The present invention relates to a process for stripping chloride impurities from hot raw metal oxide particles produced by the vapor phase oxidation of the corresponding metal chloride at an elevated temperature.

2. Description of the Prior Art.

A known method of producing metal oxides such as silicon dioxide or titanium dioxide is to react the corresponding metal chloride in the vapor phase with oxygen at an elevated temperature, e. g., 1200°F to 1500°F. The raw metal oxide particles produced in the reaction are entrained in gaseous reaction products which are separated from the metal oxide particles in a centrifugal separator or the like. The raw metal oxide particles recovered from the separation contain low concentrations of chloride impurities such as metal chlorides and hydrochloric acid. The chloride impurities are adsorbed and chemisorbed on the surfaces of the metal oxide particles, usually in amounts of from about 0.18% to about 0.36% by weight of the particles. The chloride impurities must be removed from the metal oxide particles because they are incompatible with product applications such as pigmenting paints, enamels, finishes and the like.

A variety of processes for removing chloride impurities from raw metal oxide particles have been developed which include calcining the raw metal oxide particles, contacting the raw metal oxide particles at high temperature with treating agents such as water vapor, air, oxygen, and mixtures thereof, contacting the raw metal oxide particles with steam, mixing the raw metal oxides with water whereby the chloride impurities are converted to hydrochloric acid which is subsequently neutralized with caustic followed by washing the de-chlorinated metal oxide particles, all of which are time consuming and/or expensive.

Thus, there is a need for an improved process for removing chloride impurities from raw metal oxide particles which is simple, fast and relatively inexpensive to carry out.

Summary of the Invention The present invention provides a process for stripping chloride impurities from hot raw metal oxide particles produced by the vapor phase oxidation of the corresponding

metal chloride which meets the need described above and overcomes the deficiencies of the prior art. The process is basically comprised of the following steps. The hot raw metal oxide particles containing chloride impurities from the oxidation process are introduced into the top portion of a stripper column. A heated stripping gas containing water vapor is introduced into a bottom portion of the stripper column so that the stripping gas flows upwardly in the stripper column and the metal oxide particles flow downwardly by gravity through the stripping gas. As a result, the chloride impurities are stripped from the metal oxide particles by the stripping gas. The stripping gas and chloride impurities are removed from the top of the stripper column and the chloride impurity stripped metal oxide particles are removed from the bottom of the stripper column.

Thus, it is a general object of the present invention to provide an improved process for stripping chloride impurities from raw metal oxide particles.

Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows when taken in conjunction with the accompanying drawing.

Brief Description of the Drawing In the drawing forming a part of this disclosure, a side cross-sectional view of apparatus including a stripper column which can be utilized in accordance with this invention is illustrated.

Description of Preferred Embodiments As mentioned, metal oxide particles produced by the vapor phase oxidation of the corresponding metal chloride at an elevated temperature contain chloride impurities such as metal chlorides and hydrogen chloride which are adsorbed and chemisorbed to the surfaces of the metal oxide particles. The metal oxide particles containing the chloride impurities are referred to in the art as raw metal oxide particles.

A variety of metal oxides can be produced by the vapor phase oxidation of the corresponding metal chloride including, but not limited to, titanium dioxide, silicon dioxide, aluminum oxide, zirconium dioxide, iron oxide and antimony oxide. Of these, titanium dioxide and silicon dioxide are commonly utilized as pigments. The pigmentary properties of metal oxide particles containing chloride impurities, and particularly of titanium dioxide and silicon dioxide particles, are improved by removing the chloride impurities therefrom.

In accordance with the improved process of the present invention, a major portion of the chloride impurities is removed from raw metal oxide particles by stripping the chloride impurities from the particles utilizing a heated stripping gas containing water vapor. That is, the hot raw metal oxide particles produced in the vapor phase oxidation of the corresponding metal chloride containing chloride impurities are introduced into a top portion of a vertically positioned stripper column. Simultaneously, a heated stripping gas containing water vapor is introduced into a bottom portion of the stripper column so that the stripping gas flows upwardly in the stripper column and the metal oxide particles flow downwardly by gravity through the stripping gas. As a result of the contact between the hot raw metal oxide particles and the heated stripping gas, the adsorbed and chemisorbed chloride impurities are stripped from the metal oxide particles by the stripping gas. The stripping gas and the chloride impurities are removed from the stripping gas column at the top thereof and the chloride impurities stripped metal oxide particles are removed from the stripper column at the bottom thereof. Thus, there is no opportunity for the stripped metal oxide particles to re-contact the chloride impurities and re-adsorb them after the stripping process takes place.

