Process for leaching magnesium from a material

Abstract

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Description

[0001]PROCESS FOR LEACHING MAGNESIUM FROM A MATERIAL The present invention relates to a process for leaching magnesium from a material which comprises magnesium and phosphorus. STATE OF THE ART Generally, magnesium-leaching process is carried out on a phosphate rock, which contains, amongst other compounds, phosphorus (expressed as P2O5) in a certain amount and which forms the compound of interest in the production of phosphates. Nowadays, high-grade phosphate rocks can be less attractive because of their current cost on the market, which leads the users to retrieve phosphorus from lower-grade-phosphate rocks. However, lower-grade rocks contain more impurities which can impart the quality and purity of the produced phosphate. Magnesium is one of those impurities. As a matter of fact, the leaching step should be efficient in order to sufficiently remove magnesium from the rock with a minimum impact on phosphorus loss. Typically, such processes involve an attack of the rock with an acid, then a separation step is performed to collect the solid phase depleted from magnesium, and a liquid phase. The latter can be neutralised resulting into the formation of another liquid phase along with a residue. US 3717702 discloses a process for treating phosphate ore in order to remove magnesium impurities. The process involves a reaction at a temperature comprised between 10 and about 80 °C of ground phosphate rock with sulfuric acid until partial completion. Filtration is carried out and the obtained filter cake is washed with water and dried. The liquid phase which contains magnesium impurities is discarded. Unfortunately, this teaching is limited to the removal of magnesium present in the rock. However, the liquid phase obtained after the attack of the rock still contains some magnesium which remains an impurity within the process. This additional stream is not upgraded since it is discarded or rejected as explained in the patent document. The USSR Inventor’s Certificate №711021 discloses a method of producing phosphate fertilizers from high magnesium carbonated phosphate rock. The process includes processing high magnesium carbonated phosphate rock with sulfuric acid at the temperature of 85 to 95°C with subsequent filtration of the produced slurry and production of a magnesium-containing solution. The main drawbacks of the process are running the process at an increased temperature thus requiring expenses on the additional heating of the system and formation of a liquid phase mostly consisting of water after regeneration with milk of lime, which is not suitable for reuse (recirculation) due to its insufficient purity. CN106744753 relates to a magnesium-leaching process involving phosphate rock with a certain granulometry which is mixed with water to obtain a pulp. The pulp should be preheated at 40 °C and then treated according to at least 2-stage-magnesium-removal attack at the same temperature. The attack of the pulp is carried out with sulfuric acid or phosphoric acid in several stages, followed by a filtration step. Preheating of the pulp at 40 °C is one of the main disadvantages of this process. CN 100392124 discloses the treatment of a rock in the form particles with the aim to retrieve magnesium (such as Mg(OH)2) in a high purity degree. CN 102115819 discloses a method for recovering magnesium from low grade high-magnesium phosphate ore. The process needs an agent comprising magnesium hydrogen sulfate or a mixture of concentrated sulfuric acid and magnesium sulfate. Unfortunately, known processes are not sufficiently economical in terms of the amounts of raw materials used and in terms of the parameters (temperature, duration,…) applied at each step involved in the process – energy consumption. Moreover, current processes do not valorise all produced streams which have to be further treated by additional steps or even rejected without added value for the environment, which remains disadvantageous for the user. There is therefore a need to provide a simple and efficient process for retrieving a maximum amount of magnesium from a rock, while collecting phosphorus with a high production rate. Moreover, the process should be simple, sustainable, economical and should save energy. Furthermore, valorisation of the streams produced during removal of magnesium would be beneficial for the user. INVENTION These technical issues are solved by the present invention which provides a simple and more efficient process, while ensuring a good balance between the amount of phosphorus collected and the removal of magnesium from the processed material, while upgrading some streams generated during the process. Moreover, the process of the present invention is sustainable and designed to enable reduced energy consumption over time, compared with known processes. Present invention enables: - Removing magnesium from a material with efficiency at several stages of the process; - Ensuring high amount of phosphorus in the obtained material; - Valorising the use of some added-value streams within the process (via a recycling loop) in order to provide a sustainable process over time. The present invention is directed to a process for leaching magnesium from a material, comprising the following steps: a) Providing a material which comprises magnesium and phosphorus, wherein the material comprises between 10-40 wt.% of phosphorous, expressed in wt.% equivalent P2O5, and wherein the molar ratio MgO / P2O5 of the material is of between 0.09 and 1.00, b) contacting said material with acidic ions (H+) in the presence of water with formation of a first slurry composed of a first solid phase which comprises said material depleted from magnesium and a first liquid phase containing magnesium, wherein the acidic ions (H+) are reacted with said material in amounts such that a molar ratio (H+ / CO2) of the total amount of acidic ions (H+) to the amount of C atoms (expressed as CO2 present in said material) is more than 0.60 and is less than 2; and wherein the acidic ions comprise sulfuric acid ions, c) Separating said first liquid phase containing magnesium from said first solid phase mainly comprising phosphorus, d) Providing a neutralising agent, preferably a base, e) Neutralising by addition of said neutralising agent to said first liquid phase resulting into the formation of a second liquid phase mainly comprising water and ions, and a second solid phase comprising magnesium, f) Separating said second liquid phase mainly comprising water and ions from said second solid phase, g) Recycling at least a part of said second liquid phase which mainly comprises water and ions in at least step b). According to the present invention, step b) of contacting the material and the acidic ions H+(attack of the material) leads to the formation of a first solid phase and a first liquid phase. The first solid phase mainly comprises phosphorus and the first liquid phase comprises magnesium. The first liquid phase is separated from the first solid phase and neutralised by a neutralising agent. This neutralisation reaction results into the formation of the second solid phase, which comprises magnesium, and of the second liquid phase. Said second solid phase is preferably a crystallized solid that allows good filtration conditions (faster filtration). The process of the present invention can be carried out in batches or continuously, even though continuous process is preferred. Preferably, the process of the invention is a continuous process, more preferably said second liquid phase, which mainly comprises water and ions, is continuously recycled in the process. Moreover, the process can be performed within one reactor or in multiple reactors (put in series / in parallel), advantageously with 2 reactors put in series or in parallel with respect to each other, more