Determining the Bond work index is a standardized method for characterizing the comminution performance of mineral samples. CEMTEC uses this method for designing rod and ball mills. Comminution performance, just like the desired comminution step as well as the production capacity, is crucial for determining which mill size is required.
The required minimum sample quantity is 15 kg. The test simulates a closed grinding circuit in a standard ball or rod mill. At CEMTEC, these tests are done with batch mill "ELLA". After determining the Bond work index, the specific comminution capacity is calculated using the following formula, factoring in various correction factors obtained from experience.
/please insert formula/
As the Bond test is only defined for a certain fineness and current technological requirements call for products outside of these limits, it is also possible at any time to carry out specially coordinated, modified Bond tests in order to achieve a representative measurement of the comminution resistance.
As comminution performance depends on grinding load, which is different for vertical mills as compared to ball mills, the sample characterization must be adapted, as well. For the Zeisel and Hardgrove grindability tests, the feed sample with a precisely defined particle size is crushed by pressure and friction between a grinding bowl and grinding balls. The exerted stress corresponds to that in vertical roller mills.
Basically, the test station consists of a grinding bowl with rotary mounting, the grinding media (balls), and the motorized grinding ring.
The differences between the methods according to Hardgrove and Zeisel are different grinding chamber geometries and a different dispersity skip measurement:
- Zeisel method: measurement of the specific surface increase
- Hardgrove method: measurement of the rate of passage through a 74 µm sieve
Beside the standard Bond method, it is also possible to determine grinding resistance by recording grinding curves with batch mills. This means that a product with a certain fineness is produced while measuring the energy needed. There are various options for varying this energy input:
- Mill speed
- Grinding media filling degree
- Grinding media shape
- Batching composition
These tests complement the Bond work index, so that the ideal mill can be designed.
At CEMTEC's laboratory, there are four batch mills of different sizes for these tasks. They include an iron-free drum mill with a diameter d = 0.5 m and an effective grinding length l = 0.4 m.
Formula for the required crushing power
W = Wi*(10/√P—10/√F)
W ... comminution energy needed in kWh/t
Wi ... Bond Work Index
P ... 80% mass flow rate of the product in μm
F ... 80% mass flow rate of the feed material in μm