WO2018218294A1 - Procédé de production d'oxyde de magnésium à partir de cendres volantes ou de scories alcalines - Google Patents

Procédé de production d'oxyde de magnésium à partir de cendres volantes ou de scories alcalines Download PDF

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Publication number
WO2018218294A1
WO2018218294A1 PCT/AU2018/050529 AU2018050529W WO2018218294A1 WO 2018218294 A1 WO2018218294 A1 WO 2018218294A1 AU 2018050529 W AU2018050529 W AU 2018050529W WO 2018218294 A1 WO2018218294 A1 WO 2018218294A1
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leaching
magnesium
leaching solution
fly ash
salt
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PCT/AU2018/050529
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English (en)
Inventor
Lian Zhang
Tahereh HOSSEINI
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Monash University
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Priority claimed from AU2017902088A external-priority patent/AU2017902088A0/en
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Publication of WO2018218294A1 publication Critical patent/WO2018218294A1/fr

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B7/00—Working up raw materials other than ores, e.g. scrap, to produce non-ferrous metals and compounds thereof; Methods of a general interest or applied to the winning of more than two metals
    • C22B7/02—Working-up flue dust
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B17/00—Sulfur; Compounds thereof
    • C01B17/69—Sulfur trioxide; Sulfuric acid
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B7/00—Halogens; Halogen acids
    • C01B7/01—Chlorine; Hydrogen chloride
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F11/00—Compounds of calcium, strontium, or barium
    • C01F11/46—Sulfates
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F5/00—Compounds of magnesium
    • C01F5/02—Magnesia
    • C01F5/06—Magnesia by thermal decomposition of magnesium compounds
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F5/00—Compounds of magnesium
    • C01F5/02—Magnesia
    • C01F5/06—Magnesia by thermal decomposition of magnesium compounds
    • C01F5/12—Magnesia by thermal decomposition of magnesium compounds by thermal decomposition of magnesium sulfate, with or without reduction
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F5/00—Compounds of magnesium
    • C01F5/26—Magnesium halides
    • C01F5/30—Chlorides
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F5/00—Compounds of magnesium
    • C01F5/26—Magnesium halides
    • C01F5/30—Chlorides
    • C01F5/34—Dehydrating magnesium chloride containing water of crystallisation
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F5/00—Compounds of magnesium
    • C01F5/40—Magnesium sulfates
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
    • C01G49/00—Compounds of iron
    • C01G49/14—Sulfates
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B26/00—Obtaining alkali, alkaline earth metals or magnesium
    • C22B26/20—Obtaining alkaline earth metals or magnesium
    • C22B26/22—Obtaining magnesium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B3/00—Extraction of metal compounds from ores or concentrates by wet processes
    • C22B3/04—Extraction of metal compounds from ores or concentrates by wet processes by leaching
    • C22B3/06—Extraction of metal compounds from ores or concentrates by wet processes by leaching in inorganic acid solutions, e.g. with acids generated in situ; in inorganic salt solutions other than ammonium salt solutions
    • C22B3/08—Sulfuric acid, other sulfurated acids or salts thereof
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B3/00—Extraction of metal compounds from ores or concentrates by wet processes
    • C22B3/04—Extraction of metal compounds from ores or concentrates by wet processes by leaching
    • C22B3/06—Extraction of metal compounds from ores or concentrates by wet processes by leaching in inorganic acid solutions, e.g. with acids generated in situ; in inorganic salt solutions other than ammonium salt solutions
    • C22B3/10—Hydrochloric acid, other halogenated acids or salts thereof
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B3/00—Extraction of metal compounds from ores or concentrates by wet processes
    • C22B3/20—Treatment or purification of solutions, e.g. obtained by leaching
    • C22B3/44—Treatment or purification of solutions, e.g. obtained by leaching by chemical processes
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B7/00—Working up raw materials other than ores, e.g. scrap, to produce non-ferrous metals and compounds thereof; Methods of a general interest or applied to the winning of more than two metals
    • C22B7/04—Working-up slag
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/20—Recycling

