ISIJ International
Online ISSN : 1347-5460
Print ISSN : 0915-1559
ISSN-L : 0915-1559
Regular Article
Identification of the Major Constituents of Fused Potassium Silicate Fertilizer
Yasuko YaoEtsuo HamadaKaoru SatoTakashi AkiyamaTadakatsu Yoneyama
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2014 Volume 54 Issue 4 Pages 990-993

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Abstract

The major component of fused potassium silicate (FPS) fertilizers, produced from steel-making slag, was studied. FPS compounds have received considerable attention as slow-release potassium fertilizers beneficial for crops. This major component was found to be a single phase compound, K2Ca2Si2O7, which had not been previously identified. In order to confirm the presence of the newly identified compound, we synthesized a potassium calcium silicate mixture, K2O–2CaO–2SiO2, by fusing a mixture of K2CO3, CaCO3, and SiO2. The X-ray diffraction patterns of the synthesized K2O–2CaO–2SiO2 were largely consistent with those of FPS fertilizer, and Energy Dispersive X-ray Spectroscopy indicated that this compound was a single phase with a K:Ca:Si molar ratio of 1:1:1. It is concluded that the major component of FPS fertilizer is a compound of K2O–2CaO–2SiO2, newly identified as K2Ca2Si2O7. FPS fertilizers exhibit the characteristic, controlled by their K2Ca2Si2O7 content, of slowly releasing potassium into water and soil.

1. Introduction

Currently, there is interest in fertilizers whose rate of release matches that of crop nutrient demands, in order to minimize the leaching of nutrients applied to agricultural fields into the environment. In this regard, slow-release potassium (K) fertilizers have received considerable attention as good sources of potassium for crops.1,2,3,4) Potassium silicate, potassium metaphosphate (KPO3)n, metal potassium phosphates such as KCaPO4 and KMgPO4, and coating potassium fertilizer are possible materials for controlled slow release potassium fertilizer.

There are two types of potassium silicate fertilizers: potassium silicate and fused potassium silicate. Potassium silicate fertilizers have been produced from fly ash, and collected from molten ash, produced during fine coal combustion for electric power generation. The two main potassium silicate components in this fertilizer, identified by X-ray diffraction, are K2MgSiO4 and K2(Al,Fe)2Si2O8.4)

Fused potassium silicate (FPS) fertilizers have been produced from steel-making slag.5) Potassium carbonate (K2CO3) pellets are added to desiliconized slag and melted uniformly at high temperature. The melted mixture is collected from the ladle, cooled and pulverized into granular fertilizer. This fertilizer contains potassium partly in slow-release form. However, the main potassium components have remained unidentified up to now.

In this study, a new single phase compound, featuring slow-release potassium, has finally been identified through the analysis of the major components in FPS fertilizer using chemical, X-ray diffraction, and electron microscopic methods.

2. Experiment

2.1. Preparation of Potassium Calcium Silicate and Analysis of the Structure

The chemical composition of the FPS fertilizer produced from steel-making slag was 205 g kg–1 of K2O, 281 g kg–1 of SiO2, 244 g kg–1 of CaO, 12 g kg–1 of MgO and 48 g kg–1 of MnO.

Two potassium calcium silicate mixtures, K2O–2CaO–2SiO2 and K2O–CaO–SiO2, were individually synthesized by fusing a stoichiometric mixture of K2CO3, CaCO3, and SiO2 at 1:1:1 or 2:1:1 molar ratios of K:Ca:Si at 1150°C.6) The melted mixtures were cooled on a steel plate and crushed to pass through a 1-mm sieve.

The structure of the FPS fertilizers was analyzed using X-ray powder diffraction. This diffraction data was obtained using a Philips diffraction meter (X’Pert-MPD) with a 60°h–1 diffraction speed and 1.5406 × 10–10 m λCu–kα. The X-ray tube voltage and current were 40 kV and 50 mA, respectively. The Powder Diffraction File (International Center of Diffraction Data, ICDD, 2001) was used for the interpretation of the diffraction pattern.

The phases present in the synthesized FPS fertilizers were analyzed using scanning electron microscopy (SEM) (JSM840A, JEOL, Japan), energy dispersive X-ray spectroscopy (EDX) (ISIS, Oxford, UK), and a transmission electron microscope (TEM) (Philips, CM20FEG, Nederland).7)

2.2. Analysis for Water and Citric Acid Solubility of FPS Fertilizer

Solubility of the FPS fertilizer and the synthesized K2O–2CaO–2SiO2 was determined using water (W-soluble) and a 0.2 g L–1 citric acid solution (C-soluble) according to the Japanese Official Methods of Analysis of Fertilizers.8) The W-soluble nutrients, as an index of quick release, were measured by agitating 5.0 g of fertilizer in 400 ml of deionized water for 30 min. The C-soluble nutrients, as an index of slow release, were measured by agitating 1.0 g of fertilizer in 150 ml of 0.2 g L–1 citric acid solution (pH2.1) at 30°C for 1 hour. The potassium content of the extracts were determined using an atomic absorption spectrophotometer (AA-1 MK-I, NIPPON Jarrell Ash, Japan). Potassium solubility was calculated as the percentage of soluble potassium quantity in total potassium quantity.

