Comparative performance of different urea coating ...prpi.upm.edu.my/IPR/Copyright/UPM6254_7845_copyright.pdf · Please cite this article in press as: Mohd Ibrahim, K. R., et al

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ARTICLE IN PRESSG ModelARTIC-670; No. of Pages 8

Particuology xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Particuology

jo ur nal home page: www.elsev ier .com/ locate /par t ic

omparative performance of different urea coating materials for slowelease

hairul Ridzwan Mohd Ibrahima, Farahnaz Eghbali Babadib, Robiah Yunusa,∗

Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, Selangor 43400, MalaysiaInstitute of Advanced Technology, Universiti Putra Malaysia, Selangor 43400, Malaysia

r t i c l e i n f o

rticle history:eceived 9 November 2013eceived in revised form 22 February 2014ccepted 2 March 2014

eywords:oatingreaypsumulfurlow releaserushing strength

a b s t r a c t

Approximately 70% of the applied urea fertilizer may be lost into the environment. This loss is due toleaching, decomposition and ammonium volatilization in soil, water and air. Through coating, the slowrelease technology can be used to reduce losses and to increase the fertilizer efficiency. Sulfur has beenused as a coating material, but the coating cracks easily because of its friability, sometimes being peeledoff from the urea surface. In this study, four types of materials, namely, gypsum, cement, sulfur andzeolite, were mixed and used as coating materials to search for the most effective and cheap coatingmaterials. The primary reasons for selecting these materials were improving fruit quality and preventingplant diseases, providing a plant nutrient, increasing soil fertility and water retention. The materialswere also selected based on their availability, processiblity and price. The effects of the coating materials,thickness, drying time, sieving and sealant on the crushing strength and dissolution rate of urea wereinvestigated. Coated urea with the same proportion of gypsum–sulfur exhibited high crushing strengthand lower dissolution rate. However, the performance was further enhanced by applying molten paraffin
wax on the hot urea surface. SEM images demonstrated that the micro-structure of gypsum–sulfur coatedurea after sieving resulted in a smoother coated layer. The efficiency of the coated urea was improved by26% using gypsum–sulfur (20% total coating), 3% paraffin wax and sieving the coating materials beforeapplication.
© 2014 Published by Elsevier B.V. on behalf of Chinese Society of Particuology and Institute of ProcessEngineering, Chinese Academy of Sciences.

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Urea is the most popular and economical of the nitrogenousertilizers used worldwide. Compared to other nitrogenous solidources, urea has a nitrogen (N) content of 46%, the highest con-entration of N available. The high solubility of urea in the water,articularly in areas with high rainfall, makes it be easily leachedrom the soil before plants have a chance to assimilate it. It iseported that approximately 70% of the applied urea fertilizer maye lost in regions with high, intermittent precipitation (Allison,955; Lundt, 1971). This loss is due to leaching, decomposition,mmonium volatilization in the soil, handling, and storage (Shaviv

Please cite this article in press as: Mohd Ibrahim, K. R., et al. Comparelease. Particuology (2014), http://dx.doi.org/10.1016/j.partic.2014.03

Mikkelsen, 1993). The slow release technology, by coating, cane used to reduce the dissolution rate of urea and to increase thefficiency of urea fertilizer.

∗ Corresponding author. Tel.: +60 89466266; fax: +60 386567120.E-mail address: [email protected] (R. Yunus).