advantageously step b) of the present invention is performed in series or in parallel and step e) is performed in a separate reactor. When the process of the invention is applied, it is possible to add the reactants in different orders. For instance, the addition of the neutralising agent can be performed as follows: - simultaneously adding the first liquid phase and the neutralising agent within a reactor, or - adding the first liquid phase to a reactor followed by the addition of the neutralising agent to the reactor, or - adding the neutralising agent to a reactor followed by the addition of the first liquid phase to the reactor, or - adding the first liquid phase to a reactor followed by the partial addition of one or more neutralising agent to the reactor and a second addition of one or more neutralising agent. The attack of the material (step b)) is preferably performed at a certain temperature range which enables ensuring a good balance between phosphorus collection and magnesium retrieval. Advantageously, the temperature applied at step b) above is comprised between 60 and 80 °C, more preferably between 60 and 75 °C, even more preferably between 65 and 75 °C, even more preferably between 68 and 72 °C, more advantageously around 70 °C. The present invention is sustainable since it contains a recycling loop of said second liquid phase which mainly comprises water and ions directly within the process, without the need to further process it. The produced liquid which mainly comprises water and ions is directly usable, without any further treatment, rendering the present process easy to apply. Therefore, the present process enables removing magnesium from a material with efficiency at several stage of the process, ensuring high amount of phosphorus in the obtained material, valorising the use of some added-value streams (comprising water & ions) within the process in order to provide a sustainable process over time. Moreover, the present process enables reducing MER content from a material with efficiency at several stage of the process. Concretely, this means that delivering means are provided in order to introduce said second liquid phase into one of the above steps in an easy way: storage tanks / buffer tank, pipes and pumps. According to a particular embodiment of the process, at least a part (or all of) of said second liquid phase which mainly comprises water and ions is added to: - said provided material before reaction step b), or - said provided acidic ions (H+), or - to eventual additional provided water, or - step b) wherein said material is contacted with said acidic ions (H+) in the presence of water, or - said provided neutralising agent, or - step e), wherein said first liquid phase is neutralised, or - step c) to dilute the first slurry, or - additional washing step of the solids (mainly 1stsolid phase), or - any combinations thereof. The present process offers the possibility of adding water separately in order to, for instance, dilute the acidic ions (H+). Another option can also be to add the second liquid phase to the material (phosphate rock), in order to form a slurry. The recycling loop (i.e. step g)) of the present invention enables feeding thereby some or all of the steps of the present (continuous) process, in place of using fresh water. This is particularly useful when the process is continuously running, by continuously feeding the process with said second liquid phase, which can be carried out at least one step of the process or several ones. According to certain embodiments, the second liquid phase mainly comprising water and ions can also be recycled at step(s) c) and / or f), preferably when a dilution / washing is needed, more preferably at step c). Preferably, the acidic ions (H+) in the presence of water is contacted with said material in amounts such that a molar ratio (H+ / CO2) of the total amount of acidic ions (H+) to the amount of C atoms (expressed as CO2 present in said material) is preferably more than 0.65 and is less than 1.9 (i.e., 0.65 < H+ / CO2 < 1.9), more preferably more than 0.7 and less than 1.8 (i.e., 0.7 < H+ / CO2 < 1.8), even more preferably more than 0.75 and less than 1.7 (i.e., 0.75 < H+ / CO2 < 1.7) even more preferably more than 0.8 and is less than 1.6 (i.e., 0.8 < H+ / CO2 < 1.6). It will be understood that the molar ratio of the total amount of acidic ions to the amount of C atoms (expressed as CO2present in said material) is calculated by firstly determining the CO2content of the material, and thus providing the acidic ions in the presence of water accordingly to reach the desired molar ratio (H+ / CO2), as detailed above. The amount of C atoms (expressed as CO2present in said material) can be more particularly calculated by determining the CO2content of the material, said determination is carried out by first weighing the material, subjecting the material to an acid attack with a predetermined amount of a strong acid, preferably sulfuric acid, and weighing the resulting slurry thereby obtained. It will be understood that the amount of CO2 is obtained by calculating the difference between the amount of material and acid as compared to the amount of the resulting slurry. It will be thus understood that the amount of C (expressed as CO2present in said material) of the present invention is intended to denote the amount of C coming from inorganic compounds (inorganic carbon), such as for example inorganic compounds in the form of carbonates (e.g. Ca(CO3) or Mg(CO3)), being measured as CO₂ is released during an attack with an excess of strong acid, as detailed above. Thus, the amount of C of the present invention excludes carbon from organic compounds (organic carbon), such as soil, soot, or organic matter. In simple terms, the measured carbon (expressed as CO₂ present in said material) reflects inorganic carbon only, and not organic carbon. According to one embodiment of the present invention, step b) comprises the steps of first diluting a concentrated solution of sulfuric acid in water, thereby resulting in acidic ions (H+) in the presence of water, and then subsequently contacting said acidic ions in the presence of water with the material. According to this embodiment, the acidic ions (H+) in the presence of water is produced prior to the contacting, when the concentrated solution of sulfuric acid is diluted with water. According to this embodiment, the step of first diluting the concentrated solution of sulfuric acid in water may be carried out by any means known to the skilled in the art, such as notably by using a mixing tee. According to another embodiment of the present invention, step b) comprises the steps of first mixing the material with water, and then, subsequently contacting the material in water with a concentrated solution of sulfuric acid. According to this embodiment, the acidic ions (H+) in the presence of water is produced in situ, when the material with water is contacted with the concentrated solution of sulfuric acid. According to this embodiment, the material is mixed with water, in order to form a pulp, the pulp preferably having a solid content of from 5-70 wt.%, preferably from 10-60 wt.%, more preferably from 5-50 wt.%, more preferably from 20-40 wt.%, even more preferably from 25-30 wt.%, advantageously less than 35 wt.%, more advantageously less than 30 wt.