Technical Field

This disclosure relates, in general, to processes of thermally treating minerals, particularly iron bearing minerals which are treated for enhancement of magnetic character or other upgrades of the mineral. More particularly, the disclosure relates to processes for thermally treating minerals comprising low temperature roasting with high efficiency.

Background Art

Development of processes to thermally treat minerals have been carried out for a number of reasons, typically to achieve upgrading of the mineral in some way, for example, to increase or decrease leachability of specific compounds, change the state of oxidation, devolatilise organic and or inorganic components, enhance separation properties, etc.

Historically, high temperature roasting (HTR) techniques have been employed for enhancement of magnetic susceptibility in iron bearing minerals. This is because in-situ fuel combustion has been used, so the operating temperature must be greater than the autoignition temperature of the fuel, which is typically a minimum of 750°C for safe operation. For example, the Becher process uses a very high temperature reductive roast in a rotary kiln to metallise the iron in ilmenite for the main purpose of conditioning the mineral for leaching to make synthetic rutile. The roasted mineral has a high magnetic susceptibility due to the presence of metallic iron and so this property is used to magnetically separate char, ash and some gangue minerals as part of the flowsheet.

Benelite process uses rotary kiln reduction with fuel oil to convert iron oxide in the ilmenite to FeO, followed by a hydrochloric acid leach to make synthetic rutile.

Two major processing routes exist for production of titanium dioxide (TiOs) pigment; sulfate and chloride. Feedstock (ilmenite) to sulfate pigment production must be soluble in sulfuric acid. TiOs exists in three crystalline forms, rutile, anatase and brookite. Only anatase is soluble in sulfuric acid. If anatase is heated above 650°C it will convert to rutile given sufficient time. While a number of high temperature processes have been commercialised or proposed for specific ore types, none of these can economically treat low grade ores or produce titanium feedstock suitable for both pigment production routes.

Most of the easily accessed mineral sand deposits that occurred along coast lines have either been mined or are locked up by development or national parks. This is driving exploration inland. A number of the new deposits under development have contamination issues that require a roasting step to gain acceptable mineral recovery or market acceptance of the final product(s). LTR can be applied to these resources, using locally available fuel. Traditional distributor plate and plenum designs can have significant wear problems arising from the presents of solids in the fluidisation gas stream. Secondarily, if the fluidising gas is hot, mechanical strength of the distributor plate is a difficult issue to design for. Often the only choice is to provide external cooling on the plate structure, which is achieved using dual plates with a cooling channel running between. Each tuyere pipe has to be sleeved. In a commercial fluid bed there are hundreds of tuyeres, so the fabrication cost is increased as a result.

The above references to the background art do not constitute an admission that the art forms a part of the common general knowledge of a person of ordinary skill in the art.

Summary

The disclosure relates to low temperature processing of minerals in general, but in the detailed description the process is directed to augmentation of the magnetic susceptibility of iron bearing minerals to allow efficient magnetic separation, in particular iron-titanium bearing minerals. It will be clear that the process disclosed may find application with other minerals.

In some forms, the process may alter the oxidation state of iron within naturally occurring mineral sands to approximate magnetite in composition (FeO:Fe ₂ C>3), thereby homogenising the otherwise wide range of oxidation states in the naturally occurring mineral assemblage, which in turn homogenises the magnetic susceptibilities. This may have the benefit of allowing simple magnetic fractionation of iron-titanium minerals with different proportions of titanium dioxide and iron oxide, e.g. separation of titano-magnetite, ilmenite and leucoxene, plus rejection of lower magnetic gauge minerals such as chromite, monazite, garnets, along with any non-magnetic residual minerals, i.e. rutile, zircon.

However, it will be appreciated that the process is not limited to these uses or outputs. The process handles variability in feedstock minerals and can operate over a wide range of conditions.

According to a first aspect, disclosed is a process for thermally treating a mineral feedstock, the process comprising partially combusting fuel to output hot syngas, preheating the mineral feedstock utilising the hot syngas output in the step of partially combusting fuel, thermally treating the pre-heated mineral feedstock. In some forms the step of partially combusting fuel is performed by partially combusting fuel with air. In some forms the hot syngas and the mineral feedstock are delivered to a suspension pre-heater. In some forms only a single pre-heating step is performed. In some forms waste off-gas from the step of thermally treating the mineral feedstock is delivered to an afterburner. In some forms waste heat from the afterburner is utilised for mineral drying. In some forms waste heat from the after burner is delivered to a drier.

