The continuous ball mill was applied for Germany and patented in 1891 by Konow and Davidson, with Patent No. 62871, titled Ball Mill for Center Charging and Tangential Discharge. It was described in the patent that the mill was filled with gravels and few metallic balls, and the shell was protected by a ceramic and cast iron liner, that was the earliest modern ball mill liner. In the early 20th century, the largest ball mill was φ1.2*6m, and the output of grinding shaft kiln reached 3t/h. In 1990, a φ5.8*16.0m ball mill driven by a 8.7MW ring motor was put into operation in Gaurain-Ramecriox - a cement plant in Belgium, and it was combined with a φ2.0*1.0m roller press with the installed power of 2.4MW. The grinding rotary kiln produced clinker, and the maximum output of the mill reached 360t/h when the specific surface area of cement was 380m2/kg.
It is observed that the diameter of mill becomes larger and larger and the mill liners are changed dramatically thereupon along with the high-speed development of cement industry. The liners that were made out of high manganese steel, Ni-hard cast iron and common white cast iron early now can be made from several series of materials over hundred varieties, mainly from alloy steel, as well as high or low chromium cast iron, austempered ductile iron and other materials. The liner structure now is a “device” with strong technicality and theoretical property but not a simple protective lining in the past. The liner designed with advanced structure can effectively improve the mill output, reduce power consumption, increase the specific surface area of cement, and lower down noise. Its shape is not restricted to plate shape. The columnist helped Wu Mianqi and Wang Shaoxing who were teachers from Tianjin Cement Design Institute to design angle spiral and rounded liners in 1984, and invented grooved liner together with Lu Youqin, an engineer of Hefei Cement Design Institute in 1986. All these are deviated from the initial concept of protection, especially plenty of retention rings or rings which have been widely used in the fine sieving mills in recent years. There are so many forms and shapes that are too numerous to mention. Then in production practice, which material or which structure of liner should be chosen depends on the actual working conditions. A jargon in the wear-resistant material industry goes that there is no one gun that can finish everything. It is hard to find out two mills that have similar operation parameters just like two machine tools of the same model. Even though one burdening line, two mills of the same model in one workshop will have different outputs, power consumptions and specific surface areas. This is the special characteristic of mill, and the charm that attracts so many engineering technicians to plunge into this work.
I. Classification of Liners
By used part, mill liners are generally divided into shell liner, grinding head liner, partition liner, discharge grate liner, manhole liner and special liner. The special liner is the component for special purpose such as retention ring and ring. Although it is already not a traditional protective component, we are used to call it liner.
II. Working Conditions of Liner
When the mill is rotating, the abrasive medium (steel ball, segment, rod, etc.) and materials therein are brought to a certain height by the liner and then are thrown or poured down, and the liner will undergo impact and abrasion in this process. Due to fineness control of cement industry, the length diameter ratio of mill is larger than 2.5 generally, and the mill has two to three bins (five bins at most). Typically in a mill with two bins, the first bin is used for fine crushing and coarse grinding, with input particle size at P80≤25mm (or about P80≤5mm and P80≤1.0mm for a mill matched with a fine crusher and a roller press), and it will be different under different working conditions. Because of large particle size of charged material, the largest ball diameter in the first bin is φ90-100mm, and the average ball diameter is φ70-75mm. The large-diameter steel balls crush the liner with larger impact force, the materials are rushed and squeezed by the steel balls, and its front points will intrude into the liner. At this moment the abrasion mechanism of liner is mainly from high-stress impact scraping, accompanied by squeeze scraping. For the second bin of the mill, the materials charged through the partition have P80≤1.5mm and specific surface area S≥120m2/kg commonly. Then the diameter of steel ball or steel segments in equivalent weight is commonly between φ15mm and φ50mm. Because materials charged into the rear bin are controlled at P80≤0.9mm and S≥150m2/kg, the sieving mill can be equipped with φ6*6-φ14*14 mini steel segments. As the second bin is generally provided with slightly wavy or patterned plane liner with lower capacity of carrying balls, the grinding body is pouring with small impact force to the liner, therefore, the abrasion of the second bin (fine grinding) is from low-stress cutting.
III. Main Types of Liner Damages
The liner damages mainly include fracture, abrasion, deformation and dropping. Now, let’s discuss the first one - fracture.
Fracture is the most serious problem during production and application of liners; the causes include five parts during production, besides use problem.
1. Unreasonable design of chemical components or inaccurate control of chemical components;
2. Serious metallurgical quality problems such as occluded foreign substance out of limits, and composition segregation, etc.
3. Grievous foundry defects, such as shrinkage cavity, loosening, air pores, metallurgical tissue defect;
4. Unreasonable design of technology for heating processing or inaccurate control;
5. Unreasonable stress control and relief during production, such as improper foundry technology design, wrong time to release and open the box after casting, unsuitable polishing and repair welding, and delayed tempering after quenching, etc.
There are four problems in use:
1. Incorrect installation
Because most of wear resistant materials have poor thermal conductivity, in particular to high alloy materials such as high-chromium cast iron and middle-chromium alloy steel, it is forbidden from using electrowelding or gas cutting during installation before reaching consensus with the technical department of the supplier.
2. Lack of management in installation
For high-hardness liner, high-chromium cast iron liner in especial, it is demanding on installation process due to larger brittleness, that is the surface of shell should be coated with mortar or inlaid with a 3-5mm thick asbestos or rubber blanket. In the past, some factories that wanted to save troubles just threw into several bags of cement and filled with water to rotate it for a few minutes after installation, under the centrifugal force, cement paste can perfuse at the back of liner along with the installing joints. Although it looked rough, it was really easier, and the effect was not too bad, at least it was better than do nothing. In addition, it is better to add a portion of materials first after the new liner is well installed in order to avoid malignant damages to it. It would be great to use torque wrench to install it as far as possible under standard pretightening force.
3. Unbefitting replacement cycle
Some friends who used high manganese liners summarized from practice that the liner could be used to tens of millimeters thick (not adding, but reducing) as long as there were no impact on mill output. Some factories especially asked the supplying party to make it thinner in order to enlarge the effective inner diameter of mill and add the load of grinding bodies for increasing the output. The columnist reduced the thickness of the liner for one φ1.83*6.4m mill to 25mm from 45mm in 1987, and regulated the load of grinding bodies and grading of steel balls, then the output of mill increased by 10%. But now, most of liners are made from high abrasion-proof hard brittle materials such as high chromium cast iron or middle-high chromium alloy steel, they often get fractured easily after using to half of thickness. This is why high chromium cast iron abrasion sample that shows better abrasive resistance over ten times or more than high manganese steel on a disc abrasion tester is provided with service life only 3-5 times longer than that of high manganese steel liner? One of important reasons is high manganese steel deforms but does not fracture after it was worn like a thin sheet until it was used up. But high chromium cast iron liner will be recommended to be replaced after it is abraded to 50% of original thickness.
4. Mismatching of liner material and structure design
It refers to two aspects. On one hand, the high manganese steel liner often used before, because of extremely high tenacity the liner structure is hardly designed at will to be long, thin or cavern out the back, but some cement plants usually carried the drawing only for high manganese steel liner with high tenacity to the inexperienced foundry work, and asked them to produce wearable liners with high hardness and brittleness. When two outsiders gathered, there would be problems. On the other hand, the previous liner often has two bolt holes that are square, and this is easy to cause stress concentrated inside and on the sharp corner during casting.