WO1996039359A1

WO1996039359A1 - Process for the reduction of carbochlorination residue

Info

Publication number: WO1996039359A1
Application number: PCT/US1996/008600
Authority: WO
Inventors: James A. Sommers; Robert S. Yih
Original assignee: Teledyne Industries, Inc.
Priority date: 1995-06-06
Filing date: 1996-06-06
Publication date: 1996-12-12

Abstract

Reduction of elemental carbon in the reaction zone of a carbochlorination reaction for producing zirconium chloride by periodically introducing zirconium oxide without additional carbon into the reaction zone under conditions which react portions of the carbon residue.

Description

PROCESS FOR THE REDUCTION OF CARBOCHLORINATION RESIDUE

Field of the Invention The present invention relates to the carbochlorination processes generally and more particularly in the use of such processes for the production of zirconium metal, and most particularly, to the reduction in carbon-containing residue which results from the practice of the carbochlorination process with zircon sand.

Background of the Invention The carbochlorination of refractory, zirconium- containing ores such as zircon (ZrSiO^) inevitably generates inside the chlorination reactor a normally unreactive residue, which builds up to slowly fill the reactor. This material must be periodically removed, interrupting production. Furthermore, once removed, this material is radioactive due to involatile, naturally- occurring radionuclides such as radium and thorium. Therefore, it must be stored, handled and disposed of as radioactive waste. Such measures are costly. Also, the largest fraction of the residue is unreacted carbon, which is by itself an innocuous material, which can still have chemical reduction and energy values. In present practice, a large amount of costly waste disposal volume is taken up by this carbon. Previous efforts to reduce the volume of this waste material have focused on treatment of the residue after it has been generated.

Objects of the Invention The object of this invention is to provide processes to lower the amount of zircon carbochlorination residue which requires disposal as radioactive waste.

It is a further object of the present invention to provide such processes which requires a minimum of extraneous chemicals or equipment for the further treatment of the residue. A further object of the present invention is to provide such processes while minimizing disruption of current production activity.

A further object is to reduce personnel exposure to radioactive materials present in the carbochlorination residue.

A still further objective is to provide such processes where some of the remaining reduction value of the presently-discarded carbon in the residue is utilized.

Summary of the Invention All of the foregoing objects and advantages are realized by two embodiments of the present invention. In one embodiment there is included the steps of adding reducible zirconium compounds containing oxygen to the chlorination containing residue from prior carbochlorination reactions and continue chlorination under carbochlorination conditions to produce zirconium metal chlorides while reducing the volume of the prior produced residue.

The addition of a chlorinatable zirconium compound or mixtures of compounds containing oxygen while not normally expected to react with the spent carbon- containing residue of prior carbochlorination reactions does in fact react in a manner which produces further zirconium chloride while reducing the amount of carbon in the residue in the reactor. The volume of waste generated by the prior carbochlorination is reduced easing the problem of further handling and disposal. It has been discovered that this carbon, although not reactive in the residue, is nevertheless reactive with an easier-to-chlorinate oxide, such as Zr0₂. Therefore, by adding Zr0₂ to a reactor which has an inventory of residue from the prior chlorination operation, the residue amount is decreased while useful product ZrCl is simultaneously generated.

In the second embodiment, significant residue reduction is achieved through lowering the carbon content in the solid feed mixture from current practice of about 21% to 19.3%. The carbon content is also strictly controlled so that the standard variation of the carbon concentration is at the level of +0.1% for each batch of 2000 lb. of feed mixture.

Detailed Description of the Invention In the past, carbochlorination residue has been dealt with most often by simply packaging and shipping the residue removed from the reactor to a disposal site. However, costs of such disposal have been rising rapidly and all indications are that they will continue to rise.

A further way to deal with the carbon-containing residue is to combust the carbon away with air and thereby achieve a reduction in the volume of residue. However, the residue is normally quite resistant to oxidation. Also, the reaction of water vapor in the air with the chloride salts gives rise to hydrogen chloride (HC1) , which is highly toxic and corrosive to process equipment. Likewise, the coke originally charged to the reactor normally has a sulfur content, which in combustion tends to be liberated as sulfur oxides and/or chlorides. This requires that a large amount of lime be fed to the combustor to neutralize and trap these acidic components. The combustion further generates a radioactive ash which must be confined. All these factors combine to drive up the capital and operating costs of a combustion apparatus for the carbochlorination residue.

