Separation and recovery of phosphorus from steelmaking slag via a selective leaching–chemical precipitation process

Hydrometallurgy 189 (2019) 105109

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Separation and recovery of phosphorus from steelmaking slag via a selective leaching–chemical precipitation process

T

Chuan-Ming Dua,b, , Xu Gaob, Shigeru Uedab, Shin-Ya Kitamurab ⁎

a b

School of Metallurgy, Northeastern University, Shenyang 110819, China Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan

ARTICLE INFO

ABSTRACT

Keywords: Steelmaking slag C2S–C3P solid solution Phosphorus recovery Selective leaching Chemical precipitation

Steelmaking slag, a significant by-product of steelmaking, contains a certain amount of phosphorus, which makes it a potential phosphate source. To achieve its high value-added utilization, the P-concentrated C2S–C3P solid solution in slag is commonly separated. In this study, we adopted a selective leaching–chemical precipitation process to recover P from steelmaking slag. The effects of the acid, pH, and valence of Fe in slag on the leaching characteristics of P were first investigated. Chemical precipitation was then adopted to extract P from the leachate. The characteristics of the final product were evaluated. The results indicate that the P-concentrated solid solution was readily leached, whereas the Fe-rich magnesioferrite was resistant to dissolution in each acid solution. In view of the utilization of the residue and reducing the production costs, hydrochloric acid was considered the optimum leaching agent for dissolving the slag. The dissolution of steelmaking slag in the hydrochloric acid solution increased with a decrease in pH. In addition, 81.6% of the P in slag was dissolved at pH 3, whereas the Fe leaching was negligible, exhibiting an improved selective leaching. Further, the P-removed steelmaking slag can be reutilized in a steel plant to achieve waste-free steelmaking. In the following precipitation at pH 7, most of the P in the leachate was transformed into an insoluble calcium phosphate. This process provides more than a 70% P recovery from steelmaking slag. The P2O5 content in the product obtained reached 20.2 mass%, which can be applied as a fertilizer.

1. Introduction Steelmaking slag, which is produced to remove the impurities of hot metal through oxidation and the addition of lime, is a significant byproduct of the steelmaking process. The productivity of steelmaking slag is approximately 0.11 tons per ton of molten steel (Nippon Slag Association, 2018). Because the global output of steel has exceeded 1.5 billion tons (World Steel Association, 2018), the disposal and utilization of such large amounts of steelmaking slag is an enormous challenge to society and the environment. The major components in steelmaking slag are CaO, SiO2, and FetO. Steelmaking slag also contains some valuable oxides such as P2O5 and MnO, which are considered potential sources. However, steelmaking slag is normally applied as a concrete material, or in road construction and landfills (Miki and Kaneko, 2015). The intrinsic value of steelmaking slag has yet to be properly exploited. With an increase in the environmental burden and depletion of natural resources, the extraction of valuable elements from steelmaking slag has drawn significant attention. Phosphorus is an essential element for the growth of animals and

⁎

plants and is one of the significant strategic sources of fertilizer production (Abelson, 1999). Although phosphate ores are the main raw material for industrial phosphate, they are not evenly distributed worldwide. For example, Japan has no natural phosphate ores (Matsubae-Yokoyama et al., 2009). Because the high-grade phosphate ore reserves are decreasing and the demand for fertilizer is strong, the price of phosphate ore is continually increasing. To secure a sustainable supply of P, it is critical to either recover P from waste wherever possible or develop alternative sources of P (Budhathoki et al. 2018). Although P is vital for the chemical industry and agriculture, it is detrimental to steel products. Most of the P in hot metal needs to be removed and concentrated in slag. P-bearing steelmaking slag is regarded a potential P source owing to the extremely large amount produced (Matsubae et al. 2016). To increase the efficiency of P removal and reduce the consumption of lime, hot metal dephosphorization is generally carried out using a converter during the steelmaking process. During this process, the slag is saturated with dicalcium silicate (2CaO∙SiO2) in a coexisting solid and liquid state. The P2O5 generated from the oxidization of P in metal

Corresponding author at: 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan. E-mail address: [email protected] (C.-M. Du).

https://doi.org/10.1016/j.hydromet.2019.105109 Received 25 January 2019; Received in revised form 1 June 2019; Accepted 13 July 2019 Available online 15 July 2019 0304-386X/ © 2019 Elsevier B.V. All rights reserved.