The process may have the benefit of increasing thermal efficiency, which may have the benefit of reducing fuel consumption and allowing for less overall expense.

An additional feature of the process is incorporation of a submerged distributor plate in the LTR that can handle any solids that carry-over from ilmenite preheating. This design does not need to support the full weight of the fluidised bed, so the mechanical duty is decreased, which intern means external cooling is not necessary. This further reduces heat losses and capital cost.

Brief Description

Notwithstanding any other forms that may fall within the scope of the process and apparatus as set forth, specific embodiments will now be described, by way of example only, with reference to the accompanying drawings in which:

Fig. 1 illustrates a simplified process flow chart of one embodiment of the disclosure;

Fig. 2 illustrates a more detailed process flow chart of a further embodiment of the disclosure;

Fig. 3 shows a longitudinal cross-sectional view of a fluid bed reactor of one embodiment of the disclosure;

Fig. 4 shows a lateral cross-sectional view of one embodiment of a fluid bed reactor of the disclosure;

Fig. 5 shows a lateral cross-sectional view of one embodiment of a fluid bed reactor of the disclosure;

Fig. 6 shows one embodiment of an individual gas distributor segment of one embodiment of the disclosure;

Fig. 7 shows a further embodiment of an individual gas distributor segment of a further embodiment of the disclosure; Fig. 8 shows a perspective view of a distributor plate of one embodiment of the disclosure;

Detailed Description

According to a first aspect, disclosed is a process for treating mineral feedstock, the process comprising partially combusting fuel to generate hot syngas; preheating the mineral feedstock utilising the hot syngas generated by partially combusting fuel; thermally treating the pre-heated mineral feedstock.

In some forms, the mineral feedstock comprises an iron bearing mineral. In some forms the mineral feedstock comprises ilmenite.

In some forms the step of partially combusting fuel is performed in the presence of air. In some forms the step of partially combusing fuel is performed in a gasifier.

In some forms the step of preheating the mineral feedstock is performed in a suspension preheater. In some forms hot syngas generated by partially combusting fuel is delivered to the suspension preheater.

In some forms the mineral feedstock is pre-heated in only a single pre-heating step.

In some forms the step of thermally treating the pre-heated mineral feedstock is performed at a low temperature. In some forms, the step of thermally treating the pre-heated mineral feedstock is performed at a temperature of less than 650°C. In some forms the step of thermally treating the mineral feedstock is performed in a reactor comprising a fluidised bed. In some forms, the process further comprises delivering a spent syngas from the thermal treatment step to an afterburner.

In some forms waste heat from the afterburner is utilised to dry mineral feedstock.

The advantages of this process may include:

• a thermal treatment or roasting process at a low temperature with higher thermal efficiency, resulting in a lower fuel usage;

• a process with a single stage preheating using syngas, therefore less stages of pre-heat and no indirect heat exchanger required to pre-cool the hot syngas in order to meet the mechanical requirements of the thermal treatment reactor, therefore lower capital cost. Referring now to the Figures, Fig. 1 shows a simplified process flow diagram of one embodiment of the disclosure.

The process of treating a mineral feedstock 1 illustrated in the flow diagram comprises the step of generating syngas in a syngas generator 11 . Fuel 12 and air 13 are delivered to the syngas generator 11 and the fuel is partially combusted to generate syngas.

The syngas is delivered to a pre-heating stage 14 and the mineral feedstock 15 which may be in the form of ilmenite is delivered to the pre-heating stage where it is preheated by the hot syngas generated in the syngas generator 11 . The stream from the pre-heating stage may contain solids as a result of pre-heating the ilmenite. The gas distributor is designed to overcome or improve upon this issue.

Pre-heated mineral feedstock is delivered to a reactor 16 for thermal treatment. In the illustrated form the reactor provides low temperature roasting of the mineral feedstock. Roasted mineral 17 is output. Waste material and in some forms secondary air is delivered to an afterburner 18 where it is burned and waste heat in the form of hot off-gas is delivered 19 to a drier for drying feedstock.

Referring now to Fig. 2, a more detailed process is illustrated in a process flow diagram.

The process 1 comprises delivering fuel 23 and air 24 to a gasifier 22 for partial combustion of the fuel. The process generates hot syngas 25 which is delivered to a suspension preheater 26. Mineral feedstock 27 is delivered to the suspension preheater 26. The hot syngas and mineral feedstock is combined to pre-heat the mineral feedstock resulting in pre-heated mineral feedstock that is delivered to a reactor 30 via an inert gas 29, for example nitrogen through a fluidised seal pot 42). Warm syngas 28 from the preheater 26 is delivered to the reactor 30.

