of the carbon monoxide is formed this way due to the temperature in the top half of the furnace being around 1,000 C.

I

In customary good practice, the volumetric carbon monoxide to carbon dioxide ratio leaving the zinc-producing blast furnace charge is in the order of 1.8 to 2.2 representing a practical limit to the utilisation of carbon. Moreover, the addition of oxygen to the blast air to increase furnace productivity has the disadvantage of increasing the carbon monoxide to carbon dioxide ratio in the gas leaving the lead- producing blast furnace, with consequent waste of carbon units.

In recent times, the zinc-producing blast furnace has come under increasing pressure from the high cost of metallurgical coke due to the high consumption of hot carbon to zinc which approaches a ratio of 1.0 in customary operation. The zinc-producing blast furnace also requires an oxidic feed which is predominantly prepared on a sinter machine that is expensive to operate both from mechanical and environmental aspects. Its main technical limitation however is the use of metallurgical coke. Any means of increasing zinc metal production per unit of metallurgical coke and increasing the throughput of existing installations would improve the standing of the process.

We have now discovered that zinc-producing blast furnace throughput and/or zinc production per unit of solid carbon consumed can be considerably increased by the recycle to the furnace of indirectly preheated furnace gases. It is necessary to add essentially pure, commercial oxygen to compensate for reduced quantities of blast air when furnace gases are recycled. In cases where zinc and lead feed materials are limited in supply by, for example, sinter machine operation, improved carbon efficiency can still be obtained for the maximum zinc and lead feed availability.

A metallurgical model has been constructed to replicate the performance of the zinc- producing blast furnace process and has been validated against best customary practice. The results of the case studies are presented in Table I in which the oxygen, blast air and recycled furnace gases have been adjusted to maintain a constant blast volumetric rate of 48,500Nm /hr for ease of comparison.

It should be noted that the zinc blast furnace produces moderately large active zones (raceways) in front of the tuyeres, such that fuel in the form of solids or liquids injected in the bottom of the furnace, for example pulverised coal, pulveilsed coke, liquid hydrocarbons, etc, will combust within the residence time provided in the raceway. It is already known in the art for such fuels and other charge materials to be injected into zinc- producing furnaces to provide cheaper carbonaceous and zinciferous material, but also as a feed supplement when there is insufficient sinter feed.

There is a limit to the hydrocarbon fuels that can be admitted to a zincproducing blast furnace due to the water that is produced. Water reacts with zinc vapour in the furnace top space to produce zinc oxide that lowers the zinc condensation efficiency.

Direct preheating of furnace blast with hydrocarbon fuels is also not favoured for the same reason. It is therefore preferred to preheat the recycled furnace gases in an indirect manner using such proven equipment as Cowper Stoves and tubed preheaters.

DESCRIPTION OF DRAWING

A side-elevation of a zinc-producing blast furnace is shown schematically as reference (1) on the drawing. The furnace is fed from the top with zinc and lead materials and fluxes (2) and solid carbonaceous material, usually metallurgical coke (3). Air (4) is compressed by a blower (5) and ducted through cold blast mains (6) to a preheater (7). The preheater is an indirect device and can be recuperative or regenerative. Preheated air from the preheater is passed through hot blast mains (8) admitted to the bottom of the furnace through tuyeres (9) with typically 8 tuyeres on each side of the approximately rectangular furnace bottom. Oxygen (10) is optionally added to the furnace through tubes within the tuyeres, or through separate lances into the furnace bottom in the vicinity of the tuyeres, or added to the hot blast air upstream of the tuyeres. Bullion lead and slag, are withdrawn from the bottom of the furnace, and zinc vapour together with other gases passes up through the furnace charge to the furnace top.

Cold or hot combustion air (11) from separate cold air fans or from furnace hot blast is added to the gases in the furnace top to combust some of the carbon monoxide in the gases leaving the furnace charge and thereby to control the temperature of the gases.

