Occurrence State and Dissolution Mechanism of Metallic Impurities in Diamond Wire Saw Silicon Powder

[NNadir]

The paper I'll discuss in this post is this one: Occurrence State and Dissolution Mechanism of Metallic Impurities in Diamond Wire Saw Silicon Powder (Yang, Wan, Wei, Ma, Wang, ACS Sustainable Chem. Eng. 2020, 8, 33, 12577–12587).

Although it has no bearing on the scientific integrity paper, it begins with a statement that is demonstrably untrue, yet widely believed, one of the two statements to which I have added bold below in the introductory text from the paper. Here is the introduction:

The growing concerns of the traditional energy crisis, the global energy shortage, and pollution associated with the use of traditional energy sources have gradually led to the replacement of the conventional fossil fuel-based energy structure with a renewable energy-based structure.(1?3) Consequently, with the rapid development of the photovoltaic (PV) industry, the demand for solar-grade silicon has increased sharply because of its wide applications as the predominant photoelectric conversion material.(4) Si wafer-based solar cells are the main solar-cell products in the PV industry, accounting for 95% of the total production in 2017.(5,6) The current silicon wafering technology, namely, diamond wire sawing (DWS), has the advantages of a higher feed rate, low surface roughness, and a clear operating environment.(7,8) Diamond wire saw silicon powder (DWSSP) waste is a material generated during DWS. It is a micrometer-sized powder containing numerous metallic contaminants.(9) DWSSP is also considered to be a serious threat to the workplace due to the potential hazards of exothermic oxidation, its highly inflammable properties, and the creation of dust pollution.(10) Like for red mud, the conventional treatment methods for DWSSP, including landfilling and stacking, threaten both human health and the ecosystem due to the complex nature and substantial volume of DWSSP.(11) It is well-known that the selective recovery of metal in multicomponent systems is highly desirable for sustainable metal resource recycling and refractory solid waste treatments,(12) and it is important to develop environmentally friendly processes for the valorization of industrial residues and end-of-life products.(13) Consequently, there is no doubt that the recycling of silicon is both environmentally and economically favorable.(14)

It is simply not true that so called "renewable energy" has replaced, or is replacing, gradually or otherwise, dangerous fossil fuels to any extent at all. The use of dangerous fossil fuels has continuously been growing at an alarming rate since the 20th century through the 21st. In many of my posts here I post a table I prepared from recent editions of the International Energy Agency's World Energy Outlook, which reports on the actual energy consumption of the entire human race.

Here is a table of sources of energy taken from the International Energy Agency’s 2017, 2018, and 2019 Editions of the World Energy Outlook, the last three editions published:



(In this table, I have converted MTOE in the original tables in to the SI unit exajoules in this text.)

World Energy Outlook, 2019

World Energy Outlook, 2018

World Energy Outlook, 2017

Of course, every edition of the World Energy Outlook that I have in my files for this century - probably between 10 and 15, I'm too lazy to look right now - talks about the projected growth of so called "renewable energy" in "scenarios."

I have a few editions from the 20th century, less slick, but more realistic.

The fastest growing source of energy in the 21st century has been coal. For the last year for which data is available, despite all sorts of claims that coal is dying, 2017-2018, the use of coal grew. Mostly this has been driven by China, where they are still debating adding even more coal plants.

Coal grew faster - at 2.97 exajoules for annual growth that year - than did "other renewable energy" which includes solar, wind, geothermal and tidal combined, which grew by 1.63 exajoules in that period.

Despite statements by politicians I enthusiastically support about how many jobs putative "clean energy" will create, it has proved to be the case that the overwhelming majority of the solar cells manufactured, and pretty much everything else, on this planet are manufactured in China.

One reason of course, that China is able to manufacture most of the world's "stuff" is lower worker safety standards, although as the second bold statement in the introduction above.

The second statement in bold in the introduction above lists some of the hazards to workers and to the environment connected with solar PV cell manufacture, these connected with diamond wire saw silicon power, abbreviated DWSSP throughout the rest of the paper. The hazards listed speak for themselves.

