Synthesis and Application of Urea-formaldehyde for Manufacturing a

Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 1593−1606

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Synthesis and Application of Urea-Formaldehyde for Manufacturing a Controlled-Release Potassium Fertilizer Yanle Guo,† Zhiguang Liu,*,† Min Zhang,*,† Xiaofei Tian,† Jianqiu Chen,‡ and Lingli Sun§

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†

National Engineering Laboratory for Efficient Utilization of Soil and Fertilizer Resources, National Engineering & Technology Research Center for Slow and Controlled Release Fertilizers, College of Resources and Environment, Shandong Agricultural University, Tai’an, Shandong 271018, China ‡ State Key Laboratory of Nutrition Resources Integrated Utilization, Kingenta Ecological Engineering Group Co., Ltd., Linshu, Shandong 276700, China § Zhongde Fertilizer (Pingyuan) Co. Ltd., De’zhou, Shandong 253100, China S Supporting Information *

ABSTRACT: Few studies have paid attention to highconcentration controlled release potassium (CRK) fertilizer because conventional potassium chloride (KCl) particles with the characteristics of irregular, high specific surface area, and bad fluidity are generally unsuitable to produce controlledrelease fertilizer. The objective of this study was to investigate the interacting effects of urea-formaldehyde and additives on KCl granulation. In addition, the controlled-release characteristics of CRK based on modified KCl particles were determined in our research. Results indicated that 4−8% urea-formaldehyde combined with 6−8% bentonite was used as a double binder to increase the granulation rate, smoothness, and particle hardness of KCl granules, which enhanced its characteristics for the coating process. The K release rates of modified KCl particle-based CRK were significantly lower than that of the conventional KCl particle. In conclusion, the novel KCl granulation technology has an enormous potential for large-scale applications to satisfy the increasing demand for CRK fertilizers in the future.

1. INTRODUCTION Potassium (K) is one of the essential elements for plant growth and development,1,2 but only part of the ordinary K fertilizer can be absorbed and utilized by plants in the soil after fertilization.3 Adding to this challenge, the K reserves of China are scanty,4 and soil K deficiencies have increased across 30 provinces in China.5 According to the FAO, the global demand for potash is expected to grow by 2.6% from 2014 to 2018.6 How to improve the nutrient utilization of potassium fertilizer has thus been one of the hot topics in recent plant nutrition and fertilization research.7 Meanwhile, the production of some long-growth-stage crops often incorporates K fertilizer as a basal fertilizer, resulting in asynchronous potassium supply and demand bioregulation and creating conditions for premature aging of crops due to lack of potassium at the later growth stages. In addition, it is not easy to add fertilizer at the late growth stage of crops. Moreover, increasing the fertilization is not conducive to improvement of economic benefits. The main types of potash fertilizer on the market are potassium chloride (KCl) and potassium sulfate (K2SO4). Potassium chloride has high potassium content, and its price is generally far lower than that of potassium sulfate.8 It has been the main potassium fertilizer used in modern agricultural © 2018 American Chemical Society

production. However, the salinity coefficient of potassium chloride is higher than that of K2SO4,9 and it is not generally conducive to seed germination or seedling growth when it is applied to poorly drained saline or arid soils. Moreover, as some crops are sensitive to chloride or may even be termed as “chlorine-avoiding crops”, they are thus not suitable for applications of KCl or low-quality formulations of K2SO4.10,11 As a new technology, controlled-release fertilizers (CRFs) have developed rapidly in recent years, with controlled-release nitrogen-based fertilizers leading the way.12 Furthermore, potassium-based CRFs have been shown to improve crop yields and nutrient use efficiency in a number of production systems. Controlled-release potassium chloride can improve the utilization rate of fertilizer,4 as well as meet late crop demand for fertilizer.13 Potassium chloride, with its relatively low price and plant-available K, offers an attractive opportunity for the slow release of chloride and potassium ions in the coated fertilizer, as limitations are solved in its wider application. Received: Revised: Accepted: Published: 1593

November 7, 2017 January 3, 2018 January 11, 2018 January 11, 2018 DOI: 10.1021/acs.iecr.7b04629 Ind. Eng. Chem. Res. 2018, 57, 1593−1606

