| A large amount of converter sludge(12～16 kg dry sludge/ton steel)is produced in the converter steelmaking process of steel plant.Converter sludge is a valuable secondary resource,but it contains considerable moisture and must be concentrated,dehydrated,and dried before recycling.At present,the main problem lies in the drying process.The traditional drying technologies have the problems of high energy consumption cost and low drying efficiency.Therefore,we proposed a new rotating-drum-drying technology using hot steel balls.After the steel balls are heated by the waste heat of metallurgical slag,they are added to the drum and mixed with the wet sludge.Steel balls are used as the heat source and crushing medium,and the wet sludge is broken and heated.This technology has the advantages of simple system,small floor area,low cost,high efficiency,and large treatment capacity,and can be extended to the treatment of more kinds of solid wastes.In this paper,the heat and mass transfer processes of converter sludge drying in the rotating drum were investigated from different quality/size grades to reveal the heat and mass transfer law of sludge drying and the characteristics and energy efficiency of drum drying.The main work is as follows:(1)The basic physical and chemical properties of converter sludgeBasic physical and chemical properties of converter sludge were measured,including component,particle size,density,thermal conductivity,specific heat capacity,and the relationship between apparent state and moisture content of sludge.The results show that the converter sludge is a kind of inorganic sludge.Its iron content is high(TFe:50.6～55.0 wt%),and mostly exists in the form of ferrous oxide.The skeleton density of converter sludge is large(5619 kg/m3),and the particle size is fine(0.314～586 μm).Different apparent states(solid/plastic/fluid)will appear under different moisture content,but there is no obvious water absorption expansion(or drying induced cracking)phenomenon.This part of research clarifies the characteristics of converter sludge,and these basic property data provides the values of necessary parameter for the subsequent drying study.(2)The heat and mass transfer during the drying of tiny converter sludge particleThe characteristics of moisture evaporation and energy consumption during converter sludge drying were studied by thermogravimetry analysis(TGA-DSC).The chemical stability of skeleton,drying stage division,bound energy,and water activity were determined.The results show that the chemical property of converter sludge skeleton is stable.When the dry base moisture content of converter sludge is lower than 0.32 g/g,the bound energy starts to be greater than 0.With the decrease of moisture content,bound energy first increases slowly and then increases rapidly,and the critical point of increase rate is about 0.024g/g.When the dry base moisture content is lower than 0.165 g/g,the water activity starts to be less than 1.With the decrease of moisture content,water activity first decreases slowly and then decreases rapidly,and the critical point of decrease rate is about 0.024g/g.This part of research provides basic parameters for the following group level and system level drying process research.(3)The heat and mass transfer during the drying of converter sludge groupThe key processes of drum drying(i.e.sludge group drying processes)under different working conditions were investigated and the heat and mass transfer parameters were obtained through thermogravimetric experiments and numerical simulation.To be specific,the drying process of spherical sludge particle was experimentally studied,the influence of key factors on drying process was revealed,the drying mechanism was discussed,and the parameters such as diffusion coefficient were obtained.Based on the drying mechanism,a multi-physical field coupling mathematical model of flow,heat and mass transfer was established to describe the sludge drying process.Simulation research was carried out on the key processes of drum drying to obtain the relationship between the heat and mass transfer coefficients and the key factors.The key drying processes include:①the heating process of thick-layer sludge surrounding steel ball inside the material bed,②the heating and drying process of thin-layer sludge wrapped on the surface of steel ball at the surface of the material bed,and ③the process of sludge particle heated and dried by the environment at the surface of the material bed.This part of research reveals the drying process at group level in the drum,which can help readers better understand the drying mechanism and provide the basis for the following drum system level research.The results show that:(a)For the key drying process ①,higher sludge moisture content or higher drum rotating speed leads to higher comprehensive heat transfer coefficient between steel ball and thick-layer sludge.The comprehensive heat transfer coefficient is between 170～940 W/(m2·℃)in our working conditions.