Mining refinery waste for solar energy
Byproduct metals play critical role in new solar technologies Metal Tech News Weekly Edition – April 1, 2020
Last updated 6/27/2020 at 5:38am
Solar accounted for nearly 40% of the new electricity generating capacity added in the United States during 2019. With sun-fueled electricity expected to continue growing with more residents installing panels on their roofs and utilities commissioning large solar electricity generating facilities, a group of metals that typically lurk in the shadows are now seeing the light of day.
Many of the metals needed to build out America's solar generating capacity are fairly common – aluminum, copper, molybdenum and silver for example – others, however, are not as familiar – cadmium, gallium, germanium, indium, selenium, and tellurium are among the metals in this shadowy group.
Although this second group of metals are used in relatively small amounts compared with other materials that go into solar panels, they play a critical role in the performance of semiconductors that allow photovoltaic cells to turn sunlight into electricity.
While these more obscure metals have had specialty applications prior to the advent of solar energy, the market for them was limited and they were often disregarded with the other waste materials from mining – it was just not worth the expense and effort to recover them.
Now with the rise of solar as the fastest growing electrical source in the U.S., miners and refiners re-examining the viability of recovering these by-product metals often found alongside their more widely known counterparts.
"With the increasing importance of photovoltaic systems as a source of supplying electricity for homes, industrial facilities, and supplemental grids in the United States and other countries, the demand for these mineral commodities will likely increase over the near term," the U.S. Geological Survey inked in a paper on byproduct mineral commodities used in solar cells.
Three of these metals oft used in photovoltaic cells – germanium, indium and cadmium – are typically associated with zinc and are primarily recovered as a byproduct of mining this galvanizing metal.
Gallium is currently generally recovered as a byproduct of aluminum but is almost as common in zinc deposits.
Tellurium and selenium are oft associated with copper.
Currently, however, we only capture a small amount of these solar panel byproduct metals – the rest gets tossed with the other waste at refineries. As the solar industry needs more of these mined materials, refinery operators will likely look for new ways of recovering these byproducts to meet the growing demand.
Germanium has several optical qualities – transparent to the infrared electromagnetic spectrum, it can be formed into glass, has an exceptionally high refractive index and low chromatic dispersion – that have traditionally been the primary driver of demand for this zinc byproduct.
As an intrinsic semiconductor, however, germanium also is a powerful ingredient in highly efficient but expensive solar panels.
"Germanium substrates are used to form the base layer in multijunction solar cells, which are the highest efficiency solar cells currently available," according to the USGS.
These solar arrays, which have three layers of cells that use germanium and other critical metals, are much more expensive to build than the typical photovoltaic cell that primarily uses silicon to convert light into electricity.
The high efficiency of the germanium infused solar cells make them preferred for space applications such as the Mars rovers.
"The solar cells are stacked in three layers on the rover's solar arrays and, because they absorb more sunlight, can supply more power to the rover's re-chargeable lithium batteries," NASA Jet Propulsion Laboratory explains on technologies to power space missions.
USGS said recent research could make similar high-efficiency solar arrays commercially viable on Earth, a potential driver for future germanium demand.
"Solar powerplants that use concentrator technology composed of lenses or mirrors that focus high concentrations of direct sunlight onto germanium-based multijunction solar cells have emerged as viable sources for large-scale renewable energy generation," the geological survey wrote.
The efficiency of germanium infused solar cells, however, has yet to justify the cost in a way that is making any difference in demand for this semiconductor metal.
According to the USGS, American germanium consumption has held steady at around 30,000 kilograms (30 metric tons) per year. Germanium currently sells for about US$1,900/kg.
The Red Dog zinc mine in Alaska is believed to produce significant amounts of germanium. Due to being a byproduct metal that is credited to the Canadian refinery that extracts the germanium from the zinc and lead concentrates, it is hard to pinpoint exactly how much of this semiconductor metal is produced at Red Dog each year.
Additionally, a zinc smelter in Tennessee produces and exports germanium leach concentrates produced from nearby zinc mines.
Indium has a lot in common with germanium – it has strong optical qualities, is used in higher efficiency solar panels and is often recovered as a byproduct from zinc mining.
While indium is not part of the everyday lexicon, it is almost guaranteed that you are looking at (or more accurately, through) indium right now.
This is because indium-tin oxide is used as a transparent conducting film applied to virtually every flat-panel display and touchscreen on the market. This thin coating transforms incoming electrical data into an optical form, a property that makes touchscreens touchable.
Flat-panel displays and touchscreens account for more than half of the indium consumed globally. While each of these devices only need a small amount of indium-tin oxide, the massive quantity of televisions, computer screens, tablets, smartphones and numerous other devices with liquid crystal displays adds up to a lot of this metal.
In addition to the strong adherence to glass that contributes to indium's use on electronic displays, this silvery metal is highly reflective, making it an ideal ingredient for an energy saving coating on architectural glass on high-rise buildings.
With thin-film photovoltaic cells catching on for supplying electricity to homes and businesses, indium is finding new demand in the solar markets.
Copper-indium-gallium-selenide (CIGS) solar cells can absorb much more sunlight than traditional cells, which means that a much thinner film is needed. These thinner cells can be applied on flexible materials and are less expensive to produce.
Despite these advantages, CIGS solar panels are much less efficient at converting electricity than their more rigid silicon-based counterparts. As such, CIGS only make up about 2% of the solar panel market. This is expected to climb with improved solar efficiency.
Much like germanium, solar has yet to move the demand dial for indium. In fact, USGS estimates that U.S. indium consumption dropped about 12% to 110 metric tons during 2019. The average price of this zinc byproduct last year was about US$390/kg.