Article

Industrial & Engineering Chemistry Research

most commonly used slow-release fertilizers worldwide. It has good slow release properties, it can promote the formation of good soil aggregate structure, improved soil permeability, and increased crop root penetrating power, and nitrogen use efficiency can be as high as 50% or more.28 In the soil, UF can be hydrolyzed into ammonium nitrogen, carbon dioxide, and water by microorganisms, which facilitates absorption and utilization directly by plants.29 Urea-formaldehyde fertilizer can also achieve complete degradation in soil, representing an environmentally friendly approach, so the slow-release fertilizer has a unique advantage. Previous studies of our research group have proven the ability of polymer-coated potassium chloride to increase crop yields and K use efficiency.4,7 Therefore, based on these findings, and as a next stage, a granular fertilizer suitable for the coating of potassium chloride is needed, and a suitable combination of urea-formaldehyde and additives is viewed as the best approach to providing such products. The results will provide a scientific basis for fertilizer technology innovation regarding sustainable and efficient application of K fertilizer. Taking into account earlier studies, this study aimed to examine the effect of the variables of a disc granulation process30 such as UF binder to powder mass ratios and additives to powder mass ratios, etc. Within this context, UF resin can not only be used as a potassium chloride granulation binder but also provide slow release of nitrogen. Thus, using urea-formaldehyde and an auxiliary material as binders, optimization of their combination ratio can be studied, with the appropriate adding time frame, to help ensure the optimal amount of the effective potassium content and, at the same time, improve the granulation rate, particle hardness, surface smoothness, etc. Hence, suitable potassium chloride granules need to be selected for preparation of controlled-release potassium chloride.

The development of controlled-release potassium chloride products is relatively slow, and the main cause of this is as follows. First, the existing potassium chloride is mainly extrusion granulation and crushed potash ore on the market, which has an uneven surface, irregular edges, and corners. These properties present certain difficulties when the material is processed into coated controlled-release potassium chloride. One major problem is that the nutrient release is too fast at the edges and corners. Second, in making potassium chloride powder granulation, it is very difficult because it lacks binding and plasticity properties. However, to prepare coated-potassium chloride with good controlled release effect, it is necessary to carry out proper granulation. Making the powderized KCl into 3−5 mm spherical particles,14 the coated controlled-release potassium performance would be better at controlling the release of nutrient. Moreover, the spherical granular fertilizers have advantages in storage, transportation, and seeding performance, being well suited for mechanical sowing and fertilization. Granulation is a process in which small particles accumulate and enlarge under controlled conditions. This is a key process in industries that work with products such as agrochemicals, processed foods, agricultural products, pharmaceuticals, detergents, etc. The critical particle properties have important effects on particle size distribution, product quality, and porosity.15−17 The granulation of fertilizer is a process of converting powdered material into particles accumulated and enlarged which can reduce fertilizer loss and improve utilization rates; granulized fertilizer can also be easily implemented in mechanized fertilization applications.18 Granulation also prevents dust problems during transportation and application of ash.19 The granulation process mechanism includes three phases: wetting and nucleation, growth and consolidation, and attrition and breakage.20,21 Granulation is, in essence, the set of natural and physical-mechanical processes which take place during the formation of little pieces, which exhibit distinct dimensions in terms of ranges, forms, structures, and physical properties. Granulation allows significant simplification of the storage, transport, and dosage functions; moreover, it increases powdering qualities while both eliminating dusting and improving the working conditions in the production sphere; besides that, it can regulate granule structure and related properties. The granulation process itself depends on the feedstock grain size and physical-mechanical properties.22 This process can be conducted in a granulator, and this method allows a wide variation in the composition of material in fertilizer. However, these works were concerned with highshear and drum granulation. Disc granulation has rarely been the subject of laboratory studies.23,24 Owing to the poor adhesion and plasticity of potassium chloride powder, it is necessary to add some form of binder to obtain spherical particles with smooth surfaces and high hardness. The cost of the adhesive and its effect has become a major issue of powdered potassium chloride granulation production. Urea-formaldehyde (UF) resin, a polymer formed by urea and formaldehyde, has strong adhesion properties and low cost and is used in the field of wood processing mainly.25 When its molar ratio of urea to formaldehyde is about 0.50− 0.67,26 it does not have a fertilizer effect that can release nitrogen. However, when the molar ratio of urea to formaldehyde is 1−2, UF resin can be used not only as a potassium chloride granulation binder but also as a slow release nitrogen fertilizer.27 Urea-formaldehyde has been one of the

