DK150627B - PROCEDURE FOR EXPLOITING THE MOLYBODY FROM WASTE CATALOGS - Google Patents

Description

150827

The present invention relates to a process of the kind specified in the preamble of claim 1 for the extraction of molybdenum from waste catalysts, in particular from spent catalysts based on activated alumina and containing one or more molybdenum compounds and one or more other metal compounds. The catalysts used are especially of the type used in desulfurization of oils. Catalysts of this type 4 contain a support for γ-alumina impregnated with one or more molybdenum compounds. These are generally oxides, such as MoO 2, which are themselves obtained by dissociating a salt such as ammonium molybdate. Other metal compounds, especially cobalt oxide and / or nickel oxide, are often found in the catalyst as active constituents. Finally, the catalyst contains impurities, most of which remain bound to the catalyst throughout its useful life. In particular, these are various organic compounds, such as those containing sulfur.

Prior to subjecting these waste products to chemical treatment, it is common practice to subject them to an oxidizing shake at a temperature generally below 600 ° C to remove the impregnating hydrocarbons, carbon and a portion of the sulfur in the form of volatile compounds; see e.g. German Publication No. 2,316,837. This treatment is often carried out in the same column used in the treatment of the hydrocarbons. The treatment may e.g. also performed when the chemical treatment is performed. After this roasting, molybdenum is present in the catalyst in the form of its oxide or sulfide. The molybdenum can then be separated and recovered in usable form by various methods.

In particular, reference is made to French patent specification 701 426, 150827 2 which relates to a process for treating catalysts used in the hydrogenation of coal, oil and tar substances. In addition to an alumina-based support, these catalysts contain metal compounds based on molybdenum, chromium, zinc and magnesium. It was found that if these catalysts were shaken at a temperature below 500 ° C, it was possible to solubilize molybdenum with a solution of ammonia, which allowed ammonium molybdate to be obtained while the other metals remained unaffected or only affected. to a small extent. It is then possible to precipitate molybdenic acid with hydrochloric acid at the boiling point. This process has the major disadvantage that there is a very low reaction rate between ammonia and molybdenum oxide in the catalyst. In addition, the extraction yield is low, with a significant portion of the molybdenum oxide remaining in the inert materials. Finally, molybdenum sulfide is hardly affected by the ammonia treatment.

U.S. Pat. No. 2,367,506 also discloses a method for recovering molybdenum from spent catalysts used in the degradation of naphthenes to aromatic compounds, which catalysts are based on molybdenum compounds with a carrier of activated alumina. Here, a process is described which consists of immersing the spent catalyst in a solution of sodium carbonate until it is completely impregnated, and then heating the catalyst pieces thus impregnated for 30 minutes at a temperature of 1150 ° C, e.g. in a rotary oven. Under these conditions, the alumina is rendered substantially insoluble, and the resulting sodium aluminate can then be dissolved in water, leaving only a small amount of aluminum in the form of sodium aluminate. The process thus described has the disadvantage that the solubility of the molybdate formed is reduced and that the solution of the molybdate in water becomes difficult. Finally, it has been found that it is not possible to avoid dissolution of alumina, which must then be separated by further treatment if a sufficiently pure molybdenum compound is to be recovered.

It has now surprisingly been found that the method according to the invention can avoid the above-described disadvantages of the known methods. Thus, by the process of the present invention, it is possible to completely recover the molybdenum contained in spent catalysts without simultaneously containing substantial amounts of aluminum. It is also possible to treat the catalyst after a roasting, at temperatures preferably below 600 ° C, without this roasting temperature being particularly critical, either with respect to the solubilization of the molybdenum or with respect to the extraction yield.

The present process is thus characterized by calcining the impregnated catalyst at a temperature of 600-800 ° C, treating the impregnated and calcined catalyst with a stream of carbon dioxide and then washing it with water to dissolve the formed sodium molybdate , Na2MoO4, and gradually the solution obtained by washing with water adds a quantity of nitric acid which amounts to 1.5-2.3 times the amount needed to maintain a pH of 5-6 while maintaining at the same time the temperature below 30 ° C and hydrolyzing the acid-treated solution at a temperature near the boiling point, whereupon the molybdenic acid precipitated is then purified and dried.

