The Effect of Sulphur and Phosphorus Pentoxide on the Strength And

The Effect of Sulphur and Phosphorus Pentoxide on the

Strength and Reducibility of Iron Oxide Briquettes*

By C. A. PICKLES** and P. J. T UMIDAJSKI**

Synopsis slag phase, similar trends would be expected if either The strength and reducibility of iron oxide briquettes were measuredby blast furnace gas or solid carbon was used as the the reduction-under-loadtechnique and the loss-in-weight technique, re- reductant rather than pure hydrogen gas. spectively. The effectsof sulphur and phosphorusPentoxide concentrations on the two properties were measured at elevated temperatures. Also, lime II. Literature Survey additions were made to briquettes containing either phosphorus Pentoxide Shimomura et al, have studied the effect of sulphur or iron sulphide. In general, phosphorus Pentoxide and sulphur lowered on the internal state of the lumpy and softening zones both the strength and reducibility of the briquettes. Microscopic examina- tion demonstratedthat this is due to the formation of a slag phase with of the blast furnace.4~ The ores in the lumpy and a low melting temperature range which decreases both the activity of the softening zones absorbed sulphur from the gas and iron oxide and the porosity of the briquette. The addition of lime improved the sulphur distribution in these zones matched that the strength of the briquettes containing either phosphorus Pentoxide or in the gas flow. In the softening-melting zone the sulphur. sulphur concentration increased due to the higher sulphur content of the gas phase and the improved I. Introduction conditions for sulphur absorption. Below the soften- In the past, the mechanism of formation of the ing-melting zone the slag and metal absorb a large softening-melting zone in the blast furnace was poorly quantity of sulphur. In the hearth, the metal is de- understood. Experimental furnaces are not repre- sulphurized by the slag phase. The sulphurization rate was a maximum at 1 000 sentative of the processes which occur in operating °C and in general blast furnaces and samples obtained by boring sam- , it increased with temperature and plers were limited to the lumpy zone. However, in with the reducing potential of the gas phase. Also, the last decade, Japanese workers have obtained in- when the slag and metal began to separate, the ab- formation and data regarding the distribution and the sorption of sulphur from the gas increased and de- changes in the structure and composition of burden sulphurization of the metal by the slag was initiated. materials inside the furnace. These results were ob- The sulphur which transferred to the gas phase in the tained by quenching blast furnaces after normal opera- tuyeres and in the dropping zone was absorbed by the tion.1_3) With this information, new experimental burden before it reached the top of the furnace. techniques and test procedures have been devised for Kuwano et al. added gaseous sulphides during the studying the behaviour of the burden materials in the reduction of iron oxide pellets in the temperature blast furnace. Both the strength and reducibility of range 800 to 1 000 °C.5 The reduction rate de- the briquette or pellet depend on the temperature at creased with the addition of GUS and H2S to CO which the lowest melting range liquid forms and the and H2, respectively. Dense iron sulphide shells were amount of liquid produced by the reaction between observed which reduced the rate of diffusion of gas the slag components and wustite. Thus, it is neces- to the oxide core and thus decreased the reduction sary to improve the high temperature properties of rate. Also, the penetration of the sulphide gas into iron oxide pellets by ensuring that the melting tem- the oxide core contributed to the lower reduction rate. perature range of the slag, which initially forms, is Microexamination of samples of ore reduced in the high. presence of sulphur dioxide exhibited a low-melting In this investigation, the effects of sulphur and Fe-S-O eutectic phases} This phase reduced the phosphorus pentoxide on the strength and reducibility melt-down temperature of the sample. At low of iron oxide briquettes were investigated by the reduc- concentrations of liquid phase the reduction was ac- don-under-load technique and thermogravimetry, re- celerated. It was postulated that this increase could spectively. In the blast furnace, the reductant can be be caused by the more favourable kinetics for the either solid carbon or blast furnace gas. This gas is removal of oxygen from the bed due to a liquid/gas a complex mixture of carbon monoxide, carbon di- reaction rather than a solid/gas reaction. At higher oxide, hydrogen and various other minor constituents. liquid concentrations, pores became blocked and dif- In this study it was decided to reduce the number of fusion of the reducing gas was prevented. In this variables and use either pure hydrogen, carbon mon- case, the reduction rate decreased. These results are oxide or methane as the reducing gases. Since the in good agreement with those of Takahashi et al.'s major factor affecting the softening-melting behaviour The addition of sulphur to a basic sinter also re- of the pellets or briquettes is the composition of the sulted in a decrease in the melt-down temperature.

