FIELD OF THE INVENTION

The present invention relates to a process and plant for the decomposition of a rare earth-bearing mineral.

BACKGROUND TO THE INVENTION

Rare earths or rare earth elements (REEs) are a group of 17 elements comprising lanthanides, yttrium and scandium, found in approximately 200 rare earth-bearing minerals of which bastnasite (carbonate mineral type), monazite (phosphate mineral type) and xenotime (phosphate mineral type) are of greatest commercial relevance. The importance of these rare earths in the manufacture of a myriad of contemporary technologies, ranging from the automotive industry to medical devices, and corresponding commodity price increases has led to a recent strengthening in the focus on the development of economically and environmentally viable processing routes for the conversion of rare earth-bearing minerals to saleable rare earth products.

The processing routes for these rare earth-bearing minerals generally entail the steps of beneficiation, mineral decomposition and hydrometallurgical processing to allow for a high degree of selectivity in the production of the saleable rare earth products. In this context, mineral decomposition forms an important process step as it allows for the solubilisation of rare earths in a beneficiation product such that it is suitable for subsequent hydrometallurgical processing.

At present, continuous mineral decomposition of major rare earth deposits, such as Bayan Obo in China and Mt. Weld in Australia, is achieved through acid baking in rotating cylindrical reactors known as rotary kilns. This acid baking comprises decomposing a feedstock material comprising a rare earth-bearing mineral or minerals, such as a dewatered and dried froth flotation concentrate, by means of heating a mixture of the feedstock material and sulfuric acid to a temperature above 160°C (typically however above 300°C) in the rotary kiln to allow for sulphation of the rare earth and thereby rendering it soluble during leaching, in particular water leaching, which in turn allows for further downstream impurity removal and solvent extraction.

The problem however with this rotary kiln-based decomposition process is that the associated cascading of the feedstock material resulting from its rotational movement leads to the generation of dust and fines which, together with the presence of radioactive thorium (especially in monazite and to a lesser extent in bastnasite and xenotime) and the decomposition product gasses including hydrogen fluoride (HF), sulphur dioxide (SO2), sulphur trioxide (SO3), and silicon tetrafluoride (SiF4), necessitates complex and costly dust and off-gas capturing and cleaning infrastructure, which compounds the already high costs associated with the installation and operation of rotary kilns.

Still further, due to the design of a rotary kiln, off-gas extraction from a rotary kiln is largely restricted to the kiln inlet head. This limits the extent to which internal gas recycles can be employed in the decomposition process, resulting in a comparatively larger volume of off-gas being produced and therewith necessitating greater off-gas scrubbing capacity.

OBJECT OF THE INVENTION

It is accordingly an object of the present invention to provide a novel process and plant for the decomposition of a rare earth-bearing mineral which overcomes, at least partially, the abovementioned problem and limitation and/or which will be a useful alternative to existing processes for the decomposition of rare earth-bearing minerals by means of a rotary kiln.

SUMMARY OF THE INVENTION

According to the invention, there is provided a process for the decomposition of a rare earth-bearing mineral, the process comprising the steps of: providing a bed comprising a feedstock material mixed with a source of an oxoacid of sulphur on a conveyor movable by a conveyor system, the feedstock material containing the rare earth-bearing mineral; and moving the bed provided on the conveyor past at least a first heat source and heating the bed to a temperature in a range of 160°C to 900°C to at least partially decompose the rare earth-bearing mineral in the presence of the oxoacid of sulphur to form a rare earth sulphate product and a gaseous product.

Decomposition in this context is to be understood as referring to the at least partial decomposition or cracking of the structure of the rare earth-bearing mineral such that a sulphate compound of the rare earth is formed in the presence of an oxoacid of sulphur.

An oxoacid of sulphur is to be understood as an acidic chemical compound containing, but not limited to, sulphur, oxygen and hydrogen. Preferably, the oxoacid of sulphur is sulphuric acid, and correspondingly the source of the oxoacid of sulphur may comprise dilute or concentrated sulphuric acid.

The rare earth sulphate product may comprise the sulphate compound of the rare earth together with one or any combination of: an impurity metal constituent comprising impurity metals and/or impurity metal compounds; a concomitant decomposition product constituent; an unreacted rare earthbearing mineral constituent; and a residual acid constituent. The feedstock material may comprise particles with a particle size smaller than 500pm or even smaller than 100pm.

Reference to bed in the current context refers to a uniform or non-uniform layer of material. The height of the bed provided on the conveyor may be between 5 to 50mm, preferably 20 mm.

