SMART DOG MININGTM

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VIBRATING SCREENS

This article is meant to be used with and as a backup to the information found in the SDM Screen spreadsheet found at (http://www.smartdogmining.com/tools/Software/default.html)

 Vibrating screens are primarily classification devices, used for separating a feed stream into two or more products of dif­ferent size ranges.  They are also used for dewatering clean coal and refuse products from cleaning processes, and for media recover following dense medium separation.  If they are used to remove excess of water and fines from a feed stream prior to cleaning they are often referred to as desliming screens.  The general nomencla­ture and definition of screens whether they are used for classification or dewater­ing is essentially the same.

 Clean coal and refuse from a wet cleaning device is usually accompanied by large volumes of water which must be removed from the coal.  At the same time, the coal is often separated into various size ranges for subsequent crushing or further dewater­ing action.

 Coarse coal may be sized and dewatered on the same screen.   Coarse coal to be dewatered can either be presized before cleaning or can contain all the fines.  If it is presized, with the fine coal removed it can be dewatered at openings ranging from 0.5 mm to 6 mm, depending upon the market requirements of the coal, unless it is necessary to remove the fine coal caused by degradation in the process.   Fine coal is not usually sized since the main purpose of the screen is to retain the coal solids while removing water.

 Dewatering screen selection is based on handling a bed depth thin enough to be free-draining.  The depth of coal is a function of the size of coal being dewatered.  The smaller the average particle size, the more difficult it is to drain and the thinner the bed must be.  The presence of fine coal particles tends to fill up the voids and hold the water.  Dewatering screens are usually 3.5 M (12 ft.) or longer depending upon the dewatering efficiency required and the size distribution of the coal being dewatered.  Bed depth is normally con­sidered to be no more than three times the largest particle size as a maximum. 

 When dewatering fine coal (12 mm or 6 mm x 0), sizing is not important.  The purpose of the screen is to retain the coal solids while removing the water.  The openings in the screen surface are usually small (0.25 mm to 0.5 mm) and it is necessary to provide sufficient screen area to pass the water.  In order to obtain satisfactory screen surface life, practice is to use profile wire screen surface.

 Because of the tendency of fine coal to pack, stratify, or form a blanket or cake, better dewatering can be obtained if the bed is periodically disturbed.  In order to mix up the bed of coal, cross-dams are usually used, which forces the coal to climb over the dam and makes the bed more porous permitting the free drainage of water.  Many operators use spray water to break-up the bed of coal or a combination of dams and sprays may be used. 

 If there is a large amount of free water with the feed coal, a stationary sieve or sieve bend ahead of the screen may be used to reduce the water.  The capacity of fine coal dewatering screens is influenced by the amount of water with the feed.  If the amount of water is too great, the high entrance velocity will cause the coal to flush-down the deck reducing the available area for the watering.  Under these con­ditions, the surface moisture of the dewatered product will be high and under ex­treme conditions, free water will be discharged with the coal.  In order to prevent this, the amount of water admitted to the feed must be limited.

The use of dense medium in cleaning coal is based on the difference in specific gravity between the coal and its impurities.  A suspension of finely ground media (usually magnetite) and water with the specific gravity maintained at a point be­tween the specific gravity of the coal and the refuse is used.  Media is expensive and it is desirable to recover the maximum amount possible and to recirculate it in­side the circuit.  Following the dense medium process, the clean coal and refuse products are handled separately to remove the water and recover the media.

 A media recovery screen has two sections, a drain and a rinse section.  The first portion of the screen drains the media from the stream and returns it back to the system.  The second portion washes and then dewaters the coal, or refuse.   Drain and rinse screens commonly have two compartments underneath them; the first portion the drain, and the second portion the rinse product.  The drain portion returns directly to the dense medium circuitry, often times through a control system to help maintain the gravity.  The rinse portion goes to dilute media for media recovery.  In order to perform these operations, long screens, often 5 M (16 ft.) or longer, are usually selected.  The drain section is usually the first 1 to 1.5 M (3 to 4 ft.) at the feed end of the screen.  Following the drain section, material is washed using spray water and the media recovered.  Media recovery screens are selected on the same basis as others, using the depth of bed that can be successfully drained and rinsed.  In general the finer the coal, the more difficult to drain and the thinner the bed must be.

 To reduce the moisture in a fine refuse product, recovery screens are often used to partially dewater the material before disposal.  This water is then reused or dis­carded, as necessary. 

 Equipment Sizing

Raw Coal/Desliming Screens  Raw and desliming screens are sized on the basis of product through the screen deck or the overflow discharged over the screen deck.  Spray or rinse water is added for maximum efficiency in desliming. The underflow should at least be 10% solids.   

Dense medium Drain and Rinse Screens  Screen capacity for drain and rinse screens is specified in regard to feed capacity, based on average grain size.  Rinse water is added in two locations, one immediately following the drain section, and the second half way along the rinse section.  Rinse water normally comes from two sources; partly fresh and partly as recirculated water from the dilute media recovery circuit.  At finer average grain sizes the rinse water should be increased.  For minimum magnetite loss higher flow rates of fresh water are preferred.  A general excepted factor of 2% degradation in particle size in a dense medium vessel circuit is excepted. 

 Clean Coal Dewatering Screens  Clean coal dewatering screens are sized in a similar manner to raw coal and dense medium drain and rinse screens.  Sufficient spray water should be added to bring the underflow concentration up to 10% by weight solids. 

 Refuse Dewatering  Refuse dewatering screens will be sized similar to dense medium cyclone drain and rinse screens.

 Calculations for other vibrating screen sizing can be done using the SDM Screen spreadsheet found at (http://www.smartdogmining.com/tools/Software/default.html)

 Performance

Screen performance is very intimately related to sizing and has been covered in the previous section. 

 Screen efficiency for raw coal is commonly taken at 80 to 85% of the near size material.  Following desliming stage overall efficiency should be on the order of 95%.  The moisture content of the deslimed coal will be in the range of 16 to 20% by weight.  A value of 15% could be used to be optimistic with a 18% being a good plant design factor. 

A general accepted factor of 2% degradation in particle size in a dense medium vessel circuit is used.  Media carry-over from drain to rinse section can be 2-5% of media fed to screen. 

 Experience has shown that coal can have a degradation of 6% and the refuse can have up to 2% degradation.  Media carry-over can reach 5-10%. 

 In as much as the refuse and fine clays are removed the product surface, moisture will be lower than from raw coal screening.  It is commonly expected that + 38 mm (1-1/2") dewatered clean coal will have a moisture content of 3 to 5% by weight.  38 mm  by 12 mm (1 1/2" by 1/2") coal will have a surface moisture of 5 to 7%, and 12 mm x 6 mm (1/2" by 1/4") coal with surface moisture of 15%. 

 Calculations for other screen performance for alternative configurations can be done using the SDM Screen spreadsheet found at (http://www.smartdogmining.com/tools/Software/default.html)

 

 

 

MIke Albrecht, P.E.

o   40+ years’ experience in the mining industry with strong mineral processing experience in Precious metals, copper, industrial minerals, coal, and phosphate

o   Operational experience in precious metals, coal, and phosphate plus in petrochemicals.

o   Extensive experience studies and feasibility in the US and international (United States, Canada, Mexico, Ecuador, Columbia, Venezuela, Chile, China, India, Indonesia, and Greece).