Production of Non-Ferrous Metals

The most common non-ferrous metals are copper, aluminium, magnesium, titanium, tin, lead and nickel. These metals have valuable properties and find wide application, despite of their relatively high cost, in engineering, aircraft, radio and electronic industry and in a number of other areas.

Content of various elements (including non-ferrous metals) in the Earth's crust (in mass %) are:

Oxygen-46.6                                             Nickel-0.01    

Silicon-27.7                                              Tin-0.004

Aluminium-8.0                                         Zink-0.004

Iron-5.0                                                     Lead-0.0016

Magnesium-2.3                                         Silver-0.00001

Titanium-0.6                                             Gold-0.0000005

Copper-0.01                                              Platinum-0.00000005

 

So, such metals as aluminium, iron, magnesium and titanium have the highest abundance in the Earth's crust. But copper, having low content in the crust, is used by man during from 8 to 10 thousand years, while the industrial method of aluminium production was suggested only in 1886 independently by American student Ch. Hall and French engineer Poll Eru. Industrial production of magnesium and titanium started about 50 years ago. The point is that copper is present in nature sometimes in metallic state, besides that, it may be received from chemical compounds (copper ore) relatively easily. Aluminium, titanium and magnesium have very high chemical affinity to oxygen and other elements. Consequently, it is very difficult to receive them in metallic state.

 

2.4.1. Production of Aluminium

 

Aluminium is one of the lightest of the structural materials. It has a specific gravity of
2.7 g/cm³ and melting point 660°C. Annealed aluminium exhibits low ultimate strength of 80 to
120 MPa, and reduced hardness of HB 250 MPa, but possesses high ductility with elongation ranging from 35 to 45 %. But due to low specific gravity aluminium alloys have higher specific strength (s_u/g) than some kinds of alloy steel. Aluminium has good electrical and heat conductivity, is resistant to attack of corrosion in fresh water and atmosphere. Its alloys are used for production of parts of aircrafts, rockets, cars, ships and other machines and devices. Pure aluminium is used in electrical industry as a conductor. The main aluminium ores are bauxites, which consists of aluminium hydroxides AlO(OH) and Al(OH)₃ and foreign impurities. The process of aluminium production consists of two stages:

- production of alumina Al₂O₃ from bauxites;

- production of aluminium from melted alumina by electrolysis process.

After refining process bauxites undergo leaching in autoclaves at temperature 150 to 250°C and pressure 0.5 to 1.0 MPa (5...10 atm).

The following reactions take a place:

                                        (2.25)

                                        (2.26)

 

Sodium aluminate NaAlO₂ is dissolved in water and all impurities fall out on the bottom of the autoclave.

After filtration, cooling of liquid and decrease in pressure magnitude the reaction proceeds:

                                         (2.27)

 

A1(OH)₃ particles have the form of flakes, which come to the surface of liquid. They are removed from the surface of liquid, dried at temperature 1200°C in tube furnace and converted into alumina:

                                              (2.28)

 

Alumina has high melting point, equal to 2050°C. Because of this it is dissolved in cryolite Na₃AlF₆, which has low melting point.

The dissociation (Al₂O₃®2Al+³/₂O₂) takes place in the bath at the temperature 930 to 950°C (Fig. 2.16). Liquid aluminium falls out on the bottom of the bath. Then it is removed from the bath and subjected to refining.

Fig.2.16. Scheme of electrolyzer for aluminium production: 1 - cathode; 2 – fireproof lining;

3 – metallic shell; 4 – graphite lining; 5 - rough aluminium; 6 - current lead;

7 - sintered pitch resin (anode); 9 - electrolite (90 % Na₃AlF₆+10 % A1₂O₃);

10 - graphite (carbon) bath; 11 – electrode; 12 – foundation.

