I think that it is that the rna in the first centrifugation is tiny particles coming out of solution so need to be spun hard to form a pellet. After decantation the precipitate is only slightly broken into larger conglomerates of rna so this only needs a slower spin to reform the pellet and stick it to the bottom of the tube. Using a second very fast centrifugation may cause the samples to heat up in the centrifuge so would be avoided where possible, It is not possible in the first centrifugation as rna yield would be much lower

Hello Odilija Safinaite

The centrifugation speed of 12000 g is not required during the ethanol washing step because the RNA already exists as a precipitate form in the preceding step. So, you just need to give a 70% ethanol wash to remove any residual salt from the nucleic acid pellet, and centrifuge at a lower centrifugal force for a short spin. The RNA pellet will stick happily to the bottom of the tube.

As mentioned by Paul Rutland, centrifuging the ethanol wash step at 12000g would unnecessarily invite problems.

Best.

Hello Odilija Safinaite

I am pretty sure that nothing will happen if you spun at 12000g instead of 7500g: the 70% ethanol step is to remove salt and isopropanol ...