Recovery of Phosphorus from Modified Steelmaking Slag with High P 2 O 5 Content via Leaching and Precipitation

Transcription

1 ISIJ International, Vol. 58 (2018), ISIJ International, No. 5 Vol. 58 (2018), No. 5, pp Recovery of Phosphorus from Modified Steelmaking Slag with High P 2 O 5 Content via Leaching and Precipitation Chuan-ming DU, 1) * Xu GAO, 2) Shigeru UEDA 2) and Shin-ya KITAMURA 2) 1) Graduate School of Engineering, Tohoku University, Katahira, Aoba-ku, Sendai, Japan. 2) Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai, Japan. (Received on July 31, 2017; accepted on January 5, 2018) The P contained in steelmaking slag is regarded as a potential phosphate source, especially with regard to slag with high P 2 O 5 content, which is generated from the utilization of high P iron ores. If P can be efficiently extracted from slag, the obtained P can be used as a phosphate fertilizer. Moreover, the remaining slag can be recycled inside the steelmaking process. Compared with other phases, the P-condensed C 2 S C 3 P solid solution in slag is more easily dissolved in water; therefore, selective leaching was applied to recover P from slag with high P 2 O 5 content. In this study, the effect of K 2 O modification on P dissolution in the citric acid solution was investigated, and subsequently, a process for extracting phosphate product from the leachate, via precipitation, was explored. It was determined that K 2 O modification promoted dissolution of the solid solution, resulting in a higher dissolution ratio of P. By modification, the majority of the solid solution was dissolved at ph 6, and other phases remained in residue, indicating that a better selective leaching of P occurred. As the ph decreased, the dissolution ratios of both P and Fe increased. Following leaching at ph 5, a residue with a higher Fe 2 O 3 content and lower P 2 O 5 content was obtained. When the ph of the leachate increased, the dissolved P in the aqueous solution was precipitated. Through separation and calcination, a phosphate product with a P 2 O 5 content of 30% was obtained, which has the potential to be used as a phosphate fertilizer. KEY WORDS: steelmaking slag; phosphorus recovery; selective leaching; K 2 O modification; citric acid. 1. Introduction Steelmaking slag, including dephosphorization slag, is an important by-product of steelmaking, with more than 14 million tons produced in Japan alone in ) Steelmaking slag contains considerable quantities of valuable elements, such as P, Mn, and Fe. Although P is detrimental to the deformability of steel, it is an important strategic resource for agricultural food production. In particular, increased demand for food, owed to global population growth, consequently increases the demand for P. Because of the depletion of high-grade natural phosphate ores, the P hidden in steelmaking slag is regarded as a secondary phosphate source. 2) Consequently, with regard to the steelmaking industry, it is important to develop an efficient method to recover P from steelmaking slag to lower production costs and save resources. Several methods have been explored. Li and Ishikawa et al. 3,4) attempted to reduce dephosphorization slag using a slag regenerator; P was reduced and concentrated in hot metal, and was further dephosphorized to produce fertilizer material. Yokoyama et al. 5) proposed a magnetic-separation method to separate and recycle the P-rich phase from steelmaking slag, by exploiting the differences in their magnetic properties. * Corresponding author: dcm198812@gmail.com DOI: Over the past decade, owing to the sharp increase in steel production, iron ores worldwide are steadily deteriorating in quality while prices remain at a high level. Therefore, it is inevitable that the use of low-grade iron resources will be extended in the future. Large reserves of iron ores with high P content are considered potential alternative resources. 6) During their use, the P content in hot metal will increase. After dephosphorization, the majority of the P is concentrated in slag and steelmaking slag with high P 2 O 5 content will be generated. 