Tech Talk

Most of our designs incorporate moving dry bulk materials from point A to point B. Several methods of transfer are available and some are also useful for elevating materials.

The most versatile and economical way to move large amounts of bulk material from 20 ft to several miles away is a belt conveyor. Rubber belts became practical in the 1920's with the advent of vulcanizing in the rubber industry. Belt Conveyors are simple devises that have proven over the course of time to be more cost effective than trucks and most every other method of moving large masses of dry material. They can lift material as well up to 15 to 25 degrees if the properties of the bulk material allow. With some tricks, belt conveyors have reached inclines up to 35 degrees, but in general this practice is expensive and not normal.

Belt conveyors start having problems when the materials being transferred react with the material of the belt and sticks to or burns into the surface. At higher incline angles some materials roll back and some act like liquids. In these cases we use other transfer conveyor methods or extend the range with chevron or rubber cleats.

Belt conveyors range from 12" wide to 120" wide and can be arranged on rollers to be flat, or troughed. Capacities are in the range of a few tons per hour up to giants that can move 10,000 tons per hour.

Extensions to this technology include the flex trough belt which rolls into a tube, sealing the material from wind and allowing the conveyor to make sweeping turns, sandwich belts which trap the material between two running belt surfaces to lift material and the flexible wall conveyor with rubber side walls and partitions which can transfer horizontally and lift vertically.

The oldest, tried and proven method is a screw conveyor using a technology that dates back to the time of Archimedes. Screws can transfer horizontally and lift up to 45 degrees in some cases, and while typically not much bigger than 24" in diameter, have been made as large as tens of feet in diameter. Some special projects have been successful using vertical screws. Rates are typically up to 200 tph.

Drag chains have been around almost as long, dating back to the formation of the iron and steel industry, and reaching maturity of design at the heyday of the logging industry in the late 1800's and early 1900's. Here, the chain sits over the product and drags it along to the discharge point.

As the drag chain technologies matured, combination chains were developed and became the principle mover for apron conveyors and pan conveyors. In this case, the product to be moved is over top of the chain and is carried along on overlapping steel pans riding on combination chain and oversized rollers. These steel pan and apron style conveyors are used for very heavy materials and low speed applications and relatively short distances, where impact, heat, corrosion and abrasion are the primary properties of the material to be moved. They are also capable of elevating to some extent and make exceptional feeders. Rates can be as high as 5,000 Tph with widths up to 102".

As we get more specialized, vibrating conveyors were developed to move and shake castings, which helped to cool them and remove unwanted mold sand and cores. These short conveyors spread out the loads, and can even orient parts and have been proven to be very versatile in specialty applications where moving is only part of the problem. Material can be moved up to 70 fpm with these conveyors.

Materials that take the form of powders, dusts and fine particles may exhibit properties similar to liquids so specialty transfer methods have been developed over the years to correct this. A common method relies on air injection into pipes and this technology is commonly referred to as pneumatic conveying. Pneumatic conveying is not constrained to horizontal movement only. Dilute phase transfer systems offer the highest volume of movement, but at a cost of high volume low pressure air and relatively increased levels of wear. Dense phase transfer systems offer low volume low pressure transfer of material by moving it in slugs. While the rates are somewhat lower, the consumption of air is dramatically reduced and wear is no longer a big issue. Both of these methods can be used with vacuum technology as well as pressure applications, but vacuum is meant for short distances, lower transfer rates and where leaks from a pressurized system would be detrimental or dangerous.

Moving bulk material is only part of the issue. In order to process bulk materials, it is often necessary to establish continuous and steady flow. There are many storage methods used to act as buffers and once these are set up we use feeders to remove the material in a steady and continuous process. Bulk material often has more than one size particle. Large particles mixed with fines often pass through a reduction stage to get the material closer to a standard size. In order protect the reduction equipment from over-loads a grizzly feeder is used to separate the big particles from the small. This grizzly feeder sends the large particles into the reduction equipment and sends the finer material on to another conveyor.

