Study On Detoxification Of The Chromium Residue By Two-Stage Reduction Techniqe

Chromium residues as a toxic waste are generated in the production process of chromium and chromic salts. Characters of chromium residue were different in the raw material and chromium salt production process. Chromate production in China accounted for1/10yield of the world. Chromium compounds accounted for15%of Chinese national economy. Chromium residues mainly come from the chromate sodium dichromate (NaCr2O7) producing process in China. This process can produce a lot of chromium residues, A great quantity Cr(â…¥) was left in chromium residues, and it was difficult to dispose. Because sodium dichromate market was changed and other reasons, mostly Chromate plants were closed down and meanwhile a large number of chromium residues were disordered stacked in nearly50years. About6,300,000tons chromium residues were produced, and there were3,200,000tons of peripheral storages without detoxification. In Henan Province,640,000tons of historical chromium residues were layed up, The stockpiling chromium slag had caused serious pollution problems, include of surface water, groundwater, soil, etc.Chromium slag toxicity is due to the strong oxidation of six valence chromium. Cr(â…¥) is easily soluble in water and mainly existed in the Cr2O42-and Cr2O72-two forms. Cr(â…¥) can enter cells through the cell membrane of living cells and inactivate the cell. Cr(â…¢) has stable property and is difficult to dissolve in water. Cr(â…¢) is a kind of trace elements necessary for mammals. Cr(VI)is the key of chromium slag harmless disposal. Cr(â…¥) of chromium slag exists in several forms including water soluble Cr(â…¥) and acid soluble Cr(â…¥) which are harmful and easily released into the environment, The process of detoxification chromium residues are that Cr(â…¥) is changed into Cr(â…¢), and the detoxification method of traditional include dry detoxification and wet detoxification. Dry detoxification has low cost, but it is easy to generate high energy consumption and secondary pollution. Wet detoxification has strong operability, less spaces, flexible process, and good detoxification effect. Cr(VI) of chromium residues are the premise of effective leaching wet detoxification of chromium slag, so a suitable reducing agent is choosed, that will be the key of chromium residues detoxification effect.Comprehensive utilization of chromium residues are based on component composition and physical elements in chromium slag phase. The obstacles of comprehensive utilization are magnesium oxide and Cr(VI) content in the chromium slag."Control Chromium Pollution Specification(HJT301-2007)" is the main comprehensive utilization approach of chromium residues, and it indicates the pollutant control index of different use restrictions.This topic came from the Henan Province science and technology research project (item number:092102310244) in2009. The object of study was historical chromium slag of Wulibao Chemical General Factory in Zhengzhou City. The chemical compositions of chromium slag were analyzed, and the leaching mechanisms of hexavalent chromium in chromium slag were discussed. The Cr(VI) was analyzed under the condition of chromium slag in water leaching and acid leaching. Two stage reduction process of detoxification chromium slag and hexavalent chromium reducing agent kinds were also researched. This study work also made some beneficial attempts to popularize and apply this new technology. The main content of this paper is as follows:1. Chemical composition analysis of chromium residue:Al2O39.48%, Cr2O33.22%, MgO18.45%, Fe2O35.65%, SiO216.8%, CaO30.79%, water soluble chromium0.32%, acid-soluble chromium0.49%, chromium slag water pH value>10.2. Leaching characteristics of Cr(VI) in chromium residues were analyzed. The effects of agitation strength, particle size, pH value, liquid-solid ratio and leaching solution concentration on Cr(VI) leaching conditions were discussed and experimental datas were fitted. Leaching of chromium slag tap water and leaching Cr(VI) in chromium slag were completed by two stages. The first stage was the Cr(VI) fast dissolution process from chromium slag surface. In the stage, Cr(VI) leaching kinetics model was In second stage, the internal process of chromium residues of the package was dissolved. The dynamic equation was: leaching process through the chemical reaction of solid phase and liquid phasebelonged to the membrane diffusion control. The dynamic equation was:. The concentration increase of sulfuric acid and the chromium slag particle size decrease were in favor of Cr(â…¥) leaching.3. The hexavalent chromium. Leaching effects on