GB2427200A - Methods of operating a lead-producing blast furnace - Google Patents

Abstract

In a first method of operating a lead-producing blast furnace 1, solid carbon-containing materials 3 and oxidic lead-containing materials 2 are charged into the top of the furnace 1 and a blast of directly preheated air 4 and commercial oxygen 7 are introduced into the bottom of the furnace 1. In a second method carbon-containing materials 3 and oxidic lead-containing materials 2 are charged into the top of the furnace 1 and a blast of preheated air 4, preheated furnace gas 8 and commercial oxygen 7 are introduced into the bottom of the furnace 1. In both methods preheating is done using a fossil fuel 20 and 28 and furnace gas 8 to heat to a temperature of 100-1000 {C. Water or steam 24 and 31 may be added to the blast in the first method and the recycled furnace gas in the second method.

Description

DESCRIPTION

Operation of lead-producing blast furnaces This invention relates to leadproducing 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 lead produced, compared with the prior art in which lead is smelted with lowtemperature blast air with additional limited quantities of pure oxygen.

In a lead-producing blast furnace, as customarily operated, the charge material consists of oxidised plumbiferous material and metallurgical coke. Coke is fed cold and oxidised plumbiferous material may be fed at up to 300 C. Fluxing agents, such as lime or silica, are incorporated with the plumbiferous charge in order to provide a fluid slag of low lead content for tapping from the furnace hearth. Blast air at low temperature is introduced through tuyeres near the bottom of the furnace. The air is normally at a temperature determined by the heat of compression after the air blower, say 50 C. The blast air can also be preheated if suitable heating equipment is available, such as a metallic tube recuperator. Fossil fuel, such as oil or natural gas, is necessary to indirectly preheat the air because there is insufficient heating value in the furnace offgas to use it as a fuel. Generally speaking, operators of leadproducing blast furnaces do not provide additional blast air preheating due to the additional expense of so-doing. It would also be necessary to restrict the blast temperature to less than 400 C to meet materials of construction limits, unless the blast air supply mains were lined with refractory materials and the blast air tuyeres into the furnace were water-cooled.

In a lead-producing 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. The ratio of carbon monoxide to carbon dioxide leaving the furnace charge 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+ 02=C02 (1) C02+C=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)

I

Higher up in the furnace, lead in oxidic form is reduced to metallic lead (equation 5).

PbO+COPb+C02 (5) Carbon dioxide formed by reactions 1, 4, and 5 can further react with carbon higher in the shaft to regenerate carbon monoxide (as equation 2). In practice, most of the CO is formed near the furnace bottom.

In customary good practice, the volumetric carbon monoxide to carbon dioxide ratio leaving the lead-producing blast furnace charge is in the order of 0.10 to 0.30 representing a practical limit to the utilisation of carbon. Moreover, the addition of oxygen to the blast air 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 leadproducing blast furnace increases, with consequent waste of carbon units.

In recent years, the lead-producing blast furnace has come under increasing pressure from new lead-producing technologies which have increased the intensity of lead production and enabled the direct smelting of sulphidic lead materials without first passing through an oxidation step. The lead-producing blast furnace requires an oxidic feed which is prepared on a sinter machine that is expensive to operate both from mechanical and environmental aspects. It also requires strong metallurgical coke to provide a porous furnace burden to enable the passage of furnace gases. Despite these shortcomings, the lead-producing blast furnace is still a major source of lead. lts main technical limitation is the use of metallurgical coke which is expensive and suffering supply limitations. Any means of increasing lead 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 lead-producing blast furnace throughput and/or lead production per unit of solid carbon consumed can be considerably increased by the direct heating of blast air, the addition of steam, and the recycling of furnace gases to the furnace. In all cases it is necessary to add essentially pure, commercial oxygen to replace the oxygen consumed in direct preheating and to compensate for reduced quantities of blast air when furnace gases are recycled. In cases where plumbiferOuS feed materials are limited in supply by, for example, sinter machine operation, improved carbon efficiency can still be obtained at the maximum plumbiferoUS feed availability.

