WO2011080764A1

WO2011080764A1 - Method of coating zinc sulphate monohydrate on npk granules

Info

Publication number: WO2011080764A1
Application number: PCT/IN2010/000867
Authority: WO
Inventors: Nagendra Rao Teki Nagendra Rao; Srivastava Tej Parkash Srivastava
Original assignee: Nagarjuna Fertilizers and Chemicals Ltd
Current assignee: Nagarjuna Fertilizers and Chemicals Ltd
Priority date: 2009-12-30
Filing date: 2010-12-29
Publication date: 2011-07-07
Anticipated expiration: 2012-06-30

Abstract

Zinc is one of the most essential micronutrients required for crop productivity, food and nutritional security for growing population globally. As Zn deficiency is wide spread in the soils, its application in the form of fertilizers is essential. Delivery of Zn along with other major nutrients like phosphorus through single granule in a cost effective and efficient manner has been a real challenge. A method of coating of zinc sulphate monohydrate on to commercially manufactured NPK granules was devised using a thixotropic, binding cum anticaking agent in ideal ratios of 2:1 and 3:1 zinc sulphate monohydrate: binding cum anticaking agent to achieve better adhesion of particles to granule surface facilitating loading zinc up to 2% with minimum segregation. Through this coating process, consistent zinc availability was observed even in the presence of high level of water soluble phosphorus.

Description

METHOD OF COATING ZINC SULPHATE MONOHYDRATE ON NPK

GRANULES

FIELD OF INVENTION

This invention relates to a method of coating of zinc sulphate monohydrate on to processed NPK fertilizer granules for effective binding thereby minimizing segregation / separation of particles from the coated granules and at the same time maintaining availability of zinc in presence of phosphorus even under moist conditions.

BACKGROUND OF THE INVENTION

Zinc (Zn) is one of the essential micro-nutrients required for healthy crop growth. Zn is usually taken up by crops from the soils. Zinc deficiency in soils was reported across globe causing reduced crop growth, yields and quality of produce. Substantial crop improvement due to zinc fertilization was documented (Takkar and Walker, 1993; Alloway, 2003). This has also profound influence on human nutrition because the source of food for majority of the global population is agricultural produce. External supply of this nutrient is inevitable when soils are not able to supply enough available zinc to meet crops' requirement. Westfall et al., (1999) observed that total zinc content of fertilizer is not enough to determine its effectiveness for a crop grown in such soils where the available zinc is low but, the degree of water solubility is an important factor. Apart from water solubility cost is also a major consideration for wide scale usage of these compounds as zinc fertilizers in agriculture. Zinc sulphate and zinc oxide are most commonly known carriers of zinc though several zinc sources are known and commercially available. Zinc oxide because of its limited water solubility is less effective as zinc fertilizer particularly for short duration crops. Zinc sulphate on the other hand is a proven good source of Zn and widely recommended for soil application, because of low cost per unit of zinc and better crops' response to this Zn source. However, it's usage in granulation process along with NP nutrient carriers such as Di-ammonium phosphate (DAP)/Ammonium phosphate has been restricted because, zinc sulphate heptahydrate (ZnS0₄ 7¾0) in particular is hygroscopic in nature compared to zinc oxide hence zinc ions could easily react with phosphate ions through chemical reaction forming a relatively insoluble compounds. This is particularly possible when ammonium phosphate is coming in direct contact with zinc sulphate and vice-versa in the granulation process in presence of moisture. This phenomena largely discouraged usage of zinc sulphate coatings on manufactured NPK granulation fertilizer products: This is particularly relevant to products of wet granulation process under high temperature and moisture conditions, when Di- ammonium Phosphate/ Ammonium phosphate is used with the known reaction product of insoluble zinc ammonium phosphate (ZnNH₄P0₄). The relative advantage of zinc sulphate monohydrate as a source of zinc and appropriate method of coating the material on to the NPK granules is not explored. With the growing prominence of multi-nutrient delivery through granulated fertilizers with due considerations of quality, nutrient use efficiency, cost effectiveness and handling convenience needs devising suitable methods and processes.

