Encapsulation of mineral fertilizer by polysulfone using a spraying method

Transcription

1 Desalination 198 (2006) Encapsulation of mineral fertilizer by polysulfone using a spraying method Maria Tomaszewska a*, Anna Jarosiewicz b a Institute of Chemical and Environmental Engineering, Szczecin University of Technology, Pułaskiego 10, Poland Tel. +48 (91) ; Fax: +48 (91) ; maria.tomaszewska@ps.pl b Institute of Biology and Environmental Protection, Pomeranian Pedagogical University of Słupsk, Arciszewskiego 22b, Słupsk, Poland Received 3 November 2005; Accepted 15 January 2006 Abstract Studies on the encapsulation of the commercial granular fertilizer, NPK , with polysulfone (PSF) as a coating were performed. In order to improve the properties of coatings, the granules of previously coated fertilizer (wet method) were sprayed with a polymer solution or pure solvent (N,N dimethylformamide). Concentration of the polymer in solutions used for spraying was in the range of wt%. Measurements of thickness, porosity of prepared coatings and microphotographic observation of the coatings were taken. It was observed that the porosity and thickness of additionally sprayed layers was decreased by about 30% compared with the reference coating. However, the experiments showed that the above-mentioned parameters were independent of PSF concentration in the spraying solution. The release rate of nutrients as a function of time was examined at room temperature. It was observed that the application of the method of preparation of coatings resulted in a reduction of the release rate of NPK from a coated granule. Moreover, the increase of the number of sprayed polymer layers caused a decrease in the release of each macroelement by ca. 25%. The reduction of the release rate was associated with a more compact structure and lower porosity of sprayed coating compared to the reference one. Keywords: Coated fertilizers; Slow-release fertilizers; Spraying method; Polymer coating 1. Introduction Controlling the release of active ingredients in fertilizers has proven to be useful for providing *Corresponding author. valuable advantages over conventional products. The advantages include longer duration of efficacy, reduced toxicity for the crops, reduced nutrient loss and environmental loading, and reduced application costs [1,2]. Presented at the 2nd Membrane Science and Technology Conference of Visegrad Countries (PERMEA), Polanica Zdroj, Poland, September /06/$ See front matter 2006 Elsevier B.V. All rights reserved. doi: /j.desal

2 M. Tomaszewska, A. Jarosiewicz / Desalination 198 (2006) Use of various polymer materials for controlled-release fertilizer (CRF) preparation creates the possibility of the new agricultural chemicals production. The most important materials found to be suitable for coating purposes are polyolefines [3 5], polystyrene and polyurethane [6,7], polyacryloamide [8], derivatives of cellulose and many others. The method of production of physically prepared controlled-release fertilizers has been described in numerous papers [3 9]. The general idea is to cover granules of water-soluble fertilizers with an insoluble coating. Usually, the solution of polymer is sprayed on the granule of fertilizer and then the granules are dried to remove the solvent through evaporation [4,9 12]. The treatment is repeated as often as necessary until the desired coating percentage is reached. The fertilizer can also be physically blended with a polymeric matrix. In this case the mixture is subjected to sheeting on a mill and finally cured in an oven to attain a reasonable degree of crosslinking [13]. Another method involves the formation of fluid dispersion of the water-soluble fertilizer in molten wax and then dropping the dispersion in the form of droplets into water. As each droplet of the fluid dispersion comes in contact with water, it immediately solidifies and becomes a solid particle comprising a dispersion of solid fertilizer in solid wax [7]. In the following method of manufacturing physically prepared CRF fertilizer, the granules of fertilizer were added to a polymer solution and stirred at high temperature. Later, the solvent was evaporated and condensed. The coated products were dried completely [14]. In our previous papers [15,16], the new method of controlled-release fertilizer production the inversion phase technique, wet method was described. The present work is a continuation of the investigations. Studies on the possibility of combining two techniques in the fertilizer coating preparation inversion phase with the spraying method are presented. The main object was to evaluate the application of the new method on the physical properties of coating and on the release rate of macroelements from coated granules. 2. Experimental The granular water-soluble fertilizer, NPK (Chemical Plant Police, Poland), used contained KCl, NH 4 Cl and NH 4 H 2 PO 4. The mineral fertilizer was coated with a polysulfone (PSF 1700) solution in N,N-dimethylformamide using the inversion phase technique, wet method. In this method of encapsulation the granules of fertilizer were placed in a film forming solution. Next the granules with a thin layer of the polymer solution were dropped into a precipitation bath (water temperature of 294 K) where the solvent nonsolvent exchange proceeded, resulting in the formation of coatings. During the formation of the membrane, the fertilizer dissolved slightly and was diffused into the forming coating [15]. However, polymer precipitation during the immersion step was very fast and loss of fertilizer salts was insignificant. After precipitation the coated granules were removed from the bath and then dried. The encapsulation process has been previously described [15]. Multiple coatings were prepared by the immersion of the single-coated dried fertilizer granules into a polymer solution. Thereafter, the procedure was the same as during the preparation of single-coated fertilizers. In order to improve the properties of the porous coatings, the granules of precoated fertilizer were sprayed with polymer solution or pure solvent (N,N- dimethylformamide). The concentrations of polysulfone in sprayed solutions ranged from 15 to 17 wt% (Table 1). The fertilizer with double coating prepared from 18 wt% solution of PSF was placed on a movable sieve and sprayed with solvent or an adequate polymer solution. Then the granules were dried in a warm air stream. Due to the small

