Aqueous Processing Project

8/8/2019 Aqueous Processing Project

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Manufacturing Solar-Grade Silicon from Metallurgical-Grade Silicon via Aqueous Processing

Matthew Gadinski, Joseph Hagan, Denton Reel, and Kaylan Wessels

Team Unicorn

MATSE 426 Aqueous Processing

April 30, 2010


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Introduction

Solar energy is a bright and shining topic in our world today. Ever searching for cleaner, cheaper

and renewable energy resources, the global economy is bound to turn to solar power as a viable

option with great potential. Silicon is a prime candidate for solar photovoltaics due to its high

efficiency with respect to cost and energy output. Despite this efficiency, the technology is

expensive to make, and is less likely it is to be used in practice with such high costs. Aqueous

processing is a feasible alternative to current thermochemical processing techniques; two

methods to be used to purify metallurgical-grade (MG) Si to solar-grade (SG) Si include acid

dissolution and electrowinning.

Silicon Solar Cell Technology

Though there are a wide range of materials that can be used for solar photovoltaics, silicon can

be highly efficient and is thus often selected as the prime candidate material for photovoltaic

applications. Indeed, the most efficient solar cell ever produced was done at the University of

New South Wales, the conversion efficiency of which was 25%.1

These cells are beginning to

fulfill their potential by approaching their theoretical efficiency of 29% energy conversion. Such

efficiencies produce a repayment of cost in as little as one year, and definitely within two.2

Silicon solar cells are produced as monocrystalline, polycrystalline, and thin-film photovoltaics.

Polycrystalline silicon solar cells are easier to grow and produce, but fall short of the efficiencies

of their monocrystalline counterparts (which boast the highest efficiencies of these three types).

Thin film silicon solar cells are being considered because they require less semiconductor

material than the other two, reducing the cost that is required to process, purify, and implement

this technology. Despite the increased affordability of this approach, the efficiencies are driven

down to no more than 10% energy conversion.2


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Engineering Considerations

There are many factors that influence why solar energy has a bright and shining future. These

factors span economical, environmental, and technological facets. Exploring these factors leads

us to the reason why pushing the boundaries of solar cell innovation is important.

Economically, solar cell technology is very attractive because of one factor: free, virtually

unlimited, constant energy. Even though our solar system has a small sun, the power that it

constantly shines on the Earth is virtually limitless within our time scales. An underlying tone of

this limitless energy is also reliability. Without relying on ores for power, very little maintenance

has to be done to keep the power supply, which also in turn cuts costs. On a personal level, home

owners who choose to install solar cells can achieve an economic independence from electric

companies, as well as sell the energy back to the grid. This makes producing personal energy

economically satisfying for personal consumers.

Solar cell technology is very attractive environmentally because it reduces the use of

combustible fossil fuels, thus reducing the emission of green house gasses. In this day and age

of global warming, technologies that are deemed to be green are extremely desirable and

marketable.

Technologically, solar cells are desirable because they allow for independence and advances in

other technological medium. With reliable, hearty solar cells, side walk lighting no longer has to

feed off of the power grid to maintain lighting over night. Also, with a compact technology,

personal homes can harvest their own natural plot of energy to provide owners with power.

The idea of a green technology also makes the investment in solar cell technology socially

desirable. With the idea of doing something positive for the global environment, through cutting


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down on emissions, the technology would be socially smiled upon. This means that people

would spend the money to install the solar cells with a driving factor of a positive social viewing.

Alternatives to Silicon Solar Materials

Silicon solar materials are some of the most popular systems for photovoltaics. Silicon solar

materials offer some of the highest conversion efficiencies for photovoltaics. One main

disadvantage of silicon solar materials, whether they be monocrystalline, polycrystalline, or thin

film, is that the refining of silicon is very expensive and energy intensive. There are many

alternative technologies that do not rely on silicon for the generation of electricity from solar

energy. These technologies typically do not offer energy conversion efficiencies as high as

silicon materials, though some are very close in performance. The biggest advantage to these

alternative materials is the lower cost, ease of manufacturing, and often more flexible

manufacturing constraints. Some common types of alternative technology systems are cadmium

telluride (CdTe), copper indium gallium selenide (CIGS), and dye-sensitized solar cells.3

Cadmium telluride is one of the most common alternative systems in the market right now.

