Process for co-neutral, commercial production of magnesium with later recovery of hydrogen and / or synthetic fuels

Abstract

The invention relates to a process for CO2-neutral, commercial recovery of metallic magnesium from seawater, the recovered magnesium being processed further for use as a raw material for metallurgy and also being usable as a starting material for CO2-neutral recovery of hydrogen, and the CO2 recovered or released in the entire process being dissolved and bound in seawater.

Description

[0001] Process for the CCh-neutral, technical production of magnesium with subsequent production of hydrogen and / or synthetic fuels

[0002] Description

[0003] The invention relates to a technical process for the CCh-neutral extraction of metallic magnesium from seawater.

[0004] The process is modular and consists of a verification of known technical steps. The individual modules (processes) are not carbon dioxide-neutral and are based on conventional, already technically used processes. CO2 neutrality is achieved through the inventive sequence of the individual modules. With a favorable selection of the individual process steps, the overall process has a completely carbon dioxide-neutral, even negative, balance.

[0005] The extracted magnesium can be further processed as a raw material for metallurgy. It can also be used as a starting material for CO2-neutral hydrogen production. The technical process demonstrated here also offers the possibility of producing synthetic fuels, using the extracted hydrogen as a raw material.

[0006] In contrast to known technical processes for the production of green hydrogen or synthetic fuels, the process according to the invention does not require complex electrolysis steps, which makes it very efficient in terms of energy and economy.

[0007] Seawater can absorb very large amounts of CO2. This absorption capacity increases with decreasing temperature and increasing sea depth. According to Henry's law, it is also proportional to the partial pressure of CO2 above the water in the atmosphere. Even at room temperature and atmospheric pressure, the literature assumes approximately 2 grams of dissolved CO2 per liter of water, as described in "Carbon Dioxide in Water and Seawater: The Solubility of a Non-Ideal Gas," Marine Chemistry 2 (1974), 203-215. This value is also consistent with our own measurements.

[0008] The previously unpublished patent application DE 10 2022 108 490.6 describes a process for extracting Mg(0H)2 particles from seawater. This process generates large quantities of alkaline wastewater, which can be discharged into the sea.

[0009] US Patent US 2167120 "Process of calcining finely divided calcium carbonate" describes the calcination of calcium carbonate to calcium oxide by way of example. Calcium carbonate pressed into pellets was heated for 20 minutes with an air stream at 1,100°C to 1,200°C and then separated from any impurities by sieving.

[0010] Patent EP 0 196 176 A1 "Energy conserving limestone calcining system" describes calcination starting at 900°C when working under reduced pressure and heat losses are minimized accordingly.

[0011] According to the literature, according to "Physicochemical characteristics of fired clay-limestone mixes"; Cement and Concrete Research 30, 7-11 (2000), it is also known that complete calcination is already possible at 800°C.

[0012] DE 2 637 504 "Process for slaking lime" describes the subsequent production of calcium hydroxide. Calcium oxide is added to four times the amount of water with vigorous stirring for about four minutes.

[0013] In the application DE 2 232 355 A1 "Process for the production of active magnesia" the calcination of magnesium hydroxide precipitated from seawater is described. To produce such active magnesium oxide, magnesium hydroxide precipitated from seawater or brines with lime milk is washed out, the chloride content of which is less than 0.7 wt.% NaCl, and spray dried. The dry and fine-grained magnesium hydroxide powder is fed into an indirectly heated rotary kiln at a temperature of 440-450°C exhaust gas temperature and passing through 40 m 3 / h*kg of non-preheated and undried air in cocurrent with the Mg(OH)2, calcined to active MgO. The patent US 2437815 "Process of Magnesium Production" describes the carbothermal production of metallic magnesium. To achieve the reduction of magnesium oxide with carbon, a temperature above the boiling point of magnesium is necessary in order to remove it from the reaction equilibrium in gaseous form. In a melting furnace under exclusion of air, a mixture of magnesium oxide and coal is heated to approximately 2,000°C. Since the reverse reaction of carbon monoxide with magnesium to form magnesium oxide predominates below 1,850°C, the vapor phase is quickly cooled with natural gas to approximately 200°C, thus condensing the magnesium vapor.

