GB2437958A

GB2437958A - Operating ferrous and non-ferrous bast furnaces

Info

Publication number: GB2437958A
Application number: GB0609575A
Authority: GB
Inventors: Michael William Gammon; Joseph Philip Evans; Philip John Gabb
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-05-13
Filing date: 2006-05-13
Publication date: 2007-11-14
Also published as: GB0609575D0

Abstract

A method of operating a metal producing blast furnace 1 by charging carbon containing material 3 and oxidic materials 2 into the top of the furnace 1 and introducing blast air 4, commercial oxygen 8 and directly preheated gases from the direct combustion of solid, liquid or gaseous fuel 9 and 22 into the bottom of the furnace 1. The fuel may be at least in part recycled furnace gas 9. The fuel 9 and 22 is burnt in a combustion chamber 21 external to the furnace 1 with part of the blast air 4 which may be either cold or indirectly heated. These gases are then mixed with further blast air 4 before entry into the furnace 1. Water or steam 24 may also be added to the combusted gases before entry into the furnace 1.

Description

DESCRIPTION

Operation of ferrous and non-ferrous blast furnaces This invention relates to ferrous and non-ferrous 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 metal produced, compared to the prior art in which metal is smelted with blast air with additional limited quantities of pure oxygen.

In a ferrous or non-ferrous blast furnace, as customarily operated, the charge material consists of metal-bearing oxidised material and metallurgical coke. Coke is fed hot or cold and oxidised material may be fed at up to 500 C. Fluxing agents, such as lime or silica, are incorporated with the oxidised charge material in order to provide a fluid slag of low metal content for tapping from the furnace hearth. Blast air between ambient and 1200 C is introduced through tuyeres near the bottom of the furnace. The blast air is normally preheated indirectly using suitable heating equipment such as Cowper stoves.

In a ferrous or non-ferrous blast furnace, the gas leaving the furnace charge contains as its main components nitrogen, carbon monoxide and carbon dioxide. There are also small proportions of hydrogen and water vapour determined by volatiles and water inputs in solid and gaseous charge materials. In the zinc-producing blast furnace the gas leaving the furnace charge is much hotter than for lead-producing and iron-producing blast furnace operation, and zinc vapour is also present. The ratio of carbon monoxide to carbon dioxide leaving the furnace charge vanes depending on the metal being produced, but for each type of furnace this ratio is an indicator of the efficiency of 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+ 02C02 (1) C02-fC=2C0 (2) Some water vapour reacts with carbon (equation 3), and some reacts with carbon monoxide according to the gaseous equilibrium (equation 4).

H20+CH2 CO (3) H20 + CO = H2 + CO2 (4) Higher up in the lead-producing and iron-producing furnaces, lead and iron, represented by the symbol "M", in oxidic form are reduced to metallic form (equation 5).

MO+CO=M+C02 In the zinc-producing blast furnace, zinc is produced in the bottom part of the furnace according to the same equation (5).

Carbon dioxide formed by reactions 1, 4, and 5 can further react with carbon higher in the shaft to regenerate carbon monoxide (according to equation 2). In practice, most of the CO is formed near the furnace bottom.

The addition of oxygen to the blast air, as practised in some blast furnaces to increase furnace productivity is only practical up to a point, beyond which the carbon monoxide to carbon dioxide ratio in the gas leaving the blast furnace increases, with consequent waste of carbon units.

In recent years, the blast furnace has come under increasing pressure from new technologies which have increased the intensity of metal production and enabled the direct smelting of suiphidic or oxidic materials. The ferrous and non-ferrous blast furnaces require oxidic feed which is prepared on sinter machines or other agglomerating equipment that are expensive to operate both from mechanical and environmental aspects. The blast furnace also requires strong metallurgical coke to provide a porous furnace burden to enable the passage of furnace gases. Despite these shortcomings, the blast furnace is still a major source of metal production. Its main technical limitation is the use of metallurgical coke which is expensive and suffering supply limitations. Any means of increasing metal production per unit of metallurgical coke and increasing the throughput of existing installations while reducing the quantities of greenhouse gases emitted to the environment would improve the standing of the ferrous and non-ferrous blast furnace processes.

