For new stockyard projects, operators must consider capital costs and the operational efficiency of equipment and must choose between the virtually infinite possibilities offered by the market. Homogenisers, blenders or storage only and circular or linear storage are the main decisions.
Stockyards are used in almost every bulk material handling industry for storage of raw material and act as a buffer or mixing facility for blending of materials with differing properties prior to further processing. For new projects, operators must consider capital costs and the operational efficiency of equipment and must choose between the virtually infinite possibilities offered by the market. Homogenisers, blenders or storage only and circular or linear storage are the main decisions.
Stockyard Size and Type of Operation
The first question to ask when designing a new stockyard project is the size in terms of storage capacity. The size of the storage capacity is not just proportional to the production capacity. It results from the combination of a feed and a reclaim process and is only a synchronising buffer between two different conveying flows. Even though raw materials have generally relatively low cost per tonne, the stored quantities must not be oversized unnecessarily.
Stockyards are commonly classified into two main categories according to their shape: circular (Fig. 1) or longitudinal (Fig. 2). Another classification is according to the general type of operation: continuous or dual pile (batch).
Circular stockyards based on dual-pile operation do exist but the main advantage of the circular shape is the possibility of continuous operation whereas a longitudinal stockyard can only exist in dual-pile mode.
A dual-pile operation consists of two identical stacks for one single flow of a given commodity, with each pile being alternatively stacked while the other is reclaimed (Figg. 2). This way the risk of interference between the stacker and the reclaimer is avoided. When one pile is full, the other one should be empty and the two machines cross each other (under strict conditions) to begin a new cycle.
There are cases where only one pile is required. This occurs if the time for rebuilding a stack is short compared to the time for reclaiming it completely. In such cases the daily bin capacity may be sufficient to cover the entirety of the rebuild time (during which no reclaim is allowed) or a by-pass option shall be provided to allow direct feed of the daily bin.
When working in single pile mode, it is useful to have the possibility of rebuilding a partially reclaimed pile. Otherwise deliveries have to be coordinated with great accuracy.
Optimal Sizing of a Dual-pile Stockyard
Most of the time storage capacity of a stockyard is defined by the longest possible interruption to the stacking operation. The question is: How long can the process go on while the stacking operation is interrupted? The answer to this question defines the size of the stockyard as the size of the buffer necessary for the synchronisation of the incoming and outgoing flows. As a corollary to that, the rate of stacking must be adapted so that the system shall never unprime.
Let's consider a case where the raw material feed is achieved from a nearby quarry or by a truck shuttle loop that operates (preferably) during daytime and possibly only on weekdays. On the other hand, the process is supposed to operate continuously. The storage acts as a buffer to cover the interruption of feed during night time and weekends and a reserve of two or three days should be sufficient.
Is there an advantage to have two days more ahead in stock? Over-storage is beneficial only if there is the corresponding extra feeding capacity that allows the stacker to step over the reclaimer. At the same time a larger pile is being reclaimed, a larger pile must be built.
If the quarry or the crushing is the bottleneck, it can't be balanced by a larger storage capacity. Should the two piles be empty, the reclaimer will be stopped for longer because the new pile takes longer to build. Generally speaking, larger storage capacities come at the expense of flexibility.
Another typical example is the feed from cargo ships. In this case, the size of the maximum cargo defines the maximum size of the stockyard. The rebuilding of the pile must be carried out as quickly as possible to minimise ship detention. Therefore the pile rebuild operation should not take more than a couple of days. To ensure the continuity of the process, additional capacity has to be provided to cover that period.
As a general rule the further away and the less certain the source of raw materials, the larger the stock required. A second rule is that the larger the stock capacity, the higher the stacking rate needs to be. If this is not the case, the flexibility of production will be low.
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