[1 ] The present invention relates to a process for producing magnesium oxide from fly ash or slag comprising magnesium and iron. In particular, the process comprises leaching the fly ash or slag with an aqueous mineral acid solution, precipitating iron from the leaching solution and decomposing the dissolved magnesium salt present in the leaching solution to form magnesium oxide.

Background of Invention

[2] Fly ash and slag are large scale by-products of the coal-fired power generation and steelmaking industries respectively. A proportion of these materials is utilised industrially, for example as additives for cements. However, alkaline fly ashes and slags, including those rich in alkaline earth metals such as calcium and/or magnesium, are typically unsuitable for cement applications. The accumulation of these waste materials presents environmental and health concerns, and provides a strong incentive for the development of processes which could beneficiate this low cost and underexploited resource.

[3] Australia remains heavily reliant on the use of coal, and in particular brown coal from the state of Victoria, for power generation. Over 1 million tons of Victorian brown coal fly ash is produced annually, most of which is simply stored on-site in ash ponds. With several power stations nearing the end of their life or scheduled for closure, the remediation of this accumulated fly ash is becoming increasing urgent.

[4] Victorian brown coal fly ash is a highly alkaline fly ash, containing elevated levels of magnesium, calcium and iron, while being relatively lean in alumina and silica. In comparison to other global sources of brown coal fly ash, magnesium content is particularly high (up to about 25 weight %). However, the speciation of magnesium in the fly ash can vary substantially. In some Victorian fly ashes, periclase (MgO) is the major mineral form of magnesium, while in other cases spinel structures such as magnesioferrite (MgFe ₂ O ₄ ) predominate. As a result of this compositional variability, and the high levels of other co-extractable elements (notably iron and calcium), Victorian brown coal fly ash has previously been considered an unattractive source material for production of high purity magnesium oxide relative to traditional sources like natural deposits of magnesite and dolomite.

[5] Previous efforts for exploiting Victorian brown coal fly ash have instead focused on its potential for indirect carbon dioxide sequestration, particularly in view of the abundant quantities of this material in close proximity to power station carbon- dioxide sources. Fly ash has thus been leached with regenerative leachants such as ammonium chloride or acetic acid, with the leached magnesium and calcium salts then being precipitated as carbonates to mineralise carbon dioxide and regenerate the leaching reagent.

[6] The extraction yield of magnesium in such processes is generally low, both in terms of absolute recovery and relative to the calcium leaching recoveries from the fly ash. Leaching of magnesium was found to be particularly challenging when a substantial proportion of the magnesium is in the magnesioferrite spinel mineral form. Recovery of magnesium could be increased by using hydrochloric acid as the leachant, resulting in enhanced co-leaching of both iron and magnesium together with the calcium in the fly ash. This non-selective leaching was considered acceptable for sequestration applications, considering that the primary goal was to maximise combined calcium and magnesium ionic content for reaction with carbon dioxide, and not to selectively leach or precipitate individual compounds in high purity.

[7] There is therefore an ongoing need for new processes capable of exploiting waste or by-product sources of magnesium for the production of magnesium oxide. In particular, such processes should be able to extract and convert magnesium to acceptably pure magnesium oxide in high yields, when the magnesium is present in the source material together with at least iron, and typically both iron and calcium, and when the magnesium is present in a wide range of mineral forms. Although such a process should be particularly adapted for the production of magnesium oxide from Victorian brown coal fly ash, it is preferred that the process should be compatible with a range of different source materials, including other fly ashes, steelmaking slag and the like. [8] A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that the document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

Summary of Invention

[9] The inventors have now developed a process for producing magnesium oxide from fly ash or slag materials which contain both magnesium and iron, and optionally also calcium. In particular, the process allows the production of high purity magnesium oxide by leaching fly ash or slag with aqueous mineral acids at low pH, selectively precipitating the co-leached iron salts from the leaching solution and decomposing the leached magnesium salt to form magnesium oxide. Calcium, if present in the fly ash or slag, may optionally be excluded by either selection of the mineral acid or by selective precipitation of calcium from the leaching solution. The mineral acid may optionally be regenerated and reused in the process.

[10] Therefore, in accordance with a first aspect the invention provides a process for producing magnesium oxide from fly ash or slag comprising magnesium and iron, the process comprising: leaching the fly ash or slag with an aqueous mineral acid solution to produce a leaching solution and a residue, the leaching solution comprising a magnesium salt and an iron salt; precipitating the iron salt by increasing the pH of the leaching solution to above 3.5, and separating the leaching solution from the precipitated iron salt; and forming magnesium oxide by decomposing the magnesium salt present in the leaching solution.

[1 1 ] In some embodiments, the fly ash or slag is leached at a pH below 3, preferably below 2.5, more preferably below 2, and most preferably below 1 .5. In some embodiments, the fly ash or slag may be leached at elevated temperatures, such as between 25°C and 80°C, preferably between 30°C and 60°C. It is believed that leaching with mineral acid solutions at lower pH values and/or higher temperatures improves the recovery of magnesium from alkaline fly ash or slag, particularly when a substantial proportion of the magnesium is present in the magnesioferrite spinel mineral form. [12] Longer contact times between the fly ash or slag and the aqueous mineral acid solution will in general also increase magnesium leaching yield, although it will be appreciated that the selection of a suitable leaching contact time will in practice also be influenced by counterbalancing economic considerations. The optimum contact time for a given process may thus be less than the time required to obtain maximum possible magnesium recovery from the fly ash or slag. Suitable contact times may be between 5 minutes and 24 hours, between 10 minutes and 2 hours, or between 10 minutes and 40 minutes.

[13] Any suitable mineral acid may be used to leach the fly ash or slag, including sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid and the like. In preferred embodiments, the mineral acid comprises sulfuric acid or hydrochloric acid, and may be selected from the group consisting of sulfuric acid, hydrochloric acid and mixtures thereof. Sulfuric acid and hydrochloric acid are particularly preferred as they have been found to be effective for leaching magnesium with high recoveries from brown coal fly ash, even when the magnesium is present in spinel form. Moreover, sulfuric acid and hydrochloric acid may be recovered from or regenerated from the evolved gas produced during decomposition of the magnesium salt.

[14] In some embodiments, the fly ash or slag is leached via multistage counter-current leaching. As used herein, the first stage of a multistage counter- current leaching process refers to the stage where fresh fly ash or slag is introduced for leaching. As used herein, the final stage of a multistage counter-current leaching process refers to the last stage wherein the fly ash or slag is leached before the remaining undissolved residue exits the process. The fly ash or slag is progressively transferred from the first to the final leaching stages. The fresh mineral acid leaching solution is introduced at least partially into the final stage, and the leaching solution, containing increasing amounts of leached salts and with increasing pH due to consumption of the acid, is progressively transferred from the final to the first leaching stage before being further processed to recover and/or transform the dissolved salts.

