Chapter IISIZE REDUCTION

OBJECTIVES

1. To obtain solid particles in a specific range size and specific surface area

2. To destruct mineral or crystal of chemical bounded in a solid compounds

3. To make separation of desirable components is possible

4. To reduce fibrous materials size to ease their treatments

5. To enhance specific surface area

Solid Raw Materials

Solid in fluid

processing

Industries Size reduction

Solid Materials

Cracking

Crushing

Grinding

Smaller particle

Large size raw solid particles

Synthetic solid materials

Treatable/process able solid particles

sheet, cubic, powder

CLASSIFICATON OF SIZE REDUCTION

Basic: Feed/Original Size

1. Coarse Size Reduction

Designated for hard, irregular solid particles having 2 – 96 inches or larger.

Equipments: Jaw crusher, gyratory crusher, toothed roll crusher, hammer mill

2. Intermediate Size Reduction

Designated for hard, irregular solid particles having 1 – 3 inches. Equipments: cone crusher, crushing rolls, stamp hill, disintegrator

3. Fine Size Reduction

Designated for hard, irregular solid particles having 0.25 – 0.5 inches.

Equipments: ball mill, tube mill, rollers mill

CLASSIFICATON OF SIZE REDUCTION

Basic: Product size

1. Crusher

Designated for crushing of large solid particles into smaller particles of about several inches

These equipments are divided into:

a. Primary Crusher

Designated for crushing of feed having any size of solid particles to obtain smaller particle products having 6 – 10 inches in size.

b. Secondary Crusher Designated for crushing of feed having any size of solid particles to obtain smaller particle products having 1/4 inches in size.

2. Grinder

Designated for size reduction of large solid particles produced by crusher.

These equipments are divided into:

a. Intermediate Grinder

Designated for obtaining smaller particle products having 40 mesh in size.

b. Ultra fine Grinder Designated for size reduction of feed having smaller than ¼ inch in size to obtain smaller particle products having 200 mesh in size.

3. Cutter/Disintegrator

Designated for size reduction of solid particles by cutting and tearing. Suitable for fibrous materials, such as asbestos and wood

Size reduction

Irregular particle shape

Irregular particle size (non- uniform)

Pre-processing

I WANT REGULAR SHAPE AND PARTICLE SIZE

SEPARATION

1. Free Crushing

Fine particles are discharged from the crusher immediately.

Discharging can be done by gravitational force.

Particles are blown using pressurised air, washed with water and discharged by centrifugal force.

This operation may prevent the formation of more fine particles by reducing contact intensity.

2. Choke Feeding

It is reverse of free crushing operation.

Crusher is equipped with feed catcher and maintained to be fulfilled to prevent immediate product release.

This operation may perform a very economical operation as more fine particles may be produced and reduce size reduction operation steps.

Operating Variables:

1. Moisture Content (MC)

Should be less than 3-4 %, as pasting and agglomeration may occur.

For low MC solid materials, coarse and intermediate size reduction is applicable.

For high MC (>50 %), fine size reduction is ore suitable.

2. Reduction Ratio (RR)

RR = ratio of feed to product particle diameter.

Coarser: RR 3 to 7

Fine Grinder: RR >100

3. Material Structures

Toughness, brittleness, fibrous, etc.

4. Material Hardness

Measured by Mosh Degree (1 to 10).

Scale 1 – 4 : Soft

More than 4: Hard

THE SIZE REDUCTION PROCESS

1. Batch

2. Continue

Is a sustainable operation. It Can be grouped into:

a. Continuous Open Circuit (No material is recycled to be processed)

b. Continuous Closed Circuit (Oversize products are recycled into process, economical of crushing power, uniform product size)

CrusherRaw material Products a

Raw material Crusher Products bScreen

Over Size

THE SIZE REDUCTION LAWS

During size reduction process, with the application of mechanical energy materials experience:

Distortion, Tensile/loose, and crack.

Every unit area of solid has it’s specific energy, the formation of new surface needs energy released from stress energy when the solid materials crack.

