The Overweight Railcar and the Mill's Chemistry Limit
Your yard has loaded a 4,000 lb (net) gondola of shredded auto scrap for a mill whose spec sheet caps copper at 0.25% and requires a minimum bulk density of 60 lb/ft³. The shredder's eddy-current separator has been running with a widened split since Tuesday because the belt tracking drifted; the operator compensated by increasing air flow. The car was loaded Thursday and Friday. On Friday afternoon the scale house reports the car at 4,120 lb net, and the mill's inbound inspector calls to say the last three cars averaged 0.31% copper and that this car will be probed on arrival. Freight to the mill is $2,900 per car and the mill's downgrade penalty is $18 per gross ton plus a re-inspection fee of $450. You have 14 hours before the car must be pulled or it misses the mill's weekly melt window.
1. Diagnose the most likely cause of the copper excursion and decide whether to re-sort, re-ship as-is, or hold the car. Show your reasoning and the economics.
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The timing is the tell: the widened eddy-current split began Tuesday, and the excursion appears in cars loaded after that. A wider split throws more nonferrous (including copper-bearing radiators, wire, and motors) into the ferrous stream; raising air flow to compensate makes it worse by lifting light copper into the ferrous fraction. This is an equipment-setpoint failure, not a feedstock change, so the fix is to restore belt tracking and the correct split before any more material is made.
Economics: at 4,120 lb (about 2.06 gross tons) the downgrade penalty is roughly 2.06 × $18 ≈ $37, plus $450 re-inspection - small in absolute terms, but the mill has flagged three consecutive cars, and a rejected car costs freight both ways plus relationship capital.
Decision: hold the car, do not ship as-is. The 14-hour window is enough to pull the car, run the load back across a properly set eddy-current separator or hand-pick, and re-load. The re-sort labor is cheaper than a rejection and far cheaper than losing the mill's weekly allocation. If re-sort cannot finish in time, ship with a proactive call to the inspector disclosing the setpoint issue - never let the mill discover it cold.
2. The car is 4,120 lb net but the mill's purchase order was for 4,000 lb ± 2%. What is the weight position, and what should you do about the tare and the probe sample?
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4,000 lb ± 2% is a band of 3,920–4,080 lb. At 4,120 lb the car is 40 lb over the upper limit - about 1% over, so it is out of tolerance but not egregiously. The overage likely came from loading two days of shredder output without re-weighing between shifts, and possibly from an unverified tare.
Actions: (1) Confirm the tare - a gondola's light weight can drift with retained debris, scale drift, or a mis-keyed tare; re-weigh the empty car or verify against the last certified tare. (2) If the net is genuinely 4,120 lb, the mill may accept it but will note the overage; the 40 lb excess is trivial in dollars but signals loose process control. (3) On the probe: the mill probes a car, not a whole lot, so a single probe can be unrepresentative. Ask that the probe be drawn from multiple locations and that you receive the assay. If the assay confirms >0.25% Cu, accept the downgrade rather than contest it - contesting a valid assay damages the relationship more than the $37 penalty.