Screening Notes

Screening

Minerals exist in dispersed (separate entity) or combined (host rock) forms in nature.
Differences in hardness, friability, and crushability between the mineral and host rock can lead to mineral liberation through repeated crushing and comminution processes.
Comminution produces particles of different sizes and shapes.
Valuable mineral particles are separated from the gangue over screens.
- Particles smaller than the aperture size pass through.
- Particles larger than the aperture size are retained.
Screening is a cheap and efficient method of concentration.

Purpose of Screening

Industrial screening is used:

To remove oversize material before it is sent to the next unit operation, as in closed circuit crushing operations.
To remove undersize material before it is sent to the next unit operation, which is set to treat material larger than this size.
To grade materials into a specific series of sized (finished) products.
To prepare a closely sized (the upper and lower size limits are very close to each other) feed to any other unit operation.

Screens

A screen is a surface with many apertures.
Particles placed on the screen surface will either pass through the apertures or be retained.
Screen performance is measured by:
- Capacity
- Efficiency
Capacity: Quantity of material fed to the screen per unit time per unit area of screen surface.
Screen efficiency: Perfection of separation of the material into size fractions above or below the aperture size.
Usual efficiencies for industrial screens are 60 – 85%.

Types of Screen Surfaces

Parallel Rods or Profile Bars
- Description: Rod/bar cross sections (Circular, Triangular, Wedge, etc.)
- Applications: Used for lumpy and coarser size particles
Punched or Perforated Plates
- Description: Openings (Circular, In-line and Staggered openings; Square, In-line and Staggered openings; Slot-like, In-line and Staggered openings). Slotted openings are sometimes used for fine particles
- Applications: Used for coarser and small sizes
Woven Wires
- Description: Openings (Square, Rectangle, Triple shute elongated)
- Applications: used for fairly coarse particles; used for fine particles; used for fine particles

Principal Types of Industrial Screens

Stationary Screens
- Grizzly
  - Description: Equally spaced parallel rods or bars running in the flow direction, sloped to allow gravity transport.
  - Applications: Lumpy or coarse separations, scalping before crushing, dry separation.
- Divergator
  - Description: Parallel rods running in the flow direction, fixed at one end, gap increases from fixed to free end, alternate rods diverge at 5º-6º.
  - Applications: Separations in the range of 400 to 25 mm size, self-cleaning and blockage-free, dry separation.
- Sieve Bend
  - Description: Stationary curved screen with horizontal wedge bars at right angles to slurry flow, feed slurry enters tangentially imparting centrifugal action.
  - Applications: Separations in the range of 2 mm to 45 $µm$ , wet separation.
Revolving Screens
- Trommel
  - Description: Rotating, punched or woven wire, slightly inclined cylindrical shell.
  - Applications: Separations in the range of 10 to 60 mm, dry if coarse, wet if fine, also used for scrubbing lumpy or coarse materials.
Vibrating Screens
- Vibrating Grizzly
  - Description: Similar to stationary grizzly, mechanical or electrical vibrations.
  - Applications: Coarse and dry separations, also used as feeders.
- Vibrating Screen
  - Description: High-speed motion to lift particles, mechanical or electrical vibrations, both horizontal and inclined types.
  - Applications: Separations from 200 mm to 250 $µm$ , dry if coarse, wet if fine.

Screen Performance

Factors affecting screening performance:

Feed rate: Effective screening to an almost complete separation may be achieved by low feed rates and prolonged screening duration.
Screen angle: Screen slope affects the angle at which particles will be presented to the screen surface and the speed at which the particles will be conveyed along the screen. To obtain high screen efficiencies, horizontal screens should be used.
Particle shape:
- Spherical particles will pass in any orientation.
- Irregular-shaped particles will only pass if they orient themselves in an attitude that allows passage.
- Elongated particles will pass depending on the cross-section presented.
Open area: The chances of particles passing through apertures are proportional to the percentage of screen open area.
Screening performance is affected by:
- Factors that influence the probability of particle passage through the aperture.
- Factors that influence the number of opportunities the particles are given to pass through the screen mesh.
Particle size: Particles with a size closer to that of the aperture have lesser chances of passing through that aperture. The overall efficiency of the screen is affected by the amount of these near-mesh particles.

