NZ242400A - Removal of radionuclides from titaniferous material using a pre-treatment then leaching with acid - Google Patents

Description

New Zealand Paient Spedficaiion for Paient Number £42400 242400 Priority Date(s): '.v." Complete Specificaticri Filed: Class: Publication Date: .. .?.?.?£?. .TO. P.O. Journal, No: IIK'/ NEW ZEALAND PATENTS ACT, 1953 No.: Date: COMPLETE SPECIFICATIOr fjji. EAT5MT QFFKS1 16 APR I992 R£«E1V'3 REMOVAL OF RADIONUCLIDES FROM TITANIFEROUS MATERIAL CHANGE OP H*ve •» AftftitANT ftGC Mfr&ral Sands Urd ■ We, a company incorporated under the laws of the State of Western Australia, of 45 Stirling Highway, Nedlands, Western Australia 6009, Australia hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- , - 1 -(followed by page la) 242400 * la" This invention relates to a process for facilitating the removal of radionuclides from titaniferous materials, and is particularly concerned with the removal of radionuclides such as thorium and/or uranium from weathered or 10 "altered" ilmenite.

Ilmenite (FeTi03) and rutile (Ti02) are the major, commercially-important, mineral feedstocks for titanium metal and titanium dioxide production. Although ilmenite and rutile almost invariably occur together in nature as components of "mineral sands" or "heavy minerals" (along with zircon (ZrSi04) and monazite 15 ((Ce, La, Th)P04)), ilmenite is usually the most abundant. Natural weathering of ilmenite results in partial oxidation of the iron, originally present in ilmenite in the ferrous state (Fe2+), to ferric iron (Fe3+). To maintain electrical neutrality, some of the oxidised iron must be removed from the ilmenite lattice. This results in a more porous structure with a higher titanium (lower iron) content. Such 20 weathered materials are known as "altered" ilmenites and may have Ti02 contents in excess of 60% Ti02, compared with 52.7% TiOa in stoichiometric (unaltered) ilmenite. As weathering, or alteration, of the ilmenite proceeds, impurities such as alumina-silicates (clays) are often incorporated into the porous structure as discrete, small grains that reside in the pores of the altered ilmenite. It appears 25 that radionuclides such as thorium and/or uranium can also be incorporated into the ilmenite during this process.

Most of the world's mined ilmenite is used for the production of titanium dioxide for use in the paint pigment industry. Pigment-grade titania (titanium dioxide, Ti02) has been traditionally produced by reacting ilmenite with 30 concentrated sulphuric acid, with subsequent hydrolysis of the titanium sulphate to titania - the so-called sulphate route. However this process is becoming 242 400 increasingly unpopular due to environmental considerations. The chloride route, which involves reaction with chlorine to produce volatile TiCl4, which in turn is converted to TiOz, presents fewer environmental problems and is currently the preferred method for pigment-grade titania production. However rutile or 5 "synthetic rutile" (rather than ilmenite) are the preferred feedstocks for the chloride route because the large amount of iron in ilmenite causes excessive consumption of chlorine during processing. Hence an increasing amount of the world's production of ilmenite is being converted to synthetic rutile (TiOz content > 90%) for use in the chloride-route, pigment-production process. A number of 10 different processes have been developed to upgrade ilmenite to synthetic rutile, the most widely used, commercially, being the Becher process.

The Becher process involves reducing the iron in ilmenite (preferably altered ilmenite) to metallic iron in a reduction kiln at high temperature, then oxidising the metallic iron in an aerator to produce a fine iron oxide that can be 15 physically separated from the coarse titanium-rich grains. The product finally undergoes a dilute acid leach. The titanium-rich synthetic rutile so produced contains typically > 92% Ti02.

Whether ilmenite is marketed as the raw mineral or as upgraded, value-added synthetic rutile, producers are being increasingly required to meet more 20 stringent guide-lines for the levels of radionuclides, e.g. thorium and uranium, in their products. The Becher synthetic rutile process does not reduce the levels of radionuclides in the product and so there exists an increasing need to develop a commercially satisfactory process for removal of radionuclides from ilmenite and/or from other titaniferous material such as synthetic rutile. Provision of 25 such a process is therefore an object of the invention.

