Study On The Multiphase Transformation Of Bayer Red Mud In The High Temperature Roasting Reaction And Recovery Of Iron Aluminum, And Sodium

Bauxite residue, or red mud, is the largest environmental concern of alumina refineries mainly because of the fine particle sizes of this waste stream and its caustic nature. This waste presents serious problems for storage and potential environmental pollution. The treatment and utilization of high volume red mud waste has been a major challenge for the alumina industry.For this study, the Bayer red mud sample was supplied by the No.2 alumina plant of Shandong Aluminum Company, a branch of Chinalco. Based on analysis on elements and mineral phase distributions in red mud particles with different sizes, the thesis proposed that, the separation pretreatment of red mud by particle sizes was beneficial to red mud utilization. The study focused on the recovery of Al, Na and Fe from fine red mud with soda-lime roasting under reductive atmosphere. Both of direct reductive iron (DRI) and magnetite were considered as desirable products in different reductive atmospheres. Optimizing experiments for soda-lime roasting and leaching processes were carried out. The reductive soda-lime roasting process of Bayer red mud at high temperature was modeled, and the effect of PO2 and composition of the system on phase transformation and recovery of Fe, Al and Na were discussed.(1) Based on the distributions of elements and mineral phases in red mud particles with different sizes, the thesis proposed that, the red mud sample can be divided into two parts, i.e. bulk red mud particles, with particle sizes above 0.075 mm, and fine red mud particles, with sizes below 0.075 mm. Iron ore and sand can be separated out from the bulk red mud with physical beneficial technologies. Fe, Al and Na can be recovered from fine red mud particles.(2) The optised conditions for DRI recovery from red mud are sintering temperature of 1300℃,110 min, with 18 g carbon and 6 g addition for 100 g red md. Under these conditions,81.40% Fe was recovered from red mud with 89.05% of total Fe contents (TFe). When the samples were roasted at 1100℃, over 80% Al were concentrated into the solution, however, the TFe and recovery ratio of Fe were 34.46% and 45.83%, respectively. A serial of experimental results showed that, the temperature requirements of recovery of Fe and Al were much different.(3) For recovery of magnetite from the fine red mud, the coal char was used as reductive agent. When the fine red md was roasted with coal char (0.4-0.5 g/100 g red mud) at 1000℃for 1h, spinel can be obtained as one of main phases. Under optimized soda-lime roasting and leaching conditions, the extraction of Al and Na were 75.74% and 88.42%, respectively. And 80.70% of Na in raw red mud was recovered. After magnetic separation of leached residue,50.01% of Fe was concentrated with TFe of 50.29% in concentrate. In separated residue, the content of Na was 1.55%, which would be available for production of building materials.(4) The roasting process was modeled with MPE (Multiphase Equilibrium), a thermodynamics modeling software. Some experiments were carried out to verify the modeling results. The modeling results showed that, the equilibrium phases in reductive roasting system are sodium aluminosilicate (NaAl(Al, Si)O4), melilite ((Ca, Na)2 (Al, Mg, Fe2+) [(Al, Si)SiO7]), spinel (including the solid solution of Fe3O4 and FeAl2O4), hematite (Fe2O3), wustite (FeO) and metallic Fe. The existing of iron containing phases depends on the atmospheres of the system. The PO2 (Partial Pressure of O2) should be kept between 1.0 x 10-5 atm and 1.0×10-13 atm to make the spinel phase stable in the system. Experiments proved that the spinel phase in samples were solid solution of aluminous spinel (Fe (Fe, Al)2 O4), ulvite ((Fe (Fe, Ti)2O4, or (Ti, Fe) Fe2O4)), as well as magnetite (Fe3O4). The results were agreed with MPE results.The soda-lime roasting processes of the Bayer red mud were also modeled. The calculated results showed that, the addition of CaO and Na2O in limited ranges can decrease the stability of spinel phase. And the addition of CaO may decrease the Si contents in sodium aluminosilicate, as well as the addition of Na2O decrease the Al content in spinel. The temperature range of 850℃-1000℃is suitable for the formation of desirable phases i.e. sodium aluminosilicate, larnite and spinel. However, the reaction velocity will be slow.After pretreating of Bayer red mud, iron ore and sand can be separated from the bulk red mud. Fe, Al and Na can be recovered from the fine red mud, and the residue can be used for producing building materials. The study on effects of reaction atmospheres and compositions of the system on phase transformation and recovery of Fe, Al and Na will be a theory foundation for realization of zero waste of Bayer red mud.