12 March, 2016

Old plastic to clay science: super-size Atterberg cylinder

Size fractionation for sediment and clay minerals analysis is done using a centrifuge or Atterberg sedimentation cylinder. Both methods rely on the Stokes' Law to determine the settling time. The usual size fraction for clay analysis is < 2.0 microns. The settling time is determined by the viscosity of the medium - usually a suspension prepared with demineralised water - as well as its density and that of the material (e.g. clay, sand). Densities of quartz and water may be used for general purpose analysis but should be adjusted if high-quality results are needed. The viscosity is temperature dependent. Size separation must be done at constant temperature (e.g. 20°C). Usual set-ups consists of multiple Atterberg glass cylinders of 25 to 30 cm settling height. A typical set-up is shown below.
A dozen of Atterberg cylinders and glass jars to catch the < 2.0 micron suspensions.
Ordinary Atterberg cylinders have a diameter of roughly 5 to 6 cm. This is great for daily operations. But sometimes you need more and want to produce a concentrate of the sand/silt/heavy mineral fraction, as well. Sadly, swelling clays (e.g. bentonites) are awful to sieve either wet or dry. Sieves will be clogged fast. So, with the help of the department technician and using some old, left-over plastic scrap we built a super-size Atterberg cylinder. It has the same settling height but a diameter of 16 cm! This cylinder can hold half a bucket (~ 5 litres) of clay suspension. You can see the super-sized cylinders and the results of the test run in the image below.
Test run of super-sized Atterberg cylinder using 500 g of bentonite. The bentonite was dispersed in several litres of water.
It is great for separating and keeping tens of grams of sand, silt or heavy mineral fractions that can afterwards be sieved without hassle. The clay fractions (in the bucket) can be re-used in ordinary Atterberg cylinders for a more controlled setting. Admittedly, it has a few downsides. Controlling the settling speed of fine fractions is problematic. The suspension must be poured in and cannot be shaken. The internal locomotion of currents  hinders sedimentation. The prototype has the markings on the inside. We have made a replacement from transparent Plexiglas that I will try to get a picture of in action soon.

Costs: A few hours of playing around.