SOME EXPERIMENTAL STUDIES OF COAL INJECTION INTO SLAGS

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1 (Published in 6 th EAF Conference Proceedings, USA, Nov. 1-13, 22, pp ) SOME EXPERIMENTAL STUDIES OF COAL INJECTION INTO SLAGS F. Ji, M. Barati, K. Coley and G.A. Irons Steel Research Centre, Department of Materials Science and Engineering McMaster University 128 Main Street West Hamilton, Ontario L8S 4L7 Canada , ext ironsga@mcmaster.ca Key Words: Coal Injection, Foaming Slag, Carbon-Gas Reaction, Kinetics, Slag void Fraction, Foaming Index INTRODUCTION Foamy slag practice is important for EAF steelmaking. A stable foamy slag can be of benefit in saving electrical energy, protecting the furnace lining and decreasing the consumption of electrodes. When coal is injected into slag that contain iron oxides, the iron oxides in the slag will be reduced and a foamy slag can be expected. The reactions between carbon and slag include in four separate steps: FeO transfer from the bulk phase to the slaggas interface, CO (C) reacts with FeO at the interface and the reaction product, CO 2, transfers through the gas phase to react with carbon to form CO. Those processes involve gas-liquid, gas-solid even solid-liquid reactions and mass transfer in the gas and liquid phases. The gas generated in the slag leads to a large number of bubbles which results in slag foaming. To understand those phenomena, a number of authors have studied FeO reduction in slags thermodynamically and kinetically [1-6]. For the reaction between Carbon and CO+CO 2 gas mixtures, Turkdogan gives a detailed description of the reaction rate for different carbonaceous materials 7-1. Many researchers have studied slag foaming To find the optimum conditions of coal injection into slags, a number of questions remain to be answered. In the present work, the overall reaction kinetics were measured and an estimation of carbon gasification rate under low volatile coal injected to FeO bearing slag have been carried out. Experimental Set up EXPERIMENT The experiments were carried out on a 75 kw induction furnace. A high-mgo crucible with interior diameter cm and height 3.48 cm was used for the experiments. Figure 1 shows the experimental set up. Nitrogen was passed through a digital flow meter then introduced into the feeder for coal injection. The feeder was suspended from a load cell in order to measure the variation of weight of coal during injection. Before coal injection started, the crucible was covered with a ceramic board and a stainless steel lid. The lance was introduced to the crucible through the holes in the centre of the lid and the ceramic board. During experiment,

2 the gas was exhausted through a single outlet. To take gas samples, a special port was mounted in the outgoing gas line. A filter filled with a fine screen and glass wool was installed before the sampling port to eliminate dust. The signals of carrier gas flow rate, inner pressure of the feeder and weight of coal were collected by a computer at.5 second intervals. A - Gas sample port; B - Filter; C - Lance; D - lid; E - Ceramic board; F - Steel rod; G - Slag; H - Steel; I - Feeder; J - Load cell; K - Flow rate transducer; L - Flow meter; M - A/D card terminal; N - High-MgO crucible; O - Computer. Figure 1. The experimental set up. Experimental Materials The metal used in these experiments was low carbon steel. The slags were prepared by mixing oxide powders. After the oxides had been weighed, they were mixed carefully then kept in a dry container. Experimental Procedure About 17 kg of steel was placed in the crucible. When temperature reached around 16 o C, slag was added onto the surface of the steel. Immediately prior to adding slag, about 9 grams of copper was added into the steel. The copper was used to identify the source of metal droplet in the slag. Usually, slag powder was added three batches to reach the desired depth. After the slag was completely melted, the ceramic board and the lid were set properly. The lance was then introduced into the crucible for preheating for five minutes while passing about 1 NL/min nitrogen to purge the freeboard of the crucible. A steel rod was placed in contact with the surface of slag to measure the height of foamed slag. Right before injection started, the carrier gas flow rate was increased to 1 NL/min and the first gas sample was taken. When coal injection started, gas samples were taken continuously using gas tight syringes. Metal and slag samples were also taken before and after the injection. To find the vertical distribution of carbon in slags, some triple samples at different depth of slags were taken. Gas samples were analyzed using a Perkin Elmer AutoSystem XL Gas Chromatograph.

