A METHOD FOR THE PYROPROCESSING OF POWDERS

Abstract

A method for heating a powder material to induce a crystalline phase change in the grains of the particle comprising the steps of: a. preheating the powder from the high temperature streams generated from cooling the phase changed product; b. injecting the powder into a metal tube; c. controlling the gas composition in the metal tube by injecting a gas into the reactor; d. externally heating the first section of the tube by a first furnace segment system; e. externally heating the second section of the tube by a second furnace segment system; f: quickly quenching the powder product temperature in a cold third segment of the tube; g. collecting the processed powder at the base of the tube in a bed ejecting the powder from the tube; h. cooling the powder in a heat exchanger and using the heat to preheat the powder in step a.

Claims

1. A method for heating a powder material comprising -spodumene to induce a crystalline phase change in the grains of the particle comprising the steps of a. preheating the powder from the high temperature streams generated from cooling the phase changed product and or from any hot combustion gas stream in one or more heat exchangers; b. injecting the powder into a metal tube such that the velocity of the powder flow is about 0.2 m/s throughout the tube; c. controlling the gas composition in the metal tube by injecting a gas into the reactor to displace gases that leak into the reactor and to displace gases that otherwise accumulate in the reactor; d. externally heating the first section of the tube by a first furnace segment system in which the temperature and power is distributed and controlled so that the falling powder is heated to the temperature at which the phase change commences in the grains of the particle; e. externally heating the second section of the tube by a second furnace segment system in which the temperature and power is distributed and controlled so that the phase change in the falling powder occurs at a temperature that allows the phase change in the grains of particle to be completed to the degree required during the drop of the powder through the length of this segment; f. quickly quenching the powder product temperature in a cold third segment of the tube; g. collecting the processed powder at the base of the tube in a bed ejecting the powder from the tube; h. cooling the powder in a heat exchanger and using the heat to preheat the powder in step (a).

2. The method of claim 1, wherein the degree of conversion is greater than 90%.

3. The method of claim 2, wherein the degree of conversion is greater than 95%.

4. The method of claim 3, wherein the degree of conversion is greater than 99%.

5. The method of claim 1, wherein the reactor operates in the range of up to about 1150 C. by the use of high temperature steels.

6. The method of claim 1, wherein the tube has a variable diameter or with the segments therein are separated by powder beds.

7. The method of claim 1, wherein the residence time of the particles in the bed, and the bed temperature, is controlled so that a high degree of conversion can be met.

8. The method of claim 1, wherein the temperature and power system of the furnace segments firstly limits the temperature so that the stresses along the length of the hot metal tube limits the deformation and creep of the tube to give the tube a desirably long operational lifetime, and the temperature of the particle is maintained preferably just above the phase change temperature so that secondary decomposition reactions of the particle, if any, are suppressed.

9. The method of claim 1, wherein the process conditions are controlled such that the particles are not subject to internal stresses and collisions so that decrepitation of the particles as a result of the phase transitions or heating are suppressed to the extent that is desirable for subsequent processing.

10. The method of claim 1, wherein the furnace segments of the furnace segment system are combustor, and the fuel is renewable fuel such as biomass, or hydrogen.

11. The method of claim 1, wherein the furnace segments of the furnace segment system are electrical heating elements, and the electricity is produced from renewable sources such as wind, solar or hydro generators.

12. The method of claim 1, wherein the furnace segments of the furnace segment system are a combination of combustion segments and electrical heating elements.

13. The method of claim 1, wherein the method includes a pyroprocessor segment, in which the external furnace is a combustion system, or an array of combustion systems that provide the desired wall temperature distribution and power distribution required to accomplish the phase transformation as the powder falls through the reactor.