% with respect to the total weight of said pulp. The material is thus in suspension in water. According to yet another embodiment of the present invention, step b) comprises a step of diluting a concentrated solution of sulfuric acid in water, a step of mixing the material with water, and then, subsequently, contacting the material in water with the concentrated solution of sulfuric acid in water. According to this embodiment, the acidic ions (H+) in the presence of water is produced in situ, when the material with water is contacted with the concentrated solution of sulfuric acid in water. According to this embodiment, the step of first diluting the concentrated solution of sulfuric acid in water may be carried out by any means known to the skilled in the art, such as notably by using a mixing tee. According to yet another embodiment of the present invention, step b) comprises a step of contacting concomitantly a concentrated solution of sulfuric acid, water and the material. According to this embodiment, the step of contacting concomitantly a concentrated solution of sulfuric acid, water and the material may be carried out by any means known to the skilled in the art, and more particularly by providing each of the material, the concentrated solution of sulfuric acid, and the water in a tank via three separate feeds, each feed comprising either the material, the concentrated solution of sulfuric acid, or the water. According to this embodiment, the acidic ions (H+) in the presence of water are produced in situ, when the material is concomitantly contacted with water and with the concentrated solution of sulfuric acid. According to yet another embodiment of the present invention, step b) comprises a step of contacting the material with a diluted solution of sulfuric acid. According to this embodiment, the diluted solution of sulfuric acid may originate from by-products of other processes of production of phosphoric and / or sulfuric acid and / or from washing of facilities thereof, said by-products being unsuitable to be released into the environment or being unsuitable from being used in other processes. Said by-products have typically a concentration of at least 5 wt.%, or 10 wt.%, or 20 wt.% and at most 80 wt.%, or at most 70 wt.% or at most 60 wt.% of sulfuric acid based on the total weight of the by-products. It will be understood that, said by-products may be diluted before being contacted with the material. This embodiment is particularly advantageous as it allows to valorise said by products. Within the context of the present invention, the term “concentrated solution of sulfuric acid” is intended to denote any commercially available sulfuric acid solution which is concentrated in an amount of at least 95.0 wt.%, more preferably of at least 96 wt.%, more preferably of at least 98 wt.%, more preferably of at least 99.0 wt.% based on the total weight of the concentrated sulfuric acid solution. Within the context of the present invention, the term “digestion solution” will be used hereinbelow and is intended to denote the acidic ions (H+) in the presence of water, which is produced by any embodiments as detailed above. In other words, the term “digestion solution” is intended to denote the acidic ions (H+) in the presence of water, which is either produced prior to the contacting, as detailed above, or in situ. Thus, it will be understood that in the context of the present invention, the terms “digestion solution” and “acidic ions (H+) in the presence of water” can be used interchangeably. It will be further understood that the acidic ions (H+) in water, or the digestion solution, is comprised of the acidic ions (H+) comprising sulfuric acid ions, and any of the water which may be used to either (i) dilute the concentrated sulfuric acid solution, (ii) mix the material, and / or (iii) the combinations of (i) or (ii), as detailed above. Preferably, the digestion solution is reacted with said material in amounts such that a molar ratio (SO42- / CO2) of the total amount of sulfate ions (H+) in the digestion solution to the amount of carbon atoms (expressed as CO2present in said material) is preferably more than 0.33 and is less than 0.95 (i.e., 0.33 < SO42- / CO2< 0.95), more preferably more than 0.35 and less than 0.9 (i.e., 0.35 < SO42- / CO2 < 0.9), even more preferably more than 0.38 and less than 0.85 (i.e., 0.38 < SO42- / CO2 < 0.85) even more preferably more than 0.4 and is less than 0.8 (i.e., 0.4 < SO42- / CO2 < 0.8). Preferably, the digestion solution is reacted with said material in amounts such that a molar ratio (H+ / (Mg+Ca)) of the total amount of acidic ions (H+) in the digestion solution to the total amount of Mg and Ca atoms expressed as Mg + Ca present in said material is more than 0.10 and less than 1 (i.e., 0.10 < H+ / (Mg + Ca) <1.0), preferably more than 0.15 and less than 0.9, even more preferably more than 0.17 and less than 0.75 , even more preferably comprised between 0.2 and 0.6. The aforementioned molar ratios enable further improving the reaction between the material and the digestion solution. According to certain embodiments, said digestion solution (L) comprises sulphuric acid ions (H+(SA)) and at least one mineral acid (HX) ions, wherein a ratio (H+(SA) / H+(L)) of a mole content of H+issued from the sulphuric acid ions (= H+(SA)) to a total mole content of H+ions in the digestion solution (L) (= H+(L) = H+(X) + H+(SA)) is comprised between 50 and 100 %, preferably between 70 and 98 %, more preferably between 80 and 98, even more preferably between 93 and 95 %. Preferably, the digestion solution (L) can contain up to 20 % of a mineral acid (HX) ions which is different than H2SO4. Preferably The mineral acid (HX) may be HCl, HNO3, H3PO4, HF, H2SiF6or combinations. According to certain embodiments, the ratio (H+(SA) / H+(L)) of the digestion solution, is of at least 95 mol.%, more preferably of at least 98 mol.%, even more preferably of at least 99 mol.%. More advantageously, said digestion solution (L) has a concentration of at least 5 wt.% H2SO4, preferably at least 10 wt.%, more preferably at least 15 wt. % H2SO4, even more preferably at most 25 wt.% H2SO4, advantageously at most 20 wt.% H2SO4, more advantageously of about 17 wt.%. H2SO4. As said, the process for leaching magnesium from the material of the present invention comprises a step g) of recycling at least a part of the second liquid phase which mainly comprises water and ions in at least step b). It will be understood that step g) of recycling at least a part of the second liquid phase in at least step b), as detailed above, may also be applied to any one of at least one step a) to f). In other words, within the invention, a recycling loop, which corresponds to the recycling of the second liquid phase to at least step b) and optionally additional to any of the other steps may be applied, without any further treatment and eventually at different stages of the process (that is to say at step c), at step d) or at step c) and d)), preferably in a continuous way. It is therefore possible to recycle a part of the second liquid phase at one stage of the process and another part of the second liquid phase at another stage of the process, in a handy way. In certain embodiments, as detailed above, when step b) comprises the steps of first diluting a concentrated solution of sulfuric acid in water, thereby resulting in acidic ions (H+) in the presence of water, and then subsequently contacting said acidic ions in the presence of water with the material, step g) allows to replace at least part, or all, of the water used in said step of first diluting the concentrated solution of sulfuric acid in water. In certain embodiments, as detailed above, when step b) comprises the steps of first mixing the material with water, and then, subsequently contacting the material in water with a concentrated solution of sulfuric acid, step g) allows to replace at least part, or all, of the water used in said step of first mixing the material with water. In certain embodiments, as detailed above, when step b) comprises a step of diluting a concentrated solution of sulfuric acid in water, a step of mixing the material with water, and then, subsequently, contacting the material in water with the concentrated solution of sulfuric acid in water, step g) allows to replace at least part, or all, of the water used in step said step of diluting a concentrated solution of sulfuric acid in water, in said step of mixing the material with water, or in both steps. In certain embodiments, as detailed above, when step b) comprises a step of contacting concomitantly a concentrated solution of sulfuric acid, water and the material, step g) allows to replace at least part, or all, of the water used concomitantly with the material and the concentrated sulfuric acid. In certain embodiments of the present invention, the recycling loop can be used to dilute the digestion solution or H2SO4 or any mixtures mainly containing H2SO4. In certain embodiments of the present invention, the recycling loop can be used to dilute the neutralising agent. According to certain embodiments, when the acidic ions in the digestion solution consist essentially of sulfuric acid ions, said contacting step b) between said material and said digestion solution is carried out at a ratio of 8-20 g of H2SO4(100%) for 100g of said material, preferably at a ratio of 10-15g of H2SO4(100%) for 100g of said material, more preferably at a ratio of 12-15g of H2SO4(100%) for 100g of said material, with removal of at least 30 wt.