PartTech Lecture 3 Part B Screening

Sieving

__________ is a separation technique based on the difference in particle size.

Sieve

In sieving, the __________ is responsible for retaining the larger particles.

Sieve plates

__________ have meshed or perforated bottoms which allow only particles of a specific size to pass through it.

Screening

SCREENING VS SIEVING. Identify if the characteristics below are of screening or sieving.

continuous

Sieving

SCREENING VS SIEVING. Identify if the characteristics below are of screening or sieving.

[usually] small diameter wires

Screening

SCREENING VS SIEVING. Identify if the characteristics below are of screening or sieving.

[relatively] large wire thickness

Screening

SCREENING VS SIEVING. Identify if the characteristics below are of screening or sieving.

for heavy-duty and lasts longer

Sieving

SCREENING VS SIEVING. Identify if the characteristics below are of screening or sieving.

batch

Sieving

SCREENING VS SIEVING. Identify if the characteristics below are of screening or sieving.

for light-duty, fine separations

Tyler Standard Sieve Series

Identify the scales used to classify particle sizes (Tyler Standard Sieve Series or US Standard Sieve Series)

Based on a 200-mesh screen with wires 0.0021 in (0.0053 cm) thick and openings of 0.0029 in (0.0074 cm)

US Standard Sieve Series

Identify the scales used to classify particle sizes (Tyler Standard Sieve Series or US Standard Sieve Series)

Introduced by the National Bureau of Standards

Tyler Standard Sieve Series

Identify the scales used to classify particle sizes (Tyler Standard Sieve Series or US Standard Sieve Series)

Other sizes vary by a fixed ratio of √2

US Standard Sieve Series

Identify the scales used to classify particle sizes (Tyler Standard Sieve Series or US Standard Sieve Series)

Based on a 1-mm opening (no.18 mesh) and varying by ∜2

US Standard Sieve Series

Identify the scales used to classify particle sizes (Tyler Standard Sieve Series or US Standard Sieve Series)

Arbitrary numbers

Dutch Weave

Double Crimp

Corrugated Crimp

Lock-Mesh

Lock-Mesh Flat Top

Weave Types (5)

Dutch Weave

Identify the Type of Weave

Weft wires are woven as closely together as possible to obtain a “zero mesh“, i.e., practically without an aperture. [single plain dutch weave (SPW), basket weave, reps, corduroy]

Double Crimp

Identify the Type of Weave

Wires are crimped evenly in both direction and sprung together by the weaving process to provide a rigid mesh used for most normal applications.

Corrugated Crimp

Identify the Type of Weave

Screens have additional crimps in both wrap and weft directions between intersections; used to achieve a rigid mesh where the aperture size is large in relation to the wire diameter. [double intermediate]

Lock-Mesh

Identify the Type of Weave

Similar to double crimp, but with the positive crimps of each intersection, the mesh remain firmly locked throughout its life; traditionally associated with heavy wire diameters.

Lock-Mesh Flat Top

Identify the Type of Weave

Similar to standard lock-mesh but with the crimps on the underside of the mesh only, the top surface remains smooth allowing materials to pass freely across screens.

TRUE

TRUE or FALSE

The smaller the percentage of the open space, the slower the screening.

TRUE

TRUE or FALSE

The larger the percentage of the open space, the shorter its working life.

FALSE

The larger the percentage of the open space, the faster the screening.

TRUE or FALSE

The larger the percentage of the open space, the slower the screening.

TRUE

TRUE or FALSE

The smaller the percentage of the open space, the longer its working life.

[1] particle being too large that it is easily retained

[2] particle being too small that it passes through easily

[3] particle having a critical dimension so that it would be trapped and promote blinding or clogging of the screen surface which occurs when a particle has an approximate size of 1.1D

Situations that a particle may face when it strikes a screen (3)

Aperture (A)

SCREEN CHARACTERISTICS

The clear space between the individual wires of the screen

Square Mesh

SCREEN CHARACTERISTICS

Identify what mesh or screen is being described:

Used in applications in which a cubic shape is required

Slotted Mesh

SCREEN CHARACTERISTICS

Identify what mesh or screen is being described:

Faster screening because of the reduced resistance

Harp Screen

SCREEN CHARACTERISTICS

Identify what mesh or screen is being described:

For screening wet and difficult-to-screen feed material

1 inch/25.4 mm

SCREEN CHARACTERISTICS

Calculations for the Aperture Size

From the formula, what is L and its value (in inches or mm)?

