Under certain conditions of temperature and reducing atmosphere, during a metal-containing oxide minerals in the feedstock is converted into the corresponding metal oxide or metal Low called reduction roasting. Except that the oxides of mercury and silver are heated in air at a temperature lower than 400 ° C to resolve the metal, most metal oxides cannot be reduced by thermal decomposition, only the corresponding reducing agent is used. In order to restore it. The reduction of metal oxides can be expressed by the following formula:
MO+R=M+RO
△G°=△G° RO -△G° MO -△G° R
Where MO - metal oxide;
R, RO - reducing agent and reducing agent oxide.
The above formula can be synthesized by the reaction reaction of MO and RO:
The necessary condition for the reduction of the metal oxide (MO) by the reducing agent (R) is ΔG°<0, that is, P o2(RO) <P o2(MO) , therefore, the affinity for all oxygen is lower than that of the reduced metal. A substance having a large affinity for oxygen can be used as a reducing agent for the metal oxide. Figure 1 shows the standard free energy generation curves of some metal oxides at different temperatures. From the graph, most of the metals can be oxidized by oxygen under the calcination conditions. The oxides are stable and their stability rises with temperature. High and low, the lower the metal oxide in the figure, the more stable the metal oxide is, the more difficult it is to be reduced by the reducing agent; conversely, the higher the position of the metal oxide is, the easier it is to be reduced by the reducing agent.
A solid reducing agent, a gas reducing agent or a liquid reducing agent may be used for the reduction roasting. It can be seen from Fig. 1 that the free energy of carbon monoxide formation decreases remarkably with increasing temperature, so that carbon can be used as a reducing agent for many metal oxides under higher temperature conditions. [next]
The following reactions can occur when solid carbon is burned:
1) C+O 2 =CO 2 △G° 1 =-393.76～0.0008T kJ/m [er]
2) 2C+O 2 = 2CO
△G° S =-223.21-0.175T kJ/m 3)2CO+O 2 =2CO 2 △G 3 °=-564.8+0.173T kJ/m 4)CO 2 +C=2CO △G° 4 = △ G ° -T relationship -170.54-0.174T kJ / mole CO.'s system 2 shown in figure 2, the curve in FIG. (1), (2), (3) intersect at 978K, CO CO 2 relatively stable; When the temperature is lower than 978K, CO 2 is more stable than CO.
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Carbon, carbon monoxide and hydrogen are commonly used as reducing agents in production. The reaction of carbon monoxide to reduce metal chloride is called indirect reaction:
If no solution is formed between the metal and its oxide during the calcination, the equilibrium constant of the carbon monoxide-reduced metal oxide is:
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