Alternatively, where multiple blast furnaces are operated, it may prove possible to shut down one or more furnaces without losing throughput.

A metallurgical model has been constructed to replicate the performance of the lead- producing blast furnace process and has been validated against customary practice.

The results of the case studies are presented in Tables I (efficient operation) and 2 (inefficient operation) in which the oxygen, blast air and recycled furnace gases have been adjusted to maintain a constant blast volumetric rate for ease of comparison. In some cases it would be possible to increase the oxygen addition to increase lead production, but these cases may increase coke consumption and reduce lead-to-coke ratio. The model shows that the use of direct preheating is more efficient than indirect heating due to increased levels of CO2 and H20 in the blast volume and the effect these have on 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.

It should be noted that the lead-producing blast furnace produces small active zones (racewayS) in front of the tuyeres, such that fuel in the form of solids or liquids injected into the bottom of the furnace, for example pulverised coal, pulverised coke, liquid hydrocarbonS fuel-water emulsions etc, will not completely combust due to insufficient residence time in the raceway. For these reasons no case studies with injected solid and liquid fuels have been presented in Tables I and 2, due to lack of practicality.

INTRODUCTION TO DRAWING

A schematic end-elevation of a lead-producing blast furnace is shown as reference (1) on the drawing. The furnace is fed from the top with plumbiferous material and fluxes (2) and solid carbonaceous material, usually metallurgical coke (3). Air (4) is compressed by a blower (5) and admitted to the bottom of the furnace through tuyeres (6) with typically 18 tuyeres on each long side of the approximately rectangular furnace bottom. Oxygen (7) 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 (8) are withdrawn from the top of the lead-producing blast furnace and admitted to a gas cleaning system (9) to remove fume and particles. 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 (10) which exhausts to a stack (11). 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/CO2 Commercial Gas flow prodn in Coke equivalent ratio Oxygen to tuyereS t/day charge ratio coke leaving consumption Nm3/h __________ ______ %w/w _______ wt ratio furnace Nm3Ih _________ I Base Case 649 8.08 5.27 5.27 0.36 0 23500 - Efficient ______ furnace ______ ________ ________ ___________ ________ _____________ 2 Base Case 752 7.65 5.59 5.59 0.26 849 23500 with 2.5% v/v 02 ______ enrichment ______ _______ _______ _________ ________ ___________ 3 As Case 2 838 7.30 5.88 5.46 0.32 849 23500

INDIRECT

heating to _____ 400 deg C _____ _____ _____ _______ ______ _________ _______ 4 As Case 2 845 7.22 5.95 5.58 0.26 1862 23500 with

DIRECT

heating to _ 400 C _ _ __ __ As Case 2 855 7.14 6.03 5.60 0.22 2303 23500 with

DIRECT

heating to 400 C and water _____ added ______ _______ _______ _________ ________ ___________ _________ 6 25% 896 6.55 6.61 6.17 0.11 4075 23500 furnace gas recycle,

DIRECT

heating to ______ 400 C ______ _______ _______ __________ ________ ____________ _________ 7 Max (38%) 1044 5.94 7.34 6.35 0.09 6468 23500 furnace gas recycle

DIRECT

heating to ______ 800 C ______ _______ _______ __________ ________ ____________ _________ 8 Maxfurnace 1111 5.35 8.21 7.14 0.17 7024 23500 gas recycle (97%) with Co2 removal.

DIRECT

heating to ______ 800 C _______ ________ _______ ___________ _________ _____________ __________ TABLE 2- LEAD BLAST FURNACE CASE STUDIES Inefficient Operation Case Description Lead Coke Lead! Lead! CO/CO2 Commercial Gas flow prodn in Coke equivalent ratio Oxygen to tuyeres t/day charge ratio coke wt leaving consumption Nm3/h ______ __________ ______ % wlw _____ ratio furnace Nm3/h ________ I Base Case 487 10.20 3.74 3.74 0.78 0 23500 - Inefficient ______ furnace ______ _______ ______ ___________ _________ _____________ _________ 2 Base Case 567 9.62 3.99 3.99 0.62 849 23500 with 2.5% v/v 02 ______ enrichment ______ _______ _____ _________ ________ ___________ ________ 3 As Case 2 635 9.13 4.22 3.94 0.72 849 23500

