ISIJ International
Online ISSN : 1347-5460
Print ISSN : 0915-1559
ISSN-L : 0915-1559
Regular Article
P2O5 Solubility Behavior and Resource Utilization of P-Bearing Slag
Lu LinYan-ping BaoMin WangWei JiangHan-mei Zhou
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2014 Volume 54 Issue 12 Pages 2746-2753

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Abstract

In order to recycle the phosphorus in P-bearing steelmaking slag and make it used as slag phosphate fertilizer, the citric acid solubility of P-bearing steelmaking slag was researched. The research results show that the citric acid solubility of P-bearing steelmaking slag is decreased with the increasing of P2O5 and Fe2O3 content for the CaO–SiO2–Fe2O3–P2O5 slag system. Added CaF2 into slag can easily form the fluorapatite (Ca5(PO4)2F) that can’t be dissolved in 2% citric acid solution, which makes citric acid solubility of the slag decreased obviously. And added MgO, MnO or Na2O into slag can prevent the precipitation of β-Ca3(PO4)2 phase with low citric acid solubility, which makes the citric acid solubility of slag increased. While for the CaO–SiO2–Fe2O3–P2O5–X (MgO, MnO or Na2O) slag system, MgO and MnO in slag mainly enters into RO phase, which has no effect on the phosphorus existence form in slag and has little influence on citric acid solubility of the slag. The Na2O in slag changes the phosphorus existence form in slag, but the generated Na2Ca4(PO4)2SiO4 and Na3PO4 also have good citric acid solubility, which also has little effect on citric acid solubility. Al2O3 and TiO2 modification only increases the phosphorus content in phosphorus-rich phase, does not change the phosphorus existence form in slag, which makes the citric acid solubility increased slightly. While SiO2 modification prevents the precipitation of β-Ca3(PO4)2 phase with low citric acid solubility, which makes citric acid solubility of the slag increased significantly. The modified slag without fluoride makes the citric acid solubility increased to 95% and meets the requirement of producing slag phosphate fertilizer.

1. Introduction

With the obviously increasing of high quality ore price and the development of new generation steel-making duplex process in China, high phosphorus content ore utilization in melting is inevitable. Duplex process is using two converters for production, one converter is using for dephosphorization, another one receiving the low phosphorus hot metal from dephosphorization converter is using for decarbonization High phosphorus iron ore resource is rich in China, proved reserves is up to 7.45 billion tons, accounted for above 10% of national iron ore resource reserves. Currently, due to high phosphorus content, the ore can’t be fully used. If blast furnace hot metal with phosphorus content greater than 0.3% was used for dephosphorization by duplex process, which can obtain high phosphorus slag with P2O5 content greater than 10%, and also can obtain high quality slag phosphate fertilizer with further processing, settle the environment pollution and create economic benefits. M. Ishikawa in Sumitomo Metal Industries produced the dephosphorization slag by dephosphorization furnace, and then recycled the slag by slag regeneration processing, which can get slag with P2O5 content above 10% and the slag can be directly used as slag phosphate fertilizer.1,2) JFE Steel used low silicon hot metal with 0.1–0.2% phosphorus, blowed CaO and oxygen source for dephosphorization, and produced the high phosphorus slag by dephosphorization furnace in duplex process can be directly used as phosphate fertilizer raw materials.3)

Using of slag phosphate fertilizer has hundreds years of history, in the country with rich phosphorus containing iron ore such as France, Germany etc., slag phosphate fertilizer always has big share,4) accounts for 13–16% of total phosphate fertilizer.1,5) However, blast furnace slag and steel slag used as agricultural fertilizer and soil improvement is totally 230000 tons in China 2010, accounts for 0.5% of total blast furnace slag and steel slag using amount,6) so the utilization of steel slag for fertilizer is very low.

