WO2012071618A1

WO2012071618A1 - Controlled release product

Info

Publication number: WO2012071618A1
Application number: PCT/AU2011/001559
Authority: WO
Inventors: Robert Goulston Gilbert; Colin Mckenzie; Bradley Smith; Cristy Manala Warrender; Garry William Warrender
Original assignee: The University Of Queensland; Csbp Limited
Priority date: 2010-11-30
Filing date: 2011-11-30
Publication date: 2012-06-07

Abstract

A method for producing controlled release granules containing an agriculturally active ingredient, which method comprises: granulating agriculturally active ingredients with an aqueous slurry of an exfoliated clay in the presence of a binder; and drying the granules.

Description

CONTROLLED RELEASE PRODUCT

TECHNICAL FIELD The present invention relates to the production of a product having controlled release characteristics with respect to an active ingredient, specifically an agriculturally active ingredient. The present invention also relates to the controlled release product per se, and to the use of the product in agricultural applications. BACKGROUND TO THE INVENTION

With growing populations around the world, the pressure on agricultural industries is increasing to perform and produce more and more efficiently. In this context there is a continuing need to optimise the performance and use of agriculturally active ingredients, such as fertilisers. This is also important since the treatment of crops and pasturelands.with such active ingredients, represents a significant financial outlay for farmers/producers based on overall production costs.

There are however other issues that are relevant to consider. For example, current concerns with standard fertiliser practices see dose rates and re-application regimes that if unchecked have the potential to cause severe and on-going environmental issues. Seepage and run-off of excess nutrients from agricultural origin have been responsible for eutrophication of local and downstream water bodies. The environmental cost of an oversupply of nutrients can be far-reaching and long-lasting and government bodies are now acting to restrict and regulate fertilisation practices of the agricultural industry. With this environmental cost in mind, innovative products that propose to fill the demands of regulators while providing farmers/producers with a superior product are sought.

Against this background, the present invention seeks to provide an innovative product that exhibits a controlled release characteristic with respect to an agriculturally active ingredient. Providing a product that is able to deliver active ingredients at required dosage rates over a prolonged period of time will reduce the burden of excessive treatment and reduce the impact on the environment. An added benefit of such a product would be the reduced necessity for re-application, thereby saving farmers/producers time and money. The product may have particular use in highly leaching soils with low actives holding capacity.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a method for producing controlled release granules containing an agriculturally active ingredient, which method comprises: granulating an agriculturally active ingredient with an aqueous slurry of an exfoliated clay in the presence of a binder; and drying the granules.

The present invention also provides controlled release granules containing an agriculturally active ingredient produced in accordance with the invention. The present invention also provides the use in an agricultural application of the granules of the invention. In this case it is envisaged that the granules will be used/applied in conventional manner.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates. BRIEF DISCUSSION OF DRAWINGS

Embodiments of the present invention are illustrated with reference to the accompanying non-limiting drawings in which:

Figure 1 is a photograph of experimental apparatus referred to in the Example; and

Figures 2 -5 are graphs showing the results of experiments described in the Example. Figure 2 is an accelerated release plot comparing percentage clay addition to granule with a standard MAP containing fertiliser (SMAP). Dashed lines represent the exponential line of best fit.

Figure 3 is a pot trial plot comparing percentage clay addition to granule with a standard MAP containing fertiliser (SMAP). Grey lines represent measurement of peak width and height.

Figure 4 is an accelerated release plot comparing the presence/absence of binding agent with a standard MAP containing fertiliser (SMAP). Dashed lines represent the exponential line of best fit.

Figure 5 is a pot trial plot comparing the presence/absence of binding agent with a standard MAP containing fertiliser (SMAP). Grey lines represent measurement of peak width and height.

DETAILED DISCUSSION OF THE INVENTION

In accordance with the present invention it has been found possible to produce agriculturally important granules that are mechanically robust and that have controlled release characteristics with respect to the active ingredient that is contained. The invention has the potential to provide the agricultural industry with an effective product, having reduced cost associated with application volume and re-application rate, whilst reducing the impact on the environment from excessive application of such active ingredients. The product of the invention is in granular form - this is a conventional form for controlled release products used in agriculture.

