Lecture 9

Metallurgy of refractory metals: molybdenum, titanium, tungsten, tantalum, niobium, vanadium, zirconium, hafnium, rhenium. Physical and chemical features and properties of rare refractory metals

The lecture plan:

1. Molybdenum: physical and chemical properties, occurrence in nature, minerals, technology of molybdenum concentrates processing, application.

2. Tungsten: application, the natural minerals, technology of concentration and processing of tungsten ores.

3. Vanadium: properties, application, ways of vanadium extraction from iron vanadium-containing ores and concentrates.

4. Niobium, tantalum: properties, application, ways of ore concentration, the basic stages of technological processing of concentrates.

5. Zirconium, hafnium: properties, application, minerals, technology of raw material processing.

6. Rhenium: properties, application, raw materials for rhenium production, technology of rhenium extraction from molybdenum and copper concentrates.

7. Titanium: properties, occurrence in nature, production ways, application.

The aim of the instructor: to pay the special attention to methods of refractory metals production and their application field.

Aims for students: to study the basic properties and manufacture ways of refractory metals.

Molybdenum is the silver-white metal; 6 stable isotopes are known; it possesses the high heat conductivity, high corrosion resistance. Small additives of alloying elements essentially raise mechanical properties of molybdenum, practically not affecting on physical properties. Molybdenum forms two stable oxides MoO₂ and MoO₃. The basic natural mineral of molybdenum - molybdenite MoS₂ – is soft livid crystals similar to graphite.

Molybdenite is easily floated. Molybdenum concentrate is subjected to oxidizing roasting:

MoS₂ + 3,5O₂ = MoO₃ + 2SO₂

MoS₂ + 6MoO₃ = 7MoO₂ + 2SO₂

MoO₂ + 0,5O₂ = MoO₃

Except for MoO₃ in these reactions molybdenites (CaMoO₄, CuMoO₄ and others) are formed. MoO₃ is the volatile compound which can be easily caught. Sulfide sulfur should not remain in the cinder. Absence of sulfide sulfur in the cinder is an attribute of full extraction of molybdenum from raw material.

The following stage of cinder processing is alkaline hydrometallurgy. Solutions of soda, NH₃, NaOH are used. Soda is used more often as it is the cheapest reagent. MoO₃ and molybdates are well dissolved in alkalis solutions. Concentration of soda solutions practically does not influence on extraction of molybdenum at leaching.

The basic application field of molybdenum is alloying of steel and pig-iron (tool, high temperature, high-speed, stainless and other kinds of steels). For manufacture of high-alloyed steels ferromolybdenum, less often – calcium molybdate and etc. compounds are used.

The circuit (fig. 10 p. 44) + production of pure molybdenum (fig. 16 p. 53).

Tungsten was opened in 80th years of XVIII century. In 1893 y. ferrotungsten was received in the industrial scale, it is the basic additive for alloyed steels; 1900 y. - tungstic filaments in electro lamps were made; 1914 y. - on the basis of tungsten carbide the ceramic-metal hard alloys were received surpassing on quality the best tool steel.

Application:

1. The additive for steels. Tungsten is entered in the fused steel as ferrotungsten, containing 50-70 % W. The additive of ferrotungsten in steel raises durability, elasticity, resistance to wear and impact, including at high temperatures.

2. On the basis of tungsten carbide the most productive ceramic-metal hard alloys for manufacturing of working parts of cutting and chisel tools, etc. are created.

3. Alloys of tungsten with copper and silver are applied to manufacturing of knife switches, switches, electrodes for welding (electro- and heat-conductivity of Cu, Ag + wear resistance of W).

4. Alloy W-Ni-Co, possessing in high density, is applied as protection against gamma-radiation in radiotheraphy.

5. W is part of wear resistance, acid resistance, heat resistance alloys which basis is Cr, Ni, Co.

6. W is used for covering of strongly weared details of machines (valves of aviation engines, stamps, blades of turbines).

