GB2193114A - Catalysts - Google Patents

Abstract

Iron is recovered from spent reduced iron catalyst particles by passivating the spent catalyst by contacting the reduced catalyst particles with an oxidising oxygen-containing gas at a temperature below 100 DEG C, to oxidise the surface layers of the particles without oxidation of the bulk of the particles, reacting the passivated catalyst particles with nitric acid to dissolve the bulk of the un-oxidised iron in said particles, and filtering undissolved material from the reaction mixture, thereby obtaining a nitrate solution. If cobalt is present in the spent catalyst, it will be recovered with the iron. The nitrate solution may be used for the preparation of fresh catalyst by precipitation by addition of a base followed by drying and calcination of the precipitate.

Description

SPECIFICATION Catalysts This invention relates to catalysts and in particular to iron containing catalysts.

Iron catalysts are widely used for ammonia synthesis and adiponitrile hydrogenation. After extended use, the catalysts lose their activity, generally as a result of sintering of the iron crystallites and poisoning by oxygen containing compounds, particularly carbon oxides and water. Generally the catalysts are not contaminated with poisons such as chlorine, arsenic, and sulphur compounds since the catalysts are usually well protected from such poisons by upstream catalyst beds used for other catalytic processes in the production of the feed to the iron catalyst. Provided the amounts of such poisons are less than about 200 ppm by weight of sulphur, less than about 50 ppm by weight of- chlorine and less than about 5 ppm by weight of arsenic, it would be desirable to subject the spent catalyst to a recovery operation so that the iron can be re-used.

We have devised a process whereby iron can be recovered from spent catalyst in a form suitable for re-use to make fresh catalyst.

The iron catalysts often contain quantities of other metals and/or metal oxides. Thus, aluminium, magnesium, and/or calcium oxides are often present as stabilisers or promoters, generally in relatively small proportions. Also, in the case of ammonia synthesis catalysts, small amounts of alkali metal compounds, particularly potassium compounds, are usually present as promoters.

Since these stabilisers or promoters are generally present in relatively small proportions and/or are relatively inexpensive, recovery thereof is not important. However some iron catalysts, eg for ammonia synthesis, may contain substantial propqrtions of cobalt: for example cobalt containing iron catalysts are disclosed in EP-A-993 and EP-A-174079.

Because of the relatively high cost of cobalt, recovery thereof is desirable.

In the present invention any cobalt present can be recovered together with iron.

Accordingly the present invention provides a process for recovery of iron and cobalt, if any, from a particulate spent iron catalyst in the reduced state comprising: a) passivating the spent catalyst by contacting the reduced catalyst particles with a stream of an oxidising oxygen-containing gas, the oxidising power of the gas and the flow rate thereof being controlled so that the temperature is maintained at below 100 C, thereby oxidising the surface layers of the particles without oxidation of the bulk of the particles; b) reacting the passivated catalyst particles with nitric acid to dissolve the bulk of the unoxidised iron and cobalt, if any, in said particles; and c) filtering undissolved material from the reaction mixture, thereby obtaining a solution containing iron nitrate, and cobalt nitrate if any cobalt was present in said iron catalyst.

The nitrates solution product may be used to prepare fresh iron catalyst by precipitation of iron, and cobalt, if any is present, as hydroxides and/or basic carbonates by the procedure described in aforesaid EP-A-174079.

Where structure-stabilising oxides of metals such as aluminium are present in the spent catalyst, these may largely remain undissolved in the nitric acid and so the nitrates solution may generally contain little or no nitrates of such metals. Stabilising oxides of metals such as magnesium and calcium may however be dissolved. However where the nitrates solution is to be used for the production of fresh catalyst, the presence of nitrates of such other metals in the nitrates solution.merely modifies the amount of fresh stabiliser that has to be employed.

The iron catalyst to be treated is in the form of particles which may have been derived by reduction of particles formed from a fused mixture of oxides, or, especially, formed by pelleting, compacting, or extruding a finely divided mixture of iron, or iron and cobalt, and/or oxides thereof and stabilising oxides or precursors thereto, in some cases followed by a sintering step.

One important feature of the process of the invention is the passivation of the spent catalyst. In the reduced, ie active, state of the catalyst, the active component, ie iron, or iron and cobalt, is in a microcrystalline state and is very reactive towards oxygen-containing compounds even at ambient temperatures. particularly for catalysts that are derived from precursors obtained by precipitatipn.

