19.Tell about electrometallurgy process, electro refining with example.

Electrometallurgy processing is used both in the primary extraction of metals from their ores(electrowinning) and in the subsequent refining metals to high purity (electrorefining). Both operations are accomplished in an electrolytic cell, a device that permits electrical energy to perform chemical work. The occurs by the transfer of electrical charge between two electrodes immersed in an ionically conducting liquid containing metal dissolved as positive ions. At the negatively charged cathode the metal cations acquire electrons, and deposit as neutral metal atoms. At the positively charged anode there are two possible reactions, depending upon the type of cell. In an electrowinning cell, nonmetal anions generally undergo oxidation at the anode to give gaseous products, whereas in an electrorefining cell the dissolution of the anode metal itself occurs. The more noble metals such as copper and zinc, are electrolyzed from aqueous electrolytes, whereas reactive metals such as aluminum and magnesium are electrolyzed from electrolytes of their fused salts. In contrast to elecrowinning, which faces competition from other primary extraction processes, electrorefining is virtually unrivalled in its capability to produce metal of the highest purity. Copper is the predominant metal to be electrorefined, as this is the only process for preparing metal pure enough for electrical applications. In 1908, Wohlwill modified his process by the use of a pulsed current, in fact a low-frequency a.c. superimposed on the normal d.c. supply. This pulsing allowed insoluble chloride films to flake off the anode, facilitating the treatment of higher silver content gold at elevated current densities. Wohlwill cells are usually operated at a relatively high temperature, 60-75C with significant stirring (usually by air agitation) in order to achieve acceptable gold deposits. Fume extraction to remove HCl/Cl₂ gas is essential at a cathode current density of 10cm^-1. Cell voltage is typically from – 0.9 to -2.5. the deposited gold has a purity >9939% and following extraction, washing and drying it is melted and cast into marketable bars.

Gold dissolves as stable anionic chloro-complexes:

These complexes are reduced at the cathode to yield a pure gold deposit:

The gold concentration in the electrolyte varies considerable from one installation to another, but values of 140-200 g dm^-3 encountered with a free HCl content of 100-150g dm^-3.

20.Could you explain classification of electrochemical cells according to the nature of energy transformation. In order to describe the performance of a reactor, a mathematical model may be used, a set of quantitative relationships between the important parameters. This defines the reactor characteristics and their relationships to process conditions. Hence, the model may be used to simulate process conditions, and predict behavior in practice. There are two extreme approaches to modeling. 1.The fundamental approach . a precise and detailed model is prepared from first principles using well established scientific and engineering expression and approaches.2.The second approaches empirical relationships are established from practical observation, treating the reactor as a characterized module. While the first approach is more detailed and intellectually satisfying the second often provides a simpler and more appealing description of the reactor and hence, its behavior towards major process variables its scale up characteristics an attendant optimization parameters. The reactor must fit into the overall process and it is useful to develop ‘figures of merit’ which indicate cell performance and permit a discussion of its interaction with other parts of the process. Classification of electrochemical cells according to the nature of energy transformation

Parameter	Self-driving cell	Driven cell
Energy transformation	Chemical electrical	electrical Chemical
	-	+
Thermodynamic tendency for cell reaction	Spontaneous	Not spontaneous
	+	-
Relative polarity of the electrodes cathode anode	+ -	- +
Example of application areas	Immersion plating; Electroless plating; Corrosion cell; Batteries on discharge and fuel cell; Most chromating	Chlor-alkali cells; Electrowinning and electrorefining; Inorganic synthesis; organic synthesis; industrial process recycling; electrochemical machining;electroplating