Due to the relatively low calcination temperature, the treatment with carbon dioxide, the considerable excess of nitric acid and the precipitation of molybdenic acid near the boiling point, it is obtained that the thus-obtained molybdenic acid thus obtained has a significantly higher purity, ie. a substantially lower content of aluminum than is possible by the known methods. Furthermore, the consumption of chemicals is relatively modest.

After the catalyst has been shaken to remove volatile compounds, the molybdenum content of the waste products is usually of the order of 4-12%, while the sulfur content is between 0.5 and 4%. These figures are only indicative, and certain types of catalysts may have molybdenum or sulfur content less or greater than these indications. The impregnation treatment must be carried out so that all the catalyst pellets uniformly absorb the reactant. This result is obtained e.g. by spraying a stirred bed of pellets with a solution of sodium carbonate, causing a systematic movement of the pellets during the spraying. The volume of the solution depends to a limited extent on the specific surface of the catalyst and is 200-500 ml per liter. kg of treated catalyst. The concentration of sodium carbonate varies greatly with the content of molybdenum and sulfur, as will be explained below.

This impregnation is followed by heating to a temperature between 600 and 800 ° C, preferably to a temperature of 650-750 ° C. It has been found that molybdenum present in the form of molybdenum or molybdenum sulfide in this temperature range is almost completely converted to molybdate soluble in hot water (80 °) in a short period of time, i.e. of the order of 1 hour. At this temperature, oxides of cobalt and nickel present do not react with sodium carbonate to any significant extent and remain essentially insoluble in water. However, although the reaction rate in this temperature range of the reaction rate in this temperature range for the reaction between alumina and sodium carbonate is very low, it is not possible to completely avoid the formation of small amounts of sodium aluminate. These quantities remain limited, provided the temperature conditions and processing time are kept within the limits indicated, and provided that the excess sodium carbonate supply is not excessive. After this heat treatment and the treatment with Cf described below, the catalyst pellets are stirred with hot water until the sodium molybdate is dissolved as completely as possible. This is achieved at temperatures from 60 to 100 ° C over approx.

1 hour. It is usually desirable to obtain a relatively concentrated solution, e.g. contains 45-50 g / liter molybdenum in the form of sodium molybdate. These solutions also contain sodium aluminate. Due to the operating conditions defined above, the ratio of aluminum to molybdenum in solution is generally of the order of 10 per cent, in many cases even lower and rarely above 20 per cent. As already mentioned, these solutions also contain sodium sulfate and free sodium carbonate. These solutions may contain in suspension small solid particles, primarily resulting from a partial decomposition of the waste catalyst. These particles are removed by decantation or filtration e.g. in a filter press. The most important step of the process is the separation of molybdenum in the form of molybdenic acid from the sodium aluminate and the alkali salts. Surprisingly, it has been found here that it is possible to convert the original basic solution to an acidic solution, from which the molybdenic acid is subsequently precipitated without the risk of partial hydrolysis of sodium aluminate. This seemingly complex operation is carried out in a precise and reproducible manner by a simple and novel method, which consists of treating the solution in two successive reactors of the same capacity arranged in such a way that the effluent quantities in these reactors are equal in size and constant. The first reactor receives the solution resulting from the extraction of hot water soluble salts which are contained in the catalyst after treatment with sodium carbonate and freed of any solid particles by decantation or filtration as described above.