7) Compared with traditional slags, this slag is considered more suitable for the production of phosphate fertilizer because of its high P 2 O 5 content. If it was technically feasible to recover P from slag, the utilization of high P iron ores would be extended, securing supplies of phosphate sources. Based on this information, this study focused on developing an efficient and economical method of separating and recovering P from steelmaking slag with high P 2 O 5 content. Dephosphorization slag primarily consists of a CaO SiO 2 Fe t O P 2 O 5 system; it is a typical multiphase slag, which is saturated with dicalcium silicate (2CaO SiO 2 ). The 2CaO SiO 2 can absorb the dephosphorization product of 3CaO P 2 O 5 to form a 2CaO SiO 2-3CaO P 2 O 5 solid solution (C 2 S C 3 P) at steelmaking temperatures. 8) The high distribution ratio of P 2 O 5 between the C 2 S C 3 P solid solution and matrix phase indicates that P is mainly concentrated in the solid solution, even in slag with high P 2 O 5 content. 9) Hence, 833

2 the process used to separate the C 2 S C 3 P solid solution from the matrix phase is the same as that used to separate P from Fe contained in the matrix phase. Teratoko and Kitamura 10) determined that at a constant ph, the solubility of elements from the matrix phase, in the nitric acid (HNO 3 ) solution, was lower than that from the solid solution; they clarified that it was possible to dissolve a solid solution containing P without dissolving the matrix phase. They also found that the dissolution ratio of C 2 S C 3 P decreased greatly when the P 2 O 5 content was high. Based on such selective leaching, to increase the dissolution ratio of P from slag, we conducted a series of experiments to study the dissolution behavior of C 2 S C 3 P solid solution, and a synthesis slag with high P 2 O 5 content, in aqueous solutions ) It was determined that citric acid (H 3 C 6 H 5 O 7 ) exhibited a superior performance with regard to promoting dissolution of the C 2 S C 3 P solid solution compared with that of nitric acid. By reducing cooling rate of the molten slag, as well as Na 2 O modification, the selective leaching of P from slag was facilitated. This is because the dissolution of solid solution was enhanced, and the dissolution of matrix phase was suppressed. K 2 O has similar chemical properties to those of Na 2 O; it is also an important fertilizer constituent. To investigate the effects of the modification of slag with K 2 O on the dissolution and recovery of P, in present study, a slag with high P 2 O 5 content was modified by the addition of K 2 O, and then leached in the citric acid solution under various ph conditions. Chemical precipitation methods are commonly employed during phosphate fertilizer production, and used to recover P from wastewater. 14) To recover the P dissolved in the leachate, a process for extracting the phosphate product via precipitation was explored. 2. Experimental Method 2.1. Slag Synthesis To prepare a steelmaking slag for a CaO SiO 2 Fe 2 O 3 system, reagent-grade SiO 2, Fe 2 O 3, Ca 3 (PO 4 ) 2, MgO and K 2 CO 3 were fully mixed, along with CaO, and heated in a Pt crucible under an air atmosphere. To produce CaO, reagentgrade CaCO 3 was heated at K for 10 h in an Al 2 O 3 crucible under an air atmosphere. Fe 2 O 3 was used as the iron oxide because the dissolution ratio of P from this slag was greater than that from a CaO SiO 2 FeO system slag. 10) The compositions of the slags with varying mass ratios of K 2 O are shown in Table 1. These slags were initially heated at K for 1 h to form a homogeneous liquid phase. Subsequently, the slag was cooled to K at a cooling rate of 3 K/min and held at this temperature for 20 min to precipitate solid solution. Finally, the slag was cooled in furnace at a rate of 5 K/min, and was removed from the furnace at K. The composition of each phase in the slag was measured using an electron probe micro analyzer (EPMA). In addition, the precipitated phases were determined using X-ray diffraction (XRD) analysis Slag Leaching The experimental procedure used for the leaching was similar to that used in previous studies. 