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Sintering preparation and release property of K2MgSi3O8 slowrelease fertilizer using biotite acid-leaching residues as silicon source Xi Ma, Hongwen Ma, and Jing Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02991 • Publication Date (Web): 04 Oct 2016 Downloaded from http://pubs.acs.org on October 7, 2016

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Industrial & Engineering Chemistry Research

Sintering preparation and release property of K2MgSi3O8 slow-release fertilizer using biotite acid-leaching residues as silicon source Xi Ma†‡, Hongwen Ma*‡, Jing Yang‡ †

State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administraction, Beijing, 100029, PR China

‡

School of Materials Science and Technology, China University of Geosciences, Beijing, 100083, PR China

KEYWORDS: K2MgSi3O8; biotite acid-leaching residues; solubility; slow-release fertilizer ABSTRACT: The sintering preparation of K2MgSi3O8 at the temperature from 700oC to 950oC for 2h using biotite acid-leaching residues, K2CO3, and Mg(OH)2 as the starting materials were investigated in this research. The sintered samples were characterized by X-ray diffraction, X-ray fluorescence, Fourier transformation infrared spectrometry, and scanning electron microscopy. The results indicated that the hexagonal kalsilite-like K2MgSi3O8 with an irregular blocky morphology was prepared at 900°C for 2h. The solubility of K2MgSi3O8 in HCl and citric acid solution show that the extraction of K2O is 38.94% in 0.50M HCl and 23.58% in 0.10M citric acid solution, respectively. For the optimal sample dispersed in distilled water, the accumulative release of K2O is 6.06% in first day and 44.90% in 28 days which reaches the Chinese national standard of slow-release fertilizer. All the results indicate

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that the as-prepared K2MgSi3O8 is appropriate for application as slow-release fertilizers supplied nutritional components K, Mg, and Si for crops. 1. INTRODUCTION High food productivity of agricultural products is crucial for the rapidly increasing world population. High crop yields are normally achieved by improving soil productivity through the addition of fertilizer.1,2 The efficiency of nutrients in chemical fertilizers (just approximately 30%) is low as the slower speed of adsorption nutrients by plant roots. Moreover, the high solubility of chemical fertilizers results in several agronomic and environments problems. The loss of the nutrients in chemical fertilizers contaminates the ground water and deteriorates soil structure. In this regard, the slow-release fertilizers represent an attempt to minimize the difference between solubility and uptake.3-5 Recently, agronomic and chemical researchers have focused attention on slow-release potassium fertilizers (SRKF) as good source of potassium for crops. Show-release mineral potassium fertilizer is one of important SRKF, including fused potassium silicate fertilizer K2Ca2Si2O7,6,7 potassium silicate fertilizers K2MgSiO4 and

K2MgSi3O8,8-10

zeolite-based

fertilizers

phillipsite

and

merlinoite,11,12

mechanochemical formation fertilizers K-Si-Ca-O compound and KMgPO4.13-15 Due to their slower dissolution rates, which relate to the mineral structure, these mineral potassium fertilizers are more efficient for crops than chemical fertilizers.


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It is reported that K2MgSi3O8 has a similar structure of KAlSiO4, which is known as natural or synthetic kalsilite or kaliophilite.16 The powder diffraction data of K2MgSi3O8 (PDF#19-0973) reported by Hughes show that a hexagonal structure with cell parameters of 5.208Å * 8.683Å have strong diffraction lines appeared at d(Å) = 3.12, 2.61, 4.01, 4.33, 2.50, 1.59, 2.43, 2.24, 2.19, and 2.17.17 The kalsilite (PDF#11-0579) crystallized in a hexagonal unit cell with space group of P63 and cell parameters of 5.159Å * 8.703Å, and strong diffraction lines of kalsilite are distributed in d(Å) = 3.12, 3.97, 2.17, 2.47, 4.35, 2.22, and 1.57, respectively.18 The slight deviation of cell parameters and strong diffraction lines are attributed to the substitution of Al3+↔1/2 Mg2+ + 1/2 Si4+ in the crystal structure. The careful analysis by near edge X-ray absorption spectroscopy indicate that Mg2+ and Si4+ in the crystal structure of K2MgSi3O8 are designed to occupy in the sites of Al3+ in kalsilite with the framework of SiO4 tetrahedral and MgO4 distorted tetrahedral. 19,20 The kalsilite-like K2MgSi3O8 used as slow-release potassium silicate fertilizer could provide three kinds of nutritional components K, Mg, and Si for crops. Generally, the slow-release fertilizers with main mineral composition of K2MgSi3O8 were prepared by sintering of fly ash or molten ash, oil-shale residues, lime shale, and rice husks.8-10 The solubility of the metastable kalsilite-like K2MgSi3O8 in 0.50M HCl, 0.10M citric acid, and H2O solution suggested a potential use as SRKF.10 However, the release property of nutritional components K, Mg, and Si have not been investigated in detail in the previous researches.



