To address issues in the cement grinding system—such as the unreasonable configuration of grinding media in the ball mill and high power consumption of the main motor due to long-term low-speed operation—a technical energy-saving upgrade was carried out on the ball mill. Part of the steel balls in the second chamber of the cement grinding ball mill were replaced with ceramic balls, and a frequency converter was added to the main motor for precise control of the ball mill's rotational speed. These changes significantly improved the ball mill's working efficiency and reduced energy consumption.

After the transformation, both the main motor current and the cement temperature at the mill outlet dropped noticeably, while the quality of the finished cement remained stable. Additionally, with the use of a frequency converter, the hourly output of the ball mill increased by approximately 40 tons per hour, and the specific power consumption of the main motor decreased by about 1.3 kWh per ton. This results in an annual electricity cost savings of approximately 1.06 million yuan (RMB), with a payback period of 1.01 years.

In the cement production process, cement grinding is a critical production stage. Its energy consumption has a significant impact on the overall energy usage and production cost of cement manufacturing. Notably, the energy consumption of the main motor accounts for more than 40% of the total energy consumption in the production process. Therefore, implementing energy-saving upgrades for the main motor in the cement grinding system is crucial for improving the economic efficiency of cement production.

Client A's cement grinding system uses a dual closed-circuit grinding process consisting of a roller press and a ball mill. The key equipment parameters of the cement grinding system are shown in Table 1. The system’s annual cement production capacity is 1.2 million tons, with a system-wide energy consumption of up to 27.5 kWh per ton. Of this, the ball mill—operating long-term at low speeds—consumes as much as 13 kWh per ton, accounting for nearly half of the total system energy usage. Compared to top-performing companies in the industry, this presents a significant gap. To reduce energy consumption and enhance efficiency, the company implemented a technical upgrade of the ball mill, resulting in a significant reduction in power consumption and a successful outcome.

2. Issues in the Cement Grinding System Ball Mill

2.1 Unreasonable Grinding Ball Configuration Before the technical upgrade, the grinding media in Client A’s cement grinding system ball mill consisted entirely of steel balls, with a filling rate of about 8% in the first chamber and about 30% in the second chamber. The crushing and grinding of materials by steel balls consumes a large amount of energy, which is one of the main causes of high power consumption in the ball mill.

2.2 Improper Ball Mill Speed The roller press in the system has high working efficiency, and the material entering the mill has a 45μm fineness of less than 35%. However, the ball mill operates at a relatively low speed, causing materials to remain in the mill for a longer time. As a result, the grinding efficiency is high at the beginning and low towards the end, reducing overall production efficiency and increasing system energy consumption.

3. Technical Upgrade Plan

3.1 Use of Variable Frequency Drive (VFD) on Main Motor By adopting VFD technology on the main motor, the ball mill’s rotational speed can be adjusted in real-time based on operating conditions, ensuring smooth material flow through the mill.

3.2 Use of Non-Metallic Grinding Balls While maintaining proper filling rates and grinding efficiency of the ball mill, reducing the total amount of grinding media can effectively lower energy consumption. Practice has shown that replacing part of the grinding media with a steel-to-ceramic ball ratio of 2:1 can significantly reduce power consumption while ensuring stable mill operation.

4. Implementation of the Technical Upgrade

4.1 Partial Replacement of Steel Balls in the Second Chamber with Ceramic Balls Compared to steel balls, ceramic balls have lower density and are nearly 50% lighter, effectively reducing the load on the mill and, consequently, lowering the power requirement of the main motor. Ceramic balls, being inorganic non-metallic materials, generate minimal heat during grinding and do not produce static electricity, significantly reducing electrostatic attraction between cement particles.

Additionally, ceramic grinding media exhibit extremely low wear—much lower than that of steel balls. Their microcrystalline grinding characteristics also effectively increase the proportion of 3–32μm particles in the finished cement, optimizing the particle size distribution and enhancing cement strength.

A comparison of the parameters between ceramic and steel grinding balls is shown in Table 2.

To meet the high crushing capacity requirements of the first chamber in the cement grinding system's ball mill, the technical upgrade retained the use of steel balls in the first chamber. Only a portion of the steel balls in the second chamber were replaced with ceramic balls.

Considering that the minimum diameter of the steel balls in the first chamber is 20mm, and following the principle that the maximum diameter of the grinding media in the second chamber should not exceed the minimum diameter of the grinding media in the first chamber, a combination of large ceramic balls and small steel balls was used. This approach helps maintain the grinding efficiency of the ball mill and fully leverages the synergistic effect between ceramic and steel balls.

A comparison of the second chamber grinding media gradation before and after the adjustment is shown in Table 3.

4.2 Variable Frequency Drive (VFD) Retrofit of the Ball Mill Main Motor

Under the condition of using the same volume of grinding media (steel balls and ceramic balls) in the ball mill, the specific surface area of the product ground by ceramic balls is on average about 20% lower than that of steel balls. Moreover, the mass of ceramic balls is 40%–50% less than that of steel balls. Since ceramic balls and steel balls follow different motion trajectories inside the mill, the optimal rotation speed of a ball mill using ceramic balls differs from that of a mill using steel balls.

Currently, the commonly used design principle for setting the rotation speed of ball mills in cement production is based on maximizing the normal energy of the grinding media. This principle works well for mills focused on crushing functions. However, for mills with a primary grinding function, this approach leads to significant mechanical energy surplus and considerable energy waste.