The method of the invention results in the agriculturally active ingredient being held within interstitial voids of the exfoliated clay matrix that forms the granule. The agriculturally active ingredient is preferably provided approximately uniformly throughout the granules, and this is achieved by thorough mixing of the agriculturally active ingredient with the clay during granule production.

As will be explained, use of a binder in the method of the present invention is important. In accordance with the invention structural features within each granule are believed to impede diffusion of water into and out of the granule. Th e binder enables structural features that act as diffusion barriers to be preserved in granules having suitably robust mechanical properties.

According to the invention the structural features within each granule that are believed to impede diffusion of water into and out of the granule, are provided throughout the granule. This is beneficial since the controlled release properties of each granule are not dependent upon the macroscopic shape of the granule.

The present invention also achieves controlled release properties without the need to resort to a degradable coating as demanded by other controlled release products. A coating must act in some capacity to retard the infusion of water and the outflow of active ingredient from the granule. The coating may be a semi water-permeable, or water-impermeable but degradable, layer. There tend to be cost implications with the coating approach, particularly as the coating usually has to be relatively thick to be effective. There is also the necessity for core granules to be almost exclusively spherical in nature in order to apply the coating in an efficient manner. Furthermore, the complete coverage of the core fertilizer particle is intrinsic to the efficient and effective workings in a controlled release capacity. Thus any damage or incomplete coverage of granules renders the process ineffectual. Whilst not essential, conventional coating technologies may be applied in combination with the approach adopted in the present invention.

During use it is intended that the granules will come into contact with water, for example from rain or by deliberate watering spraying. Initially, water will contact and dissolve any agriculturally active ingredient that is present at the surface of the granule thereby removing a portion of the agriculturally active ingredient from the granules. However, as the agriculturally active ingredient is provided throughout the granules, the granules are able to give up further agriculturally active ingredient on subsequent contact with water. Due to structural features deliberately created within the granules (voids and surfaces), water tends to permeate the granules relatively slowly and to be released from the granules relatively slowly also. The result is that agriculturally active ingredient is released in a controlled manner from the granules after repeated cycles of coming into contact with water. Noting the principles underlying the present invention, there is no particular limitation on the type of agriculturally active ingredient that may be used in the present invention. The agriculturally active ingredient is one that is intended to be released in a controlled manner during normal use of the product of the invention. The agriculturally active ingredient may be selected from fertilisers, herbicides, fungicides, nutrients, growth regulators, and the like. The invention is believed to have particular utility for the controlled release of nutrients from fertilisers and here particular mention may be made of phosphate fertilisers, such as ammonium phosphate fertilisers.

To be used in the methodology of the present invention the agriculturally active ingredient may be used in dry form such as a powder or crystals. Alternatively, the active ingredient may be used in wet form. One or more active ingredients may be used. The agriculturally active ingredient is also required to have at least limited solubility in water so that it may be dissolved by water contacting granules that contain the agriculturally active ingredient. The clay to be used in implementation of the invention is one that is capable of being exfoliated (expanded) as this will favour formation of desirable structural features within the granules and allow greater distribution of clay throughout the granules formed during granulation. The clay may be a 1 :1 to 3:1 type clay, preferably a 2:1 type clay. By way of example, the clay may be from the smectite group of clays. Such clays are known to swell when immersed in water. Particular clays that may be employed include montmorillonite and bentonite. The use of bentonite has been found to be particularly suitable.

The clay may be exfoliated by known techniques, such as immersion of the clay in water followed by heating to transform the water into steam. However, in a preferred embodiment the clay is exfoliated at elevated pH by addition of a strong base, such as sodium hydroxide or ammonium hydroxide, and heating whilst stirring vigorously. Typically, the pH is 12-13 and the temperature is 60-100°C. Without wishing to be bound by theory it is believed that charged ions of large radius (sodium or ammonium ions in this example) hold apart sheets of the clay once expanded. The use of ammonium hydroxide may be preferred in certain instances as this may provide an added source of nutrient to the granules. This embodiment of the invention is undertaken at elevated pH. However, a low pH approach (pH 2-3) could be employed if this better suits the pH balance of downstream processes.

