WO2000050343A2

WO2000050343A2 - Treatment of solutions comprising metals, phosphorous and heavy metals obtained from dissolution of combusted waste materials in order to recover metals and phosphorous

Info

Publication number: WO2000050343A2
Application number: PCT/DK2000/000072
Authority: WO
Inventors: Jens Østergaard JENSEN
Original assignee: Pm Energi A/S
Priority date: 1999-02-23
Filing date: 2000-02-23
Publication date: 2000-08-31
Also published as: AU2658700A; EP1175374A1; WO2000050343A3

Abstract

The present invention relates to a method for treatment of a solution comprising ions of at least one metal selected from iron and aluminium, phosphorus ions and at least one heavy metal selected from lead, cadmium, chromium, copper, mercury, nickel and zinc ions, in order to recover ions the metal(s), phosphorus and heavy metal(s). The method comprises the following four steps performed in arbitrary (however preferably sequential) order: contacting said solution with a first cation exchange material; when the solution comprises HSO4-ions, contacting said solution with a first anion exchange material; contacting said solution with a second anion exchange material; contacting said solution with a second cation exchange material. The method is useful in large scale plants for treatment of combusted waste materials. Alternatively, the two anion contacting steps can be replaced by contacting with a nano-filtration material.

Description

TREATMENT OF SOLUTIONS COMPRISING METALS, PHOSPHOROUS AND HEAVY METALS OBTAINED FROM DISSOLUTION OF COMBUSTED WASTE MATERIALS IN ORDER TO RECOVER METALS AND PHOSPHOROUS

Field of the invention

The present invention relates to a method for treating a solution comprising ions of at least one metal selected from iron and aluminium, phosphor and at least one heavy metal selected from lead, cadmium, chromium, copper, mercury, nickel and zinc in order to recover said ions from said solution. In particular, the method is characterised in that the solution to be treated is contacted with a series of different ion exchange materials and that the metallic and the phosphoric ions are optionally subsequently recovered from the said ion exchange materials. Alternatively, some of the steps involving contacting with ion exchange materials can be substituted with step(s) involving nano-filtration.

Background of the invention

Disposal of ashes from waste materials is a major problem because of it's high content of heavy metals which constitutes a major hazard to the environment. However, ashes from waste materials may also contain valuable substances which advantageously can be recovered and recycled. Many different methods for the recovery of the different components in waste materials have been pursued, however, only few methods have yet proven useful for efficient recovery of iron, aluminium, phosphorus and heavy metals from waste material, affording a solution with less than 30 mg/L of heavy metal after treatment of the waste solution.

GB 760,524 describes a process for recovery chromic or phosphoric acid. The process comprises passing the liquor through a bed of mixed cation and anion exchange materials which are respectively in the hydrogen and hydroxyl states.

EP 0 608 874 A1 describes a method for dissolving sludge and recovering constituents therefrom.

The above-mentioned prior art processes deal with recovery of constituents from relatively simple waste materials where a ion exchange material or a mixture of ion exchange materials is used. In contrast thereto, the problem to be solved by the present invention is to recover a number of different constituents from complex waste materials.

EP 0 715 603 B1 describes a method for treatment of waste water sludge, where the method comprises various liquid-liquid extractions steps. This method yields iron or aluminium for reuse as well as phosphorous compounds in precipitated form. However, it is believed that the numerous extraction steps and the use of organic solvents therefor and the numerous variations of the pH value may give rise to problems with respect to economy and dimensions of the plant involved.

The present invention provides a method which can be use not only for the separation (and up-concentration) of undesired components from a solution, but also isolation of these components in a form where the components can be more or less directly used or reused in industrial processes. It is particularly relevant that the present invention can minimise the cross-contamination between the various components intended for reuse, i.e. the metals (Fe, Al), sulphate (HSO ^"), phosphate (H₂P0₄ ^") and heavy metals (Pb, Cd, Cr, Cu, Hg, Ni, Zn).

Summary of the invention

The object of this invention is to provide a method whereby a solution originating from combusted waste materials can be purified. The main objective is to provide a method whereby iron and/or aluminium, phosphorus and heavy metals can be recovered from the solution. Iron and aluminium can be recycled, e.g. as coagulants in a waste water purification process, phosphorus can be recovered as high quality H₃PO₄ and the isolated heavy metals can be either deposited or further worked up for resale. This object can be accomplished by the present invention, and thus the present invention provides a method for treating solutions obtained from waste materials comprising ions of at least one metal selected from iron and aluminium, phosphorus and at least one heavy metal selected from lead, cadmium, chromium, copper, mercury, nickel and zinc ions, said method comprises the following four steps which, in principle, can be performed in arbitrary order:

■ contacting said solution with a first cation exchange material for separation of a substantial part of the ions of the at least one metal;

^■ when the solution comprises HSO ^" ions, contacting said solution with a first anion exchange material for separation of a substantial part of any HSO₄ ^' ions; ■ contacting said solution with a second anion exchange material for separation of a substantial part of the H₂PO₄ ^" ions;

■ contacting said solution with a second cation exchange material for separation of a substantial part of the ions of the at least one heavy metal.

