Molten grains of titanium sub-oxides and ceramic products comprising such grains

Abstract

Molten grains include titanium suboxides of the formulation Ti.sub.nO.sub.2n-1, in which the phases are principally Ti.sub.5O.sub.9 or Ti.sub.6O.sub.11 or a mixture of these two phases, the phases Ti.sub.5O.sub.9 and/or Ti.sub.6O.sub.11 representing, in total, more than 60% of the weight of the grains, the grains further including less than 30% by weight of Ti.sub.4O.sub.7.

Claims

1. Molten grains consisting essentially of phases of titanium suboxides corresponding to the formulation Ti.sub.nO.sub.2n-1, n being a positive integer, in which said phases are principally Ti.sub.5O.sub.9 or Ti.sub.6O.sub.11 or a mixture of these two phases, said Ti.sub.5O.sub.9 and/or Ti.sub.6O.sub.11 phases representing, in total, more than 60% of the weight of the molten grains, said molten grains also comprising less than 30% by weight of Ti.sub.4O.sub.7, wherein the molten grains are obtained by: melting, under reducing conditions, an initial mixture comprising titanium dioxide particles, at a temperature greater than 1700 C., cooling the molten mixture until the mixture has solidified, and grinding the solidified mixture in order to obtain molten grains of the titanium suboxides.

2. The molten grains as claimed claim 1, wherein the Ti.sub.5O.sub.9 and/or Ti.sub.6O.sub.11 phases represent, in total, more than 70% of the weight of the molten grains.

3. The molten grains as claimed in claim 1, comprising less than 25% by weight of Ti.sub.4O.sub.7.

4. The molten grains as claimed in claim 1, comprising more than 90% by weight, in total, of one or more phases of titanium suboxides corresponding to the generic formulation Ti.sub.nO.sub.2n-1, n being an integer greater than 3.

5. The molten grains as claimed in claim 4, wherein n is between 4 and 9, limits included, and wherein said Ti.sub.nO.sub.2n-1 phases represent, in total, more than 90% of the weight of the molten grains.

6. The molten grains as claimed in claim 1, essentially corresponding to an average overall formulation TiO.sub.x, in which x is between 1.95 and 1.50.

7. The molten grains as claimed in claim 6, wherein x is between 1.75 and 1.85.

8. A ceramic product or material, obtained by sintering the molten grains as claimed in claim 1.

9. The ceramic product or material as claimed in claim 8, wherein the molten grains are sintered at a temperature of between 1200 C. and 1800 C.

10. A product comprising a coating obtained by projection of the molten grains as claimed in claim 1.

11. A process for producing the molten grains consisting essentially of phases of titanium suboxides corresponding to the formulation Ti.sub.nO.sub.2n-1, n being a positive integer, in which said phases are principally Ti.sub.5O.sub.9 or Ti.sub.6O.sub.11 or a mixture of these two phases, said Ti.sub.5O.sub.9 and/or Ti.sub.6O.sub.11 phases representing, in total, more than 60% of the weight of the molten grains, said molten grains also comprising less than 30% by weight of Ti.sub.4O.sub.7, said process comprising: melting, under reducing conditions, an initial mixture comprising titanium dioxide particles, at a temperature greater than 1700 C., cooling the molten mixture until the mixture has solidified, grinding the solidified mixture in order to obtain molten grains of the titanium suboxides.

12. The production process as claimed in claim 11, wherein the initial mixture comprises a coke reducing agent.

13. The production process as claimed in claim 12, wherein the initial mixture comprises between 1% and 25% by weight of coke, relative to the total weight of the mixture.

14. The production process as claimed in claim 12, wherein the melting is carried out under air.

15. The production process as claimed in claim 11, wherein the titanium dioxide represents more than 90% of the total inorganic mass present in the initial mixture.

Description

EXAMPLES

(1) In all the examples according to the invention, the samples were prepared from a mixture of raw materials consisting of a coke powder and a commercial titanium oxide powder in rutile form comprising more than 96% of TiO.sub.2.

(2) The samples of examples 1 to 6 according to the invention are obtained by melting the mixture of the above powders, in the various proportions reported in table 1.

(3) More specifically, the mixtures of initial reagents are pre-melted in an electric arc furnace, under air. The molten mixture was then cast into ingots allowing relatively slow cooling.

