Comparison of advantages and disadvantages of fly ash grinding tube mill process and vertical mill process

Fly ash is one of the industrial waste residues with a large volume of discharge in China, and with the development of the electric power industry, fly ash emissions from coal-fired power plants are increasing year by year. Fly ash not only occupies a lot of arable land and consumes a lot of water for ash flushing, but also the secondary dust from fly ash can cause serious harm to the ecological environment.

The main ways of fly ash emission from power plants are dry collection and dry discharge, dry collection and wet collection and wet discharge. Generally, the fly ash collected by dry collection and dry discharge is called dry discharge fly ash, while the fly ash collected by dry collection and wet collection and wet discharge is called wet discharge fly ash. As there is a low season in the fly ash market, power plants can only use the wet discharge process when the storage capacity of the power plant fly ash bank is not enough during the low season.

In order to effectively deal with wet-row fly ash and reduce its pollution to the environment, the challenge we face is how to make reasonable use of wet-row fly ash and turn it into a treasure. According to the physical and chemical properties of wet fly ash and the outstanding advantages of the vertical grinding process, the vertical grinding process is adopted to make it meet the requirements of the standard fly ash used in cement and concrete.

The specific gravity of fly ash is between 1.95~2.36 g/cm3, the loose dry density is in the range of 450~700 kg/m3, and the specific surface area is between 220~588 m2/kg. Among the physical properties of fly ash, fineness and particle size are more important, and it directly affects other properties of fly ash, the finer the fly ash is, the larger the proportion of fine powder is, and the higher its activity is. The fineness of fly ash affects the early hydration reaction, while the chemical composition affects the later reaction.

When the calcium content of fly ash is only about 3%, it basically has no water-hard cementing properties by itself. Only when it acts with water in powder form can it react chemically with calcium hydroxide or other alkaline earth metal hydroxide at room temperature to produce compounds with water-hard cementing properties. Fly ash used in cement production can save clinker and improve the performance of cement, fly ash used as a concrete admixture can also save a lot of cement and has the advantages of reducing water demand, improving the compatibility, enhancing pumpability and reducing concrete creep, heat of hydration, thermal expansion and improve the permeability of concrete.

The coarse fineness, low early activity and high water demand of fly ash in its original form make it difficult to be used directly in concrete. There are two main ways to process fly ash, one is to use the separator for sorting, and the other is to grind it all fine, of which grinding is the more reasonable way.

According to the different grinding principles of the host equipment, the wet-row fly ash grinding process can be divided into the tube grinding process and the vertical grinding process.

For wet-row fly ash, it needs to go through dewatering and drying treatment before entering the tube mill grinding system. The tube mill process of fly ash grinding system mainly includes open-flow high-fine mill grinding process, closed-circuit tube mill grinding process, semi-final grinding process, open-flow micro-powder tube mill process, etc. Among them, the open-flow micro-powder tube mill process can be further divided into sorting and then open-flow grinding and direct open-flow grinding process.

The traditional drying and tube mill grinding process has the disadvantages of complex process system, large floor space, high energy consumption and pollution.

Compared with the tube mill process, the vertical mill grinding process has the following advantages:

(1) Large drying capacity. The vertical mill uses hot air to transport materials, and as long as there is enough heat, the materials with moisture up to 20% or more can be dried in the vertical mill.

(2) High grinding efficiency and low energy consumption. The power consumption of vertical mill grinding system is 20% to 40% lower than that of tube mill, and with the increase of raw material moisture and the increase of product fineness, the power saving effect is more significant.

(3) Simple process flow, small footprint and occupied space. The vertical mill itself has a powder separator, so there is no need to add another powder separator and lifting equipment, and the dusty gas from the mill is directly collected and treated by the high concentration bag dust collector, so the process is simple, occupies about 70% of the floor space of the tube mill system, and the building space is about 50% to 60% of the tube mill system.

(4) Stable product quality. The chemical composition and fineness of the product are stable. The time that the material stays in the vertical mill is only 2~3 min, while it takes 15~20 min in the ball mill, so the chemical composition and fineness of the vertical mill product can be measured and corrected quickly.

(5) Low noise and low pollution. The noise level of vertical mill is low, 20-25 dB lower than that of tube mill, and the system is operated under negative pressure because the vertical mill adopts a fully sealed system, so there is no dust and the environment is clean.

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