REFINING OF

METALS

Purity of a metal has no absolute scale and is in itself ambiguous unless the purpose of use of the metal is defined, i.e. an impurity may be negligible from the point of view of one property but may be significant for another.The above trace impurity effect can be well explained by the Hafnium in Zirconium system (where Hf is present in trace amounts). One of the reasons for such an effect is due to the tendency of the impurity element to accumulate in certain areas (viz. grain boundaries, etc.) of the crystalline structure of the matrix.

WHAT IS PURITY?

Refining is the process by which the impurity level of a particular metallic system is reduced to the required grade so as to eliminate the trace impurity effect on the property corresponding to its use. The required purity varies from 98.5% Titanium for aerospace applications to 99.95% Copper used in electrical applications to that of semiconducting materials used in electronics which demand a purity level of six nines (99.9999%).

REFINING

Metal of the type commonly referred to as commercial-purity is normally used as the starting material in ultra-purification operations. Depending on the metal in question, commercial purity usually means a purity between 99.0 and 99.95%. Commercial-purity metal can be prepared by a variety of processesThe two basic approaches to the refining of metals are as follows: Production of high purity metal compounds and their

subsequent reduction to very pure metal (useful in case of very reactive metals). This can be achieved through various means in which some are physical (sublimation, distillation, crystallization) and some are chemical (precipitation, ion exchange, solvent extraction, etc.)

Production of bulk metal from metal ore, ensuring maximum recovery, and then purify the metal to the required extent (Eg. Selective distillation, liquation, electrolytic refining, zone refining, etc.)

THERMODYNAMICS OF REFININGAssume a binary solution is formed when two elements A and B are mixed together according to the following equation—

XAA + XBB = AB (Solution)

Thus the integral molar free energy of mixing ΔGm is given by

ΔGm = ΔHm – T( ΔSmc + ΔSmxs )

where ΔSmc - configurational entropy of mixing.

ΔSmxs –excess entropy of mixing due to non-configurational factors.

Assuming a random distribution of atoms,ΔSmc = -R ( XA lnXA + XB ln XB )

ΔGm = XA + XB + RT( XA lnXA + XB ln XB ) - TXA - TXB

Assume that B is the impurity atom and A the element to be purified, XA + XB = 1 , dXA = -dXB

= - + + RT ln + T - T

As XB 0 , = 0 & = 0

= + RT ln XB - T

ln XB -∞

From the above equations ΔGm in the region where XB 0 due to the configurational term. The free energy of

pure phase is always lowered by the addition of impurity atom.

Hence the final steps of refining are very difficult to carry out as the pure metals tend to become impure by absorbing impurities from the environment.

 

PHYSICAL METHODS OF REFINING

SUBLIMATION This method is used when the metal to be refined has a solid

compound which possesses “high vapor pressure” . Purification by sublimation is specially advantageous if the metal

compound sublimes at low temperatures as the decomposition of heat sensitive impurities are avoided , which in turn minimizes corrosion.

Highly sublimating halides can be conveniently produced by chlorination . Zirconium oxide and hafnium oxide are each chlorinated in fluidized beds to give their tetrachlorides. Chlorination of the oxides is faster than that of the silicates, and can be efficiently conducted at a lower temperature in the presence of carbon.

The zirconium tetrachloride is then purified by sublimation at 350-400 °C in a nitrogen atmosphere containing 1-5% hydrogen. The hot gases pass through a filter to remove entrained particles before being cooled to condense the zirconium tetrachloride. This sublimation reduces the levels of oxide, iron, phosphorus and aluminium

DISTILLATION In a liquid solution of A and B, A can be separated from B by

partial vaporization , followed by the recovery of vapor and residue. One of the factors that determines the efficiency of separation is the relative volatility(α) of components which is given by :

α = (pa/Xa) / (pb/Xb)

= (ϒapao) / (ϒbpb

0)

where (pa, Xa) and (pb , Xb ) are the partial pressure and mole fraction of components A and B respectively. The ϒ’s and p0’s are the respective activity coefficients and vapor pressures of the components in pure states respectively. From the above we can directly conclude that α is a direct

measure of the ease with which the components can be separated by distillation. It does not vary with temperature generally as temperature has similar influence on the vapor pressure of both the elements.