CdTe thin films are not as efficient as many of the silicon solar technologies, but the

manufacturing cost is much lower. Because the thin film CdTe photovoltaics are so much less

expensive to produce, even though they are less efficient, they allow for more electricity to be

generated for the same amount of money. This cost per kilowatt-hour efficiency is a more

effective way to evaluate photovoltaics since for all intents and purposes the sun is a resource

that can never be consumed. Because of the lower cost of CdTe photovoltaics, it also lowers the

capital cost for consumers, which helps to encourage growth in the solar market, allowing for

further development of all of these solar technologies.3


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Cadmium indium gallium selenide photovoltaics are another thin-film technology that offers

lower cost than silicon technologies. CIGS photovoltaics, unlike CdTe cells, can actually

approach the efficiency of some silicon technologies, while still offering the cost saving benefits

of other thin film systems. These and other thin film technologies offer the distinct advantage of

being thinner, and therefore more flexible, which allow for lighter installations that need not be

flat in form. One place where this can be very helpful is in a system like the one employed by

Solyndra. Solyndra uses CIGS thin films and wraps them into a cylindrical array. The effect

looks similar to many long fluorescent tubes, and offers several huge benefits as compared to flat

plate solar systems.

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The cylindrical cells can collect sunlight from a greater range of angles, offering a lower peak

power generation, but a broader generation curve which allows for more power generation

during the morning and evening times which is beneficial to residential users who are typically

not home in the middle of the day when most solar panels are making peak energy.

Additionally, these cylindrical arrays allow for light to be reflected back up from below. This

means that roofs can either be painted with a reflective paint which helps to keep the house

cooler, or they can be fitted with a green roof which has many other side benefits, but serves to

reflect the green wavelengths of light to the cells. The cylindrical arrays also allow for better

cooling. Temperature increases are one of the main reasons for decreased efficiency in solar

cells, so this helps as well.

Manufacture of MG-Si and Purification

Metallurgical grade silicon is the lowest purity silicon grade, and is produced commercially by

carbon reduction of highly pure silica in electric arc furnace equipped with graphite electrodes.

The feeds for this reaction are often globular quartzite as the silicon source and wood, charcoal,


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or coal as the carbon source. This reaction is carried out at processing temperatures reaching

above 1900 C, producing liquid silicon metal that is 98.5% pure silicon.5

The pertinent chemical

reactions are found in equations 1 and 2.

SiO2 + C = Si + CO2 (1)

SiO2 + 2C = Si + 2CO (2)

The produced metallurgical grade silicon, though 98.5% pure still contains many metallic

impurities. The typical content of impurities for metallurgical grade silicon are found in Table 1.

Table 1. Impurities in Metallurgical Grade Silicon5

Impurity Concentration (wt. ppm)

Manganese 10-80

Chromium 40-220

Copper 15-40

Nickel 10-95

Iron 1500-6000

Aluminum 1000-4000

Calcium 250-620

Magnesium 60Titanium 120-275

Boron 40-60

Phosphorus 20-45

Vanadium 50-250

Carbon 1000-3000

Zirconium 15-25

From this table we see that iron, aluminum, and carbon are the most significant impurities found

in metallurgical grade silicon. Metallurgical grade silicon is being examined as feed stock for

production of solar grade silicon due to its inherently low cost, though purification is needed to

produce this medium purity form of silicon. For solar cells to be economically viable as an

energy alternative the efficiency of these cells must be optimized at a minimum cost.5


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The metallic impurities act to lower the efficiency of solar cells if not removed. It was found

that the presence of copper and nickel reduce the efficiency of the cell by degrading the p-n

junction within the cells reducing energy production. Titanium, manganese, vanadium, and

aluminum reduce efficiency by reducing the diffusion length while iron impurity was capable of

both. As such, for metallurgical silicon to be used as a feedstock to produce solar grade silicon

these impurities must be removed.5

Aqueous-Based Processing

The primary driving force behind developing aqueous processes to upgrade MG-Si to SG-Si is

cost. Inherently, thermochemical processes used to upgrade MG-Si to SG-Si are expensive due

to the large amount of energy to heat and (usually) create a vacuum for processing. Aqueous

processing is an attractive solution, because it usually does not require as much heat energy, nor

is a vacuum usually required.

