Recovery of Fe and P from CaO-SiO 2 -Fe t O-P 2 O 5 Slag by Microwave Treatment

Transcription

1 Materials Transactions, Vol. 52, No. 12 (211) pp to 2238 #211 The Japan Institute of Metals EXPRESS REGULAR ARTICLE Recovery of Fe and P from CaO-SiO 2 -Fe t O-P 2 O 5 Slag by Microwave Treatment Taeyoung Kim* 1 and Joonho Lee* 2 Department of Materials Science and Engineering, Korea University, Anam-dong, Seongbuk-gu, Seoul , Korea In order to optimize the recovering process of Fe and P from steelmaking slag by the microwave irradiation, heating rates of various substances and recovery ratios of iron and phosphorus from slag-graphite mixtures were investigated. The heating rates of the examined substances are in the order of graphite Fe 3 O 4 Fe 2 > the synthesized CaO-SiO 2 -Fe t O-P 2 O 5 slag > the industrial slag. From the maximum heating rates, it was considered that the microwave absorption ability of graphite was 4 times greater than that of the industrial slag. Consequently, it was considered that the heating rate of the graphite-slag mixture was dominantly determined by graphite. In the reduction of the synthetic slag (15 g) by graphite (.816 g), the weight of the recovered metal was maximized at 9 s under the present experimental condition (1.7 kw, 2.45 GHz). At a fixed microwave processing time of 9 s, the recovery ratio of iron increased with increasing the carbon equivalent (C eq. : the mole ratio of carbon to oxygen to reduce Fe 2 and FeO, yielding CO gas), and reached.97 when C eq. ¼ 1:69. The recovery ratio of phosphorus also increased with C eq., and showed.89 at the same condition. From the chemical analysis of the slag and metal after experiments, it was found that the phosphorus distribution ratio (L p ) strongly depended on T. Fe content in slag, namely, as FeO content decreased, L p value decreased. [doi:1.232/matertrans.m211178] (Received June 13, 211; Accepted September 8, 211; Published November 25, 211) Keywords: microwave irradiation, recovery iron and phosphorus, steelmaking slag 1. Introduction Although steelmaking slag is one of the major by-products from the steelmaking industry, it has not been effectively recycled due to the high content of iron and impurities such as phosphorus and sulfur in the slag. In 29, 7,831 k-tons of steelmaking slag was generated in Korea, and the slag generation rate is expected to grow continuously. Currently, steelmaking slag is generally utilized for land filling (43.2%) and stabilization of lower part of road construction (27.2%). Only some parts of the steelmaking slag (21.7%) are recycled in the steelmaking process, but the recovery of metallic components has not been realized yet. Therefore, it is expected that after removing Fe and P, the recycling rate of the steelmaking slag may increase and eventually the CO 2 emission from the steelmaking process can be decreased by reducing the consumption of CaC. There are several attempts to recover iron and phosphorus from the slag. 1 3 year ago, several attempts were made to recover iron and phosphorus from steelmaking slag by Shiomi et al. 1) and Takeuchi et al. 2) They reported that iron and phosphorus were successfully recovered as Fe-P-C alloy and P 2 vapor by using carbon as reducing agent. It was pointed out that due to the poor thermal and electrical conductivities of slag the conventional combustion of a carbonaceous fuel was not so effective to supply heat required for melting and endothermic reaction. Later, Morita et al. 3, suggested a carbothermic reduction of the steelmaking slag under the microwave irradiation, because the microwave may penetrate slag and makes it possible to heat materials very rapidly and uniformly. This method was very effective to recover both iron and phophorus as a form of Fe- C-P alloy in a very short time. In their work, the recovery ratio of iron was satisfactory, while that of phosphorus * 1 Graduate Student, Korea University. Present address: Central Research Center, Hyundai Motor Co. Uiwang-si , Korea * 2 Corresponding author, joonholee@korea.ac.kr needed some improvement. In the present study, similar experiments as Morita et al. were carried out to find out a way to increase the recovery ratio of phosphorus by changing the reaction time and the amount of carbon added. 