Grinding Mills

NCP supplies new Grinding Millsand all ancillary equipment to clients across the globe

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New Concept Projects (NCP), are highly regarded as leading manufacturers and suppliers of New Osborn Grinding Mills into Africa and internationally. Additionally, as suppliers of refurbished plant & mining equipment, NCP are able to meet demand and assist on projects carrying constrained budgets or accelerated delivery timing requirements.

Regarded as experts in their field, NCP are centered on the supply of Capital equipment and the management of related turnkey services, to deliver equipment to client’s exact specifications.

New Concept Projects

1. NEW EQUIPMENT

• Grinding Mills & Communition Circuits

2. REFURBISHED EQUIPMENT

• Grinding Mills & Comminution Circuit Equipment • Crushers & Crushing Plants• Thickeners • Conveyor Equipment • Pump Projects • Ventilation Fans • Shaft Compressors • Mine Winders • Power Generation Equipment • Cranes

The product range offered by NCP includes:

• ROD MILLS• BALL MILLS• ROM BALL MILLS• SAG MILLS• FAG MILLS• LINERS• GRINDING MEDIA• ANCILLARY EQUIPMENT

Product Range

NCP is actively involved with supplying and developing a wide

range of mill types with mills being identified by the type of

grinding media used. Each type of mill has characteristics

that enhance its grinding application and the operational

characteristics of each must be evaluated in relation to the

objectives of the grinding requirement.

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The primary purpose of mill liners is to protect the internal surfaces of the mill which would otherwise come in contact with the grinding charge and be subject to wear. The lining due to its position becomes the final link in transmitting the energy of the rotating mill to the mill load, therefore careful consideration must be given to the shape of the liners. Various profiles and types are used, depending on the application. (diagram below)

Shell and head liners can be made of manganese steel, chrome moly steel, other wear resistant alloys, silica blocks, rubber or ceramic. Liner bolts are generally made of forged steel with an oval head to prevent turning and loosening within the liner.

Leak proof washers are furnished with liner bolts for wet or dry grinding applications.

Liner wear results from a combination of corrosion, abrasion and impact. The relative importance of each factor varies with the grinding application. The most significant milling variables relative to wear are feed size, media size, mill speed, mill diameter, ore hardness, ore abrasiveness and corrosiveness of the pulp. Wear increases as each of these variables increase. As liners become worn, the volume within the mill increases.

The increased mill volume allows for additional ore and media within the mill. Additional load within the mill results in an increased mill power draw. Power draw can increase by up to 8% as liners wear to their replacement condition, if the charge level relative to the mill axis remains constant.

Liners

SINGLE WAVE TYPE(Normal Rod Mill)

DOUBLE WAVE TYPE(Normal Ball Mill)

GRID TYPE(Ball or Run-of-Mine Mill)

TWO PIECE RUBBER TYPE(Rod or Ball Milll)

Many factors are involved in the selection of grinding media. Including ore hardness, feed size, mill size, mill speed and feed rate. Media selection is important because the cost of media is a major expense in grinding with significant savings available by optimizing the use of grinding media. Media diameter can be selected so as to provide for the best mill performance as well as media shape selection (established shapes are either spherical or cylindrical).

Choice of materials used in grinding media is significant. Grinding balls should be made of forged steel, or cast steel having the same resistance to breakage. Rods should be hot rolled, hot sawed or sheared with standard tolerances and machine straightened.

NCP typically undertakes DEM Analysis (Discreet Element Modelling) to determine optimal Liner Design & Mill charge characteristics.

Grinding Media

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Rod Mills

Rod Mils can accept feed up to approximately 25 mm and are generally selected to grind products in the -4 mm to -0.5 mm range. Grinding action is by line contact between rods which extends the length of the mill.

Large feed particles spread the rods at the feed end and in so doing ,allow preferential grinding of the coarse fraction. A minimum amount of fines are created and a relatively uniform sized grading is produced.

Rod Mills are usually operated in open circuit. Most rod mill applications involve wet grinding where material is reduced in size from crusher product size to a size suitable for ball mill feed. Rod milling in the size range utilised is more efficient than primary ball milling, in that the desired product is obtained at a lower cost per ton. To avoid breakage or tangling of rods the L/D ratio of rod mills is kept within the limits 1.3/1 to 2/1.

NCP actively supply the three most common Rod Mill types

OVERFLOW ROD MILL END PERIPHERIAL DISCHARGEROD MILLS

CENTRE PERIPHERIAL DISCHARGEROD MILLS

1. Rod Mill

OverflowAn overflow mill is constructed with a feed opening at one end and a discharge opening at the other, both on the axis of the mill. Material leaves the mill by overflowing at the discharge end. Used for producing the finer end of the product range.

