At 600 °C you are at stone wool's ceiling (high-density wired mat, ~640 °C limit — verify grade) and inside the comfortable range of silica textiles (~1000 °C), microporous panels (~1000 °C) and ceramic fibre (~1200 °C). Typical choices: wired-mat + cladding for static surfaces if the grade's limit covers the duty with margin; silica-mat-core removable covers for components needing access; microporous where space is rationed; multi-layer (ceramic or silica hot face + stone wool outer) for furnace walls — the hot-face layer takes the temperature, the cheap outer layer does the bulk insulating.
Compute your own case: free heat-loss calculator · materials library: 12 datasheets · related: What is CUI (corrosion under insulation)? · What is economic thickness of insulation? · R-value vs k-value (λ) — what's the difference? · How hot can an exposed surface be for worker safety?
