An improved pyroprocessing method for the production of synthetic rutile

Abstract

A process of producing synthetic rutile from a titaniferous feedstock or feed material treated by pyroprocessing using a multiple hearth furnace. The pyroprocessing process comprising: oxidation roasting of the titaniferous feed material through a plurality of oxidising hearths of said multiple hearth furnace using an oxidising gas to produce an oxidised intermediate product within the furnace. Further pyroprocessing the oxidised intermediate product in a reducing environment through a plurality of reducing hearths in the same multiple hearth furnace using a reducing gas such as hydrogen to produce a reduced intermediate furnace discharge product. The process also comprises the steps of allowing the reduced intermediate product to cool in an inert or low oxygen environment to substantially prevent re-oxidation thereof; and passing the cooled / or cooling reduced intermediate product to further processing.

Description

[0001] An improved pyroprocessing method for the production of synthetic rutile

[0002] This document claims priority to AU 2024903845 filed on 22 November 2024 the contents of which are hereby incorporated by reference in their entirety.

[0003] Technical field

[0004] The present invention relates to the production of synthetic rutile from titaniferous feedstocks including ilmenite.

[0005] Background

[0006] Ilmenite is a common, naturally occurring, iron-rich titanium mineral. It is the main natural source of titanium dioxide. It is generally represented by the chemical formula FeTiCh. Ilmenite is principally an oxide of titanium and iron, but typically contains a number of minor impurities that vary in composition and concentration based on the material's host rock genesis, and may include silicon, magnesium, aluminium, manganese and vanadium, as well as trace impurities including thorium and uranium.

[0007] Leucoxene is a naturally occurring weathered ilmenite mineral, containing higher concentrations of TiOz as a result of lower levels of iron due to the partial removal of iron by natural chemical weathering processes over time.

[0008] The TiOz content of natural ilmenite can typically range from 45-70%, whereas the TiOz content of natural leucoxene can typically range from 70- 90%. Rutile is the highest grade naturally occurring titanium mineral with a TiOz content of 95+%.

[0009] Synthetic rutile is not naturally occurring but can be produced by pyroprocessing ilmenite and / or lower-grade leucoxene followed by leaching to remove the iron. The TiOz content of synthetic rutile can range from 90- 95+%.

[0010] Synthetic rutile can be a substitute for natural rutile and like natural rutile, is a desirable feedstock for the titanium tetrachloride processing route to produce pure TiC pigment which is primarily used in paints, plastics and paper. Synthetic rutile crystals are generally finer grain size and porous (from the leaching of iron) compared to natural rutile crystals.

[0011] The current commercial process used in Australia and other parts of the world for many years to produce synthetic rutile is referred to as the Becher process. This process involves the use of a rotary kiln to create a reducing environment by adding coal with ilmenite to reduce the iron contained in the ilmenite to metallic iron at a temperature of 1100-1300°C. The reduced kiln discharge material is then subject to iron leaching using ammonium chloride in solution with air sparging, followed by secondary leaching with sulphuric acid to remove residual iron and other impurities such as manganese and silicon.

[0012] It is known that an earlier version of the Becher process included a preoxidation step, however this step may have been abandoned by some processors. This move away from a pre-oxidation step, while not certain, may have been driven by a shift to a higher percentage of secondary or altered ilmenites in the feedstock which may not be as sensitive to the requirement for pre-oxidation to achieve suitable synthetic rutile quality.

[0013] A number of other processes have been proposed for upgrading ilmenite to synthetic rutile. US patent 4097574 outlines the process of using a pre- oxidation step in a rotary kiln or fluidized bed reactor at 592-870°C, followed by hydrogen reduction of iron to metallic iron at 787-845°C followed by aeration leaching and secondary acid leaching. A more recent process outlined in Australian patent 752851 describes a similar process of using a pre-oxidation step in a rotary kiln or fluidized bed reactor in the range of 925-1000°C followed by treating the oxidised material in a fluidized bed reactor in an atmosphere containing a reducing gas (preferably substantially pure hydrogen) at a temperature of 750- 900°C so as to reduce the iron to metallic iron, followed by cooling and one or more leaching treatments for separating out metallic iron and other impurities to produce a synthetic rutile product. In general, this implies the pre-oxidation step is conducted in one furnace and the reduction step in a second furnace.

[0014] The inventors in patent 752851 expound on the importance of pre-oxidation at a sufficiently high temperature above 900°C (generally 925-975°C) to ensure the titaniferous material which is of a structure in which M2O3 is the major phase prior to roasting, is converted by roasting to a structure in which M3O5 is the major phase, where M represents any metal species such as Fe, Mn, Mg, Al or Ti, and in general includes impurity metals. The inventors suggest that ensuring M3O5 is the major phase serves to enhance the removal of impurities by the leaching treatment and increases the titanium dioxide content of the synthetic rutile product.