A difficulty in determining an appropriate method for the removal of radionuclides from titaniferous material is that it is not known just how the radionuclides are present in the mineral. It is known, for example, to dissolve pure Th02 in acid [e.g. U.S. Atomic Energy Comm. TID-5223, 307-17] but 30 experimental acid leaches of ilmenite for the present applicant have achieved only limited removal of radionuclides such as thorium and uranium from the ilmenite. 242400 The thorium may not be present as Th02 or, even if ThO^ may not be in a readily dissolvable form.

US patent 5011666 discloses a process for purifying TiC>2 ore, by reducing the levels of a number of impurities including thorium and uranium. The process 5 entails subjecting the ore to two or more leaching treatments including the use of an aqueous solution of a mineral acid and an aqueous solution of an alkali metal compound selected from the group consisting of alkali metal carbonates, hydroxides and mixtures thereof. According to the claims of the reference, the acid leach is effected first but the reference also discloses an effective treatment 10 in which an alkali metal leach under severe conditions (25% NaOH at 18 atm and 210 °C), is followed by the acid leach.

While the multiple leach process proposed in US patent 5011666 may per se effectively reduce thorium and uranium content, the leaching conditions discussed are generally very severe and are intended to extract a variety of 15 impurities, and iron in particular, from the titaniferous material. The process is particular to anatase and if it were applied as disclosed to primary ilmenite, there would be loss of titanium values by reaction with the acid and/or by acid dissolution of alkali metal titanates formed during the strong alkali leach. Moreover, the levels of acid and alkali reagents required, and other process 20 conditions specified in the patents, render the process uneconomic for the purposes of the present exercise.

It has also been proposed, in Australian patent 599090, to removal various impurities including thorium and uranium by roasting Ti02 ore with an alkali metal compound to convert the thorium and uranium for subsequent leaching 25 with a non-sulphuric mineral acid. The TiC>2 ore is preferably anatase but may be ilmenite. This process, which is clposely related to a general processing system disclosed, eg, in US patent 4759916, is complex and expensive.

It has been realised that thorium and uranium removal can surprisingly still be achieved at effective levels under leaching conditions which are much milder 30 than those disclosed in US patent 5011666, but at which both iron and titanium values are substantially preserved and the process is more economically and 242400 practicably viable. More generally, it has been appreciated that removal of radionuclide;} from titaniferous material by acid leaching is greatly enhanced if the material is first treated in a manner to alter the state of the radionuclides in the . material for a more effective subsequent acid leach, and that one such treatment 5 is treatment of the titaniferous material with a base under conditions such that iron and titanium are substantially not dissolved from the titaniferous material.

According to the present invention., there is therefore provided a process for facilitating removal of radionuclides from titaniferous material which comprises the steps of subjecting said titaniferous material to a treatment in a 10 manner to alter the state or condition of radionuclides in the material for a more effective subsequent acid leach, and thereafter leaching the titaniferous material with acid to dissolve radionuclides, wherein the conditions of such treatment and said leach are such that titanium and iron are substantially not dissolved from the material.

By "substantially not dissolved" is meant herein that less than 10% is dissolved, preferably less than 5%, most preferably less than 2%.

The process preferably includes the further step of separating the radionuclides from the titaniferous material. Such separation can be effected by any suitable known method, eg filtration or cyclone separation. 20 The base may, eg be an alkali metal or alkaline earth compound, eg oxide, hydroxide or carbonate, or ammonia. However, a preferred said treatment is contact with an aqueous basic solution, most preferably an aqueous solution of a strong base eg an alkali metal oxide or hydroxide, under conditions effective to achieve said alteration of the state or condition of the radionuclides but such 25 that iron and titanium are substantially not dissolved from said material. By "strong" base, is meant herein a base substantially fully dissociated in water.

The term "state or condition of radionuclides" may refer herein to their chemical state, e.g. the compounds in which the radionuclides are primarily present, and/or to the physical state or condition. An example of the physical 30 state or condition is the structure or phase in which the radionuclides are found in the titaniferous material. By way of particular example, in altered ilmenite, *5- 242 40 0 alumino-silicates may restrict access to the thorium or other radionuclide traces and thus the acid leach may be rendered more effective by pre-treating the ilmenite to alter the condition of the thorium by partially or wholly removing the alumino-silicates and thereby enhancing access to the radionuclide traces during 5 _ the subsequent acid leach.

