Research On The Calcination Characteristic Of Medium-sized Limestone

As a kind of non-renewable resources, limestone plays an important role in many industries.Large amounts of limestone resources have been consumed in cement industry and steel industryannually. Reports showed that the existing reserves of limestone in China only can be used for26years. However, a lot of limestone, of which the size is below25mm, would be abandoned in thelimestone mining process under the condition of recent China. And in lime production processes,nearly all sorts of kilns can not make effective use of the limestone which the size is below10mm,which also make lots of limestone be abandoned. Therefore, the recycling of this kind of abandonedlimestone resources would imply great economic and social significance. In view of this, thelimestone, of which the size is below10mm, was used as the research object in this paper. Thelimestone decomposition mechanism was studied through the thermo gravimetric experiment. Theinfluence of some factors on the product characteristics was described through the static calcinationand fluidized calcination experiment. Meanwhile, the fluidized calcination process feasibility wasexplored.First of all, thermal decomposition experiments of five different particle size of limestone wereconducted to study the calcined characteristics of limestone of large particle size. The limestoneparticle size ranged from0.5to10mm, while temperature was changed from900to1050℃. Theresults revealed that the smaller the particle size was, the higher the temperature was, and the fasterthe decomposition was. And the particle size of limestone and calcining temperature had an obviousinfluence on the thermal decomposition mechanism. When the size was small and the temperaturewas high, reaction was in according with the randomly nucleating and nucleus growth model; whilereaction was in according with the phase interface mechanism, when the size was large and thetemperature was low. And when the particle size was0.5~1mm, reaction activation energy wasvery small. Meanwhile reaction activation energy increased with the increase of particle size. Whenthe particle size is in range of1~5mm, the activation energy fluctuated. And with the continuousincrease of particle size (5~10mm), the activation energy doubled. By the limestone calcinationreaction dynamics simulation, it was found that the higher the temperature was, the less thedifference between the simulation response curve and the actual response curve would be and thereaction mechanism was more close to the random nucleation model. For the limestone, of whichthe size was5~10mm, in calcination temperature of900℃, the initial stage change of cacinationwas shown to be accord with the random nucleation and subsequent growth model, and thefollowing decomposition would become phase boundary model.Then, the muffle furnace was used to statically calcine the limestone sample with the particlesize of0.5~10mm, and the influence of the different calcining temperature, calcining time, grainsize on the calcined product activity was analyzed. The results showed that: calcining temperatureand time affected lime activity. With low calcining temperature and short time, limestone can not becompletely calcined, activity of which was low; while with high temperature and long time, limestone would be easily over burning, activity of which was still not high. It was also found thatthe higher the temperature was, the sooner over burning appeared. The product of the best activitycould be obtained when the limestone sample was calcined for60minutes in950℃. The grain sizeof limestone affected calcined product activity; lime activity did not increase with particle size on asingle trend, but roughly first increased, then decreased. In this experiment the best lime activityappeared when particle size was2~3mm. At the same time, the automatic gas adsorption analyzerwas used to do N2adsorption and desorption experiment on13different samples, and to get thesurface pore structure characteristics. The influence of calcining temperature, calcining time andparticle size on the pore specific surface area and pore volume was analyzed, and the relationshipbetween the pore structure and activity of the product was explored. The results showed that, withthe increasing temperature, particle size and calcining time, the adsorption pore volume and porespecific surface area distribution of product reduced significantly. It was not a linear relationshipbetween the product activity and specific surface area or pore volume, but with the increase of thespecific surface area, product activity increased at first, then decreased; with the increase of porevolume, product activity first increased, then decreased, and then had a tendency to rise; Productactivity was not determined only by the pore structure factor.Next, the fluidized calcination of the limestone of which the particle size was small (0.5~1mm)was conducted in a fluidized bed. The product activity was analyzed, and the difference between thefluidized calcination and the static calcination was compared. The results showed that, the similar asthe static calcination, process the calcining time and calcination temperature influenced the productactivity during the fluidized calcination. As the temperature increased, the calcining time to get theproduct for the best activity would be shorten, and continued to calcine after that, product activitydeclined. Compared with the static calcination, the fluidized calcination could not only enhance thelimestone decomposition rate, but also improve the sintering rate of the product, and the lime ofhigh activity could be obtained in a short time. In this experiment, with the fluidized calcination ofthe limestone for3minutes in1050℃, it was found that the activity of the products was338ml.Finally, the fluidized experimental apparatus was used to do the fluidization experiment on thelimestone of which the particle size was5-10mm. It was found that, in the experimental process thelimestone could be steady fluidized in the bed and the critical fluidization speed was measured to be3.8m/s. Combined with the generation technology of ultra high temperature flue gas and theexperimental results, in this paper, a kind of new technology, which used the fluidized bed to rapidlycalcined the wasted limestone in the high temperature to obtain active lime, was put forward.