* * * *

ISSN 1018-5593

European Commission

technical steel research

Reduction of iron ores

Technical study into the means of prolonging blast furnace campaign life

STEEL RESEARCH

European Commission

technical steel research Reduction of iron ores

Technical study into the means of prolonging blast furnace campaign life

D. Jameson, H. Lungen, D. Lao British Steel pic

9 Albert Embankment SE1 7SN

United Kingdom

Contract No 7210-ZZ/570

1 June to 1 December 1995

Final report

Directorate-General Science, Research and Development

1997 EUR 17247 EN

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Luxembourg: Office for Official Publications of the European Communities, 1997

ISBN 92-827-9912-3

European Communities, 1997 Reproduction is authorized, except for commercial purposes, provided the source is acknowledged

Printed in Luxembourg

CONTENTS Page

OBJECTIVE 5

INTRODUCTION 5

3. OPERATIONAL PRACTICES

3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9

Productivity Burden Ti02 Addition Instrumentation and Control Tuyere Diameter Off-blast Periods Casthouse Practices Production Rules References

9 10 17 19 26 26 29 32 33

4. REMEDIAL ACTIONS 77

4.1 Gun/Spray 77 4.2 Enhanced Cooling 78 4.3 Grouting and Welding of Shell 80 4.4 Replacing of Staves and Coolers 81 4.5 Ancillary Equipment 82 4.6 References 88

5. FUTURE DESIGNS 105

5.1 Fourth Generation Staves 105 5.2 Copper Staves 1 5 5.3 Improved Refractories 107 5.4 More Comprehensive and Reliable Instrumentation 110 5.5 Throat Cooling 110 5.6 Improved Hearth and Taphole Design 110 5.7 Furnace Designs and Dimensions 112 5.8 References 118

CONCLUSIONS 133

PROLONGING BLAST FURNACE CAMPAIGN LIFE

British Steel pic

ECSC Agreement No. 7210.ZZ/570

Final Technical Report

1. OBJECTIVE

The objective of this study was to carry out a comprehensive literature review of techniques used to extend the campaign life of blast furnaces and identify important aspects of operational practices, remedial actions and future plant design, to establish cost effective methods of extending European blast furnace campaign lives.

2. INTRODUCTION

In recent years there has been an increasing requirement to extend blast furnace campaign life:

In order to reduce hot metal costs, by reducing manpower requirements and capital costs, steel companies have increased the size and reduced the number of their blast furnaces. In many cases there is no longer a standby furnace to be brought into operation during blast furnace rebuilds11'. Consequently, long campaigns with minimum reline periods are essential.

The cost of rebuilding or relining a blast furnace can be very high and may represent a large proportion of the total capital expenditure available to a company. In recent years, the level of subsidy to the steel industry has fallen, following an increasing degree of modernisation and privatisation. When funding for capital projects relies directly on business profits, the cyclical nature in steel demand and prices will have a major effect on the capital available, and it may be necessary to extend plant life until improved trading conditions are predicted. In addition, the priority for capital expenditure has tended to move towards the finishing end to improve product quality and range, to increase sales and enhance profit directly. Consequently, this results n a reduction in the proportion of capital available in the primary end.

Techniques to extend blast furnace campaign lives have often been pursued more actively outside Europe, notably in Japan, where campaign lives have been steadily increasing since the 1970's. Figure 2.1(2) indicates the operational results of Japanese blast furnaces of over 2000 m3 inner volume for campaigns completed between 1970 and 1981. Towards the end of that period the longest campaign was 7 years, with the production per unit volume approaching 5000 t/m3. Figure 2.2(3) dates from the mid 1980's and shows that there was an increasing number of furnaces, many then still operating, with a campaign output above the 5000 t/m3 maximum indicated in Fig. 2.1. More recent data, from a variety of technical literature, are presented in Fig. 2.3 which illustrates how the campaign output of Japanese furnaces has increased further, with a figure of 10000t/m3 being exceeded in 1993. Completed campaign lives are typically 11-13 years, but these values have been exceeded by furnaces whose campaigns are yet to be completed.

Using information from recent EBFC Blast Furnace Constructional Features compilations, Fig. 2.4 compares the campaign output of European and Japanese furnaces over a similar time period. Although the best European examples are approaching Japanese levels, in general the campaign life of European furnaces is significantly lower.

A shorter campaign life may result from a lower level of capital investment, as rebuilding a blast furnace for a longer campaign life involves additional capital cost. It could also result from a lower level of development of new equipment and techniques, or from inferior operation. Figure 2.2 suggests that larger furnaces tend to have a slightly higher campaign output per unit volume. This difference may result from larger furnaces generally being of more modern design or because the cost of extending the life of a large furnace is lower per tonne of output gained and therefore is more economic than for a smaller furnace. The viability of an integrated steelworks depends on a continuous supply of hot metal, which on a works with a small number of large furnaces puts great importance on long campaign life. Such furnaces are likely to be well instrumented and operated to achieve this aim. Since Japan tends to have a greater proportion of large furnaces, this could explain some of the difference between Japanese and European campaign outputs.

Traditionally, the limiting factor to blast furnace campaign life was damage in the lower shaft13'. With improvements in operational practices (e.g. improved burden distribution control), remedial actions (e.g. stave replacement, gunning, etc.) and improved designs (e.g. improved cooling and refractories), there are fewer problems in this area. Consequently, other regions which used to have less effect on furnace life become more critical, e.g. throat armour and upper stack refractories. Ultimately, as techniques are developed to extend the life of, and repair, the various regions of the furnace, the most critical region becomes the most difficult region to repair, which is generally considered to be the furnace hearth.

The techniques for prolongation of blast furnace campaign life will be reported in three categories:

Operational Practices - The control of the blast furnace process has a major effect on the life of the furnace. The furnace must be operated in a manner to maximise furnace life, compatible with production requirements. It will often be necessary to modify operating practices as the campaign progresses and in response to problem areas, to maximise the furnace life.

Remedial Actions - Once wear or damage that may affect the life of the furnace becomes evident, engineering repair techniques must be utilised or developed to maximise campaign life.

Future, Improved Designs - As improved materials and equipment are developed, these should be incorporated into future rebuilds to extend the life of critical areas of the furnace, where it is cost effective to do so.

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RELATION BETWEEN SERVICE LIFE AND PRODUCTION OF BLAST FURNACES HAVING 2000 m 3 OR MORE

INNER VOLUME IN JAPAN

FIG. 2.1

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INNER VOLUME vs CUMULATIVE PRODUCTION OF BF BLOWN IN BETWEEN '65 TO '80

FIG. 2.2

JAPANESE B.F. OUTPUT PER UNIT VOLUME Improvements with time

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FIG. 2.3

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FIG. 2.4

3. OPERATIONAL PRACTICES

3.1 Productivity

The productivity of a blast furnace can have an important effect on campaign life. It is normally expressed as an index, in tonnes of hot metal per unit furnace volume per day.

High productivity involves an increased throughput of material at higher burden descent rates, with an increased hearth activity to remove the larger quantity of molten products. The stability of operation may be affected when the furnace is being driven hard; burden descent may be less smooth and the melting zone may be higher in the furnace, both of which may affect wall wear. The increased throughput of molten products may accelerate hearth wear and present more arduous taphole conditions.

Low productivity may involve extended periods of low blast volume, which may result in reduced blast penetration and an increased gas flow up the furnace wall, unless suitable modifications to burden distribution are made. Long production pauses may have a detrimental effect on hearth condition.

The above factors will be discussed individually, in more detail, in subsequent sections. However, this brief discussion of productivity serves to illustrate that there are many interacting factors involved and it is hardly surprising that opinions differ as to the effect of productivity on campaign life.

Kawasaki Steel's Chiba No. 6 furnace (4500 m3 Inner Volume) has been operating since 1977, with a total production exceeding 52 Mt. During such a long campaign, changing steel demand and energy prices have dictated levels of productivity between 1.5 and 2.3 t/m3 I.V./d(4). The average productivity has been below 2 t/m3 I.V./d and a productivity of 2.2 t/m3 I.V./d was only exceeded for two periods: one period of two years in the early stages of the campaign and one of 6 months in the 14th year of the campaign (during No. 5 furnace reline). The reduced production levels during the majority of the campaign may have contributed towards stable operation and long life. As a result of the measures taken, a stable, high level of productivity was possible during the later stages of the campaign, that did not appear to jeopardise the campaign life.

Nisshin Steel's Kure No.1 blast furnace (2650m3 I.V.), has been operated for over 9 years of its campaign (1984-1994) at an average productivity of 2.29 t/m31.V./d. The loss of shaft brickwork in 1987 resulted in high heat loads and stave damage but, after stack grouting in 1988 and upper stack gunning in 1989, the furnace operation has been stable, with 89 consecutive months operation with productivity exceeding 2.2 t/m31.V./d(5>. Figure 3.1 illustrates this productivity and shows that the furnace stability in terms of slip index was poorer during the early part of the campaign. After grouting and gunning, and a slight reduction in productivity, a more stable operation has been achieved for several years.

