Recovery of silicon in the form of silicon tetrachloride from biochar

Abstract

The invention relates to a process for the recovery of silicon in the form of silicon tetrachloride, in which biochar having a silicon content, expressed in % by weight of elemental silicon, of at least 1% by weight is reacted with elemental chlorine at a temperature of 350 to 900°C and the silicon tetrachloride formed is separated from the exhaust gas stream, preferably by condensation.

Description

[0001] Recovery of silicon in the form of tetrachloride from biochar

[0002] The present invention relates to the recovery of silicon in the form of tetrachloride from biochar.

[0003] The desire to recover raw materials from biomass, such as sewage sludge from wastewater treatment plants, but also animal manure and plant residues, has grown in recent years, primarily for ecological and political reasons. In addition, the application of fertilizer to fields has often been increasingly restricted due to high heavy metal levels and the potential presence of pathogens and germs. While agricultural biomass, such as animal manure, especially cow or chicken manure, and liquid manure, may still be applied to fields, their application increases nitrate levels in groundwater, often exceeding locally permissible limits. The effective and cost-effective removal of nitrate from groundwater is still technically unsolved.

[0004] An alternative use of biomass, using sewage sludge from wastewater treatment plants as an example, is that it is usually partially mechanically dewatered and / or thermally pre-dried and then incinerated in incinerators, possibly together with domestic or industrial waste. The resulting ash is a primarily inorganic residue containing various compounds of elements such as phosphorus (P), iron (Fe), aluminum (Al), calcium (Ca), and silicon (Si). Disposal of this ash by spreading it into the environment without further treatment can lead to serious environmental pollution, primarily due to the presence of heavy metals and other toxic substances.

[0005] The incineration of sewage sludge in preparation for the recovery of essential recyclables represents a process step that, however, does not add any value. It merely serves to reduce the amount of waste. The heat generated by the incineration is generally used entirely to pre-dry new sewage sludge.

[0006] In view of these problems, it has become necessary to develop processes for the treatment of biomasses, in particular sewage sludge, with the aim of recovering the usable elements contained therein in the form of their various compounds and reusing them for various applications.

[0007] In fact, there are various current attempts and methods to utilize biomass such as sewage sludge as a source of valuable raw materials. The recovery of valuable materials from such biomass, for example from sewage sludge, is becoming increasingly important, particularly with regard to the utilization of phosphorus and silicon, for which there is a high demand worldwide.

[0008] Depending on the operating mode of a wastewater treatment plant, the concentration in sewage sludge is typically around 30 wt.% solids, silicon typically around 1 to 12 wt.%, and phosphorus around 1.5 to 15 wt.%, each expressed in elemental form. To date, the recovery of the valuable materials contained in biomass has typically been described for phosphorus compounds.

[0009] In the case of sewage sludge, there are various wet-chemical approaches available for the recovery of phosphorus in the form of phosphate, but not for the recovery of silicon in the form of silicon tetrachloride.

[0010] Silicon tetrachloride can and is currently preferably produced from the elements, but also from a mixture of sand and coal with chlorine. In US4604272, diatomaceous earth with a high BET surface area is converted to silicon tetrachloride with chlorine, but only with the addition of special chlorides such as nickel chloride (see Example 1) and in the presence of special types of coal with a high BET surface area, at temperatures as low as 500°C. However, conventional beach sand with a smaller surface area does not react to silicon tetrachloride under these conditions below 1290°C (see Comparative Example 1).

[0011] In addition to SiO2, SiC also serves as a starting material for the production of silicon tetrachloride. This process is carried out at temperatures ranging from 1000°C to 1600°C, as described in US2843458.

[0012] In Example 10 of EP0034897, an alkali silicate produced by melting silicate rock and Na2CO3 is reacted at 700°C in the presence of activated carbon in a chlorine gas stream, yielding silicon tetrachloride in a yield of 28%, based on the silicon used. In contrast, the use of powdered silicate with activated carbon without pretreatment with a Na2CO3 melt at 700°C resulted in only traces of silicon tetrachloride in the exhaust gas stream (see Comparative Example 6).

[0013] Processes for recovering phosphorus from sewage sludge are also known.

[0014] EP3984966A1 describes a process for the extraction of phosphorus in the form of struvite from raw sludge produced in sewage treatment plants by means of a hydrolysis step.

