Wednesday, May 11, 2011

Gradation Errors

The gradation test is the cornerstone for quality control testing.  It is the most requested test and in many ways the most useful test in determining if a product will meet the minimum requirements needed for a job.  But what happens when you get a bad test result and the gradation is out of specification?  There are two places that you should check, in the laboratory and in the plant.

The laboratory can be easily checked for deficiency while testing the product.  As I mentioned yesterday, there are several ways that material can be screened for a gradation test but the same general principal remains.  Layers of screen cloths are set up, with the largest on top and decreasing in size as they go down.  Material is deposited on top and shaken for a designated amount of time so that the smaller material can fall through each screen.  In a perfect test, each screen would only retain the size material between that screen and the one above it, i.e. a #8 screen would only retain material sizes between #8 and #4.  However, this does not always happen and you can have smaller sized material retained on each sieve because of inefficient screening. 

Think about it this way, imagine you're coming out of a baseball game at the end of the game.  There are tons of people crowding through just a few doors, which slows down the process and it takes longer for people to move through the doors than if you'd left earlier in the night.  If you'd left at the 4th inning you probably would have only tried to go through those doors with one or two people so getting to and through the doors takes a lot less time.  It's the same way with screening.  The gradation shaker is calibrated to shake for a certain amount of time, but if there is too much of one size material in your sample, there are more particles trying to get through the screen and it will take longer to shake them through to the next screen cloth.  When removing the rock from each sieve at the end of the test, the technician should be looking to see if the amount of rock on the sieve covers the entire screen cloth or not.  If it does, it is possible that the material needs to shake for a longer period of time or that the sample should be broken into two batches instead of one. 

Also while emptying the sieves, your technician should be looking at their screens to make sure there is no damage to the screen cloth that could allow larger material to pass through the screen than intended (like a hole).  Both of these double checks should take your technician less than 5 seconds and could make a huge difference in your test results. 

If you're confident in the laboratory's test results the next step is to go to the plant and see if the same issues exist on the screens there.  When the screen is not running you should take a look at the screen clothes and be sure that there are no holes in the screen.  This is a good practice for the plant operator to perform at the beginning of his shift before he turns the plant on.  Holes in screens will typically cause a coarser gradation in the product that is directly below the damaged screen, although if the hole is big enough, the extra material could overload the screen deck below and cause a finer gradation instead of a coarser one.   

Plant screens work identically to that of those in the laboratory but instead of material being deposited on top of a screen stack and shaken until it passes through, the material is deposited on the screen and moved down the screen cloth to the end.  How fast the material moves down the screen cloth depends on how fast you are running the plant and this will regulate how much time the material will have in order to pass through each of the screen cloths.  You can have the exact same problem of material not passing through the screen cloths in the plant as in the lab if the material doesn't have enough time to get through due to too much material on the screen cloth.  While you're at the plant you should take a look at how much material is on each screen deck at any given time and look to see if any of the screens look overloaded with material.  The overloading will cause finer gradations in the products that come from your upper screen decks and coarser gradations in the products that come from your lower screen decks.  To reduce overloading you can reduce the amount of material being fed onto the screen, but this will lower your tons per hour of output.  Instead, you could also reduce the angle at which the screen deck is slanted at.  The steeper the angle on the screen deck, the faster the material will move down the screen and the less time it will have to move through the screen cloths.  Reducing the angle will give the material more time to shake through the screen. 

Overloading and holes on a screen are not the only things that can change your gradations but are easy starting points before you delve further into the problem.  What other causes have you found for poor gradation results? 


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