REACTOR SYSTEM AND METHOD FOR THERMALLY ACTIVATING MINERALS

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims the benefit of the filing date of Australia

Provisional Patent Application No. 2011901545 filed 27 April, 2011, the disclosure of which is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0002] The present invention relates broadly to a reactor system and method for activating minerals by thermal processing.

2. Description of Related Art

[0003] Beneficiation of minerals by thermal activation is a common industrial pyroprocess carried out by calcination. Calcination is the chemical transformation of a mineral by the application of heat, and the specific calcination process of interest herein is one in which the chemical transformation is used to achieve a substantial change in the physical properties of the processed mineral particle - such its surface area and porosity. This application of calcination is different from most calcination processes in which the primary objective is the chemical transformation of the particle, in which case the primary specification of the calcined material is its chemical composition and especially the degree of calcination, for example, a 98% calcination.

[0004] The physical properties of calcined minerals are traditionally characterised by terms such as dead-burned, lightly burned or caustic, developed primarily by the lime industry from experience with kilns, but more generally used. [0005] By contrast, the thermal activation of a mineral is quantitatively measured by the texture of the particles, through the particle size distribution and the pore diameter distribution function. The latter is characterised by the specific surface area, and the mean pore length and porosity, which are the moments of this distribution function. For specific minerals, the effect of the activation on the mineralogy of the calcined can be deduced from the linewidths of X-ray diffraction patterns, including the small angle X-ray scattering. As the activation increases, the diffraction pattern often approaches that expected from an amorphous solid, and sintering involves the crystallisation into stable crystalline forms with growing long range order as part of the sintering process leads to the formation of stable crystalline grains. The sintering and crystallisation processes are intimately related.

[0006] Thermal activation enables many industrial processes, such as extraction of ores from the activated solid by dissolution by acids and alkalis; the adsorption of catalytically active materials such as nickel and paladium to produce supported catalysts; and to produce cementitious constituents for cement formulations. The primary benefits of these products rely on the texture of the particles produced, rather than the degree of calcination.

[0007] The first stage of the thermal activation process is to grind the feedstock to a suitable particle size distribution, thereby allowing the second stage of calcination to deliver a product with the desired texture and composition. The texture arises from an irreversible change in the physical and properties of the particles caused by the chemical processes induced by calcination. Typical thermally activated products have particle sizes ranging from about 10-1000μ, with a high specific surface area, of about 50-300 m ² gm ^"1 compared with the feedstock of 1-4 m ² gm ^"1 and a high porosity of 20-50% compared with the feedstock porosity of less than 2%. The high surface area and porosity are the signatures of activated products and they convey the benefits because gas-solid and liquid-solid chemical reactions occur more quickly when the gases and solids can readily access the surfaces through the pores generated by thermal activation. [0008] Thermal activation of a ground particle is achieved by volatilisation of a component of the mineral feedstock in a manner such that the voids from the displacement of the volatile constituents aggregate to create a network of pores. The chemical properties of the surface are determined by the mineral properties, and generally these are oxides. An exception is the activation of gypsum, CaS0 ₄ .2H ₂ 0. Methods of thermal activation include dehydration by calcining the water of hydration from hydrated minerals such as gypsum, or by removal of water by tiehydroxylation of hydroxide minerals such as bauxite and kaolmite; or by eliminating carbon dioxide from carbonate minerals such a limestone, magnesite, dolomite or other sedimentary minerals such as phosphate rocks which includes carbonate ions; or by eliminating volatile organic compounds by pyrolysis or gasification such as biomass or lignite to produce carbon black char. While the focus of the invention disclosed in this paper is on mineral processes, the applications also apply to the activation of manufactured solids designed to be thermally activated.

