From improving durability and flowability through dust reduction to a pronounced depot effect or a targeted nutrient supply via better active ingredient distribution, fluidised bed technology can deliver many benefits. The shape and size of the fertilizer granules can be determined within a defined range. The quantity of fertilizer components delivered to the soil can be adapted optimally, and significantly higher product qualities can be achieved. Solubility and protection against external influences can also be markedly improved.
» Author: Dr.-Ing. Michael Jacob, Head of Process Technology Food, Feed & Fine Chemicals, Glatt Ingenieurtechnik
» originally published in the magazine World Fertilizer, Issue 01-02/2019, Palladian Publications
Fertilizers – both liquid and powder – can pose multiple challenges during handling. Liquid solutions can be unstable and their transport can be complex in many ways. Very fine powders can behave hygroscopically and can be extremely dusty, making storage and application difficult. However, fluidized bed technology can help to overcome all of these challenges. Additionally, it offers an enormous spectrum of possibilities for optimizing the properties of fertilizers, enhancing their functionalization and broadening their scope of application across agriculture.
From improving durability and flowability through dust reduction to a pronounced depot effect or a targeted nutrient supply via better active ingredient distribution, fluidized bed technology can deliver many benefits. The shape and size of the fertilizer granules can be determined within a defined range. The quantity of fertilizer components delivered to the soil can be adapted optimally, and significantly higher product qualities can be achieved. Solubility and protection against external influences can also be markedly improved. In addition, fluidized bed processing, which has been in use since the 1960s, is one of the most effective drying techniques. The technology is particularly suitable for the manufacture of products that require safe handling and dosing – whether this is a high performance fertilizer, an artificial fertilizer or a standard fertilizer based on regular components or recycled raw materials. One of the most interesting advantages of fluidized bed technology is that all of the necessary production steps can be carried out in a single continuous process – without interruptions or machine changes.
Custom particle design
A fluidized bed occurs when process air flowing upward lifts a layer of solid particles and fluidizes them. The process air is used to generate the fluidized bed state and it also supplies the thermal energy required for particle production at the same time. In addition to the thermal treatment of solid materials, fluidized bed processes are used for drying tasks, to form granules from powders (spray agglomeration) or liquids (spray granulation), and to coat particles (spray coating). All particles are always mixed together so intensively that a uniform treatment temperature is ensured across the entire fluidized bed. This means that not only is the drying process very easy to control, but the gentle handling of temperature-sensitive materials is also ensured. Parameters such as granule size, residual moisture content and solids content can be specifically influenced to achieve a wide variety of product properties. In trials, different process conditions and recipes can be tested and compared using variables such as, for example, different process inserts, spray pressures, fluidized bed volumes, temperatures, process gas volumes, residence times and choice of raw materials.
Compact, dust-free granules from liquids
Spray granulation with fluidized bed technology is among the leading processes when it comes to particle design, precisely adjusted particle sizes, dust-free and safe handling and perfect dosing. It is an optimum process for the manufacturing of fertilizer granules from liquids and powders. The uniform round granules dissolve exactly according to the desired time and ambient conditions and do not segregate during transport or storage. Granule formation and subsequent drying can be carried out in a single process step. This facilitates the production of round pellets with a homogeneous structure, dense surface, and high resistance to abrasion. Furthermore, spray granulation enables the drying of liquids while simultaneously forming dust-free granules. Liquids are sprayed onto fluidized particles and dry on their surface, thereby creating a layered accumulation of particles (Fig. 1 + 2). The small particles required to maintain the granulate build-up are generated in the process itself: The only raw material required is the liquid. It is also possible to add specific powder form or fine particle solids to the process in order to integrate them homogeneously into the granule structure or to use them as external starter cores for granule accumulation.
