EP2556026A1

EP2556026A1 - Improved synthetic rutile process a

Info

Publication number: EP2556026A1
Application number: EP11764945A
Authority: EP
Inventors: Timothy John Mcdougall; Andre Kirwan Vaisey
Original assignee: Iluka Resources Ltd
Current assignee: Iluka Resources Ltd
Priority date: 2010-04-06
Filing date: 2011-04-06
Publication date: 2013-02-13
Also published as: WO2011123888A1; AU2011238419B2; CN103201221A; AU2011238419A1; CN103201221B; US20130022522A1

Abstract

A process for recovering titanium as synthetic rutile from an ilmenite unsuited to the standard Becher process includes the steps of treating the ilmenite unsuited to the standard Becher process in a reducing atmosphere in the presence of a carbonaceous reductant whereby to convert the ilmenite to reduced ilmenite in which iron oxides in the ilmenite have been reduced to metallic iron, and separating out the metallic iron so as to obtain a synthetic rutile product. The treatment of the ilmenite is at an elevated temperature lower than that for which the TiO₂ content of the synthetic rutile product is highest but at which there is substantially no reoxidation of the metallic iron. The carbonaceous reductant comprises coal selected for a gasification reactivity that results in an increased rate of reduction of iron oxides and titanium species effective to at least partly offset the lowered TiO₂content of the synthetic rutile product resulting from the lower elevated temperature, and to achieve a TiO₂ content of 90% or greater in the synthetic rutile product.

Description

Improved Synthetic Rutile Process A

Field of the invention

This invention relates to the recovery of titanium as synthetic rutile from titaniferous ores, and in particular from primary ilmenites, hybrid ilmenites and other ilmenites with a relatively high proportion of iron or problem impurities or a relatively low proportion of titanium.

Background of the invention

The standard process by which titanium dioxide is recovered from the ilmenite component of Western Australian mineral sands deposits is the Becher reduction process in which the ilmenite is roasted in a rotary kiln in the presence of coal and a reducing atmosphere so as to reduce iron oxides in the ilmenite to metallic iron, which is then separated by aqueous oxidation to obtain a product known as synthetic rutile, typically having a Τ1Ό₂ content of 90% or greater. The synthetic rutile is a feedstock for further processing to white paint pigment and other applications. These further processes are sensitive to a minimum Ti0₂ content, and the output of the Becher process is in turn dependent on a relatively tight ilmenite feed specification, e.g. in Western Australia an iron content measured as FeO<12%. In practical terms this limits the feedstock for the Becher process to secondary ilmenites, also known as altered or weathered ilmenites. Primary ilmenites, which have a higher iron content, are not suitable for the Becher process but in Western Australia for 16%< FeO<24% so-called sulphate ilmenites have commercial value as a feedstock for the alternative sulphate process route to Ti0₂. Between the Becher and sulphate ranges, i.e. 12%<FeO<16%, ilmenites, known in this range as hybrid ilmenites, have no commercial use. The strict upper limit on FeO content for Becher process ilmenite feedstock relates to the prevention of iron reoxidation during reduction. Circumstances that give rise to reoxidation in the kiln are difficult to measure and control but it is known that reoxidation is more significant with primary ilmenite due to its higher iron content and the resultant risk of agglomeration or sintering and boulder formation. It is known that susceptibility to reoxidation (and therefore to the formation of agglomerates) can be countered by lowering the kiln operating temperature: for example lowering the temperature from around 1100 to 1150°C, a typical Becher process range, to the vicinity of 1000-1025°C can reduce agglomerate/sinter formation to acceptable levels. The problem is that the resultant rate of synthetic rutile production is uneconomic.

The restrictive ilmenite specification for the Becher process is becoming a more urgent problem in locations where secondary ilmenite resources are diminishing. From the perspective of the owners of these resources, it has been and remains desirable to extract greater commercial returns for the resource, from both the hybrid and sulphate ranges of FeO content.