[0009] Specific examples of mineral activation by calcination include :-

(a) The activation of bauxite, generally for the production of aluminium, so that dissolution of the aluminium by sodium hydroxide in the Bayer process can occur at lower temperatures to limit the undesirable dissolution of silica and organic materials, or for low grade bauxite, to enable a fast acid dissolution process. The desirable properties of a fast alkali or acid digestion are generally attributed to the high surface area of the flash calcined mineral;

(b) The production ofmetakaolin from kaolin, where the high surface area produces a material which acts as a pozzolan for cement applications. The pozzolanic properties are generally attributed to the high surface area of the flash calcined material. At higher temperatures, large internal surfaces are produced giving rise to a high brightness when the particles are embedded in materials such as paper or cement;

(c) The production of gypsum hemihydrate and soluble gypsum anhydride from gypsum to produce a material which sets in water to form plaster products such as wallboards. The desirable properties for forming a strong product are generally associated with the high surface area of the flash calcined material. The hemihydrate is formed at low temperatures and soluble anhydride is formed at higher temperatures. Mixtures of both can be desirable for strong gypsum plasterboards;

(d) The production of exfoliated vermiculite or perlite to produce a material with a high internal porosity driven by the expansion of the particle from steam produced by flash calcination;

(e) The production of caustic oxides such as lime, magnesia, dolime and semidolime by the calcination of their respective carbonates. Applications of these materials are well established. The surfaces created have considerable chemical defects that are desirable for catalysts;

(f) The activation of sedimentary phosphate rocks by calcination of the carbonates to facilitate acid leaching of phosphate to produce superphosphate fertilizers

(g) The flash drying of lignite or biomass to reduce the water content of the mineral to increase its calorific value (lower heat value), and to preferably increase the surface area such that subsequent gasification process such as pyrolysis, combustion or hydrogasification occur more quickly.

[0010] The method of production of activated particles is most generally achieved by a flash calcination process, with a residence time of seconds compared to hours in conventional kiln or shaft calciners. The benefit of a fast (flash) process is that the high surface area porous particles are prone to sinter at high temperatures as the particles minimise their surface energy, causing a reduction of the desirable attributes of the high surface area and porosity. It follows that the preferred thermal activation method is one in which the residence time to achieve the optimum activation can be is minimised. Flash calciners have this desirable attribute. [0011] However, flash calciners that are currently used for thermal activation have been generally designed for the efficient chemical transformation of the feedstock into a product, rather than thermal activation, They generally use a direct interaction of the particles with the combustion gases, and sometimes with the combustion flames. Direct heating is adopted because it gives very efficient heat transfer to the particles. However, the high temperatures of combustion gases and flames can cause excessive sintering of the particles, which is detrimental to achievement of thermal activation. While the temperature of the combustion gas can be reduced by excess air or recirculated combustion gas, the input gas volumetric flow rates can become so large that the dimensions of the calciner can become excessive.

[0012] However, laboratory studies of thermal activation have

demonstrated that the highest thermal activation is achieved when the particles experience a monotonically increasing temperature during the calcination process from the injection point to the exhaust point of the calciner. Thus the most challenging issue for the use of direct heating to achieve a high thermal activation is that the particles are entrained by the hot combustion gases and co-flow, with the result that the temperature of the combustion gas falls as the particles heat up and the endothermic calcination reactions take place. The particle temperature will rise as the mixing with the hot combustion gas occurs, and then falls as the endothermic reaction absorbs the heat. Unless other measures are introduced, the particles leaving the reactor will not be at the highest temperature experienced during the activation process, and will not have the optimum activity. Measures have been introduced in conventional flash calciner systems to overcome this limitation.

[0013] For example, the reactor can be separated into a number of segments in which the particle temperature is raised between each co-flowing segment by injection of additional fresh combustion gases. This segmented reactor can give a counterflow of heat with the coflow of gases and particles. However, in each segment, the particle temperature will generally increase and then have a tendency to decrease depending on the thermal load from the endothermic reaction. Great care is required to optimise the secondary hot gas injectors. In another example, additional fuel is injected into the gas stream at a number of points in the reactor to combust with excess air, or additional air is injected into the gas stream to combust with excess fuel. This creates undesirable hot spots in the reactor where excessive sintering can take place. There is a need to simplify the reactor design to achieve the desirable temperature profile.