[15] For example, the fly ash or slag may be leached in two consecutive leaching stages, with counter-current flow of the leaching solution. The second (and final) leaching stage utilizes fresh mineral acid solution to produce a second stage leaching solution comprising dissolved magnesium and iron salts, and a second stage residue. The second stage residue may optionally be discarded. The first leaching stage is performed on fresh fly ash or slag, using the acidic second stage leaching solution as at least a portion of the leaching solution, to produce a second stage leaching solution and a first stage residue. The first stage residue is transferred to the second leaching stage for further leaching. The first stage leaching solution is further processed to produce magnesium oxide by decomposing the leached magnesium salts contained therein. Although a two stage counter-current leaching process has been described herein, the skilled person will appreciate that more than two leaching stages may be utilised in a multi-stage counter-current leaching process.

[16] In some embodiments, the leaching solution is separated from the residue before precipitating the iron salt. The residue and the precipitated iron salt are then produced as separate fractions in the process. The iron salt thus recovered may itself be a useful or economically valuable by-product of the process.

[17] In other embodiments, the leaching solution is separated from the residue after precipitating the iron salt, such that the residue and precipitated iron salt at least partially co-mingle. The residue and the precipitated iron salt are then produced as a single fraction in the process. This option allows both the residue and the precipitated iron salt to be separated from the magnesium-bearing leaching solution in a single separation step, thus minimising the number of unit operations in the process.

[18] In embodiments where the fly ash or slag is leached via multistage counter-current leaching, at least the leaching solution produced from the final stage of the multistage counter-current leaching comprises both the magnesium salt and the iron salt as dissolved salts. Preferably, the leaching solution produced from the final stage of the multistage counter-current leaching is below 3, preferably below 2.5, more preferably below 2, and most preferably below 1 .5. Optionally, the leaching solution produced from each of the non-final leaching stages also comprises the magnesium salt and the iron salt as dissolved salts, with the iron then being precipitated out by increasing the pH of the first stage leaching solution to above 3.5 in a separate precipitation step. Alternatively, the first leaching stage may be operated such that the pH of the first stage leachant increases to above 3.5, thereby causing precipitation of the iron salt during the leaching. The iron salt is thus separated from the ultimate leaching solution produced via multistage counter-current leaching, and may be sent for further processing to produce magnesium oxide without the need for a separate iron precipitation step.

[19] The iron salt is precipitated by increasing the pH of the leaching solution to above 3.5. Preferably, the pH is increased to between about 3.5 and 6.5, more preferably to between about 3.5 and 5, such as between about 4 and 4.5, to allow selective precipitation of the iron salt and subsequent separation of the magnesium- bearing leaching solution from the precipitated iron salt.

[20] Any base may be used to increase the pH of the leaching solution to above 3.5 so as to precipitate the iron salt. The base may be a water soluble base, such as sodium hydroxide or lime. In other embodiments, the pH is increased by adding a substantially water-insoluble base to the leaching solution. As used herein, a substantially water-insoluble base is a solid base that does not substantially dissolve in water at neutral pH, but which increases the pH of the leaching solution by reaction with the mineral acid. The selection of a substantially water-insoluble base, which only dissolves to the extent that it reacts with the mineral acid, minimises the introduction of other solutes such as sodium, potassium or calcium into the leaching solution.

[21 ] In a preferred embodiment, the substantially water-insoluble base is itself an alkaline fly ash or slag. The same fly ash or slag leached in the process may be used to increase the pH to above 3.5. In this case, further amounts of the fly ash or slag may be added to the leaching solution after a period of leaching at a suitably low pH, either before or after separation of the leaching solution from the residue. The use of the same fly ash or slag to precipitate the iron salt advantageously eliminates the need to provide an extraneous base to the process, is readily available and cheap, and does not introduce solutes to the leaching solution that are not already present. If the fly ash or slag leached in the process is insufficiently alkaline, an inconveniently large quantity of the material, or an excessively long contact time, may be required to effect the necessary pH change. In this case, another source of suitably alkaline fly ash or slag may be used to increase the pH to above 3.5. Such a material, if available, may also be cheap and will only introduce solutes, such as magnesium and calcium salts, already present in the leaching solution. As a further alternative, magnesium oxide product from the process may be used to increase the pH, via its reactivity with mineral acids.

[22] In some embodiments, the fly ash or slag has an elemental composition comprising at least 10 weight % magnesium, preferably at least 20 weight % magnesium. As used herein, the "elemental composition" refers to the weight % of elements present in a material in their most oxidised form, i.e. an elemental composition comprising 10 weight % magnesium refers to a material which would contain 10 weight % MgO if each element in the material were converted to its oxide of highest valence. In some embodiments, the fly ash or slag has an elemental composition comprising at least 10 weight % iron, such as at least 20 weight % iron. The process of the invention is useful for producing high purity magnesium oxide from fly ash or slag sources containing economically significant quantities of magnesium, even when a high content of iron is also present.

[23] In some embodiments, the fly ash or slag is brown coal fly ash, for example Victorian brown coal fly ash. In some embodiments, the fly ash or slag comprises magnesium and iron at least partly as a magnesium-iron spinel, for example in an amount of at least 10 weight % of the magnesium-iron spinel, such as at least 20 weight % of the magnesium-iron spinel. The process of the invention has been found to be suitable for extracting substantial quantities of magnesium from brown coal fly ash even where the magnesium is predominantly in the form of magnesioferrite, which is not readily leached by milder leachants such as ammonium chloride or acetic acid.

[24] In some embodiments, the fly ash or slag further comprises calcium. The fly ash or slag may have an elemental composition comprising at least 5 weight % calcium, such as at least 10 weight % calcium, or at least 20 weight % calcium. The process of the invention is suitable for producing high purity magnesium oxide from fly ash or slag sources which contain high calcium content in addition to magnesium and iron.

[25] In some embodiments wherein the fly ash or slag further comprises calcium, the mineral acid reacts with the calcium in the fly ash or slag to form an insoluble calcium salt that remains in the residue. The leaching solution is thus substantially free of or contains only minor quantities of dissolved calcium salts before the iron salt is precipitated. The use of a mineral acid comprising, or consisting of, sulfuric acid has been found to produce an insoluble calcium salt (i.e. calcium sulfate) which remains in the leaching residue.

[26] In other embodiments wherein the fly ash or slag further comprises calcium, the mineral acid reacts with the calcium in the fly ash or slag to form a soluble calcium salt dissolved in the leaching solution together with the magnesium and iron salts. The use of hydrochloric acid as the mineral acid has been found to produce a soluble calcium salt (i.e. calcium chloride) dissolved in the leaching solution. The iron salt may then be precipitated by increasing the pH of the leaching solution to above 3.5, leaving both the magnesium and calcium salts substantially dissolved in the leaching solution.