Up to now, recycling P-bearing slag as slag phosphate fertilizer has attracted widespread attention, in this regard, some scholars majorly researched on phosphorus existence in slag7,8) and phosphorus enrichment separation behavior of P-bearing slag,9,10,11,12,13) while there are few researches on citric acid solubility (for short P2O5 solubility) of P-bearing slag.14,15,16) Slag phosphate fertilizer can not be dissolved in water, but can be dissolved in 2% citric acid solution. So the slag phosphate fertilizer belongs to citrate-soluble phosphatic fertilizer, and its fertilizer efficiency is determined by P2O5 content and P2O5 solubility. P2O5 solubility means the mass fraction ratio between P2O5 content that can be dissolved in 2% citric acid solution and total P2O5 in slag, and the P2O5 solubility is the important indicator to evaluate if the dephosphorization slag can be used as slag phosphate fertilizer or not. The using of slag phosphate fertilizer provides necessary support for the using of high phosphorus ore, while also saves the mining amount of phosphorus ore resources, and instructs the resource utilization of P-bearing slag.

This paper researches the influence of different basic slag composition (basicity, Fe2O3 and P2O5), surplus calcium alkalinity and slag composition modification on the P2O5 solubility of P-bearing slag, the effect of melting modification by SiO2, Al2O3 and TiO2 addition and slag without fluoride on the slag P2O5 solubility was discussed emphatically, which provide the necessary research basis for the using of slag phosphate fertilizer.

2. Experimental Methods

2.1. Thermal Experimental of P-bearing Slag

Reagent-grade CaO, SiO2, Fe2O3, P2O5, MgO, MnCO3, CaF2 and Na2CO3 were used to produce slag in this experiment. Then, the reagents were mixed in various ratios in order to produce the CaO–SiO2–Fe2O3–P2O5 slag system. For investigating the effect of other elements on P2O5 solubility, Na2O, CaF2, MgO and MnO were added in some cases. The mixing conditions are summarized in Table 1. Figure 1 also shows the composition range of the used slag on the CaO–SiO2–Fe2O3 phase diagram that calculated by FactSage 6.2 for samples A, B, and C respectively. As shown in Fig. 1, all experimental slag are in the dicalcium silicate (C2S) primary zone, C2S was precipitated firstly during the cooling process, and so nCa2SiO4–Ca3(PO4)2 (for short nC2S–C3P) solid solution (mainly Ca15(PO4)2(SiO4)6 and Ca5(PO4)2SiO4) was easily formed. Most of the iron oxide would be in the form of FeO in steelmaking condition, but Fe2O3 was used as iron oxide in this experiment, the reasons are as follow: 1) Ito et al.17) has shown that the distribution behavior of P2O5 between solid solution and liquid phases was not different, when the iron oxide was changed from FeO to Fe2O3. 2) Shimauchi et al.13) found that FeO was solved into the solid solution but the content of Fe2O3 in solid solution was negligible small. 3) From the preliminary experiments, it was found that phosphorus enrichment behavior between solid solution and liquid phases was not affected, when the iron oxide was changed from FeO to Fe2O3.

Table 1. Composition of the synthetic slag samples (mass fraction, %).
No.CaOSiO2Fe2O3P2O5MgOMnOCaF2Na2OP2O5 solubility/%
A-145.7118.2930692.31
A-242.8617.14301084.70
A-337.1414.86301858.47
B-149.3324.6720696.90
B-246.6723.33201093.54
B-341.3320.67201876.05
C-150.4033.6010698.39
C-248.0032.00101095.51
C-343.2028.80101882.86
A-2(5%MgO)39.2915.713010579.59
A-2(10%MgO)35.7114.2930101092.81
A-2(5%MnO)39.2915.713010598.28
A-2(10%MnO)35.7114.2930101096.89
A-2(3%Na2O)40.7116.293010396.99
A-2(6%Na2O)38.5715.433010696.13
A-2(3%CaF2)40.7116.293010317.18
A-2(6%CaF2)38.5715.433010611.25
Fig. 1.

Observed slag composition in the ternary phase diagram of the CaO–SiO2–Fe2O3 system.

Some scholars18,19,20,21) modified the slag by adding some oxides of SiO2, Al2O3 and TiO2 in the P-bearing slag, and used melting-cooling method to research the phosphorus enrichment and separation, and phosphorus enrichment and separation in the slag was realized, and the separated phosphorus-rich phase was used as phosphate fertilizer. But the research on P2O5 solubility (P2O5 solubility is an important indicator for the evaluation of slag used for fertilizer) of P-bearing slag after melting modification is less.

