Grate Discharge has a steep pulp gradient for a quick discharge operation. A grate and pulp lifter assembly at the discharge end of the mill enables a ball charge of 45%. The grate at the outlet of the shell causes less excessive grinding, compared to an overflow type. Therefore, generally, it is best suited to grind materials up to particles ranging from 60 to 100 mesh.

The overflow type of Ball Mill is designed to overflow and discharge materials from the trunnion on the outlet side.  This creates less bed volume for pulp.  With more pulp in a grate mill you will need more balls, or consume them faster. Generally an overflow mill does not grind as fine as a grate mill and is better suited to fine-grind materials up to particles ranging from 150 to 200 mesh.

From SME Mineral Processing Handbook, Ed. N. Weiss, SME, 1985, page 3C-28 and surrounding article on grinding:

“In grate-discharge mills, due to the lack of pulp in the zone just in front of the grate, there is a loss in grinding efficiency and and increase on ball and liner wear in this portion of the ball mill.”

Look at the pulp filling of charge for overflow and grate discharge mills:

https://www.911metallurgist.com/blog/overflow-vs-grate-discharge-ball-mills

There is much more opportunity for metal-metal wear.

See King, Technical Notes 6. Grinding, 2000:

http://www.mineraltech.com/MODSIM/ModsimTraining/Module6/Grinding.pdf