We have now discovered that blast furnace throughput and/or metal production per unit of solid carbon consumed can be considerably increased by the addition to the furnace of externally combusted fuels, preferably in the form of recycled furnace gases. In all cases it is necessary to add essentially pure, commercial oxygen to maintain both the furnace heat balance and the total gas volume to the furnace.

We have also discovered that the use of direct preheating of the blast air or recycled furnace gases is more efficient than indirect heating due to increased levels of CO2 and H20 in the blast volume in the direct preheating case and the effect these have on the furnace shaft heat exchange. There is also no loss of preheating efficiency with a direct preheating system compared with the losses in exhaust gases of an indirect preheating system, such as preheating in Cowper stoves.

The injection of CO2 into the bottom of a blast furnace would appear to be counter intuitive. What has been discovered is that the higher heat capacity of CO2 not only helps to control temperatures in the furnace bottom but also enables higher rates of heat transfer from the tuyere zone of the furnace to the upper parts. Since energy is required in the upper zones of the furnace to reduce metal oxides, this extra energy that is transferred enables higher rates of metal production to be achieved. We have now discovered that the limit of extra metal that can be produced is determined by how much ( the CO/CO2 ratio can be lowered before the rates of metal oxide reduction become limiting.

In both lead-producing and iron-producing blast furnaces, metal can be produced when the sinter charge reaches about 600 C and by the time the sinter reaches 1000 C the majority of the metal oxides have been reduced to metal. The differences between the lead-producing and iron-producing blast furnaces are as follows: * The iron oxide reduction reaction is endothermic whereas the lead oxide reduction reaction is exothermic. This means that in the case of the lead-producing blast furnace, the sinter charge may become hotter than the local gas whereas in the iron blast furnace case the sinter temperature will always be below the local gas temperature. In the case of the lead-producing blast furnace with higher lead contents in charge, this may result in the sinter reaching its softening point before all the lead is reduced (lead oxide melts at 880 C). When this occurs furnace instability can result and gas temperatures leaving the furnace can rise due to gas channelling up through the softened charge. There is therefore an upper limit for lead content of sinter dependant on its sinter softening characteristics. This phenomenon is not so relevant with the iron blast furnace.

* Lead melts at 327 C whereas iron melts at 1450 C (carbon will reduce the melting point to around 1250 C). Thus as soon as lead is produced it will form liquid drops and drip away from the sinter and run down to the furnace bottom. Iron, on the other hand, will stay with the sinter as solid metal until the furnace reaches a temperature of around 1250 C near the bottom of the furnace.

* Thermodynamically, lead oxide can be reduced at much lower CO/CO2 ratios than iron and as a consequence the CO/CO2 ratio in the gas leaving the lead blast furnace is generally well below 1, whereas the CO/CO2 ratio leaving the iron blast furnace is generally well above 2.

The zinc-producing blast furnace differs from the lead-producing and iron-producing blast furnaces in that zinc oxide is not reduced until the temperature exceeds 1000 C.

This occurs in the bottom half of the furnace. The boiling point of zinc is 907 C, thus as soon as the metal is produced it is volatised and leaves with the gases leaving the furnace top. This higher temperature has implications for both the quality and consistency of both the sinter and coke charge. The sinter must not soften prematurely in the shaft (it's softening temperature must be above 1000 C) and coke reactivity (the rate of the reaction C+ CO2 = 2CO at around 1000 C) is more critical for the zinc-producing blast furnace because any CO generated in the upper shaft is wasted as it does not contribute to any increase in metal production.

The CO/CO2 ratio required for zinc reduction is similar to that for iron reduction, thus the CO/CO2 ratio in the gas leaving the furnace charge of both the zinc-producing and iron-producing furnace is well above 2.

To summarise, the iron-producing blast furnace operation shares features of both the zinc-producing and lead-producing blast furnaces. It is similar to the lead-producing blast furnace in that the metal is produced in the upper part of the shaft but it requires a higher CO/CO2 ratio similar to that required in the zinc-producing blast furnace. The temperatures of the gases leaving the lead-producing and iron-producing blast furnaces are similar but much lower than for the zinc-producing blast furnace.

