Experiment No.
4 Grinding (Ball Mill)
1.1. Introduction
The grinding process in mineral processing, particularly using ball mills, is a critical
operation that significantly affects the efficiency and effectiveness of mineral liberation. The
mechanism of grinding in ball mills involves the application of mechanical forces, primarily
through impact and shear, to reduce the size of ore particles. This process is characterized by
the interaction between the grinding media (typically steel balls) and the ore, where energy is
transferred to the particles, leading to their fracture and subsequent size reduction.
The fundamental theory behind the grinding process in ball mills is rooted in the
principles of comminution, which involves the breaking down of materials into smaller
fragments. The energy consumed during this process is substantial, accounting for
approximately 50-60% of the total energy used in mineral processing plants (Wu et al., 2024).
The efficiency of this energy usage is influenced by several factors, including the design of
the mill, the type and size of the grinding media, and the operational parameters such as
rotational speed and charge fill level
(Doroszuk et al., 2024; Matsanga et al., 2023; Safa & Aissat, 2023)
Ball mills operate by rotating a cylindrical shell filled with grinding media. As the mill
rotates, the grinding media is lifted and then falls under the influence of gravity, creating a
cascading effect that generates impact forces on the ore particles . The design of the lifters,
which are installed on the internal walls of the mill, plays a crucial role in enhancing the
movement of the grinding media and optimizing the grinding action (Safa & Aissat, 2023) .
The rotational speed of the mill also affects the grinding efficiency; higher speeds can lead to
increased impact energy but may also result in excessive wear of the grinding media and over-
grinding of the material (Si et al., 2021; Xu et al., 2014).
The choice of grinding media is another critical aspect of the ball milling process.
Steel balls are commonly used due to their high density and hardness, which facilitate
effective comminution (Matsanga et al., 2023; Wu et al., 2024) . The size and distribution of
the grinding media also significantly influence the grinding kinetics and the resulting particle
size distribution of the ground material (Petrakis et al., 2021) . Studies have shown that
optimizing these parameters can lead to enhanced mineral liberation and improved recovery
rates in subsequent processing stages (Doroszuk et al., 2024; Malyarov et al., 2020)
Moreover, the grinding process is not merely a mechanical operation; it is also
influenced by the physical and chemical properties of the materials being processed. Factors
such as particle shape, size distribution, and mineral composition can affect the breakage rates
and the overall efficiency of the grinding operation. By understanding these interactions and
its mechanism, it is important for optimizing the grinding process and achieving desired
outcomes in mineral processing.
1.2. Objectives
The main objectives of this experiment are to
Grind sand to a smaller size using ball mill
Understand factors influence ball mill such as grinding speed and grinding
time
Obtain size distribution of the feed and ground project
1.3. Equipment and Materials
Laboratory Scale
Streel balls
Rotary milling
Sieves
Sand
1.4. Procedure
300g of sand with the grained size between 2.36mm to 1.18mm
Observe ball mill machine components and identify the critical speed (RPM) for this
specific configuration mill
Setting up the speed (RPM) of the mill according to 50% 0f CS, 60% of CS and 70%
of CS
For each speed, all steel ball and sand to the mill container with the ratio 3:1 and run
for 10 min. after 10 min sieve the ground product using sieve 600, 300 and 150
micrometers respectively, weight and note the result
Put everything back to the mill container and run for another 10 min to represent
20min grinding time. Sieve the ground product same as previous step
Repeat for another 10min for representing 30 min grinding time.
Construct column chart between size distribution vs weight of the ground sample
1.5. Calculations
Critical Speed ( CS )=
Where: g is the gravitational constant
1
2π
×
√ g
(R−r )
×60
R is the inside radius of the
mill
× 60
r is the radius of one piece of the media (ball)
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓
𝑅𝑜𝑡𝑎𝑡𝑖𝑜𝑛𝑠
𝑅𝑃𝑀 =
𝑇𝑖𝑚𝑒𝑠 𝑖𝑛 𝑆𝑒𝑐𝑜𝑛𝑑
1.6. Results
After conducting the performance evaluation test for each 3-difference critical
speed, the results of the tests were recorded and presented graphically in fig. 1-3,
respectively.
Fig.1 The following chart presents the distribution of ground material size at 50%
critical speed, showing the weight retained at sieve sizes of 600 µm, 300 µm, and 150 µm for
grinding times of 10, 20, and 30 minutes. At the 600µm sieve, most material is retained, with
weights decreasing from 289.5 g at 10 minutes to 281.7 g at 30 minutes, showing particle size
reduction. The weight of the material at the 300µm sieve increased from 5.55 g at 10 minutes
to 7.4 g at 30 minutes. This is understandable, as increased grinding time resulted in more
passage of particles. Moving on to the 150µm sieve, this too shows increases in weight 4.58 g
at 10 minutes up to 7.23 g at 30 minutes showing the action of grinding with time. Data
indicate that longer grinding times result in finer particle distributions, moving material from
larger to smaller sieve sizes.