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Biotite, K(Mg,Fe)3AlSi3O10(OH)2, is a potential magnesium and potassium mineral resources. In our previous research, potassium sulfate and magnesium hydroxide were prepared from biotite by an innovative process of sulfuric acid leaching and alkali precipitation with ammonia. The acid-leaching residues obtained in the process of dissolution of biotite in sulfuric acid solution were normally processed for application in construction materials such as glass, ceramic, and cement industries.21 This acid-leaching residue, which is mainly composed of amorphous SiO2, can be a promising silicon source for preparation of K2MgSi3O8. For the prospect of development of slow-release potassium fertilizer and comprehensive utilization of mineral residues, we tried to prepare and characterize K2MgSi3O8 for application as slow-release potassium fertilizer using biotite acid-leaching residues as silicon source in this work. The solubility of K2MgSi3O8 in 0.50M HCl solution, 0.10M citric acid solution, and distilled water were also investigated for evaluating the nutrient release property and mechanism on K2MgSi3O8. 2. EXPERIMENTS 2.1. Materials. After dissolution of the biotite powder, which mineral and chemical composition was studied in our previous research,21 in 3.0M sulfuric acid solution with sulfuric acid of 1.47mol/100g biotite powder at 90oC for 3h in a stainless steel autoclave, biotite acid-leaching residues were discharged by filtering the mixture. The material balance of the dissolution of biotite in sulfuric acid solution is shown in 4 ACS Paragon Plus Environment

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Figure 1. Chemical composition of the biotite acid-leaching residues is listed in Table 1. It shows that the biotite acid-leaching residues mainly consist of 76.65% SiO2 and 11.73% Loss almost for chemisorbed water of amorphous SiO2. Due to a small quantity of undissolved biotite and other minerals, it contains 1.67% K2O, 4.30% MgO, 1.63% Fe2O3, and 2.47% Al2O3.

Figure 1 Material balance of the dissolution of biotite in H2SO4 solution

Table 1 Chemical composition of the biotite acid-leaching residues powder (wt%) Sample

SiO2

TiO2 Al2O3 Fe2O3 MnO

MgO

CaO

Na2O

K2O

P2O5

Loss

BR-15

76.65

0.34

4.30

1.28

0.11

1.67

0.01

11.73 100.20

2.47

1.63

0.02

Total

According to the chemical formula of biotite K(Mg,Fe)3AlSi3O10(OH)2 and product K2MgSi3O8, Just a third Mg(OH)2 prepared from the refined biotite leaching solutions in the alkali precipitation process could be used as the starting material for preparation of K2MgSi3O8.21 The sources of soluble potassium (KCl) are extremely rare in China, while insoluble potassium resources, in which the principal mineral is K-feldspar, are