2. Rod Mill

End peripheral discharge (EPD)The end peripheral discharge mill’s steeper gradient produces shorter retention time and thus a coarser product than the overflow type. Product is discharged through port openings equally spaced around the periphery of the shell at the discharge end. Grinding may be accomplished wet or dry. End peripheral discharge is used to good advantage in damp grinding of coke or steelwork sinter plants.

3. Rod Mill

Centre peripheral discharge (CPD)Feed is from both ends and is discharged at the centre through discharge ports, equally spaced around the mill periphery.

The ports are in a separate ring between the shell halves. The fact that the particles only travel half the mill length makes the CPD rod mill an outstanding unit for coarse grinding and rapid removal of finished material.

Ball Mills accept feed that ranges in size from 80% passing 15 mm to fine feeds in regrind applications. Products may be as coarse as 0.5 mm or as fine as 40 microns or below. Grinding is accomplished with spherical-shaped media generally made of steel. In a ball mill the length may be less than, equal to, or greater than its diameter. A short length mill has a lower retention time and produces a coarser product. Longer mills have a longer retention time and therefore produce a finer grind. Ball mills can be operated in an open circuit but modern practice generally utilises closed circuit operation.

Ball Mills

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1. Ball Mill - Overflow Discharge:

As the name implies, material is discharged from an overflow mill by overflowing the discharge opening. In this type of mill the balls are kept in the mill by keeping the top of the ball charge below the level of the discharge opening. A reverse spiral in the discharge trunnion liner also assists this condition.

OVERFLOW BALL MILL

2. Ball Mill - Grate Discharge:

This mill type embodies the principle of steep pulp gradient, quick discharge operation. A grate assembly at the discharge end of the mill permits retention of a higher ball charge (45%) than is possible in an overflow mill, which has a practical limit of around 40%. Behind the grates are pulp lifters, which pick up the ground material as it passes through the grates and transports it out of the mill. This quick removal of ground material produces overgrinding especially when operated in closed circuit with a classifier. The classifier removes the particles, which are within the product specification, and returns the oversize back to the mill feed chute.

GRATE BALL MILL

ROM Ball Mills

3.Run-of-Mine Ball Mills (ROM Ball Mills):

Run-of Mine Ball Mills can accept ROM up to 150 mm. The large feed particle size reduces effective Ball charge carrying capacity of the Ball Mill, thus ROM Ball Mills typically operate between 20-32% Ball charge. ROM Ball Mills carry a low aspect ratio & can generate a final Grind size in closed circuit through a single Mill.

SAG Mills

4. Semi-Autogenous Mills (SAG Mills):

SAG Mills use semi-autogenous grinding in place of autogenous grinding when the ore does not produce the necessary competent media. Forged steel balls are used to supplement the coarse ore. With this additional load by adding steel balls, more motor power is required and the physical design of the mill requires to be more robust. Most clients envisaging autogenous operation however request their mill be designed to cater for semi-autogenous loads. Ball sizes commonly used are 100 to 120 mm. The ball charge is generally in the range of 2 to 12% of the mill volume.AUTOGENOUS OR

SEMI-AUTOGENOUS MILL (US STYLE)

5. Trunnion Supported vs Shell Supported Mills:

Trunnion supported mills have been proven by many years of operation. NCP/Osborn have over 180 mills in the field with powers from 2 kW for laboratory use to 3880 kW on production units. NCP also carry shell supported designs for mills >12 ft diameter.

Shell supported designs reduce the bending moment on shells by reducing the effective distance between support points. Limitations on trunnion openings is removed, thus shell diameters can be increased.

Trunnion Supported Mill

Shell Supported Mill

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Mill Sizing and Power Draw Calculations

When selecting a grinding mill it is necessary to first calculate the amount of power (kW) required to achieve the process requirement. Once the power value has been determined a suitable mill size and shape can be selected. The methods and tables below will enable an approximate sizing to be made for project purposes.

MILL CALCULATIONS

A common method used in determining power requirements uses the “Bond” theory where the following mathematical relationship applies:

W= 10 Wi ( 1/P80 - 1/F80 )

where,

W = Kilowatt hours per short ton requiredWi = The Bond Work Index (Kwh/short ton)P80 = Size in microns which 80% of the product passesF80 = Size in microns which 80% of the feed passes

Wi is usually obtained from laboratory batch tests or records kept from previous tests. The value W can now be metricated by multiplying by 1.1 (Kwh/tonne) and then by the feed rate to give kilowatts at pinion shaft.

A series of correction factors are considered when using the Bond method, in the table to the right.

As the formulae for correction factor 3 includes the mill diameter a provisional mill size can be selected from the chart below using the power value (1).

For this a suitable L/D ratio must be assumed. The final power in kilowatts can be obtained by multiplying the previously calculated value (1) by each “applicable” correction factor.