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There was no initial acceleration in the reduction rate density of about 5.12 g/cc. This indicates that the but the amount of gaseous reduction decreased. In porosity was about 2.3 %. Care was taken to ensure these samples, it appeared that calcium silicate formed that all briquettes received the same heat treatment rather than calcium sulphide and thus lime should not so that differences in the reducibilities and hot com- prevent premature sulphur induced melt-down. pression strengths could not be attributed to the sin- There was no information in the literature regard- tering operation. The porosity for briquettes to which ing the effects of phosphorus pentoxide on the strength the various additions were made was always below or reducibility of iron oxide briquettes. However, it 2 % and this porosity would not be expected to sig- would be expected that any phosphorus pentoxide nificantly affect the mechanical strength or the re- which enters the furnace would have a significant ducibility of the briquettes. effect on the softening-melting behaviour of the slag- The briquettes were sintered by slowly raising them forming constituents in the blast furnace burden. In through the hot zone of a vertical tube furnace. The " green " briquettes were general, the phosphorus pentoxide should reduce the placed in an uncovered melting temperature range of the slag phase which sample holder which was attached to an electric forms during reduction. Thus, it would be predicted hoisting mechanism. Then the motor was started. that both the strength and reducibility of the briquettes When the sintered briquettes emerged from the top would decrease when phosphorus pentoxide is present. of the furnace, a limit switch automatically turned off the motor. A schematic diagram of the sintering III. Experimental furnace is shown in Fig. 1. The furnace consisted of The analysis of the magnetite concentrate employed a 4.13 cm. (I.D.) sillimanite tube heated by six silicon in this investigation is given in Table 1. Reagent carbide globar elements symmetrically placed around grade sulphur and phosphorus pentoxide additions it. This assembly was surrounded by K-28 refractory were thoroughly mixed with the concentrate prior to bricks enclosed in transite. The temperature was briquetting. Similarly, the lime additions were re- controlled by a platinum-platinum ; 10 % rhodium agent grade. In some tests, reagent grade iron sul- thermocouple attached to a thermovolt recorder fitted phide was added to the briquette rather than sulphur. with ambient temperature compensation. A finely This addition was made because of the higher stability adjusted high/low power circuit enabled the tem- of iron sulphide at the reaction temperatures in the perature of the hot zone to be maintained within present experiments. If pure sulphur was added then ±5 °C. The retention time in the 1200 °C constant it would be expected that the majority of the sulphur temperature zone was about 40 min and a complete would be lost either by vapourization and oxidation to heating and cooling cycle required about 6 h. sulphur dioxide under oxidizing conditions (i.e., dur- All of the sintering was performed in air, resulting ing sintering) or vapourization and production of in the magnetite being converted to hematite. The hydrogen sulphide under reducing conditions (i.e., iron sulphide additions oxidized during sintering and during reduction). The magnetite and all of the thus the sulphur content of the briquettes decreased. additions were minus 200 mesh (Tyler) and were The phosphorus pentoxide combined with the hema- dried at 110 °C before being used (except sulphur). tite and the impurities present in the briquette. Ele- Briquettes were employed (rather than pellets) be- mental sulphur additions were made only to unsin- cause their size, density, porosity, surface area and tered briquettes. texture can be easily reproduced. The magnetite was compacted in a 1.90 cm (I.D.) cylindrical mould under a load of 105 kgmf cm2. This floating mould, the movable plungers inserted from both ends, was used to produce uniform briquettes which would shrink uniformly with a minimum of tapering during the subsequent sintering. The amount of material required to produce a briquette of equal length and diameter was determined (20.64 g) and this amount was used for all the briquettes. Cylinders of equal height and diameter will have the same kinetic reduction characteristics as spheres. This experimental technique has been found to be satisfactory for funda- mental studies on the reduction kinetics of iron ores.s~ After sintering the pure hematite briquettes had a

Table 1. Analysis of the magnetite concentrate employed in the experiment. (wt%)

Fig. 1. Schematic diagram of the sintering furnace.

