Purification Of Late Transition Elements Or Compounds

Published Date: 02 Nov 2017

CHM 321: Purification of late transition elements or compounds

Prof. Tiwari

Avinaash Persaud

This series include all elements in the sub-series Lanthanoids and Actinoids of the inner-transition elements and at least part of the sub-series Transactinides, which are the elements following the actinoid series. In general these elements are known for their hardness, high density, high melting point and boiling point and heat conduction although there are exceptions.

Transition elements are commonly group into early and late transition elements. The purification of the late transition elements commonly depends upon their properties and reactiveness.

The late transition elements are:

The common sources of the late transition elements are:

Element

Source

Iron

Iron is produced starting from iron ores, principally haematite (nominally Fe2O3) and magnetite (Fe3O4).

Cobalt

Occurs in compounds with arsenic, oxygen and sulfur as in cobaltine (CoAsS) and linneite (Co3S4).

Nickel

Chiefly found in limonite (Fe,Ni)O(OH), garnierite (Ni,Mg)3Si2O5(OH) and pentlandite [(Ni,Fe)9S8] ore.

Copper

Usually copper found in such minerals as azurite (Cu3(CO3)2(OH)2), malachite (Cu2CO3(OH)2) and bornite (Cu5FeS4) and in sulfides as in chalcopyrite (CuFeS2), coveline (CuS), chalcosine (Cu2S) or oxides like cuprite (Cu2O).

Ruthenium

Found in the minerals pentlandite ((Fe,Ni)9S8) and pyroxinite

Rhodium

the metal occurs in ores mixed with other metals such as palladium, silver, platinum, and gold

Palladium

Obtained with platinum, nickel, copper and mercury ores

Silver

Found in ores called argentite (AgS), light ruby silver (Ag3AsS3), dark ruby silver (Ag3SbS3) and brittle silver

Osmium

Obtained from the same ores as platinum

Iridium

Found in gravel deposits with platinum

Platinum

Produced from ores called native platinum

Gold

Found in veins in the crust, with copper ore and natively

Purification of Iron

The practical application of pure, commercial high-purity iron has been growing dynamically in recent years, involving massive demands by magnet technology and special powder metallurgy. Ultra-high-purity (semiconductor grade) iron has long been used in basic research, and it is promising also in electronic applications, in the new optoelectronic semiconductor (#-FeSi2)

material and as the basic component in hard magnetic films and in the (Fe-Tb-Co-Cr) thin film applied in the recently developed magneto-optical (MO)recording technology. The investigation of the intrinsic and limiting properties of chemically pure iron bears supreme importance in ferrous material development, requiring the production of well-defined samples as a first step.

The starting material for purification is the conventionally produced technically pure iron. Higher purity may only be achieved by a sequence of a number of different methods.

Vacuum treatment of the liquid metal is practiced on a massive scale in the steel-making industry. A technical vacuum is effective in enhancing the reactions of degassing, decarburization or deoxidation by continuous removal of the evolved gases (N2, H2, CO and H2O, respectively). The usual levels of nitrogen, hydrogen, carbon and oxygen are 300-400ppm, 20-40 ppm, 1000-1500 ppm and 6000 7000 ppm, respectively. This treatment, however, has no or little effect on metallic impurities, of which Al, Si, Co, Cr, Mo, Sb, Sn, Ti and As have concentrations in the range 100-1000 ppm, and Cu, Mn, Ni and S are usually in the 1000 -5000 ppm range in raw iron.

In order to produce an iron material of significantly higher purity, the method of electrorefining in aqueous media can be applied. The low-carbon soft iron starting material is anodically dissolved in the FeCl2 or FeSO4 solutions - which contain supporting electrolytes and organic additives – and the Fe2+ ions are plated on the cathode. The brittle and coherent or powdery iron deposit can be separated mechanically from the starting cathode sheet. The resulting iron particles - after washing and drying - are subjected to vacuum melting or hydrogen + vacuum annealing. Hydrogen treatment serves the removal of absorbed oxygen. Electrolysis significantly reduces the concentrations of impurities, especially in the cases of Cu, Mn. Sn, C, 0 and S, leading to a purity level of 99.0 99.4% Fe. Higher purity can be achieved by further chemical and physical purification steps.

The method of reactive distillation can be implemented by reacting fine iron powder with carbon monoxide. The exothermic reaction of volatile carbony 1 formation,

Fe + 5CO Fe(C0)5,

can be reversed in a separate reactor by reducing the pressure and increasing the temperature. The process of carbonyl synthesis is significantly faster at higher temperatures, and - correspondingly - at high pressures (180-200°C,15-25 MPa). Carbonyl vapor is removed from the water-cooled autoclave and decomposed in a special tower at standard pressure. A number of other metals are also capable of forming carbonyls, the most important examples being Ni(CO)4 and [Co(CO)4]2. Differences in vapor pressures, however, allow their separation by fractional distillation.

The purifying effect of melting techniques in general is strongly limited in the case of iron, because of the high solvent power of the liquid metal. Horizontal zone melting in a container involves a high risk of contamination. Water-cooled copper or silver vessels offer pseudo-container less conditions, and this has been further improved by the introduction of the vertical floating zone technique, in which the molten zone of the iron specimen does not come into contact with any supporting material. Table 1 lists the equilibrium partition coefficients for a number of impurities in iron. Unfortunately, carbon exhibits distribution coefficients close to unity if the residual concentration is below 10 15 ppm. The removal of selectively oxidizable impurities (Ca, Mg, Al, Si, Zr, Ti and C) can be enhanced by applying a controlled oxygen concentration in the gas phase. The oxide phase is separated to the surface of the bar during the zone-melting procedure.

