High Purity Silicon Production via Silicide Phase Migration
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Solution Overview
Problem
Existing methods for producing high purity silicon, such as those described in U.S. patent No. 4,539,194, fail to completely remove calcium-silicide phase and other impurity elements isolated within silicon grains and narrow channels, limiting the effectiveness of leaching steps.
Innovation Solution
A method involving heat treatment of crushed silicon at temperatures between 1250°C and 1420°C, followed by a third leaching step with an aqueous solution of HF and HNO3, to melt and migrate impurity-containing phases to the surface, where they can be dissolved, resulting in higher purity silicon.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If calcium is added to molten silicon to form calcium-silicide phase along grain boundaries, then impurity elements are removed during leaching, but some calcium-silicide phase remains isolated within grains and narrow channels making it inaccessible to acid solutions
Solution Approach 1:
The patent applies parameter changes by heating the silicon to high temperatures (1250-1420°C) during heat treatment. This temperature parameter change causes the calcium-silicide phase to melt and migrate from isolated positions within grains and narrow channels to the grain boundaries and surface, making it accessible to acid solutions during subsequent leaching steps.
Solution Approach 2:
The patent utilizes phase transitions by heating silicon to temperatures where the calcium-silicide phase melts (above its solidus temperature). This phase transition from solid to liquid state enables the calcium-silicide to flow and migrate to accessible locations, then solidifies upon cooling, ensuring complete reaction with acid during leaching.
2Manufacturing precision
If multiple leaching steps are performed to remove impurities, then silicon purity increases, but impurities isolated within grains remain inaccessible and cannot be completely removed
Solution Approach 1:
The patent applies preliminary action by performing heat treatment before the final leaching step. This preliminary heating action melts and redistributes the calcium-silicide phase to accessible locations, ensuring that subsequent leaching steps can completely remove all impurities, including those previously isolated within grains.
3Manufacturing precision
If heat treatment is performed at higher temperatures to melt silicide phases, then impurity migration to surface is enhanced, but energy consumption increases
Solution Approach 1:
The patent optimizes the temperature parameter within a specific range (1250-1420°C) to achieve the desired effect of melting and migrating calcium-silicide phase while avoiding excessive energy consumption. This parameter optimization balances purification effectiveness with energy efficiency.
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 achieves a substantial reduction in impurity elements, with an 80% or more reduction at 1420°C, producing very pure silicon particles by effectively removing impurities that were previously inaccessible during initial leaching steps.
Implementation Method 1
during the heat treatment remaining calcium-silicide phase and FeSi 2 phase containing impurity elements melts and migrate out to the surface of the silicon particles
Implementation Method 2
migrate out to the surface of the silicon particles
Implementation Method 3
when solid silicon is heated to a high temperature the silicide phases melt and undergoes a volume expansion while the silicon undergoes a volume increase thus creating a force on the molten silicide phases squezing the molten silicide phases out from the narrow channels to the outer surface of the silicon particles
Implementation Method 4
The phases that have migrated to the surface of the silicon particles are then dissolved in the third leaching step resulting in very pure silicon particles after the third leaching step
Data Source
AI summary
The present invention relates to a method for producing high purity silicon comprising providing molten silicon containing 1-10% by weight of calcium, casting the molten silicon, crushing the silicon and subjecting the crushed silicon to a first leaching step in an aqueous solution of HCI and/or HCI + FeCI3 and to a second leaching step in an aqueous solution of HF and HNO3. The leached silicon particles is thereafter subjected to heat treatment at a temperature of between 1250°C and 14200C for a period of at least 20 minutes and the heat treated silicon is subjected to a third leaching step in an aqueous solution of HF and HNO3.