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In the sintering process the oxidation of magnetite a dial to within 0.0250 cm. Readings were taken to hematite results in an increase in the weight of the every minute for the first 5 min, and then every 2.5 briquette of about 3 wt%. Thus, the lime and phos- min. The experiment was considered complete when phorus pentoxide contents were assumed to remain the height of the briquette did not change by more constant. The sulphur contents of the briquettes than 0.0125 cm in 5 min-usually after 60 min of re- were determined using a Leco Sulphur Analyser and duction in hydrogen and about 360 min in carbon the sulphur content was reported as iron sulphide monoxide. (FeS). Figure 3 shows the typical behaviour of the bri- A schematic diagram of the reduction-under-load quettes in the experimental apparatus. The curve furnace is shown in Fig. 2. The furnace consisted of can be conveniently divided into three stages. a 3.81 cm (I.D.) alumina tube heated by six silicon Stage 1-an incubation period in which the nitro- carbide heating elements symmetrically placed around gen is being flushed from the furnace by the reducing it. This assembly was surrounded by K-28 refractory gas and the sample temperature is increasing. bricks enclosed in transite. The furnace temperature Stage 2-the briquette expands because of the vol- was controlled by a platinum-platinum: 10 % rho- ume change during the reduction of hematite to mag- dium thermocouple attached to a thermovolt con- netite. The degree of expansion of the briquette is troller fitted with ambient temperature compensation. defined as the difference in height of the briquette A finely adjusted high/low power circuit enabled the between Stage 1 and the maximum height attained temperature in the hot zone to be maintained within during expansion in Stage 2. ±5 °C. The measuring thermocouple (also Pt-Pt: Stage 3-the briquette shrinks because of the fol- 10 % Rh) was located directly above the sample. lowing factors : The briquette was placed in the graphite block i) conversion of magnetite to wustite and iron, which was supported by a silicon carbide rod, and resulting in a reduction in volume, slowly raised into the hot zone of the furnace. In a ii) softening of the slag phase, and hydrogen atmosphere, it was observed that there was iii) softening of the metallic iron. a reaction between the graphite block and the gas The shrinkage degree is defined as the difference phase which was not noted in a carbon monoxide in height between the maximum height of the bri- atmosphere. A visible reaction between the graphite quette in Stage 2 and the final height of the briquette. block and the briquette was not noted and thus the Figure 4 is a schematic diagram of the reduction entire reduction process was due to reaction with the furnace. For the purpose of description the apparatus gaseous species and not with the graphite block. The can be divided into the following four sections: hot zone temperature was 900 °C. The load 30.2 1) a vertical loss-in-weight furnace, kgmfcm2 (except where indicated) was transmitted to 2) an electrical heating circuit with temperature the upper inverted graphite block via a silicon carbide controller, rod. The furnace was flushed with nitrogen for about 3) a reducing gas purification system, and 30 min to ensure that thermal equilibrium had been 4) a transducer to measure and record weight loss achieved. The load was then applied and hydrogen as a function of time. or carbon monoxide was introduced at 101/h. The (1) Loss-in-weight Furnace change in height of the briquette was measured with The furnace consisted of an inconel tube 3.18 cm (O.D.) and 2.50 cm (I.D.), wound with a Kanthal-A wire heating element which was insulated with alumina cement. Inconel baffles were placed above and below the high temperature section. This ensured a uniform 5 cm constant temperature zone and also preheated the incoming reducing gas from the bot- tom. The sample was suspended in a stainless steel

Fig. 2. Schematic diagram of the reduction-under-load Fig. 3. Typical behaviour of a briquette in the reduction- furnace. under-load furnace.

