CA2098774C

CA2098774C - Chemical reaction

Info

Publication number: CA2098774C
Application number: CA002098774A
Authority: CA
Inventors: Rodney C. Baxter; Gerhard J. Mostert; Lynette D. Smillie
Original assignee: Council for Scientific and Industrial Research CSIR
Current assignee: Council for Scientific and Industrial Research CSIR
Priority date: 1992-06-19
Filing date: 1993-06-18
Publication date: 2000-01-11
Anticipated expiration: 2013-06-18
Also published as: GB2267845A; NZ247887A; ZA934078B; GB2267845B; AU4132493A; GB9312550D0; AU658305B2; CA2098774A1

Abstract

ABSTRACT

A method of carrying out an endothermic chemical reaction, whereby a solid reagent is reacted with a gaseous reagent at an elevated temperature of at least 70°C, is described. The method involves mixing the solid reagent with a microwave-absorbing material to form a mixture containing 1-80%
by mass of the microwave absorbing material. The mixture is then heated to the reaction temperature in the presence of the gaseous reagent by directing microwave radiation at and into the mixture.

Description

This invention relates to a method of carrying out an endothermic chemical reaction whereby a solid reagent is reacted with a gaseous reagent at an elevated temperature. More particularly it relates to a method suitable for reacting a solid mineral reagent with a gaseous reagent at a reaction temperature of 700°C or more, the method being suitable for use in the nitriding of titanium-containing minerals.
According to the invention, there is provided a method of carrying out an endothermic chemical reaction whereby a solid reagent in the form of a mineral ore is reacted with a gaseous reagent at an elevated reaction temperature of at least 700°C, the method comprising the steps of (i) mixing the solid reagent with a microwave-absorbing material, the microwave-absorbing material being effective above 700°C and forming 1 - 80% by mass of the mixture;
(ii) consolidating the mixture by subjecting it to a pressure of at least 2 MPa; and (iii) heating the consolidated mixture to the reaction temperature in the presence of the gaseous reagent, the heating comprising directing microwave radiation at and into the mixture, to supply heat to the mixture.
The mineral ore may be a mineral ore containing metal values such as titanium values which are reacted with the gaseous reagent. The consolidating may be by pelletizing the mixture.

Mixing the reagent with the microwave absorber may be in proportions such that the micr owave-absorbing material forms 20 - 60 % by mass of the mixture. Suitable microwave-absorbing materials include those which provide a carbonaceous residue upon heating to above 700°C, such as rubbers or waxes, which can act as binders in the consolidation, or carbon itself, such as graphite; mineral microwave absorbers such as magnetite;
ceramic microwave absorbers such as refractory ceramics, eg ferrite; and suitable metals. Routine experimentation will be required to select, from known microwave absorbers, both the particular microwave absorber suitable to a particular heating application, and the proportion thereof to be used, to obtain optimum or at least acceptable results. Thus, in particular, the mixing may be such that the microwave-absorbing material forms 20 -60 % by mass of the mixture, the microwave-absorbing material being selected from carbonaceous microwave absorbers, mineral microwave absorbers, ceramic microwave absorbers and metallic microwave absorbers and mixtures thereof.
Preferably, the solid reagent and the microwave absorber each have a particle size of 1 ~cm (or less) - 10 mm, preferably 4U - S00 ~m and more preferably 50 - 7~ ,~cm.
~ When the consolidation is by pelletizing, the pellets formed may have a diameter and length respectively of 2 - 20 mm, preferably 4 - 10 mm; the pelletizing may involve a pressure applied to the pellets of 2 - 10 MPa, preferably ~. - 8 MPa; and pellets will be formed of a mass typically of 0,7~ -20 g, usually S - 15 g, depending on the density of the materials forming the 2~ mixture. Thus, in a convenient embodiment of the method, the solid reagent and the microwave-absorbing material rnay each have, prior to the mixing thereof, a particle size of at most 10 mm, the method including the step, before the heating, of consolidating the mixture by pelletizing it to form pellets having a diameter and a length each in the range 2-20 mm and having a mass of 0,75-20g.

