After the acid decomposition process is the current industry standard scheelite concentrate (wolframite and requirements with low phosphorus, arsenic and other impurities, for example one of the scheelite type B) of the main method, the flow of make appropriate modifications, also It can be used to treat poor quality scheelite concentrate (containing 40% ~ 70% W0 3 and containing a small amount of soluble compounds such as CaC0 3 ), which has the characteristics of short process and low cost.
The principle flow principle of A principle is shown in the figure below.
B Tomb principle a main reaction and its thermodynamic conditions
Tremensite: CaW0 4 (s) + 2HC1 (aq) ==== H 2 W0 4 (s) + CaCl 2 (aq) (1)
Black tungsten ore: FeW0 4 (s) + 2HC1 (aq) ====H 2 W0 4 (s) + FeCl 2 (aq) (2)
MnW0 4 (s)+2HC1(aq)====H 2 WO 4 (s)+MnC1 2 (aq) (3)
The equilibrium constant of the above reaction at 25 ° C and the concentration equilibrium constant Kc (Kc = [CaCl 2 ) / [HCl] 2 ) are shown in Table 1.
Reaction number | Ka (calculated value at 25 ° C) | Kc=[CaCl 2 ][HCl] 2 (measured value) |
(1) | 10 7 | 10 4 |
(2) | 6.3×10 4 | 700 |
(3) | 2.5×10 8 |
b Reaction mechanism and factors affecting the decomposition rate Reaction mechanism Zheng Changqiong et al. showed that when the stirring speed is fast enough, the diffusion of the solid H 2 W0 4 membrane is the controlling step of the decomposition process, and the reaction in the process is different with the concentration of hydrochloric acid. The number of stages and the properties of the H 2 W0 4 film produced are not completely the same. When the concentration of hydrochloric acid is 1 to 4 mol / L, the membrane has the greatest inhibitory effect on the diffusion process, while at 8 km / L, the inhibition is smaller. For artificial white tungsten, the relationship between the decomposition fraction (x) and time (t) is subject to the following equation:
2
1 - ——x - (1 - x) 2/3 = kt
3
The apparent activation energy (40-98 ° C) of the reaction was 37.93 kJ/mol, which was controlled by a solid phase membrane (ie, H 2 W0 4 membrane). For scheelite concentrates, the apparent activation energy was measured to be 43.61 kJ/mol.
Factors affecting the decomposition rate increase the decomposition temperature, increase the amount of hydrochloric acid and prolong the time are all beneficial to increase the decomposition rate. In the case of a certain amount of acid, changing the acid concentration changes the solid-liquid ratio. On the one hand, the concentration of the reactant is large, which is favorable for the reaction; on the other hand, the solid-liquid ratio becomes large, which is not conducive to the liquid phase. Quality, in general, it is generally believed that the use of high concentrations is beneficial to increase the decomposition rate. Since the process is controlled by diffusion of the H 2 W0 4 film, reducing the particle size of the ore, correspondingly reducing the thickness of the H 2 W0 4 film, is advantageous for increasing the decomposition rate, for example, 2.5 times the theoretical amount of hydrochloric acid. The concentration of hydrochloric acid was 29.75%, 100 ° C, and stirring for 1.5 h. When the particle size was 58-74 μm and less than 58 μm , the decomposition rates were 87.3% and 95.1%, respectively.
Since the diffusion by the H 2 W0 4 film is a rate controlling step, decomposition at the same time as ball milling is advantageous for increasing the decomposition rate. Further, since the mine wolframite reaction with HCl, so that the iron-containing raw material (wolframite) is high, the decomposition rate decreases.
Impurity behavior The behavior of impurities when decomposed with hydrochloric acid is shown in Table 2.
