EP2556027A1

EP2556027A1 - Improved synthetic rutile process b

Info

Publication number: EP2556027A1
Application number: EP11764946A
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: AU2011238420B2; CN103209926A; US20130022521A1; AU2011238420A1; WO2011123889A1

Abstract

A process for recovering titanium as synthetic rutile from a titaniferous ore, for example a secondary ilmenite, includes the steps of treating the ore in a reducing atmosphere at elevated temperature above 1075°C 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 carbonaceous reductant comprises a coal selected for a moisture content below 40%, a volatiles content greater than 30%, ash content below 10%, and a gasification reactivity that results in an increased rate of reduction of iron oxides and titanium species effective to achieve a TiO2 content of 90% or greater in the synthetic rutile product.

Description

Improved Synthetic Rutile Process B

Field of the invention

This invention relates to the recovery of titanium as synthetic rutile from titaniferous ores, and is particularly though not exclusively directed to improving the economics of recovery of titanium from lower grade secondary or altered ilmenites.

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 Ti0₂ 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 Τί0₂ 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.

The restrictive ilmenite specification for the Becher process is becoming a more urgent problem in locations where secondary ilmenite resources are diminishing in respect of their Ti0₂ grade. For example, the standard feed specification for Western Australia secondary ilmenite to the Becher process is FeO<12%, 57% <Ti0₂ < 65%. From the perspective of the owners of these resources, it has been and remains desirable to extract greater or more economically attractive commercial returns for the resource, and/or to extend the life of lower grade secondary ilmenite provinces.

Whilst the ilmenite properties cannot be changed, the coal properties can still have a significant overall effect. The volatile component of coal is necessary in the ilmenite preheating stage (<800°C) where minimal reduction occurs. The char produced from the preheating stage loses its volatile component and the internal structure becomes porous in the process. The internal porosity structure created is measured in terms of micro and macro porosity. A finely porous structure has a significantly increased surface area to volume ratio which has been shown to increase reaction rates. The internal structure created during de-volatilisation largely depends on the volatile content and the molecular coal structure.

The function of the coal is however two-fold so improvements in performance in one aspect may result in a reduction in the other. Whilst a highly porous char structure assists reaction rates in the reduction zone the loss of carbon and moisture during the preheating/de-volatilisation stage leads directly to a reduction in carbon mass in the reduction zone. A trade-off therefore occurs between the amountof volatiles produced in the preheat zone, the char porosity/reactivity and the amount of carbon entering the reduction zone.

Western Australian Collie coal has long been considered as having the ideal properties as a fuel and a reductant for ilmenite reduction kilns. Its qualities include a relatively low ash content of 6%, a low volatile content of 26% and high ash fusion temperature of 1410 deg (deformation).

Collie coal is from the Gondwanan coal formation (the super continent forming part of Antartica, Australia, South Africa and India some 240 to 280 million years ago). Coals of this origin are unique in that they are of relatively low rank for their age due to being buried at relatively shallow depths.

In the coalification process vegetative and organic matter (peat) is first compressed to remove moisture to less than 70% to form brown coal or lignite. Coals with high moisture (eg Victorian brown coal) are therefore usually low rank with poor mechanical strength. In the second coalification stage volatiles are converted to fixed carbon and the remaining moisture is further reduced to less than 50% whilst the colour darkens appreciably. Such coals (eg Collie coal) are classified as sub-bituminous having a specific energy value greater than 19MJ/kg and volatile content less than 30%. There are another two stages which involve further reduction of moisture and volatiles. Bituminous coals have a further reduction in moisture to less than 4% (eg Sydney and Bowen Basins). Anthracite is formed in the very last stage of volatile removal and usually occurs only in tectonic zones. The only Australian coals approaching anthracite composition are Yarabee and Baralaba in Queensland.

Bituminous coals are considered less suitable for the Becher process due to their low volatile content and higher ash levels (>8%). Victorian Brown coal briquettes trialled in 1997 and 1998 showed only minor process improvements. No significant benefits were measured during these trials.

