WO2011025440A1 - Recovery of al from p-containing material - Google Patents

Recovery of al from p-containing material Download PDF

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Publication number
WO2011025440A1
WO2011025440A1 PCT/SE2010/050899 SE2010050899W WO2011025440A1 WO 2011025440 A1 WO2011025440 A1 WO 2011025440A1 SE 2010050899 W SE2010050899 W SE 2010050899W WO 2011025440 A1 WO2011025440 A1 WO 2011025440A1
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trivalent
scavenger
solution
aluminum
hydrochloric acid
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PCT/SE2010/050899
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French (fr)
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Yariv Cohen
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Easymining Sweden Ab
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    • CCHEMISTRY; METALLURGY
    • C01INORGANIC CHEMISTRY
    • C01FCOMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F7/00Compounds of aluminium
    • C01F7/48Aluminium halides
    • C01F7/56Chlorides
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22BPRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B3/00Extraction of metal compounds from ores or concentrates by wet processes
    • C22B3/04Extraction of metal compounds from ores or concentrates by wet processes by leaching
    • C22B3/06Extraction 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
    • C22B3/10Hydrochloric acid, other halogenated acids or salts thereof
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22BPRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B3/00Extraction of metal compounds from ores or concentrates by wet processes
    • C22B3/20Treatment or purification of solutions, e.g. obtained by leaching
    • C22B3/42Treatment or purification of solutions, e.g. obtained by leaching by ion-exchange extraction
    • YGENERAL 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
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02PCLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00Technologies related to metal processing
    • Y02P10/20Process efficiency
    • Y02P10/21Process efficiency by recovering materials
    • Y02P10/212Recovering metals from waste
    • Y02P10/234Recovering metals from waste by hydro metallurgy

Abstract

Phosphorus containing material is treated with mineral acid to form a leach-solution (7) comprising of trivalent aluminum, phosphorus and optionally trivalent iron. Aluminum and possibly iron is extracted by adsorbing in a scavenger (22) having an affinity for cations and by releasing aluminum and iron into a strip-solution during regeneration. The regeneration is performed with hydrochloric acid (31). Anionic metal chloride complexes are extracted from the hydrochloric acid strip-solution (32) by adsorbing in a scavenger (42) having affinity for anionic metal chlorides. Metal chlorides are recovered from the loaded scavenger (46) by elution with water (49). Gaseous hydrogen chloride (38) is added to the raffinate (33) from the metal chloride extraction. Crystalline aluminum chloride hexahydrate (37) is separated. The hydrochloric acid strip-solution (31) is recycled and used for stripping a scavenger (26) loaded with aluminum and/or iron. The depleted ash leach-solution (9) is treated for phosphorus recovery and preferably reused for dissolution of a phosphorus containing material.

Description

RECOVERY OF AL FROM P-CONTAINING MATERIAL

TECHNICAL FIELD

The present invention relates in general to the recovery of aluminum from solutions, and in particular to recovery of aluminum from solutions obtained from dissolution of phosphorus containing materials, such as leach solutions derived upon dissolution of ash from incinerated sewage sludge through mineral acids.

BACKGROUND

Phosphorus is an important element, and indeed essential to life. However, the release of phosphate to surface waters, and its consequent contribution to eutrophication, has also led to increasing concerns about water quality. Policies were therefore implemented throughout the world, to reduce the levels of phosphorus entering surface waters, by the implementation of technologies to remove phosphorus from domestic and industrial wastewater.

Phosphorus resources are limited and will last about 100 years, if mined by methods currently regarded as economic. This knowledge initiated an interest in technologies which facilitate the recycling and beneficial re-use of the phosphorus present e.g. in waste products in agriculture.

Fertilization with sewage sludge is gradually prohibited in an increasing number of countries due to the sludge's content of heavy metals and organic contaminants. Incineration is practiced as a solution to reduce the volume of sewage sludge before disposal.

Ash of incinerated sewage sludge contains about 8 - 14% P by weight, which is similar to the concentration of P in phosphate rock (e.g. 12 - 16% P by weight). More than 90% of the P present in sewage is found in the ash. The phosphorus present in ash is insoluble in water due to bindings with calcium, iron or aluminum. Therefore, the P-fertilizer value of ash is low. Furthermore, heavy metals are enriched in ash and limit the recirculation of ash to cropped land.

A number of methods have been developed to extract phosphorus from sewage sludge incinerator ash, as its phosphorus content is high. These methods are based on dissolution of phosphorus using acids or bases followed by a chemical and/or physical separation process to selectively recover phosphorus compounds from the solution. The separation techniques include chemical precipitation, ion-exchange, nano-filtration and liquid-liquid extraction. Most of the state-of-the-art methods do not enable recovery of iron and aluminum.

Phosphorus removal in wastewater treatment is mainly based on chemical precipitation with iron or aluminum salts. Iron and aluminum are non-volatile metals remaining in ash during incineration of sewage sludge. Therefore, iron and aluminum are major components in ash of incinerated sewage sludge amounting up to 20% by weight.

A necessary step in phosphorus recovery from ash is the dissolution of phosphorus. Dissolution with a base has a low efficiency, as only part of the phosphorus in ash is leached. In contrast, the efficiency of phosphorus leaching from ash with mineral acids is very high. For that reason, leaching with acid is the most common approach. The use of mineral acids for phosphorus leaching from ash results in the dissolution of iron and aluminum. High concentrations of iron and aluminum in ash leach-solutions are a main obstacle for recovering phosphorus. It is not possible to precipitate phosphorus in forms other than iron phosphate or aluminum phosphate unless iron and aluminum are separated, preferably prior to P recovery. Iron- and aluminum phosphates have a very low solubility in water and thus cannot release phosphorus at rates sufficient for crops when used as a fertilizer. The fertilizer value of iron phosphate and aluminum phosphate is therefore very low. Furthermore, iron- and aluminum phosphates cannot be processed by the phosphate industry since they interfere with the industrial process. High concentrations of iron and aluminum in ash leach-solutions are also an obstacle for recovering phosphorus by liquid-liquid extraction or ion exchange techniques since phosphorus extraction efficiency decrease with increasing metal ion concentration. Furthermore, it is preferred to recycle the solution from phosphorus extraction and to use it for ash dissolution in order to reduce acid consumption and to minimize production of effluents. Recycling of ash leach- solution in such a manner results in build up of iron and aluminum in the circulating solution, which renders phosphorus recovery ineffective. It is therefore, desirable to be able to separate and preferably recover iron and aluminum from ash leach-solutions in order to enable phosphorus recovery. It is desirable to recover the main components in ash in order to reduce the costs associated with ash disposal. It is further desirable to be able to recover iron and or aluminum separately and preferably in a form which is suitable for phosphorus precipitation in sewage treatment plants. This can enable the recycling of iron and aluminum from ash to be used for phosphorus precipitation and thereby reduce the need for external iron and aluminum salts.