 

2.4.2. Production of Copper

 

Copper holds one of the leading positions among the non-ferrous metals by its high thermal and electrical conductivity, enhanced ductility and good corrosion resistance. Copper is easy to work in the cold and hot state. It has density of 8.93g/cm³ and melting point 1083°C. Annealed copper has an ultimate strength of 250 MPa, relative elongation of 45 to 60 %, and Brinell hardness of 600 MPa. Because of its high conductivity, copper has wide application in electrical engineering for production of conductors, connecting wire, magnet wire and current-conducting parts of devices. But application of pure copper as a structural material is limited. Industry widely employs the copper base alloys namely, brass and bronze.

Copper ores, named copper pyrite and copper glance, usually contain small amount of copper, from 0.5 to 6 %, and require concentration (dressing). Main compounds of copper ores are: Cu₂S, CuS, Cu₂O, CuCO₄, Cu(OH)₂, FeS, SiO₂, Al₂O₃, CaO, MgO and others.

Copper production process consists of several operations:

- concentration of ore to increase copper content from 0.5…6 to 35 %:

- oxidizing roasting of copper concentrate at 750...800°C to reduce sulphur content;

- melting of concentrate and preheating it to temperature of 1250 to 1300°C to separate the slag, which consists of oxides of iron, silicon and other impurities and primary matte, which consists of sulphides of copper and iron;

- convertation of matte by air blasting through it in converter to remove sulphur and iron and receive rough copper which is 98.4 to 99.4 % pure.

- fire and electrolytic refining to remove impurities and receive copper from grade MOO that is 99.99 % pure to grade M4 having a purity of 99 %.

 

2.4.3. Production of Magnesium

 

Magnesium is the lightest structural metal produced in commercial amounts. The density is 1,74 g/cm³, melting point is 651°C, tensile strength in the as-cast state ranges from 100 to 120 MPa and elongation is 8…12%. Like aluminium, magnesium has high specific strength and is used in form of alloys with aluminium, manganese, zinc and other metallic elements as a structural material for production of parts of rockets, aircrafts, cars, ships and so on. Magnesium alloys has s_u=200...400 MPa and high resistance to corrosion.

Magnesium is obtained from carnallite MgCl₂·KC1·6H₂O, magnesite MgCOs and from dolomite MgCOs·CaCOs. It is largely produced by the electrolysis of molten salts mixture
(Fig. 2.17), which has composition: 10% MgCl₂, 45% CaCl₂, 30% NaCl, 14% KC1 and
1% (NaF+CaF₂). This composition is needed to decrease melting point of electrolyte, which ranges from 710 to 730°C. At this temperature and voltage of 2.7...2.8 V the reaction of dissociation only MgCl₂ takes a place:

                                                   (2.29)

 

Fig. 2.17. Scheme of the unit for magnesium production by electrolysis: 1 – bath; 2 – slime;

3 – anode; 4 – cathode; 5 – magnesium; 6 – chlorine; 7 – collector with pipe for removing of chlorine

 

Liquid magnesium evolves on cathodes 4 and comes to the surface of electrolite. Gas chlorine evolves on a graphite anode 3 and comes into chorine collector 7. MgCl₂ periodically is added into a bath 1.

Crude magnesium produced by this method contains from 2 to 5 % harmful impurities. The crude metal is refined by melting it in an electric furnace under refining slags to obtain magnesium of 99.82 % to 99.92 % purity.

 

2.4.4. Production of Titanium

 

Titanium has density of 4.5 g/cm³ and melting point 1670°C. Commercially pure titanium contains no more than 0.1 % impurities, has a tensile strength from 300 to 500 MPa and relative elongation from 20 to 30 %. The alloying elements added to titanium make it stronger, but less ductile.

Titanium and its alloys possess the advantages of high mechanical properties and low density combined with the resistance to attack against corrosive environments, such as nitric, hydrochloric, and hydrofluoric acids. Titanium alloys of required mechanical properties are produced by alloying titanium with chromium, aluminium, vanadium, molybdenum, tin and other metals. These alloys are rather heat-resistant and can withstand temperatures up to 600...700°C.

Ilmenite (TiO₂·FeO) and rutile (TiO₂) are the major titanium ores. The process of titanium production consists of two stages:

- the conversion of rutile in titanium tetrachloride TiCl₄;

- the reduction of titanium by liquid (molten) Mg.

