3.2.2 Producing sulfuric acid from pyrite
3.2.2.1 Production principle
The main component of iron ore is sulfur, iron sulfide, pyrite sulfur content required ≥30%, 6-8% moisture, particle size ≤3mm.
4FeS2 + 11 O2 = 2Fe2O3 + 8SO2
SO2 + O2 = SO3 ; SO3 + H2O = H2SO4
3.2.2.2 Process
3.3 Production methods:
(1) Tower method: The advantage is that the material and heat energy are fully utilized, especially the process of producing sulfuric acid by using elemental sulfur as raw material, the conversion tower is separated from the heat exchanger, and the heat energy is fully utilized.
(2) Contacting the process sulfuric acid, using vanadium pentoxide as a catalyst, increasing the reaction rate and the purity and concentration of the product.
3.4 Production process
3.4.1 Production process using sulfur as raw material (two rotations and two suctions)
(1) Pretreatment of raw materials
The sulfur is turned into a liquid by steam heating, and then the organic matter is removed by filtration.
(2) Incinerator
The air dried by 98% sulfuric acid passes through the fan and enters the sulfur incinerator and the liquid sulfur that is blown into the furnace is in full contact and burned.
Reaction equation: S + O2 = SO2
The large amount of heat of the gas is taken away by the steam from the waste heat boiler for power generation. The temperature of the gas after cooling is controlled at 420 o C.
(3) Conversion of SO2
A four-stage converter is used, and the heat exchanger is separated from the converter, and the process is roughly as shown in the following figure:
The gas is again sent to the converter and absorber to form a secondary conversion secondary absorption process. The conversion rate can reach 99.9%, so the tail gas can be directly excluded without treatment.
3.4.2 Production process using pyrite as raw material (one turn and one suction)
(1) Pretreatment of raw ore: The pyrite content of pyrite is first measured and then pretreated.
a Mineral processing: the ore should be matched according to the size of the boiling furnace.
b Drying: The temperature in the drying kiln should reach 600o C ~ 900o C to remove moisture.
c Crushing: The production of the crusher is continuous. It mainly crushes the ore, the particle size is ≤ 3mm, and the smaller the particles of the ore, the faster the reaction speed.
(2) The boiling furnace burns out SO2: 4FeS2 + 11 O2 = 2 Fe2O3 + 8 SO2
3 FeS2 + 11O2 = Fe3O4 + 6SO3
The temperature in the boiling furnace is about 850oC～900oC. The pyrite is in the boiling furnace, the gas-solid contact is good, and the gas is mixed vigorously. Therefore, the higher the burning rate in the boiling furnace, the better, and the sulfur content of the slag required to be burned is ≤0.5. %. The temperature of the slag conveyed by the reaction is about 850oC to 900oC. It is first cooled to 500oC with water in the conveyor. The waste heat is collected in the waste heat boiler for power generation.
(3) Purification and drying of flue gas:
SO2, SO3 come out from the top of the boiling furnace, the temperature is about 1000oC, and the pressure is very small, so there is an induced draft fan in the upper part of the boiling furnace. Gas mixed with water vapor, a small amount of SO3, arsenic compound, and other impurities in the gas dust in the air. These impurities corrode the pipeline, poisoning the catalyst in the contact chamber, and thus making the reaction insufficient. Therefore, a series of impurity removal work must be carried out. After dust removal (cyclone dust removal, electric dust removal, acid cleaning purification), in addition to acid and drying, the efficiency of dust absorption in the mixture can reach 99%.
a pickling tower: mainly for cooling, using circulating water to cool SO2, SO2 from the tower with water, forming an acid mist, acid mist to be removed, and the requirement is ≤0.005g / L. The advantage of the pickling method is that less waste water is produced and the removal of acid mist is relatively thorough. The downside is that its equipment is complex and the investment is large.
b The packing in the drying tower is a Raschig ring. It is required to ensure that the acid concentration is between 92.5% and 93.5%, and the temperature is controlled below 50o C.
(4) Conversion of SO2: SO2 + O2 → SO3 + Q
The Zhuhua Group uses a four-stage converter with an efficiency of about 96.5%. The requirements for the catalyst in production are: high activity, low reaction activation energy, long life, not easy to be broken, aging and poisoning, and also require easy availability of raw materials and low price. The vanadium catalyst used by Zhuhua Group uses diatomaceous earth as a carrier, vanadium pentoxide as an active material, and potassium and cesium sulfate as a promoter. They are combined to a certain extent to prevent the loss of vanadium pentoxide gasification. The activation energy of the reaction is lowered and the rate of the reaction is accelerated.
(5) Absorption of SO3: SO3 + H2O → H2SO4
The structure of the absorption tower is the same as that of the drying tower. The main purpose is to use 98% H2SO4 as the absorbent, and the absorption temperature is controlled below 55oC. However, in fact, 92.5-93% of H2SO4 is used as the absorbent. The absorption temperature is controlled at 60-80o C.
(6) Exhaust gas treatment: The SO2 standard required to be discharged into the air in the exhaust gas is ≤400ppm. There is always a small amount of SO2 in the absorption tower that is incompletely reacted, so ammonia contact, lime, sodium citrate or catalytic conversion can be used.
The plant uses ammonia contact to absorb the remaining 4% of the SO2 in the converter.
2NH3 + H2O + SO2 = (NH4)2SO3 + Q
(NH4)2SO3 = NH4HSO3 +NH3↑
(NH4)2SO3 + H2SO4 = (NH4)2SO4 + SO2↑+H2O
The remaining ammonia contact method is compressed by a SO2 compressor to become liquid SO2. Used in medicine, food industry, etc., and returned to the process of producing H2SO4. For profitable companies, tail gas treatment is costly.
Considering two methods for preparing sulfuric acid, the conversion rate of the former is much higher than that of the latter, and the process is simple and the pollutants discharged are small. In the case that environmental protection is gaining more and more attention today, the former will gradually replace the latter. But it may not be completely banned because the price of sulphur soars.
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