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SULPHURCOATEDUREA

Article·May2016

2authors,including:

PremBaboo

NationalFertilizersLtd.,India

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Retrievedon:03May2016

SULPHUR COATED UREA

Author

Prem Baboo

Sr. Manager (Prod)

National Fertilizers Ltd. Vijaipur, India

Abstract

Coating particles or granules of urea with sulphur and a sealant results in the formation of a membrane that regulates the availability of nitrogen for plant growth.Sulphur Coated Urea (SCU) fertilizer is a slow-release fertilizer that is made by coating urea with sulphur and wax that increases nitrogen efficiency, improves plant growth and reduces water pollution, compared with water soluble fast-release urea. Sulphur Coated Urea Avoid soil compaction; reduce frequency of application and reducing total cost; effectively reduce salt index, improving quality of crops; sulphur is a middle element, to provide nutrition for crops. As a hi-tech controlled/slow release fertilizer, Sulphur coated urea (SCU) has both effects of nitrogen and sulphur fertilizers.

Introduction Urea is used extensively as a high Nitrogen fertilizers but agronomic test have shown

that 15-70 % of the applied urea may be lost due to leaching and could therefore be

unavailable to crops.There has been exponential growth in the earth's population that

has now reached approximately 7.4 billion and approach 9.52 billion by 2050. Global

food requirements have also risen and the expected per capita food requirement is

likely to double by 2050. Meanwhile, arable lands diminish due to industrialization,

urbanization, and desertification and land degradation from heavy flooding. These

intimidating factors threaten global food security and demand a robust response.

Multidimensional steps have already been taken worldwide to meet the challenge of

food security with modifications to improve agricultural systems. To meet the

increasing food demands, the agricultural sector is bound to employ enormous

quantities of fertilizers that have thus far demonstrated undesirable environmental

impacts. Hence, it is of paramount importance to develop systems that boost production

and alleviate environmental problems. Controlled Release fertilizers may be one such

solution as they are believed to enhance crop yield while reducing the environmental

pollution caused by the hazardous emissions (NH3, N2O etc.) from current fertilizer

applications .Sulphur coated urea is a controlled release fertiliser produced by coating

hot urea by molten sulphur polyurethane oil or a microcrystalline wax. The ideal ratio

comprises 30%–40% nitrogen and around 20% sulphur. Sulphur coated urea is

specialised fertiliser that is coated with wither wax or polymer and releases nutrients

slowly. Sulphur coated urea finds application predominantly in agriculture, golf courses,

professional lawn care and turf, green houses, horticulture and nurseries. By region, the

global sulphur coated urea market is segmented into North America, Western Europe,

Asia Pacific Excluding Japan (APEJ), Eastern Europe, Latin America, the Middle East and

Africa, China and Japan. Of these, North America is the most prominent markets, both

collectively accounting for over 60% value share of the overall market in 2016.

Drivers & Restraints

Growing concerns over sustainability and cost minimisation in the agriculture s ector is

driving demand for sulphur coated urea globally. Stringent regulatory framework on

conventional fertilisers, concerns about Eutrophication, and preferences for spending

on luxury amenities such as household lawns and growing number of golf courses are

some of the key factors supporting growth of the global sulphur coated urea market.

Increasing urbanisation and income levels across the globe (especially in Asian

economies) are anticipated to significantly contribute to the overall demand for sulphu r

coated urea in the near future. Conversely, increasing commercialisation and product

innovation in alternative products such as polymer coated urea and higher cost of

sulphur coated urea compared to conventional urea fertilisers are some minor factors

restraining growth of the global sulphur coated urea market.

Sulphur-Coated Urea

The additional labour and equipment required to produce coated fertilizers and the cost

of the coating materials make them much more expensive than conventional N

fertilizers. Due to the simplicity and relatively low cost of using sulphur as a coating

material, sulphur-coated urea has become the most commonly used coated-urea

product.

This fact and the high N content of the product (30% to 40%, depending on the amount

of sulphur applied) have added to its popularity among coated products. In addition, as

a generic product, its cost has remained relatively low. The sulphur coating is an

impermeable layer that slowly degrades through microbial, chemical and physical

processes Figure-1. The completeness of the coating determines its effectiveness;

incompletely coated or cracked prills are immediately amenable to dissolution in soil

water and hydrolysis by urease. Because not all granules have complete integrity of

their sulphur coating, some N is quickly made available to the soil solution. In fact, the

"7-day dissolution rate" is routinely as high as 30% and may be as high as 40% to 60%

of the total N content of the product in some cases. With such high rates of dissolution, a

rapid initial effect on the crop would be expected.

Fig. No.1

The figure No 2 shows the sulphur coated urea is slow release of Nitrogen.

Fig. No.-2

Sulphur + Polymer-Coated Urea

To help solve the problem of irregular nutrient release from sulphur-coated urea, a class

of "hybrid" products has been developed. These products include a thin polymer coating

on top of the low-cost sulphur coating. This has the benefits of reducing the o verall cost

vs. that of polymer-only products, while providing a better seal than the sulfur-only

products.

