In 2002, China's magnesium output reached about 200,000 tons, which is half of the world's magnesium production, ranking first in the world. After the aluminum industry, it has become a major magnesium industry.
China's magnesium output is nearly 80% exported, but because its production technology has a certain gap compared with the world's advanced level, its profit level is not high.
In order to increase the reduction ratio of magnesium, reducing the reaction temperature, can increase the life of the reduction, shortening the cycle time reduction, studied by iron aluminum silicon as a reducing agent instead of the new technology, some plants also had a similar test, but a The price of aluminum has strengthened and the price of magnesium has weakened. This process is not reasonable considering economically, so this research was interrupted.
This study used a kaolin-type coal gangue by carbothermic production of inexpensive Al-Si-Fe alloy as a reducing agent, to study the effects of various process parameters on the rate of reduction in the vacuum thermal reduction of magnesium smelting process and obtain Better results. It is believed that this research is of no benefit to the development of new high-efficiency and low-cost processes in China's magnesium industry.
First, the experimental method
(1) Experimental device
In Figure 1, the reduction tank is made of 1Cr25Ni20Si2 to ensure the strength at high temperature. The vacuum system is connected in series with a Roots pump in front of the mechanical pump to control the different vacuum. The temperature control adopts DWT-702 precision temperature meter, and the control precision is 1000 °C ± 1 °C.
Figure 1 Vacuum heat reduction method magnesium smelting device
1- electric furnace; 2-reduction tank; 3-material; 4-Mc vacuum gauge;
5-ionized vacuum juice; 6-filter; 7-roots pump; 8-mechanical vacuum pump
(2) Reducing agents and ingredients
The composition of the reducing agent was: Al: 46.7%; Si: 45.4%; Fe: 7, 6%; Ti: 0.2%.
When formulating, use the following formula:
2MgO+2CaO+Si=2Mg+2CaO·SiO 2 (1)
21MgO+12CaO+14Al=21Mg+12CaO·7Al 2 O 3 (2)
(3) Experimental conditions
According to the experimental requirements, Table 1 gives the experimental midpoint values.
Table 1 Process conditions at the midpoint of the experiment
Reduction temperature °C | Reduction time h | Group pressure MPa | Reducing agent dosage Wt% | Vacuum pump Pa | Condenser cooling temperature |
1150 | 2.5 | 20 | 2 | 5 | in |
Each time the feed amount was 200 g, CaF 2 was added to a fixed value of 3%.
The calcined dolomite and magnesite are cooled to room temperature in a sealed iron crucible. After being taken out, it is ground and milled in 5 minutes.
Second, the experimental results and discussion
(1) Effect of temperature on reduction rate
The effect of temperature on the reduction rate is shown in Figure 2.
Figure 2 Effect of reduction temperature on magnesium reduction rate
It can be seen from Fig. 2 that the effect of the reduction temperature on the reduction rate is very remarkable, and the reduction ratio differs by about 10% between 1100 and 1200 °C. Whether it is burning coal or burning oil, it is not difficult to reach a furnace temperature of 1200 °C. The factor that limits the temperature rise appears on the material of the tank. At present, the critical point of high temperature strength of domestically produced 1Cr25Ni2OSi2 is exactly in the region of 1150 to 1200 °C. At higher temperatures, the strength of the tank is reduced, and some are even convoked, reducing the life of the tank. So in actual production, not the higher the temperature, the better.
(2) Restore time
The effect of the reduction time is shown in Figure 3.
Figure 3 Effect of restore time on reduction rate
The so-called reduction time is also the production cycle, which is determined by the heat transfer time and the time the product diffuses into the condenser. When other process conditions are fixed, the reaction time is mainly determined by the heat transfer time:
Where: t 2 - is the temperature of the outer surface of the can;
T 1 - is the material temperature;
γ 2 - outer diameter of the can;
γ i - the inner diameter of the tank;
Γ- is the radius of the unreacted material;
λ b - is the heat transfer coefficient of the tank;
Λ- is the overall heat transfer coefficient between materials, and other symbols have a conventional meaning.
If the temperature t of the reaction is a constant value, the higher the furnace temperature, the faster the reaction, the larger the radius of the tank, the longer the reaction time, and the larger the heat transfer coefficient, the more favorable the reaction proceeds.
In order to achieve higher production efficiency, it is desirable to install more materials in the tank; however, the increase in the inner diameter of the tank increases the reaction time, making the tank easier to twitch. At present, it is generally believed that the outer diameter of the can is within 285 mm to 340 mm, but this is indeed a problem that needs to be further solved by theory and practice.