Table 1

Experimental data

Phase diagram data

Approximate values

Impurity

ko

Impurity

ko

Impurity

k0

O

0.033

B

0.05

Pb, Bi, Ag

<0.1

S

0.05

Ti

0.27

As

0.1-0.2

P

0.17

N

0.28

Sb, Nb

0.3 0.4

c

0.2

H

0.32

Ge

0.4-0.5

Ta

0.43

Pd

0.55

Au. Zr

0.5-0.6

Ni

0.74

Rh

0.78

Be, Si, Sn, Cu

0.6-0.7

Mn

0.86

 

 

Mo

0.7-0.8

 

 

 

 

V, Co

0.8-0.9

 

 

 

 

Al,Cr,W,Ru,Pt

0.9-1.0

 

 

 

 

Re

> 1.0

A powerful combination of vacuum melting and zone refining is achieved by the ultra-high-vacuum electron-beam floating-zone melting technique (UHV-EBFZM). Although ultra-high vacuum may result in iron losses, experimental proof has shown that it is a powerful technique for purifying iron, although the efficiency may strongly depend on the composition of the

starting material.

A comparison of the purifying effects of different procedures is shown in Table 2 for a few of the most important impurities in iron. The tabulated data demonstrate that Ni, Mo, Cu and Co are eliminated successfully by the anion-exchange route, yielding residual concentrations that are much lower than in the cases of electrorefining and zone melting.

Table 2

Impurity

 

 

Concentration (ppm)

 

 

Starting

Electrorefining

Electrorefining +

Anion exchange +

 

material

 

 

zone melting*

electrowinning

Ni

160

75

 

50-190

1.5

Mo

19

2

 

6.3-14.5

0.7

Co

0.22

0.1

 

0.24-0.8

0.2

Cu

7.7

1.5

 

0.7-3.4

0.1

Mn

5.3

1

 

0.1-0.3

0.05

C

100

40

 

25-66

25

"The first value corresponds to the concentration in the head, and the second t«

that in the tail end of the bar.

 

 

 

For trace analysis, iron can be activated for gamma-ray spectroscopy by neutron irradiation. The application of thermal neutrons helps to avoid interference by Mn radionuclides produced in the 56Fe (n,p)56Mn reaction [31]. The effect of purification is directly evidenced by a significantly reduced background level. Comparative results of testing the anion-exchange and the melting methods are shown in Table 3.

The highest reported RRR value referring to conditions after H2 annealing (indicating the total impurity level in the bulk material) is 20 210.

Table 3

Preparation RRR

Concentration (ppm)

 

Ga

In As W Na

Mn Cu

Electrolytic (original) 80 59 AIEX Fe(II-III) 7.2 AIEX Fe(II-III) + ZM 6700 <0.26

50 5.2 1 0.65 <0.02 <0.1 <0.08 0.2 <0.01 <0.6 <0.1 <0.1

1.7 28 0.05 <0.4 0.1 <0.1

AIEX Fe(II-III), two-stage anion exchange in Fe(II) and Fe(III) form + evaporation + H2 reduction; ZM, zone melting in vacuum.

Purification of Cobalt

Cobalt is primarily used in materials for advanced applications, such as superalloys for aircraft turbine vanes and blades, alloys for powerful, high-coercive-force permanent magnets (Alnico and rare earth-cobalt types), hard metal alloys for cutting tool materials (Co-Cr-W-C and Co-Cr-W-Mo-Ni-Fe-C) and electrodeposited Ni-Co alloys for protective hard coatings. Cobalt is also used as a basic component in specialty materials, including alloys for dental and surgical implants or bone fracture fixations (Vitallium Co-Cr-Mo-Si C, Co-Cr-Ni-Ti-Mo), low thermal expansion alloys (Fe-Ni-Co, Co-Fe-Cr), fine spring alloys (Co-Cr-Ni-Fe-Mo-Mn-C-Be) and. as a recently expanded application, magnetic recording thin film materials (Co-Ni, Co-Fe-Mg-C, Co-Mo-Zr-Nb, 7-Fe203-Co), and as a constituent in the (Fe-Tb-Co-Cr) thin film used for magneto-optical (MO) recording technology. A large portion of cobalt production is converted into different chemicals that are used for glasses, ceramics, dryers, paints, varnishes, catalysts, electroplating materials, electronics and solid state devices. Black cobalt oxide, C03O4, has recently gained application as a selective coating material for high-temperature solar collectors. High-purity cobalt is also promising as a basic component in gate electrode materials (cobalt silicide) in ULSI technology. Most of these strategic uses require materials based on the production of pure cobalt.

