Abstract
Production of rare earths by metallothermic reduction and its variants is presented in an earlier study and its analysis is performed indicating major type and sequence of reactions. Diagrams required and procedures to establish them are also described. Their role and importance are also described. A comparison is also made to identify parameters of importance. This study is continuation of a previous study in which cell design to extract metals is described. Parameters to design and construct cell (metallothermic and electrothermic) are determined and used. Rate processes (heat transfer, mass transfer, charge transfer and simultaneous processes) occurring inside a cell, its operation for a specific reaction and sequence for a specific metal (Nd) are described. Outline of calculations outlining cell efficiency, output and yield are enumerated for individual (Nd, Ce, Dy) metal. Optimization and method to achieve is described. Procedures to control and production is also described. Here preliminary graphs are presented.
Keywords: Rate processes, heat transfer, mass transfer, efficiency, yield.
Introduction
Essentially, pyrometallurgical processes involves design of furnace for melting, holding, and refining of metal oxides and compounds while electrowinning and electrorefining (after pyrometallurgy) involves design and control of electrochemical cell (room or high temperature) operated by the fundamental principles of current density, its control, distribution, and electromotive force (EMF). Its detail may be found in specialized texts [1-7] and literature [8-11]. This study is continuation of previous study of author [12] on the subject describing extraction and refining of rare earths from their ores by metallothermic reduction. In this area of emphasis is cell design and rate processes (heat, mass, and activity coefficient determination) which lead towards economic production of these metals from their source. Various graphs and carts are presented lying emphasis on the efficient extraction and refining of rare earths. Nd-Fe foundry alloy is taken as example to describe construction and operation of cell. Unit operation including piping and instrumentation diagrams (P&IDs) is described to explain different (Elution and electrowinning) circuits. Lastly, a future outlook is presented to calculate individual efficiency and throughput.
Fundamentals
Cell design of rare earth electrowinning process depends on type of metal won (Nd [8], Sm, W, Th, U, Ce, Sc), its quantity required, production rate, type of electrolyte, concentration of metal in electrolyte and flow rate of electrolyte.
Cell design
Cell design primarily consists of outlining parameters of its construction based on its holding capacity. It may be carried out in various ways such as production rate, throughput, efficiency, yield but design based on holding capacity constitutes the simplest. It is presented here.
Consider a typical cell of 5 ton / day. For this, considering rectangular dimensions (as they ease out operation and maximizes yield)
Rate processes
Research progress in development and processing of rare earths may be described following [13, 14]. Density of a typical metal as a function of concentration and temperature is presented by following equation.
𝜌 = −6.53212 × 10−4𝑐2 − 0.06987𝑐 + 1.31265 × 10−5𝑇2 − 0.05297𝑇 + 1.69004 × 10−4𝑐𝑇+ 43.14013