The resulting zinc-containing gases (12) are withdrawn from the top of the zinc- producing blast furnace and admitted to a lead-splash condenser (13) to recover zinc metal. Gases from the condenser offtake (14) are admitted to a gas cleaning system (15) to remove residual fume and dust particles. The gas cleaning system is wet- based for preference to reduce the dangers associated with fires and explosions with low calorific gas.

Gases are withdrawn from the gas cleaning system by a gas booster fan (16) which exhausts to a stack (17). Under normal operations around 30% of furnace gases are exhausted to the stack, or used for other purposes such as the production of steam and electrical energy. The major part of this low calorilic gas is normally utilised to preheat metallurgical coke (18) and blast air (19). Combustion air (20) is provided from separate air fans to combust the furnace gases and the products of combustion (21) are exhausted to the atmosphere.

According to the present invention, additional equipment is inserted into the flowsheet to improve productivity and solid carbonaceous fuel consumption. These additions are referenced in the following examples.

TABLE I - ZINC BLAST FURNACE CASE STUDIES

Case Description Furnace Lead Hot Wet Zn/C Comm'l Equilib'm Zinc Bullion coke coke ratio Oxygen temp ( C) Nm!h of gas t124h 1124h t124h 1124h addition leaving _____ ____________ ________ _______ _____ _____ _____ ________ furnace I Base Case - 336 150 334 382 1.15 0 1009 ______ Good_Practice _________ _________ ______ ______ ______ _________ ___________ 2 Base Case with 387 162 390 445 1.13 2796 1014 4% by vol 02 _______ enrichment __________ __________ _______ _______ _______ __________ _____________ 3 Base Case with 389 162 334 382 1.33 6855 1065 25% furnace gases preheated _______ recycle __________ _________ _______ _______ _______ __________ ____________ 4 BaseCasewith 484 186 334 382 1.65 14470 1145 52% furnace gases preheated recycle, zero blast air, and gas cooling to _____ 40C ________ _______ _____ _____ _____ ________ __________ AsRun4but 492 193 334 382 1.68 14300 1124 with some CO2 removal and gas cooling to _____ 35C ________ _______ _____ _____ _____ ________ __________ All cases have 48,500Nm3/hr total blast rate to the tuyeres.

The zinc to carbon ratio (Zn/C) column is the furnace zinc produced by the zinc condensation system, divided by the carbon content of the hot coke charged.

EXAM PLES

Conditions of Operation with Normal Air Blast Case I of Table I illustrates good zinc blast furnace practice according to the current art, blowing 48,500Nm3/h hot air blast, with sinter at 250 C and preheated coke at 650 C, to produce around 34Otpd of zinc metal and l5Otpd of lead metal. The carbon monoxide, carbon dioxide and hydrogen leaving the furnace with zinc vapour are calculated to have an equilibrium temperature of 1009 C. This temperature represents the point below which zinc oxide will form on solid surfaces such as the walls of the furnace offiake. Furnace offtake gases are therefore controlled in practice at around 1,050 C by the addition of air (11) into the furnace top space so that zinc oxide does not form and block the furnace offiake.

Conditions of Operation with Oxygen-enriched Blast Case 2 of Table I illustrates good practice according to the current art, with 4.0 percentage points of commercial oxygen enrichment whilst maintaining the same blowing rate of 48,500Nm3/h of oxygen-enriched hot blast. In this context commercial oxygen is the essentially pure oxygen produced by a commercial oxygen plant. Coke consumption increases and zinc/carbon ratio falls to give increased zinc and lead metal productions around 39Otpd and l6Otpd respectively. The disadvantage of oxygen enrichment on conventional furnace operation is the reduction in zinc production per unit of consumed carbon as oxygen enrichment increases. This situation occurs due to higher temperatures in the lower shaft and the consequent increase in the ratio of carbon monoxide to carbon dioxide in gases leaving the furnace. Oxygen enrichment of 4% represents an upper limit in current practice.