In order that the spent catalyst can safely, and successfully, be treated to recover iron and cobalt, if any, the spent catalyst is cooled from the temperature at which the catalytic process was being operated to a temperature below 100 C, preferably to below 50"C. This cooling is effected in an atmosphere in which the catalyst will remain in the reduced state: this atmosphere may be hydrogen, nitrogen or other inert gas, or mixtures thereof. Where hydrogen, or a hydrogen/inert gas, eg nitrogen, mixture, is used as the atmosphere for such cooling, it is preferred to replace the hydrogen in that atmosphere by an inert gas, eg nitrogen, prior to passivation with the oxygencontaining gas.

Passivation is preferably conducted using oxygen, air, nitrous oxide, or carbon dioxide, or mixtures thereof, or such gases diluted with nitrogen. In particular the gas used for passivation preferably initially has an oxidising power of less than half that of air.

Passivation is effected at a temperature be low 1 00,C, and preferably at a temperature below 50"C.

After passivation, which effects superficial, but not bulk, oxidation, the particles are reacted with nitric acid. If passivation is conducted poorly, ie under too strongly oxidising conditions so that the catalyst has been bulk, rather than surface oxidised, then the passivated catalyst will not dissolve. The nitric acid employed should not be too concentrated or else oxidation of the iron, and cobalt if any is present, in the bulk of the passivated particles will occur rather than dissoiution thereof. Preferably the acid has a strength of 2 to 8 M.

Sufficient nitric acid should be used in relation to the weight of particles that at least 80% by weight of the unoxidised iron and cobalt, if any, is dissolved. The reaction with nitric acid is preferably effected at a temperature not exceeding 80"C.

The insoluble material, which will normally be a small amount of iron, or iron and cobalt, oxides together with residues of stabilising oxides such as alumina, is removed by filtration.

The invention is of particular utility where the reduced catalyst contains at least 60% by weight of iron and at least 5% by weight of cobalt.

If the nitrates solution product is to be used to make fresh catalyst, metal compounds may be precipitated from the nitrates solution, to which such additional quantity of iron and/or cobalt nitrate, as may be necessary, may have been added, by addition of a base such as an alkali metal carbonate or ammonia. While ammonia is a suitable precipitant where the proportion of cobalt is small, eg less than 5% w/w of the iron preSent, it is preferably not used where the cobalt proportion is greater since undue losses may occur as a result of the formation of sol4ble cobaltammines. Salts, eg aluminium, magnesium, and/or calcium, nitrates corresponding to desired stabiliser oxides may be added to the nitrates solution prior to precipitation. Alternatively the desired stabilisers, or precursors thereto, may be precipitated before or after the iron, or iron and cobalt.Alternatively the iron, or iron and cobalt, and the stabiliser, or precursor thereto, may be separately precipitated and the resulting precipitates mixed. After precipitation, the precipitate or mixture of precipitates is washed, dried, and calcined, preferably at a temperature in the range 200 to 500"C. Any water soluble promoter or precursor thereto, eg potassium salts, may be incorporated by impregnation of the precipitate, before or after calcination, with a solution of the promoter or precursor thereto.

The calcined precipitate may then be pelleted: before pelleting, but preferably after an agglomeration step, the calcined precipitate may be subjected to a de-oxidation step as described in EP-A-174078.

The pelleted product is a precursor to a fresh catalyst.

Reduction of the precursor to active catalyst is conveniently effected by hydrogen at a temperature in the range 300-500"C. If the catalyst is to be used for ammonia synthesis, the reducing gas used is usually ammonia synthesis gas and is passed over the precursor in the reactor in which synthesis is to take place. Precautions should be taken to keep the concentration of water vapour, which is formed during the reduction, low and to avoid back-diffusion of water vapour into contact with iron formed by the reduction and to prevent over-heating once ammonia synthesis has begun. The gas pressure is suitably in the range 1-300, for example 20-120, bar abs.

In an alternative procedure the precursor is reduced outside the synthesis reactor and passivated by cool dilute oxygen to give "prereduced" catalyst, which thereafter is charged to the synthesis reactor and therein fully reduced. If the catalyst is to be used for nitrile hydrogenation, the precursor is usually reduced with hydrogen, preferably containing a few percent of ammonia.

