Common types of grinding mills include Ball Mills and Rod Mills. This includes all rotating mills with heavy grinding media loads. This article focuses on ball and rod mills excluding SAG and AG mills. Although their concepts are very similar, they are not discussed here
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As the mill revolves, lifters assist in picking up the grinding charge and elevate it to an angle at which gravity overcomes friction and centrifugal force. The charge then cascades downward, effectively grinding particles of material within the mill by continuous, repeated impact and attrition action
Grinding Mill speed is one of the factors affecting the character of the cascading charge. As shown in the illustrations, the lower the percentage of critical speed, the smoother the flow of balls from top of charge to bottom. Higher percentage of critical speed is used for impact grinding of large feed. Lower percentage of critical speed is used for attrition grinding when a fine product is desired. The graph below will be helpful in determining percentage of critical speed when internal mill diameter and RPM are known. A Grinding Mill is a revolving cylinder loaded to approximately one-half its volume with steel rods, balls or pebbles
Grinding mills reduce particle size by impact, rolling and sliding. Of the many types in use, the cylindrical mill, which employs a cascading mass of balls or rods, is universally used for the size reduction of hard, moderate to highly abrasive materials, such as minerals, ores, stone, and chemicals. ,
A cylindrical mill, when operating under uniform conditions, will produce a uniform product. Wear on grinding surfaces has little effect on capacity or product size. Very little maintenance is required with these mills, downtime being a negligible factor in their operation. For continuity of operation, the cylindrical mill has no equal
Grinding mills of this type will give you dependable, trouble-free operation year after year, with “planned” periods of stoppage for renewal of parts. Initial cost is distributed over a long operating period. Many grinding mills are still in service after more than 40 years of almost continuous operation. Ton for ton of material handled, the cylindrical type mill has proved to be the most economical investment for reducing moderate to extremely abrasive materials
911Metallurgist sources manufacturers of all the proven mill designs in a “small” range of sizes — your assurance of getting the most suitable mill for your purpose. Best grinding efficiency and economy can be obtained only when the type and size of your mill is matched with your grinding job. Mills can also be furnished with modifications to suit any special application
The choice between wet or dry grinding is dependent upon the use of the product or the subsequent process. It is imperative to dry grind many materials because of physical or chemical changes which occur if water or a solution are added. Wet grinding with water (or with a concentrated solution of the soluble salts being ground) is generally preferred, because of the overall economies of this operation
Another factor in choosing a grinding mill is the consideration of the feed size introduced into the mill, and the product required from the mill or mill circuit. This can be best illustrated by comparing two prevailing and diametrically opposite grinds
In the manufacture of standard cement by grinding cement clinker, the clinker is reduced from 1″ or finer down to a specific surface of 1750 sq cm per gram. This area can be produced by an open or closed circuit grind. The specific area method (which indicates the square centimeters of surface exposed per gram of material ground) is a most satisfactory method of determining whether a cement product will meet an accepted standard
In a typical cement plant employing closed circuit grinding, 1750 surface can be obtained with a finish grind of between 93 and 96% passing 200 mesh. This area requirement means that fines are not only desirable but necessary, and that a size analysis must show a distribution of material from approximately 80 microns down to less than one micron
Wet grinding ball mill in mining operation. Balls within mill retain their high grinding efficiency regardless of wear. Ball mills are used extensively for grinding metal bearing ores and abrasive minerals
When grinding ore prior to concentration, on the other hand, the grind is determined by the degree of reduction necessary to unlock the valuable mineral from the gangue. This gangue is undesirable and must be separated from the desired material. For example, the full-size illustration above shows a piece of coarse-grained magnetite. The grind necessary to unlock the magnetite is about 14 mesh. At this grind, almost every individual crystal of iron oxide is freed from associated gangue and the ground material is ready for magnetic concentration
Actually then, the ideal grind would be to reduce this ore down to 14 mesh particles—not finer, for any expenditure of energy to reduce this ore beyond the unlocking mesh size is wasted. Such a grind is impossible, but any grinding circuit should be controlled so as to minimize overgrinding. Some fine material is produced, but it is tolerated rather than desired
The product resulting from the reduction of a number of particles is dependent on two distinct shapes of grinding media—the rod and the ball. Here the term ”ball” is used to cover the entire range of grinding media which is spherical in shape, or roughly so
