Method for recovering an inorganic coagulant

Abstract

The present invention relates to a method for recovering an inorganic coagulant from a sludge. The method comprises obtaining a sludge originating from a treatment process of an aqueous medium, wherein the sludge comprises organic material and an inorganic coagulant in form of a metal hydroxide. The sludge is subjected to a thermal treatment, which comprises sludge drying and a high temperature treatment having a temperature of 300 - 1000 °C for destruction of the organic material. The high temperature treatment is carried out in a presence of oxygen. A treatment residue is obtained from the thermal treatment, and the treatment residue is dissolved with an inorganic acid and the inorganic coagulant is recovered.

Description

[0001] METHOD FOR RECOVERING AN INORGANIC COAGULANT

[0002] The present invention relates to a method for recovering an inorganic coagulant according to the preamble of the enclosed independent claim.

[0003] Inorganic coagulants, mainly aluminium and iron salts, are commonly used in treatment of water. When inorganic coagulant is added to water, e.g. aluminium chloride or polyaluminium chloride, it is transformed to a corresponding metal hydroxide, which flocculates contaminants, such as bacteria, dissolved organic substances, colouring agents, suspended solids, colloidal material and other substances. The formed flocs of metal hydroxide and contaminants are then separated from the water phase, resulting in formation of sludge. In addition to metal hydroxide and contaminants the separated sludge contains large amounts of water, which is complicated to remove. After dewatering the solid content of the sludge may still only be around 10 - 25 weight-%.

[0004] Water treatment processes produce large amounts of this relatively wet sludge, and its disposal is a problem. Often sludge is incinerated, composted or disposed of to a landfill. Incineration of sludge can be problematic, at least energy intensive, due to the high water content of the sludge, and the incineration process produces great amounts of ash, which leads to new disposal problems. Composting of the sludge is not always possible due to the organic contaminants present in the sludge. Even after the composting, the obtained material may still contain too high amount of pathogens or toxic substances, e.g. residuals of herbicides, which inhibit its use for soil improvement. Traditional landfilling requires vast areas, and the general trend is against landfill disposal, manifesting itself by legislative restrictions and / or expensive fees imposed on landfilling of sludge. Consequently, there is a need to an alternative method of taking care of the sludge. In view of the growing trend of circular economy, preferably the sludge should be put to practical use and exploited in manufacture of new products.

[0005] The production of inorganic coagulants itself would benefit if the amount of recycled material or waste raw material could be increased in their production. There is an incentive to find methods that would allow the use of new or alternative sustainable raw materials for production of inorganic coagulants.

[0006] The object of the present invention is to minimize or even eliminate the disadvantages existing in the prior art.

[0007] One object of the present invention is to provide a method for inorganic metal coagulant recovery from sludge.

[0008] Another object of the present invention is to provide an efficient method for inorganic coagulant recovery, preferably avoiding precipitation steps.

[0009] These objects are attained with the invention having the characteristics presented below in the characterising part of the independent claim.

[0010] Some preferred embodiments of the invention are presented in the dependent claims.

[0011] The embodiments mentioned in this text relate, where applicable, to all aspects of the invention, even if this is not always separately mentioned.

[0012] A typical method according to the present for recovering an inorganic coagulant from a sludge comprises

[0013] - obtaining a sludge originating from a treatment process of an aqueous medium, the sludge comprising organic material and an inorganic coagulant in form of a metal hydroxide,

[0014] - subjecting the sludge to a thermal treatment, which comprises sludge drying and a high temperature treatment having a temperature of 300 - 1000 °C for destruction of the organic material, wherein the high temperature treatment is carried out in a presence of oxygen,

[0015] - obtaining from the thermal treatment a treatment residue,

[0016] - dissolving the treatment residue into an inorganic acid and recovering the inorganic coagulant. Typical use of an inorganic coagulant according to the present invention, obtained by a method according to the invention, is for producing of potable water or for treating wastewater, especially industrial wastewater.

