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Being hygroscopic, solid fertiliser tends to absorb moisture, leading to caking and handling issues. The fertiliser can also create dust, a work environment hazard. However, both these issues can be addressed by coating the fertiliser, and process oil can be used either neat or in formulated coatings, also containing wax and hydrophobic amines.
Unfortunately, there are only a few standard methods for coating fertiliser or for relevant testing of coated solid fertiliser on a lab scale, so in order to study how their naphthenic process oil products perform in this application, Nynas had to develop their own in-house coating and testing equipment and procedures.
“Fertiliser producers or their coating formulators often use their own internal methods, but we wanted to be able to prepare and test fertiliser coatings in our own lab,” explains Emma Öberg, Development Engineer at Nynas.
Having set up a testbed for the comparative evaluation of different oils, she was able to determine that naphthenic process oils offer benefits and can be used to replace diminishing volumes of Group I oils in this application. However, Nynas products also showed performance advantages compared with Group II oils.
“The study shows that naphthenic process oils keep amines in solution at lower temperatures than Group I or Group II oils. This allows an anti- caking coating to be sprayed at lower temperatures, enabling cost reductions for processors,” says Emma Öberg.
Amine and oil blends prepared with the naphthenic mineral oil NYTEX® 820 had a lower cloud point compared with Group I and Group II oils of the same viscosity, indicating higher solvent power. Figure 1. shows the correlation between the cloud point for amine and oil blends and the respective oil’s aniline point.
The study also looked at the anti-dust performance of various Nynas products. Figure 2. shows the dusting tendency of untreated and oil coated multi-nutrient NPK (Nitrogen, Phosphorous and Potassium) fertiliser. The results indicate that dust levels can be significantly reduced by coating the fertiliser with a naphthenic process oil.
“Testing different oils showed that the addition of Nynas process oil to NPK fertiliser reduced dust by 50 percent,” says Emma Öberg.
The results of Nynas’ fertiliser coatings study can be summarised in a checklist for coatings formulators or fertiliser companies.
• Naphthenic process oils have higher solvency and will keep amines in solution at lower temperatures than Group I and Group II oils.
• A higher nitrogen content will typically make the fertiliser cake more easily.
• Consider the fertiliser’s Critical Relative Humidity (CRH) value when estimating which type of coating is needed.
• Choose oils of low colour to coat Calcium Ammonium Nitrate (CAN) if it is important to keep its white colour intact.
• Ammonium sulfate (AS) has lower caking tendency, so will need mainly anti-dust coating.
Figure 1. Correlation between cloud point for amine and oil blends and aniline point of the oil. Oil aniline point: Group I (115 cSt at 40 °C, AP 116°C), Group II (106 cSt at 40 °C, AP 126°C), NYTEX® 820 (110 cSt at 40 °C, AP 86°C).
Figure 2. The dusting tendency of NPK fertiliser can be significantly reduced by coating with a naphthenic process oil.
Emma Öberg is a Development Engineer, Chemical Manufacturing Industry, Nynas Naphthenics. She joined Nynas in 2016.