Process for the recovery of molybdenum from mixtures of molybdenum compounds with other metallic compounds

The invention relates to a process for obtaining an aqueous solution of molybdic acid from mixtures of molybdenum compounds with other metal compounds.

Molybdenum compounds can be used in a variety of ways, especially as components of hydrogenation catalysts or catalysts for the oxidation of methanol to formaldehyde. The complete separation and recovery of molybdenum from such spent catalysts is a very important problem in practice because of the high cost of molybdenum and for environmental reasons.

Various methods for obtaining molybdenum compounds are known. According to British Pat. No. 350,135, mixtures of molybdenum or molybdenum compounds with other metals or metal compounds are treated with oxygen or a gas containing molecular oxygen at elevated temperatures, generally up to about 600 ° C. Then, the molybdenum oxides are separated from the mixtures obtained by this treatment. According to US Pat. No. 3,538,017, spent catalysts based on molybdenum and iron oxides are pulverized and calcined. The calcined powder is then treated with aqueous ammonia to form a molybdenum salt solution. Excess ammonia, which exceeds the stoichiometric amount required to form ammonium molybdate, is separated from this solution.

The conventional methods are disadvantageous due to the number and cost of the process steps and often lead to unsatisfactory low molybdenum yields.

The invention relates to a process for obtaining an aqueous solution of pure molybdic acid from an aqueous solution of an alkali metal molybdate obtained by leaching solid, depleted molybdenum oxide-containing catalysts.

The process according to the invention is characterized in that an aqueous solution containing from 2 to 20% by weight of sodium molybdate and having a ρθ of from 9.5 to 13 is passed through a bed of a strong cationic exchange resin in the acid form at a rate of 0.5 to 3 volumes per volume / resin per per hour at a temperature of 10 to 80 ° C percolated.

The separation of metal ions from molybdate ions by means of cation exchange resins is known. It was surprisingly found that by using such resins under appropriate conditions, pure molybdic acid can be obtained in a one step process from alkali molydates.

The inventive method is particularly suitable for the recovery of molybdenum from spent catalysts containing molybdenum oxide together with other metal oxides. Examples of such catalysts are catalysts for the oxidation of methanol to formaldehyde, which are formed from molybdenum oxide and iron oxide and optionally contain minor proportions of cobalt or nickel in the form of oxides. Such catalysts are described, for example, in the following references: U.S. Patent No. 1,913,405, Canadian Patent No. 619,043, U.S. Patent No. 3,459,807, Belgian Patent No. 601600 and U.S. Patent No. 3,464,931.

Further examples of such catalysts are hydrogenation catalysts formed from molybdenum oxide together with cobalt and / or nickel oxide, generally applied to an inert support such as alumina. In this regard, reference is made to the hydrogenation processes described in U.S. Patent Nos. 3,478,121 and 3,551,511.

The spent catalysts to be recovered by the process of the invention are preferably ground to comminute them into a powder having a particle size of the order of 300 gm.

Occasionally, thermal treatment of such powders has proven valuable. For example, in the case of catalysts supported on alumina, the aluminum oxide becomes insensitive or substantially insensitive to the alkali metal hydroxide by heating at 900 to 1000 ° C for one or more hours.

The spent, ground and optionally heat treated catalyst is treated with an aqueous solution of an alkali metal hydroxide. An aqueous sodium hydroxide solution having a concentration of from 2 to 15% by weight is preferably used for this purpose.

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- 3 used. This treatment is preferably carried out at room temperature (20 to 25 ° C) or generally without external heat input. However, it is possible to work even at temperatures above room temperature, for example up to 70 ° C. When working under these conditions for about 1 to 4 hours, the molybdenum is dissolved in the form of an alkali metal molybdate, while the other metals such as iron, cobalt and nickel are precipitated in the form of hydroxides. Therefore, the alkali metal hydroxide is used in an amount at least equal to the stoichiometrically required amount to form the alkali metal molybdate and to precipitate the other metals in the form of hydroxides. Conveniently, a p ^ value of 9.5 to 13 is maintained during this treatment. The hydroxides are separated by filtration or centrifugation and optionally washed with water.

The solution remaining after filtration, if appropriate after combination with the washing liquids, is brought into contact according to the invention with a strong cation exchange resin present in the acid form (also referred to as hydrogen form).

Resins with sulfone or methylene sulfonic acid groups are suitable as strong cation exchange resins. Among the latter resins, the most important are the exchange resins obtained by sulfonation of divinylbenzene crosslinked polystyrene.

