The free energy of the formation of the material and the free energy of the reaction are related to the temperature. Therefore, the dominant area map of the system with a temperature significantly different from 298.15K is required, and the free energy ΔG T Θ of the substance participating in the reaction at the degree of diffusion is required. The free energy change ΔG Θ value of each equilibrium reaction is calculated, and the relevant thermodynamic data in the high temperature aqueous solution system can be estimated for drawing the dominant area map.
I. Advantage map of S-H 2 O system under 423K
(1) Free energy data
As in the system at 298 K, only S Θ , H 2 S(aq), HS - (aq), S 2 - (aq), SO 4 2 - (aq) and HSO 4 - are thermodynamically stable. The components. The free energy data required to plot the system is listed in the table below.
Table 1 Free energy of each substance in S-H 2 O system 423K
substance | S 2 - | HS - | H 2 S(aq) | SO 4 2 - | HSO 4 - | H 2 O(I) | H + | H 2 | O 2 |
△G 423 Θ ∕kJ | -4.60 | 10.23 | -48.9 | -736.9 | -770.6 | -247.63 | -3.20 | -17.01 | -26.45 |
In view of the sulfide from at S 2 - the C p Θ and S Θ now uncertain, but it does not appear on the advantages of 298K zone map. This ion is not considered here.
(2) Balance equation
Similar to the previous ones, the equilibrium equations involved here are numbered separately and correspond to the numbers marked on the corresponding balance lines on the dominant area map, but here is the full battery equation.
(III) Analysis of the dominant area map
The dominant region at 423K is shown in Figure 1 with various sulfur-containing substances and H 2 S(aq) activity of 10 -1 mol∕L. Compared with 298K, there are two points worth noting. First, HSO 4 at higher temperatures - the predominant higher pH region extending direction - than the SO 4 2. Second, the stable zone of 423K elemental sulfur is much smaller than at 298K. The equilibrium formula (f) representing the boundary between HS - and S is not important even at 10 -1 mol ∕L at 298 K, but the advantage of sulfur at higher temperatures and lower sulfur-containing activity. The zone is determined by the equilibrium formula (c): HSO 4 - -S, equilibrium formula (d): SO 4 2 - -S and equilibrium formula (e): H 2 S(aq)-S. At a certain temperature, as the activity of HSO 4 - or SO 4 2 - decreases, the equilibrium E value falls to a lower value. However, as the H 2 S(aq) activity decreases, the equilibrium E value increases. At a certain temperature and activity of a group of dissolved materials, the elemental sulfur is thermodynamically unstable when the H 2 S(aq)-S line rises above the SO 4 2 - or HSO 4 - line. The ratio of HSO 4 - to SO 4 2 - is limited by the pH of the solution, and the activity of total sulfate and H 2 S (aq) can be independently regulated.
Figure 1 S-H 2 O dominant area map
(423K, S and H 2 O activity is 10 -1 )
Second, the advantageous area map of Cu-S-H 2 O system under 423K
(1) Free energy data
The free energy data of Cu-S-H 2 O system at 423 K is shown in Table 1.
Free energy data of Cu-S-H 2 O system at 423K
substance | Cu | Cu + | Cu 2 + | CuO | Cu 2 O | HcuO 2 - | CuO 2 2 - | CuS | Cu 2 S |
△G 423 Θ ∕kJ | -4.72 | 39.76 | 71.54 | -136.4 | -159.3 | -252 | -156 | -63.03 | -103.4 |
substance | S | HS - | H 2 S(aq) | SO 4 2 - | HSO 4 - | H 2 O(1) | H + | H 2 + | |
△G 423 Θ ∕kJ | -4.60 | 10.23 | -48.9 | -736.9 | -770.6 | -247.63 | -3.20 | -17.01 |
(2) Balance equation
As mentioned earlier, a full cell reaction should be used at higher temperatures to get the correct results.
(III) Analysis of the dominant area map
Sulfur-containing ionic activity 10-1 containing copper ion activity 10 -3 mol / L drawn Cu-S-H 2 O system at 423K predominance area shown in Figure 2, in this figure HCuO 2 - is pH 12.91 is in equilibrium with CuO, but CuO 2 2 - does not appear.
Figure 2 Cu-S-H 2 O dominant area map
(423K, S and H 2 S activity is 10 -1 , Cu ion activity is 10 -3 )
Advantages of the system of FIG region 423K and 298K in the most important difference is that, at lower temperatures, | Cu + | lower = 10 -6 mol / L Cu + appears still to see, while in the 423K | Cu + | = When 10 -3 mol ∕ L, there is a dominant zone of Cu + . Cu + advantages region surrounded by four line balance: Cu 2 +, Cu 2 O , Cu metal and Cu 2 S. After an equilibrium line is HSO 4 - ion reduced to produce Cu 2 S. If the activity of the sulfur-containing ions in the system is very low, or its reduction is very slow, the upper and lower boundaries of the Cu + dominant region are Cu 2 + ions and Cu metal regions, respectively, and the right side is determined by the solubility product of Cu 2 O. The limiting pH for Cu + activities of 10 -2 , 10 -1 and 1 mol ∕ L was 2.15, 1.15 and 0.15, respectively. Whether or not the dominant region of Cu + appears on the graph depends entirely on Cu 2 + or Cu + formation when Cu metal is oxidized with increasing oxidation-reduction potential at a specific activity. At 298K, when Cu + |=|Cu + |=10 -6 mol∕L, Cu 2 + is formed at a lower E value, and when the activity is 10 -3 mol ∕L at 423 K, it is a Cu + .