(4) On each sample, a specimen is taken from the molten mixture after cooling in order to measure the electrical resistivity thereof (denoted Re and conventionally given in Ohm.Math.cm) according to various protocols described below in the experimental section.

(5) The rest of the product obtained is ground and sieved so as to retain the powder at 20 m. The powder finally obtained after sieving has a median diameter d.sub.50 of 8.5 m.

(6) A first comparative sample A, not in accordance with the invention, is also synthesized from a mixture of TiO.sub.2 powder (rutile previously described) and of carbon black sold by Cabot Corporation, in a proportion equal to 4% of the total weight of the mixture of TiO.sub.2 and carbon black. The mixture is pelleted and sintered at 1450 C. under argon for 2 hours, without going as far as melting said mixture, in accordance with the process described in the prior publications U.S. Pat. No. 4,422,917 and WO 2009/024776. The product thus sintered is then ground until a powder with a median diameter d.sub.50 equal to 8.8 m (d.sub.10=1.6 m; d.sub.50=8.8 m; d.sub.90=26.2 m) is obtained.

(7) A second comparative sample B, not in accordance with the invention, is also synthesized from a mixture of TiO.sub.2 powder (anatase) and carbon black sold by Cabot Corporation, in a proportion equal to 1% of the total weight of the mixture of TiO.sub.2 and carbon black. The mixture is sintered at 1450 C. under argon, without going as far as melting said mixture, for 2 hours, and then ground as previously.

(8) The chemical composition and the crystalline phases present are analyzed using the powder of molten grains for some of the samples thus obtained. The results are reported in table 1 which follows.

(9) The resistance to corrosion of the molten grains constituting the powders is then measured for certain samples. The results are grouped together in table 2 below.

(10) The experimental protocols used for the characterization of the composition and the properties of the various samples obtained are the following:

(11) 1) The overall chemical composition of the grains in the TiO.sub.x form was determined by means of a test consisting of measuring the gain in mass of a sample brought to 1000 C. under air which will oxidize until the TiO.sub.2 stoichiometry is achieved. The heating is continued until the weight of the sample is stabilized. The final gain in weight, corresponding to the difference between the stoichiometric compound TiO.sub.2 and the initial composition, makes it possible to calculate the value of x of the general formula TiO.sub.x reported in table 1.

(12) The content of impurities is determined by X-ray fluorescence. It is thus determined that all the samples tested have a total amount of impurities of between 1% and at most 4% by weight.

(13) 2) The crystalline phases present in the refractory products were characterized by X-ray diffraction. The results obtained are grouped together in table 1 which follows. In this table, PP indicates a principal phase, MP indicates the presence of at least one other minor phase, and signifies that the phase(s) is (are) present in trace form. For the purposes of the present invention, it is considered that a phase is a principal phase when it represents at least 25% of the total weight of the grains. It is considered that a phase is a minor phase when it represents more than 5% and less than 25% of the weight of the grains, in particular more than 5% and less than 20% of the weight of the grains and preferably more than 5% and less than 15% of the weight of the grains, it being understood that the summed amount of the weight of the minor phases is normally less than 50% and preferably less than 30%, even less than 20%, of the weight of the grains. It is considered that a phase is in trace form when it represents less than 1% of the total weight of the grains.

(14) The proportions of the various phases constituting the grains were measured quantitatively from the diffractograms of the powders by means of the Rietveld method, using the EVA software and the PDF-2 Release 2005 ICDD database. More particularly, the quantitative analysis of the phases is carried out conventionally by refining of the diffractograms according to the Full Pattern Matching option proposed by the EVA software and developed in the DIFFRACplus Evaluation Package Release (2005) program series. The relative proportions by weight of the major and minor phases of the Ti.sub.3O.sub.5 (file 01-082-1138 of the database), Ti.sub.4O.sub.7 (file 01-077-1392), Ti.sub.5O.sub.9 (file 01-076-1690) and Ti.sub.6O.sub.11 (file 01-076-1266) type are measured for examples 3 to 5. The total sum of the contributions of these phases in each sample is approximated to 100%, the other phases normally being only present in trace form in these examples.