[0015] EP2176177B1 also describes a precipitation process for recovering phosphorus from biomass. EP3061725B1 describes a process for recovering phosphorus from biomass through hydrothermal carbonization, which then releases the phosphorus as phosphate by lowering the pH. All of these processes have in common that they use acid, which inevitably leads to the formation of large quantities of neutral salts during the subsequent neutralization. The downstream processing is generally laborious and the costs correspondingly high. Because aqueous suspensions are processed, the energy costs for stirring or pumping are very high. Furthermore, all processes only yield phosphate, which can be used in agriculture but not to higher-value products such as phosphorus-chlorine compounds as raw materials for the chemical industry.

[0016] The utilization of biomass such as plant residues is usually achieved through drying and dry pyrolysis to produce so-called biochar. Such processes are described, for example, in EP3919586 and result in a product such as that described in WO2023 / 275271. In addition to phosphorus, plant residues always contain considerable amounts of silicon. In the ash, this amounts to approximately 16 to 26% calculated as SiO2, and 8 to 13 wt.% calculated as elemental Si (see, for example, "Austrian Federal Ministry of Agriculture, Forestry, Environment and Water Management, BIOMASS ASH FLOWS IN AUSTRIA, Birgit Walter, Peter Mostbauer, Brigitte Karigl, REPORT REP-0561 Vienna, 2016," Table 21).

[0017] Pyrolysis of plant residues is typically performed to capture the carbon contained therein and prevent the formation of carbon dioxide through decomposition. The products are typically spread on fields as so-called biochar.

[0018] The formation and separation of volatile silicon-chlorine compounds from biomasses such as sewage sludge (with varying water content) or plant residues or animal manure, or biochar derived therefrom, is not mentioned in the literature and cannot be scientifically expected under the wet chemical conditions mentioned in the literature.

[0019] While phosphorus-chlorine compounds can be used to produce, for example, flame retardants or high-purity phosphoric acid, high-purity SiCl can be used to produce very clean SiO2 or fumed silica for the production of fiber optic cables or polycrystalline silicon for the semiconductor industry.

[0020] What is being sought is a process that eliminates the need for the laborious and expensive combustion of biomass, such as sewage sludge, in preparation for further processing and instead allows the relevant valuable materials, such as silicon, phosphorus, iron and aluminum, to be extracted directly from it.

[0021] The object of the invention is therefore to produce silicon compounds and preferably phosphorus compounds in the form of their chlorine compounds starting from a biomass base.

[0022] The invention now relates to a process for the recovery of silicon in the form of the tetrachloride, in which biochar having a silicon content, expressed in wt.% of elemental silicon, of at least 1 wt.%, in particular at least 5 wt.%, preferably from 5 to 40 wt.%, particularly preferably from 7 to 30 wt.%, is reacted with elemental chlorine at a temperature of 350 to 900°C, preferably from 400 to 850°C, particularly preferably from 500°C to 800°C and the silicon tetrachloride formed is separated from the exhaust gas stream, preferably by condensation.

[0023] “Biochar”

[0024] Biochar, which in English is also referred to as “biochar”, for example from EP3285920A1, is understood to mean the pyrolysis product of a biomass, more precisely the preferably solid pyrolysis residue of a biomass, which in the case of the present invention is the pyrolysis residue of a silicon-containing biomass.

[0025] A preferred biochar has a Si content, determined as elemental silicon, of at least 5 wt.%, preferably from 5 to 40 wt.%, particularly preferably from 7 to 30 wt.%. Preferred further inorganic components, each described as elements, may include:

[0026] Phosphorus from 2 to 19 wt.%, and in particular

[0027] Iron from 3 to 14 wt.%,

[0028] Calcium from 2 to 35% by weight,

[0029] Aluminum from 0.4 to 13 wt.%,

[0030] Potassium from 0 to 7% by weight,

[0031] Magnesium from 0.1 to 3 wt.%,

[0032] Sodium from 0.1 to 3 wt.%

[0033] Sulphur from 0 to 7 wt.%,

[0034] Carbon 3 to 45 wt% and

[0035] Titanium from 0 to 2 wt%.

[0036] A preferably used biochar contains, in addition to the described proportion of Si compounds, calculated as Si element, phosphorus from 2 to 19 wt.%, in particular from 5 to 16 wt.%, and preferably also iron from 3 to 14 wt.% and aluminum from 0.4 to 13 wt.%, each described as elements. The biochar used in the process according to the invention preferably contains a carbon content of greater than 20 wt.%. If it has a lower carbon content of 20 wt.% or less, the content can be increased by adding another carbon source.

[0037] While SiO2 compounds in the form of diatomaceous earth show the strongest reflection in X-ray diffraction at a 20-value (A(K a ) =1 ,540 Å) at 21.9° (see Journal of Inorganic and Organometallic Polymers and Materials (2022) 32:2455-2472), biochar usually has the strongest reflection at a 20 value (A(Ka) =1 ,540 Å) at 26.3°.

[0038] The biochar can be used as it is from pyrolysis or can be shaped. This can be done, for example, by extrusion, tabletting, pelletizing, granulating, or briquetting. If necessary, a binder and, if necessary, water can be added to the biochar beforehand for shaping.

[0039] Particularly preferably, shaping can also take place at the level of the previous biomass used for pyrolysis, which remains largely intact after pyrolysis.

[0040] The shaping of the biochar is optional.

[0041] Pyrolysis

[0042] It is preferred to use a biochar in the process according to the invention which was obtained as a pyrolysis residue of a biomass by heating the biomass, in particular a sewage sludge, preferably with a water content of less than 15 wt.%, in particular less than 12 wt.%, preferably < 8 wt.%, particularly preferably from 0.5 to 7 wt.% in an oxygen-poor atmosphere at temperatures of 250 to 800°C. During this pyrolysis, organic substances decompose, volatile compounds are formed, and carbon, together with other solids, remains as biochar.

[0043] Pre-drying the biomass used in pyrolysis to a specific water content is optional. However, pre-dried material can be more easily shaped. This step is optional.

[0044] In an alternative embodiment, biomass and optionally one or more carbon sources are mixed, optionally formed into molded bodies with a binder and solvent such as water, and dried. If the carbon content in the carbon source is less than 70 wt.%, pyrolysis is preferably carried out before mixing with sewage sludge or sewage sludge coal. In a particular embodiment, drying and pyrolysis can also be combined, for example by using a biomass with a water content of preferably >15 wt.%, in particular >12 wt.%, under otherwise identical conditions.

[0045] A low-oxygen atmosphere is preferably understood to mean a reaction in which the oxygen content is preferably less than 0.5 percent by volume, in particular less than 0.3 percent by volume of the atmosphere.

[0046] Pyrolysis is particularly preferably carried out under an inert gas such as nitrogen. Pyrolysis preferably takes place at temperatures of 350 to 550°C. Pyrolysis is particularly carried out until the gas formation of volatile components is less than 11 gas per 1 kg of biomass used per hour.

[0047] The biochar thus obtained preferably has a carbon content of 3 to 45 wt.%.

[0048] If the carbon content after pyrolysis is greater than 20 wt.%, the resulting biochar can preferably be diluted by adding sewage sludge ash or other silicon-containing bioashes from biomasses with lower carbon content, thereby improving the efficiency of the process.

[0049] In the context of this invention, “bioash” means the combustion residue of a biomass or biochar, preferably with a carbon content of preferably less than 3 wt.%, in particular less than 1 wt.%, particularly preferably less than 0.1 wt.%.

[0050] Biomass

[0051] The biomass preferably used for pyrolysis has a water content of less than 15 wt.%, in particular less than 12 wt.%, particularly preferably less than 8 wt.%, and most preferably less than 4 wt.%, which is preferably obtained by thermally drying a biomass with a water content of 12 wt.% or more, in particular 15 wt.% or more. The biomass can be shaped by extrusion, tabletting, pelletizing, granulating, or briquetting. If desired, a binder and optionally also water can be added to the biomass beforehand.

[0052] The shaping of the biomass is optional.

[0053] The biochar used in the process according to the invention is preferably the pyrolysis residue of a biomass, preferably sewage sludge, animal manure, also in a mixture with bedding, microalgae, aquatic plants, wood chips, wood waste, agricultural waste, straw, plants, fruits, and the like. Biomasses typically have a Si content that results in a pyrolysis residue, the biochar, with a Si content of at least 1 wt.%. In a preferred embodiment, the liquid in which the sewage sludge particles are suspended is wastewater, which preferably includes all liquids of an aqueous and / or organic nature or mixtures thereof, which preferably do not meet drinking water quality standards.

[0054] The biomass preferably contains, based on its dry weight, a Si content of at least 0.1 wt.%, in particular from 0.2 to 20 wt.%, preferably from 0.2 to 10 wt.%. Sewage sludge, alone or with other biomasses, is the preferred biomass used for pyrolysis.

[0055] Preferably, the biomass from which the biochar used in the process according to the invention is obtained contains a carbon content, expressed in wt.% of elemental carbon, of at least 1 wt.%, preferably at least 15 wt.%, in particular 15-50 wt.% carbon, based on the dry weight.

