The density of steel balls is approximately 7.6-7.8g/cm³, while the density of ceramic balls is generally 3.6-3.8g/cm³, roughly half that of steel balls. When the filling rate of the ball mill remains unchanged, the lower density results in the grinding media being half as heavy, reducing the load on the ball mill and consequently lowering the current. If the grinding efficiency and hourly production of the mill remain unchanged, the power consumption for grinding will inevitably decrease.
The ball mill operates based on the impact and abrasion actions of the grinding media. Lighter grinding media result in reduced impact and abrasion forces. Can grinding efficiency and mill hourly production remain unchanged under these conditions? This is the key question in determining whether lightweight grinding media can save energy. Below, we explore ways to avoid a decrease in hourly production:
The ball mill performs two main functions: crushing and grinding the input material. Crushing (mainly in the first chamber) relies on impact, while grinding (mainly in the second or third chamber) relies on abrasion. These two functions are distinct and are completed sequentially within a closed process, with the process capacity depending on the weaker function. To maximize the process capacity, it is necessary to balance the supply and demand of each function based on the input material's particle size, friability, and grindability, achieving an equivalent balance between the two functions.
Without changing the internal structure of the mill, simply replacing the grinding media with lighter ones reduces both impact and abrasion forces, with a greater reduction in impact force. This leads to an imbalance between the supply and demand of the crushing function and an overall imbalance between the two functions, ultimately resulting in a significant drop in hourly production.
If changing the internal structure of the mill to establish a new balance is not currently an option, the reduced impact force can be compensated by adjusting the demand side by lowering the input material's particle size. Pre-crushing the input material can be enhanced. Many cement grinding systems are equipped with a roller press, which is much more efficient than a ball mill. Therefore, it is crucial to use a roller press in grinding systems that employ ceramic grinding media, preferably in a closed-circuit system.
In a roller press combined grinding system using ceramic grinding media, the closed-circuit system of the roller press significantly reduces and effectively controls the particle size of the input material to the ball mill. This substantially reduces the demand on the ball mill's crushing function, making the grinding function more dependent on abrasion. As long as the ball mill's grinding function is not compromised, energy savings are possible.
Regarding the imbalance of the grinding function supply and demand, adjustments can be made on the supply side. One way is to increase the filling rate of the grinding chamber to compensate for the decrease in grinding function. Another is to reduce the average ball diameter in the first chamber, converting some crushing function into grinding function. The decline in grinding function is not substantial, so the filling rate increase need not be excessive—generally around 10%. In practice, using ceramic grinding media has increased the filling rate by about 6%. The ball mill's structural strength and transmission system can handle a surplus capacity of about 50% of the grinding media, allowing for a filling rate of around 60%.
To ensure that the ceramic balls do not reduce grinding efficiency despite the decreased impact force, the presence of microcrystalline minerals in the high-toughness ceramic structure is emphasized. These microcrystalline minerals have strong grinding capabilities, enhancing the abrasion effect on the material and increasing grinding efficiency, making them well-suited for the roller press combined grinding system. Therefore, it is often more effective to replace only the grinding media in the fine grinding chamber of the ball mill.
The microcrystalline minerals in ceramic balls have a hardness second only to diamond, leading to low consumption and unaffected grinding function despite wear, which metal grinding media cannot achieve. Even after prolonged use, the surface of the ceramic grinding media appears smooth, but rough microcrystalline structures are still visible under a microscope.
Additionally, the lighter weight of the grinding media reduces the load on the ball mill, allowing for an appropriate increase in the grinding media filling rate. The grinding action shifts from primarily impact to primarily abrasion, reducing reliance on the regular dropping of the grinding media. This lays the foundation for increasing the filling rate, which in turn enhances the mill's production capacity.
Moreover, since ceramic grinding media are non-metallic, they significantly reduce static electricity during the grinding process. The lighter weight and reduced lifting power of the grinding media lead to higher grinding efficiency, lower heat generation inside the mill, and a lower grinding temperature. These factors improve grinding efficiency, reduce dependency on grinding aids, lower grinding costs, and enhance cement's adaptability to concrete admixtures.
The above is theoretical analysis, now let's look at some specific cases:
Given the "zero investment, zero risk, high return" characteristics of applying ceramic grinding media technology, although large-scale trials are relatively recent, with fewer application cases and limited basic usage experience, major domestic cement groups have already begun large-scale trials and, in some cases, full-scale promotion following successful trials.