Based on the converter slag of one steel plant, reagent grade P2O5 was added into original slag and the P2O5 contents were all adjusted to 10%, reagent grade SiO2 (slag basicity R is 3.0, 2.0 and 1.0, and R=(mass%CaO)/(mass%SiO2), the same below), Al2O3 (Al2O3 content is 10% and 15% respectively) and TiO2 (TiO2 content is 10%) were added to research the effect of modification of SiO2, Al2O3 and TiO2 on P2O5 solubility in slag, the composition of modified slag is listed in Table 2.

Table 2. Chemical composition of the original slag and modified slag (mass fraction, %).
Slag sampleCaOSiO2Fe2O3P2O5Al2O3MgOTiO2CaF2MnOP2O5 solubility/%
Original slag 146.7211.6819.47101.075.460.970.374.2658.35
No. 144.7814.9318.66101.035.230.930.354.0860.50
No. 241.3520.6817.23100.954.830.860.333.7772.43
No. 333.6333.6314.01100.773.930.700.273.0798.84
Original slag 245.7215.2417.82101.095.000.890.343.9074.96
No. 441.1313.7116.0310104.500.800.313.5178.91
No. 538.4612.8215.1210154.240.760.293.3181.09
No. 641.1213.7116.00100.884.49100.313.5083.07

The mixed slag (200 g) was placed in an crucible 60 mm dia×100 mm, placed inside a graphite crucible and heated in a MoSi2 electric resistance furnace up to 1773 K, according to observing, the slag had been melted at 1773 K during the experimental process. In order to make slag fully melted, and the slag was maintained the temperature for 30 minutes. Then the slag was cooled to 1623 K at a cooling rate of 3 K/min, kept at this temperature for 1 hour in order to fully promote the precipitation of nC2S–C3P solid solution, then cooled to 1423 K at a cooling rate of 3 K/min, the furnace was then closed and the slag sample was cooled within it (see Fig. 2). After each experiment the slag was ground (less than 300 mesh (48 μm)), mineralogical phases were determined by XRD analysis. Diffraction patterns were measured in a 2θ range of 10–90° using copper Kα radiation of 40 kV and 30 mA, and the scan speed was 5°/min.

Fig. 2.

Experimental conditions for precipitation of C2S–C3P solid solution.

2.2. P2O5 Solubility Experimental of P-bearing Slag

The quinoline phosphomolybdate gravimetric method provided by national standard GB20412-2006 is used to measure the P2O5 solubility of P-bearing slag. 1.0000 g P-bearing slag ground to less than 300 meshes (48 μm) has been prepared, and then it was put in a dry 250 mL volumetric flask. Accurately added 150 mL, 28–30°C 2% citric acid solution into the volumetric flask, kept the temperature in the range of 28–30°C, and put the volumetric flask on oscillator to shake for one hour. After adding water in volumetric flask to the tick mark, the solution was blended and filtered. Certain sample solution was poured into a 500 mL beaker, and 10 mL 1:1 nitric acid solution was added, then water was added into the beaker to dilute to 100 mL. Sample solution was heated to boil. Then 35 mL quimociac was added into the beaker, and reacted with the P2O5 in sample solution and precipitated as yellow quinoline phosphomolybdate. And then the beaker was covered with watch glass, heated it again to micro-boil to make the sediment layered, and the beaker was taken out to cool to ambient temperature. The glass filter which has been put in a (180±2)°C drying oven to dry to constant weight was used to filter, then put the precipitate and glass filter in the (180±2)°C drying oven to dry for 45 min. At last, the precipitate and glass filter were taken out to cool and weight. Meanwhile, the blank experimental is also done. The main reaction in experiment is Eq. (1). Figure 3(a) is the picture of extracted quinoline phosphomolybdate precipitation, Fig. 3(b) is the blank experiment for comparison.   

H 3 PO 4 +12 MoO 4 2- +24 H + +3 C 9 H 7 N = ( C 9 H 7 N ) 3 H 3 (PO 4 12 MoO 3 )+12 H 2 (1)
Fig. 3.

Comparisons between quinoline phosphomolybdate precipitation and blank experiment. (a): quinoline phosphomolybdate precipitation; (b): the blank experiment.