3 To check the compositions of slags, some chemical analysis were carried out by XRF analysis. The contents of FeO and Fe 2 O 3 in slags were determined using a titration method 13. After injection experiments, the dusts were collected to determine the amount of carbon lost to exhaust gas. RESULTS An overview of injection experiments is listed in table 1. For injection experiments 3 and 4, the slag foamed violently over the top of the crucible, therefore, the foaming height H f could not be obtained accurately. In table 3, α, the void fractions, were calculated by equation α = 1 H s /H f 8, where H f is the height of foamed slag and Hs is the original slag height. The solid rates were calculated by weight of injected divided by injection time, which means they are average injection rates. It should be pointed that the solid rate for injection No. 1 was estimated using dry test data whereas all others were measured directly. The temperature listed in the table was measured by thermocouple before coal injection. Table 1. Overview of the Experimental Results Steel Slag Carbon Injection Exp. H f α T No. W, Depth W, Dep. Weight Time Rate, (cm) ( C) (kg) (cm) (kg) (cm) (g) (s) (g/s) Inj Inj Inj > Inj > Inj Inj Inj Inj Inj Inj Inj Inj According to SEM analysis, the copper contents between the metal droplets in slags and the sample taken from the bulk metal were.63% and.35% respectively for injection No. 5. It shows that the metal droplets originate from the bulk metal phase. However, for injection number 8, the concentration of copper in bulk metal was.82% and none could be detected in the metal droplets, which indicates these droplets were generated from iron oxide reduction in slag. Therefore, the metal droplets arise from the bulk metal phase as well as iron oxide reduction. Further work will be required to assess the contribution from each source. The quantities of dust produced during coal injections were 1.2 to 3.5 grams. Chemical analysis of the dust indicates that the content of carbon was between %. The maximum carbon weight found is.35 grams (injection number 4), for which the percentage of carbon escaping to dust is.92% of total injected carbon. Tables 2 and 3 show the chemical analysis and some of the physical chemical properties of the slags. As mentioned earlier, a titration method was adopted to determine total iron, ferrous and metallic iron contents in

4 slags. The content of ferric oxide in slags can then be calculated by iron balance. The contents of nonferrous oxides in table 2 were calculated according to the XRF analysis of slag No. 6 and 7. The FeO represents the total iron considered to be FeO. In table 3, the density ρ, viscosity µ and surface tension s, activities a FeO and activity coefficient of iron oxide γ FeO of the slags were estimated using appropriate methods The B and B1 are basicities of slags, the ratio of CaO/SiO 2 and (CaO+MgO))/(Al 2 O 3 +SiO 2 ), respectively. Table 2, Chemistries of slags Ex. TFe Al 2 O 3 CaO FeO MFe Fe 2 O 3 MgO SiO 2 FeO Inj. No. Table 3. Slag chemical and physical properties. B B1 ρ σ µ (Kg/m 3 ) (N/m) (Pa s) DISCUSSION Variations of Iron Oxides and Carbon Distributions in Slags Figures 2 to 4 show the variation of total iron, FeO and the atomic ratio of Fe 3+ /Fe 2+ in slags respectively. The TFe of slag increases during coal injections and decreases after injection stopped. The FeO in slag follows the same trends. The ratio of ferric to ferrous iron decreased rapidly during coal injection then slowly after injection stopped. Figures 2 and 3 are in accordance with each other, which suggests that the metal can be oxidized indirectly through slag, i.e. FeO is oxidized to Fe 2 O 3 on the top surface of slag while iron is oxidized to FeO by Fe 2 O 3 at the slag-metal interface. It should be recognized that there may be some deviation in slag composition, a FeO γ FeO

5 because the slag samples were taken by steel rods. It is clear that the slag surface was quite heavily oxidized before coal injection even though the crucibles were carefully covered during slag melting in a nitrogen atmosphere. TFe in slag, mass% FeO in slag, mass% End of injection End of injection Injection No.5. Injection No.7. Injection No.6. Injection No Injection No.5. Injection No.7. Injection No.6. Injection No.8. Figure 2. Variation of TFe in slags during injection experiment. Figure 3. Variation of FeO in slags during injection experiment. Figure 5 shows the distributions of carbon in slags. Inj. 11 and Inj.12a represent the slag samples taken at immediately coal injection stopped and Inj.12b is after 3 minutes of coal injection stopped. In general, the concentration of carbon in the top of slags is higher than in the bottom. Comparing to figure 6, the distributions of Fe 3+ /Fe 2+ in slag vertically, it likely is that the reaction rates of carbon with slags in the top area is faster than in the bottom, because the ratio of Fe 3+ /Fe 2+ is higher in the top area of slags. Hence, the distributions of carbon and ratio of Fe 3+ /Fe 2+ in slags should be important parameters to the reaction rates. Flow Rate of Gas and Gasification Rate of Carbon In the present study, both of gasification rates of coal in slag and slag foaming state are related to the gas flow rate. The gas flow rates were calculated based on gas compositions and nitrogen flow rate. To determine the rate of gas generation from gas analysis we require the flow rate of carrier gas and the amount of entrained air. The amount of entrained air can be determined from the amount of oxygen taking part in post combustion plug the free oxygen in the effluent gas. In order to determine the amount of oxygen involved in post combustion we must make some assumption about the CO/CO 2 ratio in gas leaving the slag.

