Balancing Diagram in the Planning of Dragline Workings

IntroductionDraglines have been abundantly used in coal

mining for decades, either as stripper or stripper and coal extractor.

High initial capital outlay for dragline makes effective and efficient operation imperative to obtain low costs of overburden striping .

Constant supervision, good overburden preparation, and preventive maintenance of dragline and selecting proper bench height of overburden for suitably selected machine should ensure efficient and effective operation of a dragline.

Improvement in dragline productivity can have a dramatic effect on overall mining operation.

What is a balancing diagram? Balancing diagram can be defined as the

graphical representation of the scheme to be adopted for determining the suitable seating position of the dragline in order to get maximum overburden accommodation in de-coaled area with

least re-handling for achieving high rate of coal exposure and ensuring

slope stability. Dragline balancing primarily means establishing

relationship between the system capacity and its workload. High capital outlay for dragline requires efficient operation to obtain low cost of OB removal.

What is the purpose of drawing a balancing diagram?

As the name suggests, the overburden to be removed by the dragline shall be balanced in the available void, of previous cut, such that the re-handling percentage is minimum, the internal dump is safe and stable, and least susceptible to failure.

A dragline-balancing diagram is prepared incorporating suitably selected machine, design and geotechnical parameters of quarry. The process of preparing such a diagram is highly interactive and time consuming. If a dragline is deployed as per the guideline balancing diagram, better safety, higher productivity and greater production can be achieved.

Key points:The balancing diagram shows:• The dragline cut sections – key cut,

first dig and re-handled section.• The dragline bench height, cut width

taken by draglines, thickness of coal seam and gradient and various slope angles.

The balancing diagram is used for calculation of:• Rate of coal exposure.• Workload distribution on each

dragline w.r.t their annual productivity.

• Percentage of re-handling.• Volume of overburden possible to be

accommodated in the de-coaled area.

Method of Drawing Balancing Diagram

• Step 1 – Decide the Width of Cut Depends on no. of shovels to be deployed, o/p parameters of dragline and system of working of dragline to be adopted.• Step 2 – Decide the Thickness of Dragline

Bench

V = Capacity of dragline in solid and re-handlingA = Width of cutH = Height of dragline benchK = Coefficient of re-handlingP= Coal Production targeth = Height of coal seamn = Coefficient of loss of coal

• Step 3 – Assume different Angles for drawing balancing diagram.

Depends upon past experience and records.• Step 4 – Deduce seating position of

dragline. Depends on the technological system of face operation and the boom length of dragline. • Step 5 – Calculate volume of OB to

be accommodated for upper dragline.

Limit of cutting depth and radius of unloading by D/L should be kept in mind• Step 6 – Calculate the volume of OB

to be re-handled.• Step 7 – Calculate the area to be

excavated by lower dragline.• Step 8 – Calculate the maximum

capacity of dump accommodation.

Case Study• The following case study has been carried out in mine 1 of a

major opencast coal mine of Northern Coalfields Ltd, Singrauli, India.

• In the mine 1, the dragline bench was divided into two vertical benches, ie, upper bench (top bench) and lower bench (main bench). The upper bench height was about 14 m and lower bench height was about 28 m.

• A dragline (dragline1) seated on upper bench excavated the overburden and casted the spoil into the de-coaled area to form a level pad to be re-handled subsequently. When the upper bench was advanced sufficiently, it provided the seat to excavate the key cut for coal exposure.

• Thereafter, the lagging dragline (dragline 2) on lower bench excavated the first dig to fully expose the coal seam subsequent to which it marched towards the spoil side to sit on the previously formed level pad, from where it re-handled the spoil to a greater height and distance.

The balancing diagram for draglines operating in vertical tandem was prepared to the scale, by using the given field parameters as registered during the field study and tabulated in the following table:Parameters Details

Dragline make and operational make One 15/90 Russian make and one 24/96 Rapier and Ransome make working in vertical tandem mode.

Bench Height, m Approx. 42

Cut width, m 85

Strip length, m 2000

Highwall slope angle, degree 70

OB bench slope angle, degree 60

Angle of repose for loose material, degree 38

Coal bench slope angle, degree 80

Gradient of coal seam, degree 3

Key cut width at top, m 38

Key cut width at bottom, m 5

Coal rib Left for full height

Fig. Balancing diagram as prepared for the case study

Fig. Sectional view of the draglines working in vertical tandem

Dragline Excavation Cut area Dump area Equivalent annual o/p ,

Mm3

15/90 leading dragline no.1

Top Cut ABCDA(1190 m2)

IJKQPI 1.725

Key Cut EFGHE(520 m2)

PQROP 0.755

24/96 lagging dragline no.2

First Dig GHIJG(1860 m2)

VWNLKRSTUV 2.67

Re-handle IJLNI {+10%}(702 m2)

1.11

Total annual output = (1.725 + 0.755 + 2.67 + 1.10) Mm3 = 6.26 Mm3

Results from the Balancing Diagram

Where, B = bucket capacity of dragline (m3); C = overall cycle time of dragline (s); K = overall utilization factor (AU); S = Swell factor for loose easy digging sandstone (0.719); F = Fill factor for loose easy digging sandstone (0.933); M = Machine travelling and positioning factor (0.8); Nh = No. of hours in a shift (8h); Ns = No. of shifts in a day (3 shifts); and Nd = No. of days in a year (365 days).

Where, CE is in Mt; PFD = annual output of the dragline from the first dig (Mm3); A = Area of first dig (m2); W = Cut width (m); T = Thickness of coal seam (m); D = Specific gravity of coal; and R = Recovery factor.

References

• A Critical Case Study on Draglines Operating in Vertical Tandem in a Coal Mine - Dr P Rai

• Opencast mine planning, deployment of HEMM for optimum utilization, total technical parameters of HEMM including limitations, and haul roads and it’s maintenance. - P.D. Rathi

• Balancing diagram—a basic planning tool for dragline operation. – V.K.Rai