I. Leaching mechanism of ferric chloride
The hydrometallurgical leaching and dissolution, can be divided into three types: simple dissolution of a soluble compound in water; chemical dissolution becomes invaluable; valuable variant of electrochemical dissolution, the ore is dissolved bismuth chloride salt Is an electrochemical dissolution reaction:
Bi 2 S 3 +6FeCl 3 =2BiCl 3 +6FeCl 2 +3S
According to the galvanic theory, the above oxidation-reduction reaction can be decomposed into two half-cell reactions. There are anode and cathode regions on the surface of the strontium ore, and the oxidation-reduction reaction of electrons is lost and obtained in each zone. The mechanism is shown in Fig. 1. Shown. From Figure 1, the anode reaction is known as:
Bi 2 S 3 â†’2Bi 3 + +3S+6e
The generated elemental sulfur covers the surface of the granules of the strontium ore, loose and porous, does not affect the migration of the solvent from the solution to the surface of the ore particles, and does not affect the diffusion of the product from the surface of the ore particles into the solution.
Figure 1 Schematic diagram of FeCl 3 leaching mechanism
In the cathode reaction zone, since ferric iron can form a complex with chloride ions, the following four parallel reduction reactions exist:
During the immersion process, the concentration of chloride ions in the leachate continues to decrease over time, depending on the rate of diffusion of chloride ions toward the surface of the leached mineral, V D . On the one hand, it depends on the speed V K of the chemical reaction of chloride ions with the impregnated mineral. When V "V D , the reaction rate depends on the chemical reaction link, that is, the reaction proceeds in the kinetic region: when V D "V K , the reaction speed depends on the diffusion link, that is, the reaction proceeds in the diffusion region; and when V K = At V D , the reaction establishes a new balance.
The main factors affecting chlorination leaching:
(1) Influence of the particle size of the immersed mineral: The smaller the particle size, the larger the specific surface area, and the more favorable the leaching rate. The degree of grinding of the immersed minerals is determined by economic factors such as grinding energy consumption.
(2) Effect of leaching temperature: The dissolution rate increases with the leaching temperature, and the effects of temperature vary due to differences in various mineral structures. If the bismuth concentrate (naturally enthalpy of Bi 2 S 3 and Bi 2 O 3 concentrate) is leached with FeCl 3 solution, leaching for 4 hours at room temperature (leaching agent containing Fe 3 + 30 g / liter, HCl 20 g / Li, liquid-solid ratio 4), the leaching rate of bismuth can reach 80% to 90%.
(3) Effect of stirring speed: The purpose of leaching stirring is to reduce the thickness of the diffusion layer. The thickness of the diffusion layer and the stirring speed can be approximated by the following formula:
Wherein the thickness of the Î´-diffusion layer;
The n-index is generally 0.6.
Stirring does not eliminate the diffusion layer wrapped around the ore particles, because the saturated solution layer close to the crystal has strong adhesion to the crystal, but the high-speed eddy current generated by the agitation will take away most of the diffusion layer. When the stirring speed reaches a certain level, increasing the stirring speed does not accelerate the diffusion speed. The optimum agitation speed for processing various materials can be determined experimentally.
(IV) Effect of solvent concentration: The concentration of solvent has a great influence on the reaction rate and dissolution. When the solvent concentration increases, the dissolution rate and solubility increase. However, when the solvent concentration is too high, a large amount of impurities will enter the solution. The most reasonable solvent concentration should be to dissolve the valuable component to be extracted quickly and in large quantities, and the impurities enter the solution as little as possible. The optimum solvent concentration must be determined experimentally.
Ferric chloride is an oxidizing agent. At a suitable temperature and concentration, ferric ions can effectively dissolve the valuable metals in the impregnated minerals, and the suspended sulfides can be oxidized to oxidize the sulfur to elemental sulfur.
The advantages of using ferric chloride leaching are:
(1) Ferric chloride has high solubility in aqueous solution and high stability, and it is difficult to form insoluble complexes of jarosite.
(2) With proper oxidation potential, the sulfur in the metal sulfide can be precipitated in the form of elemental sulfur, eliminating the pollution of SO 2 gas:
(3) Selective leaching of metals: According to the experiment, under certain conditions, the order of leaching with FeCl 3 is as follows:
â†’ molybdenite ore pyrite brass â†’ â†’ â†’ sphalerite galena Chalcocite â†’ â†’ pyrrhotite, this order is the basis for selective leaching. The difference in physicochemical properties of various metal chlorides can be utilized to separate the metal elements in the leachate, such as PbCl 2 and AgCl to form complex ions, which have low solubility in water, and BiCl 3 is a strongly hydrolyzable compound.
(4) It can be leached under normal pressure.
(5) The ferric chloride can be regenerated by electrowinning or chlorine oxidation.
The disadvantage of leaching with ferric chloride is that the amount of iron in the leachate is large, which makes it difficult to separate and purify the leachate, and since ferric chloride is a strong oxidant, the anticorrosion problem of the leaching equipment must also be solved.
