MATERIALS FOR WEARING SURFACES — Хонинговальные головки

The working surfaces of crushers and grinding mills are lined with wear-resistant materials to protect the permanent parts. The history of wear protection is an integral part of the history of comminution, because it is a major factor in the operating costs for circuits. Replaceable surfaces were used as early as the 1880s in South Africa when cast — wearing parts made with Hadfield manganese steel were placed in crushers and mills. It was a good choice, because, when manganese steel castings are exposed to continuous impacting, they grow sideways as they wear. Because of this characteristic of manganese, it has been possible to design the castings and the crushing chambers to allow for balanced wear and growth of the castings. When the wear rate of the castings is lower than the rate of their growth, the intermittent operation of primary crushers allows time to remove the growth. Consequently, replaceable manganese steel castings have proved to be satisfactory wearing surfaces for jaw and gyratory crushers.

Materials used in mill liners now include impact-resistant alloyed cast iron, abrasion — and impact-resistant alloyed cast steel, rolled alloy steel, and—since the 1960s—rubber. Where heavy impacting occurs, chrome-molybdenum alloyed steel castings are used. Iron castings containing nickel resist abrasive wear but are brittle and break under impact. Nickel-iron cast works well as shell liners in rod mills but cannot be used in ball mills when the largest balls exceed 51 mm, because rods have a line contact with shell liners and a lower impact pressure than balls that have a point contact with the mill liners. Impact-resistant shell liners made of nickel-hardened cast irons have long lives in rod mills, but the end (head) liners, which are subjected to pounding by the lateral movement of rods, are made from chrome-molybdenum steel. Cast or rolled wear-resistant alloyed steel liners are used in ball and SAG mills. The story in tumbling mills was different. Manganese steel liners worked well in pebble mills and in small-diameter rod and ball mills, but when the rod-and-ball-mill diameters exceeded 2.25 m and larger grinding media were used, the heavier impacts caused the liners to grow faster than they wore. To protect the surfaces of the rotating parts of grinding mills, clearance between liners was minimal; as a result, growth in the liners without room to expand put forces on the mill heads and shells that caused cracking. If the edges of the liners were beveled, they would climb over adjacent liners as they grew, which would cause the liner bolts to break. When grinding mill diameters reached 3.55 m, the use of cast manganese steel liners was discontinued.

In the 1960s, Swedish and American companies used rubber for mill liners, which had the advantages of being much lighter in weight, easier to install, and less noisy. The problem was that when flotation reagents, such as xanthates, were added to the mills the rubber in the liners was degraded and softened. Broken rods cut rubber liners installed in rod mills. Due to the heat in dry grinding mills, rubber liners were not used in dry grinding mills.

Rod, ball, and SAG mill shell liners are made with either waves cast into them that act as lifters or with separate lifters. These lift the outer rows of grinding media to a higher position in the mill rotation. A rule of thumb is that the number of lifters in a ball mill should be twice the diameter in feet plus two, and in a rod mill it should be twice the diameter in feet. Liner wear in grinding shells affects mill performance, because it increases the internal diameter and volume of the mill, which results in more media in the mill, higher power draw, and higher mill capacity.

CONCLUSION

The development of tumbling mills to date has been the most significant stage in the history of size reduction. The Industrial Revolution not only developed a rapidly increasing demand for size reduction, it led the way to the development of crushers and tumbling mills that rapidly increased in size and ability to produce the amount of materials needed. The roller mills discussed in Chapter 6 and the tumbling mills covered in this chapter efficiently grind materials to as fine as 20 |im. In Chapter 8, we describe grinding processes and machinery that were developed to more efficiently grind materials to finer than 20 |im, even to 1 |im. These systems were developed in response to the ever — increasing demand for finely ground materials.