Accompanying with an increasing focus on renewable energy in recent years, the solar industry has grown and developed rapidly. Particularly in Taiwan, it is extremely possible that Taiwan would become the world's first photovoltaic site followed by the semiconductor, panel and diode industries through vertical integration of upstream and downstream supply. During these two years, the need for solar cells has risen considerably since renewable energy policies were motivated in every country, especially in Germany. The shipping quantity of 2005 exceeds 1 GW in a single year so that the lack of silicon raw material causes its high-rising price (above 100$/Kg at present), and this also directly impacts the development of the solar industry. Therefore, low-cost raw materials and recovery of consumed materials would play a key role in positive development of the industry and cost reduction of solar power generation. Additionally, more and more firms joined the solar industry these years in Taiwan, such that the supply of silicon raw material is unable to meet the demand.
After completing the growth of a solar silicon crystal, its crown and tail would be cut first, followed by using a diamond wheel to perform external grinding till its diameter meets the wanted size. The silicon crystal bar is fixed in the crystallographic direction through its flat, then sliced into wafers by a metal slicing wire, followed by steps of edge profiling, lapping, polishing and the like to give the required silicon wafers for IC manufacturing process. In the above process, the most easily consumable step is the slicing step, wherein an average about 40% of silicon would be loss due to the widths of slicing wires themselves (kerf loss). The silicon slurry caused by slicing is discarded as sludge, and in view of economics and costs, this would be an incredible waste. Even though diamond wheels have been replaced by wire saws to slice crystal ingots in industry, but the kerf loss is still unavoidable due to their wire width of about 150 μm. A wafer slice would approximately get one lost.
It consumes a large amount of cutting fluids and abrasive fluids in lapping and polishing a wafer. The main compositions of these cutting/abrasive slurries are water, silicon carbide abrasive particles (5-30 μm), further containing lubricating oil with chemical composition, resins for fixing crystal bars and the consumed metal of slicing wires (iron and brass as the basis). The function of water is to dilute the abrasive particles and carry away the heat generated by cutting and lapping. The key roles, which cause the cutting/abrasive action, are silicon carbide particles suspended in the slurry. The reason for selecting silicon carbide is owing to its high hardness and low price. In spite of the cheapness of silicon carbide, most people still put emphasis on recovering the silicon carbide from wasted abrasive slurry because it is used in a high volume and takes the most fraction of wasted silicon slurry. Since a large amount of abrasive fluids are utilized in lapping wafers and they cannot be recycled in order to maintain good wafer quality as well as the most portion of these abrasive fluids is silicon carbide and the silicon content is relatively low, thus the recovery of silicon carbide is more simple and beneficial than that of silicon. Moreover, in comparison with silicon powder, some silicon carbide particles have small particle sizes (about 1 micron or less) due to the particle crush by lapping. This would lead to the difficulty of separation. Additionally, the purity required for silicon raw material is very high (6-nine to 7-nine) with allowable impurity levels below 1 ppm. Therefore, the separation of silicon from silicon carbide is quite difficult in terms of technology.