Dust Control Lecture Notes PDF, F2023

Mining Optimization Laboratory Dust Deposition in the Lungs 7 • The size of dust particles determines the location and extent of dust deposition in the lungs and influences the action of the dust: – Relatively large particles in the respiratory system will usually be carried back to the mouth by...

Mining Optimization Laboratory Dust Deposition in the Lungs 7 • The size of dust particles determines the location and extent of dust deposition in the lungs and influences the action of the dust: – Relatively large particles in the respiratory system will usually be carried back to the mouth by ciliary action. – Ultra-fine particles (<0.2 μm) are likely to pass relatively quickly and harmlessly into solution in the extra-cellular fluids of the lung tissue. – Inhaled particles within the approximate size range 0.2 - 5 μm can reach the lower regions of the lungs where they will probably be retained. • Prolonged exposure to such dusts can cause various diseases, most of them potentially serious and often resulting in permanent damage to the lung tissues. 7 Dust Classification • It is convenient to classify dusts into four categories according to their biological action: – Inert dusts: • Accumulate in the body but do not produce reaction. – Toxic dusts: • Acute or chronic effects on specific organs such as the central nervous system, the peripheral blood-forming system or the kidneys. • Usually metal compounds, such as chromates or lead compounds. – Allergenic dusts: • May cause asthma or eczema, the actual effects varying from person to person. – Fibro-genic dusts: • The most important from an occupational health point of view, as they cause the pulmonary fibrosis characteristic of pneumocomosis. 8 Ahlam Maremi 8 Mining Optimization Laboratory Dust Concentration Limits 9 • Dust concentration limits: – One of the criteria used in monitoring the compliance of companies with the Health and Safety Act is the concentration of airborne dust. – The measured concentration is compared with variously defined threshold limit values (TLVs) which are functions also of the duration of the exposure of personnel to the dust. – Commonly used definitions of threshold limit value are: TLV-TWA Time-weighed average: the concentration for a normal 8-hour workday or 40hour work week to which most workers can be repeatedly exposed, day after day, without adverse effect. TLV-STEL Short-term exposure limit: the maximum concentration to which workers can be exposed for a period up to 15 minutes provided that no more than four excursions to this value occur each day. TLV-C Threshold limit ceiling: the concentration that should not be exceeded, even instantaneously. 9 Dust Suppression – Dust Elimination • Elimination of dust: – The best solution to the problem of dust is to stop making it! – However, it is difficult to eliminate the generation of dust in manufacturing processes and during the handling of bulk materials. – An assessment of the magnitude of a potential dust problem can be made by examining the bulk material being handled, paying special attention to the fines content of that material. – Dustability: is the tendency of particles from within the bulk to become airborne when the bulk is subjected to external forces. • There is a test used to determine the dustability of a bulk particulate material; – If the test results indicate that the generation of dust will present a real problem, serious consideration should be given to methods of modifying or treating the product to reduce its dustability. 10 Ahlam Maremi 10 Mining Optimization Laboratory Dust Suppression - Elimination of Dust 11 – The first step to control dust generation is to examine carefully the various operations which may be a source of dust. • Some examples of these operations are: – Crushing and grinding processes, – Pneumatic conveying at high velocity, – Open stockpiles subjected to winds, and so on. – Try to reduce the amount of dust generation by modifying the process or the method of handling by; • Handling the product wet instead of dry, • Agglomerating the particles (pelletizing), or • Minimizing air flows which might disturb the dry product. 11 Dust Suppression - Dust Dispersion Control 12 • Control of dust dispersion: – If the total elimination of dust is not possible or not feasible, some method of controlling the dust must be used. – This means keeping the dust away from personnel and preventing its escape into the environment. – Attention must be given to dust clouds within enclosures presenting an explosion hazard. – Total enclosure of the processing and handling plant is the most desirable approach but, in addition to the high cost, there are obvious problems over accessibility. – Using some kind of partial enclosure or hood in conjunction with an exhaust system is more satisfactory. • The exhaust system removes the dust particles and keep the enclosure below atmospheric pressure so that any leakage occurring will be inward rather than outward. 