(3) Group pressure
Figure 4 The effect of the pellet pressure on the reduction rate
The melting point of aluminum is 660 ° C, the melting point of silicon is 1420 ° C, the melting point of A1 (46.7)-S1 (45.4)-Fe (7.6) alloy is about 987 ° C, and the volume of aluminum increases by 7% after melting, which is beneficial to the reaction. In particular, after the alloy is melted, the liquid is solid-solid reaction, and the volume is increased, so that the reducing agent of the liquid phase is more likely to contact with the solid phase MgO, so that the reaction speed is greatly accelerated, and of course, it is not necessary to adopt a very large Group pressure. When the reducing agent is aluminum reduction much more active than magnesium, strontium, briquetting pressure reduction is only about 1/10 of ferrosilicon magnesium, is evidence of this issue.
(4) The effect of the excess degree of reducing agent on the reduction rate
In the actual production, the reducing agent is always excessive, and Figure 5 shows the effect of the degree of reducing agent on the reduction rate.
Figure 5 Effect of reducing agent excess on reduction rate
Since the impurities contained in the raw materials consume a certain amount of reducing agent, and the reducing agent will also have a small amount of oxidation, the utilization rate thereof is not 100%, so the amount of the reducing agent is always excessive. As can be seen from Fig. 5, an excess of 4% to 5% of the reducing agent is suitable. Too much reducing agent increases the loss of reducing agent, and conversely, magnesium production is significantly reduced.
(5) The effect of system vacuum on recovery rate
The effect of vacuum on the reduction rate is shown in Fig. 6.
Figure 6 Effect of vacuum and condensation temperature on reduction rate
In vacuum heat reduction of magnesium, vacuum is an indispensable condition for this process, and its influence on production is also great. The most direct effect of vacuum is the initial temperature of reduction. For example, when the degree of vacuum is 13.3 Pa, the initial temperature of reduction is 1070 ° C, and when the degree of vacuum reaches 5 Pa, the initial reduction temperature is lowered to about 1040 ° C, which increases the reaction rate. Another important role of the vacuum is to prevent the reducing agent and product magnesium vapor from being oxidized at high temperatures. Generally, the reduction reaction lasts for about 8 hours. In such a long period of time, if there is too much residual air in the tank, oxidation of the generated magnesium vapor and the added reducing agent is unavoidable. Another effect of the degree of vacuum is to directly affect the crystallization of the product magnesium. The higher the degree of vacuum, the denser the crystal of the product, the brighter the surface, and the higher the recovery rate during refining.
It should be noted that when the vacuum system of the reduction process is determined, the degree of vacuum in the tank is determined and cannot be changed during the production process. The same is true in the lab. In this study, whether to open the Roots pump to change the degree of vacuum. After feeding at room temperature, the vacuum is 5 Pa when the Roots pump is not opened. In the case of the Kairoz pump, the vacuum is 2 Pa.
(6) Effect of crystallizer temperature on reduction rate
The effect of mold temperature on the reduction rate is shown in Figure 6. In the figure, the high temperature is indirect water cooling, which is equivalent to the current situation. The low temperature in the figure is equivalent to direct water cooling. Since the specific temperatures of "high temperature" and "low temperature" cannot be determined, "high temperature" and "low temperature" are used to distinguish.
The Mg vapor is reduced from the material and diffused, and the surface of the condenser is condensed into crystalline magnesium. The speed determines the diffusion distance and concentration difference, that is, Fick's first law:
(4)
That is, it is proportional to the diffusion coefficient, proportional to the concentration difference, and inversely proportional to the diffusion distance.
However, in the reduction process, due to the pumping action of the vacuum pump, the Mg steam has a certain flow rate, then the flow rate:
(5)
Where: P1 - P2 is the difference between Mg vapor pressure and K is the drag coefficient.
It can also be seen from this formula that whether C 2 or P 2 is determined by the surface temperature of the crystallizer, and the lower the surface temperature, the lower the vapor pressure of Mg, the larger the yield of Mg and the shorter the reduction period.
(7) Effect of reducing agent on reduction rate
For comparison, 75 # ferrosilicon was also used as a reducing agent in the experiment, and the results are shown in Fig. 7.
Figure 7 Effect of reducing agent on reduction time
It can be seen from Fig. 7 that under the same reduction time, the reduction rate of the ferrosilicon reducing agent is about 4% to 5% higher than that of the ferrosilicon reducing agent, and the reduction time can be reduced by 1/4 for the same reduction rate. .
(8) Production of low-cost Al-Si-Fe alloy
At present, the aluminum price is higher than the magnesium price. It is obvious that the Al-Si-Fe alloy is prepared from pure aluminum. The reduction of magnesium by this alloy is economically untenable, but if the production of Si-Fe alloy is used to produce Al- For Si-Fe alloys, it is entirely possible to reduce the cost of the alloy. Table 2 shows the Al-Si-Fe alloy cost table produced by carbothermal reduction in an electric arc furnace using coal gangue as a raw material.
