Tungsten is a strategic resource and is a high-yield element and a protected mineral in China. For a long time, China's exports of tungsten primary products, imported high-end products, the price of export products is only 1% of imported products, and China's economic development requirements are extremely unsuitable. In order to accelerate the research and development of tungsten new materials and realize the transformation of tungsten products from primary to high-tech content and high value-added products, the advantages of tungsten resources in China are transformed into economic advantages. The research on the preparation technology of high-performance tungsten materials has important practical significance and development. prospect.
Since the genetic relationships, ammonium paratungstate (APT) of crystal properties, such as crystal morphology, average particle size and particle size distribution, bulk density and flowability subsequent Powder Metallurgy Products - Effect of Material Properties tungsten powder, tungsten and tungsten alloy greatly . Single crystal APT is an ideal raw material for the production of high performance tungsten materials due to its excellent physical properties. Firstly, the single crystal APT powder has good fluidity. The tungsten powder prepared by calcination-hydrogen reduction of single crystal APT has small sliding friction resistance during the pressing process, and the void defect of the blank is obviously reduced, and the mechanical properties of the processed material are improved. substantial improvement. Due to the good tensile and fracture resistance, the yield of tungsten wire in the drawing process is 90%, and the yield of tungsten wire produced by using polycrystalline APT as raw material is only 70%. Therefore, single crystal APT has become a must-have powder material for producing high quality tungsten wire for vehicles. In addition, the single crystal APT powder has a high bulk density, the grain gap in the blank is small and uniform, the mechanical defects are few, the compaction density is high, and the alloy material prepared therefrom has the compressive, shearing and impact resistance. Excellent performance. For example, the life of the top hammer made from single crystal APT is 2 to 3 times that of polycrystalline APT. Due to the large weight effect, single crystal APT is an excellent powder raw material for producing high performance tungsten materials such as armored bombs, high density alloys, micro drills and numerical control blades.
Therefore, the preparation technology of single crystal APT powder and its preparation principle are a key issue. At present, the research on the performance of APT at home and abroad is more concerned with the relationship between the process conditions and the crystal characteristics such as particle size, particle size distribution, bulk density and fluidity of ammonium paratungstate. On the basis of the crystallization principle of single crystal APT, the author studied the influence of crystallization device, stirring speed and temperature on the agglomeration of ammonium paratungstate.
First, the test part
(1) Test materials and reagents
(NH 4 ) 2 WO 4 solution: a solution obtained by removing the transformation of the wolframite concentrate by alkali dissolution and ion exchange, and Ï(WO 3 )=285.66 g/L, pH=9.80, c(Cl - ) = 2.5 mol / L, trace amounts of Mo, Si, P, As impurities.
During the test, the solution crystallized to 40% of the initial solution volume.
(2) Test equipment
DF-1 collector type thermostatic magnetic stirrer (Jiangsu Jinda Zhongda Instrument Factory); 5312 digital display stirrer (Jiangsu Jintan Zhongda Instrument Factory); 0.1mg electro-optical analysis balance (Chengdu Scientific Instrument Factory); 721 Spectrophotometer (Shanghai Precision Scientific Instrument Co., Ltd.); SFC-100FL McAudio microscope; infrared fast dryer.
(3) Test device
The test device is shown in Figure 1.
Figure 1 Evaporation crystallization device for preparing spherical ammonium paratungstate
Second, the preparation mechanism of single crystal ammonium paratungstate
A prerequisite for grain agglomeration is contact. There are two ways of contacting the crystal grains: one is the deposition contact deposited on the bottom of the crystallizer; the other is the collision contact suspended in the crystallization solution. Among them, the chance of collision contact is directly related to the flow of fluid in the mold and the concentration of solid particles in the solution.
The stacking contact can be avoided by stirring the crystal particles by stirring. Therefore, under the premise of ensuring that the crystal grains are suspended, the opportunity to reduce the movement collision of the crystal grains in the solution is a prerequisite for preparing the single crystal of the ammonium paratungstate.
Due to the different stirring devices and the stirring speed, the chances of the crystal grains colliding in motion are very different. According to research, in a crystallizer with a circular cross section, when the fluid moves in a circular concentric laminar flow around the stirring shaft, the chance of grain collision is minimal.
Whether the fluid motion is laminar or turbulent depends on the flow rate, ie the agitation speed. The slower the agitation, the farther the fluid deviates from the turbulence. Therefore, under the premise of ensuring that the crystal grains are not deposited, the stirring speed is as slow as possible.