The published international patent application WO 00/50343 describes a process for recovering iron, aluminum and phosphorus from ash leach solution using ion exchange. The process includes separation of iron and aluminum from ash leach solution with a strong acid cation exchange resin such as Dowex Marathon C in a sodium or proton form. Regeneration of the cation exchange resin is performed preferably with aqueous sodium chloride, but hydrochloric or sulfuric acids are also mentioned as possible regeneration solutions. It is stated that the eluate solution containing iron and aluminum ions can be recycled to a water purifying plant where substances comprising iron and aluminum are valuable as coagulants.

The approach presented in the disclosure WO 00/50343 has a number of severe drawbacks such as high costs due to the need for a large excess of regeneration chemicals and limited value of recovered iron and aluminum due to contamination with acids or salts. Regeneration of a strong acid cation exchange resin is based on an ion exchange equilibrium reaction. The affinity of cation exchange resins towards trivalent cations such as iron and aluminum is much higher than the affinity towards monovalent cations such as sodium or protons. A very high concentration of monovalent cations is therefore required in the regeneration solution in order to shift the ion exchange equilibrium reaction so that trivalent iron or aluminum can be exchanged with monovalent sodium or protons. Therefore, regeneration of a cation exchange resin loaded with iron and aluminum requires the use of highly concentrated salt or acid solutions. The salt or acid requirement is much higher than the stoichiometric amount required for forming iron and aluminum salts. This makes the regeneration process costly as an excess of chemicals is needed. Furthermore, the eluate has a high content of residual acid or salt. Introduction of concentrated acids or salts into the wastewater treatment process is undesirable.

In addition, the above described process does not enable to recover iron and aluminum separately. In sewage treatment, phosphorus precipitation is optimized for use of either iron or aluminum salts. To achieve maximum efficiency of phosphorus removal, it is important to control the pH of the solution and to be able to adjust the metal to phosphorus ratio during precipitation. Therefore, it is desired to use a precipitation chemical with a pre-known composition, and to use either iron or aluminum salts and not mixtures of varying Fe and Al contents.

The content of iron and aluminum in ash of incinerated sewage sludge varies with time. In several cases, sewage sludge is incinerated in central facilities which receive sludge from several treatment plants using either iron or aluminum as precipitation chemicals. Since the molecular weight of aluminum and iron differ considerably and the relative content of iron and aluminum in ash varies with time, eluates produced according to disclosure WO 00/50343 have a very limited value as precipitation chemicals. In the published international patent application WO 2008/115121, a method and an arrangement for phosphorus recovery are disclosed. The method is applicable to recovery of phosphorus from ash leach solutions. Separation of iron and aluminum is performed with a strong cation exchange resin regenerated with a mineral acid. The disadvantages are similar to disclosure WO 00/50343 and include high costs due to the need for a large excess of regeneration chemicals, limited value of recovered iron and aluminum products due to contamination with acid, and it is not possible to recover iron and aluminum separately.

In the US patent 3,193,381, a process for concentration of nickel and cobalt in aqueous solutions is disclosed. In a first step, iron is extracted and subsequently stripped by hydrochloric acid forming iron chloride complexes. The iron chloride complexes are separated be a second extraction solution and subsequently stripped with water.

Despite extensive research work conducted worldwide in the field of phosphorus recovery, and attempts to apply ion exchange technology and liquid-liquid extraction for this purpose, recovery of iron and aluminum from ash leach-solutions, separately and in pure form, for use as coagulants in sewage treatment, has not been applied in the industry. There is a need for an improved method for recovery of iron and aluminum from ash leach solutions, in which the continuous need for large excess of regeneration chemicals is excluded. There is also a need for a method that enables the recovery of iron and aluminum separately and in pure form without contamination with heavy metals for use as coagulants in water/wastewater treatment plants. SUMMARY

A general objective of the present invention is to provide an efficient, low cost and environmentally friendly method for recovery of aluminum and preferably also iron from solutions obtained from dissolution of phosphorus containing materials and in particular to provide a method for recovery of aluminum and preferably also iron from leach solutions obtained by dissolution of ash from incinerated sewage sludge with mineral acid. A further objective of the present invention is to provide a cost effective method for separating aluminum and preferably also iron from ash leach-solutions in order to enable, in a subsequent step, phosphorus recovery in a valuable form for use in agriculture. Another objective of the present invention is to enable recovery of and aluminum salts and preferably also iron salts without the need for large excess of regeneration chemicals. An additional objective of the present invention is to enable recovery of aluminum salts without contamination with heavy metals. A further objective of the present invention is to enable separation of iron from aluminum to be used separately for phosphate control in treatment of waste effluents. The above objectives are achieved by methods and devices according to the enclosed patent claims. In general words, phosphorus containing material is treated with mineral acid to form a leach-solution comprising of trivalent aluminum and phosphate anions, and possibly trivalent iron and/or divalent heavy metals. At least trivalent aluminum is extracted from the leach-solution by adsorbing trivalent aluminum in a scavenger having an affinity for cations and by releasing and the trivalent aluminum into a strip-solution during regeneration of the scavenger. The regeneration is performed with hydrochloric acid. Anionic metal chloride complexes are extracted from the hydrochloric acid strip-solution by adsorbing anionic metal chloride complexes in a scavenger having affinity for anionic metal chlorides. Metal chlorides are recovered from the loaded scavenger by elution with water and the scavenger recycled to extract more anionic metal chlorides. Hydrogen chloride is added to the raffinate from the metal chloride extraction step. Crystalline aluminum chloride hexahydrate is optionally separated from the hydrochloric acid strip-solution. The hydrochloric acid strip-solution, after separation of metals, is recycled and used for stripping a scavenger loaded with trivalent aluminum. The ash leach-solution after separation of trivalent aluminum is treated for phosphorus recovery and preferably reused for dissolution of a phosphorus containing material.

