PROCESS FOR PRODUCING TECHNICAL-GRADE SILICON

Description

[0001] The invention relates to a process for producing technical silicon from a particulate raw material mixture of silicon metal-containing material and a particulate mediator by heating, whereby a liquid silicon metal phase is formed.

[0002] Silicon in technical quality (Si content <99.9 mass%, also known as technical silicon) is used today primarily in silicon thermal processes, in metal extraction, as a deoxidizer in steel production and serves as an alloying component of cast alloys of aluminum, copper, titanium and iron as well as a starting material for chemical compounds.

[0003] The silicon manufacturing and / or processing industries generate large quantities of silicon-containing waste and / or by-products, which are sometimes associated with high transport or disposal costs for the respective producers. This results in significant losses of metallic silicon.

[0004] CN109052407A describes a multi-stage process that first processes silicon waste, then converts this processed silicon waste into higher-purity silicon by adding a slag-forming agent and subsequently remelting it in a vacuum. Disadvantages of this process are the use of halogen-containing fluxes and the need to specifically produce a slag-forming component mixture with specific mixing ratios of the individual components prior to use in the described process. The latter represents additional expense. Halogen-containing fluxes can release volatile halogen compounds under operating conditions, which is associated with a number of disadvantages and potential hazards (e.g., corrosion of plant components, environmental and health damage), and thus requires additional technical effort to control these effects. Such technical solutions reduce the economic viability of the process.

[0005] CN 109 052 407 A describes a process for the recycling and purification of silicon cutting waste.

[0006] EP 0 699 625 A1 describes a process for removing impurities from molten silicon by treating molten silicon contained in a vessel with a slag capable of removing the impurities, particularly boron, from molten silicon. The slag is continuously or substantially continuously added to the molten silicon and then continuously or substantially continuously inactivated or removed from the silicon melt once an equilibrium is reached between the slag and the molten silicon with respect to the impurities or elements to be removed.

[0007] The object of the present invention was to provide an economical process which enables the production of technical-grade silicon (<99.9 mass% Si) starting from silicon-containing waste and / or by-products and to overcome the disadvantages of the process of CN109052407A.

[0008] The invention relates to a process for the production of technical silicon, in which a particulate raw material mixture containing particulate silicon-metal-containing material and a particulate mediator containing at least one of the elements C, O, Al, Ca and Si is heated to at least 1490 °C, whereby a liquid silicon-metal phase is formed, and the liquid silicon-metal phase is caused to solidify, wherein the particulate raw material mixture is described by a characteristic number K, where K has a value of 0.000 to 60 and is calculated as follows: K = m SiM ⋅ d 50 SiM + m Med ⋅ d 50 Med m SiM + m Med ⋅ φ where m(SiM) is the mass of the silicon-metal-containing material in the batch, m(Med) is the mass of the mediator in the raw material mixture, d 50,SiM is the particle size (diameter) at 50% of the mass flow rate of the grading curve of the silicon-metal-containing material, d 50,Med is the particle size (diameter) at 50% of the mass flow rate of the mediator, φ is the average porosity of the silicon-metal-containing material wherein the silicon-containing material is used without further purification steps, wherein the particles of the silicon-metal-containing material have an average particle size d 50,SiM from 0.1 to 100 mm and wherein the particles of the mediator have an average particle size d 50,Med from 0.1 to 100 mm.

[0009] The recovery of the silicon metal content from the silicon metal-containing materials contributes to a more economical and sustainable production of technical silicon.

[0010] The greatest advantage of the process according to the invention compared to known processes lies in the ability to recycle the metallic silicon components of silicon-metal-containing materials without chemical additives. This provides an ecological and economic advantage. With a focus on the circular economy, without silicon recovery, a large proportion of this raw material would no longer be available for further use. The process thus enables better utilization of technologically relevant silicon.

[0011] The technical silicon produced preferably has a Si content of at least 90 mass%, particularly preferably at least 95 mass%, in particular at least 97 mass%.

[0012] Preferably, the index K has a value of 0.01 to 40, particularly preferably 0.01 to 30, in particular 0.03 to 10.

[0013] The components of the particulate raw material mixture can be added to the furnace together or separately. The addition can be done manually or automatically. Under operating conditions, the particulate raw material mixture converts into a mixture known as the furnace charge. Upon heating, metallic silicon forms in liquid form within the furnace charge, creating a silicon-metal phase.

[0014] According to the invention, the particulate raw material mixture has the following properties: the particulate raw material mixture is preferably used as a particle mixture, wherein the particles of the silicon-metal-containing material preferably have an average particle size d 50,SiM of preferably 0.5 to 75 mm, particularly preferably 1 to 50 mm, in particular 5 to 30 mm; the particles of the silicon-metal-containing material have an average particle porosity of preferably 0 to 0.6, particularly preferably 0.05 to 0.4, very particularly preferably 0.1 to 0.35, in particular 0.15 to 0.3; the particles of the mediator have an average particle size d 50,Med of preferably 1 to 75 mm, particularly preferably 2 to 50 mm, in particular 5 to 30 mm; the mass ratio m (silicon metal-containing material) / m (mediator) preferably assumes values ​​from 0.1 to 10, particularly preferably from 0.2 to 8, very particularly preferably from 0.4 to 6, in particular from 0.5 to 5.

