EP3212577A1

EP3212577A1 - A method for treating a waste containing sulfuric acid

Info

Publication number: EP3212577A1
Application number: EP15825628.9A
Authority: EP
Inventors: Maria Fernanda Garcia; Maria De Los Angeles Garcia
Original assignee: Dell'omodarme Mario Alberto
Current assignee: Dell'omodarme Mario Alberto
Priority date: 2014-10-27
Filing date: 2015-10-27
Publication date: 2017-09-06
Also published as: WO2016067085A1

Abstract

A method for treating a waste material (20) containing sulfuric acid and metal ions, as In the case of the waste material the production of the T1O2, provides mixing to the waste material (20) an alkaline reagent (10), selected among a hydroxide of an alkaline metal such as Mg, Zn, Sn, adapted to selectively precipitate (300) the cations as hydroxides, thus obtaining a first mixture (21) having a higher pH that contains a solution (22) of a sulfate and/or oxysulfate of the metal and solid hydroxides (23) of the metal cations that are separated from the first mixture (21) and can be used in metal sulfates production process. The method also comprises mixing the solution (22) with a precipitation agent (31), thus forming a second mixture (32) in which the sulfate and/or oxysulfate precipitates and is removed (301), heated, reduced and decomposed (400) by means of a reducing agent (41) selected among elemental sulfur, hydrogen and a reducing flame, thus obtaining SO₂ (42) available for making H₂SO₄, and a solid oxide (43) of the metal of the alkaline reagent that is recycled (101) as alkaline reagent (10) in the step (200) of mixing with the waste material (20). Advantageously, a step (350) is provided of recovering precipitation agent (31) from the water of second mixture (32), after removing (301) the sulfate and/or oxysulfate, and a step (351) is also provided of recycling the regenerated precipitation agent (31), In particular, an alcohol as ethyl alcohol, used as the precipitation agent (31), can be easily regenerated by distillation, exploiting low temperature-heat, which is largely available from a possible sulfuric acid production plant close to the plant that producing the sulfuric waste material (20). The method according to the invention makes it possible to eliminate the sludge resulting from the sulfuric waste material treatment, while recovering valuable compounds.

Description

TITLE

A METHOD FOR TREATING A WASTE CONTAINING SULFURIC ACID.

DESCRIPTION

Field of the invention

[0001] The present invention relates to a method for treating a waste material containing spent sulfuric acid (H2SO4) and metals in the form of cations, such as iron, magnesium, calcium, chromium, titanium, aluminium, vanadium, manganese and the like.

Prior art - Technical problems

[0002] Several industrial chemical processes produce spent sulfuric acid as a waste, i.e. at a low concentration. Moreover, metal cations are present in such waste material.

[0003] In some of these chemical processes, sulfuric acid is used as an intermediate product, in other words the sulfur is not present in the final product. One example is the process for manufacturing titanium dioxide (T1O2) starting from a mineral such as l!menite (FeTi03) or from a Titanium-rich waste material, which is digested with concentrated sulfuric acid, In this case, the spent acid is accompanied by metal ions such as iron-ll, but also vanadium, manganese, chromium, and the like.

[0004 ] In most cases, these acid waste materials are neutralized directly, usually with lime or with alkaline products containing calcium, This way, a sludge is obtained mainly consisting of gypsum, in particular CaS04, which co- precipitates with the metal cations, if these are present; in the case of the T1O2 manufacture, this leads to the well known "red mud", The mud is then stored in large dumps.

[0005] This way to proceed has serious drawbacks, in particular large amounts of material must be moved and stored, which remarkably contributes to increase transport and treatment costs. Moreover, these dumps require wide lands, which could be exploited for other purposes, For these reasons, the waste storage costs are important, as well as costs that must be borne if a disused dump has to be decontaminated and restored. [0006] An alternative to direct neutralization, and to dumping the mud, consists in producing a "white gypsum", i,e. a mud without metal ions, which is however hard to be sold, since it is otherwise easily available, and it is hard to be produced in an economically sustainable way.

[0007 ] Processes have been also proposed to obtain sulfates of Fe, Mg, Na, Al, etc. from such waste material, but this approach meets with several difficulties:

— the large availability of sulfuric waste material would make it possible to produce sulfate amounts that cannot be brought onto the market as a single product, which would require investments to make the plants well-suited for manufacturing multiple products;

— the presence of heavy metal ions makes it difficult to obtain "pure" products, on the other hand the presence of heavy metal sulfates remarkably limits the possible uses of these products;

— concerning T1O2 manufacture, the spent acid are available at very low H2S concentrations (200-300 g/l), which requires a large amount of energy to crystallize the sulfates and also makes it hard to exploit low-enthalpy heat, which is largely available from sulfuric acid manufacture, since the higher the concentration, the higher the boiling point is during the crystallization,

[0008 ] WO2013037649 discloses a magnesium sulfate ( gSC ) production method, comprising the steps of concentrating a sulfuric waste material from T\Oz manufacture; bringing it into contact with a magnesium-based neutralizing agent, in order to cause a reaction by which an aqueous solution of MgSC is formed; crystaH^'izating the gSO-t to obtain a crystalline product to be used for agricultural purpose,

[0009] CN 101513993A discloses a process for manufacturing H2SO4 from MgS04 comprising a step of calcination, decomposition and reduction in which magnesium oxide (MgO) and sulfur dioxide (SO2) are formed. The MgO is reused for treating acid waste materials, whereas the SO2 is cooled to pre-heat a MgS04 feed, then is catalytically converted into SOs and finally is absorbed into water to form H2SO4.

[0010] EP 0125142 A2 discloses a process for treating a sulfuric waste material containing different metal cations, in which an initial step is provided of concentrating the waste material and a following treatment step is provided in which metal oxides are used. The partially neutralized final product is dried and treated with coal; this way, a granular product is obtained that contains sulfates of metals such as Fe, Mg, Al, Cr etc., which have on average 5 hydration water molecules, In the subsequent calcination step, the product receives the energy that is required for removing these water hydration molecules and for decomposing the sulfates into SO∑ and the oxides of the respective metals. One part of the oxides produced in the calcination step is recycled to the initial oxide treatment step. However, a significant amount of the oxides must be purged away, so that it does not accumulate in the process, which is environmentally relevant. Another drawback of EP0125142A2 is that a very energy-consuming concentration step is required at the beginning of the process, A further drawback is that several sulfates are involved in the calcination step, which have dissociation temperatures very different from one another, and in any case high dissociation temperatures.

[0011] The need is therefore felt of a method for treating a liquid waste material of a chemical process, the waste material containing H2$0 and metal cations such as iron, magnesium, calcium, chromium, titanium, aluminium, vanadium, manganese ions etc., in which the by-products can be treated without substantially producing calcium sulfate-containing mud, while retaining a substantial portion of the by-products, without producing materials that can hardly be brought onto the market.

Summary of the invention

[0012] It is therefore an object of the present invention to provide a method for treating a waste material of a chemical or similar process, said waste material containing H2SO4 and metal cations as iron, magnesium, calcium, chromium, titanium, aluminium, vanadium, manganese ions and the like, which makes it possible to substantially recover the whole waste material within the process that generates the waste material.

[0013] It is a particular object of the invention to provide such a method that makes it possible to effectively treat an acid waste material from the sulfate process for manufacturing T1O2. [0014 ] It is also a particular object to provide such a method for recovering the cations present in the waste material and obtaining useful products.

