METHOD FOR PURIFYING TiO2 ORE

Abstract: Process for beneficiating titanium bearing ore selected from the group consisting essentially of titaniferous ores, leucoxene, rutile, perovskite, and sphene comprising: (a) contacting said ore with an aqueous solution of a mineral acid having a concentration of about 3-30 percent by weight, said contacting taking place at a temperature of about 160-300 DEG C, until the desired amount of impurities are solubilized and a leachate is formed; (b) removing the leachate from the product of step (a).

Description: TITLE METHOD FOR PURIFYING Ti0 2 ORE CROSS-REFERENCE TO RELATED APPLICATION This is a Continuation-In-Part Application of U.S. Patent Application Serial-No. 07/613,049 filed November 15, 1990, which is a Continuation-in-Part Application of U.S. Patent Application Serial No. 07/490,195 filed March 8, 1990, now abandoned. BACKGROUND OF THE INVENTION 1. Field of the Invention: This invention relates to an improved method for purifying Tiθ2 ore which contains numerous impuri¬ ties including unacceptable levels of naturally occurring radionuclides (NORS) such as thorium and uranium. The purified ore can be used to make Ti0 2 pigment or titanium metal or be used in any other process where a purified Tiθ2 ore is required. This invention especially relates to removing impurities from titaniferous ores, leucoxene, rutile, perovskite, sphene, and their derivatives or intermediates such as blow-over in the chloride process. 2. Description of Related Art: Currently, approximately 75 percent of the titanium minerals produced in the world are utilized by the pigments industry to produce Tiθ2« In the production of Tiθ2 by the chloride process, benefici- ated ore is used which generally contains about 55-96% Tiθ2« The beneficiation processes are aimed at removing impurities such as alkali metals, alkaline earth metals, rare earth metals, iron, aluminum. siliσon, phosphorus, thorium, uranium, chromium, manganese, vanadium, and yttrium. These impurities may be present as oxides, salts, or other complex forms. Especially detrimental to the chloride proce are such ores which contain in considerable quantity the impurities of iron, calcium, aluminum, phosphoru magnesium, barium and strontium, and radionuclides such as thorium and uranium (and their daughters of radioactive decay) . For example, phosphorus can cau processing problems in the chloride Tiθ2 process, a thorium and uranium may concentrate in the Tiθ2 process and present a potential health hazard. Also, the impurities of aluminum, iron, phosphorus, thoriu and uranium are additionally a problem because they are especially resistant to removal by conventional mechanical or chemical means. Finally, alkaline earth metals can impair fluidization in the Tiθ2 fluidized bed chlorinator. Being able to remove such impurities efficiently would be highly desirable because known sources of Tiθ2 ore not containing such impurities ar becoming increasingly scarce and expensive. Moreover while other processes to purify Tiθ2 ore are known, i appears that they either require additional, more complex or more expensive processing steps or are deficient in one or more benefits as compared to the process of this invention. For titaniferous, leucoxene, rutile, perovskite, and sphene ores, impurities which are especially important to reduce to acceptable levels are iron, manganese, calcium, and radionuclides such as thorium and uranium. It is important that iron be reduced to acceptable levels (1) because it often is a major impurity which can cause substantial chlorine consumption in the chloride process for producing Tiθ2, and (2) it will form iron chlorides in the chloride Tiθ2 process, and such iron chlorides can b a disposal problem. It also is important that manganese be reduced to acceptable levels. This is because manganese is a high boiling material which c coalesce and form a hard slag on the interior of the flue exiting the fluidized bed chlorinator, which is the first step of the chloride iθ2 process. Note that manganese is commonly associated with titanif- erous ores such as ilmenite. Finally, it is importa that the radionuclides be reduced to acceptable level because they can present potential health problems. In purifying titaniferous ore, leucoxene, rutile, perovskite, and sphene, it is also important that the Tiθ2 content in the ore be upgraded to a reasonably high level so that output of Tiθ2 from the Tiθ2 process is optimized and processing problems associated with removing ore impurities from the process are minimized. Therefore, generally, the Tiθ content in the beneficiated ore should be upgraded to at least 75 percent, preferably to at least 80 percent, and most preferably to at least about 90 percent. While some prior art processes can remove some of the aforementioned impurities, they typically require prereduction or