Why do you want to store bulk materials? The capital cost of storage is high. Steve Davis explains what should be considered to get the maximum benefits with minimum problems.
Many bulk materials create problems when stored, whether this is from the bulk properties themselves and the way in which they vary or change, or from effects of ambient conditions, run off and dust management.
We hear regularly of reclaim blockages, excess dust, slumps, fires, collapsed silos and other issues. Many of the problems with storage result from incorrect assumptions in design.
Almost all bulk material storage manage surge between handling stages or processes and are part of an overall system. Examples are bins and silos to manage the disconnect between a continuous process and truck transport, and large open stockpiles for process disconnect with trains and ships.
Trucks, ships and trains are batch systems and should be loaded (or unloaded) at the highest practical rate from a pre-assembled batch for maximum efficiency.
Surge capacity would also consider the likelihood of non-linear transportation such as with shipping, where loading and unloading is interrupted by hatch changes, and where external impacts often alter schedules.
Other surge storage would be considered between two process steps to allow for up and downstream outage, difference in production method, for example, a 12-hour operation feeding a 24-hour operation.
An example is trucking in daylight hours from a 24/7 process, where production must be stored for 12 hours and reclaim ensure trucks can be loaded as they arrive during the second 12 hours. Storage will be larger than if trucking was also 24/7, as few processes can manage batch deliveries without some surge storage to smooth the flow.
An extreme surge storage situation is seen on Baffin Island, where iron ore is produced year-round, but winter ice can block shipping for six to eight months in a year.
Some storages are kept for long term contingency reasons, such as a coal fired power station where an outage through loss of supply cannot be tolerated. These stockpiles can hold several months reserve and may require a continuous management strategy.
Other storages may be used to manage market fluctuations, allowing accumulation when prices are at lows and draw down when prices are high. Iron ore is often stockpiled in this manner by end users.
Sequential storage systems are common for many of our large resources. Iron ore and coal in Australia, for example, will have storage prior to on-site sizing plant, and the type and size will depend on blending and pit configuration.
After the sizing plant there will be a stockpile prior to train loading, another stockpile at the port for train unloading combined with ship loading, and a further stockpile at the customer’s port. Typically, there are two or more products which require duplication of storage, and management strategy to suit.
Iron ore port stockpiles are sized to match several typical daily train loads of some 24,000t, and to match multiple typical ships with capacity of some 200,000t.
Storage at Australian iron ore and coal terminals is in the millions of tonnes. The sequential system is designed to maintain a relatively constant “average” throughput, which is approaching a billion tonnes of each commodity annually.
Storage must be sized to suit the system, and the feed and reclaim methods and capacities must match potential rates from up and downstream. Trucks and trains can be loaded relatively consistently and continuously, but ships loading rate depends on deballast rate, number of holds and loading plan. Assuming all operations in a system proceed without upset will result in the smallest storage provisions, however this is usually unrealistic. ‘
Making unrealistic assumptions to minimise cost through small storages will cause endless stop/start operation. Ignore maintenance, truck queuing, ship queuing, catch up provision, any blending or separation, weather etc. at risk. One feasibility included a value engineered 2000t bin in a large mine operation.
Simple math showed at least 25,000t was required for continuous operation, and a stockpile was substituted. Defining storage capacity without inclusion of the associated systems is risky. For complex scenarios a discrete event simulation provides a realistic interpretation of all variables and determines minimum size of storage. As with all simulations, output depends on input. Be realistic.
The most complex storage scenarios are represented by Australian east coast coal terminals, where several mines rail multiple products to a port where they are assembled into cargoes for different ships. There are ten coal export ports in New South Wales and Queensland. Simulation is used to design and manage these systems.
Consider what kind of storage is required: covered, enclosed or open? Covers are expensive and keep rain off and limit dust generation. Full enclosures are more expensive and offer weather contamination and dust management.
Consider fabric covers/enclosures as these are well proven and economic compared to steel and concrete. Automated fabric covered mechanical stockpiles of over 250,000m3 are in use. Multiple storage units, such as silos and stockpiles for grain, can be used.
Open stockpiles have the lowest cost per tonne stored and provide some of the largest capacities. Inloading method for the bulk may impact storage method, with belt and other conveyors being common.