A second attractive feature of using an aqueous processing method as opposed to using a

thermochemical process to refine MG-Si is the actual ease of extraction of multiple impurities

with greater ease. With thermochemical processing, several common impurities are not

removable using a common process such as zone refining.6

To give a better idea of the advantages of using aqueous processes as opposed to using

thermochemical processes, a comparison between zone refining and acid dissolution will be

discussed. Acid dissolution will also be explored further, later in this paper. Simply, acid is

used to dissolve a whole material, or a specified impurity. This solution can then either have the

desired material precipitated out, or impurities precipitated out under certain pH and Eh

conditions. A couple key advantages to using a process such as this are the ease of using raw

materials, lack of wasted product, and lack of geometrical conditions. Basically the only


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expense is buying an acid and a vessel to have the reaction take place in. There really is not

much requirement in the area of geometry of the material, nor is there much waste of the desired

material.7

A thermochemical process such as zone refining requires several cost intensive factors. First, the

material must already be in a useable shape to be zone refined, usually a long cylinder. Second,

there is a large amount of energy required to head the material, which often has to be heated over

several repetitions to get appropriate results. Third, there is a decent amount of waste of desired

material, which comes from all of the impurities ending in a concentrated location in the desired

material. The fifth con to using a process such as zone refining is that certain impurities are

thermally stable in Si such as phosphorous, boron, and carbon. This means that another process

has to be used to then remove these impurities.6

Alternate Aqueous Processing Routes

Figures 1-5 are Eh-pH diagrams for the silicon-oxygen-hydrogen system overlaid with respect to

various relevant elements to be removed through aqueous processing, the merits of which have

been discussed in the previous section. The figures are ordered from aluminum, boron, carbon,

iron, and phosphorus; they display the pH and electropotentials at which the aqueous, ionic and

solid species are stable. The diagrams for the individual systems have been taken from literature8

and altered to suit these purposes.


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Figure 1. Eh vs. pH diagram for Si-O-H-Al system; silicon is in red and aluminum is in black.


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Figure 2. Eh vs. pH diagram for Si-O-H-B system; silicon is in red and boron is in black.


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Figure 3. Eh vs. pH diagram for Si-O-H-C system; silicon is in red and carbon is in black.


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Figure 4. Eh vs. pH diagram for Si-O-H-Fe system; silicon is in red and iron is in black.


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Figure 5. Eh vs. pH diagram for Si-O-H-P system; silicon is in red and phosphorus is in black.

Purification of MG-Si via Acid Leaching

Literature review has shown that acid leaching of metallurgical grade silicon is an effective and

scientifically accepted means of silicon purification. This technique is dependent greatly on the

size of particles introduced to the acid bath. Smaller particles allow for greater amounts of

impurity dissolution, especially with regard to intermetallics and phase boundary inclusion.9

This is of course the product of increased surface area correlating with increased exposure of

impurities to the acid leach environment. This is highlighted in Figure 6.


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Figure 6. Effectiveness of an acid leach with respect to particle size for Fe, Ca and Ti

impurities.7

It was found that extraction of metallic impurities by sulfuric, nitric, and hydrochloric acid

produced purifications of 50%, 55%, and 85%, respectively.7

Though hydrochloric acid

produced the greatest result of these acids studied, addition of small amounts of hydrofluoric

acid was capable of metallic impurity extraction in excess of 95%.7

Figure 7 shows the effects of

hydrofluoric acid concentration versus major impurity concentration at a constant hydrochloric

acid concentration.