Nippon Steel's Kimitsu No. 4 blast furnace (4930m3 I.V.) produced 38 Mt during its last campaign (1975-1986), at an average productivity of just under 2 t/m3 I.V./d. Productivity exceeded the average value for only two years in the middle of the campaign'6'.

Sumitomo Metal Industries' Kashima No. 3 blast furnace (5050m3 I.V.) had produced 48 Mt in over 13 years, when it was blown out in 1990. The productivity of the furnace was consistently around 2 t/m3 I.V./d for the whole of its campaign171. Operation remained stable following the application of repair techniques and operational improvements.

The average productivity of the present campaign of British Steel's Redcar No. 1 blast furnace (4308m3 I.V.) is 2.1 t/m31.VVd. During the early years of the campaign, higher productivities were being achieved'8'. Between the third and fifth years of the campaign, some of the hearth sidewall thermocouples showed a marked rise in temperature. Corrective action was taken to reduce the temperatures, and hence hearth wear, with a series of operational practice changes which included a daily productivity limit of 9200 t/d on the 10000 t/d rated furnace. The current campaign output is over 30 Mt.

BHP's Port Kembla No. 5 blast furnace (3045m3 I.V.) was blown out in 1991 after making 25.2 Mt in a campaign lasting over 12 years. The average productivity was less than 1.9 t/m3 I.V./d, although productivities up to 2.45 t/m31.V./d were attained'9'.

When considering the level of productivity on blast furnaces that have achieved a long campaign life, it is clear that the furnaces in question have not been operated to their maximum potential for the majority of the campaign; despite a requirement for higher output whilst other blast furnaces at the plant are relined and during periods of high demand. The common factor is stable, consistent operation, with practices employed to monitor and protect the walls and hearth. Such operation is more easily achieved at production levels below maximum output. However, it is difficult to define a universal value of productivity index (t/m3/day) to achieve this, since the index is affected by factors other than furnace driving rate, e.g. furnace internal shape, state of refractory wear, local operating conditions, maintenance periods, etc.

To maximise campaign life, a strategy is required to enable the furnace to be operated in a stable, controlled manner whilst producing the required output. Many recent rebuilds have involved increasing the furnace inner volume, not to increase output but to enable production targets to be met at lower productivity levels and hence offer the potential for more stable operation and longer campaign life.

Obviously, frequent stoppages reduce the productivity of a blast furnace, but the campaign life is also reduced due to the excessive number of stop-start operations. The campaign output per unit volume is reduced disproportionally to the percentage downtime, as shown in Fig. 3.2(1', for Japanese and European blast furnaces. Long campaigns, measured by this criterion, are best achieved with continuous blast furnace operation without lengthy stoppages.

Short term reductions in productivity may also be required in response to problem areas identified on the furnace, in order to protect the integrity of the furnace, thereby avoiding a premature conclusion to the campaign. These will be discussed in more detail in the relevant section.

3.2 Burden

3.2.1 Coke Quality

For stable blast furnace operation at reasonable productivities, good quality coke is essential. It is one of the most often cited reasons for a poor period of operation. It is such periods of poor, erratic, often chilled operations that are potentially destructive to the blast furnace lining and hence to campaign life'10'.

Coke must be strong and stabilised, to support the weight of the burden with minimal mechanical breakdown. It must be sufficiently large and closely sized, with minimal fines, to create a permeable bed through which liquids can drip down into the hearth without restricting the ascending gases. A consistent size is required to avoid undesired variations in permeability and to support the concept of varying coke layer thickness across the furnace radius to control radial gas

10

flow. The coke must be sufficiently unreactive to solution loss, retain its strength under such conditions, and be low in alkalis to minimise alkali gasification in the raceway, which has a deleterious effect on coke breakdown and on furnace refractories. A low sulphur content is also required to minimise hot metal sulphur and desulphurising costs. Coke moisture and carbon content variations must be controlled to minimise their effect on the thermal state of the process. At high levels of tuyere hydrocarbon injection, used to reduce ironmaking costs and increase productivity, there is a corresponding reduction in the proportion of coke charged and consequently coke quality becomes even more important.

A universal coke quality, for stable operation compatible with long blast furnace life, is difficult to specify, since not only do different types of operation have different coke requirements but also physical properties vary according to the point of sampling between the coke ovens and the blast furnace. A typical specification for a large blast furnace is given in Table 3.1(10).

It is often the case that the coke charged to a blast furnace is sourced from more than one coke plant. In these cases, a system of blending adequately or charging discretely the different cokes is essential, as fluctuating proportions of cokes of different properties may result in unstable furnace conditions111'.

Coke in the furnace centre will gradually replace the deadman and the coke in the hearth, which must remain permeable to allow the liquids to drain across the centre of the hearth. This avoids excessive peripheral flow of hot metal in the hearth which can result in severe refractory wear at the base of the sidewall. An increase in the hearth pad centre temperatures has been observed with an increase in deadman coke size (Figs. 3.3 - 3.5(1012,13'), which indicates increased hearth centre activity. The aperture size of the coke screens is an important parameter to maintain hearth permeability. It is usually beneficial to increase the screen size and charge the additional small coke arising, mixed in with the ferrous burden, away from the furnace centreline. Further details will be discussed in a later section.

The aim of specifying high quality coke is to ensure that large coke reaches the lower regions of the furnace. To monitor this objective in the long term, it is advisable occasionally to sample coke from tuyere level to assess the coke breakdown through the furnace. This is usually carried out during planned maintenance, often in conjunction with tuyere changes. A large sample of coke is raked from a tuyere aperture and its properties compared with a sample of the corresponding feed coke'14'. In this way other factors affecting coke size can also be identified. Many operators also use probes through the tuyere aperture to obtain information on coke properties in the lower regions of the furnace.

Good, consistent quality coke and the monitoring of both stockline and bosh coke is clearly an important strategy for long campaign life.

3.2.2 Ferrous Burden Composition

Blast furnaces may be operated with a wide variety of ferrous burden components, the main ones being sinter, pellets and lump iron ore. A variety of fluxes may be used to trim the final chemistry. Smaller proportions of other iron bearing materials are often used, which may include recovered scrap, ferrous fines, mill scale, BOF slag, ilmenite, recycled waste or even directly reduced or granulated iron. The burden of an individual plant usually depends on local factors and on burden costs.

Integrated steelmaking sites usually have sinter making facilities, resulting in these blast furnaces operating on a large proportion of sinter, with the balance of the burden consisting mainly of lump

11

ore and/or pellets. This is the case for the long life blast furnaces mentioned in Section 3.1, which operated with 55-90% sinter'4'6,78,9'. Traditionally, the balance of the burden was mainly pellets, due to their superior properties. In recent times, more sophisticated blast furnace operation has enabled increasing proportions of directly charged lump ore to be used, reducing burden costs. Lump ore has been used at up to 45% of the ferrous burden at British Steel's 14 m Redcar furnace'8'.

Recent environmental concerns on emissions from sinter plants could result in older sinter plants being forced to close on economic grounds'15'. This situation necessitates an increased proportion of directly charged materials. Operation with a large proportion of pellets and lump ore may also be necessary when operating a standby blast furnace to satisfy short term increases in steel demand.

Operations with up to 100% pellets are encountered throughout the world, particularly in North America and Scandinavia, where indigenous ores suitable for pellet manufacture are plentiful. Past experience has shown that blast furnaces burdened with a high proportion of pellets suffer higher heat load variations in the lower stack and bosh'15,16', leading to excessive lower stack and bosh wear and a shorter campaign life'17). One of the main reasons for this has been inadequate control of burden distribution. Pellets have a much lower angle of repose than sinter or coke and, when landing on an inclined stockline, tend to roll easily. This may result in a relatively thick ferrous layer towards the furnace centre which will encourage excessive gas flow at the furnace wall.

This situation has improved in recent years, with the addition of high density cooling in the lower shaft and improved burden distribution equipment. BHP's Whyalla No. 2 blast furnace (1543m3 I.V.) had achieved a campaign output of 7970 t/m3 I.V. by 1993 on an 80% pellet, 20% lump ore burden'18'. The proportion of pellets was increased from 35-70% at Kobe Steel's Kakogawa No. 2 blast furnace (3850m3 I.V.) in 1991. Fluctuating lower stave temperatures, increased slipping and hot metal temperature fluctuations were initially observed, but stable operation was recovered by burden distribution control with centre coke charging and addition of small coke to the pellets'19'. Hoogovens carried out trials with a 100% pellet burden and found that productivity, fuel rate, metal quality and heat losses were worse than with the usual 50% sinter burden, although control could be maintained satisfactorily up to 80% pellets, with the sinter charged in the wall area'15'.

