Dust Collection Systems in Mineral Processing Plants

Dust collection systems are the most widely used engineering control technique employed by mineral processing plants to control dust and lower workers' respirable dust exposure. A well- integrated dust collection system has multiple benefits, resulting in a dust-free environment that increases productivity and reclaims valuable product.

The most common dust control techniques at mineral processing plants utilize local exhaust ventilation systems (LEVs). These systems capture dust generated by various processes such as crushing, milling, screening, drying, bagging, and loading, and then transport this dust via ductwork to a dust collection filtering device. By capturing the dust at the source, it is prevented from becoming liberated into the processing plant and contaminating the breathing atmosphere of the workers.

LEV systems use a negative pressure exhaust ventilation technique to capture the dust before it escapes from the processing operation. Effective systems typically incorporate a capture device (enclosure, hood, chute, etc.) designed to maximize the collection potential.

As part of a dust collection system, LEVs possess a number of advantages:

• the ability to capture and eliminate very fine particles that are difficult to control using wet suppression techniques;
• the option of reintroducing the material captured back into the production process or discarding the material so that it is not a detriment later in the process; and
• consistent performance in cold weather conditions because of not being greatly impacted by low temperatures, as are wet suppression systems.

In addition, LEVs may be the only dust control option available for some operations whose product is hygroscopic or suffers serious consequences from even small percentages of moisture (e.g., clay or shale operations).

In most cases, dust is generated in obvious ways. Anytime an operation is transporting, refining, or processing a dry material, there is a great likelihood that dust will be generated. It also follows that once the dust is liberated into the plant environment, it produces a dust cloud that may threaten worker health. In addition, high dust levels can impede visibility and thus directly affect the safety of workers.

The five areas that typically produce dust that must be controlled are as follows:

1. The transfer points of conveying systems, where material falls while being transferred to another piece of equipment. Examples include the discharge of one belt conveyor to another belt conveyor, storage bin, or bucket elevator.
2. Specific processes such as crushing, drying, screening, mixing, blending, bag unloading, and truck or railcar loading.
3. Operations involving the displacement of air such as bag filling, palletizing, or pneumatic filling of silos.
4. Outdoor areas where potential dust sources are uncontrolled, such as core and blast hole drilling.
5. Outdoor areas such as haul roads, stockpiles, and miscellaneous unpaved areas where potential dust-generating material is disturbed by various mining-related activities and high-wind events.

While areas 4 and 5 can be significant sources of dust, they are generally not included in plant or mill ventilation systems design because of the vast area encompassed and the unpredictability of conditions. Therefore, dust control by methods alternative to LEVs is required.

Dust control systems involve multiple engineering decisions, including the efficient use of available space, the length of duct runs, the ease of returning collected dust to the process, the necessary electrical requirements, and the selection of optimal filter and control equipment. Further, key decisions must be made about whether a centralized system or multiple systems are best for the circumstances. Critical engineering decisions involve defining the problem, selecting the best equipment for each job, and designing the best dust collection system for the particular needs of an operation.

For more information on dust control systems, contact Process Systems Design by visiting http://processsystemsdesign.com or calling (410) 861-6437.

Equipment Used in Crushed Stone Processing

Major rock types processed by the crushed stone industry include limestone, granite, dolomite, traprock, sandstone, quartz, and quartzite. Minor types include calcareous marl, marble, shell, and slate. Major mineral types processed by the pulverized minerals industry, a subset of the crushed stone processing industry, include calcium carbonate, talc, and barite. Industry classifications vary considerably and, in many cases, do not reflect actual geological definitions.

Rock and crushed stone products generally are loosened by drilling and blasting and then are loaded by power shovel or front-end loader into large haul trucks that transport the material to the processing operations. Techniques used for extraction vary with the nature and location of the deposit. Processing operations may include crushing, screening, size classification, material handling and storage operations. All of these processes can be significant sources of PM and PM-10 emissions if uncontrolled.

