Material separation is an inherent problem in most storage technologies. As demand for higher quality products increases, the problem of stock isolation becomes more acute.
As we all know, telescopic radial stack conveyors are the most efficient solution for stack separation. They can create inventory in layers, each layer is made up of a number of materials. To create inventory in this way, the conveyor must run almost continuously. While the movement of telescopic conveyors must be manually controlled, automation is by far the most efficient method of control.
Automatic retractable conveyors can be programmed to create custom inventory in a variety of sizes, shapes and configurations. This virtually limitless flexibility can improve overall operational efficiency and deliver higher quality products.
Contractors spend millions of dollars each year producing aggregated products for a wide variety of applications. The most popular applications include base materials, asphalt and concrete.
The process of creating products for these applications is complex and expensive. Tighter specifications and tolerances mean that the importance of product quality is becoming more and more important.
Ultimately, the material is removed from the stockpile and transported to a location where it will be incorporated into subgrade, asphalt or concrete.
The equipment required for stripping, blasting, crushing and screening is very expensive. However, advanced equipment can consistently produce aggregate according to specification. Inventory may seem like a trivial part of integrated manufacturing, but if done incorrectly, it can result in a product that is perfectly compliant with specification not meeting specification. This means that using the wrong storage methods can result in losing some of the cost of creating a quality product.
Although placing a product in inventory can compromise its quality, inventory is an important part of the overall production process. It is a method of storage that ensures the availability of the material. The rate of production is often different from the rate of product needed for a given application, and inventory helps make up the difference.
Inventory also gives contractors enough storage space to respond effectively to fluctuating market demand. Because of the benefits that storage provides, it will always be an important part of the overall manufacturing process. Therefore, manufacturers must continually improve their storage technologies to reduce the risks associated with storage.
The main topic of this article is isolation. Segregation is defined as “separation of material according to particle size”. Different applications of aggregates require very specific and uniform material grades. Segregation leads to excessive differences in product varieties.
Separation can occur virtually anywhere in the aggregate manufacturing process after the product has been crushed, screened and blended to the proper gradation.
The first place where segregation can occur is in inventory (see Figure 1). Once the material is placed in inventory, it will eventually be recycled and delivered to the location where it will be used.
The second place where separation can occur is during processing and transport. Once at the site of an asphalt or concrete plant, the aggregate is placed in hoppers and/or storage bins from which the product is taken and used.
Separation also occurs when filling and emptying silos and silos. Segregation can also occur during the application of the final mix to a road or other surface after the aggregate has been mixed into the asphalt or concrete mix.
Homogeneous aggregate is essential for the production of high quality asphalt or concrete. Fluctuations in the gradation of the detachable aggregate make it practically impossible to obtain an acceptable asphalt or concrete.
Smaller particles of a given weight have a larger total surface area than larger particles of the same weight. This creates problems when combining aggregates into asphalt or concrete mixtures. If the percentage of fines in the aggregate is too high, there will be a lack of mortar or bitumen and the mix will be too thick. If the percentage of coarse particles in the aggregate is too high, there will be an excess of mortar or bitumen, and the consistency of the mixture will be excessively thin. Roads built from separated aggregates have poor structural integrity and will eventually have a lower life expectancy than roads built from properly separated products.
Many factors lead to segregation in stocks. Since most inventory is created using conveyor belts, it is important to understand the inherent impact of conveyor belts on material sorting.
As the belt moves material over the conveyor belt, the belt bounces slightly as it rolls over the idler pulley. This is due to the slight slack in the belt between each idler pulley. This movement causes the smaller particles to settle to the bottom of the cross section of the material. Overlapping the coarse grains keeps them at the top.
As soon as the material reaches the discharge wheel of the conveyor belt, it is already partially separated from the larger material at the top and the smaller material at the bottom. When the material begins to move along the curve of the discharge wheel, the upper (outer) particles move at a higher speed than the lower (inner) particles. This difference in speed then causes the larger particles to move away from the conveyor before falling onto the stack, while the smaller particles fall next to the conveyor.
Also, it is more likely that small particles will stick to the conveyor belt and not be discharged until the conveyor belt continues to wind up on the discharge wheel. This results in more fine particles moving back towards the front of the stack.