(b)For the key drying process ②,reducing the steel ball diameter,sludge thickness and hot air flow rate,or increasing the steel ball temperature increases the comprehensive heat transfer coefficient between steel ball and thin-layer sludge.The initial moisture content of sludge only affects the comprehensive heat transfer coefficient in the early stage of drying.The influence of hot air temperature on comprehensive heat transfer coefficient can be ignored.The comprehensive heat transfer coefficient is between 180～275 W/(m2·℃)in our working conditions.The effects of steel ball diameter,steel ball temperature,sludge thickness and hot air velocity on the heat loss rate of steel ball to the environment can be ignored.The initial moisture content of sludge only affects the heat loss rate in the early stage of drying.The higher the hot air temperature is,the smaller the heat loss rate is.(c)For the key drying process ③,the drying time of sludge particle can be shortened by reducing the diameter or initial moisture content of sludge particle and increasing the temperature or flow rate of hot air.An empirical model MR=exp(-at-bt2)to describe the drying curve(R2>0.99)was proposed.Increasing the sludge particle diameter,hot air temperature and hot air flow rate,or reducing the initial moisture content of sludge can increase the effective diffusivity coefficient of sludge moisture.The effective diffusivity coefficient is between 6.60×10-9～1.78×10-8 m2/s in our working conditions.Reducing the sludge particle diameter or increasing the temperature and flow rate of hot air can increase the convective heat and mass transfer coefficient between the sludge particle and the environment.The initial moisture content of sludge only affects the convective heat and mass transfer coefficient in the early stage of drying.The convective heat and mass transfer coefficients are respectively 48～130 W/(m2·℃)and 0.05～0.15 m/s in our working conditions.At the same time,the dimensionless criterion equations of heat transfer(Nussel number Nu)and mass transfer(Sherwood number Sh)for the convective drying of spherical sludge were established(RNu2>0.995 和RSh2>0.996).(4)The heat and mass transfer during the drum dryingThe drum drying process using hot steel balls was experimentally studied,including the effects of operating parameters on the drying process and the drying energy efficiency.A mathematical model describing the drum drying process was established,including the dimensionless criterion equation for describing the drying curve and the heat transfer model for describing the temperature curve.Some heat transfer parameters in the model came from the previous study.The combination of operation parameters was comprehensively optimized and analyzed from the perspective of energy consumption and efficiency using the improved selfscaling method.Taking the production requirements of a steel plant as an example,the combination of operation parameters that can not only meet the requirements but also ensure the comprehensive optimization of energy efficiency was obtained using the drum drying model.The results show that:(a)The proposed drying technology is feasible and efficient.The converter sludge was thoroughly dried and is in the form of dry powder after 1200 s,and the dry base moisture content was lower than 0.024 g/g.(b)The sludge drying can be accelerated by appropriately reducing the treatment mass or initial moisture content of sludge,increasing the temperature,diameter or number of steel balls,and selecting an appropriate drum rotating speed.(c)In our working conditions,the system energy efficiency(ηe)is 40～84%,thermal energy utilization rate of steel balls(ηb)is 50～95%,specific energy consumption(SEC)is 0.8～1.5 kW h/kgw,and average drying rate(DR)is 1.0×10-4～1.8×10-3 g/(g·s).The steel ball temperature and sludge treatment mass are the main influencing factors of energy efficiency.Selecting appropriate steel ball diameter and drum rotating speed can guarantee high DR,ηe,and ηb and low SEC simultaneously.If the operating parameters are set reasonably,thenηe>67%,ηb>78%,SEC<1.0 kW h/kgw,and DR>9.9×10-4 g/(g·s).(d)The Weibull distribution function model MR=exp[-(t/a)b]can accurately describe the drying curve(R2>0.97).(e)The mathematical model of drum drying can accurately predict the drying process.This research provides experimental basis and theoretical guidance for the rapid application of our rotating-drum-drying technology,and provides a new way for waste heat recovery and solid waste treatment in the iron and steel industry. |