We have now constructed metallurgical models to replicate the performance of blast furnace processes and have validated these models for lead-producing and zinc-producing furnace operation for customary practice. The results of the case studies are presented in Table I (efficient lead operation), Table 2 (inefficient lead operation), and Table 3 (zinc operation). The trends found for both lead-producing and zinc-producing blast furnaces are considered applicable to the iron-producing blast furnace based on the similarities described above.

INTRODUCTION TO DRAWING

A schematic end-elevation of a ferrous or non-ferrous producing blast furnace is shown as reference (1) on the drawing. The furnace is fed from the top with oxidised material and fluxes (2) and solid carbonaceous material, usually metallurgical coke (3). Air (4)is compressed by a blower (5) and is either preheated using suitable preheating equipment such as Cowper stoves (14), or in the case of most lead-producing blast furnaces, passed directly through blast lines (6) to the bottom of the furnace through tuyeres (7). Ferrous producing blast furnaces are generally circular with tuyeres equally spaced around the perimeter, while non-ferrous producing blast furnaces are approximately rectangular with typically the same number of equally-spaced tuyeres on each tong side of the furnace bottom. Oxygen (8) 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. Oxygen can also be added upstream or downstream of the blower (5).

Furnace gases (9) are withdrawn from the top of the ferrous and non-ferrous producing blast furnace and admitted to a gas cleaning system (10) to remove fume and particles.

In the zinc-producing blast furnace the gas cleaning system is more complicated than for the other blast furnaces because additional equipment in the form of a lead splash condenser and zinc separation system is necessary to absorb and then separate the zinc metal upstream of the gas cleaning system. The gas cleaning system may be wet-or dry-based according to the preference of the individual furnace operator. Furnace gases are withdrawn from the gas cleaning system by a fan (11) which exhausts to a stack (12). Furnace gases with sufficient calonfic value may also be used to preheat blast air and metallurgical coke. The conventional flowsheet thus explained is shown with bold flow lines on the drawing.

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 -LEAD BLAST FURNACE CASE STUDIES

Efficient Operation Case Description Lead Coke Lead/ Lead! CO!C02 Commercial Greenhouse prodn in Coke equivalent ratio Oxygen gas per tlday charge ratio coke leaving consumed tonne lead %w/w wt ratio furnace Nm3/h _____ __________ _____ ______ _____ _________ _______ __________ Nm3_C02/t 1 Base Case 752 7 65 5.59 5.59 0.26 849 296 (2.5% 02 enrichment) at 50 C blast air ______ temperature ______ _______ ______ __________ ________ ___________ ___________ 2 AsCasel 838 730 588 546 0.32 849 296

INDIRECT

heating to ______ 400 deg_C ______ _______ ______ __________ ________ __________ ___________ 3 As Case 2 855 7.14 6.02 5.59 0.22 2303 305 with

DIRECT

heating to ______ 400 C ______ _______ ______ __________ ________ __________ ___________ 4 25% 896 658 6.57 629 0.09 3913 282 furnace gas recycle,

DIRECT

heating to ______ 400 C ______ _______ ______ __________ ________ ___________ ___________ Max (39%) 1137 6.04 7.21 651 0.06 7260 284 furnace gas recycle

DIRECT

heating to I 800 C ______ _______ ______ __________ ________ ___________ ____________ All cases have the same gas flow to the tuyeres of 23,500 Nm3Ih at the indicated preheat temperatures.

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TABLE 2 -LEAD BLAST FURNACE CASE STUDIES Inefficient Operation Case Description Lead Coke Lead! Lead! CO/CO2 Commercial Greenhouse prodn in Coke equivalent ratio Oxygen gas per t!day charge ratio coke leaving consumed tonne lead ______ ___________ ______ %w/w ______ wt ratio furnace Nm3/h Nm3 C02!t 1 Base Case 567 9.62 3.99 3.99 0.62 849 414 (2.5% 02 enrichment) at 50 C blast air ______ temperature ______ _______ ______ __________ ________ ___________ ___________ 2 AsCasel 635 9.13 4.22 3.94 0.72 849 414

