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<p>[QUOTE=&quot;calcol, post: 2485822, member: 77639&quot;]First, let&#039;s dispose of the exploding pennies theory.&nbsp; The thin layer of copper on the outside is not going not going to contain the expanding zinc.&nbsp; And yes, zinc does have a greater coefficient of thermal expansion than copper.&nbsp; The copper will just bubble and crack, not explode.&nbsp; And then it dissolves in the melting zinc.&nbsp; Disagree?&nbsp; Well, look at the pic.&nbsp; It&#039;s the remains of five zinc cents that had a propane torch briefly applied and was then stirred with a screw driver before it solidified again.&nbsp; Copper all gone.&nbsp; Actually, it&#039;s still there as part of a very low-grade brass.&nbsp; The gray/purple slag on top should be mostly zinc oxide and copper oxide.&nbsp; There are ways to minimize slag production, not used here though.</p><p><br /></p><p>Recyclers won&#039;t care.&nbsp; They&#039;ll be delighted to pay zinc prices for copper, mixed-in with zinc or not.&nbsp; So, from this point on, I&#039;m going to ignore the copper and consider the cents as pure zinc.</p><p><br /></p><p>Scrap buyers are very good at determining the approximate composition of metal without expensive assays.&nbsp; And we&#039;re talking about zinc, the cheapest metal except for cheap steels and lead.&nbsp; All you need to distinguish among these three is a magnet and pocket knife.&nbsp; However, scrap buyers can do it with a quick glance.&nbsp; If they make a mistake and the scrap turns out to be a more valuable metal, so much the better for them.</p><p><br /></p><p>Now, let&#039;s do some economics, and we&#039;ll ball-park it at first so as to stay in the realms of reality.</p><p><br /></p><p>Zinc ingots go for about $1/lb these days, and chances are that includes some profit for mines or recyclers.&nbsp; So any financial calculations that produce a price far above that, even for small scale operations, should be a red flag.&nbsp; So, our $10K face of pennies weighs 5500 lbs and supposedly will cost $13K to recycle.&nbsp; That&#039;s $2.36 per lb; flags are flying.</p><p><br /></p><p>Before I mention energy, let&#039;s look at transportation.&nbsp; $2343 to haul it 35 miles?&nbsp; Yee-ha!&nbsp; Tell me who&#039;s paying that to haul 5500 lbs 35 miles, and I&#039;ll be there with my truck and trailer.&nbsp; We routinely haul that much weight with my 1977 F250 and stock trailer and maybe get 7-10 miles to a gallon of gas.&nbsp; Sign me up!</p><p><br /></p><p>OK, now for energy for melting.&nbsp; Ingots of zinc don&#039;t spring fully formed from the ground or recycling plants.&nbsp; The zinc was melted, probably more than once, for those ingots to go out the door.&nbsp; So, the energy cost for a single melt has to be less than $1/lb, probably a lot less in view of the other costs of production.</p><p><br /></p><p>I&#039;ll do the calculation using propane, although there are cheaper sources of energy (e.g. coal and natural gas) available in some areas.&nbsp; We have 2500 kg (5500 lbs) of zinc to melt.&nbsp; Specific heat of zinc is 0.39 Joule/gC or 390 J/kgC; i.e. we have to pump 390 Joules of energy into 1 kg of zinc to raise its temp by 1 degree C.&nbsp; Zinc melts at 420 C, and we&#039;ll assume we start at 20 C , so there&#039;s 400 C to raise.&nbsp; Needed energy is 2500 x 390 x 400 = 390,000,000 J which is 390 MJ (megaJoules).</p><p><br /></p><p>A gallon of LP (liquid propane) has a combustion energy of 91,330 BTU per gallon (engineeringtoolbox.com), and one BTU is 0.001056 MJ (unitjuggler.com).&nbsp; So,</p><p><br /></p><p>0.001056 MJ/BTU x 91,330 BTU/gal = 96.4 MJ per gallon of LP.&nbsp; We need 390 MJ to melt our zinc, so that&#039;s 390 MJ / 96.4 MJ per gal = 4 gallons of LP.&nbsp; I&#039;m paying about $2.20/gal for LP right now, so that&#039;s $9 of LP to melt the zinc.</p><p><br /></p><p>I doubt it was a nickel&#039;s worth of propane to melt the 5 pennies and that was done very inefficiently.&nbsp; Most of the heat went into heating air and the brick upon which they sat.</p><p><br /></p><p>2500 kg (5500 lbs) of zinc has a pretty small volume (density of zinc is 7.1 kg/L); it&#039;s 12.3 cu ft or 0.46 cu yard.&nbsp; There&#039;s a galvanizing plant near me with a 800 cu ft tank of molten zinc.&nbsp; It would take 65 lots of 5500 lbs to fill it.&nbsp; If each lot took $9K of energy to melt, that would be 65 x $9K = $585K to melt it all.&nbsp; No way could they stay in business at that rate.</p><p><br /></p><p>Even on a small scale, it&#039;s possible to use excess energy from other processes to melt zinc and to re-capture energy used to melt zinc.&nbsp; An artist friend of mine has a small gas-fired kiln, but it&#039;s big enough to crawl into.&nbsp; Most of the energy is used to heat the kiln structure.&nbsp; Setting a pot of pennies in it with the artworks would take very little extra gas. Another example: If you have two melting ovens for zinc, you fire one until the zinc melts.&nbsp; Then you start the other and draw combustion air for the second through the first.&nbsp; Cools the first more quickly, and heats the second more quickly and efficiently.&nbsp; Just keep alternating.&nbsp; Countercurrent systems are used for large furnaces.&nbsp; Incoming combustion air is heated by exhaust gases.</p><p><br /></p><p>Will we get to the point that melting pennies is profitable?&nbsp; I&#039;d guess the government will discontinue cents in their present form before then.</p><p><br /></p><p>Cal</p><p>[ATTACH=full]525268[/ATTACH][/QUOTE]</p><p><br /></p>

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