Agglomeration in multi-stage spray dryers involves recirculating fine material into the nozzle zone, where dry particles collide with drying droplets to form agglomerates. This process enhances powder properties compared to methods lacking direct control over product characteristics. A mechanistic approach was utilized to detail the agglomeration mechanisms, with a marker powder added to the fines return system of a pilot plant spray dryer to quantify the incorporation of dry particles into droplets. The marker powder was found throughout the agglomerate size distribution, with surface coverage increasing alongside the total solid content of the feed solution (TS). Coalescence of fines and droplets was influenced by the drying state of the droplet, increasing as TS decreased. The properties of agglomerates suggested that nozzle zone agglomeration is affected by counterflow air classification in the drying chamber. A correlation between fines mass flow rate and agglomerate properties was observed, with a reduction in fines mass flow linked to decreased coalescence. Higher total solids resulted in more pronounced changes in agglomerate size and sphericity, creating a loose "grape-like" structure. Agglomeration efficiency improved with increased fluidized bed airflow, driven by collision frequencies, while sticking probabilities significantly influenced agglomerate structure formation. Enhanced agglomeration efficiencies led to q