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ttp://dx.doi.org/10.1016/j.partic.2014.03.009674-2001/© 2014 Published by Elsevier B.V. on behalf of Chinese Society of Particuology

There are several varieties of materials available for coatingrea, such as polymers, resin, sulfur and phosphogypsum. Variousypes of polymers, including the copolymer of vinyl chloride andinyl acetate, poly(vinyl acetate), low-density polyethylene (LDPE)Salman, 1988), ethylene-vinyl acetate copolymers, ethylene-crylic acid copolymers (Goertz, Timmons, & McVey, 1993),olyurethane (Du, Zhou, & Shaviv, 2006), poly(l-lactide) (Chen, Xie,huang, Chen, & Jing, 2008), starch/acrylic acid (Anggoro, 2011),olyacrylic acid latex (Lan et al., 2011), poly(vinylidene-chloride)Tzika, Alexandridou, & Kiparissides, 2003), paraffin and polyeth-lene waxes (Al-Zahrani, 2000), were tested to coat urea. Sulfuroated urea (SCU) has been produced for half of a century. Manytudies reported the improvement of the quality of sulfur coatedrea by using polymer (Goertz et al., 1993), dicyclopentadiene (Liu,ang, Qin, & Jin, 2008), and attapulgite clay (Gullett, Simmons, &

rative performance of different urea coating materials for slow.009

ee, 1991). Another sulfur source is phosphogypsum. It is slightlyoluble in water and therefore long-lasting in the soil (Vashishtha,ongara, & Singh, 2010). Gypsum plaster is another option con-

idered for the coating of urea. In addition, it was realized that

and Institute of Process Engineering, Chinese Academy of Sciences.

dx.doi.org/10.1016/j.partic.2014.03.009


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mailto:[email protected]


ARTICLE IN PRESSG ModelPARTIC-670; No. of Pages 8

2 K.R. Mohd Ibrahim et al. / Particuology xxx (2014) xxx–xxx

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A 50 g sample of granules was placed in 250 mL of distilled waterin an Erlenmeyer flask and sealed. The Erlenmeyer flask was thenplaced in an incubator shaker for 5 h. The temperature was set at

Table 1Operational parameters of coating urea in a rotary drum.

Urea particle size (mm) 2.80Proportion of coating (%) 20Drum speed (rpm) 52

Fig. 1. Size distribution of urea particles purchased.

ypsum coating on urea reduces the ammonia volatilization lossy approximately 40% compared to uncoated prills, and contin-ed use of gypsum coated urea can supplement the soil with sulfurRamachandran, Geevarghese, Sasi, & Rao, 1992). Gypsum is a min-ral composed of calcium sulfate dehydrate. It is also known aslaster of Paris (POP). Gypsum is used mainly in the building sector,hus making it a very common item that meets the characteris-ics required. Therefore, it can be used to enhance the efficiency ofrea fertilizer as gypsum coated urea. To close any cracks or imper-ections and seal the flaws, paraffin and polyethylene waxes weresually used on the outer surface of the coated urea to reduce theate of urea release (Ayyer, 1989, 1992).

In this paper, gypsum plaster was used as a base material to coatrea and cement, sulfur and zeolite were used to combine withypsum to produce slow release urea. The properties of the coatedrea particles were investigated by varying the coating materialhickness, drying time, use of paraffin wax and sieving the coating

aterials prior to application.

xperimental

aterials

Commercial urea granules with a nitrogen content of 46% andizes ranging from 1 to 4 mm were supplied by Petronas Agrenas.ig. 1 shows the particle size distribution for the urea granules.he materials selected as coating materials were gypsum pow-er, sulfur, white cement and zeolite. Gypsum increases the valuef organics and sulfur resists leaching or washing through theoil into the water supply. Commercial-grade gypsum and sulfurere obtained from Siam Gypsum Plaster L.P., Bangkok, Thailand

nd Palm Brand (National Est. for Agricultural and Industrial Sul-hur, Saudi Arabia), respectively. In addition, cement providesxcellent binding capacity, and zeolite improves soil fertility andater retention. Cement and zeolite were supplied from Aalborg

ortland and MB Plus, respectively. These materials were also cho-en based on their availability and price. For the analysis of ureaeleased using high-performance liquid chromatography (HPLC),cetonitrile (HPLC grade, Friendeman Schmidt) was used in a 10:90ixture with distilled water for the mobile phase.