14. The method of claim 1, wherein the powder has a particle size distribution that is in the range of 5-300 microns.

14. The method of claim 14, wherein the powder has a particle size distribution that is in range of 5-150 microns.

16. The method of claim 1, wherein an application of the method, the powder comprises -spodumene and where the phase change occurs in the range of 500 to 1000 C. where the grains in the powder convert to a mixture of -spodumene and -spodumene, and the process conditions are set to maximise the efficiency of the process for extraction of lithium by (a) minimising the decomposition of the material in the powder into materials which fuses, and (b) minimising decrepitation of the product, and (c) minimising the temperature for energy efficiency by use of a reducing gas.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0040] Embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings.

[0041] The embodiment of FIG. 1 illustrates a schematic of a system in which an externally heated vessel is used to pyroprocesses the feedstock so that both the wall temperature distribution and the gas composition can be controlled.

DESCRIPTION OF THE INVENTION

[0042] Preferred embodiments of the invention will now be described by reference to the accompanying drawings and non-limiting examples.

The Method of Pyroprocessing

[0043] The method of the invention described herein is an adaptation of the indirect heated calciner described by Horley and Sceats in WO2007112496 System and Method of Calcination of Minerals and references therein (incorporated herein by reference), and further developed Sceats et al. in WO2018076073 A flash calciner and references therein (incorporated herein by reference), where the adaptation in this invention is for the purposes of pyroprocessing of minerals, rather than calcination of minerals.

[0044] The need for a pyroprocessing reactor is illustrated by a typical example, where a calcination reaction may have an enthalpy of reaction of, say, 180 kJ/mol because bonds are broken, a pyroprocess may have an enthalpy of phase change of less than 10 kJ/mol. Most pyroprocessing reactors have been developed from traditional calciner designs, such as kilns, and perform relatively poorly compared to the invention described herein.

[0045] The example embodiments refer to the pyroprocessing of -spodumene, which is one example of the application of this invention.

[0046] FIG. 1 is a pyroprocessor in which the mineral to be processed 101 is continuously injected by a feeder 102 into the top of a tubular reactor 103 which is heated externally by a furnace 104, and an injection of desired gas 105 is injected into the reactor near the base, and the pyroprocessed powder 106 is ejected from the base of the reactor, and the exhaust gas stream 107 is ejected from the top of the reactor. In this embodiment the pyroprocessor is separated into 3 segments A, B and C.

[0047] It would be appreciated by a person skilled in the art that the energy demand for the pyroprocess is minimised by preheating the power and gas by heat extracted from the exhausted powder 106 and exhausted gas 107, and any heat extracted from the furnace 104.

[0048] The difference with the calciner applications previously disclosed is that the reactor is not required to deal with large volumes of gas that that results from a calcination reaction of the mineral. The need to introduce a gas flow is to remove small volumes of gases that invariably leak into the calciner from the devices used to inject and exhaust powders, and for removal of any gases evolved from the powder such as moisture or from volatile impurities in the mineral, including those from floatation. It is desirable that such moisture and gases are removed in the preheating of the solids, where the preheating temperature is maintained below the temperature of the desired phase transition. Small volumes of gases may be introduced either in coflow or counterflow with the particles, and it may be preferable that the counterflow option is selected because the gas quenches the temperature of the pyroprocessed solids at the base of the reactor and preheates the powder at the top of the reactor.

[0049] Other reasons to inject small volumes of gas include (a) an ability to accelerate a phase change where the kinetics of the phase change is catalysed by a gas, such as steam or CO.sub.2 and/or (b) where a control of the oxidation state of impurities or crystal defects is desired.

[0050] The heat is transferred into the reactor through steel, or other heat conductive materials, and the heat is absorbed by the gas and particles primarily by radiative heat transfer. Because the gas flow is preferably very low, the particles flow down the tube under gravity at about the terminal velocity of the particles in the nearly quiescent gas. The reactor diameter is typically the order of 2 m in diameter for a process flux of about 3 tonnes/hr/m.sup.2.

