Friday, 15 August 2014

A brief history of classification in grinding circuits

In our recent discussion, Prof. Alban Lynch expressed an opinion on the use of hydrocyclones in closed grinding circuits: "the way they are used now is an absolute nonsense, with circulating loads in some cases of well above 200%. The future is high frequency screens". This has led, unsurprisingly  to a few comments on the use of hydrocyclones and screens in milling circuits.
Prof. Lynch has been in touch this morning by email, and asks me to publish this short technical note, which he entitles Classification – the enigma in mineral processing.
An enigma is defined as a puzzling or inexplicable occurrence. The description fits the present position of wet classifiers well. We need to grind increasing amounts of low grade, harder ores to meet the demand for metals so the expenditure on mills and understanding breakage is high. Classifiers can limit circuit productivity by 10% or more yet there seems to be relatively little expenditure on ensuring that size separation in classifiers is accurate and efficient. Knowing the past is the starting point for doing better so I will briefly review the story of cyclones and size separation.
 Cyclone classifiers have been used in dry and wet grinding circuits for decades. The hydro-cyclones used in wet circuits are small, inexpensive, easy to operate, and handle changes in throughput without difficulty.  Their disadvantage is that their separation characteristics are poor and they can produce recycling loads up to 400% although the particles which require regrinding may comprise only a small fraction of this. These high loads limit the capacity of mills to grind new feed and reduce the sharpness of the split, both of which may be costly. The problem occurs in both dry and wet processes and the different approaches used in the cement and ore industries to operating centrifugal separators will be discussed.      
In cement circuits the Askham dynamic separator was used for many years from 1885. It was superior to static cyclones but the circulating load was high and to reduce it a second generation centrifugal separator was designed and built in 1960. Further reduction in the load was still needed to reduce the grinding cost further and a third generation separator was designed and built in 1985. The improvement which has occurred over three generations is shown in the table and figure below.

There was intermittent interest in hydro-cyclones for wet circuits from 1891 but the Dorr rake classifier became available in 1902 and this unit and the Akins spiral classifier were used for wet classification from 1900-1950. By then the post war minerals boom had started and this required higher capacity processing units. Hydro-cyclones were of interest because pumps were available which could create high velocity spiralling flows in cyclones and these flows provided a mechanism to separate the feed into coarse and fine streams. Performance of hydro-cyclones improved when Kelly Krebs designed the involute entry which reduced disturbances in the outer spiral.                                                                                                      
Since then the strategy to handle high capacities of grinding circuits has been to build nests of hydro-cyclones and larger hydro-cyclones and these have been effective in handling high flows. But only with high frequency screens does there seem to have been much emphasis on better separation. These screens have been successful in reducing circulating loads and improving the sharpness of separation in 300 tph grinding circuits at the Apatit mine in the Kola Peninsula, Russia.             
Screens are more expensive than hydro-cyclones and their capacity limits must be observed during operation. But these limits will be extended and they will be an important part of the future of wet classification.  


  1. Thank you for this fascinating article about the history and separation improvements. Is there any sources that compare energy requirements?
    Kate Siew, Australia

    1. With dry grinding circuits the only power used in the first generation classifiers was by the motor which drove the fan. In second and third generation classifiers there are two motors which are for the fan and the rotor. If the separation efficiency increases the circulating load decreases, the feed rate can be increased with no loss in the quality of the fines and the overall energy consumption per tonne in the grinding circuit is reduced.
      With wet grinding circuits when the hydro-cyclones are replaced with high frequency screens there is no need for high pressure, high capacity pumps. The separation efficiency of these screens so far looks to be promising and they produce much smaller circulating loads. But more work is still to be done to define their complete role in these circuits.
      Alban Lynch, Brisbane

    2. Very interesting. Now a little comparison for the wet grinding like it is shown for the dry would be helpful in order to see a difference between hydro-cyclones and high frequency screens.

    3. There will be a discussion of high frequency screens and hydro-cyclones at the Santiago, Chile, IMPC in October. The paper is based on data from several plant circuits in Peru, Russia and Turkey. Partition curves are used to describe the accuracy of the separation process. The partition curves for the 300 tph circuits in the Apatit plant in the Kola peninsula in Russia are sharper and the bypass is lower with screens than with the hydro-cyclones they replaced. Similar data from five other plants all show the same result. A better partition curve in a wet grinding circuit leads to lower energy per tonne as occurs in dry grinding circuits. The partition curve is not the only factor involved in the choice of a classifier but the long term economic advantage of having a circuit with a good partition curve must be a high priority.
      Alban Lynch

  2. Comparing screens and hydrocyclones just by their partition curve may be misleading. They behave very differently as dynamic entities in a grinding circuit. As an example: wen the ore becomes harder to grind the outcome of a hydrocyclone is a coarser product and indeed a "flatter" partition curve (e.g. a higher imperfaction) The circulating load increase is typically modest. That means that any disturbance in grindability will be seen in cyclone overflow particle size. The operation of the circuit in terms of pumping and mill operation is quite stabe but the price is paid as a poorer liberation in flotation (or any other separation stage)
    A screen behave in a totally different way. As the circulating load starts to increase, the shorter residence time on the screen starts to make the screen passing fraction finer. The partition curve will start to change towards finer cuts with the nominal aperture as the hinge point. The end result is that the circulating load increase fast (How fast and when this will happen will depend on the screen dimensioning). All disturbances remain in the grinding circuit.
    One has to be very clear and careful where to use screens or cyclones. Operations with high ore grindability variations need to think twice before using screens!
    Kari Heiskanen

    1. Thank you Kari. It will be interesting to review the partition curves and bypass of high frequency screens and cyclones which would have come from your plant data, and to compare the conclusions with those we have reached using data from 7 plants. It will be possible to do this at the Santiago IMPC in October where my colleagues and I will present a paper on the topic.



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