Collection Of Metal Carbonyls

Abstract

Gaseous streams of carbon monoxide containing nickel and iron carbonyls are contacted with cooled liquid iron pentacarbonyl to dissolve the carbonyls and to provide purified carbon monoxide. When the liquid iron pentacarbonyl has dissolved controlled amounts of nickel carbonyl, the solution is fractionally distilled to recover nickel carbonyl and liquid iron pentacarbonyl which can be recycled. The process can also be conducted under pressure and in conjunction with intermediate pressure carbonyl processes for recovering nickel.

Description

The present invention relates to the treatment of gases containing carbonyls of iron and nickel, and more particularly, to the separation of nickel carbonyl and iron carbonyl from carbon monoxide. The process is also useful when recovering nickel carbonyl under elevated pressures.

Nickel and iron carbonyls are frequently produced under dynamic conditions, i.e., carbon monoxide is continuously flowed over or through metals containing nickel and iron to react therewith. Under such dynamic conditions, nickel and iron are more rapidly carbonylated since flowing carbon monoxide continually removes products of reaction from the reaction site thereby speeding up the overall reaction rate. Although this procedure increases the overall reaction rate, it has the distinct disadvantage of producing a dilute off-gas so that the ultimate recovery of metal carbonyls is difficult and expensive. In most instances, the off-gas must be refrigerated to about 0.degree. C. and the gas volume reduced to about 1percent of its original volume in order to condense the highly volatile metal carbonyls. Refrigerating a dilute off-gas which may contain less than about 30percent metal carbonyls is inefficient, particularly when large volumes of off-gas are treated. Compressing the off-gas in order to condense the metal carbonyls is expensive, particularly when only about 90percent of the carbonyls in the off-gases are recovered by condensation, and, in addition, such compression increases the temperature of the gases being compressed thereby increasing the refrigeration required.

In order to overcome the foregoing problems, it has been suggested that off-gases containing carbonyls of nickel and iron be collected in organic solvents which have boiling points higher than either of the carbonyls. This approach works reasonably well but it involves additional reagent expense and can complicate subsequent separation of nickel and iron carbonyl from the organic solvent and from each other.

After separation from carbon monoxide, nickel and iron carbonyls are most often separated from each other for individual use. After separation, most frequently by distillation, nickel carbonyl is thermally decomposed to carbonyl nickel, which carbonyl nickel is characterized by high purity. Therefore, so carbonyl nickel will not be contaminated with iron or with other impurities resulting from iron carbonyl co-decomposition,nickel carbonyl must be separated from iron carbonyl. The separation of carbonyls of iron and nickel can be effected by chemical processes, but often the cost of such chemical processes far outweighs their results. In industrial practice, nickel carbonyl is most frequently separated from iron carbonyl by fractional distillation. Since fractional distillation depends upon one component in a solution of two or more components having a vapor pressure greater than the vapor pressure of the other components and since the vapor pressure of the component being volatilized is directly proportional to its concentration in the solution from which it is being distilled, care is exercised in the industrial treatment of gases containing carbonyls to provide solutions concentrated in nickel carbonyl. Thus, most industrial processes for producing and separately recovering iron and nickel carbonyls are specially designed, some successfully and others not, to intermediately produce, when separating metal carbonyls from carbon monoxide, carbonyl solutions rich in nickel carbonyl to expedite the recovery of pure nickel carbonyl. In so designing such processes, highly inefficient and expensive refrigeration and compression operations are resorted to. Although attempts have been made to avoid the foregoing problems and disadvantages, none, as far as we are aware, has been entirely successful when carried into commercial practice on an industrial scale.

It has now been discovered that carbonyls of iron and/or nickel can be separated from carbon monoxide in a more efficient manner, particularly when such a process is conducted in conjunction with the recovery of nickel and iron from materials containing the same by atmospheric or pressure carbonylation techniques, without unduly affecting the separation of nickel carbonyl from iron carbonyl.

The principal object of the present invention is to separate carbonyls of nickel and/or iron from each other and from carbon monoxide.