The reactor 30 may be in the form of a fluidised bed and performs low temperature roasting of the pre-heated mineral feedstock. In some forms the low temperature roasting is roasting at or below 650°C.

Roasted mineral feedstock 31 is delivered to an inert cooler 33 and provides a roasted feedstock output 32 of the process such as roasted ilmenite. The feedstock is delivered to magnetic separation. Cooling water 34 and recompressed inert gas 35 are delivered to the inert cooler for the cooling process. Inert off-gas from the cooling process is re-used 37 through filtration, cooling and recompression.

Waste off-gas 39 from the reactor 30 is delivered to an afterburner 40 along with air 41 to convert remaining chemical energy in the form of CO, Hs or CH4 to thermal energy. The thermal energy is delivered to a drier 44 and wet mineral feedstock 46 from gravity separation is delivered to the drier to be dried and delivered 27 to the pre-heater step 26. Alternatively, hot combustion gas from afterburner 40 could be utilised for additional mineral roasting steps or production of steam for power generation and/or process heating duties.

Low temperature roasting can achieve all of the desired outcomes, however fuel must be partially combusted external to the ilmenite roasting unit operation. Partial combustion of fuel with air generates a lean syngas at high temperature (>1000°C). This syngas must be fed to the ilmenite roasting unit operation. The most effective reactor design for this unit operation is a fluidised bed. Supply of hot gas (>750°C) to a fluid bed is problematic due to mechanical design constraints, particularly at large scale. Traditional options to cool the syngas were either injection of water to cool the syngas directly, however this affects the reaction equilibrium in the roasting step by adding products of reaction, or in-direct cooling using a heat exchanger. Both options remove heat from the system and in the case when a heat exchanger is utilised, a low-grade waste heat stream is recoverable, but extra equipment is necessary which increases the plant capital cost and associated maintenance costs.

Spent syngas leaving the roasting unit operation is fully combusted in an after-burner 40 with excess air to convert any remaining chemical energy in the form of CO, H ₂ or CH ₄ into thermal energy which is traditionally used for pre-heating the feed ilmenite via direct contact. Two or more stages of pre-heat are required to effectively capture this waste heat in this process.

An improved LTR process flowsheet is proposed whereby the hot syngas is used to pre-heat the feed ilmenite. In this way, only a single stage of pre-heat is required and heat is not lost from the system. Waste heat from the after-burner can then be used for mineral drying, additional roasting stages, steam generation or other process heating duties. Overall thermal efficiency is increased, reducing fuel consumption and lower capital cost plant design.

In one example a low temperature roast applied to concentrated ilmenite ore may be effective to change the iron species of trash iron from ferric (Fe2Os) to ferrous (FeO). This enables a subsequent magnetic separation of trash iron from accompanying ilmenite grains. In some forms, roasting for a longer residence time of 30-90 minutes and at higher temperature ranges but less than 600°C, has a reducing effect on the contained ferric iron component of the ilmenite itself without causing “rutilization” of the contained TiO ₂ . For example in some forms the temperature range of roasting may be between 400 and 750°C, and in some forms the temperature range may be between 515 and 625°C. This temperature range is preferably higher than other low temperature roasting temperatures.

In some forms, the roasting process requires exposing ilmenite ore and iron trash in the form of fine or sand-like particles to a hot reductant gas as a specific form of a multi-phase chemical reaction (gas - solids).

This is achieved by use of a fluid-bed design whereby the solid particles are suspended above a gas distributor within a vessel by passage of the reductant gas or syngas from below the bed at sufficient velocity to maintain agitation and exposure but limit the blowing over and loss of the particles prior to completion of the reaction.

The agitated solid and gas mix can be represented as a fluid (for example, in the sense of water) that will form a level surface, overflow a weir or flow under gravity through ducts. Exposure time or residence time, is set by the mass of bed material retained within the vessel divided by the mass feed rate. Particle movement is random resulting in a probabilistic regime for exposure. The exposure is measured as a mean residence time because the exposure is probability based. Mean residence time is designed to achieve exposure of most particles before they traverse the bed and escape from the discharge port.

In some forms, an upper fluidizing velocity of 0.6m/s may be used, dependent upon the physical properties of the mineral feedstock. In some forms an upper velocity of between 0.2 and 1 .0m/s is used.