The hot raw metal oxide particles containing chloride impurities and gaseous oxidation products produced in the vapor phase oxidation of the corresponding metal chloride are generally at a temperature in the range of from about 875°F to about 1000°F after exiting the oxidization reactor. The gaseous oxidation products are disengaged from the hot raw metal oxide particles and reactor scouring media, e. g., sand (if used). The temperature of the hot raw metal oxide particles and reactor scouring media (if used) after separation is in the range of from about 875°F to about 1000°F, more often in the range of from about 875°F to about 950°F, and most often about 900°F. After separation, the hot raw metal oxide particles and reactor scouring media (if used) are introduced into the stripper column where they flow downwardly in intimate contact with upwardly flowing stripping gas.

The heated stripping gas can be air or nitrogen containing water vapor. Generally, air containing water vapor is preferred from a cost standpoint. The heated stripping gas preferably contains water vapor in an amount of from about 0.012% by weight of the stripping gas up to saturation, most preferably in an amount of at least about 0.024% by weight of the stripping gas at the temperature to which the stripping gas is heated. The stripping gas is generally heated to a temperature in the range of from about 850°F to

about 950°F, more preferably from about 875°F to about 900°F and most preferably about 900°F.

In operation, and as a result of the high temperatures of the hot raw metal oxide particles and heated stripping gas entering the stripper column, the contact between the hot raw metal oxide particles and stripping gas in the stripper column takes place at a temperature in the range of from about 850°F to about 950°F, more preferably from about 875°F to about 925°F and most preferably about 900°F. The pressure in the stripper column during its operation is generally in the range of from about 0 psig to about 0.3 psig. This pressure is not critical so long as the residue gas will flow through one or more gas scrubbing systems downstream of the stripper column.

The weight ratio of the heated stripping gas containing water vapor introduced into the stripper column to the hot raw metal oxide particles containing chloride impurities introduced therein is generally in the range of from about 2.2: 100 to about 10.4: 100, more preferably from about 2.2: 100 to about 5.2: 100 and most preferably about 2.2: 100. The raw metal oxide particles introduced into the stripper column generally contain chloride impurities in an amount in the range of from about 0.18% to about 0.36% by weight of the raw metal oxide particles. By the process of this invention, the chloride impurity content of the metal oxide particles is reduced to a level in the range of from about 0.07% to about 0.08% by weight of the metal oxide particles.

Referring now to the drawing, an apparatus for carrying out the stripping process of the present invention is illustrated and designated by the numeral 10. The apparatus 10 includes an elongated vertical column 12 having a top end 14 and a bottom end 16. A heated stripping gas inlet connection 18 is provided near the bottom 16 of the stripper column 12 for introducing heated stripping gas containing water vapor in an upward direction therein. A stripping gas-chloride impurity outlet connection 20 is connected to the top 14 of the stripper column 12. The stripping gas-chloride impurity mixture produced is conducted from the stripper column 12 to a point of chloride recovery or disposal.

An inlet connection 22 for the hot raw metal oxide pigment is provided in the top 14 of the stripper column 12. The inlet connection 22 extends downwardly a distance within the top portion of the stripper column 12 in order to prevent carry over of the downwardly flowing metal oxide pigment with the stripping gas-chloride impurity mixture exiting the stripper column 12. As will be understood by those skilled in the art, various

arrangements of baffles or the like can also be utilized in the top portion of the stripper column 12 to eliminate or minimize such carry over. As shown in the drawing, the inlet connection 22 can be directly attached to a separator 24 for separating the hot raw metal oxide pigment which contains reactor scouring media (if used) from gaseous oxidization reaction products. A rotary or other type of isolating valve 25 can be included at the outlet of the separator 24.

Attached to the bottom open end of the stripper column 12 is an accumulator 26 for accumulating the de-chlorinated metal oxide particles produced in the stripper column 12.

