Ball, Rod, and Tube Mills WILLIAM H . WITHINGTON

Hardinget Company, Incorporated, New York, N. Y

NINE 10-FOOT CONICAL BALL MILLS WITH AIR CLASSIFIERS GRXDIKG TO 98 PER CENT THROUGH 200 MESH

N RIIAXY industries grinding or pulverizing forms the major part of the total cost of the final product. The selection of the right type of grinding machinery is

often difficult in view of the number of different makes on the market. It is out of the question to discuss the relative merits of such a large number of machines, and therefore this paper will be limited to ball and pebble mills which have a greater adaptability to modern grinding requirements than any other type of grinding equipment.

I

The Ball Mill The ball mill is one of the oldest preliminary machines that

is still on the market, and it remains an important factor in the reduction of ores and nonmetallic minerals. It is widely used for both wet and dry grinding. 9 ball mill may be described as a cylinder, generally made

of steel plate having separate heads or trunnions attached to the ends with the trunnions resting in suitable bearings for supporting the mrtchine. The trunnions are hollow for the introduction and discharge of the material undergoing reduc- tion.

Ball mills vary in size from 2 to 10 feet in diameter and with shell lengths from one to six times the diameter.

The mill shell is lined on the inside with 1- to 3-inch chilled iron, carbon steel, or manganese steel liner plates attached to the shell with countersunk bolts. These liners are made in different shapes so that the contour of the inside surface of the mill may be suited to the requirements of the application.

The conventional method of driving the mill is to attach a

ring gear to the shell or to one of the heads, engaging a pinion mounted on a countershaft parallel with the long axis of the mill.

Spur gear and pinion using countershaft and pulley drive for belting from motor or line shaft. Pulleys may be either plain or equipped with friction clutch. Spur gears are usually made of steel with cut teeth for the larger mills and of cast iron with cast or cut teeth for the smaller mills.

Spur gear and pinion using countershaft and cha,in, V belt, or reduction gear drive direct to a high-starting torque-type motor.

Herringbone gear and pinion for direct connection through flexible coupling to a slow- or medium-speed, high-starting torque- t e motor. %ingle helical gears for direct connection through flexible

coupling to motor.

Exceptions to these types are the 2- and 3-foot-diameter mills which are equipped with pulleys around the cylinder.

The feed to ball mills is introduced through one of the hollow trunnions by means of a chute, drum with internal scoop, external scoop, or helical conveyor. The discharge of the ground or partially ground material is accomplished in several ways, depending upon the type of mill and the shape and particle size desired in the ultimate product. The dif- ferent types of discharge arrangements will be described later.

As indicated by its name, the grinding in a ball mill is pro- duced by rotating a quantity of steel balls. The individual balls may vary in size from The ball charge may occupy from one-third to nearly one-half of the total internal volume of the mill shell.

The operating speed of a ball mill is governed by the diameter of the mill, the size of material entering the mill,

The types of drives most frequently used are:

to 5 inches in diameter.

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VOL. 30, NO. 8 898 INDUSTRIAL AND ENGINEERING CHEMISTRY

the mesh size and shape of product desired, the contour of the lining, and whether the material is being ground wet or dry. Mathematical formulas have been derived to calculate the theoretical speeds of ball mills, but the theoretical speed can seldom be applied in practice because of the number of variables in each case. As a rule ball mills are operated a t peripheral speeds of 250 to 650 feet per minute.

The power required for driving a ball mill depends upon the peripheral speed of the mill, the weight of the ball charge, the specific gravity of the material undergoing reduction, the contour of the lining, the load level within the mill, and the moisture content or dilution. I n general, however, the power required varies from approximately 2 horsepower per foot of shell length for a 3-foot-diameter mill up to 30 horse- power per foot of length for a mill 8 feet in diameter.

Pebble mills differ essentially from ball mills in the type of lining and grinding media used. They are also somewhat lighter in design because of the lesser weight to be supported.

The interior of a pebble mill shell is lined with blocks of Belgium silex, American quartzite, porcelain, or waved type plates of chilled iron or alloy steel. The lining blocks are cemented to the shell with Portland cement, acid-proof cements, or other suitable binders. The metal linings are attached with countersunk bolts. The silex or quartzite blocks are from 21/2 to 4 inches thick. The metal plates are from 11/2 to 2‘/2 inches thick.