2. EXPERIMENTAL SECTION 2.1. Materials. Urea (N 46%) was obtained from Shandong Luxi Chemical Co., Ltd., China. Powder potassium chloride (K2O 60%) was obtained from Qinghai Salt Lake Potash Fertilizer Co., Ltd., China. Formaldehyde solution (37%), RedBull potassium chloride (K 2 O 60%), gypsum powder, bentonite, superphosphate, and leonardite were all obtained from Zhongde Fertilizer (Pingyuan) Co. Ltd., China. 2.2. Synthetic Urea-Formaldehyde Binder. Urea-formaldehyde resin binder was prepared as follows: First, 60 g of urea was mixed with 60 g of 37% formaldehyde solution at 50 °C in a 500 mL, three-neck flask equipped with a thermometer, a reflux condenser, and a stirring rod. After the urea was dissolved, the pH of the system was adjusted to 9.0 with NaOH solution. The reaction was completed under a water bath at 50 °C for 1.5 h. The pH of the system was adjusted to 5.0 with HCl solution after first-step polymerization. After the solution was agitated and reacted for 1 h, all the products were transferred to glass dishes, dried at 90 °C in an oven, and then passed through a 16 mesh screen as binder material and for further analysis. The reaction equation is listed in Scheme 1.31,32 2.3. Granulation. A house-made disc granulator (1.0 m in diameter with a variable tilt angle from 30° to 60° and a manual speed adjustment), as shown in Figure 1, was used to perform experiments with a total of 5 kg of raw material per batch. The production flowchart was shown in Figure 2. Depending on the formula (revolving speed = 0.8 × 32 × √(sin Ø/D)) with tilt angle as 45°, the revolving speed was set as 21.5 r/min. All 1594

DOI: 10.1021/acs.iecr.7b04629 Ind. Eng. Chem. Res. 2018, 57, 1593−1606

Article

Industrial & Engineering Chemistry Research Scheme 1. Reaction between Urea and Formaldehyde

A=

Wp Wk + Was + Wuf + Wm

× 100%

where Wp is the total weight of the product 2−5 mm spherical granules, Wk is the weight of the additive potassium chloride powder, Was is the total weight of the additive auxiliary substances, Wuf is the weight of the additive UF binder, and Wm is the weight of the additive moisture. 2.4.2. Particle Hardness. Crushing strength is an index used to characterize granule hardness. For each test in our study, 20 granules were sampled from the final product. A granule hardness tester (Yinhe Instrument Factory, Jiangyan, China) was used to measure the granule crushing strength by applying an increasing compressive force on a single granule. The tester recorded the compressive force when the granule was crushed. In the fertilizer industry, the Newton is the unit used for the granule crushing strength. 2.4.3. pH. Ten grams of sample was taken and placed in a 100 mL beaker. Fifty milliliters of carbon dioxide water was stirred for 1 min and allowed to stand for 30 min, and then pH was tested by acidometer (Leici PHSJ-3F, Shanghai, China). 2.4.4. Surface and Cross-Section Characteristics of Granule. Scanning electron microscopy (SEM, JEOL JSM6700F, Japan) was used to examine the particle surface and cross-section characteristics. 2.4.5. Moisture Absorption Rate. For each experiment, 10.00 g of fertilizer particles was placed in Petri dishes, with culture conditions maintained at 60% relative humidity and 25 °C. Change of weights was recorded after 4 h. The overall moisture absorption rate B was calculated by the following equation W − W1 × 100% B= 2 W0 where W0 is the weight of fertilizer particles, W1 is the weight of the fertilizer particles and Petri dish, and W2 is the weight of the fertilizer particles and Petri dish after 4 h cultured. 2.5. Preparation of Coated Potassium Chloride. Using 1 kg of the potassium chloride granules (2−5 mm in diameter), membrane coating materials were prepared for each sample. First, all the fertilizer was heated at 70 °C in a drum for 10 min, and then 10 g of coating material (mixed with 49.9% LCS, 49.9% PM-200, and 0.2% diethylenetriamine) was added to the surface of the rotating potassium chloride granules. Then the coating material was finished through the curing reaction in 8 min.14 Six types with different coating rates of controlled release potassium treatments (CRKs) and controlled release Red-Bull potassium treatments (CRRKs) (i.e., CRK1, CRK2, and CRK3; CRRK1, CRRK2, and CRRK3) were prepared, with repetition of the coating step for 2, 3, and 4 times, respectively (Figure 2). 2.6. Characterization of Different Coated Potassium Chloride Samples. Scanning electron microscopy (SEM, JEOL JSM-6700F, Japan) was used to examine the morphologies of different coated potassium chloride samples. 2.7. Potassium Release of Coated Potassium Chloride. Next, 10.00 g of coated fertilizers (CRK and CRRK) was added into a gauze bag and placed in a glass bottle with 500 mL deionized water; each fertilizer type had 27 replicates. Maintained at a constant incubation temperature of 25 °C, three replicates were sampled at 1, 3, 5, 7, 10, 14, 28, 42, and 56 d after incubation, respectively, and the weight of the remaining fertilizer was determined until CRK and CRRK accumulated

Figure 1. Disc granulator.

