Knowledge piece onCrushing of Coal and Efficiency of

Crushers

Utsav Kant

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Importance of Cruhsing

• To liberate the minerals/ Coal particles from Gangue / Impurities

• To ease material handling

• To increase the surface area

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History and evolution of Crushers

The development of mechanical crushing hastaken place over the last two centuries and,most probably, started in Cornwall, England, aspart of the Industrial Revolution. Records showthat rolls crushers were being used in Englishmines as early as 1806.

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Cornish Rolls

These had limitations on feed input size

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Jaw Crushers

The next development in the field was theinvention of Jaw Crushers in 1840’s in NorthAmerica. Eli Blake patented the Double toggleBlake Jaw Crusher in 1858. The Blake type Jawcrusher has fixed feed opening where as thedodge type jaw crusher has fixed dischargeopening.

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Jaw Crusher Operation

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Single toggle and Double toggle Jaw Crushers

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Advantages of Double Toggle Jaw Crusher

• It is unique in that it has its hinge pointdirectly above the center of the crushingchamber.

• The advantage of this design is that when themovable jaw strokes, its motion is as nearlyperpendicular as possible to the stationaryjaw.

• This action produces a pure compressionaction that minimizes wear of the jaw plates.

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Features of Jaw Crusher

• Jaw plates can be either smooth orcorrugated.

• While one jaw is fixed, the other jaw pivotsabout a top hinge. This moving jaw is shapedto move firmly and squarely against thematerial, at 250 to 400 strokes per minute.

• There is no rubbing or grinding, onlycompression, which produces a generallycubical product with minimum fines

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Double Toggle Jaw Crusher

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Illustration of How a jaw crusher Works

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Advantages and limitations

• Wear of liners is less

• Fines generation is less

• Simple Structure and Reliable operation

• Crushing is limited to only half of the cycle

• Wider Product size distribution in in blaketype Jaw Crusher

• Choking of the Jaw crusher (dodge Type)

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Rotary Crusher

• Rotary Crusher arrangement was patented in 1869 By James Rutter but it was not success full. The first commercial rotary Crusher was patented by Charles Brown, of Gates Iron Works, Chicago in 1878

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Gates’ Gyratory Crusher

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Single Roll Crushers

• Single Roll Crushers for Stone were developedfor crushing Sticky or Muddy materials whichcould not be handles in Jaw Crushers. A.Thomas patented it in 1864

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Rotary Breakers

• Hezekiah Bradford of Pennsylvania is attributed with the invention of the first breaker in 1893 and his Bradford Breaker design is still in use today.

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Impact Crushers

• Impact Crushers developed at the end of the 19th century. One of the earliest patients was by Milton F Williams in 1895.

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Modes of Size reduction

• Impact – Sharp instantaneous impact between moving objects

• Attrition- Caused by surface sliding over another

• Shearing- Cause by shear forces generated in particle

• Compression – Caused by material trapped between two hard objects movien towards each other

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Sizer in Operation

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Hard grove grindability index

HGI is defined to classify coals on the basis oftheir easy while grinding

It is obtained by grinding 50 g of 1mmx 0.6 mmof dried coal sample in a standardised ball-andrace mill for 60 cycles at a seed of 20 r.p.m. Thesample is then sieved at 74 micro meter todetermine the weight passing through the sieve.This weight is called w.

HGI =13.6 + 6.93 w

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HGI Aparatus

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Size distribution curveSize DifferentialWt % Size Cumulative

25 100

+20 3.29 2096.71

13 8.28 1388.43

10 5.93 1082.50

6 17.50 665.00

3 14.45 350.56

1 20.49 130.06

0.5 6.05 0.524.02

-0.5 24.02 00.00

0

20

40

60

80

100

120

0 5 10 15 20 25 30

Cu

m. W

t%

Size (mm)

Cumulative Wt %

Y-Values

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Bond Work Index

𝑤 = 10𝑊𝑖1

𝑝−

1

𝑓

W = Power required in KWH to crush 1 short tonne ( 2000 pound) of materialWi is the work index of the materialp is the 80 % passing size of the productf is the 80 % passing size of the feed