For exfoliation the clay is formulated as a slurry with water, and the base if used. Generally, the amount of clay will be from 4-6 wt% based on the slurry weight as any higher concentrations may present mixing problems especially since exfoliation of the clay results in an increase in viscosity. The clay is allowed to expand in the water for about 30 minutes or as many hours as required and then heated and stirred to achieve exfoliation. This can take some time (up to 24 hours is sometimes necessary); however the levels of heating and stirring may accelerate this process. Exfoliation/expansion of the clay dramatically increases the surface area of the clay. Viscosity measurements may be made to monitor the progress/degree of exfoliation, noting that the viscosity will increase as exfoliation proceeds. Viscosity will rise from that typical of water, about 1 cP, to a significantly higher value (e.g. several thousand cP in a matter of minutes, and 8000-12000 cP can be readily achieved).

In an embodiment of the invention, after the clay has been suitably exfoliated, a flocculating agent may be added to the clay slurry. Without wishing to be bound by theory, it is believed that the flocculating agent dewaters the slurry aiding the formation of clusters of clay particles. In turn it is believed that multiple, complex voids and surfaces are created within each cluster and that these structures will trap the agriculturally active ingredient during granulation. The flocculating agent may also assist in preserving the exfoliated structure during subsequent dewatering of the clay slurry.

The flocculating agent may be a water-soluble polyacrylamide (PAM). These are known in the art and have been used in agriculture as soil conditioners. Usually, the flocculating power of the PAM will increase with molecular weight. Typically, the PAM will be non- ionic with a weight-average molecular weight of 1-30 million. PAMs useful in the present invention are commercially available. The flocculating agent may also be chosen from a wide variety of such materials, including but not limited to diallyldimethylammonium chloride (DADMAC). Depending upon the amount of clay in the slurry, the flocculating agent (e.g. PAM) is typically used as a solution of 0.05 - 0.2 wt% , for example 0.1 wt%, and is added in an amount to achieve an amount of about 0.01 wt% based on the weight of the clay in the slurry. After addition of the flocculating agent to the slurry, the slurry is stirred gently and then allowed to stand to allow flocculation to proceed. This can take a number of hours.

The next step of the process involves granulation with the agriculturally active ingredient. Granulation takes place by contacting the clay slurry and agriculturally active ingredient in, or immediately prior to, a granulator. The invention may be implemented using conventional equipment, such as a drum granulator. In the granulator the feeds are tumbled together and heat and other components such as ammonia applied to assist in product activation and granulation. The temperature at which granulation takes place must be sufficiently high to drive off water that is present in the clay slurry. Steam can be used as a source of heat and is generally delivered using a sparging lance. Alternatively, direct heat and a warm air flow may be used. Ammonia may also be used for pH adjustment.

The relative proportions of active ingredient to clay can be varied to influence the release characteristics of the resultant granules. Typically, the weight ratio of agriculturally active ingredient to clay is from 10:1 to 1000:1, for example about 40:1. The concentration of active ingredient in the final granule product is generally up to about 20% by weight, for example up to about 10% by weight. In an embodiment the agriculturally active ingredient and aqueous clay slurry are mixed together prior to introduction to the granulator. In another embodiment however mixing of these components takes place in the granulator.

Importantly, in accordance with the present invention a binder is added during the granulation step. The binder is intended to hold together clusters of flocculated clay particles in the form of a granule. It is believed that the binder works by contacting the surfaces of (clusters of) the clay and effectively sticking them together in the form of a granule. During mixing the agriculturally active ingredient permeates the clusters of clay particles and becomes trapped within the clusters. The resultant granule should have a uniform distribution of agriculturally active ingredient and structural features (multiple, complex voids and surfaces) that slow ingress of water into and egress out of the granule during use.

The binder may be co-introduced into the granulator with the clay slurry and active ingredient, although it is preferred to spray the binder into the granulator as granulation proceeds. Useful binders include water-soluble binding agents, such as guar gums and starches, modified organic binding agents, such as modified guar and modified carboxymethyl cellulose and synthetic polymers, such as polyacrylates with a preference for anionic varieties. The use of polyvinyl acetate (PVAc) is preferred. The binder is introduced into granulator as an aqueous suspension as approx. 10-50% w/v. Preferably, this is sprayed onto the mix being granulated. In this case it is important that the spraying apparatus for the binder is suitably far removed so that it does not interfere with the steam and ammonia sparges. It may also be important that the binder does not contact the hottest and most reactive areas of the granulation mix as this may cause degradation of the binder.