Flint pebbles, porcelain balls, and quartzite cubes may be used for grinding media in mills grinding materials which must not be contaminated with iron. Flint pebbles are not found in commercial quantities in this country and are there- fore imported from Denmark, Belgium, and France. Where there is no objection to a slight contamination with iron, 7/8- to 1-inch-diameter steel balls, short pieces of round rods, high-carbon steel slugs, or boiler punchings may be used in pebble mills.

The speed of a pebble mill is usually slightly higher than that of a ball mill of the same diameter in order to overcome the slip between the pebble charge and the relatively smooth silex or porcelain lining. The capacity of a pebble mill is considerably less than that of a ball mill of equivalent size or occupying approximately the same floor area. This differ- ence in capacity is due to the difference in the weights of the respective grinding charges used in these two types of mills. The weight of a cubic foot of flint pebbles is approximately

105 pounds compared with 356 pounds for the same volume of steel balls.

The power required for driving a pebble mill is reduced in almost direct proportion to the capacity or weight of the grinding charge. The reduction in power is not directly pro- portionate because of the slightly higher speed a t which the pebble mill must be operated.

Because of the greater capacity per unit of mill volume or per unit of floor area which can be obtained from ball mills, the ball mill is installed wherever possible. However, there is a definite place for pebble mills in modern grinding where purity of product, particularly freedom from iron contamina- tion, is an important considerattion.

Shapes of Mills Ball and pebble mills may be classified into two distincf

groups as distinguished by their general appearance. In commercial practice these groups are identified as: (1) cylindrical ball and tube mills and (2) conical ball and peb- ble mills. Typical arrangements of these mills are illustrated in Figure 1.

Cylindrical Ball and Tube Mills The cylindrical mill is distinguished by its cylindrical shell;

with flat or nearly flat heads attached to the ends. The cylindrical sections are made from 3 to 9 feet in diameter with lengths from one to six times the diameter. When the cylinder length is two or more times the diameter, the mill is usually designated as a tube mill. It may be a ball-tube or pebble-tube mill, depending upon the kind of lining and ginding media used.

With one or two minor exceptions, cylindrical ball and peb- ble mills are fed through one of the hollow trunnions. The discharge may be through the opposite trunnion or through slotted openings in the periphery of the shell.

Figure 2 represents two types of cylindrical overflow mills. I n the practical operation of all overflow mills with a mixed charge of balls or pebbles, the trFvel of material through the mill is accomplished by replacement. That is, the material going into the mill replaces the ground material discharged.

The value of the long tube mill as a medium fine and ex- tremely fine grinding device for both wet and dry grinding is well established. It is able to produce a larger percentage of

uniformlv fine material without an outside separatiig device than any other type of mill. It is also a perfect mixing machine where there are two or more materials re- quiring grinding and intimate blending. Its efficiency, however, as measured in terms of power input per ton output when operating in open circuit is low, and the trend is definitely towards shorter mills in closed circuit with adequate sizing devices.

Since the purpose of the pebble-tube mill is essentially that of fine grinding by attri- tion, the grinding charge should be made up of small balls or pebbles, the greater propor- tion of which are 11/4 inches in diameter or less. Because of the small balls or pebbles used, it is necessary to feed a tube mill with material which has been previously reduced {-$ {:+ ingmedia. to 8 mesh or finer to avoid “floating” the grind-

While many tube mills are installed as

----- - - - - - - - -- - - - - - - - -- - - -

trunnion overflow mills, the rate of travel of material through the mill, particularly in dry grinding, can be expedited by the use of a rapid discharge arrangement consisting of a

FIGURE 1 (Abooe). CYLINDRICAL BALL AND TUBE MILL (Left) AND CONICAL

TYPES OF CYLINDRICAL OVERFLOW MILLS

BALL AND PEBBLE MILL (Right)

FIGURE 2 (Below).