The most important impurities in cobalt are Ni, Fe, Cu. Zn and Pb. Iron and nickel are not removed efficiently by zone melting. Standard chemical separation methods, eliminating impurity metals from solutions are based on the varying solubility and stability of their compounds. Iron can be eliminated after dissolving commercial cathode cobalt in hydrochloric acid, also by a solvent extraction step in the methyl-isobutyl ketone 0.8M HC1 system. Overall solution purification in acid chloride media is possible by the application of anion exchange resins. Anion-exchange chromatography is capable of eliminating almost all of the significant impurities during a 9M HC1 rinsing step. Cobalt is subsequently eluted by applying 4- 5M HC1 as the eluent solution. However, satisfactory separation of copper may require a preliminary anion-exchange step retaining copper either in the divalent or in the monovalent state. Controlled potential electrowinning with gradually increased current densities, can further enhance separation, and produce cathode cobalt of superior purity, characterized by residual resistivity ratios well over 200. However, the concentrations of interstitial impurities may remain in the 1-5 ppm range. The steps in the anion-exchange-electrowinning-floating-zone melting process are characterized in terms of relative importance and efficiency by the corresponding increments in the measured residual resistivity ratios. Starting from an initial purity level of RRR ~ 65 (measured on the crude metal reduced directly with hydrogen from the C0CI2 raw material) it was increased to approximately 200 after a single-step anion exchange (followed by evaporation and hydrogen reduction). Application of electrowinning further increased the purity to a level indicated by an RRR value of ~ 240. Application of the floating-zone melting method to the purified metal could raise the RRR value to ~ 330. In order to assure the proper state of the impurities, the RRR was measured after annealing the specimens in dry hydrogen at 1123 K for 214 h. Proton activation analysis has shown that dry hydrogen annealing is also effective in removing C. N and O impurities.

The corresponding compositions are given in Table 4.

Element

Radioisotope and

 

Concentration (ppm)

 

-ray energy (keV)

Raw

After AIEX

After EW

After FZM

Ti

48V (983.5)

15

1.7

0.2

<0.08

Cr

52Mn (744.2)

22

<0.4

<0.09

<0.09

Fe

56Co (846.8)

21

14

3.8

3.3

Ni

61 Cu (656.0)

272

<4

<0.2

<0.2

Cu

63Zn (962.1)

45

0.08

0.01

<0.006

Zn

66Ga (833.7)

32

<2

<1

<0.3

Ga

69Ge (1106.4)

<5

<0.9

<0.04

<0.01

Ge

72As (834.0)

<4

<0.2

<0.4

<0.4

As

75Se (264.7)

<1

<1

<4

<4

Se

82Br (776.5)

<6

<2

<1

<0.7

Zr

^Nb (1129.2)

<1

<0.2

<0.05

<om

Nb

93Mo (684.7)

<0.01

<0.005

<0.003

<0.002

Mo

95TC (765.8)

<3

<1

<0.5

<0.7

Cd

1,1 In (254.4)

<30

<8

<2

<1

Sn

122Sb (564)

<3

<2

<1

<0.6

Sb

,21Te (573.1) 130i /coc i i

<0.2 <0.1

<0.02 <0.09

<0.02 <0.02

<0.02 <0.02

Te

I (536.1)

Pb

2wiBi (803.1)

<200

<40

<80

<90

Purification of Nickel

The traditional markets for nickel (stainless steel and other common ferrous and nonferrous alloys) represent little demand for high-purity metal. There is, however, a growing market for special-purpose materials, whose designed properties rely upon the quality of the raw materials used as components. Nickel is a key element in high-temperature stress- and corrosion-resistant

superalloys applied in the aerospace industry, mainly as parts in gas turbine engines. Other special uses are alloys for nuclear reactors, electrodeposited Ni-Co materials for protective hard coatings, biomaterials and low-expansion alloys. The electronics industry accounts for approximately 6-8% of nickel consumption, which is a constantly growing share. A special nickel-iron alloy is widely used in lead frames, while a Cu-Ni-Sn alloy is used in terminals, clips and springs. Pure nickel is also required for the production of hydrogenation catalysts and other chemicals.

The starting material for fine purification is the commercially available electrolytic Ni, obtained by cathodic reduction from sulfate solutions, or carbonyl Ni. Elements that impose adverse effects in technical applications primarily include Sb. As, Bi, Co, Cu, Fe, Pb, P. S, Sn and Zn. Due to their closely similar chemical and physical properties, cobalt presents great difficulties in its removal from nickel by conventional methods. Fortunately, it only exerts slightly harmful effects on the properties of the base metal. Conventional aqueous chemical techniques and standard hydrometallurgical methods are capable of purifying nickel to a significant degree. According to a practically proven method, commercial-grade nickel powder is dissolved in sulfuric acid. The dissolution is accelerated by the presence of sparged oxygen. While cobalt and iron go into solution, copper is not dissolved as long as metallic nickel is in contact with the solution. Iron is subsequently removed as ferric hydroxide, after oxidation with hydrogen peroxide or ammonium persulfate, by adjusting the pH value using ammonia. Cobalt ions can be removed by preferential electrolytic deposition on a high-speed rotating cathode, or by precipitating with l-nitroso-2-naphtol at pH 2. Nickel is extracted as pure powder from the aqueous system with hydrogen gas under pressure at elevated 180°C) temperatures (in autoclaves). The use of catalysts for nucleation (ammonium carbonate) and for densification (nickel powder) is essential. The carbon and oxygen impurities of the produced nickel powder can be removed by wet and dry hydrogen treatment at 450°C.

The purification of electrolytic or carbonyl nickel material by zone melting can take place by selective vaporization and/or segregation. The equilibrium vapor pressures of the most important impurity elements in the pure state at the temperature of molten nickel are in the 0.1-1.0 Pa range for Ni, Si and Fe, in the 20 80 Pa range for Cu and Al. but ~ 0.1 MPa for Ca and > 0.5 MPa for Mg. These data suggest that only Mg and Ca can be efficiently removed from nickel by selective vaporization. Zone melting in an atmosphere of dry hydrogen eliminates carbon, oxygen, arsenic, antimony, zinc, sodium and a number of other minor impurity elements. The removal of carbon can be assisted by preliminary oxidation. Table 5 demonstrates, in cases of electrowon and carbonyl nickel starting materials, that zone refining under a hydrogen atmosphere in a refractory container efficiently removes nonmetallic impurities, while the concentrations of the most important metallic components copper, iron, and especially cobalt are virtually unaffected, or may even increase because of contamination from the container wall and the vaporization loss of nickel. Therefore, zone melting of nickel has to be preceded by a different method of purification, one which is capable of eliminating cobalt, copper and iron.