In other ilmenite provinces, e.g. the Murray Basin of Victoria and New South Wales, the available ilmenite is not suitable as Becher process feedstock because of a high content of disadvantageous impurities, notably magnesium and chromium, and a consequent lower proportion of titanium. For example, the standard feed specification for Western Australia secondary ilmenite to the Becher process is FeO<12%, 57% <Ti0₂ < 65%. Murray Basin ilmenites typically have a Ti0₂ content around 54-56%, with Mg typically present in the range 1.5 to 2.5% and Cr around 1%.

It is accordingly an object of this invention to provide a commercially useful process for recovering titanium dioxide values from primary ilmenites, hybrid ilmenites and other ilmenites with a relatively high proportion of iron or problem impurities or a relatively low proportion of titanium. These ilmenites are collectively referred to herein as ilmenites unsuited to the standard Becher process.

Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art. Summary of the invention

For primary ilmenites, FeO>16%, it has been found that the rate of sinter formation through reoxidation at 1100°C may be as high as six times that for ilmenite of FeO<12%, and that this can only be prevented by substantially lowering the temperature of the reducing treatment below that normally employed for the Becher process. It has been further found that synthetic rutile of an acceptable grade, >93%, can still be produced at temperatures such as 1025°C, but the rate of production of synthetic rutile is unacceptably low. In accordance with the invention, it has been surprisingly found that this unacceptable outcome rendering the use of primary or hybrid ilmenites uneconomic for the Becher process can be offset and indeed overcome by the employment of a coal reductant having a gasification reactivity that results in an increased rate of reduction of iron oxides and titanium species.

The invention provides a process for recovering titanium as synthetic rutile from an ilmenite unsuited to the standard Becher process, including the steps of treating the ilmenite unsuited to the standard Becher process in a reducing atmosphere in the presence of a carbonaceous reductant whereby to convert the ilmenite to reduced ilmenite in which iron oxides in the ilmenite have been reduced to metallic iron, and separating out the metallic iron so as to obtain a synthetic rutile product. The process is characterised in that the aforesaid treatment of the ilmenite is at an elevated temperature lower than that for which the Ti0₂ content of the synthetic rutile product is highest but at which there is substantially no reoxidation of the metallic iron, and in that the carbonaceous reductant comprises coal selected for a gasification reactivity that results in an increased rate of reduction of iron oxides and titanium species effective to at least partly offset the lowered Ti0₂ content of the synthetic rutile product resulting from said lower elevated temperature, and to achieve a Ti0₂ content of 90% or greater, preferably at least 93%, in the synthetic rutile product.

It may be that the gasification reactivity of the coal is simply sufficiently high to achieve said offset, but a high value for the gasification reactivity may not be sufficient. It may be relatively high as a coal gasification reactivity, by which is meant in the context of this specification significantly higher than the average of all coals. In practical terms, this means that the gasification reactivity is towards the higher end of the range of gasification reactivity generally found in coals. The gasification ^' reactivity is preferably greater than 0.005 g-g/min at 850°C, more preferably greater than 0.01 g-g/min at 850°C, both values for coal char at atmospheric pressure. Alternatively or additionally the gasification reactivity is preferably at least twice that of typical Collie coal, more preferably at least three times that of typical Collie coal.

The elevated temperature of said treatment is preferably less than 1050°C, more preferably between 975 and 1035°C, and most preferably in the range 1000 to 1030X.

One known indicator of higher coal gasification reactivity is the level of ion-exchanged calcium, although it is thought that other impurity elements can play a similar role. The selected coal accordingly preferably has impurity levels of ion-exchanged inorganic elements sufficiently high to increase the gasification rate of the coal thus improving the reducing conditions in the process and thereby increasing the rate of reduction of iron oxides and titanium species. Such elements may include alkaline earth elements such as calcium and magnesium, or alkali elements such as sodium, or iron. Coal containing relatively high levels of ion-exchanged calcium has been found to be particularly useful.

