Dry Bulk Materials Processing, Transport and Storage
Designed primarily for use by dry bulk solids producers and shippers, dry bulk handling processes include crushing, screening, conveyance, storage, bagging, loadout, and transport. IAC’s aim is to empower bulk handling industry personnel to apply state-of-the-art dust control technology to help reduce or eliminate worker exposure to hazardous dust concentrations.
Throughout the production and processing of bulk materials, the dry solid undergoes a number of crushing, grinding, cleaning, drying, and product sizing operations they are processed into a marketable commodity. These operations are highly mechanized, and both individually and collectively these processes can generate large amounts of dust. If control technologies are inadequate, hazardous levels of respirable dust may be liberated into the work environment, potentially exposing workers. Accordingly, federal regulations are in place to limit the respirable dust exposure of mine, factory and warehouse workers.
Crushing, Milling and Screening
Materials crushing, milling, and screening operations can be major sources of airborne dust due to the inherent nature of size reduction and segregation processes. Control of dust generated by these operations can be achieved with proper analysis of the sources, identification of appropriate control technologies, and consistent application and maintenance of selected controls.
Size reduction processes will always contain at least one crushing circuit, and many times will have multiple crushers, often of different types. Selection of crushers is based primarily on material size-reduction and throughput requirements. Secondary selection considerations include the composition, hardness, and abrasiveness of the feed material(s). The most common primary crusher type is the jaw crusher which operates by compressing the feed material between a fixed and moving plate or jaw. Cone or gyratory crushers may be used as primary or secondary crushers and also operate through compression.
Hammermills, impact breakers, and roll crushers operate primarily by impaction depending upon rotor speed at which the breaking force is transmitted from the crusher to the material. Compression involves a slower energy transfer than impaction, and compressive crushers produce somewhat less dust than impact crushers.
The mechanical action of crushers can generate air movement; i.e., jaw crushers can have a bellows-type effect, and cone or gyratory crushers can act as fans, although neither of these classes of crushers operate at high speeds. In contrast, hammermills operate with higher speed components and can act as centrifugal fans.
Milling, often called grinding, is a process by which granular minerals are reduced in size by compression, abrasion, and impaction. Mills may be classified into two broad types based on how they operate: tumbling mills and stirring mills. Tumbling mills generally operate in a horizontal orientation, and the shell of the mill rotates to impart motion to the contents or charge. Stirring mills may be horizontal or vertical and motion is imparted to the charge by an internal stirring element.
Tumbling mills employ some type of medium to perform the size reduction, often rods or balls, usually manufactured from iron or a steel alloy, or high density ceramic material when metal contamination of the product is a concern. Autogenous mills use the feed stock itself as the medium, and semi-autogenous mills use a combination of the feed stock and balls. Stirring mills reduce the size of the feed stock between fixed and rotating mill components.
The most important consideration for dust control within a grinding/classification/product storage circuit is dust containment, and the second is dust collection. All of the material conveying equipment related to a milling or grinding circuit must be enclosed. In addition, all transfer points must be fully enclosed.
Screening operations can produce high levels of dust because smaller sized material is handled. Airborne dust is generated from the vibrating screen decks that accomplish the size separation. Additionally, material must fall some distance as part of the separation process, and some dust will be suspended by this process.
Dust control for screening systems is similar to that for crushers, although wet systems are generally not used due to blanking of the screen openings by the wet material. Also, screens are not normally subject to large surges of material flow as are crushers. This is due to the fact that screens operate most efficiently within a designed flow range. Overloading screens, whether through excessive feed rate or large surges of material, can cause accelerated wear of components, reducing the efficiency of the operation and potentially increasing dust emissions.
Screens should be totally enclosed, and water suppression systems (when compatible with the process) or dust collection and exhaust systems should be incorporated. Necessary openings in the screen enclosure must be minimized, and inspection and maintenance openings must be provided with tight-fitting closures.
Flexible materials (e.g., rubber or synthetic sheeting) must be used to seal openings between the moving screen components and stationary equipment and structures. Because the seal between moving and stationary components is under dynamic stresses whenever the screen is operating, it should be frequently inspected for signs of failure, on a schedule that incorporates data from its operating history.