These are not, of course, the only hazards associated with PV manufacture. Some are concerned with the fact that the reduction of silicon dioxide - the familiar chemical form of almost all the silicon on Earth, and all naturally occurring silicon - is by heating silicon with carbon. The main source of carbon on this planet is coal.

Silicon is purified by converting it into the liquid trichlorosilane which is then distilled. In 2014, a trichlorosilane distillation plant in the Mie Prefecture of Japan exploded, killing 5 workers instantly and injuring 13.

Of course, industrial accidents happen all the time, and domestic energy accidents do as well - every once in a while one will read about a house exploding and killing people. (Such an explosion killed people near where I live a few years back, and I believe I saw one reported in the media recently.) None of this is remarkable except for maybe this: The trichlorosilane explosion in Mie Prefecture instanteously killed more people than radiation from the destroyed Fukushima nuclear reactors instantaneously killed (zero), and neither event killed as many people as seawater killed.

We couldn't care less how many people are killed by seawater of course, but we never forget if people are merely exposed to industrial radiation while ignoring the fact that people are continuously bathed internally and externally by natural radiation.

Odd world; crazy world; disturbingly crazy world; no wonder it's dying.

The authors of this paper propose to mitigate the hazard of diamond wire saw silicon powder, something they propose to do by removing metals in the powder using strong acids.

To recycle silicon derived from DWSSP, increasing the yield of high-purity silicon is an imperative issue that needs to be resolved.(15) The metallurgical route has become a popular method for silicon purification due to its low cost and environmental friendliness associated with its sustainable process.(16,17) As leaching is the primary and essential stage for the recovery of metals in comparison with pyrometallurgical treatments,(18,19) the metallic impurities present in DWSSP could be removed with a cost-effective compatible acid leaching pretreatment process. Since 2019, several methods have been presented for the purification and recovery of silicon from DWSSP. It is evident that this topic has become a research hotspot and has garnered significant interest from researchers.(20) The authors’ previous studies have investigated the kinetic mechanism of Al removal via HCl leaching,(21) the dissolution and mineralization behavior of metal elements,(22) and the Si core–SiO2 shell structure.(23) Moreover, the kinetic mechanism of iron removal with H2SO4 leaching,(24) multicomponent acid purification,(25) the combined process of slag treatment and acid leaching,(26) and acid leaching followed by induction furnace melting(27) have also been investigated by other research groups...

...In this study, different leaching tests were conducted to reveal the dissolution mechanism of metallic impurities. The common impurities present in DWSSP were sufficiently removed, and two occurrence states of impurities were defined based on their different dissolution behaviors. Moreover, the corresponding dissolution mechanisms were derived...

An issue in these dissolution studies is the formation of silicon dioxide, an unacceptable impurity in solar cells and other silicon devices. This is discussed in the full paper.

Pictures from the paper:



The caption:

Figure 1. SEM characterization of the DWSSP elements mapping: (a) Micro-images for DWSSP? and (b–g) mapping images for different metallic impurities.

It is important to note that the elements discussed, with the exception of nickel, are not appreciably toxic, however the mode of contact is important. This is dust, and as such the tissue in which it is most likely to be deposited is lung tissue. I personally do not know off hand the effect of reduced silicon in lung tissue, but silicosis, a disease associated with the deposition of silica (silicon dioxide is well known. I suppose one could look up data about these issues, whether something rather like black lung disease is associated, but frankly, I don't think we'd care. Solar energy is "green."

Leaching with hydrochloric acid:



The caption:

Figure 2. Relationships between Al removal and leaching parameters: (a) HCl concentration, (b) leaching temperature, and (c) leaching duration.

Then they get a little nastier, adding an acid mixture close to aqua regia and the scary acid HF. HF dissolves silicon, and in fact, silicon compounds like silica and silica derived products like glass. Presumably the idea is to partially dissolve the silicon to get at the impurities.