[0051] The furnace is not dependent of the nature of the fuel used to provide the heat for the process, which may be from combustion of fossil fuels, waste materials, or desirably biomass, solar radiation or from the use of renewable power through electric elements that may be placed internally in the reactor. It is designed to provide heat to the powder to give effect to the segments A, B and C described below.

[0052] In this embodiment, the segment A at the top of the reactor is used to provide heat to the powder to a temperature above the phase change to activate that change, segment B is used to complete the phase change and segment C, if required, is used to extracted heat to flash quench the powder so that the reverse phase change does not have time to occur. The latter segment may be used in the case that the phase change is reversible.

[0053] The difference with the calciner applications previously disclosed is that the reactor is not required to deal with large volumes of gas that results from a calcination reaction of the mineral. The need to introduce a gas flow is to remove small volumes of gases that invariably leak into the calciner from the devices used to inject and exhaust powders, and for removal of any gases evolved from the powder such as moisture or from impurities in the mineral, or control a catalysis of a phase change, or inhibit the formation of eutectic phases. It is desirable that such moisture and gases are removed in the preheating of the solids, where the preheating temperature is maintained below the temperature of the desired phase transition.

[0054] The selection of the gas in determined by the nature of the mineral to be processed, and by the ability of the gas to absorb heat. The overall length of the reactor is determined by both the heat required to be transferred to the particles and the kinetics of the process. The residence time of the particles in the reactor is generally in the range of 10-60 seconds for pyroprocess, and the powder particles are in the range of 1-200 microns and is preferably matched to powder requirements used for separation processes such as floatation and the like.

[0055] The reactor length is typically in the range of 10-30 m to provide the residence time, and is primarily determined by the powder particle size, heat transfer rates and the kinetics of the desired phase change processes so as to achieve the desired degree of the phase change transformation, and to generally control the sintering of the processed mineral.

[0056] It is found that pyroprocesses are sensitive to the temperature distribution along the reactor wall, and control is important. This is associated with the low enthalpy of phase changes in most minerals compared to calcination reactions because the number of chemical bonds is not significantly changed, so that the settings of the reactor must be controlled with higher precision to enable the phase change to occur at the most desirable temperature, whereas in calcination reactions, the temperature within the particles is held within tight bounds by the endothermic load of the reaction. With control, the propensity of the temperature of the particle to rise substantially above the targeted phase transition temperature can push the particles towards entering reactions with impurities, such as those initiated by silica to form clinkers, eutectics, and undesirable phase changes of the minerals. It is desirable to have the control of the temperature to within 5 C. to meet product specifications that are otherwise impaired. These requirements feed into the detailed design of the furnaces to control the heat transfer rate to maintain the particle temperature within a narrow band immediately after the temperature has reached the phase transition.

[0057] In the reactor of FIG. 1, the particle temperature first rises to the phase change temperature, and is then desirably pinned at the phase change temperature until the phase change is complete, and the temperature is rapidly quenched so as to prevent the particles reverting to the original phase. This requires not only the temperature of the reactor walls to be maintained with high precision, but also the design of the particle ejection system 106. The length scale over which a uniform temperature is required to be maintained is several meters.

[0058] To maintain a relatively uniform temperature of particles across the reactor, the design of the reactor is such that the diameter of the reactor tube is limited to be near the specification stated above. For large scale processing plants, a module of tubes may be used to achieve the desired throughput of the plant. In such a configuration, multiple tubes may be deployed in a single furnace.

[0059] It will be recognised by a person skilled in the art that modifications of the process flows of embodiment of FIG. 1 may be varied to account for other factors, such as fouling and environment emissions requirements.

[0060] It would be understood by a person skilled in the art that the design of pyroprocessors based on internal combustion, for example, from a flame in the centre of a reactor as used by the current systems used in pyroprocessing cannot give the temperature profile described above, with the precision described above, that is obtained using indirect heating. In such systems, a powder will typically experience a range of temperatures from the flame temperature of say, 1400 C. and that a range of 300 C. or more is typical.