Another way to deal with the carbochlorination residue is to use aqueous processing to dissolve the salts, isolate the solution which now contains some of the radioactive components, and subsequently treat it for suitable disposal. It is then possible to treat the solids fraction in another manner such as combustion, to recover some of the coke and zircon values. Some elements of this process are described in U.S. Patent 5,039,336. This process, however, has some drawbacks: Once the residue is contacted with water, the radioactive elements largely dissolve, giving a now-radioactive aqueous stream. Spills from such a stream are very consequential for entry to other water sources, and so very rigorous measures are needed to insure against such incidents. Such liquors often have relatively high radioactivity levels of radium per liter, meaning that spills of small quantities could contaminate to above drinking water standards. Measures to guard against such incidents are expensive. Furthermore, once radium has been solubilized (and therefore mobilized) , it must be rigorously sequestered in succeeding steps, including a co-precipitation with BaS0₄. It must be further processed for radioactive waste disposal.

Further, the expected behavior of the residue after such an aqueous contacting step is that it chlorinate thoroughly upon its return the chlorinator. This may not happen, for example, if the coke or zircon was a fraction which is inherently unreactive. In such a case, this fraction will not be consumed and the amount of residue will continue to increase.

The difficulties encountered in the prior industrial practices described for treating residue can be avoided by the practice of the present invention.

Carbochlorination of zircon sand proceeds at temperatures of 900-1200*C according to the reaction:

ZrSi0₄ + 4C + 4C1₂ -> ZrCl₄ + SiCl₄ + 4CO

Therefore, nearly all of the zircon in theory is chlorinated away. However, normal commercial zircon sand contains impurity elements, either as grains of other kinds of sand with which it occurs in nature, as well as those inevitable impurity elements which are in solid solution in the zircon crystal lattice. Many of these, such as alkali, alkaline earth and rare earth elements, along with thorium and radium, have chlorides which are relatively involatile and therefore tend to remain in the chlorination bed. It is widely believed that these chlorides together tend to form a molten salt mixture which coats the unreacted feed mix, making it unreactive. A known feature of residue is that despite the presence of both unreacted zircon and coke, it is resistant to further chlorination under the conditions described. These features of residue have also been described in U.S. Patent 5,039,336. A very significant feature of the residue is that the radioactivity due to naturally- occurring radionuclides, mostly thorium, and radium, now concentrated, are present in the residue and require it to be handled, stored and disposed of as radioactive waste. This can be a significant disposal problem as well as a significant element of overall process cost. Recently mounting costs and shortage of such disposal space have forced attention on the desirability to lowering waste volume.

Residue is difficult to characterize fully, however it can be described as being mostly or substantially carbon with some zircon and miscellaneous formed salts.

As previously described, the volume of carbon in the disposed residue makes a reduction in that component desirable. In conducting zircon chlorination operations, it is known that after a certain interval of feeding a ground mixture of coke and zircon to a fluid bed reactor, together with chlorine gas, an intolerable volume of residue has accumulated. The operation of removing it, known as a residue pull, is time-consuming, hazardous, and interruptive of production. The residue is allowed to flow from the bottom of the reactor into receptacles where it cools. Personnel in attendance must have proper thermal and radiological protection. All these factors dictate that the frequency of pulls and their amount be as small as possible.

Specific embodiments of the present invention are described hereinafter. In the first embodiment, normal carbochlorination of zircon operations is carried out to the point where an accumulation of residue has occurred. Specifically, a mixture of milled zircon sand plus coke is fed to a fluidized bed chlorinator, the fluidization being maintained by the chlorine gas fed to the bottom of the bed. After an interval, the pressure drop across the bed will increase providing evidence of an increased inventory of solids therein. At this time, the previous feed of zircon plus coke is suspended and replaced by Zr0₂, or baddeleyite fed or mixtures including zircon in such predetermined amounts as to keep the ZrCl₄ production rate at some desired level. Chlorine feed rate is maintained at that equal to the stoichiometric ratio of the Zr0₂ or mixed oxide. The Zr0₂ reacts with coke, which is the largest part of the residue, simultaneously consuming it and generating product ZrCl₄.