Fig. No. 3

Polymer-coated urea fertilizers use a hydrophobic (water insoluble) coating that

temporarily isolates the urea prill from the soil environment. These polymer coatings

may be resins or mineral-based products that act as semi permeable membranes or

impermeable membranes with tiny pores. Nutrient release through these membranes is

controlled by the properties of the coating material, i.e., i ts permeability characteristics

as affected by temperature and moisture Figure No.-3&4.Thus; they are not significantly

affected by soil properties such as pH, salinity, soil texture, microbial activity, re-dox

potential or cation exchange capacity. Therefore, it is possible to predict and control the

nutrient release rate from these products more accurately than for sulphur -coated urea.

Fig No.-4

The chemical controlled-release products became available as fertilizers in the many

countries over 50 years ago. From the beginning, their high cost relative to other N

fertilizers has limited their use in large-scale production of commodity crops such as

corn, sorghum, wheat and canola. Rather, they have been more commonly used in

specialty, high-value crops such as vegetables, orchards, nurseries, seed production, etc.

Today, their use is limited but increasing in row crops, primarily due to higher grain

prices, environmental concerns and regulations, and niche uses such as foliar

fertilization. Common classification of urea coating & chemical controlled-release

products are described below in figure No.5.

Fig. No.-5

The fusion of the urea and sulphur is formed in four basic steps.

1. Urea is heated to prepare its surface for sulphur coating.

2. It then enters a rotating drum to be sprayed hydraulically first with

sulphur and then with the wax sealant.

3. The product is cooled, coated with diatomaceous earth conditioner to

prevent caking.

4. Screened and transferred to storage for testing.

The more nitrogen that is fed to and used by turf, the greater its need for sulphur.

Sulphur deficiencies in turf have the same visual signs as nitrogen deficiency; yellowing

of leaves, faint scorching of leaf tip. When used in combination with proper ratios of

nitrogen, phosphorous and potash, sulphur offers the following benefits:

1. Improve water penetration in soil;

2. Increases availability of iron, manganese, copper, zinc and boron to the plant

3. Improves soil structure

4. Builds healthy protoplasm and plant tissue to help resist drought, disease and

winter damage.

5. Enhances colour;

6. Promotes turf growth and density.

7. Aids the turf response when used in combination with nitrogen.

8. Helps keep alkalinity in balance

9. Aids nitrogen release from organic matter.

10. Improves recuperation capacity.

PROCESS DESCRIPTION

The Tennessee Valley Authority (TVA) process developed the first large scale sulphur coating which was commercialized by Canadian Industries Ltd(CIL).The TVA process consists essentially of rotary drum into which preheated about 60-750C urea is fed. Molten Sulphur temperature about 1500C is sprayed onto the urea particles and solidification rapidly thereby for mixing solid coats. This process consisted of five stages:

1. The pre heating of urea about 60-750C 2. the coating of the urea particles in a rotary drum, where liquid sulphur

temperature about 1500C was atomized on the particles, 3. A second coating stage with wax to cover imperfections on the sulphur

coating, which presented some holes through which the urea could be leached,

4. The cooling of the coated product and 5. The adequate conditioning of the product to avoid particle agglomeration.

The process is mechanically complex and the investment and maintenance costs are high.

Using the spouted bed for coating urea with sulphur. The best quality product is obtained when the bed is maintaining at 80º C. Evaluating the dissolution rate, D25%, they concluded that quality increased at higher sulphur mass flow rates and at lower atomizing air flow rates.

THE UBC SPOUTED BED PROCESS A sulphur coating process using spouted bed mode was developed by Mathur & Meisen at the University of British Colombia (UBC) starting 1975. The facilities consisted mainly of spouted bed i.e. cylindrical column with conical base filled with urea particles. A high velocity air jet entered the bed from the bottom and carried particles located in central region of the bed (spout) upwards until they reached the top of the bed(fountain) whence they fall back into the annulus as shown in the figure No.6.The annulus behaved as a slowly descending packed bed.

Fig. No. 6

MATERIALS AND METHODS Elementary sulphur is used as the coating material. Some of its physical properties are shown in Table -1 Physical Properties of Elementary Sulphur

Sr. No. Melting Point,0C 1 Rhombic 110

2 Monoclinic 145 Liquid Dynamic Viscosity, μCp

1 1200C 17 2 140 0C 8 3 158 0C 6.4 4 160 0C 5.9

Table No-1 When the temperature of the melted sulphur is increased, its viscosity first decreases, as occurs with most of the liquids, from 17 cp at 120º C to 6.4 cp at 158º C. In this temperature range the melted sulphur is yellow. Above this temperature viscosity rapidly increases to a very high value at 160º C, and at this point the melted sulphur is dark red. This behaviour of the viscosity of sulphur in relation to temperature is explained by the modification of its molecular structure. The Sauter mean diameter was obtained by granulo metric analysis in a standard sieve series and is 1.98 mm. A particle density of 1335.0 kg/m3 is determined.. With these values and the density of the air, we could verify in following diagram that urea particles are classified as a group D powder, which justifies the choice of the spouted bed as the proper gas-solids contact.