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Rosin Rammler Distribution

• 𝐹 𝑑 = 100 1 − 𝑒 −𝑑/𝐷 𝑚

F(d) is the cumulative percent of material passing size d in mm

D is the size modulus = 63.2 percent passing size

m is the distribution modulus

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Relationship between HGI and m

𝐻𝐺𝐼 = 35.5𝑚−1.54

For HGI between 48 and 106

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Size reduction equipment

Duty Feed Size Product Size

Coarse Up to 1 m3 100-150 mm

Medium Upto 150 mm 35-75mm

Light Upto 100 mm 25-50mm

Fines 25 mm 150 micron

Ultra fines 1 mm 74 micron

Coarse and medium duty crushers are generally used as Primary Crushers, Medium and light crushers are generally used as secondary crushers. Light duty crushers are generally used as tertiary crushers

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Types of Equipment

• Heavy Duty -Rotary BreakerJaw CrusherGyratory CrusherSingle Roll CrushersOpen Hammer and Ring Impactor• Medium Duty –Cone GyratorySingle Roll CrushersDouble Roll CrushersOpen Hammer and Ring Impactor• Light DutyCone gyratoryHammer millDouble roll crushersSizers

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Rotary Breaker

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Trommel Screens

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Rotary Breaker

• The forerunner of the rotary breaker was the heavy-duty trommel sizing screen.

• This type of screen, used extensively a hundred years ago, was observed to break coal down by its tumbling action whilst having little effect on shale and other harder rocks.

• The breaker consists of an outer fixed shell with an inner rotating cylindrical frame fitted with perforated Plates. This drum rotates at speeds of between 12 and 18 r.p.m. and this speed is expressed as a function of the so-called critical speed which is defined as the lowest speed that will cause rock fragments to be carried around without slippage

• Performs Screening, breaking and segregation.

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Sizer

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Sizer

Inward Rotating Outward Rotating

Inward Rotating

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Sizers

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Single Roll Crushers

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• Consists of a cylindrical toothed roll rotating towards a fixed anvil

Toothed Roll

Anvil

Teeth

Feed

Product

Toothed

RollAnvil

Double Roll Crusher

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Double Roll Crusher

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Roll Crusher

Roll crusher consists of two horizontally mounted rigid cylinders always moving in opposite direction but towards each other and downward. The ore particles to be crushed and nipped are pulled inward through the rolls by friction. The nipped particles are broken by compression. Damage of rolls surface is prevented by mounting the bearing of one of the rolls against a nest of heavy compression coil springs. There is always a considerable wear of the rolls and thus the crushing surfaces are made of replaceable tough steel shell.

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Roll Crusher

• The reduction ratio and efficiency of roll crusher is very low.

• The main advantage associated with roll crushers is small proportion of fines produced and more uniform size of product crushed, than the other crushers.

• Low capacity

• Low reduction ratio = F/P

• Sensitive to top size

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Capacity calculations

• Capacity = D π x RPM x L x S x Bulk density

• D is the Diameter of Rolls

• L is the length of the rolls

• S is the set of the crusher

• Actual capacity is 12 – 25 % of this

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Maximum feed size calculations

Cos θ/2 = (D+S)/(D+d)

• p = F Sin θ/2

• ᶙN Cos θ/2= N Sinθ/2

• ᶙ = Tan θ/2

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Impact Crusher

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Impact Crushers

2/25/2015 6:58:05 AM 42Impact Crushers

(a) Single Rotor (b) Double Rotor

Product

Feed

Feed

Product Product

① Rotor

② Hammer

③ Breaker Plates

④ Screen

① Rotor

② Hammer

③ Breaker Plate

Impact Crushers

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Types of Impactors

• Vertical Shaft Impactors

• Horizontal Shaft Impactors

• Non reversible Impactors

• Reversible Impactors

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Horizontal Shaft Impactor

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Shaft And Blowbar

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Vertical Shaft Impactors

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Effect on Products

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Factors Affecting the efficiency

• Optimum rotor Speed

• Moisture in feed

• Dispersal of feed in crushing chamber

• Feed rate control

• Input material properties like strength and abrasivity

• Wear of blowbar

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Effect of abrasively on impactors

• Most minerals have abrasive properties that limit availability of impactors and Roll crushers.

