Occurrence

The amount of chrome in the Earth's crust is 0.02% (mass). Overall its content ranks about 21st in natural abundance among the elements in crystal rocks. Chromite FeO∙Cr₂О₃ mineral is the main source of this element rich deposits of which are in Kazakhstan and the Urals.

Chrome can replace part of the aluminium or iron in many minerals, imparting to them their unique colours. The presence of chrome in the mineral beryl produces the green colour of emeralds. The red colour of ruby is due to Cr(III) for Al(III) substitution in the aluminium(III) oxide mineral.

The main natural compound of molybdenum is molybdenite, or molybdenum glance MoS₂— a mineral greatly resembling graphite and considered as graphite for a long time. The total content of molybdenum in the Earth's crust is 0.001% (mass). Large amounts of molybdenum are contained in sulfide copper ores.

Tungsten in the Earth's crust [0.007% (mass) is inferior to chrome, but is ahead of molybdenum. The natural compounds of tungsten in the majority of cases are tungstates—salts of tungstic acid H₂WO₄. For instance, the most important tungsten ore—wolframite—consists of iron and manganese tungstates. The mineral scheelite CaWO₄ is also encountered quite frequently.

Preparation

Chromite reduced by coal forms an alloy of chrome with iron — ferrochrome. Being added to steel, ferrochrome is utilized to produce the other alloy — stainless steel (12-15 % chrome).

FeO•Cr₂O₃ + 4C = Fe + 2Cr + 4CO

The pure metal can be prepared by reduction of Cr₂O₃ with powdered aluminium, silicon, or by electrolytic reduction of CrO₃. The chromite annealing with Na₂CO₃ and oxygen is the first step of the other method of metallic chrome production:

4Fe(CrO₂)₃ + 8Na₂CO₃ +7O₂ =2Fe₂O₃ + 8Na₂CrO₄ + 8CO₂

Afterwards, sodium chromate is dissolved in water. It is transformed into dichromate in the presence of H₂SO₄:

2Na₂CrO₄ + H₂SO₄ = Na₂Cr₂O₇ + Na₂SO₄ + H₂O

Chrome (VI) oxide is produced from dichromate at the action of H₂SO₄:

Na₂Cr₂O₇ + H₂SO₄ = 2CrO₃ + Na₂SO₄ + H₂O

To manufacture chrome(III) oxide, excess of chromate is usually required:

Na₂Cr₂O₇ + 2C = Cr₂O₃ + Na₂CO₃ +CO

and Cr₂O₃ then reduced by the aluminothermy or by silicon:

Cr₂O₃ + Al = A₂O₃

2Cr₂O₃ + 3Si + 3CaO = 4Cr + 3CaSiO₃

Electrolysis of CrO₃ solution in the presence H₂SO₄, HF is described by the following cathode half-reaction:

CrO₃ +6e^- +6H⁺ = Cr + 3H₂O

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The first step of metallic molybdenum production from its ore molybdenite MoS₂ is the annealing process:

2MoS₂ + O₂ =MoO₃ + 4SO₂

Molybdic acid that is produced further can be reduced by hydrogen:

H₂MoO₄ + 3H₂=4H₂O +Mo

The powdered metallic molybdenum is obtained. Compact molybdenum is mainly produced by powder metallurgy techniques. This process consists in pressing the powder into a blank and sintering the latter.

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Tungsten can be prepared by wolframite mineral sintering process with soda in the air (180-200^оС):

CaWO₄ + Na₂CO₃ = CaCO₃ + Na₂WO₄.

Tungsten in the form of sodium tungstate Na₂WO₄ can be extracted from the melt by water, whereas iron and manganese form Fe₂O₃ and Mn₃O₄ that are insoluble in water. The aqueous phase reaction with hydrochloric acid allows to isolate amorphous yellow precipitate of tungstic acid:

N a₂WO₄ + 2HC1 = H₂WO₄ + 2NaCl.

When heated, tungstic acid is transformed into tungsten trioxide WO₃. By reducing it with hydrogen at 700-850°С, a powder of metallic tungsten is obtained that is further processed like molybdenum powder to produce the compact metal:

WO₃ + 3Н₂ = W + 3H₂O.

Uses

• Metallic chrome is used for chrome plating, and also as one of the most important components of alloyed steels. The introduction of chrome into steel improves its resistance to corrosion both in aqueous media at ordinary temperatures, and in gases at elevated temperatures. Chrome steels also have an increased hardness. Chrome is a component of stainless, acid-resistant, and heat-resistant steels.

• More than half the production of chrome goes into metallic products, and about another third is used in refractories. It is an ingredient in several important catalysts. The chief use of chrome is to form alloys with iron, nickel, or cobalt. The addition of chrome imparts hardness, strength, and corrosion resistance to the alloy. In the stainless steels, chrome makes up 10 percent or more of the final composition. Because of its hardness, an alloy of chrome, cobalt, and tungsten is used for high-speed metal-cutting tools. When deposited electrolytically, chrome provides a hard, corrosion-resistant, lustrous finish. For this reason it is widely used as body trim on automobiles and other vehicles. The extensive use of chromite as a refractory is based on its high melting point, its moderate thermal expansion, and the stability of its crystalline structure.

• Chrome gives emeraldsand rubies their green and red colors. Chrome is also a catalyst in many industrial chemical reactions. Dichromates such as K₂Cr₂O₇ are oxidising agents that can be used in quantitative analysis and also in tanning leather. Lead chromate as yellow is a pigment compounds are used in the textile industry as mordants. Chrome is used by the aircraft to anodise (to put a protective oxide film on) aluminum. Chrome coats metals to make them durable .

MOLIBDENUM

• The major part of the tungsten produced is used in metallurgy for preparing special steels and alloys. High-speed tool steel contains up to 20% of tungsten and has the property of self-hardening. Such steel does not lose its hardness even when red hot. This is why the use of cutters produced from tungsten steel considerably increases metal cutting speeds.

• In addition to high-speed steels, other tungsten and chrome-tungsten steels have found broad application. For example, a steel containing from 1 to 6 % of tungsten and up to 2 % of chrome goes to make saws, milling cutters, punches, and dies.

• Being the most refractory metal, tungsten is included in a num­ber of heat-resistant alloys. Particularly, its alloys with cobalt and chrome—stellites—have a great hardness, wear resistance, and heat resistance. Alloys of tungsten with copper and with silver combine high electrical and thermal conductivity with wear resist­ance. They are used to make the working parts of knife-blade switches, circuit breakers, and electrodes for spot welding.

• Pure tungsten in the form of wire, band, and a variety of com­ponents is used in the production of electric lamps, in radio electron­ics, and X-ray equipment. Tungsten is the best material for the filaments of incandescent lamps: its elevated working temperature (2200-2500 °C) ensures a high luminous efficiency, and its very slight evaporation—a prolonged service life of filaments made from it. Tungsten wire and bars are also used as the heating elements in high-temperature furnaces (up to 3000 °C).

• Tungsten carbide WC has a very great hardness (close to that of diamond), wear resistance, and refractoriness. The most produc­tive hard tool alloys have been developed on its basis. They include 85-95% of WC and 5-15% of cobalt imparting the required strength to the alloy. Some grades of such alloys contain titanium, tantalum, and niobium carbides in addition to tungsten carbide. All these alloys are produced by powder metallurgy techniques and are em­ployed chiefly for making the working parts of cutting and boring tools.