The exchange resins suitable for the process according to the invention generally have an exchange capacity of 1 to 3, expressed as equivalents per 1 liter of resin.

According to a preferred embodiment of the invention, the resin is used in fixed bed form. After optionally subjecting the resin to conditioning and treatment to convert to the acid form, the aqueous solution of sodium molybdate is percolated through the bed. In the present invention, the aqueous solution has a sodium molybdate concentration of 2 to 20% by weight and is passed through the exchange resin at a rate of 0.5 to 3 parts by volume per 1 part by volume of resin and 1 hour. In general, this operation is carried out at room temperature (20 to 25 ° C), although the use of a wider temperature range, for example 10 to 80 ° C, is possible.

Working under these conditions gives an aqueous solution of molybdic acid which can be used directly or after conversion to the desired salt by treatment with an appropriate base. After washing with water and treating with an acid (sulfuric acid, hydrochloric acid or the like) to regenerate the acid form, the resin is ready for a new operation.

According to the process of the invention, molybdenum in the form of molybdic acid is obtained in a simple and economical manner.

The process of the invention comprises a simple percolation of an aqueous solution of an alkali metal salt of molybdic acid (generally sodium molybdate) through a strong cation exchange resin in the hydrogen form. This percolation gives strongly acidic, slightly blue colored (molybdenum blue) solutions which can reach very high concentrations (of the order of 80 g / l, indicated as metallic molybdenum). These solutions, which have a very acidic pi value (of the order of 1.5), remain stable for a very long time and can be used to prepare any desired molybdenum data. It should also be noted that these solutions can be converted to various molybdenum polyacids depending on the pi value which is adjusted by the addition of a base.

To determine the molybdenum content of the solutions obtained in the process according to the invention, a titration method with solutions having a known content of sodium hydroxide is used. When using a p ^ meter for the titration, it is possible to determine the neutralization points for the two protons of the acid.

The examples illustrate the invention. In these examples, a strong cation exchange resin is used which has the following properties: matrix of macroporous divinylbenzene cross-linked polystyrene, sulfonic acid group SO content₃H, particle size of the resin granules in the range of 0.3 to 1.2 mm and exchange capacity of about 2.5 equivalents per liter.

Prior to use, the resin is conditioned and converted to the hydrogen form in the following manner:

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- 4 1) 10 to 15 hours conditioning in deionized water and subsequent washing with deionized water to make the resin bed:

2) Treatment with a strong acid (like HCl, H₂SO "or ENT₃ in aqueous solution having an acid concentration in the range of 2 to 40% by weight at a rate of 2 to 8 parts by volume of acid solution per 1 part by volume of resin and per 1 hour;

3) Wash with deionized water until neutral p "value in n

a rate of 10 to 20 parts by volume of water per 1 part by volume of resin and per 1 hour.

After use, the regeneration of the resin into the hydrogen form is carried out by repeating the above-mentioned treatment steps 2) and 3).

Example 1: Molybdenum is recovered from a spent catalyst for the oxidation of methanol to formaldehyde. The catalyst consists of molybdenum, iron and cobalt oxides containing molybdenum oxide and 81.5% by weight.

The catalyst is ground to a powder having a particle size of the order of 300 gm. This powder is dried by heating at 200 ° C for 2 hours.

300.2 g of the dried powder are treated with 1132 g of an aqueous solution containing 12% by weight of sodium hydroxide. The entire mixture is contacted with agitation for 2 hours at room temperature (20-25 ° C) with agitation. Then the solid is filtered off and washed. This gives 1400 ml of an aqueous solution containing 163 g of molybdenum, indicated as metal. The solid residue (55.5 g) consists essentially of iron and cobalt hydroxides. The resulting solution is percolated by 4 liters of the above-described exchange resin in the acid form at a rate of 1 part by volume of solution per 1 part by volume of resin and per 1 hour. After this percolation treatment, the acid is washed with deionized water. The percolated solution and the washings are combined. 4 l of an aqueous solution of molybdic acid containing about 41 g / l of molybdenum are obtained. The total yield of molybdenum is thus 100%.

The molybdic acid obtained in this way is converted into ammonium paramolybdate. The latter compound is used as a starting material for the preparation of catalysts for the oxidation of methanol to formaldehyde.