11,12) During the procedure, 1 g of the ground sample (with particles smaller than 53 μm) was put into a Teflon vessel containing 400 ml of ion-exchanged water. The aqueous solution was agitated at a speed of 200 r/min and its temperature was maintained at 298 K using an isothermal water bath. The dissolution of the Ca from slag increases the ph; therefore, to maintain the ph at a constant value, a ph meter was immersed, and citric acid, as a leaching agent, was automatically supplied using a PC control system. A schematic of the leaching system is shown in Fig. 1. At appropriate intervals, approximately 5 ml of the aqueous solution was sampled, and filtered using a syringe filter (< 0.45 μm). The concentration of each element in the filtered aqueous solution was analyzed using inductively coupled plasma-atomic emission spectroscopy (ICP-AES). During this study, three kinds of slags were leached at ph values of 5, 6, and 7, respectively. After the experiment, the leachate was separated from residue by filtering. The residue was dried, and then its mass and composition was measured. The leachate was used in the following experiment to recover P Recovery of Phosphorus from Leachate The leachate was treated, via the following procedure, to recover P, as shown in Fig. 2. To precipitate the dissolved P, the ph of the aqueous solution should be increased. In this study, saturated Ca(OH) 2 and NaOH solutions (1 mol/l) were selected and added to the leachate to adjust the ph to a value of approximately 11, respectively. Subsequently, the cloudy solution was settled for 24 h to separate the flocculent precipitate and aqueous solution. Following the removal of the upper solution, the concentrated precipitate was dried at 373 K until a solid product was formed. To remove crystal water and get a crystalline substance, the obtained precipitate was placed in a Pt crucible and further calcined at 873 K for 2 h. The compositions of the phosphate products were analyzed using XRD, and the contents of the key elements were determined using ICP-AES. Table 1. Composition of slags with various K 2O contents (mass%). Sample CaO SiO 2 Fe 2O 3 P 2O 5 MgO K 2O 1# Slag # Slag # Slag Fig. 1. Schematic of the leaching system. 834

3 3. Results and Discussion 3.1. Slag Composition The typical mineralogical structure of each slag with various K 2 O contents, and the composition of each phase, analyzed using EPMA, is shown in Fig. 3 and Table 2, respectively. In the case of no modification, three phases were observed in slag. Except solid solution, a magnesioferrite phase was precipitated from the matrix phase during slow cooling. A high distribution ratio of P 2 O 5 was determined between the solid solution and other phases. In contrast, Fe was mainly distributed in the matrix phase and magnesioferrite phase. Similar conditions were also observed in the case of the modified slags. Moreover, in these slags, a number of small solid solution particles, which had a lower P 2 O 5 content than the large solid solution particles, were observed to surround the magnesioferrite phase. The added K 2 O was distributed into the solid solution, and its content was lower than that in the matrix phase. With the increase in K 2 O content in slag, the K 2 O content in the solid solution increased; however, the P 2 O 5 content decreased. In the case of the matrix phase, the K 2 O addition resulted in a decrease in CaO and SiO 2 contents, corresponding to an increase in the Fe 2 O 3 content. The mass fractions of each phase in slag were estimated using the above EPMA results. To simplify calculation, we disregarded a small portion of the small solid solution particles with low P 2 O 5 content. The mass balance of each Table 2. Composition of each phase in different slags (mass%). Sample CaO SiO 2 Fe 2O 3 P 2O 5 MgO K 2O Phase Fig. 2. Experimental procedure of extracting phosphate product from leachate. 1# Slag (0% K 2O) 2# Slag (4% K 2O) 3# Slag (8% K 2O) A Magnesioferrite B Matrix phase C Solid solution A Magnesioferrite B Matrix phase C Solid solution D A Magnesioferrite B Matrix phase C Solid solution D Fig. 3. Mineralogical structure of slags with different K 2O contents. 835

