First, the anode reaction mechanism
More in-depth study of Wangcheng Yan, Qiu Ding-fan and so on bismuthinite were in the anode reaction slurry electrolysis process. Through a large number of experimental studies, it is believed that the anode leaching process of stibnite is a complex reaction process, and the anode leaching process of stibnite suspended in acidic sodium chloride medium can be achieved by the following ways:
(1) Graphite is equivalent to a conductor, and stibnite is equivalent to a soluble anode. When strontium ore collides with graphite anode, it will be oxidized by the following reaction:
(2) Other oxidation reactions may occur on the graphite electrode, such as the production of Cl 2 and O 2 gas, and some of the gases reoxidize the yttrium ore.
(3) Relevant tests show that the addition of iron ions in the leaching ferry, the leaching reaction rate of the stibnite is significantly improved, and the cell voltage is significantly reduced, indicating that iron ions are also involved in the anodic leaching process of the stibnite.
In order to find out the reaction mechanism of stibnite in the leaching process of slurry electrolysis anode, the i-E curve of stibnite and stibnite in solution and the addition of 4g ∕L of Fe 2 in the upper slag were tested. + After the i-E curve in the presence of and without stibnite, see Figure 1.
Figure 1 i-E curve under different conditions
1-HCl 1 mol ∕L+NaCl 200 g ∕L;
2-HCl (1 mol∕L)+NaCl (200 g∕L)+ stibnite (-0.074 mm, L:S=10:1);
3-HCl (1 mol ∕L) + NaCl (200 g ∕ L) + Fe 2 + (4 g ∕ L);
4-HCl (1mol∕L)+NaCl(200g∕L)+Fe 2 + (4g∕L)
+ stibnite (-0.074 mm, L:S = 10:1).
In the absence of stibnite and iron ions in the HCl-NaCl solution, the graphite anode may only have the following reactions:
(1)
E 333 (1 ) =1.177-0.066pH+0.0165lgPO 2
(2)
E 333 (2 ) =1.306=0.066lg[Cl - ]+0.0333lg[Cl 2 ]
Under the condition of slurry electrolysis, pH=0, pO 2 =0.2×10 5 Pa, [Cl - ]=3mol∕L, substituting the above two equations to obtain E 333 (1 ) =1.248V, E 333 (2 ) =1.255+0 .0333lg [Cl 2], since the solution of [Cl 2] is small, therefore, the difference E 333 (1) and E 333 (2) is not, the two reactions are likely to occur on the anode. Arslan and Duby studied the anodization of pyrite in sodium oxide solution. At an anode potential of 1.4-1.5 V (SCE), t=35-40 ° C, the concentration of HClO in the anolyte can reach 0.15 smol ∕L. It is believed that HClO is produced by Cl 2 precipitated on the anode, and the oxidation reaction of water on the anode also occurs simultaneously and shares part of the charge transport. When Arslan studied the anodization of pyrite with a graphite anode, it was found that CO 2 was formed on the anode and anodic alteration occurred. Wang Chengyan and Qiu Dingfan also found that the graphite anode was altered in the slurry electrolysis expansion test. These can also prove that during the pulp electrolysis process, when the anode potential is high, simultaneous precipitation of Cl 2 and O 2 can occur on the anode.
For the reaction
Considering that iron ions can form iron-chloride complexes in solution, the actual potential will be lower (as shown in Figure 2, line 23). Therefore, when iron ions are present in the part, the above reaction may be the main anode. reaction.
Fig. 2 Bi 2 S 3 -Cl - -H 2 O system E-lg[Cl - ]
In Fig. 1, line 1 is the i-E curve measured in the feldspar-free, iron-free ionized liquid, and the current can only be generated by the reaction formulas (1) and (2), and the current magnitude should indicate The speed of the reaction. It can be seen from the figure that when the anode potential is higher than ~1.10 V (SCE), the current rises sharply, and below this potential, the anode current is extremely low and the fluctuation is small. Therefore, it can be considered that in the test solution, when the anode potential is higher than -1.10 V (SCE), a large amount of gas is precipitated on the anode of the stone, and this potential is in the vicinity of the theoretical precipitation potential of chlorine and oxygen.
