2.3 HCN gas absorption
Â Â The HCN- containing gas obtained by degassing the acidified wastewater with a gas (called a carrier gas) is neutralized with a NaOH absorption liquid to form a neutralization reaction to form NaCN . The reaction is completed in an instant, and since the HCN is a weak acid, the absorption liquid must be kept constant. The alkalinity can ensure the complete absorption. Generally , the residual NaOH in the NaOH absorption liquid is controlled in the range of 1% to 2% , and the absorption reaction is as follows:
Â NaOH+HCN(g)=NaCN(aq)+H 2 O
Â The price of NaOH is much higher than that of CaO . Therefore, there is a use of lime milk instead of NaOH , but measures to prevent fouling and block the absorption tower.
Â 2HCN+Ca ( OH ) 2+Ca ( CN ) 2 +H 2 O
After the HCN is absorbed, the carrier gas is recycled, and its advantages are as follows:
Â 1) The carrier gas is originally air, and the acidic component such as CO 2 reacts with the alkali in the absorption liquid to consume alkali. If it is recycled, the alkali solution is no longer consumed.
Â 2) When the carrier gas is recycled, the oxygen in the carrier gas is less and less, and the cyanide is prevented from being oxidized during the acid stripping process.
Â 3) If the carrier gas is not recycled, due to the contact of the carrier gas with the liquid phase, the humidity is very large and contains a small amount of HCN , which can only be discharged to the outside, but a large amount of fresh air is introduced in the winter, and the blow-off requirement is required. Temperature, inevitably consumes additional energy and wastes energy, so the carrier gas is generally recycled.
Â 3 Â Consumption of acidification recovery
Â The consumption of cyanide-containing wastewater by acidification recovery method is mainly sulfuric acid, caustic soda, lime and electricity. In winter, it is necessary to preheat the wastewater, so it also consumes steam, and it is rarely useful to heat the wastewater, so the cost is too high.
Â Â Â 3 . 1 Â Sulfuric acid consumption
Â Â The acid consumption is mainly the acid required to convert cyanide into HCN in wastewater, and the acid required to make the wastewater reach a certain acidity ( pH<2 ). Secondly, the alkaline pH of the wastewater is 9.9 to 11.5 . Acid, if the slurry is treated, the carbonate in the slurry will react with the acid to form CO 2 gas, which also consumes acid.
Â 1) Conversion of cyanide to acid consumed by HCN
Â The sulfuric acid required for the conversion of cyanide to HCN can be
Â 2) Acid consumed in acidification wastewater
Â Â Â The sulfuric acid required to acidify the wastewater to 0.2% of residual acid
Â 3) Acid consumed by alkali in wastewater
Â The pH value of wastewater is generally 10 ~ 11.5 , and the consumption of sulfuric acid is not much, less than
Â 4) Acid consumed by carbonate in waste ice
Â This part of consumption is related to the characteristics of cyanide raw materials. For cyanide raw materials with high carbonate content, the cyanide tailings slurry is not suitable for treatment by acid recovery, and the sulfuric acid consumption is too high.
Â The total acid consumption of wastewater is generally 4 ~
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