Relevant physical properties of the vessel include bed area, collapsed bed mass and required strength for the vessel and gas distributor plate structure to carry that load, weir and discharge heights.

In one example, 60t/h ilmenite concentrate (IC) feed rate, with the roasting chamber operating at 570°C, producing sulfate-grade ilmenite with 28% FeO, was used as the base case.

In some forms a fluid bed temperature of 450°C may be used.

In some forms a fluid bed syngas (FBS) generator operates under consistent conditions, as this supplies the reactive gases for reduction of iron oxides and thermal energy for the plant. This fluid bed is also limited by fluidizing velocity. Bed material used in the FBS can be manipulated to optimize bed behaviour and minimize blow-over though. In some forms the bed is composed of ilmenite, in alternative forms the bed material may comprise an alternate material. The quantity of syngas generated is a set-point condition, with the other plant flows adjusted to close the mass and energy balance.

In some forms, disclosed is a submerged distributor plate in the reactor that can handle any solids that carry-over from mineral feedstock preheating or from the fluid bed gasifier. For example pre-heating ilmenite results in some solids and the gas distributor system of the present application is designed to handle these solids. This design does not need to support the full weight of the fluidised bed, so the mechanical duty is decreased, which in turn means external cooling is not necessary. This further reduces heat losses and capital cost. A feature of the design is it is self-cleaning, therefore no accumulation of solids takes place in the plenum space.

The submerged gas distributor design principle can be utilised in the fluid bed drier also for the same reasons.

Taking advantage of the submerged gas distributor design improved mechanical strength, this design may also be utilised in the fluid bed gasifier to allow use of an in-plenum start-up burner (as opposed to an above-bed start-up burner) for bringing the process up to operating temperature. In this way the burner is continuously air cooled when in normal operation, assuming the air flow for partial combustion is maintained through the burner nozzle. An above bed burner cannot be air purged as this would cause combustion of the syngas, and must rather be inert gas purged (which also affects the syngas chemistry) or physically isolated from the reflected heat emitted from inside the fluid bed (which is mechanically difficult to achieve).

Fig. 3 shows a simple sketch of a fluid bed reactor 50 comprising a submerged gas distributor 52. The fluid bed reactor 50 comprises a steel shell 54 and refractory lining 55 along with an inlet 56 for fluidising gas and an outlet 57 for off-gas. Fluidised bed material 58 is contained within the reactor while static bed material 59 is contained below the gas distributor in the reactor. Supports 53 are positioned to support the gas distributor from the base. In some forms the reactor is supported on a concrete foundation and in some forms the supports support the reactor from the base.

Referring to Fig. 4, individual gas distributor segments 61 are arranged to receive and distribute the fluidising gas supply 56. Manifolds 62 run down the side walls of the fluid bed reactor and deliver the fluidising gas to the gas distributor segments. Segments are only connected at one end to allow for differential thermal expansion and contraction.

In Fig. 5, individual gas distributor segments 61 are arranged to receive and distribute the fluidising gas supply 56. The circumference 63 is the circumference of the fluid bed reactor vessel and gas is delivered to the gas distributor segments arranged radially.

Referring to Fig. 6, a cross section of one embodiment of an individual gas segment is shown. Fluidising gas is provided through a centrally located input 65 and exits through gas distribution holes 66. The segment in some forms has a crescent shape with a curved upper surface and the distribution holes 66 are located lower than the input 65 to allow immediate discharge of any entrained solids entering with the fluidising gas, therefore avoiding accumulation of solids which can result in excessive internal wear through abrasion or blockages.

Referring to Fig. 7, a cross section of a second embodiment of an individual gas segment is shown. Fluidising gas is provided through a centrally located input and exits through gas distribution holes 66. The segment has an upper surface with a corner or elbow and the distribution holes 66 exit downwardly and are located lower than the input to allow for movement by gravity.

Figure 8 shows a side view of an individual gas segment showing the input 65 and the distribution holes 66 located at a lower portion of the body of the segment. The gas segment is designed to avoid accumulation of solid material.

In some forms, the process may augment magnetic susceptibility of iron bearing minerals through partial reduction or partial oxidation of FeO and FesOs. The process may include heating to between 515 and 625

It will be understood to persons skilled in the art that many other modifications may be made without departing from the spirit and scope of the process, and apparatus as disclosed herein.

In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations thereof 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 process and apparatus as disclosed herein.