[0014] In all of the examples described above to mitigate the tendency for a materials co-flow to lead to temperature reduction, it is evident that the volumetric flow of gases at the- exhaust will be very large. Importantly, the dilution of the steam or carbon dioxide by the combustion gases makes it difficult to recover the steam, as water, or carbon dioxide. There is significant thermal energy in the steam, and there is a need for a process which allows the steam to be condensed to give a more energy efficient process. In the case of carbon dioxide gases, the need to reduce the carbon footprint of all industrial processes means that there is a need for thermal activation of carbonates which captures the carbon dioxide as a pure gas stream.

[0015] A need therefore exists to provide a reactor system and method for thermally activating minerals in a continuous flash calciner which provides a short residence time, a monotonic increase in the particle temperature during the course of the calcination reaction, and a means of recovery of the volatilized components - mostly water or carbon dioxide - liberated during the calcination.

SUMMARY OF THE INVENTION

[0016] In one form of the invention, a reactor system and method for thermal activation of mineral particles in a continuous entrained flow flash calciner is described in which a monotonic increase in temperature of the reacting solids is achieved by indirect heating of the solids entrained by superheated steam by a counter-flowing hot combustion gas by transferring heat from the combustion gas to the reactant stream through the reactor walls; and by condensing the water in the exhaust gas to recover the heat and recover water for generation of superheated steam, and for the case of activation of carbonate minerals, to generate a carbon dioxide gas stream. [0017] In one form, the invention provides a reactor arrangement for thermal activation of a mineral to increase porosity by flash volatilisation of gas from the mineral, comprising: a flash calciner reactor having a reaction chamber and a heating chamber separated by a reactor wall, the reaction chamber and the heating chamber being in heat transfer communication through the reactor wall;

a reaction feed comprising the mineral in particulate form entrained in steam passing through the reaction chamber;

a heating fluid flow passing through the heating chamber in counterflow to the reaction feed passing through the reaction chamber;

whereby the thermal activation of the mineral proceeds by flash volatilisation of gases from the mineral in the reaction feed by means of heat transferred through reactor wall from the counterflow of heating fluid the chamber to the reaction feed, so that temperature of the mineral feed increases to reach a maximum temperature at exhaust of thermally activated mineral from the reactor.

[0018] In a further form, the invention provides method for thermal activation of a mineral of a mineral to increase porosity by flash volatilisation of gas from the mineral, comprising: providing a flash calciner reactor having a reaction chamber and a heating chamber separated by a reactor wall, the reaction chamber and the heating chamber being in heat transfer communication through the reactor wall; passing a reaction feed comprising the mineral in particulate form entrained in steam through the reaction chamber; passing a heating fluid flow through the heating chamber in counterflow to the reaction feed passing through the reaction chamber; whereby the thermal activation of the mineral proceeds by flash volatilisation of gases from the mineral in the reaction feed by means of heat transferred through reactor wall from the counterfiow of heating fluid the chamber to the reaction feed, so that temperature of the mineral feed increases to reach a maximum temperature at exhaust of thermally activated mineral from the reactor.

[0019] In preferred forms, the heating fluid is a combustion gas mixture.

[0020] In other preferred forms, the steam in the reaction feed is superheated steam.

[0021] Yet other aspects of the invention provide means for recovery of heat from the reactor output.

[0022] Further aspects of the invention will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] 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, in which Figure 1 shows a schematic drawing illustrating a thermal activation process according to an example embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0024] The embodiments for the invention are explained by the use of the invention to thermally activate minerals in two established mineral processes. The first application is the beneficiation of bauxite for the production of aluminium metal, and the second application is the beneficiation of sedimentary phosphate rocks for the production of superphosphate, principally for fertilizers.

