A phase transition to a thermodynamically stable phase will reduce the free energy of the entire system. The generation of the new phase changes the phase number of the system from one phase to two phases. On the one hand, some atoms change from high free energy (old phase) to low free energy (new phase), which reduces the internal free energy of the system; on the other hand, the formation of the new phase requires energy, which increases the freedom of the system. can. Thus, the change in free energy of a new phase (such as the crystal of yellow iron sputum) can be expressed by the following formula:
△ F = -V △ f v Sσ
Where V is the volume of the crystallization new phase;
△ f v —— the difference between the free energy of the old phase and the new phase in the unit volume, △ f v = F liquid- F solid ;
S - the surface area of ​​the new phase;
σ —— The surface tension between the old and new phases at the interface of the unit phase, that is, the surface energy between the two phases.
Assuming the new phase is spherical, the above formula can be rewritten as:
4
△ F = - —— πr 3 n Δfv + 4 πr 2 n σ
3
Where r is the radius of the spherical grain;
n - the number of particles produced by the new phase.
As can be seen from the left figure, the change in ΔF depends on the size of the new phase particles. When the radius of the grain is smaller than the critical particle radius r k , the total free energy of the system increases; when the radius is greater than r k , the opposite is true; when the radius is equal to r 0 , the increase of ΔF is equal to zero, indicating that the formation of the new phase is formed. The interface surface energy can offset the decrease of the free energy of some atoms from the liquid phase to the lower solid phase of free energy. When the radius is greater than r 0 , the increase of free energy is negative, indicating the free energy in the whole system. Reduced. It can be seen that after the crystallization starts, there may be many crystal grains, but there are certain restrictions, and only those crystal grains which increase in particle size due to fluctuations and the like to cause a decrease in the free energy of the system can be grown, that is, the radius is larger than r. The crystal grains of k can become crystal nuclei. The figure below shows that additional seeding can greatly speed up the sinking.
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the behavior of other ions during precipitation of jarosite f jarosite process iron is mainly used for wet factors zinc refining, so the impact of zinc precipitate is first to be considered in addition. When examining the influence of zinc, it was found that even if the solution contained 100 g/dm 3 of Zn 2+ , the critical pH of the precipitation transition from crystal to amorphous was almost unchanged, that is, without Zn 2+ , the critical pH JA was:
pH JA = 0.211lg[Fe 3+ ]+1.84
After adding 100 g/dm 3 of Zn 2+ (ZnSO 4 added):
pH JA = 0.21log[ Fe 3+ ]+1.80
It is indicated that zinc can be regarded as an inert substance during the process of sedimentation. However, in the industry, it is considered that if the zinc concentration is too high, the viscosity of the solution is increased, which is unfavorable to the operation.
B. Application of yellow iron smelting method in zinc smelting a Huangshui iron slag removal method In the wet zinc smelting, the application of jatropha iron removal method is the most widely used. The use of several plants listed here is as follows.
The Reston Electric Zinc Plant in Australia places the residue in the leaching plant (as shown below). The leaching slag (concentration of 800-1000g/dm 3 ) from the zinc electrolysis system and the accumulated leaching slag (with the pre-heated to 75 ° C waste electrolyte) enter the leaching tank and leaching at 85-95 ° C 7h. The residue after leaching is classified by a hydrocyclone, and the zinc-rich slag (ZnS 80%) is separated from the lead- rich slag. Further, the leachate was neutralized to pH = 0.90 (15 g (H 2 S0 4 ) / dm 3 ), and then 25% aqueous ammonia was added to the solution. At the same time as adding calcine, it was maintained at pH=1.3-1.7 for 4.5 h to form a yellow iron sputum to remove most of the iron.
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The Norwegian Zinc Company incorporates the leach residue treatment process into the leaching system as shown on the left in the figure below. Neutral leaching is included throughout the system to dissolve 80% of the soluble zinc in the calcine while a small amount of iron and other impurities precipitate to produce a neutral solution. The separated residue was subjected to hot acid leaching at a temperature of 90 to 95 ° C and an acidity of 40 to 80 g (H 2 S0 4 )/dm 3 to dissolve the residue zinc. The solubility of different calcinations is different, so it is necessary to add concentrated sulfuric acid to control the appropriate acidity to achieve the highest extraction rate. The insoluble lead and silver residue are neutral, and 10% to 12% of the calcine is added during the leaching process. The iron in the zinc-containing and iron solution remaining after the separation is precipitated in the form of pyrite.
Ma Rongjun of Changsha Research Institute of Mining and Metallurgy has developed a wet combined process for the effective recovery of zinc and antimony for China's sorghum high-iron zinc mine resources. The key process in the process is the hot acid leaching of the calcined material - the removal of iron from the yellow iron. When the low-acid leachate is iron-plated by the ferro-dragon method, the antimony is first enriched in the iron slag, and then the indium is recovered from the slag, and a part of zinc and sodium are recovered, thereby increasing the total recovery of zinc and reducing the sodium reagent. Consumption. The law of indium and iron entering the yellow iron sputum in the multi-system in which indium and iron coexist is studied. When iron is precipitated with yellow iron, sodium (or ammonium) indium iron oxide crystals are formed, and its thermodynamic properties are similar to those of yellow iron. When calcined at 530-590 ° C, most of the iron is ferric oxide and indium is still double salt of sulfuric acid, and indium is easily leached by dilute acid. 1983 is a smelter of this process has been tested in Liuzhou non-ferrous metals industry, in September 1985 completed the process industry zinc test. The principle flow used in industrial trials is shown on the right as shown in the figure above.
Liuzhou Nonferrous Metals Smelter has carried out industrial production for many years with this process, and has obtained very good benefits, filling the gap of China's iron shovel method. At present, several wet zinc smelting plants in China have used the hot acid leaching method to produce zinc by wet method. They all draw on the experience of this achievement and occupy an important position in China's zinc smelting industry.
b Low-polluting jarosite method The existing wet-zinc smelting plant widely uses conventional pyrite method to remove iron, but some valuable metals are still lost in the iron slag.
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