[0017] Now it has been surprisingly found that when the sludge from the treatment of an aqueous medium is subjected to a high temperature treatment at a temperature of 300 - 1000 °C, it is possible to transform the inorganic coagulant to such form, either crystalline or amorphous, which is relatively easy to dissolve into an inorganic acid, leading to the effective recovery of the inorganic coagulant. Furthermore, the structure of the organic material in the sludge is at the same time effectively destroyed so that it can be removed. The inorganic coagulant recovered from the sludge with the method of the present invention shows appropriate concentration of coagulant metal and appropriate flocculation efficiency and can be used in a conventional manner. The present invention thus enables not only the reduction of sludge amount that must be disregarded, but also increases in the produced inorganic coagulant product the amount of material originating from recycled material and from waste material. The method produces effective inorganic coagulants, which can be used for various water treatment applications.

[0018] The sludge for recovery of the inorganic coagulant originates from a treatment process of an aqueous medium. The sludge comprises organic material and the inorganic coagulant, such as aluminium, in form of a metal hydroxide. The sludge may comprise 7 - 50 weight-%, preferably 12 - 40 weight-%, of inorganic coagulant, calculated from the weight of the sludge, as dry. When the inorganic coagulant is aluminium, the sludge may contain 7 - 30 weight-%, preferably 12 - 25 weight-%, of aluminium, calculated from the weight of the sludge, as dry. When the inorganic coagulant is iron, the sludge may contain 15 - 50 weight-%, preferably 20 - 40 weight-%, of iron, calculated from the weight of the sludge, as dry.

[0019] The sludge used for recovery of the inorganic coagulant may originate from treatment and / or production of potable water, from treatment of surface water and / or ground water, from treatment of industrial wastewater, especially wastewater from food or beverage industry, or from treatment of wastewater from pulp or paper industry. Preferably the sludge originates from treatment and / or production of potable water or industrial water produced from surface water and / or ground water, by using aluminium and / or iron coagulants.

[0020] The inorganic coagulant may be an aluminium coagulant or an iron coagulant. According to one preferable embodiment the method is for the recovery of an inorganic coagulant, selected from an aluminium coagulant, such as aluminium chloride, polyaluminium chloride or aluminium sulphate, or an iron coagulant, such as iron chloride or iron sulphate. Preferably the method is for the recovery of an aluminium coagulant, especially aluminium chloride or polyaluminium chloride.

[0021] According to the invention, the sludge is subjected to a thermal treatment comprising sludge drying and a high temperature treatment at a temperature of 300 - 1000 °C, which leads to destruction of the organic material, and optionally transformation of the inorganic coagulant from the metal hydroxide form into a metal oxide. The gist of the thermal treatment is to remove water and moisture from the sludge and to raise the temperature to a level where the organic material in the sludge is effectively destructed. In this manner undesired substances, such as pathogens, medicinal residues, etc., can be effectively removed and the quality of the recovered inorganic coagulant guaranteed. Furthermore, the destruction of the organic material either removes the organic material completely or transforms it to a form which is essentially insoluble in the inorganic acid and can be easily removed, e.g. by filtration in the following process steps. The thermal treatment also preferably transforms the inorganic coagulant from hydroxide form to a crystalline or amorphous form which have good solubility in the inorganic acid.

[0022] The present method is preferably free of dissolution and precipitation steps involving acids or alkaline agents before the sludge is subjected to thermal treatment. This means that the sludge originating from the treatment process of an aqueous medium is directly subjected to thermal treatment, excluding any preceding dissolution and / or precipitation steps. Before the thermal treatment, the sludge is may be subjected pre-treatment step consisting of mechanical removal of water, e.g. through filtering, pressing and / or centrifuging, as explained later.

[0023] In the thermal treatment, the sludge drying can be conducted in any suitable industrial arrangement available and by using any suitable drying method. The term “drying” indicates here removal of moisture and / or liquid from the sludge by evaporation, preferably at an elevated drying temperature. The sludge drying may employ either direct or indirect transfer of heat, or both. Suitable sludge drying techniques employ, for example, drum dryers, fluidized bed dryers, thin film dryers, belt dryers, tray dryers or the like. According to one embodiment, sludge drying may be performed in a drying temperature in a range of 50 - 250 °C, preferably 75 - 200 °C, more preferably 90 - 120 °C. The sludge drying is usually conducted until the sludge has obtained the desired solids content. For example, the sludge may be dried at the drying temperature of 50 - 250 °C, or 75 - 200 °C, or 90 - 120 °C, or 95 - 110 °C until the sludge has obtained the solids content of at least 80 weight-%, preferably at least 85 weight-%, more preferably at least 90 weight-%, calculated from the total sludge weight. After drying the solids content of the sludge may be in a range of 80 - 100 weight-%, preferably 85 - 99 weight-%, more preferably 90 - 98 weight-%. Effective sludge drying can save energy in the following method step of the high temperature treatment, where the organic material is destructed. It is possible that some part of the organic material may be destructed or at least degraded already during the sludge drying, but the destruction of the organic material mainly occurs in the high temperature treatment, which follows the sludge drying.