Another binder that may be . useful in practice of the invention is polydiallyldimethylammonium chloride (polyDADMAC). This may be used alone or in combination with another binder, such as with PVAc. In the latter case each binder will usually be delivered into the granulator separately by spraying. The amount and ratio of each binder may be optimised by experimentation. After granulation the granulated product is dried. The granules may then be passed over sizing screens that allow the granules to be sorted.

In the methodology of the present invention the main process considerations are pH and heat. The robustness/ integrity of the granules is also important as the granules must survive the rigours of granulation, drying and screening. The resultant granules should also show suitable strength and crush resistance, and they may also be required to exhibit prolonged storage stability.

The invention also relates to controlled release granules formed in accordance with the present invention. The invention also extends to the use of the granules in an agricultural application. The context of use will obviously depend upon the nature of the active ingredient that is present in the granules.

In another embodiment a coating comprising an exfoliated clay and . binder, each as described above, may be applied to conventional granules of agriculturally active ingredient to influence release characteristics. In this embodiment a slurry or suspension of the exfoliated clay and binder is first made up and then applied to the surface of the granules by known techniques. The relative proportions of clay and binder, the thickness of the coating etc may be manipulated as required to achieve desirable release characteristics with respect to the agriculturally acceptable ingredient.

Embodiments of the present invention are illustrated with reference to the following non- limiting example. Example

Preparation of Modified Granules: A suspension of 4g/L bentonite clay was prepared with deionised water and stirred for 24 h to fully separate. This suspension was basified with 50% NaOH solution to pH 13. This was left for a further 24 h with medium stirring rate and at the end of this process the exfoliated suspension was flocculated with 20 ppm of neutral charged PAM to dewater and reduce volume.

For this work a commercial monoammonium phosphate (MAP) containing fertiliser was used. This fertiliser had a MAP concentration of 50% but the present invention can be applied to fertilisers having MAP concentrations up to 100%. The MAP fertiliser was prepared by grinding (powdering) in a hammer mill to enable thorough mixing with the clay.

The powdered monoammonium phosphate containing fertiliser was then mixed to the proportions of 1% and 4% exfoliated clay w/w. Granulation was conducted in a small scale bench-top arrangement shown in Figure 1. This set up provided variable rotational speeds as well as being able to be heated indirectly from the metal of the bowl.

Granulation was achieved on 50 g batches of fertiliser/clay powders using either deionised water or a 10 % suspension of PVAc and deionised water. The PVAc suspension used during granulation was prepared by dilution from a 50% w/w colloidal suspension of particle size -100 nm. A total mass of ~18 g of dilute PVAc suspension or deionised water was used to granulate each batch of 50g fertiliser/clay mixture. All samples were sieved to collect the 2-4 mm fraction and in total a set of 4 modified granule samples with the formulation were produced. Two samples were based on a fertiliser product containing 1% exfoliated clay, one granulated with binder and another granulated without binder. The other two samples were based on a fertiliser product containing 4% exfoliated clay, one granulated with a binder and another granulated without binder.

Accelerated Release Experiments Release patterns for the 4 example products were investigated following the shaking water bath method developed for accelerated release. Formulations were dried in an 80°C oven overnight and allowed to cool before weighing. 166 mg of each different formulation was buried in 25 g of inert soil in a sealed dialysis tube. This arrangement was suspended in 440 g deionised water, in a shaking water bath with an operating temperature of 60°C and agitation of approximately 40 rpm. Four formulations were chosen to be duplicated and samples were taken at various times over a 24 h period. One sampling duplicate was collected at each sampling from a selected formulation. Samples were diluted with deionised water for analysis of Total P by ICP-OES. Pot Trials

The 4 modified granule formulations were tested in a phosphate leaching experiment, which consisted of 4 replications and 2 control samples (18 samples total). Analysis of P concentration was via the molybdate colorimetric method.

Results

The aim of the additions made to the original MAP containing formulation is to extend the delivery time of P to plants without compromising max P output. If the trialled additives were performing this function the rate of release of P would decrease from the original MAP and the length of delivery time would increase, giving a more broad release peak. There were two additive combinations included in this group looking at the effects of:

1. percent clay content (1% and 4% additions),

2. presence of binding agent (PV Ac) To observe the trends more easily and for display purposes each data set representing these three combinations were averaged.