Simple feeders act as cutoff gates from storage silos, hoppers or bins and meter the flow out of the silos or bins. Vibratory feeders hang from the structure and allow the material to cascade on to another conveyor. Free flowing material however, may not stop moving when the feeder is turned off. In this case, a screw feeder can be used to positively shut off the flow. Belt feeders can be used instead of vibratory feeders when noise and wear is an issue. They are a little more complex and more expensive and a little less maintenance friendly but handle larger discharge amounts. Apron feeders are used when the material is lumpy, hot and/or abrasive and in impact situations.

Raising material to a workable height using a minimum of work space is a job for bucket elevators. Here belts or chains are fitted with special buckets that lift the material to the height desired. Capacities can range from under 1/2 tph to over 6,000 tons per hour and heights of 150 ft. Because of these buckets, there can be product degradation. In these cases we use flexible wall conveyors that can lift at almost any angle. Rates for flexible wall conveyors are similar but the belting and construction costs are higher than bucket elevators of the same capacity. In some cases, pneumatic conveying is used which is the predominant method for handling Portland cement or flour. The rates for pneumatic conveying are relatively slower, in the order of 100 tph per hour.

In the foundry and automotive industries elevating of bulk material in batches is acceptable and can be done with skip hoists and lift dumpers. Skip hoists are economical for lump coal, ashes, stone and hot or abrasive materials. Large buckets ranging from 10 to 150 cubic foot capacity are lifted up a track and dumped into a storage container. Rates of 500 tph and lifts to 200 ft are achievable. Lift dumpers are simpler devices than skip hoists as they use a fixed pivot point to lift a bucket and dump it in the same motion using levers and cylinders. Rates are similar, but heights are limited around to around 20 ft.

Many materials need to be reduced in size in order to be processed properly. This is an energy intensive operation and takes special expertise to select the right equipment as failure to do so will result in poor performance, failure to achieve the right sizing, the right rate of production or a suitable wear life. Crushers are also the main piece of equipment that sets the locations and designs of all other conveying devises.

Typically, jaw crushers are used for primary crushing to get the bigger pieces down in size quickly. The lever action of the jaw crusher puts extreme pressure on the material without imparting rotary motion and thus abrasion. The downfall is that jaw crushers are positive displacement machines that must complete the stroke and if something is too hard to crush, the jaw crusher itself will be damaged. Impactors are a refinement to jaw crushers and use rotary plates to smash pieces off larger materials until a given size is reached. Feed can be either horizontal on HSI units or vertical on VSI units. This gives a more uniform breaking action and a more consistent sizing but at a higher price. These are often used for secondary reductions where quality of sizing is important. Cone crushers are used for secondary and tertiary reductions and give a finer finished product. Here, a rotary cone on an eccentric shaft rolls lumps into an ever tightening space, causing the material to break down in size until it can pass by the cone. This action gives a more uniform breakage and produces fewer fines than impactors.

On soft materials such as grain, roll crushers are popular and can be found in numerous configurations from double row rollers through multiple stages. These max out around 250 tph.

In some cases material does not fracture but breaks apart under plastic deformation. Hammer mills are used in these situations. Hammer mills have teeth or "hammers" mounted on either swing arms or bolted directly to a large rotary drum. As the drum or swing arms rotate the hammers beat on the material until it passes through gaps in the housing.

An improvement to hammer mill design is the Ring crusher. Single rotors or dual rotors, both with eccentric weights in the shape of rings, impact like hammers but also shear material, combining both operations to reduce total horsepower requirements and increase rates.

When material binds together during transport, and is readily friable, lump breakers are an economical choice to reduce the size and quantities of these lumps. Here, shear plates with impact teeth counter rotate against each other shearing lumps into smaller pieces. Another method, most often used in scrap metal handling, is a shredder which uses rotary shearing action to break up strings and odd shaped pieces.

A complex issue in processing is to get the right size pieces to the right spot every time. Commonly, the most economical way to grade material is to sift it through vibrating screens. Screen cloth is inserted into frames or decks and stacked to give multiple layers of sizes. Screens are offered in many materials and sizes and are typically stacked in single, double and triple arrangements.

When dealing with very large pieces, grizzly decks are used which are made from heavy wear resistant bars arranged like fingers and are used to separate the bigger pieces from the main stream.