Another object of the present invention is to minimize the inefficient steps of refrigeration and compression of gases containing carbonyls of nickel and/or iron in order to recover the carbonyls as liquids.

Other objects and advantages will become apparent from the following description taken in conjunction with the drawings in which:

FIG. 1 is a general flowsheet including the process in accordance with the present invention; and

FIG. 2 is a schematic diagram of carbonyl manufacture, collection, separation and decomposition operations of the process depicted in FIG. 1.

Generally speaking, the present invention involves a process for separating nickel carbonyl from other gases which are nearly insoluble in liquid iron pentacarbonyl. Gases, containing nickel carbonyl, are contacted with liquid iron pentacarbonyl containing less than about 1.0 percent nickel carbonyl to dissolve nickel carbonyl in the liquid iron pentacarbonyl, thereby separating nickel carbonyl from the gases. The liquid iron pentacarbonyl with nickel carbonyl dissolved therein is treated to separately recover pure nickel carbonyl.

Advantageously, the nickel-carbonyl-containing gases are maintained at a temperature below about 80.degree. C. and at a pressure of more than about 2 pounds per square inch gauge (psig), and the liquid iron pentacarbonyl is maintained at a temperature below about 25.degree. C., e.g., less than about 20.degree. C. and even lower, and at a pressure of more than about 2 psig. The nickel-carbonyl-containing gas and the liquid iron pentacarbonyl are advantageously maintained at a pressure between about 2 psig and 150 psig.

FIG. 1 is a general flowsheet showing the use of the process in accordance with the present invention in conjunction with the treatment of nickeliferous material to recover nickel as nickel carbonyl. A mixture of nickel and iron carbonyls in a stream of carbon monoxide is produced in step A by treating nickeliferous material (e.g., a nickeliferous lateritic ore that has been selectively reduced) with carbon monoxide. The gaseous stream produced in step A is scrubbed with liquid iron pentacarbonyl in step B to collect substantially all of the metal carbonyls and to provide a gas stream of carbon monoxide which contains only minor amounts of iron carbonyl which is recycled to step A. The solution of nickel carbonyl and iron pentacarbonyl is treated in step C to separate carbonyls with a portion of nickel carbonyl being thermally decomposed to metallic nickel in step E and a portion of iron pentacarbonyl being decomposed in step D. Appropriate amounts of purified carbon monoxide are recycled to steps A and C.

The process in accordance with the present invention will most frequently be employed to separate nickel carbonyl from carbon monoxide or gases containing large quantities of carbon monoxide. Generally, the gases being so treated will not contain oxidizing constituents, such as oxygen or chlorine, in amounts that will cause undue oxidation. Advantageously, the gases are substantially free of oxidizing constituents. Although gases containing only nickel carbonyl can be treated in accordance with the present invention, gases containing carbonyls of nickel and iron are advantageously treated since the use of liquid iron pentacarbonyl requires eventual separation of nickel from iron while gases containing only nickel carbonyl may be treated more economically by other methods. Nickel carbonyl will generally be present in the gases being treated in amounts between about 250 grams of nickel per standard cubic meter (gmsNi/Sm.sup.3) and 20 gmsNi/Sm.sup.3, i.e., between about 40 and 5percent, advantageously between about 80 gmsNi/SM.sup.3 and 120 gmsNi/SM.sup.3, i.e., about 18and 25percent. Higher and lower concentrations of nickel carbonyl can be treated but concentrations in the foregoing ranges are most effectively treated. Likewise, the gases will generally contain between about 250 grams of iron per standard cubic meter (gms Fe/SM.sup.3) and 20 gmsFe/SM.sup.3, i.e., about 45 and 5percent iron pentacarbonyl. It will be noted that all compositions given herein are on a weight basis unless otherwise expressly stated. Volumetric units are on a standard basis of 15.5.degree. C. and 14.7 pounds per square inch absolute (psia).