It is an object of this invention, at least in one or more embodiments, to provide one or more modifications to the standard Becher process that improve the economics of recovery of titanium into synthetic rutile. It is another object of the invention, at least in one or more embodiments, to improve the economics of recovery of titanium from lower grade secondary or altered ilmenites.

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

In accordance with the invention, it has been surprisingly found that the objects of the invention can be met, at least in part, by the employment of a sub-bituminous or lignite 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 a titaniferous ore, for example a secondary ilmenite, including the steps of: treating the ore in a reducing atmosphere at elevated temperature above 1075°C 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 carbonaceous reductant comprises a coal selected for a moisture content below 40%, a volatiles content greater than 30%, ash content below 10%, and a gasification reactivity that results in an increased rate of reduction of iron oxides and titanium species effective to achieve a Ti0₂ content of 90% or greater, preferably at least 93%, in said synthetic rutile product.

It may be that the gasification reactivity of the coal is sufficiently high to achieve said Ti0₂ content, 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 in the range 1075-1200°C, more preferably between 1100 and 1075°C, and most preferably in the range 1100 to 1 150X.

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 of any rank including bituminous, a suitable coal is typically a sub- bituminous or lignite coal. Moisture content of the coal may be a total moisture content between 5 and 40%, or an inherent moisture content in the range 5 to 25%, in either case preferably not less than 5%. In the latter case, the moisture content is preferably 20% or less. Volatiles content is preferably >40%. Ash content is preferably <5%. Ultimate hydrogen content, 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, is preferably in the range FeO<12%. 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 separation 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. An alternative or additional separation step may entail an acid leach or wash, typically employing sulphuric acid.

Examples To establish the relative performance of different coals a standard pot reduction test was used. For this test 500g samples of a secondary standard Capel ilmenite (FeO 12%) were combined with 500g of coal and reduced for 9 hours under a standard heating profile which reaches and maintains 1100°C by 6.5 hours. Coal was prepared by crushing and screening to +4mm and -9.5mm. Samples of reduced ilmenite (Ri - the product of the treatment prior to separation of the metallic iron) were extracted at timed intervals of 3.5, 4.0, 4.5, 5.0, 5.6, 6.2, 6.8, 7.4 and 9.0 hrs. The extracted samples were then analysed for metallic iron and total iron. The final 9.0 hr RI sample in each case was acid leached in 1.0 sulphuric acid at room temperature for 15 minutes and then at 60°C for a further 60 minutes. A total of 10 coals were selected for testing including Collie coal as the reference. Collie coal is commonly used in Western Australia as the solid reductant in commercial operations of the standard Becher process using secondary or altered ilmenites. Coal specifications for each of the coals selected are shown in Table 1 , while Table 2 sets out an assay for the standard Capel secondary ilmenite. Metallic iron levels were measured at the intervals discussed above, and observed metallisation rates for the different coals are shown as log cures in Figure 1.

Using the log reducibility constant derived from Figure 1 (slope of log metallisation curve), kiln feed rates were predicted via calculation in a 90 step kiln model and these are set out in Figure 2. The baseline throughput rate with Collie coal is predicted as 40.1 t hr. Other tested coals ranged from 37.7 to 49.3 t hr, save for CS3 which ranked first by an easy margin at 68.3 t/hr.

Rl samples from the pot reductions for each coal sample were acid leached at a strength of 1.0M sulphuric acid to simulate the SR grade produced. XRF assays of the SR product are shown in Table 3. SR grades produced by the test coals were on average 1.0% Ti0₂ better than Collie coal. CS3 coal produced an SR grade of 93.9% Ti0₂ compared to Collie coal at 92.4% Ti0₂.

In general better Ti0₂ grades were generated by the slower of the test coals (CS6, CS7 and CS9) where the amount of time for reoxidation at the end of the reaction is minimised. In reality the faster test coals such as CS3 and CS2 would produce better SR Ti0₂ grades if the Rl was extracted at 6.2 hrs instead of 9.0hrs.

Manganese (MnO) was notably higher in test coal reductions due to their lower_fsulphur content.

