GB1567570A - Process for recovering the molybdenum present in waste products - Google Patents

Abstract

100 kg of spent catalyst waste (containing 8 % Mo, 1.5 % S and 2 % Co) on a support based on gamma alumina, freed from volatile matter, carbon and part of the sulphur by preliminary oxidising roasting, are mixed with 37.5 litres of aqueous solution of Na2CO3 (400 g/l) at 70 DEG C. An impregnated product is obtained which is heated between 650 and 750 DEG C for one hour. The product thus treated is washed with water so as to obtain a solution of sodium molybdate which is neutralised with a solution of nitric acid in three precipitation operations separated by a clarification. A precipitate of molybdic acid is obtained while the alumina remains in solution. After drying, this precipitate has an apparent relative density of 2 and contains 61.2 % Mo and 0.004 % Al. Molybdenum yield : 85 %.

Description

(54) A PROCESS FOR RECOVERING THE MOLYBDENUM PRESENT IN WASTE PRODUCTS (71) We, METAUX SPECIAUX, S.A., a body corporate organised under the laws of France, of 23 bis rue Balzac 75008, Paris, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a process for recovering the molybdenum present in waste products, above all in spent catalysts, containing one or more oxides of molybdenum associated with alumina and other metallic oxides. The catalysts in question are in particular those of the type used for the desulphurisation of petrols.

Catalysts of this kind contain a support based on gamma-alumina impregnated with one or more molybdenum compounds.

These compounds are generally oxides, such as MoO3, which are themselves obtained by the dissociation of a salt, such as ammonium molybdate. Other metal compounds, in particular cobalt oxide and/or nickel oxide, are often present in the catalyst as active constituents thereof.

Finally, the catalysts contains impurities, most of which remain fixed on the catalyst throughout its service life. This is particularly the case with various organic compounds, such as those containing sulphur.

Before undertaking their chemical treatment, it is standard practice to subject these waste products to an oxidising roasting at a temperature generally below 6000C in order to eliminate in the form of volatile compounds the hydrocarbons, the carbon and part of the sulphur with which they are impregnated. This treatment is often carried out in the same column which is used for treating the hydrocarbons. It may also be carried out subsequently for example when the chemical treatment is applied. After this roasting, the molybdenum present in the catalyst is in the form of its oxide or sulphide. Various methods have been proposed for separating and recovering the molybdenum in a useable form.

In this connection, reference is made in particular to French Patent No. 701,426 which proposes a process for the treatment of catalysts used for the hydrogenation of coal, oils and tars. In addition to an alumina-based support, these catalysts contain metal compounds based on Mo, Cr.

Zn and Mg. The inventor found that, if these catalysts were roasted at a temperature below 500"C, it was possible to solubilise the Mo with a solution of ammonia which enables an ammonium molybdate to be obtained, the other metals remaining unaffected or only slightly affected. It is then possible to precipitate the molybdic acid with hydrochloric acid at boiling temperature. This process has the major disadvantage of a low reaction velocity between the ammonia and the molybdenum oxide contained in the catalyst. In addition, the extraction yield is low, a significant proportion of the molybdenum oxide being retained in the inert materials. Finally, the molybdenum sulphide is hardly affected by the ammonia.

US Patent No. 2,367,506 relates to the recovery of molybdenum present in spent hydroforming catalysts based on compounds of molybdenum with a support of activated alumina. It proposes a method which comprises immersing the pellets of spend catalyst in a solution of sodium carbonate until they are completely impregnated and then heating the pellets thus impregnated for 30 minutes at a temperature of 1150"C, for example in a rotary furnace. Under these conditions the alumina is rendered substantially insoluble and the sodium molybdate formed may be dissolved in water, entraining only a small quantity of aluminium in the form of sodium aluminate. The process thus described has the disadvantage of reducing the solubility of the molybdate formed and, in fact, the dissolution thereof in water becomes difficult.Finally, it has been found that it is not possible to avoid the dissolution of alumina and this has to be separated by additional treatments if it is desired to recover a sufficiently pure molybdenum compound.

The process according to the invention enables the disadvantages of conventional processes to be overcome. In particular, it enables the molybdenum contained in the spent catalysts to be completely recovered without, at the same time, entraining significant quantities of aluminium. It also enables catalysts to be treated after preroasting at temperatures of preferably below 6000C without this pre-roasting temperature level being particularly critical either from the point of view of the conditions of solubilisation of the molybdenum or from the point of view of the extraction yield. The process according to the invention also provides for a significant saving of reactants because the quantities used are strictly proportional to the composition of the materials to be treated.Finally, by virtue of the development of an original method for precipitating molybdic acid, it is possible to obtain the molybdic acid in highly pure form, its aluminium content being lower than that which is normally obtained by processes of the type described in US Patent No.

