FIELD OF THE INVENTION

The present invention relates generally to the preparation of metal nitrides of the Group IV, V and VI metals and more particularly to the preparation and recovery of metal nitride from ferrometal alloys and nickel metal alloys.

BACKGROUND OF THE INVENTION

The reaction of nitrogen with various metals and metal alloys to form nitrides and more particularly the metal nitrides of Group IV, V and VI metals is important industrially. The metal nitrides have various end use utilities as well as being important intermediates in many processes. Most particularly, the nitriding of metal alloys such as ferro metal and nickel metal alloys can form the basis for subsequent separation and recovery of the metal values. Specifically, ferroniobium, ferrovanadium, ferrotantalum and ferrosilicon alloys can be nitrided to form separate iron and metal nitride phases which can be subsequently separated to recover the specific metal values.

Exemplary of the preparation of the metal nitrides of Group IV, V and VI metals is the preparation of iron nitride and niobium nitride by the reaction of a nitrogen-containing gas with the comminuted product of a reaction between hydrogen and ferroniobium. This nitriding reaction is initiated at a temperature above about 500°C and is quite exothermic. It is possible for the temperature to rise high enough to threaten the reactor materials or to sinter the comminuted mass of hydrided ferroniobium and nitrided products. The latter condition may then require the use of special equipment to crush or otherwise comminute the sintered mass which will increase the cost of the process. Further, the sintered mass may prevent nitrogen from contacting the normally pulverulent hydrided ferroniobium thereby reducing the yield of the

reaction and making the subsequent separation of the reactants from the nitride products more difficult. On the other hand, reaction temperatures that are too low, e.g., low enough to fully protect the reactor and avoid sintering, may proceed too slowly to be economically viable.

OBJECTS OF THE INVENTION

It is, therefore, an object of the present invention to provide a system for the controlled nitridation of Group IV, V and VI metals or their ferro or nickel alloys, which system enables the operation of a high temperature reaction vessel at temperatures that avoids deleterious effects on the vessel components and the reactants and final nitrided products. It is a further objective of the present invention to maximize the rate of the previously described reaction without sintering of the reactants and products and provide a reaction environment that maximizes the yield of nitrided product while minimizing the presence of unreacted starting material.

BRIEF SUMMARY OF THE INVENTION

The foregoing objectives and advantages of the present invention are obtained by the controlled addition of pulverulent hydrided ferroniobium into a reaction vessel containing a positive pressure of up to about 2.0 psig of nitrogen gas. The reactor is maintained at a temperature of greater than about 500°C. Preferably, the reactor should be maintained at a temperature of from about 900 " C to about 1400 ^β C, and most preferentially maintained at between about 1100°C and 1400°C by the addition of heat or additional hydrided ferroniobium which reacts exothermically with the nitrogen gas which is maintained during the addition of the solid reactants in the previously described above atmospheric pressure range in the absence of oxygen.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure l is a schematic of the reactor and system for the controlled nitriding of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to Figure 1, the nitride reactor 1 is shown schematically containing nitride product 10 inside a sealed reaction vessel 9 equipped with one or more heat shields 18. The starting material, pulverized, sized, hydrided ferroniobium material is charged to feed hopper 2 which discharges to an auger drive 3 for conveying the hydride through an optional manual isolation valve 7, when open, and into the reaction vessel 9 through inlet conduit 25. Nitrogen-containing gas is introduced into the reaction vessel 9 through an inlet 4 and manual valve 5 or automatic valve 6 into the inlet conduit 25. As is described herein, the valves 5, 6 and 7 can be closed to provide a seal to the reactor during any desired stage of the reaction.

Whatever pressure is selected for operation, the pressure can be monitored by a visual pressure gauge 11 or by means of a pressure transducer 23 mounted on the conduit 21 which communicates with the interior of the reaction vessel 9. Pressure can be vented by means of automatic valve 17 or manual valve 16 and through pressure vent 20 which is in gaseous communication with conduit 21, respectively through these valves. Additionally, when it is desired to over pressure the reaction vessel with an inert gas, such as helium as shown, the inlet 15 is provided with gaseous communication with the conduit 21 through the manual valve 12 and the automatic valve 13.

With the described system, it is possible to introduce the nitrogen-containing gas into the reaction vessel 9 either intermittently or continuously during the charging of the reactor 9 with the ferroniobium hydride from the hopper 2 by means of the auger drive 3. When valves 7, 12, 13, 16, 17 and 22 are closed a desired pressure of nitrogen-containing gas can be achieved and

observed and the rate of addition of the gas controlled. This is one of the prior means of controlling the reaction rate. This mode of operation also required that the vessel be evacuated prior to the initiation of the reaction by pulling a vacuum through valve 22 with valves 5, 6, 7, 12, 3, 16 and 17 closed. Additionally, helium or any other inert gas can be alternatively introduced through valves 12 or 13 to sweep the reaction vessel of unwanted gases by alternatively pumping the vessel 9 down with a vacuum and flushing with an inert gas. Inert gas could also be introduced as a diluent for the nitrogen either with the nitrogen or by this arrangement. The reaction vessel can also be equipped with external or internal heating and/or cooling means (not shown) and the temperature monitored, for example, by means of thermocouples located in the thermocouple wells 8. The thermocouples, not shown, can be placed at any height in the wells 8 to monitor the temperature of the reactants at any preselected location, or of the vessel cavity itself. Since the reaction is exothermic, the reaction vessel 9 is provided with at least one internal heat shield 18.