In electric resistant furnace, at presence of carbon-containing material (coke, oil coke) and at temperature 600°C rutile is converted in tetrachloride by Cl:

                                               (2.29)

 

TiCl₄ has melting point of 23°C and boiling temperature equal to 136°C. It is poured in steel retort and is reduced by liquid Mg in atmosphere of argon at temperature 750...800°C:

                                               (2.30)

 

The sponge of composition: 55...60 % Ti; 25...30 % Mg, 10...15 % MgCl₂ is formed on the walls of the retort. MgCl₂, which is in liquid state, goes into electrolytic bath for producing of magnesium. The sponge undergoes vacuum distillation at temperature 900...950°C. During this operation part of impurities is evaporated, part is removed in liquid state. After that a consumable electrode is made of titanium sponge. The electrode is remelted in vacuum arc furnace to refine titanium to 99.6...99.7 % purity.

Melting and pouring of titanium and its alloys are conducted in vacuum because of high chemical activity of titanium.

 

Powder metallurgy

 

There are three types of metals and alloys (according to technological features):

- cast, that are castings and ingots;

- wrought alloys, i.e. alloys after metal forming;

- sintered alloys, that are alloys produced by methods of powder metallurgy.

Powder metallurgy uses metal and non-metal elements and their chemical compounds for manufacture of products. The powder metallurgy techniques comprises following stages:

- powders production;

- preparation of mixture of powders and technological additions;

- forming of an article by pressing process (compressing a briquette or green compact);

- sintering that renders the article proper strength.

This technique is more complex and expensive than casting or plastic working methods. But the powder metallurgy techniques attracts more and more attention since they offer ample scope for production of materials and parts with high heat and wear resistance, which display stable magnetic properties or specific physicochemical properties. Main advantage of these techniques is that mentioned properties are impossible to be obtained by casting or plastic working methods.

Powders are produced by mechanical and physicochemical methods. Mechanical methods do not change the chemical composition of material and prepare powder by two ways:

- grinding solids in ball mills, vortex chambers and vibratory mills;

- granulating the melt, that is spraying the liquid metal.

Mechanical methods are applicable only for hard and brittle materials, which are the base material of all cermets. These are the powdered carbides of such metals as tungsten titanium and tantalum the hardness of which is close to that of diamond.

Physicochemical methods enable to reduce crushed oxides (ores) or carbides to metal powders. Sizes of metal powders range from 0.005 to 0.5 mm.

Ball mills or vibratory mixers may be used to blend ingredients in required proportions.

Moulding the blend into various shapes is the process of single-action or double-action compaction (pressing) in dies by mechanical or hydraulic presses at a pressure of 150 to 800 MPa (Fig. 2.18).

Fig. 2.18. Single-action (a) and double-action (b) pressing of powders: 1-bottom; 2-powder;

3-container; 4-plunger

 

Another method of powder moulding is compaction of metal powder into strip, including bimetallic strip.

Sintering is the process of heating green compacts in vacuum furnace or in furnace with shielding gases (argon, nitrogen, hydrogen) at a temperature from 60 to 80 % of the melting temperature of the base metal and holding time from 1 to 2 hours.

To impart the parts the final shapes and desired properties, the sintered articles can be put through additional processing: heat treatment, diffusion heat treatment, and coining or sizing.

Powder metallurgy techniques are used for manufacture of:

- filters, because of porosity of articles (porosity ranges from 10 % to 50 %of volume; it is determined by moulding pressure);

- frictional materials, produced by additions of asbestos, oxides and carbides to metallic powders;

- antifrictional materials, produced by additions of graphite and plastics to metallic powders;

- cutting tools by sintering carbides of tungsten, titanium and tantalum with cobalt powder;

- fireproof materials by sintering oxides and carbides, which have high melting point;

- pseudoalloys, components of which can't form solution in liquid state (e.g. iron and lead).

 

FOUNDRY PRACTICE