Example 2: The procedure is as in Example 1 and uses 328 g of the ground and dried catalyst. This powder is treated with 1237.6 g of aqueous sodium hydroxide solution at a concentration of 12% by weight. After separation and washing of the solid residue, 1500 ml of a sodium molybdate solution containing 178.1 g of molybdenum are obtained.

The solution is percolated by 4 l of the resin which has been regenerated after the completion of the process of Example 1, at a rate of 1 part by volume of solution per 1 part by volume of resin and per 1 hour. After the percolation treatment, the column is washed with deionized water. This gives a total of 5 1 of an aqueous solution of molybdic acid with a content of 35.6 g / 1 molybdenum, indicated as metal. The percolation and washing operations are carried out at room temperature (20 to 25 ° C). The total yield of molybdenum recovery is 100%.

The molybdenum acid solution thus obtained is preserved in a cold transparent glass container which is exposed to light. After 3 months of storage, the solution shows no change.

Example 3: The procedure is as in Example 1 and using 304.1 g of the dried and ground catalyst: The powder is treated with 1147.5 g of aqueous sodium hydroxide solution with a concentration of 12 wt .-%. After separation and washing of the solid residue, 1400 ml of a sodium molybdate solution containing 165.15 g of molybdenum are obtained.

The solution is percolated by 4 l of the resin which has been regenerated after carrying out the process of Example 2, at a rate of 1 part by volume of solution per 1 part by volume of resin and per 1 hour. After the percolation treatment, the column is washed with deionized water. This gives a total of 4.5 1 of an aqueous solution of molybdic acid with a content of 36.7 g / 1 molybdenum, indicated as metal. The percolation and washing operations are carried out at room temperature (20 to 25 ° C).

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The total yield of molybdenum is 100%. The molybdic acid solution is treated with aqueous ammonia. The ammonium molybdate thus obtained is used to prepare a hydrogenation catalyst based on molybdenum and cobalt oxide applied to alumina.

Example 4: According to the procedure of Example 1, 500 ml of an aqueous solution of sodium molybdate containing 50 g of molybdenum are prepared. This solution is percolated by 1.5 L of the above strong cation exchange resin in the hydrogen form. After the percolation treatment, it is washed with deionized water. A total of 2 liters of an aqueous solution containing essentially all of the molybdenum 10 originally present in the form of molybdic acid is obtained. The acidic solution is treated with 1 l of an aqueous solution containing 127.3 g of barium chloride dihydrate.

A precipitate of barium molybdate is obtained in an amount of 154.7 g. The rest of

Solution contains 0.15 g of barium molybdate.

Example 5: The procedure is as in Example 4 and the solution obtained after the percolation treatment 15 and the washing of the resin is mixed with 1 l of an aqueous solution containing 57.8 g of calcium chloride. It precipitates a precipitate of 102.5 g of calcium molybdate.

The remaining solution contains 1.5 g of calcium molybdate.

EXAMPLE 6 The procedure is as in Example 4, and the solution obtained after the percolation treatment and the washing of the resin is mixed with 500 ml of a solution containing 20.6 g of ammonia. The resulting solution is added with 500 ml of an aqueous solution containing 50.6 g of ferric chloride hexahydrate. After filtration and separation of the precipitate gives 89.6 g of iron molybdate. The remaining solution contains 3.44 g of iron molybdate.

Example 7: Example 6 is repeated except that 500 ml of an aqueous solution containing 54.5 g of barium chloride dihydrate is used instead of the ferric chloride solution. 109.2 g of barium paramolybdate are recovered as precipitate.

Example 8: Molybdenum is recovered from a catalyst formed from 12% by weight of molybdenum oxide and 4% by weight of cobalt oxide on an alumina carrier. 10 g of this catalyst are crushed to a powder having a particle size of the order of 300 microns and treated with 200 g of an aqueous sodium hydroxide solution containing 12 wt .-%.

The mass is stirred for 4 h at room temperature (20 to 25 ° C). The resulting solid is filtered off. 170 ml of a sodium molybdate solution containing 4.2 g / l of molybdenum and 790 mg / l of aluminum are recovered.

Example 9: The procedure of Example 8 is followed except that the catalyst powder is calcined for 2 1/2 hours in an oven at 900 ° C. The resulting aqueous sodium molybdate solution contains 4.2 g / 1 molybdenum and 45.5 mg / 1 aluminum, indicated as metal.

From this solution, the molybdic acid is recovered in the manner described above.