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Due to the difference of raw materials and auxiliary materials, the production process of rare earths is not consistent. However, the exhaust gas generated in the production process are many processes, such as the fluorine-containing gas produced bastnaesite sulfuric acid baking method, a chlorine-containing exhaust gases produced by molten salt electrolysis of rare earth chlorides, rare earth iron-silicon alloy pyrometallurgical such as exhaust gas. The common feature of these processes is that the amount of exhaust gas generated is large and harmful, and the treatment process of the exhaust gas is representative.
1 The industrial waste gas produced by the treatment of fluorocarbon antimony ore by sulfuric acid roasting contains more harmful substances, mainly hydrogen fluoride, sulfur trioxide, sulfur dioxide, silicon fluoride and sulfuric acid mist. The process is as follows:
2REFCO 3 +3H 2 SO 4
â–³
→
RE 2 (SO 4 ) 3 +2HF↑+2CO 2 ↑+2H 2 O
CaF 2 +H 2 SO 4
â–³
CaSO 4 +2HF↑
→
H 2 SO 4
â–³
H 2 O↑+SO 3 ↑
→
2SO 3
â–³
2SO 2 ↑+3O 2 ↑
→
The chemical reaction of concentrated sulfuric acid to decompose rare earth concentrate is more complicated, and the reaction in the low temperature section (kiln tail) is more intense. Therefore, a part of the volatilized sulfuric acid mist is also discharged with the exhaust gas. In addition, carbon dioxide and a small amount of solid particles (smoke) are present in the tail gas of the roasting kiln.
2 The chlorine-containing waste gas produced by the rare earth chloride molten salt electrolysis is mainly chlorine gas generated by the anode. The reaction process is:
2C1 – 2e – → Cl 2 ↑
Hydrogen chloride gas is also produced during the electrolysis of crystalline rare earth rare earth or when the rare earth chloride dehydration is incompletely electrolyzed:
2RECl 3 +3H 2 O→RE 2 O 3 +6HCl↑
RECl 3 +H 2 O→REOCl+2HCl↑
3 In the process of producing rare earth ferrosilicon alloy by electric arc furnace, a large amount of flue gas is generated, and the flue gas is composed of carbon dioxide, carbon monoxide, silicon fluoride, low-valent silicon oxide, sulfur dioxide and the like. These components are mainly derived from carbon furnace lining and graphite electrode participation in the reaction, calcium fluoride and silica action, sulfate decomposition and the like.
(MeO)+C→[Me]+CO↑
2(CaF 2 )+2(SiO 2 )→(2CaO·SiO 2 )+SiF 4 ↑
(SiO 2 )+[Si]→2SiO↑
MeSO 4
â–³
MeO+SO 3 ↑
→
In addition, the flue gas also contains a large amount of solid dust particles, which is also an important pest in the production of ferrosilicon alloy exhaust gas.
4 In addition to the above process to produce waste gas, because of the chemical materials used in wet smelting, such as: hydrochloric acid, hydrofluoric acid, sodium hydroxide, nitric acid, ammonia, etc., when they react with materials, they are volatile or Hydrogen chloride, hydrogen fluoride gas, nitric acid mist, ammonia gas, etc. are discharged. These harmful gases not only have a strong corrosive effect on the production and purification equipment, but also have great harm to the human body, animals and plants, and the environmental impact is also very prominent.
The main exhaust gas produced in the production of rare earths is shown in the following table. It can be seen that the rare earth production process determines that the exhaust gas discharged from it is a mixture of solid, liquid and gaseous substances. Therefore, the treatment of exhaust gas, recovery of useful substances, reducing the harm of exhaust gas, has certain economic and environmental benefits.
The main exhaust gas is discharged from the rare earth production in the table.
Waste gas name
Exhaust gas contains harmful substances
source
Dust / (g/m3)
Fluorine / (g / m3)
Chlorine / (g / m3)
Hydrogen chloride / (g / m3)
Dust exhaust
Fluorine-containing exhaust
Fluorine and chlorine waste gas
Chlorine-containing waste gas
Hydrogen chloride-containing exhaust gas
2.10
Trace
1.50
14.00
6.45
17.36
00.20
6.00
Production of rare earth ferrosilicon alloy
Acid treatment of mixed rare earth ore
Chlorination treatment of mixed rare earth ore
Production of rare earth metals by electrolysis
Acid treatment of ionic rare earth ore