A measure of sufficiently high levels of ion-exchanged inorganic elements is the acid extractable proportion of the elements: this is desirably greater than 50%, more preferably greater than 70%, most preferably greater than 80%. Usefully, at least one such inorganic element is present to the extent of at least 0.2%db on a dry coal basis. While the coal may be of any rank including bituminous, a suitable coal comprises a sub-bituminous or lignite coal selected for a total moisture content between 5 and 40%, or an inherent moisture content in the range 5 to 25%, in the latter case most preferably about 20% or less. Volatiles content is preferably greater than 30%, most preferably greater than 40%. Ash content is preferably below 10%, most preferably below 5%. Ultimate hydrogen content of the coal, on a dry ash basis, is preferably greater than 4%. Ultimate carbon content is preferably greater than 65%. Ash fusion temperature may be above 1100°C, on an initial deformation temperature (I.D.T.) basis, above 1200°C on a hemispherical temperature (H.T.) basis (more preferably at least 1150°C and 1250°C respectively). Preferably, char is mixed with the ilmenite before it is delivered for the aforesaid treatment step. The presence of char mixed with the ilmenite has been found to further assist in reducing the rate of agglomeration or sintering arising from reoxidation.

Preferably, the sulphur content of the coal is less than 1 % w/w, more preferably less than 0.5%, most preferably less than 0.2%. Preferably, there is no additional sulphur present for most of the duration of said treatment. It has been found that sulphur contained in the coal above these preferred levels (for example by providing a. blend of low-sulphur and high sulphur coal fractions) or present by virtue of additional sulphur, adversely affects the reactivity of the ilmenite, i.e. the rate of metallisation (the speed at which iron oxide is converted to metallic iron in the reduction treatment step).

Thus, if in order to further increase the Ti0₂ content of the synthetic rutile product of the process, it is desired to deliver sulphur to the ilmenite during said treatment step, e.g. for removing manganese impurity as manganese sulphide, such delivery is effected only later during the duration of the reduction treatment, for example only during the last 3 hours of a 9 hour treatment.

The iron content of the ilmenite, expressed as FeO, may be in the range FeO>12%, for example in the range 12%<FeO<30%.

Preferably, free oxygen in the treatment atmosphere is no greater than 2.5% and preferably less than 2%, most preferably less than 1 %. Preferably, the treatment at elevated temperature in a reducing atmosphere is carried out in an inclined rotary kiln of the kind normally employed for the Becher process. The material recovered from the lower end of the kiln is known as reduced ilmenite, a mix of metallic iron and titanium dioxide with a residual content of iron and other impurities. This reduced ilmenite is cooled to prevent reoxidation of metallic iron and then passed to the separation step.

The iron removal step may be any suitable separation method employed in Becher reduction processes. A typical such method is an aqueous oxidation step in which the metallic iron is oxidised or rusted to magnetite, haematite or lepidocrocite in a dilute aqueous solution of ammonium chloride catalyst. A final stage to remove further iron and manganese impurities may entail an acid leach or wash, typically employing sulphuric acid (e.g. 1 to 2M - at least double the strength in the standard process).

It will be appreciated that the process of the invention is applicable to primary and hybrid ilmenites (however locally defined) and to other ilmenites unsuited to the standard Becher process, e.g. Murray Basin ilmenites of relatively low Ti content (e.g. 54-56%) and higher impurity content (notably Mg 1.5-2.5%, Cr 1 %).

Examples

A sequence of tests was carried out employing simple bulk samples of a number of primary ilmenites selected to have a range of FeO contents. These ilmenites were Yoganup Extended, Wagerup, Cloverdale and Waroona ilmenites from different resources in Western Australia. Yoganup Extended ilmenite was chosen for its high (27%) FeO content, which would represent a worst case scenario in sintering and reduction test results. The other three ilmenites have FeO contents within the afore- mentioned sulphate ilmenite range.

To initially test the effect of temperature, two large samples of Yoganup Extended primary ilmenite and a secondary standard Capel ilmenite (FeO 12%) were reduced using Gollie coal at the standard 1100° reduction temperature. Table 1 sets out an assays for each of the five selected ilmenites. Analysis of the ilmenite and reduced ilmenite (Rl - the product of the treatment prior to separation of the metallic iron) showed minimal sintering in the primary ilmenite during the initial reduction.