2,367,506.

The present invention provides a process for treating waste products based on active alumina containing one or more compounds of molybdenum and, optionally, one or more other metal compounds so as to recover the molybdenum in the form of a substantially aluminium-free compound, wherein, after having substantially eliminated, when present, the carbon, the hydrocarbons and part of the sulphur, the waste product is treated with sodium carbonate at a temperature of from 600"C to 800 C and then washed with water, the quantity of sodium carbonate used being in excess in relation to the stoichiometrically necessary quantity for forming sodium molybdate from the molybdenum present and for fixing the sulphur in the form of sodium sulphate, and wherein there is progressively added to the alkaline solution thus obtained a quantity of nitric acid in the range of from 1.5 to 2.5 times that quantity which would be necessary to reach a pHvalue of from 5 to 6, whilst, at the same time, keeping the temperature below 30"C, and wherein the solution thus acidified is hydrolysed at a temperature close to boiling point so as to precipitate molybdic acid monohydrate.

Preferably, the treatment with sodium carbonate comprises the following steps: adding sodium carbonate in the form of an anhydrous powder to the waste product.

thorough mixing by stirring, adding water at substantially ambient temperature in a quantity not exceeding the absorption capacity of the waste product, thorough mixing by stirring until the water is completely absorbed by the waste product, and baking the waste product thus impregnated at a temperature of from 600 to 800or Preferably, the nitric acid is added in two stages: progressive addition in a first reactor to the alkaline solution containing the Mo in dissolved form of such a quantity of HNO3 that the pH reaches a value of from 5 to 6 whilst at the same time keeping the temperature of the solution below 30"C: and then, in a second reactor, progressive addition to the solution issuing from the first reactor of a quantity of HNO3 substantially equal, within + 20 ó by weight, to that introduced into the first reactor whilst at the same time keeping the temperature of the solution below 30"C.

The molybdic acid obtained by the process of the invention preferably has an aluminium content of less than 0.01% by weight, based on Mo, an apparent density after drying at 100"C of substantially 2, and excellent filtrability.

The process according to the invention comprises, in a first step, impregnating the catalyst with a solution of sodium carbonate and then heating the catalyst thus impregnated to a temperature high enough to convert most of the molybdenum present into sodium molybdate whilst, at the same time, preventing an excessively large quantity of alumina from being converted into sodium aluminate. In order to obtain this result, we have found it necessary to have an excess of sodium carbonate in relation to the quantities which are strictly necessary for forming sodium molybdate Na2MoO4 from the molybdenum present and for fixing the sulphur in the form of Na2SO4. If the excess of sodium carbonate is too small, the conversion yield of the molybdenum present into sodium molybdate decreases, whilst on the other hand if this excess is too large, the quantity of alumina converted into aluminate increases, giving rise to a higher consumption of reactants and to greater difficulties in subsequently separating this aluminium converted into soluble form.

In practice, the molybdenum content of the waste products of the catalyst, after they have been roasted to eliminate the volatile compounds, the carbon and part of the sulphur, is generally of the order of 4 to 120,0,, whilst their sulphur content is generally from 0.5 to 40o. These values are purely by way of example and certain types of catalyst waste may have Mo or S contents which exceed these limits. We have found that it is preferable to limit the excess of sodium carbonate to l00,o by weight, based on the products to be treated, and that this excess is with advantage from 1 to 3% by weight. In order suitably to adjust this excess, it is necessary to determine with sufficient accuracy the Mo and S contents of the waste catalyst products which it is intended to treat.The impregnation treatment should be carried out in such a way that all the catalyst pellets uniformly absorb the reactant. This result may be obtained, for example, by spraying a stirred bed of pellets with a solution of sodium carbonate to produce a systematic movement of the pellets during the spraying period. The solution volume depends to a limited extent upon the specific surface area of the catalyst and is of the order of 200 to 500 cc per kg of treated catalyst. The concentration of Na2CO3 varies widely according to the Mo and S contents, as has just been explained.

