First, the main reaction and its thermodynamic conditions
(1) Black tungsten ore
The reaction of black tungsten ore with NaOH is:
(1)
(2) (3)
At 25 ° C, the thermodynamic equilibrium constants of the reaction formulas (1) and (3) are 1.9 × 10 4 and 1.7 × 10 5 , respectively, and when the temperature is higher than 102 ° C in the standard state, the reaction formula (2) proceeds to the right. The potential-pH diagram of the W-Fe-H 2 O system at 25 ° C drawn by K. Osseo-Asare is shown in Figure 1. All of the above data indicate that the reaction of the wolframite with caustic soda is easy to proceed automatically. When the temperature is higher than 102 ° C, the iron in the product is mainly in the form of FeO (Fe 2 O 3 in an oxidizing atmosphere).
Figure 1 Fe-W-H 2 O system potential-pH diagram
(25 ° C, Fe 2 + activity is 10 -3 )
(2) Baizhu Mine
The reaction of scheelite with NaOH is:
(4)
The equilibrium concentration ratio Kc of the reaction at 90 ° C and 150 ° C is shown in Table 1 and Table 2.
Table 1 Kc (90 ° C) of the reaction formula (4)
Table 2 Kc (150 ° C) of the reaction formula (4)
Second, the dynamics of the process and factors affecting the reaction rate and leaching rate
(1) Mechanism of reaction
The mechanism of reaction between black tungsten ore, scheelite and sodium hydroxide is as follows:
1. In the range of 75-105 °C, the kinetic equations for the reaction of wolframite, scheelite and NaOH are in accordance with the particle shrinkage model. For the mechanically activated wolframite, when the external diffusion rate is fast enough:
Where Dp-particle diameter, cm;
T-time, min;
R-gas constant, 8.31 J / (K·mol) - 1 ;
X-leaching score.
At the same time, the apparent activation energy of the reaction is 77.37kJ∕mol, indicating that the process is controlled by chemical reaction and the apparent reaction order is secondary.
For scheelite, a similar equation is also applied, the apparent activation energy of the reaction is 58.83 kJ ∕mol, and the apparent reaction order is secondary.
2. Mechanical activation can reduce the apparent activation energy of hematite and NaOH by 18.4kJ∕mol. Under the same leaching conditions, the activated mineral leaching rate is 10%-20% higher than that of unactivated.
The effect of AH Zelikman's pre-mechanical activation of raw materials in a planetary centrifugal mill is shown in Table 3.
Table 3 Effect of mechanical activation on leaching rate
(2) Factors affecting the leaching rate are:
1. Temperature, NaOH concentration, NaOH dosage and particle size temperature of the raw materials. Elevation, an increase in NaOH concentration, and a decrease in the particle size of the feedstock are all beneficial to increase the leaching rate of the wolframite.
Generally, for a black tungsten concentrate containing calcium ≤ 1%, the base amount is 1.4 to 1.6 times at a temperature of 150 to 160 ° C, and the decomposition rate is 99% or more when the particle size is less than 43 μm and 89% to 85%. As the temperature increases and the amount of alkali increases, the leaching rate of impurities such as SiO 2 also increases.
The leaching rate of WO 3 and the relationship between the SiO 2 concentration in the leachate and the amount of NaOH are shown in Table 4 when a plant in China is decomposing black tungsten concentrate by high pressure stirring leaching.
Table 4 Effect of NaOH dosage (multiple theoretical multiples) on WO 3 leaching rate and SiO 2 concentration in leachate
(temperature 170 ° C, NaOH concentration 240 g ∕ L, particle size less than 74 μm)
2. Calcium content and its form in raw materials. CaWO 4 generally does not react with NaOH under the conditions of normal mechanical agitation leaching (the initial concentration of NaOH is less than 250g ∕L, the temperature is lower than 190 ° C), so the CaO content in the black tungsten concentrate under stirring leaching conditions The increase will seriously affect the leaching rate. A plant in China has decomposed black tungsten concentrates with a calcium content of 0.18% to 0.82% and 2.7% to 2.9% at 170 ° C, a NaOH concentration of 240 g ∕ L, and a NaOH dosage of 1.5 times the theoretical amount. It is 97% to 97.5% and 81%.
