【作者】 陈喜平；

【导师】 周孑民； 李旺兴；

【作者基本信息】 中南大学 ， 热能工程， 2009， 博士

【摘要】 铝电解生产排出的废槽衬是极其有害的固体废物,其可溶氟化物和氰化物含量严重超标。受风吹、日晒、雨淋的作用,其毒害物质将转移,进入大气,渗入土壤和地下水,将破坏生态环境,严重影响人类健康和动植物生长。废槽衬的处理方法,分为湿法和火法两大类,相关的专利有100多件。火法处理技术因其工艺流程短、容易操作、回收利用氟化物、投资少而逐渐发展为处理废槽衬的主导技术。本文重点研究了废槽衬的火法处理技术,从理论和实践方面深入分析研究了废槽衬的火法处理过程,为中国铝业公司开发的废槽衬火法处理技术的完善及推广应用提供了理论依据和技术基础,得到了以下主要结论：1、废槽衬中氟化物主要以NaF、Na3AlF6形式存在,氟化物的分散度大,SEM和EDX分析均证明了这一点。废槽衬含有较高浓度的F-和CN-,远远超过国家允许的排放限值。废槽衬遇水时,F-和CN-容易浸出,并能与水反应生成HF、HCN,毒害性极大,外排的废槽衬是潜在的危险源。2、采用自由焓函数法分析研究了废槽衬的火法处理过程,计算了各反应的△GT0值和1200～1300K的平衡常数,氰化物的氧化分解反应平衡常数最大,氟化钠转化为氟化钙的反应平衡常数最小。各反应平衡常数从大到小的顺序是：氰化物的氧化分解反应>氟化铝的转化反应>碳的燃烧反应>氟化钠的转化反应。在石灰石、二氧化硅、空气与废槽衬同时存在的条件下,废槽衬的火法处理在热力学上是完全可行的。3、通过模拟工业条件实验,在暴露于空气中单独焙烧、与石灰石混合焙烧两种情况下研究了废槽衬中氟化物的化学稳定性,氟化钠具有好的化学稳定性,氟化铝的化学稳定性最差,按稳定性高低排序是：氟化钠>冰晶石>氟化铝。研究结果与热力学计算结果相吻合。表明在废槽衬火法处理过程中,氟化铝最容易转化,氟化钠最难转化,冰晶石的化学稳定性介于两者之间。4、以工业试验回转窑为热工对象,研究了废槽衬火法处理的传热过程。对工业试验回转窑进行了热平衡分析,验证了废槽衬火法处理工业试验方案的合理性。推导了物料自然堆角的计算公式,给出了回转窑内煤粉燃烧的释热模型,导出了物料与被覆盖窑壁间热传导的物理数学模型,给出了颗粒与受热面接触过程中通过气膜导热的计算公式,求出了物料与被覆盖窑壁间的热传导综合传热系数。建立了回转窑一维轴线温度分布数学模型,基于热工测试和窑内传热原理,编制了回转窑轴线温度计算程序。讨论了工艺参数对回转窑温度分布的影响,给出了优化的运行参数值。5、工业试验研究表明,在废槽衬中添加石灰石、工业废料PCA处理废槽衬是可行的,在进料量500 kg／h的工业规模下取得了较好的处理效果,实现了废槽衬的无害化处理,氟化物的转化率达98%以上,氰化物的氧化分解率达99.5%以上。回收的固体残渣中平均F-含量39.7 mg／L,CN-含量0.053 mg/L,低于国家环保标准允许限值。烟气中的HF浓度约1.4g/m3,可通过干法净化系统回收、生成氟化铝作为电解原料。固体残渣含氟化钙约20%,可替代荧石作为水泥烧成的催化剂。本研究开发的废槽衬火法处理技术实现了废槽衬的无害化、资源化和减量化,具有明显的经济效益和环境效益。该技术在全国推广应用后,将根除我国废槽衬的污染。更多还原

【Abstract】 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.更多还原