Mesh Number

SCREEN CHARACTERISTICS

Calculations for the Aperture Size

From the formula, what is N?

Wire Diameter

SCREEN CHARACTERISTICS

Calculations for the Aperture Size

From the formula, what is D?

Mesh Number/Mesh Count (N)

SCREEN CHARACTERISTICS

The number of openings (A) per linear inch of a screen. German, American, British

Mesh Number/Mesh Count (N)

SCREEN CHARACTERISTICS

Equal to the aperture (A) expressed to the nearest decamicron (0.01 mm). Indian

Pitch

SCREEN CHARACTERISTICS

The sum of the aperture or opening size (A) and the wire’s diameter or gauge size (D).

Open Area

SCREEN CHARACTERISTICS

Percentage of the actual opening vs the total screen area.

Open Area

SCREEN CHARACTERISTICS

This can be calculated if the mesh count and wire diameters are the same across the width and length.

Aperture Size

SCREEN CHARACTERISTICS

Calculations for % Open Area:

What is A?

Wire Diameter

SCREEN CHARACTERISTICS

Calculations for % Open Area:

What is D?

+20 mesh

SCREEN ANALYSIS

Methods of Reporting Size Fractions

Give other methods of reporting from the given size fraction:

(Ex. Given: Oversize, ¼ in. Answer: +1/4 in)

Oversize, 20 mesh

[-1/4 + 1/8] or [-1/4 / 1/8]

SCREEN ANALYSIS

Methods of Reporting Size Fractions

Give other methods of reporting from the given size fraction:

(Ex. Given: Oversize, ¼ in. Answer: +1/4 in)

Through ¼ in, on 1/8 in

[-10 + 20] or [10 / 20]

SCREEN ANALYSIS

Methods of Reporting Size Fractions

Give other methods of reporting from the given size fraction:

(Ex. Given: Oversize, ¼ in. Answer: +1/4 in)

Through 10 mesh, on 20 mesh

[-200] or [-200 + pan]

SCREEN ANALYSIS

Methods of Reporting Size Fractions

Give other methods of reporting from the given size fraction:

(Ex. Given: Oversize, ¼ in. Answer: +1/4 in)

Undersize, 200 mesh

Differential or Fractional Analysis

SCREEN ANALYSIS

Methods of Reporting Screen Analysis

Reports the mass fraction of particles passing through the bigger screen and retained on the next smaller screen.

SCREEN ANALYSIS

Methods of Reporting Screen Analysis

Example of Differential or Fractional Analysis (see image)

Cumulative ON Screen

SCREEN ANALYSIS

Methods of Reporting Screen Analysis

Reports the mass fraction of particles whose diameter is larger than the mesh or screen opening (% retained)

SCREEN ANALYSIS

Methods of Reporting Screen Analysis

Example of Cumulative ON Screen (See image)

Cumulative THROUGH Screen

SCREEN ANALYSIS

Methods of Reporting Screen Analysis

Reports the mass fraction of particles whose diameter is less than the mesh or screen opening (% passing).

SCREEN ANALYSIS

Methods of Reporting Screen Analysis

Example of Cumulative THROUGH Screen (See image)

Mass flow rate of the feed

MATERIAL BALANCE OVER SCREENS

Consider a binary mixture containing particle A and B,

From the given Diagram:

“F“ refers to _____.

Mass flow rate of the reject (Oversize)

MATERIAL BALANCE OVER SCREENS

Consider a binary mixture containing particle A and B,

From the given Diagram:

“R“ refers to _____.

