1 Quote

Dry ramming material is composed of a certain granularity of refractory aggregate, powder, binder and admixture by artificial or mechanical ramming construction, and hardened at a certain temperature of amorphous refractory material, mostly used as induction furnace lining material. Dry ramming materials for coreless medium frequency induction furnaces can be divided into acidic, alkaline and neutral three categories. Coreless induction furnace is widely used in the casting industry because of its strong heating capacity, intermittent operation, low lining cost, flexible and convenient use, and is the main equipment for melting iron and non-ferrous metals or molten heat preservation in domestic casting enterprises. At present, the lining material of many induction furnaces uses silicon dry ramming material.

Dry ramming material is loose, no adhesion before forming, only by vibration or strong ramming can obtain a dense structure, which makes the particle size distribution (reasonable gradation) become one of the important factors affecting its performance. As an important part of induction furnace, furnace lining not only directly affects the quality of smelted metal, but also affects its production cost, so the choice of furnace lining material is very important. Silica sand and neutral and alkaline refractory materials compared to the low price, the current boric acid as a sintering agent silica dry ramming charge is widely used, quartz material has high temperature resistance, low thermal conductivity and low thermal expansion (α = 0. 54 × 10-6 ℃ -1), but the siliceous material has a complex crystal transformation at high temperature, so it is necessary to consider the effect of the particle size composition of silica sand and the type of binder on the high temperature performance of the material. From the phase diagram of SiO2-B2O3, the amount of B2O3 added should be less than 2%. With the increase of raw material particle size, the compressive strength of the burnt siliceous material increases, so the particle size of silica sand added to the ramming material is appropriately large. These factors will affect the properties of the dry silica ramming samples after firing. In this paper, the effects of particle size composition of different raw materials and types of binders on the properties of silicon dry ramming materials were studied in order to determine the appropriate particle composition and composition of the materials. A good binder.

2 Verification

The raw materials used are 3 ~ 1 mm, ≤1 mm and ≤0, respectively. Quartz sand with 053 mm particle size (wSiO2 = 99. 53%) and industrial grade silica powder, boric acid, boron oxide, sodium metasilicate (0. 5 ~ 1 mm), calcium oxide. The proportion composition of siliceous dry material is shown in Table 1. Accurately weigh the material according to the ratio in Table 1, dry mix and import it into the mold for manual ramming and forming. The molded sample was kept at 1500 ℃ and 1550 ℃ for 3 h for demudding. Based on the particle gradation of S3 sample, the ratio of samples with different binders is shown in Table 2 below. After firing at 1500 ℃ and 1550 ℃, the samples were measured according to GB /T 5988-2007, GB /T 2997-2000 and GB /T 5027-2008, respectively. Linear variation, apparent porosity and compressive strength were analyzed by X-ray diffractometer.

3 Results and discussion

FIG. 1 is the performance diagram of the sample after firing at different temperatures. As can be seen from FIG. 1a and b, the porosity of samples with different particle size compositions after firing at 1500 ℃ is significantly higher than that of samples fired at 1550 ℃, and the porosity and bulk density of sample S2 after firing at different temperatures have great changes. The porosity of S3 sample was higher than that of S1 and S2 at different temperatures. Small apparent porosity, yes. Large volume density. The reason is that with the high sintering temperature, the sintering degree of particles increases, and the combination between particles and particles is tight. At the same sintering temperature, S3 samples are packed more closely and pores are filled more fully.

FIG. 1c shows the linear change rates of samples with different particle gradations after firing at 1500 ℃ and 1550 ℃. It can be seen from the figure that the linear change rate of S2 and S3 samples after firing at 1500 ℃ is significantly lower than that after firing at 1550 ℃. Linear change rate of S3 sample in the three groups of samples after firing at the same temperature. Small, line change rates were 7. 79% and 8. 03%. The reason is that the crystal transformation of the quartz phase in the particles requires a higher temperature. When the sintering temperature is high, more liquid phase has been formed in the dry ramming material, and the volume expansion of large particles will be absorbed by the liquid phase, reducing its expansion rate [10]. Therefore, considering the influence of volume stability at high temperature, the particle size composition of S3 sample is better for quartz sand as a furnace lining material.