4 oxide can be represented using Eq. (1) N N N N... (1) MOn MOn MOn MOn 1... (2) where, α, β, and γ are the mass fractions of the magnesioferrite phase, matrix phase, and solid solution, respectively, α N MOn is the MO n content in slag, and N MOn is the MO n content in phase α. The total mass balance of the phase fraction can be represented using Eq. (2). The calculated mass fractions of each phase in different slags will be described in the following discussion, as shown in Fig. 11. With the increase in K 2 O content in slag, the mass fraction of solid solution increased, and that of matrix phase decreased. There was little change in the mass fraction of magnesioferrite phase. When 8 mass% of K 2 O was added, the mass fraction of solid solution increased from 24.1% to 40.2%. As a result of the increase in mass fraction of solid solution, the P 2 O 5 condensed in the solid solution was diluted, and its content decreased; meanwhile, some of CaO and SiO 2 in the matrix phase were involved in the enlargement of solid solution, resulting in a decrease in their contents in the matrix phase Dissolution Behavior of Slag The dissolution reaction of solid solution is expressed in Eq. (3). 10) The change in the concentration of each element in the aqueous solution at ph 6 is shown in Fig. 4. The concentrations of Ca, Si, and P increased rapidly in 20 min; however, following 60 min, the dissolution rate of slag decreased. When the K 2 O addition was 4 mass%, the concentrations of Ca, Si, and P significantly increased, and a further increase in the K 2 O content resulted in further increases in their concentrations. Compared with other elements, the concentration of Fe was only several mg/l in each case. Fig. 4. Change in the concentration of each element in the aqueous solution at ph 6. 2CaO SiO2 3CaO P2O5 8H... (3) 2 5Ca H SiO 2HPO H O On the basis of the results shown in Fig. 4, the dissolution ratios of each element at 120 min were calculated using the following equation: WMaque, RM =... (4) WMslag, where, R M is the dissolution ratio of element M, W M,slag is the mass of element M in slag prior to leaching, which can be calculated using the mass and composition of the initial slag, and W M,aque is the mass of element M dissolved in the aqueous solution, which can be calculated using the mass and composition of the aqueous solution at 120 min. The calculated dissolution ratios of each element from each slag are shown in Fig. 5. In each case, the dissolution ratio of P was the highest, followed by those of Ca and Si, and the dissolution ratios of Fe and Mg were very low. Because Fe was mainly distributed in the magnesioferrite phase and matrix phase, it indicated that it was difficult to dissolve these two phases. In the case of the unmodified slag, a selective leaching of P from slag was performed, but the dissolution ratio of P was insufficient. With the increase in K 2 O content, the dissolution ratios of Ca, Si, and P increased. In the case of Fig. 5. Dissolution ratios of each element from different slags at ph 6. the slag with 8 mass% of K 2 O, 72.0% of the Ca and 84.6% of the P were dissolved, while the Fe was hardly dissolved; this demonstrates the superior selective leaching of P. The dissolution ratio of the added K was approximately 50% in this case. 836

5 To determine the optimum conditions for selective leaching, the dissolution behaviors of each slag under various ph conditions were also investigated. The dissolution ratios of P and Fe, which can represent the dissolution behavior of the solid solution and other phases, respectively, are shown in Fig. 6. This figure shows that the dissolution ratio of P was far greater than that of Fe in each case. As the ph decreased, the dissolution ratios of P and Fe both increased; however, the associated trend differed for each slag. At ph 7, with a K 2 O addition of 4 mass%, the dissolution ratio of P increased from 25.8% to 40.1%. However, a further increase in the K 2 O content did not significantly promote the dissolution of P. As described in previous studies, 13) Ca 2+ and phosphate ions react easily and form Ca 10 (PO 4 ) 6 (OH) 2 (hydroxyapatite, HAP) at higher ph condition (expressed in Eq. (5)), which determines the P concentration. Figure 7 shows the solubility line of HAP and experimental results. At ph 7, the observed points of the modified slags were located above the solubility line of HAP, indicating that the P concentration reached saturation and a further dissolution of P was hindered by HAP precipitation. Fig. 6. Fig. 7. Change in the dissolution ratios of P and Fe with ph. Solubility line of HAP and experimental results at various ph conditions. 