2. Heating Rates Test 2.1 Experimental materials The materials used in this experiment were graphite (Sigma-Aldrich, powder, synthetic regent grade), Fe 3 O 4 (Kanto chemical. Co., Inc., 99.9% purity), Fe 2 (Sigma- Aldrich, 99% purity), CaC (Junsei chemical Co., Ltd., 99.5% purity), SiO 2 (Junsei chemical Co., Ltd., 99.9% purity), Ca 3 (PO 4 ) 2 (Junsei chemical Co., Ltd., 98% purity) and industrial steelmaking slag provided by POSCO. These materials were kept in a drying machine at 373 K. The synthetic slag was prepared by mixing oxides in an iron crucible (I.D: 5.5 cm, Height: 9 cm), keeping at 183 K for 3 min under a deoxidized Ar atmosphere and quenching it in flushing N 2. The chemical composition of the slag was analyzed with X-Ray fluorescence spectrometer (S4 Pioneer, Bruker, XRF) and Inductively Coupled Plasma-Atomic Emission Spectrometry (138 Ultrace, Jobin Yvon, ICP-AES). Analysis of Fe 2þ and Fe 3þ in slag was conducted by the wet chemical analysis. The chemical analysis results are presented in Table 1. It was shown that the industrial steelmaking slag was mainly composed of CaO, SiO 2, and FeO. There were small amounts of MnO, P 2 O 5,Al 2, MgO and TiO 2. For the better understanding of the reaction mechanism, a synthetic slag was prepared of the CaO-SiO 2 -FeO-P 2 O 5 system. The composition of the synthesized slag is shown together in Table 1. Morita et al. reported that the microwave energy was most efficiently absorbed in the slag when the value of Fe 3þ =ðfe 2þ þ Fe 3þ Þ was in the range of :15:18 with the largest dielectric loss of the slag. 3) The synthetic and industrial slag showed the value of Fe 3þ =ðfe 2þ þ Fe 3þ Þ as.176 and.175, respectively. Therefore, the microwave

2 2234 T. Kim and J. Lee Table 1 Chemical composition of slags (mass%). component CaO T. Fe FeO Fe 2 SiO 2 Al 2 P 2 O 5 MgO MnO synthetic slag industrial slag analysis tool XRF Wet Chemical Anal. XRF ICP XRF Table 2 Average size of the sample powders examined in the heating experiments. material graphite Fe 3 O 4 Fe 2 synthetic slag industrial slag average size (mm) Thermocouple Sample Temperature, T / K Graphite Fe 3 O 4 Fe 2 Synthesized slag Industrial slag O-ring 3 6 Time, t / s 9 12 N 2 gas inlet Gas outlet Fig. 2 Heating behaviors of graphite, Fe 3 O 4,Fe 2, synthetic slag, and industrial slag. Refractory materials Microwave generator Alumina crucible Fig. 1 A schematic diagram of the microwave furnace used in the heating experiments. energy absorption ability of the slag was considered high enough. The average diameter of sample powders was analyzed with dynamic light scattering analyzer (ELS-8, Otsuka, Japan). Table 2 shows the average size of the powders examined. 2.2 Experimental procedure Figure 1 shows a schematic diagram of the microwave furnace (max. power: 2 kw, frequency: 2.45 GHz) used in this study. This furnace was used in our previous studies. 5,6) The heating rate was investigated for five materials (graphite, Fe 3 O 4, Fe 2, the synthesized slag and the industrial steelmaking slag). 4 g of each material was charged in an alumina crucible (outer diameter: 28 mm, inner diameter: 24 mm, height: 3 mm) except graphite (2 g, due to low density), and was covered with a refractory brick for insulating. Then, the sample was heated under the microwave irradiation with flowing N 2 gas at a flow rate of 2 ml/min STP. The microwave power was kept constant at 1.7 kw. The temperature of the sample was measured at every minute using a B-type thermocouple buried in the center of the sample. In order to remove the interference effect of the microwave on the temperature measurements, the thermocouple was protected by a double alumina lance (Pt-coated on the inner alumina lance surface). 2.3 Results and discussion In Fig. 2 are shown the temperature changes of graphite, Fe 3 O 4, Fe 2, the synthesized slag and the industrial steelmaking slag as a function of the microwave irradiation time. The heating rates are in the order of graphite Fe 3 O 4 Fe 2 > the synthesized slag > the industrial slag. Although the amount of graphite was half of the others, it was dramatically heated up to 185 K in 6 min (at a heating rate of 32 K/min). In the case of Fe 3 O 4, a typical heat-run behavior was observed at 1 min. 7) Once the heat-run initiated, temperature rapidly increased to 1316 K in 5 min (at a heating rate of 2 K/min). Thereafter, however, the heating rate decreased to approximately 14 K/min. Fe 2, the synthesized slag, and the industrial slag showed almost the same heating behavior. Since the dielectric loss changes with increasing temperature, the heating rate can be considered an indirect index of the microwave absorption. 8,9) The maximum heating rate of graphite, Fe 3 O 4,Fe 2, the synthesized slag and the industrial steelmaking slag were 32 K/min, 2 K/min, 21 K/min, 16 K/min, and 8 K/min, respectively. Therefore, the microwave absorption ability of graphite is 4 times or much larger than that of the industrial slag. Consequently, it is considered that the heating rate of the graphite-slag mixture is dominantly determined by the amount of graphite. After consuming graphite in reduction, fine metallic Fe particles generated by the reduction of FeO might play the role of the microwave heating. 3. Reduction Test under Microwave Irradiation 3.1 Experimental materials In this study, the synthesized slag of Table 1 was used. Graphite was used as the reducing agent. The amount of