The principle of grinding action, rods as compared to balls, can be best understood by making a comparison of their contact with adjacent rods or balls. Rods making up a grinding charge are nearly parallel and tend to meet adjacent rods in line contact. Rods tend to bear only on the largest particles, thereby expending most of their crushing force on the over-size and allowing the fine particles a freer passage between the rods without being ground to objectionable fineness. Balls, on the other hand, meet adjacent balls in point contact and particles of material at these points are ground to a very fine state
Concavex grinding medium is an improved type of ball grinding media which offers more surface area per unit of weight, and has found extensive use in the grinding of cement clinker. The advantage of Concavex medium is its ability to increase mill capacity because of its interlocking shape and increased density per cubic foot of grinding charge. Surface areas for Concavex grinding media are given later
Ball mills are built in Overflow and Diaphragm types. In the Overflow mill the material is discharged by new feed moving into the mill and displacing a mixture of solids and water being ground within the mill. The diaphragm arrangement in a ball mill is a positive means of pumping pulp or dry material out of the mill. The gradient is steeper than in an Overflow type mill. A Diaphragm ball mill has a higher capacity and requires more power than an Overflow ball mill of equal ball charge
The Overflow rod mill is applied to wet grinding. The Center Peripheral Discharge rod mill is also used for wet grinding but produces a coarser product than the overflow type. Either the End or Center Peripheral discharge rod mill can be used for wet or dry grinding. Whatever the type, the rod mill is used to produce a coarse product, whereas the ball mill is used to produce a finer product
Should a ball mill grind be required, the relationship of the length to the diameter of the mill is important. Feed and product screen analyses, and the type of circuit (open or closed), dictate the proper diameter to length ratio of a mill
Colored line indicates hp-hr/ton required to grind a hard ore from 1/2″ to 65% minus 200 mesh; closed circuit, wet grinding. Black line indicates hp-hr/ton required to grind average cement clinker from 0.5″ to 96% minus 10 mesh; open circuit, dry grinding
The type of mill for a particular grind and the circuit in which it is to be used must be considered simultaneously. Circuits are divided into two broad classifications, open and closed. In open circuit, the material is fed into the mill at a rate calculated to produce the correct finished product in one pass through the mill. This circuit has been popular in the cement and chemical industries, although the present trend is toward closed circuit installations. The closed circuit is generally employed in the mineral dressing industry
In the closed circuit the material is discharged from the mill into a classifying device. The classifier separates and (1) returns the oversize material to the mill for further grinding, (2) delivers the fine material as finished product of that circuit. Material returning to the mill is called circulating load, and the ratio of this material to new feed may vary from a few percent to 600 percent or more
Several types of separators are used in closed circuit grinding. Vibrating screens with screen cloth as fine as 28 mesh are used to produce a mesh product from either wet or dry grinding circuits. Fine wet grinding circuits employ a classifier to separate a product varying from 10 to 325 mesh. Fine dry grinding circuits employ an air separator for products of 65 mesh and finer
In open circuit grinding the feed rate must be low enough to permit a longer retention time per particle within the mill. This assures that each particle of the incoming feed, however large, will be broken down to product size. As a result, many particles in the product are ground to sub-sieve size. In ore dressing, these fine particles are usually undesirable, and the additional power required to produce them is a wasted expenditure. However, sub-sieve size particles are sometimes desirable, where the properties of the finished material require it. Finished cement and pottery glaze are examples of products requiring fines in the micron sizes
In a closed circuit operation no effort is made to produce all the reduction during a single pass through the mill. Instead, every effort is made to remove a particle from the circuit as soon as it reaches the required product size. Quite often one of the largest particles of the feed, partially reduced, will be discharged, separated and returned to the mill several times before being completely reduced to the desired size
Open circuits are particularly useful where simplicity of the layout may be a determining factor . . . where a product containing ultra-fines is preferred … or where the material does not lend itself to handling in any classifying device
All foregoing references to open and closed circuits apply to the ball mill. Because of the action of its grinding media, many rod mills are operated in open circuit, especially when preparing feed for ball mills. The rod mill, due to the character of its parallel grinding surfaces, simulates a slotted screen. The screening effect in the mill tends to retard the larger particles until reduced. Smaller particles slip through spaces between rods and are discharged without appreciable reduction
Due to many variables, grinding is considered an art, not a science. A number of factors which affect grinding capacity are so variable that considerable engineering experience is required for a judicious selection of the proper mill and circuit for a given operation
Some operators prefer the high speed mill, others will consider only grinding mills operating at low speeds. Grinding mill speed, it should be noted, is not absolute, not a definite rpm, but relative to a quantity called critical speed