The size of the storage will impact choice, as should the material handling properties relating to storage and time stored. Are there other material properties that must be considered? Abrasion, corrosion, chemical reactions, that prevent the use of a particular type of store.
Be realistic, a 50kt silo looks good on paper, but can it really function? Consider whether expansion allowances could be beneficial, as some storage is easier to extend.
Are redundant systems viable or valuable for large storage facilities?
We have excellent 3D tools for assessing live and total storage for any shape, yet the capacity is often overstated by using incorrect data or relying on a 2D perspective. Ground stockpiles with machine reclaim are 100 per cent live, and provided the correct repose angle and bulk density is used, capacity is simply calculated.
There are several versions of live bottom silos that also give 100 per cent live capacity for compatible bulks. Any storage with gravity reclaims including many bins and silos, conical and linear stockpile with below ground reclaim, require a detailed understanding of material properties and flow.
Experience shows many designs assume rill (drawdown) angle is the same as repose angle, and some use incorrect angles for both, compounding geometric errors by using incorrect bulk density. Storage that is filled and emptied concurrently, the situation for many surge systems, must be sized assuming a working capacity, not the maximum.
When full, the upstream system must stop, when empty the downstream system must stop.
Neither option allows the storage to fully absorb surge. A recent project where the design used repose and rill angles that were different from defined and agreed material properties showed an 18 per cent reduction in capacity over requirement.
Other errors include reclaim outlet size assumed too small, flow assumed as mass flow when funnel flow or ratholing is more likely, wear material selection incompatibility, and even poor basis for structural design. Be wary of any design with offset loading or discharge. Material properties are rarely constant, and boundary values must be defined for design.
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Using the highest compacted bulk density combined with a low repose angle for filling and reclaim will indicate a higher live volume than is practical, so storage capacity below expectation. This error is compounded when multiple in line reclaim gates are used and a single 2D view is used for calculation. All storages are 3D.
Live capacity of gravity reclaimed storage is commonly overstated. Flow from bins and silos often does not meet expectation, as design ignores flow properties.
This may look good on the Capex sheet, but as storage will be a process pinch point when too small, will not work so well for Opex and profit. Bulk material flow properties are necessary for storage design.
Discuss needs with one of our Australian test organisations. Be aware that the test(s) will be completed on a relatively small amount of the material that is available at the time, and some interpretation is necessary to design for long term.
Test results from a 40kg sample are not a finite statement of the flow properties of millions of tonnes of material over a 50-year period, rather an indication of the potential complexity of handling the material.
For most bulk solids, dust must be managed with storage. Legislation has increased in recent years as the risks from dust exposure are better understood. Open storage will see dust separation during stacking and reclaim and often continuous dust lift off from wind.
There is little difference with covered storage, although containment may be better. With enclosed storage wind effects are removed, however any dust generated may remain suspended, and air displacement can see dust blowing out of any gaps. Covered and enclosed storage structures can become encrusted with large quantities of dust to the point where structure integrity is at risk. Consider dust build up and removal in the design.
We are more concerned with fine dusts which create health impact and remain suspended longer than coarse dust. Dust can be managed, but spraying on water randomly is rarely effective with fine dust. A fully integrated dust management design is required.
This may include water, foam, mist, fog and surfactant, dust collectors or scrubbers and disposal depending on the scope. Dust management systems need maintenance, or like belt cleaning, function will rapidly deteriorate.
Dust and storage can lead to explosions, for example the grain elevator in Texas that recently destroyed itself. If the bulk forms an explosive dust, grain, coal, coke, sulphur, some fertilisers, etc., then the design must incorporate management methods to limit the probability.
This will include electrical rating, preventing dust build up on structures, provision of wash down, dust collection into suitable systems, inclusion of static management practises and specialised equipment, explosion relief and other risk management.
Flammable bulk storage must consider fire mitigation. State and local legislation will often define minimum compliance requirements for fire prevention and mitigation. Understanding the Australian Building Codes and having an independent fire assessment will reduce risk.
Safety is a key considerartion due to risks from fire, explosion, engulfment, collapse and more that must be considered during design.