Stockpile and feed bins are structures in crushing and screening plants where material is stored temporarily and transferred to the next machine in a controlled way through a feeder. The primary bin where trucks tip, the surge bins between crushers and the product loading bins act as a buffer between parts of the plant that work at different speeds.
A well-designed bin absorbs fluctuations in truck traffic, keeps the crusher continuously choke fed and increases plant availability. A poorly designed bin becomes the plant’s bottleneck through bridging, ratholing, segregation and excessive wear.
Bin Types
- Primary (truck dump) bin: Receives coarse run-of-quarry material from trucks or loaders. It has impact-resistant liners and usually an apron feeder or vibrating grizzly feeder underneath.
- Surge bin: Located between the primary and secondary crusher. It is critical for keeping cone crushers continuously choke fed.
- Storage bin: Silo-type structures where fractions are stored until loading.
- Loading bin: Used to load product into trucks quickly and with weighing.
- Mobile/modular bins: Built as a combined unit with the feeder in portable plants.
Flow Types: Mass Flow and Funnel Flow
The most important decision in bin design is how the material will flow inside the bin.
| Mass flow | Funnel flow | |
|---|---|---|
| Flow pattern | All the material moves at once; first in, first out | Material flows only through a channel above the outlet; dead zones form at the sides |
| Hopper slope | Requires steep, smooth walls | Shallower walls are sufficient |
| Height | Taller structure | Lower and more economical |
| Segregation | Low | Marked; coarse and fine can separate |
| Ratholing/bridging risk | Low | High with sticky material |
| Suitable use | Fine, wet, sticky material; product silos | Coarse, free-flowing quarry rock; primary bins |
Points to Consider in Design
- Capacity (volume): Bin volume is determined by the balance between the quantity and frequency of discharge from the supplying equipment (truck/loader) and the hourly capacity of the withdrawing equipment (feeder-crusher). A buffer of at least one or two truckloads is recommended in the primary bin, and a few minutes of the crusher’s requirement in a surge bin.
- Outlet opening: To prevent bridging, the outlet width should be at least 3–5 times the largest particle size. For sticky and fine material, the critical opening is determined by material tests.
- Wall slope and surface: Slopes steeper than the material’s wall friction angle should be used; corner fillets should be added to prevent build-up in the corners.
- Impact zone: In the truck dump zone, material should be prevented from falling directly onto the feeder; a “rock box” or bed of material absorbs the impact.
- Structural calculation: The pressures acting on the bin walls (Janssen theory), dynamic loads during discharge and truck impact must be taken into account.
- Level measurement: Ultrasonic/radar level sensors are required for feeder control and truck traffic management.
Dealing with Bridging, Ratholing and Sticking
- Bridging (arching): Coarse particles lock together above the outlet to form an arch. Solution: increase the outlet opening, change the feeder type.
- Ratholing: In a funnel-flow bin, a vertical channel opens above the outlet and the surrounding material does not flow at all. Solution: a hopper suited to mass flow, flow-modifying inserts.
- Sticking: With wet and clayey material, build-up occurs on the walls. Solution: low-friction liners such as UHMW-PE, air cannons, vibrators — vibrators should be chosen with care, as used wrongly they can compact the material and make the problem worse.
- Segregation: Filling from a single point causes coarse and fine particles to separate; distributor chutes and mass-flow design reduce this problem.
Wear Liners
| Liner type | Advantage | Suitable zone |
|---|---|---|
| Wear plate (Hardox etc., 400–500 HB) | High wear and moderate impact resistance | General bin walls, chutes |
| Manganese steel | Very high impact resistance | Truck dump impact zone |
| Chromium carbide overlay plate | Very long life under sliding wear | Flow of fine and abrasive material |
| Rubber liner | Impact damping, noise reduction | Medium-size rock, chutes and surge bins |
| UHMW-PE (polyethylene) | Low friction, prevents sticking | Wet, fine, sticky material |
Maintenance Checklist
- Weekly: Liner thickness and bolts, impact zone, cleaning of the level sensor, operation of air cannons and vibrators.
- Monthly: Cracks in the supporting structure and welds, outlet gates and their slides, bin edge protection (barriers).
- Yearly: Structural inspection; measuring wall thickness reduced by wear, corrosion checks.