Impurities and their minerals | Behavior during the decomposition of hydrochloric acid |
Iron and heavy metal sulfides | Reacts with hydrochloric acid to form the corresponding chloride salt and evolves H 2 S. In order to avoid the reduction of tungstic acid to a low-priced compound by H 2 S, it is often added to the saltpeter with a concentrate weight of 0.1% to 0.15%. |
Arsenic: As 2 S 5 , As 2 S 3 | Does not react with HCl under neutral atmosphere, in the presence of oxidant NaNO 3 , produces H 3 AsO 4 into the solution: 3AsS 3 +28NaNO 3 +10HCl+4H 2 O====6H 3 AsO 4 +9Na 2 SO 4 +10NaCl+28NO↑ |
FeAsO 4 | Partially reacted to form H 3 AsO 4 into the solution and further form heteropolyacids |
Molybdenum : molybdenite | Does not react with HCl under neutral atmosphere, when oxidant is present: 3MoS 2 +18NaNO 3 +6HCl====3H 2 MoO 4 +6Na 2 SO 4 +6NaCl+18NO |
Molybdenum: calcium molybdate ore | CaMoO 4 +2HCl====H 2 MoO 4 +CaCl 2 ; H 2 MoO 4 has a much higher solubility in hydrochloric acid than H 2 WO 4 (see Table 3); therefore, it is partially separated into tungsten and separated under high acidity, industrial conditions. The molybdenum removal rate is 50%~70% |
Phosphorus: apatite | 2Ca 5 (PO 4 ) 3 F+18HCl→6H 3 PO 4 +9CaCl 2 +CaF 2 (or HF), H 3 PO 4 and H 3 AsO 4 form heteropoly acid with tungsten, and the presence of P causes loss of tungsten The amount is increased. HF accelerates damage to equipment such as enamel |
Metal oxide ore | Most of the metal oxide ore reacts with HCl to form the corresponding chloride salt. |
Hydrochloric acid concentration | 20 ° C | 50 ° C | 70 ° C | |||
H 2 MoO 4 | H 2 WO 4 | H 2 MoO 4 | H 2 WO 4 | H 2 MoO 4 | H 2 WO 4 | |
400 | 440 | 7.02 | 551.3 | 9.45 | 535.6 | 6.48 |
270 | 192.6 | 4.32 | 270.0 | 4.86 | 265.0 | 5.25 |
200 | 101.5 | 1.7 | 124.5 | 2.50 | 135.0 | 2.16 |
130 | 29.2 | 0.65 | 18.6 | 0.69 | 2.6 | 0.67 |
80 | 10.9 | 0.25 | 6.48 | 0.28 | 13.0 | 0.25 |
40 | 3.8 | 0.13 | 2.46 | 0.09 | 4.6 | 0.01 |
C Industrial practice a Acid decomposition process equipment (1) Stirring leaching tank The inner wall of the tank and the agitator are lined with rubber resistant to hydrochloric acid corrosion, or the groove wall is lined with FRP, and then a layer of graphite brick is built to ensure that the equipment has For a sufficient service life, the acid is usually directly heated by steam. There are no special sealing measures for the equipment.
(2) The structure of the main acid equipment of the closed acid decomposition tank is the same as that of the stirring and leaching tank, but the sealing is good, the rotating shaft adopts mechanical seal, and the wear-resistant and acid-proof sealing elements are selected; the transmission system is improved to reduce the swing of the shaft rotation process, thereby It is guaranteed that there is no air leakage during the operation, the tank can be operated under the positive pressure of 50kPa, the temperature reaches 110°C, and the evaporation and consumption of HC1 are small, so it has good economic benefits and environmental benefits.
(3) The structure of the hot ball mill reactor is shown in the figure below. The material of the lining material and the ball is required to be resistant to hydrochloric acid and abrasion at the working temperature, and no material capable of satisfying the above requirements has been found yet. Former Soviet scientists suggested using fused diabase . Chinese scientists compared the materials of the grinding media and found that under the specific conditions of acid decomposition, the loss per hour of the ball and the pebbles is 0.36% and 0.1% respectively, and an aluminum ball with a diameter of 20 mm can be used for 5000 hours.
Main technical and economic indicators The process characteristics and main technical and economic indicators of some factories are shown in Table 4. Generally, when the scheelite acid is decomposed, the final mother liquor is controlled to contain HCl of about 100~150g/L. [next]
Process characteristics | Technical Parameters | Decomposition rate /% | |||
Acid dosage / theoretical amount | Solid: liquid | Temperature / °C | Time / h | ||
Hydrothermal ball milling decomposition of scheelite concentrate | 1.3~1.5 | 1:1 | 55~60 | 4 | 99 |
Styrene decomposition of sulphur concentrate | 3.5~4.0 | 1:2 | ~100 | About 99.5 | |
40%~75% WO 3 scheelite acid is decomposed and then alkali leached | 98 | ||||
Sealed acid decomposition of scheelite concentrate | 2.9~3.0 | -- | 110 | 1 | 99.7 |
b In the ammonia solution process, the crude tungstic acid is firstly pulverized with hot water, and 1.2~1.5kg of water is added according to 1kg of tungstic acid to control the pulp density of 1.6~1.65g/cm 3 (also controlled to about 1.35 g/cm 3 ) The hot slurry is added to the agitation water with a concentration of 25%~28%. 1kgW0 3 requires about 28% ammonia water 1~1.5L, the control temperature is about 60 °C, and it can be completely dissolved in about 2h. Finally, the solution is controlled. Containing about 35g/L of free ammonia, the solution obtained by filtration contains about W0 3 350g/L.
When ammonia is dissolved, H 2 W0 4 and H 2 Mo0 4 enter the solution in the form of (NH 4 ) 2 WO 4 and (NH 4 ) 2 Mo0 4 respectively, and phosphorus and arsenic also form corresponding iron salts into the solution, which is phosphorus removal, Arsenic may be added to MgCl 2 to precipitate ammonium ammonium salt . In the final ammonium tungstate solution, As/W0 3 and P/W0 3 may be reduced to 0.01% or less.
After acid decomposition, the yield of ammonia slag is about 10% to 15% of the concentrate, including W0 3 up to 5% -30%, and WO 3 should be further processed.
references:
1.Zheng Changqiong. Preliminary study on the kinetics of the decomposition of scheelite concentrate by hydrochloric acid. Rare metals, 1980(6): 11~15
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