The increase in the volume of pellets during reduction is measured in this test. The volume of the initial charge as well as that of the reduced material is measured by a mercury volumenometer. At the end of the test, the percentage increase in volume, the degree of reduction, and the compression strength of the pellets are reported.
9.8.3 Reducibility
For determining reducibility, two test procedures—relative reducibility and reducibility—are adopted. The results obtained from the first method indicate the degree of reduction as a relative rate compared with a standard sample; while in the second method, the rate of oxygen removal between 30% and 60% reduction is measured and the (dR/dt) at 40% reduction in per cent/
minute is reported. Both the methods help to assess the reduction rate when the higher oxides present in any iron oxide sample under testing are reduced to FeO.
9.8.4 Reduction under Load
The test is conducted in a vertical retort of 125 mm diameter in which the sample is exposed to reducing conditions (30% CO + 70% N2) at temperatures up to 1100°C as given in Figure 9.8. The test is particularly suitable for determining the stability of pellets, and other iron oxides, under these conditions. In addition to the extent of reduction, the shrinkage of the charge and its resistance to gas permeability are also measured. The differential pressure that is measured corresponding to 80% reduction gives an idea of the stability of the iron oxide during reduction.
9.8.5 Softening–Melting Test
In Japan and in the UK, in an effort to simulate the behaviour of iron oxides in the cohesive zone of a blast furnace, the test for reduction under load was extended up to 1500°C, using somewhat different test conditions, which are also included in Figure 9.9. This was necessary in order to study not only the reduction that takes place at low and medium temperatures in the solid state, but also the softening and melting behaviour of any iron oxide feedstock at higher temperatures in the cohesive zone of a blast furnace. The cohesive zone is the boundary between the two-phase (solid/gas) region in the upper part of the furnace, and the three-two-phase (solid/gas/liquid) region in the lower regions of the furnace.
Various parameters that are measured in the softening–melting test are shown schematically in Figure 9.9. Each of these parameters is defined below:
Weight Parameter Reduction under load Softening–melting test Sample
Size, mm 9.5–12.7 9.5–12.7
Weight, g 500 200–300
Load, kg/cm2 2 1
Reducing gas
Flow rate, Ipm 15 7.2
Composition, % CO: 30 CO: 30
N2: 70 CO2: If require N2: 70
Heating rate RT-800°C: 90 min. RT-1000°C: 10°C/min.
800–1100°C: 180 min. 1000–1500°C: 5°C/min.
?
Figure 9.8 Schematic diagram of the reduction under load test (including test conditions).
S-value
TS Tm
0 100 500 1000
DP, mmWC
Temperature, °C
Figure 9.9 Depiction of various parameters measured in the softening–melting test.
(i) Softening start temperature (Ts): It is the temperature in degree Celsius at which softening begins denoted by a pressure differential of 100 mm water column across the bed.
(ii) Melting temperature (Tm): It is the temperature in degree Celsius at which melting is completed, i.e. when the pressure drop across the bed after reaching its maximum value again comes back to 100 mm water column. If the pressure drop does not reach this value after the test, Tm is assumed to be greater than 1550°C, which is the maximum temperature that the sample can reach in the softening–melting test apparatus.
(iii) S value (expressed in kPa°C): It is determined by measuring the area under the curve generated by plotting the pressure differential (DP) across the bed against temperature (shown in Figure 9.9). It is a measure of the resistance offered by the bed to gas flow in the furnace.
(iv) Non-dripped material (%): Non-dripped material is defined as the amount of residue that is left in the crucible after the test is completed, expressed as a percentage of the total weight of the sample less the oxygen associated with iron.
