1. Dissolution of zinc calcine in dilute acid
Many acid and alkali leaching of oxides obey the shrinkage core model. A typical example is the dissolution of zinc calcine in dilute acid. It is calculated using equations (1) and (2) according to the ion diffusion coefficient and ion mobility of each chemical involved in the dissolution process. The calculation assumes that the dissolution rate is controlled by mass transfer, so the calculation used can only be used in situations where no chemical reaction is involved.
(1)
(2)
Solving equations (1) and (2) requires several boundary conditions that specify the values ​​of the various parameters in the model and correlate the flux of each species through the metering relationship of the leaching reaction.
For the sulfuric acid leaching system, the data used for the calculation include the ion diffusion coefficients and ion mobility of H + , HSO 4 - , SO 4 2 - and Zn 2 + , and the following equilibrium equilibrium constants and activity coefficients.
The mass transfer data used in the mathematical model calculation of dilute acid leaching of zinc oxide is listed in the following table.
substance | Equivalent ion conductance Λ i 0 ∕ (Ω - 1 ·cm 2 ·equ -1 ) | Ion diffusion coefficient D∕(cm 2 ·s -1 ) | Ion mobility u∕(cm 2 ·V -1 ·s -1 ) |
H + | 348.9 | 9.3×10 -5 | 3.6×10 -3 |
Zn 2 + | 53.8 | 7.2×10 -6 | 5.6×10 -4 |
SO 4 2 - | 79.0 | 1.0×10 -5 | -8.2×10 -4 |
HSO 4 - | 100.00 | 2.7×10 -5 | -1.6×10 -3 |
Several boundary conditions are
At the solid-liquid interface, ie r=r t , C i =C i s (3)
Since the slowest step in the leaching process is mass transfer through the boundary layer, it can be assumed that chemical equilibrium is reached at the interface, resulting in the following boundary conditions.
(4)
(5)
(6)
In the formula, , , Representing the equilibrium constants of reactions (a) and (b) and (c), respectively; Q a , Q b , and Q c are the equilibrium constants of reactions (a), (b), and (c), respectively, when concentration is used; γi is a substance. The activity coefficient of i.
In the solution phase, ie r = ∞, E = 0 (7)
C i =C i b (8)
The bulk concentration is calculated by the mass balance and the chemical equilibrium of the bulk phase.
(9)
(10)
(11)
(12)
(13)
In the formula, [H 2 SO 4 ] and [ZnSO 4 ] are the net concentrations of t-time sulfuric acid and zinc sulfate.
Measurement relationship (14)
Sulfate flux (15)
The mathematical model consists of the written equations (2), equations (1) and the boundary conditions derived above for each substance. Once the flux of each species is known, the dissolution rate of ZnO can be calculated.
If the spherical particles of radius r t contain Nmol of ZnO, then
(16)
In the formula, M w is the molecular weight of ZnO.
Since there is no material accumulation in the boundary layer under steady state, all dissolved zinc must be transferred to the bulk phase of the solution. Therefore, the reaction rate can be correlated with the rate of mass transfer of zinc and acid through the boundary layer as follows
(17)
Wherein J Zn - the net flux of zinc flowing away from the surface;
J H - The net flux of acid to the surface.
From equations (16) and (17)
(18)
Equation (18) is numerically integrated by the finite interval method to obtain a function of r t versus time. For single-sized particles, the relationship between r t and reaction fraction α is
(19)
This is the contracted particle model of equation (20), where r 0 is the initial radius of the solid particles.
(20)
The case of the particle size distribution can be similarly processed, and the single size fractions of the m initial radii r 0k each constitute a fraction w k of the total mass. Degree of leaching
(twenty one)
The total leaching rate is determined by the following formula
(twenty two)
In order to verify the correctness of the model and calculation, it is necessary to study the rate at which the calcined zinc sulfide concentrate is dissolved in four acids such as sulfuric acid, perchloric acid, nitric acid and hydrochloric acid. The selected agitation conditions allowed all solid particles to be suspended and the rate of dissolution independent of the rate of agitation. The experimental curve in perchloric acid and nitric acid solution agrees well with the predicted curve calculated by the model, but it is acceptable before the leaching rate is 80% in the sulfuric acid solution. The reason why the dissolution curve is not satisfactory is due to the solid particles. The dissolution is not as uniform as it is assumed and always remains spherical, and it is actually found that the partially leached calcine particles have large, deep pores. The simplified model does not consider the formation of chloride ions by the formation of zinc chloride and therefore cannot be used to predict the dissolution rate of hydrochloric acid leaching calcine. The model established earlier without considering the contribution of electromigration to mass transfer is seriously deviated even for the kinetics of 0.1 mol 高L perchloric acid leaching, reflecting the role that electromigration cannot be ignored in mass transfer.
Second, the alkali leaching dynamics of scheelite
Tungsten is a high-yield resource in China. The scheelite (CaWO 4 ) is not leached with caustic as well as the wolframite because it does not decompose in the caustic solution, and is conventionally decomposed with hydrochloric acid. Institute of Process Engineering Kejia Jun et solubility product principle, proposed to add an appropriate amount of soluble phosphate at the caustic leaching, in order to precipitate calcium scheelite is insoluble compounds HAP Ca 5 (PO 4) 3 OH (kap = 1.6 × 10 -58 ), so that the scheelite is continuously dissolved and leached in the form of sodium tungstate. In order to confirm this possibility and the speculative mechanism, the alkali leaching kinetics of scheelite was studied.
The disc samples were pressed with scheelite powder of 100% particle size <44 μm (-325 mesh), and the empirical leaching kinetic equation was obtained by the rotating disc method as follows:
(twenty three)
Where k is the apparent leaching rate constant. From the above formula, it can be seen that the apparent energy E α =42700 J∕mol, indicating that the leaching process is mainly controlled by surface chemical reactions. During the experiment, it was observed that solid suspension appeared in the liquid and the solid precipitate layer appeared on the surface of the disk sample, which is consistent with the conclusion that the leaching process derived from apparent activation energy is mainly controlled by surface chemical reaction. The results of X-ray diffraction and infrared spectroscopy of the solid suspension also confirmed the formation of calcium hydroxyphosphate in the leaching. It can be inferred that the caustic leaching of scheelite is carried out as follows:
(twenty four)
This is an example of understanding the reaction mechanism through kinetic studies.
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