[0015] The oxidation step can be represented by the following equation:

[0016] 2FeTiO3+ V2O2 Fe2TiO5+ TiO2which may be written:

[0017] 2M2O3 + V2O2 M3O5 + TiO2where M represents metal species including Fe, Ti, Mn, Mg or Al or impurity metals. The inventors in patent 752851 also suggest that the reduction step achieves greater than 80% metallization of the iron, and preferably greater than 95% and that leaching should be carried out using ammonium chloride followed by secondary leaching using sulfuric acid as used in the Bether process, or leaching using hydrochloric acid, to end with a TiO2content of synthetic rutile of 90-95%.

[0018] The reduction step reactions can be represented by the following equation: where M represents any metal species including impurity metals.

[0019] It is desirable to find improved processes of producing synthetic rutile which overcome or at least ameliorate some of the problems of the prior art, or which can provide certain advantages or serve as a useful alternative.

[0020] Summary of invention

[0021] In a first aspect there is provided a process of producing synthetic rutile from a titaniferous feed material by pyroprocessing using a multiple hearth furnace, the process comprising: oxidation roasting of the titaniferous feed material through a plurality of oxidising hearths of said furnace using an oxidising gas to produce an oxidised intermediate product within the furnace; further pyroprocessing the oxidised intermediate product in a reducing environment through a plurality of reducing hearths of said furnace using a reducing gas such as hydrogen to produce a reduced intermediate product; allowing the reduced intermediate product to cool in an inert or low oxygen environment to substantially prevent re-oxidation thereof; and passing the cooled or cooling reduced intermediate product to further processing.

[0022] In an embodiment the oxidising hearths are in the upper section of the multiple hearth furnace. In an embodiment, the reducing hearths are in a lower section of the multiple hearth furnace.

[0023] The present invention proposes pyroprocessing with oxidation and reduction steps undertaken in a multiple hearth furnace. The oxidation and reduction are undertaken in the same multiple hearth furnace. The process therefore requires the use of different gases in different parts of the same furnace. This process of using oxidising gases and reducing gases in the same multiple hearth furnace can be applied to the production of synthetic rutile from a titaniferous feed material. In a preferred embodiment natural gas is used as the oxidising gas and hydrogen is used as the reducing gas. There may be several significant advantages to the use of a multiple hearth furnace over the use of rotary kilns and fluidized bed reactors to produce synthetic rutile.

[0024] In an embodiment, the titaniferous feed material is dried prior to the oxidation and reduction steps. In an embodiment, moist titaniferous feed material can be fed to the furnace with drying occurring as an integral part of the oxidation step.

[0025] The multiple hearth furnace can be fuelled by natural gas augmented with oxygen or air sparging in the oxidation section of the furnace, and fuelled by hydrogen in the reduction section of the furnace. By using these gaseous fuels instead of coal, the overall carbon dioxide emissions may be substantially reduced when compared to the current commercial Becher process used in Australia which uses coking coal as the iron reductant. In an embodiment, the operating temperature requirement in the multihearth furnace can be less than 1,000 degrees C which is a significantly lower temperature than commercial synthetic rutile production processes in a rotary kiln currently used in Australia. kThe heat transfer to the material in a fluidized bed reactor would largely be from direct contact with the heated gases with perhaps a minor component of radiant heat from the walls of the reactor. In a multiple hearth furnace on the other hand, heat is transferred to the material from a combination of heat transfer from hot gases, and from significant radiant heat from the walls of the furnace as well as from radiant heat from the rabble arm teeth in direct contact with the material while conveying the material across the hearth floors.

[0026] The present process is undertaken in a multiple hearth furnace. Multiple hearth furnaces can be designed for the thermal treatment of materials such as ores, chemicals and metal powders. These furnaces are widely used in many industries, including metallurgy, chemical processing, and materials engineering. The primary purpose of multi-hearth furnaces is to provide controlled heating, calcination, roasting, and / or drying of materials.

[0027] A multiple hearth furnace can consist of a number of hearths (individual furnace segments) which can be stacked substantially vertically. Figure 1 shows a general arrangement of a multiple hearth furnace.

[0028] An individual hearth in a furnace consists of a single processing platform, shelf, stage or level in the furnace where a material is processed typically by being exposed to elevated temperatures, and includes the rotating rabble arms stirring and raking the material across the hearth floor. Each hearth can be circular. A multiple hearth furnace can consist of any number of hearths. In practice there are from about 2 to about 20 hearths in a single multiple hearth furnace. The total number of hearths will depend on the required feed material treatment with respect to retention time and overall throughput rate.