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Figure 7. HF concentration vs. impurity concentration for a) 16 hours at RT; b)-2 hour at 80C.7

From Figure 7 it can be observed that calcium and aluminum concentration drop, reaching a

minimum value at a hydrofluoric concentration of 2.5%, after which the concentrations begin to

climb again. A possible explanation is insoluble fluorides that may form at greater hydrofluoric

acid concentrations.7 The increase in dissolution from the hydrofluoric acid is a product of Fe-Si

and Fe-Si-Ti species that are not leached by hydrochloric acid, but leached readily in

hydrofluoric acid solutions.10 Many of the minor alloying elements are also removed by this

method. Figures 1 through 5 of the previous section are Pourbaix diagrams of the overlapped Si-

O-H and impurity elements; it is seen that dissolution occurs at Eh of 0.

The typical impurity analysis of post acid leached metallurgical grade silicon is found in Table 2.


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Table X. Acid Leach Results5

Table X shows that for all metallic impurities removal in excess of 90% is possible showing the

efficacy of this method; it does, however, fail in the removal of copper, boron, and phosphorus.

Since the concentrations of phosphorus and boron are well above the levels needed of doping of

solar grade silicon for solar cell use this method is not sufficient alone.5

To remove elements not removed by acid leach a method of reactive gas blowing may be

employed. The method proposed here is to blow Cl2 gas through a metallurgical grade silicon

melt.5

Figure 8 shows the standard free energy of formation of chlorides.


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Figure 8. Standard free energy of formation of chlorides.5

The low melting and boiling points of the produced chloride impurities allow for their

volatilization and removal from the melt. Large amounts of C, Ca, Mg, Al, B, P, and Ti are

capable of being removed by this process. In combination with the acid leach solar grade silicon

can be produced with the remainder of the impurities being removed.5

The proposed method of metallurgical grade silicon purification to solar grade silicon is found in

the flow sheet in Figure 9.


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Figure 9. Reaction flow diagram for purification of MG-Si to SG-Si through acid dissolution.

The reactive gas blowing can be done before prior to cooling the metallurgical grade silicon. In

this way the cost of reforming the melt can be avoided. Once cooled from the melt,

comminution must be done to form consistent particles for the acid dissolution that follows. The

dissolved impurities can be removed by solid liquid separation leaving the solar grade silicon.

Purification of MG-Si to SG-Si through Electrowinning

A second aqueous processing method to be considered is electrowinning, also known as

electrodepositing, the aluminum and iron from MG-Si to produce SG-Si. The pertinent reactions

are shown and described below.


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Firstly, aluminum must be dissolved into solution out of the metallurgical-grade silicon and

deposited onto an aluminum electrode (Equation 3). This equation is divided into two steps

Equations 4 and 5. For Equation 4, the pH must be less than 5 for dissolution; Equation 5 is the

reduction of aluminum ions into solution onto the corresponding electrode.

Al Al3+

(aq) Al(s) (4)

Al + 3H+ Al

3++ 3/2H2 (5)

Al3+

+ 3e- Al(s) (6)

Using the same conditions as the Pourbaix diagrams from a previous section as well as standard

cell potentials11

and equations12

, the following electrolyzing voltages for the aluminum electrode

can be determined to accompany the reaction in Equation 6.

E= -1.66 V 11

E = E + (2.303RT/zF)log[Al3+] 12

T = 298.15 K z = 3 [Al3+] = 1X10-10 M

Ecell = -1.86 V

Minimum electrolyzing voltage =1.86 V

Next, carbon must be evaporated out of the silicon to form carbon dioxide gas, as shown in

Equation 7.

C(s) + H+

+ OH- CO2 + H2 (7)

Much like the aluminum, iron can be removed for electrowinning onto its own electrode

(Equation 8).

Fe Fe2+

Fe (8)


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This summarized equation actually contains two steps, shown in Equations 9 and 10. Equation 9

is the dissolution of iron from the silicon (for which the pH and Eh must both be less than 9) and

Equation 10 shows how the ions are deposited onto an iron electrode.

Fe + 2H+ Fe

2++ H2 (9)

Fe2+

+ 2e- Fe (10)

Also similar to the process used for the aluminum, the following electrolyzing voltages for the

iron electrode can be determined to cause the reaction in Equation 10.

