I would think grind size would have quite a big effect on...

I would think grind size would have quite a big effect on recoveries with a knelson in sulphide ore guessing would need it around a P80 of sub 60 microns, and possibly the use of a Knelson is to reduce the frequency in changing out carbon columns. then there is residancy time as all ready been pointed out at the back end plus extent of agitation to prevent silting
Have to admit I am no metallurgist though just a simple country boy.
No mention of what grade the tailings are testing at to determine recoveries.