Rafal had asked, in a different thread, for more information on replenishing a fixer; this is my attempt to explain it.
What this is, and who should read it (or not): I wrote this as a primer on how a replenished fixer system works. If you want to understand this sort of thing, you may find it interesting. If you don't care, then it's not worth your time; there is nothing directly useful here. At best, you might read replenisher instructions elsewhere, and be excited to say, "Oh! Now I understand why they say to do that." I wish I could have made it interesting, but I hope I made it understandable.
I wanted to just explain how a two-stage fix system could be replenished, but after trying various approaches, I couldn't come up with a straightforward way to do it. So I decided to try it in two parts - the first is a primer on how replenishment works in a single tank. The second part will go into a 2-stage fixer system which gets more complicated because the tanks interact with each other.
Part 1, single tank fixing:
Replenishment of a single fixer tank is fairly simple to understand. When the paper goes into the fixer tank it also brings along some liquid from the previous tank. So the fixer gets slightly diluted from this "carry-in". When the paper is removed from the fixer, it takes a bit of the fixer ("carry-out") into the following tank (when I say "tank" it can also mean tray).
So two things happen, aside from the actual fixing operation: first, the fixer becomes slightly diluted, and second, some of the active fixer (and silver) is lost to the following tank. If the carry-in and carry-out are roughly balanced, the tank volume stays the same.
If we don't replenish, here's what CAN happen: the silver will gradually build up to some high level (which it cannot exceed because of the continuous dilution due to solution carry-in), and the fixing agent will gradually diminish in concentration (because of the dilution). The missing fixer (including silver) has gone into the following tank; if that tank is wash water to be discarded, all this silver will be lost.
In reality, the extreme situation above never happens because we decide when the fixer has been used enough; we dump it into the silver-recovery tank (or whatever else we choose to do), then we start again with fresh fixer.
If we DO use a replenisher, here's how it should work. Ideally the replenisher formula is overconcentrated slightly, just enough to overcome the dilution effect of the carry-in solution. Thus the fixer will always remain at normal strength. However, the volume will increase, so we should remove roughly the same volume as that of the added replenisher. (It should go into the silver-recovery tank.)
The ideal replenisher, when combined with the carry-in, would dilute the tank's silver concencentration to a specified aim point. Fortunately fixer is not very critical as to it's exact chemical makeup, so there is a lot of leeway to use whatever replenishment rate one needs.
A summary may be in order here. The main difference between a batch vs replenished system is that the batch system is always changin; it starts out "clean" and gradually becomes both diluted and exhausted. At which point it is dumped and the cyle restarts. A replenished system reaches an equilibrium, then stays there, potentially forever. In both cases, silver-bearing fixer is lost (via carry-out) to the following solution; the loss in volume is equal in both cases; the loss of silver is directly proportional to the silver concentration in the fixer tank.
*NOTE 1: there are actually a couple other things going on, which can be significant or not, depending on how you do things: evaporation will tend to concentrate the solution, and some of the preservative will be used up (typically sulfite oxidizes to sulfate).
*Note 2: to put the amount of dilution into perspective, if we use a 500 ml tank (1/2 liter; about 1/2 quart), and the carryover from a sheet of paper is 15 ml, each sheet of paper dilutes the fixer to about 97% of it's prior concentration. If you ran, for example, 5 sheets, the concentration would fall to (0.97 x 0.97 x 0.97 x 0.97 x 0.97) = 0.86, etc. (I am making up the carryover numbers; it could vary a lot.)
I think it's worth mentioning that this single tank system has two competing "goals." For low fixer costs, we want to use the heck out of our fixer, so that the silver reaches a high concentration. But for best print life, we want to keep the silver concentration below some specified limit (the reasons for this are discussed in other threads).
If your effluent is regulated, other issues come into play - the more you let your silver spread out, the more costly it's gonna be to get it back later.
Next will be part 2, covering a two-stage fixer with replenishment.
(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."
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.
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.
Just for fun (I had time on my hands), I made a crude spreadsheet model to compare how much paper could be processed in three configurations. They are single tank fixing and two-tank fixing (both with and without replenisher).
I made various assumptions regarding how much solution carry-over there is, how much silver is in the paper, etc., etc., so the results are not necessarily valid elsewhere. Consequently, I won't report the actual calculated capacity, but rather just compare the three process configurations.
I set the capacity limit where the final fix tank got to 1/2 gram/liter of silver.
A) Single tank, 1 liter, fixes one baseline unit of paper.
B) Two-stage fix, NO replenisher (1/2 liter in each tank = 1 liter)
- fixes 2.5 baseline units of paper.
C) Two-stage fix, WITH replenisher (based on steady-state replen rate)
- fixes 4.0 baseline units of paper (per liter of replenisher)
note: I'm underrating case "B" since the "exhausted" fix-2 still has more capacity in the fix-1 position. I just don't know a graceful way to put numbers to it.