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reacts with the 2CaO∙SiO2, leading to the formation of a solid solution of 2CaO∙SiO2–3CaO∙P2O5 (C2S–C3P) in the liquid phase (Suito et al. 1977). Ito et al. (1982) confirmed that the majority of P2O5 in steelmaking slag was enriched in a C2S–C3P solid solution, whereas the other phases showed a lower P2O5 content. Thus, P recovery substantially depends on a separation of the solid solution from steelmaking slag. If shown to be practicable, a sustainable and waste-free steelmaking process has the potential to be achieved. In addition to the P recovery, the P-removed steelmaking slag, which contains higher amounts of FetO and MnO, can be reutilized in a steel plant. Therefore, the extraction of the C2S–C3P solid solution from steelmaking slag is of significance in both the steelmaking and phosphate industries. Many studies have been conducted to separate the solid solution from steelmaking slag using the difference in physicochemical properties between different mineralogical phases. Kubo et al. (2010) adopted magnetic separation to extract the nonmagnetic solid solution from the Fe-rich matrix phase. Ono et al. (1980) and Li et al. (2016) attempted to separate the solid solution, which had a lower density, through floatation and centrifugalization, respectively. Miki and Kaneko (2015) used CaO to absorb the Fe-rich liquid phase using the capillary phenomenon to separate the solid solution. However, these methods are costly in terms of energy and have a lower efficiency. Because the C2S–C3P solid solution dissolves readily in water compared to other mineralogical phases, Teratoko et al. (2012) proposed the recovery of P from steelmaking slag through selective leaching of the solid solution. Numata et al. (2014) confirmed this selective leaching for the quenched steelmaking slag, whereas the extraction efficiency of P was lower. Previously, our studies principally focused on the selective leaching of P from steelmaking slag with a high P2O5 content (8 mass%), which is generated from the utilization of high-P iron ores. The C2S–C3P solid solution in this slag contains a higher P2O5 content, resulting in lower water solubility (Teratoko et al. 2012). To achieve an improved selective leaching, we should maximize the extraction efficiency of the P-concentrated solid solution while minimizing the leaching of other phases. Through a series of fundamental studies, we determined that Na2O modification, a slow cooling of the molten slag, and the use of citric acid as a leaching agent are advantageous to the selective leaching of P (Du et al. 2017a, b). For the Pcontaining leachate, we proposed a novel method that makes the soluble P precipitate as calcium phosphate by adjusting the pH. It was confirmed that most of the P is precipitated and recovered (Du et al. 2018a, b). These studies presented an efficient and energy-saving method for the comprehensive utilization of the slag with a high P2O5 content, although the process is complex and expensive. To achieve a high value-added utilization of normal steelmaking slag (containing approximately 3 mass% P2O5), selective leaching and chemical precipitation were applied to recover P from this slag. The objective of this study is to reduce the production costs and realize an improved selective leaching of P. Specifically, the effects of the acid, pH, and valence of Fe on the leaching characteristics of P from furnacecooled steelmaking slag were first investigated. Precipitation using an alkaline solution was then adopted to extract P from the leachate under a nearly neutral condition. The characteristics of the final product and its available P2O5 content were evaluated. Finally, we proposed a lowcost process for P recovery and waste-free steelmaking.

Table 1 Actual composition of steelmaking slag with different valence of Fe (mass%).

Slag A Slag B

CaO

SiO2

Fe2O3

FeO

P2O5

MgO

MnO

Al2O3

31.19 32.60

19.86 22.48

27.79 0

0 26.27

3.29 3.52

10.24 7.29

4.58 4.81

3.05 3.02

solution was then precipitated from the liquid slag during cooling. Finally, the sample was removed from the furnace at 1273 K. The atmosphere while synthesizing the slag containing Fe2O3 and FeO was air and Ar, respectively. Table 1 shows the actual composition of steelmaking slag with different valence of Fe, which was determined using inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The mineral constituent of steelmaking slag and the average composition of the mineralogical phase were determined using X-ray diffraction (XRD) analysis and electron probe micro analyzer (EPMA). 2.2. Selective leaching The synthesized slag was crushed and ground with a particle size of smaller than 53 μm. A total of 4 g of steelmaking slag was added to a Teflon container with 400 mL of distilled water. The solution was mixed using a rotary stirrer at 200 rpm for 2 h. All leaching experiments were conducted at 298 K. To achieve the selective leaching of P from steelmaking slag, we need to control the pH of the solution at a constant value. In the present study, hydrochloric acid (HCl, 1.5 mol/L), sulfuric acid (H2SO4, 0.75 mol/L), nitric acid (HNO3, 1.5 mol/L), and citric acid (H3C6H5O7, 0.75 mol/L) were selected as the leaching agents. Acid was automatically pumped into the solution using a PC-controlled system which connects with a pH meter. To study the effects of the acid and valence of Fe in slag, the pH was maintained at 3. During leaching, the solution was sampled and filtered through a 0.45 μm mixed cellulose ester membrane filter. The concentration of each element released from the slag into the leachate was determined using ICP-AES. After the reaction, the slurry was filtered. The leachate was then used for the following P recovery, and the dried residue was weighed and analyzed using XRD and EPMA. 2.3. Chemical precipitation Following leaching in the hydrochloric acid solution at pH 3, the leachates of slags A and B were used to extract P. The soluble P can be precipitated by adjusting the pH to a higher value, although it is necessary to transform the metal-P precipitates into a plant-available fertilizer by adding Ca2+ or NH4+ cations (Franz 2008). We selected a saturated Ca(OH)2 solution as the source of Ca2+ ions and to adjust the pH. When the pH of the leachate exceeded 7, a large amount of precipitate was formed. The cloudy leachate was first settled to allow a solid–liquid separation. A concentrated precipitate was then collected using centrifugal separation. After calcination at 773 K, a final phosphate product was obtained. The mineral composition of the product was characterized using XRD, and the contents of each component were determined using ICP-AES. 3. Results and discussion

