Mechanical Process Engineering - Particle Technology

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May 7, 2012 ... Process principles of particle separation in particle technology. 3.2 ... 3.3.1 Fundamentals and microprocesses of sieving. 3.3.2 Model of ...
Fig. 3.1 Prof. Dr. J. Tomas, chair of Mechanical Process Engineering

3. Particle separation 3.1 Process principles of particle separation in particle technology 3.2 Evaluation of separation efficiency by separation probability (function) 3.3 Particle separation by sieving 3.3.1 Fundamentals and microprocesses of sieving 3.3.2 Model of screening dynamics 3.3.3 Sieving machines and screens

Fig_MPE_3 VO Mechanical Process Engineering - Particle Technology Particle Separation/Screening Dr. S. Aman/Prof. Dr. J. Tomas 07.05.2012

Figure 3.1

Fig. 3.2 Prof. Dr. J. Tomas, chair of Mechanical Process Engineering

Particle Separation Principles in Particle Technology particle separation characteristic

unit operation float and sink cleaning

operation principle A F L A

channel washing

H

F L

H hydrotable separation

A

F

H density

L

aerotable separation

F

L jigging

H L

A

F

counter current separation A

H cross flow separation

L H

optical, colour, shape

hand sorting

radiation emission, reflexion, diffraction, particle shape

P2

P1

automatic sorting

P2

P1

F

magnetic susceptibility

magnetic separation M NM

electric conductivity

+

electric separation

-

FP

wetting

flotation R

size, shape molar mass 10-5

10-4

A

semipermeable membrane separation 10-3

0,01

0,1

1

10

100

103

C,R F,P

fragment or particle size d in mm Fig_MPE_3 VO Mechanical Process Engineering - Particle Technology Particle Separation/Screening Dr. S. Aman/Prof. Dr. J. Tomas 07.05.2012

Figure 3.2

Fig. 3.3 Prof. Dr. J. Tomas, chair of Mechanical Process Engineering

Separation (Grading) Efficiency Assessment feed

separation

A m

G m

F m

product G

(coarse)

product F

(fines)

A =m  G +m F m

 total mass balance

(1)

 A ⋅ µ A ,i = m  G ⋅ µ G ,i + m  F ⋅ µ F ,i m

 component balance

R m ,G =

 recovery coarse product G  recovery of valuable coarse fraction i

R m ,i =

 grading ratio

Ai =

 separation function of a particle characteristic fraction ∆ξi

(2)

G m A m

(3)

 G ,i m m µ G ,i  = G⋅  A ,i m  A µ A ,i m

(4)

µ G ,i µ A ,i

>1

(5)

 G ,i m m µ G ,i  = G ⋅ = R m ,G ⋅ A i (6)  A ,i m  A µ A ,i m ξ 25 κ= ≤1 (7) ξ 75

Ti (ξ) =

 separation sharpness

Separation/Classification of particles according to size fractions ∆di: 1

coarse product G  m q  m Ti (d ) ρ=const . = G ,i = G ⋅ G ,i = R m ,G ⋅ A i  A ,i  A q A ,i m m

perfect separation

separation function Tj (d)

κ=1 0.75

κ=

misplaced product

d 25 ≤1 d 75

0,3 < κ < 0,6 sufficient 0,6 < κ < 0,8 good 0,8 < κ < 0,9 very good

0.5

feed mass splitting

0.25 cut point dT = d50 0 d25

d50

d75

particle size d

Fig_MPE_3 VO Mechanical Process Engineering - Particle Technology Particle Separation/Screening Dr. S. Aman/Prof. Dr. J. Tomas 07.05.2012

Figure 3.3

Fig. 3.4 Prof. Dr. J. Tomas, chair of Mechanical Process Engineering

Separation Function (Grade Efficiency Curve) Input mass flow rate m A feed

G m

separation

F m

A m

product G

(coarse)

product F

(fines)

 F mass flow rate of fine particles m  G mass flow rate of coarse particles m Masse balance: A =m F +m G m Total: Component balance of each fraction i:

(1)