3. Results and Discussion

3.1. Structure of Fused Potassium Silicate Fertilizer

As shown in Fig. 1(a), the X-ray diffraction pattern of the FPS fertilizer was generally consistent with the powder diffraction pattern of K2CaSiO4 obtained from ICDD card No. 19-0943 (Fig. 1(b)). However, we also found several unidentified X-ray diffraction peaks in the fertilizer that were not recorded in the ICDD card for K2CaSiO4. The compounds responsible for these peaks remain undetermined.

Fig. 1.

X-ray diffraction of fused potassium silicate fertilizer (a), synthesized K2O–2CaO–2SiO2 (c), and synthesized K2O–CaO–SiO2 (d). The X-ray diffraction of ICDD file for K2CaSiO4 is shown at (b).

The chemical composition of the FPS fertilizer analyzed by SEM-EDX also showed a presence with a whole molar ratio of K:Ca:Si around 1:1:1 phase (Fig. 2(a), white phase, and Table 1, Point 1). Accordingly, we concluded that two potassium calcium silicate compounds, K2Ca2Si2O7 (K2O–2CaO–2SiO2) and K2CaSiO4 (K2O–CaO–SiO2), were present in the fertilizer.

Fig. 2.

SEM images of fused potassium silicate fertilizer (a), synthesized K2O–2CaO–2SiO2 (b), and synthesized K2O–CaO–SiO2 (c).

Table 1. Chemical composition of fused potassium silicate fertilizer by SEM-EDX analysis.
Molar ratio
KCaSi
Entire phase0.850.941.00
Point 11.010.991.00
Point 20.092.011.00
Point 30.840.681.00

Three points were randomly selected.

Two potassium calcium silicate compounds, K2O–2CaO–2SiO2 and K2O–CaO–SiO2, were individually synthesized and analyzed by chemical, X-ray diffraction, and electron microscopic methods to identify the major components of FPS fertilizer.

3.2. Structure of the Synthesized K2O–2CaO–2SiO2

The X-ray diffraction pattern produced by the synthesized K2O–2CaO–2SiO2, with a K:Ca:Si molar ratio of 1:1:1, showed the strongest diffraction peak at 2θ =31.6° (Fig. 1(C)). A sharp peak near 2θ = 30° was observed and there was no background in the synthesized K2O–2CaO–2SiO2, indicating the absence of an amorphous solid.

The backscattered electron images of the SEM show that the surface of the synthesized K2O–2CaO–2SiO2 particle was smooth, and the Z contrast indicated that the surface was uniform (Fig. 2(b)). The chemical composition of the entire phase and five randomly selected points of the synthesized K2O–2CaO–2SiO2 analyzed by SEM-EDX revealed that a K:Ca:Si molar ratios were 1:1:1 (Table 2). This data indicates that the synthesized K2O–2CaO–2SiO2 may be a single phase compound with a K:Ca:Si molar ratio of 1:1:1.

Table 2. Chemical composition of synthesized K2O–2CaO–2SiO2 by SEM-EDX analysis.
Molar ratio
KCaSi
Entire phase1.150.961.00
Point 11.090.981.00
Point 21.181.011.00
Point 31.080.931.00
Point 41.201.021.00
Point 51.100.951.00

Five points were randomly selected.

This finding was further supported by other important evidence from TEM observations previously reported.7)

As shown in Fig. 3, the electron diffraction pattern indicated that the synthesized K2O–2CaO–2SiO2 was a single phase and TEM-EDX analysis data showed that its K:Ca:Si molar ratio was 1:1:1. This single phase compound, K2Ca2Si2O7, had not been previously reported,9,10,11) and is therefore considered newly identified.

Fig. 3.

Transmission electron diffraction of synthesized K2O–2CaO–2SiO2 (a) and chemical composition by TEM-EDX analysis (b).

3.3. Structure of the Synthesized K2O–CaO–SiO2

The X-ray diffraction pattern of the synthesized K2O–CaO–SiO2, with a K:Ca:Si molar ratio of 2:1:1, also showed its strongest diffraction peak at 2θ =31.6° (Fig. 1(d)), which was similar to that of K2CaSiO4 (ICDD card No. 19-0943). However, broad peaks near 2θ = 30° and backgrounds were observed in the synthesized K2O–CaO–SiO2, indicating the presence of an amorphous solid. Several strong sharp peaks other than those of K2CaSiO4 (ICDD card No. 19-0943) were also observed. These peaks were generally consistent with a combination of the peaks from the synthesized K2O–2CaO–2SiO2 and K2CO3·1.5H2O (ICDD card No. 11-0655).

The backscattered electron image of the synthesized K2O–CaO–SiO2 showed a microscopically ragged surface (Fig. 2 (c)), indicating a mixture of phases. Table 3 shows the chemical composition of the synthesized K2O–CaO–SiO2 analyzed by SEM-EDX. The entire phase molar ratio of K:Ca:Si of the synthesized K2O–CaO–SiO2 was close to the chemical composition 2:1:1. However, analysis at four randomly selected points showed that composition varied. The molar ratios of K:Ca:Si at Points 1 and 2 were essentially 1:1:1 but those at Points 3 and 4 were K-rich (Table 3).