[0014] In patent US 0400309B2 "Carbothermal reduction reactor system, components thereof, and methods of using same" it is shown by using a vacuum system that magnesium can be produced by this method as early as 1500°C.

[0015] Metallic magnesium is processed in large quantities in metallurgy. However, under certain conditions, magnesium reacts with steam to form Mg(OH)2, releasing hydrogen. The conversion of elemental magnesium is described in the publication "Comparative reactivity of industrial metal powders with water for hydrogen production"; International Journal of Hydrogen Energy 40, 1026-1036 (2015). Hydrogen is produced from magnesium powder and water in a reactor system at 200°C and 2 to 20 bar overpressure. Up to 0.92 liters (0.041 mol; 0.082 g) of hydrogen per gram of magnesium have been obtained.

[0016] A long-established technical process for producing synthetic fuels is the so-called Fischer-Tropsch synthesis. It starts with CO or CO2 and hydrogen and produces a mixture of long- and short-chain hydrocarbons, which can be used, among other things, as fuel for conventional combustion engines.

[0017] As described in patent DE 293 787 C "Process for the preparation of hydrocarbons and their derivatives", hydrocarbons can be prepared according to the Fischer-Tropsch synthesis from a gas mixture of hydrogen and carbon monoxide at 120 bar pressure and 360 to 420°C reaction temperature using a suitable catalyst, e.g. cobalt oxide. Building on the above-mentioned prior art, the object of the invention is to provide a process by means of which metallic magnesium can be obtained in a CO2-neutral manner. This can then be used, among other things, to produce green hydrogen, from which synthetic fuels can also be produced in a CCh-neutral manner.

[0018] In the process according to patent application DE 10 2022 108 490.6, the seawater is first treated with Ca(OH)2 up to the saturation point. This creates a very high pH value, at which all the CO2 dissolved in the seawater precipitates as a solid and is filtered off as CaCCh. CaCCh is a commercial product sold in large quantities worldwide.

[0019] In a subsequent step, the Mg is extracted from the remaining seawater using special nozzles. ++ MgCl2 dissolved in seawater precipitates as Mg(OH)2. For each gram of Mg(OH)2, approximately 1 liter of wastewater is produced, which has a pH of approximately 10 and is largely free of CO2.

[0020] If this alkaline wastewater is discharged directly back into the sea, the much larger amount of water in the sea will balance its pH value again.

[0021] However, measurements have shown that this alkaline wastewater can absorb relatively large amounts of CO2 (approximately 2-2.5 grams of CO2 per liter) even at room temperature and atmospheric pressure. This is achieved by introducing CO2 into the water, which dissolves in it. This simultaneously neutralizes the alkaline wastewater.

[0022] If this process occurs at atmospheric pressure, equilibrium is immediately established, and the CO2 dissolved in the water will not escape. This assumption assumes that the partial pressure of CO2 in the atmosphere does not decrease and that the sea temperature remains below the discharge temperature. Since seawater is naturally slightly alkaline, the CO2 dissolved in the neutralized wastewater is further ensured. If the neutralized water is discharged into the sea at a certain depth, the lower temperature and higher pressure prevailing there will further ensure the retention of the dissolved CO2. If one compares the amount of CO2 released to extract the Ca(OH)2 from CaCO3, which is needed to produce 1 gram of Mg(OH)2, with the CO2 absorption capacity of the 1 liter of wastewater released per gram of Mg(OH)2, one will find that this capacity is far from exhausted.Calculations consistent with data from the literature have shown that, until this wastewater is neutralized, it is possible to introduce into it all the CO2 that is released in the subsequent processes through which the metallic Mg is ultimately extracted.

[0023] The process steps in the method according to the invention can be simplified by the following reaction equations:

[0024] According to the inventive method, all process steps are to be carried out at a single location to achieve CO2 neutrality of the entire process. The reactors, particularly in process steps 1 and 5, are constructed in such a way that, for example, all the CO2 released is captured in a surrounding jacket. The same applies to the exhaust gases released during the combustion of carbon-containing primary energy sources, which are required to reach the corresponding temperatures. The entire amount of CO2 released in this way is collected centrally and from there, in a specially constructed mixer, added to the wastewater generated during the extraction of Mg(OH)2 in step 3 (Fig. 4).

