Study On The Vanadium Nitride Powder Granulation

Vanadium nitride is an important vanadium alloy additive in metallurgical industry,V-N microalloying technology can increase the steel intensity significantly and reduce the cost of steel.A patent application for the production method of vanadium nitride had been filed by Panzhihua Iron and Steel Corporation in December,2001 and it initiated the industrialized process of vanadium nitride in China.VT cored wire(a kind of cored wire in which vanadium nitride powder and ferrotitanium powder are mixed in certain proportion and the powder is packed by steel strip) was invited by Panzhihua Iron and Steel Research Institute in 2005 and it was used to adjust the chemical element composition when melting steel by wire-feeding method.While there has been existing a problem of uneven powder core density since VT cored wire was commissioned,the main reason is that the fine vanadium nitride powders(the proportion of fine powders which particle size smaller than 0.6mm reach 40% to 50%)produced after crushing bridge in funnel by which to feed to steel strip, what hinders other powders downflowing at normal speed, so the phenomenon occurs.In order to improve the powder flowability and the fine powder utilization,the classification-granulation-drying technological process is used. In this dissertation, the fine powders after crushing which particle size smaller than lmm are sorted by sieve,then granulated by pan granulator and dried by drier.The green pellets water content is measured by infrared rapid moisture instrument;the surface morphologies of dry pellets are studied by stereomicroscope;the microstructures of dry pellets cross-section are analysed with scanning electron microscope(SEM);the dry pellets compressive intensity and dry pellets stirring intensity are measured by self-made equipments;the dry pellets drop number is determined by free drop experiments.The results of vanadium nitride granulation orthogonal experiments and drying experiments show that,the low-temperature starch adhesive is the proper binder for vanadium nitride powder granulation;the optimum process of vanadium nitride granulation-drying operation for the dry pellets with ideal granulating rate and higher strength are investigated as follows:the disc rotation speed is 35r/min,the binder addition amount is 1.0%,the material particle size distribution is 0.2～0.6mm,the drying temperature is 110℃,the drying time is 1h; the surface morphologies of dry pellets is different under different granulation conditional combinations through the stereomicroscope graphs;the growth mechanisms of large particle vanadium nitride pellets lie between stratification and coalescence, the small particle pellets grow up by the way of coalescence.Based on the results of drying experiments, it's found that,the smaller the green pellets initial moisture content is, the faster the total drying speed is;the finer the materials particle and the larger the binder ratio in material is, the slower the total drying speed is; the drying rate curves are influenced with the drying temperature.There are two types of vanadium nitride pellets drying rate curves:one-peak type and two-peak type.The green pellets drying process of one-peak type is composed of increasing speed stage and deceleration stage and the process changes gradually.The green pellets drying process of two-peak type is composed of the fist increasing speed stage,the first deceleration stage,the second increasing speed stage and the second deceleration stage.