The best Solution for Raw Material Storage and Blending

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Claudius Peters Technologies S.A.

Linear Homogenisers (Dual Pile)

For a linear homogeniser, a complete pile is the quantity over which the homogenisation is achieved. If three days of production is the capacity of one pile, then three days is also the time span over which the variations will be smoothed out and equalised.

With a dual-pile operation however, a pile-end effect must be taken into account by which the homogeneity of the volumes of material stored in the extremities of a stack may not be as good as it is in the current section of the same, in particular under the adverse effects of segregation. This is mainly the case where piles are built in the chevron pattern but there is a stacking procedure to reduce this inconvenience.

Instead of stopping the stacker right above the cone centre, an additional travel distance is applied, shorter and shorter as the pile is growing, to follow the slope of the end cone. This method reduces the adverse pile end effect but requires more free space between the two stacks, which comes at the expense of storage capacity.

Gallery

Gallery with 10 images

Circular Homogenisers (Continuous)

In circular blenders (continuous operation) there are no pile end effects (Fig. 7). Homogenisation is carried out within the construction ramp itself. It is an early decision as to how much space is to be allocated to the ramp. The larger the ramp, the longer the homogenisation span and the better the homogenisation.

In practice, project specifications hardly ever impose any requirements on the homogenisation span. The only mentioned criterion is the sacrosanct blending ratio, which is commonly related to just the number of material layers. As a result, common practice sets the average ramp volume to one of the following criteria:

• Average slope of the ramp (standard ~20 degree),

• Sector angle filled by the ramp in the top view (standard 100 degree),

• Fraction of the storage capacity (standard ~30 percent).

Meaning of the Homogenisation Span

Apart from the adverse effect of segregation, it is a fact that almost all stockyard systems provide some homogenisation effect, in so far as the reclaiming flow never strictly follows the reverse order of the stacking flow.

During the reclamation process, the current output flow results from a combination of particles that have been deposited at various times during the stacking process. This will automatically provide some homogenisation.

However, when homogenisation performance is sought, the aim is to maximise the volume over which homogenisation is achieved and therefore maximise the age difference of all of the particles within a given output sample. The parameter to characterise this could be called the homogenisation span.

The meaning and importance of homogensation span can be better visualised on a continuous homogeniser.

The action of such is to produce an average number out of dispersed values. For a continuous homogeniser it is more about a moving average. Spent layers are disappearing at the bottom of the cross section while new ones appear at the top. The outflow results from the mix of all layers as cut by the raking system.

The chart in Fig. 8 presents the moving average made out of the stochastic signal at three hour, 12 hour and 24 hour spans respectively, over a period of two and a half days. It clearly appears that each moving average reduces the variations according to its span.

Defining the size of the ramp also defines the time over which the moving average will be assessed. The longer the time, the more stable the moving average. Ideally users should define a typical span through which they expect the variations to smooth out but this is likely to be an unknown. As a minimum, ramp sizes should be compared and equalised between different bidders.

Bulk Material Blending

Blending is a higher level function. It aims at obtaining not only homogeneity of properties over a given mass of compound product, but also to bring such properties as close as possible to a certain targeted level. Blending implies the knowledge of the inlet material flow properties plus the possibility of uniformly altering a mix by altering the proportion of each component of the inlet flow and the order in which they are stacked out.

Blending Performance

To reach blending performance, a good homogenisation must be achieved first. For this, a consistent homogenisation span must be defined. Blending requires something more, i.e. the capability to obtain a proportioned mix by controlled variations of the input flow.

To achieve this, an important feature is that all time periods of the stacking process are equally represented at any time in every bit of the outgoing flow. For example, if one corrective material must be added in a proportion as small as 1 percent of the total stack capacity, then it must be spread over the entire stack in a few layers that will each equally contribute to the outgoing flow. Such a feature defines a true blender compared to a mere homogeniser.

Number of Layers

Once the stockyard has been defined in terms of shape, size, stacking and reclaiming modes, it is time to consider the number of layers.

The number of layers results from the ratio between the stacking rate and the stacking speed compared to the cross section area of the pile. Obviously, a higher number of layers can only improve the quality of homogenisation and increasing the number of layers can be achieved by merely increasing the speed of the stacker.

Many theoretical analyses have been conducted into this matter to find the optimum conditions. The formula established by Pierre Gy and co-workers gives a blending efficiency equal to half the square root of the number of layers. This sets 400 as the number of layers necessary to reach a 1:10 ratio, which in most cases is a reasonable goal.

Care must be taken that increasing the speed of the stacker to achieve more than 400 layers may generate unnecessary costs because a limit is set by the size of the material itself. The blending will never be better than that allowed by the grain size.

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