- To participate in the 911Metallurgist Forums, be sure to JOIN & LOGIN
- Use
**Add New Topic**to ask a New Question/Discussion about Grinding. - OR
**Select a Topic**that Interests you. - Use
**Add Reply**= to Reply/Participate in a Topic/Discussion (most frequent).

Using**Add Reply**allows you to Attach Images or PDF files and provide a more complete input. - Use
**Add Comment**= to comment on someone else’s Reply in an already active Topic/Discussion.

## Quantifying Grinding Efficiency (15 replies)

A metric often encountered in the hard rock business is a measure of the power consumed per unit of desired product e.g. kWh/t of -75 micron material in cyclone overflow (assumes you want to grind to 75 microns). You have to compare the specific electrical energy consumption required for the targeted milling final product particles size.

The milling efficiency defined as the ratio of theoretical work index by operating work index can also be used.

From an operations perspective would you say that energy savings from improving grinding efficiency offer the biggest cost savings to a typical mine?

This is correct as the grinding is the highest electrical energy consumer in concentration plants. The increase of grinding efficiency will represent important economy for the mine.

For many mineral processors, drying and smelting can be just as bad or worse, but grinding is always the most galling, as in 'theory', it should not be so bad! I fully agree that, kWh per tonne of -75 micron (or whatever size) is a good place to start.

Could you please provide a description of your crushing and grinding circuit, indicates particle size in each stage, tell me what type of hardness testing you have performed for your ore, what type of metals are you dealing with .

have you performed plant sampling campaigns while measuring energy consumption, do you have the grinding and comminution efficiency curves (based on size by size assay of the feed and discharge and on the energy consumption, do you have the classification efficiency curves for your screen and cyclones (if any).

First remove the water from slurries then grind the dry material; for removing water from the slurries i know a machine, equipments manufacturer if you want.

Thanks, however, the technology I am looking at needs to use wet grinding. Solids could possibly be increased, but it would still need to be in slurry form.

If the ball mills are in connection with hydrocyclones then the ball mills can't be evaluated by themselves but they need to be assessed as part of the grinding classification system.

In any case the evaluation is simple: You need to take a sample of the feed to the grinding-classification system and a sample of the discharge of the system. If cyclones are present then you need to include internal streams such as mill discharge, cyclone feed , cyclone underflow, cyclone overflow. You develop this sampling and measure the consumed energy during the sampling period.

You perform this sampling when you are using the reagent and when you are not using it, but you repeat this sampling at least 3 times per condition to discard that random results are making you to take the wrong conclusions.

You perform size analysis of the different streams and then calculate the grinding efficiency and classification efficiency. Make sure that every sampling campaign last at least 4 hours to make sure that the circuit is in steady state. The residence time in the mills is short as well in the cyclones but the steady state for a circuit is not reached in short time

in the method that I'm suggesting, you end calculating the Specific Selection Function for grinding in ton/KwH as a function of the particle size plus the grinding efficiency by each size fraction

Check any lab testwork for screen passing size. The bond work index actually changes for a specific ore depending on the product size you screen for. I.e. if you are comparing a 75micron product in the plant with a 125 micron product in the lab.

This means that if you are looking at how grinding efficiency varies at different product sizes you will need the BWi in the lab at similar passing size.

One way around this is to use the Morrell fine work index as your basis for comparison. This is an attempt to make the milling work index independent of product size. You can use the lab data from a standard bond ball mill test to calculate this alternative index.

Grinding efficiency plays an important part in calculating the work index of any Ore or Minerals. The Lab data should be precise before scaling to pilot scale and this should be repeated to come to final conclusion.Screening at 74 microns is a tedious affair and there should not be any clogging. Standard bond mill tests are the starting point.

I think the point is missed here.

One should grind to the requirements of the treatment needs of the ore in question

To overgrind costs a lot of money in maintenance, power, steelballs, liners and result n float losses.

Fine grind helps in gold recovery and most other leach plants, especially when it comes to leach mixing characteristic and viscosity which need to be looked at.

Do bottle roll tests to see at what grind you get the best recovery and float runs open and closed circuit using different grind ranges to see at what grind the float is optimized from a recovery point of view... the float will be more challenging as fine grind will liberate more minerals but risk residue losses, course grind could increase residue losses as well, so do the test work to get the optimum grind, after this you can worry about power draw, demand and sizing.

Grinding efficiency could be defined in many ways, useful consumption of energy usually being the parameter to be maximized. The Bond Work Index is a measure of theoretical power consumption to grind to a certain size while the Operating Work Index can be measured for an industrial system and reductions in Operating Work Index show improvements in efficiency. However, milling is a preparation for the downstream process and recovery losses occur in both coarse and fine size fractions so the target size range needs to be identified. Operating Work Index is not always able to account for this. I have been able to use Functional Performance Analysis to track increases in rate of production of desired product size range material during an optimization of a number of mills.

Some very interesting and informative comments from everyone, and thanks for that. We have just run some Bond Index studies and found a reduction in energy requirements with our additive and now need to consider the next step. It is evident from this discussion that we need to give this very careful consideration.

The best way is to quantify the absorbed energy per hour per tone ( Kwh/t) for a given Ore milling practice at a given predetermine hardness and comparing it to the theoretical Power consumption. Care has to be taken to ensure certain influential parameters are kept more or less constant during this survey. such parameters are mathematically represented as

Absorbed power ( Kw) =f( Mill diameter 3,RPM 1.27 ,Filling Degree, Length, pulp Sg)

Efficiency is an ambiguous word that needs to be defined for each specific case. For some people it means power to produce one t/h that's minus a certain size. For others it means actual power compared to predicted power from a Bond work index test to grind a ton to 80% minus a certain size. For others, it means ratio of right size material produced compared to total material ground. It's also affected as much by the sizing or classifying device used as by the mill itself.

What is the best way of measuring and quantifying the grinding efficiency of mineral slurries? I am referring to determining energy savings by improving efficiency. Would the Bond Work Index be the best way?