Choosing the Right Steel Grinding Balls for Your Mill
In the grinding process of materials in the mining and cement industries, steel balls play a vital role in converting raw minerals into fine particles and the final product. Typically, chrome or stainless steel balls are used for grinding in industrial ball mills.
Research shows that around 37% of ball mill operating costs go to supplying and replacing balls.
Therefore, optimal ball selection can significantly reduce costs while increasing mill efficiency and equipment lifespan.
Important characteristics of suitable balls include high hardness, impact strength, wear resistance, and high durability. In this article from Sepahan Foolad Atashgah, we will introduce the types of mills used in the mining and cement industries, the classification of steel balls, and the principles of their optimal selection.
Types of Mills Used in Mining and Cement Industries
Ball Mill:
A cylindrical rotating machine that produces soft or homogeneous powder by crushing materials with the impact of steel balls. These mills are used in both mineral crushers and cement clinker mills (after firing). The ball mill speed is set near its critical speed so that the balls crush materials through falling motion.
Rod Mill:
Similar to a ball mill, but instead of balls, it uses long steel rods (rods) as its grinding medium. Rods crush materials as they rotate in the mill, and their greater linear contact usually produces coarser particles.
Rod mills are ideal for grinding softer, coarser minerals or low-capacity applications and generally consume less energy than ball mills.
Semi-automatic Aggregate Mill (SAG Mill):
This type of mill is a hybrid between a ball mill and an automatic grinding mill.In a SAG mill, part of the crushing load comes from steel balls and the rest from coarse ore particles (raw feed).
Typically, balls make up 6–15% of the mill volume, with the remainder provided by ore.
The mill’s length-to-diameter ratio is around 2 or less, and it is often used as a primary mill before the final ball mill.
SAG mills are commonly used in mining—especially gold, copper, platinum, and other base metals—to reduce grinding steps.
Automatic Grinding Mill (AG Mill):
A cylindrical device in which the entire crushing load is provided by coarse ore particles. In this mill, steel balls are not used and the ore itself acts as the grinding media. AG mill can replace two to three crushing stages in case of high ore hardness and suitable capacity. For some feeds, a combination of SAG and AG is also used to increase productivity.
Vertical Roller Mill (VRM):
It is widely used in the cement industry (especially in post-clinker cement mills). In this system, the materials are crushed between rotating plates under heavy rollers. Roller mills consume less electrical energy and have the ability to dry themselves; therefore, they have become popular in modern cement production.
Review of available types of steel balls
Mill balls are divided into different categories in terms of manufacturing method and chemical composition:
Forged steel balls (blacksmithing):
They are made from low-carbon or medium alloy steel (such as 60Mn and 65Mn manganese steels). These balls have high durability and impact resistance, uniform hardness and are hard to break. For wet applications in mineral mills (with high fluid flux), forged balls are preferred. Many of the world’s major mining companies (such as Rio Tinto and BHP) use these balls.
Cast steel balls (cast iron-steel):
This category includes cast steel balls with low, medium or high chromium content. High chromium cast steel balls (with 15–35% Cr) have excellent hardness and wear resistance and are widely used in dry cement mills. However, due to their brittleness and low impact strength, they are less used in large diameter mills or in wet mines. Conversely, low manganese cast iron is cheap but has low wear resistance and is not suitable for heavy duty work.
High manganese steel balls (High-Mn):
The high manganese in the chemical composition causes the surface of the ball to be work-hardened by impact. These balls form a hard and resistant layer on their surface due to continuous contact with coarse materials, and therefore their wear resistance increases over time. High manganese steel is relatively inexpensive and is a good choice for mills that require high durability and impact strength.
Medium chromium manganese steel balls (special alloys):
Some balls are made from alloy steel with chromium and molybdenum to balance hardness and impact resistance.
For example, certain alloy steels have a martensitic structure with around HRC60 hardness and ≥15 joules impact resistance, giving them a longer service life than high-manganese balls under the same conditions.
Ceramic balls:
They are made from alumina-based ceramic materials (such as alumina and zirconia).These balls have very high hardness and excellent wear resistance, making them ideal for grinding chemicals, ceramics, and paints.
Some balls are made from special minerals (e.g., perlite) or polymers (rubber/polymer balls) for specific conditions, such as sensitive mills or to reduce noise and sparks.
Ball Coating:
In some cases, steel balls are coated with coatings such as ceramic or rubber to improve durability and performance in specific environments. For example, corrosion-resistant balls may be used in grinding mills for electronics or corrosive slurries.
Factors affecting the selection of the right ball
The selection of the right grinding ball should take into account all technical and economic aspects. The main factors are:
Ball size and shape:
Balls are usually spherical to have uniform contact with the material. An optimal distribution of ball sizes (different ranges in a flux) helps to break the particles more efficiently. Typically, large balls (e.g. 50–80 mm) are used for primary grinding of coarse particles and smaller balls (e.g. 10–30 mm) for final grinding. Depending on the type of mill (tube size and rotation speed), the
optimal ball size is determined. Incorrect selection of size can lead to low efficiency and undesirable particle size.
