Not only iron corrosion, mineral deposits and hydrometallurgical important role in both, and Fe-H 2 O system combines all four types of chemical reactions in the aqueous solution, this system is most suitable for the leaching forth Thermodynamics The basic principles and practical steps of processing.
The first step is to identify the substances that need to be considered and list their free energy of generation, as shown in the table below.
Table Fe-H 2 O system, the formation of free energy of each substance
substance | Fe 2 + | Fe 3 + | Fe(OH) 3 | Fe 2 O 3 | Fe 3 O 4 | Fe 0.947 O | H 2 O(1) | OH - |
△G Θ ∕kJ | -78.90 | -4.7 | -696.5 | -742.2 | -1015.4 | -245.12 | -237.129 | -157.244 |
In the table, Fe 2 + , Fe 3 + and OH - are simple hydrated ions; Fe(OH) 3 is precipitated solid crystal; Fe 2 O 3 is hematite; Fe 3 O 4 is magnetite; Fe 0.947 O is Square iron (for convenience, its molecular formula can be replaced by FeO); H 2 O is liquid water.
1. Reactions involving dissolved substances and solids without redox
When iron oxide is in contact with pure water, some degree of dissolution occurs:
(1)
The equilibrium constant of the reaction (the activity of pure iron oxide and pure liquid water is 1)
Since the iron oxide is only slightly soluble, the activity coefficients of the above two ions are all 1, the equilibrium constant is related to the solubility product K SO , and the subscript "SO" indicates that the iron ions and the hydroxyl ions are not coordinated in the solution. Ions.
The standard free energy of the reaction is
(2)
Obtained by equation (2)
Solubility product
(3)
This value refers to the equilibrium of iron ions and hydroxide ions with solid phase hematite α-Fe 2 O 3 , and the experimental determination of the solubility product is carried out using precipitated iron hydroxide. The nature of the precipitated iron hydroxide is determined by the conditions of the precipitation and the resulting solid is usually amorphous. The free energy change of the following reaction was calculated from the ΔG Θ data of the crystalline Fe(OH) 3 given in the previous table.
At 25 ° C lgK = -38.55
This is the solubility product of the well-crystallized Fe(OH) 3 on a hot knife.
The experimentally determined values ​​of Fe 3 + and ligand OH - lgK SO vary with the measurer. The solid phase was determined to be α-Fe 2 O 3 with an lgK SO value of -42.7. The reported values ​​for the solids of the unidentified crystal form are: -36.35, -37.7 to -39.2, -37.50, -39.43. All data were corrected for infinitely dilute solutions. The comparison between the calculation of hematite and Fe(OH) 3 and the experimental results actually shows that the amount of iron remaining in the solution after precipitation of most of the iron as an oxide or hydroxide in the solution depends on the solid phase balanced with the solution. The essence.
It can be seen from the solubility product equation (4) of α-Fe 2 O 3
(4)
(5)
Therefore, the solubility product of hematite in water is related to pH, and formula (3) actually represents the following reaction.
2. Redox reactions involving no hydrogen ions
In a chemical reaction, one substance is oxidized and the other substance is reduced. But for convenience, here is a reaction divided into two parts. One part provides electronics and the other part accepts electronics. There are two types it relates to Fe 3 + and Fe 2 + Fe ions and reaction of the metal is important in the Fe-H 2 O system. One is to cause a balance from the electrode potential, such as
The other type is the redox balance between ions in solution.
The writing convention for the redox reaction formula is that the left side of the equal sign is the oxidation state, and the right side of the equal sign is the reduced state.
Among them, Red and Ox represent a reduced state and an oxidized state, respectively. For a single ion electrode, the activity of the metal is 1, so for the case of the z-valent metal M
In the case of an anion A - , an activity of 1 is an oxidized substance, thus
For the reaction
Let △G e Θ =0, then
Since the reaction involves two electrons, z=2, the Faraday constant F is equal to 96.487 kJ ∕ (V·mol), so
Similarly, for the reaction
Therefore, the activities of Fe 2 + and Fe 3 + are equal at 0.769V.
Third, the balance between solids
Solids considered in the iron system are Fe, Fe(OH) 3 (cr), Fe 2 O 3 , Fe 3 O 4 and Fe 0.947 O. The stable zone of all these solids except Fe(OH) 3 (cr) can be determined, while the stable zone of Fe(OH) 3 (cr) is consistent with the stable zone of hematite, except that the boundary line moves slightly. The reason is that the free energy ΔG Θ of the two solids is different.
FeO-Fe
Iron oxidation to galena can be written as follows:
The above formula is written in such a form as if the left side represents the reactants and the right side represents the product, but the writing convention of controlling the redox reaction formula as described above writes the oxidation state to the left of the equal sign and the reduced state to the right of the equal sign. However, since the reaction considered is occurring in water, it is more convenient to react this reaction with a reaction that represents water balance:
Thereby getting
The above formula reflects that the reaction to form an oxide is related to pH.
