Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
  • C01G23/00—Compounds of titanium
  • C01G23/04—Oxides; Hydroxides
  • C01G23/047—Titanium dioxide
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B34/00—Obtaining refractory metals
  • C22B34/10—Obtaining titanium, zirconium or hafnium
  • C22B34/12—Obtaining titanium or titanium compounds from ores or scrap by metallurgical processing; preparation of titanium compounds from other titanium compounds see C01G23/00 - C01G23/08
  • C22B34/1204—Obtaining titanium or titanium compounds from ores or scrap by metallurgical processing; preparation of titanium compounds from other titanium compounds see C01G23/00 - C01G23/08 preliminary treatment of ores or scrap to eliminate non- titanium constituents, e.g. iron, without attacking the titanium constituent
  • C22B34/1209—Obtaining titanium or titanium compounds from ores or scrap by metallurgical processing; preparation of titanium compounds from other titanium compounds see C01G23/00 - C01G23/08 preliminary treatment of ores or scrap to eliminate non- titanium constituents, e.g. iron, without attacking the titanium constituent by dry processes, e.g. with selective chlorination of iron or with formation of a titanium bearing slag
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B5/00—General methods of reducing to metals
  • C22B5/02—Dry methods smelting of sulfides or formation of mattes
  • C22B5/10—Dry methods smelting of sulfides or formation of mattes by solid carbonaceous reducing agents

Definitions

• 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.
• 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 Ti0 2 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 ⁇ 0 2 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 Ti0 2 grade.
• the standard feed specification for Western Australia secondary ilmenite to the Becher process is FeO ⁇ 12%, 57% ⁇ Ti0 2 ⁇ 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.
• 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.
• 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.
• peat 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
• 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
• 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%).
• Contemporary Brown coal briquettes trialled in 1997 and 1998 showed only minor process improvements. No significant benefits were measured during these trials.
• 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 Ti0 2 content of 90% or greater, preferably at least 93%, in said synthetic rutile product.
• a 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 re
• the gasification reactivity of the coal is sufficiently high to achieve said Ti0 2 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.
• 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.
• 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%.
• at least one such inorganic element is present to the extent of at least 0.2%db on a dry coal basis.
• 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).
• I.D.T. initial deformation temperature
• H.T. hemispherical temperature
• 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.
• the sulphur content of the coal is less than 1 % w/w, more preferably less than 0.5%, most preferably less than 0.2%.
• the iron content of the ilmenite, expressed as FeO, is preferably in the range FeO ⁇ 12%.
• free oxygen in the treatment atmosphere is no greater than 2.5% and preferably less than 2%, most preferably less than 1 %.
• 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.
• 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.
• 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.
• Manganese (MnO) was notably higher in test coal reductions due to their lower f sulphur content.
• Elemental analyses of the coals 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%).
• 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.
• the ilmenite here was not a secondary ilmenite (FeO is above 12%)
• the Ti0 2 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.
• Table 3 SR grades from test and Collie coal reductions on standard Capel SR ilmenite ilmenite Reference SR SR Assay

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• 2011
  • 2011-04-06 AU AU2011238420A patent/AU2011238420B2/en not_active Ceased
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