Conditions of Operation with Partial Furnace Gas Recycle It has been proposed in the prior iron blast furnace art that furnace gases be separated and carbon monoxide be returned to the furnace for beneficial use in metal production (French patent FR0215316, dated 2002-12-04). It has also been proposed that furnace gases be partially recycled to the furnace after drying through a second, higher row of tuyeres with hydrogenaceous fuel injection in the bottom row of tuyeres (USA patent US5234490, dated 1993-08-10). It is not a feature of the prior art for substantial quantities of furnace gases to be recycled to existing blast furnace tuyeres without prior separation and without fuel injection.

Case 3 of Table I illustrates the case according to the present invention whereby around 25% of furnace gases in the amount of I 7,200Nm3/h is recycled to the furnace with a balance of 6,855Nm31h commercial oxygen and 24,400Nm/h blast air. A gas blower (22) compresses furnace gases from the outlet of gas cleaning fan (16) and routes them to an indirect preheater (23). Preheated furnace gases are sent to dedicated zinc- producing furnace tuyeres via blast main (24). Furnace gases (25), and combustion air (26) optionally derived from the blast air, is burnt to preheat the blast furnace gases and the products of combustion are exhausted to atmosphere (27). It should be noted that the preheated furnace gases contains some water in gaseous form equivalent to that present in the wet scrubbed furnace gases at a temperature around 40 C. The lower the scrubbed furnace gases temperature, the lower the water present and the better the zinc-producing furnace operation.

It will be appreciated that air and commercial oxygen can not be added to the preheated furnace gas prior to entry into the furnace due to the risk of explosion or early combustion of fuel gases such as carbon monoxide. Hence for situations involving partial recycle of furnace gases to the zinc-producing blast furnace, the furnace gases and blast air streams are preheated in separate preheaters, (7) and (23). A proportion of the fumace tuyeres are allocated to furnace gases with the remainder allocated to blast air. In Case 3, approximately one half of the blast gases are made up from recycled furnace gases and its share of commercial oxygen, so the tuyere split between blast air and recycled furnace gases is nominally 50:50.

Commercial oxygen is added to the recycled furnace gases at the entry to the furnace (10), and to the blast air either prior to entry to the furnace or at the entry to the furnace (10).

It will be observed that the recycle of around 25% of furnace gases increases the gas equilibrium temperature to around 1,065 C, partly due to the increased zinc content of the furnace gases and partly due to the increased water vapour content of recycled furnace gases. This increase would be compensated for by increasing the temperature of the furnace offiake gases. One way used in the current art to accomplish this is to increase the amount of top air (11), hence to increase the furnace top gas temperature. Another way to compensate for the increased gas equilibrium temperature is to increase the radiation to the furnace offlake walls by adding top air in the cold state, which has been proven to increase the radiative effect of the top air flame and thereby increase the wall temperature. Another way is to reduce the chilling effect of lead splashed onto the furnace offtake walls by improving the trajectory of the lead-splash spray. Yet another way is to reduce the water vapour content of recycled furnace gases by reducing the temperature of the gas scrubbing system water. By such means the inevitable increase in the equilibrium temperature with increasing carbon efficiency can be managed to prevent undue furnace downtime for cleaning and reduced zinc condensation efficiency.

Zinc-producing blast furnaces are provided with typically sixteen tuyeres with typically eight along each long side of a rectangular-sided furnace bottom. It is possible to segregate tuyeres, for example, such that one tuyere in every two consecutively along the furnace sides is connected to a separate blast main, one blast main containing blast air and the other containing recycled furnace gases. It will be realised that other combinations of tuyeres will be possible depending on the quantity of furnace gases to be recycled. It is also possible to connect single tuyeres to either an air blast main or a recycled gas main by means of Ypiece connections in the tuyere leads with valves inserted in each leg of the Y-pieces.