An arrangement for recovery of trivalent Al from a solution comprises digester, arranged for treating a phosphorous- containing material additionally containing Al with mineral acid. A digester separator is connected to the digester and arranged for separating solid and liquid phases of the treated phosphorous-containing material. A leach solution comprising phosphate and trivalent Al is thereby formed. An inlet for a mineral acid solution comprising trivalent Al based on the leach solution is provided connected to a first ion exchange unit. The first ion exchange unit is arranged for extracting the trivalent Al by adsorption in a first scavenger. The first ion exchange unit is further arranged for releasing the trivalent Al from the first scavenger into a hydrochloric acid solution. Anionic metal chloride complexes are thereby formed. A second ion exchange unit is connected to the first ion exchange unit. The second ion exchange unit is arranged for extracting the anionic metal chloride complexes from the hydrochloric acid solution provided from the first ion exchange unit by adsorption in a second scavenger. The second ion exchange unit is further arranged for releasing the anionic metal chloride complexes from the second scavenger into a water solution. A recirculation unit is connected between the second ion exchange unit and the first ion exchange unit recirculation unit is arranged for adding gaseous hydrochloride into the hydrochloric acid solution depleted from anionic metal chloride complexes. The recirculation unit is arranged for providing the hydrochloric acid solution depleted from anionic metal chloride complexes to the first ion exchange unit for reuse as strip solution. An arrangement for recovering of phosphate from the leach solution is provided and a recirculation arrangement is arranged for recirculating the leach solution depleted from trivalent Al and phosphate to the digester.

One advantage with the present invention is that it enables extraction of aluminum and preferably also iron from ash leach solutions in form of high quality aluminum chloride products in an environmentally friendly and cost effective way. Separation of aluminum and preferably also iron from ash leach-solutions enables to recover phosphorus, in a possible subsequent step, in a form which is suitable for use in agriculture. Aluminum can thus be recovered from ash leach solutions in a simple and cost effective way without the need for a large excess of regeneration chemicals. Another advantage of the present invention is that it enables to recover iron and aluminum separately and without contamination with heavy metals. A further advantage of the present invention is that coagulants, used for phosphorus precipitation from waste effluents, can be recovered from ashes of incinerated phosphorus containing sludge and thereafter be reused again for phosphorus precipitation from waste effluents, thus saving resources.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

FIG. 1 is a block scheme of an embodiment of an arrangement for recovery of phosphorus from ashes;

FIG. 2 is a block scheme of an embodiment of a trivalent metal recovery arrangement according to the present invention;

FIG. 3 is a diagram illustrating extraction of metals from sulfuric acid by di-ethylhexyl phosphoric acid, as a function of equilibrium pH;

FIG. 4 is a diagram illustrating extraction of anionic metal chloride complexes with tri octyl/decyl amine from hydrochloric acid as a function of chloride concentration;

FIG. 5 is a flow diagram of steps of an embodiment of a trivalent metal recovery method according to the present invention;

FIG. 6 is a flow diagram of steps of an embodiment of a phosphorus recovery method;

FIG. 7 is a block scheme of an embodiment of an aluminum recovery arrangement according to the present invention;

FIG. 8 is a block scheme of an embodiment of an arrangement for recovery of iron and aluminum according to the present invention; and

FIG. 9 is a block scheme of another embodiment of an arrangement for recovery of iron and aluminum according to the present invention.

DETAILED DESCRIPTION

Throughout the drawings, the same reference numbers are used for similar or corresponding elements.

Some often used terminology in the present disclosure is to be interpreted as follows:

Scavenger- material having affinity for solute species, e.g. material adsorbing ions, by ion association or solvation mechanisms. The term comprises different kinds of extractants contained in solvents as well as ion exchange resins.

Solvent - A liquid phase, typically organic, which preferentially dissolves extractable solute species from an aqueous solution. Extractant - An active component, typically organic, of a solvent enabling extraction.

Liquid-liquid extraction - The separation of one or more solutes from a mixture by mass transfer between immiscible phases in which at least one phase typically is an organic liquid. Diluent - A liquid, typically organic, in which an extractant and a modifier are dissolved to form a solvent Modifier - A substance added to a solvent to increase the solubility of the extractant, salts of the extractant, or ion species derived from extraction or stripping. Also added to suppress emulsion formation. Regeneration - The displacement from the scavenger of the solute species removed from the process solution to make the scavenger ready for reuse.

Stripping - Elution from a loaded solvent. Scrubbing - The selective removal of impurities from a loaded solvent prior to stripping Raffinate - An aqueous phase from which a solute has been removed by extraction

The present invention is based on the understanding that by combining and adapting different processing steps having a high degree of reuse of processing chemicals, a total process having a very high degree of chemicals reuse can be obtained. In the present invention, by connecting two ion exchange procedures with a process, which in itself is a recirculation system, synergetic effects for the entire system can be achieved. This can be illustrated by a typical embodiment presented here below. In a typical embodiment of the present invention, which however should not be considered as limiting the general scope of protection, phosphorus containing material is treated with mineral acid to form a leach-solution comprising of trivalent iron, trivalent aluminum, divalent heavy metals, and phosphate anions. Trivalent aluminum and preferably also trivalent iron is extracted from said leach-solution by adsorbing aluminum and possibly also iron in a scavenger having an affinity for cations and by releasing the trivalent metal into a strip-solution during regeneration of the scavenger. The regeneration is performed with hydrochloric acid. Anionic metal chloride complexes are extracted from the hydrochloric acid strip-solution by adsorbing anionic metal chloride complexes in a scavenger having affinity for anionic metal chlorides. Metal chlorides are recovered from the loaded scavenger by elution with water and the scavenger recycled to extract more anionic metal chlorides. Hydrogen chloride is added to the raffinate from the metal chloride extraction step. Crystalline aluminum chloride hexahydrate is optionally separated from the hydrochloric acid strip-solution. The hydrochloric acid strip-solution, after separation of metals, is recycled and used for stripping a scavenger loaded with trivalent metals. The ash leach-solution after separation of trivalent aluminum and possibly iron is treated for phosphorus recovery and preferably reused for dissolution of a phosphorus containing material.

The originally intended objective of the present invention was to provide a simple and cost effective method for recovery of aluminum and possibly iron from ash leach solutions containing trivalent aluminum and phosphorus and possibly also trivalent iron and/or divalent heavy metals. The method enables to recover aluminum and possibly also iron separately and without contamination with heavy metals. Separation of iron and aluminum enables to recover phosphorus, in a subsequent step, in a valuable form for use in agriculture. Furthermore, the method enables production of iron and aluminum salts without the need for large excess of regeneration chemicals. Below, embodiments of processes for recovering iron, aluminum and phosphorus from ash of incinerated sewage sludge are described in details. However, the present invention is not limited to recovery of aluminum from ash of incinerated sewage sludge, but is applicable to many other phosphorus containing materials. A similar process can be used e.g. for extracting aluminum from ash of incinerated biomass, ash of incinerated household waste, ash of incinerated peat, ash of incinerated coal, chemical sludge from water/wastewater treatment plants, etc.