2. Experimental method

3.1. Mineralogical composition of slag

2.1. Slag preparation

The mineralogical phases in slag with different valence of Fe were characterized using EPMA, as displayed in Fig. 1. Table 2 lists the average compositions of the mineralogical phase in different slags. Three major domains corresponding to three mineralogical phases were identified in each slag. The white phase with high FetO, MgO, and MnO contents was magnesioferrite. In addition to the large particles of magnesioferrite, large amounts of fine particles are also present in the

The slag preparation method used in this study is almost the same as that described in previous papers (Du et al. 2017a, 2018c). Two types of steelmaking slag containing Fe2O3 and FeO, respectively, were synthesized using chemical reagents. The mixing ratios of the total Fe and other oxides of each slag were virtually the same. The mixture of chemical reagents was first melted at 1823 K in a MgO crucible. The solid 2

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Fig. 2. Change in the concentrations of P and Fe in different acid solutions at pH 3.

3.2. Leaching characteristic of steelmaking slag Slag A was leached by various acid solutions at pH 3. Fig. 2 shows the change in the concentrations of P and Fe with the leaching time for different acid solutions. The leaching rate of the slag was high during the initial stage, accounting for higher concentrations of P and Fe after 30 min. Their concentrations increased slightly after 60 min, indicating that the leaching reaction approached equilibrium. The P concentration in each acid solution exceeded 105 mg/L, which was considerably higher than the Fe concentration. Citric acid exhibited a strong ability to dissolve the slag. In the citric acid solution, the P concentration could achieve a higher level within a short time, and the Fe concentration was three-times higher than that in the other acid solutions. After 120 min, the P concentration in different acid solutions was almost the same except for that in the sulfuric acid solution. The Fe concentration in the inorganic acid solutions was only approximately 8 mg/L. The extraction efficiency of element M (EM) from slag in different acid solutions at pH 3 was calculated as follows:

Fig. 1. Micrographs of steelmaking slag with different valence of Fe. Table 2 Average compositions of the mineralogical phase in different slags (mass%).

Slag A

Slag B

CaO

SiO2

FetO

P2O5

MgO

MnO

Al2O3

Phase

1 2 3

1.1 51.1 59.4

0.1 33.4 27.0

66.4 0.8 1.2

0.0 2.3 9.0

16.0 11.4 2.1

9.5 0.9 1.2

6.9 0.1 0.1

1 2 3

0.7 50.5 56.6

0.2 32.0 27.2

75.5 3.6 3.5

0.0 3.0 8.3

11.4 9.2 1.9

10.8 1.6 2.4

1.3 0.1 0.1

Magnesioferrite Merwinite (C3MS2) Solid solution (C2S–C3P) Magnesioferrite Merwinite (C3MS2) Solid solution (C2S–C3P)

EM =

CM V × 100% mM

(1)

where CM is the M concentration after 120 min (mg/L), V is the leachate volume (L) (considering the volumes of the sampled solution and the added acid), and mM is the mass of M in 4 g of slag (mg). As displayed in Fig. 3, the dominating elements extracted during leaching were Ca, Si,

Fe2O3-containing slag. The other two phases, in which almost all of the P2O5 was distributed, showed a similar color because of a low FetO content. The greyish phase rich in P2O5 was the C2S–C3P solid solution. The phase with a higher MgO content was considered merwinite whose chemical formula is Ca3Mg(SiO4)2 (C3MS2). Owing to the slow cooling, no glassy phase of the CaO–SiO2–FetO system was observed. Because the P2O5 content in these slags was approximately 3 mass%, the P2O5 content in the solid solution was not high compared to that in slag with a high P2O5 content (Shimauchi et al. 2009). For slag A, which contained Fe2O3, the P2O5 content in the solid solution was 9.0 mass%. The distribution ratio of P2O5 between the solid solution and merwinite was higher than that in the FeO-containing slag, indicating a better enrichment of P2O5 in the solid solution. For slag B, the FetO content in the solid solution was higher than that in slag A. This is because Fe2+ ions have the same ionic valence as Ca2+ ions, and the radius is smaller than that of Ca2+; thus, Fe2+ ions can replace Ca2+ ions and easily enter the C2S–C3P solid solution (Du et al. 2018c).