Both nickel and iron carbonyls are highly volatile so that in order to effect an acceptable separation of the carbonyls from the other gases the temperatures of the gases being treated and of the liquid iron pentacarbonyl must be carefully controlled. For example, if only a small quantity of liquid iron pentacarbonyl is employed and the temperature and the volume of the gases being treated are high and large, it is possible that liquid iron pentacarbonyl will be volatilized rather than scrubbing the carbonyls from the gases being treated. Thus, for the sake of efficiency and control, the liquid iron pentacarbonyl preferably is maintained at a temperature below about 5.degree. C., advantageously at a temperature lower than about minus 10.degree. C., while the gases being treated are maintained at a temperature below about 80.degree. C., e.g., about minus 10.degree. C. or lower. When the nickel carbonyl-containing gas is derived from extractive metallurgical operations which produce gases rich in nickel carbonyl,e.g.,up to 250 gmsNi/SM.sup.3, cooling the effluent gases to below about 5.degree. C. by passage through heat exchangers is highly advantageous in that a portion of the gaseous nickel carbonyl is thereby condensed as liquid from the gases and smaller amounts of iron pentacarbonyl can be employed for scrubbing in a more efficient manner. The liquid iron pentacarbonyl and the gases being treated can be maintained at even lower temperatures with the absolute minimum being that temperature at which iron pentacarbonyl, nickel carbonyl and solutions thereof begin to freeze or solidify. Although such lower temperatures provide a better separation of the carbonyls from other gases, at temperatures substantially below minus 10.degree. C. the overall efficiency of the process begins to suffer.

Overall efficiency of the process can be increased by subjecting both the gases being treated and the liquid iron pentacarbonyl to superatmospheric pressures, i.e., pressures of at least about 30 psig preferably are employed to increase processing efficiency. The greater efficiencies realized when conducting the process at superatmospheric pressures can, at least in part, be attributed to the lower volumes of gases that have to be treated. Cooling of the gases being treated is also rendered more efficient at superatmospheric pressures since (1) smaller volumes of gases must be cooled, (2) the thermal conductivity of compressed gases is greater and (3) the gases do not have to be refrigerated to such low temperatures as when superatmospheric pressures are not employed. For reasons of efficiency and economy, the liquid iron pentacarbonyl and the gases being treated are maintained at superatmospheric pressures between about 80 psig and 100 psig. Lower pressures can be employed, but the process is less efficient; higher pressures are less economical but they too can be employed.

Another important aspect of the present invention is the concentration of nickel carbonyl in liquid iron pentacarbonyl. Nickel carbonyl boils at 42.5.degree. C. and has a vapor pressure substantially higher than iron pentacarbonyl at all temperatures employed in the present invention. Therefore, to insure recovery of at least about 99percent of the nickel carbonyl in the gases being scrubbed, fresh or recycled liquid iron pentacarbonyl employed for scrubbing should not contain more than about 0.01 percent nickel carbonyl. When liquid pentacarbonyl has dissolved sufficient nickel carbonyl to have a concentration thereof of up to about 25percent e.g., about 10percent or even less, the liquid iron pentacarbonyl is removed and treated to separately recover nickel tetracarbonyl and iron pentacarbonyl.

Gases containing metal carbonyls and liquid iron pentacarbonyl are contacted in any manner which provides good gas-liquid contact. For example, a bath of liquid iron pentacarbonyl can be established and gases containing metal carbonyls passed therethrough to collect the metal carbonyls. When the latter procedure is employed, countercurrent principles are advantageously employed. Fresh or recycled iron pentacarbonyl containing less than about 0.01 percent nickel carbonyl is continuously added to the top of the bath and is recovered from the bottom of the bath containing up to about 25percent nickel carbonyl. In practice, it is advantageous to use a packed or tray-type column with cold iron pentacarbonyl being continuously added to the top and the gas to be treated being precooled and added at the bottom. Provision can be made for intercoolers between trays to remove the heat released by the condensation of the metal carbonyls.