In an example of how the invention may be carried out, iron, cobalt and aluminium compounds are precipitated by addition of a solution of sodium carbonate to a solution containing nitrates of iron, cobalt and aluminium.

The precipitate is washed until free of sodium salts, impregnated with a potassium carbonate solution, and dried. The product is then calcined and pelleted to give an oxides mixture.

A typical oxides mixture obtained by this route contains 84% by weight of iron oxide (expressed as Fe203), 12% by weight of cobalt oxide (expressed as CoO), 3% by weight of alumina, and 1% by weight of potassium oxide. The catalyst precursor pellets comprising the oxides mixture is reduced to active catalyst and used for ammpnia synthesis.

When the catalyst is spent, it is cooled from the ammonia synthesis temperature to 25"C in an atmosphere of nitrogen. Nitrogen, containing 1% by volume of oxygen, is then passed over the catalyst. The proportion of oxygen in the mixture is slowly increased over a period of hours until the oxygen content of the mixture is about 22% by volume. The rate at which the oxygen content is increased and the gas flow rate are controlled so that the temperature of the catalyst during this treatment remains below 50to.

The passivated catalyst pellets are then discharged from the reactor. 6 M nitric acid at ambient temperature is then added to the passivated pellets, while agitating, at such a rate, and with such cooling as is necessary, that the temperature is maintained at below 80"C, until about 1 litre of acid has been added for each 100 g of passivated pellets. After any induction period, nitrogen oxides evolved may be collected and scrubbed with oxygenated water to produce nitric acid which may be re used.

When the evolution of gas ceases, the mixture is cooled and filtered. About 90% by weight of the iron and about 98% by weight of the cobalt in the passivated pellets is recovered in the form of a nitrates solution suitable for re-use in catalyst manufacture, with of course, addition of such further additives as may be desired.

Claims (11)

1. A process for recovery of iron and cobalt, if any, from spent iron catalyst particles in the reduced state comprising: a) passivating the spent catalyst by contacting the reduced catalyst particles with an oxidising oxygen-containing gas, the oxidising power of the gas and the flow rate thereof being controlled so that the temperature is maintained at below 100"C, thereby oxidising the surface layers of the particles without oxidation of the bulk ofthe particles; b) reacting the passivated catalyst particles with nitric acid to dissolve the bulk of the unoxidised iron and cobalt, if any, in said particles; and c) filtering undissolved material from the reaction mixture, thereby obtaining a solution containing iron nitrate, and cobalt nitrate if any cobalt was present in said iron catalyst.

2. A process according to claim 1 wherein the spent catalyst is cooled from the temperature at which the catalytic process was being operated to a temperature below 100"C, said cooling being effected in an atmosphere of hydrogen, inert gas, or mixtures thereof.

3. A process according to claim 2 wherein the atmosphere employed for the cooling to below 50"C contains hydrogen and the hydrogen in that atmosphere is replaced by an inert gas prior to passivation by said oxygen-containing gas.

4. A process according to any one of claims 1 to 3 wherein the gas used for passivation initially has an oxidising power of less than half that of air.

5. A process according to any one of claims 1 to 4 wherein the nitric acid has a strength of 2 to 8 M.

6. A process according to any one of claims 1 to 5 wherein the reaction with nitric acid is effected at a temperature not exceeding 80 C.

7. A process according to any one of claims 1 to 6 wherein the reduced catalyst contains at least 60% by weight of iron and at least 5% by weight of cobalt.

8. A process according to any one of claims 1 to 7 wherein the spent catalyst is the reduction product of an oxidic catalyst precursor obtained by the calcination of precipitated iron compounds.

9. A process for the manufacture of an iron catalyst precursor comprising precipitating metal compounds from a solution containing iron nitrate prepared by a process of any one of claims 1 to 8, to which such additional quantity of iron and/or cobalt nitrate, as may be necessary, may have been added, by addition of a base, followed by drying and calcining the precipitate.

10. A process according to claim 9 wherein aluminium, magnesium, and/or calcium, nitrate is added to the solution containing iron nitrate prior to said precipitation.

11. A process according to claim 9 or claim 10 wherein the solution containing iron nitrate also contains cobalt nitrate, the amount of cobalt being at least 5% by weight, based on the weight of iron in the solution, and the base employed for the precipitation is an alkali metal carbonate.