Overview of molybdenum chemistry PDF

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Hebron University Faculty of sciences and technology Chemistry department Advanced Inorganic Chemistry Research Student's name: Ghadeer Amleh ID: 21520012 Dr. Hatem Maraqa 2019 The transition metal (molybdenum) Molybdenum history In ancient times a number of substances were collectively known by...

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Hebron University Faculty of sciences and technology Chemistry department

Advanced Inorganic Chemistry Research Student's name: Ghadeer Amleh ID: 21520012

Dr. Hatem Maraqa

2019

The transition metal (molybdenum)

Molybdenum history In ancient times a number of substances were collectively known by the Greek word „molybdos,‟ meaning lead-like. Molybdenite (MoS2), the most abundant molybdenumcontaining mineral, was in this class along with lead, galena, graphite and others. Though they did not distinguish between these various compounds, the ancients certainly used molybdenite. One example of their insight, a 14 th century Japanese sword, has been found to contain molybdenum as an alloying element. In 1768, the Swedish scientist Carl Wilhelm Scheele determined that molybdenite was a sulfide compound of an as-yet unidentified element, by decomposing it in hot nitric acid and heating the product in air to yield a white oxide powder. In 1782, at Scheele's suggestion, Peter Jacob Hjelm chemically reduced the oxide with carbon, obtaining a dark metal powder that he named 'molybdenum.' Molybdenum remained mainly a laboratory curiosity until late in the 19th century, when technology for the extraction of commercial quantities became practical. Experiments with steel demonstrated that molybdenum could effectively replace tungsten in many steel alloys. This change brought weight benefits, since the atomic weight of tungsten is nearly twice that of molybdenum. In 1891, the French company Schneider & Co. first used molybdenum as an alloying element in armour plate steel.

Wills Saint Claire - The first automobile to employ Mo steels in its construction (Courtesy of Climax Molybdenum Company)

Demand for alloy steels during World War I caused tungsten demand to soar, severely straining its supply. The tungsten shortage accelerated molybdenum substitution in many hard and impact-resistant tungsten steels. This increase in molybdenum demand spurred an intensive search for new sources of supply, culminating with the development of the massive Climax deposit in Colorado, USA and its startup in 1918. After the war, reductions in alloy steel demand triggered intense research efforts to develop new civilian applications for molybdenum, and a number of new low-alloy molybdenum automotive steels were soon tested and accepted. In the 1930s, researchers determined the proper temperature ranges to forge and heat-treat molybdenum-bearing high-speed steels, a breakthrough that opened large new markets to molybdenum. Researchers eventually developed a full understanding of how molybdenum imparts its many cost-effective benefits as an alloying element to steels and other systems. By the end of the 1930s, molybdenum was a widely accepted technical material. The conclusion of World War II in 1945 once again brought increased research investment to develop new civilian applications, and the post-war reconstruction of the world provided additional markets for molybdenum-containing structural steels. Steels and cast iron still comprise the single biggest market segment, but molybdenum has also proven to be

invaluable in superalloys, nickel base alloys, lubricants, chemicals, electronics and many other applications

Overview of molybdenum chemistry Molybdenum is a transition metal in group 6 of the periodic table between chromium and tungsten. Although molybdenum is sometimes described as a „heavy metal‟ its properties are very different from those of the typical heavy metals, mercury, thallium and lead. It is much less toxic than these and other heavy metals. Its low toxicity makes molybdenum an attractive substitute for more toxic materials.