INDIRECT

heating to ______ 400 C ______ _______ ______ ___________ ________ ___________ ___________ 3 AsCase2 665 8.76 4.42 4.11 0.45 2455 415

DIRECT

heating to ______ 400 C ______ _______ ______ __________ ________ ___________ ___________ 4 Max (47%) 836 6.96 5 68 5.59 0.07 6485 332 furnace gas recycle

DIRECT

heating to ______ 400 C ______ _______ ______ ___________ ________ ____________ ____________ Max (50%) 997 7.01 5.63 5.42 0 11 8775 347 furnace gas recycle

DIRECT

heating to 800 C Zero blast ______ air ______ _______ _______ ___________ ________ ___________ ____________ All cases have the same gas flow to the tuyeres of 23,500 Nm3/h at the indicated preheat temperatures. (

TABLE 3-ZINC BLAST FURNACE CASE STUDIES Case Description Furnace Lead Hot Wet Zn/C Comm'l Greenhouse Zinc Bullion coke coke ratio 0xyen gas per Nm /h tonne metal t/24h t/24h t/24h t/24h addition Nm3 C02/t 1 Base Case-336 150 334 382 1.15 0 1124 _______ Good_Practice __________ _________ _______ _______ _______ __________ 2 Base Case with 387 162 390 445 1.13 2796 1190 4% by vol 02 ________ enrichment ___________ __________ _______ ________ _______ ___________ 3 Base Case with 389 162 334 382 1 33 6855 1060 25% furnace _______ gases_recycled ___________ _________ _______ _______ _______ 4 BaseCasewith 492 191 298 340 1.88 14470 832 52% furnace gases recycled _______ Zero_blast_air ___________ _________ _______ _______ _______ __________ ____________ All cases have 48,500Nm3/hr total blast rate to the tuyeres at a preheat of 1,030 C.

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.

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EXAMPLES

Normal Conditions of Operation of Lead and Zinc Blast Furnaces Case I of Table 1 illustrates the performance of a large and efficient lead-producing blast furnace according to the present art, with 7.7% by weight coke in the charge, blowing 23,500Nm3/h of cool air which is oxygen enriched with 2.5 percentage points of commercial oxygen to produce around 752tpd of lead metal. The carbon monoxide to carbon dioxide ratio (CO/CO2 ratio) of 0.26 leaving the furnace charge reflects relatively good quality coke and metalliferrous charge materials. Quality in this sense means combinations of hardness, reactivity, chemical composition, softening-point, etc, which are well-known to those experienced in the art.

By comparison, Case 1 of Table 2 illustrates the performance of the same size lead-producing blast furnace according to the present art when operating inefficiently with 9.6% coke in the charge, for the same blowing rate of cool oxygen-enriched air, producing around 567tpd of lead metal. The CO/CO2 ratio of 0.62 leaving the furnace reflects relatively poorer quality coke and metalliferrous charge materials, hence the inefficient operation.

Case 1 of Table 3 illustrates the performance of a zinc-producing blast furnace according to the present art when operating at average efficiency for this type of furnace. A quantity of around 334tpd of hot coke is consumed to produce 336tpd of furnace zinc and I 50tpd lead bullion.

Zinc-producing blast furnaces are not normally operated with oxygen enrichment because oxygen enrichment reduces the metal output slightly per tonne of coke. Case 2 of Table 3 illustrates this point where 4% by volume oxygen enrichment has increased zinc and lead production to 387tpd and 1 62tpd respectively for the same gas flow to the tuyeres but at the increased hot coke consumption of 39Otpd. The furnace efficiency, as defined by the ratio (furnace zinc/carbon in hot coke), has deteriorated from 1.15 in Case I to 1.13 in Case 2.

Conditions of Operation with Indirect Blast Preheat to 400 C In most lead-producing furnace operations the furnace blast air and any commercial oxygen additions are blown into the furnace in a cool condition at a temperature around 50 C. It is known by analogy with other blast furnaces in the current art that preheating the oxygen-containing blast gases will increase the carbon consumption of the furnace and thereby increase lead metal production. Preheating lead-producing blast furnace blast gases, however, involves the use of additional fossil fuel as the gases leaving the furnace are not sufficiently high in calorific value to be used for this purpose.

