Currently, the gold cyanidation extraction methods still dominated, as international gold prices, China's gold industry vivacious exuberant scene. However, the damage of cyanide-containing wastewater discharged from cyanide and gold extraction to humans and the natural environment is still a common concern of the whole society.
The cyanide-removing plant contains cyanide wastewater with complex components. The main ions are: free cyanide (CN - ), hydrocyanic acid, copper cyanide complex Cu(CN) 4 2- , zinc cyanide complex Zn (CN 4 2- , ferricyanide complex Fe(CN) 3 2- and thiocyanate (SCN - ), which also contains a small amount of gold cyanide complex Au(CN) 2 - , silver cyanide complex Ag (CN) 2 - .
The direct destruction method and the comprehensive recovery method are mainly used for the treatment of cyanide-containing wastewater (see Table 1 for specific methods). The cost of directly destroying or eliminating sodium cyanide is about 0.5-1.0 US dollars per kilogram, and the cost of recovering sodium cyanide from wastewater is 0.25-0.5 US dollars per kilogram; the direct destruction method can reduce the pollution degree of wastewater, but it is seriously wasteful. Resources do not meet the development trend of clean production and recycling economy.
I. Research status of ion exchange technology
The ion exchange method can recover both useful substances and environmental pollution caused by tailings. In 1950, South Africa began to study the ion exchange process for the treatment of cyanide-containing wastewater in the gold industry. The Soviet Union also began research in 1960. In 1970, industrial plants were put into operation and achieved good results. In 1985, Lakefield Research Co., Ltd. of Canada proposed the recovery of cyanide and its complex anions by anion exchange resin method, so as to achieve the purpose of comprehensive recovery of valuable metals and cyanides while recovering gold. The process for processing a lean solution produced zinc replacement method using strong basic anion exchange resin adsorption of heavy metal cyanides, when effluent CN - the resin is regenerated when excessive pickling, sodium cyanide recovered from the eluate. Most of the eluent is regenerated and reused for acid pickling regeneration of the resin; a small portion of the eluent is neutralized and precipitated after heavy metal discharge; a resin named V912 (Vitrokele 912) invented by Devoe Holbein International, France It is more suitable for recycling cyanide in gold extraction tail liquid, especially the desorption regeneration method of resin is relatively simple.
Table 1 Classification of cyanide treatment methods
classification | method | classification | method |
Destructive Processing method | Alkaline chlorination Ozone oxidation Electrochemical method Natural degradation method Hydrogen peroxide method Sulfur dioxide-air oxidation Biochemical treatment | Resource synthesis Use method | Acidification recovery Solution extraction Liquid membrane method Activated carbon adsorption Zinc sulfate-sulfate acidification Electrodialysis Ion exchange resin method |
China's research on the recovery of cyanide in gold extraction tailings by ion exchange method is quite different from that in foreign countries. In 1987, Yinghaiyan of China conducted a semi-industrial test on the treatment of electroplating cyanide wastewater with ion exchange resin . The purification rate of Cu and CN- was over 95%, and the purified wastewater met the national emission standards. Xu Kexian of Changchun Gold Research Institute has conducted a series of studies on cyanide in resin adsorption and recovery of gold tail liquid, and applied for a patent. He has done small experiments with the gold tail liquid from the Huajian Gold Mine in Hebei Province. The results also show that this method is economically feasible. Compared with the alkaline liquid chlorine method, it can save tens of thousands of dollars per year, not only governance. Cyanide-containing wastewater, and the recovery of metals and cyanide, has good economic, environmental and social benefits. However, due to the current problems of various resins, complicated operation, and not mature technology, the ion exchange method is still in the laboratory or semi-industrial test stage.
Table 2 Experimental results of resin selection
Resin code | D320 | D296R | D301G | 201×7 | D2-1 | D320 | D418 |
Adsorption rate/% | 82.96 | 84.74 | 68.16 | 92.59 | 80.80 | 83.02 | 51.10 |
In this paper, through the selection and functional optimization of the resin, the ions in the gold extraction tail liquid were transformed, and then the adsorption, step-by-step desorption, and the cyanide and valuable metal ions in the solution were comprehensively recovered.
Second, experimental research
(1) Experimental equipment and research methods
Experimental equipment: SHY-2A benchtop constant temperature oscillator, pHS-3c type acidity meter, vacuum drying oven, electronic balance, 
10c m × 30cm ion exchange column and some glass instruments. The total cyanide concentration was determined by the dual indicator silver method, and the metal ion concentration was determined by the atomic absorption method.
Resin pretreatment: soak the resin in 4 times deionized water for 16 h, wash with water until clarification, add 4 times the amount of 1 mol·L -1 NaOH, shake at room temperature for 4 h, remove the lye, wash with water until phenolphthalein is no The color was further stirred by adding 1 mol·L -1 of HCl under the same conditions for 4 hours to remove the acid solution, and the resin was rinsed with deionized water until the end of the addition of methyl orange became yellow. Repeat the process multiple times. The pretreated resin was added to a 4 times amount of 1 mol·L -1 NaOH/HCl solution under the same conditions for 2 h to convert the resin to OH - /Cl - type, and then washed with deionized water until the phenolphthalein was colorless. After the backup.
(2) Selection of resin
The cyanide adsorption experiments were carried out on the seven resins at room temperature. The experimental results are shown in Table 2. It can be seen from the data in Table 2 that the adsorption rate of cyanide by 201×7 strong basic styrene resin can reach over 90%, and the adsorption rate of several resins such as D320, D2-1, D296R and D320 can reach 80%. %the above. Therefore, we have studied the adsorption properties of these resins in the experiment, and systematically summarized the research results of 201×7 resin.