[0025] The Mg obtained in step 5 can be used to produce green hydrogen according to the following equation. The hydrogen production described above can also be carried out decentrally at a different location than the Mg production. Based on the above equation, mobile, decentralized hydrogen generators are conceivable, which can be operated at other industrial sites, homes, or even vehicles. Since the resulting Mg(OH)2 is a commercial product, it can also be sold as such.

[0026] The process according to the invention also offers the possibility of producing CO2-neutral synthetic fuels. The starting materials would be CaCCh, seawater, and, for example, coal. The process, known as Fischer-Tropsch synthesis, follows the following equation:

[0027] In step 5, steam would be introduced into the resulting reaction gases without first separating the CO2. The CO2 neutrality of this step results from the fact that the CO2 released in step 5 is not discharged into the wastewater, but is bound in the resulting fuel. When the fuel is then burned, it releases exactly the amount of CO2 that results from step 5 but was not discharged into the wastewater. The wastewater will therefore absorb exactly the amount of CO2 from the air that the fuel releases into the air elsewhere.

[0028] Alternatively, the hydrogen obtained in step 6 could be reacted with CO2 from the air at another location and the CO2 from step 5 could be discharged into the wastewater. Since the hydrogen produced in step 6 is CO2-neutral and the amount of CO2 released by the fuel during combustion is removed from the air, the fuel would be CO2-neutral in this case too. The process according to the invention is therefore made up of known process steps, each of which is not CO2-neutral in itself and only becomes CO2-neutral when the CO2 released in the wastewater generated during the process is neutralized. For this to happen, the steps must occur in a certain order. The following reaction equations are shown in a simplified manner and describe this sequence. It can be seen from this that CO2 is released in step 1 (equation 1) and in step 5 (equation 5).This is then neutralized in the wastewater produced in process step 3 (equation 3).

[0029] Below are the equations again, with brief explanations:

[0030] Burning limestone to produce burnt lime at high temperatures. This process converts the limestone (CaCO3) into quicklime (CaO) and CO2. The resulting exhaust gases, which contain the CO2, are not discharged over the roof but are collected and neutralized in the alkaline wastewater from step 3.

[0031] Slaking of quicklime to slaked lime (Ca(OH)2). When quicklime comes into contact with water, slaked lime is formed, releasing heat. In the process according to the invention, the quicklime can also be added directly to seawater.

[0032] Precipitation of magnesium-containing particles from the seawater. Ca(OH)2 (slaked lime) is added to the seawater in quantities below its solubility limit, raising its pH to 12. This alkaline seawater reacts at the outlet of a nozzle with the magnesium chloride dissolved in a second seawater stream. The magnesium hydroxide precipitates, producing alkaline seawater with a pH of 10, in which the CO2 generated during the process is neutralized.

[0033] Burning magnesium hydroxide to magnesium oxide. Heating the Mg(OH)2 from step 3 converts it to magnesium oxide (MgO), releasing water vapor, which can be condensed and used in subsequent steps.

[0034] Thermal reduction of magnesium oxide to magnesium (Carbotherm process). Here, the magnesium oxide is heated to very high temperatures in the presence of carbon. This produces magnesium vapor and carbon monoxide. The magnesium vapor is condensed to metallic magnesium, and the carbon monoxide is eventually converted to CO2. The CO2 is collected and neutralized in the alkaline wastewater from step 3.

[0035] Oxidation of magnesium with steam (water split process). When magnesium powder is heated in the presence of steam, it reacts to form magnesium hydroxide and hydrogen.

[0036] Synthesis of hydrocarbons from hydrogen and carbon monoxide (Fischer-Tropsch process). Under certain conditions, the hydrogen from step 6 and the carbon monoxide from step 5 react to form hydrocarbons, which can be used, for example, as synthetic fuels.

