Hydrometallurgical Production of Technical Grade Molybdic Oxide From Molybdenite Concentrates

Abstract

An improved hydrometallurgical method is provided for producing technical grade molybdic oxide from molybdenite concentrates. According to this method, the molybdenite concentrates are leached in an acid medium having a nitric acid concentration between about 25 gpl and about 50 gpl and an initial sulphuric acid concentration of nil to about 750 gpl, under oxygen pressure of about 100 - 250 psig and at a temperature above 115.degree. C so as to produce technical grade molybdic oxide having not more than 0.1% sulphur. Then a liquid-solid separation of the reaction mixture is effected and the obtained leach liquor is recycled back to the leaching stage optionally after partial neutralization with a basic reagent. The solid is washed and recovered as technical grade molybdic oxide. This invention relates to a novel method of producing hydrometallurgically technical grade molybdic oxide from molybdenite concentrates. Technical molybdic oxide is usually prepared commercially by roasting molybdenum disulphide concentrates in air at temperatures of 550.degree.-600.degree. C. These molybdenum disulphide concentrates commonly contain minor amounts of silica and iron, copper and lead sulphides which are oxidized and report as impurities in the molybdic oxide product, and traces of rhenium sulphide which are also oxidized and partially volatilized in the roasting process. The volatilized rhenium can be recovered from the flue gases by known scrubbing techniques. Due to more and more stringent pollution controls, however, these roasting techniques, which produce great amounts of polluting gases, have now become unsatisfactory. It is also known that molybdic oxide can be prepared from the molybdic acid obtained by leaching molybdenum containing minerals with nitric acid in a sealed vessel under increased partial pressures of oxygen and at elevated temperatures, using less than the theoretical concentration of nitric acid. This has been disclosed, for example, by G. Bjorling and G. A. Kolta in Intern. Mineral Process. Congr., Tech. Papers, 7th, New York City, 1964 -- 127-38 and J. Chem. U.A.R. 12, No. 3, 423-435 (1969). In the leaching of molybdenum minerals with nitric acid, molybdenum disulphide is oxidized to molybdic acid and sulphuric acid, according to the following equation: however, since molybdic acid is only soluble in the acid medium to a limited extent, precipitation of a large part of the molybdic acid occurs during the course of the oxidation process, and this precipitate can be filtered off from the solution, together with siliceous gangue, and can be used directly as technical grade molybdic acid or, following a simple calcining treatment, as technical grade molybdic oxide. If the reaction is carried out under sufficient pressure and at temperatures in excess of 115.degree. C, the molybdic acid is dehydrated in situ to molybdic oxide, so that technical molybdic oxide can be obtained directly. The major disadvantage of such processes, which has rendered them uneconomic in the past, is discussed in Canadian Pat. No. 905,641 issued July 25, 1972 to Bengt O. P. Mollerstedt and Karl-Erik Backius. It lies in the difficulty of recovering the dissolved molybdic acid from the leach liquor which also contains rhenium, copper, iron and other impurities in addition to sulphuric acid, and unconsumed nitric acid. The molybdenum content of this solution can represent as much as 20 to 25% by weight of the molybdenum content of the feed material. It is known that such solutions can be treated by solvent extraction and ion exchange techniques to separate the molybdenum and rhenium values from the acid leach liquor. One such method is described in U.S. Pat. No. 3,739,057 of June 12, 1973 issued to Ellsworth W. Daugherty, Albert E. Erhard and James L. Drobnick. In this patent there is described a process using in the leaching stage a very low nitric acid concentration of between 10 and 25 gpl while controlling the temperature of the gas phase reaction zone in the pressure vessel, and then recovering from the obtained solution the molybdenum and rhenium values with an amine or quaternary ammonium type extractant. This is followed by stripping the molybdenum and rhenium values from the solvent with ammonium hydroxide, separating the rhenium from the molybdenum in the stripping solution with a quaternary ammonium type extractant, and finally recovering the molybdenum and rhenium by conventional techniques. However, it will be appreciated that such a process is relatively difficult and costly since it involves the treatment by solvent extraction of the entire liquid component of the reaction mixture, which is certainly a complex and expensive operation. In addition, nitric acid has a deleterious effect on molybdenum extraction and consequently the nitric acid content in the liquor passing to the solvent extraction stage must be kept very low. This is probably the reason for which very low concentrations of nitric acid are used according to U.S. Pat. No. 3,739,057 in spite of the fact that they will obviously result in lower conversions to molybdic oxide. Furthermore, the by-product sulphuric acid from such processes is relatively impure and does not find a ready market. It is therefore the principal object of this invention to provide a novel hydrometallurgical method for the transformation of molybdenite concentrates into technical grade molybdic oxide which would obviate or substantially diminish