Process for Selectively Chlorinating the Titanium Content of Titaniferous Materials

For many years, a great deal of attention has been devoted to techniques aimed at the effective separation of the titanium and iron constituents of titaniferous ores such as ilmenite. Nonselective chlorination techniques, i.e., in which the two metals are chlorinated simultaneously and the chlorides then separated from one another, have proven to be sufficiently effective that they are now practiced in the manufacture of titanium dioxide (TiO.sub.2) pigments, particularly by the so-called "chloride" process involving the oxidation of titanium tetrachloride (TiCl.sub.4). Such techniques are much less efficient than would be desired, however, since depending upon the iron content of the ore a considerable amount of costly chlorinating agent may be consumed in producing relatively useless iron chlorides as by-products. Conversion of the latter to metallic iron or some form of iron oxide would be deisrable to recover the chlorine content and to eliminate the waste disposal problem which has been associated with iron chlorides, but such conversion is difficult to achieve in economic fashion.

Other techniques for separating the iron and titanium constituents of ores have been devised that involve selectively chlorinating the iron content, thereby leaving an upgraded or beneficiated TiO.sub.2 fraction. While these have also achieved some measure of commercial importance, they have not served to lessen the problems associated with the formation of iron chloride by-products. Then, too, if one considers that TiCl.sub.4 is the desired intermediate, i.e., for producing a TiO.sub.2 pigment or metallic titanium, such beneficiation merely introduces an added step because the beneficiated fraction must still be chlorinated.

One approach that might appear to overcome the shortcomings of the above-noted techniques is that described in Wilcox U.S. Pat. No. 2,589,466. According thereto an ore such as ilmenite is said to be treated by chlorine in such a way that TiCl.sub.4 is produced without chlorination of iron. In repeated endeavors to follow the procedure of that patent as closely as possible, it has been found that a selective chlorination of the titanium does not occur; that is, that instead of selective chlorination the chlorine attacks iron either preferentially over titanium, or at least nearly equally, at temperatures up to essentially 1400.degree.C. That the process of the Wilcox patent would not result in a successful selective chlorination of the titanium constituent in ilmenite had been predicted on the basis of calculated equilibrium constants by S. Wilska, Suomen Kemistilehti, 29A, pages 220 to 225, 1956; Chemical Abstracts (1957), 4801(f). In C. C. Patel et al., Transactions of the Metallurgical Society of AIME, 218, pages 219-225 (April, 1960) it is further stated in reference to the Wilcox patent that "no justification can be found for the preferential chlorination at 1250.degree. to 1500.degree.C. of titania of roasted ilmenite. . ."

In the specification set forth hereinafter I describe my findings of a unique process for chlorinating the more useful titanium constituent of a titaniferous ore in a way that leaves the less useful iron constituent unchlorinated and, preferably, in the metallic state. That the titanium could be chlorinated without chlorinating the iron is contrary to what would have been expected from various technical articles, including those aforementioned as well as, more recently, S. H. Iqbal et al., Chemical Engineering World, Vol. VI, No. 8, pages 81-83 (1971). That such titanium chlorination could, moreover, be accomplished economically and efficiently using readily available starting materials is all the more unexpected.

In accordance with my invention, there is provided a reduction/chlorination process for the treatment of titaniferous materials in such a way that the titanium constituent is chlorinated but there is no appreciable net yield of iron chloride from the iron constituent. The reduction utilizes as the reductant an amount of carbonaceous material which, based on oxygen in the titaniferous material, is at least stoichiometric to produce carbon monoxide. The selective chlorination utilizes as the chlorinating agent either ferrous chloride alone or a combination of ferrous chloride (FeCl.sub.2) plus molecular chlorine (Cl.sub.2) and/or hydrogen chloride (HCl). The use of ferric chloride (FeCl.sub.3) as a part or all of the chlorinating agent is the equivalent of using a FeCl.sub.2 /0.5 Cl.sub.2 mixture. Sufficient chlorine atoms are preferably provided by the chlorinating agent to react with essentially all of the titanium in the titaniferous material. As will be apparent from the detailed description of the invention below, the amount of Cl.sub.2 and/or HCl present in the chlorination will be maintained at a sufficiently low level that there is no net yield of iron chloride, i.e., advantageously no more than about 4 atoms of chlorine being provided by Cl.sub.2 and/or HCl for each mole of FeCl.sub.2 which is present. An elevated temperature in the range of 950.degree. to 1400.degree.C. is maintained during the chlorination. Depending upon the composition of the chlorinating agent, there may be a minimum temperature within the stated range that is needed to maintain selectivity.

