Powder Technology 212 (2011) 354–358

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Mechanochemical synthesis of kaolin–KH2PO4 and kaolin–NH4H2PO4 complexes forapplication as slow release fertilizer

Solihin a, Qiwu Zhang a, William Tongamp b,⁎, Fumio Saito a

a Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira, Aoba-Ku, Sendai 980-8577, Japanb Faculty of Engineering and Resource Science, Akita University, 1-1 Tegata-Gakuen machi, Akita 010-8502, Japan

⁎ Corresponding author.E-mail addresses: tbune16x@yahoo.com, wtongamp

(W. Tongamp).

0032-5910/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.powtec.2011.06.012

a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 March 2011Received in revised form 2 June 2011Accepted 13 June 2011Available online 22 June 2011

Keywords:Mechanochemical synthesisKaolinKH2PO4

NH4H2PO4

MillingSlow release fertilizer

A milling process to reduce kaolin to amorphous phase in the presence of KH2PO4 or NH4H2PO4 and allowmechanochemical (MC) reaction for incorporation of KH2PO4 and NH4H2PO4 into the kaolin structure wasinvestigated in this work. Mixtures of kaolin and KH2PO4 and NH4H2PO4 in separate systems were preparedby milling in a planetary ball mill. Tests with kaolin contents ranging from 25 to 75 wt.% and mill rotationalspeeds from 200 to 700 rpm were performed to evaluate incorporation of KH2PO4 and NH4H2PO4 and releaseof K+, NH4

+ and PO43− ions into solution. Analyses by XRD, DTA and ion chromatography indicated that the MC

process was successfully applied to incorporate both KH2PO4 and NH4H2PO4 into the amorphous kaolinstructure. Release of K+ and PO4

3− ions from the system (kaolin–KH2PO4) when dispersed in water for 24 hreached only up to 10%. Under similar conditions for the system (kaolin–NH4H2PO4), release of NH4

+ and PO43−

ions reached between 25 and 40%. These results indicated that the MC process can be developed to allowamorphous kaolin to act as a carrier of K+, NH4

+ and PO43− nutrients to be released slowly for use as fertilizer.

@gipc.akita-u.ac.jp

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Kaolin is a type of clay mineral consisting mainly of hydratedaluminum silicate or kaolinite formed through hydrothermal alterationof feldspar and muscovite, usually containing quartz [1]. Kaolinite isusually represented by the formula Si2Al2O5(OH)4 and/or other forms asAl2O3·2SiO2·2H2O, Al2O7Si2.2H2O, which builds up by a sheet structurecomposed of SiO4 tetrahedral sheets and Al(O,OH)6 octahedral sheets([Si2O5]2− sheet and [Al2(OH)4]2+ sheet) with pseudo-hexagonalsymmetry that are created from planes, which are occupied as follows:O6–Si4–O4–(OH)2–Al4–(OH)6. Kaolin is an important industrialmineral,which is used in many applications, including paper filling and coating,refractory, ceramic, and food additives [2–5].

The intercalation or incorporation of plant nutrients (potassium,ammonium, nitrate, phosphate) into the kaolin structure to be appliedas a potential candidate for application as starting material for slowrelease fertilizer can also be considered. Fertilizers in agriculture havelong been used for promoting plant growth to achieve high crop yield.In many applications, their efficiency is low due to the unbalancespeed between nutrients released from fertilizer and the absorption ofnutrients by plant roots, resulting in loss of nutrients and contami-nation of underground water [6,7].

Preparation or synthesis of slow or controlled release fertilizers (SRFor CRF) usually by the physical methods such as dispersion of ordinaryfertilizer in the matrix or encapsulating ordinary fertilizer, in whichnutrient release is slowed down by diffusion, is widely discussed inliterature [8–14]. In some recent studies, the application of themechanochemical synthesis route to prepare complex compounds tobeused as candidates for slow release fertilizer has been reported.Makóet al.[15] have reported the intercalation of urea (NH2CONH3) into thekaolin structure by a mechanochemical process involving milling andcompared against the aqueous suspension method. Solihin et al.[16]have successfully synthesized KMgPO4 and NH4MgPO4 compoundsby mechanochemical process and Tongamp et al. [17] have reportedincorporation of thenitrate ion into theMg–Al–NO3 type layereddoublehydroxide structure by the mechanochemical process. In these in-stances the mechanochemical reaction effect allowed intercalation oftarget materials but the crystal structure of kaolin and the layereddouble hydroxide are maintained. Balaž and Dutkova [19] discussmechanical activation of the fluoroapatite (Ca5(PO4)3 F) mineralnormally used for the production of phosphorus fertilizers. AlsoMinjigmaa et al. [18] discusses fine milling in applied mechanochem-istry and its application in agriculture for fertilizer production.