[0037] The recovered Mg(OH)2 can be recycled into the process and used to produce magnesium again, which can then be sold alongside hydrogen. This is based on the following considerations:

[0038] In step 6, 1 mol of Mg, or 24 g of magnesium (molar mass of Mg: 24 g / mol), produces 1 mol of H2, or 2 g of hydrogen (molar mass of H2: 2 g / mol). The resulting hydrogen mass is therefore only about 1 / 12 of the mass of the magnesium used. Assuming current world market prices for Mg of approximately €3,000 / t and for hydrogen of approximately €6,000 / t, one would obtain €6,000 of hydrogen from magnesium worth €36,000. If the Mg(OH)2 is returned to the process and cycled through once, the process becomes very lucrative, as in addition to the magnesium, the hydrogen is obtained virtually free of charge.

[0039]

[0040] In the first pass, as shown in the graphic (number 1), hydrogen is obtained; the saleable magnesium is only produced in the second pass (number 2). After that, the process begins all over again with seawater and lime. The invention will be explained in more detail using figures and examples.

[0041] Example 1:

[0042] Fig. 1 shows a schematic representation of the process according to the invention up to the extraction of metallic magnesium.

[0043] In a first step, a saturated Ca(OH)2 solution is prepared from seawater (MW) and Ca(OH)2 (Solution 1). The required 1.7 g / liter of Ca(OH)2 was previously obtained from CaCCh by calcining it to CaO, which was converted to Ca(OH)2 upon contact with the seawater (MW). This process releases a significant amount of energy in the form of heat.

[0044] A mixture of salts, which also contains large amounts of Ca(CO)3, precipitates. This can be purified and sold as a commercial product.

[0045] According to the "Information Sheet on CO2 Factors" published by the Federal Office for Economic Affairs and Export Control, 0.89 tons of CO2 are released per tonne of Ca(OH)2 during the extraction of Ca(OH)2. Accordingly, 1.7*0.89 = 1.513 g of CO2 are released during the production of the above-mentioned 1.7 g / liter of Ca(OH)2. Solution 1 is then reacted with fresh seawater (MW) in a ratio of 2:1, precipitating approximately 1 gram / liter of Mg(OH)2. The resulting Mg(OH)2 particles are separated by filtration. This yields 3 liters of suspension, the preparation of which required the release of 2*1.513 g of CO2, or 3.026 g of CO2. Consequently, 3.026 grams of CO2 were released for the 3 grams of Mg(OH)2 contained in the suspension. The aqueous phase resulting after filtering off the Mg(OH)2 originally has a pH value of approximately 10. If its pH value is then lowered to a neutral pH of 7 by dissolving CO2, our own measurements have shown that it contains approximately 2 - 2.5 grams of CO2 are dissolved at room temperature and atmospheric pressure.

[0046] In the next step, the resulting Mg(OH)2 particles are fired into MgO particles by heating them to approximately 600°C. The MgO is then mixed with coal dust and reacted at approximately 1800°C. This creates a gas mixture containing Mg in the vapor phase and CO. The magnesium is separated from the CO gas by condensation. Instead of the aforementioned coal dust, carbon from any other source can be used. For example, biomass or products from combustion plants can be used.

[0047] According to the list of emission factors published by the German Federal Environment Agency, natural gas releases 0.055 tCO2 / GJ and coal 0.095 tCO2 / GJ. Based on these data, 0.14 g of CO2 from natural gas or 0.24 g of CO2 from coal are released per gram of Mg(OH)2 used to generate the heat required for the aforementioned processes. In addition, 0.754 g of CO2 per gram of Mg(OH)2 are produced when the MgO obtained from the Mg(OH)2 reacts with coal.

[0048] All CO2 produced during the process shown in Fig. 1 is collected and added to the alkaline wastewater produced during the precipitation of Mg(OH)2.