This impregnation is followed by heating at a temperature in the range of from 600 to 800"C and preferably at a temperature in the range of from 650 to 750"C. We have found that, in this temperature range, the molybdenum present in the form of molybdenum oxide or sulphide can be almost completely converted into molybdate soluble in hot water (800 C) over a short period of the order of 1 hour. At this temperature, the oxides of cobalt and nickel optionally present do not react with the sodium carbonate to any significant extent and remain substantially insoluble in water.

Although in this temperature range the reaction velocity of the alumina with the sodium carbonate is very low, it is still not possible to prevent small quantities of sodium aluminate from being formed. As mentioned above, these quantities remain limited provided that the temperature conditions and treatment time are respected and provided that the excess of sodium carbonate introduced does not exceed the limits specified.

After this heat treatment, the catalysts pellets are treated while stirring with hot water until the sodium molybdate has been dissolved as completely as possible. This result may be obtained at temperatures of from 60 to 1000C by treatments lasting approximately 1 hour. It is generally desired to obtain relatively concentrated solutions containing, for example, of the order of 45 to 50 g/l of Mo in the form of sodium molybdate. These solutions also contain sodium aluminate. By virtue of the operating conditions described above, the ratio of Al/Mo in solution is generally of the order of 10"by in many cases even lower and scarcely exceeds 20 o. As already mentioned, these solutions also contain sodium sulphate and free sodium carbonate.

These solutions may contain in suspension small solid particles emanating primarily from the partial disintegration of the waste catalyst products. These particles are eliminated by decantation or by filtration, for example in a filter press.

The most important step of the process is the separation of the molybdenum in the form of molybdenum hydrate from the sodium aluminate and the alkali metal salts.

In this respect, it has unexpectedly been found that it is possible to convert the initial basic solution into an acid solution from which the molybdic acid will subsequently be precipitated without any danger of the sodium aluminate being partially hydrolysed. This apparently complex operation may be carried out in a precise, reproducible manner by a simple and original method forming a preferred embodiment of the invention.

This method comprises treating the solution in two successive reactors of the same capacity which are arranged one behind the other in such a way that the input and output rates of these two reactors are equal and constant. The first reactor receives the solution emanating from the extraction with hot water of the soluble salts contained in the catalyst after treatment with sodium carbonate and freed from the solid particles which it might contain by decantation or filtration. This solution is cooled to enter the first reactor at a temperature not exceeding 30"C. A stream of nitric acid is continuously introduced into this same reactor at a rate of flow adjusted in such a way that the pH-value of the solution is in the range of from 5 to 6 and preferably of the order of 5.2 to 5.5.This rate of flow may be adjusted by known means, such as a metering pump, of which the output is continuously regulated by means of a pH-meter of which the probe is placed in the reactor itself. The nitric acid is preferably introduced in concentrated form.

Since the reaction is exothermic, the reactor should be equipped with cooling means known to the expert, such as a double jacket or refrigeration coils and, optionally, agitators, to enable the temperature of the solution to be kept at a value not appreciably exceeding 200C and, in any case, below 30"C. Under these conditions, the free sodium carbonate is neutralised and the sodium aluminate is decomposed to the point where precipitation commences, which gives the solution a slightly clouded appearance.

The solution then enters the second reactor at a constant rate of flow, for example by overflowing. A stream of nitric acid is continuously introduced into this reactor, too, at a rate of flow substantially equal to that adjusted in the first reactor, as has just been mentioned. A simple way of obtaining this result is to use a metering pump comprising two separate circuits with a single regulating system which enables two flows of equal volume to be obtained at any instant, one feeding the first reactor and the other the second reactor. It is sufficient to feed each circuit from a common reservoir so as to be certain of introducing the same quantities by weight of nitric acid. Like the first reactor, the second reactor is equipped with a cooling system enabling the temperature of the solution to be kept below 30"C.

By virtue of the excess of nitric acid thus introduced it is possible on the one hand to redissolve the alumina which tends to precipitate and, on the other hand, to create conditions which favour the precipitation of molybdic acid. In order to obtain this precipitation, it is necessary to heat the solution to a temperature close to its boiling point. This is preferably done in one or more precipitators which receive the solution issuing from the second reactor and which heat it to approximately 100"C. The precipitate obtained is then washed, rinsed and dried in the usual way. This precipitate based on molybdic acid monohydrate contains only small quantities of aluminium.

Its Mo-content, expressed in 0, of dry product, is substantially equal to or greater than 60 ó, whilst its Al-content is less than 0.1 O and may even fall to around 0.01% in some cases.

One embodiment of the process according to the invention as described above is illustrated by Example I below.

