Powdering of rare earth ferrosilicon alloy and its prevention measures

Certain rare earth alloy is placed in the middle of the air will automatically powder, especially a rare earth alloy concentrates as raw materials and the production of alloys carbothermal reduction, dusting tendency is severe, some only in a few tens of minutes The powdered alloy was once exploded into a dark gray powder. It has been observed that the gas escaping from the alloy during pulverization often has a pungent odor, which is accompanied by a cracking sound. Alloys with a tendency to chalk tend to speed up the pulverization process in humid air. Powdered alloys pose certain difficulties for traditional applications.

The easy-powder alloy is placed in a device for studying the pulverization process, and the collected gas is mainly hydrogen gas, and a small amount of PH ₃ and AsH _{3 are} reduced. The relationship between the amount of hydrogen discharged and the total gas versus time is shown in Figures 1 and 2.

Fig.1 Relationship between the amount of hydrogen catalyzed by alloy powder and time

Figure 2 Volume change of alloy powdered escaping gas

According to the literature, after the alloy is pulverized in air, its mass increases by about 0.62%. The change in phosphorus content was 0.07%, and the change in arsenic content was 0.01%.

Table 1 Analysis results of some elements before and after pulverization of the alloy Unit: %

Uses pure raw materials, smelting in vacuum induction furnace silicon iron alloy, rare earth and rare earth silicides have chalking.

Analysis of the causes of alloy powdering

There are many factors affecting alloy pulverization, such as smelting temperature impurity content, cooling rate and ambient temperature. In the long-term observation in practice, it is found that the easy-to-powder alloys generally have coarse crystal grains and loose structure, especially the alloy of the composition of the II region shown in Fig. 3, which is automatically pulverized in the air. The sideline of this area is extended to the ferrosilicon line, and its range is just the ferrosilicon easy-to-powder zone. Therefore, the main reason for alloy pulverization may be that when the alloy with easy pulverization is slowly cooled, rare earth silicide or bismuth phase ferrosilicon precipitates at the grain boundary, and the precipitated compound is oxidized by water and air in the air, and the volume is expanded to pulverize the alloy. .

When the alloy encounters water, it accelerates the pulverization, and the precipitated gas has a calcium carbide smell. Therefore, it can be inferred that the pulverization is related to the inclusion of a trace amount of carbide or slag phase in the alloy.

Measures to prevent alloy powdering

(1) Providing alkalinity of raw materials, reducing the content of harmful impurities in raw materials Increasing the alkalinity of raw materials can effectively reduce the silicon content in the alloy and make it outside the easy-to-powder zone. However, changes in alkalinity affect the entire smelting process. It is important to reduce the harmful impurities in the raw materials, especially the phosphorus content. When de-ironing the rare earth concentrate, only pay attention to controlling the temperature and alkalinity. If necessary, add some strengthening measures such as adding a small amount of ferrosilicon to ensure the phosphorus content in the slag meets the requirements. . Nanjing Institute of Metallurgy uses rare earth concentrate powder to directly smelt ferrosilicon alloy at high alkalinity, so that phosphorus enters the atmosphere and slag, and the obtained alloy is not powdered.

Figure 3 Rare earth alloy chalking area map

(2) Increasing the temperature of the furnace to raise the temperature of the furnace to prevent the inclusion of slag and other impurities in the alloy, and the separation of the slag and iron is good; after the temperature of the furnace is increased, it is also easy to carry out the treatment in the package.

(3) Casting ingot and water quenching Casting ingot and water quenching are measures to accelerate the cooling rate of the alloy, prevent alloy segregation and refine the grains, and have certain effects on suppressing alloy powdering. Especially water quenching, simple operation, little change in chemical composition of the alloy before and after water quenching, the specific data are listed in Table 2. However, after water quenching, the alloy particles are often hollow and the bulk density is small. It is necessary to carry out an in-depth test on this treatment. Beijing University of Science and Technology uses argon gas to make the alloy powder directly used in the blowing device.

Table 2 Comparison of chemical composition of alloys before and after water quenching Unit: %

Use and treatment of powdered alloy

(1) It is directly applied to the conventional method of adding rare earth in steel in the blowing method or the core wire. The particle size of the powdered alloy is too small, the surface energy is large, and it is inevitable to float in the steel to be treated. The surface of the liquid or molten iron is oxidized in vain, causing the process to fail. However, at present, with the improvement of the rare earth addition method, the blowing method and the core wire feeding method have been promoted and applied, and the bulk alloy needs to be pulverized to a certain particle size, and has a certain shape to be applicable. From a large amount of analytical data, the chemical changes before and after the powdering of the alloy are not large, and the oxide layer on the surface is also very thin, which does not affect the use.

(2) Preparation of other varieties of alloys In addition to rare earth ferrosilicon alloys, which are smelted by silicon thermal reduction, most other types of alloys are produced by melt compounding.

(3) The remelted alloy after pulverization is remelted under the protection of rare earth rich slag (or slag after smelting rare earth alloy) with a certain alkalinity, the grade of rare earth is slightly improved, and the silicon content is slightly decreased. The data is shown in Table 3. The alloy is generally no longer pulverized after remelting.

Table 3 Comparison of main chemical components before and after alloy remelting Unit: %

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