Extractive Metallurgy 1

Basic Thermodynamics and Kinetics

Alain Vignes

• Publisher: Wiley-ISTE
• Year: 2013
• ISBN:
  9781118619674
• Edition: 1
• Topic: Tecnología e ingeniería
• Subtopic: Ciencias de los materiales

This book is dedicated to the processes of mineral transformation, recycling and reclamation of metals, for the purpose of turning metals and alloys into a liquid state ready for pouring.

Even though "process metallurgy" is one of the oldest technologies implemented by man, technological innovation, with the development of processes that are both focused on product quality and economically and ecologically efficient, continues to be at the heart of these industries.

This book explains the physico-chemical bases of transformations, vital to their understanding and control (optimization of operational conditions), and the foundations in terms of "process engineering" (heat and matter assessment, process coupling: chemical reactions and transport phenomena), vital to the optimal execution and analysis of transformation process operations.

This book is addressed to students in the field of metallurgy and to engineers facing the problem of metal and alloy development (operation of an industrial unit or development of a new process).

Chapter 1

Metallurgical Thermochemistry

1.1. Introduction

The industrial processing route for the production of a metal, from its ore or concentrate to a refined metal, consists of a combination or sequence of operations (illustrated conveniently by means of a flowsheet, see [VIG 11c], Chapter 10), performed in reactors. In a unit operation a single extraction process is performed, such as the roasting of a sulfide ore or the reduction of an oxide or a transfer process (removal of a component from a phase) such as a solvent extraction. In some operations, such as in blast furnaces, several processes occur in sequence from iron ore to hot metal in the same reactor.

These operations are carried out in different conditions: discontinuous (batch, closed), continuous (open) and semi-continuous (semi-batch), see Figure 1.2.1.

Metallurgical processes, occurring in the operations of extraction and refining of metals and alloys, involve homogeneous chemical reactions or heterogeneous chemical reactions (between reactants present in two phases, the reaction occurring at the interface between the two phases).

In the first part of this chapter the physical quantities allowing the quantitative description of the state (and its evolution) of a reaction mixture undergoing a chemical reaction are defined. The second part deals with the fundamentals of thermodynamics for these reactions.

The thermodynamic analysis of reactions constitutes the first unavoidable step of the study of these extraction processes.

Thermodynamics provides three important pieces of information:

– it allows the calculation of the energy balance, i.e. the energy (thermal or electric) that needs to be provided or extracted for the reaction to occur at a certain temperature and pressure;

– it allows the calculation of the maximum possible degree of advancement (i.e. the extent) of a reaction and the maximum possible fractional conversion of the reactants, in different operating conditions, which constitute what can be called the thermodynamic modeling of a process.

– it allows the determination of operating conditions (T, P, initial composition of the reaction mixture), optimizing the maximum possible fractional conversion of the reactants.

The thermodynamic quantities and data necessary for the prediction of operating conditions of processes are of two types:

– thermodynamic functions: enthalpy, Gibbs free energy (free enthalpy), affinity of a reaction, activity of a phase’s component, law of mass action, equilibrium constant of a reaction;

– phase diagrams: graphs that show which phases (and their extension) are present in a binary, ternary system. The coordinates of these diagrams are various parameters: for instance: temperature (ordinate) and composition (abscissa), see section 1.4. These equilibrium diagrams are quantitatively related to the thermodynamic functions of the systems they describe.

A thermodynamic analysis is often sufficient to predict the maximum possible extent of reactions carried out at high temperatures, especially in the case of reactions between fluid phases, for which the process rates are not limited by slow diffusion phenomena in solid phases or by chemical reactions. It can then be considered that the reaction is indeed occurring: until the total consumption of a reactant or until it nearly reaches equilibrium state for a closed system; or is occurring through a succession of equilibrium states, in the case of a semi-batch system during the continuous injection of a reactant.

The extent of this presentation comes from the fact that in the literature (books and publications) dealing with this field, many definitions and expressions can be found for the same quantities, especially for the activities of components. All these different definitions are presented in this chapter in order to help the reader interpret and understand future readings.

1.2. Quantities characterizing the state of a system and its evolution

The progress of ...