The effective P2O5 content dissolved in 2% citric acid solution was calculated, and then the slag P2O5 solubility expression is shown in Eq. (2), where (%P2O5)Available, (%P2O5)Total denote P2O5 content that can be dissolved in 2% citric acid solution and total P2O5 content in slag respectively, (P2O5)Solubility is slag P2O5 solubility.   

( P 2 O 5 ) Solubility = ( %P 2 O 5 ) Available ( %P 2 O 5 ) Total (2)

3. Experimental Results and Discussion

3.1. The Effect of Slag Composition on P2O5 Solubility for CaO–SiO2–P2O5–Fe2O3 Slag System

Some synthetic slags were used for experiment in Table 1, to research the effect of slag basic composition on P2O5 solubility of CaO–SiO2–P2O5–Fe2O3 slag system. The synthetic slag composition of experiment is shown in Table 1.

Figure 4 shows the effect of P2O5 and Fe2O3 content on P2O5 solubility in CaO–SiO2–P2O5–Fe2O3 slag. As shown in Fig. 4, about the CaO–SiO2–P2O5–Fe2O3 slag system, when P2O5 content in the slags is same, with the increasing of slag basicity and Fe2O3 content in slag, P2O5 solubility is decreased, and the slag basicity and Fe2O3 content has greater impact on the slag P2O5 solubility with higher P2O5 content. While for the slag with same basicity and Fe2O3 content, with the increasing of P2O5 content in slag, P2O5 solubility is decreased. According to the slag ionic structure theory:22) O2– in slag is decreased with the increasing of P2O5 content in slag, and because the polarization between ions, P5+ and O2– will form polyhedral structure PO 4 3- complex ion, that is the dense group is formed on the base of O2–, phosphorus in slag is located among the gaps of O2– dense group, which makes phosphorus in slag not easily dissolved in 2% citric acid solution, thereby the P2O5 solubility of the slag is reduced.

Fig. 4.

The effect of P2O5 and Fe2O3 content on P2O5 solubility for CaO–SiO2–P2O5–Fe2O3 slag system.

3.2. Effect of Slag Composition Modification on the P2O5 Solubility of P-bearing Slag

Figure 5 shows the effect of different slag composition modification on P2O5 solubility in CaO–SiO2–P2O5–Fe2O3 slag, synthetic slag composition is shown in Table 1. The P-bearing slag is composed of phosphorus-rich phase, matrix phase and RO phase, the phosphorus in slag is mainly in the existence of nC2S–C3P solid solution in the phosphorus-rich phase. Matrix phase is mainly composed of nCaO·SiO2, the matrix phase also contains some iron and phosphorus, and the RO phase mainly consists of iron oxide or iron and magnesium oxide, the iron in the slag mainly exists in this phase. XRD results of modified synthetic slag with different composition are shown in Fig. 6. It can be seen from Fig. 5, when adding MgO, MnO or Na2O into CaO–SiO2–P2O5–Fe2O3 slag system, the slag P2O5 solubility is generally more than 90%. And adding MgO, MnO and Na2O into CaO–SiO2–P2O5–Fe2O3 slag system can break the complex net structure formed by Si–O on certain degree, and also can hinder the precipitation of β-Ca3(PO4)2 crystal with low P2O5 solubility during the melting-cooling process.15) Therefore adding appropriate MgO, MnO or Na2O content in CaO–SiO2–P2O5–Fe2O3 slag can improve the slag P2O5 solubility.

Fig. 5.

Effect of different slag composition modification on slag P2O5 solubility.

Fig. 6.

XRD results of different slag composition modification.

For A-2(5%MnO) and A-2(10%MnO) slag, MnO mainly enters into RO phase which has no effect on phosphorus existence in slag, so which has little effect on P2O5 solubility of the two slags. For A-2(5%MgO) and A-2(10%MgO) slag, the principle is the same.

For A-2(3%Na2O) and A-2(6%Na2O) slag, owing to adding Na2O into slag, Ca ions in phosphorus-rich phase (nC2S–C3P solid solution) are partly replaced by Na ions, then Na2Ca4(PO4)2SiO4 and Na3PO4 are formed, so the phosphorus existence form is changed. But these compounds (Na2Ca4(PO4)2SiO4 and Na3PO4) and phosphorus-rich phase (nC2S–C3P solid solution) have almost same good citric acid solubility, so P2O5 solubility of the two slags have little difference.