oating urea in rotary drum coater


A rotary drum coater, shown in Fig. 2, was used to coat the ureaarticles. The rotary drum coater was made from stainless steel andas 30 cm in diameter. Urea granules previously sieved to 2.8 mm

Fig. 2. Schematic diagram of a rotary drum coater.

n diameter were weighed and fed into the rotary drum rotatingt 52 rpm. Coating mixture was frequently applied onto the sur-ace of the tumbling urea. Water was introduced as a mist onto therea granules each time after a small quantity of coating powderas spread inside the rotary drum. Coated particles were collected

fter 20 min for drying and analysis. The operational parametersre tabulated in Table 1.

easurement of crushing strength

The crushing test is important to ensure that the product canithstand physical handling throughout the supply chain. The

rushing test was performed on urea and coated urea, where 20 gf granules was randomly selected from the majority populationased on the sieve test. A particle strength tester (Chatillon TCM00 tensile strength tester) with the capacity of 200 lb (100 kg,000 N) and 12.7–317.5 mm/min speed was used to measure theranule crushing strength by applying an increasing compressiveorce on a single granule. The tester recorded the compressive forcehen the granule was crushed. In this test, individual granules ofrea were subjected to measured force applied by means of a metallunger that was part of an apparatus used for the analysis calledhe strength meter. The force at which the granules fractured wasaken as a measure of strength. Usually, the test was run on particlesrom the major size fractions.

easurement of the urea dissolution rate


Mass of bed (kg) 1Spray water (%) 1.92Temperature (◦C) 30Retention time (min) 20



K.R. Mohd Ibrahim et al. / Particuology xxx (2014) xxx–xxx 3

Table 2Coating material formulas (total coating: 20% (mass of coating/mass of urea)).

Coating No. Composition (%)

Gypsum powder White cement Sulfur Zeolite

C1 20 0 0 0C2 15 5 0 0C3 10 10 0 0C4 15 0 5 0C5 10 0 10 0C6 15 0 0 5C7 10 0 0 10C8 7.5 7.5 5 0C9 7.5 7.5 0 5

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By cutting the coated particles with a sharp knife, the cross sec-tion of the coating shell could be observed under SEM. Fig. 5 showsthe micro-structure of the uncoated and coated urea granules.

C10 7.5 7.5 2.5 2.5

0 ◦C, and the rotation speed was set at 100 rpm. Every hour for fiveours, 1 mL of solution was sampled. The extracted samples werenalyzed using high performance liquid chromatography (HPLC)o obtain the urea concentration in the solution at a particularime. A 20 �L volume of samples was injected onto a LiChroCART50-4,6 Pureser STAR column mounted on the HPLC (ShimadzuC20AT-Prominence, Japan), which was fitted with a UV-vis detec-or. The oven temperature was set at 30 ◦C with the wavelength sett 210 nm. Because the dissolution rate was used to indicate theoating efficiency, urea’s dissolution at the fifth hour (h5) was useds the benchmark. With this, the efficiency, which is a comparisonf the dissolution concentration at h5 of the samples against urea,as calculated based on the formula:

fficiency (%) =(

CU − CCU

CU

)× 100 (1)

here CU = concentration of urea for uncoated urea samples at h5,nd CCU = concentration of urea for coated urea samples at h5.

esults and discussion

Ten formulations using the four selected materials were devel-ped for screening purposes. Gypsum plaster was used as a baseaterial for all formulations. The formulations are tabulated in

able 2. In this study, the naked urea was used as a control sample,hereas the sulfur coated urea (SCU) was used as the benchmark.

he coated samples produced using these formulations were ana-yzed by the crushing test. The samples which passed the initialcreening were further analyzed by the dissolution test and SEM.

trength of the coated urea

The strengths of the coated urea with different materials arehown in Fig. 3. Based on the average figures, none of the sam-les showed superior strength to urea. In comparison to sulfuroated urea, samples number 1–6 and 8 all showed higher aver-ge strength. On the contrary, samples number 7, 9, and 10 hadower average strengths compared to sulfur coated urea.