Table 2 Al-Si-Fe alloy cost estimation table
Cost name | Tons of alloy consumption | Unit price, yuan | Tons of alloy price, yuan |
Fly ash | 1.5t | 5 | 7.5 |
Slime | 1.6t | 20 | 80 |
electrode | 0.25t | 2000 | 500 |
Electric energy | 12500KWh | 0.35 (0.3) | 4375 (3750) |
wage | 500 | ||
Equipment depreciation | 200 | ||
Interest on bank liquidity | 200 | ||
other | 100 |
It can be seen from Table 2 that the production of ferrosilicon from coal gangue is similar to the price of ferrosilicon produced by electric arc furnace. The production of aluminosilicate from coal gangue has the following advantages:
1. About 10% of the fixed carbon in coal gangue can be directly used as a reducing agent, saving the amount of reducing agent.
2. Coal gangue contains about 14% of hydrocarbons. When these compounds are burned at high temperature, they increase the energy in the furnace.
3. After the organic matter in the raw material is volatilized, the ore is increased in surface area, the pores are increased, the activity of the mineral is increased, and the reduction is easy.
4. The binder used in the group is local coal slime, which is not only cheap but also has the same composition as coal gangue.
5. Use waste coal gangue (fly ash) as raw material and enjoy the national preferential policy.
(IX) Estimation of metal magnesium cost
Table 3 Silicon thermal method cost estimation
name | Tons of magnesium consumption | Unit price, yuan | Tons of magnesium cost / yuan | cost/% |
dolomite | 12.8t | 50 | 640 | 5 |
coal | 12t | 180 | 2160 | 17 |
Electricity | 1500KWh | 0.5 | 750 | 5.8 |
Ferrosilicon | 1300Kg | 5 | 6500 | 50.8 |
Still in the can (rental 60 yuan / day) | 1680 | 13.2 | ||
Auxiliary material | 200 | 1.5 | ||
Equipment depreciation | 150 | 1.2 | ||
wage | 400 | 3.1 | ||
Liquidity interest | 200 | 1.5 | ||
other | 100 | 0.8 | ||
total | 12780 | 99.9 |
As can be seen from Table 3, in the production cost of magnesium, the reducing agent ferrosilicon accounts for 50.8%, and a little more than half. And to reduce the reaction:
2MgO+2CaO+Si=2Mg+2CaO·SiO 2 (6)
1t magnesium production, 576 kg of pure silicon required; 75 # ferrosilicon is converted into 768kg, 70 # ferrosilicon was 823kg.
In ferrosilicon, a portion of silicon and iron form FeSi compounds, which reduces the activity of silicon. If the gap is to remove this part of silicon, the theoretical consumption of 70 # and 75 # ferrosilicon is 1056 kg and 924 kg, respectively.
From this, it can be seen that in the thermal reduction of silicon, the excess coefficient of the reducing agent is large, ranging from 30% to 50%. When magnesium is used as a reducing agent to reduce magnesium ore and dolomite, the utilization rate of aluminum can reach 95.24%. The excess factor is only 1.4% and does not exceed 5%. Since the ferrosilicon alloy is liquid at the reaction temperature, it is not a solid/solid reaction, but a liquid/solid reaction, so the initial reaction temperature is low, the reaction speed is fast, the utilization rate of the reducing agent is high, and the excess coefficient is small. Therefore, in the existing silicon thermal process, if aluminum ferrosilicon is used as the reducing agent, the current two reduction cycles per day can be changed to three reduction cycles per day, which should be no problem. If so, not only the production of magnesium can be added 1/3, but the production cost of magnesium is reduced by about 1,680 yuan / ton · magnesium, the economic benefits are very considerable.
Fourth, the conclusion
(1) Alumino-silicon-iron alloy is used as a magnesium-reducing reducing agent, which has the advantages of low initial reduction temperature, fast reduction rate, small excess coefficient and low group pressure compared with ferrosilicon.
(2) It has been found that increasing the condensation depth of the crystallizer is conducive to increasing magnesium production and shortening the reaction time. This research result is rarely reported.
(3) The use of kaolin-type coal gangue can directly produce low-cost aluminum-silicon-iron alloy by carbothermal reduction.
(4) Since the use of aluminum ferrosilicon as a reducing agent can reduce the reduction time, it is feasible to change the current two daily reduction cycles to three daily reduction cycles. If so, on the basis of an increase of nearly 30%, the cost is reduced by about 1,680 yuan / t · Mg, it is worth a try.Permanent Ferrite Segment,Iron Ferrite Blocks,Ferrite Block Iron,Magnets Ferrite Blocks
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