The caption:

Figure 3. Impurity removal efficiency of different acid regeants in leach II: (a) the removal efficiency for the different lixiviants and (b) the effect of the HF concentration in the mixed acid solution on the removal efficiency.

The effect on particle size and the changes as suggested by X-ray diffraction.



The caption:

Figure 4. XRD patterns of different materials for the two-stage leaching process (a) the XRD results for DWSSP? after different leaches and (b) the particle size distribution for DWSSP? after different leaches.

Silica is an impurity to be avoided of course; the whole point of all this processing is to convert impure silica into pure silicon. This graphic refers to that side product:



The caption:

Figure 6. Silicon and silicon dioxide peak obtained by XPS analysis for the two-stage leaching process: (a) the XPS spectra for DWSSP? after different leaches and (b) the average SiO2 thicknesses for DWSSP? after different leaches.

A graphic on the mechanism:



The caption:

Figure 9. Schematic diagrams of the dissolution mechanism and occurrence state of the metallic impurities in DWSSP during the two-stage leaching process.

A cartoon on the proposed process:



The caption:

Figure 10. Recommendation route for silicon recovery from DWSSP

The author's conclusions:

In this study, the occurrence state and dissolution mechanism of metallic impurities in DWSSP during the acid leaching process were determined. The main findings were obtained as follows.

1. The combined process of two-stage acid leaching with 4 M HCl and 2 M HCl + 2.5 M HF was proposed to facilitate the removal of metallic impurities from DWSSP.

2. The leaching results indicated that there are two occurrence states of metallic impurities in the DWSSP. The first type can be easily dissolved in HCl solution, while the elimination of the second type of retained impurities depends on the disintegration of the SiO2 shell, as the SiO2 layer creates a barrier to the dissolution of this type of impurity. For this reason, the removal of metallic impurities can be facilitated via HF addition.

3. The occurrence state of metallic impurities in DWSSP demonstrated the retention of metallic impurities caused by the propagation growth process of the amorphous SiO2 shell. This provides evidence to support the recommended route for the recovery of silicon from DWSSP with an effective acid leaching flow design.

Well then...

I feel like Debby Downer sometimes...

There is a wide spread belief that so called "renewable energy" will save the world. It is of course a kind of cant among us in the Democratic Party, some of us advocating for the "Green New Deal" which is certainly popular among many of us. Regrettably, I argue that the "green new deal" is neither "green" (where "green" is a code word for "sustainable" and "environmentally benign" and "safe" ) nor "new" nor much of a deal, to be honest.

The problem with so called "renewable energy" isn't silicon dust in people's lungs. That's small when compared with the vast death toll associated with the normal use of dangerous fossil fuels. The problem is that it's not working, not even close to working, to address climate change or to ameliorate the vast death toll associated with dangerous fossil fuels. This remains true despite vast cheering, incredible sums of money and huge quantities of matter thrown at it in an almost sacrificial ritual that cannot be stopped because for humanity it has always been an issue that faith can be stronger than reality until at least, reality bites.

Speaking only for myself, I believe reality is biting, big time.

The defining point of my liberalism, and I suspect the liberalism of many others, is that facts matter.

It is a fact that the huge investment in so called "renewable energy" has done nothing to address climate change. Climate change is getting worse, not better. The number of people being killed by dangerous fossil fuel waste, aka "air pollution" remains where it has been for many years, around 19,000 people a day, far more than Covid kills.

A fact..

A fact...

We in the Democratic Party, of course, very much - and rightly so - want to save the world, not only from fascism, because in a fascist world politics trumps science, but also from environmental disaster.

It is not enough to simply push the bad guys out of the way. It is essential to govern well.

Governing well does not include advancing programs that do not work.

In a way, I regret the Republican Party's suicide and its abandonment of principle in favor of personality. A principled opposition makes one stronger, not weaker, because to succeed, one must have one's ideas challenged.

I implore everyone to keep this in mind: Unchallenged ideas don't work very well as ideas, nor, for that matter, as policy.

I wish you a pleasant and relaxing Sunday in these challenging times.