3 348 M. Tomaszewska, A. Jarosiewicz / Desalination 198 (2006) Table 1 Composition of coating forming solutions Coating Phase inversion, wet method Spraying method Polymer concentration, wt% Solvent, wt% Polymer concentration, wt% Solvent, wt% 18*/DMF * *+15* *+16* *+17* *Double coating. amount of sprayed polymer solution the solvent evaporation in warm air stream was very fast. Measurements were taken of the thickness and porosity of the prepared coatings and microphotographic observations of the coatings were made. The granules were cut, and the thickness of the coatings removed from the granules was measured by a micrometer. The porosity of the coating was determined by using the gravimetric method, based on the difference in weight between the water-wetted coating and dry coating [16]. The coating morphology was examined using a scanning electron microscopy (Jeol JSM-6100). Before the measurement, the coatings were frozen in liquid nitrogen, broken to obtain cross sections and then coated with a gold layer for SEM observation. The release rate of nutrients (N, P and K) as a function of time for each of the products obtained was examined at room temperature. The dynamic tests were carried out for 5 h. For these purposes, 1 g of each encapsulated fertilizer was placed in a beaker containing 50 ml of water. The mixtures were mechanically stirred. Every hour samples of solution were taken from the beakers, and the concentrations of particular ingredients were determined. The release rate was estimated as the amount of macroelement (in percent) released after time t in relation to the initial concentration of this component in the fertilizer [16,17]. The phosphate content was determined by using the colorimetric vanadate molybdenum method, and the NH 4 + and K + contents were determined by using adequate ion-selective electrodes. 3. Results and discussion Investigations previously performed [15 17] showed that the multiple coating exhibits a positive effect on the quality of the prepared coatings. The subsequent coating repairs any defects of the first polymer layer and seals its structure. As a consequence, a slower release rate of ingredients from the coated fertilizer granule can be obtained. In order to prolong the time of fertilizer release, the double-coated fertilizer (Table 1), prepared from an 18 wt% PSF solution, was sprayed with pure N,N dimethylformamide. It was supposed that a fog of the solvent would have slightly dissolved the coating surface and, after its evaporation, a more compact layer would be created. However, the experiments showed that the DMF spraying caused a destruction of the compact top layer of coating (Fig. 1). This was a very unfavorable phenomenon because the dense skin layer acts as a barrier for mass transfer and hence reduces the rate of water diffusion into the

4 M. Tomaszewska, A. Jarosiewicz / Desalination 198 (2006) Fig. 1. SEM of cross-section of coating sprayed by DMF. Right: top layer. granule and the diffusion of ingredients outside the granule. As was suspected, the release rate of the NPK from granules additionally sprayed with the solvent increased even by almost 100%. This experiment pointed out the particular role of the dense top layer of the coating. In the next step, the coated fertilizer was sprayed with the PSF solution. The amount of sprayed polymer was so small that the solvent evaporated very fast in a stream of warm air. It was observed that the spraying of coatings previously formed by the phase inversion technique wet method made them denser. Porosity (Fig. 2) and the thickness of additionally sprayed coatings were decreased by about 25% and 30%, respectively, in comparison with the reference coating. Moreover, the experiments showed that neither of the investigated parameters was dependent on PSF concentration in the spraying solution. The thickness of the coatings was equal to 0.47 mm and the porosity was about 20% (Fig. 2), regardless of the PSF concentrations in the spraying solution applied. The SEM observation (Fig. 3) shows that the sprayed polymer solution partially dissolved the top layer of precoating and a dense barrier for fertilizer nutrients was formed. The release rate of Fig. 2. Thickness and porosity of prepared PSF coatings. N, P, and K through the polymer coating was determined during the 5-h experiments. The release was controlled by the mass transfer properties of the coating material, properties of the active agent (fertilizer) within the granule and coating structure. It can be controlled by the thickness and porosity of coating [15 17]. The polymer coating structure is a crucial parameter. This property limits the diffusion of water into the fertilizer granule and of nutrients out of the granule. Fig. 4 shows the release of water-soluble fertilizer from the coated granules. As can be seen, the applied method of preparing coatings resulted in a decrease of the release rate of N, P, and K from the coated granule. In the case of

