Grinding Mills SALA Leaflet 603 E, Sweden 1973

November 18, 2017 | Author: Tsakalakis G. Konstantinos | Category: Mill (Grinding), Crystalline Solids, Procedural Knowledge, Materials, Industries
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Application of grinding mills (ball-, rod-, pebble mills etc.) Nowadays grinding mills are almost extensively used for ...


Mills Leaflet 603 E


Ball mills

Nowadays grinding mills are almost extensively used for comm inut1on of materials ranging from 5 mm to 40 mm (3/ 16" -1 5/ 8" ) down to varying product sizes. They have vast appl ications within different branches of industry such as for example the ore dressing, cement, lime, porcelain and chem ical industries and can be designed for continuous as well as batch grind1ng.

Ball mills can be used for coarse grinding as described for the rod mill. They will , however, in that appl ication produce more fines and tramp oversize and will in any case necessitate installation of e ~ fect i ve classification. If finer grinding is wanted two or three stage grinding is advisable as for instant primary rod mill with 75-100 mm (3"-4" ) rods , secondary ballmillwith25-40mm(1"-11j2 ") balls and possibly tertiary ball mill with 20 mm (3/ 4" ) ball::; or cylpebs. To obtain a close size distribution in the fine range the specific surface of the grinding media should be as high as possible. Thus as small balls as possible should be used in each stage.

Rod mills The principal field of rod mil l usage is the preparation of products in the 5 mm-0.4 mm (4 mesh to 35 mesh) range . It may sometimes be recommended also for finer grinding. Within these limits a rod mill is usually superior to and more efficient than a ball mill. The basic principle for rod grinding is reduction by line contact between rods extending the full length of the mill , resulting in selective grinding carried out on the largest particle sizes. Th is resul ts in a minimum production of extreme fines or slimes and more effective grinding work as compared with a ball mill. One stage rod mill grinding is therefore suitable for preparation of feed to gravimetric ore dressing methods, certain flotation processes with sl ime problems and magnetic cobbing. Rod mills are frequently used as primary mil ls to produce suitable feed to the second grinding stage. Rod mills have usually a length/ diameter ratio of at least 1.4.

Fig. 1. Rod mill , two-tire type .


Tube mills Tube mills are m principle to be considered as ball mills , the basic difference being that the length/ diameter ratio is greater (3- 5) . They are commonly used for surfa ce cleaning or scrubbing action and fine grinding in open circuit.

Pebble mills In some cases it is suitable to use screened fractions of the material as grinding media. Such mills are usually called pebble mills , but the work ing principle is the same as for ball mills. As the power input is approximately directly proportional to the volume we ight of the grinding media, the power input for pebble mills is correspondingly smaller than for a ball mill.

Wet or dry grinding A dry process requires usually dry grinding. If the feed is wet and sticky, it is often necessary to lower the moisture content below 1 %. Grinding in front of wet processes can be done wet or dry. In dry grinding the energy consumption is higher, but the wear of linings and charge is less than for wet grinding, especially when treating highly abrasive and corrosive material. When comparing the economy of wet and dry grinding , the different costs for the ent1re process must be considered.

Method of discharge Overflow Rod Mill Recommended for normal wet grinding in rod mills. The diameter of the discharge trunnion is larger than that of the feed trunnion giving an easy material flow through the mill. The discharge trunnion can be furnished with a trommel screen to remove rod fragments, wooden chips etc. from the mill discharge.

Rod Mill with End Peripheral Discharge Used when a moderately coarse product is desired.

Rod Mill with Centre Preripheral Discharge Suitable for dry grinding at extremely high capacities and very coarse grinding, wet or dry. Also applicable for very viscous material and humidities of 3-15 % by weight.

Overflow Ball Mill Suitable for almost all applications where a ball mill is required . Simple and trouble-free grinding method. The discharge trunnion can be furnished with trommel screen.