Definitions

  • 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, precipitating iron from the leaching solution and decomposing the dissolved magnesium salt present in the leaching solution to form magnesium oxide.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • sulfuric acid and hydrochloric acid may be recovered from or regenerated from the evolved gas produced during decomposition of the magnesium salt.
  • the fly ash or slag is leached via multistage counter-current leaching.
  • the first stage of a multistage counter- current leaching process refers to the stage where fresh fly ash or slag is introduced for leaching.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • the iron salt is precipitated by increasing the pH of the leaching solution to above 3.5.
  • 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.
  • 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.
  • the pH is increased by adding a substantially water-insoluble base to the leaching solution.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • magnesium oxide product from the process may be used to increase the pH, via its reactivity with mineral acids.
  • the fly ash or slag has an elemental composition comprising at least 10 weight % magnesium, preferably at least 20 weight % magnesium.
  • 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.
  • 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.
  • the fly ash or slag is brown coal fly ash, for example Victorian brown coal fly ash.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • decomposing the magnesium salt comprises heating the magnesium salt to a temperature sufficient to produce magnesium oxide and an evolved gas.
  • 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.
  • 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.
  • a regenerated mineral acid solution is produced from the evolved gas.
  • 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.
  • 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.
  • the process may include suitable steps for preparing the fly ash or slag for leaching.
  • 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.
  • water-soluble components such as sodium or potassium salts
  • the process may include additional steps of grading or comminuting the fly ash or slag prior to leaching.
  • 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.
  • the leaching solution may be separated from the residue by conventional means, including thickening and filtration.
  • the precipitated salts may also be separated from the supernatant leaching solution by conventional means, including filtration.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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
  • the invention provides magnesium oxide, produced by the process according to any of the embodiments disclosed herein.
  • 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.
  • 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.
  • 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.
  • Figure 3 depicts XRD patterns for sample A (Hazelwood power station) and sample B (Yallourn power station) fly ash.
  • 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 .
  • Figure 5 depicts the XRD pattern of the residue of sample A fly ash after leaching with H 2 SO 4 in Experiment 1 .
  • 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 .
  • Figure 7 depicts the XRD pattern of the magnesium salt recovered after removal of the water from the leaching solution in Experiment 1 .
  • 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.
  • 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.
  • Figure 10 depicts the XRD pattern of the residue of sample B fly ash after leaching with H 2 SO 4 in Experiment 3.
  • 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.
  • Figure 12 depicts the XRD pattern of the magnesium salt recovered after removal of the water from the leaching solution in Experiment 3.
  • FIG. 13 depicts the XRD pattern of the magnesium salt recovered after precipitation from the leaching solution by addition of ethanol in Experiment 4.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • Fly ash 10 which is a renovated 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • fresh hydrochloric acid solution may be added to first leaching stage 53 to supplement intermediate leaching solution 55.
  • 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.
  • 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 .
  • 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.
  • 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.
  • 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.
  • 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 .
  • 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.
  • sample A for Hazelwood fly ash
  • sample B for Yallourn fly ash
  • the elemental composition of samples A and B (defined as the weight % of elements in their most oxidised state) as show in Table 1 .
  • Example 1 H2SO4 leaching of Sample A fly ash, with post-separation iron precipitation
  • 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.
  • 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).
  • 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.
  • 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 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 .

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Abstract

La présente invention concerne un procédé de production d'oxyde de magnésium à partir de cendres volantes ou de scories comprenant du magnésium et du fer, le procédé comprenant : la lixiviation des cendres volantes ou des scories avec une solution aqueuse d'acide minéral pour produire une solution de lixiviation et un résidu, la solution de lixiviation comprenant un sel de magnésium et un sel de fer ; la précipitation du sel de fer par augmentation du pH de la solution de lixiviation au-dessus de 3,5, et la séparation de la solution de lixiviation du sel de fer précipité ; et la formation d'oxyde de magnésium par décomposition du sel de magnésium présent dans la solution de lixiviation.
PCT/AU2018/050529 2017-06-01 2018-05-31 Procédé de production d'oxyde de magnésium à partir de cendres volantes ou de scories alcalines WO2018218294A1 (fr)

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CN113336252A (zh) * 2021-06-24 2021-09-03 四川顺应动力电池材料有限公司 一种从煤系固体废弃物的酸浸液中除钙的方法
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JP7139497B1 (ja) 2021-07-20 2022-09-20 株式会社ヨータイ 使用済みマグネシアスピネル耐火物のリサイクル方法
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US20220411940A1 (en) 2018-05-18 2022-12-29 California Institute Of Technology Process to convert reduced sulfur species and water into hydrogen and sulfuric acid
US12110601B2 (en) 2018-05-18 2024-10-08 California Institute Of Technology Process to convert reduced sulfur species and water into hydrogen and sulfuric acid
US11718558B2 (en) 2019-08-13 2023-08-08 California Institute Of Technology Process to make calcium oxide or ordinary Portland cement from calcium bearing rocks and minerals
CN111060417A (zh) * 2019-11-25 2020-04-24 中国石油化工股份有限公司 一种气化渣矿相的定量分析方法
CN111060417B (zh) * 2019-11-25 2023-03-21 中国石油化工股份有限公司 一种气化渣矿相的定量分析方法
CN113336252A (zh) * 2021-06-24 2021-09-03 四川顺应动力电池材料有限公司 一种从煤系固体废弃物的酸浸液中除钙的方法
JP7139497B1 (ja) 2021-07-20 2022-09-20 株式会社ヨータイ 使用済みマグネシアスピネル耐火物のリサイクル方法
JP2023015789A (ja) * 2021-07-20 2023-02-01 株式会社ヨータイ 使用済みマグネシアスピネル耐火物のリサイクル方法
US11920246B2 (en) 2021-10-18 2024-03-05 The Regents Of The University Of California Seawater electrolysis enables Mg(OH)2 production and CO2 mineralization
CN114438319A (zh) * 2021-12-30 2022-05-06 云锡文山锌铟冶炼有限公司 处理湿法炼锌过程中钙镁的方法
CN114438319B (zh) * 2021-12-30 2023-12-08 云锡文山锌铟冶炼有限公司 处理湿法炼锌过程中钙镁的方法

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