However, the following two difficulties were encountered during the working of this process: First of all, we found that impregnation of the catalyst with a strictly calculated quantity of sodium carbonate solution was relatively difficult to carry out if the solution was to be homogeneously distributed. An aqueous solution containing 400 g of Na2CO3 per litre was generally used to limit the quantity of water. This solution, initially heated to approximately 70"C, tended to crystallise during its contact with the catalyst which prevented the carbonate from penetrating into the pores of the catalyst particles. However, greater dilution was not desirable because the absorption capacity of the catalyst is limited.

Another more serious difficulty was revealed during prolonged tests carried out on a pilot scale to dissolve in hot water the sodium molybdate formed in the catalyst after the carbonate treatment. The formation of progressive deposits was observed on the walls of the containers accommodating the aqueous solution and in the pipes through which this solution circulated. This is because dissolution of the sodium molybdate is also accompanied by the dissolution of a certain quantity of sodium aluminate and the ratio by weight of Al to Mo in the solution is generally of the order of 0.1 and, in some cases, may even increase to approximately 0.2. Our investigations showed that the deposit which forms on the walls of the containers and pipes is primarily based on alumina and, more particularly, on alumina trihydrate.

The layers thus formed adhere strongly to steel, ebonite, glass and rubber. It would appear that this phenomenon is promoted by the presence of catalyst particles in suspension in the solution which act as seeds. When these layers increase in thickness they tend to separate locally and the types of solid platelets thus released are entrained by the displacement of the solutions and tend to obstruct the pipes and even to block the circulation pumps.

We have discovered original means of obviating these difficulties. We found that these means not only enable a certain number of incidents to be avoided in the working of the process, but they also and above all enable a product of more reproducible quality to be obtained in even purer form.

These new means which very significantly improve the effectiveness and reproducibility of the process according to the invention concern above all a significant modification in the first step of the process.

Instead of impregnating the catalyst particles with an aqueous solution of sodium carbonate, the catalyst particles are initially mixed with anhydrous sodium carbonate in the form of a fine powder in a mixer of any type, such as a rotary mixer. In general, mixing for a few minutes is sufficient for the particles of sodium carbonate to be distributed over the surface of the catalyst particles. It is then sufficient to add the necessary quantity of water at ambient temperature and to restir the batch in the mixer for a few minutes to obtain virtually complete absorption of the water inside the catalyst particles. Experience has shown that this penetration of water enables the sodium carbonate to penetrate inside the particles, probably by diffusion.During the following step of baking the catalyst thus impregnated, the yield of molybdenum converted into sodium molybdate is at least as high as in the case of impregnation with a hot solution of sodium carbonate as described above.

Another particularly important improvement has made it possible to solve the problem posed by the deposits of alumina trihydrate which are formed during the dissolution of the sodium molybdate in water. The improvement has resulted from the following experimental finding: when the catalyst is left standing for a few days after the treatments of impregnation with sodium carbonate and baking, the deposits which form during the dissolution in water are less abundant. Subsequent tests have shown that the reduction in these deposits was due to the action of the carbon dioxide in the air on the sodium aluminate present in the catalyst particles. An additional step of treating these catalyst particles after baking with a stream of carbon dioxide gas was then introduced into the process.The operating conditions are very simple and it is sufficient for the carbon dioxide gas to be brought into contact with the catalyst particles for a period of time sufficient to enable it to diffuse into them. The treatment with the carbon dioxide gas is preferably carried out at substantially ambient temperature by passing a stream of carbon dioxide gas through the product. This result is obtained for example, by filling a vertical column of plastics material or of sheet steel with catalyst particles and by circulating a stream of carbon dioxide gas through this column. The quantity of carbon dioxide gas required for an effective treatment is of the order of 1 Nm3 for 50 to 100 kg of product, preferably for 60 to 70 kg of catalyst. It does, of course, depend upon the quantity of alumina present as sodium aluminate in the catalyst.Although the exact nature of the physicochemical process which takes place is not entirely known, it is probable that the sodium aluminate is at least partly decomposed with the formation of carbonate. This reaction takes place at a temperature close to ambient temperature.

The other steps of the process are then carried out in the manner initially described. In this way, deposits are no longer formed on the walls of the container and pipes during the dissolution treatment by washing the sodium molybdate contained in the catalyst particles with water. In addition, the excess of sodium carbonate to be used in relation to the quantity of catalyst is no longer critical and may readily exceed the limit of 10% specified earlier on which facilitates the working of the process. The small quantities of amorphous alumina in the form of fine particles which are inevitably detached from the catalyst particles during this washing treatment are not troublesome because they do not agglomerate into solid lumps, but remain in divided form.Part is separated by decantation whilst the rest, which is in suspension in the washing solution, will be retained by filtration before introduction into the neutralisation and clarification reactors.

