Method for recovery of silicon parts

Abstract

The present invention relates to a method for the recovery of raw materials of silicon parts occurring during the recycling of electronic waste. In order to present an effective recovery process in terms of time, plant technology and energy, it is proposed to dissolve the silicon parts in an alkaline solution at a process pressure p with a first suspension temperature in the range from 50° C. to the boiling point of the alkaline solution at the process pressure p. After the silicon has dissolved, the resulting suspension is filtered to separate meta-, di- and oligosilicates, the temperature of the second suspension during the filtration being maintained in a predefined temperature range from 50° C. to below the boiling point of the alkaline solution.

Description

[Technical field]

[0001] The present invention relates generally to a method for the recovery of raw materials of silicon components generated during the recycling of electronic waste to obtain secondary raw materials, and also to the energy recovery of silicon components, in particular solar wafers generated during the recycling of photovoltaic modules. [Background technology]

[0002] Material recovery of silicon components is the recovery of materials contained in the components for use in products. Depending on the material and the effort required to separate it to the desired purity, the recovered material can be obtained in a similar form to the original raw material (raw materials recovery) or it can be recovered as improved or new material for another use (materials recovery).

[0003] By recovered silicon parts is meant components of electronic components that have been released from the housing or equivalent cover or carrier substrate, including the cover foil, and essentially only contain a layer stack consisting of silicon and metal and / or dielectric layers / conductor tracks applied thereto. In addition to semiconducting silicon, the most commonly recovered materials are aluminum, silver, molybdenum and other metals as well as dielectric layers such as silicon nitride. Depending on the electronic component, more different materials may be present. The remaining layer stack of the silicon part, whose layers are extensively interconnected, is used for the actual function of the electronic component.

[0004] There have been many attempts to obtain high purity semiconducting silicon from electronic waste, particularly silicon components.

[0005] The layer stack of the silicon components to be recovered, for example obtained after removing the housing from the electronic components, usually consists of a polymer or its coating, for example glass and / or plastic. In a photovoltaic module, the silicon components are covered with a layer of glass or plastic. For example, about 85% of the solar cell modules produced essentially consist of a composite of successive layers of material: glass plate / top plastic film / several silicon wafers adjacent in a plane parallel to the glass plate (including a metallization layer for electrical contact of the wafer / film composite). The latter consists of several plastic films. To separate the wafer layers from the foil and glass, various suitable processes can be used. For example, in WO2018 / 137735A1, the front side of the photovoltaic module is irradiated with intense visible light from a flash lamp through a front glass panel for less than one second. The incident light is absorbed by the underlying material layer formed by the adjacent silicon wafers in one plane. Absorption of light heats the material layer, and the pressure of pyrolysis gases generated at the interface between the silicon and plastic layers causes the adjacent plastic films to peel apart.

[0006] To extract the pure semiconductor, the doping layer must be removed in addition to the metal, usually using a series of chemical processes, and determining the type and level of doping is so time-consuming that recycling semiconductors at the purity required to recover silicon parts seems environmentally and economically unfeasible.

[0007] To obtain the silver present in the silicon parts, in WO2020240126A1 the silver is separated as a solid by surface etching in a preceding process, the so-called "lift-off process". [Prior art documents] [Patent documents]

[0008] [Patent Document 1] WO2018 / 137735A1 [Patent Document 2] WO2020 / 240126A1 Summary of the Invention [Problem to be solved by the invention]

[0009] The current need to replace silicon wafers, especially those in solar modules, is driving demand for recycling, but known multi-step processes make recycling too complicated and costly.

[0010] There is therefore a need for recovery processes that are time-, plant technology- and energy-efficient, capable of recovering large quantities of silicon components and allowing the recovery of an increased range of materials contained within them.

[0011] This includes that the materials resulting from the method are as separated from each other as is possible and practicable with reasonable efforts. The reasonable efforts limitation also includes that the resulting material may not be the same material or material composition that was added during the manufacture of the silicon part, such as high purity crystalline silicon. In other words, the original material is still obtained even for material compositions that are made up of multiple components of the silicon part or material compositions that were added during the utilization process.

[0012] The need for energy-efficient recovery means that the process must be managed so that only little or no primary energy supply is required. [Means for solving the problem]

[0013] To solve the problem a method is given in claim 1. Advantageous embodiments of the method are given in the dependent claims.