It is a significant feature of this invention that ferrous chloride, FeCl.sub.2, can be used under certain conditions for the chlorination and removal of only the titanium constituent of a titaniferous material such as ilmenite ore while leaving the iron as a metallic residue. Under otherwise similar conditions the use of Cl.sub.2 and/or HCl alone would result in the conversion of all or at least nearly all of the iron to iron chlorides. In any event, the invention thus makes it entirely practical to directly and effectively recover the more useful titanium constituent of titaniferous materials, e.g., as relatively pure TiCl.sub.4, without the production of appreciable quantities of unwanted iron chlorides. The advantages of the process, particularly when operated in conjunction with a chloride process for producing TiO.sub.2, will be apparent to those skilled in the art.

As indicated above, FeCl.sub.2 can be used either alone or in combination with Cl.sub.2 and/or HCl to serve as the chlorinating agent. As somewhat different processing conditions can be involved depending upon whether or not FeCl.sub.2 is used alone, a general description of each embodiment will first be given.

In connection with the various embodiments it is noted there will be used the formula FeTiO.sub.3 which is an idealized formula chosen to represent the titaniferous materials of interest. The empirical formula will vary, as is known, from one ore source to another. In this respect the term "ore" will be used herein in a general way since, while it is not essential that the titaniferous material be an ore, normally it will at least be derived from an ore source.

It is also noted that while it is not essential to selectively chlorinate every final trace of titanium in the ore, unconverted titanium represents a loss which is to be avoided as much as possible. Also it is not essential that every trace of iron in the ore be reduced to the metallic state. It is desirable to convert at least about 75% by weight of the titanium and iron in the ore to TiCl.sub.4 and metallic iron, respectively, and it is to be understood that such a conversion is intended by the term "essentially completely" when used in this connection throughout this specification. Conversions of at least 85% by weight of the titanium in the ore, while producing at least a corresponding amount of metallic iron, are decidedly preferred for most purposes and normally these can be achieved without difficulty. But as a practical matter, and indeed this is one of the significant advantages of the invention, with most ores there is no problem in effectively and directly separating as an essentially pure TiCl.sub.4 product more than 95% of the titanium in the ore, while leaving corresponding amounts of metallic iron.

Similarly, it is noted that when reference is made herein to the selective chlorination of the titanium constituent of the ore, this is not to be construed as necessarily precluding the net chlorination of minor quantities of other metals in the ore. Thus certain metals such as vanadium, which often may be found in minor percentages in titaniferous ores, tend to be chlorinated along with the titanium. Also, the desired product of the selective chlorination reaction, which is TiCl.sub.4, can itself under certain conditions chlorinate some metallic iron. Of course, any net yield of iron chloride (as opposed to metallic iron) represents a condition which is to be avoided as much as possible since it represents a chlorine and iron loss and a disposal problem. A judicious selection of the process conditions in accordance with the disclosure herein makes it readily possible to operate such that the net yield of iron chloride does not exceed about 10% by weight of the iron in the titaniferous ore, hence the term "no appreciable net yield of iron chloride" is employed herein. Most often, as is preferred, the percentage will be 5% or less, an amount which for all practical purposes can be ignored. The term "net yield" is used so as not to exclude the possible temporary formation of iron chloride during an intermediate stage of the process.

