Ball Mill

GRINDING(Grinding mills)

Introduction:

A Grinding is a unit operation in which solid material breaks into smaller pieces. There are many types of grinding mills and many types of materials processed in them. Historically mills were powered by hand, working animal, wind, water but now they are powered by Electricity. When a solid matter exposed to any mechanical force that break the structure by overcoming the bonding forces.After grinding the state of the solid changes inits grain size and in its grain shape. Grinding mills can be classified as Ball mills,Rodmills,Autogenous mills, SAG mills,Pebblemills,High pressure grinding rolls,Towermills,Vertical shaft impacter mills.

Ball Mill:

Ball mill is a type of grinder used to grind materials into extremely fine powder for use in mineral dressing process, paints, pyrotechnics, and ceramics. They are horizontal cylinders or cones in which substance, such as mineral, is grounded by rotation with steel or ceramic balls. Ball mills grinds the material to the necessary fineness by friction and impact with the tumbling balls.They operate with an approximate ball charge of 30%.Ball mills are charcterised by their smaller (comparatively) diameter and longer length, and often have a length 1.5 to 2.5 times diameter.The feed is at one end of the cylinder and discharge at the other.They are commonly used in the manufacture of Portland cement and finer grinding stages of mineral processing.Industrial ball mills can be as large as 8.5mts(28ft)in diameter with 22MW motor. The final stages of comminution are performed in tumbling mills using steel balls as the grinding medium and so designated “Ball mills”. Since balls have a greater surface area per unit weight than rods, they are better suited fine grinding. The term ball mill is restricted to thosehaving a length to diameter ratio of 1.5 to 1 and less. Ball mills in which the length to diameterratio is between 3 and 5 are designated tube mills.These are sometimes divided into several longitudinalcompartments, each having a different chargecomposition; the charges can be steel balls or rods, or pebbles, and they are often used dry to grindcement clinker, gypsum, and phosphate. Tube millshaving only one compartment and a charge of hard, screened ore particles as the grinding medium are known as pebble mills.

Since the weight ofpebbles per unit volume is 35-55% of that of steel balls, and as the power input is directly proportionalto the volume weight of the grinding medium, thepower input and capacity of pebble mills are correspondinglylower. Thus in a given grinding circuit,for a certain feed rate, a pebble mill would be much larger than ball mill, with correspondingly higher operating cost. However, it is claimed that theincrement in capital cost can be justified economically by a reduction in operating cost attributed to the lower cost of the grinding medium. This may, however, be partially offset by higher energy cost per tonne of finished product .

Ball Mill - Mineral Process Engineering
Figure

Ball mills are also classified by the nature of the discharge. They may be simple trunnion over flow mills, operated in open or closed circuit, or grate discharge (low-level discharge) mills. The latter type is fitted with discharge grates between the cylindrical mill body and the discharge trunnion.The pulp can flow freely through the openings in the grate and is then lifted up to the level of the discharge trunnion.These mills have a lower pulp level than overflow mills, thus reducing the dwell time of particles in the mill.Very little over grinding takes place and the product contains a large fraction of coarse material, which is returned to the mill by some form of classifying device. Closed-circuit grinding ,with high circulating loads, produces a closely sized end product and a high output per unitvolume compared with open circuit grinding. Grate discharge mills usually take a coarser feed thanoverflow mills and are not required to grind sofinely, the main reason being that with many smallballs forming the charge the grate open area plugs very quickly. The trunnion overflow mill is thesimplest to operate and is used for most ballmillapplications, especially for fine grinding and regrinding. Energy consumption is said to be about15% less than that of a grate discharge mill of thesame size, although the grinding efficiencies of the two mills are the same .Ball mills are rated by power rather thancapacity.

Today the largest ball mill in operation is 7.3 m in diameter with a corresponding motorpower of more than 11 MW.The trend in recent years has been to use fewer comminution machines per grinding line with theresult that units have increased considerably incapacity. For example, in the 1980s, the largestoperating ball mill was 5.5 m in diameter by 7.3 min length driven by a 4 MW motor. Today, ballmills of 5 m plus are common place, and 7 m ball mills are currently employed on at least two sites. However, there are several cases where large ballmills have not achieved design capabilities. Oneexample was the 5.5 m diameter by 6.4 m ball millsat Bougainville Copper Ltd where the coarse materialgrinding was particularly in efficient. The "post-Bougainville Copper" literaturehas been reviewed recently. Operationaldata from a wide range of large diameter ball mills were collected and analyzed. Theissues related to sizing of large diameter ball millsincluded power draw, residence time, feed size,and the applicability of Bond's equations. It wasconcluded that the power draw of large diametermills follows the same relationships that hold forsmaller diameter mills, as described in Morrell'spower equation.Ball-mill scale-up studies have been conductedin Australia and the United States, the resultsemphasizing that there are limitations to conventionalprocedures for estimating large mill requirementsfrom small-scale results Attempts have been made to use laboratoryball-mill test results to calibrate a suitable ballmill mathematical model, and a set of scale-upcriteria have been developed for scaling the parametersof the model to predict full-scale ball-millperformance .

The efficiency of grinding depends on the surface area of the grinding medium. Thus, balls should be as small as possible and the charge should be graded such that the largest balls are just heavyenough to grind the largest and hardest particlesin the feed. A seasoned charge willconsist of awide range of ball sizes and new balls added tothe mill are usually of the largest size required.Undersize balls leave the mill with the ore productand can be removed by passing the discharge overscreens. Various formulae have been proposed forthe required ratio of ball size to ore size, noneof which is entirely satisfactory. The correct sizesare often determined by trial and error, primary grinding usually requiring a graded charge of10-5 cm diameter balls, while secondary grinding requires 5-2cm. Concha et al. (1988) have developeda method to calculate ball-mill charge by using a grinding circuit simulator with a model of ball wear in a tumbling mill.

Applications: Cement, silicate, new-type material, refractory material, fertilizer, ore dressing of ferrous metal and non-ferrous metal, glass ceramics, etc,.

Questions and Answers:

(1) What is grinding?
Ans: Grinding is a unit operation designed to break solid material into smaller pieces.

(2)What are the uses of Grinding?
Ans: Increase of the surface area of the solid, manufacturing of a solid with desired grain size, pulping of resources.

(3)What are the different types of grinding mills?
Ans: Ball mill, Rod mill, Autogenous mill, SAG mill, Pebble mill, High pressure grinding rolls, Tower mill, Vertical shaft impactor mill etc.,

(4) What are the characterstics of a Ball mill?
Ans: Ball mills are characterized by their smaller (comparatively) diameter and longer length, and often have a length 1.5 to 2.5 times the diameter.

(5) What is the form of Bonds equation to predict the power for Ball mill?
Ans:
Ball Mill - Mineral Process Engineering

here
  • E is the Energy (kilowatt-hours per metric or short ton)
  • W is the work index measured in a laboratory ball mill (kilowatt-hours per metric or short ton)
  • P80 is the mill circuit product size in micrometers
  • F80 is the mill circuit feed size in micrometers.

References:

Mineral Processing Technology by B.A.Wills.

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