When the sputum mine is leached with hydrochloric acid-ferric chloride solution, various forms of strontium in the middle ore are all in solution in the form of chloride. The main reaction is:
The hydrochloric acid added during the leaching process, in addition to the action of Bi 2 O 3 , mainly maintains the acidity of the solution to prevent BiCl 3 from being hydrolyzed into BiOCl precipitate.
At the end of the leaching, the leaching residue containing 0.5% of bismuth, part of ferric ion leach solution remaining in this case if the iron powder is directly replaced, the remaining Fe 2 + only not been fully utilized, but will increase the iron powder consumption. Therefore, it is necessary to transfer the leached supernatant into another leaching tank, add a new samarium ore, and reduce the Fe 3 + in the solution to Fe 2 + by using cerium in the middle ore to avoid the replacement of iron powder in the next process. Increase the consumption of iron powder due to the presence of Fe 3 + :
Second, the mechanism of iron powder replacement sinking
The composition of the solution after the smelting of the sorghum mine by ferric chloride and the reduction of the sorghum mine is shown in Table 1.
Table 1 Liquid composition (g/L) after leaching and reduction in the middle of the mine
Displacement deposition is the use of base metals to replace the more positively charged metals from the solution. The base metal itself enters the solution, and the valuable metals are displaced from the solution.
According to the electrochemical order of the metal, in the acidic solution, the highly active iron atom is easily oxidized, and the less active trivalent europium ion is replaced by the metal ruthenium deposition, and the reaction formula is:
2BiCl 3 +3Fe=2Bi+3FeCl 2
The dissolution of the metal and deposition from the solution are determined by the opposing forces of the dissolved voltage and the osmotic pressure of the solution ions.
In the system of iron-trivalent iron ion solution and strontium-trivalent strontium ion solution, the dissolution of iron and the deposition of bismuth are carried out under the influence of the osmotic pressure and the dissolved voltage, forming a potential difference (near the electrode). The difference in charge between the electric double layer and the entire solution). The dissolved voltage of iron is greater than the osmotic pressure, the difference is negative, so iron has a negative potential; and the dissolved voltage of cerium is less than the osmotic pressure, the difference is positive, so é“‹ has positive polarity.
As the reaction of ruthenium in the iron powder displacement solution proceeds, the concentration of iron ions in the solution increases, and the concentration of ruthenium ions in the solution decreases. The negative potential of iron decreases due to an increase in ion osmotic pressure; and the positive potential of helium decreases due to a decrease in ion osmotic pressure. When the two potential values â€‹â€‹and symbols are the same, the reaction stops.
The relationship between the electrode potential of a metal and its ion activity in an actual solution can be expressed by the Nernst formula:
Wherein Ï†- is an electrode potential that is compatible with a given ion concentration in the solution;
Î¦Â°-standard electric æ“ž potential, Fe 2 + âˆ•Fe 3 + =0.771 volts;
The valence of n-metal;
A-ion activity, gram equivalent/liter.
The values â€‹â€‹of the standard potential Ï†Â° at 25 Â° C for various major metals and anions associated with wet extraction are shown in Table 2.
Table 2 Standard potential
The rate of displacement reaction, the rate at which electrons are transferred from one electrode to the other, is related to the electrode potential. The larger the phase difference between the metal potentials, the larger the potential difference, the faster the replacement speed; the smaller the phase difference between the metal potentials, the smaller the potential difference, and the slower the replacement speed. Therefore, a pair of plates with the largest potential difference should be selected for displacement deposition.
The following aspects must be noted when performing displacement deposition:
(a) The metal used for replacement or reduction should constitute a soluble compound with an anion bound to the displaced metal.
(2) The metal to be added for reduction in a solid state should be appropriately excessively because the displacement reaction is carried out on the metal surface of the reducing agent, and the larger the surface area, the faster and more complete the reaction proceeds. Therefore, the finer the metal reducing agent is ground, the better the reduction effect after the solution is added.
(3) Stirring must be carried out to remove the displaced metal deposited on the surface of the metal reducing agent to ensure that the fresh surface continues to participate in the reaction.
According to the standard potentiometer, it can be determined which metal can replace another metal from the solution, determine the electromotive force of the displacement reaction, calculate the equilibrium constant of the reaction, and determine the completeness of the displacement deposition.
The use of iron powder as a displacer for depositing ruthenium from a solution is exemplified.
Replace the hydrazine in the solution with iron as follows:
Let the concentration of bismuth and iron ions in the solution be 1 mole ion/liter, and determine the trend of the reaction of the battery composed of Feâˆ•Fe 2 + â€–Bi 3 + /Bi. The standard potential of the reaction is:
The maximum function of the reaction is calculated as follows:
Î”ZÂ° is a negative value, indicating that the reaction proceeds to the right, and iron can cause the ruthenium in the solution to be displaced and the reaction proceeds toward the formation of the sponge.
Third, ferric chloride regeneration mechanism
Chlorination leaching must take into account the recovery of chlorinating agents, which is important for both economic benefits and environmental protection. The regeneration of ferric chloride is generally carried out by a chlorine gas oxidation method and a diaphragm electrowinning method.
