Agricultural Waste Silicon Production via Carbothermal Reduction
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Solution Overview
Problem
The high cost and energy-intensive processes currently used to produce high purity silicon for photovoltaic cells are hindered by the need for expensive and environmentally harmful chlorosilanes, limiting the rapid adoption of silicon photovoltaics due to the inefficiency and pollution associated with the Siemens purification process.
Innovation Solution
A method utilizing agricultural waste products, such as rice hull ash, to produce high purity silicon through a process involving impurity extraction, carbon-to-silica ratio adjustment, and carbothermal reduction at lower temperatures, reducing the number of processing steps and energy requirements, and eliminating the need for chlorosilanes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the Siemens purification process is used to produce high purity silicon, then the required purity level is achieved, but the process becomes energy intensive and environmentally harmful due to corrosive and toxic chlorosilanes
Solution Approach 1:
The patent extracts and removes the harmful chlorosilane purification steps from the traditional Siemens process. Instead of using corrosive HCl and forming toxic chlorosilanes, the invention uses a direct carbothermal reduction process that eliminates these harmful intermediate compounds while still achieving the required silicon purity for photovoltaic applications
Solution Approach 2:
The patent converts the naturally occurring silica from agricultural waste (which would normally require extensive purification) into a beneficial starting material. By using rice hulls and other agricultural wastes as feedstock, the process transforms abundant, low-cost materials containing silica into high-purity silicon without requiring the harmful chlorosilane purification steps
2Manufacturing precision
If multiple high-energy processing steps are used to produce photovoltaic grade silicon from quartz rock, then the required purity is achieved, but the energy consumption and capital costs increase significantly
Solution Approach 1:
The patent performs preliminary purification by using agricultural waste materials that naturally contain pre-purified silica through plant uptake processes. The rice hulls and other agricultural wastes already have silica in a form that requires minimal additional purification, eliminating several high-energy processing steps required when starting from quartz rock
Solution Approach 2:
The patent changes the starting material parameters from high-purity quartz rock to agricultural waste materials containing amorphous silica. This parameter change allows the use of lower temperature carbothermal reduction (around 1000-1200°C) instead of the higher temperature processes (1500-2000°C) required in traditional Siemens processes, significantly reducing energy consumption
3Ease of manufacture
If traditional carbothermal reduction of silica is used to produce metallurgical grade silicon, then the process is relatively simple, but the purity level is insufficient for photovoltaic applications without additional costly processing steps
Solution Approach 1:
The patent uses a composite approach by combining agricultural waste materials (rice hulls, corn stover, etc.) with carbon sources to create a reactive mixture for carbothermal reduction. This composite feedstock provides both the silica source and reducing agent in intimate contact, enabling direct production of photovoltaic-grade silicon in a single step without requiring separate purification stages
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach results in a more energy-efficient, environmentally friendlier, and cost-effective production of high purity silicon, potentially increasing the throughput of electric arc furnaces and reducing the energy needed for reduction, while also providing a renewable source for silicon production.
Implementation Method 1
extracting the impurities with an aqueous acidic solution
Implementation Method 2
reacting the extracted rice hull ash with ammonium hydroxide to increase the carbon to silica ratio
Implementation Method 3
carbothermally reducing the silica to photovoltaic silicon by heating in a gaseous atmosphere at a temperature of at least about 1000°C
Implementation Method 4
heating in an electric arc furnace to about 1900°C to produce a photovoltaic grade of silicon
Data Source
AI summary
The present invention is directed to a method for providing an agricultural waste product having amorphous silica, carbon, and impurities; extracting from the agricultural waste product an amount of the impurities; changing the ratio of carbon to silica; and reducing the silica to a high purity silicon (e.g., to photovoltaic silicon).