FIELD OF INVENTION

THIS INVENTION relates to titanium carbonitride. The invention provides a method of preparing titanium carbonitride and extends to titanium carbonitride prepared in accordance with the method of the invention. BACKGROUND OF INVENTION

TITANIUM CARBONITRIDE (TiCN) has proved to be the material of choice for many engineering and structural applications, most especially in machining industries.

This material, TiCN, is often employed both as bulk and as hard protective coatings for many cutting tool inserts. It is an ardent competitor to tungsten carbide (WC) both in terms of micro-hardness and fracture toughness.

Titanium carbonitride with a stoichiometric phase, TiC1 -xNx, is a boundary solid solution of titanium carbide and nitride, which combines the excellent properties of both in one structure.

TiCN is often described with high hardness, high melting temperature, excellent electrical and thermal conductivity, low density, superb chemical and high temperature stability, good resistance to thermal shock, high wear and corrosion resistance properties etc. Compared with pure titanium carbide, titanium carbonitride is adjudged to have better properties such as higher high-temperature hardness, higher transverse rupture toughness, better resistance to oxidation, and much higher thermal conductivity. In addition, its finer grain size has made it a better resistant material to high-temperature creep deformation when compared to ordinary titanium carbide material.

Hence, titanium carbonitride has been chosen as a bride choice in many applications such as, in cutting tools industries, where high-speed milling, finishing and semi finishing of carbon steels are of paramount importance, thereby gradually replacing TiC-based cermets in many applications.

In order to make available this special material, various methods have been adopted to synthesize titanium carbonitride. Amongst these methods are, direct reaction between pure titanium dioxide and graphite under nitrogen at 1250°C; formation of titanium carbonitride by self-propagating high temperature synthesis (SHS) in nitrogen atmosphere; synthesis of titanium carbonitride by chemical vapour deposition in hydrogen-ammonia atmosphere; high energy ball milling at low temperature in nitrogen atmosphere.

Formerly, synthesis of titanium carbonitride by carbonitrothermic reduction of rutile has been the most sought method. However, this process requires the extraction and purification of titanium dioxide from its mineral ore, which itself is a costly process. Titanium dioxide (TiC ), which is a reduction product of ilmenite (FeTiOs), is extensively used as raw material in paint industries as well as in the production of welding rod coatings. Therefore, further extension of earlier work would be to use the mineral precursor of rutile, i.e. ilmenite, as the starting material for the reduction process. This will remove the necessity for the purification to T1O2, reduction to metallic titanium, and subsequent reaction to form the hard material.

South Africa, as one of the leading producers of ilmenite in the world, accounts for about 37% of 6.2 million metric tons global production, hence sourcing and processing of TiCN powder directly from South African ilmenite will be very potential. However, mechanical activation followed by carbothermic reduction of this relatively less expensive natural ore, is the most favourable production route for the synthesis of titanium carbonitride. This great advantage offered thereof, is in the use of inexpensive raw materials and carbonitrothermic reduction of its precursor, thereby reducing the number of production steps.

SUMMARY OF INVENTION

ACCORDING TO A FIRST ASPECT OF THE INVENTION, THERE IS PROVIDED a method of preparing titanium carbonitride, the method including exposing a mixture of a titanium mineral and a carbon reductant to reducing reaction conditions in a nitrogen atmosphere.

Typically, the titanium mineral is not rutile but ilmenite.

The titanium mineral and the carbon reductant may both be in particulate format, preferably being in finely divided particulate format, i.e. powder format. It is preferred that size of the carbon reductant and titanium mineral is smaller than 1 pm and not bigger than 5pm to reduce the reaction radius between the titanium mineral and the carbon reductant.

The method may include a prior milling step, in which the titanium mineral and the carbon reductant are, separately or, more preferably, jointly (i.e. as a mixture thereof) subjected to milling. The carbon reductant may, for example, be graphite and/or activated carbon.

The reducing reaction conditions may include a maximum reaction temperature, a predefined holding time and a predefined molar ratio of the titanium mineral and the carbon reductant. By“holding time” is meant the time that the mixture is exposed to the maximum reaction temperature.

The maximum reaction temperature may be at least between about 1300°C and 1450°C, preferably 1400°C. The method may include heating the mixture from a starting temperature to the maximum reaction temperature. This may occur at a predefined heating rate. The heating rate may range between 5°C/min and 10°C/min. The starting temperature may be the temperature of the mixture when exposed to ambient, or atmospheric temperature. In an embodiment, the period for heating the mixture from the starting temperature to maximum reaction temperature at the heating rate of between about 5°C/min to 10°C/min may be between 5 to 7 hours. The method may include cooling the mixture from the maximum reaction temperature. This may occur at a predefined cooling, or heat removal rate. The cooling rate may range from 5°C/min to 10°C/min.