[0024] The duration of the sludge drying depends on the drying temperature, solids content of the sludge before the sludge drying and the solids content desired after the sludge drying. Typically sludge drying is continued for 0.5 - 24 hours, or for 1 - 12 hours, or for 2 - 8 hours. Depending on the employed drying technique, the duration of the sludge drying may even be short, even <1 h or <0.5 h.

[0025] The thermal treatment comprises, in addition to sludge drying, the high temperature treatment conducted at a temperature of 300 - 1000 °C, preferably 325 - 900 °C, more preferably 350 - 800 °C, even more preferably 375 - 750 °C, for destruction of the organic material. According to one embodiment, the high temperature treatment may have the temperature in the range of 325 - 700 °C, preferably 350 - 625 °C, more preferably 375 - 600 °C. For example, the high temperature treatment may be conducted at the of 300 - 650 °C, preferably 325 - 625 °C, more preferably 350 - 600 °C, even more preferably 375 - 575 °C. According to one embodiment, also a transformation of the inorganic coagulant from the metal hydroxide form into a metal oxide occurs during the high temperature treatment. The high temperature treatment may be combustion or incineration. Any suitable combustion or incineration technique can be employed. The duration of the high temperature treatment may be 0.5 - 8 h, preferably 1 - 6 h, more preferably 1 .5 - 4 h. Depending on the employed high temperature treatment technique and the employed temperature, the duration of the high temperature treatment may even be short, even <1 h, or <0.5 h or even <15 min or <5 min.

[0026] Advantageously the high temperature treatment is carried out in a presence of oxygen. In this case the high temperature treatment is combustion, incineration or the like. When the high treatment temperature is carried out in the presence of oxygen, e.g. as combustion or incineration, a transformation of the inorganic coagulant from the metal hydroxide form into the metal oxide can effectively take place simultaneously with the destruction of the organic material. After the high temperature treatment have been carried out in a presence of oxygen, such as combustion, the treatment residue may comprise inorganic coagulant, such as aluminium or iron, mainly in oxide form, showing good solubility in the inorganic acid. However, even in this case the treatment residue may comprise the inorganic coagulant in amorphous form.

[0027] According to one embodiment the sludge drying may be performed as a separate step from the high temperature treatment. For example, sludge drying may be performed by using any of drying techniques described above, at any temperature or temperature interval described above, for obtaining a partially dried or dried sludge. This partially dried or dried sludge may then be transferred to the high temperature treatment, where the dried or partially dried sludge is subjected to a temperature of 300 - 1000 °C, preferably 325 - 900 °C, more preferably 350 - 800 °C, even more preferably 375 - 750 °C. For example, in high temperature treatment the dried or partially dried sludge may be subjected to a temperature in a range of 325 - 700 °C, preferably 350 - 625 °C, more preferably 375 - 600 °C. Possible temperatures in high temperature treatment may be in the range of 300 - 650 °C, preferably 325 - 625 °C, more preferably 350 - 600 °C, even more preferably 375

[0028] - 575 °C. The lower temperature ranges can be preferable due to reasons of process technology and process efficiency. The high temperature treatment may be combustion, incineration or pyrolysis.