Case 2 of Table 1 illustrates the case according to the known art, of indirectly heating the blast air with 2.5 volume percentage points commercial oxygen, to the current practical limit of 400 C resulting from the low-temperature steel construction of the blowing system. Blast air from the air blower (5) is routed through blast line (13) to indirect air heater (14). Fossil fuel (15) and combustion air (16) are burnt and the combustion gases after indirect heat exchange with the blast air stream are passed to atmosphere (17). Preheated blast air is passed through blast line (18) to tuyeres (7) with optional addition of oxygen (8).

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Lead metal production increases to around 838tpd at an improved coke in charge around 7.3%, but the CO/CO2 ratio increases somewhat over Case I to 0.32. This ratio increase is predicted due to the higher temperature in the furnace bottom and the resulting increase in the amount of carbon dioxide that reacts with coke to form carbon monoxide.

Taking into account the fossil fuel used for preheating the blast volume, it is observed that the (lead metal/equivalent coke) weight ratio decreases compared with Case 1.

Equivalent coke is defined as the combined furnace fossil fuel feed and blast volume preheating fuel, expressed as total fuel of the same calorific value as metallurgical coke.

Case 2 of Table 2 illustrates the corresponding inefficient practice according to the known art with the noticed deterioration in (lead metal/equivalent coke) ratio compared with Case 1 of Table 2.

There is no corresponding Case of indirect blast preheating with fossil fuel for the zinc-producing blast furnace in Table 3, as normal operation is to preheat the blast air to around 1000 C by substituting furnace gases for fossil fuel (15) in Cowper Stoves (14).

The same is true for iron-producing blast furnaces.

Conditions of Operation with Direct Blast Preheat to 400 C Hot gases can be produced by the combustion of fossil fuels introduced into cool gases and subsequently mixed. One such method for preheating blast air is a fuel/air burner inserted into the cool blast using a side stream of cool blast air to burn the fuel. Another method is by burning fuel and air in a separate combustion chamber and ducting the hot gases into the cool blast air stream.

Case 3 of Table I illustrates the performance of an efficient lead-producing blast furnace according to the present invention whereby the blast is directly preheated to 400 C by burning fossil fuel such as oil or natural gas or pulvensed fuel, and introducing the combustion products by a suitable method into the blast stream, and adding commercial oxygen, if desired, to the blast stream at a suitable point. In this case, the products of combustion dilute the blast stream and additional commercial oxygen is added to bring the oxygen content of the blast stream back to the level of Cases I and 2. Blast air from the air blower (5) is routed through blast line (26) to direct preheater (21). Fossil fuel (22) and commercial oxygen or blast air, or a mixture (23), is burnt externally or within the direct preheater (21). The combustion gases pass into the blast air stream and through blast line (25) to tuyeres (7) with optional addition of commercial oxygen (8).

It will be noticed that the CO/CO2 ratio in the gases leaving the furnace improves over Case 2 and there are beneficial changes in lead metal production and (lead metal/metallurgical coke) ratio. These benefits are predicted to result from the additional carbon dioxide and water vapour added to the furnace from direct preheating of the blast stream. First, the combustion gases have a higher sensible heat than the nitrogen they replace, so increasing the heat input to the furnace. Second, the carbon dioxide reacts with solid carbon at, or near, the entrance of blast into the furnace to produce additional carbon monoxide for reducing lead-containing materials. Third, the sensible heat of gases passing up the furnace shaft is higher than in Case 2 50 the ( charge is more effectively preheated. Fourth, the water vapour reacts with carbon monoxide exothermally, thus increasing the energy input at the furnace bottom.

Operation of the lead-producing blast furnace, according to the present invention, will commence with cold or preheated air and commercial oxygen enrichment being blown through the tuyeres in the conventional manner by a suitable blast-line route, for example through blast lines (26) and (25). Fossil fuel combustion will then be established at the direct preheater (21) to control the blast air and combustion gases at a temperature at, or below, the maximum temperature permitted by the materials of construction of the components of the blast mains, tuyeres and associated fittings, generally less than 400 C. Blast volume will be controlled at, or below, the maximum volume permitted by the furnace design by reducing blast air from the blower (5), and correspondingly increasing commercial oxygen addition at a suitable point, or points.