The conveyor may be a belt, bucket, plate or the like suitable for moving the bed provided thereon. The conveyor may be a belt, preferably a steel belt. The steel belt may be a woven steel belt, the woven steel belt comprising a woven steel, such as woven cabled wire or steel cords. The woven steel belt may comprise apertures less than 500pm, preferably 100pm, such that the bed comprising the feedstock material is retained on the woven steel belt while the woven steel belt allows for a gas to pass therethrough.

The at least first heat source may be an infrared heat source, preferably a medium wave infrared radiation heater.

The belt or a section of the belt upon which the bed is to be provided may at least partially be supported by a first underlying support structure. The first underlying support structure may comprise a framework or grid of spaced bars, the bars preferably having a square or rectangular crosssection with angular and/or rounded cornering. The bars may have a circular cross-section. The bars may be hollow. The first underlying support structure may be provided above a first underflow chute defining an inlet at an open top and a chamber, the first chute preferably having a funnel- shaped cross-section comprising an outlet disposed at a lower narrow end.

The first heat source may be disposed above the belt, the first underlying support structure and the first underflow chute. The first heat source, the first underlying support structure and the first underflow chute may form part of a first processing station.

The step of providing the bed comprising the feedstock material mixed with a source of an oxoacid of sulphur on the conveyor may be preceded by a step of feeding a mixture of the feedstock material and the source of an oxoacid of sulphur onto the conveyor to provide the bed. The step of feeding the mixture of the feedstock material and the source of an oxoacid of sulphur may comprise feeding the mixture onto the conveyor by means of a screw feeder or a feed chute, the feedstock material and the source of an oxoacid of sulphur provided as a feed to the screw feeder or the feed chute.

The feedstock material may be an ore or a concentrate containing the rare earth-bearing mineral. The concentrate may be a dewatered and dried froth flotation concentrate. Where the feedstock material is a dewatered and dried froth flotation concentrate, the step of feeding the mixture of the feedstock material and the source of an oxoacid of sulphur onto the conveyor to provide the bed may be preceded by a step of drying the feedstock material such that the feedstock material has a moisture content of less than 30wt%, in some application less than 10wt% and even less than 2wt%.

The step of providing the bed comprising the feedstock material and the source of an oxoacid of sulphur on the conveyor may be followed by a step of raking the bed so as to at least partially expose an interior of the bed.

The step of heating the bed to a temperature in a range of 160°C to 900°C, preferably in a range of 500°C to 550 °C, by means of moving the bed provided on the conveyor past at least the first heat source may comprise moving the bed provided on the conveyor underneath a medium wave infrared radiation heater.

The step of heating the bed to a temperature in a range of 160°C to 900°C to form the rare earth sulphate product and the gaseous product may comprise heating the bed to a temperate in a range of 300°C to 700°C, preferably in a range of 500°C to 550°C. The step of heating the bed may be preceded by and/or coincide with and/or be followed by a step of extracting the gaseous product from the bed. Where the conveyor is a woven steel belt with the first underlying support structure as a framework or a grid of spaced bars provided above the first underflow chute, the step of extracting the gaseous product from the bed may comprise extracting the gaseous product from the bed to the outlet by means of a suction source in fluid flow connection with the outlet.

The gaseous product may therefore pass through the apertures of the woven steel belt, the spacing between the spaced bars and the underflow chute to report to the outlet.

The step of heating the bed to a temperature in a range of 160°C to 900°C by means of moving the bed provided on the conveyor past at least a first heat source may comprise sequentially moving the bed provided on the conveyor past a plurality of heat sources. Preferably, the step of heating the bed to a temperature in a range of 160°C to 900°C by means of sequentially moving the bed provided on the conveyor past a plurality of heat sources comprises sequentially moving the bed provided on the conveyor underneath two or more medium wave infrared radiation heaters.

Each of the two or more medium wave infrared radiation heaters may be disposed above a respective support structure and a respective underflow chute comprising an outlet. The step of extracting the gaseous product from the bed may therefore comprise extracting the gaseous product from the bed to an outlet of the first underflow chute disposed below the first medium wave infrared radiation heater, the first medium wave infrared radiation heater provided subsequent to or downstream of a second medium wave infrared radiation heater relative to a feed point of the feedstock material and the source of an oxoacid of sulphur upon the conveyor and disposed above a second support structure and a second underflow chute comprising an outlet.

The step of extracting the gaseous product from the bed to the outlet of the first underflow chute may be followed by a step of introducing into the bed the extracted gaseous product at an upstream section of the bed relative to a section of the bed provided underneath the first medium wave infrared radiation heater. The upstream section may be a section of the bed at least partially underneath the second medium wave infrared radiation heater.