When material falls onto a stack, larger particles have more forward momentum than smaller particles. This causes coarse material to continue to move down more easily than fine material. Any material, large or small, that runs down the sides of a stack is called a spill.
Spills are one of the main causes of stock separation and should be avoided whenever possible. As the spill begins to roll down the slope of the spoil, the larger particles tend to roll down the entire length of the slope, while the finer material tends to settle on the sides of the spoil. Consequently, as the spill progresses down the sides of the pile, fewer and fewer fine particles remain in the billowing material.
When the material reaches the bottom edge or toe of the pile, it is composed primarily of larger particles. Spills cause significant segregation, which is visible in the stock section. The outer toe of the pile consists of a coarser material, while the inner and upper pile consists of a finer material.
The shape of the particles also contributes to side effects. Particles that are smooth or round are more likely to roll down the slope of the stack than fine particles, which are usually square in shape. Exceeding the limits can also lead to damage to the material. When the particles roll down one side of the pile, they rub against each other. This wear will cause some of the particles to break down to smaller sizes.
Wind is another reason for isolation. After the material leaves the conveyor belt and begins to fall into the stack, the wind affects the trajectory of the movement of particles of different sizes. Wind has a great influence on delicate materials. This is because the ratio of surface area to mass of smaller particles is greater than that of larger particles.
The likelihood of splits in inventory can vary depending on the type of material in the warehouse. The most important factor in relation to segregation is the degree of particle size change in the material. Materials with greater particle size variation will have a higher degree of segregation during storage. A general rule of thumb is that if the ratio of largest particle size to smallest particle size exceeds 2:1, there may be problems with package segregation. On the other hand, if the particle size ratio is less than 2:1, volume segregation is minimal.
For example, subgrade materials containing particles up to 200 mesh may delaminate during storage. However, when storing items such as washed stone, the insulation will be trivial. Since most of the sand is wet, it is often possible to store the sand without separating problems. Moisture causes particles to stick together, preventing separation.
When the product is stored, isolation is sometimes impossible to prevent. The outer edge of the finished pile consists mainly of coarse material, while the interior of the pile contains a higher concentration of fine material. When taking material from the end of such piles, it is necessary to take scoops from different places to mix the material. If you take material only from the front or back of the stack, you will get either all the coarse material or all the fine material.
There are also opportunities for additional insulation when loading trucks. It is important that the method used does not cause an overflow. Load the front of the truck first, then the rear, and finally the middle. This will minimize the effects of overloading inside the truck.
Post-inventory handling approaches are useful, but the goal should be to prevent or minimize quarantines during inventory creation. Helpful ways to prevent isolation include:
When stacked on a truck, it should be neatly stacked in separate stacks to minimize spillage. Material should be stacked together using a loader, raising to full bucket height and dumping, which will mix the material. If a loader must move and break material, do not attempt to build large piles.
Building inventory in layers can minimize segregation. This type of warehouse can be built with a bulldozer. If the material is delivered to the yard, the bulldozer must push the material into the sloping layer. If the stack is built with a conveyor belt, the bulldozer must push the material into a horizontal layer. In any case, care must be taken not to push the material over the edge of the pile. This can lead to overflow, which is one of the main reasons for separation.
Stacking with bulldozers has a number of disadvantages. Two significant risks are product degradation and contamination. Heavy equipment working continuously on the product will compact and crush the material. When using this method, manufacturers must be careful not to over-degrade the product in an attempt to alleviate separation problems. The extra labor and equipment required often makes this method prohibitively expensive, and producers have to resort to separation during processing.
Radial stacking conveyors help minimize the impact of separation. As inventory accumulates, the conveyor moves radially to the left and right. As the conveyor moves radially, the ends of stacks, usually of coarse material, will be covered with fine material. The front and back fingers will still be rough, but the pile will be more mixed than the pile of the cones.
There is a direct relationship between the height and free fall of the material and the degree of segregation that occurs. As the height increases and the trajectory of the falling material expands, there is an increasing separation of fine and coarse material. So variable height conveyors are another way to reduce segregation. At the initial stage, the conveyor should be in the lowest position. The distance to the head pulley must always be as short as possible.
Free-falling from a conveyor belt onto a stack is another reason for separation. Stone stairs minimize segregation by eliminating free-falling material. A stone staircase is a structure that allows material to flow down the steps onto the piles. It is effective but has limited application.