[0025] Consider first the thermal activation of bauxite. The production of aluminium from bauxite generally uses the Bayer process in which the aluminium is extracted from the bauxite in a hot NaOH solution. In this process, other mineral constituents such as silica, iron oxide, mineral carbonates and organic carbon impurities are substantially unreacted and are discharged as red mud. However, silica and organic products are partially soluble in the hot alkaline materials and these impurities can build up in the NaOH liquor, with undesirable effects in the Hall- Herault electrolytic process for aluminium metal production. One method of limiting these impurities is to thermally activate the bauxite prior to the Bayer process so that the time or temperature for dissolution of the aluminium can be substantially reduced. This enables the use of bauxite with higher silica content in the Bayer process. This thermal activation process calcines Gibbsite (aluminium trihydroxide Al(OH)3) and Boehmite (aluminium oxide hydroxide AIO(OH)) to produce alumina (aluminium oxide A1203) and steam. It has been established that thermal activation provides conditions for dissolution of alumimum in NaOH which suppresses the dissolution of the silica and organic impurities. For bauxite, or kaolinite, in which there is a very high fraction of silica, thermal beneficiation can be used to accelerate an acid dissolution process to eliminate the silica, which is insoluble in acids. A method of beneficiation of bauxite for the Bayer process by thermal activation has been described in the prior art by Hollit et. al. (M.Hollit, S.Grocott, J.P.Kisler and C.J. Beeby, "Feed Processing for Improved Alumina Process Performance" US pat 6,582,670, and M.Hollit, S.Grocott, G.Roe, "Feed Processing for Improved Alumina Process Performance" US pat 6,661,902) and Bhargava et al (S. Bhargava, M. Allen, M. Holloit, S.Grocott, A.Hartshorn and D.B.Akolekar, Chemistry in Australia, 6-8, June 2004). In this method, direct heating of the bauxite is accomplished by combustion gases, and they used multiple segments with injection of fresh hot combustion gas in each segment to achieve an overall counterflow of heat with a coflow of the combustion gas with the reaction gases and solids. A. Rijkebour and A.P. van der Meer ("Bauxite Roasting - An Option to Reduce the Organics Input to Bayer Plant Liquor", 3rd International Alumina Quality Workshop, 1993, pp 254-269) describe a thermal activation processes in which the organic matter was pyrolysed during a calcination process such that its dissolution in the alkali during the Bayer process was supressed. They determined that the upper limit to the partial pressure of steam in the reactor was limited to about 2 kPa. On the other hand, Hollit et al found that their flash multistage process did not require such a limitation, and the combustion gas could be substantially diluted by the steam given off by the calcination reaction. [0026] Hollit et. al. describe the ideal conditions for thermal activation of bauxite, being that the exhaust temperature of the calcined solids should be highest at the exhaust from the reactor. However, the reactor design claimed by Hollis is complicated by the necessity to inject fresh hot combustion gases are a number of points in the reactor to offset the loss of heat from the endothermic reaction. The use of reactor segments to enable co-flow of the gases and solids only approximates the ideal conditions for thermal activation. It is noted that the solids mass fraction in the exhaust is reduced by the combination of the accumulated combustion gases and the steam and other gases volatilised by the calcination reaction. The efficiency of separation of the solid and gas streams is thereby reduced by the dilution of the combustion gases, and the water it at such a low partial pressure that it is generally prohibitive to condense the steam from the gas stream.

[0027] By contrast, in the design described in this invention, an externally heated reactor is used to activate the entrained mineral feedstock in a continuous flow flash calciner reactor. In this approach, steam is used to entrain the ground minerals into the reactor, and as the reaction proceeds through transfer of heat from the combustor through the walls of the reactor, additional steam is generated from bauxite so the particles are accelerated through the reactor in an entrained flow. In this approach, the temperature of the solids rises monotonically as the particles pass through the reactor, so that the solids always exhaust from the reactor at the maximum temperature. This is the ideal condition described by Hollit et al, but only

approximately achieved in their claimed reactor design.

[0028] In the continuous flash calciner reactor arrangement of Figure 1 , the calciner reactor 116 has a reactor body having an inner calciner reaction tube 200 for carrying a stream of ground mineral to be calcined, entrained in a gas stream which comprises steam, preferably superheated steam. Surrounding the calciner tube, and in heat transfer communication therewith, is a heating/combustion gas chamber 205 through which passes a flow of combustion gas, typically a fuel gas 118 such as a syn gas made from coal or from biomass, methane or natural gas, mixed with preheated air 122, to create a heat source for the calcination reaction occurring in the calciner tube. The combustion may be a flameless combustion, whereby mixture of the feed gas with air at sufficiently high temperatures may result in combustion without a flame front.