Screen Efficiency

Overall Screen Efficiency:

$E = \frac{c (f – u)}{(1 – u) (c – f)} \cdot \frac{(1 – u)^2}{ f (c – u)^2 (1 – f)}$

Where:

$F$ = feed rate, $th^{-1}$
$C$ = overflow rate, $th^{-1}$
$U$ = underflow, $th^{-1}$
$f$ = fraction of material above cut-point size in feed
$c$ = fraction of material above cut-point size in coarse fraction
$u$ = fraction of material above cut-point size in underflow

Calculations

Example 1:
- A gold ore is screened through a 30 mm screen. The average size distribution of the feed, oversize, and undersize were determined.
- The efficiency of the screen is 83.4%.
Example 2:
- From a crushed quartz sample, the fraction less than 2 mm had to be removed by screening.
- The feed sample contained 35% of minus 2 mm material.
- After screening, the oversize fraction contained 10% of minus 2 mm size, and the undersize contained 82% of minus 2 mm size.
- The efficiency of the screen is 73.5%.

Determining Screen Efficiency from Screening Results Table

Get the sum of values in Column 1, Column 2, and Column 3. Each column should have a total of 100.
Prepare Columns 4, 5, and 6 for Cumulative weight percent passing the screen in Feed, Overflow, and Underflow, respectively.
Calculate the cumulative weight percent (increasing amount of sample that passes through the screens from pan to largest screen size).
- Example:
  - For feed, 3.3 mm screen: $100 - 3.5 = 96.5$
  - For 2.3 mm screen: $96.50 - 13.50 = 83.00$
  - For 1.5 mm screen: $83.00 - 33.00 = 50.00$
This is done down to the pan where the value is 0.00 showing that no sample passed through the pan as it is without
The cumulative weight percent value represents the increasing amount of material that passes through the screen from the total amount that was presented to the screen, the balance is what gets retained.

Cut-point

Determine $f$ , $c$ , and $u$ , fractions of material coarser than 1,5 mm in feed, overflow and underflow.
Observe the cumulative weight percent passing values on the cut-point. Subtract the value from 100%:
- $f = (100-50) / 100 = 0.50$
  - Thus, the fraction of material coarser than 1,5 mm in feed is 0,50%.
- $c = (100 - 20) / 100 = 0.80$
  - Thus, the fraction of material coarser than 1,5 mm in overflow is 0,80%.
- $u = (100 - 85) / 100 = 0.15$
  - Thus, the fraction of material coarser than 1,5 mm in underflow is 0,15.

A host rock is a rock formation in which a mineral exists in a combined form, as opposed to being dispersed as a separate entity. During screening processes, differences in hardness, friability, and crushability between the mineral and its host rock facilitate mineral liberation through repeated crushing and comminution.

In the context of screening, "coarse" refers to larger particles that are retained on a screen because they are larger than the aperture size.

The screen efficiency formula helps determine how well a screen separates materials of different sizes. Here's a simplified breakdown:

Fractions: The formula uses fractions ( $f$ , $c$ , $u$ ) that represent the proportion of material above a specific size (cut-point) in the feed, overflow (coarse), and underflow, respectively.
Input and Output: It compares what goes into the screen (feed) with what comes out (overflow and underflow) for a specific particle size.
Efficiency Score: The formula calculates a score (E) that indicates how accurately the screen separates the materials. A higher score means better separation.

I am sorry, but the provided notes do not contain information about dewatering. However, dewatering usually refers to the process of removing water from a solid material or soil by methods such as centrifugation, filtration, or evaporation. It is commonly used in various industries, including wastewater treatment, mining, and construction, to reduce the moisture content of materials for easier handling, transportation, or disposal.