FIG. 2a and b show the apparent porosity and bulk density of different binder samples after firing at 1500 ℃ and 1550 ℃, respectively. It is known from the figure that the apparent porosity of S20 sample at 1500 ℃. Low, bulk density. High. Apparent porosity of S26 sample at 1550 ℃. Low, S29 apparent porosity. High. The apparent porosity of S20 samples is significantly different at 1500 ℃ and 1550 ℃. The reason is that the binder added to the S20 sample at 1500 ℃ can promote sintering well, which is conducive to the discharge of pores in the sample and improve the density of the material. Figure 2c shows the linear change rates of the samples of different binders after firing at 1500 ℃ and 1550 ℃. After different samples were fired, the linear change rate of the samples was positive, which showed volume expansion. Linear expansion rate of S26 sample at 1500 ℃. High, is 8. 37%, the expansion rate of S20. Low, is 2. 69%. Linear expansion rate at 1550 ℃. The height is S24, which is 7. 5%, the expansion rate of the S20. Low, is 5. 23%. Therefore, it can be judged that the liquid phase formed by the fusion of the binder of S20 sample at high temperature can better reduce the expansion caused by the grain crystal transformation under the condition of the same particle size. FIG. 2d shows the compressive strength of different bond samples after firing at 1500 ℃ and 1550 ℃. Compressive strength at 1500 ℃. The high sample is S25, which is 10. 22 MPa,. The small sample is S23, ** 2. 07 MPa; Compressive strength at 1550 ℃. The highest is S20 at 9. 936 MPa,. The small sample is still S23, ** 2. 27 MPa. Compared with S21, there is no significant change in the apparent porosity and bulk density of S22 after firing, but the linear change rate and compressive strength of S22 are higher than that of S21, which may be because the increase in the addition of B2O3 promotes the sintering of quartz, and there will be more complex phase transition in the sintering process of quartz, accompanied by a certain amount of volume expansion. The linear rate of change is large, and the compressive strength of the sample is increased by promoting sintering. Through the comparison of S21, S23, S24 and S25, the bulk density of the latter three samples is not much different from that of the former, but the apparent porosity, linear change rate and compressive strength are higher than that of the former. Sodium metasilicate can be used as a binder in the process of quartz sintering. The addition of sodium metasilicate can promote the sintering of quartz sand and increase the compressive strength of the sample, but the law of apparent porosity and linear change rate is not obvious. CaO was also added as a mineralizing agent to control the crystal transformation of quartz. With the increase of CaO content, the apparent porosity, linear change rate and compressive strength of S27 are all changed, and the compressive strength of S27 is increased compared with that of S26, indicating that CaO can control the crystal type transformation (conversion of cristobalite to quartz) of SiO2 and make it develop in a direction conducive to its strength improvement. The addition of SiO2 micropowder can form a liquid phase and promote the sintering of quartz sand. However, with the addition of SiO2 micropowder, its strength decreases somewhat, which may be because the introduction of SiO2 changes its crystal type transformation to a certain extent, resulting in high expansion rate and low compressive strength. FIG. 3 shows the phase composition analysis diagram of the sample after firing. As can be seen from Figure 3a, the main phases of S20 sample at 1500 ℃ are quartz and cristobalite. Through semi-quantitative analysis, the content of quartz and cristobalite in the sample is 58% and 42%, and the conversion from quartz to cristobalite is not complete. Figure 3b shows the XRD pattern of S20 sample at 1550 ℃. There are more cristobalite peaks. Through semi-quantitative analysis, the content of quartz is 5% and the content of cristobalite is 95%, indicating that the conversion of quartz to cristobalite is relatively complete. According to the XRD pattern analysis, the expansion rate at 1550 ℃ is higher than that at 1500 ℃ because there is more conversion of quartz to cristobalite, so the expansion rate is higher. At 1550 ℃, because the transformation of quartz to cristobalite in the sample is relatively complete, the overall structural stability of the sample is relatively good, and it has higher compressive strength than other samples.

2. Properties of different bond samples after firing at 1500 ℃ and 1550 ℃ (a) apparent porosity; (b) Bulk density; (c) line change rate and (d) compressive strength

4 Conclusion

(1) After sintering at 1500 ℃ and 1550 ℃, the linear change rate, porosity and bulk density of the sample are significantly different, indicating that different particle compact packing degree has a greater impact on the performance of the product; (2) The sample contains more large particles, which can reduce the expansion rate of the sample during the sintering process, and its particle size distribution conforms to the principle of close packing. Fine powder can fully fill the porosity, reduce the porosity, and improve the bulk density of the product, thus making its compressive strength high; (3) After the binder generates a liquid phase at 1500 ℃ and 1550 ℃, it can wet the surface of the particles and reduce the large linear changes caused by particle expansion. 2486 Special Papers Silicate General Report Volume 34 After burning at 1550 ℃, the quartz to quartzite transformation will be complete, will have a better porosity and compressive strength.