TITLE A PROCESS FOR CONTROLLING THE PASSIVATION OF ALUMINUM CHLORIDE FORMED IN THE CHLORINATION OF TITANIUM- CONTAINING ORES BACKGROUND OF THE INVENTION In the production of titanium tetrachloride, raw materials, including ilmenite or rutile ores or other titanium-rich materials such as those obtained from beneficiating these ores, are reacted with chlorine under reducing conditions to yield a mixture of metal chlorides from which titanium tetrachloride may be recovered.

Aluminum chloride is present in the chlorination product of practically every case where aluminum compounds are present in the raw materials.

Aluminum chloride present in the crude titanium tetrachloride is a highly corrosive material. It both quickly and severely attacks the metal materials of construction in the purification systems.

Prior art has taught methods by which aluminum chloride present in the crude titanium tetrachloride may be passivated, meaning the aluminum chloride is converted to a less corrosive or non-corrosive compound.

US Patent No. 2,600,881 to Kay and Rick teaches the addition of water to the crude titanium tetrachloride. According to this patent, water is added in an amount that is sufficient to react with only the aluminum chloride present in the crude titanium tetrachloride. Water reacts with the aluminum chloride and converts it to a less volatile, non- corrosive hydrate or oxychloride. Although this patent teaches a method of passivating the aluminum chloride, there continued to be a major disadvantage. If water was added in excess of that amount just sufficient to remove the aluminum chloride, the excess water reacted

with the titanium tetrachloride converting it to hydrolysis products resulting in losses of titanium value. This patent offered no teaching or suggestion as to how to avoid the addition of excess water or how to accommodate the variations in aluminum chloride content of crude titanium tetrachloride that may be experienced in a continuous industrial process.

It was reported in a US Patent No. 4,125,586 to Glaeser that to successfully apply the method taught in US 2,600,881 on a commercial scale required that at least 10% excess water be added in order to effectively passivate the aluminum chloride. As an improvement to reduce losses of titanium value, US 4,125,586 taught that loss in titanium value could be essentially eliminated by adding a mixture of water and sodium chloride to the crude titanium tetrachloride. The water addition was made in an amount less than that needed to convert all the aluminum chloride present to aluminum oxychloride; but the sum of the total amounts of water and sodium chloride added was in excess of that amount needed to react with the aluminum chloride present.

While US 4,125,586 was an improvement, there was still the need to have a method that could control passivation of aluminum chloride present in the crude titanium tetrachloride at a process aim point reducing both the losses of titanium value and the corrosion.

An accurate, in process control method was needed that could respond to the demands of continuous plant operation where there are variations in the aluminum chloride content of the crude titanium tetrachloride due to (1) variations in the uniformity in the distribution of the aluminum containing minerals in the ore being processed; (2) variations in the rate at which ore is processed; (3) variations in aluminum content from one ore source to another; and (4) variation in the content of mineral species other than

those containing aluminum and titanium that consume the passivating agent.

SUMMARY OF THE INVENTION The present invention is a process control loop to passivate aluminum chloride present in crude titanium tetrachloride comprising the steps: (a) adding an aluminum chloride passivating agent at a known addition rate to the crude titanium tetrachloride to form a passivated crude titanium tetrachloride; (b) measuring the concentration of titanium oxychloride in the passivated crude titanium tetrachloride; (c) comparing the measured concentration of titanium oxychloride to that of a predetermined aim point concentration; and (d) adjusting the rate of addition of the aluminum chloride passivating agent to achieve the predetermined aim point concentration of titanium oxychloride.

The aluminum chloride passivating agent is selected from the group consisting of water, water containing solutions, water containing mixtures, and carboxylic acids.

In the present process it is preferred to measure the concentration of titanium oxychloride by an optical method selected from the group consisting of transmission filter Infrared spectroscopy, transmission Fourier Transform Infrared spectroscopy, Raman spectroscopy, Attenuated Total Reflectance Infrared spectroscopy, and Attenuated Total Reflectance Fourier Transform Infrared spectroscopy.

It is also preferred that the measurement of the concentration of titanium oxychloride be made in a frequency range of from 800 to 2000 cm-1.

It is most preferred that the concentration of titanium oxychloride is measured by diamond based

Attenuated Total Reflectance Fourier Transform Infrared at a frequency of about 820cm-1.

BRIEF DESCRIPTION OF THE DRAWING The figure illustrates a control loop of the present invention.

DETAILED DESCRIPTION The present invention is a process control loop capable of controlling, at an aim point, the passivation of aluminum chloride present in titanium tetrachloride. The phrase passivation of aluminum chloride as used herein means that the aluminum chloride is converted to a less corrosive or non- corrosive compound.

In the present process the aluminum chloride is made non-corrosive by reacting the aluminum chloride containing crude titanium tetrachloride with an aluminum chloride passivating agent. The term aluminum chloride passivating agent as used herein means an agent that reacts with the aluminum chloride to form a non-corrosive or essentially non-corrosive aluminum containing compound and also results in the production of titanium oxychloride. Passivating agents include water, water solutions, water containing mixtures, and carboxylic acids. For example, water solutions of sodium chloride or sodium hydroxide or water solutions or mixtures containing carboxylic acids are suitable as passivating agents for the aluminum chloride.

Although chemical theory offers some guidance in the selection of an aluminum chloride passivating agent, one may screen passivating agents by adding the material in question, alone or as a solution or mixture in water, to titanium tetrachloride containing a known amount of aluminum chloride to form a treated sample.

The concentration of aluminum chloride in the treated

sample is then measured to determine the amount of aluminum chloride that has reacted and the amount of titanium oxychloride formed. The reduction in corrosion is determined by comparing the corrosion of the treated sample to that of a control using metal coupons. Selection of a passivating agent may also require such considerations as cost and the material's overall acceptability for an industrial process.