Quarried stone normally is delivered to the processing plant by truck and is dumped into a bin. A feeder or screens separate large boulders from finer rocks that do not require primary crushing, thus reducing the load to the primary crusher. Jaw, impactor, or gyratory crushers are usually used for initial reduction. The crusher product, normally 7.5 to 30 centimeters (3 to 12 inches) in diameter, and the grizzly throughs (undersize material) are discharged onto a belt conveyor and usually are conveyed to a surge pile for temporary storage or are sold as coarse aggregates.

The stone from the surge pile is conveyed to a vibrating inclined screen called the scalping screen. This unit separates oversized rock from the smaller stone. The undersized material from the scalping screen is considered to be a product stream and is transported to a storage pile and sold as base material. The stone that is too large to pass through the top deck of the scalping screen is processed in the secondary crusher. Cone crushers are commonly used for secondary crushing (although impact crushers are sometimes used), which typically reduces material to about 2.5 to 10 centimeters (1 to 4 inches). The material (throughs) from the second level of the screen bypasses the secondary crusher because it is sufficiently small for the last crushing step. The output from the secondary crusher and the throughs from the secondary screen are transported by conveyor to the tertiary circuit, which includes a sizing screen and a tertiary crusher.

Tertiary crushing is usually performed using cone crushers or other types of impactor crushers. Oversize material from the top deck of the sizing screen is fed to the tertiary crusher. The tertiary crusher output, which is typically about 0.50 to 2.5 centimeters (3/16th to 1 inch), is returned to the sizing screen. Various product streams with different size gradations are separated in the screening operation. The products are conveyed or trucked directly to finished product bins, to open area stock piles, or to other processing systems such as washing, air separators, and screens and classifiers (for the production of manufactured sand).

Some stone crushing plants produce manufactured sand. This is a small-sized rock product with a maximum size of 0.50 centimeters (3/16th inch). Crushed stone from the tertiary sizing screen is sized in a vibrating inclined screen (fines screen) with relatively small mesh sizes.

Oversized material is processed in a cone crusher or a hammermill (fines crusher) adjusted to produce small diameter material. The output is returned to the fines screen for resizing.

In certain cases, stone washing is required to meet particulate end product specifications or demands.

For more information on equipment designed for processing crushed stone, visit Process Systems Design at http://www.processsystemsdesign.com or call (410) 861-6437.

The Difference Between a Feeder and Conveyor

Conveyors

In a bulk material handling system, feeders and conveyors are very important. Feeders and conveyors not only function much differently, but their design requirements are also very different. If a conveyor is misused for a feeder, increased power requirements as well as stagnant regions within the bin could occur. The decision must be made of which will be used when designing a bulk material handling system.

A feeder controls the rate of material from either a bin or a hopper. When the feeder stops, the material slow should also stop. Feeders are flood-loaded and are capable of rate control. Feeders are able to modulate the discharge rate from the vessel that is flood loading it. They also have a relatively slow speed of operation, but the speeds are not fixed. At times, more than one feeder may be needed for a particular application. Feeders are to operate when they are one hundred percent full. Belt feeders for coal, vibratory pan feeders for applying seasoning to chips, and apron feeders for ore under a stockpile are all examples of feeders.

Mechanical conveyors are used to transport bulk materials, but are not able to modulate the rate of material flow. Conveyors, unlike feeders, are not flood-loaded and are running at a constant speed. The constant speed for a conveyor is much different than a feeder; it runs at relatively higher speeds. Conveyors are to operate when they are partially full. Drag-chain conveyors for hot clinker, screw conveyors for limestone, and belt conveyors are all examples of conveyors.

A general rule in a bulk material handling system is that conveyors should not be used as feeders and vice versa. A feeder should always be used if modulation of flow rate is needed and if mass flow in the bin above is required. A combination of a feeder and conveyor should be used if the horizontal distance between the hopper outlet and discharge point is two to three times the length of the outlet. Selecting and designing a specific feeder for particular jobs is not a huge deal. On the other hand, conveyors have much stricter design rules.