materials were passed through a 1 mm sieve, and the powdered potassium chloride and additives were added to the pelletizer. Then the UF was mixed with water to prepare a mixture of urea-formaldehyde suspension and sprayed onto the material through the atomizer evenly. The moisture content was controlled at the same value as the total powdered potassium chloride. Screened once every 15−20 min as the suitable particle appeared (2−5 mm spherical granules), the granulation time in this work was about 240 min. With the appropriate timing of adding and nebulization effect, the best granulation effect can be achieved, which is a fixed proportion screened on the basis of a large number of trial tests. 2.4. Characterization of Granulated Products and Particle Selection. The equipment used to conduct the characterization included the following: field emission scanning electron microscopy (SEM Model JSM-7500F, Japanese Electronics Corp., Japan), an electric thermostatic drier (Model 101-A, Shanghai Sunshine Experimental Instrument Co., Ltd., China), a pH meter (PHS-3C, Shanghai Hongyi Instrument Co., Ltd., China), a granule hardness tester (Yinhe Instrument Factory, Jiangyan, China), standard sieves (0.88, 1.50, 2.35, 3.35, and 4.75 mm), and a constant temperature rapid extraction instrument (Tai’an, China).33,34 2.4.1. Granulation Rate Calculation. The overall granulation rate A was calculated using the following equation 1595


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Figure 2. Diagram of the production process and mechanism for manufacturing controlled release granule potassium chloride.

potassium chloride with 3% gypsum powder accretion. Thus, it can be seen that moisture is an important factor, so that under the same conditions of moisture addition amount, the granulation rate with 8% UF and 3% gypsum powder is lower than that with 6% UF and 3% gypsum powder. A critical issue was that a better rate of granulation in ensuring the presence of more effective potassium should be explored, which meant to add as little additional binder as possible, so we determined that the optimal addition amount of UF to be added was 4−6%. All samples were prepared by controlling the same addition of the moisture content (12%), thus the granulation rate decreased slightly when the UF addition was 8%. In actual production, the rate of granulation can be increased if moisture is guaranteed enough. In addition, the particle hardness of the potassium chloride increased markedly with the increase of the addition amount of UF. While the addition amount of UF slow-release coating was 0, the particle hardness was 9.49 N; and with the 3% gypsum powder on this basis, the particle hardness was yet only 15.98 N. The pH of the fertilizer granules decreased when the addition amount of UF increased. Gypsum powder can be viewed as an auxiliary substance in this test. The difference between CK1 and CK2 was that CK2 added 3% of gypsum powder, and all of them did not add ureaformaldehyde binders. Under the condition of adding UF, GUF was mixed with gypsum powder but ZUF was not. From the above, with the addition of the gypsum powder, the particle surface characteristics become coarse (Figure 4). This may be attributed to the evaporation of moisture from gypsum, leading to unstable granulation conditions and bringing about granules with rough surfaces.18 In the same way, as an auxiliary substance, granulation effect with gypsum was less than ideal. From the depiction in Figure 4 of the cross-section surface

nutrient release reached 80%. The fertilizer release period is set to the day that K release reaches 80%. 2.8. Statistical Analysis. SAS version 9.2 (SAS Institute, Cary, NC) was used for all statistical analyses, and average values of each treatment were calculated. Regression equations and coefficients were calculated between the granulation rate or particle hardness of granular potassium chloride, UF addition amount, and gypsum powder addition amount. The differences among means and correlation coefficients were considered significant when P < 0.05.

3. RESULTS 3.1. Selection of UF Binder. All the potassium chloride fertilizer granules with different UF addition are shown (Figure 3.A), and particle surface, cross-section characteristics (Figure 4) are shown, respectively. Table 1 shows the granulation results obtained from small-scale tests in the presence of different addition amounts of UF, showing the dependence of the granulation rate and hardness. Among the binders tested, the best addition amount of UF was 8% of the total powder potassium chloride, yielding a granulation rate of 31.53%, granular particle hardness of 90.12 N, and smooth particle surface. Obviously, though the granulation rate increased with the addition of the UF, the value of the granulation rate was too low yet. Adding gypsum powder as an additive binder to the powder potassium chloride granulation is practiced. Therefore, on this basis, powder gypsum, which was 3% of the total powder potassium chloride, was added to powder potassium chloride granulation (Table 1). Distinctly, the accretion of gypsum powder increased the granulation rate at the same rate as UF. On account of making the same value of the water addition, the best granulation rate was 6% of the total powder 1596


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Figure 3. (A) Photograph of various uncoated granule particles and (B) photograph of various coated fertilizers. Abbreviation interpretation: CK1 (adding nothing), CK2 (only adding 3% gypsum), ZUF (only adding UF), GUF (adding UF and gypsum), CAS (adding UF and superphosphate), HA (adding UF and leonardite), BEN (adding UF and bentonite), KIE (adding UF and kieselguhr), GES (adding UF and gypsum powder), CRK (controlled release potassium), CRRK (controlled release Red-Bull potassium).

