(This is a continuation from the previous post)
Part 2: 2-stage fixing with replenisher
This is about how a 2-bath fixer works with replenishment. It's a continuation from my prior post. Same disclaimer as before: it's to help gain an understanding, and has no real practical use. If you don't care how it works, it's probably not worth your time to read.
First, the conventional 2-bath fixer as per the attached sketch, "Paper/Solution Carryover."
Solution Carry-over ok.jpg
Obviously each solution is carried over into the following - stop bath into fix-1, fix-1 into fix-2, and fix-2 into the wash water. The normal method is to use the standard total fix time, half of it in each fix tank. The general idea is that fix-1 does the bulk of the fixing, and can build up a higher silver content; fix-2 maintains a lower silver content, so can do a good job of finishing up the fixing. Whenever the fix-2 silver content reaches a certain level, it is moved to the fix-1 position, and fresh fixer goes into the fix-2 position. (The used fix-1 can go to silver-recovery.)
If the amounts of carry-over are all similar, then the two fix tanks should keep the same volume. However the chemical makeup will gradually change. Here's a list of the main things happening in each tank:
- is diluted by carry-over stop bath
- pH is lowered by same
- builds up higher silver concentration (most fixing is done here)
- builds up a lower silver concentration (virtually no fixing is done here, but there IS some carryover of "dirty" fixer from fix-1)
- eventually becomes diluted and has pH lowered as a result of carry-over from fix-1.
The next sketch, "Replenishment Scheme," shows how this could be done with trays.
Replenishment Method ok.jpg
It's more efficient to do it in th order shown.
1) Remove some fix-1, save it for later silver recovery. (The amount removed should be the same as the replenisher addition. Something like a "turkey baster," a plastic tube with a rubber suction bulb on the top, might work well.)
2) Remove some fix-2, placing it into the fix-1 tank.
3) Add some replenisher to fix-2.
A summary of the fluid flows is this: the paper-carry-over path tries to degrade the fixer, starting in fix-1 and gradually affecting fix-2. The replenishment scheme tries to restore the fixer, starting in fix-2 and gradually being carried over into fix-1.
At the same time, our ultimate goal is to simultaneously get high silver conentration in fix-1, and low silver concentration in fix-2. High silver in fix-1 means we are using a relatively small amount of fixer, so fixer costs are low. Low silver in fix-2 means that the paper is well fixed, and simultaneously, a smaller amount of silver is being lost to the wash water.
We get closer to this goal when the replenishment rate is high compared to solution carry-over. If the opposite happens, where carryover is high compared to replenishment, the two silver concentrations will be more similar. The extreme case of this is when the replenishment falls to zero - that is, using a 2-bath system with no replenishment. Even so, this is still an improvement over a single-bath fixer.
We could improve the situation further by squeeging of the paper before advancing it to the next tank; this is about as far as we can go with these schemes. If more improvement is desired, adding on a third fix stage is an option. The trick is deciding how far you should go - if the costs of handling the material are greater than the savings then... why do it? It's the same situation as seeing a penny on the ground. If you pick it up, you definitely have more money; the question is, was it worth your effort to pick it up?
I hope at least someone finds some value in this post.
Last edited by Mr Bill; 03-23-2013 at 06:24 AM. Click to view previous post history.