Innovation in Grinding Technology
The roller press is a relatively new development in the cement manufacturing process, compared to other standards in the industry. This efficient comminution machine is applied mainly for clinker, but is also used for raw material and slag grinding. The size reduction of the material takes place under high pressure between two rotating rollers. The press can be installed in combination with a tube mill (semi-finish grinding mode) or standalone in the finish grinding mode.
The roller press operates under the principle of high pressure inter-particle comminution.
KHD was the first to run its roller presses in the finish grinding mode for raw material, slag and clinker grinding. However, especially in India, the question soon came up as to whether KHD´s roller press can be operated in the finish grinding mode with coarser fly ash added into the feed composition. (Refer to Figure 1 & Figure 2).
In the past, many people believed that the particle shape of roller press produced cement was not as good as compared to product coming out of a ball mill. However, the German Cement Research Institute (VDZ) has proven that the above statement is not true. The graph below confirms the results of tests done on semi-industrial grinding circuits.
The distribution of various particle sizes and their roundness in different machines shows that the major population between 3 to 30 microns follows almost the same roundness using either a ball mill or a roller press. This diagram shows that all grinding systems have similar particle shapes. (Refer to Figure 3).
Water demand depends on particle size distribution and not roundness. Strength also depends on many factors, but not mainly on roundness or water demand. With this in mind, the Dalmia Cement operations team began using a KHD roller press RP16-170/180 in the finish grinding mode to produce PPC. In order to meet Dalmia´s high quality standards, the Dalmia quality department monitored the cement quality at frequent intervals.
Two roller presses in one combined circuit were operated in the finish grinding mode (Figure 5) [without using the ball mill itself] by routing fly ash into the circulation elevator (at the ball mill) and feeding it into the top of the separator [common for two roller press and fly ash feeding]. The process ran in this manner for one week. In order to make clear comparisons, samples were taken from PPC cement produced in two separate 10-hour production runs in the finish grinding mode. The same was then done for PPC produced in the semi-finish grinding mode, i.e., two roller presses and one ball mill. (Figure 4). The results are summarised in the following Tables 1 to 4. From Table 2, it is clear that the RP alone in finish grinding mode is operated with same roller loading as in the semi-finish grinding mode (RP with BM). Fly ash absorption is also recorded at 27 per cent, i.e., similar to semi-finish grinding mode. The roller press operational stability is similar in finish and semi-finish modes, when comparing parameters like pressure, grinding force, vibration, kilowatts, etc. From Table 1 & 2, finish grinding mode operation for production of PPC with coarser fly ash is well established. Furthermore, finish mode operation with a specific power requirement of 25.2 kWh/t is more efficient than production in the semi-finish mode operation with specific power of 27.5 kWh/t.
Quality Parameters: Comparison Finish Mode [RP Only] and Semi-Finish [RP+BM] The particle size distribution (Table 3) shows no significant differences. PPC cement quality in the finish grinding mode of operation (Table 4) in terms of strength (1-day, 3-day,7-day and 28-day) is equal to that produced in the semi-finish grinding mode operation at 54.2 MPa strength after 28 days and 30 MPa at 3 days.
- No hot gas requirements.
- PPC cement product quality parameters show equivalent strengths in 1D, 3D, 7D and 28D in finish (RPs only) and semi-finish grinding modes (RPs plus BM).
- It is observed that the water demand (normal consistency of cement paste) is almost same in finish mode grinding, in spite of using coarser feed fly ash [200-240 M2/kg],
- Particle size distribution of product produced in the finish grinding mode with 65 per cent particle population, between 3 to 30 microns (refer to the enclosed PSD curve) results in better setting time and high early strengths (Blaine is also slightly higher).
- Separator reject distribution made at the RP feed-bin and at the V-separator discharge makes the roller press operation more stable.
- Roller press operation with studded rolls was found to generate more grip and take a better nip of the material to be ground. Therefore, consistent grinding force between the grinding rolls results in a consistent power draw by the rolls.
- The studded rolls ran with optimum power draw, even with lower specific grinding force < 4.5 N/mm². Nevertheless, it achieved 88 per cent draw power, which indicates studded rolls are better in comparison to other grinding surfaces in terms of exerting draw power on rolls.
- Total grinding circuit specific power consumption was found as 25.2 kWh/t and 22.8 kWh/t for the main equipment.
- A circuit equipped with an air chute and close transfer points makes this circuit a neat and clean circuit.
By using the latest technological configurations, like studded rolls, air chutes, RP location above the V-Separator, you can achieve the lowest power consumption in the total circuit. Therefore, the KHD roller press circuit fulfills all 3Es (Efficiency, Energy and Environmental friendly). The roller press is the most efficient comminution machine for grinding of slag, raw material and OPC, based on the above results for PPC and other composite cements.
About the authors
This article has been Authored by RA Somani, Dalmia Bharat Cement Ltd, and AK Dembla, Humboldt Wedag India Pvt Ltd. The authors convey their special thanks to the Dalmia Site team, M/s. Bhartendu Tiwari, HWI Team Ramji Gautam and Balesh K Singh, for their excellent support in making the exercise a success.