Bill, thanks for sharing your explanations, and for putting the time into making an illustration. They make perfect sense to me. The detail would be, however, in the actual concentrations. If fix 2 stays below 0.5g/l of silver, and fix 1 below 2g/l, ideally below 1.8g/l, always, and with no significant, gradual buildup of iodide, or pollutants that reduce fixer's efficiency and effectiveness—as far as I know that is part of the reason Kodak's two-bath system requires you to throw out the second bath every 5th rotation—then, I think the system would work. I realise, that Ilford publish replenishment data for their Rapid Fix, however, I thought it was designed for a system with silver recovery, rather than a system that works on the gradual carry-out alone.
Have you practiced this approach? Did you gather data on residual silver and thiosulfate compounds left in paper? Other than that, I wonder if it would be convenient enough to replace the existing approaches, but that is not the point—we are discussing the idea itself, after all, at the moment.
I will do more thinking, and I will post again. For now, I am (happily) stuck in the darkroom for another week, then travelling for three more. Thank you for taking the time to explain your thoughts.
Hi, you're welcome, I'm glad it was understandable.
My experience in these areas is mostly with color neg and print, in what you might call industrial-scale processing. In RA-4 process, regular fixer is not used on paper, rather bleach-fix is the preferred treatment. Anyway, similar modeling methods have been pretty reliable predictors. We would typically only screen results when a process was new, or for new materials, but a couple times a year we'd run "typical product" samples for image stability tests.
I doubt you'll have any problems with iodide in B&W papers, but I'm not certain. It's never an issue with color papers; replenished regeneration loops don't have any trouble with high reuse rates. Color film is a different story - multi-stage replenished fixer works fine, but any significant reuse (desilvered and regenerated) slows down the fix rate drastically (yep this is practical experience, it's presumably due to iodide build-up).
I'm not familiar with Ilford's replenishment method, so can't comment. But in general, I think one needs a certain amount of volume for replenishment to be cost effective, after that, it's got a lot of advantages.
Regarding Kodak's instructions to periodically discard (in 2-stage fixing), I've never understood why. In my experience, Kodak's info is pretty reliable, but they seem to prefer to err on the conservative (safe for the customer) side. So possibly they came across some odd customer situation and made the universal rec to discard so as to head off possible problems. But I'm just guessing.
Hope you have a good trip (includes G.E. House, as I recall).
I remember reading this recommendation in Haist's volumes on photographic processing. The rationale behind it was that fixing can suffer from impurities and processing errors. Note that with a two bath fixer setup every impurity or other problem in bath 1 gets carried over into bath 2 during use, so it will stay in the process forever, unlike in single bath fixing where all used fixer is tossed out regularly. A fresh setup after a certain number of reuses made sure that these issues don't stay around forever and, even more important, don't accumulate up until they become obvious.
Originally Posted by Mr Bill
Hi Rudi. On the surface, that sounds reasonable. But from another viewpoint, you might consider the silver to be an impurity, yet its concentration doesn't keep building up (it would if we didn't shift and dump tanks). If the impurities come from the paper, they ought to sort of track with the silver concentration.
Another way to consider this is to ask, "What could possibly cause a buillt-in impurity to become more concentrated?" Provided it doesn't settle out or stratify, evaporation is the only possibility I can see. So, aside from a slight amount of evaporation, it doesn't seem like any built-in impurity CAN increase in concentration.
A not-so-obvious way in which the impurities are removed is via solution carry-over to the wash.
Anyway, I'm not convinced that this is a plausible reason. Maybe I'll try calculating it out later tonight.
It is not only the buildup of silver in the fixer that is of concern but also halide ions such as chloride, bromide and iodide. Iodide is a particular problem since it causes the fixing bath to exhaust more rapidly. Tmax and delta type films contain more iodide than do conventional emulsions. In the past when papers contained heavy metals like cadmium their builup was also a problem. Carryover of developer by insufficient time in the stop bath can also lead to staining in prints. Most photographers view fixing as a simple and straight forward process which it is not.
I didn't get through it last night, but didn't see anything that seemed like a problem.
Originally Posted by Mr Bill
This evening, I tried a different version - how much would the carried-over stop bath concentrate in the fixer? (One could consider any contaminant in that tank to behave in a similar manner.) Result: This does not cause problems either. The unwanted stop bath contamination (in both fix baths) initially builds up fast, but by the 4th and 5th cycle (of moving fix-2 into the fix-1 position) the rate of change has almost stopped.
So I still do not see a plausible reason for the periodic dump and restart of the system. (I'm not recomending to disregard Kodak's instructions, just saying that I don't know the reason for it.)
Hi Gerald, I don't believe that the halide concentrations will behave differently than silver, since they are all results of the same mechanism. So as long as the silver content is in an "accpetable range," as per the manufacturer, so should the halides also be.
Originally Posted by Gerald C Koch
The spreadsheet-modeling that I just did, for build-up of stop bath in the fixers should also be valid for contaminants in the stop bath, such as developer residue. What I'm finding is this: if you can successfully run 4 cycles, you ought to be able to run 10 or more. In my model (depending on parameters, tank size and carry-over rate), whatever stop bath concentration exists in either fix bath on the 4th cycle, has only increased on the order about one percent (or less) by the 10th cycle.
ps: in my earlier example, modeling capacity differences between 1-stage and 2-stage fixing, my approximations were based on paper, not film. Film might typically have 5 or 10 times higher silver-loading than paper, so the ratios might shift around. I don't really know, since I didn't try it.