% of magnesium from said material. Within the context of the present invention, the term “the acidic ions (in the digestion solution) consists essentially of sulfuric acid ions” is intended to denote that any acidic ions other than sulfuric acid ions is present in the digestion solution in an amount of at most 5 wt.%, more preferably at most 2 wt.%, even more preferably at most 1 wt.% relative to the total weight of the acidic ions in the digestion solution. According to certain embodiments of the invention, the contacting step b) between said material and said digestion solution is carried out until removal of magnesium from said material by: - simultaneously adding said digestion solution and said material within a reactor, or - first adding said material within a reactor and then said digestion solution to said reactor, or - first adding said digestion solution within a reactor and then said material to said reactor. Preferably, the reaction step b) between said material and said digestion solution is carried out until removal of magnesium from said material by simultaneously adding said digestion solution and said material within a reactor. This magnesium removal is preferably comprised between 30 and 80 %, more preferably between 40 and 70 %; the magnesium removal is defined as the % ratio of ‘the Mg content in the first solid phase / the Mg content in the material’. According to a preferred embodiment of the process of the invention, said contacting step b) is performed during a period of time comprised between 2 and 180 minutes, preferably between 20 and 150 minutes, more preferably between 30 and 100 minutes. Preferably, said contacting step b) is completed when a pH comprised between 2 and 5 is reached, more preferably between 2 and 4. According to a particular embodiment of the process, said first slurry has a solid content of from 5-45 wt.%, preferably from 10-40 wt.%, more preferably from 20-35 wt.%, even more preferably from 25-30 wt.%, advantageously less than 35 wt.%, with respect to the total weight of said first slurry. Advantageously, the first slurry comprises calcium phosphate in any form known to the skilled in the art (e.g. calcium monophosphate, dicalcium phosphate, tricalcium phosphate, …), and gypsum (CaSO4.2H2O) in solid form. Advantageously, said neutralisation step h) is carried out with the addition of a (at least one) base suitable for precipitating magnesium, preferably until complete neutralisation is reached. This enables providing a cleaner second liquid phase which can be reused easily within the process of the present invention. Advantageously, the second liquid solution comprises less than 300 ppm Mg, more advantageously less than 200 ppm Mg, more advantageously less than 150 ppm Mg. More advantageously, neutralisation is reached when the pH is of at least 8, preferably at least 9, more preferably at least 10, even more preferably at least 11, advantageously at least 12. More advantageously, neutralisation is reached when the pH is of at most 11, advantageously at most 12. In one embodiment of the present invention, said neutralisation step can be performed by maintaining a temperature around the one applied in step b). This way, there is no specific needs to further increase the temperature or reduce it by cooling. Keeping the already applied temperature can be enough and advantageous. According to another embodiment, said neutralisation step can be performed at a temperature comprised between 20 and 75 °C, preferably between 25-70 °C, more preferably between 30-60 °C, even more preferably between 50 and 55°C. This way, the temperature management is initiated by the temperature applied at the reaction step b). This is advantageous because the heat generated earlier in the process can be managed in such a way that the temperature of the neutralisation step is easily handled and adjusted without necessarily involving an increase of temperature or any eventual additional cooling. The process is also improved by this technical feature regarding the efficient management of the temperature (no further heating or cooling). In other words, it means that if the reaction step b) is performed in one module and the neutralisation is carried out in another module, there is no need to cool or overheat the first liquid phase since the temperature is already optimal for the next steps of the process (read, neutralisation step). In one preferred embodiment, said neutralisation step can be performed for a period of time of less than 60 minutes, preferably less than 45 minutes, preferably less than 30 minutes, more preferably between 2- 25 minutes, even more preferably between 10-25 minutes, advantageously less than 20 minutes. Preferably, said base suitable for precipitating magnesium is selected from the group comprising NH3, calcium or sodium in the form of their oxides, hydroxides or carbonates, and combinations thereof. More preferably, the base is selected from the group consisting of NH3, Ca(OH)2, milk of lime, CaO, or combination thereof. Even more preferably the base is selected from the group consisting of Ca(OH)2, milk of lime, CaO, or combination thereof. Non-limiting examples of bases comprising CaO and Ca(OH)2 is lime. Within the context of the present invention, the term “milk of lime” or “lime milk” is intended to denote a slurry of particulate Ca(OH)2 in aqueous solution. Thus, it is understood that the terms “milk of lime” and “lime milk” can be used interchangeably. According to certain embodiments, use can be made of a composition comprising lime and water, which is comprised between 2 and 20 wt.%, more preferably between 10 and 20 wt.% with respect to the total weight of the lime milk composition. According to certain embodiments, use can be made of a composition comprising calcium hydroxide (Ca(OH)2) in water, said calcium hydroxide being present in the composition in an amount of between 2 and 20 wt.%, more preferably between 10 and 20 wt.% with respect to the total weight of the composition. It has been advantageously observed that the addition of at least one neutralising agent enables forming crystals in the second solid phase. The particle size distribution of these crystals is such that separation step is improved and facilitated through the fact that it can be carried out in a quicker way. Advantageously, the second solid phase mainly comprises insoluble magnesium salts (such as Mg(OH)2) and calcium sulfate in any form known to the skilled in the art, such as anhydride CaSO4, CaSO4. ½H2O,or gypsum (CaSO4.2H2O). More preferably, said second liquid phase mainly comprising water and ions is recycled in an amount comprised between 20 and 100 wt.%; preferably between 30 and 99 wt.%, more preferably between 40 and 99 wt.%, preferably between 60 and 98 wt.%, more preferably between 70 and 95 wt.