[0029] According to one embodiment the thermal treatment may be performed as a single step comprising both the sludge drying and the high temperature treatment. In this case, the sludge drying and the high temperature treatment occur either simultaneously and / or consecutively in a single heat treatment apparatus or furnace. For example, the thermal treatment may be carried out by using rotary kiln or other similar calcination furnace, where the sludge is fed into the furnace at its first end and the treatment residue is removed from the furnace at its second end. The temperature at the first end of the calcination apparatus may be in a range of 90 - 150 °C, preferably 95 - 120 °C, and the temperature at the second end may be 600

[0030] - 1000 °C, preferably 650 - 800 °C. According to one embodiment, the temperature at the second end may be 325 - 700 °C, preferably 350 - 625 °C, more preferably 375 - 600 °C. Possible temperatures may be, for example, in the range of 300 - 650 °C, preferably 325 - 625 °C, more preferably 350 - 600 °C, sometimes even 375 - 575 °C.

[0031] According to one preferable embodiment, the present method may be used for recovery of an aluminium coagulant, such as aluminium chloride, polyaluminium chloride or aluminium sulphate. The temperature of the high temperature treatment is selected so that the treatment residue comprises aluminium in form capable of dissolving in inorganic acid, such as amorphous form, oxide form or any mixture thereof. Such inorganic acid dissolving forms of aluminium oxide are, for example y-alumina (gamma alumina), x-alumina (chi alumina) and q-alumina (eta alumina). Preferably, the temperature of the high temperature treatment is selected so that the treatment residue comprises aluminium oxide in form of y-alumina (gamma alumina), x-alumina (chi alumina) and q-alumina (eta alumina).

[0032] The present invention makes it easy to adjust the temperature of the high temperature treatment, so that the temperature does not exceed the interval for formation of the desired aluminium oxides or amorphous aluminium. The sludge may be subjected to high temperature treatment at the temperature in the range of 300 - 750 °C, preferably 325 - 650 °C or 325 - 700 °C, more preferably 350 - 600 °C or 350 - 625 °C or 375 - 600 °C. The temperature may be, for example, in the range of 300 - 650 °C, preferably 325 - 625, more preferably 350 - 600 or 350 - 575 °C, sometimes even 375 - 575 or 325 - 550 °C, for transforming the aluminium in the sludge into x-alumina (chi alumina) or q-alumina (eta alumina). Alternatively, the temperature may be, for example, in the range of 500 - 700 °C, preferably 550 - 700 °C or 550 - 650, for transforming the aluminium in the sludge into y-alumina (gamma alumina).

[0033] According to another embodiment the method may be used for recovery of iron coagulant, such as iron chloride or iron sulphate. The temperature of the high temperature treatment is selected so that the iron is obtained in mineral form capable of dissolving in inorganic acid, such as amorphous form, oxide form or any mixture thereof. Such acid dissolving forms of iron oxide is, for example magnetite or wustite. The sludge may be subjected to high temperature treatment at the temperature in the range of 300 - 900 °C, preferably 325 - 750 °C, more preferably 350 - 700 °C. The temperature may be, for example, in the range of 400 - 700 °C, preferably 450 - 650 °C, for transforming the iron in the sludge into magnetite.

[0034] A treatment residue is obtained from the thermal treatment. The treatment residue may comprise the inorganic coagulant in form of oxide or in amorphous form. The treatment residue may further comprise other inorganic residues in form of inorganic oxides, as well as destructed organic material in form which is undissolved in inorganic acid. The organic material in the treatment residue may be given as the amount of total organic carbon. The amount of total organic carbon (TOC) for the treatment residue may be <40 weight-%, preferably <20 weight-%, more preferably <15 weight-%, calculated from the total dry weight of the treatment residue, measured as described in the experimental section. The amount of total organic carbon for the treatment residue may be in a range of 0.01 - 40 weight-%, preferably 0.1 - 20 weight-%, more preferably 0.5 - 15 weight-%, calculated from the total dry weight of the treatment residue. When the high temperature treatment is carried out in presence of oxygen, for example as combustion or incineration,, then the amount of total organic carbon in the treatment residue may be in a range of 0.01 - 15 weight-%, preferably 0.1 - 10 weight-%, more preferably 0.5 - 6 weight-%, calculated from the total dry weight of the treatment residue.

[0035] According to one embodiment, the organic material may be nearly completely absent from treatment residue. The total amount of inorganic residues selected from calcium, silica and sulphur in the treatment residue is preferably low, <5 weight-%, preferably <4 weight-%, more preferably <3 weight-%, calculated from total dry weight of the treatment residue, measured with ICP-MS, as described in the experimental section. The total amount of inorganic residues selected from calcium, silica and sulphur in the treatment residue may be in a range of 0.01 - 5 weight-%, typically 0.5 - 4 weight-%, more typically 1 - 3 weight-%, calculated from total dry weight of the treatment residue, measured with ICP-MS, as described in the experimental section. Preferably the amount of total organic carbon as well as the total amount of inorganic residues selected from calcium, silica and sulphur is as small as possible.

