Al2O3-CaO-MgO-Na2O-SiO2 Slag

Name: Al2O3-CaO-MgO-Na2O-SiO2
Diagram No.: 1131
Type of diagram: TTT
Chemical composition in weight %: See the table
Group: Slags
Note: SHTT (Single Hot Thermocouple Technique) experiments were performed on samples 1-5. All samples were melted at 1300C for several minutes. Sometimes, big bubbles which could be attributed to moisture in samples were found in samples and they were very different to remove. It was found that it is possible to quench all samples except sample 1 to temperature as low as 800C with the largest cooling rate. All sample investigated in the present work are transparent at molten state, which guarantees clear observation of crystal.

TTT diagrams for samples are constructed and shown in Fig.1 and Fig.2. All TTT diagrams are classic C-shaped curves. Only one nose was determined in diagrams, though three different crystals were found in annealed sample for XRD measurements. The crystallization process for sample 3 is very sluggish so that incubation time at investigated temperatures is more than 1000 seconds, therefore no exact incubation time is determined for sample 3. Since crystallization process for sample 1 is so fast that it is impossible to quench the liquid slag into glassy state to temperature lower than 1000C, only TTT diagram at temperature more than 1000C were determined at the present work.

As shown in Fig.1, addition of B2O3 significantly increases the incubation time of crystallization for slag samples, which means that B2O3 could decrease crystallization ability of slags. Slag with 0% B2O3 has a minimum incubation time less than 1 second at nose temperature, while slag with 10% B2O3 was not found to be crystallized within 1000 seconds.

The effect of B2O3 on crystalline fraction in solid slags was previously investigated by Chang et al. They found the crystalline fraction will be decreased if F in slag is replaced by B2O3. Fox et al. also investigated the crystallization of fluoride-free mould fluxes with Na2O and B2O3. They obtained a slag film with 52% crystallinity for flux with 1.5% B2O3 and a completely glassy film for flux with 5%. The present result for B2O3 effect is consistent with these reports.

B2O3 is generally accepted as a network forming oxide in slags. Complex networks based on [BO3] triangular unit and [BO4] tetrahedral unit could form in structure of slags containing B2O3. In borosilicate slags, borate network could also incorporate into silicate network to increase the complexity of network structure[13]. Increased complexity of network structure helps to improve the glass forming ability of molten slag and weaken the crystallization ability of slag. Therefore, B2O3 is also regarded as a good glass forming oxide. It was reported that [3] there was no break temperature found in viscosity temperature curves in slag with more than 8% B2O3 content. This indicates that no crystal formed during cooling in viscosity measurements and slag become glassy.

Effect of TiO2 on the crystallization of slag samples is not as obvious as that of B2O3. It might be due to uncertainty for TTT diagram determined by SHTT technique. For example, the first crystal is judged by eyes of investigator. Incubation time at nose temperature change slightly as TiO2 content increase from 3% to 5%, and then decrease as TiO2 content increase from 5% to 10%. It seems that crystallization of slag is promoted as TiO2 increase from 5% to 10%. It has been reported that increase of TiO2 in slag would lead to decrease of viscosity of slag. Low viscosity of slag is favorable to crystallization of slag with lower barrier for diffusion. Accordingly, increase of TiO2 in slag would promote crystallization ability of slags.
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