(15) 3) The electrical resistivities (Re) of the samples according to the invention and of comparative examples A and B are measured according to the Van Der Pauw method according to various protocols: A resistivity Re-1 is measured on disks 25 mm in diameter and 2 mm thick by taking a cylindrical core sample, 25 mm in diameter, of the molten mixture (i.e. after melting step b) according to the invention and without grinding) from which a pellet 2 mm thick of the crude material obtained by melting is cut. A resistivity Re-2 is measured for the sample according to example 4 on disks 25 mm in diameter and 2 mm thick, of a material obtained by grinding the molten mixture after cooling until a powder with a median diameter of approximately 8.5 micrometers is obtained, then sintering, at 1200 C., said grains in a graphite matrix, under a pressure of 21 MPa (3000 psi) and under vacuum, for one hour. A resistivity Re-3 is measured for the sample according to example 4 on disks 25 mm in diameter and 2 mm thick, of a material obtained according to the same principle as previously described for the measurement of Re-2, but at a sintering temperature equal to 1400 C.

(16) Since it is sought to maximize the electrical conductivity, the samples will be judged as being all the better the lower their electrical resistivity (Re).

(17) 4) The resistance to corrosion of the materials was evaluated by immersing 1 g of powder in 15 ml of 85% concentrated H.sub.3PO.sub.4 or HCl or H.sub.2SO.sub.4 solution (as indicated in table 2) at a temperature of 60 C. After a period of time as indicated in table 2, 1 ml of the solution is sampled and the content of Ti element dissolved in the solution is quantitatively determined by ICP. The various contents measured, given in mg per ml, are reported in table 2. Since, according to the invention, the strongest resistances to corrosion are sought, the samples are all the better the lower the quantity of the Ti element.

(18) TABLE-US-00001 TABLE 1 Example 1 2 3 4 5 6 Coke (% in the raw 1.0 10.1 13.8 15.3 10.4 16.7 materials) x in TiO.sub.x 1.88 1.86 1.82 1.79 1.74 1.68 Phases PP Ti.sub.9O.sub.17 Ti.sub.8O.sub.15 Ti.sub.5O.sub.9 (68) Ti.sub.5O.sub.9 (56) Ti.sub.3O.sub.5 (40) Ti.sub.3O.sub.5 Ti.sub.8O.sub.15 Ti.sub.7O.sub.13 Ti.sub.6O.sub.11 (32) Ti.sub.4O.sub.7 (35) Ti.sub.7O.sub.13 MP Ti.sub.6O.sub.11 Ti.sub.6O.sub.11 (18) Ti.sub.5O.sub.9 (24) Ti.sub.4O.sub.7 Ti.sub.3O.sub.5 (15) Ti.sub.4O.sub.7 (11) ~ Ti.sub.9O.sub.17 Ti.sub.4O.sub.7 TiO.sub.2 Re-1 ( .Math. cm) 11 .Math. 10.sup.3 9 .Math. 10.sup.3 4 .Math. 10.sup.3 Re-2 ( .Math. cm) 3 .Math. 10.sup.3 Re-3 ( .Math. cm) 4 .Math. 10.sup.3

(19) In table 1 above, the respective weight percentages of the principal phases and, where appropriate, the minor phases in the grains, as measured from the X-ray diffractogram and Rietveld analysis of the powders according to examples 3, 4 and 5, have been reported between parentheses.

(20) The analysis of the crystalline phases by X-ray diffraction and Rietveld analysis on the powder of the grains of example B show that they consist of the Ti.sub.4O.sub.7 (25% of the total weight of the grains), Ti.sub.5O.sub.9 (30% by weight), Ti.sub.6O.sub.11 (20% by weight) and Ti.sub.3O.sub.5 (25% by weight) phases. The general formula of the grains, obtained by measuring the gain in mass previously described, is TiO.sub.1.79 for comparative sample B, as it is for example 4. However, it can be seen, by comparison with the data reported in table 1, that the grains obtained according to example 4 according to the invention and those obtained according to comparative example B have very different relative percentages of the Magneli phases, although their general formulation TiO.sub.1.79 is identical.

(21) A general formula TiO.sub.1.82 is also determined for sample A, by measuring the gain in mass previously described. This formulation appears this time to be identical to that of example 3 according to the invention. A comparable electrical resistivity is also measured between sample A and the sample according to example 3 according to the invention.

