Gas Cleaning System in Metallurgy: Furnace Wet Scrubbers, Dry Gas Cleaning

Author: Michael Klepik, Chief Executive Officer

In terms of particulate composition, dust from the air purification of ferrous metals can be classified into coarse and fine categories. Coarse inclusions form due to the air flow's impact on particles of processed charge materials or metallurgical waste. Coarse dust inclusions typically have a fragmented shape with a particle size ranging from 3-10 to several hundred microns. Fine particulate matter enters the air stream through the volatilization of low-volatility components (such as zinc, cadmium, germanium, indium, and other rare and dispersed elements).

The operation of furnaces in metallurgy is accompanied by high temperatures and emissions of solid particles. For this reason, air purification is carried out using wet-type installations to capture by-products. Scrubbers and Venturi pipes have shown the highest efficiency in the furnace gas purification process.

To reduce the high temperature of the outgoing gases supplied to wet cleaning installations, waste heat boilers are required. Their design includes ring pipes forming an umbrella. Water circulates through them, facilitating heat dissipation.

Filtration of the contaminated high-temperature gas-air mixture can also be performed by industrial dry-type equipment. Cyclones, bag filters, and cartridge units are most widely used in this category.

To achieve optimal results in air cleaning for blast furnaces and similar units, scrubbers and other equipment utilizing wet particle capture methods are employed. The principle of operation of such dust and gas removal systems in metallurgy involves the absorption of secondary particles by a liquid absorbent (usually water) and their agglomeration either amongst themselves or within the liquid medium. Subsequently, the purified air is released into the atmosphere, while the spent liquid is drained from the working chamber.

The main components of a furnace exhaust scrubber include:

• The working chamber (often cylindrical in shape).
• Water supply nozzles.
• Pipes for supplying clean water and discharging dirty water.
• Various types of inserts filling the internal space of the unit (packing plates, grids, etc.).
• Outlet for draining the spent liquid (sludge).

Types of furnace scrubbers include:

• Centrifugal (nozzle-type).
• Packed bed.
• Hollow.
• Venturi furnace gas scrubber.
• Plate (foam or bubbling) scrubbers.

Examples:

Blast furnace scrubber gas cleaning achieves an efficiency of up to 99%. Such equipment is indispensable in industries where production processes involve extremely high temperatures, fire and explosion hazards, and the generation of highly electrified particles.

Wet air cleaning in metallurgy also proves effective when combined with dry methods. The combined approach effectively handles odors, impurities, and coarse dust particles.

Due to the requirement for low final dust content, blast furnace gas undergoes two to three stages of purification. Typically, the scheme includes coarse, semi-fine, and fine stages.

Coarse purification aims to capture the largest fraction of coke dust, with particle sizes exceeding 100 micrometers. Typically, this is carried out in dry devices such as radial or tangential dust collectors (cyclones). These devices utilize the principle of inertial cooling. In a radial collector with a diameter of 16-26 ft, the dirty air enters from above along the apparatus axis and is removed after the coarse dust settles, also from above. Particles precipitate due to the flow's 180° turn and a sharp decrease in velocity from 66 ft/s in the inlet pipe to 2-3 ft/s in the apparatus. The inertial effect combines with gravitational settling. Pollutants from the hopper is removed using a screw conveyor moistened with water. The coarse purification stage reduces the blast furnace gas dust content to 5-9 ppm.

In some gas cleaning schemes for furnaces operating without elevated pressure, cyclones are used in the coarse purification stage. Their efficiency is slightly higher than radial collectors, but they also incur higher pressure losses.
The semi-fine purification stage in the majority of gas cleaning systems is conducted using a wet scrubbing scheme in full nozzle scrubbers and low-pressure Venturi scrubbers, concluding in a throttle group.

At numerous metallurgical plants, the semi-fine and fine cleaning stages culminate in low-pressure Venturi scrubbers in blast furnace systems.

Feel free to reach out to us; we will perform gas cleaning system calculations and engineering. We will select the appropriate design and dimensions, calculate the technical specifications of the aspiration system. Additionally, our engineers will undertake the construction of the gas cleaning complex, leaving only the acceptance of work to you.