The extent of CaO impact in concentrates is also related to their morphology. The smaller the solubility product of the calcium compound, the less its adverse effect on leaching. Generally, calcium in the form of CaF 2 and Ca 3 (PO 4 ) 2 has little effect on leaching, but has a large influence on the form of CaCO 3 , CaSO 4 or CaWO 4 .
3. Additives. Leaching can be added during the formation of Ca 2 + and the solubility product is smaller than the anionic compound CaWO 4, CaWO 4 is advantageous for the decomposition. Commonly used additives are CO 3 2 - , PO 4 3 - , F - , which react with CaWO 4 as follows (taking sodium salt as an example):
Third, the behavior of impurities in the process of caustic soda leaching
Tungsten concentrate contains impurities such as silicon, phosphorus , arsenic , fluorine, molybdenum , tin, etc., which are mainly silicate, scorodite [FeAsO 4 ], apatite [Ca 5 F(PO 4 ) 3 ], molybdenum Ore [MoS 2 ], calcium molybdate ore [CaMoO 4 ], cassiterite [SnO 2 ], antimony tin, etc., in the process of NaOH leaching, apatite, fluorite, etc. are difficult to react, some arsenate And silicates react to varying degrees, the reaction formula is:
During the NaOH leaching process, the molybdenum and tin minerals behave as follows: calcium molybdate ore and antimony tin can react with NaOH at a relatively fast rate to form Na 2 MoO 4 and tin compounds respectively into the solution, and the leaching rate is related to the NaOH concentration. The increase and the increase of temperature increase. At a temperature of 80 ° C and a NaOH concentration of 300 g ∕ L, the leaching rates are 95.83% and 4.79%, respectively. When the temperature rises to 160 ° C, the bismuth tin leaching rate reaches 44.4%. The cassiterite is difficult to react with NaOH, and its leaching rate is only 0.46% even at 170 ° C and a NaOH concentration of 500 g ∕L. In the absence of an oxidant, molybdenite is also difficult to react with NaOH and remains in the leach residue. The leaching rate was 0.58% at a NaOH concentration of 500 g ∕L and a temperature of 160 °C. In the presence of an oxidizing agent (such as NaNO 3 or the like), sulfides such as MoS 2 and As 2 S 5 are easily oxidized and form corresponding sodium salts into the solution.
Alkaline earth metal hydroxides feedstock can inhibit leaching of phosphorus, arsenic, silicon. The phase analysis shows that the above impurities can form the following insoluble compounds with the alkaline earth metal hydroxide (or oxide) and enter the slag phase:
Arsenic: (Na, M) AsO 4 , M 3 (AsO 4 ) 2 (M stands for alkaline earth metal)
Silicon: MSiO 3
Phosphorus: M 3 (PO 4 ) 2 , M 3 (PO 4 ) 2 · xH 2 O, M 5 (PO 4 ) 3 OH
Tin: MSnO 3 , MSn(OH) 6
As the leaching temperature is increased, the above effect of suppressing impurities is improved.
4. Physical and chemical properties of Na 2 WO 4 -NaOH-H 2 O system
Solubility of Na 2 WO 4 in NaOH solution. It can be seen from Fig. 2 that when the concentration of NaOH is constant, the solubility curve shows the highest point (such as points A, B, C, D, and E) as the temperature increases, and X-ray diffraction analysis proves that at the temperature above the highest point, The equilibrium solid phase is Na 2 WO 4 crystal, and the equilibrium solid phase is Na 2 WO 4 ·2H 2 O when the highest point temperature is below.
Figure 2 Na 2 WO 4 solubility and temperature and NaOH concentration
A-M NaOH = 1 mol ∕ L; B-M NaOH = 3 mol ∕ L;
C-M NaOH = 6 mol ∕ L; D-M NaOH = 9 mol ∕ L;
E-M NaOH = 12mol ∕L
(a) Density of the Na 2 WO 4 -NaOH-H 2 O system. The relationship between the density of the system and the Na 2 WO 4 concentration, NaOH concentration and temperature can be expressed by the following formula:
Where Ï-density, g∕cm 3 ;
M-concentration, mol∕L;
T-temperature, °C.
(b) The vapor pressure of the Na 2 WO 4 -NaOH-H 2 O system. The relationship between the vapor pressure of the system and the concentration of Na 2 WO 4 , the concentration of NaOH and the temperature can be expressed by the following formula:
Where Ï-vapor pressure, kPa;
T-temperature, K.
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