With the adding of CaF2, fluorapatite phase (Ca5(PO4)3F) exists along with Ca15(PO4)2(SiO4)6 in phosphorus-rich phase of slag. As shown in Fig. 5, when the CaF2 content is respectively 3% and 6%, the slag P2O5 solubility is decreased to 17.18% and 11.25%. The adding of CaF2 makes the slag P2O5 solubility decreased significantly, because the fluorine ion can easily enter into the α-Ca3(PO4)2 crystal lattice, and fluorapatite is formed, which plays a strong role in the crystal lattice stability. Fluorapatite has low energy state and stable structure, and its generation process is a highly spontaneous process, so the fluorapatite has extremely stable structure and thus can’t be dissolved in 2% citric acid solution. Therefore, in order to improve the slag P2O5 solubility, to make the dephosphorizing converter slag produced phosphate fertilizer slag, and to finally achieve resource utilization of dephosphorizing slag, the slag forming route without fluoride should be used, and using flux such as soda, iron scale etc. instead of fluorite for slag forming.

3.3. Effect of Surplus Calcium Alkalinity on P2O5 Solubility of P-bearing Slag

The surplus calcium alkalinity of slag has an important effect on P2O5 solubility of P-bearing slag. Some thermal experimental slag in Table 1 was used for experiment, to research the effect of surplus calcium alkalinity on P2O5 solubility. The surplus calcium alkalinity expression is shown in Eq. (3), where (%CaO)T, (%MgO)T, (%SiO2)T denote CaO, MgO and SiO2 mass fraction in slag respectively, ( %CaO ) 3CaO P 2 O 5 is CaO mass fraction combined with phosphorus to form 3CaO·P2O5,%, Rθ is the surplus calcium alkalinity.   

R θ = ( %CaO ) T - ( %CaO ) 3CaO P 2 O 5 56 + ( %MgO ) T 40 ( %SiO 2 ) T 60 (3)

Figure 7 shows the effect of slag surplus calcium alkalinity on citric acid solubility, and the relationship between slag property indicators and slag P2O5 solubility is shown in Table 3. For the CaO–SiO2–P2O5–Fe2O3 slag system, when the slag basicity is constant, (%CaO)/((%SiO2)+(%P2O5)) and surplus calcium alkalinity are also increased with increasing of (%CaO)/(%P2O5) and (%SiO2)/(%P2O5), the slag P2O5 solubility is increased correspondingly.

Fig. 7.

The effect of surplus calcium alkalinity on slag P2O5 solubility.

Table 3. The relationship between slag property indicators and slag P2O5 solubility.
No.P2O5
solubility/%
(%CaO)/
(%P2O5)
(%SiO2)/
(%P2O5)
(%CaO)/
((%SiO2)+
(%P2O5))
Surplus calcium alkalinity
A-192.317.623.051.882.26
A-284.704.291.711.581.94
A-358.472.060.831.131.14
B-196.908.224.121.611.83
B-293.544.672.331.401.60
B-376.052.291.151.071.04
C-198.398.405.601.271.38
C-295.514.803.201.141.21
C-382.862.401.600.920.81
A-2(5%MgO)79.593.931.571.532.35
A-2(10%MgO)92.813.571.431.472.84
A-2(5%MnO)98.283.931.571.531.87
A-2(10%MnO)96.893.571.431.471.79
A-2(3%Na2O)96.994.071.631.551.90
A-2(6%Na2O)96.133.861.541.521.86
A-2(3%CaF2)17.184.071.631.551.90
A-2(6%CaF2)11.253.861.541.521.86

As shown in Fig. 7 and Table 3, for the CaO–SiO2–P2O5–Fe2O3–X (X represents MgO, MnO, Na2O or CaF2) slag system, except the A-2(MgO) slag, (%CaO)/(%P2O5) and (%SiO2)/(%P2O5) are increased in other slag system, (%CaO)/((%SiO2)+(%P2O5)) and surplus calcium alkalinity are also increased at the same time, the slag P2O5 solubility is increased correspondingly. Surplus calcium alkalinity is not increased with increasing of (%CaO)/(%P2O5) and (%SiO2)/(%P2O5) for A-2(MgO) slag, but slag P2O5 solubility is also increased with the increasing of surplus calcium alkalinity. So, for the same slag system, the slag P2O5 solubility is increased with the increasing of surplus calcium alkalinity.