[0015] It is also a particular feature of the invention to provide such a treatment method that involves treatment costs lower than the prior art processes, in particular lower energy costs.

[ 0016] It is also particular feature of the invention to provide such a treatment method that makes it possible to exploit the low-enthalpy heat that is available from the process itself that produces the waste material, or from a different process connected to it, as in the case of H2SO4 manufactured for making Ti02,

[0017 ] These and other objects are achieved by a method for treating a process waste material containing sulfuric acid and metal cations, comprising the steps of:

— prearranging an alkaline reagent comprising a hydroxide of a metal selected from the group consisting of: Magnesium; Zinc; Tin; a combination thereof;

— mixing the alkaline reagent with the waste material, obtaining a first mixture, which has a pH higher than the waste material, comprising a liquid sulfate and/or oxysulfate solution of the metal of the alkaline reagent and hydroxides of the metal cations as a solid;

— separating the hydroxides of the metal cations from the liquid solution;

— mixing the solution of the sulfate and/or oxysulfate with a precipitation agent for the sulfate and/or oxysulfate, creating a second mixture, and causing the sulfate and/or oxysulfate to precipitate as a solid sulfate and/or oxysulfate in the second mixture;

— removing the solid sulfate and/or oxysulfate from the second mixture;

— bringing the solid sulfate and/or oxysulfate into contact with a reducing agent selected from the group consisting of:

— elemental sulfur;

— hydrogen gas;

— an oxyacetylene flame;

— a reducing flame comprising atomic hydrogen and oxygen;

— a mixture of hydrogen, carbon monoxide and carbon dioxide, and heating up to a predetermined reduction and decomposition temperature, so as to cause a reduction and decomposition of the solid sulfate and/or oxysulfate thus obtaining;

— gaseous sulfur dioxide SO2;

— a calcined oxide of the metal of the alkaline reagent,

wherein the step of prearranging an alkaline reagent comprises a step of recycling the oxide of the metal of the alkaline reagent obtained from the reduction and decomposition of the sulfate and/or oxysulfate.

[0018 ] The above mentioned metals, g, Zn, Sn, can form water-soluble sulfates and/or oxysulfates. This way, by treating the waste material with an alkaline reagent comprising a hydroxide of one of these metals, or also a combination of such hydroxides, the above-mentioned metal cations, which are present in the waste material, are caused to almost fully precipitate in the form of insoluble hydroxides, whereas the sulfate/oxysulfate of Mg or Zn or Sn remains in the solution. For this reason, the hydroxide precipitate can be easily separated from the sulfates/oxysulfates by a simple operation, such as a filtration or a centrifugation, Therefore, the metals that are present in the waste material not only immediately leave the process, which makes it easier to recover the alkaline reagent from the acid solution before recycling it, but are also easier to be reused in several industrial processes, due to their hydroxide nature, typically for producing sulfates of the respective metals.

[ 0019 ] Moreover, the oxides of Mg, Zn, Sn, of the residue of the step of reduction and decomposition, if water is present, can provide alkaline suspensions of hydroxides, which can be used again in the step of mixing, i.e, when treating the waste material stream.

[ 0020 ] As a further advantage, with respect to EP125142A2, an initial step of waste material concentration is not necessary, which makes the process easier and reduces energy costs.

[ 0021 ] Preferably, the alkaline reagent is Magnesium hydroxide, Mg(OH)2. This makes it possible to minimize the initial costs for supplying the alkaline reagent.

[ 0022 ] Advantageously, a step is provided of adding an oxide of said metal of said alkaline reagent to the liquid solution of the sulfate of the metal of the alkaline reagent. In particular, the MgO addition is carried out in order to form an oxysulfate of the metal of the alkaline reagent in the liquid solution, by combining the oxide with already formed magnesium sulfate, In particular, a step is provided of adding magnesium oxide MgO to the liquid solution of magnesium sulfate, in order to form magnesium oxysulfate 0(MgO)zS02 by combining magnesium oxide MgO with already formed magnesium sulfate MgS04.

[00X0] In particular, the step of mixing is carried out in such a way that the liquid solution reaches a metal precipitation pH value set between 8,85 and 9,5, in particular set between 9.0 and 9,2, in order to cause most of the above- mentioned metal cations to precipitate as hydroxides, without causing Magnesium hydroxide to precipitate.

[0011] Preferably, the step of prearranging an alkaline reagent comprises a step of forming a slurry of the alkaline reagent. In particular, the alkaline reagent in the slurry comprises Mg(OH)2 and the weight amount of Mg(OH)2 in the slurry is set between 30% and 50%.

[0012] The supply of the alkaline reagent in the form of a slurry, i.e. of a thick water dispersion or suspension, has the advantage of not significantly increasing the volume of the waste material due to the mixing with the alkaline reagent, such that the size of the plant can be contained as well as the construction costs and the fluid-handling costs.

[0013] In particular, said step of forming said slurry comprises a step of combining an oxide of said metal of said alkaline reagent with water, for example a step of combining an amount of magnesium oxide with water. This way, both the hydroxide formed by reacting the oxide with water, and the oxide as such, can be present in the slurry, so that the step of adding oxide to the sulfate liquid solution takes place while mixing the alkaline reagent with the waste material, and oxysulfate is obtained in the liquid solution. In particular, this step of combining comprises combining the calcined oxide, obtained by the reduction and decomposition step, with water.

[ 0014 ] Advantageously, the step of mixing the alkaline reagent with the waste material and of forming the sulfate and/or oxysulfate solution comprises a step of progressively adding the alkaline reagent to the waste material. [0015] Preferably, the step of separating the metal hydroxides is carried out substantially at a final temperature of the step of mixing the alkaline reagent with the waste material and of forming a sulfate and/or oxysulfate solution. This way, the operating conditions are such that the solubility of the sulfate and/or oxysulfate is maximized without external heating, i.e. by using process heat only, and the metal hydroxides are substantially fully separated from the sulfate and/or oxysulfate of the first mixture.

[ 0016] Advantageously, steps are provided of:

— recovering the precipitation agent from a liquid fraction obtained from the second mixture, after the step of removing the solid sulfate and/or oxysulfate from the second mixture;

— recycling, i.e. using the regenerated precipitation agent in said step of • mixing said sulfate and/or oxysulfate solution with a precipitation agent solution of the first mixture.

[0017 ] A technique is therefore carried out of regenerating and recovering the precipitation agent, so as to minimize the costs connected with the use of the precipitation agent, in order to minimize the related purchase costs and to substantially eliminate the disposal costs.

[0018] The precipitation agent of the sulfate and/or oxysulfate can be an alcohol selected from the group consisting of: methyl alcohol; ethyl alcohol; n-propyl alcohol; isopropyl alcohol. In this case, the recovery step provides a distillation step for separating the alcohol from the water coming from the liquid fraction of the second mixture.

[0019] In an exemplary embodiment, a glycol can even be used as the precipitation agent.

[0020 ] This way, an immediate crystallization of the oxysulfate is surprisingly obtained, which yields crystals that have a size ideal for a centrifugation, and have averagely 3 hydration water molecules, which corresponds to not more than the 4-5% of the mother liquor. The obtained very pure product is dried under vacuum, whereas the alcohol vapour can be sent to a distillation plant for recovering the used alcohol. Once the distillation has come to an end, the so-recovered alcohol can be reused in the process. Concerning the low mother liquor amount separated from the alcohol, it can be discharged complying with the regulatory requirements. In other words, the steps of precipitating and of removing the sulfate and/or oxysulfate can occur without generating waste water that requires specific treatments. In particular, an alcohol such as ethyl alcohol, used as the precipitation agent for the sulfate and/or oxysulfate, has the advantage of being suitable to be recovered by an easy distillation step, from which water is obtained that can be treated in a conventional wastewater treatment plant and then discharged, and a regenerated ethyl alcohol is also obtained that can be recycled as the precipitation agent to the step of removing the sulfate and/or oxysulfate.