(1) Chlorine gas oxidation method. After the iron powder replacement and the reduction of the new bismuth ore, the iron mainly exists in the form of FeCl 2 . Before returning to leaching, the FeCl 2 must be oxidized to the ferric chloride by the chlorine gas. The reaction is:
2FeCl 2 +Cl 2 =2FeCl 3
In order to improve the efficiency of chlorine gas absorption, chlorine-passing equipment often uses several sealed steel tanks connected in series, and the exhaust gas is absorbed by water and then emptied. The regenerated ferric chloride solution is returned to the leaching.
(2) Diaphragm electrowinning method. This method regenerates ferric chloride while producing sponge crucible, and the solution is closed in a closed circuit during electrowinning, and no waste liquid is discharged.
According to the principle of bimetal electrowinning, the solution after reduction of ferric chloride and the reduction of the ore in the neodymium is used for electrowinning using a graphite plate as a cathode and an anode. The electrochemical reaction at the cathode is:
The electrochemical reaction of the anode is:
The outermost layer of the chlorine atom has seven electrons. When chlorine is present as a simple substance, in order to satisfy the typical non-polar covalent bond, a diatomic gas molecule is formed, requiring eight electrons in the outer layer of the atom, shared by two atoms. A pair of electronics. The electronic structure of a chlorine atom makes it have a large electron affinity and tends to obtain electrons from the outside, becoming a negative monovalent ion, namely:
It is difficult to lose chlorine by the chlorine atom, so chlorine is not easy to form a positive valence ion, especially a high-priced positive ion.
Compared with some other diatomic gases, chlorine is easier to dissociate. The dissociation energy of different gas molecules is shown in Table 3.
Table 3 Dissociation energy of diatomic gases (Joules per mole)
It is known that it has a large chemical reactivity from the dissociation energy of chlorine gas. When ions are formed in a chemical reaction, a large amount of energy can still be generated. which is:
Since chlorine atoms tend to acquire electrons, and metals tend to lose the valence electrons of the outer layer, so:
However, the standard potential of Fe 3 + +e=Fe 2 + is 0.771 volts, and the standard potential of Bi 3 + +3e=Bi is 0.215 volts. If ferric ions enter the cathode region, electrons are first reduced than trivalent strontium ions. Divalent iron ions. For this reason, it is necessary to use a microporous plastic plate as a separator between the anode and the anode to prevent the generated Fe 3 + from entering the cathode discharge, and preventing the sponge crucible from entering the anode region and re-dissolving after contact with the ferric ion.
Fourth, the melting mechanism of sponge enamel
The melting of the sponge is essentially a regular arrangement of helium atoms in the crystal, which loses equilibrium after being heated and transitions to a completely irregular amorphous state.
During the melting phase, when the sponge is heated, the kinetic energy of the atom increases. When the temperature reaches the melting point, it is necessary to continue to absorb heat, so that when the sponge é“‹ crystal lattice is broken, the gravity of the anti-antigen is work. Only when the sponge mash is completely melted, the temperature of the metal sputum rises again under the condition of continuing heating.
From the kinetic point of view, the sponge é“‹ melts and merges into the liquid, and must also overcome the resistance of the outer oxide film, requiring the molten mash to merge quickly, requiring the liquid mash to be quickly separated from the solid oxide film. In fact, the sponge é“‹Melting is carried out in molten sodium hydroxide.
The melting point of NaOH is 318.4 Â° C, the density is 2.13 g / cm 3 , and the sponge is melted by adding molten NaOH, which has the following effects:
(1) Separating the contact between the sponge and the air to prevent the sponge from oxidizing.
(2) The molten metal droplets sink due to the high density, and the oxide film on the surface of the sponge is absorbed by the NaOH to form a solid slag floating up, which is rapidly separated from the mash.
(3) Some impurities in the sponge mash metal oxide enters the scum, which improves the grade of sputum.
(4) Dechlorination. NaOH forms a sodium salt with residual chloride ions in the sponge.
The molten sputum ingot is stored as a raw material for pyrometallurgical refining.
5. Behavior of impurities during leaching of iron trioxide
(1) The behavior of sulfur. Suizhong Mine contains between 8% and 20% sulfur, mainly in the form of metal sulfides. During the leaching process, most of the sulfur in the sulfides is oxidized to elemental sulfur.
Only a very small amount of sulfur is oxidized to sulfate:
Therefore, the sulfur in the sulfide is leached into the slag in the form of elemental sulfur, and is separated from the leachate.
(b) The behavior of arsenic . The arsenic in the samarium ore is mainly in the form of toxic sand (FeAsS) and orpiment (As 2 S 3 ). In the leaching of ferric chloride, the arsenic minerals are not chlorinated and remain in the leaching slag.
(3) The behavior of tin . The tin in the Yuzhong mine is mainly in the state of stellite (SnO 2 ). In the leaching of ferric chloride, the cassiterite does not leaching and remains in the leaching slag.
(4) The behavior of lead. Lead is mainly present in the form of PbS in the strontium ore. In the leaching of ferric chloride and hydrochloric acid, lead is chlorinated to PbCl 2 , and the reaction is:
With normal temperature leaching, the solubility of PbCl 2 in the solution is only about 1%, and it is easy to precipitate and separate from the solution.
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