and the effects of particle hardness, surface properties, and moisture absorption are also considered. Similarly, the relationship between particle hardness, UF addition amount, and gypsum powder addition amount has a close correlation (Figure 5-B). The addition of UF could increase the particle hardness and density obviously, which could be reflected in the cross-sectional structure of the fertilizer (Figure 4). Under the same UF addition, the particle hardness decreases with the increase of the amount of gypsum powder. Different moisture absorption rates were caused by different compositions of particles (Figure 6-A1A2). All the particles

characteristics of CK1, CK2, GUF, and ZUF, it was shown that the degree of packing became compact with the addition of gypsum powder, especially in GUF, and its particle hardness improved only slightly. The granulation rate had a very close relationship (R2 = 0.91) between UF addition amount and gypsum powder addition amount (Figure 5-A). Both promoted the rate of granulation. This correlation showed that the granulation rate could be predicted by using the 3D fitting model. To achieve a better granulation effect, it is necessary to adjust the ratio of UF and additives (gypsum powder) in the next optimization design, 1597


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Figure 4. Particle surface characteristics of (A1) CK1, (B1) CK2, (C1) GUF4, and (D1) ZUF4 and the cross-section surface characteristics of (A2) CK1, (B2) CK2, (C2) GUF4, and (D2) ZUF4.

were cultivated under the conditions of 25 °C and air relative humidity of 60%. Apparently, the max value of moisture absorption rate was the CK2 after being cultivated 4 h, and the next was CK1. The minimum value of moisture absorption rate

was the GUF1, which was mixed with the maximum addition amount of UF slow-release binder. 3.2. Selection of Additives. As granulation effect of gypsum powder was not well demonstrated, the screening and 1598


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Table 1. Granulation Results Following Use with Different Addition Amounts of UF Slow-Release Binder and Gypsum Powder treatment

addition amount of UF binder (%)

addition amount of gypsum powder (%)

granulation rate (%)

particle hardness (N)

pH

particle surface characteristic

CK1 ZUF1 ZUF2 ZUF3 ZUF4 CK2 GUF1 GUF2 GUF3 GUF4 GUF5 GUF6

0 2 4 6 8 0 2 4 6 8 4 4

0 0 0 0 0 3 3 3 3 3 6 8

8.96 14.33 26.22 26.34 31.53 15.98 33.97 40.52 44.28 38.86 50.77 57.65

9.49 32.90 48.69 85.23 90.12 12.24 68.69 73.14 87.29 93.44 58.93 49.68

9.24 9.15 9.13 8.62 8.48 9.10 8.70 8.68 8.41 8.29 8.65 8.64

tiny smooth smooth smooth smooth tiny tiny tiny tiny tiny tiny tiny

Figure 5. Relationship between the granulation rate or particle hardness of granular potassium chloride, UF addition amount and gypsum powder addition amount (A, z = 9.9932 + 2.8275x + 4.6128y, with R2 = 0.91; B, z = 18.4049 + 9.8647x + 0.5336y, with R2 = 0.83; x was the addition amount of UF, and y was the addition amount of gypsum powder).

optimization of additives such as superphosphate, leonardite, kieselguhr, and bentonite, which are usually used in the chemical fertilizer industry, were studied. All the formulations of potassium chloride fertilizer granules with different auxiliary substance additions are shown in Figure 3.A, and particle surface cross-section characteristics are shown in Figure 7, respectively. Owing to the optimized addition amount of UF slow-release binder being 4−6%, the addition amount of 4% was selected in this test. Table 2 shows the granulation results obtained from small-scale tests in the presence of different addition amounts of different auxiliary substances, showing the dependence of the granulation rate, pH, and hardness. According to the results, the granulation rate with the superphosphate was much the same condition as with the gypsum powder. However, the bentonite exhibited the best granulation rate of all additives in this paper. At the same added quantity, its granulation effect was better than the gypsum powder and superphosphate, which were at the same level effect. The granulation rate of kieselguhr decreased with increased amounts, and the granulation rate of leonardite increased insubstantially with its addition. On the other hand,

the granulation effect of leonardite was poorer than the gypsum powder. Owing to the fact that all tests were prepared by controlling the same addition of the moisture content, thus the granulation rate was limited by it. In actual production, the rate of granulation can be increased if moisture is guaranteed enough. In addition, what needs to be noted is that by controlling the moisture content of potassium chloride fertilizer particles, the drying process has less energy consumption, and the prepared fertilizer particles have smooth surfaces and efficiently applied coatings. Particle hardness is an important indicator that concerns transportation, storage, and fertilizer coating. The particle hardness with gypsum powder was higher than the others at its addition amount of 3%, but the particle hardness with bentonite was higher than the others when its addition amount was 6% and 8%. Although the granulation rate with superphosphate was well enough, its particle hardness was significantly less than others. As the auxiliary substance changes, the pH of the fertilizer particles varies slightly. Especially when the auxiliary material is superphosphate, owing to its character, the granules were pH < 1599