[0029] A multiple hearth furnace can be used for thermally treating (pyroprocessing) large volumes of a variety of raw materials. The individual hearths can allow for precise temperature control throughout the furnace depending on the required number and location of gaseous fuel burners and desired temperature on each hearth.

[0030] By "multiple hearth" it is meant that the furnace has more than one hearth. Each hearth level can act as a separate processing stage in the furnace. The hearth levels can be stacked substantially vertically; however, offset from vertical could still provide function. As can be seen from Figure 1, the furnace design can allow material to move from one hearth level to another as it undergoes various stages of thermal treatment. The multi-level arrangement can enable sequential processing of materials, which is useful for applications that require different temperature zones and / or different oxidation / reduction environments for different stages of treatment.

[0031] Each hearth consists of a floor covered with refractory bricks. Each hearth can have associated with it two or more rabble arms with teeth. The rabble arms rotate on a common central shaft and the teeth mix and push the material being treated to a discharge port in the floor of the hearth. The internal walls of the furnace are also lined with refractory bricks.

[0032] As can be seen in Figure 1, the rabble arms on the first and all odd numbered hearths mix and push the material inwards towards the centre shaft to an inside discharge port. The rabble arms on the second and all even numbered hearths mix and push the material outwards to discharge ports located near the outside wall of the furnace. A multi-hearth design could also incorporate the opposite arrangement where the odd numbered hearths have an outside discharge port, and the even numbered hearths have an inside discharge port. The feed material to the furnace can enter the furnace by any means including metered gravity flow into the upper most hearth. The material can be conveyed through the furnace by the action of the rabble arm teeth pushing the material towards the individual hearth discharge ports, and then falling by gravity to the next lower floor. The heated gases in the furnace travel counter-current to the flow of material and travel upwards through the hearth discharge ports and exit the furnace at the top of the first or upper most hearth. The titaniferous feed material is thermally treated by the hot gases travelling upwards through the furnace as well as by radiant heat from the rabble arm teeth and the walls and floor of the furnace.

[0033] In practice, multiple hearth furnaces are operated on a continuous feed / processing basis. In addition, there are a variety of ancillary systems and equipment associated with multiple hearth furnaces such as rotary valves and seals to control the feed and discharge materials and prevent the escape of furnaces gases, cooling fans, temperature controllers, shaft rotation and cooling, and off-gas scrubbing and emissions control to ensure overall safe operations and environmental protection.

[0034] In embodiment, the multiple hearth furnace can be operated as a batch process. However, this is not preferred since the system would have to be cooled down between batches and the furnace opened to manually remove the material in the furnace.

[0035] The multiple hearth arrangement can enable the thermal processing of materials which require treatment at different temperatures and / or different oxidation / reduction environments at different stages of the process. The multiple hearth arrangement can also minimise short-circuiting of the material in the furnace and thereby, in embodiments, maximises effective average retention time of the material in the furnace and thereby can result in more efficient thermal treatment. The number of levels in a multi-hearth furnace can vary. The number of hearths depends on factors such as the type of materials being processed, the desired throughput, and the complexity of the thermal treatment required. In practical applications, multiple hearth furnaces typically have 2 to 20 or more levels (hearths). In the present process, there can be a drying of the feed material in the top hearth. Oxidation can be undertaken over about 3 to about 9 hearths in the upper section of the furnace. Reduction can be undertaken over about 3 to 9 other hearths in the lower section of the same furnace. The number of hearths required for oxidation and reduction can be determined in accordance with the desired total throughput rate and with retention requirements of about 30 minutes to about 4 hours each for oxidation and reduction. Accordingly, in the present process there can be anywhere between about 2 to about 20 hearths, more commonly about 8 to about 12 hearths.

[0036] The furnace feed material to be treated in the multiple hearth furnace is a titaniferous feedstock material. By "titaniferous feedstock" it can be meant that the feed material comprises feedstocks containing both titanium dioxide and iron. In an embodiment the titaniferous feedstock is a naturally occurring titaniferous ilmenite and / or leucoxene mineral. The more desirable ilmenite and / or lower-grade leucoxene feedstocks for synthetic rutile production can comprise about 55% TiC to about 65% TiC , but higher or lower concentrations of TiOz can be present.

[0037] The invention is not limited to ilmenite and / or leucoxene feed materials, a mixture of minerals with titaniferous minerals can be the target of the pyroprocessing. The titaniferous feed material can be fed into an inlet in the multiple hearth furnace. The feed material inlet can be at the top of the furnace.