Line 2 is the i-E curve measured in the strontium ore and iron-free ion solution. At this time, the current on the anode should be the oxidation reaction of the stibnite directly colliding with the electrode, and the chlorine gas and oxygen precipitation reaction. Comparing line 1 and line 2, in the range where the potential is lower than -1.10 V (SCE), the current can be considered to be due to direct electrooxidation of the stibnite on the graphite anode, which is much higher than the line 1 It is indicated that the direct electro-oxidation of stibnite can occur; the potential of the second line is more than -1.10V (SCE), and the chlorine evolution reaction plays a leading role.
Line 3 is the i-E curve measured in a solution without strontium ore and having ferrous ions. It can be seen from the figure that when the anode potential is higher than 0.5 V (SCE), the current is significantly increased. In reaction
It is near the standard potential, so it can be considered that this current is generated by the anodization of divalent iron ions. Under the condition of fixed current density less than 300A∕m 2 , the anode will not undergo chlorine evolution and oxygen evolution reaction. Only in the later stage of electrolysis, the oxidation of ferrous iron is nearing completion, and the chlorine evolution and oxygen evolution reaction may occur. Significantly rising.
Line 4 is the i-E curve measured in a solution with stibnite and having ferrous ions, which is larger than the current of line 3. The generation of this current can be thought of as anodization of divalent iron ions and contact oxidation of stibnite with an anode collision. However, line 4 is not a simple addition of line 2 and line 3, it is only slightly higher than line 3 and is similar to line 3, so it can be considered that the main reaction at this time is still the anodizing reaction of the divalent iron ion, and the stimulating Direct electrooxidation of the ore is secondary. Due to the presence of stibnite, the ferric iron formed on the anode reduces itself to divalent after oxidation of Bi 2 S 3 , and the divalent iron is anodized to trivalent. This is repeated until the oxidative leaching of the stibnite is nearly complete.
If the fixed current density is 200 A/m 2 , it can be seen from Fig. 1 that the anode potentials of line 2 and line 4 differ by about 0.7 V, that is, to obtain the same leaching reaction rate, in the presence of iron ions. In the stored solution, the anode potential is 0.7 V lower than the anode potential of the iron-free solution, and the corresponding cell voltage is also reduced by about 0.7 V, thereby reducing the power consumption of the electrolysis process.
Figure 3 is at a fixed current density of 200A ∕ m 2 , Fe 2 + is 4.0 g ∕ L, Cl - is 150 g ∕ L, H + is 1.0 g ∕ L, Bi 3 + is 10 g ∕ L, 100 g stibnite, particle size Graphite anode potential (SCE) and cell voltage as a function of time measured with <0.038 mm at 96% and L:S = 3:1.
Figure 3 Constant voltage cell voltage and anode potential change with time
Figure 3 illustrates that during the theoretical leaching electrolysis time of stibnite, the cell voltage is passively in the range of 0.8 to 0.9 V, and the anode potential fluctuates within the range of -0.5 to -0.6 V (SCE), which is in the divalent state. The standard oxidation potential of iron ions is nearby. It can be considered that the anode reaction during this time is mainly the oxidation reaction of divalent iron ions, and the leaching of the antimony concentrate is mainly due to the oxidation of ferric iron.
After the theoretical leaching electrolysis time of bismuth, both the cell voltage and the anodic potential rise sharply, the cell voltage rises to 1.6-1.8V, and the anode potential fluctuates around -1.2V (SCE). At this time, the leaching enthalpy of the stibnite is nearly complete. Almost all of the ferrous iron is oxidized to ferric iron, and the anode begins to produce a chlorine evolution reaction. The cell voltage also increases with the increase of the anode potential and the polarization of the cathode.
From the above analysis, the following conclusions can be drawn:
(1) Under the conditions used in the test, when there is no iron ion in the solution, the anode reaction is mainly in the range of -0.2V to -1.0V, and the anode reaction is mainly direct oxidation of the stibnite on the graphite anode, when the anode When the potential is greater than -1.10V, the chlorine evolution reaction plays a leading role.