[0021] Therefore, if an alcohol is used, in particular ethyl alcohol, the distillation can be carried out by exploiting low-enthalpy heat, which is largely available, in particular, from a possible sulfuric acid production plant close to the production plant that generates the sulfuric waste material.

[0022 ] Preferably, the precipitation agent of the sulfate and/or oxysulfate comprises a substantially azeotropic alcohol mixture, for example a mixture of ethyl alcohol and water that has a composition close to the azeotropic composition. This makes it even easier to regenerate the alcoholic precipitation agent. In particular, the weight amount of the substantially azeotropic alcohol mixture with respect to the second mixture is 30% or higher, more preferably the weight amount is 40% or higher, even more preferably the weight amount is close to 50%. The distillation step can be advantageously carried out up at a pressure lower than the atmospheric pressure, in order to limit the number of stages and then the costs of the separation apparatus, typically a rectification column,

[0023] In the step of removing the solid sulfate and/or oxysulfate from the second mixture, a step is advantageously provided of drying the removed sulfate and/or oxysulfate, in order to substantially fully remove any residue of the precipitation agent from it, typically any alcohol residue. The precipitation agent residue withdrawn from the solid sulfate and/or oxysulfate during the drying can be condensed and possibly used as a part of the total precipitation agent in the step of mixing the solution of the sulfate and/or oxysulfate with the precipitation agent. This way, the emissions of the precipitation agent, and the related disposal costs, are substantially eliminated, besides reducing the costs for supplying the precipitation agent, such as ethyl alcohol.

[0024 ] The reducing agent for treating the solid sulfate and/or oxysulfate can be elemental sulfur, in which case the step of treating the solid sulfate and/or oxysulfate with a reducing agent comprises a step of adding elemental sulfur to the solid sulfate and/or oxysulfate, in particular, an amount of elemental sulfur set between 5% and 15% of the total weight of the sulfate and/or oxysulfate and of the sulfur, at a reduction decomposition temperature set between 700°C and 1200°C, preferably between 850°C and 1100^eC, even more preferably between 950°C and 1050°C.

[0025] The method makes it possible to substantially eliminate the sludge that is instead produced in the conventional lime treatment of sulfuric acid-containing waste materials, typically, which produces gypsum containing sludge, since

— the sulfur contained in the sulfuric acid, and possibly also fresh elemental sulfur introduced as the reducing agents, is converted into gaseous SOzS;

— at the same time, the alkaline reagent is substantially fully regenerated by forming the sulfate and/or oxysulfate, which is an intermediate product of this treatment, and is substantially fully recycled to the waste material treatment process.

This is particularly advantageous, as described hereinafter, in the case of a potentially dangerous sludge, i.e. a sludge containing a polluting material, as in the case of the "red sludge" produced in the T1O2 via sulfate manufacture processes.

[0026] The process is particularly advantageous if SO2 is used as a raw material for making the sulfuric acid that is used in the process that produces the acid waste materials; in many cases, the plant using sulfuric acid, such as a TiOa production plant, belongs to the same industrial complex as the plant that produces and provides sulfuric acid,

[0027 ] In particular, the method makes it possible to treat a sulfuric waste material that contains metal ions, which is the case of T1O2 manufacture waste material. In this case, a precipitation agent is used as the alkaline reagent that is selective for metal cations, which causes such metals to precipitate in the form of hydroxides, while the SCV ions remain in solution. It is then advantageously provided a step of separating these metal hydroxides. This way, the metal cations are made useful for various metal sulfates production processes, and this allows a partial valorisation of the metal cations that, in the conventional treatment processes, are leaved in the above mentioned sludge,

[0028] In summary, the method according to the invention makes it possible to recover valuable compounds that can be used as raw materials in other production processes. In particular, sulfur dioxide, SO2, can be recovered for making H2SO4, and the metals, in the form of metal hydroxides, can be used for producing metal sulfates.

Brief description of the drawings

[0029] The invention will be now shown with the following description of its exemplary embodiments, exemplifying but not limitative, with reference to the attached drawings, in which:

Fig. 1 is a block diagram of a method, according to an exemplary embodiment of the invention, for treating a sulfuric waste material containing metal ions;

Fig. 2 is a block diagram of a method according to an exemplary embodiment where the alkaline reagent is Mg(OH)2, in particular in the form of a "slurry", i.e. of a Mg(OH)2 thick water dispersion or suspension; Fig. 3 is a flowchart of a possible plant actuating the method of Fig. 2.

Description of preferred exemplary embodiments

[0030] With reference to Fig. 1 , a method is described for treating a process waste material 20 comprising a diluted sulfuric acid water solution. Process waste material 20 has typically a pH value set between 0.1 and 1.

[0031] Besides sulfuric acid, process waste material 20 also contains metal cations, typically cations of at least one metal selected from the group consisting of; iron; magnesium; calcium; chromium; titanium; aluminium; vanadium; manganese, responsive to the production process from which waste material 20 comes. [0032] The method comprises a step 100 of prearranging an alkaline reagent 10 selected among the hydroxides of metals selected from the group consisting of; Magnesium, Zinc and Tin, or a combination of hydroxides of such metals, and a following step 200 of mixing alkaline reagent 10 with waste material 20, in which a heterogeneous mixture 21 is formed, i.e. a mixture consisting of a liquid phase and of a solid phase.

[0033] In fact, due to mixing 200, solid hydroxides are formed of the cations present in waste material 20. In the liquid phase of mixture 21 , the metal present in alkaline reagent 10 forms a soluble sulfate and/or oxysulfate. Therefore, mixture 21 substantially comprises a solid phase essentially formed by the hydroxides of the cations of waste material 20, and a liquid phase substantially consisting of an aqueous solution of a sulfate and/or oxysulfate of the metal of alkaline reagent 10.

[0034 ] The alkaline reagent is selected among those which, by mixing 200, are adapted to cause the cations of waste material 20 to precipitate as hydroxides, while the sulfate/oxysulfate of their metal is left in the solution. In other words, alkaline reagent 10 is a selective precipitation reagent for metal cations, which leaves the ion SOA^S in the solution of first mixture 21.

[0035] Preferably, as shown in Figs. 2 and 3, step 100 of prearranging an alkaline reagent comprises a step 110 of making a mixture 10 of the alkaline reagent and water. This mixture can be a slurry 10, i.e. a thick water dispersion or suspension of an insoluble oxide or hydroxide, such as MgO. The production of the mixture can be carried out in a conventional stirred container 410' (Fig. 3).

[0036] In particular, alkaline reagent 10 comprises MgO, and the weight amount of Mg(OH)2 in slurry 10 is set between 30% and 50%.