Operation of the zinc-producing blast furnace according to Case 3 will commence with preheated air being blown through that proportion of the tuyeres allocated to blast air.

Furnace gases, when they become available, will then be preheated and routed to the remaining tuyeres with separate oxygen enrichment. All of the furnace tuyeres may be initially allocated to oxygen-containing gases, and tuyeres may be switched individually to furnace gases by the Y-piece methodology mentioned above. By such means, the practical difficulties of changing from blast air to furnace gases in tuyeres will be overcome. The main consideration during normal operation will be to operate all tuyeres with equal carbon burning rates in order to maintain uniform charge descent in the furnace.

Conditions of Operation with Zero Blast Air Case 4 of Table I illustrates the case according to the present invention whereby the amount of recycled furnace gases are increased to the point whereby no blast air is required to maintain the total blast rate at 48,500Nm3/h. Commercial oxygen additions are increased to around 14,500Nm3/h to compensate for the removal of oxygen in blast air. For the same hot coke consumption, zinc and lead production increase by some 44% and 24% respectively over base case.

Without exception, all operating zinc-producing blast furnaces preheat the blast air using Cowper stoves. A minimum of two stoves are required, one preheating blast air and one reheating by combusting furnace gases with air. In the case whereby all zinc-producing blast furnace blast air can be replaced by recycled furnace gases and commercial oxygen, the furnace will initially start operations with preheated air being blown through all furnace tuyeres.

When full-volume furnace gases are obtained, the zinc-producing blast furnace will switch to operation with furnace gases and commercial oxygen through all tuyeres.

The Cowper stove just preheated, shown as reference (23) in the drawing, will be full of inert gas from the combustion of furnace gases and on changeover of stoves, recycle furnace gases from blower (22) will enter the stove displacing the inert gas ahead of it through the recycle blast main (24) and into the furnace. As the stove volume is much greater than that of the blast main, air within the blast main from the previous stove operation will be effectively purged to prevent risks of explosion.

At the next changeover, the Cowper stove shown as reference (7) will similarly be connected to furnace gases blower (22) through line (28) and both Cowper stoves will then be on-line in furnace gases preheating mode. Return to air blast will be the reverse procedure with inert combustion gases purging furnace gases from the hot blast main followed by blast air. In this manner a safe and simple procedure can be adopted without incurring the additional capital cost of separate preheaters and blast mains for blast air and recycle furnace gases.

Conditions of Operation with Zero Blast Air, Carbon Dioxide Removal, and Furnace Gases Drying As stated above, it is known in the art for proposals to be made to remove carbon dioxide from furnace gases before recycling high carbon monoxide-containing gases to a blast furnace. It has not been necessary in Cases 3 and 4 of the present invention to introduce gas separation to effect considerable improvement in zinc- producing blast furnace operation.

Case 5 in Table I illustrates the case according to proposals in the current art whereby some carbon dioxide is removed from the zincproducing furnace gases, the gases are dried to remove water, and a maximum quantity of the resulting high carbon-monoxide gases are recyded to the furnace to eliminate blast air. Some 60% of the resulting high strength gas is predicted to be recycled containing 48% of the carbon dioxide in the furnace offgas. Additional water vapour is removed compared with Case 4 in order to maximise the furnace gases recyded.

Despite the additional expense of installing carbon dioxide removal, the benefits of Case 5 over Case 4 are not great. There are marginal improvements in the zinc and lead production rates and a small fall in the zinc equilibrium temperature. Hence the benefit to a zinc-producing blast furnace of enriching the carbon monoxide in recycled furnace gas is considered unlikely to be of sufficient advantage to justify the installation of furnace gas separation technologies when compared with recycling unprocessed furnace offgases.

In all the above cases the furnace blowing rate has been maintained at a constant level of 48,500Nm3/h to represent similarity in the pressure drop of the furnace blowing equipment. It will be understood that some zinc-producing furnace operations may have the capability to increase furnace blowing rates above the representative level of 48,500Nm3Ih and that such increases will have a corresponding effect on the parameters indicated in the above cases.

