Huang Kaiguo Hu Tianjue
At present, zinc is recovered from the wet mill Leach Residue silver, they use silver flotation concentrate is selected from the dross floating, and then sulfating roasting - leach - precipitation of silver [1,2]. The process is long, the number of filtration is large, the solid-liquid separation is difficult, and the efficiency is not high. If the conventional cyanidation method is used to extract silver from the zinc acid leaching residue by cyanide leaching, the first one is to use a large amount of alkali to adjust the pH of the acidic zinc leaching residue to a high alkalinity to cyanide leaching; the second is cyanide leaching. Slow speed, long cycle, low leaching rate, easy to interfere with iron , lead , copper , arsenic and other ions; third is cyanide poisonous, environmental pollution. In this study, thiourea was used as a leaching agent to directly leach silver from the acid leaching residue of zinc, which overcomes the shortcomings of the cyanidation method.
The structural formula of thiourea is: 
Abbreviated as TU, soluble in water, neutral, unstable in alkaline liquid, easy to decompose to form sulfide and cyanamide, stable in acidic liquid, able to form stable complex ions with silver ions [Ag (H 2 NCSNH 2 ) 3 ] + , abbreviated as [Ag (TU) 3 ] + , and its complexation constant is 13.1 [3] . Therefore Ag 2 S, AgCl and metals such as Ag has a strong solubility.
I. Samples and test methods
(1) Samples
The sample is a acid leaching slag (pH=5.0) of wet smelting zinc in a smelting plant, and a representative sample is obtained by mixing and shrinking. The multi-element chemical analysis results of the sample are shown in Table 1. The phase analysis results are shown in Table 2. The sample size was fine, 95.8% was less than 75 μm.
Table 1 Multi-element chemical analysis results of samples
element | Ag | Zn | Pb | Cu | Fe | S | Si0 2 |
Content ω/% | 488.4 | 20.45 | 4.17 | 0.486 | 19.59 | 8.35 | 6.08 |
Note: The unit of Ag content is g/t.
Table 2 Results of phase analysis of silver in the sample
Silver sulfide | Silver oxide | Metal silver | Other forms of silver | total | |
Silver content / (g·t -1 ) | 386 | 41 | 14 | 44 | 485 |
Distribution rate /% | 79.59 | 8.45 | 2.89 | 9.07 | 100 |
(2) Test methods
Weigh a certain amount of acid leaching residue and transfer it into a 800mL beaker. Add a certain amount of TU and acidified aqueous solution, vigorously leaching, warm if necessary, and then separate by filtration. At each step, the effects of slurry concentration, leaching agent TU concentration, leaching time, temperature and pH on the silver leaching rate were studied according to the established process flow.
The silver in the leachate was replaced with zinc, and the analysis of silver was analyzed by a Z-8000 atomic absorption spectrometer (Hitachi).
Second, the analysis and discussion of test results
In this study, silver was extracted from the wet zinc sulphate leaching residue by thiourea method. The effects of slurry concentration, leaching agent TU concentration, temperature, leaching time and pH value on the silver leaching rate were discussed. The optimum process conditions were obtained.
(1) Effect of slurry concentration and leaching agent concentration on leaching rate
Experiments were arranged by orthogonal method (L 4 table) to analyze the effect of slurry concentration and leaching agent TU concentration on leaching rate (R). Two factors: leaching agent concentration (A); slurry liquid to solid ratio (B). Two levels: A 1 4g/L; A 2 8g/L; B 1 5:1; B 2 10:1. The test results are shown in Table 3.
Table 3 Two-factor two-level test results
Pilot | A ÏTU/(g·L -1 ) | B Slurry solid ratio | AB | test results R/% |
1 2 3 4 | 1(4) 2(8) 1(4) 2(8) | 1 (5:1) 1 (5:1) 2 (10:1) 2 (10:1) | 1 2 2 1 | 38.0 57.8 84.4 87.3 |
Effect γ | +11.4 | +38.0 | +8.5 |
It can be seen from the orthogonal test data in Table 3:
1. The effect γ A of the leaching agent TU concentration (ie, factor A) is +11.4, indicating that the TU concentration change has a greater influence on the leaching rate of silver;
2. The effect of slurry-solid ratio γ B is +38.0, indicating that the change of slurry concentration (ie liquid-solid ratio) has a great influence on the leaching rate of silver;
3. Comparing the effects of A and B, it can be seen that γ A <γ B indicates that the influence of the change of slurry concentration is greater than the influence of the change of TU concentration;
Effect of γ interaction of two factors 4, A, B AB less, pulp density and described TU concentration less interaction.