Sewage sludge ash is the residue produced during the incineration of dewatered sewage sludge in an incinerator. Sludge ash is primarily a silty material with some sand-size particles. The specific size range and the properties of the sludge ash depend to a great extent on the type of incineration system and the chemicals used in the sewage treatment process.

Sewage sludge ash is mainly comprised of the elements O, Si, P, Ca, Fe and Al. Phosphorus is mainly present in the form of different phosphate salts of metal cations, while the remainder of the elements is mainly present as oxides. The phosphorus concentration in the ash is usually in the range 7 - 10% by weight. The concentrations of the other major elements vary as follows: Si 9 - 21%, Ca 4 - 15%, Al 3 - 15% and Fe 1 - 14%. The sum of heavy metals such as Cd, Co, Cr, Cu, Hg, Ni, Pb and Zn usually amount to 0.1 - 0.5 % weight.

A solution is prepared by dissolving ash of incinerated sewage sludge in acid in a dissolver arrangement. Strong mineral acids such as sulfuric acid, nitric acid, or hydrochloric acid can be used. The preferred acid is sulfuric acid due to its low cost and supply in concentrated form. The preferred way of dissolving ash in acid is to first mix the ash with recycled process solution and then maintain a low pH (pH≤ 2) by continuously adding sulfuric acid in a controlled manner. The required pH level during dissolution is a function of the ash composition and thus specific for each ash. The insoluble parts of the ash, mainly silicates, non-dissolved metal oxides and gypsum, are removed by sedimentation, filtration or centrifugation.

The entire arrangement for ash dissolution and separation of insoluble solids can be seen as a pretreatment for providing a feed solution containing trivalent iron, trivalent aluminum, divalent heavy metals, and phosphorus to a recovery arrangement according to the invention.

An embodiment of a general block scheme for an arrangement 100 for recovery of iron, aluminum and phosphorus from ashes is shown in figure 1. Ash 1, acid 2, and process solution 3, preferably at least to a part recycled process solution, are provided into a digester 4. The digester 4 is arranged for treating the phosphorous-containing material, in this embodiment the ash 1, with the acid 2, in this embodiment mineral acid. An outflow 5 from the digestion unit 4 is thereafter treated for removal of insoluble matter 13 in a digester separator 6, being a solid-liquid separation unit. The digester separator 6 is thus arranged for separating solid and liquid phases of the treated phosphorous-containing material. The insoluble matter 13 is typically removed by sedimentation, filtration and/or centrifugation and a remaining liquid is fed as a mineral acid solution 7 comprising trivalent Al and/or trivalent Fe to an inlet for an arrangement 8 for recovery of trivalent metal. After removal of a substantial part of the aluminum and possibly iron, the depleted leach solution 9 output from the arrangement 8 for recovery of trivalent metal is provided as input solution of an arrangement for recovering phosphate 10, in which phosphorous recovery is performed by means, known as such in prior art, such as liquid-liquid extraction, ion exchange, chemical precipitation, etc. An effluent 11 from the arrangement for recovering phosphate 10 is recycled back to the digester 4 as the process solution 3 as a recirculation arrangement arranged for recirculating the leach solution depleted from trivalent metal and phosphate back to the digester 4. A bleed 12 of the circulating process solution 3 is removed for further treatment e.g. in order to prevent buildup of heavy metals and alkali cations.

In a preferred embodiment, after removal of a substantial part of aluminum and/or iron, the depleted solution 9 is treated for

5 phosphorus recovery by extracting phosphoric acid with organic solvents such as alcohols, trialkyl phosphates, etc. (e.g. heptanol, tributyl phosphate and mixtures of these compounds) in a liquid-liquid extraction operation. This phosphorous recovery may e.g. be performed according to the teachings of the published International patent application

WO2008/115121. It has been found that such solvents extract phosphoric acid in preference to sulfuric acid, nitric acid and hydrochloric acid from ash leach solutions. Alternatively, sulfuric acid and phosphoric acid can be extracted together using

10 solvents such as liquid amines and mixtures of liquid amines, tributyl phosphate and alcohols. Phosphorus can thus be recovered from such organic solvents by known means in a form suitable for use in agriculture.

An embodiment of the arrangement 8 for recovery of trivalent metal is schematically illustrated in Figure 2. This arrangement 8 is suitable to operate for recovery of aluminum and optionally iron from ash leach-solutions, as mentioned above. This

15 embodiment could thus be used in the arrangement 100 of Fig. 1. The feed aqueous solution is the mineral acid solution 7 comprising trivalent Al and possibly trivalent Fe. The feed aqueous solution is provided by an inlet 21 to a first ion exchange unit 20. In an extraction section 27 of the first ion exchange unit 20, the feed aqueous solution containing dissolved aluminum and possibly trivalent iron is exposed to a first scavenger 22 having an affinity for trivalent aluminum and/or trivalent iron. Aluminum and possibly iron are thereby extracted by absorption in the first scavenger 22. The first scavenger

20 26 leaving the extraction section 27 is thereby loaded with trivalent aluminum and/or trivalent iron. After removal of preferably a substantial part of aluminum and/or iron, the depleted leach solution 9 is output via an outlet 23 e.g. to be treated for phosphorus recovery. Any scavenger capable of removing trivalent aluminum and/or trivalent iron can be used. The mechanism of aluminum and/or iron extraction is based on exchange with protons. The scavenger can be a solid ion exchange resin such as styrene divinylbenzene or acrylic divynylbenzene with sulfonic or phosphonic functional groups, or a

25 liquid organic extractant such as different organic derivatives of phosphoric acid e.g. di-ethylhexyl phosphoric acid, etc.

The use of liquid scavengers such as di-ethylhexyl phosphoric acid is preferred since such scavengers enable separation of aluminum and iron from aqueous solutions by liquid-liquid extraction. Liquid-liquid extraction involves selective transfer of solute between two immiscible phases, an aqueous phase and an organic phase. The two immiscible phases are first 30 thoroughly mixed in order to facilitate the transfer of solute and then separated.

It is common to dissolve organic extractants (e.g. di-ethylhexyl phosphoric acid) in suitable diluents such as kerosene and to add substances to the solvent mixture in order to increase the solubility of the extractant and to suppress emulsion formation. Any such combinations are hereby referred to as liquid scavengers.

