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Due to the very active uranium elements and the metallogenic conditions, the uranium in the uranium ore is in a chemical state. The requirements for leaching conditions of these uranium-containing minerals vary widely. The dissolution and leaching of UO 3 can be carried out as long as there is sufficient acidity or sufficient concentration of carbonate ions; and the dissolution and leaching of UO 2 is difficult, and it is generally necessary to first oxidize a low-valent compound such as UO 2 in the ore to a high price. State before the dissolution leaching reaction with the leachant. The form of uranium in the liquid phase is related to the pH value, the concentration of the leaching agent ion, and the like. In order to improve the heap leaching rate, understand the leaching conditions, and the presence and transformation conditions of various uranium compounds in the solution, the uranium water potential-pH map can be utilized. The potential-pH diagram is carried out by plotting the equipotential and equipotential potential-pH relationship lines representing certain chemical reactions in the graphs of the potential as the ordinate and the pH as the abscissa. Therefore, the chemical reaction equation should be listed first; then the equations of the potential, pH, and material activity of the reaction are written, and the relevant data are calculated, and the corresponding straight line is drawn according to the data. As shown in Figure 1 and Figure 2. The chemical reactions and their thermodynamic relationships in uranium heap leaching are as follows:
Figure 1 U-H 2 O system potential-pH diagram
Figure 2 Au, Zn-H 2 O system potential-pH diagram
UO 3 +2H + UO 2 2 + +H 2 O
pH=7.38-0.5lg 1
UO 2 +4H + U 4 + +2H 2 O
pH=0.95-0.25lg 2
3UO 2 2 + +2H 2 O+2e U 3 O 8 +4H +
E=-0.403+0.12pH+0.089lg 3
UO 2 (OH) 2 +2H + UO 2 2 + +2H 2 O
pH=2.49-0.5lg 4
UO 2 2 + +2e UO 2
E=0.221+0.0295lg 5
UO 2 2 + +4H+2e U 4 + +2H 2 O
E=0.333-0.12pH+0.0295lg 6
Fe3++e Fe2+
E=0.77+0.0591lg 7
O 2 +4H + +4e 2H 2 O
E=1.23+0.0148lg -0.0591pH 8
MnO 2 +4H + +2e Mn 2 + +2H 2 O
E=1.23-0.118pH-0.0295lg 9
According to the actual situation of uranium heap leaching, the concentration of uranium in the leaching solution is 50-1000 mg/L, and the average is 500 mg/L, that is, aUO 2 2 + is about 2× 10-3 mol/L. The concentration of sulfuric acid averages 40 g/L, that is, aH + is about 4 x 10 -1 mol/L. The amount of manganese dioxide added is generally 4 to 5 kg/m 3 , the concentration of Mn 2 + is about 2.7 g/L, and the concentration of a Mn 2 + is about 5 × 10 -2 mol/L. Take the equilibrium potential of Fe 3 + /Fe 2 + with an activity ratio of 1. By substituting these data into various formulas, the uranium water potential-pH diagram can be plotted, as shown in Fig. 1.
From Figure 1, we can get the following biased conclusions:
First, comparing the line 135 in the figure, the conditions for the conversion of UO 3 , U 3 O 8 and UO 2 present in the ore into UO 2 2 + are not the same. The dissolution of UO 3 is only related to the pH value. UO 2 is first oxidized to be converted, while U 3 O 8 not only requires the potential to reach a certain value, but also requires the pH to reach a certain value before it can be converted into UO 2 2 + .
Second, the area consisting of the marking line 156, the cup is concerned with the thermodynamic conditions of UO 2 2 + stable stop, that is, the basic conditions necessary for dissolution and leaching. The pH value should not be higher than 3.8, the lower the pH value, the more stable UO 2 2 + ; when the potential is higher than 500mV, UO 2 2 + can be stably existed; when the potential is lower than 500mV, the UO 2 2 - and U 4 + in the solution The ratio varies with pH and potential. Therefore, in the heap leaching, if the pH of the leaching is lower than 2 and the potential is higher than 500 mV, the thermodynamic conditions in which UO 2 2 + is stably present are satisfied.
Third, in order to meet the potential conditions of UO 2 2 + stable existence, there must be sufficient oxidation environment in the heap leaching. Comparing line 789, it can be seen that both MnO 2 , Fe 3 + and O 2 can oxidize uranium in UO 2 to hexavalent uranium. However, considering the price of the reagent, the oxidation of tetravalent uranium is suitably carried out by Fe 3 + (line 7). Moreover, modern studies have shown that if there is no iron ion in the solution, MnO 2 and the like will not effectively oxidize UO 2 . The oxygen in the air is slower than the oxygen ratio of tetravalent uranium at normal pressure. The potential value characterized by the vertical distance between the reticle 7 and the reticle 5 is the driving force for the Fe 3 + oxidized UO 2 . The combined response of 7 and 5 is
2Fe 3 + +UO 2 UO 2 2 + +2Fe 2 +
Comprehensive 7 and 5
â–³=0.77-0.22+0.0295lg
According to the law of mass action, â–³E=0 at equilibrium, thus obtaining
=1×10 -18.6
Then K= =1×10 18.6
The equilibrium constant K of the reaction is large, indicating that the driving force of the reaction is large. The key issue is how to oxidize Fe 2 + in solution to Fe 3 + . In stirred leaching, pyrolusite is generally added to accelerate the leaching process. Heap leaching can add pyrolusite when the ore is broken to accelerate the oxidation of low-valent uranium in the ore. However, most uranium ore heaps are immersed in the so-called "self-breathing process", that is, by solution discontinuity and contact with ore, air and solution alternately enter the capillary cracks of the ore, and the oxygen in the air is used to oxidize the low-priced uranium minerals in the ore.
4. It can be seen from lines 1 and 1 that the dissolution and leaching of UO 3 in the ore is relatively easy, as long as the acidity is sufficient, it can be converted into UO 2 2 + . This is easy to achieve with agitation leaching because the agitation leaching liquid has a large solid ratio; while in heap leaching, especially in the early stage of heap leaching, it is not easy to maintain the acidity of the leaching solution, so UO 2 which has been leached in the upper part of the heap is often 2 + , at the bottom of the pile, as the acid of the solution is continuously consumed, the pH rises, and UO 2 2 + is hydrolyzed into UO 2 (OH) 2 · 2H 2 O to precipitate.
Fifth, in FIG. 1 between the UO 2 and U 3 O 8 and U 3 O 8 and the two lines A and B between 3 UO involves conversion between these solid phase material, with little to hydrometallurgical.
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