[0036] According to one preferable embodiment, when the inorganic coagulant is aluminium, the treatment residue comprises 15 - 60 weight-%, typically 20 - 50 weight-%, more typically 30 - 40 weight-%, of aluminium, calculated from the total dry weight of treatment residue, measured with ICP-MS, as described in the experimental section.

[0037] According to one preferable embodiment, when the inorganic coagulant is iron, the treatment residue comprises 25 - 80 weight-%, typically 30 - 75 weight-%, more typically 50 - 70 weight-%, of iron, calculated from the total dry weight of treatment residue, measured with ICP-MS, as described in the experimental section. The treatment residue is dissolved into an inorganic acid and the inorganic coagulant is recovered. When the treatment residue is dissolved into the inorganic acid, the obtained solution forms the inorganic coagulant which is ready to use. The treatment residue may be dissolved into hydrochloric acid or sulphuric acid, preferably hydrochloric acid. Sulphuric acid can only be used if the treatment residue contains a low amount of calcium and / or magnesium, so that excess formation of gypsum and related compounds can be avoided. However, hydrochloric acid is preferred. When the inorganic coagulant is aluminium, use of hydrochloric acid for dissolving the treatment residue leads to recovery of aluminium chloride or polyaluminium chloride, and when the inorganic coagulant is iron, its use for dissolving the treatment residue leads to recovery of iron chloride.

[0038] If needed, undissolved solid material can be separated after dissolving from the inorganic coagulant. The separation can be done for example by filtration.

[0039] The duration of the dissolution of the treatment residue into the inorganic acid may be, for example, 0.5 - 8 h, preferably 1 - 5 h, more preferably 1.5 - 4.5 h. The treatment residue may be dissolved in the inorganic acid under an elevated temperature and / or pressure. Typically the temperature may be in a range of 70 - 200 °C, preferably 90 - 170 °C, more preferably 110 - 165 °C and / or the pressure may be in a range of 2 - 10 bar, typically 2.5 - 7 bar, more typically 3 - 6 bar.

[0040] The sludge may be subjected to one or more optional pre-treatments before the thermal treatment. For example, the solid content of the sludge may be increased by mechanical dewatering. Mechanical dewatering indicates here removal of water by mechanical methods, such as pressing, filtering, centrifuging, suction. Any conventional dewatering method can be used. According to one embodiment the sludge used in the present method may be a dewatered sludge having a dry solids content of 10 - 35 weight-%, typically 15 - 25 weight-%.

[0041] The treatment residue may be subjected to one or more optional treatments after the thermal treatment but before the dissolving into the inorganic acid. According to one embodiment, the treatment residue may be subjected to one or more acid washes after the thermal treatment and before dissolving into the inorganic acid. The acid wash may be conducted by using diluted inorganic acid. The inorganic acid may be same or different inorganic acid that is used for dissolving. Preferably the inorganic acid may be hydrochloric acid. The concentration of the diluted inorganic acid may be 5 - 20 weight-%, preferably 7 - 15 weight-%. The acid wash(es) remove(s) effectively calcium, magnesium and other disturbing ions which have higher solubility than the inorganic metal coagulant. Especially removal of calcium ions from the treatment residue is advantageous, as it provides use of sulphuric acid in dissolving, with reduced or eliminated risk of gypsum formation.

[0042] According to one preferable embodiment the treatment residue is directly transferred from thermal treatment to the dissolving into the inorganic acid, without of any washing steps. This makes the method simple to perform, saves chemicals and water and may improve recovery of inorganic coagulant.

[0043] According to one preferable embodiment the inorganic coagulant, obtained by the present method is used for treating wastewater, especially industrial wastewater.

