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7 February 2004
The purpose of this experiment was to investigate the effect that a flocculant has on the settled volume and the settling rate of clay as it settles out of a clay-water slip. The clay used is EPK and the flocculant is Epsom salts (MgSO4.7H2O).
A series of tests were performed adding different amounts of flocculant to 50g of EPK. In each case the EPK was placed in a clear plastic quart jar. The epsom salts were mixed with 1 cup of warm water and this plus an additional cup of water were added to the quart jar contiaining the EPK. The exception here being the reference case of EPK alone to which no Epsom salts were added. In this case 2 cups of water were added to the EPK. Then the mixture was throughly shaken (with the lid on) until mixed. After this, the jar was stood on a flat table and the time monitored until the clay had settled and appeared to have reached a stable level. This level was then measured and the settling time was noted.
The table below presents the results of these tests. Note that the settled height is measured as this is proportional to the settled volume given that all tests are performed in the same jar, as they were.
| EPK (g) | Epsom salts (g) | Epsom salts (%) | Settling time(s) | Settled height(mm) | Notes |
|---|---|---|---|---|---|
| 50 | 0 | 0 | 2385 | 18 | |
| 100 | 0.1 | 0.1 | 2960 | 42 | 1 |
| 50 | 0.1 | 0.2 | 870 | 22 | |
| 50 | 0.2 | 0.4 | 1720 | 24 | |
| 50 | 0.3 | 0.6 | 2395 | 29 |
1. It should be noted that this 100g test was performed to allow for a 0.1% flocculant addition. This would have been beyond the accuracy of my equipment with only 50g clay. Its inclusion in the graph would thus be suspect and could only be used to give an indication of what a true result with 50g might have been. It is for this reason that it does not appear in the graphs.
Below are the graphs of the flocculant percentage vesus settled height and settling time.
As can be seen there is a definite drop to a minimum settling time at 0.2% (0.1g) of flocculant.
Interestingly, in this case there is a relatively steady rise in settled height (and hence settled volume) with increasing amounts of flocculant. It is not possible to determine from this small sample data set whether there will ultimately be a limit to this increase.
This experiment was inspired by Bruce Girrell in a Clayart thread, where he commented that the settled volume of a deflocculated slip would be found to be lower than that of an unflocculated one. The original post can be found in the Clayart archives here. I intend to test the deflocculated case soon.
As suggested in email by Dave Finkelnburg, the settling rate will reach a maximum as flocculant is added and then decrease as still more is added.
It should be noted that there was an attempt to perform a settling test with just 1 cup of water and a 50g of EPK. This test resulted in a very long settling time forcing it to be abandoned. Dave Finkelnburg suggested that this was likely due to "hindered settling" due to too high a slip density. It should be noted that this test was performed in a jar of smaller surface area, though it is not clear that this had any effect on the settling rate. In the end the settled volume of the unflocculated 50g in 1 cup of water and the unflocculated 50g in 2 cups was within 7%. This test may lead to other interesting tests later but is of little real interest in this test. It is mentioned for the sake of completeness only.
It should also be mentioned here that the apparatus used in this was very primitive as were the measurement methods, a wrist watch for time (with second hand), and a plastic rule with millimeter increments.
Improvements could be made in future tests through
Future experiments will be performed using a different flocculant and using deflocculants to determine if one can demonstrate experientally that a deflocculated slip of this kind will settle in a more compact fashion than either an unflocculated one or a flocculated one.