Alternatively, said method comprises the following three steps which, in principle, can be performed in arbitrary order:

■ contacting said solution with a first cation exchange material for separation of a substantial part of the ions of the at least one metal; ■ contacting said solution with a second cation exchange material for separation of a substantial part of the ions of the at least one heavy metal;

■ contacting said solution with a nano-filtration material for separating and recovering HSO₄ ^" ions and H₃P0₄ molecules.

It should be understood that the solution, after contacting with one ion exchange material, has been deprived at least a part of the component for which said ion exchange material is chosen to be specific with respect to adsorption.

Detailed description of the invention

The solution to be treated in the method of the present invention can be obtained by dissolving metallic and phosphoric components present in ashes from combusted waste water sludge, household rubbish and dock sludge. Alternatively, the solution can be obtained by dissolving dry dock sludge comprising less than 2% organic material. It is generally preferred that the content of organic components in the solution to be treated is less than about 2%, such as less than about 1%, in order to avoid any clogging of the ion exchange materials. As an example, the solution is prepared by dissolving, or partly dissolving, the ashes with an acid, preferably H₂SO₄ or HCI, in particular H₂S0₄, so as to obtain a pH value of -0.5 to 1.4, preferably 0.8 to 1.3, or alternatively -0.5 to 1 , preferably 0.5 to 0.6. Initially, the ashes or dry sludge are pulverised, using an impact pulveriser, a disk grinder, a roller or another suitable crushing machine, into a very fine dust, preferably with a particle size of 0.5-3.0 mm. The fine dust is subsequently mixed with water, to provide a sludge containing 5-15% w/w water, preferably 8-12% w/w, and more preferably around 9% w/w water. The sludge is stirred shortly (for 1-5 min.) before addition of acid to a pH value of the sludge in the range of -0.5 to 1.4, preferably 0.8 to 1.3, or alternatively - 0.5-1.0, preferably 0.5-0.8, more preferably 0.5-0.6. The acidified sludge is typically stirred for 15-90 min. at 60-150°C prior to addition of warm water at 40-80°C, preferably 50-70°C, more preferably 55-65°C, to an initial total ion concentration in the solution in the range of 10,000-130,000 mg/L. The ratio between ashes and warm water is normally in the range of 1 :4-1 :15. Finally, the aqueous solution is stirred for 30-150 min. before filtration to remove insoluble substances from the solution. The chemical process during acidification and dissolution of the sludge is expected to be as following: The metal oxides contained within the fine dust such as the iron and aluminium oxides e.g. FeO, Fe₂O₃, AI₂O₃, the heavy metals oxides e.g. PbO, CdO, CrO₃, CrO₂, CuO, HgO, NiO and ZnO, and the phosphor oxide e.g. P₂O₅ decompose under heat evolution during acidification. In the aqueous solution the metals will primarily be present as ions: iron and aluminium as Fe²⁺, Fe³⁺ and Al³⁺, heavy metals as Cd²⁺, Cr ⁺, [CrO₄]^2", etc. If H₂SO₄ is used as the acidifying agent, HSO ^" will constitute the counter ion for the metallic ions. Initially, upon treatment with acid, phosphorus oxide, P₂O₅, is transformed into pyrophosphoric acid, H P₂O₇, which upon treatment with warm water is transform into orthophosphoric acid, H₃PO₄.

The solution is preferably prepared by dissolving, or partly dissolving, the starting material in an acid, preferably H₂SO or HCI, in particular H₂SO , so as to obtain a pH value of -0.5 to 1.4, preferably 0.8 to 1.3, or alternatively -0.5 to 1 , preferably 0.5 to 0.6.

The initial total concentration of the metal(s) in the solution is typically in the range of 2,000-50,000 mg/L, in particular 4,000-40,000 mg/L. In the method according to the present invention, the total concentration of the metal(s) in the resulting solution is preferably less than 1 ,200 mg/L, preferably less than 1 ,000 mg/L, after contacting the solution with the first cation exchange material. Preferably, the concentration of metals is reduced (concentration of solution to be treated relative to the resulting solution) at least 5 times, especially at least 10 times, in particular 20 times.

