Our results confirm the established concept that oil and clay anti-caking treatments are only fully effective in preventing the caking of fertiliser that is well made. They are not designed to compensate for faulty manufacture.
We suggest limits for moisture level, granule strength and free dust for the different categories of fertiliser. However within these categories we discovered considerable variation in the required treatment. Products with the same chemical composition and NPK analysis could behave differently when manufactured on different plants. This effect was particularly marked with blended fertilisers. Considerable variations in the granule size, shape and moisture levels of the base materials were noticed. To overcome these difficulties we found that it was necessary to deal with each plant and formulation on an individual basis and to determine the best treatment to use by experimental coating in the laboratory.
From the sieve analysis of caked samples we observed that if more than 10% of the granules were less than 2.0mm in size then long term storage problems could be expected. High dust was often associated with high moisture. Blends were found to be particularly troublesome especially when some of the components consisted of hard irregular shaped granules.
Fertilisers made up with such materials tended to develop higher levels of fines during storage even though properly screened during manufacture. The analysis of the fines showed that they consisted of a mixture of fertiliser dust and coating clay.
Selection of the correct coating oil and clay is an important factor in ensuring that the most cost effective treatment is used. For many manufacturing plants an oil with a viscosity of 135 centi-Stokes at 40oC is satisfactory. However, when granules are porous to mineral oil and are liable to be stored above 30oC for extended periods, a higher viscosity oil is appropriate. In cases where the coating drum is short and the fertiliser cold or where there are inadequate facilities for oil heating a lower viscosity oil should be used.
We have shown that oils dosed with organic amine have their place in the treatment of ammonium nitrate compounds but they can present total failure to many urea compounds. Categorically we would state our experience, it is to avoid the use of amines with urea based compounds.
Laboratory coatings have indicated that oils containing organic acids are possible alternatives to amines in treatment of ammonium nitrate compounds.
Micronised talcs are another alternative to the use of organic amines in the treatment of ammonium nitrate based products and give good protection to both blends and urea compounds at application rates lower than an ordinary clay, providing there is sufficient length of track in the coating drum to ensure that the fertiliser is completely coated.
The cost of treating a tonne of fertiliser in the UK is in the order of £2.00 (1985). The cost differential between using a fuel oil or wax and an oil which is technically designed to suit the process is £0.1-0.2/tonne. When deciding on which oil, to use it is this differential that should be compared with the cost of dealing with caked product (reprocessing, compensation, lost order etc.) rather than direct price comparison between a special coating oil and either wax or fuel oil.
Good coating has to be regarded as part insurance. As well extending storage life it has to create free flowing attractive fertilisers.
P A Mackay, J O Buchanan and Co Ltd., Renfrew, UK.
K S Sharples, Grimsby, UK.
25 pages, 17 figures.