01 Introduction

The φ 3.8× (7.75+3.5) m single warehouse air sweep coal mill is used for the preparation of pulverized coal in a 4500t/d clinker production line of a company. The designed production capacity of the mill is 42t/h(80μm sieve, 2% of sieve residue), and the main motor is YKK 710-8 with a power of 1400kW. The loading capacity of grinding body is 75t, and the grinding liner adopts wear-resistant alloy graded liner and corrugated stepped liner. Since the technical transformation of other equipment in the production line, the output per hour of rotary kiln has been gradually increased, and the running time of coal mill has been gradually extended, especially when the wearing parts such as inner lining board are ground to the later stage, the output will decrease and the power consumption will increase. In daily production operation, the average running current of coal mill is 69A, the power is about 1080kW, the output per unit hour is 40t/h, and the power consumption is 27kwh/t. Sometimes, the water content of pulverized coal coming out of the mill is too high. When it is close to 3%, the output has to be reduced, and the grinding pressure difference is low. When the pressure difference is about 800～1000Pa, coal ash appears at the grinding tail. Although the lining board and other parts of coal mill are replaced every year, the effect is not ideal, the output per hour has not been improved, and the wear-resistant materials are increasing year by year. In order to meet the requirements of pulverized coal preparation in kiln system, reduce power consumption or increase output per hour, during the overhaul in February, 2020, the company carried out energy-saving renovation on the grinding liner of coal mill.

02 Energy-saving technology and characteristics of ball milling grinding body

Energy-saving technology of ball mill grinding body refers to the technology of adopting compound corrugated liner and grinding ball grading design to achieve energy saving of mill. Among them, the composite corrugated cylinder liner is a cylinder liner with various wave crest cross-sectional shapes, which can improve the crushing and grinding efficiency of the mill and reduce the energy consumption. Grinding ball grading technology accurately matches the grain size distribution and Hardness grindability index of materials, and reduces the extra ball loading without reducing the output of the mill, thus achieving the purpose of energy saving. Its characteristics are as follows:

(1) Compound waveform design to improve mill efficiency. Through calculation and simulation, the composite peak cross-section shape is designed, which makes the height and number of balls carried by the liner plate more suitable, makes the number of balls in the throwing area greatly increased (as shown in Figure 1: steel ball throwing area), and improves the crushing efficiency; It greatly reduces the running dead zone that does not produce pulverizing effect (as shown in Figure 2: the running dead zone of steel balls), reduces the number of grinding balls that do not do work, and improves the grinding efficiency.

Figure 1 Dropping area

Figure 2 Running dead area

(2) Adopt graded lining board. The middle section of the mill adopts a similar spiral grading liner, which can achieve good grading effect, and has lighter weight than the traditional conical grading liner, thus reducing mill load and saving energy.

(3) Segmented design waveform in the mill. In the axial direction of the mill, different waveforms are designed in sections. According to the distribution of grinding balls in the mill, matching waveforms are designed, so that the height of ball steel in different sections is appropriate, and the crushing and grinding efficiency is maximized.

(4) Grinding ball grading technology. The material is tested, and its particle size distribution, Hardness grindability index and other indexes are checked, so that the grinding balls can be accurately proportioned, and the excess ball loading amount can be reduced without reducing the output of the mill, thus achieving the purpose of energy saving. Grinding ball grading technology is matched according to the principle of maximum density or minimum clearance. On the premise of ensuring that large balls have enough crushing capacity, it can reduce the chance of “drilling empty” of materials, make powder exposed on the surface of grinding balls, increase effective blow and improve grinding efficiency. The use of high-performance grinding balls has good wear resistance, is not easy to be broken, wears evenly in the running process, can keep the ball shape well, and is conducive to maintaining the stability of gradation in grinding.

To sum up, after the ball milling efficiency is improved, the loading capacity of grinding balls can also be reduced, and the unit power consumption of the ball mill can be reduced. To sum up, after improving the efficiency in the mill and reducing the load of the mill, a good energy saving effect can be achieved.
03 Implementation of transformation

Under the condition that the service life of the original lining board has expired, the company will purchase and manufacture energy-saving lining board during the overhaul of the Spring Festival in 2020, and the outsourcing unit will complete the replacement work of the lining board within 6 days. The cost of the manufacturing and disassembly of the lining board is about 900,000 yuan.

(1) Replace the grinding inner lining plate with the composite waveform energy-saving lining plate, change the existing corrugated lining plate shape, the distribution and structure of the grinding inner chamber remain unchanged, and the end lining plate and the compartment plate will not be replaced. See Figure 3 and 4 for changes of coal mill lining before and after modification.

Figure 3 Schematic diagram of lining before modification

Figure 4 Sketch diagram of liner after modification

(2) The steel ball with surface hardness ≥58HRC and core hardness ≥56HRC is required to have Cr content ≥12% in the material of the ball (especially the ball with diameter below 40) to reduce the wear of the ball. At the same time, the grading of the steel ball in the mill is adjusted. The coal grinding ball loading quantity is 60t, including 53t in the initial loading, and the grading of the steel ball is adjusted 7t.

(3) Under the premise of ensuring the production of the table hour, reduce the amount of ball in the mill, reduce the load of the mill, reduce the amount of ball in the mill from 75t to 60t, optimize the grading of the steel ball and improve the quality of the steel ball, and improve the crushing and grinding ability of the mill. See Table 1 for steel ball gradation and coal mill grinding body segment energy reconstruction gradation.

Table 1 Gradation of energy-saving transformation of coal mill grinding body

04 Transformation effect and benefit analysis

After the transformation, the loading capacity of grinding body was reduced from 75t to 60t, the output per unit hour of coal mill was increased from 40t/h to 42t/h, the average running current was 60~61A, which decreased by about 8A, the average power was basically controlled at 960kW, the power consumption was reduced from 27kWh/t to 24kWh/, and the power saving rate reached 11%.

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