Mass flow rate of the product (Undersize)

MATERIAL BALANCE OVER SCREENS

Consider a binary mixture containing particle A and B,

From the given Diagram:

“P“ refers to _____.

Xf - mass fraction of A in the FEED

Xr - mass fraction of A in the REJECT

Xp - mass fraction of A in the PRODUCT

MATERIAL BALANCE OVER SCREENS

Consider a binary mixture containing particle A and B,

From the given Diagram:

Xf refers to _____.

Xr refers to _____.

Xp refers to _____.

1. Recovery of desired product

2. Rejection of undesired material

MATERIAL BALANCE OVER SCREENS

Screen Effectiveness or Efficiency

Screen effectiveness/efficiency mostly depends on two (2) aspects:

Recovery of Desired Product

MATERIAL BALANCE OVER SCREENS

Screen Effectiveness or Efficiency

Identify what aspect of screen effectiveness/efficiency is represented:

Represents the ability of the screen to separate all particles of the desired size.

Rejection of Undesired Material

MATERIAL BALANCE OVER SCREENS

Screen Effectiveness or Efficiency

Identify what aspect of screen effectiveness/efficiency is represented:

Separated product must contain minimal amount of particles having sizes other than the desired size.

Screen effectiveness or Efficiency

MATERIAL BALANCE OVER SCREENS

Measure of success of a screen in separating materials A and B

formula for Ea

MATERIAL BALANCE OVER SCREENS

Screen Effectiveness or Efficiency

One measure of the efficiency is the ratio of undersize material A that is actually in the underflow to the amount of A entering the feed (read lungs)

Formula for Eb

MATERIAL BALANCE OVER SCREENS

Screen Effectiveness or Efficiency

Can also be based on the oversize i.e, the ratio of the oversize material B that is actually in the overflow to the amount of B entering the feed (read lungs ulet)

MATERIAL BALANCE OVER SCREENS

Screen Effectiveness or Efficiency

What is the formula for overall efficiency?

MATERIAL BALANCE OVER SCREENS

Screen Effectiveness or Efficiency

The general method of classifying screen effectiveness (Ec) is _________.

where Yp and Yf are the mass fraction of the desired particle in the product and feed respectively

MATERIAL BALANCE OVER SCREENS

Screen Effectiveness or Efficiency

What is the formula for Recovery?

MATERIAL BALANCE OVER SCREENS

Screen Effectiveness or Efficiency

What is the formula for Rejection?

MATERIAL BALANCE OVER SCREENS

Screen Effectiveness or Efficiency

Substituting the formula for Recovery and Rejection in the general method for classifying screen effectiveness (Ec) (read lungs ulet)

• Mesh Size & Wire Diameter

• Screen Capacity

• Blinding

• Moisture

• Direction of Particle Approach to Screen Surface

• Cohesion

• Adhesion

MATERIAL BALANCE OVER SCREENS

Factors Affecting Effectiveness (7)

Screen Capacity

MATERIAL BALANCE OVER SCREENS

Refers to the mass of material that can be fed per unit time to a unit area of the screen.

MATERIAL BALANCE OVER SCREENS

What is the formula for Capacity or Screen Capacity?

TRUE

MATERIAL BALANCE OVER SCREENS

Capacity vs Effectiveness

TRUE or FALSE

The capacity of a screen is measured by the mass of material that can be fed per unit time to a unit area of screen.

TRUE

MATERIAL BALANCE OVER SCREENS

Capacity vs Effectiveness

TRUE or FALSE

Capacity and Effectiveness are opposing factors

FALSE

To obtain maximum effectiveness, the capacity must be small.

MATERIAL BALANCE OVER SCREENS

Capacity vs Effectiveness

TRUE or FALSE

To obtain maximum effectiveness, the capacity must be large.

FALSE

Large capacity is obtainable only at the expense of a reduction in effectiveness.

MATERIAL BALANCE OVER SCREENS

Capacity vs Effectiveness

TRUE or FALSE

Large capacity is obtainable without the expense of a reduction in effectiveness.