[0049] If we now consider all these CO2 emissions and calculate the amount of CO2 that the alkaline wastewater generated to extract 1 gram of Mg can absorb (dissolve), the following picture emerges: If the required amount of heat is generated from natural gas, according to the list of emission factors published by the German Federal Environment Agency, -0.04 g of CO2 are released per gram of Mg, whereas if the same amount of heat is generated from coal, the amount is + / - 0.00 g of CO2 per gram of Mg. The process is therefore CCh-neutral in every case.

[0050] The conditions underlying the above calculation describe a worst-case scenario. The CO2 balance of the overall process can be significantly improved by optimizing the following aspects:

[0051] 1. Fuels: If, for example, natural gas is used instead of coal to provide the heat required in the process, according to the list of emission factors published by the German Federal Environment Agency, only 0.055 t of CO2 are released per GJ of heat generated, instead of 0.095 t of CO2—just under half. It is also possible to generate the required heat using solar thermal energy in so-called solar furnaces. This would reduce the emissions for heat generation to almost zero. In this case, the CO2 balance would improve to -0.120 g of CO2 released per gram of Mg.

[0052] 2. Heat recovery (Fig. 4): Most of the thermal energy is required to extract metallic Mg from MgO. The waste heat resulting from this step could, for example, be used to operate the kilns used to produce CaO from CaCO3. The waste heat from this process step could, in turn, also be used to convert Mg(OH)2 into MgO. The waste heat generated in this process step would presumably be sufficient to dry the Mg(OH)2. The waste heat could also be used to generate steam, which could provide the electrical energy required for the process. If the waste heat is recovered from the individual process steps, approximately -0.08 grams of CO2 would be released per gram of Mg when natural gas is used as fuel, and approximately -0.06 grams when coal is used as fuel. In either case, the CO2 balance becomes negative, meaning more CO2 is bound in the process than is produced during its operation. 3.Carbon source: If carbon that does not come from fossil sources but is obtained renewably, e.g., biomass, is used in the described process, the carbon dioxide balance can be significantly improved.

[0053] Example 2:

[0054] Fig. 2 shows the process from Fig. 1, in which the obtained Mg is brought into contact with steam in a further step. This produces hydrogen and Mg(OH)2. The process for obtaining the required Mg from Fig. 1 is CO2-neutral, as the above explanations show. Since no additional CO2 is released during hydrogen production, the process from Fig. 2 is also CO2-neutral.

[0055] Example 3:

[0056] Fig. 3 shows a process schematically in which the hydrogen obtained according to the process of Example 2 is converted into fuel by a Fischer-Tropsch synthesis.

[0057] In contrast to the process in Example 2, the CO produced during the reaction of Mg with carbon is not removed from the mixture but introduced directly into steam. This produces Mg(OH)2, which is separated as a solid, and a gas mixture containing CO and H2. This gas mixture is converted into fuel using Fischer-Tropsch synthesis. The CO2 released during the process is initially bound in the fuel. It is released during its combustion. Since the CO2 released is not added to the wastewater as in Examples 1 and 2, the wastewater can absorb an equivalent amount of CO2 from the atmosphere. The fuel produced in this way is therefore also CO2-neutral.

[0058] If CO2 from other sources, such as the atmosphere, is used for the synthesis described, the immediate CO2 balance of the process can be further improved. In this case, the CO2 from the previous process steps is added to the wastewater.

Claims

Claims 1. Process for the CCh-neutral extraction of metallic magnesium from Seawater with the following steps: - Adding Ca(OH)2 to a quantity of seawater up to the saturation limit; - Precipitation of the CO2 dissolved in seawater as a solid and filtration as CaCCh; - Precipitation of Mg ++ from the MgCb dissolved in seawater as Mg(OH)2; - Obtaining an alkaline wastewater with a pH of essentially 10; - Capturing or generating the CO2 released in previous or subsequent steps of the magnesium production process; - Introducing the CO2 into the alkaline wastewater, whereby the CO2 dissolves in the wastewater and neutralises the wastewater, whereby the introduced CO2 remains bound in the wastewater; - Transfer of the now neutralised wastewater to deeper marine zones in particular to secure the dissolved CO2.

2. Process according to claim 1, characterized in that the magnesium obtained is mixed with water to obtain Mg(OH)2 and hydrogen as an energy carrier.

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