Case 3 of Table 2 illustrates the corresponding inefficient practice according to the present invention but with larger beneficial changes in lead metal production and (lead metalIcoke) ratio. These Case 3 operations illustrate the typical operating window of a lead-producing blast furnace with direct preheating of blast provided by fossil fuel combustion.

There is no corresponding Case of direct blast preheating for the zinc-producing blast furnace in Table 3, as existing operations preheat the blast air to around 1000 C using indirect heating with furnace gases in Cowper Stoves. However, it would be possible to supplement, or replace, the preheating of blast gases to zinc-producing and iron-producing blast furnaces by direct preheating in the manner described.

Conditions of Operation with Mixed Blast Air and Combustion of Partial Furnace Gas Recycle It has been proposed in prior iron-producing blast furnace art that furnace gases be separated and carbon monoxide enriched gases be returned to the furnace through existing tuyeres for beneficial use in metal production (French patent FRO21 5316, dated 2002-12-04). It has also been proposed in prior iron-producing blast furnace art 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 iron-producing blast furnace prior art for recycled furnace gases to be essentially completely combusted prior to entry to the furnace.

It has been proposed in the zinc-producing blast furnace prior art that a proportion of the furnace gases be returned to the furnace by injection into the blast for beneficial use in metal production (UK patent GB 1470722 A, dated 1977-04-21). It is not a feature of the prior zinc-producing blast furnace art for recycled furnace gases to be essentially completely combusted prior to entry to the furnace. All such prior-art recycling without prior combustion suffers from the disadvantage of a practical means of injecting flammable and toxic gases into a blast furnace. It is not advisable to add such recycled gases to the blast air supply for fear of uncontrolled combustion and even explosion.

Addition of such recycled gases to the furnace through separate tuyeres presents the problem of managing tuyere cleaning operations without leakage of toxic gases.

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Case 4 of Table 1 illustrates the case for an efficient lead-producing blast furnace according to the present invention whereb' around 25% of the furnace gas is recycled to the furnace with a balance to 23,500Nm /h made up from blast air, water vapour and commercial oxygen, all at 400 C. A gas blower (19) compresses furnace gases from the outlet of gas cleaning fan (11) and routes them via gas line (20) to a direct preheater (21). Fossil fuel (22) and commercial oxygen or a mixture of commercial oxygen and air optionally derived from the blast air system (23), is burnt externally or within the direct preheater (21). The fossil fuel and the furnace gases are preferably completely combusted by adding excess oxygen over the quantity required for theoretical combustion. The combusted gases are then mixed with blast air within the direct preheater (21) to obtain the required temperature and oxygen content of blast passing through blast line (25) to tuyeres (7) with optional addition of oxygen (8). Water or steam (24) can be added to the optimum level to the direct preheater (21) or blast line (25).

It should be noted that our calculations indicate that wet gas cleaning of furnace gases naturally provides around the optimum level of water around 12% by volume in the lead-producing furnace blast gases at the typical exit temperature from the gas cleaner, which is in practice around 50 C. There is therefore no requirement to dry the recycle furnace gases from lead-producing blast furnace wet gas cleaning systems.

It should also be noted that to obtain recycle furnace gases of essentially the same composition as gases leaving the top of the lead-producing blast furnace, a sealed furnace top will be necessary to avoid drawing in dilution air. Such furnace top sealing by means of double bell charging gear, and the like, is a feature of at least onelead-producing blast furnace and all iron-producing and zinc-producing types of blast furnace and is well-known to those experienced in the art. Air infiltration into recycle furnace gases should be avoided to prevent formation of flammable or explosive mixtures.

It will be appreciated that air and commercial oxygen can be added to the preheated furnace gases prior to entry into the furnace because essentially complete combustion of fossil fuel and furnace gases has been achieved. Hence for situations involving recycle of furnace gas to the blast furnaces in this invention, the cold blast air (26) or hot blast air (27) can be mixed with the combusted fossil fuel and furnace gases without danger of explosion or combustion in the blast mains.