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abundant and extensively distributed. The starting material K2CO3 used in this research can be prepared from potassium rocks with lower consumption of disposable mineral resource and energy by a novel hydrothermal process.22-24 In the experiments, the analytical grade reagent potassium carbonate (K2CO3), the chemical grade reagent magnesium hydroxide (Mg(OH)2), and the above-mentioned biotite acid-leaching residues powder were used as the starting materials. 2.2. Preparation. Appropriate amounts of the biotite acid-leaching residues, K2CO3, and Mg(OH)2 were mixed evenly with stoichiometric ratio of K2MgSi3O8. Thermal gravimetric analysis of the starting materials indicated that the occurrence of reactions in the interval between 700°C and 950°C. According to these initial results, the sintering experiments were executed in the range of 700-950°C (intervals of 50°C and stay 2 hours at each temperature) using program control chamber electric furnace with the heating rate of 10°C/min. The sintering reaction is as followed: 3SiO2 (amorphous) + Mg(OH)2 + K2CO3 = K2MgSi3O8 + H2O + CO2

(1)

2.3. Characterization. The chemical compositions of biotite acid-leaching residues and sample were investigated by X-ray fluorescence (XRF) in an ARL ADVANTXP XRF spectrometer. The thermal decomposition of the starting materials with stoichiometric ratio of K2MgSi3O8 was studied by differential scanning calorimetric and thermal gravimetric analysis (DSC-TGA) using a SDT Q600 V20.9 Build 20 instrument in air atmosphere at a heating rate of 10°C/min. The X-ray diffraction (XRD) pattern of samples were recorded by a SmartLab (Rigaku) X-ray 6 ACS Paragon Plus Environment

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diffractometer with Cu Kα radiation over a 2θ range of 3° to 70° with a step size of 0.02°. FTIR spectra of samples were collected by a Perkin Elmer 2000 in the 4000~400 cm-1 region using potassium bromide as the diluent and binder. The morphology of samples were examined by a Sirion 200 scanning electron microscopy (SEM) under EHT=10.0 kV. 2.4. Solubility. The solubility of sample in acid solution was measured as followed: One gram of sintered, ground, and oven-dried sample was added to 150mL of 0.50M HCl and 0.10M citric acid in 250mL volumetric flasks. The mixtures were shaken at 28-30°C for 80 min at 180 rpm in the horizontal water bath oscillator, then filtered, transferred and diluted with distilled water to 250mL volumetric flasks.10 The nutrient releases of sample were investigated via the Chinese national standard of slow-release fertilizer (GB 23348-2009). Ten gram of sample was put into the nylon mesh bag, added to 200mL of distilled water in 250mL volumetric flasks. The mixtures were held at 25°C in the biochemical incubator, filtered after standing 24h, 3d, 5d, 7d, 10d, 14d, 28d, 42d, 56d, 84d, continually, then transferred and diluted with distilled water to 250mL volumetric flasks. K2O in the solution was analyzed by gravimetric analysis using sodium tetraphenylboron, MgO by volumetric analysis using EDTA, and SiO2 by colorimetric analysis using silicon molybdenum blue. 3. RESULTS AND DISCUSSION 3.1. Preparation of K2MgSi3O8. DSC-TGA curve of the starting materials with stoichiometric ratio of K2MgSi3O8 is illustrated in Figure 2. It shows that three main 7 ACS Paragon Plus Environment


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weight losses in the temperature ranges of about 100°C, approximately 360°C, and 700~950°C are associate with three endothermic peaks at 102oC, 365oC, and 901oC, respectively. The initial weight loss of 2.6% could be caused by loss of the adsorbed water of the starting materials. The first endothermic peak at 102oC responds loss of chemisorbed water in the biotite acid-leaching residues powder. The second is relative to decomposition of Mg(OH)2. The third generates by decomposition of K2CO3 and crystallization of K2MgSi3O8. The weight losses at each endothermic peak are in good agreement with decrease of appropriate amounts of the starting materials the biotite acid-leaching residues powder, Mg(OH)2, and K2CO3, respectively. The total percentages of weight loss for the starting materials with stoichiometric ratio of K2MgSi3O8 are 23.6%. According to the DSC-TGA results, the sintering experiments for preparation of K2MgSi3O8 were performed in the range of 700~950°C for 2h.