Correspondingly, the initial total concentration of sulphate in the solution is typically in the range of 30,000-70,000 mg/L, in particular 40,000-60,000 mg/L when H₂SO₄ is used in the preparation step. In the method according to the present invention, the concentration of sulphate in the resulting solution is preferably less than 10,000 mg/L, preferably less than 7,000 mg/L, after contacting the solution with the first anion exchange material. Preferably, the concentration of sulphate is reduced (concentration of solution to be treated relative to the resulting solution) at least 5 times, especially at least 10 times, in particular 20 times.

Correspondingly, the initial total concentration of phosphate in the solution is typically in the range of 1 ,500-40,000 mg/L, in particular 3,000-15,000 mg/L. In the method according to the present invention, the concentration of phosphate in the resulting solution is preferably less than 1 ,100 mg/L, preferably less than 900 mg/L, after contacting the solution with the second anion exchange material. Preferably, the concentration of phosphate is reduced (concentration of solution to be treated relative to the resulting solution) at least 5 times, especially at least 10 times, in particular 20 times.

Correspondingly, the initial total concentration of the heavy metal(s) in the solution is typically in the range of 50-4,000 mg/L, in particular 500-2,000 mg/L. In the method according to the present invention, the concentration of the heavy metal(s) in the resulting solution is preferably less than 30 mg/L, preferably less than 10 mg/L, after contacting the solution with the second cation exchange material. Preferably, the concentration of heavy metals is reduced (concentration of solution to be treated relative to the resulting solution) at least 5 times, preferably at least 10 times, in particular 20 times.

In a preferred embodiment of the invention, the solution is contacted with the three/four different ion exchange materials in the following order:

■ a first cation exchange material;

^■ a first anion exchange material (if relevant);

^■ a second anion exchange material; and ^■ a second cation exchange material.

In the alternative embodiment of the invention, the solution is contacted with two different cation exchange materials and one nano-filtration material in the following order:

^■ a first cation exchange material; ■ a second cation exchange material; and

^■ a nano-filtration material.

It should be understood that the method may include any necessary or desirable process step between the above-mentioned four step. As will be apparent in view of the present description with claims, such process steps may include adjustment of the pH, dilution, concentration, addition of components in order to facilitate or suppress the adsorption of a specific chemical species, etc. However, typically, and preferably, the method does essentially only include intermediate steps, if necessary, for adjustment of the pH value according to the directions given herein.

Subsequently, the absorbed ions are typically recovered form the ion exchange materials with suitable regeneration to provide useful compounds for recycling or resale.

The pH of the solution to be treated is typically adjusted to a value in the range of 0.3-1.4, preferably 0.8-1.4, or alternatively 0.4-1.0, even more preferably 0.5-0.6, using an acid such as H₂SO₄, before contacting with the first ion exchange material.

The acidified solution is contacted with the first cation exchange material, which is characterised in that it specifically absorbs iron and aluminium ions within the said pH ranges. Selectivity over, e.g., heavy metal ions is of course particularly relevant. The first cation exchange material can be any commercial cation exchange material which fulfils these criteria. An example of such a material is the DOWEX Marathon C cation exchange material from Dow, with a working temperature range of 0-120°C. The first cation exchange material is typically in the H⁺ state or in the Na⁺ state.

After contacting the solution with the first cation exchange material, the pH of the resulting solution should, if necessary, be adjusted to a value in the range of 1.0-1.7, preferably 1.0-1.5, more preferably 1.1-1.3, using a base such as sodium hydroxide (NaOH). If the first cation exchange material has been washed with an aqueous sodium chloride (NaCI) solution prior to contacting with the solution, only minor adjustments of the pH value, if any, is required. Thus, such NaCI washing is preferred.

The pH adjusted solution from the first cation exchange is then, if the solution comprises HSO₄ ^" ions, contacted with the first anion exchange material, which is characterised in that it specifically absorbs the negative counter ion to the metallic ions in the solution, HSO₄ ^", within the said pH ranges, where the specificity over the absorption of phosphate ions, H₂PO₄ ^", is relevant. The material of the first anion exchange material can be any commercial anion exchange material which fulfils these criteria. An example of such a material is the DOWEX Marathon A anion exchange material from Dow, with a working temperature range of 0-60°C. The first anion exchange material is typically in the CI^" state or the OH^" state.

After passage of the first anion exchange material, the pH of the resulting solution is raised to a value of about 2.5-3.5, preferably 2.7-3.3, more preferably 2.9-3.1 , using a base such as NaOH. The solution being treated is then contacted with a second anion exchange material, which is characterised in that it specifically absorbs the phosphate ion, H₂PO₄ ^". The material of the second anion exchange material can also be any commercial anion exchange material, which fulfils these criteria. An example of such a material is the DOWEX Marathon W.B.A. anion exchange material from Dow , with a working temperature range of 0-60°C. The second anion exchange material is typically in the CI^" state or the OH^" state.

