Experimental study on calcined white vacuum carbothermal reduction

I. Overview

From 1972 to 1976, Winand and his colleagues conducted a small and expanded test of calcined white vacuum carbon hot magnesium . Under the conditions of reaction temperature above 1600K and condensation temperature of 200-500 °C, columnar magnesium crystals were obtained. The purity of magnesium was 70%～80%; Dr. Li Zhihua controls the reaction temperature above 1500°C and the condensation temperature is 300～500°C. The vacuum carbothermal reduction test of magnesium oxide is carried out to obtain the crude fibrous magnesium crystal. The optimum purity of condensed magnesium is as high as 94.38. %. On this basis, the active fumed white Jiaoyou as raw material, carbothermal reduction tests carried out under vacuum in order to obtain crystals of magnesium metal.

Second, raw materials

The test raw materials mainly include: calcined white (MgO41.12%), calcium fluoride (>99%), coal . The composition of coal is shown in Table 1.

Table 1 Composition of coal

Third, the equipment

The test device mainly consists of a vacuum carbon tube resistance furnace, a temperature detection and control system, a vacuum acquisition and detection system. The vacuum carbon tube resistance furnace includes a graphite base and a graphite heating element. In order to strengthen the heat preservation effect, the heat is concentrated to increase the heating temperature of the furnace, and the heat radiation is prevented from causing the furnace wall to overheat. A three-layer graphite heat shield is arranged outside the graphite heating element, and each two layers of the heat shield high filled between aluminum refractory fiber cotton to increase the insulation effect. The vacuum pump used had an ultimate vacuum of 6.67 x 10 ^-2 Pa. The system has a leak rate of less than 0.1 Pa l/s. When the furnace temperature is 1400 °C, the ultimate vacuum of the system reaches 15~20Pa, the ultimate vacuum of the system can reach 3Pa; when the sample is not placed in the furnace, the vacuum degree of the temperature system can meet the needs of the experiment. . The furnace temperature control accuracy is ±10 °C, the furnace temperature can be heated to above 1500 °C, and the heating system can also meet the needs of the test.

Between the graphite crucible and the condenser used as the reactor, a condensing hood can be added to block the floating fine material which may be generated by the reaction, and the escutcheon can be used as a condenser.

Fourth, the method

According to the optimal calcination conditions obtained by the experimental study, the dolomite was calcined at 1000 ° C for 2 h, and the calcined material, pulverized coal and calcium fluoride were compounded according to the ratio. Under the pressure of 2 MPa, the diameter was about 3 cm and the height was 1.5 cm. The columnar material was baked at 300 ° C for 1.5 h, and the material was vacuum-reduced at a set temperature.

V. Results and discussion

The reaction starts with the ratio of the atomic ratio of C and Mg to 1.2:1, the reaction temperature is controlled at 1500 ° C, the reaction is almost no condensation product, and the amount of C is increased so that the ratio of C to Mg atoms is 3:1. The mass percentage of Mg in the reactant after the compounding was 16.27%. The test records are shown in Table 2.

Table 2 Test record

The condensation products of the reaction No.1 and No.2 were brown, the reaction temperature and the condensation temperature were all low; the silver- white metallic magnesium particles were obviously observed in the reaction condensate No. 3; the reaction condensation products of No.5-7 were milky white or silvery white; The reaction product No. 8 is a magnesium crystal having a length of 2 to 3 mm which is a vertical condenser wall (the distance between the insulating escutcheons is 2 mm, and the inner layer is covered with holes and cracks). The reaction condensate and residue of 1, 5, 8 and 10 were selected for phase and component analysis. The results are shown in Tables 3 and 4.