(2) In the presence of iron ions, the main reaction occurring on the anode is the oxidation reaction of divalent iron ions. The oxidation of stibnite can be considered to be completed by ferric ions, and the ferric iron is originally The divalent, ferrous iron is oxidized again on the graphite anode, so that it is repeated. Of course, there is also a collisional contact oxidation of the stibnite with the anode throughout the leaching process.
(3) In the presence of iron ions, the anode potential can be reduced by about 0.7 V compared to the anode potential without iron ions, which is advantageous for reducing power consumption.
Second, the leaching reaction mechanism
The thermodynamic analysis of Figure 3 shows that the complex acid dissolution reaction of stibnite
Can occur under test conditions. Tests have shown that the reaction rate is slower in the absence of oxidant.
Wang Chengyan and Qiu Dingfan studied the oxidative leaching mechanism of stibnite during slurry electrolysis. It is believed that the oxidation of stibnite can be achieved by several different reaction processes.
(3)
(4)
(5)
The reaction formula (3) is a direct wall contact oxidation of the stibnite with the anode. The reaction formula (4) is direct contact oxidation of ferric iron with stibnite. The reaction formula (5) is that the stilbite is first subjected to complex acid decomposition reaction to form strontium sulfide, and the oxidant ferric chloride is mainly redox with hydrogen sulfide. The difference between the formula (4) and the formula (5) is here. Macroscopically, the mechanism of the leaching reaction of the stibnite can be determined by means of the analysis of the phase structure of the process leaching slag.
In general, elemental sulfur in the sulphide phase transformation products hydrometallurgical process. At leaching below the melting point of sulfur (386K), elemental sulfur is usually embedded in three forms (Fig. 4): (a) loosely porous around the sulfide ore; (b) densely packed in the form of dense fine particles around the sulfide ore. (c) The fine-grained monomer is distributed in the proposed slag and has nothing to do with the sulfide ore itself. The former is a structure in which a metal cation diffuses into a solution and remains; the latter two are structures in which a sulfide ore is first oxidized by acid decomposition to form H 2 S; whether it is (b) or (c) depends on the leaching process. Many influencing factors. The state of inclusion of elemental sulfur in the leach residue is directly related to the interpretation of the leaching process.
Figure 4 Several embedded forms of elemental sulfur
The observation of the slag sampling in the leaching process of the stibnite mine showed that the stibnite changed little when leaching for 15 minutes. At this time, there was a small amount of fine granular monomeric element sulfur in the slag, which was distributed in the leaching slag. . When the leaching time reaches 30 min, some stibnite boundaries have been eroded; the amount of elemental sulfur is slightly increased compared with the former, and it is basically present as fine-grained monomers. When the leaching time reaches 60min, the dissolution of stibnite is more obvious, the zigzag boundary is visible at any time, the elemental sulfur is mostly monomeric, and a few are finely granulated on the boundary of stibnite particles. At 90 min, the elemental sulfur attached to the boundary of the uranium ore is more common, and the particle size is obviously increased. The stibnite of the fine particles is not easily found in the slag. The leaching time is up to 130min. The sulfur beads around the stibnite mine are more and more, and almost become a sulfur bead ring. At the same time, the monomeric sulfur beads in the slag are also obviously increased. The residual stibnite has no change with the leaching time. Very obvious.
Based on the above analysis, it can be considered that the leaching reaction of stibnite in the actual pulp electrolysis process is not a simple diffusion process of sulfide metal cations. From the case where there is a large amount of fine elemental sulfur which is not densely packed with sulfide in the leaching residue, it is not a residue of metal ions diffused into the solution in the sulfide, but a re-formed product. . That is to say, there must be a sulfur-forming reaction during the leaching process of the stibnite, and there must be an acid decomposition reaction of the stibnite. According to the change of minerals in the leaching residue, the following reaction can be considered in the acid leaching process.