[27] Optionally, the magnesium oxide may be formed by decomposing both the magnesium salt and soluble calcium salt present in the leaching solution to produce a mixture comprising magnesium oxide and calcium oxide. For some commercial applications of magnesium oxide, a high amount of calcium oxide (but not iron oxide) may be tolerated. It may therefore be acceptable to produce magnesium oxide from the mixed salts present in the leaching solution after iron precipitation without the necessity of performing an additional calcium separation step.

[28] In other embodiments where the leaching solution contains a soluble calcium salt, the calcium salt is precipitated from the leaching solution by adding a source of insolubilising anion, and the leaching solution is then separated from the precipitated calcium salt before forming magnesium oxide. The calcium salt may be precipitated either before or after precipitating the iron salt, preferably after precipitating the iron salt. Optionally, the iron and calcium salts may be precipitated sequentially (or simultaneously), but separated from the magnesium-containing leaching solution in a single separation step to produce a mixed iron and calcium salt by-product.

[29] Any suitable source of insolubilising anion may be used, provided that it selectively precipitates calcium while leaving the magnesium salt substantially dissolved in the leaching solution. Suitable sources of insolubilising anions include sulfuric acid and citric acid, which precipitate calcium as calcium sulfate and calcium citrate respectively. Salts of sulfuric acid and citric acid (for example magnesium sulfate or sodium citrate) may also be suitable sources of insolubilising anions, provided that the cation thereby introduced to the leaching solution may be tolerated when subsequently forming magnesium oxide. The source of insolubilising anion may be added in stoichiometric excess relative to the calcium. Preferably, however, the amount of the source is minimised to avoid contaminating the leaching solution with additional soluble components that may be detrimental for the subsequent formation of magnesium oxide, or the purity of magnesium oxide thus formed.

[30] In some embodiments, the magnesium salt is recovered as a solid from the leaching solution before decomposing the magnesium salt to form the magnesium oxide. The magnesium salt may be recovered as a solid by precipitating the magnesium salt by at least one of i) concentrating the magnesium salt by removing a portion of the water in the leaching solution; ii) cooling the leaching solution; iii) and adding a water-miscible liquid to the leaching solution, wherein the magnesium salt is insoluble in the water-miscible liquid. Suitable water-miscible liquids include alcohols such as ethanol. The precipitated magnesium salt may then be recovered from the supernatant by a suitable means such as filtration and dried before thermal decomposition. Alternatively, the magnesium salt may be recovered as a solid from the leaching solution by any other suitable method, including evaporating off all of the water in the leaching solution.

[31 ] In some embodiments, decomposing the magnesium salt comprises heating the magnesium salt to a temperature sufficient to produce magnesium oxide and an evolved gas. In embodiments where the magnesium salt present in the leaching solution comprises magnesium sulfate (such as when the mineral acid solution comprises sulfuric acid), the magnesium sulfate thermally decomposes to produce MgO and an evolved gas comprising both SO ₂ and SO ₃ . Magnesium sulfate may be thermally decomposed at temperatures of between 600°C and 1000°C, preferably between 800°C and 950°C, such as about 900°C. In embodiments where the magnesium salt present in the leaching solution comprises magnesium chloride (such as when the mineral acid solution comprises hydrochloric acid) the magnesium chloride hydrates recovered as a solid from the leaching solution thermally decompose to produce MgO and an evolved gas comprising HCI. Magnesium chloride hydrate may be thermally decomposed at temperatures of between 300 and 600°C, preferably between 350°C and 450°C, such as about 400°C.

[32] In some embodiments, a regenerated mineral acid solution is produced from the evolved gas. Where the evolved gas comprises SO ₂ and SO ₃ , a regenerated sulfuric acid solution may be produced by catalytic oxidation of the SO ₂ component to SO ₃ , and hydration of the SO ₃ in water or weak sulfuric acid. Where the evolved gas comprises HCI, a regenerated hydrochloric acid solution may be produced by absorption of the gaseous HCI into water. The fly ash or slag may then be leached with an aqueous mineral acid solution comprised at least in part of the regenerated mineral acid solution. This advantageously reduces the overall consumption of mineral acid in the process.

[33] In some embodiments, the process may include suitable steps for preparing the fly ash or slag for leaching. For example, the fly ash or slag may be washed with water to remove water-soluble components, such as sodium or potassium salts, before leaching with the aqueous mineral acid solution. Although most suitable sources of fly ashes or slags, unlike natural magnesium ores, are advantageously already in fine particulate form, the process may include additional steps of grading or comminuting the fly ash or slag prior to leaching.

[34] The fly ash or slag may be leached in a single leaching stage or multiple leaching stages, for example a multistage counter-current leaching process as described herein. Each leaching stage may individually be a batch leaching process or a continuous leaching process. In some embodiments, the fly ash or slag is leached in at least one CSTR leaching vessel.

[35] The leaching solution may be separated from the residue by conventional means, including thickening and filtration. For embodiments including one or more subsequent and separate precipitation steps from the leaching solution (i.e. iron salt precipitation and/or calcium salt precipitation and/or magnesium salt precipitation) the precipitated salts may also be separated from the supernatant leaching solution by conventional means, including filtration. [36] The inventors have further discovered that the use of an aqueous sulfuric acid solution as leachant may be advantageous for producing magnesium oxide from fly ash or slag materials containing magnesium, iron and optionally calcium, in accordance with the invention. In particular, it is believed that calcium in the fly ash or slag is converted to insoluble calcium sulfate, thus remaining in the residue, while magnesium and iron dissolve selectively into the sulfuric acid-based leaching solution.

[37] Therefore, in accordance with a second aspect, the invention provides a process for producing magnesium oxide from fly ash or slag comprising magnesium and iron, the process comprising: leaching the fly ash or slag with an aqueous sulfuric acid solution to produce a leaching solution and a residue, the leaching solution comprising magnesium sulfate and an iron salt; precipitating the iron salt by increasing the pH of the leaching solution to above 3.5, and separating the leaching solution from the precipitated iron salt; and forming magnesium oxide by decomposing the magnesium sulfate present in the leaching solution.

[38] The inventors have further discovered that the process of the invention is particularly suited for producing magnesium oxide from fly ash or slag materials which contain each of magnesium, iron and calcium. In particular, the process allows the production of high purity magnesium oxide by leaching fly ash or slag with aqueous mineral acids, excluding calcium from the leaching solution by either selection of the mineral acid to prevent calcium dissolution or by selective precipitation of co-leached calcium from the leaching solution, selectively precipitating the co-leached iron salts from the leaching solution by increasing the pH and decomposing the leached magnesium salt to form magnesium oxide.