[0014] In the method according to the invention, at least one silicon part, or on an industrial scale a plurality of them, is dissolved in an alkaline solution.

[0015] In principle, any alkaline solution capable of dissolving silicon components can be used, for example the alkaline solutions used for etching silicon in the manufacture of electronic silicon components, or other aqueous alkaline solutions, including organic aqueous alkaline solutions, can be used, provided that their behavior towards materials, in particular silicon, noble and base metals, corresponds to the behavior of the inorganic aqueous alkaline solutions described below.

[0016] According to one embodiment of the method, an alkali or a mixture of alkalis is used, which are hydroxides of alkali metals or alkaline earth metals. The latter behave similarly to silicon and metals as the former, but in a weakened form. Alternatively, an alkali solution of ammonium hydroxide can be used. The alkali solution can be selected, for example, depending on the parameters of the water glass and / or silicon parts to be produced (material composition, layer thickness, etc.). Criteria for the selection of the alkali solution include costs, the time and energy required for the process, health hazards, etc.

[0017] For example, aqueous solutions of potassium hydroxide, magnesium hydroxide, calcium hydroxide, strontium hydroxide, barium hydroxide, lithium hydroxide, rubidium hydroxide, and cesium hydroxide can be used, but are not limited to these. In particular, caustic soda (aqueous solution of sodium hydroxide) and caustic potash (aqueous solution of potassium hydroxide) can be used. Since there are many types of alkali that can be used, it is possible to avoid the use of caustic soda (aqueous solution of sodium hydroxide), which is inexpensive and can be supplied in large quantities but is harmful to health. Caustic soda in particular has the advantage of being inexpensive and can be supplied in large quantities.

[0018] To produce caustic potash or caustic soda solutions, for example, potassium hydroxide or sodium hydroxide is dissolved in water. The chemical reaction formula for recovering silicon parts using caustic potash solution is as follows: 2Si+2KOH+3H2O→K2SiO5+4H2

[0019] In addition to the alkaline solution used and the temperature of the alkaline solution and suspension, the process is also influenced by other parameters.

[0020] Since the type, shape and size of the particles of silicon parts added to the alkaline solution also influence the method, according to a further embodiment of the method, the silicon parts can be crushed before being added to the alkaline solution, which affects the viscosity of the suspension. A particularly significant acceleration of the dissolution process is achieved with particle sizes of less than 1 cm in the largest dimension. Preferably, crushing can be carried out to particles of less than 0.5 cm, more preferably less than 2 mm, even more preferably less than 1 mm or 0.5 mm. Various methods can therefore be used to produce fragments of silicon parts.

[0021] A more effective stirring of the suspension together with the particles themselves also has a significant accelerating effect. Here, the shape, position, number and speed of the stirrers can optimize the dissolution process. The same applies to the mixing of the suspension when adding the particles. Furthermore, different process sequences can be achieved when the alkaline solution is added to the silicon parts in a ground or total form, or vice versa.

[0022] For this purpose, they are ground in an alkaline solution or, alternatively, are placed in a bath of alkaline solution, the temperature of which is above room temperature and below the boiling temperature of the alkaline solution, optionally below the boiling temperature of water. In the technical environment, a room temperature of 20° C. is generally assumed. Such an initial temperature of the alkaline solution does not prevent the temperature of the resulting suspension from rising above the boiling temperature of the alkaline solution in the course of the process, as will be explained in more detail below.

[0023] To distinguish between temperature stages in the process, the initial temperature of the alkaline solution is referred to as the alkaline solution temperature. When the silicon part decomposition process (hereinafter also referred to as the solution or dissolution process (dissolution step)) begins, the temperature changes as chemical reactions begin and a suspension is formed that also contains the reaction products of the dissolution process. The temperature that changes as the dissolution process progresses is referred to herein as the suspension temperature, specifically the first suspension temperature. This is to be distinguished from the second suspension temperature that is set or adjusted later in the process, as explained below.

[0024] The temperature range of the alkaline solution from 50° C. to a temperature well below the boiling point of the alkaline solution so that it does not begin to boil during addition of the silicon parts has proven to be favorable for an effective dissolution process.