To establish the temperature effect on sintering, the Rl samples were subjected to various temperatures and oxygen concentrations whilst being held in a furnace. Rl samples were placed on a platinum crucible and exposed to mixtures of oxygen and nitrogen of 1.2% 0₂, 2.46% 0₂ and 5.3% 0₂. Tests were conducted at 1000, 1050, 1100 and 1 150°C. Sinter production was measured by sizing the original Rl and the product removed from the platinum boat after 1 minute. Screening was initially conducted using 10 standard aperture sizing betweens 106um and 1mm. Analysis of the sizing results showed the best measure of sintering to be the increase in the amount of +250um and +1mm material. For this reason in later tests screening was carried out with only 250um and 1mm screens.

The presence of char also has an effect on sintering due to the protection it offers from reoxidation. Since the kiln may have zones of segregation of coal and Rl it was decided to test the degree of sintering in the both the presence and absence of char.

The reference feed ilmenite was a 14% FeO sample selected to form the basis of comparison. The reference ilmenite represents the highest FeO level that had been processed through nearby SR kilns without incident. Results from the reference ilmenite set a benchmark for the maximum acceptable level of sintering and by how much reduction temperatures need to be dropped to achieve the same sintering levels.

Table 2 shows the degree of sintering at 1000, 1050, 1 100 and 1150°C after 1 minute of exposure to a 5.3% oxygen/nitrogen mixture. Sizings of Rl and ilmenite are also shown for reference.

The data from Table 2 is also plotted in Figure 1. The following observations can be made regarding increased temperature in the presence of surplus oxygen:

The degree of sintering increases with higher temperatures. This is evident by a reduction in the amount of fine grained material in the 125 and 150um size range. The amount of agglomerates in the 212 and 250um size range more than doubles as temperature increases. There is also a sharp increase in the amount of +1mm sized material which represent multi-particle agglomerates compared to 2-particle agglomerates.

• The degree of agglomeration of +1mm primary (Yoganup Extended) ilmenite (Figure 2, Table 3) compared to standard Capel ilmenite was measured to be 6 times. At 1150°C the amount of +1mm sinter was 9.9% in Capel ilmenite and 67% in the primary ilmenite.

• At 1000°C the degree of sintering in both Capel ilmenite and primary ilmenite was negligible. The degree of sintering increased proportionately with temperature and time. When the exposure time was left longer than 1 minute the entire sample was found to fuse into a single lump.

Plotting the amount of +250um in Rl against temperature showed a point of inflection at around 1050 °C. At temperatures above 1000°C the rise in sintering rates was significant particularly at higher oxygen concentrations of 5.3%. However in most practical instances an oxygen concentration of 1 to 2% is the most likely scenario except in the instance of a cracked shell air tube. At lower oxygen concentrations of around 1 % to 2% the amount of +250um sinter began to increase at around 1020 °C.

Figure 3 shows the amount of plus +250um sinter formed after one minute at increasing oxygen concentrations for different reduction temperatures. It will be seen that there is a marked rate of diminution at temperatures below 1100°C for an oxygen concentration below 2.5%.

Having established that agglomeration and sintering could be minimised if the kiln temperature was in the region of 1000 to 1025°C, reduction tests were carried out respectively employing Collie coal, commonly used in Western Australia as the solid reductant in commercial operations of the standard Becher process using secondary or altered ilmenites, and a coal determined by testwork to have a high gasification reactivity. This reactive coal was found to have a gasification reactivity about five times higher than the Collie coal. A consequence is that the generation of reduction gases (CO,H₂ etc) will occur at lower temperatures than for Collie coal and it was thus thought possible that ilmenite reduction would also occur at lower temperatures, thereby allowing the option of reducing kiln operational temperatures to the desired level.

The gasification (C0₂) reactivity behaviour of char samples (200-300μm) produced from the reactive coal and the Collie coal was determined using a high-pressure thermogravimetric analyser. For samples of about 300mg, C0₂ reactivity was determined from the rate of sample mass loss due to the reaction C + C0₂ (g) <=> 2CO(g). Tests were performed under two temperature conditions at atmospheric pressure: isothermal at 850°C and a varying temperature increased from 700°C at a rate of 2°C/min. The latter test allowed the temperature dependence of the gasification reaction to he determined. The relative reactivities of the coal chars are presented in Table 4. It will be seen that, as noted above, the reactive coal was found to have a gasification reactivity at 850°C about five times higher than the Collie coal.

