Manufacturing silicon also generates toxic emissions. In 2016, New York State's Department of Environmental Conservation issued a permit to Globe Metallurgical Inc. to release, per year: up to 250 tons of carbon monoxide, 10 tons of formaldehyde, 10 tons of hydrogen chloride, 10 tons of lead, 75,000 tons of oxides of nitrogen, 75,000 tons of particulates, 10 tons of polycyclic aromatic hydrocarbons, 40 tons of sulfur dioxide and up to 7 tons of sulfuric acid mist. [10] To clarify, this is the permittable amount of toxic waste allowed annually for one New York State metallurgical-grade silicon smelter. Hazardous waste generated by manufacturing silicon in China likely has significantly less (if any) regulatory limits.
Step Three
Step Two's metallurgical-grade silicon is crushed and mixed with hydrogen chloride (HCL) to synthesize trichlorosilane (TCS) gas. Once purified, the TCS is sent with pure hydrogen to a bell jar reactor, where slender filaments of pure silicon have been pre-heated to about 2012F (1100C). In a vapor deposition process that takes several days, silicon gas atoms collect on glowing strands to form large polysilicon rodskind of like growing rock candy. If power is lost during this process, fires and explosions can occur. A polysilicon plant therefore depends on more than one source of electricityi.e. two coal-fired power plants, or a combination of coal, nuclear and hydro power. [11]
A large, modern polysilicon plant can require up to 400 megawatts of continuous power to produce up to 20,000 tons of polysilicon per year (~175 MW/hours per ton of polysilicon). [12] Per ton, this is more than ten times the energy used in Step Twoand older plants are usually less efficient. A single plant can draw as much power as an entire city of 300,000 homes.
Once cooled, the polysilicon rods are removed from the reactor, then sawed into sections or fractured into chunks. The polysilicon is etched with nitric acid and hydrofluoric acid [13] to remove surface contamination. Then, it's bagged in a chemically clean room and shipped to a crystal grower.
Step Four
Step Three's polysilicon chunks are re-melted to a liquid, then pulled into a single crystal of silicon to create a cylindrical ingot. Cooled, the ingot's (contaminated) crown and tail are cut off. Making ingots often requires more electricity than smelting. [14]The silicon ingot's remaining portion is sent to a slicer.
Step Five
Like a loaf of bread, the silicon ingot is sliced into wafers. More than 50 percent of the ingot is lost in this process. It becomes sawdust, which cannot be recycled. [15]
Step Six
Layer by layer, the silicon will be "doped" with tiny amounts of boron, gallium, phosphorus or arsenic to control its electrical properties. Dozens of layers are produced during hundreds of steps to turn each electronic-grade wafer into microprocessors, again using a great deal of energy and toxic chemicals.
Questions for a world out of balance
In 2013, manufacturers began producing more transistors than farmers grow grains of wheat or rice. [16] Now, manufacturers make 1000 times more transistors than farmers grow grains of wheat and rice combined. [17]
After I learned what it takes to produce silicon, I could hardly talk for a month. Because I depend on a computer and Internet access, I depend on siliconand the energy-intensive, toxic waste-emitting, greenhouse gas-emitting steps required to manufacture it.
Of course, silicon is just one substance necessary for every computer. As I report in letter #3 [18], one smartphone holds more than 1000 substances, each with their own energy-intensive, GHG-emitting, toxic waste-emitting supply chain. [19] One electric vehicle can have 50-100 computers. [20] When a computer's microprocessors are no longer useful, they cannot be recycled; they become electronic waste. [21]
Solar panels also depend on pure silicon. At the end of their lifecycle, solar panels are also hazardous waste. (In another letter, I will outline other ecological impacts of manufacturing, operating and disposing of solar PV systems.)
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