Milling of Materials

Ball Mills

A Ball Mill grinds material by rotating a cylinder with steel grinding balls, causing the balls to fall back into the cylinder and onto the material to be ground. The rotation is usually between 4 to 20 revolutions per minute, depending upon the diameter of the mill. The larger the diameter, the slower the rotation. If the peripheral speed of the mill is too great, it begins to act like a centrifuge and the balls do not fall back, but stay on the perimeter of the mill.

The point where the mill becomes a centrifuge is called the “Critical Speed”, and ball mills usually operate at 65% to 75% of the critical speed.

Ball Mills are generally used to grind material 1/4 inch and finer, down to the particle size of 20 to 75 microns. To achieve a reasonable efficiency with ball mills, they must be operated in a closed system, with oversize material continuously being recirculated back into the mill to be reduced. Various classifiers, such as screens, spiral classifiers, cyclones and air classifiers are used for classifying the discharge from ball mills.

Rod Mills

Rod mills are very similar to ball mills, except they use long rods for grinding media. The rods grind the ore by tumbling within the the mill, similar to the grinding balls in a ball mill. To prevent the conditions leading to rod charge tangling, the length to diameter ratio is maintained at 1.4 to 1.6. Rod mills accept feed up to about 50 mm (2 in.) and produce a product in the size range of 3000 to 270 mm (–4 to –35 mesh). Grinding action is by line contact between the rods extending the length of the mill. Rods tumble and spin in roughly parallel alignment simulating a series of roll crushers. This results in preferential grinding of coarse material and minimizes production of slimes.

Of the three main types of rod mill, overflow, end peripheral discharge, and center peripheral discharge only the overflow mill is in common usage. Wet grinding rod mills are normally used in the mineral processing industry. Dry grinding is used in some areas; however, it is confronted with problems and should be avoided except where absolutely necessary. Rod mills operate at lower speed than ball mills since the rods are rolled and not cascaded. For an equivalent grind, a rod mill uses less steel than a ball mill because of the lower speed and better contact between the media and ore. The rod charge must be maintained in good working condition, and broken and worn rods must be removed. Rod mills usually require greater operator attention. It is important that the rods stay essentially parallel to one another. If rods become misaligned, grinding action is lost and, more importantly, rod tangles occur. Maximum rod length is limited to about 6.1 m (20 ft). This in turn limits the length, diameter, and capacity of rod mills. The heavier rods acting upon the lifters and liners result in greater wear on the mill liners.

Rod mills normally carry 35 to 65% rod charge by volume. The limits on charge level are (1) keeping the feed end trunnion open so that feed will get into the mill, and (2) keeping the rod charge low so rods will not work their way into discharge openings where they can cause rod tangling.

Hammer Mill

A hammermill crushes material that is friable, by impacting it against a rotating hammer (typically traveling between 750 RPM and 1800 RPM). Then the material is forced against a rugged solid plate called a “breaker plate” which further degrades the particle size. Finally, the material is forced over a discharge grate by the hammers, where crushed finer particles drop through the discharge grate and larger particles travel around for another crushing cycle, until they fall through the discharge grid. During the entire time the material is traveling around in the hammermill it is constantly being impacted by the hammers, and the side of the mill casing, causing breakage of the particles. This sequence repeats itself between 750 – 1800 times each minute, until the particle is ground fine enough to fall through the discharge grid.

One big drawback to hammermills is they wear, due to the abrasiveness of the material being crushed and the high speed of the mill. It should not be used to grind any material harder than a medium hard limestone. They are used most in crushing coal, but are also common in limestone operations. Large particles can not usually be crushed in hammermills, so they are almost never used for run of mine material or primary crushers, but secondary or tertiary crushers. Six to eight inches is a typical top size of feed for hammermills.

Another possible drawback for using hammermills, is that they tend to produce a lot of fine material (50 mesh to -100 mesh). Crushers like jaw crushers, cone crushers, and roll crushers, which operate a much lower speeds, tend to produce very few fines (generally less than 1% finer than 50 mesh). For some processes, excess fine material is a drawback, for others, it is not a problem.

On the positive side, hammermills are relatively inexpensive, as crushers go, can produce relatively fine material (1/4″ to -10 mesh) from 6″ or 8″ feed in one step, and they take up relatively little floor space. The are easily repaired, and simple to operate. Production size hammermills can produce products ranging from 1″ to as fine as 10 mesh, or finer, depending upon the material being crushed.

Roller Crusher

Roll Crushers are compression type crushers, and were once widely used in mining. They have, within the last 10 or so years, fallen into dis-favor among mining and processing companies. The probable reason is because the large mines require very large crushed product output with minimal cost, makes the roll crusher uncompetitive. The roll crushers are not nearly as productive as cone crushers, with respect to volume, and they do have a little higher maintenance associated with them. Roll crushers do, however, give a very close product size distribution, and if the ore is not too abrasive, they do not have high maintenance costs.

Roll crushers have a theoretical MAXIMUM reduction ratio of 4:1. If a 2 inch particle is fed to the roll crusher the absolute smallest size one could expect from the crusher is 1/2 inch. Roll crushers will only crush material down to a minimum particle size of about 10 Mesh (2 mm). A roll crusher crushes using compression, with two rolls rotating about a shaft, towards the gap between the rolls. The gap between the rolls is set to the size of product desired, with the realization that the largest feed particle can only be 4 times the gap dimension.

The particles are drawn into the gap between the rolls by their rotating motion and a friction angle formed between the rolls and the particle, called the nip angle. The two rolls force the particle between their rotating surface into the ever smaller gap area, and it fractures from the compressive forces presented by the rotating rolls. Some major advantages of roll crushers are they give a very fine product size distribution and they produce very little dust or fines. Rolls crushers are effectively used in minerals crushing where the ores are not too abrasive and they are also used in smaller scale production mining of more abrasive metal ores, such as gold. Coal is probably the largest user of roll crushers, currently, though. Coal plants will use roll crushers, either single roll or double roll, as primary crushers, reducing the ROM coal. Usually, these crushers will have teeth or raised forms on the face of the roll. (Roll crushers used for minerals and metal ores have smooth faced rolls.) 

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