The first component looked at was percentage clay content (Figure 2). These results show a shift in the release profile from standard MAP containing fertiliser for both clay percentages: The concentration of P released from the modified granules, 1% and 4% was initially lower than that observed from the standard product. This small effect indicates an early impact of the modifications on reducing the rate of P diffusion in this type of test. These curves are also expressed as an exponential model [CM Warrender, GW Warrender, B Smith, C Mc enzie., RG Gilbert. Aust. J. Soil Research, 48 668-73 (2010)] which is shown in Figures 2 and 4 as dashed lines. The r² values as an indicator of fit suitability were >0.95 for all accelerated release samples.

Rate constants (k) determined at 60°C for these results (Table 1) indicate a decrease in diffusion rate for 1% clay compared to the standard MAP containing fertiliser. Although there was a shift in the curves for 4% clay there was no significant difference in rate constants between the standard product and 4% clay.

Similar results were found for percent clay additive through the pot trial method (Figure 3). A broader peak was observed for both 1% and 4% over the standard product. The increase in width is an indication of P release over a longer time frame (counted as rainfall events) from modified granules. This translates into a slower release rate for modified granules compared to the standard product. The initially lower P concentration observed for modified granules compared to the standard product seen in the laboratory release tests was also observed in the pot trial samples. However, release from the modified granules overtook that of the standard product, although as mentioned, the overall release peak for clay additives was broader (Table 1)..In the final stages of this trial the modified granule was releasing a higher concentration of P than that of the modified granule. The width of the curves was measured at the latter-stage crossing point for all three samples, as indicated by the horizontal grey line, and the percentage difference to the standard product peak width was calculated (values shown in Table 1 ). There was only a small difference in peak width and height between the 1% and 4% clay additives with the 1% inclusion being the broader of the two.

The second additive investigated was the effect of binding agent, in this case PVAc. Figure 4 shows the same trends between the standard MAP containing product and the modified granule as shown in the other two additives in that the rate of release of P from the modified granule is slower. From the exponential line of best fit there appears to be little difference in the rate of release with the presence of binder or without. The calculated rate constants are both lower than that of the standard product (Table 1).

The pot trial results (Figure 5) also show that there was little difference between the presence or absence of binder and its effect on rate of release. Rate of release was still slower than that of the standard product with the higher concentrations released in the tail section of the trial than that of the standard product.

Table 1 : Calculated rate constants (k) and peak width difference for model granule formulations and a standard MAP containing fertiliser (SMAP in Table 1).

With Without

SMAP l% Clay 4% Clay

binder binder

k/min-1 0.0022 0.0020 0.0022 0.0018 0.0020

% difference

n/a 116 108 116 116

in peak width

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A method for producing controlled release granules containing an agriculturally active ingredient, which method comprises:

granulating agriculturally active ingredients with an aqueous slurry of an exfoliated clay in the presence of a binder; and

drying the granules.

2. A method according to claim 1, wherein the agriculturally acceptable ingredient is a fertiliser.

3. A method according to claim 2, wherein the fertiliser is a phosphate fertiliser.

4. A method according to claim 1, wherein the exfoliated clay is an exfoliated 1: 1 to 3:1 type clay.

5. A method according to claim 4, wherein the exfoliated clay is an exfoliated 2:1 type clay.

. . -

6. A method according to claim 5, wherein the exfoliated clay is exfoliated 2:1 bentonite.

7. A method according to claim 1, wherein the clay has been exfoliated at elevated pH by addition of a strong base and heating whilst stirring vigorously.

8. A method according to claim 1, wherein the binder is selected from polyvinyl acetate and polydimethyldiallylammonium chloride.

9. A method according to claim 1, wherein the granules contain up to 20% by weight agriculturally acceptable ingredient.

10. A method according to claim 1, wherein the aqueous slurry of exfoliated clay comprises a flocculating agent.

11. A method according to claim 10, wherein the flocculating agent is a water-soluble polyacrylamide.

12. Controlled release granules containing an agriculturally active ingredient when produced in accordance with the method of claim 1.

13. The use in an agricultural application of controlled release granules as claimed in claim 2.