2 Ca10( PO4) 6( OH) 2 14H 10Ca 6H2PO4 2HO 2 logk (5) When the ph decreased to 6, the concentration of H + ions increased, which facilitated dissolution of the solid solution. In this case, as shown in Fig. 7, the solubility line of HAP moved to the high concentrations of Ca and P, indicating that it was difficult for HAP precipitate to form. Therefore, the dissolution ratio of P from each slag increased significantly. In the case of the unmodified slag, the dissolution ratio of P exhibited a significant improvement when the ph decreased to 5; however, the dissolution ratio of Fe also increased dramatically, reaching 20.4%. This illustrates that some of the matrix phase also dissolved, which is detrimental for selective leaching. In the case of the modified slags, the dissolution ratio of P increased slightly at ph 5, and the dissolution of Fe occurred at low level; this indicates that the dissolution of matrix phase was suppressed. Hayashi et al. 15) reported that Fe 3+ ions in silicate glasses have two possibilities of being tetrahedrally and octahedrally coordinated; the fraction of Fe 3+ in tetrahedral symmetry is larger for the SiO 2 Na 2 O Fe 2 O 3 system than that for the SiO 2 CaO Fe 2 O 3 system. Therefore, it is reasonable to speculate that the structure of matrix phase became more stable as a result of Na 2 O addition. Because alkaline oxides (Na 2 O and K 2 O) have the similar chemical properties, it is considered that K 2 O modification has the same effect on suppressing dissolution of the matrix phase. In summary, the addition of K 2 O is beneficial for selective leaching of P from slag. The microstructure and composition of the residue obtained after leaching at ph 6 are shown in Fig. 8 and Table 3, respectively. Two main phases were identified in each residue. The white-colored area, rich in Fe 2 O 3 and MgO, is the magnesioferrite phase. The grey-colored phase, consisting of a CaO SiO 2 Fe 2 O 3 system, is considered the matrix phase. Compared with the results in Table 2, the compositions of these phases in the residue were almost identical to those in the slag prior to leaching. In each residue, it is difficult to detect the solid solution, indicating that the solid solution that had contacted with the aqueous solution had dissolved. For the residue of the modified slags, some holes could be observed near the magnesioferrite phase. These areas are considered to be the small solid solution particles that were observed prior to leaching. Figure 9 shows the XRD patterns for the slags and their residues after leaching at ph 6. In each slag, the precipitated solid solution and magnesioferrite phase were observed. The crystal form of the solid solution was changed because of K 2 O modification. After leaching, the intensity of the peaks associated with solid solution weakened, and that of the magnesioferrite phase increased. In the case of the residue of the unmodified slag, peaks associated with the solid solution still existed. However, for the modified slags, the peaks associated with the solid solution almost disappeared, indicating that the dissolution and separation of the solid solution were enhanced by K 2 O modification. The average composition of each residue was analyzed using ICP, as shown in Table 4. Compared with the initial slag prior to leaching, the CaO, SiO 2, and P 2 O 5 contents in 837

6 Fig. 8. Microstructure of residue after leaching at ph 6. Table 3. Composition of each phase in the residue after leaching (mass%). CaO SiO 2 Fe 2O 3 P 2O 5 MgO K 2O Residue (1# Slag, ph = 6) Residue (2# Slag, ph = 6) Residue (3# Slag, ph = 6) A B A B A B each residue reduced; however, the Fe 2 O 3 and MgO contents increased. After leaching at ph 6, with the increase in K 2 O content, the P 2 O 5 content of the residue decreased because of an increase in the dissolution ratio of P. The undissolved Fe remained in the residue, and its content correspondingly increased. When the ph decreased to 5, the P 2 O 5 and Fe 2 O 3 contents of the residue further decreased and increased, respectively; this is beneficial for the recycling of slag in steelmaking process. From the above findings, it can be determined that the P-condensed