Table 3 Product composition analysis

Table 4 Volatilization rate

X-ray diffraction analysis of the condensate showed that the condensed products were mainly magnesium metal and magnesium oxide, and a small amount of carbon was present, and the presence of metal carbides, nitrides, calcium and silicide could not be excluded. The condensed magnesium exhibits different crystal forms due to the difference in condensation temperature, and a preferred orientation occurs. The condensing temperature is controlled to be higher (400-550 ° C), the magnesium metal is fibrous, and when the condensing temperature is slightly lower (200-300 ° C), the magnesium is powdery; at the same time, the condensate increases with the condensing temperature. The content of magnesium in the metal increases. The char phase appears in the condensate because the affinity of carbon for oxygen decreases with decreasing temperature. In the case where there is more than 1000 ° C and carbon is present, the carbon oxide is almost entirely CO, and at a lower temperature, the CO and the condenser wall are inevitably decomposed. Magnesium oxide in the condensate is reoxidized during the reaction. The vapor pressure ratio of calcium and magnesium in the reaction increases with the increase of the reaction temperature, but when the temperature exceeds 1400 ° C, calcium combines with carbon to form calcium carbide, and the amount of magnesium carbide is small. The pressure of the carbide is small, and the carbide can be stably present within the vacuum condition of the reaction and the pressure range of the CO. During the reaction, due to the presence of elemental silicon in the calcined white, silicon and oxygen will form an oxide, and the vapor of calcium and magnesium will form a corresponding silicide with silicon monoxide, but the free energy of formation of calcium silicide is greater than that of magnesium silicide. In this case, the contamination of magnesium with magnesium is small, the condensation temperature is higher, and the contamination of impurities can be reduced, thereby obtaining a better test effect.

When the reaction temperature is 1400 ° C, carbon remains in the residue after the reaction; the reaction at 1500 ° C reduces the amount of residual carbon in the slag; the content of elemental magnesium in the slag decreases with increasing temperature, from 5.71% to At 3.76%, the volatilization rate of magnesium also increased from 86.6% to 88.6%, indicating that increasing the temperature of the reaction is completely beneficial to the reaction.

According to the pressure change in the reaction process, the progress of the reaction can be judged. For reaction 10, at 1550 ° C, the pressure in the furnace is 253 Pa; when the reaction proceeds to the later stage, the pressure in the furnace is 195 Pa, and remains unchanged; when the maximum pressure in the furnace is 371 Pa, the temperature is 734 ° C. The reaction proceeds at about 700 ° C, the carbon in the material changes, the tar in the carbon volatilizes to increase the pressure in the furnace, and then the pressure in the furnace decreases. As the temperature increases, the reaction between the formation of magnesium metal and CO occurs, and the pressure in the furnace rises again. After the reaction is completed, the pressure in the furnace decreases and remains unchanged. When the maximum pressure in the furnace occurs, the temperature is about 679-765 ° C. It is necessary to reduce the coking process of the material in this temperature range in order to reduce the pumping operation of the vacuum pump.

After the material is mixed with water, it is stirred and pressed. During the calcination process, the carbonate in the dolomite is decomposed, the specific surface area of ​​the material is increased, the crystal grains are fine, and the dolomite remains after burning a certain degree of parent salt illusion, namely MgO and CaO. The Mg ²⁺ , Ca ²⁺ , O ^{2- in the} still occupying the lattice position of the original dolomite, the decomposition product has the morphology of the parent salt particles, and the structure is loose and porous. The material is mixed with water, the strong disintegration of the hydration process and the formation of hydroxides make the particles finer. During the heating process of the reaction, the hydroxide will adsorb and desorb between the two phases of MgO and CaO. A typical interfacial reaction reduces interfacial energy and causes new character defects and distortions in the particles, resulting in increased oxide activity. However, after calcination, calcium and magnesium are present in the form of oxides. Light burning of magnesium oxide is a kind of cementitious material. Whether water is used as a binder is unfavorable to the reaction, and whether other binders can be used without affecting the reaction. Whether the forming of materials under high pressure is feasible is still to be further studied and weighed.

In the test No. 8, two condensing hoods were placed in the upper part of the reaction crucible, and the distance between the two hoods was about 2 mm, the inner layer was covered with cracks, and the circular holes were circumferentially opened. After the reaction, the metal crystals at the slits and holes were grown perpendicular to the outer casing, and the condensate on the condenser was powdery. The authors believe that the temperature gradient is small at the narrow gap between the two covers, and the fine grains that are first condensed can stabilize the nucleation, and the slightly larger grains grow due to the consumption of fine crystals with a large chemical potential. To obtain the desired condensation product, not only is the temperature of the condenser high, but the temperature of the condensation space is equalized.

Conclusion

(1) The reaction proceeds smoothly, and the temperature is required to be higher than 1500 ° C. The higher the temperature, the more complete the reaction, and the higher the degree of metal volatilization.

(2) The increase of the condensation temperature can improve the condensation effect and at the same time optimize the metal quality. The temperature of the condensation should be 500 ° C or higher, and the temperature of the condensation space is equalized.

(3) The reaction can be judged according to the change of the gas pressure in the furnace. The maximum pressure appears when the char is escaping at about 679-765 ° C, and the coke formation of the material in this temperature range is necessary. .

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