Experimental System The two-dimensional spouted bed is constructed of Plexi glass to allow the observation of particle movement. Bed dimensions followed the relations to maintain spouted bed

stability and adequate solids circulation. The depth of the air inlet orifice must be equal to the depth of the bed and the width of the air inlet orifice must be within 1/6 and 1/20 of that of the bed. Figure No-6, show the two-dimensional spouted bed used in this work. Meisen and Mathur (1978) verified the possibility of using the spouted bed for coating urea with sulphur. A batch-operated pilot plant was developed and the particles were coated in a cone-cylindrical spouted bed. The authors concluded that product quality is a function of air temperature and that the product obtained was comparable to that of the TVA process for some fixed process conditions. Weiss and Meisen (1983) modified the basic equipment used by Meisen and Mathur (1978) and related product quality to the operational conditions. The best quality product was obtained when the bed was maintained at 80º C. Evaluating the dissolution rate, D25%, they concluded that quality increased at higher sulphur mass flow rates and at lower atomizing air flow rates.

Fig. No-7

The double-fluid atomizer was set at the bed base. The complete experimental assembly developed and constructed is shown in Figure No.-8

Fig. No.-8

With the objective of studying the effects of the temperature of the spouted air, Tas, the sulphur mass flow rate, Ws, and the atomizing air flow rate, Wat, on the coating process, a two-level experimental factorial design is used. The results permit the evaluation of the statistical significance of the influence of the operational variables, specified above , on product quality in terms of D25%,. Several preliminary experiments were conducted to establish the methodology and the

ranges for the variables. Table no.-2 presents the operational conditions and the process

times set for the experiments.

Operational Conditions

Tas (0C) 69 82.3 Ws(g/min) 26.7 33.8 Wat(m3/hr) 1.1 1.5

The mass of urea in the bed is fixed at 1300.0 g for all experiments and the process time is fixed in 20 minutes. The spout air flow rate set at 85% above the minimum spout flow

rate, as the mass of sulphur added to the bed causes a mass increase in the bed of about 50%.

Fig. No.-9

Methodology for Determination of Product Quality, D25%

The samples collected during the process of coating and after the end of the experiments is analysed to determine the sulphur content and the dissolution rate. The standard TVA test is used to obtain the dissolution rate after seven days: 10g of coated particles are put in a test tube containing 50ml of water and maintained at 38º C for seven days. After this period, the tube is shaken slightly and a sample of the solution is removed. The urea content is obtained by refractometry. The seven-day dissolution rate is thus given by D =100 X Mass of the urea dissolved in 7 days/Total mass of the urea in sample The dissolution rate is a function of the sulphur content of the coated material. Thus, to compare the quality of the products obtained for different operational conditions, the dissolution is evaluated for fixed sulphur content. The sulphur content usually specified is 25%. The value of D25% is found fitting the curve of D versus sulphur content and by interpolation. The TVA dissolution test described and applied here is a measurement of the average dissolution of the sample and is not adequate to determine the dissolution of individual particles.

Why consider slow-release N

1. Consider slow-release N when attempting to reduce environmental losses .

2. Slow-release fertilizer is becoming more cost effective

3. Consider your soil system and cropping system and evaluate which N losses may

be occurring and hindering efficiency.

Fig. No.10

The value of increasing efficiency

Efficiency = more N applied taken up by the crops

1. Increase in yield with same fertilizer rate

2. Maintain yield with reduction in rate.

3. Increase in yield with decrease in rate.

4. Large increase in yield with increase in rate (in each case more N is taken up

per unit applied!)

Disadvantages

Not all sulphur coated urea are created equal. That is sulphur coated urea is only as

good as the weakest point on the coating. If this coating is cracked during transportation

and blending, the content is released as water soluble urea. Some independent studies

have shown that as much as 50% of the nitrogen from sulphur coated urea could be

released within 7days of application.

CONCLUSION

Not only do SCU enhance the efficiency of nutrient utilization, they also reduce the impact on the environment and the possible contamination of the subsurface water with N. Significantly, less N to leach from controlled-release products than from soluble N sources. In the future, as a result of SCU for various crops, the demand for controlled-release products should increase. Numerous studies have been conducted on to evaluate the quantity of N lost through leaching. Almost without exception, application of sulphur Coated Urea to turf results in less total N being leached. A primary advantage to using SCU is the reduced threat of groundwater contamination with N. Polymer/sulphur-coated fertilizers (PSCF) are hybrid products that utilize a primary coating of sulphur and a secondary polymer coat. These fertilizers were developed to deliver controlled-release performance approaching that of polymer-coated fertilizers but at a much-reduced cost. Although organic nitrogen fertilizers continue to receive a great deal of attention, it is important to note that sports turf managers have alternatives to organics. Many of these alternatives areas "environmentally friendly' as SCU and their release patterns are very often, more predictable.

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