[39] Therefore, in accordance with a third aspect, the invention provides a process for producing magnesium oxide from fly ash or slag comprising magnesium, iron and calcium, the process comprising: leaching the fly ash or slag with an aqueous mineral acid solution to produce a leaching solution and a residue, the leaching solution comprising a magnesium salt and an iron salt; separating the calcium from the leaching solution by either: i) reacting the mineral acid with the calcium in the fly ash or slag to form an insoluble calcium salt that remains in the residue, or ii) reacting the mineral acid with the calcium in the fly ash or slag to form a soluble calcium salt in the leaching solution, precipitating the calcium salt from the leaching solution by adding a source of insolubilising anion, and separating the leaching solution from the precipitated calcium salt; precipitating the iron salt by increasing the pH of the leaching solution to above 3.5, and separating the leaching solution from the precipitated iron salt; and forming magnesium oxide by decomposing the magnesium salt present in the leaching solution.

[40] In accordance with a fourth aspect, the invention provides magnesium oxide, produced by the process according to any of the embodiments disclosed herein. Notably, magnesium oxide produced by the process of the invention is obtained in high purity and is free of asbestos, which is an inevitable impurity in magnesium oxide produced from natural ores such as magnesia.

[41 ] Where the terms "comprise", "comprises" and "comprising" are used in the specification (including the claims) they are to be interpreted as specifying the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

[42] Further aspects of the invention appear below in the detailed description of the invention.

Brief Description of Drawings

[43] Embodiments of the invention will herein be illustrated by way of example only with reference to the accompanying drawings in which:

[44] Figure 1 depicts a schematic flowsheet of a process for producing magnesium oxide from fly ash or slag according to an embodiment of the invention.

[45] Figure 2 depicts a schematic flowsheet of a process for producing magnesium oxide from fly ash or slag according to another embodiment of the invention.

[46] Figure 3 depicts XRD patterns for sample A (Hazelwood power station) and sample B (Yallourn power station) fly ash.

[47] Figure 4 depicts a graph of the concentration of dissolved elements in the leaching solution while leaching sample A fly ash with H ₂ SO ₄ in Experiment 1 . [48] Figure 5 depicts the XRD pattern of the residue of sample A fly ash after leaching with H ₂ SO ₄ in Experiment 1 .

[49] Figure 6 depicts the XRD pattern of the iron precipitate (recovered together with the sample A fly ash used to increase pH), that precipitated from the leaching solution in Experiment 1 .

[50] Figure 7 depicts the XRD pattern of the magnesium salt recovered after removal of the water from the leaching solution in Experiment 1 .

[51 ] Figure 8 depicts a graph of the pH and concentration of dissolved elements in the leaching solution while leaching sample A fly ash with H ₂ SO ₄ in Experiment 2.

[52] Figure 9 depicts a graph of the pH and concentration of dissolved elements in the leaching solution while leaching sample B fly ash with H ₂ SO ₄ in Experiment 3.

[53] Figure 10 depicts the XRD pattern of the residue of sample B fly ash after leaching with H ₂ SO ₄ in Experiment 3.

[54] Figure 1 1 depicts the XRD pattern of the iron precipitate (recovered together with the sample A fly ash used to increase pH), that precipitated from the leaching solution in Experiment 3.

[55] Figure 12 depicts the XRD pattern of the magnesium salt recovered after removal of the water from the leaching solution in Experiment 3.

[56] Figure 13 depicts the XRD pattern of the magnesium salt recovered after precipitation from the leaching solution by addition of ethanol in Experiment 4.

[57] Figure 14 depicts a graph of the concentration of dissolved magnesium in the leaching solution while leaching sample B fly ash with H ₂ SO ₄ at three different temperatures in Experiment 5.

[58] Figure 15 depicts a graph of the concentration of dissolved elements in the leaching solution while leaching sample A fly ash with HCI in Experiment 6. [59] Figure 16 depicts the XRD pattern of the calcium precipitate that precipitated from the leaching solution after addition of H ₂ SO ₄ in Experiment 6.

[60] Figure 17 depicts a graph of the concentration of dissolved elements in the leaching solution while leaching sample B fly ash with HCI in Experiment 7.

Detailed Description

[61 ] The present invention relates to a process for producing magnesium oxide from fly ash or slag comprising magnesium and iron. The process comprises leaching the fly ash or slag with an aqueous mineral acid solution to produce a leaching solution and a residue, where the leaching solution comprises dissolved magnesium and iron salts. The iron salts are precipitated by increasing the pH of the leaching solution to above 3.5, and the leaching solution is separated from the precipitated iron salt. Magnesium oxide is then formed by decomposing the magnesium salt present in the leaching solution. The mineral acid may optionally be sulfuric acid or hydrochloric acid, and in some embodiments is sulfuric acid. When the fly ash or slag further comprises calcium, the calcium may be excluded from the leaching solution by reacting the mineral acid (typically sulfuric acid) with the calcium in the fly ash or slag to form an insoluble calcium salt that remains in the residue. Alternatively, the calcium may be excluded by reacting the mineral acid (typically hydrochloric acid) with the calcium in the fly ash or slag to form a soluble calcium salt in the leaching solution, precipitating the calcium salt from the leaching solution by adding a source of insolubilising anion, and separating the leaching solution from the precipitated calcium salt.

[62] The inventors have found that the process of the invention may be used to obtain high recoveries of magnesium from fly ash or slag, even when a substantial proportion of the magnesium is in a spinel form such as magnesioferrite. High purity magnesium oxide may be produced in accordance with the process of the invention, despite the presence of co-leachable elements such as iron and calcium. The magnesium oxide thus produced is advantageously free of asbestos which is typically present in magnesium oxide produced from natural magnesium-bearing ores. Optionally, other co-products from the process can be usefully exploited, including the leached fly ash or slag residue (which may be suitable for cement applications as a result of the acidic leaching treatment), the precipitated iron salt, and the precipitated calcium salt (if formed). The mineral acid may advantageously be regenerated and reused in the process.

[63] An embodiment of the invention will now be described with specific reference to Figure 1 . In the process depicted in Figure 1 , fly ash 10 is leached with aqueous sulfuric acid solution 1 1 in leaching section 12. Leaching section 12 is a multistage counter-current leaching section comprising first leaching stage 13 and second (and final) leaching stage 14. Fresh fly ash 10 is introduced to first leaching stage 13 where it is subjected to first stage leaching using intermediate leaching solution 15 from second leaching stage 14 as the leachant. The leaching slurry in first leaching stage 13 is separated in solid-liquid separation unit 16 into intermediate residue 17 and leaching solution 18. Intermediate residue 17 is then transferred to second leaching stage 14 where it is subjected to second stage leaching using fresh aqueous sulfuric acid solution 1 1 as the leachant. The leaching slurry in second leaching stage 14 is separated in solid-liquid separation unit 19 into residue 20 and intermediate leaching solution 15.