%, even more preferably up to 100 wt. %, with respect to the total weight of that second liquid phase, preferably in a continuous way. This enables providing a sustainable process wherein the second liquid phase is directly used without the necessity to further process it before re-entering the process, creating thereby at least one recycling loop. With the features of the invention, it is easier to feed the one or several steps of the process with products stemming from that process in an economical way. Moreover, they can be used directly, for instance to perform the attack of the material. Advantageously, the material comprises at least 10 wt.% of phosphorus, preferably between 10-40 wt.% of phosphorus, more preferably between 10-35 wt.% of phosphorus, even more preferably between 20-30 wt.% of phosphorus, expressed in wt.% equivalent P2O5. The present invention preferably aims to provide a process dedicated to the processing of low-grade phosphate rock or any other material containing a low amount of phosphate and wherein impurities, such as magnesium, iron and / or aluminum, are present. More advantageously, said material is selected from the list comprising phosphate rock, slimes, beneficiation residues, ashes containing phosphorus such as bone ashes and / or sewage sludges ashes, pig or chicken manure, guano, or sewage sludge, and combinations thereof. Advantageously, said material is a phosphate rock. According to certain features of the invention, said material has a purity degree expressed by the Minor Elements Ratio index (MER index) comprised between 0.080-0.820, preferably between 0.081-0.500, more preferably between 0.082-0.250, even more preferably 0.150-0.220, wherein the MER index is expressed as follows: MER = (Al2O3 + Fe2O3 + MgO) / P2O5, where Al2O3, Fe2O3, MgO and P2O5 are total amount of Al, Fe, Mg and P atoms, expressed in wt.% equivalent of their oxides. The MER index as expressed hereinabove corresponds to the initial MER index of the material before being processed by the process of the present invention (read, before applying step b)). According to a particularly preferred embodiment of the invention, the molar ratio MgO / P2O5of the material is lower than 0.90, preferably lower than 0.80, more preferably lower than 0.70, even more preferably lower than 0.60, advantageously lower than 0.50, advantageously lower than 0.45. It will be understood that the molar ratio MgO / P2O5 of the material is advantageously higher than 0.10, preferably higher than 0.11, even more preferably higher than 0.12. According to a preferred embodiment of the invention, the molar ratio MgO / P2O5 of the material is of between 0.09 and 0.90, preferably of between 0.09 and 0.80, more preferably of between 0.09 and 0.70, even more preferably of between 0.10 and 0.60, even more preferably of between 0.11 and 0.50. even more preferably of between 0.12 and 0.45. More preferably, when said reaction step b) is completed, an amount of at least 50 wt.%, preferably at least 60 wt.%, more preferably at least 65 wt.%, more preferably at least 70 wt.%, even more preferably at least 80 wt.%, advantageously at least 90 wt.% of magnesium is removed from said material. According to an advantageous embodiment, when said reaction step b) is completed, an amount of up to 50 wt.%, preferably up to 40 wt. %, more preferably up to 30 wt.%, even more preferably up to 20 wt.%, advantageously up to 15 wt.%, more advantageously up to 10 wt.% of aluminum is removed from said material. More advantageously, when said reaction step b) is completed, an amount of up to 50 wt.%, preferably up to 40 wt. %, more preferably up to 30 wt.%, even more preferably up to 20 wt.%, advantageously up to 15 wt.%, more advantageously up to 10 wt.% of iron is removed from said material. Thanks to the process of the present invention, it is also possible to retrieve other impurities from the material of the invention, i.e. iron and aluminium, which positively contribute to the final purity of the material containing phosphorus (read, first solid phase). This was also surprising since several impurities can be removed by applying the process of the present invention. Preferably, at the end of said reaction step b), the purity degree of the material expressed as the Minor Elements Ratio index (MER index) is reduced in an amount of up to 30%, preferably up to 50 %, more preferably up to 60 %, even more preferably up to 70 %. This MER index can be expressed as the final MER index. The process of the present invention requires at least 2 separation steps c) and f). Preferably, separation step can be performed as follows: by centrifugation, settling or decantation or filtration, preferably with a filter selected from the group comprising filtration under pressure and under vacuum and combinations thereof. The wording ‘a reactor’ can also mean a module which is selected from the list comprising at least one reactor, one vessel, one tank and combinations thereof. In one embodiment, a recirculating loop may exist to recycle said first liquid phase in the reaction step b). The terms “MER index” (minor elements ratio) used in the context of the present invention enables determining the degree of purity of the material. The “MER index” corresponds to the sum of the oxides of the major impurities relative to the P2O5 content: MER = ((Al2O3+ Fe2O3+ MgO) / P2O5) where Al2O3, Fe2O3, MgO and P2O5are total amount of Al, Fe, Mg and P atoms, expressed in wt.% equivalent of their oxides expressed in wt.%. The initial “MER index” is measured before the reaction step(c) and the final “MER index” is preferably measured in the first solid phase. The expression “complete neutralisation” should be understood as meaning that a certain pH value is reached: pH above 7, preferably above 8, more preferably above 9, even more preferably 10, advantageously above11. In the present invention, the amount of phosphorus is expressed in wt.% equivalent of P2O5. P2O5 Yield is expressed in % according to the following equations: • P2O5Yield = (P2O5in (material) – P2O5out (filtrate / 1stliquid phase)) / P2O5in (material) According to a preferred embodiment, the present invention, the solid phase obtained after completion of step b) has a P2O5yield of more than 95 %, preferably more than 96 %, more preferably more than 97 %, even more preferably more than 98 %, advantageously more than 99 %. The present invention enables selective magnesium removal from the material, e.g. phosphate rock. The material can be slurried in water and gradually acidified in a first module (a 1streactor), preferably under agitation, with sulfuric acid to reach a pH zone in the range 2 – 4, wherein the magnesium content of the material is solubilized, but not (to a lesser extent) the phosphate content (P2O5, for instance as apatite). The resulting material (e.g. rock cake, first solid phase) obtained after filtration will have a reduced content of magnesium with a minor loss of P2O5. The filtrate (first liquid phase) is further processed in a second module (e.g. 2ndreactor) called “magnesium precipitation”, wherein magnesium is precipitated by pH increase until reaching a pH, preferably in the range from 8 to 11 A solid / liquid filtration step is carried out leading to the recovery of a residue containing the precipitated metal hydroxides (mainly Mg, second solid phase) and a water filtrate (second liquid phase). This solid / liquid separation step is highly efficient in terms of its duration, thanks to the presence of magnesium-based crystals. The present invention also covers the possibility of having preferred features relating to the neutralisation step which can take the form of a 2-stage neutralising step. More precisely, the material which comprises magnesium and phosphorus and the digestion solution mainly comprising sulfuric acid are provided. Then, at a temperature preferably comprised between 60 and 80 °C, more preferably between 60 and 75 °C, even more preferably between 65 and 75°C, a reaction