Production of finely divided metals

Application filed March 4, 1930, Serial No.

The present invention relates to the production of finely divided medals and more particularly to improvements in that process of thermally decomposing metal carbonyls in g which the metal carbonyl is thermally decomposed by introducing it, diluted, if desired, with inert gases, into a heated vessel in such a manner that the decomposition takes place substantially in the hot free space of the ves10 sle instead of by contact with the hot walls of the vessel. In the said process the temperature of the walls of the vessel should not exceed a certain value since otherwise a substantial decomposition of carbon monoxide and in 15 consequence a considerable contamination of the metal powder bv deposited carbon may take place. It has therefore been recommended to effect the decomposition of the metal carbonyl either at between about 100° 20 and 400° C. or at about 900° C. or more.

From an economical point of view, it is advisable to work in the lower of the temperature ranges indicated, and the present invention is concerned more particularly with op25 erations carried out in this range of temperatures. Since the decomposition of metal carbonyls is an endothermic reaction, it is essential that sufficient heat is supplied to the carbonyl vapor to be decomposed.

In the said method of working there is always the risk that due to an insufficient supply of heat more or less substantial amounts of the metal produced are deposited in a compact form on the hot parts of the decomposi35 tion apparatus.

We have found that this undesirable formation of compact metal during the decomposition of the carbonyl is practically avoided and a metal powder which is uniform in <0· composition and in the size of its particles, is obtained by keeping the temperature of those parts of the apparatus in which the decomposition of the carbonyl takes place substantially higher than the temperature at which 45 the decomposition of the metal carbonyl is actually carried out in the free space of the vessel.

We have observed that deposits are very readily formed on the parts of the apparatus 50 which come into contact with the carbonyl

433,170,. and In Germany March 5, 1929.

and these deposits quickly increase in size when the temperature of the said hot parts lies directly above the temperature in the free space of the vessel, but that on the contrary these deposits are no longer formed, 55 but a powdery or spongy finely divided metal is formed when the temperature of the hot parts of the apparatus is substantially higher.

The amount to which the temperature 60 must be raised in order to avoid the undesirable deposits is dependent on the decomposition conditions for the time being, for example on the dimensions and shape of the decomposition apparatus, on the size of the 65 throughput of carbonyl and the like. It is preferable to provide the hot parts of the decomposition apparatus with smooth highly polished surfaces and to avoid any accumulation of metal powder in the hot chamber, 70 for example by continually or temporarily inclining the parts of the apparatus concerned.

Another way of facilitating the transfer of heat from the hot walls of the vessel to the 75 carbonyl vapor to be decomposed consists in subjecting the substances present in the decomposition chamber to a whirling or like motion.

This whirling may be effected with a special 80 apparatus built into the decomposition chamber, for example with a stirrer, which is preferably cooled from the inside in order to avoid any separation of the metal thereon. Also by suitably introducing a stream of gas 85 or vapor, which if desired may be heated, or also by tangential introduction of the carbonyl into the decomposition chamber, a mixing of the hot layer of gas and vapor in the neighbourhood of the walls with the C3 colder layers in the middle of the chamber is effected, so that the equalization of heat is accelerated and a more rapid removal of heat from the heated walls to the interior of the furnace is rendered possible. C j

In this manner of working there is no risk of solid coherent deposits of metal being formed on the walls of the decomposition vessel, provided the intensity of the whirling, stirring or like motion is suitably ad- J.:>

1,836,732

CO justed. This is probably due to the fact that there exists an extremely thin layer of carbon monoxide on the hot walls which, when proper working conditions are employed, prevents the carbonyl from coming into direct contact with the walls. Only by whirling or stirring too intensely, the said layer or carbon monoxide is destroyed, and only then the carbonyl can come into direct contact with the walls and form a coherent deposit of metal. Therefore, the whirling or stirring should be so thorough as to effect a good equalization of heat within the vessel, but not so strong as to result in a destruction of the protective layer of carbon monoxide.

Even when employing the aforesaid measures for facilitating the supply of heat to the carbonyl to be decomposed, only a certain amount of carbonyl can be decomposed per unit of time in a furnace of a given size. The same disadvantage is inherent with another known process for the decomposition of metal carbonyls to produce metal powders which consists in bringing the said carbonyls into contact with hot inert gases. This method of working has the additional disadvantage that large amounts of hot gases are to be employed in consequence of the low specific heat of gases in order to supply the necessary heat and accordingly the metals produced are distributed in large amounts of gases and difficulties arise in the separation of the metals.