~i3~~'d"d The microwave radiation used rnay have a frequency in the range 0,9 - 3,0 GHz, typically 2,0 - 3,0 GHz and preferably 2,45 - 2,55 GHz. 'Ifie intensity of the microwave radiation (kW) used will depend on the rate of heating required, taking into account economic considerations, and routine experimentation can be employed to determine an optimum or at least acceptable combination of the microwave wavelength and power and intensity. It is expected that, usually, the microwave radiation will have a frequency of 0,9-3,OGHz, the heating being to a temperature of 700-1600°C.
Heating the mixture may be to a maximum temperature of 70 -1800°C, preferably 700 - 1600°C and more preferably 1150 -1600°C, eg 1250 - 1350°C. Heating may be by both microwave heating and radiative/convectional heating, for example by using radiative/convectional heating to heat the mixture initially, eg up to a temperature of 800 -1000°C
and using microwave heating thereafter finally to heat the mi.Yture to its reaction temperature. Thus, the heating may initially be by a method of heating selected from radiative heating, convectional heating and mixtures of radiative and convectional heating, from ambient temperature up to a temperature of 800 - 1000°C, the microwave radiation then being used to contribute to the heating until the reaction temperature is reached.
The heating may take place in a suitable reactor, preferably one which has a non-stationary load, such as a rotary furnace or a fluidized bed reactor. Instead, the mixture, eg in the form of pellets, can be moved through a reaction zone subjected to microwave irradiation on a suitable conveyor, or it can be moved through such zone as part of a vertically or horizontally moving bed. Naturally, the reactor or reaction zone preferably has microwave-reflective walls of the type found in microwave ovens.
Accordingly, the heating may take place in the interior of a reactor having microwave-reflective walls, the mixture being moved in the reactor during the heating.

The heating may be carried out for an extended period, to maintain the reagent at the reaction temperature, until all or an acceptable proportion of the reagent has reacted. This period can be determined by routine experimentation.
When the microwave absorber and/or the reagent/reagents are susceptible to unwanted oxygenation, the heating may take place in an inert or non-oxidizing atmosphere or environment, eg a nitrogen environment, which may be obtained by supplying ammonia or ammonium-containing compounds to said environment, or nitrogen gas, eg from a nitrogen/oxygen plant. Thus, the reaction may be caused to take place in a non-oxidizing environment.
While the nitriding of titanium is of particular interest to the Applicant, the invention extends to chemical reactions in general wherein the solid reagent comprises a transition metal which is reacted with the gaseous reagent analogously to titanium, eg in the nitriding thereof. The Applicant has evidence that nitriding and carbonitriding of vanadium and zirconium can proceed in analogous fashion to the nitriding and carbonitriding of titanium as described herein, and, without having tested them, believes that Ni, Fe, Cr, Co, Mn, Cu, Sc and Zn can similarly be nitrided or carbonitrided.
In a particular embodiment of the invention the solid reagent may comprise a transition metal and the gaseous reagent may comprise nitrogen, the endothermic reaction comprising nitriding the transition metal in the solid reagent by means of the gaseous reagent, and the heating being to a temperature of 110-1600°C, eg 12s0 - 130°C.
In another embodiment of the invention the solid reagent may comprise a transition metal admixed with a carbonaceous material, the gaseous reagent comprising nitrogen, the endothermic reaction comprising 0~~'~~~, carbonitriding the transition metal in the solid reagent by means of the carbonaceous material and the gaseous reagent, and the heating being to a temperature of 1150 -1600°C, eg 1250 - 1350°C.
The transition metal may be selected from the group consisting of V, 5 Zr and, in particular, Ti.
Naturally, when a carbonaceous microwave absorber is used for nitriding or carbonitriding, a sufficient amount of the absorber must be used for adequate microwave absorption, for adequate consumption of any oxygen released by the solid reagent during the heating, and for the nitriding or carbonitriding reaction. As is known in the art, nitriding and carbonitriding should, as far as practicable, take place in the absence of oxygen.
The invention extends also to a reaction product whenever produced by a method as described above.
The invention will now be described, by way of example, with reference to Examples 1 to 5, Tables 1 to 3 and the accompanying diagrammatic drawings in which Figure 1 is a schematic flow diagram of a plant or installation for carrying out the method of the present invention;
Figure 2 is a schematic sectional side elevation of a microwave furnace;
Figure 3 is a schematic three-dimensional side elevation of a batch microwave unit;
Figure 4 is an X-ray diffractogram of the product of Example 5 plotting intensity in counts per second against °20; and Figure 5 is a similar plot of a computer generated X-ray diffractogram of a standard TiN sample.

