In late July 2023, Guangxi Guigang Hengyao steel Co., Ltd successfully completed the production and acceptance inspection of their annual 300,000-ton ultra-fine high-performance cementitious admixture project. Zhejiang Tongli Heavy Machinery was responsible for providing the technical design and equipment manufacturing and commissioning for the ultra-fine powder milling.
The project involves mixing steel slag powder, mineral powder, and coal ash in certain proportions, grinding them into ultra-fine powder, and using the resulting product as a substitute for mineral powder or part of the cement in cement and concrete production. After conducting market research, Guangxi Guigang Steel Group selected Zhejiang Tongli to handle the technical design and comissioning of the closed-loop ultra-fine composite mineral admixture milling system.
The process scope of the ultrafine composite mineral admixture grinding system includes the following steps: starting from the bottom of the storage at the ultrafine composite mineral admixture grinding and mixing station and ending at the top of the storage at the ultrafine composite mineral admixture storage, which includes the processes of ultrafine composite mineral admixture grinding and transportation, ultrafine composite mineral admixture grinding, and the transport of ultrafine composite mineral admixture into storage.
This system utilizes a closed-circuit powder grinding system with a pipe mill and powder selection machine. It consists of a super fine composite mineral blending ball mill, an elevator, a powder selection machine, a high-efficiency bag dust collector, an exhaust fan, and other components. To ensure the proper functioning of the system, there is a weighing and steady-flow bin before entering the ball mill, and a material-air mixing device is installed after the ball mill.
The raw materials, including steel slag, ore slag powder, and pulverized coal ash, are mixed at the batching station. This mixture is then elevated, weighed, and stored in a stable flow silo before being sent to the ball mill for grinding. After grinding, the powdered material is mixed with air in a material-air mixing device and sent to the powder selector. Coarse powder, after selection, is transported through an air slanting chute to a flowmeter, measured, and then recirculated into the ball mill for further grinding. Fine powder is collected by a dust collector, transported through an air slanting chute, and lifted into storage by a storage elevator.
The powder selector has a primary air inlet, secondary air inlet, and tertiary air inlet. The selection of air in the powder selector is regulated by the negative pressure created by fans, with air entering through these inlets in proportion. After the material-air separation is achieved in the dust collector, the separated materials are sent to an emission pipeline by the main exhaust fan.
The ventilation system of the ball mill relies on a variable frequency speed-regulated motor driving a centrifugal fan to supply air into the primary air duct of the powder selector. The ball mill consists of a coarse grinding chamber, a medium grinding chamber, and a fine grinding chamber. The coarse grinding chamber is equipped with large steel balls for crushing and coarse grinding the pre-mixed materials in proportion. The materials that are coarse-ground are directed through partition plates into the medium and fine grinding chambers for further grinding. After grinding, the materials are transported to the powder selector through a heavy hammer flap valve and a material-air mixing device for selection.
Furthermore, the powder selector in this system is characterized by improving the production yield of the grinding system, reducing energy consumption, enhancing the quality of ultra-fine composite mineral admixtures, and requiring minimal maintenance. The powder selector operates under negative pressure, with airflow entering through the primary air duct and the secondary and tertiary air inlets. Guided by the blades, the airflow enters the powder selection area and forms a balanced and stable horizontal rotating vortex ring under the influence of rotating vortex blades and horizontal separating plates.
The common problems with the pipe mill for superfine powder grinding are as follows: rapid material flow inside the mill, low ball-to-material ratio, high internal temperature, severe “over-grinding,” unreasonable technical parameters, and a lack of targeted design for grinding media grading, lining plates, compartment plates, and discharge scraper plate structures. To address these issues, we have made significant improvements to the ball mill design:
1.Variable speed drive (4–44Hz) for flexible startup and energy-efficient operation to accommodate different specifications and loading capacities of grinding media.Multi-compartment design, with grinding media distributed from large to small in sequence throughout the ball mill.
2.Special-shaped curved lining plates, which not only increase the height of the grinding media but also prevent reverse classification of the grinding media and extend the material grinding path.
3.Anti-clogging cooling compartment plates and discharge scraper plates, a diffusion-type activation device effectively controls the material flow rate, ball-to-material ratio, and internal temperature inside the mill.