It is customary to draw the dominant area maps to set the activity of all ions containing a particular element to be the same, but in reality this is not common. Take a look at the limit activity of Cu + at a given Cu 2 + activity for 423K. This can be calculated by setting the E values ​​of the reaction formulas (h) and (i) to be equal. then
0.340+0.0420lg|Cu 2 + |=0.406+0.0839lg|Cu + |
Lg|Cu 2 + |-2lg|Cu + |=1.57
From this
Lg|Cu 2 + | | -3 | -2 | -1 | 0 |
Lg|Cu + | | -2.29 | -1.79 | -1.29 | -0.79 |
|Cu + | | 5.13×10 -3 | 1.62×10 -2 | 5.13×10 -2 | 1.62×10 -1 |
Using lg|Cu 2 + | to plot lg|Cu + | to obtain a straight line with a slope of 0.5 intercept 1.57.
Similar calculations at 298K
Lg|Cu 2 + |-2lg|Cu + |=6.04
From this
Lg|Cu 2 + | | -3 | -2 | -1 | 0 |
Lg|Cu + | | -4.52 | -4.02 | -3.52 | -3.02 |
|Cu + | | 3.02×10 -5 | 9.56×10 -5 | 3.02×10 -4 | 9.56×10 -4 |
These calculations show that in a solution that is not strongly coordinated with cuprous ions, such as sulfuric acid solution, the maximum Cu + activity is only about 10 -3 when the copper sulfate activity reaches 1 mol ∕L. Since the activity coefficient γ ± of CuSO 4 is 0.062 in a 0.5 mol solution and 0.15 in a 0.1 mol solution, the Cu + activity achievable in the solution is actually much lower than this value. At 423 K, cuprous ions can be considered as a possible reactant.
The main difference between copper sulfides in the dominant regions of 298K and 423K is that CuS is stable only in solutions with a pH below 8 at 423 K, while the stable region of CuS extends to pH = 11 at 298 K. It is worth noting that at these two temperatures, Cu 2 S is formed by reduction of HSO 4 - , SO 4 2 - or CuS. CuS is formed by the reduction of Cu 2 S from SO 4 2 - or HSO 4 - to provide additional sulfide ions. However, it is well known that, like many other elements, the addition of sulfides, usually in the form of NaHS, precipitates out of solution. This reaction is reflected in this advantage map, where the sulfides are stable in the dominant regions of H 2 S(aq) and HS − . CuS can coexist with elemental sulfur and Cu 2 S cannot coexist with sulfur.
Third, the application of the advantage zone map
In view of the importance and typicality of copper, iron sulfide and mixed sulfides in mineral resources and its wet treatment, this paper takes Cu-Fe-S-H 2 O system as an example to discuss the advantage zone in wet Application in metallurgy.
Figure 3 is a diagram showing the dominant regions of the Cu-Fe-S-H 2 O system at a total pressure of 298 K and 1 atm. This map can help mineralogists understand the relationship between the oxidation zone and the secondary enrichment zone in the deposit, and also help hydrometallurgists understand the oxidative leaching process of copper-iron sulfide minerals. The map of the dominant area of ​​the Cu-Fe-S-H 2 O system is quite complex at first glance, with a little attention, that is, seeing clues. Variety of copper, copper - iron and an iron sulfide is quite clear from the E h -pH with the oxide of copper and iron are separated, the E h -pH E h with from about 0.3, pH = 0 impose each pH The slope of the unit 60mV extends to the right, and only the copper ore extends this boundary, which is consistent with the secondary enrichment of copper ore to produce a copper ore layer. It is worth noting that Cu 2 + and Fe 2 + are produced in solution when equilibrated under acidic conditions, and Cu 2 + and Fe 2 O 3 (or Fe 2 O 3 ·H 2 are produced at slightly lower acidity). O). The reason for the separation of iron and copper during oxidation is reflected. The dominant area map identifies copper minerals such as chalcopyrite, copper blue, porphyrite and chalcopyrite, as well as pH and potential zones stabilized by pyrrhotite, pyrite and elemental sulfur. From the perspective of hydrometallurgy, the figure can also be used to determine the nature of the aqueous solution that will decompose the minerals, and also to understand what new solid or gaseous products are formed during mineral decomposition. Of course, the advantage zone map does not tell us the speed of mineral decomposition.
Figure 3 Cu-Fe-S-H 2 O system advantage zone map
(298K, 1atm, dissolved total sulfur 10 -1 mol∕L)
It can be seen from the figure that sulfide minerals may decompose in the following four different solutions.
(1) An oxidizing solution. Sulfurized minerals may oxidize to form elemental sulfur or sulfate depending on the selected oxidation potential and pH.
(2) Strong acid solution. The sulfide mineral is liberated by acid to release H 2 S to dissolve copper and iron.
(3) Reducing solution. The sulfide mineral reduction fraction liberates H 2 S or sulfide ions as well as metal low-valent sulfides or metals.
(4) Strong alkaline solution. Sulfide minerals are decomposed by alkali to produce sulfide ions and metal oxides (or low-valent sulfides).
The above four solution systems are generally applicable to all metal sulfides, but strong acid solutions and strong alkali solutions sometimes require too high a strength, and the actual aqueous solution does not. The oxidizing solution and the reducing solution can be further divided into acidic and basic systems, respectively.
Taking chalcopyrite as an example, its oxidation in the acidic region (pH = 0) can be expressed by the following chemical formula:
or
The reaction (d) is a reaction for producing copper blue, which is derived from the stable elemental sulfur in the figure. This reaction was observed by anodization in a pressurized system at 130 ° C in the laboratory. The reaction (e) is observed in a hydrothermal system at 160-230 ° C and is patented for hydrometallurgical purposes and can be considered as an activation process prior to leaching of chalcopyrite. However, visible copper blue products have never been observed in any atmospheric or pressure leaching studies.
Reaction (f) oxidizes chalcopyrite at a low potential to produce a chalcopyrite. The reaction rate is too slow and cannot be observed under laboratory conditions. However, this reaction is very important in geology, which explains the presence of chalcopyrite in copper oxide ore. It may also be an important reaction in the heap where bacteria accelerate the oxidation of chemically slow sulfur.
Reaction (f) is a common reaction in laboratory leaching experiments, especially at high pH values ​​such as in ammonia leaching. In acidic oxidative leaching, this formula explains why the oxidation of sulfur is below 1% to more than 30% due to pH and oxidant. The equations (a)-(g) explained from the dominant zone diagram do not explain the dominant reaction observed in acidic oxidative leaching. This dominant reaction can be described by the reaction formula (h):
In fact, Cu 2 + is an oxidant that oxidizes sulfur, so the above formula is the only reaction found. The irreversible oxidation of sulfur to sulfate (at least in an acidic solution) is very difficult, and the reaction rate of the reaction (h) is very advantageous compared to the previous seven reactions.
A new map that is more useful for hydrometallurgy can be obtained by omitting the equilibrium line of the unrecognized reaction on the map of the dominant area (in the laboratory rather than in the geological sense). For example, the reactions (a) to (b) are omitted, that is, H 2 S cannot be supplied as a reactant, or pyrite cannot be produced at an appropriate rate, and the stable region of the chalcopyrite is expanded. At this time, the map still contains some reactions that can only be observed at very high temperatures or long geological times. These reactions can provide a more useful tool for hydrometallurgy.
Fourth, the limitations of the advantage zone map
The dominant area map is the most common and useful tool for summarizing the thermodynamic data of aqueous chemical systems. However, some of its limitations when drawing and applying it to the actual system also need to be considered.
The drawing of the dominant area map is limited by the adequacy of thermodynamic data. All possible substances in the system and the reactions between them should be considered. If one or more important substances are neglected, the result may be a completely misleading picture. For example, in the Fe-H 2 O dominance map, if the presence of Fe 2 + is neglected, the horizontal line separating the Fe 3 + and Fe 2 + regions disappears, and the line separating the Fe 3 + and Fe 2 O 3 regions extends to The boundary line of Fe 2 O 3 and Fe 3 O 4 then changes the slope to separate the Fe 3 + and Fe 3 O 4 regions.
The correctness of the map of the dominant area is also limited by how the thermodynamic data is selected. Some free energy data can be obtained from various standard sources, but for detailed studies, the data for solution components must be calculated from the stability constant of the reaction. The stability constant needs to be chosen very carefully based on reproducibility and similarity to the medium (ionic strength and electrolyte composition) of the system under study. In many cases the stability constant needs to be corrected for the ionic strength and the activity of the substances involved in the reaction, and these ionic strengths and activities may not be known. Therefore, the advantage zone map should be updated periodically based on new or more accurate data.
The free energy data of many solution materials at high temperatures is unknown. The most common way to solve a problem is to use the Crist-Cobble correspondence principle to estimate the entropy and indirectly obtain the free energy value. This method is considered feasible for four ions: simple cation simple anion, oxyanion and oxyacid anion. However, it does not apply to certain substances, such as coordination ions, uncharged substances, and multi-plant matter.
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