The settling velocity of APT grains is related to their particle size. The larger the particle size, the easier it is to settle, and the faster the spin speed required to maintain it in suspension. Therefore, according to the case of grain growth, the crystallization process is controlled from slow to fast to determine the stirring speed, and the optimum particle size range is determined. The optimum rotational speed of APT grains neither deposit nor collision is the preparation of single crystal APT. One of the key technologies.
The collision chance of APT grains in the crystallization process is also related to the number of particles (ie solid phase concentration) and grain size per unit volume of crystal liquid. According to the previous research results, under the same conditions of the initial concentration of ammonium tungstate solution, lowering the temperature of the pre-crystallization solution and stirring the rotation speed reduces the number of nucleation (ie, lowering the solid phase concentration) and the grain growth rate (ie, reducing the grain size). The key to reducing the APT grain collision and the single crystal APT.
Third, the results and discussion
(1) Effect of crystallization unit on agglomeration of ammonium paratungstate
Reaction conditions: stirring speed 70 r/min, crystallization temperature 95 °C. The effect of the crystallization unit on the agglomeration of ammonium paratungstate was qualitatively investigated.
The crystallization apparatus is a crystallizer having a circular cross section. According to the theory of crystallization kinetics of ammonium paratungstate and the principle of fluid mechanics, in this crystallization device, the fluid moves circumferentially around the stirring shaft, and the fluid velocity at the same radius point is substantially the same, and the fluid laminar flow is basically realized. The developed crystallizer is significantly different from the normal mold fluid flow state, as shown in Figure 2.
Figure 2 Flow state of fluid in different crystallizers
Based on the above principle, the agitating slurry is improved. The device is divided into three types: A, B, and C. A has not been improved, and B and C are the improved 1 and improved 2 devices, respectively. The test results are shown in Table 1. It can be seen that the ratio of the single crystal of the obtained APT powder is significantly increased by the improved crystallization apparatus. The following tests were all carried out in a C apparatus.
Table 1 Effect of crystallization unit on agglomeration of ammonium paratungstate
Crystallization device | APT powder single crystal rate /% | APT powder size / μm | APT powder loose density / (g · cm -3 ) |
A | 46 | 46 | 2.2 |
B | 78 | 35 | 1.8 |
C | 85 | 42 | 2.1 |
(II) Effect of stirring speed on agglomeration of ammonium paratungstate
The crystallization temperature was 95 ° C, and the test results are shown in FIG.
Figure 3 Effect of stirring speed on APT powder agglomeration
It can be seen from Fig. 3 that the single crystal ratio of the APT obtained at a low stirring speed is low, and the single crystal ratio is 62% when the number of revolutions is 30 r/min. This is because when the stirring speed is slow, the APT particles are not sufficiently suspended in the solution in the solution, but are deposited in a stacked manner at the bottom of the crystallizer, which inevitably leads to the occurrence of APT agglomeration. As the stirring speed increases, the APT agglomeration phenomenon is gradually alleviated, so the APT single crystal rate is gradually increased, and reaches the optimum value at 70-90 r/min. At this time, the single crystal ratio is about 86%.
2. With the further increase of the stirring speed, the APT single crystal rate gradually decreases. This is because the increase of the stirring speed inevitably leads to the flow state of the solution in the crystallizer from laminar flow to turbulent flow, and the turbulent flow state increases the collision contact chance of the APT particles suspended in the solution, thereby causing the APT agglomeration phenomenon to occur.
3, can conclude that the stirring speed of 70 ~ 90r / min is a cut-off point. Below 70r/min, some of the APT particles in the solution are agglomerated due to insufficient stirring force; in this range, the APT particles in the solution are substantially suspended in the solution; above 90r/min, the solution is aggravated and suspended in the solution. The collision of APT particles is intensified and agglomerated. Therefore, from the viewpoint of the entire process of crystallization, the stirring speed should be controlled within the range of 70 to 90 r/min.
(III) Effect of stirring speed on agglomeration of ammonium paratungstate in different periods
From the viewpoint of the local process of crystallization, the stirring speed of 70 to 90 r/min is not an optimum value. As mentioned earlier, the optimum agitation speed is as slow as possible to ensure that the APT grains are not deposited. However, at different stages of crystallization, since the number and size of crystal grains are different, the slowest rotational speed for ensuring that APT grains are not deposited is also different. Therefore, it is necessary to further explore the optimum rotational speed for maintaining laminar flow and preventing grain deposition at different crystallization times.
From the beginning to the end of the crystallization, the size of the APT particles should increase as a whole, so the stirring speed can take a small value at the initial stage of crystallization, and the stirring speed should be gradually increased as the crystallization process progresses. In the later stage of crystallization, the stirring speed reached the optimum value of 70^90 r/min.