Conveying and Transport
There are numerous types of equipment used at processing operations to transfer material from one location to another. The material being transferred can range from raw unprocessed ore to fully processed, finished product. Each material transfer methodology has its own strengths and weaknesses. Proper selection of the correct type of equipment is a function of the specific application, taking into account the material to be transferred, transfer distance, and nature of the transfer (i.e. horizontal, vertical, incline or decline).
Belt conveyors are among the most commonly used piece of equipment at bulk processing operations. A conveyor, and the associated transfer points, can generate significant quantities of respirable dust and be one of the greatest sources of fugitive dust emissions within an operation. Operations must control these emissions by containing, suppressing, or collecting dust mechanically, either before or after it becomes airborne, giving special attention to transfer points.
There are three primary root causes for fugitive dust emissions associated with conveyor belts: spillage, carryback, and airborne dust. Control of all three primary dust sources is necessary to eliminate fugitive dust emissions.
The great challenge involved with conveyors is the number of belts involved and the total distance traveled throughout a mineral processing plant. Some belts are located outside where dust liberation is not as critical as when they are located within a building. Another challenge particular to conveyors is their ability to generate or liberate dust whether they are loaded heavily with material or nearly empty. Controlling dust from conveyors requires a constant vigilance by the maintenance staff to repair and replace worn and broken parts, including conveyor belting. Basic maintenance and inspection are required to ensure that all parts of the system are performing to their capacity.
Material can escape through chutes worn from rust or abrasion, and even small holes created by missing bolts or larger holes created from open access doors can be a pathway for fugitive dust. In some cases, it may become necessary to replace an entire loading chute to ensure proper containment.
Screw conveyors are one of the oldest and simplest methods for moving bulk materials. They consist of a conveyor screw rotating in a stationary trough. Material placed in the trough is moved along its length by rotation of the screw. Screw conveyors can be mounted horizontally, vertically, and in inclined configurations.
A typical bucket elevator consists of a series of buckets mounted on a chain or belt that operates overhead mounted on head and tail pulleys. The buckets are loaded by scooping up material from the boot (bottom) or by feeding material into the buckets. Material is discharged as the bucket passes over the head pulley.
A steel casing usually encloses the entire assembly and effectively contains dust unless there are holes or openings in the casings. Dust emissions typically occur at the boot of the elevator where material is being fed into the elevator or at the head of the elevator where material is being discharged.
Emissions at the boot of the elevator can be controlled by proper design of a transfer chute (similar to belt conveyors) between the feeding equipment and the elevator. Dust can be reduced significantly by keeping the height of material fall to a minimum and by gently loading material into the boot of the elevator.
Controlling dust emission at the discharge end of the bucket elevator can be accomplished by proper venting to a dust collector, as well as through the use of proper enclosures and chutes between the elevator discharge and the receiving equipment.
Pneumatic conveyors are tubes or pipes through which material is moved by pressure or vacuum (suction) systems. Positive pressure systems can be either dilute or dense phase.
Storage Bins, Hoppers and Silos
A storage vessel is considered as consisting of a bin and a hopper. A bin is the upper section of the vessel and has vertical sides. The hopper, which has at least one sloping side, is the section between the bin and the outlet of the vessel. Although a mass-flow bin is usually preferable to a funnel-flow vessel, the additional investment generally required must be justified. Often, this can be done by the reduced operating costs. But when installation space is limited, a compromise must be made, such as providing a special hopper design and sometimes even a feeder.
Stockpiles and open areas are common at many mining, quarrying, timber and grain handling sites. There are generally two types of stockpiles: long-term and short-term. A long-term stockpile undergoes no disturbance to the pile over a long time period (several months or more). Short-term stockpiles could consist of product or material that is being stored temporarily until it is used in a process. Surge piles are common short-term stockpiles.
Flow-Assisting Devices and Feeders
Often there are situations in which mass-flow bins cannot be installed for reasons such as space limitations and capacity requirements and bridging takes place, and unassisted mass flow is not possible. To handle these situations, a number of flow assisters are available, the most desirable of which use a feeder and a short mass-flow hopper to enlarge the flow channel of a funnel-flow bin. The following are usually as effective as the mass-flow types.