Hignett, T. P. (1964) suggested that incorporation of micronutrients into granular fertilizers was possible by adding 1-3% of oil, wax or other binders to stick the fine micronutrients materials onto the granule surface. However, the aliphatic and hydrophobic nature of these compounds prevents them forming a tenacious coat on the surface of the granule.

Walter, K. H. and Wetherley, D. (Australian Patent No. 554749) demonstrated that micronutrient incorporation was possible by treating the phosphate containing fertilizer with a mineral acid while the said granular fertilizer material is tumbled in the presence of a particulate micronutrients like copper, cobalt, manganese, zinc or mixtures, which are bounded to the surface of the fertilizer granules while said surfaces are wetted by the said acid. This method may not be suitable for fertilizer granule mixtures having urea in it.

Thus there is a need to develop an effective method to coat and physically bind preferential source of Zn fertilizer viz., zinc sulphate monohydrate with the help of suitable binding cum anticaking agents on to NPK granules manufactured through wet granulation process to achieve good retention of particles on the surface of granules at different levels of Zn concentrations, without sacrificing zinc and phosphorus availability even in presence of moisture for best usage in agriculture.

SUMMARY OF THE INVENTION

The main object of the present invention is to devise and standardize a method to effectively coat and physically bind preferential source of Zn fertilizer viz., zinc sulphate monohydrate with the help of an effective binding cum anticaking agent on to NPK granules manufactured through wet granulation process to achieve good retention of particles on the surface of granules at different levels of Zn concentrations, without sacrificing zinc and phosphorus availability even in presence of moisture for best usage in agriculture. Three important steps are involved in the zinc coating method onto NPK granules to achieve the object of effective coating of zinc on the NPK granules with a minimum separation/segregation, at the same time to maintain zinc availability in the granules even in the presence of moisture, a character which enhances the effectiveness of the fertilizer material prepared for balanced fertilization of crops. Use of specific binding cum anticaking agent having eco-friendly compounds viz.,

(i) Napthenic Oil (ii) Stearyl amine (iii) Soya licithine and (iv) Stearic acid that prevents (a) caking thereby improve shelf life and (b) having binding property that helps uniform coating and adhering of zinc sulphate monohydrate on the granule surface in a way that it maintains consistent zinc availability in the granule even in the presence of water soluble phosphorus and moisture. This specific binding cum anticaking agent has dual property of (i) spreading fast and uniform at 50-55 °C (ii) binding nature at lower temperature allows zinc sulphate monohyderate to adhere well on to the surface with minimum segregation there by preventing handling losses.

The choice of zinc sulphate monohydrate as an ideal source of zinc nutrient on the grounds of better water balance of the compound (<= 1.5%) as per FCO norms Fertilizer Control Order (FCO) having high analysis of zinc (33-35%), >99% water solubility, consistent Zn availability even in the presence of phosphorus that fits well to our chosen process of coating on to NPK granule, though the coating process was successful even with zinc sulphate heptahydrate and zinc oxide. For best results of coating process, keeping of zinc sulphate monohydrate to binding cum anticaking agent at the ratio of 2:1 for zinc loading quantity ranging between 0.5 to 2 percent or 3: 1 for zinc quantity ranging between 1 to 2 percent, that effectively binds the zinc sulphate monohydrate on to the surface of NPK granules. Coating beyond 2% Zn is not usual, hence the process found to be ideal for a normal range of zinc loading. Use of Zinc Sulphate in the present inventive process has more advantages to farming community by supplying NPK and Zn nutrients together in this form which considerably brings down the cost of fertilizer manufacture and handling as compared to solo applications. Above proven application has already been tested successfully in the field for customized fertilizers.

Thus, the use of Zinc Sulphate in the present inventive process has more advantages to farming community by supplying NPK and Zn nutrients together in this form which considerably brings down the cost of fertilizer manufacture and handling as compared to solo applications. Above proven application has already been tested successfully in the field for customized fertilizers.