An important aspect to consider when selecting individual burden components is their softening and melting characteristics. The major part of the pressure drop across a blast furnace is in the region where the ferrous burden is softening, melting and dripping down the coke bed through which the gases are ascending. A wide melting and softening range may result in an increased pressure drop and a large cohesive zone root impinging on the lower shaft brickwork, with the refractories being exposed to high temperatures over a wider area than is desirable. For example, the reducibility and melting properties of fluxed pellets are superior to acid pellets. Dofasco, operating with a 100% pellet burden, reported a decrease in mid-stack wall heat losses when replacing 75% acid pellets with fluxed pellets, resulting from their improved high temperature properties'20'. BHP reported that a change from 80% acid to 80% dolomite-fluxed pellets at Whyalla No. 2 blast furnace decreased the thermal loading on the lower stack walls and resulted in improved stability of operation'21'. This suggests a lower wall temperature and/or fewer thermal fluctuations, both of which would help to prolong the life of the shaft brickwork. The melting and softening properties of a multiple component burden will differ from those of the individual components and it is desirable to consider softening and melting test data not only of individual burden constituents but of the proposed mixture, to aid burden selection'22,23'.

12

To minimise thermal and chemical variations, a homogeneous burden is desirable. The burden components should be as intimately mixed as possible. This depends on the number of burden components and the individual charging system, but it can usually be achieved to a reasonable degree by selection of storage bunkers and the sequence of material discharge.

Clearly, it is possible to achieve stable furnace operation and long campaign life using different burdens providing that suitable consistent quality materials, adequate wall cooling capacity and distribution control are available.

3.2.3 Ferrous Burden Quality

To ensure a permeable blast furnace, essential for stable operation, it is important that the ferrous burden is strong, closely sized and efficiently screened to remove fines. It must not disintegrate excessively in the stack, which would generate additional fines, and should be sufficiently porous, reducible and of an appropriate size to allow the material to be adequately reduced by the time it reaches the softening zone. In this way the cohesive zone will be less restrictive, with less FeO rich slag, and the thermal load in the lower regions of the furnace will be lower, encouraging smooth operation. Requirements on the physical and metallurgical properties of sinter and pellets for efficient operation are discussed in more detail in reference'24', and summarised in Table 3.2'24'.

The softening and melting properties of the ferrous components have an important effect on blast furnace operation. Restrictions in the cohesive zone and poor melting characteristics may result in erratic burden descent, unstable operation and thermal fluctuations; conditions likely to shorten furnace wall life. Many operators determine the softening and melting properties of ferrous materials in laboratory tests; details of the British and German versions are given in references'22,23'. There is no standardised softening and melting test and there are many indices quoted to represent the softening and melting temperatures; start of direct reduction, pressure drop during melting, quantity of dripped material, etc. British Steel have found that, for production sinters, the quantity of material retained in the coke bed after dripping ('residual material', essentially high melting point slag) can be related to blast furnace performance and stability. A low quantity of residual material is desirable for good furnace operation'25'.

3.2.4 Chemistry : Alkalis, Zinc

It is well known that alkali metals and zinc have a deleterious effect on the blast furnace process and refractories, and their input in the burden is maintained at a minimum economic level. Although generally charged at levels of less than 5 kg/thm (tonne hot metal), by the condensation of alkali vapour on the descending burden a large recirculating load can build up in the furnace. This results in increased sinter degradation'26' and coke breakdown'10,14', and encourages the formation of wall accretions, all of which can result in irregular burden descent and unstable furnace operation.

Alkali metal compounds and zinc, in gaseous form, will penetrate cracks and pores in the furnace wall refractories. The resultant chemical attack and thermal cycling weakens the surface layer of refractory, which is eventually removed by the descending burden, allowing the process to be repeated.

Hearth dissections after the end of a campaign have shown that excessive wear may occur at the base of the sidewall and that a brittle zone is often formed between the shell and the hot face of the carbon. Zinc and alkalis have often been found at high levels in this brittle zone and various breakdown mechanisms have been proposed involving these compounds'1'. Stress and thermal cracking in the sidewall will allow gaseous zinc and alkalis to penetrate and be deposited in the

13

pores, which can lead to brick expansion, embrittlement, further swelling and ultimately destruction of the refractory mass'16'. A significant degree of refractory protection from alkalis and zinc may be achieved if an accretion or skull is frozen on the hot face of the refractory, thereby protecting the refractory from chemical attack'16'. However, zinc and alkalis are not necessarily involved in the development of a brittle layer, since minimal levels were found in the brittle zone on Nippon Steel's Muroran No. 3 blast furnace, which was blown out after 1 year and 6 months of its sixth campaign'27'.

The actual level of alkali charged depends on economic and political constraints, with best practice being around 2 kg/thm. The majority of the alkalis are removed in the slag and the remainder in the top gas. However, the furnace slag practice, thermal state and burden distribution play a major role in alkali removal'25'. A reduction in slag basicity will increase the quantity of alkali removed in the slag (Figs. 3.6, 3.7)(26), s will an increase in the thermal level of the furnace or in the top temperature, by broadening or intensifying the degree of central working'28'. In addition, for a given alkali loading, coke degradation is likely to be greater for operations with a high rate of tuyere hydrocarbon injection, due to the increased burden residence time'29'.

It is important that the balance of input and output alkali and zinc is monitored and that a furnace is operated with a thermal and chemical regime compatible with the input level of these elements, to encourage their removal in the slag and top gas.

3.2.5 Burden Distribution

Burden distribution is one of the main factors affecting blast furnace campaign life. Not only can it affect the stability of operation but, by determining the radial gas flow in the furnace, it is one of the major factors controlling the rate of wear of the furnace walls.

The two types of distribution system that enable sufficient control for high productivity and long furnace campaign life are the bell-less top using a tiltable rotating chute, most commonly that of Paul Wurth design, and a bell charging system with movable throat armour.

Primarily, radial gas flow is controlled by the proportion of ferrous burden to coke, since coke is generally much larger in size. This is most easily achieved by charging the material in discrete layers and varying the layer thickness across the furnace radius. Protection of the furnace walls is therefore achieved by increasing the proportion of the ore layer at the wall, which will result in a reduced quantity of heat removed by the wall cooling system. An example of this is shown in Fig. 3.8, for Kawasaki's Chiba No. 6 blast furnace'4'. However, there is a limit to the proportion of ferrous material close to the furnace wall, otherwise an inactive layer will form, which may encourage the formation of wall accretions and allow unprepared burden into the lower regions of the furnace and increasing tuyere losses. The proportion of coke at the centre of the furnace must be sufficient to allow stable furnace operation at the desired level of production. A large proportion of coke creates a relatively permeable region with fewer descending liquids, allowing the use of maximum blast volume without large fluctuations in blast pressure and erratic burden descent. The coke at the centre of the furnace replaces the coke in the hearth and a coke rich permeable centre will encourage a permeable hearth, as shown in Fig. 3.9(4), which relates the liquid flow across the hearth, as indicated by the hearth pad thermocouples, to the upper shaft probe etaCO at the furnace centre. The central coke chimney should not be unnecessarily wide, however, or inefficiency will result, and damage may be incurred to certain parts of the furnace top due to excessively high heat capacity of the ascending gas.

14

3.2.5.1 Split Size Charging

More sophisticated distribution systems permit additional control of burden distribution by utilising more than one size range of a given material. One of the most commonly used practices is the charging of fine ferrous materials, often from screenings of the main ferrous burden. Fines are charged separately in small quantities close to the furnace wall, to give a localised reduction in permeability and thereby protect the walls, as shown in Fig. 3.10'30'. Charging a separate small batch of finer material will usually reduce the charging capacity of the furnace. At Kawasaki's Mizushima No. 3 blast furnace, a 3-parallel top hopper system was incorporated at the 1990 rebuild to overcome this problem. Charging of small batches with a bell and movable throat armour system should cause fewer delays than with a bell-less top due to the reduced discharge time, as was achieved at NSC's Sakai No. 2 blast furnace (Fig. 3.11'31'). It may be possible to charge small quantities of finer materials to the furnace wall by charging them first into the top hopper or large bell hopper and using the corresponding initial chute angle or movable throat armour setting. However, the quantity is limited by the hopper discharge characteristics to that which will pass through the hopper without mixing with the remainder of the charge. There is also a financial benefit in using such ferrous fines directly as opposed to returning them to be re-sintered.

In a similar manner, the ferrous burden may be split into large and small sizes which are then charged over different parts of the furnace radius to control the radial permeability. Such split sized sinter charging has been practised at POSCO's Kwangyang furnaces since the mid-1980's, where the 4-12 mm sinter is charged towards the wall and the 12-50 mm sinter towards the centre, although the cut size is adjustable for maximum distribution flexibility'32'.

