Aggregate production cost per ton

Why one number is not enough

Aggregate production cost per ton can be found at month end by dividing total spend by total tonnage. The division gives a correct average and supports no decision at all: when the number rises, nothing in it says which line item rose. What makes cost manageable is performing the same division component by component.

The method is simple. Each component is expressed as an hourly cost, and the total hourly cost is divided by the real hourly tonnage. The critical choice sits in the denominator: not rated capacity, but the tonnage actually produced and actually saleable during that shift.

The denominator: real tonnage

Catalogue capacity describes a plant running without interruption on its design feed. Real tonnage separates from it through three deductions: availability, feed fullness and the fraction nobody buys. When a plant produces a high hourly figure but banks a fraction with no demand, that tonnage does not share the cost, because stock generates no revenue. For this reason the denominator is built on despatchable output, and that single choice is the most consequential decision in the whole calculation.

The energy component

Energy is the easiest item to measure and the most frequently estimated instead. The correct method is a meter: the difference between the kWh reading at the start and the end of the shift, divided by the tonnage produced in that shift, gives kWh/t. That figure is the plant's own signature and moves with material, reduction ratio and circuit design.

To anticipate the order of magnitude, the comminution energy relation is used: W = 10 · Wi · (1/√P80 − 1/√F80), where W is specific energy in kWh/t, Wi is the material's work index, and F80 and P80 are the 80 percent passing sizes of feed and product in micrometres. The relation does not replace the meter on site. When the measured value sits clearly above the expected one, the usual causes are worn jaws or blow bars, oversize slipping into the feed, or excess recirculation.

Wear, labour, maintenance and capital

Wear part cost comes from part life, not part price: the total tonnage passed with one set of jaw plates, blow bars or mantles is what divides the price of that set. Where life is not logged the item stays a guess and almost always looks smaller than it is. Labour is the hourly burden of the crew assigned to the plant and does not fall when output falls, which is exactly why stoppages inflate labour per ton. Planned maintenance is its own line, spread from an annual figure over annual tonnage. Depreciation divides the investment by the tonnage the asset will produce across its economic life; dividing by tonnes rather than by years exposes how sharply a low-output year raises unit cost.

Component table

Running the calculation as one picture

Once the table is filled, two subtotals are drawn: variable cost per hour (energy, fuel, wear) and fixed cost per hour (labour, maintenance, depreciation). Variable cost shrinks when output falls; fixed cost does not. That split explains why a low-tonnage shift lifts unit cost out of all proportion, and it sets the order of improvement: downtime first, feed fullness second, unit price negotiation last. To run the same arithmetic repeatably, the fields on the operating cost calculator follow the same component order.

Common mistakes

The first is writing rated capacity into the denominator, which always flatters the result. The second is booking wear parts as an expense of the month they were bought; the set serves several months of production, so that month looks expensive and the following ones look wrongly cheap. The third is ignoring stock: an unsold fraction carries production cost while bringing no income. The fourth is stretching the reading interval to a year, because an item nobody reads at shift level goes unnoticed exactly when it changes.

Frequently asked questions: aggregate production cost per ton

Which component improves fastest

Usually downtime loss. Fixed cost keeps running while the plant is stopped, so every recovered operating hour lowers unit cost directly.

Why can kWh/t not be compared between plants

Because material resistance, feed size and target product size differ. The comparison is only meaningful inside one plant's own time series.

How often should the calculation be refreshed

Variable items per shift, fixed items monthly. The other pieces under investment and cost connect the same data to the investment decision.