Introduction
Screening is one of the most important parts in aggregate production. It takes place before and/or after the crushing stage (or stages) and produces the specifications required of the final products.
The purpose of screening is to separate a flow of aggregate particles into uniform sizes. This is accomplished by using Vibrating Screens.
What is a Vibrating Screen?
Vibrating Screens are machines used to separate aggregates by particle size by means of an exciting force applied to the machine. The vibration lifts the material, causing it to stratify, allowing the smaller material to move from the top of the depth of material to the bottom of the material, thus exposing them to the screening surface.
Types of aggregate Vibrating Screens
There are two main types of aggregate vibrating screens for dry screening applications:
- Horizontal Screens
- Inclined Screens
The primary difference between Horizontal Screens and Inclined Screens is the Inclined Screen uses gravity to help with the flow and conveyance of the feed material.
However, both screens offer advantages for various applications.
Inclined Screens
Inclined Screens are the most popular for aggregate applications. They are installed on an incline (anywhere from 5-45 degrees depending on the type of screen and the required cut) and employ the use of gravity to propel material down the screen. Since gravity helps to move the material, this type of screen requires low energy and low stroke.
Horizontal Screens
Horizontal Screens are run flat at zero degrees but can be inclined or declined as much as 10 degrees in either direction. Because they don’t rely on gravity to move the material forward, Horizontal Screens require higher energy and a larger stroke.
These types of screens are ideal for low-profile applications where height requirements are limited, which makes them a great choice for portable plants. In stationary plants, they are often used as tertiary or finishing screens. Finer screening applications can greatly benefit from this type of screen. Read more about how McNamara Contracting, Inc. increased tonnage with a McLanahan Horizontal Screen.
Known for their efficiency and reliability, Horizontal Screens offer more accurate particle sizing. The high G-forces required to move the material ensure screen plugging is virtually eliminated, allowing for more continuous open area. The flat orientation of the Horizontal Screen allows material to be retained on the screen longer for higher efficiency. It also makes them a great choice for wet screening applications.
How to select the right aggregate screen
Because screening plays such an important role in producing the required specifications of a final product, it is imperative that the right screen is selected for the job.
Vibrating Screen selection is based on the following factors:
- The maximum tons per hour
- A gradation of the feed material
- The type and weight of material
- The desired size of separation
- Any surface moisture on the material
- Any special operation requirements, including physical characteristics of the feed or product requirements
Sizing Vibratory Screens
Vibratory Screens come in a variety of sizes. They can range from 4’ to 12’ wide and 6’ to 32’ long. The width of the screen determines the carrying capacity of the screen deck, while the length of the screen determines the overall efficiency of the screen. Typically, the length of the screen is 2.5 to 3 times the size of the width.
Every manufacturer has its own screen sizing formula. Screens are tailored to an operation based on the following factors:
- Basic capacity – how much material is going to pass through a certain opening
- Incline – how much of an incline, if any
- Deck – not all of the length of the lower decks may be utilized, as material has already traveled down some length of the upper decks before reaching the lower ones
- Oversize – how much material should go over the deck
- Half size – how much material is half the size of the media opening With a high percentage of half size, the probability of the material passing through the screen openings will be higher, and the screen will be that much more efficient.
- Slot – how large the openings in the screen are
- Condition – applies to both the feed material and the atmosphere around the plant. Is the material wet and sticky? Does it have a high percentage of clay? What is the temperature outside? Is it a wet or dry climate?
- Shape – Round or cubical particles have a higher chance of passing through the screen openings than a flat, elongated particle that has to find just the right position to fall through
- Weight – the density of the material
- Open area – the more open area on the screen, the better the chance particles will find an opportunity to pass through
How many decks do I need for my Vibratory Screen?
Vibratory Screens can be configured with one, two, three or four decks. Some fine screening operations can even have as many as eight screening decks. Three decks are pretty common in the aggregate industry, but the number of screen decks depends on the number of products the site needs to make.
In a typical application where multiple products are being made, the top deck(s) of the screen makes the coarse cut, the middle deck(s) makes a middle cut and the bottom deck(s) make the fine cut.
Each screen deck is covered with screen media containing the openings for the particles to pass through. Screen media comes in many different forms, and the type of screen media plays an important role in screening efficiency.
What type of screen media is best for my Vibratory Screen?
Screen media is all about open area, or the number of openings in the screen. The more openings in the screen, the more opportunities the particles will have to pass through and the more efficient the screen will be.
However, more openings in the screen can decrease the wear life of the media. The correct screen media for an application will balance both wear life and efficiency.
Common types of screen media for aggregate applications include:
- Woven wire
- Polyurethane
- Rubber
- Hybrid
Wove wire cloth is the most consistent, versatile performing media product. It averages 50-70% open surface area in most configurations and provides the most flexibility for operations that need to make frequent media changeouts due to varying product specifications. Overall screening efficiency is good with this type of media.
Wire media
Polyurethane is better suited for wet/wash screens and dry applications with highly abrasive materials. It provides 30-40% less open area than wire cloth; however, polyurethane offers extended wear life over wire media.
Polyurethane media
Rubber media is used for heavier, coarser materials that can damage standard wire screens. It is most often used for dry applications or abrasive materials. Rubber media has less openings than wire cloth, and therefore less throughput; however, they last much longer than wire screens.
Rubber media
Hybrid media combines wire screen with reinforced rubber or urethane strips to fit screen supports. It is highly popular in dry screening applications with high moisture materials and a high proportion of fines. This type of media works to eliminate blinding and pegging, which ends the need to stop production to clean the screen. It provides more open area than polyurethane and lasts longer than woven wire. Depending on the application, production can increase 40% using hybrid screen media.
When sizing and selecting a Vibrating Screen for an aggregate operation, be sure to provide the manufacturer with as much accurate detail about the feed material and the application as possible. From there, they can recommend what type of screen (Inclined or Horizontal) is best for the site, as well as the number of decks required and the type of screen media that will provide the best wear life and efficiency.