Figure 2 DSC-TGA curves of the starting materials with stoichiometric ratio of K2MgSi3O8

XRD patterns of samples obtained by sintering the starting materials with stoichiometric ratio of K2MgSi3O8 in the range of 700~950°C for 2h are shown in 8 ACS Paragon Plus Environment

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Figure 3. When the sintering temperature was 700°C, no mineral phase K2MgSi3O8 was generated. Amorphous silica was occurred due to the loss of chemisorbed water of the biotite acid-leaching residues powder, and MgO was produced by decomposition of Mg(OH)2. Diffraction peaks of K2MgSi3O8 started to appear at the sintering temperature of 750°C and heightened gradually with the increase of the sintering temperature from 750°C up to 900°C. On the contrary, diffraction peaks of MgO, biotite, and K2CO3 disappeared. It can be observed that the mineral phase of K2MgSi3O8 was obtained at 900°C for 2h with three main strong diffraction peaks at 2θ = 22.219°, 28.579°, and 34.360°, corresponding to the Miller Indices (101), (102), and (110), which is in good agreement with that of hexagonal structure K2MgSi3O8 reported by Hughes from powder diffraction data PDF#19-0973.16 Therefore, the sintered sample prepared at 900°C for 2h was selected for subsequent property characterization and solubility experiments.



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Figure 3 XRD patterns of the sintered samples.

3.2. Characterization of K2MgSi3O8. The chemical composition of the optimal sample (SR-1) prepared at 900°C for 2h is listed in Table 2. It can be seen that the contents of K2O, MgO, and SiO2 in the as-prepared sample are 26.12%, 12.00%, and 55.27%, respectively. The calculated K:Mg:Si mole ratio of 1.86:1.00:3.08 is close to the theoretical ratio of K2MgSi3O8. Besides, a little of CaO, Fe2O3, and SO3 derived from biotite acid-leaching residues are distributed in as-prepared sample, which are helpful for growth of crops.

Table 2 Chemical composition of the optimal sample (wt%) Sample SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 SO3 Loss Total SR-1

55.27 0.21 1.86

1.01

M.R.

0.01 12.00 0.81 0.00 26.12 0.01 1.95 0.41 99.67 1.86:1.00:3.08

M.R. is calculated K:Mg:Si mole ratio.

FTIR spectra of the optimal sample (SR-1) prepared at 900°C for 2h is shown in Figure 4A. The mineral K2MgSi3O8 has IR wave numbers of 456, 619, 745, 962, and 1068cm-1. The band at 456cm-1 is related to the Si-O bending vibration. The band at 619 and 745cm-1 belongs to Si-O-Mg symmetric stretching vibrations. The band appearing at 962 and 1068cm-1 is associated with Si-O-Mg asymmetric stretching vibrations. Compared with FTIR spectra of the metastable KAlSiO4 polymorph or kalsilite reported by Su et al25 and ABW-type KAlSiO4 reported by Becerro et al26, the deviation of peaks at 620cm-1 and the separated peak at around 1000cm-1 are attributed to the substitution of Al ↔ Mg or Si in the crystal structure of K2MgSi3O8. FTIR indicates that the K2MgSi3O8 has the same symmetry as KAlSiO4.


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Figure 4 FTIR spectra of (1) the optimal sample and (2) sample after water solubility for 84d.

The SEM images of the optimal sample (SR-1) prepared at 900°C for 2h is shown in Figure 5. Sample (SR-1) composed of pure K2MgSi3O8 exhibits an irregular blocky morphology and various particle size from 20~50μm.



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Figure 5 SEM images of the optimal sample (SR-1).

3.3. Solubility of K2MgSi3O8. Table 3 presents the analysis results carried out in the aqueous extracts of the optimal sample (SR-1) prepared at 900°C for 2h and treated in 0.50M HCl and 0.10M citric acid solution (see experimental above). The extraction or release of K2O, MgO, and SiO2 were calculated by the following formula:

i(%)= ciV/mwi

(2)

where i are components K2O, MgO, and SiO2, respectively, ci is concentration of the component i in the aqueous extracts, V=250 mL, the quality of the optimal sample m, wi are contents of component i in the optimal sample.