As the last step in the treatment procedure the remaining solution from the passage of the second anion exchange material is contacted with a second cation exchange material. The second cation exchange material is characterised in that it specifically absorbs heavy metal ions. The affinity is more relevant than the selectivity as it is important that the resulting solution comprises as little heavy metals as possible, in particular less than 5 mg/L. The material of the second cation exchange material can also be any commercial cation exchange material which fulfils these criteria. An example of such a material selective divalent cation exchange material, e.g. the LEWATIT TP 207/208 cation exchange materials from Bayer, Germany, or Purolite S 930 from Purolite, USA, or Amberlite IRC 718 from Rohm and Haas, Germany, with a working temperature range of 0-120°C. The second cation exchange material is typically in the H⁺ state or in mixed H7Na⁺ state.

Alternatively, a nano-filtration material can be used instead of the two anion exchange materials. In this alternative embodiment, the solution is typically first contacted with the two cation exchange materials as described above (where the same conditions with respect to pH values and temperature applies). It should be understood that the pH value of the solution resulting from contacting with the first cation exchange material typically needs to be adjusted before contacting with the second cation exchange material, cf. the instructions given above. After passage of those two cation exchange materials, essentially all metal ions are removed and the pH value is normally in the range of 0.8 to 1.4. The pH value should typically, e.g. after adjustment with the acids or bases mentioned above, be in the range of 2.0 to 3.0 before contacting with the nano-filtration material.

Nano-filtration is a form of filtration that uses membranes to separate different constituents of a fluid, e.g. ions. Nano-filtration is not as fine a filtration process as reverse osmosis, but the energy requirement is correspondingly lower. Nano-filtration uses a membrane that is partially permeable to perform the separation. In embodiments relevant in the present context, nano-filtration is characterised in that a solution comprising ions and molecules is pumped past/through a filter material and due to the surface tension, charged ions are retained and uncharged molecules are allowed to pass. In the interesting example, HSO₄ ^" ions are retained and H₃PO molecules are allowed to pass through the filter material (see element Z in Figure 2). The process is typically performed under a pressure of 20-40 bar.

The resulting solution after passage of the three or four ion exchange materials can be lead to a water purifying plant. The same applies for the alternative embodiment where the solution is treated with two cation exchange materials and a nano-filtration material.

The amount of ion exchange material used for a specific volume of solution can be adjusted in order to ensure that the adsorption is optimal, e.g. that the ion exchange material is not over-loaded. Typically, the amount of ion exchange material is selected so that approximately at the most 80% saturation of the ion exchange material is obtained, in particular 60-80% saturation (saturation is calculated in relation to the capacity for the material in question). The contacting times of the solutions with the ion exchange materials are typically 3-15 minutes for a vessel having a length in the flow direction of 1 m. The temperature of the acidified solution when contacted with the first (cat)ion exchange material is typically 30-60°C, preferably 35-45°C, due to the fact that the warm acidic solution is used directly from the mixer without storage. The temperature of the resulting solutions when contacted with the succeeding three ion exchange materials are contingent on the temperature of the surrounding which is generally within 0-30°C, preferably 15-25°C. For temperatures below 0°C, precipitation may make up a considerable problem. Beside the examples of ion-exchange materials mentioned above, the person skilled in the art will be able to suggest other equally applicable ion-exchange materials according to the specifications given herein with respect to pH value intervals, specificity for the specified types of ions, temperature intervals, etc.

Recovery of the absorbed metallic, phosphoric and sulphuric ions from the four ion exchange materials is performed by regeneration of each ion exchange materials with suitable regeneration solutions.

Regeneration of the first cation exchange material can be performed with aqueous NaCI, hydrochloric acid (HCI) or sulphuric acid (H₂SO ), preferably aqueous NaCI. Regeneration of the first cation exchange material with aqueous NaCI recovers iron as Fe(ll)CI₂ which has a higher solubility than Fe(ll)SO . However, contrary to Fe(ll)S0₄, Fe(ll)CI₂ is easily oxidised to Fe(ll)CI₃. The solubility of Fe(ll)CI₃ is very temperature dependent and it is therefore advisable to avoid oxidation of Fe²⁺ to Fe³⁺ to prevent precipitation of Fe(ll)CI₃ at low temperature. The regeneration solution containing iron and aluminium ions can subsequently be recycled in a water purifying plant where substances comprising iron and aluminium are valuable as a coagulants. Another advantage of using aqueous NaCI for regeneration of the first cation exchange material is that the amount of NaOH needed for neutralisation of a solution subsequently passed over the said ion exchange material can be reduced with at least 20%.

The anionic counter ion HS0 ^" absorbed by the first anion exchange material can be recovered either as KHS0 or NaHSO when the anion exchange material is regenerated with aqueous potassium chloride (KCI) or aqueous NaCI, respectively. Alternatively the first anion exchange material can be regenerated with HCI. The NaHSO solution can be lead to a water purifying plant, whereas the KHSO solution can be collected for resale.

