Concept of dust

Dust that can be easily wetted by water is called hydrophilic dust, while the opposite is called hydrophobic dust. Wet dust removal and purification is not suitable for hydrophobic dust. Certain dusts form water-insoluble hard deposits when they absorb water; such dusts are known as hygroscopic dusts. These hard deposits can cause blockages, leading to the failure of the dust removal system. Hydrophilic dust tends to agglomerate when moistened by water, resulting in an increased gravitational force that facilitates its separation from the air; therefore, wet dust removal methods can be considered for such dust ； Hydrophobic dust should not be removed using wet dust removal methods. 　　 However, some hygroscopic dusts (such as cement and lime) adhere and harden when in contact with water, thereby blocking pipes; such dusts are known as hydraulic dusts. Hydraulic dust should not be removed using wet dust removal methods. 　　 Particle size has a significant impact on the wettability of dust. Dust with a particle size of 5 um or less (especially 1 um), due to a layer of gas film adsorbed on its surface, is difficult to be wetted by water (or other liquids), even if it is hydrophilic dust. Only when there is a high relative velocity between the liquid and the dust particles can the air film be broken through to wet it. Below is some basic information about dust; it may be useful for those who are not familiar with it. Sources and classification of dust: Dust refers to solid particles that can remain suspended in the air for an extended period of time. Dust generated during the production process is called industrial dust. From the perspective of colloid chemistry, dust is an aerosol whose dispersion medium is air, and whose dispersed phase consists of solid particles. The main sources of dust in the production process are: (1) the mechanical processing or crushing of solid materials, such as metal grinding, cutting, drilling, blasting, crushing, milling, and the processing of agricultural and forestry products. (2) Dust particles formed by the condensation in air or oxidation of vapors generated when materials are heated, such as in metal melting, welding, casting, etc. (3) Particles formed by the incomplete combustion of organic substances, such as the smoke and dust generated when wood, oils, coal, etc., burn. (4) Sources of dust also include the operations such as sand turning and cleaning of castings, mixing of powdered materials, screening, packaging, and handling; as well as the resuspension of dust that has settled in the air due to vibration or air currents (resulting in secondary dust generation). Classification of dust: There are generally two methods for classifying dust. One is by the properties of the dust, and the other is by the size of its particles. Classified by the nature of the dust: (1) Inorganic dusts: including mineral dusts (such as sand, coal) and metallic dusts (such as iron, tin, lead and their compounds) ； Artificial inorganic dusts (such as emery, cement, glass fibers). (2) Organic dusts: including plant-based dusts (such as wood, tobacco, flour) and animal-based dusts (such as animal skins, keratin, hair) ； Artificial organic dusts (such as **, organic dyes, plastics, synthetic fibers) ； (3) Mixed dusts, mixtures of the various dusts mentioned above (such as a mixture of metal dust and abrasive dust during metal grinding). In occupational health work, the nature of dust is often used as a basis for preliminarily assessing its mechanism and degree of harm to the human body. Classified by the size of dust particles: (1) Dust: Dust particles have a diameter greater than 10 micrometers; in still air, they settle more rapidly and do not spread. (2) Dust mist: The diameter of dust particles ranges from 10 to 0.1 micrometers; in still air, they fall at a constant speed and do not spread easily. (3) Soot: Dust particles have a diameter of 0.1 to 0.001 micrometers; since their size is similar to that of air molecules, they undergo Brownian motion (random movement) due to collisions with these molecules, and they hardly settle at all or fall very slowly in a tortuous manner. Due to the different sizes of dust particles, they remain in the air for varying lengths of time, which directly affects the amount of time operators are exposed to dust. Different treatment methods are employed depending on the state of dust in the air. The impact of dust concentration, dispersion, solubility, shape, and hardness on occupational hazards. Dust concentration: There are two ways to express dust concentration; one is by the weight of dust per unit volume of air (milligrams per cubic meter) ； Another way is to express it as the number of particles per unit volume of air (particles per cubic centimeter). I think the former is more reasonable, as the latter involves dust particle diameter categories and sizes. Dust concentration directly determines the degree of harm that dust poses to humans; the higher the dust concentration, the greater the harm. Since free silica in dust is the cause of silicosis, the higher the silica content, the