Compounds of molybdenum which are commonly encountered have molybdenum in its highest oxidation state (VI), for example molybdenum trioxide, MoO3, sodium molybdate, Na2MoO4.2H2O, and ammonium di- and heptamolybdate, (NH4)2Mo2O7 and (NH4)6Mo7O24.4H2O. In aqueous solution molybdenum(VI) is present as the simple molybdate, [MoO4] 2- ion which is like sulfate or, depending on the concentration and pH as a polymeric polymolybdate ion. The lower oxidation state(IV), is found in the commonest ore of molybdenum the disulfide, MoS2. Molybdenum(IV) also forms an oxide, MoO2. The redox chemistry of molybdenum-oxygen compounds, as in selective oxidation catalysts and molybdenum oxidase enzymes, has molybdenum cycling between oxidation states (VI) and (IV). The chemistry of molybdenum is extraordinarily versatile: oxidation states from (-II) to (VI), coordination numbers from 4 to 8 and, accordingly a very varied stereochemistry; the ability to form compounds with most inorganic and organic ligands and bi- and polynuclear compounds containing molybdenum-molybdenum bonds and bridging ligands. It is this versatility which makes the chemistry of molybdenum challenging and exciting and the actual and potential applications of its compounds many and varied. Molybdenum is the first of the transition metals to have an extensive sulfur chemistry shown, for example, having as its principal ore molybdenum disulfide, MoS 2, its binding by sulfur ligands in molybdenum containing enzymes, application of MoS2 as an important industrial catalyst, and formation of many sulfur complexes some of which are used as soluble lubricating oil additives. Molybdenum has an extensive organometallic chemistry in its lower oxidation states. These compounds contain molybdenum-carbon bonds. A well known example is molybdenum hexacarbonyl, Mo(CO)6. These compounds are difficult to prepare and may decompose on exposure to air. They have specialized small volume uses as for example catalysts in fine chemicals synthesis. Molybdenum-based technical chemicals exploit the versatility of molybdenum chemistry in oxidation states. (VI), (V) and (IV). Materials made from molybdates are oxidation catalysts, are photoactive, and semiconducting. Many of the properties of molybdenum provide development opportunities and new commercial applications through the exploitation of its chemistry.

Molybdenum properties Molybdenum, element number 42 of the periodic table, lies in the table's second transition series, in Group 6B between chromium and tungsten. It has one of the highest melting temperatures of all the elements, yet unlike most other high-melting point metals, its density is only 25% greater than iron's. Its coefficient of thermal expansion is the lowest of the engineering materials, while its thermal conductivity exceeds all but a handful of elements. Molybdenum properties Atomic number

42

Atomic weight

95.96

Crystal structure

Body-centered cubic (BCC)

Lattice constant

a = 3.1470 Å

Density

10.22 g/cm3

Melting temperature

2623°C

Coefficient of thermal expansion

4.8 x 10-6 / K at 25°C

Thermal conductivity

138 W/m K at 20°C

When added to steel and cast irons, molybdenum enhances strength, hardenability, weldability, toughness, elevated temperature strength, and corrosion resistance. In nickelbase alloys, it improves resistance to both corrosion and high-temperature creep deformation. Molybdenum-based alloys have a unique combination of properties, including high strength at elevated temperatures, high thermal and electrical conductivity, and low thermal expansion. Molybdenum metal and its alloys are the first choice in many demanding specialized applications. Chemically, the outstanding feature of molybdenum is its extraordinary versatility: 

Oxidation states from (–II to VI)



Coordination numbers from 4 to 8



Varied stereochemistry



The ability to form compounds with inorganic and organic ligands, with particular preference for oxygen, sulfur, fluorine and chlorine donor atoms



Formation of bi- and poly-nuclear compounds containing bridging oxide or chloride ligands and/or molybdenum-molybdenum bonds.

Uses of new Molybdenum

Use 2017: 558 m lbs Mo (253,000 tonnes Mo) contained.

The chart above refers to molybdenum produced from mined ore, not to scrap material recycled by chemical processes or remelting; hence the name "new molybdenum". About 23% of this material is used to make molybdenum grade stainless steel, while constructional steel, tool and high speed steel and cast iron, taken together use an additional 56%. The remaining 21% is used in upgraded products including nickel alloys, molybdenum chemical compounds, lubricant grade MoS2 and molybdenum metal.

Catalysts: Molybdenum-based catalysts have a number of important applications in the petroleum and plastics industries. A major use is in the hydrodesulfurisation (HDS) of petroleum, petrochemicals and coal-derived liquids. The catalyst comprises MoS2 supported on alumina and promoted by cobalt or nickel and is prepared by sullfiding cobalt and molybdenum oxides on alumina. As the world supply of crude oil is further extended and low-sulfur crudes become less available, molybdenum-based catalysts will increase in use. Molybdenum not only allows for economical fuel refining but also contributes to a safer environment through lower sulfur emissions. Molybdenum catalysts are resistant to poisoning by sulfur and, for example, catalyse conversion of hydrogen and carbon monoxide from the pyrolysis of waste materials to alcohols in the presence of sulfur, under conditions that would poison precious metal catalysts. Similarly Mo-based catalysts have been used in the conversion of coal to hydrocarbon liquids. As a component of the bismuth molybdate selective oxidation catalyst molybdenum participates in the selective oxidation of, for example, propene, ammonia, and air to acrylonitrile, acetonitrile and other chemicals which are raw materials for the plastics and fibre industries. Similarly molybdenum in iron molybdate catalyses the selective oxidation of methanol to formaldehyde.