Fig. 3. Effect of acid on the extraction efficiency of each element from slag at pH 3. 3

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and P, which were mainly distributed in the solid solution and merwinite. The extraction efficiency of Fe was negligible. These results demonstrate that the P-concentrated solid solution was readily leached, whereas the Fe-rich magnesioferrite was resistant to dissolution. Approximately 30% of the Mg was also dissolved, which was attributed to a large dissolution of the merwinite. Overall, a selective leaching of P from steelmaking slag was performed in each case. In the acidic solution, phosphate and silicate ions primarily existed in the form of H2PO4− and H2SiO30 (Futatsuka et al. 2004). The major reactions during leaching include the following:

2CaO SiO2

3CaO P2 O5 + 8H+ = 5Ca2 + + H2 SiO30 + 2H2 PO4 + H2 O (2)

Ca3Mg(SiO4 )2 +

8H+

=

3Ca2 +

+

Ca2 + + C6 H5 O73 = Ca(C6 H5 O7 )

Mg2 +

+

2H2 SiO30

+ 2H2 O

(3) (4)

The mineralogical phases were leached in the citric acid solution through two mechanisms (McDonald and Whittington 2008), namely, direct displacement of the metal cations from the mineral structure by H+ ions at a low pH and the formation of soluble organometallic complexes by chelation. As described in Eq. (4), citrate ions (C6H5O73−) function as a chelator of Ca2+, which favors a crystal lattice destruction (Astuti et al. 2016). These actions lead to a higher extraction efficiency of P and Fe from the slag. When sulfuric acid is used, the dissolved Ca2+ ions combine with sulfate ions to precipitate CaSO4. Thus, the extraction efficiency of Ca is less than 40%. Because the CaSO4 precipitate wraps the undissolved slag, it hinders a continuous leaching of the steelmaking slag, resulting in the lowest P releasing capacity. Further, the residue containing a CaSO4 precipitate is difficult to be reutilized in the steelmaking process because sulfur is a troublesome element during steelmaking. Nitric acid and hydrochloric acid are both monoacids. In these inorganic acid solutions, the leaching of mineralogical phases principally depends on H+ ions, and anions such as NO3− and Cl− ions have little effect. Therefore, the leaching characteristics of each element in the nitric and hydrochloric acid solutions were virtually the same. Approximately 81% of the P was extracted, whereas the leaching of Fe was insignificant. Although the use of citric acid shows a slightly higher dissolution ratio of P, the price of citric acid is far higher than that of normal inorganic acids. Except for sulfuric acid, the price of hydrochloric acid is lower. In view of reducing the production costs and the utilization of the residue, hydrochloric acid was considered the most suitable leaching agent for extracting P from steelmaking slag. When hydrochloric acid was used as a leaching agent, the extraction efficiencies of each element from slag A at different pH conditions are as displayed in Fig. 4. The dissolution of the slag was promoted with a decrease in the pH, resulting in higher extraction efficiencies of each element. The extraction efficiencies of Ca, Si, and P were far higher than those of other elements in each case. Fe could barely dissolve, and its extraction efficiency was negligible. The leaching of P was insufficient at pH 5. When the pH decreased to 3, almost a complete Ca extraction was achieved; the extraction efficiencies of P and Si increased considerably, reaching 81.6% and 73.7%, respectively. The P extraction showed a slight improvement when the pH continued to decrease, whereas the extraction efficiency of Fe almost doubled. Therefore, the pH should be controlled at 3 to achieve an improved selective leaching of P and reduce the acid consumption. To evaluate the leaching characteristics of the mineralogical phases, we first calculated the phase fractions in different slags using the phase composition (listed in Table 2). The mass balance of each oxide and phase fraction can be described as follows:

(%MO)slag =

Xi = 1

(%MO)i Xi

Fig. 4. Extraction efficiency of each element in the hydrochloric acid solution under various pH conditions.

Fig. 5. Phase fractions of slag A and the percentage of the dissolved slag under various pH conditions.