- Safety: Entering a bin is “confined space work”; collapse of hung-up material can be fatal. Before entering a bin, the feed and discharge must be locked out, the material completely emptied and a permit-to-work procedure applied. Trying to break bridged material from below or from inside is strictly forbidden.
Frequently Asked Questions
How is bin capacity calculated?
The geometric volume is multiplied by the bulk density of the material to give capacity in tonnes. However, the usable (live) capacity of a funnel-flow bin is markedly below the geometric volume because of dead zones.
Why is material not flowing in the bin?
The most common reasons are a small outlet opening, increased cohesion due to moisture and clay, insufficient wall slope, or material compacting after standing for a long time.
When should bin liners be replaced?
A liner should be replaced before it wears down to a thickness that can no longer protect the supporting plate. Impact zone liners usually need replacing long before other areas; this zone should be monitored separately.
Capacity Calculation: An Example
We can roughly calculate the usable volume of a primary bin as follows:
- Truck capacity: 30 tonnes; average dump interval: 4 minutes (15 trucks per hour ≈ 450 t/h of incoming material).
- Crusher capacity: 400 t/h (about 6.7 tonnes per minute).
- To absorb fluctuations in truck traffic, the bin is intended to have a live capacity of at least 2 truckloads (60 tonnes).
- With a bulk density of 1.6 t/m³, the live volume should be ≈ 60 / 1.6 ≈ 37.5 m³.
In a funnel-flow bin, the geometric volume must be markedly greater than the live volume because of dead zones. The top opening and edge heights should also be checked so that a truckload discharged at once does not overflow the bin edges.
Loads Acting on Bin Walls
The pressure acting on bin walls differs from hydrostatic pressure in liquids. Because of friction between the material and the wall, pressure does not increase much beyond a certain depth (the Janssen effect). During discharge, however, especially in mass-flow bins, sudden pressure peaks (switch pressure) occur at the hopper transition. In addition:
- Impact loads at the moment of truck dumping,
- Eccentric loads from asymmetric discharge,
- Wind and earthquake loads,
- The possibility of a truck hitting the bin edge
must be taken into account in the structural design. Bin design and structural calculations must comply with the relevant standards and regulations.
Flow Aids: Correct Use
| Method | How it works | Caution |
|---|---|---|
| Air cannon | Breaks up built-up material with a sudden blast of compressed air | Position and timing must be chosen correctly; safety interlocks are needed |
| Bin vibrator | Makes material flow by vibrating the wall | Running it continuously while the bin is full can compact the material |
| Low-friction liner (UHMW-PE) | Reduces wall friction | Should not be used in the impact zone |
| Flow-modifying inserts | Prevent ratholing by changing the flow profile | The design requires calculation |
| Heating | Prevents freezing | In cold climates, with fine and wet material |
Bin Level Measurement and Automation
Radar or ultrasonic level sensors continuously measure how full the bin is. This information is used:
- In surge bins, for feed control that keeps the secondary crusher full,
- In the primary bin, for directing truck traffic (not sending trucks to a full bin),
- In storage silos, for loading planning.
In dusty environments, radar sensors usually give more reliable readings.
An Example from the Field: Ratholing in a Surge Bin
At one plant, the surge bin feeding a cone crusher had a capacity of 150 tonnes on paper, yet the crusher was often starved as if only a few minutes of material remained in the bin. The investigation showed that fine, wet material was ratholing in the funnel-flow bin and most of the material remained stationary against the walls. A low-friction liner was added to the bin hopper, the outlet was lengthened and the feeder’s draw-down profile was corrected. Live capacity increased markedly and the cone crusher began running continuously choke fed.
More Questions
How is segregation in a bin prevented?
Filling from the centre and from a low height, distributor chutes and mass-flow design reduce segregation. Keeping the bin always partly full without emptying it completely also helps.
Steel bin or reinforced concrete bin?
Steel bins offer quick installation, modularity and portability; reinforced concrete bins stand out for long life and impact resistance in large-capacity permanent plants. The choice depends on the plant’s life and capacity.
Why do bin gates jam?
Material built up on the slides, wear and corrosion, and hydraulic cylinder problems are the most common causes. Gate slides should be cleaned and lubricated regularly.
Bin Solutions from CSP Mühendislik
CSP Mühendislik designs and manufactures stockpile and feed bins suited to your plant’s flowsheet and material, and supplies bin spare parts such as wear liners, gates and feeder connections. Contact us to solve flow problems in your existing bin.