between said material with said digestion solution mainly comprising sulfuric acid is performed resulting into the formation of a first slurry composed of a first solid phase which comprises said material depleted from magnesium and a first liquid phase containing magnesium. The separation step can take place in order to separate the first liquid phase. The addition of the neutralising agent, preferably a base, to said first liquid phase is carried out until reaching a pH below 10 (first neutralisation). This results into the formation of a residue in the presence of the liquid phase. A separation step can therefore be applied in order to separate the liquid phase. Then a second neutralisation step is carried out by adding a neutralising agent to the liquid phase (obtained after separation) and when the pH of complete neutralisation is reached (above 8, preferably above 9, more preferably above 10, even more preferably above 11) another separation (corresponding to step f) of the process) is applied in order to provide the liquid phase which mainly comprises water and ions. With this embodiment, it is possible to perform neutralisation in a 2-step form. Preferably, the first step of neutralisation can be performed at a pH which is lower than the pH of the second neutralisation. Moreover, according to a preferred embodiment, the first neutralisation can be realised with a neutralising agent which is different or the same than the second neutralising agent for the second neutralisation step. With this preferred embodiment linked to the 2-stage neutralisation, an additional separation step can be carried out, which is done between the first and the second neutralisation steps. All preceding embodiments, technical features can be combined with each other, even if not specifically mentioned. One aforementioned paragraph including technical features can be combined with any other paragraphs, except if disclosed otherwise. Hereinbelow, without being linked to any theory, an example of some reactions that may occur in the context of the present invention: ^ Magnesium dissolution = Magnesium leaching of the material • MgCO3(s) + H2SO4(l) → MgSO4(s) + H2O (l) + CO2(g) MgO (s) + H2SO4 (l) → MgSO4 (s) + H2O (l) ^ Magnesium collection = neutralisation: • MgSO4(l) + Ca(OH)2→ Mg(OH)2(s) + CaSO4(s) • 2 H3PO4(l) + 3 Ca(OH)2→ Ca(HPO4) + 6 H2O Table 1 illustrates the composition of a phosphate rock according to one embodiment of the present invention. Table 1 PARAMETERS (wt.%) Lab tests Pilot tests Moisture 0.58 0.31 P2O5 Total 23.6 24.5 F 2.4 2.6 Ca expressed as CaO 40.9 41 Al expressed as Al2O3 1.27 1.10 Fe expressed as Fe2O3 0.97 0.90 Mg expressed as MgO 3.03 3.27 CO2 12 9.8 In the examples below, filtration is carried out and the collected phosphate rock (first solid phase) is washed with water (ratio ~3kg water / kg material). The water used in contacting step b) is not taken into account for magnesium removal calculation (contains 40 mg / kg of magnesium, contribution <1%). Example 1 In a first module, phosphate rock (as illustrated in table 1 above) is added at a flow rate of 3.70 kg / h to water (1L), then heated to desired temperature (70°C), and acidified with concentrated sulfuric acid (with 93 % concentration) during 60 minutes under strong agitation. Solid content of the first slurry is equal to 35 wt. % with respect to the total weight of the slurry. Filtration is carried out and the phosphate rock cake is removed. In a second module, 400 g of the first liquid phase collected after filtration (e.g. vacuum filtration) is neutralised with 20 % of Ca(OH)2at (slurry in water, at concentration of 10 % w / w), at a temperature of 50°C during about 20 minutes, until reaching a pH comprised between 5.5 and below 10.0, around 9.8. Then, the second solid phase containing Mg(OH)2(in the form of crystals) is quickly separated from the second liquid phase which mainly comprises water and ions. The total amount (100%) of this second liquid phase is added to reaction step (c) in order to feed the process with water (recycling loop). Table 2 illustrates the amount of some impurities in the 1stliquid phase when the process is applied for example 1. Table 2 Element Unit Elimination of Mg < 600 mg / kg Elimination of Ca < 620 ppm Elimination of Fe < 0.5 ppm Elimination of Al < 1 ppm The second solid phase contains Mg(OH)2 and less than 3.5 % of P2O5, less than 1000 ppm of Aluminum and less than 400 ppm of Iron. Test according to example 1 is performed with top cover on each module in order to avoid evaporation mass loss. Table 3 below represents other features stemming from example 1. Table 3 Parameters Value P2O5 recovery >95% Mg leaching 67 % Kg 100% H2SO4 / kg P2O5 0.56 Kg CaO / kg P2O5 0.17 Kg Mg cake (dry) – 1stsolid phase 0.54 Initial MER 0.214 Final MER 0.120 Molar ratio H+ / CO20.9888Same example has been reproduced wherein step (c) has been performed at 60 °C, 68 °C and 80 °C. Moreover, additional tests have also been done wherein step (c) is carried out during a period of time of 30, 60 and 90 minutes respectively. Can be noted that inventors have performed step(c) in such a way that the content of solids was equal to about 30, 35 and 40 wt. %. Regarding the molar ratio H+ / CO2 it can also be comprised between 0.6 to 2. The example 1 can also be performed at a concentration of H2SO4 of 98 wt. %. As of the purity degree of the rock (MER index), inventors obtained results in the range from 0,08 to 0,15. These tests have also been made for examples 2 and 3. Example 2 In a first reactor, phosphate rock is added at a flow rate of 3.70 kg / h to water (1L), then heated to desired temperature (70°C), and acidified with concentrated sulfuric acid (97.5 %) during 60 minutes. Solid content of the slurry is equal to 35 wt. % with respect to the total weight of the slurry. Filtration is carried out and the phosphate rock cake is removed. 400 g of the first liquid phase collected after filtration is neutralised with NH3 leading to the formation of struvite (ammonium magnesium phosphate) at a temperature of 20 °C during about 20 minutes until reaching a pH comprised between 5.5-9.8. between 0.2 and 0.4 kg NH3 / kg of rock are added. Then, the second solid phase containing struvite is separated from the second liquid phase which mainly comprises water and ions. This second liquid phase is used to feed one of the process steps water (recycling loop). Example 3 In a first reactor, phosphate rock is added at a flow rate of 3.70 kg / h to water (1L), then heated to desired temperature (70°C), and acidified with concentrated sulfuric acid (97.5 %) during 60 minutes. Solid content of the slurry is equal to 35 wt. % with respect to the total weight of the slurry. Filtration is carried out and the phosphate rock cake is removed. 400 g of the first liquid phase collected after filtration is neutralised with 10 % of Ca(OH)2 at a temperature of 50 °C during 20 minutes until reaching a pH of about 7 with precipitation of phosphate. After separation and collection of the liquid phase, a solution of 25 % of NH3is added until reaching a pH of about 10 at a temperature of 50 °C during 20 minutes with the formation of a residue which comprises magnesium. Then, an additional separation is necessary to remove the solid phase from the liquid phase which mainly comprises water and ions. 98% of this second liquid phase is added to reaction step (c) in order to feed the process with water (recycling loop). In the context of the present invention, any single article such as “a”, “the”, can be replaced by an article designating plural forms such as “a series of” or “plurality of” or “several” or “at least one” or “at least 2” or “at least 3”, etc. The wording “comprising”, “contains” or any other equivalent terms can be replaced by “consisting of” in order to define a more restrictive list or possibilities and exclude thereby other non-cited elements from the expression used.