6 Atom ratio of Fe 3+ /Fe End of injection. Depth of slags, cm time, s. Injection No.5, B1=1.6 Injection No.7, B1=4.4. Injection No.6, B1=1.6 Injection No.8, B1= Carbon in slags, mass%. Inj. 11. Inj.12a. Inj 12b. Figure 4. Variation of Fe 3+ /Fe 2+ in slags during injection experiment. Figure 5, Distributions of carbon in slags vertically. Depth of slags, cm Fe 3+ /Fe 2+. Inj.11; Inj. 12a; Inj.12b. Figure 6, Distribution of the ratio of ferric and ferrous in slags vertically. In the present study where the reaction time is relatively short, reactions at the gas-slag (CO+FeO = CO 2 +Fe, P CO2a ) and gas-carbon interface (CO 2 +C = 2CO, P CO2b ) cannot both be in equilibrium. The P CO2i in gas phase should be between those two equilibrium partial pressures and close to the faster of the two reactions. Assuming the gas phase is uniform, then one can take, P = χp + ( 1 χ) P (1) CO2i CO2a CO2b

7 Where, the value of χ is usually between and 1 and in the case of Fe 2 O 3 containing slags can be greater than 1. The equilibrium partial pressure of CO 2 for gas-slag reaction is a function of temperature and activity of FeO in slag. On the gas-carbon interface, the equilibrium partial pressure of CO 2 is a constant at a given temperature. To determine the value of χ, one has to find the reaction rates of these two reactions experimentally. However, prior to determining the rate controlling steps, P CO2 can be approximated by properly choosing χ values and, at this stage, supposing Turkdogan s 8 rate constant for metallurgical coke is reasonable for the low volatile coal used in the present experiments. Therefore, for each experiment, we can find a value of χ that best fit the experiment results. The details of these calculations will be published elsewhere. Figures 7 and 8 show two examples of the gas flow rates calculated according to the analysis of the gas for injection number 8 and 9. From the two experiments, the curves of flow rates of air entrained and total gas are significantly different because the air flow rate depends on the seal conditions. However, the curve patterns of flow rate of CO and CO 2 are basically the same. Flow rate, NL/min Total gas flow rate. Air entrained. CO+CO Flow rate, Nl/min Total gases flow rate. Air entrained. CO+CO Figure 7. Gas flow rate for the injection 8. Figure 8. Gas flow rate for the injection 9. Estimation of Carbon Accumulation in Slag From the carbon balance, the accumulation rate of carbon in slag should equal the rate of injected carbon minus the rates of reaction and floatation as well as the rate of escape as dust. This can be expressed as follows dw dt = R ( k + k + k ) w (2) i f e During coal injection, we have < t < t i, R i >. Where, w is the amount of coal in slag in gram, t the time in second, t i the coal injection termination time, R i the injection rate of coal in g/s, k the apparent first order rate constant of carbon reacted with CO-CO 2 mixture, k f the floating rate of coal and k e the escape rate of carbon in outgoing gas. By dust analysis, at the present stage, the quantity of carbon in dust collected from the outgoing gas is less than 1% of the total carbon injected as mentioned earlier, hence k e is practically neglectable. Then, inserting initial conditions t =, w =, the integrated form of equation (2) gives,