Material and chemical composition:
As mentioned earlier, the steel composition (carbon, manganese, chromium, and other alloys) determines a ball’s hardness, toughness, and wear behavior.
For example, manganese balls perform better under high-impact conditions, making them efficient in mining operations.
Full-chromium balls are preferred in dry cement mills due to their high wear resistance.
The ball’s hardness should not greatly exceed the mill liner’s hardness to avoid liner damage.
Hardness and wear resistance:
The harder the ball, the greater its wear resistance, but excessive hardness reduces impact strength. Therefore, an optimal balance between high hardness and sufficient toughness is necessary. The microstructure of the steel (such as martensite or mesosite composite) also affects the final properties. For example, high carbon-manganese alloy steels have a hardness of ~HRC60 and good toughness, and their life is longer than high manganese balls.
Effect on mill efficiency:
The balls determine the dynamic load on the particles. The size and intensity of the balls impacting the material affects the energy input to the mill and ultimately the grinding efficiency. Proper ball selection increases the efficiency of particle impact. Research shows that the optimal combination of ball size and type can reduce the energy consumption of the mill and increase productivity. In addition, proper ball distribution prevents the formation of fine particle accumulation zones (cushion effect).
Cost and Life:
The purchase price of the ball and its wear rate are important criteria in the selection. Balls that are more wear-resistant and have a longer life reduce ancillary costs. For example, a low-carbon alloy steel ball has a longer life than a low-manganese cast iron ball. On the other hand, a low-manganese cast iron ball, although cheaper, must be replaced sooner. According to reports, about 37% of the costs of a ball mill are related to the replacement of the grinding media. Therefore, it is better to choose bullets that require fewer replacements in the long run at a reasonable cost.
Key points for increasing mill efficiency by selecting the right ball
To achieve maximum mill efficiency, in addition to selecting the right ball material, the following should be observed:
Optimal ball loading:
The ball loading volume, or the space occupied by balls inside the mill, is usually set at 30–35% of the total mill volume.
If the ball load is too high, particles become compacted, creating a “cushion” effect where impact energy is wasted passing through the balls, reducing mill efficiency.
. Studies have shown that as the ball loading increases, internal wear increases and effective grinding decreases. Therefore, maintaining the right ball loading ratio is essential.
Ball size combination:
Use a combination of different ball sizes for each grinding stage. Larger balls are suitable for primary crushing and breaking up coarse stones, and smaller balls are suitable for final grinding of fine powders. This optimizes energy distribution and prevents excessive coarse particles from being sent to the final stages.
Periodic replacement and monitoring:
Ball wear is inevitable over time. To prevent a decrease in grinding efficiency, worn balls must be replaced periodically to maintain the appropriate hardness and weight of the grinding media. Continuous monitoring of the wear rate and timely replacement of balls has a significant impact on long-term productivity.
Other grinding parameters:
The mill rotation speed should be adjusted close to the optimal critical speed (about 70–80% of the critical speed) so that the balls transmit the desired force to the particles. Also, the appropriate design of the liner (shoes) and the degree of filling of the input material (creating enough space for the balls to move freely) play a role in efficient ball impact.
Correct selection of auxiliary materials:
If necessary, the addition of additives or special ceramic balls (such as in resin or ceramic mills) can improve product quality and efficiency.
Conclusion
Choosing the right steel balls is crucial for mill efficiency and productivity in the mining and cement industries.
Factors such as size, chemical composition, hardness, wear resistance, and lifespan directly impact operating costs and product quality.
Using high-quality balls tailored to each plant’s operational needs increases productivity, reduces maintenance stops, and optimizes energy consumption.
With more than three decades of continuous experience in the field of steel and cast iron casting, Sepahan Foolad Atashgah Company is one of the leaders in the production of castings and industrial balls in the country. The company, using 7 furnaces with a simultaneous melting capacity of 20,000 kilograms and an area of over 25,000 square meters, has the capacity to produce 25,000 tons of castings annually. The use of young and specialized manpower, up-to-date equipment and world-class technical knowledge has enabled Sepahan Foolad Atashgah to provide its services to the iron and steel, copper, mining and cement industries and to export its products to the Middle East and European countries in addition to the domestic market.
Relying on human capital and strategic management, Sepahan Foolad Atashgah has been able to play an effective role in reducing the dependence of domestic industries on the import of cast parts and at the same time developing international markets.
To order steel balls suitable for grinding and receive specialized advice, you can contact our colleagues in the complex or the CEO of Sepahan Foolad Atashgah Company so that the best products according to your needs are provided and the productivity of your production lines is maximized.
