Winning the pH 4–5 Grind: Multi-Alloy Steel vs Hadfield in Acidic Slurry
Acidic wet grinding is where ordinary liner logic breaks down.
At one copper concentrator, a Φ3.2 × 3.1 m ball mill was grinding ore with a hardness factor of 6–8 and about 2% sulfur — which put the pulp at pH 4–5, hour after hour. In that environment you are not fighting wear or corrosion separately. You are fighting both at once, and they multiply each other.
Why Hadfield struggles here
Two mechanisms gang up on manganese steel in acidic slurry:
- The slurry film problem. A wet pulp layer coats the liner face and cushions the ball impacts. That sounds like a good thing until you remember Hadfield steel needs impact to work-harden. The film starves it. In acidic wet grinding, surface hardness — not impact response — decides the wear rate.
- Corrosion stripping. Every fresh abrasion scar exposes new metal to an acid bath, so the surface never gets a chance to settle.
The trial steel: CHMe multi-alloy
The replacement grade, CHMe, was engineered for exactly this duty:
| Element / Property | Value |
|---|---|
| C | 0.7–0.9 % |
| Cr | 2–4 % |
| Ni | 0.3–0.5 % |
| Mo | 0.4–0.6 % |
| Cu | 0.3–0.5 % |
| Heat treatment | 950 °C air quench + 500–550 °C × 2 h temper |
| Hardness / strength / toughness | HRC 46.2 / σb 1500 MPa / ak 55 J/cm² |
Note the toughness: 55 J/cm² is genuinely high for a 46 HRC steel. Wet mill liners take continuous ball strike, so dropping impact toughness to chase hardness is how you trade a wear problem for a breakage problem.
And the properties held in the actual casting — a 263 kg liner plate with 125 mm maximum thickness measured 49.5–52 HRC across the section. Uniformity like that only happens when hardenability and quench practice are both right.
196 days of receipts
The industrial trial ran 196 days — about 4,000 operating hours — and ground 203,400 t of ore. In the same campaign, at symmetric (identical-duty) positions:
| Metric | ZGMn13 Hadfield | CHMe multi-alloy steel |
|---|---|---|
| Condition at ~4,000 h | worn through | still in service |
| Relative life | 1× | 1.3× or better |
| Wear rate | baseline | ~3/4 of Hadfield |
| Economic effect | — | ~¥1,593 additional value per tonne of liners |
A life gain of 1.3× may read modest next to chamber-two numbers, but in pH 4–5 slurry with 2% sulfur, it is the difference between planning relines and chasing failures. And the accounting matters: at roughly ¥1,593 of extra grinding value per tonne of liner steel installed, the grade paid its own premium many times over.
If your pulp is acidic
A quick checklist we use when assessing acidic-duty liner jobs:
- Pulp pH below ~6? Corrosion is a first-order factor — pick a grade with Cr and no reliance on work-hardening
- Still measurable impact? Keep toughness above roughly 50 J/cm²
- Thick sections? Verify through-hardness on the actual casting, not a test coupon
QM Mill Liners produces multi-alloy wet-grinding liners for acidic duty and can cross-check your slurry chemistry against our grade selection table. Send pulp pH, ore sulfur content and the mill drawing — we’ll match a grade before we quote a price.
FAQ
What is the best mill liner material for acidic slurry? Low-alloy steels with 2–4 % chromium, air-hardened and tempered, generally outperform Hadfield in pH 4–6 slurries because they carry intrinsic hardness and better corrosion resistance. For more aggressive acid, higher-chromium grades enter the picture (see our ZG25Cr7 corrosion case).
How much does corrosion add to mill liner wear? In near-neutral pulp, modestly. In acidic pulp, corrosion and abrasion interact so total loss can reach several times the sum of the two acting alone. This is why simply hardening a steel does not fix acidic wear.
What liner life improvement did CHMe achieve? 1.3× or better against Hadfield at identical mill positions, with wear rate at roughly three-quarters of the Hadfield baseline.