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Viability of recycling semiconductors in Dell LCD PC monitors

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Asi.png This page was part of a project for MY3701 -- an MTU class on semiconductors.

This page is now open edit -- please fix mistakes or feel free to leave comments using the discussion tab.



Authors


Overview

The technology that me and my colleagues have chosen to examine as a possible semiconductor recycling project is the LCD PC monitor. We have chosen Dell[1] to take a closer look at the viability of recycling semiconductors. Hydrogenated amorphous silicon is the typical choice of semiconductor material in the manufacturing of LCD monitors. It is the best choice of semiconductor material because it can be "grown" over large areas. The structure of this silicon is not that of a crystalline material. It lacks the long range order that is present in crystalline silicon. Within the material there are vacancies that form, leaving behind dangling bonds; these free bonds are free electrons from the silicon that have not bonded with another silicon atom. However, the addition of the hydrogen atoms reduces the amount of dangling bonds because hydrogen consists of one free electron and can attach to the vacant bonds. This can be seen in Figure 1. [2]
Fig 1: Shows the gaps and dangling bonds with hydrogen atoms in the structure of the silicon.
The process of which the hydrogenated amorphous silicon is deposited onto a substrate is called plasma-enhanced chemical vapor deposition (PECVD). This process uses silane gas which enters the chemical vapor deposition chamber and dissociates into a cloud of plasma. This plasma is induced by a radio frequency power generator; the chamber that holds this process must be in a vacuum to eliminate any impurities in the process. Similiar to an electron beam deposition process,the silicon and hydrogen atoms condense on a substrate, in LCD monitors this would be the glass, and make the thin layer of hydrogenated amorphous silicon. The setup of this process is shown in Figure 2. [3]
Fig 2: Shows the setup for the production of amorphous hydrogenated silicon

Scale of market

The production of PC LCD monitors on the worldwide market shows Dell, Samsung, and LGE at the top of the market selling most for the 12 Brands selling from as low as 10 million units in a month to nearly 15 million units a month for 2010. Dell was ranked number one in the production of LCD for 2010. (Fig 3) In Figure 3, we were capable of determining the amount of units sold by Dell, which is estimated at 22.5 million units in 2010. [4]


In the total amount of PC LCD monitors that the top 12 brands sold between January 10 2009 and January 10 2010 was approximately 139 million units. (Fig 4)[5]


The growth for the production of LCD screens is expected to slowdown in the next two years. The growth in production of LCD panels for LCD has decreased, by the slow economic recovery, which is causing consumers to buy less and is predicted to slow down further for the next few years. (Fig 5) [6]

Recycling practices

Dell


Yes, LCD monitors are recycled.

Scrap LCD monitors are sold for recycling of the actual LCD panel when its not cracked, if it is cracked, its value drops dramatically to almost 0 since the only thing that is left to recycle is the actual printed circuit board and the circuit board in the LCD monitor is usually lower grade. [7]

HOW to recycle your LCD monitors…

To prevent your old electronics from being melted down over a rudimentary stove in Guiyu, China, or being tossed into a landfill in Lagos, Nigeria, you’ll want to choose a reputable recycler. [8]

Recycling LCD monitors is easy if you do not have to go through the process yourself. There are multiple companies that allow you to send in your monitors for free, for a price or you can even drop them off at specific locations and they will handle it all for you. Some of these companies include Apple, Staples and Dell. Michigan Technological University allows people to drop off their monitors and the Apple store will recycle it all. If you want to go straight through Apple, their recycling process is to purchase any Apple computer or monitor and receive free recycling of your old computer and monitor no matter what the brand is.

WHY it should be recycled…

Electronics consist of valuable resources, such as precious metals, copper, and engineered plastics, all of which require considerable energy to process and manufacture. Recycling electronics recovers valuable materials, conserves virgin resources, and results in lower environmental emissions (including greenhouse gases) than making products from virgin materials. For example: Recycling one million desktop computers prevents the release of greenhouse gases equivalent to the annual emissions of 16,000 passenger cars. By recycling 100 million cell phones, approximately 7,500 pounds of gold could be recovered - allowing that amount of gold to go into new products. Recovering the gold from cell phones, rather than mining it from the earth, would prevent 12,000,000,000 pounds of loose soil, sand, and rock from having to be moved, mined, and processed. [9]


DANGERS of not recycling properly...