With bismuthinite decomposition of the reaction are ongoing to sulfur; with the increase of the generation amount of H 2 S, the reaction of trivalent iron and H 2 S portion of the solution, the elemental sulfur yield in disseminated bismuthinite Around, part of the H 2 S reacts with the ferric ore particles to react with ferric iron to form a single piece of sulfur beads.
After the theoretical leaching electrolysis time, the leaching of the stibnite is nearly complete, and the ferrous iron is almost completely converted into ferric iron, and the chlorine evolution reaction begins to occur.
From this, we can draw the following conclusions:
(1) In the anode leaching process, the yttrium ore is first subjected to acid decomposition reaction.
(2) The trivalent iron formed by the anode is mainly redoxed with H 2 S formed by the decomposition of stibnite, and the redox with direct contact with the stibnite is secondary.
(3) The phase analysis of the leaching slag shows that the formation of elemental sulfur is not a simple metal cation diffusion process product, but an oxidation product of hydrogen sulfide. Therefore, in the actual acid leaching process, there is both an acid dissolution reaction of sulfide ore and a sulfur-forming reaction; most of the sulfur produced is fine-grained monomer, and a small amount is adsorbed around the stibnite.
Zhang Yingjie started from the double-electrode structure of the solid-liquid interface in the electrolyte solution and the mechanical movement of the ore particles, and derived the factors affecting the anode reaction rate (current density) under a certain superpotential (the anodic chlorine reaction has not yet occurred), and the anode current density was obtained. (i) It has a linear relationship with the slurry concentration (Cs) and the square of the stirring speed (N R 2 ), regardless of the grain size. Further calculate the total surface area of ​​the ore particles attached to the 1 cm 2 anode surface at any instant:
S 0 =3C s /Ï
Wherein the total surface area of ​​the S 0 - ore particles;
Ρ-mineral density, g∕cm 3 ;
C s - slurry concentration, g ∕ mL.
According to this calculation, if C s =0.lg/mL and Ï (stibnite) = 6.4 g ∕ cm 3 , then S 0 = 0.046. That is to say, when the slurry contains Fe 2 + at the same time, only 0.046 cm 2 of the area on the surface of the anode of 1 cm 2 is subjected to collision contact oxidation of the mineral with the anode, and the remaining area is the oxidation of Fe 2 + . This is a good explanation for the fact that in the presence of Fe 2 + in the presence of Fe 2 + , the collisional contact oxidation of stibnite with the anode is not dominant.
3. Anodizing kinetics of Fe 2 +
In the process of pulp electrolysis, the iron ions in the solution play an important role. It directly participates in the electrode reaction of the anode and the oxidative leaching of the stibnite, which plays an electron transfer role. Therefore, it is necessary to study the anodization process of Fe 2 + . Wang Chengyan and Qiu Dingfan measured the polarization curves of Fe 2 + on graphite anodes and clarified the rate control process of Fe 2 + anodization.
Test conditions: 333K, NH 4 Cl is 200g ∕L, H + is lg / L, stirring speed 600r ∕ min, scanning speed 1mV / s, measured FeCl 2 concentration is 0.01, 0.02, 0.03, 0.04, 0.05mol ∕L The lower anodic polarization curve takes the current density i at the same η value as the η-lgi relationship diagram, as shown in Fig. 5.
It can be seen from Fig. 5 that η is between 60 and 10 mV, and the curve shows a significant Tafel segment, indicating that in this overpotential range, the Fe 2 + anodization process is controlled by the electrochemical reaction; when η is in the 100~ Between 18mV, η is linear with {lg(i∕i 0 )+lg[i d /(i d -i)]}, as shown in Figure 6, indicating the Fe 2 + anodization process in this overpotential range. It is a mixed reaction control; when η is between 160 and 220 mV, η has a linear relationship with lg[i d /(i d -i)], as shown in Fig. 7, indicating that Fe 2 + anodizing in this overpotential range The process is controlled by diffusion.
Figure 5 η-lgi relationship diagram for different FeCl 2 concentrations
Figure 6 η-lg(i∕i 0 )+lg[i d /(i d -i)]
Figure 7 η-lg[i d /(i d -i)] relationship diagram
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