Table 3 Solubility of the optimal sample (SR-1) treated in HCl and citric acid solution


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0.50M HCl solution (g/L) Sample SR-1

0.10M citric acid solution (g/L)

K2O

MgO

SiO2

K2O

MgO

SiO2

0.407

0.254

0.063

0.246

0.162

0.056

The calculated results showed that about 38.94% K2O, 50.48% MgO, and 2.84% SiO2 were dissolved from K2MgSi3O8 in 0.50M HCl solution. And the extraction of K2O, MgO, and SiO2 in 0.10M citric acid solution is 23.58%, 32.19%, and 2.53%, respectively. The extraction of K2O are much lower than the product α-K2MgSi3O8 with a structure belonging to the metastable silicate kalsilite (1.214g/L in 0.50M HCl, and 0.929g/L in 0.10M citric acid).10 Besides, it can be also seen from Table 3 that the extraction concentrations of K2O, MgO, and SiO2 in 0.50M HCl solution are greater than that in 0.10M citric acid solution, due to the difference of pH value (0.50M HCl solution equals to pH value of 0.3, however, 0.10M citric acid solution equals to pH value of 2.0). The lower solubility of sintered sample in acid solutions indicates that K2MgSi3O8 is a citrate-soluble mineral, probably relating to the similar hexagonal structure of kalsilite which possesses the citrate-soluble property. The extractions of K2O in acid solutions (38.94% in 0.50M HCl, and 23.58% in 0.10M citric acid) are high enough to suggest a potential utilization of the sintered sample composed of K2MgSi3O8 as a potassium fertilizer.10 The nutrient releases of the optimal sample (SR-1) prepared at 900°C for 2h in distilled water were investigated according to the Chinese national standard of slow-release fertilizer (GB 23348-2009). According to the formula (2), accumulative release profiles of nutrients K2O, MgO, and SiO2 from the optimal sample are calculated and illustrated in Figure 6. In first day, nutrients releases of K2O, MgO, and 13 ACS Paragon Plus Environment


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SiO2 are 6.06%, 0.33%, and 1.36%, respectively. The data are consistent with a very low water solubility. Accumulative releases of K2O, MgO, and SiO2 are 44.90%, 3.38%, and 18.57% in the test time of 28 days. As the extension of test days, accumulative release of K2O rises obviously, however, accumulative release of SiO2 increase much slower than that of K2O. Conversely, accumulative release percentage of MgO is close to equilibrium with little release of MgO. The K2O release of 6.06% in first day is lower than 15% confirmed in the Chinese national standard of slow-release fertilizer. Similarly, the K2O release of 44.90% in the test time of 28 days is much lower than 80% in slow-release fertilizer standard. Therefore, the K2O release of the sintered sample composed of K2MgSi3O8 has reached the Chinese national standard of slow-release fertilizer.

Figure 6 Accumulative release profiles of the optimal sample (SR-1).


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3.4. Release property and mechanism. According to the water solubility results, the release property equation for accumulative release profile of K2O was calculated using nonlinear fitting method as followed: y = −57.21 + 28.51 ln(𝑥 + 7.94) (3) where y is accumulative release of K2O (%), x is test time (d), the correlation coefficient R2= 0.998. The accumulative release profile of K2O fits the release kinetics Elovich equation.27 According to equation (3), the accumulative release percentage of K2O reaches 79.27% after water solubility for 112d, 90.19% for 168d, and 98.07% for 224d, respectively. The release property of K2O in optimal sample composed of K2MgSi3O8 indicates that the sample could be used as a slow-release potassium fertilizer with nutrient K2O release for approximately 8 months. XRD patterns of the optimal sample prepared at 900°C for 2h and the sample after water solubility for 84d are shown in Figure 7. The weakened diffraction peaks of K2MgSi3O8 after water solubility for 84d is connected to the dissolution of K2MgSi3O8 in disordered water. Amorphous diffraction peak probably represents magnesium-silicon precipitate obtained from the already release of MgO and SiO2 in distilled water, which leading to the slower release percentage of MgO and SiO2 as shown in Figure 6. Diffraction peaks corresponding to biotite indicated that mineral biotite could not be dissolved in distilled water in a short period.



