WO2003097533A1

WO2003097533A1 - Process for removal of radioactive impurities from zirconium containing materials

Info

Publication number: WO2003097533A1
Application number: PCT/AU2003/000614
Authority: WO
Inventors: Halil Aral; Kenneth John Mcdonald
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2002-05-22
Filing date: 2003-05-21
Publication date: 2003-11-27
Also published as: AUPS250102A0

Abstract

A process for reducing the amount of impurities in a zirconium containing material, said process including the step of treating the zirconium containing material with a composition, substantially including a borate salt or mineral, at a temperature and for a period of time sufficient to collect at least part of said impurities into a borate phase.

Description

PROCESS FOR REMOVAL OF RADIOACTIVE IMPURITIES FROM ZIRCONIUM CONTAINING MATERIALS

FIELD OF THE INVENTION

This present invention relates to a process for reducing the amount of radioactive impurity components in a zirconium containing material without significantly altering the grain size distribution and mineralogy of the material. The invention is particularly applicable to the removal of radioactive impurities such as U, Th, and Ra. The process also can remove non-radioactive impurities such as Fe, Al, Ti and the rare earth elements. The process of the invention is applicable to removal of impurities, from zirconium containing materials. While the process of the invention is applicable to any zirconium containing material, the following description will focus on its application to the mineral zircon. However, it is to be clearly understood that the invention is not limited to that application. BACKGROUND OF THE INVENTION

The discussion of the background to the invention herein is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge in Australia as at the priority date of any of the claims. Zircon (ZrSiO₄) is an important component of most mineral sands or heavy mineral deposits. It is typically separated from the other heavy minerals (ilmenite, rutile, leucoxene, monazite) by wet concentration using gravity processes and then dry separation using magnetic and electrostatic processes.

Zircon finds major uses in foundries as sand moulds, refractories as lining for steel ladles and furnaces, and ceramics as glazes, pigments and enamels. More than half of the commercially produced zircon is consumed in the ceramic industry as an opacifier in glazes. When used as an opacifier it is added as frit or used directly in glazes, or in the body of granito type tiles. Zircon added as frit is ground to -45 micron size, while that added to glaze is micronised to a fine powder (generally less than 5 micron).

It is often the case that the zircon industry is required to lower impurity levels to meet customer specifications as most impurities adversely affect the end use. In practice the presence of impurities in zircon is a problem in the magnetic and electrostatic separation stages of the concentration. Zircon sand in foundry applications needs to be free of any alumina and clay which otherwise could cause sintering or fusion between cast metal and the zircon mould. Light cream or white coloured ceramic grade zircon, typically low in iron oxides and titania, is preferred in many ceramics applications. All commercially available zircon sands contain impurities as surface coatings, discrete minerals, mineral inclusions and trace element substitutions. Zircon sand concentrates are, in general, ginger to light brown in colour to the eye and appear mostly as clear, colourless, yellowish or reddish grains under the optical microscope. The gingery or brownish colour is often related to iron- containing clayey surface coatings/stains but the removal of such material by attritioning or chemical-aided-leaching does not always eliminate the colour completely.

The presence of gangue minerals such as rutile, garnet xenotime, monazite, spinels, ilmenite, and aluminium silicates in the concentrate also add to the impurity levels in the zircon as well as imparting some colour to the concentrate.

While intense leaching and careful ore dressing can reduce the impurity levels in zircon, they typically do not remove them completely. Further, some impurities are locked in the crystal lattice as mineral inclusions or trace element substitutions. Even surface clean single zircon grains can be almost any colour and this diversity is related to the presence of impurities like U, Ti, Fe, Th and rare earth elements in the crystal lattice.

Currently the existing major applications for zircon do not require the removal of the radioactivity, provided it is below the 500 ppm U+Th level.

However, the presence of U and Th is a major impediment for the expansion of the zircon market into niche applications, for example biomedical, electronics and fine chemicals.