For lump iron ores, the softening start temperature (Ts) usually varies from 1150°C to 1200°C depending on its composition, whereas depending on the basicity and its gangue content, Ts of sinter is usually higher. High-quality sinter can have softening start temperatures as high as 1300–1320°C. The Ts values of mixed beds of lump iron ore and sinter are between the softening temperatures of the individual components. The final melting temperature (Tm) of iron ores and sinter varies between 1350°C and 1550°C. It is highly desirable to have as small a difference between Tm and Ts as possible, so that the mushy zone temperature range (Tm–Ts) is restricted. This temperature difference gives a direct indication of the span of the cohesive zone in a blast furnace. The narrower the cohesive zone, the easier it is for the gases to flow through the bed, resulting in better reduction kinetics of iron oxide. It is preferable to keep this difference in the range of 150–180°C, for optimum blast furnace performance.
The S-value, which indicates the resistance offered by the bed to gas flow, is usually higher for lump ore compared with sinter, because lump ores generally decrepitate more under reducing gas conditions at relatively lower temperatures. For any high-quality iron oxide burden, the resistance offered by the bed to gas flow should be as low as possible, in order to allow faster flow of gases through the bed.
The amount of non-dripped material measured at the end of the test gives an idea of the slag characteristics in the drip zone of a blast furnace. It is desirable that the fluidity of the slag formed by the iron oxide burden should be sufficiently high so that it can easily flow out of the cohesive zone. Low residue, i.e. less of non-dripped material (maximum 30–35%), is taken as an indication of adequate slag fluidity. As far as this parameter is concerned, lump iron ores which are acidic in nature fare better than sinter, which is generally basic.
9.9 RECYCLING OF MATERIALS IN THE BLAST FURNACE
In the case of most elements like Si, S, etc. the input is equal to the output, and there is no accumulation within the furnace. Carbon is also, to some extent, recycled within the furnace on
account of the Boudouard reaction (Eq. (5.5)). Indirect reduction of iron oxide by CO produces CO2. CO2 further reacts with C to form CO at the bottom of the stack. CO in excess of equilibrium deposits carbon because of the backward reaction in the upper stack at a lower temperature (Eq. (5.5)). The fine carbon thus deposited travels downwards, and gets gasified into CO again. However, there is no net accumulation of carbon in the furnace.
In contrast, there are some elements/species that tend to accumulate inside the furnace over a period of time, since the output in these cases is less than the input. Alkalies and zinc fall in this category. The alkali metals enter the furnace as constituents of the fluxes, iron oxide and coke. They are found in many coal and iron ore deposits around the world. Therefore, the iron oxide used (lump ore or sinter/pellets) as well as the coking coal used to produce coke have to be carefully chosen with stringent limitations on their K2O and Na2O contents. Otherwise, the alkali oxides would generate large volumes of potassium and sodium vapour within a blast furnace by reaction with carbon above 1500°C. This occurs principally in the tuyere zone and, to some extent, in the bosh and hearth according to the overall reaction:
2K2(Na2) SiO3(s) + 6C(s) = 4K (Na)(g) + 2Si + 6CO(g) (9.7) Of course, a part of the alkali silicates joins the slag phase. The vapour generated as per Eq. (9.7) travels upwards and reacts with the burden. Both potassium and sodium react in almost the same way and, hence, only one will be dealt with. The following reactions are of importance.
Formation of cyanides in the upper part of the stack. For example:
2K(g) + 2C(s) + N2(g) = 2KCN(g) (9.8)
The cyanide vapours get absorbed by the solid burdens and descend downwards.
Formation of carbonates in the upper part of the stack. For example:
2K(g) + 2CO2(g) = K2CO3(s) + CO(g) (9.9) These carbonates deposit on the descending burden. At a higher temperature, they either get decomposed into the respective vapours through the reverse reaction, thereby causing excessive decrepitation, or alternatively, they dissolve in the slag (which is less detrimental to the process, but gives rise to problems of slag disposal).
Since alkali metals have negligible solubility in hot metal, they can be flushed-out only through the slag as oxides. Acid slags have the capability of absorbing more alkali oxide than basic slags, and hence SiO2 in the form of quartzite and feldspar is occasionally charged into the furnace to flush-out excess alkalies.