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Figure 7 XRD pattern of (1) the optimal sample and (2) sample after water solubility for 84d.

FTIR spectrum of sample after water solubility for 84d is illustrated in Figure 4B. The band at 449cm-1 is related to the Si-O bending vibration. The band appearing at 1005cm-1 is associated with Si-O-Mg asymmetric stretching vibrations. However, the band at 619 and 745cm-1 belongs to Si-O-Mg symmetric stretching vibrations in the optimal sample composed of K2MgSi3O8 disappeared. It explained that the tetrahedral framework Si-O-Mg of K2MgSi3O8 was broken in the process of water solubility. Moreover, the band at 1645 and 3460cm-1 representing chemisorbed water and absorbed water deduces that magnesium-silicon precipitate was precipitated along with dissolution of K2MgSi3O8 in distilled water. Based on the nutrient accumulative release profiles, XRD pattern and FTIR spectrum of sample after water solubility for 84d, the nutrient release process from K2MgSi3O8 is the same to dissolution of kalsilite in sulfuric acid solution,22,24,28 which can be described by the following chemical equation:


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K2MgSi3O8 + (n+1)H2O = 2K+ + MgO3SiO2nH2O + 2OH-

(4)

Firstly, ion change was occurred between H+ ionized from distilled water and K+ occurred in interstitial of the structure of K2MgSi3O8. It results in nutrient K+ was released from K2MgSi3O8. Then tetrahedron framework of K2MgSi3O8 was destructed with breaking of Mg-O-Si and Si-O-Si. The released nutrient Mg2+ was hydrolyzed into [Mg(OH)3]- in the weak alkaline solution. The [Mg(OH)3]- and [H2SiO4]2- were precipitated to produce magnesium-silicon precipitate. If being used as slow-release fertilizer, the magnesium-silicon precipitate could be dissolved easily by organic acid from roots of plants,4,29, corresponding to the citrate-soluble property of K2MgSi3O8 and kalsilite. Therefore, three components K2O, MgO, and SiO2 in K2MgSi3O8 could be released slowly and supplied as nutrients for crops. 4. CONCLUSION A slow-release potassium fertilizer K2MgSi3O8 was prepared through a sintering process using the starting materials of biotite acid-leaching residues, K2CO3, and Mg(OH)2. The optimal sample obtained at 900°C for 2h exhibits a hexagonal kalsilite-like structure and an irregular blocky morphology with various particle size from 20~50μm. K2MgSi3O8 is a citrate-soluble mineral with the K2O extraction of 38.94% in 0.50M HCl and 23.58% in 0.10M citric acid solution. The nutrient release tests in distilled water show that the accumulative release of K2O from K2MgSi3O8 is 6.06% in first day and 44.90% in 28 days, which reached the Chinese national standard of slow-release fertilizer. The nutrient release property corresponding to the



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Elovich equation is similar to the dissolution of kalsilite in sulfuric acid solution. This suggests that K2MgSi3O8 has a potential to serve as a slow-release fertilizer. AUTHOR INFORMATION Corresponding Authors * Tel.: +86 10 82323374

E-mail: [email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This study was financially supported by Fundamental Research Funds for the Central Universities (2652015015) and China Geological Survey Project (12120113087700).

References (1) Ghosh, P. K.; Ramesh, P.; Bandyopadhyay, K. K.; Tripathi, A. K.; Hati, K. M.; Misra, A. K.; Acharya, C. L. Comparative effectiveness of cattle manure, poultry manure, phosphocompost and fertilizer-NPK on three cropping systems in vertisols of semi-arid tropics. I. Crop yields and system performance. Bioresour. Technol. 2004, 95, 77-83. (2) Ghosh, P. K.; Ajay; Bandyopadhyay, K. K.; Manna, M. C.; Mandal, K. G.; Misra, A. K.; Hati, K. M. Comparative effectivness of cattle manure, poultry manure, phosphocompost and fertilizer-NPK on three cropping systems in vertisols of


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