Grate Discharge Ball Mill This type of mill usually works with a high circulating load and thus produces very little extreme fines. The specific power consumption is therefore less than for an Overflow Mill. A Grate Discharge Mill will also have 15-25 % higher capacity per volume unit as compared with an Overflow Mill. The discharge trunnion can be furnished with trommel screen .


Fig. 2. Ball mill, dia. 2.7 X 6 m (9' X 21 ')

Mill speed Mill speed is generally referred to as a percentage of critical , which is defined as the speed at which an infinitly small particle just follows a smooth shell lining in its motion. n, =2.36 X N X (n,

1.31 X N

lfD, lfD,}

where n, is % of cr itical speed D; is the mill diameter inside shell lin ing in meters {feet) and N is the mill speed rpm . An increase in the mill speed will give a directly proportiona l increase in mill power but there seems to be a square proportional increase in the wear. Rod mills generally operate within the range of 60-75 % of critical speed in order to avoid excecclve wear and tangled rods. Bal i and pebble mills are usually operated at 70-85 % of critical speed . For dry grinding the speed is usually somewhat !ower.

Ncrit rpm



N rp m




90 10





............ ~




/' /'


v .... >< .......











!'-.... ....... ~


!""-- .......





""" ~

~ ~ 1/








1.0 10




~~~ter 30

D 1 feet

Critical speed graph . Example : A m ill with 4.5 m dia (15') N ,;, obtained = 20 rpm . If N = 15 rpm nc obtained = 75 %.

Lining The mill lining can be made of rubber or different types of steel (manganese or Ni-hard) with liner types according to the customers requ irements . For special appl ications we can also supply porcelain, basalt and other linings.

Fig. 3. Rubber lining, grate mill.


Charge volume The mill power is approximately directly proportional to the charge volume with in the normal range . When calculating a mill 40 % charge volume IS generally used. In pebble and ball mills quite often charge volumes close to 50 % are used. In a pebble mil l the pebble consumption ranges from

3-15 % and the charge has to be controlled automatically to maintain uniform power consumption . The volumetric density of a rod and a ball charge is around 4.9 ton/ m3 (31 0 lb/ cu .ft) corresponding to 37 % void space.

Sizing of mills Factors influencmg the m11l size are :

1. Type, hardness and screen analysis of feed 2. Desired product size after grinding 3. Requ ired capacity In all cases the net energy consumption per ton (kWh/ton) must be known either from previous experience or laboratory tests before mill size can be determ ined . The requ ired mill net power P kW ( = ton/h X kWh/ton) is obta ined from P = const. X :; X q X n, X L; X



where an approximate figure for the constant is 3.5 for rod mills, 4.0 for overflow ball or pebble mill s an d 4.5 for grate discharge ball or pebble mills . ~

is specific gravity of grinding media (for rods or balls normally 7.85) q is the charge volume in fraction of mill volume

(40 %= 0.40) n c is the mill speed in fractio n of critical speed L; and D; are length and diameter inside shell lining respec ti ve ly (i n meters) For a mill with more than 2.1 m (7') diameter the tota l power is obtained by dividing net power by 0.85. Fig. 4. Rod mill w ith max1mum rod charge.


