Show notes on the metallurgy of molybdenum BY WILL BAUGHMAN the metallurgy of molybdenum properly lios licis three divisions the concentration of the ores the treatment of the concentrates to t tract the oxide or one of its compounds ard the conversion of this oxide to alloys etal or other product this article deals rt ah the second division the two commercial molybdenum ores cuo buo re the sulfide and the lead the first is generally concentrated by electrostatic separation or uil oil flotation in the latter case the best practice seems to be classification with treatment of slimes in an agitation frothing machine while the sands are best treated in a surface tension machine of the woods type surface tension is not adapted to handling bandling slime may be separated from the gangue by gravity concentration either wet or dry the extraction bears a direct ratio to the amount of slime as slimed is extremely hard to recover mill tests by the writer show that by using a non sliming recovery can be increased to a very high percentage there is but little published work on the treatment of molybdenum concentrates even patent records are scarce processes in use and proposed may be classified as follows alkaline fusion processes for and electric Electr 6 furnace processes for these ores acid leaching processes for only volatilization processes for alkaline leaching processes for in all processes save those of the last class high grade concentrates are to be desired and in some cases are absolutely necessary on account of the great secrecy maintained by rare metal treatment plants details of operation are not always available and in some cases improvements of processes may now allow the treatment of lower grade or impure concentrates treatment by alkaline cushion Fus hion gin method the ore is roasted to oxide this step is of vital importance and must be done properly A detailed description is given later on of this roasting operation the ore is next fused with soda ash this forms the soda compounds of molybdenum phosphorus silica arsenic and sulphur the melt is cooled pulverized and digested with hot water all the soda compounds go into solution A large excess of hydrogen sulphide gas is passed under pressure for some time this hydrogen sulphide gas may be generated by electr W ave 30 los angeles calif any method decomposition of calcium sulphide by carbon dioxide or the heating of oil and sulphur are the most economical methods the gas converts the sodium to the compound sodium num sulphide and slowly precipitates the and silica at the same time the molybdenum precipitates the red sulphide solution is filtered off and acidified acidifier preferably with hydrochloric acid when the molybdenum precipitates as dark brown gelatinous precipitate of the composition mosa mos I 1 this precipitate is dried and charged in crucibles cruci bles in a closed muffle furnace and heated to a dull red at this temperature all arsenic sulphide is sublimed and the molybdenum tri sulphide is converted to the disulphide this di sulphite is then heated in an oxidizing atmosphere to form the trioxide the finished product may contain a little silica but is of better than commercial grade the cost of operation would appear to be somewhat higher than that of some other methods but the initial cost of the plant will be but little good results obtained As an improvement on the above the writer worked out the following procedure on a twenty pound scale with very gratifying results the sodium solution is treated with carbon dioxide gas instead of hydrocarbon sulphide this precipitates the silica and much faster is less costly of production and manipulation and is less noxious to be handled the solution must be cold as the reaction is reversible in heated solution and silica is dissolved the solution is then evaporated to a thin syrupy condition and a small amount of alcohol is added then hydrochloric acid is added with agitation the solution should be warm the molybdenum is precipitated as the trioxide and is of alloy grade its further purification is an easy matter if desired it will be noticed that the roasting of the sulphide to the trioxide with its attendant danger of formation of sulphate is avoided berzelius method proper proportions of sodium carbonate s sulphur and carbon to form on reduction a of the formula naas are mixed charged in graphite crucibles cruci bles and heated to degrees C then the powdered is added and a blue flame appears the heating at degrees is continued till this blue flame disappears temperature pera ture is then raised to degrees C and kept at this point till no more sulphurous fumes are evolved after which it is raised to degrees and kept there several hours the fusion is cooled out of contact with t air then pulverized and leached beached with boiling water the filtered solution is acidifier acidified and the precipitated treated as outlined in the gin process above this method requires a large amount of fuel and labor as well as high grade concentrates graphite crucibles cruci bles are necessary and form another source of expense treatment by alkaline fusion may be used instead of the roasted employed in the gin process above the fusion is treated in the same manner however lead oxide rapidly attacks the furnaces so the better practice is to add a proper amount of carbon so as to reduce the lead oxide to metal the molten sodium slag should then be blown in a small converter to oxidize the slag as much as possible otherwise it will be rather