Is a goethite iron oxyhydroxide, iron oxyhydroxide called [alpha] type α-FeO (OH). There are four kinds of hydroxy iron oxide homomorphisms in nature, and the other three are: tetragonal pyrite β-FeO(OH), fibrite γ-FeO(OH) and hexagonal fibrite δ-FeO(OH) . Goethite is the most common iron oxyhydroxide mineral in nature, reflecting its stability under weathering conditions. In fact, it is usually the product of the weathering of iron-bearing sulfide ore, oxidized ore, carbonate and silicate in nature. Studies have shown that goethite is the most likely product of high-iron hydrolysis in the range of pH 1.5-3.5 at the boiling point of atmospheric pressure and the total concentration of sulfate in the range of 3 mol ∕L. Most goethite contains other elements in the form of solid solutions.
Goethite may also be seen to type [alpha] - water iron oxide α-Fe 2 O 3 · H 2 O, which is structurally the same as diaspore ore, is orthorhombic. In the crystal structure of goethite, there are only Fe 3 + , O 2 - and OH - 3 ions, and the mixing ratio of the three is 1:1:1. Where O 2 - is at the apex of the octahedron, and Fe 3 + is at the center of the octahedron and is surrounded by O 2 - . O 2 - ions and ions Fe 3 + 4 phase coupling, i.e. common for between four octahedra, wherein each valence bond only 1/2 price. OH - ion is used in total between two octahedrons, and each valence bond is also 1/2. The high-iron ions located in the center of the octahedron have strong polarization, which causes the outer electron cloud of the surrounding coordination ions to shift, causing the electron clouds of the outer and negative ions to overlap and form a covalent bond. Since O 2 - is more susceptible to deformation than OH - , the coordinating oxygen ion will have a stronger covalent bond than the coordinating hydroxide ion, that is, the polarity of the bond is weak.
Thermodynamic calculations indicate that goethite has a larger lattice energy than trihydrate iron oxide, indicating that goethite is more stable than the latter. Therefore, under normal conditions (less acidity and temperature not higher than 140 ° C), the thermodynamically stable structure of the high-iron hydrolysate should be goethite rather than colloidal ferric hydroxide. However, in practice, when the high-speed iron is precipitated from the aqueous solution by the neutralization method, the precipitate obtained is a colloidal iron oxide trihydrate rather than a crystalline goethite. The main reason for this is that pH has a great influence on the degree of supersaturation of high iron in the solution. Therefore, when the hydrolysis is neutralized, the high super-iron supersaturation occurs with the increase of the pH of the solution, forming a large nucleation rate. The hydrolyzed product is precipitated as a limb. In view of the fact that it is difficult to control the supersaturation of the system due to the neutralization and hydrolysis of high-iron solution, it is necessary to avoid the precipitation of iron hydroxide in the rubber. The key is to control the high-iron concentration in the solution to a very low level, generally less than 1kg·m - 3 . The goethite method is proposed for this problem. It adopts the hydrolysis conditions of air oxidation, low supersaturation and higher temperature, which is beneficial to the dehydration and condensation of hydrates, and also facilitates the orderly arrangement of the relevant particles, so that the hydrolyzed products are crystal rather than limbs. The goethite method has two modes to control high iron concentration. One is to first reduce the high-iron ions in the solution to a low price, and then neutralize to a pH of 4.5 to 5. At this time, because the high-iron concentration is very low, colloidal iron hydroxide is not precipitated, and ferrous ions are at this pH. No Fe(OH) 2 precipitate formed under the value. Then, the ferrous iron is reoxidized into high iron at a temperature of about 90 ° C, and a small amount of high iron ions are hydrolyzed to form a small amount of crystal nuclei, and slowly develop into goethite crystals and precipitate. The relevant reaction equation is :
(1)
High-iron reducing agents can have many options, but the reducing agents used in production should be inexpensive, easy to handle, and do not introduce any hazards after oxidation. From this practical standpoint, the best reducing agent is zinc sulfide concentrates purified zinc sulfate electrolyte goethite process. As a result of reducing the high iron by zinc sulfide, the zinc in the ZnS enters the solution as Zn 2 + ions, and the sulfur remains in the slag as a solid form of elemental sulfur, without any harm to the subsequent operation. The total reaction formula of zinc sulfide to reduce high iron is:
(2)
Thermodynamic calculations show that the standard electromotive force of this redox reaction is 0.506V, which has sufficient thermodynamic driving force. Practice has shown that the reaction speed is also relatively high, generally only 3 to 4 hours at 90 ° C temperature can reach a considerable reduction depth. For example, the equilibrium constant obtained from the standard electromotive force of the reaction formula (2) is Kc = [Fe 2 + ] 2 [Zn 2 + ] ∕ [Fe 3 + ] 2 = 10 17.09 , and the activity of the zinc ion is 0.1. For mol∕L, [Fa 2 + ]∕[Fe 3 + ]≈10 9 is obtained , indicating that zinc sulfide makes the reduction of high iron more thorough.