• As a rule of Thumb, impactors seem to have about ten times the metal wearing rate of rotary and jaw type machines for same reduction ratio

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Hammer Mill

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Hammer Mill (contd.)

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Product

Feed

Hammer Mill

Product

Feed

Reversible Hammer Mill

Hammer Mill (contd.)

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Ring Granulator

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Product

Feed

Cage Mill

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Cage Mill

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57

How to select a Crusher?

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FACTORS AFFECTING THE SELECTION OF CRUSHERS

58

• Purpose of crushing (Primary / Secondary / Tertiary / Quaternary)

• Location (Fixed /Mobile)

• Type of material (Hard /Soft/Medium)

• Abrasiveness (Abrasive/Non Abrasive)

• Feed size of materials (Larger/Smaller/Medium)

• Capacity of crusher (Higher /Medium / Lower)

• Reduction ratio (Higher /Medium / Lower)

• Product Size (Fines /Coarse)

• Operation & Maintenance aspects

• Other Economic aspects

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SELECTION OF CRUSHERS (Based on Capacity)

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Factors to be considered before selection of Crushers

An size-reduction machine will be Limited by

• Size range of Feed

• Natural properties of rocks to be crushed –Hardness and Abrasivity

• Feed rate

• Moisture content

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Crushing efficiency

• Generally the efficiency of crusher has been calculated from the Power point of view.

• As per a data in US, 1 % of the energy generated is used up for Size reduction Processes

• The literature in the area have mostly talked about size energy relationships but we will look into the performance of crusher from customer point of view

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Cyclone efficiency with respect to PSD

1. Generate -0.5 mm particles to the tune of 23 %

2. Minimize + 13 mm particles in Product of the Crusher

3. Generate minimum - 3 mm + 0.5 mm fraction size to maximise the cyclone efficiency.

4. Minimum undersize in recirculating load.

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Cyclone efficiency in different feed size

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Calculation of Efficiency based on these parameters

Size Week 1 Week 2 Week 3 Week 4100.0 100.0 100.0 100.0

20 96.7 98.2 96.9 100.013 88.4 88.1 84.9 78.410 82.5 81.4 77.1 71.16 65.0 63.2 58.8 53.53 50.6 50.3 44.0 41.11 30.1 31.7 25.5 26.4

0.5 24.0 23.9 19.9 20.20 0.0 0.0 0.0 0.0

3 to 0.5 mm 26.5 26.4 24.1 21.0

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PSD Curves for different weeks on OCT

0.0

20.0

40.0

60.0

80.0

100.0

120.0

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Week 1

Week 2

Week 3

Week 4

Size (mm)

Wt

%

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For the Month of September

Sep. Week 1 Week 2 Week 3 Week 4

100.00 100.0 100.0 100.00

20 94.83 95.6 100.0 100.00

13 87.40 77.1 82.1 79.45

10 63.58 68.5 75.2 69.87

6 46.34 51.9 57.9 48.51

3 35.58 39.7 42.2 38.56

1 22.55 25.1 26.9 24.23

0.5 17.27 17.4 20.7 18.71

0 0 0 0 0

-3 + 0.5 mm 18.30 22.29 21.54 19.85

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PSD is affected by

• Feed rate

• Performance of Screen responsible for recirculating load

• Blow bar wear

• Ash content of the feed

• Moisture Content of the feed

• Position of Breaker plate in Impactor

• Physical Properties of coal

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Nordberg HS 1310

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Setting to Throughput rate relations

0

50

100

150

200

250

0 50 100 150 200 250

Min TPH

Max TPH

TPH

Crusher Setting

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0

200

400

600

800

1000

1200

1400

0 50 100 150 200 250

Min TPH

Max TPH

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TPH

Crusher Setting

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Variation of PSD Wrt Setting

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120

Wt

%

size

51 mm

76 mm

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Thank You

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