INDIRECT

heating to _ 4OC ___________ 4 AsCase2 645 8.98 4.30 4.05 0.59 1862 23500

DIRECT

heating to _ 4OC ___________ As Case 2 665 8.76 4.42 4.11 0.45 2455 23500

DIRECT

heating to 400 C and water _____ added ______ ______ ______ __________ _______ ___________ ________ 6 25% 698 8.05 4.85 4.55 0.35 4301 23500 furnace gas recycle,

DIRECT

heating to _____ 400 C _____ ______ _____ _________ _______ __________ ________ 7 Max (45%) 765 6.95 5.69 5.29 0.15 5876 23500 furnace gas recycle

DIRECT

heating to ______ 400 C ______ _______ ______ __________ ________ ____________ _________ 8 Max (54%) 896 6.33 6.28 5.37 0.15 7873 23500 furnace gas recycle

DIRECT

heating to ______ 800 C ______ ________ ______ ___________ _________ _____________ __________

EXAMPLES

Conditions of Operation with Normal Air Blast It is clear from the above description that lead-producing blast furnace throughput is dependent on the size of the furnace, a larger furnace being more productive. Thus Case I of Table I illustrates the performance of a large and efficient furnace according to the present art, with 8% by weight coke in the charge, blowing 23,500Nm3Ih cool air with warm feed materials to produce around 65Otpd of lead metal. The carbon monoxide to carbon dioxide ratio (CO/CO2 ratio) of 0.36 leaving the furnace reflects relatively good quality coke and metalliferous 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. With even higher quality feed materials, the CO/CO2 ratio can be lowered to around 0.10 to 0.15 and lead production correspondingly increased to around 700tpd, but furnaces working at this efficiency limit are relatively few.

By comparison Case I of Table 2 illustrates the performance of the same size blast furnace according to the present art when operating inefficiently with 10.2% coke in the charge, for the same blowing rate of cool air, producing around 49Otpd of lead metal.

The CO/CO2 ratio of 0.78 leaving the furnace reflects relatively poor quality coke and metalliferous charge materials, hence the inefficient operation.

Conditions of Operation with Oxygen-enriched Blast Case 2 of Table I illustrates efficient practice according to the present art, with 2.5 percentage points of commercial oxygen enrichment whilst maintaining the same blowing rate of 23,500Nm3/h of oxygen-enriched cool air. In this context commercial oxygen is the essentially pure oxygen produced by a commercial oxygen plant. Coke consumption increases and CO/CO2 ratio falls to give increased lead metal production around 75Otpd at an improved coke in charge around 7.6% by weight.

Case 2 of Table 2 illustrates the corresponding inefficient practice according to the present art, whereby the coke in charge improves to around 9.6% and lead metal production increases to around 57Otpd.

Conditions of Operation with Indirect Blast Preheat to 400 C In most leadproducing 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, however, involves the use of additional fuel as the gases leaving the furnace are not sufficiently high in calorific value to be used for this purpose.

Case 3 of Table I 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 given by the construction in steel of the blowing system. Blast air from the air blower (5) is routed through blast line (12) to indirect air heater (13).

Fossil fuel (14) and combustion air (15) are burnt and the combustion gases passed through heat exchange tubes in the blast air stream before passing to atmosphere (16).

Preheated blast air is passed through blast line (17) to tuyeres (6) with optional addition of oxygen (7).

Lead metal production increases to around 84Otpd at an improved coke in charge around 7.3%, but the CO/CO2 ratio increases somewhat over Case 2 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-to-equivalent coke ratio decreases compared with Case 2.

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 3 of Table 2 illustrates the corresponding inefficient practice according to the known art with the noticed deterioration in lead metalto-equivalent coke ratio compared with Case 2 of Table 2.