Regeneration of the second anion exchange material is performed with HCI or H₂SO₄ affording a solution of H₃PO₄ with very high purity regarding contamination of heavy metals and the product may therefore be suitable for resale. The concentration of the H₃PO₄ solution is in general in the range of 10-50%, preferably 10-30%. The heavy metal ionic solution obtained after regeneration of the second cation exchange material with aqueous NaCI, HCI or H₂SO₄ can be either deposited or further worked up for resale.

In a preferred embodiment of the invention the solution is sequentially contacted with the four, or, if the solution does not comprise any substantial amount of HSO ^" ions, three, ion exchange materials which are arranged in series allowing continuously flow of the solution from one ion exchange material to the next ion exchange material.

In an even more preferred embodiment of the invention, two portions of one or more of the ion exchange materials are arranged in a parallel manner, and where one portion is contacted with the solution being purified while the other portion is regenerated to recover the ions which have been adsorbed on to the ion exchange material in question in a previous contacting step.

A particularly preferred embodiment of the present invention relates to a method for treatment of a continuous flow of a solution, said solution comprising ions of at least one metal selected from iron and aluminium, phosphorus ions and at least one heavy metal selected from lead, cadmium, chromium, copper, mercury, nickel and zinc ions, in order to recover said ions of at least one metal, phosphorus and the at least one heavy metal, characterised in that the method comprises the following four steps performed in the following sequence:

^■ contacting said solution with a first cation exchange material for separation of a substantial part of the ions of the at least one metal; ^■ when the solution comprises HSO₄ ^" ions, contacting said solution with a first anion exchange material for separation of a substantial part of any HSO₄ ^" ions said solution having a pH value of 1 to 1.7, optionally after adjustment of the pH value with a base, prior to the contacting;

^■ contacting said solution with a second anion exchange material for separation of a substantial part of the H₂PO₄ ^" ions, said solution having a pH value of 2.5 to 3.5 after adjustment of the pH value of said solution with a base prior to the contacting;

■ contacting said solution with a second cation exchange material for separation of a substantial part of the ions of the at least one heavy metal, where the solution is sequentially contacted with the four, or, if the solution does not comprise HSO₄ ^" ions, three, ion exchange materials which are arranged in series allowing continuously flow of the solution from one ion exchange material to the next ion exchange material. In this embodiment, it is preferred that iron and aluminium ions are recovered from the first cation exchange material by regeneration of said first cation exchange material with solutions comprising NaCI, HCI or H₂SO , preferably NaCI; HSO ^" ions are recovered from the first anion exchange material by regeneration of said first anion exchange material with solutions comprising KCI, NaCI and/or HCI, preferably KCI; H₂PO₄ ^" ions are recovered from the second anion exchange material by regeneration of said second anion exchange material with solutions comprising HCI and/or H₂S0 , preferably HCI; and that heavy metal ions are recovered from the second cation exchange material by regeneration of said second cation exchange material with solutions comprising NaCI, HCI and/or H₂SO₄.

Figure 1 shows an example of a plant suitable for operation of the present invention according to the main embodiment.

B represents a tank for storage of the waste material (e.g. the ash from the incineration of waste water sludge) equipped with a distributor system and a draining system. From B the waste material is lead to a device for pulverisation C, such as an impact pulveriser, a disk grinder, a roller or another suitable crushing machine. From C the pulverised waste material is lead to a mixing tank D. The mixing tank D is made out of an acid-resisting material e.g. steel and equipped with a stirring and draining system. Addition of water to the mixing tank D takes place through valve E and addition of acid from a tank F takes place through valve G. From the mixing tank D the aqueous solution is transferred to a device for filtration Y, where insoluble substances are removed from the solution. The solution obtained from the filtration is collected in tank I and the insoluble substances are transferred to storage H. From the tank I, the solution to be treated is transferred to a vessel containing the first cation exchange material K through valve J. From K the solution is transferred to the vessel containing the first anion exchange material N. From the vessel containing the first anion exchange material N the solution is transferred to the vessel containing the second anion exchange material Q and, finally, the solution is transferred from Q to the vessel containing the second cation exchange material T. The solution for regeneration of the first cation exchange material is lead to the vessel containing the material K from a tank L. The regeneration solution containing recovered iron and aluminium ions is lead to storage or a water purifying plant for recycling through a pipeline M. The solution for regeneration of the first anion exchange material is lead to the vessel containing the material N from a tank O. Depending on the character of the recovered solution the solution is either lead to a water purifying plant through a pipeline P, which will be the case if the recovered solution contains NaHSO , or if the recovered solution contains KHSO the solution can be collected for resale. The regeneration solution for the second anion exchange material is transferred to the vessel containing the material Q from a tank R. Recovered H₃PO₄, is lead through a pipeline S to a tank for storage of the product for resale. The solution for regeneration of the second cation exchange material is lead to the vessel containing the material T from a tank U. The recovered heavy metal solution is lead to a storage tank for destruction or further work-up through a pipeline V. The remainder of the treated solution after passage through all four ion exchange materials is lead to a waste water purification plant through a pipeline X.

