Raw material stacking and reclaiming
Quarry management is an art: most quarries will probably have´good quality lime stone´from which cement can easily be made. Cement producer may also have some material that is not good enough. This might be harder to grind, or be of less suitable chemical composition.
If the ´good stuff´ is all used up first, it may be more difficult to make cement out of what is left. Careful selection on a day-to-day basis is needed to make the optimum use of all the raw materials available.
Raw materials are extracted from the quarry, crushed, stored by stacking and reclaimed in specific pattern to achieve pre-blending. The crushed material is ground with other additives as necessary to provide a final recipe for final blending. The pre-blended raw material/meal is stored in a continuous storage-cum-blending silo before being fed into the kiln. The silo stores several days´ supply of material to provide a buffer against any glitches in the supply of raw material from the quarry.
Technically, a cement producer can have almost complete control over clinker composition by pre-blending raw materials of different compositions to produce the desired result. In practice, however, clinker composition is largely determined by the compositions of the locally available raw materials, mainly lime stone which makes up bulk of the raw meal.
Supplementary materials are used to adjust the composition of the raw meal, but cost and availability are likely to determine the extent to which they are used. Transport costs in particular becomes significant in view of the large quantities of materials (limestone) used in making cement.
Promac Engineering Industries Ltd has supplied many linear stackers, bridge-type reclaimers and side scraper reclaimers.
This machine (Pictu re-1) basically has a travelling tripper and a luffable boom conveyor. The crushed material of suitable size carried from quarry is elevated and dropped onto the boom conveyor through the travelling tripper; in turn the boom conveyor drops the material on the yard. The boom conveyor can be luffed to required height, either by winch rope or by hydraulic cylinder.
Promac has supplied a number of linear stackers for various cement plants in India and abroad, like, JK Cement Fujairah (UAE); Lake Cement Ltd, Tanzania; Sagar Cements (formerly known as BMM Cement); India Cement etc; all working satisfactorily. The capacity of stackers vary from 300 TPH to 800 TPH.
In order to blend the raw material stored, it is necessary to stack the material in a specific pattern. Stacking pattern has to be decided based on the type of reclaimer we intend to use. If we use the bridge-type reclaimer, the pattern of stacking is multilayer stacking, as shown in Figure 1. More the number of layers, better the blending efficiency.
In this type of stacking method, the material is stacked layer over the layer to make a heap of required height and length to have the desired storage capacity. Here, it is ensured that different quality material is stacked in multilayers. When the Harrow of Bridge type reclaimer (pictu re-2) travels across the width of the stock pile, it grabs all the layers and feeds almost uniform mixture of raw material to the buckets of the bridge reclaimer. To achieve better homogenisation of the mixture of raw material, it is essential to know the different qualities of materials the mines is going to send to the stock yard during a particular period and to have as many ´layers´ as possible.
Promac has supplied a number of bridge-type reclaimers till now for various cement plants in India and abroad: JK Cement, Fujairah, UAE; Lake Cement Ltd., Tanzania; Falcon Cement, Bahrain; BMM Cement; India Cement; etc., all working satisfactorily. The capacity of reclaimers vary from 150 TPH to 600 TPH.
Side scraper reclaimer
In the case of side scraper reclaimer (Pict ure-3), stacking to be done in ´mult i heap stacking´ (figure 2) method. This means different quality materials have to be stacked in a heap one beside the other. So, when the side scraper grabs the material, almost constant portion of all different quality materials will be transported, forming a homogenised mixture.
Promac has supplied a number of side scraper reclaimers till now for various cement plants abroad to JK Cement, UAE and Lake Cement, Tanzania, all working satisfactorily. The capacities of reclaimers are 300 TPH to 170 TPH correspondingly.
Stacker-cum-reclaimer/Bucket wheel reclaimer
In the event, if there is no need of homogenisation and to handle considerably big volume of material, the option is to go for bucket wheel stacker-cum-reclaimer. A single machine can stack the material and can reclaim from the stockpile with the help of a bucket wheel provided at the end of the boom conveyor. Unlike linear stacker, in which the boom does not swivel around the centre point, here the boom conveyor along with the bucket swivels around the centre point. With this, one can get pyramidal frustum shape. This increases the stacking capacity for the same boom length and also, swivel enables the bucket to reach all points of the stock pile to reclaim the material.
There are two types of stacker-cum-reclaimers, namely, uni-directional stacker-cum-reclaimer and reversible stacker-cum-reclaimer. In the case of uni-directional stac ker-cum-reclaimer, the material is transported in one direction by the yard conveyor. This means, the material is fed at the tail-end of the yard conveyor and after stacking and reclaiming, it is delivered to the head-end of the yard conveyor. Whereas in reversible stac ker-cum-reclaimer, the yard conveyor is reversible in design. Therefore while stacking, the material is fed to the yard conveyor in one direction and after reclaiming, returns from the yard conveyor in the reverse direction.
Promac has supplied stacker-cum-reclaimers to the power industry and these machines are working satisfa-¡ctorily in two different coal-based thermal power plants for eight years.
The authors are associated with Promac Engineering Industries Ltd, Bengaluru.