the disadvantages of the presently known hydrometallurgical processes. It is a further object of this invention to provide a more economical method of hydrometallurgical treatment of molybdenite concentrates to produce directly technical grade molybdic oxide. Other objects and advantages of the present invention will be made obvious from the following more detailed description: Thus, it has been surprisingly found according to the present invention that the yield of technical grade molybdic oxide which can be recovered directly by filtration from the nitric acid leach process can be increased from about 75-85% to at least 95% by recycling and reusing the leach liquor (after filtering off the molybdic oxide product) to leach a further quantity of molybdenite concentrate. Since the leaching medium is already saturated with molybdic acid, the precipitation of molybdic oxide from the molybdenite feed, in the second and subsequent leach steps, becomes substantially quantitative. The method of the present invention for producing hydrometallurgically technical grade molybdic oxide from molybdenite concentrates basically comprises: a. leaching the molybdenite concentrate in an acid medium having a nitric acid concentration between about 25 gpl and about 50 gpl and an initial sulphuric acid concentration of nil to about 750 gpl, under oxygen pressure of about 100 to 250 psig. and at a temperature above 115.degree. C so as to produce technical grade molybdic oxide having not more than 0.1% sulphur; b. effecting a liquid-solid separation of the resultant reaction mixture to separate the leach liquor from the solids; c. recycling the leach liquor from the liquid-solid separation step (b) to the leaching step (a); d. washing the solids of the liquid-solid separation step (b) and recovering the technical grade molybdic oxide; and e. repeating leaching step (a) with recycled leach liquor from step (c) after adjustment of the nitric acid and sulphuric acid concentrations. It was found that in such leach liquor recycling method, a nitric acid concentration below about 25 gpl (about 0.45 lbs. of nitric acid per lb. of quadravalent molybdenum in the molybdenite concentrate) would be insufficient to give satisfactory conversion to technical grade molybdic oxide and concentrations above 50 gpl (about 0.90 lb. of HNO.sub.3 per lb. of quadravalent molybdenum in the molybdenite concentrate) would be unnecessary because they would involve useless recycling or loss of nitric acid in wash water and would render the reaction control more difficult. The initial concentration of sulphuric acid in the leach liquor could be nil or it could be adjusted to a level of up to about 750 gpl, preferably up to 400 gpl, and it should be maintained at such level (referring to pure sulphuric acid) during the entire operation in order to prevent excessive dissolution of the molybdic oxide product. In accordance with the present invention the sulphuric acid concentration can readily be maintained at the desired level. In view of the fact that during the liquid-solid separation step, which is carried out after the leaching operation, (e.g. by filtration), the moist solid phase will retain a portion of the liquor of about 10 to 15%, this portion of the liquor removed with the solids will obviously contain a good proportion of sulphuric acid which is thus eliminated from the recycled leach liquor and replaced by an equivalent amount of water, thereby continuously maintaining the sulphuric acid concentration in the system at the desired value. It will be apparent that it is unnecessary to remove more liquor than about 15% during the liquid-solid separation in order to maintain the sulphuric acid concentration at the desired level, but if, for some reason, this became desirable one could easily remove some additional liquor at this stage and replace it by water or the like. Optionally, the sulphuric acid concentration can be reduced by partial neutralization of the liquor after liquid-solid separation and washing, but prior to recycling. Such partial neutralization can be carried out with a calcium reagent such as CaO, Ca(OH).sub.2 or CaCO.sub.3 until the pH value of the liquor reaches the isoelectric point of molybdic acid (pH .congruent. 0.9). Up to that point, the dissolved molybdenum is present in solution as cationic molybdenyl species which remain in solution while calcium sulphate precipitates as gypsum. consequently the loss of molybdenum to the gypsum precipitate during such partial neutralization is negligible. The preferred pH limit for this operation is about 0.7. Moreover, since calcium perrhenate is soluble in aqueous acid solutions, there is little possibility of rhenium co-precipitating with the calcium sulphate, so that repeated recycling of the leach solution leads to progressive enrichment of the solution with rhenium. By this technique rhenium can be recovered economically from concentrates of low rhenium content, since progressive enrichment of the leach solutions in rhenium is thereby achieved. A fraction of such enriched solution (generally less than 10%) can thus be removed during each cycle, from which rhenium and molybdenum can be separated by established solvent extraction or liquid ion exchange techniques, and the rhenium can be separated from the molybdenum by sorption on activated charcoal or by an ion exchange process. This procedure, with the partial neutralization step, gives even higher direct recovery yields than without partial neutralization, namely of the order of 98%. Furthermore, it was found