AUGUST, 1938 INDUSTRIAL AND ENGINEERING CHEMISTRY 899

vertical grate or partition installed near the discharge head of the mill with lifting flights or scoops between the grate and the discharge head. The flights or scoops are arranged to dis- charge through the trunnion. The material passing through the openings in the grate is picked up by the scoops near the periphery of the shell and lifted by the rotation of the mill to a sufficient height to discharge by gravity through the hollow trunnion. By picking up the material near the periphery of t he shell, the pulp level in the mill is lowered and thus causes a more rapid flow of material towards the discharge end.

The purpose of the grate is to retain the grinding media and not to size the material. The openings in the grate are there- fore made as large as possible without permitting the small- est balls or pebbles to discharge from the mill.

FIGURE 3. SHORT CYLINDRICAL BALL MILL

In order to prepare 8-mesh or finer material for tube mills and also for producing coarser products for metallurgical and other purposes, the short cylindrical ball mill was developed (Figure 3). In this type of mill the length of the cylinder is approximately the same as the diameter. It is fed through one trunnion and discharged by means of the grate and lift- ing flights described above. Because of the short length of the cylinder the grate discharge arrangement provides a low pulp level within the mill, and the grinding is principally by impact. The mill of low pulp level was designed originally to crush 2- or 3-inch ore down to a fineness of 10 to 48 mesh with a minimum of slimes. I n order to crush the large pieces of ore by impact, the mills are charged with steel balls up to 5 inches in diameter.

The short-cylinder, low-pulp-level mills are used for wet grinding ores where moderately fine grinding is required and where the mills are operated in closed circuit with classifiers capable of handling large circulating loads. The short mills have not been used to any great extent in industrial wet and dry grinding operations.

With the development of efficient secondary reduction crushers, particularly gyratory, cone, and impact crushers which can now economically crush to inch, the need for coarse or impact crushing in ball mills has been practically eliminated. This factor, together with the demand in most industries for finer grinding (more surface area), has created a trend to mills having shells approximately twice as long as their diameters and also in quite a number of instances to the elimination of the quick discharge grates.

Compartment Ball and Pebble Mills The desire to simplify two-stage grinding operations re-

quiring a short preliminary ball mill for coarse grinding and a tube mill for finishing was responsible for the introduction of the compartment ball mill. The compartment ball mill is a long cylindrical ball mill divided into two or more com- partments by partitions or grates to segregate the grinding media into groups of approximately the same size. The largest balls are placed in the feed end compartment to crush the large pieces of incoming feed which may be as large as ll/z inches in size.

In the earlier compartment mills the partition between the sections was a plain grating of cast steel or manganese steel. In later types, in order to gain the advantage of peripheral discharge, the partitions have been made double, consisting of one grate and one solid partition with the lifting flights be- tween the two. The material discharged from one compnrt- ment flows through the grate, is picked up by the lifting flights, and is discharged through an opening in the center of the solid partition into the succeeding compartment. Com- partment mills are built in diameters from 5 to 10 feet and with shell lengths from 25 to 50 feet.

The disadvantage of the compartment mill is its inability to handle materials of varying size or grindability. There is no convenient way to vary the operations independently within the different compartments. If the mill is working on material of a certain hardness or size and the hardness or size should increase, the first compartment may not break it down fine enough for the succeeding compartment. Like- wise, if softer material or a finer feed is supplied due to bin segregation or some similar cause, the product may be over- ground with the resultant waste in power.

In recent installations of compartment mills, attempts have been made to provide greater flexibility by operating one or more of the compartments in closed circuit with out- side sizing devices. This has improved the efficiency, but the fact remains that when these mills are made of the same diameter throughout their length, the peripheral speed is the same for all compartments. If the speed is correct for impact grinding with large balls in the first compartment, it is necessarily too fast for attrition grinding in the finishing compartment, and vice versa.

Compartment mills are used for both wet and dry grinding. They are used in the cement industry for grinding raw materials and cement clinker.

As a rule the power consumption per unit of capacity is higher. when operating compartment mills in open circuit than when using two-stage grinding in Peparate units or a single mill in closed circuit with a suitable classifier.

Screen-Type Ball Mills Screen-type ball mills are those in which the cylindrical

section of the mill is surrounded by a screen through which the product must pass. The material too coarse to pass through the screen is picked up by buckets or flights and re- turned to the grinding compartment for further reduction. Screen mills are limited in capacity and are susceptible to overloading, especially when the openings in the screen be- come clogged.

