How could solar PV silicon ingot producers see the cost of Argon production reduced by 95%? Dr Rob Grant of Gas Recovery and Recycle Limited (GR2L) thinks he may have the answer.

Argon makes up almost 1% of the Earth’s atmosphere, and its chemical inertness makes it a vital technological gas. The largest use for high purity argon is in the manufacture of solar cells and microelectronics, where the most significant application is in the manufacture of silicon ingots for onward processing into solar cells. Here the silicon is smelted in vacuum furnaces purged with argon to remove the impurities. It is then either cast as a multicrystalline ingot (directional solidification or DS) or grown as a single crystal using the Czochralski (CZ) process (Figure 1). 

It is estimated that 2-4 million m3 of argon is required in the fabrication of 1GW of solar cell production. At prices of up to $2/m3 this represents a significant cost to the manufacturers who typically just vent the gas to atmosphere after passing it through the production process.

Argon recovery is therefore a potentially attractive proposition for the silicon wafer industry. However, it is only recently, with the introduction of new purification technology from Gas Recovery and Recycle Limited (GR2L), that this has been made an economic possibility. The company's ArgonØ ¬™ is the first commercial product to utilise a new breed of purification technology based on chemical looping combustion jointly developed with Cambridge University, UK, and extensively tested in solar ingot manufacturing facilities in South Korea and Taiwan.

The exhaust argon gas is contaminated at the ~5,000 ppm level; the contaminants typically comprise CO and H2 from the process along with CH4 and other hydrocarbons from the vacuum pump oil etc. These need to be removed to below 10 ppm for the gas to meet the Semi standard (PV6-1110) for solar photovoltaic argon gas i.e. N5.0, although a majority of manufacturers require higher specification gas at N5.8 (i.e. 99.9998% pure or better).

There are a number of purification technologies available in the market today but the majority of these have been developed for the microelectronics industry to purify process gases which are contaminated, at worst, at the ppm level and purify them to the ppb level. The current generation of microelectronics type purifiers would typically breakthrough in minutes at the levels of contamination observed in silicon ingot furnace exhaust gas and so are not suitable as the primary purification stage for a process taking up to 50-60 hours, although they do have a place in further cleaning photovoltaic specification gas to the higher specification microelectronics grade where that is required.

A different approach needs to be taken for argon recovery in solar ingot production - this involves a combustion step to convert the CO and H2 to CO2 and moisture followed by an adsorption or separation step to remove the CO2 and moisture leaving high purity argon. This is exactly the approach taken with ArgonØ. It uses a specially designed chemical looping combustive purification (CLCP) reactor that utilises solid state oxygen carriers to ensure oxygen-free recycled gas. A CLCP reactor typically comprises a packed bed made of a metal/metal oxide couple that is repeatedly cycled from the oxidised state (Metal Oxide or MO) to the reduced state (Metal or M) and back again as shown below for CO oxidation i.e. continuous looping between steps 1 and 2.

The process is strongly exothermic and is stoichiometric rather than catalytic in nature. The overall process is chemically the same as gas phase oxidation, although during the gas clean-up, step 1, there is no introduction of any gas phase oxygen. In fact the reactors will very effectively, and simultaneously, remove any O2 acquired from any air leaks from the gas stream. The gaseous oxygen required for regeneration, step 2, comes from ambient air and so the CLC approach does not require any additional, and potentially expensive, gas supplies.

Multiple benefits

There are several benefits for solar PV silicon ingot manufacturers:

• The ArgonØ, figure 2, is designed to recover, purify and recycle exhaust purge gas from both CZ and DS vacuum furnaces.
• The system copes with both oil sealed and dry vacuum pumped furnaces and can be retrofitted into existing installations with minimal impact on productivity.
• It is Point of Use and can connect to multiple vacuum furnaces, subject to a maximum exhaust gas recycle flow limit of approximately 15 Nm3/hr.
• In new installations the ArgonØ units can be installed in phase with the main vacuum furnaces.
• At the heart of the system is a patented chemical looping combustive purification (CLCP) reactor that stoichiometrically combusts the process contaminants along with vacuum pump oil vapour etc. to CO2 and water; these are subsequently removed via molecular sieve traps.
• The solid state oxygen carrier ensures the recycled gas is oxygen free – measured levels are better than 100 ppb, a factor of 30 better than the SEMI specification.

Figure 3 shows the typical impurity profile associated with H2 and CO from a single vacuum furnace – the profile of the contaminants can be directly linked to events occurring within the vacuum furnace such as heating, melting and pulling. These impurities will be removed to typically better than 1 ppm.

In one location an ArgonØ system connected to eight DS vacuum furnaces typically recycles approximately 11 tonnes of argon per month with a recycle rate of better than 95%. None of the ~60 high performance ingots produced showed any variation outside of normal measurement parameters.

As a Point of Use system the ArgonØ is as simple and straightforward to retrofit into an existing facility as it is to install alongside the vacuum furnaces in a new build. The commercial savings generated give a simple pay back in less than three years and in addition there is an average carbon dioxide footprint reduction per furnace of 3-5 tonnes per year – achieved through reduction in the electrical power used to separate argon from air and the associated trucking of the liquid argon to site.

The ArgonØ system can be applied in other argon intensive technological applications such as jet turbine blade manufacture and powdered metals processing. In addition the basic chemistry associated with chemical looping combustive purification can be applied to other high value inert gases such as helium and xenon.

ABOUT THE AUTHOR

Dr Rob Grant is the founder and CEO of Gas Recovery and Recycle Limited (GR2L) a cleantech company specialising in the recovery, purification and recycling of purge gases used in the photovoltaic, microelectronics and the material processing industry sectors.

FURTHER INFORMATION

Gas Recovery and Recycle Limited (GR2L): Email rob.grant@GR2L.co.uk or go to the website at
www.GR2L.co.uk