[0037] The production of solid hydroxides occurs, in particular, when the pH of mixture 21 formed during mixing 200 reaches a predetermined pH precipitation value. In particular, if Magnesium oxide or hydroxide is used as the alkaline reagent, precipitation pH is preferably set between 8.5 and 9.5, in particular it is higher than 8.85, more in particular it is higher than 9.0, even more in particular it is set between 9.0 and 9.2. In fact, a pH equal to or higher than 8.5 causes most cations possibly present in the waste material to precipitate, as it can be argued from Table 1. In fact, a pH equal to or higher than 8.5 causes most of the cations potentially present in the waste material to precipitate,

- Table 1 -

[0038] If Mg(OH)2 is used in alkaline reagent 10, the precipitation reactions which take place during and as a consequence of mixing 200 may comprise;

SOA" + Me²⁺ + Mg(OH)2^■» gSO-ι (solution) + Me(OH)₂ J, [1 ]

3S0₄ ⁼ + 2 Me³⁺ + 3Mg(OH)2-» 3MgS04 (solution) + 2Me(OH)₃ j [2] wherein Me²⁺ and Me³⁺ are ions of a bivalent metal and of a trivalent metal, respectively, Similar reactions take place in case of metal cations of different valence, and/or if oxides or hydroxides of metals different from magnesium are used such as the alkaline reagent.

[ 0039 ] The use a slurry enhances the kinetics of the oxysulfate or sulfate formation reaction, which avoids using an excess of alkaline reagent 10.

[0040 ] In particular, step 200 of mixing alkaline reagent 10 with waste material 20, and of forming a sulfate and/or oxysulfate solution 22, comprises a step of progressively adding alkaline reagent 10 to waste material 20.

[ 0041 ] In particular, in case of use of MgO as alkaline reagent 10, sulfate ions are initially present in first mixture 21 as a magnesium sulfate gSOa solution, When pH increases, due to MgO addition, in particular when the pH becomes higher than 8.85, more in particular, when the pH is set between 9,0 and 9.5, a reaction forming magnesium oxysulfate 0(MgO)2S02, i.e.

takes place.

This reaction is a combination of gS04 with MgO, i.e. it can be written as:

MgSO« + MgO -» 0(MgO)∑S02 13]·

[0042] In particular, if a Magnesium hydroxide Mg(OH)2 concentrated slurry is used as alkaline reagent 10, prepared starting from magnesium oxide MgO, an equilibrium is established between hydroxide and oxide due to their low solubility, therefore a portion of magnesium oxide MgO, which is not converted to hydroxide Mg(OH)2, is present in the slurry. This fraction of MgO contributes to oxysulfate formation according to reaction [3].

[0043] The mixing of alkaline reagent 10 with waste material 20, and the sulfate and/or oxysulfate formation reaction are exothermic steps, therefore that step 200 of mixing the alkaline reagent with the waste material, and of forming the sulfate and/or oxysulfate solution can occur with an increase of the temperature.

[0044] As shown in Fig. 3, step 200 of mixing alkaline reagent 10 with waste material 20, and of obtaining a solution 22 of a sulfate and/or oxysulfate of the alkaline reagent, may take place in a conventional stirred container 200'.

[0045] Still with reference to Fig. 1 , the method comprises then a step 210 of separating solid metal hydroxides 23 from sulfate and/or oxysulfate solution 22. Step 210 of separating metal hydroxides 23 can be carried out in any conventional apparatus. Preferably, the step of separating metal hydroxides 23 comprises a filtration step 210 or a centrifugation step 2 0', as shown in Fig. 3.

[0046] The solubility of oxysulfates in water, in particular the solubility of magnesium oxysulfate 0(MgO)2S02, is enhanced by higher temperatures, such as the temperature that can be reached due to the exothermicity of the mixing of alkaline reagent 10 with waste material 20, i.e. the exothermicity of precipitation reactions [1], [2] and of oxysulfate formation [3],

[0047] Therefore, step 210 of separating metal hydroxides 23 from solution 22 is advantageously carried out immediately after step 200 of mixing alkaline reagent 10 with waste material 20. This way, separation 210 can be carried out substantially at the final temperature of step of mixing 200. In other words, the mixing and reaction heat is used to obtain a better, substantially total separation, of metal hydroxides 23 from the sulfate and/or oxysulfate of solution 22.

[0048 ] Step 210 of separating metal hydroxides 23 can be followed by a step 220 of washing wet solid hydroxides 23 obtained in separation 210, with release of an amount of washing water 27. Washing water 27 can be sent to disposal, as shown in Fig. 1 or, as described hereinafter, it can be added to sulfate and/or oxysulfate solution 22 in order to totally recover magnesium sulfate/oxysulfate.

[0049 ] Metal hydroxides 23 can be used in a metal sulfates production process. For instance, if an iron-rich waste material 20 is treated, as it is always the case of titanium dioxide production process waste material, precipitated hydroxides 23 can be easily dissolved into the spent acid that is used for producing iron sulfate FeS04, as it is already done in some T1O2 facilities.

[0050 ] The method also comprises a subsequent step 300 of mixing sulfate and/or oxysulfate solution 22 with a suitable precipitation agent 31 , thus obtaining a second mixture 32, wherein a crystallization of the sulfate and/or oxysulfate takes place forming a solid sulfate and/or oxysulfate 33 in second mixture 32. A step 301 follows of removing solid oxysulfate or sulfate 33 from second mixture 32, in particular by centrifugation. Removal step 301 also produces a liquid stream 35 that contains the exhausted precipitation agent, which can be disposed, as shown in Fig, 1 , or recovered according to some modifications of the method, one of which is shown in Fig, 2 and described hereinafter.

[ 0051] As shown in Fig. 3, step 300 of mixing sulfate and/or oxysulfate solution 22 with a precipitation agent 31 , thus obtaining second mixture 32, may take place in a simple feed tank 300' of an equipment 301 ' used for removing, i.e. for separating solid oxysulfate or sulfate 33 from second mixture 32. [0052] Still with reference to Fig. 1 , in order to recover the metal of alkaline reagent 10, the method provides steps 390, 400, 401 of treating solid sulfate and/or oxysulfate 33 with a reducing agent 41 ,

[0053] More in detail, a step 390 is provided of contacting solid sulfate and/or oxysulfate 33 with this reducing agent 41 , which is selected among elemental sulfur, hydrogen gas, an oxyacetylene flame, a reducing flame comprising atomic hydrogen and oxygen, a mixture of hydrogen, carbon monoxide and carbon dioxide also known as water gas.

[0054 ] A step 400 follows of heating solid sulfate and/or oxysulfate 33 in the presence of reducing agent 41 until a predetermined reduction and decomposition temperature is attained, in order to cause a step 401 of reduction and decomposition of solid sulfate and/or oxysulfate 33, thus obtaining gaseous sulfur dioxide (SO2) 42 and a solid oxide 43 of the metal of alkaline reagent 10,

[0055] As still shown in Fig. 3, if elemental sulfur is used as reducing agent 41 , treatment step 390 can comprise a step of mixing that is preferably carried out in a screw mixer 390', whereas heating 400 and reduction/decomposition 401 of solid sulfate and/or oxysulfate 33 can be advantageously carried out in a rotating oven 400'.

[0056] SO2 42 obtained from step 401 of reduction and decomposition of sulfate and/or oxysulfate 33 can be used for making sulfuric acid.