phase i, and Xi is the phase fraction. The percentage of dissolved slag was calculated on the basis of the extraction efficiency and slag composition. In Fig. 5, we compared the percentages of the dissolved slag and residue under various pH conditions with the phase fractions of slag A. The mass fractions of the solid solution and merwinite were 29.6%–30.6%, respectively. As discussed above, the solid solution and merwinite were readily dissolved. The percentage of the dissolved slag was only 33.8% at pH 5, which is less than the sum of the mass fractions of the solid solution and merwinite. This indicates that the leaching of the solid solution was insufficient, and that the residue contains a portion of the undissolved solid solution. The percentage of the dissolved slag increased significantly as the pH decreased. It was slightly lower than the sum of the mass fractions of the solid solution and merwinite at pH 3, illustrating that the majority of the slag was leached, with the exception of the Ferich magnesioferrite. When the pH decreased from 3 to 2, the percentage of the dissolved slag slightly increased. Fig. 6 shows the extraction efficiencies of each element from slag with different valence of Fe in the hydrochloric acid solution. Almost a complete Ca extraction was achieved regardless of the valence of Fe in the slag. The extraction efficiency of P from the FeO-containing slag was only 67.3%, which is lower than that from the Fe2O3-containng slag. However, the leaching of Fe, Mn, and Mg was considerably improved when the Fe2O3 in the slag was transformed into FeO. The extraction efficiency of Fe reached 6.1%. Selective leaching of P from steelmaking slag worsened. In addition, a larger dissolution of Fe will have a negative influence on the P recovery from the leachate because the ferric phosphate is not suitable as a fertilizer. Therefore, for

(5) (6)

where (%MO)slag is the MO content in slag, (%MO)i is the MO content in 4

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Table 3 Residue compositions after leaching under various pH conditions (mass%). Sample

CaO

SiO2

Fe2O3

P2O5

MgO

MnO

Al2O3

Slag A (Fe2O3) Residue (HCl, pH = 2) Residue (HCl, pH = 3) Residue (HCl, pH = 4) Residue (HCl, pH = 5)

31.19 1.46 1.04 9.00 14.50

19.86 5.37 6.33 14.18 18.21

27.79 62.50 61.73 51.23 43.83

3.29 0.82 0.84 1.69 2.25

10.24 14.13 14.45 10.68 9.65

4.58 9.26 9.27 7.81 6.80

3.05 6.46 6.34 5.41 4.75

Table 3. Compared to the original slag composition, the CaO, SiO2, and P2O5 amounts in the remained slag decreased after selective leaching, whereas the Fe2O3, MnO, and Al2O3 amounts increased. To achieve a reutilization of the residue during the steelmaking process, the P2O5 content in the residue should be lowered as much as possible. When the slag was leached at pH 5, the P2O5 content in the residue was still higher, reaching 2.25 mass%. Because an improved selective leaching of P was performed at a lower pH, the residue showed a lower P2O5 content and higher Fe2O3 content. The compositions of the residues after leaching at pH 2 and 3 were almost the same. In these residues, the P2O5 content decreased to 0.84 mass% and the Fe2O3 content exceeded 61 mass%. It is possible to utilize such materials as a steelmaking flux or raw ironmaking material. Thus, the Fe2O3 and MnO remaining in the residue is recovered, and the maximum value of the steelmaking slag is exploited. Fig. 8 shows a micrograph of the residue surface of the Fe2O3-containing slag after leaching in the hydrochloric acid solution at pH 3. The average compositions of the identified domains on the residue surface are listed in Table 4. There were many holes on the surfaces of the large particles. The residual slag mainly consisted of Fe, Mn, and Mg, which was virtually identical to the magnesioferrite. In addition to the large particles, smaller particles without holes were also observed. These particles rich in Fe and Mg were also magnesioferrite. The P-containing solid solution and merwinite were not detected on the residue surface. This result demonstrates that the solid solution and merwinite were readily leached compared to the magnesioferrite. Most of the P could be extracted from the steelmaking slag. Fig. 9 shows X-ray diffraction patterns of the Fe2O3-containing slag and its residues after leaching at pH 3. Three crystalline phases were observed in the original slag, which was consistent with the above EPMA analysis. The peaks associated with the solid solution and merwinite were almost the same owing to their similar structures. After leaching in the hydrochloric acid solution, the peaks associated with the solid solution and merwinite weakened significantly, whereas the peaks of the magnesioferrite intensified. This demonstrates that the majority

Fig. 6. Extraction efficiency of each element from slag with different valence of Fe.

practical steelmaking slag, which contains both FeO and Fe2O3, an oxidization treatment is necessary to make the FeO in the slag transform into Fe2O3. In general, steelmaking slag poured from a converter is directly oxidized by blowing oxygen during the molten state. Using the phase fractions of each slag, we calculated the mass fractions of P and Fe distributed in each phase, and compared them with the extraction efficiencies of P and Fe. As shown in Fig. 7, P was only distributed in the solid solution and merwinite in each slag, whereas Fe was mostly concentrated in the magnesioferrite. For the Fe2O3-containing slag, the mass fraction of P distributed in the solid solution was approximately 80%. The extraction efficiency of P was comparable to this value, illustrating that most of the solid solution was leached, although some of the extracted P was supplied from the merwinite. Because only a small portion of Fe2O3 was distributed in the solid solution and merwinite, the dissolution of Fe was poor. For the FeO-containing slag, the extraction efficiency of P was lower than the mass fraction of P distributed in the solid solution, showing that the extraction efficiency of the solid solution was lower compared to the Fe2O3-containing slag. The mass fraction of Fe distributed in the solid solution and merwinite was approximately 8.4%, which was nearly equal to the Fe extraction efficiency. This indicates that the extracted Fe was supplied from the large leaching of the solid solution and merwinite, rather than the magnesioferrite. 3.3. Residue composition Following leaching in the hydrochloric acid solution, the average composition of the residue was determined using ICP-AES, as listed in

Fig. 7. Mass fractions of P and Fe distributed in each phase, compared with their extraction efficiencies.