[0025] The choice of the temperature of the alkaline solution depends on various conditions, such as the alkaline solution used, the amount, size, and state of the silicon parts to be added (silicon pieces or whole elements, surface characteristics, etc.), the desired reaction rate, and the desired achievable pressure in the reactor. When setting the temperature of the alkaline solution, the first suspension temperature required for the dissolution process of the material to be added must also be taken into account. Both the initial alkaline solution temperature and the suspension temperature are suitable to affect the dissolution rate of the substrate, and higher temperatures are suitable to accelerate the process. The temperature of the alkaline solution must be adapted to the application of each process by optimizing through trials.

[0026] Another parameter that influences the process is the process pressure in the reactor during dissolution. Pressure can, for example, positively influence the boiling temperature of both the alkaline solution and the suspension and can be used to influence the reaction rate of the dissolution process. Whenever the boiling point of the alkaline solution is mentioned in the description of the process, this is always understood to mean the boiling point that corresponds to the value at the process pressure selected or set in the reactor.

[0027] The term reactor is used here to describe a vessel-bounded space specifically designed and constructed to allow a particular reaction to take place under defined conditions, particularly of alkaline solution, temperature and pressure, which can be influenced and controlled.

[0028] Dissolution in this context refers to the process in which the silicon parts in question disintegrate and decompose in the alkaline solution, and as a result react to form different chemical compounds. It is preferred that the component silicon decomposes completely. The mixture of the alkaline solution and the reaction product of the dissolution process is referred to herein as a suspension. The silicon parts remain in suspension until the silicon dissolves, and the resulting reaction solution is stirred during the process according to one embodiment of the method.

[0029] The first suspension temperature is maintained throughout the dissolution process of the silicon parts in a temperature range limited at the upper end by the boiling temperature, or optionally above (exceeding) the boiling point of the alkaline solution. It is also possible to limit the temperature range of the first suspension temperature by an order of magnitude below the boiling point. It is also clear that an increase in the first suspension temperature above the boiling point, preferably only by an order of magnitude, is desirable to keep the reaction under control. The temperature an order of magnitude below or above the boiling point is possible at any value below 10 Kelvin, whereby the most advantageous value results from an optimization between reaction speed and reaction controllability.

[0030] By means of the temperature of the first suspension it is possible to influence the method with regard to its effectiveness, in particular with regard to the reaction rate, and furthermore the temperature of the first suspension can be limited by the process itself in that the energy required for the phase transformations which occur in the alkaline solution at temperatures above the boiling point of the alkaline solution is covered by the process itself, so that the temperature of the first suspension does not increase any further and the reaction can be prevented from proceeding.

[0031] Among other things, the temperature affects the rate at which the silicon parts dissolve, whereby the highest possible temperature does not necessarily give the best results. Thus, the suspension temperature during the process operated under normal pressure can be preferably in the range of 60-90°C, more preferably in the range of 70-85°C, even more preferably in the range of 80-85°C, whereby deviations in the range of a few degrees, i.e. less than 5 degrees, are possible without significantly affecting the results. At higher boiling temperatures, the range of suspension temperatures can be higher, similar to the range designations above and the stated intervals up to the boiling temperature related to the boiling temperature of water.

[0032] After the silicon parts have been dissolved, the suspension is present as a viscous reaction solution containing silicon as meta-, di- and oligosilicates. Depending on the type of silicon parts dissolved, the reaction solution may contain other components. In the case of silicon parts in electrical contact, such as wafer layers of photovoltaic modules or electronic semiconductor parts, the reaction solution may also contain precious metals as deposits and base metals as impurities. When pure wafer scrap is dissolved, the suspension contains mainly or exclusively meta-, di- and oligosilicates with no or only minor impurities.

[0033] To separate the metasilicates, disilicates and oligosilicates from the suspension, they are removed at high temperature, for example by hot suction, through suitable filters. This mixture is removed "hot" if it has a second suspension temperature, which is maintained during filtration in the range of 50° C. to below the boiling point of the alkaline solution at the process pressure p. The second suspension temperature can be the same as the first separation temperature or it can be different from it. This means that the suspension temperature is actively influenced before or during filtration or remains in a temperature range that gives the desired filtration result based on the values ​​at the start of the filtration.

[0034] A filter is suitable for this process if it has a pore size that allows the suspension to pass through as filtrate while retaining other components of the suspension, especially solid components such as precious metals and other above-mentioned impurities of silicon parts, as a filter cake. A suitable filter can be easily determined by testing or analyzing the suspension.