[0057 ] Instead, oxide 43 of the metal of the alkaline reagent is reused in step 200 of mixing and obtaining a solution 22 of the sulfate and/or oxysulfate, in particular after making a mixture with water. In other words, the step of prearranging an alkaline reagent comprises a step 101 of recycling oxide 43 of the metal of the alkaline reagent, obtained from reduction and decomposition 400 of sulfate and/or oxysulfate 33,

[0058] With reference to Figs. 2 and 3, advantageously, precipitation agent 31 for sulfate and/or for oxysulfate comprises an alcohol or a glycol that is miscible with water, and in which the solubility of the sulfate and/or oxysulfate is substantially zero. This way, step 301 of removing sulfate and/or oxysulfate 33 solid by second mixture 32 can occur up to a substantially full extent, releasing a liquid fraction 35 substantially free from the sulfate and/or oxysulfate.

[0059 ] Still with reference to Figs. 2 and 3, preferably, a step 350 is provided of recovering precipitation agent 31 from liquid fraction 35 obtained from second mixture 32, after step 301 of removing solid sulfate and/or oxysulfate 33 from second mixture 32, and also a step 351 is provided of recycling recovered precipitation agent 31 into solution 22 of first mixture 21 , in a subsequent treatment cycle. Alcohol 31 , used as the precipitation agent for the sulfate and/or oxysulfate, has the advantage of being easy to recover by a distillation step 350.

[ 0060 ] Alcohol 31 can be selected from the group consisting of: methyl alcohol, CH3OH; ethyl alcohol, CH3CH2OH; n-propyl alcohol, CH3CH2CH2OH; isopropyl alcohol, CH3(CHOH)CH3, and butyl alcohols, C4H9OH. The alcohols from methyl alcohol to propyl alcohols are preferred, because they are completely miscible in water. Instead, butyl alcohols show miscibility gaps and are therefore more difficult to recover, i.e. to be separated from the liquid fraction of second mixture 32 that is left after removing precipitated sulfate and/or oxysulfate solid 33.

[ 0061 ] Advantageously, step 300 of mixing precipitation agent 31 with solution 22, and of causing sulfate and/or oxysulfate 33 to precipitate, comprises a step of progressively adding solution 22 to a predetermined amount of precipitation agent 31 comprising an alcohol. This makes it possible to obtain an immediate precipitation of sulfate and/or oxysulfate 33, which allows to obtain a practically anhydrous sulfate and/or oxysulfate 33, This simplifies treatment steps 400,401 for restoring alkaline reagent 10, as described hereinafter. Normally, magnesium oxysulfate 33 precipitates with 3 to 11 hydration molecules.

[ 0062 ] Advantageously, alcohol 31 is ethyl alcohol. This reduces the process risks, since ethyl alcohol is less toxic than other alcohols, in particular than methyl alcohol, which is harmful also by inhalation. Furthermore, the use of ethyl alcohol as precipitation agent 31 makes recovery step 350 easier. In fact, the mixtures of ethyl alcohol and water have a maximum boiling azeotrope at 95% alcohol, the boiling point of which, 78.2°C, is relatively low. This makes it possible to exploit low-temperature heat, which is largely available, in particular, from a sulfuric acid production process, normally present in production units close to those that generate the sulfuric waste material.

[0063] Preferably, sulfate and/or oxysulfate precipitation agent 31 comprises a substantially azeotropic alcohol mixture, i.e. a mixture of ethyl alcohol and water that has a composition close to azeotropic composition, i.e. 95% alcohol.

[0064 ] In this case, the recovery step comprises a step 350 of distilling liquid fraction 35, until it reaches a concentration close to 95% ethyl alcohol, and a step 351 of recycling ethyl alcohol to mixing step 300.

[ 0065] Absolute ethyl alcohol can also be used as oxysulfate or sulfate precipitation agent 31 . In this case, the recovery step comprises, downstream of distillation step 350, an azeotropic distillation step, not shown, in order to completely remove water from the substantially azeotropic alcohol mixture obtained by distillation.

[ 0066] Preferably, the weight amount of substantially azeotropic alcohol mixture 31 with respect to second mixture 32 is 30% or higher, more preferably this ratio is 40% or higher, even more preferably this ratio is close to 50%.

[ 0067 ] In table 2 are indicated the weight ratios between azeotropic alcohol 31 and sulfate and/or oxysulfate solution 22 which are preferred for step 300 of precipitating the sulfate and/or oxysulfate. Intermediate ratios between those given in table 2 can also be used.

- Table 2 -

This way, as it is deduced from the table, after step 300 of mixing precipitation agent 31 with solution 22, and after the precipitation of sulfate and/or oxysulfate 33, the concentration of sulfate and/or oxysulfate in solution is substantially negligible. [0068 ] Advantageously, distillation step 350 is carried out at a pressure lower than the atmospheric pressure, for example at a pressure at which the substantially azeotropic alcohol mixture of ethyl alcohol and water has a boiling temperature close to 60^eC,

Moreover, recovery step 350 requires much less energy than the conventional crystallization process, which comprises a solvent evaporation step for making the solution supersaturated and a subsequent cooling step, mainly for the following reasons:

— at such weight ratios, the volume of alcohol to be evaporated is lower; — the temperature at which the evaporation of the alcohol occurs is relatively low, therefore relatively little heat is required for heating the second mixture;

— the low evaporation temperature, as anticipated, makes it possible to exploit the low temperature heat that is normally largely available at zero cost in those titanium dioxide production plants that are connected to a sulfuric acid plant,

[ 0069 ] Step 301 of removing solid sulfate and/or oxysulfate 33 from second mixture 32 can be carried out by conventional techniques like filtration or centrifugation.

[ 0070] Preferably, after step 301 of removing solid sulfate and/or oxysulfate 33 from second mixture 32, a step 310 is provided of drying removed sulfate and/or oxysulfate 33, in order to remove any residue 34 of the precipitation agent from it, typically an alcohol residue 34, Preferably, drying step 310 is carried out at a temperature of 200°C or higher, for example in a rotating oven 310' (Fig. 3), so that sulfate and/or oxysulfate 33 lose possible hydration water and are changed into anhydrous sulfate and/or oxysulfate.

[0071 ] Advantageously, a step 311 is provided of condensing precipitation agent residue 34 withdrawn from solid sulfate and/or oxysulfate 33. Precipitation agent 34 withdrawn during drying 310 can also be used in the step 300 of mixing precipitation agent 31 with solution 22 of a new amount of first solution 21 , in order to cause further sulfate and/or oxysulfate to precipitate. In particular, precipitation agent 34 withdrawn during drying 310 can be joined to liquid fraction 35 coming from removing step 301 and can be recovered in recovery step 350 along with liquid fraction 35.

[0072 ] Preferably, reducing agent 41 for treating solid sulfate and/or oxysulfate 33 is elemental sulfur 41 , and step 390 of treating solid sulfate and/or oxysulfate 33 with a reducing agent 41 comprises a step 390 of adding solid sulfate and/or oxysulfate 33 with elemental sulfur S. This is advantageous because elemental sulfur, when is oxidized, takes the oxidation state +4 like the sulfur of the sulfate and/or oxysulfate, and it also contributes to the production of sulfur dioxide (SO∑), according to the reaction:

Mg₂SOs(s) + S(s) + ½02(g) -» 2 gO(s) + 2 SOz (g) [4]

[0073] In particular, step 390 of adding solid sulfate and/or oxysulfate 33 with sulfur 41 provides a sulfur amount 41 set between 5% and 15% of the total weight of sulfate and/or oxysulfate 33 and sulfur 41 .