The test results of the effects of different pulp concentrations and different TU concentrations on the leaching rate are shown in Figures 1 and 2. Other leaching conditions were: pH 1.5 to 2.0, [Fe 3+ ] = 0.0159 mol/L, temperature 30 ° C, and leaching time 2 h.
It can be seen from Fig. 1 that at normal temperature, as the concentration of the slurry decreases, the leaching rate increases, and the leaching rate is inversely proportional to the concentration of the slurry, which is a linear relationship. When the TU change increases, the leaching rate still increases as the slurry concentration decreases. TU 6g/L, the leaching rate of Ag (65%) and TU 2g/L at a liquid-solid ratio of 7.5:1, and the leaching rate of Ag at a liquid-solid ratio of 10:1.
Figure 2 shows that as the TU concentration increases, whether the liquid-solid ratio is 5:1 or 10:1, the leaching rate begins to increase with increasing TU concentration, but increases to a certain extent and does not rise, and the slope approaches zero.
It is worth noting that the TU concentration does not represent the consumption of thiourea. According to the theory, the consumption of thiourea in silver per ton of acid leaching residue is 1032g, but the actual consumption of thiourea is 1500-1800g. The control TU concentration reaches 6g/L, which is mainly to increase the concentration of the material on the left side of the equilibrium reaction of the leaching reaction, so that the reaction equilibrium is fully moved to the right. As shown in equations (1) and (2).
Excessive consumption of thiourea is caused by two causes. The first is the consumption of copper ions to thiourea in the acid leaching residue. The complexing constant of Cu[TU] 4 2+ is 15.40 [3] . Therefore, copper ions can also form a stable complex with TU and consume it. Oxidation consumption of thiourea itself.
Ag 2 Sâ†â†’Ag + + S 2- (1)
Ag + + 3TUâ†â†’Ag[TU] 3 + (2)
(2) Effect of temperature on leaching rate
Tables 4 and 5 give the effect of temperature on the silver leaching rate.
Table 4 Effect of different temperature and slurry concentration on leaching rate
Temperature / °C | Liquid to solid ratio | Remarks | |||
3:1 | 5:1 | 7.5:1 | 10:1 | ||
25 60 | 18.80 58.60 | 38.00 75.11 | 54.00 80.50 | 84.40 85.76 | ÏTU=4g/L, other conditions are unchanged |
Table 5 Effect of different temperature and thiourea concentration on the immersion rate
Temperature / °C | Liquid to solid ratio | Remarks | ||
4 | 6 | 8 | ||
25 60 | 84.40 85.76 | 85.80 89.91 | 87.30 89.61 | The liquid to solid ratio is 10:1, other conditions are unchanged |
It can be seen from Table 4 and Table 5 that as the temperature increases, the leaching rate of silver increases correspondingly. Above 60 ° C, the temperature is further increased (to 90 ° C), and the recovery rate is very small, which is related to E. Acma et al. [4] The experimental reports are similar. According to the experimental and theoretical conclusions, the silver leaching rate increases little at high temperatures, which is related to the chemical reaction equilibrium.
Ag 2 S + 2Fe 3+ + 6TU → 2Ag(TU) 3 + + 2Fe 2+ + S 0 (3)
The chemical reaction equilibrium constant can be obtained:
(4)
Since the reaction is an endothermic reaction, the temperature rises, K increases, and the leaching rate increases. However, as the temperature increases, when the reaction reaches a certain level, the change of [Ag(TU) 3 + ] is not large. At the same time, the concentration of iron ions is limited, the change is small, and the growth rate of the leaching rate is decreased. Therefore, the temperature is increased. The leaching rate does not increase much.
(3) Relationship between reaction time and leaching rate
From the test results (see Figure 3), it can be seen that the longer the leaching time, the higher the leaching rate, the leaching rate is more than 85%, and the reaction time is increased. The leaching rate has increased. Not big, not even increasing. Therefore, the reaction time is generally taken to be 2 hours. The relationship between the leaching rate and the leaching time can be expressed by the "shrinking core model":
Where R is the leaching rate, t is the time, and k is the diffusion rate coefficient.
The conclusion of this conclusion is not detailed here because of the limited content.
When the temperature is constant, k is certain, t and In a straight line relationship, as shown in Fig. 3, this model shows that the solid film diffusion rate during the leaching process is the determined speed of the entire leaching reaction, and therefore plays a decisive role in the leaching reaction time.
