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What Is "Slurry?"

 

Slurry plays a vital role in two different processes of solar cell production. First, in the wafer-creation phase of solar cell production, the block (“ingot”) of crystalline silicon goes to a wire saw machine to be sliced into thin sections. But it isn’t really the wire saw that’s doing the cutting – it’s the cutting liquid, or “slurry.” This highly abrasive liquid wears away the surface of the silicon to create the saw cut. This ensures the wafers are cut to the precision required. A less viscous form of slurry is used in the wafer polishing process. Both of these slurries are captured and pumped through a filter press before reconditioning/recycling.

Slurry is a generic term for any type of thick suspension of solids in a liquid. Slurries are used for a variety of purposes in construction, ceramics, paper, explosives, and many other industries. In the solar manufacturing industry, slurry is used as polishing or cutting agent. It is made from a mixture of powdered silicon carbide (SiC) suspended in polyethylene, diethylene glycol, or in an oil-based fluid.

While rare on Earth, silicon carbide is remarkably common in space, where it is frequently found in the form of stardust around carbon rich stars.

Silicon carbide is an extremely hard, durable, heat-resistant compound, occurring naturally as moissanite (an extremely rare mineral). Silicon carbide is uniquely suited for the extreme operating conditions of solar cell manufacturing. These same qualities make it useful in automotive and explosives applications; grains of silicon carbide can be sintered together to form very hard ceramics for use in car brakes and bullet-proof vests.

 

Virtually all silicon carbide sold in the world is synthetic. The simplest manufacturing process for silicon carbide combines silica sand and carbon in an Acheson graphite electric resistance furnace at a high temperature. This production process was patented by Edward Goodrich Acheson in 1893 and is still used today. Companies like Saint-Gobain Abrasives in Worcester, MA, and Washington Mills, with multiple plants across the U.S., specialize in production of silicon carbide powder and other raw materials used in slurries.

Solar manufacturing customers typically purchase the silicon carbide powder “grit” separately from the glycol, then mix the components together on site at their manufacturing plant to their specifications. Since the quality of the slurry is directly correlated to the quality of the wafer produced, slurry production is a serious science and craft. Grit sizes, viscosity, and other characteristics are different for every customer.

Once the raw materials are mixed, the slurry is moved to storage tanks until it is ready for the wire saw machines. The slurry is pumped into the machines, fed along a guide wire that circulates over the rolling ingots as the grit of the slurry cuts through them to produce the wafers. Eventually, the slurry becomes polluted by silicon particles, iron, and other by-products from the sawing process, which makes it unusable.

Slurry is one of the more expensive costs of solar cell production. Silicon carbide is a relatively expensive abrasive, and the volume of slurry used by manufacturers can exceed 2,000 metric tons a month. Costs to transport the virgin and used materials are very high. That’s why many customers are investing in systems that recondition slurry for reuse. Companies such as CRS Reprocessing Services, based in Louisville, Kentucky, contract with solar cell manufacturers to recover the reusable components of the used slurry, reprocess the material on or off site of the customer manufacturing facility, then reblend the material for operations. These slurry reprocessors distinguish themselves based on how well they recover the reusable components of the used slurry. This is known as the “recovery rate;” the higher the recovery rate, the less the manufacturer must spend on new material for slurry production.

 

 Heavy-duty, reliable industrial pumps have a critical role to play in every stage of slurry production, use, and reconditioning.

 

 

 

ALMATEC Maschinenbau GmbH


Carl-Friedrich-Gauß-Straße 5
47475 Kamp-Lintfort
Germany

 

Tel. +49/2842/961-0
Fax. +49/2842/961-40
Mail: info@almatec.de