Che5700 陶瓷粉末處理 造粒Granulation To produce free flowing particles for further processing; often after powder synthesis and before forming of products, may need to add binder/ wetting agent to keep small particles together, (but not to form hard agglomerate), semi-dry granule. Principal methods: spray drying, extruding, simple pressing, mixing + perforated plate, etc; Characteristics: critical range of liquid content for granulation (for each particle system); it affects granulate size, distribution, porosity; fine particles need more liquid.

Che5700 陶瓷粉末處理 Direct Granulation Sometimes referred as “pelletizing” process; e.g. pressing, extrusion, spray granulation etc. used to produce alumina, ferrite, clays, tile bodies, porcelain bodies, conventional refractory compositions, catalyst support, and feed materials for glass or metal refining; Granules may not be spherical, could be cylindrical; Spray granulation: spray (may contain binder) and stir to make pellets

Formation of Granule Can be viewed as nucleation & growth process; Che5700 陶瓷粉末處理 Formation of Granule Can be viewed as nucleation & growth process;  At first, binder solution droplet touch particle  nucleus; capillary force and binder flocculation provide strength Growth by layering through contact and adhesion; or by nuclei agglomeration; Rubbing between granules  make granules surface smooth

Spray granulation uniformity closely related to liquid content; Hardness: mostly related to binder (and particle characteristics)

Spray Granulation Power demand = resistance to flow Liquid requirement is higher when specific surface area is high; Common liquid requirement: 20-36%

Spray Granulation 2 There is a critical liquid content for each process; Granule may need to be dried before use;

Che5700 陶瓷粉末處理 Spray Drying Main method of granulation: produce spherical particles (~20m), high productivity (e.g. ~ 10-100 kg/h); suitable for subsequent pressing process. Use hot air (co-current or counter-current flow) to dry flowing solids Droplet size ~ product size Slurry viscosity: important operation variable, should be shear thinning, shear rate at nozzle ~104/sec

Spray Drying (2) Atomization: large pressure drop at nozzle, significant wear; possibility of blockage; other variables: surface tension, feed rate Drying rate: gas temperature, contact time (usually less than 30 sec); avoid sticking to walls; Due to high temperature: should be aware of possible loss of material along with evaporation; polymer additives: possible cracking or decomposition;

Taken from TA Ring, 1996 Droplet/particle: mean residence time ~ 30 sec Three basic steps: (a)atomization, (b)droplet drying, (c) gas-droplet mixing

Spray dried samples: donut particle, temperature rise too fast, surface dried (sealed), vaporization of internal liquid  pores (viscous binder fluid may flow toward inside)

Che5700 陶瓷粉末處理 Spraying Drying (3) Foam index: bubbles in slurry  low quality of granules, use foam index to represent bubbles in slurry: foam index (%) = [T – E] 100/T; T, E = theoretical and experimental density of slurry (the latter contain bubbles) If necessary, add anti-foam agent; wall deposit problem two-fluid nozzle: to lower pressure drop and to get smaller particles Mass and heat transfer during drying, relative rate  may get dry surface with some internal liquid

Che5700 陶瓷粉末處理 Atomization Some common techniques: high pressure nozzle, two-fluid nozzle, and high speed centrifugal disc; often need to remove large particles from slurry Energy efficiency often low, also about 1% for new surface formation (breakup of steams into droplets), others for heating up the system; Jet breakup mechanism: Rayleigh instability, one dimensionless parameter, Weber number; = aerodynamic force to surface tension force; u1: interfacial velocity between gas and liquid; Ddmax = at critical Weber number, largest stable size; ratio of fluid inertia to surface tension

Droplet Size Depending on jet breakup mechanism  different equations to estimate droplet size Rayleigh breakup mechanism  Dd = 1.89 Dj; for high viscous liquid, then Dd = 1.89 Dj (1 + 31/(1 Dj g) 1/2) 1/6; (Dj = jet diameter;) Gas / liquid interfacial velocity (u1) increase, breakup mechanism more complex; critical Weber number  decide droplet size Nv = dimension-less viscosity;

Droplet Drying In theory, ideal drying (no crust), size of particle and size droplet relations (as follows): Cd & Cp: solid content in droplet and particle; (simple material balance) During solvent evaporation: temperature should decrease; Solvent evaporation  concentration increase  precipitation to get solid particles If crust formation  hollow particles

---------------- Gas-droplet mixing: maybe co-current or counter-current or even cross-current flow; decide contact time and heat and mass transfer effects.

Che5700 陶瓷粉末處理 Characteristics of dried particles: moisture adsorption; flow time; fill density; tap density/fill density ratio etc.

Che5700 陶瓷粉末處理 Classification

Principle and Techniques Che5700 陶瓷粉末處理 Principle and Techniques Wish to separate different particles according to its size, utilize the difference between differently sized particles: e.g. size (sieve opening), motion trajectory; (hydro-cyclone), or forces related to motion; gravity, drag, centrifuge); density, shape or even surface characteristics; Sometimes: feed is separated into two streams (not many streams).

Taken from TA Ring, 1996; can add some baffles, to separate large particles

Che5700 陶瓷粉末處理 Size Selectivity To evaluate performance: size selectivity: SS(d), subscript c for coarse; f for fine; F(d) = cumulative distribution Sharpness index s: ratio of size of particle entering coarse section at probability of 0.25 and 0.75 Cut size: particle over this size all enter coarse section; in reality not so ideal Apparent bypass a: feed directly enter the coarse section

取自TA Ring 1996; Cut size; bypass; Sharpness index b-b’ curve: normal case

Recovery & Yield Classifier performance: recovery R & yield Y Che5700 陶瓷粉末處理 Recovery & Yield Classifier performance: recovery R & yield Y If fines are the product: following equation (if coarse is the product, one can write a corresponding equation) Classifier efficiency: E(d) = Rf(d) – Rc(d); difference between fine and coarse streams

Che5700 陶瓷粉末處理

phase transformation during calcination Gibbsite, Bayerite Al(OH)3; Boehmite AlOOH Diaspore α-AlOOH