In a preferred embodiment of the invention, each of the four ion exchange materials (K, N, Q and T) are arranged into two separate compartment e.g. Kl, KM, Nl, Nil, Ql, Qll and Tl, TM. This arrangement allows one part of the ion exchange materials e.g. Kl, Nl, Ql and Tl, to be brought in contact with the solution being treated, while the other part of the ion exchange materials e.g. KM, Nil, Qll and Til, are being regenerated. This preferred embodiment of the invention ensures a continuously flow of the solution being treated from one ion exchange material to the next ion exchange material.

Thus, the present invention also relates to a plant for treatment of a stream of a solution, said solution comprising ions of at least one metal selected from iron and aluminium, phosphorus ions and at least one heavy metal selected from lead, cadmium, chromium, copper, mercury, nickel and zinc ions, in order to recover said ions of at least one metal, phosphorus and the at least one heavy metal, characterised in that the plant comprises the following:

■ at least two vessels for contacting said the stream with a first cation exchange material for separation of a substantial part of the ions of the at least one metal;

■ when the solution comprises HSO₄ ^" ions, at least two vessels for contacting said stream with a first anion exchange material for separation of a substantial part of any HS0 ^" ions; ^■ at least two vessels for contacting said stream with a second anion exchange material for separation of a substantial part of the H₂PO ^" ions;

■ at least two vessels for contacting said stream with a second cation exchange material for separation of a substantial part of the ions of the at least one heavy metal.

In order to render it possible to process a stream of the solution and at the same time regenerate some of the vessels comprising ion exchange materials, it is preferred that the stream can be shunted off at least one of each of the at least two vessels. In particular, one of each of the at least two vessels should be arranged in series to allow the stream of the solution to flow through all of the one of the at least two vessels. Furthermore, one of the other of each of the at least two vessels should be adapted to allow regeneration and recovery of ions adsorbed on to the ion exchange material in question. As follows from Figure 1 , wherein not all relevant and possible valves are indicated, each set of at least two vessels can be arranged in a parallel manner wherein valves are arranged to control flow of the stream of the solution through the individual vessels. In this way it is possible to handle a continuos stream of the solution and passing this stream through successive ion exchange materials arranged in series and at the same time allow some of the vessels to be shunted off so that regeneration and liberation of the useful species can be performed.

Figure 2 shows an example of a plant suitable for operation of the present invention according to the alternative embodiment.

The embodiment referring to the use of a nano-filtration material essentially follows the description of Figure 1 apart from the fact that the modules including the materials N (Nl, Nil) and Q (Ql, Qll) are omitted and that a separate nanofiltration module Z is included. The fluid from the second cation exchange material is lead to the nano-filtration module Z via valve Y. The permeate (uncharged constituents - H₃PO₄) from the nano-filtration step is lead to the vessel R whereas the retentate (charged constituents - HSO ^") is lead to the vessel O. Examples

The method according to the invention can be illustrated with the following experiment performed in a 1 :4.8 scale pilot-plant. The exact dimensions of the pilot-plant are as follow:

C: Device for pulverisation 600kg/h

D: Mixing tank 0.6 m³

I: Tank 0.6 m³

KI/KII: First cation exchange materials 50L of each arranged in 1.2m tall cylinders

NI/NII: First anion exchange materials 50L of each arranged in 1.2m tall cylinders QI/QII: Second anion exchange materials 50L of each arranged in 1.2m tall cylinders TI/TII: Second cation exchange materials 20L of each arranged in 1.2m tall cylinders

Ashes from combust waste water sludge (1000 g) was placed in a mill and pulverised into a fine dust with a particle size of 0.5-3.0 mm. The composition of the pulverised ashes is shown in table 1.

Subsequently, the pulverised ashes was mixed with 110 g of water and stirred for 3 min. at 8-15°C. Sulphuric acid (600 g) was added to the sludge and the mixture was stirred for 60 min. at 110-120°C before addition of additionally 8,700 g of water at 60°C. The mixture was stirred for another 120 min. at about 60°C and filtered to afford 9210 g of an acidified solution with pH 0.8 and 1200 g of wet sand. The composition of the obtained acidified solution is showed in table 1.

