Another 3# Experiment

In my first set of experiments, I was interested in determining how slurry affected the process of coarse grinding in 3# barrels. I found that, at least with 3# barrels and 60/90 SiC grit, you should not use a slurry thickener or reuse slurry. 60/90 grit does not need help to mix with the water, so increasing the viscosity slows down the coarse grind process.

Now I am studying how grit size relates to grinding efficiency. The prevailing wisdom is that 60/90 or 80 SiC grit should be used for the coarse grind in 3# barrels, though some use 46/70. I had no reason to doubt this until dshanpnw shared an article found in the Dec 1967 edition of the Lapidary Journal.

In that article, the authors made several discoveries. First, they found that 1 tbs of 80 SiC per pound of rock outperformed higher and lower amounts of 80 SiC grit during 10 day tumbling runs using 3# barrels. Second, they discovered that 1 tbs of 80 SiC per 1 pound of rock outperformed the same amount of different sized grits during 10 day runs in 3# barrels. Third, they determined that 1 tbs of 80 SiC grit outweighs the same volume of different sized grits. Finally, when adjusting the volume of grit used to equal the weight of 1 tbs of 80 SiC, 220 SiC and 400 SiC actually removes more material during 10 day tumbling runs in 3# barrels than 80 SiC and 46 SiC.

Given that this runs counter to what is thought to be best practice, I decided to test this out for myself. I have 2 dual barrel CE tumblers that I used in this experiment. One turns at a slightly higher rate than the other (43.5 rev/min vs 40 rev/min with fully loaded barrels), which I tried to mitigate by swapping the barrels multiple times during the 7 day tumbling run.

I loaded each of the 4 barrels with 954 - 955 grams (approximately 34 ounces) of rock (quartz and other hard rocks from the parking lot at work), 230 mL (approximately 8 ounces) of water, and 72 grams of grit (equivalent in weight to 3 level tbs of 80 SiC grit). Barrel A used 80 SiC, Barrel B used 120/220 SiC, Barrel C used 46/70 SiC, and Barrel D used 220 SiC.



Here are the result of the 7 day run:

46/70 - 7.33% reduction in weight of rocks
80 - 9.32%
120/220 - 11.11%
220 - 10.89%

Only the 46/70 grit was not used up after 7 days:



Also, the 120/220 probably spent more time on the faster tumbler than the 220. I currently have all 4 barrels tumbling to deterime if 120/220 or 220 is more effective.

What might be the explanation? After all, it is expected that coarser grit removes more material than finer grit. Well, I believe that the coarser grit requires more pressure/weight in order to remove more material than is available in a 3# tumbler. Likewise, the lesser pressure/weight does not break down the larger grit as well. Breaking down SiC creates sharp edges that keeps removing material. Thus, the much greater number of smaller grit particles, though individually removing less material, manage to outdo the larger grit. I plan to try this experiment out in 6# barrels. My expectation is that the advantage will swing to the larger grits.

I did observe something interesting. In each barrel I placed one piece of quartzite that I painted yellow:

46/70

Before



After



80

Before



After



120/220

Before



After



220

Before



After



Though the larger grits did not remove as much rock, their large sizes enabled them to remove the paint from more of the recesses of the quartzite.