[64] First leaching stage 13 and second leaching stage 14 may independently be operated in batch mode or continuous mode. Solid-liquid separation units 16 and 19 may be either separate to or integrated within the leaching vessels of the first and second leaching stages 13 and 14, respectively. Although leaching section 12 in the embodiment depicted in Figure 1 is a two stage counter-current leaching section, it will be appreciated that more than two leaching stages may be employed, and that leaching may alternatively be performed in a single leaching stage only.

[65] Fly ash 10, which is a Victorian brown coal fly ash containing magnesium, iron and calcium, may optionally be pre-treated before leaching (not shown). For example, it may be washed with water to remove unwanted water soluble components. The particle size of fly ash 10 introduced to first leaching stage 13 may also be adjusted by comminution and/or grading if required. Aqueous sulfuric acid solution 1 1 , which has a sulfuric acid concentration of between 20 weight % and 40 weight % and a pH below 1 , is formed at least in part with sulfuric acid regenerated in the process, as further described hereafter. [66] Leaching of intermediate residue 17 in second leaching stage 14 is operated such that the pH of intermediate leaching solution 15 is below 3.0, preferably below 2.5. It will be appreciated that this may be achieved by controlling, among other variables, the relative feed rates and contact time of intermediate residue 17 and fresh sulfuric acid solution 1 1 , and the leaching temperature. The skilled person, with the benefit of this disclosure, will be able to control the pH of intermediate leaching solution 15.

[67] In the embodiment depicted in Figure 1 , leaching of fresh fly ash 10 in first leaching stage 13 is operated such that the pH of leaching solution 18, while higher than the pH of intermediate leaching solution 15, remains below the precipitation pH of the leached iron salt, and is preferably also below 3. It will be appreciated that this may be achieved by controlling, among other variables, the relative feed rates and contact time of fresh fly ash 10 and intermediate leaching solution 15, the pH of intermediate leaching solution 15, and the leaching temperature. Furthermore, it is not excluded that fresh sulfuric acid solution may be added to first leaching stage 13 to supplement intermediate leaching solution 15. The skilled person, with the benefit of this disclosure, will be able to control the pH of leaching solution 18 below at least the precipitation pH of the leached iron salt, such that leaching solution 18 comprises both the magnesium salt and the iron salt leached from fly ash 10.

[68] The products of leaching section 12 include leaching solution 18 and residue 20. Residue 20 includes calcium at least partially in the form of insoluble calcium sulfate as a result of reaction between the calcium component of fly ash 10 and sulfuric acid. Residue 20 may further comprise un-extracted magnesium and iron and other elements such as aluminium and silicon. Residue 20 exits the process, and may be discarded. Alternatively, residue 20 may be suitable for use as a cement additive, considering that its alkalinity has been substantially reduced by the leaching process. Leaching solution 18 comprises dissolved magnesium salt and iron salt, at least partially in the form of magnesium sulfate and iron sulfate (believed to be mainly iron (III) sulfate) respectively, which is leached from fly ash 10 in leaching section 12. Leaching solution 18 is substantially free of or contains only minor quantities of dissolved calcium salts. [69] In the process depicted in Figure 1 , leaching solution 18 is transferred from leaching section 12 to iron precipitation section 21 , which includes iron precipitator 22. Alkaline ash 23, or alternatively magnesium oxide product from the process, is added to leaching solution 18 in iron precipitator 22 in an amount sufficient to increase the pH of leaching solution 18 to above 3.5, preferably between 4 and 4.5. The pH increase causes the iron salt to precipitate out of leaching solution 18. Iron-rich precipitate 24, including the precipitated iron salt and the undissolved portion of alkaline ash 23, is then separated from iron-depleted leaching solution 25, which exits iron precipitation section 21 . Iron-rich precipitate 24 is believed to contain the precipitated iron at least partly in the form of iron sulfate, and may further include smaller amounts of other salts precipitated from the leaching solution at elevated pH, including aluminium and silicon salts.

[70] Alkaline ash 23 is preferably the same material as fly ash 10. However, if insufficiently alkaline, a different source of alkaline ash or another suitable substantially water-insoluble base may be employed instead. Alternatively, if preferred, a water-soluble base may be used instead of alkaline fly ash 23, so that a high purity iron precipitate may be obtained as a co-product of the process.

[71 ] In the process depicted in Figure 1 , iron-depleted leaching solution 25 is transferred from iron precipitation section 21 to magnesium salt recovery section 26, which includes magnesium precipitator 27. Magnesium sulfate 28 is precipitated out of iron-depleted leaching solution 25 and recovered as a solid. Magnesium sulfate 28 may be precipitated by adding ethanol to iron-depleted leaching solution 25. Alternatively or additionally, magnesium sulfate 28 may be precipitated by removing a sufficient portion of the water (optionally by indirectly heating with the hot evolved gases produced in the decomposition section) or by cooling iron-depleted leaching solution 25. Magnesium sulfate 28 may be dried (not shown) before exiting magnesium salt recovery section 26.

[72] Solid magnesium sulfate 28 is then transferred from magnesium salt recovery section 26 to decomposition section 29, where it is thermally decomposed at about 900°C in decomposer 30 to produce high purity magnesium oxide 31 and evolved gas 32. Evolved gas 32, comprising both SO ₂ and SO _3, is catalytically oxidised in catalytic converter 33 over a vanadium (V) oxide catalyst at about 400°C to produce SO ₃ gas stream 34. Gas stream 34 is then cooled to about 120°C, optionally using the heat to precipitate magnesium sulfate 28 from iron-depleted leaching solution 25 (not shown). Gas stream 34 is then bubbled through water or weak sulfuric acid in water absorber 35 to produce concentrated regenerated sulfuric acid 36. Regenerated sulfuric acid 36 may then be included as at least a portion of aqueous sulfuric acid solution 1 1 for leaching fly ash 10.

[73] Another embodiment of the invention will now be described with specific reference to Figure 2. In the process depicted in Figure 2, fly ash 50 is leached with aqueous hydrochloric acid solution 51 in leaching section 52. Leaching section 52 is configured similarly to leaching section 12 depicted in Figure 1 , having first leaching stage 53 with solid-liquid separation unit 56 and second leaching stage 54 with solid- liquid separation unit 59. Fresh fly ash 50 is leached in first leaching stage 53 with intermediate leaching solution 55. Intermediate residue 57 is then transferred to second leaching stage 54 for further leaching, and residue 60 finally exits the process. The leaching solution flow is counter-current to the fly ash flow, with fresh aqueous hydrochloric acid solution 51 being introduced as the leachant into second leaching stage 54. Intermediate leaching solution 55 is transferred from second leaching stage 54 to first leaching stage 53, and leaching solution 58 then flows out of leaching section 52 for further processing. Aqueous hydrochloric acid solution 51 , which has a hydrochloric acid concentration of between 20 weight % and 40 weight % and a pH below 1 , is formed at least in part with hydrochloric acid regenerated in the process, as further described hereafter.