greater the harm, the more severe the resulting lesions, and the faster these lesions progress. Therefore, it is of great significance to establish the maximum allowable concentration of dust in the air of the production workshop work area. Dust dispersion: Dust dispersion is a concept that indicates the size of dust particles; the smaller the dust particles that make up the dispersed phase, the higher the dispersion degree ； Conversely, it is lower. It is expressed as a weight percentage based on the grouping of dust particles by diameter; that is, it is the percentage of the weight (in grams) of dust particles with a diameter of d (within a given diameter group) within the total weight (in grams) of the sampled dust, and this value represents the dispersion degree of that group. When dust particles are in equilibrium, the higher the dispersion degree, the slower they settle and the longer they remain suspended in the air. In still air, dust particles smaller than 1 micron take 5 to 7 hours to fall from a height of 1.5 to 2 meters to the ground, thereby increasing the chances of them being inhaled by humans. Dispersion also relates to the retention of dust in the human respiratory tract; larger dust particles are more likely to be retained in the upper respiratory tract, while smaller particles have a greater chance of passing through the upper respiratory tract and reaching the lungs, thereby posing a greater risk. Dust solubility: The relationship between the degree of dust solubility and the level of harm it poses to humans varies depending on the properties of the dust. Dusts that primarily exert chemical effects see an increase in their harmful impact as solubility rises ； In contrast, dust with a mechanical irritant effect sees its harmful effects decrease as solubility increases. All insoluble dusts can cause tracheitis and pulmonary fibrosis (pneumoconiosis). Toxic dust that is lipid-soluble (soluble in fats) and water-soluble (soluble in water) can be rapidly dissolved and absorbed through the moist upper respiratory tract; it can also enter the body via the sweat glands, sebaceous glands, and hair follicles in the skin, thereby causing toxic reactions. Shape and hardness of dust: Dust particles come in various shapes, including blocky, flaky, needle-like, spherical, and fibrous. Dust particles experience different air resistance during sedimentation depending on their shape. When dust comes into contact with the upper respiratory tract, eye mucosa, and skin, the shape and hardness of the dust particles play a certain role. Sharp and hard dust particles often cause significant mechanical damage, while soft, fibrous organic dusts tend to accumulate on the mucosa of the trachea and larger bronchi, covering the respiratory mucosa with a velvety substance and leading to chronic bronchitis and tracheitis. Properties of dust: Charge of dust particles: According to measurements and ultra-microscopic observations, 90–95% of the dust particles floating in the air are charged positively or negatively, while 5–10% of the particles are uncharged. The source of this charge is either generated by friction during crushing and movement, or by the adsorption of charged ions from the air, or by coming into contact with the surfaces of other charged objects. The same type of dust particle can be positively charged, negatively charged, or uncharged. The charge of dust particles has a certain impact on the stability of dust in the air. Like charges repel each other, which increases the time that dust particles remain suspended in the air and thus raises the amount inhaled by humans; opposite charges attract each other, allowing dust particles to coalesce upon collision and settle. We often take advantage of the charging properties of dust for dust removal. Dust explosivity: Explosiveness is a property unique to highly dispersed dusts of coal, sugar, flour, sulfur, flax, lead, zinc, aluminum, and others. The conditions for an explosion are the presence of high temperatures (flames, sparks, discharges) and a sufficient concentration of dust in the air. In the 1980s, an explosion caused by flax dust occurred at the Harbin Flax Textile Factory (the third largest in the world and the largest in Asia). Not only was nearly one-third of the factory’s assets destroyed in that incident, but even today, in the new century, the problems resulting from the large number of injuries suffered as a consequence of that accident remain very difficult to address. A few years ago, I was in charge of handling an explosion caused by aluminum dust. The incident occurred during the demolition of a workshop used for producing aluminum powder after it had been washed; the collapse of the iron plates in the ceiling caused friction with walls covered in years’ worth of aluminum dust, which triggered the explosion. In that sudden flash explosion, the victims’ muscles were almost burned to death. Water absorption of dust: The water absorption of dust is determined by factors such as its composition, size, charge state, temperature, and air pressure. Water absorption increases with increasing