Table 5

Material

 

 

Concentration (ppm)

 

 

 

Co

Fe

As

Cu

S

O

C

Cathode Ni

 

 

 

 

 

 

 

Raw

5300

1000

350

140-220

<10

80

27

Zone-melted

4500-7300

1100-1100 ~ 4

165-185

1.5-4.5

0-2

7

Carbonyl Ni

 

 

 

 

 

 

 

Raw

6.9

93

0.04

37-60

14- 40

 

52

Zone-melted

5.3-7.8

93-102

0.02-0.07

12

2.5-7.0

<1

9

The purification of nickel in chloride solutions by anion exchange is especially suitable for the purpose of eliminating these important impurities. Nickel does not form anionic chloro-complex species: thus it is entrained in the solution passing the ion-exchange column directly, while a large number of impurities (including Co, Cu and Fe) may be retained in the resin bed. High concentrations of chloride ions favor sorption and thereby elimination - of these impurities. A parallel reduction in the solubility of nickel, however, suggests that an optimum concentration (of aproximately 8.6 mole din-3) should be applied.

In order to assess the efficiency of the anion-exchange purification step. Table 6 shows neutron activation analytic results of relevant materials. Three commercial nickel grades (99.99% Johnson Matthey, electrolytic and Mond carbonyl nickels), the commercial-grade nickel oxide and chloride reagents, which were used as the starting materials for the anion-exchange purification, are compared to the nickel obtained as a result of anion exchange purification of the chloride solution and the evaporation- H2 reduction process steps. It is obvious that anion-exchange is especially effective in the removal of Fe, Co, Zn, Sb. Hg and Mo. Further purification by floating-zone melting in vacuum efficiently eliminates volatile elements, such as Na. K, Cr. Ag, Sb and Hg, in addition to the purifying effect of segregation. However, the two most important impurity elements are not affected by the vacuum-zone melting step.

Table 6

Me

 

 

Concentration (ppm)

 

 

J-M (4N)

Electrolytic Mond Ni

NiO

NiCla

AIEX Ni

Na

0.31

<0.2

0.11

-

600

7.7

K

<0.6

<21

<0.5

-

28

4.1

Sc

<0.0004

<0.004

<0.0009

0.004

<0.0004

0.0005

Cr

<0.3

<0.2

0.2

6.4

3.7

2.2

Fe

25

<32

130

460

62

3.2

Co

1.2

110

0.69

14

7.2

0.04

Zn

<0.2

72

<0.5

6.9

1.0

<0.06

Ga

<0.02

<0.6

<0.01

<3

<0.04

<0.05

As

<0.007

1

0.021

<5

<0.03

<0.02

Se

0.16

0.75

<0.1

<0.2

<0.03

<0.03

Rb

<0.1

<2

<0.4

<0.6

<0.2

0.04

Mo

0.5

<0.3

0.05

<1

0.5

<0.04

Ag

<0.03

<0.2

<0.1

<0.09

<0.04

0.02

Cd

<0.3

<0.9

<0.3

22

0.4

1.6

Sn

<5

<20

<16

<20

<4

<3

Sb

<0.004

<2

<0.002

130

2.1

0.05

Cs

<0.006

<0.06

<0.02

0.02

<0.007

<0.003

Ba

<2

<17

<5

<8

8.7

2.9

Instead of H2 reduction after evaporation, nickel may be electrodeposited from the purified ion-exchange effluent after separating the excess hydrochloric acid content. A starting concentration of 120 g Ni dm-3 may be reduced to a final value of 20 g dm-3 in a simple cell containing 110 diaphragms between the anode and cathode compartments. The required range of acidity is

pH 1-3 and the temperature should be higher than 40°C, at a current density of approximately 10 A dm-2 to obtain a pure metallic deposit. Enhanced purification by the electrowinning step is possible by applying a preliminary electrolysis at lower current densities, which dominantly deposits elements of a more noble character than nickel. The inert anode is conveniently made of platinum, while the starting cathode is made of a high-purity nickel sheet. The metal purified by the anion-exchange procedure, and electrodeposited directly from the chloride electrolyte, is characterized by significantly higher purity than that listed in Table 6, and even higher than that achieved by vacuum-zone melting of the H2-reduced metal. The concentrations of Na,Cr, Co and Fe do not exceed the 0.05, 0.14. 0.1 and 2.3 ppm levels, respectively. The highest reported purity of nickel, obtained by the anion-exchange-electrolysis-H2 annealing-EBFZM route, is characterized by an RRR value of 7000.

Purification of Copper

The electrical and mechanical properties of copper strongly depend on the degree of purity. The variations in properties are linked to the chemical and physical lattice imperfections. High-purity copper is extremely ductile, exhibiting an especially high electrical (and thermal) conductivity. Mechanically detrimental impurities, which decrease the strength at high temperatures, are principally Pb. Bi, Sb, Se, Te and S. which form oxides or other precipitated compounds at the grain boundaries during heating. Appreciable amounts of oxygen make the metal susceptible to hydrogen embrittlement, which is caused by the precipitation of water vapor at grain boundaries, arising from the reaction of dissolved oxygen with incoming hydrogen.

Demands are extremely stringent in cryogenic uses, where the high electrical and thermal conductivity of copper is indispensable for its use as a low-temperature superconductor stabilizer. Recently developed applications in microelectronics include extremely soft bonding wires, which replace the traditionally used gold wires, copper-to-ceramic bonding in high-thermal-conductivity substrates, interconnection of ULSI circuits, or pure copper alloys. Due to the low level of impurities and the immunity to hydrogen embrittlement, ultra-high-purity copper is widely used in high-vacuum technologies, such as materials for gaskets, high-energy vacuum tubes, or cavities in particle accelerators. The homogeneous grain structure of high-purity copper is an advantage in producing laser mirrors. This material is also used for hollow conductors (for high-field magnets, induction coils and generator windings) cooled with water or liquid hydrogen, or for tubing of deep-water optical cables. Special copper grades are also necessary for the production of high-fidelity audiovisual cables.

Conventional refining, which is widely applied in industrial production technology, comprises the major steps of: (i) fire refining, removing large amounts of oxidizable impurities from the crude melt: (ii) electrolytic refining, eliminating the majority of residual impurities to the anode slime or to the electrolyte solution; and, finally, (iii) cathode melting and casting. The last step can be carried out under strictly controlled reducing conditions, so as to reach a standard OFC (oxygen-free, high-conductivity) grade, where the remaining oxygen content is typically in the 1-5 ppm range, without the presence of deoxidants. The industrial refining procedure is hardly capable of achieving higher levels of purity than 99.99%.

The purification of copper in aqueous solutions by standard chemical precipitation techniques can be considered as a possibility for rejecting the larger part of the bulk impurities. The separation of copper from aqueous solutions by chemical precipitation is relatively simple. After dissolution in sulfuric acid, with the addition of hydrogen peroxide, Ag+, Hg+, Pb2+ and Tl+ ions are precipitated (in the chloride form) on adding HC1. Copper can be extracted from the solution by reducing the divalent ions to the monovalent form by the addition of dissolved glucose or formaline, which will result in the precipitation of CuCl. Copper chloride is readily reduced with hydrogen, although the temperature has to be regulated under very close control, due to the low melting point of copper chloride.

The efficiency of commercially applied purification may be increased basically by repeating the electrorefining step in a specially purified CUSO4-H2SO4 electrolyte under strictly controlled operating conditions and with the application of a diaphragm around the anode. The electrodeposited copper is stripped from the stainless steel plates and melted in the presence of oxygen to remove sulfur arising from the entrapped sulfates of the electrolyte. The impurities that remain and are additionally picked up can be removed by subsequent electrolysis in purified CUNO3 solution. The nitrate impurities are eliminated by thermal decomposition during the final melting and casting, under a protective N2 atmosphere.

The composition of the corresponding copper product is compared with that of common grades in Table 7.

Table 7

Element

 

Concentration (ppm)

 

Fire refined

Electrolytic

Oxygen-free

Special (5N)

Ag

10-40

10

10

<0.3

As

1-10

<5

3

<2

Bi

5-15

3

1

<0.1

Cd

1-10

1

Not detected

 

Co

1-10

1

~ 0

 

Fe

5-20

2

5

<0.7

Mn

<50

<0.5

0.5

 

Ni

30-80

3

6

<1

0

350

350

1.7

<0.1

P

<10

<10

Not detected

 

Pb

1050

2

6

<1

S

15

10

25

<1

Sb

<5

<2

5

<1

Se

30-70

<10

2

<1

Si

<8

<8

4

<0.1

Sn

550

5

2

<1

Te

<10

<10

1

<2

Zn

<10

<10

Not detected

 

Total

<550-800

<440

<72

<11.8

%IACS*

100

101.5

101.7

103.6

Special copper purification processes that have recently been reported, are principally based on electrolytic refining in acidic sulfate and/or nitrate solutions and zone melting. Since nitrogen, unlike sulfur, is practically insoluble in solid copper, possible nitrate inclusions from the final electrolysis step are readily eliminated by melting the cathodes. However, nitrate electrolysis has the disadvantage of poor reproducibility and extreme sensibility to the pH of the solution. The favorable cathodic process may be disturbed by the undesirable deposition of CU2O which takes place at low- acid concentrations according to the following reaction:

2Cu2+ + H20 + 2e" = Cu20 + 2H+

characterized by an electrode potential of E = 0.20 + 0.06 pH V in 1M Cu solutions. On the other hand, reduction of the nitrate ion can interfere with the cathode process at low pH:

NO3- + 4H+ + 3e- = NO + 2H20,

E = 0.95 - 0.08pH V,

Kinetic inhibition of reaction, however, makes it possible to deposit copper despite the unfavorable electrode potential.

Continuous and close control is a general requisite for any electrorefining operation. The deposition of impurities may be avoided efficiently by a proper adjustment of the cathode potential. Separation of the anode with a diaphragm is useful in preventing the formed anode slime particles from being carried over to the cathode, in which case forced circulation of the electrolyte may also be applied to enhance material transport and to diminish the harmful reduction in the copper concentration in the cathodic boundary layer. Constructing the cell with separate anodic and cathodic compartments can reduce the effects of the anodic process on the composition of the catholyte. A portion of the impure anolyte can be drained out continuously for chemical purification (e.g. precipitating Ag on Cu), followed by recirculation to the cathodic compartment. Further improvement of the chemical conditions in the electrolyte is possible by addition of reagents, such as free Cl- or H2 to decrease the Ag and S concentrations, but organic additives such as levelling agents should be avoided if ultra-high purity is desired. Further refining steps are generally necessary after electrolysis, including the conventional melting or zone refining under a protective (reactive) atmosphere or in vacuum.

