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.

Direct Granulation

Che5700 陶瓷粉末處理

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

Che5700 陶瓷粉末處理

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 flowLiquid 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;

Spray DryingChe5700 陶瓷粉末處理

Main method of granulation: produce spherical particles (~20m), 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)

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

Che5700 陶瓷粉末處理

g

DuN dWe

max1

21

u1: interfacial velocity between gas and liquid; Dd

max = at critical Weber number, largest stable size; ratio of fluid inertia to surface tension

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;

Droplet Size

Depending on jet breakup mechanism different equations to estimate droplet sizeRayleigh breakup mechanism Dd = 1.89 Dj; for high viscous liquid, then Dd = 1.89 Dj (1 + 31/(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

3/1)(pp

dd

d

p

C

C

D

D

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.

----------------

•Characteristics of dried particles: moisture adsorption; flow time; fill density; tap density/fill density ratio etc.

Che5700 陶瓷粉末處理

ClassificationChe5700 陶瓷粉末處理

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).

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

Size Selectivity

Che5700 陶瓷粉末處理

)()(

)()(

dFWdFW

dFWdSS

ffcc

cc

75.0)(

25.0)(

dSS

dSS

d

ds

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

Che5700 陶瓷粉末處理

Classifier performance: recovery R & yield YIf 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

cf

ff

ccff

fff

WW

WY

dFWdFW

dFWdR

)()(

)()(

Che5700 陶瓷粉末處理

phase transformation during calcinationGibbsite, Bayerite Al(OH)3; Boehmite AlOOHDiaspore α-AlOOH