Pigments Molybdate-based pigments are used for two properties: stable colour formation and corrosion inhibition. Molybdenum oranges are prepared by co-precipitating lead chromate, lead molybdate and lead sulfate. They are light- and heat-stable pigments with colours from bright red-orange to red-yellow and are used in paints and inks, plastic and rubber products, and ceramics. Zinc molybdate is the basis of white corrosion inhibiting pigments which are used as paint primers. Molybdophosphoric acid is used to precipitate the dyes methyl violet and victoria blue.

Corrosion inhibitors Sodium molybdate has been used for many years as a substitute for chromates for the inhibition of corrosion in mild steels over a wide range of pH. Molybdates have a very low toxicity and are less aggressive oxidants than chromates toward organic additives that may be used in corrosion inhibiting formulations. A prime application is in cooling water in air-conditioning and heating systems to protect mild steel used in their construction. Molybdates are used to inhibit corrosion in water-based hydraulic systems and in automobile engine anti-freeze. Molybdate solutions protect against rusting of steel parts during machining. Corrosion inhibiting pigments, primarily zinc molybdate, but also molybdates of calcium and strontium, are used commercially in paints. These pigments are white and can be used as a primer or as a tint with any other colour.

Smoke suppressants In electronic technology, wire and cable insulation represents a potential fire and smoke hazard to fire fighters and others in confined spaces of aircraft and hospitals. Ammonium octamolybdate is used with PVC to suppress the formation of smoke. These uses and other developments will increase as video, telephone and computing networks increase.

Lubricants Molybdenum disulfide, the most common natural form of molybdenum, is extracted from the ore and then purified for direct use in lubrication. Molybdenum disulfide because of its layered structure is an effective lubricant. When MoS2 particles are located between moving surfaces the MoS2 layers slide over each other, permitting the surfaces of steel and other metals to move fluidly, even under severe pressures, as bearing surfaces. Since molybdenum disulfide is of geothermal origin, it has the durability to withstand heat and pressure. This is particularly so if small amounts of sulfur are available to react with iron and provide a sulfide layer which is compatible with MoS 2 in maintaining the lubricating film. Molybdenum disulfide will perform as a lubricant in vacuo where graphite fails. A number of unique properties distinguish molybdenum disulfide from other solid lubricants: A

low coefficient of friction (0.03-0.06) which, unlike graphite, is inherent and not a result of absorbed films or gases;  A strong affinity for metallic surfaces;

 Film forming structure; 5  A yield strength as high as 3450 MPa (5 x 10 psi);  Stability in the presence of most solvents;  Effective lubricating properties from cryogenic temperatures

to about 350C o in air

o

(1200C in inert or vacuum conditions). A combination of molybdate and water soluble sulfides can provide both lubrication and corrosion inhibition in cutting fluids and metal forming materials. Oil soluble molybdenumsulfur compounds, such as thiophosphates and thiocarbamates, provide engine protection against wear, oxidation and corrosion. Several commercial manufactures supply these additives to the lubrication industry.

Molybdenum chemicals in agriculture Molybdenum is an essential trace element for plants and animals. It is an essential component of the enzyme nitrogenase which catalyses the conversion of atmospheric nitrogen to ammonia. Accordingly molybate is applied in fertiliser formulations.

Molybdenum grade stainless steels The generic term “Stainless Steel” covers a large group of iron-base alloys that contain chromium. The term “stainless” implies a resistance to staining or rusting in air. Stainless steels contain at least 10.5% chromium, which promotes formation of a thin, chromiumenriched surface oxide. Without this minimum amount of chromium, iron-base alloys or steels corrode in moist air, forming the familiar red rust. While chromium content determines whether or not a steel is "stainless," molybdenum improves the corrosion resistance of all stainless steels. It has a particularly strong positive effect on pitting and crevice corrosion resistance in chloride-containing solutions. Stainless steels are grouped in several different types defined by the steel's microstructure. Austenitic stainless steels account for almost 75% of all stainless steels used in the world; ferritic, about 25%; duplex (mixed austenite and ferrite), about 1%; and martensitic about 1%. Composition is the primary determinant of stainless steel microstructure.