[0029] The gases in combustion gas chamber flow countercurrent to the material flow in the calciner tube, with heat transferring across the calciner tube walls to heat the steam and solids mixture passing through the calciner tube, such that the temperature of the steam and solids mixture increases monotonically along its flow path and the exhaust temperature of the calcined matrial exiting the calciner tube is at its maximum temperature.

[0030] In Figure 1 , the calciner tube follows an inverted U-shaped path with a riser portion 210 and downer portion 215. The combustion gas chamber 205 surrounding the calciner tube has a baffle 220 which forces the combustion gas to follow a path which is countercurrent to the calciner tube.

[0031] However, it will be appreciated that the calciner reactor may take other physical forms, as long 'as the objective of countercurrent heat exchange between the combustion gas and material being calcined is achieved.

[0032] The material feed to the calciner tube may be injected tangentially

(not shown), to form a vortex flow through the calciner tube, to enhance heat exchange to the material as it travels through the tube.

[0033] In an alternative form, the calciner tube may be formed as a helical tube, similar to that shown in AU2007233570, so as to urge the particle against the side wall of the tube.

[0034] The base of the calciner reactor 116 also contains a heat exchanger for recovery of heat from the exhausted combustion gas from the chamber 205, to preheat the air intake 122, and a mixing chamber 120 in which the preheated air 122, a recycled portion of the exhausted combustion gas 124 and fresh fuel gas 118 is mixed prior to injection to the combustion gas chamber 205 at 136.

[0035] Tests conducted in the development of this invention revealed that the beneficiation of bauxite could be achieved using pure steam as the gas for entraining the solids into the reactor. This result was surprising because steam is the primary calcination product, and will induce back reactions which impede calcination. However, the degree of calcination of the product from steam entrainment was not substantially different from that obtained under identical conditions except that air was used to entrain the solids for injection into the reactor. Further, the activation of the product, as measured, for example, by the rate of dissolution by alkali, or acid, of physical measurements, was equivalent, if not superior, when steam was used to entrain the solids, compared to air. The ability to use steam to entrain the bauxite solids into the reactor has a number of substantial benefits because the exhaust gas from the reactor after separating the solids is substantially pure steam, diluted only by C0 ₂ from decomposition of carbonates, and the gases produced by

pyrolysis/gasification of organic impurities. The primary benefit is that the latent heat of condensation of the water can be recovered by condensing the water, and a portion of the water can be superheated by cooling the solids. The thermal efficiency of the reactor is very high compared to the reactor described by Hollit et al because the latent heat is substantially recovered.

(0036] The steam partial pressure is only slightly reduced by the presence of the products of pyrolysis/gasification of organic materials, and by the calcination of carbonates, if any. The calcination reaction proceeds when the partial pressure of the steam is lower than the equilibrium steam pressure of the dehydroxylation reaction. In the calcination of bauxite, the lowest temperature dehydroxylation process is the decomposition of gibbsite, which has an equilibrium temperature of about 350°C at 1 bar of steam. Above this temperature there are a sequence of dehydroxylation reactions of the reaction product, which is an amorphous form of AIO(OH) and Boehmite crystals of the same chemical composition, and their dehydroxylation reactions take place at a faster rate as the temperature is raised towards the preferred exhaust temperature of about 650°C. Experiments have found that the optimum activation of the product is set by the reactor exhaust temperature. This optimum activation occurs when the degree of dehydration is less than 100%, and is typically in the range of 80-90% for a residence time of several seconds in the reactor. It would be appreciated by persons skilled in the art that the degree of dehydration and the degree of activation, as measured by the surface texture properties is a trade-off between completion of the reaction to increase the surface area, and the sintering of the previously calcined regions of the particle. In the case of bauxite activation, the sintering occurs through either the formation of crystalline phases of AIO(OH) from the amorphous solid AIO(OH) created by the flash calcination, or from the crystallisation and sintering of AI2O3. Thermogravimetric studies show that there is a continuum of processes that occur when bauxite is heated. The incomplete removal of water per se has no impact on the Bayer process because the product is rapidly hydrated.