The reoxidation of ferrous iron in the goethite method uses oxygen in the air as the oxidant. The oxidation reaction equation is:
(3)
The standard oxidation potential of air at a temperature of 25 ° C is E = 1.22 - 0.059 pH. At pH = 4, the standard potential of oxygen is 0.984 V, and only the standard potential of the Fe 3 + ∕Fe 2 + pair (0.771 V) is 0.213 V higher. However, since Fe 3 + has been pre-reduced to Fe 2 + at this time, the actual potential E of this pair Greatly reduced. For example, when Fe 3 + /Fe 2 + =10 -4 , E Dropped to 0.538 V, the potential of the oxidation reaction (9) was increased to 0.316V. At the same time, in the hydrolyzed iron system, the high-iron high-mass produced by oxidation is hydrolyzed and precipitated immediately, so that [Fe 3 + ]/[Fe 2 + ] in the system can always be kept at a lower value.
Ferrous oxide precipitation includes two successive steps of ferrous oxidation and high iron hydrolysis. The process of oxygen ferrous oxide includes the dissolution of oxygen, the diffusion of oxygen molecules from the phase interface to the interior of the solution, the adsorption of ferrous ions to oxygen molecules, the cleavage of oxygen molecules into oxygen atoms, and the electronic exchange between ferrous ions and oxygen atoms. Wait for multiple steps. The cracking of oxygen molecules into oxygen atoms is a key step in controlling the speed. Increasing the rate of oxygen molecular cleavage reaction can be carried out in three ways: by increasing the oxygen partial pressure, such as using an oxygen-enriched blast and using compressed air and maintaining the entire reaction process at a higher pressure to increase the temperature; using catalysis, generally with Cu 2 + as a catalyst.
After the adsorbed oxygen molecules are converted into adsorbed oxygen atoms, electron transfer between the oxygen atoms and the ferrous ions occurs, and as a result, the ferrous ions are oxidized to high iron ions, and the oxygen atoms are reduced to O 2 - ions. :
The other oxygen atom will also be reduced to the ion O 2 - in the same manner, and the formed O 2 - will strongly bind to the high iron to form a complex ion such as (Fe-O-Fe) 4 + . It then with OH - ions incorporated, and further dewatered synthesis, goethite is generated:
Another model for the control of high-iron concentration by the goethite method was developed by the Australian Electrolytic Zinc Company. Instead of reducing it first, it directly adds the hot high-iron solution together with the neutralizing agent to the sedimentation tank at a controlled rate to make the high-speed rail. The concentration is maintained at 1 kg·m -3 or less. At a temperature of 70 to 90 ° C and maintaining the pH at about 2.8, goethite is continuously precipitated with the addition of high iron. The relevant reactions are:
(4)
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