Some zircon sands contain radioactivity levels well above the required level for transport as a non-radioactive material of 500 ppm U+Th, or 70 Bq/g activity limit. For example, the U+Th levels of Bangladeshi zircon sands can reach as high as 1500 ppm U+Th. This is about three times higher than the level in commercial zircon sands. The fine-particle size zircon of the Horsham (Victoria, Australia) area contains about twice the level of U+Th than other commercial zircon sands. While the zircon sand potential of the Horsham area is up to 20 million tonnes, this zircon currently is not saleable due to its high radionuclide content. Occupational health, safety and environmental concerns also puts increasingly more pressure on zircon producers to lower the radioactive impurity levels in their products. The dust arising during handling of zircon and any release of radon and thoron gases during its beneficiation are potential health hazards. The use of less radioactive or non-radioactive zircon sand would lower these risks in existing applications. Companies processing zircon to zirconia, and zirconium chemicals and pigments would welcome a low radionuclide feedstock. Consequently, countries importing zircon may prefer the importation of less radioactive material. Uranium and thorium undergo alpha decay. The alpha particles ejected from the atoms progressively damage the zircon's lattice. The accumulation of radiation damages over millions of years changes many of the zircon's properties. The radiation-damaged zircon, which is called metamict zircon, is amorphous to X- rays and is less resilient to attack by acid and alkali solutions. Leachants such as mineral acids or alkalis can be used to dissolve or partly dissolve the metamict zircon while the crystalline portions of the grains remain intact. Radiation damage expands the crystal lattice and causes cracks in the grains. However, commercial zircon sands are not metamict to such a degree that X-ray amorphous soluble zircon has formed. It is often a general practice in zircon beneficiation plants that any such "metamict" zircon is separated and discarded as a waste material. In general, the radiation-related damage is usually in the crystal lattice and no extensive fracturing is involved to channel leachants deep into the grain. The required level of about 10¹⁶ alpha events/mg for an effective amorphization is hard to achieve. The radioactive impurities in zircon crystals are believed to be partly located in the crystal lattice and partly in the bands of zoned zircon grains. The bands of zoned zircon grains are microporous and therefore expected to be accessible with some lixiviants. This may suggest that the impurities locked in the zone regions could be removed at least partially by a hydrometallurgical treatment along with those impurities adhered on grain surfaces. However, it has been found that a large part of U and Th still remains in the zircon crystals even after a harsh acid and alkali leach.

A number of prior processes have been proposed for the removal of radioactive and/or non-radioactive impurities from zircon, which variously include heat treatment and/or leaching stages. However, these processes have been of limited effectiveness and have suffered from various disadvantages, including: substantial disruption of the zircon crystal lattice, reduction in particle size, - introduction of new impurities, requirement for a further calcination step in order to remove newly introduced impurities or to reconstitute the leached product, high consumption of leaching agents, complex, multi-stage processes, - uneconomical processes.

There is accordingly a need for a process that removes impurities from zirconium containing materials which overcomes, or at least alleviates, one or more disadvantages of the prior art processes. SUMMARY OF THE INVENTION Accordingly, the present invention provides a process for reducing the amount of impurities in a zirconium containing material, said process including the step of treating the zirconium containing material with a composition, substantially including a borate salt or mineral, at a temperature and for a period of time sufficient to collect at least part of said impurities into a borate phase. The present invention also provides a purified zirconium containing material produced by a process as defined in the preceding paragraph.

Preferably, the process further includes the step of leaching the borate phase produced above in order to dissolve the impurities. Typically the leachant is an acid, preferably a dilute acid. Advantages of the process of the invention in at least some of the embodiments thereof, as applied to the purification of zircon, are that it can:

1. Produce a cleaner zircon product,

2. Produce a product with mineral constitution not significantly different from the feed zircon, 3. Produce a product with grain size distribution not significantly different from the feed zircon, 4. Allow for removal of impurity elements from the zircon without destroying the crystal lattice of zircon grains to large extent, 5. Remove impurities without introducing new impurities to the final product to any large extent,

6. Use leaching agents at low concentrations to remove the impurities, and

7. Avoid the requirement for a second calcination step to remove newly introduced impurities or to reconstitute the leached product.

