Evolution Of Particle Size Grading And Process Optimization In Granulation Of Iron Ore Fines

Up to now, the role of iron ore granulation in sinter has been shifted from the traditional homogenization of chemical composition and adjustment of particle sizes to the structure design and the grading control of the ganules. More duties are taken by granulation process so that the sinter can be operated more flexible and more economical in using various iron-bearing materials while a high efficiency, high quality of sinter production can be made. The enhancing mehods on traditional granulation technology and the development of new granulation process are the most two important parts in the improvement and enhancement of the sintering process. In this thesis, a series of granulation experiments on several sinter mixtures from sinter plants have been conducted under laboratory conditions. The evolution of particle size grading in granulation process was revealed and the effects of raw material properties and moisture content on the granulated particle sizes and permeability were analysed and discussed. By using discrete element method(DEM), the packing behavior of the granules has been simulated. The effect of size gradation on the packing porosity of the green bed of granules has been discussed, and which was also compared with the experimental results. The main conclusions of this work can be summaried as follows:① The contact angle(θ) of water on iron ore fines was a function of the volume % of goethite(φG) in iron ores: ( )Gq= arccos 0.33j +0.22. The φG shows a multilinear regression relation to the surface morphology index(SMI) and pore volume(Vpore):G0.266* 0.769*(10* ml/ g)Porej =SMI +V.② The preferred agglomeration of the pre-wetting particles was promoted by prewetting treatment, and thus the particle sizs of granules was modified. At the same moisture content,the permeability of granules in pre-wetting granulation was more than 1.2 times of that in traditional granulation. The W+3 and W+3,-7 of granules in traditional method can be improved by more than 15% with pre-wetting granulation(PWD=65%). Besides, a higher pre-wetting degree(PWD=80% for instance) can lead to a rapid increase in the content of super-coarse particles(>8 mm), which has a detrimental effect on the permeability and the sinter reactions in high temperature sintering process.③ The particle size distributions in iron ore granulation can be described by fractal mathematical models. The fractal models for optimal granules were shown as follows:max adj2.318 0.27 0.059- 0.294, R 0.838 f f MCD granule = +D raw +D raw W =The hit rates of the prediction for both the two models can be larger than 85% when the corresponding relative error is no less than 10%. This indicated that fractal theory can be applied into the gradation control of granules, and thus to improve the permeability of the packed bed.With particle sizes of raw mixture and moisture being the input parameters, BP neural network models can have a good prediction of the granulation result, but obviously these models are a little more complicated than the regression models.④ The numerical simulation of the packing process of the granules with different size grading indicated that:(a) the improvement of granulation is accompanied with the increase of porosity of the packed bed. The relationship between permeability(JPU) and porosity(ε) can be expressed as JPU =507.98′e -144.62 with Radj2 >0.99.(b) for rigid, non-deformable granules with a fractal distribution of particle sizes, the simulated porosity can be expressed as a function of fractal dimension of the granules: 0.5140 0.0665( )fe = - ′D granule, where the value of Df(granule) varied from 0.2 to 2.0.The research of this thesis can be vital to the improvement on granulation of sinter mixtures. The conclusions of this work can be applied into the size control or particle size design of the raw sinter mixtures, so that the permeability of a sinter mixture can be improved further.