The following examples further describe the process of the present invention.

Example I

A composite residue sample (composed of several residue pulls) from commercial zircon carbochlorination operations was obtained and removed to a laboratory dry box to preserve its anhydrous condition. Chlorination of small samples in boats inside a quartz tube at 1050'C was done by flowing chlorine gas at 60 ml/min for two hours. At the end of this interval, the tube was cooled and the boat, as well as the tube, in which chlorination products (mostly ZrCl ) were condensed, were weighted. From experience, we know that such an arrangement captures virtually all of the ZrCl₄ and none of the SiCl_A. The boat contents were sand chlorination residue (SCR) and additives, the latter being either Zr0₂ or zircon flour. Thus, these experiments mimic the effect of in-reactor treatments to a bed of SCR, said treatments being undertaken to lower the amount of the SCR. The results of several runs performed as described are summarized in Table 1.

TABLE 1

Run Additive Mass net g weight reduction

Additive/ weights OUT of g factor Mass SCR IN products

1 none o 4.00 3.36 0.52 1.2

2 Zr0₂ 3 5.00 0.40 7.42 2.6

3 Zrθ₂ 3.5 5.00 0.38 7.33 2.4

4 Zr0₂ 4 5.00 0.28 7.69 3.0

5 zircon 3 5.00 1.54 3.65 — flour

♦defined as

= (weightIN)x(3.36/4.00)x(1/(1+(m_adc m_SCR) ) (weight OUT)^"1

The results in Table 1 show that there is a small tendency to chlorinate the supposedly unreactive residue, under the conditions of the experiments. (Run 1 is a blank intended to correct for this.) The amount of chlorination observed, a weight loss of 100(1- (3.36/4.00) )=16%. This factor has been used to adjust the amount of residue in the boat to an unchlorinatable fraction in the reduction factor calculations.

By adding Zr0₂, (runs 2,3,4) we can make use of the coke in the SCR while simultaneously eliminating residue, as well as, generating useful ZrCl^,. The radionuclides in the residue have been concentrated by a factor equal to the reduction factor, but is still easily within the category of low-level waste. The amount of residue which must be disposed of is less by the reduction factor. By adding zircon, (run 5) some reduction is achieved. We estimate that 40% of the residue was consumed by this measure. This is considered potentially useful since zircon is the material choice for commercial production.

Example II

Zircon sand and petroleum coke were co-milled to a level of about 20 weight percent carbon and this feed mixture was fed together with chlorine gas to a bank of five production chlorinators of identical design on a continuous basis for a period of several weeks. Product ZrCl_A was condensed, collected and weighed as the primary product of this operation. Periodically, according to operating conditions, the undesirable residue was removed from the reactor and weighed. All five of the reactors were given the same feed. Reactor E, the test reactor, was operated in a slightly different manner, in that a certain amount of zircon sand, in the form of milled flour, was added to the feed hopper according to the following schedule. Following a residue removal, the chlorinator was fed as normal until about 10,000 lb of product ZrCl₄ had been collected. At that time, 500 lb of zircon flour was added to the feed hopper. Thereafter, 500 lb of zircon flour was added to the feed hopper after each further 5,000 lb of ZrCl_A was generated. About

18,000 lb of zircon flour was added to chlorinator E over the period of the test. The production figures for ZrCl₄ and residue are shown for the reactors are shown below in Table 2. TABLE 2

Reactor lb ZrCl_A lb residue lb ZrCl /lb Designation residue

A 270,378 11,387 23.7

B 316,115 17,523 18.0

C 249,489 10,395 24.0

D 279,880 14,759 19.0

E 331,904 7,074 46.9

The test reactor more than doubled the product/residue ratio of the average non-test reactors. It is now contemplated that mixtures of zirconium compounds containing oxygen, including mixtures of Zr0₂ and zircon sand will also provide substantial reductions in the volume of chlorinator residue thereby achieving the objectives of the present invention.