Figure 1 Bar Chart of Size Distribution of Ground Product at 50% CS
Fig.2 The bar chart showing size distribution of the ground material at 60%
critical speed with weight retained on the sieves after 10, 20, and 30 minutes of
grinding on 600 µm, 300 µm, and 150 µm sieves respectively. Most of the material is
retained at the 600µm sieve, where the weights have decreased from 289.15 g at 10
minutes to 281.25 g at 30 minutes, showing that particle size has reduced with
increased grinding time. On the 300µm sieve, weight increases from 3.17 g at 10
minutes to 6.21 g at 30 minutes as more particles pass through. At the 150µm sieve,
weight increases from 2.04 g at 10 minutes to 4.91 g at 30 minutes, showing
significant fine particle production. This trend indicates that 60% CS grinding yields
finer particles with longer grinding time.
Figure 2 Bar Chart of Size Distribution of Ground Product at 60% CS
Fig.3 The bar chart shows the ground material size distribution at 70% critical
speed, which indicates sample weights retained at 600 µm, 300 µm, and 150 µm
sieves over 10, 20, and 30 minutes. The 600µm sieve retains most material, and
weights drop from 287.39 g at 10 minutes to 279.28 g at 30 minutes, indicating size
reduction. At the 300µm sieve, material weight rises from 6.15 g at 10 minutes to 8.14
g at 30 minutes, showing a shift to smaller sizes. At the 150µm sieve, the weight
increases from 1.97 g at 10 minutes to 7.17 g at 30 minutes, which means there are
finer particles. This shows that grinding at 70% CS produces finer particles with
longer grinding times.
Figure 3 Bar Chart of Size Distribution of Ground Product at 70% CS
1.7. Discussion
The results indicated that grinding speed and grinding time significantly affected the
particle size distribution in the sand sample. Even at 50% of CS, the largest fraction of
particle size (600 µm) was found to dominate the sample weight despite an increase in
grinding time, which indicates that there is a limit to grinding efficiency at low speeds. At
60% CS, on the other hand, particle mass at a size of 600 µm decreased as grinding time
increased, and the smaller fractions of 300 µm and 150 µm manifested a small rise, meaning
that grinding in this respect was better in comparison with 50%
At all speeds, longer grinding times from 10 to 30 minutes resulted in a systematic
decrease in coarser particles and an increase in finer fractions, hence proving the time
dependency of the grinding process. Higher speeds increased the effectiveness of grinding,
but the speeds need to be optimized to avoid excess wear or energy consumption, which are
very important aspects in industrial settings.
1.8. Conclusion
In summary, the results showed that a 70% CS enabled the most efficient grinding
process, which resulted in the highest yield of fine fractions. A 30-minute grinding time at
this speed produced the best size reduction results. Therefore, optimization of grinding
parameters in terms of speed and time is very important to achieve effective particle size
reduction while maintaining operational sustainability.
REFERENCES
Doroszuk, B., Bortnowski, P., Ozdoba, M., & Król, R. (2024). Scaling Energy Transfer in Ball
Mills: A Scale-Agnostic Approach through a Universal Scaling Constant. Energies , 17(11).
https://doi.org/10.3390/en17112693
Malyarov, P., Dolgov, O., & Kovalev, P. (2020). Mineral raw material disintegration mechanisms
in ball mills and distribution of grinding energy between sequential stages. Mining of Mineral
Deposits, 14(2), 25–33. https://doi.org/10.33271/mining14.02.025
Matsanga, N., Nheta, W., & Chimwani, N. (2023). A Review of the Grinding Media in Ball Mills
for Mineral Processing. In Minerals (Vol. 13, Issue 11). Multidisciplinary Digital Publishing
Institute (MDPI). https://doi.org/10.3390/min13111373
Petrakis, E., Karmali, V., & Komnitsas, K. (2021). Factors affecting nickel upgrade during
selective grinding of low-grade limonitic laterites. Mineral Processing and Extractive
Metallurgy: Transactions of the Institute of Mining and Metallurgy, 130(3), 192–201.
https://doi.org/10.1080/25726641.2018.1521578
Safa, A., & Aissat, S. (2023). Analyzing the influence of lifter design and ball mill speed on
grinding performance, particle behavior and contact forces. Mechanics and Industry, 24.
https://doi.org/10.1051/meca/2023035
Si, L., Cao, Y., & Fan, G. (2021). The Effect of Grinding Media on Mineral Breakage Properties of
Magnetite Ores. Geofluids, 2021. https://doi.org/10.1155/2021/1575886
Wu, C., Chen, Z., Liao, N., Zeng, C., Wang, Y., & Tian, J. (2024). Enhancing the Grinding
Efficiency of a Magnetite Second-Stage Mill through Ceramic Ball Optimization: From
Laboratory to Industrial Applications. Minerals, 14(2). https://doi.org/10.3390/min14020160
Xu, J. L., Zhang, L., Guo, Q., Zhou, S. G., & Feng, C. (2014). Research on the Preparation of
Antimony Nanoparticles by Mechanical Ball Milling. Key Engineering Materials, 609–610,
244–249. https://doi.org/10.4028/www.scientific.net/kem.609-610.244