Elemental analyses of the coals is set out in Table 5. It will be seen that the reactive coal has materially higher levels of calcium and magnesium (a full order of magnitude difference) relative to the Collie coal and this was found to be the case also in analyses of the respective ash residues. On a dry coal basis, each is about 0.2%db. It was established that the calcium and magnesium, and also the iron, were present in an ion- exchanged form in the reactive coal. This was established by demonstrating that the acid extractable levels of Ca, Mg and Fe in the reactive coal were of the order of 85- 95%, while the Collie coal had much lower levels (less than 50%) of acid extractable Ca, Mg and Fe. The presence of ion-exchanged calcium, iron, sodium and, to a lesser extent magnesium, in coals has been found to enhance the gasification reactivity. By increasing the gasification rate of the coal, the reducing conditions in the process are improved, thereby increasing the rate of reduction of iron oxides.

Each ilmenite was reduced at 1025°C, which has been found to be the maximum desirable operating temperature from previous sintering tests. Samples were extracted from the reduction pot at 4.5, 5.0, 5.6, 6.2, 6.8, 7.4 and 9.0hrs. Titrations were carried out on each sample to determine the amount of metallic iron formed. The metallisation rate for each ilmenite sample is shown in Table 6.

Table 6 clearly shows the slower reduction rates of Collie coal compared to the reactive coal, taking nearly the full 9 hours to achieve 95% metallisation compared to the reactive coal taking just over 5.6 hours. Cloverdale ilmenite reduced significantly faster than either of the other three sulphate ilmenites with a behaviour more similar to an altered secondary ilmenite. Complete reduction was achieved in just over 5.6hrs. Cloverdale ilmenite also has the lowest FeO content of 18.4% and lowest MnO level of 0.96%.

Expected kiln throughput rates for the different sulphate ilmenites are shown in Table 7. The baseline reduction shows Yoganup Extended sulphate ilmenite (27% FeO) with Collie coal at 22.1 t/hr which is an approximate 45% reduction in capacity compared to typical throughputs. All reduction temperatures are assumed as 1025°C to minimise the likelihood of sintering.

In contrast, throughput rates of 31.8 t/hr (20% reduction) are expected for Yoganup extended sulphate ilmenite using the reactive coal at 1025°C, and throughput rates of 69.9 t/hr (75% increase) are expected for Cloverdale sulphate ilmenite using the reactive coal at 1025°C.

The expected feed rates as shown in Table 7 are depicted graphically in Figure 4 to show that two sulphate ilmenites performing above current typical feed rates and two below. The two best performing sulphate ilmenites had the lowest FeO of 18.4% (Cloverdale) and 19.1% (Waroona). The lowest performing sulphate ilmenites (>20% reduction in throughput) had FeO levels of 27% (Yoganup Extended) and 19.7% (Wagerup).

Rl samples from the reduction of sulphate ilmenite were acid leached using a 2M sulphuric acid concentration to produce a simulated synthetic rutile (SR). From previous leach tests on Yoganup Extended primary ilmenite a 2M acid concentration was found to produce the optimum SR Ti0₂ grade. The 2M strength is approximately twice the normal strength needed with altered ilmenites (0.5 to 1.0M) required to fully extract all of the iron.

Table 8 sets out the assays of the resultant synthetic rutile products. Acceptable SR Ti0₂ grades were obtained from 3 of the 4 sulphate ilmenite samples tested using the reactive coal at 1025°C. Unacceptable SR Ti0₂ grades (<90% Ti0₂) from Collie Coal (89.61%) occurred due to the slower metallisation rates and incomplete metallisation at the end of 9 hours at 1025°C. 93.3% Ti0₂ grade was achieved at 100°C due to higher metallisation rates, however the higher reduction temperatures also has a much higher risk of sintering.

Acceptable SR Ti0₂ grades (>93%) were produced from Cloverdale (95.12%) and Yoganup Extended (93.00%) primary ilmenite at 1025°C using the reactive coal. Acceptable but below specification SR grades (92.08%) were produced from Waroona sulphate ilmenite at 1025X using the reactive coal. Unacceptable SR ΤΊΟ₂ grades (88.67%) were produced from Wagerup sulphate iimenite, which was also the slowest reducing of the four sulphate ilmenites. A lower overall total metallisation completion of 96.6% (Table 6) compared to 98% resulted in a residual SR iron level of 8.76%. Figure 5 illustrates the rates of reduction of iron oxides (as measured by metallic iron formation) and titanium species, for respective kiln reductions of a primary iimenite under similar conditions with Collie coal and the reactive coal. An assay of the primary iimenite employed is provided under the graphs.