In order to achieve an overall purification of copper in a virtually single-step operation, the method of anion-exchange chromatography in hydrochloric media has been proved efficient. Proper selection of HCl concentrations and oxidizing- reducing conditions during the anion-exchange operation may result in the efficient elimination of practically all of the impurities.

Purification of RUTHENIUM 

Ru, in chemistry, a metallic element, found associated with platinum, in platinum ore and in osmiridium. The metal may be obtained from the residues obtained in the separation of osmium from osmiridium. These are washed with ammonium chloride until the filtrate is colourless, ignited, fused with caustic potash and nitre, the melt dissolved in water and nitric acid added to the solution until the colour of potassium ruthenate disappears. A precipitate of ruthenium oxide gradually separates; this is collected and ignited in a graphite crucible and finally fused in the oxyhydrogen furnace.  A purer ruthenium is obtained by heating the crude metal (obtained by other processes) in a current of oxygen until all the osmium is volatilized as tetroxide. The residue is then fused with caustic potash and nitre, dissolved in water, saturated with chlorine and distilled on the water-bath in a current of chlorine. Pure ruthenium tetroxide distils over. This is then dissolved in water, reduced by alcohol and ignited in oxygen. Ruthenium in bulk resembles platinum in its general appearance, and has been obtained crystalline by heating an alloy of ruthenium and tin in a current of hydrochloric acid gas. Its specific gravity (after fusion) is 12.063. It fuses easily in the electric arc. It oxidizes superficially when heated, but fairly rapidly when ignited in an oxidizing blowpipe flame, forming a black smoke of the oxide. It is also oxidized when fused with caustic potash and nitre, forming a ruthenate. Acids have practically no action on the metal, but it is soluble in solutions of the alkaline hypochlorites. Like most of the other metals of the group, it absorbs gases. A colloidal form has been obtained by reducing ruthenium salts with hydrazine hydrate in the presence of gum-arabic.

Several oxides of ruthenium have been described, the definite existence of some of which appears to be doubtful. The dioxide, Ru02, is formed by heating sulphate, or by heating the metal in a current of oxygen. It crystallizes in octahedra isomorphous with stannic oxide. It is insoluble in acids and decomposes when heated to a sufficiently high temperature. Fusion with caustic potash converts it into a mixture of potassium ruthenate and ruthenium sesquioxide, Ru203, which is a black, almost insoluble powder. An oxide of composition Ru409 is obtained as a black hydrated powder when the peroxide is heated with water for some time. It becomes anhydrous at about 360° C., and is unattacked by acids and alkalis. The peroxide, Ru04, is formed when a solution of potassium ruthenate is decomposed by chlorine, or by oxidizing ruthenium compounds with potassium chlorate and hydrochloric acid, or with potassium permanganate and sulphuric acid. It forms a golden yellow crystalline mass, which sublimes slowly in vacuo, and melts at 25.5° C. It blackens on exposure to moisture, and decomposes when exposed to light. It is insoluble in water, but gradually decomposes, forming a hydrated oxide, Ru205H2O. It is readily reduced. Its vapour possesses a characteristic smell, somewhat resembling that of ozone. Ruthenium dichloride, RuC12, is obtained (in solution) by reducing the sesquichloride by sulphuretted hydrogen or zinc. It is stable in the cold. The sesquichloride, Ru2Cl6, is formed when a mixture of chlorine and carbon monoxide is passed over finely divided ruthenium heated to 350° C. It is a brown powder which is readily decomposed by boiling water. It absorbs ammonia readily, forming Ru2C16.7NH. Numerous double chlorides are known, e.g.Ru2C16.4KC1; Ru2C16.4NH4C1, &c. The pure tetrachloride, RuC14, has not been isolated, but is chiefly known in the form of its double salts, such as potassium ruthenium chloride, K2RuC16, which is obtained when finely divided ruthenium is fused with caustic potash and potassium chloride is gradually added to the fused mass. It is a red-brown crystalline powder, which is soluble in water. A similar ammonium salt has been obtained. Ruthenium sulphides are obtained when the metal is warmed with pyrites and some borax, and the fused mass treated with hydrochloric acid first in the cold and then hot. The insoluble residue contains a mixture of two sulphides, one of which is converted into the sulphate by nitric acid, whilst the other (a crystalline solid) is insoluble in acids. Ruthenium sulphate, Ru(S04)2, as obtained by oxidizing the sulphide, is an orange-yellow mass which is deliquescent and dissolves in water, the solution possessing a strongly acid reaction. Rouge de Ruthene, Ru2(OH)2C14(NH 4)7, is obtained from ammonia and ruthenium sesquichloride at 40° C., the product being purified by crystallization from ammonia. It forms small brown lamellae which dissolve slowly in water to give a fuchsine-red solution possessing a violet reflex. The solution possesses a considerable tinctorial power, dyeing silk in the cold. Potassium ruthenium cyanide, K4Ru(CN) 6.3H2O, formed when potassium ruthenate is boiled with a solution of potassium cyanide, crystallizes in colourless plates which are soluble in water. A ruthenium silicide, RuSi, has been prepared by the direct combination of the two elements in the electric furnace. It forms very hard metallic-looking crystals, burns in oxygen and is not attacked by acids. Potassium ruthenate, K2Ru04. H20, obtained by fusion of the metal with caustic potash and nitre, crystallizes in prisms which become covered with a blackdeposit on exposure to moist air. It is soluble in water, giving an orangered solution which becomes green on standing, and gradually deposits the hydrated pentoxide, Ru2O5H2O

