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.