The molar ratio of carbon reductant to the titanium mineral, specifically when the mineral is ilmenite, may range from 3.08 to 4.5, more preferably from 3,25 to 3.75, for example 3.25.

The holding time may range from 15 minutes to 60 minutes. For example, the holding time which may be about 20 minutes.

The exposure of the mixture to reduction conditions may produce reduction products comprising TiCN and impurities. The impurities may, for example, include Fe, FesC, unreacted C, and the like. The reaction product may preferably be devoid of TiN and TiC.

The reaction product may preferably be devoid of unreacted C.

The method may include recovering TiCN from the reduction product. Recovering TiCN from the reduction product may include treating the reduction product with a solvent for TiCN, thus leaching the Fe, FesC and other impurities from the reduction product. For example, the solvent may be an acid, such a hydrochloric acid.

The method may include recovering TiCN from the solvent, e.g. by way of filtering the solvent to remove the TiCN from the solvent, and followed by washing and drying the recovered TiCN. ACCORDING TO A SECOND ASPECT OF THE INVENTION, THERE IS PROVIDED titanium carbonitride produced by the method of the first aspect of the invention.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described in more detail by way of non-limiting example, with reference to worked experimental examples, A1 to A8, and to the accompanying drawings, in which:

FIG. 1 shows an experimental workflow of a single step synthesis of TiCN powder in accordance with the invention;

FIG.2 shows a representation of the extent of reduction recorded from A1 to

A8;

FIG.3 shows X-Ray diffraction patterns of the reduction products of A1 to A8;

FIG.4 shows X-Ray diffraction patterns of Fe-TiCN reduction product of A3; FIG.5 shows FESEM micrographs and EDX images of Fe-TiCN produced from A3 (“synthesized TiCN”);

FIG. 6 shows X-Ray diffraction patterns of synthesized TiCN;

FIG. 7 shows particle size distribution of the synthesized TiCN;

FIG. 8 shows X-Ray diffraction patterns of synthesized TiCN and commercial

TiCN powder;

FIG. 9 shows FESEM micrograph and EDX image of synthesized TiCN; and

FIG. 10 shows Raman spectrum of the synthesized TiCN.

EXAMPLES

FEATURES OF TFIE INVENTION as described above will be apparent from the worked experimental examples that follow.

Raw materials characterization Raw materials, comprising ilmenite concentrate and TIMCAL TIMREX ® KS6 graphite, used in the experiments were obtained from Richards Bay Minerals in South Africa and Imerys Graphite & Carbon, Switzerland, respectively.

The elemental composition of the ilmenite concentrate as determined by XRF analysis is given in Table 1. The various elements detected are also expressed in the form of their stable oxides. Table 1 Elemental and chemical composite of the ilmenite.

Elemental composition (wt. %)

Fe Ti O Si Al Mn Mg Cr Ca Zr Nb P Zn

25.21 22.1 1 27.74 0.97 0.61 0.55 0.22 0.1 1 0.08 0.07 0.04 0.02 0.02

Chemical composition (wt. %)

Fe ₂ C> ₃ T1O ₂ K ₂ O S1O ₂ AI ₂ O ₃ MnO MgO Cr ₂ 0 ₃ CaO Z1Ό ₂ Nb ₂ 0s P ₂ O ₅ ZnO

48.06 45.06 0.02 2.94 1.72 0.95 0.55 0.22 0.13 0.13 0.09 0.07 0.03

Hydrochloric acid (analytical reagent) used for the preparation of acid solution for leaching 5 was obtained from Merck KGaA, 64271 Darmstadt, Germany.

Gaseous nitrogen was also used, to provide the reduction atmosphere.

Experimental procedure

Mixtures of varied molar ratios of graphite to ilmenite concentrate in accordance with Eq.0 (1 ) were milled in ethanol under still air for 2 hours at room temperature.

FeTiOs + mC + ½ N ₂ = Fe + TiCi- _x N _x + 3CO (1 ) Steel balls of 10 mm diameter and a vial were used in a planetary ball mill (PM 100, Retsch Germany), which was operated at a speed of 250 rpm throughout the milling operation. After ball milling, the resulting slurry (“as-milled” material) was subjected to evaporation in a rotary evaporator, CVC 3000, Germany, and thereafter dried by exposing the remaining material to a temperature of 120°C in a laboratory draught drying cabinet (Thermo Electron LED GmbH, Germany). The material obtained from the drying process was subjected to the method of the invention (“carbonitrothermic reduction”) in eight different experimental embodiments (“experiments”), designated A1 to A8. Fig. 1 depicts the detailed flow chart adopted in the cause of these experiments. Table 2 gives detailed experimental parameters adopted in these experiments, including sample name, carbon:FeTi03 molar ratio, reaction temperature, reaction atmosphere, and reaction (holding) time.

