In the cement final grinding process, separate grinding is employed: clinker and gypsum are ground together to meet specified standards, while blended materials such as slag or fly ash are ground separately to meet their respective standards. Then, based on the cement’s specification requirements, the two are blended in a specific ratio via a blending unit to produce the finished cement. This production process allows for the selection of different grinding specifications—and even different grinding equipment—based on material characteristics and product performance requirements. This approach improves grinding efficiency and minimizes over-grinding and under-grinding. It also enables the use of purchased blended materials (such as ultra-fine slag powder and fly ash) to increase production capacity. Additionally, by adjusting the mixing ratio, the various specifications of the cement can be precisely tailored. This achieves the goals of flexible production, reduced energy consumption, and lower clinker consumption. However, it should be noted that “proportions and mixing” are the key elements of this process; in other words, aside from metering equipment, the mixing equipment is a critical component.
The mixing described above is a physical mixing process, in which two types of powdered particles are uniformly blended together through external force and continuously discharged. Currently available equipment includes:
1) Mechanical mixers, which operate by using a horizontal main shaft to drive high-speed mixing blades that blend two or more materials entering the equipment. Simultaneously, the thrust generated by the rotation of the angled blades discharges the mixed material from the equipment. The mixing effectiveness depends on the number of mixing cycles, specifically the rotational speed of the blades and the length of the equipment;
2) Pneumatic-mechanical composite mixers, which operate by introducing air beneath the filter cloth at the bottom of the unit to fluidize two or more materials entering the unit, thereby achieving mixing. Simultaneously, a horizontal, low-speed main shaft drives the blades to agitate the materials, activating and homogenizing them while reducing the segregation caused by gravitational settling of particles; Material transport within the equipment relies on the pressure difference between the inlet and outlet, causing the material to flow toward the outlet and be discharged via overflow. This ensures a consistent material level inside the equipment, thereby guaranteeing adequate mixing time to achieve the desired mixing results. This method of material transport causes high-density particles, such as metal, to settle at the bottom of the equipment, requiring timely removal; however, this is beneficial for cement quality and subsequent processing steps. In terms of power consumption, due to the different states of the working media in the two types of equipment, the latter consumes only 10–20% of the power of the former under comparable conditions. Consequently, the latter has a much lower dead weight and requires significantly less space for installation.
(Liaoning Yingkou Golden Earth Cement Co., Ltd.) adopted a combined roller press and ball mill double-closed-circuit grinding process. The ball mill specifications are
Φ3.8 × 13 meters. The original design called for using the existing two Φ2.4 × 13-meter ball mills to grind slag into ultrafine powder, which was then metered and fed into the tail-end recirculation elevator, with the blending ratio set at approximately 15% of the mill’s output (feed rate at the mill inlet). This placed a certain burden on the classifier itself, and the portion returning to the mill inlet also affected the mill’s grinding efficiency. After one year of operation, to adapt to market changes, we planned to moderately increase the blending ratio of ultrafine slag powder. Therefore, after conducting a review and comparison and taking our actual conditions into account, we decided to select the FH800-2 pneumatic-mechanical composite mixer.
1) Equipment Installation: Given the equipment’s light weight and vibration-free operation, it will be installed directly on the floor slab at the level of the finished product chute. The mixer’s feed inlet and discharge outlet on one side will be positioned adjacent to the chute. Openings will be cut into the side of the chute, with the upper opening connected to the mixer’s inlet and the lower opening connected to the mixer’s outlet. The inclined chute remains unchanged; simply install a screw gate midway between the discharge and feed openings to block off the original passage. After being metered, the fine slag powder is conveyed to the mixer’s other inlet via conveying equipment. With proper preparation, the connection work can be completed within a single production shift.
2) Commissioning and Production: After the mill system has started up and is operating normally, start up the mixer system simultaneously. at which point the screw gate on the finished product chute remains open. Once the system has stabilized, close this gate to allow cement to enter the mixer. When the cement flows normally from the mixer outlet back into the finished product chute and into the product silo, this indicates that the system is operating normally. Then, activate the slag micropowder conveying and metering equipment to feed it into the mixer. Initially, gradually adjust the blending ratio to increase the admixture proportion and discharge the finished product into the designated silo. Monitor the process continuously and take samples for testing throughout production.
3) Mixing Effect Testing: During trial production, we tested the finished cement using visual inspection, chemical analysis, and physical testing methods. In the laboratory, samples collected at different time points were leveled and pressed onto white paper for observation; no color differences or textural abnormalities were found in the cement. The deviation values from the chemical analysis and physical testing results were virtually indistinguishable from those obtained before the mixer was used. The cement from the trial production passed inspection and was released as normal; no abnormalities have been detected in subsequent production to date, nor has the customer provided any feedback regarding this matter.
4) Equipment Operation Status: The mixer has been in operation for three years. During this period, aside from routine cleaning, maintenance, and servicing, no additional labor or spare parts have been required, and there has been no impact on the system’s operational rate, thereby achieving the goals of energy conservation and reduced consumption. During this time, the mill’s output (feed rate to the mill head) increased from 110 metric tons per hour to 115 metric tons per hour, with a maximum fine powder blending ratio of 25% (some of the fine powder was purchased externally).
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