In the current study, a milling operation was performed for thesystems; (kaolin–KH2PO4) and (kaolin–NH4H2PO4) to completelyreduce kaolin structure to amorphous phase and effect mechano-chemical reaction to obtain chemical bonding between K/NH4–Al–Si–P–O as amorphous glass phase. The new product was targeted to givea water-insoluble property so the release of K+, NH4

+, PO43− nutrients

355Solihin et al. / Powder Technology 212 (2011) 354–358

into the soil can be controlled and offered as SRF or CRF fertilizer. It iswell known that kaolin and other hydrated silicate minerals undergoamorphization against grinding operation and that alkali–alumina–phosphate glass does exist [20–27]. Different milling conditions andkaolin addition ratios were investigated to obtain milled productswith desired properties is discussed in this paper.

2. Experimental

2.1. Mechanochemical synthesis

Chemical grade reagents, potassium phosphate (KH2PO4) andammonium phosphate (NH4H2PO4) were used to mill with kaolin(Al2Si2O6(OH)4 – XRD file PDF No. 01-527) branded as white kaolin inseparate systems [(a) Al2Si2O6(OH)4−KH2PO4) and (b) Al2Si2O6(OH)4−NH4H2PO4] for the synthesis of new complex compounds to be offered ascandidate for slow release fertilizers. A 2.0 g sample mixture of startingmaterials of varying weight ratios and ball/sample weight ratio of about40 was milled in a planetary ball mill (Pulverisette-7, Fritsch, Germany),which had twomill pots (45 cm3 inner volume each) made of stainless-steel with 7 steel-balls of 15 mmdiameter. Mill speed in this experimentwas ranged between 100 and 700 rpm and milling time was fixed at120 min.

2.2. Characterization

X-ray diffraction (XRD) patterns of the solid samples wereobtained using equipment Rigaku Rint-220/PC, using Cu K-α radiationat 40 kV/20 mA. Thermal decomposition behavior of the milledproducts was performed using TG-DTA equipment (Rigaku ThermoPlus TG-8120).

Subsequent release of the nutrients (K+, NH4+ and PO4

3−) into waterfromthe complex compounds synthesizedbymechanochemical reactionwas evaluated by dissolving 1.0 g of the milled product in 20 ml ofdistilledwater for 24 h at room temperature. A liquid ion chromatographinstrument (Shimadzu L10 Series) was used to determine the concen-tration of the (K+, NH4

+ and PO43−) ions (nutrients) in the filtrate.

3. Results and discussion

3.1. Kaolin–KH2PO4 system

The XRD patterns of (kaolin–KH2PO4) sample system milled atvarying kaolin contents for 120 min at 600 rpm is shown in Fig. 1. Atlow content of kaolin (25 wt.%), characteristic peaks corresponding to

Fig. 1. XRD patterns of kaolin–KH2PO4 sample system with varying amounts of kaolin(wt%) milled for 120 min at 600 rpm.

KH2PO4 continued to appear in themilledmixture while only kaolin isreduced to amorphous state. However, when kaolin content wasincreased to 50 and 75%, milling resulted in complete amorphizationof both kaolin and KH2PO4 starting samples.

A huge amount of impact energy imparted from the milling actioncan slide the grain boundary of kaolin, resulting in amorphization ofits crystal structure, and this can allow the KH2PO4 molecule to beincorporated into the amorphous structure of kaolin. At low content ofkaolin, the distorted crystal of kaolin was not large enough to receiveall K+ and PO4

3− atoms from starting material. But when the contentof kaolin was increased, the capacity of the amorphous structure ofkaolin to receive KH2PO4 also increased. Thus all K+ and PO4

3− atomswere incorporated into the amorphous structure of kaolin. This can beexplained by the XRD patterns shown in Fig. 1.