[0044] According to one embodiment, the high temperature treatment may be pyrolysis. Any suitable pyrolysis technique can be employed. The pyrolysis is conducted in an inert atmosphere in absence of oxygen. Nitrogen atmosphere is preferable. The method for recovering an inorganic coagulant from a sludge thus comprises 1 ) obtaining a sludge originating from a treatment process of an aqueous medium, the sludge comprising organic material and an inorganic coagulant in form of a metal hydroxide; 2) subjecting the sludge to a thermal treatment, which comprises sludge drying and a high temperature treatment having a temperature of 300 - 1000 °C for destruction of the organic material; 3) obtaining from the thermal treatment a treatment residue; and 4) dissolving the treatment residue into an inorganic acid and recovering the inorganic coagulant. All conditions, embodiments and results described above, e.g. temperature ranges, fully apply when the high temperature treatment is pyrolysis. The pyrolysis of sludge minimises the formation of insoluble oxides during the high treatment step, which may improve the dissolving of the treatment residue into the inorganic acid at the later process step. After pyrolysis, the treatment residue may comprise inorganic coagulant, such as aluminium or iron, in amorphous form or in oxide form, showing good solubility in the inorganic acid. The aluminium is usually present in the treatment residue as amorphous aluminium or aluminium hydroxide. However, it was found that aluminium in these forms was easily dissolved in the inorganic acid, and the organic material was effectively destroyed. The amount of total organic carbon in the treatment residue may be in a range of 0.1 - 40 weight-%, preferably 1 - 37 weight-%, more preferably 10 - 35 weight-%, calculated from the total dry weight of the treatment residue.

[0045] EXPERIMENTAL

[0046] An embodiment of the invention is described in the following non-limiting example.

[0047] Example 1

[0048] Analysis Methods

[0049] ICP-MS: ICP-MS analyses were done according to standard SFS-EN ISO 17294- 2:2016, by using an Agilent 7700x analyzer.

[0050] Total Organic Carbon (TOC): TOC was analysed with Shimadzu TOC 5000 A instrument for solid samples, which determines TOC indirectly by measuring Total Carbon (TC) and Total Inorganic Carbon (TIC). TIC is subtracted from TC, yielding TOC. Results were calculated using TOC-Control-L software.

[0051] Sludge

[0052] Potable water treatment plant sludges were sampled from a Finnish water treatment plant and from a Swedish water treatment plant. The Finnish sludge comprised Fe and the dewatered sludge from the Swedish water treatment plant comprised Al.

[0053] Drying and incineration of sludge

[0054] The sludge samples were dried overnight at 105 °C. The dried sludge samples were incinerated at different temperatures and different incineration times. The incineration temperatures and times for the two sludge samples are shown in Table 1.

[0055] Incineration was carried out in oven in atmospheric conditions and no additional oxygen was used.

[0056] Table 1 Incineration temperatures and times for the two sludge samples.

[0057] *450 °C for 2h only for the Finnish sludge.

[0058] **900 °C for 2h and 4 h only for Swedish sludge.

[0059] Pyrolysis

[0060] As an alternative to incineration, pyrolysis was also tested with the sludge samples. The pyrolysis was performed in a tubular furnace at 350 °C, 500 °C or 550 °C for 4h under N2 atmosphere. The pyrolysis time was 4 hours, pyrolysis temperatures are given in Table 2.

[0061] Table 2 Pyrolysis temperatures. Pyrolysis time was 4 h for all samples.

[0062] Washing

[0063] Some treatment residue samples were washed with 10% HCI before the treatment residue was dissolved in acid, see Table 3.

[0064] Acid dissolution of samples into HCI or H2SO4 after incineration / pyrolysis Water was first added to the reactor (a Florence flask with a reflux condenser) and the mixing was started. The calculated amount of 34% HCI or 95% H2SO4 was added next, which raised the temperature. The needed acid amount was calculated based on the Fe and Al content, determined with ICP-MS. The temperature was allowed to drop to 30 °C before the heating was started. 20 g of the incinerated or pyrolyzed sludge sample was added to the reactor. The temperature was raised in a controlled manner to 102 °C. The mixture was dissolved under constant stirring with a magnetic stirrer for the required dissolution time.

[0065] After dissolving, the mixture was allowed to cool down to 50 °C and poured through a Buchner filter. The obtained filter cake was washed with water. The acid phase from dissolution was analysed with ICP-MS to provide the metal concentration in the recovered inorganic coagulant.