The details of one or more embodiments of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and

advantages will be apparent from the description, drawings, and also from the claims.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated into and form a part of this specification, illustrate one or more embodiments of the present disclosure and, together with the detailed description, serve to explain the principles and implementations of the disclosure.

Figures la & lb: Percent detachment of (a) zinc sulphate monohydrate and (b) zinc sulphate heptahydrate from the coated NPK granules at four levels of zinc loading and zinc sulphate : binding cum anticaking agent ratios. Figures 2a-2f: Moisture absorption by zinc coated NPK granules at air sealed and ambient (open) air conditions at four levels of zinc loading and three zinc sulphate monohydrate /zinc sulphate heptahydrate: binding cum anticaking agent ratios.

Figure 3: Relative water soluble P₂0₅ and Zn contents in the zinc coated NPK granules kept under ambient (open) air conditions at two different zinc sulphate, anticaking agent ratios.

DETAILED DESCRIPTION OF INVENTION

Method of Zinc Sulphate Monohydrate coating on the NPK granules: Generic NPK fertilizer grades manufactured through known routes of granulation methods can be used as a carrier/base in this zinc coating process. Required quantities of micro sized zinc sulphate should be added to these granules before bagging. Zinc sulphate mono hydrate (ZnS0₄ H₂0) having 33-35% Zn is preferred source of zinc due to its low hygroscopic nature, having high nutrient content compared to zinc sulphate heptahydrate. In order to facilitate proper coating of the zinc sulphate monohydrate on each NPK granule, less than 100 mesh size is ideal size of particles. A thixotropic fluid is used which is an ideal mixture of the following specific compounds formulated to acquire both the binding and anticaking properties.

(i) Napthenic Oil

(ii) Stearyl amine

(iii) Soya licithine

(iv) Stearic acid

This binding cum anticaking agent is having viscosity of 15-25 centi-poise (at 20 °C). Specifc gravity is 0.91. It is devoid of any aromatic smell. The above compounds are eco friendly in nature and no known acute toxic affects were reported. Because of low surface tension, the agent is easily dispersible in presence of water, which is a desirable character while applying the coated granules as a fertilizer to the agricultural fields.

This high viscosity compound having synergic blend of organic compounds with mineral oils is first heated to convert it into free flowing solution and the same is chosen as binding agent for zinc sulphate monohydrate. This binder also prevents direct contact of zinc sulphate powder with DAP/AP material in the granules having phosphate and also to avoid reaction of forming zinc phosphate in presence of moisture.

This specific binding cum anticaking agent has dual property of (1) spreading fast and uniform at 50-55 °C (2) binding nature at lower temperature allows zinc sulphate monohydrate to adhere properly on the surface with minimum segregation/detachment from granule surface.

In the present inventive method, the anticaking viscosity is first reduced by heating at 50-55 °C and the same is applied on the granules in a rotary drum with the help of pump and nozzle. Thus, the NPK granules are coated uniformly with binding cum anticaking agent film. The granules are allowed for a minimum 3-5 minutes contact with binding cum anticaking agent in a rotating coating drum. At this temperature, it doesn't require air or any medium to spray and the binding cum anticaking agent gets coated uniformly on it's own. The precondition for easy and uniform spread on the granule is that, the granule temperature also should be in between 40-45 °C. Before the film gets solidified at room temperature, zinc sulphate mono hydrate powder is applied at required rate on the granules. Thus the selection of specific binding cum anticaking agent is important as it should have free flowing characteristic while applying and also should be able to bind zinc sulphate monohydrate powder after attaining normal temperature without separation while handling the fertilizer material.

EXPERIMENTAL

A series of studies have been presented here as examples on our claims.

Fresh NPK fertilizer granules manufactured through wet granulation process by commercial granulator using Urea, DAP and MOP as source material having composition of 17:17: 17% N-P₂0₅-K₂0 was chosen for this study as an example.

