Environmental contamination is developing in tandem with the expansion of manufacturing groups. A contaminated environment will have a negative influence on personnel, including humans, plants, and animals, as well as the surrounding ecology and scenery.
(Air pollution, air conditioner running strange water. Can lead to lung injury underground, pleural thickening)
To create a clean living environment, first of all, the natural environment must be mentioned, in which the air environment plays the most important role. The impact from the air impacts right at the workplace affecting the entire factory. According to the research, the temperature, wind direction in general, and climate conditions in particular in factories and in our country have almost no air pollution warning system. In which, companies often do not pay attention to investing in dust extraction systems. Common types of dust such as paper fibers, silk fibers, cement dust, coal dust, metal dust, chemical dust, and flying powders are often of little concern because some of them are not easily reacted right away when working at work. That environment.
Currently, air pollution is a matter of concern when the number of industrial parks and export processing zones is increasing. Therefore, the treatment of dust and emissions in the production process is an important step in environmental protection.
I. THE FUNDAMENTAL FEATURES OF DUST
Dust’s stickiness
Dust particles tend to adhere together, and a high level of adhesion can cause a pipeline to become partially or completely blocked. The finer the dust particles, the more easily they adhere to the surface. It is easier to cause adhesion in the dust with 60% of particles less than 10mm, while dust with a higher proportion of particles bigger than 10mm is porous.
Depending on the adhesion, it is divided into 4 groups as follows:
– Non-adhesive such as: dry cement, quartz, dry soil… (dry particles)
– Weak adhesion: coke grains, manhezite, dry apatite, blast furnace dust, ash containing unburnt substances, crushed stone dust…
– Bonding: peat, moist magnesium, metal dust, dust, pyrite, lead oxide, tin oxide, dry cement, unburnt fly ash, peat ash…
– Strong adhesion: cement dust, dust separated from humid air, gypsum dust, asbestos dust, clinker dust, chemical dust..
The following are also mentioned:
Dust Abrasion: The intensity of metal abrasion at the same airflow velocity and dust concentration is defined as dust abrasion. The hardness, shape, size, and quantity of dust particles all have a role. The abrasiveness of dust must be taken into account while designing equipment.
Dust soaking: Water wetting of dust particles reduces the performance of wet separators, particularly when they are used in recirculation mode. Because the rough surface can be covered by a gas envelope that interferes with wetting, flat particles are more permeable than those with rough surfaces.
According to the wettability properties, solid objects are divided into 3 groups as follows:
– Materials that are hydrophilic: easily absorbent such as calcium, quartz, most silicate particles, oxidizing minerals, halides of alkali metals…
– Hydrophobic materials: difficult to absorb moisture such as graphic, coal, sulfur …
Completely impermeable materials: paraffin, tephanlon, bitumen,…
Dust hygroscopicity: The ability of dust to absorb moisture depends on the chemical composition, size, shape, and surface roughness of the dust particles. The hygroscopicity of dust facilitates their separation in wet-type dust separators.
The conductivity of the dust layer: This index is evaluated according to the resistivity index of the dust layer Rb and depends on the properties of each individual dust particle (surface and internal conductivity, size, shape..), particle layer structure and gas flow parameters. This index greatly affects the working ability of the filter.
Self-heating and forming explosive mixtures with air: Dust at high pressure with a sufficiently large fraction of dust particles can lead to explosions. These combustible dust are produced by combining oxygen in the air. The resulting mixture is self-igniting and explosive due to the very large contact surface of the particles (1m2/g). Explosive strength depends on chemical properties, thermal properties, size and shape of particles, their concentration in the air, humidity and composition of gases, size and temperature of the flame source, and the concentration of the gas. relative of inert dust (non-combustible dust). Types of dust that can catch fire as organic dust (corn dust, bran, fiber, plastic, paint, peas, grains…) and also some inorganic dust such as monazite, aluminum, zinc.
Dust recovery efficiency: The degree of cleaning (efficiency factor) is expressed as the ratio of the amount of dust recovered to the total matter in the air stream entering the device in a unit of time.
