Powder Segregation Unit Operations PDF

Powder segregation Segregation is the opposite effect to mixing, i.e. components tend to separate out. This is very important in the preparation of pharmaceutical products because, if it occurs, an already formed random mix may change to a non-random mix, or a...

Powder segregation Segregation is the opposite effect to mixing, i.e. components tend to separate out. This is very important in the preparation of pharmaceutical products because, if it occurs, an already formed random mix may change to a non-random mix, or a random mix may never be achieved. Care must be taken to avoid segregation occurring during handling. Variations in particle properties mean that the particles will tend to behave differently when forced to move and hence tend to separate. Particles exhibiting similar properties tend to congregate together, giving regions in the powder bed which have a higher concentration of a particular component. Segregation is more likely to occur, or may occur to a greater extent, if the powder bed is subjected to vibration and when the particles have greater flowability. 8 8 Trajectory segregation – During mixing, larger particles will tend to have greater kinetic energy imparted to them (owing to their larger mass) and therefore move greater distances than smaller particles before they come to rest. This may result in separation of particles of different size. Percolation segregation – Smaller particles tend to fall through the voids between larger particles and thus move to the bottom of the mass. – Percolation can occur whenever a powder bed containing particles of different sizes is disturbed in such a way that particle rearrangement occurs, e.g. during vibration, stirring or pouring. Elutriation segregation – During mixing, or when a material is discharged from a container, very small particles (‘dust’) in a mix may tend to be ‘blown’ upwards by turbulent air currents as the mass tumbles, and remain suspended in the air. When the mixer is stopped or material discharge is complete, these particles will sediment and form a layer on top of coarser particles. 9 9 1  Initially, rate of mixing > rate of demixing Later rate of demixing > rate of mixing An equilibrium situation arises where the two effects are balanced N.B. See Aulton’s Pharmaceutics for: Approaches to minimize segregation: Particle size fraction selection Milling Controlled crystallisation Selection of excipients with suitable densities Granulation Minimise agitation, vibration, movement after mixing Perform different processes in the same equipment to minimise movement in and out Ordered mixtures Ordered mixtures 10 10 2