(4) The influence of pH value and iron ion concentration
Figure 4 shows the leaching rate at different pH values. The leaching test conditions are: ÏTU = 6g/L, the liquid-solid ratio is 10:1, [Fe 3+ ] = 0.0157mol/L, the leaching time is 2h, and the temperature is At 60 ° C, when the pH is between 3.0 and 6.0, the leaching rate decreases. When the pH is between 3.0 and 1.5, the leaching effect is very good. At pH=2, the leaching rate reaches the highest, but at pH <1.0, the leaching rate does not increase.
This can be explained by the stability of thiourea in different acidity media and the activity of iron ions at different acidities. Generally, thiourea tends to be stable with increasing acidity, and the stable value is pH=1.78-2.0. When pH>2.0, high concentration thiourea is easily hydrolyzed, and its consumption increases. When pH<1.78, it is Easy to oxidize and decompose into bismuth disulfide. Therefore, it is preferred to use a dilute acid for the leaching reaction to maintain the leachate pH between 1.5 and 2.0. On the other hand, when Fe 3+ is pH>2.7, it is easy to be hydrolyzed to form Fe(OH) 3 precipitate. At pH<2.0, Fe 3+ is completely free. At this time, the activity is the largest and the oxidation ability is the strongest. Table 6 The redox potential of the reaction solution at different pH values. Here, since the iron content of the reaction sample is 19.59% in the form of Fe 2 O 3 , the experiment has been satisfied, and no additional addition is required. Therefore, the influence of the change in the concentration of iron ions is not considered, and it can be seen from Table 6 that oxidation is also observed. When the reduction potential is about 200 mV, the leaching rate is the highest, and at this time, the pH is 1.7.
Table 6 Relationship between pH value, potential and leaching rate of pulp
pH value | Potential / mV | Leach rate /% |
5.6 2.5 1.7 1.0 | 50 151 194 240 | 53.88 84.57 89.91 89.55 |
(5) Silver replacement
The replacement of silver is carried out under acidic conditions. When the leaching solution is used for 5 cycles, the concentration of silver ions in the liquid is increased. At this time, the pH value can be adjusted to 4.0 to 5.0, and the silver in the leaching solution is replaced with 1 g of zinc powder. The complexation constant of zinc ion and thiourea is only 1.77 [3] . Therefore, the presence of a large amount of zinc does not affect the leaching of silver and the replacement of silver. The amount of zinc powder is more than 10 times the theoretical amount, mainly because other impurity ions such as Cu 2+ and Fe 3+ consume zinc during the replacement process.
Third, the conclusion
In the leaching reaction, the main influencing factors are slurry concentration, leaching agent concentration, reaction temperature, reaction time, and acidity pH. The optimum conditions for extracting silver from thiourea by wet smelting zinc leaching residue are: liquid-solid ratio of 10:1, TU concentration of 6g/L, reaction temperature of 40-60 ° C, reaction time of 2 h, pH value For 1.5 to 2.5, iron ions are supplied from the material itself. Under this condition, the leaching rate of silver is higher than 89%.
references
1 Chen Zhifei, Shen Xiangyu, Ning Shunming, et al. Zinc indium practical metallurgy. Changsha: Central South University of Technology Press, 1996. 259~260
2 Yu Bingquan. Recovery of silver during wet zinc smelting. Proceedings of the 4th National Gold and Silver Election Conference. Kunming: Yunnan Science and Technology Press, 1993.182~183
3 Editorial Committee of Leaching Technology. Leaching technology. Beijing: Atomic Energy Press, 1994.215
4 Acma E, Arslan F, Wuth W. Silver extraction from a refractory type ore by thiourea leaching. Hydrometallurgy, 1993, 34: 263-274
SILVER RECOVERY WITH THIOUREA SYSTEM FROM ACIDIC
LEACHING RESIDUES OF ZINC HYDROMETALLURGY
Huang Kaiguo Hu Tianjue
ABSTRACT
This paper describes the process of silver extraction with acid thiourea solution from acidic leaching residues in zinc hydrometallurgy. The factors, which affect the process of leaching, such as pulp density, thiourea concentration, reacting time, and pH, are tested and discussed. The best optional is obtained, and the leaching rate reaches approximately 89%.
Key words zinc leaching residues;thiourea;silver;extraction
Originally published in the Journal of Central South University of Technology, December 1998, Vol. 29, No. 6, ☺
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