DETAILED DESCRIPTION OF THE INVENTION

Accordingly, there is provided a process for at least partially purifying a zirconium containing material, especially zircon. The zircon is typically mixed with a composition, at least largely comprised of a borate salt or mineral, and heated to a temperature and for a time sufficient to concentrate the impurity elements in a borate phase. The borate phase is then generally subjected to leaching, preferably dilute acid leaching, which dissolves the impurities into solution, from which they may be later extracted.

The borate containing composition effectively functions as a sink for the impurities in the zirconium containing material. Preferably the composition contains a minimum of 50 wt% of the borate salt or mineral. In preferred embodiments, the fluxing composition contains 70 wt% or higher of the borate salt or mineral. In one embodiment, the fluxing composition is substantially 100 wt% of the borate containing material. Preferably the borate salt or mineral contains a minimum of 10 wt% B₂O₃. In some embodiments, the borate salt or mineral contains at least 50 wt% B₂O₃. In one embodiment, the borate salt or mineral is substantially 100 wt% B₂0₃.

In some cases it may also be found beneficial to grind or micronise the zirconium containing material alone or with the composition before heating. Furthermore, the process of this invention may include a dilute caustic wash following the acid leach, rinsing with water and a drying step for removal of moisture.

Where the zirconium containing material is zircon sand, the final product of the process of this invention is essentially a pale pinkish, cream to white coloured zircon sand containing substantially low amounts of impurities such as Fe, Ti, Al, U, Ra and Th. The amount of impurities remaining in the final product is controlled by the initial impurity levels, intensity of the treatment conditions such as temperature of calcination, acid concentration, leach time and most importantly the amount of borate used in accordance to the amount of zircon sand. The integrity of the grain size is largely protected when about <20 wt% borate salt or mineral is added to the zircon sand.

Although the solids to be treated may be selected from any zirconium- containing minerals the use of zircon sand concentrate is preferred. The particle size of the zircon sand concentrate could comprise any particle size but is generally in the average size range of less than 500 microns. However, the finer the grain size the better the impurity removal as a result of exposure of a higher surface area to the borate containing material and leach solution.

Processing zirconiferous material to a higher purity zircon product is particularly applicable on the fine-grained (dso<50 microns) and U-Th-rich zircon sands of the Horsham area (Australia) and other fine-grained zircon sands, such as fine air table tails of commercial operations and metamict zircons.

The borate containing material is typically a borate salt or mineral or a mixture of borate salts and minerals. In an embodiment, the first step of the invention involves intimately mixing the zircon sand and the borate salt or mineral and calcining the mixture in a muffle furnace or rotary kiln or other suitable furnace. The borate salt or mineral used in this invention is chosen preferably from sodium, calcium, ammonium, magnesium or lithium containing borate salts or naturally occurring borate minerals. Double salts of borates can also be used, in particular sodium calcium borate (e.g. the mineral ulexite) or hydroboracite (magnesium calcium borate).

Contacting the zircon sand with the borate to obtain a uniformly mixed product may be conducted in a wet or dry state employing any means of mechanical or manual mixing. An intimate mixing may be obtained by grinding the two together. The amount of borate relative to zircon sand could vary from 0.1 to 50 wt percent, such as from 1 to 20 weight percent, although the use of 5 to 15% borate salt or mineral may be found sufficient to obtain a good level of impurity removal.