While these mills have been used for preliminary grinding in the cement industry, they are not as versatile as the trun- nion discharge types of mills and are, therefore, not as ex- tensively used.

Conical Ball and Pebble Mills The conical mill may be readily distinguished from the

cylindrical types of mills, described previously, by its dis- tinctive conical shape (Figure 1). This apparatus, which has had a revolutionary effect on milling, was invented by H. W. Hardinge in 1907. The essence of the invention was the conical shape which caused the grinding media, which were of different sizes, to arrange themselves in the mill according to their diameters with the largest towards the feed end. This classification of the grinding media and material undergoing reduction was obtained without the use of grates, partitions, or other artificial means.

The conical mill consists of a short cylindrical section from 2 to 10 feet in diameter and from 8 inches to 7 feet in length, with a 60" cone attached to the feed end and a 30" to 40" cone

900 INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 30, NO. 8

attached to the discharge end. The over-all shell length of a conical mill, including the cones, is usually from one and one-half to two times its largest diameter. The plate-steel conical ends replace the cast-steel heads required for the cylindrical types of mills. The trunnions are attached directly to the apex of the cones. This type of construction forms a trusslike structure of inherent strength without ex- cessive weight. A ring gear is attached to either the feed or discharge cone and the same methods of driving, as de- scribed previously for cylindrical mills, are used.

Conical ball mill shells are lined with segmental metal plates from 2 to 3 inches thick. The linings may consist of wave-type plat,es of chrome or manganese steel, or plates and wedge bars as illustrated in Figure 4. When the wedge-bar type of lining is used, the wedge bars are made of chrome or manganese steel and serve the threefold purpose of taking the major impact of the balls, lifting the mass, and holding the plates in place. Since the bolts do not pass through the liner plates, the plates are usually made of a special chilled iron known as “Titmite” which is extremely hard and has good wearing qualities. The plates may also be made of chrome or manganese steel.

Because of the automatic classifying action of the grinding media, the grinding charge may be made up of mixed sizes of balls from 7/8 to 5 inches in diameter. The feed may be as large as 2 inches although more efficient results are usually obtained when the feed has been previously crushed to ”4

inch or less. Conical pebble mills are lined with silex, quartzite, or por-

celain blocks. Flint pebbles, porcelain balls, or quartzite cubes may be used for grinding media. In special cases large

FIGURE 4. SECTION OF WEDQE BAR TYPE OF LINING SHOWING MANNER OF HOLDING (‘TITANITE” PLATES IN

PLACE

pieces of the material undergoing reduction, such as feldspar and barytes, are used for the grinding elements. The feed to conical pebble mills should not exceed inch.

The principle of operation of the conical ball and pebble mills is the same, but each covers a specific grinding field. Both types grind wet or dry in open circuit or in closed cir- cuit with screens, wet classifiers, or air classifiers. They may be operated as trunnion overflow high-pulp-level mills or, provided with quick discharge lifters or peripheral discharge ports, for medium- or low-pulp-level operations.

The conical mill utilizes three of the most important laws of grinding: (1) proportioning the energy to the work to be done, (2) rapidly circulating the mass within the mill to pre- vent the particles undergoing reduction from becoming im- bedded in the mass, and (3) ejecting the material rapidly to prevent cushioning of the grinding media.

Rapid circulation is maintained by the angular impact of the grinding media against the feed and discharge cones. The parabolic circulating effect produced by the rotration of the cones keeps the mass active and promotes a quick discharge of the fines and a violent agitation of the grinding media both vertically and semihorizontally.

:l?$&%ve 1 650 f t . 400 f t . 250 ft: speeds per min. per min. per min.

=43:1 =125:1

WITHIN THE MILL FIGURE 5 . EFFECT OF CONES ON THE MASS

Figure 5 illustrates the effect of the cones upon the mass within the mill. The incoming feed is crushed by the largest balls or pebbles with the greatest superincumbent weight, the greatest height of fall, and the highest peripheral speed. As the discharge end is approached, the crushing force is gradu- ally diminished since the balls or pebbles are smaller and‘are dropped from a lesser height. Power and wear on balls and lining are thus kept low.