pressure, decreases with rising temperature, and reduces as dust particles become smaller. Dust that can be easily wetted by water is called hydrophilic dust, while the opposite is called hydrophobic dust. Wet dust removal and purification is not suitable for hydrophobic dust. Certain dusts form water-insoluble hard deposits when they absorb water; such dusts are known as hygroscopic dusts. These hard deposits can cause blockages, leading to the failure of the dust removal system. Effects of dust on health: Systemic effects: Prolonged inhalation of high concentrations of dust can cause systemic diseases characterized by diffuse, progressive fibrosis in the lungs (pneumoconiosis) ； If toxic dusts such as lead, copper, and zinc-manganese are inhaled, they can dissolve on the walls of the bronchi and be absorbed, then carried by the blood to various parts of the body, causing systemic poisoning. Lead poisoning is chronic, but an acute onset of the poisoning can also occur if the affected person has a fever, or if they take certain medications and consume excessive alcohol ； Excessive inhalation of copper dust can lead to hemolytic anemia ； Zinc produces zinc oxide fumes when burned, and inhaling these fumes can cause a disease similar to malaria known as \"metal fume fever\" ； Prolonged inhalation of manganese and its oxide dusts or fumes has adverse effects on the central nervous system, respiratory system, and digestive system. Local effects: Upon contact or inhalation of dust, it first causes local irritation to the skin, cornea, mucous membranes, etc., leading to a series of pathological changes. When dust affects the respiratory tract, it can initially cause hyperfunction of the nasal mucosa and dilation of capillaries; over time, this leads to hypertrophic rhinitis, and ultimately, due to insufficient nutrition supply to the mucosa, atrophic rhinitis develops. Pharyngitis, laryngitis, tracheitis, and bronchitis can also occur. When it comes into contact with the skin, it can cause acne, folliculitis, and pyoderma. If lead dust penetrates the skin, small red spots appear, a condition known as \"lead dermatitis\". Carcinogenicity: Exposure to dusts such as nickel, chromium, and chromates can cause lung cancer ； Exposure to radioactive mineral dust can easily lead to lung cancer ； Asbestos dust can cause skin cancer. Infective effect: Some organic dusts, such as pieces of torn cloth, animal skins, and grain dust, often contain pathogenic microorganisms like fungi and Actinomycetes. When these dusts enter the lungs, they can cause pulmonary mycosis and other related diseases. The effects of dust on the lungs: Pneumoconiosis, which results from long-term inhalation of occupational dust, is a common and highly harmful occupational disease. Due to the different properties of dust, the pathological changes it causes in lung tissue also vary; lung diseases caused by dust can be divided into three main categories. 1. Pneumoconiosis. The \"List of Occupational Diseases\" issued on April 18, 2004 by the Ministry of Health and the Ministry of Labor and Social Security classifies dust-related lung diseases into 13 types based on their causes, including silicosis, welder’s lung, and caster’s lung. Pneumoconiosis is an incurable disease according to current medical standards; therefore, in the \"Evaluation Criteria for Safety and Quality Standardization in Machinery Manufacturing Enterprises,\" the degree of risk associated with exposure to occupational dusts (one of the four classification levels) and occupational health monitoring are listed as important elements of the evaluation. 2. Pulmonary pneumoconiosis. Some productive dusts, such as those of tin, barium, antimony, etc., can deposit in lung tissue after inhalation, causing general foreign body reactions; they pose little harm to human health or have no significant effects. The lesions can gradually diminish or disappear with treatment or by removing exposure to the dust. 3. Pulmonary lesions caused by organic dusts. Pulmonary inflammation caused by organic dusts, as well as conditions such as pneumoconiosis, are extremely rare in manufacturing enterprises. Due to limited research on the causes of diseases caused by organic dusts and differing opinions regarding their pathogenic mechanisms, these conditions have not yet been classified as occupational diseases. Common dust-related tasks in machinery manufacturing enterprises: during the production preparation phase – boiler operation, gas production, ash removal, and boiler maintenance; crushing and transporting solid fuels; wood processing, as well as grinding operations in mold making that may generate wood dust ； Abrasive manufacturing processes, including the preparation, crushing, fine screening, packaging of materials such as grinding wheels, emery stones, and sandpaper, as well as the operation of applying sand ； Coating operation in the welding material production process ； Ceramic work related to the technology of electrical porcelain (insulators), etc. During the blank forming stage: gas and electric cutting of metal raw materials ； Handling of