The acidification method is to add acid to the cyanide-containing solution to make the pH less than 7, and CN is hydrolyzed to a volatile HCN. The resulting HCN is very unstable in the liquid phase (the vapor pressure of HCN is 100 kPa at 26 ° C), and is easily volatilized into the gas phase. However, its acid stripping recovery rate is determined by temperature, acidity, stripping air volume and time. In China, the current operating conditions are mostly at a temperature of 35 ° C, a sulfuric acid dosage of 3.5 to 4 kg ∕ m 3 , and a blown blast of 700 to 1000 m 3 /m 3 . Under this condition, the treatment time t(h) of each batch of cyanide-depleted liquid is determined by the lean liquid volume V(m 3 ) and the blast volume Q(m 3 ∕h). which is
t=V·(700～1000)∕Q
HCN blown off from the liquid phase is a monobasic weak acid which is easily reabsorbed by lye to form NaCN or Ca(CN) 2 . After removal of cyanide ions, valuable metals such as gold , silver , and copper remaining in the solution should be recovered. Therefore, the acidification process includes four processes of acidification, volatilization, alkali absorption, and metal precipitation.
First, the chemical reaction of the acidification process.
The acidification operation usually involves adding sulfuric acid or SO 2 (from the roaster or burning sulfur) to the solution, converting sodium cyanide (calcium) or the like into sulfate, and releasing HCN:
2NaCN+H 2 SO 4 
2HCN↑+Na 2 SO 4
Ca(CN) 2 +H 2 SO 4 2HCN↑+CaSO 4
At the same time, the cyanide complex decomposes to form a cuprous cyanide precipitate and releases HCN:
Na 2 Cu(CN) 3 +H 2 SO 4 CuCN↓+2HCN↓+Na 2 SO 4
If gold and silver cyanide are present in the solution, they also undergo a similar reaction, ie, gold, silver compound precipitation and HCN evolution. Zinc cyanide and nickel cyanide release all of the CN when acidified, or a part of CN is released when hydrolyzed to form a precipitate, depending on the degree of acidification of the solution.
When cyanide in the solution is present as thiocyanide (CNS - ) or ferricyanide complex [Fe(CN) 6 4 - ), slight acidification is not obtained. Many side reactions occur at this time, including the formation of CuCNS and Cu 4 Fe(CN) 6 . The conversion of these substances not only produces HCN but also H 2 S.
The resulting HCN typically volatilizes as the solution is aerated, i.e., it is blown off the top-down solution through a series of wooden grids in the plant and a convection from bottom to top blast (Figure 1). HCN entering the gas phase is absorbed by the lime milk or lye cycle:
2HCN+CaO Ca(CN) 2 +H 2 O
HCN+NaOH NaCN+H 2 O
Figure 1 Recycling device for cyanide solution at the Flynn-Fran plant in Canada
I-deamination tower; II-absorption tunnel;
1-add acid tube; 2-de-gold solution into tube; 3-mixing tank; 4-blower; 5-rotor;
6- lye into the tube; 7-adjustment equipment; 8-water seal; 9-solution feed pump;
10-air, 11-lye; 12-closed cyanide solution tank
The H 2 S gas generated in the process can react with the lye to form Na 2 S in the absorber, so hydrogen sulfide is a harmful gas in the cyanidation process. The addition of mitochondria or lead nitrate to the solution decomposes H 2 S.
Second, the operation index of the acidification method.
According to the practice of the Flin Flon factory in Canada in 1970, the indicators are as follows:
Table 1970 Practice Index of the Flynn Fran Plant in Canada
Disposal capacity of waste liquid regeneration equipment /t·d -1 | 2150 |
Cyanide concentration in waste liquid (converted to NaCN)/% | 0.075 |
Lime content (converted to CaO) ∕% | 0.11 |
The amount of tail liquid after regeneration / t·d -1 | 2150 |
Cyanide concentration in tailings (converted to NaCN)/% | 0.006 |
Free sulfuric acid concentration /% | 0.02 |
Recovery of cyanic acid during regeneration /% | 92.0 |
Tons of liquid sulfuric acid consumption ∕kg | 1.6 |
Third, economic accounting.
AJ Gilmore et al. estimated the cost of processing the 3000t waste liquid using the Flint Fran factory on a daily basis. If the tailings of the regeneration operation contain 0.02% of cyanide ions (converted to NaCN), the treatment is terminated, which is economically advantageous. If we can work from other aspects, reduce the amount of solution that needs to be regenerated from the original 3000t∕d to 900t∕d, and the minimum allowable concentration of cyanide ion (converted to NaCN) in the tail liquid after regeneration is not higher than 0.035%. This is more economically advantageous.
A mine in China adopts the acidification method to treat the waste liquid containing (mg∕L) CN687, SCN - 608, Cu395, Au0.015, Ag0.09. After one year of operation, the waste liquid is 44,200 m 3 , and the chemical water and steam wages are used. Excluding equipment depreciation) A total of 130,000 yuan was spent, and the total value of recovered NaCN, Au and Cu was more than 320,000 yuan, and the annual surplus was 190,000 yuan. The mine's equipment and infrastructure investment in wastewater treatment is 750,000 yuan, and all investment can be recovered in 3.9 years.
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