[74] Leaching of intermediate residue 57 in second leaching stage 54 is operated such that the pH of intermediate leaching solution 55 is below 3.0, preferably below 2.5. In the embodiment depicted in Figure 2, leaching of fresh fly ash 50 in first leaching stage 53 is operated such that the pH of leaching solution 58, while higher than the pH of intermediate leaching solution 55, is below the precipitation pH of the leached iron salt, and is preferably also below 3. Optionally, fresh hydrochloric acid solution may be added to first leaching stage 53 to supplement intermediate leaching solution 55. [75] The products of leaching section 52 include leaching solution 58 and residue 60. Residue 60 may comprise un-extracted magnesium, iron and calcium, as well as other elements such as aluminium and silicon. Leaching solution 58 comprises dissolved magnesium, iron and calcium salts, at least partially in the form of magnesium chloride, iron chloride and calcium chloride respectively, which are leached from fly ash 50 in leaching section 52.

[76] In the process depicted in Figure 2, leaching solution 58 is transferred from leaching section 52 to iron precipitation section 61 , which includes iron precipitator 62. Alkaline ash 63, or alternatively magnesium oxide product from the process, is added to leaching solution 58 in iron precipitator 62 in an amount sufficient to increase the pH of leaching solution 58 to above 3.5, preferably between 4 and 4.5. The pH increase causes the iron salt to precipitate out of leaching solution 68. Iron-rich precipitate 64, including the precipitated iron salt and the undissolved portion of alkaline ash 63, is then separated from iron-depleted leaching solution 65 which exits iron precipitation section 61 .

[77] As depicted in Figure 2, iron-depleted leaching solution 65 is transferred from iron precipitation section 61 to calcium precipitation section 66, which includes calcium precipitator 67. Sulfuric acid 68 is added as a source of calcium insolubilising anion to iron-depleted leaching solution 65 in calcium precipitator 67, in an amount sufficient to precipitate the calcium salt as calcium sulfate (gypsum) 69. Preferably, only the minimum amount of sulfuric acid 68 required to precipitate the calcium is added, since a larger excess will result in subsequent recovery of mixed magnesium sulfate and magnesium chloride. Calcium sulfate precipitate 69 is then separated from calcium-depleted leaching solution 70, which exits calcium precipitation section 66. Calcium sulfate precipitate 69 may be optionally be discarded, or used or sold as a co-product of the process.

[78] Although calcium precipitation section 66 is depicted in Figure 2 as following after iron precipitation section 61 , it will be appreciated that the order of these operations may be reversed. Furthermore, if fly ash 50 contains sufficiently low amounts of calcium, or if a high amount of calcium oxide can be tolerated in the magnesium oxide product of the process, calcium precipitation section 66 may be omitted entirely.

[79] As depicted in Figure 2, calcium-depleted leaching solution 70 is transferred from calcium precipitation section 66 to magnesium salt recovery section 71 , which includes magnesium precipitator 72. Magnesium chloride 73 is precipitated out of calcium-depleted leaching solution 70 and recovered as a solid. Magnesium chloride 73 may be precipitated by adding ethanol to calcium-depleted leaching solution 70, or by other suitable methods described herein (including partial evaporation of the water). Magnesium chloride 73 may be dried (not shown) before exiting magnesium salt recovery section 71 .

[80] Solid magnesium chloride 73 is then transferred from magnesium salt recovery section 71 to decomposition section 74, where it is thermally decomposed at about 400°C in decomposer 75 to produce high purity magnesium oxide 76 and evolved gas 77. Evolved gas 77, comprising gaseous hydrochloric acid, is then cooled to about 120°C, optionally using the heat to precipitate magnesium chloride 73 from calcium-depleted leaching solution 70 (not shown). Evolved gas 77 is then bubbled through water in water absorber 78 to produce concentrated regenerated hydrochloric acid 79. Regenerated hydrochloric acid 79 may then be included as at least a portion of aqueous hydrochloric acid solution 51 for leaching fly ash 50.

EXAMPLES

[81 ] The present invention is described with reference to the following examples. It is to be understood that the examples are illustrative of and not limiting to the invention described herein.

Materials

[82] Samples of brown coal fly ash were recovered from the electrostatic precipitators at the International Power Hazelwood and TruEnergy Yallourn power stations, located in the LaTrobe Valley, Victoria, Australia. The samples (hereafter designated sample A, for Hazelwood fly ash, and sample B, for Yallourn fly ash) were obtained as dry powers with a particle size of less than 150 microns. The elemental composition of samples A and B (defined as the weight % of elements in their most oxidised state) as show in Table 1 .

Table 1. Elemental composition of samples A and B.

[83] The mineral composition of samples A and B was investigated by X-Ray Diffraction (XRD) analysis, with the XRD diffraction patterns presented in Figure 3. XRD peak fitting (and thus mineral identification) was performed using Jade software. A key difference between the samples is that the magnesium in sample A is predominantly in the form of periclase (MgO), whereas the magnesium in sample B is predominantly in the form of the spinel compound magnesioferrite (MgFe ₂ O ₄ ).

Example 1. H2SO4 leaching of Sample A fly ash, with post-separation iron precipitation

[84] A mass of 50 grams of sample A fly ash was added incrementally over 1 0 minutes to 150 ml of a solution of 15% H ₂ SO ₄ in water (initial pH of 0.46). The mixture, with a final liquid:solid ratio of 3, was stirred for a total of 30 minutes at room temperature. The pH of the leaching solution was measured as 2.4 after 1 minute. The composition of dissolved elements in the leaching solution over the leaching period is shown in Figure 4. Iron and magnesium were rapidly extracted into the acidic leaching solution, while only low levels of dissolved aluminium and calcium were detected. After 30 minutes, the leaching solution was separated from the residue by filtration. The final pH of the leaching solution was 2.81 .

[85] A further amount of sample A fly ash was then gradually added to the filtered leaching solution with stirring to raise the pH to 4.01 (5.0 g ash was required to effect this pH change). At the increased pH, a precipitate formed from the solution, and the dark red-brown colour of the leaching solution changed to light yellow. The supernatant leaching solution was separated from the combined solids (i.e. the mixture of the precipitate and the further amount of sample A ash) by filtration. The mass of combined solids recovered was 13.8 g. The composition of dissolved elements in the leaching solution, before and after the pH change, is shown in Table 2.