The acidified solution (pH 0.8) was contacted with the first cation exchange material, Marathon C, with a working temperature range of 0-120°C, for 7-8 min. at 40°C in order to absorb iron ions contained within the acidified solution. The composition of the solution after contact with the first cation exchange material is shown in table 1. The absorbed iron ions were recovered as FeCI₂ by regeneration of the first cation exchange material with a 20% aqueous solution of NaCI (total amount of NaCI used: 16,658mg).

Subsequently, the resulting solution from the passage of the first cation exchange material was contacted with the pre-washed first anion exchange material, Marathon A, with a working temperature range of 0-60°C. The first anion exchange material was washed with a 20% aqueous solution of NaCI prior to contact with the solution. The composition of the solution after contact with the first anion exchange material is shown in table 1.

The anionic counter ion HSO ^" was absorbed by the first anion exchange material from which it was recovered as KHSO using a 12% aqueous solution of KCI (total amount of KCI used: 34,628 mg)

After passage of the first anion exchange material, the pH of the resulting solution was raised to pH 3 using a 30% aqueous solution of NaOH (total amount of NaOH added: 20,600 mg). The solution was then contacted with the second anion exchange material, Marathon W.B.A., with a working temperature range of 0-60°C. The composition of the solution after contact with the second anion exchange material is shown in table 1.

The second anion exchange material absorbed the phosphate ion H₂PO ^" which was recovered by regeneration of the ion exchange material using a 30% aqueous solution of HCI (total amount of HCI used: 8,570 mg)

Finally, the solution was contacted with a second cation exchange material, LEWATIT TP 207 cation exchange materials from Bayer, Germany , with a working temperature range of 0-120°C. The second anion exchange material absorbed heavy metal ions which were recovered from the ion exchange material by regeneration of the material with a 20% aqueous solution of HCI (total amount of HCI used: 2022mg).

The composition of the remaining of the acidified solution after passage of the four ion exchange materials is shown in table 1. Table 1 : Composition of the solution throughout the purification process.

*The concentration has been calculated. The calculation was based on the analysed concentration of Cu and Zn in the solution after passage of the second cation exchange material.

Claims

1. A method for treatment of a solution, said solution comprising ions of at least one metal selected from iron and aluminium, phosphorus ions and at least one heavy metal selected from lead, cadmium, chromium, copper, mercury, nickel and zinc ions, in order to recover said ions of at least one metal, phosphorus and the at least one heavy metal, characterised in that the method comprises the following four steps performed in arbitrary order:

■ when the solution comprises HSO₄ ^" ions, contacting said solution with a first anion exchange material for separation of a substantial part of any HSO ^" ions;

■ contacting said solution with a second anion exchange material for separation of a substantial part of the H₂PO ^" ions; ■ contacting said solution with a second cation exchange material for separation of a substantial part of the ions of the at least one heavy metal.

2. A method for treatment of a solution, said solution comprising ions of at least one metal selected from iron and aluminium, phosphorus ions and at least one heavy metal selected from lead, cadmium, chromium, copper, mercury, nickel and zinc ions, in order to recover said ions of at least one metal, phosphorus and the at least one heavy metal, characterised in that the method comprises the following three steps performed in arbitrary order:

■ contacting said solution with a second cation exchange material for separation of a substantial part of the ions of the at least one heavy metal;

■ contacting said solution with a nano-filtration material for separating and recovering HSO ^" ions and H₃PO molecules.

3. A method according to claim 2, wherein the order of performing the three steps is the order given in claim 2.

4. A method according to claim 1 , wherein the order of performing the four steps is: ■ contacting said solution with a first cation exchange material; ■ when the solution comprises HSO₄ ^" ions, contacting said solution with a first anion exchange material;

■ contacting said solution with a second anion exchange material; and

■ contacting said solution with a second cation exchange material.

5. A method according to claim 1 , wherein the order of performing the four steps is:

■ contacting said solution with a first cation exchange material;

■ contacting said solution with a second cation exchange material;

■ when the solution comprises HSO ^" ions, contacting said solution with a first anion exchange material; and

■ contacting said solution with a second anion exchange material.

6. A method according to claim 4, wherein the pH value of the said solution is adjusted to pH -0.5 to 1.4, preferably pH 0.8 to 1.3, with acid, preferably H₂SO , prior to contacting said solution with the first cation exchange material.

7. A method according to claim 4, wherein the solution arising from contacting the solution with the first cation exchange material is contacted with the first anion exchange material, said solution having a pH value of 1 to 1.7, optionally after adjustment of the pH value with a base.

8. A method according to claim 4, wherein the solution arising from contacting the solution with the first anion exchange material, or, if the solution does not comprise HSO₄ ^" ions, arising from contacting the solution with the first cation exchange material, is contacted with the second anion exchange material after adjustment of the pH value of said solution with a base to pH 2.5 to 3.5.

