SAG Mill Economics: Lower kWh/t Must Preserve Throughput and Recovery

AI-generated editorial illustration; it does not depict a verified project, actual prices or chart data.

1. Choose the correct objective

A SAG circuit should not be optimized for its electricity invoice alone. A change in media, liner geometry, speed or feed preparation can reduce kWh/t while reducing ore throughput or flotation recovery. The commercial objective is recoverable contribution over the bottleneck operating time. Establish whether the comparison covers the SAG mill only, the whole grinding circuit including pebble crushing, or the complete concentrator. Moving load into another motor is not a plant saving.

2. Evidence and market context

The DOE mining bandwidth study identifies grinding as an important efficiency opportunity but separates theoretical, practical and operating performance. Its older national estimates are not 2026 SAG design values. The World Bank October report shows copper at 14326 USD/t in August and 14474 in September; that modest benchmark increase cannot justify sacrificing recovery without a settlement calculation. The October EIA outlook is a dated forecast publication, not a quotation for a mine’s delivered electricity. No publicly verified site-specific August or September SAG service price is used here.

3. Worked operating comparison

Assume an illustrative baseline of 1000 t/h ore, 28 kWh/t and 6000 operating hours/year. Electricity is 168000000 kWh/year; at hypothetical 0.12 USD/kWh it costs 20160000 USD/year, or 3.36 USD/t ore. A modified duty of 950 t/h and 24 kWh/t uses 136800000 kWh/year and costs 16416000 USD/year, or 2.88 USD/t. The invoice reduction is 3744000 USD/year, but production falls by 300000 t ore/year. At an illustrative contribution of 20 USD/t of lost ore before the modeled grinding electricity and after other variable costs, the lost contribution is 6000000 USD/year: the modification worsens annual contribution by 2256000 USD before its capital cost.

4. Tariff sensitivity

For the modified duty, electricity cost is 2.4, 2.88 and 3.6 USD/t at tariffs 0.10, 0.12 and 0.15 USD/kWh. Do not apply this tariff sensitivity to the hypothetical 20 USD/t contribution without rebuilding its cost boundary. If downstream capacity already limits feed to 950 t/h, the apparent production loss may disappear; then compare both circuits at 950 t/h rather than crediting nonexistent baseline production. If retained ore can be processed later, value the timing and capacity constraint instead of treating all deferred metal as permanently lost.

Illustrative modified mill electricity cost per ore tonne at 24 kWh/t and tariffs 0.10, 0.12 and 0.15 USD/kWh. These are hypothetical site energy tariffs; throughput and recovery must be assessed separately.

5. Specify the trial and contract

Run a controlled campaign with ore hardness, feed size, water balance, liner condition and final grind-size distribution documented. Record throughput, true motor input energy, circulating load and recovery during stable periods. An OEM proposal should guarantee a defined duty and measurement method rather than a standalone efficiency percentage. Include exclusions for ore variability, wear, downtime and changes in upstream crushing. Reconcile the trial with downstream flotation; a finer grind can help liberation while harming selectivity or increasing reagent demand.

6. Limits of the comparison

This comparison excludes tax, demand charges, media, liners, maintenance, water, reagents and capital finance. Its 20 USD/t is an explicitly hypothetical contribution after other variable costs but before the grinding electricity modeled separately here, not the copper benchmark multiplied by grade. Avoid counting an avoided variable cost again inside the electricity saving. A meter reading during an unstable start or an empty-mill period is not a representative kWh/t benchmark. The best operating point can move as the ore body changes, so retain campaign data by ore domain.

7. Frequently asked questions

Does 24 kWh/t prove better economics than 28? No; throughput, recovery and other variable costs decide. Should the project use motor nameplate power? No; use integrated input energy and matching processed tonnes. Is the 3744000 USD saving project cash flow? It is an electricity invoice difference only; the example becomes negative after the specified lost contribution.

8. Sources and related analysis

• DOE Mining Energy Bandwidth Study
• World Bank October 2026 Pink Sheet
• EIA October 2026 STEO

Compare grade and recovery and transformer loss economics; use the ROI calculator after a controlled trial.