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 4 of Table I illustrates the case according to the present invention whereby the blast is directly preheated to 400 C by burning fossil fuel such as oil or natural gas 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 2 and 3. Blast air from the air blower (5) is routed through blast line (18) to direct air heater (19). Fossil fuel (20) and combustion air (21) optionally derived from the blast air, is burnt externally or within the direct air heater (19). Commercial oxygen (22) is optionally added to combustion air (21) or directly to the direct air heater (19). The combustion gases pass into the blast air stream and through blast line (23) to tuyeres (6) with optional addition of commercial oxygen (7).

It will be noticed that the CO/CO2 ratio in the gases leaving the furnace improves over Case 3 and there are small beneficial changes in lead metal production and lead metal- to-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 it replaces and increase 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 3 so the charge is more effectively preheated. Fourth, the water vapour reacts with carbon monoxide exothermally, thus increasing the energy input at the furnace bottom.

Case 4 of Table 2 illustrates the corresponding inefficient practice according to the present invention but with larger beneficial changes in lead metal production and lead metal-to-coke ratio.

Conditions of Operation with Direct Blast Preheat to 400 C and Optimum Water Addition Case 5 of Table I illustrates the case according to the present invention whereby the blast stream is directly preheated to 400 C by the means of Case 4 and adding water to generate around 12% steam by volume in the blast such as to optimise the improvement in the production parameters. Water (24) can be added to the direct air heater (19) or blast stream (23) as a fine liquid spray, or as steam. When added as liquid an additional quantity of fossil fuel will be required to raise the water to steam at 400 C. The commercial oxygen volume increases somewhat to compensate for the oxygen in air displaced by the water, and the CO/CO2 ratio again decreases to improve the production parameters.

Case 5 of Table 2 illustrates the corresponding inefficient practice according to the present invention but with larger beneficial changes in lead metal production and lead metal-to-coke ratio.

Conditions of Operation with Direct Blast Preheat to 400 C and Partial Furnace Gas Recycle It has been proposed in the prior iron 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 FR0215316, dated 2002- 12-04). It has also been proposed that furnace gases be partially recyded 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 6 of Table I illustrates the case according to the present invention whereby around 25% of the furnace gas is recycled to the furnace with a balance to 23,500Nm3/h made up from blast air, water vapour and commercial oxygen, all at 400 C. A gas blower (25) compresses furnace gases from the outlet of gas cleaning fan (10) and routes them via gas line (26) to a preheater, preferably a direct preheater (27). Fossil fuel (28) and combustion air (29) optionally derived from the blast air system, is burnt externally or within the direct air heater (27). The combustion gases pass into the recycle furnace gas stream and through blast line (30) to tuyeres (6) with optional addition of oxygen (7). Water or steam (31) can be added to the optimum level between 12% and 20% by volume to the preheater or gas line. It should be noted that wet gas cleaning of furnace gases is preferred as this naturally provides around the optimum level of water in the furnace gases at the corresponding exit temperature from the cleaner between 50 C and 60 C. 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 sealing by means of double bell charging gear, and the like, is a feature of other types of blast furnace and is well-known to those experienced in the art.

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 recycle of furnace gas to the leadproducing blast furnace, the blast air and furnace gas streams are preheated in separate preheaters (19) and (27) respectively. A proportion of the furnace tuyeres is allocated to furnace gas with the remainder allocated to blast air. Commercial oxygen is added to the recycled furnace gas at the entry to the furnace (7), and to the blast air either prior to entry to the furnace, for example at the preheater (22), or at the entry to the furnace (7). It will be observed that the recycle of around 25% of furnace gases enables the CO/CO2 ratio in furnace gases leaving the furnace to be lowered to a level around 0.10 to 0.15 considered close to the practical limit of carbon economy.

Lead-producing blast furnaces are typically provided with thirty-six, or more, tuyeres with half on each long side of a rectangular-sided furnace bottom. It is possible to segregate tuyeres, for example, such that one tuyere in 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 a Y-piece connection in the tuyere lead with valves inserted in each leg of the Y-piece to control passage of gases from one main or the other.