Introducing the extracted gaseous product at the upstream section of the bed may comprise feeding the gaseous product from the outlet of the first underflow chute to a fluid dispensing arrangement in or at the second support structure by means of a suction blower in fluid flow communication with the fluid dispensing arrangement at the second support structure. The fluid dispensing arrangement may comprise one or more vents provided in or at the second support structure. The suction blower may therefore be in fluid flow connection with outlet of the first underflow chute and the vents at the second support structure by means of piping, allowing passage of the extracted gaseous product from the outlet of the first underflow chute via the suction blower to the vents. The piping may at least partially be located in the chamber defined by the second underflow chute.

The process may comprise a further step of extracting a concentrated gaseous product from the bed to the outlet of the second underflow chute at least partially disposed below the second medium wave infrared radiation heater. Said piping at least partially located in the second underflow chute thereby allows for heat transfer from the concentrated gaseous product in the second underflow chute to the gaseous product in the piping.

The extracted gaseous product introduced at the upstream section of the bed may therefore be a heat exchanged gaseous product.

The process may still further comprise a step of discharging the rare earth sulphate product from the conveyor for subsequent processing.

In some embodiments, the bed may be heated to and maintained at a first temperature in the range of 160°C to 900°C for a first time period to crack the feedstock material and then to a second higher temperature, to calcine. The first temperature may be about 250°C, the first time period may be about 10min and the second temperature may be about 700°C.

The invention also extends to a rare earth-bearing mineral decomposition plant comprising: a conveyor holding a bed comprising a feedstock material mixed with a source of an oxoacid of sulphur, the feedstock material containing the rare earth-bearing mineral; at least a first station comprising a heat source for heating the bed; and a conveyor system for moving the conveyor in a first direction past the heat source of the at least first station to heat the bed to a temperature in a range of 160°C to 900°C, to at least partially decompose the rare earth-bearing mineral in the presence of the oxoacid of sulphur to form a rare earth sulphate product and a gaseous product.

The conveyor, support structure, underflow chute, and/or piping of the plant may be manufactured from a suitable structural metal and/or structural metal alloy, such as a nickel-based alloy or steel, preferably a stainless steel, most preferably a grade 304 stainless steel. The stainless steel belt and/or heating source may be coated, such as with a ceramic micro coating so as to mitigate against corrosion thereof. BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described below, by way of a non-limiting example only and with reference to the accompanying drawings in which: figure 1 is a schematic representation of an embodiment of a plant and a process for the decomposition of a rare earth-bearing mineral; figure 2 is a side view of the representation of figure 1 ; figure 3 is a perspective view of a conveyor, support structures, underflow chutes, suction blowers and piping forming part of the plant; and figure 4 is a perspective view of the support structure of figure 3.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

With reference to the drawings, in which like numerals refer to like features, a plant implementing a process for the decomposition of a rare earthbearing mineral according to the invention is generally designated by reference number 10.

As a feed to the plant 10, a feedstock material, such as a beneficiated ore or a dewatered and dried concentrate containing a rare earth-bearing mineral or minerals, is mixed with a source of an oxoacid of sulphur to form a mixture 12. The oxoacid of sulphur in this embodiment is sulphuric acid with the source of sulphuric acid being concentrated sulphuric acid. The feedstock material, with a moisture content of less than 2wt% and a particle size of less than 100pm, is accordingly mixed with the concentrated sulphuric acid at ambient conditions at a ratio in a range of 1 :1 (w/w) to 3:1 (w/w) acid to feedstock material to form the mixture 12 comprising stoichiometrically sufficient sulphate ions for the at least partial decomposition of the rare earth-bearing mineral or minerals contained in the feedstock material.

The resultant mixture 12 is introduced into a feed chute 14 wherefrom the mixture 12 is gravitationally fed at a feed point A onto a conveyor 16, as a woven stainless steel belt forming part of and movable by a conveyor system 18, shown as a conveyor belt drive system, to provide at B a bed 20 of the mixture 12 at a height of approximately 20 mm upon the belt 16. A rake (not shown) can also be provided above the belt 16 subsequent the feed point where the mixture 12 is fed onto the belt 16 such that an interior of the bed 20 can be exposed, thereby increasing an exposed surface area of the bed 20 and correspondingly increasing the reaction kinetics of decomposition in the process.

As will become evident from this description, the woven stainless steel belt 16 is selected such that a mesh of the belt 16 allows for retaining the bed 20 on the belt 16 while allowing for gasses to pass therethrough. It will be appreciated that the bed 20 need not be a continuous bed, the continuity of the bed 20 being dependent on the manner and rate of feeding of the mixture 12 onto the belt 16. Furthermore, reference to the bed 20 in the current context includes reference to a section of the bed 20.

