The production of components for applications in the electronic and photovoltaic fields make use of thin silicon disks (“wafers”) obtained from either polycrystalline or monocrystalline silicon ingots by cutting (slicing) the ingot perpendicularly to its length. Typically, this slicing operation is carried out by means of a wire saw, in which a moving metal wire of considerable length and suitable mechanical resistance, wound in a system of rollers and spools, is contacted with the ingot perpendicularly to the ingot length, at the points where the cut is to be made. At the same time, a cutting slurry containing abrasive grains or particles (abrasive slurry) is fed to the contact area between the cutter wire and the ingot to aid in cutting the ingot.
The conventional abrasive slurries used for cutting ingots of silicon, quartz or other ceramic material with a wire saw typically comprise a suspending lubricant or cooling fluid in which abrasive particles of suitable hardness are suspended. The suspending fluid typically comprises a mineral oil, or a water-soluble organic liquid, e.g., a high molecular weight polyether such as polyethylene glycol (PEG). The abrasive typically comprises silicon carbide (SiC) or some other high hardness abrasive substance. Water-based slurries (i.e., in which the suspending fluid comprises mainly water) are also known.
During wire sawing of an ingot (e.g., a silicon ingot) with a cutting slurry, the slurry eventually becomes contaminated with particles from the ingot (kerf), and with metal particles from the wire. In addition, some of the abrasive grains in the slurry are broken down into smaller sizes. This combination of contamination with foreign particles and breakdown of the abrasive particles reduces the cutting effectiveness of the slurry.
Used or spent slurry containing kerf particles and degraded abrasives (exhausted or spent slurry) can be discarded; however, the spent slurry contains a large fraction of still usable abrasive grains, which are valuable. A recycling industry has developed to recover and re-use the abrasive from spent cutting slurries. Current recycling processes are designed to handle either oil-based slurries or water-soluble organic liquid-based slurries (e.g., polyethylene glycol-based slurries), collectively referred to herein as “organic-based” cutting fluids. There is an ongoing need to develop an effective recycling process for spent water-based wire saw cutting slurries. The methods of the present invention are designed to address this need.