Smelting metal flue gas and smoke purification equipment

1、 Analysis of Smoke and Dust Pollution in the Metal Smelting Industry

The pyrometallurgical process of non-ferrous metals (including drying, sintering, roasting, smelting, blowing, fuming, volatilization, melting, casting, etc.) generates a large amount of flue gas, which carries smoke and dust containing main metals and various valuable elements. To improve the smelting recovery rate, comprehensively recover valuable elements, and reduce environmental pollution, it is necessary to collect smoke and dust from the flue gas. For smelting flue gas that requires further comprehensive utilization, the dust content after dust collection must also meet the requirements of the next process, such as the acid washing process using contact method, which requires the dust content of the flue gas to be less than 100-200 milligrams per standard meter. The dust collection of non-ferrous metal smelting flue gas generally includes the cooling of flue gas, dust collection, and transportation of smoke and dust.

2、 Key technology for smelting smoke and dust treatment

1. Cooling

The temperature of non-ferrous metal smelting flue gas is generally high, with some reaching over 1000 ℃. Usually, cooling is required first to meet the requirements of dust collection processes or equipment. Smoke cooling is generally divided into indirect cooling and direct cooling. When cooling, in addition to direct cooling for flue gas that requires cooling and humidity control, indirect cooling should be used as much as possible, such as air heat exchangers, vaporization coolers, and waste heat boilers, to utilize the waste heat of the flue gas.

2. Dust removal

Smoke dust collection is divided into coarse dust collection and fine dust collection according to dust particle size and dust collection efficiency; According to the dust collection method, it can be divided into dry dust collection and wet dust collection.

2.1 Dry dust collection

Coarse dust collection equipment, commonly used mechanical dust collectors include: ① Dust settling chamber. For an expanded chamber, smoke enters the chamber, the flow velocity decreases, and larger dust particles settle under the action of gravity. ② Inertial dust collector. It is to add several partitions in the chamber to change the direction of smoke flow. When a certain velocity of smoke changes direction, the dust particles contained in it will collide with the partitions under the action of inertial force and settle down. ③ Cyclone dust collector. It is the use of centrifugal motion generated by high-speed rotating flue gas to separate flue gas and dust. ④ Cyclone dust collector. The smoke is introduced from the lower part at a speed of 25-35 meters per second and rotates upwards. The dust particles are concentrated on the outside of the gas column by centrifugal action, forming a stable rotating gas column. The concentrated dust particles are carried into the lower dust collection hopper by the secondary gas entering along the tangential direction. The first three types of dust collectors can capture dust particles with a particle size greater than 10 microns; The cyclone dust collector can capture dust particles with a particle size of 1-5 microns, but it has a large loss of airflow resistance and a complex structure.

There are two commonly used types of fine dust collection equipment: ① filter bag dust collector. It is to pass the flue gas through the filter bag to retain the dust particles, thereby achieving gas-solid separation. The smoke that flows out from inside the filter bag is called an internal filter type, while the opposite is called an external filter type. Filter bags are divided into round bags and flat bags, and materials that are temperature resistant and corrosion-resistant should be selected. Commonly used materials include tussah silk fabric and wool that can withstand temperatures below 100 ℃, polyester velvet fabric that can withstand temperatures below 130 ℃, and glass fiber cloth (plain cloth, plain cloth treated with silicone oil or graphite, polytetrafluoroethylene) that can withstand temperatures below 250 ℃. After collecting dust, the filter bag is cleaned by shaking or blowing back the dust particles attached to the filter bag. There are three common methods: mechanical vibration cleaning, back blowing cleaning, and pulse back blowing cleaning. The filter bag dust collector is an efficient dust collection device that can capture dust particles with a particle size of 0.1 microns. If the parameters are selected appropriately, the dust collection efficiency can generally reach 98-99%. The filter bag dust collector has a simple structure, strong adaptability, and low equipment investment. However, the labor conditions for inspecting and replacing filter bags are poor, and the operating costs are high. ② Electric dust collector. Using the action of a high-voltage electric field to charge dust particles and move them towards the dust collection electrode, separating smoke and exhaust gas. The cathode wire of a typical electrostatic precipitator is the corona electrode, and the anode is the dust collection electrode. Electric dust collectors come in dry and wet, vertical and horizontal types. A horizontal electrostatic precipitator is composed of several electric fields connected in series, usually 2-3 electric fields. Nonferrous metallurgical factories also have 4-5 electric fields connected in series. Electric dust collector is an efficient dust collection device. If used properly, the dust collection efficiency can reach 98-99%, and it can capture dust particles with a particle size of 0.1 microns. The dust collection efficiency of an electrostatic precipitator is related to its structure, the resistivity and composition of dust particles, temperature, the composition and flow rate of flue gas, etc. The main factor is the electrical resistivity of dust particles, which is closely related to the properties of smoke and dust, as well as the composition, temperature, and humidity of the smoke. The generally suitable resistivity is between 104 and 1010 ohms per centimeter. Since the 1970s, ultra-high voltage electrostatic precipitators have emerged, which can capture dust particles with a resistivity of 10-1014 ohms · cm. Electric dust collectors have complex structures and high costs, but they have low airflow resistance losses, good working conditions, and low production costs.