8 R i -(k+ k ) t w = [1- e f ] (3) k + k After carbon injection stops, i.e. t i t t and R i =, accumulation rate of carbon in slag is, dw dt f = ( k + k w (4) f ) It should be noted that this represents a negative accumulation. Integration of equation (4), by inserting experimental data, injection time t i in second and coal weight w i in gram and assuming k f =, gives, ( k)( t i t) w = w i e (5) Because the particles of the coal are small in the present experiments, complete pore diffusion can be assumed. Then the reaction rate constant, k, suggested by Turkdogan 8 can be used to equations above, Φ[ PCO 2 ( PCO 2 ) e] k =, min ( 1 P / φ ) CO CO 1 (6) Where, P CO2 and P CO are the values estimated for the slag bubbles. Estimation of Residual Carbon by Gas Compositions The gasification rate of carbon in slag can be estimated by analysis of outgoing gas. For the first stage coal injection experiments, the injection time was about 3-44 seconds, therefore, the injection quantities of coal is relatively small. In those cases, one can assume that the activity of iron oxide in the slag is constant, which means we can take the initial P CO2 to be constant in equation (6). Also we assume all iron in slags to be FeO and the floating coal is so little that it can be neglected. Based on those assumptions, figures 9-12 show the comparisons between predicted curves using equations (3) and (5) and calculations based on gas analysis data. These results show the apparent reaction rate of carbon with slag can be described by Turkdogan s rate expression for metallurgical coke. It was found that the appropriate values of χ lie between.65~1., which suggests that reactions at the slag-gas and gas-carbon interface both play a role in controlling the rate with the slag gas reaction being slightly faster. This result means that in order to predict the gasification rate we require independent measurements of both the slag gas and gas carbon reaction kinetics. Figure 13 shows carbon gasification rate as a function of time for injections 5-9. Usually, the maximum gasification rate can be found at around 2 seconds after injection started. The maximum gasification rates are between g/s. For comparison, slag compositions for injection 5 and 6 were almost the same, they have the same χ value and similar rate curves. Carbon Balance and Residual C for Longer Injections In an attempt to achieve the predicted steady state shown in Figures 9-12, longer injections were carried out. To determine the value of χ, carbon balance calculations were conducted. Figures 14 to 16 show the carbon balances. It seems that the value of χ is higher when FeO is higher in slags, particularly noteworthy is a value of χ is 1.2 for injection 12, which is believed to be due to the higher ratio of ferric to ferrous iron (see figure 6).

9 Residual Carbon, g Injection No. 5, (FeO)=32.3% Residual Carbon, g Injection No. 7, (FeO)=39.77% Predicted, χ=.5. Experimental, χ=.5. Predicted, χ=.4. Experimental, χ=.4. Predicted, χ=.35. Experimental, χ=.35. Coal injection stopped. Figure 9. Comparison of residual carbon predicted and experiment in the slag for injection Predicted, χ=1.. Predicted, χ=.9. Experimental, χ=1.. Injection stopped time. Figure 1. Comparison of residual carbon predicted and experiment in the slag for injection Residual Carbon, g Injection No. 8, (FeO)=43.41% Predicted, χ=.7. Experimental, χ=.65. Experimental, χ=.7. Injection stopped. Predicted, χ=.65. Figure 11. Comparison of residual carbon predicted and experiment in the slag for injection 8. Residual Carbon, g Injection No. 9, (FeO)~25% Predicted, χ=.8. Experimental, χ=.8. Predicted, χ=.85. Experimental, χ=.85. Injection stopped. Figure 12. Comparison of residual carbon predicted and experiment in the slag for injection 9.

10 Carbon gasification rate, g/s Inj 5, FeO=35.5% Inj 6, FeO=36.7% Inj 7, FeO=4.4% Inj 8, FeO=41.2% Inj 9, FeO=3.5% χ=.35. χ=.35. χ=1.. χ=.65. χ=.8. Weight of carbon, g C injected. C reacted. Residual C. C in dust. C in slag Inj. No.1, FeO'=35.11%, χ=.58. Figure 13. Gasification rate of carbon in slag as a function of time at steel making temperature. Injection time 3-44 seconds. Figure 14. Balance of carbon for injection 1. Weight of carbon, g C injected. C reacted. Residual C. C in dust. C in slag. Weight of carbon, g C injected. C reacted. Residual C. C in slag. C in dust Inj. No.11, FeO'=31.64%, χ= Inj. No.12, FeO'=45.6%, χ=1.2. Figure 15. Balance of carbon for injection 11. Figure 16. Balance of carbon for injection 12. Based on the balances of carbon, comparisons between predicted and experimental residual carbon in slags were carried out. Figures 17 to 19 show the results. One can see that the predicted and measured values are significantly different. This suggests that the equations for short injection are not applicable to those cases. Therefore, further investigations should be carried out, for example, introducing the relationship of P CO2 with slag compositions, for example Fe 3+ /Fe 2+ and basicity, to the rate expressions.