Fig. 8. Micrograph of the residue surface of the Fe2O3-containing slag after leaching. 5

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Table 4 Average composition of each domain on the residue surface of slag A (mass%). Point

Ca

Si

P

Fe

Mn

Mg

Al

1 2

0.3 0.8

1.0 0.1

0.1 0.1

45.5 45.3

7.0 7.2

10.7 12.9

5.3 5.7

Fig. 9. X-ray diffraction patterns of the Fe2O3-containing slag and its residues.

of the solid solution and merwinite were extracted from the slag through selective leaching, and the Fe-rich magnesioferrite remained. When the slag was leached in the sulfuric acid solution, the amount of the residue increased considerably. The main peaks in the residue were associated with gypsum (CaSO4), which agrees with the above discussion. The intensities of the peaks of magnesioferrite seemed weaker because the amount of the precipitated gypsum was large. As mentioned above, the existence of gypsum will restrict the reuse of residue inside a steel plant.

Fig. 10. Change in the concentration of each element in the leachate with the pH.

Standard chemical potential of each substance and activity coefficient at 298 K are used in the present work (Futatsuka et al. 2004). For the leachate containing higher concentrations of Ca and P, various calcium phosphates are able to precipitate depending on the pH and solution composition (Valsami-Jones 2001). It was reported that CaHPO4·2H2O (brushite, DCPD) is preferentially precipitated because of a fast nucleation and growth, although Ca5(PO4)3(OH) (hydroxyapatite, HAP) was the most thermodynamically stable calcium phosphate (Koutsoukos and Nancollas 1981; Lee et al. 2018). The reactions of CaHPO4·2H2O in the solution and their Gibbs-free energy are expressed in Eqs. (8) and (9) (Futatsuka et al. 2004; Iglesia 2009). Using the calculated equilibrium constant (K) and activity coefficients of ions (γ), the relationship between Ca2+ and phosphate ions in the aqueous solution can be determined, as described in Eqs. (10) and (11). The activity coefficient of the dissolved ion, i, was estimated using the Debye–Hückel theory, as expressed in Eq. (12) (Stokes and Robinson 1948).

3.4. Phosphorus extraction in the leachate Fig. 10 shows the change in the compositions of the leachates of slags A and B when the pH increased. The leachate of slag A mainly consisted of Ca, Si, and P ions, and had lower concentrations of Fe and Mn because of an improved selective leaching. As the pH of the leachate increased, a chemical precipitation occurred, causing a decrease of the concentration of each element. When the pH increased to 7, the P concentration decreased from 119.3 to 7.0 mg/L, exhibiting a sharp decrease compared with the other elements. This illustrates that most of the phosphate ions in the leachate were precipitated, whereas the precipitation of silicate ions was not as significant. When the pH continued to increase, the concentrations of P, Fe, and Mn decreased to a very low level, and the precipitation of silicate and Ca2+ ions began to occur significantly. At pH 9, the P concentration was less than 1.0 mg/L, and the Si concentration decreased to 85.1 mg/L. The concentrations of Fe and Mn in the leachate of slag B were far higher than those in the leachate of slag A. The P concentration in the leachate of slag B reached 101.8 mg/L, which is lower than that of Fe. When the pH increased to 7, the P and Fe concentrations decreased significantly compared with the Si and Mn concentrations. Their concentrations in the leachate were only 9.3–24.4 mg/L, respectively, indicating that the precipitation of the phosphate and Fe2+ ions preferentially occurred. HPO42− and H2PO4− are the major phosphate ions in the aqueous solution whose pH ranges from 3 to 9 (Recillas et al. 2012). The relationship between HPO42− and H2PO4− ions is described in Eq. (7).

