14 Tell about Hydrometallurgical processes with examples.
1. Electrowinning. Although it is possible to extract the more noble metals by electrolysis of aqueous solutions of their salts, chemical reduction methods such as smelting are also feasible. The choice of process depends upon site conditions such as the composition of the ore and the relative costs of electrical energy versus other forms of energy. Copper and zinc are the main metals extracted by electrowinning from aqueous solutions.
Before electrolysis can take place there are many processing steps. Briefly, the concentrated ore is first converted to an acid-soluble form, followed by leaching in acid, commonly sulfuric, to produce an acidic solution of metal salts. This solution is purified to remove the more readily than the metal of interest. Purification techniques include cementation and solvent extraction or ion exchange. The purified solution is then fed to electrolysis cell.
The electrolysis cells are open concrete tanks lined with plastic or rubber, approximately 1 m across, 1 m deep and 5-15 m long. Electrodes measuring somewhat less than 1 m* 1 m hang vertically at intervals of about 5cm in diaphragmless cells and about 10-15cm when diaphragms are employed. The electrodes are arranged so that they are alternately anodic and cathodic, and all anodes and cathodes in a single tank are usually connected in parallel. The anodes are made of a lead alloy; the cathodes consist of the metal being produced and are prepared by prior electrolysis onto titanium or aluminium starter sheets. The cells operate at current densities of 100-500A m-2. During electrolysis the electrolyte is made to circulate slowly between tanks.
2. Electrorefining. 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. Electrorefining is also becoming increasingly popular for recycling metals. In the electrorefining of copper, nickel and lead, it is also possible to recover silver and gold from the anode slimes by first dissolving the less noble anode impurities in acid and then casting the remaining noble metals into anodes for subsequent electrorefining. In many cases the value of the noble metals so recovered exceeds the cost of the entire base-metal refining operation.
15 Tell about active metals manufacture, sodium, magnesium etc. These metals are all produced by electrolysis of a mixture of molten metal chlorides; the electrolyte composition is selected to minimize the process temperature and to ensure that it is the desired metal that is discharged at the cathode. The estimated annual world production of sodium and magnesium is a few hundred thousand tons while that for lithium is only a few thousand tons. The major uses are: (a) sodium-manufacture of lead alkyls, isolation of titanium metal, production of several organic and inorganic substances; (b) magnesium-organic synthesis and baterries. The general technology may be illustrated by the example of sodium production in the Down’s cell. The electrolyte is a molten mixture of sodium chloride (40%) and calcium chloride (60 wt % requiring a process temperature of about 6000C. The principle of cell design is shown in Fig.4.4, although more modern cells have four anodes and cathodes in each cell. The design and materials of construction are again determined by the electrolysis medium. The electrode reactions are simple; at the cylindrical graphite anode:
2Cl - - 2e- → 2Cl2
and at the steel cathode surrounding the anode:
Na+ + e- → Na
Some separation of the products is necessary to prevent the black-reaction and a steel gauze diaphragm ensures that the chlorine gas and the liquid sodium are guided to different collection reservoirs. The sodium is much less dense than the melt and readily rises up a pipe into a reservoir; indeed, this is the basis of its separation from the small quantity of calcium which also forms at the cathode. The calcium is more dense and sinks back into the electrolyte where the equilibrium:
Ca + 2NaCl ↔CaCl2 + 2Na
ensures that its concentration remains constant and low. The operation of the cell is controlled by the rate of anode corrosion; in the presence of some trace water of oxide ions in the molten chloride medium, the graphite anode is oxidized to carbon monoxide and carbon dioxide, and, hence, precautions are necessary to minimize the rate of loss of graphite.
Down’s cells operate at about 1 A cm-2 with a cell voltage of approximately -7 V. This compares with a reversible cell voltage of approximately -3.6 V, the difference being almost entirely iR drop in the electrolyte and the electrodes since overpotentials for the electrode reactions will be small at the operating temperature. The current efficiency is about 80%, indicating that significant back reaction occurs, and the energy consumption is in the range 9000-10 000 k Wh ton-1.
16.-9.
17.
Explain process of between electrodes and electrolyte. Electrode
potentials If you put strips of two different metals into an
appropriate electrolyte solution you are likely to find that a
potential difference appears between the two strips. The system is a
galvanic cell, commonly known as a battery. The potential difference
is produced by an electromotive force associated with the chemical
reactions between both the metals and the electrolyte. The interface
between each piece of metal and the electrolyte solution forms an
electrode. Electrodes are always used in pairs. It is impossible to
make voltage measurements or pass current into an electrolyte without
using two electrodes. A simple example is a piece of copper (Cu) and
a piece of zinc (Zn) both partly immersed in the same dilute solution
of copper sulfate (CuSO4).
The copper becomes about 1 volt positive with respect to the zinc.
The potential change occurs in two steps at the electrode-electrolyte
interfaces, not in the bulk solution. (See figure 5.2.) Both metals
are negative relative to the electrolyte, but the potential of the
zinc is more negative than that of the copper. So the copper is
positive with respect to the zinc. It
is impossible to measure these absolute electrode
potentials
directly
because you can't make contact with the electrolyte without using
another electrode. If you introduce a third electrode the problem is
still there because you don't know its electrode potential. However
the cell
potential,
which is the difference between the two electrode potentials, can be
measured directly. Metal
- electrolyte
electrode.
The chemistry, electrode potential, and other characteristics of an
electrode depend not only on the solid electrode material, but also
on the solution in which it is placed. Strictly the electrode
is
not just the metal, but the combination of the metal and the
solution. Thus for example, when copper is placed in a copper sulfate
solution we refer to a copper
- copper sulfate electrode.
18 Tell about electrometallurgy process, electrowining 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. Electrowinning, also called electroextraction, is the electrodeposition of metals from their ores that have been put in solution via a process commonly referred to as leaching.In mining applications electrowinning is used to recover silver and gold from cyanide-based leach solutions. The precious metals are first dissolved or "leached" from the ore or concentrate into solution using cyanide.
When exposed to solutions of potassium or sodium cyanide, into simple salts of silver, a white curdy precipitate of silver cyanide, which easily dissolves in excess cyanide salts to form a complex. These reactions can be expressed by the following equations:
AgNO3+ KCN= AgCN+ KNO3;AgCN + KCN = KAg (CN)2 or AgCl + 2KCN = KAg (CN)2 + KC1.The resulting complex salt KAg (CN)2 is the foundation of cyanide silver plating electrolyte.
In the manufacture of pure salt electrolyte AgN03 sufficiently mixed in equivalent amounts AgNO3 solutions and KCN, and to introduce an additional electrolyte obtained potassium nitrate in an amount of 100-150 g / l. When dissolving metallic silver or silver alloys in a nitric acid AgCl precipitation operation is inevitable. It should only take into account that the freshly precipitated salt dissolved much faster and more completely than cooked in advance.