[0056] In a particular embodiment, the biomass is used as so-called primary sludge, raw sludge, excess sludge, as treated and / or stabilized sewage sludge (aerobic / anaerobic) or dried sewage sludge, also formed into pellets, for example, as it preferably arises in municipal sewage treatment plants.

[0057] Preferably, the biomass used in the pyrolysis is a sewage sludge with a water content of <15 wt.%, in particular <12 wt.%, which contains the following inorganic constituents as elements, based on the dry matter:

[0058] Silicon from 1 to 25 wt.%,

[0059] Phosphorus from 1.5 to 15 wt.%,

[0060] Iron from 2 to 20 wt.%,

[0061] Calcium from 2 to 25% by weight,

[0062] Aluminum from 0.3 to 15 wt.%,

[0063] Potassium from 0 to 8 wt.%,

[0064] Magnesium from 0.05 to 4 wt.%,

[0065] Sodium from 0.05 to 2 wt.%

[0066] Sulphur from 0 to 5 wt.%, and

[0067] Titanium from 0 to 1 wt%.

[0068] In the context of this application, “dry substance” means that the material was dried in an oven at 120°C until constant mass was reached.

[0069] The biomass used for pyrolysis is preferably sewage sludge, preferably in the form of shaped bodies such as extrudates or pellets. If the biomass has a Si content of less than 1 wt.%, this can be increased, particularly by adding or using sewage sludge or sewage sludge ash with a Si content greater than 1 wt.%.

[0070] Proceedings

[0071] In the process according to the invention, the chlorine gas can be brought into contact in various ways with the biochar or its mixture with another carbon source, preferably with a carbon content of greater than 20 wt.%, such as another biochar or with a bioash, preferably below 1 wt.%, particularly preferably below 0.1 wt.%.

[0072] The reaction is preferably carried out with an oxygen content of preferably less than 0.5 volume percent, in particular less than 0.3 volume percent of the gas phase.

[0073] It is also preferred to carry out the reaction at a water content of preferably less than 0.5 volume percent, in particular less than 0.3 volume percent of the gas phase.

[0074] Preferably, chlorine is passed over or through the biochar, which may be in the form of the mixture described above, whereby this biochar is preferably agitated during the reaction for effective conversion. This can take place in a rotary kiln or in a paddle dryer in which the biochar, which may be in the form of the mixture described above, is agitated. However, the chlorine gas can also be passed through the biochar, which may be in the form of the mixture described above, which can be achieved, for example, in a fluidized bed or fixed bed. If the addition of a further carbon source is a gaseous compound, the chlorine gas used as described above is preferably carried out together with or separately from the gaseous carbon source for the biochar, which may be in the form of the mixture described above.

[0075] The reaction with chlorine preferably takes place at a temperature of 400 to 850°C. The reaction is preferably complete when exothermicity is no longer observed in the reaction mixture, i.e., when the internal and external reactor temperatures—after deducting the calculated amount of heat transported by the gas stream—no longer exhibit a temperature difference, or when the formation of CO2 has ceased.

[0076] The residence time in the reactor during the reaction generally depends on the temperature and the possibility of contact between the material and chlorine gas. The residence time in the reactor can range from one minute to five hours, for example. The process according to the invention can be operated as a batch or continuously.

[0077] reactor

[0078] The biochar to be used according to the process according to the invention, preferably after shaping, is preferably introduced into a reactor, which is preferably provided with a layer resistant to the reaction conditions to be established. Preferred reactor materials are reactors coated with nickel or nickel-based alloys or graphite, or reactors consisting of a ceramic such as silicon carbide or aluminum oxide or aluminum oxide-silicon dioxide mixed ceramic. Tubular reactors such as rotary tube reactors or other reactors such as shaft furnaces can be used as such. Particular preference is given to reactors that allow movement of the biochar during the reaction in order to enable the most effective contact between biochar and chlorine gas. Fluidized bed reactors, rotary tube reactors, or a reaction in an extruder device with screw propulsion are preferred.A fixed-bed reactor or a shaft furnace-type reactor can also be used.

[0079] Preferably, the reactor has an outlet for the exhaust gas stream. The exhaust gas stream contains the gaseous reaction products, volatile components of the biochar at the chlorination temperature, and any excess chlorine gas, which can be discharged together from the reaction chamber.

[0080] In the case of a tubular reactor, the reactor height is preferably 0.2 to 40 m.