preoxidation followed by prereduction as an essential step. Prereduction and/or preoxidation is undesirable because it is expensive (due to substantial energy and investment requirements) and tends to make it more difficult to remove radionuclides. It therefore would be desirabl to have a beneficiation process which did not require prereduction as an essential step. The following information is disclosed whi may be of interest to this invention: U. S. Patent 4,176,159 discloses a process for the removal of impurities from rutile, ilmenite, and leucoxene ores. The process requires high temperature calcining, cooling, reducing, cooling, magnetic separation, mineral acid leaching, neutralizing, and washing. U. S. Patent 4,562,048 discloses the bene- ficiation of titaniferous ores by leaching with a mineral acid. The temperature used is 120-150"C, an the pressure used is 10-45 pounds per square inch gauge ("psig*) . An essential aspect is the venting o water vapor generated during the leaching process. Prior to leaching, the ore is reduced at about 600-1100'C. U. S. Patent 4,321,236 discloses a process for beneficiating titaniferous ore. The process requires preheating the titaniferous ore and a minera acid prior to the leaching operation. The temperatur is maintained at 110-150"C, and the pressure is maintained at 20-50 psig. For ores containing iron i the ferric state, reductive roasting at about 800-1100°C is suggested prior to leaching. U. S. Patent 4,019,898 discloses the addi¬ tion of a small amount of sulfuric acid to the leach liquor used to beneficiate ilmenite ore. The tempera ture used is 100-150"C, and the pressure used is up to 50 psig. For ores containing iron in the ferric state, the ore is reduced prior to leaching at a temperature of about 700-1200"C. U.S. Patent 3,060,002 discloses acid leaching of ilmenite and Sorel slag at temperature of 150-250'C. Prior to leaching, the ore preferably is roasted oxidatively at about 500-1000*C. SUMMARY OF THE INVENTION Process for reducing the amount of natural occuring radionuclides such as thorium and uranium a their daughters of radioactive decay in a titanium bearing ore selected from the group consisting essentially of titaniferous ores, leucoxene, rutile, perovskite, sphene, and derivatives thereof, comprising: (a) contacting said titanium bearing ore, wherein said ore is characterized as containing an unaceptable level of at least one naturally occurring radionuclide selected from the group consisting of thorium and uranium, with an aqueous solution of a mineral acid having an aci concentration of about 3-30 percent by weight, said contacting taking place at a temperature range in excess of 150"C up to about 300*C, unti the desired amount of thorium and uranium are solubilized and a leachate is formed; and thereafter, (b) removing and recovering the remaining ore of reduced level of radionuclide from the leachate. A preferred process of this invention is as follows: Process for beneficiating titanium bearing ore selected from the group consisting essentially of titaniferous ores, leucoxene, rutile, perovskite, sphene, and their derivatives comprising: (a) contacting said titanium bearing ore, wherein said ore is characterized as containing an unaceptable level of at least one naturally occurring radionuclide selected from the group consisting of thorium and uranium, with an aqueous solution of a mineral acid having an aci concentration of about 5-30 percent by weight, said contacting taking place at a temperature o about 160-300"C until the desired amount of impurities including thorium and uranium are solubilized and a leachate is formed, and thereafter, (b) removing recovering the remaining ore of reduce level of impurities from the leachate. There is also provided by this invention Tiθ2 pigment which is produced by the chloride proce from the purified Tiθ2 ore of this invention. In accordance with this invention, it has been found that most of the aforementioned impuritie in ore (and especially iron, manganese, calcium, aluminum, phosphorus, magnesium, barium, strontium, rare earths, and the radionuclides such as thorium an uranium) can readily be reduced to acceptable levels. Moreover, the process is especially useful for removing impurities which are resistant to conventional removal means, including aluminum, phosphorus, rare earths, thorium, and uranium (and their daughters of radioactive decay) to acceptable levels. Such purified Tiθ2 ore is especially suitabl for making Tiθ2 pigment by the chloride process. Finally, the process of this invention is highly useful and desirable because it can make practical th utilization of low grade, inexpensive and more abundant Tiθ2 ore which contains numerous impurities. The process is also simple and requires few steps. Moreover, the process of this invention can have considerably less energy requirements