ID Shell m (ft) 1,07 (3’- 6”) 1,37 (4’- 6”) 1,68 (5’- 6”) 2,00 (6’- 6”) 2,30 (7’- 6”) 2,60 (8’- 6”) 2,90 (9’- 6”)Mill Type Grate Over

FlowRod Grate Over

FlowRod Grate Over

FlowRod Grate Over

FlowRod Grate Over

FlowRod Grate Over

FlowRod Grate Over

FlowRod

% Charge 45 40 40 45 40 40 45 40 40 45 40 40 45 40 40 45 40 40 45 40 40

% Critical Speed (Ball)

79,2 77,7 76,6 75,5 74,7

RPM 27,1 25,8 24,3 22,7 22,2 20,4 20,5 18,5 19,1 17,1

Rod PFM 405 425 449 465 483

LENGTH Kilowatt draw at pinion shaft with new liners

m (ft)

0,914 (3) 8 7

1,22 (4) 10 9 9 12 11

1,83 (6) 15 14 13 18 16 25 48 42 45 76 67 -

2,44 (8) 21 33 64 56 60 102 90 91 151 135 214 189 -

3,05 (10) 70 75 127 112 114 189 168 163 268 27 220 363 321

3,66 (12) 84 134 137 201 196 321 284 264 435 386 348

4,27 (14) 157 235 228 331 308 508 450 406

4,88 (16) 269 379 352 - 514 464

5,49 (18) 426 - 578 522

6,10 (20) (1) Power values shown are for Rod & Ball charges

(2) Sizes marked - Are suitable for AG, SAG or Run of Mine units. Powers on application.

(3) Powers for AG, SAG & Run of Mine sizes within the range in (1)

above are also available on application.

- 643

6,70 (22)

7,31 (24)7,92 (26)

8,53 (28)

9,14 (30)9,75 (32)

10,36 (34)

10,97 (36)

11,58 (38)

12,20 (40)

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Material No. of Tests Specific Gravity

Work Index

Cement Clinker 60 3.09 13.49Cement Raw Material 87 2.67 10.57Chrome Ore 4 4.06 9.66Coal 16 1.63 11.37Coke 12 1.51 20.70Copper ore 308 3.02 13.13Dolomite 18 2.82 11.31Feldspar 8 2.59 11.67Ferrochrome 18 6.75 8.87Ferromanganese 10 5.91 7.77Ferrosilicon 15 4.91 12.83Flint 5 2.65 26.16Fluorspar 8 2.98 9.76Gold Ore 209 2.86 14.83Graphite 6 1.75 45.03Iron ore 8 3.96 15.44Magnetite 83 3.88 10.21Lead Ore 22 3.44 11.40Lead Zinc Ore 27 3.37 11.35Limestone 119 2.69 11.61Limestone for Cement 62 2.68 10.18Manganese Ore 15 3.74 12.46Mica 2 2.89 134.50Nickel Ore 11 3.32 11.88Phosphate Fertilizer 3 2.65 13.03Phosphate Rock 27 2.66 10.13Potash Ore 8 2.37 8.88Pyrite Ore 4 3.48 8.90Quartzite 16 2.71 12.18Quartz 17 2.64 12.77Silica 7 2.71 13.53Silica Sand 17 2.65 16.46Silica Carbide 7 2.73 26.17Silver Ore 6 2.72 17.30Slag 12 2.93 15.76Slag, Iron blast furnace 6 2.39 12.16Sodium Silicate 3 2.10 13.00Tin Ore 9 3.94 10.81Titanium Ore 16 4.23 11.88Uranium Ore 20 2.70 17.93Zinc Ore 10 3.68 12.42

CORRECTION FACTOR 5, CF5 (fineness of grind)

If P80 < 74 µm then

CF5 = P80 + 10.3

1,145 P80

If P80 > 74 µm then CF5 = 1,0

CORRECTION FACTOR 6, CF6

(ratio of reduction, rod milling)

Optimum Reduction Ratio:

Rr0 = 8 + 5L

D1

Reduction Ratio, Rr = F80

P80

CF6 = 1 +

(Rr - Rro)2

150

CORRECTION FACTOR 7, CF7(low ratio of reduction, ball milling)

If Rr < 3 then

CF7 = (Rr - 1,35) + 0,26

2 (Rr - 1,35)

If Rr >/= 3 then CF7 = 1,0

CORRECTION FACTOR 1, CF1 (dry grinding) = 1,3 (wet grinding) = 1,0

CORRECTION FACTOR 2, CF2 (open circuit ball milling)

Reference Correction % Passing Factor 2 50 1.035 60 1.05 70 1.10 80 1.20 90 1.40 92 1.46 95 1.57 98 1.70

If closed circuit grinding: CF2 = 1,0

CORRECTION FACTOR 3. (diameter)