[0015] The silicon metal-containing material in the dry state preferably contains at least 20 mass%, particularly preferably at least 30 mass%, very particularly preferably at least 40 mass%, in particular at least 45 mass% of metallic silicon.

[0016] Preferably, the silicon metal-containing material is silicon residues, which are preferably selected from by-products or wastes of the silicon-producing or -processing industries, e.g. which arise during the production or mechanical processing of silicon, such as poly-, multi- or single-crystalline silicon; which arise during the production of granulated silicon metal, for example in fluidised bed, centrifugal, gas atomisation or water granulation processes; which arise during the production of technical-grade silicon by means of carbothermal reduction of SiO2; which arise during the mechanical processing and optionally one or more classification processes of technical-grade silicon. Mechanical processing can in particular involve crushing and / or grinding. Typical classification processes are, for example, sieving and / or classifying; which arise during the production of silanes.For example, this can be neutralized contact mass from chlorosilane reactors, before and / or after Cu recovery; in particular, the Müller-Rochow direct synthesis, hydrochlorination, or low-temperature conversion processes for the production of silanes. Purification of these silicon residues prior to use in the process according to the invention is usually not necessary, i.e., the silicon-containing materials are used without further purification steps.

[0017] In order to specifically set certain values ​​for the K index, the silicon metal-containing material is preferably agglomerated, for example by pelletizing, briquetting, sintering and drying.

[0018] Preferably, the mediator is subjected to a comminution (e.g. grinding, crushing), classification (e.g. sieving, sifting) and / or agglomeration process (e.g. pelletizing, briquetting, sintering) in order to obtain the desired value for the K index.

[0019] According to a preferred embodiment, the particle mixture of the particulate raw material mixture is selected such that the average particle size of the silicon-containing material d 50,SiM is larger than the average particle size of the mediator d 50,Med .

[0020] According to a further preferred embodiment, the particulate raw material mixture has a water content of at most 5 mass%, preferably of at most 1 mass%, particularly preferably of at most 1000 mass ppm, in particular of at most 500 mass ppm.

[0021] The particulate mediator preferably contains the elements O, Si and Ca. The particulate mediator preferably contains at least 10 mass%, particularly preferably at least 20 mass% O. The particulate mediator preferably contains at least 5 mass%, particularly preferably at least 10 mass% Ca. The particulate mediator preferably contains at least 15 mass%, particularly preferably at least 20 mass% Si. The particulate mediator preferably contains at least 1 mass%, particularly preferably at least 3 mass% Al.

[0022] The particulate mediator preferably contains at most 1 mass%, particularly preferably at most 0.1 mass%, of halogen. The particulate mediator preferably contains at most 0.05 mass%, particularly preferably at most 0.01 mass%, of F.

[0023] It may also be preferable to selectively dope the particulate raw material mixture and / or the furnace charge with elements. This may be appropriate, for example, if the technical silicon to be produced is intended for use in the synthesis of chlorosilanes. This involves one or more of the elements from the group consisting of Al, Cu, Sn, Zn, O, and P, or one or more compounds of these elements, or mixtures of these elements and compounds.

[0024] The energy required to heat the particulate raw material mixture can be applied in any way. Electrical energy is preferred, for example, using induction technology or resistance heating.

[0025] In a preferred embodiment, the furnace in which the necessary energy input for liquefying the raw material mixture is carried out electrically is an industrial furnace, in particular a standing furnace, such as an induction furnace, in particular a vacuum induction furnace.

[0026] The particulate raw material mixture is preferably heated to at least 1500°C, particularly preferably at least 1520°C.

[0027] The particulate raw material mixture is preferably kept at the temperature at which the liquid silicon metal phase forms for at least 30 minutes, particularly preferably 1 hour.

[0028] Due to density differences, the silicon-metal phase separates from the rest of the liquefied raw material mixture. Due to its lower density, the silicon-metal phase accumulates on the surface of the liquefied raw material mixture.

[0029] After phase separation, the silicon metal is solidified by cooling. This can occur on a cooled surface or in a cooled medium. In the former, the solidified silicon metal is removed either mechanically from the cooled surface or by heating the cooled surface through melting. In the latter, the silicon metal is usually granulated, for example, using so-called atomization techniques. Devices with a cooled surface or a cooled medium can be used in a variety of ways.

[0030] The process can be carried out in ambient air, under protective gas, selected from nitrogen and argon, for example, or under reduced pressure. Furthermore, this process can be carried out in batch, semi-continuous, or fully continuous modes. The remaining slag can remain in the vessel as a mediator for multiple melting processes, allowing new particulate, silicon-metal-containing raw material mixtures to be recharged.