Separation caused by wind can be minimized by using telescopic chutes. Telescopic chutes on the conveyor’s discharge sheaves, extending from the sheave to the stack, protect against wind and limit its impact. If properly designed, it can also limit the free fall of material.
As mentioned earlier, there is already insulation on the conveyor belt before reaching the discharge point. In addition, when the material leaves the conveyor belt, further segregation occurs. A paddle wheel can be installed at the discharge point to remix this material. Rotating wheels have wings or paddles that traverse and mix the path of the material. This will minimize segregation, but material degradation may not be acceptable.
Separation can entail significant costs. Inventory that does not meet specifications may result in penalties or rejection of the entire inventory. If non-conforming material is delivered to the job site, fines can exceed $0.75 per ton. The labor and equipment costs for rehabilitating poor quality piles are often prohibitive. The hourly cost of building a warehouse with a bulldozer and operator is higher than the cost of an automatic telescopic conveyor, and material may decompose or become contaminated to maintain proper sorting. This reduces the value of the product. In addition, when equipment such as a bulldozer is used for non-production tasks, there is an opportunity cost associated with using the equipment when it was capitalized for production tasks.
Another approach can be taken to minimize the impact of isolation when creating inventory in applications where isolation can be a problem. This includes stacking in layers, where each layer is made up of a series of stacks.
In the stack section, each stack is shown as a miniature stack. The split still happens on each individual heap due to the same effects discussed earlier. However, the isolation pattern is more often repeated over the entire cross section of the pile. Such stacks are said to have greater “split resolution” because the discrete gradient pattern repeats more often at smaller intervals.
When processing stacks with a front loader, there is no need to mix materials, as one scoop includes several stacks. When the stack is restored, the individual layers are clearly visible (see Figure 2).
Stacks can be created using various storage methods. One way is to use a bridge and discharge conveyor system, although this option is only suitable for stationary applications. A significant disadvantage of stationary conveyor systems is that their height is usually fixed, which can lead to wind separation as described above.
Another method is to use a telescopic conveyor. Telescopic conveyors provide the most efficient way to form stacks and are often preferred over stationary systems as they can be moved when needed, and many are actually designed to be carried on the road.
Telescopic conveyors consist of conveyors (guard conveyors) installed inside outer conveyors of the same length. The tip conveyor can move linearly along the length of the outer conveyor to change the position of the unloading pulley. The height of the discharge wheel and the radial position of the conveyor are variable.
The triaxial change of the unloading wheel is essential to create layered piles that overcome segregation. Rope winch systems are typically used to extend and retract feed conveyors. The radial movement of the conveyor can be carried out by a chain and sprocket system or by a hydraulically driven planetary drive. The height of the conveyor is usually changed by extending the telescopic undercarriage cylinders. All of these movements must be controlled to automatically create multilayer piles.
Telescopic conveyors have a mechanism for creating multilayer stacks. Minimizing the depth of each layer will help limit separation. This requires the conveyor to keep moving as inventory builds up. The need for constant movement makes it necessary to automate telescopic conveyors. There are several different automation methods, some of which are cheaper but have significant limitations, while others are fully programmable and offer more flexibility in inventory creation.
When the conveyor begins to accumulate material, it moves radially while transporting the material. The conveyor moves until a limit switch mounted on the conveyor shaft is triggered along its radial path. The trigger is placed depending on the length of the arc that the operator wants the conveyor belt to move. At this moment, the conveyor will extend to a predetermined distance and start moving in the other direction. This process continues until the stringer conveyor is extended to its maximum extension and the first layer is completed.
When the second level is built, the tip begins to retract from its maximum extension, moving radially and retracting at the arcuate limit. Build layers until the tilt switch mounted on the support wheel is activated by the pile.
The conveyor will go up the set distance and start the second lift. Each lifter can consist of several layers, depending on the speed of the material. The second lift is similar to the first, and so on until the entire pile is built. A large portion of the resulting heap is de-isolated, but there are overflows at the edges of each heap. This is because conveyor belts cannot automatically adjust the position of limit switches or the objects used to actuate them. The retract limit switch must be adjusted so that the overrun does not bury the conveyor shaft.