3 Recovery of Fe and P from CaO-SiO 2 -Fe t O-P 2 O 5 Slag by Microwave Treatment 2235 Table 3 Experimental condition of the microwave process. Sample No. Crucible Slag weight (g) Carbon addition (g) C eq. time (min) SS1 SiO SS2 SiO SS3 SiO SA1 Al SA2 Al SA3 Al SA4 Al SA5 Al SA6 Al SA7 Al SA7 Al SS2 SiO SS4 SiO SS5 SiO SS6 SiO Slag Metal Fig. 4 Typical cross-section of the sample after experiment (Synthesized slag = 15 g, C eq. ¼ 1:56 (1.9 g), Microwave irradiation time = 15 min). 2 Microwave furnace Two-color pyrometer f =2.45 GHz, P =1.7 kw Refractory materials Wool Alumina or quartz crucible Sample Temperature, T / K Time, t / s Fig. 3 A schematic diagram of the microwave furnace used in the reduction experiments. graphite was determined based on carbon equivalent (C eq., the amount of carbon required for the reduction of FeO into pure Fe). The experimental condition is given in Table Experimental procedure Figure 3 shows a schematic diagram of the microwave furnace (MM-344 L, 2.45 GHz, 1.7 kw, LG) used in this study. 15 g of slag was well mixed with :8161:18 gof graphite. The mixed powder sample was then charged in an alumina crucible (outer diameter: 39 mm, inner diameter: 36 mm, height: 29 mm). For insulating, refractory bricks and silica wool were used. Then, the refractory assembly was placed at the center in the microwave oven and the microwave was irradiated for a predetermined period of time. During the experiments, the temperature was monitored using a two-color pyrometer (IR-HQH2, K, CHINO) through a hole on the upper part of the refractory block. The carbon equivalent varies from 1.17 to 1.69 and the microwave irradiation time was set 11, 15 and 2 min. After the experiments, the sample was cooled down in air and the reduced metallic particles were separated from the remaining slag. Figure 4 shows a typical cross-section of the sample Fig. 5 Typical heating behavior of the slag-graphite mixture (15 g slag,.816 g graphite). after experiment (Synthesized slag = 15 g, C eq. ¼ 1:56 (1.9 g), Microwave irradiation time = 15 min). The metal droplet was easily separated from the residual slag and the crucible mechanically. The size of the metal droplet was approximately 1 mm and the surface of metal was bright. Then, the recovered metal was weighed and analyzed with ICP. The residual slag was also analyzed with ICP and XRF. Carbon content in metal was analyzed using LECO-C/S analyzer (C/S-3, LECO, USA). 3.3 Results and discussion Effect of the microwave irradiation time In order to determine the process time, the effect of microwave irradiation time on the recovery ratio of iron was investigated. Here, C eq. was fixed at 1.17 (.816 g). A typical temperature profile during the carbothermic reduction under microwave irradiation is shown in Fig. 5. Temperature increased rapidly up to approximately 175 K at a heating rate of 31 K/min, which was slightly smaller than that of graphite in the section of 2.3, because graphite powders in the slag-graphite mixture were consumed in reductions. Then, the temperature maintained roughly constant. Although the surface temperature might be different from the bulk

4 2236 T. Kim and J. Lee T. Fe content in slag, (mass% T.Fe) / % Quartz crucible Alumina crucible Time, T / s Fig. 6 Typical heating behavior of the slag-graphite mixture (15 g slag,.816 g graphite, solid square: T. Fe in slag with quartz crucibles, solid circle: T. Fe in slag with alumina crucibles, open square: the weight of reduced metal with quartz crucibles, and the weight of reduced metal with alumina crucibles). T. Fe content in slag, (mass% T. Fe) / % temperature due to the selective and direct heating behavior of microwave, it could be an index of the average process temperature. 