Construction Shell of steel plate quality B.S. En. 2 B arch welded, stress relieved and with machined flanges . If desired the shell can be provided w ith manholes. Heads of S.G. iron (nodular) or steel castings w ith machined and drilled connection flanges to shell and trunnion. Trunnions of S.G. iron or steel castings w ith machined flange and bearing seat incl. device for dismantling the bearings. For smaller mills the heads Dnd trunnions are sometimes made in grey cast iron. Bearings Dnd housings of SKF anti -friction type or selfali gning ball and socket type. Gearing of spur type in S.G. iron or steel with cut teeth. Pinion of steel quality B.S. En 43 w ith cut teeth and flame hardened to su itable hardness. Pinion Shaft of quality B.S. En 43, press fitted and key locked to the pinion and carried in two SKF standard antifriction bearings. For bigger mills the pinion shaft is integral with the pinion. Speed Reducer with cast iron cas ing and shafts and gears of spe ci al steel carried in anti -friction bearings . Flexible Couplings provided between motor and speed reducer as well as between speed reducer and pinion shaft.


Mills for laboratories and pilot plants For smaller capacities and laboratory use SALA manufactures grinding mills with sizes according to the table below. These mill s are built on a steel frame , on which the complete drive is also installed. The mill runs on rubberized rollers, wh ich are driven via torque arm speed re ducers and rope drives from two standard squirrel-cage motors. The speed reducers are of standard SALA type runn ing in oil in dust-tight housings. fhe mills can be used either fo r dry or wet, rod or bnll grinding. By us1ng a separate attachment the discharge end can be changed so that the mills can be used for peripheral instead of overflow discharge. The lining is generally made as a plate of wear resistant steel w ith welded on lifters. The heads are bolted onto the shell wh ich sim;:llifie:s the change o f lin ing.

Fig . 6. Laboratory mill with peripheral discharge .

Fig. 7. Batch mill w ith water cooling jacket. Made in stainless steel fo r grinding of tungsten . Fig. 8. Mill s for pilot tests during ass embly in our workshop.







Arr. 2


Arr. 3


Arr. 1 Arr. 4

L... ....J, D









1320 4'4" 5' 1500 6' 1800 7' 2100 8' 2400 9' 2700 10' 3000 11 ' 3300 3800 12' 13' 3900 4200 14' 4500 15'




5'-16' 5'-17' 6'-18' 7'-21' 8'-24' 9'-27' 10'-27' 11'-27' 12'-27' 13'-27' 14'-27' 15'-27'


1800--5400 21 ()()..-..&QO 2400-7200 2700--8100 3000-8100 3300-8100 3600-8100 3900--8100 4200-8100 4500--8100





5200- 9000 5500--9400 6200- 9800 6700-10900 7400-12400 7800-13600 6200-13600 8800-13700 9300-13800 9800-14600 10400-15600 12400-1 6200

17'--3/J' 18'--31' 20'--32' 22'--36' 24'-.1' 26'-.&' 27'-.s' 28'-.s' 30'-.s' 32'-.&' 34'--51' 41'--53'

1) Diameters and barrel lengths are manufactured with a modulus of 300 mm (abt. 1 ') . Tube mills with greater lengths than shown in the table can be delivered . For calculations of power input and critical speed the diameter and length should be reduced with the liner thickness.







mm 3500 3500 4400

4800 6000

6200 9300.) 9300•) 9300•) 9600•) 10000•) 10700•)



H max


L ft


ft 11 '6" 11'6" 14'5" 15'9" 19'8" 20'4" 30'6" 30'6" 30'6'' 31'6" 32'10" 35'1"


mm 2900 2900 3200 3500

4200 5100 5100 5100 5100 5600 6100 6600




Total Power Hpl)

9'6" 9'6" 10'6" 11'6" 13'9'' 16'9" 16'9" 16'9" 16'9'' 18'5" 20' 21'8"

35- 115 45- 150 90- 270 170- 510 260- 780 415-1250 610-1650 870-2100 1200-2700 1550-3250 2000-3950 2500-4500


2) Ball mill grate discharge w1th 40 % charge and speed 75 % of critical. For rod mills with 40 % charge and 60 % of critical multiply power figure by 0.60. 3) Drive motor with m1n. 10 % higher power should be chosen. *) Dual dnve as per arr. 4. All measurements are for preliminary use only.

SA SALA INTERNATIONAL AB S-733 00 Sala · Sweden • Tel. 0224/132 20 Telegrams: Salamachine · Telex 7536

Sprlngfeldt/VLT Civil Sweden 1973

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