It’s assuming that you are trying to grind the same feed at the same feed rate before and after the change.

First you will get a 10% reduction in capacity all else being equal just because the mills are now overflow mills.

Second, the grates allowed the slurry level in the mill to be kept low at the low (26%) % full ball charge. Now the slurry level is higher and the balls have to fall through a lot of slurry to hit another ball. This dissipates energy. The overflow mills I'm familiar with typically run about 40% full. I don't know if your mill has the power capacity to run at 40% full.

Third, mill % solids has a huge effect on mill efficiency. What mill feed % solids were you running at before, and what is it now? There is an optimum.

Too low and the balls contact steel to steel and don't grind anything because the viscosity of the slurry is too low to keep particles on the ball surfaces where they can be ground.

Too high and the slurry layer on the balls is too thick and viscous and cushions the impact of balls on balls, reducing grinding.

Optimum is about 40 to 45 volume % solids in my experience. Using the specific gravity of the feed this can be converted to weight % solids or SpG slurry.

There may be other reasons but these are contributors. Out of curiosity, what factors drove the change to overflow mills?

Comments seems logical, the feed hardness , throughput, F80, and per solid 72, are kept constant. It will try raise ball level to 30% to reduce the Ball travel distance in slurry before collision. Installed power is 6.2kw and currently running at 4.1kw, although before we were running at less than 4.0kw and get the desired grind.

It’s found that you are floating in this plant, and would suggest that you look carefully at your grind because of that. You may find a coarser grind but improved float recovery and lower residues.

Get your engineer or the supplier of the mill and ensure that your mill is designed to carry the additional weight of steel expected on the journals and bearings and that your lubrication pressure is sufficiently enough to keep the journal oiled.

With grates out your load will increase as the volumes larger inside and then you also want to raise your filling level, so its two components you need to look at.

Look at the cost as well, steel cost money and you power draw is likely to increase as well.

What was the motivation behind the change?

Can you provide me the critical speed of that mill? What is your F80 exactly? And the F50? What size of balls are you adding in the mill? The answer will be, Cr=17.9rpm, F80=212um, F50= ? and using 50mm forged balls.

An interesting problem, unfortunately in this case, I think optimizing the grate design (and maybe the pulp lifter design) might have resulted in a better outcome.

If you are now operating at a higher power draw and still not achieving the old grinding duty, then a new source of inefficiency has been introduced to the circuit. Optimizing your ball size and cyclone configuration could be an option.

I think your problem needs to be examined in more detail and I recommend you get a third party to do some sort of review. If you would like us to help, let me know and I can generate a proposal.

Historically, grate discharge ball mills were used to avoid overgrinding and/or slimes generations when using larger ball size. At the time, smaller ball size did not exist and/or more expensive so that was the only way to achieve the grind economically. However, the paradox is that its use (grate discharge) also limits the size of grinding balls since that their smaller scraps can plug terribly the grates (and restricts and slurry flow at the same time) and/or go through the opening slots with terrible consumption.

Today, situation has changed because smaller balls exist with some better competitive price. Therefore, you shall not base the selection of your grinding media on their price only ($/kg or $/t of steel) like an accountant but rather on the wear and energy efficiency ($/t by taking into account energy reduction). Therefore, the choice of the media will not only depend on their price but also on the price of energy in your region (for which I have no idea what it might be).

Concerning grate discharge VS overflow, based on your recent answer, I did not make the calculation but if 95% of the ball mills are overflow, there is certainly a reason - for which you mentioned them. Otherwise, more grate discharge mills would be in used today.

Recently, we did try to grind below 50 microns with 2 inch balls in our pilot mill from a feed size of about 150-212 microns in a batch process (not enough material to be ground in continuous). After 45 minutes, we were still off the target - reaching it very slowly. However, the amount of every fine particle was increasing rapidly. The reason is because the breakage was decreasing rapidly on particles larger than 100 microns to be the same as the breakage rate of the 25-50 microns range. Therefore, the end result was generation of fines. When we changed the charge for smaller balls, we achieve the grind in 20 minutes and the amount of fines (<25 microns) were much less than using the 2 inch charge. Although that might be different in your process, the principal fundamental of ball size and breakage rate according to particle size remains the same. If you are using the wrong size, then the performance can be terrible - that is exactly what you have.

All this to say that from my point of view and experience, your problem is definitely the size of the ball that you are using, no way can 2 inch balls grind efficiently particle size less than 212 microns. Please stop the 2 inch additions and add smaller balls immediately and that will solve your problem.

We have done some test in our pilot plant comparing grate and overflow discharge. For the same specific energy, the grind was the same.
Grate discharge will absorb 15 to 20% power than an Overflow mill for the same ball filling degree. To achieve your grind, you should increase your ball filling degree to reach the same specific energy as you had in grate discharge.
The main advantage of converting to overflow is on the grinding media consumption, it can be significantly reduce as it allow some harder balls to be used in the mill.

Do not hesitate to contact me for more information.