Electronic waste isn’t just waste, it contains some very toxic substances, such as mercury, lead, cadmium, arsenic, beryllium and brominated flame retardants. When the latter are burned at low temperatures they create additional toxins, such as halogenated dioxins and furans – some of the most toxic substances known to humankind. The toxic materials in electronics can cause cancer, reproductive disorders, endocrine disruption, and many other health problems if this waste stream is not properly managed. Many of the toxic constituents are elements, which means they never disappear, even though they may change form. Other toxic chemicals in electronics do not break down over time and instead, accumulate in the food chain and biosphere. Not only do these toxins present risks to communities and the global ecosystem, but also to electronics recycling workers, even in developed countries.

An estimated 70-80% of the e-waste that’s given to recyclers is exported to less developed countries. Once there, primitive technologies such as open air burning and riverside acid baths are used to extract a few materials. The rest of the toxic materials are usually dumped. These operations amount to government subsidies, undermining the development of responsible private-sector recycling infra-structure and distorting the economics of recycling. [10]

What can the recycled be used for…

Flat screen LCD monitors and televisions contain toxic substances that can wreak havoc on the environment, but they may also have a silver lining -- or in this case, a silver nanoparticle and PVA lining.

British scientists have discovered that by separating the films of LCD screens, they can remove the polyvinyl-alcohol (PVA) and turn it into a disinfectant. According to the researchers, the substance can kill harmful bacteria including strains of Escherichia coli and Staphylococcus aureus.

Andrew Hunt of the University of York says, "We can add significant value this waste...that has great potential for use in biomedicine. Now we have gone a step further by enhancing its anti-microbial properties through the addition of silver nanoparticles, with the result being that it can destroy bacterial infections."

Previous research of theirs has found that pure PVA might be useful in human tissue regeneration and possibly in treatments to transport medications to specific areas of the body. By adding a dash of silver to the screen material, they hope to potentially make antibiotic cleaning solutions for hospital use.

And what to do with the rest of the toxins leaking from our technotrash? The EPA estimates that in 2005, the computers, TVs, VCRs, monitors, cell phones and other electric items ditched by Americans amounted to almost 2 million tons. That’s a whole lot of lead, mercury, arsenic, beryllium, cadmium, and other dangerous substances. That's why it's so important to recycle your old electronics. [11]


Amount of semiconductor in market

The amount of hydrogenated amorphous silicon is all dependable on the thickness of the thin film layer. These layers can range from nanometers to micrometers, and for the LCD monitors an average between the two will be used to determine the thickness. [12]


Thickness=((1x10^-9)+1x10^-6))/2=5.005x10^ -7 meters

To determine the total volume in each monitor that Dell sells, we took the average height and width of the monitors that are offered. [13]

Width=(18.5+4*21.5+3*23+24*4+27+30)/14=23.32

Height=(13.74+4*12.84+3*12.73+4*15.47+16.7+18.95)/14=14.34

Converted to meters, the average size of the screen is: .592x.364 meters

Total volume=(5.005*10^-7)*.364*.592=1.0785*10^-7 meters^3

To determine the mass of the silicon present, we took the average volume of the monitors and multiplied it by the density of silicon.

1.0785*10^-7 meters^3 * 2330000 grams/meters^3=.2513 grams per monitor

Finally, the total amount of silicon that could be possibly extracted from the 22.5 million units sold is:

.2513 average grams per unit * 22.5 million units sold=5,654,250 grams

5,654,250=12,465.487 lbs

Methods of collecting lost semiconductor materials

The methods for collecting the liquid crystals from the LCD panels are: 1. Supercritical Carbon Dioxide Fluid Technology extracts the liquid crystals from the glass. This method uses iso-thermal and a depressurization method to remove the liquid crystal from the glass panel, by converting carbon dioxide gas to its supercritical fluid state, thus dissolving the liquid crystal. Then the temperature is dropped, the carbon dioxide reverts back to gas, leaving the liquid crystal [14]. 2. Ultrasonic cleaning uses ultrasonic waves, which causes pressure against the liquid, thus removing the liquid crystals by force from the glass substrate[15]. After the liquid crystals have been extracted and removed and cleaned using a solvent, they can be recycled back into a different LCD [16] [17].