[0081] In addition to silicon tetrachloride SiCk, the exhaust gas stream also contains phosphorus chlorides, in particular phosphorus trichloride and phosphorus oxychloride, as well as possibly gaseous iron(III) chloride, and possibly also AlCh, if iron and / or aluminum were contained in the biochar used.

[0082] The formation of silicon tetrachloride begins at about 350 °C.

[0083] To recover SiCk from the exhaust gas stream, a scrubber may also be used, for example with a higher boiler such as toluene, which is completely miscible with SiCk, whereby SiCk and the higher boiler can be separated by distillation in a subsequent step.

[0084] From the exhaust stream, the phosphorus trichloride, which is gaseous at reaction temperature, and the gaseous phosphorus oxychloride, which may also be formed, as well as the gaseous silicon tetrachloride, are preferably separated from each other using a condenser. Typically, a mixture of phosphorus trichloride, phosphorus oxychloride, and silicon tetrachloride is formed, which can be further separated by distillation. This allows the phosphorus and silicon components to be recovered in very pure form.

[0085] The process according to the invention is preferably characterized in that the exhaust gas stream containing phosphorus trichloride, or the phosphorus trichloride after separation therefrom, is reacted with chlorine gas at a temperature of 20 to 160°C to form PCl5. Phosphorus pentachloride is formed during this reaction. A molar chlorine / phosphorus trichloride ratio of 1:1 to 10:1 is preferred.

[0086] The process according to the invention is preferably used for the recovery of phosphorus and silicon in the form of their chlorine compounds.

[0087] If the process according to the invention is operated without a stoichiometric excess of chlorine, and thus there is little to no chlorine in the exhaust gas stream, the exhaust gas stream containing the chlorides of phosphorus, in particular phosphorus trichloride and optionally phosphorus oxychloride, can also be introduced into water or a lye, preferably after it has been freed from possible iron chloride, in order to obtain the corresponding acids of phosphorus such as phosphoric acid and phosphonic acids, from which further phosphorus derivatives, such as their esters, can then be produced.

[0088] The resulting SiCL can, after distillative purification, be reacted with water, for example, to produce high-purity SiC>2 and hydrochloric acid.

[0089] Ferrous chloride and also AlCl3, if the sewage sludge ash contains aluminum, can be separated from the exhaust stream and separated from each other, preferably by resublimation on surfaces of different temperatures. If ferrous chloride is present in the exhaust stream together with AlCl3, the respective chlorides can also be fractionally resublimated on different surfaces at different temperatures due to their sufficiently different boiling points, thus allowing them to be separated very cleanly.

[0090] The gaseous products PCI3, POCh and SiCL are condensed and collected in condensers.

[0091] Preferred deposition temperatures for FeCh are less than or equal to 307°C, in particular 150°C to 300°C, and for AICI3 less than or equal to 150°C, in particular 110°C to 149°C.

[0092] Bioash + Carbon + Chlorine

[0093] The invention further relates to the production of SiCl, characterized in that bioash is reacted with elemental chlorine in the presence of a carbon source at a temperature of 350 to 900°C, and the SiCl formed is separated from the exhaust stream. The terms "bioash" and "carbon source" each have the meanings given above, including the preferred ranges.

[0094] Bioash

[0095] As already described above, “bioash” in the context of this invention also means the combustion residue of a biomass or biochar, preferably with a carbon content of preferably less than 3 wt.%, in particular less than 1 wt.%, particularly preferably less than 0.1 wt.%.

[0096] The preferred bioash is sewage sludge ash, the ash resulting from the incineration of sewage sludge. This can be obtained from both sewage sludge incineration and sewage sludge gasification to produce fuel gas. In addition to phosphorus compounds, sewage sludge ash preferably contains compounds of iron (Fe), aluminum (Al), calcium (Ca), and silicon (Si).

[0097] Preferably, the sewage sludge ash used in the process according to the invention has a phosphorus content, expressed as percentage by weight of elemental phosphorus in the ash, of at least 3%, preferably from 3 to 29% by weight, in particular from 3 to 15% by weight.

[0098] Preferably, the bioash used in the process according to the invention, in particular sewage sludge ash, has a silicon content, expressed as percentage by weight of elemental silicon in the ash, of at least 1%, preferably from 3 to 30% by weight, in particular from 3 to 20% by weight.