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Fig. 4. Schematic dia gram of the reduction furnace. wire basket which was supported by a stainless steel briquette, was connected to the transducer by a fine wire. The furnace tube was surrounded by K-28 thread which damped out any small vibrations in the refractory insulating brick, which was encased in support wire. transite. The transducer is temperature sensitive and thus (2) Heating and Cooling Circuitry must be maintained at a constant temperature during A chromel-alumel thermocouple in an alumina an experiment. An increase of 0.5 °C in the ambient sheath was embedded in the insulator at the center temperature is equivalent to an apparent weight in- of the hot zone. The tip of the sheath was in contact crease of 0.019 g. Therefore, the transducer was with the inconel tube. The Thermovolt Controller placed in a " hot box " with a mercury thermoregu- was identical to that used in the sintering furnace. lator and a heater with a high/low power supply cir- A constant voltage transformer with a high/low input cuit so that the transducer temperature could be con- circuit maintained the hot zone temperature to within trolled to within ±0.1 °C. ±2 °C of the desired value. During the experimental program it was observed (3) Reducing Gas Purification System that both sulphur and phosphorus pentoxide had a Commercial purity hydrogen, methane, and carbon significant effect on the softening-melting behaviour monoxide were employed as reducing gases in the ex- of the briquette in both carbon monoxide and hydro- periments. These were purified to remove oxygen gen atmospheres. In order to confirm that a similar and water vapour by passing the gas through copper behaviour would be observed in the presence of coke, turnings heated to 450 °C and then through sulphuric sulphur and phosphorus pentoxide additions were acid and calcium sulphate. The gas flow rate of 10 made to magnetite briquettes which were reduced in l/h was above the critical value necessary to ensure a bed of -200 mesh (Tyler) metallurgical coke at that the reaction rate was not dependent on the rate 900 °C. Also, mechanical mixtures of the magnetite of gaseous diffusion through the stagnant boundary powder, the metallurgical coke powder and phos- layer in the reducing zone. A three way solenoid phorus pentoxide or sulphur were reduced in a fireclay valve enabled the gas to either pass to a burner during crucible at 900 °C for 1 h. flow-rate adjustment or to the reduction furnace for the test. Iv. Results and Discussion The furnace was flushed with nitrogen both before The effect of sulphur and phosphorus pentoxide on and after a test. This prevented oxidization of the the macroscopic structure of iron oxide briquettes is briquette and also prevented air-hydrogen mixtures shown in Photos. 1 and 2, respectively. Magnetite from exploding in the furnace. The nitrogen flow briquettes shrink dramatically when sulphur is added was controlled by a two-way solenoid. This was con- to the briquettes which are reduced with either coke nected with the hydrogen valve so that the nitrogen or hydrogen. The density of the reduced briquette flow to the furnace was on when the hydrogen was increases with initial sulphur concentration and the off. number of observable cracks is reduced. Similarly, (4) The Transducer although to a lesser degree, phosphorus pentoxide A Stratham Transducer (Model G10B-3-350) was promotes densification of the briquette during reduc- employed to provide a continuous measurement of the tion. weight-loss of the sample during the reduction tests. Photograph 3(a) shows the microstructure of the The transducer was situated directly above the fur- reduced mechanical mixture of magnetite and coke nace. The stainless steel wire, which supported the powders. There was essentially complete reduction

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Photo. 1. The effect of sulphur on the structure of magnetite briquettes. (x7/l0)

Photo. 2. The effect of phosphorus pentoxide on the structure of hematite briquettes reduced with hydrogen at 900 °C. (x 4/5)

Photo. 3. The microstructure of magnetite powder ( - 200 mesh Tyler) reduced with metallurgical coke at 900 °C. (x3/5)

of the magnetite to iron and there is only a small The slag volume increased and the reduction rate amount of slag present. However, with a phosphorus decreased. Considerable iron sulphide was also ob- pentoxide addition the quantity of slag increased served. dramatically, as shown in Photo . 3(b). Qualitatively Figure 5 shows the quantitative results for the effect the reduction degree decreased and agglomeration of the concentration of phosphorus pentoxide added occurred because of the presence of phosphorus pent- prior to sintering on the strength of the briquette. oxide. Also, the addition of sulphur promoted the The initial expansion of the briquettes tended to information of slag and the reduction degree decreased crease with phosphorus pentoxide concentration. At (Photo. 3(c)). The low melting point eutectic phase the present time, the reason for this behaviour is not which is shown in the photograph is responsible for clear. However, it can be postulated that at high the sintering which occurs and the reduced porosity. concentrations of phosphorus pentoxide some reduc-