Referring to Figure 1, reference numeral 10 generally indicates a flow diagram of a plant or installation for carrying out the method of the present invention. The plant 10 comprises a reagent stockpile 12 and a microwave absorber stockpile 14, which feed respectively via flow lines 16 and 18 to a mixing stage 20. The mixing stage 20 feeds via flow line 22 to a pelletizing stage 24, the pelletizing stage 24 feeding via flow line 26 to a heating stage 28 and the heating stage 28 feeding via flow line 30 to a product stockpile 32.
Referring to Figure 2, reference numeral 40 generally indicates a microwave furnace for use in the invention. The microwave furnace 40 consists of a mica-lined steel housing 42 in which a reaction chamber 44 of an alumina silicate refractory material is mounted. The reaction chamber 44 is in the form of a hollow block of the refractory material, having an upper portion and a lower portion which engage to form the chamber 44.
An cc-alumina inlet tube 46 extends through the wall of the housing 42 and through the wall of the reaction chamber 44 to allow nitrogen to be fed directly into the chamber 44. A second u-alumina tube, shown schematically at 48, extends through the wall in the housing 42 and through the wall of the reaction chamber 44, and holds a thermocouple (not shown) for measuring the temperature inside the chamber 44. The chamber 44 is, further, provided with a nitrogen outlet indicated schematically at 50 and a source of microwave radiation (not shown). The chamber 44 has a capacity of about 0,8 m3 and holds a titanium-containing pelletized charge 51 as is described in farther detail in Example 3 below.
Referring to Figure 3, a batch microwave unit is generally indicated by reference numeral 60. The unit 60 consists of a hollow cylindrical reactor 62 which has an inlet end 64 and an outlet end 66 both of which are sealed. The reactor is provided with a nitrogen inlet 68 and a thermometer 70 which extend into the reactor 62 through the inlet end 64, and a reactant gas outlet 72 extending through the outlet end 66. An oxygen sensor 74 r L

extends into the reactor 62 through the outlet end 66. The reactor 62 has a sample inlet (not shown) with a sealable lid through which a charge 76 can be introduced into the reactor 62. The reactor contains a titanium-containing palletized charge 76 as is described in further detail in Example 4 below. The unit 60 is, further, provided with a source of microwave radiation (not shown) for irradiating the charge 76.

In an initial investigation conducted by the Applicant to demonstrate the feasibility of the method of the present invention, an ilmenite sand, and a titanium-containing stag produced by Highveld Steel and Vanadium Corporation Limited were heated, using a domestic microwave oven, operated at a frequency of 2,5 GHz and having a power output of 0,7 kW.
1~ Analyses of the ilmenite and the slag are given in Table 1.

h A NALYS FS
Constituent Ilmenite Sand Slag (As Received - % (As Received -by mass) % by mass) Total Fe 37,3 -Fe203 - 3,46 Mn0 O,S2 0,69 Cr203 < 0,01 0,19 V205 0,12 1,05 Ti02 4S,S 30,50 Ca0 0,0=1 16,50 K20 0,11 0,11 P20s 0,04 Si02 1,3 20, 75 A1203 0,7 13,65 1$ Mg0 0,3 14,10 Na20 < 0,1 -Cl < 0,01 S 0,01 -Zr 550 ppm -Nb 530 ppm -ppm - parts per million In each case powder samples and pelletized samples were prepared from starting materials having a particle size of < 100fem, the pelletized samples using, as microwave absorber, coal char obtained from Rand Carbide (Proprietary) Limited and bentonite (1 - 10% by mass of < 100~cm particle size) as binder, and the powder.samples using duff coal as microwave absorber.
Three sets of powder samples were prepared in each case, one containing an amount of carbon in the coal stoichiometrically equivalent to the amount of oxygen in the reagent, another set containing twice this amount of carbon, and another set containing three times said stoichiometric amount. In the pelletized samples (pellets of length:diameter ratio of 1:1 and diameter of l mm pressed at 6 MPa) the char ~~~f "~~~~

contained twice said stoichiometric amount of carbon. Two duplicate sets of powder samples with twice the stoichiometric amount of carbon were prepared, containing respectively 2%
and 10%by mass magnetite ( < 100fcm) as well.
Samples of powder (~0 g) and pellets (100 g) were placed in small ceramic crucibles in the microwave oven and heated for 3 minutes while their temperature was monitored using a thermocouple. Results are set forth in the Tables 2 and 3.
Table 2 shows results for the ilmenite reagent and Table 3 shows results for the slag reagent.