[0038] Prior to oxidation and reduction treatment of the feed, it can be desirable to remove moisture from the feed material. The moisture can be removed from the feed outside of the multiple hearth furnace prior to processing. In an alternative embodiment, the moisture is removed from the feed material inside the multiple hearth furnace. The drying (moisture removal) of a moist furnace feed material can be undertaken inside the furnace. The drying of the moist feed material can occur in the uppermost hearth(s). The drying can be by the residual hot gases that rise to the top hearths from the lower oxidation roasting hearths prior to those gases exiting the furnace at the top of the upper-most hearth and entering the off-gas scrubbing circuit. Following drying, the material will be completely dry due to the temperature of rising gases in the range of from about 700 degrees C to about 900 degrees C.

[0039] Multiple hearth furnaces are flexible and can be fired with gaseous, liquid or solid fuels. Gaseous fuels can be natural gas or hydrogen. Liquid fuels can be fuel oils including kerosene. Solid fuels can be coal or organic matter such as wood chips. Solid fuels can be augmented with gaseous fuels to ensure ignition. In an embodiment, the furnace is heated using natural gas as it is the lowest CO2 emissions fuel after hydrogen and the gaseous fuels are the best for sensitive temperature control. Illustrative locations of the gaseous fuel burners can be seen in Figure 1.

[0040] The atmosphere within a section of the furnace can be controlled to be oxidising. An oxidising atmosphere contains oxygen or an oxygencontaining gas, such as air. Natural gas can be used to create an oxidising environment when the gas is burned in air. Oxygen can be introduced with the natural gas by air sparging. The oxygen level in the gases in each oxidising hearth can be measured to ensure that there is sufficient oxygen to cause oxidation of the iron component of the feed material. The oxidation potential of the environment in the furnace can be calculated or measured. Measuring the oxygen potential is an effective process control element but is not critical to the working of the invention. The oxidising steps can be undertaken in the upper section of the furnace. During the oxidation roasting, the material can be mechanically stirred and conveyed by the rotating rabble arm and teeth. There can be gravity transition of the material from one hearth to the next lower hearth. There can be from 2 to 9 hearths for oxidation roasting, or more or less if required. The retention time in the oxidation roasting section of the furnace can be in the range of from about 30 minutes to about 4 hours.

[0041] The oxidising gas can be introduced into the upper part of the furnace in one or more hearths and generally with one or more burners per fired hearth.

[0042] In embodiments, the oxidation roasting step required retention time, and therefore the number of hearths required for oxidation, may be lower as the TiOz content of the feed material increases. It is important to note that the TiOz content of the feed material increases with higher grade ilmenite and increasing proportions (if any) of leucoxene. An alternative to decreasing the retention time by using a fewer number of hearths for oxidation is increasing the rotation speed of the rabble arms or increasing the throughput rate of the feed material.

[0043] The oxidation roasting step can be carried out at a temperature sufficiently above 900 degrees C for the metallic species (M) in the feed material to be converted from a structure of M2O3 as the major phase, to a structure of M3O5 in the major phase. This transition in phase is thought to enhance the effectiveness of the subsequent reduction step and may improve impurity removal in the subsequent leaching step. M represents any metal species but is primarily iron and titanium, but also includes impurity metals.

[0044] In an embodiment, the temperature in the oxidation roasting stage of the furnace is in the range of from about 900 degrees C to about 975 degrees C. This is the measured temperature within the furnace and is effectively the temperature of the heated gases in the furnace as measured by the standard use of a thermocouple in the wall of the furnace. The specific parameters may vary based on the feed material source and quality, which are worked out in bench-scale and pilot testing. The parameters can be verified in a demonstration scale and finally a commercial scale unit.

[0045] In an embodiment greater than about 80% and preferably greater than about 95% of the feed material changes phase to the M3O5 as the predominant phase. Different feed materials will likely respond differently and therefore the retention time for the oxidation step may vary depending on the percentage of impurities removed and final synthetic rutile TiOz grade. With the variation of the oxidation retention time relative to achieving desirable impurities removal and synthetic rutile TiOz grade likely associated with the about 95% phase change.

[0046] Following oxidation, the oxidised intermediate product is subjected to further pyroprocessing in the same furnace in a reducing environment. The atmosphere within a part of the furnace therefore is controlled to be reducing. A reducing atmosphere can be generated by adding hydrogen gas and or carbon monoxide gas, to create conditions where oxygen is removed from the material. Preferably, the reducing atmosphere in the further pyroprocessing step is formed by pure hydrogen.

[0047] In embodiments, it is possible that a sufficient oxygen free environment could be produced using only methane (natural) gas.

