There are at least two critical changes to your throughput when lowering mill speed:

Lower mill RPM = lower throughput due to lower circulation rate at critical comminution zone near toe of charge where ore changes direction with maximum ore particle pressure and shear work are exerted.

Lower mill RPM = lower charge lift (shoulder height) and therefore lower rock pressure at toe, which further reduces rate of comminution

Properly designed lifters will allow higher mill speed and comminution rate.
Lower speed will however reduce rate of lifter/liner wear

We model mill throughput for any lifter and liner geometry, mill end geometry mill speed, ball size and population, mill loading, ore strength, size distribution, et al.

There is a relatively simple solution to this question. A lower speed implies lower power input requirement to the mill, hence lower throughput.

We have calibrated power based models that can make these predictions almost instantaneously provided we know what the ore properties and operating conditions are. Let me know if I could be of some help.

Mill liners ultimately wear over a period of time, so the dynamics internally of the mill are changing all the time. These effects are sometimes compensated via varied process operating conditions and changing feed characteristics (selective mining and ore blending).

Just had a quick look online, you are operating a low aspect single stage SAG mill / ROM ball mill? Is this correct?

Without jumping to conclusions, may I ask for what reasons you are investigating reducing the mill speed? Do you plan to increase the ball charge? Is it for short term or long term operational reasons?

A colleague of mine wrote a paper which may be useful to you (see herehttp://is.gd/fJDSDf). The control philosophy section provides SAG mill speed guidelines for common operational scenarios.

Generally copper ores from the Katanga / Solwezi region are comparatively soft so a low speed, high ball charge can be useful if it is difficult to maintain a stable rock charge in the mill.

Please let us know some of the background information and perhaps we can provide a more detailed answer.

Is this what you want to achieve for some reason, or is it the outcome of the way you operate your mill.

If you have a proper mill control system implemented, and you are achieving your grind objectives (grind size, throughput, etc) is it a problem? A well implemented SAG control philosophy should be able to vary the mill speed, in response to variations in feed characteristics. If the feed ore becomes softer, then the mill control system should respond to reduce the mill speed. You may not need to keep adding grinding media either, if the issue persists too long. Alternatively if you do not have a good mill control philosophy implemented, then this is what you / or the operators should be doing.

Of course the problem can be made more complicated considering all process and mill parameters.

It’s very important to know why do you want reduce the speed below 75 % CS? Of course, there are a impact in throughput (reduce) and a very strong rise the specific energy consumption (global of the complex) if the reduction is too much. If the case is the bottleneck in the mine (few ore), the best option is work in campaign. Always the circuits must work at the maximum rate, to optimize the energy consumption.

If the case is a little reduces the critical speed to increase the ball mill level, the rate will be about the same and in some case to increase.

I suggest you to look at a couple of papers by Liddell &Moys in SAIMM on power mill speed & load

http://is.gd/pGdySW

http://is.gd/VuLvCE

If you have to reduce the SAG mill speed because of avoiding liner breakage, maybe it’s better to modify the shell lifter design. Low speed may cause decreasing capacity, higher steel ball and liner wear rate. D80 of SAG mill may be finer because of more cascading action and less capacity which results in more retention time in SAG mill.