Tricone Mill The tricone mill (Figure 6) is a modification of the conical

mill. This mill consists of three conical sections. The center section is a long tapering cone with 5 O to 7 O slope. The coni- cal heads have an angle of 60” to 70”. Shell lengths vary from one to five times the mean diameter. It has been found that the slight pitch of the center section is sufficient to cause classification of the grinding media, as in the standard coni- cal mill, but more time is required for the various sizes of balls to find their respective positions.

FIGURE 6. TRICONE MILL

Short-cylinder tricone mills are being successfully used for grinding coal for firing boilers, rotary kilns, and industrial furnaces. They are capable of delivering a uniformly fine product with an economical expenditure of power. The long-cylinder tricone mill with its automatic classification of the grinding media apparently has a potential field in the cement industry as a more efficient and less complicated device than the existing compartment mill.

Rod Mill The rod mill is a modification of the cylindrical ball-tube

mill and, although not strictly a ball or pebble mill, it belongs

AUGUST, 1938 INDUSTRIAL AND ENGINEERING CHEMISTRY 90 1

Low DISCHARQE LEVEL

to that general group. It was first used in Germany about forty years ago. The rod mill as now developed is a good device for grinding various ores and materials in certain fields.

The general construction of a rod mill is similar to that of a cylindrical ball mill. The shell length, however, is main- tained a t approximately twice the diameter. The grinding media consist of high-carbon steel or alloy rods from 2 to 5 inches in diameter and only a few inches shorter than the cylindrical length of the shell. Rod mills are operated a t speeds varying from 26 r. p. m. for the 4-foot diameter mill to 13 r. p. m. for an 8-foot mill. They require from 5 to 20 horsepower per foot of length.

It has been demonstrated by many tests that within its field the rod mill will produce a more uniformly sized product than a ball mill. It is a good granulator when grinding friable ores and minerals. When grinding to 10 or 20 mesh, it produces R minimum quantity of fines. Practice has also demonstrated that, when conditions are favorable to both types of mills, the rod mill can be operated with less moisture than a ball mill when grinding wet. When dry grinding, the rod mill with peripheral discharge ports (Figure 7 ) is capable of grinding damper material without packing than the equivalent ball mill. The feed to a rod mill should not ex- ceed ”* inch maximum size.

Classification of Grinding Operations It its important to consider the application of the various

types of ball, pebble, and rod mills to methods used in various industries. As may be expected, there are numerous ex- amples of overlapping due to local or special circumstances. As mentioned previously, grinding is accomplished by the wet or dry process, in open or closed circuit. A granular or fine product may be obtained in either case

Wet grinding is almost always used where the process requires the addition of water or moisture to the product of the mill immediately after the grinding operation is completed. For wet grinding, sufficient moisture must be introduced to produce a mixture that will flow like thick cream.

In dry grinding, the material may con- tain a very small percentage of moisture but it must not be sufficient to cause the material to become sticky or plastic. A bone-dry material grinds with the highest efficiency. a n exception is the air-swept ball mill in which preheated air is circulated through the mill to dry the material while it is being ground

Open-circuit grinding is that method whereby the material is fed in a t one end of the mill

WET GRINDING.

DRY GRINDIKG.

OPEN-CIRCUIT GRINDING.

of-sizing devices in most cases are entiiely different and much larger circulating loads are

permissible in wet grinding operations. Figure 8 illustrates a closed-circuit wet-grinding operation

with a ball mill in closed circuit with a classifier. The par- tially ground product discharges from the mill and flows by gravity to the classifier where sufficient water is added to overflow the material of the desired fineness. The coarser particles settle to the bottom of the classifier and are con- veyed and elevated, by the rakes or spiral flights in the classi- fier, to a sufficient height to discharge by gravity into the scoop box of the mill. This represents a typical arrangement and the operation is practically the same, regardless of the type of mill or Classifier used. As the classifier and mill are a t approximately the same elevation, the distance through which the oversize must be elevated is only a few feet. It is therefore practical in wet closed-circuit operations to main- tain circulating loads of from 300 to as much as 1500 per cent without an excessive power expenditure for handling the over- size. Products ranging from 10 to 350 mesh may be obtained.