than many prior art processes because a roasting step prior to leaching generally is optional. Especially important advantages of this invention are its ability (1) to reduce iron. manganese, and naturally occurring radionuclides suc as thorium and uranium to acceptable levels, (2) to increase iθ2 content to at least 75% and often to a least 90 percent, and (3) to produce the foregoing benefits without the use of a roasting or prereductio step. Another especially important advantage of this invention is that it can reduce thorium and uranium to less than about 200 to 250 parts per million ("ppm") , often less than about 150 parts per million, and for some ores less than about 100 parts per million. BRIEF DESCRIPTION OF THE DRAWING The Figure represents a plot of the weight percent of metal oxide remaining as a function leach temperature when treating western Australian ilmenite with HC1 according to the present invention. DETAILED DESCRIPTION OF THE INVENTION The following sets forth a detailed descrip tion of this invention. It should be noted that the process of this invention can be carried out on a batch or continuous basis. Ore Ores suitable for use in the process of thi invention include titaniferous, rutile, leucoxene, perovskite, and sphene. Preferred are titaniferous, such as ilmenite, titaniferous hematite, and titanif¬ erous magnetite. Especially preferred is ilmenite. As used herein, the term "ore* includes raw ore and beneficiates and derivatives thereof such as slags, blow-over fines from Tiθ2 chlorinators or other process streams from a Tiθ2 manufacturing process. The process is especially suitable for further pro¬ cessing of synthetic rutile, i.e., beneficiated ilmenite and chlorination blow-over solids which oft have undesirable levels of thorium, uranium and, in the latter case, phosphorus. Impurities The impurities which can be removed in accordance with the process of this invention includ alkali metals, alkaline earth metals, rare earth metals, iron, aluminum, phosphorous, thorium, uraniu chromium, manganese, vanadium and yttrium. Especial suitable for removal by the process of this inventio are the impurities of iron; phosphorus; aluminum; manganese; calcium; barium; strontium; chromium; manganese; vanadium; yttrium; lanthanide elements suc as lanthanum, cerium, and neodymium; thorium; and uranium. The impurities of phosphorus, aluminum, iron, calcium, barium, strontium, manganese, and radionuclides such as thorium and uranium are especially detrimental to the chloride process for making iθ2 pigment; such impurities can be readily reduced to acceptable levels by the process of this invention. Also, while the impurities of aluminum, phosphorus, thorium, and uranium are especially resistant to removal by conventional chemical or mechanical means, they can readily be reduced to acceptable levels by the process of this invention. By the term "impurities" is meant the foregoing metals in their elemental state, oxides thereof, salts thereof and other complexes thereof. An especially important advantage of this invention is its ability to reduce iron, manganese and radionuclides such as thorium and uranium to accept- able levels. This is important because such impurities are commonly associated with titaniferous ores, leucoxene, rutile, perovskite, and sphene. Particle Size of Ore For the process of this invention, prefera¬ bly, the ore should be in particulate form. The optimum particle size for any Tiθ2 ore desired to be processed can readily be determined by comminuting (such as by grinding, crushing, milling, etc.) the or into several different particle sizes and evaluating the amount of impurities removed by the process of this invention. Generally, it can be desirable to liberate the minerals to be separated from the ore, i.e., to comminute the ore into as fine particles as practical so that the presence of discrete minerals or nearly discrete minerals in the particles is improved. Ordinarily, the ore should have a particle size of less than about one-fourth inch. If ore treated in accordance with this invention is to be used in the chloride process for making iθ2, its particle size can be adjusted so that it is compatibl with such process. In such case, the particles preferably will fall within the range of about -20 mesh to +400 mesh. Of course, some ores in their natural state have a particle size within this range. If so, additional comminuting is not necessary. Mineral Dressing If desired, the ore can be subjected to mineral dressing prior to the leaching treatments and/or after the leaching treatment. By mineral dressing is meant mechanical processes which can remove some of the undesired impurities, including desliming (removing fine particles by a cyclone, a classifier, grating or settling process) , crushing, grinding, classification, screening, flotation, electrostatic