[0048] The reducing step can be undertaken at slightly lower temperatures than the oxidising steps. The reduction can be at a temperature in the range of from about 600 degrees C to about 900 degrees C. This is the measured temperature within the furnace and is effectively the temperature of the heated gases in the furnace as measured by the standard use of one or more thermocouples in the wall of the furnace. The specific parameters may vary based on the feed material source and quality, which are worked out in bench-scale and pilot testing. The parameters can be verified in a demonstration scale and finally a commercial scale unit.

[0049] The reduction is undertaken for a period of time (retention time) sufficient to reduce the iron in the material. In an embodiment, the iron contained in the titaniferous material is reduced to metallic iron. Preferably, greater than about 95% metallisation of the iron is achieved in the reduction step. The reduction aims to maximise the amount of metal, such as iron that can be leached and separated from the TiC residue in the subsequent leaching step.

[0050] Retention time requirements are worked out through bench-scale and pilot testing, based on a curve generated relating to retention time and synthetic rutile quality. This is then confirmed in the commercial unit. In commercial operation, the process operator is given control parameters to allow adjustment of the furnace retention time (such as by changing the feed rate of the material or rake speed) to meet changing feed material characteristics. Such control parameters may be based on specific control setpoints that have been clearly correlated with final synthetic rutile quality, such as a formally developed rapid feedback analysis.

[0051] During reduction, the oxidised intermediate product can be mechanically stirred and conveyed by the teeth on the rotating rabble arms. The material can travel from one hearth to the next lower hearth by gravity. There can be from 2 to 9 reducing hearths, or more or less if required, depending on the retention time requirements of the specific feed material.

[0052] The reducing gas can be introduced into the lower part of same furnace in one or more hearths and generally with one or more burners per fired hearth.

[0053] In the current process, the hot off-gas from the reduction stage will travel up into the oxidation hearths where any excess reductant in the off-gas will be oxidised and the heat will be preserved so as to minimise the amount of fuel required in the oxidation roasting stage. The design of the multiple hearth furnace can be such that if it is determined that the reduction offgas is not compatible with the oxidation stage or is desired to be recycled, the reduction off-gas can be removed through an exhaust port in the side of the furnace in the upper most reduction hearth and handled separately from the off-gas from the oxidation stage exiting from the upper section of the furnace.

[0054] It should be understood that there are natural advantages to having the oxidation undertaken at the top of the furnace, and reduction undertaken at the lower part of the furnace. Advantages are that hot air rises and gravity pulls the feed material downward. These advantages align well given that the feed material flow must be counter-current to the flow of hot gases from reduction to oxidation, that is the feed material must first be oxidised, then reduced as it travels through the furnace. In addition, the oxidation steps require a higher temperature, and therefore additional gaseous fuel and oxygen must be introduced to achieve the higher temperature and create the oxidizing atmosphere. Another advantage, as described, is that the reduced stage off-gas can simply pass upwards to the oxidation stage, where any excess hydrogen in the reduction stage off-gas will be oxidised to water vapour in the oxidation stage before exiting the furnace.

[0055] In embodiments, the process flows could be reversed, so long as they remained counter-current to one another. In this embodiment, the oxidation stage could be undertaken below or adjacent to the reduction stage and the feed material conveyed in a manner up or horizontally, counter-current to the flow of gases from reduction to oxidation. A key disadvantage of this arrangement being that the material would no longer be able to fall from hearth to hearth via gravity and would have to be transported or conveyed upwards or horizontally through the oxidation and reduction zones. The pressure inside the furnace during operation is preferably slightly lower than atmospheric pressure. Generally, the multiple hearth furnace is under slight negative pressure by controlling the off-gas flowrate to ensure the containment of combustion gases. In general, the lower the amount of hot gases leaving the furnace at temperature, the lower the heat losses. The furnace internal pressure can be tracked through the use of simple manometer instrumentation which can be automated to a set point by the automatic control of an off-gas damper valve as standard control technology.

[0056] Depending on the specific feed material characteristics, the retention time required can range from about 30 minutes to about 4 hours each for oxidation and reduction. Bench-scale testing of the material can allow specific furnace operating parameters including the minimum number of hearths and specific temperature requirements for each step of the process to maximise the iron removal efficiency. Accordingly, the present process may require a total of anywhere between about 2 to about 20 hearths in a single furnace, depending on the source of the titaniferous feedstock

[0057] Once the reduction step is complete, the reduced intermediate product or furnace discharge product can be cooled. The cooling can be undertaken in an inert or oxygen free environment to prevent re-oxidation of the iron. The oxygen free environment is an optimisation step. If minor amounts of iron in the material are re-oxidised this can be tolerated. However, minimising re-oxidation serves to improve overall synthetic rutile quality.