[0035] The meta-, disilicates and oligosilicates can be obtained from the filtered suspension by washing and, optionally, dried and used as water glass for various applications.

[0036] According to various embodiments of the method, when an alkaline solution of hydroxides of alkali metals or alkaline earth metals, or an alkaline solution of ammonium hydroxide, or a mixture thereof, is used to dissolve silicon, metasilicates, disilicates and oligosilicates can be converted into silicates (silicates) of different compositions and hydrate forms. For example, (Na2SiO3) can be mixed with potassium silicate (K2SiO3) and / or calcium silicate (CaSiO3). Furthermore, when the pH value is lowered, for example by acidification with acids such as H2SO4 or HCl, or by exposure to air with CO2, various oligosilicates and polysilicates can be formed, since silicic acid tends to split water between molecules and form oxygen bridges between silicon atoms. This leads to chain-extending (branched and unbranched) and chain-closing (ring-forming) condensation reactions, and the complexity of the alkali-silica reaction must be taken into account.

[0037] Water glass is known as a glassy, ​​i.e. amorphous, non-crystalline, water-soluble silicate with the composition M2O·nSiO2, n = 1 to 4. For example, it is used for the following purposes: Potassium silicates or sodium silicates (the latter) are used under conditions where appropriate measures are taken to protect against harmful effects. Water-soluble silicates are used, for example, in the construction industry, as binders, fire-retardant materials, as buffers and stabilizers for chemicals (e.g. H2O2), in coal briquetting, paper recycling, paper coating, metal degreasing and in many other areas. They are also found in detergents, bleaches, soaps, cleaning agents, cement, mortar, ceramic products, paints and cosmetics. Potassium silicates are used as binders, adhesives (mineral paints and plasters, fire-resistant mortars, building materials) and as fire-retardant materials for wood, so-called plant strengtheners.

[0038] According to a further embodiment of the method, the addition of auxiliary substances, for example, which improve the wettability of the silicon parts or their particles by alkaline solutions, can also facilitate the dissolution process. For example, isopropanol is suitable for this purpose. It is also possible to use basic surfactants or phase transfer catalysts, such as quaternary ammonium salts. Alternatively, other materials suitable for reducing the surface tension of alkaline solutions and improving wettability can be used.

[0039] In addition to effective process control with optimal reaction rates, the highest possible energy gain through exergonic reactions is also desired, which is also possible with higher suspension temperatures within the aforementioned temperature ranges.

[0040] The formation of meta-, disilicates and oligosilicates is highly exergonic, especially with the abovementioned alkalis, in particular with aqueous potassium hydroxide and aqueous sodium hydroxide. This means that the free enthalpy of the reaction products is lower than the sum of the free enthalpies of the starting materials, so that heat is released during the process. As a result, the energy for setting, i.e. generating and maintaining, the first and / or second suspension temperature, optionally the alkali solution temperature, above 50° C., as defined in the above process, can be partially, mainly or exclusively covered by energy gain. The energy usable for this purpose can be obtained from the method of the invention or, in particular in the case of the initial alkali solution temperature set, from a preceding process according to the invention.

[0041] Energy capture from the ongoing process can be realized, for example, in a continuous process sequence, as described below. Alternatively, according to an embodiment of the method, the ongoing process can be actively influenced by adjusting the suspension temperature TS1 or the suspension temperature TS2 or both by metered addition of alkali and / or metal hydroxides and / or by maintaining them during the course of the process. For example, potassium hydroxide solid has a high solution enthalpy of -57.1 kJ / mol. The reaction enthalpy is -424 KJ / mol and the reaction entropy is 159 J / K. At 80°C, the free reaction enthalpy of sodium metasilicate (Na2SiO3) is about -480 KJ / mol.

[0042] Based on this, according to one embodiment of the method, the initial temperature of the alkaline solution can be generated to a desired temperature within the above temperature range by dissolving hydroxide in water under stirring to generate the alkaline solution, optionally without an external energy supply, by simply calculating and / or monitoring the temperature during the addition of hydroxide to water.

[0043] It is likewise the case that the temperature of the first suspension is maintained during the process. For this purpose, hydroxide can also be metered into the suspension. The amount added can be determined by tests and / or temperature monitoring and / or calculation. Due to the above-mentioned process sequence and the associated influence of the second suspension temperature by the first suspension, the above also applies indirectly to the second suspension temperature. However, an external energy supply is not excluded. For example, precipitation of silicates in the filter can be avoided or at least significantly reduced by reheating during filtration.

[0044] In a further embodiment, ultrasound can be introduced into the alkaline solution or suspension by a suitable ultrasonic source to initiate the reaction in the first case and to assist or accelerate the reaction in the second case. A partial external energy supply can make large-scale processes more efficient.

[0045] If the silicon parts contain precious metals, they can be separated from the suspension in one embodiment of the method by collecting them on a filter cake when filtering the suspension. The precious metals of the silicon parts, such as silver and gold, do not react with the alkaline solution and accumulate as solids in the bed of the reactor due to their density. These precious metals are present in elemental form, i.e. in oxidation state 0, and can be separated as a "solid precipitate". According to one embodiment of the method, the precious metals are then purified and, depending on the metal, concentrated.

[0046] Purification can be carried out, for example, by electrochemical deposition on electrodes. However, solid precipitates can also be dissolved in water with nitric acid or methylsulfonic acid as salts and precipitated as halides, sulfides or sulfates. The precious metals can also be reacted with complexing agents such as cyanides, thiocyanates, thiosulfates, amines, etc., and precipitated purified with the above substances. Silver can be dissolved in a salt solution such as saline or calcium chloride solution with iron(III) chloride as a chloro complex, diluted with water and precipitated again. Other methods for purifying silver and gold and other precious metals present in silicon parts are known to those skilled in the art.

[0047] Base metals contained in silicon components, such as aluminum, tin, lead, etc., are also dissolved, preferably completely, in the alkaline solution, producing hydrogen. These materials are present in solar modules in small quantities and therefore remain as impurities in the meta-, disilicates and oligosilicates without causing any harmful effects. For example, lead and aluminum oxide from the back contact of solar wafers are harmless for the production of water glass for the building materials industry.

[0048] Another positive recovery effect of silicon components is the generation of hydrogen as an energy carrier as a result of the dissolution of silicon and base metals of silicon components in alkaline solutions.

[0049] The above equation for dissolving silicon in potassium hydroxide solution shows that 2 moles of silicon will produce 1 mole of potassium disilicate and 4 moles of hydrogen. The production of hydrogen applies to each of the alkaline solutions listed above. This reaction result has at least two advantages:

[0050] In one embodiment, dissolution of the silicon parts in the alkaline solution can be detected based on hydrogen production and the next process step can be initiated. Thus, in a further embodiment of the method, hydrogen production is monitored and its completion indicated by a sensor. Alternatively, hydrogen itself can be utilized as an energy source.

[0051] The hydrogen produced in this way is mainly produced from silicon, which takes up the largest volume: 1 kg of silicon produces about 1600 liters, or 144 g of hydrogen. Obviously, the greatest effect is achieved when using modern solar modules.

[0052] The hydrogen can be collected and extracted, for example, using a bell-shaped cover of a bath of alkaline solution. The compressed hydrogen is filled into a pressurized container and supplied to a new application. According to a further embodiment of the method, a first compression stage of hydrogen can already be realized at the outlet of the reactor for energetic utilization of the silicon parts, thus supporting the storage, transportation and liquefaction of the resulting hydrogen. For example, a compression device can be connected to the gas outlet of the reactor by a flange, and the hydrogen can be compressed to a second high-pressure level. The hydrogen can also be dried and cooled in the compression device.

[0053] In a further embodiment of the method, the starting materials and the suspension can be fed or discharged continuously without interrupting the dissolution process (continuous process). Alternatively, the process can be carried out in such a way that the starting materials and / or the final product are fed or discharged discontinuously (quasi-continuous process). Both variants are advantageous in large-scale industrial applications of the method, since, in contrast, in discontinuous processes, downtimes are inevitable, in which the reactor must be stopped after each batch, emptied and recharged with the starting materials, and the process parameters must be increased again.

[0054] The silicon pieces that have been added and are not yet dissolved float on the surface of the already formed suspension, while the silver and other precious metals settle as a sludge at the bottom, making it possible to separate the suspension to be treated in the next process step and its impurities from the silicon pieces that are still dissolved during the reaction. The rate-determining factors of strongly exergonic reactions are related to the continuous or quasi-continuous process control, in particular the rapid removal of heat, the capture of hydrogen and the associated supply of silicon pieces or other starting materials.

[0055] In summary, the material and energy advantages of the method according to the invention can be explained as follows: The method consists in dissolving, preferably completely, the semiconductor silicon from electronic waste using an alkaline solution. - Apart from the subsequent washing of the precious metals, only one chemical is required, namely an alkaline solution, e.g. potassium hydroxide or sodium hydroxide solution. Hydrofluoric acid or other chemicals for dissolving solids are not used in the method according to the invention. - In many process variants, the recovery of silicon components is linked to the extraction of various forms of water glass, a material that is widely used in industry and in many products. - Due to its high density, the precious metals, e.g. silver, settle as a solid at the bottom of the reactor as a dark precipitate, while the silicon floats to the surface of the already formed suspension, allowing the two materials to be separated by filtration. - In this process, due to the high enthalpy of solution (due to the formation of an alkaline solution) and the high reaction enthalpy of the reactants, the process can be carried out exergonic and heat can be recovered to carry out the process or for other uses. -In this process, hydrogen is generated, which can be collected and used as an energy source. Since there are a large number of solar modules produced for recycling, the amount of hydrogen that can be collected corresponds to a useful amount. -Base metals, such as aluminum on the surface of semiconductor silicon, dissolve in alkaline solutions and contribute to hydrogen production. Furthermore, since the silicon slurry suspended in the alkaline solution is separated from the suspension, the process is capable of continuous or at least quasi-continuous operation, making it suitable for large-scale industrial applications. [Brief description of the drawings]

[0056] The figure illustrates the present invention in more detail using a flow chart as an example of an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0057] The present invention will be described in more detail below using a flowchart (figure) as an example of an embodiment. The process sequence will be described only to the extent necessary for understanding the present invention. In addition, this description does not claim to be a complete description of the process sequence.

[0058] The process can start in a variety of ways. First, the hydroxide and water mentioned above are mixed, to which the silicon part fragments (hereafter also referred to as silicon pieces for short) are added in a controlled manner. Alternatively, the process can start with a suspension of silicon pieces and water, in which case the hydroxide is added in a controlled manner at a later stage in the process. In either case, water, an alkaline solution, and silicon parts are the necessary starting materials for the process, and in some cases, are the only starting materials for the process.

[0059] In the following exemplary embodiment example, the first of the two options mentioned above is explained in more detail. The second option differs from the first option in the further process sequence only in that the subsequently controlled, i.e. temperature-dependent, additive is a hydroxide instead of silicon pieces.

[0060] In an example embodiment, an alkaline solution 1 suitable for dissolving silicon, for example sodium hydroxide, is dissolved in water 2 under normal pressure by stirring 3, whereby the ratio of both starting materials is calculated so that the temperature of this solution rises by exergonic reaction to approximately T=80° C. Then, crushed fragments 4 of silicon parts, abbreviated as "BE", with a particle size <1 cm (less than 1 cm), are added in portions, monitored by temperature measurement 5 so that the suspension temperature T present now does not exceed 95° C. and so that the bubbles formed do not leave the reactor. If, during the addition of the silicon pieces 4, the suspension temperature T rises significantly above 85° C. 6, its supply is reduced 7.

[0061] After complete addition of the silicon pieces 4, the mixture is stirred for a further 4 hours at 80° C. until all solids except silver are dissolved. A suspension 8 is obtained containing sodium meta-, di-, and oligosilicates and precious metals, possibly other insignificant impurities.

[0062] The hydrogen 9 produced during the decomposition of silicon is removed for external storage. The production of hydrogen 9 is monitored 10 so that upon completion of hydrogen production, dissolution of the added silicon pieces can be detected. In the example embodiment, approximately 380 g of silicon was dissolved, resulting in the production of over 608 liters of hydrogen, as measured using a gas meter.

[0063] The silver in the silicon pieces 4 is then separated from the suspension 8 by hot filtration (high temperature filtration) 11. For example, but not by way of limitation, the silver is extracted hot through a suitable filter with a pore size compatible with the precious metal particles, washed again with hot water, i.e., water temperatures in the above-mentioned range, and dried if necessary. The filtrate free of washing water can be used as sodium meta-, di-, or oligosilicate water glass 12 in the example embodiment.