When liquid iron pentacarbonyl dissolves sufficient nickel carbonyl to contain between about 5 and 25percent thereof, the iron pentacarbonyl is removed from the absorber and is heated to fractionally distill nickel carbonyl. The solution of nickel and iron carbonyls is preheated to a temperature between about 10.degree. C. and 25.degree. C. under pressures between about 2 and 10 psig. Nickel carbonyl is distilled in a distillation column or tower from the preheated solution of carbonyls with a carrier gas, such as carbon monoxide, at a temperature below the boiling point of iron pentacarbonyl, e.g., 85.degree. C. The carbonyl solution is maintained at temperatures between about 0.degree.and 85.degree. C.; and, more specifically, when a distillation column is employed so that the carbonyl solution at the top of the column is substantially pure nickel carbonyl and the bottom of the column is substantially pure iron pentacarbonyl, the top of the column is maintained at the lower end of the temperature range and the bottom of the column is maintained at the higher part of the temperature range. Again, any multi-stage apparatus and mode of operation which provides good gas-liquid contact between the carrier gas and the liquid carbonyls can be employed. Fractional distillation is conducted in such a manner that the carbonyl vapors and carrier gas are progressively cooled and passed through carbonyl solutions progressively enriched in nickel carbonyl so that any gaseous iron pentacarbonyl is dissolved in the cooler carbonyl solutions enriched with nickel and the carrier gas vaporizes progressively more nickel carbonyl by passing through carbonyl solutions enriched with nickel carbonyl. The distilled nickel carbonyl can be heated to decompose and provide a purified nickel product. The nickel carbonyl-depleted liquid iron pentacarbonyl can be cooled and transferred to the absorber for further use. If the gas being purified also contains iron pentacarbonyl, some of the nickel carbonyl-depleted liquid iron pentacarbonyl can be bled off and decomposed to iron product in order to maintain a constant circulating load of liquid iron pentacarbonyl.

The present invention is best carried into practice as shown in FIG. 2. Referring now to FIG. 2 which is a schematic diagram showing in greater detail operations depicted in FIG. 1, nickeliferous material is carbonylated at A to produce a mixture of nickel and iron carbonyls which is collected at B, separated at C and then thermally decomposed into iron and nickel metals at D and E.

The gaseous stream of metal carbonyls in carbon monoxide exiting from apparatus 1 is passed through conduit 2 to heat exchanger 3 to partially cool the carbonyl-containing stream and to preheat stripped carbon monoxide before it is reintroduced into apparatus 1 via conduit 4. The partially cooled carbonyl-containing stream is then passed through pipe 5 to glycol-cooled heat exchanger 6 to further cool the gaseous stream. Portions of the carbonyls passing through heat exchangers 3 and 6 are condensed to the liquid state which condensed carbonyls are sent to carbonyl separation unit 7 via conduit 8.

The cooled carbonyl-containing stream exiting from heat exchanger 6 is then fed to carbonyl absorption unit 9 to strip substantially all the nickel carbonyl from the carbon monoxide stream. Carbonyl absorption unit 9 can be either a packed column or a tray-type column which may have separate coolers below all or some of the trays to compensate for the heat released by condensation of the carbonyls. Liquid iron pentacarbonyl at a temperature below about 5.degree. C., e.g., below about 0.degree. C. or even below about minus 10.degree. C., is fed to the top of carbonyl absorption unit 9 through pipe 10 while the carbon monoxide stream to be treated is fed to the bottom of the unit whereby countercurrent flow between the carbon monoxide stream and the liquid iron pentacarbonyl is established. Additional liquid iron pentacarbonyl containing dissolved nickel carbonyl from a secondary carbonyl absorption unit 11 is fed via conduit 12 to absorption tower 9 to establish a reservoir of liquid iron pentacarbonyl having dissolved therein controlled amounts of nickel carbonyl. Liquid iron pentacarbonyl collected in a reservoir of carbonyl absorption unit 9 is conveyed to carbonyl separation unit 7 through pipe 13. The carbon monoxide stream scrubbed of its nickel carbonyl content and having its iron carbonyl content lowered to less than about 10 grams of iron per standard cubic meter, e.g., about 5 grams of iron per standard cubic meter or lower, is passed through heat exchanger 3 via pipe 14 for recycle to apparatus 1. In heat exchanger 3, the stripped carbon monoxide is heated and then recompressed before being reintroduced into apparatus 1 along with makeup carbon monoxide via pipe 15 from gas storage.