Additional productivity and coke economy can be obtained by the use of higher coke and sinter qualities than assumed in the base case. Coke and sinter qualities are measured by parameters such as reactivity, hardness, fines production, softening point, etc, which are well known to those experienced in the art. The coke and sinter assumed for the base case calculation have average qualities by current standards.

However, the cases of the present invention reflect a major objective of the invention, namely to reduce dependency on high quality feed materials.

Claims

CLAIMS: 1. A method of operating a zinc-producing blast furnace comprising

the steps of charging solid carbon-containing materials and oxidic zinccontaining materials at the top of the furnace and introducing a blast of preheated air, recycled furnace gases, and commercial oxygen, at the bottom of the furnace.
2. A method according to ClaimI, in which recycled furnace gases have essentially the same dry gas composition as the furnace gases leaving the zinc condenser of the blast furnace.
3. A method according to Claim 2, in which the water content of the recycled furnace gases is essentially that given by cooling the gases to below 50 C, preferably below 40 C, in wet gas scrubbing equipment.
4. A method according to Claim 3, in which the blast is preheated to a temperature in the range 600 C to 1,200 C, preferably 1,000 C to 1,200 C.
5. A method according to Claim 4, in which the blast air and recycled furnace gases are separately and indirectly preheated in suitable apparatus, such as tubed recuperative heaters or Cowper stove regenerators.
6. A method according to Claim 5, in which commercial oxygen is added to the preheated blast air after preheating and before entry to the furnace.
7. A method according to Claim 5, in which commercial oxygen is added to the zinc- producing blast furnace separately from the preheated blast air or preheated recycled furnace gases, through tubes or lances either inserted within the preheated blast tuyeres or separately around each preheated blast tuyere.
8. A method according to Claim 5, in which recycled furnace gases are fed to a number of dedicated tuyeres in the zinc-producing blast furnace, the ratio of the number of tuyeres passing recycled furnace gases to the whole number of tuyeres being essentially equal to the ratio of recycled furnace gases to the total volume of blast gases.
9. A method of operating a zinc-producing blast furnace comprising the steps of charging carbon-containing materials and oxidic zinc-containing materials at the top of the furnace and introducing a blast of preheated recycled furnace gases, and commercial oxygen, at the bottom of the furnace.
10. A method according to Claim 9, in which recycled furnace gases have essentially the same dry gas composition as the furnace gases leaving the zinc condenser of the blast furnace.
11. A method according to Claims 10, in which the water content of the recycled furnace gases is essentially that given by cooling the gases to below 50 C, preferably below 40 C, in wet gas scrubbing equipment.
12. A method according to Claim 11, in which the recycled furnace gases are preheated to a temperature in the range 600 C to 1,200 C, preferably 1,000 C to 1,200 C.
13. A method according to Claim 12, in which the blast air and recyded furnace gases are indirectly preheated in sequence in regenerators, such as Cowper Stoves, with the changeover between blast air and recycled furnace gases being accomplished when the Cowper stoves are changed over.
14. A method according to Claim 12, in which recycled furnace gases are fed to all the installed tuyeres in the zinc-producing blast furnace, after first starting zinc production with preheated blast air.
15. A method according to Claim 12, in which commercial oxygen is added to the zinc-producing blast furnace separately from the preheated recycled furnace gases, through tubes or lances either inserted within the preheated blast tuyeres or separately around each preheated blast tuyere.
16. A method according to Claim 12, in which a minimum quantity of zincproducing blast furnace gases is recycled to the furnace of the order of 50% of the volume of

such gases, to eliminate blast air introduction.
17. A method in which furnace gases surplus to furnace recycle are used to preheat, in whole or in part, furnace carbonaceous materials and furnace blast.