5 350 M. Tomaszewska, A. Jarosiewicz / Desalination 198 (2006) Fig. 3. SEM of cross-section of double coating, twice sprayed by 17 wt% PSF solution. Right: top layer. Fig. 4. Influence of PSF concentration in spraying solution on the release rate of Σ NH 4 +, K + and phosphates. *, double coating. coated fertilizer sprayed by a 15 wt% polymer solution (signed 18*+15*), 22.3% of NH 4 +, 22.3% of phosphates and 6.8% of K + were released after 5 h of testing. In comparison with the reference fertilizer (18*), the release of NH 4 +, phosphates and K + was decreased by about 40%, 55% and 60%, respectively. The course of curves suggests that the release of fertilizer was independent of PSF concentration in a spray solution. As seen in Fig. 4, the amount of nutrients released from the coated fertilizer was the same for each case. This effect was associated with the same porosity, thickness (Fig. 2) and probably morphology of the coatings. Fig. 5 illustrates the influence of the number of sprayed layers on the release of active watersoluble nutrients. As can be seen, the increase of the number of sprayed layers forming the coating causes the macroelements to be released at a slower rate. The difference in the amounts of N, P, and K released between the single-sprayed fertilizer with 15 wt% PSF solution (18*+15) and for the double-sprayed granules of fertilizer (18*+15*) was about 25% for all measured components. The thin-skin layer of double-sprayed coating was probably more compact and dense, thereby reducing the diffusion of water into granules, the dissolution of ingredients and transfer outside the encapsulated granule.

6 M. Tomaszewska, A. Jarosiewicz / Desalination 198 (2006) rate from coated granules. The release of fertilizer was independent of the PSF concentration in the spraying solution: for every case the amount of nutrients released from the coated fertilizer was the same. This effect was associated with the same porosity, thickness and probably morphology of coatings. The increase of the number of sprayed polymer layers forming the coating caused a 25% decrease of the release rate of macroelements. References Fig. 5. Dependence of the macroelement release rate on number of sprayed polymer layers. 4. Conclusions The spraying of coatings previously formed by the inversion phase technique wet method made them denser. Porosity and thickness of double sprayed coatings were decreased by about 20% in comparison with the reference coating. It was also observed that neither of these parameters was dependent on PSF concentration in the spraying solution. The applied method of preparaing coatings resulted in a decrease of the N, P, and K release [1] K.L. Smith, B. Holmes and F. Matthew, Asymmetric microporous beads for controlled release, EP , [2] A. Ombodi and M. Saigusa, Broadest application versus band application of polyolefin-coated fertilizer on green peppers grown, J. Plant Nutr., 23(10) (2000) [3] N. Cosuge and K. Tobataku, Coated granular fertilizers, EP , [4] O.A. Salman, Polyethylene-coated urea. 1 Improved storage and handling properties, Ind. Eng. Chem. Res., 28 (1989) [5] T. Posey and R.D. Hestar, Developing a biodegradable film for controlled release of fertilizer, Plastics Eng., 1 (1994) [6] W.P. Moore, Reinforced-resin coated fertilizer granules, US , [7] G. Pipko, Method for manufacture of slow-release fertilizers, EP , [8] A.J. Rajsekharan and V.N. Pillai, Membraneencapsulated controlled-release urea fertilizers based on acrylamide copolymers, Appl. Polym. Sci, 60 (1996) [9] B. Gordonov, A. Shoviv and E. Zlotnikow, Manufacture of fan encapsulated slow-release particulate fertilizer, US , [10] J. Wolstenholme, D.G. Pauly, M. Nyborg and E. Solberg, High efficiency controlled release phosphate-based fertilizer, EP , [11] M.C. Garcia, A. Vallejlo, L. Garcia and M.C. Cartagena, Manufacture and evaluation of coated triple superphosphate fertilizers, Ind. Eng. Chem. Res., 36 (1997)

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