Basic Study On Titanium Enrichment Selectivity In Pentoxide Slag (CMSTA)

This research was focused on the thermodynamics and kinetics of Ti-enrichment selectivity in pentoxide slag system of CaO-MgO-SiO₂-TiOx-Al₂O₃.As a result, the patterns of related factors and optimum conditions on Ti-enrichment selectivity were obtained. The thermodynamic equilibrium methods of Slag-Sn(l)-C and Slag-Sn(l)-CO systems were used to determine activities of a CaO, a TiO2and a Ti2O3in molten slag. This is the first time that the a_Ti2O3was directly determined and a_TiO2 was gained by deduction from a_Ti2O3. The active coefficients of titanium solved in liquid Sn(l) were obtained as well. They are γ_Ca^o =7.06×10^-5 and γ_Ti^?=3.77×10^-3 respectively. Based on the experimental results, the expressions of basicity, a CaO and a TiO2 for the deoxidized slag were established: where, STiO₂ = ?[w( CaO)/w(SiO₂)-1]?8.57w(MgO)+5.71w(TiO₂)+4.29w(Al₂O₃). By observed results of Ti-enrichment level of different oxidized samples under microscope, it was found that the content of Ti in the slag had a less effect on the amount of precipitated Perovskite but strongly affected Ti-enrichment level. All effect on Ti-enrichment level could be attributed to both slag basicity and oxygen potential in the slag. Therefore,the high basicity and high oxygen potential are the keys to enhance Ti-enrichment level. The optimum formula to increase Ti-enrichment level is as follows. The natural additive, as the sixth component, could improve the situation of diffusion in molten slag and increase Ti-enrichment level in large scale. The experiment showed that optimum natural additive could increase Ti-enrichment level up to 85% only if slag basicity reached 0.85 at high oxygen potential. The study on the kinetics of slag oxidation included three parts. The first was static oxidation of slag in air and the second was dynamic oxidation with pure oxygen blowing into slag bath. The third is to make sure the effect of oxygen potential within slag on Ti-enrichment level . The following is in detail. 1. The EMF approach was used to determine the behavior and mechanism of slag oxidation for the first time. Experimental results showed that oxygen transmission in molten slag is the rate-limiting step for the slag oxidation. When ferrous or ferric oxide presented in the slag, the multivalence ions of iron were a key media for oxygen transmission. It accelerated the oxidation of (Ti2+ ) and ( Ti3+ ) in the slag by coupling reactions with (Ti2+ )and ( Ti3+ ). Because of the coupling reaction, the concentration changes of all multivalent ions in the slag were correlated. This correlation was decided by oxygen potential within the slag. The oxygen partial pressure, which is the expression of oxygen potential, could represent the oxidizing state of molten slag , and the rate changed with time is just the velocity of the slag oxidation. Then, the apparent rate-equation of slag oxidation could be set up as following: (P)V(P)kAVA22222 *d*OOOOO DPPdtdP = δ?=?= kh d (PO*2?PO2) and the apparent activation-energy E of oxygen diffusion in molten slag is the value from 112.4 kJ·mol-1 to 156.9 kJ·mol-1. 2. Dynamic oxidation of the slag by blowing pure oxygen into slag bath could shorten slag oxidation time and the time needed for complete oxidation only took 1/6～1/9th of that for static oxidation. The apparent rate-equation of slag dynamic oxidation is expressed as: =dtdPO2= k ×kdPO * 2 ?e?k?kdt Having molten slag be oxidized thoroughly could largely increase slag oxygen potential and thus the precipitation of Perovskite, which raised Ti-enrichment level up to 30%⁴0%. The increased precipitation of Perovskite with the increased of oxygen potential in molten slag reflected that the ability of Perovskite precipitation could be confirmed by oxygen potential. Thus, using slag oxygen potential could accomplish on line control with the end point of slag oxidation and Perovskite precipitation. . 3. Based on the results of slag oxidation , experimental phenomena and theoretical analysis, the relationship between heat releasing and temperature variety of molten slag during its dynamic oxidation was obtained. It included the following content: Without considering heat-lose, the released heat by slag oxidation could raise slagtemperature 406 K from its initial 1723 K. The chief matter of heat releasing was iron oxidization, the heat effect of which represented about 55% total heat released during slag oxidation. Heat releasing by slag oxidation resulted in a unique peak on the cooling curve, the height and width of which were related to the amount of oxygen supplied and deoxidized matter presented. Without additive presented in the slag,the heat released by the oxidation for over 10 tons slag was enough to compensate the heat-lose by radiation. This allowed the unique peak at a high level, slag temperature to stay over 1723 K for 70 min and slag-cooling rate was less than 1K/min , which was available to Perovskite precipitation and growth . With additive presented in the slag,in contrast,net heat released by slag oxidation and addtive melting could be dropped down so large that the unique peak was risen at a lower position and the time of slag temperature over 1723K was shorten. In this case, only if intensified oxygen had been supplied, would there be enough heat to melt additive and to end oxidation before Perovskite precipitation started. Effect of slag amount on its cooling rate is notable——the less the amount of slag,the more sensitive the rate would be. The sensitivity would be feeble when slag amount was over 50 tons.