"The estimated value of refined indium consumed domestically in 2019, based on the average New York dealer price, was about $43 million," USGS penned in its Mineral Commodity Summaries 2020 report.
Like its sister metal germanium, indium production is often credited to the refinery that extracts the metal from concentrates, which makes the mine source sometimes difficult to track down.
There are good indicators, however, that Red Dog Mine is the source of at least some of the indium the U.S. imports from Canada.
While most zinc smelters are not equipped to recover indium, Teck's Trail Operations in southern British Columbia is.
"Indium is produced as a co-product of the zinc smelting process at our integrated refinery in Trail," according to the Teck website.
Electricity to laser
While gallium also happens to be found in zinc deposits, this is not where most of the world's supply of this CIGS metal comes from. Instead, it is mostly recovered as a byproduct of aluminum mining.
"Almost all the world's primary gallium supply originates as an impure byproduct recovered during the processing of bauxite ore to alumina, a precursor to refined aluminum," USGS explains.
Only about 320 metric tons was produced in 2019, roughly 97% of this refinery output came from China.
Gallium-based semiconductors are able to convert electricity into laser light. Light emitting diodes (LEDs) take advantage of this ability and is the largest use for gallium.
Gallium compounds are also used in the production of highly specialized integrated circuits, semiconductors and transistors to help regulate the flow of electricity. These components are necessary for high-performance computers and smartphones.
In order to pack more computing power in smaller devices, newer models of computers and smartphones use more gallium than their predecessors. This, along with CIGS solar panels, are driving increased demand for the silvery metal that melts at 84.2 degrees Fahrenheit (29 degrees Celsius).
USGS, however, reports that most of the gallium that is mined with aluminum and zinc is discarded as waste at refineries – providing a ready source of additional metal to meet the high-tech demands.
Cadmium thin-film solar
Cadmium, which is one of two main ingredients in cadmium telluride photovoltaics, is another solar metal that is a byproduct of zinc mining.
Most of the world's primary cadmium metal was produced in Asia. Like most critical metals, China is the top producer of cadmium, accounting for roughly 8,200 metric tons, or 33 percent, of global production. Korea, the world's second largest supplier of cadmium, produced 5,000 metric tons during 2019.
Two companies in the United States produced undisclosed amounts of refined cadmium during 2019. One of these companies produced cadmium from zinc concentrates in Tennessee, the other from recycling spent nickel-cadmium batteries.
Besides NiCd batteries, cadmium metal and compounds are mainly consumed for alloys, coatings, pigments, and plastic stabilizers.
Cadmium telluride photovoltaics account for more than half of the thin-film photovoltaic market.
"CdTe solar cells are the second most common photovoltaic technology in the world marketplace after crystalline silicon, currently representing 5 percent of the world market," according to the U.S. Department of Energy. "CdTe thin-film solar cells can be manufactured quickly and inexpensively, providing a lower-cost alternative to conventional silicon-based technologies."
Due to the low cost of cadmium telluride solar cells, coupled with being less affected by dust, shading and high temperatures, this emerging technology is expected to nab larger shares of the solar market.
Tellurium in Alaska
Tellurium, the other semiconductor ingredient in CdTe solar cells, is primarily produced as a byproduct of copper mining.
This metalloid, an element that possesses the properties of both a metal and non-metal, is very rare.
"Most rocks contain an average of about 3 parts per billion tellurium, making it rarer than the rare earth elements and eight times less abundant than gold," the United States Geological Survey wrote in a 2015 report on this critical metalloid.
And because it is so scarce, very little is produced. During 2019, about 470 metric tons of tellurium was produced in global refineries, with about 62 percent being produced in China.
USGS estimates about 40 percent of the tellurium consumed last year was used to make solar panels, the balance was primarily used in thermoelectric devices for both cooling and energy generation; as a steel, copper and lead alloy; and as a vulcanizing agent and accelerator in the processing of rubber.
Though tellurium is seldom found in economic concentrations, the Pebble copper-gold-molybdenum mine project in Alaska hosts "an enormous tellurium endowment" and could provide a future source of this solar panel ingredient.
While there has never been an official calculation of how much tellurium is contained within the Pebble deposit, there are indications that it is sizable. Work completed in 2013 estimates that 2.5 to 3 percent of the more than 100 million ounces of gold identified at Pebble is hosted in telluride minerals. This gold carrying telluride, in turn is hooked to chalcopyrite, which is the main copper ore mineral at the Pebble deposit.
Of all the solar byproduct metals, selenium stands out for its food production uses and the U.S. produces most of its needs domestically.
While domestic production of selenium is withheld, USGS reports that less than 25% of the 500 metric tons of selenium used in the U.S. was produced on American soil. Like tellurium, this metal is a copper byproduct metal captured at refineries.
Selenium is an essential micronutrient and is used as a dietary supplement for both people and livestock, as well as a fertilizer additive to enrich selenium-poor soils. Selenium is also used as an active ingredient in antidandruff shampoos.
The largest use and driver of new demand for selenium is in the production of manganese and as a decolorizing agent in glass.
Like many other byproduct metals, much of the selenium is discarded as refinery waste.
"Significant improvements in recovery of selenium and tellurium at refineries will likely result in an increase in supply of selenium if prices justify recovery," USGS penned in its report on byproduct commodities in photovoltaic cells.
So, it seems that most of the semiconductor metals that have the potential to make the growing number of solar panels being installed across the U.S. more affordable, flexible and efficient are already being mined – we only need to recover them at the refinery to meet growing demand.