[0058] Cooling of the reduced intermediate product can be achieved by allowing the material to cool naturally. Alternatively, a cooling medium can be applied to cool the reduced intermediate product. The cooling can be undertaken in the furnace, although this would require that the furnace is also cooled. This in situ cooling could be done in a batch operation, but it not preferred since the furnace would be non-operational during cooling. Multiple hearth furnaces are typically continuous operations by their very nature. Accordingly, it is advantageous if the reduced material is discharged from the furnace and cooled outside the furnace.

[0059] The reduced intermediate product can be removed from the furnace and allowed to fall into or be conveyed to a cooling bin or cooling conveyor. Cooling can be indirect such as by using water jackets on the outside of the bin and or conveyor. The atmosphere in the cooling bin or cooling conveyor is preferably oxygen free. After a certain amount of cooling time, the cooled or cooling material can be stored in a storage bin for further processing.

[0060] In embodiments, the cooled or cooling material can be transferred directly to a leaching circuit. The reduced intermediate product can be further processed by another operator. Alternatively, the reduced intermediate product is processed further by the same operator. Use of the wording "intermediate" refers to the product in the context of the process as a whole as described herein. However, it should be understood that the reduced intermediate product is a furnace discharge product and can be referred to as such.

[0061] In an embodiment, the further processing is an iron leaching circuit. In an embodiment the further processing of the reduced intermediate product is leaching to remove iron and other impurities from the titanium dioxide contained in the reduced intermediate product. The leaching can be hydrochloric acid leaching. Alternatively, the leaching can be aeration leaching and secondary acid leaching as used in the current Becher process, to dissolve (leach) the iron into solution so it can be separated from the undissolved TiO2 solids, which is the synthetic rutile.

[0062] The reduced intermediate product can be discharged from the storage bin into the leaching process. The reduced intermediate product can be discharged directly from the furnace discharge into the leaching process. The leaching process can be undertaken in one or more leaching tanks. The leaching process can be principally for removal of the iron and other impurities from the TiOz in the reduced intermediate product furnace discharge material.

[0063] The leaching can be undertaken as an immediate follow-on step from the oxidation / reduction pyroprocessing to a) minimise any excessive reoxidation of the material with time and b) to take advantage of some of the residual heat from the furnace to improve the kinetics of reaction in the leaching circuit.

[0064] The iron leaching can be undertaken using an acid solution. The acid can be hydrochloric acid. The acid can be heated to increase the kinetics and effectiveness of the leaching process.

[0065] Alternatively, the iron leaching can be undertaken using an acidic salt solution. The acidic salt solution can be ammonium chloride. The leaching solution can be aerated during treatment with the ammonium chloride. Following leaching treatment with the ammonium chloride, the material can be secondarily treated with an acid solution to remove additional iron and contaminants. The acid can be sulfuric acid.

[0066] Following leaching, the leachate containing dissolved iron and impurities can be separated from the porous residual TiC (synthetic rutile or SR.) and the synthetic rutile can be washed with water to remove residual leaching reagents, dewatered and stockpiled for sale. The TiC content of the synthetic rutile product is preferably in the range of from about 90% to 95%. In an embodiment, the TiOz content can be greater than 95%.

[0067] The iron-rich leach liquor from the leaching process can then be further treated (thermally or potentially chemically) to recover the iron as a potential by-product and to recover and recycle the hydrochloric acid and potentially the acid salts. In an embodiment, the use of hydrochloric acid leaching followed by thermal recovery of the hydrochloric acid can result in the production of a very high-grade, fine-grained iron by-product. The alternative use of ammonium chloride and sulfuric acid leaching can result in the iron not being recoverable as a by-product and can result in substantially larger quantities of tailings requiring disposal.

[0068] Regardless of the leaching reagent used, post-leaching steps required to produce the final synthetic rutile product can be (1) separating the iron-rich leach liquor from the undissolved TiOz solids; (2) washing the residual leach liquor out of the TiC solids using water; and (3) drying the TiC solids (SR) either naturally while in a stockpile or in a storage bin or using heat to remove the moisture to produce a dry synthetic rutile product. The product can be packaged for sale in either bulk bags and / or shipping containers or transported in bulk via trucks or trucking and shipping.

[0069] Brief Description of the accompanying Figure

[0070] Embodiments of the invention will now be described with reference to the accompanying drawing in figure 1. The drawings may not be drawn to scale. Similar components may be represented by only one reference numeral. The drawing is exemplary only:

[0071] Figure 1 is a schematic of a basic design of a multiple hearth furnace.

[0072] Detailed Description of Embodiments of the Invention

[0073] A titaniferous feedstock or feed material is treated by pyroprocessing in a multiple hearth furnace as shown by the schematic in Figure 1. The titaniferous feed material is delivered into the furnace through the feed inlet. The feed material can be added at a steady rate continuously.