12 Ahlam Maremi Mining Optimization Laboratory Dust Suppression - Dust Dispersion Control 13 – Extractor hoods are essentially of two types: • Receptor hood: – It is designed to capture dust that is forced towards it by some external agency. • Captor hoods – It must be capable of collecting dust which would otherwise not enter it. – The size, shape and position of the hood must be chosen to ensure that the dusty air is collected, and the exhaust flow must be sufficient to ensure that at no point over the entry plane of the hood is the velocity less than that of the approaching air. 13 Gravity and Inertial Separators • The gravity chamber is used to separate solid material from a gas stream. • The velocity of the gas/solid stream is reduced, and the residence time increased, so that the particles fall out of suspension under the influence of gravity. • The rate at which the solid particles settle as well as the efficiency of separation depends on the mass of the particles (size and density). 14 Ahlam Maremi 14 Mining Optimization Laboratory Air Cleaners - Cyclones 15 • Different types of cyclones that are used to separate particles from a gas stream, • The most common type is the cone shaped reverse-flow cyclone; • The centrifugal and gravitational forces are developed by imparting a spinning motion to the incoming stream. • The gas then flows downwards, then flows back upwards and is discharged as clean gas. • The solid particles are collected from the outlet at the base of the conical lower part. 15 Cyclone Efficiency • Cyclone collection efficiency, or simply efficiency, – It is the cyclone’s ability to separate particles from the flowing gas stream. • Several factors affect a cyclone separators efficiency: – Particle density: • It the most deciding factors affecting a cyclone’s efficiency. • Dense particles can be separated with a 99% or greater efficiency. • When the particle density decreases, the efficiency decreases; – Particle size: • Larger particles can be more easily separated than smaller particles. • A reduction in particle size will give a corresponding reduction in efficiency. 16 Ahlam Maremi 16 Mining Optimization Laboratory 17 Factors Effecting Cyclone Efficiency – A separator’s geometry, Gas/solid inlet • The shape of the cyclone would be modified by: – Decreasing the cross-sectional area of the gas/solid inlet and the gas outlet; – Reduce the depth to which the gas outlet channel extends into the cyclone cylinder. • A cyclone of smaller diameter (D) is more efficient for smaller particles. Gas outlet – These modifications will reduce the diameter of the internal vortex and increasing its length, and so allowing smaller particles to be collected. 17 Factors Effecting Cyclone Efficiency 18 – Pressure drop: • It is the amount of energy required to move the gas through the separator. • It is one of the important indexes of the performance of cyclone separators. • Pressure drop is a product of the gas flow rate, gas density and cyclone geometry. – Pressure drop = the absolute pressure (inlet) - the absolute pressure (outlet) – The smoothness of the cyclone’s internal surfaces • Material selection is a very important consideration when choosing a separator for a specific application. • Some process systems may contain erosive or corrosive flowing mediums, so it is necessary to add a protection layer to the cyclone’s internal surfaces. 18 Ahlam Maremi Mining Optimization Laboratory Factors Effecting Cyclone Efficiency 19 – Large or small separator? Large cyclone separators Small cyclone separators Lower efficiency rating Higher efficiency rating Low pressure drop High pressure drop High volumetric flow rate Low volumetric flow rate Low gas velocity Very high gas velocity Large cyclone separators are not suitable for removing fine particles from a gas stream This will lead to a high level of erosion if the gas stream contains abrasive particles. 19 Advantages – Disadvantages of Cyclones • Advantages: – – – – – Cheap to purchase. Low maintenance. Suitable for high temperatures. Suitable for liquid mists. Do not require much space. • Disadvantages – Increased operating costs associated with the pressure drop (assuming large pressure drop). – Inefficient when handling small/fine particles. – Not suitable for sticky substances. • These disadvantages can be reduced in severity if the correct separator is selected for the correct application. 