Porous, optimally dispersible granules
Spray agglomeration makes it possible to produce highly soluble fertilizer granules directly from a liquid. Spray agglomerated granules can be used as a temporary condition for liquid fertilizer by first transforming the fertilizer solution into agglomerates for transportation and storage and then liquefying it again before spreading. Another interesting application is fine fertilizer agglomerates, which are applied directly to plants and dissolve immediately so that they are available to the plants on contact with water. Fluid bed spray agglomeration also improves flow behavior and eliminates negative effects such as dust formation. Fertilizer granules therefore behave in a similar way to instant products: They dissolve quickly and are applied in liquid form. In addition, the segregation effects of powder mixtures can be prevented by joining them into agglomerates. Wettable, loose granules with good wetting behavior and which dissolve very well are produced for instant applications. In spray agglomeration in the fluidized bed, powder is sprayed with liquid until sufficient bonding forces are created between the particles (Fig. 3). The agglomerate structure is directly reinforced with simultaneous drying. In accordance with the required properties of the raw material or product, water or any other liquid auxiliary material can be used for granule construction and structure formation.
By applying a coating, granules can be protected against external influences, colored or equipped with additional values – for example, for controlled release. Fluid bed spray coating covers each particle with a defined layer in order to provide optimum protection for active substances or to functionalize the particle surfaces (Fig. 4). The application of the coating material by spraying the liquid containing the solid materials onto the fluidized particles, as well as the drying and reinforcement of the film, are carried out in a single process step. A wide range of different coating layers can be realized in a variety of ways, depending on the required product properties. In hotmelt coating, for instance, the particle shell is formed by solidifying a sprayed-on melt. This method is ideal for fast layer application. Other applications require minimal layer thicknesses (Fig. 5). In this regard, solvent-based processes are an alternative to water-based methods. Suitable fertilizer coatings here would include, for example, amino acids or polymers.
Fluid bed granulators and coaters
Systems featuring a fluidized bed with a circular bottom plate are used primarily for particularly intensive mixing. The diverse array of possible configurations includes different filter systems, various nozzle and spray systems, different solid inputs, granule discharges, and the classifying discharge for continuous operation (Fig. 6 + 7). Glatt GFG fluidized bed granulators, featuring an elongated, rectangular fluidized bed, enable targeted material movement through the process chambers. If necessary, the bed can be divided into zones. This allows the particles to be subjected to various process conditions one after the other in a targeted way, enabling multiple process steps – such as granulation, drying, and cooling – to be undertaken in the same system. And of all this can be done during continuous operation. Whether it is simple particle coating using the top-spray method or functional film coating via the bottom-spray method, it is always important that the spraying of the particles is as even as possible in all coating processes. A special, cleverly designed process chamber construction, in combination with high quality spray systems, makes this possible. But whether the geometry of the apparatus is circular or rectangular, the chamber can also be divided into multiple zones or further chambers if necessary. How decisive the geometry of a multi-chamber system is for the success of a production process has already been shown in an article on methylene urea published earlier this year (World Fertilizer Issue January/February 2018, p xx).
There are numerous technologies available on the market that can produce standard fertilizers effectively and efficiently. However, when it comes to enhancing fertilizers with special properties or making them easier and safer to handle, fluidized bed technology offers virtually unbeatable advantages and freedom. Fluidized bed technologies deliver maximum flexibility in particle forming processes where precisely adjusted properties are key. And, last but not least, several manufacturing steps can be combined into one economical process – a point at which several other technologies reach their limits.
A new way forward: fluidized bed granulation produces phosphate fertilizer from sewage sludge
Phosphorus is an important raw material for every biological organism. It cannot be produced synthetically, but its salts and esters – phosphates – are recyclable. To ensure the accessibility of this precious mineral for the future, phosphate recovery became a legal requirement in Germany for precautionary reasons in October 2017. Working with a partner, plant manufacturer and engineering specialist Glatt Ingenieurtechnik has developed a two-step process that initially releases phosphate from sewage sludge and subsequently transforms it into ready-to-use fertilizer granules using a fluidized bed spray granulation process. This highly efficient system meets today’s market demands for the production of ready-to-use standard and multicomponent fertilizers based on recycled phosphate, whereby the ashes can be reused to provide a 100 percent waste-free product.