造粒 Granulation

1. 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.

2. Che5700 陶瓷粉末處理 Direct Granulation • Sometimes referred as “pelletizing” process; e.g.pressing, extrusion, spray granulation etc. • used to producealumina, 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

3. 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

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

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

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

7. Spray Drying Che5700 陶瓷粉末處理 • 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

8. 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;

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

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

11. Spraying Drying (3) Che5700 陶瓷粉末處理 • 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

12. Atomization Che5700 陶瓷粉末處理 • 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

13. Droplet Size • Depending on jet breakup mechanism  different equations to estimate droplet size • Rayleigh breakup mechanism  Dd = 1.89 Dj; for high viscous liquid, thenDd = 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;

15. 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

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

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

18. Classification Che5700 陶瓷粉末處理

19. Principle and Techniques Che5700 陶瓷粉末處理 • 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).

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

21. 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

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

23. 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

24. Che5700 陶瓷粉末處理

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