In the case where FeCl.sub.2 is used alone, the process of the invention can be expressed according to the following reaction (as in the case of all reactions in the specification, it will be referred to by Roman numeral designation): Net reduction/chlorination reaction:

reaction (I) is in effect a net reaction as it is considered to occur in a series of individual reaction steps that can be expressed by the following: Ilmenite reduction:

and TiO.sub.2 chlorination with FeCl.sub.2 :

the equilibrium conversion for Reaction (III) has been found to increase with temperature, as will be apparent from a later discussion of FIG. 4. In any case, as will be understood from the equations themselves, selective chlorination of essentially all of the titanium content of the ore would require that the atomic ratio of carbon to oxygen in the ore be at least 1:1, and the molar ratio of FeCl.sub.2 :titanium in the ore be at least 2:1. In actual practice it has been found that substantial amounts of trivalent titanium result from the reduction, and that the chlorination product often contains both TiCl.sub.4 and TiCl.sub.3. Hence it can be said that complete removal of the titanium content of the ore would require a FeCl.sub.2 :titanium molar ratio of about 1.5-2:1.

Reaction (I) is, of course, favored by using greater quantities of both carbon and FeCl.sub.2 and such practice will be advantageous in many cases, e.g., for laboratory operations where economics are not of particular importance. For most commercial purposes, however, it will be desirable from a cost viewpoint to operate as closely as possible to the specified minimum molar ratios.

Regardless of the conversion efficiency, however, and essentially independent of temperature over the range of about 950.degree. up to 1400.degree.C., there is no appreciable tendency for iron chlorides to be formed, i.e., FeCl.sub.2 will selectively attack the titanium constituent rather than the iron constituent.

Among the techniques for effecting Reaction (I) in accordance with this embodiment, one involves the use of FeCl.sub.2 in the vapor phase and another the use of FeCl.sub.2 in the melt phase. Examples 1 and 2 hereinafter represent these techniques respectively.

In the technique illustrated in Example 1, FeCl.sub.2 is vaporized (m.p. 677.degree.C. and b.p. 1026.degree.C.) and brought into contact with a mixture of ore and carbon in a reactor. The TiCl.sub.4 which forms is removed, for example, by the aid of an inert purge gas, and condensed. If desired, the ore/carbon mixture can first be heated to a temperature of 500.degree.C. or more out of contact with FeCl.sub.2 to initiate some prereduction, primarily of the iron content. While prereduction introduces an extra processing step, it can nonetheless be of advantage as will be described later.

In the technique illustrated in Example 2, the FeCl.sub.2, ore and carbon are heated together in the same reactor, to form a melt. If the temperature is in the range of about 950.degree. to 1026.degree.C., the reaction will apparently proceed in a melt phase. Above 1026.degree.C. the FeCl.sub.2 will tend to boil and the reaction will at least partially occur in the vapor phase. If desired, however, a solid/liquid reaction mass can be maintained at temperatures in excess of 1026.degree.C. by inclusion of a salt such as an alkali metal chloride or an alkaline earth metal chloride to act in raising the boiling point. Sodium chloride and potassium chloride are particularly preferred salts for this purpose because of their availability but other salts may be used provided they have a melting point below 950.degree.C. and neither boil nor decompose at the reaction temperature employed. The amount of salt employed will affect the extent to which the boiling point of the mass is raised. Normally it should constitute 10 to 90% by weight of the contents in the reactor.

Regardless of the technique employed the embodiment is readily susceptible to being carried out on a continuous cyclic basis. Thus by removing that portion of the metallic iron made from the FeCl.sub.2, it can be reacted with chlorine gas in accordance with the reaction:

followed by recycling of the ferrous chloride thus generated.

In the embodiment in which Cl.sub.2 is utilized in combination with FeCl.sub.2 as the chlorinating agent, the following generalized reaction can be given for chlorination of the titanium constituent while reducing the iron content to metallic iron:

again, however, the mechanism is considered to involve Reaction (III) above; that is, conditions are employed whereby one takes advantage of the fact that FeCl.sub.2 reacts with the titanium in preference to the iron and the additional Cl.sub.2 is allowed to serve only as an auxiliary chlorinating agent for generating a supply of FeCl.sub.2. In Reaction (V) the symbol x represents a number which is no less than about 1.0 so that the molecular chlorine provides no more than 4 chlorine atoms per mole of FeCl.sub.2.

For each value of x an optimum temperature, T.sub.p, can be ascertained at which the reaction will result in a minimum loss of iron, i.e., as iron chloride, from the iron constituent in the ore. For example, if x is 2 such that 2 moles each of FeCl.sub.2 and Cl.sub.2 are present per titanium atom in a quantity of unreduced ilmenite ore, it has been established that T.sub.p occurs at a temperature of about 1225.degree.C. Further details concerning the relationship of x and T.sub.p will be given hereinafter with reference to FIGS. 5 and 6.