Two exemplary embodiments of the process according to the invention are described hereinafter with reference to the accompanying drawings, in which: Figure 1 is a flow chart of a first embodiment of the process.

Figure 2 is a flow chart of a second embodiment of the process.

Examples 1 and 2 below describe in detail these first and second embodiments of the process.

Example 1.

In this Example the process according to the invention is carried out in accordance with the flow chart illustrated in Figure 1.

Waste products of spent catalysts have undergone preliminary oxidising roasting at a temperature of the order of 500"C which has substantially freed them from volatile materials, carbon and part of the sulphur.

They are in the form of small cylinders or small balls. These waste products now contain in combined form 8% of Mo, 1 .5?o of S and approximately 2% of Co; the support is based on gamma-alumina. 100 kg of these waste products and 37.5 litres of an aqueous solution heated to approximately 70"C and containing 400 g/litre of Na2CO3 are introduced into a rotary mixer and treated therein for about 30 minutes. The quantity of Na2CO3 amounts to 150 g per kg of waste products. Calculation shows that the quantity of Na2CO3 theoretically necessary for converting the 8% of Mo into sodium molybdate amounts to approximately 87 g per kg of waste products. Similarly, the quantity of Na2CO3 required for converting the 1.5% of S into sodium sulphate amounts to 44 g per kg of waste products.Accordingly, this leaves an excess of 19 g of Na2CO3 per kg of waste products, i.e. 1.90,, based on their weight.

The waste products are alternately impregnated in batches of 100 kg in two mixers arranged in parallel so as to enable a batch which has just been impregnated to be progressively transferred to the baking furnace whilst a second batch is being impregnated.

The baking furnace is a rotary furnace approximately 4.2 metres long and with an internal diameter of 630 mm. The furnace is heated to between 650 and 750"C by a propane burner. At one end, it is continuously fed with impregnated product at a rate of approximately 100 kg/hour. The residence time of the product in the hot zone of the furnace is fixed at about 1 hour by means well known to the expert. At the output end of the furnace, the temperature of the product is lowered to between 70 and 80"C by passage through a cooler of which the walls are cooled by the circulation of water. Approximately 95 Ó of the molybdenum contained in the product is then present in the form of sodium molybdate.

The sodium molybdate is dissolved by washing the product in a layer approximately 10 cm thick in countercurrent on a continuous belt filter with a filtering surface of 1 square meter.

The washing unit comprises 6 stages; the sixth and last stage is fed with hot water at 80"C at a rate of approximately 120 litres per hour. The concentrated solution of sodium molbdate is removed at the output end of the first stage at a rate of about 104 litres per hour with a content of from 45 to 50 g/l of Mo in dissolved form. The washed solid product which is removed from the belt after washing with pure water in the last stage contains approximately 0.27% of Mo in soluble form. Accordingly, the washing yield is of the order of 97%. The solution containing the Mo is freed from suspended solids by filtration in a filter press and then introduced into a first neutralisation reactor at a constant rate adjusted to 104 litres per hour.At the same time, a first metering pump introduces an HNO3-solution with an HNO3-concentration of 53% into the reactor. This reactor, which has a volume of about 150 litres, is equipped with cooling means in the form of a double jacket filled with circulating water so as to keep the temperature below 20"C. The operation of this first metering pump is controlled by a probe for continuously measuring pH which is placed in the reactor so that a pH in the range of from 5.2 to 5.5 can be maintained.

Under these conditions, the average output of the pump is 8 litres per hour. The solution, which is slightly cloudy because of the incipient precipitation of alumina, enters a second clarification reactor equal in volume to the first reactor where it receives another addition of nitric acid by means of a second metering pump which is controlled in dependence upon the first metering pump so that it delivers exactly the same volume of nitric acid which has the same composition because it is taken from the same reservoir. The temperature of this reactor is kept below 300 C, again by the circulation of water through a double jacket.

Under these conditions, the alumina is redissolved and clarified. The solution then enters a third precipitation reactor which has the same volume as the first two reactors and which is heated to 100"C by the circulation of steam through a double jacket to effect hydrolysis.

