Desliming and Drain & Rinse Screens

An Introduction to Desliming and Drain & Rinse Applications

 In mineral processing two common screen applications are desliming and dense media drain and rinse.  Sizing the screen area is mostly by size distribution and feed rates, the addition of wash water is also very important.  This discussion is about determining the amount of water needed for these two purposes.

 DESLIMING

In mineral processing desliming refers to the removal of very fine particles from a feed stream that can cause operational difficulties in the next and latter processing steps.  For flotation it is often done at around 50 microns.  For dense media operations it is normally done at 0.5 mm (500 microns or 28 mesh(nominal)).  The following discussion is about the second operation, desliming at 0.5 mm.

 For Dense Media applications, desliming is normally defined as the removal of the -0.5 mm fraction from the feed.  This is particularly important when feeding material that is finer than 10 mm.   If the amount of -0.5 mm in the -10 mm product is less than or equal to 6% by weight, desliming is not necessary, but may improve operations.  If the amount of -0.5 mm in the -10 mm product (for industrial minerals, ores, and coal) is equal to or less than 10% but more than 6% desliming by vibrating screens only is possible.  If the amount of -0.5 mm in the -10 mm product (for industrial minerals, ores, and coal) is more than 10% then sieve bends followed by screens is recommended.

 The normal liquid used is water, except in cases where the treated mineral is water soluble than a saturated brine solution should be used.  Since in most cases water is the liquid that is used, that  term will used for this discussion.

The amount of water used for desliming should give a solids concentration of 7% by volume in the underflow from the desliming stage. 

 

Where:

A = the amount of water required

a = % by weight of -0.5 mm in the feed -10 mm

S = amount of -10 mm in the feed in t/hr

 = the specific gravity of solids in the feed (average)

 The +0.5 mm material in the feed has no effect on the total amount of desliming water required.  In general, at least 8 m3/hr of water per 1 m of screen width should be used.

 Two methods of adding the wash water are generally used, sprays and overflow boxes (weirs). 

 For sprays either nozzles or “fish tails” are used.  When using nozzles there should be a minimum of 3 meters of head on the sprays.   And the general design is 4 nozzles per meter of screen width.  For “fish tail” spray bars, the general design is to use 2 sprays for each meter (3 foot) of screen width.

 

DRAIN AND RINSE

The float and sink products from a dense media process are processed in drain and rinse circuits that normally contain a sieve bend followed by a vibrating screen.  Alternatively two sieve bends followed by a screen or a large banana screen can be used.  The most common has been the sieve bend followed by a vibrating screen.

80% to 85% of the media is removed by the sieve bend.  The first part of the screen continues the draining with the sieve bend and drain section media going back to the correct media sump for immediate reuse.  A common practice is to take a minor split (less than or equal to 10%) of this stream to the media recovery circuit to minimize build-up of fine solids in the media.  The drain portion is normally about 25% of the screen length.

The second part of the screen is normally equipped with sprays or wash water to remove any remaining media adhering to the particles.  This goes to a media recovery circuit to reclaim the media.  Rinse is normally done in two sections, with the first using recycled water (if available) and being about 75% of the flow, and the second (final) portion being fresh water and 25% of the flow. 

Following the drain sectionthe amount of media still adhering to the material can generally be calculated by:

 1:   or  2:

Where:

Q = the amount of media adhering in liters/ton of material

d_a =  average grain  size of the material

 = the specific gravity of solids in the feed (average)

 Formula 1 is for coarse (200mm feed) applications and 2 for fine (50mm feed) applications.

 For coal the amount of rinse water can be calculated using

 per meter of width

Where:

Q = the amount of rinse water per meter of screen width (25% fresh and 75% recycled)

D_t = top size of material in mm

 Amount of rinse water for industrial minerals and ores can be calculated using one of the following.

 per ton of material

Where:

Q = the amount of rinse water per ton of material rinsed (25% fresh and 75% recycled)

d_a =  average grain  size of the material

 = the specific gravity of solids in the feed (average)

 Since the current trend is to use large screens a better formula is

 per meter of screen width

Where:

Q = the amount of rinse water per meter of screen width (25% fresh and 75% recycled)

d_a =  average grain  size of the material

 = the specific gravity of solids in the feed (average)