Slide 1

Beneficiation To improve both microscopic characteristics and macroscopic uniformity of the system, and also to modify the liquid content and rheology for forming Include processing operations such as: comminution, dispersion and mixing, particle separation and concentration, granulation; Che5700 Slide 2 Crushing and Grinding Crushing, Grinding, Comminution, Milling, Blunger, Pulverizing: several different terminology Purpose: to reduce particle size, to release impurity (from large raw material particles, e.g. minerals), to modify particle shape, size distribution, to increase fine particles, to destroy agglomerates good for: purity improvement, increase surface area(for reaction or sintering), improve flowability, to change microstructure etc. Often require screening after grinding, take out the qualified portion, remove the un-qualified particles; may be contaminated due to wear Slide 3 Slide 4 Taken from TA Ring, 1996; Comminution and classification often go together. Dry or wet operation Batch or continuous operation Slide 5 Equipment Forces acting on particles: impact, compression & shear Slide 6 Jaw Crusher; Rotary Crusher; Crushing Rollers; Hammer- mill JS Reed, 1995; Slide 7 Ball Mill * size/quantity of ball (ball volume often 50% of total volume), rotation speed, quantity of material, property of material, ball material, container material; if wet milling then viscosity also important factor Che5700 media: porcelain 2.3 g/cm 3 ; alumina 3.6; zircon 3.7; zirconia 5.5; steel 7.8; WC 15.6; Slide 8 Laboratory Ball Mills Critical rotation speed: the mill is centrifuging; n c = (1/2 )[g/(R-r)] 0.5 R = radius of mill; r = radius of grinding element; Common choice: 65 80% of critical value Slide 9 Che5700 Many different types of mills: vibratory mill, hammer mill; etc; Fluid energy mill & stirred media mill can produce smaller particles Slide 10 A special product from a company: (as compared to a conventional ball mill) claim to save quite some energy Can reduce size down to 5 m (not attainable by ball mills) Slide 11 Energy Requirement n=1 Kicks Law n=2 Ritengers law n=1.5 Bonds law Che5700 Crushing particles: mainly by shearing & impacting forces, coming from velocity difference between particles and balls; Energy required to reduce particle from size Lo to L, can be estimated by the following equations: energy used in (1) rotating the equipment; (2)crushing the particles; (3)wear of balls Slide 12 Comminution Efficiency The following two equations: for n = 1 and n = 2 (from previous page) Other empirical model: U T = Ac [1/a m 1/a o m ]; U T = energy input in producing a unit of product; Ac: efficiency constant; m = fracture constant; a = mean size Che5700 Slide 13 Energy Consumption Taken from McCabe & Smith, 6e, p. 981 One of the most inefficient operations; probably 99% go to operating the equipment, producing noise, heat, 1% creating new surface; (cooling is necessary) Slide 14 Taken from TA Ring, 1996; size on the right - feed, on the left - product Che5700 Slide 15 Fracture of Particles * Forces on a particle during grinding: compressive and shearing forces; leading to crushing of particle into several pieces, or gradually worn out due to attrition, frictional stress, or rubbing (to produce very fine particles) Che5700 Slide 16 Taken from Chem. Eng. Sci., Aug, 56-62, 1989. Terminology different here Slide 17 Fracture Strength Attrition: produce many very fine particles, yet the original size distribution roughly remains; Some particles may undergo anelastic deformation to reduce stress, difficult to crush (tough particles) Fracture strength of particle: proportional to size and defect of particle: Sf ~ K Ic / (y c 1/2 ) ; c= depth of flaw; y=constant, depending on defect geometry; K Ic material fracture toughness (influence by microstructure) Small particles, size of defect also small greater strength Che5700 Slide 18 Milling Performance Action of grinding formation of internal lattice defects of particles, e.g. dislocation, may change properties of particles, such as refractive index or phase change, etc. Chemical effect: additives (e.g. surfactants, deflocculants) can wet the new surfaces good for the milling process. CMP chemical mechanical polishing (used in IC processing) Milling performance by: [particles generated/time] = (media collision/time) x (particle impact/collision) x (particles generated/impact) Che5700 Slide 19 Milling Performance (2) Media collision frequency increase for small ball size and rotation speed; Particle impact frequency cylinder ball greater than spherical balls, better when particle stick to ball surface, wet grinding improve frequency (at the cost of more energy input) Large balls for large particles, small balls for small particles. Che5700 Slide 20 Particle Fracture Original microstructure: very important factor, defects like vacancy, dislocation will grow due to external force, if crack branching during fracture then produce finer fragments If external force: frictional stress type more attrition effect Slide 21 Milling Practices In general: ball volume ~ 50% of container volume, particle volume fill in the space between balls. Media size/feed size: ~ 25/1 Wear-resistant property of ball: important factor. size distribution often log-normal distribution after grinding; Some estimate: only 1% of energy for producing new surface. Mohr scale of hardness: 1 talc; 2 rock salt or gypsum; 3 calcite; 4 fluorspar; 5 apatite; 6 feldspar; 7 quartz; 8 topaz; 9 carborundum; 10 diamond; Che5700 Slide 22 Taken from JS Reed, 1995; ball mill vs vibratory mill: more cheap Vibratory mill: better results Slide 23 Particle Size Distribution Mostly follow log- normal distributions, but maybe skewed to large sizes Slide 24 Example 17-1 Q: compare ball mill, vibratory mill, planetary mill: particle volume fraction and their grinding efficiency ball media = mill 50% volume; material = interstices of the media (media packing fraction = 55%), particle in slurry = 40% then media charge = (1-0.55) x 0.4 = 0.18 effective volume fraction in mill = 0.18 x 0.5 = 0.09 For vibratory mill, planetary mill: media up to 80% volume then effective volume fraction = 0.8 x 0.18 = 0.14; (greater than 0.09), collision efficiency increase, grinding rate increase Slide 25 Ultrafine Grinding Difficulty in getting very fine particles: small particles more tough, less internal defect re-agglomerate (key: how to prevent this) energy utilization: low semifluid nature of fines protection of fines by large particles (cushion effect) Ways to improve: use closed-loop, continuous operation to remove fine products use grinding aids: surfactant, deflocculants, etc. Che5700 Slide 26 Effect of Ball Size Sample: BaTiO 3 ; ball: zirconia; size: 0.35 and 0.2 mm; sample conc. 1 vol% (7 wt%) Slide 27 Fluid Energy Mill Che5700 Taken from McCabe & Smith, 6e, p.978 One of ultrafine grinder Carried by high speed fluid, cause particle crushing on impact, + classification, to remove fines continuously Particles can be reduced to 0.5 10 m Slide 28 Jet-O-Mill: high pressure air to carry particles, particle- particle collision; particle-wall collision, breaking particles; small particles carried away, large ones return to system; relative velocity 100 m/sec 10- pulverizing zone; 2 classifier 6 venturi nozzle; 7 compressed air