Under these conditions, the molybdic acid precipitates whilst most of the aluminium remains in solution. An agitator enables the precipitate to be kept in suspension. The suspension is then delivered to a rotary filter on which the precipitate is collected and continuously washed with demineralised water containing 2 o by volume of concentrated HNO3. The precipitate then enters a hot air dryer where it is heated to approximately 100"C. This precipitate, based on molybdic acid, has an average Mo-content of 61,20o. Its aluminium content is only 0.004 O. Its mean apparent density is 2. The yield of Mo recovered in the form of molybdic acid based on the Mo contained in the waste products amounts to approximately 85%.

The process which has just been described in this Example may be modified in numerous ways without departing from the scope of the invention because there are numerous equivalent ways of carrying out the essential steps of the method according to the present invention. In particular, it is possible to carry out in a single stage the operations of neutralisation and clarification of the solution containing the sodium molybdate extracted by washing. In this case, it is sufficient to introduce into a single reactor a quantity of HNO3 equal to the sum of the quantities which are successively introduced into the two reactors. In that case, more difficulties are encountered in controlling the temperature of the solution which should not appreciably exceed 30"C in order to avoid irreversible precipitation of the aluminium.

We have also found that, during the subsequent precipitation of the molybdic acid, there is a greater risk of the molybdic acid being obtained in a light partly colloidal form in which it is very difficult to filter. By contrast, when acidication is carried out in two stages, a dense precipitate of molybdic acid, which is easy to wash on a filter, is normally obtained.

This is an important factor in obtaining a molybdic acid of very low Al-content which may be used in particular for the production of highly pure Mo powder by reduction with hydrogen.

Example 2.

In this Example, the process according to the invention is carried out in accordance with the flow chart of Figure 2.

The catalyst treated is a spent catalyst in the form of small rodlets based on gammaalumina which has previously been subjected to an oxidising roasting during which the hydrocarbons, the carbon and part of the sulphur present in it were eliminated. After roasting, this catalyst contains 5% by weight of Mo, 1.5% by weight of S and 2% of Co. 25 kg of sodium carbonate powder and 150 kg of this catalyst are introduced into a rotary mixer. After 10 minutes' mixing, 64 litres of water at ambient temperature are added, followed by mixing for another 15 minutes.

Thereafter, the sodium carbonate and the water are almost completly retained by the catalyst particles. The catalyst particles thus impregnated are baked in a rotary furnace maintained at a maximum temperature of 650 to 7500C by means of a propane burner.

The residence time in the hot zone is approximately 1 hour. On issuing from the furnace, the product is cooled to a temperature around ambient temperature and is then continuously introduced into the upper end of a vertical column of sheet iron filled with approximately 200 kg of catalyst particles at a rate of 60 to 70 kg per hour. In this column, a stream of carbon dioxide gas circulates upwards at a rate of approximately 1 cubic metre per hour. The catalyst is also continuously removed at the base of the column. The residence time of the catalyst particles in the column is approximately 3 hours. This catalyst is then treated in the manner described in Example 1.In order perfectly to separate the insoluble particles which may be suspended during dissolution of the sodium molybdate with hot water, these particles being in particular alumina particles, the dissolution treatment is followed by filtration in a filter press of the alkaline solution before it is introduced into the first neutralisation reactor using HNO3. After this filtration step, analysis of the solution shows that its aluminium content is less than 0.03% by weight, based on its molybdenum content.

Since the molybdenum content is of the order of 45 to 50 g/l of Mo, it can be seen that the Al-content is less than 0.015 g/l as against 5 to 10 g/l in the case of Example 1.

Under these conditions, the quantities of nitric acid used in the two successive neutralisation and clarification reactors are significantly reduced, which represents a considerable saving of reactant. However, the conditions under which the acid is introduced and under which the quantities introduced are controlled by pHmeasurement in the first reactor are unchanged. The subsequent steps of precipitation of the molybdic acid, followed by filtration, washing and drying, are unchanged.

The molybdic acid thus obtained is even purer, especially in regard to its aluminium content, than that obtained in accordance with Example 1. This is because precipitation is carried out with a solution in which the concentration of aluminium is several hundred times lower. This is a considerable advantage for certain applications of molybdic acid.

Other embodiments of the process according to the invention are possible. In particular, it is possible to carry out the process in the manner described in Example 2 whilst retaining the initial step of impregnation of the catalyst with a solution of sodium carbonate such as described in Example 1.

