Which process parameters are decisive in promoting agglomeration deliberately rather than destroying it?
Agglomeration arises from the controlled interplay of several parameters. What is decisive is energy input, liquid, temperature and particle properties. The aim is a stable build-up of agglomerates. The rates of growth must exceed the rates of destruction. At the same time the system must remain stable.
The mechanical energy input governs the balance. Shear forces create particle contacts. These are necessary for bridge formation. The shear must not, however, be too high. Otherwise the agglomerates break. Insufficient shear results in unstable structures. Excessive shear causes attrition and disintegration.
The liquid-to-solid ratio is central. The liquid forms the binding bridges. Too little liquid produces weak bonds. The agglomerates then disintegrate easily. Too much liquid leads to over-wetting. Uncontrolled lumps are formed. The bed can become unstable.
The binder properties are also important. The quantity added must be dosed exactly. The rate of addition influences the distribution. High viscosity increases the binding force. The surface tension governs the capillary forces. The wetting behaviour determines bridge formation. The degree of pore saturation influences the structure. Pendular states produce loose structures. Capillary states result in dense agglomerates.
In spray processes, droplet size is decisive. Small droplets can evaporate before they take effect. Large droplets lead to local over-wetting. The spray rate influences the local moisture. The spray pressure governs the distribution. The air-to-liquid ratio determines the droplet spectrum. The aim is uniform wetting.
Temperature governs several effects at once. It influences the viscosity of the binder. It determines the rate of evaporation. The binder must remain liquid for long enough. Only then do stable bridges form. Drying that is too rapid produces brittle structures. Drying that is too slow leads to tackiness and deposits.
The properties of the particles define the process window. A broad particle size distribution improves the packing. This increases stability. Narrow distributions result in weaker structures. Rough surfaces improve adhesion. Smooth surfaces require stronger binders. The surface energy influences wetting.
Flow and movement within the apparatus are likewise critical. The fill level influences the collision rate. Too high a fill level creates dead zones. The gas velocity governs fluidisation. Too low a velocity impairs intermixing. Too high a velocity increases breakage. The residence time determines the uniformity of the product.
Operation must take place within a stable process window. Energy input, liquid and temperature must be matched to one another. The mechanical loading must not destroy the agglomerates.
Modern processes make use of PAT systems. These measure moisture, temperature and particle size in real time. The process can thus be controlled in a targeted way. Agglomeration is built up stably. Destruction is detected early and avoided.
Which process parameters favour targeted agglomeration?
Targeted agglomeration comes about where particle contacts, liquid addition, binder action and energy input are matched to one another. The aim is to build up stable agglomerates without causing over-wetting, unwanted lump formation or excessive particle breakage.
Decisive first of all is the ratio of liquid to solid. The liquid forms binding bridges between the particles; its quantity, viscosity, surface tension and wettability influence the strength, porosity and solubility of the agglomerates. Too small a quantity of liquid produces weak structures, while local over-wetting can lead to coarse lumps or deposits. The liquid should therefore be introduced into the moving powder bed slowly, in metered form and as finely distributed as possible. For this purpose amixon® uses liquid lances that reach down into the lower mixing chamber; the liquid flow is matched to the absorption capacity and the product movement.
The energy input must likewise be chosen in a targeted way. It should permit sufficient particle contacts and uniform wetting, but must not place unnecessary stress on sensitive agglomerates, coatings or coarse constituents. Where required, cutting rotors, whirlers or de-agglomerators can be used to break up existing lumps and distribute the liquid better. These tools should, however, be operated only for as long and as intensively as the product requires.
Agglomeration can be carried out particularly economically and reliably with the amixon® multi-step mixing method. Robust powder components are first mixed intensively in a small partial batch and wetted with liquid. Further powder fractions are then added step by step and, where appropriate, sprayed with liquid again. The fill level thus grows in a controlled manner up to the final batch. Finer powders can envelop slightly over-wetted particles and thereby contribute to a more uniform, less tacky agglomerate structure.
The amixon® AM conical single-shaft mixer is particularly suitable for this procedure. Its low-mounted cutting rotor can work effectively even at low fill levels. Highly concentrated premixes, wetting, de-agglomeration and the subsequent final mixing can therefore be carried out in the same apparatus. Separate premixers, additional product transfers and the associated cleaning effort can be dispensed with.
A further advantage of the multi-step procedure is that sensitive or coarse components are added only at the end. After the intensive wetting and agglomeration step, they are folded in gently at reduced mixing intensity. Dust binding, flowability, instant properties and particle integrity can thus be taken into account together.