9. A method according to claim 4, wherein iron and aluminium ions are recovered from the first cation exchange material by regeneration of said first cation exchange material with solutions comprising NaCI, HCI or H₂SO₄, preferably NaCI.

10. A method according to claim 4, wherein HSO ^" ions are recovered from the first anion exchange material by regeneration of said first anion exchange material with solutions comprising KCI, NaCI and/or HCI, preferably KCI.

11. A method according to claim 4, wherein H₂PO ^" ions are recovered from the second anion exchange material by regeneration of said second anion exchange material with solutions comprising HCI and/or H₂SO , preferably HCI.

12. A method according to claim 4, wherein heavy metal ions are recovered from the second cation exchange material by regeneration of said second cation exchange material with solutions comprising NaCI, HCI and/or H₂SO .

13. A method according to claim 1 or 4-8, wherein the solution is sequentially contacted with the four, or, if the solution does not comprise HSO₄ ^" ions, three, ion exchange materials which are arranged in series allowing continuously flow of the solution from one ion exchange material to the next ion exchange material.

14. A method according to claim 1-11 , wherein two portions of one or more of the ion exchange materials, or nano-filtration material, are arranged in a parallel manner, and where one portion is contacted with the solution being purified while the other portion is regenerated to recover the ions which have been adsorbed on to the ion exchange material in question in a previous contacting step.

15. A method for treatment of a continuous flow of a solution, said solution comprising ions of at least one metal selected from iron and aluminium, phosphorus ions and at least one heavy metal selected from lead, cadmium, chromium, copper, mercury, nickel and zinc ions, in order to recover said ions of at least one metal, phosphorus and the at least one heavy metal, characterised in that the method comprises the following four steps performed in the following sequence:

^■ contacting said solution with a first cation exchange material for separation of a substantial part of the ions of the at least one metal;

^■ when the solution comprises HSO ^" ions, contacting said solution with a first anion exchange material for separation of a substantial part of any HSO ^" ions said solution having a pH value of 1 to 1.7, optionally after adjustment of the pH value with a base, prior to the contacting;

■ contacting said solution with a second anion exchange material for separation of a substantial part of the H₂PO₄ ^" ions, said solution having a pH value of 2.5 to 3.5 after adjustment of the pH value of said solution with a base prior to the contacting; ^■ contacting said solution with a second cation exchange material for separation of a substantial part of the ions of the at least one heavy metal, where the solution is sequentially contacted with the four, or, if the solution does not comprise HSO₄ ^" ions, three, ion exchange materials which are arranged in series allowing continuously flow of the solution from one ion exchange material to the next ion exchange material.

16. A method according to claim 15, wherein iron and aluminium ions are recovered from the first cation exchange material by regeneration of said first cation exchange material with solutions comprising NaCI, HCI or H₂S0₄, preferably NaCI; HSO₄ ^" ions are recovered from the first anion exchange material by regeneration of said first anion exchange material with solutions comprising KCI, NaCI and/or HCI, preferably KCI; H₂PO ^" ions are recovered from the second anion exchange material by regeneration of said second anion exchange material with solutions comprising HCI and/or H₂S0₄, preferably HCI; and wherein heavy metal ions are recovered from the second cation exchange material by regeneration of said second cation exchange material with solutions comprising NaCI, HCI and/or H₂SO₄.

17. A plant for treatment of a stream of a solution, said solution comprising ions of at least one metal selected from iron and aluminium, phosphorus ions and at least one heavy metal selected from lead, cadmium, chromium, copper, mercury, nickel and zinc ions, in order to recover said ions of at least one metal, phosphorus and the at least one heavy metal, characterised in that the plant comprises the following:

^■ when the solution comprises HSO ^" ions, at least two vessels for contacting said stream with a first anion exchange material for separation of a substantial part of any HSO ^" ions;

^■ at least two vessels for contacting said stream with a second anion exchange material for separation of a substantial part of the H₂PO₄ ^" ions;

^■ at least two vessels for contacting said stream with a second cation exchange material for separation of a substantial part of the ions of the at least one heavy metal.

18. A plant according to claim 17, wherein the stream can be shunted off at least one of each of the at least two vessels.

19. A plant according to claim 17 or 18, wherein one of each of the at least two vessels can be arranged in series to allow the stream of the solution to flow through all of the one of the at least two vessels.

20. A plant according to claim 17 or 18, wherein one of the other of each of the at least two vessels can be adapted to allow regeneration and recovery of ions adsorbed on to the ion exchange material in question.

21. A plant according to claim 18-20, wherein each set of at least two vessels are arranged in a parallel manner wherein valves are arranged to control flow of the stream of the solution through the individual vessels.