Wet -g r ind in g mills are also oper- ated in closed cir- cuit with trommel a n d v i b r a t i n g screens for rela- tively coarse prod- ucts. Screens are seldom used for p r o d u c t s f iner t h a n 30 mesh. When vibrating screens are used, t h e y a r e p laced above the mill so that the oversihe can be returned to the feed end of the mill by gravity and with a mini- mum quantity of water. The par- t i a l l y g r o u n d m a t e r i a l d i s -

BALL MILL IN CLOSED charged from the * CIRCUIT WITH CLASSIFIER FOR WET mill is elevated or GRINDINQ pumped t o t h e screen.

The advantages of operating dry-grinding mills in closed circuit with suitable sizing devices have been thoroughly dem- onstrated. Mill capacities have been doubled with less

FIGURE 8.

INDUSTRIAL AXD ENGINEERING CHEMISTRY VOL. 30, NO. 8 902

tban 20 per cent increase in the total power for the niill and accessories.

For products ranging from 10 to 30 mesh, dry-grinding ball, pebble, and rod mills are usually operated in conjimction with vibrating screens. A typical layoiit is shovrn in Figure 9. Screens are occasionally used for sepamtions as fine as 200 mesh, but such installations are the exception rather than the rule. It is necessary to have the material quite dry when the product is to be screened.

When dry grinding to deliver 8 finished product 7vithin:the range of 40 to 350 mesh, it is the established practice to operate the mill in closed circuit with an air-separating sys- tem. This applies particiilarly to cylinilrieal and conical types of ball and pebble mills.

There are two air-separating systems in general use with cylindrical and conical mills and a third system which is used exclusively with conicalmills. Figure IOillustrates a centrifu- gal-type air separator in closed circuit with a tube mill. The partially ground material i s discharged from tlje mill and

elevated by means of a bucket elevator to the air separator. The separator (Figure 11) removes the finished material from the circulating load, and the oversize particles are returned by gravity or by mechanical means to the feed end of the mill for regrinding. The same arrangement may also he used Nit11 conical mills.

This sy~tem is capable of prodocing a very fine product for approximately 60 per cent of the power that would be re- quired for open-circuit grinding to tlie same fineness. Its disadvantages are tlie additional moving parts introduced by the accessory equipment and the headroom required.

A totally different method of air separation is that ob- tained by passing air through the mill. Although the first patent for passing air through a ball mill was issued to .James F. Drnmmond in 1867, this system has only come into promi- nent use within the past few years. The air-swept hall niill has no mechanical disctrarge. Air is admitted through one trunnion and is drawn tlrruugh the mill by means of an in- diiced-draft fan eonne&xl to the opposite trunnioii. Tlle

FI~+CRE 1 1 . CENTRIFUGAL-TYPE Am SSPARATOR

velocity of the air is varied so that it removes only the mate- rial that has been ground to approximately tlie desired fine- ness The material-laden air is disclinrged irom tlie ran int,o a cyclone or similar type of collector to separate the niaterial from the air stream. The clean air may be returned to the mill or vented to the atmosphere as desired.

In the operation described, the niill is actually in open circuit, as no oversize material is discharged from the mill. The open-circuit air-swept mill is nsed principally fur pul- verizing coal when the fineness of the product is within the range of 40 to 100 mesh and where fluctuations in the fineness are not critical. Where accurate control of the fineness of the product is required, an air classifier ie used in conjunction with the air-swept system to remove tlir oversize particles which are automatically returned to the mill for regrinding along with the new feed.

The principle of reversed air ciirrents is used exclusively

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904 INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 30, NO. 8

with the conical mill and is illustrated in Figure 12. The fan blows the air into the mill through a pipe passing centrally through the discharge trunnion. The air current reverses itself within the grinding zone of the mill and is drawn back to the discharge end carrying the partially ground material with it: The air is withdrawn around the central pipe and passes upward between the two concentric cones within the air classifier section. At the top of the classifier section are adjustable vanes that deflect the air in such a way that the oversize material is thrown out of the air stream into the inner cone from which i t is returned by gravity to the return pipe

FIGURE 12. CONICAL BALL MILL WITH REVERSE-CURRENT AIR CLASSIFIER, ARRANGED FOR DRYING WITH HOT AIR

WHILE GRINDING

I ,

through which it is blown back into the mill. The material of the desired fineness is conveyed by the air stream from the air classifier to a product collector which may be located 60 to 80 feet above the mill. The dust-free air discharged from the product collector is drawn back into the fan to repeat the cycle. A vent is provided on the pressure side of the system to discharge any air that leaks into the system on the vacuum side.