From the perspective of crushing strength, it is satisfactory toote that the samples that had better strength compared to SCUan withstand operational handling. The highest strength obtainedas for coating 5 (C5), which was 33.34 N, whereas the lowest

trength was for coating 10, which was 1.96 N. Based on the average


trength, C5 and C6 had the highest results, where both were at2.16 N. The three samples with the highest strengths (C3, C5, and6) were further analyzed for effectiveness.

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Fig. 3. Crushing strength of uncoated and coated urea.

elease behavior of coated urea

The purpose of this analysis was to study the rate of ureaelease from the coated urea granules when they were immersedn water. The results provided a general indication of the perfor-

ance for granules with different coating parameters in retardingrea release and thus superior coating efficiency. The three sam-les were subjected to dissolution tests, and their dissolution ratesere measured accordingly based on the efficiency equation. Fig. 4

hows the results of the dissolution rate for coated urea granules,3, C5, and C6.

The dissolution patterns of all the samples were monotonicallyncreasing. C3 had the highest concentration at the first hour, which

as at 1.308 mol/L. The lowest concentration at the fifth hour,hich also represents the highest efficiency amongst the three,as C5 at 2.154 mol/L. At the end of the 5th hour, the samples at the

ottom of the beaker were poured out. Some shells still containedntact urea In terms of the efficiency, C3 and C6 were lower andpproximately at 5.9% and 4.9%, respectively. C5 had the highestfficiency of 13.8%.

orphology of the coating shell


ig. 4. Dissolution test results for C3, C5, and C6 in terns of urea concentration versusissolution time.


Please cite this article in press as: Mohd Ibrahim, K. R., et al. Comparative performance of different urea coating materials for slowrelease. Particuology (2014), http://dx.doi.org/10.1016/j.partic.2014.03.009



Fig. 5. Micro-structure of uncoated and coated urea granules, with a, b, c, and d representing respectively uncoated urea, C3, C5, and C6 granules, and a1 through d1 atmagnification of 1000× and a2 through d2 at 100×.




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ig. 6. Effect of the coating thickness on the crushing strength of C5 coated urearanules.

The long pointy crystals were tightly cemented with the finernes on the surface of the urea (Fig. 5(a1)). Some pores and gapsere also visible. As granular urea production involves agglomera-

ion, this suggests that the theoretical illustration is similar to onionayers. However, ring formation was not significantly observed. Thearticles looked jagged with clear pointy crystals seen through-ut the halved surface of the urea (Fig. 5(a2)). For C3, the particlesbserved were very edgy and rough with some bigger slabs ran-omly seen (Fig. 5(b1)). The thickness of the measured coating was43 �m, as shown in Fig. 5(b2). Fig. 5(b2) also shows that the coat-

ng layer was dense, and there was no visible sign of gaps or cavitiesn coating layer.

Relative to C3, the surface of C5 was smoother, but more cavitiesere observed, which indicated imperfection of the coating layer

Fig. 5(c1)). The larger pointy crystals observed can be concludeds the surface of urea. This indicated that the cavities provided ahannel for the urea to escape the imperfect coating. The cross sec-ion of C5 (Fig. 5(c2)) shows that the contact surface between theoating layer and the urea surface looked compact with no gaps inetween. The surface of C5 observed from this angle is relativelymooth with a lump observed at the far sight. The thickness of theoating layer was 179 �m. The surface of C6 was very coarse withery large lumps (Fig. 5(d1)). The cross section of C6, as seen inig. 5(d2), was not clearly visible. The surface was undulating. Thehickness of the coating layer was only 50 �m.

ptimization study

Based on the dissolution test results and the SEM images of theoated urea, which were used for screening the best coating mate-ial, C5 was concluded to be the best. The next step taken was toork on the effects of thickness, drying time, and wax coating on

he characteristics of C5.

ffect of the coating thicknessTo observe the effect of the coating thickness, the coating pro-

ess was performed using the same formulation of C5 but at 25%nd 30% of the total coating (%wt). The crushing strengths of theoated particles with different thicknesses are shown in Fig. 6.he crushing strength increased markedly as the coating thickness


ncreased. The highest crushing strength was achieved at the max-mum proportion of coating (30%) at 56.88 N against C5-20 at 33.34. The average results confirmed the earlier figures, where C5-30as 48.44 N and C5-10 was 22.16 N.

