STRIPPING RATIO CONSIDERATIONS

The parameter known as the stripping ratio isalmost universally used and represents the amount of uneconomic material that must be removed to uncover one unit of ore.

Fig. 13.1.1.4 represents an idealized open pitore body, dipping at angle a.

Stripping Ratio Considerations

Fig. 13.1.1.4. Overall stripping ratio representation

CUTOFF STRIPPING RATIO

A simple and widely used method of analysis toobtain pit limits is to utilize the cutoffstripping ratio.

First, the pit slope is determined fromgeotechnical and other considerations andthe stripping ratio then calculated.

CUTOFF STRIPPING RATIO

The cutoff stripping ratio is the one for whichthe costs of mining the ore and waste arematched by the revenue from that block ofore.

Factors used to determine costs should includethe added costs of mining as the pit deepensand the interest charges on the prestrippingof waste.

OVERALL STRIPPING RATIO

The production from each time period isdetermined, the costs and revenues listed,and a cash flow generated.

The profits from each year are discounted toreflect the time value of money in one ofseveral ways, depending on managementpreference

OVERALL STRIPPING RATIO

The result is considered to be the value of themine or production.

Mining is continued until it no longerincreases the value, and so a pit limit isdetermined.

The ratio of the total volume of waste to totalvolume of ore is then the overall strippingratio.

PRICES AND COSTS

With fluctuating commodity prices, increasingmining costs, and the introduction of moresophisticated mining techniques, the overallmining plan and overall stripping ratio canchange over the total life of any mine.

For this reason, it is necessary to update the long-term plan of the mining project at regularintervals.

Having determined the final pit limits and overallstripping ratio, the mining plan can be executedin a number of ways, as illustrated in thefollowing.

DECLINING STRIPPING RATIO METHOD.

This method (Fig. 13.1.15) requires that each bench of ore be mined in sequence, and all the waste on the particular bench is removed to the pit limit.

The advantages of this method are:1. the operating working space available,2. the accessibility of the ore on the subsequent

bench,3. all equipment working on the same level,4. no contamination from waste blasting above

the ore, and5. Equipment requirements are a minimum

towards the end of the mines life.

DECLINING STRIPPING RATIO METHOD

DECLINING STRIPPING RATIO METHOD

The primary disadvantage of this method isthat the overall operating costs are amaximum during the initial years ofoperation when maximum profits arerequired to handle interest and repayment ofcapital.

INCREASING STRIPPING RATIO METHOD

In this method, stripping is performed as needed touncover the ore (Fig. 13.1.1.6).

The working slopes of the waste faces are essentiallymaintained parallel to the overall pit slope angle.

This method allows for maximum profit in the initialyears of operation and greatly reduces theinvestment risk in waste removal for ore to be minedat a future date.

This method is very popular where the miningeconomics or cutoff stripping ratio is likely to changeon very short notice.

INCREASING STRIPPING RATIO METHOD

INCREASING STRIPPING RATIO METHOD

The disadvantage of this method is the

impracticality of operating a large number of

stacked, narrow benches simultaneously to

meet production needs

CONSTANT STRIPPING RATIO METHOD.

This method (Fig. 13.1.1.7) attempts to remove thewaste at a rate approximated by the overall strippingratio.

The working slope of the waste faces starts veryshallow, but increases as mining depth increasesuntil the working slope equals the overall pit slope.

This method, from an advantage and disadvantagepoint of view, is a compromise that removes theextreme conditions of the former two strippingmethods outlined.

Equipment fleet size and labor requirementsthroughout the project life are relatively constant.

CONSTANT STRIPPING RATIO METHOD.

CONSTANT STRIPPING RATIO METHOD.

In actual practice, the best stripping sequence for a largeore body is one in which the rate is low during the initialstages and towards the end of the projects life.

The advantages can be summarized as follows:

1. A good profit can be generated initially to increase cashflow.

2. The labor and equipment fleet can be increased tomaximum capacity over a period of time.

3. The labor and equipment requirements decreasegradually toward the end of the mines life.

4. Distinct mining and stripping areas can be operatedsimultaneously, allowing for flexibility in planning.

Figure A illustrates a cross section through a pit,

showing the relationship of the phases to the deposit.