Ruthenium (III) chloride (2H20

Dissolve in H2O, filter and concentrate to crystallize in

the absence of air to avoid oxidation. Evaporate the solution in a stream of HC1 gas while being heated just below it boiling point until a syrup is formed and finally to dryness at 80-100° and dried in a vacuum over H2SO4. When heated at 700° in the presence of Cl2 the insoluble a-form is obtained

Ruthenium (IV) oxide

Freed from nitrates by boiling in distilled water and filtering. A more complete purification is based on fusion in a K.OH-KNO3 mix to form the soluble ruthenatc and perruthenate salts. The melt is dissolved in water, and filtered, then acetone is added to reduce the ruthenates to the insoluble hydrate oxide which, after making a slurry with paper pulp, is filtered and ignited in air to form the anhydrous oxide

Ruthenocene [bis-(cyclopentadienyl)ruthenium]

Sublime in high vacuum at 120°. Yellow crystals which can be recrystallized from CCI4 as transparent plates.

Purification of Rhodium

This member of the platinum group of metals is prepared from the solution remaining after throwing down ammonio-platinum chloride, by precipitating by metallic iron, and fusing the deposit with 1 part lead and 2 parts litharge to form a regulus; the copper, lead, and palladium are dissolved out by dilute nitric acid, and the insoluble residue is mixed with 5 parts barium binoxide, and heated to redness for 2 hours; the mass is lixiviated with' water, and the residue is boiled with aqua-regia to volatilize the osmium tetroxide, which is condensed, the barium in excess is precipitated by adding sulphuric acid, and the filtrate is evaporated at 212° F. (100° C.) with excess of sal-ammoniac; the residue is washed with sal-ammoniac solution so long as a rose-red colour is communicated to the wash-water, and the filtrate is evaporated with nitric acid in excess to decompose the sal-ammoniac; the solid residue is heated to redness with 3 or 4 parts of sulphur, and rapidly boiled out with aqua-regia and sulphuric acid.

The resulting nearly pure metallic rhodium may be refined by fusing with 3 or 4 parts of zinc, and treating the alloy with strong hydrochloric acid; it is then dissolved in aqua-regia, and evaporated with ammonia in excess; the rhodium-ammonium chloride which separates out is purified by re-crystallization, ignited with sulphur in a graphite crucible, and finally fused in an oxyhydrogen furnace to remove traces of osmium and silicon. The pure metal is less fusible than platinum, has a sp. gr. of 12.1, and resembles aluminium in lustre and colour; it is almost insoluble in all acids, though its alloys dissolve in aqua-regia; it is attacked by chlorine more readily than any other member of the platinum group, and can be dissolved by repeated ignition with phosphoric acid, acid phosphates, or fused potash bisulphate.

Purification of Palladium Compounds

Palladium (11) acetate

Recrystallized from CHC13 as purple crystals. It can be washed with AcOH and H2O and dried in air. Large crystals can be obtained by dissolving in C6H6 and allowing to evaporate slowly at room temp. It forms green adducts with nitrogen donors, dissolved in KI solution but is insoluble in aqueous saturated NaCl, and NaOAc. Soluble in HCl to form PdC142-

Palladium (11) acetyl acetone

Recrystallized from CsH6-pet ether and sublimed

in vacuo. It is soluble in heptane, C6H6 (1.2% at 20°, 2.2 at 40°), toluene (0.56% at 20°, 1.4% at 40°) and acetylacetone (1.2% at 20°, 0.05% at 40O).

Palladium (11) chloride

The anhydrous salt is insoluble in H20 and dissolves in HCl with difficulty. The dihydrate forms red hygroscopic crystals that are readily reduced to Pd. Dissolve in concentrated HC1 through which dry C12 was bubbled. Filter this solution which contains H2PdC14 and H2PdCl6 and on evaporation yields a residue of pure PdC12.

Palladium (11) cyanide

A yellow solid, wash well with H20 and dry in air

Palladium (11) trifluoroacetate

Suspend in trifluoroaceticacid and evaporate on a steam bath a couple of times. The residue is then dried in vacuum (40-80°) to a brown powder.

Purification of Silver

Silver, called Luna and Diana, by the chemists, is a metal of a white colour, and lively brilliancy. It has neither taste nor smell, when perfectly pure. Its specific gravity is, although considerable, nearly one half less than that of gold, as it loses in the hydrostatic balance about an eleventh part of its weight. A cubic foot of this metal weighs seven hundred and twenty pounds. The tenacity of silver is also very considerable, for a wire of this metal, only one-tenth of an inch in diameter will sustain a weight of two hundred and seventy pounds, without breaking. Although gold exceed it in ductility, yet it may be drawn into wires as fine as liairt, and extended into very thin leaves; sp thin, that a grain only may be spread under the hammer, and be made to contain an ounce of water. It is inferior even to copper in hardness and elasticity and next after it the most sonorous. Under the hammer it acquires a hardness, which it may be deprived of by heating. It seems to be as fixed and indestructible as gold. Kunckci kept silver, as well as gold, in a glass-house furnace during a month, without alteration. Silver is apt to tarnish, and even to turn black, but it does not lose its property of being brightened to brilliancy. All the strong

acids are capable of dissolving it, but the muriatic and vitriolic are less powerful than the nitric, or aqua fortis.

