How do I achieve a robust scale-up for the granulation of fertilisers with a constant particle size distribution?
Robust scale-up in fertiliser granulation means transferring the process from the laboratory or pilot plant into production in such a way that the particle size distribution and the product quality remain the same. What is decisive is not the mere enlargement of the plant, but the stable maintenance of the governing process conditions.
For this, the raw materials and the reference batch must first be characterised precisely, for instance in terms of particle size, bulk density, moisture, binder behaviour and wetting. The target distribution is then established via characteristic values such as d10, d50, d90 and span.
Depending on the granulation process, different scale-up criteria apply. Particularly important are, for example, the Froude number, the fill level, the residence time and the energy input.
The Froude number relates to rotating free-fall, drum and throw mixers as well as to systems with a horizontally mounted mixing tool. In vertical mixers with forced restratification it is not a design or scale-up criterion. What is decisive there is that the mixing principle remains the same across all sizes and that the restratification covers the entire mixing chamber independently of the fill level; transferability is secured in the pilot plant with the original product.
The same particle size distribution arises only when nucleation, growth, agglomeration, breakage and abrasion interact in a controlled manner. The spray rate, drying, liquid addition and heat transfer must therefore be adjusted together.
The safest approach is a combination of pilot trials, statistical design of experiments and process-analytical monitoring. In this way the granulation process can be transferred robustly to production.
How amixon® scales ring-layer granulation (RMG) from laboratory to production scale
The amixon® ring-layer mixer-granulator RMG is suitable for the continuous granulation of fertilisers into round granules of similar size. The mixing chamber is of particularly round design and machined by cutting. This supports a uniform product flow and a reproducible particle size distribution.
Robust scale-up succeeds above all when the balance between grain build-up and grain destruction is kept stable. Precise powder and liquid dosing, defined tool clearances to the wall and a suitable circumferential speed are decisive for this. In this way the particle size distribution remains reproducible, even when the plant becomes larger.
For trials, a very small RMG 10 is also available at amixon®. Process windows can thereby be checked early and subsequently transferred to larger plants. This increases process reliability and eases the path from the pilot plant into production. amixon® can offer, manufacture and service ring-layer mixers and ring-layer mixing granulators of the RMG design; this class is not, however, developed further, because it works with a horizontally supported mixing shaft. The development focus lies on precision mixers with vertical or inclined mixing shafts.