3.2.5.2 Coke Nuts

A flexible charging system will allow the use of coke nuts. The size of coke nuts available for charging will depend on the size and efficiency of the blast furnace coke screens, the company coke requirements and the marketability of small coke, but is typically in the range 10-30 mm. The charging of nuts, mixed in the ferrous material and positioned along the mid-radius, often improves operation by improving reduction efficiency and permeability of the ore layer in the cohesive zone'33'. POSCO reported improved permeability and reduced belly temperatures with nut coke charging, as shown in Fig. 3.12'32'. At NSC's Sakai No. 2 blast furnace, the nut coke was charged at the wall, sandwiched between the two ore charges (Fig. 3.11), to prevent an inactive wall region when fine ore was charged at the wall. At Kobe Steel's Kakogawa No. 2 blast furnace, fitted with movable throat armour and using a burden of up to 70% pellets, coke nuts were added to the pellets to increase their angle of repose, Fig. 3.13, thereby reducing the proportion of ferrous material at the furnace centre'19'. The replacement of prime coke with coke nuts also reduces fuel costs.

3.2.5.3 Size Segregation

Most charging systems create some degree of size segregation in the input materials. If the initial material to discharge is finer and the final material is coarser, this characteristic may be utilised to benefit the radial size distribution, and hence the radial gas flow distribution. This type of segregation generally occurs on belt charged furnaces rather than on skip charged furnaces and is more controllable with a bell-less top'34'. Suitable modifications may also be added to the charging system to enhance the desired segregation characteristics, such as segregation control plates in the top hoppers at Mizushima No. 3 blast furnace, shown in Fig. 3.14'35'.

15

Additional radial size segregation may also occur by rolling down an inclined stockline. Size segregation may also modify the melting and softening characteristics of the burden along the furnace radius, when one component has a different size range and chemistry.

Some charging systems may result in a circumferential variation in burden distribution. These variations must be minimised by design or operation. For a parallel hopper bell-less top, this may be achieved by redesign of the lower chute and downcomer or by periodically alternating the material type charged from each bunker'30,36'. In addition, the starting position of the rotating chute should be varied for each charge'35'. With bell charged furnaces fed by skips, it is essential that the distributor operates to minimise any circumferential maldistribution, resulting from fines segregation on the bell. Improvements were made at BHP's Whyalla No. 2 blast furnace when the operation of the distributor was changed from indexing to continuous rotation'17'.

3.2.5.4 Centre Coke Charging

A large proportion of coke is often required at the furnace centre, to encourage sufficient centre working for stable operation. This is particularly so at higher productivities and when operating with high levels of tuyere hydrocarbon injection. However, to operate with entirely coke in the centre of the furnace is less fuel efficient and techniques have been developed to minimise the width of this region, by centre coke charging. On a bell-less top, this is achieved by charging a small batch of coke with the rotating chute fully lowered, resulting in layers as shown in Figs. 3.15'32' and 3.10'30'. A furnace with movable throat armour (MTA) has less control of the layer shape, since all of the falling material has to be deflected by the MTA, which is located near the periphery of the furnace. Consequently, centre coke layers such as those illustrated are not normally possible. To overcome this deficiency and enable centre coke charging on MTA charged furnaces, a technique was developed by Kobe Steel at Kakogawa No. 2 blast furnace, to charge additional coke to the furnace centre down an inclined chute located between the large bell and the MTA. The arrangement is shown in Fig. 3.16, and the reductions in furnace pressure drop, blast pressure fluctuation and slip frequency with increasing quantities of centre charged coke in Fig. 3.17'13'.

A permeable coke bed is necessary in the hearth, to encourage the flow of liquids across the centre of the hearth and reduce peripheral flow, which can cause excessive sidewall wear. The coke in the deadman and hearth is gradually replaced by coke from the furnace centre. Centre coke charging reduces the proportion of ferrous material at the furnace centre and improves hearth permeability, as indicated by increasing hearth bottom temperatures, Fig. 3.5. Larger, stabilised coke charged to the centre is likely to further improve hearth permeability.

3.2.5.5 Throat Armour Life

For long campaign life, it is important to minimise wear on the fixed throat armour caused by the direct impact of burden materials. Although it is possible to repair the throat armour or incorporate protection plates, this may involve long maintenance stoppages, which themselves may be detrimental to furnace life. Consequently, the burden distribution and the stockline height used should be chosen to avoid such burden impact.

3.2.5.6 Monitoring

Burden distribution must be monitored regularly to ensure the correct degree of wall protection and a stable, driving furnace. Changes in furnace operating parameters, e.g. changes in tuyere hydrocarbon injectant rate or blast volume, may require adjustments to burden distribution. The effect of burden distribution is monitored with various probes and instruments, which will be

16

discussed in Section 3.4. The use of a predictive mathematical model of burden distribution that has been calibrated from plant measurements is highly desirable, for assessment of existing distributions and prediction of future changes.

To maximise the campaign life, it is essential that the charging equipment is capable of accurate control of burden distribution, that adequate instrumentation is fitted to comprehensively monitor the resulting furnace operation and that the burden distribution is changed and assessed in a controlled, technical manner.

3.3 Ti02 Addition

For many furnaces worldwide, samples of the hearth lining at the end of a campaign have been found to contain titanium bearing deposits. These form a protective layer in eroded regions of the hearth sidewall, in the salamander and in brick pores and joints. The titanium is usually in the form of carbonitrides, a solid solution of titanium carbide and titanium nitride. Consequently, practice in recent decades has involved the introduction of additional titania (Ti02) into the furnace to promote these protective layers. Three methods of introducing Ti02 are by addition to the burden, injection at the tuyeres or by addition to taphole clays.

3.3.1 Introduced in Burden

Probably the simplest and most common technique has been by the addition of titaniferrous ores, commonly ilmenite, to the burden'38'. Alternatively Ti02 can be incorporated in sinter, although usually at low levels. This technique has been more prevalent in Japan, and the Ti02 source is usually in the form of iron sand. Operators using this technique have experienced sinter quality problems, with a deterioration in cold and hot strength and a reduction in sinter productivity'39,40,41'. Usinor reported an increasing reduction degradation index (RDI) with increasing Ti content in the sinter, Fig. 3.18'39', which was also experienced at NKK's Fukuyama Works, Fig. 3.19'40'. This figure shows that, although the increase in RDI reached a peak, the sinter productivity continued to fall, with a reduction of 30% at iron sand additions equivalent to a blast furnace addition rate of 10 kg TiOj/thm. BHP Port Kembla reported a deterioration in sinter quality with ilmenite sand addition of above 0.5% Ti02'41>. It is also possible to incorporate Ti02 during pellet manufacture. Kobe Steel has practised this with a pellet Ti02 of 1.7%, equivalent to a level of 9 kgTiO/thm in a 25% pellet burden'38'.

There tend to be two strategies concerning the method of burden Ti02 addition. One is remedial, commencing Ti02 additions only once high hearth temperatures are observed, indicating hearth wear. The other takes a preventative approach and adds a small quantity of Ti02 continuously, increasing the addition level if high temperatures are observed. The Ti02 intake for the preventive approach is generally 3-5 kg/thm, which usually results in up to 0.1% Ti in the hot metal and 1-1.5% Ti02 in the slag. For remedial action, the Ti02 dosage may be up to 20 kg/thm, at which level the hot metal may contain up to 0.3% Ti and the slag up to 3.5% Ti02. This creates operating problems due to high slag viscosity and scaffolding in the runner, and consequently such high Ti02 levels can only be used for short periods. An increased quantity of titania in granulated or pelletised blast furnace slag will affect its properties as a cement additive and may decrease its value and marketability. At high levels of titania addition, steelmaking difficulties may be experienced. Titanium carbonitrides can also remain in the final steel product, as unwanted inclusions.

At NKK's Fukuyama No. 5 blast furnace, Ti02 addition was used to overcome localised increases in sidewall temperature (Fig. 3.20)'42'. Ti02 addition was increased up to 20 kg/thm, with a Ti02 balance confirming the retention of up to 1.5 kg/thm. The slag basicity was increased from 1.2 to

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1.3 to counterbalance the potential slag viscosity problem. At Sumitomo Metal Industries' Kashima No. 3 blast furnace, Ti02 additions were increased to 10-15 kg/thm, after 9 years operation, to control hearth sidewall temperature (Fig. 3.21)'43'. At Wuhan Iron and Steel Co., 2-3 weeks after charging ilmenite (10.5% Ti02) at up to 10 kgTiOg/thm, the heat load on the hearth staves started to decrease and after 2-3 months it became stable (Fig. 3.22)(44'. In 1981, British Steel's Redcar No.1 blast furnace reported a decrease of 120C in hearth sidewall temperatures, following an increase in Ti02 level to 8 kg/thm, which was interpreted as evidence that a protective layer had been formed on the eroded area'45'. At Altos Homos De Mexico, ilmenite was charged at the rate of 9 kg TiO^thm'46' and within two weeks a reduction in hearth temperature of 180C was observed.