A valve 28 is utilized to discharge the de-chlorinated metal oxide particles from the accumulator 26 without allowing stripping gas to flow out of the apparatus 10 with the pigment. The stripper column 12 and the separator 24 attached thereto preferably include an outer layer of insulation 30 for reducing heat transfer through the walls thereof.

A process of the present invention for stripping chloride impurities from hot raw metal oxide particles produced by the vapor phase oxidation of the corresponding metal chloride is comprised of the following steps: (a) introducing hot metal oxide particles containing chloride impurities at a temperature in the range of from about 900°F to about 950°F into a top portion of a stripper column; (b) introducing a heated stripping gas containing water vapor having a temperature in the range of from about 850°F to about 900°F into a bottom potion of the stripper column so that the stripping gas flows upwardly in the stripper column and the metal oxide particles flow downwardly by gravity therethrough whereby chloride impurities are stripped from the metal oxide particles by the stripping gas; (c) removing the stripping gas and the chloride impurities from the top of the stripper column; and (d) removing the chloride impurity stripped metal oxide particles from the bottom of the stripper column.

The stripping gas is preferably air containing water vapor in an amount in the range of from about 0.012% by weight of the stripping gas up to saturation at the temperature at which it is injected in the stripping column. The temperature in the stripper column is generally in the range of from about 875°F to about 950°F at a pressure in the range of from about 0 psig to about 0.3 psig. The weight ratio of the heated stripping gas containing water vapor introduced into the stripper column in accordance with step (b) to

the hot metal oxide particles containing chloride impurities introduced therein in accordance with step (a) is generally in the range of from about 2.2: 100 to about 10.4: 100.

Another process of this invention for stripping chloride impurities from hot raw titanium dioxide particles produced by the vapor phase oxidation of titanium tetrachloride comprises the following steps: (a) introducing the hot titanium dioxide particles containing chloride impurities and having a temperature in the range of from about 900°F to about 950°F into a top portion of a stripper column; (b) introducing a heated stripping gas comprised of air containing water vapor in an amount in the range of from about 0. 012% by weight of the stripping gas up to saturation having a temperature in the range of from about 875°F to about 900°F into a bottom portion of the stripper column so that the stripping gas flows upwardly in the stripper column and the titanium dioxide particles flow downwardly by gravity therethrough whereby the chloride impurities are stripped from the titanium dioxide particles by the stripping gas; (c) removing the stripping gas and the chloride impurities from the top of the stripper column; and (d) removing the chloride impurity stripped titanium dioxide particles from the bottom of the stripper column.

The weight ratio of the heated stripping gas containing water vapor introduced into the stripper column in accordance with step (b) to the hot titanium oxide particles containing the chloride impurities introduced therein in accordance with step (a) is in the range of from about 2.2: 100 to about 10.4: 100, most preferably about 2.2: 100.

In order to further illustrate the improved process of this invention, the following example is given.

Example A laboratory apparatus was assembled which included a heated stripper column formed of a quartz tube connected at its bottom end to a flask by a neoprene sleeve. An inlet connection was passed through the neoprene sleeve for directing heated stripping gas comprised of nitrogen containing water vapor upwardly through the quartz tube. A heated screw conveyor was utilized to heat raw titanium dioxide pigment containing about 0.21% by weight chloride impurities and conveying the pigment particles to the top of the quartz tube where they were directed downwardly into the tube.

The screw conveyor heated the raw titanium dioxide pigment to about 900°F and the preheated nitrogen containing water vapor was heated to about 850°F. The interior temperature of the stripper column was maintained at about 940°F. The stripping gas (nitrogen) contained water vapor in an amount of about 0.012% by weight of the stripping gas. The titanium dioxide pigment was passed through the stripper column at about 7 grams per minute and the nitrogen stripping gas containing water vapor was introduced into the stripper column at a rate which allowed the titanium dioxide pigment to flow by gravity downwardly through the stripper column and through the stripping gas flowing upwardly therein. The titanium dioxide chloride impurity stripped product was cooled and a sample was analyzed for chloride content. The chloride content of the stripped titanium dioxide was 0.09% by weight thereof.

Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned as well as those which are inherent therein. While numerous changes may be made by those skilled in the art, such changes are encompassed within the spirit of this invention as defined by the appended claims.

What is claimed is: