S. J. Pickering/New process for dry granulation and heat recovery from slag

New process for dry granulation and heat recovery from molten blast-furnace slag

S. J. Pickering, N. Hay, T. F. Roylance, and G. H. Thomas

Ironmaking and Steelmaking. – 1985. – Vol.12. - No.1
A new dry granulation process for molten slag is described. The process produces a slag product of high glass content, suitable for cement manufacture, and allows heat recovery from the slag. Experimental tests have been carried out on a laboratory-scale plant to prove the new process. Empirical relationships have been derived to predict the behaviour of the granulation operation. A study of a full-scale plant has been made and shows that about 60% of the heat in the molten slag can be recovered and utilized within a blast-furnace complex. Fuel savings of the order of 2 m/year could be achieved processing half the slag from a large blast furnace of capacity 10000 t/d of iron which is operating with about 7700 t/d throughput.

INTRODUCTION

HEAT RECOVERY FROM SLAG

The production of iron and steel is an energy-intensive process; the manufacture of steel by the conventional blast furnace-basic oxygen furnace route requires an energy consumption of typically 24 GJ (~ 11 of coal) per tonne of crude steel. With ever-increasing emphasis on the efficient use of energy, new ways are sought to reduce the energy required to make iron and steel. In existing iron- and steelmaking processes this can be done by recovering and utilizing waste heat. The sensible heat in molten slag is a source of waste heat and, although it only accounts for 2-3% of the energy consumption to make crude steel, it is a very useful heat source as its temperature is over 1400°C. High-temperature heat sources mean that the efficiency of the heat-recovery process can be greater and the heat utilized in a greater variety of processes. It is because of the difficulties arising from the very high temperature and the corrosive nature of molten slag that attempts have not been made until recently to recover the waste heat from it.
Molten slag is produced in both the iron- and steelmaking processes. About twice as much iron-making slag is produced as steelmaking slag and the new process described in this paper is concerned only with the ironmaking or blast-furnace slag. A typical blast furnace produces about 0,3 t of slag for every tonne of iron produced.

USES FOR SLAG PRODUCT

In considering processes for molten slag it is important not only to recover the heat from the molten slag but also to produce a commercially useful slag product. All the blastfurnace slag currently produced in the UK is sold as a raw material for other processes; most of these are within the construction industry and involve using the slag as an aggregate or for cement manufacture.
Blast-furnace slag can have two forms in the solid state: itmay have a crystalline structure, produced by cooling the molten slag relatively slowly, or it can have an amorphous or glassy structure which is formed by cooling the slag very rapidly, usually by water quenching. The sensible heat released when molten slag is cooled to form a glassy product is about 17% less than the heat released when a crystalline product is formed. The glassy slag has properties of hydration and can be used, in certain circumstances, as a partial substitute for Portland cement. Glassy slag is the more valuable product and indirectly can save energy in the cement industry.
Conventionally, molten slag is quenched in water when a glassy granulated slag product is required. This must be dried before it can be ground for use in cement and the drying process can require up to 0,3 GJ/t of slag. (The cost of this drying process energy may be m the region of ?2/t of slag.)

OTHER WORK IN THIS FIELD

In the area of heat recovery from molten slags, some work is already 1n progress in Sweden and Japan.
In Sweden, a subsidiary of the State Steel Company, Merox Ltd, is developing a process for granulating and recovering heat from slag. The slag is; granulated by striking a falling film of slag with previously solidified slag particles. This breaks the film up into granules that then fall into a multistage fluidized bed from which heat is recovered. It is claimed that over 60% of the sensible heat in the slag can be recovered as steam by this method and that the slag product has a high glass content making it suitable for cement manufacture.
In Japan three separate schemes are under investigation. Sumitomo Metals Industries is developing a dry granulation process for blast-furnace stag where a stream of slag breaks up as it impinges on to a rotating drum. The slag particles then fall into fluidized bed where the heat is recovered. This process aims to produce a particulate slag to substitute for river sand as well as to recover about 55% of the heat in molten slag in a stream of hot air. Mitsubishi Heavy Industries and Nippon Kokan KK are developing a process for granulating basic oxygen furnace slag using a powerful air blast to break up a slag stream.4 The slag particles solidify as they travel through the air and the heat is recovered by radiation from the spray of particles and also from the bed into which the particles fall. The Kawasaki Steel Corporation is also developing a process for recovering heat from blast-furnace slag. This involves granulating the slag by mechanical agitation and recovering heat from the granulation process by radiation and later from the granulated slag particles in a bed. The sljg product is used as an aggregate for the construction industry.
In this paper a new process to produce a dry granulated slag product with a high glass content from molten blastfurnace slag is described. The sensible heat from the molten slag is also recovered and ways are suggested for utilizing this heat within an ironworks.