Stripping Process

This process involves the use of a base, usually quaternary ammonium hydroxide, a surfactant and a high boiling solvent (di- or tri- propylene glycol alkyl ether.) to strip the silicon.[18]

Mechanism of Cured Silicone Adhesive Removal with TBAF

The cured silicon can be exposed to tetrabutyammonium fluoride reagent (TBAF) in non-hydroxylic aprotic solvent. This causes a disintegration of the polymer matrix, thus removing the silicon into the solvent.[19] Table 1: Shows the chemistry, conditions for each process, and function of each process for the TBAF process. This process is extremely viable for removing the semiconductor silicon adhesive off of the TFT or many other types of substrates.


Potential for Post-Consumer Recycling

It has been decided that the semiconductor material found in Dell LCD PC monitors is a viable resource that should be recycled post-consumer.

Part A

Collection Methods

Recovered 174,633,062 Kg of computer * 0.0002513 Kg average amount of semiconductor per unit = 43,885.29 Kg

Wasted 657,164,626 Kg of computer * 0.0002513 Kg average amount of semiconductor per unit = 165,145.47 Kg

Percent Recovered: 43,885.29/(43,885.29+165,145.47)=.2099*100 = 20.99% recovered

Contaminants

Mercury is the contaminant found. Some computer monitors that use liquid crystal display (LCD) technology could contain mercury, a highly toxic metal that can cause brain or liver damage if ingested.

Mercury, in screens and monitors, is there to produce visible light when the mercury is electrically energized. When the laptop monitors are tilted, the mercury flows to either end cutting off the circuit on one end, while opening it on the other side. They often function as on/off switches[20]

At Stena Innovative Recycling they extract more than 88 percent clean units without mercury. Cleaned units are separated into iron, metals, plastics, circuit boards and glass with liquid crystals. The whole process works in a closed controlled environment. During the entire process mercury levels are continuously controlled, so Stena can ensure that the mercury is removed from all material that will be recycled. Mercury-contaminated units are sent to special hazardous waste treatment.[21]

The most efficient treatment method is to separate the dangerous backlight lamps from the panel. If the backlight lamp is not extracted before, the whole screen has to be treated as hazardous waste. Although some alternative treatment technologies were investigated for the separation of the backlight lamps, such as water jet cutting, laser cutting and circular saw, the best results were achieved by manual dismantling. Both the costs per item and the quality assessment reach the best results. [22]


The concentration of Mercury found in each unit is about 2mg.


831,797,688 Kg (Total computers)* .0000020 Kg(Concentration of Mercury per computer)=1663.60 Kg of Mercury sold from LCD monitors in 2000.

Purification Methods

The purity of silane is 99.9999 percent. Electronically active impurities, such as boron, phosphorus, and arsenic are controlled to less than 10 parts per trillion, making silane one of the purest materials on Earth.[23]



The patented, closed-loop silane manufacturing process used by REC produces consistent, ultra pure silane by conversion of metallurgical grade silicon into trichlorosilane and redistribution/ distillation to silane. The continuous-flow process recycles all hydrogen and chloride materials back to the initial reactors, while continuous distillation steps purify the gas. The entire process is a low waste, low impact and environmentally friendly procedure. [24]


In the most simple way to describe the production of silane, silicon is turned into a gas by grinding it down to a fine, sand-like consistency and heating it with hydrogen and silicon tetrachloride. After this is done it is then put through a series of reactions, as seen below, and silane and ultra-high pure polysilicon are made.