The solution of liquid carbonyls condensed from the gaseous phase in heat exchangers 3 and 6 and the solution of carbonyls from carbonyl absorption unit 9 are conveyed to carbonyl separation unit 7 wherein nickel carbonyl is separated from liquid iron pentacarbonyl. Carbonyl separation unit 7 is essentially a conventional distillation column and is constructed to provide good gas-liquid contact. Advantageously, carbonyl separation unit 7 is either a packed or a tray-type column. To provide efficient operation of carbonyl separation unit 7, the solution of carbonyls condensed in heat exchangers 3 and 6 is introduced at a higher level than is the solution of carbonyls from carbonyl absorption unit 9 since the solutions of carbonyls from heat exchangers 3 and 6 are more enriched in nickel than is the solution from carbonyl absorption unit 9. In actual practice, the solutions of carbonyls from heat exchangers 3 and 6 are passed through conduits 8 to heat exchanger 16 to cool liquid iron pentacarbonyl which is ultimately transferred to carbonyl absorption unit 9 and to partially preheat the solutions of liquid carbonyls prior to distillation in step C. The nickel-rich carbonyl solution from heat exchanger 16 is then conducted to carbonyl separation unit 7 via pipe 17 and is introduced to the carbonyl separation unit at a higher elevation as schematically shown in FIG. 2. The solution of carbonyls from carbonyl absorption unit 9 is conducted to carbonyl separation unit 7 via pipes 13 to heat exchanger 18 and pipe 19. Substantially pure liquid iron pentacarbonyl is withdrawn from carbonyl separation unit 7 by pipe 20 and is passed through independently heated heat exchanger 21.

Carbon monoxide from storage is passed through heat exchanger 21 via pipe 22 to produce a gaseous stream of carbon monoxide and substantially pure iron pentacarbonyl at a temperature of 85.degree. C. which gaseous mixture is reintroduced to carbonyl separation unit 7 by conduit 23. Unvaporized substantially pure iron pentacarbonyl is transferred from heat exchanger 21 to heat exchangers 16 and 18 by pipes 24. Sufficient liquid iron pentacarbonyl from heat exchangers 16 and 18 is conveyed by conduit 25 to glycol-cooled heat exchanger 26 to be cooled to a temperature below about 10.degree. C. before being divided into streams in conduits 27 for recycle to carbonyl absorption unit 9 and via heat exchanger 28 to secondary carbonyl absorption unit 11. The remaining liquid iron pentacarbonyl from heat exchangers 16 and 18 is sent to the iron carbonyl decomposing unit 29 via pipe 30.

Carbon monoxide containing very low amounts of iron pentacarbonyl and containing nickel carbonyl in amounts between about 700 to 800 grams of nickel per standard cubic meter is withdrawn from the carbonyl separation unit 7 through pipe 31. The carbon monoxide stream containing nickel carbonyl is conveyed to glycol-cooled heat exchanger 32 to produce a gas stream containing a controlled amount of nickel carbonyl, e.g., about 500 grams of nickel per standard cubic meter, and to condense liquid nickel carbonyl from the stream. The nickel carbonyl-containing carbon monoxide is split into two streams 33 and 34. Stream 33 is conveyed to glycol-cooled heat exchanger 35 to condense the major portion of the nickel carbonyl while stream 34 is transferred to nickel carbonyl decomposing unit 36 to be described hereinafter. Nickel carbonyl from heat exchangers 32 and 35 is transferred to liquid nickel carbonyl surge tank 37 through pipes 38. Liquid nickel carbonyl is introduced into the top of carbonyl separation unit 7 via conduit 39 to provide an effluent carbon monoxide stream substantially free of iron pentacarbonyl.