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ig. 7. Effect of the coating percentage on the dissolution rate of C5 coated urearanules.

Fig. 7 shows that the concentration of urea release from the C5-0% sample at the end of the 5th hour was at 1.972 mol/L against5-20% at 2.154 mol/L. This indicates that the thicker coating layerave better performance. The efficiency of C5-30% was at 21.1%, and5-25 was at 18.9%. A significant improvement in the efficiency wasbserved, where C5-20% was only at 13.8%. In comparison with C5-0%, C5-25% showed a 5.1% improvement, and C5-30% was slightlyetter at an increment of 7.3%.

To investigate the micro-structure of the thicker coated urea,EM analysis was conducted on the coated samples (Fig. 8). Longointy crystal structures were observed on the surface of C5-25%,hich indicates a coating layer gap that exposes the urea surface.

his can be seen in Fig. 8(a1). The cross section of the sample is seenn Fig. 8(a2). The thickness was measured as 218 �m. The coatingayer was very dense, and the contact between the coating layer andhe urea granule was thorough. From this angle, small lumps werebserved all over the surface. The surface of the C5-30% sampleooks very similar to the C5-25%. This was relevant as both weref the same coating formula mixture; thus, similar micro-structureharacteristics existed. Fig. 8(b1) shows that at a higher thicknessevel, the gaps go all the way from exposing the urea granule toompletely covering it. Fig. 8(b2) shows the cross section of C5-30%as a coating layer thickness of 154 �m.

ffect of drying timeTo investigate the effect of the drying time on the post-

roduction behavior of the samples, three cases were studied. Inhe control case, coated urea was left exposed at room tempera-ure to dry for one hour before packing it, whereas for the otherases, the coated samples were dried in an oven at 90 ◦C for 1, 2,nd 3 h before packing (let these times be denoted by H1, H2, and3, respectively). Fig. 9 shows the effect of the changing drying timen the crushing strength of C5 with different thicknesses. The crus-ing strength of the samples dried at room temperature was clearlyigher than those dried in the oven. The highest average crushingtrength obtained was for C5-30% and drying at room temperaturet 48.05 N. The lowest crushing strength was observed for the sam-le C5-30% dried in the oven for 3 h at 12.75 N. This may be becausehe base material used in the coating was gypsum powder. Gypsumlaster, which also acts as a binding agent, is a hemihydrate mate-


ial. This means it requires a curing time for the hydration to makehe material harder. Based on these analyses, it is concluded thathe material is best left to cure at room temperature rather thannducing drying for the best strength of coating.




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Effect of sieving the coating materialsBecause the size distribution of coating materials affects the

quality and uniformity of the coating, a two-step sieving was

ig. 8. Micro-structure of thicker C5 coated urea granules, with a and b representingf 1000× and a2 through b2 at 100×.

ffect of paraffin waxTo investigate the effect of sealant on the efficiency of the coat-

ng, three cases were considered. First, before applying molten waxn the surface of particles, C5 coated urea was heated to 30 ◦C,

wt.% molten wax was then applied onto the surface. For the nextample, the wax was introduced onto the C5 coated urea when theemperature reached 40 ◦C. For last case, the heat was directed out-ide the wall of the coating drum until constant temperature waseached, then the wax was introduced to the granules. (These casesre denoted as C5W-S1, C5W-S2, and C5W-S3, respectively) Fig. 10


llustrates the comparative dissolution rates of urea for samplesoated by paraffin wax.

