Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01D—COMPOUNDS OF ALKALI METALS, i.e. LITHIUM, SODIUM, POTASSIUM, RUBIDIUM, CAESIUM, OR FRANCIUM
  • C01D1/00—Oxides or hydroxides of sodium, potassium or alkali metals in general
  • C01D1/02—Oxides
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B17/00—Sulfur; Compounds thereof
  • C01B17/48—Sulfur dioxide; Sulfurous acid
  • C01B17/50—Preparation of sulfur dioxide
  • C01B17/58—Recovery of sulfur dioxide from acid tar or the like or from any waste sulfuric acid
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01D—COMPOUNDS OF ALKALI METALS, i.e. LITHIUM, SODIUM, POTASSIUM, RUBIDIUM, CAESIUM, OR FRANCIUM
  • C01D5/00—Sulfates or sulfites of sodium, potassium or alkali metals in general
  • C01D5/02—Preparation of sulfates from alkali metal salts and sulfuric acid or bisulfates; Preparation of bisulfates
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
  • C01F5/00—Compounds of magnesium
  • C01F5/02—Magnesia
  • C01F5/06—Magnesia by thermal decomposition of magnesium compounds
  • C01F5/12—Magnesia by thermal decomposition of magnesium compounds by thermal decomposition of magnesium sulfate, with or without reduction
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
  • C01F5/00—Compounds of magnesium
  • C01F5/40—Magnesium sulfates
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
  • C01G19/00—Compounds of tin
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
  • C01G19/00—Compounds of tin
  • C01G19/02—Oxides
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
  • C01G49/00—Compounds of iron
  • C01G49/14—Sulfates
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
  • C01G9/00—Compounds of zinc
  • C01G9/02—Oxides; Hydroxides
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
  • C01G9/00—Compounds of zinc
  • C01G9/06—Sulfates

Definitions

• 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.
• H2SO4 spent sulfuric acid
• metals in the form of cations such as iron, magnesium, calcium, chromium, titanium, aluminium, vanadium, manganese and the like.
• 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.
• 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,
• 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,
• 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.
• MgO magnesium oxide
• SO2 sulfur dioxide
• 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.
• 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,
• 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.
• EP0125142A2 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.
• an alkaline reagent comprising a hydroxide of a metal selected from the group consisting of: Magnesium; Zinc; Tin; a combination thereof;
• 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;
• 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.
• the above mentioned metals, g, Zn, Sn can form water-soluble sulfates and/or oxysulfates.
• 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.
• 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.
• 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.
• the alkaline reagent is Magnesium hydroxide, Mg(OH)2. This makes it possible to minimize the initial costs for supplying the alkaline reagent.
• 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.
• 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,
• 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.
• 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.
• the step of prearranging an alkaline reagent comprises a step of forming a slurry of the alkaline reagent.
• 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%.
• 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.
• 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 step of combining comprises combining the calcined oxide, obtained by the reduction and decomposition step, with water.
• 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.
• 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.
• steps are provided of:
• 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.
• 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.
• the recovery step provides a distillation step for separating the alcohol from the water coming from the liquid fraction of the second mixture.
• a glycol can even be used as the precipitation agent.
• 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.
• 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.
• a substantially azeotropic alcohol mixture for example a mixture of ethyl alcohol and water that has a composition close to the azeotropic composition.
• 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,
• 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.
• the reducing agent for treating the solid sulfate and/or oxysulfate can be elemental sulfur
• 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 e C, even more preferably between 950°C and 1050°C.
• 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 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.
• 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,
• the method makes it possible to treat a sulfuric waste material that contains metal ions, which is the case of T1O2 manufacture waste material.
• 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,
• the method according to the invention makes it possible to recover valuable compounds that can be used as raw materials in other production processes.
• sulfur dioxide, SO2 can be recovered for making H2SO4
• the metals, in the form of metal hydroxides, can be used for producing metal sulfates.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• alkaline reagent 10 is a selective precipitation reagent for metal cations, which leaves the ion SOA S in the solution of first mixture 21.
• 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).
• alkaline reagent 10 comprises MgO, and the weight amount of Mg(OH)2 in slurry 10 is set between 30% and 50%.
• the production of solid hydroxides occurs, in particular, when the pH of mixture 21 formed during mixing 200 reaches a predetermined pH precipitation value.
• 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.
• 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.
• a pH equal to or higher than 8.5 causes most of the cations potentially present in the waste material to precipitate,
• 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;
• 3S0 4 + 2 Me 3+ + 3Mg(OH)2-» 3MgS04 (solution) + 2Me(OH) 3 j [2] wherein Me 2+ and Me 3+ 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.
• 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.
• This reaction is a combination of gS04 with MgO, i.e. it can be written as:
• 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.
• 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'.
• 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.
• the step of separating metal hydroxides 23 comprises a filtration step 210 or a centrifugation step 2 0', as shown in Fig. 3.
• solubility of oxysulfates in water 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],
• 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.
• 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.
• 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.
• 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.
• step 300 of mixing sulfate and/or oxysulfate solution 22 with a precipitation agent 31 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.
• the method provides steps 390, 400, 401 of treating solid sulfate and/or oxysulfate 33 with a reducing agent 41 ,
• 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.
• 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.
• 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,
• SO2 gaseous sulfur dioxide
• 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'.
• SO2 42 obtained from step 401 of reduction and decomposition of sulfate and/or oxysulfate 33 can be used for making sulfuric acid.
• 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.
• 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.
• 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.
• 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.
• 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.
• magnesium oxysulfate 33 precipitates with 3 to 11 hydration molecules.
• alcohol 31 is ethyl alcohol.
• the use of ethyl alcohol as precipitation agent 31 makes recovery step 350 easier.
• 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.
• 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.
• 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.
• Absolute ethyl alcohol can also be used as oxysulfate or sulfate precipitation agent 31 .
• 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.
• 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%.
• 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 e C,
• 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:
• 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.
• 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,
• 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.
• 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.
• 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.
• reducing agent 41 for treating solid sulfate and/or oxysulfate 33 is elemental sulfur 41
• 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.
• 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:
• 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 .
• 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,
• 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.
• reducing agent 41 can comprise hydrogen gas.
• reducing agent 41 can comprise hydrogen gas.
• reducing agent 41 can comprise an oxyacetylene flame.
• oxyacetylene flame In this case, during reduction and decomposition 400, still in the case of magnesium oxysulfate, the following reactions take place:
• reducing agent 41 can comprise a so-called Brown's Gas, HHO, i.e. a reducing flame consisting of atomic hydrogen and oxygen.
• HHO Brown's Gas
• reducing agent 41 can comprise a so-called water gas, i.e. a mixture of hydrogen, carbon monoxide and carbon dioxide.
• 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.
• the reduction decomposition temperature is set between set between 700°C and 1200°C, advantageously between 850°C and 1 00 e C, preferably between 950°C and 1050°C, even more preferably it is about 1000°C.
• 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.
• 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 e C, 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
• 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.
• the first mixture comprised a magnesium oxysulfate water solution.
• Example 4 Treatment of an acid waste material containing H2SO4 and metal cations using Zinc hydroxide
• 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
• 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.
• 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.

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• 2015
  • 2015-10-27 EP EP15825628.9A patent/EP3212577A1/de not_active Withdrawn
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