This method is particularly effective when the relative volatility has a large value.

We now know that the fact that the most volatile component is enriched in vapor phase makes possible separation of two metals by distillation. Thus separation of A from B by distillation can be best achieved by repeating the distillation for a large number of times (as shown in the below binary phase diagram).

This “Differential Distillation” requires the continuous removal of the vapor. It is clear, however, that a 100 percent separation is impossible.

FRACTIONAL DISTILLATION OR “RECTIFICATION”

Rectification is a distillation technique for separating two components that is conducted as a continuous counter-current process.

The fractionating column consists of a long column with an assembly of large number of trays(a flat vessel with a hole at the bottom). The vapors move upward through the tray against a stream of dripping liquid and on contact with the liquid, vapors condense and transfer the latent heat to cause an overflow and lead to the partial vaporization of the liquid.

Since the column is heated on at the bottom, there develops a temperature gradient which further helps the separation.

The best example for rectification refining is the removal of silicon tetrachloride (SiCl4) from titanium tetrachloride(TiCl4) after its first purification stage (wherein the entrained dust and chlorides which are solid at room temperature are separated by means of simple distillation).

Commercial TiCl4 (b.p. 409 K) contains upto 2.5 percent SiCl4 (b.p. 330 K). In a liquid state both the chlorides are completely soluble in each other. TiCl4 can be completely separated from phosgene and SiCl4 in a stainless steel fractionating column.

“CRYSTALLIZATION” REFININGThe aim of refining by crystallization is to separate a solid phase from a liquid solution, where the solid phase may selectively accumulate either metallic values or the impurities.

The property of differential solubility of different compounds(dissolved in a solvent, usually water) is the basis of this process. A solute is precipitated from a solution when its concentration exceeds the solubility. This can be achieved by evaporation or by lowering the temperature so as to reduce the solubility limit itself.

This process is used in the purification of Platinum group metals as the solubility of their chloro-complexes vary significantly with bath temperature.

The only disadvantage encountered here is that significant amount of impurities are trapped either in the crystals themselves or in the liquid film adhering to the crystals. These impurities can be effectively eliminated by repeated crystallization.

Variation of Solubility of Pt group chloride complexes in water (x-axis) v/s Temperature(y-axis)

CHEMICAL METHODS OF REFINING

In this process metals are separated from each other through the process of differential precipitation by using suitable reagents.Eg: Bismuth phosphate process is used to separate plutonium from irradiated uranium The mixture is dissolved in nitric acid to give the

respective nitrates. Bismuth is added with phosphoric acid for the

precipitation of bismuth phosphate along with the complete co-precipitation of plutonium from its tetravalent state.

The precipitate is removed and plutonium is oxidized to its hexavalent state.

The subsequent precipitation of bismuth phosphate leaves plutonium in the solution.

Repetition of this process leads to almost 90% recovery of plutonium

CHEMICAL PRECIPITATION

Ion exchange method is used in the preparation of pure compounds rather than pure metals. A suitable resin is used to absorb the desired metal

from its compound by an ion exchange process. The absorbed ion is subsequently taken into

solution with a second ion exchange process. This is called ELUTION.

The metal is recovered from the solution by precipitation using a leach solution.

Resins are complex organicacids or bases which are insoluble in water. The organic part usuallyContain a polymeric backbone.

ION EXCHANGE

The resin may be written as RX, where R is organic part and X is the ion exchange site. The two types of resins are Cation-exchange resin: They are used to

exchange cations from the metal compound. Eg. RSO3H

CuSO4 + 2RSO3H = 2(RSO3).Cu + H2SO4

Anion-exchange resin: They are used to exchange anions from the metal compound. Eg. RNO3, RCl

[UO2(SO4)3]4- + 4RNO3 = R4UO2(SO4)3 4(NO3)-

The efficiency of ion exchange depends upon the following factors:- Size and valency of the ions being exchanged. Concentration of ions

At low concentrations of aqueous solutions, ion exchange increases with valency

For ions with same valency, ion exchange increases with atomic number.