NEW PROCESS FOR DRY GRANULATION AND RECOVERY OF HEAT FROM MOLTEN SLAG

Essentially, the process is to atomize the molten slag and then to cool the particles rapidly so as to produce a glassy slag. The atomization is done using a rotary-cup, air-blast atomizer. The particles cool as they travel through the air and are then cooled further in a fluidized bed. Both of these processes provide the rapid cooling necessary for the formation of glassy slag product. The fluidized bed is a convenient method of containing the slag particles as it prevents the agglomeration of hot particles in addition to providing rapid cooling. Figure 1 shows a design for a plant to handle 40 t/h of molten slag.

SLAG PRODUCT

Slag particles with a mean diameter of ~ 2 mm can be produced by the atomizer and so the slag product is in a form that is easy to handle. Tests have shown that the slag particles are cooled fast enough for the product to ha\e a glass content in excess of 95% Also very little slag wool is produced in the process.

CHOICE OF ATOMIZATION PROCESS

The rotary-cup, air-blast atomizer was chosen for several reasons. First, it offers fine control very easily. The particle size can be controlled by varying either the rotary-cup speed or the air-blast flow. This atomizer also produces a relatively narrow particle size range. The problem of having a rotating cup in contact with molten slag need not be limitation and can be solved with careful design and the correct choice of materials. Second, the power required to drive this atomizer is considerably less than that required by a twin-fluid atomizer. A twin-fluid atomizer needs over 20 times as much power to atomize the slag to similar-sized particles as that required by a rotary-cup, atomizer. This could affect the overall cost savings of the heat-recovery process by up to 5%. Finally, the trajectory of the slag particles is outwards and upwards. The upwards motion means that the atomizer can be used in a location where there is restricted height between the slag delivery point and the ground. This could be the case at a blast furnace where slag is supplied from the cast-house floor which may be only 7 m above the ground.

ATOMIZATION PROCESS

The rotary-aip, air-blast atomizer operates by spinning out a thin film of slag which extends radially from the cup lip. As the film of molten slag extends from the cup lip, it will break up of its own accord. (This is the method of operation of a rotary-cup atomizer without air blast.) However the presence of an annular air jet around the cup assists the break up of the slag by inducing unstable waves in the film. The air blast has the effect of producing small particles of a more uniform size and of deflecting the particles upwards to produce a cone-shaped spray moving out from the atomizer.

HEAT-RECOVERY PROCESS

Heat is recovered from the slag in several ways: (i) as the spray of slag particles moves outwards from the atomizer, heat is lost by radiation to the vessel and by convection to the air moving through the vessel; however, as the time of flight is short (of the order of 0,1 s) the temperature falls by only about 100-200 K; (ii) on impact with the vessel wall, some heat is transferred from the slag to the wall. It was found from tests that if the wall was kept relatively cool the slag particles do not stick to the wall but either bounce off immediately or fall off after a very short time. In either case the time of contact between the slag particles and the vessel wall is considerably less than 0,1 s (less than 5 ms for particles that bounce off) and again the temperature reduction of the slag is only about 150 K; (iii) more heat is lost by radiation and convection as the particles fall from the wall into the primary fluidized bed; (iv) the remainder of the heat recovery is in the fluidized beds, by heat transfer just to the fluidizing air or additionally by immersed boiler tubes or other heat-transfer surfaces. In a fluidized bed, the fluidizing air and solids leave the bed at the same temperature as the solids in the bed. The temperature of the secondary bed must be chosen carefully. If the bulk slag particle temperature is too hot recoverable heat in the slag would be wasted; if it is too cold the usefulness of the hot fluidizing air on discharge from the process would be reduced.

