(560je) Mechanism and Kinetics of Metal Dissolution in Nitric Acid

Authors: 
Vamsi Vikram, G., IIT Madras
Pushpavanam, S., Indian Institute of Technology, Madras
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text-align:center;line-height:normal;mso-pagination:none;text-autospace:none"> font-family:" times new roman mso-font-kerning:1.0pt>Mechanism
and Kinetics of Metal Dissolution in Nitric Acid 
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text-align:center;line-height:normal;mso-pagination:none;text-autospace:none"> font-family:" times new roman mso-font-kerning:1.0pt>Abstract

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justify;line-height:150%;mso-pagination:none;text-autospace:none;word-break:
break-all"> font-family:" times new roman mso-font-kerning:1.0pt>Dissolution
of metals in acid is an important step in the production of metal salts,
extraction of metals from its ore and extraction of metals from e-waste through
the hydrometallurgical route.  The
reaction mechanism and the stoichiometry of metal dissolution vary with the
concentration of acid used and stirring speed. The dissolution of metals is
reported to exhibit autocatalytic kinetics under certain operating conditions.

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justify;line-height:150%"> 150%;font-family:" times new roman mso-ansi-language:en-us>To understand the different physio-chemical mechanisms
prevailing in the system, we focus on the dissolution of copper in nitric acid
under controlled conditions for different nitric acid concentrations and
stirring speeds. Specifically we focus on the autocatalytic behavior observed
for low acid concentrations and high stirring speeds. When copper reacts with nitric acid then NOx are released depending on the concentration
of nitric acid.

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justify;line-height:150%;mso-pagination:none;text-autospace:none">During reaction, autocatalytic behavior is observed
experimentally under low concentration and high stirring speeds as shown in Fig
1. When initial concentration of nitric acid is 4M then sigmoidal behavior is
observed, confirming an autocatalytic effect. This autocatalytic behavior
arises from the nitrous acid generated. This behavior is not observed at high
concentration of nitric acid.

justify;line-height:150%;mso-pagination:none;text-autospace:none">A reaction mechanism is developed which explains the
generation of nitrous acid and autocatalytic effect behavior. The model also
predicts that at high concentrations of nitric acid there is no autocatalytic
effect observed.

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text-align:center;line-height:150%;mso-pagination:none;text-autospace:none">Fig 1. Comparison of experimental (marker) and model
prediction (lines) for Cu dissolution

text-align:center;line-height:150%;mso-pagination:none;text-autospace:none">a) in 4M HNO3                                   b) in 8M HNO3

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none"> " times new roman mso-ansi-language:en-us> 1">                                                                                                                     

justify;line-height:150%;mso-pagination:none;text-autospace:none;word-break:
break-all"> font-family:" times new roman mso-font-kerning:1.0pt> The mathematical model proposed incorporates
the change in surface area of metallic copper during the dissolution process. line-height:150%;font-family:" times new roman batang ko>This model considers multiple reactions and also accounts for change in
volume in the reactor due to sampling.  A heterogeneous model
consists of Differential Algebraic Equations (DAEs) which considers both mass
transfer and reaction is developed to predict the performance.

justify;line-height:150%;mso-pagination:none;text-autospace:none">The kinetic parameters in the proposed model were
estimated using a constrained optimization formulation based on particle swarm
optimization technique. It is shown that model predictions fits well with the
experimental data (see Fig 1). It is shown that two reactions are sufficient to
describe the different features exhibited by the dissolution kinetics of
copper.

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justify;line-height:150%;tab-stops:14.4pt">Acknowledgment

justify;line-height:150%;tab-stops:14.4pt"> 12.0pt;line-height:150%;font-family:" times new roman mincho>

justify;line-height:150%;tab-stops:14.4pt"> 12.0pt;line-height:150%;font-family:" times new roman mincho>This study was
supported by Exploratory Research Grant Work, IIT Madras.

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justify;line-height:150%;tab-stops:14.4pt">References

justify;line-height:150%;tab-stops:14.4pt"> 12.0pt;line-height:150%;font-family:" times new roman mincho>

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150%;tab-stops:14.4pt"> 150%;font-family:" times new roman mincho mso-ansi-language:en-us>[4] El-Chelkh,
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