Ion exchange remains almost same at high concentrations.

Heavy organic ions and complex metallic anions are better suited for ion exchange.

The physical and chemical nature of the resin. Temperature.

Ion exchange takes place by the migration of ion from solution into the resin and another ion in the opposite direction in stoichiometric amounts. The various steps followed are :- The transport of an ion from the solution to the resin

across a liquid film boundary. The diffusion of the ion to the interior of the resin. The main chemical exchange reaction. Diffusion of outgoing ion to the surface of the resin. Diffusion of outgoing ion across the liquid film

boundary.

Since the chemical exchange reaction is usually rapid, the overall reaction rate is controlled by one of the diffusion stages.

Ion Exchange Kinetics

Solvent extraction or liquid- liquid extraction is the separation of one or more components from a liquid by preferential dissolution in an extracting solvent. It is commercially used to separate Cobalt from Nickel, Vanadium from Uranium, Zirconium from Hafnium, etc. The basic stages in solvent extraction are :- Dissolution : The impure compound is dissolved in

suitable acid or alkaline medium Extraction and Decontamination : The extractant,

usually an organic liquid, is brought in contact with the aqueous solution. The metal ion enters the organic phase preferentially.

Partitioning : the organic and aqueous phase are separated by suitable techniques.

Stripping : The loaded solvent containing the product is introduced into another extracting unit to recover pure metal.

SOLVENT EXTRACTION

In metal purification, very often, the starting material for the refiner is the crude metal produced in the bulk. The various refining methods used in such case are :1. Selective Distillation ( Pb refined by removing

Zn)2. Liquation3. Purification by Solvent Extraction4. Electrolytic refining5. Chemical methods(fire refining and impurity

precipitation)6. Indirect distillation7. Zone-refining for production of ultra-pure

metals8. Special re-melting techniques such as vacuum

arc re-melting, electron beam melting and electro-slag refining.

LIQUATION Liquation implies selective melting of a component in an alloy. This process depends on various factors such as :

Difference in melting points of alloy components Immiscibility of the phases Difference in densities of the alloy components

The phenomenon of immiscibility is governed by the phase diagram of the respective alloy system.

Example for this process would be the separation of iron from tin .

Even after double smelting with tin ore the metal contains about 1 percent iron. From the Fe–Sn phase diagram , we can see that by cooling impure tin to above its melting point (232 degree Celsius), the compound FeSn2 will separate out as a solid phase and give a liquid with less than 0.01 percent iron.

This process may be carried out by cooling the melt in the ladle, whereby the solid compound is allowed to settle, or impure solid tin maybe heated slowly in a reverberatory furnace with a sloping hearth , whereby the almost pure liquid drains off.

A type of reverse liquation takes place when pure metal has a higher melting point than the impure melt. A typical example is given by the so-called “Pattison Process” , which was previously used for the de-silvering of lead.

De-silvering of lead is today mostly done by the “Parke’s Process” , wherein some Zn is added to the impure lead. Zn forms a number of stable solid phases with Ag , which may then be skimmed off the melt on cooling to just above the melting point of Pb.

CHEMICAL METHODS In chemical methods of refining, a reagent is introduced

into the basis metal to combine with one or more of the impurities and to form an insoluble compound or a mixture of compounds.

The impurity often forms a liquid phase with the flux and is called slag. For the removal of impurities which are less noble than the basis metal, chemical methods are usually more preferred.

Main chemical precipitation processes include: “Oxidation” which can be used to remove Na from

Zn , Zn from Au , (Pb, As, Sb, Sn, S, Mg , Al ,Fe ) from Cu , etc.

“Sulphidation” which can be used to remove Cu from metals such as Pb , Bi and Sn .

“Chlorination” which can be used to remove Zn from Pb , Pb from Sn , (Zn, Cu , Pb ) from Bi .

