Dear Py De many thanks for sharing this very interesting technical question (which is really a bunch of questions) with the RG community. I must admit that I have some difficulty with your question as I don't really know what you mean by PAF. Given your scientific background, I assume that the PAF in your question stands for Plasma Assisted Fuel Processing, right? However, when you search the internet for PAF, you find more articles about "Plasma platelet activating factor" which is also abbreviated as PAF. In fact, when you check out the following link, you will find that the abbreviation PAF can have no less than 64 different meanings:

https://acronyms.thefreedictionary.com/PAF

Unfortunately I'm not an expert enough in this field to give you a qualified answer to your original question about white and gray. What I know is that the terms "white and gray biotechnology" are frequently being used. In this context please see that following potentially useful link which has been posted as public full text on RG::

White Biotechnology

Article White Biotechnology

When you search the "Publications" section of RG for the term "gray biotechnology" you will also find a good number of potentially useful articles. For example, please have a look at the following interesting paper which is also freely available as public full text on RG:

Gray biotechnology: An overview

Conference Paper Gray biotechnology: An overview

What I want to say in the end is that you will certainly get more expert answers to your interesting technical question when you specify it a little bit and explain what you mean by PAF. In any case I cross my fingers that your question will receive more qualified answers than mine.

Good luck with your research and best wishes, Frank Edelmann

Dear Py De thank you for your kind response and explanation. First of all this case shows that the use of ambiguous abbreviations can cause quite a bit of confusion. At least now it is clear what you mean. To my knowledge cryolite (Na3AlF6) and potassium cryolite (K3AlF6) are often contaminated with calcium and iron. The iron can for example be present in the form of siderite, FeCO3, which can cause yellow, brown, or even black colorations. Thus the main differences between white and gray potassium cryolite is purity. The white version is the purest material and therefore more expensive than the gray one. For a short explanation about the differences between the two please have a look at the following potentially useful link:

http://www.bestfluoride.com/case/Potassium-cryolite-gray-and-white-difference.html

However, the explanation of the origin of the different colors given here is rather ridiculous (Citation: "The difference between the gray and white of potassium cryolite is actually the difference of a color."). Thus the article should be read with some care. However, the link provides a useful statement about the different uses of the two forms of potassium cryolite (gray: aluminum alloy additives and fluxes; white: grinding wheels, soldering fluxes and related high-end applications.

I hope you will find this information useful. Good luck with your work and best wishes, Frank Edelmann

P.S. As a side aspect I would like to mention that the correct name of potassium cryolite is not "potassium aluminum fluoride" (PAF) but "tripotassium-hexafluoridoaluminate".