The heating temperature for the borate calcination is chosen such that a complete or largely complete reaction between the borate salt or mineral and the impurities of the zircon sand is obtained. Heating the zircon sand with the borate salt or mineral could be done in a furnace in the temperature range 700° to 1400°C, preferably 800°C to 1300°C, more preferably from 900°C to 1200°C. In one preferred embodiment, the temperature range is from 1200°C to 1250°C. Heating time is chosen to be long enough to assure a complete or largely complete reaction between the borate and the impurities of the zircon sand. It is expected that heating time of 1 to 48 hours, preferably 2 to 24 hours, more preferably 2 to 6 hours would be sufficient for such reaction. The heating process may be conducted in any suitable conventional manner, including utilising batch or continuous processing in a furnace. The heating steps of the process may be conducted in a furnace of any size, shape or type including conventional and circulating type fluidised beds, microwave, and static, rotating or vibrating type furnaces. These furnaces may be heated by employing any suitable heat source.

The calcined product may be cooled in air or quenched quickly in water, acid or any other solution to provide rapid cooling and maximum disintegration of any aggregates formed during heating. The calcined product may be stirred in the solution after quenching to promote a faster disintegration of lumps, aggregates and clusters. In some cases, especially when higher borate to zircon weight ratios are used, the cooled product may require a gentle grinding and the calcined product could be passed through a roll or a ball mill to break down the lumps, aggregates and clusters formed during heating.

The acid leaching step according to the present invention may be conducted in any suitable conventional manner, including utilising batch or continuous processing in open vessels at any suitable temperature, preferably with agitation. Temperatures in the range of 10°C to 110°C may be used. However, leaching at a temperature below the boiling point of the acid is preferred.

The acid leach solution may be selected from mineral or organic acids or their mixtures. Mineral acids such as hydrochloric, sulphuric, nitric, perchloric or hydrofluoric acids or organic acids including formic, citric, acetic, oxalic or tartaric acid or mixtures thereof may be used. Preferred acids are mineral acids, such as HCI or HNO₃, although HCI is more preferred because of its lower cost. While sulphuric acid may also be used, it has the disadvantage of forming insoluble RaSO as a by product of the leaching process.

Acid strength is chosen such that the bulk of the impurities dissolve at a maximum solids content within shortest reaction time. In general the required acid strength is low due to the selective removal of only trace amounts of impurities and added borate salt or mineral. The concentration of the acid may be such that the content of the effective ion in the leaching solution to the minor impurities is slightly in excess amount. A suitable acid concentration is from 0.1 molar to concentrated acid, preferably 0.2 to 2 molar, more preferably about 1.0 molar.

The solid content of the leach may be within the range 5 to 85% solids by weight, such as from 5 to 75% solids by weight, preferably from 10 to 50% solids by weight.

The reaction vessel may be agitated to promote the reaction rate. Agitation of the leaching vessel could be in the form of mechanical stirring or any other means to provide a rapid reaction between the impurities and the acid. The acid leach step of the process according to the present invention may continue for a time sufficient for the dissolution of impurity elements in the acid solution. Although a leaching time in the range of 5 minutes to 24 hours may be used, in most cases 30 to 120 minutes, such as 30 to 60 minutes, leaching time is sufficient for the removal of impurities. The solids may be separated from the leaching solution by filtration, flotation, centrifugation or by sedimentation. The leach solution may be recycled for use with a fresh charge of zircon sand, usually following a regeneration step. During the acid recycling step, the dissolved borate in the acid may be extracted and also recycled. The residue generated in the leach step may be reacted further with a fresh leaching composition under approximately the same or milder or stronger reaction conditions, but still remaining within the limits of the claims made in this application.

A second treatment can be expected to yield a product with lower impurity levels.

The recovered solids may be washed with water until the components of the leach solution are reduced to negligible levels. A rinse with a dilute caustic solution before a water rinse may be found beneficial to neutralise and remove any residual acid remaining in the product. However, in most cases the use of a caustic leach step is found optional and may even be not necessary. The water rinsed final product may be subjected to drying at a low (e.g. 100 to 200°C) temperature to remove the moisture.