solid solution was easily dissolved compared with other phases in aqueous solutions, and could be separated from slag via leaching. Assuming that only the solid solution is dissolved, the dissolution ratios of Ca, Si, and P can be calculated using the mass fraction and the composition. The calculated values are shown in Fig. 10; these are also compared with the experimental results obtained at ph 6. The observed dissolution ratio of P was lower than the calculated value in each case, indicating that not all of the solid solution dissolved. With the increase in K 2 O content, the observed value gradually approaches the calculated values. This shows that the dissolution of solid solution was promoted. It is well Fig. 9. XRD patterns for the slags and their residues after leaching at ph

7 Table 4. Average composition of residues analyzed by ICP (mass%). Residue CaO SiO 2 Fe 2O 3 P 2O 5 MgO K 2O 1# Slag (ph = 6) # Slag (ph = 6) # Slag (ph = 6) # Slag (ph = 5) # Slag (ph = 5) # Slag (ph = 5) Fig. 10. Fig. 11. Calculated dissolution ratios of each element from solid solution comparing with the experimental results. Mass fractions of residue and dissolved part, compared with the phase fractions of each slag. known that K 3 PO 4 and Na 3 PO 4 are soluble in water, while Ca 3 (PO 4 ) 2 is insoluble; in addition, the 2CaO Na 2 O P 2 O 5 shows a higher solubility than 3CaO P 2 O 5 in the 2% citric acid solution. 16) Consequently, the introduction of K 2 O is considered to enhance the solubility of solid solution, resulting in a higher dissolution ratio. The dissolution ratios of Ca and Si were closed to the calculated values, and slightly greater than the calculated values when K 2 O modification was introduced. This shows that a portion of the dissolved Ca and Si was supplied from the matrix phase. Because the difference was not great, it is considered that the dissolution of matrix phase was not significant. The lower dissolution ratio of Fe could also confirm from this point. Following leaching, the masses of the residue and the dissolved portion, calculated using the dissolution ratio, were compared with the phase fractions of the initial slag Table 5. Concentrations of each element in the leachate and in the upper solution after precipitation (mg/l). Ca Si P Fe Mg Na K Leachate Solution (adding Ca(OH) 2) Solution (adding NaOH) in Fig. 11. In the case of the unmodified slag, the dissolved mass was lower than the mass fraction of solid solution, indicating that a portion of the solid solution remained in the residue. With the increase in K 2 O addition, the dissolved mass increased, and its value was almost identical to the mass fraction of solid solution. Combined with above analysis, it could be concluded that the majority of the solid solution had dissolved, and little dissolution of other phases occurred. Enhanced selective leaching of the P-condensed solid solution was achieved Phosphorus Recovery The leachate, after the leaching of slag with 4 mass% of K 2 O at ph 6, was used for P recovery. The composition of each element in the leachate is listed in Table 5. Owing to selective leaching, the Ca, Si, and P concentrations are high, and the Fe and Mg concentrations are low. Following the addition of the alkaline solution, and the precipitation, the concentration of each element in the upper solution were compared with those in the leachate in Table 5. This shows that the P concentration was reduced significantly. The P precipitation ratio was estimated using Eq. (6), where, Y P is the P precipitation ratio, C 1 is the P concentration in the leachate, and C 2 is the P concentration in the upper solution after precipitation. When Ca(OH) 2 was added, there was little change in the Ca concentration, and the P precipitation ratio reached 99.6%. When NaOH was added, the Ca concentration was reduced by half, and the P precipitation ratio was 96.0%. There was a little decrease in the Si and K concentrations in both cases. C1 C2 YP (6) C1 It is well known that in solutions containing Ca and phosphate ions, a number of calcium phosphate phases such as dicalcium phosphate dihydrate (DCPD, CaHPO 4 2H 2 O), octacalcium phosphate (OCP, Ca 8 H 2 (PO 4 ) 6 5H 2 O), tricalcium phosphate (TCP, Ca 3 (PO 4 ) 2 ), and HAP (Ca 10 (PO 4 ) 3 (OH) 2 ), may form depending on the ph and solution composition. 