Table 2 Experimental parameters of the carbonitrothermic reduction in nitrogen atmosphere

Sample name Carbon: FeTi0 ₃ (m) Temp. (°C) Atmosphere Holding time (min)

^~~ A1 3.08:1 1400 20

A2 3.17:1

A3 3.25:1

A4 3.42:1 A5 3.58:1

A6 3.75:1

A7 4.0:1

A8 4.5:1

The carbonitrothermic reductions - that is to say the reduction of titanium oxide and titanium suboxides, in a nitrogen gas atmosphere with a carbon reductant, to non-oxide ceramics of Titanium, in particular TiCN - were carried out isothermally, typically at a maximum reaction temperature of about 1400°C for a period of about 20 minutes, in a laboratory high temperature furnace (Thermal Tech., USA).

Powder samples of known weight were placed in graphite crucibles, which were held on an automated crucible holder before the furnace was purged with Ar gas and later switched to nitrogen gas after an inert atmosphere had been created.

The samples were heated, in a nitrogen atmosphere, to the desired temperature, typically 1400°C, at a heating rate of 5°C/min and held at that temperature for about 20 minutes, before being cooled to the ambient temperature at a heat removal rate of 5°C/min.

The reduction powder-products were measured, and the differences between the initial weight and final weight after reduction were used to compute the extent of reduction as given in Table 3. Table 3: Reduction degree of carbonitrothermic reduction of ilmenite at 1400 °C.

Sample name Extent of reduction % Reduction products

A1 83.7 Fe ₂ Ti0 ₅ , Fe, Fe ₃ C, TiN

A2 87.7 Fe ₃ Ti ₃ Oio, Fe, Fe ₃ C, TiN

A3 97.9 Fe, Fe ₃ C and TiCN

A4 97.9 Fe, Fe ₃ C, C and TiCN

A5 98.5 Fe, C and TiCN

A6 97.3 Fe, C and TiCN

A7 95.3 Fe, C and TiCN

A8 91.5 Fe, C and TiCN

To remove Fe from these synthesized Fe-TiCN powders, the powders were treated with 10% hydrochloric acid solution at a temperature of 80°C for 5h while being continuously stirred by a RW16 basic overhead stirrer. The pregnant leach solution (PLS) was then filtered to recover the powder, and the recovered powder was washed thoroughly with purified water, and then dried at 120 °C in a draught drying cabinet. This resulting powder (believed to be pure TiCN) was characterized using X-ray diffractometer (XRD, Brucker D8 Advance, Germany) with Cu - Ka radiation (l = 1.54060 A) operated on a scan rate of 0.01 (°)/s in 2 Theta (°).

The morphology and microstructure of the resulting powder were characterized using field emission scanning electron microscope (FESEM: ZEISS Gemini Nvision 40, Germany) equipped with energy dispersive spectrometer (EDS) for point mapping.

The elemental composition of the powder was analyzed using X-ray fluorescence spectrometer (Bruker S8 Corporation, Billerica, MA 01821 , England). Particle size, phase identification and Raman spectroscopic analyses (Horiba LabRAM HR Vis [400-1 100 nm], HORIBA GmbH, 64625 Bensheim, Germany) were used to compare the synthesized TiCN with the spectrum of standard of TiCN powder.

Results and discussion

Effect of carbon ratio on synthesis of Fe-TiCN composite

There was significant difference in the reduction behaviour of ilmenite concentrate when reacted with varied molar ratio of solid reductants, e.g. graphite or activated carbon. This was evident in the results of the XRD analyses at various amount of carbon reacting with ilmenite at 1400 °C as presented in Table 3.

From sample A1 , it was seen that the phases of the reduction products present after reduction for 20 min at the experimented temperature of 1400°C, were ferric pseudobrookite (FesTiOs), Fe, FesC, and TiN.

The presence of pseudobrookite indicated that the reduction process did not carry through to completion, and this was also evident from its extent of reduction of about 83.7%.