chromium residue at different leaching agent were studied Water soluble Cr(â…¥) was easy soluble in water, and acid soluble Cr(â…¥) was easily soluble in acid,. Therefore, the influencing factors of the leaching of chromium residues in water and sulfuric acid were studied respectively. The results of the optimum leaching conditions in water were shown below:liquid to solid ratio scale was20:1, chromium residues particle size was100-200MU, leaching time was4-8h, leaching temperature was30℃～40℃. The optimum leaching conditions in acid were shown below::liquid to solid ratio scale was3:1, chromium residue particle size was200MU, leaching time was3h, sulfuric acid addition0.25mL/g, The pH of leaching liquid was7.6in the end.4. The reductants of detoxification were selected. First a variety of organic and inorganic reductants were examined in due to the complex composition of selected chromium over time. The results were shown as:Cr(VI) reduction effect was obvious with the selected straw, wheat flour, glucose, organic reducing agents, only acidic conditions at60℃(straw pH<4) under acidic conditions at60℃(pH<4). But the reaction time and reaction rate could provide a theoretical reference for the chromium slag comprehensive utilization, industrial production and further research. The selected inorganic reductant had the best detoxification effect, but the agent consumption was large. Removal effects of several inorganic agent on Cr(VI) and total chromium in chromium slag leaching solution were further investigated in order to determine the two type reduction on the leaching of Cr(â…¥) by sodium metabisulfite (Na2S2O5) and ferrous sulfate (FeSO4).5. Two segment reducing process were optimized. The characteristics of chromium slag leaching of water soluble Cr(â…¥) and acid soluble Cr(VI) were considered by using water continuous leaching and acid leaching method respectively. In order to ensure maximum limit of Cr(â…¥) in chromium slag leaching, a reducing agent was added to achieve two type of Cr(â…¥) reduction in the different stages of leaching. Continuous leaching conditions were shown as follows:with ball milling of chromium slag added directly to200MU, sodium metabisulfite, leaching reaction of1h into the reaction tank, with concentrated sulfuric acid, leaching of3h, addition of ferrous sulfate, adjusting the ratio of liquid to solid3:1. The reducing agent adding order:first adding sodium metabisulfite; the best dosage is:sodium pyrosulfite chromium slag and the mass ratio of0.018:1, ferrous sulfate and quality of chromium residue ratio was0.4:1, the average content of Cr6+after detoxification chromium slag was0.02mg/L, the average concentration of total chromium was0.525mg/L.6. To observe the morphology of chromium residue after detoxification and analysis of crystal structure, Results show that two stage reduction treatment can make the chromium residue harmless and has a long-term stability, good complete detoxification7. Two sections of reduced chromium slag production process were studied. Based on the laboratory research, the result showed chromium slag leaching conditions were as follows:the size of200MU, pH value is6, and the ratio of liquid to solid4:1. Conditions:the sodium pyrosulfite, reduction reaction1hâ†'93%concentrated sulfuric acid reaction2-4hâ†'ferrous sulfate, curing time10-12h, reaction temperature60℃～80℃. Dosage:400kg/t chromium slag of sulfuric acid93%, sodium pyrosulfite30kg/t chromium slag, ferrous sulfate300kg/t chromium slag. The total chromium was removed by choosing20%lime milk, adjusting the pH9of the solution, with time>2h. Experiment indexes were tested by the Ministry of land and resources of Zhengzhou mineral resources supervision and monitoring center. The testing results looked as following: detoxification of total chromium in chromium slag≤1.5mg/L, Cr6+≤0.5mg/L, far belowed the chromium slag control index of the general process of solid waste landfill on the basis of "pollution environmental protection technical specifications (Provisional)"(HJT301-2007). The technology had been put into effect to dispose several chromium slag stockpiling in a total of643300tons in Henan Province, respectively, by the environmental protection department of Henan Province.8. Main innovations and features. An effective method of hexavalent chromium optimum leaching conditions in chromium slag and chromium residues detoxification were found. A set of feasible chromium residues detoxification technology were provided, The application of the technology has significant social, economic and environmental benefits. The technology has won the second prize of the scientific and technological progress in Henan Province.