Claims

CLAIMS 1. Process for leaching magnesium from a material, comprising the following steps: a) Providing a material which comprises magnesium and phosphorus, wherein the material comprises between 10- 40 wt.% of phosphorous, expressed in wt.% equivalent P2O5, and wherein the molar ratio MgO / P2O5 of the material is of between 0.09 and 1.00, b) contacting said material with acidic ions (H+) in the presence of water with formation of a first slurry composed of a first solid phase which comprises said material depleted from magnesium and a first liquid phase containing magnesium, wherein the acidic ions (H+) are reacted with said material in amounts such that a molar ratio (H+ / CO2) of the total amount of acidic ions (H+) to the amount of C atoms (expressed as CO2 present in said material) is more than 0.60 and is less than 2; and wherein the acidic ions comprise sulfuric acid ions, c) Separating said first liquid phase containing magnesium from said first solid phase mainly comprising phosphorus, d) Providing a neutralising agent, preferably a base, e) Neutralising by addition of said neutralising agent to said first liquid phase resulting into the formation of a second liquid phase mainly comprising water and ions, and a second solid phase comprising magnesium, f) Separating said second liquid phase mainly comprising water and ions from said second solid phase,g) Recycling at least a part of said second liquid phase which mainly comprises water and ions in at least step b).

2. Process according to claim 1, wherein step b) comprises the steps of first diluting a concentrated solution of sulfuric acid in water, thereby resulting in acidic ions (H+) in the presence of water, and then subsequently contacting said acidic ions in the presence of water with the material.

3. Process according to claim 1, wherein step b) comprises the steps of first mixing the material with water, and then, subsequently contacting the material in water with a concentrated solution of sulfuric acid, thereby resulting in acidic ions (H+) in the presence of water.

4. Process according to claim 1, wherein step b) comprises a step of diluting a concentrated solution of sulfuric acid in water, a step of mixing the material with water, and then, subsequently, contacting the material in water with the concentrated solution of sulfuric acid in water, thereby resulting in acidic ions (H+) in the presence of water.

5. Process according to claim 1, wherein step b) comprises a step of contacting concomitantly a concentrated solution of sulfuric acid, water and the material, thereby resulting in acidic ions (H+) in the presence of water.