35

Scavengers such as di-ethylhexyl phosphoric acid have different affinities towards different metals ions. The extent of metal extraction varies considerably with the pH of the aqueous solution. Therefore, by controlling the equilibrium pH during liquid- liquid extraction, the scavenger can selectively extract certain metals in preference to other metals. Extraction of metals from sulfuric acid by di-ethylhexyl phosphoric acid, as a function of equilibrium pH, is shown in Fig. 3. From sulfuric acid the order

40 of extraction as a function of pH is: Fe3+ > Al3+ > Zn2+ > Cd2+ > Ca2+ > Mn2+ > Cu2+ > Mg2+ > Co2+ > Ni2+. It must be understood that comparisons of per cent extraction-pH curves for any cationic extractant are useful only as an indication of the pH range over which metal extraction occurs. The curves shift according to metal concentration, extractant concentration, phase ratio, contact time, etc. For example, increase in extractant concentration will shift the curves toward lower pH values.

From Fig. 3 it can be understood that by controlling the equilibrium pH during liquid-liquid extraction, trivalent iron can be effectively separated from heavy metals using scavengers such as di-ethylhexyl phosphoric acid. At a certain equilibrium pH value, di-ethylhexyl phosphoric acid has a considerable capacity for extraction of trivalent iron and only a very limited capacity for divalent heavy metals. Aluminum can be co-extracted with trivalent iron in preference to heavy metals, or iron can be first extracted in preference to aluminum and heavy metals, and in a second extraction step at a higher pH level, aluminum can be extracted with some co-extracted heavy metals. Different embodiments utilizing these different possibilities are presented further below. Returning to fig. 2, the extraction section 27 of the first ion exchange unit 20 is therefore in this embodiment preferably provided with a pH control unit 29 for controlling the pH in the first scavenger 26 in the extraction section 27 of the first ion exchange unit 20.

The first scavenger 26 loaded with iron and/or aluminum is optionally scrubbed in a scrubber 19 to remove co-extracted impurities. Thereafter, the first scavenger 26 leaving the extraction section 27 is fed into a stripping section 28 of the first ion exchange unit 20. Here, the trivalent Al and/or trivalent Fe is released from the first scavenger into a hydrocloric acid solution 31 , in this embodiment recycled aqueous hydrochloric acid having a concentration of about 6-8 N. Trivalent iron and heavy metals such as zinc, copper, cadmium, etc. form anionic complexes with chlorine. Anionic metal chloride complexes are thereby formed. Trivalent aluminum do not form negatively charged complexes with chlorine and remains in a cationic form. An excess of hydrochloric acid over the stoichiometric ratio is used to enable stripping of iron and/or aluminum from the first scavenger 22. Consequently, the first scavenger 25 leaving the stripping section 28 is depleted of trivalent Al and/or trivalent Fe and is re-circulated back to the extraction section 27. The strip-solution, leaving the stripping section 28 of the first ion exchange unit 20, is thus a hydrocloric acid solution 32 consisting of eluted metals and hydrochloric acid.

The hydrocloric acid solution 32, containing anionic metal chloride complexes, hydrochloric acid and trivalent aluminum cations, is thereafter provided to a second ion exchange unit 40. An extraction section 47 of the second ion exchange unit 40 is arranged for extracting the anionic metal chloride complexes from the hydrocloric acid solution 32 by absorption in a second scavenger 42. The second scavenger 42 has an affinity for anionic metal chloride complexes. Anionic metal chloride complexes are selectively extracted by the second scavenger 42 in preference to trivalent aluminum cations and hydrochloric acid which remain in the aqueous solution. The second scavenger 46 output from the extraction section 47 of the second ion exchange unit 40 is thus loaded with metal chlorides. Any scavenger capable of removing anionic metal chloride complexes in preference to trivalent aluminum and hydrochloric acid can be used. The mechanism of metal chloride extraction is mainly based on exchange with chloride anions. The second scavenger can be a solid ion exchange resin such as styrene divinylbenzene or acrylic divynylbenzene with primary, secondary, tertiary or quaternary amine functional groups, or a liquid organic extractant such as different liquid amines, trialkyl phosphates, ketones, ethers, etc. The use of liquid scavengers such as liquid tertiary amines (e.g. tri octyl/decyl amine) is preferred since such scavengers enable separation of anionic metal chloride complexes from aqueous solutions by liquid-liquid extraction. It is common to dissolve tertiary amines in suitable diluents such as kerosene and to add substances to the solvent mixture in order to increase the solubility of the amines and to suppress emulsion formation. Any such combinations are hereby referred to as liquid scavengers having an affinity towards anionic metal chloride complexes. Fig. 4 shows extraction of anionic metal chloride complexes with tri octyl/decyl amine from hydrochloric acid as a function of chloride concentration.

Returning to fig. 2, the raffinate 33 from the extraction section 47 is an aqueous solution of hydrogen chloride depleted in metal chloride anionic complexes. A recirculation unit 34 is connected between the second ion exchange unit 40 and the first ion exchange unit 20. The recirculation unit 34 is thereby arranged for providing the hydrocloric acid solution depleted from anionic metal chloride complexes, i.e. the raffinate 33 to the first ion exchange unit 20 for reuse as strip solution 31. The stripping section 28 of the first ion exchange unit 20, the extraction section 47 of the second ion exchange unit 40, the recirculation unit 34 and the connections therebetween constitutes a recirculation system 30. However, since part of the chloride content has been removed as metal chloride anionic complexes, such chloride ions have to be replaced. To this end hydrogen chloride 38 is added to the raffinate 33 from the extraction section 47 in the recirculation unit 34 an HCI adder part 35. The raffinate 33 is there gassed with hydrogen chloride 38 to bring the concentration to the preferred concentration of about 6-8 N HCI.

The raffinate 33 comprises aluminum when the first ion exchange unit 20 is arranged for extracting trivalent Al by absorption in the first scavenger 22. Since aluminum thus is present in the raffinate 33 at relatively high concentration, addition of the hydrogen chloride in the adder part 35 results in the precipitation of crystalline aluminum chloride hexahydrate 37. In the present embodiment, the recirculation unit 34 also comprises a separator 36 for separating of precipitate of aluminum chloride hexahydrate 37 from the hydrocloric acid solution. At room temperature, the solubility of aluminum chloride decrease from 32% weight in water to about 6.5% weight in 8 N HCI (25% HCI by weight) and to about 0.7% weight in 11 N HCI (35% HCI by weight). At 00C, the solubility of aluminum chloride hexahydrate in saturated aqueous HCI (35% HCI by weight) is only 0.2 g liter1. Crystalline aluminum chloride 37 is thus separated from the aqueous hydrochloric acid solution in the separator 36 by sedimentation, filtration or centrifugation. The hydrochloric acid strip-solution 31, after separation of metals, is as mentioned above recycled and used for stripping the scavenger 26 loaded with trivalent metal of iron and/or aluminum.