As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude further additives, components, integers or steps.

It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

Claims

The claims defining the invention are as follows.

1. A process for recovering titanium as synthetic rutile from an ilmenite unsuited to the standard Becher process, including the steps of treating the ilmenite unsuited to the standard Becher process in a reducing atmosphere in the presence of a carbonaceous reductant whereby to convert the ilmenite to reduced ilmenite in which iron oxides in the ilmenite have been reduced to metallic iron, and separating out the metallic iron so as to obtain a synthetic rutile product, characterised in that said treatment of the ilmenite is at an elevated temperature lower than that for which the ΤiΟ2 content of the synthetic rutile product is highest but at which . there is substantially no reoxidation of the metallic iron, and in that the carbonaceous reductant comprises coal selected for a gasification reactivity that results in an increased rate of reduction of iron oxides and titanium species effective to at least partly offset the lowered ΤiΟ2 content of the synthetic rutile product resulting from said lower elevated temperature, and to achieve a TiO₂ content of 90% or greater in said synthetic rutile product.

2. A process according to claim 1 wherein the elevated temperature of said treatment is less than 1050°C.

3. A process according to claim 1 or 2 wherein the gasification reactivity is sufficiently high to achieve said offset.

4. A process according to claim 3 wherein said gasification reactivity of the coal is relatively high (as defined herein).

5. A process according to any one of claims 1 to 4 wherein the selected coal has impurity levels of one or more ion-exchanged inorganic elements sufficiently high to increase the gasification rate of the coal thus improving the reducing conditions in the process and thereby increasing said rate of reduction of iron oxides and titanium species.

6. A process according to claim 5 wherein the acid extractable portion of said one or more ion-exchanged inorganic elements is at least 50%.

7. A process according to any one of claims 1 to 4 wherein the selected coal has relatively high impurity levels of ion-exchanged calcium.

8. A process according to any one of claims 1 to 7 wherein the selected coal is a sub-bituminous or lignite coal.

9. A process according to claim 8 wherein the selected coal has a total moisture content between 5 and 40%, volatiles content greater than 30%, and ash content below 10%.

10. A process according to claim 8 wherein inherent moisture content of the selected coal is 20% or less, volatiles content is >40% and ash content is <5%.

11. A process according to any one of claims 1 to 10 further including mixing char with the ilmenite before it is delivered for said treatment step.

12. A process according to any one of claims 1 to 11 wherein the sulphur content of the coal is less than 1 % w/w, and there is no added sulphur present for most of the duration of said treatment.

13. A process according to claim 12, wherein the sulphur content of the coal is less than 0.5%.

14. A process according to claim 12, wherein the sulphur content of the coal is less than 0.2%.

15. A process according to claim 12, 13 or 14 further including delivering sulphur to the ilmenite during said treatment step for removing manganese impurity as manganese sulphide, such delivery being effected only later during the duration of the reduction treatment.

16. A process according to any one of claims 1. to 15 wherein the iron content of the ilmenite, expressed as FeO, is greater than 2%.

17. A process according to any one of claims 1 to 16 wherein the iron content of the ilmenite, expressed as FeO, is less than 30%.

18. A process according to any one of claims 1 to 17 wherein free oxygen in the treatment atmosphere is no greater than 2.5%.

19. A process according to any one of claims 1 to 18 wherein the Ti0₂ content achieved in said synthetic rutile product is at least 93%.

20. A process according to any one of claims 1 to 19 wherein the ilmenite unsuited to the standard Becher process is one of a primary ilmenite and a hybrid ilmenite.

21. A process according to any one of claims 1 to 19 wherein the ilmenite unsuited to the standard Becher process is a Murray Basin ilmenite of relatively low Ti content (e.g. 54-56%) and higher impurity content (including Mg 1.5-2.5%, Cr 1%).