The main harmful components of waste gas in rare earth production are fluorine and chlorine, followed by sulfur dioxide, etc. Absorption method is commonly used for the separation and purification of these harmful components. The principle is to use water or an aqueous solution containing sodium hydroxide as an absorbent. In the purification equipment, the absorbent comes into reverse contact with the gas and absorbs fluorine, chlorine, sulfur dioxide, etc., thereby purifying the waste gas. On the premise of meeting emission standards, using as few processes as possible to remove more harmful components not only saves land and equipment investment, but also reduces the cost of exhaust gas purification.

The processing scale should be determined based on the concentration (or amount) of harmful gases emitted from each production process and the efficiency of purification equipment. Both single-stage equipment purification and multi-stage equipment combined purification devices can be used. However, regardless of the type of purification method used, purification equipment must be selected based on the characteristics of rare earth production waste gas, otherwise the expected effect cannot be achieved.

1、 Process Design

In the exhaust gas of rare earth production, fluorine mostly exists in the form of HF and SiF4. When treating fluorocarbon cerium lanthanum ore by sulfuric acid roasting method, according to the theoretical amount of HF generated after complete decomposition, the decomposition of LT concentrate requires the production of 50-150kg of hydrogen fluoride. The concentration in the flue gas can reach 14g/m3, and the fluoride content exceeds the standard by 47 times. According to the characteristics of HF and SiF4, the commonly used processing methods are:

1. Cyclone dust collector

It is a dust removal equipment that uses centrifugal force for separation and purification, suitable for smoke particles with a particle size greater than 20 μ m. The dusty airflow enters the dust collector along the tangent direction from the upper side of the cylindrical body of the dust collector. It rotates in the space between the cylinder and the central exhaust pipe and descends along the spiral, causing the dust particles to be thrown to the wall of the collector by centrifugal force and lose speed. They separate from the flue gas and slide into the ash hopper. The dust removal efficiency of a cyclone dust collector is generally 70% to 80%, characterized by a simple structure, small volume, and stable effect.

2. Neutralizing Fluorinated Waste Gas with Alkaline Water Solution

Absorption reaction equation of sodium hydroxide solution:

HF+NaOH=NaF+H2O

After being absorbed and neutralized by alkali spray, the purification rate can reach 97% to 98%, and the fluoride content and sulfur dioxide in the exhaust gas can meet the emission standards.

2、 Equipment structure and characteristics

1. Cyclone dust collector

(1) There are no moving parts inside the cyclone dust collector. Easy to maintain.
(2) Production and management are very convenient.
(3) Small size, simple structure, and affordable price when dealing with the same air volume.
(4) When used as a pre dust collector, it can be installed vertically and is easy to use.
(5) It is convenient for multiple units to be used in parallel when dealing with high air volume, and the efficiency resistance is not affected.
(6) Can withstand high temperatures up to 400 ℃, and if special high-temperature resistant materials are used, it can also withstand even higher temperatures.
(7) After installing wear-resistant lining inside the dust collector, it can be used to purify flue gas containing highly abrasive dust.
(8) Dry cleaning is possible, which is beneficial for recovering valuable dust.

2. Fiberglass purification tower

Sodium hydroxide solution is used as the absorption and neutralization solution. The acid mist exhaust gas is pressed into the middle inner cylinder of the purification tower by a fan to form a pressure chamber, and then distributed to each bubble tube through the pressure weighting chamber. The exhaust gas enters the absorption and neutralization liquid in the storage tank through the bubble tube to generate bubbles. The bubbles pass through the filter ring layer, allowing the gas-liquid to fully contact again, and then are discharged into the atmosphere through the dehydration air. Based on the above design principles, the structure of a general fiberglass purification tower has been improved, and the following measures have been taken:
(1) We have adopted a new filter material with a large specific surface area, high perforation rate, uniform hole arrangement, regular arrangement, and minimal groove retention, called a circular flower wreath, to replace the commonly used Bao'er ring filter material in China.
(2) The selection of equipment materials must be scientific and reasonable.
(3) According to the working environment of each component of the purification tower, resins with good alkali resistance and high structural strength were selected, and the exterior was made of waterproof and anti-aging adhesive resin. To enhance the strength of the tower body, several longitudinal strengthening disks are used on the outer shell of the tower body, and a comprehensive structure combining segmented and segmented cylindrical double cylinder bodies is adopted according to the size of various models. Due to the long-term operation of the purification tower in an acidic or alkaline corrosive gas environment, in addition to the pressure of the tower itself and the solution pressure, it also needs to follow the wind pressure during operation. All fiberglass components are required to have good corrosion resistance and maintain high tensile and compressive strength. Therefore, after fully considering and excluding the strength reduction factor under strong corrosion and the discrete performance factors of hand lay up molding process, our company has taken into account the characteristics of the tower structure and usage environment, and maintained sufficient reserve strength for the entire fiberglass components under harsh working conditions.