Likewise, from the A2 sample, the presence of FesTisOio, which is an incomplete reduction product of ilmenite, was a clear indication of the incompleteness of the reduction reaction. The incompleteness of these reactions was attributed to the insufficient amount of reductant in the as-milled samples prepared from 3.08 and 3.17 molar ratios of carbon to 1 mole of FeTi03. Though, this presence of Fe, FesC, TiN suggested that the reaction commenced, only that it had not proceeded to completion. Flowever, sample A3 featured phases of the reduction products present as Fe, FesC, and TiCN. This provided a clear indication of a complete reduction reaction, and this was also corroborated with about 97.95 % extent of reduction.

The absence of unreacted carbon among its reduction products gave credence to the optimism of 3.25 as molar ratio of carbon to be reacted with 1 molar ratio of ilmenite in order to synthesize Fe-TiCN composite in a single step reduction synthesis. But when the amount of carbon reacting with ilmenite was increased above 3.25 (i.e. A4 to A8), there was evidence of unreacted carbon among the reduction products, which suggests that the reduction reaction might have not gone to completion.

The highest extent of reduction was recorded on A5, however, the presence of free carbon is a pointer to the fact that more reduction could still occur when one or two parameters (i.e. time or temperature) of reduction reaction are altered. Figs. 2 and 3 are the pictorial representation of the extent of reduction recorded from A1 to A8 and the diffraction peaks, respectively.

Carbonitrothermic reduction

While the ilmenite was being reacted with carbon at temperature above 1000°C, it dissociated to FeO and TiC>2.

The FeO then got reduced by carbon to metallic iron (Fe) and remained as molten iron throughout the reduction process.

Above the eutectic temperature of 1 147°C, there was dissolution of carbon in liquid iron phase, and this enhanced the transport of carbon to the reaction interface.

On the other side, the T1O2 was reduced to sub-oxides of titanium otherwise known as Magneli, Tin02n-1 , which are more stable below 1200 °C. Above 1200 °C, the cubic phase of TiN began to form in the presence of nitrogen, and as the temperature increased, carbon atoms started to replace the nitrogen atoms in the already formed cubic phase of TiN. This continued until the stoichiometric TiCi-xNx finally formed, (A3).

The overall reaction for the carbonitrothermic reduction of ilmenite has already been given in Eq. (1 ).

Characterization of the Fe-TiCN composite

Chemical and elemental analysis

The elemental and chemical composition of the synthesized composite (reduction product) is as given in Table 4.

These quantitative analyses were carried out by an X-ray fluorescence spectrometer (Bruker S8 Corporation, Billerica, MA 01821 , England). The results of the XRF showed that the synthesized composite is a high-grade composite with less than 3 % impurities.

These impurities, which are traces of Mn, Mg and perhaps Si, were assumed to have been carried over from the used ilmenite as a starting material.

Table 4 Elemental and chemical composition of the synthesized Fe-TiCN.

Elemental composition (wt%)

35.16 37.81 4.96 19.85 0.15 0.82 0.53 0.34 0.09 0.10 0.06 0.03 0.10 Chemical composition (wt.%)

TiCN Fe Other impurities

58.94 38.40 2.66

XRD analysis

The XRD patterns of the carbonitrothermic reduced products for A1 to A8 have already been presented in Fig. 3. However, detailed analysis will only be provided on A3 under this section.

Fig. 4 reveals that the ilmenite was reduced by carbon in the presence of nitrogen, to metallic iron (Fe), austenite (FesC, i.e. carbon dissolved in iron phase) and titanium carbonitride.

However, it was found that some impurities (as earlier mentioned) in trace form, dissolved in the Fe phase during the reduction process.

Microstructural observation

The morphology and microstructure of the carbonitrothermic reduction products of A3 were characterized using the field emission scanning electron (FESEM: ZEISS Gemini Nvision 40, Germany) equipped with energy dispersive spectrometer (EDS) for elemental analysis. Fig. 5 shows representative images obtained on A3 after homogenizing for 20 min at 1400 °C. These images showed that there were only two distinct regions of bright and grey phase. The micrographs primarily consist of bright and grey regions Fig. 5 (a-b), which were identified by EDX analysis, Fig. 5 (c-d) to be Fe and TiCN respectively. There was another minor dim-bright region, which was assumed to be the region containing dissolved carbon in iron phase, FesC. It was clearly observed that the cubic phase of TiCN particles was evenly distributed in the iron matrix, Fig. 5 (a). The EDX analysis revealed that the bright region is a region of metallic iron, though some impurities in trace form might have dissolved in it, but they could be effectively removed during the hydrochloric acid leaching.