The metallisation profile for CS4 showed that metallisation stopped mid way through the reaction and did not complete. Iron extraction and Ti0₂ grades were observed to suffer accordingly.

Test work was now conducted on CS3 coal to determine what might characterise its clearly better performance in the pot reductions. The gasification (C0₂) reactivity behaviour of char samples (200-300pm) produced from the CS3 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 be determined.

The relative reactivities of the coal chars are presented in Table 4. It will be seen that CS3 coal was found to have a gasification reactivity at 850°C about five times higher than Collie coal.

Elemental analyses of the coals (CS3 and Collie) are 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 above 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 of acid extractable Ca, Mg and Fe (less than 50%). 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.

One potential benefit of a more reactive coal would be to reduce the feed coal ratio and so reduce the expense of coal. Reducing the feed coal ratio helps to offset the additional coal cost but also negatively impacts on kiln capacity. A cost-benefit analysis is therefore needed to find the optimum outcome.

Simulations were performed to estimate kiln throughput rates at different feed coal ratios. Conditions could not be found where a feed coal ratio of less than 0.35 could be achieved safely (with sufficient char rates to prevent reoxidation). Attempts at further reductions in kiln temperature to reduce overall throughput rates to around 40t hr were unsuccessful in achieving the required char production rates. Higher char rates were produced at higher throughput rates compared to lower throughput rates (due to the greater overall coal input), with a 0.35 coal ratio giving the minimum acceptable result. Throughput rates and feed coal ratio data are plotted in Figure 3. At the nominal 0.42 coal ratio the predicted feed rate was 68.3 t hr. At a 0.40 coal ratio (5% saving) the predicted feed rate was 67.8 t/hr. At a 0.35 coal ratio (16% saving) the predicted rate was 66.2 t/hr. At a 0.30 coal ratio (28% saving) the predicted rate was 64.1 t hr but this does not produce minimum acceptable char rates. At low char production rates ( coal ratios of 0.30 and lower), Rl quality may become adversely affected through reoxidation.

It will be appreciated that identification of a basis for selecting an effective higher reactivity coal for the synthetic rutile process gives rise to a variety of opportunities for economic^" improvements in the process. Lower Ti0₂ grade secondary ilmenites may still be economically processed to synthetic rutile of acceptable grade. Coal feed ratios may be reduced, as discussed above, where good quality ilmenite feed remains available. Alternatively, coal ratios may be maintained to achieve higher throughput rates for given or higher Ti0₂ SR grade.

Figure 4 is provided for illustrative purposes to demonstrate how gasification reactivity can affect reduction rates. The figure illustrates the rates of reduction of iron oxides (as measured by metallic iron formation) and titanium species, for respective kiln reductions of an ilmenite under similar conditions with Collie coal and the reactive coal. An assay of the ilmenite employed is provided under the graphs. Although the ilmenite here was not a secondary ilmenite (FeO is above 12%), the Ti0₂ content is high because of low other species such as Si and Al, and the depicted comparative behaviour of the coals is valid across a wide range of ilmenites.

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. Table 1 : Coal Specifications (Typical specifications- as received from Coal suppliers)

Ash Fusion

Inherent Moisture Volatile Fixed

Ash Ultimate (%, daf) Temperature

Moisture Content matter Carbon

(degC)

% [%ar] [%ar] [%ar] [%ar] Carbon Hydrogen I.D.T. H.T.