HPO24 + H+ = H2 PO4

G1° =

(7)

41005 J/mol

Ca2 + + HPO24 + 2H2 O = CaHPO4 2H2 O G2° =

(8)

37183 J/mol

(9)

Ca2 + + H2 PO4 + 2H2 O = CaHPO4 2H2 O + H+ G3° = 3822 J/mol log K1 =

6

G1° = 2.303RT

log C Ca2 +

log

Ca2 +

log CHPO24

log

HPO24

(10)

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log K2 =

G2° = 2.303RT

log C Ca2 +

log

Ca2 +

log CH2 PO4

(11)

—pH log i =

log H2PO4

Az i2 I 1 + Ba0 I

(12)

3Fe2 + + 2HPO24 + 8H2 O = Fe3 (PO4 )2 8H2 O + 2H+ G4° =

36919 J/mol

(13)

3Fe 2+ + 2H2 PO4 + 8H2 O = Fe3 (PO4 ) 2 8H2 O + 4H+ G5° = 45091 J/mol (14) where R is a gas constant, and T is the leachate temperature. In addition, A and B are constants that depend on the temperature and solvent, z is the valence of dissolved substances, a0 is the ion-size parameter (Å), and I is the ionic strength of the solvent. Because the leachate is a diluted solution, the values of A, B, and I applied in the present work were 0.509, 0.329, and 0.676, respectively (Okabe et al., 1980). Using the above equations, we calculated the solubility lines of CaHPO4·2H2O at different pH conditions and compared the results with the experimental results of leachate A, as shown in Fig. 11. The calculated P concentration in the solution was equal to the sum of the HPO42− and H2PO4− concentrations. With an increase in the pH and Ca concentration, CaHPO4·2H2O will precipitate, accounting for a decrease in the P concentration. This confirms that the dissolved P can be extracted from the leachate through a chemical precipitation. At a lower pH condition, the pH had a significant influence on the P concentration. However, the Ca concentration principally affected the P concentration at a higher pH condition. Because the solubility of CaHPO4·2H2O was high at pH 3, the P concentration in the original leachate did not reach saturation, and the Ca2+ and phosphate ions could stably coexist. When the pH increased, the P concentration in the original leachate was far higher than its saturation concentration. Thus, CaHPO4·2H2O precipitated and the P concentration decreased to a lower level. The experiment result for the leachate at pH 7 was located slightly above the corresponding solubility line, indicating the supersaturation of P in this solution. At pH 8–9, the experimental results for the leachates also lay near the solubility lines. In summary, the majority of the soluble P was precipitated and the solubility of CaHPO4·2H2O determined the P concentration in the leachate. In the leachate B containing a higher Fe2+ concentration, the precipitation of Fe3(PO4)2·8H2O (vivianite) may occur (Iglesia 2009). To

Fig. 12. Solubility lines of Fe3(PO4)2·8H2O under various pH conditions and experimental results.

determine its formation, we also calculated the relationship between the concentrations of Fe2+ and phosphate ions under various pH conditions using Eq. (13) and Eq. (14) (Futatsuka et al. 2004; Iglesia 2009). As displayed in Fig. 12, the experimental result for the leachate B was located below the solubility line of Fe3(PO4)2·8H2O at pH 3, indicating that Fe3(PO4)2·8H2O did not precipitate, and that Fe2+ and phosphate ions can coexist. When the pH increased to 7, the solubility of Fe3(PO4)2·8H2O decreased significantly. The P concentration in the leachate was higher than its saturation concentration. Meanwhile, the experimental result at pH 7 was also located above the corresponding solubility line of CaHPO4·2H2O in Fig. 11. These results indicate that Fe3(PO4)2·8H2O and CaHPO4·2H2O both precipitated in leachate B at pH 7. The existence of the insoluble ferric phosphate is a disadvantage to the utilization of this precipitate as a fertilizer. Following the calcination of the precipitate separated from the leachate, the final phosphate product was obtained, as shown in Fig. 13. Table 5 lists the compositions of the phosphate products produced from different leachates under various pH conditions. These products mainly consisted of CaO, SiO2, and P2O5, which is similar with the C2S–C3P solid solution. The SiO2 content was the highest among these components, accounting for nearly half of each product. The P2O5 content in product A (obtained from leachate A at pH 9) was only 12.95 mass%. As

Fig. 11. Solubility lines of CaHPO4·2H2O under various pH conditions and experimental results.

Fig. 13. Image of the phosphate product obtained in this process. 7

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Table 5 Compositions of the product under various pH conditions (mass%) and P precipitation ratio. Sample Product Product Product Product

A (Leachate A, pH = 9) B (Leachate A, pH = 8) C (Leachate A, pH = 7) D (Leachate B, pH = 7)