(22) The results with respect to the tests for resistance to corrosion of these two samples according to the protocol previously set out are reported in table 2 below.

(23) TABLE-US-00002 TABLE 2 Ti having passed into solution (mg/1) Example 3 Example A H.sub.3PO.sub.4/72 hours 4.5 6.2 H.sub.3PO.sub.4/144 hours 10.0 15.0 H.sub.3PO.sub.4/216 hours 13.0 49.0 HCl/72 hours 31.0 59.0 HCl/144 hours 38.0 62.0 H.sub.2SO.sub.4/72 hours 4.3 25.0 H.sub.2SO.sub.4/144 hours 6.2 39.0

(24) The analysis of the data reported in table 2 shows the superiority of the products/materials obtained from the grains according to the invention: for a similar composition (but a very different Magneli phase distribution), it is observed that the material according to the invention exhibits a much better resistance to corrosion.

(25) The improved results obtained for the molten grains with respect to corrosion in the presence of H.sub.3PO.sub.4 make it possible to envision the use of a material obtained according to the invention as an electrode in electrolysis reactions for which the electrolyte is H.sub.3PO.sub.4, for example the electrolysis of Co(II) so as to obtain Co(III) or in the purification of H.sub.3PO.sub.4 by electrolysis in H.sub.3PO.sub.4.

(26) The improved results obtained for the molten grains with respect to corrosion in the presence of HCl make it possible to envision the use of a material obtained according to the invention as an electrode for the production of chlorine, from concentrated hydrochloric acid.

(27) The improved results obtained for the molten grains with respect to corrosion in the presence of H.sub.2SO.sub.4 make it possible to envision the use of a material obtained according to the invention in lead batteries for which the electrolyte is H.sub.2SO.sub.4.

(28) According to an example 7 according to the invention, a sample is prepared from a mixture of raw materials consisting of a coke powder and a powder of titanium oxide in anatase form and comprising more than 98% of TiO.sub.2. The coke powder represents 10.4% by weight of the initial mixture of raw materials.

(29) The sample according to example 7 is obtained, as for the previous examples 1 to 6, by melting the mixture according to the same techniques (melting using an electric arc furnace, under air, then casting into ingots and then grinding).

(30) A general formula TiO.sub.1.79 is determined for the sample according to this new example, by measuring the gain in mass previously described. This formula is identical to that of the sample according to example 4. The weight percentages of the principal and minor phases in the grains also appear to be comparable to those previously described in table 1 for example 4. As for example 4, the weight of the Ti.sub.4O.sub.7 phase in the sample according to example 7 appears in particular to be much lower than 30% of the total weight of the grains.

(31) The sample according to example 7 is formed so as to measure the previously described electrical resistivity Re-3 thereof. The measurement shows a resistivity equal to 510.sup.4 .Math.cm, which is much lower than that of the sample according to example 4.

(32) The chemical analysis of the impurities of the molten grains according to examples 4 and 7 is reported in table 3 which follows.

(33) TABLE-US-00003 TABLE 3 TiO.sub.x SiO.sub.2 Al.sub.2O.sub.3 Fe.sub.2O.sub.3 Nb.sub.2O.sub.5 ZrO.sub.2 Cr.sub.2O.sub.3 Example 4 remain- 1.25 0.35 0.8 0.4 0.7 0.2 (% by der weight) Example 7 remain- 0.8 0.2 0.2 0.2 0.3 <0.1 (% by der weight)

(34) The table illustrates the unexpected influence of the impurities present in the molten grains according to the invention and in particular of silicon or of zirconium, the previously reported electrical resistivity measured according to example 7 (in which the amount of impurities is lower) being much lower than that measured on the sample according to example 4.

(35) In the preceding examples and description, the invention was especially described in relation to the advantages that it provides with respect to use in the electrode field.

(36) However, it is quite obvious that the invention also relates to the use of the grains of the invention in other applications, in particular all those where good electrical conductivity and also good resistance to corrosion are necessary. According to the application, the size of the molten grains according to the invention may especially be adjusted, in particular by choosing a suitable grinding method.

(37) According to one possible mode, it is also possible to use the molten grains according to the invention as a filler in polymers, in particular in a process for producing batteries.