The crystallization time takes 3 points: before the appearance of the nucleus, the nucleus appears and 1 h after the occurrence of the nucleus. For time point 1, the rotational speed is 10, 20, 30r/min respectively; for time point 2, the rotational speed is 30, 40, 50r/min respectively; for time point 3, the rotational speed is 60, 70, 80r/min respectively. The crystallization temperature was 95 °C. The orthogonal test was carried out under these conditions. The test results are shown in Table 2.
Table 2 Effect of stirring speed on APT agglomeration in different crystallization periods
Test number | Time point 1 | Time point 2 | Time point 3 | APT single crystal rate /% |
1 | 20 | 40 | 60 | 89 |
2 | 30 | 50 | 70 | 93 |
3 | 40 | 60 | 80 | 91 |
4 | 20 | 50 | 80 | 92 |
5 | 30 | 60 | 60 | 88 |
6 | 40 | 40 | 70 | 92 |
7 | 20 | 60 | 70 | 91 |
8 | 30 | 40 | 80 | 92 |
9 | 40 | 50 | 60 | 90 |
It can be seen from Table 2 that the single-crystal rate of the product obtained in the No. 2 test is the highest, that is, the stirring speed is 30r/min before the appearance of the crystal nucleus; the stirring speed is 50r/min when the crystal nucleus appears; the stirring speed is 70r after the crystal nucleus appears for 1h. /min; In the later stage of crystallization, the stirring speed is controlled within the range of 70 to 90 r/min. Under this condition, the obtained product APT single crystal rate was 93%.
Late crystallization means that no new nucleation is formed in the solution, ie the supersaturation of the solution reaches a minimum. According to estimates, the density of this solution is 1.116 to 1.125 g/cm3. From the late stage of crystallization to the end of crystallization, there is still crystallization time of 5-6 h, but the change of process conditions during this period has little effect on the APT single crystal rate, because the crystals are longer and the collision is no longer easy. Reunion.
(4) Effect of temperature on agglomeration of ammonium paratungstate
The chance of collision of APT grains during crystallization is also related to the number of particles in a unit volume of solution. As described above, the stage that has the greatest influence on the APT single crystal rate is the pre-crystallization stage, that is, from the start of nucleation to the end of nucleation. Therefore, the effect of different temperatures in the early stage of crystallization on the APT single crystal rate was studied. The solution temperature was still taken at 95 ° C, and the heating was accelerated to shorten the cycle; the nucleation was completed until the end of crystallization, and the temperature was still controlled at 95 ° C.
Test conditions: before the appearance of crystal nucleus, the stirring speed is 30r/min; when the crystal nucleus appears, the stirring speed is 50r/min; 1h after the appearance of crystal nucleus, the stirring speed is 70r/min; the later stage of crystallization to the end of crystallization, the stirring speed is 80r /min. In the early stage of crystallization, the steam temperature was changed, and the test results are shown in Fig. 4.
Figure 4 Effect of crystallization temperature on the single crystal ratio of APT powder
It can be seen from Fig. 4 that the temperature of the pre-crystallization period is appropriately reduced, and the single crystal ratio of the APT powder has a large room for improvement, but the temperature cannot be lowered too much; when the temperature is 80 ° C, the APT single crystal ratio reaches an optimum value, 96%; further lowering the temperature, the crystal nucleation rate is too low, the crystal growth rate is too fast, the crystal grains are coarse, but it has a negative influence on the single crystal ratio of APT powder.
Fourth, verification test
According to the above test results, the verification test was carried out under the optimum process conditions. The results show that the APT powder single crystal ratio is greater than 95%, the bulk density is 1.5-3.0 g/cm 3 , the Fischer particle size is between 30-60 μm, and the Hall fluidity is 30-50 s/50 g. The single crystal electron microscope scan of the product is shown in Figure 5.
Figure 5 Single crystal APT electron microscopy
V. Conclusion
With the improved crystallization device, the single crystal ratio of APT powder is significantly improved. This improvement is mainly reflected in the stirring slurry, which can promote the realization of laminar flow of the solution in the crystallizer. The developed laminar APT powder preparation fluid laminar flow control technology and device can effectively reduce the collision between grains, and the single crystal APT powder has a single crystal ratio of more than 90%.
The optimum crystallization condition of APT powder is: before the appearance of crystal nucleus, the stirring speed is 30r/min; when the crystal nucleus appears, the stirring speed is 50r/min; after the crystal nucleus appears for 1h, the stirring speed is 70r/min; At the end, the stirring speed is 70-90r/min; appropriately reducing the pre-crystallization temperature, the APT powder single crystal ratio has a large room for improvement, and the optimum value is obtained when the crystallization temperature is 80 ° C, and the single crystal ratio is 96%.
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