insoluble electric furnace processes for these processes are limited to the reduction of to metallic lead and a soda slag which is treated as previously described one plant on which unfortunately no operating data are obtainable mixes the with iron turnings and carbon the whole is then reduced in a resistance type furnace the lead is first reduced to metal and then volatilizes leaving the molybdenum to combine with the iron forming in view of the fact that trioxide is reduced by carbon only with great difficulty and that furthermore it volatilizes at degrees C it seems that considerable molybdenum must be lost two electric furnace methods of producing a rather poor grade of ferro molybdenum direct from have been designed very pure concentrates are necessary as all impurities tend to pass into the product the and iron turnings are charged direct in a resistance type furnace in one method lime is used for ing and carbon for reducing another utilizes calcium carbide both as reducer and silicon may be used in the same manner as it will act both as a reduction and izing agent or may be used in which case no iron turnings are necessary acid leaching processes sulphuric acid Is used as a solvent of molybdenum I 1 in treating this is known as the elbers method the concentrates or high grade ores are digested with concentrated sulphuric acid till fumes appear the solution is then allowed to cool and after dilution is filtered off the lead remains in the residue as sulphate the molybdenum forms a blue solution nitric acid is then added and the solution evaporated the residue is dried and washed with as little dilute nitric as pos sible to remove phosphorus acid can be recovered from fumes volatilization processes molybdenum trioxide is volatile in an oxidizing atmosphere at degrees C this forms the basis of one patent which proposes roasting and then izing the trioxide in one operation and recovering the molybdenum in flue chambers gin developed the process of first producing crude cast molybdenum and then heating this in an oxidized atmosphere to form the trioxide this trioxide or a trioxide produced by any other method is then sublimed in an induction furnace using a copper resistor bath the trioxide will form a volatile compound if heated to degrees C in a current of dry hydrogen chloride gas the sublimate is soluble in water and is recovered by evaporation alkaline leaching processes for only strange as it may seem the most efficient extracting process is also the oldest 0 it was designed about years ago by the chemist berzelius As developed now it can be applied to very low grade ores that contain many detrimental impurities it seems to be in favor at most extraction plants ammonia methods improved berzelius the ore is roasted to oxide this is the key to the whole process and must be done properly roasting should be conducted in a hand furnace as a great deal is lost in flue dust when using a revolving cylindrical type roaster the roast should not exceed a dull red as the trioxide becomes volatile at degrees C at completion of the roast when no more sulphurous fumes are evolved the ore should be of a brown earthy appearance the ore is leached beached with ammonia solution of the trioxide as ammonium nha is affected in a closed di aestor with a 20 per cent solution of ammonia and by the aid of heat the solution is filtered and impurities removed ammonium sulphide is used to precipitate any zinc copper or nickle that may be present hydrogen sulphide is too difficult to handle for proper saturation magnesium chloride and a little sal ammoniac are added to precipitate phosphorus and arsenic as insoluble compounds the solution is evaporated evaporation takes place in a vessel with a removable cover at first the fumes are condensed to recover ammonia but after all free ammonia has been driven off the cover is removed and the white ammonium crystals are fished out of the blue solution as they are f ormed formed gives the anhydride the copper phosphorus and other impurities are easily removed so the only impurities will be from the formation of sulphate caused by I 1 careless roasting or excessive use of ammonium sulphide ohly proposed oxidizing with nitric believing that the increased recovery would more than offset the increased cost such a method would of course be more certain in action than any roasting leaching with other alkalis Al kalies the writer has experimented with leaching with other kalies alkalis al and while it is too early as yet to state what the working details would be results are quite encouraging is soluble in caustic potash and is re precipitated by acidifying acidify ing such a method of course is too expensive at present molybdenum trioxide combines with and becomes soluble in a solution of sodium car bonate and phosphate from which it may be recovered ye recovered covered by fractional crystallization if magnesia and molybdenum trioxide are intimately mixed treated with boiling water and digested a few hours a soluble magne slum is formed the uncombined magnesia remains in the residue evaporation ora yields long glistening crystals these crystals are treated with sufficient hydrochloric acid to remove the magnesia which leaves an impure molybdenum trioxide which can be purified by sublimation in an induction furnace mention might also be made that by reducing a hydrochloric hydro solution with zinc and then adding caustic soda to the blue solution molybdenum hydroxide moa OH 6 is precipitated along with the zinc etc |