This solution is cooled at the inlet of the first reactor to a temperature below 40 ° C. A stream of nitric acid is continuously fed to the same reactor at such a rate that the pH of the solution is adjusted to between 5 and 6, preferably between 5.2 and 5.5. This amount of nitric acid can be adjusted in known manner, such as by means of a metering pump, where the amount is continuously controlled by means of a pH meter with a probe located in the reactor itself. As the reaction is exothermic, the reactor must be provided with cooling means, such as a cooling jacket or a cooling coil, and optionally with an agitator to allow the temperature of the solution not to exceed 20 ° and in all cases below 30 ° C. Under these conditions, the free sodium carbonate is neutralized and the sodium aluminate decomposes to the point where the precipitation begins, making the solution more easily cloudy. The solution is then fed to the second reactor at a constant flow rate, e.g. via an overflow. A stream of nitric acid is continuously fed into this reactor substantially in the same amount supplied to the first reactor. A simple way to achieve this is to use a metering pump with two separate circuits and with a single control system which allows two flows of equal volume to be obtained, one flow feeding the first reactor and the second flow feeds the second reactor. It is sufficient to feed each circuit from a common reservoir so that the same weight amounts of nitric acid are supplied. As the first reactor, the second reactor is equipped with a cooling system that allows the temperature to be kept below 30 ° C. Due to the excess of nitric acid thus supplied, it is possible, on the one hand, to redissolve the alumina which tends to precipitate and, on the other, to create conditions favorable for the precipitation of molybdenic acid.

To achieve this precipitation, it is necessary to heat the solution to a temperature near the boiling point.

This is preferably done in one or more precipitation vessels receiving the solution from the second reactor, the solution being heated to approx. 100 ° C. The precipitate obtained is washed, cleaned and then dried in the usual manner. The precipitate, based on molybdenic acid monohydrate, contains only small amounts of aluminum. The molybdenum content expressed as a percentage of the dry product is equal to or greater than 60%, while the aluminum content is less than 0.1%. and in many cases around 0.01 per cent. However, during the development of the process in question, the following two difficulties are encountered:

First, it has been found that impregnating the catalyst with an accurately calculated amount of sodium carbonate solution was relatively difficult to perform if the solution were to be homogeneously distributed. An aqueous solution containing 400 g of sodium carbonate per ml. liters were generally used to limit the amount of water. This solution, which was initially heated to ca. 70 ° C, tended to crystallize during contact with the catalyst, preventing the carbonate from penetrating the pores of the catalyst particles.

Furthermore, a greater dilution was not desirable as the absorption capacity of the catalyst is thereby limited.

Instead of impregnating the catalyst particles with an aqueous solution of sodium carbonate, the catalyst particles are first mixed with anhydrous sodium carbonate in the form of a fine powder in a mixing vessel of any type, such as a rotary mixer. In general, mixing for a few minutes is sufficient for the particles of sodium carbonate to disperse over the surface of the catalyst particles. It is then sufficient to add the required amount of water at room temperature and stir again for a few minutes to obtain a substantially complete absorption of water into the catalyst particles. Studies have shown that this penetration of water allows the sodium carbonate to penetrate the particles, probably by diffusion. During the following step of heating the catalyst, the yield of molybdenum converted to sodium molybdate is at least as great as that of impregnation with a hot solution of sodium carbonate in a known manner.

Another, more severe, difficulty was found during sustained pilot scale experiments attempting to dissolve the sodium molybdate formed in the catalyst after the carbonate treatment in hot water. The formation of deposits was observed on the walls of the containers containing the aqueous solution and in the tubes through which the solution circulated. This phenomenon is due to the fact that the solution of the sodium molybdate is also accompanied by the dissolution of a certain amount of sodium aluminate, and the weight ratio of aluminum to molybdenum in solution is generally of the order of 0.1 and in some cases even approx. 0.2. Studies have shown that deposits formed on the walls of the containers and in the pipelines are primarily based on alumina and are primarily aluminum trihydrate. The layer thus formed adheres strongly to steel, ebonite, glass and rubber. This phenomenon appears to be amplified by the presence of catalyst particles in suspension in the solution, acting as germs. As these layers become thicker, they have a tendency to fall off, and the thus released plate-shaped deposits are entrained by flowing through the solution and tend to block pipelines and furthermore to block the circulation pumps.

It has now surprisingly been found that these difficulties can be avoided. It has been found that it is not only possible in the method according to the invention to avoid these accidents, but also to produce a product of a more reproducible quality and in a purer form.