Carbon monoxide from heat exchanger 35, which contains residual amounts of nickel carbonyl, after passing through nickel carbonyl surge tank 37 is transferred by pipe 40 to secondary carbonyl absorption unit 11 which, like carbonyl absorption unit 9, can be either a packed or a tray-type column as long as good gas-liquid contact is achieved and which is fitted with glycol-cooled heat exchangers 41 and 42 to remove the heat of condensation of gaseous carbonyl. Carbon monoxide stream 40 containing residual amounts of nickel carbonyl is introduced to the bottom of secondary absorption unit 11 while cold iron pentacarbonyl from glycol-cooled heat exchangers 26 and 28 via pipes 27 and 43 is introduced to the top of column 11. Scrubbed carbon monoxide from absorption unit 11 is sent by pipes 44 to gas storage. Nickel carbonyl-containing stream 34 from glycol-cooled heat exchanger 32 is introduced into nickel carbonyl decomposer 36. Carbon monoxide is removed from decomposer 36 and transferred to gas storage by conduit 45. The nickel product resulting from the decomposition of nickel carbonyl is removed from decomposer 36 at outlet 46.

A stream of liquid iron carbonyl, from conduit 30 is conveyed to the iron carbonyl decomposer unit 29. Carbon monoxide is removed from the iron carbonyl decomposer 29 and transferred to gas storage by conduit 47. The iron product resulting from the decomposition of iron carbonyl is removed from decomposer 29 via outlet 48.

In order to give those skilled in the art a better understanding of the present invention, the following illustrative examples are given:

EXAMPLE I

A nickeliferous oxide ore was selectively reduced and then carbonylated at a temperature of 55.degree. C. and under a pressure of 2 psig to provide an off-gas of carbon monoxide containing 16.9 percent nickel carbonyl and 20.3 percent iron carbonyl at a temperature of 25.degree. C. and at a pressure of 2 psig. The carbon monoxide containing the metal carbonyls was cooled to minus 12.degree. C. and was then transferred to a glycol-cooled packed tower absorber at 2 psig where nickel carbonyl vapor was scrubbed out at minus 12.degree. C. by liquid iron pentacarbonyl containing less than 0.01 percent nickel carbonyl. This treatment lowered the nickel and iron carbonyl content of the carbon monoxide to 0.03 and 3.2 percent respectively. Before equilibrium between the gas and liquid carbonyls was reached, liquid iron pentacarbonyl containing 11.6 percent nickel carbonyl, by volume, was withdrawn to separately recover the carbonyls. The mixed liquid nickel and iron carbonyls withdrawn from the absorber were delivered, via a heat exchanger and pump, to a distilation column at a temperature of 28.degree. C. and at a pressure of 5 psig. Carbon monoxide at a temperature of 60.degree. C. was passed therethrough at a rate such as to provide an off-gas containing 72.5 percent nickel carbonyl and only 0.008 percent iron pentacarbonyl at 16.degree. C. and 2 psig. The remaining liquid iron pentacarbonyl was at a temperature of 82.degree. C. and contained 0.008 percent nickel carbonyl. Part of the remaining iron carbonyl was decomposed and the remainder was cooled to minus 12.degree. C. before being returned to the glycol-cooled absorber.

EXAMPLE II

A nickeliferous oxide ore was selectively reduced and then carbonylated at a temperature of 65.degree. C. and under a pressure of 7 atmospheres to provide an off-gas of carbon monoxide containing 9.8 percent nickel carbonyl and 11.5 percent iron carbonyl at a temperature of 45.degree. C. and at a pressure of 45 psig. The carbon monoxide containing the metal carbonyls was cooled to 2.degree. C. and then transferred to a glycol-cooled packed tower absorber at 44 psig where nickel carbonyl vapor was scrubbed out at 2.degree. C. by liquid iron pentacarbonyl containing less than 0.9 percent nickel carbonyl. This treatment lowered the nickel and iron carbonyl contents of the carbon monoxide to 0.57 and 2.1 percent respectively. Before equilibrium between the gas and liquid carbonyls was reached, liquid iron pentacarbonyl containing 8.6 percent nickel carbonyl, by volume, was withdrawn to separately recover the carbonyls. The mixed liquid nickel and iron carbonyls withdrawn from the absorber were delivered, via a heat exchanger, to a distillation column at a temperature of 34.degree. C. and at a pressure of 5 psig. Carbon monoxide at a temperature of 60.degree. C. was passed therethrough at a rate such as to provide an off-gas containing 54.9 percent nickel carbonyl and only 0.01 percent iron pentacarbonyl at 5.degree. C. and 2 psig. The remaining liquid iron pentacarbonyl was at a temperature of 82.degree. C. and contained 0.84 percent nickel carbonyl. Part of the remaining iron carbonyl was decomposed and the remainder was cooled to 2.degree. C. before being returned to the glycol-cooled absorber.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the scope and purview of the invention and appended claims.

