Good blending requires formation of a lyered blended bed stockpile. Once this is available, reclaimer comes in the picture.

Reclaiming by front acting machines:

All front acting machines I.e. Machines for “end on” removal of material from stockpiles are equipped with some form of handling device which is only able to carry away the material from the toe of the pile. The material is dislodged from the pile by the action of raking device which sweeps across the cross-sectional face. Each cycle of the device removes a thin `slice` comprising of all layers of the pile, and in the process of tumbling down the slopping face the material of the various layers is mixed together.

Associated with these raking down systems there are in the main, three different types of front-acting reclaimers moving on rails on both sides of stockpile.

1.Bridge type scraping reclaimer with Harrow attachment.

There is an upper limit to the reclaiming rate,

Along the edge of the pile besides the collecting conveyor a scrape feeding shelf has to be provided, which must not be covered with material during stacking and which thus restricts the utilizable stockpiling width.

2.Bucket wheel reclaimer, bridge mounted type.

The handling rate is virtually unlimited. For high rates, two or more buckets can be mounted on the same bridge.

The rate of the material handling during transverse movement of bucket-wheel is not constant. This is compensated by variable speed of transverse movement. This requires an elaborate control arrangement

3.Drum Reclaimer.

The elaborate and expensive design features make the drum reclaimer uneconomical except for very high rates of handling (above 2000 t/hour)

This type of machine is characterized by its good homogenizing and blending effect, since the entire width of the pile is at all times acted upon. The material discharged from the face of stockpile is picked up by scoops mounted on a revolving cylindrical drum and is deposited onto a belt conveyor installed inside the drum.

Ref: Cement Engineers' Handbook by Otto Labahn – Forth English edition by B. Kohlhass & others. ISBN 3-7625-0975-1 ■

Blending efficiency depends on how you make a stockpile. Planning and working of stacker is responsible for achieving a good blend.

For assessing the homogenizing perfoemance of a blended bed, the following parameters are considered. Four of these are estimated values of the statistical variance:

Sa: overall variance of the input variations.

Sb: overall variance of the output variations

Sq: variance of output variations within the reclaimed slice &

Sx: variance of the varages of the slices

Other Parameters are

N: Number of layers

Dt: the quantity of material per layer

Dq: The quantity of material per reclaimed slice

effectiveness of a blending bed is e = Sa / Sb

The blending effect of a blended bed depends on the method of stacking and on the characteristics of the reclaiming machinery. Once these aspects have been determined and the appropriate thickness or number of layers to build up the bed has been decided, the blending effect of the stctpile as a whole will have been determined.

For estimating the output variation you can use folling equation:

Sx = Sa/sq. rt of N

Theoretically high blending effects can be attained by making the number of layers large enough.

For an iron ore concentrate stockpile we had to make 400 layers to achieve desired blending performance when reclaiming with a bridge mounted bucket wheel reclaimer. 120 layers were sufficient for a crushed limestone stockpile with similar reclaimer. In both cases chevron stacking method was used.

Once a blended stockpile is made with desired blending effect, any front acting reclaimer will perform with acceptable performance. ■