电化学Chapter 4

(2) Electrolysis processes
The electrolysis has always been run with consumable carbon anodes so that the overall cell reaction is: 2Al2O3(s) +3C(s) E= 1.18V, The carbon anode is consumed in a stoichiometric amount. 4Al(l) + 3CO2(g)
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electrode process, I can reach 105 A/m2
metal dissolution in fused salt special requirements for cell
(2) Properties and structure of fused salts
4.1 Principle of electrometallurgy
1. Cathode processes The required cathode reaction, considered simply, is in both cases the deposition of metal from a soluble, electroactive species:
an acidic metal sulphate solution: MO + H2SO4 MSO4 + H2O
3) The acidic metal sulphate solution is purified and
all metal ions which are produced more readily than the element of interest must be removed. The principal methods of purification are: a) precipitation with hydroxide or sulphide ion b) solvent extraction c) Cementation, i.e. the solution containing Mn+ ions
2. Anodic processes
(1) Electrorefinning
A soluble anode is used, dissolution occurring at
high efficiency:
MA
Mn+ + ne
Ideally, the overall cell reaction in electrorefinning is
addition
of
AlF3 ， Tm↓ ， viscosity↓,
but
conductivity↓ c) Conductivity κ Al2O3↑，κ↓ Addition of MgF2, AlF3, CaF2 Addition of LiF, NaCl, NaF κ↓ κ↑
d) Effects of the concentration of Al2O3
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structure
●
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melting point
density Conductivity viscosity
(3) Electrochemical processes of fused salt electrolysis a) Decomposition voltage
b) Less polarization
simply the transfer of metal from the impure anode into
a purified cathode deposits:
MA
MC
(2) Electrowinning An insoluble anode is used, oxygen evolution being the major reaction: 2H2O O2 + 4H+ + 4e
Chapter 4
The extraction, refining and production of metals
Electrometallurgy （ 电 冶 金 ） : applications of electrochemical methods in metallurgical industry Winning（提取） Refining（精炼） Metallurgy: dry/fire/fusion metallurgy（火法冶金） Wet/hydro-metallurgy（湿法冶金）— —( electrodeposition
a relatively high percentage of elements which were
originally minor constituents (e.g. arsenic containing liquor from Cu cells) and such elements are often extracted. In many case, the effluent liquors must be treated to remove transition-metal and heavy-metal ions before they are discharged into the environment.
Mn+ + ne
M
There are two key problems in the cathodic reduction of metal ions: codeposotion of ions electrocrystallization
( 1) Codeposition E1=E2 E=Ee + η Hydrogen evolution is the most important competing reaction:
Electrowinning is usually one of the final steps in a complex extraction procedure.
The complete extraction process : 1) Mining the ore, which is commonly a sulphide or an oxide or a mixture both and will contain several
Using inert anode
2Al2O3(s) Hale Waihona Puke Baidu=2.21V 2Al(l) + 3O2(g)
The sacrifice of the carbon anode leads to a much
reduced cell voltage and energy consumption. In practice, there is an unwanted back chemical reaction in the electrolytic process: 2Al(l) +3CO2(g) Al2O3(s) +xC(s) Al2O3(l) + 3CO(g) 2Al(l) + yCO2(g) + zCO(g)
Properties of electrolyte in aluminim electrolysis
Cryolite, Na3AlF6 or 3NaF· AlF3, melting point 1010℃
a ) Structure of alumina-cryolite solution
The high solubility of alumina in molten Na3AlF6 results from the near equality of size of fluorine and oxygen atoms in the aluminium complexes in the melt and, hence, facile formation of oxyfluoride ions on
4.2 Aluminium extraction
1. Electrolysis of fused salts
(1) Characteristics of fused salt electrolysis
a) The properties of electrolyte are different from water solution b) Electrochemical processes at high temperature:
addition of the oxide to the Na3AlF6. Indeed, the
aluminium is probably present as a mixture of several
related species.
b) melting point
10～11.5wt％ Al2O3 960~962℃
ions.
2) If the ore is a sulphide, it is roasted in air to 2MS + 3O2 2MO + 2SO2
convert it to an oxide:
which is then leached with acid (normally H2SO4) to give
is passed through a reactor containing a more
electropositive metal, M1, in powdered form :
Mn+ + M1
M + Mn+1
d) The electrolysis
e) The liquor from the electrolysis cells may contain
initial stage Al2O3, % Melting point, ℃ Density, g/cm3 Viscosity, ×10-3Pa s Conductivity, S/m 8.0 945-940 2.105-2.085 3.65-3.50 1.85-1.75 metaphase stage 5.0 960-955 2.110-2.090 3.26-3.10 2.05-1.95 end stage 1.7 975-970 2.125-2.105 2.95-2.85 2.25-2.15
2H+ + 2e
H2
How to prevent the codeposition of ions?
a) Changing the concentrations of ions in solution b) Application of additives or complexing agents c) Current and potential d) Mass transport conditions ( 2) Electrocrystallization
)
Metals: Ferrous metals（黑色金属） Fe
Nonferrous metals（有色金属）
Nonferrous metals:
heavy metals >4.5g/cm3 Cu, Pb, Zn,etc. light metals <4.5g/cm3 Al, Mg,
noble/ precious metals Au, Ag, Pt rare metals W, Mo, rare earth metals
c) Anode effect d) Dissolution of metal in fused salt
2. Principles of aluminium electrolysis
(1) Electrolysis system
anode: graphite
cathode: liquid aluminum electrolyte: Al2O3 + Na3AlF6 (cryolite) Why to select cryolite as solvent Hall-Heroult: Alumina melts at 2020℃ to give a non-conducting medium, it will dissolve to the extent of 15 wt% in molten cryolite at 1030℃ to give a conducting medium.