E= -0.41 V 11

E = E + (2.303RT/zF)log[Fe2+] 12

T = 298.15 K z = 2 [Fe2+] = 1X10-10 M

Ecell = -0.706 V

Minimum electrolyzing voltage = 0.706 V

Phosphorus and boron are desirable in SG-Si and do not precipitate out during this process. This

is useful as well as convenient as their stability is very similar to that of silicon. Figure 10 below

is a flow diagram of the electrowinning process as devised for the conversion of metallurgical-

grade silicon to solar-grade silicon.


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Figure 10. Flow diagram for conversion of MG-Si to SG-Si through electrowinning.

Economic Analysis

One of the major drawbacks of refining metallurgical grade silicon to solar grade silicon via

thermochemical processing is that it is very energy intensive. The tremendous amount of heat

that is required makes the cost of this process high, which in turn creates high costs for the end

products: the silicon photovoltaics.

Aqueous processing of metallurgical grade silicon offers a distinct advantage in this regard. By

doing the majority of the processing at room-temperature conditions, all of the energy that was

used for generating heat is not needed. The steps that have been outlined in our processes

require very little additional energy be input into the system. Since most of them are simply

aqueous steps, the most they would typically need is the application of an electrical bias, but


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never a very great potential. There are additional steps of mixing and separating, but the energy

usage for these is minimal in comparison to the savings. The only major place where a large

amount of energy would be consumed is in the roasting step, which is still a thermochemical

process. This step is short, however, so should not contribute nearly as much energy

consumption as would the traditional all-thermochemical processing route.

The chemicals needed for the aqueous steps are not particularly rare or exotic, so should not

present a large cost. Most of the chemicals are generic, and should not keep the process from

being cost-effective for use in industry.

Conclusions

Silicon solar cells have great promise for the energy market, but the thermochemical processing

techniques currently used to purify silicon for this application are expensive and energy

intensive. Aqueous processing techniques including electrowinning and acid dissolution provide

a real alternative to these expensive techniques and may be used for the purification of

metallurgical-grade silicon to solar-grade silicon with the right flow of processes.


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References

1 University of New South Wales, Highest Solar Cell Efficiency Ever Reached, Science Daily,

24 Oct 2008, 26 Apr2010.

2 M.A. Green, Third generation photovoltaics: solar cells for 2020 and beyond,Physics E:

Low-dimensional Systems and Nanostructures, 14 [1-2] 65-70 (2002).

3D. Bonnet, Chapter 6: Cadmium Telluride Solar Cells, Clean Electricity from Photovoltaics,M.D. Archer and R. Hill, Eds. Imperial College Press, 2001.

4 Solyndra - The New Shape of Solar, Solyndra, Inc. 2010, < http://www.solyndra.com/> 29

Apr 2010.

5 B.R. Bathey, M.C. Cretella, Review: Solar-grade-silicon,Journal of Materials Science, 17[11] 3077-3096 (1982).

6K. Morita and T. Miki, Thermodynamics of solar-grade-silicon refining, Intermetallics, 11

[11-12] 1111-1117 (2003).

7J. Dietl, Hydrometallurgical purification of metallurgical-grade silicon, SolarCells, 10 [2]145-154 (1983).

8N. Takeno, Atlas of Eh-pH diagrams, Geological Survey of Japan Open File Report No. 419,

National Institute of Advanced Industrial Science and Technology (2005)

9X. Ma, J. Zhang, T. Wang, T. Li, Hydrometallurgical purification of metallurgical gradesilicon,Rare Metals, 28 [3] 221-225 (2009).

10I.C. Santos, A.P. Gonalves, C.S. Santos, M. Almeida, M.H. Alfonso and M.J. Cruz,

Purification of Metallurgical Grade Silicon by Acid Leaching Hydrometallurgy, 23 [1-2]237-246 (1990).

11Department of Physics and Astronomy, Table of Standard Electrode Potentials in Aqueous

Solution at 25C, Georgia State University, 30 Jun 2007, 28 Apr 2010.

12K. Osseo-Asare, Aqueous Processing of Materials:AnIntroduction to Unit Processes with

Applications to Hydrometallurgy, Materials Processing, and Environmental Systems, to bepublished by Academic Press/Elsevier (2006).

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