that the leaching operation in accordance with the novel method of this invention should be effected under oxygen pressure of about 100 to 250 psig and at a temperature above 115.degree. C and preferably between 120.degree. and 160.degree. C for a period of time sufficient to effect the required conversion to produce molybdic oxide having not more than 0.1% sulphur. In this regard, it is well known that technical grade MoO.sub.3 should contain a maximum of 0.1% sulphur. It was found that only conversions of the order of about 99.5% or more give such low sulphur contents in the final technical MoO.sub.3 product which is directly produced by this method. It should also be noted that the leaching operation in accordance with this invention is carried out in a pressurized vessel or autoclave and, since the reaction is exothermic, the slurry which is being leached is cooled in the autoclave to maintain the temperature of said slurry between about 120.degree. and 160.degree. C. It has actually been found that there is essentially no temperature differential between the slurry and the gases in the upper part of the autoclave, indicating that the exothermic regeneration of nitric acid occurs in the slurry itself in accordance with this invention. There is thus no need for additionally controlling the temperature of the gas phase within the autoclave by providing complex means in such gas phase as this is done, for instance, in U.S. Pat. No. 3,739,057. The method of the present invention can be carried out in a regular autoclave or pressure vessel without any special additions or transformations thereof. However, the vessel or autoclave should preferably be so designed as to achieve optimum rates of oxygen transfer into the slurry. The required conversion to achieve a sulphur control of 0.1% or less has usually been obtained, under operating conditions of this invention, within about 120 to 240 minutes. In many instances this conversion was 99.9%, which is practically quantitative. The preferred operational leaching conditions in accordance with this invention are as follows: The consumption of nitric acid under such process conditions is of the order of 5g/100g of molybdenite treated, which is certainly very economical. The other conditions of the leaching operation are generally conventional. Thus, the leaching slurry may contain a suitable proportion of solids, but is usually carried out at about 10% solids. The particle size of the concentrate treated should be suitable to provide satisfactory contact surface area for the leaching operation which is carried out under agitation. The preferred particle size is about 50-80% minus 325 mesh. After separation of the liquor from the solid reaction product of the leaching step, the obtained wet solids are washed to yield directly the technical grade molybdic oxide, which contains less than 0.1% sulphur and can be marketed as such. This product, although entirely satisfactory, has however a greyish colour rather than the usual yellow colour of the roasted molybdic oxide. This greyish colour is believed to be due to a slight surface reduction of molybdic oxide during drying of the product. If desired, the greyish product can be transformed into the usual yellow technical grade MoO.sub.3, by optionally calcining it at a temperature of up to about 600.degree. C. Obviously, since the molybdic oxide product has a sulphur content of less than 0.1%, such calcining operation produces essentially no objectionable SO.sub.2 polluting gases. The wash waters will contain some recoverable metal values, particularly some molybdenum and rhenium. These recoverable metal values can be recovered from the wash waters by the usual solvent extraction or liquid ion exchange techniques and the rhenium can be separated from the molybdenum by sorption on activated charcoal or by an ion exchange process. It will be appreciated that due to the fact that only a small portion of the overall leaching solution will be so treated at a time (up to about 20%), the solvent extraction or ion exchange treatment will be much simplified and will require much smaller investment for the construction, reagent inventory and maintenance of the solvent extraction or ion exchange systems necessary for such treatments. Furthermore, it will be appreciated that repeated recycling of the leach solution will lead to progressive enrichment of the solution with rhenium and therefore rhenium concentrations will be higher than usual and thus it can be more readily and more economically recovered by conventional techniques. However, if the initial molybdenite concentrate is such that the amount of rhenium is very low and unimportant in the overall economy of the process, it need not be recovered separately from molybdenum. It will also be appreciated that other metal values present in the molybdenum concentrate will dissolve to some extent in the leach liquor so that recycling will lead to a progressive increase in the concentration of such metals. However, the removal of a small fraction of the liquor with the solids, in each cycle of the operation, as already described above, either with or without partial neutralization, will enable to control the concentration of these impurities to a satisfactory degree. After recovery of the recoverable metal values from the wash waters, the wash waters, which are acidic in nature, can be neutralized, usually with calcium oxide, calcium hydroxide or calcium carbonate, and discarded.

Patent Drawings (3 sheets)

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