Water is most preferred as a passivating agent in the present process. Water solutions or mixtures may be used as passivating agents even if the materials other than the water show no reactivity towards the aluminum chloride. To realize the full potential of the present invention, it is clear that one must avoid the use of a material that causes instability (reaction to re-form aluminum chloride) in the non-corrosive aluminum compound formed, that uncontrollably depletes the titanium value of the crude titanium tetrachloride, or produces solid deposits on equipment surfaces.

The present invention is designed for use in process. In the present control loop, it is preferred to place the addition of the aluminum chloride passivating agent as near the point of titanium tetrachloride condensation as possible since it is desired to passivate the aluminum chloride as soon as possible to reduce any chance of corrosion.

Although the mechanism of the reaction is not fully understood, the inventors have found that aluminum chloride is passivated and the extent of the passivation is indirectly indicated, and may be controlled by the monitoring the formation of titanium oxychloride. The inventors have found that titanium oxychloride is not a persistent species as long as aluminum chloride is present in the titanium tetrachloride. Therefore, the presence of titanium oxychloride in the process stream indicates that the aluminum chloride has been passivated. Since titanium oxychloride is soluble in titanium tetrachloride at the

process conditions that exist where it is desirable to passivate the aluminum chloride, the concentration of titianium oxychloride provides a means both to monitor and to control aluminum chloride passivation by in process measurement.

The formation of titanium oxychloride represents a loss of titanium value; but solubility and detectability of this oxychloride provide a means to control the addition of water or another aluminum chloride passivating agent to an amount that is only minimally above that required to react with the aluminum chloride. The presence and concentration of titanium oxychloride may be measured by use of transmission filter Infrared spectroscopy, transmission Fourier Transform Infrared spectroscopy, Raman spectroscopy, and Attenuated Total Reflectance Infrared spectroscopy, and Attenuated Total Reflectance Fourier Transform Infrared spectroscopy in a frequency range of from 800 to 2000 cm-1.

In the raw process stream from a chlorinator, for example, as much as 10% by weight of the material may be particulate material that scatters or absorbs light.

Also if water is used as a passivating agent, particles may form immediately, even at process conditions, in the passivated crude titanium tetrachloride. It is recommended to use attenuated reflectance methods for measurements made on the raw process stream where particle concentrations are high. For accuracy and precision, it is most preferred to measure the presence and the concentration of titanium oxychloride by diamond based Attenuated Total Reflectance Fourier Transform Infrared at a frequency of about 820cm-1.

Diamond based attenuated reflectance means that the infrared probe or detector placed in the process stream has a diamond element. Suitable probe units include those manufactured by ASI Applied Systems of Millersville, MD, Axiom Analytic, Inc. of Irvine, CA and others.

The probe or detector may be located in the immediate vicinity of the addition point for the aluminum chloride passivating agent or downstream. Its actual location is not critical as long as it is located in an area where the temperature ensures that titanium oxychloride will be in solution. Measurement of the concentration of titanium oxychloride may also be made using transmission methods following the in-line filtering or screening out of interfering particles. In a case where particle concentrations are less than a percent-or-so in the crude titanium tetrachloride, or are due only to the formation of the insoluble hydrolysis products on addition of water or a carboxylic acid; the presence of particles may be accommodated by the use of multiple scan averaging techniques.

In general the operation of the present invention may be described by the illustrated control loop of the Figure. The control loop comprises a control devise, a feedback controller, and an analyzer. The analyzer includes the signal generator (frequency range of from 800 to 2000 cm-1), and detector. The analyzer measures the concentration of titanium oxychloride downstream from the point of addition of the passivating agent.

The analyzer produces an output signal representing the measured concentration of titanium oxychloride. This output signal is sent to the feedback controller.

There the concentration of the titanium oxychloride measured by the analyzer is compared to the predetermined aim point. The feedback controller, based on this comparison, provides input to the control device to adjust the addition rate of the passivating agent to restore or maintain the concentration of the titanium oxychloride at the aim point. Algorithms used in the analyzer and the feed back controller to convert the data collected to a signal output are not critical.

One skilled in this art can select or design an algorithm suitable to the specific type of analyzer or

feedback controller. The control device may be any regulated flow device equipped with an automatic actuator. Typically the control device is a valve.

The present process allows control of the passivation of aluminum chloride by indirectly controlling the addition of a passivating agent in real time by monitoring the concentration of titanium oxychloride present in the crude titanium tetrachloride formed on the addition of the passivating agent. The present control loop responds rapidly to variations in the aluminum chloride content of the crude titanium tetrachloride indirectly by comparing the concentration of titanium oxychloride present at a certain time with the concentration selected as the aim point. It is desirable to set the aim point at the lowest reliable concentration of titanium oxychloride that is detected by the analytical detection devise used in the control loop. The rate of addition of the passivating agent is automatically adjusted to compensate for any increase or decrease of the titanium oxychloride that results from the increase or decrease in the aluminum chloride present in the crude titanium tetrachloride.

Any suitable analytical detection method may be used in the present invention. As stated above, it is most preferred to monitor at a frequency of 820 cm-1 using the Fourier Transform infrared method. This provides sensitivity and precision for the in-process measurements in the presence of high concentrations of particles. Using this technique titanium oxychloride in concentrations as low as 0.1% may be reliably measured.

The control loop of the present invention provides a means to reduce the addition of water or other passivating agents to an amount minimally in excess of that needed to react with all the aluminum chloride present. The resulting reductions in corrosion and deposition of aluminum containing solids in the present process are equivalent to methods of the prior art; yet, at the same time, the present process allows real time on-aim process control with controlled addition of water or other aluminum chloride passivating agent to substantially reduce the loss of titanium value.