Fertilizer International 500 Jan-Feb 2021
31 January 2021
Tailor-made additives: keeping fertilizers in shape
FERTILIZER COATINGS
Tailor-made additives: keeping fertilizers in shape
The degradation of fertilizer granules during handling and storage can be economically-costly and therefore needs to be avoided. Preventing unwanted caking and dust formation requires tailor-made coatings, as Kim ten Wolde, product manager at Holland Novochem, explains.
Holland Novochem B.V. has been helping the fertilizer industry to maintain the quality of fertilizer granules and keep these in good shape since its establishment in 1992. The company, working in collaboration with its customers, manufactures tailor-made additives specifically designed for fertilizer granules and the granulation process.
Different fertilizers, different coatings
In our view, there is no such thing as a ‘one-size-fits-all’ approach when it comes to coatings. All fertilizer granules are physically and compositionally different, having been produced from different raw materials and by different production processes. Handling conditions and storage environments for fertilizers also vary widely. Consequently, the problems that require prevention – from dust formation to caking and moisture uptake – are never exactly the same.
The selection of the wrong coating agent can also cause problems. This may increase dust formation, for example, or allow the take-up of moisture from surroundings to continue – leading to caking and ultimately the complete disintegration of granules.
Moisture holds the key
Fertilizers and water are not friends and should never be allowed to meet, except for the final moment when the product is applied to agricultural land. This is the precise moment when water is required to release valuable nutrients to the soil and crops.
Fertilizers are generally hygroscopic, being prone to the absorption of water to a greater or lesser extent. Their behaviour generally varies from very hygroscopic (e.g. magnesium-stabilised ammonium nitrate) to moderately hygroscopic (e.g. urea). Therefore, every effort needs to be taken to prevent contact between fertilizers and moisture, especially when granules are stored and handled in bulk. Almost everyone within the fertilizer industry has seen unfortunate examples of what happens after exposure to moisture – and the huge hard compact lumps that can result from the caking of bulk materials (see photo).
Moisture, the main culprit and cause of such lumping, can originate either from within the fertilizer or be introduced through uptake from the external environment. It is therefore crucial to look at both of the following factors:
- The innate/original moisture content of the fertilizer granule (product quality)
- The tendency of the fertilizer granule to take up moisture from the environment, expressed as critical relative humidity (CRH).
External moisture
Fertilizer granules will absorb water when the relative humidity of the surrounding air, at a given temperature, is higher than their CRH. When fertilizer granules are stored above their CRH for prolonged periods of time, undesirable dissolution and recrystallisation of solids can occur – resulting in crystal bridge formation and caking.
Fertilizer granules are significantly affected by moisture uptake, as can be seen clearly in Figure 1. The uptake of moisture, as well as resulting in caking, can also trigger extreme dust formation in some fertilizer types such as urea. This happens when the original smooth surface of the granule dissolves completely to be replaced by hollow tubes of recrystallised tetragonal urea crystals (Figure 2). These hollow tubes have an even stronger attraction for water due to capillary action. The presence of such crystals ultimately leads to extreme dust formation when granules are subsequently handled.
The take-up of external moisture occurs most readily in the exposed uppermost layer of piled bulk materials, eventually leading to the creation of strong crusts. These surface crusts are often so hard and impervious that they act as an effective seal, completely preventing any water uptake by the underlying fertilizer granules.
For other fertilizer grades only a very thin crust forms on the surface of piles. Upon shipment, this fragile crust can be easily pierced – allowing access to the underlying free-flowing granules.
That is not always the case, though. For other multi-component fertilizers such as calcium ammonium nitrate (CAN), for example, exposure to moisture leads to the creation of ammonium nitrate crystals in the top of stored piles, significantly increasing the amount of dust formed. By increasing the specific surface area, this creates a vicious circle by triggering repeated cycles of moisture uptake, caking and dust formation.
Internal moisture
As well as external uptake, the migration of internal moisture held within fertilizer granules can also result in significant recrystallisation and caking. Temperature differences can promote this by triggering the transfer of moisture from the interior to the surface of granules.
Consequently, when stored without sufficient drying, mobilisation of moisture within the fertilizer itself, as well as water absorption from the external environment, can create problems. Under these conditions, stalactites can form from fertilizer salts due to a dissolution-precipitation process. These stalactites can be very hard and large, making it difficult to handle and reclaim fertilizer piles from bulk storage. Scraping with large machinery, such as a wheel-loader bucket, is often necessary – although this inevitably generates a large amount of dust.
Note: Samples exposed to moisture in a climate-controlled cabinet at 80 percent relative humidity for 72 hours at 20°C.
It is clear from the above discussion that there are many different factors determining the origin and effects of moisture on fertilizer granules. It is equally clear that preventing moisture uptake is vital for maintaining good fertilizer handling properties. For that reason, the continuous development of coating agents – capable of reducing the uptake of moisture as much as possible – has always been a major priority at Holland Novochem.
