Following the publication of the October issue of International Mining, and more specifically the annual in-pit crushing and conveying feature, we took a closer look at one of the core elements that make up these systems, the apron feeder.
In mining, apron feeders play an important role in ensuring smooth operation and increasing uptime. Their applications in mineral processing circuits are very diverse; however, their full capabilities are not well known across the industry, leading to many of the questions raised.
Martin Yester, Global Product Support, Metso Bulk Products, answers some of the more important questions.
In simple terms, an apron feeder (also known as a pan feeder) is a mechanical type of feeder used in material handling operations to transfer (feed) material to other equipment or from storage inventory , box or hopper to extract material (ore/rock) at a controlled rate.
These feeders can be used in a variety of applications in primary, secondary and tertiary (recovery) operations.
Tractor chain apron feeders refer to undercarriage chains, rollers and tail wheels that are also used on bulldozers and excavators.This type of feeder dominates industries where users need a feeder that can extract materials with different properties.Polyurethane seals in the chain prevent abrasive material from entering the internal pins and bushings, reducing wear and extending equipment life compared to dry chains.Tractor chain apron feeders also reduce noise pollution for quieter operation.The links of the chain are heat treated for extended life.
Overall, the benefits include increased reliability, fewer spare parts, less maintenance and better feed control.In return, these benefits increase productivity with minimal bottlenecks in any mineral processing loop.
A common belief about apron feeders is that they must be installed horizontally.Well, contrary to popular belief, they can be mounted on slopes!This brings many additional benefits and features.When installing an apron feeder on a slope, less space is required overall – not only does the slope limit floor space, it also reduces the height of the receiving hopper.Sloping apron feeders are more forgiving when it comes to larger chunks of material and, overall, will increase the volume in the hopper and reduce cycle times for haul trucks.
Keep in mind that there are some factors to be aware of when installing a pan feeder on a slope to optimize the process.A properly designed hopper, angle of inclination, design of the support structure, and a system of passages and stairs around the feeder are all key factors.
A common misconception about operating any device is: “The sooner the better.”As far as apron feeders go, that’s not the case.Optimal speed comes from finding a balance between efficiency and shipping speed.They do run slower than belt feeders, but for good reason.
Usually, the optimal speed of the apron feeder is 0.05-0.40 m/s.If the ore is non-abrasive, the velocity can be increased to above 0.30 m/s due to possible reduced wear.
Higher speeds impair operation: if your speeds are too high, you risk accelerated wear on components.Energy efficiency also decreases due to increased energy demand.
Another issue to keep in mind when running an apron feeder at high speed is the increased likelihood of fines.There may be abrasive effects between the material and the plate.Due to the possible presence of fugitive dust in the air, the creation of fines not only creates more problems, but also creates a more dangerous work environment for employees as a whole.Therefore, finding the optimum speed is even more important for plant productivity and operational safety.
Apron feeders do have limitations when it comes to the size and type of ore.Restrictions will vary, but material should never be dumped pointlessly onto the feeder.You need to consider not only the application where you will use the feeder, but also where that feeder will be placed in the process.
In general, the industry rule for apron feeder sizes to follow is that the width of the pan (inner skirt) should be twice the size of the largest piece of material.Other factors, such as a properly designed open hopper combined with the use of a “rock flip plate”, may affect pan size, but this is only relevant in certain situations.
It is not uncommon to be able to extract 1,500mm of material if a 3,000mm wide feeder is used.Negative 300mm material extracted from crusher ore piles or storage/mixing boxes is typically extracted using an apron feeder to feed the secondary crusher.
When sizing an apron feeder and corresponding drive system (motor), as with many pieces of equipment in the mining industry, experience and knowledge of the entire process is invaluable.Apron feeder sizing requires a basic knowledge of factory data to accurately fill out the criteria required by the supplier’s “Application Data Sheet” (or the supplier receives their information).
Basic criteria that should be considered include feed rate (peak and normal), material properties (such as moisture, gradation and shape), maximum block size of ore/rock, bulk density of ore/rock (maximum and minimum) and feed and Outlet conditions.
However, sometimes variables may be added to the apron feeder sizing process that should be included.A major additional variable that suppliers should inquire about is hopper configuration.Specifically, the hopper cut length opening (L2) is located directly above the apron feeder.Where applicable, this is a key parameter not only for correctly sizing an apron feeder, but also for the drive system.
As mentioned above, the bulk density of ore/rock is one of the basic standard requirements and should include effective hoarding feeder size.Density is the weight of a material in a given volume, usually bulk density is measured in tons per cubic meter (t/m³) or pounds per cubic foot (lbs/ft³).A special note to keep in mind is that bulk density is used for apron feeders, not solids density like in other mineral processing equipment.
So why is bulk density so important?Apron feeders are volumetric feeders, which means that bulk density is used to determine the speed and power required to extract a certain tonnage of material per hour.The minimum bulk density is used to determine the speed, and the maximum bulk density determines the power (torque) required by the feeder.
All in all, it is important to use the correct “bulk” density rather than “solid” density to size your apron feeder.If these calculations are incorrect, the final feed rate of the downstream process can be compromised.
Determining the hopper shear length is a critical component in the correct determination and selection of an apron feeder and drive system (motor).But how is this certain?Hopper shear length is the dimension from the skirted hopper back plate to the shear bar at the outlet end of the hopper.It sounds simple, but it’s key to note that this should not be confused with the size of the top of the hopper that holds the material.
The purpose of finding this hopper shear length measurement is to determine the actual shear plane line of the material and where the material in the skirt separates (shears) from the material (L2) in the hopper.The shear resistance of the material is usually estimated to be between 50-70% of the total force/power.This shear length calculation will result in either underpower (loss of production) or overpower (increase in operating expenses (opex)).
Equipment spacing is essential to any plant.As mentioned earlier, the apron feeder can be mounted on slopes to save space.Choosing the correct length of apron feeder can not only reduce capital expenditure (capex), but also reduce power consumption and operating costs.
But how is the optimal length determined?The optimal length of an apron feeder is that which can meet the required task in the shortest possible length.However, in some cases, for an operation, the choice of feeder may take longer to “transfer” material to downstream equipment and eliminate transfer points (and unnecessary costs).
To determine the shortest and best possible feeder, the apron feeder needs to be positioned flexibly under the hopper (L2).After determining the shear length and bed depth, the overall length can be minimized to prevent so-called “self-flushing” at the discharge end when the feeder is idle.
Choosing the right drive system for your apron feeder will depend on the operation and goals of the feeder.Apron Feeders are designed to operate at variable speeds to extract from storage and feed downstream at a controlled rate for maximum efficiency.Materials may vary due to factors such as season of year, ore body or blasting and mixing patterns.
Two types of drives suitable for variable speed are mechanical drives using gear reducers, variable frequency motors and variable frequency drives (VFDs), or hydraulic motors and power units with variable displacement pumps.Today, variable speed mechanical drives have proven to be the drive system of choice due to technological advancements and capital expenditure advantages.
Hydraulic drive systems do have their place, but are not considered ideal between the two variable drives.
Post time: Jul-14-2022