3,4,9) Figure 6 shows the iron content in the residual slag and the weight of metallic particles after experiments with respect to the microwave irradiation time. At 15 min, the weight of the recovered metallic particle had a maximum value and thereafter it decreased. It was considered that the decrease of the weight of the recovered metal was due to the reoxidation of the sample after consuming carbon, because the sample was cooled in air. From this result, the processing time was taken as 15 min in the following experiments. It is considered that the sort of crucible (quartz and alumina) does not affect the reduction behavior Effect of the carbon equivalent Figure 7 shows the changes of iron content in slag with respect to C eq.. For comparison, data after Morita et al. were plotted together. Iron content in the slag generally decreased with increasing C eq.. Despite a longer processing time and a higher initial T. Fe content, the present results showed reasonable accordance with the previous results. Experimental scatter might be caused by the non-uniform heating characteristics of microwave. Figure 8 shows the phosphorus contents in slag and metal with respect to C eq.. It was found that the phosphorus content Fig. 7 Total Fe content in slag after 15 min microwave processing with respect to C eq. Weight of reduced metal, W / g (a) (b) (mass% P 2 O 5 ) P content in metal, [mass% P] / % C eq Fig. 8 (a) P 2 O 5 content in slag and (b) P content in metal after 15 min microwave processing with respect to C eq. C content in metal, [mass% C] / % Fig. 9 Carbon content in metal after 15 min microwave processing with respect to C eq. in the slag gradually decreased with increasing C eq., whereas that in the metal did not change so much. It is noteworthy that the phosphorus contents in metal are much higher than the previous results. The reason is not clear, but it is speculated that in the previous study the evaporation of P 2 gas might be much stronger than the present work. Figure 9 shows the carbon content in metal. Carbon in metal ranged from to 1.5 mass%, which was much lower than the previous results. In addition, the carbon content in metal increased with increasing the amount of carbon addition (C eq. ). Carbon (as graphite or soluble carbon in

5 Recovery of Fe and P from CaO-SiO 2 -Fe t O-P 2 O 5 Slag by Microwave Treatment 2237 Recovery ratio of Fe, R Fe Fig. 1 Recovery ratio of Fe with respect to C eq. Al 2 content in slag, (mass% Al 2 ) / % Fig. 12 Concentration of Al 2 in slag after microwave processing with respect to C eq Recovery ratio of P, R P Fig. 11 Recovery ratio of P with respect to C eq. metal) may react with oxygen in the air. Therefore, as the reaction time increases or the amount of carbon addition decreases, the carbon content in metal would decrease. This behavior was also investigated in the previous study. Figures 1 and 11 show the recovery ratio of iron and phosphorus as a function of the carbon equivalent, respectively. The recovery ratios of iron and phosphorus were defined as eq. (1). ðweight of i in the recovered metalþ R ¼ ðweight of i in the initial slagþ ði ¼ Fe or PÞ ð1þ As C eq. increased, the recovery ratio increased, which behavior was similar to the reported values. The maximum recovery ratio of.97 was obtained at C eq. ¼ 1:69. The recovery ratio of phosphorus also increased with increasing C eq.. In the previous study, the recovery ratio of phosphorus had a maximum value.56 at C eq. ¼ 1:25, and decreased with further increase in C eq.. However, in the present study, the recovery ratio continuously increased and had a value of.89 at C eq. ¼ 1: Effects of Al 2 and T. Fe contents in slags Figure 12 shows the alumina content in slag after experiments. In some of the present experiments, alumina crucible was used, so that dissolution of alumina occurred at the slagcrucible-gas interface due to Marangoni convection as shown Partitioin ratio of P, L P.1.1 1E Al 2 content in slag, (mass% Al 2 ) / % Fig. 13 Partition ratio of phosphorus with respect to alumina concentration in slag after the microwave processing. in Fig. 4. 1) As C eq. increased, the alumina content in slag increased. However, there was a large scatter in experimental results due to the non-uniform heating behavior of the microwave process. Figure 13 shows the partition ratio of phosphorus with respect to the alumina content in the slag after experiments. It is found that there is no considerable effect of the alumina dissolution on the distribution ratio of phosphorus (L p = (mass% P in slag)/[mass% P in metal]). When the partition ratio of phosphorus was plotted with respect to T. Fe in the slag, a meaningful relationship was obtained, namely, as T. Fe decreased, L p decreased simultaneously (Fig. 1. For better understanding, thermodynamic calculations were carried out with FactSage. 