Operation of the lead-producing blast furnace, according to the present invention, will commence with preheated air and commercial oxygen enrichment being blown through that proportion of the tuyeres allocated to oxygen-containing gases. Furnace gases, when they become available, will then be preheated and routed to the selected tuyeres with separate oxygen enrichment. In cases where furnace gases comprise a high proportion, or all, of the total blast volume, tuyeres initially allocated to blast air will be changed-over to furnace gases by means of the Y-piece connection mentioned above.

By such means, the practical difficulties of changing from blast air to furnace gas in tuyeres will be overcome. The main consideration during normal operation will be to operate all tuyeres with equal carbon burning rates by controlling commercial oxygen additions in order to maintain uniform charge descent in the furnace.

Case 6 of Table 2 illustrates the corresponding inefficient practice according to the present invention but with a higher CO/CO2 ratio in gases leaving the furnace than for efficient practice. Whereas the CO/CO2 ratio of the efficient practice of Case 6 in Table 1 is dose to the practical minimum of 0.10 to 0.15, that of the corresponding inefficient practice is still relatively high at around 0. 35. It has been found that by increasing the quantity of furnace gas recycled to the furnace to a level around 45% of the total furnace gas leaving the furnace, as illustrated in Case 7 of Table 2 according to the present invention, that the CO/CO2 ratio can be lowered to dose to the practical minimum. Lead metal production and lead metal-to-coke ratio are brought significantly closer to those parameters of efficient furnace operation in Case 6 of Table 1. Hence, the performance of inefficiently operating lead-producing blast furnaces can be improved to a greater level by recycling more furnace gas, a point of great significance for such operations.

Conditions of Operation with Direct Blast Preheat to 800 C and 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 mains. It is usual in other 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 steel components. The size of the mains becomes larger at higher temperatures due to the larger volume of gases and the thickness of refractory insulation in the blast mains. Preheat temperatures above I, 000C are used in other applications.

Case 7 of Table I illustrates the case according to the present invention whereby the blast volume is preheated to 800 C with maximum furnace gas recycle around 38% of the total to give a CO/CO2 ratio in gases leaving the furnace close to the lower practical limit around 0.10. This case gives the limit of efficient furnace performance for recycle of furnace gases without separation of gaseous species. Lead metal production is seen to increase to around I O5Otpd, an increase of 39% on base case efficient operation with air blast and oxygen enrichment of Case 2 in Table 1. Coke in charge reduces to give an overall improvement in the lead metal-to-coke ratio of 50%.

Case 8 of Table 2 illustrates the corresponding inefficient practice according to the present invention with maximum furnace gas recycle around 54% corresponding to effectively nil blast air addition to the furnace. This case gives the limit of inefficient furnace performance for recycle of furnace gases without separation of gaseous species. Lead metal production is seen to increase to around 900tpd, an increase of 58% on base case inefficient operation with air blast and oxygen ennchment of Case 2 in Table 2. Coke in charge reduces to give an overall improvement in the lead metal-to- coke ratio of 88%. 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.

Conditions of Operation with Direct Blast Preheat to 800 C and Furnace Gas Recycle after Carbon Dioxide Removal As stated above, it is known in the art for proposals to be made to remove carbon dioxide from furnace gases before recyding high carbon monoxide-containing gases to a blast furnace. It has not been necessary in the above cases to introduce gas separation to effect considerable improvement in lead-producing blast furnace operation.

Case 8 in Table 1 illustrates the case according to proposals in the present art whereby carbon dioxide is removed from furnace gases and the resulting enriched carbon monoxide gases are recycled to an efficient furnace. Indications are that effectively all of the carbon dioxide-free furnace gas can be recycled if just over half the carbon dioxide isremoved to give around 6% increase in lead metal production over Case 7 of Table 1. Coke in charge also reduces to give an overall improvement in the lead metal- to-coke ratio around 18%.

There is no corresponding case for inefficient lead-producing blast furnace operation because removal of carbon dioxide is not predicted to improve lead metal production or lead-to-coke ratio. This is believed to be due to the fact that nil air blast has already been achieved for inefficient furnace operation without removal of carbon dioxide.

Hence the benefit to a lead-producing blast furnace of enriching the carbon monoxide in recycled furnace gas is only likely to be of benefit for the most efficient furnace operations. It is considered unlikely to be of sufficient advantage in a majority of cases to justify the installation of furnace gas separation technologies.

