Roasting process optimization of molybdenite concentrates in rotary kiln

The aim of this study is processing of domestic molybdenum concentrate to produce molybdenum trioxide in rotary kiln under different air conditions and removing copper to use it for the production of various molybdenum products. Molybdenum has an atomic number 42, an average atomic weight of 95,95, belongs to the sixth group of the periodic system of elements, and occurs between chromium and tungsten vertically and between niobium and technetium horizontally. 235 thousand tons molybdenite concentrate were produced in worldwide 2015. Most of this production were made in China, Chile and U.S.A. China has produced 83 thousand tons, Chile has produced 52 thousand tons and U.S.A. has produced nearly 47 thousand tons. Turkey has produced 400 tons in 2011 but it was 900 tons in 2015. Turkey is currently producing only molybdenite concentrate, not any other molybdenum compounds. In its pure state, molybdenum is a lustrous gray malleable metal, capable of being filed and polished. It can also be turned and milled without difficulty. Molybdenum is an important refractory metal with a very high melting point (~2610⁰C). Only carbon, tungsten, rhenium, tantalum and osmium prossess still higher melting points. Its boiling point is 5560⁰C and its density is 10,22 g/cm3 at 20⁰C. The coefficient of thermal expansion is about one third to one half that of most steels. At elevated tempetatures, this low expansion provides dimensional stability and minimizes the danger of cracking. Its thermal conductivity is several times that of many high temperature alloys and approximately half that of copper. Good thermal conductivity together with low specific heat allows molybdenum to be rapidly heated or cooled, with lower reultant thermal stresses than most other metals. The electrical conductivity of molybdenum, which decreases with increasing temperature, is relatively high, being about one third that of copper. This makes the metal very suitable for electrical applications. Molybdenum is endowed with a high modulus of elasticity – one of the highest found in commercial metals. The modulus is not affected much by temperature, and even at 800⁰C, its value substantially exceeds that of an ordinary steel at room temperature. Molybdenum is stable in air at ambient temperatures. A major weakness of unprotected molybdenum is its failure to resist oxidation at elevated temperatures. There is slight oxidation at about 400⁰C. At temperatures above about 600⁰C, the oxidation of molybdenum is so rapid in air or in oxidizing atmospheres with formation of volatile molybdenum trioxide that extended use under thse conditions is impractical. This is not so in a vacuum, where uncoated molybdenum has an almost unlimited life. Molybdenum is completely inert in pure hydrogen, argon and helium atmospheres. Its numerous furnace applications provide ample testimony to this. Above a temperature of about 700⁰C, the metal is rapidly oxidized by steam to molybdenum dioxide (Mo + 2H2O  MoO2 + 2H2). Molybdenum exhibits relative inertness in carbon dioxide, ammonia and nitrogen up to temperatures of about 1100⁰C. At still higher temperatures, a superficial nitride film may form on molybdenum in ammonia and nitrogen atmospheres. At temperatures over 1100⁰C, molybdenum may become carburized by carbonaceous gases such as hydrocarbons and carbon monoxide. A varying behavior, depending upon the prevailing atmospheres is exhibited by molybdenum in sulfur containing gases. In a reducing atmosphere, molybdenum resists hydrogen sulfide at elevated temperatures very well. A thin adherent sulfide coating forms on the metal rapidly. Molybdenum is variably disposed to the halogens. It has excellent resistance to iodine vapor up to about 800⁰C, to bromine up to about 450⁰C and to chlorine up to about 200⁰C. Fluorine which is the most reactive of the halogens, attacks molybdenum evet at room temperature. Generally molybdenum metal is produced from its high grade sulphide ore through oxidizing roasting of molybdenite in order to obtain molybdenum trioxide, purification of molybdenum trioxide and hydrogen reduction of molybdenum trioxide. Molybdenum is widely used for the production of ferromolybdenum which is requred for the production of alloyed steel. Almost 80 % of molybdenum produced from molybdenum trioxide is used for steel making industries. When added to steel and cas irons, molybdenum enhances strength, hardenability, toughness, elevated temperature stregth and corrosion resistance. In nickel-base 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. The raw material which was used in total oxidizing roasting experiments is loval molybdenite concentrate sent by Özdoğu İnş. Tic. Ltd. Şti. According to Özdoğu İnş. Tic. plant production chief, the ore which has 0,4 wt.% Cu and 0,05 wt.% Mo content is subjected to a flotation process in order to obtain enriched two different concentrates which have 27 wt.% Cu and 52 wt.% Mo contents The cehmical analysis of the molybdenite concentrate matches with the company's analysis results with 52 wt.% Mo content. Technical grade MoO3 is produced by roasting molybdenite in air atmosphere in a multiple-hearth furnace. The roasted MoO3 product usually has <0,1% sulphur content. In this study roasting process were made in rotary kiln under different temperatures (500-550-600-625 and 650⁰C) and air conditions (atmospheric, stoichiometric 100 % and stoichiometric 200 %). S content were reduced to 0,063 % from 39 %. The roasted MoO3 product had 2,41 % Cu. This copper needed to remove from the MoO3. Copper is limited by 0.5 %. Leaching experiments were done after roasting experiments to remove copper content. In this study the effects of H2O2 were investigated. 20 grams of samples were leached with H2SO4. Leaching process parameters were: 1/5 solid/liquid ratio, 0,6 M acid concentration and room temperature. The samples had 52 % Mo, 0,53 % S and 2,41 % Cu. After leaching process Cu content is reduced to 0,02 % and S content is reduced to 0,0637 %. In this study, it was aimed to determine the optimum parameter for the roasting process of molybdenum concentrate to produce MoO3 in a rotary kiln and purify the MoO3 product by removing the copper content. A kinetic study were made to roasting experiments which are made under 200 % stoichiometric air conditions. After calculations, activation energy was found nearly 80 kj. The raw material and the products were characterized by using XRD and AAS.