We have now found that the efficiency of the process in which the metal carbonyl is thermally decomposed by introducing it, diluted, if desired, with inert gases, into a heated vessel in such a manner that the decomposition takes place substantially in the hot free space of the vessel instead of by contact with the hot walls of the vessel can be substantially increased without the aforesaid disadvantages by supplying the heat necessary for the decomposition partly from outside through the walls of the decomposition vessel and partly by means of hot gases or vapors or finely divided solids supplied to the interior of the vessel. This manner of working renders it possible on the one hand to avoid overheating the walls of the furnace and on the other hand to avoid too great a dilution of the carbonyl with inert gases and also to increase the yield of metal powder obtained per unit of time and per unit of volume of the decomposition chamber.

The walls of the vessel may be heated in any known manner for example by electrical resistance heating or by means of a hollow jacket surrounding the decomposition chamber through which heating gases circulate. As the source of heat in the interior, introduced hot gases which are preferably employed in a cycle may be employed as for exi ample heated carbon monoxide. Other heated substances such for example as liquid or solid substances in a finely divided state may be introduced into the decomposition chamber, and in this connection, substances with the greatest possible specific heat are the most suitable. As finely divided substances introduced into the interior in a heated condition preferably such are employed as may remain in the metal powder produced, for example the same metal in a finely divided state as is produced in the process, or those metals with which the metal produced by the decomposition of its metal carbonyl is afterwards to be alloyed. Other solid substances in a fine division which can afterwards readily be removed, for example by dilution, may also be used. Moreover by the introduction at a suitable place in the vessel of an amount of air, or better still of oxygen, insufficient for the complete oxidation of the substances to be decomposed or of the substances formed by the decomposition, an inverted flame may be used as the source of heat. Furthermore the metal particles themselves which are formed by the decomposition may be employed in many cases for supplying heat in the interior, this being effected by heating them by electrical induction.

The introduction of the vaporous or atomized carbonyl preferably takes place in the centre of the upper end of the decomposition chamber, which latter preferably has the form of a vertical cylinder.

In this process a certain whirling or stirring effect may be obtained in some cases due to the supply of heat in the interior, but it may be advantageous to employ additional whirling or stirring in the manner hereinbefore described, or to heat the walls of the vessel to a temperature substantially higher than the temperature in the free space of the vessel, or to employ both, whirling or stirring and strong heating of the walls of the vessel.

Of course, the most favorable conditions of working depend on the particular circumstances in each case, for example on the capacity and heating surface of the decomposition vessel, on the amount of carbonyl to be converted per unit of time, and on the degree of fineness and the carbon content, the product should have. They cannot, therefore, he predicted in a precise manner, but everybody skilled in the art will be able to apply the principles set forth in the foregoing to the best economy and advantage.

The process, according to the present invention is particularly valuable for the production of finely divided iron by the thermal decomposition of iron carbonyl.

An apparatus suitable for carrying out the process according to the present invention is diagrammatically illustrated in vertical section in the accompanying drawing. In the

100 ΐη-5

110

115

120

125 uo

1,836,732 said apparatus liquid iron carbonyl is supplied from a storage vessel A into a heated vessel B in which the carbonyl is vaporized. % The resulting carbonyl vapours are intro₅ duced by means of a pipe into the decomposition vessel H at the top thereof. The said decomposition vessel is constructed of iron and is externally provided with electric heating means J by means of which the tempera₁₀ ture in the interior of the vessel is maintained between 250° and 300° C. In the said vessel the iron carbonyl is decomposed into finely divided iron and carbon monoxide. The iron is carried along with the gas and ₁₅ separated in the chambers Ki, K₂ and K₃ from which it can be withdrawn by means of worm conveyors C arranged in the lower part thereof. A portion of the carbon monoxide set free during the decomposition is in2o troduced by means of the circulation pump M into an electric heating device N and is then returned into the decomposition vessel H by means of a pipe 0. In order to supply the hot carbon monoxide uniformly to the inter25 rior of the vessel H, the pipe O is connected with an annular tube P arranged in the upper part of the decomposition vessel H and provided with a large number of fine holes. The amount of cabon monoxide which is maintained in circulation can be controlled by means of the pump M and its temperature by means of the heating device N. The remainder of the carbon monoxide which is not circulated through the apparatus is with35 drawn by way of a pipe L.