Chemical Mo products Chemical Mo products available on the market include:  lubricant

grade molybdenum disulfide (MoS2), produced by drying and de-oiling unroasted high purity molybdenite concentrate;  sublimed pure molybdic oxide, produced by the sublimation of roasted molybdenite concentrate;  ammonium heptamolybdate,  ammonium octamolybdate,  ammonium dimolybdate,  calcined pure molybdic oxide, and sodium molybdate, produced by upgrading roasted molybdenite concentrate using wet chemical processes. because of their unique characteristics, molybdenum chemicals find extensive use in:  desulphurisation catalysts,  colourful pigments,

   

corrosion inhibitors, fertilizer micronutrients, flame and smoke suppressants, and lubricants for extreme pressure and temperature operating conditions.

Meltstock Mo products The following table shows various alloys and the meltstock Mo products they use. Meltstock Mo products used for alloying Superalloys

Stainless Steel

Alloy Steel

Roasted molybdenite concentrate (Technical Mo oxide)

x

x

Tool Steel & Cast High Speed Iron Steel x

Ferromolybdenum

x

x

x

Mo metal pellets

x

x

Stainless steel, alloy steel, tool steel, and high speed steel are typically melted in an electric arc furnace. The initial charge usually contains alloy steel scrap, pig iron, and a source of "Mo units" to attain the proper composition. Using scrap ensures that recycling provides an important share of the melt. Recycled material comes from in-house scrap, scrap collected from steel fabricators, and remelt stock purchased from recyclers. Depending on the steel grade and market factors, the share of molybdenum from recycled material can vary between 10 and 50%. Technical Mo oxide is usually added with the scrap charge to approach the specified Mo content. If only small amounts of new molybdenum are required, ferromolybdenum (FeMo) the normal alloy addition. After meltdown, the liquid metal is usually transferred into an AOD vessel or a ladle furnace for further metallurgical treatment. Ferromolybdenum is added at this stage to adjust the composition to the specified Mo content. FeMo is used exclusively as the source of molybdenum in cast iron. Superalloys, on the other hand, use only molybdenum pellets because of the alloys' requirement for high purity additions and vacuum melting. Molybdenum metal scrap generated by mill product manufacturers is also used for alloy additions. The balance between metal and other addition sources depends upon the relative economics and availability of the various sources of Mo units.

Roasted molybdenite concentrate (Technical Mo Oxide) Tech oxide is the principal product for adding molybdenum to alloy and stainless steels. It typically contains 56 - 58% Mo and a maximum of 0.5% Cu, and is available in the following forms and packaging options:  powder (max 400 mesh) packaged in: o cans with 10 kg or 20 lbs Mo content, o steel drums (250 kg or 500 lbs), or o bulk bags ( 1000 kg or 1500 kg); and

 pillow shaped briquettes, packaged in: o steel drums (250 kg or 500 lbs), or o or bulk bags (1000 kg or 1500 kg).

Product standards include:  ASTM

A146-04 Standard Specification for Molybdenum Oxide Products

Roasted molybdenum concentrate (Technical Mo oxide) powder (left) and briquettes (Courtesy of Molymet, Chile)

Ferromolybdenum (FeMo) FeMo can be used in any melting or refining unit to produce Mo-containing steel or cast iron. It is frequently used as ladle addition to achieve accurate final adjustment of composition. In steels with low Mo content (usually no more than 0.2% Mo) like High Strength Low Alloy (HSLA) steel, all of the Mo is added as FeMo. FeMo typically contains 65 – 75% Mo and a maximum of 0.5% Cu. The product is produced in size ranges between:  0 and 10 mm, and  10 mm on the low side and several maximum size limits between 20 and 100 mm. It is available as powder for special applications like welding electrodes. Packaging is normally in steel drums or big bags. Product standards include:  ASTM A132-04 Standard Specification for  DIN 17561, 2004-02, Ferromolybdenum

Ferromolybdenum

Ferromolybdenum (Courtesy of Treibacher, Austria)

Mo metal pellets Pure Mo metal (Mo min. 99.9%) is used for superalloys to avoid contamination with trace elements. The molybdenum powder is pressed into pellets and sintered in hydrogen to chemically reduce adsorbed oxygen and oxide films on the powder particle surfaces, densify the pressed pellets, and increase pellet strength. Sintering thus minimizes the amount of oxygen the pellets carry into the melt and increases ease of handling them in the melt shop.

Molybdenum mining & processing Molybdenum mining and processing techniques have been improved continuously since the first mine was started at Climax near Leadv...