 A ⋅ µ A ,i = m  F ⋅ µ F ,i + m  G ⋅ µ G ,i m with mass fractions µ A,i

 m = A ,i ,  A , tot m

µ F,i =

 F,i m  F, tot , m

(2) µ G ,i =

F m A m  m = G A m

Mass recovery of fines F

R m ,F =

Mass recovery of coarse G

R m ,G

Total mass balance Eq.(1) can be rewritten as: 1 = R m ,F + R m ,G and Eq.(2) as

 G ,i m  G , tot m

µ A ,i = R m , F ⋅ µ F ,i + R m ,G ⋅ µ G ,i

(1a) (2a)

Separation function: Separation function is defined as the ratio of mass of all particles with size i in the separated coarse product to mass of particles with the same size i in the feed: Fig_MPE_3 VO Mechanical Process Engineering - Particle Technology Particle Separation/Screening Dr. S. Aman/Prof. Dr. J. Tomas 07.05.2012

Figure 3.4

Fig. 3.5 Prof. Dr. J. Tomas, chair of Mechanical Process Engineering

TG (d i ) =

 G ,i m  ⋅µ m µ = G G ,i = R m,G ⋅ G ,i  A ,i m  A ⋅ µ A ,i m µ A ,i

TF (d i ) = R m , F ⋅

From Eq.(2a)

for coarse product

µ F,i µ A ,i

for fine product TF (d i ) + TG (d i ) = 1

The coarse product is used T(di) = TG(di) To find the separation function we can use Eq.(1a) and Eq.(2a) It is necessary to know a) case 1: one of R m ,f or Rm,c and two of three mass fractions

µ A ,i , µ F , i , µ G , i b)

case 2: three mass fractions µ A ,i , µ F,i , µ G ,i

separation function Tj (d)

1

coarse product G

κ=

0.75

d 25 ≤1 d 75

0.3 < κ < 0.6 sufficient 0.6 < κ < 0.8 good

0.5

0.8 < κ < 0.9 very good

0.25 cut point dT = d50 0 d25

d50

d75

particle size d

⇒ Separation function characterizes the quality or grade of separation

Additionally one may use: Fig_MPE_3 VO Mechanical Process Engineering - Particle Technology Particle Separation/Screening Dr. S. Aman/Prof. Dr. J. Tomas 07.05.2012

Figure 3.5

Fig. 3.6 Prof. Dr. J. Tomas, chair of Mechanical Process Engineering

1 ⋅ (d 75 − d 25 ) 2

ET =

ξ=

or

1 d 75 − d 25 ⋅ 2 d 50

Ideal or perfect separation: T(d) 1

0 dT

d

Fig_MPE_3 VO Mechanical Process Engineering - Particle Technology Particle Separation/Screening Dr. S. Aman/Prof. Dr. J. Tomas 07.05.2012

Figure 3.6

Fig. 3.7 Prof. Dr. J. Tomas, chair of Mechanical Process Engineering

3.3 Screening Screening (sieving) is the separation of a mixture of various grain sizes into two or more products by a semipermeable membrane

Feed A

L

G+F

Oversize overflow coarse G

v F Vib.

Undersize underflow fine F

Semipermeable membrane (screening surface) with defined opening size w acts as a multiple go-no-go gauge. The final portions consist of grains of more uniform size than those of the original mixture. Oversize, overflow, plus or coarse product is that remains on the screen. Undersize, underflow, minus or fine product is passing through the openings of screen If screen consist of more then one screening surfaces:

Fig_MPE_3 VO Mechanical Process Engineering - Particle Technology Particle Separation/Screening Dr. S. Aman/Prof. Dr. J. Tomas 07.05.2012

Figure 3.7

Fig. 3.8 Prof. Dr. J. Tomas, chair of Mechanical Process Engineering

One (n) screening or intermediate product is passing through 1th or nth surface and retained on a subsequent surface. The screening surface may consist of d • woven-wire • silk • plastic cloth • perforated or punched plate

w w+s

• grizzly bars and wedge wire sections

Aperture or screen-size opening is the minimum free space between the edges of the opening in the screening surface. Open Area of a screen is the ratio of actual openings versus total screen area Aopen/Atot Aperture

Fig_MPE_3 VO Mechanical Process Engineering - Particle Technology Particle Separation/Screening Dr. S. Aman/Prof. Dr. J. Tomas 07.05.2012