2.2 Wet dust collection

The characteristic of wet dust collection is to fully contact the smoke and water before entering the water, and then separate the smoke and dust from the water. Wet dust collection mainly has the following methods: ① Water film dust collection: It creates a flowing water film on the inner wall of the dust collector, and the flue gas rotates and flows inside the collector. The dust particles in the flue gas come into contact with the water film under centrifugal action and flow out with the water. ② Foam dust collection: make the flue gas pass through the sieve plate in the foam tower at a certain speed, and form a foam layer with the liquid flowing through the sieve plate to capture the dust particles in the flue gas. ③ Impact dust collection: The smoke passes through the nozzle and rushes towards the water surface at a speed of 40-80 meters per second, and the dust particles enter the water under the action of inertia. ④ Venturi dust collection: The Venturi dust collection device consists of a Venturi tube and a wet cyclone dust collector. The smoke passes through the throat of the Venturi tube at a high speed of 60-120 meters per second, dispersing the water sprayed into the throat into droplets, moistening and condensing the dust particles, and then entering the cyclone dust collector to capture the dust particles. Wet dust collection equipment has a simple structure, low investment, good working conditions, but significant resistance losses, high power consumption, and complex mud and water treatment. Smoke contains corrosive media, and equipment needs to take anti-corrosion measures.

3、 Process technology for treating metal smelting flue gas and dust

3.1 Working mechanism and technical characteristics of wet electric defogger

The wet electric demister equipment is composed of a cathode wire and an anode tube (precipitation electrode). Its working principle is that the flue gas passes through a high-voltage electric field, which charges the smoke and fog droplets in the flue gas, forming charged ions. The charged ions move towards the electrode with opposite charges, discharge after reaching the electrode, and form neutral dust and mist particles, which deposit on the electrode, aggregate and fall, and are removed.

In order to ensure the stable movement of charged ions in the same direction in an electric field, it is necessary to convert alternating current to direct current. Therefore, the electric defogger equipment must be equipped with a set of rectification and transformation power supply devices.

In order to improve the dust removal and defogging efficiency of electric demister equipment, a certain strength of electric field must be formed, which requires that the anode tube of the electric demister equipment must have a voltage and current greater than the corona inception voltage, and the cathode line must have a certain line current intensity.

The particle size and conductivity of dust and fog are also important factors determining the efficiency of dust removal and fog. The main particle size range of conductive fiberglass electric demister equipment for dust removal and fog is between 0.01-100um, and the specific resistance range of smoke and dust is 3 × 106-1010 ohms Centimeters.

In order to ensure the dust and mist removal efficiency of the electric demister equipment, it is necessary to regularly clean the sedimentation electrode of the electric demister equipment with water.

3.2 Technical characteristics of wet electric defogger

(1) Low resistance and low energy consumption; (2) Gas suspended particles and mist can be removed within the range of 0.01-100um, and are widely applicable to smoke and dust;

(3) The dust removal and defogging efficiency is high, generally ranging from 95-99.5%, with a maximum of 99.9%; (4) Effective treatment can be carried out for low temperature, humid saturated gas dust and mist; (4) Easy to operate and capable of implementing self-control.

4、 Main equipment and equipment characteristics and performance

The honeycomb type conductive fiberglass electric defogger produced by our company is a high-efficiency purification and defogging equipment made of high-grade corrosion-resistant meta phenylene resin as the matrix, carbon fiber, and glass fiber as reinforcement materials, and extruded through a special mold using extrusion molding technology. Its characteristics are as follows:

(1) Good conductivity and high defogging efficiency. The efficiency of electric fog can reach over 99.00%; (2) Strong corrosion resistance. Capable of withstanding corrosion from various dilute acid, alkali, and salt media; (3) Good flame retardancy. After testing by a national professional testing organization, the flame retardant level has reached V0 level. Therefore, even if an arc is generated in the electric field, it will not ignite the precipitation electrode tube; (4) Lightweight, high strength, stable external dimensions, and long service life of the equipment; (5) Compact structure. The honeycomb structure makes both the inner and outer surfaces of each polar tube become precipitation polar surfaces. There is no dead zone between the polar tubes. Therefore, for treating smoke of the same scale, it has a small volume and occupies less space; (6) The resistance of the working smoke is low, the airflow distribution is uniform, and the cathode wire remains stable, thus achieving the best effect and high defogging efficiency.