The Time that the ore spends in the grinding mill is called, RETENTION TIME.
This is a very important variable, the longer the retention time, the more the ore comes in contact with the rods, the better and finer the grind becomes. To understand this important control it must be remembered that any volume that is added to a circuit must leave the circuit at exactly the same rate. All the equipment has a pre-set size, it is capable of holding just so much, the more that you add to it the more must leave it.  As you increase the volume of the through put, the time that the ore has to react to the grinding media is lessened.

As you add ore to the mill you are also adding WATER and changing density which affects the slurry rheology and workings of the grinding action inside that mill.

This water is one- of the main controls that you have over your grind. Like many things it has its good sides and it’s had sides. The good side is that it disperses the ore allowing contact between the rods and the ore to be uniform. It also maintains an even steady flow of material through the mill. Once the ore has been mixed with water and is in a slurry form, it will flow along with the water. The bad side of the water is that it takes up space. The more water that is added, the faster the ore will go through the mill. This means the time it has to be ground will be reduced and a poor grind will result. At the other extreme, if not enough water is used, the ore will not be able to flow through the mill. The result is the ore will not be able to move into the rod load. Because there isn’t enough water to carry the ground material deeper into the mill the ore begins to pile up at the entrance. This will mean that there is more ore than surface area of the Rods. This excess amount of un-ground ore will cause the rods to lift and separate, spoiling the grinding action of the entire mill.

The amount of water that the mill requires to grind the ore, properly, varies depending upon the type of mineral and the characteristics of the Native rock that it is found in. Some concentrator plants require a very accurate control of the water addition. To do this there has to be a method of measurement. A way to gauge the amount of water that is to be added, to the ore, to maintain the correct ratio between ore and water. This ratio is termed the DENSITY of the circuit.

One method of measurement is simply to weigh the slurry to determine the amount of solids in relationship to the water. The density of the slurry is expressed as percentages. As an example, a density of thirty percent means there is thirty parts ore to seventy parts water. Here is another example, if you have a density of eighty percent that means eighty percent of the VOLUME is ore, twenty percent of the volume is water.