A product of any commercial degree of fineness may be secured with this system, and the fineness can be maintained with a variation of less than 0.5 per cent. As the material does not pass through the fan, the wear is negligible, and a curved-blade high-efficiency fan is employed. Materials having greater than normal moisture content can be ground by circulating preheated air as the drying medium. The surplus air and water vapor are discharged through the vent pipe.

The control of the reversal point of the air current within the grinding section of the mill makes i t possible to obtain an extremely fine product or a granular material practically devoid of superfines. This control feature was described by Harlowe Hardinge.1

The conical mill with reverse-current system of air classi- fication is economical in power consumption and dustless in operation. Because of the partial vacuum maintained within the mill and classifying system, there is no discharge of dust-laden air into the grinding room. This is particularly advantageous when grinding siliceous or other materials having an injurious effect upon the respiratory organs of the human body.

Grinding Applications Characteristics of different materials have to be carefully

considered before selecting the type of mill. The most common reference to any material is that it is either hard, abrasive, or soft. The common assumption is that the harder the material, the greater the power consumption per ton to

1 IND. ENG. CHEDI., 26, 1139 (1934).

grind it. Toughness is the chief factor in power consumption.

Because of their use in both wet and dry grinding opera- tions, ball and pebble mills are grinding a greater variety of materials than any other type of grinding equipment. They are used almost exclusively for grinding such hard and abra- sive minerals as silicon carbide, coke, cement clinker, feldspar, glass, and quartz, and for the hard and tough materials such as basalt, gray slate, traprock, corundum, and silica sand. They are also used practically without exception for wet grinding ores, clays, limestone, pigments, pottery slip, oxides, graphite, barytes, chalk, and similar materials.

However, it has only been since the development of the air-swept mill with an adequate air classifier to permit ef- ficient closed-circuit operation that ball and pebble mills have been able to compete favorably with high-speed roller, ball, and impact mills in power consumption per ton when dry grinding such materials as bituminous coal, talc, kaolin, bauxite, phosphate rock, gypsum, limestone, lime, and marl.

It must not be assumed, however, that power consumption is the most important item in the cost of grinding. Sim- plicity, low maintenance, continuity of operation, and long life, which are characteristic of the slow-speed mills described, are equally important.

The operating field data given in Table I represent what is being done in practice under normal operating conditions with the types of mills described.

Cost of Equipment

This is erroneous.

The cost of an installation is often underestimated because of failure to consider the necessary accessories. There is no empirical formula for estimating the cost of a mill and its accessories for a specific application. It is necessary to pre- pare a list of all the essential parts before the total cost can be determined. Much depends upon the completeness of the list.

Freight is an important item which should not be passed over hurriedly, The weights of different types of mills vary considerably. This is also true of their accessories. It should not be assumed that the freight cost will be approxi- mately the same for the several types of equipment under consideration.

An analysis of the cost of typical units for wet grinding ore shows the following relative costs of the essential items to the total factory cost of the complete unit (in per cent) :

Regulating feeder Ball mill Classifier Ball charge Motors and power-transmiasion equipment

Total

4 50 15 11 20

100 -

A similar analysis of dry-grinding units for grinding lime- stone or a material of equivalent grindability to a fineness of 85 per cent passing 200 mesh indicates that the accessories amount to more than 50 per cent of the total cost of the unit:

Regulating feeder 4 Ball mill, 42 Air classifier 25 Ball charge 8 Motors and power-tranamission equipment 21

Total mi While these figures do not apply to all types of mills and ar- rangements, they serve to emphasize the importance of in- cluding the cost of all of the accessories.

It is desirable, when conditions permit, to have one sup- plier responsible for the mill and its immediate accessories. This not only eliminates divided responsibility for the satis- factory performance of the equipment but assures the inclusion of all of the necessary parts. RECEIVED December 23, 1937.