separation and magnetic separation. Suitable mineral dressing processes are disclosed in U.S. Patent 4,243,179, which is hereby incorporated reference. If mineral dressing is used, it can be designed to reduce the ore to the desired particle size in order to satisfy both mineral liberation and Tiθ2 ore chloriήation requirements. Roasting Optionally, prior to the leaching process o this invention, the ore can be subjected to reductive roasting. It has been found that such roasting, if carried out under proper conditions, can further reduce the amounts of phosphorus compounds in the ore and lower the temperature needed for the leaching step. The most critical parameter is roasting temper ature. If reductive roasting is used, it generally will be carried out at a temperature of about 900-1600'C, in the presence of a carbonaceous reducin agent. Suitable carbonaceous reducing agents include coke, lignite char, charcoal, coal, lignite, petroleum such as residual oil, carbon monoxide, and natural gas. The roasting should take place under reductive conditions, i.e., in the substantial absence of air or oxygen or under conditions which favor reduction rather than oxidation. A preferred temperature is about 1000-1500 ' C. The most preferred temperature is about 1100-1300"C. If roasting is used, it can be carried out by any suitable means, process or device. For example, a fixed bed, rotary kiln, fluidized bed, batch or continuous process can be utilized. The time required for the roasting step can readily be determined by making several experimental trials and selecting those which produce the desired results with the lowest temperature and the least tim so that output can be optimized and energy consumptio can be minimized. Suitable times often will be in th range of about five minutes to 8 hours, preferably about five minutes to 2 hours, and most preferably about 15 minutes to one hour. Generally, it has been found that a reduc¬ tive roasting step is optional, and that usually satisfactory and often excellent results can be obtained without it. A benefit of not using a reduc¬ tive roasting step is that this can save substantial operating and investment costs. If a reductive roasting step is used, care should be exercised because it has been found that such a step can make the aluminum, thorium, and uranium impurities present in the ore more resistant to removal by the leaching step. Generally, any oxidative roasting or reduc¬ tive roasting at less than 900*C is not desirable, because at such temperatures, in addition to aluminum, thorium and uranium, phosphorus also becomes resistant to removal by the leaching step. Preleach If desired, prior to the leaching step, the ore can be subjected to a preleach operation. The purpose of the preleach step is to remove impurities which can be removed with milder conditions than the leaching step described below. Use of the preleach step could enhance the economics of the process and, in some grades of ore, could improve quality. The acids and concentration of acids described below for the leaching step can be used. Also, if desired, the spent acid from the leach step can be used as the feed for the preleach step. Suitable temperatures are about 50-100 # C, preferably about 60-90'C and most preferably 70-80"C. The pressure ordinarily will be about atmospheric. Leaching For this step of the process of this inven¬ tion, suitable acids include hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, and mixtures thereof. Especially preferred are hydrochloric acid, nitric acid, hydrofluoric acid, an mixtures thereof. Most especially preferred is hydrochloric acid. The acid should be utilized in an effective amount, i.e., an amount and concentration sufficient to solublize substantially the impurities. Analysis of the leachate, i.e., the acid solution containing the dissolved impurities, and the leached ore can readily determine whether or not the amount and/or concentration of acid is sufficient. The acid concen¬ tration should be at least 3% by weight, based on the total weight of the solution. Ordinarily, the acid will be present in an amount of about 3-30% by weight, based on the total weight of the solution. Prefera¬ bly, the concentration of the acid will be about 5-25 % and most preferably about 15-25 % by weight, based on the total weight of the solution. If sulfuric acid is used, lower concentrations within the foregoing range may be preferable because higher concentrations of sulfuric acid may dissolve undesirable amounts of Ti0 2 . The acid leaching treatment will take place at a temperature and pressure, and for a time which i sufficient to solubilize substantially the mineral impurities present. Ordinarily, the time required will be at least about 5 minutes. Typical ranges of time are about 10 minutes to four hours, preferably about 10 minutes to two hours and