[0099] The sewage sludge ash preferably contains an iron compound content of 5 to 21 wt.% (calculated as elemental iron), and preferably a calcium compound content of 4 to 38 wt.% (calculated as elemental calcium). The preferred composition of a sewage sludge ash, calculated for each element, preferably also contains:

[0100] Aluminum from 0.7 to 20 wt.%,

[0101] Potassium from 0 to 2 wt.%,

[0102] Magnesium from 0.1 to 4 wt.%,

[0103] Sodium from 0.1 to 4% by weight

[0104] Sulphur from 0 to 7 wt.%, and

[0105] Titanium from 0 to 2 wt%.

[0106] The elements together, calculated as oxides, preferably make up more than 70 wt.% of sewage sludge ash. The exact structural composition of sewage sludge ash cannot be determined using conventional analytical methods available to the expert, such as X-ray diffraction (XRD), since the diffractogram only shows reflections for the SiO2 present.

[0107] Preferably, the carbon content in the bioash, in particular in the sewage sludge ash, is less than 3 wt%, in particular less than 1 wt%, particularly preferably less than 0.1 wt%.

[0108] Carbon source

[0109] As a carbon source or additional carbon source, in principle, all modifications of carbon can be mentioned, such as graphite, soot, coal, coke, activated carbon, but also carbon-containing gases such as carbon monoxide, methane, or phosgene, as well as liquid materials such as polyethylene glycol or various oils, or solid materials such as biowaste or sewage sludge. Sewage sludge is particularly preferred. This preferably contains carbon in a proportion of at least 5 wt.%, preferably 20 to 50 wt.%, based on the dry mass.

[0110] Preferably, the sewage sludge contains a carbon content, expressed as elemental carbon, of at least 5 wt.% carbon. In a preferred embodiment, the liquid in which the sewage sludge particles are suspended is wastewater as defined herein. The term "wastewater" refers to all liquids of an aqueous and / or organic nature, or mixtures thereof, that do not meet drinking water quality standards.

[0111] In a particular embodiment, the sewage sludge is preferably present as primary sludge, raw sludge, excess sludge, treated and / or stabilized sewage sludge (aerobic / anaerobic).

[0112] The term "biowaste" refers to all organic waste of animal or plant origin that is generated in a household or factory and can be broken down by microorganisms, soil-dwelling organisms, or enzymes. Examples of such waste include straw, sawdust, waxes, fats, and bird droppings.

[0113] The carbon source can be solid, liquid, or gaseous. The use of a solid carbon source is preferred.

[0114] If the carbon content of the carbon source is less than 70 wt.%, pyrolysis is preferably carried out prior to the reaction with chlorine gas. This is preferably carried out under an inert gas such as nitrogen at temperatures of 250 to 800°C, preferably at 350 to 550°C, until the gas formation of volatile components is less than 1 / 1 kg of carbon source used per hour.

[0115] Sewage sludge is particularly preferred as a carbon source.

[0116] Preferably, the chloride content in the carbon source, in particular based on its dry weight, is less than 1 wt.%.

[0117] Mixing ratios

[0118] The ratio of the weight fractions of carbon from the carbon source to bioash, in particular to sewage sludge ash, is preferably greater than 0.01, preferably from 0.04 to 0.5.

[0119] Process / Reactor

[0120] The reaction of the bioash with the carbon source and chlorine takes place as described in the "Process" section above, with bioash and the carbon source being used instead of biochar. This also applies to the respective preferred embodiments. The same applies to the reactor preferably used for the process.

[0121] The invention further relates to the use of biochar and / or bioash, in particular sewage sludge ash, for the production of SiCl by chlorination in the presence of a carbon source. In the case of biochar, this may already contain the carbon source. Chlorination is preferably carried out with elemental chlorine at a temperature of 350 to 900°C, with the SiCl formed being separated from the exhaust stream, preferably as described above.

[0122] Analytics

[0123] The stated weight percentages were determined by an ICP-OES measurement. For this purpose, a weighed amount of the solid is first dissolved in a known amount of an acid, the concentration of the indicated elements is determined using an ICP analysis against a calibration measurement, and from this, the percentage of the solid is calculated.

[0124] Carbon analyses were carried out using a special carbon analyzer type G4 ICARUS HF.

[0125] The analysis of phosphorus and silicon compounds, particularly POCl3, PCh, and SiCl3, in the liquid phase is preferably performed using a GC method. An Agilent Technologies 7890A series instrument is used. A TCD and an FID, respectively, are used as detectors. A Gerstel MPS is used as the injection system. An HP5 column with the following dimensions is installed: length = 30 m, inner diameter = 320 μm, and a film thickness of 0.25 μm.

[0126] To determine the identities of the obtained liquids, a GC-MS is also used and the obtained mass spectra are compared with those in databases.

[0127] The water content is determined using an infrared drying scale.

[0128] Example 1 a) Drying

[0129] The biomass used is 8 kg of sewage sludge (obtained from the Wuppertal sewage treatment plant, in wt.% as respective elements):

[0130] Al: 0.86, Ca: 0.98, Fe: 0.8, K: 0.09, Cu: 0.01, Mg: 0.14, Mn 0.01, P: 0.91, Si: 2.33, Zn: 0.03, S: 0.16 wt% and other elements in trace amounts.

[0131] The sewage sludge, with a water content of 77%, is spread on stainless steel sheets and dried in a drying cabinet at up to 180°C for five days under vacuum at 20 mbar. This resulted in a friable sewage sludge with a water content of less than 1%. b) Pyrolysis

[0132] 1146 g of the dried sewage sludge according to Example 1a) was filled into a vertical quartz glass tube with an inner diameter of 80 mm, which was installed in a vertical furnace.

[0133] The furnace was heated to 500°C within two hours in a flow of 30 L / h of purified and dried nitrogen (N2 content > 99%, water content < 0.05%) and maintained at this temperature (pyrolysis step). Volatile products formed at this temperature were removed via a water scrubber. Pyrolysis was carried out for five hours under these conditions until no more volatile products were detectable (<1 L / kg of gaseous products per hour and per kilogram of dried sewage sludge used). The pyrolysis residue, the biochar, was then cooled to room temperature.

[0134] The obtained biochar showed the strongest reflection at a 20 value (A(Ka) =1,540 Ä) at 26.3° and contained (in wt.% as element):

[0135] Al: 5.6, Ca: 6.3, Fe: 5.2, K: 0.58, Cu: 0.079, Mg: 0.92, Mn: 0.061, P: 5.97, Si: 15.2, Zn: 0.21, C: 21.2 wt%, S: 1.07 wt%, and other elements in trace amounts. c) Carbochlorination

[0136] The biochar obtained according to Example 1 b) (761 g, black) was heated to 780°C as a bed in the cleaned, standing quartz tube from the pyrolysis step b) while flowing with nitrogen (30 L / h).

[0137] Once the temperature is reached, 12 L / h of chlorine are added to the 30 L / h of nitrogen, which also flowed through the sample. The nitrogen flow is not mandatory. The reaction was carried out for a total of eleven hours under these conditions.

[0138] A red-brown precipitate (33 g) formed on the glass surfaces at the reactor outlet, which were kept at room temperature. A condensate (34 g) also formed, containing the two main components, SiCl and PCL, in a weight ratio of 2 / 3 to 1 / 3. The condensate was separated into its two main components by distillation.

[0139] The chlorination residue is a gray solid (646 g), consisting primarily of soluble alkaline earth chlorides and soluble heavy metal chlorides. The solid is suspended in water and filtered. The residue can be used for further recovery of silicon and phosphorus chlorides by continuing the reaction with further treatment with chlorine.

[0140] Example 2: a) Drying and pyrolysis

[0141] 200g of sewage sludge ash from a sewage sludge incineration plant with the specified ingredients (containing, at a C content of < 1 wt.%: silicon 18.0 wt.%, calcium 9.1 wt.%, iron 6.5 wt.%, 7.4 wt.% phosphorus, 7.0 wt.% aluminum, 793 ppm copper, 0.36% potassium, 1.2% magnesium, 879 ppm manganese, each calculated as elements; color: red) are mixed with 133g of sewage sludge (45% solids content, 30% carbon content in the dry residue) and heated to 500°C in a quartz glass tube placed in an electric furnace with exclusion of air in a nitrogen gas stream of 1 l / min. The heat is maintained at this temperature for one hour until no more gaseous products are formed. The exhaust air is passed through a scrubber and disposed of. The resulting mixture (color: black) is cooled and ground and homogenized. b) Carbochlorination

[0142] 60g of the mixture produced above (measured total carbon content: 10.2 wt.%, calcium 7.5 wt.%, silicon 16.2 wt.%, iron 9.7 wt.%, phosphorus 6.6 wt.%, aluminum 6.1 wt.%, magnesium 1.1 wt.%, chlorine: <0.1 wt.%) are reinserted into the quartz glass tube, heated to 780°C, and maintained at this temperature. A stream of chlorine gas (99.8% purity) of 100ml / min is then passed over the mixture. The resulting FeCh-containing product is resublimated in a cooler at a temperature of 200°C. The product gas stream, which is gaseous at this temperature, is analyzed at irregular intervals in a gas-phase infrared spectrometer and then disposed of via a cold trap and scrubber.

[0143] After some time, an adsorption band at 501 cm appears in the IR spectrum of the gas phase, in addition to bands for CO2 and others. -1 that can be assigned to PCI3.

[0144] By far the largest band is at 617cm -1, which is a silicon tetrachloride band. The silicon tetrachloride was not isolated, and the gas stream was fed directly into a scrubber operated with sodium hydroxide solution, where a white precipitate of silicic acid was formed.

[0145] After about three hours of reaction time, the intensities of all bands observed in the IR decrease and the reaction is complete.

[0146] The chlorine gas flow is shut off and replaced with a nitrogen flow. The reactor is allowed to cool to room temperature under these conditions, and the reaction mixture (43.9 g, color: off-white, C content: 2 wt%) is removed. The residue remaining in the reactor tube is partially water-soluble. The resulting solution is an aqueous calcium and magnesium chloride solution containing heavy metals, which can be separated from any traces of heavy metals using methods known to those skilled in the art, such as ion exchange resins.

[0147] Example 3

[0148] 160g of sewage sludge ash from a sewage sludge incineration plant (containing 18.0 wt.% silicon, 9.1 wt.% calcium, 6.5 wt.% iron, 7.4 wt.% phosphorus, 7.0 wt.% aluminum, 793 ppm copper, 0.36% potassium, 1.2% magnesium, 879 ppm manganese, all calculated as elements, with a C content of <1 wt.%; color: red) are mixed with 40g of activated carbon and heated to 780°C in a quartz glass tube placed in an electric furnace with exclusion of air in a nitrogen gas flow of 1 l / min. The exhaust gas flow is monitored by an IR spectrometer.

[0149] Then the nitrogen stream is replaced by a stream of 200 ml / min chlorine gas (purity 99.8%) and passed over the mixture for 14 hours at 780°C.

[0150] In the IR spectrometer a strong band is observed at 617 cm -1visible, which can be reliably attributed to SiCl. The product gas stream, which is gaseous at this temperature, is not isolated but disposed of via a cold trap and scrubber. Silica precipitates in the scrubber as a hydrolysis product of the SiCl.

Claims

Patent claims 1. A process for the recovery of silicon in the form of tetrachloride, in which biochar having a silicon content, expressed as wt.% of elemental silicon, of at least 1 wt.% is reacted with elemental chlorine at a temperature of 350 to 900°C and the silicon tetrachloride formed is separated from the exhaust gas stream, preferably by condensation.

2. Process according to claim 1, characterized in that the biochar has a silicon content, expressed in wt.% of elemental silicon, of at least 5 wt.%, preferably from 5 to 40 wt.%, particularly preferably from 7 to 30 wt.%.

3. Process according to claim 1, characterized in that the reaction with elemental chlorine takes place at a temperature of 400 to 850°C, particularly preferably from 500°C to 800°C.

4. A process according to claim 1, characterized in that the biochar has a phosphorus content, expressed as % by weight of elemental phosphorus, of at least 2 to 19 % by weight, in particular of 5 to 16 % by weight.

5. Process according to at least one of claims 1 to 4, characterized in that the biochar has an iron content of 3 to 14 wt.% and an aluminum content of 0.4 to 13 wt.%, each expressed in wt.%, as elements.

6. Process according to claim 1, characterized in that the biochar is the pyrolysis residue of a biomass, in particular a sewage sludge, preferably with a water content of less than 15 wt.%, in particular less than 12 wt.%, preferably < 8 wt.%, particularly preferably from 0.5 to 7 wt.%, which was obtained by heating the biomass in an oxygen-poor atmosphere at temperatures of 250 to 800°C.

7. A process according to claim 4 for the recovery of phosphorus chlorides, in which the phosphorus and silicon chlorides formed are separated from the exhaust gas stream.

8. Process according to at least one of claims 1 to 7, characterized in that phosphorus chlorides formed from the exhaust gas stream, in particular phosphorus oxychloride and phosphorus trichloride and / or phosphorus pentachloride and / or silicon tetrachloride, are separated by condensation.

9. Process according to at least one of claims 1 to 8, characterized in that the residue of the chlorination is separated from insoluble components with water.

10. A process for the production of SiCU, characterized in that bioash is reacted with elemental chlorine in the presence of a carbon source at a temperature of 350 to 900°C and the SiCU formed is separated from the exhaust gas stream.

11. Use of biochar and / or bioash, in particular sewage sludge ash, for the production of SiCU by chlorination in the presence of a carbon source.