[0037] The exhaust temperature of the solids is set by the combustor temperature and flow rate for a given mass flows and temperatures of the solids and gases at the reactor input, including the temperature and pressure of the superheated steam carrier fluid which entrains the solid material, flow rates/residence time, and the temperature and proportions of the fuel gas/recycled combustion gas/air combustion fuel mixture.

[0038] The achievement of the desired exhaust temperature is a substantive indicator that the appropriate degree of calcination and thermal activation has been achieved, and the control of the reactor system is based on this result.

[0039] The temperature of the calcined material exiting the reactor 116 is measured at sensor 134, this information being used for feedback control of the process conditions for optimising the calicination reaction. It is preferred that for calcining of bauxite the exit temperature at sensor 134 is about 550-750 °C, more preferably about 650°C.

[0040] Preferred process parameters for calcining of bauxite by the present example embodiment include a particle size of the ground bauxite feed less than about lmm, and a ratio of entrained solids to the steam carrier fluid in the feed of less than about 10:1 on a mass basis.

[0041] Residence times of 3-20s are preferred, or more preferably about 3-

10s. [0042] In designing the externally heated counterflow reactor for the thermal activation of bauxite, it is preferred that:-

(a) The reactor has efficient transfer of heat between the combustion gas and the solids/gas reactor constituents through the walls that separate these streams. This is preferably achieved by using the Vortex reactor design which has been previously applied by M.G.Sceats and CJ.Horley "System and method for the calcination of minerals" AU2007233570 to the flash calcination of carbonate minerals by others reference therein for the flash pyrolysis of biomass. In this design, the gas and particles are centrifuged against the heated wall that separates the solid/gas reactor constituents from the combustion gas, to give a high convective and conductive heat transfer to supplement, at higher temperatures, the radiative heat transfer

contributions. It is recognised that, for large plants, a large heat transfer surface area must be provided and this can be accommodated by using a reactor design with an array of throats as described by Sceats and Horely. The disclosure of AU2007233570 is incorporated herein by reference.

(b) The solids be pneumatically transported through the reactor, and this is accomplished without saltation or choking by controlling the mass flow ratio of superheated steam to solids and the diameter of the reactor tube. Because steam is generated at high pressure in the reactor, it can readily accelerate the transport of the solids through the reactor by maintaining a positive gauge pressure through the reactor.

(c) The reactor has preferably only a single source of heat, and must have a means of start-up to generate the steam. This can be done using air to start the system and then reducing the air intake as steam is generated in the calciner and recirculated. The system can be air preheated before the injection of solids. Experiments have shown that, in the case of bauxite, the degree of calcination is largely independent of whether the calcination occurs in air or steam. Surprisingly, the thermal activation of the product in steam is marginally higher than that in air. (d) The temperature of the combustion gas at the exhaust end of the reactor is substantially reduced by the transfer of heat to the reactor. Excess heat from the combustion gas can be used to preheat the solids before injection into the steam to preferably above the condensation temperature of steam, nominally 100°C. The reactor operates such that steam does not condense from the reactor upon injection of the solids into the recirculated stream.

(e) The efficiency of the reactor is optimised by minimising thermal losses from the recirculated steam so that the temperature of the gas stream injected into the reactor is close to the exhaust temperature. The simplest option is to use the hot solids to boil the water and superheat the steam, and to use the hot gas exhaust from the cyclone to preheat the solids by using the gas to pneumatically convey and preheat the bauxite particles. After separating the feedstock in the steam entrainment system, this gas contains fines from the feedstock and from the reactor. These fines are preferably separated by a ceramic filter. It is noted that the digestion of small thermally activated particles in the alkali or acid processing is faster than larger particles, even if the effect of recycling small particles leads to excessive sintering. That is, there is no substantive loss of efficiency in the Bayer process incurred by filtering out the fines before they are injected into the calciner, and mixing them with the activated solids.

[0043] In the embodiment of Figure 1 described in this invention, the conditions for thermal activation of bauxite are achieved using counter flow indirect heating and steam recirculation.

