Studying On Pyro-process For Spent Potlining And Its Heat Transfer Analyzing

Spent potlining (SPL) from aluminum smelters is classified as hazardous waste. Soluble fluorides and cyanides in SPL are much higher than permitted threshold of national standard. When SPL is piled in open air and suffered from wind, sunshine and rain, toxic fluorides and cyanides in SPL will be shifted, emitting into air, penetrating into soil and underground water. SPL piled in open air will destroy the environment and do harm to human health and propagation survival.Detoxifying processes for SPL are divided into hydrometallurgical process (hydro-process) and pyrometallurgical process (pyro-process). There are more than one hundred patents related to SPL processing. Pyro-process has become leading process for detoxification of SPL due to its characteristic short flowsheet, easy operation, recycling of fluorides and low investment.Pyro-process for detoxification of SPL is focused on in the present work. Pyro-process of SPL is deeply analyzed and studied from theoretic and practical points of view. The achievement in the present work could provide technological base for optimization and industrial application of the pyro-process which developed by Chalco. Main conclusions of the dissertation are summarized as following:1. Fluorides which are widely dispersed in SPL are mainly NaF and Na3AlF6 by analysis with XRD, SEM and EDX spectra. SPL contains high level of soluble F-and CN-which is much higher than permitted threshold of national standard. If SPL meets water, soluble fluorides and cyanides would be dissolved into water as F-and CN-or react with water to emit virulent HCN and HF. Hence, SPL land-filled or piled in open air is latent danger.2. Pyro-process was studied by calculation of free enthalpy function of compounds.ΔGT0 and equilibrium constant of reactions during 1200～1300K were calculated. Equilibrium constant of cyanide decomposition reaction is the largest and equilibrium constant of the reaction for NaF into CaF2 is the smallest. According to the order from large to small, equilibrium constant of reactions are listed as:decomposition of NaCN> converting of AlF3> burning of C> NaF converting. When SPL coexists with limestone, silica and air, pyro-process of SPL is greatly feasible from thermodynamic point of view.3. Chemical stability of fluorides in SPL was studied by simulating industrial conditions. Fluorides solely baked in air and baked with limestone were discussed, respectively. NaF had good chemical stability, but the stability of AlF3 was poor. According to the order from high to low, chemical stability of fluorides is NaF> Na3AlF6> AlF3, which has perfectly similar with thermodynamic result. During pyro-process of SPL, AlF3 was easily converted, but conversion of NaF was difficult, Na3AlF6 just went after AlF3.4. Heat transfer process of SPL pyro-process is studied by taking industrial test rotary kiln as research object. Heat balance of the kiln was analyzed and rationality of SPL pyro-process scheme was validated. A calculation formula of material natural angle was deduced. Heat-releasing model of coal powder in the kiln was introduced. Physical-mathematical model of heat conduction between material and covered furnace wall was put forward. A calculation formula for contacting heat transfer between material granule and heated surface through air-film was offered. Integrative heat transfer coefficient between the material and covered kiln wall was gained. One-dimension math model of kiln temperature distribution along kiln axes was set up. A program was made for calculation of temperature distribution along kiln axes. Parameters affecting kiln temperature distribution were discussed and optimized parameters for kiln operation were obtained.5. SPL can be sufficiently detoxified by mixing limestone, coal slag with SPL which was demonstrated in industrial tests. Conversion rate of fluoride was by more than 98% and decomposition rate of cyanide was by more than 99.5% at the rate of 500 kg/h for feedstock. Average soluble F-and CN-in solid residue were respectively 39.7 mg/L and 0.053 mg/L, which is much lower than permitted threshold of national standard. Concentration of HF in tail gas from kiln was about 1.4 g/m3. HF could be recycled to produce aluminium fluoride by conditional dry scrubber system in a smelter and returned back to aluminum reduction cells. CaF2 in the residue was about 20%, which could be used as activator for sintering process of cement instead of freshly added fluorite. Therefore, pyro-process of SPL which was successfully developed in this thesis has great benefits economically and environmentally and makes detoxifying, recycling and reducing of SPL come true. It will root up pollution of SPL after its application all over the whole country.