[0061] The reactor design disclosed in this invention provides the desired control of temperature, is not adversely impacted by decrepitation, and the particle size is compatible with those obtained from flotation and required for lithium leaching. The particle size can be accommodated by the height of the reactor, and a large height for large particles can be offset by additional grinding before flotation where that process is used to remove gangue.

[0062] In consideration of the second aspect of the present disclosure with regard to temperature, the temperature of phase change can be set in an air environment to be about 1000 C. In the present invention of the pyroprocessor the pyroprocessor reactor has an array of furnace elements that provide heating for the reactant powder at the top of the steel tube to raise the temperature to that at which calcination can commence, and below that, the heating array provides the energy for calcination. An unexpected discovery is that the low enthalpy of a phase change is such that the wall temperature of the reactor requires only a small temperature above that of the phase change because the heat transfer rate into the particles is fast. In the indirectly heated pyroprocessor, FIG. 1 shows that the powder is injected at the top of the reactor, and the heat injection is intense to heat the particle up to the temperature of the phase change, and the length of the reactor below that has to be sufficient to allow the phase changes to occur, but now required very little demand for heat while avoiding a temperature rise which activates molten silica or formation of silicate eutectic coatings on the external surface and internal pores of the particle. The wall temperature can be controlled to maintain this, and the furnace power is distributed asymmetrically down the reactor. Further, the rapid quenching of the temperature can be achieved by a cold tube segment within the reactor, rapid ejection from the reactor by rotary valves, and the use of a plume heat exchanger as described by Sceats et. al. in AU 2019901169 or an air conveying system or cooled screw feeders.

[0063] A second advantage of the second aspect of the present invention arising from the external heating is that the product quality is not impaired by impurities in the combustion gas, such as bottom ash and fly ash. The absence of impurities such as CaO, MgO, Al.sub.2O.sub.3 and SiO.sub.2 from the combustion of coal or biomass removes the clinkering reactions of these with silica in the spodumene phases, which fouls the surface of the product and may interfere with the subsequent lithium hydrothermal extraction processes. The separation of such combustion ash from the product lowers the production costs because the ash generally consumes the materials used to extract the lithium ion, and also may complicate the extraction process.

[0064] A third advantage of the second aspect of the present invention is that secondary milling of the particles to break up silica or silica eutectic coatings is not required.

[0065] In consideration of the third aspect of the present disclosure with respect to the particle size, in the present invention of the pyroprocessor, the particles flow down the reactor in a dilute solids fraction flow at a low velocity dictated by friction from the near-quiescent gas. Simply, there is no combustion gas that can entrain the particles, and this difference means that issues of entrainment are not relevant.

[0066] The powder gently falls through the reactor at a velocity of about 0.05-0.2 ms.sup.1 in a low solid fraction flow. The residence time is relatively uniform because the small particles form streamers around the larger particles to minimise the drag. The particle-particle collisions are infrequent and have a low momentum. In such a flow regime, the particles do not decrepitate by particle-particle collisions or particle-wall collisions so that the particle size distribution is almost unchanged from that of the input material. The advantage to this is that the product is easy to handle as a powder for the subsequent hydrothermal processing. This is particularly true of filtering and dewatering processes. Further, the cost of disposal of material that does not contain fines is lower. Thus the advantages of the reactor described in this invention is that the slow particle velocities and streamer formation allow for uniform degree of phase change, with little decrepitation that leads to lower cost of delithiation with an input of particle sizes that matches the most desirable size from efficient gangue separation.