3.4. Effect of Industrial Slag Modification on Slag P2O5 Solubility

In order to fully recycle the phosphorus resource (used for phosphate fertilizer) in slag and further confirm feasibility of slag melting modification and separating phosphorus-rich phase from P-bearing slag by magnetic separation method, SiO2, Al2O3 and TiO2 were added into converter slag of one steel plant, and the effect of slag melting modification on slag P2O5 solubility was researched. The composition of original slag and modified slag is listed in Table 2.

The comparison of P2O5 solubility between original and modified slag is shown in Fig. 8. For the SiO2 modification, slag P2O5 solubility is increased with the increasing of SiO2 content, mainly because the increased SiO2 in slag can effectively prevent the precipitation of β-Ca3(PO4)2 phase with low P2O5 solubility, phosphorus is mainly in the existence of α-Ca3(PO4)2 phase with high P2O5 solubility in slag rapid cooling process.23) So adding SiO2 into slag can improve the slag P2O5 solubility, which is related to domestic and foreign companies are adding sand into slag to improve the SiO2 content during slag phosphate fertilizer production.23) However, When the addition of SiO2 is excess (R<1.5), the precipitation amount of 2CaO·SiO2 phase decreases significantly even disappears, CaO·SiO2 phase generates, so the generation of nC2S–C3P solid solution is thermodynamically difficult, which is adverse to phosphorus enrichment and recovery in slag. Therefore, the suitable modification basicity of slag should be 1.5–2.0,19) the XRD results of SiO2 modification slag are shown in Fig. 9.

Fig. 8.

XRD results of original slag 1 and SiO2 modified slag.

Fig. 9.

XRD results of original slag 2, Al2O3 and TiO2 modified slag.

For Al2O3 and TiO2 modification, P2O5 solubility of modified slag is increased slightly compared to original slag with the increasing of Al2O3 and TiO2 content, which is mainly due to the added Al2O3 and TiO2 reacted with Ca and Si in nC2S–C3P solid solution, which respectively promoted the precipitation of gehlenite (Ca2Al2SiO7) and CaTiSiO5, CaTiO3 during cooling process, so nC2S–C3P solid solution with high phosphorus content was formed, which is coincides with the XRD results in Fig. 10.

Fig. 10.

Effect of industrial slag modification on slag P2O5 solubility.

Al2O3 and TiO2 only make the phosphorus content of phosphorus-rich phase increased, does not change the existence form of phosphorus element in slag, so P2O5 solubility was slightly increased after Al2O3 and TiO2 melting modification. The reaction mechanism between Al2O3, TiO2 modification and phosphorus-rich phase are shown in expression (4) and (5) respectively, where subscripts (ss), (in slag) and (S) denote the solid solution, modifier in slag and liquid phase slag , respectively. So according to above mentioned description, slag melting modification of SiO2, Al2O3 and TiO2 can improve slag P2O5 solubility, but P2O5 solubility was slightly increased after Al2O3 and TiO2 melting modification   

Ca 3 ( PO 4 ) 2 -yCa 2 SiO 4(ss) + xAl 2 O 3(in   slag) = xCa 2 Al 2 SiO 7(S) + Ca 3 ( PO 4 ) 2 -(y-x)Ca 2 SiO 4(ss) (4)
  
Ca 3 ( PO 4 ) 2 -yCa 2 SiO 4(ss) + 2xTiO 2(in slag) = xCaTiSiO 5(S) + xCaTiO 3(S) + Ca 3 ( PO 4 ) 2 -(y-x)Ca 2 SiO 4(ss) (5)

3.5. Effect of Slag without Fluoride on P2O5 Solubility

To further verify the effect of fluorine on the slag P2O5 solubility, the No. 2 slag is especially treated with no fluorine, synthetic slag composition is shown in Table 4. Figure 11 is the comparison of P2O5 solubility between industrial modified slag and modified slag without fluoride.

