# How to remove Cd from relays contacts silver



## gres (Aug 3, 2015)

Hi All, I have cca. 400g of Ag ? composition from hi power relays and contactors. They were soldered to massive brass "fingers" with solder that possibly contains Cd. I want process them in HNO3 and cement out with Cu. But have fear of Cd. Somewhere on this forum I read about Cd extraction from solution with NaHCO3, what time is better to add bicarbonate, before or after cementing ?


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## Westerngs (Aug 3, 2015)

Gres,

First of all, do not for any reason try to melt these contacts before separating the silver from the copper. Cadmium and cadmium oxide fumes are very toxic. Also, it is likely the contacts themselves contain cadmium as most electrical contacts do.

Dissolving in nitric is a good idea. Once dissolved, cement out the silver first, using copper as you planned. Once all the silver has cemented out, rinse it extremely well using distilled water to remove any trace of cadmium bearing solution. Once rinsed, the silver is ready for melting, or whatever you want to do with it.

The solution will now contain all the cadmium. I suggest adding lime to raise the pH to between 10.5 and 11. This is the optimum pH range for cadmium precipitation as hydroxide. I like adding lime instead of sodium hydroxide as it provides some bulk from undissolved calcium and magnesium oxides. Also, I find that sodium hydroxide sludges are more difficult to filter.

BEFORE you do any of these things, you need to figure out what you are going to do with the cadmium sludge. You will find that it is difficult to get rid of.


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## kurtak (Aug 4, 2015)

Westerngs said:


> Gres,
> 
> First of all, do not for any reason try to melt these contacts before separating the silver from the copper. Cadmium and cadmium oxide fumes are very toxic. Also, it is likely the contacts themselves contain cadmium as most electrical contacts do.



The Cd is in the solder for the most part not the points them selves - "some" points "may" have Cd alloyed with them



> Dissolving in nitric is a good idea. Once dissolved, cement out the silver first, using copper as you planned. Once all the silver has cemented out, rinse it extremely well using distilled water to remove any trace of cadmium bearing solution. Once rinsed, the silver is ready for melting, or whatever you want to do with it.



There is no need to use D-water to wash the silver cement - tap water is just fine to use - just be SURE you have cemented ALL the silver out of the solution with the copper - if there is still silver in the solution it (tap water) will create "some" silver chloride



> The solution will now contain all the cadmium. I suggest adding lime to raise the pH to between 10.5 and 11. This is the optimum pH range for cadmium precipitation as hydroxide. I like adding lime instead of sodium hydroxide as it provides some bulk from undissolved calcium and magnesium oxides. Also, I find that sodium hydroxide sludges are more difficult to filter.



This is not even close the proper procedure for treating the waste - the only thing here that is right is that the Cd is still in the nitrate solution --- after cementing the silver out with copper you now have a mostly copper nitrate solution with other base metals that includes the Cd & likely lead, tin, etc. from the solder & metals used in the alloy of the points

You want to first test your copper nitrate (after cementing the silver) to insure you got all the silver out AND as well to see if there is any lead --- take a sample (3 ml or so) put in small beaker & ad a couple drops HCl - if you get a white precipitate (& you most likely will) it could be any one of - or both - silver chloride &/or lead chloride

If you get the white cloud from adding the HCl - make your sample a bit bigger (50 - 100 ml) & dose it with HCl - let the precipitate settle - decant - now start washing the white precipitate with HOT water - the HOT water will dissolve the lead chloride & cause the silver chloride (if you didn't get it all in the cementing) to curd up & settle - decant the hot water wash "while still hot" into another beaker & be sure what you decant is clear - if there is lead it will re-precipitate as it cools

If you got silver chloride in this test (did not dissolve with the hot water wash) you need to cement longer (so do this testing before decanting &/or washing your silver cement - incase you need to cement more)

Once you are sure you have all the silver cemented out & if there is lead in the waste copper nitrate solution you next want to ad some sulfuric to the copper nitrate to drop the lead as lead sulfate - it doesn't take that much sulfuric to get the lead out - if you don't get the lead out it will hinder the next step (this is your waste steam & should include the original Cu nitrate decant as will as all the washes from washing your Ag cement)

Once you have the lead out (with the sulfuric) you then want to cement the copper & other base metals (Cd included) out with iron --- if you don't drop the lead out first the lead will passivate the iron preventing it from cementing the copper & other base metals - wash the nitrate (iron) out of these cemented metals

You should now have a nitrate that is only iron & because of all the washes ( silver first & then copper/base metals) lime is not going to work - its to dilute & you will be lucky to get the Ph to 4 or 5 let alone high enough to drop out the iron - NaOH is the way to go here which will drop the iron as its hydroxide --- filtering hydoxides are a real pain - the best is to set up a "wick" filtering system for this

The reason for following this method of dealing with your waste is that you CAN NOT send hydroxides &/or carbonates of copper, cadmium, nickel & some other base metals to the land fill - but iron hydroxide &/or carbonate can go to the land fill --- so the other metals need to be cemented out before treating with hydroxide or carbonate

Kurt


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## Anonymous (Aug 6, 2015)

Oh and do remember that Cadmium Nitrate can be absorbed through the skin. It's not the most pleasant of contaminants to absorb.


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## Westerngs (Aug 21, 2015)

Kurtak,

My experience with cadmium bearing silver electrical contacts is very different than yours. It is my experience that there is an abundance of electrical contacts that have cadmium as a component, 90/10 Ag/Cd electrical contacts being very common. The solder is but a small portion of the cadmium present.

It is true that tap water can be used to rinse cemented crystal. I prefer to use distilled or deionized and thus always state it by default. Further, it is understood that you need to check to make sure all the silver has cemented. It is not my intent to write the entire procedure out again as it is well covered elsewhere in the forum and I will not holds hands.

It is an incorrect statement that "metal hydroxides cannot be sent to the landfill". It is a true statement for carbonate sludges. A metal hydroxide sludge passes the EPA Method 3010 (commonly called TCLP) for leachability of metals. Meaning that the sludge will not leach metals and can thus be sent to a regular landfill. You will notice that I do not advocate this. 

We treat over one thousand gallons of waste water per day from silver refining, all is treated using lime to precipitate metal hydroxide sludge (after silver separation of course). No need to deal with each metal individually (such as lead). The resulting sludge passes TCLP, and we would be able to send it to the local landfill, but since we get paid for the copper content, we gladly send it to a copper smelter.

Lime slurries have a pH of 14 so it will easily neutralize any acidic solution. Lime sludges are also vastly easier to dewater than sodium hydroxide sludges, which tend to be gel-like.

For small scale refiners, I would collect the hydroxide sludges until I could find an easy way to get rid of them, such as smelt them into a metal bar (DO NOT DO THIS WITH CADMIUM SLUDGES).

By the way, following your method, which also works but is long and tedious, what do you do with the cemented metal that contains cadmium? Smelt it into a bar? For cadmium, you too would be stuck with the metal powder.


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