most preferably about 10 minutes to one hour. The temperature should be at least 150"C. The temperature will ordinarily b about 160-300*C, preferably about 160-250'C, and most preferably about 170-210"C. The broadest temperature range is in excess of 150"C up to about 300 ' C. An especially preferred temperature range is about 190-210'C. An especially preferred temperature is about 190*C. The pressure will generally be autogeneous, i.e., that generated in a closed vessel under the leaching conditions. However, additional pressuriza- tion can be added, if desired, which may speed remova of impurities from some ores. Ordinarily, the pres¬ sure range will be about 4-100 atmospheres absolute, preferably about 5-75 atmospheres absolute, and most preferably about 10-60 atmospheres absolute. An especially preferred pressure range is about 10-25 atmospheres absolute. By the term "solubilize substantially," as used to describe the leaching treatment, is meant that the concentration of acid and conditions of tempera¬ ture, pressure, and time which will solubilize at least about 10% by weight of the total impurities. Preferably, at least 50% of the total impurities will be solubilized. Often, a graph of the concentration of the acid and conditions of temperature and time, compared to the amount of impurities removed will help to determine trends and optimizations. Re oving the Leachate Following the leaching step and the washin step, if used, the leachate is removed from the treated Tiθ2 ore. Preferably, this is done by remov ing the leachate followed by washing with water or b washing with water alone. The leachate can be remove by any suitable means, including filtrating, decant¬ ing, centrifuging or use of a hydroclone or a classif ier. Preferably, the water will be hot, i.e., up to its boiling point. The amount of washing required ca readily be determined by analyzing the wash water for the presence of impurities and acid. Use of Treated Ore After the ore has been treated in accordanc with the process of this invention, it can be used to make Tiθ2 pigment or titanium metal or be used in any process where a purified Tiθ2 ore is desired. Prefer ably, the Tiθ2 ore treated by the process of this invention can be used to make Tiθ2 pigment, and most preferably, to make Tiθ2 pigment by the chloride process. Suitable chloride processes and reactors for using the Tiθ2 ore treated in accordance with the process of this invention are disclosed in U.S. Patents 2,488,439, 2,488,440, 2,559,638, 3,203,763, 2,833,626, 3,284,159, and 2,653,078, which are hereby incorporated by reference. EXAMPLE 1 The following example illustrates this invention. Unless otherwise indicated, all percent¬ ages are on a weight basis. Untreated mechanical concentrate of a Western Australian ilmenite was leached hydrotherraally under the following conditions: 20% HCL 190'C 1 hour gentle agitation 10% pulp density no thermal pretreatment. ■ - ; The resultant products,^after filtration an washing, shows a high grade iθ2 ore beneficiate whic is summarized in Table I. It is important to note that (1) the amount of Fβ 2 θ 3 has been reduced from about 33 percent to about 3%, and (2) the naturally occurring radionuclides, thorium and uranium have bee reduced from 514 parts per million ("ppm") in the starting ore to less than 44 ppm in the beneficiate. TABLE I A mechanical concentrate of a Western Australian ilmenite ore was used as the starting material. The ore was not subjected to any thermal pretreatment. The analysis of this starting material is shown in Table II. Sixty grams of this ilmenite ore were leached hydrotherraally with 200 ml of 20% by weight HC1 for 60 minutes in a Tantalum-clad steel bomb. T pulp density was 30 %. A series of individual experiments were carried out at the following reacti temperatures: 110*C, 130"C, 150'C, 160'C, 170 * C, 190*C and 210*C. .During the reaction, the bomb was agitated by a rocking action. The results of the test are shown in Table II and Figure 1. Note that at 150 * C an inflection point appears, and that above such temperature (1) th Tiθ2 concentration increases at a faster rate and (2) the concentration of Fβ2θ3 and MnO decreases at a faster rate. * Includes 1.47Z FeO EXAMPLE 3 A fine Tiθ2 solid derived from the carbochlorination of ilmenite concentrate, called "chlorinator blow-over", was tested by the process of this invention. The chlorinator blow-over was first washed and mineral-dressed to remove the soluble meta chlorides and coke dust. The resultant solid that is rich in Tiθ2, Siθ2, P2°5. T and u , was hydrothermall leached with HCl as follows: Twenty-five grams of coke-free blow-over or were leached at 180"C in 250 ml of 20% HCl acid for 6 minutes. After leaching, the ore was separated from the leach liquor by filtration followed by washing with boiling water until the solid was free of residual