[0064] For example, a G3 type frit can be used as a filter. A frit is a filter made of porous glass or porous ceramic, in which the silver to be filtered is retained in the fine pores. The G3 type refers to the pore size, which is in the range of 16 to 40 μm. As a result, a filtrate containing dissolved silicon that passes through the filter and a residue remaining on the filter, the so-called filter cake, which in this case contains the precious metal silver-13, are obtained.

[0065] The silver-containing filter cake is dissolved in 20-30% semi-concentrated nitric acid at 60 ° C under stirring. A G4 frit (pore size 10-16 μm) is then used for suction filtration and the filtrate is mixed with physiological saline. The formed silver chloride precipitate is suction filtered through a G4 frit and thoroughly washed with water 14. The precipitate is dissolved in dilute aqueous sodium hydroxide and reduced to metallic silver with sucrose. The silver precipitate is removed and dried. The hydrogen formed in this process can also be recovered by air pressure (not shown), which can further improve the energy balance of the process. Other methods of silver extraction are possible, such as squeezing the silver-containing filter cake through an anode plate and electrolytically refining the silver. In this variant, precious metal components such as gold, platinum and palladium are concentrated in the anode sludge and are recovered therefrom as described above.

Claims

1. 1. A method for recovering materials contained in silicon parts, comprising the following successive process steps: - providing an alkaline solution in which silicon dissolves to the reactor; - adjusting the process pressure p in the reactor and the alkaline solution temperature TL, which at the process pressure p ranges from 50° C. to below the boiling temperature of said alkaline solution, - providing silicon parts to said alkaline solution and dissolving silicon of said silicon parts in said alkaline solution, generating and maintaining a first suspension temperature TS1 and forming a suspension comprising meta-, di- and oligosilicates; wherein the first suspension temperature TS1 lies in the range from 50° C. to above the boiling point of said alkaline solution at the process pressure p, - filtering said suspension and separating the meta-, di- and oligosilicates from the filtrate at a second suspension temperature TS2, wherein the second suspension temperature TS2 of said suspension is maintained during its filtration in the range from 50° C. to below the boiling point of said alkaline solution at process pressure p.

2. 2. The method according to claim 1, wherein at least one alkaline solution from the following list is used to dissolve the silicon: an alkaline solution of an alkali metal or alkaline earth metal hydroxide or an alkaline solution of ammonium hydroxide.

3. 3. The method according to claim 1 or 2, wherein the regulation of at least the first suspension temperature TS1 and / or the second suspension temperature TS2 is performed using thermal energy from the exergonic reaction of the dissolution of the silicon.

4. 3. The method according to claim 1 or 2, wherein the adjustment of the alkaline solution temperature TL is obtained using heat energy from the exergonic reaction of dissolution of the silicon of a preceding or ongoing process.

5. 3. The method according to claim 1, wherein the first suspension temperature TS1 and / or the second suspension temperature TS2 are adjusted and / or maintained by metered addition of alkali metal hydroxide and / or metal hydroxide.

6. 3. The method according to claim 1 or 2, wherein ultrasound is introduced into the alkaline solution or suspension by an ultrasound source.

7. 3. The method of claim 1 or 2, wherein the suspension is stirred during dissolution of the silicon.

8. 3. The process of claim 1 or 2, wherein the filtrate is washed to obtain meta-, di- and oligosilicates.

9. The method of claim 1 or 2, wherein the silicon parts are ground before being added to the alkaline solution.

10. 3. The method of claim 1 or 2, wherein the silicon parts contain precious metals, and the precious metals are captured in the filter cake.

11. 3. The method according to claim 1 or 2, wherein the alkaline solution is added with an auxiliary substance suitable for improving the wetting of the silicon part by the alkaline solution.

12. 3. The method according to claim 1 or 2, further comprising monitoring the production of hydrogen as a result of the dissolution of the silicon part in the alkaline solution and determining the completion of the silicon dissolution process based on the amount of hydrogen produced.

13. 3. The method according to claim 1 or 2, wherein hydrogen produced as a result of dissolution of the silicon parts in the alkaline solution is captured and supplied for energy utilization.

14. 13. The process of claim 12, carried out with two pressure stages, the second pressure stage having a higher pressure than the first pressure stage and present on the outlet side of the reactor.

15. 3. The process according to claim 2, which is carried out continuously with a continuous supply of starting material and a continuous removal of the suspension, or semi-continuously with the supply of starting material and / or the removal of the suspension being carried out discontinuously.

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