[0074 ] Reduction and decomposition 400 of sulfate and/or oxysulfate 33 in the presence of elemental sulfur 41 can be carried out in a conventional rotating oven 400' (Fig. 3), preferably an indirect heat oven, in order to minimize dust production, Preferably, a step is however provided of fine filtration of the dust, not shown, which can be carried out in an electrofilter, not shown,

[0075] Advantageously, a step is provided of cooling sulfur dioxide 42 and oxide 43 of the metal of the alkaline reagent produced in step 400 of reduction and decomposition of sulfate and/or oxysulfate 33, and preferably also a step 450 of heat exchange with cooling heat recover, advantageously, associated with a step of preheating sulfate and/or oxysulfate 33 when entering reduction and decomposition step 400.

[0076] As an alternative, or in addition to sulfur, reducing agent 41 can comprise hydrogen gas. In this case, during reduction and decomposition 400 of sulfate and/or oxysulfate 33, in the exemplary case of magnesium oxysulfate, the following reaction takes place;

Mg₂S0₅ (s) + H₂(g) 2MgO(s) + S0₂(g) + H2O (v) [5]

[0077] As an alternative, or in addition to the above, reducing agent 41 can comprise an oxyacetylene flame. In this case, during reduction and decomposition 400, still in the case of magnesium oxysulfate, the following reactions take place:

C2H2 + O2 -> 2CO (g) + H₂ (g) [6] MgaSOs (s) + 2 CO (g)+ H2 (g) + O2 (g) ->

-» 2MgO(s) + S0₂(g) + 2COz (g) + H2O (v) [7]

[0078] As an alternative, or in addition to the above, reducing agent 41 can comprise a so-called Brown's Gas, HHO, i.e. a reducing flame consisting of atomic hydrogen and oxygen. In this case, during reduction and decomposition 400, still in the case of magnesium oxysulfate, the following reaction takes place:

g₂S0₅ (s) + H H or 2MgO(s) + S0₂(g) + H₂0 (v) + ½ 02(g) [8]

[0079] As an alternative, or in addition to the above, reducing agent 41 can comprise a so-called water gas, i.e. a mixture of hydrogen, carbon monoxide and carbon dioxide.

[0080 ] The use of the sulfur and/or of a reducing gas makes it possible to carry out decomposition 400 of sulfate and/or oxysulfate 33 at a reduction decomposition temperature lower than thermal decomposition temperature of sulfate and/or oxysulfate 33. In particular, in the case of magnesium oxysulfate, the reduction decomposition temperature is set between set between 700°C and 1200°C, advantageously between 850°C and 1 00^eC, preferably between 950°C and 1050°C, even more preferably it is about 1000°C.

[0081] Even if the foregoing description relates to sulfuric acid-containing waste materials, the method can be made useful, in a obvious way for a skilled person, to the case of an acid waste material that contains anions different from sulfate ion, and also comprising metal cations in the solution state.

[0082] The process can be operated both discontinuously, i.e. batch, and continuously, with changes that are obvious for a skilled person. EXAMPLES

[00831 The invention will be illustrated below with the description of examples of a method for treating a process waste material containing sulfuric acid, exemplifying but not limitative.

Example 1

[0084] In a first container equipped with a stirring means, 5 litres of an acid waste material were arranged containing sulfates (300 g/l) and impurities including metal ions such as iron (17 g/l), manganese (5 g/l) and chromium (1 g/l), at an initial pH of 0.38.

[0085] In the same first container, 2.5 Kg of a MgO slurry, 50% by weight, were added, thus obtaining a first mixture that reached a pH value of 9.0. In these conditions, a precipitation reaction took place involving the hydroxides of the above mentioned metals. Due to the mixing heat, and to the reaction heat, a temperature of 98°C was attained at the end of the precipitation reaction. Besides the precipitated hydroxides, the first mixture comprised a magnesium oxysulfate water solution.

[0086] The precipitated solids, Fe(OH), manganese hydroxide, η(ΟΗ)ζ and chromium hydroxide Cr(OH)3, were filtered at a warm temperature, i.e. substantially at this final reaction temperature, and then were washed and dried, obtaining a dry fraction of 200 g total weight.

[0087 ] In a second container, 7.5 kg of azeotropic ethyl alcohol were arranged, i.e. a 95% alcohol hydroalcoholic mixture, and then 7.5 kg of the magnesium oxysulfate water solution were added from the first container, after separating the hydroxides. In these conditions, a solid spontaneously precipitated that was substantially formed by magnesium oxysulfate, so that the second container finally contained a liquid fraction comprising a diluted hydroalcoholic solution and a solid precipitate. The solid precipitate was removed by centrifugation, whereas the diluted alcoholic solution was distilled until it reached the substantially azeotropic concentration of 95% alcohol.

[0088] The solid magnesium oxysulfate removed from the second mixture was dried at 200°C for 2 hours, obtaining about 2.75 kg of a dry material. [ 0089 ] The dry magnesium oxysulfate was added with 0.25 Kg of sulfur, i.e. about 8.3% of the total solid, and then was heated in an oven up to 980^eC, maintaining this temperature for 2 hours. In these conditions, a magnesium oxysulfate reduction and thermal decomposition took place, with a magnesium oxide yield of 95%. More in detail, 1.15 Kg MgO and 1.4 Kg S02(g) were obtained.

Example 2 - Treatment of an acid waste material containing H2SO4 and metals using a Magnesium hydroxide slurry starting from magnesium oxide

[ 0090 ] In a first container equipped with a stirring means, 5 litres of an acid waste material were arranged containing sulfates (300 g/l) and metal ions such as iron (17 g/l), manganese (5 g/l), aluminium (1.6 g/l) and chromium (1 g/l), at an initial pH of 0.38.

[ 0091 ] In the same first container which contained the waste material, were added 2 Kg of a Mg(OH)2 slurry, 50% by weight, prepared from 0.69 Kg MgO and 1.31 Kg of water, reaching a pH value of 9.1. In these conditions, a precipitation of the above mentioned metals took place as hydroxides, and a magnesium sulfate and oxysulfate mixture remained in the solution (The presence of oxysulfate is important to enhance the decomposition of the solid during the step of calcination).

[ 0092 ] Due to the mixing heat, and to the reaction heat, a temperature of 98°C was attained at the end of the precipitation reaction.

[0093] The precipitated solids, ferrous hydroxide (Fe(OH)₂), manganese hydroxide (Mn(OH)₂), aluminium hydroxide AI(OH)₃ and chromic hydroxide (Cr(OH)₃) were filtered at a warm temperature, i.e. substantially at this final reaction temperature, and then were washed and dried, obtaining a dry fraction of 200 g total weight. The solid washing water was added to the magnesium sulfate and oxysulfate main solution to be fully recovered.

[ 0094 ] In another container, were arranged 7.5 Kg of 95% ethyl alcohol, i.e. a substantially azeotropic hydroalcoholic solution, and then were added 7.5 Kg of the aqueous solution of magnesium sulfate and oxysulfate obtained in the first container. In these conditions, a magnesium sulfate and oxysulfate mixture crystallized spontaneously, with only 3 hydration water molecules, therefore at the end a liquid fraction consisting of a diluted hydroalcoholic solution was obtained. The crystallized solid was removed by centrifugation, whereas the diluted alcoholic solution was distilled until it reached the substantially azeotropic concentration of 95% alcohol.

[0095] The removed mixture of magnesium sulfate/oxysulfate trihydrate was dried at 200^eC for two hours, obtaining 2.18 Kg of a dry material, corresponding to a mixture of magnesium sulfate/oxysulfate monohydrate.