[0031] The particle size distribution can be determined according to ISO 13320 (laser diffraction) and / or ISO 13322 (image analysis). Mean particle sizes / diameters can be calculated from particle size distributions according to DIN ISO 9276-2. For mixtures of particulate substances with particle diameters predominantly > 0.1 mm, sieve analyses are typically performed to characterize the particle mixture. The particle size distribution is determined using sieve analysis according to DIN 66165. Mean particle sizes / diameters can be calculated from particle size distributions according to DIN ISO 9276-2.

[0032] The total porosity of a material is the sum of the voids that communicate with each other and with the environment (open porosity; referred to here, in the present application, as "porosity") and the voids that are not connected to each other (closed porosity). Porosity measurements can be performed according to Archimedes' principle and are carried out according to ASTM C373-88. Furthermore, the porosity of a material can be calculated from the absolute and apparent density. The absolute and apparent densities can be determined by weight and volume measurements using gas pycnometers. The determination of the density of solids is described in DIN 66137-2:2019-03.

[0033] The temperature in the process can be measured using pyrometers, for example, using ratio pyrometers.

[0034] The chemical composition of the components of the particulate raw material mixture can be determined, for example, by means of X-ray fluorescence analysis, quantitative X-ray diffraction with an internal standard and / or ICP-OES. Examples

[0035] In a vacuum induction furnace, a high-purity graphite crucible was filled with 100 kg of charges of various K values ​​and heated to a temperature of approximately 1600 °C. After phase separation began, during which a silicon surface formed, the operating temperature was maintained at approximately 1550 °C for 3 hours. The silicon was then decanted, poured into a casting trough, and solidified in ambient air. After cooling to room temperature and mechanically removing the silicon from the trough, the mass and purity of the resulting technical-grade silicon were determined. The process is economical from a mass of 20 kg and a purity of 80% by mass.

[0036] The chemical composition of the mediator used: C (0.11 mass%), O (26.5 mass%), Al (7.31 mass%), Ca (18.9 mass%), Si (46.7 mass%); the proportion of metallic silicon was approximately 36 mass%. The remaining 0.48 mass% was distributed among Fe, Ti, Cu, Mg, P, B, V, Cr, Mn, and Ni. The mass fraction of metallic silicon in the silicon-metal-containing material was approximately 49 mass% in the dry state. Table 1 Attempt Key figure K m (silicon) [kg] Purity (silicon) [mass%, Si] 1 60 38 94 2 35 39 95 3 20 42 95 4 10 40 97 5 5 45 98 6 1 42 98 7 0,073 47 99 8 0, 064 35 99 9 0, 036 34 99 10 0, 009 28 99

[0037] The examples demonstrate that the invention provides an economical process which enables the production of technical-grade silicon (<99.9 mass% Si) from silicon-containing waste and / or by-products.

Claims

1. Method for producing technical silicon, wherein a particulate raw material mixture comprising particulate silicon metal-containing material and comprising a particulate mediator which comprises at least one of the elements C, O, Al, Ca and Si is heated to at least 1490°C, with formation of a liquid silicon metal phase, and the liquid silicon metal phase is brought to solidification, the particulate raw material mixture being described by a characteristic number K, K having a value of 0.000 to 60 and being calculated as follows: K = m SiM ⋅ d 50 SiM + m Med ⋅ d 50 Med m SiM + m Med ⋅ φ where m(SiM) is the mass of the silicon metal-containing material in the batch m(Med) is the mass of the mediator in the raw material mixture d50,SiM is the particle size (diameter) at 50% of the mass undersize of the grading curve of the silicon metal-containing material, d50,Med is the particle size (diameter) at 50% of the mass undersize of the grading curve of the mediator, φ is the mean porosity of the silicon metal-containing material, where the silicon-containing material is used without further purification steps, where the particles of the silicon metal-containing material have a mean particle size d50,SiM of 0.1 to 100 mm and where the particles of the mediator have a mean particle size d50,Med of 0.1 to 100 mm.

2. Method according to Claim 1, wherein the technical silicon produced has an Si content of at least 90% by mass.

3. Method according to one or more of the preceding claims, wherein the particles of the silicon metal-containing material have a mean particle porosity of 0 to 0.6.

4. Method according to one or more of the preceding claims, wherein the mass ratio m (silicon metal-containing material) / m (mediator) adopts values of 0.1 to 10.

5. Method according to one or more of the preceding claims, wherein the silicon metal-containing material comprises silicon residues selected from byproducts or wastes of the silicon-producing or silicon-processing industries.

6. Method according to one or more of the preceding claims, wherein the particle mixture of the particulate raw material mixture is selected such that the mean particle size of the silicon-containing material d50,SiM is greater than the mean particle size of the mediator d50,Med.

7. Method according to one or more of the preceding claims, wherein the particulate mediator comprises the elements O, Si and Ca.