Preferably, the titaniferous material is ilmenite, altered ilmenite, reduced ilmenite or synthetic rutile. The radionuclides removed preferably include at least thorium and/or uranium radionuclides.

The alkali hydroxide may be caustic soda, (sodium hydroxide) or caustic 0 potash (potassium hydroxide), but caustic soda is especially preferred, eg for economic and handling reasons. Similarly any suitable acid can be used for the acid leach, for example sulphuric, hydrochloric or nitric acid. A mineral acid is however generally preferred, and sulphuric acid is especially preferred, eg for economic and handling reasons.

It will be appreciated that the conditions used for the process of the present invention, for example the temperature, reagent concentration, and the time required for each step, will depend upon the degree of radionuclide removal required, on the nature of the titaniferous starting material used, on the particular radionuclide present, and on the original levels of the radionuclide^). An 0 appropriate set of conditions may also depend upon external factors such as economic and other practical considerations. In general, a greater degree of removal of radionuclides such as thorium and uranium can be achieved by using higher temperatures, high reagent concentrations and longer leaching times.

The process is preferably carried out at or close to atmospheric pressure. 5 The acid concentration and temperature that can be used in the acid leach are limited by the propensity of the titaniferous feed material to react with the acid, thereby resulting in loss of titanium value. For example, ilmenite has a greater propensity to react with acid than does synthetic rutile. In general, process conditions in the acid leach step are preferably chosen so as to minimise ) or eliminate loss of titanium value through reaction of titanium with the acid.

Preferably, the treatment step is performed with a caustic solution of 242 400 concentration no greater than about 15 molar, and preferably greater than 0.1 molar. A useful commercially available solution is 12.5 molar. The maximum preferred caustic concentration may be to some extent temperature dependant. The temperature of the basic solution is preferably maintained above 50 "C but 5 not greater than 125 °C, but the process may be carried out over any extended period at ambient temperatures, eg by heap leaching. The treatment with a basic reagent is preferably followed by a washing step prior to the acid-leaching step.

The maximum acid concentration if loss of titanium value is to be avoided or at least minimised will depend on the temperature and on the actual 10 titaniferous material used. The higher the temperature, the lower the maximum acid concentration. Primary ilmenite is more reactive than altered ilmenite which is more reactive in turn than anatase, rutile and synthetic rutile and thus the maximum acid concentration will be lower for altered ilmenite than for synthetic rutile. A satisfactory combination of conditions for altered ilmenite is 3 molar 15 sulphuric acid concentration at 90 "C, but it is believed that the acid concentration at 90 eC may be as high as 10 molar. A practical limit, set by general consideration of acid handling such as corrosion levels and recovery requirements, is 10 molar. It is also thought that the maximum acid concentration may vary according to the acid employed, eg higher for sulphuric than hydrochloric. 20 During the acid-leaching step the acid leach solution is preferably maintained above 50 °C but not greater than 125 ®C. The acid concentration is preferably greater than 0.1 molar.

In practice, the upper limits for reagent concentrations and temperatures are usually determined by economic considerations, but it appears that no 25 substantial additional benefit is gained for concentrations greater than 10 molar and temperatures greater than 125 "C. The time elapsed for the respective treatments is preferably at least 10 mins for the treatment with the base and h hour for the acid leach. A practical upper limit for each treatment is about 4 or 5 hours, preferably h to 2 hours.

The process of the present invention can be performed at various solids concentrations on a batch or continuous basis. For example, at concentrations no 242400 greater than approximately 75% w/w solids, the process can be performed on a stirred suspension in a heated reactor. At higher solid contents the process can be performed on an appropriately wetted solid in saggers in an oven, or in a rotary kiln.

It has been further found that treatment of titaniferous material with a base followed by an acid leach has the further benefit of removing base-soluble and acid-soluble impurities from the feed material, resulting in a product with a higher titanium content. For example, the A1203 and Si02 contents can be substantially reduced in the product of the process according to the invention. 10 It will be appreciated that the degree of removal of base-soluble and acid-soluble impurities (and hence the increase in the titanium content of the product of the present invention) will also depend on severity of the conditions used.