In order to promote the precipitation of Ti(C,N), some operators increase the Ti02 additions before a shutdown so that the metal remaining in the hearth will be saturated in Ti. As the hearth cools during the shutdown, this encourages precipitation'41'. Others find that the resumption of production is more difficult at high Ti levels, creating operational problems.

3.3.2 Tuyere Injected Fines

Several operators have injected fines containing Ti02 through the tuyeres, as an alternative to Ti02 additions to the burden'5,47,48'. Kobe Steel was developing a system at Kakogawa Works from 1984, to increase hot metal viscosity and deposit Ti(C,N) on the hearth sidewall'47). The advantages quoted include: application at localised positions, reduced cost due to lower Ti02 rate, good results from short-time injection and unchanged burden properties. When benefits were not achieved by injection directly above the area of high sidewall temperatures'49', model experiments were carried out to determine the most suitable tuyeres through which to inject, relative to the position of the damaged area and the taphole in use. Two tuyeres were used for higher injection rates and to improve protection. The injection position at No. 3 blast furnace (in 1985) is shown in Fig. 3.23, and the reduction in hearth sidewall temperatures achieved is in Fig. 3.24. The injection position at No. 2 furnace (in 1991) is shown in Fig. 3.25, and the reduction in hearth sidewall temperatures achieved is in Fig. 3.26'49'. Rutile powder (95% Ti02) was used in the earlier trials, but latterly ilmenite powder (50% Ti02) was successfully used, to reduce costs. Selection of the correct injection location is important, with the angle between the damaged portion and the injection point being 40-90, and the angle between the injection point and the taphole in use being 80-1600'49'. Nisshin Steel have injected iron sand at up to 15 kg/thm, claiming a reduction in hearth brick temperature over a longer period (Fig. 3.27)'5'. ILVA injected ilmenite and rutile at Taranto No. 5 blast furnace, reducing hearth wall temperatures (Figs. 3.28 and 3.29). The injections took place through a tuyere approximately 150 from the taphole in use to protect an area 90 from the taphole'48'.

3.3.3 Impregnated Taphole Clays

The third method of Ti02 addition is by the use of taphole clay containing Ti02. This has been tried in Japan. Although few details have been published, one such clay tried was tar bonded with approximately 10% Ti02. Clearly, the titania is bound in the taphole clay in an unreduced form, and is injected in relatively small quantities. It is questionable whether it will be reduced and dissolved in the iron in sufficient quantities to be precipitated or whether it would be reduced and bonded adequately to the hearth sidewall to be of benefit.

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3.3.4 Success and Cost Effectiveness

In a blast furnace, Ti02 is partially reduced and is dissolved in the hot metal. The solubility is greater at higher temperatures. If the Ti in the hot metal is nearing saturation and the refractory hot face temperature in eroded regions, cracks and pores is lower than the hot metal temperature, then Ti may be precipitated, as Ti(C,N). The technique is more likely to succeed at higher addition rates, but there are other factors which may interfere with this basic mechanism, including thermal state of the hearth, metal/slag chemistry and liquid flow characteristics.

Success in overcoming hearth hot spots with Ti02 additions is varied. Not only will the individual furnace conditions vary, which will affect the success of the technique, but Ti02 addition is usually carried out in conjunction with other remedial actions (which are discussed in other sections) such as reducing productivity, closing tuyeres and improving hearth cooling intensity. For example, Fig. 3.30'42' illustrates the other actions taken at Fukuyama No. 5 blast furnace, although positive effects are claimed to result from Ti02 addition. The direct effect of Ti02 addition is therefore often difficult to determine. It is essential to carry out regular, accurate Ti balances to assess the technique and modify operation to encourage Ti retention. The effect of high rate additions may even have a detrimental effect on furnace operation, negating any benefits. The delay following additions, before an observed fall in hearth temperatures, can vary from hours to weeks, as can be seen in Figs. 3.20 - 3.29.

Once initial hearth wear has stabilised, in the early stages of a campaign, the addition of a small quantity of Ti02 over a long period of time would seem a sensible preventive measure, depending on the cost of the source of Ti02. It has been calculated'38' that, provided a campaign is extended for several months by charging ilmenite at a preventive level for 6 years followed by an increased rate thereafter, it will prove financially beneficial.

The addition of Ti02 for hearth protection should be considered as part of a hearth protection plan rather than in isolation. Its use should be considered in relation to success achieved by other techniques, and different methods of its use may be required to achieve the desired benefits.

3.4 Instrumentation and Control

3.4.1 Hearth Thermocouples

Early warning of hearth problem areas is vital to maximise campaign life, and thermocouples located in the hearth sidewall and in the hearth pad are absolutely essential to monitor hearth wear. Revised operating practices and actions to protect the hearth will be taken as a result of increasing hearth temperatures. Hearth pad and sidewall temperatures can also give an indication of liquid flow in the hearth, Figs. 3.3 - 3.5, an important factor in hearth wear.

Temperatures recorded by thermocouples are influenced by only a small area round the thermocouple. It is therefore vitally important to locate the thermocouples in the critical wear areas. Of particular importance are those areas below the tapholes and around the base of the sidewalls where the so called 'elephant's foot' wear pattern is often found. An adequate number of thermocouples must be installed, in the best layout to give as complete coverage as is practical. At several locations, thermocouples should be positioned at two (duplex) or three (triplex) different depths to allow calculation of the thermal profile in the refractory and hence the thickness of residual refractory.

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Sollac's Dunkerque No. 4 blast furnace (14 m hearth) was equipped with 126 thermocouples in the hearth during its reline (Fig. 3.31 )'50'; 26 in the pad on two levels, 72 in the sidewall at three levels (50% duplex) and 7 round each of the four tapholes. ILVA's Taranto No. 5 blast furnace (14m hearth) has a total of 116 thermocouples in the hearth, at three different levels'51', which was subsequently increased to a total of 151. Alto Homos de Mexico's No.5 blast furnace (11.2 m hearth) has 72 thermocouples at six different levels (Fig. 3.32)(46). Chiba No. 6 blast furnace has 81 thermocouples on six levels, Fig. 3.33'4'.

The insertion depth of the hearth sidewall thermocouples is important and will depend on whether a hearth is stave or spray cooled. Thermocouples are usually inserted to the front face of staves or shell, in front of the conducting ramming between the staves or shell and the carbon/graphite, and a short distance into the carbon/graphite'52'. If the thermocouples are positioned too close to the hot face, they may deteriorate in the early stages of campaign'53'. Ni-Cr/Ni-AI thermocouples can become poisoned at temperatures over 600C, thereafter indicating too low a temperature'54'. Movement of carbon blocks can nip hearth pad thermocouples, causing false hot junctions or total failure. These latter problems may be overcome by fitting the thermocouples in sheaths.

Thermocouples should be positioned around the tapholes, to monitor taphole conditions and operation.

Additional thermocouples are often added part way through a campaign in areas of known refractory wear, to give a more localised picture of developing problems'4,42,48'. Similarly, thermocouples are often added to repaired areas to monitor the repair.

As an alternative to thermocouples, heat flux probes are currently under development, with 12 installed in the hearth brickwork at Bremen No. 2 blast furnace'55'. They consist of 20 fine thermocouples, mounted on a ceramic carrier which is matched to the thermal conductivity of the lining. Results indicate that they give more accurate information on refractory wear than thermocouples, and a spacing of 3 m is adequate as opposed to 1m for thermocouples. Further details are given in Section 5.4.

3.4.2 Monitor Hearth Cooling

Heat flux in the hearth pad or stave cooling water can be determined from the water flow rates and the difference between inlet and outlet water temperature, using resistance thermometers. It can be used only to give an indication of the average hearth wear and is therefore of less use than hearth thermocouples. It is particularly applicable in the later stages of a campaign, following thermocouple deterioration'53'.

Monitoring long term trends in hearth cooling water temperature may give an indication of the efficiency of the cooling system. Spray cooled hearths may require periodic cleaning of the shell'42' and staves/underhearth cooling may require blast cleaning'107' or removal of pipework deposits using acid, especially if using untreated water.

3.4.3 Furnace Wall Conditions

The process conditions at the furnace wall are vital to campaign life. The walls must not be subjected to high heat loads from an excessive quantity of gas ascending at the wall or impingement of the melting zone on the wall, which would result in more rapid deterioration of the refractory and wear of the cooling members. On the other hand the walls must not be so inactive that large accretions are permitted to form on them, which would prevent smooth burden descent,

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control of burden distribution and stable blast furnace operation. To monitor wall conditions a variety of techniques are used.

The most common technique of monitoring the walls is using inwall thermocouples, positioned in the brickwork, with the tips a short distance back from the hot face to give a good thermal response. Wall activity is monitored from the temperature level and fluctuations. Thermocouples are often fitted in the body of staves. Whilst a refractory lining is present, the stave temperatures have a low value and a damped response. Their temperature rise and increasing fluctuations indicate when the refractory is lost and thereafter they function as inwall thermocouples.