In the second embodiment, the amount of the carbonaceous residue waste is reduced by lowering the amount of carbon fed into the process with strict control of the carbon level in the solid feed mixture. The previous practice of stoichiometric carbon requirement was based on the following single equation: ZrSiO^ + 4C + 4C1₂ -> ZrCl₄ + SiCl₄ + 4C0 According to this equation, the stoichiometric need for the carbon is: 4C 48

Wt%C = = 20.8%

4C + ZrSlO. 48 + 183.3

The carbon input to the process was set in such a manner that the resulting zircon and carbon mixture has an average carbon content of about 21%. The reactor off gas data show that the carbochlorination is more complicated than the single equation shown above. The reaction may be better described by two simultaneous reactions: ZrSiO_A + 4C + 4C1₂ = ZrCl + SiCl_A + 4 CO

ZrSiO + 2C + 4C1₂ = ZrCl + SiCl_A + 2 C0₂ The temperature largely determines which reaction is predominate. This is also reflected in the relative amount of CO, C0₂ in the off gas, or the ratio of CO/C0₂. Higher temperature favors the reaction that yields CO, resulting a larger CO/C0₂ ratio. Due to the complex nature of the carbochlorination, the stoichiometric requirement of the carbon for the reaction cannot be simply described by one equation alone. It has to be derived from the CO/C0₂ ratio.

In designing the experiment to reduce the residue by lowering the carbon in the feed mixture, 19.3% was chosen not to pursue the maximum effect of residue reduced, but to take the following two points into considerations.

First, 19.3% was chosen to show that by lowering the carbon in the feed from about 21%, the residue can be reduced and to show that a small amount of change of the feed carbon percentage will have a large effect on the residue reduction. The product (crude zirconium tetrachloride) to residue ratio has a high "background noise" and the 19.3% was chosen to result in an estimated product to residue ratio that was statistically different from the previous practice. Second, 19.3% was also chosen to still have some excess carbon in the feed mixture over the stoichiometric requirement so that the graphite reactor tube was adequately protected from being used as a carbon source. The experiment can be carried out without interruption. If the mixture supplier failed to deliver the required accuracy, there would still. be a safety zone left in the lower end before the carbon content would drop below the stoichiometric requirement. By reducing the amount of carbon in the premix feed from the stoichiometric amount of about 20.8% to 19.3% there is a reduction in the amount of residue. This reduction is not just a 1.5 % difference as in the difference of feed carbon, but instead it is a unexpectedly substantial difference of 36.3 % reduction in the amount of residue waste produced.

Furthermore, one would not normally consider such a reduction in the amount of carbon. From the stoichiometric equation above, one would expect that there would be required at least the stoichiometric amount of carbon present which is 20.8 wt % of carbon. In conventional chemical operations the limiting reactant is normally chosen to be in excess of the stoichiometric amount to drive the reaction. Some industry practice utilizes between 21% to 21.9% by weight carbon. One would not expect to use below the stoichiometric amount of carbon because then one would be wasting the zircon.

Example III

A run was conducted in a commercial Chlorinator for

118 days.

The premixed feed used contained 19.3 wt% carbon in the form of milled coke which was blended with milled premium grade zircon. During the run about 2 to 3 times a month the

Chlorinator was shut down and the residue was removed.

There was a total of 9 shut downs with residue waste collection.

The residue recovered from the 9 shutdowns in the run in this example weighed 15,946 pounds. The amount of

ZrCl_A made during the run was 525,675 pounds. The weight ratio of product zirconium chloride to residue waste was about 33. The normal ratio obtained from these commercial

Chlorinators is 21. For the production of 525,675 lb of ZrCl₄ at the conventional zirconium chloride to waste ratio of 21, the amount of residue waste produced would be 25,032 lb. Since only 15,946 lb was made using the 19.3 wt % carbon premix feed in this Example III, there was a significant reduction of 9,086 lb of waste residue. Expressed in terms of the conventionally expected 25,030 lb of waste, there was a reduction of 36.3 % in residue waste.