[0044] In this configuration, the ground feedstock 100 typically at ambient temperature is fed into hopper, weigh feeder pneumatic injector 102 where it is entrained in the hot entrainment gas and fines stream 104, considered later in this description. This stream generally contains superheated steam. The mass flow rates of the entrainment gas and the feedstock are sufficient to convey the solids to the intermediate cyclone and hopper 106, where the heated solids 108 are ejected from the gas stream which, after separation, at 110, contains fines from the stream 104 and the input stream 100. The heat in the stream 104 is such that temperature of the heated solids 108 is raised above the condensation temperature of steam. If required, the solids input stream can be preheated by the combustion exhaust gas 128 to ensure this condition is preferably met.

[0045] The heated solids stream 108 is injected into the gas stream 112.

This gas stream 112 is generally superheated steam at a pressure sufficient to drive the particles through the entire reactor described herein, including the reactor, cyclones and filters. The means of generating this superheated steam is considered later in this description. The preheated solids and gas stream 114 enter the calciner reactor 116, described previously, in which heat for the calcining reaction is provided by combustion of a fuel 118 which is injected into a stream of preheated air 122 and a recirculated gas stream 124. The 130 air is preheated by the combustion gas exhausting from the reactor in the heat exchanger 126 so that the exhaust gas 128 is at a low a temperature as possible for maximum thermal efficiency of the calciner.

[0046] The product stream 132 from the reactor contains the thermally activated powder, the injection gas 112 and the gas stream ejected from the particles to activate the feedstock. In the case of bauxite, the ejected gas is predominantly steam. The important control for the calciner is the temperature of the exhaust stream from the calciner measured by the sensor 134. As described above, it is this temperature, at the exhaust, which is the maximum temperature of the reaction stream. As shown in the figure, this is achieved by counterflow of the combustion gas stream 136 against the reaction stream 114. The product stream 132 is injected into a cyclone, or a set of cyclones, 138 which separates the activated solid product 140 from the exhaust gas stream 104.

[0047] The hot exhaust gas stream 104 contains the gases as well as the product fines, and is used to preheat the input feedstock stream 100 as described above. The hot solids from the cyclone are rapidly cooled in the cyclone and cyclone hopper 142 by a water stream 144 to produce the superheated high pressure steam stream 146 described above. [0048] The water injection mass flow is sufficient to extract the heat from the solids stream 140 for maximum thermal efficiency of the reactor. If this steam is in excess of that required for pneumatic transport of the solids through the reactor, the excess can be used for other applications in the plant, including the generation of electrical power (not shown). In the case where the bauxite is used for aluminium production on site, the solids stream can be produced at a temperature required to optimise the Bayer process.

[0049] The reactor exhaust gas stream 110 from the cyclone 106 contains the fines and these are separated by the filter unit 148 into a fines stream 150 and a scrubbed gas stream 152. The steam in this gas stream is condensed by condenser 154 to produce an exhaust gas stream 156 and a condensed water stream 156. The gas stream contains residual water vapour and other non-condensable gases produced in the activation process, such as CO2 from carbonates and gases produced by pyrolysis/gasification of organic matter. Air may be injected to exhaust these gases from the condenser (not shown). The condenser may be air or water cooled, as appropriate. This water stream 158 can be treated (not shown) so that a portion of the water can be reinjected into the reactor by the regulator valve 160 to the product cooler 142 to produce the superheated steam. The excess water 162 is a by-product of the activation process. If required, fresh water is added at 164.

[0050] For start-up of the reactor, the plant is preheated using the injection of air 166 using the valve 168. After air preheating, the solids are injected at 100 and an initial water charge is provided at 164. The air is turned down as the superheated steam becomes available through control of 160.

[0051] The reactor and materials handling arrangements illustrated in

Figure 1 may be applied, with adaption of process conditions as appropriate, substantially to other thermal activation processes which achieve activation from the volatilisation of steam from a solid material. Examples include the production of gypsum β-hemihydrate or soluble anhydride from gypsum, and metakaolin from kaolin, and the activation of serpentine, bentonite, perlite, vermiculite, and clays in general. Lower temperatures are required to volatilise water as water of hydration compared to hydroxyl groups.