The second ion exchange unit 40 is in the present embodiment further arranged for releasing the anionic metal chloride complexes from the second scavenger 46 into a water solution. Metal chlorides are recovered from the loaded second scavenger 46 by elution with water 49 in a stripping section 48 of the second ion exchange unit 40. Contacting the second scavenger 46, loaded with metal chloride complexes, with water 49 results in the breakage of the anionic metal complex to form a depleted second scavenger 45 loaded with chloride ions and an aqueous solution 41 containing neutral metal chloride salts. After elution of metal chlorides, the second scavenger 45 is recycled to extract more anionic metal chloride complexes. The obtained aqueous metal chloride solution 41 has a chloride to metal equivalent ratio of about 1 without an excess of hydrochloric acid. An embodiment of a method for recovery of trivalent metal from a solution is illustrated as a flow diagram in Fig. 5. The trivalent metal is trivalent Al and optionally trivalent Fe. The procedure starts in step 200. In step 210, a mineral acid solution comprising trivalent Al and possibly trivalent Fe is provided. The trivalent Al and trivalent Fe is extracted in step 220 by absorption in a first scavenger. Preferably, this extraction comprises controlling of the pH in the first scavenger. In a particular embodiment, discussed more in detail below, the controlling of the pH in the first scavenger is adapted to reduce extraction of heavy metals into the first scavenger, whereby said metal chloride complexes are essentially ferric chloride complexes. In step 230, the trivalent Al and trivalent Fe is released from the first scavenger into a hydrocloric acid solution, whereby anionic metal chloride complexes are formed. The anionic metal chloride complexes are extracted from the hydrocloric acid solution in step 240 by absorption in a second scavenger. In step 250, the anionic metal chloride complexes are released from the second scavenger into a water solution. In step 260, gaseous hydrochloride is added into the hydrocloric acid solution depleted from anionic metal chloride complexes in an amount causing precipitation of aluminum chloride hexahydrate. In this embodiment, the method also comprises the further step of 262 of separating the precipitate of aluminum chloride hexahydrate from the hydrocloric acid solution. The hydrocloric acid solution depleted from anionic metal chloride complexes is in step 270 brought back to be reused in the step 230 of releasing the trivalent Al and/or trivalent Fe from the first scavenger. The process ends in step 279.

As mentioned further above, the recovering of trivalent metals described above can advantageously be used as a part of a method for phosphorous recovery. An embodiment of such a method is illustrated as a flow diagram in Fig. 6. The method begins in step 280. In step 282, a phosphorous-containing material is treated with mineral acid. The phosphorous-containing material additionally contains Al and/or Fe. The phosphorous-containing material is in a particular embodiment ash from incineration of sewage sludge. Solid and liquid phases of the treated phosphorous-containing material are separated in step 284. Thereby a leach solution comprising phosphate and trivalent Al and/or trivalent Fe is formed. In step 286, trivalent metal is recovered according to the above presented ideas, e.g. the method illustrated in Fig. 5. The leach solution is then used as the mineral acid solution comprising trivalent Al and/or trivalent Fe. In step 288, phosphate is recovered from the leach solution. This can be performed by any known method according to prior art, e.g. according to the principles presented in WO 2008/115121. In step 290 the leach solution depleted from trivalent metal and phosphate is brought back to be reused in the step of treating phosphorous-containing material 282. The process is ended in step 299.

The content of aluminum and iron in ash of incinerated sewage sludge may vary considerably. The kind of coagulant used for phosphorus removal at the sewage treatment process is a main factor affecting metal content in sewage sludge ash. In general, two types of ashes can be identified with reference to aluminum and iron concentrations: a) ash defined by high aluminum content and low iron content having a grayish color, and b) ash defined by high iron content and low aluminum content having a reddish brown color. In addition to iron and aluminum, the calcium content in ash of incinerated sewage sludge varies considerably and is usually between 4 - 15% by weight. Contents of iron as well as aluminum vary across this same range. The silica content (SiCte) varies between 25 - 50% by weight.

During incineration of sewage sludge at high temperatures (> 5000C), inorganic phosphate compounds can re-crystallize to form new compounds. Iron phosphate and aluminum phosphate can react with calcium compounds and silica to form acid- soluble calcium phosphates (for example whitlockite (CasfPOφ) and hydroxylapatite (Cas^OφOH) and hardly soluble compounds such as hematite (Fe2θ3), aluminum oxides (AI2O3), anorthite (CaAkSi^Oe), etc. During dissolution of ash in acid, the release of phosphorus is almost complete (> 90%) irrespective of the type of ash since most phosphate compounds (iron phosphates, aluminum phosphates and calcium phosphates) are acid-soluble. However, the release of iron (10 - 50%) and aluminum (40 - 80%) is usually limited due to the presence of hardly soluble iron and aluminum compounds. Increase in dissolution of iron and aluminum can be achieved by leaching at higher temperatures.

In general, three main alternatives for recovery of iron and/or aluminum from ash leach-solution can be identified. If the ash has high aluminum content and low iron content there is very little incentive to recover iron since the content of iron in the leach-solution is usually very low. In a similar manner, if the ash has high iron content and low aluminum content there is very little incentive to recover aluminum. However, if the ash originates from central incineration facilities which receive sludge from several treatment plants using either iron or aluminum as precipitation chemicals, there might be an incentive to recover both iron and aluminum simultaneously.

If only aluminum should be recovered from the ash leach-solution in a pure form, the arrangement 8 for recovery of trivalent metal from a solution may look as in the embodiment illustrated in Fig. 7. Here trivalent aluminum is provided in a mineral acid solution 7', e.g. from an ash leach solution. Aluminum is extracted in the extraction section 27 of the first ion exchange unit 20 using e.g. di-ethylhexyl phosphoric acid. Any trivalent iron present will be co-extracted with aluminum. The liquid- liquid extraction in the extraction section 27 of the first ion exchange unit 20 can be operated in a manner in which some heavy metals also are co-extracted together with aluminum. This is performed by adapting the pH during the extraction by the pH control unit 29. The loaded scavenger 26', loaded with at least Al, is thereafter stripped with recycled aqueous hydrochloric acid 31', giving an Al depleted scavenger 25'. The strip-solution 32' consists of eluted aluminum, possibly iron and some heavy metals, and hydrochloric acid. The eluted iron and heavy metals form anionic complexes with chloride and are selectively removed from the solution with a scavenger (e.g. tri octyl/decyl amine) 42, which removes anionic metal complexes in preference to aluminum and hydrochloric acid. Iron and heavy metals are recovered from the loaded scavenger 46' by elution with water 49 and the metal chlorides solved in water 41 are removed for disposal. Hydrogen chloride 38 is thereafter added to the raffinate 33' from the extraction section 47 of the second ion exchange unit 40, which raffinate 33' comprises aluminum. The addition simultaneously precipitates aluminum chloride hexahydrate 37. Crystalline aluminum chloride is separated in the separator 36 from the aqueous hydrochloric acid solution 31'. The aluminum chloride is optionally treated for removal of excess acid by e.g. neutralization with aluminum hydroxide. The produced solid aluminum chloride can then be used e.g. for phosphate control in treatment of waste effluents. The hydrochloric acid strip-solution 31', after separation of aluminum, is recycled and used for stripping a scavenger 26' loaded with aluminum.

