# Removing cemented values from solid precipitants



## Marcel (Oct 17, 2012)

I have experimented a little with various methods of precipitation.

I started with SMB. 
Results: 

*SMB (=Sodiummetadisulfit/germ. Natriumdisulfit)*

Plus:
[*]Gold comes selectively down nicely if all steps of the recovering are well done and clean.
[*]Process is very fast
[*]Dissolving SMB in Water will make the smallest particles precipitate fast because of the excellent distribution of the precipitant
[*]Indicates end of precipitation by changing to clear solution (SC test should nevertheless be performed)
[*]Works in AR and HC bleach process as well as in HAP (Hot AP)
Minus:
[*]Amount of SMB has to be estimated to avoid values remaining in solution (too little SMB)
[*]Overdosing can cause false positive Stannous Chloride results
[*]Overdosing will take long to evaporate nasty sulfuric gas fumes 
[*]Expensive
[*]Hard to get in some areas
[*]Dangerous unhealthy fumes (similiar to nerve gas!)
[*]Can become old and useless (=>Sodiummetabisulfit)

*Zincpowder:*

Plus:
[*]Very fast precipitation almost as effective as SMB.
[*]No dangerous fumes!
Minus:
[*]Strong exothermal reaction
[*]Will redisolve PMs (read Update below)
[*]Solution can boil over
[*]Excess zinc powder has to be redisolved in a second step
[*]Precipitates all metals from zinc upwards in the electrochemical chart. Copper can be precipitated as well if present. Need for cleaning solids in a second step.

*Copperpowder:*

Plus:
[*]Medium to low speed precipitation. Almost as effective as SMB (yield)
[*]Mild exothermal reaction
[*]Precipitates only gold and above in the electrochemical chart.
Minus
[*]Amount of Cu powder has to be estimated to avoid values remaining in solution(underdose)
[*]Amount of Cu powder has to be estimated or remaining copperpowder has to be removed by second step using dilute nitric, AP etc.(overdose)
[*]Colorchange to green is not a reliable Indicator of the end of precipitation (SC test is obligartory)

*Coppersheets/Copperwire etc:*

Plus:
[*]Secure precipitation without watching the process. Precipitation will come to a standstill as soon as all PMs have been dropped.
[*]Will only drop gold and above
Minus:
[*]Process takes much longer (overnight)
[*]Result is a much finer powder, Sometimes hard to settle and to handle/filter etc.
[*]Black precipitant will passivate the surface, causing further slowing down over time and full precipitation.
[*]Valuelosses due to cementation of the solid copper object



Now from these finding I have developed my own precipitation path. Which looks like this:

1.Using zinc powder, the majority of the PMs are dropped. Zinc powder is added until no more reaction (foaming, fizzling) occours. Solution is agitated using a glassrod or else to ensure all zinc powder has gone into solution. A state of underdosing is desired!

2.Precipitated metals are seperated (filtered or decanted) from the solution. PM metals are put aside and treated as pre-final (wash, rinse, redisolve etc) - Business as usual.

3.Copper wire formed to a spiral is added to the remaining solution.

4.The beaker with the solution is put in an ultrasonic cleaning bath.Heated water will increase speed tremendously. From time to time the ultrasonic is turned on. Cemented values will be blown off the copper and sink down as black precipitants. The copper will become clean again. Over the time new material will be collected at the copper. Turn on the ultrasonic and remove those passivations.


An alternative is to start right away with the copper wire and use the ultrasonic to keep the cementation of slowing down the process.

I have achieved excellent yields with the copperwire/ultrasonic method. It combines the best of the powder and the solid precipitants.
Precipitation now only takes minutes instead of hours. There is no overdose/underdose issue anymore. The solution is constantly agitaded by the ultrasonic force.
Once the spiral remains clean the precipitation is over!

The advantage of the zinc-copper two stage precipitation for me is, that you drop the majority of the PMs and while the rest is dropped - unwatched - using the ultrasonic, you can already process that first batch of powder. So you dont have to wait over night on the one hand and you still dont have to worry about overdosing/underdosing the process which can take another day to fix.
I tested this with gold and palladium.
The key idea is to use a solid precipitant and use the ultrasonic to constantly clean the surface of the copper while the precipitation is going on. This combine speed, ease and yield of process.
Any suggestion or correction is welcome.

UPDATE: I have found a problem when using zinc powder as preciptant:
As the zinc is very reactive it causes a very strong exothermal reaction. After the values have been dropped they look black due to the zincpowder that is adhered to them.
Now if a HCl wash is performed, fine zinc powder will react with the HCl on a very high energetic level- causing heat. This heat will redissolve some of the values precipitated and make it almost impossible to get them dropped in one go.
I found this effect very strong with palladium - since it is even further away from zinc in the rectivity chart than gold.
So remember to either keep the washing solution, remove all solids and precipitate once more. Or use the copper method instead to precipitate the washing solution, since this metal is closer to PMs in the reactivity chart and reaction will be slower but alos cooler, preventing the PMs to redissolve. Dont add H2O2 for washes! It will make gold and palldium redisolve if low Bms are also present exspecially zinc, iron,aluminum etc.
Marcel


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