The standard free energy change of the above reaction can be calculated to obtain the standard potential
When the reactant activity is not complete, the potential of the reaction is given by
(6)
Fe 3 O 4 -Fe
The reaction can be written as
(7)
Can also be obtained from the standard free energy change
(8)
Fe 3 O 4 -FeO
The reaction formula is
(9)
(10)
Fe 2 O 3 -Fe 3 O 4
(11)
(12)
Fe 2 O 3 -FeO
(13)
(14)
Fourth, the balance between ions and solids
Fe-Fe 2 +
(15)
(16)
Fe 2 + -FeO
(17)
This is not a redox reaction and does not involve electron transfer. Iron is both Fe II in both solid and hydrated ions.
(18)
Fe 3 O 4 -Fe 2 +
(19)
(20)
Fe 2 O 3 -Fe 2 +
(twenty one)
(twenty two)
Five, the balance between ions
The only ion-to-ion balance in this system is
(twenty three)
6. Drawing of the dominant area map - balanced representation
The chemical reactions and equilibrium equations of the Fe-H 2 O system at an absolute temperature of 298.15 K are discussed above. All balances are expressed by pH and redox potential. Therefore, it is convenient to draw the dominant region map with pH and E for x and y coordinates, respectively. Generally, the pH range is from 0 to 14, and E is from -1.0 to +1.0 V. It is sufficient to expand as needed.
For convenience, the map of the dominant area can be systematically performed from the lower left corner, that is, from the place with the highest acidity and the most reductive. First, the data needed to draw each balance line is summarized as follows:
Fe-FeO
Formula (6), E = -0.041 at pH = 0; E = -0.663 V at pH = 10.
Fe-Fe 3 O 4
Formula (8), E = -0.0867 at pH = 0; E = -0.678V at pH = 10.
FeO-Fe 3 O 4
Formula (10), E = -0.222 at pH = 0; E = -0.814V at pH = 10.
Fe 3 O 4 -Fe 2 O 3
Formula (12), E=0.214 at pH=0; E=-0.378V at pH=10.
FeO-Fe 2 O 3
Formula (14), E = 0.0725 at pH = 0; E = -0.519V at pH = 10.
Fe-Fe 2 +
Formula (16), the equilibrium line is plotted for the activity of iron-containing materials (Fe 2 + and Fe 3 + ) 10 -6 . |Fe 2 + |=10 -6 E=-0.587V, this value is independent of pH.
FeO-Fe 2 +
Formula (18), |Fe 2 + |=10 -6 , pH=9.21, which is the solubility product equation, independent of E.
Fe 3 O 4 -Fe 2 +
Formula (20), |Fe 2 + |=10 -6 , E=1.413-0.237pH,
E = 1.413 at pH = 0; E = -0.957 V at pH = 10.
Fe 2 O 3 -Fe 3 +
Formula (21), |Fe 3 + |=10 -6 , E=1.413-0.237pH,
E = 1.413 at pH = 0; E = -0.957 V at pH = 10.
Fe 2 O 3 -Fe 3 +
Formula (5), |Fe 3 + |=10 -6 , pH=1.38.
Fe 3 + -Fe 2 +
When (Fe), |Fe 3 + |=|Fe 2 + |=10 -6 , E Θ =0.769V.
The above data is plotted in Figure 1 as a map of the dominant regions of the Fe-H 2 O system.
Figure 1 Fe-H 2 O dominant area map
(298K, Fe ion activity is 10 -6 )
VII. Analysis of the dominant area map
In the Fe-H 2 O system, the most reductive solid is metallic iron, so it predominates in any region below the oxide formed by its oxidation. The boundary line of Fe-Fe 3 O 4 is at a more negative potential than the boundary of Fe-FeO. Since metallic iron is outside the stable limit of water, it is thermodynamically unstable in aqueous solution, and produces Fe 3 O 4 instead of galena from water. The Fe 2 + ions are generated when the metal iron is oxidized in the acid-like region of the figure, and their boundary lines are also outside the stable region of the water, and the iron is dissolved in the acidic water to dissolve. Fe(II) is oxidized to Fe(III) in a lower pH solution and only at a higher potential, ie under stronger oxidation conditions. Therefore, the Fe 3 + -containing solution has a strong oxidizing property and can be used in hydrometallurgy to oxidize some of the lower valence metals in the ore to be soluble, such as oxidizing U 4 + to U 6 + . In this leaching system, Fe 3 + is reduced to Fe 2 + , and the E value of the latent liquid can be measured by a redox electrode. When the E value of the solution may be too soft adding additional oxidizing agent such as MnO 2 or Mn ore sodium chlorate NaClO 3 will re-oxidation of Fe 2 + Fe 3 +, to improve the E-value of the solution. Solutions containing high iron salts are also used to oxidize certain metal sulfides, and the dominant zone map of the sulfur-containing system helps to understand the behavior of such leaching systems.
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