Bulk material handling expert Steve Davis dives deep into stackers, reclaimers, and the yard machinery critical for mining operations around the world.
Selecting machines for stockpiles involves much more than machine design. Stockpile capacity, layout and arrangement on available land, process needs and management of discontinuity and surge caused by upstream and downstream operations are all key to obtaining a stockpile system that meets requirements.
Stockpiles consist of many components operating in single streams, and reasonable assessment of availability is important.
Stackers are the simplest of the rail mounted yard machines, being a conveyor tripper feeding to a boom belt stacker conveyor. Luffing and slewing add to the cost and complexity of the machines, so consider what form of stacking is necessary. Full slewing allows the stacker to stack on two parallel stockpiles, and to stack in various methods to mix ore better on the stockpile. Stacker selection should be managed with reclaimer selection to get the best performance of a stockpile system. As with all rail mounted machines, AS4324.1 applies.
Other stacking methods that are commonly used include the overhead tripper conveyor. An overhead support structure is needed so these are common is storage sheds. As with a fixed boom stacker these can only provide a continuous cone stockpile.
Some stackers have bypass facilities to allow through feed, especially if there are two stackers on the same yard conveyor, or if there is a requirement to bypass the stacker for direct loading to a ship. Stackers are available with capacities well in excess of 10,000tph.
Many types of reclaimer are available. Different reclaimers work better with specific stockpiles and material management methods. Stockpiles can be considered as long term storage, such as a buffer for a coal power plant, simple turn over where no mixing or turn over order matters, batching stockpiles to feed a specific ship or train, or more complex stockpiles where some mixing is required during stacking and reclaiming to even out the variability of the material. Reclaimers are generally operated under automated control with no need to provide an operator cabin.
If mixing of the reclaim stream is required to obtain a relatively homogenous bulk, the selection of machines and stockpile arrangement should be defined as early as possible. Blending, that is the proportional blending of two or more materials, requires as many discrete stockpiles and reclaimers as there are blend materials. Blending is rarely used in the minerals industry, yet the term is frequently used when mixing is required. Both mixing and blending require that the material is stacked correctly to enable reclaim to be controlled to match.
Reclaimers are installed on stockpiles to recover bulk materials. Where the material reclaim rate is relatively constant, such as power plant feed, the reclaimer capacity needs to be only marginally more than the required rate. Bulk materials are often reclaimed from stockpiles to batch transport systems. Ships, trains and trucks are batch systems, and reclaimers must be sized well above average capacities to cover this. Loading to ships will not be continuous due to hatch changes, berthing, and other delays and the reclaimer (and shiploader) will likely have a rated capacity near twice the average loading rate. Trains and trucks will likely require a surge bin immediately prior to load out.
The boom bucket reclaimer is the most flexible machine for stockpile operation and offers the highest continuous reclaim rates. More than 10,000tph is possible with these machines. The average or aggregate reclaim rate is determined by many other factors, and a 10,000tph reclaimer may only achieve less than 5000tph average due to the method of reclaim, the size of the stockpile reclaim rate. Boom bucket reclaimers can reclaim stockpiles with 50 metre or more base width and can recover material from anywhere along the length of a linear stockpile.
There is a difference between long travel, or trench, reclaim and slew reclaim. Most bucketwheel reclaimers are installed for slew reclaim. Specialist advice should be obtained if looking at long travel reclaim.
Boom bucket wheel reclaimers are designed for slew or trench reclaim and, although they can be used for either method, are not as effective in the alternate mode.
Each reclaimer operates in four quadrants around the machine centre pivot. Understand the difference in the reclaim achieved in the different quadrants and under the two possible long travel directions. Get specialist advice. There is little machine cost saving by restricting reclaim to less than four quadrant operation, but changing after award can be expensive. Long travel reclaim can only be completed in two quadrants, and the stockpile arrangement should be different to slew reclaim.
A surge feeder or surge loading table may be required to control feed to the reclaim yard conveyor. Reclaim rate varies with the position of the bucketwheel in the stockpile for both methods of reclaim, and both design and operation strategy for the boom bucketwheel reclaimer must be assessed to match the expected overall reclaim rate. Bucketwheel discharge is not continuous and if not managed will require a larger conveyor. Surge bins may be necessary between the reclaimed bulk stream and the batch delivery system. It is common to have a small bin between reclaim and truck or train load out, but less common with ships as the bins are large to be effective. Surge bins manage the differences between reclaim and load systems and result in more consistent and higher overall rates.
