One month of zinc, reconciled
Most plants know their monthly tonnage. Fewer know how much zinc each tonne really took once the kettle level is corrected, how much left as dross and ash, and what the microns above the specification cost. Enter one month and get the scorecard, built on the formulas of the Zinc Consumption KPIs article.
Set the target gap from your own gauge scatter: wide enough that the lowest specimen average stays above the minimum on nearly every batch, and no wider. Keep reactive (Sandelin-range) steel out of the average.
Formulas and ceilings from the Zinc Consumption KPIs article (Revision 2.0): contained zinc at 95% for dross and 85% for dry ash; ceilings of about 15% dross and 14% ash of zinc consumed, used as limits, not targets. The product share is a residual, so every weighing error lands in it.
Where the month’s zinc went
Every kilogram of zinc that enters the plant ends up on the product, in bottom dross, in surface ash, in the bath as inventory, or in small unrecovered losses on jigs, hooks and the floor. The kettle level is the correction most often skipped and the largest: a few centimetres of level over a month on a large kettle moves the consumption figure by more than any process improvement. Measure it to the centimetre against a fixed mark, at the same bath temperature, every month-end.
Do not import a consumption benchmark; derive one. Zinc on the product is fixed by the surface area a tonne of your steel carries, about 255 ÷ section thickness in m²/t, and the coating you put on it. Add 0.5 to 1.2 percentage points for dross and ash and you have your own target. A plant above it has a level, weighing, dross or thickness gap, and the reconciliation shows which.
The ceilings, about 15% of zinc consumed for dross and 14% for ash, describe a population of plants. Below them is acceptable; above them is a red mark. They are two independent limits, not a split that adds up. And the largest controllable zinc cost is not dross or ash at all: it is the average coating thickness sitting above the specification, which has no recovery value.
Twelve months instead of one
The Plant Health Audit takes twelve months of your own figures — zinc, ash and dross, pickling acid, flux, fuel and rework — and returns a scorecard, your pickling baths on the Kleingarn diagram month by month, and a ranked list of what to fix first.
Questions
How is zinc consumption calculated?
Zinc consumed divided by steel galvanized, times 100. Zinc consumed is opening stock plus purchases plus alloy additions, minus closing stock, minus any increase in bath inventory. The bath inventory change is the level change in metres times the kettle’s internal length and width times about 6,620 kg/m³, the density of molten zinc at 450 °C. State whether the steel weight is black or white.
What is a good zinc consumption for a galvanizing plant?
Derive it from your work mix rather than importing one. Surface area per tonne is about 255 divided by the average section thickness in mm; multiply by the average coating in µm and by 7.14, divide by 10,000, and add 0.5 to 1.2 percentage points for dross and ash. For mixed job-shop work the observed field range is about 4 to 6% on white steel, level corrected.
How much dross and ash is too much?
The commonly cited figures, about 15% of zinc consumed for dross and about 14% for ash, are best used as ceilings: below them is acceptable, above them is a red mark. Express both as contained zinc, at about 95% zinc for dross and 85% for dry ash, and track kilograms per tonne of steel alongside.
What do extra microns of coating cost?
Extra zinc in kg per tonne is the excess thickness in µm times 7.14 times the surface area per tonne, divided by 1,000. On 8 mm work each 10 µm of unintended excess is about 2.3 kg of zinc per tonne. Thickness compliance stays at 100%; the target is only the excess above your own safe margin.