[0066] The dissolution parameters and the measured metal concentrations for the recovered inorganic coagulants from different sludge samples are shown in Table 3.

[0067] The results in Table 3 show that the recovered inorganic coagulant had an appropriate metal concentration that made possible its use as coagulant for water treatment. The disclosed method thus provides an effective way to recover inorganic coagulant from sludge.

[0068] Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims. Thus, the described embodiments are illustrative and should not be construed as restrictive. Table 3 The dissolution parameters and the measured metal concentrations for the recovered inorganic coagulants.

[0069] * I = Combustion, P=Pyrolysis

[0070] ** The addition coefficient gives the molar ratio of acid to active metal (e.g. HCI / AI). *** Target metal concentration indicates the calculated active metal concentration, based on treatment residue analysis. Measured metal concentration is the actual measured value after dissolution in acid.

Claims

CLAIMS1. Method for recovering an inorganic coagulant from a sludge, the method comprising- obtaining a sludge originating from a treatment process of an aqueous medium, the sludge comprising organic material and an inorganic coagulant in form of a metal hydroxide,- subjecting the sludge to a thermal treatment, which comprises sludge drying and a high temperature treatment having a temperature of 300 - 1000 °C for destruction of the organic material, wherein the high temperature treatment is carried out in a presence of oxygen,- obtaining from the thermal treatment a treatment residue, and- dissolving the treatment residue into an inorganic acid and recovering the inorganic coagulant.

2. Method according to claim 1 , characterised in that the inorganic coagulant is an aluminium coagulant or an iron coagulant, preferably an aluminium coagulant.

3. Method according to claim 1 or 2, characterised in that the sludge drying is performed as a separate step from the high temperature treatment.

4. Method according to claim 1 , 2 or 3, characterised in that the thermal treatment is performed as a single step comprising the sludge drying and the high temperature treatment.

5. Method according to any of preceding claims 1 - 4, characterised in that the sludge drying is performed in a drying temperature in a range of 50 - 250 °C, preferably 75 - 200 °C, more preferably 90 - 120 °C.

6. Method according to any of preceding claims 1 - 5, characterised in that the high temperature treatment has the temperature of 325 - 900 °C, preferably 350 - 800 °C, more preferably 375 - 750 °C.

7. Method according to any of preceding claims 1 - 6, characterised in that the high temperature treatment has the temperature of 325 - 700 °C, preferably 350 - 625 °C, more preferably 375 - 600 °C.

8. Method according to any of preceding claims 1 - 7, characterised in that the inorganic coagulant is an aluminium coagulant and the sludge is subjected to high temperature treatment at the temperature in the range of 300 - 750 °C, preferably 325 - 650 °C, more preferably 350 - 600 °C.

9. Method according to any of preceding claims 1 - 6, characterised in that the inorganic coagulant is an iron coagulant and the sludge is subjected to high temperature treatment at the temperature of in the range of 300 - 900 °C, preferably 325 - 750 °C, more preferably 350 - 700 °C.

10. Method according to any of preceding claims 1 - 9, characterised in that the treatment residue is subjected to an acid wash after thermal treatment and before dissolving the metal oxide of the inorganic coagulant.

11. Method according to any of preceding claims 1 - 10, characterised in that the treatment residue is dissolved into hydrochloric acid.

12. Method according to any of preceding claims 1 - 11 , characterised in that the treatment residue is dissolved in the inorganic acid under an elevated temperature and / or pressure.

13. Method according to any of preceding claims 1 - 12, characterised in that the sludge originates from treatment and / or production of potable water, from treatment of surface water and / or ground water, from treatment of industrial wastewater, especially from food or beverage industry, or from treatment of wastewater from pulp or paper industry.

14. Method according to any of preceding claims 1 - 13, characterised in that the sludge is a dewatered sludge having a dry solids content of 10 - 35 weight-%, typically 15 - 25 weight-%.

15. Method according to any of preceding claims 1 - 14, characterised in transformation of the inorganic coagulant from the metal hydroxide form into a metal oxide during the high temperature treatment.

16. Use of inorganic coagulant, obtained by a method according to any of claims 1 - 15, for producing of potable water or for treating wastewater, especially industrial wastewater.

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