Grinding & Classification Circuits

Grinding & Classification Circuits2018-06-17T09:51:34-04:00
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Grinding Circuit Overload (5 replies)

4 years ago
maxwell 4 years ago

My company using ball mill as the second stage of grinding usually experiences high mill mass. we are therefore forced to grind out in order to reduce the mill mass.

We also experience excessive overflow of materials/feed entering the ball mill from the feed launder.

What could be the causes of these problems we are experiencing.


4 years ago
Jorge 4 years ago
1 like by David

I think there are several factors to analyze. For example, you should find out if the mineralogy is different, sometimes it changes drastically in some parts of the deposit. In one Pb-Zn operation the clay content was higher than usual and the grinding circuit was overloaded. Your problem can occur in a SAG Mill or Ball Mill.

Other point to consider is the particle size distribution, it is possible to receive a finer ore from the mine and the particle size distribution of the product from the crushing circuit is different, In this case, the residence time in the tumbling mill tend to be shorter. You should try to review the historical data on the particle size distribution.

Try to check the grinding circuit feed rate, specially if there is a control loop.

4 years ago
mbalexamani 4 years ago
1 like by David

Check the following please.

  1. Input rate and output rate
  2. The size of the grinding media
  3. The mill lifters
  4. Check with the mill constructor on the specifications of the particle size distribution and capacity of your mill
  5. Properties of your ore

This may help.

4 years ago
David 4 years ago

Hi Maxwell and welcome to 911Met.

A few questions I need you to answer to help a diagnostic:

  1. What is the Horsepower installed on the mill?  
  2. What is the current typical power draw of the mill?
  3. What is the primary mill you are using?
  4. Is your ball mill as grate or overflow discharge?
  5. By feed launder, you mean ball mill feed chute (as in the mill is full) OR the launder feeding the mill (as in the material full is too viscous)?
  6. What size grinding balls are you using?
  7. What P80 grind size to you want to get from the ball mill?

Is is possible for you to post a grinding circuit flowsheet?

Chat soon.


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4 years ago
(unknown) 4 years ago
Craig Lockhart
4 years ago
Craig Lockhart 4 years ago
1 like by David

Hello Maxwell,

I have experienced the mill overload issue at numerous operations, and most often I find the transfer size from the primary grinding circuit is the culprit and may require re-configuration of the classifying device.  Another common issue with SAG mills is operating at very low density (e.g. <70% solids) - if there is room to increase SAG density, this will enable the SAG to grind finer with little or no decrease in throughput.  Consider reducing the feed rate when the overload starts rather than waiting until a grind-out becomes necessary. If you are discharging your primary mill to a trommel or vibrating screen, try reducing the aperture size by about 20%.  (Caveat - if you do not have a pebble crusher in you SAG circuit, this will impact throughput).  If SAG grate aperture controls the transfer size, then this change will require a more precise calculation so you get it right the first time.  Be aware - in the event of long-term ore changes:  If the ore gets "hard" (typically reducing throughput) it is advisable to relax aperture size in proportion with the magnitude of throughput decrease; Reduce aperture size further if the ore gets "softer". If you have a pebble crusher, maintain the minimum gap setting possible and allow the system to by-pass some un-crushed pebbles back to the SAG.  As long as you maximize the power draw on the crushing unit, you will not lose throughput as a result of crushing fewer tons. (this should also reduce the circulating pebble load in the SAG circuit). If your ball mill is drawing less than the full installed power while operating, add grinding steel - if the mass of additional steel drives the bearing pressure higher, it should be offset at least partially by reduced solids loading.  If your mill shell structure and bearings will handle additional load, increase the pressure targets (to account for the steel only) and consider upgrading your lubrication system and/or lubricant for additional protection. Increasing the ball mill media top-size will decrease the overloading behavior, but the decrease in grinding efficiency will result in a significantly coarser final product size. If you need to add water to the feed end of your ball mill to control the density, by all means do so, but make sure you decrease water addition by an equal or greater amount elsewhere in the circuit (e.g. SAG feed or cyclone feed) or the overload will continue to escalate once the immediate benefits are overtaken by increased circulating load. Take steps to understand and quantify the overloading "problem".  For instance, if the mill overloads and requires grind-out every hour at 500 tph, but will run steadily without issue at 450 tph, then the required adjustments should be relatively minor.  Even if you end up at a slightly lower "peak" operating rate, the benefits of steady-state operation should more than compensate, and potentially reveal the path to the next improvement. As for the feed end spillage, check the following:

  • Ensure there are no gaps between the seal and inlet spout - start with a tight fit and allow wear to mate the sealing surfaces.  Make sure the spout is centered in the feed opening.
  • Diameter of the feed end insert opening should be significantly smaller than the discharge end insert opening.
  • One "overload" event can destroy your seal if the slurry level in the mill approaches center.
  • The feed spout should be a closed pipe where it penetrates the seal.  Inside of the seal, the top of the spout can be opened up to accept drippings from a "scoop"
  • Some mills use a small seal water line to pour water on the contact surface between the seal and the spout - this is only to rinse away abrasive solids that can cause rapid wear of the sealing surfaces (i.e. not like gland water).
  • Liners on the feed and discharge end should be smooth (and thick) close to the end openings - lifters should only be used on the outermost 1/3 of the radius - balls and slurry can otherwise channel down the lifter surfaces and achieve an ideal trajectory for direct impact in the seal area.
  • Use a feed end insert with a deep cone shape to move the seal further outboard.
  • If you are unable to maintain a conventional seal, consider passing an open-top feed spout through a small drum feeder bolted to the flange on the feed end of the mill.

Let us know what works for you!

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