11 15 Residual C in slag, g Residual C in slag, g Experimental, χ=.58. Predicted, χ=.58. Inj. No. 1, FeO=35.11%. Figure 17. Comparison of residual carbon predicted and experimental in the slag for injection Experimental, χ=.6. Predicted, χ=.6. Inj. No. 11, FeO=31.64%. Figure 18. Comparison of residual carbon predicted and experimental in the slag for injection 11. Residual C in slag, g Experimental, χ=1.2. Predicted, χ=1.2. Inj. No. 12, FeO=45.6%. Gasification rate of C, g/s Injection 1. Injection 11. Injection Figure 19. Comparison of residual carbon predicted and experimental in the slag for injection 12. Figure 2, Gasification rate of carbon in slag as a function of time at steel making temperature. Injection time seconds. Slag Foaming A number of researchers have studied the foaming behavior of various slags. The foaming Index reported by Jiang and Fruehan 11 is:

12 115µ =. ( ρσ ) 5 h = u f s (7) Where µ is viscosity, ρ the density and σ the surface tension of slag, the h f and u s are slag foaming height and superficial gas velocity respectively The void fraction (gas hold-up) according to Gou, Irons and Lu 12 can be related to superficial gas velocity by, 2 α =.91u (1 α).57 s (8) Where, α is void fraction and u s is the superficial gas velocity. Equation 8 is based on data from smelting reduction. The foaming height can be obtained using the equation 12 : h s h f = (9) ( 1 α) where h s is static depth of slag. Using Equations 8 and 9 the void fractions and foaming heights of slags can be calculated. Figure 21 shows the comparison of predicted void fractions from Equation 8 and the present experimental data. It can be seen that the experimental data are located in the left top corner, which means the slag foaming is still in low superficial gas velocity region or the classical foaming regime 12. Figure 22 gives the relationship between slag foaming heights and superficial gas velocities calculated by foaming index reported by Fruehan et al 11. Void fraction. 1. Present Work Gou, Irons and Lu Foaming height, cm Σ=1.34 This work. µ=.5 Pa.s. µ=.12 Pa.s. µ=.5 Pa.s. Σ=.322 Σ= Superfacial gas velocity, m/s Superficial gas velocity, m/s. Figure 21 Slag void fractions as a function of superficial gas velocity. Figure 22 Foam height versus superficial gas velocity.

13 The experimental points are placed in the figure for comparison. The viscosities of slags in the present experiments are between Pa s and the foaming indexes are between.11 and.32. It is clear that the deviations between experimental and predicted values are significant. This is perhaps because the results reported by Fruehan were for lower superficial gas velocities (less than.5m/s, dash dot line box in figure 22) 11. Gou and Irons correlation fits better for high superficial gas velocities, while Fruehan et al s correlation is better at low ones. However, the present conditions appear to be in an intermediate regime, where neither model fits. Further work is underway to investigate this more thoroughly. SUMMARY AND CONCLUSIONS Based on the gas compositions, the reaction rate of low volatile coal reaction with moderate FeO slag is reasonably well described using published rate constant for gasification of metallurgical coke. The rate- limiting step is mainly carbon gasification, but there is a significant contribution from the slag gas reaction particularly for long injection times. The maximum gasification rates of carbon in slag was found to be around.3 to.55 g/s under the present conditions. For the longer injection, they can be up to 1.1 g/s. The void fractions of foamed slag lie between.64 and.82 with foaming heights 12 to 2 cm, which shows some deviation from currently available prediction methods. This requires further study. ACKNOWLEDGEMENTS The authors are deeply grateful to Dr. S. Sun, Research and Development of Dofasco for his kindly help on experimental materials and chemical analysis, and also to O. Kelly, G. Bishop, D. Angelina and Subagyo for great cooperation during the experiments. Assistance on gas chromatograph operation supplied by E. Worral is gratefully acknowledged. REFERENCES 1. H.A. Fine, D. Meyer, D. Janke and H-J. Engell, Ironmaking and Steelmaking, Vol. 12, 1985, pp J. Bygden, D. Sichen and S. Seetharaman, Steel Research, Vol. 65, 1994, pp R.H. Smith and R.J. Fruehan, Steel Research, Vol. 7, 1999, pp F. Fun, Metallurgical Transactions, Vol. 1, 197, pp J. Seo and S. Kim, Steel Research, Vol. 69, 1998, pp A. Sato, G. Aragane, K. Kamihira and S. Yoshimatsu, Transactions ISIJ, Vol. 27, 1986, pp E.T. Turkdogan, Fundamental of Steelmaking, p E.T. Turkdogan and J.V. Vinters, Carbon, Vol. 8, 197, pp

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