In the past 10 years, a large number of studies on the electrode process of thiourea to dissolve gold by electrochemical manpower (as shown in the figure) have deepened the understanding of the process of dissolving gold in thiourea, and have been confirmed and applied in practice.
1) Anodizing of thiourea The anodization of thiourea has been studied on inert platinum , graphite and lead electrodes. Through chromatographic analysis and infrared spectroscopy, the oxidation products in the anolyte were dithiocarbamate and cyanamide (CNNH 2 ) over the entire range of potential 0.9-2.1V. Through measurement and calculation, the reversible electricity generated by this platinum electrode has its standard reduction potential E Ө = 0.42V, the anode mobility coefficient β is about 0.8, and the exchange current density i 0 = (2.1~14.7) × 10 -6 A. /cm 2 . It is found that when the potential rises to 1.4V, the electrode process only produces primary oxidation of thiourea to dithiocarbamidine. At this time, the current efficiency is 100%, then the potential is increased, the current efficiency is decreased, oxygen begins to precipitate and the thiourea is secondary. The oxidation reaction takes place and the dithiomethane is decomposed to form cyanamide.
[next]
2) The anode of gold is dissolved in a degassed acidic solution of 0.001-0.030 mol of thiourea. Studies using a rotating gold disk electrode show that the dissolution of gold is fast until the anode overpotential is 0.3 V (see above). And almost reaches the maximum diffusion control speed, and the reaction is Au+2SC(NH 2 ) 2 - →Au[SC(NH 2 ) 2 ] 2 + +e -
The exchange current density is greater than 10 -6 A/cm 2 and the dissolution is carried out at a current efficiency of 100%. When the anode overpotential is 0.4V or higher, the thiourea is oxidized to dithiocarbamate and other sulfur-containing compounds, and the dissolution of gold is locally suppressed, and the gold dissolution current efficiency is remarkably lowered. The passivation of the gold surface at higher anode potentials results in a hysteresis loop in the current-potential curve. As the inhibitor is enriched in this system, the peak current on the curve decreases further, which affects the above formula. The value of the standard reduction potential E Ө . The test found that the value of E Ө on the surface of fresh gold was 0.352 V (30 ° C), but the E value on the passivated surface rose to 0.41 V. This change in the E value, even if the gold electrode is not dominated by the anode potential, will occur when the inhibitor in the solution is gradually increased, in which case the E 0 value obtained is 0.336-0.395V.
3) Cathodic reduction of gold (I)-thiourea Many studies on the electrode process of acid thiourea solution confirmed that gold (I)-thiourea was carried out according to the above formula, and the cathode overpotential was between -0.15 and 0.35 V. Diffusion control, when this value is exceeded, the reduction is slightly slowed down. At the top of the cathode current-potential curve (see above), the depositional activation energy Ee of gold is 25~29kJ·mol -1 , while the Ea value of the depressed part is 71~84 kJ·mol -1 , 30 °C The diffusion coefficient D of the Au[SC(NH 2 ) 2 ] 2 + complex ion is 1.1 × 10 -5 cm 2 · s -1 , and D is 1.17 × 10 -5 cm 2 · s -1 at 50 °C. The deposition of gold is carried out at 100% current efficiency. When the cathode overpotential is at -0.5~0.6V, the current efficiency drops significantly, and gold deposition and hydrogen precipitation occur simultaneously. Although thiourea itself does not intervene in the cathodic reaction on platinum, gold and iron , dithiocarbamate is continuously reduced on the newly deposited gold surface, which quickly causes passivation of the gold surface, and the gold plate is slightly black. In view of the anodizing of thiourea, it is not difficult to understand that the anode chamber and the cathode chamber of the electrolytic cell should be separated during electrolysis to avoid the combination of sulfur on the gold electrode, and the oxidant generated during the anode reaction is prevented from causing gold to some extent. Dissolved. [next]
The electrolyte composition used was (g/L): SC(NH 2 ) 2 90, H 2 SO 4 25 , Au 0.5-5. The maximum value obtained on the polarization curve obeys the diffusion law. This is because the thiourea decomposition product adsorbed on the electrode has a catalytic effect on the electrowinning process of gold at a potential of +0.2~-0.1V. . At this point, stirring the electrolyte will strongly affect the current density. The maximum value of the current is linear with the gold concentration. The cathode process speed has a lower temperature coefficient A=2.51-2.93kJ/mol, which indicates the gold product. Under this condition, it mainly depends on the diffusion of the discharge ions. However, when the potential is more negative than -0.1V, the current minimum is actually independent of the rotational speed of the electrode, and the cathode process speed has a higher temperature coefficient A=7.12kJ/mol. Obviously, the cathode process is mainly determined by the gold electrode. reaction. In order to examine the adsorption-catalytic properties of the thiourea decomposition product, the polarization curve obtained by comparing the electrolyte with or without the decomposition product (such as Na 2 S), and by desorbing the adsorbate from the surface of the cathode, or the cathode The surface was tested by a special mechanical device for continuous updating. The results confirmed that in the process of gold accumulating, the sulfur-containing product decomposed by thiourea did have adsorption-catalysis in the range of potential 0.2-0.1V. characteristic.
In addition, the comparison of the standard decomposition potential values ​​of gold (I)-thiourea complexes with other metals (such as silver , lead, zinc , cadmium , copper ) thiourea complexes on the silver electrode shows that the gold energy on the cathode First precipitated.
The standard decomposition potential E values ​​(in the table below) for some metal thiourea complexes are as follows:
 Standard Decomposition Potential E Value of Some Metal Thiourea Complexes | ||||||
Thiourea complex ion | Au ( TU ) 2 + | Ag ( TU ) 2 + | Cu ( TU ) 2 + | Cd ( TU ) 2 + | Pb ( TU ) 2 + | Zn ( TU ) 2 + |
Standard decomposition potential E/V | 0.38 | 0.023 | -0.119 | -0.657 ( -0.647 ) | -0.65 | -0.785 |
The main sections in this process are: Soybean Dry Method Extruding , Soybean Crushing , Soybean Meal Cooling , Soybean Smashing , soybean dehulling, etc.
Applications:
1. The product can be used as high-energy and high-protein feed additive.
2. It is also widely used for production of fodders for egg chickens, broiler chickens, baby pigs and aquatic livestock. It is able to significantly increase growth rate.
We also provide Oils And Fats Project , Low Temperature and Edible Soybean Meal Project, Soy Protein Concentrate Project ( Spc Project ), Soy Protein Isolated Project (SPI project), Full Fat Soybean Meal Project , Chili Capsanthin Project , Stainless Steel Pressure Vessel, etc.
Full Fat Soybean Powder Project
Full Fat Soybean Powder Project,Full Fat Soybean Meal Project,Soybean Dry Method Extruding,Soybean Crushing,Soybean Meal Cooling,Soybean Smashing
Shandong ChemSta Machinery Manufacturing Co.,Ltd. , https://www.oil-proteinmachine.com