Airborne dust afflicts many plants. Processes such as impaction, disintegration, fracturing, grinding, crushing, etc., generate particulates that then disperse into the air. These particulates come in a wide range of sizes and many remain in the air indefinitely. Such dusts often can harm both people and equipment. So, plants should strive to eliminate or at least minimize dust generation and emission.
Preventing the generation of dust generally is easier, cheaper and more reliable than trying to control and suppress generated dusts. Although total prevention of dust in many plants and during operations such as bulk material handling, conveying, size reduction, etc., is impossible, properly designed facilities and equipment can play an important role in reducing dust generation, emission and dispersion.
However, many facilities must resort to dust control and suppression. So, here, we will look at the two most important methods: dust collection and wet suppression (i.e., using water sprays to reduce the dust generation and capture airborne dusts).
Facilities often have many potential dust generators. For example, belt conveyors can emit dust from different points. The tail end, where material is received, usually is the main point of dust generation because of the many impacts of materials there. However, dust also can come from the conveyor skirting, the return idlers (due to carryback of fine dust on the return belt), and the head end, where material is discharged. With mills or crushers, the size reduction process itself generates significant amount of dust, which can emerge from any uncovered or unsealed parts of the equipment. In addition, material stockpiles often can create dust.
Some other major areas for dust generation are transfer points, chutes and the discharges of hoppers or bunkers. The amount of dust generated in these places depends upon the specifics of how material is handled, unloaded or loaded. Reducing dust generation often is possible. For example, material being discharged onto a belt conveyor should be loaded onto the center of the belt; the material and the belt should travel in the same direction and at the same speed whenever possible.
Where and how the material is discharged are important. Design and configuration of discharge locations should minimize the dust generation. Many facilities use devices such as skirt-boards to keep the material on the conveyor after it’s received. These skirt-boards usually have flat rubber strips or something similar to provide a dust seal between them and the moving belt. These seals require proper design; otherwise short life and operational issues will ensue.
Running operations at full capacity and equipment fully loaded can boost the amount of dusts due to many effects and issues such as spillage, increased impact, etc. This is particularly true for size reduction processes, separation units, screens and material handling systems. For instance, it’s preferable to design a belt conveyor to operate at 75%–80% of its full rated capacity; this reduces spillage, dust emission and wear on different parts such as seals, etc.
Chutes used at transfer points for moving materials from one piece of equipment (such as a conveyor) to another require serious design attention, too. Inadequate designs can result in significant dust generation. The chute should be big enough to avoid jamming of materials. It should be designed so the material falls on the sloping bottom of the chute, not on the subsequent equipment. Wherever possible, materials should fall on a local hard lining rather than on the metal surfaces; this will cut dust and noise generation, lessen wear and abrasion of the chute surfaces, absorb the impact of incoming material and, more importantly, reduce dust emission. When handling fine or abrasive materials, consider welding a number of small steel angles on the chute bottom; the oncoming material slides on the material stored in angles, greatly reducing wear and abrasion of the chute bottom.
In selecting a dust control system, you should consider many factors — e.g., desired air quality, applicable regulations or standards, interactions with the process and facilities, space or budget limitations, required reliability, etc. Conduct a thorough survey of facilities needing dust control and perform a detailed evaluation before deciding whether to opt for a dust control system, dust suppression or a combination of the two. Place emphasis on the process, operating conditions, characteristics of the equipment, associated dust problems, and the harmfulness or toxicity of the dust. Properly identify major dust emission points and conditions that occur at these points during normal operations; sometimes each point needs a specific solution.
After doing whatever is practical to prevent or reduce the dust generation, the next step is to deal with the dust that is generated. In many applications, plants automatically turn to dust collection rather than water spraying or wet suppression. The common view is that dust collection, theoretically at least, can provide reliable and efficient control over a long period; however, the capital and operating costs are very high. Wet dust suppression systems, particularly water spray systems, are somewhat less efficient, and, thus, theoretically less desirable. Such systems, are less expensive to install and operate, though. Water-spray systems are the simplest and most widely used dust suppression system but they require careful selection and planning to be suitable, effective and reliable.