It has been further found that the efficiency of the process of the present invention can be further enhanced if the acid leach is carried out in the presence 15 of added fluoride, for example in accordance with a copending patent application concurrently filed herewith also entitled "Removal of Radionuclides from Titaniferous Material". The added fluoride is present in an amount effective to further enhance dissolution of the altered radionuclides but substantially without dissolving titanium from the titaniferous material. The fluoride concentration is 20 preferably in the range 0.01 to 2.0 molar, most preferably in the range 0.05 to 0.5 molar. Any suitable additive can be used as the source of the added fluoride, for example hydrofluoric acid (unless this is the acid employed for the acid leach), sodium fluoride, ammonium fluoride, or sodium fluorosilicate.

Frequendy ilmenite concentrates contain low levels of thorium due to 25 monazite contamination. It is thought that the process of the invention does not remove macroscopic monazite grains from titaniferous materials, but rather removes microscopic radionuclide traces originally incorporated into the ilmenite grains during the weathering process. 242400 EXAMPLE 1 100 g of ilmenite (from Eneabba North, Western Australia) was reacted with 150 cm^ of 10 molar sodium hydroxide solution in a reactor fitted with a stirrer rotating continuously at 750 rev/min., a thermopocket containing a thermometer (or thermocouple) and a reflux condenser. The reactor was heated by a heating mantle that was connected via a temperature controller to the thermocouple. In this way, the reaction mixture could be maintained at the desired temperature. The mixture was heated at 115°C for 4 hr. The solid residue was then filtered, thoroughly washed with water and analysed. (This material is subsequently referred to as "caustic-treated product").

The caustic-treated product was then returned to the reactor and leached with 300 cm^ of 1 molar sulphuric acid solution at 85°C for 2 hr. The solid residue was again filtered, washed thoroughly with water, dried and analysed (This product is subsequently referred to as "caustic and acid-leached product").

The XRF analyses for the ilmenite feed material, the caustic-treated product and the caustic and acid-leached product are given in Table 1.

TABLE 1 XRF Analysis (%) Ilmenite feed (Eneabba North) Caustic treated product Caustic and acid leached product Ti02 59.99 60.52 61.31 Fe203 (total Fe) 34.12 34.50 34.62 Th02 0.051 0.049 0.012 SiOi 1.00 0.20 0.18 ai2o3 0.64 0.32 0.26 EXAMPLE 2 To demonstrate the effect of not proceeding the acid leach with a caustic treatment.

No caustic leach was performed on the ilmenite. The acid leach was performed as described in Example 1. (This product is subsequently referred to as "acid-leached product"). table 2 XRF Analysis (%) Ilmenite feed (Eneabba North) Acid leached product Ti02 Fe2C>3 (total Fe) Th02 Si02 ai2o3 59.99 60.01 34.12 34.20 0.051 0.042 1.00 0.89 0.64 0.49 242400 EXAMPLE 3 To demonstrate the effect of the concentration used in the caustic treatment.

These experiments were performed as described in Example 1 except that a) the caustic leach was performed using various concentrations at 75°C for 30 mins b) the acid leach was performed using 5 molar H2SO4 at 87°C for 2 hr TABLE 3 XRF Analyses (%) Ilmenite feed (Eneabba North) Concentration of NaOH 0.5M 1.0M 2.0M 4.0M 6.0M Ti02 59.99 61.09 61.35 62.20 62.64 61.53 ^e2®3 (total Fe) 34.12 34.15 34.04 34.47 34.46 .29 Th02 0.051 0.018 0.016 0.016 0.012 0.008 Si02 1.00 0.69 0.64 0.51 0.50 0.45 A1203 0.64 0.40 0.39 0.37 0.30 0.31 242 4 0 0 EXAMPLE4 To demonstrate the effect of the temperature used in the caustic treatment These experiments were performed as described in Example 1 except that a) the caustic leach was performed using l.OM NaOH at various temperatures for 30 min b) the acid leach was performed using 5 molar H2SO4 at 87°C for 2 hr TABLE 4 XRF Ilmenite feed Temoerature of caustic leach Analyses (%) (Eneabba North) 50°C 75°C 100°C Ti02 59.99 60.90 62.15 61.97 Fe203 (total Fe) 34.12 34.33 34.31 34.19 Th02 0.051 0.031 0.020 0.018 Si02 1.00 0.82 0.61 0.60 A1203 0.64 0.47 0.42 0.33 24240 0 EXAMPLE 5 To demonstrate the effect of the concentration used in the acid leach These experiments were performed as described in Example 1 except that a) the caustic leach was performed using 4.0M NaOH at 75°C for 1 hr b) the acid leach was performed using various concentrations of H2SO4 at 87 °C for 2 hr TABLE5 XRF Analyses (%) Ilmenite feed (Eneabba North) Concentration of HoSO/i l.OM 2.5M 5.0M Ti02 59.99 62.88 62.41 63.09 Fe203 (total Fe) 34.12 .11 .07 .03 Th02 0.051 0.022 0.018 0.012 Si02 1.00 0.35 0.34 0.45 AI2O3 0.64 0.35 0.38 0.34 242400 EXAMPLE 6 To demonstrate the effect of the time used in the caustic leach These experiments were performed as described in Example 1 except that a) the caustic leach was performed using l.OM NaOH at 75°C for various times b) the acid leach was performed using 5.0M H2SO4 at 87°C for 2 hr.