FIRE REFINING The fire refining technique is used to remove more reactive

elements from a molten metal by “Preferential Oxidation” .This technique is suitable for refining Fe , Pb, Sn and Cu.The impurities due to higher affinity towards oxygen are

preferential oxidized by blowing atmospheric oxygen through the metal or a compound which decomposes to give nascent oxygen ( e.g. NaNO3).

Frequently, a flux is added so that the impurity oxide is removed not as a solid, but dissolved in a mixture of liquid oxides (as is done during open hearth steel-making).

The oxygen may be supplied through : A gas-metal transfer ( as in Bessemer Converters) Through a slag layer ( as in open hearth ) Through a combination of the above two ( as in the

LD process for steel making ).

INDIRECT DISTILLATION Even under reduced pressures, the vapor pressures of

most metals are too low to allow distillation at ordinary temperatures (e.g. Refractory metals such as Ti and Zr )

The indirect distillation of the metal at low temperatures is usually carried out as follows :

Impure Metal + Vapor 1 = Vapor 2

Vapor 2 = Pure Metal + Vapor 1 The deposition of the metal from its vapors is known as “

Chemical Vapor Deposition”. Depending upon the nature of vapors , the variants of this

process include :Carbonyl Process : Used to purify nickel and iron by passing dry CO over the finely divided impure metal, which forms a volatile metal carbonyl and then heated further to give pure metal.Iodide Process ( Van Arkel – De Boer Process) : Used to purify refractory metals( such as V, Zr, Hf, etc.) in general . The aim of the iodide process is to form a volatile halide of the metal at low temp. and its subsequent deposition to form very pure crystalline metal.

In zone refining, a molten zone is made to move slowly from one end of a bar of impure metal to the other. During this zone pass, impurities are redistributed because of differences between the solubility limits of impurity elements (limiting impurity concentrations) in the liquid phase of the metal and the corresponding limits in the solid phase. Under equilibrium conditions, the resulting distribution is measured by the coefficient K0, which is defined as follows:

KO=

where Cs is the impurity concentration in the just-freezing solid phase and CL is the impurity concentration in the liquid phase.

At a distance ‘x’ from the starting end, after a single pass of zone length ‘L’ ,

= 1 – ( 1 – KO) exp [ - KO x / L]

where C0 is the initial concentration in the liquid phase.

Additional passes of the zone in the same direction cause further concentration of impurities at one end of the bar. After many zone passes, this end is removed and discarded.

ZONE REFINING

Vacuum arc remelting has been primarily developed for the refining of reactive and refractory metals like Ti, Zr, W, etc. An electric arc is produced between the electrode (made from the metal to be purified) and a metal base at the bottom. The electrode starts melting and a refined ingot builds progressively at the bottom. The electrode is gradually lowered gradually lowered using the sliding vacuum seal to maintain a constant spacing. A pressure of 10-3mm Hg is maintained.

VACUUM ARC REMELTING

In electroslag refining, the material to be refined id used as the electrode. The droplets of melt is refined under a reactive slag. When high current is passed , sufficient heat is generated in the slag to maintain it in the molten state and also to build sufficient superheat to raise itself to a temperature above the melting point of the metal. Thus, the electrode tip melts and droplets of molten metal fall through the reactive slag and solidify in the mould. The electrode is lowered as the tip gets consumed. The refining takes place between the electrode tip and the slag, the droplet and slag, and the metal pool and slag.

ELECTROSLAG REFINING

Extraction of Non-Ferrous Metals

H.S.Ray

R.Sridhar

K.P.Abraham Principles of Extractive Metallurgy

Terkel Rosenqvist Handbook of Extractive Metallurgy

Fathi Habashi ASM HandBook - Vol 02 - Properties and Selection

Nonferrous Alloys and Special-Purpose Materials

REFERENCES

Submitted toShashi Bhushan AryaAsst. ProfessorDept. of Metallurgical and Materials Engg

Submitted bySandeep.B.S Rakesh.R.Kamath10MT33 10MT28