7. Metal tungsten is the best material for manufacturing of filaments and spirals in the incandescent lamps. The high working temperature (2200-2500⁰С) provides the big luminous efficiency, and the small speed of evaporation determines the long service life of filaments.

8. W is applied to manufacturing of cathodes, anticathodes, x-ray tubes, emission tubes, rectifiers of the high voltage and gas-discharge tubes.

9. The tungstic-molybdenum wire is applied for manufacturing of thermocouples for measurement of temperatures in the range 1200-2000⁰С.

10. Sodium volframite, tungstic acid are used for leather etching, as component of paints and fillers.

11. Tungsten oxides are used as catalysts in the oil and chemical industry.

W – atom mass is 183,92, density is 19,35 g/sm³, it represent the powder from light grey up to black color. At normal conditions tungsten is chemically stable.

The basic tungsten minerals

Minerals	Chemical composition	A content, %		Density, g/sm3	Hardness (on Moose’s)
		WO3	W
Ferberite	FeWO4	76,3	60,5	7,5	5
Gubnerite	MnWO4	76,6	60,7	7,1	5
Volframite	(Fe, Mn)WO4	76,5	60,6	7,1-7,5	5-5,5
Sheyelite	CaWO4	80,6	63,9	5,8-6,2	4-5

The WO₃ content in tungstic ore seldom exceeds 1 % (usually 0,2-0,5 %). The saleable tungsten concentrate should contain 55-65 % WO₃ and a minimum quantity of harmful impurity (P, As, Sb, Sn, Mo, Cu, S, Si, etc.).

Ores concentration. Due to the high density volframite ores are concentrated by gravitational methods (jigging, concentration on tables and sluices). For sheyelite ores the basic concentration method is flotation. During flotation sheyelite is easily separated from barren rock (quartz) by fat acids and their soaps. The best results are achieved at flotation of the ore after preliminary processing by a liquid glass.

Production of metal tungsten passes in some stages:

1. Production of pure WO₃ or pure alkaline metals volframates.

2. Production of the metal powder.

3. Transformation of the powder in the ductile metal by methods of powder metallurgy.

Processing of tungstic concentrates.

1. Decomposition of concentrates:

- Sintering or fusion with soda and the subsequent dissolution in water:

2FeWO₄ + 2Na₂CO₃ + 0,5O₂ = 2Na₂WO₄ + Fe₂O₃ + 2CO₂

3MnWO₄ + 3Na₂CO₃ + 0,5O₂ = 3Na₂WO₄ + Mn₃O₄ + 3CO₂

CaWO₄ + Na₂CO₃ =Na₂WO₄ + CaCO₃

Temperature of process is 800-1000⁰С. A result of this reaction is the soluble in water sodium volframate Na₂WO₄;

- Processing by solutions of soda ash and NaOH;

- Decomposition by acids.

2. Sedimentation of Na₂WO₄ as the insoluble tungstic acid H₂WO₄.

3. Production of pure tungsten oxide WO₃.

4. Reduction of WO₃ up to powder tungsten by hydrogen, carbon and other reducers. (P.105, fig. 2).

5. Melting and mounding of tungsten are complicated owing to high melt temperature (3395⁰С ± 15⁰С). To preparation of compact ductile tungsten the method of powder metallurgy is applied. The reduced by hydrogen tungstic powder is pressed in steel compression moulds and sintered at high temperature in briquettes of rectangular section. Solution of glycerin in spirit is added as greasing. (P.124, fig. 10 (1 part)).

New ways: zone melting with refined tungsten single-crystals growth, etc.

Vanadium – the metal of silver-white color. The content in the earth's crust: 0,015-0,020 % (on weight), i.e. vanadium content exceeds with content of Ni, Zn, W, Mo and some other elements. Pure vanadium practically does not differ from steel that allows to use it as a constructional material; easily gives in to plastic deformation. Valencies: II, III, IV, V. Vanadium compounds with valency V have the greatest practical value.

