The waste liquid and residual material discharged from the cyanide plant always contain a certain amount of cyanide. But in most cases, the cyanide content is always low. Despite this, it is economical and necessary to recover cyanide from waste. The reason is:
(1) The cyanide-containing solution containing cyanide-depleted liquid or recovered from solid waste can be used in the process of cyanidation to maintain the balance of the solution in the factory. It is especially advantageous when the solution contains high concentration of cyanide.
(2) In addition to cyanide, the cyanide lean liquid always contains a certain amount of metal ions such as gold , silver and copper , and these metal ions are present in combination with cyanide. In order to keep the slag "clean", it is possible to recover a large amount of valuable metals such as gold, silver and copper while recovering cyanide, which is more advantageous than only recovering cyanide.
(3) In order to protect the environment from pollution, it is necessary to destroy cyanide in the discharged waste liquid. Recycling cyanide is more advantageous than simply destroying it.
An industrial method for regenerating or recovering cyanide from waste liquids, mainly including direct return of cyanide-lean liquid and acidification treatment.
First, the direct return of cyanide poor liquid
The return of cyanide-lean liquid is a direct method of recovering cyanide. In the early literature, it was considered that the lean liquid in the cyanide plant could not be returned directly. The practice of modern cyanidation has corrected this misconception. Nowadays, cyanide plants at home and abroad have returned cyanide-depleted liquid directly to the operation process instead of adding water (or returning to the ball mill grinding process), which makes it easier to achieve solution balance and reduce cyanide in cyanide plants. Consumption. Only a portion of the excess liquid is treated and discharged.
The cyanide solution of a mine in China is directly returned to the preparation of cyanide solution without being replaced by a zinc wire, and the effect is good. The practice of the mine proves that the concentration of Cu 2 + , Zn 2 + and S - ions in the solution increases after the cyanide solution is returned to use, but there is no accumulation phenomenon; the dissolution rate of gold starts slightly. Slower, but after 12 hours of leaching, the dissolution rate before the return of the lean liquid is reached.
Zhang Fengge conducted a theoretical analysis and practice verification of the cyanide lean liquid return process. He pointed out that in the cyanidation system, the sodium ion concentration can fully characterize other impurity components (mainly a variety of anions paired with sodium ions, such as CN - , OH - , Me(CN) n m - , CNS - etc.) The macro sum, the concentration of any impurity component in the system is never more than the concentration of sodium ions.
In any continuous cyanide plant that uses the lean liquid return process, sodium cyanide (ie, sodium ions) is added to the cyanide tank again and again, and it is also required to be carried out in the bottom liquid phase of the three-layer concentrator. Take the sodium ion. As the number of lean liquid returns increases, the concentration of sodium ions in the cyanide leaching tank and the circulating liquid phase will increase accordingly. At the same time, the amount of sodium ions carried away from the bottom liquid phase of the three-layer concentrator also increases. After the number of lean liquid returns reaches a certain minimum value, the amount of sodium ions carried away from the bottom liquid phase of the three-layer concentrator is exactly equal to the amount of sodium ions added to the next cyanide leaching tank. At this time, the balance of sodium ions is achieved for the entire cyanide system. In this way, the impurity components will never increase again in the liquid phase of the entire cyanide system. This laid the theoretical foundation for the application of the lean liquid return process. This balance occurs only after the lean liquid returns 4 to 5 times, and the accumulation of impurity ions and sodium ions in the system cannot be infinitely increased.
Therefore, the lean liquid return process can be realized only by purifying the adhered water brought out by the bottom stream of the three-layer concentrator, and the amount of water to be purified only accounts for less than 1/3 of the lean liquid returning process, which can save production water 2 ∕3 is an economic and reasonable gold extraction process.
And from the production practice of some lean liquids returning to the cyanide plant, whether it is the high-concentration NaCN (7kg of tons of mine) leaching of the North Gold Mine, the medium concentration of NaCN (ton of mine 2.5kg) leached Luoshan gold mine and low concentration The two chemical plants leached by NaCN (ton of calcined 0.8kg) have been running for one to several years, the production is normal, and the gold leaching rate is stable.
In foreign countries, the application of lean liquid return process is also very extensive. For example, the Carrolline Gold Mine in Canada is to return the clarified cyanide-depleted liquid exuded from the tailings instead of adding water.
Second, the regeneration and recovery of cyanide waste liquid - sulfuric acid acidification
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.
(1) 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.
(2) Operational indicators 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 |
(3) 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.
3. Regeneration and recovery of cyanide waste liquid - electrodialysis - acidification
Electrodialysis is widely used in the treatment of brackish water, fluorine-containing water, seawater desalination, salt production, boiler feed water purification, etc. due to its high salt removal efficiency. Kunming Institute of Metallurgy cyanide barren solution for both CN - from recovering, Au, Zn, Pb, Cu and the like, and to reduce the amount of processing solution tail explored lean liquid cyanide integrated process flow of FIG.
Figure 2 Electrodialysis-acidification process
Electrodialysis is the orientation movement of ions under the action of an electric field. Through the alternate arrangement of the anion and cation exchange membranes and the rational assembly of the separator, the ions in the lean liquid flowing through the desalination chamber enter the phase through the ion exchange membrane under the action of the electric field. Pro concentrated room. The solutions in the rich and depleted compartments are then passed through the respective water channel ejectors to obtain two different concentrations of solution.