The release profile of K+ and PO43− nutrients in the sample mixtures

obtained fromkaolin–KH2PO4 systemasa functionof kaolin content (wt%) (Fig. 2) showed that the release rate of the nutrients decreasedwhenkaolin content was increased. Milling time and mill rotational speedwere kept constant at 120 min and 600 rpm, respectively, duringsample preparation. The release of K+ and PO4

3− nutrients at additionsof kaolin below 20 wt.% reached over 99%. When kaolin addition wasincreased to 25 wt.%, the nutrients released decreased to 77% PO4

3− and86% K+. However, amount of K+ and PO4

3− nutrients released intosolution significantly decreased to less than 10%when content of kaolinin the samplemixtureswas increased to 50 and 75 wt.%. As discussed inprevious section, when the kaolin content is 25 wt.%, the capacity of thedistorted structure of amorphous kaolin to incorporate KH2PO4 was notsufficient, and excess KH2PO4 in the milled mixture readily dissolvedwhen dispersed in water. Sufficient kaolin content in the samplemixture increased its capacity to allow incorporation of KH2PO4 into theamorphous structure. When dispersed in water the K+ and PO4

3− ionscould not readily pass through the kaolinite structure or network beforedissolving into water, resulting in the sharp decrease in nutrientsreleased.

The effect of mill rotational speed on amorphization of the kaolin–KH2PO4 (1:1 wt ratio) sample mixture is shown in Fig. 3. Milling timefor all experimental runs was fixed at 120 min and the mill rotationalspeeds were ranged between 200 and 700 rpm. The XRD patternsshowed that at lower mill rotational speeds, ranging from 200 to300 rpm, characteristic patterns corresponding to KH2PO4 remainedin the milled products. However, at mill speeds above 400 rpm, thestarting samples were completely reduced to amorphous state, whichcould suggest that KH2PO4 has been incorporated into amorphousstructure of kaolin.

Fig. 2. Nutrients release profile of kaolin–KH2PO4 sample system with varying amountsof kaolin (wt%) milled for 120 min at 600 rpm.

Fig. 3. XRD patterns of kaolin–KH2PO4 (1:1 wt ratio) samplemixturemilled for 120 minat varying mill rotational speeds. Fig. 5.Nutrients release profile of kaolin–KH2PO4 (1:1 wt ratio) sample mixtures milled

for 120 min at different mill speeds and dispersed in water for 24 h.

356 Solihin et al. / Powder Technology 212 (2011) 354–358

Thermal decomposition behavior of the kaolin–KH2PO4 (1:1 weightratio) mixtures milled for 120 min at varying mill speeds is shown inFig. 4. The endothermic peak at 230 °C seen only for sample mixtureobtained at 200 rpm is attributed to by the decomposition of KH2PO4 toKPO3, releasing the H2O mole in its structure. For the sample mixturesmilled at mill speeds above 400 rpm, no endothermic peak at 230 °Ccorresponding to KH2PO4was observed, but a broad endothermic peak ataround 100 °C corresponding to water release appeared. As KH2PO4

enters the amorphous structure of kaolin, it releases H+ ion, whereaskaolin releases OH− ion. Both those H+ and OH− form water. Thus, thepresence of water release and the absence of KH2PO4 reaction duringthermaldecomposition indicatedcomplete incorporationofK+andPO4

3−

from the KH2PO4 sample into the amorphous structure of kaolin.The release profile of K+ and PO4

3− nutrients for the kaolin–KH2PO4 (1:1 weight ratio) sample mixtures milled at different millrotational speeds and dispersed in water for 24 h is shown in Fig. 5.For sample mixtures prepared at 100 and 200 rpm, release of both K+

and PO43− reached over 90–100% indicating that higher mill rotational

speeds are required to effect complete amorphization of startingmaterials to allow incorporation of KH2PO4 into the amorphous kaolinstructure. When mill speed was increased from 300 to 700 rpm, therelease of both K+ and PO4

3− nutrients into solution significantlydecreased, reaching around 50% at 400 rpm and remained constantaround 10% for samples obtained from milling at 500–700 rpm

Fig. 4. DTA patterns of kaolin–KH2PO4 (1:1 wt ratio) sample mixture milled for 120 minat varying mill rotational speeds.

indicating that KH2PO4 was incorporated into amorphous structure ofkaolin. The network of Al–Si–O, especially with the presence of metalssuch as Al, retards the diffusion movement of those elements [2–5].The K+ and PO4

3− dissolving in water comes from both KH2PO4 and bythose attached in the network chains of Al–Si–O.