The bed 20, as so provided on the belt 16, is then sequentially moved in a first direction C past a plurality of processing stations 21.3, 21.2 and 21.1 comprising respective heat sources 22.3, 22.2 and 22.1 . This movement is exemplified in the current description as conveying the bed 20 on the belt 16 underneath three medium wave infrared radiation heaters 22.3, 22.2 and 22.1 by means of the conveyor belt system 18. The bed 20 is accordingly heated at D to a temperature in a range of 160°C to 900°C by the medium wave infrared radiation heaters 22.

Advantageously, in this manner the plant 10 allows for the temperature to be controlled by means of the rate of movement of the belt 16 as well as the energy supply from the medium wave infrared radiation heaters 22.1 to 22.3. It is to be appreciated that the rate of movement and the temperature to which the bed 20 is to be heated is, at least in part, dependant on the composition of the feedstock material and considerations such as the decomposition and volatisation of the oxoacid of sulphur. By way of nonlimiting example, where the feedstock material largely contains the thorium and iron rich phosphate mineral monazite, this temperature may be lower as compared to the temperature for the carbonate mineral bastnasite, so as to prevent the reformation of monazite (as seen at temperatures of 800°C and higher) as well as the formation of insoluble compounds of thorium containing rare earths present in the feedstock material. By further nonlimiting example, where the feedstock material largely contains xenotime, which is known to have a slower decomposition rate at a given temperature as compared to monazite, the rate of movement of the belt 16 may correspondingly be slower to ensure a longer residence time of the bed 20 underneath the medium wave infrared radiation heaters 22.3 to 22.1 .

This heating D of the bed 20 acts to at least partially decompose or crack the rare earth-bearing mineral in the presence of the sulphuric acid to produce a sulphate compound of the rare earth. Accordingly, a rare earth sulphate product containing the sulphate compound of the rare earth is formed along with a gaseous product, and the rare earth sulphate product can therefore be discharged from the plant 10, such as for subsequent processing by water leaching.

Reaction equation (1 ), in which a rare earth element is designated by RE, generally exemplifies this decomposition with reference to a rare earth phosphate mineral in the presence of sulphuric acid:

2REPO ₄ + 3H ₂ SO ₄ /?E ₂ (SO ₄ ) ₃ + 2H ₃ PO ₄ (1) The gaseous product can comprise decomposition reaction product gasses and/or the product gasses from side reactions occurring between constituents of the feedstock material and the sulphuric acid. Reaction equations (2) and (3) exemplify select reactions according to which constituents of the gaseous product are known to be formed in monazitecontaining feedstock materials:

CaF ₂ + H ₂ SO ₄ -> CaS0 ₄ + 2HF g) (2)

SiO ₂ + 2HF g) - SiF ₄ (^) + H ₂ O (g) (3)

It is to be appreciated that, by virtue of the composition of the feedstock material and the extent of the decomposition reaction, the rare earth sulphate product can contain the resultant sulphate compound of the rare earth together with any combination of an impurity metal constituent comprising impurity metals and/or impurity metal compounds (such as compounds of thorium, calcium, iron etc.); a concomitant decomposition product constituent (such as phosphoric acid as exemplified in reaction equation (1 )); an unreacted rare earth-bearing mineral constituent; and a residual sulphuric acid constituent.

As such, sequentially moving the bed 20 past the plurality of heat sources 22.3 to 22.1 can act to ensure that the bed 20 which is subject to heating D is maintained at an intended temperature at a predetermined residence time such that the formation of an amount of the sulphate compound of any rare earths present in the feedstock material is maximised.

Figure 1 further shows that each of the medium wave infrared radiation heaters 22.3 to 22.1 of the processing stations is disposed above an associated support structure 24.3 to 24.1 underlying the belt 16 and an associated chute 26.3 to 26.1 , defining a respective inlet at an open top and chamber, as an underflow chute comprising a respective outlet 28.3 to 28.1 in fluid flow connection with a suction source or blower 30.3 to 30.1 .

Each of the support structures 24.3 to 24.1 in this regard is shown in figures 3 and 4 as a framework of hollow bars having a square cross-section, with each of the support structures 24.3 to 24.1 shown in figure 3 to at least partially underly a section of the belt 16 upon which the bed 20 is provided, thereby supporting the belt 16 against sagging under the weight of the bed 20. Figure 3 further shows each of the underflow chutes 26.3 to 26.1 as having a funnel-shaped cross-section with the outlets 28.3 to 28.1 disposed at a lower narrow end.