When dealing with molded objects, the mold sands and cores need to be separated from the product. Vibrating conveyors do a good job of moving the fines, the parts and waste into flat, evenly distributed, single layers and can be used to separate the needed from the leftovers. Perforated pans can allow fines to drop out under the conveyor as well. Rotary screens do the same by rotating the mix instead of vibrating them but add another element of cost to the process. Picking belt conveyors are also available for sorting purposes. Here, objects can be picked by hand or selectively extracted from the running surface of the belt. Objects can also be picked or swept from vibratory conveyor surfaces as well.

Continuous processing is an illusive term. Since processes have to stop, slow down or speed up, and where just-in-time delivery is not applicable, bulk materials are stored in surge bins, hoppers, silos and tanks for draw when needed by the process. These containers must be fed and must have a method of removal.

Silos are generally cylindrical towers designed to hold dry bulk materials for extended periods and to keep the material in the state it is delivered in. Capacities range from 10 tons to a few hundred tons when talking about pre-designed and road shippable silos.

Silos can be fitted with multiple discharge connections. Ward Distributers are available in manual or automated control versions to divert flow through a swivel chute to any one of up to twelve discharge chutes. In order to control the flow of material from steel silos to ensure that the screen analysis of the product remains constant and to prevent the separation of fine and coarse particulate, we provide Ward Anti-segregation Devices when requested.

Surge bins are used to take into account transient flow from the input to create a steady flow output. They range in size and shape from round to square to rectangular and are normally between 1/2 cubic yard and 5 cubic yards in size, although larger and smaller examples can be found. Hoppers are smaller than surge bins and are meant to allow material to flow from one conveyance to another without overflows. At the outlet of any silo, bin or hopper when a feeder is not needed, single leaf gates, duplex gates, diverter gates, slide gates and or combinations including manual cut-off gates to provide control of material flow out of the container.

Chutes also allow material to flow from one conveyance to another without spillage, but they do not provide surge protection. Most failures of conveying systems are directly related to design mistakes in the hoppers, chute and skirting designs. Skirts and skirt-boards are meant to retain product on belt conveyors in the loading zone and as the belt transfers from being flat to being fully troughed.

The final process in any plant is to re-package bulk material into consumer size units. These transfer points are called bagging stations, where the material is put into small packages for ease of handling and sale.

Bulk materials can become dust or very fine in nature. In order to protect our workers and the air in the plant, we need to provide a means to capture the airborne particles. When Silos are filled, the displaced air can contain large amounts of suspended fine material and when this particle laden air is exhausted into the plant or into the environment the contamination is not tolerated. Generally, silos are usually equipped with bin vents which allow the natural flow of air to pass out of the silo through a series of filter cloths, bags or socks to separate the dust. Sometimes there are small fans to assist this operation, and the filter media can be equipped with shakers or pulse jets of air to maintain their efficiency. Captured particulate duct falls back into the silo or bin, and is reclaimed.

Similar to bin vents are bag breaking stations. Here product that has been placed into a bag and it becomes necessary to put that material into the process, a bag breaking station is needed. Knives puncture the bag material and allow the product to drop by gravity into the system. A vacuum fan sucks up dust particulate, passes it through a filter media where the dust particles are captured and reclaimed.

At all transfer points, screens and vibratory equipment there is a potential for dust to be emitted from the product stream. A large vacuum fan is connected by ducting to each pickup point to recover the particulate. Before passing through the fan, a large bag house is put into the path of the dust and filter media collect the dust in the same manners as bin vents. The particulate is collected for conveying out of the bottom of these bag houses.

In special applications, where the particulate is heavier than water, and can cause serious health issues, wet scrubbers have been used so operators do not have to go into the equipment to change filter media. Devansco wet scrubbers are particularly efficient at this.

No conveying system is complete without thought in how you can clean up any spillage, before the material gets contaminated. This spillage then becomes another process and the captured material can be put back into the process to eliminate or minimize wastage. Demarco Max Vac® Industrial vacuums are built with the endurance and ruggedness to offer this type of continuous service.