Sample Type Carbon Sample mass Magnetite Temperature (Powder/Pellcts)(Stoichiometric(g) Addition Attained Equivalent) (%) (C) 1. Powder lx SO - 234 1~ 2. Powder 2x 50 - 170 3. Powder 2x ~0 2 306 4. Powder 2x ~0 10 150 5. Powder 3x 50 - 190 6. Pellets 2x 100 - 620 Sample Type Carbon Sample massMagnetite Temperature (Powder/Pellets)(Stoichiometric(g) Addition Attained Equivalent) (%) (C) 7. Powder lx 50 - 125 8. Powder 2x 50 - 12~

9. Powder 2x ~0 2 1 10. Powder 2x ~0 10 1~7 11. Powder 3x 50 - 120 L 12. Pellets2x I 100 , - I 700 I

From the results it is clear that pelletizing increases the heating rate, as do the addition of magnetite to the slag samples, and the lower (2%) addition of magnetite to the ilmenite samples.
These results demonstrate the feasibility of using microwave radiation of the type in question for heating titanium-containing reagents comprising ilmenite and slag, for use in the method of the invention.

With reference to Figure 1, in a generalized description of a continuous process, ilmenite sand or a titanium-containing slag as a reagent source of titanium to be nitrified is kept in the stockpile 12, and the microwave absorber is Duff coal or coal char, kept in the stockpile 14.
The reagent and absorber are respectively conveyed continuously along the flow lines 16, 18 (belt conveyors, screw conveyors or the like) to the mixing stage 20 (milling and m screening) where they are mixed and screened to provide a particulate mixture of a particle size of 53 - 75,ecm in which the reagent:absorber mass ratio is 4:3.
From the stage 20 this mixture is conveyed along flow line 22 (belt conveyor/screw conveyor) to the pelletizing stage 24 where the mixture is pelletized by application of a pressure of 6 MPa into pellets of 5 - 15 g mass, a diameter of 7 mm and a length:diameter ratio of 0,5:1- 1,5:1.
From the pelletizing stage the pellets are conveyed along flow line 26 (screw conveyor) into the heating stage 28 (rotary furnace) where the pellets are subjected to microwave radiation at 2,45 GHz at a power supply of 100 k~V. The furnace is provided with a nitrogen atmosphere and the rate of feed of pellets to the furnace is such that the pellets are heated to and maintained at a temperature of 1300°C. In the furnace titanium values in the reagent are nitrified, the reaction time being selected to provide an acceptable yield of TiN, taking economic considerations into account.
From the furnace 23 the nitrified pellets are conveyed along flow line 30 (metal belt conveyor) to the stockpile 32 where they are stockpiled and allowed to cool prior to further processing.

With reference to Figure 2, the titaniferous slag of Example 1, was milled to particle size range of less than 75 Vim. Duff coal was similarly milled to the same particle size range.

The milled slag (1 kg) was mixed with the milled Duff coal (0,35 kg) and bentonite (13 g) which acted as a binder. The mixture was then pelletised as described in Example 1 to produce pellets 51 which had ~n average diameter of about 10 mm.
The pellets 51 were charged into the reaction chamber 44 of the microwave furnace 40. The chamber 4:1 was purged with nitrogen and the pellets 51 were subjected to microwave heating to a temperature above 1300°C, ie 1300 - 1500°C. The temperature was maintained above 1300°C for about 2 hours. The furnace was then switched off and the sample was allowed to cool, the nitrogen flow being maintained until the sample was again at room temperature.
The nitrogen, carbon and titanium content of the sample were then analysed and it was found that substantially all of the titanium present in the sample had been converted to TiN.
Furnace details were as follows: Multimode applicator microwave;
operating freduency 2.45 GHz; maximum power, 5 k~V.

The procedure of Example 3 was followed, except that the pellets were prepared from a mixture containing the ilmenite of Example 1 (1 kg), Duff coal (859 g) and bentonite (~ 20 g).
After cooling, the samples were analysed as before. The conversion of Ti in the pellets to TiN was found to be essentially 100 %.

