New Technology And Mechanism Research On Purification And Recovery Of Calcium Components In Vanadium-bearing Steel Slag

Vanadium bearing steel slag was produced in the steel-making process of ferrovanadium-containing water.There were many treatment methods for ferrovanadium-containing water,but no matter what method was used,a considerable amount of residual vanadium would enter the semi steel to form vanadium bearing steel slag.Vanadium bearing steel slag had a wide range of sources and a large total amount.Although its vanadium content was very low,only 1%～4%（based on V₂O₅,the same below）,it was still higher than stone coal vanadium ore.It was also a valuable secondary resource for mining and metallurgy and could be used as an important raw material for vanadium extraction.However,the content of calcium component in vanadium bearing steel slag was very high,up to 40%～60%（based on CaO）.No matter what kind of vanadium extraction process,such a high content of calcium would greatly affect the index of vanadium extraction and increase the cost of vanadium extraction.Therefore,it was necessary to purify and recover calcium components before extracting vanadium from vanadium bearing steel slag,which was also the key to the successful extraction of vanadium from vanadium bearing steel slag.Firstly,from the point of view of process mineralogy,the properties of the sample（vanadium bearing steel slag of Chengde Iron and Steel Co.）were analyzed by modern testing methods:the content of CaO in the vanadium bearing steel slag sample was very high,up to42.76%,and the content of V₂O₅was very low,only 1.96%;Calcium in steel slag mainly existed in the form of calcium silicate（tricalcium silicate,dicalcium silicate）and calcium ferrite,which was the main object of purification and recovery;Vanadium mainly existed in calcium titanium oxide（the main object of vanadium extraction）,and a small part occurs in calcium silicate.Due to the low content of calcium titanium oxide,only a small part of vanadium in calcium silicate could be observed by energy spectrum detection,and calcium titanium oxide and vanadium in it could not be detected.According to the characteristics of vanadium bearing steel slag and the existing problems and shortcomings in the current purification and recovery process of calcium components in vanadium bearing steel slag,a new process of purification and recovery of calcium components in vanadium bearing steel slag was proposed innovatively,that was"selective calcium leaching without adding-rapid and efficient calcium precipitation-simple regeneration of calcium precipitation solution without adding cycle leaching",The experiment and mechanism of the new process were studied.The influence of the main factors were investigated by single factor test method,and the internal reasons were analyzed.The relevant rules of the new process were obtained:（1）In the selective calcium leaching process without adding any leaching agent,under the optimal conditions（leaching time 5h,stirring speed 350rpm,fineness-74（?）m accounts for 80%）,the leaching rate of calcium component was as high as 63.1%,and no other component was leached;Vanadium was not dissolved,but was completely enriched in the leaching residue（the grade of vanadium was increased from 1.96%to 2.30%,and the enrichment degree was 1.17 times）.（2）In the fast and efficient calcium precipitation process,the ammonium carbonate precipitation method could achieve instant calcium precipitation,when the amount of precipitant ammonium carbonate was 55.15%（the pH of the solution system was 7.6 at this time）,the precipitation conversion rate reaches more than 99%,and could obtain calcium carbonate by-products with a yield of 47.88%and a purity of more than 99%（light calcium carbonate）.（3）In the simple regeneration process of calcium precipitation solution,hydrochloric acid（HCl）was added to adjust the pH value of calcium precipitation solution to 4 to remove residual CO₃^2-and HCO₃^-,and ammonia water was added continuously to adjust the pH value of filtrate to 6.4（imitating the leaching environment of 20%concentration ammonium chloride solution）.Under the condition of not adding leaching agent ammonium chloride,the calcium precipitation solution still had good calcium leaching performance and could realize recycling leaching.The effect of the new process on the subsequent acid leaching of vanadium was studied experimentally.The results showed that the solid-liquid separation efficiency was higher when the vanadium bearing steel slag was treated by the new process,and the leaching rate of vanadium could be increased by 20%under the same acid consumption;Under the condition of ensuring the highest vanadium leaching rate,the acid consumption could be saved by 30%,and the vanadium extraction index was greatly optimized.The characteristics of the new process were compared:the new process does not need to add leaching agent,had high purification rate,good selectivity,no loss of vanadium and enriches vanadium;It could quickly and efficiently precipitate calcium and prepare high purity light calcium carbonate,which basically avoids the loss of pyrolysis volatilization of ammonium salt,and had the advantages of high efficiency and fast preparation speed;After simple regeneration treatment,the filtrate of calcium precipitation could be recycled by zero addition,which had the effect of saving and environmental protection.The mechanism of the new process was analyzed from the aspects of the changes of mineral phase and surface morphology before and after leaching,thermodynamics of leaching process and leaching kinetics.The results showed that:（1）after the purification of calcium component,the original dark gray on the surface of vanadium bearing steel slag began to fade and become bright,while the original calcium silicate and free calcium oxide in the steel slag were purified,New mineral phase vanadium oxides and calcium titanium oxides appear.The content of vanadium in the new phase was high,which means that the process of purifying calcium component promotes the exposure of vanadium,which was also the reason why the index could be greatly optimized in the subsequent acid leaching of vanadium from vanadium bearing steel slag.（2）When leaching,the temperature should be raised as much as possible to promote the reaction.Tricalcium silicate was the main object of leaching reaction.Calcium titanium oxide did not react with leaching agent,while vanadium mainly exists in calcium titanium oxide,so vanadium would not be lost.The whole leaching process was a kind of selective leaching,which was the thermodynamic basis of selective leaching.（3）The leaching process of vanadium bearing steel slag with ammonium chloride conforms to the typical shrinking core model,and the leaching process was not controlled by a single step:when the temperature was between 30℃and 70℃,the temperature had a great influence on the leaching process,and the apparent activation energy of the reaction under the chemical reaction model was 62.94 k J/mol,and the leaching process was controlled by the surface chemical reaction;The apparent activation energy of chemical reaction model was 8.10 k J/mol,and that of solid film diffusion was 6.44 k J/mol,which was mainly controlled by solid film diffusion.Based on the kinetic study,it could be seen that in order to speed up the reaction speed and shorten the leaching time,we should increase the leaching temperature and improve the mineral fineness.