The term "zero investment" means that both metal grinding media and ceramic grinding media are used, with ceramic grinding media having much lower wear rates, thus requiring minimal direct investment. "Zero risk" implies that the risk is very low; even in the event of failure, the cost is merely that of two clean-outs and reloads. "High return" refers to the fact that, currently, in successful cases, replacing only the grinding chamber can save 3-5 kWh/t of cement, while a full replacement can save 6-7 kWh/t of cement, making the energy savings benefits clear.
Case Study 1:
From July 23 to September 30, Z Cement Group tested B Company's ceramic grinding media in the second chamber of their cement mill at YN Company, in a ￠4.2×13m combined grinding system.
The original design loading of steel balls in the second chamber was 160t. According to the design, the ceramic grinding media should be 96t (60% of the design load, increasing the filling rate by approximately 1.2 times). Assuming the filling rate remains unchanged, the load should be about 80t (with the density halved). However, to prevent inter-chamber media mixing, only 69t of ceramic grinding media was loaded on July 22.
The plant is equipped with an energy management system, allowing real-time monitoring of various grinding parameters (including system power consumption). From July 23 to 26, when producing PC 32.5 cement, the following results were observed:
- Second chamber load: reduced from 160t to 69t
- Second chamber filling rate: reduced from 32% to 27.6%
- Ball mill current: reduced from 171A to 101A
- Average hourly production: reduced from 216.35t/h to 169.14t/h
- Specific surface area: increased from approximately 380m²/kg to over 400m²/kg
- Average power consumption: reduced from 27.36kWh/t to 25.86kWh/t of cement
The primary reason for the reduction in hourly production was the significant underloading. Measurements inside the mill showed that there was still about 150mm of free height from the ball surface to the center hole edge, with a filling rate of only 27.6%, compared to the original steel ball filling rate of 32%. Plans were made to further increase the load. Despite the change in grinding media, cement fineness was still controlled at the original 9.0±3%, and the actual specific surface area had increased to over 400m²/kg, about 20m²/kg higher than before. The next step was to adjust the cement fineness to 12-13%.
Over the next two months, two additional loading tests were conducted. To prevent media mixing into the double-layer partition discharge chamber, a stainless steel screen funnel was used to elevate the discharge edge by 60mm. Under the premise of ensuring quality, the hourly production recovered to 192.58t/h. The performance of producing PC 32.5 cement after the change is shown in Table 01-01.
From the end of July, with 73 tons of ceramic grinding media loaded, PO 42.5 cement production was alternated. The ball mill current showed little variation with different cement types, remaining relatively stable; however, the hourly production decreased from an average of 192.06t/h with metal grinding media to 171.27t/h with ceramic grinding media. System power consumption for grinding decreased from an average of 30.45kWh/t to 28.10kWh/t.
By the end of August, during maintenance, the funnel at the double-layer partition discharge was replaced with an inverted cone screen plate, completely sealing the media mixing channel. After this issue was resolved, an additional 3 tons of media were added. In early September, the mill resumed producing PO 42.5 cement, and by the end of September, the average hourly production had reached 190.80t/h, with system grinding power consumption dropping to 25.42kWh/t.
In summary, the initial trial of using ceramic grinding media in the second chamber of a ￠4.2×13m combined grinding system for producing PC 32.5 and PO 42.5 cement demonstrated significant energy-saving effects despite a decrease in hourly production. The overall results of the initial trial are shown in Table 01-02.
From Table 01-02, it is evident that for this ￠4.2×13m combined grinding system, replacing the 160t of metal grinding media in the second chamber with 80t of ceramic grinding media reduced the ball mill's main current from 171A to 110A. Although system hourly production decreased slightly, the average grinding power consumption for PC32.5 cement was reduced by 4.08kWh/t and for PO42.5 cement by 5.03kWh/t.
Theoretical analysis suggests that there is still room to increase the grinding media load. The trials at YN Company are ongoing, and there is potential for further increases in hourly production and reductions in grinding power consumption. We look forward to seeing these developments.
Case Study 2:
From early November to mid-December, H Cement Group conducted a full-scale trial using ceramic balls in the second chamber of their φ4.2×13m combined grinding system cement mill, as well as in both the first and second chambers of their φ3.2×13m combined grinding system cement mill. Upon achieving preliminary success, the company decided to fully promote the application of ceramic grinding media across different specifications of cement grinding systems within the group.