Accumulation of alkalies in the elemental form or as cyanides is definitely harmful to efficient blast furnace operation owing to the following reasons.
1. Increased productivity of blast furnaces requires smooth gas flow and high burden permeability. Disintegration of the oxides or the coke into smaller pieces because of reduction and reaction lowers the permeability and therefore, decreases the productivity.
Alkali metals make the coke more reactive besides leading to greater disintegration of ore, coke and sinter as shown in Figure 9.10 (Sasaki et al. 1977). The fact that the coke becomes more reactive can also have an adverse effect on productivity.
1
5
10
15
17 Tuyere
20 30 40 50 84 88 92 96 0 40 80 0 10 20
Mean coke size, mm Coke drum
strength, % Reactivity, % (K2O + Na2O) in ash, %
Figure 9.10 Variations in size, strength and reactivity of coke along the height of a blast furnace as a function of the alkali content of coke ash.
2. Alkalies and alkali cyanides enhance degradation and erosion of the refractory lining, particularly in the stack area.
3. Alkali cyanides tend to bind ore pieces forming accretions and thus cause operational problems (scaffolds in the stack which is dealt with in detail later in Chapter 10, Section 10.2.4).
Therefore, the best way to alleviate the problem of alkalies is to choose raw materials with low alkali contents to keep the alkali input below a maximum of 2 kg/thm.
REFERENCES
Chatterjee, Manoj, World Iron Ore Industry and Role of Indian Iron Ore Sector, Steel Scenario, Vol. 15 (2005) No. Q1.
Eketorp, S., Chipman Conference Proceedings, MIT Press, 1962.
Gupta, S.S. and Amit Chatterjee, Blast Furnace Ironmaking, SBA Publication, 1995.
Habishi, F., Handbook of Extractive Metallurgy: Iron, Vol. 1, Wiley–VCH, Weinhein, 1997.
Sasaki, K., et al., Trans. Iron Steel Inst. Jap., 17 (1977), 252.
10.1 INTRODUCTION
Around the year 1960, the world’s best blast furnaces were capable of producing about 2000 tpd (i.e. tonnes per day) of hot metal. Around the year 1980, the production in a few blast furnaces in Japan went up to 12,000 tpd, which was the maximum at that time. After 2000, the maximum production in the largest individual blast furnace (e.g. the Schwelgern furnaces of ThyssenKrupp in Germany) has touched 15,000 tpd. The world average per furnace is obviously lower, but has been going up steadily—for example, the average production in furnaces in Germany, which was about 1200 tpd in 1971 increased to 4700 tpd of hot metal in 1998. At the same time, it needs to be noted that the number of very large furnaces (greater than 3000 m3 working inner volume) has not gone up substantially; instead, what has happened is that many furnaces of lower capacities have been shut down.
Large blast furnaces with working volumes more than 2000 m3 which have recently been built in Asia include Baoshan No. 3 (4000 m3) in China, POSCO Kwangyang No. 5 (3000 m3) in Korea and China Steel, Kaoshung No. 4 (3000 m3) in Taiwan. Two very large furnaces (3800–4200 m3) are planned to be installed by Tata Steel over the next few years. Jindal South West Steel is also in the process of installing a similar capacity furnace. Since ironmaking and steelmaking capacity is predominantly going up in Asia, most of the twelve new blast furnaces built recently have been in Asia; whereas in the Western world, the number of furnaces in operation has been decreasing year by year.
It is interesting to note that in 1992, there were 555 blast furnaces existing in the Western world, 390 of which were in operation (around 70%) and the output amounted to 314 Mt or about 800,000 tonnes per furnace, i.e. production of 2250 tpd per furnace per year (assuming 355 days of operation, which is typical today). At present there are about 700 blast furnaces (including 300 in the Western world) in operation globally. During the past decade the number of blast furnaces available in the USA has also decreased from 83 to 43 and the operating furnaces from 48 to 40. Despite this decrease of 17% in the number of operating furnaces, the production has increased by 27% in the USA.
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