Flow and bulk density enhancements of pharmaceutical powders using a conical screen mill: A continuous dry coating device

The conical screen mill or Comil, a powder delumping device, was investigated due to its recent success as a continuously operated dry coater. The improvements in packing and flowability for different size grades of active pharmaceutical ingredients (APIs) (acetaminophen, ibuprofen, and ascorbic acid), and excipients (Avicel and lactose), were assessed as a function of Comil passes, guest particle hydrophilicity, and the silica amount. First, a limited design of experiments was performed to identify standard Comil operating conditions, such as the impeller speed and powder feed rate. Detailed investigations showed that although a single Comil pass produced sufficient flow enhancements, a second pass was suggested for more cohesive powders. The hydrophobic silica yielded better results due to improved silica de-agglomeration and coating, which could be predicted through the host-guest surface energy differential. Additionally, using silica amounts equivalent to achieving theoretical 100% surface area coverage (SAC) provided better property improvements than commonly used 1. wt% silica. The improvements in bulk density and flow function coefficient using Comil were shown to be comparable to a well-studied batch-coater, magnetically assisted impaction coating (MAIC). Granular Bond numbers of Comil dry coated powders, estimated using contact-models, were one order of magnitude lower than for uncoated powders; whereas, the concept of aggregate granular Bond number was shown to better explain the flow enhancements for finer cohesive powders. The results presented suggest that Comil is a viable continuous dry coating device for powder flow improvement and can have a positive impact on the processing of poorly flowing APIs.