chlorides and acid. As shown in Table IV, most of the P2O5, rar earths Th and U in the starting material were significantly reduced. For this ore blow-over, the reduction of Th and U is especially difficult by conventional means, such as atmospheric HCl leach for prolonged period. In this Example, Th was reduced from 3016 to 188 ppm and U from 158 to 55 ppm, as shown in column A. Column B shows the analysis of the same leached ore after its Siθ2 content was reduced by a caustic waste. TABLE III Hydrothermal HCl Leach of an Ilmenite Derivative from Chlorinator Tiθ2 Fe2θ 3 A1 2 0 3 CaO BaO SrO Cr2θ3 MgO MnO Nb2θ5 P2O5 Si0 2 Zr0 2 Y 2 0 3 a2θ3 Ce0 2 Nd 2 0 3 V2O5 Th (ppm) U (ppm) A synthetic rutile derived from a Western Australian ilmenite concentrate still contains a substantial amount of thorium (540-601 ppm) . Its uranium content was below the detection limit of the X-ray fluorasce technique (20 ppm) . It is desirable to reduce the Th level to near 200 to 250 ppm or below. Twenty-five grams of the synthetic rutile were leached hydrothermally with a 20% HCl at a pulp density of 10%. After leaching at 190"C for 60 minutes, the ore was separated from the mother liquo by filtration, and then washed with boiling water. Another sample of this synthetic rutile produced fro a different period was leached under the same conditions as described above. The experimental results are shown in Table IV. TABLE IV * S.M. * = Starting Material ** HTL = Hydrothermal HCl-leached Ore

Claims: The invention claimed is: 1. Process for reducing the amount of thorium and uranium in a titanium bearing ore selecte from the group consisting essentially of titaniferous ores, leucoxene, rutile, perovskite, sphene, and derivatives thereof, comprising: (a) contacting said titanium bearing ore, wherein said ore is characterized as containing an unaceptable level of at least one naturally occurring radionuclide selected from the group consisting of thorium and uranium, with an aqueous solution of a mineral acid having an aci concentration of about 3-30 percent by weight, said contacting taking place at a temperature range in excess of 150'C up to about 300'C, unti the desired amount of thorium and uranium are solubilized and a leachate is formed; and thereafter, (b) removing and recovering the remaining ore of reduced level of radionuclide from the leachate. 2. The process of Claim 1 wherein said reduced level of radionuclide in the remaining ore being recovered is less than about 250 ppm. 3. The process of Claim 1 wherein the temperature is about 160-250'C. 4. The process of Claim 1 wherein the temperature is about 170-210'C. 5. The process of Claim 1 wherein the pressure is about 4-100 atmospheres absolute. 6. The process of Claim 1 wherein the pressure is about 5-75 atmospheres. 7. The process of Claim 1 wherein the aci concentration is about 5-25 percent by weight. 8. The process of Claim 1 wherein the aci is hydrochloric. 9. The process of Claim 1 wherein the acid is hydrochloric, the temperature is about 160-250'C, and the pressure is about 4-100 atmospheres absolute. 10. The process of Claim 1 wherein the aci is hydrochloric, the temperature is about 170-210*C, and the pressure is about 5-75 atmospheres absolute. 11. The process of Claim 1 wherein the aci is hydrochloric, the temperature is about 170-210"C, the pressure is about 10-60 atmospheres absolute, and the acid concentration is about 3-30 percent. 12. The process of Claim 1 wherein the aci is hydrochloric, the temperature is about 170-210'C, the pressure is about 10-100 atmospheres absolute, an the acid concentration is about 5-25 percent. 13. The process of Claim 1 wherein prior t contacting with an aqueous solution of a mineral acid the ore is subjected to mineral dressing. 14. The process of Claim 1 wherein (a) prior to contacting with the aqueous solution of a mineral acid, the ore is subjected to mineral dressin and optionally a preleach treatment, and (b) optionally, after contacting with an aqueous solution of a mineral acid the ore is subjected to mineral dressing. 15. The process of Claim 1 wherein prior t contacting with the aqueous solution of a mineral acid, the ore is subjected to reductive roasting at a temperature of about 900-1600'C. 16. The process of Claim 1 wherein the temperature is about 190-210'C. 17. The process of Claim 1 wherein the temperature is about 190'C. 18. Process of Claim 1 wherein prior to contacting with an aqueous solution of a mineral acid the ore is subjected to mineral dressing and the temperature is about 170-210*C. 19. Process of Claim 1 wherein there is no oxidative or reductive roasting prior to step (a) . 20. Tiθ 2 pigment produced from beneficiated Tiθ2 ore from the process of Claim 1.

Inventors: CHAO TZE, CHAO, TZE
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1992
Main classification: C22B34
IPC codes: C01G23, C22B34