Figure 3.8

Fig. 3.9 Prof. Dr. J. Tomas, chair of Mechanical Process Engineering

Sieve Scale in SI-units (mm or µm): A sieve scale is a series of testing sieves having openings in a fixed succession; for example: the widths of the successive openings have a constant ratio of, e.g. q = 2 or 1.414, while the areas of the openings have a constant ratio of 2. Generally it is:

d n = d n −1 q = d 0 q n e.g. for n = 10 q = n 10 = 10 10 = 1.25

Or for the Tyler scale q = 4 2 or 1.189. The Tyler sieve series adopted by the National Bureau of Standards (USA)

The sieve opening is specified in millimeters, which is understood to be the free opening or space between the wires. Acc. to US-standard wire cloth can be specified by the old nonmetric mesh, which is the number of openings per linear inch counting from the center of any wire to a point exactly 25.4 mm (= 1 inch) distance. 1

2

3

n=mesh

1 inch or 25.4 mm

- In this non-metric US system mesh is used for cloth w < 12.7 mm (2 mesh) - But free openings are used for coarser cloth w > 12.7 mm

Fig_MPE_3 VO Mechanical Process Engineering - Particle Technology Particle Separation/Screening Dr. S. Aman/Prof. Dr. J. Tomas 07.05.2012

Figure 3.9

Fig. 3.10 Prof. Dr. J. Tomas, chair of Mechanical Process Engineering

Testing Sieves are generally used for characterization of particle size distribution.

Fundamentals Probability screening principle uses the fact that particles moving almost at right angles to a screening surface have a low probability to pass through when the particle size is larger than about half of the opening. Classification of screening operations: a) Scalping - Strictly, the removing of a small amount of oversize from a feed which is predominantly fines. Typically, the removal of oversize from a feed with approximately a maximum of 5% oversize Type of screen: grizzly b) Separation (coarse) - separation at w ≥ 6 mm (4 mesh) and larger. Type of Screens: Vibrating screen, horizontal or inclined with small angle. Fig_MPE_3 VO Mechanical Process Engineering - Particle Technology Particle Separation/Screening Dr. S. Aman/Prof. Dr. J. Tomas 07.05.2012

Figure 3.10

Fig. 3.11 Prof. Dr. J. Tomas, chair of Mechanical Process Engineering

Separation (fine) - separation smaller than w < 6 mm (4 mesh) and larger than w ≥ 0.6 mm (48 mesh) …. 1 mm Type of Screens: Vibrating screen, horizontal or inclined; high-frequency and low-amplitude vibrating screens.

Efficiency

Separation (very fine) - separation smaller than 0.6 …. 1 mm. Type of Screens: High-frequency low-amplitude electrically vibrating screens; ⇒ separation in a fluid flow (hydrocyclones, centrifugal wheel separators, wind shifters. ° c) Dewatering - Removal of free water from a solids-water mixture: Moisture influence on screening efficiency

Dry

moist

Wet

Moisture content

Fig_MPE_3 VO Mechanical Process Engineering - Particle Technology Particle Separation/Screening Dr. S. Aman/Prof. Dr. J. Tomas 07.05.2012

Figure 3.11

Fig. 3.12 Prof. Dr. J. Tomas, chair of Mechanical Process Engineering

Screening number (Froude number) k = a/g Throw number kv = aS,max/(g.cosβ) with aS,max β g

maximum particle acceleration perpendicular to screen surface inclination angle of screen surface gravitational acceleration

kv ≤1 1 .. 1,5 1,6 .. 1,8 2,1 .. 2,3 3,0 .. 3,5 3,3 .. 4,0 .. 8,0

Particle movement no throw, planing no throw of particle layer very low throw

Characterisation of screening behaviour very low screening efficiency, obstruction in short time slow screening, obstruction

very resting screening, easy screen able particle between low and resting screening, difficult steep throw screen able particles steeper throw Sharp screening, difficult screen able fine particles very steep throw very sharp and scarified screening

Fig_MPE_3 VO Mechanical Process Engineering - Particle Technology Particle Separation/Screening Dr. S. Aman/Prof. Dr. J. Tomas 07.05.2012

Figure 3.12

Fig. 3.13 Prof. Dr. J. Tomas, chair of Mechanical Process Engineering

Transport of bulk along the sieve Feed A

L hA

coarse G

v

G+F

F Vib.

fine F

Optimal length of sieve → L = v ⋅ τ a) intensive horizontal mixing ( D hor → ∞ ) b) ideally displacement along sieve (Dax = 0) Model is applicable only at h