molding sand during the casting process (including transportation, recycling, grinding, mixing, screening, etc.), molding, melting, pouring, demolding, as well as cleaning of castings, grinding of gates, and purging operations ； Colored die-casting operations, investment casting operations, powder metallurgy pressing operations, etc. During the component processing stage: high-speed precision turning and rough turning of castings, as well as other dry mechanical cutting operations; grinding of metal components and tools with grinding wheels; wheel polishing; wheel cutting; and welding seam polishing. In metal surface treatment: metal sandblasting, mold sandblasting, plated parts sandblasting, quartz grinding, polishing, shot blasting, degreasing and derusting, rough sandblasting operations. Welding and cutting center: tasks such as manual arc welding, gas shielded welding, TIG welding, carbon arc welding, oxyacetylene welding, carbon arc gouging, as well as welding operations during the automobile assembly phase. For the control of dust generation sources and personal protection, dust prevention measures involve optimizing and combining technical approaches such as processes, process equipment, materials, operating conditions and methods, occupational health protection facilities, and personal protective equipment, in order to implement comprehensive control. Eliminating or reducing dust generation sources: Choosing processes that do not produce dust, as well as materials that are harmless or cause minimal harm, is the fundamental way to eliminate or reduce the hazards associated with dust; in other words, dust generation sources are eliminated through the selection of appropriate processes and materials. For example, replace foundry sand with resin sand, and use wet processing methods instead of dry ones (such as water grinding instead of dry grinding, hydraulic cleaning or electro-hydraulic cleaning instead of mechanical cleaning, and the use of water mist arc welding plasers). Limit and suppress dust and its dispersion: employ enclosed pipelines for transportation and enclosed equipment for processing; or, without compromising operational efficiency, use semi-enclosed, shielding, or isolation mechanisms to prevent dust from escaping or to confine it to a specific area and reduce its spread. Reduce the height difference of materials to minimize dust generation ； For hydrophilic, weakly sticky materials and dust, measures such as humidification, spraying, and steam application should be employed as much as possible to reduce dust dispersion during transportation, crushing, screening, mixing, and cleaning. Ventilation for dust removal: Ventilation for dust removal is divided into general mechanical ventilation and local mechanical ventilation, depending on the conditions of the workplace and the surrounding environment. Ventilation involves continuously introducing clean, fresh air into the workplace to dilute the dust concentration in the air, and removing polluted air outside, thereby reducing the level of harmful dust in the working area to the corresponding maximum allowable concentration. During ventilation and exhaust, air streams containing harmful substances should not pass through the breathing zone of the workers. Install additional dust collection and purification equipment: Depending on the properties, concentration, dispersion, and amount of dust generated, appropriate dust removal and purification devices should be used to eliminate and clean the dust from the air, thereby preventing secondary dust generation. Personal protection: Implement targeted personal protection measures based on the ways in which dust can harm the human body and the pathways through which such harm occurs. There are three ways in which dust (or toxins) can cause harm to the human body: first, through inhalation, entering the body via the respiratory tract; second, through the sweat glands, sebaceous glands, and hair follicles on the skin’s surface ； Third is ingestion, entering the body through the digestive tract. Regarding the routes of exposure, personal protective measures include: first, blocking the pathways by which dust enters the respiratory system. Depending on the nature of the dust, different types of dust masks, respirators should be used; (for certain toxic dusts, gas masks are also necessary) ； Second is to prevent dust from coming into contact with the skin. Wear work clothes properly (some require pants and a hooded jacket), helmets (the human head is an area with a high concentration of sweat glands, sebaceous glands, and hair follicles), glasses, etc.; thirdly, eating, smoking, drinking water, etc. are prohibited in areas where dust is present. This post was last edited by metaled on 2009-3-18 14:07]

Well written. Are there any professional books? Could you recommend some?

The book is titled Bag Dust Removal Technology (Advanced Edition). Authors: Zhang Dianyin, Wang Chun, Yu Feilu. Publisher: Metallurgical Industry Press. It analyzes the properties of various types of dusts and the mechanisms of dust removal; it’s a very professional work