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Fig. 6. The effect of phosphorus pentoxide and lime concentrations on the height of sintered hematite briquettes in a hydrogen atmosphere at 900 °C. Fig. 5. The effect of phosphorus pentoxide concentration on the height of sintered hematite briquettes in a hydrogen atmosphere at 900 °C. tion of the oxide occurs and gaseous elemental phosphorus is produced. This phosphorus could form gas bubbles which would cause increased expansion of the briquette in the early stages of reduction. For these particular briquettes the shrinkage increased with phosphorus pentoxide concentration until about 0.75 wt% phosphorus pentoxide. Beyond this concentration the strength increases and thus the melting temperature range of the slag phase must be increasing. Thus, there appears to be a eutectic in this system at a concentration of about 0.75 wt% phosphorus pentoxide. Other constituents can have a dramatic effect on the behaviour of phosphorus pentoxide in the briquettes. For example, when lime is added with phosphorus pentoxide prior to sintering, the strength of the briquette increases as shown in Fig. 6. This can be attributed to the increased melting temperature range of the slag phase as the concentration of lime increases. When the lime addition is greater than that required to negate the effect of phosphorus pentoxide then the strength of the briquette becomes greater than that of a briquette without a phosphorus Fig. 7. The effect of phosphorus pentoxide and lime con- pentoxide addition. Figure 7 shows similar effects centrations on the height of sintered hematite when the briquette is continually heated. The addi- briquettes in a carbon monoxide atmosphere at tion of lime reduces the initial expansion during the 900 °C_ conversion of hematite to magnetite and also the total the loss in weight due to the reduction and vapouriza- shrinkage. tion of phosphorus and also possibly due to an increase The effect of phosphorus pentoxide on the reduc- in the porosity of the briquette as the vapours are ibility of the briquettes was investigated by adding released. However, after about 90 % reduction, the the oxide to magnetite which was reduced in hydro- rate decreases to a value below that for pure mag- gen at 900 °C (Fig. 8). The addition of 5 wt% phos- netite. Here, the reduction becomes slow because the phorus pentoxide produced an initial increase in the activity of the iron oxide is lowered by combination apparent reduction rate. (Here the term apparent with phosphorus pentoxide. At 10 wt% phosphorus reduction rate is employed because phosphorus vapour pentoxide the concentration of the oxide is high is also being produced.) This can be attributed to enough to dramatically reduce the activity of iron

Research Article Transactions ISIT, Vol. 23, 1983 (399) oxide even in the early stages of reduction and the Similar behaviour was observed when iron sulphide rate of reduction is below that of the pure briquette. was added prior to sintering (Fig. 10). The strength Photograph 4 shows scanning electron micrographs decreased with increasing sulphide concentration and of the structure of the briquettes both with and with- the softening was a result of the formation of the out an addition of phosphorus pentoxide. Without Fe-S-O slag phase. the addition, the briquette is porous and filaments of Since the presence of sulphur in the briquette re- iron can be observed. When phosphorus pentoxide sults in a lowered melting range of the slag, it would is added, the amount of slag phase increases, porosity be expected that the reduction rate would decrease as decreases, and the filamentary nature of the reduced the sulphur content increases. This prediction was iron is no longer apparent. confirmed in the kinetic experiments as shown in Fig. The effect of sulphur on the strength of the bri- 11. In these tests, iron sulphide was added to the quettes was studied by adding sulphur to the bri- magnetite briquettes which were reduced at 900 °C quettes prior to reduction (Fig. 9). In all cases, the without prior sintering. The addition of 2.5 wt% sulphur addition lowered the strength of the briquette iron sulphide resulted in a dramatic decrease in reduc- and the reduction in strength increased with increasing sulphur concentration. Apparently, during sintering, a slag phase containing Fe-S-O forms which lowers the melting range of the slag during reduction.

Fig. 8. The effect of phosphorus pentoxide concentration Fig. 9. The effect of sulphur on the height of magnetite on the apparent rate of reduction of magnetite briquettes in a hydrogen atmosphere at a load of briquettes in a hydrogen atmosphere at 900 °C. 1.43 kg/cm2.

Photo. 4. Scanning electron micrographs of the internal structure of magnetite briquettes reduced with metallurgical coke at 900 °C. (x 3/5)

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Fig. 12. The effect of sulphur and iron sulphid e concen- Fig. 10. The effect of iron sulphide on the height of sintered trations on the rate of reduction of magnetite hematite briquettes in a hydrogen atmosphere at briquettes in a methane atmosphere at 900 °C. 900 °C.

The effect of the addition of pure sulphur and iron sulphide on the rate of reduction of magnetite briquettes in a methane atmosphere is shown in Fig. 12. Initially, there is a slight increase in the reduction rate which remains constant for about 30 min. After this point, the rate of reduction is dramatically higher than the briquette without a sulphur addition. This is because the sulphur is uncombined prior to reduction and is removed as the test specimen is preheated. The sulphur will leave the sample as vapour and thus increase the porosity of the briquette which results in increased reduction rates especially when considerable iron has formed. Thus, there is no sulphur present to combine with the iron and oxygen which would lower the reducibility of the briquette. However, when iron sulphide is added it is not totally lost during heating and thus can affect the rate of reduction. In all cases, the addition of iron sulphide produced a decrease in the rate of reduction as was observed with briquettes to which iron sulphide was added prior to sintering. However, without sintering the iron sulphide is not intimately combined with the iron oxide and can be reduced by hydrogen. This sul- Fig. 11. The effect of iron sulphide on the rate of reduction of magnetite briquettes in a hydrogen atmosphere phide reduction increases the apparent rate of reduc- at 900 °C. tion and also increases the porosity which contributes to an increase in the reduction rate as compared to tion rate. The initial rate of reduction was not de- samples with relatively low additions of iron sulphide. creased significantly but at higher reduction degrees, Photograph 5 shows scanning electron micrographs for example above 30 % reduction, there was a sig- of the structure of a briquette with an iron sulphide nificant decrease in the rate of reduction. For this addition. As was the case for phosphorus pentoxide, particular case, the reduction essentially stopped after the addition of iron sulphide increases the slag volume 70 min. For 5 wt% iron sulphide the reduction and the filamentary nature of the reduced iron disap- stopped after 60 min and for 10 wt%, after 40 min. pears.