In operation, during the reaction, nitrogen- containing gas can be continuously introduced at inlet 4 and with the proper valve settings be vented at vent 20 to maintain a predetermined pressure. The reaction can be run either below atmospheric pressure or at or above atmospheric pressure to maintain a predetermined time- temperature profile for the reaction.

If the system described is operated with the addition of nitrogen to the reactant already in the reactor 1, all of the previously described drawbacks can be experienced. For example, when nitriding a feedstock produced by the reaction of commercially available ferroniobium with hydrogen, the reactor is evacuated and backfilled with nitrogen. The reactor and contents are then indirectly heated. At a temperature of approximately 500 ^β C, the nitriding reaction begins and as it progresses, it seeks more nitrogen which, if supplied, will increase

the temperature very rapidly and potentially uncontrollably. It has been observed that temperatures can rise to above 1500°C very quickly, sometimes in seconds, and even if the nitrogen supply is regulated or terminated or diluted, the reaction will rapidly consume the available nitrogen and can produce a vacuum in the vessel which, at temperatures above about 800°C, can threaten the integrity of the reactor walls. Thus, there is a necessity for a more controllable reaction when commercial production quantities are sought. The foregoing reaction can, in this system, be nursed to completion by careful addition of nitrogen and vacuum control, however, the product absorbs less and less nitrogen until after approximately 24 hours there is essentially no more nitrogen uptake. After the reactor is cooled, the resulting product of such a large scale run is extremely heavily sintered and can only be split apart with a large maul and chisel, or jack hammer before milling. Also, the stainless steel liner to the reactor pot adheres to the product and is most difficult to remove from the product. It has been found to be necessary in these circumstances to remove the liner by dissolving it in acid.

The distinct disadvantages of this method are the inability to control the reaction rate; the inability to control the reaction temperature; the product produced, when sintered, is extremely hard and most difficult to crush; the product is quite difficult to remove from the reactor liner and inevitably liner material is retained in the product. The product is frequently very non-homogenous due to incomplete reaction or extreme reaction conditions within the reactor, and a long "soak" time of nitrogen is required to promote the reaction completion. These drawbacks require the addition of an inert gas such as helium or argon to stop a temperature excursion, and the use of a vacuum system to empty out the reactor of inert or diluent gases after temperature excursion and nitrogen reintroduction.

Contrary to the experiences described, the reaction can be relatively easily controlled by maintaining the reaction vessel at a temperature of from about 1100 ^β C to about 1200°C, with an atmosphere of relatively pure nitrogen maintained at a slight positive pressure, e.g., from about 0.5 psig to about 1.0 psig. When the less than 140 mesh powder of hydrided ferroniobium is fed into the reactor, it reacts almost instantaneously with the huge excess of nitrogen available and being supplied and maintained at 0.5-1.0 psig. The favorable reaction temperature along with the relatively small amount of ferroniobium powder being fed at any given time limits the heat potentially released and produces a very predictable and controllable temperature produced by the exothermic reaction. As the reactant continues to be added, the powder product while still sticking together, is not a sintered mass, but as a loosely compacted chunk of product which will easily crumble and break. At such time as the desired amount of reactant has been added, the nitrogen consumption rate drops off soon thereafter and the reactor can be cooled. After cooling, the product is easily removed from the pot. The liner is not observed to stick to the product and is actually no longer needed and can therefore be eliminated from the reactor design. Little manpower or equipment is thus required for its recovery.

The experiments conducted using the system and method of control of the present invention are hereinafter designated PK-16 and PK-17. The reactor vessel was a 12" diameter cylinder 48" deep. The reactor pot was a 10" diameter pot 10" deep, lined with "0.020" 304 stainless steel. The ferroniobium powder was fed through a 1" pipe with a 3/4" diameter "spring"-type auger turned by a variable speed motor. A 20 lb bin for ferroniobium above the auger replenished continually. No evacuation or inert gas dilution was required in the run. The reactor was maintained at 1100°C to 1200°C during the addition of the hydrided ferroniobium starting material and the nitrogen

pressure was maintained at slightly above atmospheric pressure.

The product recovered from these two experimental runs was leached as a less than 20 mesh powder in 2N HC1 as is the normal leach procedure. Analytical results before and after leaching are as indicated.

The fact of the low iron level in the leached product demonstrates thorough nitriding.

This invention has been described and claimed consistent with the best mode of practice presently understood by the inventor. The scope of the invention as defined in the following claims is, therefore, to be limited only by the applicable prior art.