Silver may be purified from an allay widi oilier inferior metals, by treating it with lead, and also with nitre; the former of which methods is termed cupellation or refining, and the latter purification by nitre. SILVER that is to be purified by nitre ought first to be granulated, and then mixed with a fourth part of its weight of dry nitre, an eighth part of potash, and a little common glass, all in powder. This mixture is to be put into a good crucible, of such a size as to be only two-thirds filled with it; and it is then to be covered with a small inverted crucible, with a small hole in the bottom, and luted on fast. Several, thus disposed, may be placed in a furnace, to which a ready access of air can be admitted, in order to melt the silver. Charcoal is now to surround the crucibles even with the tops, but not above them; and the fire is to be kindled, and the vessels made moderately red, a lighted coal being placed over the little hole of the inverted crucible. If a shining light be observed round this hole, and a slight hissing noise be heard, the operation proceeds well. Let the fire be kept up equally till the appearance cease, when it is to be encreased to melt the metal thoroughly, and then removed from the furnace. The larger crucible is to be broken when it is cold, and the silver will be found at the bottom, covered with a green alkaline scoria. If the metal be not sufficiently pure and ductile, the operation must be again repeated. Some silver is apt to be lost in this operation, by the swelling and detonation

of the nitre, which often forces it through the hole in the upper crucible, unless great care be used; nevertheless, this method has its advantages, being much more expeditious than cupellation:

Purification of Osmium

Osmium is found in ores mixed with other metals such as platinum, nickel, gold and silver, making it very hard to extract. First, the ore is treated to remove the other metals. Next, the residue is melted using sodium bisulphate then extracted with water to remove some of the other metals, leaving osmium in the insoluble residue. The residue is then melted with sodium peroxide and extracted with water, leaving the osmium salts. Then the salts are mixed with chlorine gas, which turns the osmium salt into osmium oxide. Next, the osmium oxide is dissolved using alcoholic sodium hydroxide, then evaporated until it is dry and burned under hydrogen gas. The result is pure osmium.

Purification of Iridium

A method of separating iridium from at least one other metal of the group consisting of platinum, palladium, ruthenium, rhodium and gold comprising converting the PGMs and gold in a solution to lower oxidation states by treating the solution with hydrogen peroxide in acid medium at elevated temperature, adjusting the pH of the solution to approximately pH 9, separating the precipitate to leave iridium and platinum in solution, converting the platinum and iridium to their highest valency states by the action of sodium bromate in acid medium at elevated temperature and adjusting the pH of the solution to between about 6.5 and 7.5 to precipitate iridium and leave platinum in solution.

 Purification of Platinum

Preliminary treatment of the ore or base metal byproduct with aqua regia (a mixture of hydrochloric acid, HCl, and nitric acid, HNO3) gives a solution containing complexes of gold and palladium as well as H2PtCl6. The gold is removed from this solution as a precipitate by treatment with iron chloride (FeCl2). The platinum is precipitated out as impure (NH4)2PtCl6 on treatment with NH4Cl, leaving H2PdCl4 in solution. The (NH4)2PtCl6 is burned to leave an impure platinum sponge. This can be purified by redissolving in aqua regia, removal of rhodium and iridium impurities by treatment of the solution with sodium bromate, and precipitation of pure (NH4)2PtCl6 by treatment with ammonium hydroxide, NH4OH. This yields platinum metal by burning.

Platinum (11) acetylacetonate

Recrystallized from C6H6 as yellow crystals and dried in air or in a vacuum desiccator.

Platinum (11) chloride

It is purified by heating at 450° in a stream of C12 for 2h. Some sublimation occurs because the PtC12 sublimes completely at 560° as red (almost black) needles. This sublimate can be combined to the bulk chloride and while still at ca 450° it should be transferred to a container and cooled in a desiccator. A probable impurity is PtCl4. To test for this add a few drops of H20 (in which PtC14 is soluble) to the salt, filter and add an equal volume of saturated aqueous NH4Cl to the filtrate. If no precipitate is formed within 1 min then the product is pure. If a precipitate appears then the whole material should be washed with small volumes of H20 until the soluble PtC14 is removed. The purified PtCl2 is partly dried by suction and then dried in a vacuum desiccator over P2O5. It is insoluble in H20 but soluble in HC1 to form chloroplatinic acid (H2PtCl4) by disproportionation

Purification of Gold

Extraction and Purification

Because of gold's inertness some 80% of gold within ore is in its elemental state. There are several processes for extracting, and then purifying it.

Amalgamation is a mercury based process which works because of gold's willingness to be dissolved by mercury. The mercury is applied on an ore, picks up the gold, and the resulting amalgam is distilled, with the mercury being boiled off to remove it. Mercury is highly toxic and therefore environmentally sensitive, making the industrial plant to perform this type of extraction expensive.

The most important process for gold extraction is cyanidation. Sodium cyanide solution in the presence of air causes gold to enter into solution. Good quality ores give up their gold under cyanidation in what is called vat leaching. Lesser quality ores require heap leaching, which involves huge piles of ore being repeatedly re-sprayed with the cyanide solution over a prolonged period.

Relatively raw gold is purified in two main ways. The cheaper first stage of purification is the Miller process which uses chlorine gas and reaches purification of 99.5%, and then there is the more expensive Wohlwill process which electrolyses gold to purities of 99.99%.