Table 2. Dissolved elemental composition of sample A leaching solution, before and after pH increase.

[86] The solid residue from the leaching step, and the combined solids recovered from the precipitation step (precipitate plus added sample A ash), were then dried and analysed to determine both elemental composition and mineral speciation. The results are shown in Table 3 and Figures 5 and 6.

[87] As can be seen from Table 3 and Figure 5, the residue of sample A ash after low pH leaching with H ₂ SO ₄ is dominated by calcium sulfate in the form of anhydrite (CaSO ₄ ). The calcium present in sample A has thus reacted with sulfuric acid, but substantially remains as a solid in the residue due to the insolubility of the calcium sulfate product. By contrast, substantial portions of the iron and magnesium are leached from the residue into the acidic leachant as salts.

[88] As can also be seen from Table 3, the solids separated from the pH- adjusted, post-precipitation leaching solution comprise elevated levels of iron, corresponding to the dissolved iron precipitated from the leaching solution (c.f. Table 2). Figure 6 shows the XRD pattern for the combined solids recovered in the precipitation step - iron (mainly precipitated from the leaching solution) and calcium (mainly from the ash added to increase pH) are at least partially present in the form of rozenite (FeSO ₄ .4H ₂ O) and bassanite (CaSO ₄ .5H ₂ O) respectively.

[89] The filtered leaching solution (pH 4.01 ) was then evaporated by heating the solution in a water bath at 100°C. A white powder of magnesium sulfate was recovered after the water was removed. The precipitate was recovered by filtration and dried. The elemental composition of the recovered solid is show in Table 3 and the XRD pattern is depicted in Figure 7. The elemental analysis reveals that highly pure magnesium sulfate (>95%) is obtained from the process. The XRD data indicates that the magnesium sulfate is produced as a mixture including sanderite (MgSO ₄ .2H ₂ O) and kieserite (MgSO ₄ .H ₂ O).

Table 3. Elemental composition of sample A starting material, leached residue, combined solids recovered from precipitation at elevated pH, and MgSO ₄ subsequently recovered by evaporative precipitation.

Example 2. H2SO4 leaching of Sample A fly ash, with pre-separation iron precipitation

[90] Sample A fly ash was added incrementally to 100ml of a solution of 15% H ₂ SO ₄ in water (initial pH of 0.49) over a period of 15 minutes while stirring at room temperature. Fly ash was added until the pH of the leaching solution exceeded 4.0, which was achieved by adding 51 .2 grams of fly ash (final liquidisolid ratio of 2). The pH and composition of dissolved elements in the leaching solution during the course of leaching is shown in Figure 8. At the initial stages of leaching in the first 8 minutes (i.e. with low leaching pH and high liquid:solid ratio), both iron and magnesium were rapidly extracted into the acidic leaching solution, while very low levels of dissolved aluminium and calcium were detected. As the pH increased to about 4, the amount of dissolved iron in the leaching solution decreased to substantially zero, indicating that the iron precipitated at this pH onto the leaching residue.

Example 3. H2SO4 leaching of Sample B fly ash, with post-separation iron precipitation and subsequent evaporative recovery of MqS0

[91 ] A mass of 84 grams of sample B fly ash was added incrementally over 10 minutes to 100ml of a solution of 15% H ₂ SO ₄ in water (initial pH of 0.46). The mixture, with a final liquid:solid ratio of 1 .2, was stirred for a total of 35 minutes at room temperature. The pH and composition of dissolved elements in the leaching solution over the leaching period is shown in Figure 9. Iron and magnesium were gradually extracted into the acidic leaching solution throughout the 35 minutes leaching time, while only low levels of dissolved aluminium and calcium were detected. It is believed that the slow rate of leaching, relative to the sample A ash (see Example 1 ), is due to the intimate association of iron and magnesium in the less reactive magnesioferrite spinel mineral form. After 35 minutes, the leaching solution was separated from the residue by filtration. The final pH of the leaching solution was 2.52.

[92] Sample A fly ash was then gradually added to the filtered leaching solution with stirring to raise the pH to 4.19 (6.5 g ash was required to effect this pH change). Sample B ash was found to be less suitable for this task, as an excessively large amount was required due to its low alkalinity. At the increased pH, a precipitate formed from the solution, and the dark red-brown colour of the leaching solution changed to light yellow. The supernatant leaching solution was separated from the combined solids (i.e. the mixture of the precipitate and the sample A ash) by filtration. The composition of dissolved elements in the leaching solution, before and after the pH change, is shown in Table 4.

Table 4. Dissolved elemental composition of sample B leaching solution, before and after pH increase. Leaching solution Mg (ppm) Fe (ppm) Ca (ppm) Al (ppm) Na (ppm)

Before precipitation 24700 25399 419 3161 826 (pH 2.52)

After precipitation 25222 291 603 43 839 (pH 4.19)

[93] The solid residue from the leaching step and the combined solids recovered from the precipitation step (precipitate plus added sample A ash) were then dried and analysed to determine both elemental composition and mineral speciation. The results are shown in Table 5 and Figures 10 and 1 1 .

[94] As can be seen from Table 5, both iron and magnesium were leached from the sample B ash, but with much lower extraction recoveries compared with sample A ash (c.f. Table 3). Calcium was not substantially leached, remaining in the residue as calcium sulfate in the form of basanite (CaSO ₄ .5H ₂ O); see Figure 10. Other significant components of the residue identifiable by XRD were residual magnesioferrite (MgFe2O ₄ ) and quartz (S1O2).

[95] As can also be seen from Table 5, the solids separated from the pH- adjusted, post-precipitation leaching solution comprise elevated levels of iron (relative to the sample A ash onto which the precipitate formed), corresponding to the dissolved iron precipitated from the leaching solution (c.f. Table 4). Figure 1 1 shows the XRD pattern for the combined solids recovered in the precipitation step - iron and calcium are at least partially present in the form of rozenite (FeSO ₄ .4H ₂ O) and bassanite (CaSO ₄ .5H ₂ O) respectively.

[96] The filtered leaching solution (pH 4.19) was then split into two equal portions. The first portion was evaporated by heating the solution in a water bath at 100°C. A white powder of magnesium sulfate was recovered upon removal of the water. The precipitate was recovered by filtration and dried. The elemental composition of the recovered solid is shown in Table 5 and the XRD analysis is depicted in Figure 12. The elemental analysis reveals that highly pure magnesium sulfate (>95%) is obtained from the process. The XRD data indicates that the magnesium sulfate is produced as kieserite (MgSO ₄ .H ₂ O), with the hexahydrate form (MgSO ₄ .6H ₂ O) also being present.