The new type of ultrafine powder classifier developed by Tongli is a novel high-efficiency classification equipment that integrates the powder selection technology from Japan and the classification technology from Germany. It has been developed through digestion, absorption, independent innovation, and design.
1. High classification accuracy. The fineness of micro-powder can be adjusted within the range of 1 to 20 μm, and the preparation fineness of non-metallic ultrafine powder ranges from 650m2/kg to 1000m2/kg. With the use of an efficient ball mill, the material temperature during processing remains below 110°C, leading to higher milling efficiency.
2. Classification efficiency exceeds 90%. Due to the high selection efficiency, the production capacity of the classification system increases significantly. Compared to an open-circuit milling and classification system, it can increase production by 25% to 50%; the increase in production is greater as the fineness decreases.
3. Reduced power consumption. The specific electrical energy consumption of the system can be reduced by 20% to 30% compared to an open-circuit milling system.
4. Simplified process flow. It facilitates the implementation of a negative pressure operation system, resulting in dust emissions (under standard conditions) of less than 10 mg/m3, ensuring clean emissions.
5. Uniquely designed classification flow field and vertical classification wheel allow for the grading of product granularity, reducing friction between the powder and the classification wheel, significantly extending the service life of the classification wheel.
6. The material contact parts use ceramic or high-quality austenitic stainless steel for wear resistance, with rounded designs to avoid energy consumption due to vortex formation. Fluid resistance is reduced, and wear is significantly reduced. It meets GMP requirements.
7. Equipped with a ball mill feed weighing and stable flow bin. The material-to-ball ratio in the ultrafine ball mill is smaller than that in regular particle-size ball mills. It is crucial to have a stable feeding and return amount in the ingredient system to ensure the stable operation of the ball mill.
8. Equipped with an airflow dispersion and disaggregation device. For the separation of ultrafine powder and the dispersion of agglomerated materials, airflow dispersion and disaggregation devices are crucial. They provide important conditions for the high-efficiency operation of the ultrafine powder selection machine.
The debugging plan includes single-machine testing, coordinated testing, load testing, production testing, and compliance debugging. “30%, 60%, 90% load testing, testing phase duration (hours), ball mill capacity (tons), production yield (tons per hour) (adjusted by Zhejiang Tongli on-site personnel).
Initially, the fast flow rate of the grinder resulted in a low surface area of the ground material. During the debugging phase, we adjusted the airflow in the grinder, modified the compartment plate and the discharge scraper board to control the area, adjusted the amount and grading of steel balls, and ensured the efficiency of the grinder. The specific surface area of the ground material can be adjusted and controlled in the range of 500 to 600 m2/kg. Proper use of airflow improves the efficiency of the powder classifier.
30μm square-hole sieve residue, particle size distribution, trioxide sulfur, free calcium oxide, chloride ions, moisture content, stability, flowability ratio, and activity index. In accordance with the agreement requirements, the main evaluation indicators are as follows: Material composition: Steel slag powder 35%; slag powder 25%; fly ash 40%; Fineness control: 30μm square-hole sieve residue ≤ 5.0%, pass rate ≥ 85%; Specific surface area ≥ 650 m2/kg, pass rate ≥ 85%.
After this equipment commissioning phase, and with operators becoming proficient in practical operations and adjustments, a solid foundation has been laid for full-load production. Material composition: Steel slag powder 35%; slag powder 25%; fly ash 40%; Fineness control: 30μm square-hole sieve residue 1.62.0%, pass rate ≥ 100%; Specific surface area: 666687 m2/kg, pass rate ≥ 100%; Hourly output: an average of 47.1 t/h. This project can comprehensively utilize 75,000 tons/year of steel slag, 105,000 tons/year of slag, and 120,000 tons/year of fly ash. It has excellent environmental, economic, and social benefits.
Practice has shown that Zhejiang Tongli’s technology for the ball mill closed-circuit ultra-fine composite mineral blending material grinding is mature. The ball mill technology design is successful, and the classifier’s design achieves a classification efficiency of 80% to 90% at a particle size of 30μm, thereby improving the grinding capacity of ultra-fine tube mills and ensuring high production for the entire grinding system. The high-efficiency classification also ensures a more rational particle composition for the fine composite mineral blending material after separation, creating better conditions for the comprehensive utilization of waste slag in steel and power plants.