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