WHAT WE CLAIM IS: 1. A process for treating waste products based on active alumina containing one or more compounds of molybdenum and, optionally, one or more other metal compounds so as to recover the molybdenum in the form of a substantially aluminium-free compound, wherein, after having substantially eliminated, when present, the carbon, the hydrocarbons and part of the sulphur. the waste product is treated with sodium carbonate at a temperature of from 600"C to 8000C and then washed with water, the quantity of sodium carbonate used being in excess in relation to the stoichiometrically necessary quantity for forming sodium molybdate from the molybdenum present and for fixing the sulphur in the form of sodium sulphate, and wherein there is progressively added to the alkaline solution thus obtained a quantity of nitric acid in the range of from 1.5 to 2.5 times that quantity which would be necessary to reach a pH-value of from 5 to 6 whilst, at the same time, keeping the temperature below 30"C, and wherein the solution thus acidified is hydrolysed at a temperature close to boiling point so as to precipitate molybdic acid monohydrate.

2. A process as claimed in Claim 1, wherein the treatment with sodium carbonate comprises the following steps: adding sodium carbonate in the form of an anhydrous powder to the waste product, thorough mixing by stirring, adding water at substantially ambient temperature in a quantity not exceeding the absorption capacity of the waste product, thorough mixing by stirring until the water is completely absorbed by the waste product, and baking the waste product thus impregnated at a temperature of from 600 to 800"C.

3. A process as claimed in Claim 1 or 2, wherein the excess quantity of sodium carbonate used for solubilising the Mo, expressed in weight of Na2CO3, does not exceed 10% of the total weight of the waste product.

4. A process as claimed in any one of Claims I to 3, wherein, after the treatment with sodium carbonate at a temperature between 600 and 800"C, the product is

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (16)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   eliminated. After roasting, this catalyst contains 5% by weight of Mo, 1.5% by weight of S and 2% of Co. 25 kg of sodium carbonate powder and 150 kg of this catalyst are introduced into a rotary mixer. After 10 minutes' mixing, 64 litres of water at ambient temperature are added, followed by mixing for another 15 minutes.

   Thereafter, the sodium carbonate and the water are almost completly retained by the catalyst particles. The catalyst particles thus impregnated are baked in a rotary furnace maintained at a maximum temperature of 650 to 7500C by means of a propane burner.

   The residence time in the hot zone is approximately 1 hour. On issuing from the furnace, the product is cooled to a temperature around ambient temperature and is then continuously introduced into the upper end of a vertical column of sheet iron filled with approximately 200 kg of catalyst particles at a rate of 60 to 70 kg per hour. In this column, a stream of carbon dioxide gas circulates upwards at a rate of approximately 1 cubic metre per hour. The catalyst is also continuously removed at the base of the column. The residence time of the catalyst particles in the column is approximately 3 hours. This catalyst is then treated in the manner described in Example 1.In order perfectly to separate the insoluble particles which may be suspended during dissolution of the sodium molybdate with hot water, these particles being in particular alumina particles, the dissolution treatment is followed by filtration in a filter press of the alkaline solution before it is introduced into the first neutralisation reactor using HNO3. After this filtration step, analysis of the solution shows that its aluminium content is less than 0.03% by weight, based on its molybdenum content.

   Since the molybdenum content is of the order of 45 to 50 g/l of Mo, it can be seen that the Al-content is less than 0.015 g/l as against 5 to 10 g/l in the case of Example 1.

   Under these conditions, the quantities of nitric acid used in the two successive neutralisation and clarification reactors are significantly reduced, which represents a considerable saving of reactant. However, the conditions under which the acid is introduced and under which the quantities introduced are controlled by pHmeasurement in the first reactor are unchanged. The subsequent steps of precipitation of the molybdic acid, followed by filtration, washing and drying, are unchanged.

   The molybdic acid thus obtained is even purer, especially in regard to its aluminium content, than that obtained in accordance with Example 1. This is because precipitation is carried out with a solution in which the concentration of aluminium is several hundred times lower. This is a considerable advantage for certain applications of molybdic acid.

   Other embodiments of the process according to the invention are possible. In particular, it is possible to carry out the process in the manner described in Example 2 whilst retaining the initial step of impregnation of the catalyst with a solution of sodium carbonate such as described in Example 1.