Table 3. Chemical composition of synthesized K2O–CaO–SiO2 by SEM-EDX analysis.
Molar ratio
KCaSi
Entire phase1.791.101.00
Point 10.820.951.00
Point 20.910.981.00
Point 37.610.681.00
Point 49.770.911.00

Four points were randomly selected.

These results indicate that the synthesized K2O–CaO–SiO2 was not a single phase but possibly a mixture of K2Ca2Si2O7 and K2CO3·1.5H2O. It was considered that K2CO3·1.5H2O was produced by the reaction of un-reacted K2O with atmospheric CO2 and water.

It remained to be determined whether the difference of the X-ray diffraction pattern between K2CaSiO4 (ICDD card No. 19-0943) and the synthesized K2O–CaO–SiO2 might be caused by the different synthesis conditions. The synthesized K2O–CaO–SiO2 in this paper was prepared by fusing at 1150°C for 1 hour, while undergoing the desiliconization process of the hot metal during steelmaking. The compound K2CaSiO4, ICDD card No. 19-0943 submitted by Hughs,9) was synthesized by firing for 48 hours at 1100°C.

3.4. Major Component of Fused Potassium Silicate Fertilizer

It is concluded that the major component in FPS fertilizer is consistent with the powder diffraction pattern of the synthesized K2O–2CaO–2SiO2, but not consistent with that of K2CaSiO4 (ICDD card No. 19-0943) in this examination (Fig. 1). The chemical composition of the FPS fertilizer determined by chemical composition analysis and SEM-EDX analysis also showed a presence with a molar ratio of K:Ca:Si around 1:1:1. According to these results, the major component of FPS fertilizer was determined to be K2Ca2Si2O7. Minor components of FPS fertilizers, β-Ca2SiO4 and K2(Mg,Mn)SiO4 were identified from the powder diffraction pattern. Ca2SiO4 was also identified by SEM-EDX (Fig. 2(a) and Table 1, Point 2).

3.5. Potassium Solubility of Potassium Calcium Silicate Fertilizer

Potassium solubility of FPS fertilizer and the synthesized K2O–2CaO–2SiO2, in both water (W-soluble) and citric acid solution (C-soluble), was determined.

The synthesized K2O–2CaO–2SiO2 contained 10.1% of W-soluble potassium and 88.5% of W-insoluble, but C-soluble potassium (Table 4). Potassium solubility of the FPS fertilizer was similar to that of the synthesized K2O–2CaO–2SiO2 (Table 4) indicating that potassium solubility of the FPS fertilizer is controlled by its content of K2Ca2Si2O7. This FPS exhibited a potassium slow-release characteristic, which was revealed by potassium uptake measurements during agricultural testing (Chinese cabbage propagation).12)

Table 4. Potassium solubility of fused potassium silicate fertilizer and synthesized K2O–2CaO–2SiO2.
fertilizerK solubility(%)
W-solubleC-solubleC-extract
FPS fertilizer12.997.584.6
synthesized K2O–2CaO–2SiO210.198.688.5

W-soluble and C-soluble are soluble K in water and 0.2 g L–1 citric acid solution, respectively.

C-extract is the difference between W-soluble and C-soluble K.

4. Conclusion

A new compound K2Ca2Si2O7 has been identified as the major constituent in the fused potassium silicate fertilizer produced from steel-making slag.

This major component of FPS fertilizers was first thought to be a compound, K2CaSiO4 (ICDD card No. 19-0943). However, it was found that several unidentified peaks other than the registered peaks for K2CaSiO4 were present in FPS fertilizers. The chemical composition of FPS fertilizers analyzed by SEM-EDX also showed a presence with a molar ratio of K:Ca:Si around 1:1:1 phase. Accordingly, two potassium calcium silicate compounds, K2Ca2Si2O7 and K2CaSiO4, were thought to be present.

To identify the major component of FPS fertilizers, two potassium calcium silicate compounds, K2O–2CaO–2SiO2 and K2O–CaO–SiO2, were synthesized. The X-ray diffraction patterns of the synthesized K2O–2CaO–2SiO2 were largely consistent with those of FPS fertilizer. SEM observations indicated that the synthesized K2O–2CaO–2SiO2 was a single phase whose K:Ca:Si molar ratio was 1:1:1. On the other hand, the synthesized K2O–CaO–SiO2 was found to be a mixture of K2O–2CaO–2SiO2 and K2CO3·1.5H2O. Thus, the component in FPS fertilizer is consistent with that of the synthesized K2O–2CaO–2SiO2, which is newly identified as single phase compound K2Ca2Si2O7 in this study.

This K2Ca2Si2O7 contained 88.5% of water-insoluble potassium and released potassium slowly in water. The FPS fertilizer exhibits a slow-release potassium characteristic controlled by its content of K2Ca2Si2O7.

References
 
© 2014 by The Iron and Steel Institute of Japan
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