[0074] For illustration purposes, there are eight hearths (1 to 8) shown in the multiple hearth furnace of Figure 1. In the first hearth (1) or the uppermost hearth, the feed material can be dried. Drying of the feed material can be undertaken in hearth 1 using the rising hot gases. The illustrated pathways of the hot gases are shown from the burners as curved arrows in Figure 1.

[0075] Each of the eight hearths depicted have rabble arms with teeth. Select hearths can have a pair of burners located opposite each other for the injection of gaseous fuels.

[0076] In this depiction, in hearths 1 to 4, oxidation roasting is undertaken. The burners burn natural gas sparged with oxygen. The oxidation roasting step is carried out at a temperature in the range of from about 900 to about 975 degrees C with temperature to be optimized for specific titaniferous feed material to sufficiently oxidise the iron contained in the feed material to the ferric iron state so as to maximise the amount of iron that can be reduced to metallic iron in the subsequent reduction step, and to enhance the level of impurities to be removed by the post-reduction iron leaching.

[0077] The rabble arms on the first hearth mix and push the feed material inwards towards the centre shaft to an inside discharge port. The discharge port allows the material to fall by gravity to the second hearth. The rabble arms on the second hearth mix and push the material outwards to a discharge port located near the outside wall of the furnace. The discharge port allows the material to fall by gravity to the third hearth. The rabble arms on the third hearth mix and push the material inwards towards the centre shaft to an inside discharge port. The discharge port allows the material to fall by gravity to the fourth hearth. In this Figure, the second and fourth hearths each comprise a pair of fuel burners. Over the course of the oxidation roasting step, the oxidised titaniferous material becomes an oxidised intermediate product.

[0078] Further pyroprocessing of the oxidised intermediate product occurs over hearths 5 to 8. When the oxidised intermediate product falls through the discharge port from hearth 4 onto hearth 5 it transitions into a reducing environment. Hearths 6 and 8 each comprise a pair of fuel burners that introduce hydrogen into the multiple hearth furnace. As the product passes through the reducing hearths, a reduced intermediate product is formed.

[0079] While the drawing shows only two outer discharge ports in hearth 8, (same for 2, 4, and 6), in reality the ports on these hearths are multiple in a ring shape. In practice, the final hearth (Hearth 8 in this Figure) can have a single discharge port, or two or more ports with a cone collector.

[0080] The reduction step is carried out at a temperature in the range of from about 600 degrees C to about 900 degrees C with temperature to be optimized for the iron contained in the titaniferous material to be reduced to metallic iron. Preferably, greater than 95% metallization of the iron is achieved in the reduction step. The reduction aims to maximise the amount of iron that is converted to metallic iron, so as to maximise the amount of iron that can be leached and separated from the TiC residue in the subsequent leaching step.

[0081] The reduced intermediate product can be removed from hearth 8 of the furnace by being raked on the final hearth floor by the rabble arm teeth to the outer discharge ports and gravity flowing out of the furnace. In an embodiment one or more rotary valves can be used to control the discharge of material from the furnace (not shown). As the valve rotates it conveys the material out of the furnace discharge pipe, while preventing gas flow into or out of the furnace. This can be referred to an as airlock valve. The rotary valve can be formed from steel to handle the high temperatures. The material in the furnace can be conveyed from the bottom of the rotary valve through a steel auger (screw) conveyor (not shown). Any conveyor may include a water jacket for cooling. The material can be transferred into a cooling bin and or a storage bin or alternatively directly to an iron leaching circuit. The atmosphere in the conveyor, storage bin and or cooling bin is preferably oxygen free. After a certain amount of cooling time, the cooled or cooling material can be discharged from the bottom of the bin using another screw conveyor. The reduced intermediate product can be discharged from the storage bin into a leaching process. The leaching process can be undertaken in one or more leaching tanks. The leaching process can be principally for the removal of the iron and other impurities from the TiOz in the reduced intermediate furnace discharge material.

[0082] The oxidising steps describe above can be undertaken in the upper part of the hearths of the furnace. The reducing steps can be undertaken in the lower hearths of the furnace. The feed material can be passed from the upper hearths of the furnace to the lower hearths of the furnace. The oxidising gas can be introduced into the upper hearths of the furnace. The reducing gas can be introduced into the lower hearths of same furnace.

[0083] It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

[0084] In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e., to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

[0085] Any promises made in the present description should be understood to relate to some embodiments of the invention and are not intended to be promises made about the invention. Where there are promises that are deemed to apply to all embodiments of the invention, the right is reserved to later delete those promises from the description since there is no intention to rely on those promises for the acceptance or subsequent grant of a patent unless the context makes clear otherwise.