[0067] In consideration of the sixth aspect of the present disclosure with respect to the reactor efficiency, the pyroprocessor operates with a high thermal efficiency. The efficiency of the pyroprocessor system is determined by the efficiency of the reactor and the ancillaries. If a combustor is used for the external heating, the flue gas from the furnace is used to preheat the combustion air, as is usual, and excess low grade heat may be used to remove moisture and preheat the powder. The heat in the powder exhaust may be used to further preheat the powder before injection into the reactor. The efficiency of the reactor segment is impacted solely by the radiative heat losses from the furnace segment, which is determined by the thickness and quality of the refractory. The efficiency of the heat exchangers for the air preheating and powder preheating are related to the capital costs. In the case in which electrical power is used to heat the steel as shown in the embodiment of FIG. 1, the only heat exchange required is the preheating of the input powder by the hot powder exhaust because the gas flow through the reactor is very small, and there is a transformer loss for converting the electrical power to heat. The efficiency of the pyroprocessor can be optimised by use of the best available heat transfer ancillaries. There no moving parts compared to rotary kilns that lead to large heat losses. The efficiencies may be in the range of 70-90%, and increases with the scaling up of the system by the use of modules. The efficiency enhancement is further enhanced by using the lower process temperature in a reducing atmosphere, by requiring a lower consumption of energy from the furnace to heat the walls.

[0068] In consideration of the seventh aspect of the present disclosure, the external heating may be from electrical elements. The efforts to limit CO.sub.2 emissions, there has been the development of solar and wind power generators which have near zero emissions footprints, and because lithium batteries may be used to store electricity. The development of steels which can operate up to temperatures of about 1150 C. enables a design in which electrical power can be dissipated into heat by using the resistance of the metal to form the reactor steel, such that the heat is transferred directly to the powder in the reactor by radiative heat transfer. The alternative is to use such steels as electrical elements, so that heat is transferred through conventional high temperature steel. In another embodiment, the steel elements can be suspended in the reactor. In another example embodiment, the pyroprocessor may operate in a hybrid mode in which electric power is used to draw power from the grid to balance the grid power when renewable power is plentiful, and may switch to a combustion mode otherwise. In another embodiment, renewable power may be converted to hydrogen and oxygen and combusted in the furnace instead of fossil fuels. The core capability that enables these options is that the use of external heating, enabling the use of a wide variety of fuels, including electrical power, and combinations of these to provide the source of heat. In minerals processing, it is now feasible to generate renewable energy, and battery storage, close to the mine site so that many of the processes of beneficiation may be carried out at or near the mine in a continuous process.

[0069] In consideration of the eighth aspect of the present disclosure regarding scale up of production, it would be apparent that the processing of minerals in a single pyroprocessor pipe with a feed rate of about 3 tonnes/hr/m.sup.2 into the pipe is such that multiple tubes are required to process sufficient material for processing minerals. There is a limit of about 2 m diameter of a tube that arises from the principles of radiation heat transfer and the penetration depth of radiation into a gas particle cloud. There are advantages in energy efficiency to scale up production using modules of tubes, where a module has preferably a small exposed surface area to limit radiation loss. Thus clusters of tubes in an array may suffice to provide a gain in efficiency, where the tubes may share the energy from a common furnace.

[0070] Another example embodiment is that the short residence time and the use of gases to control the atmosphere may be used by bypass slow phase changes or bypass reactions that would otherwise take place at a lower temperature. For example, the formation of CaO from limestone can be suppressed in a 1 bar reactor up to about 895 C. by using CO.sub.2 as the gas and in this way, some clinkerisation reactions that would otherwise take place may be suppressed. In effect, the ability to use any gas in the reactor provides an additional degree of freedom for minerals pyroprocessing.

An Example of Pyroprocessing

[0071] Some of the benefits of the invention disclosed in this invention are considered by the application to the processing of -spodumene for the extraction of lithium. There are three pyroprocessor designs currently used to calcine -spodumene, with which this invention is compared; namely (a) a rotary kiln, (b) a flash calciner-suspension cyclone stack, and (c) fluidised bed.

