Basic Research Used For Preparing Magnesium By Thermal Reduction Of Pre-made Pellets

Magnesium and its alloys have been widely used in automobile,military industry,electronic communications and aerospace industry,etc,which is honored as the green engineering material of the 21st century.Currently,more than 95%of the primary magnesium is produced by silicothermic process(Pidgeon process).Silicothermic process mainly consists of the calcination of dolomite in the rotary kiln,briquetting of calcined dolomite after mixing and the vacuum reduction of briquettes.The traditional process has the disadvantages as the followings:in the calcination process of dolomite in the rotary kiln,5%powder materials are produced and cannot be utilized,causing the waste of resources.The CO2 produced in the calcination process is not easy to be recycled.As the calcined dolomite has higher water absorption,the compressed pellets are difficult for storage Porosity between internal particles of pellets is very low,so that the escape of magnesium vapor in the reduction process is difficult.The pellets are easy to be pulverization which will increase the cycle of reduction process and energy consumption.Aiming at the existing defects in magnesium smelting process such as the high energy consumption and the low utilization rate of resources,the new process "pelletizing-calcining-reduction" for producing magnesium metal was proposed by our group,which combines with the characteristics of the magnesium resources in China,Firstly,after mixing,the dolomite,reducing agent and additives are pelletized by balling drum.Then the pre-made pellets will be calcined in the reduction pot.After calcination process,the pellets are switched to the vacuum condition for the direct reduction.The new process with calcination and reduction "completed in the same tank" is adopt to replace the existing silicothermic magnesium production process.The CO2 recovery circuit is designed independently.The waste heat carried out CO2 in the calcination process is used to preheat other reaction tanks,improving the thermal efficiency and capacity of single equipment,reducing the production cost,improving the working conditions and reducing the CO2 emissions.The following researches are carried out around preparing magnesium by using pre-made pellets thermal reduction process in this paper:First of all,the feasibility of dolomite pelletizing into balls,the mechanism of pelletizing and the drying process of pellets are studied.The main properties such as grain size composition of pellets,moisture content,drop strength,compressive strength,packing density and bulk density are also investigated.Studies have shown that:dolomite itself into a spherical coefficient k is 0.90 or more;prefabricated pellets into a ball mainly by the coalescence process mechanism and layering mechanism control As the temperature and flow rate of drying medium(argon)are increasing,the time needed for drying gradually becomes shorter.The size distribution of dolomite pellets produced with kaolin as binder is uniform.The shatter strength is 2.5 times/0.5 m,the compressive strength is 80 N per pellet.The packing density and bulk density of pellets increase with the increase of kaolin content in pellets.Secondly,illustrates that,effects of CO2 partial pressure on the calcination of pre-made pellets were analyzed based on the predominance diagram.The calcination mechanism of pre-made pellets was discussed.The gradient calcination process was proposed.Research shows that:pre-made pellets processed by gradient calcination can get good quality of the calcined dolomite and effectively shorten the calcination time of dolomite.To meet demand for quality silicon thermal refining magnesium,The burning rate of pre-made pellets was 45.023%,the hydration activity was about 33%,calcination loss was under 3%,and water absorption rate was far less than 1%.Third,the reduction process of making metal magnesium in vacuum condition using pre-made pellets as raw material were studied.The effects of reduction temperature,reduction time,diameter of pellets,amount of reducing agent and calcium fluoride and the storage time of pellets on the reduction process and nd comparing the pre-calcined white pellets and briquettes reductive.Research shows that:the reduction temperature is 1473 k,the reduction time is 120 min,the vacuum degree is 5 Pa,the diameter of pre-made pellet is 10mm,the silicon excess is 5%and the adding amount of CaF2 is 5%and the adding amount of 65%ferrosilicon is 20%.Under the same reduction conditions,reducing contrast of prefabricated pellets and briquettes showed calcined dolomite shows that,using pre-made pellets can increase the reduction rate by 15%and reduce the reduction time by 8.0 h.The kinetics of the reduction of pre-made pellets under vacuum condition to produce magnesium was investigated.The factors affecting the reaction rate are explored.The macro kinetic equations were established.Research shows that,the reduction process conforms to first order reaction kinetics.The reaction process is controlled by the diffusion of magnesium vapor through pores between the particles in the pellets.As the diameter of pre-made pellet increases,the activation energy of the reduction reaction of pre-made pellets changes a lot.When the diameter increased to a certain range,the activation energy does not change much.Adding calcium fluoride prefabricated pellets does not change the activation energy of the reduction reaction,while it plays an important role in accelerating the reaction.Finally,based on the above studies,the new process "completed in the same tank" based on pre-made pellets is put forward,that is to say the calcination and reduction of pellets are in the same reaction tank.The CO2 produced in calcination process can preheat pellets in the next tank and then be collected for other uses;the heat in calcination process of pellets can be used in the reduction process.Compared with Pidgeon method,the new process using vacuum-thermal reduction of pre-made pellets to produce magnesium has great cost advantage(saving more than 42%cost approximately).The new process is helpful for the collection and utilization of CO2;it can reduce CO2 emissions by 26.55t for one ton of magnesium production.In conclusion,the new process using vacuum-thermal reduction of pre-made pellets to produce magnesium was researched this paper.It can be used to raise the utilization of magnesium resources,reduce the energy consumption,and relatively reduce the emissions of CO2.It's a kind of energy-saving and environment-friendly magnesium producing technology.