17) The precipitation reactions of these calcium phosphates are described in Eqs. (7) (10). 18,19) On the basis of their equilibrium constants, the solubility curves of these calcium phosphates in aqueous solutions were calculated at ph 11. Figure 12 shows the compositions of the leachate and that of the solution after precipitation. After the addition of NaOH, the solution composition was located at the solubility curve of DCPD; in the case of Ca(OH) 2 addition, it was located between the solubility curve of DCPD and OCP. This shows that the precipitated calcium phosphate may consist of DCPD or OCP. Considering the thermodynamics, 839

8 Fig. 13. Image of the precipitate and phosphate product when Ca(OH) 2 was added. Table 6. Composition of the precipitates and phosphate products (mass%). Fig. 12. Solubility curves for some calcium phosphates and the experimental results at ph 11. HAP is determined to be the most stable calcium phosphate, and the P concentration in the solution can be reduced to a much lower value. However, the precipitation of unstable compounds, as precursors, is commonly observed owing to the differences in the kinetic condition of nucleation. 17) In many natural environments, DCPD, along with OCP and TCP, plays a crucial role as a precursor or intermediate to HAP. 20) Therefore, the P concentration in the upper solution is considered to be determined by the solubility values of DCPD and OCP. 2 2 Ca HPO4 2HO 2 CaHPO4 2HO 2 log K (7) 2 3 8Ca 6PO4 2H 5H2O Ca8H(PO 2 4) 6 5H2O... (8) log K Ca 2PO Ca (PO ) log K (9) Ca 6PO4 2H2O Ca 10(PO 4)(OH) 6 2 2H log K (10) Figure 13(A) shows an image of the precipitate obtained through the addition of Ca(OH) 2. Table 6 shows the composition of the precipitate. These two precipitates mainly consist of CaO and P 2 O 5 ; the K 2 O content is very low. The precipitate also has SiO 2 and Fe 2 O 3 contents of 3 5 mass% and 1.0% mass%, respectively. When NaOH was added, the obtained precipitate had a higher P 2 O 5 content and lower SiO 2 content. However, according to the mass balance calculation, approximately 20 mass% of the precipitate was determined to be unknown. This constituent was considered to be crystal water and organic substance (citrate). Following calcination, the white precipitate transformed into a grey phosphate product, as shown in Fig. 13(B). The compositions of these phosphate products are also listed in Table 6. Compared with those of the precipitate, the contents of each constituent all increased because of the removal of the crystal water and the decomposition of the organic substance. When NaOH was added, the P 2 O 5 content in the final phosphate product reached 30.4 Precipitate Phosphate product Sample CaO SiO 2 P 2O 5 Fe 2O 3 Na 2O MgO K 2O Others Ca(OH)2) NaOH Ca(OH) 2) NaOH) mass%, which is similar with that in some commercial phosphate fertilizers. 21) Figure 14 shows the XRD patterns for the phosphate product. The main peaks of these two products are almost identical. The peaks were consistent with those of silicon substituted calcium hydroxyapatite (Ca 5 (PO 4 ) 2.85 (SiO 4 ) 0.15 (OH)) and HAP (Ca 10 (PO 4 ) 6 (OH) 2 ). This shows that, following calcination, the phosphate that was precipitated from the leachate finally existed in the form of HAP. As shown in Fig. 12, the most stable calcium phosphate is HAP. There is also evidence to show that ultimately, any calcium phosphate that is precipitated will probably transform into the thermodynamically more stable HAP. 22) In summary, the addition of Ca(OH) 2 or NaOH solution into the leachate has similar effects on phosphate precipitation, and an identical HAP product is obtained. The obtained precipitate or phosphate product has the potential to be used as a fertilizer, because they have the same components (CaO and P 2 O 5 ) as phosphate fertilizer and a high enough P 2 O 5 content. 