[0072] These reactor designs are all internally heated reactors in which the gas is a flue gas from combustion. They have a need for excess air, so that the gas is say, 5% oxygen, 15% carbon dioxide, 10% steam and the remainder is nitrogen. This is an oxidising atmosphere. It will be shown below that the processing -spodumene is benefitted by processing in a reducing atmosphere.

[0073] The rotary kiln and the flash calciner suspension cyclone stack operate the process using flames to heat the particles and when used to process -spodumene the product is covered by a layer of silica and silicates that have formed because the particles see temperatures from the flames which are too high. For example, the desired phase transition temperature is 1000 C. for generating a mix of the low density 0-spodumene and -spodumene phases, the particles will see a wide range of temperatures from the combustion temperature of 1400 C. to the refractory wall temperature of say 1000 C. The rotary kiln has a long residence time, typically of hours, and is particularly susceptible to such degradation. On the other hand, the flash calciner-suspension cyclone stack has a very short residence time of, say, 10 seconds, and to achieve the phase change in that time, the process temperature is increased above the phase transition temperature so that the unwanted reactions occur, and the product quality is degraded. It is found that the layers of silica/silicates carry a significant fraction of the lithium, up to about 15%, which cannot be extracted by the leaching processes. The economics of mineral extraction is strongly dependent on the degree of extraction, and many deposits are rendered non-viable by such a poor extraction efficiency. This is particularly true for the processing of -spodumene.

[0074] In the fluidised bed, the temperature of the bed can be controlled, but small particles are rejected from the reactor by the combustion gas flow without a phase change as they heat up, and the propensity of the spodumene to decrepitate before the phase change in the particle is complete. Thus the process also has a deficiency in terms of the extraction efficiency. However, it is found that fluidised beds require large particle sizes, which are not compatible with the optimum particle size distribution from floatation process used before pyroprocessing, and with the leaching processes post pyroprocessing. While this issue can be addressed by additional processing steps, the cost of production increases and overall process is too expensive. Many deposits are rendered non-viable by the costs of the process. A characteristic of the pyro-processor described herein is that the optimum particle size is less than 200 microns because otherwise larger particles drop through the reactor too quickly to undergo the phase change for a pyroprocessor length preferably less than 20-30 metres. The particles size for the processing of -spodumene is in the range of floatation separation. For example, the range of particles reported by Filippov et. al, in Spodumene Floatation Mechanism Minerals, 9, 372 (2019), are 80-150 microns in the top fraction and the bottom fraction is 40-80 microns. The bottom fraction is below the limit of pyroprocessing in fluidised beds. Both fractions can be processed in the invention described herein. Generally, the prior art for floatation of spodumene nominates the particles to be about 40-200 microns depending on the specific separation technique used, but many of these processes have been developed for fluidised beds.

[0075] In consideration of the fifth aspect of the present disclosure related to the powder residence time, in the present invention of the pyroprocessor, residence time is preferably 60 seconds or less. This residence time is determined by the criterion that the degree of phase conversion is as high as possible, preferably greater than 98% This residence time is determined by the time required to heat the input to the phase transition temperature at the top of the reactor, and for the completion of the phase transition in the remainder of the reactor. Too long a residence time, the length of the reactor become too long, so the temperature of the lower part of the reactor is set to achieve the conversion. There are two opposing factors that define this requirement in the lower part of the reactor. Firstly, the desire to maintain a low particle temperature to limit the formation of fused products and secondly the requirement to achieve a high degree of phase conversion. The trade-off is the length of this segment, which is desirably less than about 15-20 metres. The optimum diameter of the reactor tube is determined by the mass flow rate of about 3 tonnes/hr/m.sup.2 and the need to provide uniform heating of the powder and the gas in the reactor. The diameter may vary to maintain a desirable heat transfer rate from the steel.

[0076] Further forms of the invention will be apparent from the description and drawings.

[0077] Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms, in keeping with the broad principles and the spirit of the invention described herein.

[0078] The present invention and the described preferred embodiments specifically include at least one feature that is industrial applicable.