20 Ahlam Maremi 20 Mining Optimization Laboratory Air Cleaners - Wet Washers or Scrubbers 21 • Wet scrubbers can remove particulate matter from an industrial exhaust or outlet gas stream by capturing them in liquid droplets. – The droplets are then collected by absorption. • Absorption is very effective when: – Controlling pollutant gases present in substantial concentration. – Also, it is feasible for gases at dilute concentrations when the gas is highly soluble in the absorbent. 21 Air Cleaners - Wet Washers or Scrubbers • Water is the most commonly used absorbent, – Water may frequently contain other chemicals to react with the gas being absorbed and reduce the concentration. – Non-aqueous liquids may be used for gases with low water solubility, such as hydrocarbons or hydrogen sulfide. • To reduce the high-power usage of wet dust collectors: – Electrically augmented scrubbers providing an electrostatic charge to the dust particles or to the water droplets, or both. – Two- or three-stage venturi scrubbers. – Bubble foam scrubbers. 22 Ahlam Maremi 22 Mining Optimization Laboratory Air Cleaners - Filters 23 • Filters depend for their action on the particle size to be collected. – A representative of this class is a device uses screens/fabric filter bags. • Sieving is the mechanism used to remove particles from a stream of gas passing through a porous fabric. – Particles too large to pass through the mesh of the fabric are caught and retained on the surface of the filter and gradually build up on the filter. – The effective mesh size decreases. – The collecting efficiency of the filter will tend to be improved by use, but regular cleaning is essential. 23 Air Cleaners - Electrostatic Precipitators • Electrostatic collection involves passing the dusty gas through a high-voltage field set up between two electrodes, one of which is live and the other earthed. • When the fine solid particles have acquired a sufficient charge, they migrate towards one of the electrodes (mostly to the earthed one) from which they are periodically removed by rapping or, more rarely, by spray washing. 24 Ahlam Maremi 24 Mining Optimization Laboratory Air Cleaners - Electrostatic Precipitators 25 • The collecting efficiency varies exponentially with the area of the collecting electrodes for a fixed gas flow rate. • The capital cost of an electrostatic precipitator is very high and increases as the size increases. • The operating costs is small, and on very large installations, the combined capital and operating cost would usually be less than that of alternative systems. 25 Dust Explosion • A dust explosion is the rapid combustion of fine particles (< 200 μm) suspended in the air within an enclosed location. – It is the result of high concentrations of combustible dust particles. • When dust particles mixed with oxygen, they can be ignited when coming into contact with a spark or other ignition source. 26 Ahlam Maremi 26 Mining Optimization Laboratory Dust Explosion 27 • A wide range of particulate solids may be considered as posing an explosion hazard, including: – – – – Metals such as aluminum and magnesium; Synthetic materials such as plastics, chemicals and pharmaceuticals; Traditional fuels such as coal and wood; Sugar, flour and cocoa. • Three conditions must exist before an explosion can occur: – A suspension of combustible dust of explosive concentration, – An ignition source, – Oxygen in sufficient quantity to support combustion. • If any one of these conditions does not exist it will be impossible for an explosion to occur. • To minimizing the hazard: eliminate dust clouds and sources of ignition. Ensure that standards of 'housekeeping' are adequate is the first step 27 Dust Explosion • The complete elimination of ignition sources is impossible due to unforeseen: – – – – Mechanical failures, Electrical failures, Human failures, When powered machinery is involved. • Some dust clouds can be ignited by temperatures as low as 200°C (hot surfaces for example). • High temperatures and open flames to be encountered in processing operations or during maintenance (gas cutting, welding, grinding, etc.). 28 Ahlam Maremi 28 Mining Optimization Laboratory Dust Explosion 29 • To avoid the disastrous effects of an explosion we might allow the explosion to take its full course but ensuring that it does so safely. • The plant should be divided into small separate volumes, as far as possible, so the explosion is unable to propagate, – These volumes must be either strong enough (containment), or – Well protected to withstand the explosion. 29 Learning Outcomes • By the end of this lecture: – – – – – – – Generation of dust Hazard associated with dust emission Dust as a hazard to health Dust deposition in the lungs Dust classification Dust concertation limits Dust suppression • Elimination of dust • Dust dispersion control – Air cleaners – Dust explosion 30 Ahlam Maremi 30