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Figure 6. Moisture absorption rate with different (A1, A2) UF additions and (B1, B2) additives.

between the moisture absorption rate and the granulation rate and to improve the content of available potassium and smooth surface of particles. Because the differences between moisture absorption rate of bentonite and superphosphate were not significant, bentonite was selected as the optimized auxiliary substance. 3.3. Characteristics of Coated Potassium Chloride. 3.3.1. Morphologies of the Controlled-Release Potassium Chloride. Most of the existing potassium chloride granules under commercial production are extruded granulated or broken with potassium minerals. This kind of particle has an irregular shape, with many edges and corners. The surface of the particle is rough, easily sags, and crests and is raised or sunken. This kind of bulge or depression can easily cause the coating effect to be greatly reduced. Excessive coating material is used to fill the dent, and large bulges and edges of the coating material are not easy to coat or are too thin, resulting in a significant reduction in coating effect; in this case the initial release rate is too high and cumulative release rates are not able to reach standard requirements. This kind of fertilizer is difficult to receive direct coating in the making of coated potassium chloride. When considering the manufacture of coated potassium chloride, the process must overcome its own shortcomings in terms of lack of cohesion and plasticity and in manufacturing the granule potassium chloride first. Therefore, the optimum combination of urea-formaldehyde resin coating and auxiliary material in our test was used to prepare

7. The remaining fertilizer granules with other auxiliary substance were pH > 8. The particle surface characteristics also were different (Figure 7). From this we can see that all of the particle surface characteristics were smooth correspondingly except for the particles with leonardite, kieselguhr, and gypsum powder. The smooth surface facilitates fertilizer coating and saves coating material. When the surface is not smooth, the coated material will be filled first, which increases the use of membrane material. The cross-section surface characteristics with different additives were different too (Figure 7). Obviously, the crosssection surface characteristic with bentonite was the densest (Figure 7-A2), and this also proves the hardness of its particles. The cross-section surface characteristics of the other additives were the same as each other. Different moisture absorption rates were caused by different composition of particles (Figure 6-B1B2). All of the particles were cultivated under the condition of 25 °C and air relative humidity of 60%. The moisture absorption rates of BEN3, CAS3, KIE3, and HA3 treatments were 10.01, 9.78, 7.43, and 5.44%, respectively, and the performance of BEN3 and CAS3 was higher than the others. The results here may be caused by their water absorption characteristics. Both bentonite and superphosphate more easily absorbed water than leonardite and kieselguhr and increased their granulation rate also. The purpose of the experiment was to deal with the relationship 1600


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Figure 7. Particle surface characteristics with different additives of (A1) BEN3, (B1) CAS3, (C1) HA3, and (D1) KIE3 and cross-section surface characteristics with different additives of (A2) BEN3, (B2) CAS3, (C2) HA3, and (D2) KIE3.

granule potassium chloride, which is suitable for coating. This formulation must be considered with respect to the directcoated Red-Bull potassium chloride. The various coated potassium chloride fertilizers are shown in Figure 3-B. The SEM images illustrate the morphologies of coated potassium chloride surfaces and cross sections (Figure 8). The surface of CRK (Figure 8-A2) was relatively smoother than the CRRK (Figure 8-B2). Images indicate a good quality coating of the CRK and a poor quality coating of CRRK at the corner angles from the SEM images of their respective cross sections, and the corner angle surface of the CRK (Figure 8-C1, C2) was denser than the CRRK (Figure 8-D1, D2). The above results are caused in part by the surface of RedBull potassium chloride, which has many depressions and

irregular edges and corners (Figure 8-B1). Figure 9 shows a diagrammatic sketch of coated effects for (A) Red-Bull and (B) granule potassium chloride. At the tagged location “a” in Figures 8 and 9, depressions caused more coating material to be filled, or the coating material will be preferentially filled first. Under the condition of the same envelope release stage, the coating effect is reduced or could not reach the controlled release effect. In the same volume, the surface area of the sphere is the smallest, thus ball pellet fertilizer is more economical for the use of coated materials. At the tagged location “b” in Figures 8 and 9, irregular edges and corners make it more difficult to achieve a good envelope, causing a poor controlled-release effect owing to the “bucket effect”. At the same time, when fertilizer is manufactured, 1601


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Table 2. Granulation Results Following the Use with Different Addition Amounts of Different Additives in Laboratory Testing of Potassium Fertilizer Formulations treatment

additives

addition amount of additives (%)

granulation rate (%)

particle hardness (N)

pH

particle surface characteristic

CAS1 CAS2 CAS3 HA1 HA2 HA3 BEN1 BEN2 BEN3 KIE1 KIE2 KIE3 GES1 GES2 GES3

superphosphate superphosphate superphosphate leonardite leonardite leonardite bentonite bentonite bentonite kieselguhr kieselguhr kieselguhr gypsum powder gypsum powder gypsum powder