[0052] A second example of thermal activation is the activation of ground sedimentary phosphate rocks, where the presence of carbonates, principally CaCC>3, allows flash calcination to produce CaO and liberate carbon dioxide C0 ₂ . In doing this, the surface area of the calcined particle is increased, and this enables the efficient extraction of phosphorous by acids, such as sulphuric or hydrochloric acid through the increase in the surface area available for acid attack. In addition, organic materials that would otherwise consume the acid are pyrolysed/gasified by the calcination. In this process, steam is also generated from any water present as water of hydration or hydroxyl groups. .

[0053] Experiments have shown that the degree of calcination and activation is substantially the same in an entrained flow flash calciner with indirect heating from the combustion gas with either steam or air entrainment. The calcination can occur if the entrainment is carried out in air, steam or, surprisingly, carbon dioxide and mixtures thereof. However, the use of steam for entrainment of the particles into the calciner in the present embodiments allows for a portion of the water condensed from the exhaust gas to be recycled into the reactor, preferably by boiling and superheating the steam using a heat exchanged by cooling the solids.

[0054] The described reactor design is based on indirect heating with counter flow of the combustion gas. The general operations of the reactor in the second embodiment are substantively the same as described above for the thermal activation of bauxite, except that the temperature required for calcination of CaC0 ₃ at the exhaust must exceed about 860 °C for efficient activation.

[0055] The steam has an added benefit of catalysing the reaction. In the calcination of most phosphate rocks, the steam production is limited, so that the steam is regenerated in a separate cycle to maintain the steam partial pressure at the desired level to optimise the activation. In this case, a make-up of water is required to account for losses of steam in the cycle. [0056] The calcination of phosphate rocks may also be considered in the context of Figure 1 described above for bauxite. One difference is that preferred particle size for the ground mineral feed is smaller for carbonate calcining, for example from 20-200μιη for carbonates compared to up 1mm (i.e. 1000 μηι) for gypsum or bauxite.

[0057] For phosphate rock, the predominant calcination gases at 152 are

C0 ₂ from the CaC0 ₃ found in the mineral, some steam and pyrolysis/gasification gases from organic materials. Generally, there is insufficient steam produced in the reaction to sustain the process. In this case, make-up water is added at 164 to produce the desired flow rate of water at 158 to sustain the reaction, including a provision for blow down. The gas stream contains this water, which is then condensed as described, so that 164 provides make up water arising from losses of the steam in the reactor. These losses may arise from the uncondensed water, as well as the formation of Ca(OH) ₂ in the cyclone 138 as the temperature is reduced.

[0058] Other thermal activation processes of carbonate minerals that can use the reactor design described herein include the thermal activation of magnesium oxide from magnesia or dolomite of lime from limestone or dolomite. In the case of dolomite, the temperature of the solids and gas at the exhaust reaction can be chosen at low temperature, of about 550-750 C, to selectively calcine the magnesium site to produce semidolime (as described by C.J.Horely and M.G.Sceats "A material compound and a method of fabricating the same" AU20066303828) or a temperature of 850-950 C to calcine both the calcium and magnesium sites.

[0059] Preferred solids exit temperature for calcination thermal activation of minerals by the reactor arrangement of Figure 1 are as follows:

Bauxite 550-750 °C, preferably about 650°C

Dolomite 550-650 °C to calcine Mg only

Dolomite 850-950 °C to calcine both Ca and Mg

CaCo ₃ at least 860 °C [0060] For minerals which will calcine at relatively low temperatures, such as gypsum, other heat sources such as waste heat from industrial processes or steam may be used as the heating fluid on the combustor side of the reactor.

[0061] It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

[0062] In this specification, the word "comprising" is to be understood in its "open" sense, that is, in the sense of "including", and thus not limited to its "closed" sense, that is the sense of "consisting only of. A corresponding meaning is to be attributed to the corresponding words "comprise, comprised and comprises where they appear.

[0063] While particular embodiments of this invention have been described, it will be evident to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential

characteristics thereof. The present embodiments and examples are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. It will further be understood that any reference herein to known prior art does not, unless the contrary indication appears, constitute an admission that such prior art is commonly known by those skilled in the art to which the invention relates.