The invention is more fully described in the following examples. It should be understood, however, that the following description is illustrative only and should not be taken in any way as a restriction on the generality of the invention described above. EXAMPLE 1

This example demonstrates that the treatment of a (commercially) micronised zircon sand to a particle size of -5 micron gives a good impurity removal (see Example 1 in Table 1). 50g of zircon sand was intimately mixed with 7.5 g of finely ground natural calcium borate mineral (colemanite) and the mixture was calcined in a muffle furnace at 1200°C for 4 hours. The calcined product was cooled to ambient temperature. The relatively soft but clustered product was crushed gently with pestle and mortar then leached with 1.2M HCI at a solids content of 10 weight% in a reaction vessel at 90°C for 30 minutes while being stirred. The solids were recovered by filtration and washing using an additional 150 mL water. The residue was dried at 110°C for several hours in an oven. The product was an off white colour. The particle size of the original product remained largely unchanged.

The chemical analysis of the final product is compared with the analysis of the starting sample in Table 1. Example 1 in Table 1 shows that the ZrO₂ assay of the dry product was upgraded from 61.3% to 64.1 %. The reduction in impurity levels was significant. For example, the removal of AI₂O₃ was 97%, Fe₂0₃ 82%, TiO₂ 78%, Th 38% and U was 59%. The overall weight loss as a result of this treatment was about 7%. Table 1. Experimental and analytical data.

EXAMPLE 2

This example demonstrates that the impurity levels of a zircon plant waste (fine air table tail) could be lowered to such levels that a saleable product can be made.

50g of zircon sand was intimately mixed with 7.5 g natural calcium borate mineral (colemanite) (i.e. 15 wt% colemanite) and the mixture was calcined in a muffle furnace at 1200°C for 4 hours. The calcined product was cooled to ambient temperature. The relatively soft but clustered product was crushed gently with pestle and mortar then leached with 1.2M HCI at a solids content of 10 weight% in a reaction vessel at 90°C for 30 minutes while being stirred. The solids were recovered by filtration and washing using an additional 150 mL water. The residue was dried at 110°C for several hours in an oven. The overall weight loss as a result of this treatment was about 11 %. The results are shown in Table 1.

The test results show that the impurity levels were reduced significantly and a pale pinkish coloured product was obtained. The AI₂O₃ level in the sample was reduced from 4.6% to 0.28% which is equivalent to a 94% reduction. The reduction in Fe₂O₃ was 74% and in TiO₂ was 85%. The U+Th content was reduced from 563 ppm to the required <500 ppm level. The ZrO₂ content was upgraded from 59.7% to 65.5% ZrO₂. Particle size variation before and after the treatment was negligible as shown in Table 2.

Table 2. Particle size distribution before and after treatment for Example 2.

Finally, it is to be understood that various other modifications, and/or alterations may be made without departing from the spirit of the present invention as outlined herein.

Claims

CLAIMS:

1. A process for reducing the amount of impurities in a zirconium containing material, said process including the step of treating the zirconium containing material with a composition, substantially including a borate salt or mineral, at a temperature and for a period of time sufficient to collect at least part of said impurities into a borate phase.

2. The process of claim 1, further including the step of leaching the borate phase produced above in order to dissolve the impurities.

3. The process of claim 2, wherein the leachant is an acid, preferably a dilute acid.

4. The process of any preceding claim, wherein said zirconium containing material is zircon, preferably a zircon sand concentrate.

5. The process of any preceding claim, wherein the composition contains a minimum of 50 wt% of the borate salt or mineral, preferably the composition contains 70 wt% or higher of the borate salt or mineral.

6. The process of any preceding claim, wherein the borate salt or mineral contains a minimum of 10 wt% B₂0₃, preferably the borate salt or mineral contains at least 50 wt% B₂O₃.

7. The process of any preceding claim, further including the step of grinding or micronising the zirconium containing material alone or with the composition before heating.

8. The process of claim 2, or any one of claims 3 to 7 when dependent on claim 2, further including a dilute caustic wash following the acid leach step in order to neutralise any residual leachant.