21) In this study, a process for the comprehensive utilization of slag with high P 2 O 5 content and waste-free steelmaking was proposed, which is outlined in Fig. 15. During a conventional ironmaking process, hot metal with high P content will be generated because of the reduction of high P iron ores. First, it is dephosphorized in a converter. During this process, alkaline oxide (Na 2 O or K 2 O) is added as a flux to increase the phosphate capacity of slag. 23) Following dephosphorization, the hot metal is decarburized with a smaller amount of slag, and molten steel is produced. The slag with high P 2 O 5 content is oxidized, and then treated via selective leaching. The P in this slag is dissolved and concentrated in the leachate. The leaching residue and decarburization slag, with a lower P 2 O 5 content and higher Fe 2 O 3 content can be returned to the dephosphorization process. The soluble phosphate in the leachate is precipitated in the form of calcium phosphate, which can be used as a fertilizer. No extra slag is discharged during this steelmaking process. 840

9 Fig. 14. XRD patterns for the obtained phosphate products. the leachate, via precipitation, was explored. The following results were obtained: (1) By K 2 O modification, a portion of the K 2 O was distributed into the P-rich solid solution, and the mass fraction of the solid solution in slag increased; however, the P 2 O 5 content in the solid solution decreased. (2) K 2 O modification promoted dissolution of the solid solution in the aqueous solution, resulting in higher dissolution ratios of Ca, Si, and P. In the case of the modified slag, the majority of the solid solution was dissolved at ph 6, and other phases remained in the residue, indicating that selective leaching of P from slag was more efficient. (3) As the ph decreased, the dissolution ratios of P and Fe both increased. At ph 5, more than 80% of the P was dissolved from each slag. Following K 2 O modification, the dissolution of the matrix phase was significantly suppressed, resulting in a lower Fe dissolution ratio. A residue with a higher Fe 2 O 3 content and lower P 2 O 5 content was obtained after leaching. (4) When the ph of the leachate increased because of the addition of alkaline solution, the dissolved P in the aqueous solution was almost precipitated. Through separation and calcination, a phosphate product with a P 2 O 5 content of approximately 30 mass% was obtained; it has the potential to be used as a phosphate fertilizer. Moreover, a process for the comprehensive utilization of high P iron ores, and waste-free steelmaking, was proposed. REFERENCES Fig Conclusions A process for the comprehensive utilization of slag with high P 2O 5 content. To recover P from steelmaking slag with high P 2 O 5 content, the effects of K 2 O modification of slag on selective leaching of P in the aqueous solution were investigated, and then a process for extracting phosphate product from 1) Nippon Slag Association: (accessed ). 2) K. Matsubae, E. Yamasue, T. Inazumi, E. Webeck, T. Miki and T. Nagasaka: Sci. Total Environ., 542 (2016), ) H.-J. Li, H. Suito and M. Tokuda: ISIJ Int., 35 (1995), ) M. Ishikawa: ISIJ Int., 46 (2006), ) K. Yokoyama, H. Kubo, K. Mori, H. Okada, S. Takeuchi and T. Nagasaka: ISIJ Int., 47 (2007), ) C. Y. Cheng, V. N. Misra, J. Clough and R. Mun: Miner. Eng., 12 (1999), ) S. Kitamura and F. Pahlevani: Tetsu-to-Hagané, 100 (2014), ) K. Ito, M. Yanagisawa and N. Sano: Tetsu-to-Hagané, 68 (1982), ) K. Shimauchi, S. Kitamura and H. Shibata: ISIJ Int., 49 (2009), ) T. Teratoko, N. Maruoka, H. Shibata and S. Kitamura: High Temp. Mater. Process., 31 (2012), ) C. Du, X. Gao, S. Kim, S. Ueda and S. Kitamura: ISIJ Int., 56 (2016), ) C. Du, X. Gao, S. Ueda and S. Kitamura: ISIJ Int., 57 (2017), ) C. Du, X. Gao, S. Ueda and S. Kitamura: J. Sustain. Metall., 3 (2017), ) G. K. Morse, S. W. Brett, J. A. Guy and J. N. Lester: Sci. Total Environ., 212 (1998), ) M. Hayashi and M. Susa: Proc. 14th Japan-China Symp. on Science and Technology of Iron and Steel, ISIJ, Tokyo, (2016), ) R. P. Gunawardane and F. P. Glasser: J. Mater. Sci., 14 (1979), ) E. Valsami-Jones: Mineral. Mag., 65 (2001), ) T. Futatsuka, K. Shitogiden, T. Miki, T. Nagasaka and M. Hino: ISIJ Int., 44 (2004), ) A. L. Iglesia: Estud. Geol., 65 (2009), ) P. G. Koutsoukos and G. H. Nancollas: J. Cryst. Growth, 53 (1981), ) Ministry of Agriculture, Forestry and Fisheries: go.jp/j/kokuji_tuti/kokuji/pdf/k pdf, (accessed ). 22) W. Kibalczyc: Cryst. Res. Technol., 24 (1989), ) J. J. Pak and R. J. Fruehan: Metall. Mater. Trans. B, 22 (1991),