After the 5th hour, the C5W-S2 sample had the lowest con-entration at 1.894 mol/L, followed by the C5W-S1 sample at

ig. 9. Effect of the drying time on the crushing strength of C5 coated urea granules.Fw

ctively C5-25% and C5-30% granules, and subscripts a1 through b1 at magnification

.031 mol/L and the C5W-S3 sample at 2.262 mol/L. The dissolu-ion rates of the coated urea were relatively linear. The efficiency of5W-S2, which was the best of three, was 24.2%, a 10.3% improve-ent compared to C5 without treated with wax. This alone showed

romising improvement of adding the wax layer. For C5W-S1 and5W-S3, the efficiency was at 18.7% and 9.5%, respectively.


ig. 10. Dissolution rates of urea release for C5 granules coated with 3 wt.% paraffinax.




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ig. 11. Dissolution rate of C5 coated with sieved coating materials in comparisonith uncoated urea granules.

dapted in the preparation procedure. The first was sieving eachf the components (70 mesh) before mixing them. After thor-ugh mixing of the coating mixture, the second sieving step waserformed before applying the coating mixture to the tumblingranules in the coating drum. Fig. 11 illustrates the results of theissolution rate of urea by applying sieving before the coating pro-ess. The release curve pattern of the C5 granules coated with sieved


oating materials was similar. Fig. 11 shows that using the sievedoating materials resulted in decreasing the dissolution rate of urea.

ig. 12. Micro-structure of C5 granules coated with sieved coating materials, withhe image magnification of (a) 100× and (b) 1000×.

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Fig. 13. Efficiencies of C5 coated urea granules with different parameters.

he surface morphology of the C5 coated with sieved materials isresented in Fig. 12.

The coated urea granules with sieved material had no cracks.he surface was smooth with minimal lumps or chunks. Althoughavities were still visible, they were more randomly spread over theurface (Fig. 12(a)). At high magnification (Fig. 12(b)), the uncoatedrea surface was still exposed. This was represented by the longerrystal-like structures. It was also observed that the particles onhe surface were finer and more uniform. Moreover, bigger chunksere seldom seen.

As a result, Fig. 13 shows the progression of the improvementhat took place in this study to enhance the properties of C5 coatedrea. It can be concluded that the sieving in the preparation of theoating mixture improved the efficiency of the urea dissolutionate. The efficiency improved to 26% compared to naked urea. Its important to note that the coating of the urea granules was con-ucted at room temperature, thus consuming the least amount ofnergy. The coating materials were in powder form, and the bindinggent was water. As a consequence, it can be conclusively assertedere that the improvement in the dissolution rate was remarkableespite the low-end technology used for coating.

onclusions

The sample C5, which was formulated based on an equal pro-ortion of gypsum plaster and sulfur, was determined to have

best efficiency of 13.8%. However, when the coating materialsere sieved thoroughly prior to the coating process, the efficiency

mproved to 26%. The experimental results also showed that byncreasing the thickness of the coating material, the strength of theoated urea granules increased, but drying the coated samples inn oven had a reverse effect on the crushing strength. Better releaseehavior of coated urea can be achieved by adding wax and sievinghe coating materials. It can be concluded that the low cost gypsumased material can be used as an alternative coating material forlow-release fertilizers.

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nggoro, D. D. (2011). Producing slow release urea by coating with starch/acrylicacid in fluid bed spraying. International Journal of Engineering & Technology,11(6), 77–80.

yyer, J. (1989). Development of gypsum coated urea at GNFC. Fertilizer News, 34(10),11–13.


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ullett, L. L., Simmons, C. L., & Lee, R. G. (1991). Sulfur coating of urea treated withattapulgite clay. Fertilizer Research, 28, 123–128.


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Comparative performance of different urea coating ...prpi.upm.edu.my/IPR/Copyright/UPM6254_7845_copyright.pdf · Please cite this article in press as: Mohd Ibrahim, K. R., et al