A further way to measure the reduction in the residue waste is to measure the amount of radium in the residue. Radium from the zircon sand is retained in the

Chlorinator by the primary filters. Since the total amount of radium remains the same for a given amount of crude chloride production, a higher radium level measured in the residue corresponds to a lesser amount of residue generated. The reason for this phenomena is that there will be less carbonaceous material present to dilute the same amount of radium. The radium level in the residues from the run is in the range of 2400-3100 pCi/g. The normal range for radium is 1000-1700 pCi/gram for runs where this amount of ZrCl₄ is made. Thus the radium analysis confirms the low amount of residue present.

Reviewing the data obtained and its correlation to the CO/C0₂ ratios observed at the temperatures employed, it is contemplated that the percent by weight of carbon utilized could be as low as about 17% to 18%. An additional advantage achieved by the present invention is that the buildup of the residue in the chlorinator is much slower than the previous practice. Thus, the residue pull is much less frequent. The amount of zirconium tetrachloride produced in between the residue pulls may also be used as a good indicator. In previous practice, about 40,000-50,000 lb. of zirconium tetrachloride could be made between residue pulls. With the slower residue buildup, 60,000 lb. and even up to 96,000 lb. of zirconium tetrachloride can be produced between residue pulls.

This invention has been described with respect to the best modes contemplated for its practice and it will be appreciated that variations in the operating conditions and materials utilized are possible and are included in the scope of the appended claims as limited only by the applicable prior art.

Claims

Claims 1. In a carbochlorination process for the production of volatile metal chlorides wherein chlorine gas contacts a mixed metal oxide in a reaction zone in the presence of carbon at elevated temperatures, the improvement comprising periodically introducing metal oxide or mixed metal oxide into the reaction zone during chlorination and after the build up of chlorination residue without the addition of additional carbon to thereby react a portion of the carbon in the residue by the carbochlorination reaction, whereby the volume of residue in the reaction zone is reduced.

2. The process of Claim 1, wherein the mixed metal oxide is zircon sand and the reaction zone is maintained at a temperature of from about 900'C to about 1200"C.

3. The process of Claims 1, wherein the metal oxide is zirconium oxide or baddeleyite.

4. The process of Claim 3, wherein the mixed metal oxide is zircon.

5. The process of Claim 1, wherein the chlorination reaction is continued and additional mixed metal oxide and carbon-containing material are again jointly introduced into the reaction zone.

6. The process of Claim 5, wherein the mixed metal oxide^* is zircon sand and the reaction zone is maintained at a temperature of from about 900'C to about 1200*C.

7. The process of Claim 5, wherein the metal oxide is zirconium oxide or-baddeleyite.

8. The process of Claim 7, wherein the mixed metal oxide is zircon.

9. In a carbochlorination process for the production of volatile metal chlorides wherein chlorine gas contacts a mixed metal oxide in a reaction zone in the presence of 20 to 21 wt % carbon at elevated temperatures and a residue waste is formed, the improvement comprising reducing the amount of carbon fed to the process to between about 17 wt % to about 19.3 wt % based on the weight of the mixed metal oxide and carbon, whereby the volume of residue waste in the reaction zone is reduced.

10. In a carbochlorination process for the production of volatile metal chlorides wherein chlorine gas contacts a mixed metal oxide in a reaction zone in the presence of about 20 to 21 wt % carbon at elevated temperatures and a residue waste is formed, the improvement comprising reducing the amount of the residue waste by either:

a) periodically introducing metal oxide or mixed metal oxide into the reaction zone during chlorination and after the build up of chlorination residue without the addition of additional carbon to thereby react a portion of the carbon in the residue by the carbochlorination reaction, or

b) reducing the amount of carbon fed to the process to between about 17 wt % to about 19.3 wt % based on the weight of the mixed metal oxide and carbon,

whereby the volume of residue waste in the reaction zone is reduced.