Laboratory and Analysis Technology

GRINDABILITY

MILL TYPES

The determination of the work index according to Bond is a standardized method for characterizing the comminution performance of mineral samples. The method is applied at CEMTEC for the design of rod and ball mills. Besides the desired comminution step as well as the production capacity, the comminution performance is crucial for ascertaining the requisite mill size.

The required minimum sample quantity is 15 kg. The test simulates a closing grinding circuit in a standard ball mill or rod mill. The batch mill „ELLA“ is available for performing the tests at CEMTEC. After determining the work index as per Bond, the specific comminution capacity is calculated according to the following formula or by factoring in various correction factors gained from experience:

As the Bond test is only defined for a definite fineness and current technological requirements call for products outside these limits, specially coordinated, modified Bond tests are possible at any time in order to determine a representative measurement of the comminution resistance.

Zeisel, Hardgrove

As the comminution performance depends on the grinding load and this is different for vertical mills in comparison to ball mills, the sample characterization must also be adapted. The feed sample defined precisely in terms of particle size is crushed by pressure and friction between a grinding bowl and grinding balls, on the grindability test benches according to Zeisel and Hardgrove. The prevailing load corresponds to that which also predominates in vertical roller mills.

 

The test bench essentially comprises a grinding bowl with rotary mounting, the grinding tools (balls) and the motorized grinding ring. The difference between methods according to Hardgrove or Zeisel involves the measurement of the dispersity skip, alongside different grinding chamber geometries:

 

  • In the Zeisel method, the specific surface increase is measured.
  • In the Hardgrove method, the screen throughput at 74 µm is measured.

Batch GRINDING

Besides the standard method according to Bond, it is also possible to record grinding curves with batch mills to determine the grinding resistance. In doing so, a definite product fineness is established and the energy necessary for this measured. Various options are available to vary this energy input.

  • Mill speed
  • Grinding media filling degree
  • Grinding media shape
  • Batching composition

 

These tests round off an optimum design of the mill with the work index as per Bond.

Four differently sized batch mills are available for the activities cited in our laboratory. These include an iron-free drum mill with a diameter d = 0.5 m and an effective grinding path length l = 0.4 m.

FormULA FOR THE REQUIRED GRINDING ENERGY

W = Wi*(10/√P10/√F)

W … Required comminution energy in …

Wi … Bond  Work Index

P … 80% mass throughput of the product in µm

F … 80% mass throughput of the process material in µm