3.2. Kaolin+NH4H2PO4 system

Characterization of the kaolin–NH4H2PO4 sample mixtures withvarying kaolin contents (wt%) milled for 120 min by XRD analysis isshown in Fig. 6. At 25 wt.% of kaolin addition, clear peaks correspondingto starting NH4H2PO4 sample continue to remain in the milled product.When kaolin content was increased to 50 wt.% and 75 wt.%, bothstarting samples were reduced to amorphous state. As discussed for thekaolin–KH2PO4 sample system, at low kaolin content, the amorphousstructurewasnot large enough to incorporateNH4

+ andPO43−, butwhen

amount of kaolin was increased, more NH4+ and PO4

3− ions wereincorporated into the amorphous structure of kaolin.

The nutrient release behavior when dispersed in water for thesample mixtures milled with varying content of kaolin is shown inFig. 7. At 25 wt.% kaolin addition, release of nutrients dispersed inwater reached over 85% and closer to 100% but decreased to between30 and 50% for kaolin content at 50 wt.% and was further reduced tobelow 10% for sample mixture with kaolin content at 75 wt.%. The

Fig. 6. XRD patterns of kaolin–NH4H2PO4 sample mixtures with varying kaolin contents(wt%) and milled for 120 min at 600 rpm.

Fig. 7. Nutrient release profile of kaolin–NH4H2PO4 sample mixtures with varyingkaolin contents (wt%) and milled for 120 min at 600 rpm.

Fig. 9. Nutrient release profile of kaolin–NH4H2PO4 (1:1 wt ratio) sample mixturesmilled for 120 min at varying mill rotational speeds and dispersed in water for 24 h.

357Solihin et al. / Powder Technology 212 (2011) 354–358

decrease of nutrients release into solution indicated that NH4H2PO4

on impact of milling was incorporated into Al–Si–O network.The XRD patterns of kaolin–NH4H2PO4 (1:1 weight ratio) sample

mixture milled for 120 min at different mill speeds (Fig. 8) indicatedthat the startingmaterialswere completely reduced to amorphous statewhen milled at 400–700 rpm. Characteristic peaks of starting materialsremained inproducts prepared bymilling atmill speedsbelow400 rpm,indicating that higher mill speeds were required to reduce the startingmaterials to amorphous state and allow incorporation of NH4H2PO4 intothe kaolin structure.

The nutrients release behavior of (kaolin–NH4H2PO4–(1:1 wt ratio))samples as a functionofmill rotational speedswhendispersed inwater for24 h is shown in Fig. 9. For the samples milled at 100 and 200 rpm, NH4

+

and PO43− ions completely dissolved into water, and showed only slight

decrease for samplesmilled at 300 rpm. This result correspondswellwithXRD data (Fig. 8), where the easily soluble NH4H2PO4 patterns continuedto remain in themilledproducts and this indicated that thenutrientswerenot yet incorporated into the Kaolin structure. For the sample mixturesmilled at 400 rpm and above where complete amorphization wasobserved, the subsequent release of nutrients decreased and reached anaverage of around 40% in 24 h. This result showed that higher millrotational speeds were required to reduce the starting materials toamorphous phase and allow incorporation of the nutrients into the kaolinstructure. Thermal decomposition behavior of the (Kaolin–NH4H2PO4)

Fig. 8. XRD patterns of kaolin–NH4H2PO4 (1:1 wt ratio) sample mixtures milled for120 min at varying mill rotational speeds.

sample mixtures obtained from the milling operations showed similartrends to that of the kaolin–KH2PO4 sample system (Section 3.1). Anendothermic peak appeared at 230 °C for samples milled up to 200 rpmcorresponding to NH4H2PO4 decomposition.

4. Conclusions

The incorporation of K+, NH4+ and PO4

3− ions as plant nutrients fromKH2PO4 and NH4H2PO4 compounds into the structure of kaolin wassuccessfully achieved through mechanochemical route by a millingoperation. The amount of kaolin and its degree of amorphization playedan important role in the incorporation process. The findings in thecurrent work can be summarized as follows;

(a) A mill rotational speed of at least 400 rpm was required toreduce all starting materials to amorphous state. Sufficient ballmovement and collision was necessary for milling of thesamples charged in the mill pots.

(b) Slow release behavior of the nutrients incorporated into kaolinstructure when dispersed in water was observed for samplemixtures milled at 400 rpm and above.

(c) Nutrients (K+, PO43−) released from the kaolin–KH2PO4 sample

system reached up to only 10% when dispersed in water for24 h. For the sample system kaolin–NH4H2PO4, the nutrients(NH4

+, PO43−) released reached up to between 25 and 50% for

the same time when dispersed in water.

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