This improvement is the result of the following experimental observations: When the catalyst is left standing for a few days after impregnation with sodium carbonate and heating, the deposits formed by dissolving in water are not so extensive. Subsequent experiments have shown that the reduction of these deposits is due to the effect of carbon dioxide in the air on the sodium aluminate present in the catalyst particles. Therefore, after heating, the catalyst particles are treated with a stream of carbon dioxide gas. The operating conditions are very simple and are sufficient to bring the carbon dioxide gas into contact with the catalyst particles for a sufficient time to allow a diffusion of carbon dioxide into the catalyst particles. This result is obtained e.g. by filling a vertical column of plastic material or sheet steel with catalyst particles and circulating a stream of carbon dioxide gas through the column. The amount of carbon dioxide gas required for effective treatment is of the order of 1 Nm 50-100, preferably per 60-70 kg. catalyst. The amount, of course, depends on the amount of alumina present as sodium aluminate in the catalyst. Although the exact physicochemical process taking place is not known, it is likely that sodium aluminate at least partially decomposes to form carbonate. This reaction takes place at a temperature close to room temperature. Thereafter, the other steps are performed by the method of the invention as described. As a result, deposits are no longer formed on the walls of containers and in pipelines during the solution treatment. In addition, the excess sodium carbonate used in relation to the amount of catalyst is no longer critical, making the process simpler. The small amount of amorphous alumina in the form of fine particles that is inevitably released from the catalyst particles during the washing process causes no problems as they do not agglomerate into solid lumps but remain in finely divided form. Part of the alumina is separated by decantation while the remainder suspended in the wash solution is retained by filtration before the solution is introduced into the neutralization and clarification reactors.

In the example below, a preferred embodiment of the method according to the invention is described. Reference is made to the accompanying drawing, which is a block diagram of this preferred embodiment.

EXAMPLE

The catalyst being treated is a spent catalyst in the form of small cylinders, based on? -Aluminum dioxide, which has previously been subjected to an oxidizing shake at about 500 ° C, including hydrocarbons, carbon and a portion of the sulfur in the spent catalyst is been removed. After roasting, this catalyst contains 8% by weight. molybdenum, 1.5% by weight sulfur and 2% by weight. cobalt. 25 kg of sodium carbonate powder and 150 kg of this catalyst are introduced into a rotary mixer. After 10 minutes of mixing, 64 liters of water are added at room temperature and then mixed for a further 15 minutes. Thereafter, sodium carbonate and water are completely absorbed by the catalyst particles. The catalyst particles are then heated in a rotary oven to a temperature of 650-750 ° C by means of a propane burner.

The residence time in the hot zone is approx. 1 hour. At the end of the furnace, the product is cooled to about room temperature and fed continuously to the upper end of a vertical column of sheet steel filled with approx. 200 kg of catalyst particles, in an amount of 60-70 kg per hour.

In this column, a stream of carbon dioxide circulates at a rate of 1 m per minute. hour. The catalyst is also continuously removed from the bottom of the column. The residence time of the catalyst particles in the column is approx. 3 hours. The sodium molybdate is then dissolved by washing the product countercurrently, the product being placed in a 10 cm thick layer on a continuous filter band with a filter surface of 1 m.

The washing unit has six stages, of which the sixth and last step is fed with warm water at 80 ° C in an amount of approx.

