Cementation of cobalt, nickel, and cadmium in ammoniacal medium using zinc metal
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log C,/Co
Cementation of Cobalt, Nickel, and Cadmium in Ammoniacal Medium Using Zinc Metal
where C A V t
G. ROY CHAUDHURY and I.N. B H A T T A C H A R Y A The kinetics of Co, Ni, and Cd cementation were studied separately in an ammoniacat solution using a Zn rotating disc. It was observed that the cementation rate increased with the increase of angular velocity of the disc from 63 to 209 Rad/s, and beyond that, the cementation rate showed hardly any improvement. The cementation rate was observed to follow diffusion controlled kinetics. In the case of Cd, the cementation rate showed a downward trend in the presence of Cu, whereas the Co and Ni cementation rate showed just the opposite. Detailed studies were carded out to evaluate the effect of a Cu ion in terms of cementation rate vis-a-vis the cementation of Cd, Co, and Ni. Cementation is a process by which metal values are recovered from the solution by reacting a more electronegative metal with a less electronegative metal ion. Thus, the cementation reactions are electrochemical in nature. The cementation process is widely practiced to recover metal values from solutions, especially in the case of Cu. [u Generally, cementation reaction kinetics follow two types of behavior. In the first case, the rate of cementation is initially slow, but as the deposit grows, the cementation rate becomes faster, and this faster rate is known as the enhanced rate. The enhanced rate may be either the increase of surface area due to the deposit, [z] the separation of zero concentration and zero velocity planes near the precipitant metal surface, [3] or the formation of" eddies at the boundary layer by the deposit protrusions.t4) In the second case, the cementation rate drops down with time, perhaps due to the formation of the smooth deposit, as a result of which the diffusion of ion through this layer would be difficult. [5.6] The rate limiting step in cementation is the transfer of mass to the reaction surface. Usually cementation follows diffusion controlled kinetics, [71 but Lawson et al. tS] have recently shown a cementation process which follows a mixed kinetic model. The kinetics in a cementation process are usually evaluated by the rotating disc geometry. In the laminar flow condition, the mass transfer coefficient or specific rate constant can be shown as/3] K = 0.62D 2/3u- 1/6o.)1/2
[ 1]
where K = specific or mass transfer constant (cm/s); D = diffusivity of ion (cm2/s) 2.048~ '~ 25w-~ 75KL5; [2] u = kinematic viscosity of the cementation solution (cmZ/s); and = angular velocity of rotating disc (Rad/s).
= = = =
=
-K at/v
[31
concentration of diffusing species; effective depositing surface area (cm2); volume of solution; and time.
From Eq. [3], it is understood that the rate constant is a function of the rotating disc area. In all the cementation processes, the initial disc area is used for calculation, although the effective disc area changes with the deposit. The Regional Research Laboratory is actively engaged in the processing of complex sulf
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