Moisture protection: the key to fertilizer integrity
Over the years, fertilizer producers and coatings/additives manufacturers have been quite successful at minimising moisture uptake by taking various measures. Chemically, there have been several improvements. Moisture migration in ammonium nitrate can be prevented, for example, by adding salts such as aluminium sulphate and magnesium sulphate to improve stability. This improves the strength of ammonium nitrate granules by preventing volume changes from occurring under normal storage conditions. Technical improvements in the granulation process have also helped. The ability to create smoother and more uniform granules has reduced both moisture uptake and the number of contact points – the two factors that promote moisture migration and caking.
However, the critical relative humidity of a fertilizer – a key indicator of susceptibility to moisture – is an inherent property that cannot be changed or increased. Fortunately, however, it can be effectively mitigated against by the application of a surface coating.
Indeed, it is common practice for fertilizer producers to apply coatings to fertilizer granules to prevent moisture uptake, caking and dust formation. The general function of a fertilizer coating is very simple: it provides granules with a protective layer, so reducing both granule-to-granule and granule-to-air interactions.
Traditional coatings are composed of oils, paraffins, alkylamine and other special additives. Viscous oils, in particular, have proven to be very effective against dust formation, while the use of alkylamines significantly inhibits the caking tendency of fertilizers.
Nevertheless, Holland Novochem believes that a more sophisticated customised approach is necessary to achieve consistent moisture protection, as each type of fertilizer – manufactured via a specific production process – requires its own individual and custom-blended coating mixture. Such tailored coatings also need to take account of the caking tendency of the fertilizer. Fortunately, coatings with good water repellent properties generally possess moderate anti-caking properties, and vice versa.
Holland Novochem’s extensive range of moisture protection coatings have been widely-used globally for many years, ensuring consistent fertilizer quality for the company’s clients. These products are able to keep fertilizers free-flowing by offering reductions in moisture uptake of up to 75 percent.
Controlled moisture exposure tests help illustrate how coatings protect fertilizers. In one test, equal amounts of three calcium ammonium nitrate (CAN) samples – untreated, anti-caking treated and anti-moisture treated CAN – were weighed into cups, and then exposed to moisture by placing in a climate-controlled cabinet at 80 percent relative humidity (RH) for 72 hours at 20°C. The untreated sample has the highest moisture uptake, as can clearly be seen, while the anti-moisture coated sample remains free flowing (Figures 3 and 4).
In another test, a single layer of untreated and anti-moisture treated CAN granules were exposed to moisture in a climate-controlled cabinet at 80 percent RH for one hour at 20°C. The untreated granules absorbed moisture and partly dissolved on the surface, while the integrity of treated granules clearly benefitted from their protective surface coating (Figures 5 and 6).
While every fertilizer has different susceptibilities, anti-caking coatings are unable to completely prevent moisture uptake, as can be seen from the above photos. This is what makes tailor-made coatings, that combine both anticaking and moisture control properties, so essential. The performance of different coatings formulations for moisture control are shown in Figure 7. These results demonstrates that, in practice, not all anti-moisture coatings provide the same levels of protection, with small changes in their formulation creating large differences. This reinforces how necessary it is for coatings providers, such as Holland Novochem, to work together with fertilizer producers to formulate the most effective coating solution.
Note: Samples exposed to moisture in a climate-controlled cabinet at 80 percent relative humidity for one hour at 20°C.
The future outlook for coatings
The fertilizer industry can expect the future to herald major changes. A variety of different pressures are affecting the current business operating environment, including new legislation and a shift in public opinion on fertilizers, pesticides, climate change and renewable energy. The fertilizer industry is changing quickly in response to such pressures, especially in Europe. Newly-introduced EU fertilizer market regulations, for example, will promote both the use of biostimulants and the recovery of nutrients from waste streams. Importantly, they will also require fertilizer coatings to be biodegradable in future.
This biodegradability requirement has prompted fertilizer coatings and additives producers to look beyond mineral oils and paraffin waxes, which have a very limited biodegradability, and seek out alternative bio-based raw materials instead. This is necessary as polymer-based coatings such as PE-waxes will be fully outlawed because of the EU’s ban on microplastics in fertilizer coatings.
Bio-based anti-caking and anti-dust agents have already been rolled-out into the market with great success. Yet there is still a need to close the performance gap between some new bio-based products and traditional coatings. While variations in bio-based raw materials can still cause performance issues, big steps are being made at the moment to place products on the market that deliver even higher anti-moisture performance.
These shifts in the fertilizer coatings market are part of wider changes to the whole of agriculture. Ongoing restrictions on fertilizer use and nitrogen emissions will require a major rethink by the farming industry globally. This will make both the quality of fertilizers and their use efficiency ever more important in future.
Against this backdrop, innovations at Holland Novochem are helping to bring innovative, more efficient and value-added fertilizers to market – by adding micronutrients to coatings and developing coatings with nitrification and urease inhibitors. Such coatings add significant value, both to the coating products themselves and the fertilizers they are coated with. The adoption of controlled-release fertilizers (CRFs), by allowing fertilizers to do more from less, is another important development that should not be overlooked. The future outlook for added-value coatings in the fertilizer market is very bright in our view!