9. The process of claim 8, further including a water rinsing step, followed by a drying step for removal of moisture.

10. The process of any preceding claim, wherein said borate salt or mineral is selected from sodium, calcium, magnesium or lithium containing borate salts or naturally occurring borate minerals.

11. The process of claim 10, wherein said borate salt or mineral is a double borate salt, preferably sodium calcium borate, more preferably ulexite.

12. The process of any preceding claim, wherein the mass of borate salt or mineral is from 0.1 to 50 wt%, preferably from 1 to 20 wt%, more preferably from 5 to 15 wt% of the mass of zirconium containing material.

13. The process of any preceding claim, wherein the temperature of treatment with the borate salt or mineral is such that a complete or largely complete reaction between the borate salt or mineral and the impurities of the zircon containing phase is obtained.

14. The process of claim 13, wherein said temperature is in the range 700° to 1400°C, preferably 800°C to 1300°C, more preferably from 900°C to 1200°C.

15. The process of any preceding claim, wherein the time of treatment with the borate salt or mineral is such that a complete or largely complete reaction between the borate salt or mineral and the impurities of the zircon containing phase is obtained.

16. The process of claim 15, wherein said time is from 1 to 48 hours, preferably 2 to 24 hours, more preferably 2 to 6 hours.

17. The process of any preceding claim, wherein the step of treatment with a borate salt or mineral is followed by the treated product being cooled in air or quenched quickly in water, acid or any other solution to provide rapid cooling and maximum disintegration of any aggregates formed during heating.

18. The process of claim 17, wherein said cooled or quenched product is subjected to gentle grinding or stirring in solution, to promote disintegration of any clumps or aggregates formed during said treatment step.

19. The process of claim 2, or any one of claims 3 to 18 when dependent on claim 2, wherein the temperature of leaching is below the boiling point of the leachant, preferably between 10°C and 110°C.

20. The process of claim 2, or any one of claims 3 to 19 when dependent on claim 2, wherein the leachant is selected from mineral acids including hydrochloric, sulphuric, nitric, perchloric or hydrofluoric acids or organic acids including formic, citric, acetic, oxalic or tartaric acid or mixtures thereof.

21. The process of claim 20, wherein said leachant is a mineral acid, preferably selected from HCI and HNO_3> and is more preferably HCI.

22. The process of claim 20 or 21 , wherein said leachant is not H₂SO₄.

23. The process of claim 3, or any one of claims 4 to 22 when dependent on claim 3, wherein the concentration of acid is from 0.1 molar to concentrated acid, preferably from 0.2 to 2 molar, more preferably about 1.0 molar.

24. The process of claim 2, or any one of claims 3 to 23 when dependent on claim 2, wherein the amount of solids present in the leachant is within the range from 5 to 85% solids by weight, preferably from 5 to 75% solids by weight, more preferably from 10 to 50% solids by weight.

25. The process of claim 2, or any one of claims 3 to 24 when dependent on claim 2, wherein the leachant is agitated during the leaching step.

26. The process of claim 2, or any one of claims 3 to 25 when dependent on claim 2, wherein said leaching step is conducted for a time in the range of 5 minutes to 24 hours, preferably from 30 to 120 minutes, more preferably from 30 to 60 minutes.

27. The process of claim 2, or any one of claims 3 to 26 when dependent on claim 2, further including an acid and/or borate regeneration step after said leaching step and a recycling step after said regeneration step.

28. The process of claim 9, wherein the leached product is dried at a temperature between 100 and 200°C after the rinsing step.

29. The process of claim 13, wherein said temperature is in the range 1200 to 1250°C.

30. The process of any preceding claim, wherein the step of treating the zirconium containing material with a composition introduces substantially no new impurities derived from said composition.

31. A process for reducing the amount of impurities in a zirconium containing material, substantially as herein described with reference to Example 1 or 2.

32. A purified zirconium containing material produced by a process according to any one of claims 1 to 31.