General comments/chat, page-256

Far too early to determine. Theoretically better.

This is FYI's flowsheet and it clearly shows the 3 purification stages:

http://www.fyiresources.com.au/media/files/20180918-FYI-optimised-HPA-flowsheet.pdf



This article is the original purification process defined by Hoffman in 1946:

https://nvlpubs.nist.gov/nistpubs/jres/37/jresv37n6p409_A1b.pdf

Stage IV is the part to focus on. It's the leach part of the process with inputs being Kaolin and Hydrochloric Acid. This needs to be heated and stirred. This process takes several hours for each purification stage and power needs to be applied for the heating and stirring.

So it is perhaps power and acid (although the acid appears to be recovered and reused) that are key cost inputs as well as sustaining CAPEX for each of the purification stages ( labour for equipment maintenance, spare parts, etc ).





This article also summarises the Hoffman process:
https://pdfs.semanticscholar.org/7ec0/7cfb2c9d0a9e39cb515d174e72e3770d1dfd.pdf

1. roasting clay at about 700 ◦C;
2. digesting the roasted product with dilute hydrochloric acid;
3. filtering to separate the insoluble siliceous matter from the solution containing the aluminum and soluble impurities, such as iron and alkali salts;
4. concentrating the solution;
5. precipitating the aluminum as the hydrated chloride from the concentrated solution by adding hydrochloric acid gas;
6. removing the crystals of hydrated aluminum chloride;
7. washing the crystals to remove adhering impurities;
8. calcining the hydrated chloride to obtain alumina; and
9. recovering hydrochloric acid from the waste products at the end of the process.

The article has a nice flowchart: