PARTICLE SIZE REDUCTION –COMMINUTIONcrushing, grinding, milling, micronizing and etc

PURPOSE:

To create particles of a certain size and shape

To increase the surface area available for chemical reaction

To liberate valuable minerals held within particles

An energy intensive

Highly an inefficient process

Sugar –three different grades Granular,castor and icing

TYPES OF COMMINUTION EQUIPMENTFactors Affecting Choice of MachineStressing mechanismSize of feed and product Material propertiesCarrier medium Mode of operationCapacity Combination with other unit operation

TYPES OF COMMINUTION EQUIPMENTFactors Affecting Choice of MachineStressing mechanism-

Three stresses mechanism responsible for particle size reduction in mills.

1) Stress applied between two surfaces ( either surface –particle or particle –particle) at low velocity, 0.01 – 10 m/s

Crushing plus attrition

2) Stress applied at a single solid surface (surface –particle or particle-particle at high velocity,10-200 m/s

Impact fracture plus attrition

3) Stress applied by carrier medium –usually in wet grinding to bring about disagglomeration

Machines using mainly mechanism 1 – crushing

Jaw crusher Working principle:

The motor drives the belt pulley and the belt pulley drives the eccentric shaft to rotate, and make the moving jar approach and leave the fixed jaw periodically, to crush, rub and grind the materials repeatedly, thus to make the material slower and slower and gradually fall down and finally discharge from the discharge opening.

Jaw Crusher Features:high reduction ratio, uniform grain size, simple structure, reliable operation, simple convenience and low operation cost.

Stressing mechanism

Machines using mainly mechanism 1 – crushing

Gyratory crusher-has a fixed jaw in the form

of a truncated cone. The other jaw is a cone which rotates inside the fixed jaw on an eccentric mounting. Material is discharge when it is small enough to pass through the gap between the jaws

Stressing mechanism

Machines using mainly mechanism 1 – crushing

Crushing roll machine have a theoretical MAXIMUM

reduction ratio of 4:1. If a 2 inch particle is fed to the roll

crusher the absolute smallest size one could expect from the crusher is 1/2 inch. Roll crushers will only crush material down to a minimum particle size of about 10 Mesh (2 mm).

A roll crusher crushes using compression, with two rolls rotating about a shaft, towards the gap between the rolls. The gap between the rolls is set to the size of product desired, with the realization that the largest feed particle can only be 4 times the gap dimension.

Stressing mechanism

The particles are drawn into the gap between the rolls by their rotating motion and a friction angle formed between the rolls and the particle, called the nip angle. The two rolls force the particle between their rotating surface into the ever smaller gap area, and it fractures from the compressive forces presented by the rotating rolls.

Advantages: give a very fine product size distribution

and they produce very little dust or fines.

Rolls crushers are effectively used in minerals crushing where the ores are not too abrasive and they are also used in smaller scale production mining of more abrasive metal ores, such as gold

Coal production

Stressing mechanism

Horizontal table mill- the feed material falls on to the center of a circular rotating table and is thrown out by centrifugal force. In moving outwards the material passes under a roller and is crushed

Stressing mechanism

Mechanism 2-high velocity impact

Hammer mill-this device rotates inside a cylinder.The particles are fed into the cylinder either by gravity or by gas stream.

Stressing mechanism

Mechanism 2-high velocity impact

Pin mill-two parallel circular discs –each carrying a set of projecting pins. One disc is fixed and the other rotates at high speed so that its pins pass close to those on the fixed disc. Particles are carried in air into the center and as they move radially outwards are fractured by impart or by attrition

Stressing mechanism

Mechanism 2-high velocity impact

Fluid energy mill relies on the turbulence created in high velocity jets of air or stream in order to produce conditions for interparticle collisions which bring about particle fracture

Stressing mechanism

Mechanism 2-high velocity impactMaterial is conveyed from the grinding

area near the jets at the base of the loop to the classifier and exit situated at the top of the loop.These mills have a very high specific energy consumption and are subject to extreme wear when handling abrasive materials. These problems have been overcome to a certain extent in the fluidized bed jet mill which the bed is used to absorb the energy from the high speed particles ejected from the grinding zone

Stressing mechanism

Figure page 12.12

Machines using a combination of mechanism 1 and 2 – crushing and impact with attrition.

The sand mill-vertical cylinder containing a stirred bed of sand, glass beads or shot. The feed, in the form of slurry, is pumped into the bottom of the bed and the product passes out at the top through a screen which retains the bed material.

Stressing mechanism

Machines using a combination of mechanism 1 and 2 – crushing and impact with attrition.

Colloid mill. The motor drives the rotor by the belt to make high speed rotation against the stator, the processed materials produces the spiral impact force downwards by its own weight or outer pressure (may be produced by a pump), thus the material bears strong physical actions such as shear, friction and high frequency vibration, and gets effective emulsification, dispersion and grinding and reaches the effects of being superfine grinded and emulsified.

Stressing mechanism

Figure 12.14 p/g 325

Machines using a combination of mechanism 1 and 2 – crushing and impact with attrition.

Ball mill-a rotating cylindrical or cylindrical conical shell about half filled with balls of steel or ceramic. The speed of rotation of the cylinder –balls are caused to tumble over one another without causing cascading.

Stressing mechanism

Particle size Most important in

selecting the equipment

Size of feed and productSize range of product

Term used

1-0.1 m Coarse crushing0.1m crushing1 cm Fine

crushing,coarse grinding

1mm Intermediate grinding,milling

100 micron

Fine grinding

10 micron Ultrafine grinding

Down to 3 mm

3mm-50 micron

Less than 50 micron

crushers

Ball mills Ball mills

Table mills

Rod mills Vibration mills

Edge runner mill

Pin millsTube millsVibration mills

Sand millsPerl millsColloid millsFluid energy mills

Hardness- measured in Mohs’ scale of hardness

-measure of resistance to abrasion

Abrasiveness-very abrasive material-grounded in mills operating at low speeds to reduce wear of machine parts in contact with the material

Material PropertiesMaterial Moh’s

hardnessBond Work Index (kwh/ton)

Talc 1

Gypsum 2 7

Calcite 3

Fluorite 4 10

Glass Scratches above

11

Apatite 5

Orthoclase Feldspar

6 12

Quartz 7 14

Topaz and beryl

8

Carborundum 9 26

Toughness-material resists the propagation of cracks.

Cohesivity/adhesivity – particles of material stick together and to other surfaces. –related to moisture content and particle size. Decrease particle size or increase moisture content increases

cohesivity and adhesivity

Material propertiesFibrous nature – must be

comminuted in shredders or cutters which are based on hammer mill design

Low melting point – caused by heat generated in the mill- problem can be overcome by using cold air as the carrier medium

Other special propertiesThermally sensitive, have a

tendency to spontaneous combustion or high inflamability-ground by inert carrier medium – nitrogen

Function – serves to transport the material through the mill

Transmits forces to the particles

Influences friction and abrasion

Affects cracks formation and cohesivity/adhesivity

Influence the electrostatic charges and the inflammability of the material

Carrier medium

-gas,liquidCommon gas- air and

inert gas Common liquid- water

but sometimes oil

Batch or continuosBased on throughput,

the process and economics

Capacity batch : few gram to a few tonnes

Capacity continuous- several hundreds g /hr to several thousand tonnes / h

Mode of operation and combination with other operationsCombine with

drying, mixing or sieving

Open circuit – the material passes only once through the mill, and so the only controllable variable is the residence time of the material in the mill

-product size and distribution can be controlled over a certain range by varying the material residence time (throughput)

System is inflexible

Types of milling circuit Closed circuit- the

material leaving the mill is subjected to some form of seiving with the oversize being returned to the mill with the feed material.

-flexible since product mean size and size distribution can be controlled