Industrially, silane is produced from metallurgical grade silicon in a two-step process. In the first step, powdered silicon is reacted with hydrogen chloride at about 300 °C to produce trichlorosilane, HSiCl3, along with hydrogen gas, according to the chemical equation:

Si + 3 HCl → HSiCl3 + H2

The trichlorosilane is then boiled on a resinous bed containing a catalyst which promotes the formation of silane and silicon tetrachloride according to the chemical equation:

4 HSiCl3 → SiH4 + 3 SiCl4

An alternative industrial for the preparation of very high purity silane, suitable for use in the production of semiconductor grade silicon, starts with metallurgical grade silicon, hydrogen, and silicon tetrachloride and involves a complex series of redistribution reactions (producing byproducts that are recycled in the process) and distillations. The reactions are summarized below:

Si + 2 H2 + 3 SiCl4 → 4 SiHCl3

2 SiHCl3 → SiH2Cl2 + SiCl4

2 SiH2Cl2 → SiHCl3 + SiH3Cl

2 SiH3Cl → SiH4 + SiH2Cl2

The silane produced by this route can be thermally decomposed to produce high-purity silicon and hydrogen in a single pass. Another commercial production of silane involves reduction of SiO2 under Al and H2 gas in a mixture of NaCl and AlCl3 at high pressures:

3SiO2 + 6H2 + 2Al → 3SiH4 + Al2O3

[25]

In-Situ Analysis

Cost For Recycling Compared to Manufactoring Semiconductors

The cost to transport to recycling center is estimated at 0.0172 dollars per kilogram. From our calculated total amount of world wide recovered amount of semiconductors is 43,885.29 Kg. So the total cost of transporting semiconductors is 0.0172*43,885.29 = $745.83. The cost to sort semiconductors is 0.140 dollars per Kg. The total cost to sort semiconductors = 0.140*43,885.29 = $6,143.94 . The cost to dismantle them is 0.315 dollars per Kg. Total cost to dismantle = 0.315*43885.29 = $13,823.9 . Cost to recycle semiconductor is unknown at this current time. The total cost recycle, collect, and transport is [26] The total cost to produce 3000,000 kg of the semiconductor silane is $4,202,331.00. So we get the total cost of producing silane per kg by taking $4,2020,331.00 / 300,000 kg = 14.0078 dollars per kg. The total cost to recycle the silane must be lower than the cost to manufacture minus the cost to collect and transport the silane, thus 14.0078-(.0172+0.140+0.315)= 13.5356 > cost to recycle the silane. It must cost less 13.5356 dollars per kg to recycle silane [27].

Alternative Recycling

Alternatively instead of straight recycling the silane recovered from the LCD PC monitors back into new monitors. The silane can be put in other electronics. The silane can be used in the production solar panels, smartphones and other electronic devices which either have a LCD screen or need the silane to connect the glass to the polymer matrix in these devices.[28]

Down cycling

One of the alternatives to straight cycling of the Silane is down-cycling it into titanium implants, so the biologically inert material in the implant can attach to the titanium implant. The recovered Silane can also be used as water repellent and masonry protection. The Silane can also be used for initiating the combustion for ramjets, reaction engines and liquid fuel rockets that have carbon dioxide in it.[29]

Pros & Cons

As with any form of alternative recycling such as downcycling there are goods things and there are bad things. The good thing about downcycling is the silane is reused instead of extracting the silane from the Earth and having through the pain of purification, which all require labor, energy, and other expenses. The bad thing about downcycling the silane is ________ .

MSDS Sheet Links

Silane http://www.clean.cise.columbia.edu/msds/silane.pdf

Silicon http://dept.harpercollege.edu/chemistry/msds1/Silicon%20metal%20ScienceLab.pdf

Chloride http://www.sciencelab.com/msds.php?msdsId=9927593

Hydrogen Chloride http://www.sciencelab.com/msds.php?msdsId=9924285

Trichlorosilane http://alemis.us.airliquide.com/ChemSafe/MSDS/Image/115583_1.PDF

Silicon tetrachloride http://www.airgas.com/documents/pdf/001075.pdf

Silicon Dioxide http://www.nanoamor.com/msds/msds_SiO2_7014WJQB.PDF

Sodium Chloride http://www.sciencelab.com/msds.php?msdsId=9926835

Aluminum Chloride http://www.sciencelab.com/msds.php?msdsId=9922851

Boron http://www.sciencelab.com/msds.php?msdsId=9923126

Phosphorous http://www.sciencelab.com/msds.php?msdsId=9927697

Arsenic http://www.sciencelab.com/msds.php?msdsId=9922970

Resources

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