TABLE 6 XRF Analyses (%) Ilmenite feed (Eneabba North) Time of caustic leach (min) min min 60 min Ti02 59.99 61.31 61.35 61.21 ^e2^3 (tota* ^e) 34.12 34.26 34.04 34.12 Th02 0 0 0.021 0.016 0.016 Si02 1.00 0.64 0.64 0.53.

AloO-j 0.64 0.44 0.39 0.35 242400 EXAMPLE 7 To demonstrate the effect of the time used in the acid leach These experiments were performed as described in Example 1 except that a) the caustic leach was performed using 4.0M NaOH at 75°C for 1 hr b) the acid leach was performed using 5.0M H2SO4 at 87°C for various times.

TABLE 7 XRF Analyses (%) Ilmenite feed (Eneabba North) Time of acid leach (hr) 0.5 hr 1 hr 2hr Ti02 59.99 62.54 61.53 63.09 Fe203 (total Fe) 34.12 34.97 34.10 .03 Th02 0.051 0.018 0.016 0.012 Si02 1.00 0.42 0.43 0.45 A1203 0.64 0.37 0.34 0.34 242400 EXAMPLE 8 To demonstrate the effectiveness of the caustic/acid leach on other ilmenites Ilmenites from Eneabba West and Eneabba North were leached with 2.5 molar sodium hydroxide for 1 hour at 75°C in the apparatus described in Example 1. The products were washed and then leached with 3 molar sulphuric acid for 2 hours at 95°C. These products are referred to as "caustic/acid" in Tables 8 and 9 below.

Other samples of the same ilmenites were treated as described above except that the acid leach was not proceeded by a sodium hydroxide leach. These products are referred to as "acid only" in Tables 8 and 9 below.

TABLE 8 Leaching procedure XRF Analysis Ilmenite feed caustic/acid acid only (%) Eneabba West Ti02 62.6 63.5 62.6 Fe203 (total Fe) 32.9 31.2 31.0 ai2°3 0.83 0.49 0.63 Si02 1.10 0.56 0.99 Th 0.0311 0.0073 0.0153 U 0.0009 0.0008 0.0011 TABLE 9 Leaching procedure XRF Analysis Ilmenite feed caustic/acid acid only (%) Eneabba North Ti02 60.4 61.8 60.7 Fe203 (total fe) 34.0 34.0 33.5 ai2o3 0.62 0.36 0.50 Si02 0.99 0.46 0.93 Th 0.0535 0.00142 0.0277 u 0.0012 0.0005 0.0006 242400 EXAMPLE 9 To demonstrate the effect of carrying out the acid leach in the presence of added fluoride. 100 g of ilmenite was leached with 150 cm^ of 0.2 molar sodium hydroxide solution at 75 °C for 90 mins using the apparatus described in Experiment 1. The product was thoroughly washed with water, then leached with 300 cm^ of 5 molar sulphuric acid containing 0.1 molar sodium fluoride, at 87 °C for 120 mins in the 10 same apparatus. The product was again washed with water. This product is referred to as "caustic/acid/fluoride" in Table 10 below.