Zinc sulphate monohydrate (ZnS0₄ H₂0) and zinc sulphate heptahydrate (ZnS0₄ 7H₂0) having 33%, 21% Zn respectively coated on to those granules using the mentioned procedure to load 0.5%, 1%, 1.5% and 2% of Zn on to granules. The zinc compounds and binding cum anticaking agent used in the composition of 1 : 1, 2: 1, 3: 1 and 4:1 ratio (w/w) at all the four zinc levels. The total composition matrix of NPK granules + zinc sulphate monohydrate/zinc sulphate heptahydrate + anticaking agent (expressed in percentage) is shown in table 1.

After preparation of the samples, a portion of the material was subjected to physical shaking followed by sieving (1 mm) to quantify the percent of coated zinc compounds detached from the granules. Other portion was put in airtight bags and equivalent portion of the material was put under ambient (open) conditions. The moisture gained by the samples was recorded till eight days at two days frequency (for 4 times). Under ambient condition, the temperature was 25 °C and relative humidity recorded was in between 70-74%. The first study was to show ideal Zn fertilizer anticaking ratio considering Zn particle retention on the granules after subjecting the Zn coated granules to consistent shaking simulating fertilizer material handling after bagging. The second study was to quantify the extent of moisture absorption under air sealed conditions and open conditions. Third study was to exemplify relative P and Zn availability in presence of moisture at 1% Zn loading level and at two most ideal zinc sulphate monohydrate/zinc sulphate heptahydrate: anticaking agent ratios. Zinc content in the granule was estimated through a method using ICP and water soluble phosphorus was estimated by standard AO AC methods of analysis. Table 1. Composition matrix of NPK granule, ZnS0₄ H₂0/ ZnS0₄ 7H₂0, binding cum anticaking agent at four levels of zinc contents.

7 1.5 93.18 4.55 2.27 89.29 7.14 3.57

8 2.0 90.91 6.06 3.03 85.71 9.52 4.76

3:1 Zinc sulphate : Binding cum anticaking agent ratio

9 0.5 97.97 1.52 0. 1 96.83 2.38 0.79

10 1.0 95.96 3.03 1.01 93.65 4.76 1.59

11 1.5 93.93 4.55 1.52 90.48 7.14 2.38

12 2.0 91.92 6.06 2.02 87.30 9.52 3.17

4:1 Zinc sulphate : Binding cum anticaking agent ratio

13 0.5 98.10 1.52 0.38 97.02 2.38 0.60

14 1.0 96.21 3.03 0.76 94.05 4.76 1.19

15 1.5 94.31 4.55 1.14 91.07 7.14 1.79

16 2.0 92.42 6.06 1.52 88.10 9.52 2.38

EXAMPLES

Example 1:

The zinc loading on to NPK granules beyond 1% concentration, zinc sulphate monohydrate and zinc sulphate hyptahydrate were effectively bound to NPK granules at all zinc sulphate : binding cum anticaking agent ratios except with 4: 1 ratio which is least effective at 0.5 and 1 % zinc loading levels. Based on cost considerations, effectiveness and handling conveniences, 2: 1 and 3:1 ratios were observed to be most ideal ratios. For the requirement of low levels of Zn loading (0.5%), 2: 1 ratio of zinc sulphate monohydrate : binding cum anticaking agent ratio was observed effective as particle detachment was low at all levels of zinc loading(Diagrams la & lb). This study indicates that there is a threshold level of binding cum anticaking ratio below and above which it is not very effective.

Example 2: As the moisture content was below one percent in the granules at air sealed conditions even upto eight days at all levels of zinc loading signifies storage life of the coated granules under air tight conditions. The observed shelf life was quite high under sealed conditions. The granules at all levels absorb moisture effective from day two under open conditions at 70% relative humidity. This study shows that zinc sulphate hepta hydrate coated granules absorbed more moisture than zinc sulphate monohydrate coated granules at longer days of exposure beyond 6 days particularly at^' low levels of zinc loading. Where as, the moisture absorption was almost uniform at all levels of zinc sulphate monohydrate loading as shown in diagrams 2a through 2f. For effective dissolution of Zn coated granules in presence of moisture, the inherent property of NPK granule should not be altered even after coating with zinc at the same time the coating process using binding cum anticaking agent should not have any interference with the granule dissolution process in presence of moisture. This could be achieved in this process.