The electrodialyzer for the experiment has a separator of 100 mm × 200 mm and δ = 2 mm. A plastic window gauze is attached to the double-circuit flow channel, and the flow channel is 33 mm wide and 150 mm long. It is assembled into eight stages by eight pairs of anion and cation exchange membranes. The fresh water circulation flow is indicated by the rotameter. After circulating to concentrated water to a certain concentration, it is discharged and subjected to acidification and CN - treatment.
The lean liquid used in the experiment contained NaCN 540 mg ∕L. When the power consumption was 3 kW·h/m 3 , the NaCN removal rate in the produced desalinated water was more than 90%. The amount of desalinated water accounts for 75% of the total amount of treated lean liquid, and is used during the return to cyanide operation. Concentrated water accounts for 25% of the total amount of lean liquid. Containing NaCN up to 2090 mg ∕L, enriched 3.9 times.
The acidification and de-cyanide test of concentrated water uses an electrically heated three-necked bottle, and the steam is passed through a first-stage buffer bottle and a two-stage lye absorption bottle in series, and then discharged through an air pump. Concentrated water is injected into the three-necked bottle to add sulfuric acid to adjust pH=1.5, the temperature is about 35 °C, and the vacuum degree during the process is 17 kPa (580 mmHg, local atmospheric pressure is 600-610 mmHg), and 0.5~1 h, the residual in the steam residue NaCN < 50 mg ∕ L, CN recovery rate is greater than 93%. The cyanide anion such as Au, Cu, Zn in concentrated water releases CN-, and the sulfate is enriched in the vapor residue, and lime is added to hydrolyze them to form a hydroxide and precipitate together with CaSO 4 . The chloride is then used to leach all of the metal in the precipitate and then separated and purified.
Fourth, activated carbon adsorption method
The cyanide waste liquid of Vaal Reefs in South Africa contains 0.1-0.5g/t of gold and less than 10g/t of cyanide. It is adsorbed by three activated carbon columns in series. The carbon column is a swirling tower type with 4.5t of carbon per column. The waste liquid passes through the carbon column at a flow rate of 80-130 t∕h. After 12 months of operation, the amount of gold contained in the batch of carbon reaches 5-6 kg ,t, and the recovery rate of gold is 97%. After calcining this batch of charcoal, 72 kg of gold is obtained, and only the value of gold can recover the entire investment. After the waste liquid is treated, the water quality is as clear as the river water, and it can be returned to use. It can also save 8,000 rand of water per month.
5. Ion exchange resin adsorption method
In the former Soviet Union, studies on the treatment of cyanide-containing waste liquids and sewage by ion exchange resins have been extensively carried out. The results showed that: AB-17 pair of CN - adsorption capacity of up to 30mg / g. The adsorption capacity of AM-2B, AII-2, and AH-18 is also between 15 and 20 mg ∕g. In order to find a new type of ion exchange material which is cheap and has good selectivity to cyanide, it has been found that the β β,γ -12 П ionization group resin containing quinoline not only has an adsorption capacity for CN - up to 35 mg ,g, but also Cu, Zn. The cyano complex of Fe is higher from 112 to 160 mg ∕g. The ABφ-12П ionized group resin containing methylpyridine adsorbs CN - up to 40 mg ∕g, and the cyano complex of Cu, Zn and Fe also reaches 31-81 mg ∕g. And the adsorption capacity of the latter two to thiocyanate is also 56-58 mg ∕g.
Sixth, zinc sulfate - sulfuric acid acidification
This method is to add zinc sulfate to the cyanide-containing solution to convert cyanide ions into zinc cyanide precipitate:
2NaCN+ZnSO 4 Zn(CN) 2 ↓+Na 2 SO 4
2NaCu(CN) 2 +ZnSO 4 Zn(CN) 2 ↓+Cu 2 (CN) 2 +Na 2 SO 4
The complexes of iron, copper and the like contained in the cyanidation solution also react with zinc sulfate to form a similar precipitate. Then add sulfuric acid to liberate Zn(CN) 2 to release HCN:
Zn(CN) 2 +H 2 SO 4 2HCN↑+ZnSO 4
Figure 3 shows the process flow chart for the regeneration of cyanide from zinc sulphate in Japan. The economic indicators of the plant's daily treatment of waste liquid by batch method are:
Cyanide concentration in waste liquid (based on KCN) ∕g·m 3 1510
Cyanide concentration in solution after precipitation with zinc sulfate (in terms of KCN) / g · m 3 50
Recovery of cyanic acid from zinc sulfate precipitation /% 96.7
Volatilization rate of cyanic acid during acidification of sulfuric acid /% 92.3
Recovery of cyanic acid from lye absorption /% 95.6
Total recovery of cyanic acid during regeneration /% 88
Figure 3 cyanide regeneration and recycling process
7. Other methods for recycling and recycling cyanide waste liquid
E. Goldblatt studied the recovery of cyanide from the cyanide waste liquid by IRA-400 styrene-type strong basic anion exchange resin. It was pointed out that when the waste liquid passed through two exchange columns in series, it was free. The complex anion of cyanide ion and metal cyanide can be adsorbed by a sulfate-containing anion exchange resin.
In addition, it has been suggested to use a chemical regeneration method in which sodium sulphide is precipitated from a cyanide complex, and an electrolytic regeneration method.
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