There should be a good coverage of thermocouples both vertically and circumferentially to monitor the walls adequately'6,51'. Seven levels of thermocouples, each with eight circumferential positions, is typical. With a large number of thermocouples it is difficult for the operator to monitor the variation of them all. By using the temperatures at many points, an isothermal map can be generated, identifying regions of high or low temperatures which may relate to refractory wear, asymmetrical operation or accretion formation. The dynamic temperature behaviour can also be utilised to predict the formation or loss and extent of an accretion'56"57'.

Throat or skin thermocouples are often installed around the periphery, just below the fixed throat armour. The thermocouple tips are installed level with the hot face of the refractory, to record gas temperature. These give a direct measure of the gas flow at the wall and are usually unaffected by deposition of material, unlike inwall thermocouples lower in the stack.

The heat load at the walls, or heat flux, is often calculated from the temperature rise and flow rate of the shaft cooling water. This usually gives the total heat flux for the stack or stack and bosh, and is considered a good indication of burden distribution, gas flow and wall wear (e.g. Figs. 3.34, 3.35 and 3.36)'4,8,58'. If the cooling system is designed with a series of ring mains serving banks of a small number of coolers or staves, it is possible to determine the heat load profile down the shaft, which may relate to different types of operation. British Steel's Port Talbot No. 4 blast furnace, which is plate cooled, has such a system with 13 zones of heat flux'59'.

There are also systems for determining the remaining thickness of wall refractory, besides the basic method of drilling and measuring. These usually involve embedding sensors in the brickwork and periodically measuring their remaining length. They can only measure the remaining thickness of the original brickwork and not the build up of a protective layer on the hot face as can be inferred from thermocouple readings. At Hoogovens Umuiden Works, lining wear is measured ultrasonically by ceramic rods incorporated in the refractory. The rods wear with the lining and their length is measured during furnace stops'60'. Other systems used include; time domain reflectometry, where a pulse is sent down a signal cable, the time taken for the reflection giving the length of the remaining cable'53,61'; a system using a series of fine thermocouples in a single sheath, the innermost thermocouples failing as the refractory wears back'62'; and a similar system using a series of elements, with lining thickness relating to electrical resistance'63'. Infra-red thermography may also be carried out on the furnace shell to indicate the refractory wear pattern'52'. It is important to monitor the lining thickness as it not only protects the shell directly in uncooled regions of the periphery or where cooling members have failed, but a worn lining in the upper stack may result in layer mixing and reduce the control of burden distribution at the wall. This will ultimately result in increased thermal loads at the wall and affect the stability of operation, which will accelerate wall wear.

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3.4.4 Radial Measuring Probes

It is accepted worldwide that the use of retractable probes is one of the most important techniques to monitor and optimise burden distribution, and hence campaign life. Such probes are the only method of measuring the variation in operating characteristics along the furnace radius, as opposed to relying solely on wall measurements. They are essentially of two types: overburden and underburden. A schematic arrangement of such probes on a furnace is shown in Fig. 3.37(64).

Overburden probes have many functions. The simplest type is usually fixed, water cooled and measures the radial or diametrical top gas temperature profile and, in some instances, the gas analysis. Most retractable probes measure the stockline layer profile and may be of a mechanical type, where a weight is lowered to the stockline'65' or a non-contact type, using radar'66', microwaves'64', lasers'9', etc. The radial variation in layer thickness can be measured and the ore/coke ratio across the radius calculated. This can be related to the charging sequence and used to calibrate burden distribution models. It is of less value if layer sequences which encourage the disturbance of a coke layer by the subsequent ferrous material are in use. Top gas velocity can also be physically determined to measure the quantity of gas flow'65', and top gas analysis and temperature measurement will often be carried out in conjunction with the other functions. Probes are also used to determine the trajectory of material off the rotating chute or movable throat armour, for calibration of burden distribution predictive models and to determine the effect of charging chute wear (Fig. 3.38)'36'. For experimental purposes, probes have been constructed to introduce feeding type vertical probes at different radial positions, to determine temperature and gas analysis as the burden descends, since the more usual type of vertical probe is only possible at the furnace periphery.

Underburden, or in-burden, probes sample gas and measure temperature at a number of radial positions. They are generally positioned in the upper stack, typically 3-6 m below the stockline, although some furnaces have a lower stack probe which may be 10-15 m below the stockline. These probes are generally of two types. The consumable type, is typically 50 mm in diameter, bends with the descending burden and is straightened on withdrawal for subsequent re-use'36'. An example of the other type is the DDS probe, which is oval in section, typically 550 mm by 210 mm, water cooled and resists bending whilst in the furnace'66'.

Since the top gas has to pass from the stockline up one of the four offtakes, the gas flow pattern begins to distort near the stockline. A large degree of gas mixing then occurs above the burden, and overburden probes must be positioned close to the stockline, and preferably inclined, to give acceptable temperature and gas profiles. The consensus of opinion is that upper stack underburden probes are more sensitive and give superior results to overburden probes. In addition, fixed overburden probes can be quite large and, depending on the stockline height, can create a 'shadow' and distort the burden distribution beneath them, which may create unrepresentative results.

Typical gas efficiency and temperature profiles for a furnace operating at good productivities compatible with a long campaign life may be of the type shown in Figs. 3.39, 3.40, 3.41, 3.42(6,36,65,67). Many operators use only the temperature profile, which tends to be approximately the inverse of the gas efficiency profile. Although a direct measure of temperature may be desirable, particularly near the furnace wall, it may not give as accurate a measure of furnace working, particularly at the centre. The thermocouple in the tip of a retractable probe takes several minutes to reach equilibrium after initial insertion, although this time is reduced at subsequent positions.

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The ideal probe profile for long campaign life at the required productivity is achieved by control of burden distribution, i.e. by a combination of fines/coarse placement and ore/coke ratio. Since every furnace has different top geometry, charge material characteristics and material flow patterns, the ideal profile must be developed for each furnace. The correct charging sequence for one furnace is not necessarily the best for a similar furnace and the sequence must be optimised and continually checked and refined as other operating factors change'36'.

Probes, especially underburden probes, are therefore essential tools for prolonging blast furnace campaign life.

3.4.5 Hearth Models

In recent years, with increasing computing power available, many mathematical and numerical techniques have been developed to predict blast furnace hearth erosion and liquid flow in the hearth.

3.4.5.1 Wear Profile

Hearth lining wear may be calculated by mathematical model, using temperature measurements from embedded thermocouples in the hearth bottom and sidewall'46,52,68"77'. The modelling techniques generally calculate the isotherms in the hearth, with typically the surface of the lining or solidified layer being represented by the 1100-1150C isotherm, equivalent to the temperature of solidified hot metal. The lining thickness will be calculated from the maximum temperatures achieved at a given point during a campaign and the thickness of solidified layer deduced, should the temperature at a given point be below its peak value.

For this technique to be accurate, a good coverage of thermocouples is required and their depth of insertion needs to be known precisely, together with the thermal properties and geometry of the lining. The accuracy may also be affected by parameters that may change with time, such as the conductivity of ramming, thermal contact between courses of brickwork and the development of a brittle zone in the refractory, which may significantly change its conductivity. The use of the aforementioned heat flux probes may improve the accuracy of lining wear prediction.

Although hearth temperatures alone give a direct indication of hearth wear, this type of model combines information from the thermocouples, at differing distances from the hot face, to predict the extent of wear and solidified layers more accurately.

Direct measurement of hearth lining wear is difficult and undesirable since this requires test borings and embedded sensors through the full refractory thickness. It is of interest to note that a Romanian and Indian blast furnaces have embedded radioactive sensors in the hearth pad and sidewall to determine when the hearth wears to that level (Fig. 3.43)'75,69).

3.4.5.2 Liquid Flow

Models to predict the flow of hot metal and slag in the hearth have also been developed'77"81'. These involve predicting the configuration of the deadman and hearth permeability to calculate the hot metal flow direction and velocity, thus enabling the degree of cross hearth and peripheral flow to be determined. Practical work to back up these models is described and includes physical models, the use of radioactive tracers introduced through the tuyeres and the interpretation of hearth pad and sidewall temperatures.

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The use of such models improves the understanding of the conditions that cause excessive peripheral flow in the hearth, which lead to wear of the hearth refractory by abrasion and dissolution, and helps to avoid them by operational changes and improving future designs.

3.4.6 Artificial Intelligence

The blast furnace process is a complex one, with a large number of process variables. Modern, well instrumented furnaces have hundreds of sensors which require to be monitored by a decreasing number of operators. Consequently, computerised systems are being developed to process the primary information available and give secondary advice to the operators'4,8,32,57,82,83'. This may be based on a set of operating rules, statistical analysis of data, identifying trends that compare with historical data and use of intelligent techniques such as fuzzy logic'84' and neural networks'85,86'. The aim of these systems is to predict deviation from steady operation and to quantify the change in control parameters required to minimise the deviations in production and quality. This will result in more stable blast furnace operation, avoiding major operating problems such as erratic burden descent and chilled conditions, which is a primary requirement for long campaign life.