3 6 8 3 6 8 3 6 8 3 6 8 3 6 8

41.93 51.64 55.65 25.71 27.30 32.15 53.45 59.47 70.97 47.90 36.85 28.48 40.52 50.77 57.65

10.66 22.14 25.13 30.53 43.58 44.58 21.97 65.20 69.18 46.21 63.24 68.43 73.14 58.93 49.68

6.60 6.14 6.06 8.68 8.83 9.16 8.50 8.49 8.54 8.31 8.35 8.37 8.68 8.65 8.64

smooth smooth smooth rough rough rough smooth smooth smooth rough rough rough rough rough rough

can be improved by using urea-formaldehyde and also make the moisture absorption rate decreased. This is related to the properties of its polymer compounds38 and is consistent with the study of H. M. Goertz.39 However, under the present test, when the urea-formaldehyde content was increased to 8%, the granulation rate decreased, which may be due to the limitation of moisture. Moisture is an important factor in the granulation process.40−42 If the moisture content is enough, the granulation rate will be further improved. Additionally, the total nitrogen content of modified KCl granules was 2.40−3.20%, due to that of UF fertilizer binder being 40.01%. Besides, the N stated release longevity of UF fertilizer binder was 56 days in our research. Under the test condition with a 25 L reactor, 12.0 kg of UF binder was obtained after 20 kg of raw materials produced for 3.5 h, namely the productivity was 3.43 kg/h on the basis of dry basis. Single KCl granulation just requires urea-formaldehyde, but this form is not sufficient and requires accessory substances for effective use as a fertilizer.43,44 The use of accessory substances (clay, bentonite, others) can increase the rate of fertilizer granulation and is a common practice in granulation development.45 In this experiment, the additives (superphosphate, leonardite, bentonite, kieselguhr, gypsum powder) were used as the object of study. Bentonite shows excellent effects (in terms of particle size, particle hardness, surface and cross-sectional properties), followed by superphosphate and gypsum powder. This is due to the excellent nature of bentonite, exhibiting swelling and adsorption after water absorption, so that small particles are easily gathered.46,47 This is consistent with the study of J. Kamińska.48 At the same time, as the process is also controlled by the moisture content, the resulting granulation rate has a lot of room for improvement. The proportions of urea-formaldehyde and accessory substance, adding time,49 water supply,50 speed and angle of disc granulator, etc., all affect the granulation effect. This experiment controlled for other factors; for example, all tests controlled added moisture factors, resulting in a low rate of granulation (the highest granulation rate was 70.97% in our test). Owing to all tests being prepared by controlling the same addition of the moisture content, thus the granulation rate was limited by it. In actual production, the rate of granulation can be increased if moisture is guaranteed enough. In addition, what needs to be noted is that by controlling the moisture content of

transported, and applied, the coated membrane material at the irregular edges and corners is easily damaged or destroyed. 3.3.2. Composition and K Release Characteristics of CRK and CRRK. The coating content and total K content of the coated fertilizers are presented in Table 3. Obviously, the initial release rate and subsequent cumulative release rate decreased as the amount of coating increased. Initial release rate of nutrient was an important indicator indicating the coated effect. The respective K nutrient release rates of CRK1, CRK2, and CRK3 were 58.45%, 34.30%, and 11.20% in 24 h, and their corresponding amounts of coating were 1.95% (CRK1), 3.19% (CRK2), and 4.07% (CRK3), respectively. The cumulative release rate of potassium was 89.10%, 70.63%, and 52.73% at the 28th day (Figure 10). Similarly, the nutrient release characteristics of CRRKs have significant effects on different coating rates. Within 24 h, the respective K nutrient release rates were 91.75% (CRRK1), 83.65% (CRRK2), and 75.50% (CRRK3), which were far above the international controlled-release fertilizer standard (ISO 18644:2016 [E]), and their corresponding amounts of coating were 2.26%, 3.25%, and 4.18%. The initial release rate of nutrients was far above the standard. Obviously, all the cumulative release rates of CRRKs reached 80% at third day. Correspondingly, the cumulative release rate of granule potassium chloride, with the same coating, is obviously increased (Figure 10). The above results showed that the CRRKs’ release rates were significantly higher than those of CRKs’ at the same coating thickness, which was due to the thinner coating at CRRK edges; that means a lower coating efficiency of CRRKs. Since CRKs were spherical and uniform particles, the nonuniform coating situation did not appear, so the release cycle was longer, thus saving membrane material under the same controlledrelease period.