To address this issue, a variable frequency drive (VFD) was added to the ball mill’s main motor, allowing precise control of the mill's rotation speed to optimize energy utilization.

In addition, the retrofit included the use of a high-voltage VFD to drive the ball mill transmission system. This effectively resolved challenges such as high-power motor startup, grid connection, and load adjustment. The specialized VFD system for the ball mill enables variable-speed operation with ceramic grinding media in large-scale cement grinding systems, ensuring long-term, stable operation.

This retrofit employed the CHIC2000 series high-voltage “AC-DC-AC” VFD designed specifically for ball mill speed control. It uses a power cell series structure to achieve direct high-voltage output. The system’s electrical control and protection logic can be centrally managed on-site or conveniently operated through a dedicated coordinated control system cabinet that comes with the VFD system.

A schematic diagram of the ball mill main motor system’s electrical control retrofit is shown in Figure 1.

5. Technical Transformation Results

5.1 Effect of Replacing Grinding Media in the Second Compartment

A comparison of the ball mill's operating parameters before and after partially replacing the steel balls in the second compartment with ceramic balls is shown in Table 4.

According to Table 4, after replacing part of the steel balls with ceramic balls in the second compartment, the hourly output of the ball mill slightly decreased, and the grinding efficiency of the second compartment also showed a slight reduction—but not significantly. The main motor current of the cement grinding ball mill system dropped significantly by approximately 20A, which is directly related to the reduction in grinding media load. Additionally, the temperature of the cement exiting the mill decreased notably by around 5°C. This is attributed to the material properties of the ceramic balls. As inorganic non-metallic materials, ceramic balls generate minimal heat during grinding and produce almost no static electricity, thereby lowering the overall grinding temperature.

A comparison of finished cement performance indicators before and after replacing the grinding media in the second compartment with ceramic balls is shown in Table 5.

As shown in Table 5, after replacing part of the steel balls in the second compartment with ceramic balls, the specific surface area, cement strength, and particle size distribution of the finished cement changed very little. All metrics still met the finished cement standards, further confirming the feasibility of the grinding media adjustment plan.

5.2 Effect of Adding a Variable Frequency Drive to the Ball Mill Main Motor

The production output and specific energy consumption of the main motor before using the variable frequency drive (VFD) are shown in Table 6. The impact of motor frequency on the hourly output and power consumption when producing P·O42.5 cement after using the VFD is shown in Table 7.

According to Table 7, when the main motor frequency is 52.5 Hz, the ball mill achieves its highest hourly output, averaging 268.38 t/h. When the frequency is 51.5 Hz, the specific energy consumption of the main motor is optimal, averaging 11.69 kWh/t. However, when the motor frequency exceeds 52.5 Hz, the cement flow rate inside the mill becomes too fast, preventing the cement from reaching the particle fineness required by the high-efficiency classifier. This results in excessive return powder, forcing a reduction in the mill’s hourly output. Therefore, considering both the hourly output and energy consumption of the cement grinding system, the optimal motor frequency is determined to be 51.5 Hz.

Compared to before the transformation, after installing the variable frequency drive (VFD), the hourly output of the ball mill when producing P·O42.5 cement increased by about 40 t/h, and the main motor's specific energy consumption dropped by approximately 1.3 kWh/t.

5.3 Economic Benefits and Energy-Saving Results

In this technical transformation:

• The cost of the VFD system for the main motor is approximately 700,000 RMB.

• The cost of ceramic grinding media (calculated at 15,000 RMB/ton for 2.5 tons) is approximately 375,000 RMB.

• The total investment is approximately 1.075 million RMB.

After the transformation, the electricity consumption of the cement grinding system decreased from 27.5 kWh/t to 26.2 kWh/t, achieving a power savings rate of 4.73%. This equates to an annual electricity saving of 1,560,000 kWh. Based on an annual cement production of 1.2 million tons, this results in a savings of 1.0608 million RMB per year, yielding a payback period of approximately 1.01 years.

5.4 Precautions for the Transformation

During the ceramic ball grinding process, due to the higher brittleness of ceramic balls, breakage can easily occur. This can lead to grate blockages, increased internal negative pressure, and delays in material discharge, ultimately resulting in mill overload and reduced output.

To avoid these issues:

• Use dry-grinding alumina balls specially designed for cement grinding, which offer higher strength and better wear resistance, reducing the risk of breakage.

• Avoid mixing ceramic balls with steel balls, as this increases the risk of ceramic ball fracture.

• Regularly inspect the internal grate gaps and promptly remove blockages to maintain proper ventilation and smooth material flow.

6. Conclusion

This transformation involved replacing part of the steel grinding media in the second compartment of the cement ball mill with ceramic balls, and adding a VFD to the main motor to precisely control the mill speed. The result was a significant improvement in grinding efficiency, a notable reduction in energy consumption, and minimal impact on cement product quality and key performance indicators. The specific energy consumption of the main motor was reduced by approximately 1.3 kWh/t, delivering substantial economic benefits to the enterprise.

For this technical upgrade, we supplied Customer A with 92% alumina dry grinding balls, specially designed for dry grinding in cement production. These are harder than standard alumina balls and are less prone to breakage.

In addition to providing high-quality ceramic balls, we can also dispatch a technical team to offer on-site support, if needed.