Operation of the lead-producing blast furnace, according to the present invention, will commence with cold or preheated air and any commercial oxygen enrichment being blown through the tuyeres in the conventional manner. Furnace gases, when they become available at the exit of the gas cleaning plant, will then be combusted with additional fossil fuel and mixed with the said cold or preheated air, and commercial oxygen, to achieve the required blast temperature, volume and oxygen content. By such means the practical difficulties of adding recycled furnace gases to blast air to the blast furnace will be overcome.

Case 4 of Table 2 illustrates the corresponding inefficient practice for lead-producing blast furnaces according to the present invention. As the percentage of recycled furnace gases increases, the CO/CO2 ratio of the furnace gases decreases. Since the COICO2 ratio is higher for inefficient furnaces then it is possible to recycle more furnace gases before the CO/CO2 ratio is lowered to the practical minimum of around 01. For

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the example of inefficient furnace as shown in Case 4 in Table 2 the amount of furnace gases recycled has been increased to around 47% to achieve a CO/CO2 ratio of 0.07 leaving the furnace, which is close to the practical limit. Lead metal production and lead metal-to-coke ratio are brought significantly closer to those parameters of efficient furnace operation in Case 4 of Table 1. Hence, the performance of inefficiently operating lead-producing blast furnaces can be improved by recycling more combusted furnace gas, a point of great significance for such operations.

Case 3 of Table 3 illustrates the effect of recycling 25% of furnace gases for zinc-producing blast furnaces according to the present invention. Furnace zinc production is estimated to increase from 336t1d to 389t1d. Oxygen enrichment of the blast is required to replace the oxygen consumed from the blast in the complete combustion of the recycled furnace gases and has been adjusted to achieve the same hot coke consumption of 334t/d as Case I in Table 3.

Operation of zinc-producing and iron-producing blast furnaces differs from most lead-producing blast furnaces by preheating the air blast in devices such as Cowper stoves.

Thus, operation of zinc-producing, or iron-producing blast furnaces according to the present invention, will commence with the preheated air and any commercial oxygen being blown through the tuyeres in the conventional manner. Furnace gases when they become available at the exit of the gas cleaning plant will have sufficient calonfic value to be combusted with air, or oxygen-enriched air, although it may be preferable to add some fossil fuel such as methane as a pilot burner to stabilise combustion. The resulting combusted gases will be mixed with the preheated air, and added commercial oxygen, to achieve the required blast temperature, volume and oxygen content.

Existing blast preheat equipment can therefore be retained in the present invention to enable the blast furnace to start up and to supplement directly preheated recycled furnace gases.

Conditions of Operation with Direct Blast Preheat and maximising Furnace Gas Recycle The above cases have considered operation of the lead-producing blast furnace at a maximum preheated blast volume temperature of 400 C due to the limitation brought about by the construction materials of the blast equipment. It is usual in zinc-producing and iron-producing blast furnace operations for the blast volume to be preheated to much higher temperatures to gain higher efficiencies. Such higher temperatures are obtained by constructing the blast mains from refractory and high heat resistance, or water-cooled, steel components. Preheat temperatures above I,000C are used in these applications.

Case 5 of Table I illustrates the case for efficient lead-producing blast furnace operation according to the present invention whereby the blast volume is preheated to 800 C with maximum furnace gas recycle around 39% of the total to give a CO/CO2 ratio in gases leaving the furnace close to the lower practical limit. Lead metal production is seen to increase to around 11 37tpd, an increase of 51% on base case efficient operation with air blast and oxygen enrichment of Case I in Table 1. Coke in charge materials reduces to give an overall improvement in the (lead metal/actual coke) ratio of 29%.

Case 5 of Table 2 illustrates the corresponding inefficient practice according to the present invention with maximum furnace gas recycle around 50% corresponding to

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effectively nil blast air addition to the furnace. This case gives the limit of inefficient furnace performance for recycle of combusted furnace gases without separation of gaseous species. Lead metal production is seen to increase to around 997tpd, an increase of 76% on base case inefficient operation with air blast and oxygen enrichment of Case I in Table 2. Coke in charge reduces to give an overall improvement in the (lead metal/actual coke) ratio of 41%. Thus there is an increasing advantage from the recycling of furnace gases as furnace operation becomes more inefficient by virtue of poorer quality feed materials.