Claims

CLAIMS1. A process of producing synthetic rutile from a titaniferous feedstock or feed material treated by pyroprocessing using a multiple hearth furnace, the pyroprocessing process comprising: oxidation roasting of the titaniferous feed material through a plurality of oxidising hearths of said multiple hearth furnace using an oxidising gas to produce an oxidised intermediate product within the furnace; further pyroprocessing the oxidised intermediate product in a reducing environment through a plurality of reducing hearths in the same multiple hearth furnace using a reducing gas such as hydrogen to produce a reduced intermediate furnace discharge product; allowing the reduced intermediate product to cool in an inert or low oxygen environment to substantially prevent re-oxidation thereof; and passing the cooled / or cooling reduced intermediate product to further processing.

2. The process of claim 1, wherein the titaniferous feed material is naturally occurring titaniferous minerals such as ilmenite and / or leucoxene mineral.

3. The process of claim 1 or claim 2, wherein the process further comprises a step of drying moist titaniferous feed material in a topmost hearth of the multiple hearth furnace prior to oxidation roasting step or at the same time as the oxidation roasting step.

4. The process of any one of the preceding claims, wherein the method further comprises the step of further processing the cooled reduced intermediate furnace discharge product by leaching to remove iron and other impurities from a titanium dioxide (TiOz) component of the cooled or cooling reduced intermediate furnace discharge product.

5. The process of any one of the preceding claims, wherein the hearths of the multiple hearth furnace are arranged in a substantially vertical configuration with the oxidation roasting being undertaken in the upperhearths, and the reduction roasting being undertaken in the lower hearths located underneath the upper hearths.

6. The process of any one of the preceding claims, wherein the furnace is fuelled by natural gas injected through one or more burners arranged on one or more of the oxidation hearths.

7. The process of any one of the preceding claims, wherein the oxidation roasting includes air sparging with the natural gas to provide oxygen.

8. The process of any one of the preceding claims, wherein there are about 3 to 9 oxidising hearths arranged in vertical series.

9. The process of any one of the preceding claims, wherein the oxidation roasting is undertaken at a temperature in the range of from about 900 to about 975 degrees C.

10. The process of any one of the preceding claims, wherein the retention time for oxidation roasting is in the range of from about 30 minutes to about 4 hours depending on the requirements of the titaniferous feedstock as determined by bench-scale testing.

11. The process of any one of the preceding claims, wherein the further pyroprocessing in a reducing environment (reduction roasting) is provided by injecting hydrogen gas through one or more hydrogen burners optionally located opposite each other on the one or more of the reducing hearths.

12. The process of any one of the preceding claims, wherein there are about 3 to 9 reducing hearths arranged in vertical series.

13. The process of any one of the preceding claims, wherein the further pyroprocessing is undertaken at a temperature in the range of from about 600 to 900 degrees C.

14. The process of any one of the preceding claims, wherein the retention time for reduction roasting is in the range of from about 30 minutes to about 4 hours depending on the requirements of the titaniferous feedstock as determined by bench-scale testing.

15. The process of any one of the preceding claims, wherein the multiple hearth furnace is of the type comprising one or more rabble arms in each hearth wherein each rabble arm comprises multiple teeth.

16. The process of any one of the preceding claims when dependent on claim 4, wherein the leaching is undertaken using hydrochloric acid.

17. The process of any one of the claims 1 to 15 when dependent on claim 4, wherein the leaching is undertaken by an acidic salt solution such as ammonium chloride, optionally with air sparging, and secondary leaching using sulphuric acid.

18. A system for producing synthetic rutile from a titaniferous feed material by pyroprocessing treatment, the system comprising: a multiple hearth furnace configured to allow: oxidation roasting of the titaniferous material feed through a plurality of oxidising hearths of said furnace using an oxidising gas to produce an oxidised intermediate product; further pyroprocessing the oxidised intermediate product in a reducing environment through a plurality of reducing hearths of said furnace using a reducing gas such as hydrogen to produce a reduced intermediate product; a cooling bin for receiving the reduced intermediate product and allowing it to cool in an inert or low oxygen environment to substantially prevent re-oxidation thereof.

19. The system of claim 17, wherein the cooling bin discharges cooled or cooling reduced intermediate product into a leaching tank for further processing in a leaching process followed by separation of a leachingsolution from the undissolved TiOz, washing TiOz to remove residual leach solution, and stockpiling or packaging TiOz as a synthetic rutile product.

20. A synthetic rutile product when prepared by the process according to any one of claims 1 to 17 or a system according to claim 18 or 19.

21. A multiple hearth furnace when used in a process according to any one of claims 1 to 17 or a system according to claim 18 or 19.

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