CaO

SiO2

P2O5

Fe2O3

MnO

MgO

Al2O3

Others

P precipitation ratio

25.77 24.08 28.90 18.12

50.80 52.70 41.88 49.35

12.95 13.11 20.17 13.76

0.33 0.32 0.49 10.22

2.38 2.25 2.76 3.37

2.37 1.13 0.89 0.84

0.59 0.59 0.71 0.81

4.81 5.82 4.20 3.53

0.959 0.921 0.863 0.888

the product, and M is the molar mass. As listed in Table 5, most of the soluble P in the leachate was precipitated during the chemical precipitation. The P precipitation ratio in leachate A increased from 86.3% to 95.9% when the pH changed from 7 to 9. Using the product of the P extraction efficiency from slag and the P precipitation ratio, we estimated the total recovery ratio of P from the steelmaking slag in this selective leaching–chemical precipitation process. When a chemical precipitation was conducted at pH 7, this process provided more than a 70% P recovery from steelmaking slag. The P2O5 content in the product obtained reached 20.2 mass%, which can be applied as a fertilizer. A novel process for P recovery and waste-free steelmaking was proposed on the basis of this study, as displayed in Fig. 15. Following dephosphorization of hot metal, a P-bearing steelmaking slag is generated and then treated through selective leaching. The P in steelmaking slag is leached and concentrated in the leachate. Through chemical precipitation, most of the P in the leachate is precipitated under a nearly neutral condition. The phosphate product obtained with a higher content of citrate-soluble P2O5 has the potential to be utilized as a fertilizer in agriculture. The P-removed residue with a higher Fe2O3 content can be reutilized in steelmaking process. No extra slag is discharged, and the P contained in the steelmaking slag is recovered through an efficient and low-cost process.

Fig. 14. X-ray diffraction patterns of the phosphate products obtained under different conditions.

4. Conclusions

the pH decreased to 7, the precipitation of silicate ions was significantly suppressed compared with that of phosphate ions, resulting in an increase in the P2O5 content in the product. The P2O5 content in product C exceeded 20 mass%, and the CaO content was 28.9 mass%. Fig. 14 shows X-ray diffraction patterns of the phosphate products. The peaks of Ca5(PO4)3(OH) were clearly found in products A and C. This is because the CaHPO4·2H2O that precipitated during the chemical precipitation was transformed into thermodynamically stable Ca5(PO4)3(OH) after calcination (Valsami-Jones 2001). The intensity of these peaks was higher in product C than in product A, indicating a higher content of calcium phosphate. Furthermore, an amorphous phase was also confirmed in the X-ray diffraction patterns of these products, with the broad peaks at between 20° and 35° being attributed to an amorphous nature of the silicates (Jo et al. 2017). The P availability was measured using the solubility of P compounds in a 2% citric acid solution (Shiba and Ntuli, 2017). Approximately 85% of the P2O5 in product C was leached, although the leaching of SiO2 was extremely low. It can therefore be concluded that the majority of P in the product is bioavailable for plant uptake. To produce a high-quality fertilizer, future studies will focus on the removal of SiO2 from the product. The Fe2O3 content in product D was higher because a large amount of Fe was precipitated in leachate B. However, the P2O5 and CaO contents were lower. As shown in Fig. 14, the peaks associated with Ca5(PO4)3(OH) was extremely weak in product D, whereas some peaks of Fe2O3 (hematite) were observed. The formation of Fe2O3 may be caused by the decomposition of Fe(OH)3 during calcination. This product with lower P2O5 and higher Fe2O3 amounts is not suitable as a fertilizer.

PR =

Selective leaching and chemical precipitation were adopted to recover P from the normal steelmaking slag. The effects of the acid, pH, and valence of Fe in slag on the leaching characteristics of P were first investigated. The P precipitation in the leachate and the characteristics of the final product were then evaluated. The following conclusions were drawn: (1) The P-concentrated solid solution was readily leached whereas the Fe-rich magnesioferrite was resistant to dissolution in each acid solution. In view of reducing the production costs and utilizing the

2MP m wP2O5 MP2O5 V CP

(15)

The P precipitation ratio (PR) in the leachate was evaluated using Eq. (15), where m is the total product mass, wP2O5 is the P2O5 content in

Fig. 15. A novel process for P recovery and waste-free steelmaking. 8

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C.-M. Du, et al.

residue, hydrochloric acid was considered the optimum leaching agent to dissolve the steelmaking slag. (2) The dissolution of steelmaking slag in the hydrochloric acid solution increased with a decrease in pH. In addition, 81.6% of the P in the slag was dissolved at pH 3, whereas the Fe leaching was negligible, exhibiting an improved selective leaching. A residue containing lower P2O5 and higher Fe2O3 amounts can be reutilized in the steelmaking process. (3) The FeO-containing slag showed a lower extraction efficiency of P and a higher extraction efficiency of Fe in the hydrochloric acid solution, which is a disadvantage to the P recovery. Thus, an oxidization treatment is necessary to transform the FeO in slag into Fe2O3. (4) During chemical precipitation at pH 7, most of the soluble P in the leachate was transformed into an insoluble calcium phosphate. A higher pH resulted in a higher P precipitation ratio, whereas the P2O5 content in the product decreased. This process provides more than a 70% P recovery from steelmaking slag. The P2O5 content in the product obtained reached 20.2 mass%, which can be applied as a fertilizer.

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