11) The calculations were made by the step-wise consumption of carbon to reduce the slag at 18 K. Figure 15 shows a typical example of the calculated composition changes in slag and metal with the consumption of carbon. Here, alumina dissolution from the crucible was assumed to be 2 mass%. The reduction of iron oxide firstly occurred, and then as the concentration of iron oxide decreased the reduction of phosphate started. When the iron oxide was fully reduced, the reduction of SiO 2 might occur. It is noteworthy that carbon content in the reduced iron was as low as approximately.1 mass% when T. Fe was around 1 mass%. Moreover, when further reduction occurred and 1 C eq. was consumed, the carbon content increased up to 4.11 mass%. As the T. Fe in Morita s work was in the range

6 2238 T. Kim and J. Lee L P Thermodynamic cal. (2mass% Al 2 dissolusion) Thermodynamic cal. (no dissolution) 1E (mass% T. Fe) Fig. 14 Partition ratio of phosphorus with respect to T. Fe concentration in slag after the microwave processing. (a) s% M x O y ) (mas (b) ass% M] [m P Al, Ca, O of 1:55:5, it is considered that the reactions in their experiments were unstable and non-uniform. The present experiments showed that the highest reduction ratio of Fe was.97 at C eq. ¼ 1:69. In the thermodynamic calculation, the carbon consumption was.5 C eq. to obtain the same reduction ratio. Therefore, it is considered that about three times of carbon was consumed in the experiments. The calculated relationship between T. Fe and L p was drawn in Fig. 14 for comparison. Although there were large deviations in low T. Fe content region, the calculated results for T. Fe > 2.5 mass% agreed well with the experimental Fe ΔC eq. CaO Al 2 SiO 2 1 FeO Fe 2 P 2 O ΔC eq. Fig. 15 Change of (a) slag and (b) metal compositions with the consumption of carbon at 18 K (calculated with FactSage)..6.8 C Si 1. data. It is speculated that the diffusion of phosphorus became too slow to reach equilibrium value as the phosphorus content in slag decreased. In this figure, the effect of alumina dissolution was examined together. It is found that the alumina dissolution only slightly increased L p at the low T. Fe region. On the contrary, the effect of T. Fe was more remarkable than any other variables. Consequently, it is considered that in order to obtain a high recovery ratio of phosphorus, a high recovery ratio of iron is essential. 4. Conclusions (1) The heating rates of the examined substances are in the order of graphite Fe 3 O 4 Fe 2 > the synthesized CaO-SiO 2 -Fe t O-P 2 O 5 slag > the industrial slag. (2) The heating of the graphite-slag mixture was dominated by the graphite heating. (3) In the reduction of the synthetic slag (15 g) by graphite (.816 g), a maximum recovery rate was obtained at 15 min under the microwave irradiation (1.7 kw, 2.45 GHz). ( At a fixed microwave processing time of 15 min, the recovery ratios of iron and phosphorus increased with increasing the carbon equivalent (C eq. ). When C eq. ¼ 1:69, the recovery ratio of iron was.97 and that of phosphorus was.89. (5) Alumina dissolution showed negligible effect on the distribution ratio of phosphorus, whereas T. Fe in the slag showed strong effect, namely, as T. Fe content decreased, L p value decreased remarkably. Acknowledgement This subject is supported by Ministry of Environment, Korea as The Eco-technopia 21 project. We thank Prof. Kazuki Morita, the University of Tokyo and Dr. Youngjo Kang, POSCO for their kind discussion. We also thank Pro. Youn-Bae Kang, Postech for his kind help in thermodynamic calculations. REFERENCES 1) S. Shiomi, N. Sano and Y. Matsushita: Tetsu-to-Hagané 63 (1977) ) S. Takeuchi, N. Sano and Y. Matsushita: Tetsu-to-Hagané 66 (198) ) K. Morita, M. Guo, Y. Miyazaki and N. Sano: ISIJ Int. 41 (21) K. Morita, M. Guo, N. Oka and N. Sano: J. Mater. Cycles Waste Manag. 4 (22) ) S. Cho and J. Lee: Met. Mater. Int. 14 (28) ) E. Kim, S. Cho and J. Lee: Met. Mater. Int. 15 (29) ) A. Birnboim, D. Gershon, J. Calame, A. Birman, Y. Carmel, J. Rodgers, B. Levush, Y. V. Bykov, A. G. Eremeev, V. V. Holoptsev, V. E. Semenov, D. Dadon, P. L. Martin, M. Rosen and R. Hutcheon: J. Am. Cer. Soc. 81 (1998) ) M. Gupta and E. Wong: Microwaves and Metals, (John & Wiley & Sons (Asia) Pte Ltd., Singapore, 27) pp ) J. Lee and T. Kim: Advances in Induction and Microwave Heating of Mineral and Organic Materials, (Ed. by Stanisław Grundas, InTech, Rijeka, 211) pp ) K. Mukai: Phil. Trans. R. Soc. Lond. A 356 (1998) ) S. A. Decterov, Y.-B. Kang and I.-H. Jung: J. Phase Equil. Diffu. 3 (29)