Mills Leaflet 603 E

Application Nowadays grinding mills are almost extensively used for comminut1on 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 chemical industries and can be designed for continuous as well as batch grind1ng.

Rod mills The principal f ield 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-t ire type .

2

Ball mills 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.

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 surface cleaning or scrubbing action and f ine 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 working principle is the same as for ball mills. As the power input is approximately directly proportional to the volume weight 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.

3

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 lfD, (n, 1.31 X N 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 crit ical speed. For dry grinding the speed is usually somewhat !ower.

4

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

100 90 10

1'0

60

so

30

20

IS

10

Ncrit rpm

..... /' ~ ~

/' , ............ /' ...... ~ v .... ~ >< L .......

1.0

22 /'./

f"': .... """ 1/ ~

/' ......

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

10

Critical speed graph .

N m rp 20 II 16 10 12 10 , ./"

v ./" .... ~ "'

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

~~~ter 1s 20 25 30 D 1 feet

Example : A mill 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 requirements. 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

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 determined. 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 Dt 5

where an approximate figure for the constant is 3.5 for rod mills, 4.0 for overflow ball or pebble mill s and 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 fraction of critical speed

L; and D; are length and diameter inside shell lining respective 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.

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.

Fig. 4. Rod mill w ith max1mum rod charge.

5

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 with manholes.

Heads of S.G. iron (nodular) or steel castings with machin­ed and drilled connection flanges to shell and trunnion.

Trunnions of S.G. iron or steel castings with 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 -fr iction 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 with 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 casing and shafts and gears of special steel carried in anti -frict ion bearings .

Flexible Couplings provided between motor and speed re­ducer as well as between speed reducer and pinion shaft.

6

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 running in oi l 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 with welded on lifters. The heads are bolted onto the shell wh ich sim;:llifie:s the change of lin ing.

Fig. 8. Mill s for pilot tests during assembly in our workshop.

,,....,..,........,. ..

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 .

7

H

A

L

T

D B

Arr. 1

L ... ....J, barrel len~· -:·~ .. :·~:," ., A

D L

mm ft mm ft I mm

1320 4'4" 1500--5000 5'-16' 5200- 9000 1500 5' 1~100 5'-17' 5500--9400 1800 6' 1800--5400 6'-18' 6200- 9800 2100 7' 21 ()()..-..&QO 7'-21' 6700-10900 2400 8' 2400-7200 8'-24' 7400-12400 2700 9' 2700--8100 9'-27' 7800-13600 3000 10' 3000-8100 10'-27' 6200-13600 3300 11 ' 3300-8100 11'-27' 8800-13700 3800 12' 3600-8100 12'-27' 9300-13800 3900 13' 3900--8100 13'-27' 9800-14600 4200 14' 4200-8100 14'-27' 10400-15600 4500 15' 4500--8100 15'-27' 1 2400-1 6200

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.

SA SALA INTERNATIONAL AB

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

ft

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

Arr. 2 Arr. 3

Arr. 4

"" r~' . ; ... ~:: .,,, -~

MdP) ~;:

Bmex H max Total - Power

I mm I ft I mm I ft Hpl)

3500 11 '6" 2900 9'6" 35- 115 3500 11'6" 2900 9'6" 45- 150 4400 14'5" 3200 10'6" 90- 270 4800 15'9" 3500 1 1'6" 170- 510 6000 19'8" 4200 13'9'' 260- 780 6200 20'4" 5100 16'9" 415-1250 9300.) 30'6" 5100 16'9" 610-1650 9300•) 30'6" 5100 16'9" 870-2100 9300•) 30'6'' 5100 16'9'' 1200-2700 9600•) 31'6" 5600 18'5" 1550-3250

10000•) 32'10" 6100 20' 2000-3950 10700•) 35'1" 6600 21'8" 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.

Sprlngfeldt/VLT Civil Sweden 1973