EXPERIMENTAL WORK ON ATOMIZATION OF MOLTEN SLAG

Experimental work was undertaken with the initial aim of investigating whether or not the atomization process would produce small slag particles and whether the particles had a glassy structure. A rotary-cup, air-blast atomizer was built, with a cup of 100 mm dia. and a speed range of 500-1500 rev/min. The slag flowrates used in the tests varied from 0-2 kg/s (the minimum flow required to produce a slag film at the cup lip) to 0-5 kg/s. The tests were restricted to a duration of about 2,5 min because of the limited capacity (~30 kg) of the laboratory slag-melting furnace.
The initial tests proved that the process did work; small particles with 95% glass content were produced. Further tests were then carried out to investigate the particle sizes and spray trajectories produced by the atomizer.

PARTICLE SIZE

A sieve analysis was carried out on the slag particles produced by the test atomizer. The results were plotted on a log-probability grid from which the mass median diameter dm could be determined. (The mass median diameter dm is the particle size which splits the distribution in half by weight.)

RECOVERY AND UTILIZATION OF SENSIBLE HEAT

Having established that the slag granulation process was effective on a laboratory scale, an analysis was made of the heat-recovery process that might be used in practice. The plant that would be used is that shown in Fig. 1.

Ideally the heat recovery and dry granulation plant should be positioned adjacent to a blast furnace, so that slag can be taken directly from the furnace without the necessity for intermediate handling. A blast furnace where only one taphole is used need only have one heat-recovery vessel; however, on large furnaces with four tapholes where two are in operation one on either side of the furnace, two heat-recovery vessels would be needed. This is because of the difficulty of transporting the slag from one side of the furnace to the other.

The analysis was made using data from a large blast furnace (14 m hearth dia.) with a maximum output of 10000 t/d of iron, while operating at 77% capacity. The furnace had two tapholes in operation, but in the analysis the use of slag from one taphole only is considered.

PROBLEMS OF 1NTERMITTENCY AND A PROPOSED SOLUTION

Slag is tapped intermittently from a blast furnace. The corresponding intermittent operation of the heat recovery and granulation process would entail significant extra losses at each start up and shut down and the thermal cycling would cause maintenance problems in addition to the problems, of trying to utilize the heat recovered intermittentl. For the furnace under consideration, each slag cast lasts approximately 1,6 h with an interval of 2,7 h between each cast. During each cast the average slag flowrate is 35,8 kg/s. Owing to daily fluctuations in the slag tapped from the furnace, a very large slag accumulator would be needed to utilize all the slag and provide a steady continuous flow to the granulation process. A statistical analysis showed that a slag accumulator of 150t capacity would permit a continuous slag flow of at least10 kg/s for 95% of the time and a flow of at least 11,5 kg/s for 70% of the time. The average slag flowrate from the accumulator vessel would thus be 11 kg/s. If the heat-recovery plant has a maximum capacity of 11,5 kg/s then 86% of the slag from one taphole could be utilized. The remaining 14% of the slag would flow to waste when the accumulator is full. This system with a slag accumulator of 150 t is adopted in the analysis that follows.

PROPORTION AND FORM OF HEAT RECOVERED

The sensible heat in the molten slag relative to 30°C as it leaves the blast furnace at 1500°C would be about 1,8 MJ/kg. Of the total slag flow, about 14% would overflow the slag accumulator to waste as mentioned above. From the remaining 86%, 32% of the heat content cannot be recovered because: (i) the latent heat of crystallization is not released when a glassy slag is formed; (ii) the solid slag product is discharged from the heat-recovery process at 250°C; and (iii) heat losses occur in the slag accumulator.