Table 5. Elemental composition of sample B starting material, leached residue, combined solids recovered from precipitation at elevated pH 4, and MgSO ₄ subsequently recovered by evaporative precipitation.

Example 4. H?SQ ₄ leaching of Sample B fly ash, with solvent-mediated precipitation of MqS0 ₄

[97] The second portion of the filtered leaching solution (pH 4.19) from Example 3 was mixed with ethanol, at a leaching solution :ethanol ratio of approximately 1 :1 . The resulting white precipitate was recovered by filtration and dried. The elemental composition of the recovered solid is show in Table 6, and the XRD analysis is depicted in Figure 13. The analysis reveals that highly pure magnesium sulfate (>95%) is produced, predominantly as kieserite (MgSO ₄ .H ₂ O), with the hexahydrate form (MgSO ₄ .6H ₂ O) also being present.

Table 6. Elemental composition of magnesium sulfate recovered by solvent- mediated and evaporative precipitation from leaching solution. Solid SiO ₂ AI ₂ O ₃ CaO MgO Fe ₂ O ₃ Na ₂ O K ₂ O SO ₃ P2O5

Ethanol- 0.51 0.74 0.40 53.94 0.05 n.d. 0.43 43.50 0.15 precipitated

MgSO ₄

Evaporation- 0.47 0.74 0.45 53.03 0.07 n.d. 0.37 44.47 0.15 precipitated

MgSO ₄

Example 5. H?SQ ₄ leaching of Sample B fly ash at elevated temperatures

[98] In three comparative experiments, a mass of 25 grams of sample B fly ash was added incrementally over 8 minutes to 37.5 ml of a solution of 15% H ₂ SO ₄ in water (initial pH of 0.46) while maintaining the leaching temperature at room temperature (29°C), 40°C and 60°C respectively. The mixtures were stirred for a total of 40 minutes at room temperature. The dissolved magnesium content in the leaching solution over the leaching period is shown in Figure 14 for the three reactions. The rate of magnesium leaching increased at elevated temperatures, with the best result being obtained at 40°C. In both the 40°C and 60°C experiments, the leached magnesium precipitated out of solution before the end of the experiment, with the leaching reaction at 60°C possibly being affected by a loss of water due to evaporation.

Example 6. HCI leaching of Sample A fly ash

[99] A mass of 50 grams of sample A fly ash was added incrementally over 8 minutes to 75 ml of a solution of 32% HCI in water (initial pH of 0.96). The mixture, with a final liquidisolid ratio of 1 .5, was stirred for a total of 30 minutes at room temperature. The composition of dissolved elements in the leaching solution over the leaching period is shown in Figure 15. Iron, calcium and magnesium were extracted into the acidic leaching solution. After 30 minutes, the final pH of the leaching solution was 2.25.

[100] A further amount of sample A fly ash was then gradually added to the unfiltered leaching solution with stirring to raise the pH to 4.02 (25.8 g ash was required to effect this pH change). At the increased pH, a precipitate of iron formed from the solution and the red colour of the solution disappeared. The supernatant leaching solution was separated from the combined solids (i.e. the mixture of the iron precipitate and the undissolved sample A ash) by filtration. The mass of combined solids recovered was 35.0 g.

[101 ] An amount of 10 ml of a 30% H ₂ SO ₄ solution was then added to the filtered leaching solution. A white precipitate formed immediately in solution. The white precipitate (6.9 g) was recovered by filtration from the leaching solution and dried. The XRD of the precipitate is depicted in Figure 16, and reveals that the precipitate is predominantly calcium sulfate.

[102] The composition of dissolved elements in the leaching solution, after 30 minutes of leaching (final pH = 2.25), after increasing the pH to 4.02 (iron precipitation), and after adding H ₂ SO ₄ (calcium precipitation), is shown in Table 7. It is evident that successive precipitation of Fe and Ca from the leaching solution, by pH increase to greater than 3.5 and addition of H ₂ SO ₄ as a source of insolubilising anion, resulted in the formation of a high purity magnesium salt solution.

Table 7. Dissolved elemental composition of sample A leaching solution, after leaching at low pH, after pH increase to 4, and after adding H ₂ SO ₄ .

Example 7. HCI leaching of Sample B fly ash

[103] A mass of 50 grams of sample B fly ash was added incrementally over 8 minutes to 75 ml of a solution of 32% HCI in water (initial pH of 0.96). The mixture, with a final liquidisolid ratio of 1 .5, was stirred for a total of 30 minutes at room temperature. The composition of dissolved elements in the leaching solution over the leaching period is shown in Figure 17. Iron, calcium and magnesium were extracted into the acidic leaching solution. After 30 minutes, the final pH of the leaching solution was 1 .95.

[104] Sample A fly ash was then gradually added to the unfiltered leaching solution with stirring to raise the pH to 4.72 (64.0 g ash was required to effect this pH change). At the increased pH, a precipitate of iron formed from the solution and the solution became transparent. The supernatant leaching solution was separated from the combined solids (i.e. the mixture of the iron precipitate and the undissolved sample A and B fly ash) by filtration. The mass of combined solid recovered was 75.4 g.

[105] An amount of 10 ml of a 30% H ₂ S0 ₄ solution was then added to the filtered leaching solution. A white precipitate formed immediately in solution. The white precipitate (3.1 g) was recovered by filtration from the leaching solution and dried. The XRD analysis of the precipitate revealed that the precipitate is predominantly calcium sulfate.

[106] The composition of dissolved elements in the leaching solution, after 30 minutes of leaching (final pH = 1 .95), after increasing the pH to 4.72 (iron precipitation), and after adding H ₂ SO ₄ (calcium precipitation), is shown in Table 8. It is evident that successive precipitation of Fe and Ca from the leaching solution, by pH increase to greater than 3.5 and addition of H ₂ SO ₄ as a source of insolubilising anion, resulted in the formation of a high purity magnesium salt solution.

Table 8. Dissolved elemental composition of sample B leaching solution, after leaching at low pH, after pH increase to 4.7, and after adding H ₂ SO ₄ .

Leaching solution Mg (ppm) Fe (ppm) Ca (ppm) Al (ppm) Na (ppm)

Before Iron 29850 56833 8661 3677 4365 precipitation (pH

1 .95)

After precipitation Fe 23009 3 14300 13 2859 (pH 4.72)

After Ca precipitation 16379 0.49 700 22 2171 [107] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.

[108] Future patent applications may be filed in Australia or overseas on the basis of or claiming priority from the present application. It is to be understood that the following provisional claims are provided by way of example only, and are not intended to limit the scope of what may be claimed in any such future application. Features may be added to or omitted from the provisional claims at a later date so as to further define or re-define the invention or inventions.