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September 2012

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From:
Robert Kondner <[log in to unmask]>
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Date:
Mon, 17 Sep 2012 12:54:26 -0400
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Brian,

 Eurocents or Drachmas? :-)

 About dross intermix with solder:

  I have no idea for sure but the density of dross seems so much lower than molten solder I would think the dross would rise to the top. Do we see any such process when a wave machine is turned off? The circulation pumps stops but I never recall seeing dross rise to the top? 

 Is there any chemical reason to expect dross to catalyze the formation of more dross?

Thanks,
Bob Kondner


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
Sent: Monday, September 17, 2012 11:36 AM
To: [log in to unmask]
Subject: Re: [TN] Recycled solder?

A couple of Eurocents worth and, here, I'm talking Sn/Pb as I have no experience of other alloys.

Alpha went to great lengths with Vaculoy, Multicore with Extrusol etc. 
What were these bars all about? Both processes were to reduce the dross in the bars to near-zero. When you first filled a pot with either, there was a minimal quantity of dross floating on the top. The wave was clean and stayed that way for quite some time. If you used either injected or surface water-soluble oil, you could stay almost dross-free for months (OK you had to skim the polymerised oil/dross residues, but very little solder was lost). Then, quite suddenly, dross quantities increased and the Sn:Pb ratio dropped. If you used a poor quality cast solder bar, with metallic impurities well within specs, you would get much larger dross quantities on both the first melt and in daily service.

So, what am I saying? It is my belief that dross begets dross or, to put it less biblically, microscopic dross particles in suspension in the solder, when they see oxygen, will cause the dross formation to accelerate. I speculate that there is a threshold effect, possibly dependent on the size of the suspended dross particles. This is not a scientific theory but an observational hypothesis.

Dross is a mechanical mixture of tin oxide and solder. Metal reduced from dross is tin-rich, which is why the pot is tin-poor after much removal of dross. IMO, metal recovered from dross should ideally be refined to virgin elements and re-alloyed before re-use.

The point I wish to make is that re-using the metal recovered from dross may contain more suspended dross than the pot and thus may be the cause for an acceleration of dross formation. It will probably be cheaper to send dross for true refining, rather than to reuse metal extracted from it.

That's 4 eurocents, not 2.

Brian



On 14/09/2012 16:56, Pete wrote:
> My understanding is that they will do the recycling themselves, on site.  They said the recycling would be mechanical, not chemical, so maybe it is just squeezing out the droww.  They also said it was an effort to be environmentally conscious, but they are in China, so I'd think it's really cost.
>
> We are among their smaller customers, so I can't make a lot of demands.  They asked if we would accept boards made with recycled solder.  I suppose the best I can do is reply with a list of concerns and ask for test results.
>
> Thanks!
>
> Pete
>

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