Third, the results and discussion
(1) The influence of acidity
The adsorption experiments were carried out with solutions at different pHs. The experimental results are shown in Figure 1. It can be seen from Fig. 1 that the adsorption rate of cyanide in the resin decreases with the increase of the pH of the solution during the adsorption process, and the adsorption rate is the highest when the pH is 10-11. Considering that the gold extraction tail liquid generally has a pH between 10 and 12, and the cyanide is hydrolyzed when the pH is low, the pH of the solution used is maintained at 10.5 to 11.
(II) Determination of apparent adsorption rate constant and activation energy of adsorption process
The adsorption rate of cyanide by 201×7 resin is very fast, and the equilibrium is basically reached in 15 minutes. According to the liquid film diffusion formula:
Where: F is the degree of exchange; Q t -t is the amount of adsorption at the moment, mg·ml -1 ; Q ∞ is the amount of adsorption at equilibrium, mg·ml -1 ; k is the adsorption rate constant, s -1 . The time t is plotted as -ln(1-F), and the result is shown in Fig. 2.
It can be seen from the linear relationship of Fig. 2 that the adsorption of cyanide by 201×7 resin is mainly controlled by liquid film diffusion. From the slope of the line, the rate constant of the adsorbed cyanide of 201×7 resin is k=1.01×10 -2 s -1 , and the adsorption speed is fast. It can be seen from the results of Fig. 3 that the desorption process also conforms to the Body liquid film diffusion formula, and the desorption rate is very fast. The rate constant of desorption is k=2.12×10 -2 s -1 from the slope of the straight line.
(III) Influence of temperature on adsorption effect and determination of thermodynamic parameters
By the Claeyron-Clausius equation:
Lnc e =-ln(K 0 )+â–³H/RT
Where R is the gas constant (8.3142 J·mol -1 ·K -1 ), T is the absolute temperature (K), and K 0 is a constant.
The lnc e is plotted against 1/T and linearly fitted, as shown in Figure 2. The enthalpy change ΔH can be obtained from the slope; and if the adsorption conforms to the Freundlish isotherm equation, the free energy can be obtained from ΔG=-nRT; Find ΔS.
Table 3 adsorption thermodynamic parameters of 25 ~ 40 ° C
Type of resins | ΔH/(kJ·mol -1 ) | Temperature/°C | ΔG/(kJ·mol -1 ) | ΔS/(J·mol -1 ·K -1 ) |
201×7 | 0.949 | 25 30 35 40 | -4.88 -4.96 -5.04 -5.12 | 19.56 19.51 19.46 19.41 |
It can be seen from the experimental results in Table 3 that the adsorption of cyanide by 201×7 resin is an endothermic reaction, and the increase of temperature is favorable for the adsorption, but the effect is not large, and adsorption at room temperature can be selected. In addition, as the temperature increases, the absolute value of ΔG increases, indicating an increase in the adsorption tendency, which is consistent with the experimental results of increasing the adsorption amount of the temperature increase. Further, ΔH is less than 40kJ·mol -1 , and ΔG changes little with temperature, indicating that adsorption belongs to the category of physical adsorption, and the reduction of free energy and the increase of entropy are the driving forces of the adsorption.
(4) Desorption of loaded resin
In this experiment, stepwise desorption is used. First, zinc and cyanide adsorbed on the resin are desorbed by sulfuric acid, and then copper supported on the resin is hydrolyzed by ammonia.
1. Desorption of Zn and cyanide
The zinc and cyanide on the resin are desorbed by H 2 SO 4 , and the escaped HCN is absorbed by the NaOH solution, and the obtained NaCN solution has a cyanide concentration of 10 g·L -1 and can be recycled; the zinc-containing solution after desorption, zinc The concentration is 0.6-0.8 g·L -1 , which can be directly recovered by a chemical method. The desorbed solution can also adsorb zinc ions through a cation exchange column, and the remaining H 2 SO 4 can be returned for desorption. A single factor experiment was performed on the desorption time, concentration and desorbent volume of H 2 SO 4 . Find the optimal unwashing conditions. It can be seen from Fig. 5 that the desorption time is basically balanced after 3 hours. The desorption rate of total cyanide can reach more than 85%, and the desorption rate of Zn is over 90%.
2. Desorption of Cu
The resin desorbed with H 2 SO 4 was washed to neutrality and then hydrolyzed with ammonia. The results show that ammonia can effectively desorb Cu, because the Cu desorbed by H 2 SO 4 is supported on the resin in the form of CuCN, which is soluble in ammonia. Because Cu(NH 3 ) 4 2+ is formed . Thereby Cu elutes from the resin in the form of a cation.
Fourth, the conclusion
(1) Through adsorption experiments on several resins, the results show that when pHE10.5～11, the adsorption at room temperature is carried out, and the adsorption rate of cyanide in the gold extraction tail liquid of 201~7 resin can reach more than 90%.
(2) Kinetic experiments show that the adsorption of cyanide by 201×7 resin is the main control step of liquid film diffusion. The adsorption rate constant is k=2.12×10 -2 s -1 , the adsorption speed is very fast, and the desorption rate constant k=2.12×10 -2 s -1 .
(3) Thermodynamic experiments show that the adsorption of cyanide by 201×7 resin is endothermic, but the effect of temperature on adsorption is not large, and adsorption at room temperature can be selected. It can be seen from the thermodynamic parameters that the reduction of free energy and the increase of entropy are the driving forces of the adsorption.
(4) When H 2 SO 4 is desorbed, the desorption rate of total cyanide can reach more than 85%, and the desorption rate of Zn is over 90%. The desorption rate of Cu can reach more than 90%.
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