Thus, only 58,5% of the sensible-heat content of the slag discharged from one side of the furnace would be recovered. This corresponds to a heat-recovery rate of 13-35 MW or 0-3 GJ/t of iron produced from one side of the blast furnace or a saving of about 1% of the furnace energy requirement. Given this distribution of the recovered heat, two schemes for the recovery plant have been envisaged. In scheme 1, heat is removed by blowing a large volume of air through the beds. In scheme 2, a minimum volume of fluidizing air is used and the heat is removed by using boiler tubes in the bed to raise steam. A detailed study of the heat recovered and power requirements for each scheme was made and the results obtained were
Scheme 1
Heat-recovery rate from hot air at top of vessel 18,7 Nm3 /s at 432°C 10,30 MW 77%
Heat-recovery rate from cooled walls by generating saturated steam at 16 bar 1,12 kg/s 3,05 MW 23%
Total heat recovery 13,35 MW
Power required to drive blowers for fluidized beds, atomizer, etc. ~750 kW.
Scheme 2
Heat-recovery rate from hot air at top of vessel 8,64 Nm3/s at 516°C 5,73 MW 43%
Heat-recovery rate by steam generated from cooled wallsand tubes immersed in fluidized beds - Saturated steam up to 16 bar 2,84 kg/s 7,62 MW 57%
Total heat recovery 13,35 MW
Power required to drive blowers for fluidized beds, atomizer, etc. ~ 370 kW.

UTILIZATION OF RECOVERED HEAT

The most satisfactory way of utilizing the recovered heat is to use it on the blast furnace. This means that the heat does not need to be transported for long distances and also that the supply of recovered heat will match the blast-furnace demand. The energy flows in an integrated steelworks are complex. The main purchased primary fuels are coal, oil, natural gas, and electricity; however, there are also internally generated fuels, i.e. coke, coke-oven gas, blast-furnace gas, and some electricity. Obviously, supply and demand of each of the fuels must be carefully matched and the uses to which the internally generated fuels are put will vary somewhat from works to works. The particular use to which the recovered heat is put will depend on which fuels are displaced, to what extent, and on the consequent cost savings. The savings described in this section apply to the particular blast furnace described above; however, the same approach would yield savings of a similar proportion at other works.

USES FOR STEAM

Steam is used on a blast furnace primarily to humidify the hot blast. The quantity of steam generated by heat-recover) scheme 2 is approximately the same as the humidification requirement. Using the steam generated by the heat-recovery process to humidify the blast would release steam from this duty at the works power station. More steam would then be available to generate electricity. Alternatively, less steam could be generated in the power station and so blast-furnace gas would be saved.

USES FOR HOT AIR

The heat recovered as hot air by scheme 1 represents about 8% of the heat needed to produce the hot blast. However, using this heat directly to preheat the cold blast would result in higher temperatures at the bottom of the blast stoves which would consequently reduce their efficiency and so there would be little overall saving. Preheating the combustion air for the stoves with the hot air available offers three methods for saving fuel: method 1, by reducing the coke-oven gas enrichment in the stove combustion gas; method 2, by reducing the quantity of enriched stove combustion gas; and method 3, by increasing the heat input to the blast. These three methods are shown in comparison with the base case in Fig. 6 and are discussed below in more detail.
Method 1 - Reducing coke-oven gas enrichment
The hot blast stoves burn blast-furnace gas (bf gas) enriched with coke-oven gas (co gas) to obtain a higher flame temperature. Blast-furnace gas has a low calorific value (~3 MJ/m3) compared with coke-oven gas (~ 18 MJ/m3). Coke-oven gas can be used in a greater number of processes than bf gas and it is thus a more desirable fuel to be saved. The use of preheated combustion air in the stoves can reduce considerably the amount of co gas enrichment required, while maintaining constant flame temperatures. Calculations show that by utilizing the 10-3 MW of heat recovered from the slag to preheat the stove combustion air, the co gas enrichment can be reduced by 28-7 MW. The same total heat input to the stoves (156 MW) is then maintained by using extra bf gas, equivalent to 18-4 MW, see Fig. 6. The volume of gas passing through the stoves is shown by calculation to be the same after the reduction in co gas enrichment. Thus, the stove efficiency remains constant. This is a good use for the recovered heat as it allows a low-grade fuel to be substituted in place of a high-grade fuel.
Method 2 - Reducing combustion-gas consumption
Alternatively, the preheated combustion air could be used to replace some of the mixed combustion gas. The use of preheated air without changing the composition or flowrate of the mixed combustion gas could result in higher flame temperatures and a greater heat input to the stoves. The heat input can then be reduced to that required by reducing the flowrate of mixed combustion gas. This reduces the volume flowrate of the combustion products through the stove. The higher flame temperatures and lower volumes of hot gases result in greater stove efficiencies. Calculations have shown that the use of 10,3 MW of preheat in the combustion air can result in savings of 17,0 MW of mixed combustion gas while still controlling the hot blast to the same temperature.
Method 3 - Increasing heat input to blast
If the combustion air for the blast stoves is preheated and the same amount of combustion gas used, then the hot blast temperature can be increased. This can result in savings in coke but less bf gas would be produced. Preheating the stove combustion air with 10,3 MW of recovered heat could increase the hot blast temperature from 1100 to 1169°C. This would produce savings in coke of 17,2 MW; however, 12,3 MW less of bf gas would be produced.