Claims

We claim:

1. A process for separating nickel carbonyl from other gases that are substantially insoluble in liquid iron pentacarbonyl which comprises: contacting a gaseous stream containing nickel carbonyl with liquid iron carbonyl containing less than about 1 percent nickel carbonyl to scrub nickel carbonyl from the gaseous stream and to produce a carbonyl solution of nickel carbonyl dissolved in liquid iron pentacarbonyl, and treating the carbonyl solution to recover substantially pure nickel carbonyl.

2. The process as described in claim 1 wherein the dissolved nickel carbonyl is recovered from the carbonyl solution by fractional distillation and liquid iron pentacarbonyl is recycled to scrub more nickel carbonyl from the gaseous stream.

3. A process for separating nickel carbonyl from other gases that are substantially insoluble in liquid iron pentacarbonyl which comprises: contacting a gaseous stream containing nickel carbonyl with liquid iron pentacarbonyl containing less than about 1 percent nickel carbonyl to scrub nickel carbonyl from the gaseous stream and to produce a carbonyl solution of nickel carbonyl dissolved in liquid iron pentacarbonyl, and fractionally distilling substantially pure nickel carbonyl from the carbonyl solution by passing a stream of carbon monoxide through the carbonyl solution.

4. The process as described in claim 3 wherein the gaseous stream contains nickel carbonyl, iron pentacarbonyl and carbon monoxide.

5. The process as described in claim 3 wherein the gaseous stream is maintained at a temperature below about 80.degree. C. and at a pressure of more than about 2 psig.

6. The process as described in claim 5 wherein the liquid iron pentacarbonyl is maintained at a temperature below about 25.degree. C. and at a pressure of more than about 2 psig.

7. The process as described in claim 6 wherein the liquid iron pentacarbonyl is maintained at a temperature below about 20.degree. C.

8. The process as described in claim 7 wherein the liquid iron pentacarbonyl is maintained at a temperature below about 5.degree. C.

9. The process as described in claim 8 wherein the liquid iron pentacarbonyl is maintained at a pressure between about 2 and 150 psig.

10. The process as described in claim 9 wherein the gaseous stream is maintained at a pressure between about 2 and 150 psig.

11. The process as described in claim 3 wherein the gaseous stream contains between about 20 grams per cubic meter and 250 grams per cubic meter of nickel as nickel carbonyl and between about 20 grams per cubic meter and 250 grams per cubic meter of iron as iron pentacarbonyl.

12. The process as described in claim 11 wherein the liquid iron pentacarbonyl is maintained at a temperature below about minus 10.degree. C.

13. The process as described in claim 3 wherein fractional distillation is conducted in a column which contains a solution of carbonyls that is substantially pure liquid nickel carbonyl at the top of the column and substantially pure liquid iron pentacarbonyl at the bottom of the column and is maintained at a temperature range between about 0.degree. C. and 85.degree. C. with the solution at the top of the column being maintained at the lower end of the temperature range and the solution at the bottom of the column being maintained at the upper end of the temperature range and carbon monoxide is introduced at the bottom of the column so that the carbon monoxide and carbonyl vapors are progressively cooled to condense iron pentacarbonyl therefrom and to provide a gaseous stream of carbon monoxide containing substantially pure nickel carbonyl at the top of the column.