The basic mass of vanadium (85-95 %) is used in metallurgy. Introduction of small amounts of vanadium in steel sharply raises their durability, a limit of fluidity, hardness, resistance of weariness and plastic properties. Structural steels with the vanadium content of 0,15-0,2 % possess the high durability and good plastic properties. Tool steels, containing 1-2 % of vanadium, keep hardness at the high temperatures. Introduction of 0,03-0,05 % of vanadium in the steel, intended for production of sheet metal for deep stamping, eliminates propensity of sheet metal to ageing. Vanadium in quantity of 10-13 % is part of steels for manufacturing constant magnets.

Plastic properties of alloys of vanadium with titan are higher, than at vanadium of high cleanliness. Properties of these alloys even more raise as the result of additive Al, Cr, Si, C. Ductile vanadium and alloys on its basis are applied in instrument making, the nuclear, power, defensive industry and rocket technics.

The vanadium compounds are used in the chemical industry. Five-oxide V₂O₅ - the catalyst at sulfuric acid manufacture by contact way and in the industry of organic synthesis. Vanadium possesses the high corrosion resistance in the diluted solutions of sulfuric and hydrochloric acids, the sea water, in solutions of salts, in liquid fusible metal mediums. The additive of vanadium to rubber raises density of rubber, reduces porosity, such rubber is impenetrable for oils.

In the textile industry the vanadium compounds are used at dyeing, preparation of varnishes and paints. Vanadic dyes are applied in the glass and ceramic industry.

In agriculture the vanadium compounds are applied as pesticides and micro fertilizers.

Iron vanadium-containing ores and their concentration. Independent deposits of vanadium meet seldom; from 50 known minerals of vanadium practical value have some.

The basic mass of vanadium is connected to igneous rocks. Thus owing to affinity of ionic radiuses of trivalent iron along with vanadium vanadium in deposits of this type does not form independent minerals, and isomorphically replaces iron in magnetite lattice. The vanadium-containing titanium-magnetite ores have the greatest distribution

Average composition of titanium-magnetite ores: 17-53 % Fe, 5-16 % TiO₂, 0,13-0,96 % V₂O₅.

As a result of ores aeration the titanium-magnetite sand (0,36-0,38 % V₂O₅) are formed. The vanadium-containing titanium-magnetite ores are concentrated by dry and wet magnetic separation.

There are two basic ways of extraction of vanadium from iron ores and concentrates:

- Roasting of raw material with reagent additives and subsequent hydrometallurgical preparation of V₂O₅. Raw material is subjected to oxidizing roasting with addition of sodium salts. During roasting of titanium-magnetites the supreme oxides of iron and vanadium and soluble sodium vanadate are formed. The subsequent water or acid leaching is transferred sodium vanadate in solution. A degree of vanadium extraction - up to 80 %;

- Extraction of vanadium in converter slags and their hydrometallurgical processing. Under certain conditions of domain melting vanadium, containing in ore part of charge, can be reduced and transferred in pig-iron.

At the subsequent converter processing of pig-iron vanadium can be oxidized and transferred to in slag. At domain melting except iron oxides the oxides Cr, Mn, V, Ti, P are reduced too. Extraction of vanadium in pig-iron makes 80-85 %.

The converter method of devanadation of vanadic pig-iron creates conditions at which reaction of vanadium oxidation becomes practically irreversible. Blowing is made by air or oxygen. Temperature of process is 1230-1350⁰С.

After transition of vanadium in slag the vanadium content in metal makes 0,04-0,05 %. Metal is drained from under slag and directed for the further processing, and slag is taken from the converter and given on roasting and hydrometallurgical processing. Average V₂O₅ content in slag - 15,9 %. The output: 40 kg of slag on 1 t of pig-iron.

Niobium was opened in 1801, tantalum was opened in 1802, but owing to affinity of properties these two elements over 40 years were considered identical. The uniformity of electronic structure of niobium and tantalum atoms, the affinity of ionic radiuses and crystal structure explain the exclusive similarity chemical and physical properties of these elements.