EXAMPLE S
With reference to Figure 3, a sample of the powdered titanium-containing slag of Example 1 was intimately mixed with Duff coal, in a ratio (m/m) of 100:35, and bentonite clay (1 %, m/m) and the material was consolidated as described above by pelletising to farm pellets 76.
The pellets 76 (~ 200 g) were loaded into the reactor 62 which was sealed, and was then alternately evacuated and flushed with high purity nitrogen. This cycle was repeated until the oxygen sensor 74 indicated less than 0.1 % O,. The sample was then irradiated with microwaves at a frequency of 2.4 to 2.5 G Hz, with a peak radiation intensity of 5 k Watts.
In different embodiments of the invention, the intensity of radiation employed depended on the rate of heating required.
A constant flow of nitrogen was maintained through the reactor 62 throughout the warm-up to the reaction temperature during the reaction, and during the cool down periad after the reaction. The pellets were maintained at 1300°C (the preferred reaction temperature) for 3 hours. The temperature was constantly monitored during the reaction.
The mass loss of the pellets was found to be 24.8 % of the total mass. The X-ray diffractogram of the product is given in Figure 4.
The diffractogram clearly shows, by comparison with the standard diffraction of Figure 5, that TiN was present in the sample. It was found that substantially all of the titanium in the sample was present as TiN.

~~~~ s'~-The Examples clearly demonstrate the feasibility of the present invention, at least with reference to the nitriding of titanium.

Claims

1. A method of carrying out an endothermic chemical reaction whereby a solid reagent in the form of a mineral ore is reacted with a gaseous reagent at an elevated reaction temperature of at least 700°C, the method comprising the steps of:
(i) mixing the solid reagent with a microwave-absorbing material, the microwave-absorbing material being effective above 700°C
and forming 1 - 80% by mass of the mixture;
(ii) consolidating the mixture by subjecting it to a pressure of at least 2 MPa; and (iii) heating the consolidated mixture to the reaction temperature in the presence of the gaseous reagent, the heating comprising directing microwave radiation at and into the mixture, to supply heat to the mixture.

2. A method as claimed in claim 1, in which the mixing is such that the microwave-absorbing material forms 20 - 60% by mass of the mixture. The microwave-absorbing material being selected from carbonaceous microwave absorbers, mineral microwave absorbers, ceramic microwave absorbers and metallic microwave absorbers and mixtures thereof.

3. A method as claimed in claim 1, in which the solid reagent and the microwave-absorbing material each have, prior to the mixing thereof, a particle size of at most 10 mm, the method including the step, before the heating, of consolidating the mixture by pelletizing it to form pellets having a diameter and a length each in the range of 2 - 20 mm and having a mass of 0,75 - 20g.

4. A method as claimed in claim 1, in which the microwave radiation has a frequency of 0,9 - 3,0 GHz, the heating being to a temperature of 700 - 1600°C.

5. A method as claimed in claim 1, in which the heating is initially by a method of heating selected from radiative heating, convectional heating and mixtures of radiative and convectional heating from ambient temperature up to a temperature of 800 - 1000°C, the microwave radiation then being used to contribute to the heating until the reaction temperature is reached.

6. A method as claimed in claim 1, in which the heating takes place in the interior of a reactor having microwave-reflective walls, the mixture being moved in the reactor during the heating.

7. A method as claimed in claim 1, in which the reaction is caused to take place in a non-oxidizing environment.

8. A method as claimed in claim 1, in which the solid reagent comprises a transition metal and the gaseous reagent comprises nitrogen, the endothermic reaction comprising nitriding the transition metal in the solid reagent by means of the gaseous reagent, and the heating being to a temperature of 1150 - 1600°C.

9. A method as claimed in claim 1, in which the solid reagent comprises a transition metal admixed with a carbonaceous material, and the gaseous reagent comprises nitrogen, the endothermic reaction comprising carbonitriding the transition metal in a solid reagent by means of the carbonaceous material and the gaseous reagent, and the heating being to a temperature of 1150 - 1600°C.

10. A method as claimed in claim 8, in which the transition metal is selected from the group consisting of vanadium, zirconium and titanium.

11. A method as claimed in claim 9, in which the transition metal is selected from the group consisting of vanadium, zirconium and titanium.

12. A reaction product, whenever produced by a method as claimed in claim 1.