Example 3. The third study established relative preference of zinc sulphate monohydrate over zinc sulphate heptahydrate as a source of Zn for the coating process as the zinc content on the zinc coat was relatively consistent when zinc sulphate monohydrate was used as a source (Diagram 3) of Zn in contrast to zinc sulphate heptahydrate which appears to be reacting with phosphorus there by reducing its availability. This study was performed at 1% zinc load as an example to show whether the process has any effect on reactivity with phosphorus. This objective could be achieved with zinc sulphate monohydrate because of its compatibility with the coating process.

The main limitation is that this process is quite effective only when ideal temperature conditions for the binding cum anticaking agent is maintained at 50-55 °C and the NPK granule temperature is above 40 °C. Otherwise, the coating process is not uniform on the NPK granules. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosure pertains. Although any methods and materials similar or equivalent to those described herein can be used in the practice for testing of the disclosure(s), specific examples of appropriate materials and methods are described herein. All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited.

The examples set forth above are provided to give those of ordinary skill in the art a complete disclosure and description of how to make and use the embodiments of the devices, systems and methods of the disclosure, and are not intended to limit the scope of what the inventors regard as their disclosure. Modifications of the above-described modes for carrying out the disclosure that are obvious to persons of skill in the art are intended to be within the scope of the following claims. All patents and publications mentioned in the specification are indicative of the levels of skill of those skilled in the art to which the disclosure pertains. All references cited in this disclosure are incorporated by reference to the same extent as if each reference had been incorporated by reference in its entirety individually.

The entire disclosure of each document cited (including patents, patent applications, journal articles, abstracts, laboratory manuals, books, or other disclosures) in the Background of the ¹ Invention, Detailed Description, and Examples is hereby incorporated herein by reference.

While specific embodiments of the subject disclosures are explicitly disclosed herein, the above specification and examples herein are illustrative and not restrictive. It will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Many variations of the disclosures will become apparent to those skilled in the art upon review of this specification and the embodiments below. The full scope of the disclosures should be determined by reference to the embodiments, along with their full scope of equivalents and the specification, along with such variations. Accordingly, other embodiments are within the scope of the following claims. References Cited

Alloway, B. J. (2003) Zinc in soils and crop nutrition, International Zinc Association. Ppl l4. (hhtp://www.zinc-crops.org)

Takkar, P. N. and Walker, CD. (1993) The distribution and correction of zinc deficiency. In: Zinc in soils and plants (Ed. Robson, A.D.), ppl55-165 (Kluwer Acad. Publ:

Dordrecht, The Netherlands.

Westfall, D. G., Amrani, M., and Peterson, G.A. (1999) solubility of zinc fertilizer: does it matter? Better Crops 83(2): 18-21.

Claims

CLAIMS We claim:

1. A method of coating of zinc sulphate monohydrate on processed NPK fertilizer granules, said method comprising of zinc source, NPK fertilizer granules, binding cum anti-caking agent and said method comprising the steps of: i. Heating the binding cum anticaking agent at 50 - 55 °C; and ii. Applying molten binding cum anticaking agent on NPK fertilizer granules in the rotary drum with the help of pump and nozzle.

2. A method as claimed in Claim 1, wherein the zinc source is zinc sulfate monohydrate and zinc sulfate heptahydrate.

3. A method as claimed in Claim 1, wherein the binding cum anticaking agent is the mixture of napthenic oil, stearyl amine, soya licithine and stearic acid.

4. A method as claimed in Claim 1, wherein the NPK fertilizer granules to be zinc coated shall be maintained at 40 - 45 °C temperature.

5. A method as claimed in Claim 1, wherein the NPK fertilizer granules are allowed for a minimum of 3-5 minutes contact with binding cum anticaking agent under rotation of coating equipment.

6. A method as claimed in Claim 1, wherein the zinc compounds and binding cum anticaking agent are in the ratio (w/w) of 1 : 1, 2:1, 3: 1 and 4: 1.