This configuration allows the gaseous product to be extracted E through an inlet at the open top of the first chute 26.1 to the outlet 28.1 of the first chute from a section of the bed 20 provided underneath the first medium wave infrared radiation heater 22.1 . This first medium wave infrared radiation heater 22.1 is provided subsequent to or downstream of the second medium wave infrared radiation heater 22.2 with reference to the first direction C. The second medium wave infrared radiation heater 22.2 of the second processing station 21.2 is again disposed above the second support structure 24.2 and the second underflow chute 26.2.

The gaseous product is so extracted E by means of the suction blower 30.1 causing a negative pressure in the first underflow chute 26.1 , thereby causing the gaseous product to pass through the apertures of the woven belt 16, the spacing between the spaced bars of the first support structure 24.1 and the first underflow chute 26.1 and to report to the outlet 28.1 of the first underflow chute.

This extracted gaseous product is then introduced back into the bed 20 at an upstream section of the bed 20 relative to the section of the bed 20 provided underneath the first medium wave infrared radiation heater 22.1 . This is achieved by feeding by means of the suction blower 30.1 the gaseous product from the outlet 28.1 of the first underflow chute 26.1 to a fluid dispensing arrangement, shown as a series of vents 32, of the second support structure 24.2 in figure 4. The suction blower 30.1 is therefore provided in fluid flow connection with the outlet 28.1 of the first underflow chute 26.1 and the fluid dispensing arrangement 32 of the second support structure 24.2 by means of piping 34. The fluid flow of the gaseous product is shown at F in figures 1 , 2 and 4. Figures 1 and 2 show the upstream section of the bed 20 at which the gaseous product is introduced as the section of the bed 20 which is underneath the second medium wave infrared radiation heater 22.2.

In addition to this introduction or “recycle” of the gaseous product as described above, the plant and process further allows for the extraction G of a concentrated gaseous product from the section of the bed 20 provided underneath the second medium wave infrared radiation heater 22.2. This extraction G of the concentrated gaseous product occurs in a similar fashion to that described with reference to the gaseous product above, but now the concentrated gaseous product reports to the outlet 28.2 of the second underflow chute 26.2 by virtue of the second suction blower 30.2.

Advantageously and as best shown in figure 2, at least a portion of the piping 34 through which the gaseous product flows to the vents of the fluid dispensing arrangement 32 of the second support structure 24.2 can be provided within the second underflow chute 26.2, which in turn allows for heat transfer from the concentrated gaseous product extracted G through the second chamber 26.2 to the gaseous product in the piping 34 such that the gaseous product introduced at the vents 32 is a heat exchanged gaseous product. Those skilled in the art will appreciate that this not only allows for an increase in the energy efficiency of the process but that, most advantageously, this recycle can be further employed across any plurality of heat sources 22 upstream from a first heat source 22.1 . This may significantly decrease the volume of an off-gas discharged H from the process while increasing the concentration of harmful gasses such as HF, SOXs and SiF4 therein, which may greatly reduce the complexity and size of the subsequent off-gas treatment infrastructure required.

The invention provides a novel process for the decomposition or “acid baking” of rare earth-bearing minerals without requiring the cascading movement of a feedstock material as seen in the currently employed rotary kiln acid bake applications. By maintaining a bed 20 comprising the feedstock material on a belt 16 movable by means of a conveyor belt drive system 18, the process reduces or avoids the generation of harmful and even radioactive dusts and fines, such as that seen from the existing rotary kiln applications, and the dust capturing and scrubbing infrastructure of which is therefore at least partially avoided.

It is further to be appreciated that, due to the aggressive environment brought about by the presence of at least sulphuric acid in the plant and process, the belt 16, support structures 24, underflow chutes 26 and piping 34 are to be manufactured from a structural metal and/or structural metal alloy which is resistant to corrosion by sulphuric acid, such as exotic nickel- based alloys. However, due to the further avoidance of an erosive environment such as that brought on by the cascading motion of a rotary kiln, steels and/or stainless steels coated in a micro-ceramic coating can also be used in the plant 10, thereby further reducing equipment costs as compared to the conventional methods of acid baking by means of rotary kilns.

It will be appreciated by those skilled in the art that the invention is not limited to the process as exemplified herein and that many variations are possible without departing from the scope of the invention. As such, the present invention extends to all functionally equivalent processing equipment, structures, methods and uses that are within its scope. In particular, the steps of the process provided for need not necessarily be executed sequentially. Furthermore, it is envisaged that the steps of the process provided for need not necessarily be executed in the order listed herein.