3.4.7 Furnace Top Sensors

Since the late 1970's, many furnaces have been equipped with infra-red cameras viewing through windows in the top cone, to measure stockline temperature profile'87"90'. This technique overcomes some of the disadvantages of fixed overburden temperature probes: the falling burden is not scattered as with probes, leading to a more symmetrical burden distribution, and by measuring material temperature the effects of stockline to probe distance, which can result in gas mixing and desensitises the temperature profile, are avoided. A further benefit is that the rotation of the distribution chute in the furnace can be observed. However, these systems are expensive, difficult to maintain and experience problems in keeping the viewing window clean, due to the moist, dusty top gas. Problems have been experienced with the dust in the top gas also affecting the temperature distribution. In addition, a large amount of data processing is required to process the output, which sometimes proved a problem in the late 1970's, when this technique was developed. Consequently, these cameras are not a standard fitment and many operators have abandoned them'91' in favour of radial probes.

Some furnaces are equipped with non-contact stockline profile measurement systems installed in the furnace top cone. SSAB's Lulea No. 2 blast furnace is fitted with a gamma radar type'92' and BHP was, in 1991, developing a top cone mounted laser profilemeter'9'. These systems effectively replace a retractable overburden probe and, although expensive, have the advantage that they measure over a larger proportion of the stockline than the single radius of a probe.

3.4.8 Thermography

The use of thermal imaging cameras to detect hot spots, on the furnace shell'52', top gas system, tuyere stocks, stoves, hot blast and bustle mains and other ancillary plant, is important. Not only does it enable early detection of problem areas and permit their systematic rectification, but it also helps prevent catastrophic failures, in which the furnace has to be taken off-blast in a sudden uncontrolled manner followed by an often difficult recovery, which would have a detrimental effect on campaign life.

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3.4.9 Leak Detection

An efficient system of detecting water leaks into the furnace from tuyeres and other cooling members is essential. Undetected water leaks may chill the furnace, resulting in erratic operation and difficult recovery from chilled conditions. Water leakage will directly affect furnace campaign life if it damages the refractories. Water leaks in lower, hotter regions of the furnace, which are lined with carbonaceous materials, will inevitably result in oxidation of the refractories. Ratholes in the hearth refractories may result, which can lead to breakouts. Water leakage may also result in taphole problems which may disrupt operations.

Tuyere leak detection systems are often used. Following the installation of a graphitic hearth, bosh and lower stack, Dofasco installed a comprehensive leak detection system to protect its integrity, incorporating a system of magnetic flowmeters with computer analysis of the differential flows'93'. Another good system of leak detection uses a pressurised closed circuit water system incorporating make-up tanks; the make-up frequency indicating the severity of a leak. Other systems involve observation of gas bubbles or dissolved CO content in the water, differential pressure measurements, etc. Some plants install wall thermocouples beneath the tuyeres to warn of otherwise undetected water leaks.

A good leak detection system will often warn the operator of a water leak in its early stages, before an immediate off-blast is required. This gives the opportunity for the leaking member to be isolated prior to the furnace being taken off in a controlled manner, with reduction in tuyere hydrocarbon injection and ore/coke ratio adjustments, thereby minimising detrimental effects resulting from the subsequent stoppage (as detailed in Section 3.6).

3.4.10 Hot Metal Quality

When operating without a protective skull in the hearth, the hearth carbon may be removed by solution attack of iron and slag. Early carburisation of the iron, before it contacts the hearth refractory, will minimise such hearth wear.

For early carburisation, an extended period of contact between liquids and coke is required. At a given productivity, this may be encouraged by a taller dripping zone and deadman, with a higher cohesive zone. This usually results in an increase in hot metal silicon. Figure 3.44'94' shows that the carbon saturation level decreases with increasing silicon content. As a result, the hot metal will be closer to saturation at higher silicon levels, for a given furnace size and hot metal temperature. In addition, an increase in hot metal silicon increases the hot metal liquidus temperature and thereby reduces its fluidity. This will tend to reduce the flow velocity in the hearth and encourage the formation of a solidified layer on the hearth refractory.

At lower hot metal temperatures, the carbon saturation level of the iron is lower and will be achieved earlier. Low hot metal temperature has the added benefit of increased iron viscosity which will reduce peripheral flow, reducing the tendency to dissolve protective skulls and penetrate fine cracks and pores.

Higher hot metal silicon and lower hot metal temperature are difficult to achieve together, as a higher cohesive zone usually results in a warmer furnace, but the overall effect is for the hot metal entering the hearth to become closer to carbon saturation. A reduction in high top pressure is likely to result in a slight increase in silicon, without affecting the thermal state of the furnace.

The probability of dissolution of hearth carbon is therefore lower at higher silicon levels.

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3.5 Tuyere Diameter

Tuyere diameter is chosen to ensure adequate blast penetration for given operating conditions and to prevent excessive gas ascending the furnace walls. The selection of tuyere size will influence the degree of centre working of the furnace and the degree of protection of the bosh and lower shaft walls. It may be necessary to vary the tuyere diameter around the furnace to ensure circumferential balance of gas flow, which can be determined given a method of individual blast flow measurement such as pressure tappings or cast-in venturis in the tuyere stocks. Figure 3.45 shows the variation in tuyere size and blast volume distribution at NKK's Fukuyama No. 5 blast furnace'42'. A similar effect was achieved at NKK's Keihin No. 2 blast furnace, where hot blast control valves were fitted to individual tuyere stocks'95'.

Although tuyere sizes are carefully chosen, a significant increase in diameter is often observed when a tuyere is changed, particularly when long lives are achieved. This may affect both of the above factors and it is advantageous in terms of campaign life to change tuyeres after a given period, not only to minimise the effect of tuyere wear but to reduce the likelihood of water leakage into the furnace and the number of unscheduled off-blast periods to change failed tuyeres. Kawasaki Steel have developed an eccentric, high-velocity tuyere with a three layer plasma-deposited refractory coating and a welded protective layer on the nose. Tuyere failures at Mizushima Works have reduced to 0.3 tuyeres/million tonnes of hot metal, with service lives in excess of 1000 days'96'.

The diameter of tuyeres directly above the taphole is often reduced, or the tuyeres even closed, to promote smooth casting and reduce the iron make above the taphole. The effect of closing taphole tuyeres is illustrated by the trials with hot blast control valves which showed that if those valves above the taphole were closed when the taphole started to blow, the cast duration could be extended considerably, with casting rates still greater than the production rate'95'.

Tuyere diameter is often reduced locally, in response to high hearth sidewall temperatures, to reduce the dripping liquids and hearth activity in the problem area'8,42,46'. This may be done by the addition of tuyere inserts or by tuyere replacement. In severe cases, or as a short term emergency measure, the tuyeres in question may be closed by plugging with clay'1,51'. This often has a rapid effect in reducing the corresponding hearth sidewall temperatures.

3.6 Off-blast Periods

The number of off-blast periods, particularly unplanned ones, has a major effect on campaign life in terms of output per unit volume, which is reduced disproportionally to the percentage downtime. This is illustrated in Fig. 3.2'1), for Japanese and European blast furnaces. Wall damage may result from an increased degree of wall working at the lower blast volumes encountered whilst coming off and on blast, cooling and reheating of the refractories or erratic operation during recovery from the stoppage.

Dofasco's No. 4 blast furnace suffered a hearth breakout following a period of increased downtime (7.6-14.5%), which increased tuyere losses and contributed to premature ageing of the hearth'104'. It is notable that during the campaign of Kashima No. 3 blast furnace, which produced 48 Mt over 13.5 years, the longest stoppage was of 53 hours, with annual stoppage time of about 200 hours'1'.

Some operators indicate that off-blast periods 'rest' the hearth and allow a protective skull to form or thicken. In fact, taking the furnace off-blast is often an emergency procedure, at later stages of

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the campaign, when high temperatures are detected within the hearth refractory. An example of the reduction in sidewall temperature achieved as a result of such a stoppage is shown in Fig. 3.46

where possible, and to give the option of a compensated burden. The torpedo ladle fleet or steelworks' mixers should be used as a buffer for short stoppages. In certain circumstances, when there is minimal advance warning of a shutdown, the furnace is not dry and there is little empty ladle capacity, it may be preferable to plate the iron, despite the cost penalty involved.