As will be apparent from Reaction (V), an essentially complete reaction is not possible unless the atomic ratio of carbon to oxygen in the ore is at least 1:1. While also seemingly there would be required at least 4 chlorine atoms from the chlorinating agent for reaction with each titanium atom in the ore, between 3 and 4 are actually required since, as mentioned before, some TiCl.sub.3 is often produced. Preferably these conditions are constantly maintained throughout the duration of the reaction.

The amount of Cl.sub.2 which is allowed to react with the ore must not be greater than 2 moles of Cl.sub.2 per titanium atom in the ore, i.e., any amount of Cl.sub.2 in excess thereof would be reacting with the iron constituent of the ore, even when operating at temperatures above T.sub.p. Greater amounts of Cl.sub.2 could, of course, be fed to a reactor provided that the dwell time therein was sufficiently short that the amount chemically combining with the ore did not exceed a molar ratio of 2 Cl.sub.2 per titanium atom in the ore. While a selective chlorination of titanium is possible if the molar ratio of Cl.sub.2 per titanium atom in the ore is less than 2, it is possible that there will result a net consumption of part of the FeCl.sub.2 according to Reaction (I). Should the amount of FeCl.sub.2 become unduly low while Cl.sub.2 is present, iron chlorination could then result. Hence the ratio of 2 Cl.sub.2 molecules per titanium atom in the ore represents a theoretically optimum operating ratio.

In the case where Reaction (V) is effected such that the values of x and T.sub.p assure selective chlorination of the titanium constituent of the ore, it will be noted that no net consumption of or production of FeCl.sub.2 is involved. In effect, for each two moles of FeCl.sub.2 consumed in producing a mole of TiCl.sub.4 in accordance with Reaction (III), two moles of FeCl.sub.2 are produced from iron and Cl.sub.2 in accordance with Reaction IV. This is especially significant, for in practical terms it means that predetermined operating conditions can be selected which will enable a fixed quantity of FeCl.sub.2 to be maintained while Cl.sub.2, a widely available industrial chemical, is the only reactant that has to be furnished to an ore/carbon mixture.

As FeCl.sub.2 and Cl.sub.2 are known to be involved in an equilibrium reaction with ferric chloride, FeCl.sub.3, at elevated temperatures in accordance with the following:

it will be apparent that this embodiment can likewise be practiced using FeCl.sub.3 and that Reaction (V) may be rewritten accordingly, e.g., as

the utilization of FeCl.sub.3 as a starting material can be of advantage from the standpoint that it is frequently available as an industrial by-product. As it has a comparatively low boiling point, 315.degree.C., it can be employed in the vapor phase for contacting an ore/carbon mixture. Such practice is illustrated in Example 3, hereinafter. The generation of a stream of vaporous FeCl.sub.3 from the reaction of FeCl.sub.2 and Cl.sub.2 in accordance with Reaction (VI) is also entirely practical, as will be illustrated hereinafter in Examples 4-9. It will be apparent that the procedures using FeCl.sub.3 are readily susceptible to being carried out on a continuous cyclic basis, e.g., by removing FeCl.sub.2 and reconverting it to FeCl.sub.3 via Reaction (VI).

The comments given above with respect to the use of a FeCl.sub.2 /Cl.sub.2 combination are largely pertinent, but with certain differences to be noted below, to the corresponding use of a FeCl.sub.2 /HCl combination. In the latter case the reaction can be expressed by the following:

with x being, again, no less than about 1.0. One obvious difference is that hydrogen gas, H.sub.2, occurs in the gaseous reaction product. This can be a disadvantage from the standpoint of the problem associated with the handling and disposal of a mixture of CO/H.sub.2.

Perhaps a more significant difference is that when using HCl, as contrasted to using Cl.sub.2, the conversion of HCl to TiCl.sub.4 is incomplete. In other words the amount of chlorinating agent employed to chlorinate a given quantity of ore must be such as to furnish more than 4 chlorine atoms per titanium atom in the ore. For example at a temperature of 1150.degree.C., no less than 6.4