How I chemically remove scale from steel ...the clean way recycling chemicals

Many videos show how to remove mill scale by pickling metal, but few to none show how to deal with the spent chemicals. In this video I address this problem and show the procedure I used to chemically remove the mill scale from steel generating little waste and with some ideas on how to recycle the acidic liquor or to make it safe and easily disposable.
When the mill scale is consistent and the steel is meant to stay indoor, the removal of the scale can be safely disregarded. This was the case when I made the frame of my hydraulic press that you can see here:    • Hydraulic Press (brake) DIY - 12 ton??  . But in other cases it is really necessary to remove it.
The mill scale is a mix of various iron oxides that adhere to the surface of the metal forming a tough layer. Mechanical methods or heating can be used to remove the scale, but they may sport some undesirable drawbacks. No method is perfect but pickling is quite effective particularly on smaller parts and where no deformation and thermal stress is required.
So this video focuses on simple chemistry, applied to process mild steel in metalworking.
I just followed a class in chemistry, but I'm not a chemist therefore, even though I felt the need to explain the reactions showing the related formulas, I hope no major errors slipped in the show.
CAUTION-DISCLAIMER
This video is not meant to be a tutorial, it just shows my experience and experiments.
Working with chemicals requires precautions: always wear safety goggles, gloves and wear proper clothing to protect yourself. Wear closed shoes. Remove rings and jewels. DO NOT eat or drink while working with chemicals. Keep children and animals out from the area. Some chemicals may generate very harmful fumes so good ventilation is of paramount importance, and sometimes still not enough requiring a specially designed fume hood.
Also some chemicals may unexpectedly react with the materials the furnitures are made.

Credits for photos of oxides:

By http://en.wikipedia.org/w/index.php?t... - http://en.wikipedia.org/wiki/Image:Ir..., Public Domain, https://commons.wikimedia.org/w/index...

By Benjah-bmm27 - Own work, Public Domain, https://commons.wikimedia.org/w/index...

By Leiem - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index...

Other images shown in the video, when not proprietary, come from public domain.