Desired <10 <25 <5 >40 >41 80 >5 >1200 >1250

Max <15 <10 >35 >75

Collie 27 27 6 26 41.0 74.9 4.6

CS1 10 18 4.5 38 47.5 76.7 5.3 1150 1210

CS2 14.5 25 1 42.5 42.0 74.3 5.6 1200 1260

CS3 16.5 24.5 2.5 43.5 37.5 77.4 5.8 1200 1250

CS4 5 9.5 4 39 52.0 80.5 5.4 1200 1300

CS5 4.4 11 12 40.5 43.1 78.2 6.1 1450 1510

CS6 14 26 5 39.5 41.5 75.6 8.2 . 1200 1400

CS7 12 20 5 40 41.0 ^' 84.0 4.9 1200 1230

CS8 12 1.1 43.8 43.0 68.5 4.8 1260 1410

Table 2: ilmenite assay

Table 3: SR grades from test and Collie coal reductions on standard Capel SR ilmenite ilmenite Reference SR SR Assay

Collie

MC376 Coal CS1 CS2 CS2R CS3 CS4 CS5 CS6 CS7 CS8 CS9

Coal

Repeat

57.5 92.4 92.4 93.8 94.0 93.0 .93.9 88. 93.9 93.8 ^' 94.1 94.7 93.4

Fe203 38.60 6.01 5.42 ^{' '} : ^; 2,81 2.90 3.75 3.43 7.40 2.90 3.09 2.88 2.50 3. 8 ^'

St02 0.90 0.65 0.65 1.13 0.72 0.74 0.81 1.07 0.82 0.94 0.96 0.92 0.92

Zr02 0.11 0.06 0.06 0.11 0.07 0.07 0.07 0.08 0.07 0.08 0.07 0.07 0.07

P205 0.05 0.02 0.02 0.02 0.03 0.03 0.04 0.01 0.03 0.01 . 0.01 0.01 0.01

AI203 0.62 0.77 0.77 0.88 0.83 0.84 0.88 0.91 0.88 0.88 0.93 0.87 0.91

Nb205 0.16 0.25 0.25 0.25 0.25 0.25 0.25 0.24 0.25 0.25 0.25 0.25 0.25

Cr203 0.04 0.08 0.08 0.13 0.13 0.12 0.10 0.10 0.11 0.14 0.11 0.14 0.13

MgO 0.22 0.36 0.36 0.41 0.40 0.41 0.43 0.40 0.41 0.41 0.42 0.44 0.42

V205 0.18 0.27 0.27 0.28 0.27 0.29 0.26 0.25 0.27 0.27 0.28 0.26 0.26

MnO 1.21 1.40 1.75 1 :93 1.94 1.92 1.95 1.88 1.96 . 1.36 1.38 1.65 1.65: .

S 0.02 0.03 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.02 0.01 0.01

Th 103 123 113 157 152 121 100 106 107 126 118 98 94 u 11 13 17 14 12 13 14 15 16 11 11 14 14

FeO = '14% Table 4: Char-C0₂ Gasification Reactivity

Table 5: Elemental Analysis (%dry coal basis)

Claims

Claims:

1. A process for recovering titanium as synthetic ruti!e from a titaniferous ore, for example a secondary ilmenite, including the steps of: treating the ore in a reducing atmosphere at elevated temperature above 1075°C 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 carbonaceous reductant comprises a coal selected for a moisture content below 40%, a volatiles content greater than 30%, ash content below 10%, and a gasification reactivity that results in an increased rate of reduction of iron oxides and titanium species effective to achieve a Ti0₂ 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 in the range 1075-1200°C.

3. A process according to claim 1 or 2 wherein , the gasification reactivity is sufficiently high to achieve said Ti0₂ content.

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 relatively high impurity levels of one or more ion-exchanged inorganic elements that 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 ilmenite 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 inherent moisture content of the selected coal is 20% or less, volatiles content is >40%, and ash content is <5%.

10. A process according to any one of claims 1 to 9 further including mixing char with the ilmenite before it is delivered for said treatment step.

11. A process according to any one of claims 1 to 10 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.

12. A process according to claim 11 , wherein the sulphur content of the coal is less than 0.5%.

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

14. A process according to claim 11 , 12 or 13 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.

15. A process according to any one of claims 1 to 14 wherein the iron content of the ilmenite, expressed as FeO, is less than 12%.

16. A process according to any one of claims 1 to 15 wherein free oxygen in the treatment atmosphere is no greater than 2.5%.

17. A process according to any one of claims 1 to 16 wherein the Ti0₂ content achieved in said synthetic rutile product is at least 93%.