Table 4. Chemical composition of industrial modified slag without fluoride (mass fraction, %).
Slag sampleCaOSiO2Fe2O3P2O5Al2O3MgOTiO2CaF2MnORP2O5 solubility/%
Original slag 146.7211.6819.47101.075.460.970.374.264.058.35
No. 241.3520.6817.23100.954.830.860.333.772.072.43
No. 2 slag without fluoride41.5020.7617.29100.954.850.8603.782.095.95
Fig. 11.

Effect of slag without fluoride on slag P2O5 solubility.

As shown in Fig. 11, the P2O5 solubility of original slag 1 is 58.35%, the P2O5 solubility of No. 2 slag is increased to 72.43% after SiO2 modification. And after the No. 2 slag without fluoride, the slag P2O5 solubility is increased to 95.95%, which is mainly that fluorapatite that can’t be dissolved in 2% citric acid is not generated after the No. 2 slag without fluoride, the XRD results of industrial modified slag without fluoride are shown in Fig. 12. Therefore, fluorine content in slag has an important influence on the slag P2O5 solubility, and the fluorine content in slag must be controlled to obtain higher slag P2O5 solubility.

Fig. 12.

XRD results of industrial modified slag without fluoride.

4. Conclusion

The P2O5 solubility of P-bearing steelmaking slag was fully researched; the effect of different conditions on the P2O5 solubility of P-bearing slag was analyzed. The key findings are as follows:

(1) For the CaO–SiO2–Fe2O3–P2O5 slag system, when slags have same basicity, slag P2O5 solubility is decreased with the increasing of P2O5 content. As for the slags with other same factors, slag P2O5 solubility is decreased with the increasing of Fe2O3 content and slag basicity.

(2) When adding MgO, MnO and Na2O into CaO–SiO2–P2O5–Fe2O3 slag system, the citric acid solubility of slag is generally more than 90%. And adding MgO, MnO and Na2O into CaO–SiO2–P2O5–Fe2O3 slag system can break the complex net structure formed by Si–O on certain degree, and also hinder the precipitation of β-Ca3(PO4)2 crystal with low citric acid solubility during the melting-cooling process, so adding appropriate MgO, MnO or Na2O content in CaO–SiO2–P2O5–Fe2O3 slag can improve the slag P2O5 solubility. While for CaO–SiO2–P2O5–Fe2O3–X (X represents MgO, MnO or Na2O) slag system, different amounts of MgO, MnO or Na2O has little effect on the P2O5 solubility, which is mainly due to MnO mainly enters into RO phase which has no effect on phosphorus existence form in slag, and Na ions in Na2O can partly replace Ca ions in phosphorus-rich phase (nC2S–C3P solid solution), then Na2Ca4(PO4)2SiO4 and Na3PO4 are formed, and they and phosphorus-rich phase (nC2S–C3P solid solution) have almost same good citric acid solubility.

(3) Adding CaF2 into slag can easily form the fluorapatite that can’t be dissolved in 2% citric acid solution, so the slag P2O5 solubility was significantly decreased. Adding 0.5% of CaF2 into slag, the slag P2O5 solubility is decreased to lower than 80%, when the added CaF2 content is more than 3%, the slag P2O5 solubility is decreased to lower than 20%. Slag forming route with no fluoride or less fluoride (the fluoride content in slag is lower than 0.5%) should be used in the steel-making process to make the dephosphorizing converter slag produced slag phosphate fertilizer, and achieve resource utilization of dephosphorizing slag.

(4) Al2O3 and TiO2 modification only increases the phosphorus content in phosphorus-rich phase, does not change the existing form of phosphorus in slag, thus the slag citric acid -solubility of melting slag is increased slightly after Al2O3 and TiO2 modification. However, adding suitable modifier SiO2 prevents the precipitation of β-Ca3(PO4)2 with low P2O5 solubility, slag P2O5 solubility is increased significantly. So considering the phosphorus enrichment and resource utilization recovery during modification process, the suitable slag bacisity is 1.5–2.0.

References
 
© 2014 by The Iron and Steel Institute of Japan
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