If iron and aluminum should be recovered simultaneously, two alternatives exist: a) removal of iron and aluminum in preference to heavy metals in a single extraction step, and b) removal of iron in preference to heavy metals in a first extraction step (some co-extraction of aluminum may occur) followed by removal of aluminum with some co-extraction of heavy metals in a second extraction step.

In a first alternative, illustrated by an embodiment in Fig. 8, iron and aluminum are selectively extracted from a mineral acid solution 7"' by a scavenger (e.g. di-ethylhexyl phosphoric acid) 22 in preference to divalent heavy metals which remain in the solution 9 into a Fe and Al loaded scavenger 26"'. The di-ethylhexyl phosphoric acid is thereafter stripped with hydrochloric acid 31"'. The strip-solution 32"' consists of eluted iron and aluminum and hydrochloric acid. Trivalent iron form anionic complexes with chloride while aluminum remains in cationic form. Anionic iron chloride complexes are selectively extracted from the solution 32'" with a scavenger (e.g. tri octyl/decyl amine) 45 which remove anionic iron chloride complexes in preference to aluminum and hydrochloric acid. Ferric chloride 41"' is recovered from the loaded scavenger 46"' by elution with water 49 and can e.g. be directly used for phosphorus precipitation in sewage treatment plants. Hydrogen chloride 38 is thereafter added to the raffinate 33"' from the iron chloride extraction step of the extraction section 47 of the second ion exchange unit 40 simultaneously precipitating aluminum chloride hexahydrate 37. Crystalline aluminum chloride is separated in the separator 36 from the aqueous hydrochloric acid solution 33"'. The produced solid aluminum chloride 37 can then be used for phosphate control in treatment of waste effluents. The hydrochloric acid strip-solution 31"', after separation of iron and aluminum, is recycled and used for stripping a scavenger 26"' loaded with iron and aluminum.

In a second alternative, illustrated by an embodiment in Fig. 9, trivalent iron is selectively extracted in the extraction section 27" of the first ion exchange unit 20" of a first arrangement 8" for recovery of trivalent metal from the ash leach solution 7". The extraction takes place in this embodiment with e.g. di-ethylhexyl phosphoric acid as the first scavenger 22 in preference to heavy metals (some co-extraction of aluminum may occur). The solution 9" depleted in Fe is provided as input mineral acid solution T of a second arrangement 8' for recovery of trivalent metal. In an extraction section 27' of the first ion exchange unit 20' of the second arrangement 8' aluminum is removed with some possible co-extraction of heavy metals using e.g. di-ethylhexyl phosphoric acid as the first scavenger 22. A solution depleted in both Al and Fe 9' is output for further treatment. This embodiment thus comprises two serially connected arrangements for recovery of trivalent metal, where the first one essentially recovers the Fe and the second one recovers the Al. The selective extraction of Fe and Al is performed by controlling the pH in the respective extraction section 27', 27" of the respective first ion exchange unit 20', 20". In the first arrangement for recovery of trivalent metal 8", the pH is thus controlled such that it is adapted to reduce extraction of trivalent Al. Trivalent Al is instead extracted from the mineral acid solution depleted from trivalent Fe by absorption in a scavenger in the second arrangements for recovery of trivalent metal 8'.

From the first arrangement for recovery of trivalent metal 8", pure ferric chloride 41" is recovered by stripping the iron loaded scavenger 26" with recycled hydrochloric acid 31", extraction 47"of anionic iron chloride complexes with e.g. tri octyl/decyl amine 45 into an iron chloride loaded scavenger 46", followed by elution of ferric chloride 41" with water 49. Gaseous hydrogen chloride 38" is thereafter added to the raffinate 33" from the iron chloride extraction 47". Possible co-extracted aluminum is precipitated in form of aluminum chloride hexahydrate 37" and separated. The hydrochloric acid solution 31", after separation of iron and co-extracted aluminum, is recycled and used for stripping a scavenger 26" loaded with iron.

From the second arrangement for recovery of trivalent metal 8', pure aluminum chloride hexahydrate 37' is recovered by stripping the aluminum loaded scavenger 26' with recycled hydrochloric acid 31', separation of anionic metal chloride complexes with e.g. tri octyl/decyl amine 45, followed by precipitation of aluminum chloride hexahydrate 37' by addition 35' of gaseous hydrogen chloride 38'. The aluminum chloride hexahydrate 37' is separated from the hydrocloric acid solution. The hydrochloric acid strip-solution 31', after separation of aluminum, is recycled and reused for stripping a scavenger 26' loaded with aluminum. In such a manner, iron and aluminum can be recovered in an efficient, simple and cost effective way from ash leach solutions without the need for an excess of regeneration chemicals. Even though an excess of hydrochloric acid is needed for stripping of iron and aluminum from a loaded scavenger, the consumption of hydrochloric acid, in the method according to the invention, equals about the stoichiometric amount required for forming iron and aluminum chloride salts. Thus, the costs of regenerating the scavenger are reduced considerably. Furthermore, the method according to the invention enables to recover iron and aluminum separately and without contamination with heavy metals. The recovered iron and aluminum salts are water-soluble and suitable for use as coagulants in water/wastewater treatment plants. The circulation of hydrochloric acid in a closed system, according to the invention, results in a regeneration process which does not consume a large excess of hydrochloric acid beside the amount required for forming valuable iron and aluminum products. Coagulants used for phosphorus precipitation in sewage treatment can be recovered from ash of incinerated sewage sludge and reused for phosphorus precipitation in sewage treatment plants and thereby reduce the need for external iron and aluminum salts. The different part processes are thus combined in a manner that evidently gives large synergetic effects, and in particular when also applied in a phosphorus recovery system. The detailed embodiments described above are only a few examples of how a method and arrangement for recovery of iron and aluminum can be arranged. In the described examples, iron and aluminum are extracted with liquid scavengers using a liquid-liquid extraction separation technique, but there are also other possibilities. Iron and aluminum can be extracted from leach solutions using solid scavengers by suitable separation techniques such as fixed bed column operation, etc. In conclusion, the embodiments described above are to be understood as illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. The scope of the present invention is, however, defined by the appended claims.