II. DUST-CONTROL STRATEGIES
Methods to treat particulate matter pollutant
A. DUST COLLECTION BY DRY METHOD
There are different types of dry-type mechanical devices for dust cleaning thanks to different deposition mechanisms:
Sedimentation: dust settling chambers
Inertia: change the direction of motion of the gas flow
Centrifugal sedimentation: single, double, and group cyclones, vortex, and kinetics…
These are devices with a structure that is not too complicated, easy to manufacture, but their processing efficiency is not very high, so it is only used as a preliminary settling device.
B. DUST COLLECTION BY DUST FILTRATION METHOD:
Filter cloth filter characteristics: filtration efficiency, aerodynamic resistance, and operating cycle time before changing or reverting.
Filter materials such as cloth, tangled yarn, and cartons are used to make the tape mixed with asbestos cellulose fibers consisting of fibers of different diameters.
Filter equipment is divided into 3 types, depending on the function and dust concentration in and out:
Purification device (high efficiency): to collect micro dust with great efficiency of about 99%
Air purification equipment: filter air with dust concentration less than 50mg/m2. The filter material is recoverable.
Industrial filter equipment (fabric, particles, dry yarn): cleaning industrial gas with dust concentration up to 60g/m2 with particle size can be as large as 0.5um (1/2000 mm), the filter material is usually recovered collection back.
III. INTRODUCTION OF SLEEVE DUST FILTER DEVICE:
Technological techniques of the dust collection system by a pulse (developed from Germany)
A. WORKING PRINCIPLE:
The filter body consists of 3 main parts:
Air outlet after filter
The filter body includes a dust filter bag
Part of the dust chamber at the bottom.
The filter bags are held by the filter bag frames. When dust is sucked into the filter, where the gas and dust particles will slow down by the shield, and the gas is evenly dispersed in the filter chamber. When the dust airflow decreases in the filter chamber, the dust particles with a large density will fall into the dust chamber below. When the dust is sucked into the filter chamber, the clean air passes through the dust filter bags, the dust particles stick to the dust filter bag body, and the clean air is brought outdoors through the clean air chamber.
The filter bags are cyclically cleaned by pulses of compressed air blowing directly and the dust filters from the clean air chamber side (back blowing ensures that the sticky dust falls into the chamber). The pneumatic bellows are arranged in rows above the rows of bags. The air pulses are directed down to the filter bags because the filter bag frame mouths are installed vertically along with the bag from above.
The gas pulse timing control is set in a circular cycle. The pressure differential device between the filter chamber and the clean air chamber helps the operator to check the status and cleaning cycle of the system.
B. STRUCTURE OF SLEEVE DUST FILTER SYSTEM:
The device consists of many cylindrical sleeves 125-300mm in diameter and 2.5-3.5m in height that are fastened to the pipe mesh and (maybe) equipped with a dust collector (so-called a hand filter). shirt).
C. FILTER CLOTH MUST SATISFY THE FOLLOWING REQUIREMENTS:
+ Material: normal dust filter cloth, water-resistant dust filter cloth, heat-resistant dust filter cloth, electrostatic or oil-resistant dust filter cloth …
+ Diameter of fabric pipe: d= 120-300mm
+ Length: L = 1m – 2m or more
+ High dust capacity and immediately after recovery ensures high filtration efficiency.
+ Maintains optimal air permeability
+ High mechanical strength in high temperature and corrosive environment
D. GENERAL PRINCIPLE OF THE SYSTEM:
The basic dust filter system has created 5 types of filters in the system such as:
- Dust filters installed at the tops of silos or raw material tanks
- Portable mini dust filter for increased locations
- Filter dust in the storage compartment
- Filter dust at the filter bag
- Reverse dust filter
Principle methods of dust removal for dust filter bags:
- Shake off the dust by shaking or mechanical knocking
- Shake off the dust by mechanical vibration
- Shake off the dust by combining vibrating and back blowing compressed air
- Shake off the dust by blowing compressed air
Each method has different effects. Currently, most of the mechanical dust shaking methods have been replaced by compressed air dusting methods, mostly applied in the cement, coal, food powder industry, etc. high dust particles
