Thanks Rich. The screenshot makes it perfectly clear which item to use and how to use it. It also works with Windows 10. Learned something new. 
After colour desaturation, this is a way to remove light gray from white backgrounds in black and white pictures without losing much detail. In my case I used a curved line from 0,0 to 245,255 and the result is pure white (#FFFFFF). I had to do 1243 pictures. With gimp and batcher : happy days.
After using Mono Mixer the backgrounds of some images were not white, so I tried the curves in Gimp 3.0. 4 with the curved lines 0, 0 and 245, 255 and the background was pure white. I tried these curves with Batcher and the background of most images was pure white (those were after using Mono Mixer), some were not (those were not after using Mono Mixer). So no happy days. Apparently Batcher does not work with curves in my case. Two examples that don't work :
After colour desaturation, this is a way to remove light gray from white backgrounds in black and white pictures without losing much detail. In my case I used a curved line from 0,0 to 245,255 and the result is pure white (#FFFFFF). I had to do 1243 pictures. With gimp and batcher : happy days.
After using Mono Mixer the backgrounds of some images were not white, so I tried the curves in Gimp 3.0. 4 with the curved lines 0, 0 and 245, 255 and the background was pure white. I tried these curves with Batcher and the background of most images was pure white (those were after using Mono Mixer), some were not (those were not after using Mono Mixer). So no happy days. Apparently Batcher does not work with curves in my case. Two examples that don't work :