The company has a total of 84 cement grinding systems, and as of the end of May 2019, over 50 of these systems have adopted ceramic grinding media. Most of the remaining systems are not suitable for use due to conditions such as non-closed-circuit roller presses or the absence of roller presses altogether.
The company's promotion speed has been quite fast, indicating substantial accumulated experience. However, possibly due to commercial confidentiality, the company is reluctant to publicize detailed promotion information. Therefore, the following account is based on some side information.
In early November 2017, the first trial was conducted on a φ4.2×13m cement mill, testing only the second chamber. After 10 days of testing, the summary showed that for the production of PO 42.5 cement, the hourly production rate decreased by about 5t/h, while the electricity savings exceeded 4kWh/t of cement.
In early December 2017, the second trial was conducted on a CF company's φ3.2×13m cement mill, producing PC 32.5 cement. Both chambers were fully replaced with ceramic grinding media. After 10 days of testing, the summary indicated that the grinding power consumption decreased by approximately 6kWh/t, achieving a high level of less than 20kWh/t of cement.
CF Company has two Ф3.2×13m open-circuit ball mill combined grinding systems, equipped with roller presses and dispersion classifiers before grinding. The first mill was designated as the test mill for ceramic grinding media. Before the conversion, the feed material fineness was less than 20% residue on the 80um sieve, with an hourly production rate of about 150t/h and a grinding process power consumption of about 25kWh/t. The goal of replacing both chambers with ceramic grinding media was to reduce the grinding process power consumption to below 20kWh/t of cement.
The trial started in early December 2017, and after several adaptive adjustments, the following results were achieved by January 2018: with a relatively unchanged specific surface area, the average hourly production was 140 tons, down from the original 150 tons. The process power consumption decreased from the original 25kWh/t to 19.5kWh/t. Since the conversion, the mill has been operating normally for over half a year, with minimal wear on the ceramic grinding media and no need for media replenishment to date.
It is noteworthy that the grinding system's power consumption was already at a very advanced level of around 25kWh/t for producing PO 42.5 cement, both domestically and internationally. The key factor was controlling the feed material fineness to be relatively fine (less than 20% residue on the 80um sieve), typically around 17-19%.
What does it mean to have an 80um sieve residue of less than 20%? This is not something that can be easily achieved by anyone. It requires the user to have a thorough understanding of the energy-saving effects of the roller press system and for the system design and equipment to possess the corresponding capabilities. This aspect is worth contemplating.
Case Study 3:
From March 13 to April 12, 2016, S Cement Group conducted a trial using J Company's lightweight grinding media in the second chamber of the cement mill in their SD company's φ3.2×13m combined grinding system. The loading amounts and gradations of the grinding media before and after the trial are shown in Table 01-03, and the main technical indicators of the grinding system and the produced cement before and after the trial are shown in Table 01-04.
As seen in Table 01-04, after using the ceramic grinding media, although the filling rate slightly increased, the reduction in load weight led to a significant drop in the main motor current of the cement mill, from 105A to 70A, and a noticeable reduction in cement grinding power consumption. Specifically, the power consumption decreased by 4.39kWh/t for P·O42.5R, 7.84kwh/t for P·II42.5R, and 8.9kwh/t for P·O52.5R.
In terms of capacity, although the hourly production rate decreased, the reduction was within 10%, and adaptive adjustments have nearly restored the original levels. In terms of quality, under unchanged conditions, all types of cement exhibited a trend where both the 45um sieve residue and the specific surface area decreased, the 3-day strength slightly declined, and the 28-day strength slightly increased. This indicates a reduction in over-grinding and a more reasonable particle size distribution of the cement.
Regarding the impact on the performance of the cement, the particle size distribution of the produced cements shows a decrease in particles ≤3μm and ≥32μm, and a significant increase in particles of 3-32μm. This resulted in a reduced water demand of the cement, a noticeable decrease in the temperature of the cement leaving the mill, and a reduction in the use of grinding aids. These changes, which are due to the reduction in over-grinding, improved the compatibility of the cement with concrete admixtures, thus making it more welcome by concrete companies.
During the trial period, the company took the opportunity of maintenance to inspect the mill. It was found that the ceramic grinding media had virtually no breakage, confirming the high strength, toughness, and wear resistance of the grinding media. This alleviated concerns about the potential drawbacks of non-metal grinding media, such as reduced output, increased wear, and susceptibility to breakage.
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