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Today, a colleague asked me why, in the process of producing acid from flue gas, the purification system uses dilute sulfuric acid for absorption, rather than water or sulfuric acid with a higher concentration. That’s how I answered: I said that smoke and dust contain heavy metals and various harmful substances such as As, and that weak acids are more easily absorbed compared to water; they can dissolve certain metal compounds that water cannot dissolve ； Considering the issue of equipment corrosion, a low-concentration dilute acid is used for absorption. I’m not sure if my answer is correct Or there may be other reasons; please ask an expert for an explanation. Thank you.
Water cannot be used because the dissolution of SO3 is exothermic, which results in the formation of acid mist
It is the SO3 in the flue gas that automatically increases the concentration； The reason for not using water is mainly the high water consumption and severe pollution it causes ； Unbalanced due to high-concentration sulfuric acid solution.
I said that smoke and dust contain heavy metals and harmful substances such as As; compared to water, dilute acids are more easily absorbed and can dissolve some metal compounds that water cannot dissolve； Considering the issue of equipment corrosion, it is appropriate to use a low-concentration dilute acid for absorption. . . . . . . .
It’s water after all; it just becomes a weak acid as it circulates
Initially, purification washing used water for purification; due to the large volume of wastewater, acid absorption methods were also developed. At that time, material advancements led to significant corrosion issues； These days, it is basically done through closed-system purification and absorption using dilute acids ； Moreover, when the system is started, the circulation tank is filled with water, which is used to wash out sulfur trioxide from the flue gas; as a result, the concentration of acid gradually increases, and the acidic impurities become more concentrated. Waste liquid is discharged regularly based on the level of impurities in the acid. The acidity of the acid produced from pyrite is generally low ； The production of acid from smelting flue gas is affected by the smelting process, resulting in a high acidity level, which can reach up to 30% ；
It is originally absorbed by water, and after absorbing acid mist, it becomes dilute acid
Water absorption is mainly due to high pressure in the wastewater; later, purification was achieved through dilute acid washing. If concentrated acid is used, dry absorption towers are required. Excessive water in the flue gas prevents maintaining the proper acid concentration, which leads to equipment corrosion and results in high costs, making it uneconomical