[0096] The dry solid was added with 0.25 Kg sulfur, i.e. about 10.3% of the total solid, and then was heated up to 950°C, and maintained at this temperature for 2 hours. In these conditions, a magnesium sulfate/oxysulfate reduction and thermal decomposition took place, with a magnesium oxide yield of 95%.. More in detail, 0.655 Kg MgO and 1.43 Kg SO2 (g) were obtained.

Example 3 - Use of hydrogen gas as reducing

[0097] In a first container equipped with a stirring means, 5 litres of an acid waste material were arranged containing sulfates (270 g/l) and impurities including metal ions such as iron (17 g/l), manganese (5 g/l) and chromium (1 g/l), at an initial pH of 0.38.

[0098] In the same first container, 2.4 Kg of a MgO slurry, 50% by weight, were added, thus obtaining a first mixture that reached a pH value of 9.1. In these conditions, a precipitation reaction took place involving the hydroxides of the above mentioned metals. Due to the mixing heat, and to the reaction heat, a temperature of 98°C was attained at the end of the precipitation reaction.

Besides the precipitated hydroxides, the first mixture comprised a magnesium oxysulfate water solution.

[ 0099] The precipitated solids, Fe(OH), manganese hydroxide, Mn(OH)z and chromium hydroxide Cr(OH)3, were filtered at a warm temperature, i.e. substantially at this final reaction temperature, and then were washed and dried, obtaining a dry fraction of 180 g total weight.

[00100] In a second container, 7.5 Kg of azeotropic ethyl alcohol were arranged and then 7.5 Kg of the magnesium oxysulfate water solution were added from the first container, after separating the hydroxides. In these conditions, a solid spontaneously precipitated that was substantially formed by magnesium oxysulfate, so that the second container finally contained a liquid fraction comprising a diluted hydroalcoholic solution and a solid precipitate. The solid precipitate was removed by centrifugation, whereas the diluted alcoholic solution was distilled until it reached the substantially azeotropic concentration of 95% alcohol.

[ooioi] The solid magnesium oxysulfate removed from the second mixture was dried at 200°C for 2 hours, obtaining about 2,45 kg of a dry material.

[00102] The dry magnesium oxysulfate was treated under a hydrogen gas stream in an oven heated up to 985°C, and was maintained at this temperature for 2 hours. In these conditions, a magnesium oxysulfate reduction and thermal decomposition took place, with a magnesium oxide yield of 94%. More in detail, 1.15 Kg MgO and 0.92 Kg S02(g) were obtained.

Example 4 - Treatment of an acid waste material containing H2SO4 and metal cations using Zinc hydroxide

[00103] In a first container equipped with a stirring means, 4 litres of an acid waste material were arranged containing sulfates (270 g/l) and metal ions such as iron (17 g/l), manganese (5 g/l) and chromium (1 g/l), at an initial pH of 0.4.

[00104] In the same first container which contained the waste material, were added 3.15 Kg of a 35% Zn(OH)2 slurry (prepared from 0.9 Kg ZnO and 2.25

Kg of water), reaching a pH value of 8.9. In these conditions, a precipitation of the above mentioned metals took place as hydroxides, and a Zinc sulfate and oxysulfate mixture of remained in the solution. The presence of the oxysulfate improves the conditions of the final solid calcination.

[00105] Due to the mixing and reaction heat, at the end of the precipitation the solution attained a temperature of 97°C.

[00106] The precipitated solids, ferrous hydroxide (Fe(OH)2), manganese hydroxide (Mn(OH)2) and chromic hydroxide (Cr(OH)3) were filtered at a warm temperature, i.e. substantially at this final reaction temperature, and then were washed and dried, obtaining a dry fraction of 149 g total weight. The solid washing water was added to the main Zinc sulfate and oxysulfate solution to be fully recovered.

[00107] In another container, 7.4 Kg of 95% ethyl alcohol were arranged, to which 7,4 Kg of the Zinc sulfate and oxysulfate water solution were added. In these conditions, a mixture of Zinc sulfate and oxysulfate spontaneously crystallized with only 3 molecules of hydration water, and a liquid fraction comprising a diluted hydroalcoholic solution was left. The crystallized solid was removed by centrifugation, whereas the diluted alcoholic solution was distilled until it reached the substantially azeotropic concentration of 95% alcohol.

[00108] The removed mixture of Zinc sutfate/oxysulfate trihydrate was dried at 200°C for two hours, obtaining 1.9 Kg of a dry material, corresponding to a mixture of Zinc sulfate/oxysulfate monohydrate.

[00109] The dry solid was added with 0.15 Kg sulfur, i.e. about 7.3% of the total solid, then it was heated up to 850°C, and was maintained at this temperature for 2 hours. In these conditions, a Zinc sulfate/oxysulfate reduction and thermal decomposition took place, with a process yield of about 96%. More in detail, 0,86 Kg ZnO and 0.93 Kg SO2 (g) were obtained.

Example 5 - Use of hydrogen gas as reducing agent

[00110] In a first container equipped with a stirring means, 5 litres of an acid waste material were arranged containing sulfates (270 g/l) and impurities including metal ions such as iron (17 g/l), manganese (5 g/l) and chromium (1 g/l), at an initial pH of 0.38.

[00111] In the same container, were added 2.5 Kg of a Mg(OH)2 slurry, 35% by weight, prepared from 0.6 Kg MgO and 1.9 Kg of water, thus obtaining a first mixture that reached a pH value of 9.0. In these conditions, a precipitation reaction took place involving the hydroxides of the above mentioned metals. Due to the mixing heat, and to the reaction heat, a temperature of 98°C was attained at the end of the precipitation reaction. Besides the precipitated hydroxides, the first mixture comprised an aqueous solution of magnesium sulfate and oxysulfate.

[00112] The precipitated solids, ferrous hydroxide (Fe(OH)2), manganese hydroxide (Mn(OH)2), aluminium hydroxide AI(OH)3 and chromic hydroxide (Cr(OH)3) were filtered at a warm temperature, i.e. substantially at this final reaction temperature, and then were washed and dried, obtaining 185 g of a dry fraction. The washing liquid was added to the filtered solution of magnesium sulfate/oxysulfate to be fully recovered. [00113 ] In a second container, 7,8 Kg of 95% ethyl alcohol, i.e. a substantially azeotropic hydroalcoholic solution, were arranged, and then 7.8 Kg of the of magnesium sulfate/oxysulfate water solution were added from the first container, after separating the hydroxides. In these conditions, a mixture of magnesium sulfate and oxysulfate crystallized spontaneously, with only 3 hydration water molecules, so that the second container finally contained a liquid fraction comprising a diluted hydroalcoholic solution and a solid precipitate. The crystallized solid precipitate was removed by centrifugation, whereas the diluted alcoholic solution was distilled until it reached the substantially azeotropic concentration of 95% alcohol.

[001141 The mixture of magnesium sulfate/oxysulfate trihydrate removed from the second mixture was dried at 200°C for 2 hours, obtaining about 1.95 kg of a dry material, corresponding to magnesium sulfate and oxysulfate monohydrate.

[ 00115 J The dry magnesium sulfate/oxysulfate monohydrate was treated under a hydrogen gas stream in an oven heated up to 985^eC, and was maintained at this temperature for 2 hours. In these conditions, a magnesium oxysulfate reduction and thermal decomposition took place, with a process yield of 93%. More in detail, 0.56 Kg MgO and 0,84 Kg S02(g) were obtained.