Claims

1. Method for recovery of bivalent Al from a solution, comprising the steps of:
treating (282) a phosphorous-comprising material, additionally comprises Al, with mineral acid;
separating (284) solid and liquid phases of said treated phosphorous-containing material, thereby forming a leach solution comprising phosphate and bivalent Al;
providing (210) a mineral acid solution comprising bivalent Al1 from said leach solution;
extracting (220) said trivalent Al by adsorption in a first scavenger;
releasing (230) said trivalent Al from said first scavenger into a hydrochloric acid solution,
adding (260) gaseous hydrochloride into said hydrochloric acid solution in an amount causing precipitation of aluminum chloride hexahydrate;
separating (262) said precipitate of aluminum chloride hexahydrate from said hydrochloric acid solution;
reusing (270) said hydrochloric acid solution depleted from trivalent Al in said step of releasing (230) said trivalent Al from said first scavenger;
reusing (290) said leach solution depleted from trivalent Al and phosphate for said step of treating (282); and recovering (288) phosphate from said leach solution.
2. Method according to claim 1, characterized in that said step of extracting (220) said trivalent Al comprises controlling of the pH in the first scavenger.
3. Method according to claim 2, characterized in that
said phosphorous-containing material additionally comprises iron;
said step of separating (284) forming a leach solution comprising at least phosphate, trivalent Al and trivalent Fe; said step of controlling the pH in the step of extracting (220) is performed to adsorb trivalent Al and trivalent Fe in said first scavenger;
said step of releasing (230) comprises releasing of trivalent Al and trivalent Fe from said first scavenger into a hydrochloric acid solution, whereby anionic metal chloride complexes are formed;
and by the further steps of:
extracting (240) said anionic metal complexes from said hydrochloric acid solution by adsorption in a second scavenger; and
releasing (250) said anionic metal chloride complexes from said second scavenger into a water solution;
said step of adding (260) being performed on said hydrochloric acid solution depleted from trivalent Fe.
4. Method according to claim 2, characterized in that
said phosphorous-containing material additionally comprises iron;
said step of separating (284) forming a leach solution comprising phosphate, trivalent AI and trivalent Fe; extracting trivalent Fe from said leach solution by adsorption in a third scavenger;
controlling of the pH in said third scavenger to reduce extraction of trivalent Al;
providing said mineral acid solution depleted from trivalent Fe by said adsorption in said third scavenger as said mineral acid solution.
5. Method according to any of the claims 2 to 4, characterized in that
said phosphorous-containing material additionally comprises heavy metals;
said step of separating (284) forming a leach solution comprising at least phosphate, trivalent Al and heavy metals;
said step of controlling the pH in the step of extracting (220) is performed to adsorb at least trivalent Al and heavy metal ions in said first scavenger;
said step of releasing (230) comprises releasing of at least trivalent Al and heavy metal ions from said first scavenger into a hydrochloric acid solution, whereby anionic metal chloride complexes are formed;
and by the further steps of:
extracting (240) said anionic metal complexes from said hydrochloric acid solution by adsorption in a second scavenger; and
releasing (250) said anionic metal chloride complexes from said second scavenger into a water solution;
said step of adding (260) being performed on said hydrochloric acid solution depleted from heavy metals.
6. Method according to any of the claims 2 to 4, characterized in that
said phosphorous-containing material additionally comprises heavy metals;
said step of separating (284) forming a leach solution comprising at least phosphate, trivalent Al and heavy metals;
said step of controlling the pH in the step of extracting (220) is performed to adsorb at least trivalent Al but not heavy metal ions in said first scavenger.
7. Method according to any of the claims 1 to 6, characterized in that said phosphorous-containing material is ash from incineration of sewage sludge.
8. Arrangement (8) for recovery of trivalent Al from a solution, comprising:
digester (4), arranged for treating a phosphorous-containing material with mineral acid, said phosphorous- containing material additionally containing Al;
a digester separator (6) connected to said digester (4) and arranged for separating solid and liquid phases of said treated phosphorous-containing material, thereby forming a leach solution (7) comprising phosphate and trivalent Al; inlet (21) for a mineral acid solution comprising trivalent Al based on said leach solution (7);
a first ion exchange unit (20) connected to said inlet (21);
said first ion exchange unit (20) being arranged for extracting said trivalent Al by adsorption in a first scavenger (22);
said first ion exchange unit (20) being further arranged for releasing said trivalent Al from said first scavenger
(22) into a hydrochloric acid solution (32), whereby anionic metal chloride complexes are formed;
a second ion exchange unit (40) connected to said first ion exchange unit (20);
said second ion exchange unit (40) being arranged for extracting said anionic metal chloride complexes from said hydrochloric acid solution (32) provided from said first ion exchange unit (20) by adsorption in a second scavenger (42); said second ion exchange unit (40) being further arranged for releasing said anionic metal chloride complexes from said second scavenger (42) into a water solution (41);
recirculation unit (34) connected between said second ion exchange unit (40) and said first ion exchange unit (20);
said recirculation unit (34) being arranged for adding gaseous hydrochloride into said hydrochloric acid solution depleted from anionic metal chloride complexes (33);
said recirculation unit (34) being arranged for providing said hydrochloric acid solution depleted from anionic metal chloride complexes (31) to said first ion exchange unit (20) for reuse as strip solution;
an arrangement (10) for recovering phosphate from said leach solution (7); and
recirculation arrangement (3) arranged for recirculating said leach solution (7) depleted from trivalent Al and phosphate (11) to said digester (4).
9. Arrangement according to claim 8, characterized in that said first ion exchange unit (20) comprises a pH control unit (29) for controlling the pH in the first scavenger (22) during extraction of trivalent Al.
10. Arrangement according to claim 8 or 9, characterized in that
said recirculation unit (34) being arranged for adding gaseous hydrochloride into said hydrochloric acid solution in an amount causing precipitation of aluminum chloride hexahydrate (37);
said recirculation unit (34) further comprises a separator (36) for separating of said precipitate of aluminum chloride hexahydrate (37) from said hydrochloric acid solution (33).
11. Arrangement according to any of the claims 8 to 10, characterized by further comprising:
a third ion exchange unit (20") connected to said digester separator (6);
said third ion exchange unit (20") being arranged for extracting trivalent Fe from said leach solution (7) by adsorption in a third scavenger (22");
said leach solution (7) depleted from Fe being supplied to said inlet (21') as said mineral acid solution.
12. Arrangement according to any of the claims 8 to 11, characterized in that said phosphorous-containing material is ash from incineration of sewage sludge.
PCT/SE2010/050899 2009-08-31 2010-08-20 Recovery of al from p-containing material WO2011025440A1 (en)

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