In all the above cases the furnace blowing rate has been maintained at a constant level of 23,500Nm3Ih 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.

Claims (21)

1. A method of operating a lead-producing blast furnace comprising the steps of charging solid carbon-containing materials and oxidic leadcontaining materials at the top of the furnace and introducing a blast of directly preheated air, and commercial oxygen, at the bottom of the furnace.

2. A method according to Claim 1, in which water or steam is added to the blast within the range 5% to 20% by volume, more preferably 10% to 15% by volume.

3. A method according to Claims I or 2, in which the blast is directly preheated with a fossil fuel such as natural gas, oil, coal, producer gas, and low calorilic value furnace gas.

4. A method according to Claim 3, in which the blast is preheated to a temperature within the operating range of normal blast main components, from 100 C to typically 400 C maximum.

5. A method according to Claim 3, in which the blast is preheated to a temperature in the range 100 C to 1000 C.

6. A method according to Claim 5 in which commercial oxygen is added with the preheating fuel to preheat the blast air.

7. A method according to Claim 5 in which commercial oxygen is added to the preheated blast air after preheating with fuel.

8. A method according to Claim 5 in which commercial oxygen is added to the lead- producing blast furnace separately from the preheated blast air, through tubes or lances either inserted within the preheated blast air tuyeres or in tubes or lances external to each preheated blast air tuyere.

9. A method of operating a lead-producing blast furnace comprising the steps of charging carbon-containing materials and oxidic lead-containing materials at the top of the furnace and introducing a blast of preheated air and a proportion of preheated furnace gases, and commercial oxygen, at the bottom of the furnace.

10. A method according to Claims 1, 2, 3, 4, 5, 6, 7, and 8, of producing preheated air for use in conjunction with preheated furnace gases.

11. A method according to Claims 9 and 10, in which the blast air and recycled furnace gases are directly preheated in separate apparatus, such as combustion chambers or ducting with attached combustion chambers, prior to entering the lead- producing blast furnace.

12. A method according to Claims 9, or 11, in which water or steam is added to the recycled furnace gases within the range 5% to 20% by volume, more preferably 10% to 15% by volume.

13. A method according to Claim 12 in which water is added to the recycled furnace gases at a level controlled by the temperature of the saturated recycled gases, typically 50 C.

14. A method according to Claims 9, or 11, or 12, or 13, in which the recycled furnace gases, before preheating, have essentially the same ratio of carbon monoxide, carbon dioxide, hydrogen, and nitrogen, as furnace gases leaving the top of the lead- producing blast furnace charge line.

15. A method according to Claim 14 in which a maximum of lead-producing blast furnace gases is recycled to the furnace dependent on the efficiency and preheat of the furnace, ranging from maxima of 20% to 40% for an efficient furnace, and 40% to 60% for an inefficient furnace.

16. A method according to Claim 15 in which performance of an efficient lead- producing blast furnace is maximised by around 40% increase in lead production and around 50% improved lead metal-to-coke ratio at a blast volume preheat around 800 C.

17. A method according to Claim 15 in which performance of an inefficient lead- producing blast furnace is maximised by around 60% increased lead production and around 90% improved lead metal-to-coke ratio at a blast volume preheat around 800 C.

18. A method according to Claim 14 in which recycled furnace gases are preheated with fossil fuel, such as natural gas, oil, coal, producer gas, and low calorific value furnace gas, including lead blast furnace gas.

19. A method according to Claim 18 in which the preheating fuel is combusted with preheated blast air, or commercial oxygen, or both, to a temperature within the operating range of normal blast main components, from 100 C to typically 400 C maximum.

20. A method according to Claim 19 in which preheated furnace gases are produced in the range 100 C to 1,000 C.

21. A method according to Claim 19 in which commercial oxygen is added to the lead-producing blast furnace separately from the preheated recycled furnace gases, through tubes or lances either inserted within the preheated furnace gas tuyeres or in tubes or lances external to each preheated furnace gas tuyere.