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Photo. 5. Scanning electron micrographs of the internal structure of a magnetite briquette containing sulphur (5.0 wt%) reduced at 900 °C with metallurgical coke. (x3/5)

When lime is added with the iron sulphide prior to sintering the strength of the briquette increases (Fig. 13). This is due to the increased melting temperature range of the slag phase which prevents premature melt down. When comparing the effects of iron sulphide and phosphorus pentoxide on the strength of briquettes, it can be seen that for equal weight additions (prior to sintering) phosphorus pentoxide has a stronger effect. However, a large proportion of the iron sulphide, which is added, will be oxidized during sintering and thus the effect will be dramatically reduced. Hence, it would be expected that when additions of both compounds are made to the briquette, the strength will be controlled by phosphorus pentoxide. Thus, when mixtures are added as shown in Fig. 14, the shrinkage degree is intermediate between the values for the pure compounds, but approaches more Fig. 13. The effect of lime and iron sulphide concentrations on the height of sintered hematite briquettes in closely the values for the pure phosphorus pentoxide a hydrogen atmosphere at 900 °C. addition. However, there is no increase in strength with the high phosphorus pentoxide additions, as was observed for the pure oxides. This can be attributed to the stronger role played by iron sulphide at higher concentrations.

V. Conclusions (1) The presence of phosphorus pentoxide in iron oxide briquettes promotes the formation of a slag phase, which decreases the rate of reduction and lowers the strength of the briquette. (2) The decrease in the rate of reduction is due to the decreased activity of the iron oxide since it is combined with the slag and also the lower porosity due to the low melting temperature range of the slag. (3) The reduction in slag melting point lowers the strength of the briquette and thus under blast furnace conditions the decrease in reduction rate would be more pronounced. (4) High sulphur levels in briquettes result in the formation of an Fe-S-O slag phase which has a low melting temperature range. (5) Thus, sulphur lowers the reducibility and strength of the briquette under reducing conditions.

(6) When equal proportions of the two com- Fig. 14. The effect of iron sulphide and phosphorus pent- pounds are added prior to sintering, phosphorus pentoxide concentrations on the height of sintered oxide controls the strength since it is present at higher hematite briquettes in a hydrogen atmosphere at concentrations after sintering than the iron sulphide. 900°C.

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(7) The addition of lime to phosphorus pentox- ISIJ, 17 (1977), 371. ideor sulphur-containing hematite briquettes results in 3) K. Narita, T. Sato, M. Maekawa, S. Fukihara, H. Kana- an increase in strength which can negate the effects yama and S. Sasahara: Tetsu-to-Hagane, 66 (1980), 1975. of the two detrimental impurities. 4) Y. Shimomura, K. Nishikawa, S. Arino, T. Katayama, Y. Hida and T. Isoyama: Trans. ISIJ, 17 (1977), 381. 5) R. Kuwano, T. Oku and Y. Ono : Tetsu-to-Hagane, 66 REFERENCES (1980), 1622. 1) K. Sasaki, F. Nakatani, M. Hatano, M. Watanabe, T. 6) G. Clixby: Ironmaking Proc. AIMS, 39 (1980), 370. Shimoda,K. Yokotani,T. Ito and T. Yokoi: Trans.ISIJ, 7) R. Takahashi, Y. Omori, Y. Takahashi and T. Yagi: 17 (1977),252. Ironmaking Proc. AIMS, 38 (1978), 78. 2) K. Kanbara, T. Hagiwara, T. Shigemi, S. Kondo, Y. 8) P. K. Strangway, H. 0. Lien and H. U. Ross: Can. Met. Kanayama, K. Wakabayashiand N. Hiramoto: Trans. Quarterly, 8 (1969), 235.

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