Bucket wheels should be supported on the boom by two bearings and not cantilevered. This arrangement is referred to as a fork boom end. The fork end has less structural safety issues, and the bucketwheel should not be able to fall off if something fails. The bucketwheel drive should have reversible creep speed and locking/lockout facilities for maintenance work. The boom conveyor will need a splice station and belt clamping facilities.
The boom conveyor drive and take up are usually at the tail end of the boom and can be extremely complex and difficult for maintenance if not well designed. The reclaimer centre chute is often a constrained design. It is in a difficult location and receives ore from many directions. Flow models should be completed for all slew and luff angles of use.
Bridge (single or multiple bucket wheel, drum or scraper) and portal (scraper) reclaimers are often cheaper than boom bucket reclaimers, but don’t offer the flexibility or reclaim rate of the boom bucket reclaimer. These machines run on rails on both sides of the stockpile and span the stockpile. Bridge and portal reclaimers generally operate on relatively smaller stockpiles. Bridge reclaimers can be fitted with rakes to promote flow down the face of the stockpiles.
If multiple parallel stockpiles are produced, it is possible to use a transfer car to move bridge type reclaimers to work on several stockpiles, giving a potentially cheaper solution overall. This does require excellent stockpile management practices.
Bridge bucket, bridge drum, and portal scraper reclaimers have specific benefits and negatives in operations. Reclaim rate is lower than a boom bucket reclaimer in most applications. Bridge and portal reclaimers reclaim the full width of the stockpile and are excellent for mixing the material as it is reclaimed. The more usual type of bridge reclaimer cannot be repositioned to other parts of the stockpile from where it is working, however there are some that have a lifting bridge which permits full access to the stockpile. Boom bucket and portal reclaimers can move to any part of a stockpile, as required.
Portal reclaimers can have a single scraper reclaim, or two on the larger machines. The scraper systems are hung from the portal structure on winch cables and pull material to the edge of the stockpile where it falls onto a reclaim conveyor. In dual scraper machines, one scraper pulls material to the second and then to the edge of the stockpile. Portal reclaimers can have special configurations for use in storage sheds where the stockpile has a high wall on the non-reclaim side. This improves storage capacity in a smaller footprint than a symmetrical pile. The largest portal reclaimer I have seen has two parallel double scrapers and operated on a 60m-wide stockpile. This machine also operates as a stacker by reversing the scrapers.
Combined machines are also available. Boom bucketwheel stacker reclaimers are commonly used where stockpiling design and operation allows such. The combined machine is more expensive than a boom bucketwheel reclaimer, but cheaper than separate machines. Boom bucketwheel stacker reclaimers are all slewing machines and can be used to stack and reclaim two parallel stockpiles. Portal scraper machines can also be used to stack with some bulk materials, however there are some compromises with the scraper, and this may not work as well as expected. Both types of stacker reclaimer can have a bypass facility for direct loading.
Because mechanical reclaim with rail-mounted machines is efficient, it is usual for the live capacity of these stockpiles to be 100 per cent of the total stacked capacity.
A process stockpile, for example, where stacking and reclaiming can occur at the same time such as in many operations cannot be run at 100 per cent fill. In most cases the usable live volume is about 50 per cent of the total live volume. The ‘spare’ 50 per cent capacity is to allow continued stacking for a period of time when there is an interruption of the reclaim or reclaim when there is an interruption to stacking. This means that the total volume of the stockpile should be around twice as much as the required operating live volume. Because product can be stacked and reclaimed at the same time it is rarely possible to use a combined machine for the duties.
A batch stockpile, where material is stacked prior to being reclaimed, can be sized to match the required live volume. As the product is only stacked or reclaimed, it is feasible to have a single combined machine. Pushed out stockpile storage, often considered for upsets in operation, is by definition 100 per cent dead storage, as it can only be built and reclaimed through mechanical intervention using a dozer or similar machine. Simulation modelling will assist in selecting correct size of the stockpile and best method of operation.
It is good practice to have a yard location that allows the stacker and reclaimer to be maintained in a discrete bay with access platforms under the complete machine including the boom. If this is not available some aspects of maintenance will need to be completed from the ground, which is inherently more difficult and unsafe. It is common for this bay to become a safe tie down area for use in storm events.