[ 00116 ] The foregoing description of exemplary specific embodiments will so fully reveal the invention according to the conceptual point of view, so that others, by applying current knowledge, will be able to modify and/or adapt for various applications such embodiment without further research and without parting from the invention, and, accordingly, it is therefore to be understood that such adaptations and modifications will have to be considered as equivalent to the specific embodiments. The means and the materials to realise the different functions described herein could have a different nature without, for this reason, departing from the field of the invention, it is to be understood that the phraseology or terminology that is employed herein is for the purpose of description and not of limitation.

Claims

1. A method for treating a process waste material (20) containing sulfuric acid and metal cations, comprising the steps of:

— prearranging ( 00) an alkaline reagent (10) comprising a hydroxide (10) of a metal selected from the group consisting of: Magnesium; Zinc; Tin; a combination thereof;

— mixing (200) said alkaline reagent (10) with said waste material (20), thus obtaining a first mixture (21) comprising:

— a liquid sulfate and/or oxysulfate solution (22) of said metal of said alkaline reagent;

— hydroxides (23) of said metal cations as a solid;

— separating said hydroxides of said metal cations (23) from said liquid solution (22);

— mixing (300) said solution (22) of said sulfate and/or oxysulfate with a precipitation agent (31) for said sulfate and/or oxysulfate, creating a second mixture (32), and causing said sulfate and/or oxysulfate to precipitate as a solid sulfate and/or oxysulfate (33) in said second mixture (32);

— removing (301) said solid sulfate and/or oxysulfate (33) from said second mixture (32);

— bringing (390) said solid sulfate and/or oxysulfate (33) into contact with a reducing agent (41 ) selected from the group consisting of;

— elemental sulfur;

— hydrogen gas;

— an oxyacetylene flame;

— a reducing flame comprising atomic hydrogen and oxygen;

— a mixture of hydrogen, carbon monoxide and carbon dioxide, and heating (400) up to a predetermined reduction and decomposition temperature, so as to cause a reduction and decomposition (401 ) of said solid sulfate and/or oxysulfate (33) thus obtaining:

— gaseous sulfur dioxide SO2 (42) ;

— a calcined oxide (43) of said metal of said alkaline reagent, wherein said step of prearranging (100) an alkaline reagent (31 ) comprises a step (101) of recycling said oxide (43) of said metal of said alkaline reagent obtained from said reduction and decomposition (400) of said sulfate and/or oxysulfate (33).

2. The method according to claim 1 , wherein said alkaline reagent (10) is Magnesium hydroxide, Mg(OH)2.

3. The method according to claim 1 , wherein a step is provided of adding an oxide of said metal of said alkaline reagent (10) to said liquid solution (22) of said sulfate of said metal of said alkaline reagent (10), in order to form an oxysulfate of said metal of said alkaline reagent (10) in said liquid solution (22).

4. The method according to claim 2, wherein a step is provided of adding magnesium oxide to said liquid solution (22), in order to form Magnesium oxysulfate in said liquid solution (22).

5. In particular, said step of mixing (200) is carried out in such a way that said liquid solution (22) reaches a precipitation pH value of said metals set between 8.85 and 9.5, in particular set between 9.0 and 9.2.

6. The method according to claim 1 , wherein said step of prearranging (100) an alkaline reagent comprises a step (1 10) of forming a slurry (10) containing said alkaline reagent.

7. The method according to claim 6, wherein said alkaline reagent (10) comprises Mg(OH)z, said mixture is a slurry (10) and the weight amount of Mg(OH)2 in said slurry (10) is set between 30% and 50%.

8. The method according to claim 6, wherein said step of forming said slurry (10) comprises a step (1 10) of combination of an oxide of said metal of said alkaline reagent with water (44).

9. The method according to claim 8, wherein said step (1 10) of combination comprises the combination with water (44) of said calcined oxide (43) obtained by said step (401 ) of reduction and decomposition,

10. The method according to claim 1 , wherein said step (200) of mixing said alkaline reagent (10) with said waste material (20) and of forming said solution (22) of sulfate and/or oxysulfate comprises a step of progressively adding said alkaline reagent (10) to said waste material (20).

11. The method according to claim 1 , wherein said step (210) of separating said metal hydroxides (23) is carried out substantially at a final temperature of said step (200) of mixing said alkaline reagent (10) with said waste material (20) and of forming said solution (22) of said sulfate and/or oxysulfate, so as to obtain a substantially full separation of said metal hydroxides (23) from said sulfate and/or oxysulfate.

12. The method according to claim 1 , comprising the steps of:

recovering (350) said precipitation agent (31) from a liquid fraction (35) obtained from said second mixture (32), after said step of removing (301 ) said solid sulfate and/or oxysulfate (33) from said second mixture (32);

using (351 ) said precipitation agent (31 ) recovered in said step (300) of mixing said sulfate and/or oxysulfate solution (22) with said precipitation agent.

13. The method according to claim 12, wherein said precipitation agent (31 ) of said sulfate and/or oxysulfate is an alcohol (31 ) selected from the group consisting of: methyl alcohol; ethyl alcohol; n-propyl alcohol; isopropyl alcohol, and said recovery step comprises a step of distilling (350) the alcohol from said liquid fraction (35).

14. The method according to claim 13, wherein said precipitation agent (31 ) of said sulfate and/or oxysulfate comprises a substantially azeotropic alcohol mixture, in particular a mixture of ethyl alcohol and water that has a composition close to an own azeotropic composition.

15. The method according to claim 14, wherein the weight amount of the substantially azeotropic alcohol mixture (31 ) with respect to said second mixture (32) is 30% or higher, more preferably said weight amount is 40% or higher, even more preferably said weight amount is close to 50%.

16. The method according to claim 9, wherein said distillation step (350) is carried out up at a pressure lower than the atmospheric pressure.

17. The method according to claim 1 , wherein a step is provided of drying (310) said sulfate and/or oxysulfate (33) removed in said step of removing (301 ) said solid sulfate and/or oxysulfate (33) from said second mixture (32), in order to substantially fully remove any residue (34) of said precipitation agent therefrom, in particular an alcohol residue (34).

18. The method according to claim 17, wherein a step is provided of condensing (31 1 ) said precipitation agent residue (34) withdrawn from said solid sulfate and/or oxysulfate (33).

19. The method according to claim 17, wherein said precipitation agent (34) withdrawn during said step (310) of drying is used in said step of mixing (300) said solution (22) of said sulfate and/or oxysulfate with a precipitation agent (31 ).

20. The method according to claim 1 , wherein said reducing agents (41) for treating said solid sulfate and/or oxysulfate (33) is elemental sulfur (41 ), and said step of treating (390) said solid sulfate and/or oxysulfate (33) with a reducing agent (41 ) comprises a step of adding (390) said elemental sulfur (41 ) to said solid sulfate and/or oxysulfate (33).

21. The method according to claim 20, wherein said step of adding (390) said sulfur (41) to said solid sulfate and/or oxysulfate (33) provides a sulfur amount (41 ) set between 5% and 5% of the total weight of said sulfate and/or oxysulfate (33) and of said sulfur (41), at a reduction decomposition temperature set between 700°C and 1200°C.

22. The method according to claim 21 , wherein said reduction decomposition temperature is set between 850^eC and 1100°C.

23. The method according to claim 21 , wherein said reduction decomposition temperature is set between 950°C and 1050°C.