4. DISCUSSION Urea-formaldehyde has become one of the most commonly used slow-release fertilizers worldwide;35 similarly, it has been widely used as a wood binder.36,37 It is a long-chain macromolecular compound synthesized by the condensation of urea and formaldehyde under certain reaction conditions, and it has cohesiveness and high hardness. The experimental results showed that the granulation rate and particle hardness 1602


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Figure 8. Particle surface characteristics of (A1) uncoated granule potassium chloride, (A2) coated granule potassium chloride, (B1) uncoated RedBull potassium chloride, and (B2) coated Red-Bull potassium chloride. The cross-section surface characteristics of (C1, C2) CRK and (D1, D2) CRRK.

Table 3. Composition of Various Coated Potassium Chloride Fertilizers

Figure 9. Diagrammatic sketch of coated effects of (A) Red-Bull and (B) granule potassium chloride.

potassium chloride fertilizer particles, the drying process has less energy consumption, and the prepared fertilizer particles have smooth surface and easy coating. The role of water should not be neglected.51 On the other hand, a low amount of urea-

fertilizers

design of the coating content (%)

actual coating content (%)

total K2O (%)

CRK1 CRK2 CRK3 CRRK1 CRRK2 CRRK3

2 3 4 2 3 4

1.95 3.19 4.07 2.26 3.25 4.18

52.40 51.66 51.13 58.64 58.05 57.49

formaldehyde was used when the auxiliary substance was selected and studied. The purpose of the study was not only to 1603


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successfully combined with bentonite, which exhibits a better granulation rate and particle hardness as 70.97% and 69.18 N. Because of its smooth surface and better fluidized performance, the potassium release rates at 24 h and 28 days of modified KCl particle-based CRK were significantly lower than that of the conventional KCl particle with the same polyurethane coating quantity. Consequently, the application of 4−8% UF binder in combination with 6−8% bentonite is recommended to increase the granulation rate and particle hardness of KCl particles, which showed good slow K release behavior as controlled release fertilizer core material. In conclusion, the novel KCl granulation technology has a huge potential for large-scale applications to satisfy the increasing demand for controlledrelease KCl fertilizers in the future, which can enhance the sustainability of agricultural systems and environment.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 10. Release curves of K from fertilizers coated with different coating content at 25 °C in water.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b04629. Photo of disc granulator; pilot production line of granulated potassium chloride granulation; photo of CRK-coated granule potassium chloride and CRRKcoated Red-Bull potassium chloride; size data report of the sample; appearance of the sample (A: granule KCl, B: Red-Bull irregular KCl) (PDF)

ensure as much effective potassium content as possible but also to increase the granulation rate. In summary, the granulation rate results will be increased, while the moisture and ureaformaldehyde adhesive are adequately added. The coating material of this test was heat convertible resin.14 The results indicated that granular fertilizer showed an excellent coating effect under the condition of equal coating amount; this is consistent with the shape of the fertilizer used in the S. Zhang’s study,52 as the irregular granule fertilizer cannot be directly coated which was mainly attributed to the irregular granules with higher surface area and roughness. With the increase of the CRK coating material amount, controlled release period was extended, which is consistent with the study of S. Zhang.52 In practical applications, the coating amount can be further increased by more than 4%, and a longer release cycle of controlled-release potassium chloride can be obtained. The prepared potassium chloride with a good controlled release period can not only increase fertilizer utilization rate but also reduce the times of fertilizer application and increase economic benefits.4,7,53 In order to solve the problem of the low fertilizer utilization rate, inconvenient follow-up fertilization, and high price of potassium sulfate, an effective way to solve the problem is provided and has a wide application prospect. Although the addition of UF binder increased the cost of KCl granules, it decreased the expenditure due to high production efficiency of granulation. Moreover, the good granulation properties and great fluidization of modified granules made less use of coating materials, which further reduced the cost of controlled-release potassium chloride. However, the coordination of water and binder to increase the granulation rate still needs further study and further optimization in the later study.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +86-538-8241531. Fax: +86-538-8241531. E-mail: [email protected] (M.Z.). *E-mail: [email protected] (Z.L.). ORCID

Min Zhang: 0000-0002-7838-3393 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The study was supported by the National Key Research and Development Program of China (Grant no. 2017YFD0200706) and the National Natural Science Foundation of China (Grant no. 41571236) and the Key Projects in the National “948” Program during the Twelfth Five-year Plan Period (Grant no. 2011-G30) and the Youth Foundation of Shandong Agricultural University (No. 24026).

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REFERENCES

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5. CONCLUSIONS The optimized addition amount of UF binder was 4−8%, which improved the granulation rate by 2.43−3.52 times and particle hardness by 5.13−9.49 times, compared with the unmodified treatment at the same addition rates of additives. The optimized additive was bentonite with an optimum addition amount of 6−8%, which could therefore improve the granulation rate by 1.15−2.49 times and particle hardness by 1.01−2.94 times in comparison with other additive treatments at the same addition rates. Moreover, the UF binder was 1604


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