   WHAT WE CLAIM IS: 1. A process for treating waste products based on active alumina containing one or more compounds of molybdenum and, optionally, one or more other metal compounds so as to recover the molybdenum in the form of a substantially aluminium-free compound, wherein, after having substantially eliminated, when present, the carbon, the hydrocarbons and part of the sulphur. the waste product is treated with sodium carbonate at a temperature of from 600"C to 8000C and then washed with water, the quantity of sodium carbonate used being in excess in relation to the stoichiometrically necessary quantity for forming sodium molybdate from the molybdenum present and for fixing the sulphur in the form of sodium sulphate, and wherein there is progressively added to the alkaline solution thus obtained a quantity of nitric acid in the range of from 1.5 to 2.5 times that quantity which would be necessary to reach a pH-value of from 5 to 6 whilst, at the same time, keeping the temperature below 30"C, and wherein the solution thus acidified is hydrolysed at a temperature close to boiling point so as to precipitate molybdic acid monohydrate.
2. 2. A process as claimed in Claim 1, wherein the treatment with sodium carbonate comprises the following steps: adding sodium carbonate in the form of an anhydrous powder to the waste product, thorough mixing by stirring, adding water at substantially ambient temperature in a quantity not exceeding the absorption capacity of the waste product, thorough mixing by stirring until the water is completely absorbed by the waste product, and baking the waste product thus impregnated at a temperature of from 600 to 800"C.
3. 3. A process as claimed in Claim 1 or 2, wherein the excess quantity of sodium carbonate used for solubilising the Mo, expressed in weight of Na2CO3, does not exceed 10% of the total weight of the waste product.
4. 4. A process as claimed in any one of Claims I to 3, wherein, after the treatment with sodium carbonate at a temperature between 600 and 800"C, the product is

   exposed to the action of carbon dioxide gas before washing with water.
5. 5. A process as claimed in Claim 4, wherein the treatment with the carbon dioxide gas is carried out at substantially ambient temperature by passing a stream of carbon dioxide gas through the product.
6. 6. A process as claimed in Claim 4 or 5, wherein the quantity of carbon dioxide gas used is substantially 1Nm3 for 50 to 100 kg of product.
7. 7. A process as claimed in Claim 3, wherein the excess, expressed in weight of Na2CO3, is from I to 3 Ó of the total weight of the waste product.
8. 8. A process as claimed in any of Claims 1 to 7, wherein the nitric acid is added in two stages: progressive addition in a first reactor to the alkaline solution containing the Mo in dissolved form of such a quantity of HNO3 that the pH reaches a value of from 5 to 6 whilst at the same time keeping the temperature of the solution below 30"C; and then, in a second reactor, progressive addition to the solution issuing from the first reactor of a quantity of HNO3 substantially equal within + 20%, by weight, to that introduced into the first reactor whilst at the same time keeping the temperature of the solution below 30"C.
9. 9. A process as claimed in Claim 8, wherein the pH value reached in the first reactor is from 5.2 to 5.5.
10. 10. A process as claimed in any of Claims 1 to 9, wherein the temperature at which the Mo is solubilised by the action of sodium carbonate is in the range of from 650 to 750"C.
11. 11. A process as claimed in any of Claims 1 to 10, wherein the waste product is a spent catalyst.
12. 12. A process as claimed in any of Claims 1 to 11, wherein the precipitated molybdic acid monohydrate is rinsed and dried.
13. 13. A process for treating spent catalysts based on active alumina containing one or more compounds of molybdenum and, optionally, one or more other metal compounds, so as to recover the molybdenum in the form of a substantially aluminium-free compound, wherein, after having substantially eliminated, when present, the carbon, the hydrocarbons and part of the sulphur, the spent catalysts are treated with sodium carbonate at a temperature of from 600"C to 8000 C, the quantity of sodium carbonate used being in excess in relation to the stoichiometrically necessary quantity for forming sodium molybdate from the molybdenum present and for fixing the sulphur in the form of sodium sulphate, and the product is subjected to the action of carbon dioxide gas, followed by washing with water, and wherein there is progressively added to the alkaline solution thus obtained a quantity of nitric acid in the range of from 1.5 to 2.5 times that quantity which would be necessary to reach a pIt- value of from 5 to 6 whilst, at the same time, keeping the temperature below 30"C, and wherein the solution thus acidified is hydrolysed at a temperature close to boiling point, the molybdic acid monohydrate thus precipitated then being rinsed and dried.
14. 14. A process as claimed in Claim 1, substantially as herein described with reference to either of the Examples and/or the accompanying drawings.
15. 15. Molybdic acid monohydrate obtained by a process as claimed in any of Claims 1 to 14.
16. 16. Molybdic acid as claimed in Claim 15, wherein its aluminium content is less than 0.0loo by weight, based on Mo, wherein its apparent density after drying at 1000C is substantially 2 and wherein its filtrability is excellent.