Hydrochloric acid solution treatment

The hydrochloric acid solution used for the removal of Fe and some other minor impurities dissolved in the iron phase showed a nearly 100 % effectiveness, as neither the peaks of metallic iron nor those of the impurities were present after the leaching operation.

Fig. 6 showed the X-ray diffraction patterns of the leached product, hereby called synthesized TiCN. As shown in Fig. 6, Fe and the dissolved impurities in it have been totally removed in accordance to the Eq. (2).

Fe(s) + TiCN(s) + 2HCI(i) = TiCN(s) + FeCl2(i)+ H2(g) (2)

Characterization of the as-leached pure TiCN

Particle size analysis (PSA)

The graph of the particle size distribution of the synthesized TiCN is as presented in Fig. 7. It was observed that its particle size distribution range falls within 1 - 17 pm. Its d(o.i), d(o.5) and d(o.9> were found to be 1.460 pm, 4.687 pm and 1 1.048 pm respectively. Though averagely greater than those of the commercially available TiCo.7No.3, which was found to be having d(o.s) of 2.10 pm. XRD analysis

Fig. 8 compared the XRD patterns of the commercially available TiCN to those of the synthesized TiCN powder. A close observation showed that the TiCN powder obtained after the acid treatment of the synthesized Fe-TiCN is highly compatible with the commercially available TiCN. Both only showed the peaks of (1 1 1 ), (200), (220), (31 1 ) and (420), identified to be peaks of pure TiCN using the cubic rules.

The absence of Fe peaks in the synthesized powder confirmed that the hydrochloric acid solution treatment was effective for the complete removal of the elemental iron. To further unravel the identities of these peaks, lattice parameter analysis was performed comparing the lattice parameters (a) and inter-planar spacing (d) of the synthesized TiCN (measured) with the standard values taken from the standard PDF card, International Centre for Diffraction Data (ICDD database), with reference number 01 -076-2484 under the space group of F 4/m - 32/m.

As presented in Table 5, the measured values from the synthesized TiCN powder favourably compete with the standard values. Flowever, the difference in the lattice parameter in Table 5 could be ascribed to the difference in the C/N ratio in the synthesized TiCo 2No.8 and the commercially available TiCo.7No3. Table 5 comparative figures of lattice parameter and inter-planar spacing of the synthesized TiCN and standard values

S/N hkl value d (A) 2 Theta (°) Lat. parameter

(a = b = c) A

standard Observed Standard Observed Standard Observed

1 1 1 1 2.480 2.457 36.224 36.572 4.297 4.256

2 200 2.148 2.128 42.074 42.483

3 220 1 .519 1.505 61.016 61.643

4 31 1 1 .295 1.283 73.066 73.855

5 420 1 .240 1.229 76.889 77.734

Microstructural analysis

FESEM images of the synthesized TiCN powder are shown in Fig. 9. Fig. 9 (a) showed the morphology of the TiCN particles, which are not uniform in size and shape. The non uniformity in size and shape of these particles was attributed to the effects of the mechanical milling on the starting materials prior to the carbonitrothermic reduction.

However, the clustering of the particles was attributed to the fine grains of the synthesized TiCN powder. The EDX analysis showed in the Fig. 9 (b) revealed the presence of Ti, C and N. This further confirmed that the product obtained after the hydrochloric acid treatment was pure TiCN.

Raman analysis

The Raman spectrum of the synthesized TiCN powder is shown in Fig. 10. In the spectrum, three major peaks at 266.3, 402.5 and 609.7 cm ^-1 were observed, and a very weak peak of carbon was also found at 1587.9 cm ^-1 and there were no peaks of other phases such as impurities of Si, Al, Mn, Mg, etc. This further confirmed that the synthesized TiCN is a powder of high grade. The absence of strong peaks of carbon and some other peaks confirmed the purity of the synthesized TiCN powder.

Conclusions

THE INVENTION AS DESCRIBED provides for the synthesis of a high-grade titanium carbonitride (TiCN) powder, using low grade titanium ore as the staring material.

In the invention, TiCN powder is synthesized via carbonitrothermic reduction of ilmenite concentrate.

Based on the observed results, the following conclusions were hereby drawn; ^■ Optimum molar ratio of carbon to ilmenite for the effective synthesis of Fe-TiCN composite was determined to be 3.25.

^■ Composite of Fe and titanium carbonitride was obtained via carbonitrothermic reduction of ilmenite.

■ Flydrochloric acid solution treatment (leaching) was found to be effective to obtain high grade titanium carbonitride powder with negligible impurity.

^■ The average particle size of the synthesized TiCN powder was found to be 4.687 pm, which is averagely comparable with the commercially available one.