These are one of the most important and versatile flow assisters. They are used to enlarge the storage bin opening and to cause flow by breaking up material bridges. Two basic types of vibrating hoppers are common: the gyrating kind, in which vibration is applied perpendicularly to the flow channel, and the whirlpool type, which by providing a combined twist and lift to the material, causes bridging to break.
These specialty screw conveyors can be used to assist in bin unloading and in producing uniform feed. Of importance here is the need for a variable-pitch screw to produce a uniform flow to occur.
Belt, Apron and Vibratory Feeders
These specialized conveyors control discharge from the bin or silo by tapering the outlet in the horizontal and vertical planes. To ensure the flow of non-free-flowing solids along the front bin wall, a sloping striker plate at the front of the hopper is typical. Particles are placed into motion from their static stored state through the motion of the belt or vibratory feeder.
Provisions should be made for field adjustment of the space between the skirt and the feeder to provide uniform flow along the entire length and to prevent escape of particles and dust.
Bulk Loading, Unloading and Bagging
During the bulk loading of product, dust contained within the product can be liberated and emitted into the ambient air as the product falls from the loadout area to the transport container (truck, rail car, barge, or ship). This dust-laden air can expose workers to respirable dust, as well as create nuisance dust problems.
A number of factors impact the severity of dust liberation during bulk loading, including: the type of product and its size distribution, moisture content of the product, volume of the product being loaded and the loading rate, the falling distance, environmental factors such as wind velocity and rain, and physical characteristics of the receiving vessel.
The loading spout is designed to transfer product from a plant/storage container into the vehicle that will be used to transport the product. Typically, these spouts have the capability to extend down to the vehicle being loaded and then be retracted to allow the vehicle to move away from the loading station. Spout travel can be as little as a few feet and can extend up to 100 feet. Telescoping loading spouts are equipped with a series of telescoping cups or pipes that extend and retract. The spout typically has an outer shroud which encases the product transfer section of the spout. This outer shroud shields the product from the elements (rain, wind, etc.) and helps to contain any dust that is liberated.
The loading of product into some type of container is normally called “bagging”. The stacking of these bags of product onto pallets for shipment to customers is typically called “palletizing”. There is a wide spectrum of different types of bags that are used to ship product to customers, ranging from 50-pound to over one-ton bulk bags. Only the smaller type bags (100 pounds or less) are typically palletized because the larger bulk bags are in most cases, individually shipped. Both the bagging and palletizing process can be performed manually or through some type of semi-automated or totally automated process.
Flexible Intermediate Bulk Containers (FIBCs), also called “bulk bags”, “semi-bulk bags”, “mini-bulk bags”, and “big bags”, have become more popular over the recent years for shipment of product material. They are often more cost-effective than the 50-, 80-, or 100-pound bags for both the producer and the end user.
The most effective method to control the dust liberated during filling of FIBCs is by using an expandable neoprene rubber bladder in the fill spout of the bagging unit. This bladder expands against the interior of the FIBC loading spout and completely seals it, eliminating any product and dust escaping from the spout during loading. When using this expansion bladder, the feed spout must also incorporate an exhaust ventilation system to exhaust the excess pressure from the FIBC during loading.
IAC can provide rapid flexible engineering, procurement, and construction of dry bulk processing installations utilizing our own equipment manufacture and a nationwide network of steel fabrication services to minimize shipping costs.
IAC Solutions for Dry Bulk Solids Producers
We can provide a design and installation that optimizes the performance of each component, while maximizing product storage, conveyance, yield, and flexibility. Our designs focus on ease of maintenance, dust control, and high percent uptime.
IAC designs and manufactures our own line of dry particulate collectors for process and nuisance dust control at bulk materials processing plants and mines. With recent changes in OSHA regulations concerning respirable silica, upgrades and additions to plant air pollution control (APC) components will become necessary for many of these industries.
With more than 30 years of industry leading experience, IAC is expert in the processing and bulk material handling industries. Our personnel have experience in nearly every bulk processing environment including aggregates, frac sand, ethanol, grain and feed, food and beverages, plastics and their processing; and have provided engineering, construction and equipment to the cement, coal, foundry, and steel processing industries as well.
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