CF3 = (8/D1)0,2

where D=Dia inside liners

(or 0,914 minimum value)

CORRECTION FACTOR 4. (oversized feed)

CF4 = Rr + (Wi - 7) (F80 - F0) if F80 < F0

Rr

where

Rr = F80

P80

F0 (Ball milling) = 4000 13/Wi

F0 (Rod milling) = 16,000 13/Wi

If F80 < F0 then CF4 = 1,00

MATERIAL SPECS

3,20 (10’- 6”) 3,66 (12’- 0”) 4,27 (14’- 0”) 4,57 (15’- 0”) 4,88 (16’- 0”) 5,49 (18’- 0”) 6,10 (20’) 7,00 (23’) ID SHELL

Grate Over Flow

Rod Grate Over Flow

Rod Grate Over Flow

Rod Grate Over Flow

Rod Grate Over Flow

Rod Grate Over Flow

Grate Grate Mill Type

45 40 40 45 40 40 45 40 40 45 40 40 45 40 40 45 40 45 45 % Charge

73,9 72,9 71,7 71,2 70,7 69,5 68,3 68,0 % Critical

17,9 15,9 16,5 14,8 14,9 13,2 14,3 13,0 13,8 12,8 12,7 11,8 11,0 RPM

499 535 560 592 623 Rod FPM

Kilowatt draw at pinion shaft with new liners Length

m (ft)

0,914 (3)

1,22 (4)

- - 1,863 (6)

- - - - - - 2,44 (8)

472 418 - - - - - - 3,05 (10)

567 502 800 707 - - 3,66 (12)

662 585 518 934 825 733 1370 1212 - - - - - 4,27 (14)

756 669 592 1067 943 837 1566 1386 1862 1648 2194 1942 - - - 4,88 (16)

- 753 666 1200 1061 942 1762 1559 1359 2094 1854 1659 2468 2185 3321 2939 - - 5,49 (18)

- 836 740 - 1179 1047 1957 1732 1510 2327 2060 1843 2742 2428 2217 3690 3266 - - 6,10 (20)- 920 - 1297 - 1903 2560 2266 3017 2670 - 3593 - - 6,70 (22)

- 1415 - 2078 - 2472 3291 2913 - - - 7,31 (24)

- 1532 - 2252 - 2678 - 3156 - - - 7,92 (26)

- 2425 - 2884 - 3398 - - - 8,53 (28)

- 2598 - 3090 - 3640 - - - 9,14 (30)

- 3296 - - - - 9,75 (32)

- - - - 10,36 (34)

- - - 10,97 (36)

- - - 11,58 (38)

- - 12,20 (40)

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Application Data Sheet

Detailed below are the criteria used to size a mill for a specific application.Please complete and send to projects@newconcept.co.za for evaluation.

MILL - DESIGN CRITERIA

Item Info Supplier Description Details Units Measurement

1 SITE INFORMATION

1.1 Client

1.2 Project Name

1.3 Country Located

1.4 Air Temperature Maximum deg C

1.5 Minimum deg C

1.6 Altitude m above SL

1.7 Plant Design Life Years

1.8 Mill Locations Inside Building/Outside

2 PROCESS CHARACTERISTICS

2.1 Ore Type

2.2 Precious metal recovered

2.3 Moisture Content %

2.4 Ore feed Rate to Mill Fresh feed Dry t/h

2.5 Bond Ball Mill Index Nominal kWh/t

2.6 Design kWh/t

2.7 Ore Specific Gravity Tons/m

2.8 F100 Max feed size µm

2.9 F80 80% Feed size passing µm

2.10 P80 80% Product size passing µm

2.11 Level of Corrosivity in Feed ph

2.12 Extraction Process CIL, CIP, CIC, floatation

2.13 Feedsize Analysis Please supply details if available

2.14 Mill Availablilty % & hrs/day

2.15 Circuit Open/Closed

2.16 Circulating Load %

2.17 Feed Material Temperature deg C

2.18 Ball Charge Max Operating Ball Charge %

2.19 Discharge Arrangement Grate/overflow

3 ELECTRICAL INFORMATION

3.1 Power Supply to Plant Overhead/Generated Power

3.2 Supply Voltage Large Drives V

Small Drives V

3.3 Control Voltage V

3.4 Cycles Hz

PO Box 9411Edenglen1613South Africa

Tel: +27 87 353 6791Cell: +27 82 889 8881Fax: +27 86 743 7580E-mail: projects@newconcept.co.za

www.newconcept.co.za

www.osborn.co.za

Head Office:57 Jansen Road, Elandsfontein

PO Box 8182Elandsfontein, 1406JohannesburgSouth Africa

Tel: +27 11 820 7600 Fax: +27 11 388 1136E-mail: osborn@osborn.co.za