Terms:

1. Grindability: amount of solid reduced into a specific size using a specific equipment, (Ton/Hour).

2. Relative Grindability: ratio of grindability of a solid material to grindability of a standard material

1. Kicks Law

Energy required to grind a specific material is constant for the same reduction ratio irrespective of it’s original size

Hp = Energy, K= constant, D= initial diameter, d = final diameter

Energy to grind particle 1 inch into 0.5 inch = Energy to grind particle 3 inch into 1.5 inch

d

DKHp log

2. Rittinger Law

Energy required to grind a specific material is proportional to the new surface are formed.

The Rittnger numbers are shown in table 2.1

DdKHp

11

Minerals Rittinger Number

in2/ft-lb cm2/ft-lb cm2/kg-cm

Quartz (SiO2)

37.7 243 17.56

Pyrite (FeS2)

47.7 314 22.57

Sphalerite (ZnS)

121 780 56.2

Galena (PbS)

201.5 1300 93.8

Calcite (CaCl2)

163.3 1053 75.9

Table 2.1 The Rittnger numbers of some minerals on Drp Weight Crusher

The two laws only consider the energy required for the size reduction for a given reduction ratio.

In fact, the total energy should be summation of energy to drive the machine, reduction size energy and friction energy.

3. Bond law

Energy required to grind a specific material into a specific particle size is proportional to square root of product’s surface area to volume ratio.

Dd

EHp i

11100

Ei = Bond’s working index

Ei = Nett energy in KWH/ton of feed required to reduce solid material into a specific size so that 80% of products can through 100 µ screen

General Equations:nX

dXCdE X = particle size

p

f

X

XCE log

fp XXCE

11

fp XXCE

11

Xf = feed particle size

Xp= product particle size

N = 1,

Kick’s Law

N = 2,

Rittinger Law

N = 3,

Bond Law

SIZE REDUCTION EQUIPMENTS

Machines for size reduction are divided into two groups, namely:

1. Coarse crusher for hard materials, such as: jaw crusher, gyratory crusher or disk crusher

2. Coarse crusher for soft materials, such as : hammer mill or toothed roll crusher

1. Jaw Crusher

Consists of Blake type and Dodge type crushers, Blake type is easily encountered in industries, but not for the Dodge type

1. Blake Type Jaw Crusher (BTJC)

- Developed by E. W. Blake (1858)

- usually also called as Double Toggle Crusher

- suitable for reduction of hard materials having 2-96 inches in size

- the biggest size is 8 x 6 ft, cap.1250 ton/hr, requires 400hp (300KW), able to prepare 18 inches particle.

- made of cast steel lined with a tough abrasion resistant metal such as Manganese Steel

- operational cost: expensive

- advantages: able to crack hard materials, large capacity, seldom product discharge clogging

Working Principles of Blake Type Jaw Crusher

- It works based on PRESSURE

- Two main parts of BTJC:

a. Frame: function as a fixed Jaw

b. Movable Jaws is pivoted at the top ad operated by eccentric, pitman, and toggles

The pitman is given a nearly Vertical motion by eccentric, and since one of the toggles is mounted in rigid journals at one end of the crusher frame, the reciprocating motion of the pitman causes he other toggle to move the jaw back and forth.

The jaw is held against the toggle by a tension link and spring. Crushing is accomplished if the movable jaw moves toward the fixed jaw. That means an intermittent power requirement. In order to equalize this, one or two heavy wheels are mounted on the main shaft of the crusher. The machine is driven by flat belts or V-belts

2. Dodge Crusher

- subject to uneven stresses inherent in its design

- only designed in SMALL size

- Movable jaw is pivoted at the bottom and the width of the discharge opening remain constant, therefore yielding a more closely sized product.

- No toggles are required.

- The jaw is operated through pitman by eccentric.

- If only one size reducing machine is employed, the uniformity of product size is an advantage, otherwise it is of limited use.

- The power is applied through along lever, if the crusher becomes clogged enormous stress are set up in the members which become excessive in machines with gape above 11 inch.

- The constant opening of the jaws at the discharge end gives this equipmnt an annoying tendency of CLOG….Not happen in BTJC

Dodge Crusher