Stack spray techniques have advanced, enabling the walls to be gunned in a relatively short stoppage, by blowing the burden down to a low level. Although this allows a large quantity of coke to be charged at the lower levels of the furnace to aid start-up, there is often difficulty due to the quantity of rebounded refractory falling into the furnace. Start-up will be easier if a low rebound material is used and the furnace is blown down to tuyere level, enabling the rebound material to be raked from the furnace'99'. This may be achieved more effectively by the use of T-shaped sheets of corrugated sheeting (Fig. 3.47), inserted rolled up through the tuyere cooler apertures'100'. The blow-in burden chemistry must also be adjusted to give a slag chemistry that will enable the residual rebound material to be melted.

There is a difference of opinion as to whether or not cooling water flows should be decreased for stoppages of greater than a given duration. Some operators prefer reduced flows to maintain refractory temperatures. The majority prefer the hearth cooling water on full flow to promote a thicker protective skull'41', whilst others who reduce the water flow suggest that by removing less heat it assists a smooth start-up.

Another factor which may affect the recovery from a stoppage is the removal of an accretion from the furnace walls, resulting from the additional wall working and erratic burden descent. This will encourage chilled conditions at a time when they are least desirable. If an accretion is known to have formed, it is preferable to try and remove it before a long stoppage. A good system of accretion monitoring will give immediate warning should this occur, to enable thermal compensation as soon as possible.

3.6.2 Plant Maintenance

All maintenance work possible should be carried out during production, thereby reducing the off-blast time necessary. To minimise the duration of a planned stoppage required for engineering maintenance, good planning and advance preparation are essential. Although these factors are obvious for economics and to maximise plant output, their long term effect on furnace life is not always considered.

Preparations should always be in hand for maintenance to be carried out if the furnace comes off-blast unplanned for other reasons. For instance, if the furnace is off for a tuyere change, it may be possible for work to be carried out on the charging system. If the furnace is off-blast for problems at the steelworks, then it may be possible for more extensive maintenance to be performed. In this way, the total number of stoppages during a furnace campaign can be reduced and their duration minimised.

Effective engineering maintenance operations will reduce the number of breakdowns which result in unplanned stoppages. This will involve routine maintenance, regular inspections, periodic checking of important instrumentation, condition monitoring, e.g. vibration and thermal monitoring'101'. This is most important at later stages of a campaign, as ancillary equipment gets older and less reliable. An example is the monitoring system for Chiba No. 6 blast furnace bell-less top, using vibration and displacement sensors to monitor wear in the chute rotation and tilt, shown in Fig. 3.48'4'. Similarly, improved casthouse maintenance techniques may reduce off-blast time, e.g. extension of the life of the main iron runner on a single taphole furnace will reduce downtime'102'.

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3.6.3 Long Stoppages

Depending on the duration of the stoppage, the furnace may be filled with a coke blank and a low ore/coke burden, or the burden may be blown down to tuyere level. For stoppages of several weeks or longer, the salamander or bear should be tapped'1'. If this is not done, not only will a considerable amount of process heat be required, during the recovery, to melt it, but it will expand whilst still solid and create undue stresses on the hearth refractories and shell, shortening their life. This will be even more important with newer furnaces, where the sump depth has been increased to reduce peripheral iron flow in the hearth. It is desirable to monitor these stresses with strain gauges attached to the hearth jacket, Fig. 3.49'52', and to develop procedures to minimise such stresses.

To recover from longer stoppages, when the furnace is in a cold condition, it is necessary to' warm the hearth and establish an early link between the taphole and the tuyeres to allow liquids to be removed. This may be done by the use of a blast pipe at the taphole'99,103' or the use of an oxy-fuel lance'104,105'. It is important to prevent the oxygen, entering at the taphole, damaging the hearth carbon, which could directly shorten the campaign life. Recovery from chilled hearth conditions, following major water ingress during a routine maintenance stop, has been reported to have resulted in severe hearth erosion'9'. Recovery from long shutdowns, with a large quantity of solid metal in the hearth and an impermeable deadman, may result in excessive peripheral flow in the hearth with accelerated hearth sidewall erosion'106,107'.

3.7 Casthouse Practices

Casthouse practices play an important part in controlling liquid flow in the hearth and avoiding high liquid levels which may impinge on the raceway, affecting blast distribution or even causing tuyere or blowpipe damage; factors which can affect the stability of operation, result in periods off-blast and potentially affect campaign life.

3.7.1 Taphole Length

With a longer taphole, the molten products are not only drawn from lower in the hearth, but also from a point closer to the centre of the hearth. This reduces peripheral flow near the taphole and hence wear of the hearth sidewall. To extend taphole length, it is necessary to increase the quantity of clay injected over a period of time, to progressively increase the size of the mushroom on the inside of the furnace, which will also protect the refractory below the taphole. The reduction in hearth sidewall temperature at Chiba No. 6 blast furnace, after increasing the quantity of taphole clay, is shown in Fig. 3.50'108'. A change in taphole clay resulted in an increase in taphole length at Kakogawa No. 2 blast furnace, which is shown by a decrease in hearth wall temperature, Fig. 3.51'49). Figure 3.52 shows how the predicted hearth wall circumferential temperature distribution, on a plane beneath the taphole, changes with reducing ratio of taphole length to hearth radius'79'. With a short taphole length and casting alternately from widely spaced tapholes, sidewall temperature fluctuations would increase, potentially increasing refractory erosion. A temperature trend of hearth sidewall temperature during casting is shown for Kobe No. 3 blast furnace (Fig. 3.53) and the hearth sidewall temperature distribution for Kakogawa No. 3 blast furnace (Fig. 3.54), both of which show peak temperatures at 20-50 from the tapholes, in agreement with the prediction. Figure 3.55 shows an increase in hearth bottom temperature, corresponding to the decrease in hearth sidewall temperatures achieved when the taphole length was increased at Kakogawa No. 2 blast furnace, indicating a reduction in peripheral flow and thereby a reduction in hearth sidewall erosion.

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High hearth pad temperatures may be experienced due to the loss of a frozen layer and/or hearth carbon dissolution, whilst hearth sidewall temperatures are satisfactory. At Kakogawa No. 3 blast furnace, the relationship between hearth pad and hearth wall temperatures was found to be cyclical as shown in Fig. 3.56'109'. In such cases, it may be necessary to shorten the taphole, by decreasing the quantity of clay injected and possibly by a reduction in the taphole inclination. This will help reduce hot metal flow near the furnace centre and increase the liquids retained on the hearth pad'107'.

3.7.2 Taphole Diameter

The taphole diameter necessary to sustain a given productivity will depend on the furnace parameters, such as the proportion of time casting, top pressure, slag volume, hearth coke size, liquid viscosities and taphole clay properties. Should' the taphole be too small for a given production rate, then it will not be possible to cast the furnace dry. If the taphole is too large, fewer molten products may be removed from the furnace during a cast as the taphole will blow prematurely, since the liquids above the taphole will be removed before the liquids on the opposite side of the hearth can descend through the coke bed'110'. In both of these cases, the liquid level in the hearth will remain high and will ultimately affect stable furnace operation. Consequently, the optimum size of taphole must be arrived at from experience.

When a single taphole is used, the size must be chosen to enable the furnace to be cast dry and give sufficient time for the clay to cure between casts. On a furnace where alternate tapholes are used, different sized tapholes may be required under certain operating conditions, to ensure drainage across the furnace.

On a multi-taphole furnace that is experiencing wear in the hearth pad, it may be desirable to increase the taphole diameter. This, together with a decrease in taphole length, should reduce the iron flow across the hearth pad and increase the residual iron in the hearth at the end of cast, thereby encouraging the formation of a frozen layer on the hearth pad'107'.

3.7.3 Soaking Bar Practice

The use of soaking bars has been practised for several years. A pointed bar or tube is driven into the freshly injected taphole clay, a few minutes after gunning-up. The characteristics of the clay are selected to suit the practice. The main benefit is the rapid curing of the taphole by the heat conducted down the bar from the hotter part of the taphole. This creates a consistent, stronger, more durable taphole which permits longer casts at a more even casting rate. This results in improved liquids removal and more stable operation.

3.7.4 Taphole Clays

The taphole clay properties are vital to blast furnace operations. The clay must set rapidly and cure fully between casts to create a strong, durable taphole. For soaking bar operation, it must be sufficiently plastic to allow bar insertion yet be capable of being drilled out without difficulty, to retain its dimensions and avoid casting delays. The clay must have good adhesion properties, to build up a strong, permanent structure that will resist the flow of liquids and also protect the hearth refractories below the taphole. Figure 3.51 illustrates how hearth sidewall thermocouples directly below the taphole can be used to monitor the properties of taphole clay and the build-up of the mushroom inside the furnace'49'.

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3.7.5 Number, Position and Performance

High productivity can be attained on a medium sized, single taphole blast furnace, e.g. Thyssen's 10.2 m Hamborn No. 9 blast furnace produced an average 4589 t/day in 1994, a productivity of 2.5 t/m3 W.V./d'111'. However, there are advantages when more than one taphole is available, this being a necessity at higher pr