6. Process according to any one of the preceding claims, wherein step b) is carried out at a temperature comprised between 60 and 80 °C, preferably between 60 and 75 °C, more preferably between 65 and 75°C.

7. Process according to any of the preceding claims, wherein the molar ratio MgO / P2O5 of the material is of between 0.09 and 0.90, preferably of between 0.09 and 0.80, more preferably of between 0.09 and 0.70, even more preferably of between 0.10 and 0.60, even morepreferably of between 0.11 and 0.

50. even more preferably of between 0.12 and 0.

45.

8. Process according to any one of the preceding claims, wherein the acidic ions (H+) in the presence of water are reacted with said material in amounts such that a molar ratio (H+ / CO2) of the total amount of acidic ions (H+) to the amount of C atoms (expressed as CO2present in said material) is comprised between 0.8 and 1.6 (i.e., 0.8 ≤ H+ / CO2≤ 1.6).

9. Process according to any one of the preceding claims, wherein the acidic ions (H+) in the presence of water are reacted with said material in amounts such that a molar ratio (H+ / (Mg+Ca) of the total amount of acidic ions (H+) to a total amount of Mg and Ca atoms expressed as Mg+Ca present in said material is more than 0.10 and less than 1 (i.e., 0.10 < H+ / (Mg+Ca) <1.0), and is preferably comprised between 0.2 and 0.

6.

10. Process according to any one of the preceding claims, wherein said acidic ions (H+) in the presence of water comprise sulphuric acid ions (H+(SA)) and at least one mineral acid (HX) ions wherein a ratio (H+(SA) / H+(L)) of a mole content of H+issued from the sulphuric acid ions (= H+(SA)) to a total mole content of H+ions in the acidic ions (H+) in the presence of water is comprised between 50 and 100 %, preferably between 70 and 98 %, more preferably between 80 and 98 %, even more preferably between 93 and 95 %.

11. Process according to any one of the preceding claims, wherein said acidic ions (H+) in the presence of water has a concentration of at least 5 wt.% of H2SO4, preferably at least 10 wt.% of H2SO4, more preferably at least 15 wt. % H2SO4, even more preferably at most 25 wt.% H2SO4, advantageously at most 20 wt.% H2SO4, more advantageously of about 17 wt.% H2SO4.

12. Process according to any one of the preceding claims, wherein the contacting step b) between said material and said acidic ions (H+) in the presence of water is carried out until removal of magnesium from said material by: - simultaneously adding said acidic ions (H+) in the presence of water and said material within a reactor, or - first adding said material within a reactor and then said acidic ions (H+) in the presence of water, or - first adding said acidic ions (H+) in the presence of water within a reactor and then said material.

13. Process according to any one of the preceding claims, wherein said contacting step b) is performed during a period of time comprised between 2 and 180 minutes, preferably between 20 and 150 minutes, more preferably between 30 and 100 minutes.

14. Process according to any one of the preceding claims, wherein said first slurry have a solid content of from 5-45 wt.%, preferably from 10-40 wt.%, more preferably from 20-35 wt.%, even more preferably from 25-30 wt.%, advantageously less than 35 wt.%, more advantageously less than 30 wt.% with respect to the total weight of said first slurry.

15. Process according to any one of the preceding claims, wherein said neutralisation step is carried out with the addition of a base suitable for precipitating magnesium, preferably until complete neutralisation is reached.

16. Process according to claim 15, wherein neutralisation is reached when the pH is of at least 8, preferably at least 9, more preferably at least 10, even more preferably at least 11, even more preferably at least 12.

17. Process according to claim 15 or 16, wherein said base suitable for precipitating magnesium is selected from the group comprising NH3, calcium or sodium in the form of their oxides, hydroxides or carbonates, and combinations thereof, preferably, the base is selectedfrom the group consisting of NH3, Ca(OH)2, milk of lime, CaO, or combination thereof, even more preferably the base is selected from the group consisting of Ca(OH)2, milk of lime, CaO, or combination thereof.

18. Process according to any one of the preceding claims, wherein said second liquid phase mainly comprising water and ions is recycled in an amount comprised between 20 and 100 wt.%; preferably between 30 and 99 wt.%, more preferably between 40 and 99 wt.%, more preferably between 60 and 98 wt.%, more preferably between 70 and 95 wt.%, even more preferably 100 wt.%, with respect to the total weight of that liquid, preferably in a continuous way.

19. Process according to any one of the preceding claims, wherein said second liquid phase mainly comprises water and counter ions of said acidic ions (H+) in the presence of water.

20. Process according to any one of the preceding claims, wherein the material comprises between 10-35 wt.% of phosphorus, even more preferably between 20-30 wt.% of phosphorus, expressed in wt.% equivalent P2O5.

21. Process according to any one of the preceding claims, wherein said material is selected from the list comprising phosphate rock, slimes, beneficiation residues, ashes containing phosphorus such as bone ashes and / or sewage sludges ashes, pig or chicken manure, guano, sewage sludge and combinations thereof.

22. Process according to any one of the preceding claims, wherein said material has a purity degree expressed by the Minor Elements Ratio index (MER index) comprised between 0.080-0.820, preferably between 0.081-0.500, more preferably between 0.082-0.250, even more preferably between 0.150-0.250, wherein the MER index is expressed as follows: MER= (Al2O3 + Fe2O3 + MgO) / P2O5, where Al2O3, Fe2O3, MgO and P2O5 are total amount of Al, Fe, Mg and P atoms, expressed in wt.% equivalent of their oxides.

23. Process according to any one of the preceding claims, being a continuous process, preferably wherein said second liquid phase which mainly comprises water and ions is continuously recycled in the process.

24. Process according to any one of the preceding claims, wherein, when said contacting step b) is completed, amount of at least 50 wt.%, preferably at least 60 wt.%, more preferably at least 65 wt.%, more preferably at least 70 wt.%, even more preferably at least 80 wt.%, advantageously at least 90 wt.% of magnesium is removed from said material.