120 liters per liter. hour. The concentrated solution of sodium molybdate is removed at the end of the first step in an amount of approx. 104 liters per liter. hour with a content of 45-50 g / liter molybdenum in dissolved form. The washed solid product, which is removed from the filter after washing with clean water in the last step, contains approx. 0.27 per cent. molybdenum in soluble form. As a result, the yield is in the order of 97 per cent. To perform a perfect separation of insoluble particles which can be suspended during dissolution of the sodium molybdate with hot water, the solution treatment is followed by a filtration of the alkaline solution on a filter press before being introduced into the first neutralization reactor using nitric acid. After this first step, an analysis of the solution shows that the aluminum content is less than 0.03 per cent. based on the weight of the molybdenum. Since the molybdenum content is of the order of 45-50 g / liter, it can be calculated that the aluminum content is less than 0.015 g / liter. In the first reaction vessel, a nitric acid solution with a nitric acid concentration of 53 per cent is introduced by means of a dosing pump. This reactor has a volume of 150 liters and is provided with cooling means in the form of a cooling jacket filled with circulating water, keeping the temperature below 20 ° C. . The operation of this first dosing pump is controlled by means of a probe for continuous measurement of pH and this is placed in the reactor. Thus, a pH of 5.2-5.5 is maintained. Under these conditions, the pump speed is approx. 8 liters per hour. The solution, which is slightly cloudy due to initial precipitation of alumina, is fed to the second clarification reactor with an equal volume as the first reactor, where an additional amount of nitric acid is supplied by a second dosing pump controlled by the first dosing pump so that exactly the same amount of nitric acid with the same composition is supplied as the first stream of nitric acid. The temperature of this second reactor is kept below 30 ° C, also by circulating water through a cooling jacket.

Under these conditions, the alumina is redissolved and the solution is clear. The solution is then fed into a third precipitation reactor having the same volume as at the first two reactors and heated to 100 ° C by means of a steam jacket. Under these conditions molybdenoic acid precipitates, while most aluminum remains in solution. A stirring allows the precipitate to be kept in suspension. The suspension is then passed to a rotary filter, whereupon the precipitate is collected and washed continuously with demineralized water containing 2% by volume. concentrated nitric acid.

The precipitate is then led to a hot air dryer, where it is heated to approx. 100 ° C. The precipitate based on molybdenic acid has an average molybdenum content of 61.2 per cent. The aluminum content is only 0.004 per cent. The average density is 2. The yield of molybdenum in the form of molybdenic acid based on molybdenum in the waste product is approx. 85 per cent ..

The process described in this example can be modified in various ways without departing from the scope of the present invention, as there are many similar ways of performing the essential steps of the present process. In particular, it is possible in a single step to carry out the neutralization and clarification treatment of the solution containing sodium molybdate. In this case, it is sufficient in a single reactor to introduce a quantity of nitric acid at least equal to the sum of the two quantities successively supplied to the two reactors, in this case it is more difficult to control the temperature of the solution which must not exceed 30 ° C, to avoid an irreversible precipitation of aluminum.

Finally, during the particular precipitation of molybdenic acid, it has been found that there is a greater risk that the molybdenic acid is obtained in a partially colloidal form which is more difficult to filter. By contrast, when the treatment with acid is carried out in two steps, a heavy precipitate of molybdenic acid which is easy to wash on a filter is usually obtained.

14

This is an important factor in obtaining a very low aluminum molybdenic acid which can be used especially in the preparation of extremely pure molybdenum powder by reduction with hydrogen.

Claims (3)

150627 A process for recovering substantially aluminum-free molybdenum as molybdenum acid from waste catalysts based on activated alumina and containing one or more molybdenum compounds and one or more other metal compounds, after removing, if necessary, hydrocarbons and a portion of the sulfur content, treat the catalyst with sodium carbonate, calcine the treated catalyst and wash it with water to dissolve the formed sodium molybdate, characterized by calcining the impregnated catalyst at a temperature of 600-800 ° C impregnated and calcined catalyst with a stream of carbon dioxide, then washed with water to dissolve the formed sodium molybdate and gradually added to the solution obtained by washing with water a quantity of nitric acid which is 1.5-2.5 times the amount necessary to maintain a pH of 5-6 while keeping the temperature below 30 ° C and hydrolyzing The acid-treated solution thus looks at a temperature near the boiling point, whereupon the molybdenic acid precipitated is then purified and dried. Process according to claim 1, characterized in that the catalyst treated for the removal of carbon, hydrocarbons and part of the sulfur content is impregnated by mixing with anhydrous sodium carbonate in the form of a fine powder followed by a humidification with water. Process according to claim 1 or 2, characterized in that the impregnated catalyst is calcined