In all the above cases the furnace blowing rate has been maintained at a constant level of 23,500Nm3/h for purposes of comparison. It will be understood that some lead-producing furnace operations may have the capability to increase furnace blowing rates above the representative level of 23,500Nm3/h and some may not be able to reach this level. Such variances will have a corresponding effect on the parameters indicated in the above cases.

Case 4 of Table 3 illustrates the effect of recycling the maximum of furnace gases of around 52% for zinc-producing blast furnaces according to the present invention. At this level of recycle, zero blast air is required at the tuyeres but air is still used for air added at the top of the furnace -a feature of zinc blast furnace operation. Furnace zinc production has increased to 492t/d an increase of 46% on base case operation of Case I in Table 3. Zinc production per unit of coke has increased to 1.88 an increase of 63% compared to base case operation.

It is clear from the foregoing examples that considerable benefits accrue to ferrous and non-ferrous blast furnace operation by the direct combustion of fossil fuels and recycled furnace gases in combustion devices external to the blast furnace. Such operation is a novel departure from the existing art and provides the basis for our patent request.

Claims

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CLAIMS

1. A method of operating a ferrous or non-ferrous producing blast furnace comprising the steps of charging carbon-containing materials and oxidic materials at the top of the furnace and introducing at the bottom of the furnace proportions of blast air commercial oxygen and directly preheated gases from the direct combustion of fuels.

2. A method according to Claim I in which the directly preheated gases are provided by the direct combustion of solid liquid or gaseous fossil fuels.

3. A method according to Claim 2 in which fossil fuels supplement the operation of blast furnaces without the need to inject such fossil fuels directly into the blast furnace.

4. A method according to Claim 1 in which the directly preheated gases are provided by the direct combustion of recycled furnace gases.

5. A method according to Claim I in which the directly preheated gases are provided by a mix of combusted fossil fuels and combusted recycled furnace gases.

6. A method according to the above Claims in which the direct combustion is accomplished in an apparatus such as a combustion chamber external to the blast furnace.

7. A method according to Claim 6 in which a proportion of blast air is added to the combustion apparatus to combust the fuel or control the combustion temperature or both.

8. A method according to Claims 6 and 7 in which the combusted and heated gases from the combustion apparatus mix with supplementary cool or preheated blast air prior to entry to the blast furnace.

9. A method according to Claim 8 in which the supplementary blast air is directly preheated or cooled by mixing with the directly combusted and heated gases.

10. A method according to Claim I in which the total preheated furnace blast is produced in the range 100 C to 1,300 C.

11. A method according to Claim 4 in which the water content of the recycled furnace gases is naturally controlled around the optimum level for improved furnace operation by the temperature of the saturated recycled furnace gases typically 50 C.

12. A method according to Claims 4 and 5 in which gaseous species within recycled furnace gases are not removed from the said gases before combustion and preheating.

13. A method according to Claims 2, 4 and 5 in which water or steam is added to the directly combusted gases to achieve the optimum for improved furnace operation.

14. A method according to the above Claims in which oxygen is added at a suitable point or suitable points in the supply of blast furnace air and directly preheated gases to control the furnace heat balance and maintain the blast furnace within its maximum blowing rate.

15. A method according to the above Claims in which blast furnace gas recycle to the furnace is maximised.

16. A method according to Claim 15 in which the maximum recycle of blast furnace gases is determined by the practical limit of the carbon monoxide to carbon dioxide ratio leaving the specific furnace type or by reaching the limit of zero blast air.

17. A method according to the above Claims in which metal production of blast furnaces is maximised ranging from an increase of 30% to 80% depending on the type of blast furnace and its efficiency.

18. A method according to the above Claims in which metal-to-coke ratio of ferrous or non-ferrous producing blast furnaces is maximised ranging from an increase of 25% to 65% depending on the type of blast furnace and its efficiency.

19. A method according to the above Claims in which Greenhouse Gas emissions per unit of metal produced are reduced by comparison with current operations by up to 30% depending on the type of blast furnace and efficiency of operation.