3540735577 by Unknown

Author:Unknown
Language: eng
Format: epub
Published: 2008-11-13T05:52:38+00:00

9.3 · Further Rules-of-Thumb

103

9.3.2.4

Rules-of-Thumb for Open Pit Operating Costs

Operating costs per tonne of ore in open pit mining depend to a very large extent on

the waste/ore ratio (see Sect. 1.2.6), whereas costs per tonne of material moved are

relatively constant. For small open pit mines (1 000–5 000 t / day) US $1.50 / t can be

expected, for big mines US $1.00/ t. Loaders and heavy trucks have capacity limits. If

the capacity in open pit mining is to be increased, more loaders and trucks have to be

purchased.

Example. To illustrate this point, let us take an open pit mine with 10 000 t / day and a waste: ore ratio of 1 : 1. We have to move 1 t overburden for 1 t ore, i.e. a total

of 2 t. At a cost of 1 US $ / t of moved material the open pit operating costs amount to

2 US $ / t. If the mine has a waste:ore ratio of 10 : 1, an additional 10 t of waste has to be moved for each 1 t of ore, i.e. a total of 11 t, and open pit operating costs become

11 US $ / t of ore.

Assignment. Calculate the operating costs for a massive sulphide deposit to be mined to the level of –100 m in an open cut. A typical cross-section is given in Fig. 9.4. The

open pit reserves amount to 6 million t. Envisaged lifetime of the mine is 10 years. The

orebody dips at 45°. Density of the sulphide ore is 4, of the host rock 3, of the alluvial cover 2 g/ cm3.

Step 1: The waste: ore ratio is calculated from the sectional areas. For the open pit wall an angle of slope of 45° is assumed:

Area I

= 5 949 m2

Area II

= 4 900 m2

Area III = 1 980 m2

Fig. 9.4. Vertical section to calculate the waste:ore ratio

104

Chapter 9 · Calculation of Cost Data

From these areas and the density we calculate the tonnage per m of section thick-

ness:

Area I

= 5 949 × 2 = 11 898 t

Area II

= 4 900 × 3 = 14 700 t

Area III = 1 980 × 4 = 7 920 t

Areas I and II represent waste. Hence our waste: ore ratio W: O is

At both ends of the mine additional waste will occur in the direction of strike since

the walls are at an angle. For a rough calculation the waste is increased by 10% to 3.7 : 1.

Step2: Calculation of the operating costs of the open pit mine. The deposit in the open pit area has reserves of 6 million t to be mined over a period of 10 years,

i.e. 600 000 t / a. With 300 working days / year this amounts to 2 000 t / day. With a

waste: ore ratio of 3.7 : 1, a total of 4.7 t have to be moved for each tonne of ore, i.e.

9 400 t / day. This is a medium-sized open pit mine. Medium operating costs of

1.30 US $ / t of moved material can be assumed.

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