I. Overview

With the increasing maturity and promotion of comprehensive utilization technology for fly ash in our country, the comprehensive utilization of fly ash is no longer limited to environmental protection requirements, and the tremendous economic benefits have been realized. In the southeastern coastal areas and some developed regions of our country, the finished fly ash fines are even in short supply. Currently, the fly ash from a large number of coal-fired power plants, with fineness values generally ranging from 20% to 50% (325 mesh residue), is not fully utilized. Power plants typically use dry separation technology for fly ash to achieve coarse and fine separation to obtain finished fine fly ash, thus generating economic benefits. However, the coarse fly ash (usually with a fineness of about 65%) after separation is not fully utilized and is usually discharged locally or sold at a low price, even becoming a burden for enterprises. By using a special ultra-fine grinding machine for fly ash, the original fly ash or the coarse fly ash after separation, as the main mixed material, is ultra-finely ground to impart it with certain hydraulic activity. This produces ultra-fine fly ash admixture that can be used to produce high-performance concrete, achieving the goal of complete utilization of fly ash and creating greater economic benefits.

The use of ball mills for powder grinding has been applied in the cement industry for many years, with mature technology and relatively standardized mill products. Drawing on the principles and structures of various high-fineness and high-output cement ball mills both domestically and internationally, the research department of Yancheng Yinfu Company has successfully developed a ball mill specifically for fly ash fine grinding in collaboration with the School of Materials Science at Nanjing University of Technology, a renowned institution for comprehensive application of fly ash in China. This mill grinds the original or coarse fly ash from coal-fired power plants to achieve the desired fineness of finished fly ash. A sorting device is added after the ball mill to sort the ground coal ash. Post-sorting, the coarse ash is recycled back to the ball mill for ultra-fine grinding, producing admixtures (micro-fine powders) that can be used to formulate high-performance concrete. This significantly enhances the economic benefits of comprehensive utilization of fly ash, enabling the full utilization of fly ash.

Section 2: Overview of Fly Ash Grinding System 

1. Introduction to Fine Grinding System Process

The FY series ultra-fine fly ash ball mill system mainly consists of the raw ash storage, auxiliary material storage, spiral feeders, electronic weighing scales, and fly ash-specific ball mills.SifterAir chamberPulse Jet Bag FilterComprising induced draft fans, screw conveyors, bucket elevators, feeders, control systems, etc.

The system directly extracts fly ash from the original gray storage, feeds it through a spiral feed hopper, weighs it with an electronic scale, and then transports it into the mill head hoist via an air chute. The hoist feeds the classifier for pre-screening before grinding, and the coarse ash is then sent into the mill feed port via an air chute. It enters the ball mill for grinding. The material after grinding contains a large amount of fine ash, which is re-fed into the classifier by the hoist for further separation. The separated fine ash is then sent to the finished ash storage via subsequent conveying equipment. The mill tail end is equipped with a dust collection system. Compared to the open-loop (without a classifier) system, this closed-loop grinding process can increase the hourly output by 30-40% or more, making it an ideal equipment for the deep processing of fly ash.

2. Grinding System Process Flow Diagram (Appendix)

Fly ash grinding and sorting processDescription: The fly ash powder grinding system mainly consists of the original ash storage, impeller feeders, electronic weighing scales, fly ash-specific ball mills, classifier, and pneumatic pulse box.Baghouse dust collectorThe system consists of fans, spiral conveyors, bucket elevators, feeders, control systems, etc. The system directly extracts ash from the original ash storage, feeds it through a impeller feeder, weighs it with an electronic scale, and then transports it to the mill tail elevator via a spiral conveyor. The fly ash is then fed into a classifier for pre-screening before being ground. The coarse ash after screening is sent into the mill feed hopper via a spiral conveyor, where it is ground in a ball mill. The material after grinding contains a large amount of fine ash, which is then re-fed into the classifier by an elevator for further separation. The separated fine ash is conveyed to the finished ash storage via a subsequent spiral conveyor. The mill ventilation and dust removal system, as well as the classifier's fine powder bag.Dust CollectorUtilizing this closed-circuit grinding process, compared to the open-circuit (no separator) system, the hourly output can be increased by 30-40% or more.

Section 3: Conversion of standard ball mill into fly ash-specific grinding mill

The mill cylinder is generally divided into two compartments. To address the particle size composition and grindability of fly ash or coarse ash, and the mixture, a special ultra-fine grinding modification for fly ash inside the ball mill is implemented: A system modification is carried out on the ball mill using our ZG series double-screen separator plate, specifically designed for the fly ash mill, making it more suitable for the closed-loop production process of fly ash grinding.

YF series spiral-shaped fly ash mill double-sieve separator chamber plate specifications:

Grating, perforated guard plates:

Casting Material: Manganese Chrome Alloy Sieve Opening: 5mm

② Rack:

Fly ash grinding专用, made of imported wear-resistant steel plate.

③ Sieve plate:

Stamping Material: δ3mm Stainless Steel   Screen Aperture: 1.2～1.5mm

⑷ Standard Parts: (Connecting bolts and nuts for cylinder and sieve frame) vary according to mill specifications.

II. Specific Reform Measures:

⑴ We replace the original separator plate with a special fly ash double-layer screen separator plate, featuring a 5mm sieve gap for the separator plate and a 1.2mm sieve gap for the middle stainless steel sieve plate. This effectively controls the particle size of the material entering the second bin, accelerates the transfer of qualified particles from the first bin to the second for grinding, and reduces over-grinding in the first bin.

⑵ Determine the length of the first bin of the mill based on the grindability and moisture content of fly ash. Typically, φ20 to φ50 steel balls are used for balling in the first bin, while φ8 to φ16 small-sized steel ingots are used in the second bin. Due to the larger surface area of micro-forged steel, it can grind fine-grained materials more effectively. Additionally, reducing the diameter of the grinding media can slow down the flow rate of materials within the mill, increasing the residence time of the materials in the mill, and enhancing the grinding process.

③ Adding activated lining plates inside the mill's tail stock can effectively slow down the flow rate of materials within, while also enhancing the grinding function of the small forging, thereby increasing the specific surface area of the product.

⑷ The discharge grate of the mill tail is a special 5mm small gap grate (can be modified on the original basis), adjusting the diameter of the thrower plate to control the flow rate of the material out of the mill. The reverse side of the double-layer separator plate is equipped with a protective panel with ventilation grates, which not only protects the stainless steel sieve plate from being worn by the grinding media but also enhances the internal ventilation of the mill, promoting the timely discharge of qualified fine powder and reducing the phenomenon of over-grinding.

Select an appropriate internal ventilation speed for the mill, with an ideal wind speed of 0.8 to 1.0 m/s. This reduces the residence time of qualified fine materials within the mill, promotes the timely discharge of heat generated from micro-powder and grinding, and enhances the efficiency of the powder grinding process.

⑹ A flip gate lock wind device should be added at the tail stock cutting area, and the device should operate smoothly.

Generally, the circular flow process is used, which can significantly increase the hourly output of the system, reduce the electricity consumption per unit product, and lower production costs. The mill employs a pulse bag dust collector for dust removal, and the fan motor uses variable frequency speed control to adjust the internal mill airspeed, controlling the material flow rate and fineness.