FINANCIAL SAVINGS

The financial savings would depend on the cost of each type of fuel. The typical costs of purchased fuels are readily available but it is very difficult to price internally generated fuels in a way that is realistic. The values of internally generated fuels may vary considerably from works to works. Nevertheless using typical fuel costs the net savings with either of the heat-recovery schemes, (in both cases using the hot air to save stove combustion gas) would be ?l-5m-?2m/year. This allows for the cost of electricity to operate the heat-recovery plant. If the higher blast temperature method was used instead, the saving would be smaller at about ?lm/year. In addition to these savings, the production of a dry slag granulate would save the energy of the drying operation that is required when a wet granulate is produced. The cost of the drying operation is up to ?3/t of slag. It is thus likely that the additional savings of the drying operation could be up to ?lm/year. Thus, the total savings in fuel costs could be in the region of ?2m-?3m/year for a blast furnace with a capacity of 10000t/d of iron operating at 77% capacity by processing the slag from one side of the furnace.

CONCLUSIONS

The new process described in this paper produces a dry granulated product, which has a high glass content, from molten blast-furnace slag. Also a significant amount of energy is recovered from the sensible heat in the molten slag.
Experiments have demonstrated that a rotary-cup, air-blast atomizer can be used to atomize the slag and equations which describe the particle size and trajectory of the spray in the atomization process have been obtained.
A theoretical study has been made of a full-scale plant and it was shown that if the process is integrated with an ironworks almost 60% of the sensible heat in the molten slag can be recovered and utilized. The study has shown that for a blastfurnace producing on average 7700 t/d of iron an annual saving on fuel costs of the order of ?2m-?3m could be achieved. The granulated slag is a saleable commodity; it has a high glass content and has an advantage over conventional granulated slag in that it is produced dry.
The process as investigated so far holds promise and is worthy of further development. The next stage would be the construction and testing of a full-scale pilot plant to prove the granulation process on a large scale and also to determine the heat recovery that can be obtained in practice.

ACKNOWLEDGMENTS

The theoretical work described in this paper was done in the Department of Mechanical Engineering, University of Nottingham, and the experimental work at the Teesside Laboratories of the British Steel Corporation. The authors would like to acknowledge with thanks the use of these facilities and the help and advice given by BSC personnel and also for permission by BSC to publish this paper.

REFERENCES

  1. The iron and steel industry', Energy Audit Series No 16, Department of Energy, London, April 1982.
  2. 'A new granulation technique for heat recovery from molten slag', Energy Technology, No. 4, National Swedish Board for Technical Development, 1979.
  3. M. YOSHINAGA et ai\ Trans, Iron Steel Inst. Jpn, 1982, 22, 823-829.
  4. J. ANDO et al:. Mitsubish Heavy Ind. Tech. Rev., June 1981, 18, (2), 133-141.
  5. UK Patent GB 2 091 397 A, 1981.
  6. R. P. FRASER et al.: J. Inst. Fuel, Aug. 1963, 316-329.
  7. R. p. FRASER et al: Br. Chem. Eng., Sept. 1957, 496-501.
  8. N. DOMBROWSKI and T. L. LLOYD: Chem. Eng. J., 1974, 8,
  9. T. HATCH and s. CHOATE: /. Franklin Inst., 1929, 207 : 369.
  10. R. P. FRASER et aL Chem. Eng. Sci., 1963, 18, 339-353.