Another sample of the same ilmenite was treated as described above except that the caustic leach was omitted. This product is referred to as "acid/fluoride" in 15 Table 10.

A further sample of the same ilmenite was treated as described above for the "caustic/acid/fluoride" product except that the acid leach did not contain sodium fluoride. This product is referred to as "caustic/acid" in Table 10.

TABLE 10 Leaching Procedure XRF analysis % ILMENITE FEED (ENEABBA NTH) caustic/ acid/ fluoride acid / fluoride caustic/ acid TiOz 61.51 63.25 63.12 63.16 Fe203 (total Fe) .04 34.98 .22 .13 Th02 0.047 0.008 0.017 0.024 - Si02 1.08 0.71 0.97 0.84 Al2°3 0.61 0.29 0.34 0.47 242400

Claims (26)

WHAT WE CLAIM IS:

1. A process for facilitating removal of radionuclides from titaniferous material which comprises the steps of subjecting said titaniferous material to a 5 treatment in a manner to alter the state or condition of radionuclides in the material for a more effective subsequent acid leach, and thereafter leaching the titaniferous material with acid, to dissolve radionuclides, wherein the conditions of said treatment and said leach are such that titanium and iron are substantially not dissolved from the material.

2. A process according to claim 1 further comprising separating the radionuclides from the titaniferous material.

3. A process according to claim 1 wherein said treatment comprises a 15 treatment of the titaniferous material with aqueous basic solution under conditions effective to achieve said alteration of the state or condition of the radionuclides but such that iron and titanium are substantially not dissolved from said material. 20

4. A process according to claim 3 wherein said basic solution is an aqueous solution of an alkali metal oxide or hydroxide.

5. A process according to claim 3 wherein said basic solution is an 10 25

6. A process according to claim 4 wherein said solution is of a concentration no greater than 15 molar. aqueous solution of sodium hydroxide.

7. A process according to claim 4 wherein said solution is of a 30 concentration greater than 0.1 molar.

8. A process according to claim 3 wherein said treatment includes U 240 - 18 - maintaining said basic solution at a temperature greater than 50 °C.

9. A process according to claim 8 wherein said temperature is not greater than 125 °C.

10. A process according to claim 1 wherein said radionuclides removed include thorium radionuclides and/or uranium radionuclides.

11. A process according to claim 1 wherein said titaniferous material 10 comprises ilmenite, altered ilmenite, reduced ilmenite or synthetic rutile.

12. A process according to claim 1 wherein said acid is a mineral acid.

13. A process according to claim 1 wherein said acid is sulphuric acid. 15

14. A process according to claim 1 further including the step of washing the titaniferous material after said treatment and before said acid leaching.

15. A process according to claim 1 wherein said acid leach is effected with 20 an acidic solution of concentration no greater than 10 molar.

16. A process according to claim 1 wherein said acid leach is effected with an acidic solution of concentration greater than 0.1 molar. 25

17. A process according to claim 1 including maintaining the acid leach solution at a temperature greater than 50 °C.

18. A process according to claim 17 including maintaining the acid leach solution at a temperature not greater than 125 °C. 30

19. A process according to claim 1 wherein the solids concentration is not more than substantially 75% w/w solids, and the process is performed on a stirred 15 242400 - 19- suspension in a heated reactor.

20. A process according to claim 1 wherein the solids concentration is greater than 75% w/w solids and the process is performed on an appropriately 5 wetted solid in saggers in an oven, or in a rotary kiln.

21. A process according to claim 1 wherein said treatment and/or acid leach are also effective to dissolve impurities including at least one of AI2O3 and Si02. 10

22. A process according to claim 1 wherein said acid leach is carried out in the presence of added fluoride in an amount effective to further enhance dissolution of the altered radionuclides but substantially without dissolving titanium from the titaniferous material.

23. A process according to claim 1 carried out at or close to atmospheric pressure.

24. Titaniferous material from which radionuclides have been removed by 20 the process of claim 1.

25. A process according to claim 1 substantially as hereinbefore described with reference to the accompanying examples.

26. Titaniferous material according to claim 24 substantially as hereinbefore described with reference to the accompanying examples. -A;cO i": ;:S DAY A. J. SON 19 Q4 APPLICANTS